PURIFICATION OF HIGHLY SALINE FEEDS

Abstract

A process for separating solvent from a feed solution, said process comprising contacting the feed solution with one side of a semi-permeable membrane, applying hydraulic pressure to the feed solution, such that solvent from the feed solution flows through the membrane by reverse osmosis to provide a permeate solution on the permeate-side of the membrane, separating solvent from the permeate solution to provide a stream comprising the solvent and a residual solution having an increased osmotic pressure than the permeate solution, and recycling the residual solution to the permeate-side of the semi-permeable membrane, whereby the osmotic pressure on the permeate-side of the semi-permeable membrane is lower than the osmotic pressure of the feed solution.

Claims

1. A process for separating solvent from a feed solution, said process comprising: contacting the feed solution with one side of a semi-permeable membrane, applying hydraulic pressure to the feed solution, such that solvent from the feed solution flows through the membrane by reverse osmosis to provide a permeate solution on the permeate-side of the membrane, separating solvent from the permeate solution to provide a stream comprising the solvent and a residual solution having an increased osmotic pressure than the permeate solution, and recycling the residual solution to the permeate-side of the semi-permeable membrane, whereby the osmotic pressure on the permeate-side of the semi-permeable membrane is lower than the osmotic pressure of the feed solution.

2. The process as claimed in claim 1, wherein the residual solution from the residual-side of the membrane is withdrawn from the residual-side of the membrane, and wherein a portion of the withdrawn solution is recycled as part of the feed solution to the membrane.

3. The process as claimed in claim 1, wherein solvent is separated from the permeate solution by: contacting the permeate solution with one side of a second semi-permeable membrane, and applying hydraulic pressure to the permeate solution, such that solvent from the permeate solution permeates through the second semi-permeable membrane by reverse osmosis to provide a residual solution having an increased osmotic pressure on the retentate-side of the second semi-permeable membrane and a second permeate solution on the permeate-side of the second semi-permeable membrane.

4. The process as claimed in claim 3, wherein the residual solution is recycled from the retentate-side of the second semi-permeable membrane to the permeate-side of first semi-permeable membrane.

5. The process as claimed in claim 3, which further comprises contacting the second permeate solution with one side of a further semi-permeable membrane, and applying hydraulic pressure to the second permeate solution, such that solvent from the second permeate solution permeates through the further semi-permeable membrane by reverse osmosis to provide a further residual solution on the retentate-side of the further semi-permeable membrane and a further permeate solution on the permeate-side of the further semi-permeable membrane.

6. The process as claimed in claim 5, wherein the further residual solution is recycled from the retentate-side of the further semi-permeable membrane to the permeate-side of first semi-permeable membrane and/or the permeate-side of the second semi-permeable membrane.

7. The process as claimed in claim 1, wherein, prior to contact with said semi-permeable membrane(s), the permeate solution is introduced into a reservoir from which permeate solution may be drawn and contacted with said semi-permeable membrane(s) at a pre-determined rate.

8. The process as claimed in claim 1, wherein, prior to being recycled to the permeate-side of said semi-permeable membrane(s), the osmotic pressure of the residual solution(s) is adjusted.

9. The process as claimed in claim 1, wherein the hydraulic energy on the retentate side of any of the semi-permeable membranes employed is recovered.

10. The process as claimed in claim 8, wherein said adjustment occurs by the addition of osmotic agent to the residual solution.

11. The process as claimed in claim 10, wherein the osmotic agent is selected from a salt.

12. The process as claimed in claim 1, wherein, prior to being recycled to the permeate-side of said semi-permeable membrane(s), a portion of the residual fluid is discarded or treated to balance the salt passage between the first membrane and the second membrane.

13. The process as claimed in claim 1, wherein the osmotic pressure of the feed solution is in excess of 30 bar at 25 degrees C.

14. The process as claimed in claim 1, wherein the osmotic pressure difference across the semi-permeable membrane(s) is within 5 to 70 bar.

15. The process as claimed in claim 1, wherein the feed solution is a salt solution.

16. The process as claimed in claim 1, wherein the semi-permeable membrane(s) are reverse osmosis membranes or nanofiltration membranes.

17. The process as claimed in claim 1, wherein a feed comprising solid osmotic agent or a feed solution comprising osmotic agent is added to the permeate-side of at least one of the semi-permeable membranes.

18. The process as claimed in claim 17, wherein a feed solution comprising osmotic agent is contacted with the permeate-side of the semi-permeable membrane as a draw solution to initiate the reverse osmosis step.

19. The process as claimed in claim 17, wherein the feed solution comprising osmotic agent is formed by dissolving an osmotic agent in water.

20. The process as claimed in claim 1 wherein solvent is separated from the permeate solution by using any thermal separation method and to provide a residual solution having an increased osmotic pressure to be recycled to the permeate side of the first semi-permeable membrane.

