12 Centrifugation Celeste L. Todaro 1.0 INTRODUCTION The solids-liquid separation process can be accomplished by filtration or centrifugation. Centrifuges magnify the force of gravity to separate phases, solids from liquids or one liquid from another. There are two general types of centrifuges: Sedimentation Centrifuges-where a heavy phase settles out from a lighter phase, therefore requiring a density difference and Filtering CentriBges-where the solid phase is retained by a medium like a filtercloth, for example, that allows the liquid phase to pass through. 2.0 THEORY Centrifuges operate on the principle that a mass spinning about a central axis at a fixed distance is acted upon by a force. The force exerted on any mass is equivalent to the weight of the mass times its acceleration rate in the direction of the force. 558 Centrifugation 559 F=ma m = mass a = acceleration rate F = force This acceleration rate is zero without a force acting upon it, however, it will retain a certain velocity, v. If forced to move in a circular path, a vector velocity v/r exists as its direction is continually changing. where a, = centrifugal acceleration v = velocity r = radius w = angular velocity Should a mass be rotated within a cylinder, the resulting force at the cylinder wall is called a centrifugal force, F,. Eq. (4) F, = mw2r this is away from the center of rotation. The equal and opposite force: is the centripetal force. This is the force required to keep the mass on its circular path. Ifa cylindrical bowl holding a slurry is left to stand, the solids will settle out under the force of 1 g orgravity. By spinning the bowl the solids will settle under the influence of the centrifugal force generated as well as the force of gravity which is now negligible. Solids will collect at the wall with a liquid layer on top. This is an example of a sedimentation in a solid bowl system. 560 Fermentation and Biochemical Engineering Handbook By perforating the bowl or basket and placing a filtercloth on the inside wall, one has now modeled a filtering centrifuge similar in principle to an ordinary household washing machine. This amplification ofthe force ofgravity is commonly referred to as the number of g's. The centrifugal acceleration (a,) referenced to g is w2r/g which is given by the equation: Relative Centrifugal Force (G) = 1.42 x n2 Di n = speed in revolutions/minutes Di = diameter of the bowl in inches The L.iving force for separation is a function of the square oL the rotational speed and the diameter of the bowl; however, there are restrictions in the design of centrifuges that will limit these variables. An empty rotating centrifuge will exhibit a stress in the bowl called a self-tress, S, . where w = angular velocity ri = radius of the bowl pm = density of the bowl material The contents of the bowl also generate a stress or pressure on the inner wall of the bowl. Assuming the radius of the bowl (ri) is equal to the outer radius of the bowl contents (r2), we have where t = thickness of the bowl p, = density of contents of the bowl r1 = inner radius of the bowl contents (solids and liquid) r2 =outer radius of the bowl contents (solids and liquid) Centrifugation 561 The total stress in the bowl wall is: with Di = 2ri and in common units: Eq. (9) Centrifbges are designed such that S, is 45 to 65% of &. Increasing the bowl speed and its diameter increases the g force, but also increases the self stress and the stress induced by the process bowl. The design is, therefore, really limited by the material of construction available, however, for a given bowl stress, the centrifugal acceleration is an inverse function of bowl diameter. For example, doubling the rotational speed, and halving the bowl diameter, doubles the acceleration while keeping the total stress relatively constant. It is for this reason that the smallest diameter centrifuges operate at the highest g forces. Tubular centrifuges operate at 2-5 inches diameter withg forces over 60,000. Disk centrifuges operate at 7-24 inches at 14,000 to 5500 g’s, while continuous decanter centrifuges with helical conveyors are designed with bowl diameters of 6-54 inches and g forces of 5,500-770 g’s. Filtering centrifuges with diameters of 12 to 108 inches have corresponding g forces of 2000 to 260. 3.0 EQUIPMENT SELECTION Upon review of Table 1, it is evident that there are several types of equipment that can be used for the same application. There are also many 562 Fermentation and Biochemical Engineering Handbook equipment vendors that can be consulted. In consulting with vendors to narrow the choices, proprietary information may be divulged regarding the nature of one’s process. Be sure to sign a secrecy agreement to protect all confidential information. Table 1. Product Recovery Fermentation DRUM FILTER CENTRIFUGE FILTRATION INCINERATED (LIQUID) OR ANIMAL (WASTE) FEEDSTOCK EXTRACTION EXCHANGE ABSORPTION 11- WASTE I LIQUID PRODUCT SOLIDS-LIQUID LIQUID-LIQUID CHROMATOGRAPHY -1 CRYSTALLIZATION OR [TIORFl (SMALL SCALE HIGH PURITY PRODUCT dk 1 OR FI OR F] I FILTER DRY FINAL Centrifugation 563 3.1 Pilot Testing Preliminary data taken in the laboratory regarding the separation characteristics of a product can be beneficial when beginning the equipment selection process. If one is retrofitting an existing unit with an identical system, it will not be as complex and time consuming as designing a “grass roots” facility. Pland data can then be helpful. However, should the process be altered, one should evaluate the effect on centrifugation. Careful attention should be taken to ensure that the existing tank design, peripheral pumps, piping and agitators do not provide shear that will cause particle degradation. Capital dollars spent in this area on crystalliza- tion studies, the selection of the correct pumps, etc., will directly impact the capacity of your equipment as particle degradation will significantly affect throughput and final residual moisture adversely, as it does filtration. (See Ch. 8 for a more detailed discussion.) If a process is existing and in-plant expertise is available to optimize the equipment, it would be advisable to do so before a purchase. Upon review of the existing design, sufficient improvements can often be made in an older piece of equipment thereby avoiding a more costly investment. Vendors usually offer this type of assistance at no charge from their office or at a daily rate in the field. 