Molecular sieve abstract
This invention relates to a process for treating an engine exhaust
gas stream. The process involves first flowing the engine exhaust
stream, which is relatively cool, over an adsorbent zone which comprises
an adsorbent bed, i.e., a molecular sieve bed, capable of preferentially
adsorbing pollutants such as hydrocarbons. This provides a first
exhaust stream which is flowed over a primary castalyst which converts
the pollutants to innocuous compounds and then discharging the resultant
treated exhaust stream to the atmosphere. When the adsorbent bed
reaches a temperature of about 150.degree. C., the entire engine
exhaust stream is completely diverted over the primary catalyst.
When the inlet temperature to the primary catalyst has reached about
350.degree. C., a minor portion of the engine exhaust stream is
diverted over the adsorbent bed to desorb the pollutants adsorbed
on the bed and carry them over the primary catalyst where they are
converted to innocuous components. After a certain amount of time,
the entire engine exhaust stream is again diverted over the primary
catalyst, thereby isolating the adsorbent bed to minimize deterioration.
The adsorbent zone may also have a secondary catalyst bed arranged
immediately after it.
Molecular sieve claims
We claim as our invention:
1. A process for treating an engine exhaust gas stream containing
hydrocarbons and other pollutants comprising directing the engine
exhaust gas stream over an adsorbent zone comprising a molecular
sieve bed which preferentially adsorbs the hydrocarbons over water,
to provide a first exhaust stream, flowing the first exhaust stream
over a primary catalyst to convert substantially all the pollutants
contained in the first exhaust stream to innocuous products, thereby
providing a treated exhaust gas stream and discharging the treated
exhaust stream into the atmosphere, said process being carried out
for a time until the molecular sieve bed temperature is about 150.degree.
C., at which time the engine exhaust gas stream is diverted completely
away from the adsorbent zone and routed directly over the primary
catalyst until such time as the primary catalyst reaches its operating
temperature, at which time the engine exhaust gas stream is divided
into a major and minor portion, flowing the major portion of the
engine exhaust gas stream over the primary catalyst and then discharging
the treated exhaust gas stream from the primary catalyst into the
atmosphere, flowing the minor portion of the engine exhaust gas
stream over the adsorbent zone for a time sufficient to desorb substantially
all the hydrocarbons adsorbed on the molecular sieve bed and provide
a second exhaust gas stream containing desorbed hydrocarbons, flowing
the second exhaust stream over the primary catalyst to provide a
treated exhaust stream and discharging the treated exhaust stream
to the atmosphere and after such time as necessary to desorb substantially
all the hydrocarbons from the molecular sieve bed, the engine exhaust
gas stream is completely directed over the primary catalyst to provide
a treated exhaust stream and then discharging the treated exhaust
stream to the atmosphere; the molecular sieve characterized in that
it is selected from the group consisting of molecular sieves which
have: 1) a framework Si:Al ratio of at least 2.4; 2) are hydrothermally
stable; and 3) have a hydrocarbon selectivity (.sup..alpha. HC--H.sub.2
O) greater than 1 where .sup..alpha. HC--H.sub.2 O is defined by
the following equation: ##EQU2## where .sup.X HC is the hydrocarbon
co-loading on the molecular sieves in equilibrium with the hydrocarbon
water vapor mixture in the gas phase over the molecular sieve adsorbent;
.sup.X H.sub.2 O is the water co-loading on the zeolite in equilibrium
with the water and hydrocarbon vapor mixture in the gas phase over
the molecular sieve adsorbent; [H.sub.2 O] is the concentration
of water and [HC] is the concentration of hydrocarbon.
2. The process of claim 1 where the engine is an internal combustion
engine.
3. The process of claim 2 where the internal combustion engine
is an automobile engine.
4. The process of claim 1 where the engine is fueled by a hydrocarbonaceous
fuel.
5. The process of claim 4 where the fuel is a hydrocarbon.
6. The process of claim 4 where the fuel is an alcohol.
7. The process of claim 1 where the molecular sieve is selected
from the group consisting of silicalite, faujasite, clinoptilolites,
mordenites, chabazite, zeolite ultrastable Y, zeolite Y, ZSM-5 and
mixtures thereof.
8. The process of claim 7 where the molecular sieve is faujasite.
9. The process of claim 7 where the molecular sieve is ultrastable
zeolite Y.
10. The process of claim 1 where the molecular sieve bed is a honeycomb
monolithic carrier having deposited thereon a molecular sieve selected
from the group consisting of molecular sieves having a Si:Al ratio
of at least 2.4 is hydrothermally stable and has a hydrocarbon
selectivity (.sup..alpha. HC--H.sub.2 O) greater than 1.
