Cross current staged reverse osmosis
Systems and processes for purifying and concentrating a liquid feed stream are disclosed. In the systems, the concentrated liquid output from the high pressure side of a reverse osmosis stage is used as the draw solution in the low pressure side of the reverse osmosis stage in a configuration called osmotically assisted reverse osmosis. This reduces the osmotic pressure differential across the membrane, permitting high solute concentrations to be obtained, hastening the purification of the liquid. Reduced system pressures are also obtained by arranging multiple osmotically assisted reverse osmosis stages in a cross-current arrangement. Overall system energy consumption is reduced compared to conventional thermal processes for high concentration streams.
1. A method for purifying a liquid feed stream to obtain a permeate performed under a system pressure, comprising:
feeding the liquid feed stream to a system comprising:
(a) an initial reverse osmosis stage having: an initial high pressure side with an initial high pressure side inlet for receiving the liquid feed stream and an initial high pressure side outlet for outputting an initial recycle stream and an initial concentrate stream, and an initial low pressure side with an initial low pressure side inlet for receiving the initial recycle stream from the initial high pressure side outlet, and an initial low pressure side outlet for outputting a diluted stream, wherein the initial high pressure side outlet is split to form the initial recycle stream that is fed to the initial low pressure side inlet, and the initial concentrate stream;
(b) an intermediate section having one or more intermediate reverse osmosis stages arranged in series, wherein each intermediate stage has: an intermediate high pressure side with an intermediate high pressure side inlet and an intermediate high pressure side outlet, an intermediate low pressure side with an intermediate low pressure side inlet and an intermediate low pressure side outlet, an intermediate recycle stream and an intermediate concentrate stream exiting the intermediate high pressure side outlet, the intermediate recycle stream being sent to the intermediate low pressure side inlet, and an exit stream exiting the intermediate low pressure side outlet; and wherein the intermediate section receives the diluted stream from the initial reverse osmosis stage and outputs a purified stream, wherein the intermediate high pressure side outlet is split to form the intermediate recycle stream that is fed to the intermediate low pressure side inlet, and the intermediate concentrate stream; and
(c) a final reverse osmosis stage having: a final high pressure side having a final high pressure side inlet for receiving the purified stream and a final high pressure side outlet for outputting a reject stream, and a final low pressure side that outputs the permeate.
2. The method of claim 1 , wherein the reject stream of the final reverse osmosis stage is combined with the diluted stream from the initial reverse osmosis stage and received by the intermediate section.
3. The method of claim 2 , further comprising increasing the pressure of the diluted stream prior to combining the diluted stream with the reject stream.
4. The method of claim 2 , wherein a solute concentration of the diluted stream is about equal to a solute concentration of the reject stream prior to combining.
5. The method of claim 1 , wherein the intermediate section has a first intermediate reverse osmosis stage and a second intermediate reverse osmosis stage,
the first intermediate reverse osmosis stage receives the diluted stream from the initial reverse osmosis stage,
the intermediate high pressure side of the second intermediate reverse osmosis stage receives the exit stream of the first intermediate reverse osmosis stage,
the purified stream is the exit stream of the second intermediate reverse osmosis stage; and
the reject stream of the final reverse osmosis stage is combined with the exit stream of the first intermediate reverse osmosis stage and received by the second intermediate reverse osmosis stage.
6. The method of claim 1 , wherein the method of purifying a liquid to obtain the permeate is performed under a maximum system pressure of 300 psig.
7. The method of claim 1 , wherein the liquid feed stream has a solute concentration of about 3 wt % to about 12 wt %; and the concentrate stream of the initial reverse osmosis stage has a higher solute concentration of about 7 wt % to about 25 wt %.
8. The method of claim 1 , wherein the system further comprises a feed pump for pressurizing the liquid feed stream received by the initial reverse osmosis stage, an interstage pump for pressurizing the diluted stream that is received by the intermediate section, and a final pump for pressurizing the purified stream received by the final reverse osmosis stage.
9. The method of claim 8 , wherein the operating pressures of the feed pump, the interstage pump, and the final pump are within 50 psig of each other.
10. The method of claim 8 , wherein the operating pressures of the interstage pump and the final pump are about equal.
11. The method of claim 8 , wherein the operating pressure of the interstage pump is at least 50 psig less than the operating pressures of the feed pump and the final pump.
12. The method of claim 1 , wherein a solute removed from the liquid feed stream is salt, fluoride, boron, ammonia, or a nitrate.
13. The method of claim 1 , further comprising pretreating the liquid feed stream prior to feeding the liquid feed stream to the system.
14. The method of claim 13 , wherein pretreating the liquid feed stream includes performing capacitive deionization or forward osmosis on the liquid feed stream.
15. The method of claim 14 , further comprising applying ultrasonic frequencies to the pretreatment concentrate stream generated as a result of pretreating the liquid feed stream.