WO2010003498A1

WO2010003498A1 - Method and system for producing melamine

Info

Publication number: WO2010003498A1
Application number: PCT/EP2009/004132
Authority: WO
Inventors: Frank Castillo-Welter; Christoph Steden; Martin MÜLLER-HASKY; Georg Ehring; Dominic Walter
Original assignee: Lurgi Gmbh
Priority date: 2008-07-10
Filing date: 2009-06-09
Publication date: 2010-01-14
Also published as: CA2726488A1; DE102008032425B4; MY171000A; EA021649B1; CA2726488C; NO20110075A1; NO343428B1; EA201100140A1; DE102008032425A1

Abstract

The invention relates to producing melamine from urea, by catalytically converting the urea at least partially to melamine in a fluidized bed reactor at 350 to 450°C, feeding the gas mixture arising in the reactor, optionally after filtering in a gas filter, into a crystallizer, in which the melamine crystallizes at a reduced temperature, separating and drawing off the melamine from the process gas in a separator, and then recirculating the process gas, and fed into the fluidized-bed reactor as a fluidizing gas after washing in a urea washer, precipitating droplets, and compressing. According to the invention, the process gas is pre-warmed after precipitating the fluid droplets and after entry into the compressor.

Description

Process and plant for the production of melamine

The invention relates to a process for the preparation of melamine from urea, wherein the urea is catalytically reacted in a fluidized bed reactor at 350 to 450 ⁰ C at least partially to melamine, wherein the resulting gas mixture in the reactor, optionally after filtering in a gas filter, fed to a crystallizer in which the melamine crystallizes at reduced temperature, wherein the melamine is separated and withdrawn from the process gas in a separator, wherein the process gas is recirculated and fed to the fluidized bed reactor as a fluidizing gas after scrubbing in a urea scrubber, droplet separation and compression, and a plant to carry out this process.

Melamine (2,4,6-triamino-1, 3,5-triazine; empirical formula: CaH ₆ N ₆ ) with the structural formula

is obtained technically by the trimerization of urea, for which high-pressure (> 8 MPa) and low-pressure processes (up to 1 MPa) exist. Today, melamine is used essentially in the form of melamine resins obtained by polycondensation with formaldehyde in the production of laminates, plastic moldings or the coating of wood-based materials. A low-pressure process of the aforementioned type is the so-called BASF process, as described in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition 1998 Electronic Edition, WILEY-VCH ₁ chapter "Melamine and Guanamines", Section 4.1.1. In a one-step process, molten urea is fed to a fluidized bed reactor in which the reaction takes place catalytically at 395 to 400 ⁰ C and ambient pressure. In a crystallizer is precipitated from the reactor leaving the gas mixture by lowering the temperature melamine and then separated. The essentially melamine-free gas stream is returned to a scrubbing tower in which it is washed with urea. The clean gas is partially supplied to the reactor as fluidizing gas after separation of liquid droplets. For this purpose, the gas is passed directly to a compressor after liquid separation and then heated in a heat exchanger to the desired reactor temperature.

In order to minimize the precipitation of liquids or solids, a corresponding insulation and / or heat tracing must be provided on the suction side of the compressor, which should prevent a temperature drop. In some cases, the compressor itself is carried out with a heat tracing or at least extensively isolated. The complete isolation and heating of the compressor but is very complex and can not be fully implemented because the operational waste heat of the compressor at appropriate points (eg. In the field of bearings) must be removed to allow proper operation of the compressor. In practice, at least at the suction inlet of the compressor unwanted deposits and settlements, resulting from failed substances, still not completely avoid, so that regular cleaning intervals with appropriate stoppages are necessary. By heating / insulation of the compressor while the transit times between the required Reduce cleaning cycles, but complete isolation or heating of the compressor results in significant additional costs, as a special design is required. Another disadvantage arises in the prior art from the fact that due to the moisture of the gas to be compressed and the associated higher corrosivity especially on the suction side of the compressor according to high quality materials must be used to ensure adequate service life of the compressor.

The object of the invention is therefore to achieve a prolonged running time of the compressor of a melamine plant in a cost effective manner.

The object is achieved by the invention in a method having the features of claim 1 essentially in that the process gas is preheated after the deposition of liquid droplets and before entering the compressor

Due to the appreciable increase in the gas temperature upstream of the compressor, the separation of liquid droplets or solids, for example due to dew point undershoot or desublimation, due to irreversible cooling (for example due to cold bridges on the compressor housing), is avoided. Accordingly, the running times between the cleaning cycles can be considerably extended. More important than the absolute temperature here is the overheating of the saturated gas with liquid, ie the increase (difference) of the temperature in order to avoid dew formation. In addition, with the preheat / overheat of the gas aspirated by the compressor, the compressor can be made from lower grade materials without compromising the compressor's life. In principle, the overheating of the gas to be compressed also causes disadvantages in the form of a poorer efficiency of the compressor and a higher effective volume flow. For this reason, pre-cooling or intermediate cooling usually takes place or provided during the compression process. By contrast, the invention intentionally provides a higher temperature in the compressor to reliably prevent the deposition of liquid droplets.

