CA2051552A1 - Four phase process for controlling emissions during cold start of internal combustion engine - Google Patents
Four phase process for controlling emissions during cold start of internal combustion engineInfo
- Publication number
- CA2051552A1 CA2051552A1 CA 2051552 CA2051552A CA2051552A1 CA 2051552 A1 CA2051552 A1 CA 2051552A1 CA 2051552 CA2051552 CA 2051552 CA 2051552 A CA2051552 A CA 2051552A CA 2051552 A1 CA2051552 A1 CA 2051552A1
- Authority
- CA
- Canada
- Prior art keywords
- adsorbent
- bed
- molecular sieve
- exhaust stream
- engine exhaust
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Abstract
"FOUR PHASE PROCESS FOR CONTROLLING EMISSIONS DURING COLD START OF INTERNAL COMBUSTION ENGINE" A pollutant containing engine exhaust stream is treated upon cold startup by first flowing the engine exhaust stream, which is relatively cool, over an adsorbent zone ¢5! 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 catalyst ¢7! 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 150 to 200.degree.C, the entire engine exhaust stream is completely diverted over the primary catalyst ¢7!. When the inlet temperature tothe primary catalyst ¢7! has reached 350 to 400.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 ¢7!, thereby isolating the adsorbent bed to minimize deterioration. The adsorbent zone ¢5! may also have a secondary catalyst bed arranged immediately after it.
Description
2 ~
"FOUR PHASE PRO~ESS FQ~ CONTROLLING EMlSSlONS
DURIN~; CC~D ~TART C)F INJERhlAL. COMBU~STION E~' FIELD OF INVEI`JTION
This invention rel~tes to a rlq-o,~l four phass combination of an adsorption 5 step and a catalytic conversion step for minimizing emissions during cold start of an engine.
BACKGROUND OF THE- INVENTION
Gaseous waste products resulting from the combustion of hydrocarbona-o ceous fuels, such as gasoline and fuel oils, comprise carbon monoxida, hydro-carbons and nitrogen oxides as products of combustion or incomplete combus-tion, and pose a serious health problem with respect to pollution af the atmo-sphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air 15 pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protec-tion 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 resul~ed in the use of catalytic converters to 20 reduca the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hy-drocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ra~io control means which ~unctions inresponse ~o a feedback signal from an oxygen sensor in the engine exhaust sys-25 tem. Although these three component control catalysts work quite well aft~r thayhave reached operating temperature o~ about 300C:, 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 30 other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. How~ver, new - : . .. . .
- ~ ~
.
.
' 2 20~a~2 hydrocarbon emission standards have been proposad which can not be met with the current sta~e of the art three component control catalysts.
A unique solution to this cold start start emission problem has now been found which involves the use of an adsorbent bed to adsorb the hydrocarbons 5 during the cold start portion o~ the engine cycle. Although the process will b~
exemplified using hydrocarbons, ths instant irwention can also bc used to tr~at exhaust streams from alcohol fueled 0ngin~s as will b~ shown in detail. The invention involves taking the ~xhaust stream which is discharged from an engine during the initial star~up phase of the engina cycle (cold start) and diverting it o through an adsorbent bed which preferentially adsorbs hydrocarbons relative towater under the conditions present in the exhaust stream. The exhaust ~tream discharged from the adsorbent bed then flows over a primary catalyst and is discharged into the atmosphere. After a certain amount of time, the adsorbent bed has reached a certain temperature (150 to 200C~ at which the bed is no 15 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 in a second phase 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~o during which the primary catalyst has reached its operating temperature, the engine exhaust stream is divided in a third phase into a major and minor portion.
The major portion of the engine exhaust stream is flowed directly over the prirnary catalyst while the minor portion of the engine exhaust stream is flowed over theadsorbent bed thereby desorbing the hydrocarbons and any other pollutants that 25 were adsorbed on the bed. The stream from the adsorbent bed during this thirdphase flows over the primary catalyst and is then discharged into the atmosphere.
When all ~he hydrocarbons have been desorbed frorn the adsorbent bed, the engine exhaust stream is completely directed over the primary catalyst in the fourth phase of the r,ycle. This ensures that the adsorbent bed is not exposed to
"FOUR PHASE PRO~ESS FQ~ CONTROLLING EMlSSlONS
DURIN~; CC~D ~TART C)F INJERhlAL. COMBU~STION E~' FIELD OF INVEI`JTION
This invention rel~tes to a rlq-o,~l four phass combination of an adsorption 5 step and a catalytic conversion step for minimizing emissions during cold start of an engine.
BACKGROUND OF THE- INVENTION
Gaseous waste products resulting from the combustion of hydrocarbona-o ceous fuels, such as gasoline and fuel oils, comprise carbon monoxida, hydro-carbons and nitrogen oxides as products of combustion or incomplete combus-tion, and pose a serious health problem with respect to pollution af the atmo-sphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air 15 pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protec-tion 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 resul~ed in the use of catalytic converters to 20 reduca the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hy-drocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ra~io control means which ~unctions inresponse ~o a feedback signal from an oxygen sensor in the engine exhaust sys-25 tem. Although these three component control catalysts work quite well aft~r thayhave reached operating temperature o~ about 300C:, 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 30 other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. How~ver, new - : . .. . .
- ~ ~
.
.
' 2 20~a~2 hydrocarbon emission standards have been proposad which can not be met with the current sta~e of the art three component control catalysts.
A unique solution to this cold start start emission problem has now been found which involves the use of an adsorbent bed to adsorb the hydrocarbons 5 during the cold start portion o~ the engine cycle. Although the process will b~
exemplified using hydrocarbons, ths instant irwention can also bc used to tr~at exhaust streams from alcohol fueled 0ngin~s as will b~ shown in detail. The invention involves taking the ~xhaust stream which is discharged from an engine during the initial star~up phase of the engina cycle (cold start) and diverting it o through an adsorbent bed which preferentially adsorbs hydrocarbons relative towater under the conditions present in the exhaust stream. The exhaust ~tream discharged from the adsorbent bed then flows over a primary catalyst and is discharged into the atmosphere. After a certain amount of time, the adsorbent bed has reached a certain temperature (150 to 200C~ at which the bed is no 15 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 in a second phase 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~o during which the primary catalyst has reached its operating temperature, the engine exhaust stream is divided in a third phase into a major and minor portion.
