ASPEN relative humidity
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Recently, while using APSEN to simulate relative humidity, I found that when non-ideal gas properties (such as SRK) are selected for the property calculations, using flash evaporation to simulate relative humidity results in a relative humidity that never reaches 100%, no matter how much water is added. This seems to conflict with the concept of relative humidity. I then went through some books on principles of chemical engineering and found that in many of them, the definition of relative humidity is partial pressure divided by the saturated vapor pressure of water (as given in saturation vapor pressure tables). In some other books, it is defined as partial pressure divided by the maximum value that the water vapor pressure can reach at a certain pressure and temperature (which seems to be a more meaningful definition). Finally, the ASPEN help files were consulted, and this point was also mentioned: when dealing with non-ideal gases (for which Dalton’s law of partial pressures does not apply), the maximum saturated partial pressure of water in the gas is not equal to the saturated vapor pressure of water (the value listed in the saturation vapor pressure tables). If relative humidity is still calculated using the saturated pressure from those tables as the denominator, it will result in the situation mentioned earlier – no matter how much water is present, the gas cannot reach 100% relative humidity – even though in reality the gas is already saturated at that point. I wonder what other sailors think?
Chemical engineering is actually closer to physics and mathematics; therefore, when looking at the definition, it is essential to pay attention to the boundary conditions. The data corresponding to the vapor pressure gauge values is for the pure water–air system. What is mentioned later is actually the saturated vapor pressure, which generally applies to mixed solution systems; in such cases, it depends on the properties of each component, involving multiple interaction coefficients.
The concept of relative humidity is actually an imprecise one; in my understanding, humidity is always saturated
This is an understanding that fails to take boundary conditions into account. It depends on the relative object. For example, in the drying process, compared to the desiccant, the adsorption sites on the gas-solid surface can indeed be considered saturated. However, the relative humidity as we define it is actually relative to a specific vapor equilibrium state. Therefore, under these conditions, an unsaturated state naturally exists.
That makes a lot of sense. So in fact, for non-ideal systems, even if there are slight deviations, the value calculated according to the definition doesn’t have much practical significance. For example, I also used ASPEN to select an ideal equation of state in order to calculate the relative humidity of air when it is saturated at 133 kPa. The air temperature was around 65 degrees, and the partial pressure of air at this temperature was approximately 28 kPa. According to steam tables, the saturated vapor pressure of water at this temperature is about 25 kPa; thus, ASPEN gave a relative humidity of around 112%. From this perspective, liquid is already present in the air at this point, but in reality the air is exactly saturated at this time. Therefore, to maintain consistency with the definition of relative humidity, ASPEN uses the 28/25 formula for calculating relative humidity. However, from a practical standpoint, it makes more sense to use the value 28/28, as the air is indeed saturated at this point, and 100% would be a more appropriate figure.
Yes, relative humidity, by definition, doesn’t hold much practical value as a reference for what we need. So another question: what about the dew point? According to the ASPEN calculations, with the ideal gas equation of state chosen as well, and under the same conditions as above – a temperature of 65 degrees – ASPEN calculates a dew point of 67 degrees. Since 65 degrees represents a saturated state, yet the dew point is 67 degrees, water must indeed be present. However, ASPEN indicates that the gas phase fraction is 1. Therefore, it’s possible that ASPEN’s dew point calculation also uses the saturation temperature from a steam table corresponding to the partial pressure; this contradicts the thermodynamic method of calculating dew points, according to which the dew point should be 65 degrees. I wonder what everyone thinks.
I calculated it using a flash vaporization module; the dew point temperature is determined based on the saturated temperature, which is in line with the thermodynamic definition, so there should be no problem.
There is another issue: the saturated vapor pressure of water should be measured under vacuum conditions, which differs from the assumption of an ideal gas at certain pressures; moreover, complex equation of state relationships at higher pressures lead to calculation errors