February 2021

Process Optimization

Successes and challenges installing the first proprietary closed-coke slurry system

The authors’ companya licensed a two-drum delayed coking technology process unit to Grupa LOTOS in Gdansk, Poland.

Kaminski, M., LOTOS Asfalt; Manral, V., Heeswijk, B. V., Chevron Lummus Global; Graeter, F., Knuedel, S., TRIPLAN GmbH

The authors’ companya licensed a two-drum delayed coking technology process unit to Grupa LOTOS in Gdansk, Poland. At the engineering design stage of the project, a closed-coke handling system was selected in lieu of a conventional open pit-pad system. The main reason for selecting the proprietary closed-coke slurry systemb (CCSS) was to mitigate the issue of coke fines and volatile organic compounds (VOCs) emissions. The unit was successfully commissioned in 2019. The first implementation of this proprietary CCSS presented a few challenges that were overcome by carrying out some targeted engineering modifications. This article provides a comparison between the conventional coke handling system and the proprietary CCSS. This article also details the successful integration of the CCSS in the delayed coking unit (DCU).

Conventional coke handling system

Either a pit or pad system can be designed for a DCU. Typically, both are considered a “combined pit and pad system.” The combined coke pit and pad system consists of a pit and pad with a bridge crane for movement, storage and reclamation of green coke (FIG. 1). The coke pit and pad have retaining walls on all four sides, with one side being the coke drum structure. The system allows further dewatering of the green coke but requires additional plot space to include the pad next to the pit.

FIG. 1. Schematic flow scheme of an open pit-and-pad and railcar system.

The coke pad is typically designed to store up to three days of coke production. The pit, located just below the coke drum structure, captures most of the coke cutting water and slopes the water towards the maze system. The maze system is designed to provide sufficient residence time to allow the coke fines to separate from the coke cutting water and settle to the bottom of the maze. An overflow weir—with sump pumps—is located at the end of the last maze compartment. The water that separates from the coke fines overflows the weir into the sump, where it is pumped to the hydrocyclone located in a neighboring structure near the maze area. The hydrocyclone further separates the coke fines from the water. The clarified water flows from the hydrocyclone to the clear water tank for re-use in the drilling operation. An overhead cab-operated bridge crane with clamshell bucket spans the entire coke pit and pad. The coke is spread out over the coke pad to dry out and dewater, usually taking up to 24 hr. In turn, the coke is reclaimed after dewatering and is transported by bridge crane to the inlet hopper, crusher, discharge hopper, belt feeder, belt conveying system and then either to a railcar or truck for shipment to the customer.

A different type of coke handling system involves the direct loading of the coke from the coke drum into railcars. However, this is not the recommended system due to many environmental and safety concerns, including hot material and water spillage during railcar loading, and a contamination trail of coke fines, water and dust during railcar movement throughout the plant.

The pit and pad system is a proven system that is widely used in the industry. Emissions from the pit and pad system can be controlled by the application of dust separation systems and good housekeeping.

The alternative to the conventional pit and pad system is the proprietary CCSS, which is further described in detail.

CCSS

The CCSS handles the coke produced in a DCU as a zero-emissions, reliable and safe system. The elimination of emissions of fines to the atmosphere is achieved by the enclosing of all operational steps in a “closed system.” FIG. 2 shows a schematic of the CCSS.

FIG. 2. Schematic of the CCSSb.

In the closed system, the coke and water from the coke drum discharge to a special coke crusher via a transition piece. The transition piece is a permanent and sealed connection between the coke drum bottom unheading device and the coke crusher, capable of tolerating the thermal expansion of the coke drum. The inline coke crusher reduces the size of the coke particles suitable for processing, storing and selling the green coke product. FIG. 3 shows a picture of the transition piece and the coke crusher.

FIG. 3. View of the transition piece and crusher.

Downstream of the coke crusher, a slurry basin collects all incoming streams from the coke drum; water is added to the coke to form a pumpable slurry. A special slurry pump transports the water-coke mixture to closed dewatering bins, where coke and water are separated. Each bin is equipped with internal screens and can hold one batch of coke. The majority of fines produced from the cutting procedure are trapped instantly in this coke bed, resulting in clean water filtrate. The clean water filtrate is collected in the drain water basins and routed to the water settling and clean water tank for final cleaning and re-use for the coke cutting equipment, slurry transport and quenching.

After completion of dewatering, the coke is discharged from the dewatering bin via a vibration feeder, directly onto a belt conveyor, which transports the coke product to the coke storage area.

