Re-liquefaction Process Research Articles

Energy-efficient process design and optimization of dual-expansion systems for BOG (Boil-off gas) Re-liquefaction process in LNG-fueled ship

Energy efficiency of the conventional boil-off gas re-liquefaction process in the LNG-fueled ships, which is based on a single expander cycle, can be improved with structural modifications, such as a dual expander cycle. Herein, a structural optimization framework concept is proposed to systematically screen a wide range of configurational options for dual expander processes, and to evaluate their techno-economic impacts in a holistic manner. Ten possible process configurations are embedded in the framework, with which the strategic use of design options, including splitting and/or mixing of streams, and introduction of extra hot stream in the exchanger, are fully investigated. Furthermore, a cascaded cycle is compared as a possible candidate for improving the energy efficiency of the single expander process. The optimum process design is investigated by optimizing process variables to achieve maximum energy efficiency with aid of GA (Genetic Algorithm). The optimized performance of the dual-expander process is 23% better than the single expander system in terms of energy efficiency. The advantage of the proposed modeling and optimization framework is its adaptability to other liquefaction processes.

Design and analysis of a process for directly Re-liquefying BOG using subcooled LNG for LNG carrier

A process for directly re-liquefying boil off gas (BOG) using subcooled liquefied natural gas (LNG) for LNG carrier is designed. A part of LNG, pumped out from the bottom of the LNG storage tank is subcooled by the reverse Brayton nitrogen cycle and sent back into the tank by spraying to re-liquefy the BOG. The process is simulated by Aspen HYSYS and optimized by the genetic algorithm in order to find the minimum specific energy consumption (SEC). The optimization results show that the SEC and the exergy efficiency (FOM) of the BOG re-liquefaction process are 1.101 kWh/kgLNG and 25.40%, respectively. Furthermore, the flexibility study of the process is used to compare the system performance for different mass flows of the BOG. The results show that it can also satisfy the system requirements only by adjusting the flow rate of the refrigerant and LNG. In addition, the SEC and the FOM of the six different conditions are all about 1.10 kWh/kgLNG and 26%. Finally, the comparison of the SEC, FOM and the exergy losses of the process in this paper with other systems, demonstrates that the process after compression by the cryogenic BOG compressors has lower energy consumption and exergy losses and higher efficiency.

Operability study on small-scale BOG (boil-off gas) re-liquefaction processes

This study aims at analyzing dynamic behavior for the full operation cycle considering various alternatives and constraints related to small-scale and seaborne environment for a BOG re-liquefaction process in LNG-fueled ships, leading to significant enhancement of its operability. The design framework includes steady-state modeling, dynamic modeling and control system, with which system-wide dynamic characteristics are readily investigated and operating guidelines for achieving optimal process operations are obtained. Equipment sizing with the appropriate choice of design parameters is made such that the dynamic model developed in this study simulates non-steady-state characteristics in a realistic manner. 8 different process control schemes are proposed, and their technical gains as well as dynamic characteristics are investigated in details. The control algorithms are assessed through controllability analysis, including open loop test, open loop disturbance test and dynamic response against disturbances. For improving process efficiency, operation procedure for shut-down and start-up is designed with constraints imposed such as rotating speed variation and inert material filling for the effective and practically-adequate control for a refrigeration cycle, with which feasible heat transfer can be readily maintained. The case study presented fully demonstrates the validity and applicability of the control system proposed in this study.

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.

Optimal design of boil-off gas reliquefaction process in LNG regasification terminals

Boil-off gas (BOG) generation in liquefied natural gas (LNG) regasification terminals is substantial and unavoidable. Most terminals employ a cost-intensive BOG reliquefaction process using the send-out LNG. In this work, we study the preliminary design of this process with the objective of minimizing its total annualized cost (TAC). We present a comprehensive superstructure for the reliquefaction process that incorporates several process options for cooling BOG using LNG at different pressures, and allows recondensation in multiple stages. Then, we develop custom simulation/sizing modules for the process units in our superstructure, and implement a procedure to reduce explicit constraints during optimization. Considering realistic design specifications and operational constraints, we optimize a case study terminal for various BOG rates and conditions. While the TAC increases substantially with BOG rate, two-stage recondensation is always optimal. A 2-recondenser scheme with BOG cooling by the high-pressure LNG before the first recondenser is optimal for most cases.