11. The process of claim 1 where the molecular sieve has deposited
thereon a metal selected from the group consisting of platinum,
palladium, rhodium, ruthenium and mixtures thereof.
12. The process of claim 11 where the metal is platinum.
13. The process of claim 11 where the metal is palladium.
14. The process of claim 11 where the metal is a mixture of platinum
and palladium.
15. The process of claim 1 where the adsorbent zone comprises a
molecular sieve bed followed by a secondary catalyst bed arranged
in tandem.
16. The process of claim 15 where the secondary catalyst bed is
a honeycomb monolithic carrier having deposited thereon a gamma
alumina support which has dispersed thereon palladium metal.
Molecular sieve description
BACKGROUND OF THE INVENTION
Gaseous waste products resulting from the combustion of hydrocarbonaceous
fuels, such as gasoline and fuel oils, comprise carbon monoxide,
hydrocarbons and nitrogen oxides as products of combustion or incomplete
combustion, and pose a serious health problem with respect to pollution
of the atmosphere. While exhaust gases from other carbonaceous fuel-burning
sources, such as stationary engines, industrial furnaces, etc.,
contribute substantially to air pollution, the exhaust gases from
automotive engines are a principal source of pollution. Because
of these health problem concerns, the Environmental Protection Agency
(EPA) has promulgated strict controls on the amounts of carbon monoxide,
hydrocarbons and nitrogen oxides which automobiles can emit. The
implementation of these controls has resulted in the use of catalytic
converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide,
hydrocarbon and nitrogen oxide pollutants, it has become the practice
to employ catalysts in conjunction with air-to-fuel ratio control
means which functions in response to a feedback signal from an oxygen
sensor in the engine exhaust system. Although these three component
control catalysts work quite well after they have reached operating
temperature of about 300.degree. C., at lower temperatures they
are not able to convert substantial amounts of the pollutants. What
this means is that when an engine and in particular an automobile
engine is started up, the three component control catalyst is not
able to convert the hydrocarbons and other pollutants to innocuous
compounds. Despite this limitation, current state of the art catalysts
are able to meet the current emission standards. However, California
has recently set new hydrocarbon standards (these standards most
probably will be promulgated nationwide) which can not be met with
the current state of the art three component control catalysts.
Applicants have found a solution to this problem which involves
the use of an adsorbent bed to adsorb the hydrocarbons during the
cold start portion of the engine. Although the process will be exemplified
using hydrocarbons, the instant invention can also be used to treat
exhaust streams from alcohol fueled engines as will be shown in
detail. Applicants' invention involves taking the exhaust stream
which is discharged from an engine during the initial startup of
the engine (cold start) and diverting it through an adsorbent bed
which preferentially adsorbs hydrocarbons over water under the conditions
present in the exhaust stream. The exhaust stream discharged from
the adsorbent bed (first exhaust stream) is flowed over a primary
catalyst and then discharged into the atmosphere. After a certain
amount of time, the adsorbent bed has reached a certain temperature
(about 150.degree. C.) at which the bed is no longer able to remove
hydrocarbons from the engine exhaust stream. That is, hydrocarbons
are actually desorbed from the adsorbent bed instead of being adsorbed.
At that point the engine exhaust stream is diverted such that the
engine exhaust stream completely bypasses the adsorbent bed and
flows directly over the primary catalyst bed.
After an additional amount of time during which the primary catalyst
has reached its operating temperature, the engine exhaust stream
is divided into a major and minor portion. The major portion of
the engine exhaust stream is flowed directly over the primary catalyst
while the minor portion of the engine exhaust stream is flowed over
the adsorbent bed thereby desorbing the hydrocarbons and any other
pollutants that were adsorbed on the bed. The stream from the adsorbent
bed (second exhaust stream) is again flowed over the primary catalyst
and then discharged into the atmosphere. When all the hydrocarbons
have been desorbed from the adsorbent bed, the engine exhaust stream
is completely directed over the primary catalyst. This ensures that
the adsorbent bed is not exposed to high temperatures which may
damage the adsorbent bed.
The adsorbents which may be used to adsorb the hydrocarbons may
be selected from the group consisting of molecular sieves which
have 1) a Si:Al ratio of at least 2.4;2) are hydrothermally stable;
and 3) have a hydrocarbon selectivity greater than 1. Examples of
molecular sieves which meet these criteria are silicalite, faujasites,
clinoptilolites, mordenites and chabazite. The adsorbent bed may
be in any configuration with a preferred configuration being a honeycomb
monolithic carrier having deposited thereon the desired molecular
sieve.
