NL8105027A - Converting gaseous urea (deriv.) into carbon di:oxide and ammonia - by contacting with catalyst in presence of water vapour - Google Patents

Abstract

Process for converting into ammonia and carbon dioxide of gaseous urea and gaseous cpds. with one or more N atoms and one or more C atoms which can be formed (in)directly from urea, comprises contacting one or more of these cpds. in the presence of water vapour with a catalyst. Pref. catalyst is chosen from active carbon, oxide cpds. and phosphates of boron and aluminium. Pref. oxide cpds. are SiO2, ZnO, TiO2, MgO, Al2O3 and mixts. and/or cpds. of these. The water vapour is used in at least an amt. sufficient to convert all the carbon present in the hydrolysable cpds. into CO2. The process is pref. carried out at 100-500 deg.C esp. 120-420 deg.C with a contact time between the catalyst and the gas mixt. of at least 0.01 sec., esp. 0.1-20 sec. Process is cheap and simple to operate and is esp. useful for treating vapour mixts. obtd. during the concn. of aq. urea solns. and contg. e.g. large amts. of water vapour, small amts. of CO2 and NH3 and cyanic acid and isocyanic acid obtd. by the hydrolysis of urea. The resulting CO2 and NH3 contg. stream can be recycled to the low pressure part of a urea prodn. process.

Description

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l, STAMICARBON B.V.

Inventors: Rudolf VAN HARDEVELD in Geleen Mario G.ll.T. DE COOKER in Beek Dominique J.J.S.M. MOREAU at Sittard 1 PN 3336

METHOD FOR CONVERTING IN AMMONIA AND CARBON DIOXIDE OF GASEOUS COMPOUNDS CONTAINING NITROGEN AND CARBON ATOMS

The invention relates to a process for converting to gaseous urea and gaseous compounds containing one or more nitrogen atoms and one or more carbon atoms which can be formed directly or indirectly from urea into ammonia and carbon dioxide.

Compounds which can be formed directly or indirectly from urea are here understood to mean compounds which can be obtained during the decomposition of urea or the conversion of urea into urea-sequential products with the incorporation or splitting off of water and / or ammonia and / or carbon dioxide, as well as the compounds which can be obtained from these urea-follow-up products by reaction of the urea-follow-up products with one another or with urea, with or without incorporation or splitting off of water and / or ammonia and / or carbon dioxide.

This group of compounds includes: the aliphatic urea decomposition products such as cyanic acid, isocyanic acid, diisocyanic acid, guanyl isocyanate, biuret, triuret, cyanamide, dicyandiaraide, dicyanimide, guanidine, cyanourea and guanylurea; melamine and hydroxytriazines such as ammeline, ammelide and cyanuric acid; reaction products of melamine such as guanyl melamine, cyanomelamine and ureidomelaraine; melamine deamination products such as melara, melem 20 and melon.

In the preparation of various products based on ammonia and carbon dioxide such as urea, melamine, cyanuric acid and the like, the situation often occurs that gas mixtures are formed which, in addition to ammonia and optionally carbon dioxide and / or water vapor, still contain an amount of one or more the above compounds. The presence of these compounds often hinders the further processing of these gas mixtures. For example, the presence of melamine, ammeline, ammelide, melem, melam and / or melon, which compounds may be present in the waste gas mixture originating from the so-called 'dry catch of melamine', is disturbing if one wants this gas mixture. 8105027 2 ϊ * ^ Λ recirculate to a urea synthesis. In the preparation of cyanuric acid, a waste gas mixture is obtained which, in addition to ammonia, also contains cyanic acid, isocyanic acid, cyanamide and, if appropriate, small amounts of ammelide, ammeline, cyanuric acid and melamine, which gas mixture can also not simply be fed to a urea synthesis. The presence of cyanoic acid and / or isoyanoic acid in the gas mixture released upon the evaporation of a urea solution into a urea melt complicates the removal of ammonia from the process condensate formed during the condensation of this gas mixture. If one were to simply blow off such gas mixtures, this would mean the loss of valuable raw materials and product on the one hand, while the environment would be burdened on the other hand to an extent that is no longer permitted by the authorities in many countries.

The object of the invention is now to provide a simple and inexpensive method for converting said kind of compounds into ammonia and carbon dioxide. According to the invention, this object is achieved when the gas mixture is contacted with a catalyst in the presence of water. It has been found that this makes it possible to convert the said type of compounds almost quantitatively in a relatively short period of time into ammonia and carbon dioxide.

