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 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 - 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 crystal- For 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 materi- als, usually having poor filterability in contrast to purified end products that are postcrystallization. These products precipitate from solutions as defined crystals. 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 prob- lems 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, 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 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 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. 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’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 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 fine- tuning 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. 252 Fermentation and Biochemical Engineering Handbook Table 3. Guide to Filter Selection Slurry Characteristics Cake Formation Vormal concentration Settling rate Leaftest rate, IbMsq ft Filtrate Rate, gaVminIsq ft Filter Aoolication Sontinuous Vacuum Llulticompartment drum Pingle-compartment drum >orrco 4opper dewaterer rop feed Scroll-discharge rilting-pan 3elt-discharge >ontinuous vacuum disk :ontinuous vacuum Precoa 2ontinuous pressure Preca 3atch vacuum leaf 3atch nutsche 3atch pressure filters plate-and-fiame vertical leaf tubular horizontal plate cartridge edge Fast Filtering inlsec. 20% rapid, dfl1cult to suspend 500 5 Medium Filtering inlsec 10 to 20% fast 50 to 500 0.2 to 5 Slow Filtering 0.05 to 0.25 inlmin 1 to 10% slow 5 to 50 0.01 to 0.02 very Dilute 0.05 inlmin 5% slow 5 0.01 to 2 Dilute no cake 0.1% - - 0.01 to 1 Filtration 253 -:- ~ ~ s"~ ~ ~ ~ "' .5E~'15- ~~;: ~~tQ I 254 Fermentation and Biochemical Engineering Handbook The cylinder is divided into compartments like pieces of a pie (see Fig 2), and drainage pipes carry fluid from the cylinder surface to an internal manifold. Filter diameters range from three to twelve feet, with face lengths of one totwenty-four feet, andup to 1000 ft2 offiltration area.i51 Filtration rates range from 5 GPH per square foot to 150 GPH per square foot. Moisture levels are, of course, dependent upon particle size distribution and tend to range from 25% to 75% by weight and cake thickness tends to be in the 1/8-1/2 inch range, as most applications are for slow-filtering materials. With the exception of the precoat applications, RVF’s do not usually yield absolutely clear filtrate. Although still widely used, rotary vacuum filters are, in some cases, being replaced by membrane separation technology as the method of choice for clarification of fermentation broths and concen- trating cell mass. Membranes can yield more complete filtration clarifica- tion, but often a wetter cell paste. The drum is positioned in a trough containing the agitated slurry, whose submergence level can be controlled. As the drum rotates, a panel is submerged in the slurry. The applied vacuum draws the suspension to the cloth, retaining solids as the filtrate passes through the cloth to the inner piping and, subsequently, exiting the system to a vapor-liquid separator with high/low level control by a pump. Cake formation occurs during submer- gence. Once formed, the cake dewaters above the submergence level and is then washed, dewatered and discharged. Discharge mechanisms will vary depending upon cake characteristics. Friable, dry materials can use a “doctor” blade as in Fig. 3. Difficult filtrations requiring thinner cakes incorporate a string discharge mechanism. This is the primary method for starch and mycelia applications. A series of 1/2 inch spaced strings rest on the filter medium at the two o’clock position. The cake is lifted from the drum as shown in Fig. 4. Fermentation broths containing grains, soybean hulls, etc., are applications for this type of discharge mechanism. The solids may be used for animal feed stock, or incinerated. String or belt discharge mechanisms facilitate cake removal and, therefore, can eliminate the need for filter aid. Continuous belt discharge (Fig. 5) is employed for products that have a propensity for blinding the filter medium. A series of rollers facilitate cake removal in this case. Precoated rotary vacuum drum filters (Fig. 6) are used by filtering a slurry of filter aid and water first, then subsequent product filtration. Difficult filtering materials, which have a tendency to blind, are removed with a doctor blade. Precoat is removed along with the slurry to expose a new filtration surface each cycle. Filtration 255 256 Fermentation and Biochemical Engineering Handbook DRUM-/ W CAKE Figure 3. Cake discharge mechanism. CAKE Figure 4. String discharge. Filtration 25 7 SOLIDS DISCHARGE Figure 5. Belt discharge. AD VAN CIN G KNIFE Figure 6. Precoat rotary vacuum filter. 