Gas concentration method

Abstract

Provided herein is a method for improving a gas recovery rate during generation of a high-purity gas. The method includes providing three or more adsorption towers filled with an adsorbent that adsorbs an adsorption target gas. Performing a pressure lowering equalization process in a first adsorption tower in which an adsorption process has been finished, and in a source gas supply state in which a source gas is supplied to at least a second adsorption tower in which a pressure increasing equalization process has been finished and the adsorption process is to be subsequently performed; and transferring a non-adsorbed gas from an upper portion of the first adsorption tower to the upper portion of the second adsorption tower, thereby performing an adsorption and pressure lowering equalization process in the first adsorption tower and an adsorption and pressure increasing equalization process in the second adsorption tower.

Claims

1. A gas concentration method comprising: providing three or more adsorption towers each filled with an adsorbent that adsorbs an adsorption target gas; and performing, for each of the adsorption towers, a PSA cycle of repeating in order: an adsorption process of receiving supply of a source gas containing the adsorption target gas from a lower portion of the adsorption tower, adsorbing the adsorption target gas on the adsorbent, and releasing a non-adsorbed gas composed mainly of a non-adsorbed component from an upper portion of the adsorption tower; a pressure lowering equalization process of transferring the gas in the adsorption tower in which the adsorption process has been finished and which is in a high pressure state to another adsorption tower that is in a lower pressure state so as to bring an inside of the adsorption tower into an intermediate pressure state; a decompression process of, after the pressure in the adsorption tower has been lowered by the pressure lowering equalization process, further decompressing the adsorbent into a low pressure state so as to desorb the adsorption target gas adsorbed on the adsorbent, and recovering said adsorption target gas from the lower portion of the adsorption tower; and a pressure increasing equalization process of, after finishing the decompression process, receiving the gas from an inside of another adsorption tower that is in a higher pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, the PSA cycle being successively performed for the three or more adsorption towers with shifted timing, wherein the method comprises a step of, prior to performing the pressure lowering equalization process in a first adsorption tower in which the adsorption process has been finished, and in a source gas supply state in which the source gas is supplied to at least a second adsorption tower in which the pressure increasing equalization process has been finished and the adsorption process is to be subsequently performed, transferring the non-adsorbed gas from the upper portion of the first adsorption tower to the upper portion of the second adsorption tower, thereby performing an adsorption and pressure lowering equalization process in the first adsorption tower and an adsorption and pressure increasing equalization process in the second adsorption tower.

2. The gas concentration method according to claim 1, wherein the source gas supply state is a state in which the source gas is simultaneously supplied to both the first adsorption tower in which the adsorption process has been finished and the pressure lowering equalization process is to be subsequently performed and the second adsorption tower in which the pressure increasing equalization process has been finished and the adsorption process is to be subsequently performed.

3. The gas concentration method according to claim 1, wherein the source gas supply state is a state in which while the source gas is not supplied to the first adsorption tower in which the adsorption process has been finished and the pressure lowering equalization process is to be subsequently performed, the source gas is supplied to the second adsorption tower in which the pressure increasing equalization process has been finished and the adsorption process is to be subsequently performed.

4. The gas concentration method according to claim 1, wherein four or more of said adsorption towers are provided, a plurality of different pressure states of the pressure in the adsorption towers are set as the intermediate pressure state, the pressure lowering equalization process includes: a high pressure-side pressure lowering equalization process of transferring the gas in one of the adsorption towers that is in the high pressure state to another one of the adsorption towers that is in the intermediate pressure state at a pressure lower than the pressure of the one of the adsorption towers so as to bring the pressure in the one of the adsorption towers into a high pressure-side intermediate pressure state; and a low pressure-side pressure lowering equalization process of transferring the gas in one of the adsorption towers that is in the intermediate pressure state at a pressure higher than the low pressure state to another one of the adsorption towers that is in the low pressure state so as to bring the pressure in the one of the adsorption towers into a low pressure-side intermediate pressure state, the pressure increasing equalization process includes: a low pressure-side pressure increasing equalization process of receiving, into one of the adsorption towers that is in the low pressure state, the gas in another one of the adsorption towers that is in the high pressure-side intermediate pressure state so as to bring the pressure in the one of the adsorption towers into the low pressure-side intermediate pressure state; and a high pressure-side pressure increasing equalization process of receiving, into one of the adsorption towers that is in the low pressure-side intermediate pressure state, the gas in another one of the adsorption towers that is in the high pressure state so as to bring the pressure in the one of the adsorption towers into the high pressure-side intermediate pressure state, and the gas is transferred from one of the adsorption towers in which the pressure lowering equalization process is performed to another one of the adsorption towers in which the pressure increasing equalization process is performed, the gas being transferred from the upper portion of the one of the adsorption towers to the upper portion of the other one of the adsorption tower.

5. The gas concentration method according to claim 1, wherein the source gas comprises at least one gas selected from coal mine gas, biogas, reformed gas, and natural gas, and a gas to be purified comprising methane.

6. The gas concentration method according to claim 1, wherein the adsorbent comprises at least one of activated carbons, molecular sieve carbons, zeolites, and porous metal complexes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a gas concentration apparatus (four towers) for performing a gas concentration method.

(2) FIG. 2 is a diagram showing changes in internal pressure of the adsorption towers (four towers) due to the gas concentration method.

(3) FIG. 3 is a schematic diagram of a gas concentration apparatus (three towers) for performing the gas concentration method.

(4) FIG. 4 is a diagram showing changes in internal pressure of the adsorption towers (three towers) due to the gas concentration method.

EMBODIMENTS OF THE INVENTION

(5) Hereinafter, a gas concentration method according to embodiments of the present invention will be described. Although preferred examples will be described below, the examples are described for more specifically illustrating the present invention. Various modifications may be made without departing from the gist of the invention, and the present invention is not limited to the following description.

(6) (Gas Concentration Apparatus)

(7) As shown in FIG. 1, a gas concentration apparatus used for a gas concentration method of the present invention is provided with adsorption towers A1 to A4 filled with adsorbents A11 to A41, a supply unit L1 and a recovery unit L2 for supplying biogas (an example of a source gas) from a source gas tank T1 and recovering methane that has not been adsorbed on the adsorbents A11 to A41, a decompression unit L3 for decompressing and recovering miscellaneous gases other than methane that have been adsorbed on the adsorbents A11 to A41, and a controller C that controls the operations of the supply unit L1, the recovery unit L2, and the decompression unit L3.

