Membrane and electrodialysis based seawater desalination with salt, boron and gypsum recovery

Abstract

A method of producing desalinated water and recovering minerals from the feedwater uses membrane separation and electrodialysis brine concentration. This process can recover all of the minerals as high purity industrial minerals, including capturing the calcium and sulfate as agricultural grade gypsum and boron as high purity boric acid. In addition the process allows the use of low cost lime or dolime to produce valuable magnesium hydroxide.

Claims

1. A system, comprising: a nanofiltration (NF) unit configured to separate seawater into a first non-permeate and a first permeate, wherein the first non-permeate comprises first minerals and the first permeate comprises second minerals; a first stage mineral recovery unit disposed downstream from and fluidly coupled to the NF unit, wherein the first stage mineral recovery unit is configured to receive the first non-permeate and to capture at least a portion of the first minerals; a second stage mineral recovery unit disposed downstream from and fluidly coupled to the NF system and the first stage mineral recovery unit, wherein the second stage mineral recovery unit is configured to capture second minerals, wherein the second stage mineral recovery unit comprises an electrodialysis (ED) system configured to receive at least a portion of the first permeate and to generate a diluate and a concentrate from the first permeate, and the concentrate comprises the second minerals, wherein the ED system comprises at least one non-selective ED unit; a first flow path fluidly coupling the first stage mineral recovery unit and a dolime source, wherein the first flow path is configured to supply the first stage mineral recovery unit with dolime such that at least one of the first minerals recovered in the first stage mineral recovery unit is magnesium hydroxide; and a first reverse osmosis (RO) system fluidly coupled to the ED system, wherein the first RU system is configured to receive the concentrate from the ED system and to generate a first brine and a second permeate.

2. The system of claim 1, comprising a second flow path between the first RO system and a second RO system, wherein the second flow path directs the second permeate from the first RO system to the second RO system, and wherein the second RO system is fluidly coupled to the NF unit and the second stage mineral recovery unit.

3. The system of claim 2, wherein the second RO system comprises a plurality of RO units, wherein at least one RO unit of the plurality of RO units is fluidly coupled to the NF unit and a second RO unit of the plurality of RO units is fluidly coupled to the ED system.

4. The system of claim 2, comprising a brine tank fluidly coupled to the second RO system and to the second stage mineral recovery unit, wherein the brine tank is configured to receive a second brine from the second RO system and to supply the second brine to the ED system.

5. The system of claim 1, wherein the ED system comprises a plurality of ED units, wherein at least one ED unit of the plurality of ED units comprises a monovalent cation selective permeable membrane.

6. The system of claim 5, wherein at least one ED unit of the plurality of ED units is fluidly coupled to the first stage mineral recovery unit.

7. The system of claim 1, comprising a brine tank disposed within the second stage mineral recovery unit and fluidly coupled to the ED system, wherein the brine tank is configured to receive the concentrate from the ED system.

8. A system, comprising: a nanofiltration (NF) unit configured to separate seawater into a first permeate and a first non-permeate, wherein the first non-permeate comprises first minerals and the first permeate comprises second minerals; a first stage mineral recovery unit disposed downstream from and fluidly coupled to the NF unit, wherein the first stage mineral recovery unit is configured to receive the first non-permeate from the NF unit and to capture at least a portion of the first minerals; a second stage mineral recovery unit disposed downstream from and fluidly coupled to the NF unit, wherein the second stage mineral recovery unit is configured to receive at least a portion of the first permeate and to capture the second minerals, the second stage mineral recovery unit comprises a multi-stage electrodialysis (ED) system comprising a first ED unit and a second ED unit, the first ED unit is configured to separate the first permeate into a diluate and a concentrate, the concentrate comprises the second minerals, wherein at least one of the first ED unit, the second ED unit, or both is a non-selective ED unit; a first reverse osmosis (RO) system fluidly coupled to the second stage mineral recovery unit, wherein the first RO system is configured to receive the diluate from the first ED unit and to generate a first brine and a second permeate; and a first flow path between the first RO system and the second ED unit, wherein the first flow path is configured to supply the first brine to the-second ED unit.

9. The system of claim 8, comprising a second flow path fluidly coupling the first stage mineral recovery unit and a dolime source, wherein the second flow path is configured to supply the first stage mineral recovery unit with dolime such that at least one of the first minerals recovered in the first stage mineral recovery unit is magnesium hydroxide.

