Dividing Wall Distillation Column Research Articles

Enhanced reaction extraction distillation via multi-objective optimization and 4E analysis for sustainable recovery of organic compounds from wastewater

Resource utilization and sustainable treatment of wastewater, which can achieve efficient energy utilization and environmental protection, have received hot social attention. Tetrahydrofuran and methanol are the two most common substances in pharmaceutical wastewater. It is very necessary to explore the wastewater treatment process containing these two substances. In this study, extractive distillation and reactive distillation were proposed to effectively separate the tetrahydrofuran-methanol-water component of pharmaceutical wastewater. To further reduce energy consumption, two strengthening processes, double-column reactive-extractive distillation and reactive-extractive dividing wall column distillation were designed. Taking the minimum operating and equipment cost as the objective function, the processes were optimized by using the multi-objective genetic optimization algorithm to obtain the best process parameters. Four processes were evaluated in terms of total annual cost, energy consumption, gas emissions and thermodynamic efficiency. Compared to the traditional three-column extractive distillation process, the above-mentioned four aspects of the reactive-extractive dividing wall column process was reduced by 40.63%, 43.13%, 43.14% and 17.71%, respectively. The reactive-extractive dividing wall column distillation process is of great significance for the efficient purification of valuable organic matter in wastewater and environmental protection.

Multi-objective optimization and composition control of Sargent dividing-wall distillation columns

Sargent dividing-wall distillation columns (DWDCs) can significantly reduce the energy consumption required for separating quaternary mixtures. However, their effective design and strict product purity control can be challenging to achieve, which severely limits their development and application in the industry. To address this problem, a systematic study on their design and control was conducted in this article based on the case of separating a quaternary mixture of methanol, ethanol, 1-propanol, and n-butanol. Firstly, a multi-objective optimization framework was proposed, in which the total annual cost (TAC), CO2 emission, and condition number (CN) were taken as objective functions, and NSGA-III and TOPSIS algorithms were employed to search for the optimal design of the Sargent DWDC. To avoid the convergence difficulty issue of the rigorous model of the Sargent DWDC, a Random Forest surrogate model was used in the optimization process to predict its steady-state and dynamic performance accurately and rapidly. Secondly, a novel composition control structure, consisting of seven control loops, was proposed to achieve strict product purity control of the Sargent DWDC. Three composition difference control loops, located at the prefractionator and the middle column, are responsible for maintaining their composition profile shapes and thus mitigating the impact of feed disturbances on the main column. Additionally, four composition control loops, located at the main column, are responsible for strictly controlling the purities of the distillate, upper sidestream, lower sidestream, and bottom products. Dynamic simulation results showed that the Sargent DWDC can be operated stably under the proposed composition control structure for various feed composition and feed flow rate disturbances. These results not only confirmed the capability of the proposed composition control structure to strictly maintain product purities in the Sargent DWDC but also validated the effectiveness of the proposed multi-objective framework in achieving an energy-efficient and controllable Sargent DWDC.

Integrated process design and control of divided‐wall distillation columns using molecular tracking

AbstractThis work explores the dynamic and control behavior of dividing wall distillation columns from two different steady‐state design approaches (molecular tracking and optimization method) for three different mixtures. The controllability of the six design cases was evaluated using singular value decomposition and the closed‐loop performance was evaluated using integral absolute error in Aspen Dynamics. The results demonstrate that the side‐draw location obtained by molecular tracking (MT) provides optimal controllability. As a result, there is a slight advantage in control properties while obtaining designs by reducing the time to find the optimal solution through the MT method.

