Iron-manganese medium-temperature desulfurizer

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5.4 Iron-manganese desulfurizer 5.4.1 Chemical reaction equations The iron-manganese desulfurizer is a dual-functional desulfurizing agent of the conversion and absorption type, whose main components are iron oxide and manganese oxide, along with promoters such as zinc oxide. Before use, natural gas to which 1–5% hydrogen has been added is subjected to reduction at pressures of 98–490 kPa, with a space velocity of 300–500 h-1, and at temperatures not exceeding 450°C. As a result, Fe2O3 and MnO2 are reduced to Fe2O3 and MnO respectively, which possess desulfurization activity. The reactions are as follows: 3Fe2O3 + 3H2 → 2Fe3O4 + H2O (5-50); MnO2 + H2 → MnO + H2O (5-51). On iron-manganese desulfurizers, organic sulfur compounds such as RSH, RSR, COS, and CS2 can undergo hydrolysis to produce H2S. RSH and RSR may also undergo pyrolysis to yield H2S and olefins. The reaction equations are given in Sections 5.1.1, equations (5-1) to (5-5) of this chapter. COS + H2 → CO + H2S (5-1); C2H5SH + H2 → C2H6 + H2S, ΔH298 = –70.2 kJ/mol (5-2); C6H5SH + H2 → C6H6 + H2S (5-3); CH3SC2H5 + H2 → CH4 + C2H6 + H2S (5-4); C2H5SSC2H5 + 3H2 → 2C2H6 + 2H2S, ΔH298 = –117.2 kJ/mol (5-5); C4H8S + 2H2 → C4H10 + H2S, ΔH298 = –120.2 kJ/mol (5-6); C4H4S + 4H2 → C4H10 + H2S, ΔH298 = –280.3 kJ/mol (5-7). The H2S generated through hydrolysis or pyrolysis can be absorbed by the main active components of the desulfurizer via the following reactions: 3H2S + Fe3O4 → 3FeS + 4H2O (5-52); H2S + MnO → MnS + H2O (5-53); H2S + ZnO → ZnS + H2O (5-54). RSH and RSR can also be directly absorbed by Fe3O4 or MnO to form FeS or MnS, thereby being removed. 5.4.2 Physicochemical properties of ferromanganese desulfurizers Currently, there are two types of ferromanganese desulfurizers produced in China, and their components and physicochemical properties are shown in Table 5-20. Table 5-20 Physical and chemical properties of ferromanganese desulfurizer. Models: MF-1, MF-2. Shape and specifications, in mm: φ9×5 or φ5×5; φ9×5–6 or φ5×5, cylindrical shape. Components: Mn + Fe + Zn, weight%, ≥35, ≥45. Bulk density, kg/L: 1.35–1.40, 1.20–1.30. Side compressive strength, N/cm: ≥80, ≥80. Wear rate, %: ≤14, ≤15. 5.4.3 Quality criteria and testing methods. The quality criteria for ferromanganese desulfurizer are shown in Table 5-21. 5-21 Quality specifications for iron-manganese desulfurization agents
Model: MF-1, MF-2
Standard numbers: DB/5105G57005-88, QB/510501XN.013-90
Product grades: First class, Second class, Third class; First class, Second class, Third class

Initial desulfurization activity, ppm ≤ 0.5, 0.5, 0.5, 0.2, 0.2, 0.2
Mn + Fe + Zn, weight % ≥ 35, 34, 33, 44, 43, 42
Side crushing strength, N/cm ≥ 160, 100, 80, 160, 100, 80
Wear rate, weight % ≤ 11, 13, 14, 11, 14, 15

