Generally, gases rich in nitrogen (hereafter referred to as “nitrogen-enriched gas”) which have been separated and purified from raw material air containing nitrogen (hereafter referred to as simply “raw material air”) using the pressure swing adsorption (PSA) method are used in many different applications, including as purge gases for explosion prevention and as atmospheric gases for heat treatment furnaces.
In the separation and purification of a nitrogen-enriched gas by the PSA method, electricity is used exclusively as the power source, but in recent years, there have been growing demands for greater power saving, aimed at reducing running costs and achieving better energy saving.
Conventionally known examples of this type of nitrogen-enriched gas manufacturing apparatus using the PSA method (hereafter also referred to as a “nitrogen PSA apparatus”) include the type of apparatus illustrated in FIG. 7.
As illustrated in FIG. 7, a nitrogen PSA apparatus 101 includes a raw material air compressor 102 for pressurizing the raw material air, two adsorption tanks (a first adsorption tank 103A and a second adsorption tank 103B), and a product tank 104.
Further, automatic switching on-off valves 111a, 111b, 116a and 116b are provided on the side of the adsorption tanks 103A and 103B to which the raw material air compressor 102 is connected (hereafter referred to as “the upstream side”), and the side of the adsorption tanks 103A and 103B to which the product tank 104 is connected (hereafter referred to as “the downstream side”) respectively, and the nitrogen PSA apparatus 101 is also provided with automatic switching on-off valves 112a, 112b, 113 and 115, and a flow rate regulating valve 114 which can regulate the flow rate. Further, an automatic switching on-off valve 106 is also provided at the outlet side of the product tank 104.
Furthermore, the two adsorption tanks 103A and 103B are packed with an adsorbent 105 which preferentially adsorbs unwanted components such as oxygen and carbon dioxide within the compressed raw material air delivered from the raw material air compressor 102.
One known method for separating a nitrogen-enriched gas from a raw material air using this type of nitrogen PSA apparatus 101 is a method that involves repeating a pressurization and adsorption step, a depressurization and equalization step, a depressurization and regeneration step, and a pressurization and equalization step.
In the case of this method, when one of the first adsorption tank 103A and the second adsorption tank 103B is performing the pressurization and adsorption step, the other is performing the depressurization and regeneration step, and when one is performing the depressurization and equalization step, the other is performing the pressurization and equalization step. Accordingly, in the first adsorption tank 103A, when the pressurization and adsorption step, the depressurization and equalization step, the depressurization and regeneration step, and the pressurization and equalization step are performed in that order, in the second adsorption tank 103B, the steps are performed in the order of the depressurization and regeneration step, the pressurization and equalization step, the pressurization and adsorption step, and the depressurization and equalization step. The following description describes the steps for the first adsorption tank 103A.
First, in the pressurization and adsorption step, the compressed raw material air that has been pressurized by the raw material air compressor 102 is introduced into the first adsorption tank 103A, the inside of the first adsorption tank 103A is pressurized, and the unwanted components within the raw material air are adsorbed preferentially to the adsorbent 105, yielding a nitrogen-enriched gas. Next, in the depressurization and equalization step, the relatively high-pressure gas remaining inside the first adsorption took 103A is introduced into the second adsorption tank 103B.
Subsequently, in the depressurization and regeneration step, the first adsorption tank 103A is opened to the atmosphere, thereby reducing the pressure inside the first adsorption tank 103A, desorbing the unwanted components adsorbed to the adsorbent 105, and discharging these unwanted components outside the first adsorption tank 103A. At this time, the nitrogen-enriched gas extracted from the downstream side of the second adsorption tank 103B, which is performing the pressurization and adsorption step, is preferably introduced into the first adsorption tank 103A through the downstream side of the first adsorption tank 103A, thereby accelerating the desorption of the unwanted components.
In the pressurization and equalization step, the relatively high-pressure gas remaining inside the second adsorption tank 103B, which has completed the pressurization and adsorption step, is introduced into the first adsorption tank 103A, which has completed the depressurization and regeneration step.
In a more detailed description based on FIG. 7, when the first adsorption tank 103A is performing the pressurization and adsorption step (namely, when the second adsorption tank 103B is performing the depressurization and regeneration step), the on-off valves 111a, 112b and 116a are open, and the other on-off valves are closed.
Accordingly, the compressed raw material air that has been compressed by the raw material air compressor 102 passes through the on-off valve 111a and into the first adsorption tank 103A.
The unwanted components such as oxygen and carbon dioxide within the compressed raw material air that has been introduced into the first adsorption tank 103A are adsorbed to the adsorbent 105, thereby producing a nitrogen-enriched gas that is rich in nitrogen, and this nitrogen-enriched gas passes through the on-off valve 116a and is fed into the product tank 104 as a product gas. At this time, a portion of the nitrogen-enriched gas passes through the flow rate regulating valve 114 and into the second adsorption tank 103B, where it is used for regenerating the adsorbent 105 inside the second adsorption tank 103B.
Subsequently, when the first adsorption, tank 103A enters the depressurization and equalization step and the second adsorption tank 103B enters the pressurization and equalization step, the on-off valves 113 and 115 are opened, and the other on-off valves are closed.
Accordingly, in this depressurization and equalization step, the relatively high-pressure gas remaining inside the first adsorption tank 103A is supplied from the first adsorption tank 103A to the second adsorption tank 103B through the on-off valves 113 and 115.
Next, by opening the on-off valves 111b, 112a and 116b, and closing the other on-off valves, the first adsorption tank 103A enters the depressurization and regeneration step, and the second adsorption tank 103B enters the pressurization and adsorption step.
In these steps, residual gas inside the first adsorption tank 103A is released into the atmosphere through the on-off valve 112a, and as the pressure inside the first adsorption tank 103A decreases, the unwanted components adsorbed to the adsorbent 105 inside the first adsorption tank 103A desorb. At this time, a portion of the nitrogen-enriched gas discharged from the second adsorption tank 103B passes through the flow regulating valve 114 and into the interior of the first adsorption tank 103A, and is used as a purge gas for regenerating the adsorbent 105 inside the first adsorption tank 103A.
Subsequently, by opening the on-off valves 113 and 115, and closing the other on-off valves, the first adsorption tank 103A enters the pressurization and equalization step, and the second adsorption tank 103B enters the depressurization and equalization step.
In these steps, the relatively high-pressure gas (pressure equalization gas) inside the second adsorption tank 103B is supplied from the second adsorption tank 103B to the first adsorption tank 103A.
By repeating the above steps, a nitrogen-enriched gas is separated from the raw material air.
By the way, during the press urination and equalization step and the depressurization and equalization step, because the on-off valves 111a and 111b are closed, the compressed raw material air cannot be supplied from the raw material air compressor 102 to the adsorption tanks 103A and 103B, meaning a rapid increase in pressure occurs at the outlet of the raw material air compressor 102.
Accordingly, in order to prevent this rapid increase hi pressure caused by the closing, of the on-off valves 111a and 111b of the nitrogen PSA apparatus 101, a compressed raw material air tank 107 is sometimes provided downstream from the raw material air compressor 102 as illustrated in FIG. 8.
Furthermore, in one known method of effectively utilizing the compressed raw material air using the compressed raw material air tank 107, the pressure inside the compressed raw material air tank 107 is continually monitored, and the rate of revolution of the motor of the raw material air compressor 102 is controlled accordingly (see Patent Document 1).