Process for the removal of dinitrogen oxide in process off-gas

Abstract

A process for the removal of nitrous oxide (N.sub.2O) contained in a process off-gas in an axial flow reactor. The process includes the steps of (a) adding an amount of reducing agent into the process off-gas; (b) in a first stage passing in axial flow direction the process off-gas admixed with the reducing agent through a first monolithic shaped catalyst active in decomposing nitrous oxide by reaction with the reducing agent to provide a gas with a reduced amount of nitrous oxide and residual amounts of reducing agent; and (c) in a second stage passing the gas with a reduced amount of nitrous oxide and residual amounts of the reducing agent in axial flow direction through a second monolithic shaped catalyst active in oxidation of the residual amounts of the reducing agent.

Claims

1. A Process for the removal of nitrous oxide (N2O) contained in an off-gas from the manufacture of nitric acid, the process being performed in an axial flow reactor and comprising the steps of: (a) adding an amount of reducing agent into the off-gas from the manufacture of nitric acid; (b) in a first stage, passing in axial flow direction the off-gas from the manufacture of nitric acid admixed with the reducing agent through a first monolithic shaped catalyst having a composition active in decomposing nitrous oxide with the reducing agent to provide a gas with a reduced amount of nitrous oxide and residual amounts of reducing agent; and (c) in a second stage, passing the gas with a reduced amount of nitrous oxide and NOx and residual amounts of the reducing agent in axial flow direction through a second monolithic shaped catalyst having the same composition active in decompositing nitrous oxide as the composition of the first monolithic shaped catalyst to further reduce the amount of nitrous oxide in the gas, with the addition of an oxidation catalyst for oxidizing the residual amounts of the reducing agent, the first monolithic shaped catalyst having a catalyst volume 3-5 times larger than the catalyst volume of the second monolithic shaped catalyst.

2. The process according to claim 1, comprising the further step of passing the process off-gas in axial flow direction through a third monolithic shaped catalyst active in the decomposition of nitrous oxide prior to step (a).

3. The process according to claim 2, wherein the third monolithic shaped catalyst active in the decomposition of nitrous oxide comprises a metal exchanged zeolite, in which the metal comprises Fe, Co, Ni, Cu, Mn, Zn or Pd or mixtures thereof.

4. The process according to claim 1, wherein the first monolithic shaped catalyst comprises a metal exchanged zeolite, in which the metal comprises Fe, Co, Ni, Cu, Mn, Zn or Pd or mixtures thereof.

5. The process according to claim 4, wherein the metal exchanged zeolite is selected from the group consisting of MFI, BEA, FER, MOR, FAU, CHA, AEI, ERI and/or LTA.

6. The process according to claim 4, wherein the metal exchanged zeolite is Fe-BEA.

7. The process according to claim 1, wherein the second monolithic shaped catalyst is selected from V, Cu, Mn, Pd, Pt or oxides thereof or combinations thereof.

8. The process according to claim 1, wherein the first monolithic and second monolithic shaped catalyst comprise an extruded monolithic shaped carrier.

9. The process according to claim 8, wherein the extruded monolithic shaped carrier consists of cordierite.

10. The process according to claim 1, wherein, in step (b), the process off-gas admixed with the reducing agent is passed through stacked layers of the first monolithic shaped catalyst.

11. The process according to claim 1, wherein the second monolithic shaped catalyst comprises a monolithic shaped carrier zone coated with catalyst.

12. The process according to claim 1, wherein the reducing agent is ammonia.

13. The process according to according to claim 12, wherein the first monolithic shaped catalyst is active both in decomposing nitrous oxide by reaction with ammonia and in selective catalytic reduction of NOx by reaction with ammonia.

14. The process according to claim 1, wherein the reducing agent is a precursor of ammonia.

15. The process according to claim 1, wherein the reducing agent is a hydrocarbon.

16. The process according to claim 1, wherein the reducing agent is methane.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) A process according an embodiment of the invention is performed in a nitric acid process downstream of an absorption tower, after reheating of the process off-gas but before an expander. Ammonia is injected into the gas stream and mixed into the off-gas. The off-gas admixed with the ammonia enters a reactor with a first stage with the first monolithic shaped catalyst installed. In the first stage the N2O reacts with the ammonia according to the reaction:
3N.sub.2O+2NH.sub.3.fwdarw.4N.sub.2+3H.sub.2O

(2) NOx reacts by the well-known SCR reactions which also require ammonia. The catalyst volume in the first stage is such that the off-gas with significantly reduced NOx and N.sub.2O and a NH.sub.3 slip of around 10 ppm remains in the gas after the first stage.

