Multi-Stage Reverse Osmosis Systems and Methods

Abstract

Improved reverse osmosis (RO) systems include at least first and second stages wherein each stage has at least one RO membrane, each stage has a feed stream inlet, a permeate stream outlet, and a concentrate stream outlet, the feed stream inlet of the second stage is coupled to the concentrate stream outlet of the first stage, the second pressure is greater than the first pressure, and pressure exchangers associated with each of the first and second stages are configured to recover energy from the second stage concentrate stream. The systems include M reverse osmosis membranes in the first stage and N reverse osmosis membranes in the second stage, wherein M≥N. The first pressure and second pressure are configured so that spatial variance in flux of the first stage permeate stream relative to flux of the second stage permeate stream is minimized.

What is claimed is: 
     
         1 . An improved reverse osmosis separation system, the system having a first stage and a second stage, the first and second stages each having (i) at least one reverse osmosis membrane, and (ii) a feed stream inlet for a feed stream, a permeate stream outlet for a permeate stream, and a concentrate stream outlet for a concentrate stream, wherein the feed stream inlet of the second stage is coupled to the concentrate stream outlet of the first stage, the feed stream entering the first stage is pressurized to a first pressure and the feed stream entering the second stage is pressurized to a second pressure, and the second pressure is greater than the first pressure, wherein the improvement comprises:
 M number of reverse osmosis membranes in the first stage and N number of reverse osmosis membranes in the second stage, wherein M≥N;   a first pressure exchanger that is fluidically coupled to (a) the concentrate stream of the first stage, (b) the concentrate stream of the second stage, and (c) a first booster pump, the first booster pump being fluidically coupled to the feed stream of the second stage;   a second pressure exchanger that is fluidically coupled to the first pressure exchanger and fluidically coupled to a second booster pump, the second booster pump being fluidically coupled to the feed stream of the first stage; and   the first pressure and the second pressure are configured so that flux of the permeate streams of the first stage and the second stage has a spatial variance that is minimized.   
     
     
         2 . An improved reverse osmosis separation system according to  claim 1 , wherein M>N. 
     
     
         3 . An improved reverse osmosis separation system according to  claim 1 , wherein the spatial variance is within a designated amount of a minimum value of the spatial variance. 
     
     
         4 . An improved reverse osmosis separation system according to  claim 2 , wherein M≤3N. 
     
     
         5 . An improved reverse osmosis separation system according to  claim 2 , wherein M=6 and N=2. 
     
     
         6 . An improved reverse osmosis separation system according to  claim 2 , wherein M=5 and N=3. 
     
     
         7 . An improved reverse osmosis separation system according to  claim 1 , wherein-the reverse osmosis membranes are configured to have a permeability of greater than 1.2 L/m 2 -h-bar. 
     
     
         8 . An improved reverse osmosis separation system according to  claim 1 , wherein the reverse osmosis membranes are configured to have a permeability of greater than 2 L/m 2 -h-bar. 
     
     
         9 . An improved reverse osmosis separation system according to  claim 1 , wherein the system is configured to have a permeability of approximately 1 L/m 2 -h-bar.