MESPON 2017 ADGAS Das Island Sulphur Recovery Unit Shutdown procedure improvement Presenters: ADGAS: Hamad Ibrahim Al-Ali Email haialali@adgas.com Sulphur Experts: Jamie Swallow jamie.swallow@sulphurexperts.com
Schematic of ADGAS SRU process BFW STEAM STEAM COMB AIR MAIN WASTE REACTOR-1 CONDENSER-1 FUEL GAS COMB. HEAT CHAMBER BOILER ACID GAS REHEATER-1 (H 2 S &CO 2 ) BFW Tr1&2 : 30% bypass Tr3: 50% bypass STEAM REACTOR-2 CONDENSER-2 REHEATER-2 BFW STEAM REACTOR-3 CONDENSER-3 Superclaus REHEATER-3 process BFW LIQUID SULPHUR TO TAIL GAS BFW H2S ~ 0.4-0.55% STORAGE INCINERATOR CONDENSER-4 FUEL GAS SULPHUR PIT BFW COMB AIR STACK 2 Sulphur Recovery Unit
ADGAS SRUs – Train 1 and 2 • Mid 1970s • 3 stage modified Claus • Capacity 550 TPD • Fired Reheaters • Mothballed till 1993 – Train 3 • 1993 • 3 stage modified Claus • Capacity 500 TPD • Steam Reheaters – Superclaus • Added dedicated Superclaus stage to all units in 2006
SRU Shutdown Challenges – Non Routine Operation • Often Limited Experience – Extensive procedure • Sulphur needs to be removed to ensure the plant does not plug off. – High Risk Operation • Higher than normal gas temperature • Risk of temperature excursion and fires
Typical Example of Sulphur left in plant elsewhere (NOT ADGAS)
Example of High Temperature Damage Melted MCC / Reaction furnace Refractory (NOT ADGAS)
(NOT ADGAS)
Shutdown – Industry Experience – wide variety of procedures in use; – procedures often site specific based on SRU design, type of TGU, etc. – procedures should be based on proper understanding of risks and benefits • For each step the purpose and risk associated need to be understood
Key Steps – How to remove or minimise sulphur from the catalytic reactors • When is this task completed ? – When to Introduce free oxygen
Optimum Shutdown procedure – Catalyst Preparation (Heat Soak) • Acid Gas firing • Reducing amount of sulphur in the catalyst beds – Sulphur Removal (Strip, Sweep, Sweep-out, Sweat) • Natural Gas or Fuel Gas Firing • No free Oxygen allowed – Forced Cool Down • Fuel Gas Firing
Catalyst Preparation (Heat Soak) • Heat Soak to reduce sulphur in catalyst beds – increase Converter 2 and 3 temperatures 30 degree C (50 degree F) for 24 to 48 hours before switch to fuel gas – reduce plant load (if possible) as it limits overall efficiency losses • Rejuvenation , run off ratio to lower catalyst sulphate levels – conducted simultaneously with heat soak if efficiency losses can be tolerated – increase H 2 S:SO 2 ratio for 24 – 48 hours – typically 4:1 to 16:1 or greater (-1.5% air demand or greater) • not strictly necessary if sulphation unlikely or catalyst to be replaced
Sulphur Removal / Fuel Gas Firing • when removing acid gas, provide an inert gas that will: • avoid combustion (oxygen breakthrough) of elemental sulphur • avoid soot formation • avoid drastic temperature changes in furnace slightly sub-stoichiometric fuel gas burn in main burner • • 90 to 99% stoichiometry; • low end of range if Amine based TGTU on line downstream; upper end if slightly higher emissions or O 2 breakthrough acceptable • temperature moderation is required to control flame temperature, typically 4:1 steam or N 2 to fuel • total flow should be 20 to 30+% of design plant flow to avoid channelling in vessels and minimize heat losses no risk of damage to TGU if properly done, • bypass TGU if concerned and if allowed
Sulphur Removal (“strip”, “ “sweat”, “soak”, “sweep out” etc) desorb all elemental sulphur from plant • catalyst beds are full of absorbed sulphur vapour – desorbed sulphur will condense in condensers – remaining sulphur will cause plugging, corrosion, fire hazards – maintain normal catalyst bed inlet temperatures • continue fuel gas firing until • all rundowns stop running – all catalyst bed temperature profiles are flat – tail gas analyzer H 2 S and SO 2 values near zero – (sulphur will back react to H 2 S and SO 2 – reverse Claus) • typically 8 to 36 hours – dependant on fuel/air/steam volume • can be 48+ hours at low flows or if sulphur can’t drain •
