Conventional Pressure-swing Distillation Research Articles

Sustainable investigation and process intensification on the separation of ternary organic wastewater with heterogeneous characteristics

The tetrahydrofuran (THF)/water/n-propyl acetate (PROAC) mixture, which is categorized into class 2.0-2b-C, exhibits pressure-sensitive property along with heterogeneous regions in its phase diagram. Conventional pressure-swing distillations with different separation sequences (PSD-PTW, PSD-PWT) and a novel three-column heterogeneous azeotropic pressure-swing distillation (T-HAPSD) are designed to complete the separation. The PSD section of T-HAPSD combines three columns into two columns. The HAD section of T-HAPSD is more energy-efficient owing to the automatic separation in the decanter. T-HAPSD is more economical than conventional pressure-swing distillation. Heat integration and Self-heat recuperation technology (SHRT) are then introduced to reduce energy consumption and costs. Finally, the five processes are evaluated from four aspects (economy, energy, environment and exergy). The best of five processes is T-HAPSD-SHRT with 37.83% reduction in TAC, 51.17% reduction in TEC, 48.83% reduction in gas emissions and 52.63% reduction in exergy destruction when compared with PSD-PTW.

Design and energy-saving strategy of sustainable pressure-swing distillation with thermally and electrically coupled intensification for separating ternary mixture with multiple azeotropes

The separation of ternary mixtures that form multiple azeotropes is frequently encountered in industry. Benefiting from the pressure-sensitive nature of the azeotropic mixture, the pressure-swing distillation (PSD) is promising to realise the separation without introducing foreign solvents. However, the wide application of PSD seems challenging from an energetic point of view. This work has developed a sustainable PSD process with thermally and electrically coupled intensification to decrease energy consumption, taking the separation of homogeneous acetonitrile/ethanol/water mixture as a demonstrating example. Two conventional PSD processes with different product sequences are designed through the ternary phase diagram and further optimised using the NSGA-II, considering multiple objectives, including total capital cost, total operating cost, and CO2 emissions. The optimal solution is selected by the TOPSIS analysis for each sequence and is then intensified with thermal heat integration and electricity-driven heat pump techniques. The results show that, in comparison to the non-heat-integrated PSD process, the proposed intensified design can dramatically reduce the total annual cost by up to 48%, simultaneously decreasing CO2 emissions by up to 60% and boosting energy efficiency. The present study can be of reference for designing a sustainable PSD process in ternary azeotropic separation.

Energy-saving design and multi-objective optimisation of triple-column pressure-swing distillation configuration for separating n-propanol/benzene/water mixture with multiple azeotropes

The separation of ternary azeotropic mixtures with multiple azeotropes is often encountered in industrial wastewater treatment. This study explores separating the n-propanol/benzene/water mixture with three binary azeotropes and one ternary azeotrope via pressure-swing distillation (PSD). Two conventional PSD schemes are designed based on the ternary phase diagram. The multi-objective genetic algorithm facilitates the optimisation task for each sequence, taking the total annual cost, CO2 emissions, and thermodynamic efficiency as the objective functions. Then, the selection of the optimal solution from the Pareto front is performed using the technique for order preference by similarity to the ideal solution. Subsequently, energy-saving methods, including direct heat integration, heat pump, and heat exchanger network, are introduced into each optimal non-heat-integrated PSD process to address the drawback of tremendous total energy consumption. Finally, the proposed 12 energy-efficient PSD designs are compared with the conventional PSD cases in multiple aspects. The optimisation results highlight the importance of operating pressures as variables in the multi-objective optimisation problem. In addition, the W-B-N sequence shows superiority over the W-N-B sequence in decreasing cost and gas emissions and enhancing energy efficiency. The proposed energy-saving PSD design with direct heat integration, heat pump, and heat exchanger network gives the lowest cost amongst the 14 processes, being up to a reduction of 39% compared to the corresponding conventional PSD design.

