710W - Steam Ref Op Man Rev3a

Operating Manual Catalysts for Steam Reforming Natural Gas, Associated Gas & LPG

Contents Page Introduction TM

KATALCO

3 25-series, KATALCO 57-series and KATALCO 23-series 23-series catalysts

4

Catalyst storage, handling, charging and discharging

6

Health and safety precautions precautions

9

Start-up

10

Operation

15

Reformer tube appearance

17

Catalyst poisons

19

Nickel carbonyl hazard

21

Shut-down

22

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 i t is for the User to satisfy itself of the suitability of the product for its own own particular purpose. Johnson Matthey Catalysts Catalystsgives 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 excluded except to the extend that exclusion exclusion is prevented by law. Johnson Matthey Catalysts accepts no liability for loss or damage (other than that arising from death or personal injury caused by Johnson Matthey Catalysts’ negligence or by a defective Product, if proved), resulting from reliance on this information. Freedom under Patent, Copyright Copyright and Designs cannot be assumed. assumed.

701JM/103/0AMOG © Johnson Matthey Group 2003

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Introduction Johnson Matthey steam reforming catalysts are designed for operation in all current steam reforming furnaces. They can handle a wide range of feedstocks from natural gas through o o LPGs to naphthas with a final boiling point up to 225 C (435 F). Johnson Matthey steam reforming catalysts are available in a wide range of types and sizes to suit specific steam reformer designs and feedstocks. As the hydrocarbon feedstock becomes heavier (e.g., LPG) the catalysts must be promoted with potash or lanthana to resist carbon deposition. This also allows reforming of lights feeds (e.g., natural gas, off-gas) at steam to carbon ratios lower than the unpromoted natural gas reforming types. Thus combinations of promoted and unpromoted catalysts can be provided for all duties, ranging from reforming natural gas with little or no heavier hydrocarbons, natural gas containing significant proportions of heavier hydrocarbons, mid-range feedstocks such as LPG, or even use in plants which are prone to “hot band” formation. The steam reforming of saturated hydrocarbons can be represented by the three reactions: CnHm + nH2O

→

nCO + (n+m/2) H 2

CO + 3H2

→

CH4 + H2O

CO + H2O

→

CO2 + H2

An excess of steam is required to suppress carbon formation and promote the reforming reaction, so that the steam ratio is normally in the range 2.5 – 3.5 for ammonia and methanol plants, 2.5-5.0 for hydrogen plants, and 1.6-2.5 for H 2/CO plants. Also despite the thermodynamics of the process, reformers operate at high pressure to optimize the overall economics of the process. With light feeds, KATALCO 57-series or KATALCO 23-series catalysts can be used successfully. As feeds become heavier or at lower steam to carbon ratios, combinations of catalysts are usually required. The commonest combinations are to use either alkalized KATALCO 25-series catalysts in the top 40-50% of the reformer tube, with KATALCO 57-series or KATALCO 23-series in the remainder of the tube. KATALCO 57-series and KATALCO 23-series are produced by impregnating a strong, porous calcium aluminate support with nickel. KATALCO 23-series is produced by impregnating a strong alpha alumina support with nickel. These techniques provide catalysts with high strength and activity. KATALCO 57-series and KATALCO 23-series catalysts are unalkalized. Johnson Matthey Catalysts will make appropriate recommendations for the proportions. These catalysts must be reduced before use to convert nickel oxide to the active nickel metal. This manual discusses the principles of start-up, operation and shut down for the Johnson Matthey catalysts for steam reforming natural gas through to LPG feedstocks. The information provided is sufficient for the preparation of the detailed operating instructions which of necessity will be plant-specific.


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KATALCO 25-series, KATALCO 57-series, KATALCO 23-series catalysts All the catalysts are based on calcium aluminate or alumina ceramic supports, impregnated with nickel oxide, and are available in a range of sizes to allow selection of the optimum for any particular application. For use with heavier feedstocks, or with light hydrocarbon feeds at low steam to carbon ratios, the KATALCO 25-series catalysts are specially promoted with potash. Composition

KATALCO 25-series

nickel oxide dispersed on a promoted calcium aluminate ceramic support.

KATALCO 57-series

nickel oxide dispersed on a calcium aluminate ceramic support.

KATALCO 23-series

nickel oxide dispersed on an alpha alumina support.

These catalysts can be supplied pre-reduced.

