Filtration

2,148 views
1,995 views

Published on

Filtration
0 INTRODUCTION
1 The Theory Underlying Filtration Processes
1.1 The Mechanism of Simple Filtration Systems
1.1.2 Cake Filtration
1.1.3 Complete Blocking
1.1.4 Standard Blocking
1.1.5 Intermediate Blocking
1.2 Cake Filtration – Models and Mechanisms
1.2.1 Classical Theory for the Permeability of Porous Cakes and Beds
1.2.2 The Rate of Filtration through a Compressible Cake – The Standard Filtration Equation
1.2.3 The Compression or Consolidation of Filter Cakes – Ultimate degree of dewatering
1.2.4 The Rate of Consolidation
1.2.5 Useful Semi-Empirical Relations for Constant Pressure and Constant Rate Cake Filtration
1.2.6 Constant Pressure Filtration
1.2.7 Constant Rate Filtration
1.2.8 Multiphase Theory of Filtration
1.3 Crossflow Filtration
2 The Range and Selection of Filtration Equipment Technology
2.1 Scale
2.2 Solids Recovery, Liquids Clarification or Feed stream Concentration
2.3 Rate of Sedimentation
2.4 Rate of Cake Formation and Drainage
2.5 Batch vs Continuous Operation
2.6 Solids Loading
2.7 Further Processing
2.8 Aseptic or “Hygienic” Operation
2.9 Miscellaneous
2.10 Shear versus Compressional Deformation
2.11 Pressure versus Vacuum
3 Suspension Conditioning Prior to Filtration
3.1 Simple Filtration Aids
3.2 Mechanical Treatments
4 Post-Filtration Treatments and Further Downstream Processing
4.1 Washing
4.1.1 Air-Blowing
4.1.2 Drying
5 Testing and Characterization of Suspensions
5.1 Introduction – Suspension
5.2 Properties relevant to Filtration Performance
5.2.1 Pre-Filtration Properties of Suspension
5.2.2 Properties of Filter Cake
5.2.3 Laboratory Scale Filtration Rigs
5.3 Means of Monitoring Flocculant Dosage
5.4 Filter Cake Testing
5.4.1 Strength Testing (See also piston press described earlier)
5.4.2 Cake Permeability or Resistance
5.4.3 Rate of Cake Formation
6 Examples of the Application of the Forgoing Principles
6.1 Dewatering of Calcium Carbonate Slurries
6.2 Dewatering of Organic Products – Procion Dyestuffs
6.3 Filtration of Biological Systems – Harvesting a Filamentous Organism

References
Tables
Figures

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

  • Be the first to like this

No Downloads
Views
Total views
2,148
On SlideShare
0
From Embeds
0
Number of Embeds
5
Actions
Shares
0
Downloads
139
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Filtration

  1. 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-SPG-300 FILTRATION 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 information for its own particular purpose. GBHE gives no warranty as to the fitness of this information 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 or damage 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. 2. Process Engineering Guide: FILTRATION CONTENTS 0 INTRODUCTION 1 THE THEORY UNDERLYING FILTRATION PROCESSES 1.1 The Mechanism of Simple Filtration Systems 1.1.2 1.1.3 1.1.4 1.1.5 1.2 Cake Filtration Complete Blocking Standard Blocking Intermediate Blocking Cake Filtration – Models and Mechanisms 1.2.1 Classical Theory for the Permeability of Porous Cakes and Beds 1.2.2 The Rate of Filtration through a Compressible Cake – The Standard Filtration Equation 1.2.3 The Compression or Consolidation of Filter Cakes – Ultimate degree of dewatering 1.2.4 The Rate of Consolidation 1.2.5 Useful Semi-Empirical Relations for Constant Pressure and Constant Rate Cake Filtration 1.2.6 Constant Pressure Filtration 1.2.7 Constant Rate Filtration 1.2.8 Multiphase Theory of Filtration 1.3 Crossflow Filtration 2 THE RANGE AND SELECTION OF FILTRATION EQUIPMENT TECHNOLOGY 2.1 2.2 2.3 2.4 2.5 2.6 Scale Solids Recovery, Liquids Clarification or Feed stream Concentration Rate of Sedimentation Rate of Cake Formation and Drainage Batch vs Continuous Operation Solids Loading 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. 2.7 2.8 2.9 2.10 2.11 3 SUSPENSION CONDITIONING PRIOR TO FILTRATION 3.1 3.2 4 Simple Filtration Aids Mechanical Treatments POST-FILTRATION TREATMENTS AND FURTHER DOWNSTREAM PROCESSING 4.1 5 Further Processing Aseptic or “Hygienic” Operation Miscellaneous Shear versus Compressional Deformation Pressure versus Vacuum Washing 4.1.1 Air-Blowing 4.1.2 Drying TESTING AND CHARACTERIZATION OF SUSPENSIONS 5.1 Introduction – Suspension 5.2 Properties relevant to Filtration Performance 5.2.1 Pre-Filtration Properties of Suspension 5.2.2 Properties of Filter Cake 5.2.3 Laboratory Scale Filtration Rigs 5.3 Means of Monitoring Flocculant Dosage 5.4 Filter Cake Testing 5.4.1 Strength Testing (See also piston press described earlier) 5.4.2 Cake Permeability or Resistance 5.4.3 Rate of Cake Formation 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. 6 EXAMPLES OF THE APPLICATION OF THE FORGOING PRINCIPLES 6.1 6.2 6.3 Dewatering of Calcium Carbonate Slurries Dewatering of Organic Products – Procion Dyestuffs Filtration of Biological Systems – Harvesting a Filamentous Organism REFERENCES TABLES FIGURES 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. 5. 0 INTRODUCTION For the purposes of this process engineering guide, filtration will be regarded as the process whereby solids are separated from liquids by the use of a porous medium. This definition then deliberately excludes the filtration of gas streams. Filtration together with gravity separation forms the basis for nearly all unit operations to accomplish the dewatering of suspensions [3-5]. It is therefore worthwhile considering the broad factors that favor filtration over gravity separation methods for a given system [1]. Perhaps the most important of these involves the use of a porous medium to effect the operation. The nature of the former may be tailored and designed to best suit the requirements of both phases of the suspension and the sort of dewatering action required. In contrast, all techniques based on gravity separation are completely dependent upon the density difference, ∆ρ, between solid and liquid phases. Since ∆ρ must be regarded for many systems as an invariant, (It may be slightly perturbed by a change in operating temperature), a small value for the quantity almost Invariably means that gravity separation will prove difficult. This Is often the case for biological particles (see Section 3.8). For such cases filtration is then often to be preferred. The penalty that has to be paid for this versatility of filtration process design is usually greater expense and additional complexity when continuous or automated operation is desired. It must be stressed that the above statements are based on broad, generalized principles. For both filtration and gravity separation, the ingenuity of solid/liquid separation equipment designers has led to means of at least partly circumventing many of the disadvantages associated with each [8]. The above definition of filtration encompasses a large number of possible operations ranging from the clarification (or even sterilization> of a very slightly loaded suspension to the removal of a product in the form of a solid cake. Other variations permit filtration to be used as a thickening operation where a feed stream is concentrated in the suspended phase without the formation of a cake or the deposition of solids. Since this section of the manual falls within the dewatering section, most emphasis will be given to processes where it is the suspended phase that contains the desired product and Is usually to be further processed. Clarification by filtration will be discussed briefly but, for greater detail, the reader is referred to the separate chapter on that subject (GBHE-PEG-SPG-400 Centrifugation) and to references [1,12] 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
  6. 6. Finally It is appropriate to set the context for the rest of this guide. In Section 3.5.2 some of the basic, and largely classical, theory of filtration will be presented. The range and selection of filtration equipment Is then briefly discussed in Section 3.5.3. In this and all parts of this section emphasis will be placed much more upon relating material properties to the process design than in providing a comprehensive survey of the available technology. To Illustrate process interactions, In Section 3.5.4 a brief consideration is given to those operations most likely to follow filtration in a complete process. In Section 3.5.5, methods for testing and characterizing the filtration properties of suspensions are described together with the interpretation of the results in terms of the theory previously given. To conclude the section, examples are given of the processing of suspensions containing inorganic, organic and “biological” particles. It is hoped that these will illustrate many of the principles previously developed. 1 THE THEORY UNDERLYING FILTRATION PROCESSES The purpose of this part is to provide the necessary theory on which the rest of the section is based. It is intended that each of the following topics should be self-contained and can therefore be read in isolation. The theory pertaining to washing and dewatering by air-blowing is postponed until Section 3.5.4. 