Reduction of the conversion catalyst

Conversion and medium-shift catalyst reduction 6.4.1 Purpose of conversion and medium-shift catalyst reduction 6.4.1.1 Conversion catalysts are in an oxidized state at the time of leaving the factory; before use, the NiO contained within them must be reduced to active metallic nickel. The reduction reaction is as follows: NiO + H2 → Ni + H2O. The heat effect of this reaction is very small, so it can be ignored in industrial installations. Since the equilibrium constant for this reaction is very large, the reaction proceeds relatively easily. 6.4.1.2 Medium-temperature catalyst: At the time of leaving the factory, the active component of the medium-temperature catalyst, iron oxide, exists primarily in the form of Fe2O3. Before use, it needs to be reduced to Fe3O4 using H2 or CO in order to exhibit high activity. The reduction reactions are as follows: 3Fe2O3 + H2 → 2Fe3O4 + H2O, with ΔH298 = -9.6 KJ/mol; 3Fe2O3 + CO → 2Fe3O4 + CO2, with ΔH298 = -50.8 KJ/mol. Before reaching the reduction temperature, it is necessary to supply an adequate amount of steam first; only then can reducing gases be introduced for the reduction process. Otherwise, deep reduction occurs, resulting in the formation of metallic iron: Fe3O4 + 4H2O → 3Fe + 4H2O; ΔH298 = 150 KJ/mol. Fe3O4 + 4CO → 3Fe + 4CO2; ΔH298 = -14.8 KJ/mol. 6.4.2 Selection of reduction conditions for conversion and intermediate catalyst reduction 6.4.2.1 Reduction medium Hydrogen is generally used as the reduction medium. When reducing the conversion catalyst, the wet hydrogen reduction method is generally used in industry. The catalyst activity when reduced with pure hydrogen is higher than that when reduced with wet hydrogen. However, the wet hydrogen reduction method has many advantages: it helps to increase the inlet temperature of the catalyst bed ； Increasing the gas flow rate ensures uniform distribution of the reducing agent within each conversion reactor tube ； Prevent carbon deposition resulting from the cracking of small amounts of hydrocarbons in the reducing medium ； Prevent excessive reduction of the medium-temperature catalyst. In fact, the catalyst is used under a reducing atmosphere with a high water vapor partial pressure; by using wet hydrogen for reduction, the catalyst’s activity at this point better reflects its activity in actual use. The ratio of steam to hydrogen required for the reduction of the conversion catalyst changes with temperature. Figure 6-4-1 shows the redox curve of NiO obtained from the experiment. As can be seen from the graph, a temperature of up to 840°C and a PH2O/PH2 ratio below 7 are required to maintain the condition in the reduction zone. Therefore, when reducing the conversion catalyst, PH2O/PH2 should be kept below 7.5. For medium-temperature shift catalysts, to prevent excessive reduction, a PH2O/PH2 ratio of >0.2 is sufficient. 6.4.2.2 Reduction temperature: Increasing the temperature facilitates the reduction of the conversion catalyst. During the entire reduction period, the inlet temperature of the catalyst bed should be as high as possible. The inlet temperature should be maintained at 470–500°C, and the outlet temperature at 800±10°C. Medium-temperature shift catalysts can typically undergo reduction reactions with CO and H2 at 200–250°C. 6.4.2.3 Reduction pressure: The reduction pressure is generally between 0.8 and 1.0 MPa. 6.4.2.4 Reduction space velocity: The hydrogen space velocity is >300 h-1. 6.4.3 Heating process of the conversion system: When heating the conversion system, if the desulfurization system has not been fed with gas or it has been confirmed that no higher hydrocarbons are being carried away with the gas, then the heating of the conversion system can be carried out simultaneously with that of the desulfurization system, thereby creating a large-scale circulation that includes the feed preheater (F101) as well as the hydrodesulfurization reactors (R101, R102A.B). If the desulfurization system contains hydrocarbons, it should be isolated, and a conversion system should be set up to raise the temperature separately. Under such circumstances, it is absolutely not allowed for the desulfurization section to leak into the system before steam conversion. Since the natural gas feedstock lacks higher hydrocarbons, a system cycle can be established as shown in Figure 5-1. 6.4.4 Methods for heating the conversion system There are generally two methods for heating the conversion system: 6.4.4.1 Heating in an oxidizing atmosphere: The conversion catalyst bed is first heated by circulating nitrogen, and once the temperature reaches 20°C above the dew point of water vapor, it can be directly heated with water vapor. However, since the temperature of the medium-temperature catalyst bed is low at this time, and to prevent water vapor from condensing and damaging the catalyst, water vapor is usually used for heating only when the temperature of the medium-temperature catalyst bed is 20°C above the dew point temperature of the water vapor. 6.4.4.2 Heating in a reducing atmosphere: When the conversion catalyst is in a reduced state or partially reduced, heating of the conversion bed must be carried out in a reducing atmosphere. Once the temperature is raised to a level where that of the medium-bed layer is 20°C above the dew point temperature of water vapor, water vapor is first introduced into the conversion system, along with hydrogen, while controlling the H2O/H2 ratio

Hydrogen is generally used as the reducing agent. Five conditions must be met. Temperature, hydrogen purity, water-to-gas ratio, water-to-hydrogen ratio, hydrogen space velocity.

Hydrogen is used as the reducing medium; attention should be paid to the reaction temperature, water-to-hydrogen ratio, temperature rise in the reaction bed, water vapor ratio, and hydrogen circulation rate.