Filtration Celeste. todaro 1.0 INTRODUCTION The theoretical concepts underlying filtration can be applied towards practical solutions in the field. Comprehension of the basic principles is necessary to select the proper equipment for an application Theory alone, however, can never be the basis for selection of a filter Filtration belongs to the physical sciences, and thus conclusions must be based on experimental assay. It is, however, helpful in understanding why a slurry is more suitable for one design of filtration equipment than another Methods of optimization in the field can also be predicted by having a background in the theory Slurries vary significantly in filtration characteristics. Even batch to batch variation in product particle size distribution and slurry concentration will greatly influence filterability and capacity of a given filter. It is therefore, essential to evaluate a slurry in laboratory tests at a vendors facility or at one's plant with rental equipment to prove the application There three(3)types of pharmaceutical filtrations: depth, cake, and membrane. Cake and depth are coarse filtrations, and membrane is a fine final filtration. Membrane filtration and cross-flow filtration are discussed Ch. 7
Filtration Celeste L. Todaro 1.0 INTRODUCTION The theoretical concepts underlying filtration can be applied towards practical solutions in the field. Comprehension of the basic principles is necessary to select the proper equipment for an application. Theory alone, however, can never be the basis for selection of a filter. Filtration belongs to the physical sciences, and thus conclusions must be based on experimental assay. It is, however, helpful in understanding why a slurry is more suitable for one design of filtration equipment than another. Methods of optimization in the field can also be predicted by having a background in the theory. Slurries vary significantly in filtration characteristics. Even batch to batch variation in product particle size distribution and slurry concentration will greatly influence filterability and capacity of a given filter. It is, therefore, essential to evaluate a slurry in laboratory tests at a vendor’s facility or at one’s plant with rental equipment to prove the application. There are three (3) types ofpharmaceutical filtrations: depth, cake, and membrane. Cake and depth are coarse filtrations, and membrane is a fine, final filtration. Membrane filtration and cross-flow filtration are discussed in Ch. 7. 242
Filtration 243 1.1 Depth Filtration Examples of depth filtration are sand and cartridge filtration. Solids are trapped in the interstices of the medium. As solids accumulate, flow approaches zero and the pressure drop across the bed increases. The bed must then be regenerated or the cartridge changed For this reason, this method is not viable for high solids concentration streams as it becomes cost prohibi- tive. Cartridge filtration is often used as a secondary filtration in conjunction with a primary, such as the more widely used cake filtration 2.0 CAKE FILTRATION Rates of filtration are dependent upon the driving force of the piece of equipment chosen and the resistance of the cake that is continually forming Liquid flowing through a cake passes through channels formed by particles of irregular shapes 3.0 THEORY 3.1 Flow Theory Flow rate through a cake is described by Poiseuilles equation Ade V= volume of filtrate A= filter area surface 0= time P= pressure across filter medium a= average specific cake resistance weight of cake r resistance of the filter medium
Filtration 243 1.1 Depth Filtration Examples of depth filtration are sand and cartridge filtration. Solids are trapped in the interstices of the medium. As solids accumulate, flow approaches zero and the pressure drop across the bed increases. The bed must then be regenerated or the cartridge changed. For this reason, this method is not viable for high solids concentration streams as it becomes cost prohibitive. Cartridge filtration is oflenused as a secondary filtration in conjunction with a primary, such as the more widely used cake filtration. 2.0 CAKE FILTRATION Rates of filtration are dependent upon the driving force of the piece of equipment chosen and the resistance of the cake that is continually forming. Liquid flowing through a cake passes through channels formed by particles of irregular shapes. 3.0 THEORY 3.1 Flow Theory Flow rate through a cake is described by Poiseuilles’ equation: dV P V = volume of filtrate A = filter area surface 8 = time P = pressure across filter medium a = average specific cake resistance w = weight ofcake r = resistance of the filter medium u = viscosity
