Mixed Refrigerant Liquefaction Process Research Articles

The Design and Optimization of Natural Gas Liquefaction Processes: A Review

As the energy crisis intensifies, the global demand for natural gas is growing rapidly. Liquefied natural gas (LNG) technology is among the delivery solutions with flexible and reliable application prospects and is already a significant field of research in energy utilization. The performance of natural gas liquefaction process has a major influence on the production capacity, energy consumption, economics, and safety of the entire supply chain. Many scholars have conducted numerous studies on various LNG processes and designed many classical processes. This paper summarizes and discusses current research status and development level in the design and optimization of natural gas liquefaction processes in recent years, mainly focusing on cascade liquefaction process, expansion liquefaction process, and mixed refrigerant liquefaction process. The advantages and disadvantages of various liquefaction processes are compared and analyzed in terms of liquefaction capacity, energy consumption, economy, safety, and adaptability. In addition, the rapid development of pressurized liquefaction technology in recent years and its application outlooks are also introduced in detail. Finally, the present situation and industrial demand of LNG process are analyzed, and reasonable suggestions and future research prospects are put forward.

Performance evaluation and comparison of commonly used optimization algorithms for natural gas liquefaction processes

The genetic algorithm (GA), particle swarm optimization (PSO) algorithm, and BOX algorithm have been used in natural gas liquefaction process optimization. Three algorithms all can find a solution by adopting different strategies and computational efforts. Therefore, it is necessary to compare their performance. This article presents a performance comparison of the GA, PSO, and BOX algorithms for the optimization of four natural gas mixed refrigerant liquefaction processes. The results show that PSO has the best optimization performance, reducing the specific energy consumption (SEC) to 0.3233 kWh/kg, 0.2351 kWh/kg, 0.2489 kWh/kg, and 0.2427 kWh/kg for single mixed refrigerant (SMR), dual mixed refrigerant (DMR), propane pre-cooling mixed refrigerant (C3MR), and mixed fluid cascade (MFC), respectively. Furthermore, PSO also improved the exergy efficiency of the four processes to 35.34%, 48.59%, 45.90%, and 47.07%. The composite curve analysis shows that the heat transfer efficiency of the heat exchanger optimized by PSO is more efficient. The study also discovered that the total optimization performance of PSO and GA is better than BOX algorithm, and the GA optimization performance is second only to PSO. This research would greatly assist process engineers in making the right decision on process optimization to overcome energy efficiency challenges.

Optimization Design and Analysis of Single-Stage Mixed Refrigerant Liquefaction Process

Small-scale natural gas liquefaction processes have several clear advantages, particularly in the exploitation of ‘unconventional’ natural gas (NG) from sources such as difficult-to-access and offshore gas fields. Moreover, conventional liquefaction processes have a number of disadvantages such as high energy consumption, large cooling loads required in the refrigeration cycle, and non-uniform matching of cold and hot flows in liquified natural gas (LNG) heat exchanger (HE). The main objective of this study was to optimize the most commonly used mixed refrigerant process. The liquefaction performance of the optimized process was analyzed and the influence of gas parameters on the power consumption, exergy loss, freezing mixture circulation, and cooling water load were investigated. The results show that compressor power consumption can be reduced by 29.8%, the cooling water load can be reduced by 21.3%, and the system exergy efficiency can be increased by 41% with the optimized process. Furthermore, throttling and compression of the freezing mixture were increased during the refrigeration stage. It can be concluded that reducing the feed gas temperature and increasing the feed gas pressure can reduce the total power consumption, exergy loss, freezing mixture circulation, and cooling water load, which can significantly improve liquefaction performance.

Thermodynamic and economic assessment of cyano functionalized anion based ionic liquid for CO2 removal from natural gas integrated with, single mixed refrigerant liquefaction process for clean energy