(8) Note that the adsorbents A11 to A41 are not limited as long as they can selectively (preferentially) adsorb miscellaneous gases, such as carbon dioxide and nitrogen, other than a flammable gas such as methane gas, but the use of adsorbents A11 to A41 capable of selectively adsorbing miscellaneous gases under atmospheric pressure and at 298 K allows miscellaneous gases to be sufficiently adsorbed on the adsorbents A11 to A41 even under atmospheric pressure and at 298 K.

(9) It is preferable to use adsorbents A11 to A41 that are composed of at least one selected from the group consisting of molecular sieve carbons, activated carbons, zeolites, silica gels, and organometallic complexes that have, at a pore size of 0.38 nm or more, the pore size being determined by an MP method, a pore volume (V.sub.0.38) of not more than 0.01 cm.sup.3/g at that pore size and have a pore volume (V.sub.0.34) of equal to or more than 0.20 cm.sup.3/g at a pore size of 0.34 nm.

(10) (Adsorption Towers)

(11) The adsorption towers A1 to A4 are filled with the adsorbents A11 to A41, respectively. Also, gas lines L11 to L41 for supplying biogas as a source gas from the source gas tank T1 by a supply pump P1 are provided below the adsorption towers A1 to A4 to form the supply unit L1. Gas lines L12 to L42 for releasing methane that has been concentrated as a non-adsorbed gas as a result of mainly nitrogen and carbon dioxide contained in the biogas supplied to the adsorption towers A1 to A4 being adsorbed as adsorption target gases are provided above the adsorption towers A1 to A4 to form the recovery unit L2. With this configuration, the biogas is supplied from the supply unit L1 to the adsorption towers A1 to A4, and the non-adsorbed gas that has not been adsorbed on the adsorbents A11 to A41 is discharged to the recovery unit L2. Thus, the adsorption target gases can be adsorbed on the adsorbents A11 to A41 and separated from the non-adsorbed gas. Further, for the adsorption towers A1 to A4, gas lines L13 to L43 for discharging the adsorption target gases adsorbed on the adsorbents A11 to A41 are provided below the adsorption towers A1 to A4 to form the decompression unit L3. With this decompression unit L3, high-concentration adsorption target gases that have been adsorbed on the adsorbents A11 to A41 and concentrated can be collected from the biogas supplied from the supply unit L1. The decompression unit L3 is configured to collect the adsorption target gases from the adsorption towers A1 to A4 via the gas lines L13 to L43.

(12) Furthermore, gas lines L14 to L44 for providing connection between the adsorption towers A1 to A4 are connected to the upper portions of the adsorption towers A1 to A4 to form a pressure equalization unit L4 for transferring the gas inside each of the adsorption towers A1 to A4 to another one of the adsorption towers A1 to A4, from the upper portion of each of the adsorption towers A1 to A4 to the upper portion of the other one of the adsorption towers A1 to A4.

(13) Note that switching valves V11 to V44 are provided in the gas lines L11 to L44, thus providing a configuration that enables the controller C to perform overall control of the switching between supply, discharge, and suspension of the gas to the adsorption towers A1 to A4 by the operation of the supply pump P1.

(14) (Methane Concentration Method)

(15) As shown in Table 1, the controller C controls the switching valves V11 to V44 and the supply pump P1 to control the operation of the adsorption towers A1 to A4 so as to perform, with respect to the adsorption tower A1, the following processes in order:

(16) an adsorption process of receiving supply of biogas from a lower portion of the adsorption tower A1, adsorbing gases other than methane on the adsorbent A11, and releasing a non-adsorbed gas composed mainly of methane from an upper portion of the adsorption tower A1;

(17) an adsorption and pressure equalization (pressure lowering) process of, in a source gas supply state in which the source gas is simultaneously supplied to both the adsorption tower A1 in which the adsorption process has been finished and a pressure equalization (pressure lowering) process is to be subsequently performed and the adsorption tower A2 in which a pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, transferring the non-adsorbed gas from the upper portion of the adsorption tower A1 to the upper portion of the adsorption tower A2 so as to bring the adsorption tower A1 into a slightly high pressure state;

(18) a standby process;

(19) a high pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the slightly high pressure state into the other adsorption tower A3 that is in a low pressure-side intermediate pressure state at a pressure lower than that of the adsorption tower A1 so as to bring the pressure in the adsorption tower A1 into a high pressure-side intermediate pressure state;

(20) a low pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the high pressure-side intermediate pressure state, the gas having an adsorbed gas concentration that has been slightly increased with respect to that in the high pressure-side pressure equalization (pressure lowering) process, to the other adsorption tower A4 that is in a low pressure state so as to bring the pressure in the adsorption tower A1 into a low pressure-side intermediate pressure state;

(21) a decompression process of, after the pressure in the adsorption tower has been lowered by the low pressure-side pressure equalization (pressure lowering) process, further decompressing the adsorbent A11 into a low pressure state so as to desorb the miscellaneous gases adsorbed on the adsorbent A11 and recovering the miscellaneous gases from the lower portion of the adsorption tower A1;

(22) a low pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure state, the gas in the adsorption tower A2 that is in the high pressure-side intermediate pressure state so as to bring the pressure in the adsorption tower A1 into the low pressure-side intermediate pressure state;

(23) the standby process;

(24) a high pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure-side intermediate pressure state, the gas in the other adsorption tower A3 that is in the slightly high pressure state so as to bring the pressure in the adsorption tower A1 into the high pressure-side intermediate pressure state;

(25) the standby process; and

(26) an adsorption and pressure equalization (pressure increasing) process, which corresponds to the adsorption and pressure equalization (pressure lowering) process, of receiving supply of the source gas and the non-adsorbed gas from the adsorption tower A4 that is in a high pressure state so as to bring the adsorption tower A1 into the slightly high pressure state. Although the same operation will also be performed for the other adsorption towers A2 to A4 in a phase (timing) shifted manner, the detailed description thereof is omitted and replaced by the description with reference to Table 1 to avoid redundancy.

(27) In Table 1, hollow circles indicate that the corresponding switching valves V11 to V44 are open or the corresponding pump P1 is in operation.