3.2 Data Collection The first step is to collect pertinent information to the process, including a process flow sheet, product information and completion of a typical questionnaire, as shown in Table 2. Knowledge of the most critical aspect of the process can guide the sometimes difficult selection process. For example, the requirement ofa very dry product with strict impurity levels suggests a filtering Centrifuge. A product with a feed rate of 150 gpm, without wash requirements, would lead us to a continuous sedimentation centrifuge. A simple Buchner funnel test will indicate fast, medium or slow filtration. Slow filtering materials that have inordinate quantities of particles passing the filter paper will be submicron and difficult to capture in a filtering system. Therefore, a sedimentation centrifuge should be considered. A phenomenon called “cake cracking” can occur and will be evident in this simple test. Not all materials exhibit this. It depends upon the surface tension of the product and its tendency to shrink as dewatering occurs. Amorphous, thixotropic materials will exhibit this more than rigid solids. 564 Fermentation and Biochemical Engineering Handbook Table 2. Product Questionnaire: Centrifbges, Filter Press. (Courtesy Heinkel Filtering Systems Inc.) Centrifugation 565 If the cake cracks, a liquid level must be maintained on top of the cake before washing to prevent channeling of the wash liquors. A crude estimate of the wash ratio, gallons of wash per pound of dry cake, can also be made in the laboratory. The filter cake developed on a Buchner funnel will have certain characteristics; the product may appear to have a defined crystal structure or be more amorphous. Microscope studies will indicate the shape ofparticles, which will be helpful in trial runs. Needle crystals, for example, may break easily at high filling speeds, and during discharge on a basket centrifuge with plough platelets tend to pack in compressible beds and may be better suited to sedimentation than filtration. A particle size distribution will help in this analysis and is also required for cloth selection. (See Ch. 6.) Cake compressibility is the ability of a cake to reduce its volume, Le., porosity, when stress is applied. The resulting cake will display an increase in hydraulic resistance. This is not necessarily caused by an average change in porosity, as a porosity gradient can occur by the redistribution of the solid material. Rigid granular particles tend to be incompressible and filter well even with thick cakes. Materials that are easily deformed such as amorphous or thixotropic materials will respond well to mechanical pressure or operation with thin cakes. (See Ch. 6 on Cake Compressibility.) Laboratory test tube centrifiges can determine if there is a sufficient density difference between the two phases to consider sedimentation as an alternative. If there is a sharp separation, one can anticipate the same in the field. One can also answer the following questions. Do the solids settle or float? Is the solid phase granular or amorphous? What is the moisture content? The characteristics ofthe solids indicate the solids discharge design required, i.e., scroll in decanters, or in disk centrifuges, flow-through nozzles or wall valves. A laboratory centrifige such as the Beaker design (Heinkel) can also simulate the operation of a filtering centrifuge and verify product character- istics, filtration rates and wash requirements. Various filter cloths can also be tried using only one liter samples. With these data summarized, one can now discuss the application with vendors or consultants with expertise in centrifuge operations to help simplify the selection process. Pilot plant testing can be done with 10-25 gallons at the vendor’s facility or with a rental unit for an in-plant trial. If sufficient material is available, semiworks tests are recommended as more data can be taken for scaleup. Equipment manufacturers should be questioned about how they are scaling up, whether it is based upon volume, filtercloth area, etc., and what accuracy can be expected. Critical to a trial’s success is how representative 566 Fermentation and Biochemical Engineering Handbook a slurry sample is. This is especially important with fermentation processes that change over time and in-plant trial of reasonable scale may be mandatory. There can be clear advantages to using a centrifbge over a filter, such as a drier product or amore effective separation. This will be dependent upon the application. However, there can be applications where a nutsche or even several types of centrifbges appear, from small scale testing, to yield similar product quality results. Ultimately, the decision will depend upon “operabil- ity,” that is installation, maintenance, and day-today operation. For these purposes, it is strongly recommended that the plant invest the time and relatively small capital costs for a rental unit to run an in-plant test on the product. Production scale rental units can be operated by taking a side stream from an existing process. This facilitates comparisons ofthe new equipment to existing plant operations. Rental cost for one month will be approximately three percent of the purchase price of the test equipment, credit for part of the rental is usually offered against the purchase of a production unit. Rental periods may be limited, however lease options are available. 