Weekly Topic on Ammonia Synthesis Production: Why is the temperature increase and reduction using medium-temperature catalysts carried out in several stages – namely, with air, steam, and gas?
Thread Content
Why is it necessary to carry out temperature rise and reduction using air, steam, as well as gas and steam in separate stages for medium-temperature shift catalysts? In accordance with the forum rules, please do not send hidden replies when responding, fellow sailors!
Previously, for medium-temperature shift conversion, we used a combustion furnace to heat the air. After switching to electric heaters in 1999, we began using nitrogen for heating and gas reduction instead; this method is both convenient and safe. In 2001, we changed to a medium-to-low temperature shift process, and in 2007, it was changed to a fully low-temperature shift process.
First, heat it to around 120 degrees using air, because using steam directly will cause condensation on the catalyst, thereby damaging it. Add more steam to raise the temperature to around 330 to 360 degrees. Finally, gas is added gradually; this is quite difficult, and if not controlled properly, it can damage the catalyst.
Generally, air is used to raise the temperature to around 150; thereafter, steam heating is employed. At this point, using air again would affect the catalyst’s activity. Eventually, gas is used instead
Medium-temperature catalysts are iron-chromium catalysts based primarily on iron oxide; they possess no catalytic activity on their own. During production, they must first be reduced to ferrite, which is iron oxide tetraoxide, in order to acquire high catalytic activity. The reduction method involves using CO and H2 present in semi-water gas; the reduction reactions are as follows: 3Fe2O3 + CO = 2Fe3O4 + CO2 + Q, 3Fe2O3 + H2 = 2Fe3O4 + H2O + Q. Preparatory work before carrying out the reduction at elevated temperatures: 1. Determine an appropriate heating and reduction plan based on the properties of the catalyst used, draw a temperature rise curve, prepare operation records, and simultaneously check that the electric furnace sowie all electrical instruments are in good working condition before proceeding. 2. Carefully check whether all blind flanges in the system have been removed, whether the system has been purged, whether the pressure testing and gas replacement have been completed successfully, and whether all valves in the system are in their correct positions. 3. There should be a clear division of responsibilities among the operators involved in the temperature rise and reduction of the catalyst; a technically skilled chief operator should handle the furnace temperature control. II. Temperature-raising reduction procedure
1. Method of temperature-raising reduction: First, raise the temperature using hot air heated by an electric furnace heater; then introduce semi-water gas for reduction. The entire heating and reduction process is divided into three stages: air heating, steam displacement, and CO-over-reduction. 2. Determine the process and routing for temperature rise and reduction to ensure they are unobstructed and reasonable. After completing all preparatory work prior to the temperature rise, air can be supplied to the shift conversion system. 3. Start the Roots blower or compressor to pass air through the temperature-raising reduction system at the maximum flow rate. The desired space velocity should be 200–300 NM3/hm3. Provided that the outlet temperature of the electric furnace and the heating rate are maintained, the higher the space velocity, the better. Fully open the air valves to keep the system pressure as low as possible. 4. Send a set of electric furnaces for trial use to start air heating. The outlet temperature of the electric furnace and the heating rate must be strictly controlled in accordance with the plan; the temperature should not be too high, nor should the heating rate be too fast. The control method for the outlet temperature and heating rate of the electric furnace involves coordinating changes in gas flow with adjustments to the electric furnace power. The operation first ensures a high space velocity, and then adjusts the electric furnace power. 5. Minimize the axial temperature difference within the catalyst layer as much as possible; a temperature difference of 50–80°C is acceptable. Maintaining a constant temperature of 120°C is primarily aimed at reducing the axial temperature difference in the catalyst layer, which facilitates the slow evaporation of free water, thereby ensuring a steady temperature rise of the catalyst and protecting its strength. To maintain a temperature of 200°C, the lowest temperature in the catalyst layer must be kept at least 20°C above the steam leakage point temperature. At a system pressure of 0.05–0.1 MPa, the lowest temperature of the catalyst should be above 120–130°C, in order to prepare the conditions for steam displacement. Constant-temperature operation first ensures a large steam volume. Secondly, the power of the electric furnace must be adjusted appropriately; its outlet temperature must be kept strictly within the range specified in the plan. Generally, it is not advisable to raise the outlet temperature of the electric furnace excessively in order to increase the minimum temperature of the catalyst. 6. Once the lowest temperature in the catalyst layer is above the steam dew point, steam displacement heating can be employed. During the process of heating the air to steam through displacement, it is necessary to take care to protect the electric furnace, ensure a steady rise in temperature, and guarantee the safe and normal operation of the Roots pump and compressor. Excess system pressure is strictly prohibited, as is the introduction of water into the system, especially into the catalyst layer. On the premise of maintaining the electric furnace outlet temperature and heating rate, while keeping the system pressure as low as possible, it is preferable to have a lower amount of steam. The steam should completely displace the air from all the equipment and pipelines in the heating and reduction system (including dead corners such as side lines and the bottoms of equipment). If a reverse process is required for replacement, attention should be paid to the electric furnace; it is necessary to ensure a steady increase in the outlet temperature of the furnace and to avoid backflow of the gas. Condensate water from various parts of the system should be drained regularly. 