(28) TABLE-US-00001 TABLE 1 Adsorption Steps towers 1 2 3 4 5 6 7 8 A1 Adsorption Adsorption Standby High Low Decompression and pressure pressure-side pressure-side equalization pressure pressure (pressure equalization equalization lowering) (pressure (pressure lowering) lowering) A2 Standby High Standby Adsorption Adsorption Adsorption pressure-side and pressure and pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) lowering) increasing) A3 Decompression Low Standby High Standby Adsorption pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A4 Standby High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1 Adsorption Steps towers 9 10 11 12 13 14 15 16 A1 Decompression Low Standby High Standby Adsorption pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A2 Standby High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) A3 Adsorption Adsorption Standby High Low Decompression and pressure pressure-side pressure-side equalization pressure pressure (pressure equalization equalization lowering) (pressure (pressure lowering) lowering) A4 Standby High Standby Adsorption Adsorption Adsorption pressure-side and pressure and pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1

(29) By such a control, the changes in the internal pressures of the adsorption towers A1 to A4 transition as shown in FIG. 2.

(30) This will be described more specifically using the adsorption tower A1 as an example: the operation is controlled in the following manner. Note that in the following description, an X-th step is abbreviated as <X>.

(31) <1 to 3> Adsorption Process

(32) Biogas is introduced into the first adsorption tower A1 as the source gas from the source gas tank T1. At this time, while the pressure in the first adsorption tower A1 is increased from the slightly high pressure state to the high pressure state and maintained in the high pressure state as shown in FIG. 2, gases other than methane contained in the biogas supplied from the source gas tank T1 via the switching valve V11 of the gas line L11 of the supply unit L1 are adsorbed on the adsorbent A11 of the first adsorption tower A1, and methane is discharged via the switching valve V12 of the gas line L12 of the recovery unit L2.

(33) Note that in the present embodiment, as shown in FIG. 2, the pressure in the first adsorption tower A1 is supposed to change between the high pressure state, the slightly high pressure state, the high pressure-side intermediate pressure state, the low pressure-side intermediate pressure state, and the low pressure state in descending order of pressure.

(34) Note that at this time, as shown in Table 1, <1, 3> the standby process and <2> the high pressure-side pressure equalization (pressure increasing) process are performed in the second adsorption tower A2.

(35) Also, in the third adsorption tower A3, <1, 2> the decompression process and then <3> the low pressure-side pressure equalization (pressure increasing) process are performed.

(36) Furthermore, in the fourth adsorption tower A4, <1> the standby process, and then <2> the high pressure-side pressure equalization (pressure lowering) process and <3> the low pressure-side pressure equalization (pressure lowering) process are performed.

(37) <4> Adsorption and Pressure Equalization (Pressure Lowering) Process

(38) In the first adsorption tower A1 in which the adsorption process has been finished, the adsorption and pressure equalization (pressure lowering) process is performed between itself and the second adsorption tower A2 in which the high pressure-side pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed. That is, in a source gas supply state in which the first adsorption tower A1 receives supply of the biogas from the source gas tank T1 via the switching valve V11 of the gas line L11 of the supply unit L1, and the second adsorption tower A2 also receives supply of the biogas from the source gas tank T1 via the switching valve V21 of the gas line L21 of the supply unit L1, any non-adsorbed gas in the first adsorption tower A1 is discharged via the switching valve V14 of the gas line L14 of the pressure equalization unit L4, and is transferred to the second adsorption tower A2 via the switching valve V24 of the gas line L24 of the equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the second adsorption tower A2 that is in the high pressure-side intermediate pressure state, and the first adsorption tower A1 transitions from the high pressure state to the slightly high pressure state.

(39) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, the standby process is performed in the third adsorption tower A3, and the decompression process is performed in the fourth adsorption tower A4.

(40) <5> Standby Process

(41) Next, the first adsorption tower A1 enters the standby state, and the slightly high pressure state is maintained, while keeping a balance in time with the adsorption process, which requires the longest time. At this time, the adsorption process is performed in the second adsorption tower A2, the third adsorption tower A3 is also in the standby process, and the decompression process continues in the fourth adsorption tower A4.

(42) <6> High Pressure-Side Pressure Equalization (Pressure Lowering) Process

(43) In the first adsorption tower A1 in which the adsorption and pressure equalization (pressure lowering) process has been finished, the high pressure-side pressure equalization (pressure lowering) process is performed between itself and the third adsorption tower A3 in which the high pressure-side pressure equalization (pressure increasing) process is performed. That is, the non-adsorbed gas in the tower is discharged via the switching valve V14 of the gas line L14 of the pressure equalization unit L4, and is transferred to the third adsorption tower A3 via the switching valve V34 of the gas line L34 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the third adsorption tower A3 that is in the low pressure-side intermediate pressure state, and the first adsorption tower A1 transitions from the slightly high pressure state to the high pressure-side intermediate pressure state.

(44) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, the adsorption process is performed in the second adsorption tower A2, and the decompression process is performed in the fourth adsorption tower A4.

(45) <7> Low Pressure-Side Pressure Equalization (Pressure Lowering) Process

(46) Next, in the first adsorption tower A1, the low pressure-side pressure equalization (pressure lowering) process is performed between itself and the fourth adsorption tower A4 in which the decompression process has been finished and the low pressure-side pressure equalization (pressure increasing) process is performed. That is, any non-adsorbed gas in the tower and an initially desorbed gas composed mainly of air from the adsorbent A11 are discharged via the switching valve V14 of the gas line L14 of the pressure equalization unit L4, and are transferred to the fourth adsorption tower A4 via the switching valve V44 of the gas line L44 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the fourth adsorption tower A4 in which the decompression process has been finished and which is in the low pressure state, and the first adsorption tower A1 transitions from the high pressure-side intermediate pressure state to the low pressure-side intermediate pressure state.

(47) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, the adsorption process is performed in the second adsorption tower A2, and the standby process is performed in the third adsorption tower A3.

(48) <8 to 10> Decompression Process

(49) The first adsorption tower A1 that has reached the low pressure-side intermediate pressure state is in a state in which high-concentration miscellaneous gases are adsorbed on the adsorbent A11 in the tower, and the high-concentration adsorption target gases (miscellaneous gases) adsorbed on the adsorbent A11 are discharged by performing a decompression process of decompressing the inside of the tower from the low pressure-side intermediate pressure state to the low pressure state. That is, the adsorption target gases are collected via the switching valve V13 of the gas line L13 of the decompression unit L3. Consequently, as shown in FIG. 2, the first adsorption tower A1 transitions from the low pressure-side intermediate pressure state to the low pressure state.