3.3 Materials of Construction Various materials are available as with all process equipment, ranging from carbon steel coated with rubber, Halar or Kynar, to stainless steel 304, 3 16 and higher grades, or more expensive alloys such as titanium, Hastelloy C-22, C276, or C4. These grades of Hastelloy will, however, double the capital outlays. Coatings should be avoided if possible as product “A” can diffuse into the surface and potentially reverse its path. It therefore has the potential to contaminate product “B.” Being permeable, they are also subject to peeling. Coatings can be used most effectively on stationary parts dedicated to liquid use that are, therefore, not exposed to maintenance tools, etc. Working with an existing process will provide the most reliable data for choosing the most economical material for the service required. A new process may require a corrosion study by an in-plant metallurgist, if questionable. Test coupons are a relatively inexpensive way to conduct testing on a small scale and can be obtained from equipment vendors or from the materials manufacturers themselves. Companies such as Haynes, Allegheny, etc., will also have descriptions of the suitability oftheir different alloys for various processes. An overview of corrosion testing and materials is presented in Perry S Chemical Engineer s Handbook, Sixth Edition, Sec. 23. Centrifugation 567 4.0 COMPONENTS OF THE CENTRIFUGE Centrifuges consist of the following components: - Rotor (bowl or basket) that rotates and contains the product Solids discharge unloading system, plough, scroll, inverting basket, nozzle system, etc. Drive system to rotate the bowl including main bear- ing shaft with seals, etc., and motor for electric or hydraulic operation - Frame to support unit - Enclosure to contain rotor - - 5.0 SEDIMENTATION CENTRIFUGES Sedimentation centrijkges are commonly known as solid bowl sys- tems, i.e., perforated bowls that are used to separate materials such as cream frommilk, sludges from water in waste water treatmentplants, and, ofcourse, the biotechnology materials. The basic principle of sedimentation is that a fluid consisting of two or more phases is subjected to a centrifugal-force field. As the heavier phase travels away from the axis of rotation, there is an ever increasing centrifugal force. The centrifuge increases the settling rate to clarify one phase, while simultaneously concentrating the other (usually solids). There is no flow of liquid through a cake, hence difficult filtrations are typical applications. How quickly phases separate will depend upon many factors. Capacities and performance are dictated by the particle size, distribution, solids concentra- tion, and particle shape. Adjustments for these changing factors can be achieved only through experiment, testing the particular application with its deviations. 6.0 TUBULAR-BOWL CENTRIFUGES Used often in the laboratory, this unit is limited to 4.5 kgs. of solids loading with an estimated 10-15 gallonshour liquid feed rate. Applications include stripping small bacteria or viruses from a culture medium. 568 Fermentation and Biochemical Engineering Handbook 6.1 Operation The simplest sedimentation centrifuge design is the tube type, con- structed of a tube 2 to 5 inches in diameter and spun at 62,000 g's. Slurry enters at the bottom of the tube through a feed nozzle and the effluent discharges over a dam at the top. Solids are deposited along the walls as particles intersect the bowl wall and are removed from the fluid. The bowl is suspended from an upper bearing and drive assembly. There is a loose damping assembly at the bottom. By installing two different liquid discharge ports at different radii and elevations, it is feasible to separate two different liquid phases and a solid phase. Solids are unloaded manually when clarity diminishes. 7.0 CONTINUOUS DECANTER CENTRIFUGES (WITH CONVEYOR) Typical applications in fermentation are thick fermentation broths with high solids concentrations where a relatively drier cake is required. However, protein precipitate cannot be sedimented and animal cell debris, due to their slimy nature, can render scrolling ineffective. Solids and liquids are discharged continuously in this type of design which can process coarse particles that would blind the discharge system and disks of disk bowl machines. The principle of operation is shown in Fig. 1. This unit, often referred to as a decanter, is constructed with a conical bowl and an internal rotating scroll conveyor to propel solids or beach them along the inclined wall bowl to then be discharged. The scroll rotates slower than the bowl at a differential speed of 1/20 to 1/160 of the bowl speed; this differential speed causes translation of the solids along the bowl. Particularly, soft solids can be conveyed with low conveyor differential speeds should higher differential speeds cause resuspensions. Units will haveg forces up to 6000 and range in diameter from 6" to 48".(') The solids discharge outlet is usually smaller than the liquid discharge outlet at the opposite end. By varying the liquid discharge outlet size the pool level or depth of the pond can be controlled. The lower the level, the greater the length of the dry beach section. These units operate below their critical speeds between fixed bearings attached to a rigid frame. Mechanical seals are available for pressure operation up to 150 psig. Operating temperatures are from -87°C to +260°C Centrifugation 569 570 Fermentation and Biochemical Engineering Handbook 7.1 Maintenance Bearings are the primary concern in this design, however, lifetime will depend upon the service and hours of operation. Abrasive materials can cause excessive wear along the feed zone, the conveyor leading to the beach, and the solids-discharge ports. Refacing with replaceable hard surfacing materials such as Hastelloy or tungsten may be required.