Feed Composition Disturbances Research Articles

Dynamic control investigation of an energy-saving double-thermally coupled ternary extractive distillation process

Currently, little literature has focused on the dynamic control of the economic and energy-saving double-thermally coupled ternary extraction distillation (DTCTED) systems, which demonstrates difficulties due to complex interactions between the main column and two rectifiers. The novel contribution of this paper is proposing effective and robust control structures to improve the dynamic controllability of the DTCTED process. To maintain the product purities more precisely, the key control loops employ composition-temperature cascade controls to regulate the side-vapor flowrates because the reliance on temperature control alone (CS1) is insufficient. Although this structure (CS2) can achieve the control of product purity, there could be severe oscillation in the control process. Therefore, one of the composition-temperature cascade controls is paired with the reflux ratio of the side rectifier to relax the operating freedom of the main column. The obtained structure (CS3) can brilliantly handle ±20 % throughput and various feed composition disturbances, which ensures the product purities and reduces oscillation. Additionally, control strategies based on the relative gain array (RGA) pairing criterion are explored and compared with the empirical ones. The proposed structures significantly enhance the controllability and stability of the DTCTED process, making it highly suitable for industrial applications.

Dynamic control analysis of energy-efficient side-stream extractive distillation process for separating close-boiling mixture

To separate the close-boiling mixture comprising heptane and toluene, this study introduces an environmentally-friendly method of side-stream extractive distillation by utilizing aniline as the separating agent. This approach modifies traditional extractive distillation by adding a side rectifier, converting two-way vapor and liquid communication into one-way liquid transfer using an additional distillation section along with corresponding reboiler. Our present study investigates the dynamic control behavior for a strongly non-ideal system without and with heat integration. It focuses on achieving stable and reliable regulatory control by closely keeping the dual-product composition to their original design standards. Key control loops are pinpointed, with a specific emphasis on those centered around adjusting reboiler heat duty and side-stream flow rate within extractive column C1 to regulate the temperatures of Stages 48 and 36. Furthermore, control effectiveness is enhanced by introducing a feedforward action of reboiler heat duty-to-feed flowrate. By developing a new and robust control architecture, high feed flowrate and composition disturbances can be managed efficiently, solving the issues of oscillation and large transient response.

Process intensification and dynamic control of a vapor-recompression-assisted heterogeneous azeotropic distillation for separating N-butanol/water

An ecologically beneficial process of vapor-recompression-assisted distillation was proposed, based on the conventional heterogeneous azeotropic distillation, for separating n-butanol and water. The heat integration was also adopted to improve the energy utilization rate of the process. The sequential iterative approach optimized the parameters relating to all those processes. The heat-integrated vapor-recompression-assisted heterogeneous azeotropic distillation arrangement reflects its economic superiority, for reducing total annual cost by 15.121 % and CO2 emissions by 78.860 % and enhancing second-law efficiency by 102.156 %. An exergy analysis was performed on the intensified processes, which showed that the exergy increases in the configuration coupled with vapor recompression and heat integration were higher than those with only vapor recompression. Dynamic control characteristics were investigated for these three intensified configurations, and both product compositions were well controlled when confronting 20% production rate and feed composition disturbances, and no composition measurement loops were noted in the proposed control schemes.

Novel dynamic control structure of reactive distillation process for isopropanol production via transesterification

Due to the complicated interaction between reaction and separation in a column, it was inherently challenging to design a control structure for the reaction distillation process. In this research, the control structures of the hybrid reactive distillation process combining pressure-swing distillation (RDC-HDC) and its thermal coupling (RDWC-HDC) processes, were explored for producing isopropanol (IPA) by isopropyl acetate (IPAC) with methanol (MeOH) transesterification. The control performances of only temperature control and temperature difference control schemes for RDC-HDC process were first investigated. The research results displayed that double temperature control (CS3) and temperature difference control schemes (CS4–1, CS4–2 and CS5) for the RDC-HDC process could shorten the maximum transient deviation, amplitude of oscillations and setting time in face with ±20% throughout and ±5% feed composition disturbances. To further improve control characteristics and make control structures concise, a novel control strategy (CS6) by a proportional controller adjusted by reverse action of an IPAC component controller in 34 trays for keeping IPAC composition at a certain range in the RDC-HDC process was developed based on the closed loop analysis before and after disturbance. The application of the innovative control structures (CST4) to the RDWC-HDC process was equally feasible in face of ±10% throughout and ±5% feed composition disturbances. The analysis of integral of squared error (ISE) indicated that a novel control strategy (CS6 and CST4) had better robustness and dynamic performance. These studies contributed to the development of controllability for hybrid reactive distillation processes to produce chemicals for reaction and separation.

