Urea solution recycling process

Fig. 2.2-1. Flow sheet of a conventional solution recycling process for urea manufacture. [Taken from Fromm, D. and Liitow, D., Modern Processes in the Heavy Chemicals Industry Urea , Chemie in unserer Zeit 13, 78-81 (1979).]...

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 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. ... 

The heat of condensation is recovered as 7.0 bar steam for use in downstream process steps. Urea solution leaving the IDR loop contains unconverted NH3, C02, and carbamate. These unconverted compounds are recycled to the synthesis loop108,110 ... 

The Isobaric Double-Recycle (IDR) urea process was developed by Montedison. In this process most of the unconverted material leaving the reactor is separated by heating and stripping at synthesis pressure using two strippers in series. The unconverted ammonia, C02, and carbamate in the urea solution are recycled to the synthesis loop.108110... 

Application To produce aqueous formaldehyde (AF) or urea formaldehyde precondensate (UFC) from methanol using Haldor Tbpspe A/S FK-Series iron/molybdenum-oxide catalysts. Description The process is carried out in a recirculation loop at low pressure (0 to 6 psig) (1 to 1.5 bar abs). Vaporized methanol is mixed with air and recycle gas that were boosted by the blower (1). The mixture may be preheated to about 480°F (250°C) in the optional heat exchanger (2) or it may be sent directly to the reactor (3). In the reactor, methanol and oxygen react in the catalyst-filled tubes to make formaldehyde. Reaction heat is removed by an oil heat transfer medium (HTM). The reacted gas exits the reactor at approximately 540°F (290°C) and is cooled in the LP steam boiler (4) to 260°F (130°C) before entering the absorber (5). In the absorber, the formaldehyde is absorbed in water or urea solution. Heat is removed by one or two cooling circuits (6, 7). From the lower circuit (6)... 

In the Stamicarbon process, the NH3/CO2 molar ratio in the reactor (operated at 140bar and 180-185°C) is 2.95. In this step, approximately 60% of C02 and 40% of NH3 are converted, leading to an NH3/C02 ratio of about 4.5 in the unreacted gas. The effluent solution from the reactor is stripped at high pressure and temperature with C02 (reestablishing the proper NH3/C02 ratio) and the gases are recycled to the reactor. The urea solution is further purified from residual NH3 and C02 and finally taken through an evaporation step yielding 99.7% urea melt. 

Even though this process is the simplest of the urea processes, it is the least flexible and cannot be operated unless some provision is made to utilize the large amount of off-gas ammonia. Thus, it must be used in conjunction with the coproduction of some other material, e.g., ammonium sulfate, ammonium nitrate, nitric acid, or ammonium phosphate, for which the ammonia can be used, One case in which the system can be used is the production of urea-ammonium nitrate solution the unconverted NH3 is used to make ammonium nitrate solution, which is then mixed with the urea solution. Even in this case, most new plants use a total-recycle processr-... 

The Mitsui Toatsu partial-recycle process flowsheet is typical of the first method and is shown in Rgure 9.2. Liquid NH3 and gaseous CO2 are pumped to the urea -reactoi aL20(X-atra.-The-temperature of the reactor is maintained at about 185°C by proper balance of excess NH3 and carbamate solution recycle feed. About 100%-110% excess NH3 is used about 70% of the NH3 and... 

The Allied Chemical—C.P.I. process, which was developed by T. O. Wentworth,is based on recycle of NH3 that is separated from the reactor effluent gases by scrubbing with monoethanolamine. The carbon dioxide is absorbed in the amine, and the gaseous ammonia is recycled to the reactor. (See Fig. 28.20.) The reactor effluent first is decomposed then the urea solution is removed and concentrated. A specially designed reactor... 

The first reaction is exothermic and goes almost to completion, while the second, endothermic, reaction converts less than 70% of feedstocks to urea. Consequently, the once-through process of urea synthesis had to be combined with production of other nitrogen compounds. Modern syntheses use a more expensive total recycle process in which all the unreacted CO2 and NH3 is separated from the effluent solution and returned to the reactor. Conversion rates range between 65 and 67% for each pass, and the overall reaction yield is on the order of 99%. Three commercial processes—Dutch Stamicarbon, Italian Snamprogetti, and Japanese Toyo—dominate today s world market for large (up to 2,000 t/day) urea plants using the total recycle process. ... 

Urea Technologies Inc. (Maxrovic), which offers the Heat Recycle Urea Process (HRUP), whose main feature is the hot recirculation of the aqueous carbamate solution, which enhances the energy balance. This is a conventional technique, operating at 20 to 22.10 Pa absolute. 190 to 200 C, N/C ratio s 4, and once througb CO2 conversion s 71 to 72 per cent... 

The unconverted ammonium carbamate and unreacted ammonia have to be removed from the reaction mixture which consists of an aqueous solution of urea, ammonium carbamate and ammonia. The numerous industrial processes particularly differ in the way in which this separation and the recycling of ammonia and carbon dioxide are carried out, the minimizing of the energy consumption of these large plants (up to 1700 t urea/d) being crucial. 

Since the amount of ammonium carbamate that can be absorbed in the absorber solution described above is limited by its solubility in the system H20-urea-NH3, part of the ammonia and carbon dioxide cannot be recycled and must be used in the production of a coproduct nitrogen material, As in the once-through process, the operation of the urea plant still must coincide with that of the coproduct plant. 

The dehydration of ammonium carbamate is appreciable only at temperatures above the melting point (about 150°C) and this reaction can only proceed if the combined partial pressure of ammonia and carbon dioxide exceeds the dissociation pressure of the ammonium carbamate (about 100 atmospheres at 160°C and about 300 atmospheres at 200°C). Thus commercial processes are operated in the liquid phase at 160—220°C and 180—350 atmospheres. Generally, a stoichiometric excess of ammonia is employed, molar ratios of up to 6 1 being used. The dehydration of ammonium carbamate to urea proceeds to about 50—65% in most processes. The reactor effluent therefore consists of urea, water, ammonium carbamate and the excess of ammonia. Various techniques are used for separating the components. In one process the effluent is let down in pressure and heated at about 155°C to decompose the carbamate into ammonia and carbon dioxide. The gases are removed and cooled. All the carbon dioxide present reacts with the stoichiometric amount of ammonia to re-form carbamate, which is then dissolved in a small quantity of water and returned to the reactor. The remaining ammonia is liquefled and recycled to the reactor. Fresh make-up ammonia and carbon dioxide are also introduced into the reactor. Removal of ammonium carbamate and ammonia from the reactor effluent leaves an aqueous solution of urea. The solution is partially evaporated and then urea is isolated by recrystallization. Ammonium carbamate is very corrosive and at one time it was necessary to use silver-lined equipment but now satisfactory alloy steel plant is available. Urea is a white crystalline solid, m.p. 133°C. 

There now are three prevailing process designs for urea manufacture, licensed by Stamicarbon, Mitsui Toatsu, and Snampro-getti. The main differences between these processes are in the methods used to handle the converter effluent, to decompose the carbamate and carbonate, to recover the urea, and to recover the unreacted ammonia and carbon dioxide for recycle, with the objective being minimum expenditure of energy and a maximum recovery of heat. In some processes, a liquid is used to recycle a solution of carbamate, carbon dioxide, ammonia, and water. In others, the amount of water recycled is minimized, and only carbon dioxide and ammonia are recycled. In the older plants, or once-through plants, the off-gases are used as feed to ammonium nitrate or ammonium sulfate plants. 

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