Urea solution

Place about 0 2 g. of jack-bean meal in a test-tube, add 2 ml. of water and about 5 drops of phenol-red. Mix thoroughly and allow the faintly yellow solution to stand while the urea solution is being made up. 

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. 

The urea solution is evaporated in a two-stage system (99.8%) if the final product is prills, and a single-stage system (+95%) if granules are to be provided. 

The urea solution out of the stripper bottom flows to a single-stage low pressure recirculation section (0.4 MPa, 4 bar). The stripper off-gas is sent to the carbamate condenser. 

The urea solution is then evaporated to 99.8% for prilling (2 stages) or plus 95% for granulation (1 stage). 

The urea solution leaving the stripper bottom contains about 12 wt% of NH and is further purified in the 1.8 MPa (18 bar) and 0.2 MPa (2 bar) recovery sections of the plant. The resultant NH and CO2 separated in the decomposers is absorbed and returned to the synthesis section by the high pressure centrifugal carbamate pump. 

The urea solution stream is then fed to the vacuum concentrator unit which operates at 17.3 kPa (130 mm Hg abs) and produces 88.7 wt % urea. It then goes to either two-stage evaporators if prills are made, or a single-stage unit for granule production. 

Urea processes provide an aqueous solution containing 70—87% urea. This solution can be used directiy for nitrogen-fertilizer suspensions or solutions such as urea—ammonium nitrate solution, which has grown ia popularity recentiy (18). Urea solution can be concentrated by evaporation or crystallization for the preparation of granular compound fertilizers and other products. Concentrated urea is sohdified ia essentially pure form as prills, granules, flakes, or crystals. SoHd urea can be shipped, stored, distributed, and used mote economically than ia solution. Furthermore, ia the soHd form, urea is more stable and biuret formation less likely. 

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. 

Manufacturing Processing and Uses. In commercial production, aqueous urea solution is mixed with acetaldehyde in 1 1 molar ratios. An acid catalyst is introduced into the reaction mixture which is staged at various process temperatures. After neutralization with a base, the CDU is separated from the mother hquor by filtration or spray drying. 

Klotz, I. M., 1996. Equilibrium constants and free energies in unfolding of proteins in urea solutions. Proceedings of the National Academy of Sciences 93 14411-14415. 

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. ... 

Urea solution + /i-alkane — urea-n-alkane clathrat + H2O... 

A modified DP method with urea as a precipitating agent, called the DP urea method, has been developed for Ti02 support [27]. Deposition of gold NPs by this method was appHed to AI2O3 [28], Ce02 [28], and Mn203 [29] supports. Aqueous urea solution produces hydroxyl ions above 60 °C as foUows ... 

Industrial liquid urea is known as Adblue . This industrial liquid [chemical formula (NH2)2CO] is an aqueous solution of 32.5% wt urea solution (see Figure 7.14). Adblue is corrosive and requires stable materials for the components like tank, pipes, injector, etc. Moreover, this solution is not suitable for year-round use in the Northern countries the freezing point is at —11°C. Urea hydrolysis requires at least 180°C. 

Urea is usually applied as an aqueous solution. If urea solution is atomized into the hot exhaust gas stream, the first step is the evaporation of water from the droplets thus leading to molten urea ... 

Also the thermohydrolysis of the urea solution after the injection into the hot exhaust gas upstream of the SCR catalyst has been investigated at the diesel test rig. Urea solution was atomized about 3 m upstream of the SCR catalyst into the hot exhaust equivalent to a residence time in the pipe section of 0.1 s at 440°C. As expected for the thermolysis reaction, ammonia and isocyanic acid were found at the catalyst entrance at all temperatures (Figure 9.3). The 1 1 ratio of both components shows that only the thermolysis but not the hydrolysis is taking place in the gas phase. It can also be seen that the residence time of 0.1 s is not sufficient for the quantitative thermolysis of urea, as appreciable amounts of undecomposed urea were always found. The urea share even raises with lowering the flue gas temperature, although the residence time... 

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] ... 

Fig. 7. Dependence of uncorrected (A) diffusion coefficient (D) and (B) number of particles in the observation volume (N) of Alexa488-coupled IFABP with urea concentration. The data shown here are not corrected for the effect of viscosity and refractive indices of the urea solutions. Experimental condition is the same as in Figure 6. 

Fig. 8. Dependence of (A) corrected diffusion coefficient (D), (B) steady-state fluorescence intensity, and (C) corrected number of particles in the observation volume (N) of Alexa488-coupled IFABP with urea concentration. The diffusion coefficient and number of particles data shown here are corrected for the effect of viscosity and refractive indices of the urea solutions as described in text. For steady-state fluorescence data the protein was excited at 488 nm using a PTI Alphascan fluorometer (Photon Technology International, South Brunswick, New Jersey). Emission spectra at different urea concentrations were recorded between 500 and 600 nm. A baseline control containing only buffer was subtracted from each spectrum. The area of the corrected spectrum was then plotted against denaturant concentrations to obtain the unfolding transition of the protein. Urea data monitored by steady-state fluorescence were fitted to a simple two-state model. Other experimental conditions are the same as in Figure 6.

Robertson et al. (1996) were successful in synthesising cytosine and uracil in amazingly high yields. They started from cyanoacetaldehyde (obtained via hydrolysis of cyanoacetylene) and, again, urea. Cytosine is not formed in detectable concentrations unless concentrated urea solutions are used then, however, the yields are between 30 and 50%. Uracil was obtained by hydrolysis of cytosine solutions. 

Shi SR, Chaiwun B, Young L, et al. Antigen retrieval technique utilizing citrate buffer or urea solution for immunohistochemical demonstration of androgen receptor in formalin-fixed paraffin sections. J. Histochem. Cytochem. 1993 41 1599-1604. 

Kay, C.M., and Edsall, J.T. (1956) Dimerization of mercaptalbumin in the presence of mercurials. III. Bovine mercaptalbumin in water and in concentrated urea solutions. Arch. Biochem. Biophys. 65, 354. 

We would expect this urea solution to boil at (100.00+ 0.105=) 100.105 °C under 760.0 mmHg atmospheric pressure. Since it boils at a lower temperature, the atmospheric pressure must be lower than 760.0 mmHg. 

Nitrite solution is added to an acidified urea solution. Use of urea for the removal of nitrous acid, e.g. in the preparation of ethyl nitrate (p. 148). 

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