Pressure-activated membrane processes 75 coeffici p between the fluxes and driving forces are described by phenomenological relations ients, which are nevertheless connected with the mechanism. Such phenomenological models provide an empirical description of the transport of molecules through membranes, but they do not give an explanation for the separation mechanism Gekas(1992)discusses these types of model in more detail for membrane processes Cheryan(1989)has concluded that none of the proposed models can satisfactorily explain the rejection characteristics of all solvent-solute-membrane reverse osmosis sys On the other hand, ultrafiltration is regarded as a sieving process and is often mod led as a bundle of capillaries, with flow through the capillaries being described by the Hagen-Poiseuille equation Q=d4△Px/128L where Q= volumetric flow rate, AP= pressure drop; u=dynamic viscosity, L= capillary th and d= capillary diameter. (Note that this equation is often expressed as flux rate/unit area(d AP/32uL.) Additional terms may be introduced to account for porosity and tortuosity effects These equations predict that the flow will be strongly biased toward the largest pores Such pores may plug quicker because of the higher degree of polarisation encountered lue to the higher flux.( See concentration polarisation. )Also, as pore size increases, the flow rate through the membrane will also increase. The flux for most food solutions is lower than that for water and other pure solvents for a number of reasons. The viscosity of the feed solution may be higher and this further increases throughout the concentration process. Also concentration polarisation and fouling occur. Factors which influence concentration polarisation and fouling are the prevailing flow conditions and the composition of the material being processed. Some of these factors are discussed in more detail in Chapter 4 3.7 MEMBRANE EQUIPMENT Membrane suppliers now provide a range of membranes, each with different rejection haracteristics; for reverse osmosis salt rejection values are quoted, e.g. from 99%o rejec tion of sodium chloride down to 60% rejection of sucrose, and for ultrafiltration, different molecular weight cut-offs in the range 1000 to 500 000. For example, it is implied that a membrane with a molecular weight cut-off of 20 000 will reject components with a molecular weight greater than that value. Such figures are provided for guidelines only (see Section 4.2.1). Tight ultrafiltration membranes have a molecular weight cut-off value of around 1000-5000, whereas the more 'open or 'loose membranes have a value in excess of 100 000. Thus molecular weight cut-off value is related to porosity and rejec tion characteristics; as membranes become more permeable to solutes, their molecula weight cut-off values increase. However, because there are many other factors that affect ection, molecular weight cut-off should only be regarded as giving a relative guide to its pore size and true rejection behaviour. Experimental determinations should always be made on the system to be validated, at the operating conditions to be used
Pressure-activated membrane processes 75 relationship between the fluxes and driving forces are described by phenomenological coefficients, which are nevertheless connected with the mechanism. Such phenomenological models provide an empirical description of the transport of molecules through membranes, but they do not give an explanation for the separation mechanism. Gekas (1992) discusses these types of model in more detail for membrane processes. Cheryan (1989) has concluded that none of the proposed models can satisfactorily explain the rejection characteristics of all solvent-solute-membrane reverse osmosis systems. On the other hand, ultrafiltration is regarded as a sieving process and is often modelled as a bundle of capillaries, with flow through the capillaries being described by the Hagen-Poiseuille equation: Q = d4 AP n/128pL (3.10) where Q = volumetric flow rate, AP = pressure drop; p = dynamic viscosity, L = capillary length and d = capillary diameter. (Note that this equation is often expressed as flux rate/unit area (d2AP/32pL).) Additional terms may be introduced to account for porosity and tortuosity effects in the membrane. These equations predict that the flow will be strongly biased toward the largest pores. Such pores may plug quicker because of the higher degree of polarisation encountered due to the higher flux. (See concentration polarisation.) Also, as pore size increases, the flow rate through the membrane will also increase. The flux for most food solutions is lower than that for water and other pure solvents for a number of reasons. The viscosity of the feed solution may be higher and this further increases throughout the concentration process. Also concentration polarisation and fouling occur. Factors which influence concentration polarisation and fouling are the prevailing flow conditions and the composition of the material being processed. Some of these factors are discussed in more detail in Chapter 4. 3.7 MEMBRANE EQUIPMENT Membrane suppliers now provide a range of membranes, each with different rejection characteristics; for reverse osmosis salt rejection values are quoted, e.g. from 99% rejection of sodium chloride down to 60% rejection of sucrose; and for ultrafiltration, different molecular weight cut-offs in the range 1000 to 500 000. For example, it is implied that a membrane with a molecular weight cut-off of 20000 will reject components with a molecular weight greater than that value. Such figures are provided for guidelines only (see Section 4.2.1). Tight ultrafiltration membranes have a molecular weight cut-off value of around 1000-5000, whereas the more ‘open’ or ‘loose’ membranes have a value in excess of 100 000. Thus molecular weight cut-off value is related to porosity and rejection characteristics; as membranes become more permeable to solutes, their molecular weight cut-off values increase. However, because there are many other factors that affect rejection, molecular weight cut-off should only be regarded as giving a relative guide to its pore size and true rejection behaviour. Experimental determinations should always be made on the system to be validated, at the operating conditions to be used
