WO2009083593A1

WO2009083593A1 - Reactor and process for the decomposition of nitrogen oxides in gases

Info

Publication number: WO2009083593A1
Application number: PCT/EP2008/068376
Authority: WO
Inventors: Eva Grimbergen; Onno Leendert Maaskant
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2008-01-02
Filing date: 2008-12-30
Publication date: 2009-07-09
Also published as: CA2711090A1; EP2240266A1

Abstract

A reactor for the reduction of nitrogen oxides in gases, comprises a gas inlet, leading to a first reactor section which reactor section comprises a plurality of first fixed beds defining between them first passages, wherein, alternately, first passages are closed off at one end of the first fixed beds thereby providing passageways from the gas inlet through the first fixed beds into other first passages that are in fluid communication with a second reactor section which second reactor section comprises a plurality of second fixed beds which define second passages and wherein alternately second passages are closed off thereby providing passageways from the second reactor section to a gas outlet. The concentration of nitrogen oxides in a gas is reduced in a process, comprising: (a) passing the gas with a gaseous reducing agent in lateral flow through a plurality of first fixed beds containing nitrogen oxides-reducing catalyst to obtain partly reduced gas; (b) passing the partly reduced gas inmlateral flow through a plurality of second fixed beds containing nitrogen oxides-reducing catalyst to obtain nitrogen oxides-depleted gas.

Description

REACTOR AND PROCESS FOR THE DECOMPOSITION OF NITROGEN

OXIDES IN GASES

The present invention relates to a reactor for the decomposition of nitrogen oxides in gases and to a process for such decomposition.

A number of processes are known that result in off- gases or waste gases that contain undesirable nitrogen oxides. Such processes include hydrocarbon combustion processes, e.g. for power generation, and waste incineration, but also caprolactam production or the manufacture of nitric acid by the oxidation of ammonia. The nitrogen oxides may include NO (nitric oxide) and NO₂ (nitrogen dioxide) , together referred to as NOx, but also N₂O (nitrous oxide) which may contribute to depletion of the ozone layer.

Since the harmful effects of nitrogen oxides have become more known it has become increasingly desirable to reduce the level of such oxides in waste gases and off- gases. Several processes have been developed for the reduction of nitrogen oxides. One of such processes has been described in EP-A 768110, disclosing a process in which an off™gas that contains NOx is passed over a catalyst together with a reducing agent. The catalyst comprises a carrier onto which vanadium, tungsten and/or molybdenum has been loaded. The reducing agent used can be ammonia. Other processes and catalysts are described in

US-A 2003/0143142. This document refers to the Shell DeNOx system that is based on a lateral flow reactor principle. According to this principle gas is exposed to a large amount of catalyst surface at a low pressure drop. Gas enters the lateral flow reactor in numerous gas inlet channels which are blocked at the opposite end. The gas then must travel laterally through catalyst layers to reach the outlet channels. The document relates to a process for reducing the NOx and ISI₂O concentrations in residual gas from a nitric acid plant in two stages. The first stage reduces the NOx content and the second stage reduces the N₂O content. In the example a reducing agent was used in the first stage, whereas in the second stage the residual gas was passed over an iron-containing zeolite without a further reducing agent.

Another process in which an off-gas was treated in two stages has been disclosed in US-A 2006/0051277. In this process the off-gas is first passed through a first catalyst bed and subsequently through a second catalyst bed, whilst between the catalyst beds a reducing agent is being added. However, in this case the first bed serves to reduce the N₂O content and the second bed serves to reduce the NOx content. Hence, the reducing agent is present only in the second catalyst bed. In the patent application it is acknowledged that it is necessary to mix the reducing agent and the NOx-containing off-gas intimately to ensure a high efficiency. This seems to be achieved via a complicated mixer. Therefore, it is an objective of the present invention to provide a two-stage process and a two-stage device for the decomposition of nitrogen oxides wherein the mixing of reducing agent and nitrogen oxide-comprising off-gas is not a problem, and which allows for an efficient reduction of the nitrogen oxides content.