244 Fermentation and Biochemical engineering handbook In other words Flow Rate Fo Unit Area Viscosity([Cake Resistance Filter MediumResistance 3.2 Cake Compressibility The specific cake resistance is a function of the compressibility of the where constant As s goes to 0 for incompressible materials with definite rigid cryst line structures. a becomes a constant For the majority of products, resistance of the filter medium negligible in comparison to resistance of the cake, thus eq (1)becomes dy Eq1(3) de ua(W/A) Incompressible cakes have flow rates that are dependent upon the pressure or driving force on the cake. In comparison, compressible cakes, i.e where s approaches 1.0, exhibit filtration rates that are independent of pressure as shown beloy de ua(WlA) The above equations are detailed in Perry s Chemical Engineer's a00 [ Compressible cakes are composed of amorphous particles that are easily deformed with poor filtration characteristics. There are no defined channels to facilitate liquid flow as in incompressible cakes Fermentation mashes are typical applications of compressible materi- als, usually having poor filterability in contrast to purified end products that are postcrystallization. These products precipitate from solutions as defined crystals
244 Fermentation and Biochemical Engineering Handbook In other words, Flow Rate - Force Unit Area - Viscosity[ CakeResistance + FilterMediumResistance] 3.2 Cake Compressibility The specific cake resistance is a function of the compressibility of the cake. where a’ = constant As s goes to 0 for incompressible materials with definite rigid crystalFor the majority of products, resistance of the filter medium is line structures, a’ becomes a constant. negligible in comparison to resistance of the cake, thus Eq. (1) becomes AP - dV de ,ua(WlA) -_ Incompressible cakes have flow rates that are dependent upon the pressure ordrivingforce onthecake. Incomparison, compressiblecakes, i.e., where s approaches 1.0, exhibit filtration rates that are independent of pressure as shown below. The above equations are detailed in Perry’s Chemical Engineer ’s Handbook.[’] Compressible cakes are composed of amorphous particles that are easily deformed with poor filtration characteristics. There are no defined channels to facilitate liquid flow as in incompressible cakes. Fermentation mashes are typical applications of compressible materials, usually having poor filterability in contrast to purified end products that are postcrystallization. These products precipitate from solutions as defined crystals
ation 245 4.0 PARTICLE SIZE DISTRIBUTION Modification and optimization of a slurry, whether amorphous or crystalline, in the laboratory can yield significant improvements in filtration rates. By modeling the process in the laboratory, one can model what is occurring in the plant It is evident that attention paid in the laboratory to the factors affecting particle size distribution will save on capital investments made for separation equipment and downstream process equipment. Specific cake resistance( can be determined in the laboratory over the life of a batch, to evaluate if time in the vessel and surrounding piping system is degrading the product particle size to the point it impedes filtration, washing and subsequent drying Factors such as agitator design, agitation rates, pumps, slurry lines and other equipment, which can unnecessarily reduce the particle size, should be taken into consideration. Increasing the particle size in the slurry, and narrowing the particle size distribution will result in increased flow rates Large variations in particle size will increase the compressibility of a cake per unit volume. Since small particles have greater total cumulative surface areas, they will have higher moisture contents. For example, flour and water, when filtered with the same pressure or driving force as sand and water, will have a higher residual moisture level, thereby increasing the downstream dryer size. In the plant, the type of pump and piping system used to feed the filter are often of great importance, as time spent on crystallization and improving crystal size and particle size distributions can be quickly undone through particle damage. Recirculation loops and pumps for slurry uniformity may not always be necessary A review of the most commonly used process pumps are discussed Diaphragm pumps. These offer very gentle handling of slurries and are inexpensive and mobile. However, the pulsating flow can cause feeding and distribution prob- lems in some types offiltration systems, e.g., conventional basket centrifuges. They can also interfere with proces instrumentation e.g., flowmeters and loadcells Centrifugal pumps. Probably the most common source of particle attrition problems is the centrifugal pump. the high shear forces inherent to these pumps, particularly in the eye of the impeller, make some crystal damage
