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