(50) Accordingly, the pressure in the first adsorption tower A1 during this decompression process can be lowered even more, and thus, during the decompression process, the amount of non-adsorbed gas that remains in the first adsorption tower A1 can be reduced. That is, the amount of non-adsorbed gas containing the gas to be purified that is discarded from the inside of the adsorption tower A1 in the decompression process will be only an extremely small amount of non-adsorbed gas that remains in the first adsorption tower A1 in the low pressure-side intermediate pressure state. Therefore, in the present embodiment, the amount of non-adsorbed gas that is discarded in the decompression process is one-fourth of that in the case where the adsorption and pressure equalization process and other pressure equalization processes are not performed at all. It can be understood that this is an extremely effective configuration in terms of the recovery rate of the non-adsorbed gas, because the amount of non-adsorbed gas that is discarded in the decompression process can be significantly reduced compared with the amount of non-adsorbed gas that would conventionally be discarded in the case where four pressure equalization processes (one high pressure-side pressure equalization (pressure increasing) process, one pressure equalization (pressure lowering) process, one low pressure-side pressure equalization (pressure increasing) process, and one pressure equalization (pressure lowering) process) are performed in a single cycle and that would be one-third of the amount of non-adsorbed gas discarded in the case where no pressure equalization process is performed.

(51) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, in the second adsorption tower A2, <8> the adsorption and pressure equalization (pressure lowering) process is performed between itself and the third adsorption tower A3, and then, <9> the standby process is performed, followed by <10> the high pressure-side pressure equalization (pressure lowering) process.

(52) Also, in the third adsorption tower A3, <8> the adsorption and pressure equalization (pressure increasing) process is performed, and then <9, 10> the adsorption process is performed.

(53) Furthermore, in the fourth adsorption tower A4, <8, 9> the standby process is performed, and then <10> the high pressure-side pressure equalization (pressure increasing) process is performed.

(54) <11> Low Pressure-Side Pressure Equalization (Pressure Increasing) Process

(55) In the first adsorption tower A1 which has entered the low pressure state and in which the adsorbed miscellaneous gases have been released and the adsorbent A11 has been regenerated, the low pressure-side pressure equalization (pressure increasing) process is performed between itself and the second adsorption tower A2, thereby restoring the pressure in the tower and receiving the exhaust gas which has been discharged in the low pressure-side pressure equalization (pressure lowering) process in the second adsorption tower A2 and in which the methane concentration has been increased by the initially desorbed gas from the adsorbent A21. That is, the first adsorption tower A1 receives the gas in the tower discharged from the second adsorption tower A2 that is in the high pressure-side intermediate pressure state via the switching valves V14 and V24 of the gas lines L14 and L24 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, the first adsorption tower A1 restores its pressure from the low pressure state to the low pressure-side intermediate pressure state.

(56) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, the adsorption process continues in the third adsorption tower A3, and the standby process is performed in the fourth adsorption tower A4.

(57) <12, 13> Standby Process

(58) Next, the first adsorption tower A1 enters the standby state, and the low pressure-side intermediate pressure state is maintained.

(59) Note that at this time, in the second adsorption tower A2, <12, 13> the decompression process is performed; in the third adsorption tower A3, <12> the adsorption and pressure equalization (pressure lowering) process is performed, and then <13> the standby process is performed; and in the fourth adsorption tower A4, <12> the adsorption and pressure equalization (pressure increasing) process is performed, and then <13> the adsorption process is performed.

(60) <14> High Pressure-Side Pressure Equalization (Pressure Increasing) Process

(61) In the first adsorption tower A1 that has restored its pressure to the low pressure-side intermediate pressure state, the pressure in the tower is further restored by performing the high pressure-side pressure equalization (pressure increasing) process between itself and the third adsorption tower A3 in which the adsorption and pressure equalization (pressure lowering) process has been finished and the high pressure-side pressure equalization (pressure lowering) process is performed. That is, the first adsorption tower A1 receives the gas in the tower discharged from the third adsorption tower A3 that is in the slightly high pressure state via the switching valves V14 and V34 of the gas lines L14 and L34 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, the first adsorption tower A1 restores its pressure from the low pressure-side intermediate pressure state to the high pressure-side intermediate pressure state.

(62) Note that at this time, through the operation of the open/close valves and the like as shown in Table 1, the decompression process is performed in the second adsorption tower A2, and the adsorption process is performed in the fourth adsorption tower A4.

(63) <15> Standby Process

(64) Next, the first adsorption tower A1 enters the standby state, and the high pressure-side intermediate pressure state is maintained, while keeping a balance in time with the adsorption process, which requires the longest time. At this time, in the second adsorption tower A2, the low pressure-side pressure equalization (pressure increasing) process is performed between itself and the third adsorption tower A3, and in the third adsorption tower A3, the low pressure-side pressure equalization (pressure lowering) process is performed. In the fourth adsorption tower A4, the adsorption process continues.

(65) <16> Adsorption and Pressure Equalization (Pressure Increasing) Process

(66) Next, in the first adsorption tower A1, the adsorption and pressure equalization (pressure increasing) process is performed between itself and the fourth adsorption tower A4 after the end of the adsorption process and prior to the start of the pressure equalization (pressure lowering) process. That is, in the first adsorption tower A1, the adsorption and pressure equalization (pressure increasing) process is performed in which, in a source gas supply state in which the first adsorption tower A1 receives supply of the biogas from the source gas tank T1 via the switching valve V11 of the gas line L11 of the supply unit L1, and the fourth adsorption tower A4 also receives supply of the biogas from the source gas tank T1 via the switching valve V41 of the gas line L41 of the supply unit L1, the non-adsorbed gas from the fourth adsorption tower A4 is received in the first adsorption tower A1 via the switching valves V14 and V44 of the gas lines L14 and L44 of the pressure equalization unit L4. At this time, the standby process is performed in the second adsorption tower A2, and the decompression process is performed in the third adsorption tower A3. Consequently, as shown in FIG. 2, the first adsorption tower A1 restores its pressure from the high pressure-side intermediate pressure state to the slightly high pressure state.

(67) Due to the above-described processes, the pressures in the adsorption towers change as shown in FIG. 2. Note that although the steps in the time cycle shown are illustrated to have equal widths, the steps actually proceed with the time proportions below.

(68) <1, 5, 9, 13> 132 seconds

(69) <2, 6, 10, 14> 6 seconds

(70) <3, 7, 11, 15> 6 seconds

(71) <4, 8, 12, 16> 1 second

EXAMPLE

(72) As described in the embodiment above, four adsorption towers A1 to A4 below were prepared, and adsorbents A11 to A41 below were filled therein. The adsorption towers A1 to A4 were connected by piping as shown in FIG. 1, thus preparing a methane concentration apparatus. To this methane concentration apparatus, a simulated biogas was supplied at 25.1 L/min, and the methane gas concentration operation shown in Table 1 and FIG. 2 was performed under the following operating conditions.

