Urea, production

Urea is largely used as a fertilizer (ISy ), and as a non-protein feed supplement for sheep and cattle. The most important chemical use, which however accounts for only a small part of urea production, is in the manufacture of urea-formaldehyde resins. U is also used in the manufacture of adhesives, pharmaceuticals, dyes and various other materials. U.S. production 1981 7 0 megatonnes urea resins 1983 6 megatonnes. 

The U.S. urea production from 1976 to 1980 (18,19) is given in Table 8. During this period, urea became the most important sohd nitrogen fertilizer. Urea solution production almost doubled from 1977 to 1978, proving its importance as a hquid fettihzet. 

The technology of urea production is highly advanced. The raw materials requited ate ammonia and carbon dioxide. Invariably, urea plants ate located adjacent to ammonia production faciUties which conveniently furnish not only the ammonia but also the carbon dioxide, because carbon dioxide is a by-product of synthesis gas production and purification. The ammonia and carbon dioxide ate fed to a high pressure (up to 30 MPa (300 atm)) reactor at temperatures of about 200°C where ammonium carbamate [111-78-0] CH N202, urea, and water ate formed. 

Because an excess of ammonia is fed to the reactor, and because the reactions ate reversible, ammonia and carbon dioxide exit the reactor along with the carbamate and urea. Several process variations have been developed to deal with the efficiency of the conversion and with serious corrosion problems. The three main types of ammonia handling ate once through, partial recycle, and total recycle. Urea plants having capacity up to 1800 t/d ate available. Most advances have dealt with reduction of energy requirements in the total recycle process. The economics of urea production ate most strongly influenced by the cost of the taw material ammonia. When the ammonia cost is representative of production cost in a new plant it can amount to more than 50% of urea cost. 

Only about 10% of the total urea production is used for amino resins, which thus appear to have a secure source of low cost raw material. Urea is made by the reaction of carbon dioxide and ammonia at high temperature and pressure to yield a mixture of urea and ammonium carbamate the latter is recycled. 

Zirconium is totally resistant to corrosion by organic acids. It has been used in urea-production plants for more than two decades. 

Melamine is produced by heating urea under pressure in the presence of a catalyst. The result is a ring stmcture as shown below. The reaction by-products, ammonia and carbon dioxide, can be recycled for urea production. 

Urea and ammonium sulfate [7783-20-2] are coated by Chisso Co. under the trade names LP Cote and Meister. AH U.S. consumption of these products is sourced from Japan. Chisso-Asahi products are marketed through very specific distribution channels (Table 5). Coated N—P—K products are marketed primarily to commercial nurseries and greenhouses. Coated urea products are marketed in blends to commercial nurseries, as weU as to professional turf and strawberry growers. 

The second reaction represents the decomposition of the carbamate. The reaction conditions are 200°C and 30 atmospheres. Decomposition in presence of excess ammonia limits corrosion problems and inhibits the decomposition of the carbamate to ammonia and carbon dioxide. The urea solution leaving the carbamate decomposer is expanded by heating at low pressures and ammonia recycled. The resultant solution is further concentrated to a melt, which is then prilled by passing it through special sprays in an air stream. Figure 5-3 shows the Snamprogetti process for urea production. ... 

Aminosulfonyl ureas were constructed from a sulfonylcarbamate linkage (Scheme 31) [72], Reaction of chlorosulfonyl isocyanate (CSI) with Wang resin provided a chlorosulfonylcarbamate 63 which was then converted to substituted amino sulfonylcarbamate compounds by reaction with excess amines. The final aminosulfonyl urea products were cleaved from the resin by treatment with amines in HF at reflux temperature for overnight. 

Reductive amination of an aldehyde with excess primary amine, using a support-bound borohy-dride, provides the desired secondary amine contaminated with the primary amine precursor. Covalent capture of the primary amine with a support-bound aldehyde provides the pure secondary amine. Treatment with excess isocyanate yields the final urea product, which is purified by reaction with a support-bound amine to remove unreacted isocyanate. For the full potential of this method to be realized, further development of support-bound reagents and scavengers for most of the important chemical transformations will be necessary. Al-... 

Phenothiazines (e.g., promethazine, chlorpromazine, ethopropazine etc.) on the contrary causes a significant decrease in BUN levels due to lowering of urea production from the liver, and... 

However Henseleit showed there was a correlation between the concentration of ornithine and the magnitude of the effect on urea production. That the absolute amounts of ornithine were so small provided Krebs with arguments for the idea that the ornithine acted catalytically. 

The combinatorial library synthesis of a diverse set of trisubstituted ureas has been described [64]. The synthetic pathway involves the prehminary preparation of various nitrophenylcarbamates from commercially available nitrophenyl chlorofor-mate and a selection of amines allowing for wide scope in the divergence of the final urea products. In a further reaction of the nitrophenylcarbamates with a second amine, the urea was generated. Simultaneous addition of an electrophilic and basic scavenger resin removed all by-products, again allowing rapid isolation of the products in excellent yield and purity (Scheme 2.43). 

