Improvement of the Liquefied Natural Gas Vapor Utilization System Using a Gas Ejector

The production, transportation, and storage of liquefied natural gas (LNG) is a promising area in the gas industry due to a number of the fuel’s advantages, such as its high energy intensity indicators, its reduced storage volume compared to natural gas in the gas-air state, and it ecological efficiency. However, LNG storage systems feature a number of disadvantages, among which is the boil-off gas (BOG) recovery from an LNG tank by flaring it or discharging it to the atmosphere. Previous attempts to boil-off gas recovery using compressors, in turn, feature such disadvantages as large capital investments and operating costs, as well as low reliability rates. The authors of this article suggest a technical solution to this problem that consists in using a gas ejector for boil-off gas recovery. Natural gas from a high-pressure gas pipeline is proposed as a working fluid entraining the boil-off gas. The implementation of this method was carried out according to the developed algorithm. The proposed technical solution reduced capital costs (by approximately 170 times), metal consumption (by approximately 100 times), and power consumption (by approximately 55 kW), and improved the reliability of the system compared to a compressor unit. The sample calculation of a gas ejector for the boil-off gas recovery from an LNG tank with a capacity of 300 m3 shows that the ejector makes it possible to increase the boil-off gas pressure in the system by up to 1.13 MPa, which makes it possible to not use the first-stage compressor unit for the compression of excess vapours.

CHARACTERISTICS OF HEAT TRANSFER DURING COOLING DOWN PROCESS IN A SINGLE CARGO TANK OF LNG CARRIER

A deep understanding of heat transfer characteristics is essential in evaluating risk and putting forward any option for the Liquefied Natural Gas (LNG) tank cooling down process. A novel Computational Fluid Dynamics (CFD) model was built to perform the flow and heat transfer simulation of the process. The predicted results agreed well with the test data from a prototype LNG tank. Then the heat transfer characteristics of the process were analysed. It was found that the vapour temperature and density were linearly varying and became stable after 2.3 hours. A sudden pressure drop risk was identified during the process, which will cause the inwards collapse risk of the invar membrane. Then the proposals to prevent the risks of the inwards collapsing membrane are presented. The heat transfer characteristics of the vapour and different membrane layers were analysed in detail, and if the suggested option was to be implemented this could save about 39% of LNG consumed.

The Mass and Heat Transfer Characteristics Study of the Natural Convection Evaporation of an LNG Droplet in BOG

The evaporation process of LNG droplets in BOG is closely related to the cooling down process of the LNG tank, but there isn’t an available droplet evaporation model at present. Been prepared based on the conservation of mass, momentum, and energy, a CFD model of natural convection evaporation of a single LNG saturated droplet in the BOG was developed and applied. The results show that:①There are two distinguished zones around the droplet surface, where the local temperature boundary layer of the droplet gradually thickens and rapidly thickens with the increase of the angle of inflow from 0 ° to 90 ° and from 90 ° to 180 °, respectively; ② With the increase of droplet size, the average thickness of temperature boundary layer increases gradually, which leads to the decrease of relative evaporation rate;③“blowing effect” remains almost unchanged with the increase of droplet size.

Transient Response Analysis of Multi-layer Sloshing Fluid in LNG Tank

The stratified LNG is the potential cause of rollover, so the analysis of the interface behavior is very important. In this work, a three-layer dynamic simulation model of BOG, LNG, and liquid nitrogen of sloshing stratified LNG in a tank was built. The transient behaviors of the layers were simulated, and the effects of factors such as the period of sloshing (T = 0.2, 0.4, 0.6, 0.8 and 1.0 s) and the amplitude of sloshing (A=0.01~0.06 rad) were analyzed. It was found that instability of the interface was aggravated over time. There was a specific period of sloshing in which the layers of fluid were relatively stable, away from this period the fluid layer became less stable. The influence of sloshing amplitude on the behavior of the interface presented a wave characteristic. The simulation results of the coupling with different sloshing periods and sloshing amplitude showed that the interface behavior of stratified sloshing was the most severe when the sloshing period was 0.2 s and the sloshing amplitude was 0.05 rad.

Impact Assessment of Flammable Gas Dispersion and Fire Hazards from LNG Tank Leak

This study conducts an impact assessment of flammable gas dispersion and fire hazards from LNG tank leak. The release source model is used to estimate LNG release rate. A CFD (computational fluid dynamics) based 3D model is established to simulate dispersion behavior of flammable gas from the phase transformation of LNG. Subsequently, a FDS (fire dynamics) based model is built to simulate the pool fire due to LNG tank leak. The impact of gas dispersion and fire on personnel and assets is assessed based on simulation results, which can provide a theoretical basis and method support for major accident assessment of tank leakage in large LNG receiving station. The results show that the dispersion of flammable gas from LNG tank leak has an obvious stage characteristic. The flammable gas reached a steady state around 300 s, and the corresponding coverage area is about 16250 m2. The pool fire simulations indicate that the steady flame is formed at 20 s. The flames flow along the wind, and the maximum temperature of the fire reaches 670°C, and the maximum thermal radiation reaches 624 kW/m2. According to the fire damage criteria, the pool fire from LNG tank leak may pose a serious threat on the safety of adjacent assets and personnel.

Numerical Study on the Tank Heel Determination Using Smoothed Particle Hydrodynamics

Tank heel minimization is a significant issue in the design of LNG fuel tanks because it is associated with stable suction pump operation and thermal shock requirements during LNG bunkering. This study examined how the LNG tank heel is minimized, maintaining a suction pump fully submerged in LNG during dynamic vessel motion. The study assumed two LNG fuel tanks mounted on the forward deck of a 50,000 deadweight class oil product carrier. Information on the dimensions and shape of the LNG fuel tank was determined from Wartsila’s brochure, and the specifications of Vanzetti’s suction pump were referred to. The LNG fuel tank and LNG heel were modeled as rigid elements and hydrodynamically smoothed-particles, respectively. The number of particles could be determined by performing even keel analyzes by adding or subtracting particles until the target head was satisfied under the gravity load. To simulate the motion of the LNG fuel tank, the pitch and roll periods and amplitudes of the ship were calculated using the DNV classification rules. Visual observations of the dynamic flow during the pitch and roll motions with respect to the ship’s center of mass showed that the roll motion was more critical from the viewpoint of the LNG heel than the pitch motion. After performing the simulations for three cycles of roll and pitch motions, the suction pump submergence was reviewed in the last cycle. Under the conditions assumed in this study, a filling ratio of 15% was determined as the minimum LNG tank heel. Although the LNG heel has customarily been determined, the LNG heel needs to be determined through hydrodynamic analyses of each vessel because it depends on the shape of the fuel tank and the vessel motion characteristics.