Comparative study of boil-off gas re-liquefaction processes for liquid ethylene vessels

A re-liquefaction plant is essential for a liquid ethylene gas (LEG) vessel to liquefy the boil-off gas (BOG) and return it to the cargo tank. In this study, three potential BOG re-liquefaction processes for LEG vessels have been compared in terms of coefficient of performance (COP) and exergy efficiency. The effects of the ethylene storage pressure, propane condensing temperature, propane evaporating pressure, intermediate pressures of refrigeration, and liquefaction cycles on the system performances were analysed. The parameters of the processes were optimised individually with the aim to minimise the system power consumption. The results showed that a cascade process denoted as Case-I is superior to other two processes. The optimal COP of Case-I is about 0.767 and the maximum exergy efficiency is 63.1%. This work could provide some reference for selection of the BOG re-liquefaction processes for LEG vessels.

Energy-efficient design and optimization of boil-off gas (BOG) re-liquefaction process for liquefied natural gas (LNG)-fuelled ship

Liquefied natural gas (LNG) fuelled shipping systems adopts the boil-off gas (BOG) re-liquefaction process to maintain the pressure of storage tank and to minimize methane loss. However, most of previous studies on on-board BOG re-liquefaction process were made for large-scale applications of LNG carriers. This paper focuses on small-scale BOG re-liquefaction process used for LNG fuelled ship. In order to improve energy efficiency of BOG re-liquefaction, process design and optimization study are carried out for the refrigeration process in LNG fuelled ship. The reverse Brayton cycle using nitrogen as a refrigerant fluid is selected and two different configurations without using cryogenic compressor are considered, namely, i) re-liquefaction of BOG feed stream without compression and ii) pre-heating of BOG feed to use compressor operating in ambient temperature. Thermodynamic analysis provides conceptual insights into the key operating variables on the performance of BOG re-liquefaction process, while energy-efficient strategy for achieving minimum power consumptions can be systematically obtained through process optimization. Sensitivity analysis is also performed to understand how the variation of operating conditions affects on system performance of BOG re-liquefaction process under different design conditions and constraints. The case study also illustrates how process modeling and optimization framework proposed in this paper can be effectively utilized to improve energy efficiency in BOG re-liquefaction process.

Design of boil-off CO2 re-liquefaction processes for a large-scale liquid CO2 transport ship

This paper studies re-liquefaction properties of CO2 and introduces feasible designs of boil-off CO2 re-liquefaction processes suited for a large-scale transport ship. There are additional design considerations besides re-liquefaction efficiency due to the ship. Firstly, re-liquefaction facilities should be located within a limited space of a transport ship, which induces the number of main equipment as a constraint. Secondly, if an external closed-loop refrigeration is necessary, refrigerants should be re-chargeable and safe. From this perspective, CO2 would be the most appropriate one due to the availability in a liquefied CO2 ship and its safety if it can have similar performance to conventional refrigerants. According to CO2 liquefaction simulation, CO2 as a refrigerant shows similar performance to C3H8 and NH3 with additional 5–7% power consumptions. With this preliminary investigation, four boil-off CO2 re-liquefaction processes for a large-scale transport ship are designed. A re-liquefaction process which employs a distillation column shows 99.7% of re-liquefaction fraction but it has disadvantages of power consumption and the number of main equipment. On the contrary, another re-liquefaction process which has an external closed-loop refrigeration cycle with a CO2 refrigerant meets the re-liquefaction target (>70.0%) with the lowest power consumption. This process also requires nominal level of the number of main equipment and the capital expenditure. Given these results, an external closed-loop CO2 refrigeration cycle based re-liquefaction process is expected to be appropriate for a large-scale liquid CO2 transport ship. Hopefully, the results addressed in this study will contribute to further development of liquefied CO2 transport ships in near future.

