Filtration, continuous cake discharge

Continuous Pressure Filters These filters consist of conventional drum or disk filters totally enclosed in pressure vessels. Filtration takes place with the vessel pressurized up to 6 bar and the filtrate discharging either at atmospheric pressure or into a receiver maintained at a suitable backpressure. Cake discharge is facilitated through a dual valve and lock-hopper arrangement in order to maintain vessel pressure. Alternatively, the discharged filter cake can be reslurried within the filter or in an adjoining pressure vessel and removed through a control valve. 

Cake filters separate relatively large amounts of solids by forming a cake on the surface of the filtration medium. Cake filters may be operated by applying pressure on the upstream section of the filter medium or vacuum on the downstream section of the filter medium. The operation of the filters may be continuous or discontinuous. However, most pressure filters are discontinuous since the operation of the filter under positive pressure needs to be stopped to facilitate the removal and discharge of solids. Examples of cake filters include filter press, vacuum filter, and centrifugal separator. 

CONTINUOUS FILTRATION. In a continuous filter, say, of the rotary-drum type, the feed, filtrate, and cake move at steady constant rates. For any particular element of the filter surface, however, conditions are not steady but transient. Follow, for example, an element of the filter cloth from the moment it enters the pond of slurry until it is scraped clean once more. It is evident that the process consists of several steps in series—cake formation, washing, drying, and discharging—and that each step involves progressive and continual change in conditions. The pressure drop across the filter during cake formation is, however, held constant. Thus the foregoing equations for discontinuous constant-pressure filtration may, with some modification, be applied to continuous filters. 

A vertically oriented press where two endless, slightly off-vertical, filter cloths move continuously over a series of rollers. These moving belts are sealed at their edges by two other stationary belts in a manner that allows the feed suspension to be mechanically squeezed and cake filtration to occur. The maximum squeeze pressure is restricted to -250 kPa and controlled as appropriate via the gap between the moving cloths at the base of the unit. Typical cake discharge thickness ranges between 6 and 8 mm. The tower press as described here, which should not be confused with the similarly named, but very different, unit described in Section 1.4.2.5, has now been largely superseded by the belt press (see earlier in this section). 

A variation on this type of filter is the double tipping pan filter, which is a semi-continuous type consisting of two rectangular pans fitted with a filter cloth and pivoted about a horizontal axis. Slurry is first fed onto one pan, which is turned over for cake discharge at the end of the cycle. The second pan is used for filtration while the first is being discharged. 

Filtration on a continuously operating filter is a series of individual operations, essentially made up by formation of the filter cake, filtrate draining and discharge of the filter cake. The factors which control the course of every individual operation are thus also decisive for the selection of filter type. 

---