WO2019115810A1

WO2019115810A1 - Evaporative emission control canister system

Info

Publication number: WO2019115810A1
Application number: PCT/EP2018/085068
Authority: WO
Inventors: Andrzej Kalina; Maciejg. BOJKOWSKI; Justyna SOBOL
Original assignee: Delphi Technologies Ip Limited; Delphi France Sas
Priority date: 2017-12-14
Filing date: 2018-12-14
Publication date: 2019-06-20
Also published as: GB201720905D0; GB2569353A

Abstract

An evaporative emission control canister system comprises an initial adsorbent volume (24) located on a tank port side (18); and subsequently, at least one low bleed adsorbent volume (26), wherein the initial adsorbent volume and said at least one low bleed adsorbent volume are located within a single canister (12) or the initial adsorbent volume and the at least one low bleed adsorbent volume are located in separate canisters (12, 30, 40) that are connected to permit sequential contact by fuel vapours. The low bleed adsorbent volume (26) has a low purge ratio of less than 75% and a heel of at least 2 g/dL.

Description

EVAPORATIVE EMISSION CONTROL CANISTER SYSTEM

Technical field

[0001] The present invention generally relates to the field of fuel emission control in internal combustion engines. More specifically, the invention concerns an evaporative emission control canister system for use in gasoline engines to reduce emissions from fuel systems.

Background of the Invention

[0002] Evaporation of gasoline fuel from motor vehicle fuel systems is a major potential source of hydrocarbon air pollution. Such emissions can be controlled by canister systems that employ activated carbon to adsorb the fuel vapor emitted from the fuel systems. As it is known, when a vehicle is parked in a warm environment during the daytime heating (i.e., diurnal heating), the temperature in the fuel tank increases resulting in an increased vapor pressure in the fuel tank. A mixture of fuel vapor and air from the fuel tank enters the canister through a fuel vapor inlet of the canister and diffuses into the adsorbent volume where the fuel vapor is adsorbed in temporary storage and the purified air is released to the atmosphere through a vent port of the canister. Under certain modes of engine operation, the adsorbed fuel vapor is periodically removed from the activated carbon by purging the canister systems with ambient air to desorb the fuel vapor from the activated carbon. The regenerated carbon is then ready to adsorb additional fuel vapor.

[0003] A conventional type of canister is, e.g., disclosed in EP 0 899 450.

[0004] The purge air does not desorb the entire fuel vapor adsorbed on the adsorbent volume, resulting in a residue hydrocarbon, known as “heel”, that may be emitted to the atmosphere. In addition, that heel in local equilibrium with the gas phase also permits fuel vapors from the fuel tank to migrate through the canister system as emissions. Such emissions typically occur when a vehicle has been parked and subjected to diurnal temperature changes over a period of several days, commonly called “diurnal breathing losses”, DBL.

[0005] Over more than a decade, the design of fuel canisters has been mainly inspired by the guidelines set forth in an SAE paper relating to “Impact and Control of Canister Bleed Emissions” by Roger S. Williams et al. (Society of Automotive engineers, no. 2001-01 -0733).

[0006] As reported in the SAE paper, DBL is a major concern for automotive manufacturers, also because of regulations taken by some countries or states. At the time the paper was published, the California Low Emission Vehicle Regulations made it desirable for DBL emissions from canister systems to be below 10 mg (“PZEV”) for a number of vehicles from 2003 model year and below 50 mg, typically below 20 mg, (“LEV-N”) for a larger number of vehicles beginning with the 2004 model year. The new LEV-III Regulation is even more restrictive on DBL.

[0007] A first observation is that canister bleed emissions during diurnal testing are due to carbon diffusion from areas having higher concentrations to areas of lower concentrations. The concentration of hydrocarbons in the section of the carbon bed near the atmosphere port will therefore increase with time. Relying on Fick’s law of diffusion, it appears that canister bleed emissions are affected by several parameters such as soaking time, purge volume and canister geometry.

[0008] According to the authors, the effect of the concentration gradient on bleed emissions is twofold. First, the concentration affects the diffusion rate, which can be controlled primarily by the amount of purge. Second, the amount of purge also influences the equilibrium, respectively concentration gradient, between the hydrocarbons adsorbed in the carbon pores and the hydrocarbons in the gas phase.

[0009] Thus, a well-purged canister filled with a high-capacity, low-heel carbon is desired to reduce bleed emissions.

[0010] As is well known today, one efficient solution reported in the SAE paper is the use of an auxiliary chamber, which is also often referred to as “scrubber” or“low bleed element” that constitutes a subsequent adsorbent volume located after a standard, initial adsorbent volume. An auxiliary chamber connected to the vent port of a standard base canister significantly reduces bleed emissions. However the geometry of such auxiliary chamber needs to be optimized. Granular carbon material (pellets and the like) can be used in such auxiliary chamber, but the use of carbon monoliths has become very popular because they have less flow restrictions than conventional pellets.

[0011] The SAE paper concludes by listing the canister design parameters that allow reducing bleed emissions, namely:

increasing the purge volume

Optimizing the canister geometry

Using high-capacity /low-heel wood-based carbons

Utilizing an auxiliary chamber

Optimizing the auxiliary chamber

[0012] This conclusion has served as guideline for the modern design of canisters. The consequence is that the market has been dominated by manufacturers of activated carbon, pushing forward the use of high- capacity /low-heel wood-based carbons in canister systems for their excellent purgeability.