Experimental Investigation of Heat Losses in a Pilot-Scale Multiple Dividing Wall Distillation Column with Three Parallel Sections

For an in-depth investigation of the separation process in small-scale distillation columns, knowledge about the exact vapor load inside the column is highly important. However, since columns with small diameters have a comparatively high surface-to-volume ratio, heat losses have a significant impact on fluid dynamics, as they lead to unwanted condensation, and thus, to changes in the internal flows. This work presents a procedure used to measure heat losses in a 9.6 m high distillation column with three partially parallel segments (multiple dividing wall column). The evaporator is made of stainless steel, and the column walls are made of double-walled, evacuated, mirrored glass, and additionally, these can be heated. It is found that significant amounts of heat are lost in the evaporator. Throughout the column height, around 0.8 kW are additionally lost, even with external wall heating. To determine the main reason for this significant loss, thermal images are taken, indicating that the problem mainly arises because of the flanges. Based on this, it can be concluded that proper insulation and additional heating jackets for the column walls are highly recommended for small-scale distillation columns in order to increase their thermal efficiency.

Co-process intensification in the synthesis and design of reactive double dividing-wall distillation columns

In most cases, the reactive single dividing-wall distillation column (R-SDWDC) and its corresponding conventional reactive distillation system (CRDS) fail to yield the best performance in the separations of reacting mixtures with unfavorable reaction kinetics and/or unfavorable thermodynamics properties. In this work, a novel methodology of co-process intensification is proposed for the synthesis and design of reactive dividing-wall distillation columns. The methodology leads to the establishment of a reactive double dividing-wall distillation column (R-DDWDC), and the included intermediate separating section plays a vital role in coordinating these two kinds of process intensification and consequently guarantees its optimum performance. In terms of two example systems involving the separations of the ideal quaternary reversible reaction with a somewhat unfavorable ranking of relative volatilities (A + B ↔ C + D with αA > αC > αB > αD) and the transesterification of methyl acetate with n-propanol, the proposed methodology is evaluated. The R-DDWDC is found to yield much better performance than the CRDS and R-SDWDC in the separations of the reacting mixtures with somewhat unfavorable ranking of relative volatilities, and such outcomes definitely corroborate the effectiveness and advantages of the proposed methodology for process development. Although the proposed methodology is developed solely based on the separations of the reacting mixtures with somewhat unfavorable ranking of relative volatilities, it is of great implication to the separations of the other reacting mixtures with unfavorable reaction kinetics and/or unfavorable thermodynamics properties.

Reinforced temperature control of a reactive double dividing-wall distillation column

The mass and thermal coupling makes the control of the reactive double dividing-wall distillation column (R-DDWDC) an especially challenging issue with a highly interactive nature. With reference to the separation of an ideal endothermic quaternary reversible reaction with the most unfavorable ranking of relative volatilities (A + B ↔ C + D with α A > α C > α D > α B ), 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 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.

An effective procedure for the development of vapor recompressed Kaibel dividing-wall distillation columns

While combining the Kaibel dividing-wall distillation column (KDWDC) with vapor recompression heat pumps (VRHPs) represents a promising way of process intensification, it faces great complexity, computational intensity, and tediousness. In this work, based on its thermodynamic characteristics, an effective procedure is proposed for the development of the VRHP assisted KDWDC (VRHP-KDWDC) processing a wide boiling-point quaternary mixture. The VRHP-KDWDC should be derived following the decreasing order of temperatures of the available heat sources. More specifically, while the first step is to explore the economic feasibility of process intensification by the vapor stream of the second intermediate product, the second step by the vapor stream extracted from the common rectifying section. The procedure allows maximizing the performance of the VRHP-KDWDC with rather low complication, computational intensity, and tediousness. The separations of four wide boiling-point quaternary mixtures, including benzene, toluene, o-xylene, and tri-methyl-benzene and n-pentane, n-hexane, n-heptane, and n-octane with and without component dominance in feed, are studied to evaluate the proposed procedure and the outcomes all demonstrate its simplicity, effectiveness, and efficiency. The proposed procedure is also applicable to the development of the VRHP-DWDC processing a ternary mixture and other complicated VRHP-KDWDCs processing a mixture with more than four components.