Physical properties (typical) Catalyst Form

23-4

23-4M

23-4G

23-4Q

23-4MQ

23-4GQ

Optimized 4-hole shape



Optimized Quadralobe



15 11 3

20 19 5.5

17.5 13 3.3

13 10.5 2.7

20 16 4.4

1200 75

1100 70

1090 68

1140 71

1050 66

25-4

25-4M

25-4G

25-4Q

25-4MQ

25-4GQ







15 11 3

20 19 5.5

17.5 13 3.3

13 10.5 2.7

20 16 4.4

900 56

900 56

870 54

950 59

790 49

57-4

57-4M

57-4G

57-4Q

57-4MQ

57-4GQ







15 11 3

20 19 5.5

17.5 13 3.3

13 10.5 2.7

20 16 4.4

900 56

900 56

850 53

900 56

760 47

19 Length (mm) OD (mm) 14 4 Hole ID (mm) Typical charged bulk density 3 1100 (kg/m ) (lb/ft³) 70

Catalyst Form

Length (mm) 19 OD (mm) 14 4 Hole ID (mm) Typical charged bulk density 3 1000 (kg/m ) (lb/ft³) 65

Catalyst Form

Length (mm) 19 OD (mm) 14 Hole ID (mm) 4 Typical charged bulk density 3 1000 (kg/m ) 62 (lb/ft³) 701JM/103/0AMOG © Johnson Matthey Group 2003

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By special arrangement, the catalysts may also be supplied as Raschig rings in which case the instructions below may be applied and further product information will be supplied on request.

Important Note The charged density values are typical values. Depending on reformer type, loading technique etc, actual achieved density may be different from these values .


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Catalyst storage, handling, charging and discharging Before charging, discharging and handling steam reforming catalysts any potential risk to health during these activities should be assessed and appropriate precautions taken. In addition the Johnson Matthey brochure on “Catalyst Handling” should be consulted.

Drum storage Steam reforming catalyst is generally supplied in mild steel drums fitted with polythene liners, having the following packaging details. These parameters may change but precise information will be recorded in the documentation covering the goods. Catalyst 25-4/57-4 25-4G/57-4G 25-4M/57-4M 23-4 23-4G 23-4M 25-4Q/57-4Q 25-4GQ/57-4GQ 25-4MQ/57-4MQ 23-4Q 23-4GQ 23-4MQ

Net weight (kg) 155 155 175 200 200 200 145 145 170 200 200 200

Tare (kg) 18 18 18 18 18 18 18 18 18 18 18 18

Drum dimensions (cm) 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia 88 height x 59 dia

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. Catalyst that is supplied pre-socked will be contained in wooden crates. Storage guidelines are similar to drum storage.

Pre-reduced catalyst storage Pre-reduced catalysts will be supplied in mild-steel drums as described above. All drums should be inspected when they first arrive on site. Any catalyst in damaged drums should be repacked in clean, dry lined steel drums and sealed to restrict access to air and protect the catalyst from moisture. The catalyst drums must be kept cool, dry and away from direct sunlight and sources of heat. o o The stabilization layer will break down at temperatures in excess of 100 C (210 F). With adequate access to air, the heat generated will accelerate the breakdown. The temperatures o o generated can exceed 1000 C (1830 F).


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The stabilization layer can also be broken down by contact with specific chemicals. Water is the greatest risk here, hence the strict requirement to keep the catalyst dry especially during catalyst loading and storage. Given the potential to generate heat, these catalysts should be stored away from combustible materials.

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 containers for either catalyst drums or palleted drums eases shipment and further reduces the likelihood of damage in transit but, again, suitable fork-lift trucks must be provided at the reception point. 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.

Pre-charging checks Before charging begins, the support grids must be checked and both the inlet and exit connections should be tested to ensure that they are clear. It is common practice to blow each tube with air and then to carry out a visual inspection using a boroscope. When the support grid is clear the tube top should be covered to prevent debris from dropping in. A useful check for obstructions in the inlet and exit connections can be made with a pressure drop device which is described in the Johnson Matthey “Catalyst Handling” brochure.

Charging reformer tubes The target in all steam reforming charging operations is to ensure that each tube has an identical pressure drop such that the process flow down each tube is uniform across the whole reformer. In charging, it is important to ensure that the catalyst is not dropped by more than 50 cm (20 ins) as this can lead to damage of the catalyst pellets. It is also important that the catalyst pellets must not be allowed to form bridges across the tubes as this leads to empty regions in the tubes and overheating during operation. There is a variety of established reforming catalyst charging techniques. The most common TM one in use today is UNIDENSE loading licensed from Norsk Hydro. The UNIDENSE technique uses steel springs or brushes in the tubes to prevent excessive free fall of the catalyst that might otherwise cause damage. The patented technique, which is licensed by Johnson Matthey Catalysts, eliminates the need to use socks. Experience with the UNIDENSE loading technology has found the process to be quicker and to give more even loading of the tubes. More even tube loading has been shown to give less temperature variation at the tube exits, resulting in better overall reformer performance. An alternative loading method is the use of polythene or canvas socks, which allow the catalyst to be lowered slowly down the tube until there is only a short free-fall height for the catalyst. The catalyst is then allowed to spill out of the sock by applying a short jerk to the rope to which the sock is attached. This method is described in more detail in the Johnson Matthey Catalysts brochure on “Catalyst Handling”. To save time at site with filling the socks with catalyst, the catalyst can be purchased pre-socked.

