Catalyst Breakage in Reformer Tubes

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Introduction

Catalyst breakage is a well known phenomena that occurs during operation and transients such as reformer trips, whether this be due to,

• Normal in service breakage,
• Breakage due to carbon formation/removal,
• Breakage due to steam condensation or carry over,
• Breakage during a trip.

The effect of catalyst breakage can be observed in a number of ways,

• Hot bands,
• Speckling and giraffe necking,
• Catalyst breakage and settling.

Published in: Technology, Business

Catalyst Breakage in Reformer Tubes

  1. 1. 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 Web Site: www.GBHEnterprises.com GBH Enterprises Ltd. 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, C 2 PT, Catalyst Process Technology gives no warranty as 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, C 2 PT accepts no liability for loss or damaged caused by, or resulting from reliance on this information. Freedom, under Patent, Copyright and Designs cannot be assumed.
  2. 2. 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 Web Site: www.GBHEnterprises.com Topic: Understanding Catalyst Breakage in Reformer Tubes Catalyst Breakage in Reformer Tubes Introduction Catalyst breakage is a well known phenomena that occurs during operation and transients such as reformer trips, whether this be due to, • Normal in service breakage, • Breakage due to carbon formation/removal, • Breakage due to steam condensation or carry over, • Breakage during a trip. The effect of catalyst breakage can be observed in a number of ways, • Hot bands, • Speckling and giraffe necking, • Catalyst breakage and settling.
  3. 3. 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 Web Site: www.GBHEnterprises.com These effects are illustrated below, Figure 1 – Tube Appearance In the worst case, catalyst breakage will lead to carbon formation and hence a deterioration of the observed problem. This document intends to detail the following, • The types and causes of breakage that can occur, • The effect on tube appearance, • The effect of changing out the worst affected tubes and what this means in terms of the performance of the rest of the reformer. This document will restrict itself to catalyst breakage and damage in primary reformers only.
  4. 4. 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 Web Site: www.GBHEnterprises.com Types of Breakage There are a number of different types of breakage that can be observed in a steam reformer which are detailed below. Effect of Catalyst Design If the catalyst has been designed such that on breakage, it forms a large number of small fragments, the pressure drop will rise rapidly. An example of this below. GBHE C 2 PT’s VULCAN Series VSG-Z101/102 Primary Reforming catalyst; VSG-Z101 4 Hole catalyst, similar to the JMC 4-Hole catalyst Is an example of a catalyst with good breakage characteristics, in that when it does break it forms large fragments which mean that the pressure drop is relatively small. This is because pressure drop is inversely proportional to effective pellet diameter – therefore if the fragments formed are large, then the effective pellet diameter only increases marginally. Furthermore, pressure drop is related to voidage by the following term (1-e)/e³ and therefore any decrease in voidage will cause large increases in pressure drop Figure 2 – Good Breakage Characteristics
  5. 5. 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 Web Site: www.GBHEnterprises.com An example of a catalyst with poor breakage characteristics if that of the Sud Chemie Wagon Wheel (the extended Wagon Wheel – EW, with thicker ligaments may be better) and Haldor Topsøe’s seven hole catalyst, Figure 3 – Poor Breakage Characteristics Breakage of the catalyst in a tube will lead to a high resistance to flow and therefore, the flow through the tube will be low. This will cause the tube to operate hot – a similar effect is caused by variability in the loaded voidage. Causes of Breakage Trips Excessive trips cause expansion and contraction of the tubes; the contraction of the tubes causes large stresses to build up on the pellets and these stresses can only be relieved by movement of the catalyst axially in the tube or pellet breakage. In reality, only the catalyst at the top of the tubes can move and the catalyst towards the bottom of the tube, where the temperature changes will be the greatest, are locked in position. Therefore, the only possibility is for the catalyst to fracture. Figure 4 – Example of Catalyst Breakage
  6. 6. 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 Web Site: www.GBHEnterprises.com Settling Care should be taken to allow for the effect of tube expansion. Sufficient catalyst must be charged into the reformer tube when cold to make sure that when operating, and therefore hot, the catalyst does not settle down so far as to expose empty space at the top of the reformer tube. Figure 5 – Catalyst Settling Insufficient Catalyst Loaded In some cases, it is possible that insufficient catalyst is loaded into the reformer tube and when the plant is started up, due to the radial tube expansion (see above), the total fired volume of the tube increases. Under such circumstances, it is possible that the top of the catalyst falls below the bottom of the roof refractory and this section of tube will become hot since there is no catalyst to support the steam reforming reaction to keep the tubes cool. Milling Milling typically occurs within a primary reformer at the tube inlet where the high gas velocities at the inlet of the reformer tube can cause movement of the catalyst pellets and hence attrition. This has been observed on a number of plants a Eastern European Plant. In this instance, the catalyst had been installed too high up the tube and the jet from the inlet pigtail had moved the catalyst such that they were turned in sphere. Carbon Lay down and Removal There are a number of forms of carbon, but the most serious form in terms of catalyst damage is polymeric since this carbon forms within the pore structure of the pellet. As the carbon structure grows within the pellet, stresses are generated and once these become sufficiently great, then they can cause the pellet to fracture.