(73) Adsorption tower: Cylindrical (inside diameter: 54 mm, volume: 5.726 L) :Four towers

(74) Adsorbent: Molecular sieve carbon

(75) Molecular sieve carbon having a pore size distribution in which, at a pore size of 0.38 nm or more, a pore volume (V.sub.0.38) at this pore size is about 0.05 cm.sup.3/g, and a pore volume (V.sub.0.34) at a pore size of 0.34 nm is 0.20 to 0.23 cm.sup.3/g, the pore size distribution being determined by the MP method.

(76) Simulated biogas: Methane 59% :Carbon dioxide 40% :Nitrogen 0.6%

(77) Operating Conditions

(78) Temperature: 55 C.

(79) Flow velocity: 25.1 L/min

(80) Gas adsorption pressure (gauge pressure): 0.75 MPa

(81) Gas desorption pressure (gauge pressure): 0.1 kPa

(82) Condition of ending the adsorption process: when 145 seconds have elapsed from the start of the adsorption process

(83) As a result, methane having a methane concentration of 98.1 vol % was obtained at 12.6 L/min as a product gas. The recovery rate ((amount of methane in product gas/amount of methane in source gas)100) was 82.9%.

Comparative Example

(84) To evaluate the performance of the methane concentration method of the present invention, the gas concentration operation was performed using a PSA time cycle in which the adsorption and pressure equalization (pressure increasing) process and the adsorption and pressure equalization (pressure lowering) process were not performed. Specifically, the methane concentration operation was performed using the gas concentration apparatus in FIG. 1 and a time cycle shown in Table 2. Note that in Table 2, processes denoted by the same process names as those in Table 1 are the same as those of the example above, and their description is omitted; however, the adsorption and pressure equalization (pressure increasing) process was replaced by the adsorption process, and the adsorption and pressure equalization (pressure lowering) process was replaced by the standby process.

(85) TABLE-US-00002 TABLE 2 Adsorption Steps towers 1 2 3 4 5 6 7 8 A1 Adsorption Standby High Low pressure-side pressure-side pressure pressure equalization equalization (pressure (pressure lowering) lowering) A2 Standby High Standby Adsorption pressure-side pressure equalization (pressure increasing) A3 Decompression Low Standby High Standby pressure-side pressure-side pressure pressure equalization equalization (pressure (pressure increasing) increasing) A4 Standby High Low Decompression Low pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1 Adsorption Steps towers 9 10 11 12 13 14 15 16 A1 Decompression Low Standby High Standby pressure-side pressure-side pressure pressure equalization equalization (pressure (pressure increasing) increasing) A2 Standby High Low Decompression Low pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) A3 Adsorption Standby High Low pressure-side pressure-side pressure pressure equalization equalization (pressure (pressure lowering) lowering) A4 Standby High Standby Adsorption pressure-side pressure equalization (pressure increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1

(86) The gas concentration was performed in the same manner as in the example above by performing this time cycle. As a result, methane having a methane concentration of 98.3 vol % was obtained at 12.1 L/min as a product gas. The recovery rate was 80.2%.

(87) Note that in this comparative example, the specific time cycle for the adsorption towers was set as follows so as to adjust the balance between the example above and the comparative example.

(88) <1+2, 5+6, 9+10, 13+14> 142 seconds

(89) <3, 7, 11, 15> 6 seconds

(90) <4, 8, 12, 16> 6 seconds

(91) Condition of ending the adsorption process: 154 seconds after the start of adsorption

(92) A comparison between the results of the example and the comparative example showed that in both of these examples, the methane concentration was performed to a methane purity of approximately 98 vol % or more, but the recovery rate of the example was 82.9%, whereas the recovery rate of the comparative example was 80.2%. Therefore, in the example, while the gas concentration to an extremely high purity was performed, the recovery rate was about 3% higher than that of the comparative example, and it became clear that an even more efficient gas concentration method was performed.

Another Embodiment

(93) In the above-described embodiment,

(94) in the step in which <4> the adsorption and pressure equalization (pressure lowering) process is performed in the first adsorption tower A1, the second adsorption tower A2 was brought into the source gas supply state by simultaneously supplying the source gas to both the first adsorption tower A1 in which the adsorption process has been finished and the pressure equalization (pressure lowering) process is to be subsequently performed and the second adsorption tower A2 in which the pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed;

(95) however, it is also possible to supply the source gas to the second adsorption tower A2 in which the pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, with no source gas being supplied to the first adsorption tower A1 in which the adsorption process has been finished and the pressure equalization (pressure lowering) process is to be subsequently performed, thereby bringing the second adsorption tower A2 into the source gas supply state.

(96) That is, a configuration such as that described below can be adopted as the methane concentration method.

(97) As shown in Table 3, the controller C controls the switching valves V11 to V44 and the supply pump P1 to control the operation of the adsorption towers A1 to A4 so as to perform, with respect to the adsorption tower A1, the following processes in order:

(98) an adsorption process of receiving supply of biogas from the lower portion of the adsorption tower A1, adsorbing gases other than methane on the adsorbent A11, and releasing a non-adsorbed gas composed mainly of methane from the upper portion of the adsorption tower A1;

(99) an adsorption and pressure equalization (pressure lowering) process of, in a source gas supply state in which the source gas is supplied to the adsorption tower A2 in which a pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, with no source gas being supplied to the adsorption tower A1 in which the adsorption process has been finished and a pressure equalization (pressure lowering) process is to be subsequently performed, transferring the non-adsorbed gas from the upper portion of the adsorption tower A1 to the upper portion of the adsorption tower A2 so as to bring the adsorption tower A1 into a slightly high pressure state;

(100) a standby process;

(101) a high pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the slightly high pressure state to the other adsorption tower A3 that is in a low pressure-side intermediate pressure state at a pressure lower than that of the adsorption tower A1 so as to bring the pressure in the adsorption tower A1 into a high pressure-side intermediate pressure state;

(102) a low pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the high pressure-side intermediate pressure state, the gas having an adsorbed gas concentration that has been slightly increased with respect to that in the high pressure-side pressure equalization (pressure lowering) process, to the other adsorption tower A4 that is in a low pressure state so as to bring the pressure in the adsorption tower A1 into a low pressure-side intermediate pressure state;

