Single Mixed Refrigerant Process Research Articles

Single mixed refrigerant biomethane liquefaction plant integrated with solar energy: Dynamic simulation for the decarbonization of the heavy road transport sector

This works proposes a dynamic thermoeconomic analysis of a liquefied biomethane production plant to meet the fuel demand of a fleet of heavy duty trucks in the south of Italy. The biomethane is obtained from the upgrading of the biogas produced by means of anaerobic digestion through a plug flow reactor fed by organic fraction of municipal solid waste. The upgrading of the biogas is realized using a three-stage membrane compression process, producing a 96 % pure biomethane. The biomethane liquefaction is realized using a single-mixed refrigerant process and compared to a Linde cycle process. The whole system is assisted by solar energy to reduce the fossil energy consumption of the process and feed-in tariffs are considered as funding policy. The models for the anaerobic digestion, the biogas upgrading, and the biomethane liquefaction are in detail developed in MatLab. The anaerobic digestion model is based on the ADM1 biological model, integrated with a suitable heat transfer model. The biogas upgrading model is based on a simplified Fick model. The liquefaction model is based on an equivalent two heat-exchangers model, taking into account the transient heat transfer. All the components are then integrated in TRNSYS to perform the dynamic simulation for one operating year of the whole system. Results from the thermoeconomic analysis are outstanding in terms of profitability, showing a payback period of less than 2 years and a Net Present Value of the system of 402 M€. The great environmental impact is also confirmed by a Primary Energy Saving of 91 % and a dramatic reduction of 86 % of the CO2 equivalent emissions.

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.

Enhancing Energy Efficiency and Reliability in Floating LNG Operation: A Hydrofluoroolefin-Based SMR Cycle with Thermo-Economic Assessment and Uncertainty Analysis

Natural gas liquefaction is the most energy-intensive technique in floating liquefied natural gas (FLNG) operations. Hence, a hydrofluoroolefin-based single mixed refrigerant (SMR) technology was proposed to achieve energy-efficient and eco-friendly operations. The study was perfomed to enhance the SMR process by substituting expansion valves with hydraulic turbines to improve process reversibility and enhance energy efficiency. The proposed process was optimized using a sine–cosine concept-based algorithm that tuned the design variables which yielded an overall compression energy consumption of 0.2501 (kWh/kg-LNG), resulting in a 21.43% saving as compared to the conventionally used mixed refrigerant-based process. This resulted in a reduction of about 38.81% in overall exergy destruction. The economic analysis results draw curtains that the process saves about 26.02% of total annualized cost as compared to conventional study. Owing to the appropriate mixed refrigerant component mixture ratio and process design improvement. To determine the process reliability under uncertain conditions the optimal compression power and minimum approach temperature were determined. Further, uncertainty analysis reveals that the methane and butane refrigerant flow rates along with evaporation, and condensation pressure, were among the most influential parameters that influence the process most that may affect the whole operation in uncertain process inlet conditions. In summary, our approach presents immense promise for achieving energy efficiency, environmental responsibility, and economic viability in FLNG operations, offering a robust solution for the challenges of offshore plants.

A novel small-scale biomethane liquefaction process: Assessment through a detailed theoretical analysis

The use of biomethane as fuel in the transport and power generation sectors is widely considered an effective tool to pursue the goals of carbon neutrality and energy security. Therefore, it is very important to promote not only the production and upgrading processes of biomethane from carbon neutral biomass, but also biomethane liquefaction processes, which can allow effective transportation of biomethane, thanks to the fact that the density of the liquid is higher than that of the compressed gas. Biomethane liquefaction plants can benefit from the technologies already used for the liquefaction of natural gas, but the size of biomethane liquefaction plants must be significantly smaller because of the limited quantities of biomethane generated by production and upgrading plants. Biomethane liquefaction plants have still limited diffusion; the most used technologies for the biomethane liquefaction are the Reverse Brayton cycle, the Lynde cycle, and the heat exchange with liquid Nitrogen. Single mixed refrigerant (SMR) processes, which are very effective for the liquefaction of natural gas, have a high potential for the biomethane liquefaction too, but the high specific energy consumption and high plant costs (mainly caused by typical multi-flow heat exchangers employed) are the major issues associated with SMR processes when applied to the very small size of biomethane liquefaction plants, characterised by very low biomethane flow rates. The aim of this paper is to propose a novel SMR process for biomethane liquefaction, which can overcome the above-mentioned issues, contributing towards the implementation of SMR processes for the liquefaction of biomethane and providing a valid alternative to currently used biomethane liquefaction plants; the proposed solution consists in using simple pipe-in-pipe heat exchangers in place of the more complex multi-flow heat exchangers. The validity of this solution is theoretically assessed using a detailed thermodynamic model based on well-stablished equations. Using this tool, it is shown that, by adjusting crucial parameters, such as the molar composition of the refrigerant, the heat exchange is satisfied with an affordable length of the heat exchangers, negligible pressure drops and good levels of the coefficient of performance with reference to such a small-scale application. The predicted specific energy consumption (accounting also for the biomethane compression and pre-cooling) for a biomethane flow rate of 0.03 kg/s is of the order of 0.8 kWh/kgCH4, which is a very good performance level compared to current commercial biomethane liquefaction processes.