Filtration 245 4.0 PARTICLE SIZE DISTRIBUTION Modification and optimization of a slurry, whether amorphous or crystalline, in the laboratory can yield significant improvements in filtration rates. By modeling the process in the laboratory, one can model what is occurring in the plant. It is evident that attention paid in the laboratory to the factors affecting particle size distribution will save on capital investments made for separation equipment and downstream process equipment. Specific cake resistance (a) can be determined in the laboratory over the life of a batch, to evaluate if time in the vessel and surrounding piping system is degrading the product’s particle size tothe point it impedes filtration, washing and subsequent drying. Factors such as agitator design, agitation rates, pumps, slurry lines and other equipment, which can unnecessarily reduce the particle size, should be taken into consideration. Increasing the particle size in the slurry, and narrowing the particle size distribution will result in increased flow rates. Large variations in particle size will increase the compressibility of a cake per unit volume. Since small particles have greater total cumulative surface areas, they will have higher moisture contents. For example, flour and water, when filtered with the same pressure or driving force as sand and water, will have a higher residual moisture level, thereby increasing the downstream dryer size. In the plant, the type of pump and piping system used to feed the filter are often of great importance, as time spent on crystallization and improving crystal size and particle size distributions can be quickly undone through particle damage. Recirculation loops and pumps for slurry uniformity may not always be necessary. A review of the most commonly used process pumps are discussed below: Diaphragm pumps. These offer very gentle handling of slurries and are inexpensive and mobile. However, the pulsating flow can cause feeding and distribution problems in some types offiltration systems, e.g., conventional basket centrifuges. They can also interfere with process instrumentation e.g., flowmeters and loadcells. CentriJGgal pumps. Probably the most common source ofparticle attrition problems is the centrifugal pump. The high shear forces inherent to these pumps, particularly in the eye of the impeller, make some crystal damage
246 Fermentation and Biochemical Engineering Handbook inevitable in all but the toughest crystals. This damage is exacerbated on recirculation loops, which involve mul tiple passes through the pump. Recessed impellers will reduce this damage, but will often still degrade particles to the point where filtration becomes very difficult Positive displacement pumps. The minimal shear opera- tion of progressing cavity or lobe pumps make them ideal for slurries. The non-pulsating flow is beneficial in most processes, but they are significantly more expensive and less portable than diaphragm pumps Additionally, a significant amount of attrition can be caused by the particles"rubbing against each other. Therefore, long lengths of pipe, 900 elbows, throttling valves, control valves, and restrictions of any kind, should be avoided where possible. However, the type of pump employed is usually more significant Ifthe feed vessel can be mounted directly above the filter(to reduce the possibility of blockages), then gravity feeding with some pressure in the vessel is normally the best and least expensive arrangement. minimal shear agitators should be used at speeds sufficient to enhance the solids in the slurry and provide uniformity. Unnecessarily high speeds here can degrade the The"harder" the crystal, the more brittle and easier to break. Particle ape will also play a part, i.e spherical crystals dont break easily, needles do etc In general, this will lessen the problem of particle size deterioration and the fewer lines and shorter runs will reduce luggage 5.0 OPTIMAL CAKE THICKNESS As the cake thickness of a product varies, filtration rates and capacity will also change. Equation 4 shows that rates increase as the cake(W/a)mass decreases; thus, thin cakes yield higher filtration rates. This is particularly the case with amorphous materials or materials with high specific cake resistance. As a increases, maximizing dv/de requires W/a to decrease In continuous operations this can be done easily. In batch operations however, often filtration equipment cannot efficiently operate with extremely thin cakes. The long discharge times required to remove residual product in preparation for the next cycle, etc., make operation at a products optimal
