Dual Mixed Refrigerant Process Research Articles

Novel dual-mixed refrigerant precooling process for high capacity hydrogen liquefaction plants with superior performance

Liquid hydrogen is a superior alternative for the current energy storage methods and energy carriers as it has higher energy density and cleanliness. However, hydrogen liquefaction is an energy-intensive process. In particular, the precooling process of hydrogen consumes a tremendous portion of about 30 % of the total compression power of the plant. Several previous studies introduced various pure-refrigerant and single mixed refrigerant (SMR) precooling processes, however, their specific energy consumption (SEC) still very high especially at large-scale capacities. Therefore, this study presents a novel, efficient, and large-scale dual-mixed refrigerant (DMR) process to precool the hydrogen from 25 °C to -192 °C at a pressure of 21 bar. New heavyweight-based mixed refrigerant MR1 and lightweight-based mixed refrigerant MR2 are developed for the DMR process using a new-proposed systematic approach. The proposed DMR process is capable of handling a wide range of hydrogen flow from 100 TPD to 1000 TPD with SEC of 0.862 kWh/kgH2Feed, which is 20.33 % lower than the most competitive SMR process available in the literature. Based on the sensitivity analysis, further optimization of the DMR operating parameters reduced the SEC to 0.833 kWh/kgH2Feed at an optimal capacity of 500 TPD. Furthermore, the COP of the new process is improved by 14.47 % and the total annualized cost is reduced by 12.24 %. Compared to five other technologies that use the pure-refrigerant and other SMR precooling processes, the DMR reduces the SEC by 39.0 % to 63.0 %. The novel precooling process presented herein has the potential to drive the development of large-scale hydrogen liquefaction processes.

Teaching-learning self-study approach for optimal retrofitting of dual mixed refrigerant LNG process: Energy and exergy perspective

This study unfolds the advanced process configuration modification in the evolution of a dual mixed refrigerant (DMR) process for natural gas liquefaction, followed by its optimization through a unique approach i.e., teaching–learning self-study optimization (TLSO). The DMR process is improved by replacing Joule Thomson valves with the isentropic cryogenic turbines. To ensure the maximum possible thermodynamic performance of the retrofitted DMR process, the TLSO paradigm is used and evaluated. The energy, exergy, coefficient of performance, and figure of merit are determined and compared with conventional bench-scale DMR process to find the performance improvement opportunities in the proposed cryogenic turbine-retrofitted DMR process. The performance analysis revealed that the proposed optimal retrofitted DMR process could produce LNG using 28.57% less energy than the base case. The detailed thermodynamic evaluation revealed that the proposed DMR process has 64.68% exergy efficiency, 2.42 coefficient of performance, and 41.6% figure of merit, which are 13.37%, 19%, and 11.9%, higher than the conventional DMR process, respectively. This study would significantly help process engineers overcome the challenges of relating energy efficiency of the LNG plants for both onshore and offshore applications.

Design and analysis of dual mixed refrigerant processes for high-ethane content natural gas liquefaction

Recovery and purification of ethane has a significant impact on economic benefit improvement of the high-ethane content natural gas. However, current LNG-NGL integrated processes mainly focus on conventional natural gas, which are not applicable to natural gas with high ethane content. To fill this gap, three dual mixed refrigerant processes are proposed for simulation study of high-ethane content natural gas liquefaction. The proposed processes are optimized by a combination method of sequence optimization and genetic algorithm. Comparatively analysis is conducted to evaluate the three processes from the energetic and exergetic points of view. The results show that the power consumption of Process 3 which compressing natural gas after distillation is the lowest. For safety or other considerations, some common compositions of the mixed refrigerant may need to be removed under certain circumstances. Considering this, case studies of mixed refrigerant involving six composition combinations are carried out to investigate the effects of refrigerant selection on the process performance.

Dual-effect single-mixed refrigeration cycle: An innovative alternative process for energy-efficient and cost-effective natural gas liquefaction

Typical liquefaction processes are considered to be energy and cost-intensive. The dual mixed refrigerant (DMR) process (with two independent refrigeration cycles for cooling and subcooling) produces liquefied natural gas (LNG) at relatively high energy efficiency. However, it exhibits a high degree of configurational complexity and high sensitivity to operational conditions, and it also incurs a large capital investment. These factors eventually reduce the overall competitiveness of the liquefaction process, particularly for offshore applications. To address these issues, an energy- and cost-efficient dual-effect single mixed refrigerant (DSMR) process is proposed herein, and it employs a single loop refrigeration cycle to generate the dual cooling and subcooling effect, separately. The DMR process and the proposed DSMR process are simulated (with same design parameters) using well-known commercial simulator Aspen Hysys v10. Then, both processes are optimized using modified coordinate descent algorithm. The specific energy consumption of DSMR is 0.284 kWh/kg-NG, which is equivalent to an energy saving of 22.89% when compared to the conventional DMR process. The exergy efficiency of DSMR is 36.62%, which is 29.67% higher than that of the classical DMR process. Furthermore, the economic feasibility of the proposed DSMR process is evaluated in terms of its total annualized cost, which is 18.52% lower than that of the DMR process. Thus, the proposed DSMR process offers remarkable energy and exergy efficiencies with minimal capital investment. Therefore, DSMR could replace the classical DMR process, as well as other complex mixed refrigerant-based liquefaction processes.