7. Once the temperature rise through steam displacement is complete, and the temperature becomes stable with an oxygen content in the system of ≤0.5%, the catalyst reduction stage can be initiated. It is advisable to use compressors with a low volume of air compression to supply semi-water gas to the system; generally, the outlet pressure of such compressors is around 0.05–0.1 MPa. If a Roots pump is used for gas reduction, the pressure should be as high as possible, provided that the safe operation of the Roots pump is maintained, in order to ensure that the reduced gas can enter the heating and reduction system. If the valve for gas reduction is too large, a bypass with a smaller pipeline can be considered to adjust the amount of reduction gas more accurately. 8. A very small amount of gas should be added when there is a high proportion of steam. When starting the reduction of the catalyst, the operator must be extremely careful and prudent. It is necessary to avoid excessive amounts of reducing gas, which could lead to an overly rapid reduction rate and excessively high temperatures. The catalytic reduction time should be long, exceeding 30 minutes. When there are various methods available to increase the amount of gas, it is safer to use only one of them at a time; in the initial stage of reduction, the steam/gas ratio should be greater than 5:1. During the temperature rise and reduction process of the catalyst, if there is a sudden spike in the catalyst temperature, it is necessary to appropriately and promptly reduce the amount of gas while increasing the amount of steam. A method of controlling the sharp rise in the catalyst bed temperature by adjusting the inlet temperature and using a gas bypass to lower the inlet temperatures at other sections. Where conditions permit, relevant steam bypass lines and **humidified cooling methods can be appropriately employed to control the sharp rise in catalyst temperature. An excess of steam is introduced during the catalytic reduction stage to ensure the proper progress of the conversion reaction, thereby controlling the reduction temperature and rate of the catalyst from becoming too high, in order to avoid excessive reduction of the catalyst and reduce the occurrence of side reactions. 9. During the reduction phase, the temperature at the outlet of the electric furnace should be increased gradually to 300°C and maintained at 300±10°C, in order to avoid large fluctuations in the catalyst temperature caused by variations in the furnace outlet temperature. 10. During the catalytic heating and reduction process, it should be avoided that the heating medium takes a shortcut and enters the second and third layers of catalysts directly. 11. Ensure that the O2 content in semi-water gas is ≤0.5%; during the reduction stage, it is necessary to strengthen the analysis of the gas composition at the inlet and outlet of the medium-frequency furnace. 12. The reduction end-point temperature should be slightly lower than the normal operating temperature of 450°C. When the temperature of the catalyst layer approaches 450°C, it should be maintained at this level for 4 hours. Once the temperature of the catalyst layer stabilizes and the CO content in the vented gas meets the process specifications for normal production, the reduction process is considered complete. Thereafter, the feed gas and steam were gradually increased, and normal production was resumed.
1. Method of temperature-raising reduction: First, raise the temperature using hot air heated by an electric furnace heater; then introduce semi-water gas for reduction. The entire heating and reduction process is divided into three stages: air heating, steam displacement, and CO-over-reduction. 2. Determine the process and routing for temperature rise and reduction to ensure they are unobstructed and reasonable. After completing all preparatory work prior to the temperature rise, air can be supplied to the shift conversion system. 3. Start the Roots blower or compressor to pass air through the temperature-raising reduction system at the maximum flow rate. The desired space velocity should be 200–300 NM3/hm3. Provided that the outlet temperature of the electric furnace and the heating rate are maintained, the higher the space velocity, the better. Fully open the air valves to keep the system pressure as low as possible. 4. Send a set of electric furnaces for trial use to start air heating. The outlet temperature of the electric furnace and the heating rate must be strictly controlled in accordance with the plan; the temperature should not be too high, nor should the heating rate be too fast. The control method for the outlet temperature and heating rate of the electric furnace involves coordinating changes in gas flow with adjustments to the electric furnace power. The operation first ensures a high space velocity, and then adjusts the electric furnace power. 5. Minimize the axial temperature difference within the catalyst layer as much as possible; a temperature difference of 50–80°C is acceptable. Maintaining a constant temperature of 120°C is primarily aimed at reducing the axial temperature difference in the catalyst layer, which facilitates the slow evaporation of free water, thereby ensuring a steady temperature rise of the catalyst and protecting its strength. To maintain a temperature of 200°C, the lowest temperature in the catalyst layer must be kept at least 20°C above the steam leakage point temperature. At a system pressure of 0.05–0.1 MPa, the lowest temperature of the catalyst should be above 120–130°C, in order to prepare the conditions for steam displacement. Constant-temperature operation first ensures a large steam volume. Secondly, the power of the electric furnace must be adjusted appropriately; its outlet temperature must be kept strictly within the range specified in the plan. Generally, it is not advisable to raise the outlet temperature of the electric furnace excessively in order to increase the minimum temperature of the catalyst. 6. Once the lowest temperature in the catalyst layer is above the steam dew point, steam displacement heating can be employed. During the process of heating the air to steam through displacement, it is necessary to take care to protect the electric furnace, ensure a steady rise in temperature, and guarantee the safe and normal operation of the Roots pump and compressor. Excess system pressure is strictly prohibited, as is the introduction of water into the system, especially into the catalyst layer. On the premise of maintaining the electric furnace outlet temperature and heating rate, while keeping the system pressure as low as possible, it is preferable to have a lower amount of steam. The steam should completely displace the air from all the equipment and pipelines in the heating and reduction system (including dead corners such as side lines and the bottoms of equipment). If a reverse process is required for replacement, attention should be paid to the electric furnace; it is necessary to ensure a steady increase in the outlet temperature of the furnace and to avoid backflow of the gas. Condensate water from various parts of the system should be drained regularly. 