JP2007517818A - Method for producing catalytic melamine to improve melamine yield - Google Patents

Abstract

A method for increasing melamine yield and improving dust separation in melamine production from urea in a fluidized bed catalytic process comprising fluids containing melam, melem and other polymeric nitrogen compounds in addition to melamine unconverted isocyanate The bed reactor process gas is transferred to a filtration reactor consisting of one or more annular reactors packed with catalyst, where unconverted isocyanic acid is converted to melamine, and polymeric nitrogen compounds, particularly melam and A process characterized in that melem is reconverted to melamine by reacting with ammonia in the process gas to remove catalyst fines still present in the fluidized bed reactor process gas.

Description

  Currently, the raw material for melamine production is mainly urea, which decomposes into ammonia and isocyanic acid at a first temperature of> 350 ° C. in reaction (1). The formed isocyanic acid reacts to further melamine after formula (2).

6H ₂ N-CO-NH ₂ → 6 HNCO + 6NH ₃ (1)
6HNCO → C ₃ H ₆ N ₆ + 3 CO ₂ (2)
On an industrial scale, both reactions are either non-catalytic high pressure processes at 390-400 ° C. and about 8 MPa, or catalytic low pressure processes at 0.1-1.0 MPa in a> 380 ° C. fluidized bed reactor. Done in either. Examples of low pressure catalytic processes include DSM processes or BASF processes. A detailed description of both methods is given in NITROGEN, No. 228, p 43-51, July / August l997 and Ullmann's Ency-clopedia of Industrial Chemistry, Vol. A 16,5t Edit.p 171-185 (1990). ing.

  Naturally, fluidized bed reactors exhibit the characteristics of a stirred vessel and the conversion per pass is not perfect. Urea is sure to decompose completely according to formula (1), but the isocyanate formed will not react entirely to melamine. Indeed, the conversion of urea to melamine is in the range of 75-90% per pass, depending on fluidization conditions and catalyst activity. This means that the process gas from the reactor, to varying degrees, contains a large amount of unreacted isocyanate and has not reached a thermodynamically possible equilibrium.

For the understanding of the present invention, the principle of the catalyst low pressure process is shown in FIG. 1 at hand. Liquid urea (11) is converted to melamine in a fluidized bed reactor (12) at 390-400 ° C. As the flowing gas (14), either pure ammonia formed during the reaction or an NH ₃ / CO ₂ mixture is used. The formed melamine is discharged from the reactor with process gas (= fluid gas + reaction gas) together with unconverted isocyanate and by-products such as melam (C ₆ H ₉ N ₁₁ ) and melem (C ₆ H ₆ N ₁₀ ) Is done. In addition to these compounds, the process gas contains jet catalyst fines that are not separated in the reactor cyclone.

  To separate melamine from the hot process gas, the gas can be cooled with water at line (15) in FIG. 1, as practiced with the DSM method. An aqueous suspension of melamine, polymeric by-products, oxotriazine and catalyst dust is formed. From this suspension, melamine is obtained by re-dissolution, filtration of insoluble compounds and recrystallization. For details of this process, see Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 16, p 176, (1990).

One drawback of this process is that, by cooling with water, unconverted isocyanic acid is hydrolyzed to NH ₃ and CO ₂ , and catalyst by-products and polymer by-products such as melem must be separated by filtration. In addition, oxotriazine is formed by hydrolysis of melamine.

  In the BASF process, these disadvantages are avoided by cooling the hot process gas in an extended cooler (16) that depolymerizes polymer by-products such as melam and melem, but not melamine. . Most of the desublimated compound is deposited on the catalyst dust and is removed together with the hot gas filter (17).

  Melamine purified by such a method from the process gas is desublimed in a gas quench (18) and separated from the gas in a cyclone (19). The almost melamine-free gas from the cyclone (19) is cooled by washing with liquid urea in the scrubber (20), as a cooling gas to the gas quench (18), and the reactor (12). Recycled as fluid gas to. Excess gas is treated in the separation gas unit.

In the urea scrubber, the unconverted isocyanic acid is remixed with urea along with ammonia according to a Wohler-Reaction.
NH ₃ + HNCO → [NH ₄ ] NCO → H ₂ N-CO-NH ₂ (3)

The disadvantages of the BASF method are
a) Since unconverted isocyanic acid reacts again with urea in the urea scrubber, there is certainly no loss. However, in the fluidized bed reactor, the energy for the decomposition of the total urea fed must be high regardless of the melamine yield at low conversion rates.
b) A significant amount of unreacted isocyanic acid in the process gas may form unwanted by-products in the gas quench.
c) The temperature of the outlet of the gas cooler must be adjusted to a temperature level at which the majority of the polymer compounds in the process gas are desublimated. On the other hand, since melamine does not sublimate, it means that the melamine partial pressure in the process gas is limited by the temperature of the outlet of the gas cooler. This is a considerable economic disadvantage. This is because the melamine partial pressure allowed in the process gas is low, so that a high volume of flowing gas is required to discharge melamine out of the reactor.

