Process for preparing melamine from urea

#The invention relates to a process for preparing melamine from urea at elevated temperature and in the presence of a catalyst, in which a gaseous product stream is obtained which is contacted with a liquid coolant in a cooling zone.

#A similar process is disclosed in for example WO-96/20933. This describes the preparation of melamine by supplying urea and ammonia to a reactor at a pressure of between 1.4 MPa and 2.0 MPa and a temperature high enough for virtually complete conversion of urea into melamine in the presence of a catalyst. In the process there is obtained a gas stream containing melamine, ammonia and carbon dioxide. In WO-96/20933 this gas stream is cooled with an aqueous coolant in what is known as a quench pipe with evolution of a vapour-liquid mixture, which mixture is virtually free from solid constituents. This vapour-liquid mixture is separated in this quench pipe into an aqueous melamine product stream and a vapour stream. The vapour stream from the quench pipe is virtually free from urea and melamine and consists essentially of ammonia, carbon dioxide and water vapour. The aqueous melamine product stream is virtually free from solids and contains dissolved ammonia and carbon dioxide. After the dissolved ammonia and carbon dioxide are removed with the aid of steam in a stripping section, the aqueous melamine product stream is passed to the melamine purification where the melamine is recovered. In this stripping section evolves also a vapour stream consisting essentially of ammonia, carbon dioxide and water vapour. The vapour stream from the quench pipe, together with the vapour stream from the stripping section, is scrubbed in a scrubbing section with an aqueous solution (mother liquor) from the melamine purification in order to remove melamine residues still present in the vapour stream. This aqueous solution may contain ammonia, carbon dioxide and melamine. The quench pipe and scrubbing section make up the cooling zone in the process according to WO-96/20933. Next, the gas stream from the scrubbing section is passed to an absorption zone where it is contacted with an aqueous ammonia stream and liquid ammonia, in which process there is obtained a solution of concentrated aqueous ammonia and carbon dioxide (carbamate solution) and ammonia vapour virtually free from water and carbon dioxide. In WO-96/20933, this ammonia vapour is condensed and partly returned to the absorption zone, the remainder after evaporation being used as fluidization gas for the reactor. The aqueous solution from the scrubbing section is passed to the quench pipe and used as coolant there.

#The concentrated aqueous carbamate solution from the absorption zone, which WO-96/20933 reports contains 20-35% by weight of water, is for example supplied to a urea plant. Thus, in WO-96/20933 the gas mixture coming from the reactor is cooled with the mother liquor from the melamine purification, which liquor is passed to the quench pipe via the scrubbing section.

#Melamine was prepared in a cylindrical fluidized bed with an inside diameter of 1 meter and a height of 15 m. The catalyst was fluidized by introducing ammonia through a gas distribution plate and was heated by heat exchanger tubes in the reactor through which molten salt flowed. Liquid urea was sprayed into the reactor with the aid of a two-phase sprayer using ammonia as atomizing gas. The reactor was operated at 390° C. and a total pressure of 0.7 MPa (Example I), 1.7 MPa (Example II) and 2.0 MPa (Example III). Urea was sprayed at the rate of 1.4 tons/hour with 0.7 ton of ammonia per hour via the two-phase sprayers. Ammonia was supplied through the fluidization plate at the rate of 0.7 ton/hour. The conversion of water-free urea to melamine relative to equilibrium was higher than 98%. The gas stream from the reactor contained NH3, CO2, melamine vapour and traces of by-products and was cooled in the quench pipe with liquid coolant. The water content of the liquid coolant was reduced by cooling a dilute aqueous carbamate solution from the scrubbing section. The temperature in the scrubbing section was lowered here to the temperature stated in Table 1. This cooling was effected in a heat exchanger with the aid of cooling water whereupon the cooled liquid was returned to the scrubbing section. The carbamate stream coming from the absorption zone was supplied to the adjacent urea plant direct. The water concentration in this carbamate stream is given in Table 1.

#Analogously to Examples I-III, melamine was prepared except that the temperature in the scrubbing section was not lowered. The carbamate stream coming from the absorption zone was too dilute for it to be supplied to a urea plant without an intermediate step. Refer to Table 1.

#TABLE 1 Example I II III A Pressure in Mpa 0.7 1.7 2.0 1.7 Temperature in scrubbing 110 130 135 158 section in ° C. Water content of carbamate from 29 26 25 49 absorption zone in % by weight Amount of exported water 0.95 0.77 0.74 2.5 in kg per kg of melamine