The major portion of the engine exhaust stream is flowed directly over the prirnary catalyst while the minor portion of the engine exhaust stream is flowed over theadsorbent bed thereby desorbing the hydrocarbons and any other pollutants that 25 were adsorbed on the bed. The stream from the adsorbent bed during this thirdphase flows over the primary catalyst and is then discharged into the atmosphere.
When all ~he hydrocarbons have been desorbed frorn the adsorbent bed, the engine exhaust stream is completely directed over the primary catalyst in the fourth phase of the r,ycle. This ensures that the adsorbent bed is not exposed to
3 o 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:AI ratio of at least 2.4:1; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity greater than 1. Examples of molecular sieves which meet these criteria 35 are silicalite, faujasites, clinoptilolites, mordenites and chabazite. The adsorbent bed may be in any conf~guration with a preferred configuration being a 2 0 ;~ 1 5 ~) 2 honeycomb monolithic carrier having deposited thereon the desired molecular sieve.
The prior art reve~ls several references dealing with the use of adsorbent beds to minimize hydrocarbon emissions during a cold S start engine operation. One such reference is U.S. Patent No.
3,699,683. Another reference is U.S. Patent No. Z,942,932.
Finally, Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol ~ueled automotive vehicle.
Applicant's invention differs in several ways from the processes described in the prior art. First, the adsorbent bed used in applican~'s process is a selective adsorbent bed which is a molecular sieve bed. What this means is that hydrocar-bons and other pollutants are pre7erentially adsorbed relative to water; the adsorbent bed does no~ thus 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 in the rang0 of 150 to 200C, the engine exhaust stream is diverted completely away from the adsorbent bed and routed directly over the primary catalyst. Once the three componen~ con~rol 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 ~o 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 thr~e 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 through water, the volume of the adsorbent bed is much smaller versus adsorbsnts in the prior art which do not selectively adsorb pollutants. A smaller adsorbent bed means a smaller heat sinkwhich means that a hotter exhaust gas stream contacts the catalyst. The molecular sieves which are used as the adsorbents also exhibit good 3 o hydrothermal stability, thereby minimizing replaeement 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 thecold start operation of an engine. Accordingly, one embodiment of the invention ~O~lS~2 is a four phase process for treating an engine axhaust gas stream containing pollutants comprising directing in the firs$ phase the en~ine exhaust gas streamupon cold startup of the engine through an adsorbent zone comprisin~ a molecular sieve bed which preferentially adsorbs the pollutants over water, to 5 provide a first exhaust stream, flowin~ the first exhaust stream over a primary catalyst to convert substantially all the pollutants contained in the 7irst axhaust stream to innocuous products, thereby providing a treated exhaust gas s~ream and discharging the treated exhaust stream into the atmosphere. Tha first phase of the process is carried out for a time until the adsorbent bed temperature is 150 10 to 200C, at which time in the second phase the engine exhaust gas stream is diverted completely away from the adsorb~nt zone and routed directly over the primary catalyst until such time as the primary catalyst reaches its operating temperature. At this time the engine exhaust gas stream is divided in a third phase into a ma~or and minor portion, the ma]or portion of the engine exhaust gas 15 stream is passed over the primary catalyst and the minor portion of the engine exhaust gas stream is charged to the adsorbent zone ~or a time sufficient to desorb substan~ially all the pollutants adsorbed on the molecular sieve bed and provide a second exhaust gas stream containing desorbed pollutants. The second exhaust stream is then passed over the primary catalyst to provide a 20 trea~ed exhaust stream, the third phase is r,ontinued until such time as necessary to desorb substantially all the pollutants from the adsorbent bed and then in the fourth phase the engine exhaust gas stream is cornpletely directed over the primary catalyst to provide a treated exhaust stream.
In a specific embodiment, the molecular sieve bed is a honeycomb mono-25 lithic carrier having deposlted thereon a molecular sieve selected from the groupconsisting of molecular sieves having a Si:AI ratio of at least 2.4:1, is hydrothermally stable and has a hydrocarbon selectivity ~ H~H2o) greater than 1.In another embodimen~, the adsorbent zone comprises a molecular sieve bed followed by a secondary catalyst bed arranged in tandem.
3 o BRIEF DESCRIPTION OF THE DRAWINGS
Figure 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.
~ ' 2 ~ a 2 Figure 2 is a 0raph of percent hydrocarbon retention versus ~im~ on which are presented three plots showing the results for three clifferent molecular sie\t0 adsorbents.
DETAILED DESCRIPTIQN QF Tl-IE INVEhlTlt:)N
Referring now to Figure 1, the engine 1 consists of any internal or cxternal 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 engina may b0 fueled by 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 en~ina are the mixtures of hydrocarbons which mak~ up gasolina or diesel fuel.
The alcohols which may be used to fuel engines includ~ ethanol and methanol.
Mixtures of alcohols and mixtur~s o~ alcohols and hydrocarbons can also be used. Engine 1 may consist of a jet engine, ~as 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 Figure 1, engine 1 is initially operating at a relatively reduced temperature, such as a cold engins at startup or warmup whichproduces 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 oonvenience the description will use hydrocarbon as the fuel ~o exem-plify the invention. The use of hydrocarbon in the subsequent description is not to be construed as limitins the invention to hydrocarbon fueled engines.
The engine exhaust gas stream under startup conditions is generally at a temperature below 500C and typically in the range of 200 to 400C, and con-30 tains 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 th~ exhaust stream. For example, when the fuel is a hydrocarbon fuel, hydrocarbons, nitro-gen oxides, carbon monoxide and other combustion byproducts will be found in , ~
6 ~051~2 the exhaust gas stream. The engine exhaust stream is produced at ~his relativeiylow temperature during the initial period or phase of engine operation, typically for the first 30 seconds to 120 seconds after startup of a cold engine. The engina exhaust str0am will typically contain, by volume, 500 to 1000 ppm hydrocarbons.