  7. 7. 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 Web Site: www.GBHEnterprises.com As is well known, once carbon has been laid down, it is possible to remove it by conducting what is known as a steam out. Although a steam out does have the beneficial effect of removing the carbon by gasification, this does have an effect on the catalyst. As the carbon is gasified, it is converted from a solid to a gas there is a huge volume expansion which can lead to some pellet breakage due to the large stresses generated within the pellet. Hydration Some catalysts (Haldor Topsøe) suffer from hydration of the catalyst support (MgO) if subjected to steaming conditions between 450-650°C. The hydration of the MgO to Mg(OH)2 causes a volume expansion within the pellet structure and this generates stresses which can lead to excessive catalyst breakage. Effects of Water Water can affect the primary reforming catalyst in a number of different ways; these are detailed below, • Water Carry Over - One problem associated with water is the carryover from the steam drum, where the liquid is not fully dis-engaged from the steam. If this liquid is not vaporized in the steam superheater, then it is possible for boiler salts to be carried over to the reformer where it can be poisoned or a crust of salts can be formed on the catalyst. • Water Soaking – On a Brazilian Ammonia plant, the operator managed to fill the bottom section of the reformer tubes with water. Upon restart, the pressure drop across the reformer was high and this lead to a shut down. After discharging the catalyst it was found to have had the edges sheared off as shown below, Figure 6 – Catalyst Damage
  8. 8. 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 Web Site: www.GBHEnterprises.com The cause of this was when the catalyst was heated up, the water could not escape from the centre of the ligaments, which represents the thickest part of the catalyst pellet, before it was vaporized. As soon as the water vaporized, there was a huge volume expansion which caused these sections to break away from the rest of the pellet. • Condensation - On a plant trip it is very possible that steam can condense and the sit in dead legs or low points in the feed header system. On a plant restart, it is possible that the water is carried forward on to the catalyst. The catalyst is normally hot at this stage, and as the cold water hits the hot catalyst, the catalyst will be rapidly cooled and the stresses induced can shatter the catalyst. This problem can be prevented by eliminating low points and dead legs during the design of the plant – it is usual that this kind of problem will be picked up during the plant HAZOP review. Suitable positioning of drains and correct start up procedures will also help in minimizing the risk. The inlet headers and associated pipe work from the mixing tee to the tube inlet shall be designed such that there are no dead legs where condensate (feed or steam) can collect. If there are low points then drains should be installed such that this condensate can be removed. Operations procedures should clearly state that these drains are opened during start ups. • Passing Steam Valve - If the process steam valve passes during a shut down or whilst the plant is shut down, then it is possible for water to condense on the catalyst. On restart this can lead to a number of problems such as shattering of the catalyst and potential formation of concrete. • Effect of Water Carry Forward - If water is carried forward either from a saturator or from the process steam, it is possible to generate an extreme thermal shock due to the quenching of the inside of the reformer tubes. This creates both a high tensile stress on the inside of the tubes, and reduced ductility leading to sudden, deep cracking, or even shattering of tubes. • Effect on the Catalyst and Tube - In some cases where the catalyst has been wetted, the support material can be leached out and deposited on the inside of the tube walls. When this residue is dried out, a hard coating is formed on the inside of the tube wall which is very difficult to remove. A device known as a ‘frapper’ can be used to remove this coating; this device consists of a pear shaped metal head attached to a high speed rotating shaft by a hinge. This problem occurred at a Gulf Coast Plant in the late 1990’s and took three days to clean out.