['] A variation on this design is a screening bowl machine. After the solids have been pulled from the pool of liquid they will pass under a section of a wedge-bar screen to allow for additional dewatering as well as washing the solids more effectively. This design can of course only be used with particles of 80-100 microns or greater as smaller solids will pass into the effluent. 7.2 Typical Problem For Continuous Decanter Centrifuge with Conveyor The process has a feed rate of 150 gpm ofa fermentation broth at 1.5% solids. We would like to concentrate the feed to 8% solids with no more than 0.2% loss of solids in the effluent. What must the solids phase (underflow) and liquid phase (outflow) throughputs be? Two mass balance equations must be solved simultaneously. U+Q = 150 U(.080) + Q (.002) = 150 (.015) (150-0) (.080) + Q (.002) = 150 (.015) Overflow = 125 gpm Underflow = 25 gpm (25) (.Of9 % solids Recovery = (150) (0.015) = 89% U = Underflow Q = Overflow Centrifugation 571 8.0 DISK CENTRIFUGES Applications for the disk centrifuge (Fig. 2) can overlap the continuous decanter, but will typically be lower in solids concentration and often finer in particles. Examples are: 1. Cell harvesting, broth clarification for recovery of antibi- otics and hormones from the culture medium, for ex- ample, mycelia 2. Fractionation of human blood plasma 3. Separation of microorganisms and their fragments when processing fermentation products such as: bakers yeast, single cell proteins, vaccines, amino acids and enzymes 4. Isolation and purification of cell proteins 5. Bacterial cells (E Coli) for enzymatic deacylation of 7. Mammalian cells penicillin G FEED DS Figure 2. Disk-bowl centrifuge. 572 Fermentation and Biochemical Engineering Handbook 8.1 Operation Solid wall disk centrifuges were designed initially as cream separators. It is a solid bowl design containing a set of stacked disks. Bowl diameters in a standard disk centrifuge range from 7 to 24 inches and centrifuge g-forces of 14,200 to 5,500. A continuous nozzle discharge centrifugehas diameters from 10 to 30 inches andg-forces of 14,200 to 4,600. The unit rotates on a vertical shaft as slurry is introduced and pumped down a central pipe beneath the disk stack in close proximity to the bowl wall. The slurry then flows into the disks as particles settle on the underside of the inclined disks and slide to collect along the bowl wall. Liquid continues to move upwards until it overflows a weir and exits the unit. In a manual design, the bowl is one piece and the system must be stopped and opened up to discharge the collected solids. In a continuous operation, such as a wall-valve-discharge centrifuge, the bowl is made of two cones, a top and a bottom, which periodically separate to release the solids at full rotational speed. In a nozzle discharge centrifuge, the bowl is a solid, two-cone design. Orifices are located along the maximum diameter to allow solids to flow continuously. Liquid loss must be minimized and orifice sizes are therefore closely matched to the solids capacities. Thickened solids can be recycled to satisfy nozzle flow to maintain a dry effluent. This also circumvents plugging when using larger than usual orifices. Solids concentrations can range from 15 to 50%. For smaller machines where solids content varies, the intermediate solids discharge design (wall- valve), is preferred. Solids must be ofwet toothpaste consistency to flow from these types of disk machines. With the intermittent discharge, however, the solids can be wetter as it is mechanically feasible to open and close the bowl quickly enough to avoid liquid passage. 8.2 Maintenance To cool the rotor, air is pulled into the casing of the unit and leaves through a frame drain. This air can be a biohazard depending upon the nature of the product. It is therefore recommended to seal the solids discharge frame drain to the solids collection vessel. This air can then be exhausted and handled as waste. Bearings of course, as with any centrifuge, will need to be changed, preferably on a preventive-maintenance basis. Centrifugation 573 9.0 FILTERING CENTRIFUGES VS. SEDIMENTATION CENTRIFUGES Cakes will be more compacted in a filtering centrifuge as compared to a sedimentation centrifuge. As solids build at the filter medium surface, a pressure drop is induced by this cake resistance, similar to that in pressure filtration. Sedimentation therefore has an inherent advantage, particularly with difficult filtering, compactible materials. Applications for filtering centrifuges are usually products granular in nature and relatively incompressible. Centrifugal filters, through the use of a filter medium such as a cloth, are capable of retaining particles down to 1-10 microns. Those using screens as the filter surface will be able to retain 80 micron material as the smallest, without recycling. As many filtering centrifuges leave a residual heel of product after solids discharge, finer retention than 70 microns is possible. Sedimentation centrifuges can even separate to submicron levels as long as there is a density difference. Commercial centrifuges successfully separate one micron particles when specific gravity differences of 0.1 exists. High speed disk and tubular centrifuges can separate with specific gravity differences as low as 0.02 between phases.