shift conversion catalysts - operating manual

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

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GBH Enterprises, Ltd.

Shift Conversion Catalysts - Operating Manual



Contents

Introduction High temperature shift catalysts Low temperature shift catalysts Catalyst storage, handling, charging and discharging Health and safety precautions Reduction and start-up of high temperature shift catalysts Operation of high temperature shift catalysts Reduction and start-up of low temperature shift catalysts Operation of low temperature shift catalysts 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. GBH Enterprises, Ltd., Catalyst Process Technology 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. GBH Enterprises, Ltd., Catalyst Process Technology accepts no liability for loss or damage, resulting from reliance on this information.



INTRODUCTION The water-gas shift reaction plays a major role in ammonia and hydrogen plant design and operation. Good performance of the shift catalysts, and attainment of a close approach to equilibrium and hence minimization of the CO slip from the catalyst system is critical to the efficient and economic operation of the plant and ensures maximum hydrogen production from the hydrocarbon feedstock. The water gas shift (or shift reaction) is highlighted below. CO + H2O ⇔ CO2 + H2 The reaction is exothermic and high conversions are favoured by low temperature and high steam ratio. Ammonia plants usually operate a two stage system – a High Temperature Shift (HTS) followed by a Low Temperature Shift (LTS) – with a suitable form of inter-bed cooling. Hydrogen plant designs feature a number of differing shift conversion sections. Commonly there is a high temperature shift stage followed by a PSA unit to recover the product hydrogen. On occasions an intermediate temperature shift (ITS stage) is used in preference to high temperature shift. On older hydrogen plants, a two-stage system is often utilized in which an HTS is followed by an LTS stage with suitable inter-bed cooling. Modern catalysts for the high temperature shift stage operate typically in the temperature range 300-450oC (570-840oF). Corresponding operating temperatures for the low temperature shift section are 180-270oC (355-520oF). This manual discusses the principles of start-up, operation and shut-down of shift converters, and the information provided is sufficient for the preparation of the detailed operating instructions which of necessity will be plant-specific.



High temperature shift catalysts GBH Enterprises, Ltd., Catalyst Process Technology supplies its HTS catalyst VULCAN Series VSG-F101 in two sizes. The catalysts are formulated from iron oxide, chromia and copper oxide, and feature enhanced activity and efficient operation at low steam to carbon ratios. Composition VSG-F101 iron oxide, chromium oxide, copper oxide Physical and Chemical Properties

Appearance brown cylindrical pellets

Diameter 9 & 6 mm

Length 5 & 3 mm

Bulk Density ~1.5 kg/l

Radial crushing strength (before reduction)

196(long), N/cm

100(short), N/cm

S content 200 ppm

Fe2O3 80 %

Cr2O3 5.0-9.0 %

CuO 2.0 %

Ignition Loss 10.0 %



Low temperature shift catalysts VULCAN VSG-C111 catalysts are based on copper oxide supported on a matrix of zinc oxide and alumina. The established product, VSG-C111 is also available in a smaller pellet size, to allow optimization of performance and pressure drop. GBH Enterprises also offers VSG-C111 which is based on the standard catalyst but is promoted with alkali oxides to minimize methanol by-product formation. A smaller pellet size is also available. In all cases, the copper oxide must be reduced to its active metal state before use. This critical step in catalyst activation is highly exothermic and the temperature of the bed must be strictly controlled to ensure maximum catalyst activity. An inert gas such as natural gas or nitrogen should be used to dilute the hydrogen used for the reduction reaction. All gases used in the reduction must be free of catalyst poisons. The use of steam as an inert diluent during reduction must be avoided as steam sinters the copper crystallites and therefore deactivates the catalyst. Composition: VSG-C111 Copper oxide, zinc oxide, and alumina (W/ proprietary promoters) Chemical and Physical properties (typical)

Appearance black cylindrical pellets

Diameter 5.0~6.0 mm

Length 2.5 & 5 mm

Bulk Density 1.2 - 1.4 kg/l

Radial Crushing Strength (before reduction)

65 (long), N/gr

60(short), N/gr

CuO 40±2.0 %

ZnO 43±2.0 %



Catalyst storage, handling, charging and discharging Before charging, discharging and handling shift catalysts any potential risk to health during these activities should be assessed and appropriate precautions taken. In addition the GBH Enterprises Catalysts brochure on “Catalyst Handling” should be consulted. Drum storage Shift catalysts are generally supplied in mild steel drums, fitted with polythene liners, and having the following packaging details. Precise information will be recorded in the documentation covering the goods when supplied. Drums must not be stacked on their sides or stacked more than four drums high, even when held on pallets. Taller stacks tend to be unstable and there is the risk that the top drums may fall off the stack, and the lower drums can be crushed due to the weight of the drums above them. The metal drums are usually suitable for outside storage for a few months but should be protected against rain and standing water. If prolonged storage is expected, they should be kept under cover and away from damp walls and floors. The lids should be left on the drums until just before the catalyst is to be charged. If the lids are removed it is important that they should be replaced as soon as possible, so that contamination of the catalyst is avoided. If the drum lid cannot be replaced, then the catalyst should be redrummed without delay. If any contamination occurs it is difficult to assess the extent of any damage without full examination of the catalyst. If there is any doubt about the state of the catalyst it is best not to charge it to the reactor. Drum handling Catalyst drums should be handled as carefully as possible. They must not be rolled. Catalyst drums are often supplied on pallets, which reduces the likelihood of damage in transit but requires suitable fork-lift trucks and a paved area to handle the pallets. The fork-lift truck to be used for dismantling the pallets should be fitted with rim or body clamps to avoid damage to the drums. The use of shipping containers for either catalyst drums or palleted drums eases shipment and further reduces the likelihood of damage in transit. It is important not to use standard forks to lift the drums under the rolling hoops, as damage to the drums and catalyst is almost inevitable.



