Filtration

Filtration

INTRODUCTION

A student of pharmacy will have used filtration extensively in the collection of precipitates in chemical analyses or in the preparation of parenteral fluids and will, therefore, anticipate the definition of filtration as the removal of solids suspended in a liquid or gas by passage through a pervious medium on which the solids are retained. The pervious medium or septum is normally supported on a base, and these, together with a suitable housing providing free access of fluid to and from the septum, comprise the filter.

The applications of filtration are diverse. They may, however, be classified as either clarification or cake filtration.

1. Clarification

Very high standards of clarity are imposed during the production of pharma-ceutical solutions. The aim may be simply the presentation of an elegant product, although complete freedom from particulate matter is obviously nec-essary in the manufacture of most parenteral solutions. The solids are unwanted and are normally present in a very small concentration. Clarification may be carried out by the use of thick media, which allow for the penetration and arrest of particles by entrapment, impingement, and electrostatic effects. This leads to the concept of depth filtration in which particles, perhaps a hundred times smaller than the dimensions of the passages through the medium, are removed. For this reason, such filters are not absolute and must be designed with suffi-cient depth so that the probability of the passage of the smallest particle under consideration through the filter is extremely small.

Depth filtration differs fundamentally from the use of media in which pore size determines the size of particle retained. Such filters may be said to be “absolute” at a particle diameter closely related to the size of the pore, so that there is a relatively sharp division between particles that pass the filter and those that are retained. An analogy with sieving may be drawn for this mechanism. The life of such filters depends on the number of pores available for the passage of fluid. Once a particle is trapped at the entrance to the pore, the pore’s con-tribution to the overall flow of liquid is very much reduced. Coarse straining with a wire mesh and the membrane filter employ this mechanism.

Sterilization of liquids by filtration could be regarded as an extreme application of clarification in which the complete removal of particles as small as 0.3 x 10^-6 m must be ensured.

2. Cake Filtration

The most common industrial application is the filtration of slurries containing a relatively large amount of suspended solids, usually in the region of 3% to 20%. The septum acts only as a support in this operation. The actual filtration is carried out by the solids deposited as a cake. In such cases, solids may completely penetrate the septum until the deposition of an effective cake occurs. Until this time, cloudy filtrate may be recycled. The physical properties of the cake largely determine the methods employed. Often, washing and partial drying or dewatering are integral parts of the process. Effective discharge of the cake completes the process. The solids, the filtrate, or both may be wanted.