Methanol Loop Start Up and Shut Down
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The document outlines the process for catalyst reduction and oxidation within a chemical reactor, emphasizing the importance of temperature control and inert gas specifications to prevent catalyst damage. It details the procedures for both catalyst activation through hydrogen reduction and subsequent oxidation of spent catalyst, including necessary monitoring and operational adjustments. The entire process takes approximately 24-36 hours and requires careful management of conditions to ensure safety and efficiency.
Methanol Loop Start Up and Shut Down
- 1. Gerard B. Hawkins Managing Director, CEO
- 2. The reduction is highly exothermic and temperatures must be controlled carefully during this period to avoid damaging the catalyst. Reduction is carried out using hydrogen or hydrogen/carbon monoxide mixtures in an inert carrier gas such as nitrogen or natural gas. The inert gas must be free of catalyst poisons. The hydrogen plus carbon monoxide in the circulating gas should be adjusted to control the reactor temperatures, but will typically be in the range of 1.5-2.5% at the inlet to the converter.
- 3. During the reduction, approx' 15Nm3 of CO2 is evolved from 1 ton of methanol synthesis catalyst. This must be purged from the loop to keep the pressure constant. Water is also released from the hydrated salts in the catalyst as well as from the reduction of the copper oxide by hydrogen. The total quantity of water produced amounts to approx' 15% of the weight of catalyst installed. Less water is produced if the reduction gas contains CO. Once the catalyst has been reduced it can be brought on line immediately. Alternatively, it can be put on standby and commissioned at a later date.
- 4. An on line hydrogen analyzer A flow meter that shows the rate of hydrogen or synthesis gas addition. Provision to analyse for CO2 content every hour, if possible by an on-line analyzer. Converter temperatures, cold shot flows, heater temperatures etc., should be within the control room. A start up heater specified to be able to handle the maximum load under reduction conditions as well as any duties in normal operation. Circulator - this must be specified, or at least checked out with the machine vendor, for reduction gas flow, temperature, pressure and composition in addition to the normal circulating gas.
- 5. Hydrogen for reduction is normally obtained from synthesis gas by starting up the reformer. The theoretical quantity of hydrogen required for the reduction of VSG-M101is 145 - 160 Nm3/ton (4900 - 5400 SCF/ton) of catalyst installed. If synthesis containing CO and/or CO2 is used as a source of hydrogen, this will increase the need to purge from the loop and there may be an increased loss of hydrogen due to this extra purging.
- 6. If cracked ammonia is used as the source of hydrogen, the quantity required for reduction is unchanged. The extra N2 accumulating in the loop will have to be purged out and will carry CO2 with it. This will reduce the amount of N2 required from other sources. The maximum concentration of NH3 in the circulating gas must not exceed 10 ppm and so ammonia cracking conditions must be selected to achieve this. An NH3 content of 200 ppm in the cracked gas is normally sufficient and can be easily achieved.
- 7. Reduction is carried out in an inert gas such as nitrogen or natural gas. The inert gas must not contain excessively high levels of catalyst poisons, and a specification for inert gas is: Oxygen <1000 ppm v/v CO < 1.0% v/v H2 < 1.0% v/v S < 1.0 ppm v/v NH3 < 10 ppm v/v Unsaturated Hydrocarbons traces only Other poisons e.g. chlorides absent
- 8. Purge the synthesis loop with inert gas to less than 1% v/v oxygen and then increase the pressure to the appropriate level. For plants designed to operate at 1450 psig, the reduction pressure should be approx' 100 - 115 psig. For the ICI lozenge design, circulate the inert gas through the converter ensuring that the cold shot valves are closed. The circulation rate should be sufficient to give a minimum space velocity of 300-600 Hr-1. The upper limit of the circulation rate will be fixed by pressure drop through the system and the characteristics of the circulator. The advantage of increasing the circulation rate is that a more rapid and more even reduction is obtained.
- 9. In tube cooled converters, gas should be sent directly to the top of the bed, with no flow up the tubes since this would lead to uneven reduction, as the catalyst close to the tubes would be below the reduction 'strike' temperature. Heat the converter to 130°C inlet at a rate not exceeding 50°C/hr Introduce about 1 % of H2 or (H2 + CO) as a single shot, to check the calibration of the hydrogen analysers and to calibrate the reduction gas flow meter. Increase the inlet converter temperature to 180°C at a rate of approx' 20°C/hr. Reduction will commence between 150 and 160°C.
