• Minimize noise, air and water pollution
• Condense and collect steam and hydrocarbon (HC) vapors generated during the steam stripping, water quenching and back-warming steps of the decoking cycles • Condense and collect the steam and heavy HC vapors
generated during coker startup and shutdown, as well as during coke drum overpressure upsets.
The modern CBS was selected for the revamp of the Rom- petrol unit. FIG. 2 shows the sections modified in this project
as shaded in blue. The isolation valves around the coke drums were replaced to allow for safer, more efficient offline drum op- erations. New tie-ins from the CBS to the fractionation section were also done.
PROJECT DEFINITION AND EXECUTION
Rompetrol contracted a technology licensor to do the revamp design of the DCU and replace the existing open-blowdown sys- tem with a new CBS.1 In addition to the blowdown system, other
parts of the unit were upgraded. The manual coke drum isola- tion valves were replaced with MOVs; top unheading valves were installed; and the steam/air decoking system was automated. Other sections of the unit were targeted for upgrading, but were not implemented at this stage. The project goals were to:
• Eliminate the environmental problems
• Upgrade the safety of the DCU by installing modern isolation valves and top unheading valves; bottom unheading valves were targeted for a later implementation • Improve safety by installing an interlock system for safe
coke drum valve operations
• Recover all liquid HCs from the offline coke drum operations for reprocessing within the unit and eliminate regular flaring or venting of gas from the blowdown system
• Recover all water from the offline coke drum operations for recycling through an existing sour water stripper (SWS).
DESIGN BASIS
The DCU’s design fresh feedrate was 143.8 tph (21,937 bpsd) of a mixture of vacuum residue (VR) and FCC slurry oil (95 wt%/5 wt%). The unit products included offgas to the amine plant, coker naphtha, light coker gasoil (GO), heavy cok- er GO and specialty coke. In this case, the specialty coke was not anode grade, but sponge coke that was required to meet spe- cific market requirements.
To determine the new blowdown system capacity, yields were developed for the design feedstock, and the coke drum capacity and cycle time were confirmed. The blowdown system was also designed to process two additional streams:
• An external refinery slop-oil stream with a maximum feedrate of 15 m3/h
• An emergency purge-oil stream from the existing coker heater, which had previously been sent to the open blowdown system. The residence time had to be made available in the quench tower for this potential emergency stream.
FIG. 3 shows a simplified flow diagram of the new CBS.2
Design features. A revamp is frequently more demanding than a grassroots design. Experience is necessary to avoid pit- falls and find the most cost-efficient path. Such projects require team work from the technology provider and the operating company during the project.
CBS. The CBS recovered all offline drum effluents and elimi-
nated flaring. This system recovered all HCs for reprocessing in the unit and water for recycling through a SWS. The CBS was also required to condense and collect the steam and heavy HC vapors generated during coker startups and shutdowns, as well as the external refinery slop oil and heater coil emergency purge.
Relief through the CBS. The CBS also served as a relief
system for the discharge of the coke drum relief valves. Other systems were considered for the coke drum relief valves, such as following the existing practice of discharging the relief valves to the fractionator. If the relief valves open, this practice can result in a major cleanup of the fractionator bottom section, and it may result in tray damage.
A preferable solution is to discharge the coke drum relief valves to the quench tower, where a similar cleanup may be required. However, with simpler and more robust internals, the quench tower should be more readily cleaned and able to avoid damage.1
In addition, the existing flare-header design temperature was low, and cooling of the coke drum relief load was required. Relief through the CBS condenser allowed this consideration to be met.
Dual-duty blowdown condenser. The relief load cooling
requirement required a disproportionately large blowdown con- denser. This became an issue due to space limitations. However, the location of the large blowdown condenser and associated piping was carefully managed during detailed engineering and was fitted into the available plot space.
Relief devices in the CBS. Normally, because of the robust
nature of coke drums, the set pressure of the coke drum relief valves is high enough so that relief valve settings in the CBS do not pose a backpressure problem. Typically, the coke drum relief valves discharge to the CBS, and the relief valves in this system discharge to the flare. For a revamp situation, the set pressure of the existing coke drum relief valves are frequently lower than
Fractionation and preheat section Gas recovery plant Furnace and coke drum section Closed blowdown system Fresh feed Furnace charge Coke drum (offline) Unstabilized naphtha Lean sponge oil
Stabilized naphtha Fuel gas C3/C4 Heavy coker GO Light coker GO Rich sponge oil
Coke drum vapor Sour water Sour water Fuel gas Velocity steam Offgas Sour water Steam from water quench Backwarm effluent
Quench water Steam-out
steam Coke product Light slop oil Heavy slop oil
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that for a grassroots unit, and care must be taken to ensure that there will be no backpressure problems due to the relief valve settings in the CBS.
