Precautions for desulfurization using tannin method

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1 Principles of chemical reactions: Alkaline aqueous solutions absorb H2S. Na2CO3 + H2S = NaHCO3 + NaHS (1) The pentavalent vanadium complex ion oxidizes HS– to release sulfur, while the pentavalent vanadium is reduced to a tetralent vanadium complex ion. 2V5++HS=2V4++S+H+ (2) Meanwhile, the quinone-form tannin oxidizes HS– to release sulfur, and the quinone-form tannin is reduced to the phenol-form tannin. TQ+HS－=THQ+S (3) The glucoside form of tannin oxidizes the tetravalent vanadium complex ions, thereby regenerating them. TQ+V4++2H2O=V5++THQ+OH－ (4) Oxidation of phenolic tannin in air leads to the regeneration of the tannin, with H2O2 being produced as a byproduct. 2O2 + THQ = TQ + H2O2 (5) H2O2 oxidizes tetravalent vanadium complex ions and HS–: H2O2 + HS– = H2O + S + OH– (6) The gas contains CO2, HCN, O2, as well as side reactions caused by H2O2. 2 Pretreatment of the gum solution: The viscosity and tendency to foam of the gum solution are detrimental to the processes of desulfurization and sulfur recovery. They can cause difficulties in melting sulfur and filtering, leading to an increase in the suspended sulfur content in the desulfurization solution, intensified side reactions, increased consumption, and a decline in the effectiveness of the desulfurization solution. The aforementioned phenomena occur when an untreated tannin solution is introduced into the system; although it gradually returns to normal over time as operation continues, the impact on production cannot be ignored. The process of preparing an alkaline tannin solution with a specific composition, then oxidizing it by exposing it to air under certain conditions in order to eliminate its stickiness and foaming properties, as well as converting the phenolic tannins present in it into quinonic tannins, is known as the pretreatment of the solution. Based on the properties of the double-layer structure in colloidal solutions, as the pH value of the solution increases, the hydrogen ion concentration decreases. Cations in the adsorption layer move into the diffusion layer, leading to the degradation of particles and the breakdown of the solution’s viscosity. When the solution is heated and exposed to air for oxidation, tannins undergo degradation reactions; large molecules become smaller, and surface-active substances turn into surface-inactive ones, resulting in a decrease or even disappearance of the solution’s viscosity. During this oxidation process, the phenolic structure of tannins transforms into a quinone structure, endowing the solution with activity. The recommended pretreatment conditions are as follows: (pretreatment with Na2CO3) Size concentration: 10–30 g/l; Alkalinity (Na2CO3): 10–25 g/l; Oxidation temperature: 70–90°C; Air volume: sufficient so that the solution does not spill out of the preparation container. To determine whether the solution pretreatment meets the desulfurization requirements, the extinction value of the solution can be measured; a value stable around 0.45 indicates that the goal has been achieved. 3 Production control conditions
Solution composition: Total alkalinity – 0.4 mol/l; Na+. Among these, Na2CO3: 0.1 mol/l (5.0 g/l); NaHCO3: 0.1 mol/l (25 g/l). Gum: 1.0–2.0 g/l; NaVO3: 1.0–1.5 g/l. pH value: 8.5–9.0. Suspended sulfur: 0–0.5 g/l; Na2S2O3: 0–10.0 g/l. Sulfur content in the solution: 0.1 g/l. Liquid-to-gas ratio for absorption: 0.1–0.015 L/m3. Operating temperature: Gas temperature entering the desulfurization tower – 30–40°C; liquid temperature is 3–5°C higher than the gas temperature. Regeneration temperature: 35–40°C. Heating temperature in the sulfur melting tank: 130–150°C. Before desulfurization of semi-water gas, H2S level ≤ 2 g/l; after desulfurization, H2S level ≤ 0.1 g/l. After desulfurization of transformed gas, H2S level ≤ 10 mg/l. Residence time of the solution in the spray regeneration tank: 5–10 minutes. Blowing intensity in the spray regeneration tank: 70–120 m3/m2·h. Liquid velocity at the nozzle of the sprayer in the spray regeneration tank: 15–20 m/s (the liquid pressure before entering the sprayer should be ≥ 0.32 MPa). Pressure inside the sulfur melting tank: < 0.8 MPa. Reference values for consumption: Alkali: 1.5–2.0 kg/t NH3; NaVO3 (V2O5): 0.02–0.03 kg/t NH3; Gum: 0.05–0.1 kg/t NH3.

