| 主权项 |
1. A methane gas concentration method comprising:
providing four or more adsorption towers each filled with an adsorbent that adsorbs methane gas in a methane-containing gas; and performing, for each of the adsorption towers, a PSA cycle of repeating in order: an adsorption step of receiving supply of the methane-containing gas in a high pressure state near atmospheric pressure from a lower portion of the adsorption tower, adsorbing the methane gas on the adsorbent, and releasing offgas composed mainly of air from an upper portion of the adsorption tower; a pressure equalization (depressurizing) step of transferring the gas in the adsorption tower that has finished the adsorption step and is in the high pressure state to another adsorption tower in a lower pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state; a decompression step of, after the internal pressure of the tower has been lowered by the pressure equalization (depressurizing) step, further decompressing the adsorbent to a low pressure state so as to desorb the methane gas adsorbed on the adsorbent, and recovering said methane gas from the lower portion of the adsorption tower; a pressure equalization (pressurizing) step of receiving, after finishing the decompression step, the gas from 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; and a pressurizing step of, after the internal pressure of the tower has been increased by the pressure equalization (pressurizing) step, further supplying a pressurizing air into the adsorption tower from an upper portion of the adsorption tower so as to restore the adsorbent into a high pressure state capable of selectively adsorbing the methane gas, wherein the adsorbent has a property of selectively adsorbing the methane gas contained in the air in the high pressure state near atmospheric pressure and desorbing the adsorbed methane gas in the low pressure state, and a property of preferentially desorbing the air when desorbing the methane gas, a plurality of different pressure states of the internal pressures of the adsorption tower are set as the intermediate pressure state, the method includes, as the pressure equalization (depressurizing) step, an initial pressure equalization (depressurizing) step 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 an 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 final pressure equalization (depressurizing) step of transferring the gas in one of the adsorption towers that is in the high pressure-side 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 equalization (pressurizing) step includes: an initial pressure equalization (pressurizing) step of receiving the gas in one of the adsorption towers that is in the high pressure-side intermediate pressure state into 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 the low pressure-side intermediate pressure state; and a final pressure equalization (pressurizing) step 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 that performs the pressure equalization (depressurizing) step to another one of the adsorption towers that performs the pressure equalization (pressurizing) step from the upper portion of the one of the adsorption towers to the upper portion of the other one of the adsorption towers. |