ANALISA MENURUNNYA KERJA CARGO HANDLING SYSTEM PADA PROSES RELIQUEFACTION MUATAN GAS AMMONIA (NH3) DI MT. PUPUK INDONESIA

Reliquefaction adalah proses pencairan kembali vapour gas yang bertujuan untuk menjaga temperatur dan tekanan di dalam tangki. Pengoperasian sistem ini sebagai upaya yang dilakukan untuk mengatasi kendala-kendala dalam rangka mempertahankan muatan Ammonia tetap pada keadaan cair. 
 Dalam metode penulisan deskriptif dengan alat urgency, seriously, growth diambil penilaian tertinggi dari prioritas dan spesifik masalah serta dari perbandingan tiap masalahnya. 
 Dari rumusan masalah yang ada penulis menyimpulkan sesuai kondisi yang terjadi di MT Pupuk Indonesia saat melakukan penelitian tentang menurunnya kerja cargo handling system pada proses reliquefaction gas ammonia faktor-faktor penyebab menurunnya kerja reliquefaction plant berdasarkan penilaian tertinggi spesifik masalah diakibatkan oleh kurangnya kompresi kompresor kargo karena ausnya suction and delivery valve, kurang maksimalnya pendingin glycol, serta keausan piston dampak yang terjadi adalah proses reliquefaction memakan waktu lebih lama dan kondisi tangki tidak terkontrol upaya memaksimalkan kerja compressor adalah pengecekan rutin sesuai jam kerja, serta melakukan perawatan dan perbaikan sesuai manual book.

Optimal design and operating condition of boil-off CO2 re-liquefaction process, considering seawater temperature variation and compressor discharge temperature limit

Low-temperature liquid CO2 could boil off during ship transportation because of the heat ingress from the surroundings to inside the tank, which causes the tank pressure to increase. To maintain the operating pressure range of the tank, the re-liquefaction process is indispensable. Three design alternatives to the re-liquefaction process using boil-off CO2 as a refrigerant are proposed and compared. A systematic procedure to find the optimal design of CO2 re-liquefaction is provided considering operational constraints such as the cooling water temperature and compressor discharge temperature. The optimal operating conditions of the proposed processes are determined by solving nonlinear programming. The compressor power consumption as the operation energy for the CO2 re-liquefaction ranges from 60 to 120kW/t CO2 given the operational constraints. As the seawater temperature is lower and the discharge temperature limit is higher, the proposed Alternative 2 design consumes less power than the other designs.

The development and comparison of CO2 BOG re-liquefaction processes for LNG fueled CO2 carriers

CO2 carriers are considered as a solution for CO2 transport as a part of Carbon Capture and Storage. CO2 carriers' design is similar to that of commercial liquefied gas carriers and core technology of liquefied gas carriers is to handle boil-off gas during voyages. This paper introduced newly developed CO2 boil-off gas re-liquefaction processes making use of cold LNG fuel and investigated economic feasibility of newly developed processes by comparison with a conventional boil-off gas re-liquefaction process. New processes enabled considerable main equipment costs saving and additional fuel cost saving for re-liquefying boil-off gas. Simulation study results showed total amount of boil-off gas in a LNG fueled CO2 carrier could be re-liquefied by LNG fuel consumed during the voyage and comparative cost assessment results demonstrated those newly developed processes could be applied into a LNG fueled CO2 carrier with insignificant additional cost. The finding of this paper confirms those newly introduced processes are technically and commercially feasible at present and will contribute to growing the potential market of LNG as a marine fuel as well as commercializing CCS technology by providing more cost-effective solution for CO2 shipping.

An ejector-enhanced re-liquefaction process (EERP) for liquid ethylene vessels

Summary For liquid ethylene gas (LEG) vessels, developing high-efficient re-liquefaction plant to handle the continuously yielded boil-off gas (BOG) could significantly improve its energy efficiency. In this study, an ejector enhanced re-liquefaction process (EERP) for LEG vessels is proposed to improve the performance of the conventional re-liquefaction process (CRP) with a cascaded two-stage compression refrigeration cycle. In Aspen HYSYS, an ejector model was developed to analyse the performance of the proposed cycle system. The effects of the evaporating pressure of upper cycle pR0 ' and the intermediate pressures of upper and bottom cycles (pR3 and pE3) on the performances of both the CRP and EERP were studied. The results show that better performances can be reached with lower pR0 ' for the CRP and EERP. In order to achieve the best performances, the optimal pR3 and pE3 are, respectively, lifted to 510 kPa and 630 kPa by the EERP. When the proposed cycle system with the EERP is employed to re-liquefy 3000 kg/h of ethylene BOG, its coefficient of performance (COP) and exergy efficiency can be improved by 2.95–5.31% and 2.70–4.86% over that of the cycle system with the CRP. Correspondingly, the EERP could reduce the total power consumption of 15.7–27.9 kW compared to the CRP. Copyright © 2016 John Wiley & Sons, Ltd.

