Module 3. Separation equipment

Lesson 9

9.1 Introduction

Filtration may be defined as that unit operation in which the insoluble solid component of a solid-liquid suspension is separated from the liquid component by passing the latter through a porous membrane or septum which retains the solid particles on its upstream surface, or within its structure, or both. A broader definition could be that the fluid may be a liquid or a gas; the valuable stream from the filter may be the fluid, or the solids, or both. The solid – liquid suspension is known as the feed slurry or ‘prefilt’, the liquid component that passes through the membrane is called the filtrate and the membrane itself is referred to as the filter medium. The separated solids are known as the filter cake, once they form a detectable layer covering the upstream surface of the medium. Often the feed is modified in some way by pre-treatment to increase the filtration rate, as by heating, re-crystallizing, or adding a “filter aid” such as cellulose or diatomaceous earth.

Filter, must provide a support for the filter medium, a space for the accumulation of the solids, channels for the introduction of the feed. A means of inducing the flow of filtrate through the filter and medium must also be provided.

9.2 Classification on Basis of Force

The fluid flows through a filter medium by virtue of a pressure differential across the medium. The flow of filtrate may be brought about by means of gravity alone, by the application of a pressure greater than atmospheric pressure upstream of the medium ( pressure filtration), by applying a vacuum downstream of the medium (vacuum filtration) or by means of centrifugal force (centrifugal filtration).

Filtration under the influence of gravity alone is limited in application to slurries containing very free-draining solids or with very low solids contents. It has a very limited use in the food industry but is applied to the water and sewage treatment.

The applications of filtration in the food industry may be considered to fall into three categories. First category is one in which slurries containing appreciable amounts of insoluble solids are present. In such operations, a cake is formed on the upstream surface of the medium and the process is known as cake filtration. The second category is termed clarification and involves removing small quantities of insoluble solid from a valuable liquid. Here the object usually is to produce a clear liquid and the solids are generally unwanted. The third category, often referred to as microfiltration, involves the removal of very fine particles, of the order of 1m or less, and is generally directed at removing microorganisms from liquid foods.

In the Dairy Industry, the application of filtration is during.

1. Filtration of raw milk at dump tank, either at milk collection centre or at chilling centres.

2. During processing, while passing through pasteurizer

3. Reconstitution of milk powder

4. Removal of ghee residues from ghee

5. Separation of whey during cheese manufacture

9.3 Vertical Plate Pressure Filters (Filter Presses)

Fig. 9.1 Plate and frame filter

A vertical drainage plate supporting a filter medium is the basic filtering element in a vertical plate press. A commonly used design is the plate-and-frame press. In this type of filter grooved plates covered on both sides with filter medium alternate with frames in a rack. The assembly of plates and frames can be squeezed tightly together by a screw, hydraulic or pneumatic mechanism to form a liquid-tight unit. The filter medium also acts as a gasket, preventing leakage between the plates and frames. Both plates and frames are provided with openings at one corner and when the press is closed these openings form a channel through which the feed slurry is introduced. In addition, the hollow centre of each frame is connected by an auxiliary channel to this feed channel.

Fig. 9.2 Filter plate and wash plate

The feed slurry enters the frames and the cake builds up in the hollow centre of the frames. The filtrate passes through the medium and on to the grooved surface of the filter plates from where it is removed via an outlet channel in each plate. Filtration is continued until the flow of filtrate drops below a practical level or the pressure reaches an unacceptably high level, due to the cake packing tightly in the frames. After filtration, washing of the cake may be carried out by replacing the flow of feed slurry with wash liquid. However, more effective washing is obtained by the use of special wash plates. These are arranged in the press so that every second plate is a wash plate. During filtration these wash plates act as filter plates. During washing, the outlets from the wash plates are closed and the wash liquid introduced on to their surfaces through a special inlet channel. The flow path for both filtration and washing when wash plates are used. The cake is removed manually after opening the press.

Fig. 9.3 Filtering and washing

Another common design of filter press is known as the recessed-plates press. In this filter the cake accumulates within recesses in the plates and no frames are used. The feed is usually introduced centrally and removed via an outlet on the corners of each plate. For washing, the feed slurry is replaced by wash liquid entering through the same inlets. Many other types of vertical plate filters are available. Larger presses have facilities for lifting or moving plates and frames mechanically. Presses may be jacketed for temperature control.

The vertical plate filter has found very wide application is industry. It is simple in design and operation, compact, flexible and can be used to handle a wide variety of types of slurry. It is relatively cheap initially. On the other hand labour costs and filter cloth consumption are high and washing of the cake is not always efficient.

9.4 Rare of Filtration

The basic filtration equation is

Last modified: Wednesday, 3 October 2012, 6:41 AM