Rotary vacuum drum filters applications

Filtration is a unit operation commonly employed nowadays in biotechnological processes. In this unit operation, a filter medium acts as a physical barrier to particles larger than its pores. Traditional filtration devices such as filter presses and rotary vacuum drum filters have so far found no application for the separation of animal cells. Nevertheless, membrane filters are commonly employed, as well as some alternative filter designs such as spin-filters. In the next sections, the most common types of filters used for animal cell separation will be discussed. 

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. 

The rotary vacuum filter (RVF, Rovac) comprises a revolving perforated drum that supports a filter medium and is suspended into a vessel containing slurry (Figure 4.7). Liquor is drawn through the drum by the application of an... 

The operation of the continuous rotary vacuum filter is illustrated in Figure 4.17. The filter drum is immersed in a bath of slurry from where the liquor is drawn through the filter medium by the pressure drop caused by the application of a vacuum within the drum. During filtration cake builds up on the outside surface of the drum between points A and B as if by constant pressure batch operation. It then travels to point C where it is removed and the cycle repeated. 

Horizontal belt filters working on the vacnnm principle were originally developed for process applications where intensive washing is required, but are now also employed for a wide variety of filtering reqnirements. They have become such a versatile processing tool that they have taken a considerable share of the market from the former workhorse of the vacuum filter business, the rotary drum filter, and also quite a share from traditional filter press applications. 

This equipment consists of a horizontal axis revolving drum. The surface of the drum is segmented, with compartments fitted with drainage pipes leading inside the drum to a common manifold and a rotary valve located at one end of the drum. A filter medium is attached to the external surface of the drum. When the drum is mounted with part of its surface in a tank of slurry, the application of a vacuum behind the filter medium pulls the filtrate through. The solid material is retained on the filter surface and, as the drum turns slowly past a knife, is dislodged into a collection hopper for disposal or further treatment. 

Raw fermentation broth is an example of a large volume production. Rotary drum vacuum filters (RVF s) have traditionally been found in this service. Slow-settling materials or more difficult filtrations with large scale production requirements are typical applications for this type of equipment. For an overview of filter selection versus filtering rates, see Table 3, which is excerpted by special permission from Chemical Engineering/Deskbook Issue, February 15, 1971, by McGraw Hill, Inc., New York, NY 10020. 

Application of the models to washing on fiiU-scale eqiiipment raises the question of washing efficiency. One approach involves the use of a strai tforward efficiency fector which is related principally to the type of filter to be used and to the appUcation, as mentioned above. Purchas and Wakeman [1986] noted the relationship between the washing efficiency and the type of system found by Choudhury and Dahlstrom for rotary drum vacuum filters and proposed a correction of the diversion parameter rather than the use of an efficiency ctor. This correction may be expressed as ... 

The SulFerox sulfur-cake matrix tends to be quite compressible and reportedly lends itself well to filtration via plate-and-fiame filters or automated batch filter presses with recessed membranes for post-filtration sulfur-cake squeezing. It is claimed that only small amounts of wash water are needed to displace the residual process solution from the filtercake. Rotary drum vacuum filters were used in all early SulFerox applications, but variations in feed slurry characteristics due to differing inlet gas contaminants and required additive levels made their performance inconsistent. It is repotted that filtration by pressurized-feed filter presses has eliminated the problem of cake quality variation and has substantially reduced iron chelate losses (Anon., 1994). The sulfur filtercake from pressurized-feed filter presses is reported to contain 10 to 23 wt% moisture (Allen, 1995). 

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