天然气水合物在水力提升管道中的分解特性

为了指导海底天然气水合物(以下简称水合物)绞吸式开采水力提升管道系统参数设置,研究了水力提升管道内水合物的分解特性和流动参数变化对其的影响。基于热力学和流体力学,采用数学建模的方式建立了水合物水力提升管道温压模型、水合物分解传质模型和管道多相流模型,分析了固液两相流转变成固液气三相流过程中不同影响因素下管道流体温压、水合物颗粒物质的量、分解面位置与海水深度的关系。结果表明:(1)随着管道流量增大,水合物分解速度减慢,分解面少量上移;(2)颗粒直径对管流温压、相平衡压力、水合物分解面基本没有影响,但只有直径小于0.2 mm的水合物颗粒才能在管道中完全分解,直径大于2.0 mm后,颗粒分解量忽略不计;(3)出口回压为正压且增加时,水合物分解面上移,分解速度减慢,而出口回压为负压且增大时,水合物分解面下移,分解速度加快;(4)随着采矿深度的增加,水合物分解速度变慢,分解面上移,但在与海面距离超过1 500 m后采矿深度对水合物分解速度、分解面无影响;(5)实验验证与数值仿真规律基本一致,表明所建立的模型具有较高的可信度。结论认为:绞吸式开采水合物时,控制合理的流量和出口回压能够调节分解面高度以及分解速度,并且不用考虑颗粒直径和采矿深度对产气量的影响。

Decomposition characteristics of natural gas hydrates in hydraulic lifting pipelines

For the sake of guiding parameter setting of the hydraulic lifting pipeline system for cutter-suction mining of natural gas hydrates in sea beds (hereinafter, hydrates for short), the decomposition characteristics of hydrates in hydraulic lifting pipelines and the effects of flow parameters on decomposition characteristics were studied in this paper. A temperature–pressure model for the hydrate hydraulic lifting pipeline, a hydrate decomposition mass transfer model and a pipeline multi-phase flow model were established using mathematical modeling method according to thermodynamics and fluid mechanics. Then, the relationships of the temperature and pressure of pipeline fluid, the amount of hydrate particulate matter and the decomposition surface vs. the seawater depth under the effect of different influencing factors during the transformation from solid–liquid two-phase flow to solid–liquid–gas three-phase flow were analyzed. And the following research results were obtained. First, the decomposition of hydrate slows down and the decomposition surface moves upward slightly with the increase of flow rate in the pipeline. Second, particle size basically has no effects on the temperature and pressure of pipeline fluid, the phase equilibrium pressure and hydrate decomposition surface. However, only the hydrate particles whose diameter is smaller than 0.2 mm can be completely decomposed in the pipeline while the decomposition of those whose particles size is greater than 2.0 mm is negligible. Third, if the back pressure at the outlet is positive, the decomposition surface moves upward and the decomposition of hydrate slows down with the increase of the back pressure. And if the back pressure at the outlet is negative, the decomposition surface moves downward and the decomposition of hydrate speeds up with the increase of the back pressure. Fourth, the decomposition of hydrate slows down and the decomposition surface moves upward with the increase of mining depth. However, the decomposition velocity and decomposition surface are basically unchanged when the mining depth is below 1 500 m under water. Fifth, the experimental results are basically consistent with the numerical simulation results, and it is indicated that the newly established models are of high reliability. In conclusion, decomposition surface height and decomposition velocity can be adjusted by controlling flow rate and outlet back pressure rationally during the cutter-suction mining of hydrates while the influences of particle diameter and mining depth on gas production rate need not be taken into consideration.