Design And Control Of Dividing-wall Distillation Columns

Although the dividing-wall distillation column(DWDC)outperforms the conventional direct or indirect separation sequence because of its reduced equipment investment and operation cost,the material and thermal coupling within it are substantially enhanced and result in a fairly complicated control behavior.In order to overcome this drawback,on the one hand,it is necessary to try to improve the controllabililty of the DWDC in its synthesis and design;on the other hand,it is also necessary to study systematically the operation and control of the DWDC.These are favorable to promote the applications of the DWDC in process industry.Since the high price and serious time delay of online composition analyzers lead to concentration control systems are seldom used in controlling various processes,the current work focus mainly on the temperature inferential control of the DWDC and the main research contents can be summarized as follows:1.In order to overcome the black-hole problem that prohibits assigning four specifications on the top,intermediate,and bottom products of the DWDC,feed splitting strategy is employed to coordinate the relationship between the prefractionator and main distillation column involved and a systematic procedure is proposed to derive the optimal design of the DWDC without the black-hole problem.The design and operation of a DWDC fractionating an equi-molar mixture of ethanol,propanol,and butanol are examined to evaluate the systematic procedure proposed.Both open-and closed-loop controllability assessments demonstrate that feed splitting can serve as an effective means to eliminate the black-hole problem.2.For suppressing the black-hole problem,a feed thermal condition adjustment strategy,achieved by the installation of a pre-heater in feed pipeline,is also attempted.Through the strong influence to the overall mass and energy balance of the DWDC,the feed thermal condition adjustment can alter the interlinking flows between the thermally coupled prefractionator and main distillation column and work effectively to coordinate their relationship.A DWDC separating a benzene,toluene,and o-xylene mixture is chosen to ascertain the feasibility of the philosophy proposed.The static and dynamic studies demonstrate that the feed thermal condition adjustment can completely suppress the black-hole problem and elevate consequently process flexibility and operability.3.A simplified double temperature difference control(SDTDC)scheme,including 2 temperature control(TC)and 2 double temperature difference control(DTDC)loops,is proposed to achieve tight product purity control of the DWDC.While the 2 TC loops are designed,respectively,to regulate the top and bottom products,the 2 DTDC loops to control tightly the prefractionator and intermediate product.The design strategy makes deliberately use of the operating characteristics of the DWDC and can consequently facilitate its product control and robustness to the operating condition changes.The control of a DWDC separating an ideal ternary mixture and a DWDC fractionating a benzene,toluene,and o-xylene mixture are studied to compare the SDTDC scheme against the temperature difference control(TDC)and DTDC schemes.Although sharing an equal number of temperature measurements,the SDTDC scheme is superior to the TDC scheme with comparable or even reduced static offsets in the three products and enhanced capability of handling feed composition disturbances.The SDTDC scheme leaves slightly greater static offsets than the DTDC scheme,but its reduced cost and alleviated complication justify it a competitive alternative for the operation of the DWDC.The SDTDC scheme can be further reinforced through the formation of TDC loops in the rectifying and/or stripping sections with the available temperature measurements in the two sections along the dividing wall and this increases the diversity of the proposed control strategy.4.The potential of asymmetrical temperature control(ATC)structures with each control loop involving a different number of temperature measurements is examined with reference to a frequently studied DWDC fractionating a benzene,toluene,and o-xylene mixture.With the perfect maintenance of the four controlled product compositions on their specifications,the three kinds of controlled variables,i.e.,temperature,temperature difference,and double temperature difference,are assessed with reference to all kinds of feed composition disturbances and the one with the minimum averaged variability is screened out for each control loop.In combination with the interaction analysis,three ATC schemes are finally derived.Closed-loop evaluations of these ATC schemes are carried out through in-depth comparison with the DTDC scheme and the former ones all show comparable transient responses and improved steady-state behaviors in comparison with the latter.Although the three ATC schemes result in relatively great steady-state discrepancies in the intermediate product purity,the drawback can be suppressed completely with the addition of two temperature measurements in the intermediate control loop.These findings demonstrate the feasibility and potential of adopting the ATC schemes for the tight inferential control of the DWDC.5.A novel method for configuring the DTDC scheme is proposed that employs the newly defined metric,the averaged absolute variation magnitudes(ASVM),to determine the two reference stages in each DTDC loop.The ASVM measures the variations of temperature differences(TDs)between the sensitive stage and the remaining ones with the assumption of complete rejection of all disturbances concerned and can thus reflect the inherent characteristics of coupling between the controlled product qualities.For each DTDC loop,while the first reference stage should be chosen to cope with its coupling with the other control loops,the second reference stage should be to coordinate the two TDs involved,thereby yielding a favorable effect to the inference of the controlled product qualities.Four example systems,including one conventional distillation column separating a binary mixture of ethanol and butanol,two conventional distillation columns separating a ternary mixture of ethanol,propanol,and butanol,and one dividing-wall distillation column separating a ternary mixture of ethanol,propanol,and butanol,are used to assess the proposed method by means of in-depth comparison with the conventional method.While they display comparable dynamic performances,the former leads to considerably smaller steady-state deviations in the controlled product qualities than the latter.These striking outcomes demonstrate evidently that the proposed method can be a promising alternative for the pursuit of tight temperature inferential control of various distillation columns.