It is already known that in the preparation of cyanuric acid from urea, the presence of water gives rise to a reduction in the yield due to the formation of ammonia and carbon dioxide (see Dutch patent 135,682). It is also known in the preparation of melamine from urea, before the melamine is recovered from the reaction gases, to inject water into the gas mixture in order to hydrolyze cyanoic acid and any other decomposition products of urea, so that the melamine is not contaminated during extraction. with these intermediates (see U.S. Patents Nos. 3,386,999 and 3,414,571). However, a relatively long contact time is required for the non-catalyzed complete hydrolysis of the cyanic acid and other decomposition products of urea, so that these known processes also yield off-gases from said US patents, which still contain an amount of the above-mentioned products.

The process according to the invention for the conversion to gaseous urea and gaseous compounds of ammonia and carbon dioxide 8105027 3 ψ * containing one or more nitrogen atoms and one or more carbon atoms which can be formed directly or indirectly from urea is characterized in that a gas mixture which one or more of said compounds contains in the presence of water vapor contact with a catalyst.

As catalyst, any catalyst can be used which, under the chosen reaction conditions, effects the conversion of said compounds into ammonia and carbon dioxide. Good results were obtained with activated carbon, oxidic compounds and phosphates of boriura or aluminum. Suitable oxidic compounds are SiO2, ZnO,

TiO 2, MgO, Al 2 O 3 and mixtures and / or compounds thereof.

In the process according to the invention, the gas mixture can be contacted with the catalyst in known manner. Bee . Preferably, a fixed or fluidized bed of catalyst particles is used, through which the gas mixture is passed.

The amount of water during the conversion should be at least such that all the carbon present in the compounds to be hydrolyzed is converted into carbon dioxide. It is preferable to ensure that some excess water is present. If one starts from a gas mixture which contains little or no water, such as the waste gases from the dry melamine catch, the water can be added in liquid form or as steam. If sufficient water is already present in the gas mixture, for example, as in the gas mixture that is obtained when evaporating urea solutions, no additional water needs to be added.

The temperature at which the method according to the invention is carried out is not critical and can vary within wide limits, for instance between 100 and 500 ° C. In any case, it is preferred that the water required for the hydrolysis be available in vapor form 30, so that temperatures above 100 ° C are preferred.

It is recommended that the temperature be chosen so that no deublimation of the compounds present in the gas mixture occurs, so that deposition of these compounds on the catalyst particles can reduce the activity of the catalyst. The temperature at which the gas mixture to be purified is available will often be chosen. Normally, the temperature of said gas mixtures does not exceed 500 ° C. Usually the decomposition will be carried out at temperatures between 8105027 4 i τ 120 and 420 ° C. A suitable temperature range for treating the gas mixture obtained on evaporation of urea is 125-150 ° C.

The contact time between the catalyst and the gas mixture can also vary. This depends on the nature of the compounds to be hydrolyzed in the gas mixture, the selected catalyst and the temperature at which the decomposition is carried out. If it is desired to convert isoeyanoic acid to ammonia and carbon dioxide in accordance with the process of the invention in an existing gas mixture, a satisfactory to good decomposition is already obtained with contact times of 0.01-10 seconds using the above-mentioned catalysts. If the gas mixture contains compounds such as melamine, melamine deamination products and hydroxytriazines, a longer contact time between the gas mixture and the catalyst is required - to effect the desired decomposition, which can vary, for example from 0.1 -20 seconds, depending on the temperature used and the catalyst used. When activated carbon is used as a catalyst, if melamine, deamination products of melamine and / or hydroxytriazines are present in the gas mixture, a longer contact time is required to achieve the same result at the same temperature as when using oxidic catalysts.

The method according to the invention is also suitable for treating the gases obtained when evaporating urea solutions and / or crystallizing urea. These operations produce a gas mixture which, in addition to large amounts of water vapor and small amounts of ammonia and carbon dioxide, also contains cyanoic acid and isoeyanoic acid, which have been formed by hydrolysis of urea. If such a gas mixture is contacted in accordance with the process of the invention with one or more of the above-mentioned catalysts, the cyanoic acid and isoeyanoic acid in the gas mixture are converted almost completely in a short period of time at relatively low temperatures of, for example, 100-200 ° C into ammonia and carbon dioxide. Preferably, a temperature between 125 and 150 ° C and a contact time of 0.01 to 10 seconds, more particularly between 0.1 and 4 seconds, are used. From a mixture of excess water vapor, ammonia and carbon dioxide thus obtained, a gas stream rich in ammonia and carbon dioxide can be separated in a known manner, for instance by condensation of the gas mixture followed by desorption of ammonia and carbon dioxide from the condensate formed 8105027 ♦ -5 . Such a gas flow can then be returned to the low-pressure part of a urea plant.

