Catalyst Preactivation Technology for Tail Gas Units                                              Authors:               D...
INTRODUCTIONStarting up SCOT (Shell Claus Off-gas Treatment) units for tail gas treatment can be a challenging anddelicate...
PRODUCT DESCRIPTIONTo achieve full sulfiding and activation of TGU catalysts, the oxides of molybdenum and cobalt aretrans...
ACTIVATED CATALYST HANDLING and STORAGE RECOMMENDATIONSFully pre-activated catalysts need specific care with respect to ha...
•     N2 feed point.          •     If O2 level reaches or exceeds 1 vol% at any of these points, the loading should be ha...
•   The in-line burner must be lit and immediately switched to sub-stoichiometric operation in order    to create a reduci...
TROUBLESHOOTING TGU OPERATIONSThe following operation conditions can result in unsatisfactory performance of the loaded ca...
TROUBLESHOOTING: RISK ASSESSMENT SUMMARYThe following table summarizes the issues and concerns that may occur during the v...
CONCLUSIONS  •   For Tail Gas Units, fully preactivated CoMo catalysts are an excellent alternative to in-situ      activa...
COMMERCIAL REFERENCES    Customer      Year    Catalyst   Pennsylvania   2010    TG-136      Texas       2010     C-234   ...
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Advantages of loading fully preactivated TGU catalyst in startup compared to oxide or even pre-sulfurized solutions

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61 lrgcc norman_ok_feb2011

  1. 1. Catalyst Preactivation Technology for Tail Gas Units Authors: Dr. Nilanjan Brahma, Business Development Manager, Eurecat US, Inc. 1331 Gemini Street, Suite 310, Houston TX 77058-2794 Phone: (832) 284-0609, Email: nilanjan.brahma@eurecat.com AUTHOR/PRESENTER Randy Alexander, Business Manager for Sulfiding Technologies, Eurecat US, Inc. 1331 Gemini Street, Suite 310, Houston TX 77058-2794 Phone: (832) 284-0612, Email: randy.alexander@eurecat.com CO-AUTHOR Dr. Pierre Dufresne, R&D Director, Eurecat SA Quai Jean-Jaurès, BP-45, F-07800 la Voulte sur Rhône, FRANCE Phone: + (33)-475-620402, Email: p.dufresne@eurecat.fr CO-AUTHOR ABSTRACTStarting up SCOT-TGU units is greatly facilitated by using totally sulfided and preactivated CoMocatalysts. 49
  2. 2. INTRODUCTIONStarting up SCOT (Shell Claus Off-gas Treatment) units for tail gas treatment can be a challenging anddelicate operation since oxidic CoMo catalysts must be sulfided without exceeding strict sulfur emissionlimits. In-situ activation of the catalyst is typically carried out by introducing effluent gas from theSulfur Recovery Unit (SRU) to the tail gas unit (TGU). The rate of sulfiding and activation is controlledby the quantity of H2S and the H2/CO ratio in the acid gas. In order to kinetically favor the sulfidingreaction, these gases are often introduced at high temperatures, making it difficult to limit H2S (andSOx) emissions to the atmosphere and leading to the potential for temperature excursions in the unit.Catastrophic results caused by uncontrollable temperature runaways have been reported.EURECAT offers a proprietary ex-situ full sulfiding and total preactivation technology calledTOTSUCAT® G. With TOTSUCAT G, the catalyst is fully activated prior to loading. Startup consistsof heating the unit with hot nitrogen to the desired operating temperature of the TGU. Hot SRU effluentgas is subsequently directed to the preactivated catalyst system without any risks of temperatureexotherms, H2S emissions, or activity loss. Since 2003, The TOTSUCAT® technology has beensuccessfully applied in TGU applications over 25 times without any performance issues.TOTSUCAT® G for CoMo Tail Gas Unit catalysts is a safe and reliable alternative to in-situ sulfidation.Pre-activated catalysts do require some care in handling to maintain maximum catalyst performance.The primary issues associated with the loading and startups of TGU catalysts pre-activated withTOTSUCAT® G are:1. Load catalysts under inert conditions using nitrogen (<0.5% oxygen content).2. Start up and maintain sub-stoichiometric burner conditions during heat up.3. Respect the metallurgical constraints of the unit to determine the maximum heat up rate. 50