246 Fermentation and Biochemical Engineering Handbook inevitable in all but the toughest crystals. This damage is exacerbated on recirculation loops, which involve multiple passes through the pump. Recessed impellers will reduce this damage, but will often still degrade particles to the point where filtration becomes very difficult. Positive displacement pumps. The minimal shear operation of progressing cavity or lobe pumps make them ideal for slurries. The non-pulsating flow is beneficial in most processes, but they are significantly more expensive and less portable than diaphragm pumps. Additionally, a significant amount of attrition can be caused by the particles “rubbing” against each other. Therefore, long lengths of pipe, 90° elbows, throttling valves, control valves, and restrictions of any kind, should be avoided where possible. However, the type of pump employed is usually more significant. If the feed vessel can be mounted directly above the filter (to reduce the possibility of blockages), then gravity feeding with some pressure in the vessel is normally the best and least expensive arrangement. Minimal shear agitators should be used at speeds sufficient to enhance the solids in the slurry and provide uniformity. Unnecessarily high speeds here can degrade the particles. The “harder” the crystal, the more brittle and easier to break. Particle shape will also play a part, Le., spherical crystals don’t break easily, needles do, etc. In general, this will lessen the problem of particle size deterioration and the fewer lines and shorter runs will reduce pluggage. 5.0 OPTIMAL CAKE THICKNESS As the cake thickness of a product varies, filtration rates and capacity will also change. Equation 4 shows that rates increase as the cake ( W/A) mass decreases; thus, thin cakes yield higher filtration rates. This is particularly the case with amorphous materials or materials with high specific cake resistance. As a: ‘ increases, maximizing dV/de requires W/A to decrease. In continuous operations this can be done easily. In batch operations however, often filtration equipment cannot efficiently operate with extremely thin cakes. The long discharge times required to remove residual product in preparation for the next cycle, etc., make operation at a product’s optimal
Filtration 247 cake thickness inefficient. Thus, if it requires a significant portion of the cycle time to unload the solids and only a 1/4-1/2"of cake is in the equipment, the effective throughput will be reduced, compared to operating with a cake thickness of 3-4 inches or greater 6.0 FILTER AID For amorphous materials, sludges or other poor filtering products improved filtration characteristics and/or filtrate clarity are enhanced with the use of filter aids. Slurry additives such as diatomaceous silica or perlite (pulverized rock), are employed to aid filtration. Diatomite is a sedimentary rock containing skeletons of unicellular plant organisms(diatoms).[12] Thes can also be used to increase porosity of a filter cake that has a high specifi cake resistance Volume of voids Volume of Filter Cake Addition of filter aid to the slurry, in the range of 1-2% of the overall slurry weight, can improve the filtration rates. Another rule of thumb is to add filter aid equal to twice the volume of solids in the slurry. By matching the particle size distribution of the filter aid to the solids to be filtered optimum flow rates are achieved. One should also use 3% of the particles above 150 mesh in size. to aid in filtration [3) Precoating the filter medium prevents blinding of the medium with the product and will increase clarity. Filter aid must be an inert material however, there are only a few cases where it cannot be used. For example waste cells removed with filter aid cannot be reused as animal feed. Filter aid can be a significant cost, and therefore, optimization of the filtration process is necessary to minimize the addition of filter aid orprecoat. Another possible detriment is that filter aid may also specifically absorb enzymes A typical application for these filter aids is the filtration of solids from antibiotic fermentation broths, where the average particle sizes 1-2 microns and solids concentration are 5-10%. Being hard to filter and often slim fermentation broths can also be charged with polymeric bridging agents to agglomerate the solids, thereby reducing the quantities of filter aid required
