Using alcohol as a working fluid in boilers

Discussion in 'General Science & Technology' started by trevor borocz johnson, Sep 19, 2022.

  1. trevor borocz johnson Registered Senior Member

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    Mixing alcohol with water as the working fluid of a boiler would reduce the amount of heat needed to get the same pressures from the boiler. A Organic Rankine Cycle is when they use a condenser to convert the steam from a boiler back into liquid and pump it back into the boiler. Obviously if you were to use 100% alcohol as working fluid you would want to use the Rankine cycle so you aren't allowing flammable gas everywhere. I found some information on experiments with differrent working fluids in boilers most around the turn of the century and no word the amount of energy burned to create pressures, just that they were generally low pressure systems. Here's a Rankine cycle that I invented:

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  3. exchemist Valued Senior Member

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    Yes, the Rankine Cycle applies to any closed cycle heat engine that works on converting liquid to to vapour and condensing it again.
     
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  5. billvon Valued Senior Member

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    Yep. Ammonia also works. There are a lot of working fluids out there you can use. Note that choosing a different working fluid may optimize temperatures and pressures, but will not get around basic issues of thermodynamic efficiency like the Carnot limit.
     
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  7. trevor borocz johnson Registered Senior Member

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    For most part they use the Rankine Cycle in power plants to recycle heat from WATER though, I have yet to read about any information on using alcohol or gasoline or any mixture. By turn down the heat I mean burn less coal to get the same results.
     
  8. trevor borocz johnson Registered Senior Member

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    Making the part at 2 in the drawing a long alcohol filled hose would increase pressure in the system.
     
  9. billvon Valued Senior Member

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    You are making the mistake of assuming "lower boiling point" = "more efficiency." You can achieve exatly the same thing by operating at lower pressures with water. But it will not improve efficiency.
     
  10. trevor borocz johnson Registered Senior Member

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    Lower boiling point and higher volatility. The pressure in the boiler would be the same while burning less fuel to obtain those pressures. I don't get what you're getting at about efficiency.
     
  11. billvon Valued Senior Member

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    You said "burn less coal to get the same results." That means an increase in efficiency.
     
  12. trevor borocz johnson Registered Senior Member

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    Are you saying at the boiling point of alcohol 173 F that you would get the same pressures as water at the same temperature?
     
  13. exchemist Valued Senior Member

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    My thermodynamics gets hazy at time these days but isn't the latent heat input, to change from liquid at atmospheric pressure into vapour at atmospheric pressure, and which is subsequently rejected as waste heat in the condenser, something that is basically wasted from the viewpoint of the thermodynamic cycle?

    In which case, using a working fluid with a lower latent heat of vaporisation should improve the efficiency of the cycle, shouldn't it?
     
  14. billvon Valued Senior Member

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    Nope.
     
  15. billvon Valued Senior Member

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    Well, keep in mind that any thermal plant is designed to work best with one working fluid. If, for example, you decrease the boiling point, you transfer less heat to the fluid in the boiler - and that has downstream effects (like less energy available later.)

    Also note that no modern thermal plant works like an old fashioned steam engine as depicted in a science book. Superheat is usually used, as is heat recovery / economizers. All of that increases the efficiency of the engine, and all of that has to be carefully matched to the working fluid.
     
  16. exchemist Valued Senior Member

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    OK but I'm not talking about the boiling point. I'm talking about the Latent Heat of Vaporisation: the amount of heat you have to add to convert a given amount of liquid to vapour at the same temperature and pressure. Water has a high latent heat of vaporisation, and all the heat put in to vaporise it has to be be extracted again and wasted in the condenser part of the steam circuit. I should have thought that a different fluid with lower latent heat would cut down on this waste.

    However, thinking about this more, I see your point about pressure. Enthalpy of vaporisation is ΔU + pΔV, so if you are evaporating against the back-pressure of superheated fluid, it is a lot more than just the internal energy change. I also see that, for example, n-heptane which boils at 98C has an enthalpy of vaporisation (at 1bar) of 36kJ/mol whereas that of water is 41kJ/mol, so the difference is not that great. (I use the molar values because a mole produces the same volume of vapour.)

    So maybe the difference is fairly negligible- and of course handling water and steam is less hazardous that some flammable organic fluid.
     
  17. trevor borocz johnson Registered Senior Member

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    no real way of knowing without experiments, the design above could be readily made and tested.
     
  18. billvon Valued Senior Member

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    Well, you can get a very good sense by running through the thermo for the process, but yes - testing is key for systems like this to validate those calculations.
     
  19. Beaconator Valued Senior Member

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    Nope. Just nope.
     
  20. trevor borocz johnson Registered Senior Member

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    Beak, whats got you befuddled so?
     
  21. Beaconator Valued Senior Member

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    What type of power plants?
     
  22. trevor borocz johnson Registered Senior Member

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    The Rankine cycle or Rankine Vapor Cycle is the process widely used by power plants such as coal-fired power plants or nuclear reactors. In this mechanism, a fuel is used to produce heat within a boiler, converting water into steam which then expands through a turbine producing useful work.

    Rankine cycle - Energy Education
     
  23. trevor borocz johnson Registered Senior Member

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    what?
    I am typing sometimes as fast as I can to keep up with the race.
    and the rest. Beak if you're not in the right frame of mind to say forum things on a forum that is on an internet forum, maybe doctor's could put a you in a room with a rabbit that talks and plays and is nice and you would get better with your answers here at this here forum.
     

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