Experimental Investigation of Constant and Concentration-dependent Diffusivity of a Hydrocarbon Solvents-Heavy Oil System: A Comparative Study

Abstract

This work is concerned with the experimental investigation of mass transfer, which occurs during diffusion of hydrocarbon solvents in heavy oil based on constant and concentration-dependent diffusion coefficient. Here, a series of free fall diffusion experiments have been conducted in sealed test tubes at fixed temperatures. Pentane, hexane, and octane were used as solvents and were placed on heavy oil in tubes. The diffusivity of heavy oil into solvents was monitored by a digital camera. The image analysis technique was applied to extract concentration profiles in diffusion zones of solvents by means of calibration curves. Fick's second law was used to obtain both constant and concentration-dependent diffusion coefficient. The measured diffusion coefficients are in agreement with the data reported in the literature for different heavy oils. The results show that the diffusion coefficient experiences an increasing trend with oil concentration till it reaches its maximum and then tails off to a constant value. The observed maximum happens around 0.12 for pentane and around 0.05 for both hexane and octane. Moreover, all transient concentration profiles as a function of lie on a unique line. For all solvents, it has been found that the average values of measured concentration-dependent diffusion coefficients were smaller than that of constant diffusion coefficients. The results of this work illustrate that constant diffusion coefficient assumption overestimated the total mass transfer; therefore, it may lead to unrealistic predictions of solvent penetration in solvent-based enhanced oil recovery processes.

Keywords:

• diffusion coefficient
• Fick's law
• heavy oil
• hydrocarbon solvent
• mass transfer