Vapor Recompression Heat Pump Research Articles

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.

Improved Organic Rankine Cycle System Coupled with Mechanical Vapor Recompression Distillation for Separation of Benzene-Toluene Mixture

Herein, we propose three improved Organic Rankine Cycle (ORC) Systems to separate the benzene-toluene mixture by the combination of ORC with Mechanical Vapor Recompression (MVR) heat pump distillation process. A chemical process software, Aspen plus, was used to simulate all the processes. The improved ORC system was employed to recover waste heat from the MVR heat pump system. The ORC waste-heat power generation process was obtained based upon the net power output and cycle’s thermal efficiency. Finally, energy saving was considered as primary criteria for the ORC system selection. The results showed that the ORC system by exhaust stream regenerative cycle (EGC) coupled with MVR heat pump distillation process saves 21.29% electrical energy. The ORC system by EGC coupled with MVR heat pump distillation process has more energy-saving advantages.

Improving the performance of heterogeneous azeotropic distillation via self-heat recuperation technology

Azeotropic distillation is one of the most energy intensive processes and widely used to separate thermos-sensitive azeotropes. In this work, several heterogeneous azeotropic distillation processes based on the vapor recompression heat pump (VRHP) and self-heat recuperation technology (SHRT) are proposed to study the energy-saving performance. The results show that the heterogeneous azeotropic distillation process based on self-heat recuperation technology (HAD-SHRT) has better performance in economic and environment. In HAD-SHRT process, the sensible heat and latent heat of the vapor streams are recuperated and supplied to the reboilers. The remaining sensible heat is supplied to the preheaters through the heat exchanger network (HEN). Adding a preheater can reduce the duty of compressor by increasing the initial temperature of the stream. Compared with the conventional heterogeneous distillation process, the HAD-SHRT process can save 38.50% of TAC, 93.30% of CO2 emissions, and 80.92% of energy consumption. The thermodynamic efficiency can increase to 13.51%.

Vapor recompression with interreboiler in a ternary dividing wall column: Improving energy efficiency and savings, and economic performance

This work deals with a ternary dividing wall column (DWC) for improving its energetic and economic potential through thermal intensification route. In this light, a vapor recompression heat pump system in conjunction with an interreboiler (IR) is proposed to configure externally with the classical DWC scheme that allows heat transfer through the dividing wall. Formulating a variable manipulation policy at dynamic state, it is investigated that the vapor recompressed DWC with inter and bottom reboiler (VRDWCIBR) shows a better performance than the VRDWC with either bottom reboiler or interreboiler(s). For a ternary hydrocarbon system, the VRDWCIBR secures the best performance, providing a 81.05% savings in utility consumption (which is double to that of DWC) with a thermodynamic efficiency of 55.60% and a payback time of 2.82 years. The proposed heat integration arrangement can be retrofitted with a little modification in the classical DWC column.

Towards an energy-friendly and cleaner solvent-extraction of vegetable oil

The extraction of vegetable oils is an energy-intensive process. It has moreover a significant environmental impact through hexane emissions and through the production of organic-loaded wastewater. A rice bran oil process was selected as the basis, since full data were available. By using Aspen Plus v8.2 simulation, with additional scripts, several improvements were examined, such as using heat exchanger networks, integrating a Vapor Recompression Heat Pump after the evaporation and stripping, and examining a nitrogen stripping of hexane in the rice bran meal desolventizing unit followed by a gas membrane to recover hexane. Energy savings by the different individual and combined improvements are calculated, and result in a 94.2% gain in steam consumption and a 73.8% overall energy saving. The power consumption of the membrane unit reduces the overall energy savings by about 5%. Hexane separation and enrichment by gas membranes facilitates its condensation and re-use, while achieving a reduction of hexane emissions by over 50%. Through the considerable reduction of required steam flow rates, 61% of waste water is eliminated, mostly as organic-loaded steam condensate. Through overall energy savings, 52% of related CO2 emissions are eliminated.

MVR heat pump distillation coupled with ORC process for separating a benzene-toluene mixture

In this work, a process consisting of Mechanical Vapor Recompression (MVR) heat pump distillation coupled with Organic Rankine Cycle (ORC) was proposed to separate the benzene-toluene mixture. Aspen Plus (version 7.3) was used to study the steady state simulations of the process. An optimized ORC system was applied to recover waste heat from the MVR heat pump system. In addition, the optimized parameters of ORC waste-heat power generation process were obtained based upon the net power output and cycle’s thermal efficiency. All other parameters, including operating and equipment parameters for the whole process were based upon the total annual cost (TAC). The results showed that the MVR heat pump distillation coupled with ORC process was more competitive than the conventional distillation process. Additionally, the proposed process was found to be more economical than the MVR heat pump distillation process.

