WO2015034365A1

WO2015034365A1 - Process for heterogeneous gas phase reactions and apparatus for carrying out said process

Info

Publication number: WO2015034365A1
Application number: PCT/NL2014/050613
Authority: WO
Inventors: Chepa ROJER
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 2013-09-09
Filing date: 2014-09-08
Publication date: 2015-03-12

Abstract

Heterogeneous catalyst systems and their use in gas phase reactions are disclosed. Two or more gaseous reactants are contacted with a catalyst, which catalyst is present on the outer surface of a solid porous support. The outer surface of the solid porous support is shaped to form walls that define one or more elongated channels. At least one of said reactants is fed through at least a part of said wall and wherein at least one other reactant is fed through said one or more channels. Efficient mixing of reactants and contact with the catalyst is thus obtained, resulting in compact reactors, which are characterized by a low pressure drop.

Description

Title: Process for heterogeneous gas phase reactions and apparatus for carrying out said process

The invention is in the field of catalysis. More specifically, the invention pertains to supported heterogeneous catalyst systems used in gas phase reactions. The invention will be described with reference to selective catalytic reduction of nitrogen oxides (SCR of NOx), but can be applied to other gas phase reactions as well.

SCR of NOx is frequently carried out using urea as a reducing agent. To this end, a solution of urea is injected in a gas exhaust stream. Due to the elevated temperatures, urea decomposes and forms inter alia NH3, which is a very effective reducing agent. The NH3 is subsequently contacted with the nitrogen oxides in the presence of a solid catalyst, present on a support, where they react into less harmful compounds.

In the coming years it is expected that the focus of the manufacturer of internal combustion engines will shift from emission reduction back to better fuel economy and by that to CO2 reduction.

Efficient internal combustion engines give lower exhaust gas temperature, which becomes a challenge for current Selective Catalytic Reduction (SCR) technology that performs best at higher exhaust gas temperatures.

Therefore an SCR technology must be created that performs best at lower temperatures.

The general SCR is given in reaction (1). 2 NH₃ + NO + NO₂→ 2 N₂ + 3 H₂O (1)

This reaction performs best when the fast SCR reaction is promoted by increasing the ratio between NO2 and NOx towards 0.5. This can be realized by using an oxidation catalyst coated with precious metals, such as platinum and/or palladium (also referred to as the diesel oxidation catalyst, DOC). However, at low temperatures this oxidation catalyst is not active enough and the NO2 to NOx ratio will remain low. As a consequence the fast SCR reaction will consume all NO2 and what remains is NO that can only be removed by the standard SCR reaction, which is given in reaction (2):

4 NH₃ + 4 NO + O₂→ 4 N₂ + 6 H₂O (2)

Reaction (2) is considerably slower than the fast SCR reaction (1). The present inventors realized that to achieve higher NOx conversion at low temperature a much higher residence time, which is in practice reflected in a longer SCR catalyst, would be needed. This is practically not possible due to packaging limitations. Just increasing the DOC length such that it delivers a NO2 to NOx ratio of 0.5 at lower temperature is not an option, because at intermediate or higher

temperatures the ratio will become higher than 0.5 and then the slow SCR reaction, reaction (3), is needed:

8 NH₃ + 6 NO₂→ 7 N₂ + 12 H₂O (3) This reaction is slower at low temperatures than the standard

SCR reaction (2) and needs an even longer SCR catalyst to achieve higher NOX conversions.

A possible solution is to arrange the NOx reduction unit by placing combining each SCR unit with an oxidation catalyst (OC) unit in series. The OC can boost the NO2 to NOx ratio after each section, and by that continuously promotes the fast SCR reaction, which will improve the overall NOx conversion efficiency effectively at low temperatures.

Furthermore the different OCs can now be optimized individually such that they deliver a NO2 to NOx ratio of around 0.5 at different temperatures. The OC does not only oxidize NO to NO2 but also oxidizes unconverted ammonia (ammonia slip) after the SCR. In order to supply ammonia to the next unit, an additional ammonia injection point may be added before each new SCR catalyst portion (also referred to as slice).

It was found that a standard urea dosing system cannot be used for injection of the ammonia gas into this NOx reduction unit. The main reason is that urea is an aqueous ammonia solution that must be

decomposed into ammonia gas and water. A good functioning urea decomposition solution for one single injection point is difficult to design and in general a good performing urea decomposition solution takes some decomposition length, which is not favorable for compact design.

A direct ammonia gas injection system for the system outlined hereinabove would be highly desirable. One of the challenges to meet is the proper mixing of ammonia gas with the exhaust gas. Good mixing is normally realized by using static gas mixers, which need a mixing length of 4 to 5 times the pipe diameter to fully mix the ammonia gas with the exhaust gas. This is not in favor of compact design. Furthermore mixing of gas in general results in an additional pressure drop that results in a less fuel efficient operation.