The study proposes a novel integrated process in which ionic liquid is utilized to control carbon dioxide (CO2) emissions from the natural gas combined with a single mixed refrigerant-based liquefaction process to assist safe transportation over long distances providing a sustainable and cleaner energy. Commercially amines are utilized for CO2 sequestration, but amines entail energy-intensive regeneration with elevated process costs. The present study offers a solvent screening mechanism based on important parameters such as heat of dissolution, viscosity, selectivity, working capacity, vapor pressure, corrosivity, and toxicity. The selected solvents’ performance is computed by sensitivity analysis suggesting imidazolium-based cation 1-hexyl-3-methylimidazolium[Hmim] functionalized with tricyanomethanide(tcm) as anion a potential natural gas sweetening solvent in comparison with commercially used solvent monoethanoloamine(MEA), conventional ILs 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][Pf6] and 1-butyl-3-methylimidazolium methyl sulfate [Bmim][MeSO4]. The obtained sweet gas is liquefied using a single mixed refrigerant-based process providing 0.99 mol fraction of liquefied CH4 with less overall specific compression power requirement of 0.41 kW/kg of natural gas. Moreover, an exergy analysis demonstrates that the [Hmim][tcm] based process has lower total exergy destruction of 7.49 × 103 kW and is found to utilize less overall specific energy consumption 0.49 kWh/kg of NG in contrast to other studied solvents. Furthermore, a detailed economic analysis establishes [Hmim][tcm]-based CO2 integrated with liquefaction technology offers 50.7%, 74.4%, and 85.8% of total annualized cost (TAC) savings compared with the MEA-,[Bmim][Pf6]-, and [Bmim][MeSO4], respectively. Hence, [Hmim][tcm] for CO2 removal and integration with liquefaction process will incur unit cost based on the total annualized cost to be $2.2 × 104/kmol of purified NG.

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.

Improvement of offshore adaptability of main cryogenic heat exchanger in FLNG dual mixed refrigerant liquefaction process

The dual mixed refrigerant liquefaction process and its key equipment applied in FLNG (floating liquefied natural gas) suffer performance deterioration due to the offshore conditions. In order to improve the offshore adaptability of FLNG dual mixed refrigerant liquefaction process, the experimental device of FLNG dual mixed refrigerant liquefaction process, the numerical simulation model and the visualization experimental device of falling film flow are developed. Multi-scale marine adaptability is studied from the aspects of the liquefaction process and its key equipment (meter scale) and liquid film thickness distribution (micron to millimeter scale). The results show that the effect of offshore conditions is more obvious on the spiral wound heat exchanger (SWHE) than the plate-fin heat exchanger (PFHE) of precooling circuit and the low temperature throttle valve of natural gas. It is recommended to use the corrugated tube with the diameter difference of 2 mm in the FLNG spiral wound heat exchanger to improve the offshore adaptability of dual mixed refrigerant liquefaction process.

Simulation study of deep eutectic solvent-based biogas upgrading process integrated with single mixed refrigerant biomethane liquefaction

Deep eutectic solvents (DESs) comprise ChCl/urea, in combination with water, have been considered in removing acid gases (CO2 and H2S) from biogas. The evaluation of DES for biogas upgrading at relatively high pressure (i.e., >8.0 bar) has not been reported before. The aqueous DES performance has also not been analyzed compared to conventional amines-based solvent (MEA) and ionic liquid (IL). To the best of our knowledge, this is the first study that presents the integration of DES-based biogas upgrading with a mixed refrigerant liquefaction process to facilitate the safe and economical transportation of biomethane over long distances. The biogas considered in this study consisted of 60% CH4, 39% CO2, and 1% H2S. The aqueous ChCl/urea (70 wt%) results in biomethane with ≥99.0 wt% purity and ≥97.0 wt% recovery. Then, this biomethane was liquefied with ≥90% liquefaction rate. Based on the results obtained herein, overall capital, operating, and total annualized cost savings of 2.8%, 25.82%, and 14.26% were achieved using the 70% DES-based integrated process in comparison with the MEA-based integrated process. Whereas 1.41%, 16.85%, and 8.71% capital, operating, and total annualized costs could be saved in comparison with the IL (i.e., [Bmim][PF6])-based integrated process. It could be deduced that the overall cost of the biomethane value chain can be reduced using the proposed approach.

Propane and iso-butane pre-cooled mixed refrigerant liquefaction process for small-scale skid-mounted natural gas liquefaction

The development of a small skid-mounted natural gas liquefaction plant can economically, environmentally-friendly, and efficiently collect natural gas in mountainous areas where are not easy to lay pipelines. A propane and iso-butane pre-cooled mixed refrigerant liquefaction process (C3&C4-MR) is proposed in this paper. Compared with the propane pre-cooled mixed refrigerant liquefaction process, the improved liquefaction process can reduce system energy consumption and increase exergy efficiency greatly. Genetic algorithm is applied to optimize energy consumption of C3&C4-MR process. The optimized specific energy consumption STotal is 0.301 kW·h/kg, which is 18.0% lower than the optimized C3-MR process. And compared with the optimized R410a pre-cooled mixed refrigerant liquefaction process, STotal of C3&C4-MR process is 5.64% lower than that of R410a-MR process. Besides, mixed propane and isobutane as pre-coolant is easier to obtain from the feed gas. By analyzing the temperature sensitivity of mixed refrigerant and calculating the unit compression power consumption of a single refrigerant, the guidance and recommendations for mixed refrigerant ratio are summarized. Due to the property of iso-butane, the optimal percentage of iso-butane in the pre-coolant was analyzed. In conclusion, C3&C4-MR process is energy-saving and advanced in terms of system operating costs to recycle scattered natural gas in remote areas.

