An Experimental Study Of Diffusion In The Bitumen-Toluene System

Abstract

Abstract Diffusion in the bitumen-toluene system was studied by a free diffusion method using a vertical cell with closely spaced flat windows. An optical method using infrared light was employed to measure the concentration distributions in the vertical direction. A pulsed GaAlAs laser was used as the light source with a silicon semi-conductor diode detector. The concentration distribution curves differed markedly from the normal S-shape associated with constant diffusivity; this was particularly noticeable for bitumen concentrations above 60 vol.%. The over-all diffusion coefficient was calculated as a function of concentration. It was found to vary by an order of magnitude over the concentration range (0 to 100%). It climbed from low values at each end of the range to a maximum 4.8 × 10–6 cm2 s−1 at about 54 vol. % bitumen. The observations can be explained by considering the effect of viscosity and also the difference in the size of bitumen and toluene molecules. Introduction With the diminution of conventional oil reserves, attention has turned to heavy oil production and enhanced oil recovery (EOR). Alberta possesses abundant reserves of heavy oils and tar sands. Different methods have been tried to produce the heavy oil from the ground. In situ recovery processes which involve bitumen dilution have received wide interest in recent years. Some examples are processes using CO(2) stimulation, injection of steam mixed with CO(2) and/or naphtha and the use of solvents for the establishment of a communication path between an injector and a producer. In ail of the processes, the high viscosity of bitumen is decreased and its mobility increased. Although various laboratory, pilot and field exponents have been carried out(1), little research has been done to understand the mechanism occurring during dilution of bitumen with solvents. In the modelling of miscible processes, diffusion is either neglected (an instantaneous equilibrium is assumed) or the diffusion coefficient is assumed to be constant at various concentrations of solvent in bitumen. This may misrepresent the role of diffusion during the recovery of bitumen. Only a few experimental values of the diffusion coefficient of bitumen into various organic substances are available in the open literature (Funk(2), Fu and Philips(3), Baltus(4). All of these values were obtained in a very narrow concentration range or for a single concentration and were assumed to be constant. To understand better the mass transport phenomena, more experimental data are necessary. Therefore, the objective of this work was to investigate the diffusion of bitumen into a solvent; how the concentration and permeability affect the diffusivity. Toluene was chosen as a solvent because of its complete miscibility with bitumen. There are several theories for the estimation of diffusion coefficients, e.g. Eyring's theory, the hydrodynamic theory, etc. However, none of these theories is quite satisfactory in predicting the correct diffusivities because a number of arbitrary assumptions had to be introduced into the theoretical treatment of kinetic phenomena in liquids. Eyring's theory of absolute reaction rates assumes a cubic lattice configuration model for a fluid in liquid state. The diffusion occurs in a series of jumps from "hole to hole" which are scattered in the lattice(5)