Filtration 247 cake thickness inefficient. Thus, if it requires a significant portion ofthe cycle time to unload the solids and only a 1/4"-1/2" of cake is in the equipment, the effective throughput will be reduced, compared to operating with a cake thickness of 3-4 inches or greater. 6.0 FILTER AID For amorphous materials, sludges or other poor filtering products, improved filtration characteristics and/or filtrate clarity are enhanced with the use of filter aids. Slurry additives such as diatomaceous silica or perlite (pulverized rock), are employed to aid filtration. Diatomite is a sedimentary rock containing skeletons of unicellular plant organisms (diatoms).[2] These can also be used to increase porosity of a filter cake that has a high specific cake resistance. Volume of Voids Porosity = Volume of Filter Cake Addition of filter aid to the slurry, in the range of 1-2% of the overall slurry weight, can improve the filtration rates. Another rule of thumb is to add filter aid equal to twice the volume of solids in the slurry. By matching the particle size distribution of the filter aid to the solids to be filtered, optimum flow rates are achieved. One should also use 3% of the particles, above 150 mesh in size, to aid in filtration.[3] Precoating the filter medium prevents blinding of the medium with the product and will increase clarity. Filter aid must be an inert material, however, there are only a few cases where it cannot be used. For example, waste cells removed with filter aid cannot be reused as animal feed. Filter aid can be a significant cost, and therefore, optimization of the filtration process is necessary to minimize the addition offilter aid orprecoat. Another possible detriment is that filter aid may also specifically absorb enzymes. A typical application for these filter aids is the filtration of solids from antibiotic fermentation broths, where the average particle size is 1-2 microns and solids concentration are 5-10%. Being hard to filter and often slimy, fermentation broths can also be charged with polymeric bridging agents to agglomerate the solids, thereby reducing the quantities of filter aid required
248 Fermentation and Biochemical Engineering Handbook 7.0 FILTER MEDIA Filter media are required in both cake filtration and depth filtratio Essential to selection of a filter medium is the solvent composition of the slurry and washes, and the particle size retention required of the solids Choice of the fabric, i.e., polypropylene, polyester, nylon, etc, is dependent upon the resistance of the cloth to the solvent and wash liquor used Chemical resistance charts should be referenced to choose the most suitable fabric. The temperature of the filtration must also be considered Fabrics are divided into three different types of yarns: monofilament, multifilament, and spun. They can be composed of more than one of these types of fabric. Monofilaments are composed of single strands woven together to form a translucent or opaque fabric. Very smooth in appearance its weave is conducive to eliminating blinding problems Multifilament cloths are constructed of a bundle of fibers twisted together. Only synthetic materials are available in this form, since long continuously extruded fibers must be used. Spun fabrics are composed of short sections of bound fibers of varying length. Retention of small particles is increased as the number of fibers or filaments in a bundle increases the greater the amount of twist in the yarn, the more tightly packed the fabric, which contributes to retention. This twist will also increase the weight of the fabric and frequently extends filter cloth lifetime Polyester, nylon and polypropylene are common materials found monofilament, multifilament and spun materials. Natural fibers such as cotton and wool are found only as spun material. This results in a fuzzy appearance. The effect of the type of yarn on cloth performance is shown Table I Table 1. Effect* of Type of Yarn on Cloth Performance. 14 (Courtesy of clark, J.G., Select The Right Fabric, Chemical Engineering Progress, November 1990) Maximum Filtrate Resistance Moisture Clart To Flow In Cake Discharge To blind Monofil Monofil Multifil Multifil Multifil Multifil Monofil Monofil "In decreasing order of preference
248 Fermentation and Biochemical Engineering Handbook 7.0 FILTER MEDIA Filter media are required in both cake filtration and depth filtration. Essential to selection of a filter medium is the solvent composition of the slurry and washes, and the particle size retention required of the solids. Choice of the fabric, i.e., polypropylene, polyester, nylon, etc., is dependent upon the resistance ofthe cloth to the solvent and wash liquor used. Chemical resistance charts should be referenced to choose the most suitable fabric. The temperature of the filtration must also be considered. Fabrics are divided into three different types of yams: monofilament, multifilament, and spun. They can be composed of more than one of these types of fabric. Monofilaments are composed of single strands woven together to form a translucent or opaque fabric. Very smooth in appearance, its weave is conducive to eliminating blinding problems. Multifilament cloths are constructed of a bundle of fibers twisted together. Only synthetic