Heat integration is important for energy-saving in the process industry.It is linked to the persistently challenging task of optimal design of heat exchanger networks(HEN).Due to the inherent highly nonconvex nonlinea...Heat integration is important for energy-saving in the process industry.It is linked to the persistently challenging task of optimal design of heat exchanger networks(HEN).Due to the inherent highly nonconvex nonlinear and combinatorial nature of the HEN problem,it is not easy to find solutions of high quality for large-scale problems.The reinforcement learning(RL)method,which learns strategies through ongoing exploration and exploitation,reveals advantages in such area.However,due to the complexity of the HEN design problem,the RL method for HEN should be dedicated and designed.A hybrid strategy combining RL with mathematical programming is proposed to take better advantage of both methods.An insightful state representation of the HEN structure as well as a customized reward function is introduced.A Q-learning algorithm is applied to update the HEN structure using theε-greedy strategy.Better results are obtained from three literature cases of different scales.展开更多
Crude distillation unit(CDU)is regarded as the main energy consumer in the entire refinery process.In this paper,the process simulation software and the energy management software are used to simulate the flowsheet an...Crude distillation unit(CDU)is regarded as the main energy consumer in the entire refinery process.In this paper,the process simulation software and the energy management software are used to simulate the flowsheet and analyze the energy consumption,respectively.Stream data obtained from an existing CDU are applicable in the pinch analysis.To reduce the amount of cross-pinch heat transfer,three approaches of resequencing,repiping,and adding heat exchangers are adopted.Compared with the existing CDU,the results demonstrate that the inlet temperature of the furnace can be increased by 25.4℃,the amount of hot and cold utilities can be reduced by 15.1%and 19.6%,respectively.The economic evaluation indicates that the operating cost is saved by 8×106$/a,and the payback period is about 9 months.展开更多
Diesel hydrotreating unit(DHT)is an integral part of the refinery,and its energy-saving optimization is of great significance to the enterprise.In this paper,process simulation software and energy management software ...Diesel hydrotreating unit(DHT)is an integral part of the refinery,and its energy-saving optimization is of great significance to the enterprise.In this paper,process simulation software and energy management software are used to simulate the flowsheet and analyze the energy consumption,respectively.Stream data obtained from an existing DHT are applied in the pinch analysis for retrofitting the heat exchanger network(HEN)to achieve maximum energy utilization by using pinch analysis.Since DHT is constrained by pressure,the pressure factor is considered in the process of retrofitting.The results show that the amount of cross-pinch heat transfer is reduced,the inlet temperature of the furnace is increased by 55℃,and the amount of hot and cold utilities can be reduced by 70.25%and 50.16%,respectively.The economic evaluation indicates that the operating cost is saved by 4.39×10^(6)$/a,and the payback period is about 2 months.展开更多
The heat exchanger network(HEN)in a syngas-to-methanol process was designed and optimized based on pinch technology under stable operating conditions to balance the energy consumption and economic gain.In actual indus...The heat exchanger network(HEN)in a syngas-to-methanol process was designed and optimized based on pinch technology under stable operating conditions to balance the energy consumption and economic gain.In actual industrial processes,fluctuations in production inevitably affect the stable operation of HENs.A flexibility analysis of the HEN was carried out to minimize such disturbances using the downstream paths method.The results show that two-third of the downstream paths cannot meet flexibility requirements,indicating that the HEN does not have enough flexibility to accommodate the disturbances in actual production.A flexible HEN was then designed with the method of dividing and subsequent merging of streams,which led to 13.89%and 20.82%reductions in energy consumption and total cost,respectively.Owing to the sufficient area margin and additional alternative heat exchangers,the flexible HEN was able to resist interference and maintain production stability and safety,with the total cost increasing by just 4.08%.展开更多
Synthesis of heat exchanger networks including expansion process is a complex task due to the involvement of both heat and work.A stream that expands through expanders can produce work and cold load,while expansion th...Synthesis of heat exchanger networks including expansion process is a complex task due to the involvement of both heat and work.A stream that expands through expanders can produce work and cold load,while expansion through valves barely affects heat integration.In addition,expansion through expanders at higher temperature produces more work,but consumes more hot utility.Therefore,there is a need to weigh work production and heat consumption.To this end,an enhanced stage-wise superstructure is proposed that involves synchronous optimization of expander/valve placement and heat integration for each pressure-change sub-stream in stages.A mixed-integer nonlinear programming(MINLP)model is established for synthesizing sub and aboveambient heat exchanger networks with multi-stream expansion,which explicitly considers the optimized selection of end-heaters and end-coolers to adjust temperature requirement.Our proposed method can commendably achieve the optimal selection of expanders and valves in a bid for minimizing exergy consumption and total annual cost.Four example studies are conducted with two distinct objective function(minimization of exergy consumption and total annual cost,respectively)to illustrate the feasibility and efficacy of the proposed method.展开更多
