In a process for preparing nitric acid, nitrogen oxides are first produced in an ammonia combustion plant and cooled in a condenser to form a nitric acid-containing solution. The nitric acid-containing solution is then supplied to at least one absorption tower in which the nitrogen oxides are brought into contact with water and oxygen, wherein the nitrogen-containing gas mixture reacts with the water and the oxygen at least in part to form an aqueous nitric acid-containing solution which accumulates at the base of the absorption tower and is then compressed and recycled via a conduit back into the absorption tower. In order to minimize the concentration of nitrogen oxides in the offgas from such a plant and to increase the efficiency of the process, the invention proposes injecting ozone into a connection conduit which leads from the condenser to a first absorption tower and conducts the nitric acid-containing solution.

An absorption tower for production of nitric acid by the Ostwald process may include sieve trays that are arranged on top of one another and each spaced apart from one another, a water inlet in an upper region of the absorption tower, an inlet for gaseous nitrogen oxides in a lower region of the absorption tower, and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray and is divided by a dividing wall into a first, radially inner region and at least a second, radially outer region. Nitric acid that trickles down from the lowermost sieve tray with a higher concentration can be collected in a middle region. The less-concentrated nitric acid that then effluxes from sieve trays higher up can then be collected separately in a region farther outward.

An absorption tower for production of nitric acid by the Ostwald process may include sieve trays that are arranged on top of one another and each spaced apart from one another, a water inlet in an upper region of the absorption tower, an inlet for gaseous nitrogen oxides in a lower region of the absorption tower, and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray and is divided by a dividing wall into a first, radially inner region and at least a second, radially outer region. Nitric acid that trickles down from the lowermost sieve tray with a higher concentration can be collected in a middle region. The less-concentrated nitric acid that then effluxes from sieve trays higher up can then be collected separately in a region farther outward.

A process for catalytic oxidation of ammonia gas by way of an oxygen-containing gas in a presence of a noble metal-containing catalyst may be employed to give nitrogen monoxide. A temperature of an ammonia/air mixed gas may be optimized in respect of nitrogen monoxide selectivity of the reaction before contact with the catalyst. Examination of catalytic NH.sub.3 oxidation according to 4NH.sub.3+5O.sub.2.fwdarw.4NO+6H.sub.2O revealed that an optimum mode of operation of an NH.sub.3 burner in an HNO.sub.3 plant is not to be achieved by maintenance of a constant gauze temperature of the catalyst gauze by automatic setting of the NH.sub.3:air ratio. Rather, there is an optimum temperature for each process condition that should be set not by changing the NH.sub.3:air ratio but instead by altering the temperature of the NH.sub.3/air mixed gas before contact with the catalyst gauzes.

A vertical absorption column comprising a liquid distributor comprising a feed box having a serrated weir for distribution of a liquid through upward-pointing serrations of the serrated weir into perforated trays of the liquid distributor and located directly above a structured packing, a structured packing, a plate packing comprising a plurality of horizontal plates, provided with cooling means, an inlet for the addition of oxygen to the lower part of the vertical absorption column, an inlet for the process gas comprising nitrogen oxides from an ammonia oxidation process at the lower part of the vertical absorption column, an inlet for an aqueous solution at the upper part of the vertical absorption column, at least one nitric acid outlet at the bottom of the vertical absorption column and an outlet for tail gas comprising nitrogen oxides at the top of the vertical absorption column.

A vertical absorption column comprising a liquid distributor comprising a feed box having a serrated weir for distribution of a liquid through upward-pointing serrations of the serrated weir into perforated trays of the liquid distributor and located directly above a structured packing, a structured packing, a plate packing comprising a plurality of horizontal plates, provided with cooling means, an inlet for the addition of oxygen to the lower part of the vertical absorption column, an inlet for the process gas comprising nitrogen oxides from an ammonia oxidation process at the lower part of the vertical absorption column, an inlet for an aqueous solution at the upper part of the vertical absorption column, at least one nitric acid outlet at the bottom of the vertical absorption column and an outlet for tail gas comprising nitrogen oxides at the top of the vertical absorption column.