1.1 The Mechanism of Simple Filtration Systems Various authors have proposed schemes for classifying the diverse mechanisms that may operate during filtration operations. The simplest classification for solids retaining systems discriminates between cases where the solids build up on top of a cake and those where they are retained within the filter medium. A useful classification based on this approach was provided a long time ago by Hermans and Bredee [14] who distinguished four mechanisms, which are sketched schematically in Figure 1: 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. 7. 1.1.2 Cake Filtration (Figure 1(a)) This process involves the removal of solids by the formation of a filtered “cake” on the surface of the medium. It Is the cake itself which effects the subsequent filtration. Depending upon whether the suspended particles are smaller or larger than the pores of the medium, this process Is usually preceded by a bridging or straining process in order for cake formation to ensue. Hermans and Bredee proposed the following equations to describe the time dependence for cake filtration: where V is the volume of filtrate at time t, Qo. the Initial flow rate and k an empirical constant. Alternatively in terms of a mean (cumulative) flow rate, q(t): Equations (1) and (2) clearly represent a gross oversimplification of the cake filtration of real systems. Modified forms of these are presented In subsequent sections. Graphical representations of Equations (l)-(2) are shown In Figure 2(a). 1.1.3 Complete Blocking (Figure 1(b)) This mechanism Involves a straining process either at the medium surface or within Its internal structure. This implies that the solids particles are larger than the. local size of the pores of the medium. This then results in completing blocking of pores as the filtration proceeds with a linear decrease in the flow rate with volume: 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. 8. In surveying a large number of suspensions and filter media, Hermans and Bredee found this type of filtration to be rather rare. It also falls to yield a useful and physically plausible volume-time relationship when integrated. The concept of "complete blocking" as depicted in Figure l(b) still retains some merit for classification purposes, however. 1.1.4 Standard Blocking (Figure l(c)) This is the mechanism most pertinent to depth filtration where particles may pass through the pores of the medium but are retained by eventual adhesion to it. Hermans and Bredee's model ascribed a "fouling" process where the internal volume of the pores decreased linearly with V. Thus they obtained equation (4). Since the subject of deep bed filtrations falls outside the intended scope of this guide, no further discussion of this mechanism or topic will be presented. More Information may be found in the guide on clarification or In the books by Svarovsky (Chapter 11 of [1] and Purchas (Chapters 3 and 6 of [12]. In addition, the role of the particle zero-potential has recently been considered by Raistrick amongst others (J H Raistrick in [94]). The equations (l)-(4) were developed to allow volume, flow rate and time correlations to be tested for each mechanism under conditions of constant pressure filtration. A simpler diagnostic means of distinguishing and interpreting them was provided by the dependence of the rate of change of total filtration resistance, r, (i.e. medium plus accumulated solids) with filtration volume, dr/dV, with r. The three mechanisms described so far were attributed the following dependence: 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. 9. In order to encompass the sometimes observed relationship, dr/dV a r, Hermans and Bredee's classification allowed for a fourth mechanism: 1.1.5 Intermediate Blocking Where, This mechanism may be physically viewed as being intermediate between Figures l(b) and l(c). 1.2 Cake Filtration – Models and Mechanisms In this section of the dewatering chapter, greater prominence will be given to cake filtration than to the other mechanisms just described. In part this reflects the frequency with which the formation and properties of a cake dominates a separation operation. However, the other reason for this emphasis derives from the necessity to understand and control the influence of the suspension itself as opposed to the hardware; for a properly conceived cake filtration the suspension properties are dominant and the filtration medium of secondary importance. Before presenting some of the basic theory for cake filtration it is important to delineate the factors which relate to the fundamentals of dewatering presented In Section 3.2. In general for a given cake filtration system one might want to ask the following two questions: (i) What degree of dewatering Is attainable for a given suspension In a cake filtration rig? How is this ultimate dewaterabillty influenced by changes in the suspension, filtration conditions and driving pressure? (ii) What are the kinetics of the filtration process, ie do they allow the process to operate near to or at the ultimate limit as in (1) above. In general theories describing the permeability of a cake or its rate of deposition are very much aimed at addressing the second sort of question. 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. 10. However, when one is trying to understand, for example, the expression of liquid in a filter press, it is the ultimate dewaterability that is usually critical though kinetics may again be limiting. For a comprehensive understanding of filtration processes It is therefore necessary to have a quantitative picture of both the kinetics of the process and the maximum degree of dewatering that can be obtained. Both these Issues will be tackled In the following pieces of theory. Additional aspects of the theory of filter cakes and sediments have been discussed In more detail by Tiller and other workers [11]. Other relevant references may be found in the sedimentation section of this chapter (3.3). 1.2.1 Classical Theory for the Permeability of Porous Cakes and Beds From an observation of the rate of flow of liquids through beds of sand, Darcy [15] suggested an empirical correlation between the fluid velocity, u, the pressure drop across the bed, ∆P, and the bed thickness, L. The result, Darcy's Law, may be expressed in the simple form: Some insight into the nature of the constant, K1, is gained by assuming a result from the Poiseuille [16] Equation for the flow of a liquid through a capillary tube of radius, r, and length, L: This result enables equation (6) to be modified thus: where the Influence of the fluid viscosity, Q, has been explicitly Included. dimensions of (length) 2. K2 has 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. 11. A combination of the equations of Darcy and Polseuille together with the Incorporation of two properties of the bed itself (SA and Ɛ, led to the equation attributed to Kozeny and Carman [17-18]: In this equation the bed properties SA and Ɛ represent the specific surface area (m-1) and the fractional voidage, ie 1 - ɸ, where ɸ is the dimensionless volume fraction of particles In the bed or filter cake. The Kozeny-Carman Equation, although the precursor for the most commonly used filtration equations, suffers from a number of restrictions and oversimplifications. Basically It Is a sound description for the drainage rate of viscous flow of a clear liquid through a porous bed of constant permeability VULCAN VGP systems the permeability at a given point may be a function of pressure drop, time and the height of that point within the bed. Further discussion of these features is provided later. The constraint of viscous laminar flow is also sometimes not strictly applicable In practice. Where turbulence becomes significant a correction to the rearranged equation may be made as follows, after Burke and Plummer [20]: This equation was derived for beds of uniform spherical particles of diameter, d; hence SA = 6/d. It can be seen that the leading term for the pressure drop per unit length is the simple, viscous Kozeny-Carman contribution. The second term is the kinetics energy loss to the pressure drop through turbulent flow. Whether such a kinetic energy modification is necessary for a given bed and flow rate may be judged by a plot of the Reynolds Number, 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. 12. against permeability (eg Morgan's work with sintered metal pores [21]. Fortunately it Is often the case that the simpler, purely viscous treatment of permeability Is sufficiently accurate for practical applications. 1.2.2 The Rate of Filtration through a Compressible Cake – The Standard Filtration Equation From the somewhat idealized equations for the permeability of porous beds, a straightforward modification, to allow for medium resistance, Rm, yields a general expression for cake filtration rates: Rc, the cake resistance (units of m-l) may In turn be related to, w, the weight of solids per unit volume of filtrate (kg rnm3) and a quantity, r, the specific resistance of the cake (le resistance/weight of solids per unit area or m kg-1): As pointed out in (i) for many real systems it is necessary to take account of the finite compressibility of the filter cake under an applied pressure. A simple empirical correction to resistance has been very widely used (1,12). 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