The prior art reveals several references dealing with the use of
adsorbent beds to minimize hydrocarbon emissions during a cold start
engine operation. One such reference is U.S. Pat. No. 3699683
in which an adsorbent bed is placed after both a reducing catalyst
and an oxidizing catalyst. The patentees disclose that when the
exhaust gas stream is below 200.degree. C. the gas stream is flowed
through the reducing catalyst then through the oxidizing catalyst
and finally through the adsorbent bed, thereby adsorbing hydrocarbons
on the adsorbent bed. When the temperature goes above 200.degree.
C. the gas stream which is discharged from the oxidation catalyst
is divided into a major and minor portion, the major portion being
discharged directly into the atmosphere and the minor portion passing
through the adsorbent bed whereby unburned hydrocarbons are desorbed
and then flowing the resulting minor portion of this exhaust stream
containing the desorbed unburned hydrocarbons into the engine where
they are burned.
Another reference is U.S. Pat. No. 2942932 which teaches a process
for oxidizing carbon monoxide and hydrocarbons which are contained
in exhaust gas streams. The process disclosed in this patent consists
of flowing an exhaust stream which is below 800.degree. F. into
an adsorption zone which adsorbs the carbon monoxide and hydrocarbons
and then passing the resultant stream from this adsorption zone
into an oxidation zone. When the temperature of the exhaust gas
stream reaches about 800.degree. F. the exhaust stream is no longer
passed through the adsorption zone but is passed directly to the
oxidation zone with the addition of excess air.
Finally, Canadian Patent No. 1205980 discloses a method of reducing
exhaust emissions from an alcohol fueled automotive vehicle. This
method consists of directing the cool engine startup exhaust gas
through a bed of zeolite particles and then over an oxidation catalyst
and then the gas is discharged to the atmosphere. As the exhaust
gas stream warms up it is continuously passed over the adsorption
bed and then over the oxidation bed.
Applicant's invention differs in several ways from the processes
described in the prior art. First, the adsorbent bed used in applicant's
process is a selective adsorbent bed which is a molecular sieve
bed. What this means is that hydrocarbons and other pollutants are
preferentially adsorbed over water which means that the adsorbent
bed does not have to be very large in order to adsorb sufficient
quantities of hydrocarbons and other pollutants during engine startup.
Another distinguishing feature is that when the adsorbent bed exceeds
a temperature of about 150.degree. C., the engine exhaust stream
is diverted completely away from the adsorbent bed and routed directly
over the primary catalyst. Once the three component control catalyst
bed reaches the desired operating temperature, the exhaust stream
is divided into a major and minor portion with the minor portion
being flowed over the adsorbent bed, thereby desorbing the hydrocarbon
and any other pollutants adsorbed thereon, while the major portion
is directly flowed over the catalyst. Additionally, no excess air
or oxygen is added to the catalyst. Applicant's process has the
advantage of allowing the three component control catalyst to warm
up much faster because the size of the adsorbent bed is minimized.
That is, because the molecular sieves used in the adsorbent bed
selectively adsorb pollutants over water, the volume of the adsorbent
bed is much smaller versus adsorbents in the prior art which do
not selectively adsorb pollutants. A smaller adsorbent bed means
a smaller heat sink which means that a hotter exhaust gas stream
contacts the catalyst. The molecular sieves which are used as the
adsorbents also exhibit good hydrothermal stability, thereby minimizing
replacement of the adsorbent bed.
SUMMARY OF THE INVENTION
This invention generally relates to a process for treating an engine
exhaust stream and in particular a process for minimizing pollutant
emissions during the cold start operation of an engine. Accordingly,
one embodiment of the invention is a process for treating an engine
exhaust gas stream containing pollutants comprising directing the
engine exhaust gas stream over an adsorbent zone comprising a molecular
sieve bed which preferentially adsorbs the pollutants over water,
to provide a first exhaust stream, flowing the first exhaust stream
over a primary catalyst to convert substantially all the pollutants
contained in the first exhaust stream to innocuous products, thereby
providing a treated exhaust gas stream and discharging the treated
exhaust stream into the atmosphere, said process being carried out
for a time until the adsorbent bed temperature is about 150.degree.
