Regions of Optimality for Separation System Synthesis Using Rigorous Modeling, Stochastic Optimization, and Column Stacking

Abstract

Alternatives for the separation of three-component zeotropic mixtures are compared using simulation optimization with the total annual cost as the objective function. Conventional two-column sequences with and without column stacking are compared with three types of dividing-wall columns (DWCs). All processes are modeled with a rigorous process simulator and optimized using simulated annealing. The results are represented using ternary maps and used to regress parameters of a model that predicts the relative cost of process alternatives as a function of the feed composition, relative volatilities, and ease of separation index. The results highlight the importance of considering column stacking when choosing a process alternative for the ternary separation: in some cases, the region where the stacked two-column configuration is preferable to a dividing-wall column is significantly larger than the area where a conventional two-column configuration is preferable to a dividing-wall column.

Comparison of Separation Alternatives for Two Industrial C6–C7 Aliphatic Hydrocarbon Mixtures Including Stacked Complex Sequences

“…There is no built-in models for DWCs in Aspen Plus, therefore multiple Radfrac blocks are used to represent a single DWC, as shown in Figures S1−S3. 17 Abbreviations for all configurations considered in this work are shown in Table 1. All distillation columns are rigorously simulated using the Radfrac model in Aspen Plus V12.…”

“…Although DWCM can reduce TAC by 1.7% compared with the optimal unstacked conventional sequence (DS), CISH can reduce TAC by 13.1% compared with the optimal unstacked conventional sequence (DS), which is significantly greater than DWCM. Lin et al 17 and Tang and Ward 22 found that DWCU is preferable for mixtures with ESI (ease of separation index 31 ) larger than one, DWCL is preferable for mixtures with ESI less than one, and DWCM is preferable for mixtures with ESI around one. For mixture MCM (ESI < 1) studied in this work, the rank order of TAC for DWCs is DWCU > DWCL > DWCM.…”

“…There are three types of DWCs often used to separate the multicomponent mixtures into three product streams, as shown in Figure : DWC with upper partition (DWCU), DWC with middle partition (DWCM), and DWC with lower partition (DWCL). There is no built-in models for DWCs in Aspen Plus, therefore multiple Radfrac blocks are used to represent a single DWC, as shown in Figures S1–S3 . Abbreviations for all configurations considered in this work are shown in Table .…”

“…Several studies have shown that CS can save a significant amount of energy, even more than DWCs in some cases. 17 In this work, CS is considered for conventional and complex distillation sequences. CS cannot be applied to a three-component thermally coupled system because vapor communicates between all sections in the thermally coupled system and therefore the pressures everywhere in the coupled system must be nearly the same.…”

“…This can be accomplished by adjusting the operating pressure of the two columns so that the top temperature of the high-pressure column (HPC) is greater than the bottom temperature of the low-pressure column (LPC). Several studies have shown that CS can save a significant amount of energy, even more than DWCs in some cases . In this work, CS is considered for conventional and complex distillation sequences.…”