10. The system of claim 8, comprising a second RO system fluidly coupled to the NF unit, the first RO system, and the second stage mineral recovery unit, wherein the second RO system is configured to receive the third second permeate from the first RO system and at least a portion of the first permeate from the NF system, to generate desalinated water and a second brine stream, and to supply the second brine stream to the multi-stage ED system.

11. The system of claim 8, comprising an air stripper fluidly coupled to the NF unit, wherein the air stripper is configured to remove carbon dioxide from the seawater.

12. The system of claim 8, wherein at least one of the first ED unit or the second ED unit comprises at least one monovalent cation selective permeable membrane or at least one anion selective permeable membrane.

13. The system of claim 8, comprising a third flow path between the first stage mineral recovery unit and the second stage mineral recovery unit, wherein the third flow path is configured to direct at least a portion of the concentrate from the first ED unit to the first stage mineral recovery unit.

14. The system of claim 8, comprising a brine tank fluidly coupled to the second stage mineral recovery unit, wherein the brine tank is configured to receive and to supply the first permeate to the second stage mineral recovery unit.

15. The system of claim 8, comprising a brine tank disposed within the second stage mineral recovery unit and fluidly coupled to the multi-stage ED system, wherein the brine tank is configured to receive the concentrate from the multi-stage ED system.

16. A system, comprising: a first stage mineral recovery unit disposed downstream from and fluidly coupled to a nanofiltration (NF) unit, wherein the first stage mineral recovery unit is configured to receive a first non-permeate having first minerals from the NF unit and to capture at least a portion of the first minerals; a first flow path fluidly coupling the first stage mineral recovery unit and a dolime source, wherein the first flow path is configured to supply the first stage mineral recovery unit with dolime such that at least one of the first minerals recovered in the first stage mineral recovery unit is magnesium hydroxide; a second stage mineral recovery unit disposed downstream from the NF unit and the first stage mineral recovery unit, wherein the second stage mineral recovery unit is configured to capture second minerals from a first permeate generated by the NF unit, and the second stage mineral recovery unit comprises a multi-stage electrodialysis (ED) system, wherein the multi-stage ED system comprises at least one non-selective ED unit; and a first reverse osmosis (RO) system disposed downstream from and fluidly coupled to the second stage mineral recovery unit, wherein the first RO system is configured to receive a diluate generated by the multi-stage ED system and to generate a first brine and a second permeate.

17. The system of claim 16, comprising a second RO system fluidly coupled to the NF unit, the second stage mineral recovery unit, and the first RO system, wherein the second RO system is configured to receive the second permeate, to generate a second brine and desalinated water, and to direct the second brine to the second stage mineral recovery unit.

18. The system of claim 16, comprising a second flow path between the first RO system and the second stage mineral recovery unit, wherein the second flow path directs the first brine to the multi-stage ED system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a standalone desalination system utilizing EDR-RO integration for mineral recovery.

(2) FIG. 2 is a detail of the reverse osmosis (RO) loop.

(3) FIG. 3 is a diagram of an alternative standalone desalination system incorporating mineral recovery.

(4) FIG. 4 is an example diagram for incorporating the mineral recovery system of the subject invention is an existing desalination plant.

DETAILED DESCRIPTION

(5) The following abbreviations and acronyms are used throughout the application:

(6) BWRO: brackish water reverse osmosis

(7) ED: electrodialysis

(8) ED(R): electrodialysis reversal

(9) g/l: grams per liter

(10) MGD: million gallons per day

(11) MVR: mechanical vapor recompression

(12) NF: nano filtration

(13) RO: reverse osmosis

(14) SWRO: seawater reverse osmosis

(15) sTPD: short tons per day

(16) TDS: total dissolved solids

(17) Standalone Systems

(18) Referring to FIG. 1 of the drawings, seawater or brackish water at input 12 is routed to a solids removal pretreatment section 10 which uses a combination of settling, filtration, and ultra-filtration to produce a feedwater stream essentially free of suspended solids. The pH of the feed water is adjusted to pH 6-6.5 as part of the pretreatment process using hydrochloric acid, sulfuric acid or caustic as necessary, as indicated at 14. The pH adjustment minimizes soluble silica in the feedwater to prevent downstream membrane fouling. Coagulants, flocculants and periodic disinfectant (NaOCl, chlorine) addition may be required in the pretreatment section depending on the feedwater quality. Solid amendment/landfill solids are released as indicated at 16. The pretreated feed water is preheated using low level heat to 85-113.degree. F. (preferably above 100.degree. F.) and additional hydrochloric or sulfuric acid is added as indicated at 24 to reduce the pH to 4 converting essentially all of the bicarbonate to free C02.

