Coker Retardado

CHAPTER 12.2

FW DELAYED-COKING PROCESS
Howard M. Feintuch Kenneth M. Negin
Foster Wheeler USA Corporation Clinton, New Jersey

The delayed coker is an important residue conversion process, or so-called “bottom-ofthe-barrel” process where residues from heavy, high-sulfur crudes are converted to transportation fuels. Statistically, in terms of number of units installed and total currentoperating capacity, it is quite easy to show that delayed coking is the residue-conversion process which is most often used today. In addition, because of its wide commercial acceptance, delayed coking has been referred to as the yardstick against which other, less commercially proven, processes must be measured.2 Despite its wide commercial use, only relatively few contractors and refiners are trulyknowledgeable in delayed-coking design, so that this process carries with it a “black art” connotation.3 The year in which delayed coking was first developed is given in historical listings of petroleum advances as 1928.4 We know that in early refineries severe thermal cracking of residue would result in the deposit of unwanted coke in the heaters. By evaluation of the art of heater design,methods were found by which it was possible to raise rapidly the temperature of the residue above the coking point without depositing the coke in the heater itself. Provision of an insulated surge drum downstream of the heater so that the coking took place after the heater, but before subsequent processing, resulted in the name “delayed coking.”5 The next step was to add a second coke drum, whichdoubled the run length and led to the development of the art of switching coke drums while still maintaining operation.6 In the early 1930s the drums were limited in size to 10 ft in diameter.7 Coke drums as large as 30 ft in diameter have recently been installed. Figure 12.2.1 shows the general trend in the growth in coke-drum diameter from 1930 to 1995. As of Jan. 1, 1995, there were in operation inthe United States 45 delayed cokers with the capacity to process 1,598,000 BPSD of fresh feed.8 Table 12.2.1 shows coking-plantcapacity statistics for the United States from 1946 to 1995.9,10 Figures 12.2.2 and 12.2.3 present two delayed cokers, one operating on the United States Gulf Coast and the other which had operated in the Netherlands. (Abbreviations are defined in Table 12.2.17 at the endof the chapter.)
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VISBREAKING AND COKING

FIGURE 12.2.1 Trend in delayed-coker maximum drum size.

TABLE 12.2.1 Plant Statistics Year 1946 1950 1960 1970 1973 1982 1995

United States CokingUnited States coking capacity, BPSD 102,000 158,000 475,000 835,000 1,008,000 1,118,100 1,598,000

Source: W. L. Nelson, Oil Gas J., 74, 60 (May 24, 1976); Oil Gas J., 80, 81, 130(Mar. 22, 1982); Oil Gas J., 92, 51, 53 (Dec. 19, 1994).

PROCESS DESCRIPTION
Figure 12.2.4 is a process-flow diagram showing the coking, fractionator, coker blowdown, and steam-generation sections of a typical delayed coker. The associated vaporrecovery unit is shown separately in Fig. 12.2.5 and the coke-calcining plant separately in Figs. 12.2.6 and 12.2.7. A brief description of each of thesesections is given below.

FW DELAYED-COKING PROCESS

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FIGURE 12.2.2 Delayed coker in the Gulf Coast region of the United States. Center left and right each show four coke drums with drilling platforms for hydraulic decoking. Lower left shows two heaters and four associated stacks. Center right shows two heaters and their associated stacks.

Coking Section Reduced-crude orvacuum-residue fresh feed is preheated by exchange against gas oil products before entering the coker-fractionator bottom surge zone. The fresh feed is mixed with recycle condensed in the bottom section of the fractionator and is pumped by the heater charge pump through the coker heater, where the charge is rapidly heated to the desired temperature level for coke formation in the coke drums. Steam is often...