DE102004014770A1 - Process to increase melamine yield from urea in fluidized-bed catalytic processes, comprises transferring process gas containing high molecular nitrogen compounds into a filter-reactor and reconverting the compounds to melamine - Google Patents

Abstract

Process to increase melamine yield and to improve dust separation in the melamine production from urea in fluidized-bed catalytic processes. Process to increase melamine yield and to improve dust separation in the melamine production from urea in fluidized-bed catalytic processes comprises: transferring the process gas (containing melamine, non-converted isocyanic acid, melam, melem and other higher molecular nitrogen compounds) of the fluid-bed reactor into a filter-reactor (consisting of one or more ring-reactors filled with catalysts) to convert isocyanic acid to melamine; and reconverting higher molecular nitrogen compounds (especially melam and melem) to melamine by reaction with ammonia in the process gas (where the catalyst fines present in the process gas are removed).

Description

The starting material for the production of melamine today is exclusively urea, which decomposes at temperatures> 350 ° C, first after reaction (1) in ammonia and isocyanic acid. The resulting isocyanic acid reacts further in a subsequent reaction (2) to melamine: 6 H ₂ N-CO-NH ₂ → 6 HNCO + 6 NH ₃ (1) 6 HNCO → C ₃ H ₆ N ₆ + 3 CO ₂ (2)

Technically the implementation takes place These reactions today either in the non-catalytic high-pressure processes at about 390 - 400 ° C and printing above 8 MPa or in catalytic low-pressure processes (0.1-1 MPa) in a fluidized bed reactor at temperatures> 380 ° C. Examples for the latter Processes are the processes of DSM and BASF AG. A detailed Description of said processes can be found in NITROGEN, No. 228, pages 43-51, July / August 1997 and in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 16, 5th Ed., Pp. 171-185, (1990).

It is in nature a fluidized bed reactor, which in terms of Turnover has the characteristics of a stirred tank reactor, that the conversion of the urea is not complete in a single pass. Though the urea will react according to (1) completely decomposes the formed isocyanic acid is only incomplete to melamine after reaction (2). In practice, one expects urea sales of 75-90 % to melamine, i. the process gas from the reactor contains depending on the operating conditions of the reactor more or less high proportions of isocyanic acid, the thermodynamically possible Balance is not set.

To understand the invention is in 1 the basic structure of a catalytic production process for melamine briefly described: Hamstoffschmelze ( 1 ) is in a fluidized bed reactor ( 2 ) at 390-400 ° C converted to melamine. As fluidizing gas ( 4 ) serves either pure ammonia or the obtained during the reaction NH ₃ / CO ₂ mixture. The melamine formed is discharged out of the reactor together with unreacted isocyanic acid and the by-products formed, in particular melam (C ₆ H ₉ N ₁₁ ) and melem (C ₆ H ₆ N ₁₀ ) through the process gas (= fluidizing gas + resulting reaction gas). In addition, the process gas contains after catalyst abrasion, which was not deposited in the cyclones of the fluidized bed reactor.

To recover the melamine from the hot process gas, the gas can be quenched directly with water, as in the DSM process. 5 ) in 1 , The result is an aqueous suspension of melamine, higher condensation products of melamine such as melam and melem, hydrolysis products of melamine (oxotriazines) and catalyst dust. From this suspension, the melamine is recovered by re-dissolution, filtration of the insoluble constituents and recrystallization. For details on this procedure, see 3 in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A16, page 176, (1990).

A disadvantage of this method is that when quenching with water, the unreacted isocyanic acid is hydrolyzed to NH ₃ and CO ₂ and the catalyst dust must be separated by filtration. In addition, oxotriazines are formed by hydrolysis of the melamine.

These disadvantages are avoided in the process of BASF by the hot process gas from the reactor first in a cooler ( 6 ) is cooled as far as the higher-condensed by-products such as melam and melem desublimate, but the melamine still remains in the gas phase. The desublimierenden connections are largely reflected on the catalyst fine dust still contained in the gas and are together with this in the filters ( 7 ) separated.

From the thus purified gas stream, the melamine by means of a gas quench ( 8th ) and in the cyclone ( 9 ) separated from the process gas. The latter is done by washing with liquid urea in the washer ( 10 ) and recycled as cooling gas for the gas quench and as fluidizing gas in the process.

