The Science of Coker Heaters

      During the heating of the crude, heavy molecules and asphaltenes are converted to lighter products and coke. Delayed coking is so named because the process is designed to separate out the coke from the lighter components in a large holding drum that can then be directed to other parts of the refinery. The coke is then removed from the holding coke drum.

     The coke stuck in the heater tubes must be cleaned out. One popular process used is mechanical pigging. Pigging involves shooting a compressible, solid cylinder with tungsten carbide teeth through the tubes that tear off or clean out the coke. On average, this cleaning process typically takes four days per shutdown, and is required every three to 12 months, depending upon the type of oil used at the refinery and the process conditions. The length of the downtime created by these cleaning operations costs the refinery millions of dollars in lost output from each delayed coker heater every year. There are about 200 coker heaters in North America and over 375 worldwide.

      Coker heaters can produce between $30 and $360 million in profit per year at a refinery, making it the highest profit center at the refinery. Coker heaters are able to utilize the heavier crudes that refineries are now having to process. Heavy crudes contain long hydrocarbon chains that must be heated to a high temperature in order to be turned into useful products. Heating the oil until it separates at a molecular level is called thermal cracking. This is done by running the heavy oil, called “resid” through approximately 3,600 feet of metal tubes in a very large heater, which heats the oil to about 900-950 degrees F.

      Generally, fouling often occurs because sulfur in the resid chemically attaches to the surface of the tube ("sulfidation"). It is this sulfide layer that is a precursor to coke formation. Once coke begins to form, it continues to build up in ever increasing layers. Since coke is an effective insulator; its build up inside the heater tube requires the exterior of the tube to experience temperature increase so that the internal temp of the tubes can be maintained at the required temp for thermal cracking. Once the outside temperature of the tube reaches between 1300 and 1350 degrees F, the refinery becomes concerned that higher heat could cause metal fatigue and deformation, so refiners shut down production for an average of three to four days for cleaning and refiring. If these temperatures are exceeded, the tube can fail causing a fire and potentially catastrophic damage. C2’s surface treatment prevented sulfidation and stabilized the attachment of coke to the inside of the heater tubes.