DE10319902A1 - Process and apparatus for the production of melamine under high pressure - Google Patents

Abstract

The invention relates to a process for the preparation of melamine from urea under high pressure, with a phase change device (1) for transferring a melamine melt in the gas phase, the phase change device (1) heat over at least one heat exchanger (2) and at least one stream (3) as Stripping medium is supplied, characterized in that at least a portion of the melamine melt is conducted in liquid form in the phase change device (1) and is guided there in countercurrent to the ascending gaseous phase or fluidized beds. The invention also relates to a phase separation device for the method. This makes it possible in a simple manner, a purification of the gaseous phase before deposition.

Description

The The invention relates to a method according to the preamble of the claim 1 and an apparatus for carrying out the method according to claim 14th

The Preparation of melamine from urea is e.g. in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A 16, page 174ff).

The known melamine production process can be in low pressure (using catalysts) and high pressure (without Use of catalysts).

at Low-pressure process (for example the BASF, Chemie-Linz, DSM / Stamicarbon process) leaves melamine the reactor in gaseous Shape.

In high pressure processes (eg the Montedison or Nissan process) the melamine leaves the reactor in liquid form. The high-pressure processes have the advantage that the reactor can be made considerably more compact. In high pressure processes, the liquid melamine reaction product must be converted to a solid form. This is done, for example, in WO 95/01345 A1 in that the melamine melt is first evaporated in an evaporator and then cooled in a quencher, whereupon crystalline melamine is formed. It is disadvantageous that the melamine melt CO ₂ and other oxygen-containing substances (eg ammelides, ammeline, ammonium (iso) cyanate, etc.) contains. The CO ₂ can react in the quencher with the ammonia present therein to carbamate, which is undesirable.

In a modification of this method, WO 00/71525 A1 describes that a CO ₂ stripper is arranged in front of the evaporator and the evaporator is operated at high temperatures. This should avoid the disadvantages of CO ₂ input.

However, the process described in WO 00/71525 A1 relates to two apparatus in each of which only one process step is carried out, namely a stripper for reducing the content of CO ₂ and oxygen-containing compounds and an evaporator. It is described, inter alia, that gaseous ammonia is introduced together with the melt into the evaporator and flows in cocurrent through the evaporator. The convection is improved by the gaseous ammonia, which leads to a better heat transfer.

A such procedure is not optimal in terms of cleaning the rising in the evaporator Gas phase.

Of the present invention is based on the object, a high-pressure process to produce high purity melamine, that in simple Way a purification of gaseous Phase before deposition.

These The object is achieved by a Method solved with the features of claim 1.

Thereby, that at least part of the melamine melt in liquid form is guided in a phase change device and there in countercurrent to the ascending gaseous Phase is led Efficient high melamine purity is achieved. As phase change devices can basically apparatuses be considered, in which at least one liquid phase is converted into a gaseous. In the present case can this e.g. Be an evaporator.

there it is advantageous if the guided in countercurrent melamine melt on entry has a lower temperature in the phase change device as the body of the highest Temperature in the phase change device.

Further it is advantageous if the melamine melt stream from a melamine reactor at least in two partial streams is split and at least a partial stream of melamine melt is conducted in countercurrent to the ascending gaseous phase. By splitting the streams can e.g. set the volume of the phase change device targeted are, i. it is set in which area of the phase change device a gas and a liquid phase is present.

The Task is also performed by a method having the features of the claim 4 solved.

there the melamine melt according to the invention at least a first fluidized bed to evaporate the melamine melt and a second fluidized bed fed to solidify the melamine from the gas phase.

Further it is advantageous if a molten salt as a heat transfer medium, the at least a heat exchanger flows through is used. This can be the for needed the phase change Energy supplied efficiently become.

In an advantageous embodiment of the method according to the invention, the melamine melt is first led by the melamine reactor into a CO ₂ stripper and then into the phase change device. Thus, impurities present in the feed of the melamine reactor can be discharged early from the process the.