[0013] Such adsorbent materials are considered primary (and sometimes the only) choice for canister applications where canister bleed emission shall be limited to levels from 150 mg/day down to 20 mg/day and less. This is even more so the case for applications with low canister purge levels, i.e. 200 bed volumes of air and less.

Object of the invention

[0014] The object of the present invention is to provide an evaporative emission control canister of alternative design, which can be manufactured at lower costs than conventional canisters while still meeting the requirements of current legislations on DBL.

[0015] This object is achieved by an evaporative emission control canister as claimed in claim 1. General Description of the Invention

[0016] As explained above, modern automotive canisters and adsorbents use wood based activated carbons to ensure low bleed emission performance required by new evaporative emission legislation CARB Lev III, EPA Tier III, Euro 6 c/d and China 6. Such adsorbents have been used for their very good butane purge ratios with the objective of maximizing the working (useful) capacity of adsorbent, reducing carbon heel, eliminating capacity deterioration in process of ageing with fuel vapor and ensuring low bleed emissions.

[0017] It should be noted that activated carbon manufacturing depends strongly on raw material source and on the method of carbon activation. In principle, a competitive product can be provided when the raw material is a byproduct from another high scale manufacturing process, as is for example the case for saw dust derived from wood chopping.

[0018] The present invention has been developed based on the finding that the choice of wood (and hard wood especially) is unnecessarily narrowing the potential raw material supply base, and that there is hence a need to challenge common design rules making extensive use of high- capacity/low-heel wood-based carbons, i.e materials having a high butane activity and exhibiting a low butane retentivity. Indeed, raw material processing methods can create barriers in geographical regions with regard to access to adsorbent choice. Broadening the choice of raw materials would facilitate access to technologies and reduce costs.

[0019] The present invention proposes an evaporative emission control canister system comprising an initial adsorbent volume located on a tank port side; and, subsequently, at least one low bleed adsorbent volume, wherein the initial adsorbent volume and the at least one low bleed adsorbent volume are located within a single canister or the initial adsorbent volume and the at least one low bleed adsorbent volume are located in separate canisters that are connected to permit sequential contact by fuel vapor. [0020] According to the present invention, the low bleed adsorbent volume has a low purge ratio of less than 75% and a heel of at least 2 g/dL

[0021] It shall be appreciated that the present evaporative emission control canister system has proved, in a surprisingly and unexpected manner, to be efficient to reduce DBL to a range between 10 to 100 mg/day. These results have been obtained following the Bleed Emission Test Procedure (BETP) according to CALIFORNIA EVAPORATIVE EMISSION STANDARDS AND TEST PROCEDURES FOR 2001 AND SUBSEQUENT MODEL MOTOR VEHICLES, Adopted: August 5, 1999, Last Amended: September 2, 2015.

[0022] The initial tests carried out with the inventive canister systems have confirmed that the use of alternative adsorbents, i.e. low bleed adsorbent volumes as prescribed herein, show improvements on evaporative control emissions that are comparable to prior art solutions.

[0023] Furthermore, these performances make the present canister system suitable under current legislations such as CARB Lev III, EPA Tier III, Euro 6 c/d, and China 6 / Beijing 6.

[0024] In a particularly remarkable manner, the present evaporative emission control canister system distinguishes from conventional canister design in the art. As explained in the above“background” section, the modern design of evaporative canisters has been driven by the guidelines of the 2001 SAE paper of Williams et al. and the products developed on the basis of this paper.

[0025] As already explained, due to these guidelines, for more than a decade canister systems have been manufactured using high- performance carbons, namely wood-based carbons, for their high-capacity (i.e. high butane activity) and low-heel (i.e. low butane retentivity), as well as a good purgeabiliby. Indeed, these carbons were used to ensure the high purgeability of the adsorbent volume.

[0026] Against common beliefs in the art (strongly tied to assumption that wood-based high purgeability carbons are required to achieve low DBL emissions), it has however been surprisingly discovered that the use of other carbon types with low purgeability and significant heel can perform satisfactorily to reduce diurnal bleed emissions from evaporative emissions control canisters.

[0027] According to the invention, the provision in a canister of at least one low bleed adsorbent volume having a low purge ratio of less than 75% and a heel of at least 2 g/dL does permit reducing DBL to a range of 10- 100 mg per day, in particular with low canister purge levels of 75 to 210 bed volumes (BV), as determined using BETP.

[0028] As used herein, the term“heel” conventionally designates the residual amount of hydrocarbons that remains on the adsorbent as per a predetermined load-purge cycle procedure, and is also referred to as ‘butane retentivity’. In the context of the present invention, heel is preferably determined from data obtained by test method D5228 ASTM, in which heel is specifically referred to as’butane retentivity’. If required, the test method D5228 can be adapted to be applicable to non-granular material, such as monoliths and foams.

[0029] The term“purge ratio” or“purgeability”, as used herein, refers to the amount of Hydrocarbon that can be removed from an adsorbent as per a predetermined load-purge cycle procedure. Here also, the purge ratio is preferably determined from data obtained by test method D5228 ASTM (BWC and Butane activity), which may be adapted to take into account the form of the adsorbent.

[0030] The term“adsorbent volume” or“adsorbent element”, as used herein, refers to an adsorbent material or adsorbent containing material along vapor flow path, and may consist of a bed of particulate material, a monolith, honeycomb, adsorbent foam, sheet or other material, or combinations thereof.

