Comparison and Selection between NHD and Low-Temperature Methanol Wash for Acid Gas Removal
Thread Content
Chemical production using coal as a raw material results in a large amount of acidic gases in the crude syngas, such as CO2, H2S, COS, etc. These acidic gases are detrimental to the synthesis reaction, and it is necessary to remove them before they enter the synthesis loop. The desulfurization and CO2 removal purification processes are among the key components in ammonia synthesis production, and the choice of process design has a crucial impact on the economic reliability of the entire ammonia synthesis plant. With the rapid development of the coal chemical industry, China has invested considerable research efforts in methods and processes for removing acidic gases from syngas. The methods for removing CO2 can be broadly divided into 3 categories: chemical absorption, physical absorption, and physicochemical absorption. For synthetic ammonia feed gas derived from coal, which contains a large amount of CO2 (up to about 45%), the physical absorption method is recommended by various parties due to its lower energy consumption. In recent years, for pressurized coal gasification processes such as Texaco’s water-coal slurry pressurized gasification process and Shell’s pulverized coal pressurized gasification process, the acid gas removal technologies used include the polyethylene glycol dimethyl ether method (NHD) developed by Nanhua Company, as well as the low-temperature methanol washing process developed by Lurgi and Linde companies. The solvents used in the low-temperature methanol washing process and the NHD process exhibit strong selective absorption capabilities for H2S and CO2; the solubility of CO2 in these solvents is high, which results in a lower volume of solvent that needs to be recycled and thus lower energy consumption. These processes are gradually showing advantages in the purification of coal-derived gas with high levels of sulfides and CO2
The cost of solvents in the NHD process is too high.
Comparison and Selection between NHD and Low-Temperature Methanol Wash for Acid Gas Removal 1. Absorption Capacity and Solvent Circulation Rate: A smaller amount of methanol solvent is required to absorb an equal volume of acid gas; the absorption capacity of methanol for CO2 is more than 4 times that of NHD solution. Additionally, the heat required for regeneration in the NHD process is much higher than that in low-temperature methanol wash. 2. Comparison of effective gas loss and purity: The H2 loss in the NHD process is greater than that in the low-temperature methanol washing process ； Its purity is also lower than that of methanol washing, so a precision desulfurization unit must be added after NHD. 3. Comparison of regeneration temperatures: The amount of steam required for regeneration in the NHD process is **higher** than that in the low-temperature methanol washing process; however, the higher operating and regeneration temperatures result in reduced electricity consumption for the chillers. 4. Comparison of solvothermal stability and chemical stability: The solvothermal stability and chemical stability of the NHD solvent are both inferior to those of the low-temperature methanol washing process. 5. Comparison of solvent foaming: The NHD solvent tends to foam during production, and antifoaming agents generally need to be added; methanol does not have this problem. 6. Comparison of volume price and sources: The price of NHD solvents is much higher than that of methanol, and its sources are highly limited. 7. Comparison of solvent heat and mass transfer properties: The methanol washing process exhibits better heat and mass transfer performance. 8. Maturity and reliability of process technology: At present, the largest facility using NHD is one with an annual capacity of 180,000 tons of synthetic ammonia, while there are no limitations for methanol washing. 9. Investing in NHD results in lower costs in terms of technical royalties and total investment compared to low-temperature methanol washing. 10. Comparison of operating costs: Taking various factors into account, for medium-sized ammonia or methanol production plants, the operating costs of the two are similar. However, for large-scale ammonia or methanol production, low-temperature methanol washing remains the more economical option.