Catalyst discharge


Page 7

The catalyst is usually discharged from the reforming tubes with vacuum extraction units. In some instances the design allows gravity discharge.

Disposal of discharged catalyst Through the “Catalyst Care” Programme, Johnson Matthey offers the environmentally safe disposal of their complete product range.


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Health and safety precautions Before charging, discharging and handling steam reforming catalysts any potential risk to health during these activities should be assessed and appropriate precautions taken.

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 skips 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 and it is a prudent precaution to spread the catalyst thinly over the ground until the oxidation is complete. 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. Product safety data sheets 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, materials and lifting equipment. Personnel should be aware of these and appropriate precautions taken.

Note: The European Union has decided that it is prudent to classify all insoluble nickel compounds as Category 1 carcinogens. Appropriate information is contained in the Material Safety Data Sheet sent with all orders.


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Start-up When the steam reformer is commissioned, the furnace designer’s instructions should be observed, especially those concerning the rate of increase of temperature and the heating procedure. Instructions concerning the burner pattern and the rate at which the firing is increased should be followed. As a general rule, whenever the throughput is increased, the change in steam flow should always precede the change in hydrocarbon flow to ensure that the steam to carbon ratio is always held at, or above, the design value. Throughout the start-up, regular visual checks must be made on the furnace, because the instrumentation provided to monitor furnace performance is designed primarily for operation under normal running conditions. It is virtually impossible to provide sufficient instrumentation for start-up conditions that will give a t rue measure of the process conditions during start up and shut downs and other similar transient operations. The appearance of the tubes, flames and refractory lining is the best guide to the actual furnace conditions during start-up. Care should be taken to maintain safe conditions in the heat recovery section because mechanical failures are often a major cause of delay during the start-up period. Although details will vary from plant to plant, the general procedure described will be sufficient for most modern plants. The reduction of the nickel oxide content in a steam reforming catalyst charged to a steam reformer is possible using either hydrogen, off-gases containing hydrogen, natural gas, LPG’s, ammonia or methanol. For most seam reformers, the catalyst should preferably be reduced in the presence of hydrogen before feedstock is introduced. This will achieve maximum catalyst activity, particularly at the tube inlet where the temperature is lowest. If no hydrogen is available then natural gas or other alternates may be used.

Warm-up rate The warm-up rate is determined by the need to minimize both the thermal stresses on plant equipment as well as the energy consumption during start-up. Minimum thermal stress is favoured by a low rate of temperature rise while, of course, minimum energy consumption is o o favoured by rapid heating of the reformer. Traditionally, a warm-up rate of 50 C (90 F) per hour has provided a reasonable compromise between the two objectives. Johnson Matthey reforming catalysts can however, withstand a more rapid rate of heating. Consequently, if the o o constraints on plant equipment allow, warm-up rates as high as 150-170 C (270-305 F) per hour can be used to minimize energy consumption. The effects of rapid temperature rise on plant equipment must be known before the appropriate warm-up rate can be selected. Particular attention should be given to the thermal expansion of inlet/exit pipes, reforming tube tensioners and refractory linings. 1

Purge the plant free of oxygen using nitrogen, and heat the reformer above the condensation temperature while still circulating nitrogen. Steam may be added as o o soon as possible after the temperature of the exit header is at least 50 C (90 F) above the condensation temperature of steam. As an alternative, freshly charged catalyst may be heated in air during the initial o o reformer warm-up period. As soon as the exit header is 50 C (90 F) above the condensation temperature the plant must always be purged free of oxygen with steam. Note, however, that this is not recommended for catalyst which has previously been reduced, or been supplied in a pre-reduced form.

2

The steam flow should be increased to 40-50% of the design rate as soon as possible to allow even firing of the furnace.

3

Nitrogen circulation may be stopped at any convenient time after steam has been added and before hydrocarbon feedstock is introduced.

4

Excessive delays should be avoided before starting the catalyst reduction.


Page 10

For KATALCO 23-series, KATALCO 25-series and KATALCO 57-series catalysts, steaming for more than 24 hours before proceeding with the reduction is not recommended.

