Will an excessive amount of nitrogen used for stripping also strip away hydrogen sulfide?
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As is well known, in the low-temperature methanol washing process, the nitrogen used for gas stripping is primarily intended to remove carbon dioxide gas while also supplying additional cooling capacity to the system! Some people question whether an excessive amount of nitrogen gas used for stripping could also lead to the release of hydrogen sulfide gas; high levels of hydrogen sulfide in the exhaust gases not only pollute the environment but also cause corrosion of the components inside the exhaust gas scrubbing tower! ! Please discuss and see if his doubts are justified! !
This makes no sense. 1. The function of stripping is to reduce the molar fraction of the gases in the gas phase. During stripping, CO2 and H2S are both removed from the mixture; however, the H2S that is removed is reabsorbed by methanol as it rises, thereby ensuring that the H2S concentration in the gas at the top of the tower does not exceed the allowable level. 2. Analyze based on the amount of H2S and the amount of methanol. The amount of H2S is extremely small, which theoretically ensures that it can be completely absorbed. 3. From a process perspective, the role of methanol in the flash tank located at the top of the CO2 separation tower and at the top of the H2S concentration tower (this tank is present in the Linde process; it’s not clear whether it exists in other processes) is to ensure that the H2S concentration at the top of the H2S concentration tower does not exceed the specified limits. In production, it is common to encounter situations where the H2S level at the top of the H2S concentration tower exceeds the allowable limit. When analyzing the reasons for this, the first thing considered is whether there is not enough methanol in the upper part of the H2S concentration tower (that is, whether there is a problem with the control valve that regulates this amount of methanol). I have conducted experiments in which the nitrogen gas lift was increased from 7,000 Nm3/h to over 10,000 Nm3/h, yet the H2S level in the overhead gas still did not exceed the allowable limits.
I completely agree with the first point mentioned by the brother upstairs! As the amount of nitrogen used for stripping increases, the partial pressure of H2S decreases, so the amount of desorption increases. On the second point, there are different opinions: I mainly believe that when a company – especially an established one like Linde – designs a new tower, it does not include too much margin. When the load increases, it is generally necessary to adjust the reflux volume at the top of the tower. If simply increasing the amount of nitrogen in the stripping is possible. . . Thirdly, depending on the different processes, the adjustment criteria for the reflux valve vary. The adjustment of this valve in our plant is based on the liquid level of the V2 flash tank. Additionally, increasing the amount of nitrogen stripped (within a certain range) can result in a lower temperature in the lower column. However, the reabsorption of excess CO2 and H2S in the upper column may cause the temperature of that column to rise. Resulting in a series of temperature changes.
It will have some effect, but it’s not very significant; its performance is far inferior to that of thermal regeneration. Using too much nitrogen results in poor economic efficiency, and it also makes it easy for liquid to be carried along!
Under normal circumstances, hydrogen sulfide is removed first, followed by carbon removal; levels generally do not exceed the specified limits, as the saturated vapor pressures of the gases are different. So the amount of hydrogen sulfide carried away is very small.
This is a good question. The problem isn’t serious, but we need to take cost accounting into consideration; therefore, the amount of nitrogen used must be controlled within certain limits. If too much nitrogen is used, it represents a waste, as most nitrogen is purchased from outside sources.
The amount of nitrogen used for stripping should be increased as much as possible in order to recover as much cooling capacity as possible. However, there is a limit to how much this amount can be increased; it cannot be set arbitrarily. The most important thing is to choose the most economical level of nitrogen usage. First, costs must not be wasted. Second, an excessive amount of nitrogen for stripping can lead to gas-liquid entrainment. As for removing hydrogen sulfide, this is necessary in the concentration stage, but all the lean methanol in the reflux stream going upward to the tower gets absorbed. Only if the amount of reflux flowing upward is too small, or due to valve problems, could the hydrogen sulfide levels in the exhaust gases become excessive