Integrating a vapor recompression heat pump into a lower partitioned reactive dividing-wall column for better energy-saving performance

In this study, four integrated lower partitioned reactive dividing-wall columns (LRDWCs) with vapor recompression heat pump (VRHP) models are developed to reduce the energy consumption of a reactive dividing-wall column. Energy-saving, economic, and environmental performances of the integrated LRDWC with VRHP models are compared with the corresponding LRDWC. The results indicate that the integration of VRHP into the LRDWC significantly reduces the energy consumption and CO2 emissions associated with the process. The application of a preheater further improves the energy-saving and economic performance of the process. With the aid of a preheater, the integrated LRDWC with the VRHP model saves 49.86% energy consumption and 13.76% total annual cost with a payback period corresponding to five years, and reduces all CO2 emissions when compared with the corresponding LRDWC.

Intensifying reactive dividing-wall distillation processes via vapor recompression heat pump

Recently, reactive dividing-wall distillation (RDWD) has attracted much attention due to its capacity of process intensification. Nevertheless, its energy efficiency and economic benefit are likely to be enhanced by employing vapor recompression heat pump (VRHP) to reuse the energy of its overhead vapor. In this study, we explore the feasibility and effectiveness of this technology in two representative RDWD processes with different operation characteristics through in-depth evaluations of the steady-state performance of the VRHP reinforced RDWD (i.e., the proposed RDWD-VRHP). The first process involves the esterification of mixed acid (acetic acid and propionic acid) with methanol to yield methyl acetate, methyl propionate, and water. The second process involves the reaction of glycerol and hydrochloric acid to produce 1,3-dichlorohydrin, 2,3-dichlorohydrin, and water along with the intermediate products of 1-monochlorohydrin and 2-monochlorohydrin. Different configurations of the RDWD-VRHP are proposed for these two reaction systems and a systematic design procedure is developed to determine the optimum combination of the VRHP and the RDWD. Simulation results demonstrate that the RDWD-VRHP can substantially reduce utility consumption of RDWD. Especially, much more economic benefit can be secured by VRHP for the RDWD column with small column temperature difference and high latent heat in the overhead vapor.

Multi-Effect Evaporation Coupled with MVR Heat Pump Thermal Integration Distillation for Separating Salt Containing Methanol Wastewater

Due to the high energy consumption for separation of salt containing methanol wastewater, in this work, the multi-effect evaporation coupled with mechanical vapor recompression (MVR) heat pump and thermal integration technologies were raised for the first time. The ELECNRTL thermodynamic model is used to simulate and optimize the evaporation rectification process. Energy consumption and total annual cost (TAC) are taken as objective functions. The results show that multi-effect evaporation coupled with conventional distillation process can save energy consumption and TAC by 44.12% and 39.14%. The multi-effect evaporation coupled with distillation process based on MVR heat pump technology can save energy consumption and TAC by 55.27% and 47.49%, which is super to three-effect evaporation coupled with conventional distillation process. The three-effect evaporation coupled with MVR heat integration process can save energy consumption and TAC by 81.32% and 58.55%, which is more economical than other processes. It can be clearly seen that three-effect evaporation coupled with MVR heat integration process is more competitive to deal with the salt containing methanol wastewater.

Performance Enhancement of Reactive Dividing-Wall Column via Vapor Recompression Heat Pump

In this work, a novel energy-efficient distillation technology combining reactive dividing-wall column (RDWC) with vapor recompression heat pump (VRHP) is proposed. The integrated RDWC with VRHP models with intermediate reboiler or bottom reboiler are designed for reaching better energy-savings and economic performance. The results showed that the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler is more attractive than the reactive dividing-wall column with vapor recompression heat pump and bottom reboiler in this study. Adding a preheater to the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler (RDWC-VRHP-IR2) can further reduce the compression ratio and reach better performance. The RDWC-VRHP-IR2 saves 57.06% of total utility consumption, 31.31% of total annual cost with the payback period of 5 years, and 61.52% of CO2 emissions compared with the conventional two-column reactive distillation system.

Improving the Performance of Heat Pump-Assisted Azeotropic Dividing Wall Distillation

In this work, three vapor recompression heat pump (VRHP) models are designed for the azeotropic dividing-wall column (A-DWC) system to study the energy-saving effect. During the determination of these three models, how the top stream pipeline designs affect the energy consumption is discussed. As adding a preheater can cut down compressor pressure ratio and splitting top stream can decrease the feed flow of the compressor, all these methods can obviously help reduce compressor work for the process. Evaluation results show that the VRHP A-DWC systems have a better performance in energy consumption and environmental sustainability. Especially for Model 3, the savings of total operating cost (TOC) is 47.77% and that of the total annual cost (TAC) is 32.22%, compared with the conventional two-column azeotropic system. CO2 emissions in the process of model 3 can be reduced by 63.79%, and the thermodynamic efficiency increased to 22.64%.

