Use and Applications of Membranes

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Use and Applications of Membranes

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 GENERAL
4.1 What is a Membrane Process?
4.2 What does a Membrane look like?
4.3 Why use Membranes?
4.4 Membrane Types and Polymers Used

5 REVERSE OSMOSIS
5.1 Principles of Reverse Osmosis
5.2 Limitations
5.3 Performance
5.4 Costs
5.5 Worked Example
5.6 Applications

6 MICROFILTRATION AND ULTRAFILTRATION
6.1 Microfiltration
6.2 Ultrafiltration

7 PERVAPORATION
7.1 Classes of Application
7.2 Characteristics
7.3 Costs
7.4 Example - Lurgi Design
7.5 Application - Stripping Organics from Water

8 GAS SEPARATION AND VAPOR PERMEATION
8.1 Gas Separation
8.2 Vapor Permeation

9 LESS COMMON MEMBRANE PROCESSES
9.1 Dialysis
9.2 Electrodialysis
9.3 Electrolysis
9.4 Salt Splitting

10 BIBLIOGRAPHY

TABLES

1 UTILITY CONSUMPTION AND COST COMPARISON

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Use and Applications of Membranes

  1. 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-MAS-615 Use and Applications of Membranes 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 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: Use and Applications of Membranes CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 GENERAL 3 4.1 4.2 4.3 4.4 What is a Membrane Process? What does a Membrane look like? Why use Membranes? Membrane Types and Polymers Used 3 5 6 7 5 REVERSE OSMOSIS 7 5.1 5.2 5.3 5.4 5.5 5.6 Principles of Reverse Osmosis Limitations Performance Costs Worked Example Applications 7 8 9 9 9 10 6 MICROFILTRATION AND ULTRAFILTRATION 11 6.1 6.2 Microfiltration Ultrafiltration 11 13 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. 7 PERVAPORATION 15 7.1 7.2 7.3 7.4 7.5 Classes of Application Characteristics Costs Example - Lurgi Design Application - Stripping Organics from Water 15 16 16 16 18 8 GAS SEPARATION AND VAPOR PERMEATION 19 8.1 8.2 Gas Separation Vapor Permeation 19 19 9 LESS COMMON MEMBRANE PROCESSES 20 9.1 9.2 9.3 9.4 Dialysis Electrodialysis Electrolysis Salt Splitting 20 20 21 22 10 BIBLIOGRAPHY 22 TABLES 1 UTILITY CONSUMPTION AND COST COMPARISON 18 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  4. 4. FIGURES 1 MEMBRANE "FILTER" PROCESS 4 2 DEAD END FILTRATION 4 3 CROSSFLOW FILTRATION 4 4 PLATE AND FRAME MODULE 5 5 SPIRAL WOUND MODULE 5 6 REVERSE OSMOSIS OF SEA WATER 7 7 THE REVERSE OSMOSIS SYSTEM AND NOMENCLATURE 8 8 TYPICAL EFFECT OF FEED PRESSURE ON PERMEATE 8 9 CANADIAN SITE CLEAN-UP 11 10 LANDFILL LEACHATE TREATMENT PROCESS 11 EFFECT OF PRESSURE DIFFERENCE, VELOCITY AND SOLIDS CONTENT ON FLUX 12 12 RETENTION OF SOME "DIAFLOW" MEMBRANES 14 13 EUROPEAN DYES PROCESS 15 14 FLUX AS A FUNCTION OF CONCENTRATION 17 15 THE LURGI PERVAPORATOR PILOT PLANT 17 16 LURGI PERVAPORATION PLANT COMPARED WITH ENTRAINER DISTILLATION PLANT 18 17 LIQUID EFFLUENT CLEAN-UP 18 18 GAS PERMEATION PILOT PLANT - MERCEDES BENZ 20 19 ORGANIC STORAGE TANK VENT SCRUBBING 20 11 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. 20 ELECTRODIALYSIS 21 21 SALT SPLITTING 22 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. 0 INTRODUCTION/PURPOSE Membrane processes are a large family of techniques that can provide spectacular separations in certain cases. Unfortunately such cases are relatively rare. Therefore the number of engineers familiar with the principles involved and opportunities open is relatively small. The purpose of this Process Engineering Guide (PEG) is help the reader consider if there might be a benefit from successfully employing a membrane technique for a separation task. The aim is not to make the reader an expert but to get to the stage where the ideas can sensibly be taken to one of the experts. 1 SCOPE This Process Engineering Guide presents an overview of membrane processes and, for the more common ones, some ball-park throughput and cost data, as well as worked examples. It does not cover the design of membranes. 2 FIELD OF APPLICATION This Guide applies to the process engineering community and others involved in process development in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: Osmotic pressure The applied pressure required to prevent the flow of a solvent across a membrane which allows the passage of the solvent but not the solute and which separates a solution from pure solvent. Permeate The stream which passes through (permeates) the membrane, i.e. the efflux which has passed through the membrane. Recovery(S) Mass ratio of flowrates of Permeate stream to feed stream. Rejection(R) 1 - ratio of concentrations of solute in Permeate to feed (this term is usually specific to Reverse Osmosis (RO)). 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. Retentate The stream which is retained by the membrane, i.e. the efflux which has not passed through the membrane. 