Chemical Forums
Specialty Chemistry Forums => Chemical Engineering Forum => Topic started by: kostasgr84 on November 26, 2014, 02:45:34 PM
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Hello everybody!
I want to make a cold bath using dry ice, ethanol and glycol just like this...
http://www.che.psu.edu/faculty/rioux/group/group_info/references/dry_ice_bath_based_on_ethylene_glycol_mixtures.pdf
This paper provides information about the volume fraction of glycol but it doesn't mention the quantity of dry ice, for example: How much dry ice do i need for a mixture of (200ml ethanol + 200ml glycol) so that it will reach -50 as the diagram forecasts? Do i need special equipment or a simple glass container is enough?
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I'm not sure how much dry ice, but the answer is probably: more than you thought. I would make it in a dewar; a simple glass container would lose heat too quickly (and be too cold to touch).
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People don't generally use the heat capacity of the solutions in the ice bath, the volume, and the initial temperature, to find out exactly how much ice is need, just to bring the bath to temperature. Like mjc123:, you put an abundance, and as the mixture gins heat from its surroundings, you're always ready to add more. Please have decent ventilation for the dense cloud of CO2 you'll be generating, and don't crouch down next to it, lest you take a sudden nap you never wake up from. If you don't have a dewar, a small picnic cooler will work to prevent excess heat gain.
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How does this work? I'm not inclined to doubt the authors when they claim it's temperature stable for 5 hours, but it seems counter-intuitive. The point of a traditional single-solvent slush bath is that with solid and liquid in equilibrium the temperature is fixed; absorbed heat goes into melting the solid, not raising the temperature. With a mixed solvent you have a degree of freedom, the melting point varies with the liquid composition (how much of the higher-melting component has solidified), so absorbing heat leads to some solid melting and the equilibrium temperature rising. (Follow line a1-a2-e on the diagram.) If the thermal mass is large enough this will be slow, but still....
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How does this work? I'm not inclined to doubt the authors when they claim it's temperature stable for 5 hours, but it seems counter-intuitive. The point of a traditional single-solvent slush bath is that with solid and liquid in equilibrium the temperature is fixed; absorbed heat goes into melting the solid, not raising the temperature. With a mixed solvent you have a degree of freedom, the melting point varies with the liquid composition (how much of the higher-melting component has solidified), so absorbing heat leads to some solid melting and the equilibrium temperature rising. (Follow line a1-a2-e on the diagram.) If the thermal mass is large enough this will be slow, but still....
I'm not sure, but what if they chose the eutectic composition? Would T still vary?
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It shouldn't, but they have a whole graph of bath temperature versus solvent composition.
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Thank you all! All answers were usefull and i finally did it.