Because of its paramount importance in the successful industrial control strategy of a given heat exchanger network(HEN),the control structure designs for providing appropriate manipulated variable(MV)and controlled v...Because of its paramount importance in the successful industrial control strategy of a given heat exchanger network(HEN),the control structure designs for providing appropriate manipulated variable(MV)and controlled variable pairings have received considerable attention.However,quite frequently HENs with such control structures face the problem of hard constraints,typically holding the HENs at less controlled operating space.So both the MV pairings and the above control pairings should be considered to design a control structure.This paper investigates the systematic incorporation of the two pairings,and presents a methodology for designing such two-tier control structure.This is developed based on the sequential strategy,coupling an indirect-tier with direct-tier control structure design,wherein the intention is realized in the former stage and the latter is implemented for further optimization.The MV identification and pairing are achieved through variations in heat load of heat exchangers to design the indirect-tier control structure.Then the direct-tier control structure is followed the relative gain array pairing rules.With the proposed methodology,on the one hand,it generates an explicit connection between the MV pairings and the HEN configuration,and the quantitative interaction measure is improved to avoid the multiple solutions to break the relationship among all the control pairings into individuals;on the other hand,a two-tier control structure reveals control potentials and control system design requirements,this may avoid complex and economically unfavourable control and HEN structures.The application of proposed framework is illustrated with two cases involving the dynamic simulation analysis,the quantitative assessment and the random test.展开更多
To simultaneously improve the quantity and quality of heat recovery in a heat exchanger network(HEN),this study conducts a theory analysis based on the first and second laws of thermodynamics.Under the premise of maxi...To simultaneously improve the quantity and quality of heat recovery in a heat exchanger network(HEN),this study conducts a theory analysis based on the first and second laws of thermodynamics.Under the premise of maximizing the heat recovery quantity of HEN,Ġdiss is used as an evaluation index to optimize the quality of heat recovery.Meanwhile,the total annual cost(TAC)is considered as another optimization objective to ensure the economic feasibility of the HEN.A superstructure-based multi-objective mixed integer non-linear programming approach is put forward to solve the trade-off between minimizingĠdiss and minimizing TAC.This allows for the optimum HEN structure to be obtained.A well-studied example is solved to highlight the benefits of the proposed method.展开更多
The rapid development of computational technology and the increasing energy demand have improved heat exchanger network(HEN)synthesis.The HEN synthesis involves several optimizations of matches,distributions of heat l...The rapid development of computational technology and the increasing energy demand have improved heat exchanger network(HEN)synthesis.The HEN synthesis involves several optimizations of matches,distributions of heat loads,and stream splitting of heat units.Thus,obtaining good results at high efficiency has been the main standard for evaluating the techniques in the research area of HEN synthesis.This paper first summarizes and analyzes the main contributions of the existing HEN synthesis techniques.To compare related data quantitively,information on ten typical cases is presented in this paper.Furthermore,recently improved solutions for commonly encountered existing literature cases demonstrate the evolution and competition trends in the field of HEN synthesis.The comparison data presented in this paper not only provide a useful reference for future research but also present the optimization directions.Based on the findings of this study,it is noted that there is still a large room for improvement,and current approaches are incapable of dealing with all HEN cases.Moreover,it is still difficult to escape a local optimum and overcome structural constraints when seeking the global optimum.As a follow-up to the current work,the parallel computing mode and adaptively coordinating the ratio of global and local searching abilities are major development trends for future investigation.展开更多
Work exchange is a promising innovative technology in recovering residual pressure energy. However, at the systematic level, the comprehensive utilization of different energy resources in an energy system has become a...Work exchange is a promising innovative technology in recovering residual pressure energy. However, at the systematic level, the comprehensive utilization of different energy resources in an energy system has become an issue of concern. In this work, a systematic approach is proposed, one that successively integrates heat, work and adjusts operation parameters. A detailed procedure for building a heat-work coupling transfer network is provided. The synthesis mainly consists of constructing a work exchange sub-network with pinch analysis based on positive displacement type work exchangers. Simultaneously, another kind of sub-network based on turbine-type work exchangers is built as a schematic comparison. The influence of applying a positive displacement work exchanger on the system is investigated. Finally, as a case study, a renovation design of a typical rectisol process in the coal-water slurry gasification section of an ammonia plant is presented. The results show that the added work exchanger has little impact on the existing heat exchange sub-network. Moreover,extra pressure energy is recovered by coupling the transfer network. It is concluded that the heat-work systematic design is a promising and powerful method to use energy more efficiently.展开更多