[0031] A merit of the present invention is to have demonstrated that the choice of adsorbent for the vent side of the canister should not necessarily follow the rules applicable to the choice of initial adsorbent. For the initial adsorbent volume, adsorbent material of low heel and high activity has been the preferred choice as it provides high working capacity under repetitive adsorption / desorption cycles, helping to efficiently utilize the canister volume (and reduce the mass of adsorbent used) in contact with high concentration of fuel vapors at canister inlet. However, the present inventors consider that such good purgeability adsorbent at the vent side of the canister has the disadvantage of low adsorptivity and vapour retention at low vapour concentrations. This is especially the case at lower levels of purge, when non-purged vapours are not retained and are easily released causing bleed emissions. According to the present invention, the use of high retentivity adsorbent in the low bleed element promotes efficient adsorption and retention of vapours under low fuel vapour concentration typical at the canister vent side. In contrast to the initial adsorbent volume, which has typically to adsorb up to 200 grams of fuel vapours, the low bleed adsorbent at the canister vent side is intended to adsorb significantly lower amounts of fuel vapour (100-200 mg) released from the initial adsorbent and base section of a canister. Therefore, the high heel of such low bleed adsorbent is not a concern, and in fact the high fuel vapour retentivity helps retaining vapour and prevents bleeding under emission test procedure.

[0032] In general, the adsorbents suitable for use in the adsorbent volumes may be derived from many different materials and may be in various forms, as will be explained in more detail further below. It is however clear that, concerning the low bleed adsorbent volume(s), any appropriate materials and forms may be used, to the extent that the low bleed adsorbent volumes exhibit the desired properties of purge ratio and heel, as prescribed herein.

[0033] Low bleed adsorbent volumes are advantageously manufactured from standard- to high-capacity, low purgeability activated carbon. The purge ratio may namely be below 65%, particularly below 60%, and more particularly between 50 and 58%.

[0034] Low bleed adsorbent volume may have a heel of at least 2.3 g/dL, in particular above 2.6 g/dL and more particular about 3.0 g/dL.

[0035] In embodiments, the low bleed adsorbent volume has a carbon content of between 5% and 75%, preferably between 10% and 70%, even more preferably between 30% and 60%. [0036] The monolith form is considered to be particularly convenient for use in supplemental canisters. It typically offers good and uniform flow conditions and has a geometry that can be easily integrated in a cylindrical casing. Nevertheless granular or pelletized forms, of any various shapes, are not to be excluded.

[0037] In embodiments, the supplemental canister may have a shape ratio (Length / Diameter) of at least 1.2, preferably at least 1.4.

[0038] Regarding the materials, any appropriate materials may be used. However, activated carbons manufactured from coconut shells have proved to be of particular relevance in the context of the present invention. Indeed, without willing to be bound by any theory, it is believed that the pore structure of coconut based activated carbons leads to the desired performance in terms of purgeability and heel.

[0039] In embodiments, the initial adsorbent volume may conventionally include activated carbon having a butane working capacity of at least 9 g/dL, in particular above 11 g/dL or 15 g/dL. The purge ratio may be above 85%, preferably above 90% or above 95%.

[0040] In embodiments, the canister system includes:

a base canister containing the initial adsorbent volume, the base canister having a tank port, a purge port and a vent port; and a supplemental canister having an inlet port in fluid communication with the vent port of the base canister and an outlet port for connection of the canister system to the atmosphere, the supplemental canister comprising the at least one low bleed adsorbent volume, in particular one low bleed adsorbent volume or two low bleed adsorbent volumes arranged in successive chambers.

[0041] Another parameter of relevance identified by the present inventors is herein referred to as‘effective emissions’, which reflects the ability of a bleed element to retain hydrocarbons under low purge conditions and reduce emissions during diurnal test. Effective emission is defined as the average mass (in mg) of hydrocarbons released by a bleed element at 40±1 °C after n-butane load to 2g breakthrough followed by 150 SLPM purge (standard liters per minute). The bleed element is preconditioned before test (minimum 2 cycles of n-butane load to 2g breakthrough followed by 750 SLPM of purge). The mass is given as equivalent mass of n-pentane.

[0042] Preferably the low bleed element has effective emissions less than 20 mg at 150 SLPM purge.

[0043] Still another parameter of relevance identified by the present inventors is herein referred to as‘effective bleed emissions’, which reflect the ability of the adsorbent in the bleed element to retain hydrocarbons under low purge conditions and reduce emissions during diurnal test. Effective bleed emission is defined as the average mass of hydrocarbons released by a bleed element at 40±1 °C after n-butane load to 2g breakthrough followed by 150 SLPM purge. The mass (expressed in mg/g) is given as equivalent mass of n-pentane per 1 g of adsorbent in bleed element. It is normalized to 1g of adsorbent as its content and size of element may be application-specific and the adsorbent is source of bleed emissions rather than the binder of the element.

[0044] Preferably the effective bleed emissions for said low bleed element are below 1.4 mg/g at 150 SLPM purge.

[0045] According to another aspect of the invention, an evaporative emission control canister system comprises an initial adsorbent volume located on a tank port side; and, subsequently, at least one low bleed adsorbent volume, wherein the initial adsorbent volume and the at least one low bleed adsorbent volume are located within a single canister or the initial adsorbent volume and the at least one low bleed adsorbent volume are located in separate canisters that are connected to permit sequential contact by fuel vapor.

