Natural Gas Liquefaction Process Research Articles

Experimental and numerical simulation study on the offshore adaptability of spiral wound heat exchanger in LNG-FPSO DMR natural gas liquefaction process

For studying the performance of spiral wound heat exchangers (SWHEs) applied in the LNG-FPSO (LNG Floating Production Storage and Offloading unit) dual mixed refrigerant (DMR) liquefaction process, an experimental device and a numerical simulation model of DMR liquefaction process are established. Based on the experimental results and the REFPROP software, the effects of sloshing on the performance of SWHE are selected as the disturbances to test the responses of the DMR liquefaction process. The results show that the sloshing-induced reduction of SWHE heat transfer performance is within 50% for all sloshing cases. The decrease of specific power consumption of DMR liquefaction process is within 13%, when the SWHE is under sloshing angles from 3° to 9° and sloshing periods from 6 s to 30 s.

Thermodynamic analysis of natural gas liquefaction process with propane pre-cooled mixed refrigerant process (C3MR)

Liquefaction processes plays an important role for transmission of natural gas in long distances. Production of liquefied natural gas (LNG) is classified as an energy consuming process. LNG is a complicated thermodynamics process and the thermodynamics analysis for the key points of the system seems necessary. The goal of this study is to enhance the overall performance and heat recovery from outlet streams of LNG process by changing the process thermodynamics variables. Simulation and sensitivity analysis for two objective functions; specific energy and compressor power consumption were done by Aspen HYSYS process simulator. The investigated variables in this study are four pressure levels, inner and outlet pressures of main compressor and the flow rate of propane in precooling cycle. By considering the process limitations, the possible range for each variable was obtained and then sensitivity analysis was done for each parameter. The obtained results show more improvement on objective functions compared to the other investigations; specific energy for this process was obtained 845 kJ/kg, which shows 30% improvement compared to initial model and the total energy consumption reached to 62.7 MW while it was 71 MW for initial model.

Design and optimization of natural gas liquefaction process using brazed plate heat exchangers based on the modified single mixed refrigerant process

The multi-stream plate-fin heat exchangers (PFHEs) are widely used as the core heat transfer equipment in small-scale natural gas liquefaction processes. However, the PFHEs have disadvantages such as the strict pre-treatment standard and the complicated engineering and transportation. The brazed plate heat exchangers (BPHEs) have been suggested to replace the PFHEs to overcome such disadvantages. In this paper, the methodology for transforming PFHE processes into BPHE processes was introduced and a novel liquefaction process using BPHEs was proposed based on the modified single mixed refrigerant process. The key parameters of the new process were optimized by using the genetic algorithm, and the optimal operating conditions were compared with the base case. The results showed that the specific energy consumption (SEC) of the optimized case was 10.3% lower than that of the base case, while it was slightly higher than the SEC of the original PFHE process. Additionally, the exergy losses of the major equipment in the process were analyzed. The exergy losses were reduced by 7.6%, 19.5%, 29.5% and 15.3% respectively for the compressors, the heat exchangers, the Joule-Thompson valves and the water coolers in the optimized case. The total exergy loss was lower by 14.7% after the optimization.

Offshore adaptability of the nitrogen expander liquefaction process with pre-cooling

Nitrogen refrigerant has the advantages of simple preparation and safe operation, so it is suitable for use in offshore liquefied natural gas (LNG) production under sloshing condition. However, this process has a low efficiency and small processing scale, which limits its use in LNG Floating Production Storage and of Floating Unit (FLNG) devices. In order to solve this problem, the method of adding a pre-cooling process to the high-temperature region is generally adopted. In this study, firstly, different nitrogen expander liquefaction processes with pre-cooling were simulated. The pre-cooling refrigerants used in the simulation process were propane, carbon dioxide and mixed refrigerant, and the corresponding pre-cooling temperatures were −35 °C, −45 °C, and −85 °C, respectively. Subsequently, the performance of the three liquefaction processes with pre-cooling were analyzed and summarized. Finally, in order to verify the offshore adaptability of the pre-cooling nitrogen expander natural gas liquefaction process, a small liquefaction experimental device was built. The working performance of the plate-fin heat exchanger and expander under different working conditions was analyzed through the experiment of the surge and roll of the cold box. The results showed that the nitrogen expander liquefaction process with pre-cooling can be applied to medium-scale FLNG devices.

