Page 241 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.181

COKE

Coke is conventionally manufactured from coal by using the beehive
process—a small batch process that tends to produce very large
amounts of pollutants.
Coke is also produced from coal in the by-product ovens in which the
coal charge is heated on both sides so that heat travels toward the center
and thus produces shorter and more solid pieces of coke than are made in
the beehive oven. Air is excluded so that no burning takes place within the
oven, the heat being supplied completely from the flues on the sides.
About 40 percent of the oven gas, after being stripped of its by-products,
is returned and burned for the underfiring of the battery of ovens, and
some is used for fuel gas locally.
Coke is also produced from petroleum, and it is the residue left by the
destructive distillation of petroleum residua.
The composition of petroleum coke varies with the source of the crude
oil, but in general, large amounts of high-molecular-weight complex
hydrocarbons (rich in carbon but correspondingly poor in hydrogen)
make up a high proportion. The solubility of petroleum coke in carbon
disulfide has been reported to be as high as 50 to 80%, but this is in fact a
misnomer, since the real coke is the insoluble, honeycomb material that is
the end product of thermal processes.
Petroleum coke is employed for a number of purposes, but its chief use
is in the manufacture of carbon electrodes for aluminum refining, which
requires a high-purity carbon, low in ash and sulfur free; the volatile mat-
ter must be removed by calcining. In addition to its use as a metallurgical
reducing agent, petroleum coke is employed in the manufacture of carbon
brushes, silicon carbide abrasives, and structural carbon (e.g., pipes and
Rashig rings), as well as calcium carbide manufacture from which acety-
lene is produced:

Coke → CaC
2
CaC + H O → HC≡CH
2 2

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