Partial pressures of components and phase equilibrium problems
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This post was last edited by jinjinaaaaaaaa on 2017-12-22 08:10. The following problem has arisen now: A solution (a non-electrolyte) contains three main components, a, b, and c. In order to obtain component a after evaporation and condensation, let the operating pressure during evaporation be P. The inert gas resulting from the separation of component a through condensation is then returned to the evaporation system, which reduces the partial pressure of that component; assume this partial pressure to be 0.5*P. Assume that the interactions between the three substances a, b, and c can be ignored. Question: In this solution, should the boiling point of component a be the saturation temperature at 0.5*P, or at P? In my opinion, it should be the saturation temperature at 0.5*P, because the phase equilibrium of component a is only related to its own vapor. Thank you. . .
This post was last edited by jinjinaaaaaaaa on 2017-12-22 09:21. I’ve come up with an explanation, which is explained in conjunction with the image below. When the partial pressure of component a in the gas phase is low, it shifts from point H to point 1; the liquid bubble point corresponding to point 1 is the temperature t5 corresponding to point 2, and since t5 is lower than the operating temperature t3, the phase equilibrium is not maintained. It needs to go from 2 back to 3; point 3 corresponds to the gas phase point H. In point H, the concentration of component a in the gas phase is greater than that at point 1. When component a is removed from the gas phase through condensation, it returns from point H to point 1, and this cycle repeats over and over again.
I think there is no concept of boiling point in a closed system; the partial pressure of component A reaches at most the saturated vapor pressure corresponding to its temperature. By introducing an inert gas to remove component A from the gas phase, the unsaturation level of A in the gas phase can be increased, which facilitates the evaporation of A
Why doesn’t a closed system have a boiling point? There must be a boiling point corresponding to a certain pressure
The temperature corresponding to the saturation pressure; just state the temperature, not the boiling point. The boiling point generally refers to the temperature at which a system at atmospheric pressure reaches boiling
According to what you said, regarding my question on the 1st floor: is the saturation temperature of component A the one at a vapor phase partial pressure of 0.5*P?
During the initial pre-cooling of the LNG tank, before LNG is introduced into it, BOG is used for pre-cooling while simultaneously removing N2 from the tank; only after the N2 level has been reduced to a certain level can LNG be actually introduced into the cooling tank. According to standard specifications, the purpose of controlling the N2 content is to prevent potential LNG subcooling (a temperature lower than the tank’s design temperature of -165). In my opinion, the presence of N2 reduces the partial pressure of NG inside the storage tank, which results in a lower saturation temperature for LNG at that reduced partial pressure; this temperature may drop below the design value, posing a risk. This principle is similar to what was discussed above.
I have never come across the term “saturation temperature” in any materials. One should not create their own concepts; if what one creates is misunderstood by others, it can lead to misunderstandings and the problem will change. If the component in the liquid phase is mainly a, then when a non-condensable component b is introduced into the gas phase, under sealed and non-flowing conditions, the partial pressure of a in the gas phase reaches equilibrium at its saturated vapor pressure, which is independent of component b. If b is also condensable, then the saturation vapor pressure of a in the liquid phase, xa*a, equals the partial pressure of a. If the content of N2 in LNG is high, the expansion energy of N2 can result in temperatures lower than -165°C; this has little to do with the partial pressure of LNG
All the theoretical knowledge was completely ignored.
What the original poster means is that LNG shouldn’t be mixed with N2, right? I’ve never worked with LNG, so I’m just guessing here – N2 is added to prevent the volatilization of LNG; phase changes absorb heat, and high pressure prevents the volatilization of LNG, thereby suppressing phase changes and stabilizing the temperature.
Thank you for the help, I understand! Regarding the issues with LNG and nitrogen, the condition I’m referring to is not the nitrogen contained within the LNG liquid, but rather the nitrogen used for nitrogen purging after the storage tank is completed. During the pre-cooling of the tank, it is necessary to reduce the level of nitrogen to a certain amount before proceeding with the pre-cooling of the LNG. This reason should be the same as what you mentioned.