Ind. Eng. Chem. Res. 1994,33, 1879
1879
Response to Comments on “Shortcut Procedure for Simulating Batch Distillation Operations” Lawrence B. Evans Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Sir: Diwekar states that the claim that our model is formulated so it can be used to model existing columns is unsubstantiated. This statement is incorrect. Our algorithm accepts as input the number of stages and the feed flow rate, boil-up rate, and reflux ratio as a function of time. These are the variables that are usually specified to simulate the performance of an existing column. The model calculates the composition of overhead product and still-pot as a function of time (or percent distilled). Our model was used to perform more than 100 simulations, of which 33 are presented in the paper. The results were compared with the rigorous simulation of a batch column using BATCHFRAC under the same conditions. An application of the model was presented in Sundaram and Evans (1993), where the model was successfully used to determine the optimal operating procedure for an existing batch distillation column to achieve a desired separation. These examples demonstrate that the model can be used to simulate an existing column. Our model is very different from the model of Diwekar and Madhavan even though both are based on the FenskeUnderwood-Gilliland assumptions. Their model as described in their paper is unclear. It accepts as input the initial distillate composition as the start of distillation and calculates the number of stages required to achieve this composition. Such a model is in the form that might be used to design a column rather than simulate an existing column. It is unlikely that one would use a shortcut
0888-588519412633-1879$04.5010
method to design a column that would then be constructed for a specific separation. It is more likely that a separation by batch distillation would be performed in an existing, multipurpose column. The test of a shortcut model is how well it compares with rigorous calculation, not how well it matches experimental data. Diwekar and Madhavan (1991) present only a single comparison with the rigorous BATCHFRAC model based on an example problem published by Boston et al. (1981). The value of our contribution compared with that of Diwekar and Madhavan will be determined ultimately by researchers and practitioners who choose which method to use for column simulation. We leave it to others to read both papers and determine for themselves which is easier to understand and apply for simulation of a batch column.
Literature Cited Boston, J. F.; Britt, H. I.; Jirapongphan, S.; Shah, V. B. An Advanced System for the Simulation of Batch Distillation Operations. Foundations of Computer-Aided Chemical Process Design;Mah, R. S . H., Seider, W., Eds.; Engineering Foundation: New York, 1981; Vol. 2, p 203. Diwekar,U. M.; Madhavan, K. P. Multicomponent Batch Distillation Column Design. Znd. Eng. Chem. Res. 1991,30, 713. Sundaram, S.; Evans, L. B. Synthesis of Separations by Batch Distillation. Znd. Eng. Chem. Res. 1993, 32, 500.
0 1994 American Chemical Society
