How to calculate the amount of water vapor in a gas that has reached saturation humidity?
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If the pressure of gas in the gas pipeline is 1117.325 kPa, the temperature is 70°, and the gas is at saturation humidity, how can one determine the amount of water vapor contained per unit volume of gas? Is this calculation feasible? At 70°C, the humidity of the gas is saturated, which means that the partial pressure of water vapor in the gas equals the saturated vapor pressure at that temperature. According to tables, the saturated vapor pressure at 70°C is 311.61 kPa. According to the pressure division formula, P_vapor/P_gas = V_vapor/V_gas = M_vapor/M_gas. Then the volume of gas per unit volume is 31.161/(101.325+16)/22.4*18. I would appreciate guidance from those experienced in this field.
The approach is correct, but 1. There’s an issue with your pressure data: 1117.325 is not equal to (101.3225 + 16); the order of magnitude is wrong. The saturated vapor pressure at 70°C is 311.6 kPa, and again the order of magnitude is incorrect – it’s higher than atmospheric pressure. 2. The 22.4 L value applies under standard conditions; it needs to be converted to the value at 70°C.
Reply to 2# Han Guang Duan Shui: Thank you for the correction. I made a mistake in the data given in the original question. The pressure of the gas in the gas pipeline is 117.325 kPa, the temperature is 70°, and the gas is at saturation with respect to humidity. How can we determine the amount of water vapor contained per unit volume of gas? Is this calculation feasible? At 70°C, the humidity of the gas is saturated, which means that the partial pressure of water vapor in the gas equals the saturated vapor pressure at that temperature. According to tables, the saturated vapor pressure at 70°C is 31.161 kPa. According to the pressure division formula, P_vapor/P_gas = V_vapor/V_gas = M_vapor/M_gas. Then the ratio of water vapor to gas at 70°C is 31.161/(101.325+16). Is this calculation correct? I don’t think it’s necessary to convert it to 0°C; after all, when the temperature and pressure of a gas change, its volume should remain constant, right?
This post was last edited and replied to by HanGuangDuanShui on 2010-12-14 09:23. Reply 3# huazhimaomao: It’s indeed not necessary to reduce it to 0 degrees. However, in the calculation process shown on the first floor, there is a value of 22.4*18; what I mean is that the 22.4 in that calculation should be replaced with a value corresponding to 70 degrees, as 22.4 represents conditions under standard conditions. The ratio of water vapor to gas should be (water vapor partial pressure) / (gas partial pressure). The calculation you performed used the total pressure, resulting in a volume percentage of water vapor, rather than the volume of water vapor divided by the volume of gas. 31.161/(101.325+16-31.616) is the correct one. #
Reply to 4# Han Guang Duan Shui: Got it, thanks.