Cool-down • Short term maintain normal furnace and catalyst bed temperatures • ready for immediate reintroduction of acid gas • Long Term forced cool-down (fuel gas) • continue same reaction furnace air:fuel:steam ratios • turn off reheaters; catalyst takes 4 to 12 hours to cool • possibly replace condensers with cold water for extra cooling • continue with fuel gas firing down to 100 to 150°C (210 to 300°F), – avoid crossing water dewpoint of combusted fuel gas • slowly increase air:fuel ratio or excess oxygen value; – continue until flame can’t be sustained, then air only • pyrophoric fires possible at any temperature, be prepared to cut back on air and add steam / nitrogen • furnace cool down rates 30 to 50 C/hour (50 to 80 F/hour )
ADGAS Old Shutdown procedure Steps as per original Manual / Licensor – Catalyst Preparation (Heat Soak) • Acid Gas firing – Sulphur Removal • Fuel Gas Firing – Regeneration (introduction of oxygen on hot catalyst) • Fuel Gas Firing • Excess Air / Free Oxygen – Forced Cool Down
Old Regeneration Procedure Keep Reheater on and introduce “controlled” amount of oxygen when catalyst beds are hot “BURN OUT” NOT RECOMMENDED Do not attempt to increase air to fuel gas ratio and “burn-out” any remaining sulphur: • Temperature Excursions • Fires • Catalyst sulphation / deactivation
ADGAS Sweep-out and Regeneration Reheater Switch off in optimized procedure Forced Cool Down Reheater Switched Off Regeneration Sweep Out
Old Regeneration Procedure • Increases Risk of – Equipment Damage – Unit Trips • Longer Shutdown Procedure • Could start cool down after the sweep – Regeneration took approx. 72 hours
New Adgas Procedure • Regeneration removed - Cool down in two steps 1. Reheater switched off on completion of sweep Maintain Main burner sub-stoichiometry • Bed cools down to approx. 150°C-160°C • 2. Cool Down slowly increase air:fuel ratio or excess oxygen value of main • burner; Drain condensers once temperature drops • Add nitrogen to enhance further cooling • Reduces shutdown time – 48-72 hours
ADGAS Sweep-out NO Regeneration (New) Reheater Switch off in new procedure
ADGAS “Back Steaming” – Sulphur in condensers • Bottom Rows of Tubes full of Sulphur – Hard to Clean • Hydro drilling • Time Consuming (Extending the shutdown) – BACK STEAMING (novel method) • Live Steam added to Condenser
Bottom Rows of tubes plugged with Sulphur Burned catalyst support Oxygen Breakthrough to Catalyst Bed
ADGAS Sweep-out and NO Regeneration (New) Back steaming
ADGAS “Back Steaming” Procedure Added at end of cooldown operation – Air flowing through unit – Water side from Condensers drained • (to prevent reheating of process gas) – Now live steam is added to the Condenser waterside • Increases condenser temperature (165°C) – Re-melts sulphur from bottom rows – Increases process gas temperature – extends shutdown procedure
Back Steaming Clean Condenser Tubes
ADGAS “Back Steaming” Results Bottom Rows of Tubes were absolutely clean – Impact on Shutdown • Adds approx. ½ day to the shutdown procedure – Impact on Turn Around • NO need for hydro drilling / cleaning • Reduced turnaround duration by 5 – 7 days – Adopted and added to the shut down procedure
ADGAS Continuous Improvement Further improvement of shutdown procedure is possible, looking at – Ensure sub-stoichiometric firing at Shutdown • Ait to Fuel Gas Ratio Check at Start Up • Use of Oxygen analyser – Increase fuel gas firing • Larger gas volume will speed up the procedure – Currently at 10 to 12 % of the design plant flow – Optimum is 20 to 30+ % of design plant flow • MCC / Reaction Furnace Refractory is limiting factor – N2 Availability • Final cooling of reactors is taking a long time – Warm ambient air does not cool – Looking at using more nitrogen N 2 to speed this up
THANK YOU We hope that at the next Mespon conference we can present on further improvement of the procedure QUESTIONS ?
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