An Analysis of the Primary Energy Consumed by the Re-Liquefaction of Boil-off Gas of LNG Storage Tank

The pressure in liquefied natural gas (LNG) storage tank continues to increase due to the heat transfer from ambient air to low temperature LNG, which raises safety concerns. This research numerically analyzed the energy consumption of the re-liquefaction of BOG using the following four approaches: 1) a Claude cycle driven by electrical motor with the electricity produced by burning coal; 2) a Brayton cycle driven by a gas turbine fuelled by BOG released; 3) a Claude cycle driven by a SI engine fuelled by BOG; 4) using liquefied nitrogen produced in industry. With the efficiencies of the power system derived from industry data, the energy consumed of the re-liquefaction process was explored. The impact of heat transfer coefficient, LNG tank configuration, size, and percentage of LNG stored in tank on the rate of BOG and energy needed for the re-liquefaction of methane vapor were investigated. The primary energy ratio (PER), defined as the ratio of the energy recovered to the energy consumed, was examined and compared. The highest PER was observed using the Claude cycle driven by internal combustion engine fueled with BOG while the lowest PER was observed when BOG was cooled using liquefied nitrogen approach. The data presented in this paper provides the guideline for the management of pressure development in LNG storage tank and the design of accessory LNG storage system capable of re-liquefying the BOG gas to LNG.

液化天然气接收站内BOG再液化装置研究

液化天然气(LNG)作为天然气的一种存在形式，具有储存和运输的优势便利。目前我国已在沿海城市建设了多个液化天然气接收站和卫星站来接收国外进口LNG。接收站运营过程中将产生大量的蒸发气体(BOG)，当没有或者仅有少量出口运输需求时，当天然气终端和外输管网建设不同步的时候，过量的BOG将进入火炬燃烧掉，造成极大的浪费。在接收站内建设一个撬装式的BOG再液化装置，将站内产生的部分BOG再液化后返回LNG储罐，这对接收站的安全和经济运营具有重要的意义。本文以国内某接受站为例，对站内撬装式的BOG再液化装置进行了描述，介绍了混合制冷剂再液化流程、压缩机和换热器等主要设备选型、装置能耗、占地面积和开停车方案。 Liquefied natural gas (LNG) is an existing form of natural gas, which has the advantages of storage and transportation. Nowadays, many LNG terminals and satellite stations have been built along coastal city in our country to receive foreign LNG. Lots of BOG (boil of gas) will be produced during the operation process. Excess BOG burns through the torch when there is no or a small amount export demand, or when the construction of LNG terminal and export network cannot be completed at the same time. This leads to great waste. Part of BOG is re-liquefied and returns to LNG tank through a skid mounted BOG re-liquefied installation in the terminal, which is meaning for the safety and economic operation of LNG terminal. A skid mounted BOG re-liquefied installation in a LNG terminal is described in this article. Also, the mixed refrigerant re-liquefaction process, the main equipment such as compressor and heat exchanger, the consumption, the floor space and open parking scheme are introduced.

저열유속에서 상변화를 수반하는 메탄의 유동거동특성

A liquefied natural gas(LNG) in cryogenic liquid is converted back into gaseous form for distribution to residential and industrial consumers. In this re-gasification process, LNG supplies a plenty of cold thermal energy about 83.7×10 kJ/kg. The LNG cold thermal energy is utilized for the re-liquefaction process of cryogenic fluids such as Nitrogen, Hydrogen and Helium, and ice manufacturing process and air-conditioning system in some advanced countries. Therefore, it is also necessary to establish the recovery systems of the LNG cold thermal energy around Incheon, Pyungtaek and Tongyung LNG import terminals in our country. Methane is used as working fluid in this paper, which is the major component of LNG over 85 % by volume, in order to investigate the flow behavior characteristics of LNG with phase change at low heat flux. This paper presents the effects of pipe diameters, pipe inclinations and saturation pressures on the flow boundaries of methane flowing in a cryogenic heat exchanger tube, together with those of nitrogen, propane, R11 and R134a. The outcomes obtained from this theoretical researches are also compared with previous experimental data. It was also found that the effect of pipe inclination on the methane flow boundaries was significant.

