Experimental investigation of concentration dependent non-ideal diffusion in hydrocarbon systems

Evulukwu, Ebubechi Azubuike (2015) Experimental investigation of concentration dependent non-ideal diffusion in hydrocarbon systems. Masters thesis, Memorial University of Newfoundland.

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Abstract

Solvent extraction technology (Vapor extraction/VAPEX) has drawn a lot of industry attention due to its potential to be an alternative to Steam assisted Gravity Drainage (SAGD) in heavy oil production. However the mass transfer mechanisms involved is yet to be fully comprehended. Reliable oil production rate data is scarce, hence the reluctance from oil companies to implement the technology on a field/commercial scale. More work is required at the experimental level to fully understand the intricacies of the technology and hence facilitate its commercialization. Experiments were conducted to evaluate the one-dimensional diffusivity of butane solvent in Athabasca bitumen at varying temperatures. Given diffusion is driven by concentration gradient, the diffusivity cannot be assumed constant throughout the whole diffusion process. Hence the diffusivity was found as a function of butane solvent concentration (mass fraction). Diffusivity functions for ideal mixing and non-ideal mixing were computed. Butane vapor temperature (24.00⁰C and 34.65 psi) is kept constant while the bitumen temperature is varied at 5 levels (27.00⁰C, 30.25⁰C, 33.50⁰C, 36.75⁰C and 40.00⁰C). Assuming ideal mixing between hydrocarbons in VAPEX experiments is prevalent in the field. This is because finding a parameter in the solvent-bitumen mixing system that accounts for non-ideal mixing without upsetting the system is difficult. This work accounts for non-ideal mixing by constantly measuring the bitumen liquid hydrostatic pressure via pressure differential transmitters as diffusion occurs. With bitumen height change and amount of diffused butane solvent being monitored, the real density reduction (non-ideal density reduction) can be computed. Results showed that assuming ideal mixing over-estimates the density reduction. The deviation between ideal and non-ideal mixing density values increase as temperature increases. This is supported by most literature in the field. As temperature tends to standard temperature (25.00⁰C), the effects of non-ideal mixing become insignificant. A MATLAB model is used to predict the ‘bitumen growth’ (bitumen swelling), this is compared to ‘bitumen growth’ observed experimentally. The difference between the two (experimental – predicted) is minimized by optimizing the diffusivity function coefficients. Results showed that the diffusion values (obtained via diffusivity functions) decreases as temperature increases. There was no ‘live oil’ drainage in this experiment so diffusion is governed by the butane solvent solubility in the bitumen. This butane solvent solubility decreases with increasing temperature. At equal mass fractions (ωs) all non-ideal mixing diffusivity functions yielded higher diffusion values than ideal mixing diffusivity functions. This is logical because diffusion is driven by concentration gradient. Ideal mixing scenarios over-estimate density reduction on mixing and hence provide a smaller concentration gradient compared to non-ideal mixing. The assumption of ideal mixing conditions clearly underestimates the real diffusivity values. The deviation between ideal and non-ideal diffusivity functions also increased as temperature increased. This follows the same trend as the deviation between ideal and non-ideal mixing density results. A macroscopic mass balance was used to independently validate the diffusivity functions. This mass balance predicted the change in solvent height after ‘bitumen growth’ had been resolved for the full experimental time. This is an independent validation because change in solvent height data was not used to obtain the diffusivity functions. All but one of the diffusivity functions (40.00⁰C) was independently validated. Lack of validation in the 40.00⁰C run was due to technical issues while running the experiment. For all validation data, the non-ideal diffusivity functions provided a better fit for the experimental data than the ideal diffusivity functions. Finally, the experimentally determined butane slope decrease, bitumen slope increase and non-ideal mixing coefficients for all varied temperature conditions were used as input values to make models in Design Expert (DE). These models were used to predict the aforementioned parameters at a random bitumen temperature (28.50⁰C). An extra experiment was run at this temperature (28.50⁰C) to compare the diffusivity functions coming from the experimental data to those from (DE) predictions. The DE originated diffusivities functions showed a good fit with the experimentally originated diffusivity functions. The model is therefore a robust model and can be used to predict diffusivity of butane at 24.00⁰C, within a given bitumen temperature range of 27.00⁰C - 40.00⁰C, while also accounting for non-ideal mixing and concentration dependency.

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