materials are available in this form, since long continuously extruded fibers must be used. Spun fabrics are composed of short sections of bound fibers of varying length. Retention of small particles is increased as the number of fibers or filaments in a bundle increases. The greater the amount of twist in the yam, the more tightly packed the fabric, which contributes to retention. This twist will also increase the weight ofthe fabric and frequently extends filter cloth lifetime. Polyester, nylon and polypropylene are common materials found in monofilament, multifilament and spun materials. Natural fibers such as cotton and wool are found only as spun material. This results in a fuzzy appearance. The effect of the type of yam on cloth performance is shown in Table 1. Table 1. Effect* of Type of Yam on Cloth Perf~rmance.[~] (Courtesy of Clark, J. G., Select The Right Fabric, Chemical Engineering Progress, November 1990.) Maximum Minimum Minimum Easiest Maximum Least Filtrate Resistance Moisture Cake Cloth Tendency Claritv ToFlow In Cake Discharge Life To Blind SPW Monofil Monofil Monofil Spun Monofil Multifil Multifil Multifil Multifil Multifil Multifil Monofil Spun SPW SPW Monofil Spun *In decreasing order of preference
Filtration 249 Three fabric types are available, i.e., woven, nonwoven and knit Woven fabrics are primarily what is used industrially. Yarns are laid into the length and width at a predetermined alignment. The width is called the fill direction and the length, the warp direction. They are at 900 angles and usually the yam count in the warp direction is the higher figure 41 Different weaving patterns of these materials will also vary cloth performance. Plain, twill and satin weaves are three of the most common Their effect on cloth performance is shown in Table 2 Table 2. Effect* of Weave Pattern on Cloth Performance[4l (Courtesy of Clark, J. G, Select the Right Fabric, Chemical Engineering Progress, November 1990) Maximum Resistance moisture Cl To Flow In Cake Di 。“ Twill Twill Twill Plain Twill Plain In decreasing order of preference A nonwoven material, for example, would be a felt. They are pads of short nonrandom fibers, made of rigid construction suitable for many types of filtratior The particle size distribution of the material and the clarity required will dictate the micron retention of the medium. fabrics tend to have a nominal micron retention range as opposed to an absolute micron retention rating. When using precoat on a machine that leaves a residual heel of solids a more open cloth can be used As discussed in the theory section of this chapter, the filter medium is an insignificant resistance to flow, in comparison to the cake. However, if the filter medium retains a high amount of fines, the subsequent cake that builds up becomes more resistant to filtration, thus the degree of clarity required in the filtrate can be a trade-off to capacity Air permeability is a standard physical characteristic of the mediums porosity and is defined as the volume of air that can pass through one square
Filtration 249 Three fabric types are available, i.e., woven, nonwoven and knit. Woven fabrics are primarily what is used industrially. Yams are laid into the length and width at a predetermined alignment. The width is called thejll direction and the length, the warp direction. They are at 90° angles and usually the yam count in the warp direction is the higher figure.L41 Different weaving patterns of these materials will also vary cloth performance. Plain, twill and satin weaves are three of the most common. Their effect on cloth performance is shown in Table 2. Table 2. Effect* of Weave Pattern on Cloth Performance[4] (Courtesy of Clark, J. G., Select the Right Fabric, Chemical Engineering Progress, November 1990.) Maximum Minimum Minimum Easiest Maximum Least Filtrate Resistance Moisture Cake Cloth Tendency Clarity To Flow InCake Discharge Life ToBlind Plain Satin Satin Satin Twill Satin Twill Twill Twill Twill Plain Twill Satin Plain Plain Plain Satin Plain * In decreasing order ofpreference. A nonwoven material, for example, would be a felt. They are pads of short nonrandom fibers, made of rigid construction suitable for many types of filtration equipment. The particle size distribution of the material and the clarity required will dictate the micron retention of the medium. Fabrics tend to have a nominal micron retention range as opposed to an absolute micron retention rating. When using precoat on a machine that leaves a residual heel of solids, a more open cloth can be used. As discussed in the theory section of this chapter, the filter medium is an insignificant resistance to flow, in comparison to the cake. However, ifthe filter medium retains a high amount of fines, the subsequent cake that builds up becomes more resistant to filtration, thus the degree of clarity required in the filtrate can be a trade-off to capacity. Air permeability is a standard physical characteristic of the medium’s porosity and is defined as the volume of air that can pass through one square