Dynamic control strategy of the ternary azeotrope separation process assisted by reactive-extractive distillation for ethyl acetate/isopropanol/water

The double-column reactive-extractive distillation (DCRED) process is a promisingly intensified compactness configuration for separating ternary water-containing azeotrope(s) by introducing the external reactions, such as the typical common adopted epoxy ethane (EO) hydration reaction. However, limited investigation has focused on the dynamic control analysis for the highly strong coupled DCRED process. In this paper, the efficient and robust control structure is proposed to achieve the dynamic operation of the DCRED process for the system of ethyl acetate/isopropanol/water. The composition controller is applied to precisely control the feed ratio of EO and water, thereby ensuring the qualified products when facing ± 10 % feed flowrate disturbances and ± 5 % feed composition disturbances. This work provides an alternative control strategy for the DCRED process, which is beneficial for its practical industrial applications.

Proportional-Integral Control and ARX-based Model Predictive Control for Extractive Dividing Wall Column

Extractive dividing wall column (EDWC) is a typical process intensification technology and possesses significant energy-saving prospects, while the poor dynamic controllability hinders its industrialization goal. In this work, taking the benzene and cyclohexene azeotrope separation as an example, a proportional-integral (PI) control structure based on four-point temperature control loops is proposed for EDWC by considering temperature difference as the discriminant criterion. Subsequently, to improve the control performance of EDWC, two model predictive control (MPC) structures based on auto-regressive with external input (ARX) model obtained from closed-loop identification and linear time-invariant state-space (LTI-SS) model, respectively, are studied for EDWC. The performance of three control structures is tested by ±20 % feed flow rate and composition disturbances, and further quantitatively evaluated by integral absolute error (IAE). The dynamic responses demonstrate that the ARX-based MPC structure achieves the best control performance in the presence of feed flow rate and composition disturbances, which can be attributed to the capability of the identified ARX model to track the dynamic features of EDWC accurately.

Dynamic control of an energy-saving process with two extractive dividing-wall columns for separation of acetone/methanol/butanone/tert-butyl alcohol mixtures

In the chemical and petrochemical industries, an azeotropic mixture of acetone/methanol/butanone/tert-butanol needs to be separated for high-purity products. Following a previous work in which an energy-saving process composed of two extractive dividing-wall columns (EDWCs) was developed for the quaternary system, this paper focuses on the issue of dynamic control for this process. Several control strategies in combined with temperature control, temperature-difference control and composition control are established to maintain the purity of all products in the face of disturbances in feed flow and feed composition. Dynamic simulations demonstrate that a conventional temperature control structure provides very poor control while a control strategy that used three-temperature-difference control for both EDWCs is the most effective.

Control structure selection of increased‐pressure extractive distillation process for DMC‐MeOH azeotropic mixture

AbstractProducing dimethyl carbonate (DMC) as a green chemical with the desired purity is important in the industry. Although studies on the steady‐state design of energy‐efficient extractive distillation processes are important for the purification of DMC‐methanol (DMC‐MeOH) azeotropic mixtures, the dynamic controllability of these processes is also critical in the case of feed condition changes, and it should be investigated carefully. Results of the limited studies in the literature show that changing the operating pressures in extractive distillation processes might have different effects on the dynamic controllability of different systems. Thus, in this study, alternative control strategies are developed for a recently proposed increased‐pressure extractive distillation process to separate DMC‐MeOH mixture. All control structures are designed using inferential temperature controllers, which have a general acceptance in industrial applications. Effects of different ratio controllers are investigated by evaluating the dynamic responses of control structures for disturbances in feed flowrate and composition. Two metrics including integral absolute error and steady‐state deviation of purities are used in the evaluation of alternatives. Results of dynamic simulations show that a control structure including reflux ratio controller is not a suitable strategy for this process. It is demonstrated that a control structure including reflux to feed ratio controller for both distillation columns is necessary for the robust and efficient control of a pressure‐increased extractive distillation process. These efficient dynamic results support the economic advantage of increased‐pressure extractive distillation process separating DMC‐MeOH azeotropic mixtures.