In a further advantageous embodiment of the method according to the invention becomes the gaseous Current from the phase change device for solidification in at least a fluidized bed device is guided.

Ammonia is advantageously used as the stripping medium in the CO ₂ stripper and / or in the phase change device.

In a further advantageous embodiment of the method according to the invention becomes the gaseous Melamine from the phase change device after a pressure release led to solidification in a separator. It is also advantageous if the separator is a coolant, especially liquid Ammonia, water and / or gaseous Fed to ammonia becomes.

The process can be made particularly effective if a gaseous stream of ammonia from the separator in the phase change device and / or the CO ₂ stripper is performed. For this purpose, the gaseous ammonia stream originating from the separator is advantageously heated in at least one heater. To save energy, it may be advantageous if at least one heater is heated by hot urea melt, wherein the urea melt is used in particular as starting material for the melamine reactor.

The Task is also achieved by a phase change device with the features of claim 14 solved.

By a means of leadership at least part of the melamine melt countercurrent to in the Phase change device ascending gaseous phase, becomes a thermodynamic achieved particularly effective temperature profile.

there it is particularly advantageous if the phase change device Means for intensive contacting of liquid melamine melt with gaseous Melamine and / or ammonia. In particular, a stripping section a tray column.

Also advantageously serves at least a first fluidized bed for Evaporation of melamine melt. Where additionally or alternatively at least a second fluidized bed used to solidify the melamine melt becomes.

The Invention will be described below with reference to the figures of Drawings on several embodiments explained in more detail. It demonstrate:

1 a first embodiment of the method according to the invention with a counterflow of a melamine melt and a gas phase in an evaporator;

2 a second embodiment of the method according to the invention 1 with a CO ₂ stripper;

3 a third embodiment of the method according to the invention with a division of the melamine stream in a phase change device;

4 A fourth embodiment of the method according to the invention with a division of the melamine stream in a phase change device for a CO ₂ stripper;

5 a fifth embodiment of the method according to the invention with a fluidized bed apparatus for solidifying gaseous melamine;

6 a sixth embodiment with a fluidized bed apparatus for evaporation and subsequent solidification.

In 1 a process flow diagram for a first embodiment of the method according to the invention is shown. The method relates in particular to melamine deposition from the gas phase.

The pure melamine is at pressures of 50 to 200 bar, in particular at pressures between 55 and 80 bar in a melamine reactor 4 synthesized by the thermal conversion of urea (HST). The temperature in the melamine reactor 4 is just above the melting point of melamine, maximum at 480 ° C. In the melamine reactor 4 is a heating element for supplying the necessary heat of reaction 13 arranged, which is flowed through by molten salt.

The melamine reactor 4 is used as starting material urea melt from a urea scrubber 10 fed. In the urea scrubber 10 Melamine is washed out of the offgas. The urea melt is used as a detergent. The resulting gaseous fraction leaves the urea scrubber 10 at the head and is led to a urea plant, not shown here.

At the bottom of the urea scrubber 10 is urea melt through a pump 11 to the melamine reactor 4 deducted.

At the bottom of the urea scrubber 10 In turn, off-gases (especially NH ₃ , CO ₂ , residues of melamine) from the top of the melamine reactor 4 initiated. The pressure of the off-gases is before the introduction into the urea scrubber 10 through a first throttle valve 12 reduced.

At the top of the melamine reactor 4 melamine melt is withdrawn and placed in the head of a phase change device 1 given; the melamine melt is in the phase change device 1 transferred from the liquid phase to the gas phase. In the phase change device 1 the melamine melt is in countercurrent to a gaseous ammonia phase as a stripping medium 3 guided; wherein the stripping medium 3 from below into the phase change device 1 to be led. The ammonia supplied in excess and countercurrently as stripping agent relative to the melamine causes the melamine in the phase change device 1 cleaned. In the bottom of the phase change device, there is a strong turbulent mixing due to the large amounts of gas introduced.

