WO1996036423A1

WO1996036423A1 - Catalytic method

Info

Publication number: WO1996036423A1
Application number: PCT/US1996/007060
Authority: WO
Inventors: William C. Pfefferle; Subir Roychoudhury
Original assignee: Precision Combustion Inc.
Priority date: 1995-05-18
Filing date: 1996-05-16
Publication date: 1996-11-21

Abstract

A catalytic reactor (15) for comprising an assembly of minilith catalytic elements (10, 11 and 12) having flow channels no longer than about three mm in length and spaced apart by monolith elements (A and B) of greater channel diameter. The catalytic elements are a mix of elements and differ from each other by the catalytic activity of the catalysts selected.

Description

CATALYTIC METHOD BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to improved catalytic reaction systems and to methods for catalytic reaction of carbon containing compounds. In one specific aspect the present invention relates to catalytic conversion of multiple pollutants in exhaust gases. Brief Description Of Related Art

Automotive emissions are sti_l very much a major environmental problem in spite of a more than ninety percent reduction in emissons of hydrocarbons and carbon monoxide brought about by use of catalytic converters. As a consequence of increased useage, at least a ninety-nine percent reduction is now required. Unfortunately, although present catalytic converters could readily achieve ninety-nine percent conversion of hydrocarbons and carbon monoxide at full operating temperature, at start-up, emissions are not controlled at all until the catalyst reaches its light-off temperature. As a result, the major portion of emissions occurrs during start-up.

In addition, it is now recognized that emissions of nitrogen oxides (NO_χ) during high speed operation must be reduced, and thus addition of a higher speed cycle to the FTP test is being considered. Accordingly, conversion efficiency for N0_χ needs to be improved.

Inasmuch as N0_χ conversion requires both a low oxygen concentration and the presence of reductants, eg. hydrocarbons in the gas stream, it is important that reductant conversion not be completed prior to attainment of a high conversion of N0_χ. In present commercial converters, a single three-way catalyst entity is employed for conversion of all three of the pollutants. With such converters, the conversion of N0_χ is typically significantly less than that for carbon monoxide or hydrocarbons. Hydrocarbon and carbon monoxide conversions, nevertheless, are lower than if N0_χ conversion were not required. With such converters running the engine richer to improve NO_χ conversion results in reduced hydrocarbon and carbon monoxide conversion. Ideally, it is desireable to decouple the individual reaction rates for optimum results. Mixtures or alloys of different catalyst agents have been employed to treat exhaust gases. For example, U.S. Patent 3,944,504 (Ford et al.) describes the removal of exhaust pollutants employing as a conversion catalyst, alloys of platinum and palladium. It is known in the art that even minor variations in composition, metals ratios or addtion of trace components, can result in significant differences in catalyst reactivity for a specific reaction. Thus, specific compositions are selected for their reactivity in regard to the intended application.

The platinum group metals are all oxidation catalysts, but within the group they vary in optimum selectivity for a given reaction. Thus, rhodium is more effective than platinum in reducing N0_χ to nitrogen. Palladium, on the other hand, has greater activity for the conversion of methane and other hydrocarbons than does platinum. Typical three-way catalysts represent a compromise formulation of all three precious metals.

The present invention makes possible catalyst assemblies which utilize a multipicity of catalyst compositions for optimized conversion of all exhaust pollutants. In particular, the present invention makes possible rugged, fast light-off catalytic converters for automotive engine exhaust control which provide optimum conversion of individual pollutants within a mixture of exhaust pollutants. The present invention also provides a means for selective catalytic partial oxidation of chemicals. SUMMARY OF THE INVENTION Definition of Terms In the present invention the terms "monolith" and "monolith catalyst" refer not only to conventional monolithic structures and catalysts such as employed in conventional catalytic converters but also to short channel length structures of enhanced mass transfer efficiency such as woven fabrics and catalyst coated expanded metal screens. In the present invention the term "minilith¹¹ refers to monolith elements having flow channels of less than three millimeters in length and more than forty channels per square centimeter. For the purposes of this invention, the term "catalyst brick" refers to an assembly of minilith catalyst elements having channel flow passages less than three millimeters in length and having more than forty channels per square centimeter and spaced apart by monolith elements of larger channel size. The terms "carbonaceous compound" and "hydrocarbon" as used in the present invention refer to organic compounds and to fluid streams containing fuel values in the form of compounds such as carbon monoxide, organic compounds or partial oxidation products of carbon containing compounds.

