The Growth of Oil Slicks and Their Control By Surface Chemical Agents
Authors: Cochran Robert A. , Scott Paul R. 
Published: December 2007

A calculation method for predicting the rate at which an oil slick will spread on water can also be used to predict the behavior of a slick that has been treated with a surface chemical collecting agent. Field and laboratory tests show that only a small quantity of the agent is needed to contain a slick for a considerable time.

Introduction

Highly important in designing an oil pollution control system is a knowledge of the growth rate of the oil slick. This information is particularly necessary if there is an impending spill so that a cleanup strategy can be planned before the spill occurs. This knowledge can also be applied to predict theoretically the behavior of an oil slick that has been treated with a surface chemical agent. A surface chemical agent is a compound that, when applied to the water surface, forms a thin layer that changes the surface tension characteristics of the water-air interface and thereby affects the spreading rate of oil on the water surface When a quantity of oil is added to a smooth water surface that has been treated by one of these compounds, the oil may form a stable circular lens. For such a stable lens to form, it is necessary that the spreading pressure, defined as w, - o, - ow, be negative. (Here w, o, and ow, represent the water-air, oil-air, and oil-water interfacial tensions, respectively.)

Garrett and Barger have proposed the application of suitable surface chemical agents to prevent the uncontrolled spread of oil slicks on the open sea. If such slicks are left untreated, they will spread quickly, and on a calm sea several barrels of oil will spread as much as a mile a day. These large, thin oil slicks are very difficult to recover mechanically, but a suitable surface chemical agent applied to the perimeter will drastically affect their growth. If a slick is thicker than the stable lens, its growth decreases and eventually stops when the thickness equals that of the stable lens. If at first the slick is thinner than the stable lens, treating it with the agent mill cause it to shrink until its thickness equals that of the stable lens. For a typical crude oil the stable lens is about 1/2 cm thick, and for a 1,000-bbl spill this would correspond, assuming the slick is circular, to a stable diameter of only about 200 m. Because it is thicker, this slick can be removed by mechanical skimmers more easily than one that is left untreated.

The chemicals used to control oil spills must be nontoxic and water insoluble and must have a high spreading pressure. Some chemicals that meet these requirements are the high-molecular-weight water-insoluble alcohols and acids such as steric acid and oleic acid. Additional substances are cited in Ref. 1.

To understand how these surface chemical agents affect the spreading of oil on water it is necessary to identify and to understand the forces governing the growth of a slick. In the following sections we shall deal with the important forces, develop a related mathematical model, compare the model predictions with field test results, and predict the effects of surface chemical agents on the growth behavior of an oil slick.

Forces Important in the Spreading of Oil

Fig. 1 shows a cross-section of the oil-water-air interface of a growing oil slick. It shows that the forces acting on the slick are surface tension, pressure, and viscosity. pressure, and viscosity.

JPT

P. 781