Abstract: An improved process and apparatus for the concentration of an aqueous urea solution by evaporation of water in a single pass shell and tube heat exchanger having a substantially vertical tube bundle within the shell. The aqueous urea solution is introduced into the top of the vertical tube bundle and caused to fall as a film down the inside of said tubes whereby it is heated and water evaporated therefrom to form a concentrated urea solution. The heat for this evaporation is provided by condensing at least a part of a gas mixture containing NH.sub.3, CO.sub.2 and H.sub.2 O in the shell side of the heat exchanger. This gas mixture is introduced into a lower portion of the shell wherein it is caused to flow upwardly in shell in contact with said tube bundle, counter-current to the flow of the aqueous urea solution within the tubes whereby heat from said condensation is transferred to said aqueous urea solution flowing down the inside of the tubes.

Abstract: A method and apparatus for determining the saturation temperature of a solution containing a dissolved substance, the crystals of which are optically anisotropic. The solution is placed in an optical measurement vessel under conditions wherein crystals of said substance are present, and the temperature of said crystal containing solution is gradually increased to at least the temperature at which all of the crystals dissolve. The temperature of the solution is continuously measured. Simultaneously with the temperature increase, a beam of linearly polarized light is directed into the optical measurement vessel in a manner such that it is transmitted through said solution; the transmitted beam of light, after having passed through the solution, is directed through a light polarizing analyzer having a direction of polarization normal to that of said beam of light; and at least a portion of any light transmitted through said analyzer is directed to a light measuring photodetector.

Abstract: A process for determining the composition of an aqueous solution of ammonia and carbon dioxide from which ammonium carbamate or ammonium carbonate crystallizes out upon cooling to below the saturation temperature. The density and the saturation temperature of the solution are measured, and the ammonia, carbon dioxide, and water composition is determined by reference to densities and saturation temperatures of aqueous solutions of ammonia and carbon dioxide of known composition. The composition thus determined can be used to effect control over the composition of such aqueous solutions.