Electrified Separation Processes in Industry

Abstract

For any separation procedure in the chemical industry, a certain amount of reversible work in the form of free energy is required, as dictated by the second law of thermodynamics. Classical techniques for effecting liquid-phase separations, in particular distillation, essentially supply this energy in the form of heat which is converted to required free energy in the distillation column. The latter is in itself a heat engine, converting the heat source into work in the form of a stream of vapor moving down a temperature-pressure gradient. In general, the columns are very inefficient as heat engines, expressed as a percentage of their theoretical Carnot efficiency.

The total percentage of energy used in separation processes, particularly distillation, in the chemical and related industries is very considerable. The majority of the energy used for these separations is thermal input in the form of the low heating-value of oil or gas. From the national viewpoint, it would be advantageous to examine the possible use of energy derived from abundant indigenous sources (coal, nuclear, hydro) to replace this hydrocarbon fuel input. The only practical energy vector for this application is electricity. Since the latter is work, it is important that it be used to perform separations directly, so that the maximum possible conservation of primary energy input can occur.

The purpose of this paper is to discuss practical separation processes in which free energy is used directly. Such processes include those which allow a direct separation by means of phases with favorable free energy changes for separation which are accessible to available work inputs. Such processes might involve liquid-liquid, liquid-solid or liquid gas separations, but all involve a phase transition or phase boundary. The best known of such types of processes involve membrane separation techniques, but other possible schemes will be discussed in this paper. In particular, the potential for the incorporation of these techniques into more traditional chemical preparations to allow higher purity and more concentrated products with a lower total primary energy input will be discussed.