7. Once the temperature rise through steam displacement is complete, and the temperature becomes stable with an oxygen content in the system of ≤0.5%, the catalyst reduction stage can be initiated. It is advisable to use compressors with a low volume of air compression to supply semi-water gas to the system; generally, the outlet pressure of such compressors is around 0.05–0.1 MPa. If a Roots pump is used for gas reduction, the pressure should be as high as possible, provided that the safe operation of the Roots pump is maintained, in order to ensure that the reduced gas can enter the heating and reduction system. If the valve for gas reduction is too large, a bypass with a smaller pipeline can be considered to adjust the amount of reduction gas more accurately. 8. A very small amount of gas should be added when there is a high proportion of steam. When starting the reduction of the catalyst, the operator must be extremely careful and prudent. It is necessary to avoid excessive amounts of reducing gas, which could lead to an overly rapid reduction rate and excessively high temperatures. The catalytic reduction time should be long, exceeding 30 minutes. When there are various methods available to increase the amount of gas, it is safer to use only one of them at a time; in the initial stage of reduction, the steam/gas ratio should be greater than 5:1. During the temperature rise and reduction process of the catalyst, if there is a sudden spike in the catalyst temperature, it is necessary to appropriately and promptly reduce the amount of gas while increasing the amount of steam. A method of controlling the sharp rise in the catalyst bed temperature by adjusting the inlet temperature and using a gas bypass to lower the inlet temperatures at other sections. Where conditions permit, relevant steam bypass lines and **humidified cooling methods can be appropriately employed to control the sharp rise in catalyst temperature. An excess of steam is introduced during the catalytic reduction stage to ensure the proper progress of the conversion reaction, thereby controlling the reduction temperature and rate of the catalyst from becoming too high, in order to avoid excessive reduction of the catalyst and reduce the occurrence of side reactions. 9. During the reduction phase, the temperature at the outlet of the electric furnace should be increased gradually to 300°C and maintained at 300±10°C, in order to avoid large fluctuations in the catalyst temperature caused by variations in the furnace outlet temperature. 10. During the catalytic heating and reduction process, it should be avoided that the heating medium takes a shortcut and enters the second and third layers of catalysts directly. 11. Ensure that the O2 content in semi-water gas is ≤0.5%; during the reduction stage, it is necessary to strengthen the analysis of the gas composition at the inlet and outlet of the medium-frequency furnace. 12. The reduction end-point temperature should be slightly lower than the normal operating temperature of 450°C. When the temperature of the catalyst layer approaches 450°C, it should be maintained at this level for 4 hours. Once the temperature of the catalyst layer stabilizes and the CO content in the vented gas meets the process specifications for normal production, the reduction process is considered complete. Thereafter, the feed gas and steam were gradually increased, and normal production was resumed.
First, heat it to around 120 degrees using air, because using steam directly will cause condensation on the catalyst, thereby damaging it. Add more steam to raise the temperature to around 330 to 360 degrees. Finally, gas is added gradually; this is quite difficult, and if not controlled properly, it can damage the catalyst.
In large ammonia synthesis plants that use natural gas as a raw material, the temperature increase and catalyst reduction using a variable-bed catalyst generally involve first using N2 to raise the temperature above the dew point, and then introducing the mixture from the previous stage (the two-stage converter) to continue raising the temperature until the catalyst is fully reduced. Of course, the equipment requires a corresponding N2 circulation and heating pipeline network, and there must be an ample supply of N2.
The level of catalyst activity in the medium-pressure conversion unit directly affects the stable operation of this system; it is also crucial for energy savings and the stable functioning of subsequent systems. Only by identifying the factors that affect the catalyst activity in this unit and taking appropriate protective measures during its use can the catalyst perform at its best, extending its service life and reducing production costs. I. Effect of temperature on catalyst activity 1. Significant fluctuations in the temperature of the catalyst bed cause stress changes in the catalyst crystals, which serve as a structural support; as the temperature rises, the crystal structure expands ； As the temperature decreases, the crystal structure contracts; these repeated changes cause the catalyst strength to decline continuously, and the number of active centers decreases as well. On the other hand, the expansion and contraction of gases within the catalyst pores due to temperature fluctuations reduces the strength. As the strength decreases, part of the catalyst gradually pulverizes, which affects the internal diffusion of gases through the catalyst and leads to gas maldistribution, thereby severely impairing the catalytic activity. 2. During normal operation, if the temperature rises above 560°C, the medium-temperature catalyst will melt and sinter, losing its active micropores and thus its catalytic function; this leads to an excessive flow of gases and a decrease in activity. When the operating temperature exceeds 510°C, a side reaction occurs in which the active component Fe3O4 is excessively reduced to Fe. The higher the temperature, the greater the equilibrium constant of this reaction, resulting in more Fe being produced and thus lower catalyst activity. The reaction is as follows: Fe3O4 + H2 → 3Fe + 4H2O + Q. 3. During production, temperature drops or moisture enters the system, causing steam condensation on the surface and inner surfaces of the pores of the catalyst. The subsequent large-scale evaporation of water damages the crystal structure, leading to severe pulverization of the catalyst and a decrease in its activity. II. The effect of oxygen in semi-water gas on catalyst activity. Fe3O4, which serves as the active component in medium-temperature catalysts, is an extremely unstable substance; it can be oxidized to Fe2O3 at temperatures of 50–60°C, thereby losing its catalytic properties. 