WO-2013/127473-A1 relates to a method to remove NOx compounds from engine exhaust gas by a series of two SCR catalysts, separated by a diesel particulate filter (DPF), and an oxidation catalyst (DOC) in front of the first SCR. It is described that at the cold start phase (gas temperatures of 160-220 °C) ammonium is injected together with the exhaust gas. After the cold start phase, at higher temperatures (>200 °C), ammonia addition is ceased and urea is injected between the filter and the second SCR catalyst and the ammonia, which is formed by decomposition, will reduce the NOx under the action of the second SCR catalyst. The single injection point between the filter and the second SCR catalyst is not in favor of compact design. EP-1992398-A1 describes an apparatus comprising an engine connected to a series of catalysts. The first catalyst is an oxidation catalyst, which is optionally connected to a soot filter. Subsequently connected is an SCR catalyst, which usually contains two modules in the form of the well- known honeycomb structure with parallel channels. To reduce NOx, ammonia is injected through a single point which is not in favor of compact design for the reasons mentioned above.

Surprisingly, the present inventors found that when gaseous reactants are fed through the porous material making up the support, which support can be e.g. a monolith or the like, a very efficient and compact design is obtained. Thus, in a first aspect, the present invention is directed to a process for carrying out a heterogeneously catalyzed gas phase reaction, comprising contacting two or more gaseous reactants with a catalyst, which catalyst is present on the outer surface of a solid porous support, wherein said outer surface of said solid porous support is shaped to form walls that define one or more elongated channels, wherein at least one of said reactants is fed through at least a part of said wall and wherein at least one other reactant is fed through said one or more channels. The support is typically a so-called monolith structure, viz. a block of porous ceramic material, such as cordierite e.g. with formulation EX-20, EX-22, EX-32, EX- 80 provided with parallel channels, each having substantially the same cross sectional size and shape, which channels run in the length direction of the block. Typically such a single block contains several hundreds to more than thousands of such channels, e.g. cell densities from 400 to 1200 cells per square inch (cpsi), preferably 400 to 600 cpsi.

The present inventors realized that the material making up the porous support is generally sufficiently accessible to gases. Typically the material that makes up the porous support, including the walls has a porosity ranging from 22% to 50% to allow a flow that is sufficient to maintain the reaction. In accordance with the present invention efficient mixing of reactants and contact with the catalyst is obtained, which makes compact reactor designs possible. As a result of the limited size a low pressure drop can be realized.

In case of SCR of nitrogen oxides as the gas phase reaction the invention is preferably carried out using ammonia (NH3) gas as the reducing agent. Suitable catalysts for this purpose are vanadium compounds, such as T1O2 based particles, which are impregnated with vanadate compounds, and zeolites, in particular zeolite ZSM-5, optionally exchanged e.g. with one or more metals selected from Cu, Pt, Mn, Fe, Co, Ni, Zn, Ag, Ce and Ga.

In a preferred embodiment, the process of the invention comprises a combination of an oxidation step and an SCR step, in which combination said SCR reaction is preceded by an oxidation reaction, which is carried out by contacting said gas stream containing said nitrogen oxides with an oxidizing agent, preferably oxygen from air, in the presence of an oxidation catalyst. The oxidation catalyst may also be present on a solid support.

Suitable catalysts vary, depending on the type of reaction. For the oxidation step in combination with the SCR reaction precious metal catalyst, such as Pt, Pd, Au or other compounds like Cu, Sr, CaTiOe and combinations thereof are very suitable.

The OC-SCR combination can be advantageously repeated a number of times in series, depending on the desired conversions. Due to the compact design, such repeated combinations can be constructed relatively easily.

An apparatus for carrying out the process of the invention, typically, comprises a catalyst on a solid porous support, wherein the outer surface of said solid porous support is shaped to form walls that define one or more elongated channels. It furthermore contains a gas pump for feeding gaseous reactants through at least a part of said wall and an inlet for feeding other gaseous reactants into said channels. The present invention allows for much more efficient and versatile dosing systems in SCR than conventional urea based systems. In particular the distribution of the reducing agent ammonia is much better with the present invention. Additional means to improve reactant mixing, such as static mixers, are no longer necessary in accordance with the present invention. As a result, pressure drop of the SCR unit coupled to the engine's exhaust gas system can be considerably lower, which is reflected in improved performance of the combustion engine.

Figure 1 is a schematic representation of a possible arrangement of an apparatus in accordance with the invention.

Figure 2 shows a schematic representation of a different possible arrangement in accordance with the present invention.

Figure 3 shows a schematic representation of yet another different possible arrangement in accordance with the present invention.

Figure 4 shows a schematic representation of yet another different possible arrangement in accordance with the present invention.

As shown schematically in figure 1, portions (for instance in the form of slices) with an SCR catalyst 601 are preceded and followed by portions (also possibly in the form of slices) of an oxidation catalyst (OC) in series (501,502,503, etc.). Only three SCR portions and four OC portions are shown in figure 1, but it will be understood that this figure may be varied, while the corresponding components will vary mutatis mutandis. The purpose of the OC is to increase the NO2 NOX ratio after each OC section. As a result the fast SCR reaction is continuously promoted. This will improve the NOx conversion efficiency effectively at low temperatures.