Reduction with hydrogen 1

Hydrogen or hydrogen-rich gas may be added at any time after the plant has been purged of oxygen. The steam to hydrogen ratio should be controlled in the range for reducing conditions and additional hydrogen should be supplied to maintain this ratio throughout the reduction period. Hydrogen must always be free of catalyst poisons. For KATALCO 23-series, KATALCO 25-series and KATALCO 57-series catalysts, the steam to hydrogen ratio for reduction is in the range 6-8 to 1.

2

Increase the reformer exit temperature to the design value. This may be done before the hydrogen concentration has attained the required level. At all critical stages during reduction it must be emphasised that the temperatures referred to are those at the actual tube exit. Temperatures indicated by control room instruments are inevitably lower than the true value because of heat losses and allowance must be made for any discrepancy. Depending on the location of the thermocouples, o o indicated values may be 25-100 C (45-180 F) lower than actual temperatures. Regular visual checks on the tubes should be conducted to ensure that the reformer tubes are not being overheated.

3

During catalyst reduction, the tube inlet temperature should be as high as possible to promote maximum reduction at the inlet of the tubes.

4

For KATALCO 23-series, KATALCO 25-series and KATALCO 57-series, once the reformer has reached the design exit temperature, the catalyst reduction will be virtually complete and there will be no need to maintain reducing conditions for more than two hours unless the steam to hydrogen ratio has been outside the appropriate limit during the warm-up.

5a

Introduce natural gas. Hydrogen addition should continue, if available, until the hydrogen recycle loop to the desulphurizer can be established. The natural gas rate should be increased to 25% design rate as quickly as possible. The steam to carbon ratio must be at least 5:1 and the steam rate should be increased as necessary to maintain this condition as the natural gas rate is increased. The firing rate should be adjusted to maintain the design exit temperature as the reforming reaction begins. The steam rate should always be increase before increasing the feed rate such that the steam to carbon ratio is always higher than 5:1 and therefore the potential for carbon formation is minimized.

5b

With higher molecular weight gaseous hydrocarbon feedstocks, the minimum steam to carbon ratio should be at least 10:1.

6a

Increase the flow of natural gas until the design steam to carbon ratio is reached. Check the methane concentration of the exit gas during this period to ensure that it reaches a low, steady, level. If the methane concentration increases or the tubes show hot zones, then restore reduction conditions with hydrogen for 1-2 hours, or increase the steam to carbon ratio to 7:1 and hold the decreased natural gas flow steady for 2-3 hours before increasing the rate once again.

6b

With higher molecular weight gaseous hydrocarbon feedstocks, it is recommended that the hydrocarbon flow be increased more slowly in stages during a period of at least three hours until the design steam to carbon ratio is reached. Again, check the methane concentrations of exit gas after each change in rate to ensure that it reaches a low steady level. If the methane concentration increases or the tubes show hot zones, restore reduction conditions with hydrogen for 1-2 hours, or increase the


Page 11

steam to carbon ratio to 7:1 and hold the hydrocarbon flow steady for 3-4 hours before increasing the rate once again. 7

Throughput may now be increased to the design level in regular stages that depend on the overall plant commissioning schedule. Always increase the steam flow before the hydrocarbon flow to maintain the steam to carbon ratio at or above the design value.

Hydrogen source Hydrogen or hydrogen-rich gas needed for catalyst reduction is supplied from either high pressure cylinders, from another plant, or by cracking ammonia or methanol. The source of hydrogen must be free of poisons such as sulphur and chlorine. Attention must also be paid to the nature of any hydrocarbons in the hydrogen source and the composition of these hydrocarbons. If the hydrogen source contains carbon oxides, then additional checks have to be made since these carbon oxides can methanate across the hydrodesulphurisation catalyst. At o o temperatures above 250-300 C (480-570 F) any carbon oxides could methanate over the unsulphided cobalt molybdate or nickel molybdate catalyst in the hydrodesulphurizer, causing an excessive temperature rise. If the hydrogen contains more than 3% (vol) of carbon monoxide or more than 13% (vol) of carbon dioxide, it is advisable to bypass the hydrodesulphurizer. If the hydrogen source contains hydrocarbons, then these will be converted to carbon oxides in the reformer. The start-up procedure may well include a recycle through the desulphurization system and thus a similar limit to carbon oxides applies or bypass of the hydrodesulphurizer is required.

Reduction with ammonia Ammonia can be used as a source of hydrogen by cracking it to a hydrogen/nitrogen mixture in a conventional ammonia cracker or in the steam reformer tubes. Liquid ammonia should be injected upstream of the primary reformer, usually before the process steam superheater. When it is introduced before the feedstock heater, the desulphurizer should, if possible, be bypassed. The reduction procedure described for hydrogen should then be followed except o o that the reformer exit temperature should preferably be as high as 800 C (1470 F) to achieve maximum ammonia cracking. Johnson Matthey will provide more detailed advice on the procedure on request.

