IE43571B1 - Waste gas treatment - Google Patents

Description

The invention relates to a method of waste gas treatment and in particular the removal of nitrogen oxides NO an NOg from exhaust gases containing a high proportion of oxygen and/or oxygen compounds, such as dinitrogen oxide, particularly from the waste gases of nitration plant or plant using nitric acid as on oxidant, such as an adipic acid plant.

The waste gases of adipic acid plant contain large volumes of nitrogen oxides NO and NOg. Concentrations as high as 10,000 vp.p.m. may occur. Exhaust gases of such a kind must not be discharged into the atmosphere because they would create a considerable pollution hazard.

The waste gases from adipic acid plants not only contain considerable volumes of nitrogen oxides, NC and NOg, they also have high contents of oxygen and dinitrogen oxide, the concentrations of which are usually in the neighbourhood of 16% by volume.

It is an object of the: present invention to provide a method which can be economically used of removing nitrogen oxides NO and NOg, even vihen the composition of the waste gas is one favouring oxidation.

It has now been found that, surprisingly, a method in which the nitrogen oxides NO and NOg are reduced by ammonia at elevated temperature in the presence of a catalyst and the latter is a mixed oxide comprising an iron oxide and at -24 3 ΰ Ύ ί least one chromium oxide leads to a reduction of the content of the NO and NO^ leaving concentrations of for instance only 100 p.p.m.

Thus according to the present invention a method of removing nitrogen oxides KO j.-s frofit a stccu. c••j i*.i .>»· are reduces in which the nitrogen oxides NO and NO./by gaseous ammonia at a temperature of at least 260°C, e.g. 260°C to 310°C in the presence of a catalyst comprising a mixed oxide of an iron oxide and at least one chromium oxide, e.g. a mixed oxide of iron (III) oxide and at least one chromium in which oxide, and divided into several beds, and/a branch stream of gaseous ammonia is introduced into the feed stock prior to the entry of the feed stock into each of the beds of catalyst. The amount of gaseous ammonia in each branch stream is desirably proportional to the amount of NO and NOg in the feedstock at that stage. The feedstock may contain up to 10,000 e.g. 6000 to 10,000 volume parts per million of NO or NO^ or mixtures thereof and a larger amount e.g. at least 5% or at least 10% by volume e.g. about 16% by volume of oxygen, the bulk of the feed stock being nitrogen.

Preferably the catalyst is a mixture of a major proportion by weight of iron oxide and a minor proportion by weight of at least one chromium oxide.

The catalyst may include a mixture of CrIII oxide and CrVI oxide, and the CrIII oxide may form a major part by weight of the mixture of chromium oxides. ι > The ratio by weight of CrIII oxide to CrVI oxide may be 5:1. The ratio by weight of iron oxides to chromium oxides may be at least 7:1.

The advantages afforded by the invention reside not only in that nitrogen oxides can be removed from exhaust gases which have a pronounced oxidising character, but more specifically in that contrary to expectations the space velocity can be raised. In other words, the volume of catalyst needed for treating a given waste gas stream is less than would have been predicted. The result is a saving in cost because a required throughput of waste gas calls for a smaller volume of catalyst and consequently also for a smaller capacity reactor.

The invention may be put into practice in various ways and one specific -3£3 57,1 embodiment will be described by way of example with reference to the accompanying drawing.

The drawing represents a nitric acid plant, the feedwater and steam cycles being omitted. An air compressor 1 compresses air and pumps it through a heat exchanger 2 into a mixer 3 which is also supplied with ammonia gas from an evaporator 4. The ammonia and air mixture enters a converter 5 where it is oxidised to nitrogen oxides in conventional manner. The reduction mixture is then passed through a heat exchanger 6 and a gas cooler 7 before it enters an absorption tower 8 where the water required for the process is also added. The product collects in the bottom of the absorber.

The waste gas charged with the residual nitrogen oxide leaves the absorber overhead and is taken through a pipe 9 and the two heat exchangers 6 and 2 for expansion in a gas turbine 10 before beihg discharged into the atmosphere from a waste gas stack.

In conventional nitric acid plant the waste gas is taken directly from the heat exchanger 2 to the gas turbine 10 wherein it is expanded before being exhausted into the stack. The gas turbine 10 generates part of the energy needed for driving the air compressor 1. The remainder of the energy is provided by a steam turbine 11.

The proposed apparatus 12 for reducing the nitrogen oxides in the waste gas is interposed between the heat exchanger 2 and the gas turbine 10; It comprises a reactor 13 which contains the catalyst disposed in for example three beds 14, 15 and 16 arranged in tiers. Pipes 17, 18, 19 discharge ammonia vapour into the spaces above the catalyst beds 14, 15 and 16. The entry into the reactor 13 is connected to the heat exchanger 2 through an interposed further heat exchanger 20 and possibly a supplementary burner 21. The supplementary burner 21 may be operated with a mixture of air and natural gas. The exit side of the reactor 13 is connected via heat exchanger 20 to the expanding gas turbine 10.

Example 1.

A stream of waste gas leaves the heat exchanger 2 in an amount of 20,000 m -44 3 5 71 (S.T.P.) per hour, its temperature being 180°C and its pressure 2.9 atm.abs.

The combined NO and NO,, concentration was 2500 p.p.r·.; the combined oxygen and dinitrogen oxide concentration was about 5* by volume. This stream is heated at in heat exchanger/20 to 270 C. The supplementary burner 21 is used to raise the temperature up to 320°C. About 42 kg/h of ammonia gas are now introduced into the reactor 13. The division between the three pipes 17, 18 and 19 is in the proportions of 70:20:10. The proportions by volume of the catalyst in the beds 14, 15 and 16 are 15:20:65. The reaction between the nitrogen oxides and the ammonia is exothermic. The resultant heat is transfered in heat exchanger 20 to the waste gas from heat exchanger 2. The gas stream leaving the heat exchanger 20 amounts to 20,500 m3 (S.T.P.) per hour, its temperature being 180°C and its pressure 2.8 atm.abs.