Control of fully heat-integrated pressure-swing distillation with strict pressure manipulation: A case study of separating a maximum-boiling azeotrope with small pressure-induced shift

Pressure-swing distillation (PSD) is an alternative configuration used to separate pressure-sensitive azeotropes and avoid product contamination(s) in extractive or heterogeneous azeotropic distillation via the entrainer(s). Full heat integration between high- and low-pressure columns (HPC and LPC) can provide significant energy savings compared to the conventional case, while it also loses the control degree of freedom, trim condenser or reboiler heat load, via the integrated condenser/reboiler exchanger. Typically, the pressure of HPC is left uncontrolled, and additional pressure-compensated control strategies are implemented, which may create safety issues. To address this, the study proposes adding an auxiliary cooling-water-driven condenser to the fully heat-integrated PSD system, providing an extra manipulated variable to regulate the pressure of HPC. Compared to the conventional PSD system, the new modified process could achieve a 26.31% reduction in total annual cost. The research uses the maximum-boiling azeotropic mixture of acetone/chloroform as a case study which makes the distillation system characterized by high reflux ratios and a large recycle flow rate, leading to the unwelcome snowball effect. To achieve effective process control, robust control structures are proposed that rigorously regulate the pressure of the column by manipulating the heat removal in the trim condenser, attenuating or eliminating potential overpressure issues. Product purity and other evaluation indicators, such as product transient deviations, offsets, integral absolute error, and oscillation degrees, are also considered. Results show that the proposed control schemes can efficiently control the pressure of HPC, weaken the snowball effect, and maintain product purity. This study highlights the importance of pressure control for fully heat-integrated PSD systems, particularly for pressure-sensitive azeotropes with small pressure-induced shifts.

Energy-saving reactive pressure-swing distillation process for separation of methanol - dimethyl carbonate azeotrope via reacting with propylene oxide

Pressure-swing distillation (PSD) is a widely used in the chemical industry to separate azeotropes without the introduction of a any third component; however, the process involves high energy consumption and operating costs are required owing to the change in pressure. For the separation of methanol–dimethyl carbonate azeotropes, an intensified reactive pressure-swing distillation (R-PSD) process has been proposed, wherein a reactive distillation column is used, instead of a high-pressure column, and the propylene oxide (PO) is introduced into the system as a reactant to reacts with methonal. To compare the feasibility of the proposed R-PSD process with the conventional heat-integrated PSD process, three indicators—total energy requirements, total annual cost, and CO2 emissions—were used. The results show that the R-PSD process performs better than the conventional heat-integrated PSD process in terms of energy, economics and environmental owing to the partial consumption of methanol and the simultaneous co-production of high-value propylene glycol methyl ether (PGME). Moreover, at 50% methanol consumption, the total energy consumption, total annual cost, and the CO2 emissions were reduced by 46.0%, 34.3%, and 45.0%, respectively, compared with the conventional heat-integrated PSD process. This implies the improved R-PSD process has significant energy-saving potential.

Study on energy saving of heat integrated distillation process for methyl acetate–methanol–tetrahydrofuran–water

ABSTRACT Based on the characteristics of binary azeotropic composition in the mixture of methyl acetate–methanol–tetrahydrofuran–water sensitive to pressure, pressure swing distillation (PSD) and double-solvent extraction distillation (DSED) were proposed to study the separation of the quaternary system. Numerical simulations were performed by ASPEN PLUS software (version 10.0) environment under the WILSON thermodynamic model. The economy of PSD and DSED process was performed, and the heat integration of two distillation processes was compared based on total annual cost (TAC). Research results of PSD indicated that compared with conventional PSD (CPSD), heat-integrated PSD (HIPSD) can save energy consumption and TAC by 33.87% and 30.70%, respectively, and moreover increase thermodynamic efficiency by 1.77%, so it had obvious energy-saving effect and economic advantage. Simulation results of DSED showed that water (H2O) and ethylene glycol (EG) were most suitable extractants for the system, and the suitable solvent ratios for H2O and EG were 0.28 and 0.38, respectively. Compared with conventional double-solvent ED (CDSED), heat-integrated double-solvent ED (HIDSED) can save energy consumption and TAC by 43.29% and 28.12%, respectively, and moreover increase thermodynamic efficiency by 3.25%, so it was superior to CDSED. The results indicated that the energy consumption and TAC of HIDSED are respectively 63.56% and 44.01% less than the HIPSD process, and the thermodynamic efficiency of the former is 3.9% larger than the latter, so the HIDSED process shows better economic and energy-saving effect.

