Patent Application
20090084362
The present disclosure relates to materials used in evaporative emission control systems, and more particularly, concerns a polymeric catalyst for such systems.
Many automotive vehicles are equipped with an evaporative emission control system that prevents volatile components in the fuel tank from escaping to the atmosphere during periods of vehicle inactivity. Typical evaporative emission control systems include a canister comprising a bed of activated carbon that serves as fuel vapor adsorbent. The canister is connected to the fuel tank and engine air intake such that when the engine is not running, volatile components from the fuel tank flow through the canister and are adsorbed on the activated carbon. Thus, the hydrocarbon content of any gas discharged to the atmosphere is significantly reduced. When the engine is operated, atmospheric air is periodically introduced in the canister, resulting in the desorption of adsorbed hydrocarbon vapor components from the activated carbon so that the activated carbon can again be used during the next cycle of vehicle inactivity. The desorbed hydrocarbons are routed to the engine where they are combusted.
Activated carbon is typically an expensive component that accounts for a significant proportion of the evaporative emission control system's overall cost. Thus, it is desirable to improve the efficiency with which the fuel vapor adsorbent adsorbs and desorbs vapor components. Thus, a need has arisen for an evaporative emission control system that addresses the foregoing.
Referring now to the discussion that follows and also to the drawings, illustrative approaches to the disclosed systems and methods are shown in detail. Although the drawings represent some possible approaches, the drawings are not necessarily to scale and certain features may be exaggerated, removed, or partially sectioned to better illustrate and explain the present invention. Further, the descriptions set forth herein are not intended to be exhaustive or otherwise limit or restrict the claims to the precise forms and configurations shown in the drawings and disclosed in the following detailed description.
Evaporative emission control system 26 includes a vapor outlet line 38 that is connected to an engine air intake line 36. Air intake line 36 and vapor outlet line 38 combine to form engine air inlet line 34. Valve 40 controls the flow of air and/or desorbed vapor components (see below) supplied from evaporative emission control system 26 into engine 22. Valve 40 may be operated by a suitable control system (not shown) to regulate the flow of air into engine 22.
In the embodiment of
Vapor contacting material 29 comprises a vapor storage medium, which in the illustrated system of
Fuel vapor outlet 56 is also located at first end 50 of canister 27 and is connected to vapor outlet line 38. Canister 27 also may include filter 46 for filtering particulate matter from discharged vapors before they exit canister 27 through fuel vapor outlet 56 and fuel vapor outlet line 38. As explained below, in one preferred approach, fuel vapor contacting material 29 is granular in nature and may become entrained in the discharged vapor stream. Filter 46 reduces the amount of entrained particulates exiting evaporative emission control system 26. In one example, filter 46 is a screen member having a mesh size less than that of vapor contacting material 29. In another exemplary illustration, filter 46 is formed from urethane or non-woven fabric. Filter 46 also aids in retaining vapor contacting material 29 within canister 27.
Second end 52 of canister 27 also comprises a filter 44 which is similar to filter 46. In the illustrated system of
As indicated above, vapor contacting material 29 includes fuel vapor adsorbent 29a and at least one catalyst 29b. As shown in
As shown in
In one preferred approach, fuel vapor adsorbent 29a comprises activated carbon. As is known to those skilled in the art, activated carbons may be classified according to their “Butane Working Capacity” or “BWC,” which indicates their ability to adsorb butane under certain specified test conditions. The BWC is defined as the difference between the butane adsorbed at saturation and the butane retained per unit volume of carbon after a specified purge. Unless otherwise specified, as used herein the terms “Butane Working Capacity” or “BWC” refer to the ASTM (“American Society of Testing and Materials”) Standard D-5228 test method. In one exemplary illustration, fuel vapor adsorbent 29a comprises a BAX 1100 activated carbon supplied by the Specialty Chemicals Division of MeadWestvaco Corporation of Covington, Va. BAX 1100 activated carbon has a nominal BWC of 11.3 g butane/100 cc activated carbon. In another example, fuel vapor adsorbent 29a comprises a MeadWestvaco BAX 1500 activated carbon having a nominal BWC of 15.0 g butane/100 cc activated carbon. The foregoing activated carbon products are exemplary only, and other fuel vapor adsorbents with other BWC values may also be used without departing from the spirit and scope of the invention.
Fuel vapor adsorbents such as activated carbon are typically expensive and constitute a substantial portion of the cost of evaporative emission control system 26. Thus, it would be beneficial to provide a vapor contacting material 29 that reduces the required amount of fuel vapor adsorbent 29a. It has been discovered that catalyst 29b can be added to fuel vapor adsorbent 29a to increase the amount of hydrocarbon that can be removed for a given amount of fuel vapor adsorbent 29a.
As used herein the term “catalyst” refers to a single constituent or a combination of constituents that improves the efficiency (i.e., the amount of hydrocarbon adsorbed/desorbed per unit volume of fuel vapor adsorbent 29a) of the hydrocarbon adsorption/desorption process and is not limited to materials that increase the rate of reaction.