  The present invention is directed to a low and moderate pressure catalytic process for the production of melamine from urea, wherein the melamine is produced in a fluidized bed reactor and is processed by either water quench or gas quench. It is recovered from the gas.

  The main objective of the present invention is to increase the urea conversion, reconvert polymer by-products, and simultaneously remove catalyst fines from the process gas, thereby reducing the economics of the catalytic processes described above, particularly the DSM and BASF processes. It is to avoid the drawbacks.

  The above and other objects of the present invention surprisingly increase filtration of isocyanic acid according to formula (2), reconvert polymeric by-products to melamine, and at the same time remove catalyst fines from the process gas. This is achieved by installing the reactor.

  A special feature of a filtration reactor is that it consists of one or more annular reactors. Each annular reactor consists of two concentric cylinders with perforated walls. The annular space between the two cylinders contains a catalyst.

  FIG. 2a shows an overview of the structural principle of the annular reactor used as an element in the filtration reactor, and FIG. 2b shows the filtration reactor (7) with four annular reactors (2) inside. A plan view is shown.

The hot process gas from the fluidized bed reactor is fed directly (1) to a filtration reactor (7) which may consist of one or more annular reactors (2).
Process gas flows radially through the outer perforated wall (3) of the annular reactor through the catalyst bed (4) and exits the annular reactor through its similarly perforated inner wall. From the central pipe or gas sampler (5) of the filtration reactor, gas flows to the melamine separation unit.

  Individual annular reactors are hermetically fixed to the top (6) of the filtration reactor and can be easily replaced if necessary. Since the conversion of isocyanic acid to melamine according to formula (2) is an exothermic reaction, the outer wall of the filtration reactor is sufficiently insulated to keep the reaction temperature in the filtration reactor at the required level.

  In addition to facilitating the start of the process, in order to regenerate the catalyst or to raise the temperature of the filtration reactor above the temperature in the fluidized bed reactor, for example, it can be heated with hot gas. It is recommended to use a heavy wall filtration reactor (7).

  The number and diameter of the individual annular reactors in the filtration reactor depends on the capacity and conversion rate of the fluidized bed reactor and the catalytic activity in the annular reactor. To avoid vibrations and facilitate the exchange of individual annular reactors, a length of 1000 to 4000 mm is recommended, and in special cases other diameters can be optimized. For practical reasons, the outer diameter of the annular reactor should be selected from 200 to 900 mm and the diameter of the inner cylinder should be in the range of 150 to 300 mm. However, other diameters can be used, for example, due to the technical or physical conditions of the process.

  Based on the large cylindrical outer surface and the radial flow of process gas through the catalyst bed, the catalyst is optimally used and the pressure drop across the catalyst bed is minimized. For similar reasons, dust in the process gas is deposited only on the outer surface of the catalyst bed and periodically backs hot ammonia, flowing gas or steam to individual annular reactors via valves (8). It can be easily removed by flashing. Also, by regenerating the vapor, organic deposits on the catalyst are removed. The catalyst dust can be discharged from the bottom of the filtration reactor.

Isocyanic acid still contained in the process gas reacts with melamine according to equation (2). Melem and other polymeric nitrogen compounds react with ammonia on the catalyst back to melamine. For example,
C ₆ H ₆ N ₁₀ + 2NH ₃ → 2C ₃ H ₆ N ₆ (4)
Based on these reactions, the gas remaining in the individual cyclic reactors contains only a very small amount of isocyanic acid, and the amount of melem is reduced to a level of less than 100 ppm in the final melamine product.

Suitable catalysts for the filtration reactor are all the catalysts used or known for the production of melamine, pure y-AI ₂ O ₃ , 10-90% SiO ₂ alumina-silicate or pure SiO _{2 2} included. Particularly suitable are catalysts contained in a matrix of alumina-silicate 20-80% molecular sieves having an average pore size greater than about 6 Angstroms in diameter.

  In order to avoid new formation of catalyst dust in the annular reactor, it is important that the inserted catalyst has a high abrasion resistance. For this reason, it is preferable to use a molded catalyst instead of irregular particles. Also, regularly shaped catalysts have the advantage of lower pressure drops within the individual annular reactors.