76 M.J. Lewis Other desirable features for membranes to ensure commercial success with food com- ponents are listed as Reproducible pore size from batch to batch, offering uniformity in terms of both their permeate rate and their rejection characteristics High flux rates and sharp rejection characteristics Compatibility with processing, cleaning and sanitising fluids Resistane An ability to withstand temperatures required for disinfecting and sterilising surfaces which is an important part of the safety and hygiene considerations, Extra demands placed upon membranes used for food processing include: the ability to withstand hot acid and alkali detergents(low and high pH), temperatures of 90 C for disinfecting or 120.C for sterilising and/or widely used chemical disinfectants, such as sodium hypochlorite, hydrogen peroxide or sodium metabisulphite. The membrane should be designed to allow cleaning both on the feed /concentrate side and the permeate side. See Section 3.8 Membrane processing operations can range in their scale of operation, from laboratory bench-top units, with samples less than 10 ml, through to large commercial scale operations, processing more than 50 mh". Furthermore, the process can be performed at ambient temperatures, which allows concentration without any thermal damage to the feed components The process can be batch or continuous; the fluid can be static or in motion, either agitated in a stirred cell or moving across the surface of the membrane. The membrane tself can be configured in a variety of forms. 3.7.1 Membrane configuration The membranes themselves are thin and in most cases require support against the high pressure. The support material itself should be porous. The membrane and its support, together, are normally known as the module There are a number of criteria that have to be satisfied in the design of a pressure activated membrane module. It must provide a large surface area in a compact volume must be able to support the membrane and the configuration must allow suitable onditions to be established, with respect to turbulence, high wall shear stresses, pressure losses, volumetric flow rates and energy requirements, thereby minimising concentration polarisation. Hygienic considerations are important; there should be no dead spaces and the module should be capable of being in-place-cleaned on both the concentrate and the permeate side. The membranes should be readily accessible, for both cleaning and eplacement. It may also be an advantage to be able to collect permeate from individual membranes in the module to be able to assess the per The two major configurations which have withstood the test of time are the tubular and the flat-plate configurations. The main features of these configurations will be further Tubular membranes come in a range of diameters. In general tubes offer no dead spaces, do not block easily and are easy to clean. However, as the tube diameter creases,they occupy a larger space, have a higher hold-up volume and incur higher
76 M.J.Lewis Other desirable features for membranes to ensure commercial success with food components are listed as Reproducible pore size from batch to batch, offering uniformity in terms of both their permeate rate and their rejection characteristics. High flux rates and sharp rejection characteristics. Compatibility with processing, cleaning and sanitising fluids. Resistance to fouling. An ability to withstand temperatures required for disinfecting and sterilising surfaces, which is an important part of the safety and hygiene considerations. Extra demands placed upon membranes used for food processing include: the ability to withstand hot acid and alkali detergents (low and high pH), temperatures of 90°C for disinfecting or 120°C for sterilising and/or widely used chemical disinfectants, such as sodium hypochlorite, hydrogen peroxide or sodium metabisulphite. The membrane should be designed to allow cleaning both on the feed/concentrate side and the permeate side. See Section 3.8. Membrane processing operations can range in their scale of operation, from laboratory bench-top units, with samples less than 10 ml, through to large commercial scale operations, processing more than 50 m3 h-’. Furthermore, the process can be performed at ambient temperatures, which allows concentration without any thermal damage to the feed components. The process can be batch or continuous; the fluid can be static or in motion, either agitated in a stirred cell or moving across the surface of the membrane. The membrane itself can be configured in a variety of forms. 3.7.1 Membrane configuration The membranes themselves are thin and in most cases require support against the high pressure. The support material itself should be porous. The membrane and its support, together, are normally known as the module. There are a number of criteria that have to be satisfied in the design of a pressureactivated membrane module. It must provide a large surface area in a compact volume; it must be able to support the membrane and the configuration must allow suitable conditions to be established, with respect to turbulence, high wall shear stresses, pressure losses, volumetric flow rates and energy requirements, thereby minimising concentration polarisation. Hygienic considerations are important; there should be no dead spaces and the module should be capable of being in-place-cleaned on both the concentrate and the permeate side. The membranes should be readily accessible, for both cleaning and replacement. It may also be an advantage to be able to collect permeate from individual membranes in the module to be able to assess the performance of each one individually. The two major configurations which have withstood the test of time are the tubular and the flat-plate configurations. The main features of these configurations will be further discussed. Tubular tnembratzes come in a range of diameters. In general tubes offer no dead spaces, do not block easily and are easy to clean. However, as the tube diameter increases, they occupy a larger space, have a higher hold-up volume and incur higher