Accordingly, the present invention provides a reactor for the reduction of the concentration of nitrogen oxides in gases, comprising a gas inlet at one end and a gas outlet at another end, wherein the gas inlet leads to a first reactor section which reactor section comprises a plurality of first fixed catalyst beds defining between them first passages, in which reactor, alternately, first passages are closed off at one end, closest to the gas inlet, by means of first closing plates extending to the edges of first catalyst beds adjacent to the first passage in question, and other first passages are closed off at the other end, farthest from the gas inlet, by first closing plates extending to the edges of the first fixed catalyst beds adjacent to the first passage in question, thereby providing passageways from the gas inlet through the first fixed catalyst beds into other first passages that are in fluid communication with a second reactor section which second reactor section comprises a plurality of second fixed catalyst beds that define second passages between them, in which second reaction section, alternately, second passages are closed off at one end, farthest from the gas outlet, by second closing plates extending to the edges of second fixed catalyst beds adjacent to the second passage in question, and other second passages are closed off at the other end closest to the gas outlet by first closing plates extending to the edges of the first fixed catalyst beds adjacent to the first passage in question, thereby providing passageways from the second reactor section through the second fixed catalyst beds to the gas outlet. Further, the invention provides a process for the reduction of the concentration of nitrogen oxides in gases, comprising: - A -

(a) passing the gas with a gaseous reducing agent in lateral flow through a plurality of first fixed beds containing nitrogen oxides-reducing catalyst to obtain partly reduced gas; and (b) passing the partly reduced gas in lateral flow through a plurality of second fixed beds containing nitrogen oxides-reducing catalyst to obtain nitrogen oxides-depleted gas.

The present invention provides for an efficient use of the catalyst beds wherein the nitrogen oxides component, such as a NOx component, in the nitrogen oxide-comprising gas is completely converted. Even if some nitrogen oxide slips through the first fixed beds it will be converted in the second reactor section. Some of the reducing agent may then also slip through the first fixed beds so that the reduction of NOx may take place with such reducing agent. The addition of the reducing agent can take place easily in known and conventional manners outside of the reactor. It will be appreciated that a usual way of operating the reactor is in a vertical position so that the fixed beds have been vertically arranged and the passages are all vertical. However, such is not required; it is also possible to operate the two reactor sections is series in a horizontal, or even slanted reactor vessel.

One of the known advantages of using the fixed bed with catalyst in the above-described fashion, which is known as a lateral flow fashion, is the low pressure drop. However, when a reactor is divided into two reactor sections, as is the case in the present invention, there is the risk that turbulences occur in the region between the outlet passages of the first reaction section and the plurality of second fixed beds. Such turbulence may "~ O —

increase the pressure drop to unsatisfactory levels. Therefore, the plurality of second fixed beds is preferably carried out such that the second closing plates have been provided with flow-deflecting means. The flow-deflecting means may have any suitable shape.

Suitably, they will have an apex in the direction of the outlet passages of the first reaction section. Hence, a suitable form would be a triangular-shaped bar. Also, they may be shaped in a concave, straight or convex form. More preferably, the flow-deflecting means has a rounded- off shape. Hence the bar may have a cross^section in the shape of a triangle in which the apex has been rounded off. Another suitable cross-section is the shape of part of an oval. Suitable flow-deflecting means are pipe elements which have a cross-section in the shape of a segment of a circle or a semi-circle. That means that suitably the partly reduced gas from step (a) of the above process is passed along flow-deflecting means provided on part of the plurality of second fixed beds, before being passed through the plurality of second fixed beds.

The number of catalyst beds per plurality of fixed beds may vary in accordance with the size of the reactor and the desired pressure drop over the catalyst beds. Suitably, the number of second fixed beds varies from 4 to 50 beds. Preferably, the number varies from 6 to 20 beds. Such numbers allow for an acceptable pressure drop over the bed, whilst small catalyst particles can be used, thereby increasing the effectiveness of the catalyst. It will be appreciated that in the preferred embodiments where flow-deflecting means are arranged, the number of flow-deflecting means will be half of the number of beds. The skilled artisan will further appreciate that the numbers of beds in the plurality of first fixed beds and in the plurality of second fixed beds may be the same or different. Suitably, either plurality contains from 4 to 50 catalyst beds. Generally the gas inlet has been arranged such that the gas turbulence does not pose a problem. However, if desired the skilled person can arrange for flow- deflecting means to be attached also to the first closing plates in the first reactor section. Such can be advantageous if the gas flow does incur a pressure drop due to turbulence and if the pressure drop has a detrimental effect on the performance of the decomposition .

The catalyst in the present process may be any suitable catalyst for the reduction of nitrogen oxides.

