EP0000902A1 - Process for the preparation of hydroxyl ammonium salts - Google Patents

Abstract

Process for the preparation of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in dilute aqueous solutions of mineral acids in the presence of suspended platinum catalysts at elevated temperature, the reaction mixture being circulated, passing through the nitrogen monoxide and hydrogen-containing gas mixture located in the gas space above the reaction zone introduces at least one liquid jet from the nozzle opening, directed from top to bottom, of the circulating reaction mixture into at least one circulation zone located in the reaction zone, which is open on both sides and extends in the direction of the incoming liquid jet and is completely covered by the reaction mixture, which flows out from below the reaction mixture containing unconverted gases emerging from the circulation zone is deflected, in the annular zone surrounding the circulation zone leading upwards and at the same time from a zone below the rea ktionszone located separation zone low-gas reaction mixture.

Description

The invention relates to a process for the preparation of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in dilute aqueous solutions of mineral acids in the presence of suspended platinum catalysts at elevated temperature, the reaction mixture being circulated.

The production of hydroxylammonium salts by reduction of nitrogen oxide with hydrogen in dilute mineral acids in the presence of platinum-containing catalysts is carried out in stirred tanks, as described, for example, in DE-AS 24 47 972. Here, space-time yields of up to 0.43 mol of nitrogen monoxide per liter of liquid reaction mixture in the reaction zone and hour are achieved. The disadvantages of the stirred tank are due to the technical design. With increasing size of such agitator containers, the sealing of the agitator shaft passage through the container becomes increasingly difficult. The container and lid construction must be carried out much more complex in order to absorb loads from the stirrer gear and motor as well as the rotational forces of the stirrer. In addition, the equipment suspension must be reinforced to absorb the vibrations of the entire system.

This leads to technically very complex solutions. The aim is therefore to achieve the highest possible space-time yield with a small reaction volume, if possible without using stirring devices.

DE-AS 11 93 923 describes a procedure in which the reaction is carried out in a sieve tray column, the reaction solution being pumped continuously. According to the method known from the published documents of NL patent application 69 08 934, a reaction method working according to the mammoth pump principle is used. Furthermore, from DE-OS 15 42 219 a method of operation is already known in which an intimate gas-liquid-solid mixture is brought about by nozzles and the dispersion is introduced tangentially into the reaction zone. The procedures listed above are still in need of improvement with regard to the space-time yields of nitrogen oxide achieved.

It was therefore the technical task to increase the space-time yield of nitrogen oxide at the respective pressures used in the synthesis of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen and at the same time to avoid the disadvantages mentioned above.

This technical problem is solved in a process for the preparation of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in dilute aqueous solutions of mineral acids in the presence of suspended platinum catalysts at elevated temperature with recycling of the reaction mixture, the nitrogen monoxide and hydrogen located in the gas space above the reaction zone containing gas mixture ^r _d urch at least one from the top downward, 'from a nozzle orifice emerging liquid jet of the circulating reaction mixture in at least one in the reaction zone located on both sides open circulation zone extending in the direction of the entering jet of liquid and is completely covered from the reaction solution , introduces, deflects the unreacted gas-containing reaction mixture emerging from the bottom of the circulation zone, conducts upward in the annular zone surrounding the circulation zone and at the same time returns gas-poor reaction mixture from a separation zone located below the reaction zone.

The new process has the advantage that better space-time yields, based on nitrogen oxide, are achieved under the pressures used in each case. This means that higher throughputs can be achieved with devices of existing size, or smaller devices can be selected for a given throughput. Furthermore, the new method has the advantage that it can be easily transferred to a larger scale.

As a rule, a molar ratio of hydrogen: nitrogen monoxide of more than 1, preferably from 1.5: 1 to 6: 1, is maintained. Particularly good results are obtained when care is taken that in the reaction zone has a molar ratio of hydrogen: S _t ickmonoxid from 3,5 to 5: is maintained. 1

Strong mineral acids, such as hydrochloric acid, nitric acid, sulfuric acid or phosphoric acid, are generally used. Their acidic salts, such as ammonium bisulfate, are also suitable. Sulfuric acid is particularly preferably used. As a rule, 4 to 6 normal aqueous acids are used and are left in the course of the reaction ^f the acid concentration does not fall below 0.2 normal. '

The reaction is preferably carried out at temperatures from 30 to 80 ° C. Temperatures of 40 to 60 ° C have proven particularly useful. The reaction generally takes place at atmospheric pressure. However, it has proven advantageous to carry out the reaction under increased pressure, e.g. up to 300 bar.