The Invention It has now been discovered that monolithic catalytic converter perfomance can be significantly improved by optimizing catalyst reactivity at selected points along the reacting flow path. By use of a reactor made up of individual catalytic monolith elements of different catalysts it has been found possible to tailor the catalytic reacivity along the reaction flow path to significantly enhance N0_χ conversion without impairing conversion of hydrocarbons. Minilith catalytic reactors are especially useful in the the present invention.

In one version, the invention comprises a catalytic reactor for the chemical conversion of a plurality of different compounds which are present together in an admixture, as for example the pollutants present in automotive engine exhaust. In another implementation, the invention comprises a catalytic reactor for the selective production of chemical compounds.

Preferably, the reactor comprises a minimum of five or more minilith catalyst elements, in series, most preferably ten or more, and has catalyst elements of at least two different cayalysts and more preferably elements of three or more different compositions. For automotive exhaust converters, thirty or more catalytic elements are preferred. Preferably the minilith elements are spaced apart by short monolith catalyst spacers having a channel diameter at least about fifty percent greater and more preferably more than double but less less than five times that of the spaced apart minilith catalyst elements. It is possible with this arrangement to avoid the blockage of channels from stacking of same size miniliths without the penalty of large unsupported minilith areas to achieve spacing of diverse catalyst elements. Thus it is possible to clamp a minilith catalyst assembly together between two heavier screens or short monoliths to form a rugged catalyst brick having internal elements differing in composition, one or more of which can be mounted in a housing to form a highly efficient, fast thermal response catalytic converter. The catalysts of my prior U.S. patent 5,051,241, incorporated herein by reference thereto, are especially advantageous for inclusion in the catalyst bricks of the present invention.

The low pressure drop, rugged catalyst bricks of high conversion efficiency, selectivity and fast thermal response of the present invention make possible not only as much as a ten fold or more reduction in catalyst mass as compared to that required to achieve the same conversion in mass transfer limited reactions of hydrocarbons using conventional monolith catalysts, but also catalytic converters with lower emissions of nitrogen oxides and higher selectivity partial oxidation reactors. As noted in the above referenced patent, it has been found that the specific mass transfer rate increases as the ratio of channel length to channel diameter of a monolith catalyst is reduced below about five to one or more preferably below about two to one and especially below about one to one. Mass transfer of reactants to the surface becomes sensitive to the inlet flow rate rather than being significantly limited by the diffusion rate through a thick laminar flow boundary layer as in conventional monolith catalysts, whether ceramic or metal. In conventional automotive monolith catalysts, the amount of pollutants oxidized is essentially independent of exhaust gas flow rate and thus percent conversion decreases with increase in flow rate. In contrast, in the minilith catalyst assemblies of the present invention, the amount of reactants oxidized typically increases with increase in flow rate. Thus if the inlet flow velocity is high enough, the reaction rate can even approach the intrinsic kinetic reaction rate at the given catalyst temperature without imposing an intolerable pressure drop. This means that it is practical to design catalytic converters for much higher conversion levels than is feasible with conventional catalytic converters. Conversion levels of 99.9% or even higher are achievable in for example, an automotive catalytic converter smaller in size than a lower conversion level conventional catalytic converter. Even conversion levels high enough for abatement of toxic industrial fumes are achievable in compact reactors. With the short flow paths, spaced apart catalysts of the present invention, pressure drop is low permitting the use of much smaller channel diameters for a given pressure drop, further reducing catalyst mass required. The rigid structure of the catalysts bricks of the present invention allows placement of a converter close to engine exhaust ports for more rapid heatup on starting an engine at low ambient temperatures. It has also been found that channel walls as thin as 0.1 mm or even less than 0.03 mm are practical with small channel diameters thus permitting high open areas even with such small channel diameters. Thus, as many as several thousand flow channels per square centimeter or even more are feasible without reducing open area in the direction of flow below sixty percent. Open areas greater than 65, 70 or even 80 percent are feasible even with high channel density miniliths.