The prior art has proposed several different catalysts. A list of suitable catalysts have been referred to in US-A 2006/0051277, and include amorphous vanadium- containing titania catalyst, but also zeolitic catalysts containing copper or iron, in particular iron-exchanged zeolite beta. The zeolitic catalyst may comprise other metals, e.g., noble metals such as platinum, ruthenium, palladium, osmium or rhodium. Preferably, the catalyst is one disclosed in EP-A 768110. Therefore, the nitrogen oxides-reducing catalyst advantageously comprises a titania carrier and one or more metal compounds selected from the metals vanadium, molybdenum and tungsten, vanadium being particularly preferred. Since it is advantageous to keep the pressure drop over the fixed beds minimal, it is desirable to select the most effective shape of the catalyst. The catalyst may be in the form of a powder. However, it is preferred that the nitrogen oxides-reducing catalyst is in the form of trilobes, rifled trilobes or cylinders. The catalyst particles may be solid or hollow.

The reducing agent is also known in the art. Generally, the reducing agent is selected from hydrogen, carbon monoxide and ammonia. Preferably, the reducing agent is ammonia.

The catalysts may be the same in both reactor sections. Alternatively, different catalysts may be used. This may be advantageous if different species of nitrogen oxides are to be decomposed, e.g. NO₂ in the first reactor section and any remaining NO₂ plus ISf₂O in the second reactor section. The skilled person may then select the most suitable catalyst for each decomposition. Suitable catalyst arrangements include an embodiment wherein the first reactor section comprises catalyst particles containing metals vanadium, molybdenum and/or tungsten on a titania carrier and the second reactor section comprises either the same catalyst or an iron- containing zeolitic catalyst, e.g. such as those described above.

The reaction conditions include, in general, a reaction temperature above 100 ⁰C, preferably ranging from 150 to 600 ⁰C, more preferably from 350 to 500 ⁰C. As to the reaction pressure it is desirable that that initial pressure of the gas is sufficiently high to provide for an effective flow through the fixed beds of catalyst. However, since the pressure drop over the fixed beds is not very big, the initial pressure of the gas does not need to be very high, and preferably ranges from 1 to 15 bar, more preferably from 2 to 8 bar. This relatively low pressure is very beneficial since in most cases the gas that comprises the nitrogen oxides does not — Q —

need to be compressed, which establishes a major cost savings.

The present reactor and the present process may be applied in the treatment of gases that contain a wide variety of nitrogen oxides concentrations. Such gases may contain from 10 to 10,000 ppm by volume of nitrogen oxides. The nitrogen oxides may comprise nitrogen monoxide, nitrogen dioxide and dinitrogen oxide. The gases may also comprise other contaminants, such as sulphur dioxide, carbon monoxide and carbon dioxide, and water vapour, in addition to oxygen and nitrogen. Based on the amount of nitrogen oxides in the gas, the skilled artisan may be able to design the required amount of catalyst, based on the reaction conditions and nature of the catalyst used.

The gaseous reducing agent may suitably be ammonia or carbon monoxide. Preferably the amount of reducing agent present is such that the molar ratio of reducing agent to nitrogen oxides is up to the stoichiometrically required ratio. Especially when one of the contaminants is dinitrogen oxide, the decomposition of this contaminant can also occur in the absence of a gaseous reducing agent.

The device of the present invention is very suitable if a de-bottlenecking of a reactor vessel is contemplated. In many cases a radial flow reactor with relatively large catalyst particles, e.g., with a smallest diameter of 4 to 8 mm, can suitably be replaced by lateral flow reactors containing relatively small catalyst particles, e.g., with a largest diameter of 1 to 2 mm. The smaller diameters allow for more active catalyst particles. The lateral flow reactor allows for a small pressure drop. Hence, the set-up of the present invention is excellently suited for de-bottlenecking if a radial flow reactor requires more activity.

The invention will be further illustrated by means of the figure. The figure shows a schematic version of the reactor according to the present invention.

A reactor vessel 1 has been provided with a gas inlet 2 at the top and a gas outlet 3 at the bottom. The reactor 1 has been divided in two reactor sections 4 and 5 by means of a plate 15. The reactor section 4 is provided with a plurality of first fixed catalyst beds 6. The fixed catalyst beds are closed at both ends. The side walls of the fixed catalyst beds are gas permeable. Between adjacent fixed beds first passages 7 and 8 are defined. Passages 7 are in fluid communication with the gas inlet 2, whereas the passages 8 are in fluid communication with the second reactor section 5. The arrangement of passages is caused by first closing plates

