Write a post explaining the need for vacuum distillation and discuss the utility of Watson Characterization for selecting the vacuum distillation temperature.

Vacuum distillation is essential in petroleum refining. Through this method the distillates can be separated into light vacuum oils, heavy vacuum oils, and vacuum residue. The lighter components of the crude can then be removed out of the top of the distillation column so that the heavier non-volatile compounds remain on the bottom. A vacuum is needed for this method in order have properly met temperatures and pressures within the distillation column. It also is needed in order to avoid thermal cracking.

The heaviest and most contaminated portion of the crude oil is known as the vacuum distillation residue, found on the very bottom. This residue is actually able to be upgraded into a usable fuel source through various processes such as deasphalting, visbreaking, and coking. Deasphalting requires hydrotreating whereas both visbreaking and coking require thermal cracking with coking being more severe.

In catastrophic instances a distillation unit is shut down. This puts a refinery in a very bad position. Very major coking cases can actually block the path of flow in a distillation column and force it to be shut down. Because of this reason it is very important to select the proper temperature in a vacuum distillation column. Doing so will control the risk of coking within the column. The Watson Factor, known as Kw, is used to estimate the upper temperature limit for vacuum distillation to avoid coking and ensure that the process within the distillation column occurs smoothly without any clogging. This is essential to keep a refinery up and running and not have to stop for maintenance.

Write a blog post that reviews the utility of different distillation methods and their applications in petroleum refining.

There are three common different distillation methods. These methods are very important because they allow us the opportunity to obtain a better perspective of crude oil. Using these methods crude oil can be distilled into different oils which have varying characteristics. Each method is a different level of separation. These three methods are:

True Boiling Point Distillation (TBP): In this batch distillation method only one distillation occurs based upon boiling point. This method allows for the most efficient separation. It uses a large number of stages for liquid to vapor contact in the distillation column with a very high reflux ratio while raising the temperature. When a certain boiling point is hit that only applies to a specific portion of the crude, that specific portion will begin to evaporate. Once the portion of the crude with this boiling point changes phases through vaporization it can then be extracted, thus in total now having two different oils. This form of distillation has no standard test methods.

ASTM Distillation: This method is also a batch operation but it does not use contact plates and has a reflux ratio of zero. This method uses a heater and cooling water and instead of distilling after hitting one certain temperature it gradually distills over time. It has much more of an exponential distillation curve for a binary mixture where for TBP distillation it was completely linear with one jump in the graph. This is the standard test method used for refinery products to determine property calculations and correlations.

Equilibrium Flash Vaporization (EFV): This distillation method uses a heater to heat up the liquid and then a flash drum to separate the liquid and vapor phases. This provides data that is useful for determining proper flashing operations in a refinery. By looking at the distillation curve for this method it can be noted that this gives the lowest degree of separation.

The boiling point reached for separation to occur decreases from TBP to ASTM to EFV. The degree of separation also decreases for the same order. Each distillation method is very important and has its own application in petroleum refining.

Atmospheric Distillation (CDU) separates the heavy and light crude In Atmospheric Distillation (CDU), it process with a network of heater to heat the crude to about 550 F. Then in the further process the temperature will be raised to about 750 F in furnace. In flash zone, the vapor fraction would go toward the top of the column and the liquid fraction would be send to vacuum distillation (VDU).  The vacuum distillation (VDU) is needed due to the unwanted high temperature. Such high temperature will cause cracking and coking on the surface. In order to lower the temperature, a vacuum chamber is designed to lower the pressure which to lower the temperature.

From the temperature vs Watson Characterization Factor plot that post on class website, it provides us a range of temperature which helps to avoid coking.  The risk of coking is negligible when the operation temperature is low than the band of temperature that showed on plot. However, when it operates under the temperature that is within the area of band, it is possible to cause coking during the process. From the plot, we can also see that higher Watson K factor leads to lower temperature band. Refer to what we learned in lesson 2, Watson K factor is calculate depend on physical properties. Paraffinic has higher Watson K factor than naphthenic and aromatic. It means paraffinic is easier to cause coking than naphthenic and aromatic at high temperature.

Three distillation methods that introduced in lesson 4 are True Boiling Point Distillation (TBP), ASTM Distillation (ASTM) and Equilibrium Flash Vaporization (EFV). Since the method of each distillation is different, each of them has their own utility, application in refining and separation performance.

The best possible separation is achieved in True Boiling Point Distillation (TBP). True Boiling Point Distillation (TBP) used more than 100 theoretical plates and a high reflux ratio (R/P) of 100 in the separation process. Since it has such outcome, it is usually used to characterize crude oils and constitute a significant component of crude essay.

Compare to True Boiling Point Distillation (TBP), ASTM distillation (ASTM) also used batch operation. However, the operation incorporates exclude contact plate and a reflux ration of 0. A slight reflux may involve due to the condensation of vapor on the tube. In petroleum refining, the method is used to refine products, calculate properties and correlate distill fractions.

Equilibrium Flash Vaporization (EFV) probably provides the lowest degree of separation among the three distillation methods. It contains a heater to heat a flowing feed. In the flash drum, the separation of liquid and vapor take place. In refinery, Equilibrium Flash Vaporization (EFV) always provides clear and useful data for flashing operation for us to interpret.

Before the crude can enter the vacuum distillation unit (VDU), it must first be introduced into the Crude Distillation Unit (CDU) flash zone in order to be separated into vapor and liquid streams. Separation of light and heavy crudes is achieved through Atmospheric Distillation (CDU). Vapor fractions rise to the top of the column while the liquid fraction can be sent to VDU after it is introduced to stripping by steam, in order to recover the components dissolved in the heavier liquid that has settled at the bottom of the CDU. This vacuum chamber is vital since there must be a pressure drop which ultimately results in a decrease in temperature. High temperatures are unwanted since this would bring about cracking and inevitably coking on the column’s metal surfaces, interfering with fractionation.

To control the risk of coking, the ideal vacuum distillation temperature in the vacuum distillation column can be found using the Watson Characterization Factor (Kw). This will calculate the upper bound temperature limit for vacuum distillation to avoid coking. Below this range of temperatures, coking risk would be considered negligible while the upper bound represents uncertainty of the probability of coking. To be sure, the distillation temperature would be set lower than this temperature band. One should also take into account the hydrocarbon composition of the crude as paraffinic crudes are more susceptible to coking.

There are three different distillation methods that are used in crude oil refineries, and those are True Boiling Point Distillation (TBP), ASTM Distillation (ASTM), and Equilibrium Flash Vaporization (EFV). Each of these processes vary in the way distillation is achieved, so some methods are more effective than others.

True Boiling Point Distillation (TBP) is the most ideal process. It is a batch distillation operation which utilizes over 100 theoretical plates and a high reflux ratio of 100. This high number of plates along with a high reflux ratio allows for the best possible separation in distillation processes. These parameters also yield a pure compound since the temperature remains constant during evaporation of the unwanted compound.

The next method is ASTM Distillation. This process also uses a batch operation, but it does not include contact plates and it therefore has a reflux ratio of 0. Reflux inside the fractionation column flows downwards and cools vapors flowing upwards. The more reflux provided for a given number of plates results in more effective distillation, which is why this process is not as good as TBP.

The third distillation method is Equilibrium Flash Vaporization (EFV). This method is known yield the lowest degree of separation out of the three. EFV involves separation of the liquid and vapor in a flash drum after heating the flowing feedstock.