Steam reforming details with catalyst performance.ppt

Problems in Primary Reformer

1. Presentation on Steam Reforming Primary Reformer 1

2. Contents Introduction Steam Reforming Reaction Catalyst use in steam reforming Catalyst properties Catalyst storage , handling and discharge Health and safety precautions Startup of Katalco catalyst Reformer tube appearance Catalyst poisons Side Reaction in reformer 2

3. Introduction steam reforming (SR) is a process of producing hydrogen by combining steam and hydrocarbon and reacting in a reformer at temperatures above 500oC in the presence of a metal-based catalyst. The basic reforming reaction for a generic hydrocarbon CnHm . C n H m + nH 2 O nCO + (n+m/2) H 2 - Heat 3

4. Introduction Steam reforming (SR), is a method of producing hydrogen from hydrocarbons. The steam reforming of saturated hydrocarbon can be represented by three reactions: C n H m + nH 2 O nCO + (n+m/2) H 2 - Heat CO + H 2 O CO 2 + H 2 ∆H = -41.2 KJ/g-mole CH 4 + H 2 O CO + 3H 2 ∆H = +206.3 KJ/g-mole  CH 4 + 2H 2 O CO 2 + 4H 2 ∆H = +165.1 KJ/g-mole 4

5. Catalyst In steam reforming Ni based catalyst are used. Johnson Matthey provides KATALCO JM series catalyst for steam reforming. 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 feedstock, or with light hydrocarbon feeds at low steam to carbon ratios, the KATALCO JM 25-series catalysts are specially promoted with potash. Composition : KATALCO JM 25-series Nickel oxide dispersed on a promoted calcium aluminate ceramic support. This is a lightly alkalised version of the 57 series catalyst 5

6. KATALCO JM 57-series Nickel oxide dispersed on a calcium aluminate ceramic support. KATALCO JM 23-series Nickel oxide dispersed on an alpha alumina support. Catalyst performance is most influenced by two key factors. One is the chemical formulation and the other the size and shape of the catalyst particle. Influence of tablet shape and size on steam reformer catalyst performance: activity pressure drop at start of run and over time potential for carbon formation heat transfer strength packing characteristics. 6

7. Activity: Catalyst activity is very important since this defines the rate of reaction and hence the approach to equilibrium (ATE) and in turn the methane slip. A more active catalyst reduces ATE and methane slip at constant exit temperature and rate. Alternatively, operation at a constant methane slip and rate is achieved at lower exit temperature, which leads to less fuel use, a lower maximum tube wall temperature and increased tube life. Activity is directly proportional to the geometric surface area (GSA) of the catalyst. The GSA is defined as the surface area per unit volume. It is a key factor in catalyst activity as the rate-determining step for the steam reforming reaction is diffusion of the reactants through the laminar gas layer around the catalyst particle. Once the reaction gases reach the catalyst surface, reaction is very fast and as such does not rely on diffusion into the pore structure of the catalyst 7

8. Pressure drop: Pressure drop through the front end of the plant can be a limiting factor in defining the maximum plant rate and may also influence product recompression costs. The steam reformer contributes significantly to the overall pressure drop. Pressure drop over the catalyst is important in two ways- 1) Pressure drop (start of run): This relates to the inherent pressure drop over the catalyst and is a direct function of the voidage in the tube. A larger voidage leads to a smaller pressure drop and voidage is a direct function of the catalyst particle geometry. 2) Pressure drop (over operational life): A rapid rate of pressure drop rise as a result of catalyst breakage may limit plant rate and even lead to interrupted production. 8

9. Potential for carbon formation: Carbon formation is a limiting factor to steam reformer catalyst life particularly when the feedstock is a heavier hydrocarbon such as naphtha. Net carbon formation occurs during normal operation both when the rate of thermal or catalytic cracking exceeds the rate of carbon gasification and when, during a transient, feedstock passes into the steam reformer with too little steam. Carbon formation can be prevented by a number of means : 1) Inclusion of potash promoted catalyst in the top section of the reformer. The promoter increases the rate of carbon gasification by reaction with steam and prevents net carbon laydown. 9