  3. 3. PRODUCT DESCRIPTIONTo achieve full sulfiding and activation of TGU catalysts, the oxides of molybdenum and cobalt aretransformed into their corresponding sulfides according to the following reactions: MoO3 + 2 H2 S + H2 → MoS2 + 3 H2 O (1) CoO + H2 S → CoS + H2 O (2)Fully activated catalysts are black in color, odorless, and have a dry free-flowing aspect. Since theactive metal sulfides have been formed, the catalyst is reactive to oxygen at ambient temperatures.Catalysts treated with TOTSUCAT are delivered in a ready-to-use condition. Special attention at start-up is no longer required to achieve high catalyst activity. In addition, water formation resulting from thesulfiding reaction will be negligible since no sulfiding reactions will occur upon startup. Bycomparison, the quantity of water formed during in-situ sulfiding is equivalent to roughly 10% of thetotal catalyst weight.TGU OPERATIONS The function of the TGU catalyst is to convert all sulfur containing species back to H2S, CO, and H2 via the water-gas shift reaction. The conversions of COS, CS2 and CO are limited by equilibrium. The conversion of SO2 to H2S is the most exothermic of these reactions and will essentially go to completion.Typically, the following reactions will be taking place in the TGU: Metal-sulfide catalyzed reactions: S + H2 → H2S (3) SO2 + 3 H2 → H2S + 2 H2O (4) CO + H2 O ↔ CO2 + H2 (5) Surface catalyzed reactions: COS + H2O ↔ CO2 + H2 S (6) CS2 + 2 H2 O ↔ CO2 + 2 H2 S (7) Undesirable side reactions: SO2 + 3 CO → COS + 2 CO2 (8) S + CO → COS (9) H2S + CO ↔ COS + H2 (10) 51
  4. 4. ACTIVATED CATALYST HANDLING and STORAGE RECOMMENDATIONSFully pre-activated catalysts need specific care with respect to handling, storage and reactor loading.These materials are typically supplied in an unpassivated state and are self-heating (not pyrophoric)under the UN 3190 classification with respect to the transport of spontaneously combustible solids.Exposure to air should be avoided and inert loading techniques must be utilized.Fully pre-activated catalysts are typically supplied to the end-user in sandblasted and sealed catalystflow bins or in steel drums with heat-sealed metalized liners. The catalyst should remain stored in thismanner until the customer is ready to load the catalyst into the reactor. If the catalyst is packaged indrums, the catalyst should be transferred from the drums into an inert hopper and then loaded directlyinto the reactor using an acceptable dense-loading or sock-loading procedure. If the catalyst needs to betransferred to other vessels to facilitate reactor loading, the transfer should only be done at the actualtime of loading and under an inert atmosphere.Personnel involved in handling and loading of the catalyst must use the proper protective equipment asspecified by the operating site; e.g. long-sleeved shirts, gloves, face shields, goggles and/or safetyglasses. Furthermore, they should review and be familiar with the Material Safety Data Sheet suppliedwith the treated catalyst and the caution labels on the drums/bins. Any person who enters an enclosedarea containing the catalyst (e.g. inside the reactor) should utilize a self-contained breathing apparatus(SCBA).CATALYST LOADING GUIDELINES 1. Vessel should be purged with Nitrogen to less than 0.5 vol% Oxygen prior to loading. 2. Check the purity and volume of Nitrogen available. If there is risk of having Nitrogen containing more than 0.5 vol% O2 or there is an insufficient quantity of nitrogen to purge the reactor, a supplemental Nitrogen source from vaporized liquid or pipeline should be utilized. 3. Nitrogen connection should be as far upstream of the vessel as is practical to prevent O2 migration into the vessel at start-up. 4. Vessel should be kept at a slightly positive pressure and under constant N2 purge during the loading. 5. All hoses, hoppers, and adapters attached to the loading equipment should be purged and periodically checked for O2 content while loading. 6. Continuous O2 monitoring is desired at the following points: • Reactor vessel • Transfer machinery • Loading chute (or sock) 52