['] Solids from centrifugal filters are drier and powder-like compared to wetter, viscous consistencies from sedimentation units. Of course this is also related to the nature of the products, usually due to particle size. However, if operating with the same material, a centrifugal filter will provide a firmer, drier cake and the wash will also be far more effective due to residence times and a more effective separation. Capacities or economics are usually the controlling factors in deciding which piece of equipment to use and, even though a filtering centrifuge may yield a superior product quality, the number of machines required may not be justifiable. Production capabilities of sedimentation centrifuges are stagger- ing, over 24,000gaVh in some cases. Screening centrifuges, pushers, etc., not covered here, process large volumes of materials but of particles greater than 100 microns, 10.0 FILTERING CENTRIFUGES The driving force for separation in a filtering centrifuge is centrifugal The basic force, unlike filtration where pressure or vacuum is used. 5 74 Fermentation and Biochemical Engineering Handbook principals of Poiseuilles’ cake filtration equation can be applied by substitut- ing P (pressure) with the stress or pressure induced by the bowl contents, which is a function of centrifugal acceleration. (See Eq. 8.) 10.1 Cake Washing A wash can be introduced to fulfill different requirements: 1. Remove original slurry liquid (mother liquid) 2. Dissolve impurities 3. Alter pH 4. Displace mother liquor with another liquid (often to facilitate drying of a solvent with a lower vapor pressure or eliminate toxic solvent) Distinctly different types of wash are: 1. Displacement-Removal of one liquid in favor of another 2. Diffusion-Dissolved materials retained in the capillary liquid and in the surface liquor are transported by the wash medium 3. Dissolution-Components of the solid which is com- posed of different materials of varying solubility are dissolved in the wash medium One or all of the steps can be used on a product or occur simulta- neously. Several steps can be used often, first adisplacement wash to remove mother liquors and any associated impurities, followed by a dissolution or diffusion wash. Two different methods of introducing the wash are: 1, Flood Washing-With this method, the wash medium is fed at a faster rate than the case is dewatering, thus a liquid level forms on the top of the cake. This ensures distribution of the wash fluid over the entire cake. Posi- tive displacement is the most effective form of washing. Carried out in a plug flow manner, clean wash fluid contacts the solids without backmixing. Except where retention time is required to allow for mass diffusion of the impurities through the solids, positive displacement washes are more efficient then reslurrying. Redilution of Centrifugation 5 75 the impurities occurs as reslurrying backmixes impurities into the fresh medium. The cake should be even to achieve this displacement wash, as the wash fluid will seek the path of least resistance on nonuniform cakes. Vertical basket centrifuges in particular have uneven cakes due to the feeding method and can require copious quantities of wash to compensate for the uneven cake. 2. Spray Washing-Liquid is supplied via spray nozzles. It is the only effective way of working cakes which are uneven from the top to the bottom for basket centrifuge. Peeler centrifuges, being unaffected by the force of grav- ity in the distribution of the cake, tend to have more even cakes, although nonuniformity can still occur due to the feeding mechanism. 11.0 VERTICAL BASKET CENTRIFUGES 11.1 Applications Vertical basket centrifbges have been the “work horse” for the pharmaceutical industry for many years for intermediate and final filtration steps. Chemical and specialty chemical productions use this type of equipment in a wide range of applications In fermentation, typically, post-crystallization steps are processed on this unit, crystalline products that are free-draining. Different designs are available, the simplest being a manual design or under-driven topdischarge. A perforated basket with cloth or screen is the filter media. Filtrate passes through to a filtrate chamber as shown in Fig. 3. Solids must be dug out or the entire filter bag hoisted out with the solids. Labor intensive with operation exposure a problem, this design is more often employed in pilot than production plants. G-forces up to 800 g’s are attainable. Introduction of a traversing plough mechanism to this design enables automatic solids discharge through the basket bottom. Speeds are variable in this design The cycle is similar to that of a household washing machine, filling, spinning or dewatering, washing, a final spin and unloading. The feed is fed off to one side. This, coupled with the force of gravity in the vertical basket through 800 g’s. 576 Fermentation and Biochemical Engineering Handbook design, can cause an uneven slurry distribution, Le., a thicker cake at the bottom of the basket or in the middle where the feed pipe is located. Hence, reduced filling speeds may be required depending upon the product. Cones for 360' feed distribution are available for more even loading. Figure 3. Vertical basket centrifuge (manual unloading). Operating batch-wise, basket centrifuges in general are optimized best when operated almost continuously by feeding the machine at the same rate as the slurry is dewatering, thus maximizing solids concentration. Loading the basket infinitely fast will only produce a basket with the same solids concentration as the feed tank, The wash can be quantified by mass or volumetric flow to ensure product quality. The intermediate and final spins are usually timed if the basket is automated, however, if out of balance conditions exist during the feed cycle, an operator is often required throughout the operation. Operator judgement is often required to determine when the liquid level on the cake disappears. This may be required, for example, to remove all mother liquor, introduce the wash, particularly for difficult filtrations, or to be sure the liquid level remains on the cake to prevent cracking and preferential channeling of the wash liquid. Variable solids concentrations or particle Centrifugation 577 sizes distributions will make it more difficult to fully automatic a standard basket centrifuge as operatorjudgment may be required at several points in the cycle. Consistent, uniform batches with every filtration can, however, be automated on a time basis. 