In the urea scrubber, unreacted isocyanic acid recombines with urea in reversal of Wöhler's urea synthesis with ammonia: NH ₃ + HNCO → [NH ₄ ] NCO → H ₂ N - CO - NH ₂ (3)

• a) Die nicht umgesetzte Isocyansäure reagiert zwar im Wascher (10) wieder zu Harnstoff, im Wirbelbettreaktor muss aber die Zersetzungsenergie für den insgesamt zugeführten Harnstoff aufgebracht werden, ohne dass dem eine entsprechende Melaminmehrausbeute entgegensteht.
• b) Hohe Gehalte an nicht umgesetzter Isocyansäure im Prozessgas können zur Bildung unerwünschter Nebenprodukte bei der Desublimation des Melamins im Gasquench führen. Die Temperatur im Gasquench muss deshalb so hoch eingestellt werden, dass die Reaktion (3) nicht stattfindet, was wiederum einen Melaminverlust zur Folge hat, da es entsprechend seinem Dampfdruck mit dem Gas aus dem Zyklon (9) ausgetragen wird.
• c) Die Austrittstemperatur aus dem Gaskühler (6) muss so niedrig gewählt werden, dass die höher kondensierten Stickstoffverbindungen wie Melem nahezu vollständig desublimieren. Sie würden sonst gasförmig durch die Filter (7) gehen und zusammen mit dem Melamin im Gasquench abgeschieden werden, was zu Qualitätsproblemen führt. Andererseits darf unter den Bedingungen des Gaskühlers noch kein Melamin auskristallisieren, d.h. der Partialdruck des Melamins im Prozessgas darf seinen Sättigungsdampfdruck bei der gewählten Kühleraustrittstemperatur nicht überschreiten.

Disadvantages of this method are:

• a) The unreacted isocyanic acid reacts in the scrubber ( 10 ) again to urea, but in the fluidized bed reactor, the decomposition energy must be applied to the total urea supplied, without that opposes a corresponding Melaminmehrausbeute.
• b) High levels of unreacted isocyanic acid in the process gas can lead to the formation of undesirable by-products in the desublimation of the melamine in the gas quench. The temperature in the gas quench must therefore be set so high that the reaction ( 3 ), which, in turn, results in loss of melamine, since it reacts with the gas from the cyclone, in accordance with its vapor pressure ( 9 ).
• c) The outlet temperature from the gas cooler ( 6 ) must be chosen so low that the higher-condensed nitrogen compounds such as melem almost completely desublimate. Otherwise they would be gaseous through the filters ( 7 ) and are deposited together with the melamine in the gas quench, which leads to quality problems. On the other hand, under the conditions of the gas cooler no melamine must crystallize out, ie the partial pressure of the melamine in the process gas must not exceed its saturation vapor pressure at the selected cooler outlet temperature.

By this physical law the volume of melamine which can be discharged with the fluidizing gas is limited economic disadvantage, since high fluidizing gas volumes per ton of melamine required are.

It was now surprising found that the disadvantages of a direct water quench or the previously described combination of gas cooler and Avoid filters during the process of BASF AG before the gas quench, if that's hot Process gas from the fluidized bed reactor before quenching with water or before cooling off in the gas cooler and subsequently Filtration is passed directly into a so-called. Filter reactor, in continue the still contained in the process gas isocyanic acid converted to melamine and catalyst dust is retained.

Mark This filter reactor is that it consists of one or more ring reactors in which the catalyst in an annulus between two concentric cylinders with perforated walls is arranged.

2a shows schematically the structure of such a ring reactor as an element in a filter reactor, 2 B a top view of a filter reactor ( 7 ) with four ring reactors ( 2 ). The hot process gas from the fluidized bed reactor occurs ( 1 ) tangentially into the one or more ring reactors ( 2 ) containing filter reactor ( 7 ). Through the outer, perforated wall ( 3 ) of the individual ring reactors, the gas flows radially through the catalyst bed ( 4 ) and leaves via the likewise perforated inner cylinder or the central tube ( 5 ) the ring reactor towards melamine separation and recovery.

The individual ring reactors are at the upper holding plate of the filter reactor ( 6 ) gas-tight, and can be easily changed if necessary. For temperature maintenance in the filter reactor, it is sufficient to isolate the outer wall of the filter reactor, since the reaction of isocyanic acid still contained in the gas after reaction (2) is exothermic.

To facilitate the starting process and for occasional regeneration of the catalyst bed, it is useful, the housing ( 7 ) of the filter reactor double-walled to be able to heat this example by circulating hot gas can.

number and dimensions of the individual ring reactors in the filter reactor naturally according to the capacity and the turnover in the fluidized bed reactor and after the activity of the catalyst in the ring reactor. To avoid vibration and suspension as well To facilitate the replacement of the ring reactors is a length between 1000 and 4000 mm optimal, although in special cases depending on requirements these measurements both exceeded how to fall below. The diameters of the outer cylinder of the ring reactor are preferably used for practical reasons 200 and 900 mm chosen, the inner cylinder resp. of the central tube between 150 and 300 mm. But here too from spatial or other procedural circumstances other dimensions are chosen.

Due to the large, cylindrical flow surfaces and the radial flow through the catalyst filling over the length of the entire ring reactor, the catalyst filling is optimally utilized in the process gas catalyst dust still contained is retained upon entry into the catalyst bed, and can by pe Riodisch backwashing with hot ammonia, fluidizing gas or steam again removed and discharged through a discharge at the bottom of the filter reactor.

