Prediction of VLE and VLLE of Systems Containing Hydrocarbon Reservoir Fluids and Aqueous Methanol Solutions With a Cubic EOS
Authors: Dandekar A.Y. , Chen G.J. , Guo T.M. , Khataniar S. , Patil S. 
Published: March 2010

With the purpose of accurately representing the phase equilibria of hydrocarbon reservoir fluids, water, and methanol, the revised modified Patel-Teja model (MPT2 model) has been extended to predict the vapour-liquid (VL) and vapour-liquidliquid (VLL) equilibria of hydrocarbon/water/methanol systems. The prominent feature of the work presented in this paper is the minimum usage of binary interaction parameters (BIPs) in the prediction of VLE and VLLE data. The accuracy and reliability of the extended MPT2 model has been demonstrated by extensively testing against diverse experimental data on VLE and VLLE of ternary, quaternary, five component, natural gas, and reservoir oil systems containing water and methanol. The extended MPT2 model can be easily applied to production schemes of natural gases from hydrate formations, hydrocarbon flowlines, and processing units.

Introduction

The production of natural gases from hydrate formations can be potentially accomplished by shifting the hydrate equilibrium curve through heating, pressure reduction, or by injection of an inhibitor. Water-soluble compounds such as methanol, ethanol, ethylene glycol, diethylene glycol, and triethylene glycol are commonly used as hydrate inhibitors. The other applications of hydrate inhibitors include production and transportation of hydrocarbon streams that also contain water, where there is always a risk of pipeline or processing equipment plugging owing to hydrate formation, particularly when such operations are carried out at low temperature conditions.

However, one of the most worrisome problems associated with the application of hydrate inhibitors is the partial dissolution of usually lighter hydrocarbon and other components from natural gas and oil streams in, for instance, the aqueous methanol phase, thereby reducing its effectiveness as a hydrate inhibitor. Moreover, the solubility of such lighter hydrocarbon components in the aqueous inhibitor phase also has a significant effect on the thermodynamic prediction of hydrate formation conditions. Also, from a thermodynamic perspective, the complexity of phase behaviour of hydrocarbon/water systems is even more pronounced when methanol is added as the most widely accepted hydrate inhibitor. Therefore, a thermodynamic model capable of representing the phase behaviour of all phases in a consistent manner for simultaneous phase equilibrium calculations on mixtures of hydrocarbon, water, and methanol is vital for evaluating its application as a hydrate inhibitor. Herein lies the purpose of this work.

Cubic equations of state (EOS) with quadratic mixing rules have been successfully applied to hydrocarbon fluids in a wide range of thermodynamic conditions. However, the application of cubic EOS in their basic form to polar-nonpolar systems, i.e., hydrocarbon- water or hydrocarbon-aqueous methanol solutions, is inadequate. Traditionally, the so-called activity coefficient models, such as UNIQUAC and NRTL equations, have handled polar systems. These activity models are not applicable to vapour phase and are unreliable at higher pressures. Therefore, polar-nonpolar systems are generally handled by incorporating unconventional mixing rules in the cubic EOS to improve their performance for such systems.