A comparative study of controlling the externally heat-integrated double distillation columns (EHIDDiC)

Abstract

The externally heat-integrated double distillation columns (EHIDDiC) is a newly proposed scheme featuring complete heat integration between the rectifying section of a high pressure distillation column (HPDC) and the stripping section of a low pressure distillation column (LPDC). In terms of its structural characteristics, three decentralized control systems are devised, which avoid using the pressure difference between the HPDC and LPDC as a manipulated variable and ease consequently the interaction between the control loops involved. While the first one attempts to control the composition of the blended top products of the HPDC and LPDC, the second one the composition of their blended bottom products, thereby simplifying the control structure from 4×4 to 3×3 system. The third one focuses on the simplified EHIDDiC with only three heat exchangers between the HPDC and LPDC (S-EHIDDiC) and their heat duties are employed as a combined manipulated variable. These control systems are evaluated in terms of the separation of a binary mixture of benzene and toluene and it is found that they outperform exclusively the conventional control system with the pressure difference as a manipulated variable. Both the top-mixed and bottom-mixed control systems appear to be superior to the one for the S-EHIDDiC and conventional double-effect distillation column, implying the advantages of the simplified design of decentralized control systems. The obtained results are considered to be of general significance and can be used to guide the design and operation of the EHIDDiC (S-EHIDDiC).