Figure 8.15 k plot of leucine oxidation against leucine intake. The plot illustrates a linear relab onship between leucine intake and oxidab on. The leucine intake is calculated from the protein intake. It is assumed that the same relab onship would hold for other amino acids. A similar response is seen for urea production. Data from Young et ai, 2000. 

Figure 8.16 The control of amino acid breakdown and protein synthesis in liver. The factors in regulation are as follows (i) the amino acid concentration in the blood regulates the rate of urea production (Chapter 10) (ii) the amino acid leucine, and the anabolic hormones increase the rate of protein synthesis. Mass action is a term used to describe the effect of concentration of substrate on the reaction rate. The control of protein synthesis is discussed in Chapter 20. Control by leucine has been studied primarily in muscle.

The concept of the ornithine cycle arose from the observation that ornithine, citrulline and arginine stimulated urea production in the presence of ammonia without themselves being consumed in the process. 

Table 10.2 Rate of urea production in adult humans on normal, low protein and restored normal diets...

Diet Time on diet (days) Rate of urea production (g/day)... 

An increase in the plasma concentration of amino acids, induced artificially by infusion, increases urea production not only in a normal but in a damaged liver. (Table 10.3). 

Table 10.3 Effect of increases in blood amino acid concentration on rate of urea production in normal subjects and in patients suffering from cirrhosis of the liver...

An increase in the protein content of the diet in rats increases the maximnm activities of all the enzymes of the cycle in the liver. It is assnmed that this represents increased amonnts of these enzymes in the liver (Table 10.4). Since a chronic increase in the protein in the diet in hnmans increases urea production over a long period and also a decrease in protein in the diet decreases urea production, it is assnmed that, as in the rat, this is due to changes in the concentrations and therefore activities of urea cycle enzymes. 

For assessment of the potential to predict granule moisture content, a large 1032-object data set recorded dnring 5 months of urea production was used. The first 900 objects were used for calibration and the last 132 as a validation test set [2]. The data matrix was resampled to allow acoustic data to be calibrated against laboratory tests of moisture content, which were only available with a relatively low sampling rate however, plenty of results were at hand to allow a full assessment of the multivariate prediction performance for granule moisture. The validated prediction results can be seen in Figure 9.11. 

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of urea production, urea recycling, and interorgan nitrogen transfer in living systems. 

Several lines of investigation assert to the inability of canal ine to function as an effective ornithine antagonist. Ornithine interaction with canaline has been evaluated with the ornithine carbamoyl transferase (EC 2.1.3.3) of human liver. Neither canal ine nor ornithine inhibited this enzyme when the other member of this set served as the carbamoyl group recipient (29). The ornithine antagonist, 2,4-diaminobutyric acid drastically reduced urea production in the rat this reflected curtailment of the ornithine carbamoyl transferase-mediated conversion of ornithine to citrulline. Yet, canaline had no such effect on urea formation in this mammal (30). 

Chemical mechanism for reaction of a peracid with a N-hydroxyguanidine to generate the urea product and nitroxyl. This reaction is a chemical model for the second phase of the proposed NOS reaction, which involves nucleophilic attack by an activated oxygen species, C-N bond scission, and incorporation of molecular oxygen. 

To a 2 liter Hoke pressure cylinder are added 20.0 gm (0.21 mole) of aniline, 13.8 gm (0.43 mole) of sulfur, 100 ml of methanol, and 82 ml of an 8.75 M (0.73 mole) aqueous dimethylamine. The cylinder is pressurized with 100 psig of carbon monoxide, and then heated for 2 hr at 100°C. The cylinder is then vented and the contents removed from the cylinder by washing with hot methanol. The combined product and methanol washings were filtered hot and evaporated to dryness from the methanol. The urea product is recrystallized from 400 ml of water to give 27.8 gm (79%) of l,l-dimethyl-3-phenylurea, m.p. 130°-133°C. 

In ureotelic organisms, the ammonia deposited in the mitochondria of hepatocytes is converted to urea in the urea cycle. This pathway was discovered in 1932 by Hans Krebs (who later also discovered the citric acid cycle) and a medical student associate, Kurt Henseleit. Urea production occurs almost exclusively in the liver and is the fate of most of the ammonia channeled there. The urea passes into the bloodstream and thus to the kidneys and is excreted into the urine. The production of urea now becomes the focus of our discussion. 

The urea-product solution, leaving the first decomposition stage and still containing some unreacted carbamate and excess NH3, is let down in pressure and steam heated in the second-staged decomposition section, which operates at about 2 atm and 120°C. Practically all of the residual carbamate is decomposed and stripped from the urea-product solution together with the residual excess ammonia. The 74—75 wt % urea solution thus obtained is further processed to solid urea... 

Correct answer = D. Methionine is the precursor of cysteine. An increase in gluconeogenesis releases increased ammonia and results in increased urea production. The essential amino acids leucine and lysine are ketogenic. Ornithine and citrulline are amino acids that are intermediates in the urea cycle, but are not found in tissue proteins. 

Which of the following compounds, if added to an active tissue preparation, might be expected to yield the greatest increase in urea production in terms of moles of urea produced per mole of added compound ... 

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