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Improving the melamine yield of
catalytic melamine production processes

Background of the invention

Today, the starting material for the production of melamine is mainly urea which decomposes first at temperatures > 350°C in accordance with reaction (1) into ammonia and isocyanic acid. The formed isocyanic acid reacts further after equation (2) to melamine:

6 H2N-CO-NH2 → 6 HNCO + 6 NH3 (1)

6 HNCO → C3H6N6 + 3 CO2 (2)

In an industrial scale both reactions are carried out either in non-catalytic high-pressure processes at temperatures of 390 to 400°C and pressures above 8 MPa or in catalytic low-pressure processes of 0,1 - 1,0 Pa in a fluidized bed reactor > 380°C. Examples of low-pressure catalytic processes are the DSM-process or the BASF-process. A detailed description of both processes is given NITROGEN, No. 228, pages 43 to 51, July/Augustl997 and in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 16, 5th Edit, pages 171 to 185 (1990).

It is in the nature of a fluidized bed reactor, which shows the characteristic of a stirred vessel, that the conversion per pass is not complete. It's true the urea is completely decomposed in accordance with equation (1) but the formed isocyanic acid will not entirely react to melamine. In practice, the urea conversion to melamine is in the range of 75 to 90% per pass depending on the fluidisation conditions and the activity of the catalyst. That means the process gas from the reactor contains more or less big amounts of un-reacted isocyanic acid, the thermodynamically possible equilibrium is not reached.

For understanding the invention on hand figure 1 shows the principle of a catalytic low pressure process: Liquid urea (11) is converted to melamine in a fluidized-bed reactor (12) at 390 to 400°C. For fluidizing gas (14) either pure ammonia or the NH3/CO2-mixture formed during the reaction is used. The

NH3 + HNCO → [NH4]NCO → H2N-CO-NH2 (3)

Disadvantages of the BASF-process are:

a) It is true the nonconverted isocyanic acid reacts in the urea washer again to urea and therefore there is no loss. But in the fluidized-bed reactor the energy for the decomposition of the whole urea feeding must be raised without a corresponding melamine yield in case of a low conversion rate.

b) Higher amounts of unreacted isocyanic acid in the process gas may cause the formation of undesirable by-products in the gas quench.

c) The outlet temperature of the gas-cooler must be adjusted at a tempera- ture level at which the higher molecular compounds in the process gas largely de-sublimates. On the other hand no melamine may de-sublimate, that means the melamine partial pressure in the process gas is limited by the outlet temperature of the gas cooler. This is a considerable economical disadvantage because due to the low melamine partial pressure allowed in the process gas high volumes of fluidizing gas are necessary to carry out the melamine out of the reactor.

Objective and Description of the Invention

The present invention is directed to the low and medium pressure catalytic processes for producing melamine from urea wherein the melamine is pro-duced in a fluidized bed reactor and recovered from the process gas either by a water quench or by a gas quench.

Primary object of the present invention is to avoid the economical disadvantages of the above mentioned catalytic processes, especially of the DSM- and BASF- processes, by increasing the urea conversion, re-converting higher mo-lecular by-products to melamine and removing at the same time catalyst fines from the process gas.

The aforesaid and other objectives for this invention are surprisingly accomplished by the installation of a filter-reactor, which increases the conversion of the isocyanic acid according to equation (2), re-converts higher molecular by -products to melamine and which removes simultaneously the catalyst fines in the process gas.

Particular characteristic of the filter-reactor is that it consists of one or more ring -reactors. The individual ring-reactor consists of two concentric cylinders with perforated walls. The annular space between the two cylinders contains the catalyst.

Figure 2a shows schematically the structural principle of a ring-reactor as used as an element in a filter-reactor, figure 2b shows a top view of a filter-reactor (7) with four ring-reactors (2) inside.

The hot process gas from the fluidized bed reactor is directly fed at (1) to the filter-reactor (7) which may consist of one or more ring-reactors (2).

The process gas flows radial through the outer perforated wall (3) of the ring-reactors penetrates the catalyst bed (4) and leaves the ring-reactor through its inner wall which is perforated as well. From the central pipe or gas sampler (5) of the filter-reactor the gas flows to the melamine separation unit.

The individual ring-reactors are gastight fixed at the upper part (6) of the filter reactor and can be exchanged easily if necessary. To keep the reaction temperature in the filter reactor at the required level it is sufficient to insulate the outer walls of the filter-reactor because the conversion of the isocyanic acid to melamine according to equation (2) is an exothermic reaction.

But to make it easier for start-up procedures, regeneration of the catalyst or i increasing the temperature in the filter-reactor above the temperature in the fluidized-bed reactor, it is recommended to use a filter-reactor (7) which is double-walled so that it can be heated by hot gases, for example.

