PROCESS FOR PREPARING MELAMINE FROM UREA

Abstract: 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 and in which a proportion of the concentrated aqueous carbamate solution from the absorption zone is returned to the cooling zone.

Description: PROCESS FOR PREPARING MELAMINE FROM UREA The invention rrla.es to a process for preparing meiamine 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 meiamine 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 meiamine in the presence of a catalyst In the process there is obtained a gas stream containing meiamine, 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 meiamine product stream and a vapour stream The vapour stream from the quench pipe is virtually free from urea and meiamine and consists essentially of ammonia, carbon dioxide and water vapour The aqueous meiamine 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 meiamine product stream is passed to the meiamine purification where the meiamine 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 meiamine purification in order to remove meiamine residues still present in the vapour stream This aqueous solution may contain ammonia carbon dioxide and meiamine 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 from the meiamine purification 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 meiamine purification, which liquor is passed to the quench pipe via the scrubbing section WO-96/20933 states that the water content of the carbamate solution from the absorption zone is so low, i e 20 - 35 % by weight, that a concentration step, in which water is removed from the carbamate solution, is not needed before the carbamate solution is supplied to a urea plant Experiments carried out by the applicant in accordance with the process described in WO-96/20933 indicate, however, that it is advantageous to remove water from the carbamate solution if the aim is to operate the combination of meiamine plant and urea plant in the most economical manner In a meiamine plant water is used among others as a component of the liquid coolant A proportion of the water eventually ends up in the carbamate solution from the absorption zone which is supplied to for example a urea plant Experiments and calculations by the applicant indicate that in the process according to WO-96/20933 the amount of water in the carbamate stream discharged to the urea plant is about 2 5 tonnes of water per tonne of meiamine In an economically optimum process, such as the Stamicarbon process described in Nitrogen No 139, Sep/Oct 1982, pp 32-39, in which the excess water is removed in a concentration step, the amount of water in the carbamate solution supplied to a urea plant is about 0 5 - 1 0 tonne of water per tonne of meiamine In the Stamicarbon process the gas mixture coming from the meiamine reactor is cooled with a liquid coolant in the quench columns The mixture of vapour, liquid and, possibly, solid matter, is separated in the quench columns into a vapour phase and a liquid phase The vapour phase is passed to an absorption zone and the liquid phase to the meiamine recovery The aforementioned tonnes of water per tonne of meiamine may be converted to a water concentration in the carbamate solution from the absorption zone, if the NH 3 /C0 2 ratio of the carbamate solution exported is determined If the plant according to WO-96/20933 is operated in an economically optimum manner, this ratio is minimum, for example 1 3 kg of NH 3 per kg of CO 2 This means that the water concentration in the carbamate solution from the absorption zone in the process according to WO-96/20933 is 45 - 50% by weight In the aforementioned Stamicarbon process this is 20 - 25 % by weight For supplying this 45 - 50 % by weight of water-containing carbamate stream to a urea plant it is economically attractive to further concentrate the carbamate solution by removing water from this solution The drawback hereof is that this entails additional investments and that the process becomes more costly due to increased usage of steam, cooling water and electricity It has been found that this drawback can be overcome by returning a proportion of the concentrated aqueous carbamate solution from the absorption zone to the cooling zone In particular, a proportion of the concentrated aqueous carbamate solution from the bottom of the absorption zone is returned to the cooling zone In WO-96/20933 this cooling zone is made up of the quench pipe and the scrubbing section, in the Stamicarbon process this cooling zone consists of the quench columns In both processes, the remainder of the concentrated aqueous carbamate solution from the absorption zone is supplied to for example a urea plant, preferably a high-pressure section of a urea plant, without any further processing In an embodiment of the invention the gas stream coming from the quench columns or the scrubbing section is cooled in a condenser ahead of the absorption zone Here, the gas stream is cooled by at least 5°C, preferably at least 10°C and in particular at least 15°C The dilute carbamate solution coming from the condenser ahead of the absorption zone is passed to the cooling zone In this condenser evolves a gas which is enriched with ammonia and carbon dioxide and which is passed to the absorption zone In the absorption zone then evolves a concentrated aqueous carbamate solution which contains less water and which is partly returned to the cooling zone The liquid coolant preferably consists of an aqueous carbamate solution composed of a proportion of the concentrated aqueous carbamate solution from the absorption zone to which mother liquor from the meiamine puπfication (backend section) may be added, and ammonia, carbon dioxide and water condensed in the cooling zone 20-40% by weight of the gas going to the absorption zone is returned in condensed form to the cooling zone In the specific case where the bottom product from the absorption zone is returned, this means that 30-70% by weight of the concentrated carbamate solution from the absorption zone is used for cooling the gas stream coming from the reactor and preferably 35-65% by weight The remainder is supplied to for example a urea plant but may also be used for other purposes such as a fertilizer plant or production of ammonia It has been found that in the process of the invention the water content of the concentrated carbamate solution from the absorption zone eventually amounts to 20-35% by weight This does render the proportion of this carbamate solution which is returned to a urea plant suitable for direct processing This means that the concentration step is superfluous Furthermore, it was found that the process of the invention is particularly suitable for so-called gas-phase meiamine plants operating at a pressure of 0 6-2 5 MPa, more particularly at pressures of between 0 7 MPa and 2 2 MPa The process of the present invention is particularly suitable for modifying existing meiamine processes such as those described in WO-96/20933 and the Stamicarbon process as described in the aforementioned Nitrogen publication The invention is illustrated by the following examples Examples I - III Meiamine was prepared in a cylindrical fluidized bed with an inside diameter of 1 metre 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 metered at the rate of 1 4 tonnes/hour with 0 7 tonne of ammonia per hour via the two-phase sprayers Ammonia was supplied through the fluidization plate at the rate of 0 7 tonne/hour The conversion of water-free urea to meiamine relative to equilibrium was higher than 91 %. The gas stream from the reactor contained NH 3 , CO 2 , meiamine vapour and traces of by-products and was cooled in the cooling zone with liquid coolant. A proportion of the concentrated aqueous carbamate solution from the absorption zone was returned to the cooling zone. The remainder of the concentrated carbamate solution was supplied to the adjacent urea plant. The proportion of the carbamate from the absorption zone that was returned to the cooling zone and the amount of water in the carbamate stream from the absorption zone are stated in Table 1. Comparative example A Analogously to Examples I - III, meiamine was prepared except that no carbamate from the absorption zone was returned to the cooling zone. The carbamate stream coming from the absorption zone was then too dilute for it to be supplied to a urea plant without an intermediate step. Refer to Table 1. Table 1

