Temperature Characteristics of Natural Gas Hydrate Formation in LNG Storage Tanks Yang Bo 1 Li Xiaosen 2 Li Maodong 1 Du Nansheng 1 Lin Jinmei 1 Wang Wenfeng 1 1. Guangzhou Institute of Special Pressure Equipment Inspection, Guangzhou 510663, China; 2. Natural Gas Hydration, Guangzhou Institute of Energy, Chinese Academy of Sciences Research Center, Guangzhou 510640-dimensional simulation device successfully generated methane hydrate from methane gas and aqueous solution, and studied the whole process of methane hydrate formation in porous media through temperature-pressure changes. The injection of the vertical center well causes the temperature in the central region to rise and spreads around, and the hydrates gather from the center well. Due to the capillary action of the porous medium, the methane hydrate produces a wall-hanging effect in the porous medium, and the outer hydrate is formed more. In the porous medium, there is an uneven distribution of ice-like water (gas, gas, gas, salinity, pH, etc.) generated by natural gas (gas phase) and water (liquid phase) under conditions of 13 刖 刖 (suitable temperature, pressure, gas saturation, water salinity, pH, etc.) , non-stoichiometric □ cage crystal compound 丨 solid phase) 13 NGN existing in nature, its main Methane (>90%), so it is also known as methane hydrate (MethaneHydrates). 13 Natural gas hydrates are mainly found in porous media. For example, seabed sediments and permafrost zones in high latitudes indicate that China's seas and its surroundings There are a large number of natural gas hydrates in the region, including the northern slope of the South China Sea, the East China Sea slope, the Nansha Trough, and the Qinghai-Tibet Plateau and the Qilian Mountains. Currently, the methods for monitoring natural gas hydrate formation mainly include direct observation and pressure judgment. Among them, acoustic detection, CT technology and magnetic resonance imaging technology, etc., direct observation method and method for inferring hydrate formation by pressure change are low in cost, but the precision is low; although optical inspection is low in cost and good in performance, only The hydrate in the visual kettle can be detected, and there is no power for the formation of hydrates in the whole porous medium; although the acoustic detection technology has good cost performance, it cannot generate visual images; CT technology and magnetic resonance imaging (MRI) High-precision experimental research can be done, but the price is higher for porous media The monitoring technique of the hydrate formation process is also the resistance method and the temperature-pressure method. The hydrate formation simulator is equipped with a plurality of electrodes, and a given electric field is used to monitor the hydrate by measuring the change in resistivity between different electrodes. The temperature-pressure method is a relatively mature method for monitoring the hydrate formation process. The method is intuitive and the experiment cost is low. The measurement principle is the temperature change caused by the change of the natural gas hydrate formation. Liu Chunyang et al. On the self-designed gas hydrate formation test device, it was found that the lower the reaction temperature, the faster the reaction rate of hydrate. 13 Pang Weixin et al., through a 10 L static reactor, found that the gas temperature induced the formation and formation of methane hydrate. The speed also has an important effect. The lower the gas temperature, the shorter the induction period of methane hydrate formation and the faster the formation rate. 13 At the same time, the heat of methane hydrate formation cannot be taken away in time, and the temperature of the reaction liquid will rise rapidly, resulting in the formation of methane hydrate. The speed is quickly reduced or even stopped. Yang Xin et al. passed the 7L reactor at 0 ° C, 0 ° C and below 0 ° C. Methane hydrates were formed at different temperatures. It was found that methane hydrate formed faster at temperatures above 0 °C, and the gas storage was large near °C. The hydrates were evenly distributed throughout the sand layer, and hydrate formation was mainly below 0 °C. Due to the diffusion control of methane gas, it is possible to determine whether there is natural gas hydrate formation by analyzing the temperature data, and also to qualitatively analyze the distribution of natural gas hydrate according to the change of temperature. The hydrate formation experiment was carried out in the 3D simulation experimental device, and the fund project was studied through the temperature-pressure change: National 973 Plan Project (2009CB219507); The Chinese Academy of Sciences Major Scientific Research Equipment Development Project (YZ200717) The process of gas hydrate formation in the gas storage tank l3 1Experiment 1.1 Experimental device Based on the experimental device for the study of natural gas hydrate in porous media at home and abroad, a large-scale three-dimensional natural gas hydrate formation system can be used to carry out the experiment of methane hydrate formation in porous media. Liquid supply module, regulated gas supply module, Gas tank, environmental simulation module and data acquisition module 3 The liquid supply module mainly includes electronic balance and flat flow pump. The electronic balance is ALH-30 type, the measuring range is 30kg, and the measuring accuracy is ±2g. It is used to accurately measure the liquid quality injected into the gas storage tank. The advection pump is a P MPal3 regulated gas supply module including a methane gas cylinder, a pressure reducing valve, etc. The gas used in the experiment is a pure methane gas storage tank with a volume content of 99.9%, with a pressure of 25 MPa, and an effective volume of 117.75 Ll3 in the LNG Storage Tanks. There are temperature sensor and pressure sensor, respectively, to record the temperature and pressure change in the gas tank with time in real time. 13 The temperature sensor is Pt1000 platinum resistance, the range is -50~200° (, the accuracy is ±0.1° (the pressure sensor range 301) \, the fine gas flow rate ± 0.1% 13 electrodes are distributed in the gas storage tank, and ensure that the electrode is in contact with quartz sand.