Description

[0036] These and other aspects of the present invention will now be described with reference to the accompanying figures, in which:

[0037] FIG. 1 is a schematic diagram of a system for carrying out a first embodiment of the process of the present invention;

[0038] FIG. 2 is a schematic diagram of a system for carrying out a second embodiment of the process of the present invention;

[0039] FIG. 3 is schematic diagram of a system for carrying out a third embodiment of the process of the present invention;

[0040] FIG. 4 is identical to FIG. 1 except that the process streams are annotated using the annotations used in Table 1 of the Examples;

[0041] FIG. 5 is identical to FIG. 1 except that the process streams are annotated using the annotations used in Table 1 of the Examples;

[0042] FIG. 6 is a schematic diagram of a system for carrying out the reverse osmosis process of Example 1 showing the annotations used in Table 1 of the Examples; and

[0043] FIG. 7 is a schematic diagram of the process of FIG. 1 with an additional recycle stream.

[0044] Referring to FIG. 1, this diagram depicts a system 10 comprising a first reverse osmosis unit comprising a first semi-permeable membrane 12 and a second reverse osmosis unit comprising a second semi-permeable membrane 14.

[0045] In operation, feed water 16 is contacted with one side of the first semi-permeable membrane 12. Hydraulic pressure is applied using pump 18, causing water to flow through the first semi-permeable membrane 12 by reverse osmosis. The permeate solution 20 on the permeate-side of the first semi-permeable membrane is withdrawn from the first reverse osmosis unit via line 22 and contacted with one side of the second semi-permeable membrane 14. Hydraulic pressure is applied via pump 24, such that water from the solution permeates through the second semi-permeable membrane 14 by reverse osmosis. This provides a residual solution 26 having an increased osmotic pressure on the retentate-side of the membrane 14 and a second permeate solution 28 on the permeate-side of the second semi-permeable membrane. The residual solution 26 on the retentate-side of the membrane 14 is recycled via line 30 to the permeate-side of the first semi-permeable membrane 12. As the average osmotic pressure on the permeate-side of the first semi-permeable membrane 12 is lower than the average osmotic pressure of the feed water 16 and the reject water 34, hydraulic pressure is still required to induce water to flow across the first semi-permeable membrane 12 by reverse osmosis. However, the average osmotic pressure on the permeate-side of the first semi-permeable membrane 12 is higher than what it would be in the absence of the recycle via line 30. Accordingly, the hydraulic pressure required to be applied to the feed water 16 to maintain a predetermined flux across the semi-permeable membrane 12 is less than that which would be required in the absence of the recycle via line 30.

[0046] The permeate 28 through the second semi-permeable membrane 14 may be withdrawn as product water 32, while the retentate on the retentate-side of the first semi-permeable membrane 12 may be withdrawn as reject water 34. Optionally, a portion of the reject water 34 may be recycled as feed to the membrane 12 (not shown), for example, by pump 18. This can increase the concentration of the feed water that is contacted with the membrane 12 e.g. above a threshold value and increase the recovery rate. This can improve flow distribution across membrane 12. Optionally, as shown in FIG. 7, a portion 38 of the reject water 34 may be recycled by pump 40 as a portion 42 of the feed to membrane 12. This can increase the concentration of the feed water that is contacted with the membrane 12 e.g. above a threshold value and increase the recovery rate. This can improve flow distribution across membrane 12.

[0047] The embodiment of FIG. 2 depicts a system 100 that is similar to that of FIG. 1. Like numerals have been used to label like parts. In the system 100 of FIG. 2, however, the permeate 28 through the second semi-permeable membrane 14 is withdrawn via line 132 and this is contacted with a third semi-permeable membrane 134. Hydraulic pressure is applied to the permeate via pump 136 such that water flows across the membrane 134 by reverse osmosis. The permeate 138 through the third semi-permeable membrane 134 is withdrawn as product water, while the retentate 140 on the retentate-side of the third semi-permeable membrane 134 is recycled via line 142 to the permeate-side of the second semi-permeable membrane 14. As the average osmotic pressure on the permeate-side of the second semi-permeable membrane 14 is lower than the average osmotic pressure on the retentate-side of the membrane 14, hydraulic pressure is still required to induce water to flow across the semi semi-permeable membrane 12 by reverse osmosis. However, the average osmotic pressure on the permeate-side of the second semi-permeable membrane 14 is higher than what it would be in the absence of the recycle via line 142. Accordingly, the hydraulic pressure required to be applied to the permeate from the first semi-permeable membrane in line 22 to maintain a predetermined flux across the semi-permeable membrane 14 is less than that which would be required in the absence of the recycle via line 142.