C., at which time the engine exhaust gas stream is diverted completely
away from the adsorbent zone and routed directly over the primary
catalyst until such time as the primary catalyst reaches its operating
temperature, at which time the engine exhaust gas stream is divided
into a major and minor portion, flowing the major portion of the
engine exhaust gas stream over the primary catalyst and then discharging
the treated exhaust gas stream from the primary catalyst into the
atmosphere, flowing the minor portion of the engine exhaust gas
stream over the adsorbent zone for a time sufficient to desorb substantially
all the pollutants adsorbed on the molecular sieve bed and provide
a second exhaust gas stream containing desorbed pollutants, flowing
the second exhaust stream over the primary catalyst to provide a
treated exhaust stream and discharging the treated exhaust stream
to the atmosphere and after such time as necessary to desorb substantially
all the pollutants from the adsorbent bed, the engine exhaust gas
stream is completely directed over the primary catalyst to provide
a treated exhaust stream and then discharging the treated exhaust
stream to the atmosphere.
In a specific embodiment, the molecular sieve bed is a honeycomb
monolithic carrier having deposited thereon a molecular sieve selected
from the group consisting of molecular sieves having a Si:Al ratio
of at least 2.4 is hydrothermally stable and has a hydrocarbon
selectivity (.sup..alpha. HC--H.sub.2 O) greater than 1.
In another embodiment, the adsorbent zone comprises a molecular
sieve bed followed by a secondary catalyst bed arranged in tandem.
Other objects and embodiments will become more apparent after a
more detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of one embodiment of this invention
showing an internal combustion engine and the process of this invention
during the cold start operation.
FIG. 2 is a graph of percent hydrocarbon retention versus time
on which are presented three plots showing the results for three
different molecular sieve adsorbents.
DETAILED DESCRIPTION OF THE INVENTION
As stated this invention generally relates to a process for treating
an engine exhaust stream and in particular a process for minimizing
emissions during the cold start operation of an engine. Referring
now to FIG. 1 the engine 1 consists of any internal or external
combustion engine which generates an exhaust gas stream containing
noxious components including unburned or thermally degraded hydrocarbons
or similar organics. Other noxious components usually present in
the exhaust gas include nitrogen oxides and carbon monoxide. The
engine may be fueled by a hydrocarbonaceous fuel. As used in this
specification and in the appended claims, the term "hydrocarbonaceous
fuel" includes hydrocarbons, alcohols and mixtures thereof.
Examples of hydrocarbons which can be used to fuel the engine are
the mixtures of hydrocarbons which make up gasoline or diesel fuel.
The alcohols which may be used to fuel engines include ethanol and
methanol. Mixtures of alcohols and mixtures of alcohols and hydrocarbons
can also be used. Engine 1 may consist of a jet engine, gas turbine,
internal combustion engine, such as an automobile, truck or bus
engine, a diesel engine or the like. The process of this invention
is particularly suited for hydrocarbon, alcohol, or hydrocarbon-alcohol
mixture, internal combustion engine mounted in an automobile. Under
the conditions of FIG. 1 engine 1 is initially operating at a relatively
reduced temperature, such as a cold engine at startup or warmup
which produces a relatively high concentration of hydrocarbon vapors
(when a hydrocarbon fuel is used) in the engine exhaust gas stream.
When an alcohol is the fuel, the exhaust stream will contain unburned
alcohol.
For convenience the description will use hydrocarbon as the fuel
to exemplify the invention. The use of hydrocarbon in the subsequent
description is not to be construed as limiting the invention to
hydrocarbon fueled engines.
The engine exhaust gas stream under startup conditions is generally
at a temperature below 500.degree. C. and typically in the range
of 200.degree. to 400.degree. C., and contains pollutants including
high concentration of hydrocarbons as well as nitrogen oxides and
carbon monoxide. Pollutants will be used herein to collectively
refer to any unburned fuel components and combustion byproducts
found in the exhaust stream. For example, when the fuel is a hydrocarbon
fuel, hydrocarbons, nitrogen oxides, carbon monoxide and other combustion
byproducts will be found in the exhaust gas stream. The engine exhaust
stream is produced at this relatively low temperature during the
initial period of engine operation, typically for the first 30 seconds
to 120 seconds after startup of a cold engine. The engine exhaust
stream will typically contain, by volume, 500 to 1000 ppm hydrocarbons.
The engine exhaust stream is flowed through exhaust pipe 2 and
through diverting valve 3 which directs the stream through exhaust
pipe 4 and through adsorbent zone 5 to provide a first exhaust stream.
Adsorbent zone 5 contains one or more beds of a suitable adsorbent
for hydrocarbons. The adsorbents which can be used for the practice
of this invention are molecular sieves as characterized herein.