Advanced natural gas liquefaction and regasification processes: Liquefied natural gas supply chain with cryogenic carbon capture and storage

Natural gas combined cycle power plants generate substantial amounts of CO2. Also, since natural gas is produced in limited sites, the liquefied natural gas (LNG) supply chain consumes considerable energy in transporting natural gas overwide. Herein, advanced natural gas liquefaction and regasification processes using an LNG supply chain with cryogenic CO2 capture and storage (CCS) are proposed using Aspen Plus® V11. This solves three significant problems associated with conventional LNG supply chains. First, through the utilization of LNG cold energy, the cryogenic CCS process reduces the efficiency penalty of power generation from 14.34 to 4.45%. Second, solid CO2 from cryogenic CCS is used as an additional cold source in the natural gas liquefaction process by conveying it to the liquefaction site via the returning LNG ship. As a result, the power consumption in the refrigerant cycle is reduced by up to 67.17% than single mixed refrigerant (SMR) process. Finally, the LNG supply chain with cryogenic CCS minimizes the sustained energy waste between the natural gas liquefaction and regasification stages owing to the geographical separation. LNG cold energy from the regasification stage is utilized in the form of solid CO2 in the CCS and liquefaction stages. Consequently, the exergy efficiency in the overall LNG supply chain increases from 18.18 to 46.39%. The findings of this study can potentially provide environmentally friendly design guidelines for various LNG processes that use LNG cold energy in CCS and natural gas liquefaction.

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.

A generalized disjunctive programming model for multi-stage compression for natural gas liquefaction processes

The primary driver of operating costs in natural gas processes is the energy consumption of the compression system. Multistage compression configurations are commonly employed and hence play a vital role in optimization of natural gas processes. In this study, a generalized disjunctive programming model for multistage compression is formulated. The model is useful for both synthesis and optimization of multistage compression configurations. By using this approach, we further seek improvements in shaft work savings. The model relies on thermodynamic equations and is designed to minimize the consumption of shaft work. The model is handled by employing the logic-based branch and bound algorithm, eliminating the need for explicit conversion into a MINLP, which in turn leads to improved convergence and faster computational performance. The model solution yields optimal pressure levels, and hence stage shaft work consumptions. A case study of multistage compression for a prior optimized single mixed refrigerant (SMR) process obtained from literature is used to test the proposed model. The model’s outcomes are validated through simulation using the Aspen Hysys software. Savings in shaft work of atmost 0.0088%, 0.4433%, and 1.2321% are obtained for the two, three, and four stage compression systems respectively against the optimized base cases from literature.

Comparative analysis of cryogenic distillation and chemical absorption for carbon capture in integrated natural gas liquefaction processes

Compared with the widely used chemical absorption, cryogenic distillation has a lower thermal energy consumption and smaller footprint of the carbon capture unit. However, the power consumption of cryogenic distillation is higher. This study aims to use the azeotropic properties of ethane-CO2 mixtures to achieve CO2 capture by cryogenic distillation for cleaner LNG production. A single mixed refrigerant LNG process integrated with cryogenic distillation (CD-LNG) or chemical absorption (CA-LNG) is proposed. These technologies are comprehensively compared with respect to energy, economy and environment. The results show that when the feed gas contains 1.8–17 mol% CO2, the power consumption of the CA-LNG process is 0.37–0.40 kWh/Nm3 NG with an exergy destruction of 0.20–0.23 kWh/Nm3 NG. The power consumption of the CD-LNG process is 0.41–0.42 kWh/Nm3 NG with an exergy destruction of 0.18–0.22 kWh/Nm3 NG. Due to the much higher heat load for chemical solvent regeneration, the overall exergy efficiency of the CA-LNG process (49.0–53.8%) is lower than that of the CD-LNG process (50.6–56.0%). In addition, the total annualized cost (TAC) in a 20-years lifespan of the CD-LNG and the CA-LNG for a case study with a process scale of 179,800 t/a and a specified feed gas composition (79 mol% CH4, 10 mol% C2H6 and 11 mol% CO2) are 8,441,117 $ and 9,860,792 $ respectively. The GWP analysis shows that the CO2-eq of the CA-LNG process is 37–67% higher than that of the CD-LNG process.