- 10. With an inlet temperature of 180°C, introduce reduction gas initially to maintain a H2 + CO concentration inlet the converter between 1.0 and 1.5 %.Once the temperatures have stabilized, this can be increased to between 1.5 and 2.5 %. If the reduction proceeds too slowly the inlet temperature may be increased to 200°C. The best inlet temperature is that which gives peak catalyst temperatures of about 230°C and a H2 concentration at the outlet <0.1 %. During the reduction maintain a constant pressure by purging or making up with extra inert gas as required. Ensure that water produced during reduction is drained from the catchpot. It can be useful to measure the quantity as an aid to determining when complete reduction has occured.
- 11. If, despite precautions, peak temperatures exceed 240°C, stop the hydrogen addition. Do not open the cold shot valves, as this will merely cause overheating to occur elsewhere in the converter. When there is no further sign of reaction (no temperature point reading higher than the one above it and no difference in the inlet and exit hydrogen analysis) raise the inlet temperature to 240°C or as near as possible. If there is still no sign of reaction, raise the H2 content to 10 - 20 %. The increase must be stopped immediately if there are signs of further temperature rise. Holding the catalyst under high concentrations of H2 and at high temperature is called 'soak'. this period should last for 4 to 6 hours.
- 12. Reduction is now complete. The whole process, excluding the time taken to heat the catalyst to reduction temperatures (say 150°C) will take about 24 - 36 hours.
- 13. Reduce the converter inlet temperature to 210 - 220°C and wait for all catalyst bed temperatures to stabilise at this temperature. All converter temperatures must be between 210 and 220°C. Start introducing synthesis gas, which will cause the pressure to rise. The rate of increase of pressure should be about 10% of design operating pressure per hour. This constraint is imposed by the equipment in the synthesis loop and not by the catalyst, and is only an estimate. Different pressurisation rates may be acceptable.
- 14. The introduction of synthesis gas will cause the converter temperatures to rise as the methanol reaction commences. The partial pressure of methanol vapour will increase, but at first this vapor will not condense and an equilibrium concentration of methanol will be established. Consequently, the temperature rise will only persist as long as synthesis gas is being introduced. Aim to keep the inlet converter temperature at 210 - 215°C.
- 15. When the pressure reaches 145 - 175 psig, the exact value will depend on the temperature of the cooling water and the water content of the first methanol made, methanol will start to condense. This will allow a larger continuous rate to be fed in for the same rate of pressure rise. At this stage a temperature rise will consistently appear across the catalyst. When methanol first condenses, commission the cold shot to the second bed with the control point inlet that bed set at 210 - 215°C. Commission the cold shot to the subsequent bed once the temperature at the exit of the bed is steady. The control point should be 210 - 215°C at each bed inlet.
- 16. In tubular converters as methanol starts to form, the flow up the tubes should be gradually increased as the peak bed temperature rises. For steam raising converters the steam pressure should be set to give a cooling temperature of about 215°C. This will be approximately 300 psig. As methanol is formed the steam pressure can be raised to its flowsheet value. As methanol starts to form in sufficient quantity, commission the level controllers of the separator and the letdown vessel. Commission the purge of flash gas from the letdown vessel.
- 17. When the concentrations of nitrogen and methane reach flowsheet values, commission the purge from the synthesis loop. It is usual anyway to have a small purge from the time that methanol first starts to condense. Maintain the bed inlet temperature at 210 - 215°C by commissioning the interchanger bypass and admitting cold gas to the converter inlet gas stream, or by making suitable adjustments to the heat recovery system. When the reaction is autothermal the start up heater can be shut down.
- 18. If the start up cannot follow soon after reduction, sweep out the loop with an inert gas until the H2 concentration is less than 1% and cool down the converter. When the catalyst is subsequently started up, the following procedure is used.
- 19. Pressurise the loop with inert gas to 7 - 8 barg. Establish circulation with a space velocity of at least 300-600 hr -1. Heat the reactor inlet temperature to 200°C with the loop start up heater. Allow all reactor temperatures to reach a minimum of 180°C. Introduce Synthesis gas to the loop and allow the pressure to rise.