Sieving catalyst Shift catalysts are screened before they are packed into drums for dispatch, hence sieving on site is not usually required, but in some instances attrition can occur in transit if the drums are roughly handled. In this case some form of screening is advisable before charging, especially if the catalyst appears to contain dust on delivery. A good method of sieving is to pass the catalyst over a simple inclined screen. This is often the most satisfactory method, since vibrating screens can cause additional unnecessary damage and loss. The screen should contain provision to collect the dust, and at the same time avoid generating a dusty atmosphere. The mesh spacing should be about half the smallest dimension of the catalyst pellet. While the catalyst is being poured over the screen, the use of a vacuum system situated close to the sieve will control the dust effectively. Pre-charging checks Before the catalyst is charged it is important that the condition of the catalyst support grid in the vessel and any supporting materials such as inert balls are checked. Any support or hold down material in the HTS converter should be of a low silica type to prevent the possibility of silica poisoning of the HTS catalyst. Some form of light metal shield or “spider” fitted into the discharge manhole prevents an uncontrolled discharge of catalyst, when the manhole cover is removed. The vessel should be clean, dry and free from loose scale and debris. It is important to ensure that the charging level is clearly defined, so as to avoid under filling or overfilling. The desired level can be marked with chalk before charging is commenced. It is strongly recommended that the operation of the thermocouples is checked and their position noted to allow for temperature profile analysis during operation of the catalyst. This can be done before charging commences by warming them in turn to ensure that the correct indication is given on the instrument panel.



Charging the shift converter The catalyst may be loaded directly from the drums or from intermediate bulk containers. The general rules for charging catalysts into vessels are:

• The catalysts should have a free fall of between 50 and 100 cm (20-40 inches) to ensure a suitable packed density is achieved. (More than 100 cm/40 inches may damage the catalyst)

• The catalyst must be distributed evenly as the bed is filled, with a maximum height difference of 15 cm (6 inches) across the bed when completed.

Special procedures are required for loading tubular isothermal reactors. GBH Enterprises, Catalysts Process Technology will advise on these procedures on request. Discharge of high temperature shift catalyst The catalyst is usually discharged with large mobile vacuum extraction units. Occasionally it may be discharged by gravity flow from the bottom of the converter. No special oxidation procedure is required before discharge. After cooling in steam the catalyst is not pyrophoric although water hoses should be available in case the catalyst overheats for any unexpected reason. If catalyst has been operated in the fully sulfided state care will be needed during discharge because iron sulfide is pyrophoric. The sulfur must either be removed by steaming, which may take 2-7 days, or the catalyst should be discharged under nitrogen or after drenching with water. The normal shut-down procedure for inert discharge is as follows 1 Reduce pressure in the reactor at a maximum rate of 1-2 bar (15-30 psi)

per minute, or as governed by the mechanical design of the equipment. Purge the reactor free of process gas with steam and cool to 150oC (300oF).

2 Replace steam with inert gas and cool to ambient temperature that is to

say below 40°C (105°F).



3 Discharge the catalyst under a positive pressure of inert gas. Alternatively the procedure is as follows: 1 Reduce pressure in the reactor at a maximum rate of 1-2 bar (15-30 psi)

per minute. Purge the reactor free of process gas with steam and cool to 150oC (300oF)

2 Replace steam with inert gas and cool to ambient temperature. That is to

say below 40°C (105°F) 3 Fill vessel with water and immediately drain off. Air can then be allowed to

enter the vessel as required in order to achieve an atmosphere where life support is not required.

Discharge of intermediate or low temperature shift catalyst The catalyst is usually discharged with large mobile vacuum extraction units or by gravity flow from the bottom of the converter. Reduced LTS catalyst is potentially pyrophoric and care must be taken when it is to be discharged from the reactor. The usual procedure is as follows: 1 Reduce the pressure in the reactor at a maximum rate of 1-2 bar (15-30

psi) per minute, or as governed by the mechanical design of the equipment.

2 Purge the vessel with nitrogen and cool to less than 40oC (100oF). 3 Discharge the catalyst under a positive pressure of nitrogen. This may be

done by vacuum extraction or by gravity flow from the bottom of the converter. In the latter case as catalyst falls from the bottom manhole it is sprayed with water, collected and dumped on a suitable site where it is allowed to oxidize slowly.

