M.J Lewis It can be determined experimentally for each and every component in the feed, by sampling the feed and permeate at the same time and analysing the component in ques tion. It is a very important property of a membrane, as it will influence the extent (quality )of the separation that can be achieved Rejection values normally range between 0 and 1; sometimes they are expressed as percentages(0-100%) when c E 0: nt is retained in the feed when c =c R=0; the component is freely permeating An ideal RO membrane would give a rejection value for all components of 1, whil an ideal UF membrane, being used to concentrate a high molecular weight component or remove a low molecular weight component would give respective rejection values of 1 and 0. If the concentration factor and rejection value are known, the yield of any component, which is defined as the fraction of that component present in the feed, which is recovered in the concentrate, can be estimated. Obviously for reverse osmosis, the yield for an ideal membrane is 1.0. Rejection data for membranes and their effects on eld and separation performance will be discussed in greater detail in Chapter 4 3.4 MEMBRANE CHARACTERISTICS The membrane itself is crucial to the process. The first commercial membranes were made of cellulose acetate and these are termed first-generation membranes. For food processing applications, they had some limitations, with temperatures below 30C and PH range of 3-6. These were followed in the mid-1970s by other polymeric membranes cond-generation membranes), with polyamides and, in particular, polysulphones being widely used for foods. The resulting improvements in cleaning and hygiene are covered in Section 3.8. It is estimated that over 150 organic polymers have now been investigated for membrane applications, Inorganic membranes based on sintered and ceramic materials are also now available. The physical structure of these membranes is complex and as most of them are used for microfiltration. their structure is described in more detail in Chapter 5 The main terms used to describe membranes are microporous or asymmetric. Microporous membranes have a uniform porous structure throughout, although the pore size may not be uniform across the thickness of the membrane. They are usually charac terised by a nominal pore size and no particle larger than this will pass through the nembrane. In contrast to this. most membranes used for ultrafiltration are of a type, having a dense active layer or skin of 0.5-1 um in thickness, and a further support layer which is much more porous and of greater thickness(Fig. 3.3). Overall the porosity of these membranes is high, although the surface porosity may be low, with quoted alues in the range 0.3-15%(Fane and Fell, 1987). Often the porous path may be quite tortuous, the distance covered by the solvent or solute being much greater than the thickness of the membrane; the term tortuosity has been used as a measure of this property. The pores are not of a uniform size, as can be seen when viewed under the electron microscope, and are best characterised by a pore size distribution. This70 M. J.Lewis It can be determined experimentally for each and every component in the feed, by sampling the feed and permeate at the same time and analysing the component in ques￾tion. It is a very important property of a membrane, as it will influence the extent (quality) of the separation that can be achieved. Rejection values normally range between 0 and 1; sometimes they are expressed as percentages (0-1 00%). when cp = 0; when cp = CF R = 1; all the component is retained in the feed R = 0; the component is freely permeating. An ideal RO membrane would give a rejection value for all components of 1, whilst an ideal UF membrane, being used to concentrate a high molecular weight component or remove a low molecular weight component would give respective rejection values of 1 and 0. If the concentration factor and rejection value are known, the yield of any component, which is defined as the fraction of that component present in the feed, which is recovered in the concentrate, can be estimated. Obviously for reverse osmosis, the yield for an ideal membrane is 1.0. Rejection data for membranes and their effects on yield and separation performance will be discussed in greater detail in Chapter 4. 3.4 MEMBRANE CHARACTERISTICS The membrane itself is crucial to the process. The first commercial membranes were made of cellulose acetate and these are termed first-generation membranes. For food￾processing applications, they had some limitations, with temperatures below 30°C and a pH range of 3-6. These were followed in the mid-1970s by other polymeric membranes (second-generation membranes), with polyamides and, in particular, polysulphones being widely used for foods. The resulting improvements in cleaning and hygiene are covered in Section 3.8. It is estimated that over 150 organic polymers have now been investigated for membrane applications. Inorganic membranes based on sintered and ceramic materials are also now available. The physical structure of these membranes is complex, and as most of them are used for microfiltration, their structure is described in more detail in Chapter 5. The main terms used to describe membranes are microporous or asymmetric. Microporous membranes have a uniform porous structure throughout, although the pore size may not be uniform across the thickness of the membrane. They are usually charac￾terised by a nominal pore size and no particle larger than this will pass through the membrane. In contrast to this, most membranes used for ultrafiltration are of asymmetric type, having a dense active layer or skin of 0.5-1 pm in thickness, and a further support layer which is much more porous and of greater thickness (Fig. 3.3). Overall the porosity of these membranes is high, although the surface porosity may be low, with quoted values in the range 0.3-15% (Fane and Fell, 1987). Often the porous path may be quite tortuous, the distance covered by the solvent or solute being much greater than the thickness of the membrane; the term tortuosity has been used as a measure of this property. The pores are not of a uniform size, as can be seen when viewed under the electron microscope, and are best characterised by a pore size distribution. This