Conventional Distillation Sequences Research Articles

Impact of liquid split ratio on separation performance of a multiple dividing wall column pilot plant

Multiple dividing wall columns are a promising intensified process to reduce energy consumption of conventional distillation sequences by up to 55 %. Their operation is of course more complex, in particular with regard to the internal split ratios of the liquid and vapor flows at the dividing walls. Theoretical studies have shown that the split ratios may not need to be at a specific value but in a certain range, which would simplify operation. However, until recently no experimental setup existed to prove this observation in reality. This work presents the first comprehensive experimental study in this regard obtained from an approximately 10 m high multiple dividing wall column pilot plant. The plant has two dividing walls, so the column consists of a prefractionator, a middle and a main column. The results validate the theoretical observations. With the chosen mixture in particular the middle column is found to be operable in a certain range. Temperature profiles prove that internally different separations are performed, while the resulting product compositions are unaffected.

Automated hybrid distillation sequence synthesis framework of mixed azeotropes and nonazeotropes

Distillation technology is a crucial, energy-intensive component of energy-supply systems, thereby motivating the improved design of distillation systems. This study investigated an automated framework for synthesizing hybrid distillation sequences, providing a systematic approach for the design and optimization of distillation sequences involving azeotropes. The proposed method separated mixed azeotropes and nonazeotropes by replacing the simple columns in the superstructure of a conventional distillation sequence with extractive distillation columns. The automated calculation of the superstructures was implemented by calling the external interface of the process simulator. The results of these calculations were used to formulate a mixed-integer linear programming problem that was optimized with the objective function of the total annualized cost (TAC). Finally, the optimal solution and all feasible alternatives were obtained. Moreover, the reliability and efficiency of the proposed framework were demonstrated through two case studies. The results show that the separation position of the azeotrope in the sequence and the presence or absence of heat integration significantly impact the TAC of the system. However, using an existing component versus an external component as an entrainer makes very little difference in the TAC for the optimal sequences.

The start-up of a multiple dividing wall column – A theoretical and experimental study

Multiple dividing wall columns are a promising extension of the concept of dividing wall columns offering energy savings of up to 55 % compared to conventional distillation sequences for four products. This paper presents the first systematic study on the start-up of a multiple dividing wall column. The study aims to develop a start-up strategy for the first pilot multiple dividing wall column worldwide commissioned at Ulm University. The start-up process is investigated using a dynamic simulation model that facilitates the investigation of different start-up strategies and the influence of different factors on the start-up process. Based on these dynamic simulations a viable start-up procedure for the column is found. Through improvements to the procedure a significantly shortened start-up process is achieved. The dynamic response of the column shows that the cold and dry plant is able to reach the desired operating point within few hours. Simulation results are confirmed with experimental data from the pilot column.

Separation of Ternary System 1,2-Ethanediol + 1,3-Propanediol + 1,4-Butanediol by Liquid-Only Transfer Dividing Wall Column

This study focuses on separating a mixture consisting of 1,2-ethanediol (1,2-ED), 1,3-propanediol (1,3-PD), and 1,4-butanediol (1,4-BD). Vapor–liquid equilibrium (VLE) data for 1,2-ED + 1,4-BD and 1,3-PD + 1,4-BD are determined at 101.3 kPa using a modified Rose equilibrium still. The consistency of the VLE data is checked with both Redlich–Kister and Fredenslund tests. The VLE data are fitted by the Wilson, NRTL, and UNIQUAC activity coefficient models. All three models can effectively correlate the VLE data. Then, the separation of the mixture is designed with the NRTL model and its correlated binary interaction parameters. A liquid-only transfer dividing wall column (LDWC) is investigated on the basis of a direct conventional distillation sequence (DCDS). For a fair comparison, both DCDS and LDWC are optimized to minimize total annual cost using sequential iterative optimization procedures. After optimization, LDWC exhibits a 16.87% reduction in total annual cost, while cooling and heating utility consumptions are reduced by 28.40% and 19.24% compared to DCDS.

