Does water vapour absorbtion depend on H2O conc or RH?

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Discussion Overview

The discussion revolves around the absorption of water vapor by anhydrous CaCl2 and how this process may be influenced by factors such as air temperature and water vapor concentration. Participants explore the relationship between these variables in the context of desiccant dehumidification and related thermodynamic principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the absorption rate of water vapor by CaCl2 changes with varying air temperatures, assuming a constant concentration of water vapor.
  • Another participant confirms that the absorption rate decreases as temperature increases, noting that at high temperatures, absorption may cease and the material may begin to reject moisture.
  • A third participant introduces two additional questions regarding the behavior of air in a heated horizontal tube, specifically addressing density changes and the distribution of water vapor along the tube.
  • A subsequent reply asserts that while pressure remains constant, the density of air will be lower at the heated end, and that absolute humidity remains constant while relative humidity decreases with temperature.

Areas of Agreement / Disagreement

Participants express differing views on the effects of temperature on absorption rates and humidity distribution, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Some assumptions regarding the ideal behavior of materials and the specific conditions of the experiments are not fully detailed, which may affect the applicability of the discussed principles.

Who May Find This Useful

This discussion may be of interest to those studying thermodynamics, material science, and environmental engineering, particularly in the context of humidity control and desiccant materials.

Roger44
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Hello
If I blow air containing a fixed kg/m3 of water vapour over anhydrous CaCl2, will the absorption rate change if the air temp is different? To simplify, we neglect exotheric heat generated by hygroscopy.

Thanks for your answers to this rather simple question.
 
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Roger44 said:
Hello
If I blow air containing a fixed kg/m3 of water vapour over anhydrous CaCl2, will the absorption rate change if the air temp is different? To simplify, we neglect exotheric heat generated by hygroscopy.

Thanks for your answers to this rather simple question.
Yes. You are describing the exact material and process sometimes used for for desiccant dehumidification. The absorption rate and capacity drops as temperature rises and above a certain temp, the absorption stops and it starts rejecting. You should be able to find charts giving exact performance for a theoretically perfect process. I found this, for example, but it doesn't have cacl2...
https://www.sorbentsystems.com/desiccants_charts.html
 
Last edited:
May I first wish you and your family a Happy Xmas. Father Christmas and the grandchildren visited us yesterday so I'm free today to think ...

I'll come back on absorption rates when I've finished a few observations here, but in the meantime could I again ask for your opinion on two schoolboy topics:

1. A closed air-containg horizontal tube externally heated towards one end. Will air inside be at a lower density at the heated end?
2. If the air is moist, will the g of water wapour per kg of dry air be different along the tube?
 
1) The air will be at the same pressure, but lower density at the heated end.
2) The absolute humidity (grams H20 per grams of dry air) will be the same, the relative humidity will be lower at the heated end.

You might find it interesting to spend some time studying a psychrometric chart, which shows the relationships between temperature, water vapor concentration, relative humidity, and density.
 

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