The initial desulfurization activity is determined by using 20 ml of samples with particle sizes of 2–5 mm. Under a pressure of 0.5 MPa and a space velocity of 500 h-1, the samples are heated to 380–400°C and held at that temperature for 24 hours. Afterwards, under a pressure of 2.0 MPa, a space velocity of 1000–2000 h-1, and a temperature of 380–400°C, the sulfur content at the outlet is measured using gaseous hydrocarbons containing 300–500 mg/m3 of total sulfur (by standard) and 2–5% hydrogen. The anti-crushing strength and wear rate of the desulfurizer are determined in accordance with GB-3635 and GB-3636-83 respectively. 5.4.4 Filling and Activation: Before filling, the ferromanganese desulfurizer should be screened to remove dust. The bottom of the desulfurization tank is filled with alumina balls of φ15–20 to provide compression, with a screening plate at the very top. The desulfurizer discharge pipe should extend outside the tank above the lower screen in order to discharge the waste desulfurizer. During loading, the free fall height of the desulfurizer should be less than 600 mm; it can be loaded using chutes and bag conveyors. The desulfurizer uses nitrogen or purified feed gas to which 1–5% hydrogen has been added as the reducing medium; the pressure is maintained at 0.1–0.5 MPa, and the linear velocity should be greater than 0.25 m/s. The temperature is raised to 180 °C as quickly as possible and then held constant for 4–8 hours. Once the temperature difference across various points in the bed is less than 30 °C, the temperature is increased at a rate of 20–40 °C/h, and it is held constant at 250 °C for 2–4 hours, to ensure that the highest temperature in the bed does not exceed 450 °C. Finally, the temperature of the inlet gas is raised to 350–400°C, and the pressure is increased to the normal operating pressure; normal operation can be resumed once the concentrations of CO2, O2, and H2S in the outlet gas are within acceptable levels. 5.4.5 Normal operating conditions The normal operating conditions for iron-manganese desulfurizers are shown in Table 5-22. Table 5-22 Normal operating conditions for iron-manganese desulfurizers Model LS-2 MF-1 MF-2 Operating pressure/MPa 0.1~4.0 0.1~4.0 0.1~4.0 Operating temperature/℃ 200~250 350~400 350~400 Space velocity/h-1 100~500 100~1000 100~1000 Hydrogen content in feed gas/% Oxygen content/% Water content/% 30 Linear velocity/(m/s) 0.3~ 0.3~0.6 Sulfur capacity per dual-tank/(weight %) >7 15 18~20 Sulfur capacity per single-tank/(weight %) 5~7 ≥11 5.4.6 Typical application examples A medium-sized nitrogen fertilizer plant first removes part of the sulfur from natural gas using dilute ammonia water combined with PDS in a wet desulfurization process, and then employs two tanks in series filled with MF-2 type iron-manganese desulfurizers for secondary desulfurization. The sulfur content in natural gas varies significantly; generally, H2S levels range from 200 to 1500 mg/m3 (on a standard basis), while organic sulfur levels range from 10 to 30 mg/m3 (on a standard basis). All desulfurization tanks are of φ1600×14 specification, with an inner diameter of 1200 mm, and each is equipped with 4.55 m3 of desulfurizing agent. During the heating process of the first-stage furnace, the desulfurizer is heated to 350–400°C at a rate of ≤60°C/h, using the natural gas preheated in the convection section as the heating medium; the trace amount of hydrogen present in the natural gas is utilized to reduce the iron-manganese desulfurizer. After being reduced to 250 ℃, it is necessary to prevent a sudden rise in bed temperature caused by too low an air velocity, too high a hydrogen concentration, or excessive pressure. The normal operating pressure is 1.4–1.8 MPa, the inlet gas temperature is 380–400°C, the temperature of the bed in the upstream desulfurization tank is 360–370°C, that of the downstream desulfurization tank is 340°C, and the outlet gas temperature is 290–300°C. The H2S level at the inlet of the desulfurization tank is 70–100 mg/m3 (standard), with a maximum of 3400 mg/m3 (standard); the level at the outlet is generally below 0.1 mg/m3 (standard), with some cases reaching around 0.5 mg/m3 (standard). Each cell can operate continuously for over a year, but sulfur capacity data has not been determined.