Comparison and analysis of two processes for BOG re-liquefaction in LNG carrier with normal-temperature compressor

Two processes for boil-off gas (BOG) re-liquefaction of liquefied natural gas (LNG) carrier with normal-temperature compressor are proposed and compared: (1) the re-liquefaction system with the single mixed refrigerant process (SMRP), (2) the re-liquefaction system with the dual mixed refrigerant process (DMRP). The main thermodynamic parameters to be compared are specific energy consumption, exergy efficiency, coefficient of performance, heat transfer curves and exergy losses of main equipment in optimized case and base case. The result shows that the performance of the DMRP is better than that of the SMRP and the specific energy consumption of DMRP is 0.5635 kWh/kgLNG. In addition, by comparing the thermodynamic parameters of the re-liquefaction system with the normal-temperature compressor and the cryogenic compressor, it can be found that not only the cooling capacity of BOG is used effectively, but also the energy consumption is reduced and the efficiency is improved for the normal-temperature compressor system. Therefore, for the two mixed refrigerant re-liquefaction systems, the performance of the normal-temperature compression system is better than that of the low-temperature compression system.

Optimization of a dual mixed refrigerant process using a nonsmooth approach

This article uses a nonsmooth flowsheeting methodology to create simulation and optimization models for dual mixed refrigerant processes. New improved operating conditions are obtained using the primal-dual interior-point optimizer IPOPT, with sensitivity information calculated using new developments in nonsmooth analysis to obtain generalized derivative information using a nonsmooth generalization of the vector forward mode of automatic differentiation. Several optimization studies are performed with constraints on both the minimum temperature difference (ΔTmin) and total heat exchanger conductance (UAmax) used to represent the trade-offs between energy consumption and the required heat transfer area. In addition, comparison is made with the conventional process simulator Aspen HYSYS using particle swarm optimization. Results show that the nonsmooth model was able to reduce the required compression power by 14.4% compared to the initial feasible design for the dual mixed refrigerant process, and by 20.4–21.6% for the dual mixed refrigerant process with NGL extraction. Furthermore, the solutions obtained from the nonsmooth model were 1.9–8.1% better than the design obtained by particle swarm optimization. Multistart optimization also shows that IPOPT converges to the best known solution when starting from an initial feasible design.

Dual mixed refrigerant LNG process: Uncertainty quantification and dimensional reduction sensitivity analysis

The dual mixed refrigerant (DMR) liquefaction process is complicated and sensitive compared to the competitive propane pre-cooled mixed refrigerant liquefied natural gas (LNG) process. When any uncertainty is introduced to the process operation conditions, it is necessary for the DMR process to be re-optimized to maintain efficient operation at a minimal cost. However, in actual operation, re-optimization is a challenging task when the process operational input variables are varied, typically owing to the lack of information regarding the nature, impact, and levels of uncertainty. Within this context, this study investigates the uncertainty levels in the overall energy consumption and minimum internal temperature approach (MITA) inside LNG heat exchangers with variations in the operational variables of the DMR processes. Moreover, a global sensitivity analysis is conducted to identify the influence of random inputs on the process performance parameters. The required energy is significantly influenced by the variations in the variables in the cold mixed refrigerant (approximately 63%), while changes in the warm mixed refrigerant (WMR) section only slightly affect the uncertainty of the required specific energy. Furthermore, the probability distribution of the approach temperature (MITA1) inside the WMR exchanger is mainly affected by changes in the compositions of methane, ethane, and propane, as well as the high pressure of the cold mixed refrigerant (approximately 97%). Conversely, the flow rate of ethane and low pressure of the WMR significantly affect the uncertainty of the approach temperature (MITA2) inside the cold mixed refrigerant exchanger.

