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 may6 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 them￾selves may be discrete entities, for example a series of stirred tank reactors, or there may A. S. Grandison and M. J. Lewis