4 GENERAL 4.1 What is a Membrane Process? A membrane process is where a fluid mixture is placed on one side of a thin sheet whose properties are such that one or more components in the mixture pass through it more easily than others. The actual process occurring can be adsorption, solution, diffusion, evaporation or a combination of these. However, many membrane processes can be regarded as "fine filters". Some are able to filter out or fractionate at molecular level, as illustrated in Figure 1. FIGURE 1 MEMBRANE "FILTER" PROCESS Other membrane processes such as gas separation, pervaporation, dialysis and electrodialysis also operate in the lower regions of Figure 1. Processes operating in the upper reverse osmosis (RO) region or the lower ultrafiltration (UF) region are sometimes called Nanofiltration (NF). 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. Dead end filtration (see Figure 2) is that commonly used for in-line filters and laboratory filtrations. All of the feed must either pass through the filter medium or build up as a filter cake. Membrane processes do not usually operate this way. FIGURE 2 DEAD END FILTRATION In crossflow filtration (see Figure 3), the feed travels across the face of the filter medium. Some material passes through the medium to form the Permeate. Not all permeable material is allowed to pass through. It carries the concentrated solute or solids out of the filtration (or membrane) unit in the form of a concentrate or Retentate. FIGURE 3 CROSSFLOW FILTRATION 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. 4.2 What does a Membrane look like? In its most usual form, the membrane is a flat sheet of polymer base with a thin layer of semipermeable material on one side. These layers might be the same material but are usually different, the base layer being porous, to allow easy fluid flow whilst supporting the active layer. The semi-permeable layer is very thin to give good mass transfer rates. Sheet membranes can be mounted in a plate and frame arrangement like a plate heat exchanger (see Figure 4). However, they are more commonly wound into a spiral giving a cylindrical appearance (see Figure 5). The required duty influences the choice of configuration and construction details. A second common form is extruded tubular material, in which case the membrane is usually homogeneous. The tube diameter can vary from under 1 mm to several mm. These are usually secured into a cylindrical housing to result in what looks like a miniature shell and tube heat exchanger. A third common form is more like a normal shell and tube heat exchanger. Here the metal tubes are covered in sheet membrane material which they support, and they have many holes in them to allow the Permeate to pass through. This arrangement is good for high viscosity liquids where other arrangements have excessive pressure drops at the fluid velocities needed to maximize the Permeate flow rate. FIGURE 4 PLATE AND FRAME MODULE 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. FIGURE 5 SPIRAL WOUND MODULE 4.3 Why use Membranes? (a) Low energy use Depending on the application, a membrane process employed to concentrate an aqueous solution can use as little at 1% of the energy of an evaporation process. (b) Novel separations Separations can be performed which are not possible by other means. For example, certain azeotropes can be separated, aromats can be separated from aliphats, and fractionations on molecular weight are possible. (c) Waste recovery Effluents, both liquid and gaseous, can be cleaned up using membrane extraction processes more economically than by using other technologies. 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. (d) Displacement of chemical equilibrium In certain reactions, byproducts can be removed continuously, leading to improved reaction conversions; e.g. esterifications, where water is continuously removed to favor the forward reaction. 4.3.1 Reasons for Benefits Most membrane processes do not require a change in state in the process fluid. Energy input is only required to overcome the Osmotic Pressure and flow losses. Hence they have a very low energy input compared to evaporation, and especially when compared to distillation using a high reflux ratio and minimal heat integration with other process items. Membranes involve a number of transport processes and these are not limited by vapor liquid equilibria. 4.3.2 Constraints on Use of Membranes There are a number: • In reverse osmosis, the Osmotic Pressure and membrane strength limit the concentration of dissolved inorganic salts to about 5 wt% (although higher molecular weight species can be concentrated more). • Membranes are not compatible with all chemicals, and are usually prone to fouling by small particles and by microbiological activity. • Membranes need to be tested before accurate design in any new application is possible. • Membranes sometimes have to be developed for new separation processes which adds to the cost and time needed. • Membranes are not always economic when alternative techniques exist, especially on a large scale as the normal process plant economy of scale does not apply due to their modular construction. However the world market for membranes is over $3 billion/year, and growing about 15%/year (2005 figures). Most of this is in dialysis membranes (see 9.1). 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
