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The raw materials for producing hexamine are formaldehyde and ammonia. There are liquid phase and gas phase two different production processes. Liquid phase method is to use 37% formaldehyde water solution to react with ammonia gas. The formed hexamine solution is dehydrated through evaporation, then, centrifuged, dried to receive final product. Certain amount of steam is consumed in the vaporization, concentration and drying process, therefore, the energy consuming is high. The advantage of this process is mature technology, simple operation. Because evaporation port is used in this process, with relative long crystallization time, the product particle size is bigger, which is more welcomed by customers.
Gas phase method of hexamine process is that the methanol is firstly converted into formaldehyde gas, then, the formed formaldehyde is directly sent to the ammoniation reactor to react with ammonia gas in the saturated hexamine mother liquid to form hexamine product. The feature of this process is to utilize formaldehyde latent heat and hexamine reaction heat. Under vacuum condition, the water is evaporated, therefore, large amount of steam is saved and high purity product received.
1.Liquid phase production process
Liquid phase production process is that 37% of formaldehyde water solution is reacted with ammonia gas to form hexamine and water. It is exothermic reaction. The reaction equation is as following:
6CH2O(liquid) + 4NH3£¨gas£©¡ú£¨CH2£©6N4 + 6H2O + 81Kcal/mol
The process is divided into formaldehyde condensation reaction with ammonia, hexamine evaporation, concentration, centrifuging and drying.
Ammonia gas, after filtration and liquid formaldehyde are mixed in certain proportion (ammonia excess 0.8-1.2%£©and sent to the reactor. The reaction product is 24-27% hexamine; the heat generated from the reaction is transferred out by circulation cooler. Control reaction temperature at 60-65¡æ range. Hexamine solution formed in the reaction is sent to film evaporator by vacuum to be concentrated to 60-65%. The moisture is condensed by the condenser; whole process is under vacuum condition.
The concentrated liquid is sent to the evaporator port for continuous dehydration and concentration under vacuum condition, when crystal grains are formed, sent to the centrifuge to separate mother liquid. The solid materials are sent to the gas flow dryer. After cyclone separation, final product hexamine is received.
Evaporation is continuous process, while, evaporation port operation is batch form with high labor intensity and more waste water in the liquid phase reaction process. Because liquid formaldehyde is used and concentration process is a batch form, impurities are brought in and more side reactions occurred, which has bad impact to the product purity, therefore, mother liquid filter must be set in the process line.
2.Gas phase method production process
Formaldehyde and ammonia gas has condensation reaction in the alkaline solution condition in gas phase method production process.
Reaction equation is like this£º
6CH2O(gas) + 4NH3£¨gas£©¡ú£¨CH2£©6N4 + 6H2O +745.29¡Á103Kcal/mol
According to the reaction equation, 6 mole of formaldehyde reacted with 4 mole of ammonia to form one mole Hexamine and 6 mole of water, and generate 745.29 ¡Á 103Kcal/mol heats. In order to promote the reaction to proceed towards hexamine forming direction and avoid side reactions to affect product quality and increase consumption, reaction temperature must be carefully controlled and excess ammonia must be assured. In other words, free ammonia must exist in the reactant solution, in this way to avoid reverse reaction occurrence and resist trimethylamine forming. Trimethylamine has stink odor and bad impact to the product purity. |
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Flow chat of gas phase method production process
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Gas phase method of hexamine has short processing line with less side reactions. High purified products are received in this process. Recover excess ammonia gas and methanol can eliminate environmental pollution and decrease consumption. Heat energy is comprehensively utilized; production is continuously run with high automation.
Production Process£º
(1)High temperature formaldehyde gas from oxidation is directly sent to the ammoniation reactor by vacuum, ammonia gas after filtration and measuring is also sent to the ammoniation reactor. Formaldehyde gas and ammonia gas are bubbled in the mother liquid to react each other to form hexamine.
(2)Use formaldehyde latent heat and hexamine reaction heat under vacuum condition to evaporate water. Reaction temperature is controlled at 75-80¡æ.
(3)The formed hexamine in the ammoniation reactor is sent to the receiving tank through sealing port. The mother liquid is returned to ammoniation reactor, while concentration liquid is sent to the centrifuge to separate liquid phase.
(4)Solid hexamine is sent to the dryer for further dehydration. After that, final product is received
(5)Excess ammonia along with the evaporated gas from reactor is sent to the ammonia absorption tower for absorption. Then, condensation and vacuumed, finally, the off-gas after scrubbing enter to incinerator for burning.
(6)The diluted ammonia solution is sent to ammonia stripper tower to recover ammonia, then, it returns to ammoniation reactor; the diluted methanol after methanol recovery in the recover tower is returned to evaporator.
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