The Nelson complexity index (NCI) is a measure to compare the secondary conversion capacity of a petroleum refinery with the primary distillation capacity.^[1] The index provides an easy metric for quantifying and ranking the complexity of various refineries and units.^[2] 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.^[3]

History

It was developed by Wilbur L. Nelson in a series of articles that appeared in the Oil & Gas Journal^[4] 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).

Formula

${\displaystyle {\text{NCI))=\sum _{i=1}^{N}F_{i}\cdot {\frac {C_{i)){C_{\text{CDU))))}$^[5]

Where:

• ${\displaystyle F_{i))$ is a complexity factor
• ${\displaystyle C_{i))$ is a unit capacity
• ${\displaystyle C_{\text{CDU))}$ is a capacity of crude distillation unit
• ${\displaystyle N}$ is a number of all units

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.

In the second edition of the book Petroleum Refinery Process Economics (2000), author Robert Maples notes that U.S. refineries rank highest in complexity index, averaging 9.5, compared with Europe's at 6.5. The Jamnagar Refinery belonging to India-based Reliance Industries Limited is now one of the most complex refineries in the world with a Nelson complexity index of 21.1.^[6]

The Oil and Gas Journal annually calculates and publishes a list of refineries with their associated Nelson complexity index scores.

Complexity factors

Some factors for various processing units:

Example

If an oil refinery has a crude distillation unit (100 kbd), vacuum distillation unit (60 kbd), and catalytic reforming unit (30 kbd), then the NCI will be 1*(100/100) + 2*(60/100) + 5*(30/100) = 1.0 + 1.2 + 1.5 = 3.7.