  12. 12. 4.4 Membrane Types and Polymers Used Membranes come in 2 main physical forms, porous and non-porous. Porous membranes, as used for microfiltration, have microscopic holes in them. In nonporous ones, as used in reverse osmosis, the separating layer is a dense film without holes. They come in as many polymer types as can be processed into a membrane. Polymer choice is dictated by the separation needed and processing conditions. Cellulose acetate, polyvinyl alcohol, and polysulfones are the commonest polymers used. Some specialized ceramic membranes are also available. 5 REVERSE OSMOSIS Osmosis has been observed for centuries. Reversing the process to produce a purified water from a contaminated one was first observed around 1900. It became a commercial process in the 1960s when asymmetric membranes were developed which could produce potable water from brackish water or sea water at a good rate. Figure 6 shows the reverse osmosis of sea water diagrammatically. FIGURE 6 REVERSE OSMOSIS OF SEA WATER Membranes for these duties are now highly developed and available as proprietary items in a variety of package types. 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. 5.1 Principles of Reverse Osmosis All solutions exhibit an Osmotic Pressure, but it is very low unless the molecular weight of the solute is small: Sea water has an Osmotic Pressure of about 27!bar g. Purification systems are characterized by their yield and product quality. In RO systems, these are expressed as the Recovery (S) and Rejection (R). Both salt and water traverse the membrane, but by different mechanisms and under different driving forces. The water flow rate is approximately proportional to the difference between the applied pressure and Osmotic Pressure differences between feed and Permeate sides, whereas the salt transport is concentration driven, and is independent of the applied pressure. Therefore, RO membrane give their best quality product at high transmembrane pressure and low product Recovery. As the applied pressure drops, the water rate drops, and the Permeate salt concentration increases (see Figure 8). As the Recovery increases, the outlet concentration increases, and so the salt transport rate at the outlet increases, deteriorating the average product quality. The deterioration of product quality depends on the membrane selectivity. 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. FIGURE 7 THE REVERSE OSMOSIS SYSTEM AND NOMENCLATURE Using the symbols given in Figure 7, S and R are defined as: FIGURE 8 TYPICAL EFFECT OF FEED PRESSURE ON PERMEATE 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. 5.2 Limitations Modern RO membranes are usually capable of withstanding a transmembrane pressure difference of 70 bar (1000 psi) and temperatures of about 70°C. Some are able to withstand low levels of free chlorine, but many are not. All these membranes are very sensitive to fouling and pretreatment is absolutely essential. This will involve removal of fine particles, removal of sparingly soluble salts which might precipitate out as the feed concentration increases, and pH adjustment. Increasing Osmotic Pressure limits the concentration to which a solution can be concentrated using RO. As Osmotic Pressure varies with temperature and molecular weight, no specific upper concentration can be quoted, but a concentration such that a driving force of at least 100!psi is left would be reasonable. For NaCl, using 1000 psi feed pressure this would be about 8.0!wt%. 5.3 Performance The production rate (flux) will drop from an initial value and become fairly stable, typically at 50 - 100 kg/m2 h at 40°C and 40 bar pressure difference. Periodic cleaning with a commercial cleaning solution will be necessary to maintain this. Rejection varies with the dissolved species, but typical figures would be the following, which are for a commercial Desal-5 membrane: These performances are based on certain feed flow rates. If the flow rate is reduced, the performance deteriorates as the boundary layer resistance increases. This is known as membrane polarization. Membrane flux varies exponentially with temperature, and it is worthwhile operating close to the membrane temperature limit if possible. 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. 5.4 Costs Membrane spiral wound modules cost typically $15 - 35 per m2 before installation. However, the pumping, storage and control equipment costs have to be added to this. This cost will then typically be at least doubled by the need to provide feed pretreatment. The details vary with each application, but a total installed cost in the order of $ 35 - 65 per m2 membrane area should be anticipated. 