The Influence of Freezing-Thawing On the Stability of Crude Oil-in-Water Emulsions

Abstract

Abstract The effect of freezing-thawing on the stability of crude oil-in-water emulsions was studied. Four different types of crude at three freezing temperatures were investigated. The experimental methods were the same as those used in a previous investigation. It has been found that freezing-thawing causes an increase in the droplet size and lowers the oil-drop concentration in the emulsion. INTRODUCTION THE STABILITY of crude oil-in-water emulsions was the subject of a previous investigation(J). It has been found that crude oil and water form fairly stable oil-in- water emulsions if sufficient mixing energy is available. This means that when crude oil spills on water, a persistent oil-in-water emulsion may form due to the actions of currents, tides and waves. If the spillage occurs on an arctic water, where the temperature is above freezing point in daytime and below freezing during the night, the emulsion will then be subjected to freezing and thawing cycles. What is the fate of this emulsion? Will it behave differently from the emulsions not subjected to freezing and thawing? The present work attempts to obtain some answers to these questions. Experimental The experimental methods employed were the same as those used in the previous investigation(l), except that a Model T Coulter counter was used instead of the Model Z. The ''Model T is capable of counting simultaneously the number of droplets at 15 selected size ranges and thus completing an analysis within a few seconds. A 50-pm aperture tube was used and droplets down to 1 pm in size were counted. Four different crudes, two from the Northwest Territories and two from Alberta, were investigated. The physical properties of the crudes are given in Table 1. The properties of Crude No, 3 are said to be close to those of the Prudhoe Bay crude. Identical crude oil-in-water emulsions were prepared with an initial volume ratio (oil to water) of 0.05 and stored in wide-mouth, 16-ounce glass bottles. One sample was kept at room temperature throughout the period of the experiments, Other samples were allowed to stand at room temperature for one day and then were subjected to one or two freezing-thawing cycles. Freezing was carried out by placing the emulsions in a Revco Freezer for one day at −5, −15 find −40°C, respectively. Temperatures of the emulsion during freezing was recorded continuously by means of a probe which was placed centrally in the bottle. (Figure in full paper) The freezing took place as follows; the emulsion was first supercooled to a temperature T5 (T5 was about – 12, −8, −11 and −4ºC for Crude No. I, 2, 3 and 4, respectively) and began to solidify (at this point, the temperature rose suddenly to 0º C). The frozen emulsion remained at its freezing point, 0ºC, for several hours before it cooled down gradually to the temperature of the surroundings. It should be noted that at −5ºC, only the Crude No. 4 emulsion was frozen. Thawing was allowed to proceed at room temperature.