Sublimation of urea

By-product formation can also be reduced by use of a stripping gas or vacuum to faciUtate removal of ammonia (88) however, sublimation of urea becomes excessive if the pressure is too low. Addition of ammonium salts (eg, CU, NO7, or ) (89—91), acids, or pyrolysis of preformed urea salts, eg,... 

Temperatures below 170° lead to slightly lower yields, and temperatures above 180° cause excessive sublimation of urea. Although the reaction is nearly complete after 2 hours, the yield appears to be improved by additional heating. 

Other post-combustion NO removal techniques include the injection of urea ([NH2]2CO) and cyanuric acid ([HOCN]3). The latter is termed the RAPRENO process [37a], When heated, cyanuric acid sublimes and decomposes to form isocyanic acid HNCO. Similarly, urea reacts to form NH3 and HNCO. Thus its NO reduction path follows that of the thermal DeNO route as well as that of the RAPRENO route to be discussed. 

A second example of the application of heating stage ESEM is the observation of a sublimation reaction in situ. Figure 14 shows the sublimation pressure-temperature diagram of urea (modified after [46]). Under normal conditions at high temperature urea decomposes into ammonia and isocyanic acid (see e.g. [47]). 

Figure 14. Sublimation diagram of urea as function of temperature and sublimation pressure (modified after [46]).

Emel yanenko VN, Kabo GJ, Verevkin SP (2005) Measurement and Prediction of Thermochemical Properties Improved Increments for the Estimation of Enthalpies of Sublimation and Standard Enthalpies of Formation of Alkyl Derivatives of Urea. J Chem Eng Data 51(l) 79-87... 

Ferro D, Barone G, Della Gatta G et al (1987) Vapour pressures and sublimation enthalpies of urea and some of its derivatives. J Chem Thermodyn 19(9) 915-923... 

We used a very large data set for the optimization of the in-termolecular energy parameters for the amide crystals. It included the structural data (unit cell parameters and the orientation of the molecules in the unit cell) for 10 amide crystals, the heat of sublimation of 6 amide crystals, and the dipole moments of urea, formamide aind N-methyl-acetamide. The experimental data on the geometry of the hydrogen bond were rather varied, including 0...H dis-... 

C8H10N4O2. An alkaloid occurring in tea, coffee and guarana, from which it may be prepared by extraction, It is also manufactured by the methylation of theobromine and by the condensation of cyanoacetic acid with urea. Crystallizes with H2O or anhydrous from organic solvents. M.p. (anhydrous) 235"C, sublimes at 176 C. Odourless, and with a very bitter taste. Caffeine acts as a stimulant and diuretic, and is a constituent of cola drinks, tea and coffee. 

Currently, there is continuing work on an iadustry standard method for the direct determination of monomer, dimer, and trimer acids. Urea adduction (of the methyl esters) has been suggested as a means of determining monomer ia distilled dimer (74). The method is tedious and the nonadductiag branched-chain monomer is recovered with the polymeric fraction. A micro sublimation procedure was developed as an improvement on urea adduction for estimation of the polymer fraction. Incomplete removal of monomer esters or loss of dimer duriag distillation can lead to error (75). 

Another characteristic of this solution is its proneness to crystallization and polymerization. When parts of the exhaust system are constantly welted by Adblue on the same spot, undesired urea crystals or polymers may form if the exhaust line temperature is lower than 300°C. This phenomenon will result in uncontrolled ammonia production when the crystals or polymers melt or sublimate after being heated at significantly higher temperatures (T > 350°C). This may result in ammonia release. Furthermore, the crystals or polymers can also have an impact on the SCR catalyst cells by reducing the catalyst surface and thus reducing the catalyst performances. 

The baking process, particularly the batch variety, presents a number of serious disadvantages. Not only does the reaction produce solid urea decomposition products, but it also releases large amounts of ammonia and ammonium salts which escape by sublimation. The foam which is thus formed makes for a porous reaction mixture, which in turn even prevents heat conduction. Moreover, the reaction mixture tends to adhere to the surface of the reaction vessel and the stirring unit a phenomenon which adds to the complexity of the problem. 

Resonance of the carbonate type occurs in urea, CO(NHs), and guanidine, CNH(NH )2. For urea the thermochemical value of the resonance energy is 37 kcal/mole, and for guanidine 47 kcal/mole, the latter being calculated with use of the estimated value 24 kcal/mole for the heat of sublimation. 

Calcium Cyanamide. ]CAS 156-62-7). CaCN-. white solid, funned (11 by heating cyanamide or urea with calcium oxide, sublimes at 1.05(1 C. (2) by heating calcium carbide at 1.100-I.200 C in a current of nitrogen. Decomposes in water with evolution of NFL. 

Lowering the reaction temperature or adding urea or basic solvents decreases the extent of chlorination. Solvents such as nitrobenzene, trichlorobenzene, alcohols, glycols, pyridine, and aliphatic hydrocarbons are employed. By using substituted phthalic acids such as 4-chlorophthalic acid anhydride, 4-sulfophthalic acid anhydride, or 4-nitrophthalimide, phthalocyanines with inner substitution can be produced. The products can often be purified by sublimation in vacuo at 300-400 °C. Soluble Pc s can be purified by recrystallization. 

K. Suzuki, S, Onishi, T. Koidc, and S. Seki. Bull. Chem. Soc. Japan 29,127-31 (1956) (in English). Vapor pressure AH of sublimation, urea, diformylhydrazinc. 

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