11.2 Solids Discharge For solids unloading, a plough cuts out the solids at reduced speeds of 40-70 rpm, and traversing action must be slowed down to prevent the basket from stalling. For certain products, the cake can be sufficiently difficult to remove that the plough cannot remove all of the solids due to tolerances and possibility of damaging the filtercloth, thus a residual heel of solids is left for some products. For some products, this is not a problem and the next cycle can begin. For others, the heel can glaze over and reduce filtration rates on subsequent batches. It must be scraped out manually, dissolved, or an air knife can be used, depending upon the hardness of the heel. Depending upon how problematic the residual heel, even the automated vertical basket can be labor intensive. 11.3 Operational Speeds An average cycle would be filling at 600 rpm, washing at 800 rpm and dewatering at 1000 rpm. With a 48" basket, these are g forces of 240,426 and 667, respectively. Discharge by a plough occurs at less than 100 rpm, or, if manually unloaded, at zero speed. 11.4 Maintenance If there are significant out-of-balance operating conditions, mechani- cal parts such as the plough or cake detection can vibrate loose. The bearings and shaft seal components will also have limited lifetimes, depending on the operation. 12.0 HORIZONTAL PEELER CENTRIFUGE 12.1 Applications The horizontalpeeler centrifuge (Fig. 4) is a variation of the vertical basket. Up to 80 inches in diameter and producing as much as 100 tons per 5 78 Fermentation and Biochemical Engineering Handbook day of product applications, this machine has been prevalent in the isolation of beet sugar and starch. The design, characterized as Ter Meer, after the inventor, is sometimes used in bulk pharmaceutical productions. Dedicated productions of relatively easy filtrations being processed are applications for this type of equipment. Plough Solids dis Figure 4.- Peeler centrifuge. 12.2 Operation Solids Discharge. This is carried out by an automatic plough or knife. Since the knife cannot contact the filter medium, a heel of product remains in the basket after each discharge. This can prevent fines from passing, but, like in a vertical basket, may become glazed and impervious to filtration. Backwashing the heel or redissolving may be possible. Even changing the depth at which the blade cuts the cake may help. In the Ferrum design, high pressure air forces the cake off the screen during discharge. This will work in some applications. Solids exit a chute, but can also discharge by screw conveyor. Centrifugation 579 Feed Mechanism. A cake detection device, pneumatic in some designs, activates the feed valve closure when the desired cake depth is reached. This cake depth is monitored by a proximity switch. This device can also act as a cake distributor to level the load during feeding. Alternative feed designs are available depending upon the vendor. Wash. The entire cycle is operated automatically on a pre-pro- grammed basis, all by time. The wash can, of course, be by time or volumetric basis, monitored by air in-line flow meter and totalized. During the entire sequence of loading, deliquoring, and unloading, a constant bowl speed is maintained. “Dead time,” associated with acceleration and deceleration, is minimized. Maxi- mum operating speeds depend upon bowl diameter, the larger the diameter the lower the speeds. Sizes range from pilot scale 450 mm to 1430 mm diameter withg-forces from 3200 to 1200. Basket speeds range from 3000 rpm to 1200 rpm. Specially designed vibration-damping systems will minimize plant structural supports required. Each manufacturer’s design must be evaluated as to what is required. Special cement foundations are often a necessity. Operational Speeds. 13.0 INVERTING FILTER CENTRIFUGE Originally designed by the firm Heinkel, inverting filter technology has revolutionized the concept of filtering centrifuges since their introduction into the pharmaceutical market in the early 1980’s. The design eliminated the inherent problems in the conventional centrifuge design of the solids dis- charge process and balance problems long associated with centrifuges. Effecting a filly automatic solids discharge, an inverting filter removes all product from the cloth, thereby eliminating any residual heel. This permits the separation of a wider range of materials than conventional basket centrifuges. Amorphous through crystalline products can be separated in this type of centrifuge as there are no residual solids left on the cloth that can blind or glaze over. Extremely difficult filtrations are therefore possible. Small volumes of fermentation broth through post-crystallization steps are found on this type of unit. 580 Fermentation and Biochemical Engineering Handbook Figure 5. Inverting Filtering Centrifuge. (Courtesy of Heinkel Filtering Systems, Inc.) 13.1 Operation Solids Discharge. The centrifuge is horizontally mounted and the cycle is similar to a vertical or peeler centrifuge, i.e., feeding, washing, dewatering and solids discharge. The basket, however, is in two parts, a bowl and a bowl insert. By fixing the end of the filtercloth under a clamping ring on the bowl insert, the filtercloth can be inverted by axially moving the bowl insert. This is shown in Fig. 6. Rotation of the bowl and bowl insert in unison at reduced speed ensures a complete solids discharge. Centrifugation 581 SUSPE INLET \ CENTRIFUGATION FILTER DISCHARGE Figure 6. Inverting Filter Centrifuge. (Courtesy of Heinkel Filtering Systems, Inc.) Feed Mechanism. An open-ended pipe centered in the bowl allows feeding of the slurry 360’ around the cloth. The Inverting Filter Centrihge is horizontally mounted like the peeler, so g-force does not effect the distribution of the cake. In addition, bars connecting the front plate to the back plate of the bowl insert serve as a distribution mechanism. Slurry passing the bars is evenly dispersed providing for auniform cake. As a result, outsf-balance conditions are minimized. A special cement foundation or vibration isolator normally required for centrihges is not necessary. Without this vibration, a load cell can be used in lieu of cake detectors or “feelers” to monitor the cycle and prevent overfilling the bowl. A typical cycle is shown in Fig 7. 