6 A.S. Grandison and M. J. Lewis forces in these applications are pressure and density differences. As for all processes separation rates are very important and these are affected by the size of the driving forces involved In situations where a second phase or stream is involved, mass-transfer considerations become important; these involve the transfer of components within the food to the oundary, the transfer across the boundary and into the bulk of the extraction solvent. It is also important to increase the interfacial area exposed to the solvent. Therefore, size eduction, interfacial phenomena, turbulence and diffusivities all play a role in these processes. In many applications this additional stream is a liquid, either water or an organic solvent; more recently supercritical fluids, such as carbon dioxide, have been investigated(see Chapter 2). However, in hot-air drying the other phase is hot air, which supplies the energy and removes the water. Mass-transfer considerations are important also in some membrane applications and adsorption processes, where the additional ream is a solid. Other examples of driving force are concentration differences and chemical potential, which are involved in these operations (Loncin and Merson, 1979; Gekas, 1992) In some processes, both heat and mass transfer processes are involved. This is especially so for separation reactions involving a change of phase, such as evaporation or sublimation. Heat is required to cause vaporisation for evaporation, dehydration and distillation processes. Water has a much higher latent heat of vaporisation(2257 kJ/kg) han most other organic solvents. With solid foods the rate of heat transfer through the food may limit the overall process; for example in freeze-drying the process is usually limited by rate of heat transfer through the dry layer. Separation processes may be batch or continuous. A single separation process, for example a batch extraction, involves the contact of the solvent with the food. Initially concentration gradients are high and the rate of extraction is also high. The extraction rate falls exponentially and eventually an equilibrium state is achieved when the rate becomes zero. The extraction process may be accelerated by size reduction, inducing turbulence nd increasing the extraction temperature, Equilibrium is achieved either when all the material has been extracted. in situations where the volume of solvent is well in excess of the solute or when the solvent becomes saturated with the solute, i.e. when the solubility limit has been achieved when there is an excess of solute over the solvent however the attainment of equilibrium may take some considerable time. Batch reactions may operate far away from equilibrium or close to it. Equilibrium data is important in that it provides information on the best conditions that can be achieved at the prevailing conditions. Equilibrium data is usually determined at fixed conditions of temperature and pressure. Some important types of equilibrium data are: solubility data for extraction processe apour/liquid equilibrium data for fractional distillation partition data for selective extraction processes rater sorption data for drying Continuous processes may be single-or multiple-stage processes, The stages them- elves may be discrete entities, for example a series of stirred tank reactors, or there may
6 forces in these applications are pressure and density differences. As for all processes, separation rates are very important and these are affected by the size of the driving forces involved. In situations where a second phase or stream is involved, mass-transfer considerations become important; these involve the transfer of components within the food to the boundary, the transfer across the boundary and into the bulk of the extraction solvent. It is also important to increase the interfacial area exposed to the solvent. Therefore, size reduction, interfacial phenomena, txbulence and diffusivities all play a role in these processes. In many applications this additional stream is a liquid, either water or an organic solvent; more recently supercritical fluids, such as carbon dioxide, have been investigated (see Chapter 2). However, in hot-air drying the other phase is hot air, which supplies the energy and removes the water. Mass-transfer considerations are important also in some membrane applications and adsorption processes, where the additional stream is a solid. Other examples of driving force are concentration differences and chemical potential, which are involved in these operations (Loncin and Merson, 1979; Gekas, 1992). In some processes, both heat and mass transfer processes are involved. This is especially so for separation reactions involving a change of phase, such as evaporation or sublimation. Heat is required to cause vaporisation for evaporation, dehydration and distillation processes. Water has a much higher latent heat of vaporisation (2257 kJ/kg) than most other organic solvents. With solid foods the rate of heat transfer through the food may limit the overall process; for example in freeze-drying the process is usually limited by rate of heat transfer through the dry layer. Separation processes may be batch or continuous. A single separation process, for example a batch extraction, involves the contact of the solvent with the food. Initially concentration gradients are high and the rate of extraction is also high. The extraction rate falls exponentially and eventually an equilibrium state is achieved when the rate becomes zero. The extraction process may be accelerated by size reduction, inducing turbulence and increasing the extraction temperature. Equilibrium is achieved either when all the material has been extracted, in situations where the volume of solvent is well in excess of the solute or when the solvent becomes saturated with the solute, i.e. when the solubility limit has been achieved, when there is an excess of solute over the solvent. However, the attainment of equilibrium may take some considerable time. Batch reactions may operate far away from equilibrium or close to it. Equilibrium data is important in that it provides information on the best conditions that can be achieved at the prevailing conditions. Equilibrium data is usually determined at fixed conditions of temperature and pressure. Some important types of equilibrium data are: solubility data for extraction processes; vapour/liquid equilibrium data for fractional distillation; partition data for selective extraction processes; water sorption data for drying. Continuous processes may be single- or multiple-stage processes. The stages themselves may be discrete entities, for example a series of stirred tank reactors, or there may A. S. Grandison and M. J. Lewis