Furthermore the different OCs can now be optimized individually such that they deliver a NO2/NOX ratio of around 0.5 at different temperature regimes. The OC does not only oxidize NO to NO2 but also oxidizes the remaining ammonia slip after the SCR. In order to supply ammonia to the next slice an additional ammonia injection point can be present at each new slice entrance (201,202,203, etc.). In the configuration depicted in figure 1, NH₃ is fed through common rail (1) via valves (301, 302, 303, etc.). The common rail is depicted here outside canning (5) but can also be placed inside the canning (5).

NOx containing gas (2) is fed through first OC portion (501) and subsequently to SCR portion (601) where NH3 is adsorbed onto the catalytic sites. By using the setup of figure 1, gaseous ammonia can be distributed efficiently over the different SCR portions. After passing through SCR portions (601,602,603) gas (4) exits a final oxidation catalyst (504).

In the setup of figure 2 a possible ammonia dosing configuration is shown, which is based on a catalytic monolithic structure. In the configuration of figure 2 both ends of a monolith channel (901) are plugged (701,702) so that an isolated volume is created. Through one of the plugged sides a hollow tube or needle (2001) penetrates the plugged channel. In this way ammonia can be injected through hollow needle (2001). By increasing injection pressure ammonia will be forced through the channel wall

(801,802,803,804) as indicated by the small arrows (1001) and be contacted with the catalytic sites. NOx containing gas (2) is fed through a different hollow tube or needle (2002) into channel (902) where it reacts with the catalytic sites fed by ammonia that flows through the walls, as indicated by small arrows (1002).

A further possible configuration is shown in figure 3, which is based on a catalytic monolithic structure (7) with integrated ammonia delivery system. Here both ends (701,702) of channel (901) are plugged to form an isolated volume where ammonia gas (1001) can be injected under pressure. The ammonia gas will be forced through the substrate wall (804) as indicated by small arrows (1002) and supply the catalytic sites with ammonia.

NOx containing gas (2) is fed through channel (902) where it reacts with the catalytic sites located on the substrate wall (804). This direct ammonia injection method gives a very precise ammonia delivery method that guarantees that all catalytic sites distributed over the slice will be supplied with ammonia homogeneously.

Yet another configuration is shown in figure 4, which is also based on a catalytic monolithic structure (8). In this configuration the ammonia gas is delivered through the porous vena contracta of multiple Venturis (10). Only four Venturis are shown in figure 4, but in reality the number of Venturis may range widely, e.g. from 100 to 2000, e.g. in matrices of 10 x 10, 40 x 40, etc. The ammonia gas is supplied through channel (904). Under pressure the gas will be forced through the porous venturi wall (805) to channel (903) where the ammonia gas (1002) will mix with the NOx containing gas (2). The mixed gas (9) will be supplied to the SCR portions (601,602, etc.).

Claims

1. Process for carrying out a heterogeneously catalyzed gas phase reaction, comprising contacting two or more gaseous reactants with a catalyst, which catalyst is present on the outer surface of a solid porous support, wherein said outer surface of said solid porous support is shaped to form walls that define one or more elongated channels, wherein at least one of said reactants is fed through at least a part of said wall and wherein at least one other reactant is fed through said one or more channels.

2. Process according to the previous claim, wherein said gas phase reaction is the selective catalytic reduction (SCR) of nitrogen oxides, which comprises contacting a gas stream containing said nitrogen oxides and a reducing agent in the presence of a catalyst.

3. Process according to the previous claim, wherein said catalyst is selected from the group consisting of vanadium, zeolite and combinations thereof.

4. Process according to claim 2 or 3, wherein said reducing agent is ammonia.

5. Process according to claim 2-4, comprising a combination of an oxidation step and an SCR step, in which combination said SCR reaction is preceded by an oxidation reaction, which is carried out by contacting said gas stream containing said nitrogen oxides with an oxidizing agent, preferably oxygen from air, in the presence of an oxidation catalyst.

6. Process according to the previous claim, wherein said oxidation catalyst is present on a further solid support.

7. Process according to claim 5 or 6, wherein said oxidation catalyst is selected from Pt, Pd, Au, Cu, Sr, CaTiOe and combinations thereof.

8. Process according to claim 5-7, comprising two or more of said combinations are connected in series.

9. Ap aratus for carrying out a process according to any of the previous claims, comprising a catalyst on a solid porous support, wherein said outer surface of said solid porous support is shaped to form walls that define one or more elongated channels, a gas pump for feeding gaseous reactants through at least a part of said wall and an inlet for feeding other gaseous reactants into said channels.

10. Apparatus according to the previous claim, wherein said catalyst is an SCR catalyst selected from the group consisting of vanadium, zeolite and combinations thereof.

11. Coupled oxidation/SCR reactor, comprising an oxidation reactor, which comprises oxidation catalyst that is present on a porous support, which is connected on its effluent side to an SCR reactor according to claim 9 or 10.

12. Combinations of two or more coupled oxidation/SCR reactors according to the previous claim, wherein said influent side of each further oxidation/SCR reactor is connected to the effluent side of the preceding one.