Rotary Vacuum Filters

Large rotary vacuum filters are commonly used by industries which produce large volumes of liquid which need continuous processing. The filter consists of a rotating, hollow, segmented drum covered with a fabric or metal filter which is partially immersed in a trough containing the broth to be filtered (Fig. 10.10). The slurry is fed on to the outside of the revolving drum and vacuum pressure is applied internally so that the filtrate is drawn through the filter, into the drum and finally to a collecting vessel. The interior of the drum is divided into a series of compartments, to which the vacuum pressure is normally applied for most of each revolution as the drum slowly revolves (~ 1 rpm). However, just before discharge of the filter cake, air pressure may be applied internally to help ease the filter cake off the drum. A number of spray jets may be carefully positioned so that water can be applied to rinse the cake. This washing is carefully controlled so that dilution of the filtrate is minimal.

It should be noted that the driving force for filtration (pressure differential across the filter) is limited to one atmosphere (100 kN m 2) and in practice it is significantly less than this. In contrast, pressure filters can be operated at many atmospheres pressure. A number of rotary vacuum drum filters are manufactured, which differ in the mechanism of cake discharge from the drum:

Fig. 10.9b. Rings for metafilter (Coulson and Richardson, 1991).

(22 mm external diameter, 16 mm internal diameter and 0.8 mm thick) are normally made from stainless steel and precision stamped so that there are a number of shoulders on one side. This ensures that there will be clearances of 0.025 mm to 0.25 mm when the rings

(i) String discharge.

(ii) Scraper discharge.

(iii) Scraper discharge with precoating of the drum.

(i) String discharge

Fungal mycelia produce a fibrous filter cake which can easily be separated from the drum by string discharge (Fig. 10.11). Long lengths of string 1.5 cm apart

Direction of rotation

Outlet

Stationary automatic valve ring

Filtered cake Vent to atmosphere

Direction of rotation

Outlet

Strings returning to drum

Outlet

Level of material to be filtered

Fig. 10.10. Diagram of string-discharge filter operation. Sections 1 to 4 are filtering; sections 5 to 12 are dewatering; and section 13 is discharging the cake with the string discharge. Sections 14, 15 and 16 are ready to start a new cycle. A, B and C represent dividing members in the annular ring (Miller et al., 1973).

Outlet

Level of material to be filtered

Filter medium

Strings returning to drum

Filter medium

-Cake discharge

Air blows billows medium

Knife blade

Fig. 10.12. Cake discharge on a drum using a scraper (Talcott et al., 1980).

Air blows billows medium

Knife blade

-Cake discharge

Fig. 10.10. Diagram of string-discharge filter operation. Sections 1 to 4 are filtering; sections 5 to 12 are dewatering; and section 13 is discharging the cake with the string discharge. Sections 14, 15 and 16 are ready to start a new cycle. A, B and C represent dividing members in the annular ring (Miller et al., 1973).

are threaded over the drum and round two rollers. The cake is lifted free from the upper part of the drum when the vacuum pressure is released and carried to the small rollers where it falls free.

(ii) Scraper discharge

Yeast cells can be collected on a filter drum with a knife blade for scraper discharge (Fig. 10.12). The filter cake which builds up on the drum is removed by an accurately positioned knife blade. Because the knife is close to the drum, there may be gradual wearing of the filter cloth on the drum.

Fig. 10.12. Cake discharge on a drum using a scraper (Talcott et al., 1980).

lem is overcome by precoating the drum with a layer of filter-aid 2 to 10 cm thick. The cake which builds up on the drum during operation is cut away by the knife blade (Fig. 10.13) which mechanically advances towards the drum at a controlled slow rate. Alternatively, the blade may be operated manually when there is an indication of 'blinding' which may be apparent from a reduction in the filtration rate. In either case the cake is removed together with a very thin layer of precoat. A study of precoat drum filtration has been made by Bell and Hutto (1958). The operating variables studied included drum speed, extent of drum submergence, knife advance speed and applied vacuum. The work indicated that optimization for a new process might require prolonged trials. Although primarily used for the separation of micro-organisms from broth, studies have indicated (Gray et al., 1973) that rotary vacuum filters can be effective in the processing of disrupted cells.

(iii) Scraper discharge with precoating of the drum

The filter cloth on the drum can be blocked by bacterial cells or mycelia of actinomycetes. This prob-

et al., 1980)

Cross-flow filtration (tangential filtration)

In the filtration processes previously described, the flow of broth was perpendicular to the filtration mem

Fig. 10.13. Cake discharge on a precoated drum filter (Talcott et al., 1980).

brane. Consequently, blockage of the membrane led to lower rates of productivity and/or the need for filter aids to be added, and these were serious disadvantages.

In contrast, an alternative which is rapidly gaining prominence both in the processing of whole fermentation broths (Tanny et al., 1980; Brown and Kavanagh, 1987; Warren et al, 1991) and cell lysates (Gabler and Ryan, 1985; Le and Atkinson, 1985) is cross-flow filtration. Here, the flow of medium to be filtered is tangential to the membrane (Fig. 10.14(a)), and no filter cake builds up on the membrane.

The benefits of cross-flow filtration are:

Efficient separation, > 99.9% cell retention. Closed system; for the containment of organisms with no aerosol formation (see also Chapter 7).

Separation is independent of cell and media densities, in contrast to centrifugation. No addition of filter aid (Zahka and Leahy, 1985).

The major components of a cross-flow filtration system are a media storage tank (or the fermenter), a pump and a membrane pack (Fig. 10.14(b)). The membrane is usually in a cassette pack of hollow fibres or flat sheets in a plate and frame type stack or a spiral cartridge (Strathmann, 1985). In this way, and by the

Slurry to be filtered

Flow tangential to membrane surface

Retentate

Filtration membrane

Filtrate or Permeate

Fig. 10.14a. Schematic diagram of cross-flow filtration.

introduction of a much convoluted surface, large fih tion areas can be attained in compact devices types of membrane may be used; microporous mem° branes with a specific pore size (0.45, 0.22 ¡xm etc) an ultrafiltration membrane (see later section) with^ specified molecular weight cut-off (MWCO). The tvn8 of membrane chosen is carefully matched to the product being harvested, with microporous and 100,000 MWCO membranes being used in cell separations.

The output from the pump is forced across the membrane surface; most of this flow sweeps the membrane, returning retained species back to the storage tank and generally less than 10% of the flow passes through the membrane (permeate). As this process is continued the cells, or other retained species are concentrated to between 5 and 10% of their initial volume. More complex variants of the process can allow in-sitii washing of the retentate and enclosed systems for containment and sterilization (Mourot et al, 1989).

Many factors influence filtration rate. Increased pressure drop will, up to a point increase flow across the membrane, but it should be remembered that the system is based on a swept clean membrane. Therefore, if the pressure drop is too great the membrane may become blocked. The filtration rate is therefore influenced by the rate of tangential flow across the membrane; by increasing the shear forces at the membrane's surface retained species are more effectively removed, thereby increasing filtration rate. Higher temperatures will increase filtration rate by lowering the viscosity of the media, though this is clearly of limited application in biological systems. Filtration rate is inversely proportional to concentration, and media constituents can influence filtration rate in three ways. Low molecular weight compounds increase media viscosity and high molecular weight compounds decrease shear at the membrane surface, both leading to a reduction in filtration rate. Finally, broth constituents can 'foul' the

Media storage tank (or fermenter)

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