(639e) Microfluidics Studies of Hydrocarbon-Water Systems At High Pressure and Temperature

Supercritical water upgrading of heavy crude oils is a combined physical/chemical process involving chemical reaction, phase behavior, and mixing. Fundamental understanding of each of these major phenomena is lacking. The purpose of this study is to obtain experimental data on phase behavior and mixing for heavy oil model compounds and water near its critical point.  Lab-on-chip microfluidic devices are ideal for mixing and phase behavior studies because of their excellent optical and spectroscopic access. In this study, the dissolution rate measurements of hydrocarbons in the water at conditions near the mixture critical point were studied through a droplet-based approach and the findings were compared with the microscopic mixing models described in Guang Wu’s 2012 papers [J. Supercrit. Fluids, 67, p:29 and 72, p:150].

Toluene and n-dodecane were chosen as a model compounds. The miscibility of a single component hydrocarbon droplet (toluene or n-dodecane) or a mixture of toluene and n-dodecane at varying ratios (1:3, 1:1 and 3:1) in the water was investigated over the temperature range 50 - 400^oC at p = 100 bar in a Silicon-Pyrex microreactor. The phase behavior of the hydrocarbon droplet during “stop-flow” conditions as a function of temperature was observed with an Olympus microscope equipped with a digital camera. The composition of each phase was measured using a confocal Raman spectrometer (Thermo Scientific DXR Raman) with a 532 nm wavelength excitation laser operating at 10 mW output power. Measurable water solubility in the hydrocarbon starts above 150^oC at which temperature the hydrocarbon droplet swells due to thermal expansion and water absorption. When the water concentration in the droplet reaches the solubility limit, a third phase consisting of hydrocarbon and water mixture was observed. On the other hand, the Raman analysis of the water phase showed negligible hydrocarbon solubility in water.  

Studies were performed with “small” (define) and “large” hydrocarbon droplets. Small toluene droplets swell with increasing temperature up to 200^oC, shrinks above 200^oC, and dissolves completely at 310^oC. These temperatures are shifted higher for large toluene droplets, presumably a consequence of lower mass transfer rates. For larger toluene droplets, a third phase was observed at 300^oC; the droplet shrinks and disappears at 340^oC. The behavior of dodecane droplet in water is similar to toluene, but the solubility of dodecane in water is lower. Namely, small dodecane droplets swell with increasing temperature up to 350^oC, then shrink and disappear at 390^oC. Large dodecane droplets remain stable up to 400°C, the limit of the current method. The addition of dodecane into toluene increases the miscibility temperature from 320^oC to 400^oC, depending on ratio of toluene to dodecane.