In plants where there is insufficient available nitrogen for it to be used during catalyst discharge air must not be allowed to enter the converter when it contains reduced catalyst otherwise gross or localized overheating will take place. In these situations it may be convenient to fill the converter with water, drain and discharge the catalyst wet.



With this technique catalyst should not be allowed to sit in water for any length of time otherwise catalyst breakdown can occur. Under these circumstances it is advisable to drain the vessel as soon as possible after filling. Also note that when reduced catalyst is wetted hydrogen will be generated; hence ensure that precautions are taken to ensure an explosive atmosphere cannot occur and that sources of ignition are controlled. Special procedures are required for the discharge of tubular isothermal reactors. GBH Enterprises, Catalysts Process Technology will advise on the procedures on request. Disposal of discharged catalyst GBH Enterprises, Catalysts Process Technology offers advice on the environmentally safe disposal of its complete product range. Health and safety precautions Before charging, discharging and handling shift conversion catalysts any potential risk to health during these activities should be assessed and appropriate precautions taken. Entry into inert gas atmospheres Extreme care is needed during a shut-down when an entry has to be made into a vessel containing an inert gas. Such atmospheres do not support life and personnel entering must wear a suitable breathing apparatus. Failure to do so will result in loss of consciousness within seconds of breathing the atmosphere followed within minutes by death. To avoid accidental entry of the vessels openings must be kept closed. When personnel have to work inside the vessel, prominent warning notices must be displayed. Everyone working within the area should be made aware of the nature and dangers of asphyxia. They should know how to affect a rescue and resuscitation of anyone who may be overcome. An integrated life support system is essential with adequate back up. If a company has no experience in such activities then the work is often best done by a specialized service firm.



Discharged pyrophoric catalysts Catalysts discharged in the pyrophoric state must be kept separate from flammable materials. Transport of such catalyst should only be in metal containers or metal-sided trucks. Dumps of the catalyst should be within reach of water hoses so that any overheating that occurs can be controlled. High temperatures can build up in heaps of discharged catalyst and it is a prudent precaution to spread the catalyst thinly over the ground until the oxidation is complete and under no circumstances should personnel be allowed to walk over the catalyst until it has been fully stabilized. Dust exposure Short term exposure to the metals and metal oxides used in catalysts may give rise to irritation of the skin, eyes and respiratory system. Over-exposure can give rise to more serious effects. Material Safety Data Sheets (MSDS) should be consulted for information. Catalysts should be handled as far as possible in well ventilated areas and in a way that avoids the excessive formation of dust. Operators who handle catalyst must wear suitable protective body clothing, gloves and goggles. Inhalation of dust should be avoided, and the appropriate occupational exposure limits should be strictly observed. If these limits are likely to be exceeded then respiratory protection should be used. Everyone involved in the handling operation should clean up afterwards and, in particular must wash before eating. Clothing should be changed at the end of each shift, and more frequently if contamination is heavy. Ergonomics Physical hazards arise from the handling of drums, material and lifting equipment. Personnel should be aware of these and appropriate precautions taken.



Reduction and start-up of VULCAN Series VSG-F101 When the reactor has been charged the high temperature shift catalyst must be reduced before it can be used. The reduction of high temperature shift catalyst is invariably carried out with process gas under conditions that allow the haematite to be converted to magnetite without further reduction to metallic iron. Reduction also converts any of the small quantity of residual hexavalent chromium (CrO3) to trivalent chromium (Cr2O3).

3Fe2O3 + H2 → 2 Fe3O4 + H2O ΔH = -16.3 kJ/mol 3Fe2O3 + CO → 2 Fe3O4 + CO2 ΔH = +24.8 kJ/mol 2CrO3 + 3H2 → Cr2O3 + 3H2O ΔH = -684.7 kJ/mol 2CrO3 + 3CO → Cr2O3 + 3CO2 ΔH = -808.2 kJ/mol

It is very important that steam should be present during the reduction procedure in order to prevent over-reduction of the catalyst. It can be shown that if the H2O/H2 ratio exceeds 0.18 at 400oC (750oF) or 1.0 at 550oC (1020oF) then the desired magnetite is the stable phase. Similarly, the CO2/CO ratio should exceed 1.16 at 400oC (750oF) or 1.0 at 550oC (1020oF). The graph below summarizes the conditions necessary to prevent the reduction of Fe3O4 to metallic iron in hydrogen and steam mixtures. Figure 1 Minimum H2O to H2 Ratio for HTS Catalyst Reduction



During catalyst reduction it is preferable to avoid the condensation of water in the catalyst bed. If possible, therefore, the catalyst should be heated in an inert gas stream to a temperature that will prevent the condensation of steam before process gas is admitted to the reactor. All HTS catalysts contain a small amount of residual sulfate that is converted to H2S during the reduction procedure. Low temperature shift catalysts and some CO2 removal systems are sensitive to sulfur and it may be necessary to include a desulphurization step during the start-up of some HTS catalysts. The level of residual sulfur is so low in VSG-F101, that no special desulphurization step is usually needed. Reduction and start-up In plants based on steam reforming of hydrocarbons no separate reduction procedure is required for the HTS catalyst as the introduction of process gas serves to activate, desulfurize and commission the catalyst bed. It is therefore convenient to bring the catalyst on-line as follows: 1 Purge the reactor free of air with an inert gas and heat the catalyst above

the condensation temperature at a rate of about 50oC (90oF) per hour. 2 Care should be taken to ensure that the catalyst is not dried excessively

prior to reduction. This can occur if the catalyst is held in hot nitrogen circulation for an excessive period (24 hrs+), for example if there are problems elsewhere in the plant during start-up. When wet process gas is introduced to the dried oxidized catalyst, structural changes that are exothermic can be initiated, leading to temperatures in excess of 450oC (840oF).