5.5 Worked Example How much membrane area is needed to produce 1200 m3/day of pure water (Na2SO4 concentration < 500 ppm) from process water containing 2% dissolved Na2SO4 at 50°C using reverse osmosis? A single stage process is envisaged using a feed pressure of 35 bar g and a Recovery of 30%. The membrane has the following characteristics: Retentate concentration is given by: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  17. 17. Hence the product specification is achieved using this membrane and assumed Recovery. Other Osmotic Pressures can now be calculated: Note: The concentration in use is much higher than that in the specification, so a test to examine the effect of this on Rejection would be necessary. The Recovery might then have to be reduced to keep the Permeate in specification as the Rejection would probably be less under these conditions. 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. 5.6 Applications 5.6.1 Site clean-up - Canada A large site in Canada with a problem; a dump of gypsum and 500 million gallons of ponds contaminated with ammonia, phosphates and dinitrotoluene, the legacy from a previous phosphates plant complex. Rainfall was causing these materials to be washed out into local water courses. Lime treatment and passage through ponds did not improve the runoff water to a level suitable for irrigation or cooling tower makeup. Evaporation and reverse osmosis were examined as means to purify the treated water. RO was selected as it offered the lowest overall cost (see Figure 9). FIGURE 9 CANADIAN SITE CLEAN-UP A system of pretreatment was developed which removes suspended solids, kills algae and reduces hardness. The reverse osmosis final stage then produces water to specification, with the Retentate being recycled at present to the ponds. Water is produced at a rate of 335 Imperial gpm (91 m3/h). This represents removal of water from the ponds at over 5 times the rainfall rate, and would empty the ponds in about 3 years. Frequent changes in the condition of the feed water make it difficult to pretreat consistently. However, the RO system is working very well, and the Permeate has been used as cooling tower makeup water, having < 200 ppm dissolved solids. The concentrate, with > 28000 ppm solids, will in future be evaporated to dryness, contributing to their target of becoming a zero discharge site. The membranes used are from Fluid Systems (San Diego) type 8221HR which are cellulose acetate, spiral wound units. 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. 5.6.2 European Landfill (see Figure 10) Reverse osmosis can take quite crude waste streams and ground waters and turn them into potable quality. Similar plants exist taking contaminated water and cleaning it, e.g. taking wash water from plating plants to produce a concentrated effluent and clean wash water for reuse. FIGURE 10 EUROPEAN LANDFILL LEACHATE TREATMENT PROCESS Flow: 35 m3/h Total cost: $USD 3/m3 Effectiveness: > 95% of BOD, COD and Cl - removed 6 MICROFILTRATION AND ULTRAFILTRATION 6.1 Microfiltration Microfiltration (MF) is a process which can remove particles from suspension down to about 0.02 microns. Hence it uses porous membranes with pore diameters suitable to retain the solute, or the dissolved species to be removed. It might be selected such that some materials pass through the membrane, hence giving a fractionation. It is pressure driven with the differential pressure quite low, up to 2 or 3 bar. Asymmetric membranes are used to maximize the flux. In contrast to dead-end filters (see Figure 2) such as strainers, belts filters, etc., these filters operate with a continuous, recycling flow of feed across the membrane face. This is known as crossflow filtration (see Figure 3). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  20. 20. The flow minimizes the boundary layer thickness and maximizes the flux. Hence the feed can be allowed to increase to very high levels of suspended solids, the limiting factor usually being pressure drop along the membrane due to the viscosity of the slurry. A diagram is given in Figure 11. FIGURE 11 EFFECT OF PRESSURE DIFFERENCE, VELOCITY AND SOLIDS CONTENT ON FLUX The flux increases with increasing crossflow velocity and with increasing transmembrane pressure drop until a plateau is reached. The flux also varies with solids content and temperature. Hence experimentation is usually needed before a system can be designed. It is normal to arrange more than one filtration stage for large changes in concentration to optimize flowrates and pressure drops. Also the optimum membrane geometry will vary for different Concentrations, and later stages handling high viscosity slurry will have different membranes and modules than earlier ones. Fouling is a perennial problem with membranes, and provision has to be made to allow for regular cleaning. This will be done by taking the unit off-line and circulating cleaning agents. These are usually surfactant solutions with appropriate cleaning additives. 