Two-Stripper/Flash/Distillation Column System Design, Operation, and Control for Separating 2-Pentanone/4-Heptanone/Water Azeotropic Mixture via Navigating Residue Curve Maps and Balancing Total Annual Cost and Product Loss

A novel method for the synergetic production of 2-pentanone and 4-heptanone has been explored recently at the Institute of Coal Chemistry, Chinese Academy of Sciences (ICC, CAS). The collected mixture, containing mainly 2-pentanone/4-heptanone/water/carbon dioxide, presents heterogeneous azeotropes. The separation of this quaternary mixture is the main problem urgently to be effectively solved. In this work, a two-stripper/flash/column flow sheet is proposed to achieve the separation, in which the heterogeneity of the system is fully utilized by using an overhead vapor–liquid–liquid (VLL) flash. A general total annual cost (GTAC) for balancing total annual cost (TAC) and product loss (PL) is proposed as an optimization function, and several optimization sequences are presented to facilitate the optimization. Four candidate sequences (S1, S2, S3, and S4) are derived from navigating the ternary and residue curve maps, of which the S4 cuts the GTAC with a marked margin (83.9%) compared with the S1. The rigorous economic design and optimum operation of the flow sheet are further investigated in detail. Moreover, several control structures (TCS1, TCS2, and DTCS, CTCS, C&TCS) are successively explored with consideration of ordinary-purity and high-purity separations. Dynamic control shows that whereas the flow sheet can be controlled by controlling one tray temperature in each stripper/column for ordinary-purity separation, an expensive and high-maintenance online composition control should be considered for high-purity separation.