Purification Technology Edition: One Question per Week (Fourth Week of August 2017)

High gas purification efficiency； It can remove various impurities ； Methanol has good stability ； Selective exit is possible, resulting in minimal loss of useful gases

1. Under low-temperature and high-pressure conditions, methanol has a high absorption capacity for H2S and CO2, but a lower solubility for components that are difficult to remove (such as CO and H2). In other words, methanol exhibits high selectivity as an absorbent for the gases to be absorbed. 2. Methanol washing is effective at absorbing H2S and CO2, resulting in a high level of purification; after methanol treatment, the H2S level in the output stream is less than 0.1 PPm, while the CO2 level is less than 20 PPm. 3. At low temperatures, methanol has low viscosity and good fluidity, which can reduce pressure losses during flow and improve the efficiency of the absorption tower’s trays. 4. Methanol is readily available and relatively inexpensive. 5. Methanol can be regenerated, with low heat consumption and low power consumption during regeneration, as well as low operating costs.

1. Under low-temperature and high-pressure conditions, methanol has a high absorption capacity for H2S and CO2, but a lower solubility for components that are difficult to remove (such as CO and H2). In other words, methanol exhibits high selectivity as an absorbent for the gases to be absorbed. 2. Methanol washing is used to absorb H2S and CO2, resulting in a high level of purification; after methanol washing, the H2S level in the output stream is less than 0.1 PPm, while the CO2 level is less than 20 PPm. 3. At low temperatures, methanol has low viscosity and good fluidity, which can reduce pressure losses during flow and improve the efficiency of the absorption tower’s trays. 4. Methanol is readily available and relatively inexpensive. 5. Methanol can be regenerated, with low heat consumption, low power consumption, and low operating costs during regeneration.

1. Under low-temperature and high-pressure conditions, methanol has a high absorption capacity for H2S and CO2, but a low solubility for components that are difficult to remove (such as CO and H2). In other words, methanol exhibits high selectivity as an absorbent for the gases to be absorbed. 2. Methanol washing is effective at absorbing H2S and CO2, resulting in a high level of purification; after methanol treatment, the H2S level in the output stream is less than 0.1 PPm, while the CO2 level is less than 20 PPm. 3. At low temperatures, methanol has low viscosity and good fluidity, which can reduce pressure losses during flow and improve the efficiency of the absorption tower’s trays. 4. Methanol is readily available and relatively inexpensive. 5. Methanol can be regenerated, with low heat consumption, low power consumption, and low operating costs during the regeneration process.

It features low power consumption, low steam consumption, inexpensive solvents, and low operating costs

(1) It has a strong capacity to absorb acidic gases, especially at high pressures and low temperatures, demonstrating particularly strong absorption capabilities for high-concentration acidic gases. Chongqing methanol can absorb and purify the high-concentration acidic gases in raw gas through a single purification step. This technology can also remove impurities such as COS, HCN, H2O, and naphtha from raw gas. Therefore, the CO conversion can be placed before low-temperature methanol washing, thereby shortening the gas purification process. 　　(2) For methanol, vacuum flashing and heating regeneration are used, which is a simple method. During the vacuum regeneration process of methanol-rich liquid in Chongqing, a throttling effect occurs due to the sudden drop in pressure, which causes the temperature of the solution to decrease; this heat is then transferred to the regenerated solution. Moreover, the raw gas from the artificial stream is efficiently cooled using the low-temperature off-gas generated during the regeneration of the clean gas, resulting in minimal loss of cooling capacity in the entire system. Since the solubilities of H2, CO, and CH4 in methanol are very low, minimal loss of useful gases occurs during the regeneration process. 　　(3) It facilitates the recovery and utilization of H2S, reducing environmental pollution. During the repeated vacuum regeneration of methanol in Chongqing, the off-gas containing H2S is reabsorbed using methanol, which leads to the concentration of H2S. Once the H2S concentration increases, the methanol rich in H2S is then used for regeneration, resulting in H2S gas with a concentration of 30%–35%. This gas is sent to Claus units to recover sulfur, thereby eliminating the environmental pollution caused by sulfur-containing waste gases. At present, the Claus equipment in our plant is under commissioning and testing. When the equipment is not in operation, the Claus gas is sent to the plant’s own power station for combustion. 　　(4) Since methanol has a low boiling point (64.7°C), methanol used in Chongqing can take advantage of low-pressure steam, which serves as a low-temperature heat source in the thermal regeneration of methanol, thereby helping to save energy and reduce consumption. 　　(5) At low temperatures, this technology exhibits comparable efficiency in absorbing naphtha and aromatic compounds from raw gas. During the pre-washing and regeneration process, the principle that methanol is soluble in water but insoluble in oil is utilized to extract the oil-containing methanol-water mixture, thereby obtaining naphtha product. The methanol-water mixture is then heated and distilled to recover methanol and reduce its loss. 　　(6) Methanol has a low viscosity, good stability, does not form bubbles, and is non-corrosive. 　　(7) Chongqing has an adequate supply of methanol raw materials, and the prices are low, which helps to reduce costs. 　　(8) The combined use of low-temperature methanol washing and liquid nitrogen washing results in a more economical and reasonable technical process.

Answer: The low-temperature methanol washing process uses cold methanol as the absorption solvent, taking advantage of the excellent property of methanol to have a very high solubility for acidic gases (such as CO2, H2S, COS, etc.) at low temperatures, in order to remove these acidic gases from the feed gas; it is a physical absorption method. The low-temperature methanol washing process is currently recognized both domestically and internationally as the most economical and effective gas purification technology. It has advantages that other desulfurization and decarbonization methods cannot match, such as high-quality and highly purified output gas, the ability to selectively absorb H2S, COS, and CO2, inexpensive and readily available solvents, low energy consumption, low operating costs, as well as stable and reliable performance during operation.

The process gas achieves a high level of purification with good selectivity; desulfurization and decarburization of the gas can be carried out selectively in stages within the same tower.

(a) Solvents have an extremely strong absorption capacity for acidic gases such as CO2, H2S, and COS at low temperatures; as a result, the amount of solution that needs to be circulated is small, resulting in reduced power consumption. (b) The solvent does not oxidize or degrade, and it possesses excellent chemical and thermal stability. (c) The purified gas has good quality and a high purification level; CO2