[0046] The low bleed adsorbent volume meets one or more of the following parameters:

its purge ratio is low, namely less than 75%;

its heel is of at least 2 g/dL;

its effective emissions, as defined herein, are of less than 20 mg @ 150 SLPM purge; its effective bleed emissions, as defined herein, are below 1.4 mg/g at 150 SLPM purge.

Brief Description of the Drawings

[0047] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 and 2: are principle cross-sectional views of two embodiments of the present canister system, with a single canister; and

Figures 3 and 4: are principle cross-sectional views of two further embodiments of the present canister system, wherein the canister system has a main canister and a supplemental canister.

Description of Preferred Embodiments

[0048] The present invention now will be described more fully hereinafter, but not all embodiments of the invention are shown. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the principle of the invention.

[0049] Presently described are evaporative fuel emission control systems that are surprisingly and unexpectedly able to reduce DBL emissions to the range 10 to 100 mg/day, even when using relatively low purge volumes as determined using BETP. The design of the present evaporative fuel emission control systems is remarkable in that it differs from prior art solutions by the use of low bleed adsorbent volumes having low purgeability and high heel.

[0050] The evaporative emission performance of the disclosed evaporative emission control systems may be within a range of 10 to 100 mg, under purge conditions according to BETP. The term“low purge,” as used herein, refers to a purge level at or below 210 bed volumes applied after the 40 g/hour BETP butane loading step.

[0051] In general, the evaporative emission control systems as described herein include one or more canisters comprising an initial adsorbent volume connected to or in communication (e.g., vaporous or gaseous) with at least one subsequent low bleed volume.

[0052] The initial volume of absorbent is situated on the fuel side, i.e. closest to the canister tank port to be first on the flow of fuel vapors out of the tank. The subsequent volume(s) of low bleed adsorbent material are situated after the initial volume (or downstream in consideration of the fuel vapor flow from the tank into the canister and toward the vent port).

[0053] FIG. 1 illustrates one embodiment of the evaporative emission control canister system 10 having an initial adsorbent volume and a subsequent adsorbent volume within a single canister. Canister system 10 includes a canister casing 12 (simply ‘canister’), a front screen 14, a support screen 15, a partition wall 16, a fuel vapor inlet 18 from a fuel tank, a vent port 20 opening to an atmosphere, a purge outlet 22 to an engine. The partition wall 16 divides the canister 12 into two chambers 12i and 12₂. The canister 12 contains an initial adsorbent volume 24, located on the fuel source side, i.e. close to the fuel port 18, and a subsequent adsorbent volume 26 designed as low bleed adsorbent volume. Conventionally, the adsorbent material is held by means of the front screen 14 and support screen 15. The bottom of the canister, below support screen 15, forms an air gap in communication with both chamber 12i and 122, through which air and fuel vapors can exchange; chambers 12i and 122 are thus connected in series.

[0054] As it will be understood, the arrangement of an initial adsorbent volume on the fuel source side and of one or more subsequent low bleed adsorbent volumes implies predetermined flow paths:

- a fuel vapor flow path from the fuel port 18 to the initial adsorbent volume 24 toward the subsequent, low bleed adsorbent volume(s) 26, and the vent conduit; and

- an air flow path from the vent conduit to the subsequent, low bleed adsorbent volume(s) 26 toward the initial adsorbent volume 24 and the purge port 22.

[0055] As it is known in the art, the canister system 10 is designed to be associated with an internal combustion engine having an air induction system and supplied with fuel from a fuel tank. The engine further conventionally comprises a fuel vapor inlet conduit connecting the fuel port 18 of the canister 10 to the fuel tank; a fuel vapor purge conduit connecting the purge port 22 of the canister 10 to the air induction system of the engine; and preferably a vent conduit connected at one end to the canister vent port 20 and open at the other end to the atmosphere, for venting the canister 10 and for admission of purge air, or both.

[0056] When the engine is off, the fuel vapor from the fuel tank enters the canister system 10 through the fuel vapor inlet 18. The fuel vapor diffuses into the initial adsorbent volume 24, and then into the subsequent adsorbent volume 26, before being released to the atmosphere through the vent port 20 of the canister system. When the engine is turned on, ambient air is drawn into the canister system 10 through the vent port 20. The purge air flows through the subsequent adsorbent volume 26 and then the initial adsorbent volume 24, and desorbs the fuel vapor adsorbed on the adsorbent volumes before entering the internal combustion engine through the purge outlet 22.

[0057] In any of the embodiments of the instant invention, presented herein or not, the evaporative emission control canister system may include more than one low bleed adsorbent volume. Also, the initial adsorbent volume may itself consist of one or more volumes of adsorbent. The one or more low bleed adsorbent volumes are however always located, having regard to the fuel vapor flow path, after an initial adsorbent volume. Where several low bleed adsorbent volumes are used, they may not necessarily be of the same type, but are configured to have or exhibit a low purgeability and high heel as prescribed herein. Also, conventional bleed elements, e.g. conventional monoliths, can also be additionally used as subsequent adsorbent volumes, in addition to the one or more low bleed elements 26.

[0058] Turning to Fig.2, the shown canister system 100 here comprises two low bleed adsorbent volumes 26i and 26₂ in the same canister 12 as the initial adsorbent volume 24. The low bleed adsorbent volumes are separated by an air gap 28, but such air gap could be omitted. [0059] In the embodiments of Figs. 3 and 4, the canister system includes more than one canister.