250 Fermentation and Biochemical Engineering Handbook foot filter medium at 1/2 inch water column pressure drop of water pres- sure.4 Increasing air permeability often decreases micron retention, but doesn'tnecessarily haveto. Twomaterials with the same air permeabilitycan have different micron retentions. Weave pattern, yarn count(threads/inch) yarm size, etc, all contribute to retention. Heat treating or calendaring a material will also influence the permeability as well as the micron retention Filter cloth manufacturers can provide assistance in fabric selection as well as information on fabric permeability and micron retention 8.0 EQUIPMENT SELECTION More than one equipment design may be suitable for a particular application. Often the initial approach is to replace it in kind. However, it is wise to evaluate the features of the present unit's operation in light of the process requirements and priorities. For example, is it labor intensive?Are copious volumes of wash required? Ever-increasing environmental concerns may make it necessary to evaluate the existing process to reduce emissions, operator exposure, limit waste disposal of filter aid, or reduce wash quantities requiring solvent recovery or wash treatment. Breakdown of an old piece of equipment often provides the opportunity and justification to improve plant conditions. New grass roots" designs may have the tendency to revert to industry standards This is also the opportunity to improve conditions or substantiate the current equipment of choice 8.1 Pilot Testing Various small scale test units and procedures are available to determine slurry characteristics and suitability for a particular application. buchner funnel, and vacuum leaf test units can be purchased or rented from vendors to perform in-house tests, or one can have tests conducted at the vendor facility. Pilot testing on the actual equipment would be the optimum with a rental unit in the plant. In either case, slurry integrity must be maintained to ensure accurate filtration data Slurry taken fresh from the process in-house will yield the best results as product degradation over time, process temperature, effects of process agitators, pumps, etc., must be taken into consideration when shipping product to vendors for conducting tests. Should the particles suddenly be smaller. slower than usual filtrations will be seen and vice versa
250 Fermentation and Biochemical Engineering Handbook foot filter medium at 1/2 inch water column pressure drop of water pressure.[4] Increasing air permeability often decreases micron retention, but doesn’t necessarily have to. Two materials with the same air permeability can have different micron retentions. Weave pattern, yarn count (threaddinch), yam size, etc., all contribute to retention. Heat treating or calendaring a material will also influence the permeability as well as the micron retention. Filter cloth manufacturers can provide assistance in fabric selection as well as information on fabric permeability and micron retention. 8.0 EQUIPMENT SELECTION More than one equipment design may be suitable for a particular application. Often the initial approach is to replace it in kind. However, it is wise to evaluate the features of the present unit’s operation in light of the process requirements and priorities. For example, is it labor intensive? Are copious volumes of wash required? Ever-increasing environmental concerns may make it necessary to evaluate the existing process to reduce emissions, operator exposure, limit waste disposal of filter aid, or reduce wash quantities requiring solvent recovery or wash treatment. Breakdown of an old piece of equipment often provides the opportunity and justification to improve plant conditions. New “grass roots” designs may have the tendency to revert to industry standards. This is also the opportunity to improve conditions or substantiate the current equipment of choice. 8.1 Pilot Testing Various small scale test units and procedures are available to determine slurry characteristics and suitability for a particular application. Buchner funnel, and vacuum leaf test units can be purchased or rented from vendors to perform in-house tests, or one can have tests conducted at the vendor’s facility. Pilot testing on the actual equipment would be the optimum with a rental unit in the plant. In either case, slurry integrity must be maintained to ensure accurate filtration data. Slurry taken fresh from the process in-house will yield the best results as product degradation over time, process temperature, effects of process agitators, pumps, etc., must be taken into consideration when shipping product to vendors for conducting tests. Should the particles suddenly be smaller, slower than usual filtrations will be seen and vice versa
