JP2005238160A - Gas adsorption method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a more efficient gas adsorption method employing a gas adsorption material. <P>SOLUTION: The gas adsorption method comprises the use of the gas adsorption material, where an absorption and desorption isotherm to at least one kind of gases shows a hysteresis loop, to allow a gas in which the absorption and desorption isotherm to the gas adsorption material shows the hysteresis loop to be absorbed by the gas adsorption material. When the gas is adsorbed by the gas adsorption material, the temperature of the gas adsorption material is controlled. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gas adsorption method using a gas adsorbent. More specifically, the present invention relates to an adsorbent in which an adsorption isotherm and a desorption isotherm relating to gas exhibit different behaviors. These gas adsorbents are suitably used for gas storage devices.

  The gas adsorbent has a characteristic of storing a large amount of gas at a low pressure as compared with pressurized storage and liquefied storage. For this reason, in recent years, development of gas storage devices using a gas adsorbent has been active.

As the gas adsorbent, activated carbon, metal complex, and the like are known. As a metal complex, for example, a metal complex having a composition of [Co (4,4′-bpy) _1.5 (NO ₃ ) ₂ ] _n has been proposed (see, for example, Patent Document 1). In recent years, it has been found that fine carbon such as carbon nanotubes has a hydrogen gas storage capacity, and its practical application has been studied.

However, conventionally proposed gas adsorbents have not been sufficiently satisfactory in terms of gas adsorption amount and workability.
Japanese Patent Laid-Open No. 9-227571

  Therefore, an object of the present invention is to provide a more efficient gas adsorption method.

  The inventors of the present invention have found that excellent gas adsorption can be realized by using, as a gas adsorbent, a compound whose adsorption / desorption isotherm relating to at least one gas exhibits a hysteresis loop. Moreover, it discovered that more efficient adsorption | suction was realizable by controlling the temperature at the time of adsorption | suction.

  The present invention has been completed based on these findings. Specifically, by using a gas adsorbent whose adsorption / desorption isotherm for at least one gas exhibits a hysteresis loop, the gas adsorbent adsorbs a gas whose adsorption / desorption isotherm for the gas adsorbent exhibits a hysteresis loop, A gas adsorption method, wherein the temperature of the gas adsorbent is controlled when the gas is adsorbed on the gas adsorbent.

  If a gas adsorbent having a specific gas adsorption characteristic is used, efficient gas adsorption is possible. In addition, the gas can be adsorbed more efficiently by controlling the temperature of the gas adsorbent.

  In the gas adsorbent used in the gas adsorption method of the present invention, the adsorption / desorption isotherm relating to at least one gas exhibits a hysteresis loop. That is, as shown in FIG. 1, the gas pressure-gas adsorption amount curve at the time of adsorption is different from the gas pressure-gas adsorption amount curve at the time of desorption.

  The peculiarity of the gas adsorbent in which the adsorption / desorption isotherm shows a hysteresis loop will be described with reference to the drawings. FIG. 1 is a graph showing the relationship between gas pressure and gas adsorption amount in a gas adsorbent showing a hysteresis loop. FIG. 2 is a graph showing the relationship between gas pressure and gas adsorption amount in a general gas adsorbent. In the figure, the horizontal axis indicates the gas pressure, and the vertical axis indicates the gas adsorption amount per unit mass of the adsorbent. The gas pressure-gas adsorption amount curve is also referred to as an adsorption / desorption isotherm in the present application.

In the conventional gas adsorbent, the gas adsorption amount increases as the gas pressure increases, and the pressure-adsorption amount curve during adsorption coincides with the pressure-adsorption amount curve during desorption (FIG. 2). For example, the amount of adsorption adsorbed to conventional gas adsorbent when in G _1, it is necessary to pressurize the gas pressure to the P _1, the pressure to hold the adsorption amount to the G ₁ to P ₁ Need to hold. To hold the adsorption amount G ₂ or more is 95% of the adsorption amount G ₁ is must be kept pressure on P ₅ or more.