(103) a decompression process of, after the pressure in the tower has been lowered by the low pressure-side pressure equalization (pressure lowering) process, further decompressing the adsorbent A11 into a low pressure state to desorb miscellaneous gases adsorbed on the adsorbent A11 and recovering the miscellaneous gases from the lower portion of the adsorption tower A1;

(104) a low pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure state, the gas in the adsorption tower A2 that is in the high pressure-side intermediate pressure state so as to bring the pressure in the adsorption tower A1 into the low pressure-side intermediate pressure state;

(105) the standby process;

(106) a high pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure-side intermediate pressure state, the gas in the other adsorption tower A3 that is in the slightly high pressure state so as to bring the pressure in the adsorption tower A1 into the high pressure-side intermediate pressure state;

(107) the standby process; and

(108) an adsorption and pressure equalization (pressure increasing) process, which corresponds to the adsorption and pressure equalization (pressure lowering) process, of receiving supply of the source gas and the non-adsorbed gas from the adsorption tower A4 that is in the high pressure state so as to bring the adsorption tower A1 from the high pressure-side intermediate pressure state into the slightly high pressure state. Although the same operation will also be performed for the other adsorption towers A2 to A4 in a phase (timing) shifted manner, the detailed description thereof is omitted and replaced by the description with reference to Table 3 to avoid redundancy.

(109) In Table 3, hollow circles indicate that the corresponding switching valves V11 to V44 are open or the corresponding supply pump P1 is in operation.

(110) TABLE-US-00003 TABLE 3 Adsorption Steps towers 1 2 3 4 5 6 7 8 A1 Adsorption Adsorption Standby High Low Decompression and pressure pressure-side pressure-side equalization pressure pressure (pressure equalization equalization lowering) (pressure (pressure lowering) lowering) A2 Standby High Standby Adsorption Adsorption Adsorption pressure-side and pressure and pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) lowering) increasing) A3 Decompression Low Standby High Standby Adsorption pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A4 Standby High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1 Adsorption Steps towers 9 10 11 12 13 14 15 16 A1 Decompression Low Standby High Standby Adsorption pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A2 Standby High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) A3 Adsorption Adsorption Standby High Low Decompression and pressure pressure-side pressure-side equalization pressure pressure (pressure equalization equalization lowering) (pressure (pressure lowering) lowering) A4 Standby High Standby Adsorption Adsorption Adsorption pressure-side and pressure and pressure pressure equalization equalization equalization (pressure (pressure (pressure increasing) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1

(111) By such a control, the changes in the internal pressures of the adsorption towers A1 to A4 transition in the same manner as shown in FIG. 2 of the above-described embodiment, although the absolute values of pressure are slightly different.

(112) This will be more specifically described below using the adsorption tower A1 as an example: in the fourth and sixteenth steps, the operation is controlled in the following manner.

(113) <4> Adsorption and Pressure Equalization (Pressure Lowering) Process

(114) In the first adsorption tower A1 in which the adsorption process has been finished and the pressure equalization (pressure lowering) process is to be subsequently performed, the adsorption and pressure equalization (pressure lowering) process is performed between itself and the second adsorption tower A2 in which the high pressure-side pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed. That is, in a source gas supply state in which while the first adsorption tower A1 does not receive the biogas from the source gas tank T1, the second adsorption tower A2 receives supply of the biogas from the source gas tank T1 via the switching valve V21 of the gas line L21 of the supply unit L1, any non-adsorbed gas in the first adsorption tower A1 is discharged via the switching valve V14 of the gas line L14 of the pressure equalization unit L4, and is transferred to the second adsorption tower A2 via the switching valve V24 of the gas line L24 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the second adsorption tower A2 that is in the high pressure-side intermediate pressure state, and the first adsorption tower A1 transitions from the high pressure state to the slightly high pressure state.

(115) Note that at this time, through the operation of the open/close valves and the like as shown in Table 3, the standby process is performed in the third adsorption tower A3, and the decompression process is performed in the fourth adsorption tower A4.

(116) <16> Adsorption and Pressure Equalization (Pressure Increasing) Process

(117) Next, in the first adsorption tower A1 in which the high pressure-side pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, the adsorption and pressure equalization (pressure increasing) process is performed between itself and the fourth adsorption tower A4 in which the adsorption process has been finished and the high pressure-side pressure equalization (pressure lowering) process is to be subsequently performed. That is, in a source gas supply state in which while the fourth adsorption tower A4 does not receive supply of the biogas from the source gas tank T1, the first adsorption tower A1 receives the biogas from the source gas tank T1 via the switching valve V11 of the gas line L11 of the supply unit L1, any non-adsorbed gas in the fourth adsorption tower A4 is discharged via the switching valves V14 and V44 of the gas lines L14 and L44 of the pressure equalization unit L4, and is transferred to the first adsorption tower A1 via the switching valve V44 of the gas line L44 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the fourth adsorption tower A4 that is in the high pressure state, and the first adsorption tower A1 transitions from the high pressure-side intermediate pressure state to the slightly high pressure state. At this time, the standby process is performed in the second adsorption tower A2, and the decompression process is performed in the third adsorption tower A3.

Another Example

(118) The gas concentration was performed in the same manner as in the example above by performing this time cycle. As a result, methane having a methane concentration of 98.2 vol % was obtained at 12.3 L/min as a product gas. The recovery rate was 81.7%.

(119) A comparison between the results of the other example and the comparative example showed that in both of these examples, the methane concentration was performed to a methane purity of approximately 98 vol % or more, but the recovery rate of the other example was 81.7%, whereas the recovery rate of the comparative example was 80.2%. Therefore, in the other example, while the gas concentration to an extremely high purity was performed, the recovery rate was about 2% higher than that of the comparative example, and it became clear that an efficient gas concentration method was performed as in the case of the example above (recovery rate: 82.9%).

Another Embodiment 2

(120) In both of the above-described embodiments, the present invention has been described using an example in which four adsorption towers are used; however, the same gas concentration method of the present application can also be performed when three adsorption towers are used (FIG. 3). Note that in FIG. 3, those portions that have the same configurations or functions as the configurations in FIG. 1 are denoted by the same reference numerals, and their detailed description is thereby omitted. Moreover, the same gas concentration method of the present application can also be performed when five or more towers are used, but a description of such configurations is omitted because it is obvious from the configuration examples in which three or four towers are used.