According to a preferred embodiment of the invention, the process gas is heated in front of the compressor to a temperature which is at least 1 ⁰ C above the gas outlet temperature from the urea scrubber. Preferably, the heating of the process gas is carried out to a temperature which is 3 to 100 ⁰ C, preferably 5 to 50 ° C and in particular about 10 ⁰ C above the gas outlet temperature from the urea scrubber. This can ensure that no significant separation from the gas to be compressed takes place through local cold bridges. However, a considerably higher preheating than 10 ^° C must be taken into account in the design of the compressor so that considerations of economic efficiency set limits.

Regardless of the temperature at the outlet of the urea scrubber preheating the process gas according to the invention is carried out to 135 to 200, in particular 143 to 148 ⁰ C.

Expediently, the overheating of the process gas is carried out in a heat exchanger whose heating medium according to the invention at about 200 ⁰ C (maximum 450 ⁰ C and 145 ⁰ C minimum) works to ensure proper heat transfer. It can also be used electrical heating to ensure the desired overheating, for example in the form of an electric heater, in which the gas is passed in the annular gap to an inserted heating element (heating cartridge).

The invention also extends to a plant for the production of melamine from urea, which is suitable for carrying out the process according to the invention and a fluidized-bed reactor in which the urea at 350 to 450 ⁰ C is at least partially catalytically converted to melamine, a crystallizer in which the melamine is crystallized at a reduced temperature, a separating device, in which the melamine is separated from the process gas, a urea scrubber, to which at least part of the process gas is recirculated to be washed with molten urea, a droplet separator for removing liquid droplets from the washed process gas and a compressor, with which the process gas is compressed prior to introduction into the reactor. According to the invention, a heating device, preferably a steam-heated heat exchanger, for heating the process gas is provided in front of the compressor. The use of other heating media, such as thermal oil or heat carrier salt, is also possible. An electric heater can also be used.

According to a preferred embodiment of the invention, the preheating takes place via a double-tube heat exchanger or a double-walled pipe line in order not to unnecessarily increase the pressure loss on the suction side of the compressor. In principle, however, it is of course also possible to use another heat exchanger.

Further developments, advantages and applications of the invention will become apparent from the following description of an embodiment and the drawing. All described and illustrated features, alone or in any combination, form the subject matter of the invention, regardless of their summary in the claims or their back-reference.

The single FIGURE shows the structure of a system for carrying out the method according to the invention. In the plant shown in the drawing molten urea is fed to a fluidized bed reactor 1, in which it reacts on a catalyst at a temperature of 390 to 410 ⁰ C, preferably 395 to 400 ⁰ C ₁ and at a pressure of 0.1 to 1 MPa, preferably 0.2 to 0.7 MPa ₁ is converted substantially to melamine. The fluidization is carried out with the process (ab) gas, a mixture consisting essentially of ammonia (NH ₃ ) and carbon dioxide (CO ₂ ).

In the reactor 1, the temperature is maintained by an internal heater 2 at about 395 ⁰ C. The gas leaving the reactor is essentially a mixture of gaseous melamine, traces of Meiern, urea decomposition products, ammonia and carbon dioxide. In addition, fine catalyst particles may be contained, while coarser particles are held in the reactor 1 by means of a separation cyclone 3. The mixture is cooled in a gas cooler 4 to a temperature of about 330 to 350 ^c C, in which only high molecular weight by-products (such as Meiern) are crystallized, which are then separated in a gas filter 5.

The filtered gas mixture enters the head of a crystallizer 6, in which it comes in countercurrent with recirculated process gas (135-140 ⁰ C) in contact, so that the temperature drops to 190 to 220 ⁰ C and melamine largely crystallized completely. The fine melamine crystals are separated in a separator (separation cyclone) 7, have a purity of at least 99.8% and can be used without further treatment.

The substantially melamine-free gas stream is returned by means of a blower 8 to a urea scrubber (scrubbing tower) 9 in which it is washed in cocurrent with molten urea and simultaneously cooled. For this purpose, 9 urea is withdrawn from the bottom of the urea scrubber and fed by means of a pump 10 partially the head of the urea scrubber 9, wherein it is brought in a heat exchanger 11 to a temperature of about 135 ⁰ C. The main stream of urea is fed by means of the pump 10 to the reactor 1.