[0060] Referring to the canister system 200 of Fig.3, a main canister 12 of same construction as in Fig .1 is shown, which comprises an initial adsorbent volume or volumes 24 that fills the two chambers 12i and 12₂ of the canister 12. Reference sign 30 designates a supplemental canister having an inlet port 32 and an outlet port 34, and containing a low bleed adsorbent volume 36. The inlet port is in communication via a conduit 38 with the vent port 20 of the main canister 12. The outlet port 34 of supplemental canister 30 is a vent port, and will thus typically be connected to the atmosphere via a vent conduit of the engine.

[0061] The canister system 300 of Fig .4 comprises a main canister 12 similar to the one of Fig.3 and a supplemental canister 40 comprising a housing with two low bleed adsorbent volumes 42i and 42₂. Supplemental canister 40 includes a front screen 44, a support screen 46, a partition wall 48, an inlet port 50 and an outlet port 52. The vent port 20 of main canister 12 is connected to the inlet port 50 of supplemental canister 40 via duct 54. The outlet port 52 of supplemental canister 300 forms a vent port that is in practice connected to the atmosphere via a vent conduit of the engine. The base canister thus contains an initial adsorbent volume, whereas the supplemental canister 40 contains two low bleed adsorbent volumes 42i and 42₂ arranged in respective chambers 40i and 40₂.

[0062] In general, the adsorbents suitable for use in the adsorbent volumes may be derived from many different materials and may be in various forms. It may be a single component or a blend of different components. Furthermore, the adsorbent (either as a single component or a blend of different components) may include a volumetric diluent. Non- limiting examples of the volumetric diluents may include, but are not limited to, spacer, inert gap, foams, fibers, springs, binder, filler or combinations thereof.

[0063] Any known adsorbent materials may be used including, but not limited to, activated carbon, carbon charcoal, zeolites, clays, porous polymers, porous alumina, porous silica, molecular sieves, kaolin, titania, ceria, or combinations thereof. Activated carbon may be derived from various carbon precursors. By way of non-limiting example, the carbon precursors may be wood, wood dust, wood flour, cotton linters, peat, coal, coconut, lignite, carbohydrates, petroleum pitch, petroleum coke, coal tar pitch, fruit pits, fruit stones, nut shells, nut pits, sawdust, palm, vegetables such as rice hull or straw, synthetic polymer, natural polymer, lignocellulosic material, or combinations thereof. Furthermore, activated carbon may be produced using a variety of processes including, but are not limited to, chemical activation, thermal activation, or combinations thereof.

[0064] A variety of adsorbent forms may be used. Non-limiting examples of the adsorbent forms may include granular, pellet, spherical, honeycomb, monolith, pelletized cylindrical, particulate media of uniform shape, particulate media of non-uniform shape, structured media of extruded form, structured media of wound form, structured media of folded form, structured media of pleated form, structured media of corrugated form, structured media of poured form, structured media of bonded form, non- wovens, wovens, sheet, paper, foam, or combinations thereof. The adsorbent (either as a single component or a blend of different components) may include a volumetric diluent. Non-limiting examples of the volumetric diluents may include, but are not limited to, spacer, inert gap, foams, fibers, springs, binder, filler or combinations thereof. Furthermore, the adsorbents may be extruded into special thin-walled cross-sectional shapes, such as pelletized cylindrical, hollow-cylinder, star, twisted spiral, asterisk, configured ribbons, or other shapes within the technical capabilities of the art. In shaping, inorganic and/or organic binders may be used.

[0065] The honeycomb and monolith adsorbents may be in any geometrical shape including, but are not limited to, round, cylindrical, or square. Furthermore, the cells of honeycomb adsorbents may be of any geometry. Floneycombs of uniform cross-sectional areas for the flow- through passages, such as square honeycombs with square cross- sectional cells or spiral wound honeycombs of corrugated form, may perform better than round honeycombs with square cross-sectional cells in a right angled matrix that provides adjacent passages with a range of cross-sectional areas and therefore passages that are not equivalently purged. Adsorbent volumes with similar non-uniform flow as a square cell grid in a cylindrical monolith include, for example, a particulate or extrudate adsorbent fill in a relatively narrow cross-sectional filter container. (The looser local packing of the particulate or extrudate at and near the container walls enables preferential flow at the wall, compared with flow towards the center line of the flow path.) Another example is a wound or stacked sheet adsorbent volume, or a square cross-section extruded adsorbent volume that, by virtue of design or fabrication, has a distribution of cell sizes, despite in theory allowing for a uniform air and vapor flow distribution. Without being bound by any theory, it is believed that the more uniform cell cross-sectional areas across the honeycomb faces, the more uniform flow distribution within the part during both adsorption and purge cycles, and, therefore, lower DBL emissions from the canister system.

[0066] Concerning the low bleed adsorbent volume(s), any of the above described materials and forms may be used, to the extent that the low bleed adsorbent volumes exhibit the desired properties of purge ratio and heel, as prescribed herein. It may be noted that the monolith form is considered to be particularly convenient for use in supplemental canisters. It typically offers good and uniform flow conditions and has a geometry that can be easily integrated in a cylindrical casing.

[0067] Regarding the materials, activated carbons manufactured from coconut shells have proved to be of particular relevance in the context of the present invention. Indeed, it is believed that the pore structure of coconut based activated carbons leads to the desired performance in terms of purgeability and heel.