Number and dimension of the individual ring-reactors within the filter-reactor are depending on the capacity and the conversion rate of the fluidized bed reactor and on the activity of the catalyst in the ring-reactor. To avoid vibration and to make easier an exchange of the individual ring-reactors, a length between 1000 and 4000 mm is recommended, although in special cases other dimensions may be optimal. For practical reasons, the outer diameter of the ring-reactor is chosen between 200 and 900 mm, the diameter of the inner cylinder should be in the range 150 to 300 mm. But here too other dimension can be used, for example, for process-technical reasons or physical conditions.

On basis of the big cylindrical outer surface and the radial flow of the process gas through the catalyst bed the catalyst is optimally used and the pressure drop over the catalyst bed is minimized. For the same reason catalyst dust in the process gas is deposited only on the outer surface of the catalyst bed and can be easily removed by periodical back-flushing of the individual ring-reactors via valve (8) with hot ammonia, fluidizing gas or steam. Steam regeneration also removes organic deposits on the catalyst. The catalyst dust can be discharged at the bottom of the filter-reactor.

The isocyanic acid, still contained in the process gas, reacts to melamine according to equation (2). Melem and other higher condensed nitrogen com-pounds react on the catalyst with ammonia back to melamine, for example:

C6H6Nιo + 2 NH3 → 2 C3H6N6 (4)

On basis of these reactions the gas leaving the individual ring-reactors contains only very little amounts of isocyanic acid and the amount of melem is so far reduced that its content in the final melamine product is below 100 ppm.

Suited catalyst for the filter-reactor are all catalysts used or known for the production of melamine, consisting of pure y - Al203, alumina - silicates with 10 to 90% SiO2, or pure SiO2. In particular suited are catalysts which contains in a matrix of alumina silicate 20 to 80% molecular sieves with a mean pore size diameter above 6 Angstrom.

It is important that the installed catalysts have a high attrition resistance to avoid new formation of catalyst dust in the ring -reactors. For this reason shaped catalyst are preferred instead of irregular grains. Regular shaped catalysts also have the advantage of lower pressure drop within the individual ring-reactors.

The catalysts in the filter-reactor can have the same chemical composition as the catalyst in the main reactor. But it is also possible to use quite different catalysts with respect to chemical composition, acidity or surface area in the filter-reactor, because in the filter-reactor only the reaction of isocyanic acid to melamine and the re-conversion of the higher molecular nitrogen compounds to melamine must be catalysed.

If, for example, the fluidized-bed main reactor contains a catalyst of pure alumina, it can be advantageous to use silica-alumina catalyst in the filter-reactor.

Also the filter-reactor can be operated at the same pressure as the fluidizing-bed main reactor or it can be run at a lower pressure.

It is possible to combine this process with the process according to EP Applica-tion 04 000 931.8 in which the process gas containing the melamine is cooled by contacting it with an organic liquid, preferably polyalcohols or aminoalco-hols. Preferred organic liquids are ethyleneglycol, propyleneglycol, glycerol, methyldiethanolamine, mono-, di-, or triethanolamine or mixtures thereof. This step avoids the contact of the melalamine with water, which partly is hydrolysed again.

With the following examples and figure 3 a preferred embodiment of the process is described in more detail.

Example 1

In the fluidized-bed reactor (1) urea is decomposed at 400°C and 3 bar. The catalyst consists of pure Y - alumina. As fluidizing gas the reaction gas obtained at the formation of melamine from urea is used, a mixture of about 70 vol.% ammonia and 30 vol.% carbon dioxide. The gas coming from the fluidized-bed reactor is directly fed to the filter-reactor (2). The filter- reactor contains four ring-reactors (3), (for example only) with an outer diameter of 600 mm each and a length of 4000 mm. The catalyst in the ring-reactors is a spherical alumina -silicate catalyst (d = 4,5 mm), modified with 32 weight-% of a rare earth metal oxide.

Temperature and pressure in the filter-reactor is the same as in the fluidized-bed reactor.

To avoid that too much catalyst fines are entrained with the fluidizing gas, for example in case of a disturbance of the fluidized-bed reactor, a cyclone (5) may be installed between the fluidized-bed reactor and the filter-reactor.

Table 1 shows the gas composition before and after entering the filter-reactor (without inert components).

Table 1
Component Before Filter-Reactor After Filter-Reactor
(kmol/h) (kmol/h)

Ammonia 1277,35 1277,30
Carbondioxide 567,32 567,11
Melamine 35,03 37,63
Isocyanic Acid 18,77 3,19
Melem 0,16 >0,01 About 83% of the isocyanic acid has been converted to melamine in the filter-reactor. This corresponds to a daily surplus production of 7,8 t of melamine. Melem and other higher molecular nitrogen compound could not be detected any more, as well the gas was free of catalyst dust.

After leaving the filter - reactor the process gas can be cooled down to generate high-pressure steam in the cooler (4). The extent of the cooling is limited by the partial pressure of the melamine in the gas after the filter reactor.