Inherently Safer Process Design of Natural Gas Liquefaction Processes through Multiobjective Optimization—Part I. With Inherent Safety Indexes

Inherent safety approach is the systematic strategy to achieve safety in the chemical plant by avoiding or reducing hazards through designing a safer process at the preliminary design stage. In thi...

Experimental studies on a natural gas liquefaction process operating with mixed refrigerants and a rectifying column

Small-scale natural gas liquefiers are developing rapidly in China due to the scattered and remote distribution of natural gas sources with small reserve. Compared to the complexity of cascade process and the relatively higher power consumption of expander process, the mixed refrigerant process is a preferable choice for small-scale liquefaction systems considering its relatively simple structure and moderate power consumption. In this paper, a single-cycle natural gas liquefaction process operating with mixed refrigerants and a rectifying column was proposed for small-scale applications, which could utilize a commercial oil-lubricated compressor to realize the liquefaction of natural gas. Experiments were conducted to study the influences of the operating pressures and mixed refrigerant compositions on the overall performances. Relatively optimum SPC values of 4.49 kW h/kg, 1.98 kW h/kg, 1.61 kW h/kg and 1.29 kW h/kg were obtained for the liquefactions of 0.1 MPa, 0.3 MPa, 0.5 MPa and 0.7 MPa pure methane, respectively.

The energy performance of a single‐screw compressor for natural gas liquefaction process: Effects of the lubricating oil flow rate

An experimental test system including the oil circulation system and the gas circuit system has been designed and built to explore the effects of the lubricating oil flow rate on the performance of a single-screw compressor, used in a portable natural gas liquefaction process. Outlet pressure, outlet volumetric flow rate, volumetric efficiency, and specific power rate are the parameters for evaluation of the energy performance of the single-screw compressor. The experimental results indicate that the outlet pressure and outlet volumetric flow rate can be effectively increased by increasing the flow rate of the lubricating oil at different rotational frequencies and that a certain amount of lubricant oil is helpful to improve the volumetric efficiency. In terms of the specific power rate, the optimum oil-gas ratios are 0.7%, 0.93%, and 1.79%, and the corresponding outlet pressures are 0.45, 0.45, and 0.48 MPa at the conditions of 50, 40, and 30 Hz, respectively.

Performance analyses on four configurations of natural gas liquefaction process operating with mixed refrigerants and a rectifying column

Natural gas (NG) fields of low pressure and small reserve are usually located remotely and dispersedly in China, for which pipeline transport in gaseous state is not economic. As a result, liquefying natural gas with small-scale liquefiers is a more preferable choice. In this paper, four configurations of natural gas liquefaction process (NGLP) operating with mixed refrigerants (MRs) and a rectifying column were proposed. The mixture was composed of R728, R50, R1150, R290 and R600a. In the rectifying column, the volatile and less volatile components of the MR were better separated, and the entrained lubricating oil of the compressor was trapped, thus avoiding blockage in the low temperature section. Simulations were conducted for these configurations using modified PT equation and Virial mixing rule in vapor-liquid equilibrium (VLE) calculation of MR. The influences of operating pressures, refrigerant composition and process configuration on system performance indicators, such as specific power consumption (SPC) and discharge temperature, were detailly investigated. It was found by simulation that SPC first decreases and then increases with the decrease of suction pressure in all configurations, while the minimum is achieved at around 250 kPa. The simulation results also suggested keeping the molar fraction of R600a the highest among the components, and increasing the molar fractions of R50 and R728 as much as possible provided that the compressor discharge temperature does not exceed the limit. Preliminary experiments have been conducted to compare the performances of four configurations under specified MR composition and NG flow rate.