The engine exhaust stream is flowed through exhaust pipe 2 and through diverting valvs 3 which directs the stream through exhaust pipc 4 and through adsorbent zone ~ to provide a first exhaust stream. Adsorbent zone 5 contains one or more beds of a suitable adsorbent ~or hydrocarbons. The adsorbents which can be used for the pra~ice of this invention are molecular sieves as char-1C acterized 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 zon0 may contain a secondary catalyst bed in a tandem arrangement with the molecular sieve bed, i.e., immediately aft~r 15 the adsorbent bed. The function of the secondary catalyst bed is to oxidize the hydrocarbons and carbon monoxide in the exhaust stream. This secondary cat-alyst is characterized in that it operates at a lower temperature than the primary catalyst. Bo~h catalysts will be more fully described herein.
The first exhaust stream which is discharged from the adsorbent zone is 2 0 now flowed through exhaust pipe 6 to exhaust pipe 11 and through primary cata-lyst bed 7 to provide a treated exhaust stream. The function of the primary cata-lyst is to conver~ the pollutants in the first exhaust gas stream to innocuous com-ponents. When the engine is fueled by a hydrocarbon, the primary catalyst is referred to in the art as a ~hree component control catalyst because it can simul-25 taneously oxidize any residual hydrocarbons presen~ in the first exhaust stream orengine 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 nitricoxide to nitrogen and oxygen, e.g., when an alcohol is used as the fuel. In this30 case the catalyst is called an oxidation catalyst. Because of the relatively low temperature of the exhaust stream, this primary catalyst does not ~unction 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 3 5 discharge into the atmosphere the treated exhaust stream may be flowed through a muffler or other sound reduction apparatus well known in the art.
:
7 2f3~lrj5l~
The temperature at the exit of the adsorbent bed 5 is measured by temper-ature sensing elernent 9 which typically consists of a thermocouple or other tem-perature sansing devic0 which transmits an electrical signal to a micrr~processor located on the engine. At a preset adsorbent bed temperatura usually in the range of 150C to about 200C, the mieroprocessor sends a message to control valve 3 thereby completely closing control valve 3, which bypasses adsorbent zone 5 and allows the sntire engine exhaust stream to be diverted in the second phase of the process cycle through exhaust pipe 11 and flow through the primary catalyst bed 7.
e The gas temperature at tha entrance to the primary catalyst bed is mea-sured 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 when the primary catalyst reaches ItS operatiny temperature, which is typically in the range of 350 to 400C the microprocessor sends a signal to valve 3 to partially open valve 3 such that in the third phase of the cycle 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 sec-ond exhaust stream and which flows through exhaust pipes 6 and 11 to the pri-mary catalyst bed 7 where the pollutants are converted to innocuous compounds 25 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 initiate the fourth phase of the cycle and divert all the 3 o en~ine 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 atmosphers 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 thetemperature measured by sensor 9 reaches a temperature of 650C.
The adsorbent which is used in adsorbent zone 6 is a molecular sieve which has a high selectivity for hydrocarbon versus water. in par~icular, the 8 ~ rj~
molecular sieves which can be used in this invention have the following charac-teristics. 1) a framework Si:AI ratio of at least 2.4:1; Z) are hydrotherrnally stable and 3) have a hydrocarbon selectivity taH~H2) greater than 1Ø By hydrother-mally stable is meant the ability of the molecular sieve to maintain its str~Jcture 5 after thermal cycling in tha exhaust gas stream. One method of measuring hydrothermal stability is to look at the temperature at which 50/O 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 m~ant a molecular sieve which has a T(50) of at least 750C. The hydrocarbon selectiv-10 ity ~ is defined by the following cquation:
C~HC-H20 = XHC . ~H20]
H20 ~HC]
XHc = the hydrocarbon co-loading on the molecular sieve in equilibrium with the hydrocarbon water vapor mix~ure in the gas phase over the zeolite adsorbent;
XH;~o = the water co-loading on the molecular sieve in equilibrium with the water 2 o and hydrocarbon vapor mixture in the gas phase over the molecular sieve adsor-bent;
[H2O] = 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 25 hydrocarbons over water is dependent upon the exhaust gas stream tempera-ture, the particular hydrocarbon species of interest and the relative concentrations of water vapor and hydroearbon.
In order to calculate XHc and XH2o one needs to first de~ermine the intrin-sic adsorption strength of the molecular sieve. Intrinsic adsorption strength can 30 be described by reference ~o 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 distri-bution with Gibbs free energy chan~e as follows:
X = Liq. dens * VC) * exp(-B * G~G~
where X is the loading expected, VO is the pore volume (cc/g), B is a constant 9 2 (3 .~ 2 that is dependent on the sorbent and sorbate, and G is the Gibbs Free Energy chan~e. 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.) For ideal gases G = RT In (P/P).
The constant B is then inversely relat~d $o the intrinsic adsorption strength. For example, if the hydrocarbon is b~nzene, a vaiua of B of 0.04 for both benzene and water gives good results. The estimates of water and hydrocarbon co-loadirl~s are made in the following way:
1) each individual component loadinç~ 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 den-sity), the vapor pressure as a function of temperature, and the actual con-centration of the species in the gas.
2) Once each pure component loading is calculated, the function ~ is cal-culated as, = X/XO/(1 - X/XO) where X/XO is the loadin~ ratio or fraction of the pore volume filled by each component if it were present alone. q~ then represents ~he ratio of occu-2 o pied pore volume to unoccupied porc volume.
3) The co-loadings are then calculated, accounting for each species pre-sent, by the formula, XmC=XO*~ /(1 +~
Xmc is the co-loading of each component on the z001ite. This procedure follows the Loadin~ Ratio Correlation, which is described in "Multicomponent Adsorption Equilibria on Molecular Sieves", C.l\/l. Yon and P.H. Turnock AICHE Symposium Series, No. 117, Vol. ~7 (1971).
Both natural and syn~hetic molecular sieves may be used as adsorbents.