Optimal operational analysis of metamodel based single mixed refrigerant cryogenic process for floating liquefied natural gas plant technology

Since natural gas liquefaction operation is an energy-intensive technology. Research has been published for the optimization of the process by utilizing the first principal complex nonlinear SMR model. The process is computationally expensive and requires several hours or even days for optimization. To address this issue, this study proposes a metamodel for natural gas liquefaction featuring a single mixed refrigerant cryogenic cycle. The SMR model was modeled and simulated by utilizing Aspen Hysys software. However, the metamodel was developed by utilizing radial basis function methodology and optimization work using Matlab. The results summary comparison for specific compression duty for metamodel was 0.3863 kW/kg-NG. The first principle-based published study of this duty was 0.3625 (kW/kg-NG) which is approximately 6.5% lower than the current study. However, a huge reduction in computational time was obtained. The metamodel building and optimization time-lapse was 292.96 s while the same first principle-based model lapse 201.24 h using 300 iterations. In comparison, it can be inferred that metamodel could capture almost all-important characteristics of the firs-principle based model. Hence, the proposed study could be considered as an alternative to the first principle model for optimization purposes. This research may prove to be more vital especially in any abrupt changing or uncertain conditions or for real-time plant optimization.

Optimization and analysis of a hydrogen liquefaction process: Energy, exergy, economic, and uncertainty quantification analysis

To reduce the specific energy consumption (SEC) of the hydrogen liquefaction process, a conceptual design of a hydrogen liquefaction process that integrates a modified natural gas single mixed refrigerant process and a hydrogen expansion cycle. The energy efficiency of the process is improved by operating each refrigeration cycle at different pressures. The process is optimized using a particle swarm optimization algorithm. Furthermore, the uncertainty levels of the SEC and the minimum internal temperature approach of the heat exchanger with varying parameters are investigated by applying uncertainty quantification (UQ). The results indicate SEC is 5.963 kWh/kg with 52.61% exergy efficiency. The thermodynamic evaluation shows that the coefficient of performance and figure of merit are 0.2218 and 0.526. The UQ analysis reveals that the SEC is strongly influenced by changing the flow rate and pressure of the sub-cooling refrigerant, with a distribution of SEC between 5.479 and 6.451 kWh/kg. Finally, the economic evaluation yields a total annualized cost of $25.42 million/y. This study is expected to provide an insight into designing an energy-efficient hydrogen liquefaction process by introducing a natural gas mixed refrigerant process.

Novel integrated energy-efficient dual-effect single mixed refrigerant and NGLs recovery process for small-scale natural gas processing plant

The design of natural gas liquefaction process operated at offshore regards space as a design variable and requests an integrated and compact process due to space limitation. This study proposes an integrated process for natural gas liquefaction and NGLs recovery based on single mixed refrigerant (SMR) cycle. The considered SMR refrigeration cycle functions as dual mixed refrigerant (DMR) to provide two separate refrigeration levels for natural gas precooling and subcooling. NGLs recovery section includes a conventional column and a heat integrated distillation column in indirect sequence to produce lean gas, ethane liquid, LPG and gasoline. Liquefaction operating parameters were optimized using genetic algorithm in search of optimal values for profitable process with optimal energy consumption. NGLs recovery section of the proposed process is flexible, operable in both ethane recovery and ethane rejection modes. Based on base case process, the proposed process lowers energy consumption by 14%. The key features of the proposed process are cold utility recovery through self-refrigeration and ethane liquid recovery.

Energy, economic and environmental analysis of a BOG re-liquefaction process for an LNG carrier

Due to tighter environmental regulations, newly built liquefied natural gas (LNG) carriers are equipped with a re-liquefaction system to minimize combustion of surplus boil-off-gas (BOG). Thus, this paper comparatively analyzes the re-liquefaction system for a low-pressure gas injection engine according to the refrigerant (no external refrigerant or single mixed refrigerant) with three key performance indicators: energy, economic, and environmental aspects. For an energy efficiency analysis, we proposed several process alternatives and optimized them to minimize the specific power consumption required to liquefy BOG. In economic analysis, minimizing total annualized cost is the objective. For an environmental analysis, CO2 emissions at each optimal point is calculated and comparatively analyzed. The results show that the process without external refrigerant has 10% better performance in terms of economy, while the single mixed refrigerant process is suitable in terms of energy efficiency (6%) and environmental (15%) impact.