[0068] In the present canister system, the initial adsorbent volume may typically have a conventional working capacity. That is, the initial adsorbent volume exhibits a BWC of at least 9 g/dL, preferably in the range of 11 to 15 g/dL, or higher. Preferably, the initial adsorbent volume also has a good purgeability, typically with a purge ratio above 85%. The initial adsorbent volume can thus consist of any appropriate adsorbent material, in any appropriate form, as discussed above. Conveniently, the conventional granular activated carbon with a BWC between 11 and 15 g/dL can be used, such as e.g. BAX 1100, BAX1100 LD, BAX 1500 commercialized by Ingevity (formerly known as MeadWestvaco, a division of WestRock Company, USA) or NORIT® CNR 115, CNR 115LB commercialized by Cabot Corporation (Georgia, USA).

Examples - Determination of Diurnal breathing loss.

[0069] A plurality of evaporative emission control systems were assembled with selected amounts and types of adsorbents as specified in TABLES 1 to 3 below, for the Examples referenced 1 to 10 (herein after also EX. 1 , EX. 2 to EX. 10).

[0070] For all Examples, a canister having an adsorbent capacity of 3.15 L and filled with BAX1100 activated carbon only, was used as base canister containing the initial adsorbent volume. That is, the base canister did not include any low bleed adsorbent volume. NUCHAR® BAX 1100 is a wood-based activated carbon product, commercially available from Ingevity.

[0071] In EX. 1 , the canister system includes only the base canister. The configuration is similar to that of Fig.1 , with the difference that the canister was filled solely with BAX1100 activated carbon (both chambers). There is no low bleed adsorbent volume.

[0072] The canister system configuration for EX. 2 to 10 comprises two canister housings as shown in Fig .4. The base canister 12 was filled solely with 3.15 L of BAX 1100. For Ex. 2-5, 9 and 10, one subsequent volume of adsorbent (bleed element #1 ) was arranged in the supplemental canister 40, namely in the first chamber 40i. For EX.6-8, two subsequent volumes of adsorbent were arranged in the supplemental canister 40, namely bleed element #1 in first chamber 40i and bleed element #2 in second chamber 40₂. [0073] All Examples were submitted to the Bleed Emission Test Procedure (BETP) according to CALIFORNIA EVAPORATIVE EMISSION STANDARDS AND TEST PROCEDURES FOR 2001 AND SUBSEQUENT MODEL MOTOR VEHICLES, Adopted: August 5, 1999, Last Amended: September 2, 2015 in order to determine the bleed emissions, incorporated herein by reference.

[0074] Each example was uniformly preconditioned by repetitive cycling of gasoline vapor adsorption using a certified fuel and 75 to 205 nominal bed volumes of dry air purge based on the main canister. The same preconditioning protocol was used for all examples, using the same parameters, amongst which: fuel, vapor load rate, cycles, purge rate, soaking time.

[0075] The DBL emissions were subsequently generated by attaching the tank port of the base canister for each Example to a fuel tank filled with 40 vol. % (based on its rated volume) with CARB Phase III fuel (7 RVP, 10% ethanol). Prior to attachment, the filled fuel tank had been preconditioned (temperature stabilization and venting). Two tanks were used for comparison: design A and design B exhibiting different vapour generation during BETP test.

[0076] Upon connection of each Example to the fuel tank, a soaking time of 24 hours was allowed. Before that, each Example had been loaded with 50%-50% butane - nitrogen up to breakthrough (according to BETP procedure); and purged with a volume of air as indicated under line “Purge” of the Tables, with the corresponding bed volumes (line“purge BV”, calculated with respect to the base canister).

[0077] The tank and the Example were then temperature-cycled per CARB's two-day temperature profile, and the emission samples were collected at prescribed time intervals. Following CARB's protocol the day with the highest total emissions was reported as“2-day DBL emissions.” In all cases, the highest emissions were on Day 2. This procedure is generally described in SAE Technical Paper 2001-01-0733 cited in the Background section and in CARB's LEV III BETP procedure (section D.12 in California Evaporative Emissions Standards and Test Procedures for 2001 and Subsequent Model Motor Vehicles, Mar. 22, 2012), which are incorporated herein by reference.

[0078] Also indicated in the tables, for each low bleed adsorbent volume (bleed element #1 and #2) is the purge ratio, BWC and heel.

[0079] Let us now have a look at the DBL performances. EX.1 does not contain any low bleed adsorbent and is only cited to show the behavior of the former generation of canisters. EX.2 is cited as a kind of reference, since it corresponds to the modern design of evaporative emission control canister system. Such canisters comprising a main canister filled with BAX 1100 (or higher) and a supplemental canister comprising a carbon monolith have been very popular over the last decade, for their good performance. Both the BAX 1100 and monolith are manufactured from wood-based activated carbon, having a high purge ratio and low heel.

[0080] Comparing EX.1 and EX.2, one can see the benefit of adding a supplemental canister with a monolith on DBL: the 2-day DBL emissions decrease from 70 mg down to 15 mg.

[0081] By contrast, in all of EX.3 to EX.10, the first low bleed adsorbent volume (bleed element #1 ) was a coconut-based monolith. The purge ratio was between 53 and 55% and all bleed elements #1 had a heel above 2.3 g/dL.

[0082] Activated carbon derived from coconut shells is available from a variety of manufacturers, such as e.g. Barnebey Sutcliffe (Ohio, USA - Division of Calgon Carbon Corporation), Carbon Activated Corp. (California, USA) or General Carbon Corporation (Paterson, NJ, USA). Methods to manufacture monoliths are well known in the art, see e.g. US 5,914, 294 and can be applied with coconut based activated carbon.

