| Single Bed PSA |  |
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| Versions of Single Bed PSA | Time Cycle of a Single Bed PSA |
Gaseous products that are amenable to recovery by PSA and do not require high purity, can be separated successfully using simple PSA systems. For example in the recovery of oxygen and nitrogen from air by PSA, simpler PSA systems can be used. These simpler PSA systems are much cheaper capitalwise and operating costs wise than multiple-bed systems. The simplest of these PSA systems for air separation, uses just one adsorber bed in conjunction with one or more gas storage vessels. This ensures constant product flow and gives gas for adsorber purge and pressurization during the regeneration portion of the PSA cycle.
A large number of single-bed PSA processes have been developed well-suited for the separation of air, preferably to produce an oxygen product that contains 50 to 95 mole % oxygen. Other than oxygen, the process is used for the separation of other gas mixtures for applications requiring moderate product recovery/purity. The simplest of the single-bed PSA system must contain at the minimum the following sets of equipment. An adsorbent bed, a product surge or a storage tank, a feed blower, and the pipes and valves for directing the gas flow during the cycle steps.
Many modified versions of single bed PSA systems are used in the industries, which are described below:
- Single bed PSA system with surge tank:
This process typically comprises Single-bed PSA system with an adsorber and a surge tank. It gets operated with a feed compressor in a three step cycle process. In the beginning, compressed feed air gets introduced into the adsorber. This increases the pressure in the adsorber, and as a result adsorber effluent is withdrawn into the surge tank. A substantial portion of the gas is withdrawn from the surge tank as oxygen-enriched product. After this the adsorber feed is discontinued and the adsorber is vented out countercurrently (via the adsorber feed end) into the atmosphere.
During this step of venting, purge gas available in the surge tank is thrown into the product end of the adsorber. Finally upon completing the vent/purge step, the adsorber as well as the surge tank are both pressure equalized. This is done through the adsorber product end (i. e. countercurrently). The steps are repeated in a cyclic manner till the end product is obtained. - Single bed PSA system with switch valves:
This method of Single bed PSA system has an adsorber along with a surge tank, feed compressor, and switch valves to carry out the process of separation. Compressed feed air is first introduced into the adsorber. It does two things. First, it increases the pressure in the adsorber. Second, adsorber effluent is withdrawn into the surge tank. Some amount of the gas is taken out from the surge tank as an oxygen-enriched product. The adsorber feed is stopped and the adsorber outlet is also closed.
The adsorber is vented out countercurrently to the atmosphere. Right after this, gas from the surge tank is allowed to come into the adsorber counter-currently (i. e. from the adsorber product end). Both the pressures in the adsorber as well as in the surge tank are equalized. With the feed air, the adsorber is then pressurized through the feed end. The pressure is also equalized with the surge tank. Further, the adsorber is again pressurized till the pressure is higher than that in the surge tank. Lastly the adsorber and surge tank are again pressure equalized. These steps of the PSA cycle are repeated. - Single bed PSA system with multiple gas storage tanks:
Yet another variety of the single-bed PSA process is the one that employs multiple gas storage tanks to separate air. The process works like this, compressed air is first fed from an air feed tank into the adsorber. This adsorber is already saturated with oxygen-rich gas from an earliers cycle. The adsorber effluent is made to flow into a collector tank. A portion of the gas is then taken out as an oxygen-rich product from there. The outlet of the adsorber is closed and the pressure equalized with the air feed tank. Next in the process, the adsorber is thoroughly rinsed with nitrogen-rich gas from a nitrogen tank. The gas that gets displaced is stored in the air feed tank. The adsorber(nitrogen-saturated) is depressurized countercurrently into the nitrogen tank. This gives an option to withdraw Nitrogen as a product if desired. In the end the adsorber is purged countercurrently with the oxygen-rich gas available from the product collector tank to displace the nitrogen there. It is then pressurized countercurrently with the oxygen-rich gas to the adsorption pressure. All the steps are repeated in a cyclic fashion.
A typical PSA separation process operation employing single bed and three step PSA cycle usually has the following time cycles, though these can vary depending on other conditions:
- Total Time cycle: Total time cycle is between 30 and 240 seconds.
- Adsorption step-duration: This is preferably between 5 to 60 seconds.
- Evacuation step: preferably between 15 and 160 seconds.
- Pressurization step: Ranges from 2 and 20 seconds. The short pressurization period contributes to minimize the total cycle time. This in turn results in smaller adsorber beds for a particular production rate.