Process for reducing the content of NOx and N2O from a tail gas of a nitric acid process

Abstract

Process for reducing the content of NOx and N2O from an input tail gas of a nitric acid process, said input tail gas having a temperature lower than 400 C., the process comprising an abatement stage at least including a deN2O stage and deNOx stage and providing a conditioned tail gas having a temperature greater than the input tail gas, wherein, prior to submission to said abatement stage, said input tail gas is pre-heated to a temperature of at least 400 C. by indirect heat exchange with at least a portion of said conditioned gas.

Claims

1. A process for reducing the content of NOx and N2O from an input tail gas produced in a nitric acid process, said input tail gas having a temperature lower than 400 C., the process comprising: an abatement stage including at least a stage of catalytic N2O decomposition (deN2O) over an iron-loaded zeolite catalyst and a stage of catalytic NOx reduction (deNOx), said abatement stage providing a conditioned tail gas having a temperature greater than the input tail gas, wherein: prior to submission to said abatement stage, said input tail gas is pre-heated to a temperature of at least 400 C. by indirect heat exchange with at least a portion of said conditioned tail gas; said deN2O and deNOx stages being carried out: in separated catalytic beds and said deNOx stage being carried out over at least one of: a vanadium catalyst, a copper-loaded zeolite catalyst, or an iron-loaded zeolite catalyst, or concurrently in the same catalytic bed over at least an iron-loaded zeolite catalyst.

2. The process according to claim 1, wherein said input tail gas has a temperature from 300 C. to 370 C.

3. The process according to claim 1, wherein said input tail gas has a temperature from 330 C. to 370 C.

4. The process according to claim 1, wherein said input tail gas is pre-heated to a temperature greater than 410 C.

5. The process according to claim 1, wherein said input tail gas is pre-heated to a temperature greater than 420 C.

6. The process according to claim 1, wherein: said nitric acid process includes oxidation of a stream of ammonia, obtaining a gaseous stream containing NOx and N2O, and absorption of NOx into an absorption medium, obtaining nitric acid and a tail gas containing nitrogen, N2O and residual NOx; and said input tail gas is provided by at least part of the tail gas obtained from said absorption.

7. The process according to claim 1, wherein: said nitric acid process includes oxidation of a stream of ammonia, obtaining a gaseous stream containing NOx and N2O; absorption of NOx into an absorption medium, obtaining nitric acid and a tail gas containing nitrogen, N2O and residual NOx; work-expansion of at least a portion of said tail gas, producing power; and said input tail gas is provided by the tail gas obtained after said work-expansion.

8. The process according to claim 1, wherein said deNOx stage is carried out downstream or upstream of said deN2O stage.

9. The process according to claim 1, wherein one of said deNOx stage or deN2O stage is a first stage and the other of said deNOx stage or deN2O stage is a second stage downstream said first stage, and a portion of said input tail gas bypasses said first stage and is sent directly to said second stage.

10. The process according to claim 1, wherein all the heat transferred to said input tail gas during the pre-heating of said input tail gas is taken from said conditioned gas and no heat is transferred to said input tail gas from an external heat source such as a fired preheater.

11. The process according to claim 1, wherein the NOx content of the input tail gas is at least 300 ppm (volume) volume.

12. The process according to claim 1, wherein the NOx content of the input tail gas is at least 500 ppm.

13. The process of claim 1, wherein the input tail gas in the deN2O and deNOx stages has a space velocity from 5,000 to 50,000 1/h, based on the catalyst volume in m3 and the flow rate in Nm3/h.

Description

DETAILED DESCRIPTION

(1) FIG. 1 shows a block scheme of a process according to the invention.

(2) Block 1 represents a tail gas pre-heater and block 2 represents an abatement system performing a stage of catalytic N2O decomposition (deN2O) and a stage of catalytic NOx reduction (deNOx).

(3) A tail gas 10 mainly containing NOx and N2O is supplied to the pre-heater 1. For example, said tail gas 10 is the effluent of the absorption tower of a nitric acid plant (not shown).

(4) Said tail gas 10 has a temperature lower than 400 C. and is heated to a temperature higher than 400 C. by heat exchange with a hot medium 12 inside said pre-heater 1. Said pre-heater 1 is for example a shell-and-plate heat exchanger, wherein the input tail gas flows shell-side and the hot medium flows plate-side. The pre-heater 1 is an indirect heat exchanger where the hot medium and the cold medium do not mix.

(5) The pre-heated tail gas leaves the pre-heater as stream 11.

(6) The stream 11 is supplied to the abatement system 2, wherein it is submitted to a deNOx stage and to a deN2O stage either in a single catalytic bed or in subsequent catalytic beds (not shown), providing a conditioned gas 12 with a lower content of NOx and N2O.

(7) Due to the exothermicity of the deNOx and deN2O stages, said conditioned gas 12 has a temperature greater than the input stream 11 and is used as hot medium in the pre-heater 1.

(8) After heat exchange, the hot medium leaves the pre-heater 1 as stream 13 at a lower temperature and is sent to an expander (not shown) for energy recovery.

EXAMPLE

(9) A tail gas 10 contains 1000 ppm of N2O and 1000 ppm of NOx (NO2/NOx molar ratio of about 0.5). Said tail gas enters the pre-heater 1 at a temperature of 360 C. and leaves the pre-heater 1 at a temperature of 425 C. as stream 11.

(10) The preheated tail gas 11 enters the abatement system 2, wherein an abatement of 90% of NOx and 90% of N2O is achieved, thus providing a conditioned gas 12 containing 100 ppm of N2O and 100 ppm of NOx.

(11) Said conditioned gas 12 leaves the abatement system 2 at a temperature of 435 C. and is fed as hot heat exchange medium to the pre-heater 1.

(12) The conditioned gas leaves the pre-heater as stream 12 at a temperature of 370 C.