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.

Introducing a hybrid oxy-fuel power generation and natural gas/ carbon dioxide liquefaction process with thermodynamic and economic analysis

Oxy-fuel power generation and carbon dioxide capture as a downstream process is a near-zero emission energy system. The captured carbon dioxide can be liquefied but carbon dioxide liquefaction process needs considerable amounts of energy. Liquefied natural gas plays an important role in natural gas transportation. However, its production is known as an energy-intensive process. In this study, a novel integrated process of liquefied natural gas production, oxy-fuel electrical power generation cycle and cryogenic carbon dioxide capture and liquefaction is proposed and investigated by chemical process simulators. Effective operating parameters of the process and its components are thermodynamically analyzed. In this regard, effect of the turbine inlet temperature and pressure ratio, oxidant, oxygen and propane flow rates on the power generation and electrical efficiency of the oxy-fuel cycle, carbon dioxide liquefaction temperature and pressure and liquefied natural gas production rate are examined. The obtained results indicate that, the proposed process can be used to design of the actual plants with optimal operating performance. Specific power consumption of the liquefied natural gas process is about of 0.281 kWh/kg LNG. Also, to verify the accuracy of the process performance, the exergy analysis was performed for the process and all of its components. Through this analysis the exergy efficiency of the oxyfuel power cycle and natural gas liquefaction process are gained 69% and 51% respectively. Economic analysis of the process shows that prime cost of the product and period of return are 0.299US$/kgLNG and 2.392 years respectively.

An innovative vortex-tube turbo-expander refrigeration cycle for performance enhancement of nitrogen-based natural-gas liquefaction process

Liquefied natural gas (LNG) has attracted global attention as a more ecological energy source when compared to other fossil fuels. The nitrogen (N2) expander liquefaction is the most green and safe process among the different types of commercial natural gas liquefaction processes, but its relatively low energy efficiency is a major issue. To solve this issue, an energy-efficient, safe, and simple refrigeration cycle was proposed to improve the energy efficiency of the N2 based natural-gas liquefaction process. In the proposed refrigeration cycle, vortex tube as an expansion device was integrated with turbo-expander in order to reduce the overall required energy for LNG production. A well-known commercial simulator Aspen Hysys® v9 was employed for modeling and analysis of proposed LNG process. The hybrid vortex-tube turbo-expander LNG process resulted in the specific energy requirement of 0.5900 kWh/kg LNG. Furthermore, the energy efficiency of the proposed LNG process was also compared with previous N2 expander-based LNG processes. The results demonstrated that the proposed hybrid configuration saved up to 68.5% (depending on feed composition and conditions) in terms of the overall specific energy requirement in comparison with previous studies.

P84 Co-Polyimide Based-Tubular Carbon Membrane: Effect of Heating Rates on Helium Separations

Helium is one of the most valuable gases with unique features and properties as well as widely used in various applications. Generally, most of the helium sources was extracted from natural gas and it is very crucial to develop efficient technology for helium recovery from natural gas sources, in order to overcome the deficit of the helium supply. Up to now, there are various available traditional separation methods for helium recovery, however these methods possessed several disadvantages such as expensive in cost and energy intensive. Recently, gas separation by using membranes have been utilized and showed potential in recovering and purifying helium from natural gas. This method directly separating the helium from the methane through natural gas liquefaction process where in this process the helium is recovered from the nitrogen rejection unit (NRU) exit gas. Due to the potential benefits that can be obtained from this membrane-based separation method, this current study is aiming to provide more comprehensive scientific reports on the effects of preparation parameters on the performance of tubular carbon membranes (TCMs) for helium separation. In this study, the carbonization heating rate was varied from 1 to 7°C/min by controlling the final temperature at 800°C under Argon environment for all polymeric tubular membranes. The permeation performance of the resultant TCMs have been determined by using a single permeation apparatus. It is necessary to fine-tuning the carbonization conditions in order to obtain the desired permeation properties. From the results, it can be concluded that the most optimum heating rate was found to be at 3°C/min with 463.86±3.12 selectivity of He/N2separation.