Configuring topologically optimum vapor recompressed dividing-wall distillation columns to maximize operating efficiency

For dividing-wall distillation columns (DWDCs) separating a heavy-component dominated and wide boiling-point ternary (HCDWBT) mixture, a significant amount of excessive heat exists inevitably in stripping the heavy-component from the intermediate-component and it can be employed to initiate the development of vapor recompression heat pump (VRHP) assisted DWDC (VRHP-DWDC). Despite dividing wall may locate in the top, middle, and bottom, the optimum VRHP-DWDC is found to involve uniformly-two VRHP circles. While the first one serves to compress and transform the excessive heat resulted from the separation of the heavy-component from the intermediate-component, the second one to compress and transform the overhead vapor stream of the light-component pre-heated sequentially with the condensate from the first one and the bottom product stream of the heavy-component, both releasing the temperature-elevated latent heat to the pre-fractionator’s or common stripping section. The processing of two HCDWBT mixtures of benzene/toluene/o-xylene and n-pentane/n-hexane/n-heptane are selected to assess the derived optimum topological configurations of the VRHP-DWDC and their optimality is confirmed through detailed comparisons with the DWDC and two VRHP-DWDCs involving only one VRHP circle. The proposed strategy helps to tap the full potential of the VRHP-DWDC with considerably alleviated complication in process development.

Operability assessment and systematic PI control of a class of Extractive Dividing Wall Distillation Columns: Case of ethanol dehydration

In this work, the practical control capability assessment and the input-output pairing in the design of a control system for a class of Extractive Dividing Wall Distillation Columns is addressed through a parameter called Operability Index. The configured control system is equipped with PI conventional controllers, which all of them are simultaneously and systematically tuned through relationships coming from a pole-stable assignment approach. By considering the bioethanol dehydration as a case of study, known for its difficulty in achieving high purities, the control system’s performance is illustrated. The advantage of this control system design way lies on: (1) the Operability Index smartly provides the physical perception of the control effort that the input will likely perform to cause a change to the related output, (2) a systematic tuning framework that sidestep the typical procedure in distillation columns of tuning the controllers in a sequential way, and (3) the simplest calculations on the basis of the simplest parameters that can be identified in a complex process as a distillation column is: static gains, nominal value of the inputs and the one of the outputs, and settling times.

An Effective Temperature Control Method for Dividing-Wall Distillation Columns

Temperature control is widely perceived to be superior to direct composition control for the control of dividing-wall distillation columns (DWDCs) due to its advantages in dynamic characteristics. However, because of the limited estimation accuracy to the controlled product purities, the former cannot eliminate the steady-state errors in the maintained product purities as completely as the latter. In order to reduce the steady-state deviations in the maintained product purities, an effective temperature control method is proposed in the current article by means of a kind of simple but effective product quality estimator (PQE). For the proposed PQE, temperatures of three stages located in the controlled column section (TI1, TI2, and TI3) are employed as inputs, and a linear sum of these three inputted stage temperatures (α × TI1 + β × TI2 + γ × TI3) is given as output. A genetic algorithm with an elitist preservation strategy is used to optimize the locations of the three stage temperatures and the values of α, β, and γ to ensure the estimation accuracy of the PQE. Concerning the controls of two DWDCs, i.e., one Petlyuk DWDC separating an ethanol/propanol/butanol ternary mixture and one Kaibel DWDC separating a methanol/ethanol/propanol/butanol quaternary mixture, the effectiveness of the PQE is assessed through comparing the performance of the temperature inferential control scheme using the PQE and the double temperature difference control scheme. According to the dynamic simulation results obtained, the former control scheme displays not only smaller steady-state deviations in the maintained product purities, but also better dynamic characteristics as compared with the latter control scheme. This result fully demonstrates that the proposed PQE can be a useful tool for the temperature inferential control of the DWDC.

