Refinery size
usually is measured in terms of distillation capacity. Relative size, however,
can be measured using refinery complexitya concept developed by W.L. Nelson in
the 1960s.A review of
complexity calculations, and an explanation of how indices have changed,
provide a simple means of determining the complexity of single refineries or
refining regions. The impact of complexity on product slate also will be
examined.
The Nelson index
Nelson developed
the complexity index to quantify the relative cost of components that make up a
refinery. It is a pure cost index that provides a relative measure of the
construction costs of a particular refinery based on its crude and upgrading
capacity.
The Nelson index
compares the costs of various upgrading unitssuch as a fluid catalytic
cracking (FCC) unit or a catalytic reformerto the cost of a crude distillation
unit. Computation of the index is an attempt to quantify the relative cost of a
refinery based on the added cost of various upgrading units and the relative
upgrading capacity.Nelson assigned a
factor of 1 to the distillation unit. All other units are rated in terms of
their cost relative to this unit.
For example,
assuming a crude distillation unit costs $400/b/cd to construct, a 50,000 b/cd
unit would cost $20,000,000. If another component costs $1,200/b/cd to build,
this unit would have a complexity factor of 3.
The complexity
rating of a refinery is calculated by multiplying the complexity factor for
each downstream unit by the percentage of crude oil it processes, then totaling
these individual factors. To illustrate, consider the case of a refinery with
50,000 b/cd of crude capacity and 30,000 b/cd of vacuum distillation capacity.
The throughput of
the vacuum tower relative to the crude distillation capacity is 60%. Given a
vacuum unit complexity factor of 2, the contribution of the vacuum unit to
overall refinery complexity is 2 x 0.6, or 1.2.
