Abstract: Since the minimum-boiling azeotropes of C2-C8 alcohols with water and high-water content (up to 95% (mass)) in the Fischer-Tropsch aqueous by-products, the separation is energy-intensive and challenging. The energy-saving strategy for the complete separation of the Fischer-Tropsch aqueous by-products has received massive attention in recent decades. In this study, a stripper-sidestream decanter process is proposed by exploiting homogeneous azeotropes (C2-C3 alcohols-water) and heterogeneous azeotropes (C4-C8 alcohols-water). The introduction of the stripping column for pre-dehydration avoids the re-vaporization of the mixture, and energy carried by the overhead vapor is conserved instead of being removed in a condenser. The precise fraction cutting of C1-C3 alcohol-water mixture, C4-C8 alcohols, and water is realized by the sidestream distillation column. The C4-C8 alcohols rich mixture withdrawn from the sidestream flows into the decanter to break the distillation boundary, where the organic phase returns to the sidestream distillation column to obtain the dehydrated C4-C8 alcohols, and the aqueous phase enters the stripping column. Steady-state optimization based on total annual cost (TAC) minimization shows that the stripper-sidestream decanter process reduces TAC by 17.00% and saves energy by 21.27% compared with the conventional three-column distillation process. Further, a control structure of the process is established, and dynamic simulations show that the control structure combining a differential controller with a low-selector exhibits robust control. This study provides a novel design scheme and deepens the insights into the efficient separation of aqueous by-products of the Fischer-Tropsch synthesis.

Key words: Fischer-Tropsch synthesis, Mixed alcohols, Azeotrope, Separation, Simulation, Control

摘要： Since the minimum-boiling azeotropes of C2-C8 alcohols with water and high-water content (up to 95% (mass)) in the Fischer-Tropsch aqueous by-products, the separation is energy-intensive and challenging. The energy-saving strategy for the complete separation of the Fischer-Tropsch aqueous by-products has received massive attention in recent decades. In this study, a stripper-sidestream decanter process is proposed by exploiting homogeneous azeotropes (C2-C3 alcohols-water) and heterogeneous azeotropes (C4-C8 alcohols-water). The introduction of the stripping column for pre-dehydration avoids the re-vaporization of the mixture, and energy carried by the overhead vapor is conserved instead of being removed in a condenser. The precise fraction cutting of C1-C3 alcohol-water mixture, C4-C8 alcohols, and water is realized by the sidestream distillation column. The C4-C8 alcohols rich mixture withdrawn from the sidestream flows into the decanter to break the distillation boundary, where the organic phase returns to the sidestream distillation column to obtain the dehydrated C4-C8 alcohols, and the aqueous phase enters the stripping column. Steady-state optimization based on total annual cost (TAC) minimization shows that the stripper-sidestream decanter process reduces TAC by 17.00% and saves energy by 21.27% compared with the conventional three-column distillation process. Further, a control structure of the process is established, and dynamic simulations show that the control structure combining a differential controller with a low-selector exhibits robust control. This study provides a novel design scheme and deepens the insights into the efficient separation of aqueous by-products of the Fischer-Tropsch synthesis.

关键词: Fischer-Tropsch synthesis, Mixed alcohols, Azeotrope, Separation, Simulation, Control