The phase change device 1 has a heat exchanger 2 on which flows through a molten salt.

At the head of the phase change device 1 gaseous ammonia (with melamine) is withdrawn and through a second throttle valve 14 in a separator 8th relaxed in it. The separator 8th has the purpose of solidification and separation of gas flow from solid components. The separation of the solid components is carried out, for example, with a cyclone device. In addition, the separator 8th liquid ammonia supplied as a coolant. Alternatively, gaseous ammonia or water as a coolant in the separator 8th be introduced.

At the bottom of the separator 8th Crystalline melamine is stripped off. At the head of the separator 8th gaseous ammonia is withdrawn, wherein a partial flow of ammonia through a compressor 15 at higher pressure and by a first heater 16 is brought to a higher temperature. This ammonia stream then becomes the phase change device 1 supplied below as Strippmittel.

The excess ammonia gas does not become the compressor 15 but discharged from the process and reprocessed, for example, in a fractional condensation, resulting in two phases that can be easily processed further. Alternatively, it can also be absorbed in water.

The second embodiment according to 2 is a modification of the first embodiment, so related to the corresponding description 1 Reference is made.

The second embodiment also has a CO ₂ stripper over the first embodiment 5 on, with which the CO ₂ and other by-products from the melamine reactor 4 stripping melamine melt.

This is done in the lower part of a CO ₂ stripper 5 gaseous ammonia passed. The CO ₂ stripper 5 may in particular be designed as a two-stage apparatus with two Ammoniakeinspeisestellen.

In the present case, the ammonia comes from the separator 8th , wherein after a compression 15 after a first heater 16a the ammonia stream is shared.

One part becomes, as in the first embodiment, a second heater 16b back to the phase change device 4 guided. The other part is via a valve 19 in the CO ₂ stripper 5 guided. The valve 19 serves the volume control.

In 3 a modification of the first embodiment is shown, ie, it is made a melamine without the interposition of a CO ₂ -Strippers. Another difference is a modified recycling of part of the separator 8th accumulating ammonia.

Melamine melt from the melamine reactor is analogous to the first embodiment 4 withdrawn and in the phase change device 1 guided.

However, the flow of melamine melt before being introduced into the phase change device 1 divided.

Part of the melamine melt becomes the bottom of the phase change device 1 supplied to which also a heat exchanger 2 provides the necessary energy supply via a molten salt as a heat transfer medium. At the bottom of the phase change device 1 becomes a part of the separator 8th recovered ammonia as stripping medium 3 fed.

The other part of the melamine melt is the head as a tray column 9 trained liquid-gas mixer, here in the phase change device 1 is integrated. Alternatively, the phase change device 1 be distributed over several apparatuses, so that evaporation and mixing of the phases can be carried out in separate apparatuses. Also, instead of the tray column 9 another means for mixing a liquid and a gas may be used.

The melamine melt flows in the tray columns 9 in countercurrent to the ascending stripping agent and melamine vapors. Through the tray column 9 There is a particularly intensive mixing of liquid and gaseous phase. It forms a temperature profile.

The gaseous melamine is then mixed with the second throttle valve 14 in the separator 8th relaxed. This melamine crystallizes and can be discharged from the process.

That in the separator 8th accumulating gaseous ammonia is partially in a compressor 15 brought to a higher pressure level. Subsequently, liquid ammonia is added and the mixture in a first NH ₃ heater 16a heated. Alternatively, the admixture of gaseous ammonia is possible, while the total amount in the evaporator is essential. There must be enough ammonia to deliver the melamine.

After a relaxation in the third throttle valve 17 the ammonia flow is via a second NH ₃ heater 16b in the phase change device 1 directed.