Examples of natural molecular sieves which can be used are faujasites, clinoptilo~
lites, mordenites, and chabazite. Examples of synthetic molecular sieves which can be used are silicalite, Zeolite Y, ultrastable zeolite Y, ZSM-5. Of course mix-tures of these molecular sieves both natural and synthetic can be used.
~O~j~)'2 The adsorbent bed used in the instant invention can be conveniently employed in particulate form or the adsorbent, i.~., molecular sieve, can be deposited onto a solid monolithic carrier. When particulate form is desired, theadsorbent can be formed into shapes such as pills, pellets, granules, rings, 5 spheres, etc. In the employment of a monolithic form, ~ is usually most conve-nient to employ the adsorbent as a thin film or coating deposited on an inert car-rier material which provides the structural support ~r the adsorbent. The inert carrier material can be any r0fractory material sucn as ceramic or metallic materi-als. It is desirable that the carrier material be unreactive with the adsorbent and o not be degraded by the gas to which it is exposed. Examples of suitab!e ceramic materials include sillimanite, petalite, cordierite, mullite, ~ircon, zircon mullite, spo-dumene, aiumina-titanate, etc. Additionally, metailic materials which are within the scope of this invention include metals and alloys as disclosed in U.S. Patent No.
3,920,583 which are oxidation resistant and are otherwise capabie of withstanding 15 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 20 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. Patent Nos. 3,785,998 and 3,767,453.
The molecular sieve is deposited onto the carrier by any convenient way 25 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 monollthic honeycomb by dipping the honeycomb into the slurry, removing the excess slurry by draining or blowing out the channels, and heating to 100C. 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 tha exhaust stream discharged from the engine is adsorbed.
11 2f3 jl~i52 Generally, this means that the size of the adsorbent bed varies from 1 to 3 liters.
When the adsorbent is deposited on a monolithic honeycomb carrier, the amount of adsorbent on the carrier varies from 100 to 450 grams. It is dssirable to optimize the volume of the adsorbent bed such that the primary catalyst 5 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 th~ exhaust stream are adsorbed on the adsorbent bed. It is preferred that th~ adsorbent be deposited on a monolithic honeycomb carrier in order to minimize the size of theadsorbent 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 honey-comb 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 15 the adsorbent are th~ noble metals which consist of platinum, palladiurn, rhodium, ruthenium, and mixtures thereof. The desired noble rnetal may be deposited onto the adsorbent, which acts as a support, in any suitable manner well known in theart. One example oF a method of dispersing the noble metal onto the adsorb~nt support involves impregnating the adsorbent support with an aqueous solution of 20 a decomposable compound of the desired noble metal or metals, drying the adsorbent which has the noble rnetal compound dispersed on it and then calcin-ing in air at a temperature of 400 to 500C: for a tima of 1 to 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 25 be used are set forth in U.S. Patent, No. 4,791,091 which is incorpora~ed by reference. Preferred decomposable compounds are chloroplatinic acid, rhodium trichloride, chloropalladic acid, hexachloroiridate (IV) acid and hexachlororuthenate. It is prefer~ble that the noble metal be present in an amoun~ ranging from 0.01 to 4 weight percent of the adsorbent support.
30 Specifically, in the case of platinum and palladium the range is 0.1 to 4 w~ight percent, while in the case of rhodium and ruthenium the range is frorn 0.01 to 2weight percent.
These catalytic metals are capable of oxidizing the hydrocarbon and car-bon monoxide and reducing the nitric oxide components to innocuous products.
3 5 Accordingly, the adsorbent bed can act both as an adsorbent and as a catalyst.
12 20~1~ )2 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 ofprimary catalysts are those described in U.S. Patent Nos. 4,528,279; 4,791,091;
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:AI ratio of at least 2.4:1; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity greater than 1. Examples of molecular sieves which meet these criteria 35 are silicalite, faujasites, clinoptilolites, mordenites and chabazite. The adsorbent bed may be in any conf~guration with a preferred configuration being a 2 0 ;~ 1 5 ~) 2 honeycomb monolithic carrier having deposited thereon the desired molecular sieve.
The prior art reve~ls several references dealing with the use of adsorbent beds to minimize hydrocarbon emissions during a cold S start engine operation. One such reference is U.S. Patent No.
3,699,683. Another reference is U.S. Patent No. Z,942,932.
Finally, Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol ~ueled automotive vehicle.
Applicant's invention differs in several ways from the processes described in the prior art. First, the adsorbent bed used in applican~'s process is a selective adsorbent bed which is a molecular sieve bed. What this means is that hydrocar-bons and other pollutants are pre7erentially adsorbed relative to water; the adsorbent bed does no~ thus 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 in the rang0 of 150 to 200C, the engine exhaust stream is diverted completely away from the adsorbent bed and routed directly over the primary catalyst. Once the three componen~ con~rol 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 ~o 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 thr~e 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 through water, the volume of the adsorbent bed is much smaller versus adsorbsnts in the prior art which do not selectively adsorb pollutants. A smaller adsorbent bed means a smaller heat sinkwhich means that a hotter exhaust gas stream contacts the catalyst. The molecular sieves which are used as the adsorbents also exhibit good 3 o hydrothermal stability, thereby minimizing replaeement 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 thecold start operation of an engine. Accordingly, one embodiment of the invention ~O~lS~2 is a four phase process for treating an engine axhaust gas stream containing pollutants comprising directing in the firs$ phase the en~ine exhaust gas streamupon cold startup of the engine through an adsorbent zone comprisin~ a molecular sieve bed which preferentially adsorbs the pollutants over water, to 5 provide a first exhaust stream, flowin~ the first exhaust stream over a primary catalyst to convert substantially all the pollutants contained in the 7irst axhaust stream to innocuous products, thereby providing a treated exhaust gas s~ream and discharging the treated exhaust stream into the atmosphere. Tha first phase of the process is carried out for a time until the adsorbent bed temperature is 150 10 to 200C, at which time in the second phase the engine exhaust gas stream is diverted completely away from the adsorb~nt zone and routed directly over the primary catalyst until such time as the primary catalyst reaches its operating temperature. At this time the engine exhaust gas stream is divided in a third phase into a ma~or and minor portion, the ma]or portion of the engine exhaust gas 15 stream is passed over the primary catalyst and the minor portion of the engine exhaust gas stream is charged to the adsorbent zone ~or a time sufficient to desorb substan~ially all the pollutants adsorbed on the molecular sieve bed and provide a second exhaust gas stream containing desorbed pollutants. The second exhaust stream is then passed over the primary catalyst to provide a 20 trea~ed exhaust stream, the third phase is r,ontinued until such time as necessary to desorb substantially all the pollutants from the adsorbent bed and then in the fourth phase the engine exhaust gas stream is cornpletely directed over the primary catalyst to provide a treated exhaust stream.
