(403b) Towards More Accurate Transport Models for Non-Ideal Mixtures of Hydrocarbons and Supercritical or Near-Critical Water Using Generalized Maxwell-Stefan Expression

Authors: 
He, P., Massachusetts Institute of Technology
Raghavan, A., Massachusetts Institute of Technology
Ghoniem, A. F., Massachusetts Institute of Technology

The mixing of a hydrocarbon droplet, containing a mixture of three pseudocomponents of the crude oil constructed based on Lee-Kesler correlations and predictive Peng-Robinson 78 equation of state model, in a reservoir of supercritical or near-critical water (SCW/NCW) is modeled. This study is an extension of our previous works on the mixing of a single-component or two-component hydrocarbon droplet in SCW. As in our previous works, transport, thermodynamics, and phase equilibrium sub-models are used to estimate the relevant physical properties. The current study uses a generalized Maxwell-Stefan (MS) expression to model the multi-component mass transfer process. The diffusion driving forces in Maxwell-Stefan equations, e.g. terms of fugacity gradients, account for effects of the mixture's non-ideality on mass fluxes, and provide microscale information, such as multi-component partitioning, and insight into large scale mixing in applications that include supercritical water oxidation, supercritical water desulfurization, etc. Results show that fractionation (i.e. the preferred dissolution of a specific hydrocarbon component into the water-rich phase), which characterizes the transport and mixing process at various water temperatures, is predicted to be noticeably enhanced using the MS model with complete driving force expressions including non-ideality terms (referred as model 2) compared to that using the MS model with one single driving force term computed from the species' concentration gradients (model 1). We find that when the water temperature is greater than the lowest water-HC binary upper critical solution temperature (UCST) but lower than the overall UCST, model 2 predicts longer mixing time and higher molar fractions of heavy HC species on HC-rich phase than those predicted using model 1 during the entire mixing process. This study shows that, because of high non-ideality, a generalized MS model with complete driving force expressions is important to accurately predict the mixing process of SCW/NCW and HCs.