  13. 13. where ro is the specific resistance at zero pressure drop and the exponent “s” is known as the compressibility factor. This factor ranges from 0 for a perfectly incompressible material to unity a highly compressive cake. Thus a general expression for the rate of cake filtration may be written as: Equation (15) may be Integrated to yield the total filtration volume after a specified time provided that the functional dependence of the flow rate or the pressure drop with time is known. It is, however, once again very important to note the simplifications and approximations that are Inherent In this equation. Firstly, It is very common to assume that the medium resistance, Rm, is a constant in time. Physically this corresponds to no blinding or trapping of solids within the medium, ie a complete absence of Hermans and Bredee's mechanisms (ii), (iii) and (iv) of Section 3.5.2(a). In reality the medium resistance is quite likely to change during the initial stages of cake formation. Once a cake of any substantial thickness has been formed, the cake Itself will largely prevent any further particulate matter from reaching the filter medium or support. Hence Rm will usually thereafter indeed be a constant unless further solids are leached into the medium from the underside of the cake. Thus it is often reasonable to treat Rm as a constant during the cake filtration but it may prove erroneous to deduce its value from a measurement of the resistance to "clean" suspension medium alone. (For an alternative perspective to this Issue, see (b)(v).) The specific cake resistance, r or r0 will not be a simple constant for a given suspended phase and medium. It will depend critically upon the colloidal properties of the suspension and the consequent structure of the filter cake. It may also depend upon the mode and rate of cake lay-down. 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. 14. Factors such as particle size and distribution, particle shape, particle surface properties, the presence and nature of any flocculating agent, etc, etc, will all strongly influence r. Thus there arises the ability to control the filtration process by conditioning the suspension by the use of pretreatments and filtration aids. These will be discussed in more detail later. One further reservation concerning equations (14) and (15) must be expressed. It should not be presumed that the simple power law dependence of r on the pressure drop ΔP will apply over a very extended range of pressures. 1.2.3 The Compression or Consolidation of Filter Cakes – Ultimate degree of dewatering Many of the physical principles pertaining to the consolidation process have already been derived in previous sections of the manual (3.2 - 3.4). The subject is sufficiently central to many filtration situations, however, that an outline of the theory will be reproduced here. Consolidation of a structured filter cake will occur during its laydown and the filtration process proper. Additionally it may be exploited following cessation of the actual filtration by the application of pressure to express liquid from the pores of the cake. This latter may involve cake collapse, that is consolidation, or displacement by gas. Pneumatic dewatering is briefly considered in a later section (3.5.5(b)). The ultimate attainable degree of dewatering of a filter cake through consolidation is calculated by considering the two opposing forces on the cake. On the one hand, above a certain solids content the cake will possess a structural resistance to densification which may be quantified In terms of Its uniaxial, compressional yield point, Py(ɸ). Methods for measuring Py(ɸ) and a description of its application are given later. This Internal resistance to densification operates against the externally applied pressure differential across the filter cake, ΔP. Consider a filter cake of solids finit undergoing constant pressure consolidation. Initially, provided that Py(ɸ init) is less than the applied consolidating pressure, ΔP, the cake will be compressed and liquid expelled from it. As this process continues, the concentration of solids in the cake, ɸ, and hence the function, Py(ɸ), will increase. 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. 15. The compressional yield point, Py(ɸ), is in fact a very strong function of solids content (see Examples 3.5.7) and at some later time it will be of sufficient magnitude to match the applied pressure and consolidation will cease. This defines that the ultimate degree of dewatering will occur when: when this condition Is reached the internal stresses of the cohesive cake are large enough in magnitude to fully resist the applied pressure ΔP. Thus predicting the ultimte dewaterability simply requires a knowledge of the function Py($) and this may be measured by a simple laboratory scale determination. Examples of this procedure are given later and in Section 3.2.6 of the manual. 1.2.4 The Rate of Consolidation Equation (16) enables the easy estimation of an equilibrium degree of dewatering for a given filter cake and consolidating pressure. The question of how fast that ultimate solids content is attained is a more complicated one involving additional physical factors such as the drag forces exhibited by the consolidating network on the liquid being expressed from the cake. A full analytical description of the kinetics of such processes are not presently available. The Consolidation Model of Buscall and White, based on the Yield Stress (Py) concept applied to sedimentation, has already been discussed in some detail in Sections 3.2 and 3.3. This model automatically incorporates the ultimate dewatering limit of Equation (16) for consolidation. This follows from the choice of constitutive equation relating the time evolution of the concentration of solids in the cake (the substantive derivative) in terms of the yield stress parameter: 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. 16. A number of attempts have been made to couple equation (17) with continuity equations in order to derive scaling relationships for the physically distinct problem of filtration (cf the results for sedimentation, Sections 3.2 and 3.3 of this chapter). Possible strategies are outlined in references [25-273]. Although such an approach has not yet proved entirely successful a number of comments may be made: (1) As for sedimentation the equation (17) encompasses the notion that the driving force for solid/liquid separation is attenuated by the elastic stress in the cake as described by Py(ɸ). (2) Thus at low driving pressures (ΔP), Py(ɸ)) the rate of cake consolidation may be enhanced by manipulation of those factors (Sections 3.2.6 and 3.3) that reduce Py(ɸ). Strategies for suspension conditioning may utilize this type of reasoning and are considered later in Sections 3.5.3 and 3.5.4. (3) In contrast at relatively higher driving pressures (ΔP >> Py(ɸ)), the main factors controlling consolidation will involve properties of the primary particles such as drag coefficients, together with dynamic drag coefficients for the network (λ(ɸ) in Equation (17)). The pore structure and cake permeability will therefore be relevant and hence In this case the controlling factors are similar to those affecting the specific cake resistance as discussed earlier (3.5.2(b)). Until such a time as a full analysis of filtration in terms of the yield stress concept has proved possible, the kinetics of consolidation of filter cakes will remain a largely experimental science with understanding being at best qualitative. Possible experimental approaches to the problem are given In Section 3.5.6 and examples discussed in Section 3.5.7. 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