Hereinafter, the adsorbent bed will be referred to as a molecular
sieve bed. The hydrocarbons and other noxious components are selectively
adsorbed, i.e., preferentially over water, in the molecular sieve
bed. In addition to the molecular sieve bed, the adsorbent zone
may contain a secondary catalyst bed in a tandem arrangement with
the molecular sieve bed, i.e., immediately after the adsorbent bed.
The function of the secondary catalyst bed is to oxidize the hydrocarbons
and carbon monoxide in the exhaust stream. This secondary catalyst
is characterized in that it operates at a lower temperature than
the primary catalyst. Both catalysts will be more fully described
herein.
The first exhaust stream which is discharged from the adsorbent
zone is now flowed through exhaust pipe 6 to exhaust pipe 11 and
through primary catalyst bed 7 to provide a treated exhaust stream.
The function of the primary catalyst is to convert the pollutants
in the first exhaust gas stream to innocuous components. When the
engine is fueled by a hydrocarbon, the primary catalyst is referred
to in the art as a three component control catalyst because it can
simultaneously oxidize any residual hydrocarbons present in the
first exhaust stream or engine exhaust stream to carbon dioxide
and water, oxidize any residual carbon monoxide to carbon dioxide
and reduce any residual nitric oxide to nitrogen and oxygen. In
some cases the primary catalyst may not be required to convert nitric
oxide to nitrogen and oxygen, e.g., when an alcohol is used as the
fuel. In this case the catalyst is called an oxidation catalyst.
Because of the relatively low temperature of the exhaust stream,
this primary catalyst does not function at a very high efficiency,
thereby necessitating the adsorbent bed 5. The treated exhaust stream
that is discharged from catalyst bed 7 is then flowed through exhaust
pipe 8 and discharged to the atmosphere. It is understood that prior
to discharge into the atmosphere the treated exhaust stream may
be flowed through a muffler or other sound reduction apparatus well
known in the art.
The temperature at the exit of the adsorbent bed 5 is measured
by temperature sensing element 9 which typically consists of a thermocouple
or other temperature sensing device which transmits an electrical
signal to a microprocessor located on the engine. At a preset adsorbent
bed temperature usually in the range of 150.degree. C. to about
200.degree. C., the microprocessor sends a message to control valve
3 thereby completely closing control valve 3 which bypasses adsorbent
zone 5 and allows the entire engine exhaust stream to be diverted
through exhaust pipe 11 and flow through the primary catalyst bed
7.
The gas temperature at the entrance to the primary catalyst bed
is measured by another temperature sensing element 10 which also
sends a signal to the same microprocessor. At a preset catalyst
gas inlet temperature from sensor 10 typically in the range of
350.degree. to 400.degree. C. the microprocessor sends a signal
to valve 3 to partially open valve 3 such that a minor portion of
the engine exhaust stream is flowed through exhaust pipe 4 through
adsorbent zone 5 and then through the primary catalyst bed 7 while
the major portion of the engine exhaust stream from valve 3 is flowed
through exhaust pipe 11 and then through the primary catalyst bed
7.
The minor portion of the now hot engine exhaust gas stream which
flows through adsorbent zone 5 desorbs the hydrocarbons and any
nitric oxide and carbon monoxide (pollutants) adsorbed on the adsorbent
bed to provide a second exhaust stream and which flows through exhaust
pipes 6 and 11 to the primary catalyst bed 7 where the pollutants
are converted to innocuous compounds to provide a treated exhaust
stream which is then discharged to the atmosphere via exhaust pipe
8. After a period of time in which substantially all the pollutants
are desorbed from the adsorbent bed, (by substantially is meant
at least 95% of the pollutants), generally about 3 to about 5 minutes,
the microprocessor sends a signal to control valve 3 to divert all
the engine exhaust stream directly over the primary catalyst bed
7 via exhaust pipe 11 to provide a treated exhaust stream which
is then discharged to the atmosphere via exhaust pipe 8. Instead
of waiting for a predetermined time, valve 3 may be closed, i.e.,
divert all the engine exhaust stream over catalyst bed 7 when the
temperature measured by sensor 9 reaches a temperature of about
650.degree. C.
The adsorbent which is used in adsorbent zone 5 is a molecular
sieve which has a high selectivity for hydrocarbon versus water.
