Urea processes

In this process a mixture of urea, sodium hypochlorite and sodium hydroxide is converted into hydrazine, sodium chloride and sodium carbonate. 

The urea synthesis is another etample of a large-scale industrial process. The urea production is realized under high pressure between 145 and 204 bar. In contrast to the ammonia process, described in Section 3.2, no catalyst is used so far. 

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Fig. 1. Snamprogretti thermal stripping urea process. BL = battery limits.

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

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. 

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. 

Reciprocating Compressors. Prior to 1895, when Linde developed his air Hquefaction apparatus, none of the chemical processes used industrially required pressures much in excess of I MPa (145 psi) and the need for a continuous supply of air at 20 MPa provided the impetus for the development of reciprocating compressors. The introduction of ammonia, methanol, and urea processes in the early part of the twentieth century, and the need to take advantage of the economy of scale in ammonia plants, led to a threefold increase in the power required for compression from 1920 to 1940. The development of reciprocating compressors was not easy Htfle was known about the effects of cycles of fluctuating pressure on the behavior of the... 

Urea Process. In a further modification of the fundamental Raschig process, urea (qv) can be used in place of ammonia as the nitrogen source (114—116). This process has been operated commercially. Its principal advantage is low investment because the equipment is relatively simple. For low production levels, this process could be the most economical one. With the rapid growth in hydrazine production and increasing plant size, the urea process has lost importance, although it is reportedly being used, for example, in the People s RepubHc of China (PRC). 

The estimated world production capacity for hydrazine solutions is 44,100 t on a N2H4 basis (Table 6). About 60% is made by the hypochlorite—ketazine process, 25% by the peroxide—ketazine route, and the remainder by the Raschig and urea processes. In addition there is anhydrous hydrazine capacity for propellant appHcations. In the United States, one plant dedicated to fuels production (Olin Corp., Raschig process), has a nominal capacity of 3200 t. This facihty also produces the two other hydrazine fuels, monomethyUiydrazine and unsymmetrical dimethyUiydrazine. Other hydrazine fuels capacity includes AH in the PRC, Japan, and Russia MMH in France and Japan and UDMH in France, Russia, and the PRC. 

Industrial production of copper phthalocyanine usually favors either the phthaUc anhydride—urea process (United States, United Kingdom) (1,52,53) or the (9-phthalodinitrile process (Germany, Japan) (54,55). Both can be carried out continuously or batchwise in a solvent or bake process of the soHd reactants (56). 

Some references cover direct preparation of the different crystal modifications of phthalocyanines in pigment form from both the nitrile—urea and phthahc anhydride—urea process (79—85). Metal-free phthalocyanine can be manufactured by reaction of o-phthalodinitrile with sodium amylate and alcoholysis of the resulting disodium phthalocyanine (1). The phthahc anhydride—urea process can also be used (86,87). Other sodium compounds or an electrochemical process have been described (88). Production of the different crystal modifications has also been discussed (88—93). 

Pressure Swing Adsorption. Carbon dioxide can be removed by pressure adsorption on molecular sieves. However, the molecular sieves are not selective to CO2, and the gases must be further processed to achieve the high purity required for "over the fence" use as in the urea process. Use of pressure swing adsorption for CO2 removal appears most appHcable to small, stand-alone plants (29). 

Reactions. The reactions of dicyandiamide resemble those of cyanamide. However, cycUzations take place easily and the nitrile group is less reactive. Under pressure and ia the presence of ammonia, dicyandiamide cyclizes to melamine. Considerable toimages of melamine have been made ia this manner however, melamine is produced chiefly by the urea process (43). 

By now the dehydration condensation of urea [57-13-6] has displaced the dicyandiamide process (see Urea). Although the latter is stiU used occasionally, the urea process predominates in North America. A flow sheet is shown in Figure 2 (43). 

Urea possesses negligible basic properties (Kb = 1.5 x 10 l4), is soluble in water and its hydrolysis rate can be easily controlled. It hydrolyses rapidly at 90-100 °C, and hydrolysis can be quickly terminated at a desired pH by cooling the reaction mixture to room temperature. The use of a hydrolytic reagent alone does not result in the formation of a compact precipitate the physical character of the precipitate will be very much affected by the presence of certain anions. Thus in the precipitation of aluminium by the urea process, a dense precipitate is obtained in the presence of succinate, sulphate, formate, oxalate, and benzoate ions, but not in the presence of chloride, chlorate, perchlorate, nitrate, sulphate, chromate, and acetate ions. The preferred anion for the precipitation of aluminium is succinate. It would appear that the main function of the suitable anion is the formation of a basic salt which seems responsible for the production of a compact precipitate. The pH of the initial solution must be appropriately adjusted. 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

Neutrinos are also generated by purely nuclear processes involving weak interactions, e.g. in the Sun. Such neutrinos can be an important cause of energy losses in compact stars through the Urea process, in which an inverse / -decay is followed by a normal fS-decay resulting in a neutrino-antineutrino pair. 

