Modeling Study On Phase Equilibria Of Semiclathrate Hydrates Of CH₄,CO₂,N₂ And Gas Mixtures In Tetra-n-butyl Ammonium Halogen Aqueous Solutions

Quaternary ammonium salts?QASs?such as tetra-n-butyl ammonium bromide?TBAB?and fluoride?TBAF?can form semiclathrate hydrates with water molecules at atmospheric pressure.The semiclathrate hydrates have attracted more and more attentions for their broad applications in storage and transportation,separation,absorption,and capture of natural gas or coal bed methane.The research on phase equilibrium condition of semiclathrate hydrate is of importance for theoretically guiding the development and application of hydrate technology.In this work,the experimental data from literatures were gathered,and then modeling investigations were conducted for the TBAB and TBAF semiclathrate hydrates with different gases such as CH₄,CO₂,N₂,and their mixtures.The main contents in this work are as follows.?1?Proposing the thermodynamic model: the Chen-Guo hydrate model was extended to the TBAB/TBAF systems,and then the program was coded using FORTRAN language for predicting calculation.To determine the fugacities of hydrate formers in gas phase,the Patel-Teja equation of state?PT EOS?was employed.The PT EOS combined with a Debye-Huckel electrostatic contribution term was chosen to describe the thermodynamic properties of the electrolyte solution portion.?2?Fitting the parameters in the EOS: the parameters in EOS for TBA⁺ cation were determined by fitting the activity of water in the TBAB aqueous solution.?3?Fitting the parameters in the Chen-Guo hydrate model: the parameters of TBAB and TBAF in the hydrate model were obtained from correlating the equilibrium data of semiclathrate hydrates.The calculated and predicted results using the proposed model were compared with available experimental data from literatures.The average absolute deviation of semiclathrate hydrate formation temperature was 0.43 ? for the 372 experimental data on the TBAB/TBAF systems.The extensive tests show that the calculation results are in acceptable agreement with the experimental data on phase equilibrium of semiclathrate hydrate systems and satisfy the need for the practical engineering applications.