Petrolem Coke Petrography
Delayed coking is a thermal cracking process. The feed is generally petroleum derived. The carbonization reactions involve dehydrogenation, rearrangement and polymerization. Two of the common feedstocks are vacuum, residues and aromatic oils. The vacuum residues contain asphaltic compounds which are mostly heterocyclic molecules. The aromatic oils such as decant oil and pyrolysis residues have concentrations of polynuclear aromatics mainly with 6-carbon aromatic rings. They produce graphitic structures.
In the delayed coker the feed enters the bottom of the fractionator where it mixes with recycle liquid condensed from the coke drum effluent. It is pumped through the coking heater then to one of two coke drums through a switch valve. It is 480° to 500°C. Cracking and polymerization take place in the coke drum in a nominal 24-hour period. Coking is a batch operation carried out in two coke drums. Coking takes place in one drum in 24 hours while decoking is carried out in the other drum. A complete cycle is 48 hours. Coke is cut from the drum using high pressure water. Large drums are 27' in diameter and 114' flange to flange.
There are various amounts of sponge, needle and shot coke produced in delayed cokers. Virgin petroleum feedstocks have a large number of cross-linkages with less than 6 carbon atoms. These feedstocks tend to produce isotropic or amorphous cokes and when they are visibly very porous they are called sponge coke. The highly aromatic fractions from refinery cracking remain plastic longer during carbonization allowing crystals to form needle-like ocicular structures. They have a low coefficient of thermal expansion (CTE) and maintain high current densities in products (high conductivity). Shot coke is an abnormal type of coke resembling small balls. It is believed that residum high in asphaltines and low API favor shot coke formation. Blending aromatic materials with the feedstock and/or increasing recycle ration reduces shot coke. Mesophase and its precursors have a wide size range depending on how rapid or slow components react and solidify. Fluidization in the coke drums may cause shot coke. It tends to be more isotropic, or fine granular in texture and is hard with a low porosity and high density which makes it difficult to crush. Coking severity decreases from bottom to top in the drum so bottom coke is more dense with lower VCM than top coke. Gas bubble perculation may account for some porous or spongy cokes.
Fluid coking is a continuous process in which heated coker feeds are sprayed into a fluidized bed of hot coke particles which are maintained at 20-40 psi and 500°C. The feed vapors are cracked while forming a liquid film on the coke particles. The particles grow by layers until they are removed and new seed coke particles are added.
Coke for the aluminum industry is calcined to less than 0.5% volatiles at 1300 - 1400°C before it is used to make anodes.
Description of Petroleum Coke CarbonForms
|Needle Coke||Ribbon like parallel ordered anisotropic domains that can also occur as folded structures|
|Lenticular/granular||Lenticular anisotropic domains of various sizes that are not aligned parallel to the particle surface|
|Mixed Layer||Ribbon and lenticular anisotropic domains of various sizes in curved and irregular layered arrangements|
|Sponge||Porous microstructure with walls that are generaly anisotropic but with pores and walls that vary in size|
|Shot||Ribbon and lenticular anisotropic domains arranged in concentric patterns to form shot-like coke|
|Layered||Anisotropic carbon domains aligned in concentric layers parallel to the particle surface similar to an onion-like pattern|
|Non-Layered||Anisotropic domains are not aligned parallel to the particle surface|
|Aggregates||Fragments of anisotropic domains|
|Amorphous||Isotropic carbon form closely associated with parent liquor. Higher in volatile matter than incipient mesophase|
|Incipient Mesophase||Initial stage of mesophase formation. Transition stage between amorphous and mesophase|
|Mesophase||Nemitic liquid crystals. Lower in volatile matter than incipient mesophase|
Gray, R. J. and De Vanney, K. F., 1986, Coke carbon forms: microscopic classification and industrial applications: Int. J. Coal Geol., v.6, p.277-297.
Gray, Ralph J., 1991, Some petrographic applications to coal, coke, and carbons: Org. Geochem., v. 17, no. 4, p.535-555
Gray, Ralph J. and Krupinski, Ken C., 1997, Pitch Production: Supply, coking, optical microscopy and applications: in Marsh, Harry, ed., Introduction to Carbon Technologies, Universidad de Alicante, p. 329-423.