(19) A multistage air or nitrogen stripper 22 is then used to reduce the C0.sub.2 content in the feed water to below 5 ppm (preferably below 1 ppm). Mol sieves may be optionally used to reduce the C0.sub.2 content in the stripping air.

(20) Sodium bisulfite is added to the stripped feed water, as indicated at 26, to neutralize any residual hypochlorite, or hypobromite and the water is fed to a nanofiltration unit 28. The nanofiltration unit removes nearly all (90-99%) of the magnesium, calcium, and sulfate in a non-permeate stream, as indicated at 30 and removes very little of the boron. This stream is processed for minerals at the mineral recovery unit 32. An example of a mineral recovery system is shown and described in co-pending patent application Ser. No. 12/818,740, filed on Jun. 18, 2010, entitled: Zero Discharge Water Desalination Plant with Minerals Extraction Integrated with Natural Gas Combined Cycle Power Generation, which is fully incorporated by reference herein. The mineral recovery unit, or MVR 32, produces agricultural grade gypsum from the sulfate and uses lime or dolime to precipitate high purity magnesium hydroxide. The MVR 32 comprises a non-selective anion and cation membrane with optional downstream diluate RO. Excess calcium is removed from the NF brine stream as a calcium chloride brine, flake or pellet product. A release stream 33 from MVR 32 comprises condensate at 2 MOD.

(21) The NF permeate at 34 and the purge from the NF non-permeate minerals recovery system at 36 containing the monovalent salts is adjusted to pH 9-10 using caustic and routed to a seawater RO system 38. The elevated pH and temperature allow optimized flux and boron removal with low pressure drop RO membranes. Multiple stages of permeate and nonpermeate RO may be used depending on the feed water quality and desal water purity requirement.

(22) The reject brine stream from the SWRO system at 40, preferably with 70-80 g/l TDS, is routed to an electrodialysis or electrodialysis reversal ED(R) unit 42, which operates at pH 9-10. The elevated pH converts most of the uncharged boric acid to monovalent borate, and the uncharged silica to silicate, allowing the ED(R) to capture most of these ions in the concentrate stream. The ED(R) diluate stream, at 44, preferably with 20 g/l TDS is pH adjusted to 9-10 using caustic and routed to a BWRO system 46, which produces desalinated water. The release stream 39 from SWRO 38 is RO permeate at 18 MGD.

(23) The elevated pH and temperature allow optimized flux and boron removal. Multiple stages of permeate and non-permeate may be used depending on the feed water quality and desal water purity requirement. The BWRO membrane allows a lower pressure drop and higher recovery than the SWRO membranes, since it is optimized for the lower TDS in the feed and reject brine.

(24) The reject brine from the BWRO system is recycled back to ED(R) unit, as indicated at 48. Silica and boron do not build up in the recycle loop since most of the silica and boron fed to the ED(R) 42 is rejected with the concentrate due to the elevated pH.

(25) ED(R) concentrate at 50, preferably with 230 g/l TDS, is routed to a brine storage tank 52 which facilitates off-peak salt production from the concentrate using mechanical vapor recompression (MYR) evaporators and crystallizers. The brine storage also allows the desalination plant to continue to operate when the downstream MVR units 54 experience a forced or planned outage. Brine storage is also provided within the NF non-permeate minerals recovery system to allow desalination plant operation when one of the components in the NF non-permeate minerals recovery system is down.

(26) The ED(R) concentrate stream at 56, from the brine tank, is processed for minerals recovery as explained in aforementioned co-pending application Ser. No. 12/818,740. The MVR 54 produces high purity salt and recovers boron as high purity boric acid.

(27) The RO detail is shown in FIG. 3. The NF permeate at 34 is introduced into the RO 38 as indicated in FIGS. 1 and 2, and is at a temperature of about 104? F. The RO 38 typically comprises the SWRO 90, the BWRO 96 and the SWRO 98. The stream at 34 is at pH 6 which is mixed with caustic and recycle stream 100 to increase the pH to 8.9. This is introduced into RO 90 which operates at pH 8-9. A primary release stream 97 is desalinated water. The secondary stream 93 is at pH 8-9 and is introduced into SWRO 98. The outlet stream 99 is combined with the primary release stream 97 and introduced into BWRO 96, which is at pH 10. A bypass loop 94 is also provided. The outlet stream from BWRO is at 39, as also indicated in FIGS. 1 and 2 as the outlet from SWRO system 38. Stream 100 from BWRO 96 is reintroduced into stream 34. Outlet stream 40 is introduced into ED 42 in FIG. 1 and brine tank 52 in FIG. 2.