On the other hand, in the gas adsorbent used in the present invention, the adsorption / desorption isotherm shows a hysteresis loop (FIG. 1). Description will be made assuming a compound having a gas adsorption amount G ₁ at the pressure P ₁ . To the gas adsorption amount of G _1, it is necessary to increase the pressure applied to the gas adsorbent to a P _1. However, the pressure - adsorption amount curve is to draw a hysteresis loop, rapidly gas adsorption even when reduced pressure from P ₁ is not reduced. For example, the may be maintained 95% of the gas adsorption amount G _1, it is possible to greatly reduce the gas pressure from P ₁ to P _3.

  Such a gas adsorbent whose adsorption / desorption isotherm exhibits a hysteresis curve can be applied to various uses by utilizing special properties.

  One application is a gas storage device. A gas adsorbent whose adsorption / desorption isotherm exhibits hysteresis is less likely to reduce the amount of gas adsorbed even when the gas pressure decreases. For this reason, the pressure during conveyance and preservation | save of gas storage apparatuses, such as a business gas tank, a consumer gas tank, and a vehicle fuel tank, can be reduced dramatically. As an effect brought about by reducing the gas pressure during transportation or storage, an improvement in the degree of freedom in shape is first mentioned. In the conventional gas storage device, the gas adsorption amount cannot be maintained high unless the pressure during storage is maintained. However, in the gas storage device of the present invention, a sufficient gas adsorption amount can be maintained even if the pressure is lowered. For this reason, the pressure | voltage resistance of a gas storage apparatus can be made low and the shape of a gas storage apparatus can be designed to some extent freely. This effect is great when the gas storage device of the present invention is used as a fuel gas tank for vehicles such as automobiles. When the gas storage device of the present invention is used as a fuel tank, restrictions on pressure resistance are relaxed as described above, and the shape can be designed freely to some extent. Specifically, the shape of the gas storage device can be adjusted so as to fit the shape of wheels, seats, and the like in the vehicle. As a result, it is possible to obtain various benefits such as miniaturization of the vehicle, securing of luggage space, and improvement of fuel consumption by reducing the weight of the vehicle.

  In the case of application to a gas storage device, the shape, material, type of gas valve, and the like do not have to be particularly used, and those used in the gas storage device can be used. However, improvements of various devices are not excluded. The application to which the gas adsorption method of the present invention is applied is not limited to the gas storage device, and may be applied to an application using other gas adsorption.

  In the gas adsorption method of the present invention, by controlling the temperature of the gas adsorbent when adsorbing the gas to the gas adsorbent, the relationship between the gas pressure and the gas adsorption amount of the gas adsorbent is changed, and more Enables efficient gas adsorption. Generally, when the temperature of the gas adsorbent is lowered, as shown in FIG. 3, the hysteresis curve shifts to the low gas pressure / multiple gas adsorption amount side (Hys1 → Hys2). As a result of the shift of the hysteresis curve, gas can be adsorbed at a low pressure. In general, when handling high-pressure gas, it is necessary to pay attention to the aspects of equipment and regulations, which increases costs. If the method of the present invention is used, the gas can be adsorbed at a low pressure, thereby reducing the cost.

  Since the hysteresis loop shown in FIG. 3 differs depending on the type of gas adsorbent and the type of gas, it is reasonable to determine the optimum temperature by investigating the hysteresis loop of the gas used in advance. It is. Specifically, when the temperature of the gas adsorbent is lowered to preferably 15 ° C. or less, more preferably 5 ° C. or less, a suitable hysteresis curve can be easily obtained for gas adsorption. The lower limit of the gas temperature is not particularly limited, but the temperature of the gas adsorbent is preferably −70 ° C. or higher in consideration of equipment and energy necessary for greatly reducing the temperature. The temperature of the gas adsorbent is measured using a general temperature measuring means such as a thermocouple thermometer.

  The means for controlling the temperature of the gas adsorbent is not particularly limited. In order to cool the temperature of the gas adsorbent, a refrigerant may be passed around the gas adsorbent to cool the gas adsorbent, or a low-temperature gas is supplied to the gas adsorbent and adsorbed to the gas adsorbent. The gas adsorbent may be cooled by the gas.

  When the gas adsorbent is directly cooled by the refrigerant, a pipe for allowing the refrigerant to pass therethrough is disposed inside the adsorption tank or the like where the gas adsorbent is arranged, and the refrigerant supplied from the refrigerator or the like is disposed. It is good to let it pass. However, the technical scope of the present invention is not limited to such an embodiment.