258 Fermentation and Biochemical Engineering Handbook The progressively advancing blade moves 0.05 to 0.2 mm per- revolution. Vacuum is maintained throughout the cycle, insteadofjust during submergence, so that the precoat is retained. Once the precoat is expended, the RVF must be thoroughly cleaned, and a fresh coat reapplied. 10.2 Optimization Pressure leaftests are used to model the operating cycle of a RVF. The cycle, consisting of cake formation, dewatering, washing, dewatering and discharge, is simulated by the apparatus shown in Fig 7. The test leaf is immersed in the agitated slurry for cake formation, then removed for drying. If washing is required, the leaf is placed in the wash liquor and then dried again. Discharge from the leaf will indicate type of discharge mechanism required. By varying the time of the portions of the cycle, rotational speeds can be simulated. It is recommended that optimization be camed out by developing three different cake thicknesses. From this, a capacity versus cake thickness curve can be developed. Additional parameters that have to be evaluated are vacuum level, wash requirements, slurry concentration, and slurry tempera- ture. If cake cracking occurs, the wash should be introduced earlier to avoid channeling. Several leaftests should be performed for repeatability. Data collected will permit scaleup to plant scale operations. Significant data will be pounds of dry cake per square foot per hour, gallons of filtrate per square foot per hour, filtrate clarity, wash ratios, (pounds of solids/gallon of wash), residual moistures, filter media selection, knife advance time, precoat thickness, solids penetration into precoat, and submergence level should also be evaluated. For the optimization equation, refer to Peters and Timmerhaus, and Tiller and Crump. 11 .O NUTSCHES 11.1 Applications The nutsche filter is increasingly prevalent in postcrystallization filtrations. It would not be used directly from the fermenter. Relatively fast filtrations with predictable crystal structures, often found in the intermediate and final step purifications of antibiotic drugs, work well on this batch filter. Batch sizes range from 100 to 7500 gallons. Filtration 259 260 Fermentation and Biochemical Engineering Handbook 11.2 Operation The term nutsche is derived from the German word for sucking. Vacuum is applied at the bottom of a vessel that contains a perforated plate. A filter cloth, screen, perforated plate, or porous ceramic plate may be the direct filtration medium (see Fig. 8). Subsequently, products should have lower cake resistances and well-defined crystal structures to facilitate separation. The driving force for the separation is vacuum andor pressure. With an agitated vessel, the blade can be used to smooth or squeeze the cake, eliminating cracks, when rotated in one direction or for reslurrying and or discharging the cake when rotated in the opposite direction. The rotation of the agitator can be by electric motor with variable speed drive; however, the translational movement is achieved by a separate hydraulic system. The agitator requires a stuffing box or mechanical seal for pressure or vacuum operation ofthe unit. Filling is accomplished by gravity feed or pump. Large cakes, in the 10-12 inch range, are developed. When plug flow displacement washing is not effective, and as diffusion of impurities through the cake becomes difficult, reslurrying is the required method. Displacement washing is more efficient and minimizes wash quantities, however, may not always be possible. Filtering, reslurrying and refiltering can all be accomplished in the same unit, thus achieving total containment. See Fig. 9. The vessel can also be jacketed for heating andor cooling and the agitator blade heated. This design can now be a reactor in combination with a filter-dryer or alone as a filterdryer (Fig. 10) (see also Chapter 17). This is particularly advantageous for dedicated production of toxic materials requiring an enclosed system. Operator exposure and product handling are minimized. The nutsche can have limitations for difficult filtrations, as the thick filter cakes can impede filtration. Atwo-stage system for filtration and drying can offer greater flexibility in plant operations, especially if either the filtration or drying step is rate-limited. Predictable crystals that filter and dry well are the best applications for this all-encompassing system. Mechanical discharge incorporating the agitator facilitates solids removal centrally or a side discharge is possible. A residual heel of product will be left as the agitator is limited on how close to the screen or filter medium it can go. Residual heels can be reworked by reslurrying or remain until the campaign changes. For frequent product changes, the nutsche can be provided with a split-vessel design. Upon lowering the bottom portion, free access to the inside of the vessel and the filter bottom itself for cleaning purposes is possible. Somemanufacturers have air-knife designsthat remove the residual heel. Heels as low as one-quarter inch can be obtained. Filtration 261 Figure 8. Agitated nutsche type pressure filter. (Courtesy ofCOGEIM SpA). 