6 M.J. Lewis compounds such as sugars. These pressure-activated processes can also be regarded as a continuous spectrum of processes, with no obvious distinct boundaries between ther However, it should be noted that the sizes of the components being separated range over several orders of magnitude, so it is highly likely that the separation mechanisms and hence the operating strategies may change as we move through the spectrum 3.2 TERMINOLOGY The feed material is applied to one side of a membrane, The feed is usually a low viscosity fluid which may sometimes contain suspended matter and which is subjected to a pressure. In most cases the feed flows in a direction parallel to the membrane surface and the term cross-flow filtration is used to describe such applications. Dead-end systems are used, but mainly for laboratory scale separations. The stream which passes through the membrane under the influence of this pressure is termed the permeate(filtrate). After removal of the required amount of permeate, the remaining material is termed the con centrate or retentate. The extent of the concentration is characterised by the concentration factor () which is the ratio of the feed volume to the final concentrate volume(see equation (3.5)) The process can be illustrated simply in Fig. 3. 2(a). From a single membrane process ing stage, two fractions are produced, named the concentrate and permeate. The required extent of concentration may not be achieved in one stage, so the concentrate may be returned to the same module for further concentration or taken to other modules in a cascade, or multistage process. The permeate may also be further treated in a separate process In terms of size considerations alone, one extreme is a membrane with very small pore diameters(tight pores). In this case the permeate will be pure water because even small molecular weight solutes will be rejected by the membrane; high-pressure driving forces are required to overcome frictional resistance and osmotic pressure gradients, If the permeate 1s pi then the process is known as reverse hyperfiltration; it is similar in its effects to evaporation or freeze-concentration. A con centrate will be produced, in which there is virtually no alteration in the proportion of the Feed moss Fig. 3.2. Separation of feed into a concentrate and permeate stream66 M. J.Lewis compounds such as sugars. These pressure-activated processes can also be regarded as a continuous spectrum of processes, with no obvious distinct boundaries between them. However, it should be noted that the sizes of the components being separated range over several orders of magnitude, so it is highly likely that the separation mechanisms and hence the operating strategies may change as we move through the spectrum. 3.2 TERMINOLOGY The feed material is applied to one side of a membrane. The feed is usually a low￾viscosity fluid, which may sometimes contain suspended matter and which is subjected to a pressure. In most cases the feed flows in a direction parallel to the membrane surface and the term cross-flow filtration is used to describe such applications. Dead-end systems are used, but mainly for laboratory scale separations. The stream which passes through the membrane under the influence of this pressure is termed the permeate (filtrate). After removal of the required amount of permeate, the remaining material is termed the con￾centrate or retentate. The extent of the concentration is characterised by the concentration factor df), which is the ratio of the feed volume to the final concentrate volume (see equation (3.5)). The process can be illustrated simply in Fig. 3.2(a). From a single membrane process￾ing stage, two fractions are produced, named the concentrate and permeate. The required extent of concentration may not be achieved in one stage, so the concentrate may be returned to the same module for further concentration or taken to other modules in a cascade, or multistage process. The permeate may also be further treated in a separate process. In terms of size considerations alone, one extreme is a membrane with very small pore diameters (tight pores). In this case the permeate will be pure water because even small molecular weight solutes will be rejected by the membrane; high-pressure driving forces are required to overcome frictional resistance and osmotic pressure gradients. If the permeate is predominantly water, then the process is known as reverse osmosis or hyperfiltration; it is similar in its effects to evaporation or freeze-concentration. A con￾centrate will be produced, in which there is virtually no alteration in the proportion of the Concentrate Feed :-+ pi L 1: IR I Permeate + ++- Osmosis Reverse osmosis I (4 (b) Fig. 3.2. Separation of feed into a concentrate and permeate stream