Distillation of crude oil is separation of the components of crude oil at different boiling points. This is done in two stages. They include: vaccum and atmospheric stages.
The process of distillation is a separation process not chemical process. It is one of the unit operation in chemical engineering processes. This separation is carried out on a distillation or fractionating column. In a chemical plant, there are series of columns because separated components of crude are processed more for further improvement. This process may be reforming or cracking processes.
In crude oil distillation, as temperature rise the lighter components vaporize. As the vapor rises in the column, products are collected along the column and stored in the storage tank while some are condensed back to column as liquid.
The use of fractional distillation can be more effective. fractional distillation is combined with rectification process. Rectification means counter flow contact with vapor formed and liquid condensed as products are eliminated. Notice that on a tray, the vapor leaving is in Equilibrium with the liquid leaving the plate. Also, the liquid returning to the column has equal Temperature ( know about WBT) and Composition of the distillate. All these are applicable in distillation of crude oil.
In the design of columns, considerations are made in the theoretical number of plates for a particular tower; and the plate efficiencies as operation progresses. The theoretical number of plate can be determined by the relationship between the vapor and liquid composition of the feed.
Operating condition in distillation
Column pressure: pressure is not considered as an operating variable. Higher pressure will require higher temperature. For vaporization, pressure of a saturated vapor of a liquid is the pressure developed by the vapor at a given temperature in equilibrium with the liquid.
Flow rate: this is the speed or velocity at which a unit volume of feed is passed into the column. This depends on the operating demand of production on the firm.
Reflux: this is the ratio of the distillate returned to the distillate withdrawn from the column. It serves as liquid in the rectifying section to cause vapor liquid contact. Also, it is to maintain the temperature gradient of the column. Reflux takes place on the tray. No reflux no rectification. When a reflux ratio is high, there is higher separation between lighter and heavy components.
Temperature: this is the major factor to be considered in distillation. At the top of the column, the temperature should be sufficient to allow complete vaporization of the overhead product. This is because; compounds are separated base on their individual boiling points. The temperature of the top section is known as the dew point. The flash point of first side stream product; example; heavy Naphtha can be improved by increment in temperature. The temperature at the bottom is called the bubble point temperature. This is the control quality of downstream. In the column, temperature is fixed and this depends on the vaporization required at pressure of the heated feed.
Stability of column operation: which include; pulsation, dumping, coning, blowing entrapment and flooding. read more