Composite material comprising activated carbon and expanded...

Details Composite material comprising activated carbon and expanded... Composite material comprising activated carbon and expanded...

2002-07-26

2004-09-14

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

C423S44500R, C502S418000

Reexamination Certificate

active

06790390

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite material comprising activated carbon and to a process for its preparation.
2. Description of the Related Art
Activated carbon has a great many and very old applications as carbonaceous adsorbent. It is conventionally prepared by a process consisting either in carbonizing a carbonaceous precursor and in then activating the carbon obtained using a physical activating agent or in activating the carbonaceous precursor directly using a chemical activating agent, the activation stage being in both cases accompanied by a heat treatment. Such a process generally results in a pulverulent solid product. However, the use of pulverulent activating carbons as adsorbent has many disadvantages. The pulverulent nature leads to head losses in the systems or the processes in which the activated carbon is used. In addition, the pulverulent activated carbon, if it is used as such, has a tendency to escape from the reactor in which it is used.
In addition, the activated carbons of the prior art, and the stationary beds of activated carbons, exhibit poor thermal conductivity, which limits their use in numerous industrial applications.
Very exceptional activated carbon precursors can be activated uniformly in the form of crushed grains. Thus, M. Lopez et al. (Preparation of activated carbon from wood monolith, Carbon, 1996, 34/6, 825-827) describes a process which consists in uniformly impregnating a carbon precursor (wood, almond or coconut shell, and the like) by steeping in an aqueous solution at 80° C. comprising an activating agent (for example, H
3
PO
4
or ZnCl
2
) or directly in concentrated acid (80% H
3
PO
4
). The impregnated precursor is subsequently dried and (or directly) activated at a temperature of the order of 500° C. for approximately one hour. The residual activating agent is subsequently removed by rinsing with water. However, the time necessary for the impregnation phase is very long, which limits the industrial and economic advantage of the process. Furthermore, the thermal stage of the activation process induces kinetic gradients in the grains and consequently textural gradients in the adsorbent. The grains thus obtained therefore do not possess uniform properties. The scale of the problem increases with the size of the grains.
Various processes for the preparation of an activated carbon in the form of a macroscopic composite material have been studied. In some processes of the prior art, preexisting activated carbon is used as starting material. Mention may be made, for example, of the process disclosed by T. K. Bose et al. (U.S. Pat. No. 4,999,330, 1991). In this process, particles of superadsorbent activated carbon are coated with a binder by suspending in a solution of said binder in a solvent, said solvent subsequently being removed by evaporation. The remaining product is compressed and heated. A portion of the binder is decomposed and approximately 10% of the porosity of the activated carbon is inhibited. Such a process has several disadvantages. The stage of coating the activated carbon with the binder is lengthy and expensive in energy because of the stage of evaporation of the solvent. In addition, a portion of the porosity of the activated carbon is lost, the adsorbent capacity of the composite material thus being lower than that of an initial pulverulent activated carbon. Another process of this nature is described by S. Follin [“Approche de Dubinin pour la caractérisation de charbons actifs et évaluation des performances de cycles frigorifiques à adsorption. Application à la production de froid par le couple charbon actif—CO
2
” [Dubinin approach for the characterization of activated carbons and evaluation of the performance of adsorption refrigeration cycles. Application to the production of cold by the activated carbon—CO
2
couple], Thesis, IMP, University of Perpignan,1996]. This process is derived from the abovementioned Bose process, insofar as it consists in coating particles of activated carbon with a binder using a solution of the binder in a solvent and in then compressing after removal of the solvent. The improvement introduced lies in the addition of expanded natural graphite to the coated activated carbon after removal of the solvent and before compression. The presence of particles of expanded natural graphite substantially improves the thermal conductivity of the adsorbent composite material obtained. However, in this case too, a substantial loss of the adsorbent capacity, resulting mainly from the presence of the binder, is found. This loss can be of the order of 15 to 30%, according to the binder used.
In other processes, the activated carbon is prepared during the preparation of the composite material. For example, K. P. Gadkaree (Carbon honeycomb structures for adsorption applications, Carbon, 1998, 36/7-8, 981-989) describes a process which consists in preparing a support of honeycomb type made of extruded porous ceramic. This support is subsequently impregnated with a polymeric resin, for example a phenolic resin, of low viscosity. The assembly is subsequently dried, carbonized and activated, for example by carbonization under N
2
at 900° C. and then activation under CO
2
with burn offs of 25 to 30%. The composite material thus obtained forms a continuum and has very good mechanical strength. However, such a process can only be applied to polymeric precursors. In addition, the content of activated carbon and the thermal conductivity of the material obtained are low.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a macroscopic composite material, essentially comprising activated carbon and exhibiting good thermal conductivity, which can be easily shaped in order to be adjusted to various industrial uses. Another aim of the invention is a process for the preparation of such a material.
The composite material according to the invention comprises activated carbon and expanded graphite. It is characterized in that :
it constitutes a block within which the activated carbon is distributed homogeneously and uniformly in the form of microporous particles ;
the texture of the microporous particles is characterized by a micropore volume W
0
of between 0.1 cm
3
.g
−1
and 1.5 cm
3
.g
−1
and a mean size of the micropores L
0
of between 2 Å and 30 Å;
the particles of activated carbon have substantially the same texture, whatever their location in the composite material ;
the thermal conductivities of the material are between 1 and 100 W m
−1
K
−1
.
The composite material according to the invention can be provided in the form of a matrix of expanded graphite, within which the particles of activated carbon are uniformly distributed, said material being thermally anisotropic. It can also be provided in the form of a dispersion of graphite and of activated carbon within one another, said material then being thermally isotropic. In all cases, the macroporosity of the expanded graphite is between 10 &mgr;m and 800 &mgr;m. The activated carbon/expanded graphite ratio by weight in the material of the invention can vary within a very wide range. It is preferably between 50% and 99%.
The composite material according to the invention is obtained by a process which comprises a stage during which an activating agent and a mixture of expanded graphite and of an activated carbon precursor are subjected to a heat treatment at a temperature and for a time sufficient to produce a burn off of the activated carbon precursor of between 5 and 70% by mass. The term “burn off” is understood to mean a loss in mass of the activated carbon precursor during the heat treatment.


REFERENCES:
patent: 6045915 (2000-04-01), Bou et al.
patent: 0 837 116 (1998-04-01), None
Chemical Abstracts Service, Japanese Publication No. 60-068559, Apr. 19, 1985.
Chemical Abstracts Service, Japanese Publication No. 61-032358, Feb. 15, 1986.
Lopez, M., et al., “Preparation of activated carbon from wood monolith”, CARBON,

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