In a specific embodiment, the molecular sieve bed is a honeycomb mono-25 lithic carrier having deposlted thereon a molecular sieve selected from the groupconsisting of molecular sieves having a Si:AI ratio of at least 2.4:1, is hydrothermally stable and has a hydrocarbon selectivity ~ H~H2o) greater than 1.In another embodimen~, the adsorbent zone comprises a molecular sieve bed followed by a secondary catalyst bed arranged in tandem.
3 o BRIEF DESCRIPTION OF THE DRAWINGS
Figure 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.
~ ' 2 ~ a 2 Figure 2 is a 0raph of percent hydrocarbon retention versus ~im~ on which are presented three plots showing the results for three clifferent molecular sie\t0 adsorbents.
DETAILED DESCRIPTIQN QF Tl-IE INVEhlTlt:)N
Referring now to Figure 1, the engine 1 consists of any internal or cxternal 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 engina may b0 fueled by 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 en~ina are the mixtures of hydrocarbons which mak~ up gasolina or diesel fuel.
The alcohols which may be used to fuel engines includ~ ethanol and methanol.
Mixtures of alcohols and mixtur~s o~ alcohols and hydrocarbons can also be used. Engine 1 may consist of a jet engine, ~as 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 Figure 1, engine 1 is initially operating at a relatively reduced temperature, such as a cold engins at startup or warmup whichproduces 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 oonvenience the description will use hydrocarbon as the fuel ~o exem-plify the invention. The use of hydrocarbon in the subsequent description is not to be construed as limitins the invention to hydrocarbon fueled engines.
The engine exhaust gas stream under startup conditions is generally at a temperature below 500C and typically in the range of 200 to 400C, and con-30 tains 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 th~ exhaust stream. For example, when the fuel is a hydrocarbon fuel, hydrocarbons, nitro-gen oxides, carbon monoxide and other combustion byproducts will be found in , ~
6 ~051~2 the exhaust gas stream. The engine exhaust stream is produced at ~his relativeiylow temperature during the initial period or phase of engine operation, typically for the first 30 seconds to 120 seconds after startup of a cold engine. The engina exhaust str0am will typically contain, by volume, 500 to 1000 ppm hydrocarbons.
The engine exhaust stream is flowed through exhaust pipe 2 and through diverting valvs 3 which directs the stream through exhaust pipc 4 and through adsorbent zone ~ to provide a first exhaust stream. Adsorbent zone 5 contains one or more beds of a suitable adsorbent ~or hydrocarbons. The adsorbents which can be used for the pra~ice of this invention are molecular sieves as char-1C acterized 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 zon0 may contain a secondary catalyst bed in a tandem arrangement with the molecular sieve bed, i.e., immediately aft~r 15 the adsorbent bed. The function of the secondary catalyst bed is to oxidize the hydrocarbons and carbon monoxide in the exhaust stream. This secondary cat-alyst is characterized in that it operates at a lower temperature than the primary catalyst. Bo~h catalysts will be more fully described herein.
The first exhaust stream which is discharged from the adsorbent zone is 2 0 now flowed through exhaust pipe 6 to exhaust pipe 11 and through primary cata-lyst bed 7 to provide a treated exhaust stream. The function of the primary cata-lyst is to conver~ the pollutants in the first exhaust gas stream to innocuous com-ponents. When the engine is fueled by a hydrocarbon, the primary catalyst is referred to in the art as a ~hree component control catalyst because it can simul-25 taneously oxidize any residual hydrocarbons presen~ in the first exhaust stream orengine 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 nitricoxide to nitrogen and oxygen, e.g., when an alcohol is used as the fuel. In this30 case the catalyst is called an oxidation catalyst. Because of the relatively low temperature of the exhaust stream, this primary catalyst does not ~unction 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 3 5 discharge into the atmosphere the treated exhaust stream may be flowed through a muffler or other sound reduction apparatus well known in the art.
:
7 2f3~lrj5l~
The temperature at the exit of the adsorbent bed 5 is measured by temper-ature sensing elernent 9 which typically consists of a thermocouple or other tem-perature sansing devic0 which transmits an electrical signal to a micrr~processor located on the engine. At a preset adsorbent bed temperatura usually in the range of 150C to about 200C, the mieroprocessor sends a message to control valve 3 thereby completely closing control valve 3, which bypasses adsorbent zone 5 and allows the sntire engine exhaust stream to be diverted in the second phase of the process cycle through exhaust pipe 11 and flow through the primary catalyst bed 7.
e The gas temperature at tha entrance to the primary catalyst bed is mea-sured 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 when the primary catalyst reaches ItS operatiny temperature, which is typically in the range of 350 to 400C the microprocessor sends a signal to valve 3 to partially open valve 3 such that in the third phase of the cycle 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 sec-ond exhaust stream and which flows through exhaust pipes 6 and 11 to the pri-mary catalyst bed 7 where the pollutants are converted to innocuous compounds 25 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 initiate the fourth phase of the cycle and divert all the 3 o en~ine 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 atmosphers 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 thetemperature measured by sensor 9 reaches a temperature of 650C.
