FIGURE 1 

A SWRO desalination plant requires seawater to be pressurized to about 900 PSI against a semi-permeable membrane where about 40% of the water molecules pass through the membrane that withholds 99.99% of the salt. The remaining 60% is a concentrate or brine at around 870 PSI that needs to be returned to the sea and represents a large loss of pumping energy unless utilized or recovered by some means.

The pressure exchanger has a rotor with multiple co-axial ducts where about half of the ducts are exposed to low pressure and the other half to high pressure. In between there are sealing areas where the ducts are shut off from any flow communication.

If the rotor is not rotating, concentrate at 870 PSI will flow straight through the rotor ducts on the high pressure side into a booster pump that raises the pressure to 900 PSI, which is shown on Figure 1.

FIGURE 2

At the same time feed seawater at about 15 PSI will flow through the ducts on the low pressure side and simply return back to the sea, which is in effect the opposite of what is desired. Now, in order to feed seawater at high pressure to the membrane as shown on Figure 2, the rotor must rotate at a speed where concentrate does not flow through the ducts before the ducts by being rotated into a sealing area that closes each end of the duct.

After passing the sealing area, the ducts enters the low pressure side where seawater flows in the opposite direction, forcing the concentrate back out to the sea at atmospheric pressure. The ducts are then filled with seawater before entering the second sealing area that stops the flow inside the ducts before entering the high pressure side. Here the concentrate is allowed to push out the seawater at about 855 PSI towards the booster pump that increases the pressure to the necessary membrane feed pressure of 900 PSI.