Fractionation of multi-component hydrocarbon droplets in water at supercritical or near-critical conditions

Abstract

The mixing of a hydrocarbon droplet, containing a mixture of toluene and either n-decane or tetralin, in a reservoir of supercritical or near-critical water is modeled. This study is an extension of our previous work on the mixing of a single-component hydrocarbon droplet in SCW. The current study provides microscale information, such as multi-component partitioning, and insight into large scale mixing in applications that include supercritical water oxidation, supercritical water desulfurization, etc. Transport, thermodynamics, and phase equilibrium sub-models are used to estimate the relevant physical properties. Results show that fractionation, i.e., the preferred dissolution of a specific hydrocarbon component into the water-rich phase, characterizes the transport and mixing process at various water temperatures. We find that the upper critical solution temperature (UCST) for binary water–HC mixtures control the mixing or fractionation of ternary water–HC–HC mixtures. Fractionation is: modest at water temperatures less than the water–toluene binary UCST (which is the lower UCST); substantial at water temperatures greater than the water–toluene binary UCST but less than the water–decane binary UCST (which is the higher UCST); and negligible at water temperatures greater than the ternary UCST. At water temperatures when fractionation is noticeable, the less water-soluble hydrocarbon (e.g., decane) composition in the droplet may increase over time and the water fraction in the droplet may go through a maximum, as more of the more water-soluble hydrocarbon (e.g., toluene) diffuses into the water-rich phase than the less water-soluble hydrocarbon. Varying the initial toluene concentration can either increase or decrease the droplet radius depending on the water temperature relative to the UCSTs.