In particular, the molecular sieves which can be used in this invention
have the following characteristics: 1) a framework Si:Al ratio of
at least 2.4; 2) are hydrothermally stable and 3) have a hydrocarbon
selectivity (.sup..alpha. HC--H.sub.2 O) greater than 1.0. By hydrothermally
stable is meant the ability of the molecular sieve to maintain its
structure after thermal cycling in the exhaust gas stream. One method
of measuring hydrothermal stability is to look at the temperature
at which 50% of the structure is decomposed after heating for 16
hours in air. The temperature is referred to as T(50). Accordingly,
as used in this application, by hydrothermally stable is meant a
molecular sieve which has a T(50) of at least 750.degree. C. The
hydrocarbon selectivity .alpha. is defined by the following equation:
##EQU1## X.sub.HC =the hydrocarbon co-loading on the molecular sieve
in equilibrium with the hydrocarbon water vapor mixture in the gas
phase over the zeolite adsorbent;
X.sub.H.sbsb.2.sub.O =the water co-loading on the molecular sieve
in equilibrium with the water and hydrocarbon vapor mixture in the
gas phase over the molecular sieve adsorbent;
[H.sub.2 O]=the concentration of water vapor in the exhaust gas
stream; and
[HC]=the concentration of the hydrocarbon species in the exhaust
gas.
The above definitions show that the selectivity of molecular sieves
for hydrocarbons over water is dependent upon the exhaust gas stream
temperature, the particular hydrocarbon species of interest and
the relative concentrations of water vapor and hydrocarbon.
In order to calculate X.sub.HC and X.sub.H.sbsb.2.sub.O one needs
to first determine the intrinsic adsorption strength of the molecular
sieve. Intrinsic adsorption strength can be described by reference
to the Dubinin Polanyi model for adsorption. The model says that
the sorption expressed as the volume of the sorbent structure occupied
by the sorbate is a unique function of the Gibbs Free Energy change
on adsorption. Mathematically this relationship takes the form of
a Gaussian distribution with Gibbs free energy change as follows:
where X is the loading expected, VO is the pore volume (cc/g),
B is a constant that is dependent on the sorbent and sorbate, and
G is the Gibbs Free Energy change. The product of liquid density
and VO equates to the saturation loading, XO, for any pure compound
by the Gurvitsch Rule. (see Breck, Zeolite Molecular Sieves, page
426.)
The constant B is then inversely related to the intrinsic adsorption
strength. For example, if the hydrocarbon is benzene, a value of
B of 0.04 for both benzene and water gives good results. The estimates
of water and hydrocarbon co-loadings are made in the following way:
1) each individual component loading is estimated by use of the
Dubinin Polanyi model as outlined above. For each compound present
one needs to know the liquid phase density (approximating the sorbed
phase density), the vapor pressure as a function of temperature,
and the actual concentration of the species in the gas.
2) Once each pure component loading is calculated, the function
.PHI. is calculated as,
where X/XO is the loading ratio or fraction of the pore volume
filled by each component if it were present alone. .PHI. then represents
the ratio of occupied pore volume to unoccupied pore volume.
3) The co-loadings are then calculated, accounting for each species
present, by the formula,
X.sub.mc is the co-loading of each component on the zeolite. This
procedure follows the Loading Ratio Correlation, which is described
in "Multicomponent Adsorption Equilibria on Molecular Sieves",
C. M. Yon and P. H. Turnock AICHE Symposium Series, No. 117 Vol.
67 (1971).
Both natural and synthetic molecular sieves may be used as adsorbents.
Examples of natural molecular sieves which can be used are faujasites,
clinoptilolites, mordenites, and chabazite. Examples of synthetic
molecular sieves which can be used are silicalite, Zeolite Y, ultrastable
zeolite Y, ZSM-5. Of course mixtures of these molecular sieves both
natural and synthetic can be used.
The adsorbent bed used in the instant invention can be conveniently
employed in particulate form or the adsorbent, i.e., molecular sieve,
can be deposited onto a solid monolithic carrier. When particulate
form is desired, the adsorbent can be formed into shapes such as
pills, pellets, granules, rings, spheres, etc. In the employment
of a monolithic form, it is usually most convenient to employ the
adsorbent as a thin film or coating deposited on an inert carrier
material which provides the structural support for the adsorbent.
The inert carrier material can be any refractory material such as
ceramic or metallic materials. It is desirable that the carrier
material be unreactive with the adsorbent and not be degraded by
the gas to which it is exposed. Examples of suitable ceramic materials
include sillimanite, petalite, cordierite, mullite, zircon, zircon
mullite, spodumene, alumina-titanate, etc. Additionally, metallic
materials which are within the scope of this invention include metals
and alloys as disclosed in U.S. Pat. No. 3920583 which are oxidation
resistant and are otherwise capable of withstanding high temperatures.
