Coke is produced from residues of crude oil distillation. The crude oil is first water-washed in a de-salter to remove solids and salts. The desalted crude oil is preheated, and then vaporized in a fired heater. The vaporized oil is fed to a distillation tower, where it cools and condenses as it flows up the tower.
The crude oil remaining at the bottom of the distillation tower, knows as “bottoms” or “reduced crude” is routed to another unit. In this unit the bottom oil is reheated and subjected to vacuum in another distillation tower. The vacuum causes the oil to boil at a lower temperature and allows distillation of additional intermediates, without thermally decomposing the still valuable reduced crude to carbon (or coke).
Once the bottoms from the vacuum distillation operation cannot be further distilled, the “vacuum residuum” is routed to the next unit in the process, the Coker.
In the delayed coker, the incoming residuum is mixed with coker gas oils (CGO) then fed to heaters. In the heaters, the heavy oil is thermally cracked into vapor and liquid. The vapor/liquid from the heaters flows into a coke drum, where the liquid drops solidify (eventually filling the drum), and the vapors are returned to the distillation towers. By progressively condensing the vapor coming from the coke drums in a distillation towers, useful intermediates are collected for further processing. Once the coke fills the drum it must be drilled out.
Coke drums are usually operated in pairs with an associated heater. While one coke drum is filling, the other is being drilled. So, when one drum is full, the flow from its associated heater is routed into the other drum that is empty and ready for filling. The full drum is isolated and cooled. The collected “green coke” (also known as sponge coke or shot coke) is drilled out of the drum with high-pressure water. Once the drum has been completely drilled out, it is preheated with vapors from the associated heater and it is then ready for refilling as soon as the other drum is full.
The green coke is collected in a containment basin, where it is allowed to drain. Then it is reclaimed from the “coke pit” with clamshell bucket gantry cranes, and conveyed to covered storage facilities.
Green coke with high metals content is used for burning and will not be calcined. This green coke is called fuel grade coke.
Green coke with low metals content (needle coke) is referred to as anode grade coke. The green coke must have sufficiently low metals content in order to be used as anode material.
Anode grade coke is calcined and used to make anodes for the aluminum, steel and titanium smelting industry.
The calcining process takes anode grade green coke from the oil refining process and converts it to almost pure carbon, with a defined structure. This is essentially a time-temperature function with the most important control variables being heating rate. To obtain the calcined coke properties required by the industries, the coke must be subjected to temperatures of 1200-1350 Deg C or higher to refine its crystalline structure. This is usually done using a rotary kiln.
When the hot calcined coke leaves the kiln, it is transferred to a rotary cooler. In the cooler, the hot coke is quenched by water, sprayed from a number of nozzles. The exit temperature is controlled at approximately 150 degrees C to assure a moisture-free product.