Experimental investigation of the sloshing influence on FLNG liquefaction system

ABSTRACT With the exploitation of oil and gas resources in the ocean, the application of FLNG is more and more extensive. The FLNG ships will face with sloshing, which will exert an influence on the performance of the system. In order to study the effect of sloshing on the double mixed refrigerant liquefaction process, a sloshing platform of six degrees of freedom is built to perform the sloshing experiment of DMR system. The experimental on yaw, roll and pitch is carried out by measuring the temperature, pressure, flow and pressure difference at key nodes to analyze the effects of three different sloshing forms on the overall liquefaction performance and the equipment. It can be found that sloshing will reduce liquefaction performance and increase power consumption. The sloshing has an adverse effect on the distribution of fluid in the heat exchanger and reduces the heat transfer effect. In addition, the sloshing has a greater impact on the wound tube heat exchanger than the plate fin heat exchanger. The effect of the yaw is negligible and the effect of the roll is biggest. The research results can provide reference and guidance for offshore FLNG production.

Experimental study of flow distribution in plate–fin heat exchanger and its influence on natural gas liquefaction performance

The mal-distribution of gas–liquid mixtures has serious influences on the heat-transfer performance of plate–fin heat exchangers (PFHEs), which would result in an adverse effect on the stable and efficient operation of a natural gas liquefaction plant. Therefore, it is necessary to investigate the phase distribution performance of PFHEs and its influence on the natural gas liquefaction performance. In this study, a two-phase flow distribution experimental system was built to investigate the flow distribution characteristics of a PFHE under different working conditions. The effects of gas–liquid ratio, tilt angle, and sloshing on the gas–liquid distribution were studied. In addition, the influences of mal-distribution of the feed gas, nitrogen refrigerant, and mixed refrigerant (MR) on the heat transfer and natural gas liquefaction performance were analyzed through the Aspen Muse software. The results showed that the greater the gas–liquid ratio and tilt angle, the more uneven the liquid flow distribution. The efficiency of the MR process in the tilt condition was reduced by 5.2% to 18.5% compared to that in the horizontal condition. Compared with the tilt working condition, the sloshing could reduce the unevenness of the flow distribution. To satisfy the natural gas liquefaction rate of more than 90%, the critical standard deviation values of flow distribution unevenness of the feed gas, nitrogen refrigerant, MR, and MR under the tilt condition were 8.33, 2.95, 4.42 (liquid phase), and 1.42 (liquid phase), respectively. In the process design, to ensure the output and liquefaction rate, the amount of the refrigerant circulation and power consumption should be kept at the margin of approximately 6% and 4% for the nitrogen expander and MR liquefaction processes, respectively.

A comprehensive optimization and comparison of modified single mixed refrigerant and parallel nitrogen expansion liquefaction process for small-scale mobile LNG plant

This paper aims to offer some criterions on selecting liquefaction process for small-scale mobile LNG plant. To this end, a comprehensive optimization and comparison was presented to show quantitive results on modified mixed refrigerant liquefaction process (MSMR) and parallel nitrogen expansion liquefaction process (PNEC) from the perspectives of specific energy consumption, exergy efficiency, techno-economy and operational flexibility. First, the MSMR and PNEC were optimized by using two different objective functions, namely minimizing specific energy consumption and total investment. The results showed that the minimum specific energy consumption of MSMR and PNEC were 0.411 kWh/kg and 0.618 kWh/kg, respectively. Then, the exergy efficiency comparison demonstrated that the exergy efficiency of MSMR reached 49.96%, while that of PNEC was only 33.19%. Furthermore, the techno-economic comparison was investigated on the small-scale mobile LNG plant by using total investment (TI) model in 20 years life cycle. The total investiment of MSMR was $6,126,133, while that of PNEC was $8,379,177. The total investiment of MSMR was 26.88% lower than that of PNEC, which indicated that MSMR had a better techno-economic performance than PNEC. Finally, the flexibility study was adopted to compare the capability of handling different feed gas conditions for liquefaction. The comparison results showed that MSMR had a lower specific energy consumption, higher exergy efficiency, lower total investment, and higher flexibility than PNEC. In conclusion, MSMR is a better choice for small-scale mobile LNG plant from the four perspectives.

