Microlith catalytic reaction system

A catalytic reactor for oxidation of carbonaceous fuels comprising at least one microlith catalytic element having flow channels with a flow path length no longer than about two times the diameter of the largest flow channel. The initial catalyst element is advantageously electrically conductive to permit electrical heating.

BACKGROUND OF THE INVENTION 
1. 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 quick lightoff, fast 
thermal response catalysts for use in catalytic exhaust gas reactors and 
in catalytic fuel combustion systems. 
In one still more specific aspect, this invention relates to low thermal 
mass electrically conductive catalysts suitable for rapid electrical 
heating to operating temperature. 
2. Brief Description of the Prior Art 
Automotive emissions are still a major environmental problem in spite of 
the major advances brought about by the use of catalytic converters. One 
factor limiting the performance of catalytic converters is that pollution 
is not controlled during the thirty or so seconds required to bring the 
converter catalyst to its operating temperature. In present converters, 
warm-up is dependent on heating of the catalyst by hot engine exhaust 
gases. Although electrical heating could be utilized to preheat the 
catalyst prior to engine operation, the power and the time delay required 
with present catalyst structures, ceramic or metal, have been deemed 
unacceptable. 
Subsequent to catalyst light-off, surface reactions on conventional 
monolithic catalysts such as are used in catalytic converters are mass 
transfer limited. Thus, the catalyst mass required for a given conversion 
level is much higher than if no mass transfer limitation existed at the 
given operating conditions. The high catalyst mass required for the 
required conversion level results in the relatively long heat-up times 
experienced, even with electrical heating. In addition, this mass transfer 
limitation is such that the conversion level of present automotive exhaust 
catalytic converters is limited to relatively low levels, typically not 
much more than about 95%, even with the relatively small catalyst channels 
sizes employed. Higher conversion levels would be advantageous. 
The need to reduce catalyst warm-up time of the conventional ceramic 
monolith automotive catalysts to reduce emissions during the warm-u period 
has led to increased interest in metal monolith catalysts. However, merely 
substituting metal for ceramic in a conventional monolith structure yields 
catalysts which still have much too high a thermal mass. Although metal 
monoliths are electrically conductive and could therefore be electrically 
preheated, fast enough heat up times have not yet been demonstrated as 
feasible. Furthermore, thermal shock damage would likely be a problem if a 
conventional metal monolith were heated as rapidly as needed for 
elimination of start-up emissions. There is a critical need for a catalyst 
system which can control hydrocarbon emissions during initial engine 
operation. 
For catalytic combustors the problem is not just emissions but the ability 
to function in certain applications. For example, an automotive catalytic 
combustor gas turbine must start in roughly the same time frame as present 
automotive engines. 
The present invention provides catalysts and systems which make possible 
much more rapid warm-up of converter catalysts without electrical heating 
and near instantaneous electrical heating of catalysts in combustors and 
catalytic converters. Moreover, catalysts of the present invention enable 
much higher conversions and improved selectivity in many chemical 
conversion processes by virtue of improved mass transfer to and from the 
catalyst surface. 
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 any equivalent 
unitary structure such as an assembly or roll of interlocking sheets or 
the like. 
For the purposes of this invention, the terms "microlith" and "microlith 
catalyst" refer to high open area monolith catalyst elements with flow 
paths so short that reaction rate per unit length per channel is at least 
fifty percent higher than for the same diameter channel with a fully 
developed boundary layer in laminar flow, i.e. a flow path of less than 
about two mm in length, preferably less than one mm or even less than 0.5 
mm and having flow channels with a ratio of channel flow length to channel 
diameter less than about two to one, but preferably less than one to one 
and more preferably less than about 0.5 to one. Channel diameter is 
defined as the diameter of the largest circle which will fit within the 
given flow channel and is preferably less than one mm or more preferably 
less than 0.5 mm. Microlith catalysts may be in the form of woven wire 
screens, pressed metal or ceramic wire screens or even pressed thin 
ceramic plates and have as many as 100 to 1000 or more flow channels per 
square centimeter. Flow channels may be of any desired shape. For wire 
screens, flow channel length is the wire diameter and thus advantageously 
may be shorter than 0.3 mm or even shorter than 0.1 mm. 