In Rompetrol’s case, the existing coke drum relief valves were set at 5 barg. To satisfy the backpressure requirements on these relief valves and the maximum pressure drop through the CBS during relief, it was decided to use a high-integrity pressure sys- tem (HIPS) valve on the settling drum. As long as the HIPS valve is set up appropriately, it will have an equal or higher reli- ability than a regular relief valve.
Gravity-drain blowdown header. A major advantage of the
new design is that the condensate drum and pumps used for send- ing backwarm liquid to the quench tower are not needed. This system is known to be prone to operating difficulties that were simply avoided in this revamp. Instead, a gravity-drain blowdown header was installed so that the coke drums could be drained to the quench tower. To achieve this, the elevations of the coke drums, the new blowdown header and the inlet to the new quench tower had to be carefully evaluated during an onsite review.
Gravity-drain backwarm to fractionator. Gravity-draining
the backwarm condensate to the existing fractionator was also assessed as feasible, and it was included in the design. Gravity- draining the coke drums to the quench tower and the fraction- ator is a proven concept.1
Low-cost depressuring of coke drums. In many DCU re-
vamps, an ejector is included in the CBS to depressure the coke drums before venting to atmosphere. A vapor line from the set- tling drum is typically tied into the flare header. In Rompetrol’s case, the existing flare-gas-recovery compressors were used to reduce the pressure in the CBS and coke drum prior to isolating the coke drum from the CBS and venting to atmosphere. The ejector option was not required.
Safety interlock matrix. A safety interlock system was in-
stalled for the coke drum isolation valves. This was based on the matrix provided in the licensor design package, and it was expanded during detailed engineering by Rompetrol and the local contractor. The provided matrix is the minimum required to avoid sending HCs to atmosphere, and it is frequently ex- panded to avoid upsets due to operator error. This is fine as long as the system does not become so complex that it limits operational flexibility.
Automation of coke drum structure operations. The level
of automation of the coke drum switch and isolation operations was considered. It was recommended that the board operator and structure operator work together to confirm via both DCS screens and local observation that the coke drum operations are conducted safely.
The board operator supervises the operation and is in radio contact with the structure operator. This operator acknowledg- es that required procedures have been met and then authorizes the structure operator to activate the appropriate motor-oper- ated valves from local panels on the switchdeck. The structure operator also manually turns the appropriate small steam-purge valves associated with the major valves and piping. The struc- ture operator performs the actuation of valve movement from local switchdeck panels, and not the board operator. The pro- cedure is done so that proper valve movement can be visually confirmed. The board operator should confirm the new valve position on the DCS screens.
In Rompetrol’s case, not all coke drum isolation valves were automated at this stage (e.g., drain, steam and water valves). The structure operator was still required to manually turn some of the isolation valves and report to the board operator when completed.
Water handling. Modifications to the existing water han-
dling system including the fines settling basin and quench water storage tank were proposed. These changes were not required by Rompetrol.
Operating guidelines. The licensed package included op-
erating guidelines from which Rompetrol developed its own detailed operating instructions. The addition of the new CBS required a philosophical change in the way some operations were completed. For example, backwarming procedures for coke drum warmup during startup and normal operation were significantly impacted. The coke drum quench procedure was also changed. Instead of overflowing the coke drums at the end of the water quench, they were now filling the drums to about 2 m above the coke bed, pressure draining while adding top wa- ter, then venting to atmosphere at less than 0.14 barg.
Inspections and startup. Piping modifications were made
to accommodate the new MOVs and the tie-ins to the new CBS. Licensor inspection required changes primarily to ensure that the risk of plugging with batch usage was minimized, particu- larly for the switchdeck piping. Also, coke drum thermal growth issues were identified at the cutting deck that required and re- ceived attention. Rompetrol worked extremely efficiently in making some piping changes and adding steam purges prior to commissioning and startup.
Rompetrol and the DCU technology licensor worked to- gether during precommissioning and startup, and a smooth startup was achieved on April 30, 2013.1 The performance test,
conducted on Oct. 29–31, 2013, comfortably demonstrated that the new CBS could support the targeted fresh feedrate. Unit performance. The main factors affecting CBS perfor- mance, for a given coke drum size, are the quench time and the backwarm time. During the performance test, the time duration for these operations did not exceed the design time durations.
