THERMODYNAMIC ANALYSIS OF UREA PHYSICAL VAPOR TRANSPORT

The vaporisation-condensation scheme of urea has been investigated in the temperature region below the melting point, in order to establish the thermodynamic reactions on which the physical vapour transport (PV T) of urea is based. By making use of both reported data and experimen tal results of MS, DTA, DTG, and HPLC analysis, it was shown that PVT relevant reactions are basically molecular sublimation and partial dec omposition in solid biuret and ammonia. MS analysis of urea crystals a nd source residues have also shown that no decomposition can be observ ed in the growing crystals, unless reheated during growth. The tempera ture dependence of the decomposition pressure has been found to be:log (10)P(NH3) [torr] = 38 - 13840/T[K]. The use of this relation in diffu sive mass transport calculations brought to linear growth rate values not in disagreement with the experimental values found in closed tube PVT grown Crystals.