JPS5928550B2 - Cooling method for reaction waste gas from melamine synthesis - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Urea or its pyrolysis products are treated at atmospheric pressure or at an elevated pressure below about 10 atmospheres with a catalyst and added ammonia or a gaseous mixture containing the same, such as a mixture of ammonia and carbon dioxide. It is already known to produce melamine by heating a mixture, for example to a temperature of 350-450° C., in the presence of reaction waste gases of the melamine synthesis itself.

Steam melamine is produced from a reaction gas at 150~
It is separated by fractional condensation by cooling to a temperature of 250°C.

The gas from which the melamine has been removed by means of vapor pressure still contains unreacted urea, which, as is also known, can be converted into a melt of urea or a mixture of urea and biuret (and where appropriate other than urea). (which may contain decomposition products such as cyanuric acid), unreacted urea can be washed away, in which case the gas is subjected to further cooling. The urea required for this treatment is preferably recycled and brought into direct contact with the reaction gas. The reaction waste gas cooled in this way is then fed to the urea melt to produce the components absorbed by urea from this gas or the decomposition products of urea formed on heating in the melt (melamine, urea, cyanuric acid, Biuret, etc.) are separated without leaving any residue. The reaction gas purified and cooled in this way is used in part as fluidizing gas for the catalyst in the synthesis reactor and/or as cooling gas for the melamine separation. By removing the urea required in the synthesis reactor from this circulation, the components precipitated in it during the scrubbing of the reaction gas as well as those formed by heating are re-supplied to the melamine synthesis, thus increasing the yield. Furthermore, the level of secondary components in the urea circulation is kept constant and low. The heat that the recycled urea absorbs during the cooling and purification of the reaction gas can be used, for example, to melt the new urea required for the synthesis, and the excess or even all heat can also be removed by indirect cooling in a cooler. can do.

In the conventional method, precipitation on the cooling surface of the urea cooler cannot be avoided, and this necessitates periodic cleaning of the cooler.

Temperature control in the urea cooler is difficult because no part of the heat exchange surface must be below the freezing point of the urea melt. This is hindered by the constantly changing freezing point of the urea melt. This is because the content of secondary components such as biuret in the melt also varies, which has a significant influence on the freezing point. It is therefore an object of the present invention to obtain a reaction waste of melamine synthesis obtained by catalytically converting urea at elevated temperatures and then separating by condensation the melamine formed in the conversion from the resulting synthesis gas. The gas can be cooled without the aforementioned drawbacks by treating the reaction waste gas after melamine separation with a urea-containing melt whose temperature is kept slightly above its melting point by indirect cooling. The purpose was to provide a method.

The inventors carried out the cooling of the melt, at least in part together with the treatment of the reaction waste gases, in a co-current scrubber, in which case the melt introduced into the co-current scrubber is 100% lower than its melting point.
It has been found that this object can be achieved by exhibiting a temperature as high as 0.degree. C. and by providing a cooling medium with a temperature at least 50.degree. C. lower than the temperature of the respective melt. In other words, in the process of the invention, the cooling of the urea melt or the urea-containing melt used for the treatment of the reaction waste gas is replaced at least partly directly within the treatment chamber from outside the treatment chamber for the reaction waste gas. .

The treatment is carried out in a customary manner in a washing column, the walls of which also serve for heat removal, for example by injection.
According to the method of the invention, the gas and melt enter the column from above, they are led in parallel flow through the cooling surface and commonly impart their heat to the cooling surface.

If the urea melt is generally heated only slightly in this case, the result is that, contrary to hitherto known methods, the formation of urea-insoluble thermal decomposition products is significantly suppressed. In the operating method according to the invention, therefore, lower melt and gas temperatures can be achieved, so that, as a further advantage, smaller equipment volumes or higher separation capacities can be obtained in the fractional condensation of melamine. . The reaction waste gas introduced into the treatment chamber usually has a temperature of 150-250°C, preferably 190-220°C.

Suitable melts for the treatment of reaction waste gases include not only pure urea melt, whose freezing point (melting point) is 132.7° C., but also urea and its thermal decomposition products, such as biuret and/or cyanuric acid. It is possible to use a melt containing the urea, which has the advantage that the melting point of the urea is lowered.

For example, a urea melt with a biuret content of 20% has a melting point of 120°C, and a urea melt with a biuret content of about 40%, which corresponds to a eutectic mixture, has a melting point of about 110°C. Thus, for example, about 110°C and 132.
A melt with a melting point of between 7° C. is used, and the melt may contain a larger amount of biuret than the eutectic mixture.
As mentioned above, the urea-containing melt may contain other decomposition products of urea, such as cyanuric acid or melamine itself, or other compounds that do not react with the melt or with the components of the reaction waste gas, such as ammonium carbamate, ammonium sulfate, etc. and ammonium nitrate. In all cases the freezing point of the melt lies within the abovementioned range. The urea or urea-containing melt is introduced into the column at a temperature slightly above its freezing point in each case.