Inasmuch as heat transfer and mass transfer are functionally related, an increase in mass transfer results in a corresponding increase in heat transfer. Thus, not only is catalyst mass reduced by use of the minilith catalysts of this invention, but the rate at which an automotive exhaust catalyst is heated by the hot engine exhaust is correspondingly enhanced.

The reduced catalyst mass together with the increased heat transfer rate enables a short channel catalyst to reach operating temperature much sooner than would a conventional automotive catalyst. If placed sufficiently close to the engine exhaust manifold, a minilith catalyst element can even reach operating temperature in less than ten seconds without the need for electrical heating. Many alloys are commercially available which are suitable for metal miniliths of the present invention including Haynes alloy 25, Inconel 600, and even certain stainless steels. With metal microliths, alloy selection is often determined primarily by oxidation resistance at the maximum operating temperature required by the given application.

The low pressure drops possible with catalytic converters based on the present invention makes it possible to utilize a large number of small diameter elements, even as many as two hundred in a one inch length, such that the converter diameter is not significantly larger than the engine exhaust pipe. This makes it much easier to place the converter catalyst at the exit of or even in the engine exhaust manifold, resulting in even faster catalyst warm up without electrical heating, and allows use of screens of different catalyst composition to achieve optimum hydrocarbon and N0_χ control. In fume abatement applications, the large number of different catalyst elements feasible means that it, practical to achieve whatever conversion levels are needed, even as high as 99.999 or better.

Although this invention has been described primarily in terms of automotive emissions control, the high mass transfer rates of minilith catalyst bricks offer higher conversions and improved selectivity in many catalytic conversion processes. In particular, minilith catalyst bricks employing ultra-short channel length minilith catalysts offer superior performance in highly exothermic reactions such as the conversion of methane and other hydrocarbons to partially oxidized species such as the conversion of methane to methanol and the conversion of ethane to ethylene.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a cross-sectional side view of a catalyst brick of the present invention showing spaced apart minilith catalyst elements of different catalyst activity.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention is further described in connection with the drawing. As shown in the sectional view, in one preferred embodiment a minilith catalyst brick 15 comprises a plurality of minilith catalyst elements 10, 11 and 12 each set of a differing catalyst and all having 100 or more flow channels per square centimeter and an open flow area of greater than sixty percent, separated by spacer screens 20, 21, and 22 respectively each having 35 flow channels per square centimeter and an open area of at least sixty percent. End screens 10 and 12 are also separated from high open area clamping grids 16 by a spacer. Clamping grids 16 are anchored to enclosing container wall 14 to provide a rugged high open area minilith catalyst brick with minimal blockage of catalyst flow passages. In catalyst bricks for automotive catalytic converters separator screens are advantageously catalyst coated and clamping grids are screens with an open area of at least 65 percent area. For clarity, only a few screens have been shown in the drawing, the spaces A and B having any number of catalyst elements. Advantageously, the catalyst brick has an open area in the direction of flow of at least 60 percent and more preferably greater than 65 percent. However, open areas as low as 50 percent are desirable in certain fume abatement applications. Minilith catalyst flow channel diameters less than about one millimeter are preferred. The catalysts of the present invention are readily made using known catalytic agents. The following examples describe means of making minilith catalysts but are not to be construed as limiting. A minilith catalyst of the present invention is made by vacuum sputtering a platinum group metal such as platinum onto a stainless steel screen which has been cleaned by heating in air to 750K. Typically the platinum coating may be thinner than 100 angstroms but may be thicker for greater catalyst life. Advantageously, a similarly thin layer of ceria or alumina may be deposited prior to deposition of the platinum. Catalysts containing palladium, iridium, rhodium or other metals can be similarly prepared. In many applications, a wire screen formed from a catalytic alloy, such as a platinum doped alloy, is a sufficiently active catalyst without additional coating. Although metal miniliths are preferred, ceramic miniliths can be made such as by slicing of ceramic honeycomb extrudates prior to firing. Such ceramic honeycomb extrudates advantageously may contain an organic binder to facilitate production of thin slices. However, ceramic miniliths are most advantageously in the form of fiber mats or screens composed of long fibers spun from any desired ceramic composition, preferably catalytic ceramics. As necessary for sufficient low temperature catalytic activity, ceramic and metal miniliths may be catalyzed using various techniques well known in the art. EXAMPLE I