9 that alternately close off the passages as defined between two fixed beds and extend over the adjacent catalyst beds 6. Since the side walls of the fixed beds are carried out as permeable walls the gas will flow via first passages 7 through the fixed beds 6 into the first passages 8, and from there into the reactor section 5. In reactor section 5 a plurality of second fixed beds 10 with catalyst is provided, similar to the plurality of fixed beds in the first reactor section 4. The fixed beds

10 define second passages 11 which are in fluid communication with the passages 8, and passages 12 which are in fluid communication with the gas outlet 3. Reactor section 5 is separated from the gas outlet 3 via a plate 16. The fixed catalyst beds 10 are closed at both ends and the passages 12 are closed at the upper end by second closing plates 13. These second closing plates 13 have been provided with flow-deflecting means 14. In the figure these means have the shape of a pipe element with a cross-section of the shape of a semi-circle. As indicated above, these means may also have the shape of a concave pyramidal bar, of a part of a cylinder, of which the cross-section is oval or has the shape of a circular segment. These bars extend to cover at least part of the upper ends of fixed catalyst beds 10. The flow-deflecting means 14 may be attached to the second closing plates 13. Alternatively, they may be provided at a small distance, e.g., from 1 to 25 cm, upstream of the second closing plates. Although this may not have a technical advantage, it may facilitate the attachment of the plurality of flow-deflecting means upstream of the second closing plates.

The gas that is introduced into the reactor vessel 1 flows via passages 7 through the catalyst beds 6 to the passages 8. From the passages 8 the gas flows into reaction section 5. The gas flows along the flow- deflecting means 14 with minimal turbulence and subsequently through the permeable walls and catalyst particles of the catalyst beds 10 into the passages 12. From there the gas is withdrawn from the reactor vessel 1 via gas outlet 3.

Claims

C L A I M S

1, Reactor for the reduction of the concentration of nitrogen oxides in gases, comprising a gas inlet at one end and a gas outlet at another end, wherein the gas inlet leads to a first reactor section which reactor section comprises a plurality of first fixed catalyst beds defining between them first passages, in which reactor, alternately, first passages are closed off at one end, closest to the gas inlet, by means of first closing plates extending to the edges of first catalyst beds adjacent to the first passage in question, and other first passages are closed off at the other end, farthest from the gas inlet, by first closing plates extending to the edges of the first fixed catalyst beds adjacent to the first passage in question, thereby providing passageways from the gas inlet through the first fixed catalyst beds into other first passages that are in fluid communication with a second reactor section which second reactor section comprises a plurality of second fixed catalyst beds that define second passages between them, in which second reaction section, alternately, second passages are closed off at one end, farthest from the gas outlet, by second closing plates extending to the edges of second fixed catalyst beds adjacent to the second passage in question, and other second passages are closed off at the other end closest to the gas outlet by first closing plates extending to the edges of the first fixed catalyst beds adjacent to the first passage in question, thereby providing passageways from the second reactor section through the second fixed catalyst beds to the gas outlet.

2. Reactor according to claim 1, wherein the second closing plates have been provided with flow-deflecting means .

3. Reactor according to claim 2, wherein the flow- deflecting means is a pipe element which has a cross- section in the shape of a segment of a circle or a semicircle .

4. Process for the reduction of the concentration of nitrogen oxides in a gas, comprising:

(a) passing the gas with a gaseous reducing agent in lateral flow through a plurality of first fixed beds containing nitrogen oxides-reducing catalyst to obtain partly reduced gas; (b) passing the partly reduced gas in lateral flow through a plurality of second fixed beds containing nitrogen oxides-reducing catalyst to obtain nitrogen oxides-depleted gas.

5. Process according to claim 4, wherein the partly reduced gas from step (a) is passed along flow-deflecting means provided on part of the plurality of second fixed beds, before being passed through the plurality of second fixed beds.

6. Process according to claim 4 or 5, wherein the nitrogen oxides-reducing catalyst comprises a titania carrier and one or more metal compounds selected from the metals vanadium, molybdenum and tungsten.

7. Process according to any one of claims 4 to 6, wherein the nitrogen oxides-reducing catalyst is in the form of trilobes, rifled trilobes or cylinders.

8. Process according to any one of claims 4 to 7, wherein the reaction temperature ranges from 150 to 600 °C.

9. Process according to any one of claims 4 to 8, wherein the initial pressure of the gas ranges from 1 to 15 bar, preferably from 2 to 8 bar.

10. Process according to any one of claims 4 to 9, wherein the reducing agent is ammonia.