To avoid this phenomenon, introduce process gas as soon as possible after the catalyst is hot enough, or suspend nitrogen circulation whilst problems elsewhere are attended to once the bed is up to temperature.

Should an exotherm occur when process gas is admitted, continue introducing feed to remove the heat generated, and keep the vessel at low pressure. If excessive drying is suspected, it is possible to rehydrate the catalyst by controlled addition of steam, obviously monitoring temperatures carefully whilst small amounts of steam are introduced.



It should be noted that this phenomenon only occurs on the initial start up of the HTS catalyst and does not occur on subsequent start-ups.

3 Establish a flow of process gas or steam through the reactor at a wet gas space velocity in the range 200-1000 h-1. Allow any water that does condense on the catalyst to drain from the vessel. VSG-F101 reduction will start at about 150oC (300oF), if hydrogen is present and so process gas can be utilized at an early stage during heating.

4 Increase the catalyst inlet temperature at a rate of 50oC (90oF) per hour

until the bed temperature reaches 300oC (570oF). Reduction will continue gradually until the normal operating temperature is reached.

5 The high temperature shift reaction will gradually begin at temperatures in

the range 300-320oC (570-610oF) and a temperature profile will develop through the bed. The temperature rise will be about 13.5oC (24oF) for every 1% of carbon monoxide (in dry process gas) that is converted. It is important, therefore, to restrict the amount of carbon monoxide and/or the bed inlet temperature to prevent the bed outlet temperature exceeding 500oC (930oF) during the reduction procedure.

6 If required any residual sulfur in the catalyst will be converted to hydrogen

sulfide at bed temperatures in the range 350-400oC (660-750oF). It is therefore necessary to maintain the catalyst bed at these temperatures for a period long enough for the sulfur to be completely reduced. Bed inlet temperature should be increased to at least 370oC (700oF). VSG-F101 contains less than 0.025% w/w sulfur and the desulphurization period should be only about 4 hours from the first introduction of process gas. It is usual to bring the catalyst on line without the need for a special desulphurization step. It may be prudent to check the sulfur content in the outlet stream before bringing the LTS on-line.

7 Increase the process gas rates and adjust the bed inlet temperature to the

start of run operating value. The above procedure is chosen as a reasonable compromise between energy use and stress on the plant equipment. It should be used during the first reduction of a new catalyst in order to avoid condensation on the catalyst, which can leach any soluble chromium (Vl) from the catalyst, weakening its structure and reducing its life. During subsequent start-ups, plant equipment permitting, steam or normal process gas can be used to warm up the catalyst from cold, and heating rates of 100-150oC/h (180-270oF/h) can be employed without any detrimental effect to the catalyst.



Greater care must be taken if the catalyst has been wetted during the shut-down, and in this case the catalyst must be warmed up slowly at first to allow the pellets to dry out. Once this has been achieved, heating rates can be increased to 100-150oC/h (180-270oF/h). Operation of VULCAN Series VSG-F101 Plants based on steam reforming usually incorporate an HTS stage followed either by a PSA unit or a LTS, CO2 removal and methanation stages. Whatever the plant design, it is usual to operate the HTS catalyst to give maximum carbon monoxide conversion. In plants with more than one shift reactor, a more flexible operation is possible and bed temperatures must be carefully optimized. Optimum conditions can usually be determined by trial and error. When requested, GBH Enterprises, Ltd., Catalyst Process Technology will give advice based on calculations using its own specialized computer programs. The HTS reactor is integrated with the process heat recovery system. It is usually preceded by, and in many modern plants also followed by, a waste heat boiler. The flexibility of the HTS inlet temperature can therefore be limited by steam requirements and boiler performance so that operation under optimum conditions will not always be possible. The normal life of HTS catalysts in ammonia and hydrogen plants is 3-5 years although in some cases it can be longer. End of life may be indicated by an increase in carbon monoxide slip and the end of the temperature profile moving towards and through the end of the bed. It is normal practice, at the start of life, to take advantage of the high initial activity of these catalysts by running at a low inlet temperature (around 300oC/570oF), although in some cases this cannot be achieved due to limitations with the upstream or downstream heat recovery requirements. As the catalyst ages and loses activity over its operational life, it is necessary to raise the inlet temperature gradually to maintain the minimum CO slip, which corresponds to the maximum CO conversion and maximum temperature rise across the bed. Over the life of the catalyst the inlet temperature would typically rise 30-40oC (54-72oF) depending on the initial inlet bed temperature. Catalyst operating life may also be shortened as a result of high pressure drop caused by the accumulation of deposits on the top of the catalyst bed. Depending upon the position of the deposits in the bed, it is sometimes possible to remove these deposits by using a vacuum device during a convenient plant shut-down.



