In previous post, there are three general types of filters, which are cake filters, clarifying filters, and crossflow filters. In this post, I want to share specifically about many types of cake filters.
When slurry enters cake filters, some solid particles enter the pores of the medium and are immobilized, but soon others begin to collect on the septum surface. A visible cake of significant thickness builds up on the surface and must be periodically removed. Cake filters are used almost entirely for liquid-solid separations.
Cake filters may operate with above-atmospheric pressure upstream from the filter medium or with vacuum applied downstream. Either type can be continuous or discontinuous, but most pressure filters are discontinuous.
Figure below shows many types of cake filters.
1 – Discontinuous Pressure Filters
Pressure filters can apply a large pressure differential across the septum to give economically rapid filtration with viscous liquids or fine solids. The most common type of pressure filters are filter presses and shell-and-leaf filters.
A filter press contains a set of plates designed to provide a series of compartments in which solids may collect. The plates are covered with a filter medium, such as canvas.
After assembly of the press, slurry is admitted from a pump or pressurized tank at a pressure of 3-10 atm usually. Filtration is continued until liquor is no longer flows out the discharge or the filtration pressure suddenly rises.
Shell-and-leaf filters may be used for filtering under higher pressures than are possible in a plate-and-frame press. In the horizontal-tank design, a set of vertical leaves is held on retractable rack. During operation, the leaves are inside the closed tank. Feed enters through the side of the tank and filtrate passes through the leaves into a discharge manifold.
2- Automatic Belt Filter
The Larox belt filter is a discontinuous pressure filter that separates, compresses, washes, and automatically discharges the cake. Filtration takes place in from 2-20 horizontal chambers, set one above the other. A belt of filter cloth passes through the filter chambers in turn.
3- Discontinuous Vacuum Filters
Pressure filters are usually discontinuous, vacuum filters are usually continuous. A discontinuous vacuum filter is sometimes a useful tool. A vacuum nutsche is a little more than a large Buchner funner, with diameter of 1-3 m and forming a layer of solids 100-300 mm thick.
Nutsches are uncommon in large-scale processes because of the labor involved in digging out the cake.
4- Continuous Vacuum Filters
In all continuous vacuum filters, liquor is sucked through a moving septum to deposit a cake of solids. The cake is moved out of the filtering zone, washed, sucked dry, and dislodged from the septum.
The pressure differential across the septum is not high, between 250-500 mmHg.
4.1- Rotary-Drum Filters
Rotary-drum filter is the most common type of continuous vacuum filter. A horizontal drum with a slotted face turns at 0.1 to 2 rpm in an agitated slurry trough. Many variations of the rotary-drum filter are commercially available. In some designs, there are no compartments in the drum; vacuum is applied to the entire inner surface of the filter medium.
5- Rotary-Drum Pressure Filters
Continuous rotary vacuum filters are sometimes adapted to operate under positive pressure up to about 15 atm for situations in which vacuum filtration is not feasible or economical. This may be the case when the solids are very fine and filter very slowly or when the liquid has a high vapor pressure. However, the mechanical problems or discharging the solids from these filters, their cost and complexity, and their small size limit their application to special problems.
6- Precoat Filters
A precoat filter is a rotary-drum filter modified for filtering small amounts of fine or gelatinous solids that ordinarily plug a filter cloth. A layer of porous filter aid, such as diatomaceous earth, is first deposited on the filter medium.
7- Horizontal Belt Filter
Horizontal belt filter is suitable when the feed contains coarse fast-settling particles of solid. Belt filters are especially useful in waste treatment, since the waste often contains a very wide range of particle sizes.
8- Centrifugal Filters
Solids that form a porous cake can be separated from liquids in a filtering centrifuge. slurry is fed to rotating basket having a slotted or perforated wall covered with a filter medium as canvas or metal cloth. Pressure resulting from the centrifugal action forces the liquor through the filter medium, leaving the solids behind.
- McCabe, Warren L, and Julian C. Smith, “Unit Operations of Chemical Engineering Seventh Edition”, McGraw-Hill, 2005.