Boil-off Gas Reliquefaction System Research Articles

Thermodynamic Analysis of an Ethylene Reliquefaction System Using the Entropy-Cycle Method

In this study, a boil-off gas reliquefaction system that is a part of liquid ethylene gas (LEG) carriers is evaluated. The reliquefaction system is formed by two thermally interconnected two-stage refrigeration cycles. The working fluid of the bottoming cycle is ethylene; the working fluid of the topping cycle is propylene. The research is based on determining the irreversibilities in the reliquefaction system cycles using the entropy-cycle method of thermodynamic analysis. The impact of the process performance in the main components on the reliquefaction system energy efficiency has been evaluated by the entropy-cycle method. The greatest thermodynamic irreversibility is observed in the two-stage compression process of the bottoming cycle (9%), total throttling irreversibility in the reliquefaction system (8.5%), and vapor superheating at the suction into the low stage of the two-stage compressor of the bottoming cycle (8%). The results of the study showed that it is necessary to improve the design of expansion devices using the replacement of throttle devices with ejectors when designing cascade ethylene reliquefaction plants. In addition, when operating such systems much attention should be paid to the condition of the insulation of cargo pipelines and the parameters of the cooling system of the cargo compressor.

A novel boil-off gas reliquefaction system using liquid air for intercontinental liquefied natural gas transportation

The boil-off gas (BOG) reliquefaction of cryogenic liquefied natural gas (LNG) for its intercontinental transport is challenging. This paper proposes a novel BOG reliquefaction system using liquid air and two different process designs: (i) low-pressure liquid air (LPLA) and (ii) high-pressure liquid air (HPLA). In the proposed system, an open refrigeration system using liquid air reliquefies BOG, thereby replacing the current commercial closed refrigeration cycle. As the temperature of liquid air is in the cryogenic range, the energy consumption to reliquefy BOG onboard approaches zero. However, the additional total weight of the ship is required to store liquid air, thus necessitating increased propulsion energy. Nevertheless, the reduction in energy needed for BOG reliquefaction exceeds the energy requirement for the added engine load. Thus, the LPLA shows a massive energy reduction from 4,913.07 to 642.67 kW compared to the commercial BOG reliquefaction process involving the nitrogen expander cycle. The HPLA process shows further improved results of 436.34 kW by expanding the vaporized high-pressure air. In addition, the proposed HPLA design exhibits an exergy efficiency of 77.4 %, which is 37.6 % higher than that of the commercial nitrogen expander process. A techno-economic analysis, including the purchase and transport cost of liquid air, is conducted to reflect an effort on on-shore liquid air production and transportation. Despite the additional liquid air tanks, the overall process configuration is much simpler than that of the current commercial BOG reliquefaction system. Thus, the proposed HPLA process is more economical than commercial processes for all representative voyage duration scenarios (8, 13, and 21 days).

Optimization and analysis of thermodynamic performance of boil-off gas reliquefaction system with multiple refrigerant combinations

Boil-off gas (BOG) reliquefaction plays an important role in the process of LNG transportation and storage. In this paper, an improved BOG reliquefaction system to fit the different refrigerant, consisting of two evaporative refrigeration cycle subsystems and one BOG subsystem, was investigated. Three cases with different refrigerant compositions and combinations were studied to explore the most suitable scheme for the proposed system. All cases were simulated using Aspen HYSYS and optimized using a multi-objective genetic algorithm (GA) with specific energy consumption (SEC) as the objective function, followed by exergy and energy analyses. The results showed that the performance of all three cases improved to different degrees after optimization, and that all optimized cases compared favorably to the reference system. In particular, the optimized Case 3, which used mixed refrigerants in both the high-temperature refrigeration subsystem (HTRS) and the low-temperature refrigeration subsystem (LTRS), showed the best performance, with a coefficient of performance (COP) of 0.322, an exergy efficiency (η) of 0.519, and an SEC of 0.506 kW·h/kgLNG In addition, a comparative evaluation was carried out between the composite curves and temperature differences in the main heat exchangers and the exergy analysis results, which shows different matching degrees according to different refrigerant types and is related to heat transfer performance, and can provide guidance for the selection of different refrigerants in different pressure and temperature ranges in follow-up research.