If the deposits are lying in the top section of the bed then this technique can be very effective, and an extension of the operating life may be achieved. If the deposits have migrated down into the main body of the bed, then vacuuming will be of limited use. Loss of activity under normal conditions is usually caused by slow thermal sintering, in which the small magnetite crystals agglomerate together in spite of the stabilizing effect of the chromia. The larger magnetite crystals have a lower active surface area, and hence the catalyst activity decreases. The higher the temperature the greater the rate of sintering. In addition, the effects of certain poisons such as silica can reduce catalyst activity and life. Temperature profile Performance of the catalyst may be monitored during operation by the slope of the temperature profile together with the corresponding increase of outlet carbon monoxide concentration towards the end of life. The temperature profile should not move down the bed unless there are unusual problems. These may be deposition of solids such as soda, silica, potash etc from upstream equipment (such as a waste heat boiler leak or high silica refractory), which block the bed and interfere with gas flow. The most common symptom of blockage is increasing pressure drop. Common problems can usually be identified from routine measurement of bed temperatures, pressure drop through the bed and analysis of outlet carbon monoxide concentration. Advice should be requested from GBH Enterprises as soon as any unusual conditions are experienced. Deposition of solids in the catalyst bed If any solids are deposited on the top of the catalyst bed causing increased pressure drop they should be removed. It is then possible to purge the reactor with inert gas and vacuum extract any contaminated catalyst together with the deposit from the top of the bed. Caution: great care should be taken and procedures well defined before a person enters a vessel containing an inert atmosphere. Depending on the quantity of catalyst that has been contaminated by the deposit it may be necessary to replace with an equivalent volume of new catalyst. No special reduction procedure will be required for the new catalyst.



Sulfur During use the catalyst will establish an equilibrium with any sulfur that is present in the inlet gas. Any unexpected sulfur entering the catalyst vessel will be retained by the catalyst as iron sulfide and then slowly released as normal conditions are resumed. In steam reforming flowsheets the inlet sulfur level should be much less than 1 ppm. However, a HTS catalyst may be used downstream of coal-based or partial oxidation units where the sulfur levels may be significantly higher. For concentrations of sulfur compounds less than 200 ppm in the inlet gas there should generally be no significant effect on the catalyst. Above this level bulk FeS will be formed which has only about half the activity of magnetite and allowance for this must be made in the initial design calculations. Frequent cycling between sulfiding and non-sulfiding conditions should be avoided although the catalyst is strong enough to withstand occasional cycling during plant mal-operation. GBH Enterprises can also offer VULCAN Series VIG SGS201/202/203, a cobalt molybdenum catalyst, which has been developed for shift conversion in a high sulfur environment. Details are available from GBH Enterprises, Ltd. Catalyst Process Technology’s web site, at www.gbhenterprises.com . Shut-down During a short shut-down VSG-F101 may be left in an atmosphere of process gas or steam at operating pressure and temperature. This can result in a partial oxidation of the catalyst that will be reduced rapidly during restart. If the vessel is likely to cool during the shut-down period it should be purged with an inert gas to prevent condensation of water. In addition the vessel drains should be checked and any accumulation of condensate within the vessel drained off.

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Reduction and start-up of VULCAN VSG-C111 catalysts LTS catalysts must be reduced with hydrogen before use. This procedure converts the stable copper oxide component of the new catalyst into reactive copper metal. During reduction and operation both the zinc oxide and alumina components are unchanged and act as a support, which stabilizes the copper metal crystallites and as a reservoir for poisons.

CuO + H2 → Cu + H2O ΔH = -81 kJ/mol Since the reaction is exothermic, the reduction generates large quantities of heat and depends on having equipment available to pass diluent inert gas through the LTS reactor. The easiest procedure is to pass a continuous stream of inert gas, usually methane or nitrogen, through the catalyst bed on a “once through” basis. Although this method can be relatively expensive it has the advantage of allowing a high space velocity during reduction, which will complete the procedure in about 12-24 hours. The alternative procedure is to recycle inert gas, usually nitrogen, through the catalyst bed via a special reduction loop, which also includes a recycle compressor and start-up heater. Space velocity will be limited by the capacity of the recycle compressor but should preferably be at least 300 h-

1 Care should be taken to ensure that the inert carrier gas is free from reducing components (such as hydrogen or CO) and oxidizing components (oxygen). In the event that natural gas is used as the inert carrier the quantity of heavier hydrocarbons should be minimized as such hydrocarbons can crack over copper catalysts. The carrier gas should also be free of catalyst poisons such as sulfur or chloride. With recycle systems there are several important points to remember

1 If the reformer is being used as the start-up heater, then carbon dioxide, evolved from residual carbonates in the LTS catalyst, may methanate and the product methane can crack on the nickel based reforming catalyst in the reformer and thereby deposit carbon. There are various procedures to prevent this from happening and GBH Enterprises, Ltd. Catalyst Process Technology can provide recommendations if required.