Analysis and intensification of energy saving process for separation of azeotrope by ionic liquid extractive distillation based on molecular dynamics simulation

• IL mixed entrainer was firstly used in the separation of alkyl-ester azeotrope. • The proportion of mixed entrainer was determined from microscopic mechanism. • The energy-saving intensification of ED with mixed entrainer strategy was studied. • The TAC of VRAED with mixed entrainer can be saved by 10.81%. • The environmental performance of VRAED with mixed entrainer can be reduced by 21.03%. N-heptane and ethyl acetate are important solvents for chemical intermediates such as hexanitrohexaazaisowurtzitane. Based on molecular dynamics simulation, the interaction between mixed extractant and azeotrope was discussed through microscopic mechanism, and the optimal mixing ratio was determined. Based on the process, the influence of different ratio on the separation effect was further verified. It is found that the increase of the proportion of organic solvents can effectively promote the interaction between ionic liquids and esters. In addition, the separation processes of common distillation, extractive dividing wall column distillation and vapor recompression assisted extractive distillation were studied. Based on the sequential iterative algorithm, the process was optimized with the minimum annual total cost as the objective function, and the economy of vapor recompression assisted extractive distillation with mixed entrainers was the best. Compared with traditional organic solvent extractive distillation, the annual total cost can be reduced by 10.81%. According to the optimization results, the economic performance of carbon dioxide, nitrogen oxides and sulfur dioxide emissions was calculated. Through calculation, the technology of vapor recompression assisted extractive distillation with mixed entrainer is significantly better than other processes, and the gas emission can be reduced by 21.03%. In conclusion, the use of mixed entrainer in extractive distillation process can greatly improve the energy-saving effect than a single entrainer. This provides some guidance for industrial separation of other azeotropes.

Simulation study of the production of high purity ethanol using extractive distillation: Revisiting the use of inorganic salts

High purity ethanol can be obtained using extractive distillation with several entrainers; for instance, organic solvents, ethylene-glycol, glycerol, and ionic liquids. However, some of these entrainers are considered toxic; for that reason, in this paper, pilot conventional and dividing wall distillation columns are simulated to test the separation of the ethanol-water mixture using inorganic salts as entrainers for an experimental afterwards evaluation, considering the operational requirements.First, the electrolyte-NRTL model was selected, and the binary interaction parameters were obtained by using reported experimental vapor-liquid equilibrium data. With the selected thermodynamic model, the best operating conditions, of an existing distillation column, are determined by simulation considering several inorganic salts, resulting the calcium chloride as the best option for obtaining high purity ethanol with the lowest energy requirement in the reboiler.Finally, a pilot dividing wall distillation column was simulated for the dehydration process using a mixture of ethanol-water-calcium chloride as entrainer. The results show that it is possible to obtain high purity ethanol (mass purity around 0.99) using the pilot dividing wall distillation column. Also, calcium chloride, with 16% wt in a mixture of ethanol-water, can be reconsidered in extractive distillation processes for the production of high purity ethanol.

CFD study of oscillatory motion effect on liquid flow distribution into structured packed divided wall column

Packed distillation columns are key equipment in natural gas pre-treatment process within LNG production process train operated at offshore. In this work, the impact of offshore operating conditions and that of divided wall column shape were explored via developing transient three-dimensional CFD model for a divided wall distillation column filled with structured packing. Inter-phase drag force, porous resistance force, mass transfer, and liquid dispersion were incorporated into volume averaged equations to map the distortion in liquid distribution in response to column oscillatory motion. The results show that introducing a dividing-wall into distillation column affects fluids partition and distribution. Column motions induce complex fluid flows in radial direction that result in oscillatory patterns around the steady-state solution of vertical column. Furthermore, liquid distributions in a divided wall column under influence of column motion differ with respect to column system positions throughout rotational axes, mass transfer also fluctuates according to column oscillations. A position with a dividing wall making an angle of 45˚ with rotational axis gives a scenario in which rolling motion resembles to pitching motion. The thereof position leads to moderate liquid maldistribution in comparison to liquid maldistribution from scenario with dividing wall parallel to rotational axis.

