Vacuum residue, coking product yields

Because of the increased sulfur and impurity levels in crudes currently being processed, refiners in recent years have been considering residue desulfurization units upstream of the delayed coker. In addition to the reduction in sulfur content, residue desulfurization units also lower the metals and carbon residue contents. Due to the reduction in the carbon residue, the liquid product yield is increased and the coke yield reduced. In addition, the coke produced from a desulfurized residue may be suitable for use as anode grade coke. Table I shows the yields and product properties after coking Medium Arabian vacuum residue, with and without upstream residue desulfurization. 

Table II shows the product yield for coking a vacuum residue at 3 temperatures and the percent of gas and liquid produced by the slow and fast reactions, respectively. Coking at 835°F gives proportionately more liquid in the total volatiles as a result of the slow reaction than at 915°F and 1035 °F. The data available sure insufficient to determine an overall rate constant for the process, equation 3.

Yields of Gas and Liquids Products and Their Rates of Formation for the Coking of Vacuum Residue at Different Temperatures... 

Figure 6. Overall product yields for coking a vacuum residue at different temperatures. Key G, gas L, liquid and C, coke.

Coke can account for up to 30wt% of the product. For instance, the coke yield when a vacuum residue coming from a crude oil with 22°API and 10.79 wt% CCR is about 12.2 wt%, while for a vacuum residue from a crude oil with 12.7°API and 16.4 wt% CCR it is about 22.3 wt%. Coke can be shipped by rail, truck, or conveyor belt to a calciner, which converts green fresh coke from the drum into various grades of petroleum coke. Green coke can also be used for fuel. 

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