Simulation of Dual Mixed Refrigerant Natural Gas Liquefaction Processes Using a Nonsmooth Framework

Natural gas liquefaction is an energy intensive process where the feed is cooled from ambient temperature down to cryogenic temperatures. Different liquefaction cycles exist depending on the application, with dual mixed refrigerant processes normally considered for the large-scale production of Liquefied Natural Gas (LNG). Large temperature spans and small temperature differences in the heat exchangers make the liquefaction processes difficult to analyze. Exergetic losses from irreversible heat transfer increase exponentially with a decreasing temperature at subambient conditions. Consequently, an accurate and robust simulation tool is paramount to allow designers to make correct design decisions. However, conventional process simulators, such as Aspen Plus, suffer from significant drawbacks when modeling multistream heat exchangers. In particular, no rigorous checks exist to prevent temperature crossovers. Limited degrees of freedom and the inability to solve for stream variables other than outlet temperatures also makes such tools inflexible to use, often requiring the user to resort to a manual iterative procedure to obtain a feasible solution. In this article, a nonsmooth, multistream heat exchanger model is used to develop a simulation tool for two different dual mixed refrigerant processes. Case studies are presented for which Aspen Plus fails to obtain thermodynamically feasible solutions.

Risk and efficiency analysis of dual mixed refrigerant liquefaction process configurations for floating liquefied natural gas at conceptual design stage

Liquefied natural gas (LNG) floating production storage offloading, or floating liquefied natural gas (FLNG), is an offshore unit used to produce LNG from offshore gas reservoirs. The liquefaction is critical process for liquefying natural gas (NG) into LNG. Among NG liquefaction technologies used in the industry, single mixed refrigerant, dual mixed refrigerant (DMR), and the nitrogen expansion liquefaction process have been considered for FLNG on account of its space limitations and higher safety standards. In particular, the DMR liquefaction process is preferred for a large FLNG because of its high efficiency. Many studies have been suggested about an efficiency of DMR, but a few studies have been conducted on their process safety although different configurations in process concepts can cause meaningful differences in operating conditions and safety. In this study, two DMR process configurations were optimized to maximize the efficiency and conceptual explosion risk was analyzed to compare their risk at the conceptual design stage. The results showed a difference between the explosion risks by the differences in the optimal mixed refrigerant compositions and number of devices, with similar efficiencies. These results can provide insight for a risk management strategy at the conceptual design stage, to minimize the unexpected cost generation. © 2018 American Institute of Chemical Engineers Process Process Saf Prog 38: 87–98, 2019

Hydrofluoroolefin-based novel mixed refrigerant for energy efficient and ecological LNG production

To satisfy the worldwide demand for energy, the liquefied natural gas (LNG) industry has grown significantly in the past three decades owing to its low CO2 emissions and high thermal efficiency compared to the other available energy resources. However, the process of natural gas liquefaction is generally considered to be energy-intensive. In this context, a novel hydrofluoroolefin (HFO-1234yf)-based mixed refrigerant, with the advantages of zero ozone depletion and minimal global warming potential, is proposed to liquefy natural gas in an ecological and energy-efficient manner. A new liquefaction cycle using the HFO-based mixed refrigerant is developed to fully utilize its potential. The results reveal that the overall energy requirement for natural gas liquefaction can be reduced by 46.4% compared with a single mixed refrigerant process, 42.5% compared with a dual mixed refrigerant process, and 36.3% compared with the Linde–single mixed refrigerant process. Economic analysis based on the capacity parameters of each equipment is also performed to emphasize the commercial feasibility of the proposed LNG process. The proposed HFO-based mixed refrigerant system provides an innovative solution to improve the ecological aspects and energy efficiency of natural gas liquefaction processes.

Evaluation of novel process configurations for coproduction of LNG and NGL using advanced exergoeconomic analysis

Advanced exergoeconomic analysis is applied on two novel process configurations for co-production of LNG and NGL. Dual mixed refrigerant (DMR) and mixed fluid cascade (MFC) refrigeration systems are used for supplying the required refrigeration. Based on the avoidable cost of exergy destruction in DMR process configuration, C-300 compressor with 504.43($/h) and in MFC process configuration, C-200A compressor with 251.05($/h) should be modified first. Based on the avoidable endogenous/exogenous part, three strategies are proposed for reducing exergy destruction cost in the process components. Cost of exergy destruction and investment in most of the process components are endogenous. So interactions among the components in these processes is not strong. Investment cost of turbo expander and compressors are unavoidable due to technological and economic limits while air coolers and heat exchangers have potential for improvement. Cost of exergy destruction of the air coolers and heat exchangers are unavoidable while turbo expander and compressors are avoidable.

