Process for the production of nitric acid

Details Process for the production of nitric acid Process for the production of nitric acid

2001-03-26

2001-07-24

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Oxygen containing

C423S393000

Reexamination Certificate

active

06264910

ABSTRACT:

The invention in question refers to a process for the production of nitric acid with integrated energy recovery.
In particular, a process for the production of nitric acid using the dual-pressure process is stated in which the ammonia is burnt at an initial low pressure with the aid of compressed process air and the nitrous gases formed during combustion are at least partly absorbed by water at a second pressure which is higher than the first. This causes nitric acid to be produced and the pressure of the residual gas, which is not absorbed, is reduced to atmospheric pressure in a residual gas expander for the purpose of winning compression potential.
In the first stage of the nitric acid production process, ammonia NH
3
is combined with air in a reaction to produce nitrogen oxide, NO:
4NH
3
+5O
2
→4NO+6H
2
=+907.3kJ
The nitrogen oxide, NO, produced is then oxidised to form nitrogen dioxide, NO
2
:
2NO+O
2
→2NO
2
+113.1kJ
The nitogen dioxide, NO
2
is then absorbed in water resulting in the formation of nitric acid:
4NO
2
+O
2
+2H
2
O→4HNO
3
+256.9./.390.3kJ
To ensure that as much nitrogen dioxide as possible is absorbed by the water, the absorption process takes place at elevated pressure, preferably at pressures of between 4 and 14 bar.
The oxygen required to convert the ammonia feedstock is supplied as atmospheric oxygen. For this purpose the process air is compressed until the pressure complies with both the oxidation reaction and the absorption reaction.
The energy for compressing the air is produced by reducing the pressure of the residual gas leaving the absorption process to atmospheric pressure and by recycling the heat released during the chemical reactions.
Each nitric acid plant is adapted to the special requirements of its particular site.
Single-train nitric acid plants usually have a rated capacity of between 100 and 1,000 tpd nitric acid. Doubling the reaction capacity enables up to 2,000 tpd nitric acid to be produced in a single-train plant.
If the actual daily output required is very small, or if the site benefits from comparatively low energy costs, the nitric acid plant is operated using the single high-pressure process. In this process both the ammonia combustion process and the absorption of nitrogen oxide take place at a pressure of approx. 10 bar.
If, however, large nominal capacities and/or higher acid concentrations are required, the most economically feasible solution is provided by a nitric acid plant based on the dual-pressure process.
In the dual-pressure process the ammonia is combusted at an initial pressure which is lower than the absorption pressure. After cooling, the nitrous gases formed during combustion are pressurised in a nitrous gas compressor until the second pressure, the absorption pressure, is achieved.
An article by W. Hanggeli entitled “Expansionsturbinen fur die Herstellung von Salpetersaure” is published on pages 29 to 31 of the corporate pamphlet “Technische Rundschau Sulzer” No. 2, 1986.
FIG. 2
of this particular article shows a process flow diagram for a nitric acid production plant which operates according to the dual-pressure process.
The air compressor and the nitrous gas compressor in this nitric acid plant form a set of machines which are all driven by the same shaft coupled to a steam turbine and a residual gas expander. These four turbomachines, which are all coupled to the shaft, have an axial construction. The driving fluids intersperse the machine casing in a direction which is more or less parallel to the axis.
FIGS. 1
to
7
show the overall length of the said set of machines. As this type of long, coupled turbogenerator requires a considerable amount of space, it often prevents the utilisation of a compact-design nitric acid plant. In addition, the transportation and/or alignment of these long machines can also be somewhat problematic.
A type of rotary compressor, the shaft length of which is considerably shorter than that of a compressor with an axial construction, has been known of for approx. 4 decades. At the centre of these so-called geared centrifugal compressors is a gear box with a large gear wheel which is moved by the drive-shaft speed of the driving mechanism e.g. an electromotor. The teeth of the large gear wheel mate with the teeth of at least one pinion shaft thereby increasing the drive shaft speed to that of the pinion shaft.
Up to four fast-running pinion shafts with differing transmission ratios can be arranged in the gear box around the large central gear wheel.
The turbocompressor's rotors, which convey media radially/centrifugally, are mounted at the ends of the pinion shafts. The rotors' overhang-type assembly means that the gear box also supports the compressor. The spiral casings of the compressor are flange-mounted to the gear box.
Up to four pinion shafts can be arranged on a multishaft machine with a large gear wheel, two pinion shafts being arranged horizontally, one being placed above the gear wheel and the fourth being placed below the gear wheel. In theory it is therefore possible to accommodate eight rotors in an overhang-type assembly.
Multishaft geared centrifugal compressors can be closely adapted to conform with the operational data required. The medium to be compressed is pressurised in stages until the maximum rated pressure is achieved. Each compression stage has its own rotor. The rotors are deployed in various sizes and with various blade mountings depending upon the operational requirements. The semi-concave rotors permit high flow rates as the blades are twisted and lean backwards. Fully-closed rotors achieve maximum efficiency.
Multishaft geared centrifugal compressors are now available in a variety of different designs.
European patent specification EP-0 440 902-B1 describes a geared centrifugal compressor in which an additional intermediate gear wheel is positioned next to the large, centrally situated gear wheel. Four rotors are driven by the large gear wheel coupled to two pinion shafts. Other pinion shafts are driven by intermediate gear wheels. This type of geared centrifugal compressor can be used to attain pressure ratios in excess of 80. However, such high pressure ratios do not occur in plants used for the production of nitric acid.
The German registered design entry G 92 01 858 also describes a geared centrifugal compressor for higher pressure ratios of between 60 and 80 and above. This geared centrifugal compressor is not particularly suited for use in nitric acid plants as such high pressure ratios do not occur in these plants.
The German publication of an unexamined application DE 42 39 138 deals with a compressor unit in which multishaft geared centrifugal compressors are used for low-pressure stages, whilst the high-pressure compressor stages are driven by a separate turbine which is fed by process fluids diverted from one of the low-pressure stages.
This type of compressor unit is not suitable for use in a nitric acid plant either as these high pressure ratios are not required in said plants.
The compressor unit described in DE 42 39 138 can be driven by an electromotor, but can also be driven by a gas turbine or a steam turbine. If a turbine is used to drive the unit, the large centrally located gear wheel can be driven via an additional reducing pinion coupled to the turbine shaft, provided that the speed of the said wheel is slower than that of the turbine.
In nitric acid plants based on the dual-pressure process, the drive mechanisms for both the steam turbines and the gas turbines are interconnected in parallel, hence the single-drive mechanism described in DE 42 39 138 is not suitable for these plants.
A further multishaft geared centrifugal compressor for conveying media at high pressure ratios of 60 and above is described in the European patent application EP 0 602 491. In this particular configuration the pinion shaft supporting the rotor is driven by the high rotational speed generated by the sun wheel of a planet gear.
This type of geared centrifugal compressor can

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