PROCESS FOR PREPARING NITRIC OXIDE

Abstract: A process is claimed for preparing nitric oxide by catalytic combustion of ammonia with molecular oxygen in the presence of a protective gas and in which process nitrogen dioxide, formed during the combustion and/or oxygen still remaining in the reaction gas are removed selectively in whole or in part by a catalytic reduction using hydrogen; the process is characterized in that a part of the reaction gas released from the combustion is used as protective gas while care is being taken that in the feed to the catalytic combustion the ratio between the amount of ammonia, the amount of oxygen and the amount of protective gas is such that the percentage by volume of ammonia is lower than 14 %; the rest of the reaction gas released from the combustion is subjected to the catalytic reduction with hydrogen, upon which the desired nitric oxide is recovered from this treated reaction gas.

Description: The invention relates to a process for preparing nitric oxide by catalytic combustion of ammonia with molecular oxygen in the presence of a protective gas and in which process nitrogen dioxide, formed during the combustion and/or oxygen still remaining in the reaction gas are removed selectively in whole or in part by a catalytic reduction using hydrogen. Such a process is known from the NL-B 149.761, while this process is mentioned also in Ullmanns Encyklopädie der Technischen Chemie 4(13) , page 170. Nitric oxide can generally be prepared by catalytic combustion of ammonia with oxygen at elevated temperature according to the reaction equation: 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O. In the manufacture of nitric acid this reaction is used as the initial step. In order to preclude explosion of the gas mixture (the lower explosion limit of an NH₃-O₂ mixture is at about 14 % (vol) NH₃) and overheating of the catalyst (above 900 ◊C the strength of the catalyst decreases strongly, so that the catalyst is affected), the NH₃-O₂ mixture is diluted with an inert gas. In the manufacture of nitric acid N₂ is used as diluting or protective gas (air is used as O₂ source). For the preparation of NO preference is given to choosing a diluent that can easily be separated off from the NO. Technically, for the preparation of pure NO the dilution with water vapour is applied, which is described in, inter alia, US-A-3110563. In such a process, in which the reaction gases are cooled out, so that the water vapour present is condensed, and in which practically pure NO is obtained from the remaining gas, which NO is contaminated with a small amount of nitrogen and nitrous oxide, a large quantity of water is used, which must be supplied as a protective gas in the form of steam and which is released from the process after the combustion as a highly dilute nitric acid solution. It has now been found that this disadvantage can be avoided by effecting the dilution of the reactants in the combustion with a part of the reaction gas as protective gas in the preparation of NO by catalytic oxidation of NH₃ with molecular oxygen. The process according to the invention is consequently characterized in that a part of the reaction gas released from the combustion is used as protective gas while care is being taken that in the feed to the catalytic combustion the ratio between the amount of ammonia, the amount of oxygen and the amount of protective gas is such that the percentage by volume of ammonia is lower than 14 % and while the rest of the reaction gas released from the combustion is subjected to the catalytic reduction with hydrogen, upon which the desired nitric oxide is recovered from this treated reaction gas. For the preparation of nitric acid it is known per se to recirculate a part of the reaction gas to the combustion. This is mentioned in the DE-C 514,392 of 11th December 1980. In that process the NH₃ combustion however, is so set that the reaction gas substantially contains nitrogen dioxide, which is the desired component for the preparation of nitric acid. Later it has been found, from the study of A.P. Zasorin and V.I. Atroshchenko, decribed in J. Appl. Chem. USSR 40(7) 1364-8 (1967), that combustion of NH₃ for the purpose of preparing nitric acid, in the presence of NO, gives a lower combustion yield, because undesired reaction products are formed owing, inter alia, to a reaction of NO with NH₃ to form nitrous oxide and nitrogen. It has now surprisingly been found, however, that in the combustion of NH₃ to substantially NO the adverse effects of a recycling of NO through the use of a part of the reaction gas as protective gas are more than compensated by the economic advantages of omitting the supply of fresh steam as protective gas. The advantages of the dilution with reaction gas are, inter alia: saving of steam and consequently energy, in the process a much smaller cooler-condenser is required, while a more profitable use is made of the oxygen fed to the combustion, which is partly returned in the recycle gas to the combustion. As