Evolution and optimization of the dual mixed refrigerant process of natural gas liquefaction

This study unfolds the important developments in the evolution of dual mixed refrigerant (DMR) process of natural gas (NG) liquefaction followed by its optimization. The initial designs of DMR process involve direct intermixing of non-equilibrium streams that causes thermodynamic irreversibility and reduces efficiency. Major developments that improved DMR process efficiency were the use of coil-wind type heat exchangers followed by three stage throttling in NG pre-cooling and direct utilization of cold energy available to the boil-off gas. After enumerating major developments a generic design of DMR process is selected and optimized for specific compression energy (SCE) and overall heat transfer coefficient (UA) using Box methodology and controlled elitist genetic algorithm. Single objective optimization of SCE and UA with Box methodology results in savings of 36% and 15% respectively. There exists a partial trade-off between SCE and UA, thus the savings in SCE are offset by the increase of UA. Consequently a multi-objective optimization is performed that results in a simultaneous decrease of 24% and 3% in the SCE and UA values, compared to the base case.

Advanced exergoeconomic analysis of the multistage mixed refrigerant systems

Advanced exergoeconomic analysis is applied on three multi stage mixed refrigerant liquefaction processes. They are propane precooled mixed refrigerant, dual mixed refrigerant and mixed fluid cascade. Cost of investment and exergy destruction for the components with high inefficiencies are divided into avoidable/unavoidable and endogenous/exogenous parts. According to the avoidable exergy destruction cost in propane precooled mixed refrigerant process, C-2 compressor with 455.5 ($/h), in dual mixed refrigerant process, C-1 compressor with 510.8 ($/h) and in mixed fluid cascade process, C-2/1 compressor with 338.8 ($/h) should be considered first. A comparison between the conventional and advanced exergoeconomic analysis is done by three important parameters: Exergy efficiency, exergoeconomic factor and total costs. Results show that interactions between the process components are not considerable because cost of investment and exergy destruction in most of them are endogenous. Exergy destruction cost of the compressors is avoidable while heat exchangers and air coolers destruction cost are unavoidable. Investment cost of heat exchangers and air coolers are avoidable while compressor’s are unavoidable.

Design and analysis of multi-stage expander processes for liquefying natural gas

Multi-stage expander refrigeration cycles were proposed and analyzed in order to develop an efficient natural gas liquefaction process. The proposed dual and cascade expander processes have high efficiency and the potential for larger liquefaction capacity and are suitable for small-scale and offshore natural gas liquefaction systems. While refrigeration cycles of conventional expander processes use pure nitrogen or methane as a refrigerant, the proposed refrigeration cycles use one or more mixtures as refrigerants. Since mixed refrigerants are used, the efficiency of the proposed multi-stage expander processes becomes higher than that of conventional expander processes. However, the proposed liquefaction processes are different from the single mixed refrigerant (SMR) and dual mixed refrigerant (DMR) processes. The proposed processes use mixed refrigerants as a form of gas, while the SMR and DMR processes use mixed refrigerants as a form of gas, liquid- or two-phase flow. Thus, expanders can be employed instead of Joule-Thomson (J-T) valves for refrigerant expansion. Expanders generate useful work, which is supplied to the compressor, while the high-pressure refrigerant is expanded in expanders to reduce its temperature. Various expander refrigeration cycles are presented to confirm their feasibility and estimate the performance of the proposed process. The specific work, composite curves and exergy analysis data are investigated to evaluate the performance of the proposed processes. A lower specific work was achieved to 1,590 kJ/kg in the dual expander process, and 1,460 kJ/kg in the cascade expander process. In addition, the results of exergy analysis revealed that cycle compressors with associated after-coolers and companders are main contributors to total exergy losses in proposed expander processes.

Steady-state Optimality Analysis for Investigating the Energy Optimal Operation of Representative Natural Gas Liquefaction Cycles

This study examined the energy optimal operation of representative natural gas liquefaction cycle processes such as propane precooled mixed refrigerant (C3MR) process, dual mixed refrigerant (DMR) process, and modified single mixed refrigerant (MSMR) process. Steady-state optimality analysis in dynamic simulation environment was conducted to explore the operational behavior of each cycle. From this analysis, a steady-state optimality map that describes the relation between cost function and decision variable is obtained. By exploring this map a promising optimizing variable is discovered which further can be used to develop an energy optimizing control structure for the liquefaction process. Despite the same basic working principles, the operational behavior of the three cycles is dissimilar. The DMR has the narrowest optimal operation range while in the MSMR cycle the optimum value of cost function spans in relatively wide range of decision variable. The feasible operation of C3MR and DMR is bounded by the suction temperature of mixed refrigerant compressor while in the MSMR cycle this constraint is inactive. Based on the steady-state optimality analysis the temperature difference between the warm-end inlet and outlet MR streams (TD) were proposed to be a promising optimizing variable for the C3MR and DMR process while for the MSMR process the optimizing variable is the flow rate ratio of heavy and light mixed refrigerant (HK/LK ratio).

Synthesis of an Optimizing Control Structure for Dual Mixed Refrigerant Process

Synthesis of an Optimizing Control Structure for Dual Mixed Refrigerant Process