[0083] In EX.7 the second chamber of the supplemental canister was filled with a similar coconut-based monolith. In EX.6 however the second chamber of the supplemental canister was filled with a wood-based monolith having a 78% purge ratio and low heel (0.53 g/dL). In EX.8 the second chamber of the supplemental canister was filled with a carbon foam having a purge ratio below 75%. [0084] Turning back to EX.3 to 5, it can be seen that DBL emissions down to below 50 mg and even in the 10 to 20 mg/day range can be achieved using low bleed adsorbent volumes as specified herein, i.e. having low purge ratio and high heel. Furthermore, such low emissions can be achieved with purge levels in the range 100 and 200 BV.

[0085] In EX. 6 to 8, having a supplemental canister with two low bleed elements, the purge corresponded to 110 bed volumes: the DBL was fairly low, varying between 17 to 37 mg/day and improved vs. Example 1 & 3 for corresponding purge levels.

[0086] Finally for EX.9, for a canister system having a supplemental canister having only one low bleed element consisting of a coconut-based monolith having a purge ratio of 53% and a heel of 2.98 g/dL, the DBL emissions were as low as 13 for the same purge level of 110 BV. The DBL emissions were also fairly low for EX.10, containing the same monolith but purged to a lesser extent with 75 BV.

[0087] Table 3 exemplifies the values for the parameters Effective emissions and Effective bleed emissions. Example 2 uses a wood based low bleed adsorbent whereas Examples 9 and 9a use a coconut based low bleed adsorbent. It may be noted that Example 9a is the same as Example 9, however the carbon content for the low bleed adsorbent had been reduced from 60% to 30%. It can be seen that for Example 9 using coconut based carbon the performances are comparable to a convention canister, and even better for example 9a. Determination of purge ratio, BWC, butane activity and heel (butane retentivity)

[0088] ASTM D5228 was used as main reference for determining the adsorbent properties of the low bleed adsorbent volumes, i.e. Bleed element #1 and Bleed element #2. a) Butane Working Capacity (BWC)

[0089] The standard method ASTM D5228 may typically be used to determine the butane working capacity (BWC) of the adsorbent volumes containing particulate granular and/or pelletized adsorbents.

[0090] As is known in the art, the computation of the BWC requires knowledge of apparent density, which is typically carried out following ASTM D 2854 (Test method for apparent density of activated carbon).

[0091] Both D-5228 and D-2854 are drawn up for activated carbon in particle/granular form.

[0092] A modified version of ASTM D5228 method may be used to determine the nominal volume butane working capacity (BWC) of the honeycomb, monolith, and/or sheet adsorbent volumes. The modified method may also be used for particulate adsorbents, where the particulate adsorbents include fillers, voids, structural components, or additives. Furthermore, the modified method may be used where the particulate adsorbents are not compatible with the standard method ASTM D5228, e.g., a representative adsorbent sample may not be readily placed as the 16.7 ml_ fill in the sample tube of the test.

[0093] The modified version of ASTM D5228 method is as follows. The adsorbent sample is oven-dried for a minimum of eight hours at 110±5° C., and then placed in desiccators to cool down. The dry mass of the adsorbent sample is recorded. The mass of the empty testing assembly is determined before the adsorbent sample is assembled into a testing assembly. Then, the test assembly is installed into the a flow apparatus and loaded with n-butane gas for a minimum of 25 minutes (±0.2 min) at a butane flow rate of 500 ml/min at 25° C. and 1 atm pressure. The test assembly is then removed from the BWC test apparatus. The mass of the test assembly is measured and recorded to the nearest 0.001 grams. This n-butane loading step is repeated for successive 5 minutes flow intervals until constant mass is achieved. The test assembly may be a holder for a honeycomb or monolith part, for the cases where the volume may be removed and tested intact. Alternatively, the nominal volume may need to be a section of the canister system, or a suitable reconstruction of the volume with the contents appropriately oriented to the gas flows, as otherwise encountered in the canister system.

[0094] The test assembly is reinstalled to the test apparatus and purged with 2.00 liter/min air at 25° C and 1 atm pressure for a set selected purge time (±0.2 min) according to the formula:

Purge Time (min)=(719xNominal Volume (cc))/(2000 (cc/min)).

[0095] The direction of the air purge flow in the BWC test is in the same direction as the purge flow to be applied in the canister system. After the purge step, the test assembly is removed from the BWC test apparatus. The mass of the test assembly is measured and recorded to the nearest 0.001 grams within 15 minutes of test completion. b) Apparent Density

[0096] As indicated above, the standard method ASTM D2854 may be used to determine the nominal volume apparent density of particulate adsorbents, such as granular and pelletized adsorbents of the size and shape typically used for evaporative emission control for fuel systems.