36 MTD급 천연가스 BOG 재액화 플랜트 기본설계

. Abstract : In this paper, we carried out the basic design of 36 MTD natural gas BOG re-liquefaction system to recover the generated natural gas during performance test of LNG pump and natural gas compressor. The re-liquefaction process of natural gas is designed to have 1500 kg/h of liquefaction rate with reverse Brayton refrigeration cycle. With the designed process, the variat ion of liquefaction rate is calculated for various inlet conditions of feed gas. From results, the liquefaction rate is more sensitive for inlet temperature than gas composition. The specifications of equipments such as gas blower, natural gas compressor, cryogenic heat exchanger and nitrogen compander are determined on the basis of the designed process. The requirement of power consumption and cooling water are also determined through the basic design.

Thermodynamic Analysis‐Based Improvement for the Boil‐off Gas Reliquefaction Process of Liquefied Ethylene Vessels

AbstractA new optimization design of the boil‐off gas (BOG) reliquefaction process for liquefied ethylene (LEG) vessels is proposed in order to reduce the reliquefaction process energy cost and improve its cold exergy efficiency. The exergy loss of each component is calculated and the efficiency of the available energy utilization is evaluated on the basis of a detailed thermodynamic analysis. The exergy analysis results indicate that the exergy efficiency of the improved BOG reliquefaction process is about 19.0 % higher than that of the existing process, and the amount of refrigerant used in the improved process is reduced by about 44.9 % per hour. The power consumption could be decreased by 16 %. The circulation volumes of the refrigerant and BOG are both significantly reduced, thus lowering the equipment and operation costs of the BOG reliquefaction process.

CO2 Storage Terminal for Ship Transportation

In this article, an intermediate CO2 storage system for long-distance ship transportation was modeled. The storage terminal links the continuous CO2 liquefaction process to discrete marine ship transportation and performs as a buffer between them. It is composed of four distinct processes: a CO2 input process, a storage tank and loading process, a recirculation process, and a BOG (boiled-off gas) reliquefaction process. The entire system should be operated as a liquid phase. Consequently, operation conditions, tank capacity, insulation specification, and streamflow rates play a major role in operating the storage terminal securely. The goal of this study is to design a base case of the storage terminal and propose its appropriate operation condition which makes the terminal operate with minimum operation energy. Results of the base case simulations are compared with improperly insulated systems on the pipeline and tanks that generate more BOG than the base case. The total operation energies of the base ca...

Novel design of LNG (liquefied natural gas) reliquefaction process

This paper presents an investigation of novel LNG reliquefaction process where the cold exergy of subcooled LNG is utilized to recondense the vaporized light component of LNG after it is separated from the heavier component in a phase separator. The regeneration of cold exergy is especially effective as well as important in thermodynamic sense when a cryogenic process is involved. To verify the proposed idea, we performed an experimental study by facilitating liquid nitrogen apparatus to mock up the LNG reliquefaction process. Subcooled liquid nitrogen is produced for a commercial transportation container with a house-made atmospheric liquid nitrogen heat exchanger and then, having subooled degree of up to 19 K, it simulates the behavior of subcooled LNG in the lab-scale reliquefaction experiment. Recondensation of the vaporized gas is possible by using the cold exergy of subcooled liquid in a properly fabricated heat exchanger. Effect of heat exchanger performance factor and degree of subcooling on recondensation portion has been discussed in this paper. It is concluded that utilizing pressurized subcooled liquid that is obtained by liquid pump can surely reduce the pumping power of the vaporized natural gas and save the overall energy expenditure in LNG reliquefaction process.

Design of a boil-off natural gas reliquefaction control system for LNG carriers

Onboard boil-off gas (BOG) reliquefaction is a new technology that liquefies BOG and returns it to the cargo tanks instead of burning it off during a voyage. For the commercial development of this technology, an object-oriented dynamic simulation is presented which facilitates the design of the plant and control system for the thermal process. A reliquefaction process based on the reverse Brayton cycle has been designed, and its static thermodynamic states at the design BOG load are presented. To make the cycle work for any BOG load, an idea was sought that would achieve a heat balance with the work extracted by the expander. Dynamic simulations were conducted for all operating modes, including start-up and idle. It was found that the expander exit temperature is the key process variable for control and that the process control works successfully when three actuators are activated in three different BOG load regimes. The study also shows that control of the separator pressure to keep the vapor fraction at the throttle valve exit as low as possible is an efficient method for purging nitrogen from BOG.