In petroleum refining, there are many processes that are import to achieving the overall fuels necessary for a refinery to sell. One of the main processes crude must undergo is distillation, where the crude is heated to a desired temperature in order to separate the crude into desired feedstock to undergo different treatments to get out different products. There are two types of distillation, the first is atmospheric distillation which is done at atmospheric pressure. The second is vacuum distillation.

The reason that vacuum distillation is so vital to the process of refining is due to the fact that if the temperature used in atmospheric distillation were to be raised any higher, it would cause thermal cracking which is not ideal. The reason thermal cracking is not desired is because it would compromise the ending product or fuel in many cases, and lead to coking. In order to avoid thermal cracking, and changing the chemical composition of the feedstock, vacuum distillation is used in succession to atmospheric distillation. The atmospheric residual oil is further distilled in the vacuum distillation column. This distillation must be performed at very low pressures, as low as 10-40 mmHg, in order to further distil the oil without the presence of thermal cracking and eventual coking. Vacuum distillation towers are very large, as tall as 164 feet with a 46 foot diameter. The reason they are so large is due to the expansion of the oil under such low pressures.

In order to complete vacuum distillation, you must know what temperature to perform the distillation at. This is done with the use of the Watson Characterization Factor. The factor is the ratio of mean average boiling point and specific gravity of the oil. This factor is used to further determine the chemical makeup of an oil. Higher characterization factors such as 12.5 or greater indicate a compound is mostly paraffinic, while a lower factor tells us it is naphthenic or aromatic. Once this has been determined, you can better select the temperature at which to Vacuum distil.

 References:

http://en.wikipedia.org/wiki/Vacuum_distillation

http://petrowiki.org/Crude_oil_characterization

In this lesson we looked at three different types of distillation methods. They included true boiling point distillation, ASTM distillation, and Equilibrium Flash Vaporization. True boiling point distillation is done in a batch method. In this method more than 100 theoretical plates are used as well as a very high reflux ratio of 100. This method achieves the best possible separation in distillation. In this distillation the component with the lower boiling point is distilled off completely without any contamination from the other substance, Then the compound is distilled off as a pure compound. The second distillation method we reviewed was ASTM distillation. This is also a batch method but operates without any contact plates as well as a reflux ratio of zero. ASTM achieves better separation then EFV and TBP achieves better separation then ASTM. EFV uses a steady flow that is heated and then flows into a flash drum. Within this flash drum the flowing heated feed is separated into a liquid and a vapor. This method gives the lowest degree of separation of the three distillation methods. Each method is used for a specific reason. TBP is used to characterize crudes oils, ASTM is usually used for refinery products and property calculations of the components, and EFV provides data for flashing operations in refineries.

When processing crude oil vacuum distillation is needed in conjugation with Atmospheric distillation as well. If vacuum distillation was not used the outlet temperatures of the furnace would be so high that thermal cracking would occur. This thermal cracking would cause a loss of some of the product as well as equipment fouling. To lower the outlet temperature you have to lower the boiling point of the products. This is done be creating a vacuum. The pressure within the distillation column is lowered to about 10 mmHG. This is done by using a combination of vacuum pumps as well as an addition of steam to the furnace inlet and at the bottom of the vacuum tower. Under this low pressure the products will boil at a much lower temperature, which will prevent thermal cracking and equipment fouling. The Watson characterization factor can be use to estimate the upper temperature limit of crude oil to avoid coking in vacuum distillation. There is an empirical correlation between temperature and Watson characterization factor. From this correlation a line of where coking will occur can be drawn. But do to the variability of the composition of crude oil coking may occur below this limit. So a lower limit has to be drawn. The area between these lines is known as the decomposition zone. Where coking may occur depending on the composition of the crude oil. The temperature must be kept below the lower line to avoid coking.

The three most commonly used distillation methods are true boiling point distillation, American Society of Testing and Materials (ATSM) Distillation, and Equilibrium Flash Vaporization. Each has its own benefits and drawbacks, but for the most part, the degree of separation between distillation fractions greatly decreases with the order of methods I just mentioned. True Boiling Point distillation consists of at least 100 theoretical plates and boasts a high reflux ratio of 100, making this method great for characterizing crude oils and constituting a significant component of crude. When processing binary mixed crudes, the temperature remains constant until the higher percentage compound is evaporated. Contrary to TBP distillation, ATSM distillation contains zero theoretical plates and has a reflux rate of zero. These characteristics of this method prove that it is perfect for refinery products and property calculations & correlations in distillate fractions. Finally, the least effective and most different from the other two methods, Equilibrium Flash Vaporization heats a flowing feed and separates the liquid and vapor in a flash drum, which is installed between the feed heat exchangers and the atmospheric furnace. Since EFV has the produces the lowest degree of separation, it is mostly used for data collection in finishing operations.

Increasing the furnace outlet temperature further in the atmospheric distillation unit in order to process the heavier fractions of crude oil would result in the breaking of chemical bonds (thermal cracking) causing loss of product and equipment problems due to coking. Since boiling point decreases with a lowered pressure, transferring the heavy crudes to a vacuum distillation tower allows the refinery to separate these at lower temperatures, thereby reducing the amount of energy invested into the process. Pressures in the vacuum tower range from 25mmHg – 40mmHg, but can be lowered to 10mmHg by the addition of steam into the furnace. It is ideal for the difference in pressures at the top and bottom of the tower to be at a minimum so special packing materials are used instead of trays to improve fractionation. With all this change in pressure, it is also important to keep check on the exact temperatures being used. The Watson Characterization Factor (K_w) is used to estimate the upper temperature limit. Graphing K_w versus Temperature creates a band with an upper and lower line. It is necessary to stay below the top line, although if you need to be extremely careful (especially with paraffinic oils) it is best to stay below the bottom line as well.

Vacuum distillation is necessary to separate the heavier components of crude. This is due to the temperature limit imposed in order to avoid cracking of hydrocarbons, an desirable precursor to accumulation of carbonaceous solids (coking) in the distillation column. The Vacuum Distillation Unit receives the residue of the Atmospheric Distillation Unit and operates between 10 to 30 mmHg. The presence of a vacuum essentially lowers the vapor pressure of the atmospheric residue, allowing fractional distillation of heavy distillates while still preventing cracking. The Watson Characterization Factor provides an upper temperature limit for vacuum distillation, but actual temperatures used are typically lower in order to avoid uncertainties and risks of coking. Higher values of the Watson Characterization Factor require lower temperatures and lower values of the characterization factor allow for higher temperatures. This is due to the nature of the hydrocarbons associated with the Watson Characterization Factor – values less that 10 indicate a highly aromatic composition, high stability, and a lower tendency to crack; values between 10 to 11 indicate a napthenic crude with moderate stability and moderate tendency to crack; and values ranging from 11 to 12.9 indicate a more paraffinic composition, which is relatively easier to crack and has the lowest stability among the three.