A multi-element catalytic microlith automotive exhaust reactor having forty minilith catalyst elements of 250 flow channels per square centimeter is constructed using a eight catalyst elements of 70% open area screening of platinum coated stainless steel wires having a diameter of 0.10 mm with each screen spaced apart by a downstream screen having four channels per square centimeter with platinum coated wires 0.25 mm in diameter, followed by twenty similar catalyst elements coated with a rhodium rather than platinum for enhanced N0_χ conversion, and followed by twelve catalyt elements with a palladium only coating. The assembly is clamped between two heavier screens of 1.5 mm diameter wires having one channel per square centimeter to form a catalyst brick. Installed at the exhaust manifold outlet of a four cylinder automotive engine, catalyst light-off is within ten seconds of engine starting and thus exhaust emissions are controlled during initial operation of the engine. In use, inlet exhaust gases first pass through the platinum catalyst elements, which are formulated for high carbon monoxide conversion and take advantage of the high resistance of platinum to poisoning. The heat liberated by the combustion of carbon monoxide raises the gas temperature thus rapidly bringing the downstream elements to an effective operating temperature. The hot partially reacted exhaust gases, still containing high concentrations of reductants required for NO_χ, next pass through the rhodium catalyst elements to achieve a high conversion of the NO_χ present. Finally, the exhaust gases pass through palladium catalyt elements for maximum conversion of hydrocarbons. For engine exhaust gas applications at high maximum operating temperatures, it is often advatageous to use a palladium catalyst for the inlet (carbon monoxide oxidation) catalyst elements.

Claims

WHAT IS CLAIMED IS:

1. A catalytic reactor for the chemical conversion of a plurality of compounds which are present together in an admixture, which comprises; a plurality of five or more monolith catalyst elements in series, at least four of said catalyst elements being a different catalyst composition from the remainder of said elements.

2. The reactor of claim 1 wherein said monolith catalyst elements are minilith catalysts.

3. A catalytic reactor of claim 2 comprises a multiplicity of minilith catalytic elements spaced apart from one another by monolith elements having flow channels at least fifty percent larger in diameter than the flow channels of said minilith catalyst elements.

4. The reactor of claim 2 in which the minilith elements are clamped between monolith endplates to form a catalyst brick, said endplates having an open area of at least 65 percent.

5. The reactor of claim 2 in which the flow channel diameter of said miniliths are less than about one millimeter.

6. The reactor of claim 4 in which the number density of said minilith catalyst flow channels is greater than 100 channels per square centimeter and the open area of said catalyst brick in the direction of flow is greater than about 60 percent.

7. The reactor of claim 2 wherein said minilith elements have at least about 250 flow channels per square centimeter and greater than about 65 percent open area in the direction of flow.

8. The reactor of claim 2 wherein the initial catalyst elements are platinum coated.

9. The reactor of claim 8 wherein said platinum coated elements are followed by rhodium coated elements for high conversion of N

Ox in exhaust gases.

10. The reactor of claim 9 wherein said rhodium coated elements are followed by palladium coated elements.

11. The reactor of claim 10 comprising thirty or more minilith catalyst elements.

12. The reactor of claim 2 wherein the initial catalyst elements are palladium coated.