[303]

Hi, I'm an mechanical engineer that is relatively new to chemistry but am exploring using chemical reactions as a solution to one of my heat transfer problems. I am looking for a reversible endothermic reaction whose reactants and products are both liquids. From my research, I have found that I could use the depolymerization of Paraldehyde into Acetaldehyde. The advantages of this reaction being that both Paraldehyde and Acetaldehyde are liquids and the enthalpy change is large (110 kJ/mol). Unfortunately Paraldehyde is toxic.

A reference paper discussing the reaction and application can be found here: http://www.sciencedirect.com/science/article/pii/S135943119800043X

Does anyone know of any other chemical reactions that fit my requirements? The requirements being: reversible, endothermic, and liquid reactants/products. Secondary requirements that don't necessarily have to be met are continuous cycle of reactions and non-toxic. If not could anyone point out some resources where I could look for these types of reactions?

Thanks

[AlphaScent]

I am sorry but I do not off the top of my head.  That would take some research.  Paraldehyde is a trimer.  Is the tetramer toxic?  I am sure the reaction going from the tetramer to acetaldehyde is also endothermic.

[AlphaScent]

Paraldehyde is not acutely toxic and is only harmful if swallowed.  It is flammable, so that could be an issue.  What is your application?? 

[Enthalpy]

The linked paper wants to use the reaction in a cooling aggregate. For most other uses, there may be easier methods. The monomer <-> trimer reaction must be incomplete and its equilibrium shift only over a wide range of conditions (temperature).

In case the desired "heat transfer" is a conduction, heat pipes for instance are excellent.
http://en.wikipedia.org/wiki/Heat_pipe

[zsinger]

A good quote which I learned from my all time chemistry mentor……."You don't get something for nothing!"  For instance, hydrogen cars…..Yes, hydrogen is clean, but think of the energy input to split water to create H2 gas (more coal burning).  That might be a bad enthalpy change going the other way sir!
             -Zack

[303]

My application is cooling of heated surfaces, i.e. electronics. I am using liquid cooling and want to enhance convective heat transfer by incorporating a chemical reaction within the cooling fluid.

Paraldehyde is not acutely toxic and is only harmful if swallowed.  It is flammable, so that could be an issue.  What is your application??

[303]

Thanks for the suggestion, however the heat transfer I am considering is purely convection heat transfer. I am trying to avoid phase change as this would add complexity to my cooling system as I would need to condense the vapor. Furthermore, my understanding is that reversible chemical reactions have higher energy densities than reversible phase change reactions. Therefore I expect that I would be able to achieve higher heat transfer rates without having to resort to phase change.

The linked paper wants to use the reaction in a cooling aggregate. For most other uses, there may be easier methods. The monomer <-> trimer reaction must be incomplete and its equilibrium shift only over a wide range of conditions (temperature).

In case the desired "heat transfer" is a conduction, heat pipes for instance are excellent.
http://en.wikipedia.org/wiki/Heat_pipe

[303]

I agree. Since my application is thermal management of high density electronics, this problem is not too critical. In design, the reverse exothermic reaction will take place at a heat sink where temperature does not matter and the surface area is much larger.

A good quote which I learned from my all time chemistry mentor……."You don't get something for nothing!"  For instance, hydrogen cars…..Yes, hydrogen is clean, but think of the energy input to split water to create H2 gas (more coal burning).  That might be a bad enthalpy change going the other way sir!
             -Zack

[Enthalpy]

A heat pipe can be bought, so complexity if any is not your worry. It costs very few dollar for >100W. It works without a pump, advantage over a liquid flow and a chemical reaction.

Over a trimerization, they have the fundamental advantages that vaporization occurs over zero temperature diofference, is complete, and happens quickly (which offsets any comparison of absorbed heat). The trimer <-> monomer would have all these drawbacks.

All this makes heat pipes the standard solution, especially for processors in computers. If you don't find the pipes, buy complete coolers and throw the rest away.

Don't try to develop your own heat pipes: it takes time. For instance, sealing the pipe without evaporating the liquid is less than obvious. Treating the pipe's inner face for capillary circulation isn't neither.

See the three copper pipes on the picture? There, or at competitors, since all use the same technology:
http://www.arctic.ac/de_en/freezer-7-64-pro.html
30€ unit consumer price, maybe 3€ manufacturing cost, this must leave 0.20€ per pipe - universal solution because it's the best technology. I wouldn't even try to compete against that.