Heat (energy), water (mass), and work (pres- sure) are the most fundamental utilities for operation units in chemical plants. To reduce energy consumption and diminish environment hazards, various integration me...Heat (energy), water (mass), and work (pres- sure) are the most fundamental utilities for operation units in chemical plants. To reduce energy consumption and diminish environment hazards, various integration methods have been developed. exchange networks (HENs), The application of heat mass exchange networks (MENs), water allocation heat exchange networks (WAHENs) and work exchange networks (WENs) have resulted in the significant saving of energy and water. This review presents the main works related to each network. The similarities and differences of these networks are also discussed. Through comparing and discussing these different networks, this review inspires researchers to propose more efficient and convenient methods for the design of existing exchange networks and even new types of networks including multi-objective networks for the system integration in order to enhance the optimization and controllability of processes.展开更多
Around the world energy sustainability and environment protection face many challenges due to the continuously increasing population and energy demands, where the demanding rate of energy increases by at least 2.3% pe...Around the world energy sustainability and environment protection face many challenges due to the continuously increasing population and energy demands, where the demanding rate of energy increases by at least 2.3% per year. According to statistical data, until 2035, fossil fuels still are the main source of energy consumption. Burning fossil fuels produces the greenhouse gas of carbon dioxide as a byproduct. CO<sub>2</sub> emissions have a dangerous effect on both human health and the natural environmental balance. There are many types of clean energy like solar, biofuel, and wind energy. The major limitations of using these types, their availability depends on climate conditions and their production rate is inadequate for energy demand. For any country, energy resources’ availability and economic conditions imposed on projects’ prioritization. Application of energy management and emissions control techniques for industrial unit can limit fuel combustion environmental effects and transfer this fossil fuel to an eco-friendly type. In this work, we applied Green Energy Model GEM to the Hydrotreater Unit of the refinery, GEM is composed of four techniques. Which are Heat Exchangers Networks Synthesis [HENS], Fuel Switching, Thermal Insulation application, and Carbon Captures Storage [CCS]. Where they reduce energy consumption by the rate of 3% - 34% and control CO2 emissions by the rate of 26% - 90%. This model is a radical way to face climate change challenges and practical solutions for both energy and environmental crises.展开更多
In times of increasing global warming,enormous efforts are required to rapidly reduce greenhouse gas(GHG)emissions.Due to the EU’s target of climate neutrality by 2050 and the even more ambitious goal of becoming cli...In times of increasing global warming,enormous efforts are required to rapidly reduce greenhouse gas(GHG)emissions.Due to the EU’s target of climate neutrality by 2050 and the even more ambitious goal of becoming climate-neutral in Germany by 2045,it is necessary to systematically increase energy efficiency and decarbonize the industrial heat sector.The methods of heat integration can be used to exploit existing potentials for waste heat utilization and to integrate renewable technologies for heating and cooling.By using a non-stationary,multiperiod approach,additional energy savings can be achieved by integrating a thermal energy storage(TES)that enables heat transportation over time.This paper presents a simultaneous approach for thermal energy storage integration into multiperiod heat integration problems.The approach can be used to minimize energy demand,costs and CO 2 emissions and is demonstrated in two case studies.In case study 1,it is shown that the presented approach is capable of integrating a TES properly into a simple multiperiod heat integration problem with two periods.In case study 2,a simplified example from a cosmetics manufactory is investigated.The total utility demand can be reduced by up to 44.3%due to TES integration and the energetic optimal storage size can be determined as 125 m 3.The savings are strongly dependent on the constellation of heat flows between the periods,on the temperature levels and on the storage size.Significant reductions of energy demand,costs and CO 2 emissions can be achieved with TES being properly integrated into a suitable operating environment.展开更多
基金The financial support provided by the Project of National Natural Science Foundation of China(U22A20415,21978256,22308314)“Pioneer”and“Leading Goose”Research&Development Program of Zhejiang(2022C01SA442617)。
文摘Heat integration is important for energy-saving in the process industry.It is linked to the persistently challenging task of optimal design of heat exchanger networks(HEN).Due to the inherent highly nonconvex nonlinear and combinatorial nature of the HEN problem,it is not easy to find solutions of high quality for large-scale problems.The reinforcement learning(RL)method,which learns strategies through ongoing exploration and exploitation,reveals advantages in such area.However,due to the complexity of the HEN design problem,the RL method for HEN should be dedicated and designed.A hybrid strategy combining RL with mathematical programming is proposed to take better advantage of both methods.An insightful state representation of the HEN structure as well as a customized reward function is introduced.A Q-learning algorithm is applied to update the HEN structure using theε-greedy strategy.Better results are obtained from three literature cases of different scales.