1.2 Experimental procedure In the methane hydrate formation experiment, quartz sand with a particle diameter of 0.30-0.45 mm was selected as the porous medium. First, methane gas was injected into the gas storage tank until the pressure was raised to 1 MPa, and it was vented and repeated three times. The vacuum pump is used to evacuate the remaining gas and water in the kettle to ensure that the air content in the gas tank is very small, and has no effect on the experimental results. 13 Next, the outlet valve is closed, and a certain amount of deionized water is injected into the gas storage tank. Methane gas is injected through a high-pressure steel cylinder to bring the pressure in the autoclave to 20 MPa; the temperature of the constant-temperature water bath is adjusted to 8 ° C); after a sufficiently long reaction time, when the pressure in the LNG Storage Tanks is no longer changed, the hydrate synthesis is completed, throughout Record and save data in real time during the process. 13 2 Results and discussion Pressure-temperature versus time curve in the formation of methane hydrate in porous media. See p in the gas tank as a function of time. It can be seen that when gas is injected into In the gas storage tank, the pressure reached 20.06 MPa, and the hydrate began to form immediately. No hysteresis was observed, which was related to YanLJ et al. in activated carbon. The observed phenomenon of methane hydrate is consistent. 13 According to the hydrate experiment conducted by ChaSB et al. on the hydrophilic surface, hydrates are more likely to nucleate and hydrates more easily due to the ordered arrangement of water molecules on the adsorption surface. The r. is the water jacket temperature around the gas storage tank, and T is the average temperature of each measuring point in the gas storage tank. It can be seen that in the gas injection stage, due to the work of the booster pump and the Joule-Thomson effect The temperature inside the gas tank rises sharply to 14.4 ° C, and then the temperature inside the gas tank is gradually reduced due to the influence of the ambient temperature. The whole methane hydrate formation process, the temperature is basically maintained near 8.7 ° C, due to the gas storage tank The formation of exothermic methane hydrate and the pressure and temperature change temperature of methane hydrate formation in the porous medium in the surrounding environment cause the temperature in the entire gas tank to fluctuate slightly.
In order to minimize the error caused by the temperature probe in the gas tank, the temperature difference between the real-time temperature value and the hydrate formation (1140h) during the experiment is used as a comparison parameter. It can be seen from a) that the gas injection is injected from the vertical center well, causing the temperature in the central region to rise and spread around; at the same time, the temperature around the inner wall of the gas tank is slightly lower. It can be seen from b) that when the pressure reaches the specified pressure of 20.06 MPa, the methane gas is accumulated in the upper middle part of the gas storage tank, and the temperature is higher than the gas temperature due to the gravity of the water accumulating at the bottom. Taking a certain period of time in the generation process as an example, the average temperatures in the gas storage tanks of 698, 702 and 707h are 8.7, 8.6 and 8.7 °C, respectively, compared with f), from c) to e) The temperature around the vertical center well rises, indicating that the hydrate formation gathers from the center well; the temperature around the wall of the gas storage tank decreases, indicating that the hydrate around the gas tank is unstable and is easily decomposed by the temperature of the water jacket. And it can be seen that the hydrate formation and decomposition changes in the upper part of the gas tank are stronger than the bottom.
Also, the spatial distribution of temperature during the formation of methane hydrate is in the process of methane hydrate formation, and the temperature in the gas storage tank has obvious changes, indicating that hydrate formation occurs in the gas storage tank, and there is no hydrate formation. Blind area, but the temperature around the gas tank varies greatly with respect to the central area. This is because the heat transfer performance around the chamber is better. Due to capillary action, the internal water body is “twitched” to the outside to form a layered hydrate body. It is difficult to achieve uniform and stable formation in a block-like distribution, which is regarded as the "wall climbing effect" of porous media.
3 Conclusions a) The methane hydrate solid phase was successfully formed from the gas phase and liquid phase in a self-made gas hydrate three-dimensional simulation device, and the methane hydration was detected by monitoring the hydrate formation process by using a temperature-pressure method in a porous medium. The distribution area of the object can prepare for the simulated mining of hydrate in the three-dimensional system in the future.
b) Gas injection from the vertical center well causes the temperature in the central area to rise and spread around, and the hydrates gather from the center well.
c) Due to the capillary action of the porous medium, the methane hydrate produces a "wall climbing effect" in the porous medium, and the outer hydrate is formed more and is unevenly distributed in the porous medium.
