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Table XI. Component Flow Rates of Distillate and Bottoms. Comparison of calculations based on UNIFAC and Raoult´s law. Full version. a Benzene (1) – 1-Propanol(2)-Toluene(3)-Acetic Acid (4)
Reflux ratio: 2.0; distillate: 50 mol/h; number of stages: 15; feed stage: number 7; feed: (1): 25 mol/h; (2): 10 mol/h; (3): 10 mol/h; (4): 55 mol/h.
The UNIFAC correlation correctly predicts, water hasmuch higher activity in acetic acid than 1-propanol has, and hence most of the 1-propanol goes into the bottoms product. In these calculations, we have neglected the effect of the relatively slow esterification of the alcohol with acetic acid.
Figure 8 shows a summary of results for a complex column, where Murphree stage efficiencies of 0.7 were included. This required five iterations and 17.4 scomputer time on the IBM 370/165 computer.
The system Benzene(1)-1-Propanol(2)-Toluene(3)-Acetic Acid(4). Table XI shows results for the case of an uneven feed: 25 mol/h benzene, 10 mol/h each of 1 – propanol an toluene, and 55mol/ or acetic acid. These results show the effects of successively including correction for liquid-phase nonideality (II), vapor-phase nonidelity (III), and excess enthalpyeffects (IV), it is essential to include corrections for both liquid and vapor-phase nonideality, whereas the inclusion of enthalpy balances has only limited effect on the calculation. For most of the systems we have studied, the inclusion of enthalpy balances has little effect on the product compositions. They may, however, be of much importance in the determination of internal column flowrates.
This work has indicated that UNIFAC provides a powerful tool for the chemical design engineer. As outlined and illustrated here, it is possible to construct a completely computerized producer for process design of a multicomponent distillation column, including estimation of phase equilibria. The UNIFAC method for calculating activity coefficients interfaces easily with iterationtechniques for distillation-column computations.
While the engineering utility of UNIFAC has been demonstrated, its utility inevitably rests on the quantity and quality of UNIFAC group-interaction parameters. These, in turn, depend on the quatity and quality of experimental phase-equilibrium data. The essence of UNIFAC is that it interprets and correlates such data to enable predictions of phaseequilibria in previously studied mixtures under new conditions and in mixtures that have not been studied at all experimentally.
Figure 8. distillation results for the system ethanol(1)-1-propanol(2)-water(3)-acetic acid(4).
Omissions in the group-parameter table clearly indicate where new experimental data are needed. As new and better phase-equilibrium data become available, the UNIFAC method,appropriately revised, will increase its range of applicability. However, even at this incomplete state of development, it is evident that UNIFAC may be useful in at least some of those commonly encountered industrial situations where required data are lacking and where, for commercial reasons, a reasonable design there is always a need for good data seasoned judgment, and sound experience. UNIFACis no substitute for these requirements but it can contribute toward meeting their demands.
A description and print-out of the UNIFAC programs for estimating activity coefficients and the computer programas which interface UNIFAC with distillation-column design is given by Fredenslund et al. (1977)
The authors thank Mr. Vang Hansen for carrying out the “full version” of theNaphtali-Sandholm column calculations, Mr. Thomas Anderson for his assistance in reducing to practice the correlation indicated in Appendix C, the National science foundation and Statens teknisk-videnskabelige Forskningsraad for partial financial support, and Professor Ulfert Onken (University of Dortmund) for his active interest in this work.
Appendix A
The UNIQUAC and UNIFAC Models. Equiations...
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