6.1.1 Costs MF membranes are more expensive than RO ones as they are arranged in tubular form. Complete in a housing, they cost from 350 to 650 $USD/m2. Costs of pumps, tanks, piping and instrumentation have to be added to this. The specific costs gets less up to modules of about 10 m2, and with bulk purchases, but the economy of scale is not high. Fluxes depend on all the above variables, but values in the region of 300 to 1000 kg/h/m2 are typical. 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. 6.1.2 Worked Example Design an MF system to concentrate 3.6 m3/h of a slurry from cf = 5 % w/v to cr = 20% w/v using a single stage process and MF modules each containing 19 tubes in parallel, 1" ID and 42" long, giving 1.62 m2 filtration area. The maximum allowed transmembrane pressure difference is 3 bar. Assume Permeate pressure is 0 bar g. Laboratory trials have resulted in the following data: Flux J = 2 × 10-5 V 0.75 . ln (cg/cr).dP.e-0.3dP m3/sec.m2 Retention = 100% Pressure drop at 1 m/s = 0.2 bar Note: This model follows the gel layer model where fouling and concentration polarization are modeled by regarding them as the formation of a physical layer which limits the upper concentration which the Retentate can reach, cg. Assume a value of cg = 30% Assume operation in a circuit where the feed is approximately at product concentration. This design will look at pressure differences ΔP in the region of 1 to 3 bar, and velocities V from 1 to 3 m/s. For 1 m/s with the modules in parallel, the pressure drop down the tubes is small. However at 3 m/s and/or modules in series, the effective ΔP will be much less than the feed pressure, leading to a low average flux. Ideally the flow over a membrane will be turbulent. However, at the high viscosity which slurries like this one can reach, this is often not possible. Calculation shows that in this case, for a slurry viscosity of 360 cP, Re = 71 at 1 m/s, i.e. the flow is streamline. 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. (a) Modules in parallel, V = 1 m/s, 1 bar g feed (b) Modules in parallel streams each containing 4 modules in series, with V = 3 m/s, and 3!bar g feed Clearly this is a better design, and further alternatives could be explored using more modules in series (but at necessarily lower velocity due to pressure drop) and then considering the total costs allowing for the modules, pump, pipework, recirculation tank (if any), instrumentation, power consumption, etc. A further option is to split the duty into 2 stages, the first from say 5% to 10% w/v slurry. This is more effective as the first stage works at a higher flux due to less polarisation, the 3 bar pressure is generated twice, and flows can be better optimised. Although the cost of two pumping and recirculation systems has to be borne, this is frequently a more cost-effective arrangement. 6.2 Ultrafiltration Ultrafiltration is arranged in crossflow mode rather like MF. However, the membrane is usually non-porous or has very fine pores, and so a larger transmembrane pressure drop is needed to achieve a realistic flux. Typically, the flux of UF units would be in the range 20 - 150 kg/m2/h. The pressure drop used would be from 2 to 10 bar. 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. UF membranes are usually characterised by their 'molecular weight cutoff' (MWCO). This is determined by passing solutions of globular proteins, which are spherical molecules, through the membrane, and finding the relationship between retention and molecular weight. The MWCO is defined as the molecular weight which has a retention of 90% under the test conditions. In practice, you can only use this to select which membranes to test, as performance varies with more than just molecular weight (for example, molecular shape and pH), and two different membranes with the same MWCO may give different results for your system. Also, the sharpness of the cutoff varies between membranes, and depends on the pore size distribution. UF is often used for oil-water separations. If the oil in water is a stable emulsion, it can be removed down to about 10 - 50 ppm. If the feed is over about 2% oil, a centrifugal pretreatment is usual. Figure 12 shows how some commercial UF membranes retain test solutions of varying molecular weights. FIGURE 12 RETENTION OF SOME "DIAFLOW" MEMBRANES 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. 