UlSD Hydrotreater Challenges Overcome to Improve on Stream Factor - MEPEC 2013
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Transcript of UlSD Hydrotreater Challenges Overcome to Improve on Stream Factor - MEPEC 2013
Striving for Excellence
1
ULSD Hydrotreater Challenges Overcome
to Improve On-Stream Factor
Alpesh Gurjar
The Bahrain Petroleum Company (Bapco) Kingdom of Bahrain
2 October 2013
Striving for Excellence
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Agenda
• Introduction
• Background
• Simplified Process Flow Sketch
• Challenges
• Lessons Learned and Concluding Remarks
Striving for Excellence
Introduction
• Bapco operates the Bahrain Refinery - capacity 265,000 BPD
• Produces 100,000 BPD of ULSD, of which 70% comes from the ULSD
Hydrotreater (2HDU)
• 2HDU history and operation:
– Commissioned as VGO Hydrotreater (52,000 BPD) in 1972
– Revamped to Mild Hydrocracker in 1983
– Revamped to 70,000 BPD ULSD Hydrotreater in 2007
• Sustained and reliable operation of 2HDU has great influence on refinery performance
Sulfur PNA, wt% Cetane Index T95%, °C
Feed 1.70 wt% 13 50 367
Diesel Product <10 wppm 2 55 min 357
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Striving for Excellence
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Background to 2HDU Revamp
• 2HDU was revamped with minimum capital investment - maximum re-use
of existing equipment, including recycle gas compressor (RGC)
• Two additional reactors and a stabilizer added, product stripper revamped
• Catalyst target life - 2 years with 4 years unit operating cycle
• Unit has been onstream for 6 years without T&I or catalyst changeout
• T&I in 2014 - 6½ year cycle - unprecedented in 40 years of unit’s history
• Outstanding on-stream factor achieved, with a proven safety record
Striving for Excellence
Rich Amine
LP Amine
Absorber
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Process Flow Sketch - Reaction Section
Feed Heater Feed Heater
Feed
Feed Filters
550# Steam
Condensate
Surface Condenser
PSA
Lean Amine
HP Absorber
HPS
LPS
To Stripper
Water Injection
Make-up H2
RGC
Turbine
Second Stage Reactors
First Stage Reactors
Recycle H2
Water Injection
New Equipment
Existing Equipment
LPS
Striving for Excellence
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Process Flow Sketch - Recovery Section
From LPS Stabilizer
Stripping Steam
Stripper
To DEA Unit
Diesel Product
Wild Naphtha
Stabilized Naphtha
To Flare
New Equipment
Existing Equipment
Striving for Excellence
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The Challenges
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1. Feed Filtration • Problem
– High frequency of filter fouling → low feed pump suction pressure
– Unit slowdown → increased operating cost
– Frequent filter cartridge replacement → high maintenance costs
– Filter fouling worsens due to crude unit shutdowns & start-ups
• Corrective Actions
– Filter cartridge changed from depth to pleated type - 50% cheaper
– Separate 2HDU feed and SR diesel rundown tanks - enhanced settling time
• Results
– Increased life with use of pleated type filter cartridge
– Feed tank switchovers reduced fouling and filter cartridge replacement
– Minimised impact of crude unit shutdowns/start-ups
Striving for Excellence
2. Reactor Feed/Effluent Exchanger Fouling
• Problem
– Original design → makeup H2 pre-mixed with feed downstream of
feed/effluent exchangers
– Fouling in preheat trains → unit constraint towards End-of-Run
– Increases feed heater load, could force a unit shutdown unless mitigated
• Action
– Make-up and recycle H2 routings modified → Makeup H2 and a part of recycle H2 pre-mixed with feed upstream of feed/effluent exchangers
Reactor Effluent
To Feed Heater
Makeup H2
Diesel Feed
To Feed Heater
Makeup + Recycle H2
Reactor Effluent
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Diesel Feed
Striving for Excellence
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2. Reactor Feed/Effluent Exchanger Fouling (cont’d)
• Results
– Effect of H2 mixing with feed upstream of feed/effluent exchangers:
– Improved velocity → reduced shellside fouling
– Improved heat transfer coefficient
– Satisfactory performance of preheat train → exchangers not cleaned since 2007
Striving for Excellence
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3. Reactor MPT Limitation
• Problem
– Minimum Pressurisation Temperature (MPT) – temperature to which
reactor wall must be heated before pressuring to >25% of design
pressure to avoid brittle fracture
– 1st stage reactors are new with low MPT (100°F @ 300 psig)
– 2nd stage reactors are old with high MPT (350°F @ 330 psig )
