Your SlideShare is downloading. ×
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Centrifugation
Upcoming SlideShare
Loading in...5
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×
Saving this for later? Get the SlideShare app to save on your phone or tablet. Read anywhere, anytime – even offline.
Text the download link to your phone
Standard text messaging rates apply

Centrifugation

2,402

Published on

SYNOPSIS …

SYNOPSIS
The principles underlying centrifugal separation of particulate species are briefly considered, and the main types of separator available are noted. The procedures available for scale-up from laboratory or semi-technical data are then discussed in detail with particular reference to perhaps the most important class of machine for fine particle processing: the disc-nozzle centrifuge.

Starting with the basic concepts behind their design, discussion follows to explain the factors which may limit centrifuge performance. It is shown how a few simple; laboratory scale tests can give a valuable insight into the design and operation of full-scale industrial machines.

Published in: Technology, Business
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total Views
2,402
On Slideshare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
Downloads
456
Comments
0
Likes
1
Embeds 0
No embeds

Report content
Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
No notes for slide

Transcript

  • 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE SPG PEG 304 CENTRIFUGATION Process Information Disclaimer Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss, damage or personnel injury caused or resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 2. Process Engineering Guide: Centrifugation CONTENTS Synopsis 1 INTRODUCTION 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Scope of the Section Centrifuge Types What is a Disc Centrifuge and How Does It Work? Operating Regimes – What Limits the Machine’s Output? 1.4.1 The Mechanics of separation 1.4.2 Nozzle Characteristics 1.4.2.1 Effect of Solids Throughput 1.4.2.2 Effect of Nozzle Diameter 1.4.2.3 Theory of Separation Thickening capability (solids-limitation) Clarification capability (Hydraulic limitation) Hindered settling (Flux limitation) Comments 2 USEFUL LABORATORY TESTS 2.1 Background 2.1.1 Basic Suspension Properties 2.1.2 Settling Kinetics 2.1.3 Network Strength Measurements 2.1.4 Floc Stability Assessment Test Techniques 2.2 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 3. 3 Scale-up Theories 3.1 Scale-Up When Clarification is the Limiting Process 3.1.1 Qualitative 3.1.2 Quantitative 3.1.3 Validity 3.1.4 Application 3.2 Scale-Up When Thickening is the Limiting Process 3.2.1 Background 3.2.2 Scale-Up Factor 3.2.3 Three Methods of Application 3.2.3.1 Semi-Empirical Approach I 3.2.3.2 Semi-Empirical Approach II 3.2.3.3 Ab Initio Approach 3.2.4 Estimation of Necessary Parameters and the Problems Encountered 3.2.4.1 Relationship between ṫ and P 3.2.4.2 Volume of Slurry in centrifuge 3.2.4.3 Pressure (or “field”) Factor 3.2.4.3 Concept of Ultimate Thickness 3.3 Scale-Up When Hindered is the Limiting Process 3.3.1 Background 3.3.2 Gravity Thickeners – Behavior 3.3.3 Batch Thickeners 3.3.3.1 Continuous Thickeners 3.3.4 Gravity Thickeners – Interpretation of Batch Settling Test 3.3.5 Application to Centrifuges 3.3.5.1 Batch Flux Curve 3.3.6 Discussion Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 4. 4 A SYSTEMATIC APPROACH TOCENTRIFUGE SCALE-UP 4.1 Validity of Different Methods Table 1: Table 2: Table 3: Table 4: Table 5: Table 6: Table 7: Steps in Ab Initio Scale-Up for Clarification Limited Systems Steps in Semi-Empirical Scale-Up for Clarification Limited Systems Prediction of Ultimate Thickness for Solids Limited Systems Steps in Semi-Empirical Scale-Up for Solids Limited Systems Steps in Ab Initio Scale-Up for Solids Limited Systems Steps in Semi-Empirical Scale-Up for System Limited by Hindered Settling Systems Steps in Ab Initio Guide Calculations for Systems Limited by Hindered Settling Systems 5 Worked Examples 5.1 5.2 Solids-Limited Scale-Up: Background to Examples Solids-Limited Examples: Semi-Empirical and Ab Initio Methods 5.2.1 Semi-Empirical Approach to Solids Limited Operation (simplified method) 5.2.2 Standard Semi-Empirical Approach 5.2.3 Ab Initio Approach to Solids Limitation 5.3 Calculation of Thickening Ultimate Limit Example: “Structural” limit for Thickening in “Pruteen” Centrifugation Scales-Up for Clarification Limited Systems 5.4.1 Ab Initio Clarification Calculation for P2 Type Single Cell Protein 5.4.2 (Ab Initio) Clarification Limit for Unflocculated Bacterial Cells 5.4.3 Semi-Empirical Scale-Up for Clarification Limited Systems Ab Initio Calculation for Hindered Settling Limitation Comments 5.4 5.5 5.6 REFERNCES Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 5. SYNOPSIS The principles underlying centrifugal separation of particulate species are briefly considered, and the main types of separator available are noted. The procedures available for scale-up from laboratory or semi-technical data are then discussed in detail with particular reference to perhaps the most important class of machine for fine particle processing: the disc-nozzle centrifuge. Starting with the basic concepts behind their design, discussion follows to explain the factors which may limit centrifuge performance. It is shown how a few simple, laboratory scale tests can give a valuable insight into the design and operation of full-scale industrial machines. Various scale-up theories are presented and worked samples given, using data drawn from GBHE Agricultural Division experience in biological separations. 1 INTRODUCTION 1.1 Scope of the Section Centrifugal separation of solids is governed by the action of a centrifugal field in either enhancing settling of particles or promoting deliquoring by filtration. The former effect is the more important for fine particle processing (species < 10 µm in size > and thus we have chosen to focus attention on such operations in this section. Filtration in general is considered in GBHE SPG PEG 300 Filtration. The discussion is centered upon the scale-up and operation of (continuous) disc-nozzle machines: a significant amount of data is available for this very important class of centrifuges to demonstrate the strength and limitations of scale-up procedures. It is considered that many of the concepts described can be readily extended to batch or intermittently discharging machines, and also to Centrifuges of radically different design, such as decanters. However at the current time there are insufficient working data to warrant detailed discussion of the use of scale-up techniques in these other circumstances. To exemplify the various test and calculation methods we have selected a number of “case histories*' from bioseparation experience at GBHE. This is one of the most demanding duties conceivable for a centrifuge due to the small particle size and the cohesive nature of many of the suspensions concerned. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 6. 1.2 Centrifuge Types In centrifugal separators the rate and degree of disengagement of solids from a liquid medium is enhanced by a centrifugal field due to rotation of portions of the machine. Equipment of this type falls into two main classes: “centrifuge filters” and settling centrifuges”. In “centrifugal filters” the phenomenon which gives concentration of the solids is a filtration mechanism. The particles are trapped by a filter medium of some description whilst passage of the liquid is accelerated by the field. Estimates of the performance of such operations are made by appropriate modification of standard filtration theory, based on Darcy’s Law (see Sections 3.2 and 3.5)) to allow for the “g” force. Procedures of this kind are described in references such as [1], [7] and [10]. Though far from satisfactory, these predictive techniques are the best that are currently available. As a rule centrifugal filters are not used for separation of very fine particles: about 10 µm is the usual lower limit on particle size whilst in most applications for which filtering centrifuges are employed the species are =: 100 µm or greater. Accordingly, such processes are on the margin of the scope of this manual and will not be further considered. In centrifuges based upon a settling principle, it is migration of the solid particles through the liquid phase that is enhanced by the centrifugal field. As will be seen later, this may involve either sedimentation of single particles in a Stoke’s Law type of “free fall” or consolidation of a structured (thickened> sludge. As noted above, settling rather than filtering centrifuges are appropriate for fine species; particles down to the order of 0.01 µm can be dealt with in appropriate circumstances. Machines come in very many designs – disc nozzles, decanters and so on - all of which have their particular operating regimes. Details of the principal kinds of settling centrifuge are given in reference [10]. It should also be noted that centrifuges may have intermittent or continuous solids discharge or may be operated in batch mode. Figure 1 represents schematically the continuous disc-nozzle centrifuge which Is employed extensively in fine particle separations. Feed Is introduced into the many separation channels via distribution holes In the stack of discs. The whole assembly is rotating at high speed and the strong centrifugal field produced flings solids to the outside of the bowl. Here they are discharged through nozzles. Hydrostatic pressure forces the less-dense liquid phase to flow against the gravitational field and towards the centre of the machine where it overflows a level-controlling weir. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 7. Figure 1 In this section we will focus our discussion on this kind of process. We will consider the way that the output of a given centrifuge is limited by one of three main mechanisms, viz: (i) Failure to capture the fine particles contained in the otherwise clear centrate. (ii) Failure to give sufficient time for the concentrated stream to adequately thicken. (iii) Flux limitation caused by particle:particle interaction. The first of these is likely to be the limiting mechanism when centrifuging weak suspensions of discrete, compact particles of small diameter. The second occurs when attempting to obtain a thick concentrate from an open, flocculated system having a tendency to form a cohesive structure at low concentrations. The third concerns a transition region between particle capture and thicks consolidation and is most likely to occur with a dilute feed of slowly sedimenting particles. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 8. It will be demonstrated how a knowledge of the free fall velocity under a known gravitational pull, the resistance to compression of the thickened matrix and some typical centrifuge parameters (dimensions, speed, etc) allow a ready assessment of the likely limiting mechanism. A strobe-centrifuge test, a measurement of elastic modulus and some machine specifications are (theoretically) all that are required. Armed with these data it is then possible to predict the performance which would be obtained on any commercial centrifuge. An insight into the limiting mechanism will also allow a quantitative assessment of the benefit of modifying process conditions (e.g. feed concentration > or machine parameters (e.g. number of discs) without the need for time consuming, expensive trials. An informed judgment on the suitability of centrifugation for a given duty can also be readily obtained. Currently, the battery of scale-up techniques available for disc-nozzle machines have not been properly extended to, for example, decanters. It is probable that the concepts of Section 3.4.3 are applicable in such circumstances. Further work In this area will be desirable In the future, especially as existing procedures, based exclusively on Ʃ theory (see Section 3.4.3 and reference [16] ), are obviously inadequate. In particular, decanters are perhaps at best advantage for thickening to high solids (when their method of solids discharge is more appropriate than that of the disc-nozzle design). In such a regime it will be probable that the process Is solids rather than clarification limited as is assumed in the Ʃ approach. 1.3 What is a Disc Centrifuge and How Does It Work? First, consider a simple clarifying tank in which a single, dense particle is settling out under the force of gravity: Figure 2 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 9. The particle will have a downwards velocity of ug m/s (terminal velocity for free fall under gravity). It will also be carried along horizontally by the bulk liquid at velocity v = L/bh m/s. In order to reach the solids discharge point, the particle must have fallen distance h before traversing distance 1 Therefore the maximum feed rate that the tank can clarify is given by i.e. Max Liquid Rate = Tank Surface Area x Settling Velocity under gravity The performance of the tank can be improved in one of two ways: (i) By increasing the gravitational field. An n-fold increase will give an n-fold Increase in settling velocity and hence allow an n-fold increase In throughput. (ii) B y reducing t h e height the particle has to fall, e.g. by partitioning the tank as shown below: Figure 3 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 10. Drawn apart for clarity, this arrangement effectively gives twice the tank area for the same physical volume, ie the tank separation capacity has been doubled. Taking the above comments to their logical conclusion would give a conceptual centrifuge design as below, In which a series of concentric cylinders (5 tanks) are placed In an enhanced (rotational) gravitational field. Figure 4 This is not a practical arrangement because there is no convenient means of separating solid and liquid flows. In practice, discs are Inclined to the gravitational field with the feed entering through holes in the discs. The solids migrate along the underside of the discs to the bowl periphery whilst the clarified supernatant flows inwards. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 11. Figure 5 Liquid inventory in the centrifuge is kept constant by a weir arrangement as shown below, the liquid finding Its own level with respect to the applied gravitational field. The thickened sol ids discharge through a number of nozzles at the periphery of the bowl, the discharge rate being controlled (for some machines) by varying the nozzle diameter; less sophisticated machines may not have on-line control but different sizes of nozzles will generally be available. It will also be noted that the feed enters the machine axially, typically from above. Ribbed rotating surfaces in the feed zone help accelerate the Incoming liquid to bowl velocity. Any necessary further acceleration will be imparted by the rotating disc-stack. As a further variation, the solids may be transferred through tubes from the bowl apex back towards the centre of the machine before reaching a nozzle and being discharged. The reduced hydrostatic pressure here (smaller radius of rotation and lower liquid depth > mean that larger nozzles can be used for the same discharge rate, thus reducing the risk of blockage by thickened material. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 12. Figure 6 1.4 Operating Regimes – What Limits the Machine’s Output? 1.4.1 The Mechanics of separation Despite engineering complexities (see Figure 6) the mechanics of separation in a disc-nozzle centrifuge are straightforward. Feed Introduced to the disc stack Is conveyed to the periphery by the strong centrifugal field consequent on the rotation of the disc stack and bowl. The thickened solids are discharged through nozzles whilst the less dense liquid is forced out at the weir by hydrostatic pressure. The specific capacity of a centrifuge to concentrate a particular suspension is however determined by details of the machine's geometry and other characteristics: the properties of the feed, and the degree of thickening needed for a given duty. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 13. 1.4.2 Nozzle Characteristics 1.4.2.1 Effect of Solids Throughput It is important to realize that, for a given rotational speed and nozzle diameter, the volumetric discharge rate of thicks through each nozzle is fixed. This Is because the pressure drop across the nozzle is determined solely by the liquid depth and centrifugal field In the machine. Increasing either the feed rate or feed concentration will increase the quantity of solids to be discharged in this constant volume, and hence increase the thicks concentration. This represents one of those rare occasions when driving a piece of equipment harder will improve its performance! Eventually, however, the machine's separation capacity will be exceeded (for one of a number of reasons to be discussed later) and solids will be lost into the centrate. The point at which this occurs is called the breakthrough point. 1.4.2.2 Effect of Nozzle Diameter Increasing the nozzle diameter (or the number of nozzles) will allow a greater volumetric rate of thicks to pass. It is now possible to reach a new breakthrough point where the solids throughput is higher. Generally, however, the solids concentration will have been reduced (see later). The locus of breakthrough points which can be achieved by changing the nozzle diameter Is termed the breakthrough curve. By using a variable diameter nozzle it is possible to operate with clear centrate at any point on or below this curve. The above points are sketched below, where thicks concentration is plotted against solids throughput. The latter represents a measure of both feed rate and concentration. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 14. Figure 7 1.4.2.3 Theory of Separation The detailed hydraulics of high-speed centrifuges are horrendous. Even “simplified” treatments are often mathematically and conceptually complex despite the gross assumptions that are usually made. Only concepts directly relevant to separation power will therefore be discussed. For the purposes of many applications the separating power of a centrifuge may be defined as the rate and concentration of thicks attainable at a given solids throughput while maintaining a nominally clear centrate. This immediately suggests two separate mechanisms which could limit centrifuge performance, i.e. the ability to thicken concentrate and the ability to clarify centrate. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 15. 1.5 Thickening capability (solids-limitation) When solids are flung to the bowl periphery they will rapidly form a structured mass from which supernatant has to be squeezed. The rate and extent to which this compression can occur will be roughly proportional to the applied force - a greater gravitational field will speed up the thickening process and also Increase the maximum concentration that can be achieved at low rate (the ultimate thickening or “structural” limit of Section 3.2). Figure 8 The gravitational field increases with the square of the rotational speed and is proportional to the radial distance from the axis of the machine. The time available for compaction will be roughly proportional to the volume of the peripheral space and is inversely proportional to throughput. Two points then follow:(1) A larger machine will generally pull a greater gravitational field and so improve the degree of thickening potentially available. (2) Increasing the throughput will decrease the residence time and reduce the actual thickening achieved. This is one possible cause of the trade-off between rate and concentration referred to earlier. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 16. Relatively quick laboratory-scale stroboscopic centrifuge and pulse shearometer Tests allow the compaction characteristics of the solids to be readily quantified (see Section 3.4.2). A knowledge of a machine's dimensions then allows a reasonable prediction of its thickening potential. Wlth true solids-limited operation the feed concentration entering the centrifuge is relatively unimportant. Compression times tend to be dominated by the residence time available at the higher concentrations (where resistance to further compaction is greatest). The disc stack is also comparatively unimportant for solids-limited applications; the discs provide surface area for clarification rather than residence times for solids compaction. If a machine is thought to be solids limited, there Is therefore little point In Installing a pre-thickening stage or modifying the discs. 