(28) Retrofit System

(29) Another embodiment of this invention is used as a retrofit on all or a portion of the reject brine stream of an existing desalination plant, as shown in FIG. 4. The reject brine feed stream 110 is adjusted to pH 6-6.5 using hydrochloric acid, sulfuric acid or caustic and is filtered (cartridge filter, or ultrafiltration) to remove any suspended solids, as indicated at the pretreatment unit 10. Then, as with the previously described embodiments, stream 18 is heated at preheater 20 and introduced into the air stripper 22. The NF 28, MVR 32, ED 42, BWRO 46, brine tank 52 and MVR 54 perform as previously described.

(30) The residual filter purge solid stream is routed back to the RO unit solids disposal line (not shown) or to a solids settling and filtration system included in the pretreatment section that produces byproduct solids. The pH adjustment minimizes soluble silica in the feedwater to prevent downstream membrane fouling.

(31) The feed water stream is processed the same as the standalone embodiment except the seawater RO block 38 is not used. The elevated TDS NF permeate stream 34 is fed directly to the ED(R) unit 42. The flow scheme downstream of the ED(R) unit is the same as the standalone embodiment. No streams are recycled to the existing desalination plant.

(32) Calcium and Magnesium Purge System

(33) Another embodiment of this invention is used to produce a salt brine with a further reduced calcium and magnesium content, using higher efficiency ED membranes for the bulk of the salt concentration, and follows the configuration of FIG. 2.

(34) This embodiment is substantially identical to the embodiment of FIG. 1 from the sea water intake at 12 through the SWRO unit 38. However, in this embodiment the reject brine stream 40 from the SWRO 38 is first introduced into the brine tank 52 and from there into ED 60, which comprises a monovalent selective cation membrane for Na and K passage and a non-selective anion membrane for Cl, Br, B(OH).sub.4, CO3, and SO4 passage. This releases a stream 62 at 7.7 MGD and 20 g/l TDS into the RO 64, and a second stream 80 at 2.5 MGD and 270 g/l TDS for introduction into the MVR 54. The RO produces a stream at 66 which is at 1.9 MGD and 70 g/l TDS. This is introduced into a series of ED's 68, 72 and 76. The release streams from these ED's is as follows:

(35) ED 68: stream 70 at 1.5 MGD and 15 g/l TDS; stream 71 at 0.4 MGD and 270 g/l TDS.

(36) ED 72: stream 74 at 1.2 MGD and 70 g/l TDS; stream 75 at 0.35 MGD and 270 g/l TDS.

(37) ED 76: stream 77 at 1.1 MGD and 0.2 g/l TDS; stream 78 at 0.1 MGD and 10 g/l TDS.

(38) Stream 75 is fed back into the first minerals recovery system 32.

(39) Streams 71 and 78 are combined with stream 80 from ED 60 to form combination stream 82, which includes Na, K, SO.sub.4, HCO.sub.3, B(OH).sub.4, and is at a pH 6-7. This is introduced into the MVR 54 for recovering high purity salt at approximately 3300 s TPD, high purity potassium chloride at 100 s TPD, high purity bromine at 8 s TPD and high purity Boric acid at 2 s TPD. The release stream 55 from MVR 54 comprises condensate at 3.5 MGD

(40) ED 60, and ED 68 comprise a monovalent selective cation membrane for passing Na and K and a non-selective anion membrane for Cl, SO4, HC0.sub.3, Br passage. ED 72 comprises a monovalent selective anion membrane for Cl passage and a non-selective cation membrane for Na, Ca and Mg passage. ED 76 comprises a non-selective anion and cation membrane with optional downstream diluate RO (with the RO brine recycled back to ED).

(41) First Stage ED 60:

(42) The reject brine 40 from the RO unit is pH adjusted with HCl to pH 8 and is fed to a brine tank 52 that is filled during on-peak power price conditions (typically 8 hours per day) and emptied during off-peak power conditions. During off-peak power conditions RO brine from the tank is fed to the first stage ED 60 which is equipped with monovalent cation permselective membranes and monodivalent anion permselective membranes. The first stage ED 60 is operated at pH 8-8.5 which allows higher efficiency anionic ED membranes, which are not compatible with high pH operation. Most (>85-98%) of the calcium and magnesium, boric acid and silica remain in the diluate stream. Most (70-90%) of the salt, potassium chloride, sodium bromide are extracted into the brine concentrate stream.