6.2.2 Application – European Dye Manufacturer European Dye Manufacturer makes a range of reactive dyes.The dyes are made as an aqueous solution, then precipitated by the addition of salt and filtered, The filter cake is re-slurried and the larger part of the production is evaporated to dryness. However, many customers prefer the product as a strong solution, and this can only be made if all the salt and other impurities are removed. Therefore a process was developed where the solution is passed over a membrane which was chosen to allow the salt and other small molecules to Permeate, but to retain the larger dye molecules, whose molecular weight is typically 650 to 1300. The process has been refined and enlarged over the years. It started production as a 50 m2 membrane area installation, making 300 te/yr of liquids. It now stands as a 375 m2 plant, making 2000 te/yr (see Figure 13). The process of making these liquid dyes would not be possible without this nanofiltration process. FIGURE 13 EUROPEAN DYES PROCESS 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. 7 PERVAPORATION Pervaporation (PV) is a permeation process in which the Permeate evaporates as it passes through the membrane. It uses non-porous membranes, and the mechanism is one of solution of one or more species in the feed into the membrane, followed by diffusion through the membrane. The phase change occurs inside the membrane, as the material leaves the non-porous active layer. Hence a separation can occur based on solubility differences for the feed materials in the membrane. The primary driving force is concentration difference not pressure, and so upstream pressure does not affect the process much, whereas downstream pressure has a strong effect on pervaporation rate. 7.1 Classes of Application Three classes of pervaporation have emerged: (a) Drying of alcohols, ketones and organic acids These often exhibit an azeotrope which poses problems for conventional processes. PVA membranes have been used in pervaporation processes to dry sub-azeotropic mixtures to points above the azeotrope. Ethanol drying plants are the largest PV plants in operation, with capacities up to 50,000 te/yr. The feed cannot be more than 20% water to avoid damage of this membrane material (other materials do not have this limit, but have a poorer selectivity). Studies have shown that PV is economically similar to azeotropic distillation at this scale, and considerably better at smaller ones. (b) Stripping organics from water These operate at under 10% organics in water. Examples of stripping benzene, CFCs, and solvents have been reported but no large installations are presently in operation. (c) Separating mixtures of organics Some interesting separations are possible which are presently only possible by adsorption and solvent extraction. For example, separation of aromatics from aliphatics, and alcohols from similar ketones has been reported. 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. 7.2 Characteristics As latent heat has to be supplied, the feed cools as pervaporation proceeds. As flux declines at lower temperatures, it is necessary to remove the feed and reheat it if a significant amount of Permeate is required. Low pressure drop on the Permeate side is important, and so modules tend to be specifically made spiral wound or plate and frame types. 7.3 Costs Due to the inherent complexity of the plant with vacuum pump, condensers and other auxiliaries, PV plant can typically cost $USD 350 per m2 membrane area, but for a special membrane this can reach $USD 250000 per m2. The membrane flux depends heavily on the outlet concentration, and so therefore low target concentrations lead to large membrane areas. Typically, a PVA membrane will pass 1 kg/h/m2 of water from a solution of 1% water in ethanol, falling to zero at 0.1 % water. Consequently, many studies have shown that hybrid plants are the most economic option, with conventional distillation being used for the bulk of the separation, and PV being used for surpassing the azeotrope. Treatment of 3 m3/h effluent containing 1000 ppm benzene, to discharge at < 10 ppm benzene, has been shown to cost a total of about $USD 3.5 per m3. Another company quotes treatment of 40 te/day of effluent containing 1000 ppm trichlorethane as costing capital of 1500 to 3500 $USD/m3/day capacity, and running costs of $USD 1.5 – 3.5 per m3 effluent. 7.4 Example - Lurgi Design The following example is based on work published by Dr. Ulrich Sander of Lurgi GmbH. Laboratory experiments have to be carried out initially to establish the flux obtained and the relationship between separation coefficient and feed concentration, all at different temperatures. Flux is strongly dependent on temperature. Strictly, these also need to be measured for different Permeate pressures, although again only the flux should be sensitive to this. In nonobvious cases, different membrane materials, perhaps even similar ones but from different sources, or made by different methods, need to be screened in the laboratory. 