Hydrocarbon processing in today’s petrochemical industry demands greater process control, as well as an increase in process efficiency and safety. Coking is a refinery unit operation that produces petroleum coke. According to the U.S Energy Information Administration (EIA) web site, the two types of coking processes are delayed coking and fluid coking. Both processes thermally crack feedstock into products such as naphtha and distillate, leaving behind petroleum coke. With delayed coking, coke drums are used to hold the heated feedstock while the cracking takes place. Coke is deposited in the coke drum as a solid and is removed by hydraulically cutting the coke using water and by diverting the hot feed from one coke drum to another, alternating the drums between coke removal and cracking.
Many operators of coke drums require a general overview of the height of materials and the rate of level as the process changes. The purpose of controlling the contents within a coke drum is to:
- maximize the outage (amount of hydrocarbons treated during each batch),
- minimize the anti-foaming quantity to be used, and
- avoid “foam over.”
Two technologies are usually applied to coke drums to determine their contents: neutron backscatter and gamma transmission. Neutron backscatter can detect vapors, foams, liquids and solids through any vessel thickness to provide rapid indication of process changes. Gamma transmission offers the ability to look at the height of foam, coke and water during the coking cycle, while providing vapor density compensation. When neutron backscatter and gamma transmission methods are used in conjunction, called “Hybrid Coking Level,” they can offer superior insight into the process changes than if either method was used alone.
A hybrid approach like this:
offers the advantages of both systems. At the top of the span, an 8 foot or 10 foot continuous level gauge can track the foam/air interface to help the operators against foam over. Redundancy at the top is highly recommended due to the cost associated with “foam over.” That is, there should be a neutron backscatter devise and a gamma transmission continuous level device. The neutron gauge will identify what products are at that level and the transmission device will provide information on the progression of the front of material to the operators. Also, if one gauge fails, the operators are left with another measuring system to handle the coker. In many cases, operators wish to track the rate of foam increase over a larger area, up to 48 feet. Gamma transmission is used in conjunction with neutron backscatter for the larger area coverage. Multiple gamma detectors are cascaded to cover the increased level span along with three to four neutron backscatter units.
Using neutron backscatter in conjunction with gamma transmission (Hybrid Level) offers coke drum operators the most efficient and safe drum operations via:
- Monitoring rate of foam buildup
- Measuring density of foam for proper anti-foaming use age
- Ability to use higher levels of drum utilization (closer to chord line)
- Offer preventive monitoring controls for foam over protection.
Read the next installment in this series which reviews the neutron backscatter measurement principle in more detail, or read the application note, Hybrid Solution for the Analysis for Coke Drum Applications. Learn about density and level instruments that offer the best monitoring scheme for both exact control and an overview of what is happening in the delayed coking process.