Catalyst 29b preferably has a specific heat that is higher than the specific heat of fuel vapor adsorbent 29a. As a result, the overall specific heat of vapor contacting material 29 will be higher than if fuel vapor adsorbent 29a alone were used. Without wishing to be bound by any theory, it is believed that the increase in specific heat reduces the overall temperature increase in vapor contacting material 29 during adsorption/loading in accordance with the following relationship:
ΔHadsorption=CpΔT−Qlosses (1)
In formula 1, ΔHadsorption refers to the heat released due to adsorption. Cp refers to the specific heat of vapor contacting material 29. ΔT refers to the temperature increase in vapor contacting material 29, and Qlosses refers to heat losses to the surroundings. As the temperature of vapor contacting material 29 increases, it is believed that the efficiency of adsorption decreases while heat losses to the surroundings increase. Accordingly, it is further believed that by increasing the overall specific heat of vapor contacting material 29, more heat can be recovered from the adsorption process and made available for use in desorption. It is believed that the increase in available heat for desorption increases the extent of desorption, thereby making additional active sites on fuel vapor adsorbent 29a available to adsorb hydrocarbons in subsequent adsorption/loading cycles. Thus, for a given volume of fuel vapor adsorbent, more hydrocarbon can be adsorbed and desorbed. Put differently, by increasing the specific heat of vapor contacting material 29, a desired level of hydrocarbon removal can be achieved using less fuel vapor adsorbent 29a.
Catalyst 29b preferably comprises a polymeric material. In one exemplary illustration, the polymeric material is a polyamide (nylon) resin. In another example, the polymeric material is polypropylene. Catalyst 29b may also include additives such as heat stabilizers, dyes, impact modifiers, etc. However, such additives are preferably present in a sufficiently small amount so that their effect on adsorption and/or desorption of hydrocarbon vapors is negligible. In certain exemplary envisioned approaches, fuel vapor adsorbent 29a comprises an activated carbon having a specific heat of from about 0.2 to less than 0.4 cal/g/° C., and catalyst 29b has a specific heat of at least about 0.4 cal/g/° C. Of course, catalyst specific heats greater than 0.4 cal/g/° C. are also suitable. In an illustrated example wherein fuel vapor adsorbent 29a is MeadWestvaco BAX 1100 activated carbon, fuel vapor adsorbent 29a has a specific heat of about 0.31 cal/g/° C.
In another exemplary approach, catalyst 29b comprises unfilled nylon 66. As is known to those skilled in the art, nylon 66 is a condensation polymer of hexamethylene diamene and adipic acid. One suitable type of nylon 66 is Zytel® 103FHS, a product of the DuPont Company. Zytel® 103FHS is a heat stabilized lubricated nylon 66 with a specific heat of about 0.4 cal/g/° C. In yet another example, catalyst 29b is polypropylene.
It has been further discovered that the relative amounts of catalyst 29b and fuel vapor adsorbent 29a affect the adsorption capacity of fuel vapor adsorbent 29a, i.e., the amount of hydrocarbon adsorbed per unit volume of adsorbent. More specifically, as increasing amounts of catalyst 29b are added to a given amount of fuel vapor adsorbent 29a, the adsorption capacity of fuel vapor adsorbent 29a per unit volume increases. However, it has also been observed that the percentage change in adsorption capacity ultimately reaches a maximum and then declines as the percentage of catalyst is increased.
The volume of catalyst 29b as a percentage of the total volume of vapor contacting material 29 ranges generally from about one (1) to about forty (40) percent. In a preferred illustration, catalyst 29b is present in a volume ranging from about ten (10) to about twenty (20) percent by volume of the total volume of vapor contacting material 29. In an especially preferred approach currently contemplated, catalyst 29b comprises about fifteen (15) percent by volume of the total volume of vapor contacting material 29.
Fuel vapor adsorbent 29a and catalyst 29b are preferably provided in pelletized or granular form to allow them to be mixed together. In one envisioned example, catalyst 29b comprises non-uniform granules that are about 0.5 mm to about 5 mm in length, width, and thickness. However, a variety of granule shapes and sizes may be used. In an example wherein catalyst 29b comprises Zytel® 103FHS, catalyst 29b comprises non-uniform granules of about 1 mm to about 3 mm in overall length, width, and thickness. In certain embodiments, fuel vapor adsorbent 29a preferably comprises pellets having a length that is from about 2 mm to about 5 mm and a diameter that is from about 2.1 mm to about 2.5 mm. In another example wherein fuel vapor adsorbent 29a comprises pelletized Westvaco BAX1100 activated carbon, fuel vapor adsorbent 29a has a mesh size of about 2 mm and a mean particle diameter of about 2.2 mm. However, granular fuel vapor adsorbents such as granular activated carbon may also be used. The foregoing are merely exemplary illustrations; a wide variety of pellet and granule geometries and sizes can be used.