The invention will be further elucidated by means of the examples without, however, being limited thereto.

5 Example 1

A series of experiments were conducted in which urea prils were decomposed by heating in an ammonia-fluidized bed of an inert material, water vapor was metered into the decomposition products formed, and the mixture of decomposition products and water vapor was then passed through a fixed catalyst bed, which was placed on a constant temperature was maintained. The amount of ammonia was 10.2 mol per hour in all cases. The amount of urea dosed to the fluid bed of inert particles was completely converted into urea decomposition and follow-up products. This amount corresponded to the amount to be derived from the experiments via the reaction stoichiometry. The amount of carbon dioxide formed in the experiments, related to the total amount of carbon compounds in the gases obtained during the hydrolysis, is a direct estimate of the amount of hydrolysed starting product. In experiments 1-4, an S102-A1203 catalyst was used, which 75%

Contains SiO2 and 25% Al2O3, and is commercially available under the name Ketjen HA. 3T '. In experiment 5, activated carbon, known commercially under "Norit ROW 0.8 Supra", was used. The results of the experiments are shown in Table 1.

25 Table 1

Exp. cat. Temp, con- H2O Product (mmol / h) no. (° C) tact (mmol / h) _ time CO2 HNCO NH2CN melamine (sec.) 30 1 SiO2 “Al2 O3 390 0.21 655 553 1.6 <0, 3 <0.2 2 SiO2-Al2 O3 390 0.36 1594 441.7 0.1 <0.3 <0.1 3 SiO2-Al2 O3 350 0.37 1600 600.8 0.1 <0.3 <0, 1 4 SiO2-Al2 O3 300 0.39 1672 276.3 0.1 <0.3 <0.1 5 act. coal 390 0.42 808 707.5 0.54 <0.6 <0.2 8105027 6 vi '1 v

It follows from table 1 that the conversion of urea decomposition products in the presence of at least the stoichiometric amount of water proceeds almost quantitatively.

Example 2 In the same manner as described in Example 1, raelamine crystals were metered into an ammonia-fluidized bed of an inert material to rapidly and completely vaporize the melamine. The amount of ammonia in all experiments was 10.2 mol / hour. The amount of melamine corresponded to the amount to be derived from the experiments via the reaction stoichiometry.

Water vapor was supplied to the resulting vapors and the mixture was then passed over a fixed catalyst bed. The results of the experiments are shown in Table 2.

Table 2 15 Exp. cat. Temp, con- H2O Product (mmol / h) no. (° C) tact (mmol / h) ______ time CO2 HNCO NH2CN melamine (sec.) 1 Si02-Al203 390 0.38 1484 537.2 5.4 1, 0 29.4 20 2 SiO2-Al2 O3 390 0.20 1508 402.1 12.3 2.4 96.5 3 SiO2-Al2 O3 390 0.10 1480 227.5 12.9 3.1 132.3 4 Act. cabbage 390 0.41 1646 241 37 18 90

It follows from table 2 that the conversion of melamine in the presence of at least the stoichiometric amount of water increases with the contact time. It also appears that the activity of achieve carbon is less than that of the SiO2-Al2O3 catalyst.

8105027 7

Example 3

In the same manner as described in Example 1 and Example 2, urea and melamine, both with and without supply of water vapor, were heated in the absence of a catalyst. The results are shown in Table 3.

Table 3

Exp. Temp. Time Dosage H2O Product (mmol / h) no. (° C> (sec.) (Mmol / h) CO2 HNCO NH2CN melamine 1 390 0.39 urea - 5.3 451.4 7.1 0.5 10 2 390 0.37 urea 694 22.5 467.2 6.6 0.4 3 400 9.1 melamine - 7.6 - 3.2 286 4 400 8.9 melamine 267 10.4 - 3.0 278

From Table 3 it follows that without catalyst there is practically no hydrolysis of the compounds present in the vapor mixture.

15 Example 4

Through a fixed catalyst bed containing tablets of a 3 nanosize 75 wt% SiO 2 and 25 wt% Al 2 O 3 catalyst (known commercially as "Ketjen H.A.

3T "), at a temperature of 130 eC at a speed of 20 0.1 m / sec. a gas mixture, consisting mainly of water vapor and isocyanic acid. The contact time and concentration were varied.

Almost complete hydrolysis of the isocyanic acid was obtained in a short time. The measurement results are shown in table 4.