Design and control of an energy-efficient intensified process for biobutanol purification from isopropanol-butanol-ethanol (IBE) fermentation broth

Biobutanol is one of the most preferable alternative fuels because of its superior fuel properties compared to other biofuels. In recent years, biobutanol production through isopropanol (IPA)-butanol-ethanol (IBE) fermentation has gotten more attention than acetone-butanol-ethanol (ABE) fermentation. However, the downstream separation process of this production is very energy intensive due to the several azeotropes. In this study, an energy-efficient process configuration including a reactive distillation column is proposed to break the azeotropes. In the proposed configuration, water present in the azeotropic mixture is consumed by a water-ethylene oxide (EO) reaction. Steady-state results show that the proposed configuration provides a 30% decrease in energy consumption and a 26.7% reduction in total annual cost (TAC) compared to the configurations in the literature, while ethylene glycol is obtained as a by-product besides the biobutanol of 99.5% purity. Additionally, two plantwide control structures are developed, and the dynamic controllability of the configuration is examined. Dynamic results illustrate that a robust control is provided with the proposed control structure 2 (CS2) when it is tested against disturbances in feed flowrate and feed composition.

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.

Design of a multiproduct reactive distillation system for DEC and EMC production

Multiproduct processes are gaining interest as part of the development of intensified structures. In this work, an optimal design of a flexible diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) multiproduct process is developed. DEC is a main product that is commonly produced through multi-reaction dimethyl carbonate (DMC) transesterification. However, EMC, an intermediate product, also has a significant market interest. This work explores the use of a multiproduct process that integrates a reactive and a separation column to produce either DEC or EMC. The design was obtained aided by a process optimization task with genetic algorithms tied to Aspen Plus rigorous simulations. The results show that a common design for the production of both products can be obtained, with total annual cost savings of 22% with respect to a base design taken from the literature. Additionally, an initial dynamic analysis shows that the multiproduct process can be controlled in the face of feed flow and composition disturbances.

Heat integration and dynamic control of reactive pressure-swing distillation for separation of tetrahydrofuran/ethanol/water

In this paper, heat integration and dynamic control of reactive pressure-swing distillation (RPSD) for separating tetrahydrofuran/ethanol/water was investigated from the aspects of energy consumption, environmental protection and practical applications. By exploring the heat exchange between separating tetrahydrofuran and ethanol distillation columns, the full heat integration process (FHI) was designed. Robust control of heat and non-heat integrated processes was realized based on the dual temperature control structure. All processes were resistant to ±20% feed flow and composition disturbances. Finally, these processes were compared from three aspects: economy, environment and dynamic performance. The results show that compared with the non-heat integration process, the FHI process can reduce the total annual cost by 24.14%, the CO2 emission by 20.21% and the SO2 and NO emission by 34.38%, meanwhile, having the best dynamic performance. This study is valuable and instructive in promoting the integrated optimal and control design of the RPSD process.

Dynamic Control Analysis of Internally Heat Integrated-Top Dividing Wall Column

Although The Dividing Wall Column (TDWC) with a top partition is an effective alternative for the separation of ternary mixtures dominated by the lightest component, it can be further intensified by introducing the Internal Heat Integration-The Dividing Wall Column (IHI-TDWC). However, strong internal mass and energy integration complicates the process dynamics and influences the system controllability. In the current work, a kind of decentralized temperature control scheme involving four temperature control loops is developed. The separation of the ternary mixture with methanol, ethanol, and n-propanol is chosen as an illustrative example to evaluate the controllability of the IHI-TDWC through open-loop and closed-loop dynamic process simulation. The results show that the IHI-TDWC keeps good rejection of the throughput and feed composition disturbances.