2 The concentration of hydrogen entering the LTS catalyst bed should not exceed 0.5% v/v during the early stages of reduction in order to limit the temperature rise if unreacted hydrogen builds up in the recycle loop.



3 In some cases the “minimum gas density limit” of the compressor may restrict

the maximum hydrogen concentration in recycle gas during the final stages of reduction.

4 Water evolved from the catalyst during reduction must be removed from the

closed recycle loop and not be recycled through the catalyst bed. 5 Hydrogen and nitrogen streams need to be free from water and oxygen that

will interfere with the reduction. Nitrogen should be free of hydrogen as this can lead to excess hydrogen being fed to the catalyst.

Reduction procedure The following reduction procedure is recommended for use in plants with facilities for either “once-through” or “circulating recycle” systems for catalyst reduction.

1 Purge the reactor with inert gas until all oxygen has been removed. Establish a flow of inert gas and heat the catalyst bed to 120oC (250oF) at a rate of 50oC (90oF) per hour or as governed by the mechanical design of the equipment. Any convenient pressure, up to operating pressure, may be chosen for the catalyst reduction. In a circulating system a high pressure is normally preferred as it allows a higher gas flow to be achieved in the system, and the higher partial pressure of hydrogen helps the reduction.

2 Increase the inert gas flow rate to the maximum space velocity possible. Ensure that both the hydrogen flow meter and analyzer are operating satisfactorily as the temperature approaches 130oC (265oF). Continue heating the catalyst until the top of the bed is at 180oC (355oF). The temperature of the inert gas should not exceed 210oC (410oF) during the initial heating. If the inert gas space velocity is less than 300 h-1 more care is necessary as there can be poor gas distribution that can lead to localized overheating. Start recording bed temperatures during warm-up to confirm that the thermocouples are responding correctly and that the gas is well distributed through the bed.

3 When at least the top third of the catalyst bed has reached 160oC (320oF) hydrogen should be introduced into the carrier gas entering the bed up to a maximum of 1.0% v/v. Once the reduction reaction has started it will be necessary to record the temperature at different points in the catalyst bed to determine the progress of the temperature profile at regular time



intervals. If the reduction reaction is slow with a bed inlet temperature of 180oC (355oF) then the inlet temperature should be raised cautiously to 190-200oC (375-390oF) and held steady at the temperature which gives a satisfactory reduction rate.

4 Once reduction has started and a steady temperature profile has been

established, the hydrogen concentration should be increased. With nitrogen as carrier gas the hydrogen concentration can be increased to 1.5% v/v and with natural gas as carrier gas the hydrogen concentration may be increased to 2.0-2.5% v/v. The peak temperature in the bed should not, however, exceed 230oC (445oF) and the hydrogen concentration should be changed as necessary to control the temperature rise and thereby limit the peak bed temperature. Once reduction has started it may be possible to decrease the temperature of inlet gas entering the catalyst bed to 180oC (355oF) or less. The temperature rise for 1% hydrogen is typically 30oC (54oF) in nitrogen and 20oC (36oF) in natural gas.

5 As the reduction proceeds, the temperature profile will move down the

catalyst bed. The temperature rise will decrease when most of the copper oxide has been converted to copper. At this point the catalyst bed inlet temperature may be raised to 200oC (390oF). The inlet hydrogen concentration can also be increased to 3-5% v/v provided that the maximum temperature limit of 230oC (445oF) in the catalyst bed is not exceeded.

6 When the catalyst reduction appears to be complete the catalyst bed inlet

temperature should be raised to 225-230oC (435-445oF) and then if possible, the inlet hydrogen concentration in the inert gas should also be increased to 20% v/v. This procedure should take not less than two hours. No temperature rise should be observed and the maximum catalyst temperature should not exceed 230oC (445oF). Analysis should indicate that the hydrogen concentration inlet and exit of the catalyst bed are within 0.2% of each other.

7 The catalyst reduction is complete and the reactor should be

commissioned.



