The application of the Geddes fractionation index to crude distillation units



The degree of separation effected by a fractionator can be measured by a fractionation index n. The index is the equivalent number of theoretical plates, operating at total reflux, which would effect the same component separation as the fractionator.

The value of the fractionation index is readily calculated from the Fenske-Underwood equation when component analyses of the feed and products are available.

In the case of crude oils, the number of components is so great that resolution in the laboratory is not practicable, and the feed and products are customarily analyzed by high efficiency laboratory fractionators which give their true boiling point characteristics. In this form, the data have been broken down into short fractions which have then been treated as pseudo components in subsequent column calculations.

This paper presents a method for treating the product TBP curves as algebraic functions without the necessity for breaking the feed TBP data into arbitrary short fractions. The fractionation index can be derived directly from product TBP data, or the TBP data can be calculated when the fractionation index is known.

Data from performance tests on three commercial units have been analyzed. The TBP distillation analyses of the crude oils and the products enabled the fractionation indices pertaining to each section of the fractionators to be evaluated.