[curiouscat]

Would it be viable to concentrate a dilute solution of Urea, say 10% (w/w) up to 40% by evaporating off the water using heat? My defaults say there shouldn't be any trouble but just wanted to check if there are any unforeseen hurdles like decomposition etc?

If temperature induced decomposition is an issue I could always apply a vacuum to boil off the water at a lower temperature.

[Babcock_Hall]

At around 50 °C or so, the equilibrium concentration of ammonium cyanate in 8 M urea is about 20 mM.  I am going on memory here, so don't take that number very seriously.  Also, I don't know anything about the rate at which cyanate is produced.

[curiouscat]

At around 50 °C or so, the equilibrium concentration of ammonium cyanate in 8 M urea is about 20 mM.  I am going on memory here, so don't take that number very seriously.  Also, I don't know anything about the rate at which cyanate is produced.

Ah! Thanks. So not so simple as I thought, eh? I must consider the decomposition equilibrium.

[Babcock_Hall]

I do see a potential complication.  However for rotary evaporation with a good vacuum, maybe the problem is not severe.

[DrCMS]

Babcock I'm guessing as it is curiouscat asking the question this is not a lab rotary evaporator we are talking about.  My guess is that you will decompose enough of the urea to cause you problems but try it out and see.

[curiouscat]

Babcock I'm guessing as it is curiouscat asking the question this is not a lab rotary evaporator we are talking about.  My guess is that you will decompose enough of the urea to cause you problems but try it out and see.

DrCMS is right: At this stage it is lab experiments but the goal is to scale the process to tonnage quantities.

Any tips how to test this in the lab? I've already made a dilute solution & re-evaporated but I don't have a good assay for urea in solution. And since there's always some losses it's hard to verify decomposition from an overall material balance.

PS. Some decomposition isn't a problem per se because I can add make up urea. I just need to reliably quantify & control the process.

[curiouscat]

I suppose the equilibrium I really need to worry about is the one between urea vs  ammonia and (iso)cyanic acid?

(NH₂)₂CO  HNCO + NH₃

Liquid phase equilibrium shifts I shouldn't worry about because after cooling the conc. solution the species will revert.

But a gas phase product like NH3 would escape during evaporation.

Must I also model this further decomposition of isocyanic acid?

HNCO + H₂O  CO₂ + NH₃

[Enthalpy]

as it is curiouscat asking the question this is not a lab rotary evaporator we are talking about. [...]

An evaporator with a bigger area then? 
http://www.chemicalforums.com/index.php?topic=56452.0
bubbles or a shower would offer a big area too but are more prone to clogging.

[Enthalpy]

You should definitely consider reverse osmosis
https://en.wikipedia.org/wiki/Reverse_osmosis

It usually serves to extract pure water from seawater or wastewater and leave brine. In your case, the concentrated solute would be the sought intermediate, pure water a byproduct, possibly with a value.

Clogging may be a difficulty. Reversing the flow shortly from time to time could be an answer. For sure, this worry is known and addressed, not only for desalination. For instance, the International Space Station recycles the wastewater, and, well, you get it.

Reverse osmosis is presently the process that consumes the least energy for water purification. It works at room temperature, just from a small pressure difference.

Experimenting it for your purpose looks easy: buy a home-sized unit and try it on urea.

[Babcock_Hall]

This may be tangential to your interest, but there is a test for cyanate using copper and pyridine.  It was described by Emil Werner.  http://pubs.rsc.org/en/content/articlelanding/1923/ct/ct9232302577/unauth#!divAbstract  He mentions heating a 5% solution of urea to 100 °C for 5 minutes, if I understand correctly, and seeing a positive reaction.

[Enthalpy]

If using rotating disks to evaporate a solvent and concentrate a solution, a method that looks less prone to clogging, one may prefer to condense the solvent afterwards or catch the odours. On the appended sketch, a second set of rotating disks interleaved with the first set shall do that. It brings a second liquid to close distance to the first one over a big area where the vapour of the first liquid condense.

The second liquid can be cold to favour the condensation. It can be of the same nature of the first solvent.

The second liquid can also catch the first solvent's vapour by a strong interaction. Hygroscopic substances are known to absorb water vapour for instance.

One hygroscopic substance is ammonia solution, whose vapour might (or not?) impede the transfomation of urea into poisons if heat is used.

Both liquids can move slowly through the machine, for instance parallel to the rotation axes here, preferably in opposite directions so the second liquid enters the machine fresh at the end where the first liquid exits concentrated.

A good casing lets choose the operating pressure.

The disk profile can have a groove at mid-thickness if this avoids drops falling in the other liquid.

Marc Schaefer, aka Enthalpy