  5. 5. • N2 feed point. • If O2 level reaches or exceeds 1 vol% at any of these points, the loading should be halted until an acceptable value of 0.5 vol% is reached. A documented record of O2 measurements at each measuring point should be maintained. • Another useful parameter is to monitor the temperature of the catalyst where oxygen ingress may be suspected. It should be ensured that the temperature be kept under 122F (50C) in order to prevent oxidation of the preactivated catalyst. 7. Periodic monitoring of sulfur dioxide (SO2) and hydrogen sulfide (H2S) is desired at a sample point near the loaded catalyst bed. 8. A light, easily removable barrier should be placed at the vessel entrance to prevent diffusion of O2 into the reactor at the loading point. 9. Only designated personnel should open drums or flow bins. All containers should remain closed until immediately before they are emptied. Drums with specially designed liners may be opened but the liners should remain sealed until dumping. All personnel involved with handling or loading the fully preactivated catalyst should be briefed as to the possible hazards of exposing the catalyst to air including the potential magnitude of heat release and SO2 emissions. 10. Adherence to all inert entry and confined space rules and regulations is mandatory.START-UP GUIDELINESPressure test (if required)For some CoMo TGU units, a leak test under pressure may be required prior to the introduction ofCLAUS off-gas feed. A main advantage of fully pre-activated catalysts is that they are stable under sub-stoichiometric burner ratio gas flow up to 390°F (200°C). This stability prevents deterioration of thecatalyst and thermal effects prior to reaching start of run (SOR) temperatures.Typical start-up guidelinesA typical start-up will resemble the restart of a unit after a temporary shutdown. The catalyst is deliveredwith the metals in the active sulfide form and ready for use. Thus, no reactions involving activation ofthe metal phase will take place in-situ during the start-up. Moreover, the uncertainty of incompleteactivation will be eliminated allowing the restart of the unit without unnecessary delays. The quantity ofuntreated gas will be greatly reduced and excess H2S emissions will be eliminated.The guidelines of a typical TGU start-up procedure could be: 53
  6. 6. • The in-line burner must be lit and immediately switched to sub-stoichiometric operation in order to create a reducing atmosphere containing H2 and CO. Typically an oxygen stoichiometry in the range of 95-99% should be established. This can be checked using an oxygen analyzer. Failure to maintain sub-stoichiometric conditions will likely result in oxidation of the pre-activated catalyst and subsequent deactivation.• Increase the temperature to the desired operating conditions. Since the loaded catalyst has already been fully activated, there is no need to hold at specific temperature levels as required for in-situ activation.• One can decide to heat up the TGU reactor (with nitrogen present) up to the desired operating temperature prior to introducing the feed effluent from the Sulfur Recovery Unit (SRU), or immediately introduce feed from the SRU if there are no issues with regulatory emission specifications 54
  7. 7. TROUBLESHOOTING TGU OPERATIONSThe following operation conditions can result in unsatisfactory performance of the loaded catalyst.These conditions are unrelated to the type of sulfiding and activation procedure used on the catalyst (i.e.ex-situ or in-situ). • Insufficient hydrogen content: As described by reactions (3) and (4) it is essential to have excess reducing agent present in order to achieve complete reduction of sulfur (S) and SO2 species in the system. In the case of insufficient reducing agents in the system, unconverted S and SO2 will pass into the quench and amine systems. This will result in fouling of the quench system by sulfur and destruction of the amine by SO2. • Hydrogen purity: Some TGU’s are equipped to take advantage of the introduction of supplemental refinery hydrogen. If this hydrogen contains hydrocarbon contaminates, soot formation can occur in the catalyst bed. In a worst case scenario, sooting can cause unacceptable pressure drop, resulting in shutdown of the unit. • Catalyst bed temperature: Typical operation temperatures for optimal performance of the TGU system is anywhere from 510 to 580F (270-300C), although new “low temperature” tail gas catalysts can operate as low