Description of the purification process flow

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Process flow description: Process characteristics. The low-temperature methanol washing process has the following main characteristics: (1) It can simultaneously remove components such as H2S, COS, RSH, CO2, HCN, NH3, NO, as well as paraffins, aromatics, and crude gasoline from the feed gas. It also enables dehydration to thoroughly dry the gas, and the useful components that have been absorbed can be recovered during the methanol regeneration process. (2) The purity of the gas is very high. The total sulfur content in the purified gas can be reduced to below 0.1 ppm, and CO2 can be reduced to below 10 ppm. (3) It has a relatively high selectivity for absorption. H2S and CO2 can be absorbed separately in different devices or at different locations within the same device, and then recovered separately under different devices and conditions. Since the solubility of both H2S and CO2 in methanol is high at low temperatures, the circulation volume of the absorption solution is small, especially when the pressure of the feed gas is relatively high. Furthermore, at low temperatures, the solubility of gases such as H2 and CO in methanol is low, and the vapor pressure of methanol is also low, which helps to keep the loss of useful gases and the solvent at low levels. (4) Methanol has good thermal and chemical stability. Methanol is not degraded by components such as organic sulfur or cyanides; it does not form bubbles during operation, and pure methanol does not corrode equipment or pipelines. Therefore, the equipment and pipelines can mostly be made of carbon steel or low-alloy steel resistant to low temperatures. Methanol has low viscosity; at -30°C, its viscosity is comparable to that of water at room temperature. Therefore, it is favorable for transfer processes at low temperatures. Furthermore, methanol is also relatively cheap and easy to obtain. (5) It becomes even more reasonable when low-temperature methanol washing and liquid nitrogen washing are used together. Liquid nitrogen washing needs to be carried out at a temperature of around -190°C, and the gas must be completely dry; whereas the purified gas obtained through low-temperature methanol washing has both dry properties and a temperature of around -60°C, which reduces investment costs and energy consumption. Process principle: Low-temperature methanol washing is a typical physical absorption process. The fundamental difference between physical absorption and chemical absorption lies in the different forces acting between the absorbent and the gas solute molecules. In physical absorption, the forces acting between molecules are van der Waals forces ； In chemical absorption, it is the force of chemical bonds. The differences between the two lie in their absorption equilibrium curves, heat of absorption, effect of temperature on absorption, absorption selectivity, and solution regeneration. In physical absorption, the gas-liquid equilibrium relationship follows Henry’s law at the beginning; the concentration of the component being absorbed in the solution is essentially proportional to its partial pressure in the gas phase. In chemical absorption, once the reaction between the active component of the solution and the component to be absorbed reaches equilibrium, further dissolution of the component to be absorbed in the solution can only occur through physical absorption. In physical absorption, the absorption capacity of the absorbent increases as the partial pressure of acidic components rises, while the solution circulation rate is related to the amount of feed gas and the operating conditions. The operating pressure increases, the temperature decreases, and the solution circulation rate reduces ； In chemical absorption, the absorption capacity of the absorbent is related to the content of the active component in it. Therefore, in chemical absorption, the solution circulation rate is proportional to the amount of acidic components to be removed; that is, it is closely related to the concentration of acidic components in the gas, but it is essentially independent of pressure. In low-temperature methanol washing, the absorption of acidic gases such as H2S, COS, and CO2, the regeneration of the solution after absorption, and the desorption curves of useful gases with low solubility such as H2 and CO are all based on the different solubilities of various gases in methanol. At low temperatures, the absorption of acidic gases by methanol is highly favorable. When the temperature drops from 20°C to -40°C, the solubility of CO2 increases by about 6 times, and the amount of absorbent required can also be reduced by approximately 6 times. At low temperatures, such as -40 to -50°C, the solubility of H2S is roughly 6 times greater than that of CO2, which makes it possible to selectively remove H2S from the feed gas, while recovering CO2 by desorption during solution regeneration. At low temperatures, the solubility of H2S, COS, and CO2 in methanol is at least 100 times greater than that of H2 and CO, and about 50 times greater than that of CH4. Therefore, if the low-temperature methanol washing unit is designed to remove CO2, all gases with solubilities comparable to or greater than that of CO2, such as COS, H2S, NH3, and other sulfides, are removed together, while useful gases such as H2, CO, and CH4 are lost to a lesser extent. Typically, the operating temperature for low-temperature methanol washing is between -30 and -70°C. The relative solubility of various gases at -40°C is shown in the table below: Table 3-1 Relative Solubility of Various Gases in Methanol at -40°C. Gas | Solubility in Methanol / Solubility of H2 | Solubility in Methanol / Solubility of CO2: H2S, COS, CO2, CH4, CO, N2, H2 – 2540, 1555, 430, 12, 5, 2.5, 1.0, 5.9, 3.6, 1.0. When CO2 is present in the gas mixture, the solubility of H2S in methanol is reduced by approximately 10% to 15% compared to when there is no CO2 present. The higher the CO2 content in the solution, the more significant the decrease in the solubility of H2S in methanol. When H2 is present in the gas, the solubility of CO2 in methanol decreases. When methanol contains moisture, the solubility of CO2 also decreases; when the water content in methanol is 5%, the solubility of CO2 in it is about 12% lower compared to anhydrous methanol. Similarly, depending on the different solubilities of gases in methanol, methods such as staged depressurization, inert gas (nitrogen) stripping, or heating regeneration desorption can be employed to recover the dissolved useful gases, as well as to obtain CO2 product gas, H2S acidic gas, and off-gas. Desorption at medium pressure can recover the small amount of H2 and CO dissolved in methanol. The lower the medium pressure, the more gas can be recovered; however, an increase in the volume of gas to be recycled leads to greater compression work, which in turn results in higher electricity consumption. Therefore, it is necessary to find a balance between these two factors in order to determine the optimal medium pressure value. The dissolved CO2 gas is desorbed at low pressures (atmospheric pressure, or even negative pressure); the lower the pressure, the greater the amount of CO2 that can be desorbed, resulting in more CO2 product that can be recovered. However, extremely low pressures make it difficult to transport the CO2 gas, and operating under negative pressure requires electrical power to function. The use of nitrogen stripping can further reduce the partial pressure of CO2 dissolved in methanol, enabling more thorough desorption of CO2; this is equivalent to operating under negative pressure (near vacuum). The more nitrogen used for stripping, the greater the reduction in the CO2 partial pressure and the more complete the desorption. Finally, temperature elevation is employed for desorption, that is, thermal regeneration, to completely regenerate the methanol and obtain methanol-poor fluid, while also producing H2S as an acidic gas. H2S has the highest solubility in methanol, which makes it the most difficult to desorb; an external heat source is used to heat the methanol to boiling point, and distillation is employed to completely remove H2S. The greater the amount of heating medium used, the more thorough the methanol regeneration. Based on the above process principles, a low-temperature methanol washing system should at least include units such as the absorption of feed gas, medium-pressure desorption of the useful gases, low-pressure desorption and nitrogen stripping, as well as thermal regeneration.