Rotary vacuum precoat filters

There are some liquids with solids so slimy that it is impossible to filter them on a pressure or vacuum leaf filter. Other liquids contain such a high volume of solids that the filters described previously would quickly become filled with cake, resulting in an uneconomical amount of time spent in cleaning and precoating. The rotary vacuum precoat filter was designed to meet these conditions. 

Precoating techniques recently developed by Manville can substantially increase filtration efficiency in terms of reduced precoat usage and increased flow rate. 

The filter consists of a horizontal drum, 30-50% of which is submerged in the unfil-tered liquid. The drum is covered with a septum capable of retaining filter aid. Vacuum is applied to the surface of the drum by means of internal piping, which emerges through the drum trunnion and valve on one end of the filter. 

From the valve, the filtrate goes to a receiver where liquid and air (or other gas) are separated, the liquid usually being removed by a centrifugal pump on the bottom of the receiver, and the gas by a vacuum pump and/or condenser. 

In operation, a precoat of filter aid up to 6 in. thick is built up on the drum by pumping a slurry of filter aid from a precoat tank through the filter and either back to the precoat tank or on to the process. After the precoat is built up, unfiltered liquid is introduced into the filter. As the drum rotates, a blade running across the face of the drum above the liquid level continually advances toward the drum, peeling off solids plus a very small amount of precoat. This continues until the knife is within 1/4-3/8 in. (6.3-9.5 mm) of the drum, at which time the drum is cleaned and precoated again. 

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A mixture of the hydrolyzate with diatomaceous earth, added as a filter aid, is then filtered on rotary, vacuum, precoated filters. The filtrate is a clear, slightly yellow, sweet-tasting liquor containing about 0.1% of protein and 0.2% of ash. The rest is carbohydrate, made up of 95.0% of D-glucose, 2.0% of maltose, 1.6% of isomaltose, and 1.4% of tri- and higher saccharides. 

There are two types (a) the vacuum leaf filter and (b) the rotary vacuum precoat filter (Figs. 16 and 17). 

Doctor Blade (knife) A sharp, hard blade that cuts the cake off the surface of a filter. Usually found on rotary vacuum precoat filters. 

RVPF (Rotary Vacuum Precoat Filter) A drum filter that is coated with a thick (up to 6 in.) precoat of filter aid, such as CELITE or equivalent. 

In the precoat mode, filter aids allow filtration of very fine or compressible soHds from suspensions of 5% or lower soHds concentration on a rotary dmm precoat filter. This modification of the rotary dmm vacuum filter uses an advancing knife continuously to skim off the separated soHds and the... 

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. 

Comments vacuum belt, screens, belts, top feed drum tilting pan, vacuum drums or disks continuous filters vacuiun filter, rotary vacuum, pressiue filters batch cartridge, precoat drums, deep bed... 

The precoat drum shown schematically in Figure 1.43 is similar in basic form and operation to the bottom fed rotary vacuum drum filter (see Section 1.4.1.5). Prior to filtration of the feed, a suspension of precoat is filtered under vacuum onto the drum surface to a depth between 4 and 15 cm. The feed is then introduced and filtration proceeds to produce thin, sometimes... 

Disc filters are similar in principle to rotary filters, but consist of several thin discs mounted on a shaft, in place of the drum. This gives a larger effective filtering area on a given floor area, and vacuum disc filters are used in preference to drum filters where space is restricted. At sizes above approximately 25 m2 filtration area, disc filters are cheaper but their applications are more restricted, as they are not as suitable for the application of wash water, or precoating. 

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. 

Fig. 20.9 A beer and yeast recovery system. 1. Fermenter 2. Beer clarifier 3. Surge tank 4. Yeast suspension tank 5. Filter aid mixer 6. Yeast (autolysis) tank 7. Rotary vacuum filter 8. Top feed applicator 9. Precoat applicator 10. Yeast suspension spray 11. Defoamer 12. Vacuum pump 13. Beer recovery tank 14. Pasteurizer 15. Yeast drier A. Wort preparation B. Beer treatment C. Dried yeast. (Based on drawing of Alfa-Laval Ltd.)...

FQtration with Filter Aids. A wide range of pressure and vacuum cake filters can be used with both precoat and body aid fihration. Most of these operate in the batch mode and include the filter press, the vertical and horizontal leaf filters with sluice or centrihigal disduuge and the vertical tubular fih . The rotary drum vacuum fitter works in a semi-batch shion in that the precoat is removed progressively as the fitration proceeds. 

Hofiinger, W. and Hacki, A, 1990, Evaluation of optimal knife advances for precoat filtration on rotary vacuum filters, Filtrat. and Separat, 27, pp 110-113. 

Rotary-type enclosed continuous filters are used Almost exclusively in solvent dewaxing because the solvents are valuable and because they usually, form explosive mixtures with air. The principle of operation is the same as that outlined on page 321 with regard to the Oliver precoat filter. However, a precoat is not employed in wax filtering, and the rotating drum is enclosed in a shell so that pressure can be applied (Fig. 12-4). Some plants employ vacuum rather than pressure. The wax cake is sprayed or washed with solvent while it is still on the drum. Closed filtering systems are required because the solvents must be... 

A rotary vacuum drum precoat filter is a specially designed version of the vacuum drum designed to use a thick layer of precoat material to clarify liquid flows. Initially a slurry of the precoat material is prepared and filtered to produce a cake of up to 100mm thickness on the filter cloth. Then the feed trough is drained and refilled with the suspension to be filtered, which is now filtered in the normal way through the precoat layer. The filtered cake can be washed if necessary, and is then discharged by means of a full-width scraper blade. The blade removes the whole of the cake, together with a thin slice (about 0.5 mm) of the precoat layer. 

A precoated rotary drum vacuum filter is the only filtration equipment from which extremely thin filter cakes can be positively and continuously removed in a semi-dry state. 

Rotary Drum Vacuum Filters. This filter can accommodate a large thickness of cake and can take a precoat of filter aid which is up to 150 mm in depth. The feed su ension is filtered onto the precoat and the filtered solids together with a thin layer of precoat is scraped off the cake sur ce by a horizontal knife situated just before the surface passes back into the feed suqiension tank, thus e qposing a fi esh surface of precoat. The scraper knife advances automatically until it reaches a mioimum distance fi-om the cloth, usually 5-10 mm. At this point the cycle is finished and the filter is cleaned of used precoat. 

Rotary drum vacuum filters are a widely used piece of equipment for this filtration step, due to its simple operation and low investment costs. The drum is typically precoated (e.g., with diato-maceous earth, sUica, perlite) to improve the process performance [7]. Additional filter aid is often added to improve the filtrate qnality hence, the amount of slndge generated from this unit operation is relatively large. Several other filtration devices such as filter automates and cross-flow filtration devices are also nsed for the filtration. One of the benefits of the latter is that they can be operated withont addition of filter aids and thus, the amount of generated sludge can be reduced. 

Prime advantages and applications General use for maximum clarity. Reduced dosage on pressure vacuum filters. Good on rotary filters Excellent for precoating coarse screens. Highest purity for absorption of oil from condensate and removal of iron from caustic. 

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