Abstract:
The invention relates to a method for the purification of melamine-containing gaseous NH 3 , by means of partial condensation, whereby pure NH 3  gas and liquid NH 3  enriched with melamine are obtained.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a National Phase Patent Application of International Application Number PCT/EP01/12691, filed on Nov. 2, 2001, which claims priority of Austrian Patent Application Number A 1888/2000, filed Nov. 8, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The application relates to a process for purifying gaseous melamine-containing ammonia. The inventive process is suitable in particular for purifying ammonia gases originating from high-pressure melamine plants using anhydrous melamine workup.  
         BACKGROUND  
         [0003]    In high-pressure processes for preparing melamine, urea is converted to melamine in an endothermic liquid-phase reaction. After removal of NH 3  and CO 2 , a melamine melt under high pressure is obtained which is then solidified in what are called wet workup processes by quenching with water. In what are called the dry processes, as described, for example, in U.S. Pat. No. 4,565,867, WO 95/01345, WO 97/20826 and WO 99/38852, the melamine melt is solidified by quenching with ammonia, by expanding the NH 3 -saturated melamine melt or a melamine/NH 3  suspension at a temperature just above the melting point of melamine, by sublimation with subsequent desublimation or by cooling in a fluidized bed. However, in these processes, sometimes very large amounts of gaseous NH 3  are produced which, depending on pressure and temperature of the gas produced, comprise greater or lesser amounts of melamine. This already CO 2 -free gas can be returned to the melamine plant only in part and sometimes only with difficulties. The melamine present in the NH 3  gas causes difficulties and faults, in particular in the compressors, if the NH 3  gas needs to be compressed for liquification. A need therefore exists to free melamine-containing NH 3  gas from melamine in a simple process.  
         SUMMARY OF THE INVENTION  
         [0004]    The invention is directed to a method comprising partly condensing the melamine-containing gaseous NH 3 . This produces firstly melamine-containing liquid NH 3  and, secondly, purified melamine-free NH 3  gas which is separated off from liquid NH 3 .  
           [0005]    More particularly the invention relates to a process for purifying melamine-containing gaseous NH 3 , which comprises partly condensing melamine-containing gaseous NH 3  by cooling, the melamine accumulating in the liquid NH 3  and the purified NH 3  gas being taken off. 
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0006]    A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing, wherein:  
         [0007]    [0007]FIG. 1 is a schematic view of an exemplary plant in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0008]    The melamine-containing gaseous NH 3  can be cooled using any cooling device, for example in a cooling column or by means of liquid NH 3 , liquid NH 3  being mixed with the NH3 to be purified, for example by spraying liquid NH 3  into the NH 3  to be purified or by passing the NH 3  to be purified through liquid NH 3 . Preferably, the melamine-containing gaseous NH 3  is passed through liquid NH 3 , in which case it cools and partly condenses. The resultant purified NH 3  gas is taken off and can if appropriate be condensed, and the melamine-enriched liquid NH 3  is discharged. The amount of liquid NH 3  discharged is preferably replaced by the condensing NH 3  gas, but it can also be replaced by feeding fresh NH 3 .  
         [0009]    The melamine-containing NH 3  gas to be purified is already CO 2 -free and preferably originates from dry melamine processes as set forth above, for instance from processes for solidifying liquid or gaseous melamine under NH 3  pressure or a melamine/NH 3  suspension using liquid supercritical or gaseous NH 3 . In the dry melamine processes the solidification is performed, for example, by quenching with NH 3 , with NH 3  and liquid or gaseous melamine being sprayed into a quencher. In an exemplary embodiment, the melamine-containing NH 3  gas to be purified originates from the fluidized bed of a plant for solidifying melamine, into which the liquid or gaseous melamine is introduced into a fluidized bed made up of solid melamine, or solid inert particles and solid melamine, and maintained using NH 3  gas.  
         [0010]    Preferably, the process is carried out continuously. The gaseous melamine-containing NH 3  is, for example, partly condensed using an air- or water-cooled cooler or via direct contact with liquid NH 3 . Preferably, the gaseous, melamine-forming NH 3  here is passed through liquid NH 3 , with melamine precipitating or dissolving and accumulating in the liquid NH 3 . Depending on from which plant for the dry melamine workup the gas to be purified comes, the gas can be of varying pressure and varying temperature and, as a function thereof, differing melamine content. The lower limit of temperature is preferably just above the respective condensation temperature at the respective pressure, and the upper limit at the operating temperatures of the plant section from which the NH 3  to be purified originates.  