That is, it usually exhibits temperatures up to about 10°C, preferably up to 5°C. In the treatment of reaction waste gases according to the invention, heat is removed from the gas and melt to such an extent that, on the one hand, the temperature of the melt does not drop below the freezing point of the melt, and on the other hand, the temperature of the melt is not substantially increased. can be taken away.

Due to the intimate contact of the melt and the reaction waste gas, the latter is cooled down to the temperature of the melt, which at the same time absorbs the unreacted urea contained in the gas and any melamine still present. Of course, a slight temperature increase compared to the inlet temperature of the melt is permitted, in which case a further reduction in the temperature of the melt takes place outside the processing zone. This type of operation is preferred if the heat absorbed by the melt and not removed in the processing zone is to be used for other purposes, for example for melting fresh urea which is destined for synthesis. However, in any case the melt should exhibit a temperature on leaving the zone that is not more than 4° C. higher than the temperature at the entrance to the zone. In this case, the advantages of the method according to the invention are still fully valid. Unexpectedly, it has been found that internal coolers placed within the scrubbing tower do not produce thin layers, even during continuous operation for several months.

However, even at essentially larger temperature differences, for example when using fresh water whose temperature is approximately 0° C. to 30° C. depending on the season, unexpectedly no deposits of urea decomposition products or solidified melt occur. Based on this fact, it is therefore possible to advantageously use cooling media whose temperature is at least 50° C. lower than the solidification temperature of the melt in each case. This temperature has a lower limit depending on the freezing point of the cooling medium, which in the case of water is approximately 0°C. The ability to maintain a substantially higher temperature difference between the cooling medium temperature and the melt temperature in the process of the invention clearly has the further advantage that a smaller cooling surface is required.

In order to avoid tension crack corrosion, it is advantageous to use a chlorine-free or chloride-free cooling medium, preferably a condensate, instead of fresh water.

For this purpose, the heat absorbed in the washing column can itself be removed again with fresh water. Examples: A) A urea-biuret melt 6007 exhibiting a temperature of 130° C. where 100,000 m3 of reaction waste gas per hour are fed at a temperature of 225° C. to the top of a co-current scrubber and circulated there.
Wash with rI/hr.

The temperature of the melt is increased to 136° C. during this time. Heat is removed from the melt in a cooler arranged outside the cleaning device and designed as a boiling cooler. In this case, the temperature of the water must not be lower than 120° C. in order to prevent crystallization of urea during a short period of time. The gas taken out from the tower is 1
Indicates a temperature of 38°C. The required cooling surface is approximately 6, calculated from the obtained logarithmic average temperature difference value △t-13°C and the average heat transmission coefficient value K5OOKcal/wl・hr・°C.
It is 500Kca1/M2.

The cooler must be cleaned once a month. (B) The gas is cooled as in (A), but the heat is removed by a cooler placed in the co-current scrubber.

This cooler is fed with 200 m@3 /h of condensed water, which is heated from 20'C to 31.5°C. The melt leaves the cleaning equipment at a temperature of 130 °C and the gas at 1
It is removed at a temperature of 32°C. The cooling surface load is approximately 29000 Kca1/M3 according to the measured values.

Claims (1)

[Claims] 1. Cooling of the urea-containing melt is carried out at least in part together with the treatment of the reaction waste gas described below in a co-current washing device, in which case the melt introduced into the co-current washing device is catalytically converting the urea at an elevated temperature, characterized in that it exhibits a temperature of up to 10° C. above its melting point and that the cooling medium exhibits a temperature that is at least 50° C. below the temperature of the respective melt; The reaction waste gas of the melamine synthesis, obtained by condensing the melamine produced in the conversion from the synthesis gas obtained in this case, is then converted into a urea-containing melt (the temperature of this melt is lower than its melting point by indirect cooling). A method for cooling the reaction waste gas of melamine synthesis by treating it with a slightly elevated 2 The reaction waste gas introduced into the parallel flow cooling device is 150 to 2
2. Method according to claim 1, characterized in that it exhibits a temperature of 50<0>C. 3 Urea as a melt, or with a melting point of 110 °C and 13
3. Process according to claim 1, characterized in that a urea-containing melt is used which is between 2.7[deg.]C. 4. The method according to any one of claims 1 to 3, characterized in that the melt contains, in addition to urea, biuret and/or cyanuric acid and/or melamine. 5. Claims 1 to 4, characterized in that the melt removed from the co-current cooling device exhibits a temperature that is at most 4° C. higher than the temperature it had when entering the device. The method described in any of the above.