[0048] The embodiment of FIG. 3 depicts a system 200 that is similar to that of FIG. 1. Like numerals have been used to label like parts. In the embodiment of FIG. 3, however, the permeate 20 from the first semi-permeable membrane is introduced via line 22 to a reservoir 210 from which solution may be drawn and contacted with the second semi-permeable membrane 14 at a pre-determined rate. Furthermore, lines 212 and/or 214 may be provided to withdraw a portion of the residual solution 26 (e.g. as bleed) or to treat the residual solution to balance the salt passage between the first semi-permeable membrane 12 and the second semi-permeable membrane 14. The addition of osmotic agent 214 allows the start-up of the process in particular where the osmotic pressure of the feed solution 16 is greater than the maximum hydraulic pressure that can be applied to the semi-permeable membrane 12, by lowering the differential osmotic pressure between the feed solution 12 and the permeate solution 22.

EXAMPLES

[0049] In the following non-limiting Examples, we consider three processes for the desalination of a feed water stream consisting of a sodium chloride solution using reverse osmosis. The maximum hydraulic pressure that can be applied to the reverse osmosis membrane is 69 barg (Dow Filmtec membrane SW30HR-380). The process parameters are summarised in Table 1, with all values being approximate.

[0050] Example 1 considers the situation where the feed water has a concentration of sodium chloride of 80,000 mg/l. This solution is contacted with a reverse osmosis membrane and a hydraulic pressure of 69 barg is applied (i.e. the maximum hydraulic pressure that the reverse osmosis membrane can withstand). The process scheme for this reverse osmosis process is shown in FIG. 6. Because the feed water has a calculated value of osmotic pressure of 70.7 bar at 25 degrees Celsius, there is no solvent flow. This is tabulated in Table 1 below.

[0051] Example 2 is an example of an embodiment of the invention operated in accordance with the system depicted in and described with reference to FIG. 1. FIG. 4 is identical to FIG. 1 except that the process streams are annotated using the annotations used in Table 1 below. The feed water composition is the same as that used in Example 1. The results are tabulated in Table 1 below. As can be seen from the Table, desalination can be achieved despite the feed pressure to RO1 being less than the osmotic pressure of the feed water stream. In this case, stream d, may be initially prepared by the addition of a sodium chloride solution with a TDS of 36096 mg/l.

[0052] Example 3 illustrates a second embodiment of the invention operated in accordance with the system depicted in and described with reference to FIG. 2. FIG. 5 is identical to FIG. 2 except that the process streams are annotated using the annotations used in Table 1 below. In this case the feed solution has a concentration of sodium chloride of 120,000 mg/l, with an equivalent osmotic pressure of 116 bar at 25 degrees Celsius. In this case two steps are used to produce the solvent and/or concentrate the initial feed stream. In this case, stream d, may be initially prepared by the addition of a sodium chloride solution with a TDS of 68184 mg/l and stream g may be initially optionally prepared by the addition of a sodium chloride solution with a TDS of 15786 mg/l.

TABLE-US-00001 TABLE 1 Example No. 1 2 3 FIG. No. 6 4 5 Example Example of the of the A single invention invention reverse using two using three osmosis stages as stages as membrane shown in shown in only. FIGS. 1 FIGS. 2 Osmotic and 4. All and 5. All pressure of RO feed RO feed feed solution pressures pressures greater than below the below the maximum maximum maximum hydraulic set for this set for this pressure- example at example at Stream hence no 69 barg 69 barg Description No solvent flow (FIG. 4) (FIG. 5) Feed TDS as NaCl (mg/l) a 80000 80000 120000 Osmotic Pressure (bar) a 70.7 70.7 116 at 25 C. Feed flow (m3/h) a 100 100 100 Feed pressure to RO1 a 69 69 69 (barg) Solvent flow from RO1 b 0 40 23.3 (m3/h) Retentate flow from RO1 k 100 60 76.7 (m3/h) Retentate TDS as NaCl k 80000 133413 156516 (mg/l) from RO1 Retenate osmotic k 70.7 133.3 166.3 pressure (bar) from RO1 RO1 Concentrated c Not 65000 100000 Osmotic Agent TDS as applicable NaCl (mg/l) RO1 Concentrated c Not 50 50 osmotic agent flow applicable (m3/h) RO1 Dilute osmotic d Not 90 73.3 agent flow (m3/h) applicable RO1 Dilute osmotic d Not 36096 68184 agent TDS (mg/l) applicable Feed pressure to RO2 d Not 58 62.2 (barg) applicable Solvent flow from RO2 e Not 40 23.3 (m3/h) applicable RO2 Concentrated f Not Not 25000 Osmotic Agent TDS as applicable applicable NaCl (mg/l) RO2 Concentrated f Not Not 40 osmotic agent flow applicable applicable (m3/h) RO2 Dilute osmotic g Not Not 63.3 agent flow (m3/h) applicable applicable RO2 Dilute osmotic g Not Not 15786 agent TDS (mg/l) applicable applicable Feed pressure to RO3 g Not Not 18 (barg) applicable applicable Solvent flow from RO3 h Not Not 23.3 (m3/h) applicable applicable Overall system recovery 0% 40% 23.30% (Net solvent out/Feed flow in)