Process synthesis and simultaneous optimization of extractive distillation system integrated with organic Rankine cycle and economizer for waste heat recovery

The separation of close-boiling and azeotropic components by extractive distillation (ED) is a costly process. Because ED columns usually have relatively large top/bottom temperature difference, traditional energy-saving measures like heat integration/heat pump, are limited in reusing the system inherent waste heat. However, this opens an otherwise door for organic Rankine cycle (ORC), capable of converting waste heat into power regardless of the large temperature span. Consequently, several integrated ED-ORC systems with or without economizer(s) are therefore proposed, aiming at recovering both latent and sensible waste heat. An azeotropic n-heptane/isobutanol separation is used to illustrate the performance of these integrated systems. R227EA is selected as the ORC working fluid from 11 chlorine free candidates for the highest thermal efficiency of 8.23%. Compared to the basic case, the optimum integrated system without economizer only reduces TAC by 3.01%. While reusing the system inherent waste heat by adding economizer(s) can dramatically enhance the energy efficiency, leading to TAC saving up to 30.30%. The TAC reduction is attributed to dramatic hot utility consumption saving (∼ 45%). Sharing close technical parameters, the ED process will benefit more when retrofitted into the integrated systems under minimized column modifications.