ABSTRACT
Crude petroleum oil is a mixture of hydrocarbons. The hydrocarbon gases, methane, ethane, propane, and butane, are present in crude oil in its dissolved state. Methane has a high vapour pressure and it escapes from crude oil unless pressure above the vapour pressure is maintained. Usually it separates out from crude oil itself from the well and is collected separately as natural gas and a trace of it may be found in crude oil under atmospheric pressure. Though ethane has a higher vapour pressure than propane and butane, traces of it are usually found in crude oil right from the well. Propane and butane are present in the liquid state at slightly above atmospheric pressure owing to their low vapour pressure and high solubility in crude oil. The propane-butane mixture is separated from crude oil and is used as liquifi ed petroleum gas (LPG). The remaining liquid hydrocarbons can be separated as boiling fractions such as naphtha cut (boiling up to 140°C), kerosene cut (boiling between 140°C and 270°C), gas oil or diesel cut (boiling between 270°C and 350°C), by heating and vaporising the crude oil by a gradual increase in temperature followed by collection after condensation. When crude oil is heated in a distilling fl ask, vapours start emanating as the temperature rises and these vapours are collected after condensation using ice cold water. The temperature of vapour giving the fi rst drop of condensate is reported as the initial boiling point (IBP), which may be above or below 0°C depending on the presence of the lowest boiling hydrocarbon in crude. This vaporizing phenomenon is so fast at the beginning that temperature measurement is quite uncertain as the vapour of the fi rst drop is immediately followed by the mixture of vapours with increasing boiling points, hence this must be noted in the shortest possible period (within 5-10 min) after the charge is heated. As heating is continued, more and more hydrocarbon vapours with increasing boiling temperature are separated from crude and collected as condensates. This process is continued with gradual heating until no further vaporisation takes place. The vapour temperature and the volume of liquid condensates (boiling fractions) collected are measured and reported as the distillation analysis of crude oil. In the laboratory, such a batch distillation is carried out for a specifi ed amount of crude oil (500 cc, 1 litre or more). During distillation at atmospheric pressure, the rate of vaporisation decreases gradually after 40-45% of crude is distilled and, fi nally, negligible or no vaporisation takes place. If attempts are made to increase the temperature by further heating, the residual crude in the fl ask may undergo thermal cracking (break down of hydrocarbon compounds present originally in the charge), which is a reaction phenomena other than distillation. Alternatively, when heating of the residual oil is continued under vacuum, vaporisation is restored
and condensates are collected. Distillation, in fact, is done at atmospheric pressure and then followed under vacuum. The separation of hydrocarbons can be improved if the vapour and its condensates (refl ux) are in intimate contact for some time during distillation and a reproducible distillation analysis is possible. Such a method of distillation is known as true boiling point (TBP) distillation. A refl ux ratio of 5:1 is usually sought as the standard separation. Separation of fraction by 1% volume can be obtained at a very close temperature difference and such a close separation is comparable with a distillation tower of 10-12 plates. A routine crude distillation test is carried out in a standard TBP distillation apparatus in a packed column of specifi c size and packing material with a certain amount of refl ux ratio (refl ux/vapour ratio). A TBP apparatus is shown in Figure 2.1 and a typical TBP analysis curve is shown in Figure 2.2.