基金This work was supported by the National Natural Science Foundation of China(Grant:21878333).
文摘Crude distillation unit(CDU)is regarded as the main energy consumer in the entire refinery process.In this paper,the process simulation software and the energy management software are used to simulate the flowsheet and analyze the energy consumption,respectively.Stream data obtained from an existing CDU are applicable in the pinch analysis.To reduce the amount of cross-pinch heat transfer,three approaches of resequencing,repiping,and adding heat exchangers are adopted.Compared with the existing CDU,the results demonstrate that the inlet temperature of the furnace can be increased by 25.4℃,the amount of hot and cold utilities can be reduced by 15.1%and 19.6%,respectively.The economic evaluation indicates that the operating cost is saved by 8×106$/a,and the payback period is about 9 months.
基金This work was supported by the National Natural Science Foundation of China(Grant:21878333).
文摘Diesel hydrotreating unit(DHT)is an integral part of the refinery,and its energy-saving optimization is of great significance to the enterprise.In this paper,process simulation software and energy management software are used to simulate the flowsheet and analyze the energy consumption,respectively.Stream data obtained from an existing DHT are applied in the pinch analysis for retrofitting the heat exchanger network(HEN)to achieve maximum energy utilization by using pinch analysis.Since DHT is constrained by pressure,the pressure factor is considered in the process of retrofitting.The results show that the amount of cross-pinch heat transfer is reduced,the inlet temperature of the furnace is increased by 55℃,and the amount of hot and cold utilities can be reduced by 70.25%and 50.16%,respectively.The economic evaluation indicates that the operating cost is saved by 4.39×10^(6)$/a,and the payback period is about 2 months.
基金This work is financially supported by"the Fundamental Research Funds for the Central Universities"(2020XJHH01)the Yueqi Distinguished Scholar Project of China University of Mining and Technology(Beijing)(2020JCB02).
文摘The heat exchanger network(HEN)in a syngas-to-methanol process was designed and optimized based on pinch technology under stable operating conditions to balance the energy consumption and economic gain.In actual industrial processes,fluctuations in production inevitably affect the stable operation of HENs.A flexibility analysis of the HEN was carried out to minimize such disturbances using the downstream paths method.The results show that two-third of the downstream paths cannot meet flexibility requirements,indicating that the HEN does not have enough flexibility to accommodate the disturbances in actual production.A flexible HEN was then designed with the method of dividing and subsequent merging of streams,which led to 13.89%and 20.82%reductions in energy consumption and total cost,respectively.Owing to the sufficient area margin and additional alternative heat exchangers,the flexible HEN was able to resist interference and maintain production stability and safety,with the total cost increasing by just 4.08%.
