Progressive freeze concentration of naphthenic acids
Reynolds, Dane Sterling
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Canada’s oil reserves are among the largest in the world with over 1.7 trillion barrels of bitumen in place with confirmed measurements indicating there are 173 billion barrels of recoverable oil under current economic conditions and technology. These oil sands are unlike traditional oil reserves, they are typically composed of sand or clay, water, and bitumen. As a result, the oil sands must be heavily processed to separate the bitumen from the sand and clay before further refining may occur. This process requires tremendous amounts of water; as much as 4.2 m3 of water per m3 of bitumen, an astounding 7% of Alberta’s total water use. As a result large volumes of oil sands process affected waters (OSPW) are formed containing high concentrations of suspended and dissolved organic and inorganic contaminates. These waters have been reported to be toxic to a wide range of aquatic and terrestrial organisms. Evidence suggests that Naphthenic acids (NAs) are the primary cause of toxicity. Current investigated treatment options have shown mixed results for treatment. This investigation examines the potential for progressive freeze concentration as an alternative method for the treatment of NAs in OSPW. Two methods of freezing were investigated, power ultrasonic and mechanical progressive freeze concentration. Laboratory experiments were carried out to examine the effect of freezing temperature, the initial feedwater NAs concentration, chemical nature of NAs, mixing intensity and the freezing methods on treatment efficiency of freeze concentration. Behaviour of NAs during freeze concentration was also examined by calculating the partition coefficients to predict the incorporation/ rejection of NAs during formation of ice crystals based on the experimental conditions. Synthetic wastewater samples with various NAs (cyclohexanecarboxylic acid, trans-4-pentylcyclohexanecarboxylic acid, cyclohexanepentanoic acid and a synthetic NAs mixture) were used as a feedwater for the freeze concentration processes. Experimental results showed that both ultrasonic and mechanical freeze concentration methods were equally effective for the removal of NAs at concentrations of 20mg/L to 120mg/L. An average 98% reduction of chemical oxygen demand (COD) concentration, 95% total solids (TS) and 99% conductivity were observed in the ice samples. A lower freezing temperature (-25⁰C) reported higher contamination in the solid phase as compared to a warmer freezing temperature (-15⁰C). Overall, the experimental results suggest that progressive freeze concentration has great potential as an effective treatment method for OSPW.