Reduction with methanol Methanol will decompose in the primary reformer to give a mixture of hydrogen and carbon monoxide and can also be used as the source of hydrogen during catalyst reduction. The flow of liquid methanol should be regulated to give a steam to hydrogen ratio between 6:1 and 8:1 at the reformer exit. Cracked gas should not be recycled because of the possible methanation of carbon oxides. Johnson Matthey will provide more detailed advice on the procedure on request.

Reduction with natural gas 1

After following the recommended warm-up procedure, and at the stage where there is no nitrogen circulation but with 40-50% design rate steam flow, increase the reformer exit temperature. For KATALCO 23-series, KATALCO 25-series and KATALCO 57o o series catalysts, increase the temperature to 750 C (1380 F).


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2

Introduce desulphurized natural gas at about 5% design rate. Increase the gas rate to give the required steam to carbon ratio during a period of 2-3 hours, and then hold this ratio for the specified period. The heat requirement in the furnace will increase as the endothermic steam reforming reaction begins; this will cause a reduction in the reformer exit temperature and therefore the furnace firing should be increased to maintain the necessary exit temperature. For KATALCO 23-series, KATALCO 25-series and KATALCO 57-series catalysts, the steam to carbon ratio should be 7:1. While the natural gas rate is being increased to this level, the reformer exit temperature should be increased to the design level. These conditions should be held, usually for 4-8 hours, until reduction is complete which can be checked for by monitoring the methane slip.

3

During the reduction period, maintain the tube inlet temperature as high as possible to ensure catalyst reduction at the tube inlet.

4

For the required steam to carbon ratio, a steam flow of about 40-50% of the design value usually corresponds to a natural gas flow rate of 20-25% design. As the catalyst gradually reduces, more methane will be reformed. During this stage the exit methane concentration should be checked at hourly intervals. Reduction should be complete when the exit methane concentration reaches a low steady value.

5

When the catalyst reduction is complete, increase the flow of natural gas in stages until the design steam to carbon ratio is reached. Always increase the steam rate before the natural gas rate to maintain the steam to carbon ratio at or above the design value. Check the methane concentration in the reformer exit gas after each change to ensure that it stays at a low steady value. If the methane concentration increases or the tubes show hot zones, continue the reduction for a further period at a steam to carbon ratio of 6-7:1, before once again increasing the natural gas flow to the design steam to carbon ratio.

6

Increase the throughput in stages to the design level depending on the overall plant restart schedule. Always increase the steam rate before the natural gas rate to maintain the steam to carbon ratio at or above the design value.

Reduction with other hydrocarbons Hydrocarbons, other than methane or light refinery gases may be used as an alternative hydrogen source for catalyst reduction, but there is an increased risk of carbon formation. Ethane, propane or butane streams could be used, but great care is required as the risk of carbon formation is very high, and it is essential that instrumentation is capable of monitoring the correct steam and hydrocarbon flow during the catalyst reduction (e.g., low pressure, low flows etc). In addition, the purification unit must be at a sufficient temperature to desulphurize the feed. If organic sulphur species are likely to be present, then some hydrogen is required for complete hydrodesulphurization. However, the general use of hydrogen addition to the hydrocarbon feed will greatly assist the reduction process and help prevent carbon formation. For KATALCO 23-series, KATALCO 25-series and KATALCO 57-series catalysts, the procedure described for reduction with natural gas should be used but the hydrocarbon flow during reduction should be controlled in the range 8-10:1.

Reduction with hydrogen and hydrocarbon mixtures When the reforming catalyst is required to be reduced with hydrogen/hydrocarbon mixtures (e.g. refinery off-gas feeds), consideration should be given to both the steam to hydrogen ratio as well as the steam to carbon ratio. For KATALCO 23-series, KATALCO 25-series


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and KATALCO 57-series the steam to hydrogen ratio should not be less than 6:1 and the steam to carbon ratio should not be less t han 7:1.

Pre-reduced catalyst As pre-reduced catalyst is used in conjunction with oxidized catalyst, the catalyst reduction procedure described above still applies. However, in some circumstances, use of prereduced catalyst can significantly reduce the reduction time.

Re-start When catalyst has been steamed during a plant shut-down, or upset, re-reduction will be required to restore full activity. The procedure is the same as for new catalyst except that for KATALCO 23-series, KATALCO 25-series and KATALCO 57-series catalysts the reduction procedure is faster. When reducing with hydrogen, the catalyst activity will be recovered as soon as the appropriate operating temperature is reached. When reducing with natural gas however full catalyst activity will normally be achieved in less than two hours after reaching the operating tube exit temperature. On re-heating, the holding period on steam only should not exceed 72 hours.