Analysis and assessment of partial re-liquefaction system for liquefied hydrogen tankers using liquefied natural gas (LNG) and H2 hybrid propulsion

Boil-off gas (BOG) is inevitable on board liquefied hydrogen tankers and must be managed effectively, by using it as fuel, re-liquefying it or burning it, to avoid cargo tank pressure issues. This study aims to develop a BOG re-liquefaction system optimized for l60,000 m3 liquefied hydrogen tankers with an LNG and hydrogen hybrid propulsion system. The proposed system comprises hydrogen compression and helium refrigerant sections with 2 J–Brayton cascade cycles. Cold energy recovery from the fuels and feed BOG exiting the cargo tanks was used. The system exhibits a coefficient of performance (COP) of 0.07, a specific energy consumption (SEC) of 3.30 kWh/kgLH2, and exergy efficiency of 74.9%, with the hydrogen BOG entering the re-liquefaction system at a feed temperature of −220 °C. The theoretical COP and SEC values at ideal conditions were 0.09 and 2.47 kWh/kgLH2, respectively. The effects of varying the hydrogen compression pressure, inlet temperature of the hydrogen expander, feed hydrogen temperature and helium compression pressure were investigated. Additionally, the LNG-to-hydrogen fuel ratio was adjusted to satisfy the Energy Efficiency Design Index (EEDI) Phase 2 and 3 emission requirements.

A new boil-off gas re-liquefaction system for LNG carriers based on dual mixed refrigerant cycle

A new boil-off gas (BOG) re-liquefaction system for LNG carriers has been proposed to improve the system energy efficiency. Two cascade mixed refrigerant cycles (or dual mixed refrigerant cycle, DMR) are used to provide the cooling capacity for the re-liquefaction of BOG. The performance of the new system is analysed on the basis of the thermodynamic data obtained in the process simulation in Aspen HYSYS software. The results show that the power consumed in the BOG compressor and the high-temperature mixed refrigerant compressor could be saved greatly due to the reduced mass flow rates of the processed fluids. Assuming the re-liquefaction capacity of the investigated system is 4557.6 kg/h, it is found that the total power consumption can be reduced by 25%, from 3444 kW in the existing system to 2585.8 kW in the proposed system. The coefficient of performance (COP) of 0.25, exergy efficiency of 41.3% and the specific energy consumption (SEC) of 0.589 kWh/kg(LNG) could be achieved in the new system. It exhibits 33% of improvement in the COP and exergy efficiency in comparison with the corresponding values of the existing system. It indicates that employing the DMR based BOG re-liquefaction system could improve the system energy efficiency of LNG carriers substantially.

Enhancement of a Boil-off Gas Re-liquefaction System with Cascade Cycle

Enhancement of a Boil-off Gas Re-liquefaction System with Cascade Cycle

Comparison between reverse Brayton and Kapitza based LNG boil-off gas reliquefaction system using exergy analysis

LNG boil-off gas (BOG) reliquefaction systems in LNG carrier ships uses refrigeration devices which are based on reverse Brayton, Claude, Kapitza (modified Claude) or Cascade cycles. Some of these refrigeration devices use nitrogen as the refrigerants and hence nitrogen storage vessels or nitrogen generators needs to be installed in LNG carrier ships which consume space and add weight to the carrier. In the present work, a new configuration based on Kapitza liquefaction cycle which uses BOG itself as working fluid is proposed and has been compared with Reverse Brayton Cycle (RBC) on sizes of heat exchangers and compressor operating parameters. Exergy analysis is done after simulating at steady state with Aspen Hysys 8.6® and the comparison between RBC and Kapitza may help designers to choose reliquefaction system with appropriate process parameters and sizes of equipment. With comparable exergetic efficiency as that of an RBC, a Kaptiza system needs only BOG compressor without any need of nitrogen gas.

Review on Boil-Off Gas (BOG) Re-Liquefaction System of Liquefied CO2 Transport Ship for Carbon Capture and Sequestration (CCS)

This paper presents the review on the boil-off gas re-liquefaction system of liquefied CO2 transport ship for carbon capture and sequestration (CCS). It is assumed that liquefied CO2 is transported between South Korea and Middle East for enhanced oil recovery (EOR) and enhanced gas recovery (EGR), which is similar to the oil and LNG transportation route. The concept of open re-liquefaction cycle which means that the boil-off CO2 is compressed, cooled, and expanded before being piped back into the tank is suggested. The effects of impurities on the re-liquefaction cycle is investigated under the condition of fixed cooling capacity in terms of cycle performance and reliability. Specifications of LCO2 transport ship about storage tank and re-liquefaction system is reviewed. In addition, economic feasibility is investigated. As for the case of the present study, negative effects of impurities on the cycle performance and reliability should be considered due to its considerable effects. Conservatively calculated internal rate of return (IRR) of the re-liquefaction system shows 4.1%, therefore, it can be guessed that the BOG re-liquefaction system will have economic validity in the future CCS chain.