Claims: CLAIMS 1. Process for preparing meiamine 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, characterized in that a proportion of the concentrated aqueous carbamate solution from the absorption zone is returned to the cooling zone. 2. Process according to Claim 1 , characterized in that the gaseous product stream coming from the reactor is cooled with a proportion of the carbamate solution from the absorption zone, to which solution mother liquor from the meiamine purification and ammonia, carbon dioxide and water condensed in the cooling zone may be added. 3. Process according to Claims 1-2, characterized in that 30-70% by weight of the concentrated aqueous ammonium carbamate stream from the bottom of the absorption zone is used for cooling the gas stream coming from the reactor. 4. Process according to Claims 1-3, characterized in that the gas stream coming from the quench columns or scrubbing section is cooled in a condenser ahead of the absorption zone. 5. Process according to Claims 1-4, characterized in that the gas coming from the reactor has a pressure of between 0.6 and 2.5 MPa. 6. Process according to Claim 5, characterized in that the gas coming from the reactor has a pressure of between 0.7 and 2.2 MPa. 7. Technique for modifying existing meiamine plants by applying the process according to Claims 1-6. 8. Process as defined by the description and the example.

Inventors: DE WIT NORA ANNA, GULPERS ROB KASIMIER, DE WIT, NORA, ANNA, GULPERS, ROB, KASIMIER
Applicants: DSM NV, GULPERS ROB KASIMIER, WIT NORA ANNA DE, DE WIT, NORA, ANNA, DSM N.V., GULPERS, ROB, KASIMIER
Application year: 2001
Main classification: C07D251
IPC codes: C07D251