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.

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.