  9. 9. 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 Web Site: www.GBHEnterprises.com Effects of Liquid Hydrocarbons There have been a number of instances of liquid hydrocarbons being passed to the reformer with out steam. This causes gross carbon formation which can lead to a loss of activity and also significant catalyst breakage. Localized Overheating There are a number of causes of flame impingement on the tubes, for example, mis-aligned burners, flue gas mal-distribution and poor burner maintenance. The effect of these problems is to rapidly cycle the tube and catalyst temperatures up and down and in so doing causes catalyst breakage Up Flow Fluidization Problems The majority of reformers have the process gas flowing downwards and hence there are no issues associated with fluidization of the catalyst, however, there are a number of up flow circular reformer. If the design of the reformer is poor or the plant has been up-rated, then is it possible to achieve process side velocities that are sufficiently high to fluidize the catalyst. This will lead to catalyst attrition and breakage which will cause excessively high pressure drop and fouling of downstream equipment by catalyst dust. A potential solution to this problem is to install a hold down device with sufficient mass to resist the fluidization force. A typical design is shown below. Figure 7 – Hold down Plant
  10. 10. 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 Web Site: www.GBHEnterprises.com Poor Catalyst Loading Ensuring a good catalyst loading is fundamental in ensuring efficient operation of the primary reformer. Any deviations in resistance to flow through the tubes will result in differential flows between tubes and this in turn will lead to tube wall temperature differences as illustrated to the right. A good catalyst loading will cause even process gas distribution and hence even tube wall temperature distribution as shown below, Figure 9 – Good Catalyst Loading Another effect is that there will be process gas exit temperature spreads on the reformer which will artificially increase the methane slip from the reformer. The effect of this effect is illustrated below. Figure 8 – Poor Catalyst Loading
  11. 11. 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 Web Site: www.GBHEnterprises.com Figure 10 – Effect on Flow and Pressure Drop of Poor Loading -25 -20 -15 -10 -5 0 5 10 15 20 25 -20 -10 0 10 20 Pressure Variation (%) FlowVariation(%) -25 -20 -15 -10 -5 0 5 10 15 20 25 Flow Variation (%) TWT Variation (°C) GBHE C 2 PT Catalyst Process Technology recommends the use of a pressure drop measurement device, which allows for tubes pressure drops to be measured at various points during catalyst loading. The results of this allow the operator to determine which tubes have a low resistance to flow (a low pressure drop) which need further vibration and those with a high resistance to flow (a high pressure drop) which need reloading. Also the method of loading is very important. The traditional sock loading, can when applied correctly, give a very good catalyst loading. However, the more modern dense loading methods can give a loading where little or in some cases no remedial action is required during and after catalyst loading to achieve a uniform catalyst loading. Voids Furthermore, a poor loading can give rise to localized voids within the tube which will be seen as hot spots on the tube. This can then limit the reformer performance since to keep these tubes cool, the firing around these tubes with hot spots has to be reduced. This will lead to high methane slip from the affected tubes and therefore a high overall methane slip from the reformer.
  12. 12. 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 Web Site: www.GBHEnterprises.com Effect on Tube Appearance The above problems can cause a wide variety of tube appearances; these are defined in the following table, Parameter Tube Appearance Poor breakage characteristics - Minor damage - Severe damage Hot bands Hot patches & high ∆P Trips - Minor damage - Severe damage Hot patches Hot bands & high ∆P Whole tube hot & high ∆P Settling Top of the tube is hot Insufficient catalyst loaded Top of the tube is hot Milling High ∆P Carbon Lay down - Minor - Severe Hot bands Long hot bands & high ∆P Hydration Hot bands & high ∆P Water - Carry over - Water soaking - Condensation Poisoning and hot bands High ∆P Catalyst breakage, hot bands and high ∆P Liquid hydrocarbons Carbon formation, hot bands and severe breakage on steaming Localized overheating Catalyst breakage and hot bands Up flow and fluidization Catalyst breakage, carryover of dust to WHB Poor catalyst loading Variety of effects including hot bands through to whole tubes appearing hot
