Re-liquefaction System Research Articles

LNG carrier two-stroke propulsion systems: A comparative study of state of the art reliquefaction technologies

Transportation of Liquefied Natural Gas (LNG) by sea has been intensified in the current shipping environment, opening new markets and trade routes. LNG carriers are inherently complex vessels, featuring a high degree of integration of their energy conversion systems, all operating under time and load varying mission profiles, making design decisions non-trivial. To this end, DNVGL COSSMOS (Complex Ship Systems Modelling and Simulation) in-house process modelling framework is used to build digital twins of the propulsion and cargo module of LNG carriers, modelling the entire energy conversion process from cargo tanks to useful energy required for propulsion, electricity and heat. Emphasis is given on reliquefaction systems and the improvement they provide in performance, comparing the currently available technologies and giving deeper insight for the Joule-Thomson systems. The overall ship system performance is improved by 5–15% along the low vessel speed range and 25–40% for the anchorage loaded port condition, when partial reliquefaction systems are considered, depending on the configuration they are compared to. Engine technology also plays an important role, with high-pressure engines exhibiting 3–10% better performance, depending on the reliquefaction technology coupled with, along the high vessel speed range, mainly because of their inherent better performance.

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.

LNG boil-off gas reliquefaction by Brayton refrigeration system – Part 1: Exergy analysis and design of the basic configuration

Heat inleak through insulation in the storage tanks produces boil-off gas (BOG) in LNG-carrying ships. Reverse Brayton cycle (RBC) with nitrogen is often chosen as the refrigeration cycle to reliquefy BOG to prevent loss of valuable gas and environmental pollution. In this paper, parametric evaluations of a basic RBC-based reliquefaction system are done based on exergy analysis. The analyses revealed that formation of liquid at turbine exit and close minimum temperature approach/temperature pinch in the BOG condenser plateaus out the improvement of performance of the RBC based reliquefaction system. The specification of equipment and operating parameters are determined to derive the highest savings in terms of power consumption and recovery of BOG. If RBC is operated in the range of 10–50 bara, close to 93% of BOG is reliquefied. Total reliquefaction is possible only if the RBC is designed with compressor suction at 4 bara. However, it increases the sizes of pipelines, compressor and heat exchangers. All parameters are non-dimensionalized to facilitate application of the results to any capacity of LNG-carrying ship. Part 2 of this paper presents the analyses on thermodynamically improved configurations of reliquefaction systems.

LNG boil-off gas reliquefaction by Brayton refrigeration system – Part 2: Improvements over basic configuration

Exergy analysis of reliquefaction system based on Reverse Brayton cycle (RBC), which is used to condense boil-off gas (BOG) generated in LNG carrier ships, was presented with its basic configuration in Part 1. In this paper, three modifications of the basic system are evaluated. One modification is based on intercooling and precooling of BOG before compressor suction. Others involve ambient compression of BOG. Effects of pressure ratio of RBC, mass flow rate of nitrogen, thermal sizes of heat exchangers and entry temperature of BOG to reliquefaction system on each configuration have been evaluated. Sensitivity analysis shows that 7.8% improvement of rational exergy efficiency is obtained for systems with ambient compression. When BOG enters as superheated gas, modified configurations could keep up their performance while basic system deteriorated. Higher adiabatic efficiencies of compressors and turbines improve the performance of the reliquefaction systems across the board without any additional weight or space. Design and operating parameters have been optimized with the aid of in-built optimiser tool of Aspen HYSYS® V8.6. It resulted in an improvement of rational exergy efficiency of configurations by 1%–3% and decrease of total UA by 5–7% compared to best values predicted by sensitivity analysis.

Process design and economic optimization of boil-off-gas re-liquefaction systems for LNG carriers

Recent LNG carriers are equipped with high pressure gas injection engines. However, there has been a lack of research on liquefaction processes for boil-off gas (BOG) on LNG ships driven by high pressure fuel. Thus, this paper investigates the economic feasibility of the additional BOG liquefaction facilities in the high pressure fuel supply system on the vessels. To utilize the existing BOG compressor for fuel production, the liquefaction was conducted by the Joule Thomson (JT) cycle, which can use the pressurized BOG as a working fluid. For the comparison of the fuel supply system and its variations with BOG liquefaction, they are optimized with respect to total annual cost (TAC) as the objective function. With an LNG price of 5 USD/MMBtu, the optimization results show that the use of BOG liquefiers on LNG vessels reduces the TAC by at least 9.4% compared to the high pressure fuel supply system. The use of a liquid turbine in the liquefaction configurations also resulted in 2.4% savings in TAC compared to the JT cycle based process. However, a sensitivity analysis with different LNG prices indicates that the liquefaction systems are not economical compared to the fuel supply system when the LNG price is lower than 4 USD/MMBtu.

