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Refinery size usually is measured in terms of distillation capacity. Relative size, however, can be measured using refinery complexity-a 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 units-such as a fluid catalytic cracking (FCC) unit or a catalytic reformer-to 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.




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