Supersonic liquefaction properties of natural gas in the Laval nozzle

In view of the excellent performance of the supersonic separator for natural gas dehydration, a new type of natural gas liquefaction process using the Laval nozzle is proposed in this paper. Theoretical and numerical studies of the supersonic flow and liquefaction process of the methane-ethane binary system in this nozzle are carried out. The effects of the inlet temperature, inlet pressure, back pressure and component composition on the liquefaction process are analyzed. The results show that the critical liquefaction temperature and pressure of the methane-ethane binary system decrease under low inlet temperature or high inlet pressure conditions and the range of the liquid phase region increases, which promotes the liquefaction process. With the increase of the back pressure of the nozzle, the position of the shock wave moves forward and the liquefaction environment is more completely destroyed. For a multi-component natural gas, in which the heavy hydrocarbon content is high, natural gas is more easily liquefied using the Laval nozzle. The liquefaction efficiency range of the newly designed liquefaction process with the Laval nozzle are 0.0795–0.1321 (HYSYS results) and 0.0718–0.1505 (MATLAB results) when the inlet pressure of the process is 2–5 MPa. The nozzle more easily achieves liquefaction compared with a throttle under the same conditions.

Development and use of exergy efficiency for complex cryogenic processes

In this paper, exergy efficiencies, which are effective performance parameters for cryogenic processes, are categorized based on their exergy decomposition levels. However, the existing efficiencies are not standardized for a variety of unit operations. Thus, the extension of the exergy transfer effectiveness (ETE) has been suggested with a general mathematical expression. The extended ETE is defined by decomposing both thermo-mechanical and chemical exergy to the chemical component level. A case study with a complex natural gas liquefaction process and its optimization has also been performed. The results indicate that the extended ETE brings consistent and accurate results for all types of units, also properly reflecting the changes in process performance after optimization. Other efficiencies, however, struggle to measure the performance improvement for some equipment, even showing decreases in their efficiency values.

Improved Configurations For Liquefied Natural Gas Cycles

The most important challenge in a natural gas liquefaction plant is to improve the plant energy efficiency. A process topology should be implemented, which results in a considerable reduction of energy consumption as the natural gas liquefaction process consumes a large amount of energy. In particular, system design focusing on configuring cold part cycle is an attractive option. In this study, various energy recovery-oriented process configurations and the potential improvements of energy savings for small- & midscale liquefied natural gas plants were proposed and compared with almost exclusively commercial trademarks processes. These improved simulation based investigations were validated under the variation in feed gas pressure, mixed refrigerant cooling reference temperature and the pinch temperature of cryogenic plate fin heat exchanger. The simulation results exhibited considerable reduction of specific total energy consumption. Therefore, the proposed liquefaction cycles have a simple topology, hence lower capital cost and compacter plant layout, which is compatible for power-efficient, offshore, floating liquefied natural gas liquefaction plants.

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.

Numerical study on the condensation flow and heat transfer characteristics of hydrocarbon mixtures inside the tubes of liquefied natural gas coil-wound heat exchangers

Coil-wound heat exchangers (CWHEs) have been widely used in natural gas liquefaction plants nowadays. But few studies on the condensation heat transfer and frictional pressure drop of mixed hydrocarbon refrigerants inside the tubes of CWHEs have been conducted under practical conditions. In this study, numerical simulations were carried out to investigate the condensation flow and heat transfer characteristics of mixed hydrocarbon refrigerants inside the tubes of liquefied natural gas (LNG) CWHEs under practical operating conditions. A combinational numerical method based on computational fluid dynamics (CFD) simulation and modified Silver method was proposed to predict the condensation heat transfer coefficients of mixtures. The numerical results showed good agreement with experimental data and calculation results by the empirical correlation in the literature. The relative errors of void fraction, heat transfer coefficient and frictional pressure drop were predicted within ±5%, ±15% and ±15% respectively. The simulation results indicated that the heat transfer coefficients of mixed refrigerants varied slightly with the increase of vapor quality, but significantly with the increase of mass flux. And the frictional pressure drop increased with the increase of both vapor quality and mass flux. This study is instructive to the design and optimization of CWHEs used for natural gas liquefaction process.