Energy, exergy, economic, and environmental analysis of a novel liquid-only transfer dividing wall column with vapor recompression

The Kaibel dividing wall column (KDWC) uses a thermal coupling technology for separating quaternary mixtures, which exhibits less energy consumption and more economic benefits than a conventional distillation sequence (CDS). However, it is hard to control the vapor split ratio, which limits the industrialization of KDWC. The liquid-only transfer dividing wall column (LDWC) doesn’t involve the vapor split ratio. It has been verified as equivalent to KDWC. To improve LDWC’s performance further, four novel vapor recompression-assisted LDWCs are proposed according to the column grand composite curve analysis. And CDS and LDWC are taken as the base cases for comparison. Then, the processes are optimized by the sequential iterative optimization procedures to minimize the total annual cost. The performance of optimized processes is systematically evaluated through energy, exergy, economic, and environmental (4E) analysis. The results demonstrate that the SCVRC-IR-LDWC2, equipped with a side condenser and intermediate reboiler involved in lower vapor side stream withdrawal, performs well in the total annual cost, energy consumption, gas emissions, and thermodynamic efficiency.

Transforming conventional distillation sequence to dividing wall column: Minimizing cost, energy usage and environmental impact through genetic algorithm

This work aims at proposing a multi-objective optimization (MOO) strategy for dividing wall column (DWC) that can provide energy-efficient and cost-effective operation for ternary feed mixtures compared to its conventional two-column distillation sequence. Today, DWC has emerged as a promising industrially relevant thermally integrated configuration. If its design and operating conditions are truly optimized, the cost can further be reduced. At the same time, it is also equally, if not more, important for a distillation column to consider its product purity, productivity and environmental standard. With this, a multi-objective genetic algorithm based optimization framework is formulated to include all these crucial factors. Due to the presence of various conflicting objectives, multiple optimal solutions are inevitable. With this, the proposed optimization strategy works with the three steps: (i) sensitivity analysis to find the decision variables, (ii) a non-dominated sorting genetic algorithm (NSGA-II) to generate multiple optimal solutions, and (iii) the technique for order of preference by similarity to an ideal solution (TOPSIS) method to select one optimal point. Further, the Aspen Plus flowsheet simulator is integrated with MATLAB to simultaneously perform extensive simulation and optimization. This potential integration leads to reducing the total run time substantially. Finally, two industrially relevant ternary systems, namely Benzene-Toluene-Xylene (BTX) and Benzene-Toluene-Ethylbenzene (BTE) are used to illustrate the proposed strategy for investigating the performance improvement of the DWC over its conventional distillation sequence (CDS). Simulation results show that the DWC can secure up to 23% savings in capital cost, 45% in operating cost and reduce the rate of carbon dioxide emission by about 45%.

Design and Control of Extractive Dividing Wall Column for Separating Dipropyl Ether/1-Propyl Alcohol Mixture

The focus of this work is the study of the extractive dividing wall column (EDWC) for separating the azeotropic mixture of dipropyl ether and 1-propyl alcohol with N, N-dimethylacetamide (DMAC) as the entrainer. Three separation sequences are investigated, including a conventional extractive distillation sequence (CEDS), EDWC and a pressure swing distillation sequence (PSDS). The static simulation results showed that the EDWC with DMAC as the entrainer is more economically attractive than CEDS and PSDS. Subsequently, a control structure CS1 based on a three-temperature control loop and a control structure CS2 with the vapor split ratio as the manipulated variable are investigated for the EDWC. Their dynamic control performances are evaluated by facing large feed flow rates and composition disturbances. The results showed that the CS1 can deal with feed flow rate disturbance effectively. However, the transient deviation is large and the settling time is too long when facing feed flow composition disturbances. The CS2 can quickly and effectively deal with feed flow rate and composition disturbances, and it can maintain the two products at high purity.

Synthesis framework for distillation sequence with sidestream columns: Application in reaction-separation-recycle system

In this work, a comprehensive design method for three-section sidestream columns along with a novel State Task Network representation including both conventional and sidestream distillation sequence configurations is present. The distillation network is embedded in the superstructure-based framework for reaction-separation-recycle process synthesis to explore the mass interaction in different sub-systems. In the superstructure, complete interconnections among reactor modules as well as necessary connections between reactor modules and distillation columns are involved to guarantee opportune coupled degree. Based on this, component distribution in distillation through Underwood equations is introduced to provide desired products with required purity and multiple component mixtures as feedback to the reactor network, achieving the optimization of mass interaction in the whole flowsheet. Furthermore, Fenske-Underwood-Gilliland shortcut method to design columns with and without sidestream, and finite element with orthogonal collocation approach to design PFRs are employed, so that reasonable tradeoff between model accuracy and solution difficulty is achieved. Finally, two literature examples are performed to demonstrate the feasibility and validity of the proposed methodology.