Blowdown quench tower Condensed water to SWS Settling drum From coke drums FC FC
FC Heavy slop oil to coke drum overhead quench oil
To fractionator overhead condenser
Light slop oil to fractionator or quench tower LC LC To flare PC Blowdown condenser PC FIG. 3. New CBS.2
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As long as the design time durations are not exceeded, and there is no overlap in offline drum operations, the CBS should be ca- pable of supporting higher fresh feedrates to the unit, with an associated lower cycle time. TABLE 1 lists the performance test
durations for offline coke drum operations.
Coke drum capacity. The selection of the type of coke drum isolation valve (ball or wedge plug) can impact the amount of fines carried over from the coke drums. With the installation of steam-purged ball valves, steam-purged unheading valves and ad- ditional line steam purges, the amount of steam entering the coke drums can be higher than in previous operations. This impact can be much higher if the unheading valve steam purge increases over time due to seal wear and upsets. The increased steam con-
sumption is usually more of a concern for the bottom unheading valves. Regardless, the total amount of steam entering the coke drums should be carefully determined. In Rompetrol’s case, the additional steam resulted in a slightly increased coke drum vapor velocity that did not increase fines carryover significantly. Coke morphology. Shortly after startup, shot coke was pro- duced for several drums and represented a major issue as the coke market required sponge coke. The shot coke produced was formed due to a high percentage of asphaltenes in the feedstock.
To suppress the shot coke formation, in addition to adjust- ing coke drum operating conditions, FCC slurry was introduced into the feedstock so that sponge coke was again produced. Al- though not part of the initial scope of work, Rompetrol consult- ed with the DCU licensor on this issue.1 The traditional “rule of
thumb” for predicting whether shot or sponge coke will be made is to calculate the mass ratio of CCR to asphaltenes in the feed. If this ratio is less than 2, production of shot coke is likely. This rule is neither accurate nor particularly useful in Rompetrol’s case be- cause asphaltene analysis of the feed was not typically done, and it would take about three days to perform.
A more reliable approach to determine the morphology of the produced coke is based on parameters that are readily avail- able for most feedstocks. For unusual or specific feeds, labora- tory analysis is performed on the DCU feedstock to confirm the predictions and to account for commercial operation.
Review. The DCU revamp was successfully executed by Rom- petrol, and is presently meeting all the project goals. The success of this revamp project was largely due to the effective teamwork between Rompetrol, local contractors and the DCU technology licensor. The cooperative nature of this team allowed the proj- ect to be defined and executed efficiently. More importantly, the unit started up safely and performs satisfactorily.
NOTES
1 Bechtel Hydrocarbon Technology Solutions (BHTS) purchased ThruPlus tech-
nology from ConocoPhillips in 2011.
2 BHTS closed blowdown system.
CRISTIAN BOLOHAN is the process director within Rompetrol Rafinare, company member of The Rompetrol Group. He joined the Rompetrol team in 2003. During the development and modernization of the Petromidia refinery, as progect manager, he coordinated and implemented projects. Mr. Bolohan graduated from the Faculty of Physics, Chemistry and Technology of Processing Crude Oil and Petrochemicals within Ovidius University, Constanta, Romania, and he holds an MS degree in oil and gas management. In his present position, he is responsible for the design, coordination and success for the development plan of the Petromidia refinery, in accordance with the Rompetrol Group strategy, as well as the optimization of the refining processes within the business unit.
LUMINITA MANAFU has over 30 years of experience in operations and process engineering for the delayed coker, amine and sulfur recovery, flare-gases recovery units with Rompetrol Rafinare S.A. She graduated in petrochemical engineering from the Oil and Gas Institute, located in Ploiesti, Romania. She has been involved in many projects, startup and performance testing at the Petromidia refinery. JOHN D. WARD has over 35 years of experience in process engineering design and operations, particularly in refining and petrochemical operations. He has specialized in the ThruPlus delayed coking technology for more than 15 years, and has been involved in many delayed coker startups and performance tests. As a coking technologist with Bechtel Hydrocarbon Technology Solutions, Inc. (BHTS), he has led the production of process design packages for licensees and provided technology support for business development. Mr. Ward holds a BS degree in chemical engineering from the University of Manchester, Institute of Science and Technology, UK.
TABLE 1. Performance test times for offl ine drum operations
Steam out 2 hours
Water quench 5 hours 20 minutes
Drain 2 hours (ranged from 1.4 to 2.7 hours)
Unheading and coke cutting 3 hours
Standby 2 hours
Reheading and pressure test 1 hour
Backwarm to CBS 2 hours (ranged from 1.5 to 2.5 hours)
Backwarm to fractionator 6 hours (ranged from 5 to 7 hours)
Total 23.5 hours (ranged from 22.6 to 24 hours)