582 Fermentation and Biochemical Engineering Handbook .---_--------------.----------.-.--------.---.------.---.-.----------------....- Figure 7. Filling control system. (Courtey of Heinkel Filtering Systems, Inc.) Multipurpose Applications. Cake thickness is varied, dependent upon the application. Thin cakes from finer particles or more amorphous, compressible materials versus thick cakes for hard, easy filling crystals. Discharge time is less than one minute, and even cake distributions allow for higher filling, washing anddewatering speeds, thus the overall cycle is shorter. One can therefore “efficiently” operate with a thin cake as low as 1/4”, if necessary, as opposed to 3-6 inches on a conventional basket. If operating with a thin cake on a basket, the residual heel still exists and, as it requires sufficiently longer times for processing, it would be inefficient to operate with such a thin cake. As a result of relatively thinner cakes and higher g-forces, filtration rates per unit filter cloth area can be as high as 20-30 times that of typical basket centrifuges, For that reason, a smaller volume Inverting Filter Centrifuge can replace a larger basket centrifuge. By optimizing based upon cake thickness (see Fig. S.), higher productivities will be reached. Centrifugation 583 Figure 8. Optimal cake thickness curve. (Courtey of Heinkel Filtering Systems, Inc.) 13.2 Maintenance By operating with minimal vibration, wear of mechanical parts is reduced. Bearings and shaft seals are changed on a preventive maintenance basis every three to four years. Regular maintenance is required for the filtercloths and product contacted O-rings. These must be chosen in materials of construction compatible with the product. They are usually changed at the end of a campaign before switching to a new product. For dedicated processes, lifetime will depend upon the product. 14.0 MAINTENANCE: CENTRIFUGE All rotating equipment will exhibit certain harmonic frequencies upon acceleration and deceleration ofthe unit. It is the speed at which the frequency of rotation equals the natural frequency of the rotating part.(') They can be 584 Fermentation and Biochemical Engineering Nan dbook calculated from the moment of inertia, but are best found by experiment, running the unit from zero to maximum speed and noting any increase in vibration or noise ofthe unit. Operating conditions should pass through these speeds, however, never maintain them for any period oftime as, at this speed, any vibration induced by the imbalance in the rotor is compounded resulting in abnormally high stresses. True critical speeds are well above the allowable operating speeds. 14.1 Bearings As a rotating unit, bearings changes should be planned on a preventive maintenance basis every few years. Bearing noise monitoring systems can prevent emergency shutdowns. Ninety percent of bearing failures can be predicted months in advance. Ten percent are still unforeseen. Bearing factories produce the highest caliber of any manufactured goods. Defects are not the primary cause of failures. Failures usually stem from: 1. Contamination, including moisture. 2. Overstress 3. Overuse of lubrication including mixing incompatible greases. 4. Defects created on installation or transportation and to a much less extent, insufficient lubrication. Bearing temperature probes are also available at each bearing point although they will not provide the advance notice that a noise monitoring system will. Scheduled shutdown and changing of shaft seals is recom- mended on at least a tri-annual basis. Over-greasing of bearings and mixing of incompatible bearing greases can cause more problems than under- greasing. High temperatures ofover 100°C will also be exhibited when over- greasing or under-greasing. The trend towards elimination of hydraulics in pharmaceutical process areas has turned maintenance over to the instrumentation and electrical specialists, as variable frequency drives become the standard. The Inverting Filter Centrifuge (Heinkel) eliminates all hydraulics from the centrifuge design to satisfjr increasingly stringent cleanroom requirements by pharma- ceutical companies. The risk of hydraulic oil in the process area has been a concern with respect to contamination with the product. Centrifugation 585 15.0 SAFETY Vibration Detection System. To monitor vibration levels, every centrifuge should be equipped with a vibration detection device. Usually mounted on the filtration housing itself, a local transducer will send a signal back to a vibration monitor. After exceeding a certain vibration setpoint (2- 3 inches per second for a standard basket centrifuge, 0.75 incheshec. for an inverting filter) the controller will close all process valves and decelerate the machine to a stable operating condition, therefore, any mother liquors can be spun off. Should the vibration levels still exceed the setpoint, the machine will be given an emergency stop signal. Out of balance conditions usually occur due to feeding