Urea recycling

These data do not provide strong support for the heat water stress/urea recycling model (Ambrose 1991). This model may be incorrect or inapplicable to rats. Were the experimental conditions inappropriate, with temperatures too low and/or protein levels too low or too high In the heat stress experiments that inspired this research animals were kept at a temperature of 40°C for 12 hours each day rather than 36°C in this study. In our heat and water stress experiments the protein content of the diets were set at 20% and 70%. These are relatively high levels compared to those in herbivore diets. It would be necessary to repeat the experiments with ruminant herbivores or lower protein diets to conclusively determine if rats are an inappropriate model. 

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. 

The urea produced is normally either prilled or granulated. In some countries there is a market for Hquid urea—ammonium nitrate solutions (32% N). In this case, a partial-recycle stripping process is the best and cheapest system. The unconverted NH coming from the stripped urea solution and the reactor off-gas is neutralized with nitric acid. The ammonium nitrate solution formed and the urea solution from the stripper bottom are mixed, resulting in a 32—35 wt % solution. This system drastically reduces investment costs as evaporation, finishing (priQ or granulation), and wastewater treatment are not required. 

Other Processes. Flow sheets for typical partial-recycle process and typical once-through urea process are given in Figures 9 and 10, respectively. 

For each mol of urea produced in a total-recycle urea process, one mol of water is formed. It is usually discharged from the urea concentration and evaporation section of the plant. For example, a 1200 t/d plant discharges a minimum of 360 t/d of wastewater. With a barometric condenser in the vacuum section of the evaporation unit, the amount of wastewater is even higher. Small amounts of urea are usually found in wastewaters because of entrainment carry-over. 

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. 

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

Phenolics are consumed at roughly half the volume of PVC, and all other plastics are consumed in low volume quantities, mosdy in single apphcation niches, unlike workhorse resins such as PVC, phenoHc, urea—melamine, and polyurethane. More expensive engineering resins have a very limited role in the building materials sector except where specific value-added properties for a premium are justified. Except for the potential role of recycled engineering plastics in certain appHcations, the competitive nature of this market and the emphasis placed on end use economics indicates that commodity plastics will continue to dominate in consumption. The apphcation content of each resin type is noted in Table 2. Comparative prices can be seen in Table 5. The most dynamic growth among important sector resins has been seen with phenoHc, acryUc, polyurethane, LLDPE/LDPE, PVC, and polystyrene. 

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. 

This ammonia is recycled to the reactor via a compressor and a heater. Liquid ammonia is used as reflux on the top of the absorber. The net amount of carbon dioxide formed in the reactor is removed as bottom product from the absorber in the form of a weak ammonium carbamate solution, which is concentrated in a desorber-washing column system. The bottom product of this washing column is a concentrated ammonium carbamate solution which is reprocessed in a urea plant. The top product, pure ammonia, is Hquefted and used as reflux together with Hquid makeup ammonia. The desorber bottom product, practically pure water, is used in the quench system in addition to the recycled mother Hquor. 

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). 

Only one melamine molecule is formed from six urea molecules, whilst three molecules of ammonia carbamate are formed. Whilst this can be recycled to urea the conversion from urea to melamine per cycle is at most 35%. Both the main route and the recycling operation involve high pressures and the low process efficiency offsets some of the apparent economic attractions of the route compared to those from dicy . 

Urea Plant 6. Use total recycle processes in the synthesis process reduce microprill formation and carryover of fines in prilling towers. 

Urea is made by a process tliat combines cuumonia witli aubon dioxide mider pressure to form ammonimn aubamatc, n liicli is tlien decomposed into urea and water. The uiircactcd carbon dioxide and aimnonia are recovered and recycled to tlic syntliesis operation. 

The main difference between tlie various urea syntliesis processes are in tlie metliods used to handle tlie converter effluent, to decompose the carbamate and carbonate, to recover tlie urea, and to recover tlie mireacted aimnonia and carbon dioxide for recycle with a nia. inium recovery of heat. The amiutil production rate is approximately 4.5 million metric tons. End use is... 

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. ... 

Glycolysis is the most promising approach for the chemical recycling of polyurethanes.1 The chemistry of PUR depolymerization is complicated by the presence of other chemical groups in the polymer, such as ureas, allophanates, and biurets. 

Nitrogen isotope ratios ( N/ " N) inerease from plants to herbivores to eami-vores and ean be used to estimate the degree of camivory in human diets. Some field studies observe a greater differenee in 5 N between trophie levels in dry, hot habitats than in wet, cool ones. Two hypotheses have been proposed to explain this variation in difference in 8 N between trophic levels. (1) Elevated excretion of -depleted urea in heat/water-stressed animals (2) recycling of nitrogen on protein-deficient diets. Both predict increased diet-tissue 8 N difference under stress. 

Protein stress and recycling of nitrogen could also have the opposite effect, however. If less N-depleted N is excreted as inea, then there should be less overall enrichment in the nitrogen available for tissue synthesis. Moreover, if urea itself is recycled for protein synthesis under protein stress, which often occurs in herbivores, then the diet-tissue difference should be smaller than in unstressed individuals because urea has a substantially lower 8 N value than the diet. 

A plant is to be designed for the production of 300,000 kg per day of urea by the reaction of ammonia and carbon dioxide at elevated temperature and pressure, using a total-recycle process in which the mixture leaving the reactor is stripped by the carbon dioxide feed (DSM process, references 1 to 4). 

Fic. 1.—Proton-decoupled, Natural-abundance, 13C-N.m.r. Spectra of Native and Denatured A. niger Glucoamylase at 67.9 MHz. [Spectra required 16,384 scans, with a recycle time of 2 s. (A) Native glycoprotein (1.1 mM) in 50 mM phosphate buffer, pH 5.1 (B) denatured glycoprotein (0.6 mM) in 25 mM phosphate buffer, 7.8 M urea, pH 6.2. Spectra were taken from Refs. 25 and 60 reproduced by courtesy of Marcel Dekker, Inc., New York.]... 

---