1.6 Clarification capability (Hydraulic limitation) The following sketch illustrates the paths followed by particles in the disc stack. Figure 9 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 17. The gravitational field throws the particles onto the upper disc where they are captured and can subsequently migrate to the bowl periphery. As shown, particle E just fails to reach the disc before being swept out with the centrate; all other particles are captured. Increasing the centrate flow will increasingly sweep more particles out In the centrate stream, le the breakthrough point has just been passed. This clarification limit depends on centrate flowrate and therefore the machine's solids handling capacity will be strongly dependent on feed concentration. Disc stack configuration is similarly important: doubling the number of discs will roughly halve the distance each particle has to travel before being captured. This will theoretically double machine capacity, although other effects, e.g. disc thickness, flow maldistribution, etc, may be significant. Clarification limitation can be readily identified If different feed concentrations are fed to a machine because a whole family of breakthrough curves will be obtained, one for each feed. This follows from a simple mass balance If It Is recognized that breakthrough will always occur at a constant centrate rate. A comparison between solids and clarification limitation Is shown below: Figure 10 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 18. A knowledge of the terminal velocity of particles falling through supernatant under a known g-force should allow a quick estimate of the clarification power of a given centrifuge. Such knowledge can be obtained from a strobe centrifuge test (see later). 1.7 Hindered settling (Flux limitation) This limitation is less obvious than the previous two but may be equally as important. It is a well known effect in gravitational thickeners (see Section 3.3.4(c)). Settling of solids through supernatant requires particles to move relative to liquid. The relative velocity is generally proportional to the applied gravitational field but decreases as the solids concentration builds up. This latter phenomenon is called “hindered settling”. It results from increasingly significant effects of hydrodynamic interaction between settling species as the average distance between them is reduced by rising solids content. For a constant volumetric flowrate of fluid the solids flux must, in the absence of particle-particle Interactions, be proportional to solids concentration. If hindered settling is taken into account, however, It can be shown that there is an absolute maximum to the solids flux that can be obtained. The effect is typified in gravity settlers by a critical concentration which fills the settler; solids In excess of the maximum flux are lost to the clarified liquor. The critical concentration reached depends on the feed concentration but will typically be before compression effects come into play. There are two possible places where a maximum solids flux may limit centrifuge performance: (1) In the disc stack where solids are settling out of the clarifying centrate. (2) In the bowl periphery where solids are migrating towards the nozzles. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 19. Analysis is complicated by the geometry of centrifuges and the lack of knowledge of flow regimes which pertain. Considerable work on point (1) was carried out in 1979 C [18] which showed that the net effect is not dissimilar from clarification limitation. A full analysis of point (2) has yet to be completed. Further work in this area is required. However, there is no fundamental reason why the maximum solids handling capacity of a given machine should not be predicted from knowledge of particle terminal velocity as a function of concentration. Such information should be yielded by a strobe centrifuge test. 1.8 Comments (1) The underlying principles of centrifuge operation are relatively easy to comprehend even If the mathematics are frightening. (2) A few strobe centrifuge and pulse shearometer tests of material on a laboratory scale can theoretically yield all the information required to predict centrifuge performance. Extra rheological measurements aid interpretation and provide a useful check on the validity of the models used. (3) Reality is actually much more complicated than the previous discussion suggests (maldistribution, vortex formation, counter-current shear, etc). A truly reliable theoretical model is therefore a remote possibility. However, the general principles discussed should hold to some extent and an estimate of likely centrifuge limitations should be possible. In subsequent sections we detail useful laboratory tests for prediction of centrifuge performance; develop scale-up theories; and describe and exemplify the procedures necessary for their application. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 20. 2 2.1 USEFUL LABORATORY TESTS Background The centrifugation characteristics of a particular suspension may, of course, be assessed empirically by carrying out trial runs in a centrifuge of similar design to that proposed for use in the separation process. Alternatively, and generally more desirably, various procedures can be employed to characterize the material in the laboratory and estimate behavior over a range of regimes. The second route Is, as a rule, less expensive and time-consuming and it gives a predictive capacity unattainable by the empirical approach. Also, when novel solids separation problems appear, it provides the only method of obtaining order-ofmagnitude values for centrifuge requirements, other than a completely "hit or miss" series of tests with a variety of machines. We would caution, however, that complete dependence on laboratory characterization is unwise: because of the complexity of the effects involved in centrifugation It is prudent to back up smallscale assessment of suspension behavior with at least a few semi-technical runs to test predictions. The laboratory tests used to characterize the centrifugation of a suspension are relatively few in number, and simple in concept. They are best understood by reference to the basic phenomena underlying all separations dependent upon solids settling, whether accelerated or not. Figures 11 and 12 illustrate these effects. The first of the diagrams shows typical settling curves under normal gravity and under enhanced "g". In the Initial stages the sedimentation graph tends to be linear, then settling slows as particle interaction or "crowding" occurs. Eventually, solids content reaches a limiting value as the forces promoting consolidation are balanced by the internal strength of the sediment. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 21. Figure 11 Typical settling curves for a suspension sedimenting under "lg" and "ng” Figure 12 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 22. Modulus (i.e. network strength > versus solids content for a typical cohesive suspension. In many cases there are attractive particle-particle interactions in the suspension being centrifuged, either due to deliberate flocculation or to a naturally occurring aggregated state. Here the ultimate solids content achievable (“the structural limit") is a consequence of the buildup of a cohesive structure between the interacting floes (see Section 3.4.3(b)). In some cases the suspension being centrifuged is colloidally stable, i.e. there are repulsive interactions between the particles. Here the ultimate solids content achievable is when the particles form a close packed cake. The effect of increasing acceleration to "ng" is two-fold: settling rates are enhanced (usually linearly with n In the early stages) because o f the greater driving force for separation, and the "structural limit" is pushed up as greater sediment strength (i.e. solids content> is required to resist the increased compression. The settling curves are dependent upon suspension characteristics (e.g. degree/strength of flocculation) and the physical set-up of the experiment used to derive the data < e.g. the number of "g" employed; height of column of sediment used). In principle, a family of such graphs contain u the information on a particular suspension needed for prediction of its large-scale centrifuge behavior. Initial settling rates allow calculation of throughput in the clarification regime, whilst those for later stages of consolidation are needed for estimation of centrifuge requirements under solids-limitation conditions. The behavior of the "structural limit", as a function of *ng” also enables calculation of the maximum solids that could ever be attained from a particular size/speed of machine, irrespective of the manipulation performed to increase residence time available for consolidation. In practice, it Is useful to have a bit more data than simply the raw settling curves for a sequence of accelerating conditions. There are a number of reasons for this, perhaps the most important being the difficulty in identifying transitions from, say, " free fall " of particles to a hindered settling or compression regime (Section 3.3.1). However, this problem is readily circumvented by measuring the shear modulus (explained In 3.2 and which Is a measure of strength of any cohering structure> as a function of solids. Figure 12 shows the general form of modulus/solids content curve observed for almost all suspensions. In the absence of structure the system is almost certainly going to be clarification limited, though conceivably, if the zone extended to relatively high solids contents (say > 10-15% solids by volume) hindered settling phenomena could begin to matter. Equally, where moduli are large 0 104 dynes cm-2) the rate determining effect in centrifugation will probably be sediment consolidation and the material will display solids limitation in the machine. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 23. Two final points are apposite before a description of actual test methods: (i) There is some evidence [2] that at extreme values for sediment cohesion thickening becomes limited by the material becoming too viscous to pump (a “rheological limit”). The position of the limit is difficult to define precisely as it is dependent upon machine configuration, and as It is really a function of viscosity at high shear rates, the latter quantity being only indirectly related to modulus [3,4]. However, it is reasonable to expect potential problems once the modulus rises to ~ 106 dynes/cm2 and the material starts to exhibit substantial (stiff) paste-like qualities. It should be noted that the “rheological limit” for , say, a scroll discharge decanter machine will be greater than that for a disc-nozzle centrifuge due to the difference in method of sol ids discharge. (ii) Because of the asymptotic manner in which the “structural limit” Is approached, it appears not to be feasible to operate a centrifuge to thicken anywhere close to this value: required residence times become impractically long. Thus if calculations suggest that a proposed centrifuge is just capable of thickening to the required degree, without any substantial “margin”, it is likely that it will not prove satisfactory in service. In Figure 11 we have indicated the kind of “practical” thickening limit often seen in operation. 2.1.1 Basic Suspension Properties Before carrying out any other measurements, it is essential to obtain certain basic data concerning the feed, viz the particle size, the density of the solids, the state of aggregation of the species. Simple procedures, such as optical microscopy, are often entirely appropriate to the purpose. Further details on methods which can be applied are given In reference [19]. For biological particles such as bacterial cells the density may often be accurately measured via a silica suspension density gradient [2]. 2.1.2 Settling Kinetics The standard, and so far best, approach to measurement of settling kinetics involves use of a stroboscopic centrifuge. Essentially this is a laboratory (batch) centrifuge with synchronized stroboscope which allows observation of the rate of settling of the material over time. Suitable machines can either be bought from a commercial manufacturer, or can be readily developed by simple adoption of standard laboratory centrifuges. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 24. Although the strobe centrifuge allows measurement of required rate data for calculation of centrifugation behavior in both thickening and clarification limited regimes, some precautions are necessary for extraction of the appropriate data In the latter case. What is required is the "free fall" velocity of a particle or individual floe. Thus, for example, settling velocity must be measured at several dilutions to ensure that weak (concentrationdependent) flocculation between the particles, is not perturbing the Stokesian single-particle sedimentation. Alternatively the Stokesian sedimentation rate can be estimated from first principles (Section 3.3.1). If the suspension has a wide particle size distribution It is also important to ensure that the appropriate velocity is measured. With a dilute suspension of fairly uniform (non-cohering) particles a sharp descending interface forms In the initial stages of the centrifugation experiment whereas for a hetero-disperse system the larger particles will often have formed cakes at the bottom of the tube before the smaller ones have settled appreciably. These systems require the appropriate sedimentation velocity of a given clarification criterion: e.g. if the separation criterion Is 100% clarity of centrate then the sedimentation velocity of smallest particles is the most appropriate quantity for calculation and so on (see Example (d) Section 3.4.5). Further details on the precautions to be observed in the measurement and interpretation of settling kinetics are given in reference [19]. Though strobe centrifuge measurements provide the best approach to characterizing settling kinetics, we would note that information obtained from even a basic bench centrifuge is better than nothing. Indeed this appears to be the procedure followed by centrifuge manufacturers under such names as "spin test" [6]. 2.1.3 Network Strength Measurements These can be made in two ways: from slow-speed centrifuge experiments or by employment of the so-called pulse shearometer. Details of these techniques are provided in Section 3.2 of this manual. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 25. 2.1.4 Floc Stability Assessment Fine particle, particularly biological, suspensions are usually flocculated prior to centrifugation to achieve acceptably rapid separation. It is necessary, of course that the Flocs are sufficiently robust to stand up to the disruptive forces present in the centrifuge otherwise the benefits of size enlargement are dissipated. Unfortunately, current tests for floe stability are rather limited in diagnostic capacity and can only act as a rough guide to actual behavior In a large scale machine. The main reason for this state of affairs Is the difficulty in characterizing the complex shear and elongational stresses prevalent in a disc-nozzle centrifuge. Test apparatus cannot be designed because of lack of information on the situation likely to prevail In practice. However, some assessment can be made of the stability of a flocculated suspension towards breakup, albeit of a rather empirical nature. The standard approach involves measuring a quantity dependent on Floc size, subjecting the suspension to a prescribed amount of agitation from some kind of mixer, and then remeasuring the property. The degree to which the two values differ is an Index of Floc (in) stabllity. Settling rate (as determined by, say, the strobe centrifuge) Is a good Flocsize-dependent characteristic to observe, though filtration times are also employed [8,9]. (Filtration is generally faster the larger the Flocs. If significant floe breakup occurs, filtration rates slow dramatically due both to reduced cake permeability and to blinding effects. ) The problems arise with respect to prescription of the appropriate degree of agitation for the reasons mentioned above. Commercial apparatus (ex Triton Electronics Limited) Is available for the task (see reference [81, p179] but a simple agitator in a beaker probably serves as well. Clearly the method cannot work on an absolute basis but satisfactory results can be obtained by comparison. In addition a relatively new instrument produced by Rank Brothers, the Photometric Dispersion Analyzer, allows the correlation of Floc size and strength with flow-, and hence, shear rate. For example, suspensions obtained by use of different flocculants can be assessed in order to determine the most effective agent In terms of Floc robustness. Similarly, the stability of new systems can be compared with that of a material known to retain integrity in the large-scale centrifuge under consideration. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 26. Finally we would note that a number of other laboratory techniques can play a useful, but lesser role in providing data for prediction of centrifugation behavior. Rheological methods help to identify when the flow behavior of the material may be limiting [2] whilst optical microscopy can be of great value in diagnosing variations in aggregate size which can lead to erratic performance [2, 4]. The latter is probably also the only way that the presence of attached gas bubbles - a great hindrance in centrifuging biological suspensions due to their buoyancy effect - can be proved. Simple sedimentation tests also have their place - settling usually stops under “lg” just where network strength becomes significant. Thus the suspension concentration process up to this point will probably be clarification limited. 2.2 Test Techniques Laboratory test methods resolve themselves into four main classes:(i) Basic suspension properties; (ii) Procedures for measuring settling kinetics; (iii) Network strength characterization methods; (iv) Techniques for assessing the likelihood of aggregate breakup in the machine. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 27. 3 SCALE-UP THEORIES The appropriate scale-up theory depends on the controlling mechanism likely to pertain, e.g. clarification, compression or flux limitation (see Section 3.4.1(d) for details. These will be considered In turn. 3.1 Scale-Up When Clarification is the Limiting Process 3.1.1 Qualitative Referring back to Section 3.4.1(c), if the solid particle is unable to reach the base of the gravity settling tank before leaving, then it will pass out in the supernatant stream, ie full clarification of the liquid overflow will not be achieved. It was shown that: Max Liquid Rate = Tank Surface Area x Terminal Velocity under one Gravity The clarification capacity of a disc centrifuge can similarly be quantified and is given the symbol Ʃ . The Ʃ value of a centrifuge represents the area of a simple gravity settling tank (m2) which would be required to give the same separation capacity. 3.1.2 Quantitative Calculation of the Ʃ value of a known centrifuge is reasonably straightforward and the standard derivation is outlined below: For a single disc Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 28. Figure 13 Area of disc between r and r+ δ r is given by Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 29. Gravitational field resolved perpendicular to disc 3.1.3 Validity Although Westfalia Separator use the above equation for Ʃ, Alfa Laval use an empirically “corrected” version of Ʃ, called KQ, where Centrifuge capacities may be compared by means of their relative KQ values. However, unlike Ʃ, KQ has no absolute physical interpretation. The use of either Ʃ or KQ is widespread and indeed often considered to be the only scale-up parameter of importance. It is, however, only meaningful when clarification is the limiting process. It has no relevance to solids-limited applications. Other mechanical factors may also reduce the applicability of the above formula, e.g. too narrow disc spacing or Inadequate feed acceleration. Other factors that should ideally be considered include the significant Coriolis forces present, and the inevitable spread in actual terminal velocities. 