The combined NO and NOg concentration is less than 200 p.p.m. This gas stream is expanded in a gas turbine, generating power, and then discharged through the stack.

The apparatus 12 is very flexible in operation and can be readily adapted to the conditions applying to the waste gas generated in nitric acid plant.

For instance, the supplementary burner 21 can be dispensed with if the waste gas is at a more elevated temperature. In the illustrated example the catalyst might also be divided between two beds. Moreover, the arrangement permits waste gases to be purified which contain a substantially higher or lower concentration of nitrogen oxides than the gas in the above described illustrative example.

The catalyst is a mixed oxide of iron oxide and chromium oxide as described below in Example 2 and it is provided in granular form.

Example 2.

In an experimental plant 5,000 litres (S.T.P.)/h were passed through a reactor. The waste gas from the adipic acid plant had the following compositions:0g 16 by volume Hg 2.4 % by volume Ng 67 % by volume NOg 0.24 % by volume NO 0.36 % by volume 002 0.3 2 by volume N20 16 2 by volume This represents an overall nitrogen oxide concentration of 6,000 vp.p.m., neglecting the Ν,,Ο which is entirely harmless. The high proportion of oxygen and di nitrogen oxide is striking.

Before being introduced into the reactor the waste gas was heated to a temperature of about 265°C. Three catalyst beds of different depths as described in Example 1 were provided inside the reactor. Branch streams of ammonia, which were related to the volume of catalyst in each bed were sent through each of the beds.

Preferably the volume of gas for the first bed of catalyst is greater than the volume of the catalyst e.g. 3 to 5 times greater; the volume of gas for the second bed is roughly the same as the volume of catalyst e.g. 0.9 to 1.1 times the volume of the catalyst and the volume of the gas for the third bed of catalyst is less than the volume of catalyst in the third bed e.g. 6 to 7 times less.

The catalyst consisted of: 85% Fe(III) oxide 102 Cr(III) oxide Cr(VI) oxide v/ith admixtures of silica, alkalies and alkaline earths.

An NO, NOg-content of 100 vp.p.m. was measured at the reactor exit. The maximum space velocity of the catalyst was found to be m3 (S.T.P.)/h 3 000 m3 This value is surprisingly good. The predicted space velocity would have been about m3 (S.T.P.)/h 1,000 --m3 Other tests were then performed on a waste gas having NO, NOg-concentrations up -64 3 S 7 i to 10,000 vp.p.m. These waste gases were heated to temperatures from 260°C to 3TQ°C. The NO, [^-concentrations at the reactor exit were found to be in the order of 200 vp.p.m. The maximum space velocity rose to as high as m3 (S.T.P.)/h ,000 -=nr This rise in space velocity can be explained by assuming that the oxygen and the di nitrogen oxide unexpectedly cooperate in a direction favouring the reduction.

Attention is directed to Patent Specification No. 43572 which describes and claims a method of removing nitrogen oxides from a gas stream containing NO and NO^ by heating the gas under pressure and reacting it with gaseous ammonia in the presence of an iron oxide chromium oxidp catalyst, and a nitric acid plant arranged to carry out the method.

Claims (15)

1. A method of removing nitrogen oxides NO and NOg from a feed stock containing oxygen and/or dinitrogen oxide, in which the nitrogen oxides are reduced NO and NOg/by gaseous ammonia at a temperature of at least 260 C in the presence of a catalyst comprising a mixed oxide of iron oxide and of least one chromium oxide, and divided into several beds, and in which a brandstream of gaseous ammonia is introduced into the feed stock prior to the entry of the feed stock into each of the beds of catalyst.

2. A method according to Claim 1 in which the catalyst is a mixed oxide of iron (III) oxide and at least one chromium oxide.

3. A method as claimed in Claim 1 or Claim 2 in which the amount of gaseous ammonia in each branch stream is proportional to the amount of NO and N0 2 in the feed stock at that stage.

4. A method according to any one of the preceding Claims in which the feed stock is heated to a temperature from 260° to 310°C.

5. A method as claimed in any one of Claims 1 to 4 in which the feed stock contains up to 10,000 volume parts per million of NO or NOg or mixtures thereof and a larger amount of oxygen, the bulk of the feed stock being nitrogen.

6. A method as claimed in Claim 5 in which the feed stock contains 6,000 to 10,000 volume parts per million of NO or NOg or mixtures thereof.

7. A method as claimed in Claim 5 or Claim 6 in which the feed stock contains at least 5% by volume of oxygen.

8. A method as claimed in Claim 7 in which the feed stock contains at least 10% by volume of oxygen.

9. A method as claimed in Claim 7 or Claim 8 in which the feed stock contains about 16% by volume oxygen.

10. A method as claimed in any one of Claims 1 to 9 in which the catalyst is a mixture of a major proportion by weight of iron oxide and a minor proportion by weight of at least one chromium oxide.

11. A method as claimed in any one of Claims 1 to 10 in which the catalyst -8includes a mixture of CrIII oxide and CrVI oxide.

12. A method as claimed in Claim 11 in which the CrIII oxide forms a major part by weight of the mixture of chromium oxide.

13. A method as claimed in Claim 12 in which the ratio by weight of CrIII oxide to CrVI oxide is 5:1.

14. A method as claimed in any one of Claims 10 to 13 in which the ratio by weight of iron oxides to chromium oxides is at least 7:1.

15. A method as claimed in Claim 1 substantially as specifically described herein with reference to the Examples.