Application of heat pump technology to recover 1,4-dioxane and acetonitrile from wastewater via triple-column distillation

• An effective method to recover 1,4-dioxane and acetonitrile from wastewater. • Heat pump technology is applied for thermal efficiency enhancement. • Temperature-entropy diagram is taken for illustrating the heat pump process. • The splitting arrangement is the optimal process. An effective method of triple-column pressure-swing distillation is introduced to recover 1,4-dioxane and acetonitrile from wastewater in this study. In order to save energy, heat pump technology is applied for utilizing the latent heat of the overheads. Besides, the heat exchange network is used for optimizing the heat exchange of the process. For better analysis of the heat pump process, the temperature-enthalpy diagram is taken to present the utilization, heat recovery and pinch point. And the temperature-entropy diagram is adopted to illustrate the thermodynamic parameters of the overhead during the heat exchange process. Several different processes with heat pump are designed and the triple-heat pump assisted pressure-swing distillation with splitting arrangement is the optimal process with the second-law efficiency of 1.45%. Compared with the conventional pressure-swing distillation, the savings of total annual cost and total energy consumption are 45.83% and 65.82% and the reduction of greenhouse gas emissions are 62.36% in CO 2 , 63.72% in SO 2 and 63.72% in NO x .

Differential temperature control in heat-integrated pressure-swing distillation for separating azeotropes to deal with operating pressure fluctuations

For heat-integrated pressure-swing distillation (PSD) in which the heat release of overhead vapor condensation of high-pressure column (HPC) is completely utilized to supply the reboiler duty of low-pressure column (LPC), the operating pressure (OP) of the HPC cannot be controlled. Pressure-compensated temperature control (PCTC) is widely used to hold the controlled temperature under OP fluctuations to achieve effective control. In this paper, differential temperature control (DTC) is applied to control the sensitive-stage temperature under OP fluctuations of the HPC in the control of a variety of heat-integrated PSD processes, including both conventional and entrainer-assisted PSD, both partially and fully heat-integrated PSD, both LPC->HPC and HPC->LPC sequence, and separating both minimum-boiling and maximum-boiling azeotropes. IAE of the product purities of all the processes are calculated. Systematic comparison between DTC and PCTC is implemented. The results show that effective control can be achieved when implementing DTC in these processes and the dynamic performances of DTC and PCTC are almost the same. Therefore, DTC can be applicable in the control of heat-integrated PSD for separating binary azeotropes, which is also a good inspiration for other heat-integrated distillation processes.

Optimization and heat integration of hybrid R–HIDiC–PV process with the series–parallel arrangement of PV modules and recycle streams for TAME production

Tert–amyl methyl ether (TAME) production process is consisted of a reactor, a reactive distillation (RD) column and the methanol recovery section. The bottom product of the RD column is TAME while the distillate is consisted of the azeotropic mixture of methanol and isopentane (i-pentane) that can be separated by either the pervaporation (PV) membrane or conventional pressure swing distillation (PSD) and extractive distillation (ED) methods. The PV process allows the use of both series and parallel arrangements. The series arrangement has a lower membrane area compared to the parallel arrangement, while the parallel arrangement has a lower energy cost. A series–parallel arrangement with recycle streams was used in the present study to take advantage of both the series and parallel arrangements. The HIDiC method was used for reducing energy consumption in the RD column. Moreover, heat integration between PV streams and internal flows in the RD column was used to reduce PV energy consumption. The number of modules in each row, the recycle stream, the heat exchange ratio between the rectifying and stripping sections in the RD column and the heat exchange ratio between PV streams and internal RD streams were obtained by a hybrid GA–PSO algorithm. Furthermore, all RD and PV parameters were considered optimization variables and most importantly, all process variables were optimized simultaneously. According to the results, the proposed process reduced the total annual cost (TAC) by 52% and led to a 57% energetic gain compared to the conventional hybrid RD–PSD process. The proposed process also reduced TAC by 48% and led to a 40% energetic gain in comparison with the hybrid RD–PV process. This can be related to the reduced membrane area compared to the series and parallel arrangements as well as reduced energy costs of PV heaters through heat integration between PV steams and internal flows of the column.

