Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing
Reexamination Certificate
2000-10-20
2003-07-29
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Inorganic carbon containing
C502S182000, C502S183000, C502S202000, C502S208000, C502S417000, C502S423000, C502S425000, C502S427000, C423S282000, C423S299000, C423S447500, C423S460000
Reexamination Certificate
active
06599856
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to formed activated carbon and process for producing the same. More particularly it relates to formed activated carbon for a fuel vapor collecting device, what is called “canister”, and a process for producing the same.
BACKGROUND ART
An evaporative control system having a fuel vapor collecting device using activated carbon for preventing fuel vapor from dissipating in the air is known. This system is designed to have fuel vapor generated from a fuel system, such as a fuel tank, once adsorbed onto activated carbon of the fuel vapor collecting device and, when the engine is driven to introduce the air, the fuel vapor collecting device is purged with the air to desorb the fuel vapor, which is burnt in the engine.
The activated carbon exhibits higher adsorptivity for fuel vapor at a lower temperature, and higher desorptivity at a higher temperature. Adsorption of fuel vapor is an exothermic reaction, while desorption is an endothermic reaction. Therefore, with the progress of adsorption, the temperature of activated carbon elevates to gradually reduce the adsorptivity. Similarly, the temperature of activated carbon drops to reduce the desorptivity with the progress of desorption.
To solve this problem, JP-A-55-149622 proposes providing an activated carbon chamber with fin(s) for heat release to efficiently dissipate the generated heat thereby preventing temperature rise in adsorption or temperature drop in desorption to improve the adsorption and desorption efficiency.
JP-A-64-36962 proposes a fuel vapor collecting device packed with a collecting material comprising activated carbon having dispersed therein a heat accumulating solid filler having a higher specific heat than activated carbon. The fuel vapor collecting device is prepared by kneading coal powder and a heat accumulating solid filler together with a binder, forming, grinding, carbonizing, and activating.
According to JP-A-64-36962, however, since the heat accumulating solid filler is dispersed in activated carbon, the formed activated carbon hardly manifests a sufficiently increased specific heat, fails to have sufficient mechanical strength, and is still unsatisfactory in adsorptivity and desorptivity.
In the practice a treatment for increasing the bulk density is required for obtaining increased adsorptivity per unit volume. An activation treatment following forming has difficulty in securing sufficient mechanical strength or density consistently with high adsorptivity. A method comprising, forming with an organic binder after activation (see JP-B-56-37164, JP-B-55-43402, and JP-B-52-13517) and a method comprising forming with an inorganic binder (see JP-B-45-12565 and JP-B-63-242343) have been proposed but are still insufficient in securing high adsorptivity and desorptivity.
SUMMARY OF THE INVENTION
Under these circumstances, activated carbon having moderate strength and enhanced adsorptivity and desorptivity has been sought. An object of the present invention is to provide activated carbon meeting such a demand.
The present inventors have conducted extensive studies to improve adsorptivity and desorptivity of activated carbon while retaining heat resistance, strength, and density. As a result, they have found that formed activated carbon which has sufficient forming strength and can take full advantage of the high specific heat of a heat accumulating solid filler can be obtained by kneading, forming and firing a mixture of activated carbon powder, a heat accumulating solid filler whose particle size is relatively close to that of the activated carbon powder, clay, and specific compounds.
The present invention provides formed activated carbon having a Kiya crushing strength (hereinafter defined) of 1 kg or more and a specific heat of 0.4 J/k·cc or more at 25° C.
The present invention also provides a process of producing formed activated carbon comprising kneading 100 parts by weight of activated carbon powder with (A) 10 to 100 parts by weight of clay, (B) 5 to 200 parts by weight of a metal powder and/or a metal oxide powder, and (C) 2 to 20 parts by weight of a boron compound and/or a phosphorus compound, forming the mixture (plastic body), and firing the green body.
The present invention further provides formed activated carbon produced by the above process.
In preferred embodiments of the invention, the Kiya crushing strength is that of formed activated carbon having a diameter of 2.5 mm and a length of 4 mm; the metal powder is aluminum powder and/or magnesium powder; the metal oxide powder is alumina powder and/or magnesium oxide powder; the boron compound is boric acid and/or diboron trioxide (B
2
O
3
); the formed activated carbon has an average particle size of 0.5 to 5 mm; the firing temperature is 500 to 900° C.; and/or the formed activated carbon is for a fuel vapor collecting device.
The activated carbon powder which can be used in the invention includes various species obtained from coal, coconut shell, wood, lignin, etc. by activation with steam or chemicals, such as phosphoric acid, zinc chloride or an alkali metal. Phosphoric acid-activated wood-based carbon powder is preferred. From the standpoint of forming performance and formed body strength, the activated carbon powder usually has a particle size of 0.5 mm or smaller, preferably 0.05 to 0.15 mm. It is preferred for the activated carbon powder to comprise 200-mesh undersize particles in a proportion of 60 to 95%, particularly 60% or more of 100-mesh undersize particles and not more than 50% of 325-mesh undersize particles. It is still preferred that the activated carbon powder comprises 80% or more of 100-mesh undersize particles and not more than 40% of 325-mesh undersize particles, particularly 80 to 90% of 100-mesh undersize particles and 20 to 40% of 325-mesh undersize particles.
The activated carbon powder for use in the invention usually has a specific surface area of 500 to 2500 m
2
/g, preferably 1000 to 2000 m
2
/g, still preferably 1500 to 2000 m
2
/g. Activated carbon having too small a specific surface area tends to have insufficient adsorptivity, while one having too large a specific surface area tends to have insufficient strength.
The clay which can be used in the invention preferably includes bentonite, such as sodium bentonite and calcium bentonite, with sodium bentonite being particularly preferred. Sodium bentonite, having a particle size ranging from 1 to 100 &mgr;m, is not allowed to enter and clog the smaller pores than 300 Å of activated carbon which participate in gas or liquid adsorption and therefore secures high adsorptivity of activated carbon.
The heat accumulating solid filler which can be used in the invention includes powders of metals, such as iron, aluminum, magnesium, copper and lead (or alloys thereof), oxides or carbonates of one or more of these metals, ceramics, or glass. Preferred of them are powders of metals or metal oxides.
In order to suppress the temperature fall of activated carbon while purging fuel vapor from the activated carbon, it is desirable for the heat accumulating solid filler to have a higher specific heat than activated carbon. Such heat accumulating solid fillers include metal powders, e.g., aluminum powder, aluminum alloy powder, and magnesium powder; metal oxide powders, e.g., alumina powder, magnesium oxide powder, and boron oxide powder; and metal carbonate powders, e.g, calcium carbonate powder and magnesium carbonate powder. Preferred of them are aluminum powder, alumina powder, and magnesium oxide powder. These metal powders, metal oxide powders and metal carbonate powders can be used either individually or as a mixture of two or more thereof. Metal oxide powders are preferred for their chemical stability. From this viewpoint, alumina powder and magnesium oxide powder are the most preferred.
Alumina species of alumina powder includes &agr;-alumina, &bgr;-alumina, &ggr;-alumina, &dgr;-alumina, &khgr;-alumina, &eegr;-alumina, &thgr;-alumina, and &kgr;-alumina, with &agr;-alumina being preferred.
The heat accumulating solid fille
Matsuura Kazushi
Suzuki Mitsuo
Takeda Yoshitaka
Uchino Masashi
Yamada Eiji
Bell Mark L.
Hailey Patricia L.
Tennex Corporation
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