3.1.4 Application The Initial settling rate of particles under a known gravitational field can be determined using a strobe centrifuge test. If the terminal velocity is u (m/s) under n gravities, then the gravitational terminal velocity Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 30. It has previously been shown that a gravity settling tank of area Ʃ (m2) can cope with a liquid throughput of Similarly, a centrifuge of equivalent settling area Ʃ (m2) could cope with a liquid throughput (i.e. centrate) of A more empirical approach may be preferable if data from e.g. semitechnical centrifuges are available. If the performance of a small centrifuge is known the likely throughput on a large machine may be predicted from the ratio of the respective Ʃ values for the two: It must be emphasized that this empirical approach should be used whenever possible. To scale from the behavior of a single particle in a spinning test-tube to a commercial scale machine may be a useful scouting technique but Is less than accurate. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 31. 3.2 Scale-Up When Thickening is the Limiting Process 3.2.1 Background A rather different scale-up method is required when the centrifuge may be in a solids-limitation regime, i.e. the key kinetic process is the (slow) consolidation of the network of Flocs in the partly thickened sludge. In this case the principal quantity of importance is ṫ, the residence time essential to give thickening to the required solids content, under the given centrifugal field, This is solely a property of the material and thus the suspension properties require characterization before accurate scale-up estimates can be made. To reach a given thickness, a minimum residence time ṫ, must be allowed. For a given radius, this then limits the volumetric rate of the concentrate stream to be where V is the volume of solids held in the machine. The calculation of ṫ and V will be discussed shortly. 3.2.2 Scale-Up Factor Ideally, a simple scale-up factor analogous to the Ʃ value in clarification theory is desirable. Such a factor, F, is described below. The concentrating pressure, P, developed in a centrifuge can be defined as Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 32. The time required for a given degree of consolidation will clearly depend on the concentrating pressure available. It is reasonable to assume that The effective capacities of two centrifuges can now be directly compared if their volume and concentrating pressures are known, viz The R.H.S. of the equation is thus the required scaling factor, F. 3.2.3 Three Methods of Application 3.2.3.1 Semi-Empirical Approach I If data on a given material is available on the semi-technical scale, then the performance of a full-scale centrifuge can be predicted directly from equation (16) provided that the machine's geometry and speed are known, and provided that a value for y can be estimated (see below). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 33. 3.2.3.2 Semi-Empirical Approach II A check on the above method is available if the capacities of the two centrifuges are known on another similar type of material. In these circumstances the ratio of concentrating pressures will be dominated by differences in machine configuration rather than differences in feed material; the volume term is clearly material independent. For centrifuges 1 and 2, and feed materials A and B, It then follows that I.e. use scale-up data obtained on material A to predict how material B will behave in the larger centrifuge. 3.2.3.3 Ab Initio Approach Insight into the compaction characteristics of a material can be gained quickly from small quantities of feed by using a strobe centrifuge, where the time required to achieve a given compaction under a known concentrating pressure can be found. Knowing the concentrating pressure and volume of a larger centrifuge, the maximum allowable nozzle discharge rate can be readily calculated. 3.2.4 Estimation of Necessary Parameters and the Problems Encountered 3.2.4.1 Relationship between ṫ and P The value of y in equation (15) is difficult to establish a priori. Fortunately, the concentrating pressures developed by both semitechnical and full-scale centrifuges (e.g. 'Westfalia NA7, Alfa Laval FEUX 320) are not dissimilar, since they are limited by materials of construction, and putting y = 1 is probably sufficiently accurate for the above semi-empirical approaches. Note that this choice (y = 1) is essentially the basis of the scaling rule presented earlier in Section 3.2.7. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 34. At present, however, strobe centrifuges give lower concentrating pressures and extrapolation to full-scale is thus liable to major inaccuracies. The Ab initio approach, therefore, although useful as an initial scouting technique, should not be relied on for detailed work. 3.2.4.2 Volume of Slurry in centrifuge The total volume which might be filled by thickened slurry (including, for example, the volume occluded by the spinning discs) can be evaluated by simple geometry from the known dimensions of the centrifuge. As a first approximation it appears reasonable to take the maximum allowable inventory of thickened slurry as, say, half this volume. (In the semi-empirical approach errors in this assumption will largely cancel but the Ab initio approach is potentially more sensitive to the basis at this part of the calculation.) 3.2.4.3 Pressure (or “field”) Factor Estimates of this factor are based upon equation (14). For the strobe centrifuge the quantities to be used are self evident and are detailed in the worked examples (see later) but for the continuous machine some explanation is needed. (1) Ho, the depth of the bed of consolidating sediment, is taken as the distance between the disc stack periphery and the nozzle. (2) “ng" is some mean of the centrifugal field at the disc periphery and at the machine wall. For the present example we employed an arithmetic average of the two values in question. (3) Ø.... n is taken as the solids content where the system starts to display a perceptible cohesive structure, eg ~0.1 weight fraction solids in the case of flocculated "Pruteen" suspensions. This is because thickening to the so-called "gel point" will generally be a much faster process than the subsequent consolidation of the cohesive sediment. Note that, strictly, volume rather than weight fractions should be used for i, but as the same conversion factor will be used in all cases, and we are only interested in ratios of pressures, it is satisfactory to approximate f by a weight fraction. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 35. 3.2.4.3 Concept of Ultimate Thickness It is often useful to establish the maximum solids content which could be attained with a particular centrifuge given an infinitely long residence time in the machine (NB this is the structural limit, rather than the kinetic limits discussed hereto). This can readily be predicted from laboratory-scale tests using a pulse shearometer or a slow-speed centrifuge (see Section 3.2 of this manual or Appendix A, reference [19]). The calculated ultimate thickness allows one to rapidly decide whether a desired concentration can sensibly be achieved by centrifugation. 3.3 Scale-Up When Hindered is the Limiting Process 3.3.1 Background The terminal velocity of a swarm of particles is generally less than that of a single particle falling in an infinitely wide pond; the more crowded the particles, the slower they fall. This is called "hindered settling". The Ʃ theory (Section 3.4.3(a)) implicitly deals with the initial capture of isolated particles from virtually clear supernatant. For capture to be completed, however, these particles need to be consolidated into a film on the underside of the discs so that they can subsequently migrate towards the bowl periphery. If, as a result of hindered settling, a "traffic-jam" prevents this consolidation, the disc-space will fill up with solids and lead to carry-over in the centrate. As with the derivation of Ʃ, the problem of hindered settling will first be analyzed in terms of a simple gravity thickener (cf Section 3.3.4). This will then be related to centrifuge performance. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 36. 3.3.2 Gravity Thickeners – Behavior The theory of even "simple" gravity thickeners is complex. An outline of the key points is given in reference [14]. What follows is a simplified, overall summary. (Further discussion may be found under Sedimentation, Section 3.3.4(c) as well as many relevant references.) 3.3.3 Batch Thickeners The flux-rate of settling particles is proportional to their settling velocity and their concentration. However, because their velocity will fall and their concentration increases, the following relationship results: Figure 14 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 37. 3.3.3.1 Continuous Thickeners In a continuous thickener there is a net bulk downflow caused by the continuous removal of sludge at the base of the thickener which gives an extra downward flux (proportional to the solids concentration) .This modifies the above plot to give: Figure 15 There will often be a minimum in this curve, shown occurring at concentration G* in the above plot. If, as is frequently the case, the feed concentration is lower than G* and the sludge concentration higher, then G* represents the maximum solids flux that the thickener can handle. If a greater solids loading is imposed, then the thickener can no longer cope. It will fill up with material of concentration G* and excess solids will be lost to the previously clear overflow. 