appears from the reaction equation of the oxidation, the stoechiometric O₂/NH₃ ratio in the combustion is 1.25 moles/mole. It has been found, however, that in metering NH₃ and O₂ in this ratio the combustion yield is low. This is due to a substantial part of the NH₃ being converted then into N2 via the reaction: 4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O. A good combustion yield is obtained with an O₂/NH₃ ratio ≧ 1.3, so with an excess of O₂. Now, by recycling a part of the reaction gas to the oxidation, O₂ and NH₃ can be supplied in a ratio of about 1.25 moles per mole, while, owing to the recycle, the required O₂/NH₃ ratio at the catalyst is yet ≧ 1.3. An O₂/NH₃ ratio higher than 1.5 will result in an increased formation of nitrogen dioxide. Preference is therefore given to setting the molar O₂/NH₃ ratio in the feed to the combustion at a value between 1.3 : 1 and 1.5 : 1. The temperature at which the feed to the combustion is set according to the process is determined on the one side by the fact that below about 300 ◊C there is a chance of ammonium nitrite deposits and on the other by the fact that because the NH₃ combustion is an adiabatic process and a combustion temperature above 900 ◊C is to be avoided, an ever diminishing percentage of NH₃ must be admitted to the feed as the feed temperature rises in order not to exceed the maximum end temperature. Preference is therefore given to supplying the mixture of protective gas, oxygen and ammonia to the combustion at a temperature between 300 and 450 ◊C. At the maximum end temperature of about 900 ◊C and with an adiabatic temperature increase of 600 ◊C this means an NH₃ concentration in the feed of about 9 % (vol). The hot reaction gas of the catalytic combustion of ammonia also contains, in addition to NO and H₂O, about 1 % (vol) O₂. This is due to the metering of an excess of O₂, which is necessary to restrict the formation of N₂ and N₂O at the catalyst. Depending on the temperature and the contact time, a certian portion of this O₂ will react with NO according to 2NO + O₂ → 2NO₂. Consequently, a part of the O₂ is present in the form of NO₂. Sometimes O₂-equivalents are spoken of: O₂ eq = O₂ free + > NO₂. In the process as described in US-A-3,110,563, in which the combustion is followed by cooling-condensation of the reaction gases, the excess of O₂ is removed with the condensate as nitric acid via the formation of NO₂. An alternative method for removing the O₂-equivalents is to reduce them by means of a fuel, for instance with hydrogen. Such a process is known from, inter alia, the NL-B 149.761. In that process the combustion gas is mixed with H₂ and passed over a catalyst with the following reduction reactions taking place: H₂ + NO₂ → NO + H₂O 2 H₂ + O₂ → 2 H₂O. At the same time a part of the NO is converted into the undesired byproducts N₂ and N₂O. After removal of H₂O from the treated reaction gas via condensation, the end product remains, which substantially consists of NO. This reaction can be carried out at a reaction pressure of 0.1 MPa and higher. In principle also carbon monoxide, methane (natural gas) and gaseous hydrocarbons are eligible as reductants, next to hydrogen. Thermal reduction, too, is possible in principle. In practice these other reductants are less suitable for the preparation of pure NO, because a major disadvantage of these is that the reaction product formed is CO₂, which must be removed separately. Moreover, CO, which is hard to remove, may also find its way into the reaction gas. The thermal reduction (650-1300 ◊C) produces substantially more unfavourable results. Catalytic reduction is applied also for the purpose of removing NO x from the residual gas of nitric acid plants. That knowledge cannot be used, because in that process the only object will in principle be the selective conversion of NO x into N₂ and to a lesser degree of NO₂ into NO (discolouration). This means that from the point of view of saving on fuel it is desired to restrict the reduction of the free O₂. This NO x reduction therefore differs in principle from the reduction process desired for the preparation of pure NO. Catalytic reduction using H₂ is preferred to the reaction of O₂ to NO₂, because during the reduction the desired NO is formed and a much smaller amount of nitric acid is formed, which is an undesired product. As it is desirable for the preparation of pure NO to reduce all O₂-equivalents, the temperature at which the reduction is carried out is preferably taken in the range of 350-400 ◊C. At lower temperatures it has been found that the NO₂ is, but the free O₂ is not removed, while higher temperatures are undesirable owing to a strong formation of N₂O and/or N₂. In applying a process according to the invention it is possible to reach a purity of the NO prepared in this way higher than 95 % (vol). Such a purity is particularly suitable for one of the most important uses of the NO, namely in the catalytic reduction to hydroxyl amine, which as such is a raw material for the