Separation processes-an overview 7 many stages built into one unit of equipment, for example a distillation column or a screw extractor. The flow of the two streams can either be co-current or counter-current although counter-current is normally favoured as it tends to give a more uniform driving force over the length of the reactor as well as a higher average driving force over the reactor. In some instances a combination of co-current and counter-current may be used for example in hot air drying the initial process is co-current to take advantage of the high initial driving rates, whereas the final drying is counter-current to permit drying to a lower moisture content Continuous equipment usually operates under steady state conditions, i.e. the driving force changes over the length of the equipment, but at any particular location it remains constant with time. However, when the equipment is first started, it may take some time achieve steady-state. During this transition period it is said to be operating under unsteady state conditions. In continuous processes the flow may be either streamline or turbulent Consideration should be taken of residence times and distribution of residence mes within the separation process; the two extremes of behaviour are plug flow, with no istribution of residence times, through to a well-mixed situation, with an infinite distribution of residence times. More detailed analysis of residence time distributions is by Levenspiel (1972) How close the process operates to equilibrium depends upon the operating conditions, flow rates of the two phases, time and temperature. These conditions affect the efficiency of the process, such as the recovery and the size of equipment required Finally, all rates of reaction are temperature dependent. Physical processes are no exception, although activation energies are usually much lower than for chemical reaction rates. Using higher temperatures normally increases separation rates However, there are limitations with biological materials: higher temperatures increase degradation reactions, causing colour and flavour changes, enzyme inactivation, protein denaturation, loss of functionality and a reduction in nutritional value. Safety issues with respect to microbial growth may also need to be considered A brief overview of separation methods is now considered in this chapter, base primarily on the nature of the material or stream subjected to the separation process, i.e whether it is solid, liquid or gaseous. Other possible classifications are based on unit operations; e. g. filtration, evaporation, dehydration etc. or types of phase contact, such as lid-liquid or gas-liquid ng pre fore detailed descriptions of conventional techniques can be found elsewhere -e.g Brennan et al. (1990), Perry and Green(1984), King(1982) 1.2.2 Separations from solids Most solid foods are particulate in nature, with particles having a large variety of shapes and sizes. Separations involving solids fall into two categories. The first is where it is required to separate or segregate the particles; such processes are classified as solid-solid separations. The second is where the requirement may be to selectively remove one or veral components from the solid matrix. Other processes involving solids are concerned with the removal of discrete solid particles from either liquids or gases and vapours(but these will be discussed in other sections)
Separation processes -an overview 7 be many stages built into one unit of equipment, for example a distillation column or a screw extractor. The flow of the two streams can either be co-current or counter-current, although counter-current is normally favoured as it tends to give a more uniform driving force over the length of the reactor as well as a higher average driving force over the reactor. In some instances a combination of co-current and counter-current may be used; for example in hot air drying, the initial process is co-current to take advantage of the high initial driving rates, whereas the final drying is counter-current to permit drying to a lower moisture content. Continuous equipment usually operates under steady state conditions, i.e. the driving force changes over the length of the equipment, but at any particular location it remains constant with time. However, when the equipment is first started, it may take some time to achieve steady-state. During this transition period it is said to be operating under unsteady state conditions. In continuous processes the flow may be either streamline or turbulent. Consideration should be taken of residence times and distribution of residence times within the separation process; the two extremes of behaviour are plug flow, with no distribution of residence times, through to a well-mixed situation, with an infinite distribution of residence times. More detailed analysis of residence time distributions is provided by Levenspiel (1972). How close the process operates to equilibrium depends upon the operating conditions, flow rates of the two phases, time and temperature. These conditions affect the efficiency of the process, such as the recovery and the size of equipment required. Finally, all rates of reaction are temperature dependent. Physical processes are no exception, although activation energies are usually much lower than for chemical reaction rates. Using higher temperatures normally increases separation rates. However, there are limitations with biological materials: higher temperatures increase degradation reactions, causing colour and flavour changes, enzyme inactivation, protein denaturation, loss of functionality and a reduction in nutritional value. Safety issues with , respect to microbial growth may also need to be considered. A brief overview of separation methods is now considered in this chapter, based primarily on the nature of the material or stream subjected to the separation process, i.e. whether it is solid, liquid or gaseous. Other possible classifications are based on unit operations; e.g. filtration, evaporation, dehydration etc. or types of phase contact, such as solid-Iiquid or gas-liquid contacting processes. More detailed descriptions of conventiopal techniques can be found elsewhere -e.g. Brennan et at. (1990), Perry and Green (1984), King (1982). 1.2.2 Separations from solids Most solid foods are particulate in nature, with particles having a large variety of shapes and sizes. Separations involving solids fall into two categories. The first is where it is required to separate or segregate the particles; such processes are classified as solid-solid separations. The second is where the requirement may be to selectively remove one or several components from the solid matrix. Other processes involving solids are concerned with the removal of discrete solid particles from either liquids or gases and vapours (but these will be discussed in other sections)