Investigation of Mass Transfer Effects on Naphtha Surrogates Partitioning from Bitumen to Aqueous Phases in Athabascan Oil Sands Tailings Ponds
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Bitumen , Oil sands tailing ponds , Naphtha , Greenhouse gas emissions , Fluorescence spectroscopy , Mass transfer , Athabasca , Mature fine tailings , Clark hot water extraction process
Abstract
The Athabascan oil sands deposits are one of the largest in the world. They have been the backbone of the Albertan economy and a major source of energy and petrochemical products in North America for several decades. Approximately half of the region’s bitumen production is generated from open-pit mining and is processed with the Clark’s hot water extraction process (CHWEP). The CHWEP produces and stores significant waste in oil sands tailings ponds (OSTPs). From these OSTPs, there are significant CH4 and CO2 emissions from biogenic activity, such as the biodegradation of diluent naphtha. These greenhouse gases (GHGs) are of substantial environmental concern. A better understanding of the mechanisms affecting their emission rates could be used to mitigate these GHG emissions while balancing other environmental concerns within the OSTPs. Compositional analysis of naphtha is complex, costly and time-consuming. The use of fluorescence spectroscopy with a single wavelength steady-state excitation over a broad emission spectrum range was found to reduce the cost, speed, and complexity of analysis for laboratory experiments to study aqueous soluble naphtha surrogates for bitumen-to-aqueous phase mass transfer experiments. The bio-accessibility of naphtha to microorganisms limits GHG emission rates. The mass transfer of naphtha surrogates from bituminous to aqueous phases was experimentally found to be severely impacted by the saline concentration of the aqueous phase. A combined theoretical and empirical model indicated that experiments with agitation achieved system equilibrium. The initial concentration of naphtha solubilized within bitumen droplets had minimal impact on mass transfer rates and equilibrium concentrations achieved over sessile 2D diffusional experiments. Sessile diffusional experiments achieved lower apparent equilibria than model predictions or results obtained in agitated (forced equilibria) experiments. Aqueous phase replacement experiments resulted in lower lumped mass transfer rate coefficients and apparent equilibria. These observations indicate that limitations of naphtha transfer to an aqueous phase exist due to low diffusivity rates within the bitumen droplets, developing concentration gradients over time, and/or the formation of an interfacial film between the bituminous and aqueous phases.