What is low-temperature methanol washing?

The excellent property of methanol, which exhibits a very high solubility for acidic gases at low temperatures, is utilized to remove acidic gases from the feed gas. It has an extremely strong absorption capacity for acidic gases such as CO2, H2S, and COS. The low-temperature methanol washing process is well-developed, with excellent performance in industrial applications; it is widely used in gas purification systems for ammonia and methanol synthesis, as well as for other carbonyl synthesis processes, urban gas production, industrial hydrogen production, and natural gas desulfurization, both domestically and internationally.

The low-temperature methanol washing process is a method for purifying feed gas that was jointly developed in the 1950s by the German companies Lurgi and Linde. After the 1960s, with the emergence and development of large-scale ammonia synthesis plants that used residue oil and coal as raw materials, this technology came to be widely used in the ammonia and methanol industries. Low-temperature methanol washing refers to the process of removing acidic gases such as CO2, H2S, COS, and thiols from the shift gas under certain pressure and low temperature conditions. Since no chemical reaction occurs during the absorption of acidic gases by methanol, it is a case of physical absorption. The principle behind this absorption process is to use low-temperature methanol as an absorbent, taking advantage of the fact that CO2, H2S, and COS have higher solubility in this absorbent compared to H2 and CO. The regeneration of the solution after absorption is achieved through simple flash distillation to release CO2, H2S, and other gases. The absorption of acidic gases by methanol is a physical absorption process; at the initial stage, the gas-liquid equilibrium relationship follows Henry’s law (P=KX). The absorption capacity of the absorbent increases as the partial pressure of the acidic components rises, while the circulation rate of the solution is dependent on the volume of the feed gas and the operating conditions. The operating pressure increases, the temperature decreases, and the solution circulation rate drops. Features: It ensures that the levels of H2S and CO2 in the purified gas meet the requirements for syngas, and it enables complete removal of sulfides from the syngas, thereby extending the lifespan of the methanol synthesis catalyst. The low-temperature methanol washing method can remove various components from gases. At temperatures ranging from -30°C to -70°C, methanol is capable of removing H2S, COS, CS2, RSH, C4H4S, CO2, HCN, NH3, NO, as well as paraffins, aromatics, and crude gasoline from the gas. It also enables the dehydration of the gas, resulting in complete drying. The useful components that have been absorbed can be recovered during the methanol regeneration process. Low-temperature methanol has a high absorption capacity for CO2 and sulfides, and its low solution circulation rate allows for smaller pipe and equipment sizes, thereby reducing system consumption. Methanol enriched with CO2 and sulfides can be recycled through flash evaporation, stripping, and thermal regeneration. The CO2 generated through flashing and gas stripping can be discharged in compliance with standards, and the acidic gases produced can also be discharged in compliance with standards after sulfur recovery, yielding sulfur as a by-product. Thermoregenerative wastewater containing trace amounts of methanol can be used in coal slurry preparation, reducing the system’s water consumption and the burden on wastewater treatment. The purity of the gas is very high; the total sulfur content in the purified gas can be reduced to below 0.1 cm3/m3, while CO2 levels can be lowered to below 10 cm3/m3. The low-temperature methanol washing method is suitable for chemical manufacturing processes where strict requirements are imposed on sulfur content. H2S and CO2 in the feed gas can be selectively removed and recovered separately. Since the solubility of H2S, COS, and CO2 in methanol is high at low temperatures, the energy consumption is low. Meanwhile, at low temperatures the solubility of gases such as H2 and CH4 in methanol is low, and the vapor pressure of methanol is also low; this helps to keep losses of useful gases and the solvent at low levels. Methanol has good thermal and chemical stability. At the same time, methanol is also relatively cheap and easy to obtain. Shortcomings: To reduce energy consumption and recover cold energy, a large number of heat exchange devices are used, which lengthens the process; in particular, the regeneration process is quite complex. Since the process is carried out at low temperatures, high requirements are placed on the material of the equipment. (The manufacturing cost of a methanol scrubber with a production capacity of 1 million tons is approximately 80 million yuan.) Methanol is toxic and can affect human health. Methanol is highly toxic; inhaling 10 ml can cause blindness, while inhaling 30 ml can be fatal. The allowable concentration of methanol in the air is 50 mg/m3. Therefore, strict quality requirements apply to equipment manufacturing and pipeline installation, and operations must be carried out with great care to prevent accidents such as leaks.

