Urea cyanurate

Ammonia reacts vigorously with phosgene. The products are urea, biuret, ammeUde (a polymer of urea), cyanuric acid, and sometimes cyameUde (a polymer of cyanic acid). The secondary products probably arise through the very reactive intermediate carbamyl chloride [463-72-9] NH2COCI (see... 

Cyanuric acid can also be prepared by pyrolysis of urea derivatives. Biuret and triuret give less aminotria ines due to reduced ammonia evolution. Urea cyanurate also provides a higher assay product. 

Operabihty (ie, pellet formation and avoidance of agglomeration and adhesion) during kiln pyrolysis of urea can be improved by low heat rates and peripheral speeds (105), sufficiently high wall temperatures (105,106), radiant heating (107), multiple urea injection ports (106), use of heat transfer fluids (106), recycling 60—90% of the cmde CA to the urea feed to the kilns (105), and prior formation of urea cyanurate (108). 

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

Ammonia-type reductants, which include ammonia, urea, cyanuric acid and different ammonium salts, have been applied so far mainly for stationary sources. The apparent preference for a hydrocarbon reductant for a mobile deNOx system is mainly based on the following reasons (i) Hydrocarbons are always present in a combustion exhaust (ii) Equipping mobile sources witli an ammonia tank is considered unsafe (iii) Ammonia-type reductants require a stringent injection control to prevent ammonia slip (< 5 ppm required [12]) (iv) Ammonia is found to be converted to NO and N2O at liiglier temperatures (> 400 °C) over various SCR catalysts like V2O5 [12 - 14], WO3 [13] and CrjOs [15] (v) SO2 present in a combustion exhaust is often converted to SO3 to produce acidic particulates [13]. 

Ethyl carbamate Ethyl ethylcarbamate Urea Cyanuric acid 3330 3353 3440 3350 (3440) none (3490)... 

At atmospheric pressure and at its melting point, urea decomposes to ammonia, biuret (1), cyanuric acid (qv) (2), ammelide (3), and triuret (4). Biuret is the main and least desirable by-product present in commercial urea. An excessive amount (>wt%) of biuret in fertiliser-grade urea is detrimental to plant growth. 

Reaction of lime and urea forms calcium cyanate [6860-10-2] (26) which is then converted to calcium cyanurate [32665-90-0], the latter gives calcium cyanamide at a higher temperature ... 

A convenient laboratory synthesis of high purity CA is hydrolysis of cyanuric chloride (7). On a commercial scale, CA is produced by pyrolysis of urea [57-13-6]. When urea is heated at - 250 ° C for about an hour, it is converted to crude CA with evolution of ammonia. 

Cyanuric acid can also be prepared from HNCO (100). Isocyanic acid [75-13-8] can be synthesized directiy by oxidation of HCN over a silver catalyst (101) or by reaction of H2, CO, and NO (60—75% yield) over palladium or iridium catalysts at 280—450°C (102). Ammonium cyanate and urea are by-products of the latter reaction. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

The hydroxyl derivative of X-CN is cyanic acid HO-CN it cannot be prepared pure due to rapid decomposition but it is probably present to the extent of about 3% when its tautomer, isocyanic acid (HNCO) is prepared from sodium cyanate and HCI. HNCO rapidly trimerizes to cyanuric acid (Fig. 8.25) from which it can be regenerated by pyrolysis. It is a fairly strong acid (Ka 1.2 x 10 at 0°) freezing at —86.8° and boiling at 23.5°C. Thermolysis of urea is an alternative route to HNCO and (HNCO)3 the reverse reaction, involving the isomerization of ammonium cyanate, is the clas.sic synthesis of urea by F. Wohler (1828) ... 

Below 200°C, reliable urea thermohydrolysis is very hard to achieve, therefore urea dosage is usually stopped in real-world urea-SCR systems in this temperature regime. Another serious problem connected with the urea injection at low temperatures is the formation of white to yellowish deposits, which are observed when urea solution is injected at very low exhaust gas temperatures or if the urea spray forms a thick film at the walls of the SCR system. The analysis of these deposits [26] showed that they mainly consist of urea and some biuret at low temperatures and of cyanuric acid and some biuret at higher exhaust gas temperatures around 350°C. From laboratory investigations of the urea decomposition, it is known that biuret is easily formed from 150 to 190°C [27], whereas the formation of cyanuric acid is predominant from 200 to 300°C, according to the following reactions [12] ... 

Schaber, P.M., Colson, J., Higgins, S., et al. (1999) Study of the Urea Thermal Decomposition (Pyrolysis) Reaction and Importance to Cyanuric Acid Production, American Laboratory,... 

The reaction usually used to produce cyanuric acid (48) is the thermolysis of urea (47) between 180 °C and 300 °C (Scheme 7.11) [58]. The reaction occurs with formation of ammonia, which itself can react with 48 to give secondary products. It is, therefore, necessary to eliminate NH3 and to operate with an open reactor. 

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. 

Exciting motives obtained by hydrogen bonding of calixarenes bearing 2-pyridone [32b], carbonyl and pyridyl [32c], urea [32d], melamine and cyanuric acid [32e] and other acceptor and donor groups have been recently reviewed by Bohmerand Shivanyuk [32a]. 

Cyanuric fluoride has been used to modify tyrosine residues, substituting the phenolic hydroxyl group. A maximum of 3 residues in RNase was found to react at pH 10.9 and 25° (148a). However, some mystery surrounds this number, as with other estimates of accessibility, since alkaline-denatured material where all tyrosine residues are available still showed the reaction of only 3 residues with cyanuric fluoride. However, similar observations have been made on iodination in 8 Af urea (11 )- At pH 9.3, Takenaka et al. (149) found that only 2 residues reacted and that 115 was not one of them. Two more reacted after alkali denaturation. Two were resistant under all conditions tested. No enzymic activity data were reported. 

By coupling an ultrasonic probe with a microwave reactor and propagating the ultrasound waves into the reactor via decalin introduced into their double jacket design, Chemat et al. studied the esterification of acetic acid with propanol and the pyrolysis of urea to afford a mixture of cyanuric acid, ameline and amelide (Scheme 9.19)136. Improved results were claimed compared to those obtained under conventional and microwave heating. The MW-US technique was also used to study the esterification of stearic acid with butanol and for sample preparation in chemical analysis137,138. 

Amino acids, amino groups, amino sugars, and nucleic acid derivatives usually account for >95% of the organic N in soils (Anderson et al., 1989), and many other N-containing compounds have been reported in trace amounts (Stevenson, 1994). Anderson et al. (1989) have found traces of L-phosphatidic acid, choline, ethanol-amine, and uric acid (the end product of N metabolism of many animals), which can be oxidized to allantoin, cyanuric acid, and urea. 

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