Vacuum distillation is needed when the temperatures required for atmospheric distillation of the heavier crude oils are so high that thermal coking would be possible. Thermal coking in a distillation column has the negative effects of loss of product, plugging of the furnace pipes with coke, and other equipment damage. In order to prevent this problem, the mixture is distilled under a vacuum since the boiling point temperature decreases with the decrease of the pressure in the process. The absolute pressure in the column is set from 25 to 40 mmHg. In order to increase vaporization, steam is added to lower the pressure to less than 10 mmHg. Steam addition also benefits the process by increasing the tube velocity in the furnace, which minimizes the formation of coke and also decreases the partial pressure of the mixture in the column. In order to minimize the pressure difference between the top and bottom of the column, packing is added to increase the contact between the liquid and vapor so that more fractionation occurs.

The Watson Characterization factor (Kw) can be used to determine whether vacuum distillation is needed and at what temperatures it should be implemented at in order to control thermal coking in the process. Kw is calculated using the physical properties of the crude oil. This factor could determine the temperatures for which above them a significant amount of coking production would occur. The Watson Characterization factor also can determine if the mixture is paraffinic, naphthenic, or aromatic, which also gives insight into what temperatures will need to be used. This allows the operators to know what temperatures they need the run the column at to prevent harmful thermal coking.

References

1.)    Gary, James H., Handwerk, Glenn E., Kaiser, Mark J. Petroleum Refining Technology and Economics. Taylor and Francis Group. Florida. 2007. Print.

The three common distillation methods that are tested than can be implemented at an oil refinery are True Boiling Point Distillation (TBP), ASTM Distillation (ASTM), and Equilibrium Flash Vaporization (EFV). They all have their advantages and disadvantages and specific uses at the oil refinery. True Boiling Point Distillation uses a batch distillation operation that uses more than 100 theoretical plates and a high reflux ratio of 100. These conditions are used mainly for crude oils and not petroleum products, and the distillation conditions allow for the lower boiling component to be distilled off without any contamination of the other components in the mixture. ASTM distillation also uses a batch process but does not include any theoretical plates or reflux in the operation. However there still might be some unavoidable reflux due to condensation of the vapor on the sides of the distillation column. This still allows for the different components in the column to be separated based upon their boiling points. This method is used for refinery products and property calculations and correlations for distillate fractions. The last method used is that of Equilibrium Flash Vaporization in which a flash drum is used to separate the liquid and vapor components of a mixture that is being heated while the mixture is at equilibrium. This used for obtaining data for flashing operations in the refinery. TBP distillation achieves the highest degree of separation of the mixture due to the plates and reflux being used. ASTM distillation does not have as good of separation that TBP distillation does, but it is better than EFV. Therefore depending on what information you need to obtain, you can choose the necessary method.

Vacuum Distillation Units process the fuel’s heavy distillates after settling on the bottom of the Atmospheric Distillation Unit and not vaporizing like the lighter fractions. Atmospheric Distillation is used for separation of light and heavy crudes. The heavy distillates require a vacuum because of how temperature and pressure affect molecules. Creating a vacuum chamber will greatly lower the pressure and uses Ideal Gas Law, one can use the correlation with a pressure drop then the required temperature will drop as well. This is desired because high temperatures will cause vibrations and allow for thermal cracking or destruction of bonds within a given molecule carbon chain.  Cracking the molecules would lead to coking on the metal surface of the distillation column and interfere with fractionation in distillation.

The Watson Characterization Factor gives refineries an idea of the range of temperatures for vacuum distillation to help avoid coking. A graph is comprised of temperature vs. Watson K factor and a band is used for plotting this range on the graph. In most cases, the refineries will operate below the lower end temperature to avoid as much accumulation of carbonaceous material as possible.  Higher K factors are at lower temperatures (paraffins), which makes sense because paraffins are easily cracked. In comparison to aromatics, such as benzene, which are stabilized molecules and require higher temperatures to dissociate the molecules bonds.

There are multiple distillation methods used in the industrial processes of refining crude oil. Three notable processes of distillation are True Boiling Point, ASTM Distillation, and Equilibrium Flash Vaporization. The best process in petroleum refining for distillation is determined by three factors, the number of plates, the reflux ratio, and the steam-to-feed ratio. These parameters are what dictates why the order of the distillation methods previously mentioned are in descending order of best separation methods.

True Boiling point Distillation (TBP) uses a distillation operation that utilizes more than 100 theoretical plates and a high reflux ratio. This is used in petroleum refining not only as the idealized method for best possible separation in distillation but also is used to characterize crude oils and create a significant component of crude essay.

ASTM Distillation is not as good of a separation method as TBP but is usually used for refinery products and property calculations and correlations for distillate fractions. This method does not operate in the presence of contacts plates and has a reflux ratio of 0. This method works in contrast with the TBP Distillation method.

Equilibrium Flash Vaporization has a heater that heats a flowing feed.  The heated feed then is deposited into a flash drum and involves the separation of the liquid and vapor. Equilibrium Flash Vaporization is useful for obtaining data for flashing operations within a refinery.