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 to twenty-four feet, and up to 1000 ft2 of filtration area.[5 J 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 concentrating cell mass. Membranes can yield more complete filtration clarification, 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 submergence. 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 filiations 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.
The progressively advancing blade moves 0.05 to 0.2 mm perrevolution. Vacuum is maintained throughout the cycle, instead of just during submergence, so that the precoat is retained. Once the precoat is expended, the RVF must be thoroughly cleaned, and a fresh coat reapplied.
Pressure leaf tests 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 carried 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 temperature. If cake cracking occurs, the wash should be introduced earlier to avoid channeling.
Several leaf tests 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, filtermedia 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.
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.
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 and/or 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 of the 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 and/or cooling and the agitator blade heated. This design can now be a reactor in combination with a filter-dryer or alone as a filter-dryer (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. A two-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. Some manufacturers have air-knife designs that remove the residual heel. Heels as low as one-quarter inch can be obtained.
Movable Bottom Side Discharge Permanent Agitator 100* Jacketed
Drive - Bon-Heated
Single Or Multi-Layer Filter Cloth
Figure 9. Agitated nutsche type pressure filter. (Courtesy of C0GE1M SpA).
Single Or Multi-Layer Filter Cloth
Figure 9. Agitated nutsche type pressure filter. (Courtesy of C0GE1M SpA).
Materials of construction can vary widely depending upon the application. Typically, 304 or 316 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, Ra, is the arithmetical average of the surface roughness in microinches. The rms is the root mean square of the surface roughness in microinches; rms =1.1 Ra.
A mechanical finish of400 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 of400 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 perunit area offiltration.'11 Thoseproducts that tend to blind filter media, i.e., colloidal slurries, gelatinous and protein compounds, will require alternate equipment, filtration or centrifugation.
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 maintenance 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.
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