Experimental research on the adaptability of liquid natural gas spiral wound heat exchanger in dual mixed refrigeration liquefaction process

As the key equipment of natural gas liquefaction process, the performance of spiral wound heat exchanger (SWHE) influences operating costs and reliability of the whole system. In order to investigate the adaptability of LNG SWHE in dual mixed refrigeration (DMR) liquefaction process, an experimental device using DMR liquefaction process and a multi-phase multi-stream thermal model for SWHE are both constructed. The variations of feed gas flow rate are selected as disturbances to test the dynamic responses of the LNG SWHE. And the results show that when the feed gas mass flow rate decreases to 0 kg h−1, slug flow may occur in the outlet pipeline of mixed refrigerant in shell-side. And during the cooling process of feed gas in SWHE, two critical values (−80 and −106 °C) were both obtained. In addition, with an adequate mass flow rate of mixed refrigerant, the LNG SWHE in DMR liquefaction process could overcome some disturbances and can be operated at a stable state.

A novel hybrid exergy/pinch process integration methodology

The trend of global energy demand forecasts a continuous increase in the forthcoming years together with a diversification and differentiation of energy sources. In order to ensure the energy systems transition towards a sustainable functioning mode while avoiding early depletion of energy resources, industrial activities need to foster process improvement. Among energy efficient process design methodologies, pinch analysis and exergy analysis are two powerful methods but each one shows drawbacks. This paper introduces a new methodology that uses the Jacobian Matrix of exergy destruction as a mathematical indicator to couple the Pinch and the exergy analysis and overcome their individual limitations. In the proposed approach, exergy analysis is used not only to assess the exergy losses but also to guide the changes needed in industrial processes structures and operating conditions. And while the pinch analysis considers only heat integration, the proposed methodology allows including other recoverable exergy in a process through thermodynamic conversion. In this paper, the guidelines of the methodology are first detailed. When applied on a basic natural gas liquefaction process, the proposed approach will numerically result in a new improved process comparable to the existing, most widely used natural gas liquefaction process. The main advantage of the methodology is time saving compared to holistic operating conditions improvement as in the traditional process design methods and automatic process structure modification leading to better heat integration.

Innovative propane-nitrogen two-phase expander refrigeration cycle for energy-efficient and low-global warming potential LNG production

Nitrogen (N2) expander liquefaction process has the highest ecological and safety advantages over different types of available commercial natural gas liquefaction processes. However, its relatively low energy efficiency is a major issue. In this context, the optimum flow rate of propane as a high-boiling component with low-global warming potential was mixed with conventional refrigerant N2, resulting in a two-phase single mixed refrigerant appearing at the suction point of the conventional turbo expander. The potential application of a two-phase cryogenic expander was investigated to generate a cooling effect through the expansion of the high-pressure two-phase propane-nitrogen refrigerant. The proposed study was modeled using Aspen Hysys® and optimized by adopting a MATLAB coded particle swarm optimization approach that was linked to Aspen Hysys® using the ActiveX (also known as COM) functionality. The results revealed that the specific energy consumption and required refrigerant flow rate for liquefied natural gas (LNG) production can be reduced up to 46.4% and 27.7%, respectively, in comparison with the conventional N2 single expander LNG process. Furthermore, the overall energy can be reduced from 79.2% to 29.5% as compared to previously reported N2 single expander LNG processes, depending on feed conditions, composition, and design parameters. An exergy analysis of the proposed LNG process revealed that the compressors and LNG heat exchanger have the highest exergy loss, i.e., 34.0% and 29.7%, respectively.