The carrier material can best be utilized in any rigid unitary
configuration which provides a plurality of pores or channels extending
in the direction of gas flow. It is preferred that the configuration
be a honeycomb configuration. The honeycomb structure can be used
advantageously in either unitary form, or as an arrangement of multiple
modules. The honeycomb structure is usually oriented such that gas
flow is generally in the same direction as the cells or channels
of the honeycomb structure. For a more detailed discussion of monolithic
structures, refer to U.S. Pat. Nos. 3785998 and 3767453.
The molecular sieve is deposited onto the carrier by any convenient
way well known in the art. A preferred method involves preparing
a slurry using the molecular sieves and coating the monolithic honeycomb
carrier with the slurry. The slurry can be prepared by means known
in the art such as combining the appropriate amount of the molecular
sieve and a binder with water. This mixture is then blended by using
means such as sonification, milling, etc. This slurry is used to
coat a monolithic honeycomb by dipping the honeycomb into the slurry,
removing the excess slurry by draining or blowing out the channels,
and heating to about 100.degree. C. If the desired loading of molecular
sieve is not achieved, the above process may be repeated as many
times as required to achieve the desired loading.
The size of the adsorbent bed is chosen such that at least 40%
of the hydrocarbons in the exhaust stream discharged from the engine
is adsorbed. Generally, this means that the size of the adsorbent
bed varies from about 1 to about 3 liters. When the adsorbent is
deposited on a monolithic honeycomb carrier, the amount of adsorbent
on the carrier varies from about 100 to about 450 grams. It is desirable
to optimize the volume of the adsorbent bed such that the primary
catalyst downstream from the adsorbent bed is heated as quickly
as possible while at the same time ensuring that at least 40% of
the hydrocarbons in the exhaust stream are adsorbed on the adsorbent
bed. It is preferred that the adsorbent be deposited on a monolithic
honeycomb carrier in order to minimize the size of the adsorbent
bed and the back pressure exerted on the engine.
Instead of depositing the molecular sieve onto a monolithic honeycomb
structure, one can take the molecular sieve and form it into a monolithic
honeycomb structure.
The adsorbent which is a molecular sieve may optionally contain
one or more catalytic metals dispersed thereon. The metals which
can be dispersed on the adsorbent are the noble metals which consist
of platinum, palladium, rhodium, ruthenium, and mixtures thereof.
The desired noble metal may be deposited onto the adsorbent, which
acts as a support, in any suitable manner well known in the art.
One example of a method of dispersing the noble metal onto the adsorbent
support involves impregnating the adsorbent support with an aqueous
solution of a decomposable compound of the desired noble metal or
metals, drying the adsorbent which has the noble metal compound
dispersed on it and then calcining in air at a temperature of about
400.degree. to about 500.degree. C. for a time of about 1 to about
4 hours. By decomposable compound is meant a compound which upon
heating in air gives the metal or metal oxide. Examples of the decomposable
compounds which can be used are set forth in U.S. Pat. No. 4791091
which is incorporated by reference. Preferred decomposable compounds
are chloroplatinic acid, rhodium trichloride, chloropalladic acid,
hexachloroiridate (IV) acid and hexachlororuthenate. It is preferable
that the noble metal be present in an amount ranging from about
0.01 to about 4 weight percent of the adsorbent support. Specifically,
in the case of platinum and palladium the range is 0.1 to 4 weight
percent, while in the case of rhodium and ruthenium the range is
from about 0.01 to 2 weight percent.
These catalytic metals are capable of oxidizing the hydrocarbon
and carbon monoxide and reducing the nitric oxide components to
innocuous products. Accordingly, the adsorbent bed can act both
as an adsorbent and as a catalyst.
The primary catalyst which is used in this invention is selected
from any three component control or oxidation catalyst well known
in the art. Examples of primary catalysts are those described in
U.S. Pat. Nos. 4528279; 4791091; 4760044; 4868148; and 4868149
which are all incorporated by reference. Preferred primary catalysts
well known in the art are those that contain platinum and rhodium
and optionally palladium, while oxidation catalysts usually do not
contain rhodium. Oxidation catalysts usually contain platinum and/or
palladium metal. These catalysts may also contain promoters and
stabilizers such as barium, cerium, lanthanum, nickel, and iron.
The noble metals and promoters and stabilizers are usually deposited
on a support such as alumina, silica, titania, zirconia, alumino
silicates, and mixtures thereof with alumina being preferred. The
primary catalyst can be conveniently employed in particulate form
or the catalytic composite can be deposited on a solid monolithic
carrier with a monolithic carrier being preferred. The particulate
form and monolithic form of the primary catalyst are as described
for the adsorbent above.
