Nelson compexity factors are an indices of factors for the major refining process units that reflects the capital cost of a process unit type (per barrel of capacity) relative to the capital cost of distillation.  For example, a unit with a complexity of 2 will cost twice as much to construct as a distillation unit of the same capacity.

The factors are used to estimate an overall capacity weighted complexity index (NCI) for a refinery.

Typical complexity factors for the major process units in a refinery as defined by Nelson:

Unit NCI Factor

Atmospheric distillation 1.0

Vacuum distillation 2.0

Vacuum flash 1.0

Thermal cracking 3.0

Visbreaking 2.0

Coking 5.5

Calcining 108.0

FCC (60% conversion) 5.0

FCC (80% conversion) 6.0

Reforming 5.0

Hydrocracking 6.0

Hydrorefining 3.0

Hydrotreating 1.7

Alkylation 11.0

Aromatics 20.0

Isomerization 3.0

Polymerization 9.0

Lubes 60.0

Asphalt 1.5