Performance Enhancement of Heat Pump with Preheater-Assisted Pressure-Swing Distillation Process

A heat pump is applied in pressure-swing distillation (PSD) for separating an isobutanol/isobutyl acetate mixture in this paper. To ensure dry compression and prevent compressor damage, a preheater is added before the compressor based on the dry fluid property of the isobutanol/isobutyl acetate mixture. In addition, the optimal preheated temperature of the overhead vapor stream may further improve the performance of the heat pump-assisted PSD process. Through optimization and analysis, the optimum superheat values of overhead vapor streams from two columns are 12 and 8.4 °C, respectively. The coefficients of performance of heat pumps achieve maximum values of 17.18 and 20, and the total annual cost of the process is minimum. Moreover, a heat exchanger network (HEN) is applied in the heat pump with preheater-assisted PSD (HPPSD) to reduce the use of steam in preheaters. Compared with the conventional PSD, the total annual costs, energy consumption, and CO₂ emission of the HPPSD-HEN process are decreased by 69.8, 83.99, and 79.58%, respectively.

Electrical-driven self-heat recuperative pressure-swing azeotropic distillation to minimize process cost and CO2 emission: Process electrification and simultaneous optimization

Azeotropic separation by pressure-swing distillation (PSD) is one of the most costly processes in the chemical industry. To minimize process cost and CO2 emission, process electrification based on self-heat recuperation technology (SHRT) has been implemented in many thermal-driven processes. Enlightened by these facts, a novel electrical-driven PSD-SHRT process is proposed and compared with conventional thermal-driven PSD (PSD-CONV) and PSDs integrated with heat integration and heat pump. Two typical binary systems – tetrahydrofuran (THF)/water with minimum-boiling azeotrope and acetone/chloroform with maximum-boiling azeotrope – are selected to investigate the potential economic and environmental benefits of process electrification. To make a fair comparison, all the processes are optimized to a minimum in total annualized cost (TAC) based on a simulation-based optimization framework combining Aspen Plus (process simulator) and MATLAB (external optimizer). The optimization results indicate PSD-SHRT to be the lowest in both TAC and CO2 emission of all the alternative processes. In THF/water system, PSD-SHRT triumphs over PSD-CONV by 23.72% in TAC and 83.67% in CO2 emission, respectively. Corresponding values increase to 47.82% and 92.90% in the acetone/chloroform system. These improvements verify the significant advantages of process electrification and also encourage more originations to introduce SHRT into other processes.

Performance enhancement of pressure-swing distillation process by the combined use of vapor recompression and thermal integration

The intensified pressure-swing distillation (PSD) process is explored by applying vapor recompression and thermal integration. The evaluation indicators of second-law efficiency and CO2 emissions are adopted to rank different pressure swing-top vapor recompressed (PSDVRC) configurations. Compared to the conventional PSD process, the economically optimum flowsheet is the thermal integrated PSDVRC process with overhead vapor splitting since it can reduce 67.05% in energy consumption, 72.66% in CO2 emissions and 34.14% in total annual cost (TAC) as well as enhance 159.06% in thermodynamic efficiency. Besides, PSDVRC processes with overhead vapor splitting (PSDVRC-VS) are also superior to other PSDVRC configurations where economic-efficient process is PSD process (PSDSVRC) with single vapor recompression. Dynamic controllability for the highly integrated and interacting economic-efficient process is also investigated. An effective control structure is developed that only handles small (5%) disturbances in throughput and feed composition. The modified process is further developed to achieve robust control performance in terms of peak dynamic transients, settling time, oscillation and steady-state offsets. Meanwhile, control performance comparisons for PSDVRC-VS and PSDSVRC processes are also studied and showed that there is conflict between the steady-state economics and dynamic controllability.

Comparison of continuous homogenous azeotropic and pressure-swing distillation for a minimum azeotropic system ethyl acetate/n-hexane separation

Abstract Continuous homogenous azeotropic distillation (CHAD) and pressure-swing distillation (PSD) are explored to separate a minimum-boiling azeotropic system of ethyl acetate and n-hexane. The CHAD process with acetone as the entrainer and the PSD process with the pressures of 0.1 MPa and 0.6 MPa in two columns are designed and simulated by Aspen Plus. The operating conditions of the two processes are optimized via a sequential modular approach to obtain the minimum total annual cost (TAC). The computational results show that the partially heat integrated pressure-swing distillation (HIPSD) has reduced in the energy cost and TAC by 40.79% and 35.94%, respectively, than the conventional PSD, and has more greatly reduced the energy cost and TAC by 62.61% and 49.26% respectively compared with the CHAD process. The comparison of CHAD process and partially HIPSD process illustrates that the partially HIPSD has more advantages in averting the product pollution, energy saving, and economy.