10. 2) This increases the steam reforming rate and therefore converts more hydrocarbons higher up the tube to lower the driving force for carbon formation in the carbon-forming zone . Heat transfer: The rate of heat transfer from the fluegas to the process gas may limit the rate of highly endothermic reaction. Therefore, the catalyst must be designed to maximize the heat transfer rate thus minimizing tube temperature while maximizing reaction rate. A major factor in heat transfer characteristics of the catalyst is its geometry. Smaller tablets will better disrupt the stagnant gas film at the inside tube wall through which heat must pass, while the shape can influence the ability of the heat to migrate through the refractory base of the catalyst. 10

11. Strength: This parameter is intimately linked with pressure drop. Catalyst tablets must endure the rigours of catalyst loading, normal and transient operation. A key factor in ensuring high catalyst strength is to ensure that there are no internal stresses within the pellet. Increased strength can be achieved in a number of ways. Increasing the strength of the support structure. Increasing the ligament length by either making the pellet larger or by reducing the size of the holes, although both of these reduce the GSA of the catalyst. Increased firing temperature during manufacture, although this may cause chemical changes to the support which may affect other aspects of the catalyst performance. 11

12. Catalyst packing: Good packing of catalyst in the tubes is required to avoid voids. Over-dense packing may lead to pressure drop issues. Conversely, poor packing may lead to hot spots on the tube wall adjacent to the voids in the tubes where there is a gap in the catalyst packing. The hot spots shorten tube life and may interrupt production if localized temperatures exceed design limits. 12

13. In CFCL, Ammonia-II plant , Primary reformer filled by following catalyst: Catalyst Katalco JM 25-4GQ ICI-46-6 Katalco JM 57-4GQ Type & Shape of catalyst Domed, Fluted Cylinder with 4 Holes Domed, Fluted Cylinder with 4 Holes Domed, Fluted Cylinder with 4 Holes Length (mm) 17 17 20 OD (mm) 13 14 16 Hole ID (mm) 3.5 4 4.4 Nominal Tapped Bulk Density (Kg/l) 0.88 0.85 0.77 13

14. Catalyst storage & handling Steam reforming catalysts are generally supplied in mild steel drum fitted with polythene liners. Drum must not be store in stacked on their sides. Metal drum are usually suitable for outside storage for few months but should be protected against rain and standing water. The catalyst drum must be placed in cool, dry, and away from direct sunlight and sources of heat if it in pre-reduced condition. The stabilization layer will breakdown at temp of 100ºC with adequate access of air, the heat generated will accelerate the breakdown. The temp generated can exceed 1000ºC. 14

15. Charging of Reformer catalyst 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 15

16. There is a variety of established reforming catalyst charging techniques.. The most common one in use today is UNIDENSE TM 1) Sock Method 2) Norsk Hydro Unidense™ Loading Method Sock Method: 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. 2) Norsk Hydro Unidense ™ Loading Method: The main principle behind the UNIDENSE loading method is a slow and continuously controlled catalyst loading so that all the particles come to rest in stable positions not influenced by other pellets loaded at the same time. 16

17. A rope with a set of metal brushes, somewhat similar in appearance to a group of chimney sweep brushes joined together head to tail, is inserted into the catalyst tube rope should be long enough to reach the bottom of the tube while at the same time allowing enough length on top to get a firm grip for ease of movement up the tube. The brushes are made out of flexible springs and the catalyst pellets are allowed to trickle down the tube or catalyst surface by jerking the rope. The tube is gradually filled with the use of a funnel to pour the catalyst The metal brushes have the function of reducing the speed of the catalyst pellets as they are poured into the tube. They also ensure a limited catalyst free fall of 0.5 - 1.0m (1.5-3ft), thus ensuring sufficient packing and even catalyst distribution while minimizing catalyst breakage. 17