The terms "carbonaceous compound" and "hydrocarbon" as used in the present 
invention refer to organic compounds and to gas 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 found that use of the microlith catalysts of the present 
invention makes possible 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 
monoliths. 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 such 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 microlith catalysts 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 microlith fume abatement reactors for 
much higher conversion levels than is feasible with conventional catalytic 
converters. Conversion levels of 99.9% or even higher are achievable in a 
microlith automotive converter smaller in size than a lower conversion 
level conventional catalytic converter. Conversion levels high enough for 
abatement of toxic fumes are achievable in compact reactors. 
With the short flow paths of 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. 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 microliths. 
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 microlith 
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 microlith catalyst to reach operating temperature much sooner 
than would a conventional automotive catalyst. If placed sufficiently 
close to the engine exhaust manifold, a microlith catalyst element can 
even reach operating temperature in less than five seconds without 
electrical heating. Effective operating temperature for automotive exhaust 
microlith precious metal catalysts are as low as 650 or even as low as 550 
degrees Kelvin. However, an important feature of microlith catalysts is 
that high enough operating temperatures are achievable prior to or during 
engine cranking to permit effective use of base metal catalysts. It has 
been found that a metal microlith composed of a high temperature alloy 
containing a catalytic element such as chromium, cobalt, copper, 
manganese, nickel or a rare earth metal is catalytically active if heated 
to a temperature of about 800 degrees Kelvin, a temperature readily 
achieved in less than one second with electrical heating. Many such alloys 
are commercially available and include 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 mass of microlith catalyst elements can be so low that it is feasible 
to electrically preheat the catalyst an effective operating temperature in 
less than about 0.50 seconds if a thin channel wall electrically 
conductive catalyst, e.g. a metal microlith, is used. In catalytic 
combustor applications the low thermal mass of catalyst elements of the 
present invention makes it possible to bring a combustor catalyst up to a 
light-off temperature as high as 1000 or even 1500 degrees Kelvin in less 
than about five seconds by electrical heating and even in less than about 
one or two seconds using the power from a conventional automotive battery. 
Such rapid heating is allowable for microlith catalysts because 
sufficiently short flow paths permit rapid heating without the consequent 
thermal expansion resulting in destructive stress levels. 
Typically, in automotive exhaust systems of the present invention the 
catalyst elements preferably have flow paths of less than about one 
millimeter in length and may be less than about 0.1 millimeter in length 
with as little five high channel density elements required to greatly 
exceed the start-up performance of a 150 millimeter long conventional 
monolith. The short channels result in a low pressure drop even with 
channels as small as 0.25 millimeters in diameter. However, if 
particulates are present channel size must be large enough to avoid 
plugging. In catalytic combustor applications, where unvaporized fuel 
droplets may be present, flow channel diameter is often large enough to 
allow unrestricted passage of the largest expected fuel droplet. Therefore 
in catalytic combustor applications flow channels may be as large as 1.0 
millimeters in diameter whereas in automotive catalytic converter 
applications, flow channel diameter often can be as small as 0.5 to 0.25 
millimeters or even smaller. If desired, one, two or three microlith 
catalyst elements may be placed in front of a conventional monolith 
catalyst element to serve as a light-off reactor for the monolith. This 
approach is useful for retrofit applications. 
Although as few as one or two catalyst elements advantageously may be used 
in a given catalytic converter application to improve the cold start 
performance of conventional monolith catalysts, the low pressure drops 
possible with catalysts of 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 composition to 
achieve both hydrocarbon and NOx control. In other fume abatement 
applications, the large number 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 microlith catalysts 
offers higher conversions and improved selectivity in many catalytic 
conversion processes. In particular, microlith 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 or and the conversion of ethane to 
ethylene.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The present invention is further described in connection with the drawings. 
