Working principle of low-temperature methanol washing
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Working principle of low-temperature methanol washing: Rectisol is a gas purification process that was developed in the early 1950s by the German companies Linde and Lurgi. The first low-temperature methanol washing unit was installed in 1954 by LuChi Company at the synthetic fuel plant in Sasol, South Africa. To date, there are over 100 such industrial units worldwide, of which more than a dozen have been introduced in China. This process is a typical physical absorption method that uses low-temperature methanol as the absorption solvent, taking advantage of the high solubility of methanol for acidic gases at low temperatures to remove such gases from the feed gas. Due to the high vapor pressure of methanol, the low-temperature methanol washing process operates at low temperatures (around –60°C). At such low temperatures, the solubility of CO2 and H2S increases significantly as the temperature drops; as a result, less solvent is required, and the equipment needed for the process is also smaller. At –30°C, the solubility of H2S in methanol is 6.1 times that of CO2, thereby enabling selective removal of H2S. This process achieves a high level of gas purification, capable of reducing CO2 in the shifted gas to less than 20 ppm and H2S to less than 0.1 ppm; desulfurization and decarburization of the gas can be carried out in the same tower
Could you briefly explain desorption and the Claus sulfur recovery process again? The author’s simple and easy-to-understand explanations are great; there’s no need to go into too much detail regarding the principles, reaction equations, and equipment! Thank you again! ! !
The difficulty lies in the complexity of the process flows for analysis and cold energy recovery, but it is not complicated
Principle of the low-temperature methanol washing process: 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 effective. 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. When CO2 is present in the gas, the solubility of H2S in methanol decreases by about 10% to 15% compared to when there is no CO2. The higher the CO2 content in the solution, the more significant the decrease in the solubility of H2S in methanol.
The main chemical reaction in the Claus process is as follows: All the acidic gases coming from the desulfurization unit enter the combustion furnace (see figure). One-third of the hydrogen sulfide in these gases can be oxidized to sulfur dioxide, and this sulfur dioxide, along with the unoxidized hydrogen sulfide, enters the converter where it undergoes catalytic conversion. To complete the partial combustion reaction, the air supplied to the combustion furnace must be strictly controlled, which is the key to the operation of the Claus process. The temperature of the combustion furnace is approximately 1200°C. In addition to sulfur dioxide, water, and nitrogen, the combustion products also contain a small amount of elemental sulfur generated directly from the decomposition of hydrogen sulfide. To recover heat, the combustion products are first converted into steam in a waste heat boiler before entering the converter. The converter is a fixed-bed reactor filled with alumina catalyst (see metal oxide catalyst), with the inlet temperature controlled at 220–240°C. Since the process is an exothermic reaction, the outlet temperature is 270–300°C. The reaction products exiting the converter enter the condensation cooler, while the liquid sulfur flows to the sulfur tank. To achieve a higher sulfur recovery rate, industrial plants generally are equipped with secondary, tertiary, or even quaternary converters. To achieve a high conversion rate in the converter, it is crucial to control the molar ratio of H2S to SO2 at 2, and to use an alumina catalyst with good performance. With two-stage conversion, the sulfur recovery rate can reach 93%–95%; with three-stage conversion, it can reach 94%–96%; and with four-stage conversion, it can reach 95%–97%. The exhaust gas discharged from the Claus plant also contains a certain amount of sulfur dioxide (8,000–18,000 ppm). In accordance with environmental regulations, the exhaust gases also need to be treated so that the sulfur dioxide content in the exhaust gases released into the atmosphere is around 300 ppm, achieving a total sulfur recovery rate of approximately 99.8%.
Regarding the excessive values of the control parameters at the top of the tower: I think it’s possible to rule out potential issues in a step-by-step manner, starting from the most obvious ones. First, system-related problems need to be ruled out. This includes addressing any issues with the instruments, such as instrument failures, data interruptions, transmission problems; and most importantly, eliminating the possibility of false readings. Next, equipment-related problems should be considered: 1. The tower may not be installed vertically, or the gas-liquid distributor may not be installed horizontally; 2. The packing or sieve plates might be damaged; 3. The packing or sieve plates could be clogged, although the sieve plates seem to be in good condition; 4. The gas-liquid distributor might be blocked, resulting in insufficient amounts of lean methanol; 5. Severe fouling on the walls; 6. Leaks in the heat exchanger, allowing gas to mix with the syngas. What remains are then problems related to the process itself. Regarding gas-related factors: 1. A high pressure difference across the tower, which indicates an excessive load, and this can also lead to flooding； 2. Changes in the composition of gas: high carbon dioxide content ； II. Temperature factors: 1. High gas temperature leads to poor absorption ； 2. Insufficient cooling capacity of the system, resulting in a reduced absorption capacity ； III. Absorption issues: 1. If the moisture content or other impurities in methanol are too high, the absorption capacity is reduced ； 2. Incomplete regeneration of methanol absorption ； IV. Operational issues: 1. Insufficient methanol absorption circulation rate ； 2. The valve controlling CO2 absorption is opened too wide. 3. Insufficient amount of nitrogen gas, resulting in desorption issues. 4. Incomplete thermal regeneration
Low-temperature methanol washing is a method that can both decarbonize and desulfurize; it is also a process of absorption and desorption, and is relatively complex in structure. It is also a typical physical absorption process.