Pressure Differential Filtration

Although all the techniques are effective, in industrial appHcations there is rarely time to achieve an equiHbrium reduced saturation state (see Filtration), so variables that affect only the kinetics of dewatering and not the equiHbrium and residual moisture are also very important. The most important kinetic variables in displacing the Hquid from the soHd are increases in pressure differentials and viscosity reduction. 

When the space above the suspension is subjected to compressed gas or the space under the filter plate is under a vacuum, filtration proceeds under a constant pressure differential (the pressure in the receivers is constant). The rate of filtration decreases due to an increase in the cake thickness and, consequently, flow resistance. A similar filtration process results from a pressure difference due to the hydrostatic pressure of a suspension layer of constant thickness located over the filter medium. 

If the suspension is fed to the filter with a reciprocating pump at constant capacity, filtration is performed under constant flowrate. In this case, the pressure differential increases due to an increase in the cake resistance. If the suspension is fed by a centrifugal pump, its capacity decreases with an increase in cake resistance, and filtration is performed at variable pressure differentials and flowrates. 

A plate-and-frame filter press contains 16 frames and operates at a constant flow rate of 30 gpm. Each frame has an active filtering area of 4 ft2, and it takes 15 min to disassemble, clean, and reassemble the press. The press must be shut down for disassembly when the pressure difference builds up to 10 psi. What is the total net filtration rate in gpm for a slurry having properties determined by the following lab test. A sample of the slurry is pumped at a constant pressure differential of 5 psi through 0.25 ft2 of the filter medium. After 3 min, 1 gal of filtrate has been collected. The resistance of the filter medium may be neglected. 

The efficiency of the filtration process should not be significantly affected by the pressure differential across the surface of the membrane or pressure fluctuations produced by the pumping of fluids through it. 

Filters used for the prefiltration and final filtration should be maintained to operate to their design specifications. Deterioration of filters is caused by leakage and/or accumulation of particles. The former is tested by periodical integrity test (usually dioctylphthalate DOP test), and the latter is tested by the increase of air pressure differentials between the upstream and downstream sides of the fdter. 

The final method of operation to consider is that of filtration at constant pressure and constant rate. This method is employed when pure liquid is filtered through a cake of constant thickness and a constant pressure differential exists across the cake. Cake washing by displacement may be considered as filtering of a washing liquid through a constant cake thickness at constant pressure and flowrate. The rate of... 

Conventional pressure or vacuum filtration techniques are widespread in industry for separating cells and other biological materials from a liquid phase which can be solvent based or aqueous. A pressure differential between the dirty and clean sides of the filter, created with over pressure or vacuum, provides a driving force for the liquid to be forced through the filter material which retains solids above a particular size. This type of filter is often used in conjunction with a precoat material on the filter to improve the separation characteristics. 

The salient features of rotary-drum filtration are illustrated in Fig. 14.9, where a cylindrical drum having a permeable surface is revolving counterclockwise partially submerged in a slurry. A pressure differential is usually maintained between the outer and inner surfaces by means of a vacuum pump. However, the drum might be enclosed and operated under pressure. In addition to the vacuum or pressure, each point on the periphery of the drum is subjected to a hydrostatic head of slurry. 

Becanse V is the volnme of the filtrate collected at any time t, Vs is (dVldt)IS also, expressing as cV (where c is the mass of cake collected per unit volume of filtrate), substituting in Equation (7.60), and integrating under the assumption of constant pressure differential operation yields... 

Vacuum filtration is normally conducted under constant pressure differential hence, -AP has been made constant. In addition, a may vary over time hence, its average value a has been used. 

Pressure filtration—A method of filtration in which the pressure differential used to drive the filtration is induced by impressing a high pressure on the inlet side of the filter medium. 

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