Design and comparison of conventional and side-stream extractive distillation sequences for separating the methanol-toluene binary azeotrope with intermediate boiling entrainer

The side-stream extractive distillation (SSED) system is attracting widespread attention due to its high eco-efficiency. In the article, three double-column SSED (DC-SSED) sequences and two single-column SSED (SC-SSED) sequences with different side-stream extraction locations are proposed for separating the methanol-toluene azeotrope with intermediate boiling entrainer triethylamine, which are compared with two conventional extractive distillation (CED) processes including direct and indirect CED sequences. The genetic algorithm is employed to optimize seven extractive distillation sequences to minimize the total annual costs (TAC). CO2 emissions and thermodynamic efficiency of the optimized processes are quantified as well as TAC to calculate the Eco-efficiency Comparison (ECI) Index for eco-efficiency analysis. The results illustrate that two DC-SSED sequences demonstrate the eco-efficiency superiority compared to the optimal CED process (direct CED), and their side-streams are located in the rectifying section and extractive section of the side extractive distillation column, while their ECI indexes reach 97.38% and 97.60%.

Optimal design of Kaibel dividing wall columns based on improved particle swarm optimization methods

Dividing wall columns (DWCs) are able to reduce operating costs and save capital costs for distillation columns, which verifies that DWC is a useful strategy in terms of distillation process intensification. DWCs are better choices than the corresponding conventional distillation sequences from both economical and environmental point of views. As DWC saves energy considerably and reduces CO2 emissions, it proves to be a breakthrough towards sustainable distilling. The four-product Kaibel DWC can further intensify the distillation process, and save energy cost about 40%. However, due to the complicated structures and strong interactions, the optimal design of the four-product Kaibel DWC is challenging to solve through conventional optimization methods. Therefore, this paper investigates the applicability of probabilistic global optimization algorithms, including standard particle swarm optimization (PSO), chaotic PSO, cellular PSO, and quantum-behaved PSO algorithms. This paper aims to explore the most appropriate improved PSO algorithm for the Kaibel DWC. The standard PSO and chaotic PSO show the premature convergence in the optimization process, while the cellular PSO and quantum-behaved PSO can prevent premature convergence. The chaotic PSO algorithm is more appropriate for the optimization of the continuous function, and it is less suitable for the optimization of the Kaibel DWC. The quantum-behaved PSO algorithm is relatively not appropriate for the optimization of the Kaibel DWC because of the long calculation time for rigorous simulation. The cellular PSO algorithm, based on the concept of cellular neighborhood, is the most appropriate algorithm for the optimization of the Kaibel DWC.

Optimal design and effective control of Kaibel column with liquid-only transfer streams for quaternary distillation

The Kaibel column (KC) is considered to be energy effective and capital economical replacement of conventional distillation sequences for quaternary distillation. However, the industrialization of such a structure is still hesitant due to increased complexity and related uncertainties. Compared to the highly integrated KC, distillation configurations with liquid-only transfer streams (LTS) are more easily understandable and acceptable to the industrial community. A KC-LTS configuration is therefore investigated through an optimal design and controllability comparison to the conventional direct sequence (DS). When separation of aromatics is proposed as a case study, the results demonstrate the economic superiority of KC-LTS to DS in terms of the total annual cost. Considering the controllability using composition controllers, the former structure performs as favorable as the latter when tackling a ±20% step changes in throughput and feed composition. An inferential temperature control strategy is also established for KC-LTS. Good controllability is generally realized in most cases at handling relatively small (10%) throughput and feed composition disturbances, except for large product offsets when resisting large (20%) feed perturbations. To further improve its dynamic control effectiveness, an intensified control option with three composition control loops is successfully proposed to handle 20% step feed disturbances. Overall, this work expands LTS to quaternary distillation, suggesting a competitive substitute of the well-known KC for both academic and industrial purposes.

Enhanced separation of mixing diols by distillation in dividing wall column

Dividing wall column (DWC) was used to separate the system (1,2-propylene glycol + 1,3-butanediol + 1,4-butanediol). No research in a DWC has focused on such systems which were difficult to be separated for their great similarity in both molecular structures and properties. The rigorous steady-state and dynamic simulations, as well as the optimization of DWC alternative were performed with Aspen Plus and Aspen dynamics. Sequential quadratic programming (SQP) optimization method and the efficient sensitivity analysis tool were used to minimize the total reboiler and condenser duties. Moreover, a flow rate-composition cascade control structure was first developed to maintain targeted product purity under various external disturbances as feed flow rate and side product composition changed. DWC was compared with both a side rectifier configuration and conventional two-column distillation sequence in energy requirements and total annual costs (TACs) with the same number of stages. It showed that the energy requirements and TAC by DWC were reduced 47.60 % and 35.40 % respectively, compared with conventional distillation sequence, energy savings of 48.00 % and 34.11 % lower TAC compared with side rectifier configuration. This study provided a potential solution for the distillation of the systems with similar molecular structures and properties in a DWC.

