Urea reactor

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

NH1CONH2 + H2O. The processing is complicated because of the severe corrosiveness of the reactants, usually requiring reaction vessels that are lined with lead, titanium, zirconium, silver, or stainless steel. The second step of the process requires a temperature of about 200 C to effect the dehydration of the ammonium carbamate. The processing pressure ranges from 160 to 250 atmospheres. Only about one-half of the ammonium carbamate is dehydrated in the first pass. Thus, the excess carbamate, after separation from the urea, must be recycled to the urea reactor or used for other products, such as the production of ammonium sulfate. 

Urea Reactor effluent co2, nh3 Water Formation of ammoniumn carbonate solution, which is recycled to the reactor Stripping not practiced... 

Most of the unconverted material in the reactor effluent is separated by heating and stripping at synthesis pressure using two strippers in series. Whereas all the carbon dioxide is fed to the plant through the second stripper, only 40% of the ammonia is fed to the first stripper. The remainder goes directly to the reactor for temperature control. The ammonia-rich vapors from the first stripper are fed directly to the urea reactor. The C02-rich vapors from the second stripper are recycled to the reactor via the carbamate condenser, irrigated with carbamate solution recycled from the lower-pressure section of the plant110. 

The urea reactor is usually a multi-layered vessel that is made of carbon steel with a corrosion-resistant interior liner. Reactor liner materials are usually 316L stainless steel (urea grade) with a lower ferrite content (<0.6%) for pipes and (<1%) for forgings), (see Table 11.4. for the composition of urea grade 316L stainless steel88. 

Titanium and zirconium can also be used to line urea reactors. The line must be continuously passivated with oxygen to resist corrosive action by adding air (0.5 to 0.8 vol%) in the C02 feed stream88. 

In Japan, ureaform is also produced as a component of compound fertilizers by the addition of formaldehyde solution to the urea solution fed to the granulator.49 In one process a methylene urea reactor is used to react urea and formaldehyde solution to produce a slurry that feeds directly to a granulation plant with other materials to make compound fertilizers.50 Some slow-release nitrogen is formed, and the physical properties of the products are improved. 

With Stamicarbon s pool condenser technology, condensation can be done very efficiently by reversing the former high-pressure carbamate condenser s process and steam side. The entire heat exchanging part is submerged in condensed carbamate. This pool-type condensation enables higher heat transfer, while staging two-thirds of the entire synthesis section s urea conversion in its liquid holdup. Thus, the urea reactor... 

Taking this a stage further, Stamicarbon invented the pool reactor (Fig. 2), which effectively combines the pool condenser and the downstream urea reactor into a single piece of high-pressure equipment. Pool reactor type of urea plants are currently offered for capacities up to 2,300 metric tpd. 

The minor part of the CO, entering the synthesis as a feed, enters the urea reactor at the bottom in order to produce sufficient heat for the endothermic urea reaction. 

Downstream from the pool condenser, the urea-carbamate liquid enters the vertical reactor, if required, located at ground level. Here, the final part of the urea conversion takes place. The urea solution then leaves the top of the reactor, all by gravity flow via an overflow funnel, before being introduced into the HP stripper. Gases leaving the urea reactor are directed to the pool condenser. 

About 70% of the urea solution leaving the urea reactor flows to the HP CO2 stripper. The remainder is fed into a MP recirculation section. 

In the partial-recycle process, part of the off-gas ammonia and carbon dioxide from the carbamate strippers is recycled to the urea reactor. Recycling is accomplished by absorbing the stripper gases in a recycle stream of partially stripped urea effluent, in process-steam condensate, or in mother liquor from a crystallization finishing process. In this manner, the amount of NH3 in off-gas is reduced. Any proportion of the unreacted ammonia can be recycled typically, the amount of ammonia that must be used in some other process is reduced to about 15% of that from a comparable once-through unit. 

Unreacted NH3 and CO2 are separated from the urea solution in the high-pressure separator and in two to three steam-heated carbamate strippers at successively lower pressures. The off-gas from the separator and the first-stage stripper is absorbed in the high-pressure absorber by a side stream of partially stripped reactor effluent from the high-pressure separator. Heat evolved in the absorber reaction is removed (to increase absorption capacity) by the addition and expansion of part of the liquid ammonia feed at this point. Pure gaseous ammonia from the top of the absorber is also recycled to the urea reactor after being condensed. 

The urea reactor effluent contains urea and water rer sultir from the synthesis reaction it also contains unconverted carbamate and excess ammonia. These ingredients must be separated to give a urea solution reasonabty free from the other materials and to recycle the CO2 and NH3 to the synthesis reactor. In order to separate carbamate from urea, it must be decomposed according to the equation ... 

A later paper gave a detailed description of a granulation plant that produces urea-ammonium phosphate OJAP) grades, such as 28-28-0, 22-22-11, 18-18-18, etc. [40]. The main raw materials are crushed urea prills, spray-dried ammonium phosphate (12-50-0), and potash salts. One unusual feature of the plant is a methylene-urea reactor, which is used for production of some NPK... 

Melamine production from urea reactor and internals 450-500°C nitriding... 

Urea 28 355-360 179-182 S — plus 32.2 percent ammonia. 20.5 percent water. 19 percent CO . 0.3 percent inerts, plus air. In liquid urea reactor 3 ft. below top head. Alloy C = 17 mpy... 

Urea Reactor Mixture Mixture 193 < 1 58 Urea 17NH3 15CO2 10 H,0... 

World War II the technology of building layered vessels has improved substantially. Today layered vessels are used in a wide range of high-pressure applications in the petrochemical industry such as heat exchangers, urea reactors, ammonia converters, autoclaves, and coal gasification reactors. 

The application of the new (composite) materials can be combined with the concept of multilayer vessels (see Section 3.4.1). In the urea synthesis, the idea of multilayer vessels is often applied to the urea reactor due to its large dimensions. In doing so, the choice of the inner wall material can be focused more on corrosion... 

The Mitsui Toatsu Chemical (MTC) process was the prevailing process into the early 1970s and generally serves as a base for comparing new processes. The urea reactor operates at high pressure (250 atm). The reactor liquid effluent is let down in stages, and steam is used to decompose carbamate and strip the CO2 and NH3 in each stage. The CO2 and NH3 are absorbed in water for recycle to the reactor. 

The Snamprogetti process is based on using ammonia to strip CO2 and NH3 out of the urea reactor effluent. One of the impurities that affects the performance of urea as a fertilizer is biuret. To avoid a yield loss and minimize product contamination, it is important to minimize biuret formation. The con-... 

Off-gases from the quench system contain ammonia, carbon dioxide, and water vapor. The CO2 is removed as ammonium carbonate, and the ammonia is returned to the urea reactor. 

The design of the urea reactor is critical. The wall temperature must not be too high, or corrosion is a problem. This can be avoided by using stationary layers of catalyst along the inner wall, held in place by corrosion-resistant flights secured to the periphery at suitable intervals. With this... 

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