Mixed Refrigerant Natural Gas Liquefaction Research Articles

Transcritical [formula omitted] precooled single mixed refrigerant natural gas liquefaction process: Exergy and Exergoeconomic optimization

This study proposes a two-stage transcritical CO2 pre-cooled single mixed refrigerant process to investigate the potential improvement in exergy efficiency and exergoeconomic performance. Two different objective functions, namely maximizing exergy efficiency and minimizing the unit cost exergy of LNG, were optimized using the particle swarm optimization algorithm to compare the performance of the CO2 pre-cooled single mixed refrigerant process with single mixed refrigerant process. The results of the first objective function indicate a higher exergy efficiency of the CO2 pre-cooled single mixed refrigerant process, with an increment of 7.56%. This improvement can be attributed to not only the lower total power consumption but also the better matching of the temperature-difference composite curve, particularly the location of the pinch temperature at the warm end of the heat exchanger. Additionally, the exergy destruction of the CO2 pre-cooled single mixed refrigerant process was 30.12% lower than that of the single mixed refrigerant process. The second objective function demonstrated that the CO2 pre-cooled single mixed refrigerant process had a unit exergy cost value of 7.46 $/GJ, which was 11.98% lower than the single mixed refrigerant process. In conclusion, the proposed process contributes to energy saving throughout the LNG chain. Previous studies on the transcritical CO2 precooling cycle were limited to only one-stage configuration paving the way for thermodynamic efficiency enhancement by modifying the design of the process. This study also considers the effects of two objective functions maximizing exergy efficiency and minimizing the unit exergy cost of LNG on the optimization of decision variables which has not been discussed yet in the literature. Moreover, innovative exergoeconomic analysis presents a comprehensive criterion for evaluating economic performance, providing guidance for the initial construction and sustainable development of LNG plants.

Integrated ethane recovery and cryogenic carbon capture in a dual mixed refrigerant natural gas liquefaction process

Carbon capture by direct cryogenic distillation in conventional natural gas liquefaction plants is quite difficult due to the easy sublimation characteristics of CO2. The freeze–out temperature of CH4–CO2–C2H6 ternary mixtures is analyzed in this study, and the results show that with the azeotropic properties of ethane-CO2 mixtures, CO2 in natural gas can be reduced to below 50 ppm by cryogenic distillation. Based on thermodynamic analysis, a novel dual mixed refrigerant (DMR) liquefaction process integrated with ethane recovery and carbon capture is proposed for natural gas that is rich in ethane. CO2 is first captured from natural gas along with ethane by cryogenic distillation, and then further separated from the CO2–C2H6 mixture by extractive distillation to obtain ethane. The proposed process is simulated in HYSYS and optimized by a method combining a genetic algorithm (GA) and stepwise search. Energy analysis results show that when the ethane content is 2–20 mol%, the specific power consumption corresponding to the maximum allowable CO2 content with ethane recovery rate and purity both reaching 99.5 % is around 0.40 kWh/Nm3 (NG). In addition, the comparative analysis results show that the proposed process has better performance than chemical absorption in both energy consumption and cleaner production aspects.

A new strategy for mixed refrigerant composition optimisation in the propane precooled mixed refrigerant natural gas liquefaction process

The natural gas liquefaction process occupies a considerable part of the energy consumption in the LNG industry chain, and efficient optimisation methods should be developed to improve the economic and environmental performances of this process. In this work, a new optimisation strategy based on the particle swarm optimisation (PSO) algorithm and bisection method was developed to optimise the propane precooled refrigerant process (C3MR). In the proposed optimisation strategy, the molar fractions of the mixed refrigerant (MR) components were directly adopted as variables in the PSO algorithm, and the MR flow rate was independently adjusted by the bisection method to satisfy the minimum temperature approach constraints. The results show that the optimal operating conditions reduced the energy requirement of the C3MR process by 4% and decreased the consumption of MR by 31.9%, compared to traditional using the flow rates of MR components as variables. The correlations among the properties of key streams were calculated based on the intermediate results of the proposed method and visualised with the heat map. The results show that the energy requirement of the C3MR process decreased with the increase of the low boiling temperature components and increased with the high boiling temperature components of MR.

