What is the hydrogen content in carbon dioxide from low-temperature methanol washing?
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In 300,000 tons of synthetic ammonia production, low-temperature methanol washing is used for purification; the hydrogen content in the carbon dioxide obtained is approximately 0.9%–1.0%. The hydrogen content in the exhaust gases from the urea plant is around 11%, and since oxygen is also present, there is a risk of explosion. Please share the hydrogen content in the carbon dioxide from your plants How to reduce it?
We are a methanol plant, and the hydrogen content in the CO2 product gas is only 0.08%.
In our factory, it is generally sufficient to keep the H2 content in CO2 below 1% (it has recently often reached 1.1% as well). When this level is slightly higher, the people in charge of urea production will raise concerns, but it seems they don’t really care that much either. To control the H2 content, it is necessary to take into account the solubility characteristics of H2 in methanol, which are the opposite of those of CO2 and H2S; the lower the temperature, the lower the solubility of H2 in methanol. Therefore, in the CO2 separation tower, as long as the CO2 meets the requirements, there is no need to pursue excessive purity for CO2. When temperature is reduced to increase CO2 purity, the H2 content also increases accordingly.
May I ask the original poster: Oxygen is present in the exhaust gases from the urea plant – where does this oxygen come from? ?
The effect of temperature is two-way: when the overall temperature is low, less hydrogen dissolves in the absorption tower, but more hydrogen is released in the carbon dioxide recovery tower; vice versa. It is difficult to keep the temperature low in the absorption tower and high in the carbon dioxide recovery tower at the same time throughout the system.
The urea plant requires anti-corrosion air, so oxygen is present.
The high hydrogen content in carbon dioxide after low-temperature methanol washing may be due to a high pressure in the medium-pressure flash tower, a high pressure in the absorption tower, and a large circulation volume of methanol. These are all the reasons for the excessive H2 content in CO2.
I have a different view: that is, the solubility of H2 in methanol does not decrease as the temperature drops； Rather, the solubility of H2 in methanol changes little with temperature. So in V2 and V3, H2 and CO2 are cooled first; as the temperature decreases, the solubility of H2S and CO2 increases, resulting in less desorption ； The effect of temperature on CO and H2 is less significant. I think if there is a high amount of hydrogen in the CO2, then V2 and V3 will flash, and the pressures of H2 and CO won’t be under control!
Most of the hydrogen in methanol has been recovered! It mainly depends on the conditions of primary medium-pressure flashing~! The hydrogen content of carbon dioxide in our factory is only 0.08, while it is 0.13 in the exhaust gases! The flow rate of the exhaust gas is more than 4 times that of the carbon dioxide product stream! The hydrogen in the exhaust gas is primarily the hydrogen from methanol, which is stripped in the concentration tower and emitted through two stages of low-pressure flashing! The hydrogen content in the product carbon dioxide is also related to the operating pressure of the C2 tower! High pressure means low hydrogen content, while low pressure means high hydrogen content
1. The pressures during flashing and desorption should be controlled properly. 2. Cross-contamination of gases must be prevented; if there is cross-contamination in the flashing tank, it may lead to an increase in the H2 content in the CO2 gas.
With Linde’s technology in our company, the concentration in the exhaust gases is 0.2. It is recommended that if hydrogen removal is to be carried out, the flashing pressures at V2 and V3 should still be well controlled, with an effort to flash off the hydrogen at V2 and V3
We are involved in ammonia and urea production facilities; the quality requirement for our CO2 product gas is: H2 + CO ≤ 1.2% (mol)
The main reason for the high hydrogen content in carbon dioxide is the need to control the flashing pressure properly; too high a flashing pressure can lead to a high hydrogen content in the resulting carbon dioxide. Additionally, it is necessary to control the temperature of the material entering the flash tank, as high temperatures can also result in an elevated hydrogen content in the carbon dioxide product