Framework for embedding black-box simulation into mathematical programming via kriging surrogate model applied to natural gas liquefaction process optimization

This paper presents a framework to solve the constrained black-box simulation optimization problem that arises from the optimal energy-efficient design of single-mixed refrigerant natural gas liquefaction process using reliable process simulator. Kriging surrogate model is used to introduce simple, computationally inexpensive, and effective algebraic formulations with reliable derivatives to the black-box objective and constraints functions. The algebraic surrogate optimization problem is embedded into a nonlinear programming (NLP) model in General Algebraic Modeling System (GAMS). The NLP problem is solved using efficient multi-start gradient-based optimization with CONOPT local solver to determine a candidate of decision variables for which the true functions are calculated in the rigorous simulation. The single-mixed refrigerant process is analyzed considering one-to-three-stage expansion and phase separation to assess potential energy savings. The present approach results show that more expansion stages can provide energy savings from 12.02 to 14.70 % comparing two-stage and three-stage expansion system with single-stage. This optimization framework is more effective and consistent than Particle Swarm Optimization and Genetic Algorithm given the same budget of simulation evaluations for the considered simulation optimization problems, resulting in 13.57 to 53.26 % of energy savings.

Mixed refrigerant components selection criteria in LNG processes; thermodynamic analysis and prioritization guidelines

The most important operating variables for the optimization of natural gas liquefaction processes are the flow rate of the mixed refrigerant (MR) components and operating pressures. In this paper, for the first time, the thermodynamic principles of selecting MR components were investigated. For this purpose, the single mixed refrigerant process was simulated in thirteen cases with different MR components and optimized with the genetic algorithm to minimize process power consumption. The optimization results indicated that the boiling point temperature (Tb), specific heat capacity (cp), and latent heat of vaporization (λ) were the most essential thermodynamic properties. The lower Tb and cp in the selection of lightweight components among nitrogen, methane, ethane, and ethylene, and higher Tb, cp, and λ in the selection of heavy components among propane, i-butane, and i-pentane caused better performance. The results of the process optimization with MR, including nitrogen, methane, ethane, propane, and i-butane, showed that the process power consumption would be reduced up to 11.28% and 8.91% by replacing ethane with ethylene and i-butane with i-pentane, respectively. In addition, MR components must be selected in such a way that the difference between their Tb is not significant. Therefore, propane and ethylene are a priority for heavy and lightweight components, respectively.

Neural network-inspired performance enhancement of synthetic natural gas liquefaction plant with different minimum approach temperatures

In this study, a state-of-the-art neural network algorithm (NNA) was explored to improve the overall competitiveness of the single mixed refrigerant (SMR) process for synthetic natural gas (SNG) liquefaction. The NNA approach is inspired by the functions of biological and artificial neural networks. This is the first study to implement the NNA approach, especially to find the energy and cost-saving opportunities in the SMR SNG liquefaction process. Optimized SNG liquefaction processes were analyzed and compared to a recently published SNG liquefaction process optimized by a single-solution-based vortex-search approach. The robustness of the NNA was evaluated against different values of the minimum internal temperature approach (MITA). It is observed that the SMR process corresponding to MITA values of 1.0 °C and 3.0 °C consumes approximately 16% and 2.4% less energy, respectively, compared with the base case. The exergy efficiencies of the optimized process with MITA values of 1.0, 1.5, 2.0, 2.5, and 3.0 °C are 18.52, 13.45, 11.98, 9.60, and 2.24 % higher than the base case, respectively. An economic analysis in terms of total capital investment (TCI) and TAC was also performed. The analysis showed high TCI savings of 3.3% for an MITA value of 3.0 °C compared to the base case, whereas savings in TAC were 6.6%, 7.2%, 8.1%, 7.1%, and 2.7% respectively for MITA values of 1.0, 1.5, 2.0, 2.5, and 3.0 °C. This study will help practitioners design cost-effective liquefaction technologies that would provide clean and affordable energy.