4Fe3O4 + O2 == 6Fe2O3 △H = -514 KJ/mol. An increase in the oxygen content in semi-water gas causes the active component Fe3O4 to be oxidized to Fe2O3, thereby reducing its activity. When the oxygen content decreases, Fe2O3 can be reduced back to Fe3O4 by H2 or CO present in the gas, restoring its activity. However, repeated fluctuations in the oxygen content cause the iron in the catalyst to continuously absorb and release oxygen; this leads to increasing and decreasing repulsive forces between oxygen atoms, severely damaging the strength of the catalyst and reducing its activity. Furthermore, the oxidation reaction of Fe3O4 is a highly exothermic reaction; even a 0.1% increase in the oxygen content in semi-water gas can cause the furnace temperature to rise significantly. If not controlled properly, overheating may occur, leading to a decrease in the activity of the medium-temperature catalyst. In production, semi-water gas always contains a certain amount of oxygen. The reaction between Fe3O4 and oxygen is an equilibrium reaction; as long as oxygen is present, the exothermic reaction 4Fe3O4 + O2 == 6Fe2O3 occurs, leading to a decrease in activity. Moreover, the heat effect associated with the oxidation of Fe3O4 to Fe2O3 (-514 KJ/mol) is significantly higher than the heat of reaction for the CO shift reaction (CO + H2O == CO2 + H2, △H = -41.19 KJ/mol). Under the combined effects of phase change stress and temperature variation stress, the pulverization of the medium-temperature shift catalyst is exacerbated. III. Effects of sulfur content in semi-water gas
When the H2S concentration in semi-water gas exceeds 100 mg/m³, the shift catalyst absorbs sulfur, resulting in temporary poisoning. When the steam-to-gas ratio increases or the H2S concentration decreases significantly, the catalyst releases sulfur, alleviating the poisoning effect; however, it cannot be completely eliminated. The reaction is as follows: Fe3O4 + 3H2S + H2 → 3FeS + 4H2O. When FeS comes into contact with O2, the following reaction occurs: 6FeS + 13.5O2 → 2Fe2(SO4)3 + Fe2O3. When the oxygen content decreases, the following reactions take place: FeSO4 + 4H2 → FeO + 3H2O + H2S; FeSO4 + 4CO + H2 → FeO + 3CO2 + H2S. Fe3O4 has a cubic crystal structure, while FeS has a hexagonal crystal structure. During the reduction process, the catalyst transforms from hexagonal FeS to cubic Fe3O4. Conversely, when the sulfur content in the feed gas increases, the catalyst reverts from cubic Fe3O4 back to hexagonal FeS. Particularly when there are significant fluctuations in H2S concentration, these phase transitions occur repeatedly, leading to severe damage to the crystal structure and ultimately causing the catalyst to powderize. Data shows that for the reaction Fe3O4 + 3H2S + H2 → 3FeS + 4H2O, the activity indicator is given by: when the H2S concentration ranges from 30 to 100 ppm, K ∝ (H2S)^-0.35 ； When the H2S concentration is between 70 and 1250 PPm, the activity parameter K∝(H2S)--0.55. IV. Effects of the steam/gas ratio: During normal production, a relatively high steam/gas ratio has no significant direct effect on catalyst activity. However, fluctuations in the steam/gas ratio cause relative changes in the concentrations of sulfur and oxygen in the gas, leading to a series of reactions as described in point “III” above; this in turn indirectly affects catalyst activity. When the operating system is shut down, if steam is stopped first and then there is a prolonged period during startup in which only coal gas is supplied—resulting in a low steam/gas ratio (<0.5)—hundreds of PPM of by-products are generated, which directly impair catalyst activity. Because carbon deposition reactions and deep reduction reactions occur. The reactions are as follows: 2CO == C + CO2 (1) CO + Fe3O4 == 3FeO + CO2 (2) CO + FeO == 3Fe + CO2 (3) 2CO + 3Fe == 3Fe3C + CO2 (4) In reaction (1), the carbon produced blocks the active channels of the catalyst, causing it to lose its activity. Reactions (2), (3), and (4) involve gradual deep reduction, which transforms the active component Fe3O4 in the catalyst into other substances that cannot regain their activity, resulting in complete loss of catalyst functionality. At high temperatures, feed gas with a severely insufficient gas-to-vapor ratio, when passing through a reduced catalyst, undergoes disproportionation reactions with CO to produce carbon and undergoes deep reduction of the catalyst, while also undergoing methanation reactions. CO + 2H2O = CH4 + CO2 – a highly exothermic methanation reaction. V. The influence of gas quality on catalyst activity: 1. The presence of arsenic, phosphorus, and chlorides in semi-water gas reacts with the active component of the catalyst, Fe3O4, to form iron arsenides, phosphides, and chlorides; in particular, arsenides and phosphides cause permanent poisoning of the catalyst, resulting in the loss of its activity. 2. The quality of the condensate water from steam is low; calcium, magnesium, and phosphate ions in it undergo a series of side reactions with CO2, resulting in the formation of precipitates such as CaCO3 and MgCO3. These form a hard salt layer on the surface of the catalyst, accumulating on its active surface and blocking the active pathways, thereby increasing the resistance of the system. Ca2++CO2→CaCO3; Mg2++CO2→MgCO3. 3. Negative effects resulting from the simplicity of the gas filtration system: The simple stainless steel wire demisting devices and coke filtration systems are only capable of removing large particles of impurities as well as some oil and water contaminants. Large-molecule tar, mechanical oil, small particle dust, sulfides, and other such substances contained in semi-water gas cannot be removed at all; they end up on the surface of the catalyst layer, blocking the active channels, reducing the available active surface area, increasing the system’s resistance, and leading to higher energy consumption. The continuous introduction of impurities can also cause severe blockages and corrosion in heat exchange equipment. VI. Catalyst Loading: Before loading the catalyst into the furnace, no screening was carried out, or the powder remained moist and could not be fully screened out; as a result, dust blocked the channels and affected the catalyst’s activity. Furthermore, failure to level the layer properly during loading, resulting in uneven density of the catalyst layer and causing gas flow deviation, as well as improper screening of the refractory balls on the bed surface leading to uneven gas distribution, will all lead to a decrease in catalyst activity. VII. Selection of the furnace construction method for medium-frequency converters: If a medium-frequency converter is not a hot-wall furnace, internal insulation measures must be implemented. If