18. UNIDENSE TM technology is use full for - Avoiding the pre socking of catalyst Avoiding the vibration of tubes. Avoiding weight to the catalyst. Makes lower skin temperature Decrease hot spots Increase unit capacity Decrease time for loading primary reformer tube. Contribution to better operation economy. 18

19. health and safety precaution during catalyst charging and discharging – Short-term exposure to the metals and metal oxides used in catalysts may give rise to irritation of the skin, eyes and respiratory system. Catalysts should be handled as far as possible in well-ventilated areas. Must wear suitable protective body clothing, gloves and goggles. Catalysts discharged in the pyrophoric state must be kept separate from flammable materials. Transportation of such catalyst should only be in metal skips or metal-sided trucks. Dumps of the catalysts should be within reach of water hoses so that any overheating that occurs can be controlled. 19

20. Reformer tube appearance Pyrometers provide a good guide for visual inspection of steam reformer tubes and temperature measurements of the catalyst inside the tubes. During normal operation tube wall temperature profile of all tube should be similar. Tubes will look hot when the steam reforming reaction is inhibited. Several typical forms of overheated tubes are: Tiger tailing Hot bands Giraffe necking Hot tubes 20

21. Reduction of catalyst Reformer catalyst can be reduce by following methods: Reduction with hydrogen Reduction with ammonia Reduction with natural gas 21

22. 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. Hot bands: These are not so well defined as the hot zones of ‘tiger tailing’ and are generally the result of catalyst deactivation. Catalyst deactivation may be due to 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. 22

23. Giraffe necking: These are random hot zones or patches which can occur for three main reasons-1)Channeling through the catalyst. 2)extensive deactivation from catalyst poisoning or a surface deposit. This cools the tube where the gas is still reacting and overheats the tube where the flow of gas is impeded. Channeling can be caused by pockets of broken catalyst and dust, or by accumulation of carbon. 3)Catalyst may be too old for further use. 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 23

24. Catalyst poisons Sulphur: Sulphur, which is present in hydrocarbon feedstock, 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. the sulphur content of feedstock to well below the acceptable level of 0.1 ppmv. Chlorides: Chloride poisoning of steam reforming catalysts is also reversible. Large quantities of chloride in the catalyst are, however, les When chlorides are known to be present in feedstock, usually present as hydrogen chloride (HCl) or organic chlorides, s easily removed by steaming than sulphur. The recommended maximum chloride content in the dry process feed is 0.1 ppmv. 24

25. Arsenic: Arsenic in very small quantities will irreversibly destroy the activity of all hydrocarbon reforming catalysts. Levels as low as 5 ppbv can cause problems over prolonged periods. Special care should be taken to avoid contamination of boiler feed water with carbon dioxide removal liquor that contains arsenic. Lead: Lead is sometimes found in liquid reformer feedstock transported in tankers previously used for leaded petrol. Mercury: Mercury is also a severe poison and although not too common, can be found in a wide range of feedstock from natural gas to naphthas. Mercury is best removed at ambient temperatures using a specialized absorbent provided by Johnson Matthey Catalysts.

26. 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.

27. Nickel carbonyl hazard Catalysts containing metallic nickel must not be exposed to gases containing carbon 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, odorless gas which is stable at low 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. Ni + 4CO Ni(CO) 4 27

28. Carbon formation Carbon is an unwanted by-product that is formed in all steam reformers and may occur for a number of different reasons, for example: Steam to carbon ratio is too low Catalyst is not active enough Higher hydrocarbons are present Tube walls are too hot (high heat flux) Catalyst has poor heat transfer characteristics Low flow due to poor loading 28

29. Carbon formation reaction CH 4 2H 2 + C 2CO CO 2 + C 2CO + H 2 H 2 O + C 2CO 2 + 2H 2 2H 2 O + C 29

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