Total-Recycle Urea Process

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. 

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

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). 

Preparation. The current basic comml procedure, the once-through process (Fig 1), uses liq ammonia and liq carbon dioxide at 1750 to 3000psiand 160—200°. These constituents react to form amm carbonate which is decompd at about 80psi to urea and w. Several variations of the process include a partial recycle process and, the most commonly used technique, the total recycle process. Fig 2 is a diagram of the latter process as adapted for use by the Chemical... 

The processes of the following licensors are worth mentioning among the remaining total-recycle processes for urea synthesis ... 

Montedison, which originally participated in the Momecaiini-Fauser joint venture to market a number of total-recycle variants, and now presents its IDR (Isobaric Double Recycle process) technique. TTiis process operates at 20.10 Pa absolute, around 190 to 200, with an N/C ratio of4 to 5 in the reactor. Unconverted carbamate is decomposed in two successive gas stripping steps, one with ammonia, and the second with CO 2. in the pressure conditions of urea s nthesis, i.e.. 10 Pa absolute. 

Derivation Liquid ammonia and liquid carbon dioxide at 1750-3000 psi and 160-200C react to form ammonium carbamate, NH4C02NH2, which decomposes at lower pressure (about 80 psi) to urea and water. Several variations of the process include once-through, partial recycle, and total recycle. 

It is worth mentioning that both processes employ total recycle of C02 and NH3 and very efficient methods of heat recovery. As a consequence, discharge of chemicals into the environment is kept at a very low level, characterized by 1 ppm urea and 1 ppm... 

Mole Ratio of NH3iC02 - Excess NH3 above the stoichiometric mole ratio of 2 favors the rate of the reaction. The percentage of CO2 converted to urea is increased but, of course, the percentage of NH3 converted to urea is decreased. Because recycling of excess NH3 is relatively simple (as compared with that of. CO2, which remains as carbamate), most processes use 50% or more excess ammonia (a mole ratio of 3 1 or more). Today all processes account for a balance of both CO2 and NH3 conversion to reduce total recycling to a minimum plantwide (i.e., reactor and synthesis). 

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-... 

Figure 3.8 Block flow diagram of conventional total recycle process for urea synthesis with consecutive production of solid urea.

However, the handling of the very corrosive ammonium carbamate represents a major challenge [16]. In addition, the aqueous recycle increases the water concentration in the synthesis part, thus impeding the urea formation (see Section 3.3.1). For this reason, the total recycle process is often operated at high molar NH3 CO2 ratio up-to 4—5 to increase carbon dioxide conversion and minimize the aqueous recycle [13]. 

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. ... 

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