In one currently preferred illustration, vapor contacting material 29 comprises a substantially homogeneous mixture of fuel vapor adsorbent 29a and catalyst 29b. The use of a substantially homogeneous mixture is believed to allow catalyst 29b to more uniformly absorb the heat generated when hydrocarbons are adsorbed by fuel vapor adsorbent 29a and to more uniformly release the absorbed heat during a desorption process. In turn, the efficiency gains in adsorption and desorption are more uniformly distributed throughout vapor contacting material 29, thereby minimizing the occurrence of localized hot spots that experience reduced adsorption capacity.
A variety of different processes can be used to combine fuel vapor adsorbent 29a and catalyst 29b. In one embodiment, catalyst 29b is ratably injected into a flow of fuel adsorbent 29a to obtain the desired relative amounts of each component. The combined flow of fuel vapor adsorbent 29a and catalyst 29b may be directly injected into individual canisters, or it may be injected into secondary receptacles from which the canisters are subsequently filled.
Table 1 presents adsorption data for various relative amounts of fuel vapor adsorbent 29a and catalyst 29b at a constant volume of fuel vapor adsorbent 29a. Butane Working Capacity values were calculated for four different volume percentages of catalyst: 0%, 10%, 15%, and 20%. The fuel vapor adsorbent 29a that was used was Westvaco BAX1100 activated carbon. The catalyst 29b that was used was Zytel® 103FHS nylon 66. At each different percentage of catalyst 29b, fresh activated carbon and nylon 66 were mixed and added to a test canister to obtain a bed of vapor contacting material 29 comprising a substantially homogeneous mixture of activated carbon and nylon 66.
In generating the data in Table 1, the ASTM D-5228 test method was not used. Instead, a 50/50 mixture (by volume) of butane and nitrogen was fed to the test canister until a saturation condition was observed in which 2 g of butane broke through the bed. To desorb the adsorbed hydrocarbons, 300 BV (“bed volume,” where 1 BV equals the volume occupied by vapor contacting material 29) of dry air was fed to the test canister at a flow rate of 22.7 liters/minute. To account for aging effects, at every different percentage of catalyst, the adsorption/loading and desorption/purging cycles were performed 13 times. At the 12th and 13th cycles, an adsorbed butane value was determined by calculating the difference between the amount of butane adsorbed at saturation (i.e., when 2 g of butane broke through the bed) and following purge. Average values for the amount of adsorbed butane at the 12th and 13th cycles were calculated and appear in column 2 of the table. The adsorbed butane was then divided by the volume of carbon (which was held constant at 2.3 liters) to obtain a BWC value in units of g/liter carbon. The BWC values for the various amounts of catalyst were compared to the zero catalyst BWC data to determine the percentage change in BWC for that amount of catalyst.
As the data in Table 1 indicate, as the amount of nylon 66 was increased, the Butane Working Capacity of the activated carbon increased, indicating that the addition of nylon 66 yielded more efficient adsorption for a given amount of carbon. While the data in Table 1 show a continual increase in Butane Working Capacity as the amount of catalyst is increased, the percentage increase exhibits a maximum at about fifteen (15) volume percent of catalyst. Thus, the further addition of catalyst would likely result in progressively smaller incremental gains in Butane Working Capacity.
As Table 1 suggests, by adding a catalyst that increases the overall specific heat of vapor contacting material 29, a desired amount of hydrocarbon adsorption can be achieved using less fuel vapor adsorbent. For example, in the illustrative approaches used to generate Table 1, the addition of fifteen (15) percent (by volume) of catalyst provided a 9.34 percent change in Butane Working Capacity. Thus, at fifteen (15) volume percent catalyst, the amount of fuel vapor adsorbent needed to remove a target amount of hydrocarbon is reduced by over nine (9) percent. Many polymeric catalysts are more readily available, including from recycled materials, than are fuel vapor adsorbents such as activated carbon. Thus, by using polymeric catalysts and decreasing the required amount of fuel vapor adsorbent, the overall cost of evaporative emission control system 26 may be correspondingly reduced.
The present invention has been particularly shown and described with reference to the foregoing examples, which are merely illustrative of the currently envisioned best modes for carrying out the invention. It should be understood by those skilled in the art that various alternatives to the illustrated approaches described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. The illustrations given above should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing examples are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.
With regard to the processes, methods, heuristics, etc. described herein, it should be understood that although the steps of such processes, etc. have been described as occurring according to a certain ordered sequence, such processes could be practiced with the described steps performed in an order other than the order described herein. It further should be understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted. In other words, the descriptions of processes described herein are provided for illustrating certain embodiments and should in no way be construed to limit the claimed invention.
Accordingly, it is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the invention should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the arts discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In sum, it should be understood that the invention is capable of modification and variation and is limited only by the following claims.
All terms used in the claims are intended to be given their broadest reasonable constructions and their ordinary meanings as understood by those skilled in the art unless an explicit indication to the contrary is made herein. In particular, use of the singular articles such as “a,” “the,” “said,” etc. should be read to recite one or more of the indicated elements unless a claim recites an explicit limitation to the contrary.