Novel propane-free mixed refrigerant integrated with nitrogen expansion natural gas liquefaction process for offshore units

The presence of propane inventory in offshore liquefaction processes increases the concerns for platform safety. To address this, we propose a novel liquefaction process that integrates propane-free mixed refrigerant and nitrogen expander cycles (MR-N2). The proposed design adopts the advantages of both nitrogen expander and single mixed refrigerant (SMR) processes. The process was rigorously simulated using Aspen HYSYS, and the specific energy consumption was optimized as an objective function using the genetic algorithm technique. Additionally, the MR-N2 process was investigated using exergy and sensitivity analyses to compare the obtained results with those of the previous studies. The results verify that both liquefaction and exergy efficiencies are improved by 27.15% and 14.92%, respectively, in comparison with those of the base case. Moreover, the MR-N2 process exhibits enhanced energy efficiency compared to that of the various existing nitrogen expander-based processes. The energy savings of the proposed method varies between 3.2% and 61.7%, and the cycle capacity is 22.43% better than that of the SMR process.

Optimal Process Design of Small Scale SMR Process for LNG Vessel

Recently, due to regulations on emissions of vessels, fuel is changing to liquefied natural gas (LNG). When using LNG as fuel, it is advantageous in terms of fuel saving and boil-off gas control if a small-scale liquefaction process is installed on the ship. However, due to the limited space, the small-scale liquefaction process for ships has to consider not only efficiency but also simplicity and compactness. In this respect, it is different from the process in onshore liquefaction plants, and research on this is insufficient. Therefore, this paper performs a comparative analysis in terms of efficiency by simplifying the composition of the mixed refrigerant in the liquefaction process. Additionally, a single mixed refrigerant process is used to pursue the compactness of the process. For comparative analysis, the liquefaction process is designed and simulated, and the specific power consumption calculated as the power required to liquefy the unit LNG is used as the objective function to optimize. As a result, it is confirmed that when the number of refrigerants is reduced from 5 to 4, the efficiency is only about a 1% difference, but when it is reduced to 3, the efficiency decreases by 23%, resulting in a decrease in performance.

Design and optimization of energy-efficient single mixed refrigerant LNG liquefaction process

The optimal design of single mixed refrigerant (SMR) natural gas liquefaction process is decisive for upgrading the liquefied natural gas (LNG) value chain and can be achieved using process simulators and derivative-free optimization techniques in simulation–optimization frameworks. The process work consumption can be reduced by modifying process flowsheet and manipulating refrigeration cycle thermodynamic conditions and refrigerant composition. The goal of this paper is the design of energy-efficient SMR liquefaction process for LNG production using simulation–optimization approach, in Aspen HYSYS® and MATLAB®, that considers more decision variables to be optimized with a Nelder–Mead simplex algorithm and small flowsheet modifications, such as hydraulic turbines and flash separators in between compression stages. The present approach resulted in liquefaction process with network duty of 750.2 kJ per kg of natural gas, which is considerably smaller than those reported in the literature.

Black Hole-Inspired Optimal Design of Biomethane Liquefaction Process for Small-Scale Applications

Biomethane is regarded as a promising renewable energy source, with great potential to satisfy the growth of energy demands and to reduce greenhouse gas emissions. Liquefaction is a suitable approach for long distances and overseas transportation of biomethane; however, it is energy-intensive due to its cryogenic working condition. The major challenge is to design a high-energy efficiency liquefaction process with simple operation and configuration. A single mixed refrigerant biomethane liquefaction process adopting the cryogenic liquid turbine for small-scale production has been proposed in this study to address this issue. The optimal design corresponding to minimal energy consumption was obtained through the black-hole-based optimization algorithm. The effect of the minimum internal temperature approach (MITA) in the main cryogenic heat exchanger on the biomethane liquefaction process performance was investigated. The study results indicated that the specific energy consumption of modified case 2 with MITA of 2°C was 0.3228 kWh/kg with 21.01% reduction compared to the published base case. When the MITA decreased to 1°C, the specific power of modified case 1 reduced to 0.3162 kWh/kg, which was 24.96% lower than the base case. In terms of exergy analysis, the total exergy destruction of the modified cases 1, 2, and 3 was 31.28%, 22.27%, and 17.51% lower than the base case, respectively. This study’s findings suggested that introducing the cryogenic liquid turbine to the single mixed refrigerant-based biomethane liquefaction process could reduce the specific energy consumption and total exergy destruction significantly. Therefore, this study could provide a viable path for designing and improving the small-scale biomethane liquefaction process.

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.