Supported catalysts are advantageously used as platinum catalysts; Carbon carriers have proven particularly useful. Platinum-graphite supported catalysts have gained particular technical importance. Such a catalyst advantageously contains 0.3 to 5% by weight of platinum. The supported catalysts are present in a fine distribution so that they are suspended in the reaction medium. Advantageously, the catalysts additionally contain one or more elements of the 5th and / or 6th main group of the periodic system with an atomic weight> 31, also lead and / or mercury as poisoning agents. Suitable catalysts and their production are described, for example, in DE-PS 10 88 037, DE-PS 920 963, 956 038, 945 752. 1 to 400 g of supported platinum catalyst are generally used per liter of aqueous mineral acid. The gas mixture containing hydrogen and nitrogen oxide in the gas space above the reaction zone is separated by at least one liquid jet, directed downwards and emerging from an opening in the nozzle, of the circulating reaction mixture, which essentially consists of aqueous mineral acid, catalyst, optionally from already formed hydroxylammonium salts and any By-products as well as dissolved and / or finely divided gases exist in at least one circulation zone located in the reaction zone, which is open on both sides and extends completely and in the direction of the incoming liquid jet is covered by the aqueous reaction mixture. In addition to hydrogen and nitrogen monoxide, the gas mixture can also contain gases which are formed as by-products, such as N ₂ O and inerts.

It is possible to place one or more, e.g. introduce three jets of liquid. On the other hand, when enlarging reaction zones, it is advantageous to use more than one circulation zone, e.g. a central circulation zone surrounded by six other circulation zones. At least one liquid jet is then directed into each of the circulation zones from top to bottom. In the simplest case, the circulation zone is a tube inserted vertically centrally into the reaction zone. The circulation zone or circulation zones are open on both sides and extend in the direction of the incoming liquid jet or jets. The circulation zone is also completely covered by the reaction mixture. The reaction zone generally has a height to diameter ratio of 2 to 30: 1, preferably 5 to 15: 1. The diameter ratio of the circulation zone to the reaction zone is generally about 0.7 to 0.1 and is preferably around 0. 5.

The velocity of the liquid jet introduced is advantageously from 5 to 40 m / sec, preferably from 10 to 30 m / see. The amount of recycled reaction solution is generally from 10 to 500, in particular 100 to 4 ₀₀ m ³ / m ^{3 of} the reaction zone and hour. The speed of the liquid in the circulation zone should advantageously be a factor of 1 to 6, preferably 2 to 4 times as large as in the annular zone surrounding the circulation zone. In this way, the gas content in the circulation zone and in the annular zone are approximately the same size and the density differences are small.

The nozzle opening is advantageously at a distance from the 'upper end of the circulation zone which is 0.1 to 3, preferably 0.3 to 0.9 times the diameter of the circulation zone.

The reaction mixture emerging from the bottom of the circulation zone and the gas mixture not used up to that point are redirected. The deflection is brought about by a deflection plate arranged transversely below the circulation zone. This is advantageously at a distance from the lower end of the circulation zone which is 0.1 to 1.0 times, preferably 0.2 to 0.7 times the diameter of the circulation zone. Unused gas mixture and part of the liquid rise in the ring-shaped zone surrounding the circulation zone. The liquid reaction mixture is moved down again in the circulation zone by the liquid jet entering from above, and at the same time the gas mixture located above the liquid level is introduced into the circulation zone and finely distributed by the liquid jet directed downwards.

The gas entry from the gas phase above the liquid level is regulated automatically by immersing the nozzle mouth of the nozzle opening in the liquid, ie if the liquid level is lower, more gas is introduced until the liquid level rises and vice versa if the liquid level is above the nozzle mouth, so less gas is introduced so that the liquid level drops. In this way, the content of finely divided gas bubbles in the reaction mixture can also be regulated in a simple manner. This is expediently carried out in such a way that the amount of liquid in the reaction zone is lowered by draining off after the circulation has started. It is then entered until the liquid level has reached the nozzle opening.

Here, the liquid level does not increase due to the increased supply of liquid reaction mixture, but rather through increased gas absorption, which is present in the liquid in a fine distribution. A gas content (gas hold-up) of 5 to 50%, based on the liquid reaction mixture, is advantageously maintained in the liquid reaction mixture in the reaction zone. In addition to the gas phase above the reaction zone, the fresh gas supply can also advantageously take place under the deflection plate at the point where the reaction mixture is deflected. In addition, it is also possible to feed gas mixture into the annular space surrounding the circulation zone.

