預測液化天然氣程序中碳氫化合物在甲烷中的溶解度

Prediction of Solubility of Heavy Hydrocarbons in Methane for Liquefied Natural Gas Process

魏雲杰(Yun-Jie Wei)

指導教授 ： 林祥泰

國立臺灣大學/工學院/化學工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701318

摘要

頁岩氣的發現減輕了數十年來逐漸高升的能源需求。從頁岩氣中分離出的液化天然氣在工業中扮演著重要的角色。天然氣經過液化程序利用冷卻的方式從氣態轉換為液態以方便運送及儲存。若是操作的溫度或壓力有所變動，其中的高碳數烷類極有可能就會析出，造成管線或是液化裝置阻塞。阻塞的管線會造成輸送產率的下降，工廠亦必須要花額外的成本來清理或更換管線和裝置。因此防止高碳數烷類析出是非常重要的課題。未經過完整程序的天然氣主要成份為甲烷，而高碳數烷類在甲烷內的溶解度會受到壓力、溫度影響，由於高碳數烷類包含直鏈碳烷類(如丙烷至壬烷)和芳香烴(如苯和二甲苯)，近十五種物質共存的系統會導致計算過於複雜，故我們將問題簡化為雙成份系統，計算固液相平衡中，定壓下直鏈碳的烷類在甲烷中的溶解度和溫度的關係。此外，若溫度升高，甲烷可能汽化而使系統轉為固氣相平衡，同時高碳數烷類的溶解度會大幅降低。為了避免這類問題的發生，我們進行固液氣三相平衡的計算。

我們利用MHV1混合律結合PRSV狀態方程式與COSMOSAC活行系數模型，進行各種相平衡的預測，找出在定壓下，直鏈碳的烷類與芳香烴在甲烷中的溶解度和溫度的關係和三相共存的溫度、組成並和既有實驗值做比較。溶解度預測方面，隨著溫度降低，預測結果會越趨失準，在主碳數8以上的烷類預測和實驗值相比約莫有50-70%誤差且和實驗值趨勢不同；三相計算方面，在低溫下中碳數烷類系統有良好的預測，但在高碳數、或有特殊結構的烷類系統如辛烷和二甲苯及其異構物並沒有得到非常好的成果，多數的實驗結果為發散或收斂至錯誤的區域。另外，我們檢查在稀薄溶液的系統中，沸點上升的依數性質是成立的。

關鍵字

固液相平衡 ； 三相計算 ； 狀態方程式 ； 液氣相平衡

並列摘要

Discovery of shale gas has alleviated the rising demand of energy in the past decades. Liquefied natural gas (LNG) separated from shale gas plays a role in industries. Natural gas is liquefied during the liquefaction process for the sake of delivery and storage. As the pressure or temperature in pipelines change, high-carbon number hydrocarbons in LNG might convert into solids, blocking the cooling devices. Blocked pipelines results in decrease of yields and causes extra cost to clean up or replace. In general, natural gas is composed of mainly methane and other hydrocarbons. Solubility of heavy hydrocarbons (HHCs), which is dilute in natural gas, is influenced by temperature and pressure. HHCs include straight-chain ( propane, nonane, etc.) and aromatics ( benzene, xylene, etc.).

The system with almost 15 components will lead the calculation to be complicated, so we simplified the problem to be binary system. We calculate the solubility of straight chain hydrocarbon-methane solid-liquid equilibrium (SLE) system with different temperature at fixed pressure to discuss the relationship between solubility and temperature. Also, as temperature rises, methane is prone to vaporize and the system changes into solid-vapor equilibrium (SVE), bringing about drop of HHCs solubility. To prevent this problem, we need to discuss solid-liquid-vapor equilibrium (SVLE), or three-phase equilibrium in other words.

In this work, we use MHV1 mixing rule combining with PRSV equation of state and COSMOSAC model to predict different types of phase equilibrium. At constant pressure, we discuss straight-chain HHCs and aromatics-methane for solubility and three-phase temperature predictions, comparing to experimental data. For solubility prediction, as temperature decreases, HHCs with carbon number over 8 has errors of %. For three-phase calculation, middle-chain HHCs systems have satisfying results at cryogenic temperature. However, in ling-chain HHCs and alkanes with cyclic structure our model doesn’t have well predictions. Most of the data points diverge or converge to wrong region. Furthermore, we find out in dilute-solute system, the colligative property of bubble point elevation is valid.

並列關鍵字

solid-liquid equilibrium ； three-phase calculation ； PRSV+COSMOSAC equation of state ； vapor-liquid equilibrium

參考文獻

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