8105027 4 8

Table 4

Exp. no. Concentration HNCO (ppm) Contact time before cat. bed ni cat. bed (sec.) 5 1 400 <1 3.4 2 400 <1 3.1 3 1030 <1 1.2 4 1020 <1 3.6

Example 5 A substantially water vapor gas stream containing 1300 ppm of isocyanic acid was passed through a catalyst bed at 190 ° C. r in which the same catalyst was charged as described in Example 4. The contact time was 1.2 seconds. After leaving the catalyst bed, the content of isocyanic acid in the 13 gas stream was less than 1 ppm.

Example 6

A number of catalysts were tested at 130 ° C to decompose isocyanic acid which was present in a mainly water vapor gas stream. The concentration of isocyanic acid in the gas stream was always 1300 ppm.

The rate of gas flow through the fixed catalyst bed was 0.1 m / sec. The results are shown in Table 5.

8105027

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Table 5

Type of catalyst contact time cone. HNCO HNCO conversion (sec.) (Ppm) ηδ (Z) kac. bed 5 MgO (tablets) 0.5 35 97

SiO 2 / Al 2 O 3 (10: 1) (tablets); 1.0 <1 100 commercially known under: * Zeolon Z 900 H ')

TiO2 (granules) 0.5 <1 100 10 ZnO (granules) 0.8 <1 100

Activated carbon (tablets; 1.1 <1 100 commercially known under: rNorit ROW 0.8 Supra ')

Example 7 A number of experiments were carried out for the decomposition of isocyanic acid using a fixed catalyst bed in which the catalyst consisting of 75% SiO 2 and 25% Al 2 O 3, commercially known under the name 'Ketjen H.A. 3T ', whereby relatively short contact times were applied. The temperature was 130 ° C. The 20 results of the experiments are shown in Table 6.

Table 6

Exp. Contact time HNCO conversion no. (Sec.) (%) 1 0.047 71.8 25 2 0.042 72.9 3 0.042 66.2 4 0.10 89.2 5 0.14 98.8 6 0.14 97.0 8105027

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Example 8

In the same manner as described in example 7, a number of experiments were carried out with cylindrical clay tablets (length 3 mm, diameter 3 mm) as a catalyst. The results are shown in Table 7.

Table 7

Experiment Contact time HNCQ conversion no. (Sec.) (%) 1 0.043 92.7 10 2 0.045 88.6 3 0.402 100 - f ------------—- 8105027

Claims (13)

A method for converting gaseous urea and gaseous compounds containing one or more nitrogen atoms and one or more carbon atoms which can be formed directly or indirectly from urea into ammonia and carbon dioxide, characterized in that a gas mixture is which contains one or more of these compounds in the presence of water vapor in contact with a catalyst. 2. Process according to claim 1, characterized in that the catalyst used is at least one component from the group of activated carbon, oxidic compounds and phosphates of boron or aluminum. 3. Process according to claim 2, characterized in that at least 6 components of the group SiO 2, ZnO, TiO 2, MgO, Al 2 O 3 and mixtures and / or compounds thereof are used as the oxidic compound. 4. Process according to one or more of claims 1-3, characterized in that at least such an amount of water vapor is provided that all carbon present in the compounds to be hydrolyzed is converted into carbon dioxide. 5. Process according to one or more of claims 1-4, characterized in that the temperature during contacting of the gas mixture with the catalyst is maintained at a value between 100 and 500 ° C. 6. Process according to claim 5, characterized in that the temperature during contacting of the gas mixture with the catalyst is maintained at a value between 120 and 420 ° C. Process according to one or more of Claims 1 to 6, characterized in that the contact time between the gas mixture and the catalyst is maintained at a value of at least 0.01 seconds. 8. Process according to claim 7, characterized in that the contact time between the gas mixture and the catalyst is maintained at a value between 0.1 and 20 seconds. Process according to one or more of Claims 1 to 8, characterized in that a vapor mixture obtained when concentrating aqueous urea solutions is contacted with the catalyst at a temperature between 100 and 200 ° C. 8105027 < Method according to claim 9, characterized in that the conversion temperature is between 125 and 150 ° C. 11. Process according to condusion 9 or 10, characterized in that the contact time between the vapor mixture and the catalyst is maintained at a value between 0.01 and 10 seconds. 11 PN 3336 '% Process according to claim 11, characterized in that the contact time between the vapor mixture and the catalyst is maintained at a value between 0.1 and 4 seconds. 13. A method of converting gaseous urea and gaseous compounds containing nitrogen and carbon atoms into ammonia and carbon dioxide essentially as described and illustrated in the examples. - r JL / WR A 8105027