Energy-saving design and control of a hybrid extraction/distillation system for the separation of pyridine and water

Abstract

In this study, design and control of a hybrid extraction/distillation system for the separation of pyridine and water are studied. The same separation has been studied in the previous paper (Wu et al, 2009) by using the design of a heterogeneous azeotropic distillation system where a two-column heterogeneous azeotropic column system was devised making use of the new minimum-boiling azeotrope between toluene and water. A further intensified dividing-wall column was recently proposed by Wu et al. (2014) to thermally-couple the two columns for further reduction of the overall reboiler duty. In the hybrid extraction/distillation design, pyridine is extracted from aqueous solution by using N-propyl formate as a solvent in an extraction column. Combining two strippers with the extraction column, a simple hybrid extraction/distillation process can be devised achieving the same product purity specifications as the original design. By comparing the optimized design of this newly proposed system with the previously mentioned heterogeneous azeotropic design, significant reductions of 58.8 % in the total annual cost and 65.6 % in the steam cost can be obtained. Furthermore, in comparison with the complex design of dividing-wall column system, this proposed hybrid extraction/distillation system can still significantly save total annual cost and steam cost by 41.2 % and 50.8 %, respectively. The plant-wide control of the proposed system is also studied. Overall control strategy with tray temperature control loops are proposed for the plant-wide control structure. The dynamics results show a trade-off between economical design and controllability due to selecting of an optimal solvent flow rate at low constraint in the design stage. However, the case with 20% more solvent (which results in 6.15 % more total annual cost than the optimized case) is capable of holding product qualities under various feed disturbances by a proposed plant-wide control structure.