of an unbal- anced load or uneven cake. This is more likely in vertical basket centrifuges than horizontally mounted systems. It is less common in an Inverting Filter Centrifuge due to the central feed distribution. Inerting System with Oxygen Analyzers. Operations with solvents require an inert atmosphere, usually nitrogen, or in some cases, carbon dioxide. A purge of the bearing housing, shaft sealing system and process areas are required in critical blanketing of the system, based upon time to allow for a certain number of volume changes to be performed or the preferred method of blanketing, until a low oxygen setpoint is needed. Oxygen analyzers continuously monitoring a sample gas stream from a vent on the unit confirms the safe oxygen operating level. This level must be below the lower explosive limit for solvent. Three conditions must occur for a fire, a spark, an oxygen rich atmosphere, and the fuel, Le., solvents. Although a spark is not expected, static electricity can occur in lines, etc. One should choose an oxygen analyzer with a wet sampling system, Le., precondition ofthe sample gas with aprefilter. Entrained solvents in the sample gas can affect readings. A scrubber may be required to remove noxious gases to prevent corrosion to the sampling system. Oxygen analyzers should be evaluated as some systems fail to the unsafe condition of 0% 0,. 16.0 PRESSURE-ADDED CENTRIFUGATION It is more efficient to mechanically dewater solids than thermally, due to costly energy requirements. Filters such as pressure or vacuum units are 586 Fermentation and Biochemical Engineering Handbook used for solidsfliquid separation, providing high forces to drive the liquid through the cake. Recently, equipment designed to combine both centrifuga- tion and pressurization has lead to increased dewatering of solids beyond what either process would do alone. (See Fig. 9.) Feed/ Presw - re .--- PSI Figure 9. Inverting filter centrifuge with pressurization. (Courtey of Heinkel Filtering Systems, Inc.) A centrifugal field achieves mechanical separation of slurries by emptying the liquid in the capillaries between the solids. Larger particles will exhibit faster drainage of these capillaries. Liquid in the interstices of the solids is retained due to high capillary forces in the micron pores and cracks in the particle. These capillary forces are so high that they can only be removed thermally. This contributes to a certain capillary height of liquid that is independent ofthe packed bed weight. After dewatering for an extended time, an equilibrium point is reached. Only changing the driving force by increasing centrifugal force will overcome and reduce this equilibrium saturation point. Product with a smaller particle size distribution will have higher capillary forces and thus a higher equilibrium saturation point or residual moisture. By addition of the driving force pressure, or pressure differential across the packed bed, additional liquid is forced through the capillaries below the equilibrium saturation point, thus reducing the residual moisture. Centri$ugation 587 Initial pressurization of the basket alone, thus avoiding pressurizing the entire centrifige, can decrease final dewatering steps by as much as 80%. By blowing through the cake at a certain temperature, volume of gas, and pressure, drying will be achieved. Products that are crystalline and easy to filter can be dried in a relatively short period of time, not adding significantly to the overall cycle. Difficult filtering, amorphous materials may see overall cycle times reduced or products previously wet and sticky now easily handled at lower moisture levels going into a dryer. (See Table 3.) Downstream drying equipment can then be reduced in size, or possible eliminated. The Inverting Filter with Pressure-Added Centrifugation has proved to dry products to 0.008% residual moisture using hot gas. With heated gas, it is possible to break the upper surface ofthe moisture film, aiding in dewatering, or to dry or strip solvents. Steam washing can reduce wash quantities required. Table 3. Pressure-Added Centrifbgation Product A-Unnamed pharmaceutical intermediate Exua processing time for drying (% of filtration cvcie) Product E-unnamed DharttIaCeUtiCd intermediate LOD% under centrifugal force alone (1,200 g’s) LOD% under nitrogen at 50°C (6 bar g) Extra processing time for drying (% of filtration cycle) 12 % 0.05% . 50% Mother liquor Toluene Product c-unnamed pharmaceutical intermediate n Note: This extremely difficult filtering product went from behaving as a very sticky solid at 45% LOD, to a free flowing friable powder at 15% 588 Fermentation and Biochemical Engineering Handbook 17.0 MANUFACTURERS 17.1 Filtering Centrifuges Inverting Filter Centrifuges Heinkel Filtering Systems, Inc. Heinkel Industriezentrihgen GmbH+CO Bridgeport, New Jersey Bietigheim-Bissingen, Germany Comi-Condur SpA Italy Perforated Basket Centrifuges/ Vertical or Horizontal Bir Machine Co., Inc./Ketema Walpole, MA Don-Oliver (Acquired by Krauss-Maffei) Stamford, CT Broadbent England Krauss-Maffei, Inc. Krauss-Maffei Verfahrenstechnik GmbH Florence, KY Munich, Germany Robatel Pittsfield, MA Robatel France Western States Machine Company, Inc. Hamilton, OH 17.2 Sedimentation Centrifuges Baker Perkins, Inc. Saginaw, MI Bird Machine Company, Inc. South Walpole, MA Centrico, Inc. (Westfalia) Northvale, NJ Centrifugation 589 DeLaval (SharplesE'ennwalt) Warminster, PA 17.3 Oxygen Analyzers Neutronics Exton, PA Orbisphere Laboratories, Inc. Emerson, NJ Servomex Company, Inc. Nonvood, MA REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 Perry, R. H., Green, D. W., andMaloney, J. 0. (eds.), Perry'sChemical Engineers Handbook, Sixth Edition, Sections 2 1,27, McGraw-Hill Book Co., New York (1984) Vogel, H. 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