as 430 to 450F (220-230C). The lowest temperature required to achieve the emission quality goals should be utilized to take advantage of: • Minimizing hydrothermal ageing of the catalyst (sintering) • Minimizing COS, CS2 and CO in the outlet by taking advantage of the equilibrium benefits of lower temperatures • Sulfur content: if a Claus sulfur condenser drain becomes plugged, excessive (elemental) sulfur can enter the TGU, consuming all the available hydrogen while getting transformed to H2S. Fouling of the quenching system can subsequently occur. Corrosion can radically shorten the life of the quench tower.Upsets that will destroy the catalyst: • The introduction of large quantities of oxygen into a hot and active (sulfided) system could result in complete loss of catalyst activity. Large slugs of air will result in huge exotherms (up to 2000F (1100c) or more that will basically turn the catalyst into dust and may result in significant structural damage to the reactor system. • Low levels of oxygen may also be detrimental as this condition could result in the formation of sulfates on the catalyst surface. Sulfate formation is difficult to detect initially and may result in the loss of catalyst activity over time. Low levels of oxygen can be the result of: o Poor mixing in the burner. o Operation of the burner at an Air/Fuel ratio that is above 90% of the stoichiometry required. 55
  8. 8. TROUBLESHOOTING: RISK ASSESSMENT SUMMARYThe following table summarizes the issues and concerns that may occur during the various operations aswell as their potential impact and mitigation. Operations Potential impact Mitigation/Resolution 1 Shipping, storage, handling • Catalyst preconditioned according to the • Personnel handling catalyst must take TOTSUCAT® procedure can react in appropriate safety precautions and follow • Contact of the catalyst with air air handling guidelines either during storage or • O2 adsorption starts at around 120°F • Make nitrogen available to inert the reactor handling (50°C) prior to, during, and after loading the reactor • Reactions may initiate a temperature rise • Keep the catalyst temperature below 120°F in a drum, container or hopper. (50°C) during loading to prevent oxidation • SO2 emissions are possible if the catalyst • Make sure breathing apparatus is available for contacts air catalyst loaders • Personnel injury by toxic gases • Develop action plan if SO2>15 ppm (check refinery specs) 2 Catalyst loading • SO2 emissions may be detected • Follow recommended loading guidelines • Excessive temperature rise in the reactor • Load under nitrogen • Transfer in air should be or loading equipment • Monitor temperature, oxygen, and SO2 levels at avoided • Injury to personnel and equipment several points • Develop action plan if SO2>15 ppm (check refinery SOC specs) • Keep temperature below 50°C (122°F) to prevent SO2 emission. 3 Start up • No impact on quality of sulfided active • Stop heating phase: catalyst is already activated. • Evaluate if loss of H2 circulation could have • Hydrogen make-up compressor caused coke formation on the catalyst. loss • Resume operations where they were stopped. 56
  9. 9. CONCLUSIONS • For Tail Gas Units, fully preactivated CoMo catalysts are an excellent alternative to in-situ activation of the catalyst bed. • Preactivated catalysts offer a simple and straightforward startup procedure that minimizes the risk of temperature exotherms and excess sulfur emissions. • Proper handling of the activated catalyst will ensure optimum performance of the catalyst bed. • Numerous successful commercial experiences (see next page). 57
  10. 10. COMMERCIAL REFERENCES Customer Year Catalyst Pennsylvania 2010 TG-136 Texas 2010 C-234 Axens 2009 TG-136 Pennsylvania 2009 HDMax 213 Tennessee 2009 C-234 Kansas 2009 HDMax 213 Washington 2009 KF-142 California 2008 TK-220 Texas 2008 TK-220 Louisiana 2008 TG-136 Louisiana 2008 TG-136 Texas 2008 C-234 Texas 2007 TG-107 Kansas 2007 TK-527 Texas 2007 TG-107 Texas 2007 TG-107 Wyoming 2007 C-234 Ohio 2007 TK-220 Texas 2006 C-534 Axens 2006 TG-103 Tennessee 2006 C-29 Texas 2005 TK-220 Texas 2005 TG-107 Tennessee 2004 C-534 Arkansas 2003 C-534 Texas 2003 C-234 Texas 2003 KF-124 LD 58

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