  13. 13. 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 Web Site: www.GBHEnterprises.com Options for Rectification The key option for rectification of all of the above problems is to replace the affected catalyst. However, this is not as simple as it first appears since deciding which tubes to replace is not just a matter of considering those tubes that appear hot. Why is this? Well this will be explained in the sections below. Pressure Drop Theory In order to fully understand the effect of breakage, it is important to understand the theory behind the calculation of pressure drop through a catalyst bed and more specifically the pressure drop through a reformer tube. The pressure drop across a reformer tube is defined by the equation (Eqn 1), ( ) 3 2 x ε2 ε1Qρ RePDCΔP × −×× ××= Eqn 1 Where, ∆P is the pressure drop, PDC is a constant representing the performance of a particular catalyst, Re is the Reynolds number which is defined as, ( )ε1μ duρ Re c −× ×× = Eqn 2 ρ is the gas density, Q is the gas flow rate, ε is the catalyst voidage, u is the superficial gas velocity, dc is the effective channel diameter which is inversely proportional to the pellet size, μ is the gas viscosity This can be rearranged to give, ( )ε1ρRePDC εΔP2 Q x 3 −××× ×× = Eqn 3 Which in turn can be simplified (after inclusion of the Reynolds number) to the following, ( ) ( )( )x−+ −×××× ××× = 1 ε1duρPDC μεΔP2 Q x c xx1 x3 Eqn 4
  14. 14. 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 Web Site: www.GBHEnterprises.com Which can then be simplified to (by removing parameters that can be considered constant and replacement of the channel diameter as inversely proportional to the pellet diameter), ( )( )x− − ×× ×= 1 ε1 dεΔP CQ x p 3 Eqn 5 Where C is a constant representing parameters that for a single point in a tube can be considered constant, that is to say, ( ) x x1 x uρPDC μ2 C ×× × = + Eqn 6 What does this tell us? As we change the resistance to flow (due to catalyst breakage), i.e.: reduced equivalent pellet diameter and voidage, the flow rate through a tube will be decreased. Equation 1 can also be rewritten to give, ( )( ) x p 32 x12 dεC ε1Q ΔP ×× −× = − Eqn 7 From this a “resistance to flow” term can be defined as follows, ( )( ) p x dε ε ce to Flowsis × − = − 3 1 1 tanRe Eqn 8 And also, 2 2 C Qce to flowsis ΔP × = tanRe Eqn 9 When the resistance to flow increases, due to catalyst breakage so does the pressure drop. Why is this, well firstly the effective pellet diameter is decreased since there are now some small fragments of pellets as well as whole pellets and so the effective pellet diameter is reduced. Since resistance to flow is inversely proportional to the effective pellet diameter. Also these small fragments tend to fill the void spaces between the pellets and thereby reduce the voidage again increasing the resistance to flow and hence the pressure drop. At this point a common fallacy will be put to rest; it often said that the pressure drops through the reformer tubes is different when there has been some breakage of the catalyst. This is in fact untrue. Given that the pressure at the end of the feed header when is passes process gas into the sub headers is the same and that the pressure at the collection points on the transfer headers is the same, the differential pressure drop across the reformer is the same.
  15. 15. 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 Web Site: www.GBHEnterprises.com Therefore, if there is a variation in breakage, and hence voidage and effective pellet diameter, the only variable that can change is the flow rate. Conceptualization To understand more fully the problem associated with catalyst breakage and the effect it has on the performance of the reformer, consider a reformer that has been loaded well, and then consider the distribution of tubes with respect to deviation of the PD Rig pressure drop from the average. The following figure illustrates this relationship assuming that there are a large number of tubes and that the relationship can be approximated by a “Normal Distribution”, Figure 11 – Normal Distribution Frequency Chart What does this Conceptualization Mean? We can take the above conceptualization and apply it too many different situations; for example, • Poor catalyst loading, • Gross carbon formation, • A slug of water, • Excessively fast trips, • Catalyst milling, • Localised overheating.