Introduction to the low-temperature methanol washing process: LINDE’s low-temperature methanol washing units from Germany. The “one-step five-column process” is adopted, which means that five columns are included in the process. The five towers are as follows: C-401 washing tower – Function: It uses methanol at low temperatures to wash and remove substances such as CO2, H2S, and COS from the process gas, thereby purifying the process gas ； Float valve tower: It has a total of 86 trays. C-402 CO2 separation tower: Function – to depressurize and desorb CO2 dissolved in methanol, thereby producing sulfur-free CO2 product gas while recovering cooling energy. Float valve tower: It has a total of 68 trays. The function of the C-403 H2S concentration tower is to further desorb CO2 dissolved in methanol and recover the cooling energy ； At the same time, concentrate H2S dissolved in methanol. The floating valve tower has a total of 87 trays. C-404 is used for thermal regeneration: it carries out thorough thermal regeneration of methanol, allowing H2S and CO2 to be completely desorbed, thereby producing lean methanol and acid gas rich in H2S. The floating valve tower has a total of 30 trays. C-405 is used for the separation of methanol and water; its function is to separate methanol from water, recover methanol, and at the same time maintain the appropriate water content in the circulating methanol. The sieve plate tower has a total of 51 trays. A brief analysis of the low-temperature methanol washing process: Abstract: It focuses on introducing the principle of the low-temperature methanol process as well as its production process. Keywords: low temperature ； Methanol ； Process flow ； Protection Devices – Introduction: This project uses high-sulfur coal as raw material; the feed gas is produced through Texaco gasification. Subsequent partial shift at moderate temperatures, along with low-temperature methanol washing, are used for desulfurization and decarburization, ensuring that the total sulfur content in the output gas is ≤0.1 ppm and the CO2 content is ≤3%. The purified gas is then pressurized to 8.2 MPa using a combined compressor before being fed into the synthesis tower to produce crude methanol. The indicators of gas purification directly affect the yield of methanol and the service life of the synthesis catalyst; therefore, it is very important to ensure proper gas purification. 1 Introduction to the Low-Temperature Methanol Wash Process 1.1 Brief Introduction to the Process Principle The purpose of the purification unit is to remove acidic gas components from the shift gas. This process is a physical process that uses low-temperature methanol as a washing liquid (absorbent). At the design temperature of -50°C, methanol has high solubility for CO2, H2S, and COS. During physical absorption, the liquid load containing any component is proportional to the partial pressure of that component. The controlling factors in absorption are temperature, pressure, and concentration. Rich methanol is regenerated by flashing and stripping with the vapor generated in the reboiler. The flashing of rich methanol provides additional cooling for this process. The flash vapor is compressed in a cycle and then recycled back to the absorption tower, resulting in minimal losses. The methanol-water separation tower maintains the water balance in the methanol cycle. The exhaust gas scrubber minimizes the loss of methanol along with the exhaust gas. The ammonia scrubber in the shift gas cooling section maintains the ammonia level in the shift gas at the level that results in the lowest amount of methanol being released. Acidic gases are sent to the Claus gas plant for further purification. 1.2 Advantages of the process: (1) It has a strong ability to absorb acidic gases, especially at high pressures and low temperatures, where its capacity to absorb high-concentration acidic gases is particularly excellent. A single purification step is sufficient to completely absorb the high-concentration acidic gases from the raw gas. This process can also remove impurities such as COS, HCN, H2O, and naphtha from crude gas. Therefore, the CO conversion can be placed before low-temperature methanol washing, thereby shortening the gas purification process. (2) Methanol is regenerated by vacuum flashing and heating, which is a simple method. During the depressurized regeneration process, the rich liquid experiences a throttling effect due to the sudden drop in pressure, which lowers the temperature of the solution; this cold energy is then transferred to the regenerated solution. The raw gas from the artificial unit is then efficiently cooled using the low-temperature off-gas generated during the regeneration of the clean gas, resulting in minimal heat loss throughout the system. Since the solubilities of H2, CO, and CH4 in methanol are all very low, the loss of