Purifying Knowledge: [Daily Question] 2011-03-14

1. NHD belongs to thermal desulfurization and decarburization, while low-temperature methanol washing belongs to cold desulfurization and decarburization. 2. Low-temperature methanol washing provides high purification efficiency; CO2 can be reduced to 20 ppm, and total sulfur can be reduced to below 0.1 ppm ； Meanwhile, NHDCO2 can only reduce it to below 100 ppm, and total sulfur to below 1 ppm. 3. NHD is expensive; its pure consumption cost is higher than that of low-temperature methanol washing, resulting in high operating costs. 4. The NHD method can concentrate hydrogen sulfide in acidic gases to 25%, which is sufficient for Claus sulfur recovery; the low-temperature methanol washing method can achieve a concentration of hydrogen sulfide in acidic gases of over 30%, and this figure can exceed 40% when high-sulfur coal is used. 5. The low-temperature methanol washing method requires a higher initial investment compared to the NHD method; some of the materials needed for it have to be imported, whereas all the materials required for the NHD method can be produced domestically. 6. The low-temperature methanol washing method is more suitable for use in large-scale installations.

The main difference lies in the different properties of the solvents, which suit different scales

I think low-temperature methanol washing would be better!

Come and learn*. My answers: 1. In terms of device scale, the low-temperature methanol washing process is clearly superior to the NHD process for larger-scale installations, as it has a higher absorption capacity, which results in a lower circulation volume (for example, a 200,000-ton coal-to-methanol plant requires a circulation volume of 200 for low-temperature methanol washing, compared to 900 for NHD). Thus, low-temperature methanol washing is better suited for large-scale applications than NHD. 2. Regarding material selection, the requirements for materials in the low-temperature methanol washing process are much higher than those in the NHD process; for installations with the same production capacity, the investment cost for low-temperature methanol washing is approximately twice that of NHD. 3. In terms of operation, low-temperature methanol washing is more difficult to operate than NHD (especially during startup and shutdown, as sudden changes in temperature can significantly affect the lifespan of the materials), but once the system is operating normally, the difference in operational complexity isn’t significant. 4. In terms of hazards, low-temperature methanol washing poses greater risks than NHD, as methanol is harmful to humans, whereas polyethylene glycol dimethyl ether is harmless. 5. In terms of energy consumption, since the circulation volume required for low-temperature methanol washing is lower than that of NHD, its energy consumption is much lower

1. Absorption capacity and solvent circulation volume: When absorbing an equal amount of acidic gases, a smaller volume of methanol solvent is required. Methanol’s ability to absorb CO2 is more than 4 times that of NHD solution, and the heat required for regenerating the NHD process is also much higher than that in the low-temperature methanol washing process. 2. Effective gas loss and purification efficiency: The amount of H2 lost in the NHD process is greater than that in the low-temperature methanol washing process； The purity level is also lower than that achieved by methanol washing; therefore, a desulfurization unit must be added after the NHD process. 3. Regarding regeneration temperature, the amount of steam required for regeneration in the NHD process is higher than that in the low-temperature methanol washing process. Moreover, the operating and regeneration temperatures are higher, which results in increased electricity consumption for the chillers. 4. In terms of thermal stability and chemical stability, the solvent used in the NHD process is inferior to that used in 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 restrictions for methanol washing. 9. In terms of technical royalties and total investment, investing in NHD is lower than that of low-temperature methanol washing. 10. Comparison of operating costs: Taking various factors into account, for medium-scale ammonia or methanol plants, the operating costs of the two are similar.

1. In terms of device scale-up, the low-temperature methanol washing process is clearly superior to the NHD process, as it has a higher absorption capacity, which results in a lower circulation volume (for example, a 200,000-ton coal-to-methanol plant requires a circulation volume of 200 for low-temperature methanol washing, whereas NHD requires 900). Therefore, low-temperature methanol washing is more advantageous for scaling up compared to NHD processes. 2. Regarding material selection, the requirements for materials in the low-temperature methanol washing process are much higher than those in the NHD process; for plants with the same production capacity, the investment cost for low-temperature methanol washing is approximately twice that of NHD. 3. In terms of operation, low-temperature methanol washing is more difficult to operate than NHD processes (especially during startup and shutdown, as sudden changes in temperature can significantly affect the lifespan of the materials), but once the plant is operating normally, the difference in operational complexity is not significant. 4. In terms of hazards, low-temperature methanol washing poses greater risks than NHD, as methanol is harmful to humans, while polyethylene glycol dimethyl ether is harmless. 5. In terms of energy consumption, since the circulation volume in low-temperature methanol washing processes is lower than that in NHD processes, the energy consumption is much lower. Additionally, low-temperature methanol washing can be integrated reasonably with the cold boxes used in ammonia synthesis processes.

1. In terms of device scale-up, low-temperature methanol washing is clearly superior to NHD systems, as it has a higher absorption capacity, which results in a lower circulation volume (for example, a 200,000-ton coal-to-methanol plant requires a circulation volume of 200 for low-temperature methanol washing, whereas NHD requires 900). Thus, low-temperature methanol washing is more advantageous for large-scale applications compared to NHD systems.

2. Regarding material selection, low-temperature methanol washing imposes much stricter requirements on materials than NHD systems. For plants with the same production capacity, the investment cost for low-temperature methanol washing is approximately twice that of NHD systems.

3. In terms of operation, low-temperature methanol washing is more difficult to operate than NHD systems, especially during startup and shutdown, as sudden changes in temperature can significantly affect the lifespan of the materials. However, once the system is operating normally, the difference in operational complexity is not significant.

4. In terms of hazards, low-temperature methanol washing poses greater risks than NHD, as methanol is harmful to humans, whereas polyethylene glycol dimethyl ether is harmless.

5. In terms of energy consumption, since the circulation volume required for low-temperature methanol washing is lower than that of NHD systems, its energy consumption is much lower.

They can all remove CO2 and H2S. Low-temperature methanol washing employs low-temperature physical absorption; NHD uses high-temperature chemical absorption

1. In terms of device size enlargement, low-temperature methanol washing is significantly better than NHD. Because low-temperature methanol washing has a high absorption capacity, the circulation volume is small. 2. In terms of material selection, the requirements for materials in low-temperature methanol washing are much higher than those in NHD units. For units with the same production capacity, the investment cost for low-temperature methanol washing is approximately twice that of NHD. 3. Operation aspects. In terms of operation, the start-up and shutdown procedures for low-temperature methanol washing are more difficult than those for NHD units. 4. Hazards: LMW is more hazardous than NHD, as methanol is harmful to the human body whereas polyethylene glycol dimethyl ether is harmless. 5. Energy consumption: Since the circulation volume in low-temperature methanol washing is smaller than that in NHD units, the energy consumption is much lower.