  16. 16. 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 Web Site: www.GBHEnterprises.com In the next sections, the above conceptualization will be applied to each of these situations. Bear in mind that the pressure drop deviation is as measured by a PD Rig (see below for an explanation) and that the actual pressure drop across all of the reformer tubes during normal operation will always be the same. Poor Catalyst Loading If the loading was deemed to be poor then the frequency plot would be as follows, Figure 12 – Poor Distribution Frequency So what does this difference mean in terms of real performance? In order to understand this question, we need first to understand what the above plot really means in terms of the reformer operation under normal conditions. Although the PD rig does actually report a pressure drop through the tested tubes, what this really represents is in fact a resistance to flow. Why is this true? Well, due to the design of the PD Rig, the flow through the orifice plate is sonic and therefore for a fixed upstream pressure, the actual flow rate will be constant. Since pressure drop is proportional to the square of the flow rate times by a term that could be called a “resistance to flow”, it can be inferred that the PD Rig actually measures resistance to flow. Therefore, tubes with a high pressure drop actually have a high resistance to flow whilst those with a low pressure drop have a low resistance to flow. Tubes with a high resistance to flow will therefore under normal operating conditions have,
  17. 17. 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 Web Site: www.GBHEnterprises.com • A low process gas flow rate which if the heat flux to the tubes (i.e.: the firing surrounding the affected tubes) is not reduced will lead to higher tube wall temperatures, • A higher potential for carbon formation – since the process gas temperature within the tubes is higher. This therefore means that the carbon potential of the process gas has increased which in turn means that the rate of carbon lay down increases and therefore the rate of carbon formation increases. Gross Carbon Formation Under these circumstances it is assumed that all the tubes have suffered from carbon formation and that this has then been steam off. The following graph illustrates what will happen to the pressure drop variation post carbon formation, Figure 13 – Effect on Pressure Distribution of Carbon Frequency Distribution 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -15 -10 -5 0 5 10 15 Pressure Drop Deviation FractionalNumberofTubes Base Case Carbon Formation As can be seen the distribution has moved to the right through a decrease in both effective pellet diameter and voidage and therefore the total pressure drop across the reformer will rise.
  18. 18. 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 Web Site: www.GBHEnterprises.com A steam out can be conducted to remove this carbon, however, during a steam out, any carbon within the pores of a pellet will be gasified and this leads to a huge volume expansion leading to a high stress on the pellet and breakage of the pellets. The effect of this is highlighted below, Figure 14 – Effect of a Steam Out on Frequency Distribution Frequency Distribution 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -15 -10 -5 0 5 10 15 Pressure Drop Deviation FractionalNumberofTubes Base Case Carbon Formation Post Steam Out A Slug of Water Slugs of water can be passed to the reformer either due to condensation on the feed header or due to carry over of water from the steam drum. The effect of this on the reformer depends on how the water is distributed to the tubes; the first way is when the water affects all the tubes within the reformer. In this case, the catalyst at the top of the tubes will all suffer significant breakage. The effect is that all tubes are affected the same and as such the resistance to flow and hence the pressure drop across all the tubes rises by the same amount. In reality, due to the inherent variation in such an effect, there will always be an increase in the pressure drop variation. Overall, the effect is the same as detailed in figure 13. This is typical of the effect of condensation within the tubes or back flow of water into the tubes due to a waste heat boiler failure. The second alternative is that the water is passed to a small section of the reformer; the location of this depends on the gas velocity through the feed headers and the droplet size. In terms of velocity, as the process gas passes down the feed header and portions of the process gas enter the tubes, the gas velocity is gradually reduced. Whether a droplet enters a particular tube, is a function of the gas velocity and the droplet size.
  19. 19. 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 Web Site: www.GBHEnterprises.com The relationship between droplet size and gas velocity is complex, however, it can be explained as follows. Small droplets have a relatively low momentum and therefore are relatively easily turned into a tube, no matter what the process gas velocity. Large droplets have a relatively high momentum and therefore are relatively difficult to turn into a tube when the gas velocity is high but will turn more easily when the gas velocity is low. If the droplets are small, then they will tend to affect the tubes at the “feed” end of the reformer (i.e.: that end of the reformer where the feed enters the reformer). Whereas if the droplets are large then they will tend to pass to the non feed end (i.e.: the opposite end). The effect both of these on the distribution plot is the same, in that the normal distribution as defined in figure 12, is changed such that a “double hump” is formed as highlighted below, Figure 15 – Double Hump due to Localized Water Effects Frequency Distribution 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -15 -10 -5 0 5 10 15 Pressure Drop Deviation FractionalNumberofTubes Base Case Effect of Small Water Droplets As can be seen there is now a double hump in the distribution which highlights that some of the tubes now have a higher resistance to flow than they did have. This flags up that only a portion of the reformer has been affected by the water droplets. If (almost) all the tubes had been affected then the whole distribution profile will have moved to the right. Excessively Fast Trips Specific shut downs such as loss of MP steam, tripping of the flue gas (ID fan) or combustion air (CA fan) can lead to fast plant shut downs, which in turn leads to rapid temperature transients of the reformer tubes. During these transients, the tubes contract rapidly, causing rapid reductions in tube diameter and this leads to very high stresses on the pellets and hence a high degree of catalyst breakage. Such breakage is exemplified by figure 13. In this case we see that (almost) all the tubes now have a higher pressure drop that the base case.