  17. 17. 1.2.5 Useful Semi-Empirical Relations for Constant Pressure and Constant Rate Cake Filtration Constant Pressure Filtration [1,10,12] As a starting point, the general cake filtration equation, (15), is rearranged in the following form: Integration of this simple form of the equation allows the relationships between filtrate volume, time and Instantaneous filtration rate to be deduced. The results are: 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. 18. Both the specific resistance and the medium resistance can be evaluated from the general equation, (18), via a plot of reciprocal rate, dt/dV, as a function of cumulative filtrate volume, V. Such a plot has a slope equal to the first bracketted term In the equation from which r o may be evaluated (at a given pressure); the intercept yields the medium resistance, Rm. It Is quite common with cake filtrations for the extrapolated data to pass through the origin such that the medium resistance Is negligible compared with that of the cake. These simple principles are Illustrated schematically in Figure 3. Likewise, for compressible cakes a series of reciprocal rate versus cumulative volume at various pressures yields the relationship between ΔP and specific resistance from which the exponent, S b of equation (14) can be evaluated (see Figure 3). Once the specific and medium resistances are known, from laboratory (or plant) measurements of V-l versus V, the relations (19)-(22), and those that follow for constant rate filtrations, may be applied In a predictive fashion. It is, however, Important to recall the predictions and restrictions that apply to equation (18) and were discussed in Section 3.5.2(b) (II). The two most important caveats in this context involve the scaling up of the quantities r o and Rm. The medium resistance Rm may be an important parameter and may not hold the same value at plant-scale as measured in the laboratory. Likewise, r o depends critically upon the mode of cake formation and so also may vary with scale, Initial filtration rate. 1.2.7 Constant Rate Filtration For a constant rate filtration, the pressure drop will increase as the cake builds up. In practical situations there will be a limit to the magnitude of ΔP that can be applied or tolerated. Hence it is necessary to know the cumulative volume, V*, and time, t*, associated with a given limiting pressure drop, ΔP*. The volume- time relationship is trivial: cumulative volume is given by the product of the time and constant rate. The other relations are again derived simply from equation (15): 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. 19. The equations (19)-(23) have been derived for strict conditions of constant pressure or rate. In reality many dewatering configurations operate in regimes which are intermediate between these two whereupon a more involved numerical integration of equation (15) will be required in order to derive volume-time relationships. A common configuration involving both extreme cases utilizes constant rate filtration until the pressure drop has reached Its maximum attainable (or tolerable) value whereupon the filtration continues at constant pressure until the rate falls to an unacceptable level. 1.2.8 Multiphase Theory of Filtration The theory and equations that have been presented in outline here, have long been accepted as a reliable though somewhat empirical description of the cake filtration process. However, more recently (- 1975 onwards), various workers have re-examined this so-called "two resistance* approach (ie r and ), and contrasted its basis with an alternative description, the "multiphase filtration theory" [22,23]. The latter Involves a lengthy derivation of equations based on a continuum mechanics approach. This detail will not be presented here but may be found In the references. Rather an outline of some of the results will be discussed and compared with the "two resistance" formulation. It is first worthwhile recapitulating on the Interpretation of data analyzed via the *two resistance" approach. In essence this method attributes the resistance to filtration to two additive contributions: that of the cake which may be a function of time and pressure etc and that of the septum or medium which is rigorously regarded as constant (and often negligible). For this case a linear relationship between the inverse filtration rate V-l and cumulative filtrate volume, V, Is taken to imply the following: (1) The local porosity and cake resistance are uniform. (2) The average porosity and cake resistance are constant. 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. 20. Where non-linear reciprocal rate data is encountered, the following conclusions are inferred from the apparent compressibility of the filter cake: (1) lion-uniformity of local porosity and cake resistance. (2) Time and other dependence for average porosity and cake resistance. (3) Dependence of cake resistance on slurry concentration, pressure and septum. In essence the “multiphase” description is based upon local continuity and motion equations for both suspending and particulate phases In the cake and septum. Application of dimensional arguments together with estimates of the magnitude of the relevant groupings indicate which terms dominate V-l. Briefly it is the pressure driven and drag forces that control the process whilst inertial and viscous forces may be largely neglected. At the end of the analysis the following multiphase cake filtration equation is gained: where G Is a function of slurry concentration, solid and liquid densities, the filter area (A), and the average porosity. For the case where the cake height is a linear function of filtrate volume, V, G can be shown to be Independent of V. Then the reciprocal rate equation depends upon the three quantities: K o (V) :- Septum permeability J o (V) :- Septum pressure gradient Po (V) :- Cake pressure drop and now deviation from a linear relationship between V-l and V for constant Po, are attributed to changes in the permeability and pressure gradient developed In the septum and its Interface with the filter cake. Willis et al have demonstrated that this theory is plausible by showing that a given cake (formed by the filtration of a Lucite slurry> can be made to undergo a transition from apparent “incompressible” to “compressible” filtration merely by changing the nature of the septum. 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. 21. In fact, Willis and co-workers cited only one experimental verification of their theory and so It Is unwise to speculate on Its generality. Perhaps the most important feature of the analysis Is that the attribution of non- linear reciprocal rate data to “compressible” cake formation without further corroborating evidence may prove fallacious if septum fouling, blinding or compression Is occurring. Means of obtaining this corroborative evidence and of quantifying cake compressibility without a septum present are considered later in Section 3.5.6. 1.3 Crossflow Filtration [8 -44] Although a detailed discussion of the various applications and some variants of crossflow filtration will be provided in Section 3.9 of this manual (“Pressure Driven Membrane Separation Processes”), a brief outline of the principles will be given here, The basic objective of a crossflow configuration is to achieve the limited dewatering of a suspension (or solution> using a permeable membrane but without the retention or immobilization of the solid phase. This is achieved by maintaining a tangential flow of the suspension across the surface of the membrane. Thus, Ideally, cake formation is totally avoided and the particulate phase remains evenly distributed throughout the concentrating suspension as a result of the convective effects of the flow [28,35,391. At a simple level the first requirement for an understanding of crossflow dewatering would be a model relating the permeate flux, ie the rate of concentration to the primary process variables: temperature, driving pressure, starting concentration and transmembrane velocity. For a perfectly ideal case, the flux relationship could be simply derived from treating the septum as a non-blocking bed of constant permeability. The flux would then be directly proportional to the driving pressure gradient as predicted by the Darcy or Kozeny-Carman relationships, equations (6)(9), In all real cases the behavior of the flux is by no means that simple. An alternative approach that has been applied, also largely unsuccessfully, Is to use some sort of modified cake filtration model. In reality the factors that control the flux of permeate are various, subtle and almost Inevitably time dependent and hence It Is no surprise that simple models are Inappropriate. some useful generalizations may, however, be made. 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. 22. In many real examples of crossflow filtration, particularly In ultrafiltration [31-37], the flux rate Is limited by a mechanism called concentration polarization. This arises because the layers of suspension closest to the membrane are those which suffer depletion from permeate. There Is therefore a local increase In concentration (but not necessarily a “cake”) which inevitably leads to a fall in the permeate flux. Acting In opposition to this mechanism are the effects of diffusion and laminar or turbulent convective flows. Clearly the diffusion process Is very dependent on the size and nature of the particulate (or dissolved) phase. Some useful relationships have been given to correlate the effects of the various factors that may operate when this mechanism is dominant. The following have been used for dewatering of biological suspensions by ultrafiltration [32-33]: It must be stressed that equations such as (25) and (26) are by no means applicable to all crossflow situations. In addition to the above problems Involving particle concentration gradient, another common source of permeate flux decline is the phenomenon referred to as "fouling" [35-36]. Fouling encompasses a whole series of processes whereby permeate flux falls as a function of time as a result of changes in the membrane Itself. Commonly these changes might Involve deposition of material on the surface or interior of the membrane often leading to a time-dependent decrease in Its porosity. Beyond these generalizations lies an enormous number of experimental studies and observations but unfortunately there Is as yet only relatively poor understanding of the phenomenon and how 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. 