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. Lurgi have made many PV alcohol dehydration plants, and a typical plant takes 94 wt% ethanol, aiming to dehydrate it to 99.5 wt%. The heat and mass balance shows that the feed cools too much before the product specification is met for this to be a single stage process. The design can have up to 10 stages, i.e. the feed is preheated to 100°C, then removed at 90 - 95°C for reheating to 100°C, before returning to the next stage, 10 times. The Permeate from each stage is condensed at 10 - 15 mbar. Lurgi prefers plate and frame type units made from stainless steel. The membrane chosen is PVA composite, which is always used for dehydrations like this. The relationships between Permeate flux and composition and the feed composition at various temperatures can be shown on a graph. From the mass and heat balance, lines can be drawn on it to show the temperature and composition profile, assuming perfect mixing, no polarisation and equating the sensible heat change in the feed to the latent heat in the water permeated. These show that a single stage process is ineffective as it works at too low a temperature and hence too low an average flux. Hence the feed is allowed to cool to, say, 90°C, after which it is removed from the first stage, reheated to 100°C, and returned to the second stage. Clearly, there is some optimum between keeping the feed hot with many reheating stages, and hence minimum membrane area, with the converse. The calculation needs to be done several times to determine this optimum. Figure 14 shows that about 5 stages would result if the feed were removed when the temperature drops to 90°C or the flux drops to 0.1 kg/h/m2. 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. FIGURE 14 FLUX AS A FUNCTION OF CONCENTRATION The area can be roughly calculated by considering each stage as working at an average flux between the values for the inlet and outlet streams for the stage. Using this method, the following areas are derived for a unit making 500 kg/h of dehydrated alcohol: Stage Area/m2 1 41.0 2 58.7 3 42.9 4 67.2 5 130.6 Total area: 340 m2 Figure 15 shows a typical PV plant flowsheet. 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. FIGURE 15 THE LURGI PERVAPORATOR PILOT PLANT Figure 16 shows how a PV/distillation hybrid system compares to an entrainer distillation system for an azeotropic mixture such as ethanol/water. Table 1 shows their relative economics. FIGURE 16 LURGI PERVAPORATION PLANT COMPARED WITH ENTRAINER DISTILLATION PLANT 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. TABLE 1 UTILITY CONSUMPTION AND COST COMPARISON 7.5 Application - Stripping Organics from Water Pervaporation can remove organics from aqueous wastes. Whether it is the most cost effective way of doing this for any particular waste cannot easily be decided, but needs evaluating (see Figure 17). FIGURE 17 LIQUID EFFLUENT CLEAN-UP 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. 8 GAS SEPARATION AND VAPOR PERMEATION 8.1 Gas Separation Gas separation is a diffusion process, and hence is done by porous membranes. It has been commercially in use for many years. The technology breakthrough which revolutionized this was the use of a silicone layer on polysulphone membranes to plug pinholes which cause severe performance loss with gaseous materials. Monsanto invented this, and their patented 'PRISM' system is installed worldwide, typically in ammonia plants. Flowrates up to 84,000 m3/h are processed. Separation occurs mainly due to molecular weight differences. Hence it is most commonly used in extracting hydrogen from mixed streams. However, water, H2S, CO2 and O2 all diffuse more quickly than N2 and CH4, say, in PRISM membranes. Gas separation is also used to separate nitrogen from air, and +2500 units worldwide produce this at lower cost than a cryogenic process. They usually operate with high feed pressure. Membranes made from polyimide or polyether ketone allow higher operating temperatures of 150°C and 200°C respectively, than the PRISM ones which are limited to 100°C. 8.2 Vapor Permeation Vapor permeation differs from gas separation in that it is a solution/diffusion process and uses non-porous membranes, at close to atmospheric pressure, with the Permeate under vacuum. These systems are much less common than gas separation, but are being offered for duties such as scrubbing organic-laden air caused by breathing of stock tanks. Units extracting vinyl chloride monomer from air are operating in the USA. Fluxes are lower than for gas separation due to the higher molecular weight of the Permeate and the nature of the membrane, although appropriate membrane choice such that relative solubilities favour the desired extraction can improve them. 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. 8.2.1 Application - Mercedes Benz A good study of extracting valuable dimethyl isopropylamine from exhaust air was carried out by Mercedes Benz. They studied the optimization of refrigeration temperature and vacuum pressure to produce a design which scrubbed their casting shop exhaust air from 4000 ppm to under 1000 ppm (which then went to a caustic scrubber). The plant processes 1000 m3/h air, cost a total of $USD 200K, produces 99% DMIP for re-use, and paid for itself in under 3 years (see Figure 18). 