Controlling the catalyst reduction The reduction procedure has been designed to limit the temperature rise in the catalyst bed by restricting the hydrogen concentration. This ensures that the maximum temperature in the bed does not exceed 230oC (445oF) and the maximum catalyst activity is achieved. The reduction reaction is indicated by the temperature profile which moves from the inlet to exit of the catalyst bed at a rate which depends on inert gas space velocity, hydrogen concentration and bed inlet temperature. During the whole of the reduction period it is important that operators should determine the inlet and exit hydrogen concentration at regular intervals. The difference between these two measurements during the time of the reduction represents the volume of hydrogen consumed. Any oxygen present in carrier gas will also react with hydrogen to form water. Normally the volume of hydrogen required for the reduction is 185 Nm3/m3 (195 scf/ft3) for VULCAN Series VSG C111 catalysts. A comparison of the hydrogen consumed against the theoretical consumption should be made as a cross check against the progress of the reduction. The volume of water forming during the reduction procedure will also provide an indication of the progress. Measurement of water produced should only be used as a rough check on hydrogen uptake. VULCAN Series VSG C111 catalysts will produce 240 kg water/m3 (15 lb water/ft3) catalyst, from the reduction process. Water formed from oxygen present in the inert gas should not be included in any estimate. Again this can be used as a cross check against the progress of the reduction. Catalyst reduction is virtually completed when the inlet and outlet hydrogen concentrations are the same and the whole bed is above a temperature of 225oC (435oF). The volume of hydrogen consumed should confirm this. It may be difficult to achieve exactly equal hydrogen concentrations at inlet and outlet of the bed and reduction may be considered complete when the difference between the two measurements has been less than 0.5% v/v for more than four hours. Any complex copper-zinc basic carbonates present in the catalyst decompose during reduction and release carbon dioxide. Carbon dioxide can be purged from the recycle system but if for any reason the catalyst reduction procedure is halted, or the catalyst bed isolated at reduction temperature, then any further carbon dioxide evolution will lead to an increase in reactor pressure. Pressure should therefore be monitored during the time that a reactor is isolated, when it contains partially or freshly reduced catalyst, and any increase in pressure controlled by venting.



In addition, if the reactor is to remain isolated for any length of time after the reduction is completed but before it is commissioned, catalyst bed temperatures should be monitored frequently. If any increase in temperature is detected the reactor should be immediately purged with inert gas to avoid any rapid temperature rise. Hydrogen source Almost any gas containing hydrogen is suitable for the reduction e.g. methanator gas, carbon dioxide removal or high temperature shift reactor effluent gas. Hydrogen should be free of sulfur or chlorine and, if any carbon monoxide is present, allowance should be made for the extra temperature rise during reduction. Natural gas Natural gas is used as the inert carrier gas during reduction in many natural gas/steam reforming plants. Any high molecular weight hydrocarbons in the natural gas can crack in the pre-heater at temperatures below 300oC (570oF) to produce hydrogen. Most types of natural gas have been safely used at a maximum catalyst temperature of 230oC (445oF) so it is recommended that care should be taken in measuring the hydrogen concentration carefully at the catalyst bed inlet and that the bed temperature be carefully controlled. Start-up If the catalyst has already been reduced but is cold, the bed should be warmed to a temperature above the dew point with inert gas before process gas is introduced to the reactor. When process gas first contacts the catalyst the bed temperatures will usually increase rapidly as the reaction comes to equilibrium with process conditions. The peak temperature may reach 260oC (500oF) or higher at this stage but there will be no damage to the catalyst because the peak will quickly pass through the bed. The high temperature can be moved quickly through the bed by increasing the flow of process gas to design rates as soon as possible. The catalyst bed inlet temperature should also be held as low as possible provided that it is at least 20oC (35oF) above the dew point. For most duties this corresponds to an inlet temperature of about 200oC (390oF). If there are particular reasons for avoiding a temperature peak there are several ways by which it can be minimized.



1 By increasing reactor pressure to design level with inert gas before

introducing process gas. 2 Introducing process gas at low pressure while venting gas at the reactor exit.

This is particularly easy after reducing catalyst with a ‘once-through’ flow of natural gas by gradually replacing the flow of natural gas by process gas and then opening the inlet and exit valves fully while closing the vent to commission the reactor.



Operation of VULCAN Series VSG-C111 catalysts It is important to operate the LTS catalyst under optimum conditions to achieve the potential savings in plant costs. The LTS catalyst is sensitive to changes in operating conditions but it is not difficult to maintain fixed steam ratio, pressure and gas composition so that the only real variable is the catalyst inlet temperature. During the commissioning procedure the bed inlet temperature is gradually increased until the carbon monoxide concentration in exit gas falls to the minimum level for the conditions. This is the optimum level for maximum CO conversion and at higher inlet temperatures the carbon monoxide level will again increase. As the catalyst ages or is poisoned it will be necessary to increase the inlet temperature to maintain the minimum carbon monoxide concentration in the exit gas. LTS catalysts often operate close to condensation conditions during the early part of the catalyst life. To avoid condensation of water either in the catalyst pores or onto the bed the inlet temperature should be at least 20oC (35oF) above the dew point at all times. This may mean that operation will be at temperatures higher than the optimum until catalyst activity has fallen sufficiently for the actual and optimum operating temperatures to correspond. This is not a problem because at temperatures in the range 200-205oC (390-400oF) the difference between the equilibrium outlet carbon monoxide concentration and the optimum will be very small and the actual outlet concentration will remain constant for a long period. During the normal operating life of the catalyst, optimum operating conditions can be maintained by a gradual increase of the bed inlet temperature as soon as the carbon monoxide level increases slightly. Whenever changes in steam ratio or gas composition occur the bed inlet should be checked to ensure that it is still at the optimum level. This should be done by increasing or decreasing the bed inlet temperature by 5oC (10oF) and then checking the carbon monoxide concentration at the bed outlet when conditions have stabilized. If a decrease in the carbon monoxide concentration is detected the procedure is repeated until the minimum level has been reached. A simple way of determining CO slip is to observe the methanator temperature rise if the flowsheet features this reactor. Minimum CO slip from the low temperature shift will correspond to the minimum temperature rise across the methanator. Towards the end of the catalyst life the bed exit temperature will often reach the design level, which might occasionally correspond to the specified maximum operating temperature of 250oC (480oF). This is however a conservative figure and operation up to at least 270oC (520oF) will still be possible.