Catalyst wetting If the catalyst has been subjected to severe wetting (for example, due to condensation of steam during start-up or shut-down), then it is essential that the catalyst should be dried out slowly, by holding the temperature just below or at the condensation temperature until the catalyst is dry. This is to avoid the risk of rapid vaporization of water in the catalyst pores, leading to break-up of the catalyst. To ensure complete and thorough drying, it is recommended that the catalyst is then heated o o to 500 C (930 F), and held at constant temperature for a period of one hour, before commencing the start-up procedure. This drying process should preferably be carried out using nitrogen. Alternatively, air may be used, but only with a fresh charge of catalyst which has not previously been reduced.


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Operation Johnson Matthey steam reforming catalysts operate successfully over a wide range of plant conditions and withstand most minor plant upsets. Catalyst life will however be affected by a large number of factors related to the purity of feedstock and the ability to maintain sufficient steam to prevent carbon deposition. Any variation in these two major operating conditions will eventually lead to loss of catalyst activity and overheating of the tubes. Plant operation is mainly concerned with the maintenance of approved conditions to avoid catalyst poisoning and/or carbon formation .

Carbon formation Under normal conditions, the rate of carbon gasification is greater than the rate of carbon formation and therefore the overall rate of carbon formation is essentially zero. Complete loss of reaction steam during plant incidents, however, will result in massive deposition of carbon and a rapid increase in pressure drop. In the worst gas, the resistance to flow is so great that it is impossible to steam the catalyst and complete catalyst replacement will then be required. Between these extremes, operation with a small steam deficiency or a slightly reduced catalyst activity can result in slow carbon deposition. This can cause increased pressure drop and lead to higher than normal tube temperatures that will often give a patchy tube wall appearance. To prevent this from occurs, it is important that the operating conditions are such that net carbon formation is prevented; it is typical therefore the steam to carbon ratio or hydrogen recycle is increased once carbon formation starts to occur. Carbon deposition in the pores of a catalyst, as a result of unfavourable equilibrium (thermodynamic carbon), can cause complete physical disintegration and must be avoided by the choice of suitable operating conditions. Carbon deposition from cracking reactions usually forms on the catalyst surface. This will weaken the surface so that when the carbon is removed, by steaming, the surface layers can fall off giving an eroded appearance. To protect the catalyst from carbon erosion caused by operating at low steam to carbon ratio, reformers have alarm settings and trip systems connected to the steam to carbon ratio controller. The choice of setting will depend on the design of the plant, the nature of the feedstock, the type of catalyst used, and the operating conditions. For a typical top-fired reformer running on natural gas with KATALCO 23-series or KATALCO 57-series catalysts, normal operation may be at a steam to carbon ratio of 3.3:1, with the alarm set at 2.8, and the trip at 2.5. Significantly lower settings are possible with the use of KATALCO 25-series catalysts.

Regeneration When small amounts of carbon have been deposited on the catalyst in the tubes so that hot spots or hot bands have developed and the pressure drop has increased, it is possible to regenerate the catalyst by steaming. This can be done by shutting off the flow of hydrocarbon feed whilst maintaining the flow of steam. During this time the exit temperature should be o o kept as high as possible – preferably around 7500 C (1380 F). The progress of carbon removal can be monitored during the regeneration procedure by measuring the concentration of carbon dioxide and methane at the reformer tube exit. Addition of a small constant flow of nitrogen into the steam assists the measurement of carbon oxides when taking dry gas samples. If the plant has an LTS bed downstream of the primary reformer, then the effluent from the reformer should be vented prior to the LTS bed to prevent sulphur poisoning of the LTS bed. For larger amounts of carbon on reforming catalysts, it may be necessary to add a small quantity of air to the steam. This must be done under carefully controlled conditions, and the concentration of air in steam should not exceed 2%. The downstream high temperature shift catalyst should normally be bypassed to prevent oxidation if air is to be used. 701JM/103/0AMOG © Johnson Matthey Group 2003

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Considerable care should be taken to ensure that the oxidation reaction does not proceed too quickly and cause overheating of the catalyst. The catalyst will be reoxidized during regeneration and to restore full activity the catalyst should be reduced before further operation.

Sulphur poisoning Under normal conditions, the feedstock purification system will remove sulphur from the feed such that the sulphur slip to the reforming catalyst has minimal impact on the life of the catalyst. Occasionally, due to changes in feedstock or mal-operation of the purification system etc, the reforming catalyst may be severely poisoned by sulphur. It is possible to remove most of the sulphur by steaming the catalyst as described under regeneration. Temperatures should be kept as high as possible and the progress of sulphur stripping can be monitored by measurement of hydrogen sulphide (H 2S) in the reformer effluent gas. As with measurement of carbon oxides, an addition of a small constant flow of nitrogen into the steam assists the measurement of H 2S when taking dry gas samples.