8.2.2 Application - Process Vent Clean-up Vapor separation membranes can extract organic materials from air streams to produce a vent suitable for atmospheric emission. If the specification is very low, scrubbing might be needed as a polishing stage. Most projects would be justified on environmental grounds, but for high value materials, a cost-effective Recovery can be demonstrated over and above this (see Figure 19). FIGURE 18 GAS PERMEATION PILOT PLANT - MERCEDES BENZ 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. FIGURE 19 ORGANIC STORAGE TANK VENT SCRUBBING 9 LESS COMMON MEMBRANE PROCESSES 9.1 Dialysis Dialysis accounts for the majority of membrane material used in the world today. It is a concentration driven process, where the process fluid passes over a membrane, the other side of which is swept by a suitable fluid, the 'dialyte'. The membrane is chosen such that undesired materials in the process fluid will pass through, driven by their low concentration in the dialyte, where they might react or just be diluted. Other materials are retained by the membrane, and so the process fluid is purified. The mass transfer coefficient is usually low, even if high flow rates across the membrane surface are used. The membranes are usually not very selective, and so the process is not likely to be useful to large scale processes. However over 100,000 people owe their lives to dialysis; it is the separating process used in artificial kidney machines to remove toxins from blood. 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. 9.2 Electrodialysis The membrane process uses an electric field as the driving force for the separation. It therefore has characteristics quite unlike the other membrane processes. It is only loosely related to dialysis, in that process fluid sweeps both sides of the membrane. Membranes can be made such that they are very selective to ionic charge differences. Cationic membranes allow the passage of positively charged ions, but reject negatively charges ones. An electrodialysis unit (ED) consists of a large stack of alternating cationic and anionic membranes (see Figure 20). If an electric field is impressed over the whole stack, ions try to move towards the oppositely charged end of the stack. However, the membranes interfere with this, with the result that the channels between the membrane pairs become alternately enriched and depleted in the ionic species. If the two types of product stream are physically separated as they flow out of the cell, a separation has been achieved. A European Institute has done a lot of work on ED, in their assessment of using ED to purify dilute aqueous waste, (that 8% sodium nitrate solution could be concentrated to 18%. The unit also produced a depleted stream containing 1% sodium nitrate. The power consumption was 0.26 kWh per kg of NO3 in the concentrate. ED appears to be more economic than RO in some circumstances, and is used in many large brackish water purification plants worldwide. It has also been used to remove acids from effluent water, remove salt from sugar and cheese whey solutions, and remove tartaric acid from wine. It can concentrate the feed material to a much higher concentration than RO as it does not suffer from Osmotic Pressure limitations. Hence, sodium chloride can be concentrated up to about 20 wt%. This is used in Japan as a preconcentration stage for the production of solid salt. 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. FIGURE 20 ELECTRODIALYSIS 9.3 Electrolysis Membranes have been used in brine electrolysis since the invention of the diaphragm cell. The modern way to make chlorine is using a membrane cell in the plate and frame arrangement. 9.4 Salt Splitting A variant on electrolysis and electrodialysis is salt splitting (see Figure 21), where a salt such as sodium nitrate is electrolyzed and combined with water to produce nitric acid and sodium hydroxide solution, using a bipolar membrane. It was estimated that 11% waste sodium nitrate solution could be converted into 3717 te/yr of 65% nitric acid and 2360 te/yr of 50% caustic soda for a variable cost of $USD 330 per te of NaOH, from a plant costing $USD7.92M. This plant consists of evaporation as well as the membrane operation. 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. FIGURE 21 SALT SPLITTING 11 BIBLIOGRAPHY "Introduction to Membrane Processes" by Mulder "Handbook of Industrial Membrane Technology" by Porter "Effective Industrial Membrane Processes" by Turner "Membrane Separation Technology" by Scott "Membrane Processes" by Rautenbach & Albrecht "Handbook of Industrial Membranes" by K Scott 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. 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

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