Nelson complexity index

The Nelson complexity index (NCI) is a measure to compare the secondary conversion capacity of a petroleum refinery with the primary distillation capacity. The index provides an easy metric for quantifying and ranking the complexity of various refineries and units. To calculate the index, it is necessary to use complexity factors, which compare the cost of upgrading units to the cost of crude distillation unit.

It was developed by Wilbur L. Nelson in a series of articles that appeared in the Oil & Gas Journal from 1960 to 1961 (Mar. 14, p. 189; Sept. 26, p. 216; and June 19, p. 109). In 1976, he elaborated on the concept in another series of articles, again in the Oil & Gas Journal (Sept. 13, p. 81; Sept. 20, p. 202; and Sept. 27, p. 83).

${\displaystyle {\text{NCI}}=\sum _{i=1}^{N}F_{i}*{\frac {C_{i}}{C_{CDU}}}}$

Where:

The NCI assigns a complexity factor to each major piece of refinery equipment based on its complexity and cost in comparison to crude distillation, which is assigned a complexity factor of 1.0. The complexity of each piece of refinery equipment is then calculated by multiplying its complexity factor by its throughput ratio as a percentage of crude distillation capacity. Adding up the complexity values assigned to each piece of equipment, including crude distillation, determines a refinery’s complexity on the NCI.

The NCI indicates not only the investment intensity or cost index of the refinery but also its potential value addition. Thus, the higher the index number, the greater the cost of the refinery and the higher the value of its products.

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