Comparison and analysis of two nitrogen expansion cycles for BOG Re-liquefaction systems for small LNG ships

To maintain the pressure of cargo tank and to minimize natural gas loss, two nitrogen expansion cycles for boil-off gas (BOG) re-liquefaction systems, specifically designed for small LNG ships are proposed and optimized: 1) Case 1, re-liquefaction process with the parallel nitrogen expansion, 2) Case 2, re-liquefaction process with the serial nitrogen expansion. The simulation and analysis of the two processes are carried out by using Aspen HYSYS and the genetic algorithms are selected as the optimization method. It can be found that the specific energy consumption (SEC) of 0.7333 kWh/kgLNG, the coefficient of performance (COP) of 0.2538 and the figure of merit (FOM) of 0.2827 are achieved in Case 1 and the SEC of 0.7539 kWh/kgLNG, the COP of 0.2464 and the FOM of 0.2757 are achieved in Case 2 by comparing the two optimized cases. Moreover, the total exergy losses for Case 1 and Case 2 are 153.34 kW and 160.69 kW, respectively.

Methane Slip During Cargo Operations on LNG Carriers and LNG-Fueled Vessels

Abstract In this paper was presented the problems of methane leakages during cargo operations on LNG carriers. Also the leakages are possible on LNG fueled vessels. Due to green-house effect from methane on the atmosphere it should be done some measures to avoid it. Building the cargo tanks with very high capacity, utilization of better thermal insulations limits the quantity of boil-off (BOG). It is used as a fuel in marine power plant, only the overage should be liquefied again. The leakages attend all cargo operations which methane goes directly to the atmosphere through pressure-vacuum valves and gas freeing installation to mast riser or by the ventilation system from cargo pump or compressor room. To minimize the slip on LNG carriers the re-liquefaction systems are installed. They are based on cooling systems which boil-off gas (mainly methane) is liquefied at ambient pressure in temperature about −161.5°C by pre-cooled nitrogen gas at temperature about −180°C. Compressed nitrogen to a pressure about 25MPa through multistage compressors with intercooling systems is expanded step by step (in intercoolers) to pressure about ambient reaches the temperature about −180°C. The re-liquefaction system needs delivering a lot of electric energy. The total level of methane leakages from mining to the last consumer may be different and sometimes very high. The leakage level starts as minimal 1% and may be raised up to 10%. It was indicated the undertaken actions and next possibilities of methane slip limitations.

한⋅중 조선 산업의 제품 아키텍처와 조직역량에 관한 연구

As companies seek lower cost and superior quality at the same time, which depend on improvement in product architecture, they need to critically consider product architecture as part of corporate strategy. This research investigated how product architecture and organizational capability affect innovative outcomes with using architecture framework. As a result, we were able to find out Korean shipbuilding company has put much effort on integral works such as development of FGSS(Fuel gas supply system), PRS(Partial Re-liquefaction System) and weight lightening for improving fuel efficiency. And this kind of integral ability was realized by organizational capability of Korean shipbuilding company based on interactive relationship with plant workers. In contrast, Chinese shipbuilding companies focused excessively on the standard design and the convenience of research and development made by central government, overlooking the need for fine-tuning. As a result, the fuel efficiency of Chinese LNG ships turned out to be 7-10% lower than those of South Korea with using the same modules and components.

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

Optimization of layer patterning on a plate fin heat exchanger considering abnormal operating conditions

Cryogenic processes such as natural gas (NG) liquefaction or boil off gas (BOG) re-liquefaction systems require complex heat transfer networks to achieve high thermal efficiency. A plate fin heat exchanger (PFHE) is a kind of multi-stream heat exchanger that can make a system simpler while saving space and performing the complex network functions within a single piece of equipment. However, if the layer stacking pattern that determines the PFHE cross section temperature profile is inadequately composed, the equipment can be damaged by increased thermal stress between the layers. Layer stacking patterns have mainly been determined by inefficient trial and error methods, and only considered design operating conditions. The objective of this study is to obtain a highly efficient thermal layer stacking pattern in a design condition, while also lowering thermal stress in abnormal conditions, using a Genetic Algorithm (GA). The proposed GA in this study has a checking module that reduces iteration and saves calculation time. In addition, maximum metal temperature difference is used to evaluate fitness for considering abnormal conditions. According to the proposed algorithm, PFHE metal temperature profile improved about 30% compared to trial and error methods under design condition in a single mixed refrigerant LNG liquefaction process. The optimal layer stacking pattern is determined by a fitting function that applies temperature difference between plates under abnormal and design conditions.