  When supplying a low-temperature gas to the gas adsorbent, when using a material having a very low boiling point such as LNG (−162 ° C.) as a gas source, the cold heat of the gas itself may be used. When cold heat such as LNG cannot be used, the periphery of the gas adsorbent may be cooled by cooling and introducing the gas to be adsorbed. In the case of cooling, it is economical to set a temperature region that can be efficiently cooled by a heat pump using an external atmosphere.

A specific gas adsorption method at a low temperature will be described with reference to FIG. Controlling the gas adsorbent low T _2, a relatively low pressure of the gas pressure P _6, a large amount of gas G ₄ is adsorbed. In contrast to the high temperature T ₁ , the gas adsorption amount remains at G ₃ even if the pressure is increased to P ₈ , which is a high pressure.

If it is difficult to hold the adsorbent at a low temperature T _2, or if it is not economical, after adsorbing the gas in the gas adsorbent, a part of the adsorbed gas by increasing the temperature of the gas adsorbent The atmospheric pressure around the gas adsorbent may be increased by desorption. There is no particular limitation on the temperature to be raised, but it is preferable to raise the temperature to the ambient temperature around the gas storage device in which the gas adsorbent is disposed because the cost for temperature control can be reduced. For example, while controlling the pressure of the gas storage device adsorbent is disposed, is heated to T ₁ is a temperature around the gas storage apparatus. At this time, gas adsorbed at low temperature T ₂ are released to a high temperature, the pressure in the gas storage device is increased. It has been experimentally confirmed that if the pressure in the gas storage device is controlled to be P ₇ or more, the gas adsorption amount G _{3 at} the high temperature T ₁ is maintained at a pressure of P ₇ or more.

This utilizes the property that more gas is adsorbed at a low temperature, and if the temperature rises, the gas that could not be adsorbed is released from the adsorbent. At this time, if the pressure inside the tank is controlled by a release valve or the like, the pressure inside the tank rises to a set pressure by the released gas. In other words, the energy in the state of low temperature T ₂ is recovered by the pressure energy as a self pressure. This is one of the points of this embodiment.

In the control of FIG. 3, the state changes from point A to point B. At point B, cooling energy is not required as compared to point A, and energy for cooling can be omitted, so the gas storage device is used efficiently. When the gas is adsorbed, it is also advantageous that the compressor for increasing the gas pressure can be omitted by utilizing the decrease in the gas adsorption pressure due to the low temperature (P ₈ → P ₆ ). For example, assume that an adsorbent that needs to be increased to a pressure of 55 atm to adsorb a predetermined amount of methane at 20 ° C. is assumed. Even if the same adsorbent is used, if it is adsorbed at 0 ° C. and then the gas storage device is heated to release methane from the adsorbent, before being supplied to the gas storage device, There is no need to increase the pressure of methane up to 55 atm in advance. In particular, city gas gasified from LNG (−162 ° C.) can be easily controlled at −30 to 0 ° C., and can be easily adsorbed and stored in a gas tank without using a large booster. It is.

  Next, the adsorption characteristics, composition, and structure of the gas adsorbent used in the present invention will be described.

  The mechanism by which the pressure-adsorption force curve during gas adsorption exhibits a hysteresis loop has not yet been fully understood. As a possible mechanism, the ligand binding at the molecular level has a structure that sticks and leaves like a joint valve of a magnet, and only a molecule having a predetermined kinetic energy can pass through. Since the valve structure exists, the adsorption amount suddenly increases when a certain pressure is reached. Examples of the valve structure at the molecular level include a hydrogen bond with a loose bond force or a valve having a bond energy similar to that of a hydrogen bond, which can be attached and separated like a small magnet, and therefore has a predetermined kinetic energy. Only gas molecules with a pass through the valve. A hydrogen bond is a bond that is an order of magnitude weaker than a coordination bond, a covalent bond, or an ionic bond, and thus can function as a valve structure at a molecular level. The molecular level valve structure is not limited to hydrogen bonds alone, but may be other weak bonds that fall within a range of ± 30% in hydrogen bonds and bond energy. Even such a bond is thought to be able to act as a “valve”, similar to a hydrogen bond.

