Distillation in the petroleum refinery is different from most distillation operations in two principal ways:
* Thousands of molecular species are present. The separations are roughly by molecular weight, thus yielding a differentiation of physical properties.
* In many cases, the distillation feed contains species of high molecular weight, which cannot be readily boiled in shell-and-tube heat exchangers such as those used in most distillation processes.
The first problem makes it difficult to predict distillation separations because the large number of species would overload computational systems. The problem is solved by first analyzing feed stocks by precision batch analytical distillation or equivalent gas chromatography. The stocks are then divided (on paper) into groups of narrow boiling-point range. As volatility is represented fairly accurately by atmospheric boiling point, each boiling-point range, e.g. 200-225°F, can be represented as a single pseudocomponent at the fraction's mean boiling point. Of course, in the lower boiling-point ranges, individual chemical species can be analyzed and entered as such into the distillation model.
Then the refinery distillation operation can be studied as a familiar multicomponent distilling calculational model. The second problem, providing reboil heat for heavy (high molecular weight) stocks, is handled by using fired heaters with short thermal exposure time to preheat the feedstocks. The distilling column is then set up primarily as a rectifying column with side-stripped draw streams to separate the products. Usually superheated steam is used to strip volatiles from each side-draw stream and the bottom product.
Less often the strippers may be reboiled. In some operations, the feed preheat may be supplied by a catalytic or thermal reactor. Product separation of the heaviest fractions can be carried further by reheating the bottoms from the first distillation and rectifying the distillate under vacuum. In the interest of concise presentation, this article will be primarily devoted to refinery distillation separations handled by the methods described above, with a brief discussion only of other separations that use mostly well-known general distillation processing methods.
Keywords: Distillation; Refining; Vacuum columns; Petroleum; Gasoline; Fuel oil; Cracking; Crude oil; Lubricating oil
Keywords: Distillation; Refining; Vacuum columns; Petroleum; Gasoline; Fuel oil; Cracking; Crude oil; Lubricating oil