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

Krill-Herd-Based Investigation for Energy Saving Opportunities in Offshore Liquefied Natural Gas Processes

For offshore natural gas liquefaction operation, the single mixed refrigerant process is considered as one of the best and most suitable processes. However, multivariable nonlinear thermodynamic interactions among operating conditions and flow rates of mixed refrigerant ingredients lead to energy losses, which ultimately contributes to high energy consumption for liquefied natural gas production. The present work investigates the energy saving opportunities in the single mixed refrigerant liquefaction process through “krill-herd” strategy which is based on the biological flocking (herding) behavior of individual krill. To find the energy saving opportunities, the krill-herd approach effectively reduced the exergy losses of the compression units and cryogenic heat exchanger up to 18.6 and 41.1%, respectively, as compared to the published liquefaction process. The figure of merit was found as 27.0% in the krill-herd-optimized single mixed refrigerant process, whereas it was 22.2% in the base case.

Surrogate-assisted modeling and optimization of a natural-gas liquefaction plant

In this study, surrogate-assisted modeling and optimization of the single mixed refrigerant process of natural-gas liquefaction is presented. The mixed refrigerant liquefaction process is highly nonlinear owing to the involved thermodynamics that increase the computational burden of any optimization algorithm. To address the computational-burden issue and obtain the results in a reasonable time for the complex single mixed refrigerant process, an approximate surrogate model was developed using a radial basis function combined with a thin-plate spline approach. Even with the reduced model, all the results obtained were comparable with those by rigorous first-principle models. This confirms that all the important characteristics of the model are correctly captured, and the surrogate models of the liquefaction plant are acceptable replacements of first-principle models, especially in computationally demanding situations.

Effects of varying the ambient temperature on the performance of a single mixed refrigerant liquefaction process

This paper presents the optimal design and operational solutions for a single mixed refrigerant liquefaction process under varying ambient temperatures. Given that the ambient temperature can affect the performance of air coolers and gas turbines, the correlation between changes in the ambient temperature and the performance of a single mixed refrigerant liquefaction process should be investigated. A simulation model and a mathematical model were devised and linked to find the optimal design and operational solutions for a single mixed refrigerant liquefaction process, and the particle swarm optimization method was adopted for used with the mathematical model. The results of this study can provide insight about taking advantage of low ambient temperatures for process engineers associated with LNG liquefaction plants.

Optimization of modified single mixed refrigerant process of natural gas liquefaction using multivariate Coggin’s algorithm combined with process knowledge

The optimization of a mixed refrigerant liquefaction process is a challenge because of its non-linear characteristics with stringent multiple process constraints. This study proposes a novel hybrid approach for the optimization of a newly developed, modified single mixed refrigerant process of natural gas liquefaction targeted for offshore applications. This contribution focuses on interpreting the geometric pattern of a plot of the temperature difference between the hot and cold composite curves in a cryogenic heat exchanger to understand the profound effects of the flow rates of the individual refrigerant components and the operating pressure on the liquefaction efficiency. From this, an effective method to generate a proper initial approach temperature profile was developed to ensure robust convergence of the main optimization step. An enhanced coordinate descent methodology was implemented in the main optimization procedure to accelerate the optimization of the modified single mixed refrigerant liquefaction process. The proposed knowledge-inspired hybrid optimization approach showed a robust convergence on determining the optimal design condition. The total energy requirement for natural gas liquefaction cycle was reduced by 21.9% compared to the base case. The proposed methodology can be extended directly to solve optimization problems for other mixed refrigerant based natural gas liquefaction processes.

Dynamic simulation of mixed refrigerant process for small-scale LNG plant in skid mount packages

The utilization of small natural gas reservoirs has attracted more and more attention. Most of the optimization studies concentrated on mixed refrigerant liquefaction process at steady-state simulation. Only a few studies have reported dynamic simulation of mixed refrigerant LNG (liquefied natural gas) process. The main purpose of this study was to design a dynamic model of mixed refrigerant LNG process for small-scale LNG plant in skid mount packages and to investigate the dynamic behaviors of this process. The variations of natural gas temperature, composition, pressure, flow rate and water-cooler temperature were selected as disturbances to test the stability and dynamic responses of the process. The dynamic responses of LNG temperature and total compressor energy consumption were the criteria to investigate the influences of disturbances on the process. Finally, the dynamic responses of disturbances were obtained and discussed. The simulation results showed that LNG temperature could go back to its set point value after a few minutes, while compressor duty varied with the disturbances. It indicated that the mixed refrigerant liquefaction process for small-scale LNG plant in skid mount packages could overcome some disturbances and operate at stable state.

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.