(441f) Synthesis of Optimal Hybrid Separation Processes

While distillation operations are the primary vehicles for separation within the chemical industry, they incur high utility costs due to their low thermal efficiencies [1]. Over 50% of energy required for distillation is used for purifying the last 5–10% of the distillate product [2]. This results in significant burden on the economics as well as on the environmental footprint of the overall operation when high-purity end products are targeted. Alternative techniques with membrane and adsorption-based separation technologies require the development of highly selective materials amenable to cost-effective scale up to compete with conventional standalone distillation equipment. Hybrid separation systems combining aspects of both technologies can be optimally designed to reduce energy input and costs while retaining advantages of conventional distillation units. Design of such hybrid systems requires process systems engineering approaches to identify the optimal topology of the separation sequence along with the operational conditions where the synergy is maximum. Determining the optimal topology, e.g. feed location for the distillation column, sequence and connectivity of the different operations involved, etc., in a sequential manner with scenario-based approaches may result in either combinatorial explosion or omission of potential favorable alternatives in an effort to restrict to a tractable number of possible scenarios [3]. Recently, the building block superstructure [4] representation has been demonstrated to provide a systematic methodology for process synthesis and intensification [5-6].

In this work, we extend the use of the building block representation approach for the design and synthesis of hybrid distillation-based separation schemes incorporating membrane and adsorption technologies. For a given design problem, the proposed approach starts with identification of the separation tasks that need to be accomplished. These problems can belong to either completely new process designs or part of an existing flowsheet. Identification of the separation tasks also helps into define targets for improvement. One major target for the hybrid scheme is that all the incorporated separation techniques operate at their highest efficiencies and none of them alone can do the same task better than the hybrid one. After the design targets are set, building block-based superstructure is formed for a hybrid scheme involving distillation, membrane, adsorption, etc. and solved as a synthesis-design-intensification problem addressing the identified separation task and design targets with the optimal design and operating conditions. This is facilitated by solving a mixed integer nonlinear programming (MINLP) problem describing the building block superstructure. To demonstrate the applicability of the proposed approach, optimal membrane-based schemes are synthesized for several hybrid separation schemes intended for solvent recovery applications. An adsorption-based hybrid distillation scheme is also designed for a cryogenic air separation to reduce the high energy input required especially for achieving >95% purities for non-hybrid distillation schemes. The resultant optimal solutions featuring hybrid schemes are examples of intensification that create special hybrid modules for use in processing routes where similar separation tasks exist.

References:

[1] Tian, Y., Demirel, S. E., Hasan, M. M. F., Pistikopoulos, E. N. (2018). An Overview of Process Systems Engineering Approaches for Process Intensification: State of the Art. Chemical Engineering and Processing: Process Intensification, 133, 160-210.

[2] Tula, A.K., Befort, B., Garg, N., Camarda, K.V. and Gani, R., (2017). Sustainable process design & analysis of hybrid separations. Computers & Chemical Engineering, 105, pp.96-104.

[3] Marquardt, W., Kossack, S. and Kraemer, K., (2008). A framework for the systematic design of hybrid separation processes. Chinese Journal of Chemical Engineering, 16(3), 333-342.

[4] Demirel, S. E., Li, J., and Hasan, M. M. F., (2017). Systematic Process Intensification using Building Blocks, Computers and Chemical Engineering, 105, 2-38.

[5] Li, J., Demirel, S.E. and Hasan, M.M.F., (2018). Process synthesis using block superstructure with automated flowsheet generation and optimization. AIChE Journal, 64(8), 3082-3100.

[6] Demirel, S. E., Li, J., and Hasan, M. M. F., (2019). A General Framework for Process Synthesis, Integration, and Intensification. Industrial & Engineering Chemistry Research. DOI: 10.1021/acs.iecr.8b05961.