Application of Vapor Recompression to Heterogeneous Azeotropic Dividing-Wall Distillation Columns

Although heterogeneous azeotropic dividing-wall distillation columns (HADWDCs) can achieve considerable reductions in capital investment and operating cost as compared to conventional heterogeneous azeotropic distillation systems, their steady-state performance is likely to be greatly enhanced by the application of vapor recompression heat pump (VRHP) technology owing to their unique process configuration of one top condenser and two bottom reboilers and favorable steady-state behavior of relatively small top-to-bottom temperature elevations. In the current work, the feasibility and effectiveness of reinforcing the HADWDC with the VRHP (HADWDC-VRHP) are examined. A systematic procedure is developed to effectively determine the optimum combination of the VRHP and the HADWDC in order to minimize the total annual cost. Two example systems for the separation of isopropyl alcohol and water and pyridine and water with cyclohexane and toluene as entrainers, respectively, are studied to evaluate the steady-state ...

Distillation column controllability analysis through heat pump integration

Understanding the dynamic behavior of distillation column assisted heat pump has received considerable attention, since this integrated process is used to improve thermodynamic and energy efficiency. This work studied controllability of two processes of distillation column assisted bottom flashing heat pump and vapor recompression heat pump. Integration caused extra degrees of freedom, in which some manipulated-controlled variable pairings were promising. Controllability tools such as set-point tracking and disturbance rejection were applied to compare the input-output pairings and select pairings with the best controllability and disturbance rejection indices under decentralized control in a steady state condition and frequency domain. The results indicated that controllability of recompression of the top vapor stream was better than flashing of the bottom liquid stream of the column. In addition, results showed that reboiler heat duty and reflux rate in a distillation column could be replaced with other variables. Besides, when a heat value was used in a heat exchanger for keeping the temperature of the outlet stream at a specific value, heat inputs could be replaced with the corresponding temperatures for the manipulated variables. Therefore, using this method demonstrated improved controllability indices.

Application of Mechanical Vapor Recompression Heat Pump to Double-Effect Distillation for Separating N,N-Dimethylacetamide/Water Mixture

A coupled separation system involving a mechanical vapor recompression heat pump (MVRHP) and double-effect distillation (MED) was proposed. In a case of separating N,N-dimethylacetamide (DMAC)/water mixture, four distillation schemes (i.e., conventional distillation, top MVRHP distillation, double-effect distillation, and double-effect distillation with double MVRHP) were conceptually constructed and simulated by using Aspen Plus with the binary interaction parameters of the NRTL thermodynamic model. We evaluated these four schemes to choose the best one with the goal of the minimum total annual cost (TAC). Compared to the conventional distillation, the TACs for double-effect distillation with double MVRHPs, top MVRHP distillation, and double-effect distillation were decreased by 47.76, 32.23, and 5.45%, respectively. It indicates that the double-effect distillation with double MVRHPs has obvious advantages over the other three distillation schemes in terms of economic efficiency.

Simulation and Optimization of Distillation Processes for Separating a Close-Boiling Mixture of n-Butanol and Isobutanol

Separation of close-boiling mixtures by conventional distillation consumes a large amount of energy because of the very high reflux ratio required. Mechanical vapor recompression heat pumps (MVRHPs) can recycle the energy of the vapor and can thus be used in such distillation processes to save energy. Three different distillation schemes, namely, conventional distillation, top MVRHP distillation, and bottom-flashing MVRHP distillation, were simulated for the separation of the close-boiling mixture of n-butanol and isobutanol using Aspen Plus to determine the economically best option. The research results indicate that, compared to conventional distillation, the energy savings for bottom-flashing MVRHP distillation and top MVRHP distillation can reach 67.92% and 72.92%, respectively, and the TACs correspondingly decrease by 71.74% and 75.57%.