Validation of Differential Temperature Control for a Dividing Wall Distillation Column

A novel dividing wall distillation column (DWC) control strategy was investigated using pilot plant experimentation and commercial scale column dynamic simulation. Results from both indicated that the differential temperature, PID based, control structure successfully rejected flow and composition disturbances. A four-component test system was utilized with three primary components and one trace component. Desired purities were achieved and steady state operation was maintained. The simulation results qualitatively matched the pilot plant data with discrepancies being attributed to equipment-scale dependent heat transfer effects.

Temperature control of a Kaibel, Agrawal and Sargent dividing-wall distillation columns

Intensified multi-partition dividing wall distillation columns (DWCs) have attracted the attention of academy and industry as they present benefits in the capital and operating costs over traditional and single-partition distillations systems. This work investigates and compares the economic and dynamic performance of commonly used four-product DWCs, i.e., Kaibel, Agrawal, and Sargent columns, to separate an industrially relevant mixture of benzene, toluene, xylene, and heavies. Optimal economic designs were obtained considering product specifications and compared in terms of the total annual cost (TAC). Remarkably, the Sargent column requires 45 and 28 % less energy requirements compared with the Agrawal and Kaibel columns, respectively, which translated into significant TAC savings. A dynamic study is also presented, considering conventional and differential temperature control schemes. The three systems' controllability is adequate as the proposed differential temperature control schemes allowed obtaining stable product specifications, close to their design values after the considered feed and composition disturbances. Also, the single and multi-partition DWCs can effectively handle disturbances without using vapor splits as manipulated inputs.

Some insights in experimental studies on the start-up operation of a reactive dividing wall column

The design, simulation, optimization and control of dividing wall distillation columns have been reported in several theoretical studies, and important energy savings are obtained. Also, important information has been reported in the experimental and industrial implementation of this complex distillation option; but for the case of the reactive option, the information is limited. As a result, in this paper, we report the start-up and continuous operation of a reactive dividing wall distillation column for the production of methyl oleate using an esterification reaction of methanol and oleic acid, catalyzed by sulfuric acid. This experimental DWDC has been used previously for the separation of reactive and azeotropic mixtures in batch operation. The experimental results can be useful for the practical implementation and operation of reactive dividing wall distillation columns.

Model predictive control of azeotropic dividing wall distillation column for separating furfural–water mixture

Dividing wall distillation columns (DWDCs) are effective technologies for distillation process intensifications. Azeotropic DWDCs (ADWDCs) are effective process intensification technologies to further intensify distillation process through combination of azeotropic distillation (AD) with DWDC. Investigations on the controllability and operation ability of ADWDC are mainly employing the traditional proportional–integral (PI) control schemes. Because of the inherent complicated configuration of ADWDC, the PI control leads to relatively poor dynamic performances. So as to enhance the dynamic performances, composition control scheme employing the advanced model predictive control (MPC) for an ADWDC separating furfural and water is studied in this paper. Although there are several studies employing MPC for DWDC, few investigation has been done on using MPC for ADWDC which is more interactive than DWDC. The dynamic results obtained in this paper using MPC are compared with the corresponding PI control scheme. Although the steady-state deviations and numbers of oscillations are similar employing PI control and MPC, the settling time and maximum deviations are smaller using MPC. MPC can considerably suppress the maximum deviation in face of +20% feed flow rate disturbance from -1.449% to -0.013%, and the corresponding settling time is reduced from 4.28 h to 2.20 h. These prove that MPC is more suitable than PI control for ADWDC. This paper provides new effective control scheme for the complex and highly interactive ADWDC, and verifies that MPC is a promising method for ADWDC to provide better dynamic performances with reduced maximum deviations and shortened settling time.

