The three distillation methods introduced to us in this lesson include True Boiling Point Distillation (TBP), ASTM Distillation, and Equilibrium Flash Vaporization (EFV). These methods are commonly used to generate laboratory data on crude oil, and their distillation fractions have varying degrees of separation. The largest degree of separation can be obtained by TBP distillation – this method utilizes a high reflux ratio (100:1) and a high number of theoretical plates (>100) for liquid vapor contact. The temperature remains constant during evaporation of each component, providing the least overlap between distillation fractions. ASTM distillation provides the second best degree of separation – in contrast to TBP distillation, the ASTM method operates without plates, has a reflux ratio of essentially zero, and the temperature does not remain constant during this process. EFV is the final method of distillation and utilizes a flowing feed (rather than a batch feed), where separation of liquid and vapor occurs in a flash drum. Distillation occurs at various heater outlet temperatures, providing the least amount of separation between fractions of the three methods. TBP distillation is essentially used for crude assay; the ASTM method is used for determining properties of refinery products; and EFV conveniently provides data for flashing operations.

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