基金the financial support provided by the National Natural Science Foundation of China(No.21776035)China Postdoctoral Science Foundation(No.2019TQ0045)。
文摘Synthesis of heat exchanger networks including expansion process is a complex task due to the involvement of both heat and work.A stream that expands through expanders can produce work and cold load,while expansion through valves barely affects heat integration.In addition,expansion through expanders at higher temperature produces more work,but consumes more hot utility.Therefore,there is a need to weigh work production and heat consumption.To this end,an enhanced stage-wise superstructure is proposed that involves synchronous optimization of expander/valve placement and heat integration for each pressure-change sub-stream in stages.A mixed-integer nonlinear programming(MINLP)model is established for synthesizing sub and aboveambient heat exchanger networks with multi-stream expansion,which explicitly considers the optimized selection of end-heaters and end-coolers to adjust temperature requirement.Our proposed method can commendably achieve the optimal selection of expanders and valves in a bid for minimizing exergy consumption and total annual cost.Four example studies are conducted with two distinct objective function(minimization of exergy consumption and total annual cost,respectively)to illustrate the feasibility and efficacy of the proposed method.
基金financial support from Jiangsu Collaborative Innovation Center for Cultural Creativity (XYN1911)the National Natural Science Foundation of China (22008023+1 种基金21776035)Natural Science Foundation of Jiangsu Education Department (20KJB510041)
文摘Because of its paramount importance in the successful industrial control strategy of a given heat exchanger network(HEN),the control structure designs for providing appropriate manipulated variable(MV)and controlled variable pairings have received considerable attention.However,quite frequently HENs with such control structures face the problem of hard constraints,typically holding the HENs at less controlled operating space.So both the MV pairings and the above control pairings should be considered to design a control structure.This paper investigates the systematic incorporation of the two pairings,and presents a methodology for designing such two-tier control structure.This is developed based on the sequential strategy,coupling an indirect-tier with direct-tier control structure design,wherein the intention is realized in the former stage and the latter is implemented for further optimization.The MV identification and pairing are achieved through variations in heat load of heat exchangers to design the indirect-tier control structure.Then the direct-tier control structure is followed the relative gain array pairing rules.With the proposed methodology,on the one hand,it generates an explicit connection between the MV pairings and the HEN configuration,and the quantitative interaction measure is improved to avoid the multiple solutions to break the relationship among all the control pairings into individuals;on the other hand,a two-tier control structure reveals control potentials and control system design requirements,this may avoid complex and economically unfavourable control and HEN structures.The application of proposed framework is illustrated with two cases involving the dynamic simulation analysis,the quantitative assessment and the random test.
基金The authors would like to gratefully acknowledge the financial support provided by the National Key R&D Program of China(2017YFE0116300,2019YFE0122100)the Youth Innovation Promotion Association,CAS(2017353)the DNL Cooperation Fund,CAS(DNL202023).
文摘To simultaneously improve the quantity and quality of heat recovery in a heat exchanger network(HEN),this study conducts a theory analysis based on the first and second laws of thermodynamics.Under the premise of maximizing the heat recovery quantity of HEN,Ġdiss is used as an evaluation index to optimize the quality of heat recovery.Meanwhile,the total annual cost(TAC)is considered as another optimization objective to ensure the economic feasibility of the HEN.A superstructure-based multi-objective mixed integer non-linear programming approach is put forward to solve the trade-off between minimizingĠdiss and minimizing TAC.This allows for the optimum HEN structure to be obtained.A well-studied example is solved to highlight the benefits of the proposed method.
基金supported by the National Natural Science Foundation of China(Grant Nos.21978171 and 51976126)the Capacity Building Plan for some Non-military Universities and Colleges of Shanghai Scientific Committee(Grant Nos.16060502600 and 20060502000)。
文摘The rapid development of computational technology and the increasing energy demand have improved heat exchanger network(HEN)synthesis.The HEN synthesis involves several optimizations of matches,distributions of heat loads,and stream splitting of heat units.Thus,obtaining good results at high efficiency has been the main standard for evaluating the techniques in the research area of HEN synthesis.This paper first summarizes and analyzes the main contributions of the existing HEN synthesis techniques.To compare related data quantitively,information on ten typical cases is presented in this paper.Furthermore,recently improved solutions for commonly encountered existing literature cases demonstrate the evolution and competition trends in the field of HEN synthesis.The comparison data presented in this paper not only provide a useful reference for future research but also present the optimization directions.Based on the findings of this study,it is noted that there is still a large room for improvement,and current approaches are incapable of dealing with all HEN cases.Moreover,it is still difficult to escape a local optimum and overcome structural constraints when seeking the global optimum.As a follow-up to the current work,the parallel computing mode and adaptively coordinating the ratio of global and local searching abilities are major development trends for future investigation.
