Manufacture of urea

Urea was the first compound significant to life that was prepared in the laboratory. It is, however a very important compound in its own right and is purchased in considerable bulk. 

The most common method of manufacture is to react ammonia and carbon dioxide 

The first stage is carried out by passing liquid ammonia (in excess) and carbon dioxide gas together with recycled ammonium carbonate into an autoclave under high pressures, typically 200-400 atmospheres, and at 180-210 °C. There is about 60% conversion of CO2 into the carbonate. The carbonate is passed into low pressure decomposers to allow final conversion to urea. Efficiency of the plant is improved by recycling unused ammonia and carbon dioxide and the carbonate which remains undecomposed . The resultant solution can be concentrated to about 99.7% purity when it is packaged. This is achieved under vacuum or by prilling. 

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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. 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 sHp required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availabiUty of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

There are several approaches to the manufacture of urea, but the principal method is that of combining carbon dioxide with ammonia to form ammonium carbamate (Figs. 1 and 2) ... 

Urea. H2NCONH2, is excreted in the urine as the chief nitrogen-containing end product of protein metabolism. It is synthesized on a large scale for use as a fertilizer and as a raw material in the manufacture of urea-formaldehyde plastics and of drugs. 

Urea is used in the manufacture of urea-formaldehyde plastics, adhesives, polymers, synthetic fibers, dyes etc. 

Finally, it should be mentioned that plants with roller presses can be easily adapted to the manufacture of urea supergranules for deep placement in wetland rice production.In this case, the roller surface must be modified (installation of pocketed rollers) and the flake breaker (9 in Figure 412) bypassed. The crushing cycle (granulator (16) in Figure 412) is inactivated and oversized material (briquettes) becomes product. 

Before the advent of oxygenate ethers like MTBE, formaldehyde production was the largest single application of methanol, with at least 16 manufacturers and consuming over 30% of the methanol produced. Formaldehyde is used in the manufacture of urea-formaldehyde resins, phenol-formaldehyde resins, melamine-formaldehyde resins, acetal resins, acetylenic chemicals, etc. The reactions involved in formaldehyde synthesis are ... 

EINECS 213-674-8 HSDB 5776 1-(Hydroxymethyl)urea Methylol urea Methylolurea Methylolureas Mono-(hydroxymethyl)urea Monomethylolurea N-(Hydroxy-methyl)urea N-Methyiolurea NSC 13181 Urea, (hydroxymethyl)-, Used In manufacture of urea-formaldehyde resins. Colorless crystals mp = 111 soluble H20, EtCH, MeOH, AoOH, insoluble in Et20. 

On June 10,1980, the Commission proposed a rule under Section 27(e) that would have required manufacturers of urea-formaldehyde foam insulation to alert purchasers and prospective purchasers to certain possible acute health effects. 45 Fed. Reg. 39434. The Commission s rule was never adopted because the Agency subsequently decided to propose a ban on urea-formaldehyde insulation based on evidence indicating that formaldehyde may be carcinogenic. The Commission finalized that ban in a rule published on April 2, 1982. 47 Fed. Reg. 14366. In the preamble to the final rule, the Commission explained why warnings relating to the carcinogenic hazards presented by urea-formaldehyde would not be sufficient to protect consumers. 47 Fed. Reg. 14401. 

HCOH) A colorless gaseous aldehyde. It is manufactured by the oxidation of methanol (500°C and a silver catalyst) 2CH3OH + 02 2HCOH + 2H2O The compound is used in the manufacture of urea-formaldehyde resins. A solution of methanal (40%) in water is called formalin. It is extensively used as a preservative for biological specimens. 

Supervision Field supply Harvesters Tractor haul Irrigation move Irrigation pumping Manufacture of urea Manufacture of P2O5 Manufacture of K2O Ground operations Aircraft operations Fertilizer transport... 

Haq, A.N., An Integrated Production-Inventory-Distribution Model for Manufacturing of Urea A Case , International Journal of Production Economics, 39(1991), 39-49. 

A further important improvement, which allows the manufacture of ureas with concomitant reduction of waste at source (i.e. avoiding the production of large amounts of saline by-products, which represent the main constituent of chemical waste) has been the application of carbon dioxide. This strategy combines the use of a non-toxic reagent with the benefit of reducing the emission of CO2 in a direct way by fixation of the molecule into other molecules [36-38]. 

All current processes for the manufacture of urea are based on the reaction of ammonia and carbon dioxide to form ammonium carbamate which is simultaneously dehydrated to urea ... 

Ammonia is easily liquefied, and the liquid is used as a nitrogen fertilizer. Ammonium salts, such as the sulfate and nitrate, are also sold as fatilizers. Large quantities of ammonia are converted to urea, NH2CONH2, which is used as a fertilizer, as a livestock feed supplement, and in the manufacture of urea—formaldehyde plastics. Ammonia is also the starting compound for the preparation of most other nitrogen compounds. 

Fumasoni S, Pochetti F, Roberti G (1974) Simultaneous manufacture of urea and glycol, Ger Offen 2,318,327, CA, 80, 14593J... 

Chemical industry. Thick wall titanium tubes have been used in high-temperature, high-pressure, tubular heat exchangers for processes, such as manufacture of urea and for cooling sodium and potassium chloride brines up to 25% concentration. Titanium has an impressive success history in the chemical industry. Titanium has also been used as construction material in process plants and columns. 

Ammonia is also produced in ammonia plants as a raw material for the manufacture of urea and other nitrogen-based fertilizers. Ammonia in synthesis gas at temperatures between 450 and 500 °C causes nitridation of steel components. When synthesis gas is compressed to up to 34.5 MPa (5000 psig) prior to conversion, corrosive ammonium carbonate is formed, requiring various stainless steels for critical components. Condensed ammonia is also corrosive and can cause stress corrosion cracking (SCC) of stressed carbon steel and low-alloy steel components. 

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