A question about the relationship between saturated vapor pressure and temperature.
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Taking methane (as the medium) and LNG as an example, if the pressure of the LNG onboard the ship is 16–18 Kpag with a temperature of -160°C, then the pressure in the storage tanks on land is 11 Kpag, at the same temperature of -160°C. After the liquid on board enters the storage tank, the pressure first rises and then falls. When the subcooled liquid on board enters the tank, it will flash due to the pressure difference; so how does the temperature change? Or will the temperature of the storage tank change? I have questions and can’t figure it out, so I’ve come to ask for advice.
May I ask what the saturated vapor pressure of methane is at -160°C?
Whether the temperature changes depends on whether there is sensible heat transfer； The transfer process can be analyzed in two steps: In the first step, LNG evaporates due to pressure differences; phase change involves heat absorption, and this heat comes from sensible heat. As a result, the temperature of the LNG entering the storage tank decreases during the initial stage ； In the second step, LNG continues to be supplied; the steam becomes saturated, evaporation stops, and sensible heat is transferred due to the temperature difference, causing the temperature to rise again ； (Without considering the resistance losses during transfer and external heat transfer, the temperature rise will not exceed -160°C.) Based on the conditions provided by the poster, the pressure difference is not very large; it is estimated that the overall temperature change of the storage tank will not be significant ； Of course, the amount of temperature change is related to the evaporation rate and speed, as well as the size of the storage tank.
Isn’t the definition of sensible heat that no chemical reactions or phase changes occur?
Although it is the most basic definition, it shouldn’t be memorized mechanically... Think about it; for example, in the process of water evaporating: evaporation is a phase change that requires heat absorption. Here’s a glass of water; adiabatic, evaporation occurs, with a phase change! Where does the heat come from? From this glass of water! When water releases heat to water vapor, its temperature drops, resulting in sensible heat transfer. Understanding this properly and analyzing the microscopic processes will make it easier to comprehend.