Duplex PSA

Working of it

Comparison

Duplex PSA is a highly sophisticated version of the PSA system. The separation mechanism it employs is rather unique. Unlike conventional PSA, it operates under moderate pressures. In the system of Duplex PSA, the mass transfer zones is spread over the full bed length. Also, the concentration profiles does not get destroyed and reformed in each of the cycle, as in the conventional PSA. The profiles only get shifted a bit depending upon the pressure ratio employed. Also the separation process is very clean without waste stream as in conventional PSA.

Working of a Duplex PSA:
The working of a Duplex PSA process is described here for capturing of Carbon dioxide gas. The process employs two beds and four cycles. To start the process, the Bed that is marked 1 is at higher pressure while the bed marked 2 is at lower pressure. Feed gas is then introduced in the bed. Some portion of the gas that was drawn from the PSA is recycled into bed 2 while the balance gets drawn as the raffinate product. Similarly some of the gas that was drawn from the bed 2 is recycled into bed 1 while the balance amount of gas gets drawn as the extract product. In the next step, pressures at both the beds are equalized. This can be done by connecting the bottom or upper end of the beds. The bed 2 is pressurized to high pressure from the gas drawn from bed 1. This completes the second step of the cycle.



In the third step the feed gas is entered into Bed-2. The end streams are then recycled as already discussed in the beginning. And finally in the fourth and final step, pressure equalization and the resetting of pressures as in the second step is made. This completes one cycle of the process.

However studies revealed limitations of Carbon dioxide capture in this way. Composition of the Carbon dioxide in the Bed 2 in the first step of the cycle gradually decreased with the time. The mole fraction of the Carbon dioxide gas that was drawn in the beginning of the step was significantly higher than the one drawn at the end of the step. To do away with this limiting factors a modified duplex PSA cycle was made.

Modified Duplex PSA Process:
In the modified duplex PSA the basic process remains the same but the process withdrawal of the extract was improved upon. The modifications made were like this. The extract was first drawn and the remainder was then recycled to bed 2. As can be clearly seen from the following table, there was considerable increase the purity of the products.

Comparison of original and modified cycles with purge reflux ratio