Distillation Terminology

Distillation Terminology sxr133 Tue, 05/21/2013 - 11:41

It is important to use the correct terms to clearly represent the fractionation of crude oil by distillation. The distillation temperatures, or cut points, are used to delineate the distiilate fractions and define commercial fuels and solvents.

Cut Points

Cut Points sxr133 Tue, 05/21/2013 - 11:42

Using a crude TBP curve, cut points are defined as the temperatures that represent the limits of a distillate fraction, as illustrated in Figure 4.11. For example, for kerosene, fraction Ta represents the lower cut point, and Tb represents the upper cut point in Figure 4.11.

$Temperature on y, % distilled on x. Cut points intersect with BP line showing starting point of the distillate fraction$

Figure 4.11. Cut points and definition of distillate products as boiling ranges.

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

Boiling ranges between the cut points represent distillate products, such as naphtha, kerosene, light gas oil, etc. The difference between the cumulative volume percent at upper and lower cut points is reported as the yield (in volume %) for the particular distillate fraction. For example, for the crude represented in Figure 4.11, the kerosene yield can be calculated as 40%(at Tb) -20% (Ta) = 20% by volume. Table 4.1 shows the TBP cut points for crude oil distillate fractions.

Table 4.1 TBP cut points for crude oil distillate fractions.
Distillate Product	Boiling Range
Butanes and Lighter
Light SR Naphtha	90 - 190^o F (32-88^o C)
Heavy Naphtha	190 - 380^o F (88 - 193^o C)
Kerosene	380 - 520^o F (193 - 271^o C)
Light Gas Oil	520 - 610^o F (271 - 321^o C)
Heavy Gas Oil	610 - 800^o F (321 - 425^o C)
Light Vacuum Gas Oil	800 - 950^o F (425 - 510^o C)
Heavy Vacuum Gas Oil	950 - 1050^o F (510 - 564^o C)
Vacuum Residue	> 1050^o F (>565^o C)

Separation in Fractional Distillation

Separation in Fractional Distillation sxr133 Tue, 05/21/2013 - 11:42

The quality of separation in fractional distillation can be calculated using the designated cut points (at 5%_vol and 95%_vol) for the two adjacent fractions, termed as light and heavy. As shown below, the difference in temperature (ΔT) between 5% _vol temperature of the heavy fraction and 95%_voltemperature of the light fraction is used to define the quality of separation. A positive value of ΔT(termed ASTM gap) indicates good separation, while a negative value of ΔT (termed ASTM overlap) points to a bad separation.

ASTM distillation temperatures and separation in a distillation process

Fractionation

What defines a good separation?

The relationship between the ASTM distillation temperatures at 95% vol and 5% vol of two adjacent fractions, light and heavy, respectively.

ASTM 5% _volT (heavy fraction) - 95% _volT (light fraction) = ∆T

(e.g., LGO) (e.g., kerosene)

if ∆T >0, called ASTM gap (good separation)

if ∆T<0, called ASTM overlap (bad separation)