Process and reactor for catalytic oxidation of ammonia

Abstract

A process for the catalytic oxidation of ammonia, comprising: passing an ammonia-containing gas, in the presence of oxygen, over a catalyst contained in a reactor, obtaining a process gas containing nitrogen oxides, and cooling said process gas with a heat exchanger accommodated in the reactor, wherein a portion of said process gas, located in the shell side, bypasses the heat exchanger and forms a hot current which mixes with cooled gas downstream the heat exchanger, and the bypass is regulated on the basis of a target outlet temperature of the mixed process gas.

Claims

1. A reactor adapted for catalytic oxidation of ammonia, the reactor comprising: a catalyst layer, which is adapted to promote the oxidation of ammonia in a presence of oxygen; a heat exchange apparatus that is situated in the reactor downstream the catalyst layer, and is suitable to cool a product gas effluent after passage over said catalyst layer, wherein said heat exchange apparatus has a tube side arranged to be traversed by boiling water and a shell side arranged to be traversed by process gas; at least one bypass channel arranged to provide a bypass route which bypasses at least partially said heat exchange apparatus; a mixing zone where the product gas effluent from said at least one bypass channel mixes with cooled process gas passed through the heat exchange apparatus to give a mixed gas; valve means arranged to control a gas flow rate through said at least one bypass channel; and a control system which is configured to control said valve means, and therefore the bypass flow rate in the at least one bypass channel, based on a target temperature of the mixed gas.

2. The reactor according to claim 1, wherein said heat exchange apparatus has a radial symmetry and said at least one bypass channel includes a bypass channel arranged axially at the centre of the heat exchanger and/or at the periphery of the heat exchanger.

3. The reactor according to claim 1, wherein said heat exchange apparatus includes a plurality of separate modules and said bypass channel is arranged to bypass at least one of said plurality of separate modules.

4. The reactor according to claim 3, wherein said plurality of separate modules are arranged in series, so that said plurality of separate modules are traversed sequentially by the product gas, and said second gas stream bypasses only a subset of consecutive modules, which includes a last module or only the last module.

5. The reactor according to claim 1, further comprising at least one temperature sensor disposed to detect a reactor outlet gas temperature wherein the control system is configured to control the flow rate in the at least one bypass channel, based on the detected reactor outlet gas temperature.

6. The reactor according to claim 1 wherein the catalyst layer is in a form of a platinum-rhodium gauze.

7. The reactor according to claim 1 wherein the catalyst layer includes a layer of catalyst for abatement of N.sub.2O.

Description

DESCRIPTION OF FIGURES

(1) FIGS. 1 to 5 are schemes of ammonia burners according to some embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(2) FIG. 1 illustrates an ammonia burner 1 including: a cylindrical shell 2, a gas inlet 3, a gas outlet 4, an appropriate internal catalyst, such as for example a catalyst gauze 5, a heat exchanger or waste heat boiler (WHB) denoted by 6, a bypass channel 7, a mixing zone 8.

(3) The catalyst gauze 5 is preferably a Platinum-Rhodium fine mesh gauze.

(4) The bypass channel 7 is provided at the centre of the WHB 6, which has substantially a cylindrical shape and radial symmetry. Said channel provides a bypass route of the WHB 6 for the gas leaving the catalyst 5.

(5) A flow control valve 9 is provided to control the flow rate in the bypass channel 7. In the example, the valve 9 is located at the bottom of the channel 7. The double arrow in FIG. 1 indicates that the valve 9 can open or close the bottom opening of the channel 7.

(6) The mixing zone 8 is located downstream the WHB 6 and before the gas outlet 4.

(7) In operation, a fresh charge 10 containing ammonia and oxygen is fed to the gas inlet 3. Oxygen may be provided with a suitable carrier such as air or oxygen-enriched air, or as pure oxygen.

(8) Said charge 10 reacts catalytically over the catalyst 5 forming a NOx-containing process gas. Part of the process gas, denoted by flow lines 11, traverses the WHB 6 resulting in a cooled gas 12 which enters the mixing zone 8.

(9) Said WHB 6 has heat exchange elements, e.g. tubes or plates, traversed by a boiling water or another cooling medium (not shown).

(10) A bypass portion 13 of the process gas bypasses the heat exchange elements of the WHB 6 via the channel 7 and reaches directly the mixing zone 8. Said bypass portion 13 is substantially a non-cooled portion and is therefore hotter than the gas 12. The amount of the bypass portion 13 flowing through the channel 7 is regulated by the position of the valve 9.

(11) In the mixing zone 8, the hot bypass gas 13 mixes with the cool gas 12. Mixing of the gas streams 12 and 13 results in a gas 14 which leaves the burner 1 via the outlet 4. The temperature of the resulting outlet gas 14 is therefore controlled by the bypass flow rate, that is by the position of the valve 9.

(12) FIG. 2 illustrates a variant wherein bypass channels are locate at the periphery of the WHB. For example FIG. 2 illustrates two bypass channels 7.1 and 7.2 and relevant valves 9.1 and 9.2. Each valve 9.1, 9.2 controls separately the flow rate 13.1, 13.2 of the respective bypass channel 7.1, 7.2.

(13) FIG. 3 illustrates an embodiment similar to FIG. 1, with axial bypass channel, wherein the WHB 6 includes two separate stages 6.1 and 6.2, and a bypass channel 7 is provided which bypasses only the second stage 6.2.

(14) FIG. 4 illustrates a variant of FIG. 2 with a two-stage heat exchanger 6 including stages 6.1 and 6.2.

(15) FIG. 5 illustrates a reactor as in FIG. 1 with a temperature control loop. A sensor 15 detects the temperature of the reactor outlet gas 14 and provides a signal 16 to a control system 17. The control system 17 calculates a position of the valve 9 based on the signal 16 and a target outlet temperature and governs the valve 9 via a valve position signal 18. The position of the valve 9 determines the magnitude of the bypass flow rate 13 and, therefore, the temperature of the gas 14 resulting from the mixing of the non-cooled flow 13 and cooled flow 12.

(16) It can be appreciated that the invention provides a real-time control of the outlet temperature and is therefore able to maintain the outlet temperature within a narrow range from the target. The control loop of FIG. 5 is applicable to other variants of the invention, e.g. as in FIGS. 1-4.