  Another mechanism is considered to be a shift of the complex layer. As a complex, a complex having a so-called 2D square grid layer constituted by coordination of a ligand with a metal ion as an intersection is known (Zawarotoko, MJ Crystal Engineering 2 (1999), 37). . In such a complex, the relative position of each layer overlaps to increase or decrease the porosity of the lattice, increasing or decreasing the amount of gas adsorption, and the hysteresis phenomenon with sudden gas absorption and release. Is considered to occur.

  However, these are merely mechanism estimates. That is, even if the mechanism is not followed, it can be included in the technical scope of the present invention as long as a hysteresis loop is exhibited with respect to a predetermined gas.

  The gas adsorbent used in the present invention is a material in which the adsorption / desorption isotherm relating to the gas exhibits a hysteresis loop, but it is sufficient that a hysteresis loop is exhibited for at least one kind of gas, and the hysteresis loop is exhibited for all gases. It is not necessary.

  The gas that the gas adsorbent needs to exhibit a hysteresis loop varies depending on the application of the gas adsorbent of the present invention. For example, when a gas adsorbent is used in a methane gas storage device, it is necessary to show a hysteresis loop for methane gas. Generally speaking, if the gas adsorbent of the present invention is used for storing gas used in various applications, the gas in which the gas adsorbent of the present invention exhibits a hysteresis loop is hydrogen, hydrocarbon (methane, ethane). , Propane, butane, isobutane, etc.), carbon monoxide, carbon dioxide, oxygen, nitrogen and the like. Of course, a hysteresis loop may be shown for these two or more gases so that a mixture of a plurality of hydrocarbon gases such as LNG can be stored.

  What is necessary is just to select the specific material used as a gas adsorbent according to the pressure to be stored and the gas to be stored. Although it does not restrict | limit in particular, The metal complex which has a transition metal is mentioned.

  The metal complex is preferably a metal complex having a periodic crystal structure in consideration of ease of control of gas adsorption amount and molecular structure. As an aspect of the periodic crystal structure, a structure in which a two-dimensional sheet is laminated through a ligand is considered preferable in consideration of gas adsorption characteristics. That is, a two-dimensional sheet formed by a bond mediated by a predetermined ligand has a ligand such as pyrazine, 4,4′-bipyridine, trans-1,2-bis (4-pyridyl) ethylene. A stacked structure as a pillar ligand (Piller Ligand) is preferable. When the metal complex constituting the gas adsorbent has a periodic crystal structure in which the two-dimensional sheet is laminated via a ligand, the molecular level valve structure is laminated not only within the lattice-like two-dimensional sheet. It may be present in the ligand.

As a specific aspect of the laminated structure via the pillar ligand, the two-dimensional sheet is laminated in the order of 2D ₁ , 2D ₂ , 2D ₃ , 2D ₄ , and the adjacent two-dimensional sheet is a pillar ligand ( (PL) and the aspect joined. That is, the junction sequence _{is 2D 1 -PL-2D 2 -PL-} 2D 3 -PL-2D 4.

  When the two-dimensional sheet has a structure laminated via a ligand, at least a part of the ligand is not between adjacent two-dimensional sheets but between two-dimensional sheets separated by one or more layers, Two two-dimensional sheets may be combined. When the two-dimensional sheet separated by at least one layer has a structure bonded by a ligand, the distance between the two-dimensional sheets becomes small. For this reason, it has been found by synthetic experiments of various metal complexes that a valve structure at a molecular level is densely present and a hysteresis loop is more easily developed. In the metal complex having a three-dimensional structure, it is considered that the larger the total number of valve structures at the molecular level, the easier it is to develop a hysteresis loop in gas adsorption characteristics.

