Preventing emissions in coke removal
A closed, automated coke removal system aims for gains in environmental, economic and safety performance
ARTUR KRUEGER, BERND LANKERS and JOSEF WADLE
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Conventional delayed coker units feature open pit/pad coke handling methods, utilising a range of mechanical equipment including bucket cranes, front loaders, offsite crushers, dusty loading facilities and blackened railroad shunting or truck loading areas.
In contrast, the Closed Coke Slurry System (CCSS) developed by TriPlan in Germany offers the operator a modern process that is completely enclosed. It is environmentally friendly by eliminating coke dust emissions, and it also provides effective water management by cleaning and recycling the water used in the coke quenching and cutting process.
The CCSS has completely automated operations, the plant components are designed and fabricated with a view to high reliability, resulting in significantly enhanced overall plant reliability. The safety aspects attained in this design find a high acceptance level by the operators as well as the workforce. Streamlined operations result in reduced cycle times for coke drum unloading and dewatering, providing an interesting pay-back to the operator.
In the hydrocarbon processing industry, the conversion processes, from crude to clean products, are continuous. Residues from atmospheric or vacuum units are converted by a number of different processes into added-value products, to obtain maximum benefit from the bottom of the barrel. One of these conversion methods is the delayed coker unit (DCU). The product obtained from this unit is commonly known as petroleum coke which serves as the feed for a number of different applications.
Unlike the other processing units in the refining business, delayed coking is a discontinuous batch operation. Full coker drums are opened on a regular basis for unloading. Challenges come from the process itself, the metallurgy of the equipment, and safety, health and environmental issues in view of the high degree of mechanical handling steps that are typical for a traditional pit/pad system. Consequently, field operators face more manual work in an uncomfortable, and sometimes risky, work environment that is not known in other refinery units. Staffing figures for operators, maintenance staff and additional housekeeping contractors are traditionally high and result in a substantially increased workload.
These cost elements are considered unavoidable and are tolerated by operating companies. However, coker outages due to breakdown and repair of mechanical equipment mean loss of coke production capacity, and clean product uplift value deteriorates dramatically, eating into the unit’s revenue stream.
Traditional processing in coke drums
The process step from liquid coker feed to the solid coke formed in the coke drum is independent, whether it involves the established pit/pad design or CCS technology for handling the coke after cutting.
The feed is heated to commence cracking, ultimately to extinction to carbon due to the long residence time in a drum. Once a drum is filled up, the feed stream is switched to the next empty drum and so on. Therefore a delayed coker consists at least of two, sometimes of four or even six drums.
After switching, the full drum is isolated from the rest of the unit for further processing. With injection of steam into the coke bed, light hydrocarbons are stripped out, then the bed is hardened and cooled. Further cooling is achieved through repeating quenching (filling and emptying) with water. Once the bed temperature falls below 100°C, the drum top and bottom flanges are manually opened (more recently, remote operated slide valves are commercially available to replace the risky flange operation).
Coke removal takes place with a high pressure cutting water circuit at 250-300 bar. First, a vertical hole is drilled into the coke bed with a downward water jet nozzle. This is followed by the cutting operation in which a horizontal water jet slices top down to ream out the drum up to the wall until the drum is empty. The resulting coke chunks are flushed through the drilled hole.
Open coke handling is a burden on the environment
Delayed coker operators with a pit/pad system have made efforts, together with mechanical equipment suppliers, to improve the mechanical steps in the process, in particular with installation of top and bottom unheading equipment. Unit on-time capacity has been increased, contributing to the unit’s cash flow. At the same time, environmental concerns have been addressed, as well as the creation of a healthier workplace.
In the traditional pit/pad process, coke handling from the outlet of the coke drum after cutting, to loading onto trucks/railcars, happens in open space. The free stacking of hot coke during the cutting operation and storage of the coke from one coke drum (typically 1000 t/d) result in a highly visible steam plume for several hours. During this time, the coke stack cools down and drains excess free water to the ground or into a receiving maze, usually uncovered. It is known from investigative reports that the steam contains coke fines and aerosols, and also aromatics and other polycyclic hydrocarbons.
US federal and state regulations, and European Community directives on environmental issues in the hydrocarbon processing industry, call for a critical review of all possible sources of air emissions and ground pollution, such as the mechanical equipment mentioned above, including black water laden with coke dust and fines, throughout all stages of coke handling and storage.
Technical solutions to alleviate these problems, for example the enclosure of coke piles, water/oil spraying, are all aimed at fighting the problems at the back end rather than at the source. They are costly but can still be insufficient.
Closed coke slurry technology
In Europe, coker operators have to demonstrate that their current practice is in accordance with best available control technique (BACT). This is where TriPlan CCSS technology comes into play. It is a completely enclosed system, avoiding the mechanical, environmental and health drawbacks which are inherent in conventional pit/pad systems.
In all coke handling steps, from the discharge point at the bottom of the coke drum to the coke to load-out, the water circuits are designed for re-use of contaminated water in the process after clean-up, and the effective separation and removal of coke fines. These steps have been streamlined at all stages of the process, making it no different from any other typical process unit in the refinery. This is achieved by pumping the coke slurry stream in a fluid state. With consistent improvement of metallurgies in the equipment that forms part of the CCSS, the process is robust and stable.
An installation has been operating for more than six years without interruption for repair or maintenance work in service in a refinery in Germany. The instrumentation in this process permits a fully controlled operation and allows the console operator to view the status of the process at any time.
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