- 20. As reactions start the reactor temperatures will rise and methanol will begin to condense in the catchpot, (when the pressure reaches about 10 - 12 barg). • As the inlet temperatures of the lower beds begin to rise commission the shot flows to these beds controlling the inlet temperatures at around 210 - 215 °C • When sufficient heat generation is taking place the loop start up heater can be decommissioned.
- 21. After shutdowns of a short duration the catalyst may be hot enough to allow the reactor to be started without additional heating. ◦ Bring the circulator up to normal speed as quickly as possible consistent with the vendor's instructions. When the reactor temperatures are above 210 °C bring the circulator up to normal speed. Begin to introduce fresh synthesis gas. As temperatures begin to rise commission shot flows as appropriate.
- 22. In a start up situation reaction heat for heating of the reactor inlet gas is not available. • Steam from the reforming section is used via the loop start up heater • Gas is redirected through the start up heater via valve in the HL interchanger inlet and HIC007. • Minimum stop valve to prevent isolation of HL interchanger
- 23. As reaction occurs reactor exit gas temperature rises ◦ Heat will be picked up in HL interchanger • As exit temperature continues to rise HL interchanger valve can be adjusted to allow more gas to flow through HL interchanger • When temperature exit HL interchanger is high enough to sustain reaction gas flow through start up heater can be stopped
- 24. E1113E1112 E1110 V1107 Shot Gas Converter Inlet Gas 180 °C 0 0 0 0 Shot Flows kNm3/hr From C1102 210 °C E1111 Loop start Up Heater 100 bar steam TIC 060 HIC 007 240 °C ShutOpen 180 °C 210 190 230 100 220 210 240 150 25 % 50 %
- 25. Stop the synthesis gas • Continue to circulate the loop gas over the catalyst to react all the carbon oxides • When the temperature starts to fall commission the start up heater and maintain the catalyst temperature above 200 °C • Maintain these conditions until synthesis gas is available again
- 26. Carry out the short period shutdown procedure • Reduce the circulator to minimum speed and reduce loop pressure. Allow the catalyst to cool at a rate of around 50°C/hr • While the circulator is running purge the loop with N2 until the H2 content is below 1 %.
- 27. While shutdown the loop should be kept under an inert atmosphere to prevent the catalyst contacting oxygen
- 28. In the event of a trip the loop pressure should be reduced by 10 % – This prevents the catalyst overheating due to continuing reaction of residual carbon oxides • The purge can then be isolated and loop can be left to depressurise slowly until ready to start up again. – If the trip will last more than 24 hours then the loop should be shutdown in accordance with the prolonged shutdown procedure
- 29. This procedure has been developed due to the development of the ICI ARC converter. The new ARC converter has individual catalyst beds which make it very difficult to discharge the catalyst from the converter. ◦ To compensate for this GBHE has developed an in-situ oxidation process that will completely oxidize the catalyst while it is still inside the reactor. This eliminates both the need for any specialist procedures for catalyst removal as well as the need to double handle the material after its discharge. ◦ The oxidized catalyst is suitable for immediate loading into drums or other suitable containers for shipping off-site.
- 30. The process has been carried out several times around the world. The procedure takes approx' 24 to 36 hours. A significant feature of the oxidation procedure is that the catalyst is not completely oxidized after just one pass of the exotherm front through the catalyst bed. It is only 90% oxidised, although the exact extent of oxidation is a function of the temperature at which it takes place. The catalyst can never be completely oxidised, operators need to be aware that the catalyst still has the capability to absorb O2 from the atmosphere, so appropriate precautions must be taken during entry into a vessel containing oxidised catalyst.
- 31. At completion of production, cool the converter to about 150°C Letdown the synthesis loop pressure. Purge the synthesis loop with N2 according to the standard plant procedure. Hydrogen concentration in the loop must be reduced to below 2 mol.% before any oxygen is introduced. The loop H2 analyser may need recalibrating to a range of 1 - 10 %
- 32. Maintain the loop with inert N2 at just above atmospheric pressure to prevent ingress of air. ◦ Ensure that all lines around the synthesis loop not associated with the oxidation procedure are closed to eliminate the unwanted flow of fluids in or out of the loop. The level of isolation required are double block and bleed, slip plating or physical blinding. ◦ Any methanol left in the catchpot/separator should be completely drained from the vessel before isolation. ◦ If air is to be supplied to the converter via an external air compressor, an indication of flow will be needed, either at the air compressor or via a plant gauge that has been recalibrated for the purpose.