(121) Specifically, as shown in Table 4, the switching valves V11 to V34 and the supply pump P1 are controlled to control the operation of the adsorption towers A1 to A3 so as to perform, with respect to the adsorption tower A1, the following processes in order:

(122) an adsorption process of receiving supply of a source gas from the lower portion of the adsorption tower A1, adsorbing gases other than methane on the adsorbent A11, and releasing a non-adsorbed gas composed mainly of methane from the upper portion of the adsorption tower A1;

(123) an adsorption and pressure equalization (pressure lowering) process of, prior to performing a pressure equalization (pressure lowering) process in the first adsorption tower A1 in which the adsorption process has been finished, supplying the source gas to at least the second adsorption tower A2 in which a pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, and transferring the non-adsorbed gas from the upper portion of the first adsorption tower A1 to the upper portion of the second adsorption tower A2 so as to bring the adsorption tower A1 into a slightly high pressure state,

(124) where although the second adsorption tower A2 in this process is in the source gas supply state, the first adsorption tower A1 may be or may not be in the source gas supply state (indicated by hollow triangles in Table 4);

(125) a standby process;

(126) a pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the slightly high pressure state to the other adsorption tower A3 that is in a lower pressure state at a pressure lower than the pressure of the adsorption tower A1 so as to bring the pressure in the adsorption tower A1 into an intermediate pressure state;

(127) a decompression process of, after the pressure in the adsorption tower has been lowered by the pressure equalization (pressure lowering) process, further decompressing the adsorbent A11 into a low pressure state to desorb the gases other than methane that have been adsorbed on the adsorbent A11 and recovering the gases from the lower portion of the adsorption tower A1;

(128) a pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure state, the gas in the adsorption tower A2 that is in the intermediate pressure state so as to bring the pressure in the adsorption tower A1 into the intermediate pressure state; and

(129) an adsorption and pressure equalization (pressure increasing) process, which corresponds to the adsorption and pressure equalization (pressure lowering) process of the adsorption tower A3, of receiving supply of the source gas and the non-adsorbed gas from the adsorption tower A3 that is in the high pressure state so as to bring the adsorption tower A1 into the slightly high pressure state. The same operation can also be performed for the other adsorption towers A2 and A3 in a phase (timing) shifted manner.

(130) Note that in the present embodiment, as shown in FIG. 4, the pressure in the first adsorption tower A1 is supposed to change between the high pressure state, the slightly high pressure state, the intermediate pressure state, and the low pressure state in descending order of pressure.

(131) TABLE-US-00004 TABLE 4 Adsorption Steps towers 1 2 3 4 5 6 7 8 9 A1 Adsorption Adsorption Standby Pressure Decompression Pressure Adsorption and pressure equalization equalization and pressure equalization (pressure (pressure equalization (pressure lowering) increasing) (pressure lowering) increasing) A2 Decompression Pressure Adsorption Adsorption Adsorption Standby Pressure Decompression equalization and pressure and pressure equalization (pressure equalization equalization (pressure increasing) (pressure (pressure lowering) increasing) lowering) A3 Standby Pressure Decompression Pressure Adsorption Adsorption Adsorption equalization equalization and pressure and pressure (pressure (pressure equalization equalization lowering) increasing) (pressure (pressure increasing) lowering) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 P1

(132) (Reference Configuration)

(133) A case was examined in which, instead of the adsorption and pressure equalization process in the above-described embodiments,

(134) only a pressure equalization process (hereinafter referred to as supply suspension and pressure equalization process) is performed between the first adsorption tower A1 and the second adsorption tower A2, with no source gas being supplied to the first adsorption tower A1 in which the adsorption process has been finished and the pressure equalization (pressure lowering) process is to be subsequently performed, and no source gas being supplied to the second adsorption tower A2 in which the pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed.

(135) That is, a configuration such as that described below can be adopted as the methane concentration method.

(136) The controller C controls the switching valves V11 to V44 and the supply pump P1 as shown in Table 5 to control the operation of the adsorption towers A1 to A4 so as to perform, with respect to the adsorption tower A1, the following processes in order:

(137) an adsorption process of receiving supply of biogas from the lower portion of the adsorption tower A1, adsorbing gases other than methane on the adsorbent A11, and releasing a non-adsorbed gas composed mainly of methane from the upper portion of the adsorption tower A1;

(138) a supply suspension and pressure equalization (pressure lowering) process of, in a state in which no source gas is supplied to the adsorption tower A1 in which the adsorption process has been finished and a pressure equalization (pressure lowering) process is to be subsequently performed, and also no source gas is supplied to the adsorption tower A2 in which a pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, transferring the non-adsorbed gas from the upper portion of the adsorption tower A1 to the upper portion of the adsorption tower A2 so as to bring the adsorption tower A1 into a slightly high pressure state;

(139) a standby process;

(140) a high pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the slightly high pressure state to the other adsorption tower A3 that is in a low pressure-side intermediate pressure state at a pressure lower than the pressure in the adsorption tower A1 so as to bring the pressure in the adsorption tower A1 into a high pressure-side intermediate pressure state;

(141) a low pressure-side pressure equalization (pressure lowering) process of transferring the gas in the adsorption tower A1 that is in the high pressure-side intermediate pressure state, the gas having an adsorbed gas concentration that has been slightly increased with respect to that in the high pressure-side pressure equalization (pressure lowering) process, to the other adsorption tower A4 that is in a low pressure state so as to bring the pressure in the adsorption tower A1 into a low pressure-side intermediate pressure state;

(142) a decompression process of, after the pressure in the adsorption tower has been lowered by the low pressure-side pressure equalization (pressure lowering) process, further decompressing the adsorbent A11 into a low pressure state so as to desorb miscellaneous gases adsorbed on the adsorbent A11, and recovering the miscellaneous gases from the lower portion of the adsorption tower A1;

(143) a low pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure state, the gas in the adsorption tower A2 that is in the high pressure-side intermediate pressure state so as to bring the pressure in the adsorption tower A1 into the low pressure-side intermediate pressure state;

(144) the standby process;

(145) a high pressure-side pressure equalization (pressure increasing) process of receiving, into the adsorption tower A1 that is in the low pressure-side intermediate pressure state, the gas in the other adsorption tower A3 that is in the slightly high pressure state so as to bring the pressure in the adsorption tower A1 into the high pressure-side intermediate pressure state;

(146) the standby process; and

(147) a supply suspension and pressure equalization (pressure increasing) process, which corresponds to the adsorption and pressure equalization (pressure lowering) process, of receiving supply of the gas from the adsorption tower A4 that is in the high pressure state so as to bring the adsorption tower A1 from the high pressure-side intermediate pressure state into the slightly high pressure state. Moreover, although the same operation will also be performed for the other adsorption towers A2 to A4 in a phase (timing) shifted manner, the detailed description thereof is omitted and replaced by the description with reference to Table 5 to avoid redundancy.