Temperature of operation For many duties, it will be found that the design methane concentration at the tube exit can be achieved with the reformer operating at temperatures lower than the design level for most of the catalyst life. Operation at temperatures below the design level is normally attractive because of reduced fuel consumption but also because of the increased reformer tube life. It should be remembered, however, that sufficient heat must be provided to the reforming furnace to satisfy the requirements of the heat recovery system within the fluegas duct and hence the overall plant steam balance. For any operating condition, the furnace firing should be adjusted to give an even tube wall temperature for all the tubes.


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Reformer tube appearance Visual inspection of steam reformer tubes and temperature measurements with pyrometers provide a good guide to the performance of the catalyst inside the tubes. During normal operation the tube wall temperature profile of all the tubes should be similar. Tubes will look hot when the steam reforming reaction is inhibited and they cannot absorb the heat input from the furnace. Some guidance in assessing the cause of problems can be obtained from the tube appearance. Several typical forms of overheated tubes are:

Tiger tailing Clear, well defined, hot rings alternating with cooler rings which develop at random on individual tubes are known as ‘Tiger Tailing’. These indicate voids in the catalyst packing, and are often associated with bridging in the catalyst as a result of improper charging. If vibration or careful hammering does not remove the voids, the tube should be discharged and refilled at the earliest opportunity. Hammering with a rubber mallet is possible during plant operation but does involve some risk which could make the problem worse. Care is essential.

Hot bands These are not so well defined as the hot zones of ‘Tiger Tailing’ and are generally the result of catalyst deactivation. The deactivation may be caused by a general decrease in catalyst activity either because the catalyst is past its useful life, has been poisoned or if the catalyst surface becomes coated by a thin layer of carbon or some other deposit. Hot bands may often be removed by steaming, but if this has no effect the catalyst in the affected tubes should be discharged and replaced with new catalyst when a suitable opportunity is available.

Giraffe necking These are random hot zones or patches which can occur for three main reasons: •

•

•

An extreme form of the hot band problem which is caused by extensive deactivation from catalyst poisoning or a surface deposit. Channelling through the catalyst. This cools the tube where the gas is still reacting and overheats the tube where the flow of gas is impeded. Channelling can be caused by pockets of broken catalyst and dust, or by accumulation of carbon. Catalyst may be too old for further use, so that part may be falling below the minimum required activity level. It should therefore be replaced.

Hot tubes Where extensive carbon deposition or catalyst breakdown restricts the flow of gas a whole tube can become overheated. It may be possible to remove carbon in such tubes by steaming but this is usually ineffective because of preferential flow through the unaffected tubes. Catalyst in hot tubes should be discharged at the next opportunity and replaced. When all tubes are hotter than expected with no obvious flow restriction, it is possible that the charge has not been properly reduced or that temperature indications are inaccurate. If so the proper corrective actions should be taken.


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Tube appearance

Hot band

Tiger tailing


Giraffe necking

Hot tube

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Catalyst poisons The most common poisons for steam reforming catalysts are sulphur, chlorine, arsenic and in addition for the steam naphtha reforming catalysts, occasionally, lead. . The hot patchy appearance of the reformer tubes will also give an indication of decreased catalyst activity. When the catalyst activity has been sufficiently decreased, cracking of the hydrocarbons will take place near the top of reformer tubes. The carbon deposited can block the catalyst surface and cause a further increase in tube temperature. With severe catalyst poisoning, the methane concentrations of reformer exit gas may increase.

Sulphur Sulphur, which is present in hydrocarbon feedstocks, is a poison for all steam reforming catalysts. The effect of sulphur poisoning is reversible and normal activity can be restored once sulphur has been removed from the feedstock. In cases of severe sulphur poisoning it will be necessary to remove the sulphur from the catalyst, by steaming and re-reduction, to restore normal catalyst activity. This procedure will also remove any carbon which has been formed as a result of the sulphur poisoning. Standard Johnson Matthey desulphurization systems will reduce the sulphur content of feedstock to well below the acceptable level of 0.1 ppmv.

Chlorine Chlorine poisoning of steam reforming catalysts is also reversible. Large quantities of chlorine in the catalyst are, however, less easily removed by steaming than sulphur. For very severe cases of chlorine poisoning it may be more economic to change the catalyst rather than to embark on a protracted programme of steaming and re-reduction. When chlorides are known to be present in feedstocks, usually present as hydrogen chloride (HCl) or organic chlorides, it is recommended that they be removed by pre-treatment with an absorbent material provided by Johnson Matthey. The recommended maximum chloride content in the dry process feed is 0.1 ppmv. Use of cleaning fluids that contain chlorides should be avoided in any part of the plant.