(3) This off-gas enters subsequently the second stage containing the second monolithic shaped catalyst where the majority of the ammonia slip is oxidized and thereby forms either N2 or NOx.

(4) Any NOx being formed by the oxidation of the NH3 in the second stage is not a problem, as the NOX emission from the first stage is very low (almost 0 ppm) and the NH3 slip from the first stage into the second stage is still kept at a level so low, that reduced selectivity would still only lead to a limited NOx emission. Assuming a 10 ppm slip from the first stage and a selectivity of the second stage of 100% to NOx, then still only 10 ppm NOx is formed in the second stage.

(5) Temperatures are typically in the range of 380−550° C. Pressure is typically in the range of 4-12 bar g, but can be both higher or lower. A higher pressure increases activity of NOx and N2o conversion in the first stage and it increases NH.sub.3 conversion in the second stage.

(6) Normal hourly space velocity of the first and second stage depends on temperature, pressure and required performance, but is typically in the range of 10000-40000 h.sup.−1 in the first stage and 80000-120000 h.sup.−1 in the second stage.

(7) Typically, the catalyst volume of the first monolithic shaped catalyst employed in the first stage will be 3-5 times larger than the catalyst volume in the second stage.

(8) In an embodiment, the second stage comprises catalyst active for NH3 oxidation along with catalyst active for SCR reactions. The second monolithic shaped catalyst has a similar composition to the first monolithic shaped catalyst in the first stage with the addition of a catalyst active for NH3 oxidation, such as Cu, Mn, Pd or Pt. Thereby, if NOx is formed by oxidation of NH3, the NOx can be removed by reaction with not yet converted NH3 on the catalyst active for SCR reactions thus further reducing NOx emissions.

(9) If a catalyst similar to the catalyst of the first stage is also present in the second stage, then the reactions reducing the N2O will continue in the second stage also.

(10) The second stage can optionally be performed with the second monolithic shaped catalyst arranged as zone coat on a monolithic shaped carrier.

(11) As an example, when comparing a conventional installation to operating the first and second stage of the present invention, then the N.sub.2O reduction activity can be maintained with a significantly reduced catalyst volume. The first stage can be operated with an ammonia slip of for example 11 ppm and the second stage oxidizing the ammonia will then significantly reduce the ammonia slip to a level below the 5 ppm. The result is typically that the catalyst volume in the first stage can be reduced with about 40%, whereas the second stage typically requires less than 50% of the catalyst volume saved in the first bed, resulting in a total catalyst volume reduction in more than 20% with the present invention.

(12) Additionally, by subsequently removing most of the ammonia slip from the first stage, the requirements to the mixing of ammonia with the off-gas are significantly reduced.

(13) In a conventional reactor it is important to provide highly effective mixing of NH.sub.3 and off-gas to obtain high degrees of NOx and N.sub.2O removal with a low slip of ammonia. When the mixing of the NH.sub.3 with the off-gas is insufficient, there will be regions where there is not enough NH.sub.3, meaning that NOx and N2O conversion will suffer in these regions and there will be regions with too much NH.sub.3 meaning that there will be a higher slip from these regions. To obtain high conversion of NOx and N.sub.2O in combination with a low slip of NH.sub.3, very good mixing is required. Static mixers are typically required, causing additional cost and pressure drop in the installation.

(14) By the invention, slightly higher amounts of NH.sub.3 can be dosed into the off-gas so that the average NH.sub.3 slip from the first stage is for instance 10 ppm instead of 2 ppm if that is the requirement to avoid ammonium sulfate formation in cold spots downstream the catalytic NOx and N.sub.2O conversion.

(15) The oxidation of NH.sub.3 in the second stage is independent of the presence of NOx and N.sub.2O. Further, the oxidation of NH.sub.3 is about first order so at a given catalyst volume, the oxidation of ammonia is substantially constant in the second stage. This means that a greater NH.sub.3 slip from the first stage will result in that more NH.sub.3 is removed in the second stage (the percentage conversion is more or less the same) and the overall slip from the second stage will be lower and less variable.