This method, described in Figure 4.7a and b, uses a batch distillation operation that incorporates more than 100 theoretical plates and a high reflux ratio (R/P) of 100, as described in Figure 4.7a. This is an idealized method to achieve the best possible separation in distillation, made possible by a large number of theoretical plates (stages) for liquid vapor contact in the column and an extremely high reflux ratio. As an example, consider distillation of a binary mixture of compounds A (70% by volume) and B (30% by volume), with boiling points Ta and Tb, respectively. Figure 4.7b illustrates the distillation curve that would be obtained if this mixture were distilled using the TBP method, with perfect separation of A and B as pure compounds. Because of the TBP distillation conditions, first the lower boiling component A is distilled off without any contamination with B, and following the complete vaporization of A, B is distilled off as a pure compound. Note that because a large number of plates and a high reflux ratio in the column, temperature remains constant during evaporation of A until all of this compound is boiled off, as would be seen in the distillation of a pure compound.

Figure 4.7a. TBP distillation apparatus

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 4.7b. Distillation curve for perfect separation of the components A and B in a binary mixture.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

ASTM distillation also uses a batch operation, but in contrast to TBP, it operates without the presence of a contact plate and a reflux ratio (R/P, or RR) of zero, as shown in Figure 4.8a. There may be a slight unintentional reflux because of the condensation of the vapor on the tube that connects the flask to the condenser.

Figure 4.8a. ASTM distillation apparatus.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 4.8b. Distillation curve for ASTM distillation of the binary mixture A and B.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Figure 4.8c. TBP distillation curve for distillation of the same binary mixture.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Equilibrium flash vaporization involves heating a flowing feed and the separation of the liquid and vapor in a flash drum. A distillation curve may be obtained by conducting this distillation at varying heater outlet temperatures. Figure 4.9 shows the diagram of an EFV set up and compares the distillation curves from the three methods, TBP, ASTM, and EFV. From the comparison of the curves and the relationship between IBP and EP obtained in each case, one concludes that EFV gives the lowest degree of separation between A and B, even lower than that given by the ASTM distillation.

Figure 4.9. Equilibrium Flash Vaporization (EFV) and comparison of distillation curves for EFV, ASTM, and TBP distillation of the binary mixture A and B.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

The TBP, ASTM, and EFV distillation methods achieve different levels of separation for a given sample, as related to the different techniques used in these analyses. Figure 4.10 shows TBP, ASTM, and EFV curves for a middle distillate fraction crude oil, showing significant differences in IBP and EP of the three curves [6]. Note that three curves converge near 50% volume distilled. TBP distillation achieves a higher degree of separation than ASTM and ASTM achieves better separation than EFV, as can be seen in the curves in Figure 4.10. Empirical correlations have been developed to convert one set of distillation data to another [7].

Figure 4.10: TBP, ASTM, and EFV curves for a middle distillate fraction crude oil.

Credit: Refining Overview - Petroleum, Products and Processes, AIChE, 2000.

Each distillation method discussed in this section has an application in petroleum refining. TBP distillation is used to characterize crude oils and constitute a significant component of crude essay. ASTM methods are usually used for refinery products and property calculations and correlations for distillate fractions. EFV provides useful data for flashing operations in the refinery.

There are no standard methods for TBP distillation, but ASTM D-2892 method is used to approximate the TBP distillation. This method is also referred to as 15-5 distillation, because of 15 theoretical plates and a reflux ratio of 5 used in the distillation. A simulated distillation method described in ASTM D2887 may also be used to obtain TBP data for crude oils.

ASTM D86 (atmospheric distillation) and ASTM D1160 (vacuum distillation) are used for low-boiling, and high-boiling fractions, respectively.