Managing Shot Coke: Design & Operation

Shot coke is characterized by green delayed coke forming as small round pellets (BB’s) typically ranging 1.5 to 4 or 5 mm in diameter which are frequently loosely bound together. The term, “shot”, is believed to derive from the fact that the pellets are reminiscent of those used in shotgun ammunition. Frequently, the BB’s bond into “ostrich egg” sized pieces. While shot coke may look like it is entirely made up of shot, much shot coke is not 100% shot. Interestingly, even sponge coke may have some embedded shot coke. The test for such a quantitative measurement is not precise but a very low shot coke percentage is often specified for anode grades of petroleum coke, typically less than 5%. In the case of fuel coke production, there is a general belief that shot structure of coke means that the coker is being run at the most economic conditions based on upgrading of heavy lower cost crudes. Shot coke types fall into several categories generally referred to as: • Bonded (or agglomerated) shot coke: This form is characterized by the shot BB’s being stuck to each other and, mostly, not loose until broken apart by mechanical or hydraulic forces. These BB’s can be formed into the classic ostrich egg or cannonball structure which when broken will be seen as a mass of bonded shot BB’s. • Loose shot coke: Free flowing shot BB’s which form loosely without bonding in the drum. Loose shot BB’s are also frequently formed by attrition of bonded shot coke by flow dynamics, especially hydraulic cutting, and by shattering as the coke drops from the drum. • Embedded shot coke: Shot BB’s embedded in and often hidden in a matrix of sponge or amorphous coke. The latter mixture can be expected to have poor quench characteristics due to low porosity and permeability. While changes in operating conditions can increase or decrease shot coke formation, the characteristics of a feedstock are dominant factors. Parameters such as microcarbon residue (MCR), heptane insoluble asphaltenes (HI), metals and heteroatom contents, especially nitrogen content, solids content as well as knowledge of the crude origin are used to predict the tendency of a residue to form shot coke. A well known rule of thumb is that shot coke is likely to be produced when the ratio of MCR: HI is less than 2. A method with greater validity is to calibrate based on ratios of coke yield to HI for residues with specific crude origins. These approaches need to be tempered for general use by experience and account for some of the other factors mentioned above. The impact of asphaltene properties and relative solubility also needs to be considered as these will vary based on crude origin or source of residue, e.g. fluid catalytic cracker slurry oil.