(415a) Analysis of Mixing Regimes for Refinery Hazardous Oily Sludge Biodegradation

Historic waste ponds containing oily sludge are commonly found at older oil refineries, and many of these ponds are not remediated or are still in minimal use at currently operating facilities. A common method for treating this sludge is by solidification / stabilization, if the waste unit is no longer in use. However, for currently existing waste ponds at operating refineries in which the units not under closure requirements or orders, biodegradation can be a viable and less-costly treatment option. In a quiescent wastewater pond environment, the oily sludge is present typically as a single phase. To promote aerobic biodegradation of the material, extensive mixing and aeration of the liquid will be necessary.

The research reported here investigated the level of mixing energy imparted to an oil phase present in a larger aqueous phase, in order to create oil droplets that would be amenable to biodegradation. The effectiveness of the mixing was measured using the dispersion effectiveness technique, which is an extraction approach using dichloromethane. The tests were conducted at both a laboratory scale (<0.5 liters) and a pilot scale (tens of liters), and with different types of mixing impellers. The experimental results indicated that higher mixing speeds led to a greater extent of oil dispersion, as is expected. However, there was a large drop in dispersion effectiveness with mixing speeds below approximately 150 rpm. Additionally, the energy dissipation was calculated with different mixing regimes (mixer speed and impeller type) and was compared with the dispersant effectiveness. This analysis indicated that similar levels of dispersant effectiveness were achievable using a helical impeller at low mixing speeds as compared with a marine impeller at higher mixing speeds. The success with the lower mixing speed of the helical impeller suggest that larger pond environments may be effectively mixed using less power with this type of mixing device.