Published October 1971
Petroleum coking technology has developed over some 60 years, with only two processes currently in use; delayed coking and fluid coking. Essentially, it involves thermal decomposition of heavy residuals from atmospheric and vacuum crude oil processing units into gas, light and middle distillates, and coke.
Although petroleum coke is produced in large tonnage in U.S. refining operations, it usually has been treated as a by-product. Petroleum refiners are interested in coking principally as a means of disposing of residuals for which there is a limited market, and/or upgrading the residuals to lighter distillates and thereby improving overall refinery economics. Although these residuals have been satisfactory as industrial fuel, the demand for this grade of fuel oil has not kept pace with the growth of the refinery industry. Consequently, coking operations have become of increasing importance to the refining complex.
The increased concern for reducing air pollution is further incentive for processing residuals through a coking unit. In general, the residuals have high sulfur contents, which end up as atmospheric pollutants in the products of combustion when the residuals are used as fuel. In the coking process, a significant portion of the sulfur is distributed in the gas and distillate products in a form whereby they can be removed economically. The balance remains in the coke as a contaminant. Until recently, high sulfur cokes have been used as low grade fuels.
Petroleum coking has not been practiced extensively in the free world outside the United States. This is attributed to a stronger market for heavy oils as industrial fuels in these countries. However, due to the concern over atmospheric pollution, refiners in some of these countries are now showing greater interest in coking when planning new facilities.
The major feedstock to coking units is the bottoms from refinery crude units, containing essentially all the metal or ash components of the crude oil, a good part of the sulfur, and the bulk of the asphaltic materials. Another feedstock is tar from thermal cracking units which is highly aromatic and refractory. Also used as feedstock is decant oil from catalytic cracking, which is highly aromatic. While these are the major feedstocks for coking units, numerous other materials are also processed concomitantly. Indeed, the coking unit has been treated as the refinery garbage can by many refiners, as a convenient means to dispose of off-grade or otherwise undesirable oils. Coking unit operations are adjusted to optimize distillate yield and quality, principally of gas oil fractions. As a result, the coke obtained is inconsistent in quality.
Coke consumers have been working with the problem of inconsistent quality and have developed considerable know-how on the best ways to use coke. Although much of their knowledge is proprietary, some information has recently been applied to refinery operations, resulting in a general improvement in the quality of coke produced.
Delayed coking and fluid coking are not readily comparable in that the yield and quality of the products, particularly the coke, are substantially different. The choice between the two processes, then, is made from a consideration of the refinery processing scheme and future plans, and is sometimes influenced by a company's greater familiarity with one of the processes.
This report includes an evaluation of both processes, as currently practiced, and discusses possible process modifications and innovations. Processing of the coker unit gas and distillate products is excluded. Some discussion of handling and processing of coke is included, specifically calcining and desulfurization operations and handling of delayed coker coke.
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