From the urea scrubber 9, in which the temperature is adjusted by means of a heat exchanger 12, the gas flow at the bottom exits at a temperature of about 138 ° C (133-143 ° C). From the clean gas 13 droplets of liquid are then deposited in at least one subsequent separator.

A partial flow of the process gas is then used to fluidize the reactor 1 and this heated after the liquid separation in a steam-heated heat exchanger 14, preferably a double tube heat exchanger or a double-walled pipe line to a temperature of 139 to 170 ⁰ C. The gas temperature then is at least 1 ⁰ C, pre preferably about 10 ⁰ C above the gas temperature when leaving the urea scrubber. 9

The preheated process gas is then fed to a compressor 15 and then heated in a further heat exchanger 16 to the desired gas temperature of about 400 ⁰ C before it is fed as fluidizing gas to the fluidized bed reactor 1. The heat introduced into the gas flow in the heat exchanger 14 before compression must no longer be supplied after compression, which leads to a corresponding reduction of the heat output of the downstream heat exchanger 16 to be transferred. Although the compressor 15 should continue to be insulated, especially when installed outdoors, but now the insulation can be adapted to the needs of the compressor 15 in terms of the required heat dissipation or cooling. A costly special design of the compressor can thus be omitted. Another part of the process gas is fed as a cooling gas to the crystallizer 6 and the remaining part is conducted as exhaust gas of a not shown exhaust gas treatment.

LIST OF REFERENCE NUMBERS

1 fluidized bed reactor

2 heating

3 separation cyclone

4 gas coolers

5 gas filters

6 crystallizer

7 separation cyclone

8 fans

9 urea scrubbers

10 pump

11 heat exchangers

12 heat exchangers

13 separators

14 heat exchangers

15 compressors

16 heat exchangers

Claims

claims

1. A process for the production of melamine from urea, wherein the urea is catalytically reacted in a fluidized bed reactor at 350 to 450 ⁰ C at least partially to melamine, wherein the resulting gas mixture in the reactor, optionally after filtering in a gas filter, a crystallizer is fed, in which the melamine crystallizes at reduced temperature, wherein the melamine is separated and withdrawn from the process gas in a separator, and wherein the process gas is recirculated and fed to the fluidized bed reactor as fluidizing gas after scrubbing in a urea scrubber, droplet separation and compression, characterized in that the process gas is preheated after the separation of the liquid droplets and before entering the compressor.

2. The method according to claim 1, characterized in that the process gas is heated before entering the compressor to a temperature which is at least 1 ⁰ C above the gas outlet temperature from the urea scrubber.

3. The method according to claim 2, characterized in that the process gas is heated before entering the compressor to a temperature, the 3 bis

100 ° C, preferably 5 to 50 ⁰ C, above the gas outlet temperature from the urea scrubber.

4. The method according to claim 2, characterized in that the process gas is heated before entering the compressor to a temperature which is about

10 ⁰ C above the gas outlet temperature from the urea scrubber.

5. The method according to claim 1, characterized in that the process gas is heated to a temperature of 134 to 170 ° C before entering the compressor

6. The method according to any one of the preceding claims, characterized in that the heating of the process gas takes place in a heat exchanger and that the temperature of the heating medium in the heat exchanger between 145 and 450 ⁰ C _1, preferably between 180 and 300 ⁰ C and in particular at about 200 ⁰ C. lies.

7. The method according to any one of the preceding claims, characterized in that the heating of the process gas by means of steam, heat transfer oil or heat carrier salt takes place.

8. The method according to any one of claims 1 to 5, characterized in that the heating of the process gas is effected by means of electrical heating.

9. plant for the production of melamine from urea, in particular for carrying out a method according to one of the preceding claims, with a fluidized bed reactor (1), in which the urea is at least partially catalytically converted to melamine at 350 to 450 ⁰ C, a crystallizer (6 ) in which the melamine crystallizes at a reduced temperature, a separator (7) in which the melamine is separated from the process gas, a urea scrubber (9) to which at least a portion of the process gas is recirculated, a separator (13) for removal of liquid droplets from the washed process gas and a compressor (15) with which the process gas is compressed prior to introduction into the reactor (1), characterized in that a heating device for heating the process gas is provided in front of the compressor (15).

10. Plant according to claim 9, characterized in that the heating device is a preferably steam-heated heat exchanger (14).

11. Plant according to claim 10, characterized in that the heat exchanger (14) is a double tube heat exchanger or a double-walled pipe line.

12. Plant according to claim 9, characterized in that the heating device is an electric heater, wherein the gas is guided in the annular gap around an inserted heating element.