At higher temperatures however the deactivation rate for partially poisoned catalyst is faster and the carbon monoxide equilibrium level becomes increasingly unfavorable. Operation with high outlet carbon monoxide concentrations will become increasingly expensive. It is usually more economic to plan a catalyst change before the performance deteriorates beyond the design level. By-product formation Methanol and, to a lesser extent amines formed from methanol produced in the LTS converter and any nitrogen compounds inlet the converter (such as ammonia formed in the secondary reformer), are formed in low temperature shift catalyst beds, particularly in the early stages of life when catalyst activity is at its maximum. By-product formation is very sensitive to temperature and can be minimized by running with a low inlet temperature. This is consistent with maximizing CO conversion. As ageing occurs, by-product formation is reduced. If operators require ultra-low methanol by-product formation, then GBH Enterprises, Ltd. Catalyst Process Technology should be consulted. Temperature profile The temperature profile through the catalyst bed is a useful indicator to follow changes in catalyst activity especially when the outlet carbon monoxide concentration is at the equilibrium level. For a fresh catalyst most of the reaction and the corresponding temperature rise will be at the top of the bed. Loss of catalyst activity (or catalyst deactivation) during operation is largely due to poisoning. Because the catalysts are “self-guarding” poisons accumulate at the top of the catalyst bed. The temperature profile will therefore gradually move from the inlet towards the exit of the catalyst bed as more poisons are absorbed. Towards the end of the catalyst life when the reaction zone has reached the bottom of the bed and the outlet carbon monoxide level has started to increase from the equilibrium concentration, the catalyst should be changed. Any variation from a typical temperature profile will indicate abnormal conditions. 1 A slow increase in bed temperature giving a flatter than average profile

can indicate that the whole catalyst bed has been partially deactivated. This may be due to the presence of liquid water in the bed that would block the catalyst pores and wash poisons from the top of the catalyst down to the middle or bottom levels. The catalyst may also have been overheated.



3 If the temperature profile appears to be normal but the outlet carbon monoxide is higher than expected then gas may be bypassing part of the catalyst bed.

Steam The use of steam alone should be avoided as far as possible to prevent condensation of water in the catalyst bed. During plant upsets, short periods of steaming may be unavoidable but it is far better to isolate the low temperature shift reactor and reduce pressure to depress the dew point. The reactor should then be purged with an inert gas. Shut-down During an extended plant shut-down, when the reactor can cool down, process gas must be purged from the reactor to avoid the condensation of water on to the catalyst. This could damage the catalyst by washing poisons from the top to the bottom part of the catalyst bed onto fresh unpoisoned catalyst lower down the bed. Pressure should therefore be decreased to atmospheric, before the temperature falls below the dew point, and the vessel purged with an inert gas to remove all steam. Catalyst poisons Sulfur and chloride are the most serious poisons for LTS catalysts. Of the two, chlorides are the more virulent; however, sulfur tends to be present in greater concentrations in the process gas and therefore often determines the catalyst life. Chlorine compounds are often present in process gas streams in extremely small concentrations that cannot be detected by typical analytical procedures. The poisoning effect is cumulative so that any concentration of chlorine in process gas will eventually poison the catalyst bed and detection is only possible by the analysis of samples then from discharged catalyst. The formulation of GBH Enterprises, VULCAN Series catalysts to provide thermally stable structures also enhances the ability of these catalysts to absorb poisons. VSG-C111 series catalysts can absorb chlorides at the top of the bed and guard active catalyst in lower layers, and so extend operating time.



Strict attention is necessary however to maintain steam purity and to avoid contamination of feedstocks or process air by chlorine compounds. Solvents containing chlorine should not be used for cleaning any items of plant equipment. If chloride poisoning is an issue then GBH Enterprises, Ltd. Catalyst Process Technology should be consulted. Sulfur compounds also affect the operation of VSG-C111 series catalysts but are much less virulent poisons than chlorine compounds. GBH Enterprises, Ltd. VULCAN Series catalysts are self-guarding against sulfur compounds provided that the typical levels found in ammonia or hydrogen plants based on steam reforming are not exceeded for long periods. Silica is also present in most process gas streams and is absorbed by the catalyst bed and gradually deactivates the catalyst. Small amounts of silica are deposited on the catalyst surface but larger quantities react with the catalyst to form zinc silicate. Silica is not a typical catalyst poison but has the effect of decreasing the catalyst’s capacity for other poisons and therefore allows chlorine and sulfur to pass further into the catalyst bed. Hydrogen and ammonia plants should always be designed to include sufficient catalyst volume to operate satisfactorily with average levels of poisons present in feedstocks. Adjustments can be made when increased levels of poisons are detected either by using extra catalyst volume or by installing appropriate guard beds.