The adsorbent which is used in adsorbent zone 6 is a molecular sieve which has a high selectivity for hydrocarbon versus water. in par~icular, the 8 ~ rj~
molecular sieves which can be used in this invention have the following charac-teristics. 1) a framework Si:AI ratio of at least 2.4:1; Z) are hydrotherrnally stable and 3) have a hydrocarbon selectivity taH~H2) greater than 1Ø By hydrother-mally stable is meant the ability of the molecular sieve to maintain its str~Jcture 5 after thermal cycling in tha exhaust gas stream. One method of measuring hydrothermal stability is to look at the temperature at which 50/O 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 m~ant a molecular sieve which has a T(50) of at least 750C. The hydrocarbon selectiv-10 ity ~ is defined by the following cquation:
C~HC-H20 = XHC . ~H20]
H20 ~HC]
XHc = the hydrocarbon co-loading on the molecular sieve in equilibrium with the hydrocarbon water vapor mix~ure in the gas phase over the zeolite adsorbent;
XH;~o = the water co-loading on the molecular sieve in equilibrium with the water 2 o and hydrocarbon vapor mixture in the gas phase over the molecular sieve adsor-bent;
[H2O] = 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 25 hydrocarbons over water is dependent upon the exhaust gas stream tempera-ture, the particular hydrocarbon species of interest and the relative concentrations of water vapor and hydroearbon.
In order to calculate XHc and XH2o one needs to first de~ermine the intrin-sic adsorption strength of the molecular sieve. Intrinsic adsorption strength can 30 be described by reference ~o 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 distri-bution with Gibbs free energy chan~e as follows:
X = Liq. dens * VC) * exp(-B * G~G~
where X is the loading expected, VO is the pore volume (cc/g), B is a constant 9 2 (3 .~ 2 that is dependent on the sorbent and sorbate, and G is the Gibbs Free Energy chan~e. 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.) For ideal gases G = RT In (P/P).
The constant B is then inversely relat~d $o the intrinsic adsorption strength. For example, if the hydrocarbon is b~nzene, a vaiua of B of 0.04 for both benzene and water gives good results. The estimates of water and hydrocarbon co-loadirl~s are made in the following way:
1) each individual component loadinç~ 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 den-sity), the vapor pressure as a function of temperature, and the actual con-centration of the species in the gas.
2) Once each pure component loading is calculated, the function ~ is cal-culated as, = X/XO/(1 - X/XO) where X/XO is the loadin~ ratio or fraction of the pore volume filled by each component if it were present alone. q~ then represents ~he ratio of occu-2 o pied pore volume to unoccupied porc volume.
3) The co-loadings are then calculated, accounting for each species pre-sent, by the formula, XmC=XO*~ /(1 +~
Xmc is the co-loading of each component on the z001ite. This procedure follows the Loadin~ Ratio Correlation, which is described in "Multicomponent Adsorption Equilibria on Molecular Sieves", C.l\/l. Yon and P.H. Turnock AICHE Symposium Series, No. 117, Vol. ~7 (1971).
Both natural and syn~hetic molecular sieves may be used as adsorbents.
Examples of natural molecular sieves which can be used are faujasites, clinoptilo~
lites, mordenites, and chabazite. Examples of synthetic molecular sieves which can be used are silicalite, Zeolite Y, ultrastable zeolite Y, ZSM-5. Of course mix-tures of these molecular sieves both natural and synthetic can be used.
~O~j~)'2 The adsorbent bed used in the instant invention can be conveniently employed in particulate form or the adsorbent, i.~., molecular sieve, can be deposited onto a solid monolithic carrier. When particulate form is desired, theadsorbent can be formed into shapes such as pills, pellets, granules, rings, 5 spheres, etc. In the employment of a monolithic form, ~ is usually most conve-nient to employ the adsorbent as a thin film or coating deposited on an inert car-rier material which provides the structural support ~r the adsorbent. The inert carrier material can be any r0fractory material sucn as ceramic or metallic materi-als. It is desirable that the carrier material be unreactive with the adsorbent and o not be degraded by the gas to which it is exposed. Examples of suitab!e ceramic materials include sillimanite, petalite, cordierite, mullite, ~ircon, zircon mullite, spo-dumene, aiumina-titanate, etc. Additionally, metailic materials which are within the scope of this invention include metals and alloys as disclosed in U.S. Patent No.
3,920,583 which are oxidation resistant and are otherwise capabie of withstanding 15 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 20 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. Patent Nos. 3,785,998 and 3,767,453.
The molecular sieve is deposited onto the carrier by any convenient way 25 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 monollthic honeycomb by dipping the honeycomb into the slurry, removing the excess slurry by draining or blowing out the channels, and heating to 100C. 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 tha exhaust stream discharged from the engine is adsorbed.
11 2f3 jl~i52 Generally, this means that the size of the adsorbent bed varies from 1 to 3 liters.
When the adsorbent is deposited on a monolithic honeycomb carrier, the amount of adsorbent on the carrier varies from 100 to 450 grams. It is dssirable to optimize the volume of the adsorbent bed such that the primary catalyst 5 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 th~ exhaust stream are adsorbed on the adsorbent bed. It is preferred that th~ adsorbent be deposited on a monolithic honeycomb carrier in order to minimize the size of theadsorbent 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 honey-comb 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 15 the adsorbent are th~ noble metals which consist of platinum, palladiurn, rhodium, ruthenium, and mixtures thereof. The desired noble rnetal may be deposited onto the adsorbent, which acts as a support, in any suitable manner well known in theart. One example oF a method of dispersing the noble metal onto the adsorb~nt support involves impregnating the adsorbent support with an aqueous solution of 20 a decomposable compound of the desired noble metal or metals, drying the adsorbent which has the noble rnetal compound dispersed on it and then calcin-ing in air at a temperature of 400 to 500C: for a tima of 1 to 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 25 be used are set forth in U.S. Patent, No. 4,791,091 which is incorpora~ed by reference. Preferred decomposable compounds are chloroplatinic acid, rhodium trichloride, chloropalladic acid, hexachloroiridate (IV) acid and hexachlororuthenate. It is prefer~ble that the noble metal be present in an amoun~ ranging from 0.01 to 4 weight percent of the adsorbent support.
30 Specifically, in the case of platinum and palladium the range is 0.1 to 4 w~ight percent, while in the case of rhodium and ruthenium the range is frorn 0.01 to 2weight percent.
These catalytic metals are capable of oxidizing the hydrocarbon and car-bon monoxide and reducing the nitric oxide components to innocuous products.
