Vacuum distillation is a necessary process because the outlet temperatures of the atmospheric pressure distillation are so high that thermal would begin the break down the crude oil. In order to continue distilling the heaviest crude fractions of the atmospheric distillation process, the pressure must be reduced to 25 to 40mmHg. Cracking, the breaking of the chemical bonds between carbon atoms would cause coking on the metal surfaces in the column, which interferes with distillation.

Watson Characterization is used to determine an upper temperature limit for the vacuum distillation process, which avoids coking. If severe coking occurs it can plug the flow of the distillation column, which would lead to the shutting down of the entire refinery. By comparing the Watson characterization factor (Kw) to temperatures, which relate to coking propensity, a safe temperature can be determined. Bands of temperatures show where coke formation is negligible as well as uncertain. Generally the temperature that is chosen is lower than the lower temperature line of the band.

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.

There are two broad categories of distillation which are atmospheric and vacuum distillation. Within these two categories there are three distillation methods: True Boiling Point distillation (TBP), ASTM distillation, and Equilibrium Flash Vaporization distillation (EFV). As the names imply, atmospheric distillation runs at atmospheric pressure while vacuum distillation runs lower than atmospheric pressure that allows very high boiling point compounds to boil at lower temperatures.

The first of the three distillation methods, TBP, distills the crude in a batch operation. As the crude is heated each component converts to vapor at its specific boiling point. The vapor is separated from the other liquids using what are known as theoretical plates of which there are more than 100 in the column. Another aspect of this method is that the products are stringently purified using a reflux technique which involves heating and cooling vaporized distillates while in the distillation unit or just outside the unit and put back in. By combining the high number of theoretical plates with a reflux ratio, meaning the ratio of amount of reflux to amount of products, this procedure provides the greatest purity of product of the three distillation types.

ASTM works similar process to TBP however it does not use the theoretical plates or have reflux (and therefore no reflux ratio). In the end this process looks much more like a typical distillation unit which heats a liquid to a boiling point and cools and collects the condensate. This process provides slightly lower quality products.

EFV is similar to ASTM however it heats a flowing feed of crude rather than heating a batch. Once heated the feed enters a flash drum where the crude that has boiled off to vapor will separate from the other liquids. This provides the lowest quality products of the three methods.

Vacuum Distillation is an important part of Petroleum processing in the United States. According to the EIA, approximately 80% of the refiners operating in the US have a Vacuum distillation unit (VDU).¹ This is a secondary processing unit added to petroleum refineries, consisting of vacuum distillation columns.² This process helps to produce petroleum from that of heavier oils that are left over from traditional atmospheric distillation.¹ In the refining process, the atmospheric distillation unit (ADU) separates the lighter hydrocarbons from heavier oils based on boiling process. The ADU can reach boiling point of crude oil up to 750 F, above which oil would usually thermally crack, or break apart, which hinders the distillation process.² The bottoms left over from the ADU then can be run through a vacuum distillation column to further refine and improve yield.¹ The products from vacuum distillation are slightly heavier than middle distillates, but can be further refined to make products such as gasoline and naptha.¹

To determine the temperature at which vacuum distillation can be used tp further process the petroleum, the Watson Characterization factor can be utilized. The plot of the Watson characterization is information showing the range of temperatures which can be used to avoid cracking, which would slow the refinery process. The Watson Characterization factor defines the upper bound temperature limit for vacuum distillation as a means to avoid cooking of the feed stock. Below the indicated bound, cooking risk would be minimal and above it, cooking could be a potential risk.

1. Vacuum distillation is a key part of the petroleum refining process http://www.eia.gov/todayinenergy/detail.cfm?id=9130

Crude oil is a raw material which is composed of different chemical with varying chain lengths of hydrogen and carbon atoms. To separate the hydrocarbons into their different fractions, distillation techniques can be used. There are commonly three different distillation methods used in petroleum refining. The first of these methods is True Boiling Distillation (TBP). With petroleum processing the boiling rang of the crude gives an indication of the quantities of the various products present.¹ TBP provides a reliable tool for characterizing of crude oil and petroleum in terms of their boiling point distribution.¹ It completes this by separating the individual components of the crude into individual mixtures components in order of boiling point.¹ Refiners use this information to model the crude distillation column.¹ However TBP is costly and takes significant time to analysis so it’s not always the best method or most practical for daily monitoring of crude distillation unit operation.¹

The second method of distillation is ASTM Distillation. In this process atmospheric distillation of petroleum products is performed using batch distillation unit to determine quantitatively the boiling range characteristics of the products.² By performing a simple batch distillation the boiling range can be found.² The information generated then can be used to find composition, properties, and the behavior of the fuel during storage and use. It also creates limits of use of petroleum products in commercial contract agreements, process refinery, control application, and for compliance to regulatory rules.

The last form of distillation is Equilibrium Flash Vaporization (EFV). This process Involved heating a flowing feed and separating of liquid and vapor in a flash drum.³ This process generates information about the vapor-liquid equilibrium, which can then be used for the design of equipment to separate the different components of a crude hydrocarbon mixture.⁴ This type of data is often reported as an isobaric curves on a plot showing vaporization vs. temperature.

References

1. BOILING POINT DISTRIBUTION OF CRUDE OILS BASED ON TBP AND ASTM D-86 DISTILLATION DATA Petroleum & Coal ISSN 1337-7027 Petroleum & Coal 53 (4) 275-290, 2011 Angel Nedelchev, Dicho Stratiev, Atanas Ivanov, Georgy Stoilov Lukoil Neftochim Bourgas

2. Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure

3. 3.FSC 432: Petroleum Processing- Equilibrium Flash Vaportization- Course Webpage

4. D. F. Othmer, E. H. Ten Eyck, S. Tolin Equilbrium Flash Vaporization of Petroleum Crude Oils or Fractions- Method and Apparatus for Determination-