Continuous Treatment of Oil-Sands Produced Water By Electrocoagulation

Abstract

The treatment performance of electrocoagulation (EC) was investigated using continuous flow reactor for simultaneous removal of silica and hardness causing ions calcium and magnesium from produced water. The amount of charge passed per unit volume of produced water treated (i.e., the charge loading, C L- 1) was adjusted either by varying the electrode area or the current density, under a range of inlet flowrates (20―120 mL min-1) using Fe and Al electrodes. It was found that direct current (DC) EC operation with Fe electrodes was able to remove about 87% of the total silica, whereas with Al electrodes more than 95% silica removal was achieved. These removals were achieved with an influent silica concentration of 60 mg L- 1, a charge loading 2000 C L- 1, current density 8 mA cm- 2, pH of 7.75 and feed flow rate 60 mL min-1 (corresponding to a Reynolds number of 116). In addition, the calcium removal was observed to be around 85% with both Fe and Al electrodes. Al electrodes were found to be more effective for removal of magnesium achieving about 70% removal in comparison to 39% removal achieved with Fe electrodes at the same operating conditions.The average faradaic efficiency of DC-EC with Fe and Al electrodes was observed to be 99.9% and 149% respectively over the range of charge loading between (600―2000 C L- 1) and flow rate. The super faradaic efficiency of Al was due to the chemical dissolution of Al cathode which provides additional Al coagulant [1]. The specific energy consumption in DC-EC operation was found to be 1.79 and 1.76 kWh m- 3 for Fe and Al respectively at 2000 C L- 1, at a current density of 8 mA cm- 2 and a fixed Reynolds number of 116. During DC-EC operation, the cell voltage was found to increase in time, however this increase was reduced with the application of polarity reversal (PR-EC) [2]. It was found that a longer appropriate polarity reversal duration of 10 min provided sufficient time to remove the fouling / passivation layers from the electrode surface [1], [3]. Ca and Mg precipitates form a passivating layer in the alkaline pH on the cathode surface. This layer can be dissolved from the electrode surface by the acidic pH generated at the interface of the anode after polarity reversal [1]. This results in reduced cell voltage and hence energy consumption in comparison to DC-EC. PR-EC with 10 min reversal time reduced the energy consumption to 1.53 kWh m- 3 and 1.57 kWh m- 3 for Fe and Al electrodes respectively at the same operating conditions as for DC-EC discussed above. Under these conditions (charge loading = 2000 C L- 1, current density = 8 mA cm- 2), the silica removal was found to be only 65% with Fe electrodes, compared to above 95% with Al electrodes. Finally, the use of hybrid electrodes (Fe/Al) for DC-EC in which Fe was anode and Al as cathode produced a slightly higher cell voltage than non-hybrid Fe and Al, probably due to the more negative redox potential of the Al cathode. However, for PR-EC with a 10 min reversal time using a combination of electrodes (Fe/Al) had a similar cell voltage than for Fe or Al-EC. This hybrid electrode system had an energy consumption of about 1.53 kWh m- 3 while achieving 96% Si, 84% Ca and 73% Mg removal. The results indicate that EC can effectively remove silica and hardness causing ions from synthetic produced water while reducing the cell voltage with the application of PR. As shown in Fig. 1, a lower operating cost was estimated for Fe-EC due to the low cost of the metal consumption, since the cost of Al is around 2.8 times higher than the cost of Fe. The cost of metal consumption for the hybrid combination of Fe/Al electrodes was significantly lower than for Al-EC, while sustaining more than 95% Si removal.