Filtration 251 Of course, if equipment is presently in operation at the plant on the particular product, invaluable data can beobtained. Optimization ofthe filter should be done, perhaps with the vendor's help, to be sure that over-sizing of the next piece of equipment does not occur. Variance of precoat, cake thickness, wash, etc., if not already done on the process, will enable fine- tuning of the process as well as confirm the data for the next systems design 9.0 CONTINUOUS VS BATCH FILTRATION Continuous and batch equipment can be used in the same process by incorporating holdup tanks, vessels or hoppers between them. However, the overriding factor is often one of economics. High volume throughputs in the order of magnitude of a several hundred gallons per hour or greater usually require continuous separation. The size of batch equipment escalates in cases, resulting in tremendous capital outlay. It is for this reason the rotary vacuum filter has been historically used in the fermentation industr 10.0 ROTARY VACUUM DRUM FILTER 10.1 Operation and Applications Raw fermentation broth is an example of a large volume production Rotary drum vacuum filters(RVFs)have traditionally been found in this service. Slow-settling materials or more difficult filtrations with large scale production requirements are typical applications for this type of equipment For an overview of filter selection versus filtering rates, see Table 3, which is excerpted by special permission from Chemical Engineering/ Deskbook Issue, February 15, 1971, by McGraw Hill, Inc, New York, NY 10020 The basic principle on an RVF is a hollow rotating cylindrical drum driven by a variable speed drive at o 1-10 revolutions per minute. One-third of the drum is submerged in a slurry trough. As it rotates, the mycelia suspension is drawn to the surface of the drum by an internal vacuum. The surface is the filter medium mounted on top of a grid support structure Mother liquor and wash are pulled through the vacuum line to a large chamber and evacuated by a pump Applicable to a broad range of processes, e.g., pharmaceutical, starch, ramics, metallurgical, salt, etc, many variations of the rvf have beer developed, however, the fundamental cylinder design remains the same
Filtration 251 Of course, if equipment is presently in operation at the plant on the particular product, invaluable datacan be obtained. Optimization ofthe filter should be done, perhaps with the vendor’s help, to be sure that over-sizing of the next piece of equipment does not occur. Variance of precoat, cake thickness, wash, etc., if not already done on the process, will enable finetuning ofthe process as well as confirm the data for the next system’s design. 9.0 CONTINUOUS vs. BATCH FILTRATION Continuous and batch equipment can be used in the same process by incorporating holdup tanks, vessels or hoppers between them. However, the overriding factor is often one of economics. High volume throughputs in the order of magnitude of a several hundred gallons per hour or greater usually require continuous separation. The size ofbatch equipment escalates in these cases, resulting in tremendous capital outlay. It is for this reason the rotary vacuum filter has been historically used in the fermentation industry. 10.0 ROTARY VACUUM DRUM FILTER 10.1 Operation and Applications Raw fermentation broth is an example of a large volume production. Rotary drum vacuum filters (RVF’s) have traditionally been found in this service. Slow-settling materials or more difficult filtrations with large scale production requirements are typical applications for this type of equipment. For an overview of filter selection versus filtering rates, see Table 3, which is excerpted by special permission from Chemical EngineeringIDeskbook Issue, February 15, 1971, by McGraw Hill, Inc., New York, NY 10020. The basic principle on an RVF is a hollow rotating cylindrical drum driven by a variable speed drive at 0.1-10 revolutions per minute. One-third of the drum is submerged in a slurry trough. As it rotates, the mycelia suspension is drawn to the surface of the drum by an internal vacuum. The surface is the filter medium mounted on top of a grid support structure. Mother liquor and wash are pulled through the vacuum line to a large chamber and evacuated by a pump. Applicable to a broad range of processes, e.g., pharmaceutical, starch, ceramics, metallurgical, salt, etc., many variations of the RVF have been developed, however, the fundamental cylinder design remains the same