3.3.4 Gravity Thickeners – Interpretation of Batch Settling Test In a batch gravity test, the sediment Interface height above the vessel base is measured as a function of time: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 38. Figure 16 (subscript o refers to the initial conditions) The Yoshioka Method can now be used to determine the limiting flux rate. This method is detailed in reference [10] and only the mechanistic steps are shown below: From the plot of Interface height against time A more time consuming, but potentially more accurate, method would be to make up feeds of varying concentration and measure the initial settling rate. This may well provide a useful check on the quicker, less reliable construction used above. From the required discharge concentration, co, construct a tangent to the batch settling curve and read offs the intercept: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 39. Figure 17 3.3.5 Application to Centrifuges 3.3.5.1 Batch Flux Curve The batch settling test will typically be performed in a strobe centrifuge. The equivalent gravity test can be obtained by expanding the time scale proportionately to the mean g-force prevailing. The preceding analysis will then give the minimum allowable flow area in a gravity settler to permit the desired concentration to be obtained. The equivalent settling area of a centrifuge is given by Ʃ. The solids flux calculated above may therefore be multiplied by Ʃ to test whether a given centrifuge can cope with the required duty. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 40. 3.3.6 Discussion (1) The gravity settling theory is fraught with assumptions and by no means universally valid. To further extend this to the complex flow regimes of a centrifuge must be questionable. (2) It is not clear what concentration to choose for the .underflow" In the above graphical construction, It is probably ML the nozzle discharge concentration but this should give a conservative result. (3) The identification of points a and b is generally far from straightforward given the quality of data experienced in the real world. Point b is the most important to determine as the analysis breaks down beyond this. Possibly it may be most appropriately determined experimentally as the concentration to which particles will separate under gravity in the absence of applied or hydrostatic pressure. An alternative approach to the problem utilizes a log-log plot based on Figure 16 to identify the compression point or the so-called Roberts plot (see reference [3] of Section 3.3, pp112-114). (4) Scale-up from strobe to production centrifuges is not to be recommended. However, the Ʃ ratio between two disc centrifuges probably reflects their relative capacities for hindered settling limitation. This comes as a great relief it is a simple result from a complex situation. 4 A SYSTEMATIC APPROACH TOCENTRIFUGE SCALE-UP The following steps are recommended: (1) Unless there is strong supporting evidence one way or the other, It is dangerous to jump to a conclusion regarding the limiting mechanism In centrifuge operation. (2) Carry out a strobe test and modulus measurements on the material in question. This will provide useful information on the likely mode of limitation. Follow the three "ab initio" calculation methods given later , and estimate ultimate thickening limits, to establish what is likely to limit centrifuge performance. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 41. (3) Whenever possible, carry out semi - technical trials. By varying parameters such as nozzle diameter, feed concentration and number of discs, the limiting mechanism can be checked and quantified. (4) If possible, compare results with those for any similar materials where large-scale operating data may exist. Use the semi-technical machine on the other materials as well so that the 'semi-empirical" calculation methods can be employed. (5) Use the scale-up methods to predict how many centrifuges of a given type will be needed to just cope with the envisaged duty. (6) It must be remembered that operational centrifuges generally run away from their theoretical maximum to ensure centrate remains clear despite minor process fluctuations. Allow, say, 20% installed spare capacity. (7) Further allowances for longer term deviations from ideal plant conditions may be appropriate. (8) CIP sequences (e.g. up to 1 hour per shift) need to be accommodated. (9) Finally, some spare capacity to cope with the inevitable mechanical breakdowns should be considered. See next section for some worked examples. 4.1 Validity of Different Methods Certain of the procedures - e.g. the Ab lnitio approach to calculation of centrifuge performance in a clarification limited regime - give a definite upper band to performance. However, this kind of useful relationship does not hold for the semi-empirical technique based on Ʃ theory unless one is working rigorously within a clarification regime. Outside of the latter, thickening becomes relatively independent of number of discs [11-13] whereas Ʃ and KQ methods assume a linear dependence on this factor. Thus scaling from semi-technical data, derived from a centrifuge with closely-spaced discs, to predict performance of a machine with more Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 42. widely-separated elements can give misleading results. . An example of this phenomenon is given later in the examples. There are no particularly obvious ways round this problem but the best elementary precaution is to vary the disc-spacing on the semi-technical machine. If this has an effect, then the separation is likely to be either clarification or hindered settling limited. In either case, the Ʃ or KQ theories should be valid. Manipulation of acquired data and close observation of the trials may themselves provide valuable insight. We suggest that all scale-up limitations be tested and quantified as far as possible, but that commonsense and intuition are still important when working out the implications. The need for a representative number of trials before committing large sums of capital cannot be overstressed. The overall scale-up procedure is displayed diagrammatically in Figures 18 and 19 whilst details on how to perform individual scale-up methods are provided in Tables l-7. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 43. Figure 18 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 44. Figure 19 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 45. Table 1 Steps in Ab Initio Scale-Up for Clarification Limited Systems (see Section 3.4.3(a) I Determine free fall velocity of a single particle under a known gravitational field from a strobe centrifuge test (Section 3.4.2 (b)). II Calculate the free fall velocity under a single gravity (equation (8)). III Calculate the C theory for the centrifuge under consideration (equation (6)) using manufacturer’s specifications. IV Calculate the maximum centrate rate per machine (equation (10)). V Calculate the maximum operational feed rate by simple mass balance, given the desired thicks concentration. VI Compare with other scale-up methods. Table 2 Steps in Semi-Empirical Scale-Up for Clarification Limited Systems (see Section 3.4.3(a) I Calculate I values for the semi-tech and production centrifuges (equation (6)). II Scale centrate rates achievable on semi-tech centrifuge using equation (11) to give maximum centrate rate possible on the production centrifuge. III Check semi-tech behavior as a function of number of discs or feed concentration, le establishes whether clarification is the limiting factor. IV Calculate possible speed of maximum feed rates. V Compare results with those from other methods. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 46. Table 3 Prediction of Ultimate Thickness for Solids Limited Systems (see Section 3.4.3(b)) I Use pulse shearometer to measure shear modulus of thickened solids as a function of concentration (Section 3.4.2). II Calculate the internal pressure required to give the desired solids concentration (reference [19]), III Calculate the maximum internal pressure that can be generated by the centrifuge being considered (equation (14)). IV Check whether the available pressure is sufficient for the required duty. If not, choose a more powerful centrifuge or modify the feed material. If it is, check the clarification and compaction kinetics. Table 4 Steps in Semi-Empirical Scale-Up for Solids Limited Systems (see Section 3.4.3(b) I Determine breakthrough solids content as a function of nozzle rate for a small-scale centrifuge, of comparable design to the subject (possible production) machine. II Estimate relative maximum allowable Inventories of thickened slurry for (a) small -scale machine; (b) subject centrifuge (see text for details). III Evaluate relative values of P for (a) small-scale machine; (b) subject centrifuge, from their known speed, dimensions and solids loading. IV From II and III estimate the scale-up factor between the centrifuges as discussed In Section 3.4.3(b). V Estimate maximum allowable nozzle rate, to give required thickening in subject centrifuge, from equation (16). VI Calculate the maximum feed - rate, given the known feed concentration. VII Compare with other scale-up methods. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 47. Alternatively, where scale-up data for the two machines is known for a similar material, the simplified procedure described in Section 3.4.3(b) (Iii) can be substituted for steps I-V. Table 5 Steps in Ab Initio Scale-Up for Solids Limited Systems (see Section 3.4.3(b) I Measure settling curve(s) in strobe centrifuge. II From I evaluate ṫ, i.e. the residence time, to give required solids content In strobe centrifuge. III Evaluate P (the consolidating pressure) in strobe centrifuge from its known speed, dimensions, and solids loading (reference [19]). IV Evaluate P in subject centrifuge from its known speed, dimensions and solids loading. V Using equation (15), evaluate required residence time for subject centrifuge (assuming y = l), VI From centrifuge dimensions evaluate an approximation to maximum achievable Inventory at thickened slurry in subject machine (see text for details). VII Estimate maximum allowable nozzle rate, to give required thickening in subject centrifuge, from equation (13). VIII Calculate the maximum feed-rate from simple mass balance. IX Compare with other scale-up methods. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 48. Table 6 Steps in Semi-Empirical Scale-Up for System Limited by Hindered Settling Systems# (see Section 3.4.3(c) I Calculate I values for the semi-technical and production centrifuges (equation (6)). II Scale “thicks” rate achievable on the semi-technical centrifuge (for desired degree of concentration) to give maximum probable rate on production machine by multiplying flux by Ʃ production I semi-tech. III Calculate the corresponding maximum feed rates. IV Compare results with those determined by other techniques. # NB Ab Initio scale-up from strobe centrifuge data should be treated with caution but if such a calculation is needed as a guide it should follow the lines of Table 7. Table 7 Steps in Ab Initio Guide Calculations for Systems Limited by Hindered Settling Systems# (see Section 3.4.3(c) I Determine batch settling flux for a known centrifugal field using strobe centrifuge and the calculation procedure detailed in Section 3.4.3(c). II Convert above results to their equivalent for a single gravity settler. III Calculate the E value for the centrifuge under consideration (equation (6)) using manufacturer’s specifications. IV Calculate maximum thicks flux from product of I, and the single gravity flux for the considered machine. V Calculate maximum allowable feed rate corresponding to IV. VI Compare results with those from other methods. # See Table 6 re caveats concerning the results of this technique. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 49. 5 WORKED EXAMPLES The following examples are taken from recent GBH Enterprises experience with biochemical separations which are currently amongst the most demanding centrifugation duties. 5.1 Solids-Limited Scale-Up: Background to Examples (1) A European Pruteen plant separates biomass from fermenter liquor (3 wt %) by flocculating the cells, concentrating the cells by flotation to 10X, and subsequently centrifuging the cells to 17 wt %. The preferred process now uses a different flocculation technique, giving Flocs which are more amenable to centrifugation. The flotation stage would be omitted. It was necessary to estimate how many centrifuges would be required for a full-scale plant using the new process, and to quantify the possible benefits of using a larger but untested centrifuge. (2) The existing plant has three Alfa-Lava1 FEUX 320 disc-nozzle centrifuges, each capable of producing up to 2.9 te/h (dry basis> of 17 wt % material (see Figure 20). Semi-technical trials on a Westfalia NA7 machine (see Table 8) were carried out on both the existing and the proposed new flocculation routes, called Pl and P2 respectively. A possible alternative to the FEUX 320 machines was the new Westfalia HDA 300 disc-nozzle centrifuge, a more expensive but larger machine. (3) Experience has shown that increasing the number of discs in the FEUX 320 machines had no effect on instantaneous capacity, i.e. the process is probably solids limited. (Indeed, narrow disc spacing was detrimental due to blockages.) The semi-technical results showed that roughly similar throughputs on the NA7 were possible for the two process routes, despite the great difference in feed concentration. Given the broadly similar nature of the smaller biomass debris, this also suggests that clarification is not a problem. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 50. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 51. 5.2 Solids-Limited Examples: Semi-Empirical and Ab Initio Methods 5.2.1 Semi-Empirical Approach to Solids Limited Operation (simplified method) The NA7 could produce 0.07 te/h (dry basis> o f 17% thicks o n P1 material, 0.096 te/h on P2 material. New plant throughput required = 11.4 te/h (db). On the old P1 material, the FEUX can produce 2.9 te/h (db) of 17% material. Generally, plants operate centrifuges away from breakthrough conditions, and so in practice 4-5 machines will be required to be online at any one time. Regular CIP sequences Cup to 1 hour per shift) and breakdowns means that 6-7 machines should be installed. This figure agrees well with the estimate obtained from the a priori approach (see later> and is In fair accord with the (supposedly) slightly more refined semi-empirical (1) technique: 5.2.2 Standard Semi-Empirical Approach Limiting solids fluxes for an Alfa-Lava1 FEUX 320 could be readily predicted for the Pl and P2 routes from the known Westfalia NA7 centrifugation characteristics and the easily calculated “volume” and “field” terms (Table 8). The latter gave scale-up factors of 27.5 and 56.1 respectively. Results are displayed in Figures 21 and 22. For the P1 system plant characterization data are available and so a check can be made of the accuracy of the calculation method. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 52. The observed scale-up factor for 17% material Is M 2.910.07 or 41 and (as expected) the use of only the “volume” term underestimates limiting flux. However, taking account of both “volume” and “field” factors seems to Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 53. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 54. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 55. give over-optimistic answers. There are probably three reasons for this: (1) Maldistribution and/or complex flow patterns invalidating the theory. (2) The appearance of hydraulic limitations for low degrees of thickening. (3) Over simplistic assumptions, for example concerning the “volume” term. However, no further elaboration of the basis of the calculation is justified at present. This requires advance in the fundamental science underlying the problem. However, where full scale plant characterization data are available, the mean of the two calculation methods was in good accord with experience (see Figure 21). Agreement between this approach and the more simplified approach, previously discussed, was also satisfactory. The approach has also been applied to prediction of the performance of a Westfalia HDA 300 centrifuge relative to that of an Alfa-Lava1 320 for "Pruteen" feed cream concentration. The data provided in Table 8 suggests a scale factor of circa 1.5 for thickening to, say, 17% solids. Results from actual operation of such a machine Indicate a scale-up of the order of 1.4 is observed, provided a close-spaced disc stack (" 1 mm) is employed. Unfortunately this rather satisfactory agreement is somewhat erred by the further observation that with a 2.5 mm stack the relative performance of the HDA 300 fell to only ~ 1.15 of that of the FEUX 320. At this stage the reasons for this effect are not clear. However, it is perhaps most likely that due to particular design features comparatively more discs are needed in the HDA 300 to achieve satisfactory particle acceleration and a reasonable approach to the theoretical limit to performance given by the calculation. Also, the machine i s r e l a t i v e l y taller and so maldistribution may be a problem: the extra discs may act as a manifold. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 56. 5.2.3 Ab Initio Approach to SolidsLimitation (Host suitable for cases where only a minimal quantity of material is available.) For “Pruteen 1" centrifuge feed cream it was found that 16% solids was attained in a 6.25 minutes on a laboratory-scale Trlton VRC centrifuge (1000 rpm speed) whilst consolidation to 17% solids was observed to take ~ 7.5 minutes. Using equation (15), Section 3.4,3(b), with y = 1, one estimates required residence times of - 7.8 and 9.4 seconds for these degrees of thickening in a FEUX 320 machine which can deliver 48 times the concentrating pressure of the stroboscopic instrument. Taking the maximum allowable inventory of thickened slurry in the equipment to be ca 1/2 of machine internal volume predicts limiting fluxes of 4.0 and 3.3 tonnes / hour of solids. This is in good agreement with observed performance. NB key approximation used, le required time is inversely proportional to applied pressure, is certainly not a good one for consolidating sediments close to their limiting solids content but, empirically, It has been found to hold reasonably well for a variety of flocculated systems in the earlier stages of concentration 5.3 Calculation of Thickening Ultimate Limit Example: “Structural” limit for Thickening in “Pruteen” Centrifugation As previously noted, It is useful to be able to estimate the ‘structural” limit on thickening to be expected under a particular centrifuge regime, even though practical constraints over available residence times may prevent such solids contents being approached closely. In particular, such calculations are likely to be valuable in Identifying when the kinetics of consolidation of cohesive sediment are critical in determining thickening efficiency. “Pruteen” suspension centrifugation illustrates this well: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 57. Shear modulus measurements were made at a range of solids contents for P1 and P2 type materials. Pressure/solids content curves were then generated from these results by integration, the only difference from standard procedure being that an .allowance had to be made (a value of - 3 parts water to 1 part cell solids was taken, in line with such estimates as are available of this quantity) for the amount of water In the cells so that (measurable) solids contents could be converted to cell volume fractions. As a check on the calculation, and to allow an estimate to be made of an unknown, the cell density, settling curves were determined using the Triton VRC Frozen Image Centrifuge. Examples for the P2 material are shown in Figure 22. On taking the unknown cell density as ca 1.07 g/cmj, agreement between prediction and observed limiting, strobe centrifuge solids contents were satisfactory (Table 9). (Equation (4)) Appendix A of reference [19] was used to calculate values for the pressure applied by the centrifuge using known values for speed, rotor diameter and so on.) Using the pressure/solids content curves, and estimating the pressure exerted in Westfalla BA7 and Alfa-Lava1 FEUX 320 centrifuges from various geometric characteristics (equation (14)), predictions were made of the solids contents above which one could never thicken the materials in centrifugal fields of these strengths (Table 9). These proved to be well beyond the values (e.g. - 17% by weight) to which concentration was required and hence it was concluded that strength of centrifugal field per se would not be a problem. It should be noted that extrapolation of actual centrifuge data (eg Figure 20) to zero flow (i.e. infinite residence time in the field) gave ultimate limits of the order of those calculated (Table 9). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 58. Table 9 Limiting solids contents for various "Pruteen"-organism suspensions as predicted for a number of centrifuge configurations from suspension modulus/solids content relationships. Also shown are experimentally observed values (where these are known). * The unknown parameter, density of the suspended solids, was adjusted to give exact correspondence between calculation and observation in this case. This value (" 1.075/cm3) was then used unchanged for the remainder of the calculations. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 59. b From extrapolation of known plant characterization data* NB plant material contains an unknown amount of entrained gas and thus limit may be pessimistic. 5.4 Scales-Up for Clarification Limited Systems It is instructive to consider two examples of calculation of clarification limit regime, the first Involving the "Pruteen" system discussed immediately previously, the second Involving recovery of Isolated cells of the organism, Alcaligenes Eutrophus: 5.4.1 Ab Initio Clarification Calculation for P2 Type Single Cell Protein The initial settling rate for the P2 material, for a particular "g" was given by the tangent at the origin to the settling curve, Figure 23. This was equal to 0.0435 cm/s at 180 g. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 60. Bow as can be seen from Figure 20, the observed maximum throughput is * 2.9 te/h at this degree of thickening , held far below the clarification mechanism value by solids limitation. Hence calculation confirms that clarification is not going to be a problem in operation of the FEUX 320 centrifuges. It should be noted that KQ has no fundamental meaning and so use of 1 is preferable in the above Ab lnitio approach. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 61. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 62. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 63. 5.4.2 (Ab Initio) Clarification Limit for Unflocullated Bacterial Cells The continuous centrifugation of unflocculated bacterial c ells provides a good example of clarification limited operation. The cells are very small in size (~1.0 - 1.5 µm) and usually have a very low wet density (~1.05 g/cm3). The following data was obtained from a “Pruteen” Pilot Plant fermentation of the bacterium Alcaligenes Eutrophus to make the biological polymer poly-3-hydroxybutyrate (PHB) the material is accumulated by the bacteria as little intracellular granules and the fermentation is continued until at least 70% of the suspended dry matter is polymer. Since the normal solid density of PHB is a 1.25 g/cm3, the wet density of these bacteria at the end of the accumulation is probably abnormally high (”1.10 g/cm31 for this type of system. Stroboscopic centrifuge measurements using the Triton WRC machine at 180 g gave 0.3 cm sedimentation of a clear upper interface in 30 minutes#. This is equivalent to a gravitational free sedimentation velocity of 3.3 x 10-5 m/hr at 9.2 x 10-7 cm/set, which can be combined with the appropriate Ʃ: value from Table 10 (10.9 x 103 m2 to give 360 liter/hr for the maximum calculated centrate rate for total recovery on the BRPX 207 centrifuge. For this particular example, this would also be the approximate limiting feed rate as feed concentrations would be very low (see Figure 25). Figure 24 shows the proportional recovery of thickened cell culture as a function of feed rate on the Alfa Lava1 BRPX 270s disc nozzle centrifuge. The machine was set up with 118 discs at 0.5 mm spacing and three 0.45 mm nozzles (probably the minimum number that can be safely used due to risk of blockage etc). The recovery curve clearly indicates that feed rates as low as - 500 liter/hr are needed before -100% retention of solids is achieved. Clearly there is remarkably good agreement between observation and the performance predicted from calculation, particularly when one considers the difficulty in obtaining a reliable value for the sedimentation velocity. Figure 25 shows the concentrate solids loading as a function of increasing feed rate. The inlet concentration of cells was approximately 13.5 g/l and the nozzle rate averaged 170 l/hr. These values may be used to calculate the theoretical concentrate line by simple mass balance. This is also displayed in the Figure. The experimental concentrate line only departs from the theoretical line when the solids recovery drops below - 50% (Figures 24 and 25). This is Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 64. largely due to the sedimentation properties of this particular culture In which a substantial portion of the material sediments much more rapidly than the “upper Interface” settling measurements In the stroboscopic centrifuge suggests. # By this tire a substantial quantity of cells had sedimented to the bottom of the tube to form a visible cake, Notice it is the rate of fall of the upper interface (to give a clear layer) which must be measured in this case as we are interested in total recovery, Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 65. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 66. 5.4.3 Semi-Empirical Scale-Up for Clarification Limited Systems As has been shown, the "Pruteen" system is not clarification limited and meaningless results will be obtained for scale-up. These may over PL under-predict performance (see Section 3.4.4). However, the calculation steps which would be applicable for semi-empirical calculation clarification are given below for completeness: Note that "capacity" here refers to centrate volumetric flowrate. Note also that use of KQ (rather than Ʃ) is equally valid when comparing two centrifuges for clarifying duties and would suggest a capacity ratio of 12.8. Sample Calculation: NA7 can handle 1.5 te/h of 10% feed when concentrating up to 13%. What could the FEUX 320 handle? In clarification limited duties, breakthrough always occurs at the same centrate rate (Section 3.4.1(d)). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 67. When concentrating to 17% from10%, 4.3 te/h centrate implies i.e. the centrifuge can process 1.04 te/h (db) solids (cf actual capacity of 2.9 te/h). This demonstrates the great importance of feed concentration on clarification limited duties, even though it is often irrelevant for solids limitations. It should be noted that these calculations under -predict actual performance. This has little significance as the calculation method is invalid for solids limited applications - one cannot scale-up a solids limited breakthrough curve using clarification limited theory. We would re-emphasize that this semi-empirical approach does not give a bound (either upper or lower) to performance. 5.5 Ab Initio Calculation for Hindered Settling Limitation Finally, it is useful to use the P2 route single cell protein example to illustrate use of the method for calculating hindered settling limitation: Rates of settling versus solids content could be found by drawing tangents to the curve in Figure 23. From this a batch settling curve (Figure 26) was partly constructed following equations (19) and (20 :). This gives the falling rate part of the curve viz: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 68. with the flux being the product of the two expressions. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 69. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 70. Fig 27 Definition of Terms for Settling Rate Calculation However, at low solids contents the rate of settling does not vary and is not, as assumed in the hindered settling theory, a function of concentration. Thus below the critical concentration where the falling rate zone commenced γmin was taken as C x settling velocity, where the latter was, of course, a constant.# Row for thickening to 17% by weight of total solids (= 15.5 wt. % of suspended solids) the intercept γ min = 0.235 x 10-3 g/cm2/sec, if we chose Co = 15.5%. Note that this is for 180 g and thus, scaling already, γmin (1 g) = 0.131 x 10-5 g/cm2/sec. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 71. # This difference in treatment in the two regimes leads to a slight mismatch in the flux curve around the concentration at which hindered settling commences, This 1s unimportant with regard to estimation of γmin 5.6 Comments The above calculation apparently yields about the correct magnitude for the centrifuge capacity (see previous examples). Although all data suggest that the machine is solids-limited it must be remembered that the regions of hindered settling and compression very much overlap. Idealized separation into different regimes is generally only seen in textbooks [5]. Examination of the settling curves in the form of solids content versus time backs this up (Figure 22). We would remind the reader, too, that it is rarely practical to operate a centrifuge deep In the compression zone where consolidation rates are very slow (see Figure 11, Section 3.4.2(a)). Instead, it is much more satisfactory as a rule to work somewhere near the boundary between "hindered settling" and compression where the kinetics will be much more favorable. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 72. It is evident, too, that the form of the flux/concentration curve displayed in Figure 26 departs a good deal from model behavior as in Figure 17. This again is due to a lack of very clear cut transitions from free fall through hindered settling to consolidation. It should be noted that such behavior appears the standard rather than the exception 151. However, this inevitably makes the construction embodied in Figure 26 have fairly wide error bounds, emphasizing that the method is essentially an order-of-magnitude technique. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 73. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 74. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com

×