(148) Note that in the reference configuration, the pressure in the first adsorption tower A1 is supposed to change between the high pressure state, the slightly high pressure state, the high pressure-side intermediate pressure state, the low pressure-side intermediate pressure state, and the low pressure state in descending order of pressure.

(149) In Table 5, hollow circles indicate that the corresponding switching valves V11 to V44 are open or the corresponding supply pump P1 is in operation.

(150) TABLE-US-00005 TABLE 5 Adsorption Steps towers 1 2 3 4 5 6 7 8 9 A1 Adsorption Supply Standby High Low Decompression suspension pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) lowering) A2 Standby High Standby Supply Adsorption Supply Standby pressure-side suspension suspension pressure and pressure and pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) lowering) A3 Decompression Low Standby High Standby Supply Adsorption pressure-side pressure-side suspension pressure pressure and pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A4 Standby High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1 Adsorption Steps towers 10 11 12 13 14 15 16 A1 Decompression Low Standby High Standby Supply pressure-side pressure-side suspension pressure pressure and pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) increasing) A2 High Low Decompression Low Standby pressure-side pressure-side pressure-side pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) increasing) A3 Adsorption Supply Standby High Low Decompression suspension pressure-side pressure-side and pressure pressure pressure equalization equalization equalization (pressure (pressure (pressure lowering) lowering) lowering) A4 High Standby Supply Adsorption Supply pressure-side suspension suspension pressure and pressure and pressure equalization equalization equalization (pressure (pressure (pressure increasing) increasing) lowering) V11 V12 V13 V14 V21 V22 V23 V24 V31 V32 V33 V34 V41 V42 V43 V44 P1

(151) By such a control, the changes in the internal pressures of the adsorption towers A1 to A4 transition in the same manner as shown in FIG. 2 of the above-described embodiment, although the absolute values of pressure are slightly different.

(152) The differences from the above-described embodiments will be more specifically described below using the adsorption tower A1 as an example: in the fourth and sixteenth steps, the operation is controlled in the following manner.

(153) <4> Supply Suspension and Pressure Equalization (Pressure Lowering) Process

(154) In the first adsorption tower A1 in which the adsorption process has been finished and the pressure equalization (pressure lowering) process is to be subsequently performed, the supply suspension and pressure equalization (pressure lowering) process is performed between itself and the second adsorption tower A2 in which the high pressure-side pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed. That is, in a state in which the first adsorption tower A1 does not receive supply of the biogas from the source gas tank T1, and the second adsorption tower A2 also does not receive the biogas from the source gas tank T1, the non-adsorbed gas in the first adsorption tower A1 is discharged via the switching valve V14 of the gas line L14 of the pressure equalization unit L4, and is transferred to the second adsorption tower A2 via the switching valve V24 of the gas line L24 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the second adsorption tower A2 that is in the high pressure-side intermediate pressure state, and the first adsorption tower A1 transitions from the high pressure state to the slightly high pressure state.

(155) Note that at this time, through the operation of the open/close valves and the like as shown in Table 5, the standby process is performed in the third adsorption tower A3, and the decompression process is performed in the fourth adsorption tower A4.

(156) <16> Supply Suspension and Pressure Equalization (Pressure Increasing) Process

(157) Next, in the first adsorption tower A1 in which the high pressure-side pressure equalization (pressure increasing) process has been finished and the adsorption process is to be subsequently performed, the supply suspension and pressure equalization (pressure increasing) process is performed between itself and the fourth adsorption tower A4 in which the adsorption process has been finished and the high pressure-side pressure equalization (pressure lowering) process is to be subsequently performed. That is, in a state in which the fourth adsorption tower A4 does not receive supply of the biogas from the source gas tank T1, and the first adsorption tower A1 also does not receive the biogas from the source gas tank T1 via the switching valve V11 of the gas line L11 of the supply unit L1, the non-adsorbed gas in the fourth adsorption tower A4 is discharged via the switching valves V14 and V44 of the gas lines L14 and L44 of the pressure equalization unit L4, and is transferred to the first adsorption tower A1 via the switching valve V44 of the gas line L44 of the pressure equalization unit L4. Consequently, as shown in FIG. 2, pressure equalization is performed between the first adsorption tower A1 and the fourth adsorption tower A4 that is in the high pressure state, and the first adsorption tower A1 transitions from the high pressure-side intermediate pressure state to the slightly high pressure state. At this time, the standby process is performed in the second adsorption tower A2, and the decompression process is performed in the third adsorption tower A3.

Reference Example

(158) The gas concentration was performed by performing this time cycle. As a result, methane having a methane concentration of 98.3 vol % was obtained at 12.3 L/min as a product gas. The recovery rate was 80.9%.

(159) Note that in this reference example, the specific time cycle for the adsorption towers was set as follows to adjust the balance between the example above and this reference example.

(160) <1, 5, 9, 13> 131 seconds

(161) <2, 6, 10, 14> 6 seconds

(162) <3, 7, 11, 15> 6 seconds

(163) <4, 8, 12, 16> 1 second

(164) Condition of ending the adsorption process: 143 seconds after the start of adsorption

(165) A comparison between the reference example and the comparative example showed that in both of these examples, the methane concentration was performed to a methane purity of approximately 98 vol % or more, but the recovery rate of the reference example was 80.9%, whereas the recovery rate of the comparative example was 80.2%. Therefore, it became clear that in the reference example, while the gas concentration to an extremely high purity was performed, the recovery rate was able to be slightly improved, as compared with the comparative example.

INDUSTRIAL APPLICABILITY

(166) The gas concentration method of the present invention can be used for a gas concentration apparatus for recovering a high-purity gas at a high recovery rate.

DESCRIPTION OF REFERENCE SIGNS

(167) A1 to A4: Adsorption towers (first to fourth adsorption towers) A11 to A41: Adsorbents C: Controller L1: Supply unit L2: Recovery unit L3: Decompression unit L4: Pressure equalization unit L11 to L44: Gas lines P1: Supply pump T1: Source gas tank V11 to V44: Switching valves