In this embodiment, the first heater 16a heated by hot urea melt. After heating the ammonia in the first heater 16a The thereby cooled urea melt is in the urea scrubber 10 recycled. The second heater 16b should ensure that the temperature of the stripping medium 3 when entering the phase change device 1 stable and sufficiently high for melamine discharge.

The fourth embodiment according to 4 may be understood as a combination of the second embodiment with the third embodiment. As in the third embodiment, a phase change device with a tray column 9 used to bring melamine melt and rising gases into intimate contact with each other. The melamine melt is divided and partly the head of the tray column 9 , partly the bottom of the phase change device 1 fed.

However, the melamine melt does not come directly from the melamine reactor as in the third embodiment 3 , Rather, analogously to the second embodiment, the melamine melt from the melamine reactor 4 deducted and a CO ₂ stripper 5 fed. From the CO ₂ stripper 5 the melamine melt then becomes the phase change device 1 guided, wherein the current is divided in advance as in the third embodiment.

In the return of the separator 8th accumulating gaseous ammonia is part of the compressor 15 compacted. After the supply of liquid ammonia (alternatively, gaseous ammonia), this stream is via a first heater 16a guided. This first heater 16a is heated with hot urea melt as in the third embodiment.

The thus heated ammonia stream is divided. One part will be in the third throttle valve 17 relaxed and over the second heater 16b the phase change device 1 fed.

The other part of the ammonia stream from the first heater 16a is in a fourth throttle valve 18 relaxed, over a third heater 16c fed to the CO ₂ stripper as a stripping agent.

In 5 is shown as a fifth embodiment, a variant of the fourth embodiment, so that for the common features on the corresponding description can be made. Like the fourth embodiment, the fifth embodiment has a CO ₂ stripper 5 and a phase change device 1 with two melamine melt feed streams on. Also, the return of the gaseous ammonia to the phase change device 1 and the CO ₂ stripper 5 corresponds to the fourth embodiment, since the first heater 16a is heated with urea melt.

In contrast to the fourth embodiment, the gaseous product of the phase change device 1 not directly into the separator 8th guided. Rather, the gaseous product is first in a fluidized bed device 6 with inert bodies (eg titanium) and liquid ammonia, whereby a solidification of the melamine occurs. The melamine solidifies on the inert bodies and is then abraded.

The solidified melamine is then in from the fluidized bed apparatus 6 to a separator 8th in which the gas stream is separated from the solid melamine particles. The solid melamine is discharged.

In the 6 illustrated sixth embodiment is a variant of the second embodiment. Like the second embodiment, the sixth embodiment has a CO ₂ stripper 5 on. The one from the separator 8th recovered ammonia is added to the CO ₂ stripper 5 and the phase change device 1 returns, with the first heater here 16a not heated with hot urea melt.

The phase separator 1 has two fluidized beds 21 . 22 on.

The CO ₂ stripper 5 Melamine melts coming down into the first fluidized bed 21 the phase change device 1 fed.

In the lower part serves the first fluidized bed 21 the evaporation of melamine melt; So the phase change. This melamine is down in the first fluidized bed 21 entering gas phase leads, in particular sprayed. The gas phase here is recycled ammonia gas that by maintaining (fluidizing) the first fluidized bed 21 is used. The heat is supplied to the inert bed via a heat exchanger 2 ,

In the upper part of the phase change device 1 the temperature is in a second fluidized bed 22 By injecting liquid ammonia so lowered that any by-products and / or by-products are precipitated. In a downstream filter 23 the by-products and / or by-products are separated. If there are no by-products, the filter can 23 basically omitted and the melamine directly in the second fluidized bed 22 be solidified.

In the present case, the solidified melamine product passing out of the filter is introduced into the separator 8th guided and then separated from the gas stream.

The Restricted invention in their execution not to the preferred embodiments given above. Rather, a number of variants are conceivable that of the inventive method and the device according to the invention also in principle different types Make use.

