(134b) Environmentally Friendly Heterogeneous Azeotropic Distillation System Design: Integration of Ebs Selection and Ips Recycling for Retrofitting

In chemical industries, waste solvents are considered to be the main source of pollution, whether it involves a batch process or a continuous process. Thus the separation of in-process solvents (IPS) from waste solvent streams which is considered as process design, and the selection of environmentally benign solvent (EBS) to retrieve IPS, which is considered as product design, are two important issues in waste solvents treatment problems. Because these two procedures could interact, it is necessary to integrate them in one framework based on concerns of cost, environmental quality, etc., to ensure better performance.

Computer-aided molecular design (CAMD)^1,2 is one commonly used technique for solvent selection. CAMD, which works as the reverse use of group contribution methods, can automatically generate promising EBS molecules from their fundamental building blocks or groups^3,4,5. The majority of available methods for selecting a possible process resort to geometric method with graphical representation of azeotropes, residual curve maps (RCMs), and distillation boundaries. A graph theoretical approach, namely the process graph (P-graph) based approach^6,7,8,9, can find all possible sets of structures from candidate operating units by which the desired products and necessary intermediate products leading to final products are produced. However, the ?possible? here only means combinatorially possible. Further, P-graph analysis assumes sharp split in distillation column without considering reflux effect, which is an important factor in distillation operation. Therefore, considering P-graph alone is insufficient to identify feasible and practical structures, further analysis facilitated with RCMs is needed.

In this work, we present an approach that utilizes three different methodologies at three different steps. In the process design step, the combination of Residual Curve Maps (RCMs) analysis and P-graph technique are used to identify a separation superstructure. In the product design step, the Computer-Aided Molecular Design (CAMD) results obtained from previous work³ are used as replacements for conventional solvents like ethyl acetate. A new Multi-Objective Optimization (MOP) framework under uncertainty, in which process design and product design are combined together and with solvents recovery rate, process operability, process flexibility, energy intensity, and environmental impacts like LC₅₀, LD₅₀, and Bio Concentration Factor (BCF) are included in the objective function set, is developed in the Aspen Plus simulator. The methodology is presented in the context of retrofitting a continuous separation process of acetic acid ? water system. More Pareto optimal solutions have been identified in this work compared to earlier work of Kim and Diwekar⁷.

Reference: (1) Joback, K.G.; Stephanopoulous, G. Designing Molecules Processing Desired Physical Property Values. In Foundations of Computer-Aided Process Design. 1989, 363-387, Snowmass Village, CO. (2) Gani, R.; Nielsen, B.; Freemasons, A. A group contribution approach to computer-aided molecular design. AIChE J. 1991, 37, 1318-1332. (3) Kim, K.-J.; Diwekar, U. M. Efficient Combinatorial Optimization under Uncertainty. 2. Application to Stochastic Solvent Selection. Ind. Eng. Chem. Res., 2002, 41, 1285-1296. (4) Kim, K.-J.; Diwekar, U. M. Integrated Solvent Selection and Recycling for Continuous Process. Ind. Eng. Chem. Res. 2002, 41, 4479-4488. (5) Kim, K.-J.; Diwekar, U. M. Efficient Combinatorial Optimization under Uncertainty. 1. Algorithmic Development. Ind. Eng. Chem. Res. 2002, 41, 1276-1284. (6) Feng, Gangyi; Fan, L. T.; Seib, P. A.; Bertok, B.; Kalotai, L.; Friedler, F. Graph-Theoretic Method for the Algorithmic Synthesis of Azeotropic-Distillation Systems. Ind. Eng. Chem. Res., 2003, 42, 3602-3611. (7) Feng, Gangyi; Fan, L. T.; Friedler, F.; Seib, P. A. Identifying operating units for the design and synthesis of azeotropic-distillation systems. Ind. Eng. Chem. Res., 2000, 39, 175-184. (8) Feng, Gangyi; Fan, L. T.; Friedler, F. Synthesizing alternative sequences via a P-graph-based approach in azeotropic distillation systems. Waste Management. 2000, 20, 639-643. (9) Novaki, S.; Bertok, B.; Friedler, F.; Fan, L. T.; Feng, G. Rigorous algorithm for synthesizing azeotropic distillation systems. Chemical Engineering Transactions. 2003, 3, 1123-1127.