The integration of the closed system includes the intake of all streams, which are normally routed towards the pit/pad. Simultaneoulsy, the closed system provides clean water to the DCU for quenching and cutting.

The main advantages of the CCSS vs. conventional open systems are:

  • The elimination of dust emissions to the atmosphere due to the closed design
  • Low workforce requirements due to automation
  • Reduced water consumption
  • The reduction of equipment footprint
  • More flexibility in equipment arrangement.

The design made for the Grupa LOTOS Gdansk refinery was a grassroots project and the first of its kind. All new CCSS structures may be installed in the pit/pad area, and integration and re-use of existing equipment can be investigated with a tailored customization. Several revamp studies, with concepts for maximum modularization and pre-fabrication, have already been executed and are of high interest to all units considering the elimination of fine emissions.

Startup challenges and troubleshooting

The Grupa LOTOS DCU, featuring the proprietary CCSS, was commissioned in September 2019. Other than some challenges with the hydraulic decoking system, which had some impact, this first-ever commercial demonstration of the CCSS was an opportunity to detect any other unanticipated issues, which can now be addressed in subsequent designs and implementation. These issues—which mostly involved plugging in parts of the CCSS—and how they were handled are furthered detailed in the next section.

Hydraulic decoking system. During the initial period, the coke cutting pump tripped several times due to high bearing temperature, which caused delays in the decoking time. These delays impacted the coking/decoking schedule, and the unit could not be operated at the unit design throughput. The hydraulic decoking vendor was contacted, and upon the replacement of the shaft, impeller and drilling nozzle, the coke cutting pump tripping issue was resolved.

Plugging of the slurry pump and slurry line. During commissioning, plugging of the slurry pump and slurry line was observed. An additional flushing connection was provided on the suction side of the slurry pump, which helped avoid cavitation and improved stability of the flow pattern. In addition, the flow control logic of transport water was simplified, which contributed to a more stable operation.

Plugging and leaking of the transition piece. During initial operation, plugging was observed in the transition piece, which pushed some coke into the body of the bottom unheading device. The plugging forced a back-up of water and eventually caused a breakthrough, in which a large amount of water and coke would suddenly discharge from the coke drum. This caused the slurry basin to overflow and carry coke particles into the neighboring drain water basin.

From the drain water basin, coke particles were pumped into the water settling tank, which resulted in the blocking of the desludging line. Moreover, some sludge even carried over into the clean water tank. Because of this, routine monitoring of the sludge levels in the water tanks was implemented. This step helped detect abnormal levels of sludge in the water tanks and allowed for corrective action. The desludging line was unblocked, and both water tanks were flushed. It was also recommended to install a coarse screen in the overflow between the basins to keep the coke particles inside the slurry basin. Future slurry basins will be designed to withstand a sudden discharge or breakthrough of the coke drum.

In addition, leakages were observed in the flanges of the transition piece. This issue was resolved by adding hydraulic clamps to the flexible connection of the transition piece, which are activated/tightened during the water quench and coke cutting steps.

Plugging of feed line. During operation, plugging was observed in the feed line. This was mitigated by modifying the decoking permissive to allow water through the feed line during cutting.

Instrumentation. In some events, the sudden filling of the slurry basin resulted in the radar-type level measurement sensor to be “dipped” in the liquid. The radar sensor can also be affected by the presence of steam. These events caused the measurement to freeze at its previous value for approximately 20 min each time and resulted in the level control being ineffective and taken to manual control. Instead of a radar sensor, a second differential pressure sensor can be implemented to alleviate this issue.

Dewatering/coke transport. During startup, the coke discharged to the transport system was still wet and partly unsuitable for inclined conveying.

The quality of coke has an effect on the cutting and dewatering times. Harder coke requires extended coke cutting, and powdery coke needs more time for dewatering. Both effects result in a longer dewatering cycle.

After gaining some experience with the dewatering rates, an improved operating procedure was established, and coke moisture reached an acceptable level.

Takeaway

The first implementation of the proprietary CCSS was successfully integrated with a licensed DCU. Successful execution in commissioning showcased a path to improved sustainability that refiners can follow as an alternative to a conventional open pit/pad coke handling process. The lessons learned during this project have helped improve CCSS engineering design practices. Future projects are expected to have more robust designs that present a much cleaner alternative to an open pit/pad coke handling system. All of the existing open pit/pad systems where refiners wish to minimize coke fine emissions may become candidates for revamp to the environmentally friendly CCSS. HP

NOTES

        a Chevron Lummus Global
        b TRIPLAN’s Closed Coke Slurry System (CCSS

The Authors

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