It has also proven to be advantageous if a power density of 1 to 50, in particular 5 to 30, kW / m ^{3 of} liquid reaction mixture located in the reaction zone is maintained in the reaction zone. The necessary power density is brought into the reaction mixture by the liquid jets. It is a function of the circulation volume and the pressure drop at the nozzle opening.

Immediately below the reaction zone is a separation zone, in which entrained gas bubbles separate from the liquid reaction mixture. The gas bubbles move upward against the liquid flow, while the liquid reaction mixture is drawn off and returned to the nozzle opening or openings.

In the claimed method, a jet drive is used in which the nozzle works from top to bottom. The driving jet brings energy for the circulation in the reactor. Since the nozzle mouth is on the surface of the liquid, the propellant jet introduces gas into the liquid, which is very finely dispersed in the circulation pipe. This makes the slip between liquid and gas very small and one high rotational speed generated. '

Most of the gas bubbles rising in the annular zone are carried into the circulation zone by the liquid flow. In the equilibrium state, the propellant jet therefore only introduces so much gas that the liquid level reaches the nozzle opening. This also limits the gas content in the liquid and can never become so great that the circulation of the gas-liquid flow in the reactor can be at risk of collapsing due to the formation of large bubbles. The advantage of the process according to the invention is accordingly the uniform distribution of gas and also of the suspended catalyst in the entire reaction zone. When the reactor is switched off, the nozzle opening cannot become blocked by the deposited catalyst.

The process is suitable for continuous and discontinuous working methods. In the continuous mode of operation, the aqueous mineral acid to be reacted, optionally in a mixture with hydroxylamine salts, is continuously introduced into the reaction zone. The reaction product is continuously removed from the external liquid circulation. Amounts of catalyst removed with the reaction liquid are supplemented by appropriate additions of catalyst to the liquid feed. The desired gas hold-up can be regulated by varying the liquid inflow or outflow. The gas hold-up can e.g. by determining the apparent density of the liquid-solid gas mixture or the amount of liquid in the reaction space.

Hydroxylammonium salts are used for the production of cyclohexanone oxime, a precursor for the production of caprolactam.

The process according to the invention is illustrated by the following examples.

Examples

In an immersion jet device according to FIG. 1, consisting of a reactor vessel (1) with circulation pipe (2) and baffle plate (3) and an external liquid circulation (4) with pump (5) and heat exchanger (6) for removing the heat of reaction and immersion jet nozzle (7) , Gas supply (8, 9) and gas discharge (10), 4.3 n sulfuric acid and 40 g of a platinum-graphite catalyst with a content of 0.5 wt.% Pt are added per liter of sulfuric acid used. After the recirculation has been started up, enough reaction medium is removed from the reactor that the desired gas hold-up is achieved. The pumped suspension is expanded into the reactor via the thaw jet nozzle.

After the air has been displaced with H2, a gas mixture consisting of 36% by volume NO and 63% by volume H2 (rest of intergas) is introduced into the reactor at 40 ^° C. The escaping exhaust gas is measured and analyzed. When the sulfuric acid in the reaction solution is almost used up, the reaction is terminated, the hydroxylamine solution is separated from the catalyst and the amount of hydroxylamine formed is determined.

Claims (5)

1. A process for the preparation of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in dilute aqueous solutions of mineral acids in the presence of suspended platinum catalysts at elevated temperature, the reaction mixture being circulated, characterized in that the gas in the gas space above the reaction zone is Gas mixture containing nitrogen monoxide and hydrogen by means of at least one liquid jet emerging from a nozzle opening directed downwards from the circulating reaction mixture into at least one circulation zone located in the reaction zone and open on both sides, which extends in the direction of the incoming liquid jet and is completely covered by the reaction mixture , introduces, which redirects the reaction mixture containing unconverted gases emerging from the circulation zone at the bottom, in the annular zone surrounding the circulation zone on the outside and at the same time returns a low-gas reaction mixture from a separation zone located below the reaction zone. 2. The method according to claim 1, characterized in that nitrogen oxide and hydrogen are fed at the nozzle opening. 3. The method according to claim 1, characterized in that nitrogen oxide and hydrogen are fed just below the point at which the reaction mixture emerging from the circulation zone is deflected. 4. The method according to claims 1 to 3, characterized gekennzeiichnet that the reaction mixture in the reaction zone has a gas content of 5 to 50 vol.%. 5. The method according to claims 1 to 4, characterized in that a power density of 1 to 50 kW / m ^{3 of} liquid reaction mixture is maintained in the reaction zone in the reaction zone.

Images