Dynamic Controllability Study of Extractive and Pressure‐Swing Distillations for Tetrahydrofuran/Water Separation

AbstractControllability features of the extractive distillation process and the pressure‐swing distillation process were studied with dynamic tools and compared in the case study of separation of a mixture of 50 mol % tetrahydrofuran and 50 mol % water. Process simulations with Aspen Plus and Aspen Plus Dynamics were carried out to obtain 99.9 mol % product purity. Control structures for the two distillation techniques were determined. Load rejection studies were completed for ±10 % feed throughput and ±10 % feed composition disturbances, for both processes. The comparison of the dynamic responses, integral absolute errors, and integral square errors shows that the pressure‐swing distillation process has better controllability features than extractive distillation.

Novel Reactive Distillation Process for Cyclohexyl Acetate Production: Design, Optimization, and Control.

A side-reactor column (SRC) configuration, comprising a vacuum column coupled with atmospheric side reactors, is proposed to overcome the thermodynamic restriction in the esterification of cyclohexene with acetic acid to produce cyclohexyl acetate. Meantime, this configuration can avoid the utilization of the high-pressure steam and provide enough zone for catalyst loading. In order to obtain the minimum total annual cost (TAC), the process is optimized by a mixed-integer nonlinear programming optimization method based on the improved bat algorithm. The results indicate that the optimized SRC configuration saves about 44.81% of the TAC compared to the reactive distillation process. Based on the optimized SRC process, dynamic control is carried out. The dual-point temperature and temperature-composition control structures are proposed to reject throughput and feed composition disturbances. The dynamic performances demonstrate that the temperature-composition control structure is better in maintaining product purity.

New single-column extractive distillation with heavy entrainer to separate binary azeotropic mixtures

Extractive distillation with heavy entrainer is widely used in the separation of binary azeotropic mixtures. While a large amount of energy is always consumed in conventional two-column processes. A new flash tank-assisted single-column extractive distillation with heavy entrainer (FT-SCED) is proposed in this paper. The side vapor-phase stream with high heavy component content is withdrawn from the extractive column, and then it is partly condensed in a flash tank. By this means, the high-purity light and heavy products can be achieved from the overheads of column and flash tank, respectively. To obtain the proposed process, an initial single column extractive distillation with heavy entrainer is designed firstly. Then the flash tank is added into the process on the basis of the thermodynamic feasibility analysis. Genetic Algorithm procedure is employed to optimize the independent variables of the obtained process. Two case studies show that the proposed process has lower annual total cost compared with conventional extractive distillation and extractive dividing wall column. PI control strategies are established for the proposed processes in two cases. The dynamic control results show that these control strategies handle feed flowrate and composition disturbances excellently.

Control of heat-integrated indirect triple-column extractive distillation for separating ternary azeotropic mixture tetrahydrofuran-ethyl acetate-water

Most of the design and control studies for extractive distillation of the ternary azeotropic separation today focus only on the direct sequence extractive distillation (DED) while the design study for indirect sequence extractive distillation (IED) is rarely reported. Furthermore, there is no control study that worked on the IED. To bridge this gap, we analyze the control performance of the heat-integrated (HI) IED, exemplified using the separation of tetrahydrofuran (THF)/ethyl acetate (EtAc)/water as a case study. A single point temperature control (TC) (CS 1), dual TC (CS 2), and feed forward dual TC (CS 3) are developed in this work and their performances in rejecting feed flow and composition disturbances are tested. Overall, CS 3 exhibits the best control performance in returning all the three products purities to a value close to the products specification and provides the lowest transient deviation under feed flow and composition disturbances. The CS 3 also provides the lowest sum of integral absolute error with respect to CS 1 and CS 2, further reflecting its dynamic superiority.