  20. 20. 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 Web Site: www.GBHEnterprises.com Catalyst Milling Catalyst milling is seen on only a few plants and typically affects all the tubes since the root cause is a consistent one that, is that the jet from the pigtail impacts on the catalyst and rolls the pellets around. Since there will always be a variation in the outage (or distance between pigtail inlet and catalyst surface), some tubes will be affected more than others and so not only is the frequency distribution moved to the right, but the distribution is smeared. This is as per figure 13 above. Localized Overheating Localized heating due to poor burner design, maintenance, and installation or flue gas mal- distribution effects will lead to some tubes being affected by flame impingement. In some cases, the flame itself does not impinge on the tube, but the hot jet of gas associated with the flame impinges on the tube. Although this latter effect is not as bad as the former, it still leads to rapid process gas temperature cycles which in turn lead to catalyst temperature cycling. This can then lead to catalyst breakage and an increased resistance to flow and hence increased pressures drop (as measured by a PD rig). The magnitude of the effect depends on what the root cause is and hence how many tubes are affected. Clearly a single burner that is mal-performing will affect only one or two tubes whilst flue gas mal- distribution will affect both rows along the side wall of the reformer. In principle the effect will be seen on the frequency chart as highlighted as below, Figure 16 – Localized Overheating Frequency Distribution 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -15 -10 -5 0 5 10 15 Pressure Drop Deviation FractionalNumberofTubes Base Case Poor Burner Maintainence Fluegas Maldistribution
  21. 21. 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 Web Site: www.GBHEnterprises.com As can be seen, for the case with poor burner maintenance, only a few tubes are affected and we see the formation of a small hump with high pressure drop deviations. It should be noted that if we could measure this effect with time, we would see this small hump moving from left to right as the breakage in the affected becomes worse and worse. For the flue gas mal-distribution effect, the number of tubes affected is much greater since we are considering both outside rows of tubes (on a 10 row reformer this is 20% of the tubes). Again we see a double hump formed and this again will move to the right with time. Replacing Catalyst Once catalyst breakage has been identified as a root cause of the observed visual effects in the reformer, then it is important to consider what actions to take. In some cases, it is possible to continue to run the reformer until the problem becomes so severe that action has to be taken. In other cases, the damage is already so severe that action has to be taken immediately. The classic action to take when suffering from excessive catalyst breakage is to shut the plant down, measure the pressure drops (using a PD rig) across all tubes and cross reference the measured PD’s against the visual observations (i.e.: which tubes appeared hot during operation) made whilst the reformer was on line. This cross check is performed to ensure that on restart all the problem tubes will have been recharged. Once these problem tubes are identified then they should be discharged (partially or completely); the effect on the frequency distribution is highlighted below, Figure 17 – Tubes to Change Out So if we change out the tubes that have a high breakage (i.e.: were hot during normal operation), what happens? The tubes with the fresh catalyst will have little or no breakage and therefore will have a relatively low resistance to flow (see equation 8 above). This is illustrated below,
  22. 22. 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 Web Site: www.GBHEnterprises.com Figure 18 – Effect on Frequency Distribution of Replacing Catalyst Frequency Distribution 0.00 0.05 0.10 0.15 0.20 0.25 0.30 -15 -10 -5 0 5 10 15 Pressure Drop Deviation FractionalNumberofTubes Base Case Carbon Formation Post Change Out Since the pressure drop across all tubes has to be the same, then the tubes with a low resistance to flow will see a higher gas flow rate. Now these tubes will receive the gas that was flowing through the tubes before the change out, but this is insufficient to fulfill the requirement to give the same pressure drop across all tubes. So these tubes will take some flow from the rest of the tubes. Unless the firing is reduced around these tubes, then they will be hotter as there is less flow through the tubes and therefore less heat sink (in terms of both the sensible heat load and the reaction heat required). It will therefore appear that the hot tubes have moved and that the problem has not been eliminated by changing out the catalyst. This effect has been seen on a number of plants who have performed a partial catalyst charge out.