[0097] Furthermore, the apparent density of an adsorbent volume may be determined using an alternative apparent density method, as defined below. The alternative method may be applied to nominal adsorbent volumes that have apparent densities that are not comparably or suitably measured by the Standard Method. Additionally, the alternative apparent density method may be applied to particulate adsorbents in lieu of the Standard Method, due to its universal applicability. The alternative method may be applied to the adsorbent volume that may contain particulate adsorbents, non-particulate adsorbents, and adsorbents of any form augmented by spacers, voids, voidage additives within a volume or sequential similar adsorbent volumes for the effect of net reduced incremental volumetric capacity. b.1) Apparent Density of Honeycombs, Monolith, or Foam Adsorbents

[0098] The apparent density of cylindrical honeycomb absorbents may be determined according to the following procedure. The volume of adsorbent is a multiple of the cross-sectional area (A) and the length (h) of the adsorbent. The length (h) of the adsorbent is defined as the distance between the front plane of the adsorbent perpendicular to vapor or gas flow entering the adsorbent and the back plane of the adsorbent where the vapor or gas exits the adsorbent. The volume measurement is that of the nominal volume, which is also used for defining bed volume ratios for purge. In the case of a cylindrical honeycomb adsorbent of circular cross- section, the adsorbent cross-sectional area is determined by nd<2>/4, where d is the average diameter measured at four points on each end of the honeycomb. The apparent density is calculated as follows:

Apparent density = mass of the part / (length x cross-sectional area) b.2) Pleated, Corrugated and Sheet Adsorbents

[0099] For pleated and corrugated adsorbents, the nominal adsorbent volume includes all the void space created by the pleats and corrugations. The volume measurement is that of the nominal volume, which is also used for defining bed volume ratios for purge. The apparent density of adsorbent is calculated as indicated in the previous formula. c) Purge ratio

[0100] The Purge ratio is determined based on BWC, butane activity and butane retentivity values obtained per modified version of D-5228 as described above. The Purge ratio is % ratio of BWC (Butane working capacity) to Butane activity, where BWC is difference between Butane activity and Butane retentivity (per modified D-5228). This parameter specifies reversible capacity as percentage of total adsorbent activity. The value is lower for adsorbent demonstrating strong vapour retentivity and preventing from bleed emissions under BETP conditions. [0101] As indicated before, the heel corresponds to the residual amount of hydrocarbons that remains on the adsorbent as per a predetermined load-purge cycle procedure. Heel is typically referred to as “butane retentivity” in ASTM 5228, and can thus be determined based on the method described therein, as is known to those skilled in the art. d) Heel.

[0102] As indicated before, the heel corresponds to the residual amount of hydrocarbons that remains on the adsorbent as per a predetermined load-purge cycle procedure. Heel is typically referred to as “butane retentivity” in ASTM 5228, and can thus be determined based on the method described therein, as is known to those skilled in the art.

Table 1

Table 2

Table 3

Claims

Claims:

1. An evaporative emission control canister system comprising:

an initial adsorbent volume (24) located on a tank port side; and

subsequently, at least one low bleed adsorbent volume (26; 36; 42), wherein the initial adsorbent volume and said at least one low bleed adsorbent volume are located within a single canister (12) or the initial adsorbent volume and the at least one low bleed adsorbent volume are located in separate canisters (12, 30, 40) that are connected to permit sequential contact by fuel vapor;

characterized in that said low bleed adsorbent volume (26) has a low purge ratio of less than 75% and a heel of at least 2 g/dl_.

2. The canister system according to claim 1 , wherein said low bleed adsorbent volume (26) is manufactured from standard- to high-capacity, low purgeability activated carbon.

3. The canister system according to claim 1 or 2, wherein said low bleed adsorbent volume has a purge ratio below 65%, particularly below 60%, and more particularly between 50 and 58%.

4. The canister system according to claim 1 , 2 or 3, wherein said low bleed adsorbent volume has a heel of at least 2.3 g/dL, in particular above 2.6 g/dL and more particular about 3.0 g/dL.

5. The canister system according to any one of the preceding claims, wherein said low bleed adsorbent volume exhibits effective emissions of less than 20 mg at 150 SLPM purge.

6. The canister system according to any one of the preceding claims, wherein said low bleed adsorbent volume exhibits effective bleed emissions of below 1.4 mg/g at 150 SLPM purge.

7. The canister system according to any one of the preceding claims, wherein said activated carbon used for said low bleed adsorbent volume (26) is mainly derived from coconut shells.

8. The canister system according to any one of the preceding claims, wherein said at least one low bleed adsorbent volume is in the form of a monolith.

9. The canister system according to any one of the preceding claims, wherein said canister has diurnal breathing losses in the range of 10-100 mg/day for a purge level between 75 to 200 bed volumes applied after a BETP butane loading step

10. The canister system according to any one of the preceding claims, wherein the cumulative volume of said low bleed adsorbent volume(s) is between 1 and 30% of said initial adsorbent volume.

11. The canister system according to any one of the preceding claims, wherein said low bleed adsorbent volume has a carbon content of between 5% and 75%, preferably between 10% and 70%, even more preferably between 30% and 60%.

12. The canister system according to any one of the preceding claims, wherein said initial adsorbent volume includes activated carbon having a butane working capacity of at least 9 g/dL, in particular above 11 g/dL or 15 g/dL and a purge ratio above 85%, preferably above 90% or above 95%.

13. The canister system according to any one of the preceding claims, wherein said canister system includes:

a base canister (12) containing said initial adsorbent volume, said base canister having a tank port, a purge port and a vent port; and

a supplemental canister (30, 40) having an inlet port (32, 50) in fluid communication with said vent port (20) of said base canister and an outlet port (34; 52) for connection of the canister system to the atmosphere, said supplemental canister comprising said at least one low bleed adsorbent volume, in particular one low bleed adsorbent volume (36) or two low bleed adsorbent volumes (42i, 42₂) arranged in successive chambers.

14. The canister system according to claim 13, wherein said supplemental canister has a shape ratio of at least 1.2, preferably at least 1.4.