If the phase transition is somewhat stronger than we have discussed in the previous subsection, the initial temperature is higher, To 10 MeV. Neutrinos are trapped for a while in the interior of the newly formed quark star. For lower densities, where the quark matter contains trapped neutrinos the direct Urea process is operative and neutrino cooling is a surface effect. 

Only at smaller temperature V 1 MeV when the emissivity of the direct Urea process has dropped according to the typical T6 dependence, most of the neutrino energy will be released within a small time. Neutrinos in the diffusion regime can escape from the surface in the time... 

Only two techniques have gained commercial importance. The phthalonitrile process, developed in England and Germany, is particularly important in Germany, while the phthalic anhydride/urea process has stimulated more interest in... 

Crude Copper Phthalocyanine Blue which is prepared by the phthalonitrile or urea process typically evolves as the -modification with a coarse particle size. 

There is an interesting technique which makes it possible to introduce carboxylic acid groups into a copper phthalocyanine structure by an economical route. Carrying out the phthalic anhydride/urea process in the presence of a small amount of trimellitic acid or another benzene polycarboxylic acid will afford a car-boxylated pigment. 

The phthalic anhydride/urea process may also be employed to convert tetra-chloro phthalic anhydride to green copper hexadecachloro phthalocyanine by condensation. In this case, titanium or zirconium dioxides, particularly in the form of hydrated gels, are used instead of the molybdenum salts which are used in the phthalic anhydride process [23]. There is a certain disadvantage to the fact that the products lack brilliance and require additional purification. 

Copper Perbromo Phthalocyanine Green may also be obtained from tetra-bromo phthalic anhydride by the phthalic anhydride/urea process in the presence of titanium or zirconium catalysts. This route has not yet been introduced on a commercial scale. 

Practically feasible extents of supersaturation or subcooling are fairly small and depend on the substance and the temperature. Some data appear in Table 16.2. Since the recommended values are one-half the maxima listed, they rarely are more than 2°C or so. This means that very high circulation rates through heat exchangers are needed. Thus, in the urea process of Example 16.1, the temperature rise is 2°F, and the volumetric circulation rate is about 150 times the fresh feed rate. 

The second example of process intensification at DSM is the urea process (5). The history of the urea process at DSM is rather long, as shown in Table 5. Urea is produced in a two-step process. The first step is the formation of carbamate from NH3 and C02. This reaction is exothermic. The second step is the decomposition of carbamate into urea and water. This second reaction is slightly endothermic. Both reactions are equilibrium reactions. The conversion to urea in equilibrium is about 60%. This means that substantial recycle flow is necessary to obtain sufficient overall conversion. In the reaction section the main unit operations are ... 

The layout of the new plant concept with the integrated and intensified pool reactor now has a height of 18 m, a height reduction by a factor of 3. Also, the number of units has been decreased. This is illustrated in Figure 11, in which the development of the urea process is depicted. In this figure it can be seen that even an old proven bulk chemical process can be intensified, resulting in a much more compact and economical plant. 

Two processes are commonly used for the production of copper phthalocyanine the phthalic anhydride-urea process patented by ICI [33,34] and the I.G. Farben dinitrile process [48], Both can be carried out continuously or batchwise in a solvent or by melting the starting materials together (bake process). The type and amount of catalyst used are crucial for the yield. Especially effective as catalysts are molybdenum(iv) oxide and ammonium molybdate. Copper salts or copper powder is used as the copper source [35-37] use of copper(i) chloride results in a very smooth synthesis. Use of copper(i) chloride as starting material leads to the formation of small amounts of chloro CuPc. In the absence of base, especially in the bake process, up to 0.5 mol of chlorine can be introduced per mole of CuPc with CuCl, and up to 1 mol with CuCl2. 

An example of a process with an intermediate product is the urea process. The second step, starting from ammonia, is over 90% efficient, whereas the total process, having ammonia just as an intermediate product, shows an efficiency of only half of this value. For the nitric acid process, Figure 14.2,... 

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