Enhancement of energy performance in a boil-off gas re-liquefaction system of LNG carriers using ejectors

An ejector-enhanced Liquefied Natural Gas (LNG) boil-off gas (BOG) re-liquefaction system is proposed to improve the energy efficiency of the existing system. In the new system, two ejectors are respectively used to reduce the energy loss in the expansion of the pressurized BOG and inject a part of fuel BOG into the compression system, and a recuperator is employed to recover the cold energy of the BOG exited from LNG tank. The performance improvement of the proposed system is analysed on the basis of the simulation in Aspen HYSYS. In the case of the re-liquefaction capacity of 4557.6kg/h, the coefficient of performance (COP) and exergy efficiency can be increased by 28%, and the specific energy consumption (SEC) reduced from 0.756 to 0.59kWh/kg(BOG) compared to the conventional BOG re-liquefaction system. Correspondingly, the power consumption of 754.1kW is saved. This means that applying ejectors can effectively improve the energy efficiency of the existing BOG re-liquefaction system for LNG carriers.

Optimization of UA of heat exchangers and BOG compressor exit pressure of LNG boil-off gas reliquefaction system using exergy analysis

Boil-off gas (BOG) generation and its handling are important issues in Liquefied natural gas (LNG) value chain because of economic, environment and safety reasons. Several variants of reliquefaction systems of BOG have been proposed by researchers. Thermodynamic analyses help to configure them and size their components for improving performance. In this paper, exergy analysis of reliquefaction system based on nitrogen-driven reverse Brayton cycle is carried out through simulation using Aspen Hysys 8.6®, a process simulator and the effects of heat exchanger size with and without related pressure drop and BOG compressor exit pressure are evaluated. Nondimensionalization of parameters with respect to the BOG load allows one to scale up or down the design. The process heat exchanger (PHX) requires much higher surface area than that of BOG condenser and it helps to reduce the quantity of methane vented out to atmosphere. As pressure drop destroys exergy, optimum UA of PHX decreases for highest system performance if pressure drop is taken into account. Again, for fixed sizes of heat exchangers, as there is a range of discharge pressures of BOG compressor at which the loss of methane in vent minimizes, the designer should consider choosing the pressure at lower value.

Analysis and efficiency enhancement of a boil-off gas reliquefaction system with cascade cycle on board LNG carriers

In this paper, an LNG boil-off gas (BOG) reliquefaction plant operating in accordance with cascade vapor compression cycles, using propylene and ethylene as refrigerants, on board LNG carriers is investigated. As consequence of the analysis results, a new and original design is proposed to reduce power consumption and improve its exergy efficiency. Through energy and exergy analysis, a thermodynamic model is carried out to analyse and evaluate operating conditions as well as to obtain performance values such as the Coefficient of Performance (COP), exergy efficiency, irreversibilities and specific energy consumption. The thermodynamic analysis is performed using the Engineering Equation Solver (EES) software environment. The results of the improved design implemented on the reliquefaction plant for LNG tank conditions of -160.82°C, a plant BOG input temperature of −125°C and 25°C seawater, give COP values of 0.22 and an exergetic efficiency of 37%, such values being 22.22% and 19.35% greater than the original design. The specific energy consumption decreases 14.66% to 0.64kWh per kg/s of natural BOG. The proposal for improving efficiency is founded on BOG cold energy recovery and BOG compression heat rejection with cooling water in the intercoolers.

Optimum liquefaction fraction for boil-off gas reliquefaction system of semi-pressurized liquid CO2 carriers based on economic evaluation

This investigation studied the optimum reliquefaction fraction of a boil-off gas reliquefaction system for semi-pressurized liquid CO2 carriers. To prevent the overpressure of cargo tanks, the boil-off gas should be properly treated in suitable ways: to be vented into the atmosphere or to be liquefied in a dedicated liquefaction system. Because the former accompanies the penalty of carbon tax and the latter requires energy consumption, there is an optimum reliquefaction fraction minimizing the total treatment expense. To estimate the optimum reliquefaction fraction, the total treatment expense was composed of the three terms: the reliquefaction expense, the emission penalty, and the benefit of CO2 sale, which were expressed by the reliquefaction fraction and other variables. The reliquefaction expense increased rapidly with the reliquefaction fraction, while the other two showed a linear dependence. Even though each term of the total treatment expense varied with the crude price, the CO2 sale price, and the carbon tax, the optimum reliquefaction fraction was found to stay within a narrow region, roughly between 95.7% and 98.2% for all the plausible scenarios for the variables.