the internal insulation method is not chosen properly or the insulation material is inappropriate, gas leakage will occur within the furnace’s insulation layer, allowing gas to take shortcuts. To ensure the stable operation of subsequent processes, it is necessary to increase the amount of steam used; however, this increase in steam leads to the series of reactions mentioned in point “IV”, resulting in a significant rise in energy consumption. Furthermore, under conditions of air leakage, adding more steam to maintain production will only exacerbate the air leakage, increase the resistance in the medium-pressure conversion system, and disrupt the normal operation of both the upstream and downstream systems. VIII. Effects of catalyst heating and reduction: During the heating process of medium-temperature catalysts, improper control of the heating medium, heating rate, holding time, and hydrogenation reduction concentration can easily lead to overheating or significant temperature fluctuations, thereby severely affecting catalyst activity. 1. Effects of overheating during the air heating phase: When medium-temperature catalysts are formed into tablets, some graphite is added as a lubricant. When 2% graphite is included in the catalyst, at an air velocity of 500 h-1, the temperature rise of the catalyst bed reaches 515°C. For divalent iron (with Fe3O4 expressed in terms of a molar ratio of 1/2 to Fe2O3), the temperature rise is 275.8°C at an air velocity of 100 h-1, and 195.7°C at an air velocity of 500 h-1. Additionally, the reaction in which CrO3 and graphite undergo incomplete combustion to produce CO is a highly exothermic reaction; if such overheating occurs and is not controlled, it can lead to catalyst sintering and loss of activity. 2. Effects caused by an excessively high heating rate or insufficient holding time. An overly high heating rate can lead to the breakdown of the catalyst’s strength, while insufficient holding time prevents the removal of all the saturated water and crystalline water present in the catalyst layer; both situations result in a decrease in the catalyst’s strength and prevent it from exhibiting its proper activity. 3. The impact of improper control of the hydrogenation reduction concentration and selection of the hydrogenation reduction gas: When using semi-water gas for hydrogenation reduction, CO has a strong reducing capacity; as a result, it is difficult to control the furnace temperature, which tends to fluctuate greatly and can easily exceed safe levels. The relevant reaction equations are as follows: 3Fe2O3 + CO == 3Fe3O4 + CO2, ΔH = –50.8 KJ/mol; 3Fe2O3 + H2 == 3Fe3O4 + H2O, ΔH = –9.6 KJ/mol. At the beginning of the reduction process, since the catalyst temperature is low, if one focuses on increasing the gas concentration in an attempt to maintain a stable reaction temperature, gas accumulation may occur within the catalyst’s active channels. Once the temperature rises to a certain level, intense reactions take place, causing the temperature of the catalyst layer to soar rapidly. Furthermore, there is some CrO3 present in the medium-temperature catalyst; after hydrogenation, the following reactions occur: 2CrO3 + 3CO == Cr2O3 + 3CO2, with ΔH = -808.2 KJ/mol, and 2CrO3 + 3H2 == Cr2O3 + 3H2O, with ΔH = -647.8 KJ/mol. These reactions release large amounts of heat, and if proper consideration is not given and the gas concentration is not controlled appropriately, the heating process can cause severe damage to the catalyst, resulting in a significant reduction in its activity. Based on the operational characteristics of medium-temperature shift catalysts and the factors that affect their activity, the following measures should be taken to protect them during use: First, strict screening must be carried out when loading the catalyst, and it should be installed in accordance with technical specifications, ensuring that the density of the catalyst is consistent across different sections of the tower. II. Clean air should be used for the initial heating of the catalyst, and the heating rate must be strictly controlled. The typical heating rate is 10–15°C per hour, with the temperature difference between the gas and the bed remaining less than 50°C. Since some graphite is added as a lubricant during the pelletizing process of the medium-temperature shift catalyst, it is necessary to allow it to burn off completely during the air-heating phase, so as to prevent the graphite from blocking the active channels. 2C + O2 == 2CO + Q C + O2 == CO2 + Q. These reactions begin to take place at temperatures above 180°C, and they release a large amount of heat; therefore, it is necessary to increase the air flow rate as much as possible in order to remove this heat. Moreover, during the heating process, it is not advisable to start using steam for heating too early, as this can result in incomplete combustion of the graphite and thus a reduction in catalyst activity. At the same time, it is important to avoid overheating caused by these reactions, as that could also lead to a decline in catalyst activity. Furthermore, in the early stage of temperature increase (before 120°C), the catalyst undergoes a dehydration process and thermal expansion; therefore, the temperature should be increased slowly during this phase. III. Hydrogen reduction using purified gas is employed; the temperature-rise reduction reactions of the catalyst are as follows: 3Fe2O3 + CO == 3Fe3O4 + CO2, △H = -50.75 KJ/mol; 3Fe2O3 + H2 == 3Fe3O4 + H2O, △H = -9.61 KJ/mol. From these reactions, it can be seen that CO has a stronger reducing ability than hydrogen. The heat released during reduction using CO is 5.29 times that released when using hydrogen. If gas with a high CO content (around 30%) is used for reduction, it is difficult to control the temperature, leading to large fluctuations and a risk of overheating, which significantly affects the catalyst’s activity. Therefore, it is necessary to use purified gas with a CO content of 4.5% in combination with hydrogen for reduction. In the early stage of hydrogenation reduction, due to the low temperature, the reduction reaction proceeds slowly; therefore, one should not rush to increase the hydrogen concentration just because the temperature of the catalyst layer is rising steadily, and the hydrogen concentration must be strictly controlled. Initially, the concentrations of CO and H2 are set at 0.5%, 1%, 2%, etc. As the bed temperature rises gradually, it is safer to employ the “stabilization method”. The temperature rise and reduction space velocity should be controlled at 200–300 Nm³/hr·m³ (catalyst). The reduction temperature should be maintained at around 430°C; it is strictly prohibited for the temperature of the catalyst layer to exceed 500°C. IV. Strictly control the sulfur content in the semi-water gas fed into the medium-pressure shift system. It is necessary to avoid significant fluctuations in the sulfur content of this gas, as such fluctuations can cause repeated changes in the intergranular structure of the catalyst, thereby reducing its strength and affecting its performance. V. Strengthening the purification of gas, steam, and water
