Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2001-09-05
2003-09-02
Smith, Duane (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S115000, C095S143000, C095S901000, C502S416000, C502S526000
Reexamination Certificate
active
06613126
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for storing natural gas and to an improved adsorbent for use in this method.
2. Description of Related Art
A method for storing natural gas which comprises filling a container with an adsorbent such as activated carbon, zeolite, or silica gel, and then adsorbing and storing a natural gas or the like in the container has been proposed in order to store a large amount of a fuel gas such as natural gas under a relatively low pressure.
For example, in Japanese Patent Application Laid-Open No. 258961/1986, there is disclosed the application of such a storage method for use in automobiles.
However, in this conventional method of storing natural gas by the use of the adsorbent, while a large amount of gas can be adsorbed when pure methane is stored, when a natural gas such as Japanese 13A town gas (the main component of which is methane and which also contains additional hydrocarbons such as ethane, propane, butane) is adsorbed and stored, storage density (V/V
0
) remarkably decreases. This phenomenon is believed to occur because higher carbon components such as propane and butane contained in the natural gas are liquefied in the pores of the adsorbent and clog these pores, thereby impeding the adsorption of methane.
In an example shown in
FIG. 15
components of a natural gas enter a pore
52
in an adsorbent
50
, such as activated carbon, and are adsorbed. It is intended that the diameter of the pore gradually decrease toward the inside, but, when large molecules
54
, being hydrocarbons with larger particle diameters such as propane, butane, and the like enter inside of small molecules
56
of methane or ethane, the large molecules
54
are caught midway in the pore
52
, where it is difficult to desorb these trapped large molecules
54
. Because the large molecules
54
of propane, butane, and the like have slower molecular velocities, and stronger affinity for the wall of the adsorbent
50
, the large molecules
54
are more difficult to desorb than the small molecules
56
of methane or ethane. Additionally, the pressure in the pore
52
is reduced before the adsorption of the natural gas, and, once the inside of the pore
52
is clogged with the large molecules
54
, the pressure difference between the inside and the outside of the pore
52
further impedes desorption of the large molecules
54
. In this manner, when the inside of the pore
52
is clogged with the large molecules
54
, a space is produced at the tip end of the pore
52
because the large molecules
54
cannot advance into the innermost part of the pore
52
. Because the component molecules of the natural gas are not adsorbed in this open space, the effective volume of the pore
52
is decreased, thereby decreasing the amount of gas adsorbable by the adsorbent
50
.
This decrease becomes especially remarkable as the adsorption/desorption of the natural gas is repeated because additional large molecules
54
clog the pores
52
each time the adsorption/desorption of the natural gas is repeated.
Therefore, the conventional adsorption storage method as described above has a significant problem making its practical use difficult.
Activated carbon is commonly used as an adsorbent for adsorbing and storing natural gas. An improved technique for adsorbing and storing natural gas in activated carbon is disclosed in Japanese Patent Application Laid-Open No. 55067/1994.
Generally, reduction of pore diameter is known to be effective for lowering the potential of natural gas adsorbed in the pores of an adsorbent such as activated carbon and for thereby stabilizing adsorption and storage. Therefore, activated carbon of the smallest available pore diameter is commonly used. In the above-mentioned art, activated carbon with a pore diameter on the order of 5 to 25 angstroms is disclosed, and it is further described elsewhere that the pore diameter about twice the diameter of a methane molecule, that is, of about 11.6 angstroms is preferable.
When the pore diameter is reduced as in the above-described conventional activated carbon, at a pressure as low as about several atmospheres a larger amount of natural gas can be stored than when the natural gas is simply compressed. However, when the pore diameter is small, there is a problem that, even when the storage pressure is raised to increase the storage amount, the adsorption amount does not greatly increase. This is because, when the pore diameter of activated carbon is set to an extremely small value of the order of 5 to 10 angstroms, the adsorption phenomenon becomes saturated at a relatively low pressure. This saturation pressure tends to lower as the pore diameter of the activated carbon decreases.
Moreover, when the activated carbon pore diameter is reduced, it becomes difficult to desorb the natural gas adsorbed in the pores of the activated carbon, so that a step of heating the activated carbon during the desorption or another method must be employed. Therefore, when activated carbon with a small pore diameter is used, there is also a problem that the adsorbed and stored natural gas cannot readily be used.
The present invention has been developed in consideration of the above-described problems, and an object thereof is to provide an adsorption storage method of a natural gas and an adsorbent for use in the method in which, even when a practical natural gas is used, a high storage density (V/V
0
) can be secured.
SUMMARY OF THE INVENTION
To attain the above-described object, according to the present invention, there is provided an adsorption storage method of a natural gas which comprises the steps of separating the natural gas into a low carbon component and a high carbon component, and independently adsorbing and storing in an adsorbent the low carbon component under a high pressure and the high carbon component under a low pressure. Moreover, in the adsorption storage method of the natural gas, there are provided a first adsorption tank containing the adsorbent to adsorb and store the low carbon component, and a second adsorption tank containing the adsorbent to adsorb and store the high carbon component, the pore diameter of the adsorbent contained in the second adsorption tank being smaller than that of the adsorbent contained in the first adsorption tank, wherein the natural gas is supplied to the first adsorption tank via the second adsorption tank.
Furthermore, in the adsorption storage method of the natural gas, the second adsorption tank may be provided with cooling means.
Additionally, in the adsorption storage method of the natural gas, after the natural gas is temporarily introduced into the second adsorption tank, the pressure may be once lowered before the natural gas is introduced again.
Moreover, in the adsorption storage method of the natural gas, it may be preferable that, when the stored natural gas is desorbed and used, the gas desorbed from the first adsorption tank be removed via the second adsorption tank.
In an additional aspect of the present invention, an adsorption storage method of a natural gas comprises the steps of adsorbing a gas having a smaller molecular size than propane in the adsorbent, and adsorbing the natural gas in the adsorbent.
Additionally, in the adsorption storage method of the natural gas, the adsorbent may be heated to 20° C. or more.
Moreover, in the adsorption storage method of the natural gas, the temperature of the adsorbent may be lowered as the natural gas is adsorbed.
An adsorption storage method of a natural gas according to a further aspect of the present invention is characterized in that, when the natural gas is adsorbed and stored in an adsorbent, the natural gas is adsorbed as it is caused to flow through a gap between the adsorbents.
Additionally, an adsorption storage method of a natural gas by adsorption to an adsorbent may comprise steps of first adsorbing a gas with a smaller molecular size than that of propane into the adsorbent; and subsequently adsorbing the natural gas to the adsorbent.
Moreover, in the ad
Hasegawa Hiroshi
Hibino Kouetsu
Shinozawa Tamio
Tange Kyoichi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lawrence Frank M.
Smith Duane
Toyota Jidosha & Kabushiki Kaisha
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