Thermodynamic Analysis and Optimization of the C3/MRC Liquefaction Process

In the natural gas liquefaction process, the mixed refrigerant natural gas liquefaction process is widely used in LNG liquefaction plants because of its advantages of low energy consumption. This paper focuses on the influences of important parameters in the C3/MRC liquefaction process, that is, the comparison between propane precooling temperature and the number of moles of methane in mixed refrigerant, power consumption and loss. In addition, the total process was optimized with the optimizer and manual adjustment in HYSYS software to minimize the total power consumption. The results show that with increasing propane precooling temperature, the propane flow rate is almost unchanged, while the mixed refrigerant flow rate decreases significantly, and the loss of the heat exchanger increases significantly. The power consumption of the propane precooling cycle and hybrid refrigeration cycle increases with increasing methane content in the refrigerant, so the power consumption of the whole process increases accordingly. The effect of the methane content in the mixed refrigerant on the process evaluation index is more significant than that of the propane precooling temperature.

Energy saving and production increase of mixed refrigerant natural gas liquefaction plants by taking advantage of natural cold sources in winter

Generally speaking, natural cold sources, such as seawater or air, are often used in natural gas liquefaction processes to cool the compressed refrigerant and natural gas. Since both the required refrigeration and the performance of compressors are closely related to ambient temperature, natural gas plants are designed according to summer operating conditions, because the highest ambient temperature in summer results in the highest refrigeration requirement of the system. However, in winter conditions, with the decline of ambient temperature, precooling by natural cold source can reach lower temperature. When the cooling capacity of natural cold sources is fully utilized, the already built natural gas liquefaction plant can not only reduce energy consumption, but also increase production in winter. To realize cleaner production, this paper comparatively investigated the potential of three mixed refrigerant liquefaction processes, which includes the single mixed refrigerant (SMR), dual mixed refrigerant (DMR) and propane precooled mixed refrigerant (C3MR) processes, for energy reduction and production increase in winter. Based on the optimal conditions under different ambient temperature obtained by genetic algorithm, the specific power consumption, heat transfer performance, as well as changes of typical process parameters such as the flow rate and composition of mixed refrigerants are studied to analyze the effect of natural cold source on the process. When the ambient precooling temperature drops from 40 °C to 10 °C, the specific power consumption of the three processes declines by 20.84%–24.97%, and when it drops to −10 °C, the specific power consumption of the three processes can be reduced by 34.31%–40.53%, which means huge benefit of power-saving. While keeping the volume flow of the compressor in winter the same as that in summer, the SMR process and C3MR process have the largest (26.6%–54.6%) and smallest (16.1%–25.4%) production increase potentials, respectively. Further study is conducted to analyze the load changes of key equipment and economic benefits after changing into energy saving mode or production increase mode in winter. The results indicate that both mode in winter is feasible for the processes with the help of natural cold source, and this may bring considerable increase in economic profits.

A novel propane pre-cooled mixed refrigerant process for coproduction of LNG and high purity ethane

For high ethane-containing natural gas, such as shale gas, separation and purification of ethane during liquefaction can improve economic benefits. A novel process of propane pre-cooled mixed refrigerant natural gas liquefaction integrated with cryogenic distillation for ethane separation is introduced and analyzed, which produce both liquefied natural gas (LNG) and liquefied ethane at the same time. The chemical engineering software HYSYS is used to simulate the process, and the liquefaction operation parameters of the system are optimized by genetic algorithm to reduce energy consumption. The results show that the proposed process can effectively achieve ethane separation, with the purity of ethane product over 99.5% and ethane recovery rate higher than 99.5%. As the ethane content in the feed gas increases, the specific power consumption and exergy efficiency of the system decrease slightly. When ethane content is in the range of 10%–40%, the optimized specific energy consumption is around 0.44 kWh/Nm3(natural gas), and the exergy efficiency is around 47%.

Design of single mixed refrigerant natural gas liquefaction process considering load variation

This study presents a comprehensive design approach to determine optimal equipment size and operating conditions while considering process load variation. We applied the suggested approach to PRICO® Single Mixed Refrigerant (SMR) process to take account of the feed gas load reduction owing to depletion of natural gas fields. The suggested approach differs from a traditional one in that it performs design and optimization with several steady-state operation regimes depending on the load variation. The economics of each design approach is evaluated by the economic assessment model that reflects the annual profit under the varying production rate according to the actual production profiles of the gas field wells, Maui and Kapuni in New Zealand. The proposed design approach makes a loss in the compressor equipment cost. However, it reduces the operation cost over a wide range of operations, leading to the overall improvement of economics in a gas well along its lifetime production. This study also conducts a quantitative analysis between the load capacity that a single train must bear and the key economic variables through a case study on two-train operation. This provides insight into the economics and operability of the process depending on the number of trains.