Comparing Composition Control Structures for Kaibel Distillation Columns

Although Kaibel distillation columns are superior to conventional distillation sequences owing to smaller equipment investment and operation cost, they display high nonlinearity and this greatly increases the difficulty of achieving their tight control. To overcome this problem, four decentralized composition control structures, i.e., the CSR/QR, CSR/B, CSD/QR, and CSD/B structures, are proposed and compared based on the control of a Kaibel distillation column fractionating a methanol/ethanol/propanol/butanol quaternary mixture. These four composition control structures all include five composition control loops. While the four of them are employed to maintain the purity of the top, upper sidestream, lower sidestream, and bottom products, the remaining one is employed to minimize the energy consumption of the Kaibel distillation column by maintaining the composition of propanol at the first stage of the prefractionator. Dynamic simulation results show the CSR/QR and CSR/B structures can tightly maintain the purity of the controlled products with a small overshoot and short settling time after facing various disturbances in feed conditions, but the CSD/QR and CSD/B structures lead to oscillatory responses (the latter even shows divergent responses under individual disturbances). At the end of the article, some effective guides for developing composition control systems are given.

Feasibility study on replacement of atmospheric distillation column with new sequences in a natural gas condensate refinery

The atmospheric distillation unit is the main separation unit in crude oil and natural gas condensate refineries. An atmospheric distillation column in a natural gas condensate unit usually consists of a single column with 3 pump-arounds and 2 side strippers. In this work a new column sequences have been synthesised for the first time using the modified separation matrix method and the results had been compared to other well-known configuration through economic, exergy and exergoeconomic analysis. The results illustrate that the conventional atmospheric distillation configuration has a medium performance compared to other sequences. As an example, comparing the results obtained from the direct sequence, to the conventional distillation sequence, leads in almost 30% less annual cost, 54% less exergy loss rate and 5% more exergoeconomic factor. However, the 84.87% exergoeconomic factor for the conventional configuration could justify the application of conventional sequence in the refinery.

Controllability Comparison of the Four-Product Petlyuk Dividing Wall Distillation Column Using Temperature Control Schemes

An effective process intensification strategy based on dividing walls shows promising energy-saving results for distillation processes. The three-product Petlyuk dividing wall distillation columns (DWDCs) are able to save approximately 30% energy in comparison with the traditional distillation columns. Furthermore, the four-product extended Petlyuk DWDC reduces about 50% of operation costs than conventional distillation sequences. Although researchers have extensively studied control schemes for the three-product Petlyuk DWDC, relatively little work has been done on the four-product extended Petlyuk DWDC. This paper studies feasible temperature control schemes containing temperature control scheme (TC), simplified temperature difference control scheme (STDC), and simplified double temperature difference control scheme (SDTDC) for the four-product extended Petlyuk DWDC. STDC and SDTDC are introduced so as to improve the dynamic performances with simple control schemes. All three control schemes are tested against a series of feed compositions and feed rate disturbances. Dynamic performances prove that the proposed STDC and SDTDC schemes are better at handling the inserted feed disturbances. These are very encouraging results for industrialization of the four-product extended Petlyuk DWDC in the future.

Multi-objective optimization of intensified processes for the purification of levulinic acid involving economic and environmental objectives

Levulinic acid obtained from acid hydrolysis of lignocellulosic biomass is considered within the twelve main chemicals from biomass in terms of economic potential due to its large number of applications. As product of hydrolysis is obtained a quite diluted stream which contains a lot of water. Consequently, the processing cost is high and it has limitations for further scaling up at industrial level using conventional separation schemes. In the downstream process proposed in this work, initially the water content is removed using a liquid-liquid extractive column, then it is purified by means of a distillation columns-based process. Recently, a set of separation designs has been implemented, including the use of decanters and intensified columns, which has reduced the cost of the process. However, these studies have only focused on optimizing the total annual cost, losing sight of the environmental impact of the process. In addition, some improvements may be applied from the process intensification point of view either by thermally couplings or considering a single or multiple walls in a column. Therefore, in this work four LA purification designs are studied, a conventional distillation sequence, a dividing wall column arrangement and a decanter, a dividing wall column configuration with a decanter and thermal coupling, and two dividing wall column and decanter. Those schemes were designed and optimized under a rigorous optimization process by means of a multi-objective hybrid algorithm, differential evolution with tabu list considering two objectives: 1) the total annual cost as the economic criterion and 2) the eco-indicator 99 as the environmental index. The results indicated that the intensified design (using a single dividing wall column) presents better results, with economic savings about 8.42% and a decrease in 10.94% the Eco-indicator 99 in comparison with the optimal conventional sequence.