– High MPT causes problems during start-up:
Risk of MPT violation during feed cut-in → safety issue
Potential risk of catalyst reduction at T’s >700˚F
RGC speed limitation at lower than specified suction pressure
Extended start-up duration
Striving for Excellence
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3. Reactor MPT Limitation (cont’d)
• Corrective Actions
– Conservative margin above MPT – for cooling effect due to feed cut-in
– DMDS injection into recycle gas loop at outlet of feed heater
• Results
– Minimised risk of MPT violation
– Potential catalyst reduction avoided
• Path Forward
– 2nd stage reactors permanently bypassed – sufficient volume in 1st stage
reactors to meet diesel specs with latest generation ULSD catalyst
– Bypassing 2nd stage reactors will:
Eliminate RGC speed limitation, enhance safety and reliability
Reduce start-up duration by approximately one day - cost benefit
Striving for Excellence
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4. NH4Cl Formation in Reactor Effluent Exchangers
• Problem
– Chloride (0.5 wppm) in feed causes ammonium chloride (NH4Cl)
deposition in effluent side (tubeside) of exchangers
Substantial increase in pressure drop (~80 psi)
Reduced H2/Oil ratio - increased catalyst deactivation rate
Could force unit shutdown unless mitigated
• Corrective Actions
– Initially NH4Cl was removed by hot hydrogen stripping
During hot strip, feed cut out for 4 days – significant economic penalty
– Online water wash facility installed - used intermittently for NH4Cl removal
Striving for Excellence
• Corrective Actions (cont’d)
– Challenges encountered in design of water wash system include:
Selection and orientation of injection nozzle, space constraints, water corrosion and potential exchanger metallurgy upgrade
• Results – Eliminating feed outages – Increased average H2/Oil ratio
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4. NH4Cl Formation in Reactor Effluent Exchangers (cont’d)
Water Injection Point Water Injection Nozzle
Striving for Excellence
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5. 2HDU operation with 99.9% makeup H2 – RGC Limitation
• Problem – Design makeup gas purity is 95% maximum
– Unavailability of 93% H2 from #1H2 Plant → 2HDU shutdown
– Use of 99.9% H2 from #2H2 Plant reduces recycle gas SG <0.19
– Unstable RGC operation at SG <0.19
• Corrective Actions – Gas SG maintained >0.19 using N2 injection into recycle gas loop
– Purge rate reduced and adjustment of HP absorber operation
• Results – 2HDU operates successfully at 70% throughput when #1H2 Plant
shutdown → higher profits
– Increased unit reliability → elimination of intermediate shutdowns and startups
Striving for Excellence
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6. Product Stripper Operation and Control
• Problem
– Designed to operate with total reflux → no “Wild Naphtha” product
– Design top temperature → 90 degF above water dewpoint
– In practice, top temperature falls below water dewpoint at total reflux
Risk of corrosion
• Corrective Action
– Top temperature increased to provide 15 degF margin above dewpoint
• Results
– Minimised potential risk of corrosion in column overhead
– Undesirable “Wild Naphtha” produced but manageable
Striving for Excellence
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7. Stabilizer Operation
• Problems
– Limitation in reboiler firing to achieve required column bottom
temperature - diesel water spec (50 wppm) could not be met
– Column operation unstable at low throughput
• Corrective Actions – Ultimate capability of burners established
Allow reboiler outlet temperature increase
– Column now operated at lower than design operating pressure
• Results – Required diesel water specification is achieved
– Column operation stable but root causes of instability at low
throughput still being investigated
Striving for Excellence
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Lessons Learned and Concluding Remarks
• Ensure feed impurities are fully defined and impact on operations fully
understood and addressed at design stage
• Always take start-up requirements into account during design phase
• 2HDU’s success over the last 6 years was achieved because we were
able to solve difficult problems mostly on the run
• Detailed process monitoring and optimisation, and prompt troubleshooting
of operational challenges paved the way for improved on-stream factor
• It is vital to have technically strong and experienced process engineers
who have an intimate knowledge of the units, their history, design,
operation and constraints
Striving for Excellence
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Thank You