3 5 Accordingly, the adsorbent bed can act both as an adsorbent and as a catalyst.
12 20~1~ )2 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 ofprimary catalysts are those described in U.S. Patent Nos. 4,528,279; 4,791,091;
4,760,044; 4,868,148; and 4,868,149, which are all incorporated by reference.
5 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 pallatlium metal. These catalysts may also contain promoters and stabilizers such ~s bar-ium, cerium, lanthanum, nickel, and iron. Th~ noblo metals and promoters and l0 stabilizers are usually deposited on a support such as alumina, silica, titania, zir-conia, alumino silicates, and mixtures thereof with alumina being preferred. Theprimary 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 particulata form and monolithic form of the primary catalyst 15 are as described for the adsorbent abovs.
Another embodiment of ~he invention is an adsorbent bed in a tandem arrangement with a secondary catalyst bed, i.e., immediately after the adsorbentbed. This secondary catalyst bed will contain a catalyst which is different from the primary catalyst. This secondary catalyst has the characteristic that it can 20 function more effectively at lower temperatures. Also its major function is to convert hydrocarbons and carbon monoxide to carbon dioxide and water. Addi-tionally, 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 700C.
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, e~c. 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 anatogous to those described for preparing a three component control or oxidation catalyst.
The following examples are presented in illustration of certain aspects 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.
, !
2 0 ~ 2 ~3 A slurry was preparad using Y 54 and Ludox AS-40 binder. Y-54 is an ultrastable sodium Y zeolite with a SiO2/A1203 ratio of 5, an Ao of 24.6&~ and aNa/AI ratio of 0.93. Ludox AS-40 is an ammonium stabilized colloidal silica containing 40 weight pereent solids with 20 micron spherical SiO2 particles and i~
available from DuPont Corp. To 141 grams of distilled water, there was ~dded 100 grams of Ludox AS-40. To this mixture thera were added 191 grams of Y-54 zeolite and then 551 grams of water. This mixturs was sonifled for 10 minutes using a Sonlfier 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 o~t and allowed to drain. The wee honeycomb was air dried and then heated at 100C for 1 hour. The monolith contained 4.1 grams of z001ite plus binder. This sample was designated sample A.
A monolithic honeycomb was prepared as in Exampla 1 except that the adsorbent used was Y-84. Y-84 is the ammonium form of stabilized Y zeolite with an Ao of 24.55A, an NH4/AI of 0.3 and a Na/AI of less than 0.01. This sample contained 4.2 grams of zeolite plus binder and was designated sample B.
A monolithic honeycomb was prepared as in Exampl~ 1 except that thc adsorbent used was SA-15. Sa-15 is a steamed form of Y-84 with an Ao of 24.2~
and NH4/AI and a Na/AI ratio of less than 0.01. This sample containEd 5.5 grams of zeolite plus binder and was designated sample C.
Samples A, B and C were tested to determine their hydrocarbon adsorp-tion properties by using the following test procedure. The sample to be tested, measurin~ 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 30 a gas stream containin~ 998 ppm of propylene, 17,570 ppm of water and the :
~ ;
14 7~5~
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 StandardLiters Per Minute (SLPM) over the adsorbent while heating the ~as stream from 25C to 360C in approximately 400 seconds.
The hydrocarbon retention was calculated by integrating the differenc~
between the instantaneous mass flow of hydrocarbons into and out of the adsor-bent. The percentage of the hydrocarbons r etained 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 pre-o sented in Figure 2.
The results presented in Figure 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 s~lec-tively adsorb hydrocarbons during tha cold-start phase of an automobile engine.
Another embodiment of ~he invention is an adsorbent bed in a tandem arrangement with a secondary catalyst bed, i.e., immediately after the adsorbentbed. This secondary catalyst bed will contain a catalyst which is different from the primary catalyst. This secondary catalyst has the characteristic that it can 20 function more effectively at lower temperatures. Also its major function is to convert hydrocarbons and carbon monoxide to carbon dioxide and water. Addi-tionally, 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 700C.
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, e~c. 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 anatogous to those described for preparing a three component control or oxidation catalyst.
The following examples are presented in illustration of certain aspects 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.
, !
2 0 ~ 2 ~3 A slurry was preparad using Y 54 and Ludox AS-40 binder. Y-54 is an ultrastable sodium Y zeolite with a SiO2/A1203 ratio of 5, an Ao of 24.6&~ and aNa/AI ratio of 0.93. Ludox AS-40 is an ammonium stabilized colloidal silica containing 40 weight pereent solids with 20 micron spherical SiO2 particles and i~
available from DuPont Corp. To 141 grams of distilled water, there was ~dded 100 grams of Ludox AS-40. To this mixture thera were added 191 grams of Y-54 zeolite and then 551 grams of water. This mixturs was sonifled for 10 minutes using a Sonlfier 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 o~t and allowed to drain. The wee honeycomb was air dried and then heated at 100C for 1 hour. The monolith contained 4.1 grams of z001ite plus binder. This sample was designated sample A.
A monolithic honeycomb was prepared as in Exampla 1 except that the adsorbent used was Y-84. Y-84 is the ammonium form of stabilized Y zeolite with an Ao of 24.55A, an NH4/AI of 0.3 and a Na/AI of less than 0.01. This sample contained 4.2 grams of zeolite plus binder and was designated sample B.
A monolithic honeycomb was prepared as in Exampl~ 1 except that thc adsorbent used was SA-15. Sa-15 is a steamed form of Y-84 with an Ao of 24.2~
and NH4/AI and a Na/AI ratio of less than 0.01. This sample containEd 5.5 grams of zeolite plus binder and was designated sample C.
Samples A, B and C were tested to determine their hydrocarbon adsorp-tion properties by using the following test procedure. The sample to be tested, measurin~ 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 30 a gas stream containin~ 998 ppm of propylene, 17,570 ppm of water and the :
~ ;
14 7~5~
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 StandardLiters Per Minute (SLPM) over the adsorbent while heating the ~as stream from 25C to 360C in approximately 400 seconds.
The hydrocarbon retention was calculated by integrating the differenc~
between the instantaneous mass flow of hydrocarbons into and out of the adsor-bent. The percentage of the hydrocarbons r etained 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 pre-o sented in Figure 2.