1. Purification of raw water-gas before it enters the system: The traditional methods of simple filtration using stainless steel wire mesh for defoaming and coke filtration have been replaced by newer filtration technologies and materials. Currently, the most advanced separation technology utilized in China is high-efficiency integrated membrane separation technology. The filtration materials used are sintered stainless steel felt and non-metallic silicon-boron fiber materials; the fiber contact points are sintered together to form a gradient pore structure. This constitutes a fixed-type deep filtration medium with irregular pore sizes. There is no migration of the filter medium nor any impact-induced unloading, thus preventing secondary pollution. The filter element has a cylindrical or folded tubular structure, characterized by high strength, excellent sealing properties, and a high porosity. It offers great operational flexibility and can withstand a pressure difference of △P ≥ 5 bar. It effectively separates mechanical impurities ranging from 1–10 μm up to those larger than 10 μm, as well as oil, water, sulfides, etc. The overall filtration efficiency reaches 99.85%. Moreover, this device can operate continuously for up to 10,000 hours. It can reduce system resistance, extend the service life of the catalyst, and improve the heat transfer efficiency of heat exchangers. By equipping the filter with cleaning and vibration devices, the filter element can be reused, allowing normal production during cleaning. 2. Sulfur, arsenic, phosphorus, and halides in the feed gas can poison the catalyst; they should be avoided as much as possible. 3. Demineralized water shall be used for steam production and system makeup water. VI. Controlling an appropriate steam-to-gas ratio: A medium steam-to-gas ratio of around 0.7 to 1.2 is generally considered reasonable. When the steam-to-gas ratio is less than 0.6, hundreds of ppm of CH4, C2H4, C2H6, and C3H8 are produced; for domestically produced catalysts, the steam-to-gas ratio should generally not be lower than 0.4. VII. Selection of the furnace construction method for medium-frequency furnaces: Renovation of the insulation layer in old medium-frequency furnaces by adopting a furnace construction structure that combines an insulating layer with a refractory layer. Adhering to the inner wall of the converter is a calcium silicate fiber board, which provides excellent insulation and can prevent the transmission of high temperatures. In contact with the catalyst are high-strength, lightweight refractory bricks, which possess good strength and high heat resistance. This enables the inner wall of the furnace to withstand high temperatures and the pressure exerted by the catalyst without deforming. It also prevents gaps from forming between the catalyst and the internal insulation layer, thus avoiding any short-circuiting of gases and ensuring that gases pass through the catalyst bed uniformly. VIII. Startup, shutdown, and normal operation of the system: During the startup and shutdown of the system, the process of pressurization or depressurization should not be carried out too quickly; it is necessary to ensure that the time taken for depressurization and pressurization is at least 40 minutes. It is strictly prohibited to allow water to reach the catalyst layer during startup, and care must also be taken to prevent the formation of condensate from medium-temperature steam. In summary, during the use of shift catalysts, effective protective measures must be taken to extend their service life, thereby ensuring the safe and stable operation of the shift system in a state of maximum energy efficiency. Reported by Yan Qiusheng, No. 1
When the H2S concentration in semi-water gas exceeds 100 mg/m³, the shift catalyst absorbs sulfur, resulting in temporary poisoning. When the steam-to-gas ratio increases or the H2S concentration decreases significantly, the catalyst releases sulfur, alleviating the poisoning effect; however, it cannot be completely eliminated. The reaction is as follows: Fe3O4 + 3H2S + H2 → 3FeS + 4H2O. When FeS comes into contact with O2, the following reaction occurs: 6FeS + 13.5O2 → 2Fe2(SO4)3 + Fe2O3. When the oxygen content decreases, the following reactions take place: FeSO4 + 4H2 → FeO + 3H2O + H2S; FeSO4 + 4CO + H2 → FeO + 3CO2 + H2S. Fe3O4 has a cubic crystal structure, while FeS has a hexagonal crystal structure. During the reduction process, the catalyst transforms from hexagonal FeS to cubic Fe3O4. Conversely, when the sulfur content in the feed gas increases, the catalyst reverts from cubic Fe3O4 back to hexagonal FeS. Particularly when there are significant fluctuations in H2S concentration, these phase transitions occur repeatedly, leading to severe damage to the crystal structure and ultimately causing the catalyst to powderize. Data shows that for the reaction Fe3O4 + 3H2S + H2 → 3FeS + 4H2O, the activity indicator is given by: when the H2S concentration ranges from 30 to 100 ppm, K ∝ (H2S)^-0.35 ； When the H2S concentration is between 70 and 1250 PPm, the activity parameter K∝(H2S)--0.55. IV. Effects of the steam/gas ratio: During normal production, a relatively high steam/gas ratio has no significant direct effect on catalyst activity. However, fluctuations in the steam/gas ratio cause relative changes in the concentrations of sulfur and oxygen in the gas, leading to a series of reactions as described in point “III” above; this in turn indirectly affects catalyst activity. When the operating system is shut down, if steam is stopped first and then there is a prolonged period during startup in which only coal gas is supplied—resulting in a low steam/gas ratio (<0.5)—hundreds of PPM of by-products are generated, which directly impair catalyst activity. Because carbon deposition reactions and deep reduction reactions occur. The reactions are as follows: 2CO == C + CO2 (1) CO + Fe3O4 == 3FeO + CO2 (2) CO + FeO == 3Fe + CO2 (3) 2CO + 3Fe == 3Fe3C + CO2 (4) In reaction (1), the carbon produced blocks the active channels of the catalyst, causing it to lose its activity. Reactions (2), (3), and (4) involve gradual deep reduction, which transforms the active component Fe3O4 in the catalyst into other substances that cannot regain their activity, resulting in complete loss of catalyst functionality. At high temperatures, feed gas with a severely insufficient gas-to-vapor ratio, when passing through a reduced catalyst, undergoes disproportionation reactions with CO to produce carbon and undergoes deep reduction of the catalyst, while also undergoing methanation reactions. CO + 2H2O = CH4 + CO2 – a highly exothermic methanation reaction. V. The influence of gas quality on catalyst activity: 1. The presence of arsenic, phosphorus, and chlorides in semi-water gas reacts with the active component of the catalyst, Fe3O4, to form iron arsenides, phosphides, and chlorides; in particular, arsenides and phosphides cause permanent poisoning of the catalyst, resulting in the loss of its activity. 2. The quality of the condensate water from steam is low; calcium, magnesium, and phosphate ions in it undergo a series of side reactions with CO2, resulting in the formation of precipitates such as CaCO3 and MgCO3. These form a hard salt layer on the surface of the catalyst, accumulating on its active surface and blocking the active pathways, thereby increasing the resistance of the system. Ca2++CO2→CaCO3; Mg2++CO2→MgCO3. 3. Negative effects resulting from the simplicity of the gas filtration system: The simple stainless steel wire demisting devices and coke filtration systems are only capable of removing large particles of impurities as well as some oil and water contaminants. Large-molecule tar, mechanical oil, small particle dust, sulfides, and other such substances contained in semi-water gas cannot be removed at all; they end up on the surface of the catalyst layer, blocking the active channels, reducing the available active surface area, increasing the system’s resistance, and leading to higher energy consumption. The continuous introduction of impurities can also cause severe blockages and corrosion in heat exchange equipment. VI. Catalyst Loading: Before loading the catalyst into the furnace, no screening was carried out, or the powder remained moist and could not be fully screened out; as a result, dust blocked the channels and affected the catalyst’s activity. Furthermore, failure to level the layer properly during loading, resulting in uneven density of the catalyst layer and causing gas flow deviation, as well as improper screening of the refractory balls on the bed surface leading to uneven gas distribution, will all lead to a decrease in catalyst activity. VII. Selection of the furnace construction method for medium-frequency converters: If a medium-frequency converter is not a hot-wall furnace, internal insulation measures must be implemented. If the internal insulation method is not chosen properly or the insulation material is inappropriate, gas leakage will occur within the furnace’s insulation layer, allowing gas to take shortcuts. To ensure the stable operation of subsequent processes, it is necessary to increase the amount of steam used; however, this increase in steam leads to the series of reactions mentioned in point “IV”, resulting in a significant rise in energy consumption. Furthermore, under conditions of air leakage, adding more steam to maintain production will only exacerbate the air leakage, increase the resistance in the medium-pressure conversion system, and disrupt the normal operation of both the upstream and downstream systems. VIII. Effects of catalyst heating and reduction: During the heating process of medium-temperature catalysts, improper control of the heating medium, heating rate, holding time, and hydrogenation reduction concentration can easily lead to overheating or significant temperature fluctuations, thereby severely affecting catalyst activity. 1. Effects of overheating during the air heating phase: When medium-temperature catalysts are formed into tablets, some graphite is added as a lubricant. When 2% graphite is included in the catalyst, at an air velocity of 500 h-1, the temperature rise of the catalyst bed reaches 515°C. For divalent iron (with Fe3O4 expressed in terms of a molar ratio of 1/2 to Fe2O3), the temperature rise is 275.8°C at an air velocity of 100 h-1, and 195.7°C at an air velocity of 500 h-1. Additionally, the reaction in which CrO3 and graphite undergo incomplete combustion to produce CO is a highly exothermic reaction; if such overheating occurs and is not controlled, it can lead to catalyst sintering and loss of activity. 2. Effects caused by an excessively high heating rate or insufficient holding time. An overly high heating rate can lead to the breakdown of the catalyst’s strength, while insufficient holding time prevents the removal of all the saturated water and crystalline water present in the catalyst layer; both situations result in a decrease in the catalyst’s strength and prevent it from exhibiting its proper activity. 3. The impact of improper control of the hydrogenation reduction concentration and selection of the hydrogenation reduction gas: When using semi-water gas for hydrogenation reduction, CO has a strong reducing capacity; as a result, it is difficult to control the furnace temperature, which tends to fluctuate greatly and can easily exceed safe levels. The relevant reaction equations are as follows: 3Fe2O3 + CO == 3Fe3O4 + CO2, ΔH = –50.8 KJ/mol; 3Fe2O3 + H2 == 3Fe3O4 + H2O, ΔH = –9.6 KJ/mol. At the beginning of the reduction process, since the catalyst temperature is low, if one focuses on increasing the gas concentration in an attempt to maintain a stable reaction temperature, gas accumulation may occur within the catalyst’s active channels. Once the temperature rises to a certain level, intense reactions take place, causing the temperature of the catalyst layer to soar rapidly. Furthermore, there is some CrO3 present in the medium-temperature catalyst; after hydrogenation, the following reactions occur: 2CrO3 + 3CO == Cr2O3 + 3CO2, with ΔH = -808.2 KJ/mol, and 2CrO3 + 3H2 == Cr2O3 + 3H2O, with ΔH = -647.8 KJ/mol. These reactions release large amounts of heat, and if proper consideration is not given and the gas concentration is not controlled appropriately, the heating process can cause severe damage to the catalyst, resulting in a significant reduction in its activity. Based on the operational characteristics of medium-temperature shift catalysts and the factors that affect their activity, the following measures should be taken to protect them during use: First, strict screening must be carried out when loading the catalyst, and it should be installed in accordance with technical specifications, ensuring that the density of the catalyst is consistent across different sections of the tower. II. Clean air should be used for the initial heating of the catalyst, and the heating rate must be strictly controlled. The typical heating rate is 10–15°C per hour, with the temperature difference between the gas and the bed remaining less than 50°C. Since some graphite is added as a lubricant during the pelletizing process of the medium-temperature shift catalyst, it is necessary to allow it to burn off completely during the air-heating phase, so as to prevent the graphite from blocking the active channels. 2C + O2 == 2CO + Q C + O2 == CO2 + Q. These reactions begin to take place at temperatures above 180°C, and they release a large amount of heat; therefore, it is necessary to increase the air flow rate as much as possible in order to remove this heat. Moreover, during the heating process, it is not advisable to start using steam for heating too early, as this can result in incomplete combustion of the graphite and thus a reduction in catalyst activity. At the same time, it is important to avoid overheating caused by these reactions, as that could also lead to a decline in catalyst activity. Furthermore, in the early stage of temperature increase (before 120°C), the catalyst undergoes a dehydration process and thermal expansion; therefore, the temperature should be increased slowly during this phase. III. Hydrogen reduction using purified gas is employed; the temperature-rise reduction reactions of the catalyst are as follows: 3Fe2O3 + CO == 3Fe3O4 + CO2, △H = -50.75 KJ/mol; 3Fe2O3 + H2 == 3Fe3O4 + H2O, △H = -9.61 KJ/mol. From these reactions, it can be seen that CO has a stronger reducing ability than hydrogen. The heat released during reduction using CO is 5.29 times that released when using hydrogen. If gas with a high CO content (around 30%) is used for reduction, it is difficult to control the temperature, leading to large fluctuations and a risk of overheating, which significantly affects the catalyst’s activity. Therefore, it is necessary to use purified gas with a CO content of 4.5% in combination with hydrogen for reduction. In the early stage of hydrogenation reduction, due to the low temperature, the reduction reaction proceeds slowly; therefore, one should not rush to increase the hydrogen concentration just because the temperature of the catalyst layer is rising steadily, and the hydrogen concentration must be strictly controlled. Initially, the concentrations of CO and H2 are set at 0.5%, 1%, 2%, etc. As the bed temperature rises gradually, it is safer to employ the “stabilization method”. The temperature rise and reduction space velocity should be controlled at 200–300 Nm³/hr·m³ (catalyst). The reduction temperature should be maintained at around 430°C; it is strictly prohibited for the temperature of the catalyst layer to exceed 500°C. IV. Strictly control the sulfur content in the semi-water gas fed into the medium-pressure shift system. It is necessary to avoid significant fluctuations in the sulfur content of this gas, as such fluctuations can cause repeated changes in the intergranular structure of the catalyst, thereby reducing its strength and affecting its performance. V. Strengthening the purification of gas, steam, and water
1. Purification of raw water-gas before it enters the system: The traditional methods of simple filtration using stainless steel wire mesh for defoaming and coke filtration have been replaced by newer filtration technologies and materials. Currently, the most advanced separation technology utilized in China is high-efficiency integrated membrane separation technology. The filtration materials used are sintered stainless steel felt and non-metallic silicon-boron fiber materials; the fiber contact points are sintered together to form a gradient pore structure. This constitutes a fixed-type deep filtration medium with irregular pore sizes. There is no migration of the filter medium nor any impact-induced unloading, thus preventing secondary pollution. The filter element has a cylindrical or folded tubular structure, characterized by high strength, excellent sealing properties, and a high porosity. It offers great operational flexibility and can withstand a pressure difference of △P ≥ 5 bar. It effectively separates mechanical impurities ranging from 1–10 μm up to those larger than 10 μm, as well as oil, water, sulfides, etc. The overall filtration efficiency reaches 99.85%. Moreover, this device can operate continuously for up to 10,000 hours. It can reduce system resistance, extend the service life of the catalyst, and improve the heat transfer efficiency of heat exchangers. By equipping the filter with cleaning and vibration devices, the filter element can be reused, allowing normal production during cleaning. 2. Sulfur, arsenic, phosphorus, and halides in the feed gas can poison the catalyst; they should be avoided as much as possible. 3. Demineralized water shall be used for steam production and system makeup water. VI. Controlling an appropriate steam-to-gas ratio: A medium steam-to-gas ratio of around 0.7 to 1.2 is generally considered reasonable. When the steam-to-gas ratio is less than 0.6, hundreds of ppm of CH4, C2H4, C2H6, and C3H8 are produced; for domestically produced catalysts, the steam-to-gas ratio should generally not be lower than 0.4. VII. Selection of the furnace construction method for medium-frequency furnaces: Renovation of the insulation layer in old medium-frequency furnaces by adopting a furnace construction structure that combines an insulating layer with a refractory layer. Adhering to the inner wall of the converter is a calcium silicate fiber board, which provides excellent insulation and can prevent the transmission of high temperatures. In contact with the catalyst are high-strength, lightweight refractory bricks, which possess good strength and high heat resistance. This enables the inner wall of the furnace to withstand high temperatures and the pressure exerted by the catalyst without deforming. It also prevents gaps from forming between the catalyst and the internal insulation layer, thus avoiding any short-circuiting of gases and ensuring that gases pass through the catalyst bed uniformly. VIII. Startup, shutdown, and normal operation of the system: During the startup and shutdown of the system, the process of pressurization or depressurization should not be carried out too quickly; it is necessary to ensure that the time taken for depressurization and pressurization is at least 40 minutes. It is strictly prohibited to allow water to reach the catalyst layer during startup, and care must also be taken to prevent the formation of condensate from medium-temperature steam. In summary, during the use of shift catalysts, effective protective measures must be taken to extend their service life, thereby ensuring the safe and stable operation of the shift system in a state of maximum energy efficiency. Reported by Yan Qiusheng, No. 1
Precautions for heating the catalyst: 1. Pay attention to the heating rate; 2. Control the gas-to-vapor ratio and inlet temperature; 3. Pay attention to pressure control; 4. Control the gas flow rate