How can I calculate the concentration of ethanol vapor in an airstream?

The steady state generation flow rate (of the vapor) is 625 ft^3/min. The ambient temp is 50-60 F (we can use avg temp 55 F).

These vapors would come from liquid ethanol.

What more information do I need to calculate this concentration in kg/m^3 or ppm?

I apologize as I do not have a thorough chemistry background.

  • $\begingroup$ Would you not need to know the rate of evaporation from the liquid ethanol? If the evaporation rate is very slow this will limit the amount of ethanol in the airstream, if high it may be the equilibrium vapour pressure. $\endgroup$ – porphyrin Aug 30 '17 at 13:08
  • $\begingroup$ Are you saying that ethanol vapor is being generated at a rate of 625 $\ce{ft^3}$/min or that liquid ethanol is being generated at a rate of 625 $\ce{ft^3}$/min. $\endgroup$ – J. Ari Aug 30 '17 at 13:55
  • $\begingroup$ @J.Ari Ethanol vapor. $\endgroup$ – VasNL Crafting Aug 30 '17 at 14:32
  • $\begingroup$ @porphyrin ok, how shall I find the evaporation rate of the ethanol at this temp? $\endgroup$ – VasNL Crafting Aug 30 '17 at 14:33
  • $\begingroup$ So if you know how much ethanol vapor you are generating you need to know what else is in the stream and measure the amount of the stream. Calculating a concentration at that point becomes trivial. $\endgroup$ – J. Ari Aug 31 '17 at 20:03

You can't. Evaporation rate is not a thermodynamic property. It depends on the specifics of the process it is occurring in. The vapor pressure at a given temperature is determined AT EQUILIBRIUM - meaning the process/history is unimportant. The rate that a specific process occurs is obviously process dependent. It is true that various organics have had cited evaporation rates. These rates are generally a weight ratio relative to some standard liquid (ethyl ether, acetone, benzene, etc.). The reason they are relative is because no two investigators are likely to have sufficiently similar experimental processes to state weight loss on an absolute basis and get the same results. I'm just saying the same thing in another way; that your absolute weight loss per unit time is dependent on the specifics of your process. You've two realistic options: 1. find some Chemical Engineering (etc.) literature which has measured that for a "sufficiently similar" process and interpolate. 2. Measure it yourself. There are organic vapor monitoring devices, weight loss or tracer methods available. (There are other ways as well, depending on the accuracy required and the processes involved.) Note: a simple example of the difficulty in determination of evaporation rate involves a wet surface. The evaporation rate from that surface will clearly depend on the air flow across the surface, even at a constant temperature. It will also depend on the humidity of the air. Demonstrating that the rate depends on air flow proves that the relationship is going to be complex since only the air in contact with the surface will influence it, while air flow rates are for the bulk flow through a volume (not across a surface).


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