useful gases during the regeneration process is minimal. (3) It facilitates the recovery and reuse of H2S, reducing environmental pollution. During the repeated vacuum regeneration of methanol, the off-gases containing H2S are repeatedly reabsorbed by methanol, which allows for 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 a Claus unit for sulfur recovery, thereby eliminating the environmental pollution caused by sulfur-containing waste gases. At present, the Claus unit in our plant is in the commissioning phase. When the unit is not in operation, the Claus gas is sent to the plant’s own power station for combustion. (4) Due to the low boiling point of methanol (64.7°C), the low-pressure steam, which serves as a low-temperature heat source during the thermal regeneration of methanol, can be utilized, thereby helping to save energy and reduce consumption. (5) At low temperatures, this process achieves relatively thorough absorption of naphtha and aromatic compounds in the 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) Methanol is readily available at low prices, 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 process flow. 1.3 Brief introduction to the process flow: In the unit, medium-temperature methanol is desulfurized and decarburized in the main scrubbing tower (5.4 MPa); thereafter, the rich liquid enters the medium-pressure flash tower (1.6 MPa) for flashing. The flashed gas is compressed and then recycled back to the main scrubbing tower. The liquid rich in solvents after flashing enters the reabsorption tower, where it flashes and is stripped under atmospheric pressure to achieve partial regeneration. The methanol-rich liquid then enters the thermal regeneration tower, where it is thermally regenerated using steam generated in the reboiler. Once fully regenerated, the methanol-poor liquid is pressurized by the main circulation flow pump and sent to the main washing tower. 2 Key Operating Points 2.1 Circulating Methanol Temperature: The lower the temperature, the greater the solubility; therefore, a lower temperature for the methanol-poor stream is the goal of operation (the temperature for the methanol-poor stream is -50°C). The system is equipped with an acrylic refrigeration system to provide additional cooling, with the cooling capacity generated by the flashing (stripping) of exhaust gases being used to achieve the required operating temperature. The main factors affecting the temperature of circulating methanol are: a. Cooling capacity supply from the propylene refrigeration system ； b. Nitrogen stripping flow rate ； c. Ratio of the flow rate of circulating methanol to the flow rate of shift gas. 2.2 Methanol circulation rate: To control the gas composition parameters in the output section (ΣS≤0.1ppm), the methanol circulation rate is the primary means of regulation. The system is equipped with a proportional control system that ensures the circulation volume is proportional to the gas volume, thereby producing high-quality purified gas. 2.3 Pressure (operating pressure of the main scrubber): According to Henry’s law, the higher the pressure, the better the absorption effect. The pressure in the purification main wash tower depends on the pressure of the syngas generated by gasification. The system uses Texaco gasifiers for gas production, and the pressure of the syngas entering the system is 5.4 MPa; due to this high pressure, the absorption efficiency is significantly improved. 2.4 Concentration (water content, regeneration degree of methanol) The water content in lean methanol is an important control parameter in normal production; the system is controlled to maintain a water content of ≤1%. A higher water content not only affects the efficiency of methanol absorption but also increases corrosion of the equipment. To achieve the recycling of methanol and obtain good absorption results, it is essential to effectively regenerate methanol. The methods used in the system for methanol regeneration include flashing, stripping, and thermal regeneration. Use a methanol-water separation tower to control the water balance in the solution system. 2.5 Parameters of the converted gas (temperature and gas composition) The parameters of the converted gas directly affect the operation of the purification cycle; the system controls the composition of the converted gas via the gas treatment section, and adjusts the HPC ratio by controlling the temperature of the carbon scrubber tower. The composition of the transformed gas entering the system’s stage is 44% H2, 19% CO, 34% CO2, and 1.3% H2S. 3 Main control parameters: Temperature of lean methanol – the temperature of lean methanol entering the main washing tower is to be controlled at -50°C; the concentration of COS+H2S in the gas exiting the main washing tower must be ≤0.1 ppm, while CO2 concentration must be ≤3%. Water content in the lean methanol:

