ABSTRACT
Foaming occurs when air mixes with oils. Air entrainment is a dispersion of small air bubbles in oil. The first comprehensive account of interfacial phenomena in liquid hydrocarbons covered various topics, including foaminess, foam inhibition, and air entrainment (Ross, 1987). The constant velocity with which a spherical particle falls in a liquid is expressed by a relatively simple law which is known as Stoke’s equation. The literature reported that pure liquids allow entrained air to escape with no delay other than the rate of rise described by Stoke’s equation. The rate of air rise is controlled by the diameter of the bubble and the viscosity of the bulk liquid (Ross, 1987). Stoke’s theory was extended to gas bubbles and it is one of the methods used to measure the viscosity of liquids. According to the literature, pure liquids allow entrained air to escape with no delay. Certain solutes are able to form a thin layer, known as lamella, at the liquid=air interface and adversely affect the air separation properties of liquids. If these solutes are present, the escape of the entrained air bubbles is delayed and stable foam is produced (Ross, 1987). The review of early literature on foaming of petroleum oils reported that the presence of certain acids and phenols increased foaming of crude oils. It was also reported that an increase in foam stability of medicinal grade paraffin oils and engine oils was found to increase with an increase in their kinematic viscosity (Zaki et al., 2002). The effect of conventional refining on inorganic and organic polar content of mineral base stocks is shown in Table 8.1.
