Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4↔H2N—CO—NH2+H2O
The extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone.
In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.
Different urea production process exist. These processes, and by analogy the plants in which these processes are conducted, generally provide for the following stages: synthesis, recovery of unreacted starting materials, downstream processing, and finishing. Thereby synthesis and recovery sections are applied that are connected with each other so as to form a synthesis loop, whereby starting materials (ammonia and carbon dioxide, particularly in the form of ammonium carbamate) are recovered and recycled back to synthesis stage. The output of the synthesis loop is generally a purified aqueous urea stream, having a concentration of 50 wt. % urea or higher, generally up to 75-80 wt. % before said stream is subjected to final concentration step(s).
The downstream processing generally refers to one or more sections, zones, or units in which the aforementioned aqueous urea stream is further concentrated. Such further concentration is typically conducted by evaporation, and the concentration section is frequently referred to as an evaporation section.
The output of the concentration section is a concentrated urea aqueous stream that is often referred to as a urea melt. This melt is suitable to be converted in a urea finishing section into a solid urea product. The urea melt typically has a urea concentration of greater than 90 wt %, preferably greater than 95 wt %, such as greater than 97 wt %. From urea finishing generally a gas stream is emitted that comprises ammonia. In order to prevent ammonia emissions, modern urea production plants comprise an ammonia-abatement section (also known as an ammonia-removal section), such as an ammonia-neutralizing section. Such a neutralizing section typically comprises one or more acid scrubbers.
One of the challenges in urea production concerns controlling the amount of biuret formed as a by-product, and generally present in urea products such as prills or granules. Biuret is dimer of urea, and is formed under release of ammonia. The amount of biuret is an indicator of the urea quality as can be sold. Typically, a worldwide standard specification for biuret in urea products, is below 1 wt. %. E.g., for fertilizer purposes, the amount of biuret is generally below 0.9 wt. %. For other applications, such as the use of an aqueous urea solution in a unit for the reduction of NOx in diesel exhaust gases (particularly known as Diesel Exhaust Fluid, traded as AdBlue®), the biuret content is required to be still lower.
In urea plants operating on the basis of old, once-through technology the formation of biuret is not a significant problem. Modern plants, such as urea stripping plants, however tend to result in a higher amount of biuret formed. It remains desired to better control biuret production.
An additional problem is that it is more difficult to produce urea according to desired biuret specifications, in the event that the plant in which the urea is produced, is not operated on full capacity. Generally, biuret levels are guaranteed for a plant operating at full capacity. In practice, this means that manufacturers operating their plants at reduced capacity, run a risk that the products produced do not meet specifications for all end-uses. It would be desired to provide a urea manufacturing process, and a plant suitable for such process, that allows controlling biuret formation also in the event that the plant in which the urea is produced is operated at a reduced capacity. Further, it would be desired to provide a method of controlling biuret formation that can be implemented in an existing urea production plant without substantive, expensive modification of such a plant.
U.S. Pat. No. 3,211,788 discloses a method for the production of solid urea from anhydrous urea melt, and aims at retaining and transferring the molten urea at minimum biuret formation. According to that process an anhydrous solution of ammonia and urea is formed at the point where anhydrous urea melt is formed by evaporation of aqueous urea solution from a synthesis process. Thereto a stream of the anhydrous melt, having a temperature in the range of 135-145° C., and a pressure preferably above 200 psi, is fed to the ammoniator. An ammonia feed stream is fed to the ammoniator as well. The balance of undissolved ammonia is removed along the top of the ammoniator. Such removal along the top of the reactor can only take place provided the ammonia is in the gas phase. The ammonia-urea solution is removed along the bottom of the ammoniator and passed to solidification means, which may be physically located at a considerable distance. The fact that the urea melt is transferred to the solidification means in the form of an ammonia-urea solution permits minimizing biuret formation during such transfer.
GB959.358 discloses a process for producing urea prills which permits to reduce biuret formation. According to the process of GB959.358, urea containing degasified reactor effluent is passed from the primary purification zone to a second purification zone wherein the effluent is heated under specified conditions of temperature and pressure. Biuret formation is minimized by maintaining the urea melt in the conversion zone under an ammonia pressure of 10-100 atm., and a temperature of 272-375° F. for a period of time sufficient to achieve equilibrium between the ammonia-biuret-urea, to obtain a melt containing 0.1-0.3 wt. % biuret, which is then passed to the prilling zone. The biuret concentration is further reduced by contacting the urea melt with an ammonia containing gas at a temperature above the melting point of pure urea, for a period of time sufficient to achieve equilibrium between the reacting ammonia and biuret and urea.