Thus the energy consumption in the case of the use of EtAc or MTBE as extraction agents ought to be the lowest. The energy consumption, however, also depends on the vaporisation enthalpy of the azeotropic mixture, which in turn is determined by the proportion of water in the azeotrope. According to this, the reflux ratio in the case of all the extraction agents listed here ought to be in the same order of magnitude. The difference in boiling temperatures between the pure acetic acid (118☌) and the azeotropic point provides clue on the size of the reflux ratio. The economic viability of the overall process greatly depends on the energy requirements of the solvent rectification, which in turn depends on the reflux ratio and thus on the performance of condenser and evaporator. As a result, all the extraction agents listed here must be regarded as practically equivalent with regard to the extraction. The average distribution coefficients do not differ essentially from one another. Table 1 shows a selection of extraction agents suitable for the recovery of acetic acid, with details on the average distribution coefficient between the organic and aqueous phase, density of the extraction agent at 20☌ enthalpy of vaporisation and boiling point temperature of the extraction agent and the proportion of water and temperature of the binary azeotrope between extraction agent and water. Characteristics like solubility in water, absorption capacity, distribution coefficient, price, availability and composition of the azeotrope, and requirements in terms of environmental and health protection must be taken into account for the purpose of this selection. Normally, more low boiling extraction agents are used. This technical-information leaflet deals with the recovery of acetic acid by means of liquid-liquid extraction and downstream azeotropic rectification. The recovery method using extraction must be taken into consideration, irrespective of the concentration, if supplementary impurities in the initial mixture, like salts, would be likely to cause problems during direct recovery by distillation. In the case of acetic-acid concentrations of below 40 wt%, the acetic acid is initially extracted from the aqueous solution with a suitable extraction agent, before pure recovery occurs during the rectification of the azeotropic mixture.
The addition of an auxiliary substance means that the volatility of the water is increased, which in turn means that separation can be achieved with lower energy consumption. In practice, therefore, azeotropic rectification has asserted itself and operates either with or without the extraction stage, depending on the respective acetic-acid concentration. This would necessarily involve high costs for energy and high operating costs, if rectification with vapor condensation is not used. You would then need towers having a large number of stages, which would have to be operated with a high reflux ratio. Separation of the acetic acid and water mixture by simple rectification or adjustment is ruled out in this respect, since this mixture has a very small separating factor. High content of acid in extraction column
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