The isocyanic acid still contained in the process gas is converted to melamine as it flows through the catalyst bed. Melem and other higher molecular nitrogen compounds are partially converted back to melamine by reaction with ammonia C ₆ H ₆ N ₁₀ + 2 NH ₃ → 2 C ₃ H ₆ N ₆ (4)

The from the ring reactor exiting gas contains by this reaction significantly less isocyanic acid and the proportion of melem is reduced so much that his salary in Melamine is below 100 ppm.

Suitable catalysts for the ring reactors installed in the filter reactor are all types of catalysts known for melamine production, consisting of either pure gamma-Al ₂ O ₃ , aluminum silicates with 10 to 90% by weight SiO ₂ or pure SiO ₂ . Especially suitable are catalysts which contain from 20 to 80% by weight of molecular sieves in an aluminum silicate matrix and whose mean pore diameter is greater than 6-8 angstroms.

Important is that the catalysts have a high abrasion resistance, so that when flowing through the Catalyst bed no new abrasion arises. To fulfill this Demand and to reduce the pressure loss are particularly suitable shaped catalysts in spherical or tablet form or coarse-grained catalysts with diameters of 0.5 to 5mm and above.

The In the filter or ring reactor catalysts used can have the same chemical composition as in the fluidized bed reactor . used It can but also completely other catalysts are used, as in the filter reactor no Urea needs to be decomposed more, but only the reaction of isocyanic to melamine and the reverse reaction the higher molecular weight Compounds must be catalyzed to melamine. If, for example, the Fluidized bed reactor containing a catalyst of pure alumina can it may be advantageous in the after-reactor, a catalyst based on aluminum silicate use.

With reference to the following embodiment and 3 the procedure is described in more detail.

Embodiment 1

In the reactor ( 1 ) urea is reacted at 400 ° C and 3 bar in a fluidized bed of alumina catalyst to melamine. As fluidizing gas is used in the reaction resulting gas mixture consisting of about 70 vol .-% NH ₃ and 30 vol .-% CO ₂ . The process gas leaving the reactor is fed directly into the filter reactor without intermediate cooling ( 2 ), the four ring reactors ( 3 ) having an outer diameter of 600 mm and a length of 4000 mm. The catalyst used for the ring reactors is a spherical (d = 4.5 mm) aluminum silicate catalyst containing 32 wt .-% of a molecular sieve modified with Re ₂ O ₃ .

Table 1 shows the composition of the gas (without inert) before and after entry into the filter reactor: TABLE 1

From 83% of the isocyanic acid still present in the process gas became melamine implemented, this corresponds to a daily increase of 7.8 t melamine. Melem and other higher nitrogen compounds are no longer detectable, as was the gas after the filter reactor free of catalyst dust.

After leaving the filter reactor and before entering the melamine separation by water or gas quench, the process gas in the cooler ( 4 ) are cooled to obtain HD vapor. However, the cooler outlet temperature is limited by the saturation vapor pressure of the melamine, and the partial pressure of the melamine in the process gas must not exceed the saturation vapor pressure.

Claims (5)

Process for increasing the melamine yield and improving the separation of dust in catalytic processes for producing melamine from urea in a fluidized bed reactor, characterized in that the melamine-containing process gas leaving the fluidized bed reactor after passing internal or external cyclones, in addition to unreacted isocyanic acid, melam, melem and other high molecular weight nitrogen compounds May contain traces of catalyst abrasion, is passed directly into a so-called. Filter reactor containing one or more ring-reactors filled with catalyst, wherein the still contained in the process gas due to incomplete conversion in the fluidized bed reactor isocyanic acid is further reacted on the catalyst to melamine and the higher molecular weight contained in the process gas Nitrogen compounds, in particular melam and melem, are converted back into melamine by reaction with ammonia, and the particulate matter contained in the gas through the catalyst bed from the gas e is removed. Method according to claim 1, characterized in that that the ground substance for the catalysts used in the filter reactor of aluminum oxides, Aluminum silicates or pure silica consists and these Catalysts additionally Molecular sieves may contain in the range of 20-80 wt .-%, wherein the mean pore diameter of these molecular sieves is greater than 6 angstroms should. Method according to claim 1, characterized in that that the one or more incorporated in the filter reactor ring reactors two concentrically arranged cylinders of different diameters exist whose walls perforated, and the catalyst in the annulus between the two Cylinders is arranged. Method according to claim 1, characterized in that that the process gas enters tangentially into the filter reactor and through the perforated outer walls of the or the built-ring reactors radially through the catalyst bed and through the central tube formed by the inner cylinder again from the filter reactor exit Method according to claim 1, characterized in that that particulate matter still contained in the process gas is at the outer entry layer of the ring reactor settles and by periodic backwashing with Ammonia or water vapor can be completely or partially removed can.