Synthesis,Design And Control Of Re-Enhanced Dividing-Wall Distillation Column

Thanks to the high internal coupling,the dividing-wall distillation column(DWDC)can effectively lower irreversibility of the separation operation,thereby considerably improving operational efficiency.It has achieved the triple effect of reducing equipment investment cost,installation space and energy consumption,and also has been widely used in many industries.It shows off the great superiority of process intensification.But this does not mean that DWDC has no energy-saving potential to excavate.Continuing to implement re-enhanced design for DWDC through technologies such as multiple dividingwall and heat pump can achieve wider energy-saving possibilities.However,the synthesis,design and control of the re-enhanced DWDC become a very challenging problem because the re-enhanced process of DWDC involves a large number of complex structural and manipulated variables.For the sake of settling above-mentioned issues,we bring forward a kind of systematic synthesis,design and control philosophy,that is,by means of pre-determining the optimal topological structure of the re-enhanced DWDC,the structure can be decoupled from design and manipulated variables,so that its synthesis,design,and control issues are transformed into issues of structural and manipulated variable searching and control structure design under a specific topological structure,which can lower considerably the complicated and timeconsuming work of process modeling,structural and operating parameter searching,as well as process and control structure screening in process development.Re-enhanced DWDC includes but is not limited to development of complex DWDC,vapor recompression of DWDC and development of reactive DWDC.This paper highlights synthesis,design,and control of the DWDCVRHP and R-DWDC.(i)Synthesis,design,and control of the DWDC-VRHP.Although vapor recompressed heat pump(VRHP)can significantly improve the thermodynamic efficiency and economic profit for the DWDC,due to the influence of DWDC’s double-column and multi-stage features,there are many possibilities for the development of DWDC-VRHP,for instance,single,multiple,multi-level VRHPs etc.coupled with the DWDC,which significantly exacerbates complexity and tediousness of the system development and may greatly deteriorate the process controllability.For the sake of settling abovementioned issues,we study the development of DWDC-VRHP by a kind of systemic philosophy.(ii)Synthesis,design,and control of the R-DWDC.The reaction operation is further coupled into the DWDC,which can excavate maximally the potential of coupling between the reaction and separation operations.However,due to the diversity of the number and position of the dividing-wall caused by the introduction of the dividing-wall technology as well as the different arrangements of the reaction sections,etc.,the development of R-DWDC has many possibilities,including single,double,multi-dividing-wall etc.It greatly increases complexity and tediousness of the system development and is highly likely to deteriorate dynamic characteristics of the R-DWDC.For the sake of settling above-mentioned issues,we study the development of RDWDC by a kind of systemic philosophy.The following shows the specific research contents:1.The topological structure of double columns and multiple separating sections of the DWDC makes the application of VRHP a very complex task.Although many researchers reveal the great advantages of the DWDC-VRHP,actually they ignore these inherent problems in process development.In the current research,a light-component dominated ternary wide boiling-point mixture was investigated.Owing to the separation between the light-and intermediate-/heavy-component causes the major energy consumption,so we ought to arrange VRHP for lowering the energy consumption and this causes the arise of the optimal DWDC-VRHP,i.e.,a DWDC plus a two-stage VRHPs.Due to these two advantages,the first one is that the temperature difference between the feed and top is small and the second one the mass/heat transfer can be facilitated by feed splitting,the first-stage VRHP ought to preheat the feed by recompressed overhead vapor.The second-stage VRHP is used in the stripping section of pre-fractionator(PF)to further lower the energy consumption.The optimality of this structure is demonstrated by two examples in the benzene series and the alkane series.Any DWDC can adopt this strategy(regardless of where the dividing-wall is placed).Pre-determining the optimal design for the DWDC-VRHP considerably decreases the complex and tedious tasks of modeling,parameter search optimization,as well as craft and control structure screening for process development.2.For the intermediate-component dominated ternary wide boiling-point mixture,due to the maximized coupling exists between the PF and main distillation column(MDC),the excessive heat always exists around the location of the intermediate product,the excessive heat can be effectively used to help the PF/common stripping section’s separation operation by VRHP.This leads to the optimal design of the DWDC-VRHP,a DWDC facilitated by two VRHPs.Due to the temperature difference between the intermediate product and common stripping section is small,the first VRHP ought to compress sideline vapor to heat the latter,helping to lower its irreversibility.The second VRHP serves to further reduce the operation irreversibility by compressing the overhead vapor pre-heated by the high temperature condensed liquid from the first VRHP and release its latent heat to the PF/common stripping section.For the DWDC with a top or bottom partition,the optimum design of the DWDCVRHP can also be derived in the same manner.The design philosophy’s feasibility and effectiveness are demonstrated by two examples in the benzene series and the alkane series.3.The quaternary reversible reactions’ separations with the MURRV typically require a multi-column design consisting of a reactive separation column(RDC)plus one or two conventional separation columns(CDCs).The multi-column design implies that the reaction/separation process involves a mass of energy consumption,but also reveals process intensification’s huge potential.Based on the two-column design(a RDC involving an external recycle(ER-RDC)+a CDC(ER-RDC+CDC)),we deduced a reactive double dividingwall distillation column(R-DDWDC)by carefully coordinating the coupling between the ER-RDC and CDC.According to the characteristics of the heat of reaction,we install the left dividing-wall at the top or bottom,which can intensify the coupling between the ER-RDC and CDC and can also help to enhance reaction process of the reactive part.We install the right dividing-wall at the middle to maximize coupling potential between the ER-RDC and CDC as well as the separation operations.The deduced R-DDWDC inherently has maximum process intensification.So,it is the most energy-efficient design for the quaternary reversible reactions with the MURRV.Two cases studies on the exothermic and endothermic reversible reactions with the MURRV illustrate the optimality of the R-DDWDC,and they also illustrate the validity and feasibility of this design principle.4.The high coupling makes the control of the R-DDWDC an especially challenging issue with a highly interactive nature.With reference to the separation of an ideal quaternary endothermic reversible reaction with the MURRV,the operation rationality of the R-DDWDC is studied in this contribution.The four-point single temperature control system leads to great steady-state discrepancies in the compositions of products C and D and the reason stems essentially from the failure in keeping strictly the stoichiometric ratio between reactants A and B.A temperature plus temperature cascade control scheme is then employed to reinforce the stoichiometric ratio control and helps to secure a substantial abatement in the steady-state discrepancies.A temperature difference plus temperature cascade control scheme is finally synthesized,which greatly improves the dynamic performance and controllability of the R-DDWDC and leads even to better performance than the most effective double temperature difference control scheme.These outcomes reveal not only the operation feasibility of the R-DDWDC but also the general significance of the proposed temperature difference plus temperature cascade control scheme to the inferential control of any other complicated distillation columns.