Filtration continuous cake filters

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. 

Filtration equipment will be briefly reviewed for the following categories batch cake filters, continuous cake filters, and clarifying filters. 

Filtration is the separation of two phases, particulate form, ie, soHd particles or Hquid droplets, and continuous, ie, Hquid or gas, from a mixture by passing the mixture through a porous medium. This article discusses the more predominant separation of soHds from Hquids. Filtration of soHd particles or Hquid droplets from gases is dealt with elsewhere (see Airpollution controlmethods). The oldest recorded appHcations of filtration are the purifications of wine and water practiced by the ancient Greeks and Romans. Cake filters, such as the rotary vacuum filter and the filter press, were developed much later from the necessity to filter sewage. 

Most continuous pressure filters available (ca 1993) have their roots in vacuum filtration technology. A rotary dmm or rotary disk vacuum filter can be adapted to pressure by enclosing it in a pressure cover however, the disadvantages of this measure are evident. The enclosure is a pressure vessel which is heavy and expensive, the progress of filtration cannot be watched, and the removal of the cake from the vessel is difficult. Other complications of this method are caused by the necessity of arranging for two or more differential pressures between the inside and outside of the filter, which requires a troublesome system of pressure regulating valves. 

Despite the disadvantages, the advantages of high throughputs and low moisture contents in the filtration cakes have justified the vigorous development of continuous pressure filters. 

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. 

The third category, cake filters, although well developed in many wastewater treatment applications, are the least developed of the filtration equipment use by the Biotech Industry. In the organic synthesis laboratory sometimes very simple equipment like a funnel and filter paper is used to accomplish this operation. Some other operations used for this filtration step in the lab are more sophisticated, but many are very labor intensive and limit the capacity of the overall production process itself. As a result, there is a need for optimization of the cake filtration equipment used in biotechnology. Cake filtration equipment is available in batch and continuous modes. Following are several examples of cake filtration units ... 

A continuous rotary filter is required for an industrial process for the filtration of a suspension to produce 0.002 m3/s of filtrate. A sample was tested on a small laboratory filter of area 0.023 m2 to which it was fed by means of a slurry pump to give filtrate at a constant rate of 0.0125 m3/s. The pressure difference across the test filter increased from 14 kN/m2 after 300 s filtration to 28 kN/m2 after 900 s, at which time the cake thickness had reached 38 mm. What are suitable dimensions and operating conditions for the rotary filter, assuming that the resistance of the cloth used is one-half that on the test filter,... 

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