23. to avoid It. Current active research Is being carried out in various centers in the world with notable British contributions being made at Bath 1351 and at Varren Spring Laboratory - the latter under the auspices of a BIOSEP project [39-41]. This project has utilized various electron microscopy based investigations to probe the whereabouts of protein foulants, identified by staining, during biological separations. In the future, this sort of experiments should at least assist in the elucidation of the mechanisms of fouling In various cases thereby enabling the application of collold and surface science to avoid such problems. Another engineering based strategy for reducing fouling, the deliberate production of transmembrane, turbulent vortices, has recently been investigated by Hltchell I951. Again with respect to potential future developments, it is interesting to note that developments are being made in new filtration-based dewatering strategies involving the use of electric fields [38,40]. Examples of these process operations Include electrophoresls, electrodecantation, electroflltratlon, electro-osmosis and others. Some of these will be discussed in more detail in Section 3.9. However, a particularly worthwhile technology target In the present context involves the concept of harnessing a dielectrophoretic effect to prevent fouling or concentration polarization during crossflow filtration processes [40]. 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. 24. 2 The Range and Selection of Filtration Equipment Technology [1,12] It is not the purpose of this short section to attempt to provide a comprehensive equipment selection guide for filtration-based solids/liquid separation operations. There are already established sources for such Information; see, for example, Chapter 9 of reference [12] and Chapter 20 of reference [1] . Rather it is intended to Indicate how an understanding of both the properties of the material and the rest of the envisaged process train will facilitate a choice from the available filtration-based options. The main factors that Influence the choice of technology are: 2.1 Scale The scale of the operation is not normally too stringent a constraint since most devices are available in a range of sizes to handle a variety of capacities. In ‘general, however, very small scale separations will not usually command the most expensive filtration plant If thermal drying can follow the mechanical dewatering stage. For high value feedstreams (e.g. pharmaceuticals etc) other factors may override this option, however. 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. 25. 2.2 Solids Recovery, Liquids Clarification or Feedstream Concentration As a generalization most solids recovery dewatering operations will Involve the formation of a filter cake whilst clarification (Chapter 4) procedures will often avoid cake formation in order to maintain a high flux of liquid. Where feedstream concentration is required two options arise. Either a cake may be formed which Is then reslurried to a higher solids content, or a continuous thickening process may be employed. Very often a crossflow filtration arrangement will be appropriate for such a continuous thickening arrangement. 2.3 Rate of Sedimentation The rate of sedimentation of a suspension can have various effects on the choice of filtration plant. For example a bottom fed rotary drum filter may not be suitable for slurries containing a fraction of very large or very dense particles since these may settle out to form a "heel" well before they can be transported to the bottom of the drum. The sedimentation behavior Is also often critical In determining the structure of a filter cake closest to the septum. Thus If the initial filtration rate Is properly controlled, the bottom of the cake consists of the largest, fastest settling solids which may help to trap the finer end of the particle size distribution and thus reduce blocking and blinding. A third area in which the suspension settling properties are of paramount importance Is where a filtering centrifuge Is being considered as dewatering device. For these machines, the mechanism of operation entails a rapid settling of the solids phase In the centrifuge bowl followed by flow of the supernatant through the, hopefully, porous bed. The way In which this bed is formed and the properties that result will thus depend on the settling characteristics of the suspension. 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. 26. 2.4 Rate of Cake Formation and Drainage The rate at which the height of a filter cake rises can easily be assessed using simple laboratory filtration tests (see next section). It will depend on both the solids loading and the porosity and structure of the cake itself. This property has obvious repercussions on the geometry and necessary dimensions of suitable filtration equipment. 2.5 Batch vs Continuous Operation This is clearly a critical question which must be addressed by looking at the solids loading and rate of cake build-up, etc. 2.6 Solids Loading As already explained, this factor will affect (Ill), (iv) and (v) above. In addition it will strongly Influence the flow properties and hence the rate at which the suspension can be presented to the filter If this proves to be limiting. 2.7 Further Processing It is necessary to consider the Influence of additional operations which may either accompany the filtration or follow it in further downstream processing. Possibilities include washing, air blowing and thermal drying. The physical nature of the final product may also be relevant here (e.g. in re-dispersible systems). 2.8 Aseptic or “Hygienic” Operation When handling biological materials for pharmaceutical, food or other products, the necessity to be able to clean and sterilize a filter my impose particularly stringent demands. A detailed discussion of the relevant Issues and the suitability of various filters (and other plant) to aseptic operation Is given in the BIOSEP Report SAR 1 1401. 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. 27. 2.9 Miscellaneous Various other factors are likely to Influence decisions about choice of dewatering filter device. Of these the economics of the whole process Is probably the most important. Such considerations will not be considered here but are discussed In reference [45]. It is, however, worth pointing out that process decisions cannot be taken on the basis of economic factors in isolation. Very often physical constraints (e.g. those discussed in Section 3.5.2(b)) render an otherwise economically attractive strategy impossible. In order to illustrate the influence of the factors described in (i) to (ix) above, Table 1 presents an impression of the range of suitability for commonly available filtration devices. Having briefly considered the main factors influencing a choice of filtration technology, a short discussion of two related topics is appropriate here. These are the relative merits of dewatering by shear versus compression and by vacuum versus positive applied pressure filtration. 2.10 Shear versus Compressional Deformation During the latter stages of cake filtration, further dewatering is often achieved by the application of direct mechanical pressure to the cake itself - this Is the consolidation or expression process described In Section 3.5.2(b). Such a densification of the cake, In order to expel further occluded liquid, may be promoted by either an applied shear or uniaxial compressional deformation. For either case no change In the structure will result until a critical stress, the yield stress, has been exceeded. Figure 4 compares the yield stress for both shearing cay) and uniaxial compressional (Py) deformations for samples of BaC12-coagulated, polystyrene latex suspensions. The latter provide a convenient model which mimics a typical flocculated cohesive filter cake [46-47]. It can be seen that shearing forces are effective (In the sense of exceeding the relevant yield stress) at much smaller stresses (by some 1-2 orders of magnitude); these shearing motions will often enable densification In their own right via structural rearrangement and the concomitant collapse of the cake structure. The advantages of dewatering by shear or a combination of shear and compression are already exploited in many filtration rigs, e.g. counter-moving belt filters [49], but there are almost certainly further gains still to be made in this area. 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. 28. 2.11 Pressure versus Vacuum There are a number of hardware-based factors which favor a choice of pressure over vacuum filtration or vice versa and these are fairly simple to assess. Thus, in general, positive pressure filtration, being capable of yielding larger trans-septum driving forces, can yield greater filtration rates and hence reduce the size of dewatering plant. On the other hand vacuum filters have the advantage of simple construction and ease of continuous discharge in operation. They are, however, normally limited to total driving pressure drops of - 0.8 bar and, In the normal way, unsuitable for the filtration of suspensions containing volatile solvents. The above factors relate to the actual filters. In addition, there are more subtle factors, some of them less well understood, that pertain to suspension properties. Of these the most important is the cake compressibility. For a perfectly incompressible cake (s = 0) and a constant pressure filtration, equation (20) indicates that the filtration time for a given slurry volume is inversely proportional to the driving pressure. Thus potentially large gains in rate may be expected by the use of positive pressure drops greater than a bar compared with the vacuum configurations. For compressible cakes (s > 0) the same equation predicts that the advantage to be gained may be considerably attenuated by the pressure dependence of the cake resistance. An assessment of cake compressibility, for example by using the methods described later, Is therefore highly desirable if the efficiency of increasing the trans-septum pressure drop is to be predicted. Finally, to illustrate the subtlety of some of these effects, attention Is drawn to recent membrane (but not crossflow) filtration studies of Leaver and Bewdick 1421. Studying the filtration of protein (USA) solutions these workers have observed twice the permeate flux for vacuum compared with positive pressure filtration even though the trans-membrane pressure drops were apparently identical. The reason for this behavior is unclear, but presumably involves some sort of membrane fouling. 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. 29. 3 Suspension Conditioning Prior to Filtration Suspension conditioning may involve a simple mechanical treatment of the suspension, the addition of a so-called filtration aid, or a combination of both. The range of possible treatments may be conveniently divided into these two categories: 3.1 Simple Filtration Aids Using the term "filtration aid" in its broadest sense there are three general classes of aid. The first class contains those pretreatment chemicals which are added to modify the state of flocculation or coagulation of the suspension prior to filtration [50,51]. Commonly these additives may be inorganic, e.g. Al or Fe salts or polymeric, e.g. starches, gums, polyelectrolyte’s etc. The conventional purpose of such aids is normally to enhance filtration via one of the following: (i) Production of open aggregates so as to yield a porous filter cake thereby achieving fast filtration rates [50-52]. (ii) To yield strong aggregates so as to prevent wash-off and attrition; blinding and septum fouling is therefore reduced [50-52]. (iii) To improve the suspension rheology (Chapter 7). (iv) To modify the wetting behavior of the medium on the suspended phase. It should, however, be borne in mind that if further, mechanical dewatering of the filter cake by compression is ultimately to be sought, then factors (i) and (ii) will later prove deleterious. A compromise must then be struck to enable a structured cake that may be compressed under modest driving pressures yet retain sufficient porosity during the actual filtration for reasonable flow rates to ensue. Since the selection and action of flocculants is discussed in detail in a separate section of this chapter (3.7), no further mention of these will be made here. 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. 30. Additionally the reader may wish to refer to Chapter 2 (Sections 2.4 and 2.5) for details of flocculation mechanisms and the resulting floe structures. The other two classes of filter aids are the so-called *pre-coat" and "body aid" additives (1, 12). The purpose of the former is obvious and serves to provide an enhanced filter medium surface on which a cake may be laid down. It is usually formed by re-circulating a pre-coat slurry through the filter (typically a rotary vacuum device or similar) prior to the application of the suspension of Interest. A Body Feed on the other hand is completely mixed with the suspension requiring filtration before it reaches the filter device. It serves to Increase the porosity of the developing filter cake (i.e. Factor (i) above > and hence to lengthen the filter cycle time. An indication of the efficiency of either pre-coat or body-feed filtration aids may be gained by incorporating these additives in a small scale laboratory filtration trial such as those described in Section 3.5.6, In the main the function of the former may be assessed by its effect on the measured septum resistance, The body-feed aid on the other hand should have the effect of reducing the specific resistance of the filter cake. The properties of some commonly encountered pre-coat and body-feed filter aids are presented in Table 2. Further detailed discussion of the use of these is provided in references [1, 12, and 52]. Finally it is worth noting that surfactants are often employed in order to reduce the ultimate moisture contents of filter cakes [53]. More information on this aspect of chemical pre-treatments may be found in a later part of the chapter, Section 3.7.4 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. 31. 3.2 Mechanical Treatments A large number of options are available for suspension pre-treatments that do not necessarily involve inert or chemically active additives. The following list, plus a few pertinent references, covers some of the most commonly used methods: (i) Shear Treatment - often employed to reduce the apparent viscosity of the suspension [46-48]. (II) Degassing - more frequently employed prior to gravity separations. It may be necessary before the filtration of certain biological products, however (see Section 3.8). (iii) Suspension Ageing - like (i), (iv), (v), this technique is aimed at improving filtration performance via a modification of the flocculated structure of the suspension, e.g. in the manufacture of catalyst supports. (iv) Heat Treatment/Freeze Thaw I543. (v) Acoustic Methods - generally used for biological systems (see Section 3.8) [55]. It is important to note the immense potential value of suspension conditioning to filtration operations. The field of biotechnology covers many examples where such conditioning has either a profound influence on the process economics, or is absolutely essential to the Integrity of the product. For example, to avoid protein denaturation, degassing may be an imperative conditioning step. A full and valuable review of many aspects of conditioning relevant to bio-separations is provided in BIOSEP SAR Report "Primary Solid/Liquid Separation" [40]. Finally in terms of mechanical treatments it Is appropriate here to mention for completeness a technology development program being carried out by Batelle into "Combined Fields Separation Processes". The objective of this sort of approach Is to identify combinations of separation means such as electric and acoustic fields, such that synergistic advantages In dewatering may be achieved. 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. 32. In the cases of electro-acoustic and ultrasonic-assisted dewatering, Batelle claim highly significant Improvements In the rate and degree of filtration for suspensions containing particles such as coal, biological’s, paper pulp and food materials. Unfortunately technical details are not yet available though a number of patents have been filed. Although some of these treatments do not strictly involve suspension conditioning, It Is clear that there is considerable potential for the exploitation of filtration-based processes combined with other separation fields in this way. Post-Filtration Treatments and Further Downstream Processing [56] An outline of the influence and theory of three typical post-filtration operations, the washing of filter cakes. air blowing and thermal drying, serves to illustrate process interaction with the filtration operation. 4.1 Washing [56, 59] Filter cake washing is usually employed to effect a purification of the cake by removing entrained soluble’s, or less frequently to recover the mother liquor where the latter is of high value. The two main parameters of interest are the quantity of wash liquor required to achieve the required level of solute removal and the period of time taken for this degree of washing to be attained. Probably the simplest approach to calculating the required quantity of wash liquor has been provided by Vakeman. He distinguishes between filter cakes still holding filtrate in the voids, i.e., “saturated” cakes, and those that have been blown dry, the unsaturated cakes. For both cases Vakeman has analyzed the various mechanisms influencing the washing process and produced charts of the fraction of recovered solute as a function of the wash ratio, (i.e. the volume of wash liquor X the cake voidage) and one other dimensionless parameter. These then permit a very simple way of calculating the volume of wash liquor from small scale laboratory tests. Further details are not relevant here but good accounts of the use of the tests and theory, together with the charts, are provided in the references [1,12,56-59]. 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. 33. Once the volume of liquor has been calculated, the washing time is very straightforwardly estimated from the final filtration rate that was observed following cake build-up. Inasmuch b both the wash time and volume depend upon cake porosity and tortuosity, it will be appreciated that the factors that influence the mode of cake lay-down (including the various possible pre-treatments) will be very relevant to the washing performance. 4.1.1 “Air-Blowing” The use of "air-blowing" as a method of dewatering filter cakes is strictly not restricted to air alone; other gases or vapors, for example, nitrogen or even steam may be used. For biological or food suspensions the latter may provide an additional role for purposes of sterilization (see Section 3.5, 7(c)). The gas is propelled through the cake in a fashion appropriate to the filtration mode, hence for vacuum driven systems atmospheric air is commonly sucked through the cake (deliberately or otherwise) following "breakthrough". With filter presses, pressure nutches etc, compressed air is forced through the pores of the cake in order to displace as much moisture as possible. By using heated air or nitrogen some additional drying action is available; such techniques are, however, normally restricted to small scale or high value products usually having special problems of toxicity etc such that normal drying techniques are difficult to apply* The fundamental guiding principle in "air-blowing" is that the applied gas pressure must be sufficient to overcome the capillary forces tending to hold liquor within the pores of the cake (see Sections 3.2.9 and 3.5.7(a)). Probably the best current model for this process has been provided by Vakeman. Unfortunately, in terms of real operating experience, the predictions that it provides are of limited accuracy even for near-ideal systems containing hard particles of quasi-spherical geometry. Worse than that, for suspensions of high aspect ratio particles (e.g. needles or plates), or for compressible cakes or those prone to cracking, Wakeman's method is of little practical value. In terms of more empirical approaches, experimental work on a laboratory or semi-technical scale can be used to make predictions of dewatering time, final product moisture content and the air flow required. A number of cautionary points should, however, be noted. 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. 34. Since the final moisture content can be very sensitive to changes in the particle size distribution and the way that the cake Is formed, it is very Important to use identical material for the laboratory tests and to ensure that factors such as air flow rate and cake thickness are reproduced as closely as possible. Even so, as a "rule of thumb", it should be noted that small scale characterization tests tend to yield an optimistic figure for final moisture content since effects such as cake compression and cracking tend to be more prevalent on large scale. Further discussion of most of the above features as well as some more practical examples are provided in the references [60-65]. 4.1.2 Drying [67 -76] It is not our intention to treat the subject of drying in any detail here. However, a short discussion is included for completeness to highlight the importance of considering the interaction between the filtration operation and further downstream processes. It is hoped that a future release of the Suspension Processing Manual will contain a more detailed chapter (Chapter 10) based on those aspects of drying that will be alluded to In the present context. A general guiding principle that is invoked for most large scale dewatering trains is to remove as much water as possible by mechanical means (i.e. the filtration process here). This then minimizes the expenses of the energy-intensive downstream drying operation. However, it is normally the case that physical constraints imposed by the mechanical dewatering step will intervene before the hypothetical economic optimum is reached (see Section 3.10 - “Process Synthesis”). There is a large literature, both Internal and external to the Company, based on drying. A recent report by the FCMO drying team I661 described three typical regimes of “paste preparation prior to a drying operation”: a. Where there is no requirement for pipe flow. An example of this situation is where a filter cake is discharged at high solids content and is transported, perhaps by conveyor, to say an agitated vacuum oven for final drying. It will be typical here to obtain the maximum, physically-possible dewatering during the filtration. b. Where a filter cake is re-slurried in order to deliver it by pipe flow to typically a spray-drier. Clearly it is pointless in this case 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
  35. 35. mechanically dewater to the ultimate physical limit. The filtration step should be tailored towards facilitating the re-slurrying o f the filter cake to a manageable suspension. c. Where the paste is formed into a chosen, stable, physical shape to accelerate the subsequent thermal drying. For such cases the requirement of the filtration stage is to provide a paste with rheological properties that allow this shaping process, e.g. by extrusion. This situation is relevant to the formation of catalyst supports and ceramic materials in general. The sorts of interactions between mechanical and thermal dewatering indicated in (a - c) above are variously discussed in the drying literature [67-76]. It would appear, however, that relatively little Is known of how the morphology of the filter cake influences the rate of thermal drying. For example the relationship between, say, filter cake porosity and the necessary residence time in an oven drier would be a useful one to establish. Thus such Interactions would usefully be the subject of future research. Finally the subject of drying as part of a solids Isolation process is very critical when a redispersible solid is desired. This latter topic is treated in detail in Chapter 13 of the manual. 5 Testing and Characterization of Suspensions 5.1 Introduction – Suspension 5.2 Properties relevant to Filtration Performance In order to best utilize the principles and theory that have thus far been presented, it is necessary to know as much as possible about the "colloidal" properties of the suspension requiring filtration. Both the properties of the pre-filtration suspension and those of any filter cake that is formed are of importance. All or any of the following are likely to be relevant: 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. 36. 5.2.1 Pre-Filtration Properties of Suspension (i) Suspension viscosity including any tendencies towards shear degradation, thixotropic or any other structural modification following shear flow 146-481. (ii) Suspension medium viscosity and wetting characteristics on the solid (Chapter 2). (iii) Settling properties of the suspension, particularly the rate of sedimentation. Relative density of solid phase. Floe size and structure. (Chapter 2, References [50,5]) (iv) The size distribution of particles and/or aggregates that are present (Chapter 2). (v) The ease with which flocculated structure, and in particular the above size distribution, may be modified by mechanical treatments or inert/chemical additives. Such modifications will, of course, also influence the other suspension properties above. 5.2.2 Properties of Filter Cake (i) The mechanical strength of the cake and hence its resistance towards consolidation and the variation of this property with degree of consolidation (Section 3.7, References [50-52]). (ii) The porosity of the cake as a function of voidage, that Is the tortuosity of the path that supernatant must follow through the cake. This property then is correlated with the cake resistances [51]. (iii) The influence of mechanical treatments and additives to the suspension and the actual filtration conditions, e.g. rate of cake laydown, on the cake strength and resistance. Once a representative number of the above suspension properties has been determined so as to enable a good understanding of its "colloidal" behavior, the knowledge may be applied to the following targets: (i) Identification of the most appropriate plant and scale for the filtration unit operation or suggestion of a better, alternative dewatering means other than filtration (see Section 3.10). 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
  37. 37. (ii) Optimization of the way that the operation is carried out. For filtration this will include the choice of filtration conditions and details of cycle time as well as their Impact on further downstream processing. (iii) Prediction of optimal plant operation. Thus, for example, it is essential to know what level of performance in terms of rate and degree of dewatering can be expected under a given set of conditions. This Is of paramount Importance for scaleup calculations (see Section 3.2). (iv) Removal of process "bottlenecks" and correction of plant operating problems. This again relies heavily on (iii) and the identification of "benchmarks" for optimal performance. (v) To suggest where conditioning techniques 'and/or filtration aids may be desirable or appropriate. Whereas the means and optimal extent of pre-treatment should ideally be estimated from smallscale experimentation. 5.2.3 Laboratory Scale Filtration Rigs [77-80] A number of small-scale rigs exist and these may be applied to the measurement of filtration rates, filter cake properties and the Influence of suspension properties on them. These rigs are commonly used for the Initial derivation of data for scale-up purposes. If there is any doubt, they may also be applied to the question of identifying the filtration mechanisms of Section 3.5.2, although they are predominantly applied to cake filtration tests. Apparatus for measuring filtration rates on a small scale have been described by various workers [77-80]. The rigs of Allen & Stone [77], Gregory [78] and Bridger [80] are representative and of straightforward construction. The Allen & Stone apparatus, is well automated and their paper describes its mode of operation in detail. A reproduction from their paper is given In Figure 5 from which the basic operating principles are easily deduced. The original objective of the rig was to obtain data for scale-up purposes. In contrast to this, the equipment of Gregory was initially developed in order to assess the value of polymer flocculants as additives to filtration slurries and to derive optimum polymer dosages by experiment. The report of Brldger and Tadros uses a test rig to investigate 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. 38. some fundamental aspects of the Influence of suspension properties on the mechanistic details of cake filtration. The "equilibrium" and kinetic aspects of the consolidation process (Sections 3.2 and 3.5.2) may also be studied on a laboratory scale. Tests may be carried out on a small-scale variable volume filter such as the Piston press supplied by Triton Electronics, for example. With these devices it Is possible to measure the solids content of a consolidating filter cake as a function of pressure and also the rate at which this degree of consolidation is approached. In the same vein a gas-pressure driven pressure filter for laboratory scale tests from 0- "10 bar is now available from Schenk. 5.3 Means of Monitoring Flocculant Dosage The means of selecting appropriate flocculants and assessing optimal dosages is dealt with more fully in Section 3.7 of this manual. However, a recent addition to the range of portable, small-scale testing methods is well worth a mention in the present context. The new test method is an online monitor for flocculation control [81-82]. Its operating principles are based on the measurement of turbidity fluctuations In the flowing suspension of interest. Gregory [82] has shown that the root mean square fluctuation intensity can be related to the suspended particle size distribution via a semiempirical relationship. This conclusion enables the RMS signal to be used as a fast and sensitive Indicator of floe formation. The device, marketed by Rank Brothers of Bottlsham, is relatively cheap (ca $7.5K at the time of writing) and constructed in a way that makes it ideal for portable use and for continuous monitoring. Gregory has described applications where the device has been tested both in clarification and in achieving flocculation of more concentrated suspensions such as those requiring filtration. The method may also be comfortably applied to suspensions that tend to foul the sample cell simply by monitoring the ratio of both the RMS fluctuations and the average light transmission. This ratio has been shown to be relatively invariant to the deposition of modest quantities of material on the surfaces of the sample cell. A recent ad hoc, trial of the Rank Brothers monitor has been made. The device proved a sensitive indicator of flocculation In bacterial suspensions to which high molecular weight cationic polyacrylamides had been added. The response time was also fast demonstrating the potential of such instruments as continuous dosage monitors. 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. 39. 5.4 Filter Cake Testing For the general case of the suspension processing of fine solids the most common application of filtration involves cake formation and treatment. It is therefore appropriate to consider the parameters and means by which the properties of the filter cake may be characterized. The three principal properties that define the behavior of the cake are its strength, its permeability or, conversely, resistance, and the rate at which it is laid down. Methods for determining these will now be given. 5.4.1 Strength Testing (See also piston press described earlier) This is relevant both to an understanding of the influence of the pressure drop on the ordinary compressible cake filtration rate (as described by equation (14)) and to the subject of compression dewatering following filtration. Although a number of empirical measures of cake "strength" exist, the most suitable and fundamental parameter to use is the uniaxial modulus of compression, K [83] or the compressional yield function Py(Ø) described earlier. The former my be defined In terms of the effect of pressure on a cake volume (V) or concentration (Ø) change: The modulus, K, is a very strong function, (K ~ Ø3-4 of concentration, Ø, and depends upon the nature, shape and size distribution of the priory particles as well as the structure of the cake and the Interparticle forces. K Is related to the function Py(Ø) and is also very similar numerically to the conventional infinitesimal modulus of shear G(Ø). This fact enables its determination by straightforward laboratory techniques. (For further details see Section 3.2,4.) Arguably the simplest of these to use is the Pulse Shearometer Cell (Figure 8 of Section 3.3). This device enables the rapid determination of G (~ K) for a small sample of slurry or filter cake by measuring the propagation time of a low strain (~ 10-6) shear wave between two discs mounted on piezo-electric crystals in the cell. Calculation of G requires only the propagation speed of the wave, u (from the disc spacing and propagation time), and the density, p, of the cake or slurry: 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. 40. The shearometry technique is generally restricted to the range 103 < G < 106 dynes crn-2 but this is not usually a problem. Where cakes of higher strength need testing an alternative strategy may be adopted by measuring the compressional yield point, Py(Ø), in a centrifuge. The filter cake must now be formed in situ from the slurry (to be filtered) in a centrifuge tube. A measurement of the height of the equilibrium sediment as a function of gravitational field enables the evaluation of Py(Ø) over a range of concentrations (Figure 9 of Section 3.3). The upper bound of Py(Ø) measurable by this technique is constrained mainly by the gravitational field that the centrifuge is capable of (safely) producing and the density of the solid phase. Measurements of either G or K may then be used to evaluate the pressure, Pt, which must be applied to the cake in order to concentrate it to concentrations, (Ø)*: This then assumes a long enough contact time such that kinetics will not prove limiting. That Is It represents the equality Ps = Py(Ø), the ultimate or structural limit. For the centrifuge technique Py(Ø) may, in principle, be calculated from a single experiment. Using the shearometer cell a series of determinations at different slurry concentrations must be made. In both cases equation (29) is solved either by graphical or numerical integration. An example of the calculation is provided in the next section. Finally it may be noted for completeness that K may also be measured directly In a compression cell (84) but, for practical purposes, one of the two methods described above is usually more straightforward and of sufficient accuracy. For further clarification of the definition, interpretation and measurement of G(Ø), K(Ø) and Py(Ø) the reader is referred to Section 3.2. 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

×