Intensifying Kaibel dividing-wall column via vapor recompression heat pump

The four-product Kaibel dividing-wall column (KDWC) can save about 40% energy compared with the conventional distillation sequences. Nevertheless, the energy efficiency of KDWC are likely to be increased further by employing vapor recompression heat pump (VRHP) technology. The focus of this paper is to explore effective configurations to intensify the four-product KDWC via vapor recompression heat pump. Separation of two kinds of mixtures including n-pentane, n-hexane, n-heptane and n-octane and benzene, toluene, o-xylene and tri-methyl-benzene are chosen as illustrative cases. The standard vapor recompression heat pump assisted KDWC configuration (KDWC–SVRHP) and the vapor recompression heat pump assisted KDWC configuration with an intermediate reboiler (KDWC–VRHP-IR) are proposed in this paper and compared with the conventional distillation sequences and the original KDWC configuration. The feasibility and effectiveness of intensifying KDWC via vapor recompression heat pump technology is verified and the energy saving ability is explained by the exergy loss analysis. Results prove that KDWC–VRHP-IR is able to substantially reduce the operating cost of KDWC.

Optimal Operation of a DWC by Self-Optimizing Control: Active Vapor Split Approach

Dividing Wall Column(DWC) offers the large potential for operating and capital cost saving in compared with conventional distillation sequence. In the studied DWC in this study, the aid of Vmin diagrams, it is shown that without a suitable value for vapor split fraction bellow the dividing wall in different operating conditions, the energy requirement increases from optimal value and it will lead to the suboptimal operation. Accordingly, a control structure based on the self-optimizing concept is designed using vapor split fraction as the control degree of freedom (active vapor split). To find the best self-optimizing Controlled Variables (CV) the exact local method is used. It is shown that the value of loss with the aid of active vapor split is lower than 0.7 percent of nominal value with using the conventional single measurement self-optimizing CV, which is a reasonably small value, and using a more complex combination of measurement as a self-optimizing CV provides a little enhancement in reducing the loss. So, including vapor split fraction in the self-optimizing control structure of the DWC can save energy and in the meantime avoid the complexity of combination of measurement. Moreover, the dynamic simulation studies show that the proposed control structure with the simple decentralized control loops can conveniently stabilize the plant and reject the effects of disturbances.

A cost-effective retrofit of conventional distillation sequence to dividing-wall prefractionator configuration

A dividing-wall prefractionator configuration was investigated for a safe and economic retrofit of the conventional sequence using simple distillation columns. In the proposed retrofit configuration, the first column was modified to a dividing wall column as a prefractionator to supply prefractionated multi-feeds to the subsequent column. To investigate the effectiveness of the proposed configuration, nine near-ideal feed mixtures were considered for analysis. The proposed configuration was then compared with several other alternative configurations. The proposed dividing-wall prefractionator efficiently generates prefractionated multi-feed streams avoiding feed mismatch and remixing effect with low modification cost. Moreover, because the proposed retrofit configuration allows for flexible switching between the dividing-wall prefractionator and the conventional operating mode, a safe retrofit is also ensured by reducing the operational risks. Several industrial retrofit cases were studied to validate the proposed dividing-wall prefractionator configuration.

Design and Control of Reactive Distillation Sequences with Heat-Integrated Stages To Produce Diphenyl Carbonate

Reactive distillation (RD) in quaternary systems has gained importance when one of the reagents is in excess. In this work, higher energy savings in a reactive distillation sequence is addressed to produce diphenyl carbonate, which is a crucial precursor of polycarbonate. Energy savings were attained through heat integration between the high-pressure RD column and the low-pressure separation column. The design of sequences with heat-integrated stages was done by combining simulation and optimization. The sequence with one heat-integrated stage realized the minimum energy consumption with energy savings around 22% and cost savings around 12% in comparison with the conventional reactive distillation sequence. Also, two control schemes for the best design configuration have also been studied in this work. The results showed that the control scheme that manipulates the reboiler duty and reflux ratio to control two stage temperatures can provide excellent responses under throughput and feed composition disturb...