  23. 23. 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 Web Site: www.GBHEnterprises.com Tube Appearance How can we tell what has happened within the tube based on the appearance of the outside tube For reference purposes, here is figure one again. Figure 19 – Tube Appearance Hot Bands Hot bands typically appear in Top Fired reformers around one third of the way down the tube. Their formation is associated with carbon formation due to poisoning, operating the catalyst past its effective end of life or catalyst bridging. Carbon formation and the effect it has on tube appearance is more fully discussed in “Basics of Reforming, Shapes and Carbon” . The first effect noted above, of operation of catalyst poisoning, occurs because as the catalyst activity is reduced, there is less reforming. This causes a rise in process gas temperature and more hydrocarbons slipping further down the tube. Both of these effects contribute to an increase in the carbon forming potential of the process gas. Once the carbon pinch point is reached carbon formation will start to occur and this leads to hot bands as illustrated below,
  24. 24. 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 Web Site: www.GBHEnterprises.com Figure 19b – Hot Bands The second effect noted above, of operation of a catalyst past its useful end of life, is essentially the same as the first effect in that the activity of the catalyst is too low to prevent carbon formation. The third effect is discussed below under giraffe necking.
  25. 25. 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 Web Site: www.GBHEnterprises.com Speckling Speckling is a very mild form of giraffe necking and operators often comment on it and worry about it since they think it is the start of hot band formation. Speckling occurs with all types of catalyst and on all types of reformers. It occurs when there is a small void formed near the inside wall of the tube. In this zone there is no reaction and so the tube is not cooled by transfer of heat to the process gas and hence the outside tube wall appears hot. The following figure illustrates the effect, Figure 20 – Tube Speckling
  26. 26. 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 Web Site: www.GBHEnterprises.com Giraffe Necking Giraffe necking occurs when there are large voids close to the inside of the tube wall. This is similar to speckling as noted above, but is somewhat more severe. In the worst case, giraffe necking can lead to catalyst bridging which causes a localized hot spot on the tube since in this zone there is no catalyst to support the reforming reactions and therefore reduce the process gas temperature. This is illustrated below, Figure 21 – Giraffe Necking and Catalyst Bridging If the void is sufficiently large, then the hot patch will become a hot band all the way around the tube. Catalyst Settling Catalyst settling leads to a large void at the top of the tube which since there is no catalyst to support the reforming reaction, means that this section of the tube becomes hot.
  27. 27. 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 Web Site: www.GBHEnterprises.com Excessive Breakage Excessive breakage has two effects, the first is that the resistance to flow has increased and so there is less flow through the tube and hence the tube appears hot. The second effect is that since the voidage has been reduced, then the catalyst will tend to settle and the loaded height drops giving the problems detailed above. Tube Hot on one Side There are a number of reasons why one side of a tube may appear hotter than the other side, • Misaligned burner – the flame deviated from the vertical and either the flame or the jet associated with the flame impinges on the tube surface. The opposite side of the tube is shaded and therefore does not become hot and hence change colour. • Flue gas mal-distribution – here the flame is moved from the vertical and as noted in the previous bullet point can impinge on the tube leading to a hot and cold face. • Over firing in one row – if one row is being over fired compared to the adjacent row, then the tube surface on one side of the tube will appear to be hotter due to the higher heat flux. • Insufficient combustion air – if one row of burners has too little combustion air, then this can lead to excessively high flame temperatures which in turn cause the outside tube surface to appear hot. On many furnaces, the tube coloration varies very rapidly, cycling from appearing hot to appearing cold. This is normally due to the impingement of the jet associated with the burner flame or the flame itself. Afterburning Afterburning can also cause excessive catalyst breakage since the tube temperature is rapidly cycled as combustion occurs, increasing the temperature and then stops as the combustion stops. This can lead to temperature transients within the tube and hence damage to the catalyst. After burning is normally observed as flames flickering on the tube surface. Terminology In order to be able to describe problems on a reformer, it is important to be clear as to what the various colors of reformer tubes mean. • Brown/Black – if a tube appears brown or black then the tube/catalyst is okay, • Brown with a slight orange colouration – in this case the tube appears to be brown but with a slight hint of orange. This is typical of tubes were problems are just starting to occur. • Brown with significant orange colouration – in this case the tube appear to be an orange/brown. This is typical of tubes were there are problems. • Orange with a hint of white – this is typical of tubes where there are significant problems. • White – this is typical of a tube with a very serious problem.
  28. 28. 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 Web Site: www.GBHEnterprises.com

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