基金supported by the National Natural Science Foundation of China (No. 20936004 and No. 21376187)
文摘Work exchange is a promising innovative technology in recovering residual pressure energy. However, at the systematic level, the comprehensive utilization of different energy resources in an energy system has become an issue of concern. In this work, a systematic approach is proposed, one that successively integrates heat, work and adjusts operation parameters. A detailed procedure for building a heat-work coupling transfer network is provided. The synthesis mainly consists of constructing a work exchange sub-network with pinch analysis based on positive displacement type work exchangers. Simultaneously, another kind of sub-network based on turbine-type work exchangers is built as a schematic comparison. The influence of applying a positive displacement work exchanger on the system is investigated. Finally, as a case study, a renovation design of a typical rectisol process in the coal-water slurry gasification section of an ammonia plant is presented. The results show that the added work exchanger has little impact on the existing heat exchange sub-network. Moreover,extra pressure energy is recovered by coupling the transfer network. It is concluded that the heat-work systematic design is a promising and powerful method to use energy more efficiently.
文摘Heat (energy), water (mass), and work (pres- sure) are the most fundamental utilities for operation units in chemical plants. To reduce energy consumption and diminish environment hazards, various integration methods have been developed. exchange networks (HENs), The application of heat mass exchange networks (MENs), water allocation heat exchange networks (WAHENs) and work exchange networks (WENs) have resulted in the significant saving of energy and water. This review presents the main works related to each network. The similarities and differences of these networks are also discussed. Through comparing and discussing these different networks, this review inspires researchers to propose more efficient and convenient methods for the design of existing exchange networks and even new types of networks including multi-objective networks for the system integration in order to enhance the optimization and controllability of processes.
文摘Around the world energy sustainability and environment protection face many challenges due to the continuously increasing population and energy demands, where the demanding rate of energy increases by at least 2.3% per year. According to statistical data, until 2035, fossil fuels still are the main source of energy consumption. Burning fossil fuels produces the greenhouse gas of carbon dioxide as a byproduct. CO<sub>2</sub> emissions have a dangerous effect on both human health and the natural environmental balance. There are many types of clean energy like solar, biofuel, and wind energy. The major limitations of using these types, their availability depends on climate conditions and their production rate is inadequate for energy demand. For any country, energy resources’ availability and economic conditions imposed on projects’ prioritization. Application of energy management and emissions control techniques for industrial unit can limit fuel combustion environmental effects and transfer this fossil fuel to an eco-friendly type. In this work, we applied Green Energy Model GEM to the Hydrotreater Unit of the refinery, GEM is composed of four techniques. Which are Heat Exchangers Networks Synthesis [HENS], Fuel Switching, Thermal Insulation application, and Carbon Captures Storage [CCS]. Where they reduce energy consumption by the rate of 3% - 34% and control CO2 emissions by the rate of 26% - 90%. This model is a radical way to face climate change challenges and practical solutions for both energy and environmental crises.
文摘In times of increasing global warming,enormous efforts are required to rapidly reduce greenhouse gas(GHG)emissions.Due to the EU’s target of climate neutrality by 2050 and the even more ambitious goal of becoming climate-neutral in Germany by 2045,it is necessary to systematically increase energy efficiency and decarbonize the industrial heat sector.The methods of heat integration can be used to exploit existing potentials for waste heat utilization and to integrate renewable technologies for heating and cooling.By using a non-stationary,multiperiod approach,additional energy savings can be achieved by integrating a thermal energy storage(TES)that enables heat transportation over time.This paper presents a simultaneous approach for thermal energy storage integration into multiperiod heat integration problems.The approach can be used to minimize energy demand,costs and CO 2 emissions and is demonstrated in two case studies.In case study 1,it is shown that the presented approach is capable of integrating a TES properly into a simple multiperiod heat integration problem with two periods.In case study 2,a simplified example from a cosmetics manufactory is investigated.The total utility demand can be reduced by up to 44.3%due to TES integration and the energetic optimal storage size can be determined as 125 m 3.The savings are strongly dependent on the constellation of heat flows between the periods,on the temperature levels and on the storage size.Significant reductions of energy demand,costs and CO 2 emissions can be achieved with TES being properly integrated into a suitable operating environment.