A machine train for producing nitric acid includes: a steam turbine having a steam turbine rotor rotating at a first rotational speed; a first compressor having a first compressor rotor rotating at a second rotational speed; a second compressor having a second compressor rotor rotating at a third rotational speed; and an expander having an expander rotor rotating at a fourth rotational speed. The steam turbine drives the first compressor. The rotor of the first compressor drives the second compressor. The expander drives the second compressor. The second compressor is configured and efficiency optimized with respect to its third rotational speed such that during operation of the machine train the first rotational speed of the steam turbine, the second rotational speed of the first compressor, the third rotational speed of the second compressor and the fourth rotational speed of the expander are equal,

A machine train for producing nitric acid includes: a steam turbine having a steam turbine rotor rotating at a first rotational speed; a first compressor having a first compressor rotor rotating at a second rotational speed; a second compressor having a second compressor rotor rotating at a third rotational speed; and an expander having an expander rotor rotating at a fourth rotational speed. The steam turbine drives the first compressor. The rotor of the first compressor drives the second compressor. The expander drives the second compressor. The second compressor is configured and efficiency optimized with respect to its third rotational speed such that during operation of the machine train the first rotational speed of the steam turbine, the second rotational speed of the first compressor, the third rotational speed of the second compressor and the fourth rotational speed of the expander are equal,

The invention relates to a process and apparatus for preparing nitric acid by catalytic oxidation of NH.sub.3 by means of oxygen and subsequent reaction of the NO.sub.x formed with an absorption medium in an absorption tower, which comprises a catalyst bed for N.sub.2O decomposition arranged in the process gas downstream of the catalytic NH.sub.3 oxidation and upstream of the absorption tower in the flow direction and a catalyst bed for NO.sub.x reduction and effecting a further decrease in the amount of N.sub.2O arranged in the tailgas downstream of the absorption tower in the flow direction, wherein the amount of N.sub.2O removed in the catalyst bed for N.sub.2O removal arranged in the process gas is not more than that which results in an N.sub.2O content of >100 ppmv and a molar N.sub.2O/NO.sub.x ratio of >0.25 before entry of the tailgas into the catalyst bed for NO.sub.x reduction and the catalyst bed for NO.sub.x reduction and effecting a further decrease in the amount of N.sub.2O arranged in the tailgas contains at least one iron-loaded zeolite catalyst and NH.sub.3 is added to the tailgas before entry into the catalyst bed in such an amount that an NO.sub.x concentration of <40 ppmv results at the outlet from the catalyst bed and the operating parameters are selected in such a way that an N.sub.2O concentration of <200 ppmv results.

The invention relates to a process and apparatus for preparing nitric acid by catalytic oxidation of NH.sub.3 by means of oxygen and subsequent reaction of the NO.sub.x formed with an absorption medium in an absorption tower, which comprises a catalyst bed for N.sub.2O decomposition arranged in the process gas downstream of the catalytic NH.sub.3 oxidation and upstream of the absorption tower in the flow direction and a catalyst bed for NO.sub.x reduction and effecting a further decrease in the amount of N.sub.2O arranged in the tailgas downstream of the absorption tower in the flow direction, wherein the amount of N.sub.2O removed in the catalyst bed for N.sub.2O removal arranged in the process gas is not more than that which results in an N.sub.2O content of >100 ppmv and a molar N.sub.2O/NO.sub.x ratio of >0.25 before entry of the tailgas into the catalyst bed for NO.sub.x reduction and the catalyst bed for NO.sub.x reduction and effecting a further decrease in the amount of N.sub.2O arranged in the tailgas contains at least one iron-loaded zeolite catalyst and NH.sub.3 is added to the tailgas before entry into the catalyst bed in such an amount that an NO.sub.x concentration of <40 ppmv results at the outlet from the catalyst bed and the operating parameters are selected in such a way that an N.sub.2O concentration of <200 ppmv results.