Patent EA-003323-B1 (EA)
Title: Method for preparing melamine
Inventor: TJIOE TJAY TJIEN
IPC: B01J19/30
Filed: 1999-06-10
Abstract: 1. Method for preparing melamine by causing urea to react in the melt, the resulting melamine melt being separated from gaseous products formed in the reaction, said melamine melt then being treated in a column with ammonia in order to remove the carbon dioxide dissolved in the melamine melt, the amount of ammonia used in the column being between 0.02 and 3 tonnes per tonne of melamine, the column being operated at a pressure of between 1 and 40 MPa, the temperature in the column being between the melting point of melamine at prevailing pressure and 450 degree C., and the residence time of the melamine melt in the column being between 1 minute and 10 hours, characterized in that said column is a bubble column filled with a packing or divided into a plurality of compartments or provided with both packed regions and a plurality of compartments, and that the bubble column has a liquid hold-up of at least 35%, said liquid hold-up being defined as: liquid hold-up =(dynamic liquid volume in desorption zone) / volume of column 2. Method according to claim 1, characterized in that the bubble column is provided with a packing. 3. Method according to claim 2, characterized in that the packing has a specific surface area of between 10 and 3000 m/m. 4. Method according to claim 1, characterized in that the treatment with ammonia takes place in a bubble column having from 2 to 100 compartments. 5. Method according to claim 4, characterized in that the compartments are situated next to one another and are separated by 1 or more partitions with optionally 1 or more perforations in the plates. 6. Method according to any one of claims 4-5, characterized in that the compartments comprise separate pipes which are entirely or partially filled with melamine melt. 7. Method according to claim 4, characterized in that the compartments are situated one below the other and are separated by 1 or more plates positioned virtually horizontally, the plates optionally having 1 or more perforations. 8. Method according to any one of claims 5-7, characterized in that the number and the size of the perforations are chosen such that the gas velocity through the holes which are designed for the passage of gas is between 0.2 m/s and 20 m/s. 9. Method according to any one of claims 5-8, characterized in that the size of the holes, which are designed for the passage of gas is between 0.5 mm and 100 mm. 10. Method according to any one of claims 5-9, characterized in that the total area of the perforations, which are designed for the passage of gas, is between 0.02% and 30% of the column cross-section. 11. Method according to any one of claims 1-10, characterized in that the ammonia gas throughput and the column diameter are chosen such that the superficial gas velocity based on the total column cross-section is between 0.001 and 0.2 m/sec. 12. Method according to any one of claims 1-11, characterized in that the melamine melt in the bubble column is cooled, at the same time, to a temperature between the incoming temperature and the crystallization temperature of the melamine melt. 13. Method according to claim 12, characterized in that the cooling is effected using ammonia gas having a temperature of between 150 degree C. and 350 degree C. 14. Method according to any one of claims 1-13, characterized in that the amount of oxygen-containing compounds in the melamine end product is less than 0.7 wt %. 15. Method according to any one of claims 1-14, characterized in that the amount of melamine content of the end product is less than 1.5 wt %. 16. Method according to any one of claims 1-15, characterized in that the melamine content of the end product is greater than 98.5 wt %.