Coker

Also known as:

Delayed coker, DCU, Flexicoker, Fluid coker

The coker is the most extreme of the bottoms upgrading refinery 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 (petcoke).

The major contribution from the coker is that it allows a refiner to eliminate most or all of the low valued bottoms from crude 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 a thermal process, using heat to cause the heavy feed stock 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.

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 the vast majority 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. FCC slurry generates a high amount of coke, so it is typically not fed to the coker unless there is no other outlet.

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)

  • petcoke - Essentially a byproduct from the coker. This solid carbon is typically sold as a fuel for power plants and cement plants.

  • coker 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.

Economics

The value of a coker comes from its ability to completely eliminate the heaviest distillation fractions, converting the mostly to lighter fractions that can feed other conversion units, with the remainder converted to refinery gas and petroleum coke.

A coker allows a refiner to process cheaper, heavy crude oil and still make little to no low value residual fuel oil. This makes the margin for a coker very sensitive to the light-heavy differential. This spread between light products and heavy fuel oil also sets the spread between light products and heavy crude. Heavy crude discounts essentially represent the lower value from the fuel oil that would be made from processing these crudes (in the absence of upgrading). A times when the differential has been at its widest, cokers have been extremely profitable to refiners.

The loss of product volume across the coker, stemming from the large about of volume that is lost to refinery gas and pet coke production, makes coker margins sensitive to absolute prices of crude and product. When prices are high, this volume loss translates to a higher cost of running the coker and requires even wider light-heavy differentials for the coker to maintain profitability.

While the byproduct coke produced by the coker has some value, it is far less than the fuel oil value of the resid that was used to make it. So refiners will always try to minimize the production of coke in a fuels refinery. The exception to this are the very limited number of small cokers that are dedicated to making specialty grades of coke used to make anodes and electrodes.

Technolgy types and licensors

Variations on the delayed coking process are the flexicoker and the fluid coker. A flexicoker includes a coke gasifier that continuously converts the petcoke 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 provided fuel for the coker itself.

A number of companies license coking technologies

  • Chevron Lummus Global (CLG) - Delayed coking technology

  • UOP - Delayed coking technology

  • Amec Foster Wheeler - Delayed coking technology

  • Bechtel - Thru Plus delayed coking technology

  • ExxonMobil - Flexicoking fluidized bed coking and integrated gasification