  The catalyst in the filtration reactor can have the same chemical composition as the catalyst in the main reactor. However, filtration reactors must catalyze only the reaction of isocyanic acid to melamine and the reconversion of polymeric nitrogen compounds to melamine, which makes the chemical composition, acidity or surface area quite different in filtration reactors. It is possible to use a catalyst.

For example, if the fluid bed main reactor contains a pure alumina catalyst, it may be advantageous to use a silica-alumina catalyst in the filtration reactor.
Also, the filtration reactor can be operated at the same pressure as the fluid bed main reactor or it can be run at a lower pressure.

  This process can be combined with a process according to European patent application 04000931.8 in which a process gas containing melamine is cooled by contact with an organic liquid, preferably a polyalcohol or aminoalcohol. Preferred organic liquids are ethylene glycol, propylene glycol, glycerol, methyl diethanolamine, mono-, di- or triethanolamine or mixtures thereof. This step avoids contact between melamine and water, which partially hydrolyzes again.

The following example and FIG. 3a illustrate the preferred embodiment of this process in more detail.
Example 1
In a fluid bed reactor (1), urea is decomposed at 400 ° C. and 3 bar. The catalyst consists of pure y-alumina. As the flowing gas, a reaction gas obtained by forming melamine from urea, a mixture of 70% by volume of ammonia and 30% by volume of carbon dioxide is used. Gas from the fluid bed reactor is fed directly to the filtration reactor (2). The filtration reactor contains four annular reactors (3), each with an outer diameter of 600 mm and a length of 4000 mm (just an example). The catalyst in the annular reactor is a spherical alumina-silicate catalyst (d = 4.5 mm) modified with 32% by weight of rare earth metal oxide.

The temperature and pressure in the filtration reactor are the same as in the fluid bed reactor.
In order to avoid carrying excess catalyst fines with the flowing gas, for example in the case of disturbances in the fluidized bed reactor, a cyclone (5) can also be installed between the fluidized bed reactor and the filtration reactor. Good.
Table 1 shows the gas composition before and after introduction into the filtration reactor (inactive components are omitted).

イソシアン酸の約８３％が、ろ過反応器中でメラミンに変換している。これはメラミン７．８ｔの毎日の余剰に相当する。メレム及び他の高分子窒素化合物はもはや検出されず、同様にガスは触媒ダストがなかった。

About 83% of the isocyanic acid is converted to melamine in the filtration reactor. This corresponds to a daily surplus of melamine 7.8t. Melem and other polymeric nitrogen compounds were no longer detected, and the gas was likewise free of catalyst dust.

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

It is the schematic which shows the principle of the catalyst low pressure process in a prior art. It is sectional drawing of the cyclic | annular reactor of this invention. It is a top view of the annular reactor of the present invention. It is the schematic for demonstrating the process of this invention.

Claims (8)

A method of increasing melamine yield and improving dust separation in melamine production from urea in a fluid bed catalytic process,
In addition to melamine unconverted isocyanic acid, the fluidized bed reactor process gas containing melam, melem and other polymeric nitrogen compounds is transferred to a filtration reactor consisting of one or more annular reactors packed with catalyst, There, the unconverted isocyanic acid is converted to melamine, which is then converted back to melamine by reacting the polymeric nitrogen compounds, in particular melam and melem, with ammonia in the process gas, and still in the process gas of the fluidized bed reactor. Removing the catalyst fines present.   The process of claim 1, wherein each individual annular reactor in the filtration reactor consists of two concentrically arranged cylinders each having a perforated wall and contains a catalyst or catalyst mixture in the annular space between the two cylinders.   The process of claim 1 wherein process gas enters the filtration reactor tangentially and flows radially through the catalyst bed of the annular reactor in the filtration reactor.   The method of claim 1, further comprising a cyclone mounted between the fluidized bed reactor and the filtration reactor.   The process of claim 1, wherein the filtration reactor is operated at the same or higher temperature as the fluidized bed reactor, at the same pressure or lower pressure (AP = 1 to 7 bar) compared to the fluidized bed reactor.   The process of claim 1 wherein the catalyst in the fluidized bed reactor can be regenerated with steam and backflushed with ammonia, ammonia / carbon dioxide mixture or steam to remove the deposited dust.   The process of claim 1 wherein the catalyst used in the filtration reactor consists of either alumina, alumina-silicate or pure silica or a mixture of these components. The catalyst or catalyst mixture can comprise 20-80% by weight molecular sieve having pores with a diameter greater than about 6 angstroms.   Process according to claim 1, wherein the melamine-containing process gas is cooled by contact with an organic liquid, preferably a polyalcohol or aminoalcohol.

Images