Design optimization of single mixed refrigerant LNG process using a hybrid modified coordinate descent algorithm

Design optimization of the single mixed refrigerant (SMR) natural gas liquefaction (LNG) process involves highly non-linear interactions between decision variables, constraints, and the objective function. These non-linear interactions lead to an irreversibility, which deteriorates the energy efficiency of the LNG process. In this study, a simple and highly efficient hybrid modified coordinate descent (HMCD) algorithm was proposed to cope with the optimization of the natural gas liquefaction process. The single mixed refrigerant process was modeled in Aspen Hysys® and then connected to a Microsoft Visual Studio environment. The proposed optimization algorithm provided an improved result compared to the other existing methodologies to find the optimal condition of the complex mixed refrigerant natural gas liquefaction process. By applying the proposed optimization algorithm, the SMR process can be designed with the 0.2555 kW specific compression power which is equivalent to 44.3% energy saving as compared to the base case. Furthermore, in terms of coefficient of performance (COP), it can be enhanced up to 34.7% as compared to the base case. The proposed optimization algorithm provides a deep understanding of the optimization of the liquefaction process in both technical and numerical perspectives. In addition, the HMCD algorithm can be employed to any mixed refrigerant based liquefaction process in the natural gas industry.

Energy optimization for single mixed refrigerant natural gas liquefaction process using the metaheuristic vortex search algorithm

A metaheuristic vortex search algorithm was investigated for the optimization of a single mixed refrigerant (SMR) natural gas liquefaction process. The optimal design of a natural gas liquefaction processes involves multivariable non-linear thermodynamic interactions, which lead to exergy destruction and contribute to process irreversibility. As key decision variables, the optimal values of mixed refrigerant flow rates and process operating pressures were determined in the vortex pattern corresponding to the minimum required energy. In addition, the rigorous SMR process was simulated using Aspen Hysys® software and the resulting model was connected with the vortex search optimization algorithm coded in MATLAB. The optimal operating conditions found by the vortex search algorithm significantly reduced the required energy of the single mixed refrigerant process by ≤41.5% and improved the coefficient of performance by ≤32.8% in comparison with the base case. The vortex search algorithm was also compared with other well-proven optimization algorithms, such as genetic and particle swarm optimization algorithms, and was found to exhibit a superior performance over these existing approaches.

Enhancement of single mixed refrigerant natural gas liquefaction process through process knowledge inspired optimization and modification

This study examined the enhancement of the single mixed refrigerant (SMR) natural gas liquefaction process. The effects of the main parameters, such as mixed refrigerant (MR) composition and operating pressures on the compression energy requirement were investigated. A process knowledge inspired decision-making method was exploited for liquefied natural gas process optimization. The results showed that the proposed optimization methodology is simple and effective in determining the optimal operating conditions and could save up to 30.6% in terms of the compressor duty compared to the base case. In addition, the proposed optimization methodology provides process understanding, which is essential to process engineers. Another benefit of the proposed methodology is that it can be applied to any MR liquefaction cycle. The use of heavier refrigerants, such as isobutane and isopentane, and the addition of a NG compressor were examined to improve the energy efficiency of the SMR process. The effect of the intercooler outlet temperature on energy saving was also considered. The synergistic effects of those modifications on improving the performance of the liquefaction process were investigated.

