Wednesday, February 18, 2009

Investigating the Need of a Pre-Concentrator Column for the Isopropyl Alcohol Dehydration Process

Saiful Arifin and I-Lung Chien. National Taiwan University of Science and Technology, No. 43, Keelung Road, Sec. 4, Taipei, 106, Taiwan

The design and control of a complete heterogeneous azeotropic distillation column system using cyclohexane (CyH) as entrainer has been presented by Chien, et al. (2004). In that study, the feed composition to the heterogeneous azeotropic column is assumed to be near to the isopropyl alcohol (IPA) and water azeotropic composition (IPA at 69 mol% in that paper). In that study, two-column system containing a heterogeneous azeotropic column and a recovery column was recommended. However, in a typical waste IPA stream in semi-conductor industry where IPA is used as a cleaning agent, the water content is much higher than 31 mol%. Taking a typical waste IPA stream with equal molar of IPA and water as an example, the two-column system as proposed in Chien, et al. (2004) will result in very high recycle flow from the second recovery column to the first heterogeneous azeotropic column. This can easily be seen from the conceptual material balance lines in Figure 7 of Chien, et al. (2004). The main reason is because the line drawn from the points of fresh feed and the organic reflux is too close to the point of ternary azeotropic composition of IPA/H2O/CyH. Thus, the top vapor flow rate will be much larger than the flow rate of the bottom IPA product stream.

An obvious design to solve the above problem is to include a pre-concentrator column in the upstream of the heterogeneous azeotropic column to concentrate the fresh feed composition to 69 mol% IPA. In this way, the previous conclusions in Chien, et al. (2004) can be applied. However, the overall process will include three columns, thus the Total Annual Cost (TAC) may not be competitive to other designs. An alternative process design is to still using two columns with a column (C-2 in Figure 1 below) acting as both the pre-concentrator column and the recovery column. This design flowsheet can be seen in Figure 1 and was studied by Ryan and Doherty (1989) for the ethanol/water/benzene system. In their study, they concluded that this two-column system has lower capital costs but higher operating costs than the three-column system so that the total annualized cost is about the same for both systems. However, they also cautioned that no generalizations should be made from this one example to other systems.

In this study, the IPA dehydration system for fresh feed with equal molar of IPA and water is investigated. The TAC is used as the objective function to be minimized for the above two alternative process design flowsheets. A comparison of the TACs will be made for the optimized two systems. Because feed composition of the waste IPA stream can vary widely, the overall control strategy for these two alternative design are critically important in rejecting this feed composition disturbance. The operability of these two process systems under proper overall control strategy will also be compared. Aspen Plus and Aspen Dynamics are used in the above design and control studies.

Literature Cited

Chien, I. L.; Zeng, K. L.; Chao, H. Y. Design and Control of a Complete Heterogeneous Azeotropic Distillation Column System. Ind. Eng. Chem. Res. 2004, 43, 9, 2160-2174.

Ryan, P. J.; Doherty, M. F. Design/Optimization of Ternary Heterogeneous Azeotropic Distillation Sequences. AIChE J. 1989, 35, 1592-1601.

Keywords: Isopropyl Alcohol Dehydration, Heterogeneous Azeotropic Distillation, Design and Control.

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