4 Side reactions, by-products, and their control
1) Sulfated sulfides are intermediate products formed during the absorption of H2S; they are generated when H2S comes into contact with oxygen. This involves one absorption reaction and one oxidation reaction:
Na2CO3 + H2S + NaHCO3 + NaHS → 2NaHS + 2O2 + Na2S2O3 + H2O
Both the absorption and oxidation reactions take place within the absorption tower. In other words, the higher the oxygen content in the gas, the more by-products are generated, and the more alkali is consumed. Therefore, controlling the oxygen content in gas is the key to reducing the formation of by-products. 2) If V5+ is lacking in the tannin solution, the oxidation of HS– cannot proceed completely. (NaHS+NaHCO3+NaVO3, NaV2O5+S+H2S+Na2CO3), where HS— is taken to the regeneration tank where it comes into contact with air; more thiosulfate is produced when it serves as a reactant for oxidation. If a sufficient amount of V5+ is maintained to ensure its reaction time with HS-, the formation of by-products will also be reduced. 3) Thiocyanate is formed in semi-water gas when hydrogen cyanide is absorbed by a solution and reacts with S; the reaction equation is: Na2CO3 + HCN + NaCN + NaHCO3 → NaCN + S → NaCNS. The hydrosulfide produced also consumes alkali. It is required that the levels of the by-products sodium thiosulfate and sodium hydrosulfide not exceed 100 g/l each; if these levels are exceeded, the by-products must be removed, otherwise the desulfurization capacity of the solution will decrease. When the concentration of by-products reaches a certain level, it can affect the dissolution of the alkali; in fact, salts and alkalis may precipitate, leading to blockages. To maintain normal deoxygenation and desulfurization without affecting the by-products, it is necessary to discard a portion of the solution; this not only increases the consumption of chemical materials but also causes environmental pollution. Therefore, improving operations and reducing the formation of by-products are key to mitigating environmental pollution and lowering material consumption. Imbalance and control of solution components: When the proportions in the tannin-based desulfurization solution are not balanced, NaHCO3 cannot be converted into Na2CO3 in a timely manner. The absorption reaction is as follows: Na2CO3 + H2S → NaHCO3 + NaHS. The reaction involving V2O5 to precipitate sulfur along with the conversion of NaHCO3 to Na2CO3 is: NaHS + NaHCO3 + NaVO3 → NaV2O5 + S↓ + H2S + Na2CO3. In the tannin-based desulfurization solution, the role of tannin is to oxidize V4+ to V5+, and V5+ then oxidizes HS– to elemental sulfur. If V5+ is lost in the solution or falls below the process specifications by too much, too little of the NaHCO3 in the desulfurization solution will be converted to Na2CO3. To achieve satisfactory gas desulfurization, a large amount of Na2CO3 must be added to the solution, resulting in an increasing accumulation of NaHCO3 in it. Also. A low amount of tannin or poor quality of tannin prevents the oxidation of V4+ to V5+ during the catalyst reaction. V4+ tends to form S-O-V precipitates with O2 and elemental sulfur, which hinders the regeneration of the solution, results in a dark brown color, and reduces its activity. The total alkalinity should also not be too high, as this leads to an increase in Na2S2O3 by-products and an increased absorption of CO2 by the solution. 6 Regeneration effect: The regeneration of the desulfurization solution is influenced by factors such as the composition of the solution, the regeneration temperature, the amount of air drawn in by suction, and the residence time of the solution in the regeneration tank. Generally, it is appropriate to control the regeneration temperature at around 40°C. At excessively high temperatures, side reactions increase sharply; the sulfur particles in the regeneration tank grow larger and sink due to intensified collisions, thereby entering the desulfurization tower through the lean liquid tank, which can easily lead to clogging of the packing ； If the temperature is too low, it slows down the rate of the main reaction, which is not conducive to regeneration. The amount of air drawn in is directly proportional to the outlet pressure of the regeneration pump; it is necessary to ensure that the pressure at the liquid inlet of the ejector is ≥ 0.32 MPa. Additionally, the number of injectors activated is determined based on the liquid level in the regeneration tank. (The liquid flow rate allowed per injector is 40–50 m3/h; the solution must remain in the regeneration tank for at least 5 minutes.) Furthermore, it is essential to ensure that the sulfur foam in the regeneration tank overflows in a timely manner.