- 33. Connect up an oxygen analyser upstream and downstream of the converter. This can be done with one analyser and a field switch. Lag time must be minimised in obtaining samples. Establish loop circulation with N2. Pressure up to about 100 - 150 psig. Commission start up heater to maintain the catalyst bed at 150°C. This temperature should be maintained inlet the converter for the first stage of the oxidation. Check H2 concentration in the loop. H2 concentration should be monitored continuously throughout the procedure.
- 34. If H2 concentration is below 2 mol.% (NB H2 absorbed will be released from the catalyst, so purging may be required to reduce H2 below 2 mol.%), connect air supply and admit air to a concentration of 0.5 mol.% O2 at the inlet to the converter. Check that no O2 slip is being seen exit the converter. ◦ The exotherm associated with the reaction is equivalent to about 100- 120°C per mol.% of O2 inlet the converter. A temperature rise will be seen after only a few minutes, which will go progressively down the converter - similar to that seen during catalyst reduction. ◦ Monitor temperature rises, O2 and H2 analysis and air rate regularly and that they are giving consistent information. Do not increase the oxygen concentration until inconsistencies have been satisfactorily resolved.
- 35. Once air addition has started, the loop will either require periodic purging, or a continuous small purge, to maintain the pressure at about 100 - 150 psig. Continue at these conditions until the exotherm is monitored exit the final catalyst bed. Increase air addition rate in step-wise intervals, 0.5% increments over say 30 to 60 minutes, towards a maximum of 1.5 mol.%, while not exceeding a maximum average temperature at any level of 250°C and any individual temperature exceeding 275°C.
- 36. As the oxidation proceeds, the reaction front moves down the catalyst beds. As the reaction front passes, the temperature will fall at each level. When the temperature exit the final bed begins to fall the reaction has almost proceeded to completion. Isolate the air. Raise the converter inlet to 200°C and maintain at this temperature. Repeat from 'If H2 concentration is below 2 mol.%........' When the final bed exit temperature falls after the second air cycle, the oxygen level exit the converter should be observed, for oxygen slipping from the converter. Maintain O2 at its current level by reducing the air flowrate in step- wise amounts, then isolate.
- 37. Recalibrate O2 analyser to read say 2 mol.% to 20 mol.% range. Slowly add O2 to the loop to raise the concentration to 10 mol.% at 200°C. This should be done in increments of 2 mol.% allowing time at each stage to check that conditions are stable. Maintain inlet temperature at 200°C and loop pressure 100-150 psig. Ensure that the average peak temperature at any level does not exceed 250°C, and any single thermocouple does not exceed 275°C. Maintain O2 level at 9-10 mol.% by batchwise addition of air, until there is no longer movement on the thermocouples. Isolate and shut down the air compressor. The converter needs to be cooled down prior to catalyst discharge. This should be done, as much as possible, with O2 level maintained at 10 mol.%. Allow converter to cool at up to 50°C/hr.
- 38. Fully reduced Cu crystallite First exposure to oxygen. Outer layers start to oxidize. As oxidation continues, oxygen penetrates throughout. Later, the oxide layer at the surface virtually stops any further oxygen diffusion.
- 39. Raising the temperature speeds up the solid diffusion process and so oxidation resumes... only to stop again when the thickness of the oxide layer has increased and halts any further oxygen diffusion. Complete Oxidation Can Never Occur
- 40. Cool catalyst as much as possible, at least to less than 50°C, preferably to less than 40°C. The above procedure should result in a converter full of oxidised catalyst. However, COMPLETE oxidation CANNOT be guaranteed, and the charge of catalyst may still be highly active. Appropriate precautions should be taken during vessel entry and catalyst discharge. In the event of any unusual conditions, high exotherm, high H2 levels etc., the air supply should be isolated immediately.
- 41. Process Information Disclaimer Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss or damage resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.