Vacuum distillation is necessary in the distillation of crude oils because of the complexity of compounds that make up crude oil. Being that there are thousands of different compounds in crude that will all boil at their own boiling point, there is a very broad range of boiling temperatures. If particular compounds are heated up beyond a certain temperature they will begin to crack or break apart creating smaller hydrocarbon chains also known as coke. This coke will begin to build up on the surfaces of distillation units and closely associated units such as strippers and pump arounds. Coking can cause a myriad of different problems including heat exchangers not working correctly or plugging necessary valves, nozzles, or even pipes. In order to avoid reaching this critical coking temperature the fractions of crude with the highest boiling points are removed from the atmospheric distillation column and put into a vacuum distillation column where the pressure has been reduced which allows for the liquids to boil at lower temperatures.

As explained previously in the course, a Watson Characterization factor (K_w) can be calculated for crude oils to show their general chemical makeup when it comes to hydrocarbon type. Of the three main types of hydrocarbons, paraffinic hydrocarbons have the highest K_w values followed by naphthenic hydrocarbons and aromatic hydrocarbons. Due to the fact that paraffins are the most likely and first to initial coking a correlation between the K_w factor and vacuum distillation temperature can be seen: the higher the K_w factor, the lower the temperature must be in the vacuum distillation unit must be and vice versa. This relationship is a result of chemical bonding strengths of which aromatics have the strongest bonds and therefore require the most heat to crack. It is also known that based on the chemical makeup, below a given temperature coking will not occur so the best bet is to run below this temperature as much as possible and never run above the critical coking temperature.