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Coker
Also known as: delayed coker, DCU
In a refinery, the coker is the most extreme of the bottoms upgrading process units. A coker takes the lowest value bottoms material (vacuum resid) and cracks it to the point that all of the resid is eliminated, yielding only lighter fractions and solid carbon (pet coke).
The major contribution from the coker is that it allows a refiner to eliminate most or all of the low-value bottoms from crude oil to achieve higher yield of lighter products. This in turn allows the refiner to process cheaper heavy crudes that have a high proportion of resid material, and still achieve high yield of valuable light products.
How it works
Most cokers are delayed cokers. This is purely a thermal process (non catalyst), using heat to cause the heavy feedstock to crack into a range of lighter components as well as a significant amount of petroleum coke (solid carbon). The mixture of converted material flows into large drums, where the solid coke is allowed to settle and the lighter liquid/vapor is drawn off and sent to a fractionator for separation of the liquid products. When a drum fills with coke, feed is switched to another drum. The full drum is cooled with water, then opened so that the solid coke can be drilled out using high-pressure water jets.
Variations on the delayed coking process are the flexicoker and the fluid coker. A flexicoker includes a coke gasifier that continuously converts the pet coke into a fuel gas, avoiding the need to accumulate coke in a drum and creating a significant source of (low Btu) fuel for the refinery. The fluid coker is a scaled-back flexicoker that gasifies only enough coke to provide fuel for the coker itself.
Inputs
The inputs to the coker are heavy resid material from the vacuum unit and some conversion units. Typical feeds include:
- Vacuum resid - This bottoms fraction from the vacuum tower makes up most of the feed to the coker
- FCC slurry - This bottoms material from the FCC is sometimes fed to the coker if there is no higher value use for it in fuel oil blending
Products
The major products of the coker are largely feedstocks to other refinery process units. These are:
- Coker naphtha - A heavy naphtha range material that is fed to the reformer, usually via a naphtha hydrotreater to saturate the high level of olefins in coker naptha
- Coker distillate - A cracked diesel range material that must go through a hydrotreater before blending into finished diesel
- Coker gasoil - VGO type material that is fed to the the cracking units (FCC and/or the hydrocracker)
- Pet coke - Essentially a byproduct from the coker. This solid carbon is typically sold as a fuel for power plants and cement plants
- Cracked gas - Gases from the coker contain a mixture of C1 to C4 material (including unsaturated olefins) as well as hydrogen (H2) and CO2. Coker gas is typically sent to the unsaturated gas plant for separation