Towards sustainable separation and recovery of dichloromethane and methanol azeotropic mixture through process design, control, and intensification

AbstractBackgroundHere we analysed the possibility of improving the sustainability performance for the recovery of dichloromethane and methanol from a binary azeotropic mixture using different energy‐intensified extractive distillation‐based processes: side‐stream extractive distillation (SSED), thermally coupled extractive distillation (TCED), and extractive dividing wall column (EDWC). The sustainability performance of the different processes was analysed based on three main factors: total annual cost (TAC), CO2 emissions, and condition number.ResultsThe EDWC was found to give the best improvement in terms of TAC and CO2 emissions by about 18% and 21%, relative to conventional extractive distillation (CED). These however were traded‐off by the increase in conditional number (CN) by 186 times, signifying a complex dynamic characteristic for the EDWC. Thus, the SSED was suggested as an alternative sustainable option as it also provides significant improvement in TAC and CO2 emissions by about 11%, and 18% with respect to the CED, whilst providing the least reduction in operational controllability, as evidenced by the marginal increase in the CN of about 1.5 times. We also investigated the dynamic performance of the SSED and found that the SSED provides identical dynamic performance in handling both ±10% throughput and ±5% feed composition disturbances as those of the CED.ConclusionAmong the different processes, SSED is the best sustainable alternative that provides compromised steady‐state (i.e. TAC and CO2 emissions) and dynamic (i.e. control) performance for the recovery of dichloromethane and methanol. © 2022 Society of Chemical Industry (SCI).

Dynamic controllability strategy of reactive-extractive dividing wall column for the separation of water-containing ternary azeotropic mixture

The reactive-extractive dividing wall column (REDWC), which combines conventional reactive distillation, extractive distillation, and dividing wall column, is a highly integrated process with a compact physical space and strong interrelations between variables. It is challenging to design robust control strategies for the REDWC process. However, there are no investigations reported on the control of REDWC. In this study, through analyzing open-loop controllability indicators of the steady-state process, the basic control structure (CS1) is constructed. Subsequently, four proportional-integral control schemes are further established on the CS1: the proportional control structure (CS2), the internal composition control structure (CS3), the feedforward control structure (CS4), and the three-point temperature control structure (CS5) with feedforward loops. The dynamic performance tests are conducted with ± 10 % feed flow rate and ± 10 % feed composition disturbances. The qualitative research demonstrates that the CS5 offers the best dynamic performance in terms of settling time, steady-state deviations, and overshoots without the use of expensive composition controllers. With the integral absolute error (IAE) as an evaluation criterion, it is quantitatively demonstrated that the CS5 has the best robustness and should be the preferred control scheme. Another key finding shows that the vapor split ratio and the stoichiometric ratio of the reactants are two crucial degrees of freedom to guarantee product quality in the REDWC. The major contributions of this work are to provide a meaningful reference for the control of the REDWC and even more complicated distillation.

Revisiting the binary azeotropic separation containing tetrahydrofuran and ethanol: Design and control of extractive distillation using dimethyl sulfoxide as alternative solvent

This study reexamined the possibility of improving the separation of binary azeotropic mixture containing tetrahydrofuran (THF) and ethanol from previous work (J Chem Technol Biotechnol 2015; 90: 1463–1472) that rely on the extractive distillation (ED) using ethylene glycol (EG) as solvent. Here, dimethyl sulfoxide (DMSO) is proposed as an alternative solvent for the ED, and its feasibility is preliminary screened and compared against the usage of EG. The conceptual ED using DMSO is designed by manipulating all the design variables until the minimum product specifications (i.e. purity) are achieved. Then, the conceptual design is further optimised using particle swarm algorithm to obtain the ideal column configuration and the performance is compared against the ED using ethylene glycol (EG) and pressure swing distillation (PSD) (i.e. best process) from previous work based on economic and CO2 emission. Overall, the optimised ED using DMSO provides 36% and 37% lower economic and CO2 emission with respect to the ED using EG. In comparison to PSD (i.e. best process) from previous work, it provides 24% and 25% reduction in TAC and CO2 emission. Lastly, a control structure is developed for the proposed ED using DMSO that can effectively handle ± 10% throughput and ± 5% feed composition disturbances without the need of a composition controller as in the case of previous work.