1

Phase-change device

2

heat exchangers

3

stripping

4

melamine reactor

5

CO ₂ stripper

6

Fluidized bed device

8th

separators

9

tray column

10

urea scrubber

11

pump

12

first throttle valve

13

heating element

14

second throttle valve

15

compressor

16a, b, c

Ammonia heater

17

third throttle valve

18

fourth throttle valve

19

Valve

21

first fluidized bed

22

second fluidized bed

23

filter

Claims (17)

Process for the production of melamine from urea under high pressure, with a phase change device ( 1 ) for converting a melamine melt into the gas phase, wherein the phase change device ( 1 ) Heat via at least one heat exchanger ( 2 ) and at least one stream ( 3 ) is supplied as Strippmedium, characterized in that at least a portion of the melamine melt in liquid form in the phase change device ( 1 ) is guided and is conducted there in countercurrent to the ascending gaseous phase. Process according to Claim 1, characterized in that the countercurrently conducted melamine melt has a lower temperature on entry than the point of highest temperature in the phase change device ( 1 ). A method according to claim 1 or 2, characterized in that a melamine melt stream from a melamine reactor ( 4 ) is divided into at least two partial streams and at least a partial stream of the melamine melt is conducted in countercurrent to the ascending gaseous phase. Process for the production of melamine from urea under high pressure, with a phase change device for transferring a melamine melt into the gas phase, wherein the phase change device heat is supplied via at least one heat exchanger and at least one stream as Strippmedium, characterized in that at least a portion of the melamine melt in liquid form in the phase change device ( 1 ), wherein the phase change device ( 1 ) a first fluidized bed ( 21 ) for the evaporation of the melamine melt and a second fluidized bed ( 22 ) to solidify the melamine melt from the gas phase. Method according to at least one of the preceding claims, characterized by a salt melt as heat transfer medium, the at least one heat exchanger ( 2 ). Method according to at least one of the preceding claims, characterized in that the melamine melt from the melamine reactor ( 4 ) first into a CO ₂ stripper ( 5 ) and then into the phase change device ( 1 ) to be led. Method according to at least one of the preceding claims, characterized in that the gaseous stream from the phase change device ( 1 ) for solidification in at least one fluidized bed device ( 6 ) to be led. Process according to at least one of the preceding claims, characterized in that ammonia is used as stripping medium in the CO ₂ stripper ( 5 ) and / or in the phase change device ( 1 ) is used. Method according to at least one of the preceding claims, characterized that the gaseous melamine from the phase change device ( 1 ) after a pressure release for solidification in a separator ( 8th ) to be led. Method according to claim 9, characterized in that the separator ( 8th ) is supplied to a coolant, in particular liquid ammonia, water and / or gaseous ammonia. Method according to 9 or 10, characterized in that a gaseous stream of ammonia from the separator ( 8th ) in the phase change device ( 1 ) and / or the CO ₂ stripper ( 5 ) to be led. A method according to claim 11, characterized in that the from the separator ( 8th ) gaseous ammonia stream in at least one heater ( 16 . 16a . 16b . 16c ) is heated. Method according to claim 12, characterized in that at least one heater ( 16 . 16a . 16b . 16c ) is heated by hot urea melt, the urea melt in particular as starting material for the melamine reactor ( 4 ) is used. Phase change device for transferring melamine melt into the gas phase, with at least one heat exchanger ( 3 ) for the supply of heat and a supply means for a stripping medium ( 2 ), characterized by a means for guiding at least part of the melamine melt in countercurrent to the phase change device ( 1 ) ascending gaseous phase. Phase change device according to claim 14, characterized by a device for intensive contacting of liquid melamine melt with gaseous Melamine and / or ammonia, in particular a stripping section of a Tray column. Phase change device according to claim 14 or 15, characterized by at least one first fluidized bed ( 21 ) to evaporate the melamine melt. Phase change device according to at least one of Claims 14 to 16, characterized by at least one second fluidized bed ( 22 ) to solidify the melamine melt.