This project uses high-sulfur coal as raw material; the feed gas is produced through Texaco gasification, followed by medium-temperature partial reforming and low-temperature methanol washing to remove sulfur and carbon. The total sulfur content in the output gas is controlled at ≤0.1 ppm, while the CO2 level is kept at ≤3%. The purified gas is pressurized to 8.2 MPa using a combined compressor and then fed into a synthesis tower to produce crude methanol. The indicators of gas purification directly affect the yield of methanol and the service life of the synthesis catalyst; therefore, it is very important to ensure proper gas purification. 1 Introduction to the Low-Temperature Methanol Wash Process 1.1 Brief Introduction to the Process Principle The purpose of the purification unit is to remove acidic gas components from the shift gas. This process is a physical process that uses low-temperature methanol as a washing liquid (absorbent). At the design temperature of -50°C, methanol has high solubility for CO2, H2S, and COS. During physical absorption, the liquid load containing any component is proportional to the partial pressure of that component. The controlling factors in absorption are temperature, pressure, and concentration. Rich methanol is regenerated by flashing and stripping with the vapor generated in the reboiler. The flashing of rich methanol provides additional cooling for this process. The flash vapor is compressed in a cycle and then recycled back to the absorption tower, resulting in minimal losses. The methanol-water separation tower maintains the water balance in the methanol cycle. The exhaust gas scrubber minimizes the loss of methanol along with the exhaust gas. The ammonia scrubber in the shift gas cooling section maintains the ammonia level in the shift gas at the level that results in the lowest amount of methanol being released. Acidic gases are sent to the Claus gas plant for further purification. 1.2 Advantages of the process: (1) It has a strong ability to absorb acidic gases, especially at high pressures and low temperatures, where its capacity to absorb high-concentration acidic gases is particularly excellent. A single purification step is sufficient to completely absorb the high-concentration acidic gases from the raw gas. This process can also remove impurities such as COS, HCN, H2O, and naphtha from crude gas. Therefore, the CO conversion can be placed before low-temperature methanol washing, thereby shortening the gas purification process. (2) Methanol is regenerated by vacuum flashing and heating, which is a simple method. During the depressurized regeneration process, the rich liquid experiences a throttling effect due to the sudden drop in pressure, which lowers the temperature of the solution; this cold energy is then transferred to the regenerated solution. The raw gas from the artificial unit is then efficiently cooled using the low-temperature off-gas generated during the regeneration of the clean gas, resulting in minimal heat loss throughout the system. Since the solubilities of H2, CO, and CH4 in methanol are all very low, the loss of useful gases during the regeneration process is minimal. (3) It facilitates the recovery and reuse of H2S, reducing environmental pollution. During the repeated vacuum regeneration of methanol, the off-gases containing H2S are repeatedly reabsorbed by methanol, which allows for 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 a Claus unit for sulfur recovery, thereby eliminating the environmental pollution caused by sulfur-containing waste gases. At present, the Claus unit in our plant is in the commissioning phase. When the unit is not in operation, the Claus gas is sent to the plant’s own power station for combustion. (4) Due to the low boiling point of methanol (64.7°C), the low-pressure steam, which serves as a low-temperature heat source during the thermal regeneration of methanol, can be utilized, thereby helping to save energy and reduce consumption. (5) At low temperatures, this process achieves relatively thorough absorption of naphtha and aromatic compounds in the 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) Methanol is readily available at low prices, 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 process flow. 1.3 Brief introduction to the process flow: In the unit, medium-temperature methanol is desulfurized and decarburized in the main scrubbing tower (5.4 MPa); thereafter, the rich liquid enters the medium-pressure flash tower (1.6 MPa) for flashing. The flashed gas is compressed and then recycled back to the main scrubbing tower. The liquid rich in solvents after flashing enters the reabsorption tower, where it flashes and is stripped under atmospheric pressure to achieve partial regeneration. The methanol-rich liquid then enters the thermal regeneration tower, where it is thermally regenerated using steam generated in the reboiler. Once fully regenerated, the methanol-poor liquid is pressurized by the main circulation flow pump and sent to the main washing tower. 