For example, in the case where two-dimensional sheets are laminated in the order of 2D ₁ , 2D ₂ , 2D ₃ , 2D ₄ , if every two-dimensional sheets are joined by pillar ligands, the joining order is: “2D ₁ -PL-2D ₃ ” and “2D ₂ -PL-2D ₄ ”. When every other two-dimensional sheet is joined by a pillar ligand, the PL joining 2D ₁ and 2D ₃ passes through the inside of the 2D ₂ molecule. Further, as another aspect of the laminated structure, the 2D square grid layer structure described above can be given (Zawarotoko, MJ Crystal Engineering 2 (1999), 37). In the 2D square grid layer structure, gas can be occluded in the lattice and between layers, and it is assumed that a hysteresis loop appears due to the displacement between layers. However, it is not limited to these structures. As long as the adsorption / desorption isotherm relating to a predetermined gas, which is a characteristic of the gas adsorbent of the present invention, exhibits a hysteresis loop, it is included in the technical scope of the present invention. Although the mechanism is not clear, the gas storage capacity of the gas adsorbent of the present invention when such two-dimensional sheets are joined by pillar ligands every other or when it has a 2D square grid layer structure. Increases significantly.

[Manufacture of gas adsorbent]
The method for preparing the gas adsorbent differs depending on the type of the gas adsorbent and cannot be uniquely determined. Here, however, [Cu (bpy) (BF ₄ ) ₂ (H ₂ O) _2. (Bpy) )] A case where _n is synthesized will be described as an example.

  First, an aqueous solution containing copper (II) tetrafluoroborate hydrate is added while refluxing an acetonitrile solution of 4,4'-bipyridine (hereinafter abbreviated as bpy). The resulting precipitate is filtered, the vapor of diethyl ether is diffused into the mother liquor using a nitrogen stream, and the mother liquor is stored at room temperature for several days. Subsequently, the precipitate is obtained by filtration and dried under reduced pressure for several hours. The object can be obtained by such work. However, it is needless to say that the method is not limited to the above method, and it may be synthesized using various methods.

<Synthesis Example 1>
Copper (II) tetrafluoroborate hexahydrate (Aldrich; 0.674 g) was dissolved in pure water (50 ml), and a 0.04 M solution of copper (II) tetrafluoroborate (A ₃ solution) was added. Got ready. Separately, 4,4′-bipyridine (special grade manufactured by Wako Pure Chemical Industries, Ltd .; 0.625 g) was dissolved in acetonitrile (50 ml) to prepare a 0.08M solution (B ₃ solution) of 4,4′-bipyridine. did. Under reflux using a water bath of 357K, it was added dropwise over 1 hour _{A 3} solution _{B 3} solution. This gave a solution containing a slightly grayish precipitate. After completion of the dropwise addition, the precipitate as a side reaction product was removed by filtration.

  Next, while stirring, the vapor of diethyl ether was diffused into the mother liquor using a nitrogen stream until the volume reached 150 ml. Subsequently, the mother liquor was stored at room temperature for 2 days and the solvent (acetonitrile, diethyl ether) was evaporated by heating to 353K.

The residue is filtered, and water is removed by drying under reduced pressure of 10 ⁻³ Pa or less for 4 hours, and [Cu (bpy) (BF ₄ ) ₂ (H ₂ O) ₂ that is the gas adsorbent of the present invention. -(Bpy)] _n (0.3g) was obtained.

<Example 1>
Using the system in FIG. 4A, an attempt was made to adsorb natural gas as an adsorbed gas. Gas adsorbent [Cu (bpy) (BF ₄ ) ₂ (H ₂ O) ₂ (bpy)] _n 130 kg previously subjected to a vacuum drying treatment at 110 ° C. in an adsorption tank having a capacity of 100 liters as a gas storage device Particles (1 to 2 mm in diameter) were placed, and a heat exchange Cu pipe with a diameter of 3 mm was arranged in a spiral shape between the gas adsorbents. The periphery of the gas adsorbent was cooled to about 0 ° C. with a −5 ° C. refrigerant (R-500) delivered from the freezer, and natural gas was boosted to 8 atm and introduced therein. 19 m ^{3 of} natural gas mainly composed of methane gas was adsorbed in an adsorption tank having an internal capacity of 100 liters in about 5 minutes. Next, when the operation of the refrigerator is stopped by controlling the pressure valve to 30 atm, the atmospheric temperature in the tank rises to 20 ° C., which is the temperature of the outside air, and the tank pressure is 30 atm and the tank internal volume is about 100 liters. The tank was filled with 18 m ³ methane gas which is 180 times larger.