Development of a robust refrigerant mixture for liquefaction of highly uncertain natural gas compositions

To export natural gas to overseas, it has to be liquefied and transported by the LNG tankers. Some LNG plants will receive their natural gas from different shale gas reservoirs so that one of the key challenges in developing a refrigerant mixture is the variation the of natural gas compositions. Previous attempts to create refrigerant mixtures for LNG plants were focused on implementing deterministic optimization methods. However, we demonstrate in this paper that these optimized refrigerants are so sensitive to natural gas composition that a slight variation in natural gas composition makes them unsuitable for the liquefaction process. To demonstrate the Gradient Assisted Robust Optimization technique capabilities, we developed a refrigerant mixture for propane precooled mixed refrigerant natural gas liquefaction cycles with an exaggerated variation in feed gas composition. This refrigerant is relatively unaffected by the variation of the natural gas compositions. We compared the performance of the new refrigerant to five other refrigerants found in literature and found the newly developed refrigerant is the only one satisfying the design constraints for all of the tested natural gas mixture compositions. This technique can be used for developing refrigerant for any LNG cycle that has a variation in feed composition.

Advanced exergoeconomic evaluation of single mixed refrigerant natural gas liquefaction processes

Conventional exergoeconomic analysis cannot recognize part of exergy destruction cost and investment cost that can be avoided. Also this analysis does not have ability to determine interactions between the components. In this study advanced exergoeconomic analysis is studied on two single mixed refrigerant processes. Costs of investment and exergy destruction are divided into avoidable/unavoidable and endogenous/exogenous parts for the components with high inefficiency. According to the avoidable exergy destruction cost in Linde process, E-1 heat exchanger with 13,232.4 ($/h) and in APCI process, C-1 compressor with 476.9 ($/h) should be considered for modification. Because these devices have higher improving potential than the other devices. The results show that costs of investment and exergy destruction in most of the process components are endogenous. Interactions between the process components do not affect the inefficiencies significantly. Cost of exergy destruction in the compressors is avoidable while investment costs in these components are unavoidable. Cost of exergy destruction in heat exchangers and air coolers is unavoidable while investment costs in these components are avoidable.

Exergoeconomic evaluation of single mixed refrigerant natural gas liquefaction processes

Exergy and exergoeconomic analysis is performed for single mixed refrigerant Linde and Air Products and Chemicals Inc, processes, which are among the most important and popular natural gas liquefaction processes. Cost of exergy destruction, exergoeconomic factor, exergy destruction and exergy efficiency are calculated. Results of exergy analysis demonstrates that exergy efficiency of Linde process is around 40.2%, and its total exergy destruction rate is 93,229kW. The exergy efficiency and exergy destruction rate for Air Products and Chemicals Inc, process are 45.0% and 72,245kW respectively. Results of exergoeconomic analysis suggests that maximum exergy destruction cost for Linde process is related to E-2 heat exchanger which is 34,072$/h and for Air Products and Chemicals Inc, process maximum exergy destruction cost is related to E-2 heat exchanger with the value of 4125kW. Sensitivity of cost of exergy destruction and exergoeconomic factor to operating variables of the processes are studied and analyzed.

Design optimization of single mixed refrigerant natural gas liquefaction process using the particle swarm paradigm with nonlinear constraints

The particle swarm paradigm is employed to optimize single mixed refrigerant natural gas liquefaction process. Liquefaction design involves multivariable problem solving and non-optimal execution of these variables can waste energy and contribute to process irreversibilities. Design optimization requires these variables to be optimized simultaneously; minimizing the compression energy requirement is selected as the optimization objective. Liquefaction is modeled using Honeywell UniSim Design™ and the resulting rigorous model is connected with the particle swarm paradigm coded in MATLAB. Design constraints are folded into the objective function using the penalty function method. Optimization successfully improved efficiency by reducing the compression energy requirement by ca. 10% compared with the base case.

Optimization of single mixed refrigerant natural gas liquefaction plant with nonlinear programming

ABSTRACTThe liquefaction of natural gas (NG) in a mixed refrigerant (MR) system is an energy‐demanding process. Much energy is wasted because of its irreversibilities and its nonoptimal execution. The most important factors affecting this process's performance are the refrigerant's composition and flow rate, the suction and evaporation pressures, and the extent of refrigerant vaporization. They should be adjusted to optimize the overall operation. The adjustment of one of these variables will affect the other because of their highly nonlinear interactions. This work reports the optimization of a single MR (SMR) process of NG liquefaction. The SMR process was modeled in the UniSim Design commercial process plant simulator, and the model was optimized for compression energy with nonlinear programming (NLP) while satisfying constraints. The base case for optimization was selected by mesh searching, and case study demonstrates that NLP can reduce energy use and improve the process's efficiency. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.