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Vaporization of Mixture at constant temperature

  1. Dec 1, 2012 #1
    How is it possible to completely vaporize a mixture at constant temperature? I have seen listed values for ΔH(Vaporization); however, this doesn't quite make sense to me. If a mixture (with two largely diffent boiling points) is at its boiling point in the liquid state and I want to completely vaporize the mixture, won't the final vapor mixture be at a higher temperature than the mixture boiling point?

    This is my rationale: boiling is obtained when the combined vapor pressures reach atmospheric pressure. However, following a vapor-liquid equalibrium curve, it becomes apparent that the compenent with the higher boiling point will be left in the liquid state (regardless of how little is left). Therefore, to completely evaporate the mixture, doesn't the temperature need to be increased to evaporate the last bit of high boiling component??

    Thanks for the help
     
  2. jcsd
  3. Dec 2, 2012 #2

    Borek

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    Staff: Mentor

    You are right, boiling temperature is a function of composition. But I don't see how it is related to ΔHVaporization in the context of your question, please elaborate.
     
  4. Dec 2, 2012 #3
    For a mixture to vaporize (say at atmospheric pressure) it must be at its boiling point. ΔH(vap) is specified for a specific temp and pressure of a mixture, typically at its boiling point. However, what I am wondering is how it is possible to completely vaporize a mixture at a constant temperature (as is implied by the ΔH(vap) values)? Or is there just an assumed small error in calculations involving ΔH(vap) values?

    I'll give an example: I have a process that states that a mixture enters a heat exchanger at its boiling point, while in the liquid state. The liquid exchanges heat with steam and exits the heat exchanger at its boiling point, while in the gaseous state. How is it possible for this process to occur at one temperature??
     
  5. Dec 2, 2012 #4

    Borek

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    Staff: Mentor

    I am not sure I understand what the problem is, but here is my take. As long as the composition is constant, boiling point is constant as well. Does all mixture vaporize, or is it just losing part of the most volatile components? If so, temperature difference can be relatively small, just several degrees C, and the "boiling point" can mean "something close enough to the boiling point".
     
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