262 Fermentation and Biochemical Engineering Handbook lbwbleBOttam side Mschqp Pernranent Agitator Drive - Eon-Heated loo% Jaclreted spray Eozzles or Wti-Lnyer Filter Figure 9. Agitated nutsche type pressure filter. (Courtesy of COGElMSpA). Filtration 263 Figure 10. Agitated nutsche type filter/dryer. (Courtey of COGEA4SpA). Materials of construction can vary widely depending upon the appli- cation. Typically, 304 or 3 16 stainless steel, and Hastelloy are supplied, although many other types of material of construction are available. Metal finishes, in keeping with good manufacturing practices (GMP), particularly for areas in contact with final products, require welds to be ground smooth. Finishes can be specified in microns, Ra, or grit. The unit, Ray is the arithmetical average ofthe surface roughness in microinches. The rms is the root mean square of the surface roughness in microinches; rms = 1.1 Ra. 264 Fermentation and Biochemical Engineering Handbook A mechanical finish of 400 grit is an acceptable pharmaceutical finish, however, mechanical polishing folds the surface material over itself. When viewed under a microscope, jagged peaks and crevices are visible. Product on the micron level can be accumulated in these areas. Electropolishing of the surface is often used to eliminate these peaks and valleys to provide a more cleanable surface. A layer of the surface material is removed in this case. A mechanical finish of 400 grit is achieved by progressively increasing the grit spec from 60 up to 400. If a 400 grit surface was to be electropolished, the amount of material removed would result in an equivalent 180-220 grit surface roughness. Therefore, a mechanical finish of approximately 180- 220 grit need only be specified when electropolishing. A considerable cost savings is realized. It is always advisable to specify the Ra value of the surface whether electropolishing is specified or not. (See Table 4.) Filter areas will range from 0.5 to 16 m2. For large-scale processing, significant floor area is occupied per unit area of filtration.['] Those products that tend to blind filter media, i.e., colloidal slurries, gelatinous and protein compounds, will require alternate equipment, filtration or centrifugation. 11.3 Maintenance When used for dedicated production, maintenance is minimal. The agitator sealing system (usually a stuffing box or mechanical seal), however, must be maintained. Filter cloth change and O-ring changes would be the primary mainte- nance required. This depends on the filter design. The split vessel design allows for easy access. A removable bottom which can be fixed to the vessel through a bayonet closure system is completely hydraulically controlled and can be lowered in 1-2 minutes. By first using the spray nozzles and flushing the system with a solvent that the product is soluble in, operator exposure will be minimized. Cleaning between final products for 99% validation, can take 1-2 twelve-hour shifts. Screen lifetime will depend upon the type of screen used. Various types of filter cloths or monolayer metal screens can be used. A multilayer sinterized filter screen is also available. Installation of filter cloths and screens is usually by the use of clamping rings and hold-down bars screwed on the bottom. Filtration 265 Fermentation and Biochemical Engineering Handbook Figure II. Agitated nutsche type pressure filter. (Courtesy ofCOGEIM SpA). 12.0 HP-HYBRID FIL TER PRESS Applications A batch unit, the HP-hybrid filter press is typically used for products with "specialty chemicals volumes" in the range of500-3000 gallon batches. Products are processed in an enclosed atmosphere without operator or environmental exposure. FDA requirements, and conformation to good manufacturing practices (GMP) requiring containment of product, are continual issues. Applications are replacement of plate and frame filter presses, and products that are compressible and amorphous in nature with high specific cake resistances. Fermented products in the post -broth stage, where volumes are smaller , can be applications as production rates are limited in this design. Postcrys- tallization can also be an application if solids are found to be compressible. 