As stated, another embodiment of the invention is an adsorbent
bed in a tandem arrangement with a secondary catalyst bed, i.e.,
immediately after the adsorbent bed. This secondary catalyst bed
will contain a catalyst which is different from the primary catalyst.
This secondary catalyst has the characteristic that it can function
more effectively at lower temperatures. Also its major function
is to convert hydrocarbons and carbon monoxide to carbon dioxide
and water. Additionally, since the secondary catalyst will not be
exposed to high temperatures, it is not necessary that the secondary
catalyst be stable at high temperatures, e.g., greater than 700.degree.
C.
These catalysts are known in the art and usually comprise platinum
and/or palladium dispersed on a high surface area support such as
a gamma alumina. Promoters such as lanthanum, cerium, etc. may be
added to the catalyst. This secondary catalyst can be either in
particulate form or can be deposited onto a solid monolithic carrier
as described above for the primary catalyst. The methods used to
prepare this secondary catalyst are analogous to those described
for preparing a three component control or oxidation catalyst.
The following examples are presented in illustration of this invention
and are not intended as undue limitations on the generally broad
scope of the invention as set out in the appended claims.
EXAMPLE 1
A slurry was prepared using Y-54 and Ludox AS-40 binder. Y-54 is
an ultrastable sodium Y zeolite with a SiO.sub.2 /Al.sub.2 O.sub.3
ratio of 5 and A.sub.o of 24.68 .ANG. and a Na/Al ratio of 0.93.
Y-54 is produced and was obtained from UOP. Ludox As-40 is an ammonium
stabilized colloidal silica containing 40 weight percent solids
with 20 micron spherical SiO.sub.2 particles and is available from
DuPont Corp. To 141 grams of distilled water, there was added 100
grams of Ludox AS-40. To this mixture there were added 191 grams
of Y-54 zeolite and then 551 grams of water. This mixture was sonified
for 10 minutes using a Sonifier Cell Disruptor 350.
A ceramic monolithic honeycomb carrier manufactured by Corning
Glass Works measuring 28 mm in diameter by 50 mm in length was dipped
into the slurry, pulled out and allowed to drain. The wet honeycomb
was air dried and then heated at 100.degree. C. for 1 hour. The
monolith contained 4.1 grams of zeolite plus binder. This sample
was designated sample A.
EXAMPLE 2
A monolithic honeycomb was prepared as in Example 1 except that
the adsorbent used was Y-84. Y-84 is the ammonium form of stabilized
Y zeolite with an A.sub.o of 24.55 .ANG., an NH.sub.4 /Al of 0.3
and a Na/Al of less than 0.01. Y-84 was also obtained from UOP.
This sample contained 4.2 grams of zeolite plus binder and was designated
sample B.
EXAMPLE 3
A monolithic honeycomb was prepared as in Example 1 except that
the adsorbent used was SA-15. Sa-15 is a steamed form of Y-84 with
an A.sub.o of 24.29 .ANG. and NH.sub.4 /Al and a Na/Al ratio of
less than 0.01. This sample contained 5.5 grams of zeolite plus
binder and was designated sample C.
EXAMPLE 4
Samples A, B and C were tested to determine their hydrocarbon adsorption
properties by using the following test procedure. The sample to
be tested, measuring 28 mm in diameter by 50 mm in length and having
a volume of 30.8 cc was placed in a tubular glass reactor. Over
this adsorbent bed there was flowed a gas stream containing 998
ppm of propylene, 17570 ppm of water and the remainder nitrogen.
The test was run by starting with a cold (room temperature) adsorbent
bed and gas stream flowing the gas stream at a flow rate of 7 Standard
Liters Per Minute (SLPM) over the adsorbent while heating the gas
stream from about 25.degree. C. to about 360.degree. C. in approximately
400 seconds.
The hydrocarbon retention was calculated by integrating the difference
between the instantaneous mass flow of hydrocarbons into and out
of the adsorbent. The percentage of the hydrocarbons retained was
calculated by dividing the net hydrocarbon retention by the integral
of the hydrocarbons flowed into the bed. Plots of hydrocarbon retention
versus time for samples A, B and C are presented in FIG. 2.
The results presented in FIG. 2 show that sample A has the largest
initial value of hydrocarbon retention, but the retention falls
off quickly. Samples B and C have lower initial retention but fall
off more slowly with sample B being the best. It is clear from this
test that any of the three zeolites tested can be used to selectively
adsorb hydrocarbons during the cold-start phase of an automobile
engine.
Thus, having described the invention in detail, it will be understood
by those skilled in the art that certain variations and modifications
may be made without departing from the spirit and scope of the invention
as defined herein and in the appended claims. |