Extractive distillation calculations

For extractive distillation the presence of the second feed, (solvent) presents some computational complications in maintaining stable convergence in the solution by computers of the appropriate system of equations—i.e., material and enthalpy balances and equilibrium relationship. The algorithms that can inherently cope with multiple feeds are matrix oriented, and the Newton-Ramphson procedure of solving these equations shows the maximum degree of stability (4). Several papers discuss computational approaches for extractive distillation calculations (Amud-son and Pontinen (26), Naphthali (27), Roche (4), Bruno et at. (28), Black and Ditsler (29), and others). 

G. T. Atkins and C. M. Boyer Application of the McCabe-Thiele Method to Extractive Distillation Calculations. Chem. Eng. Prog., 45 553 (1949). 

The results received form the optimization using inherent safety as the objective function are somewhat different compared to those calculated with an economic objective function earlier (Hurme, 1996). With the inherent safety objective function the simple distillations were favoured more than with the economic function. Exceptions are cases where the extractive distillation could improve separation very dramatically. This is because in simple distillations only one column is required per split, but in extractive distillation two columns are needed, since the solvent has to be separated too. This causes larger fluid inventory since also the extraction solvent is highly flammable. The results of the calculation are well justified by common sense, since one of the principles of inherent safety is to use simpler designs and reduce inventories to enhance safety. 

The theoretical and mathematical treatment of azeotropic data has been covered by several authors, including Benedict and coworkers (1), and Scheibel and Friedland (50). Licht and Denzler (37) have discussed the thermodynamic conditions necessary and sufficient for azeotropism. Colburn (8) has reviewed the calculations associated with azeotropic and extractive distillation, and Hodgson (24) stresses the relationship between azeotropic distillation, extractive distillation, and liquid-liquid extraction. 

Bruno, J.A., Yanosik, J.L., and Tierney, J.W. Distillation Calculations with Nonideal Mixtures, Extractive and Aziotropic Distillation, Advances in Chemistry Series 115, ACS, Washington, DC... 

Measurements of binary vapor-liquid equilibria can be expressed in terms of activity coefficients, and then correlated by the Wilson or other suitable equation. Data on all possible pairs of components can be combined to represent the vapor-liquid behavior of the complete mixture. For exploratory purposes, several rapid experimental techniques are applicable. For example, differential ebulliometry can obtain data for several systems in one laboratory day, from which infinite dilution activity coefficients can be calculated and then used to evaluate the parameters of correlating equations. Chromatography also is a well-developed rapid technique for vapor-liquid equilibrium measurement of extractive distillation systems. The low-boiling solvent is deposited on an inert carrier to serve as the adsorbent. The mathematics is known from which the relative volatility of a pair of substances can be calculated from the effluent trace of the elutriated stream. Some of the literature of these two techniques is cited by Walas (1985, pp. 216-217). 

The iterative calculation procedure is outlined in Figure 14.10. The method is an adaptation to extraction by Tsuboka and Katayama (1976) of the distillation calculation procedure of Wang and Henke [Hydrocarb. Proc. 45(8), 155-163 (1967)]. It is also presented by Henley and Seader (1981, pp. 586-594). 

If the extract E calculated indirectly lies between 3 and 4, such volume of the wine is measured out from a burette as will give not more than 1 5 gram of extract, distilled water being then added to bring the volume up to 50 c.c. The volume of wine to be taken is found by dividing 150 by the value of E found in the table. The diluted wine is evaporated in a tared dish as described above and the extract per litre of the wine calculated. 

In conclusion, recent developments in solvent selection, phase nonideality description, and tray-to-tray calculation schemes have greatly facilitated the design of extractive and azeotropic distillation schemes, and use of salts give new methods for extractive distillation separations. Finally, the work of Gerster (30), Black and Ditsler (29), and Black et al. (25) compare these two schemes. 

Extractive distillation has been extensively used for nearly three decades in laboratory, pilot plant, and commercial plant operations. Calculation or prediction of phase equilibria for such separations has often been discussed (I, 2, 3). Some have discussed the selection of solvents for extractive distillation (4, 5). Others have discussed its recent application to particular separations (6, 7, 8). A comparison of extractive distillation, as a separation method, with azeotropic distillation and with liquid-liquid extraction has recently been discussed briefly by Gerster (9). 