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.

A Novel Boil-Off Gas Re-Liquefaction Using a Spray Recondenser for Liquefied Natural-Gas Bunkering Operations

This study presents the design of a novel boil-off gas (BOG) re-liquefaction technology using a BOG recondenser system. The BOG recondenser system targets the liquefied natural gas (LNG) bunkering operation, in which the BOG phase transition occurs in a pressure vessel instead of a heat exchanger. The BOG that is generated during LNG bunkering operation is characterized as an intermittent flow with various peak loads. The system was designed to temporarily store the transient BOG inflow, condense it with subcooled LNG and store the condensed liquid. The superiority of the system was verified by comparing it with the most extensively employed conventional re-liquefaction system in terms of consumption energy and via an exergy analysis. Static simulations were conducted for three compositions; the results indicated that the proposed system provided 0 to 6.9% higher efficiencies. The exergy analysis indicates that the useful work of the conventional system is 24.9%, and the useful work of the proposed system is 26.0%. Process dynamic simulations of six cases were also performed to verify the behaviour of the BOG recondenser system. The results show that the pressure of the holdup in the recondenser vessel increased during the BOG inflow mode and decreased during the initialization mode. The maximum pressure of one of the bunkering cases was 3.45 bar. The system encountered a challenge during repetitive operations due to overpressurizing of the BOG recondenser vessel.

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.

Thermodynamic analysis of hydrocarbon refrigerants-based ethylene BOG re-liquefaction system

The present study aims to make a thermodynamic analysis of an ethylene cascade re-liquefaction system that consists of the following two subsystems: a liquefaction cycle using ethylene as the working fluid and a refrigeration cycle operating with a hydrocarbon refrigerant. The hydrocarbon refrigerants considered are propane (R290), butane (R600), isobutane (R600a), and propylene (R1270). A computer program written in FORTRAN is developed to compute parameters for characteristic points of the cycles and the system’s performance, which is determined and analyzed using numerical solutions for the refrigerant condensation temperature, temperature in tank, and temperature difference in the cascade condenser. Results show that R600a gives the best performance, followed by (in order) R600, R290, and R1270. Furthermore, it is found that an increase in tank temperature improves system performance but that an increase in refrigerant condensation temperature causes deterioration. In addition, it is found that running the system at a low temperature difference in the cascade condenser is advantageous.

LNG선박용 BOG 부분재액화 시스템 특성 연구

LNG선박용 BOG 부분재액화 시스템 특성 연구

Effects of impurities on re-liquefaction system of liquefied CO2 transport ship for CCS

For EOR and EGR in the CCS, CO2 must be captured from large-scale CO2 emission sources like power plants, and transported to depleting oil and gas wells to maintain the reservoir pressure and enhance the recovery rate. To achieve this, CO2 should be transported over long distances from the emission sources of developed countries to oil and gas producing regions by pipeline or ship. Compressed or liquefied CO2 transport ship can be used when pipeline is not recommendable. In case of liquefied CO2 transport ship, the BOG should be treated during sailing using methods such as re-liquefaction and adsorption.In the present study, the effects of representative impurities of N2 and O2 on the re-liquefaction cycle of liquefied CO2 transport ship were investigated by simulation. The concentration of impurities with boiling points lower than that of CO2 increased much more in the boil-off gas phase than in the liquid phase. This phenomenon led to low cycle performance compared to that of pure CO2 condition, and the decrease rate according to the concentration of the impurities was significant. In the mixture, the discharge pressure of the highest stage in the cycle increased much more than that of the pure CO2 condition. Therefore, when designing re-liquefaction cycle for treating the BOG of liquefied CO2 transport ship, consideration should be given to the effects of impurities on the cycle performance and safety problem in terms of the design pressure. However, impurities did not significantly increase the discharge temperatures of the compressors to such an extent to cause reliability problem of oil degradation in the compressors. With the same composition in the mixture of the liquid phase, nitrogen had a more negative influence than oxygen.

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.

NGL 분리식 BOG 재액화 공정 고안 및 해석

The boil-off-gases(BOG) in cryogenic LNG storage tanks are generating continuously due to the heat leakage and need to be re-liquefied by the effective way. As the present method to reliquefy BOG is using LNG cold energy to be supplied after low pressure primary pump, the demand of LNG flow rate should be over 10 times of BOG produced rate to reliquefy it. This research invented new effective re-liquefaction system having only 3~4 times of LNG flow rate against unit BOG, that the pre-liquefaction process of NGL and the use of high pressure LNG cold energy after secondary pump. By the analysis, it could be high efficient reliquefying system for all amount of BOG treatment even during the summer time, and improvement of operation safety and efficiency of LNG terminal.