Entrainer-Assisted Pressure-Swing Distillation for Separating the Minimum-Boiling Azeotrope Toluene/Pyridine: Design and Control

The separation of toluene/pyridine by distillation still remains an obstacle in industry because of not only the formation of a minimum-boiling azeotrope but also their relative volatilities, whose ratio is close to unity throughout the whole composition range. Extractive distillation is still not feasible because it is hard to find suitable entrainers. Heterogeneous azeotropic distillation might require high energy consumption to obtain both purified products. Conventional pressure-swing distillation is also not feasible. In this work, we apply entrainer-assisted pressure-swing distillation to separate toluene/pyridine (95 mol %/5 mol %) by introducing n-propanol as an entrainer. Based on the different compositions of the recycling stream sent back to the azeotropic column, a two-column sequence and a three-column sequence are established. Both sequences are partially heat-integrated. The steady-state designs of both sequences are optimized based on the total annual cost (TAC). The results reveal that th...

Design and control of pressure-swing distillation for azeotropes with different types of boiling behavior at different pressures

The mixture of n-heptane and isobutanol presents minimum- and maximum-boiling azeotropes as the pressure changes. Two different pressure-swing distillation (PSD) processes are available for this mixture if the pressure of the low-pressure column is set as atmospheric. The steady states of the two available PSDs are optimized by minimizing the total annual cost (TAC) following a sequential iterative optimization procedure. On the basis of the steady-state results, the dynamic control of the two available PSDs and a comparison between them are presented. The conventional PSD (CPSD) is more economical but less controllable than the unusual PSD (UPSD), which has seldom been studied in the published literature. These two different PSD processes should be considered for the separation of this type of mixture according to their advantages.

Separating an azeotropic mixture of toluene and ethanol via heat integration pressure swing distillation

A procedure is suggested to separating a minimum-boiling azeotrope of toluene and ethanol via heat integration pressure swing distillation (PSD), and an optimized separation configuration is obtained via taking the minimization of the total annual cost (TAC) as an objective function. The result demonstrates that PSD with heat integration is more economical than conventional PSD without heat integration. Based on steady-state simulation results, several control structures were explored using Aspen Dynamics. The results indicate that the composition/temperature cascade control structure and the pressure-compensated temperature control of a PSD process with partial heat integration with stage 21 selected as the control stage in the low pressure column can handle disturbances well. As for the PSD with full heat integration, stage 20 of the low pressure column can act as the control stage because of its more efficient controllability under feed flow rate and feed composition disturbances.

A hybrid heat integration scheme for bioethanol separation through pressure-swing distillation route

In this contribution, a hybrid thermal integration scheme is proposed for a pressure-swing distillation (PSD) column by combining an internally heat integrated distillation column (HIDiC) with vapor recompression column (VRC). The purpose of this article is three fold: first, it designs a HIDiC for a PSD system with a reduced number of internal heat exchangers. Second, it develops a hybrid column configuration by integrating that HIDiC, which is devised based on the thermal driving force existed between the two diabatic columns, and VRC that is devised based on the thermal driving force existed between the top and bottom of the same high pressure column, yielding an ideal HIDiC–VRC configuration. Third, it provides a comprehensive comparison between the HIDiC-alone and its hybrid HIDiC–VRC structure with reference to a conventional standalone PSD column. To evaluate the performance of all these configurations, we estimate the two performance indexes, namely energy consumption and total annual cost, for an example of a bioethanol dehydration system.

Adding rectifying/stripping section type heat integration to a pressure-swing distillation (PSD) process

This paper studies the economical effect of considering rectifying/stripping section type heat integration in a pressure-swing distillation (PSD) process separating a binary homogeneous pressure-sensitive azeotrope. The schemes for arranging heat integration between the rectifying section and the stripping section of the high- and low-pressure distillation columns, respectively, are derived and an effective procedure is devised for the conceptual process design of the heat-integrated PSD processes. In terms of the separation of a binary azeotropic mixture of acetonitrile and water, intensive comparisons are made between the conventional and heat-integrated PSD processes. It is demonstrated that breaking a pressure-sensitive azeotropic mixture can be made more economical than the current practice with the conventional PSD process. For boosting further the thermodynamic efficiency of a PSD process, it is strongly suggested to consider simultaneously the condenser/reboiler type heat integration with the rectifying/stripping section type heat integration in process synthesis and design.