As shown in FIG. 1, in one preferred embodiment a microlith catalyst 
element 10 comprises a plurality of square flow channels 11 with 
electrical leads 15 connected to bus bars 16. Bus bars 16 are welded at a 
forty five degree to metallic flow channel walls 12 to ensure even heating 
of catalyst 10. Advantageously, catalyst element 10 is in the form of a 
catalytic metal screen of at least about 400 flow channels per square 
centimeter with a wire diameter sufficiently small to yield an open area 
of at least about 70 percent. Using the power of a standard automotive 
battery the catalyst may be brought to an effective operating temperature 
in less than one second, often in significantly less than 0.50 seconds. 
Thus in automotive exhaust gas service, electrical power need not be 
applied till just after start of cranking thus limiting maximum drain on 
the battery. Advantageously, electrical power is applied prior to 
termination of engine cranking. Typically, an automotive microlith 
catalyst element is heated to an effective operating temperature within 
one to two seconds of start of cranking. This rapid heating is important 
in that no delay in engine starting is required to achieve emissions 
control. Typical reactors may have from one to ten or more such 
microliths. 
FIG. 2 shows a sectional view of a three element microlithic catalyst 
reactor 20 suitable for either automotive exhaust gas treatment or for 
catalytic combustor service. Microlith catalyst elements 21 having 400 
flow channels per square centimeter are spaced apart a distance equal to 
or greater than the length of the flow paths 22 to provide for some mixing 
of gases flowing between elements 21. Catalyst elements 21 are held in 
reactor 20 by retaining rings 26 and separated from each other by spacers 
27. A microlith catalyst reactor such as shown in FIG. 2, depending on the 
application, may contain any desired number of microlith elements. With 
fine wire microlith screens, as many as one hundred or more can readily be 
placed in a one inch long reactor. 
The microlith catalysts of the present invention are readily made using 
known catalytic agents. The following examples describe means of making 
microlith catalysts but are not to be construed as limiting. An microlith 
catalyst as per figure one is made by vacuum sputtering 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, especially with electrical 
heating, a wire screen formed from stainless steel or other alloy is a 
sufficiently active catalyst without additional coating. Although metal 
microliths are preferred, ceramic microliths 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 microliths 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 microliths may be catalyzed using various techniques well known 
in the art. 
EXAMPLE I 
A three element catalytic microlith automotive exhaust reactor having about 
2500 flow channels per square centimeter is constructed using a five 
centimeter wide strip of 70% open area screening of platinum coated 
stainless steel wires having a diameter of 0.03 mm spaced 0.20 mm apart 
and installed in the exhaust pipe of a four cylinder automotive engine. 
During engine cranking electrical power from the battery is applied 
heating the microlith catalyst elements to a temperature of 700 degrees 
Kelvin within one second whereby hydrocarbon emissions are controlled 
during initial operation of the engine. 
EXAMPLE II 
An electrically heated ten element microlith catalytic combustor is 
constructed using a screen fabricated with 0.076 mm wires of Kanthal. 
Ambient temperature air is passed through the reactor at a flow velocity 
greater than the laminar flame velocity of the fuel to be burned. The 
catalyst is then heated electrically to a temperature of 1000 degrees 
Kelvin and an intimate admixture of fuel and air is formed by spraying jet 
fuel into the air passing into the reactor. Plug flow combustion of the 
fuel is achieved. 
EXAMPLE III 
A fume abatement reactor six centimeters in length is constructed using 300 
microlith elements of screening with about thirty 0.050 mm wires of 
platinum coated nichrome per centimeter (nominally 900 flow channels per 
square centimeter). Fumes containing 50 ppm by volume of benzene in air 
are preheated to 700 degrees Kelvin and passed through the microlith 
reactor. Better than 99.9 percent conversion of the benzene is achieved.