Why aren't the draw off temperatures close to the IBP, FBP of the products? For example, the draw temperature of kerosene is 193degC where as the lab results suggest 140degC IBP and 240degC FBP.
Jake Gotham, InSite Technical Services, email@example.com
The draw-off temperature is the bubble point at the tower operating conditions. The IBP is an approximation to the bubble point under the conditions of the laboratory test. There are several factors which contribute to the difference in the figures:
1. The tower operates with a positive pressure. This increases the bubble point and hence draw temperature.
2. The tower operates with stripping steam. This counter-acts item 1 and reduces the bubble point / draw temperature.
3. The kerosene then flows into a side-stripper which removes some of the lighter material, increasing the bubble point.
4. The laboratory test is an indication of the boiling curve of the product, but is by no means exact. The error in the test is greatest at IBP and FBP, so these values in particular need to be treated with caution. You will notice quite different values for IBP depending on the test method chosen (D-86 vs D-2887 vs D-2892) which illustrates the fact that the reported figure is a function of the laboratory test as well as the kerosene itself.