12.1 Filtration 267 Replacement of cartridge filter systems where high filter replacement costs occur as well as low volume waste treatment streams can also be processed. Residual moistures will be reduced in comparison to standard plate and frame units and RVF's by up toone-third, due to the high driving force created by the hydraulic membrane of up to 375 psi. Particles can be retained to one micron, which can eliminate the need for a precoat and save on waste disposal. As the cake developed in the pressure chamber is relatively even, and the wash delivered is also consistent, washing efficiency is high. (See Fig. 12.) Figure 12. HP hybrid filter press. (Courtesy ofHeinkel Filtering Systems. Inc.) 12.2 Operation This unit is a fully automated, totally enclosed filter press. The core of the system is a pressure chamber. It can be connected to peripheral equipment, such as a dryer or bin, for a totally contained system. The pressure chamber consists of a perforated candle filter. On top of the screen is a filter cloth and a membrane constructed ofEPDM, BUNA, or Viton. At present these are the only available materials of construction. The 268 Fermentation and Biochemical Engineering Handbook membrane pressure is achieved hydraulically with water. Chargingthe slurry and washing the cake take place as the vessel toggles 180 degrees to ensure an even cake and wash distribution. Vacuum pulls the membrane back to allow entry of the slurry. Pressing occurs after the feed, then washing (if required), repressing and finally, solids discharge. Pressure can be varied depending upon the product. Figure 13 depicts the operation and cross- section of the pressure chamber. Inverting the membrane and reversing air flow through the cloth while slowly rotating the system 180" back and forth releases all cake from the cloth. No operator attention is required for discharge of the solids, as no residual heel is left on the cloth. Vapors and product are contained. PRESSURIZATION C EXPOSING ME FILTER CAKE SOLIDS DISCHARGE Figure 13. HP cross-section. (Courtesy ofHeinke1 Filtering Systems, Inc.) Filtration 269 12.3 Maintenance Filter cloth changing and replacement of product-contacted 0- rings-are required when cleaning between products. Wear parts are the 0- rings and filter cloths, These should be changed on a preventive maintenance basis approximately every two to three (2-3) months. The membrane has a lifetime of approximately one (1) year and, of course, must be chemically compatible with the solvents as is the filter cloth medium. Preventative maintenance is required for the vacuum and hydraulic (water) system. 13.0 MANUFACTURERS Rotary Drum Vacuum Filters Bird Machine Company South Walpole, MA Denver Equipment Company Colorado Springs, CO Dorr-Oliver, Inc. Stamford, CT Eimco Division of Envirotech Salt Lake City, UT Komline-Sanderson, Inc. Peapack, NJ Peterson Fuller, Inc. Salt Lake City, UT Nutsches Cogeim Cogeim Charlotte, NC Dalmine, Italy Rosenmund, Inc. Rosenmund Charlotte, NC Switzerland 270 Fermentation and Biochemical Engineering Handbook Hybrid Filter Press Heinkel Filtering Systems, Inc. Bridgeport, NJ Heinkel Industriezentrifugen GmBH Co Bietigheim-Bissingen Germany REFERENCES 1. Perry, R. H., Green D. W., and Maloney, I. 0. (eds.), Perry’s Chemical Engineer’sHandbook, SixthEd. pp. 19-65,19-89, McGraw HillBook Co., New York (1984) 2. Rees, R. H. and Cain, C. W., Let Diatomite enhance your filtration, Chemical Engineering, 8:76-79 (1990) 3. Om, C., (ed.), Filtration Principles and Practices, Part I; p. 370, Marcel Dekker, Inc., New York (1977) 4. Clark, J. G., Select The Right Fabric, Chemical Engineering Progress, (November 1990) 5. Vogel, H. C., (ed), Fermentation and Biochemical Engineering Handbook, First Edition, pp. 163-173, Noyes Publications, New Jersey (1983) 6. Dahlstrom, D. A., Encyclopedia of Chemical Process Equipment (W. J. Mead, ed.), pp. 417-438, New York, Reinhold (1964) 7. Silverblatt, C. E., Risbud, H., and Tiller, F. M., Batch, Continuous Processes For Cake Filtrations, Chemical Engineering, 4: 127-136 (1974) 8. Tiller, F. M., et al., Theory andpractice ofSolid-LiquidSeparation Second Ed., University of Houston (1975) 9. Smith, G. R. S., How to Use Rotary, Vacuum, Pre-coat Filters, Chem. Eng. 83:84-94 (February 16, 1976) 10. Cain, C. W., Putting the principals to work filter-cake filtration, Chemical Engineering, 72-75 (1990) 11. Lloyd, P. J., Particle Characterization, Chemical Engineering, 4: 120-122 (1 974) 12. Cheape, D. W., Jr., Leaf tests can establish optimum rotary-vacuum-filter operation, Chemical Engineering, 5: 141-148 (1982) 13. Peters M. S. and Timmerhaus, K. D., Plant Design and Economics for Chemical Engineers, Second Edition, McGraw Hill; pp. 478-90 (1968) 14. Tiller, F. M.. and Crump, J. R., How to increase filtration rates in continuous filters, Chemical Engineering, 5: 183-187 (1977)