Regarding the issue of nitrogen sealing
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I thought it was quite simple in the past; I just thought of it today. It was previously believed that the pressure of nitrogen is higher than that of volatile gases, so it would cling to the volatile gases and prevent them from escaping through the breathing valve. But today I suddenly thought that gases are mixtures; if nitrogen and volatile gases are mixed together, how is it possible to prevent the volatile gases from escaping? PS: We often read in books that certain gases have a high density and thus sink to the bottom. Leaving aside the reason why they don’t mix thoroughly and remain at the bottom, why don’t the various gases in the air form layers due to their different densities?
It depends on whether the gases are miscible with each other. Just like two liquids with different densities, if they are miscible, there will be no separation between them. Gases rarely fail to be miscible with one another; therefore, in most cases, there is no distinct boundary line between two gases of different densities, unless the difference in density is very large. The principle behind using nitrogen sealing is not to prevent the evaporation of organic liquids, but rather to use the inertness of nitrogen to protect organic gases and liquids from coming into contact with air and thereby avoiding potential risks
Well, I didn’t express myself clearly. In fact, nitrogen sealing is generally used for flammable and volatile gases. When the pressure of the liquid inside the tank decreases, nitrogen is introduced to maintain the pressure; when the pressure is high, nitrogen is released. My question is: since nitrogen is mixed with the volatile gases, how can we ensure that what is exhaled is pure nitrogen? If what is exhaled is a mixture, there must be some risks as well, right? Additionally, if the nitrogen pressure is high, can it prevent air from entering? After all, gases are in motion; so can’t low-pressure gases enter if there is high-pressure gas present? Additionally, I believe nitrogen sealing can reduce liquid evaporation
The nitrogen sealing you mentioned is probably intended for use with the liquid inside the tank. The pressure of the liquid in the tank depends on the pressure of the nitrogen sealing system and the height of the liquid level. So, as I understand it, when the pressure of the liquid in the tank decreases, nitrogen is introduced to maintain that pressure; in other words, nitrogen is introduced when the liquid level in the tank drops； If the liquid is volatile, the gas discharged through the nitrogen purge will definitely be a mixed gas; therefore, the outlet pipe of the breather valve must be connected to a suitable location for safe discharge (such as to a flare stack). The nitrogen purge serves to protect the space inside the tank by maintaining a certain pressure within it, thereby preventing external air from entering the tank and posing a risk. When there is a certain pressure from the nitrogen purge, it is possible to reduce the volatility of the liquid. However, the pressure from the nitrogen purge is usually at a slight positive level
The presence of nitrogen reduces the meteoric partial pressure at the liquid surface, thereby decreasing volatilization. However, volatilization cannot be completely prevented; if it involves toxic or harmful substances, the nitrogen-sealed exhaust gas needs to be treated, either by feeding it into a flare or using an activated carbon adsorption tank. As for why the gas does not stratify, the large distances between gas molecules, the intense irregular movements, and the small differences in gas density make it difficult for stratification to occur.
Numerically, there will certainly be a few molecules moving from low pressure to high pressure, but such movement has no meaning on a macroscopic scale. For example, intense spontaneous reactions also involve reversible reactions, but this doesn’t tell us anything.
Does the presence of nitrogen reduce the meteoric partial pressure at the liquid surface?
In fact, in the situation you described, the greater importance of nitrogen sealing lies in the fact that when pressure drops, what is drawn in is nitrogen, rather than air or other non-inert gases. What the people upstairs were talking about regarding nitrogen sealing refers more to the use of nitrogen sealing in equipment (mechanical equipment).
The statement is incorrect; it should be that it reduces liquid evaporation, rather than lowering the vapor pressure at the liquid surface.
So what is your basis? If one doesn’t seek a thorough understanding, mistakes will only be passed on.
I’m sorry; I took it for granted. Nitrogen sealing is only intended to prevent oxygen from the air from entering and creating an explosive atmosphere; it does not reduce the evaporation of the medium.