         [0011]    If the melamine-containing NH 3  originates, for example, from a process for solidifying gaseous melamine using, or in the presence of, ammonia, the pressure is, for example, about 1-20 barand about 1.5-15 bar, respectively, and the temperature is, for example, about 290-520° C. If the melamine-containing NH 3  originates, for example, from a process for solidifying liquid melamine, lower temperatures down to room temperature are also possible, preferably from about 100° C. to just below the pressure-dependent melting point of melamine, for example from about 100 to 340° C., more preferably from about 200 to 320° C.  
         [0012]    Higher pressures are also possible up to about 500 bar, preferably from about 5 to 250 bar, more preferably from about 10 to 100 bar.  
         [0013]    Pressure and temperature in the partial condensation must be chosen in such a manner here that the NH 3  can be condensed using the coolant available. For example, when water is used as the coolant, the pressure shall not be less than 10 bar, in accordance with the ammonia condensation curve.  
         [0014]    The process can be preferably and particularly simply carried out in such a manner that the melamine-containing gaseous NH 3  is passed through liquid NH 3  and partly condensed. The NH 3  gas thus purified can then be taken off and the melamine-enriched liquid NH 3  can be discharged. The amount of melamine-containing liquid NH 3  discharged is continuously replaced by condensing NH 3 . However, it is also possible to feed in part fresh liquid NH 3 . The partial condensation of the gaseous NH 3  can take place either directly on its introduction into the liquid NH 3 , or else by condensation of the purified NH 3  continuously forming, for example using a downstream cooler.  
         [0015]    On passing the NH 3  to be purified through the liquid NH 3 , melamine-enriched liquid NH 3  is formed which, depending on pressure and temperature, contains greater or lesser amounts of melamine. It can be saturated with melamine or subsaturated, or else solid melamine can already have formed;in other words, melamine-containing liquid NH 3  is to be taken to mean not only a solution, but also a suspension, of melamine in NH 3 .  
         [0016]    In a continuous process in which the melamine-enriched NH 3  is continuously discharged via an overflow, the melamine-enriched liquid NH 3  is customarily recirculated to a suitable section of the melamine preparation process. This can be the reactor or a subsequent separator. Advantageously, the recirculation is into a device in which liquid melamine is allowed to stand under ammonia pressure (“aging”) or into a device in which already-solidified melamine is allowed to stand under ammonia pressure (“tempering”).  
         [0017]    Preferably, the melamine-containing liquid NH 3  is recirculated to a plant in which liquid melamine is quenched (solidified) under expansion using liquid ammonia or the gaseous ammonia formed on expansion of the liquid ammonia. Preference is likewise given to recirculating the melamine-containing liquid NH 3  to a plant in which a mixture of gaseous NH 3  and gaseous melamine is quenched and solidified using ammonia.  
         [0018]    In another embodiment, the melamine-containing liquid NH 3  is recirculated to a fluidized bed that comprises solid melamine, or solid melamine and solid inert matter, and is maintained by NH 3  gas.  
         [0019]    A possible plant for carrying out the inventive process is shown diagrammatically in FIG. 1. In the figure are shown the following:  
         [0020]    ( 1 ) fluidized bed,  
         [0021]    ( 2 ) pressure vessel,  
         [0022]    ( 3 ) cooler,  
         [0023]    ( 4 ) intermediate vessel,  
         [0024]    ( 5 ) pump,  
         [0025]    ( 6 ) melamine-enriched liquid NH 3 ,  
         [0026]    ( 7 ) gaseous melamine-free NH 3 ,  
         [0027]    ( 8 ) gaseous melamine-containing NH 3 .  
       EXAMPLE 1  
       [0028]    A melamine-containing gaseous NH 3  stream  8  of 424 kg/h coming from the fluidized bed  1  of a melamine solidification plant is passed, at a temperature of 280° C. and a pressure of 10 bar, from the bottom through the liquid NH 3  in a pressure vessel  2  equipped with a cooler  3  which is filled with liquid NH 3 . A portion of the NH 3  condenses and the melamine separates out in the liquid NH 3    6 . To maintain a constant liquid level in the pressure vessel, 384 kg/h of liquid melamine-containing NH 3    6  are discharged and recirculated to the melamine plant. The purified NH 3  gas  7  vaporizing in the pressure vessel partly condenses in the attached cooler, and the non-condensed portion is taken off as pure NH 3  gas  7  at a rate of 40 kg/h.  
       EXAMPLES 2-4  
       [0029]    In a similar manner to example 1, melamine-containing NH 3  gas was purified at various pressures and temperatures.  
         [0030]    The values for Examples 1 to 3 are listed in Table 1.  
                                   TABLE 1                                   NH 3  to be                   Temp.   Pressure   purified   Recirculated   Pure NH 3         Example   (° C.)   (bar)   (kg/h)   NH 3  (kg/h)   (kg/h)                   1   230   10   424   384   40       2   280   40   740   650   87       3   340   10   380   340   40