Rotary Vacuum Filter Design
Rotary Vacuum Filter Maintenance
Rotary Vacuum Filter Specifications
Rotary Vacuum Filter In Sugar Industry

An Auto-Vac® utilizes a liquid ring vacuum pump that pulls water through a precoat drum filter while sucking moisture out of the suspended solids. The inside of the drum is hollow; the outside consists of a wedge wire screen covered by a polypropylene cloth. The drum rotates in a trough [filter pan] where one-third is submerged in liquid and two-thirds are exposed to ambient air. When activated, and before filtration, the drum surface is coated with a filter aid 'cake' of Diatomaceous Earth (DE) or Perlite. This consumable filter media is porous, allowing air and water to pass through while capturing micron size particles.
Once a full cake is built on the drum, the wastewater is pumped into the pan at a controlled rate. The vacuum sucks the water [liquid] through the drum and pumps it away. Suspended solids, precipitated metals, or de-emulsified fat, oil and grease larger than one-micron will accumulate on the filter cake surface. These solids will build with each revolution of the drum until the vacuum cannot draw any more water through. At this point, a stepping motor activates a variable speed knife blade that removes the top layer of solids with lathe-like precision. The knife also shaves off a slight amount of filter aid cake as it advances toward the drum; leaving a clean layer of media to grab more solids.
The drum size, filtration rate, dryness of solids and filter aid consumption varies with the characteristics of the incoming liquid. Contact ALAR to help determine the Auto-Vac® that best suits your application.

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.

Rotary Vacuum Drum Filter Designs Include: Scraper Discharge is the most common drum filter discharge. After dewatering, the cake is removed from the filter cloth by a scraper blade just prior to the re-submergence of the drum. The scraper serves mainly as a deflector to direct the cake, dislodged by an air blowback, into the discharge chute.

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.

Rotary Vacuum Drum Filter Designs Include: Scraper Discharge is the most common drum filter discharge. After dewatering, the cake is removed from the filter cloth by a scraper blade just prior to the re-submergence of the drum. The scraper serves mainly as a deflector to direct the cake, dislodged by an air blowback, into the discharge chute. Vacuum filters – These include rotary drum filters and horizontal belt filters. Both are continuous operations. Both are continuous operations. Rotary Drum filters – these are horizontal drums with perforated exteriors covered with a filter cloth and internal vacuum pipes that are designed to be partially submerged in a trough of liquor.

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.

Figure 2. Rotary vacuum filter schematic.

Figure 3. Cake discharge mechanism.

Figure 4. String discharge.

Figure 6. Precoat rotary vacuum filter.

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.

10.2 Optimization

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.

11.0 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.

Rotary Vacuum Filter Design

Figure 7. Pressure leaf test.

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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