The results presented in Figure 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 s~lec-tively adsorb hydrocarbons during tha cold-start phase of an automobile engine.
Claims (7)
1. A four phase process for treating an engine exhaust gas stream containing pollutants comprising the steps of: a) directing in the first phase the engine exhaust gas stream upon cold startup of the engine through an adsorbent zone [5] containing a molecular sieve bed which preferentially adsorbs the pollutants relative to water to provide a first exhaust stream; b) flowing the first exhaust stream over a primary catalyst [7] to convert substantially all the pollutants contained in the first exhaust stream to innocuous products, thereby providing a treated exhaust gas stream; and c) discharging the treated exhaust stream into the atmosphere, said first phase of the process being carried out for a time until the temperature of the adsorbent bed used in sleep a) is 150 to 200°C, at which time in the second phase the engine exhaust gas stream is diverted completely away from the adsorbent zone [5] and routed directly to step b) untilsuch time as the primary catalyst reaches its operating temperature, at which time the engine exhaust gas stream is in the third phase divided into a major and minor portion and the major portion of the engine exhaust gas stream is charged to step b) and the minor portion of the engine exhaust gas stream is charged to step a) until substantially all the pollutants from the adsorbent bed are desorbed and thereafter in the fourth phase the entire engine exhaust gas stream is charged to step b).
2. The process of Claim 1 where the molecular sieve bed used in step a) is selected from the group consisting of molecular sieves which have: 1) a framework Si:Al ratio of at least 2.4:1; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity (.alpha.HC-H2O) greater than 1 where .alpha.HC-H2O is defined by the following equation:
where XHC 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; XH2O 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; [H2O] is the concentration of water and [HC] is the concentration of hydrocarbon.
where XHC 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; XH2O 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; [H2O] is the concentration of water and [HC] is the concentration of hydrocarbon.
3. The process of Claim 1 or 2 where the molecular sieve used in step a) is selected from the group consisting of silicalite, faujasite, clinoptilolites, mordenites, chabazite, zeolite ultrastable Y, zeolite Y, ZSM-5 and mixtures thereof.
4. The process of Claim 1 where the molecular sieve bed used in step a) 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 (.alpha.HC-H2O) greater than 1.
5. The process of Claim 1, 2, 3 or 4 where the molecular sieve used in step a) has deposited thereon a metal selected from the group consisting of platinum,palladium, rhodium, ruthenium and mixtures thereof.
6. The process of any of Claims 1 to 5 where the adsorbent zone [5] used in step a) comprises a molecular sieve bed followed by a secondary catalyst bed arranged in tandem.
7. The process of Claim 6 where the secondary catalyst bed is a honey-comb monolithic carrier having deposited thereon a gamma alumina support which has dispersed thereon palladium metal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2051552 CA2051552A1 (en) | 1991-09-17 | 1991-09-17 | Four phase process for controlling emissions during cold start of internal combustion engine |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2051552 CA2051552A1 (en) | 1991-09-17 | 1991-09-17 | Four phase process for controlling emissions during cold start of internal combustion engine |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2051552A1 true CA2051552A1 (en) | 1993-03-18 |
Family
ID=4148384
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2051552 Abandoned CA2051552A1 (en) | 1991-09-17 | 1991-09-17 | Four phase process for controlling emissions during cold start of internal combustion engine |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2051552A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6383463B1 (en) | 1996-07-10 | 2002-05-07 | Mitsubishi Heavy Industries, Ltd. | Exhaust gas denitration method |
-
1991
- 1991-09-17 CA CA 2051552 patent/CA2051552A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6383463B1 (en) | 1996-07-10 | 2002-05-07 | Mitsubishi Heavy Industries, Ltd. | Exhaust gas denitration method |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US5078979A (en) | Molecular sieve bed/catalyst to treat automotive exhaust | |
| US5051244A (en) | Use of a molecular sieve bed to minimize emissions during cold start of internal combustion engines | |
| US5142864A (en) | Process for treating an engine exhaust stream employing a catalyst, an adsorbent bed and a turbocharger | |
| US5582003A (en) | Temperature actuated zeolite in-line adsorber system | |
| US6171556B1 (en) | Method and apparatus for treating an engine exhaust gas stream | |
| EP3261746B1 (en) | Passive nox adsorber | |
| US4985210A (en) | Exhaust gas purifying apparatus for automobile | |
| JP3311051B2 (en) | Exhaust gas purification method and apparatus | |
| EP3274077B1 (en) | Passive nox adsorber comprising a noble metal and a molecular sieve having an off framework type | |
| AU666235B2 (en) | Exhaust gas purification system for reducing hydrocarbon emissions during the cold start of combustion engines | |
| JPH10174846A (en) | Waste gas cleaning catalyst and its use | |
| JPH06182219A (en) | Device and method for denaturation of gas mixture | |
| WO1996039244A1 (en) | Diesel engine exhaust gas purification system | |
| CA2351223C (en) | Exhaust gas purifying catalyst and method for purifying exhaust gas | |
| EP0716877A1 (en) | Catalytic purification of engine exhaust gas | |
| EP0592713A1 (en) | Engine exhaust system with reduced hydrocarbon emissions | |
| CZ288953B6 (en) | Process of catalytic purification of automobile exhaust gases | |
| US6089014A (en) | Engine exhaust system with reduced hydrocarbon emissions | |
| CA2127478A1 (en) | Modified zeolites for trapping hydrocarbons | |
| EP0677142B1 (en) | Method and apparatus for treating an engine exhaust gas stream | |
| EP0639400A1 (en) | Modified large pore zeolites for trapping alkenes | |
| US5531068A (en) | Combination catalyst/adsorber system for treating an engine exhaust gas stream | |
| US5814287A (en) | Polymerization catalyst enhanced hydrocarbon trapping process | |
| EP0593904A1 (en) | Improvement in exhaust gas conversion system using zeolite positioned close to exhaust gas source | |
| CA2051552A1 (en) | Four phase process for controlling emissions during cold start of internal combustion engine |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |