Kambiz Manafi
National Iranian Oil Co.
Tehran
A simple method can be used to calculate a single refining complexity number for one refinery, a refining area, or for the entire world.
It is more comprehensive than crude-conversion calculations because it includes additional process units.
The complexity concept was first introduced in the 1940s by the late W. L. Nelson, one of the world's foremost authorities on petroleum refining. Since then the concept has gained international acceptance and has become a useful tool for determining the ability of a refinery to produce different products.
In other words, the higher the refining complexity number, the greater the ability of a refinery to convert crude oil into products, in terms of the process operations conducted in that refinery. In determining refinery complexity, the original capital cost of a crude distillation unit is assigned a value (usually equal to one).
The crude distillation unit is used as the reference point because all of the crude oil processed in that refinery goes through that unit. But only a portion of the crude is sent through the other processing units, such as crude vacuum distillation, reforming, treating, conversion, and coking units.
The throughput for each unit changes with time, depending on the products needed and the unit availability.
Therefore, the complexity of any refinery also changes with time and it is desirable to update complexity calculations at intervals of 6 months, or at most a year.
Complexity theory is also one of the best known ways to determine and compare the present value of different refineries at any given time, using the Nelson itemized refining cost indexes. 1
In addition, complexity numbers can be used to calculate and compare the operating costs of different refineries.
However, this is confined to the aspect of the complexity theory related to how the capital cost of a refinery determines its ability to convert crude to products.
COMPLEXITY COMPUTATION
All of the complexity factors for the evaluation of process operations depend on the capacities of the process units in relation to the capacity of the crude distillation unit.
The conventional crude distillation unit is assigned a complexity number of 1.0 (the basis of the complexity concept). The complexity contribution of each of the other units is determined by comparing their capital costs to that of the crude unit.
For example, a crude unit has a capital cost of say $10 million. The hydrocracking unit of the same refinery has a capital cost of $400 million, and its throughput is about 15% of the total feed, on average. The contribution of the hydrocracker to the overall refinery complexity is then derived by the following simple relation:
Complexity contribution of hydrocracker =
(400 x 0.15) divided by 10 = 6
The same method is used to calculate the complexity contribution of each refinery process unit at the interval needed.
As for off site units such as tank farms and utilities, Nelson suggests that, depending on the regular complexity of the process units (that is the total refinery before off site units), a multiplier factor from 1.77 to 3.25 must be used to arrive at the refinery's total complexity number.
Experience has revealed that off site capital costs are not directly proportional to the complexity of the process units.
Rather, off site costs are larger for simple refineries and smaller for more complex ones.
For example, a simple refinery's process unit complexity of 3 must be multiplied by 3.25 to arrive at the complete refinery complexity number, and a very complex refinery's process unit complexity of 16 must be multiplied by 1.77. 2
SHORTCUT METHOD
The method described in the preceding is the original computation method suggested by Nelson in 1976. However, a simpler, or shortcut method can be used to compute refinery complexity at any given time. That is simply to use one of two sources:
- A table originally worked out by Nelson to represent the complexity of each process unit as compared to the crude distillation unit. 2
- The updated table prepared by G. L. Farrar. 3
Although this method involves some amount of approximation, the overall results will not be so different as to misguide one in drawing conclusions.
The original method is suggested for the purpose of determining the exact changes that have occurred in a single refinery, or in comparing a few refineries in a short period of time.
However, if the prime concern is changes in the complexity of refineries in certain refining areas over a longer span of time, then the short cut method will usually be accurate enough.
In order to examine the Nelson concept, the short-cut method was used to compare the complexity numbers of different refining areas of the world in 1980 and 1990, to illustrate how the configuration of these refining areas has changed during the decade (Tables 1 and 2).
CONCLUSION
The results of Tables 1 and 2 show that refining complexity has increased in North America, Western Europe, Middle East, Asia/Far East, and Oceanic, while for Latin America and Africa, the complexity number has remained static or has increased very little over the decade.
A look at crude distillation capacity reveals that world crude distillation capacity has decreased by about 8.5 million b/d (excluding the former centrally planned economies).
The increase in the complexity number-especially in major consuming areas-reveals the tendency toward using more complex units to produce ends.
Although the same conclusions could have been reached by looking at the conversion capacity of the refining areas, the complexity number calculation is more comprehensive because it includes vacuum distillation, catalytic reforming, hydrorefining, and hydrotreating units.
These units are not considered part of conversion capacity because by definition, conversion capacity refers to thermal cracking, catalytic cracking, and hydrocracking units.
Another advantage of computing a single refining complexity number is that it can be used by management as a quick reference or for summary reports.
REFERENCES
- OGJ, Oct. 7, 1991, p. 92.
- Nelson, W. L, "How to apply complexity," OGJ, Sept. 27, 1976, p. 83.
- Farrar, G. L., "Interest reviving in complexity factors," OGJ, Oct. 2, 1989, p. 90.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
