(277c) Synthesis of Distillation-Based Separation Networks Using Block Superstructure

Distillation is the most widely-used technology for mixture separation among other alternatives, accounting for about 40% of the total energy consumption in chemical industries [1]. Process intensification enables the advancement of the process performance through improvements in controllability, sustainability and energy efficiency [2-3]. Using intensification principles, distillation operations are evolving into more energy-efficient processes through distillation sequence screening [4], heat integration [5], thermal coupling [6] and etc. Furthermore, novel distillation column configurations, such as divided-wall columns, provide more degrees of freedom for sustainable operation of distillation columns [7]. Although many advances have been made in modeling distillation column at task level and phenomena level [8-9], systematic methods for identifying rigorous distillation networks involving various alternatives, e.g., thermally coupled columns, divided-wall columns, are in high demand.

Previously, building block superstructure [2] has been applied to address systematic process intensification, e.g., membrane reactors, reactive distillation systems, etc., by utilizing fundamental phenomena building blocks instead of equipment-based representation. Later on, this approach is applied to address general process synthesis and integration problems [9-11]. In this work, we apply the block-based approach for systematic synthesis of distillation networks. Block superstructure is comprised of blocks positioned on a two-dimensional structure. Interaction of blocks with material and energy flow is facilitated by the direct connecting streams between adjacent blocks and ‘jump connecting streams’ among all blocks. The boundary between each block can be used to represent vapor-liquid equilibrium phenomena, or designated as completely restricted boundary indicating restriction of material flow (i.e. wall). Also this boundary can be unrestricted for achieving splitting composition requirements. With this, distillation column trays and dividing walls separating different regions of the distillation columns can be represented with multiple adjacent blocks. To achieve heat integration among all separation alternatives, each stream connecting adjacent blocks is assigned with heaters/coolers. The ensemble of these blocks leads to various type of distillation network representations. We formulate this block superstructure for distillation network synthesis as a mixed integer nonlinear programming optimization (MINLP) problem to minimize the total annual cost. The proposed model is applied to various distillation network synthesis problems, involving simple distillation sequences and heat-integrated distillation networks. Among these case studies, various type of distillation alternatives can be identified such as dividing wall columns and thermally coupled columns.

References:

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