藉理論計算模型預測水合物相平衡行為

Abstract

氣體水合物為類似冰塊之固態結晶物，由水分子所形成之立體結構空隙中，藉由填入甲烷、乙烷、二氧化碳等氣體分子以達到穩定狀態。本研究之研究動機即針對氣體水合物建立熱力學理論計算模型，藉由計算模型準確預測氣體水合物之氣相-液態水相-水合物相三相平衡。 在本研究所建立之理論計算模型，選擇以van der Waals and Platteeuw (vdW-P) Model來描述水合物相之相行為，並分別以Peng-Robinson-Stryjek-Vera Equation of State (PRSV EoS)與Cubic Plus Association Equation of State (CPA EoS)計算氣相與液態水相之相行為。在vdW-P Model中，藉由Langmuir吸附行為模型來描述氣體填入水合物結構之過程，過去相關研究中，多以球形對稱位能函數來估算Langmuir常數，本研究藉由一包含兩個參數之溫度函數來估算Langmuir常數，以簡化理論計算模型之計算流程。 本研究之熱力學理論計算模型可準確計算單一氣體水合物之相平衡，並可進一步應用於混合氣體水合物與添加醇類等抑制劑與促進劑之氣體水合物系統，預測其相平衡條件與相平衡狀態下系統組成，均具有相當高的準確性。

Gas hydrates are ice-like crystalline solids in which water molecules form a three-dimensional structure that is stabilized by the encapsulation of small gas molecules like methane, ethane and carbon dioxide. The motivation of this work is to develope a calculation model to predict vapor-liquid-hydrate-three-phase equilibrium condition with a better degree of predictability. In this study, the van der Waals and Platteeuw (vdW-P) model is applied to describe the hydrate phase and both Peng-Robinson-Stryjek-Vera (PRSV) equation of state (EoS) and Cubic Plus Association equation of state (CPA EoS) are chosen to describe the vapor and liquid phases. In the vdW-P model, the Langmuir adsorption behavior is assumed to model the adsorption of gas molecules in the hydrate cages and the Langmuir constant is defined as a function of spherically symmetric cell potential function. In this study, the Langmuir constant is described by a two-parameter temperature-dependent function. It is found that the vapor-liquid-hydrate-three-phase equilibria can be well described by this simple model. And the model also can be applied to predict the competition of different gases when forming binary-guest gas hydrate and predict the phase equilibrium condition of gas hydrate with inhibitor.