<Example 2>
Using the system of FIG. 4B, an attempt was made to adsorb natural gas as an adsorbed gas. Gas adsorbent [Cu (bpy) (BF ₄ ) ₂ (H ₂ O) ₂ (bpy)] _n 130 kg previously subjected to a vacuum drying treatment at 110 ° C. in an adsorption tank having a capacity of 100 liters as a gas storage device Particles (1 to 2 mm diameter). A cooling heat exchanger was installed in front of the adsorption tank, and a refrigerant (R-500) at −5 ° C. was circulated from the freezer to cool the natural gas supplied to the adsorption tank. The natural gas was pressurized to 8 atm, cooled to about 0 ° C. while passing through the cooler, and introduced into the adsorption tank.

In about 5 minutes, 19 m ³ of natural gas was adsorbed. Next, the operation of the refrigerator was stopped, the pressure control valve was set to 30 atm, and the temperature of the adsorption tank was waited to rise to 20 ° C., which is the temperature of the outside air. In the adsorption tank, 18 m ³ of natural gas remained and was adsorbed.

<Example 3>
Using the system of FIG. 4C, an attempt was made to adsorb natural gas as an adsorbed gas. LNG was introduced from a liquefied natural gas (LNG) tank into a vaporizer, and was vaporized into natural gas at about -70 ° C by a heat medium. Hot water was used as a heating medium, and an electric heater was used as a temperature regulator. When natural gas at −70 ° C. was introduced into the same adsorption tank as in Example 2 at 5 atm, approximately 19 m ³ of natural gas was filled in the adsorption tank. Next, the pressure control valve was set to 30 atm and waited for the temperature of the adsorption tank to rise to 20 ° C., which is the temperature of the outside air. In the adsorption tank, 18 m ³ of natural gas remained and was adsorbed.

  As shown in Examples 1 to 3, by cooling the gas adsorbent, a large amount of gas can be adsorbed even at a low pressure. As shown in Example 1, the gas adsorbent may be directly cooled by a refrigerant, or indirectly through an adsorbed gas supplied to the gas adsorbent as shown in Example 2. It may be cooled to LNG, or the cold energy of LNG may be used as shown in the third embodiment. After the gas is adsorbed at a low temperature, if the temperature is raised to the same atmospheric temperature as the outside air while controlling the pressure, the pressure in the tank rises to the predetermined pressure by the desorbed gas, and the gas adsorption is maintained by that pressure. Is done. If such a mechanism is used, a large amount of gas can be stored by the self-pressure rising due to desorption from the gas adsorbent, without requiring a high-pressure compressor as in the prior art. It can also be used at the same temperature as the outside air.

It is a graph which shows the relationship between the gas pressure and gas adsorption amount in the gas adsorbent which shows a hysteresis loop. It is a graph which shows the relationship between the gas pressure and gas adsorption amount in a common gas adsorbent. It is a graph which shows the relationship between the gas pressure and gas adsorption amount in a low temperature atmosphere, and the relationship between the gas pressure and gas adsorption amount in a high temperature atmosphere. 1 is a schematic diagram of a system used in an embodiment of the present invention. 4A is a schematic diagram of the system used in the first embodiment, FIG. 4B is a schematic diagram of the system used in the second embodiment, and FIG. 4C is a schematic diagram of the system used in the third embodiment. FIG.

Claims (5)

The gas adsorption method uses a gas adsorbent in which an adsorption / desorption isotherm for at least one gas exhibits a hysteresis loop, and adsorbs a gas in which the adsorption / desorption isotherm for the gas adsorbent exhibits a hysteresis loop on the gas adsorbent. And
A gas adsorption method comprising controlling the temperature of the gas adsorbent when adsorbing the gas to the gas adsorbent.   The gas adsorption method according to claim 1, wherein the gas adsorbent is cooled to 15 ° C. or lower when the gas is adsorbed.   The gas adsorption method according to claim 2, wherein the gas adsorbent is cooled by passing a refrigerant around the gas adsorbent.   The gas adsorption method according to claim 2, wherein the gas adsorbent is cooled by a gas adsorbed on the gas adsorbent.   The gas adsorbent is made to adsorb the gas, and then the temperature of the gas adsorbent is increased to desorb a part of the gas to increase the atmospheric pressure around the gas adsorbent. The gas adsorption method according to any one of the above.

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