The use of digital computers to carry out complete calculations in the design of separation processes has been the goal of many. To do this effectively, suitable methods for phase equilibria and tray-to-tray distillation calculations are required. Results calculated by the application of such methods to dehydrate aqueous ethanol mixtures using ethylene glycol as the extractive distillation solvent is discussed below. A brief review of the methods used for phase equilibria and enthalpies is followed by a discussion of the results from distillation calculations. These are compared for extractive distillation with corresponding results obtained by azeotropic distillation with n-pentane. 

Calculations for the extractive distillation of aqueous ethanol mixtures containing 85.64% m ethanol have been carried out with the aid of a UNIVAC 1108 computer. The computer program calculates all phase equilibria and tray-to-tray material and heat balances for each component... 

If the top product (water) from the solvent recovery column is to be discarded, the two distillation columns would be operated to reduce ethanol and ethylene glycol to low concentrations, as illustrated in the calculations shown here. However, where the overall plant scheme is such that the water product might be recycled and used—e.g, as solvent to an aqueous extractive distillation, it might under some conditions be more economical to leave more ethanol in the water product. The ethanol would be recovered in the series of separation steps which follow in the flow scheme. Water might be rejected at a more suitable point in the flow scheme than from the top of the solvent recovery column. The best operating conditions can be determined only when the entire plant flow scheme is known. 

The results of calculations for the two separation methods are summarized in Table VIII. Fewer trays are required in the azeotropic distillation column than the extractive distillation column. The heat loads are also smaller. The quality of the ethanol product is also slightly better for the azeotropic distillation method. Including the stripper for processing the aqueous phase, the total heat load for reboilers for the azeotropic distillation method is less than half that for the extractive distillation method. The total condenser load is roughly two-thirds that for the extractive distillation method. 

Although the azeotropic distillation scheme, using n-pentane, operates at a higher pressure, comparative calculations indicate this to be better than the extractive distillation scheme using diethylene glycol to dehydrate aqueous ethanol. 

Cost calculations were made for this modified extractive distillation process they are shown by the dashed curves on Figures 5 and 7. The proposed heat exchange system provides considerable reduction in the annual costs of the extractive distillation process. However, the extractive distillation costs are still greater than those for a binary propane-propylene distillation process as indicated on Figures 5 and 7. 

The design of azeotropic or extractive distillation columns, as with con-A ventional columns, demands a knowledge of the vapor-liquid equilibrium properties of the system to be distilled. Such knowledge is obtained experimentally or calculated from other properties of the components of the system. Since the systems in azeotropic or extractive distillation processes have at least three components, direct measurement of the equilibrium properties is laborious and, therefore, expensive, so methods of calculation of these data are desirable. 

Figure 13.24. Composition profiles and flowsketches of two extractive distillation processes, (a) Separation of methylcyclohexane and toluene with phenol as solvent (data calculated by Smith, 1963). (b) Separation of aqueous ethanol and isopropanol, recovering 98% of the ethanol containing 0.2 mol % isopropanol, employing water as the solvent. Flow rates are in mols/hr (data calculated by Robinson and Gilliland, 1950).

At this point aniline can be isolated. You could reduce the solubility of aniline by dissolving in the steam distillate 0.2 g of sodium chloride per milliliter, extract the aniline with 2-3 portions of dichloromethane, dry the extract, distill the dichloromethane (bp 4rC), and then distill the aniline (bp 184°C). Or the aniline can be converted directly to acetanilide. The procedure calls for pure aniline, but note that the first step is to dissolve the aniline in water and hydrochloric acid. Your steam distillate is a mixture of aniline and water, both of which have been distilled. Are they not both water-white and presumably pure Hence, an attractive procedure would be to assume that the steam distillate contains the theoretical amount of aniline and to add to it, in turn, appropriate amounts of hydrochloric acid, acetic anhydride, and sodium acetate, calculated from the quantities given in Experiment 2. 

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