Performance enhancement of vapor recompression heat pump

The vapor recompression heat pump (VRHP) has the potentials of reducing the energy requirements of fractionating close-boiling mixtures. It improves the quality of low grade heat with the aid of heat pump to provide heat input to the reboiler. However, this technology does not utilize heat efficiently resulting in appreciable heat loss in the condenser. In this study, enhanced VRHP models were developed to reduce the heat loss and heat pump size. The strategies adopted rely on reducing the heat differential across the heat pump by utilizing external and utility streams, and process stream within the system. The thermoeconomic and environmental performances of the developed models were compared with the base case VRHP and the conventional distillation process. The results showed that the developed models yielded considerable energy savings. Considering the present trend of short process modification payback time, the use of an external process stream is recommended as the most preferred option to boost the plant performance. However, in situation where such streams are not available within the plant premises or uneconomical due to their influence in the chosen exchanger network, the utilization of process streams within the system will be a much more attractive alternative option.

Operation Characteristic of a Mechanical Vapor Recompression Heat Pump Driven by a Centrifugal Fan

A mechanical vapor recompression heat pump driven by a centrifugal fan is designed together with falling-film evaporation. Based on theoretical analysis, experimental research is applied to study the fan type of MVR. Choosing water as the experimental medium, the operation characteristic of MVR applied to low evaporation is examined. Practically, the pressure difference of the unit is likely to keep stable while its evaporation pressure goes up. After the system performance is tested and analyzed, it shows that the total evaporation water and total input energy increase as its evaporation pressure grows. Further, some calculation is done and the result indicates that its SMER and COP decrease while the evaporation pressure rises. The reason of this phenomenon is: the leakage loss of the fan inside goes up and its displacement efficiency reduces as the evaporation temperature and pressure is high; finally, it brings forth the drop of the systems adiabatic efficiency. Finally, the trend of average input work for compressed vapor is compared in three different terms. The trend of average input work by calculation is the same as that in theory; that is to say, both of them descend when the evaporation pressure ascends. Because of displacement efficiency, the trend of average input work by measure is different from that in theory; that is to say, the average input work by measure grows slightly when the evaporation pressure goes up.

Recuperative vapor recompression heat pumps in cryogenic air separation processes

Vapor recompression heat pumps are used in distillation processes to reduce energy consumption. For sub-ambient distillation, one of the largest difficulties is to produce liquid reflux since the condensation of highly volatile components requires expensive refrigeration energy. Thermally coupled distillation columns have been commonly applied to produce reflux in cryogenic air separation processes. In order to condense the nitrogen vapor for reflux by the oxygen liquid, all the air feed has been compressed to a pressure considerably above ambient pressure. In cases where the oxygen product does not have to be at elevated pressure, this causes unnecessary compression of the oxygen in the air feed. One such large scale application is oxy-combustion in coal based power plants. A scheme of recuperative vapor recompression heat pumps is developed to substitute the thermally coupled distillation columns, and this scheme is applied in cryogenic air separation processes in this paper. A portion of the vapor nitrogen is compressed and condensed for reflux, thus the mass flow through the compression stages is reduced. The power consumption has been significantly reduced compared to conventional double-column distillation cycles, and further reduced when the principle of distributed reboiling is applied to reduce irreversibilities in distillation columns.

Design and optimization of thermally coupled distillation schemes for the trichlorosilane purification process

The trichlorosilane (TCS) purification process has significant energy requirements to achieve a TCS purity of 10 N. In this study, a dividing wall column was used to improve the performance of the TCS process. A response surface methodology was applied to the design of the dividing wall column. The conventional dividing wall column and top dividing wall column have significant benefits, e.g. decreasing the operating cost and minimizing the total annual cost. Incorporating a heat pump in the top diving wall columns was also proposed to enhance the energy efficiency further. Furthermore, a column grand composite curve was used to evaluate the thermodynamic feasibility of implementing the heat pump system into DWC. The operating cost could be reduced by 83% by novel combinations of internal and external heat integration: top dividing wall columns using a top vapor recompression heat pump. A compact integrating TCS purification process was finally proposed.

Quick selection of industrial heat pump types including the impact of thermodynamic losses

Making a rough performance estimate for conventional vapor compression and vapor recompression heat pumps is straight forward: Dividing the Carnot efficiency by 2 results in a reasonable estimate. Still, actual performance of heat pumps could easily vary to a large extent.With new and innovative heat pumps the discrepancies between the rough estimate and actual performance might be even larger as the Carnot efficiency is not the upper limit anymore due to the use of temperature glides. Lack of a simple method to determine the approximate performance of a heat pump will hinder the implementation of these novel types in industry.In this study a performance map is presented and it is shown that, for mechanical heat pumps, making use of the available temperature glide increases performance and reduces the payback period. While at low glides heat driven absorption heat pumps and vapor (re)compression heat pumps show the smallest payback times, mechanical heat pumps with large glides show to be more effective at higher temperature lifts when temperature glides are available. Due to improved performance, these mechanical heat pumps are able to achieve better economical results over their technical life time although they require higher initial investment.