Enhanced temperature difference control of distillation columns based on the averaged absolute variation magnitude

Temperature difference control (TDC) schemes can clearly suppress the adverse influence of pressure variations on product quality control of various distillation columns (DCs) by employing temperature differences (TDs) between the sensitive stage temperature (TS) and reference stage temperature (TR), i.e., TS − TR, to infer the controlled product qualities. However, because the TDC scheme has failed to specially take the corresponding relationship between the TD employed in each control loop and the controlled product quality into account, it may suffer from relatively large steady-state errors in the controlled product qualities. To address this problem, an enhanced TDC (ETDC) scheme is proposed in the current article, in which an enhanced TD (ETD), i.e., TS − α × TR, is employed to replace the conventional TD for each control loop. While the locations of the sensitive and reference stages of the ETD are respectively determined according to sensitivity analysis and SVD analysis, the adjusted coefficient α is set to be the ratio between the averaged absolute variation magnitudes (AAVMs) of the TS and TR so that the relationship between the TS and TR can be appropriately coordinated. With reference to the operations of three different distillation systems, i.e., one conventional DC distilling an ethanol (E)/butanol (B) binary mixture, one conventional DC distilling an E/propanol (P)/B ternary mixture, and one dividing-wall distillation column distilling an E/P/B ternary mixture, the performance of the ETDC scheme is assessed by compared with the conventional TDC scheme and the double TD control (DTDC) scheme. The dynamic simulation results show that the ETDC scheme is better than the conventional TDC scheme with reduced steady-state errors in the controlled product qualities and improved dynamic responses, and is comparable with the DTDC scheme despite the less temperature measurements are employed.

Energy-saving heat integrated extraction-azeotropic distillation for separating isobutanol-ethanol-water

In the production of isobutyl alcohol by hydroxyl synthesis process, a large number of industrial effluent rich in isobutyl alcohol and ethanol would be produced, and the direct discharge could cause double harm to resources and environment. Therefore, it has important economic and environmental value to develop efficient and energy-saving separation methods to realize the recovery of high value-added chemicals. Based on the conventional four column extractive distillation scheme (CED), this paper developed a more energy-saving extractive-azeotropic distillation alternative (EAD), which reduced energy consumption and TAC by 16.14% and 22.25%. Then, the steam recompression and variable pressure heat exchange were introduced to complete the heat integration strengthening design (HI1-EAD, HI2-EAD), which saves 54.32% and 54.36% energy consumption respectively. In order to further improve the energy efficiency, the dividing wall column distillation strengthening configuration (EADWC) was embedded into the EAD scheme. Compared with the CED scheme, TAC of the EADWC scheme and its heat integration configuration (HI-EADWC) decreased by 23.21% and 36.90%, respectively. However, the high bottom temperature of the main column leads to a significant increase in the duty and equipment investment, which makes the HI-EADWC scheme not show enough attraction. Through comprehensive evaluation, the HI2-EAD alternative presents more application potential.

Vapor recompressed dividing-wall distillation columns: Structure and performance

Abstract Due to the topological structure of double columns and multiple separating sections in dividing-wall distillation columns (DWDCs), the development of vapor recompressed dividing-wall distillation columns (DWDC-VRHPs) represents a challenging issue with great complexities and tediousness. For the separations of light-component dominated and wide boiling-point ternary mixtures, because the purification of the light-component from the intermediate- and heavy-components incurs the primary energy dissipation, the application of vapor recompressed heat pumps (VRHP) should be aimed to reduce the irreversibility and this leads to the generation of the optimum topological structures of the DWDC-VRHPs, i.e., a DWDC plus a two-stage VRHP. The first-stage VRHP is to preheat feed, not only taking the advantages of the small temperature elevation available but also favoring the mass transfer between the vapor and liquid phases through feed splitting. The second-stage VRHP is to reduce further separation irreversibility. The philosophy can be applied to any DWDCs no matter where the dividing wall locates. Two case studies on the separations of ternary mixtures of benzene, toluene, and o-xylene and n-pentane, n-hexane, and n-heptane demonstrate the economic optimality of the proposed DWDC-VRHPs and reveal the inherent interplay between internal and external process integration.