2 Key Operating Points 2.1 Circulating Methanol Temperature: The lower the temperature, the greater the solubility; therefore, a lower temperature for the methanol-poor stream is the goal of operation (the temperature for the methanol-poor stream is -50°C). The system is equipped with an acrylic refrigeration system to provide additional cooling, with the cooling capacity generated by the flashing (stripping) of exhaust gases being used to achieve the required operating temperature. The main factors affecting the temperature of circulating methanol are: a. Cooling capacity supply from the propylene refrigeration system ； b. Nitrogen stripping flow rate ； c. Ratio of the flow rate of circulating methanol to the flow rate of shift gas. 2.2 Methanol circulation rate: To control the gas composition parameters in the output section (ΣS≤0.1ppm), the methanol circulation rate is the primary means of regulation. The system is equipped with a proportional control system that ensures the circulation volume is proportional to the gas volume, thereby producing high-quality purified gas. 2.3 Pressure (operating pressure of the main scrubber): According to Henry’s law, the higher the pressure, the better the absorption effect. The pressure in the purification main wash tower depends on the pressure of the syngas generated by gasification. The system uses Texaco gasifiers for gas production, and the pressure of the syngas entering the system is 5.4 MPa; due to this high pressure, the absorption efficiency is significantly improved. 2.4 Concentration (water content, regeneration degree of methanol) The water content in lean methanol is an important control parameter in normal production; the system is controlled to maintain a water content of ≤1%. A higher water content not only affects the efficiency of methanol absorption but also increases corrosion of the equipment. To achieve the recycling of methanol and obtain good absorption results, it is essential to effectively regenerate methanol. The methods used in the system for methanol regeneration include flashing, stripping, and thermal regeneration. Use a methanol-water separation tower to control the water balance in the solution system. 2.5 Parameters of the converted gas (temperature and gas composition) The parameters of the converted gas directly affect the operation of the purification cycle; the system controls the composition of the converted gas via the gas treatment section, and adjusts the HPC ratio by controlling the temperature of the carbon scrubber tower. The composition of the transformed gas entering the system’s stage is 44% H2, 19% CO, 34% CO2, and 1.3% H2S. 3 Main control parameters: Temperature of lean methanol – the temperature of lean methanol entering the main washing tower is to be controlled at -50°C; the concentration of COS+H2S in the gas exiting the main washing tower must be ≤0.1 ppm, while CO2 concentration must be ≤3%. Water content in the lean methanol:

Low-temperature methanol washing is a physical absorption process in which, under low temperature and high pressure, methanol absorbs carbon dioxide, hydrogen sulfide, and COS in an absorption tower. The methanol solution after absorption is depressurized through throttling, releasing carbon dioxide, which is then vented at a high altitude via the boiler chimney. The methanol solution is then used to desorb hydrogen sulfide from it while still at high temperature, and the regenerated methanol is reused. The regenerated hydrogen sulfide exhaust gas is concentrated before being sent to the sulfur recovery unit.

We use the nine-tower process flow of the Luchi process. . .

We use a seven-tower Lindlar process, one of which is dedicated to producing carbon monoxide!

Friends from Jiu Ta and Qi Ta, could you share some of your information with me? My email address is jianguo1116@yahoo.com.cn

Reply to 8# Botian: Haha, we produce carbon monoxide and methanol, which are used exclusively by Celanese for the production of acetic acid.

That’s another washing tower! The purified process gas needs to undergo H2/CO separation!