Arsenic Arsenic in very small quantities will irreversibly destroy the activity of all hydrocarbon reforming catalysts. Levels as low as 5 ppb can cause problems over prolonged periods. All arsenic should therefore be excluded from the steam reformer. Special care should be taken to avoid contamination of boiler feedwater with carbon dioxide removal liquor that contains arsenic. Poisoned catalyst should be discharged and replaced. Reformer tubes and any other pipework which have also been contaminated with arsenic should be carefully cleaned to remove all traces and prevent further contamination of new catalyst.

Lead Lead is sometimes found in liquid reformer feedstocks transported in tankers previously used for leaded petrol. The amounts involved are generally small enough to be absorbed by the hydrodesulphurization catalyst. Lead poisoning is, however, irreversible and in severe cases catalyst must be discharged and replaced .


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Mercury Mercury is also a severe poison and although not too common, can be found in a wide range of feedstocks from natural gas to naphthas. Mercury is best removed at ambient temperatures using a specialized absorbent provided by Johnson Matthey Catalysts.

Silica Silica and other typical solids carried into the reformer and deposited onto the catalyst have no severe poisoning effect but can block the pores of the catalyst when present in large enough quantities. This may lead to a partial loss of activity, and an increase in pressure drop.

Other poisons Impurities such as copper, iron, phosphate etc will accumulate at the top of the reformer tubes and may in extreme cases, affect catalyst performance. If necessary, the contaminated catalyst can be replaced with an equivalent volume of new catalyst.


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Nickel carbonyl hazard Catalysts containing metallic nickel must not be exposed to gases containing carbon o o monoxide at temperatures below 200 C (390 F). Observation of this rule avoids the risk of the formation of nickel carbonyl, Ni(CO)4 an extremely toxic, odourless gas which is stable at low temperature. Catalysts used in the steam reforming process normally operate well above this temperature. During a plant shutdown the catalysts are normally steamed and are usually in the oxidized state when they pass through the critical temperature regime. If this is not the case and nickel metal is present then stringent precautions must be observed so that there is no risk to personnel. Nickel carbonyl formation and variation with temperature and pressure


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Shut-down It is important to avoid overheating of either the reformer tubes or the catalyst when bringing a steam reformer off-line. Frequent visual checks of the steam reformer are recommended at all stages of the shutdown procedure. Pressure may be reduced at any time during the process of shutting down. 1

Decrease the fuel rate to the furnace so that the tube exit temperatures fall to about o o 750 C (1380 F).

2

Decrease the hydrocarbon and steam rates in stages to about one third of the design values. Always decrease the hydrocarbon flow before the steam flow to maintain the steam to carbon ratio at or above the design level. The furnace firing should also be adjusted throughout this operation to maintain the tube exit temperature at about o o 750 C (1380 F). In an ammonia plant the secondary reformer is normally taken offo o line when the tube exit temperatures are about 700 C (1290 F).

3

Keep the steam flow constant and shut off the hydrocarbon feed, adjusting the furnace firing to compensate for the decreasing heat demand. The exit temperature o o should be maintained at about 750 C (1380 F) until the system is free of hydrocarbons.

4

Decrease the steam reformer exit temperature at a rate of up to 100 C (180 F) per o o hour until it reaches about 550 C (1020 F).

5

The steam flow may now be shut off and a flow of nitrogen introduced before cooling is continued. If nitrogen is not available and natural gas is to be used in the final o o cooling stage, the temperature should be decreased below 200 C (390 F) and the burners turned off before the natural gas is added. This eliminates any possibility of carbon formation due to thermal cracking.

6

It is usual to isolate the steam supply before condensation is possible to avoid the inconvenience of liquid water in the steam reformer. After a wet shutdown, care is needed to dry the catalyst thoroughly and prevent any water refluxing in tubes during the restart, as this can lead to breakdown and caking of catalyst. Refer to the section on catalyst wetting.

7

The catalyst may be discharged (if desired) as soon as the temperature reaches 50 C o (120 F). When natural gas has been used in the final cooling stage, special care should be taken to ensure that the catalyst is cool enough for the methane to be purged safely with air before the reformer is opened to the atmosphere.

o

o

o

If the catalyst has been caked either with carbon or as a result of physical breakdown, extraction can be a lengthy operation. Special methods are available for the extraction of catalyst in these circumstances and Johnson Matthey will give advice on request.


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UNIDENSE is a trademark of Norsk Hydro. Catalyst Care is a servicemark of the Johnson Matthey Group of companies


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710W - Steam Ref Op Man Rev3a

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