preparation of caprolactam, the monomer of nylon-6. The invention will be further elucidated by means of the following non-restrictive experiments. Comparative experiment A For the combustion of ammonia a quartz glass reactor was used with an inside diameter of 2.3 cm. This reactor contained 4 nets of 90 % platinum, 10 % rhodium with a wire thickness of 60 µm and, per net, 1O24 meshes per cm². To this reactor were metered, via a preliminary column heated at 300 ◊C: 36.0 g/hour NH₃ 61.8 l(NTP)/hour O₂ 290.0 l(NTP)/hour steam so that the molar oxygen-ammonia ratio was 1.30. The temperature of the nets was 880 ◊C. After the reactor the gas was cooled to about 150 ◊C by means of a water-cooled condenser and subsequently passed through a 20 % KOH solution. The gas obtained after the passing through was subjected to a gaschromatographic analysis and the amounts of N₂ and N₂O produced were determined along with the off-gas yield. On this basis the combustion yield to NO + NO₂ was determined. 1.14 l(NTP)/hour N₂ and 0.24 l(NTP)/hour N₂O were formed, while in the reaction gas no NH₃ could be demonstrated any more. Of the amount of NH₃ used 94.2 % was converted into NO + NO₂, the yield of N₂ and N₂O was respectively 4.8 % and 1.0 %. Comparative experiment B Under the same conditions as in comparative experiment A were metered to the reactor: 36.0 g/hour NH₃ 68.9 l(NTP)/hour O₂ 290.0 l(NTP)/hour steam, so that the molar oxygen-ammonia ratio was 1.45. 0.47 l(NTP)/hour N₂ and 0.17 l(NTP)/hour N₂O were formed, while in the reaction gas no NH₃ could be demonstrated any more. Of the amount of NH₃ used 97.3 % was converted into NO + NO₂, the yield of N₂ and N₂O was respectively 2 % and 0.7 %. Example-experiment I The recirculation of reaction gas as protective gas in the combustion of ammonia was simulated by metering extra NO and steam to an NH₃-O₂ mixture, the NO/steam ratio corresponding with that in the off gas of the reactor of comparative experiment A. Unlike comparative experiment A, in which a fresh supply of steam is used, reaction gas is used in this example-experiment containing steam obtained as reaction product of the combustion. The gas flows were fed to the reactor at a temperature of 380 ◊C. The O₂-NH₃ molar ratio in the feed to the reactor was 1.45; as a total recirculation includes the recirculation of residual O₂, the said O₂-NH₃ ratio of 1.45 corresponds with an O₂/NH₃ molar ratio in the NH₃-O₂ makeup gas of about 1.3 as used in comparative experiment A. To the reactor operated at a net temperature of 880 ◊C were metered: 29.9 g/hour NH₃ 57.2 l(NTP)/hour O₂ 256.0 l(NTP)/hour steam 172.2 l(NTP)/hour NO. With total combustion of NH₃, 1.02 l(NTP)/hour N₂ and 0.55 l(NTP)/hour N₂O were formed. Of the amount of NH₃ used 92.0 % was converted into NO + NO₂, the yield of N₂ and N₂O was 5.2 % and 2.8 % respectively. Example-experiment II Under further identical conditions as in example-experiment I the O₂/NH₃ ratio in the feed to the reactor was set at 1.35, which corresponds, in case of total recirculation, with an O₂/NH₃ ratio in the makeup gas of about 1.25. To the reactor were metered: 29.9 g/hour NH₃ 53.2 l(NTP)/hour O₂ 256.0 l(NTP)/hour steam 172.2 l(NTP)/hour NO. With total combustion of NH₃, 5.57 l(NTP)/hour N₂ and 2.91 l(NTP)/hour N₂O were formed. Of the amount of NH₃ used 89.2 % was converted into NO + NO₂, the yield of N₂ and N₂O was 7.1 % and 3.7 % respectively.

Claims: 1. Process for preparing nitric oxide by catalytic combustion of ammonia with molecular oxygen in the presence of a protective gas and in which process nitrogen dioxide, formed during the combustion and/or oxygen still remaining in the reaction gas are removed selectively in whole or in part by a catalytic reduction using hydrogen, the process being characterized in that a part of the reaction gas released from the combustion is used as protective gas while care is being taken that in the feed to the catalytic combustion the ratio between the amount of ammonia, the amount of oxygen and the amount of protective gas is such that the percentage by volume of ammonia is lower than 14 % and while the rest of the reaction gas released from the combustion is subjected to the catalytic reduction with hydrogen, upon which the desired nitric oxide is recovered from this treated reaction gas. 2. Process according to claim 1, characterized in that the feed to the combustion is so set that the molar ratio of oxygen to ammonia is 1.3 : 1 to 1.5 : 1. 3. Process according to any one of claims 1-2, characterized in that the mixture of the protective gas with the fresh ammonia and the fresh oxygen is supplied to the catalytic combustion with a temperature of 300-450 ◊C. 4. Process according to any one of claims 1-3, characterized in that the catalytic reduction with hydrogen is carried out at a temperature of 350-400 ◊C.

Inventors: CLEMENT, GERRIT PIETER, GREVING, GEERT, MAESSEN, JOHANNES GERARDUS HENDRIKUS
Applicants: STAMICARBON B.V.
Application year: 1986
Main classification: C01B21
IPC codes: C01B21