(43) The first stage ED brine concentrate is routed to an evaporation section where salt, potassium chloride, bromine and boric acid are recovered from the brine. The first stage ED diluate is pH adjusted to pH 8 with HCl and is routed to a two stage RO unit.

(44) Two Stage RO Unit 64:

(45) The first stage of RO 38 removes essentially all (>99%) of the dissolved salts (NaCl, KCl, NaBr, CaHC0.sub.3, MgHC0.sub.3) and most of the boric acid (75-85%). The permeate from the first stage is pH adjusted to pH 10 with caustic and fed to the second stage RO. Operation of the second stage at pH 10 enables near complete removal (98%) of the residual boric acid as sodium borate. Calcium or magnesium scale does not form in the second stage due to the low residual concentrations of calcium, and magnesium. The second stage RO brine stream 66 is pH adjusted to 8 with HCl and recycled to the first stage RO 38. The second stage RO permeate 65 is routed to the desalinated product water.

(46) The first stage RO brine is fed to the second stage ED unit 68. The first and second stage RO units remove approximately 65% of the water from the first stage ED diluate as desalinated product water. During on-peak operation the primary ED unit does not produce diluate (internal recirculation only) and a portion (10-20%) of the seawater RO feed is routed to the RO unit. This minimizes on-peak RO power consumption due to the increased membrane area, and enables continuous operation of both seawater and ED diluate RO units.

(47) Second Stage ED 68:

(48) The first stage RO brine at pH 8 is fed to the second stage ED 68 which uses the same membrane types as the first stage ED 60. The second stage ED concentrate brine is also routed to the evaporation section with the first stage ED brine.

(49) Third Stage ED 72:

(50) The second stage ED diluate 70 is routed to the third stage ED unit 72 which also operates at a pH 8 and uses monodivalent cation permselective membranes and mono anion permselective membranes. The third stage ED concentrate stream contains calcium, magnesium, and sodium chloride is routed to the magnesium and calcium recovery section of the plant. The diluate stream from the third stage ED diluate stream containing sodium chloride and residual sulfate, boron (as undissociated boric acid), and silica is pH adjusted to 9.5-10.5 using caustic and routed to the fourth stage ED unit.

(51) Fourth Stage ED 76:

(52) The fourth stage ED unit 76 uses monodivalent cation permselective membranes and monodivalent anion permselective membranes to produce a desal product water diluate stream. Optionally the desal product water diluate can be routed to a polishing RO unit to further reduce the sodium borate content in the product desalinated water, with the RO brine recycled back to the fourth stage ED unit 76. The fourth stage ED concentrate stream 78 containing mainly sodium chloride, sodium borate and sodium silicate is routed to the evaporation section, MVR 54.

(53) Optionally an ion exchange resin operating at pH 9-10 (not shown in FIG. 2) can be added upstream of the fourth stage ED to capture boric acid and the silica rich fourth stage ED concentrate stream 78 recycled to the inlet pretreatment section. The boric acid purge stream from the ion exchange resin is pH adjusted to 6-7 with NaOH and fed separately into the minerals recovery section for boric acid recovery, as explained in aforementioned co-pending application Ser. No. 12/818,740. This reduces the silica and boric acid loading in the MVR and increases the salt content in the MVR feed.

(54) Key Benefits

(55) The key benefits of this invention are summarized below:

(56) 1) Uses a dedicated low pressure drop brackish water RO at pH 9-10. to produce high quality desalinated water from ED(R) diluate at lower energy consumption than seawater RO membranes.

(57) 2) Provides zero discharge seawater desalination without buildup of boron which can cause contamination of the desalinated water.

(58) 3) Provides zero discharge seawater desalination without buildup of silica which can cause membrane scaling and fouling.

(59) 4) Does not require specialty monovalent ED(R) membranes.

(60) 5) Can be used to as a retrofit system on an existing RO plant. The system would divert all or a portion of the RO reject brine stream from disposal to an ED(R)/BWRO based system that uses an acidic air stripping, NF and Katana NF brine mineral recovery system to pretreat the ED(R) feed.
6) Produces desalinated water low in boron with the option to recover boron as byproduct boric acid.
7) Recovers sulfate as an agricultural grade gypsum byproduct.
8) Recovers essentiality all of the magnesium as high purity magnesium hydroxide (without boron or gypsum contamination) using low cost dolime to precipitate the magnesium hydroxide.
9) Does not produce a mixed calcium carbonate, magnesium hydroxide softener sludge for disposal. Essentially all of the calcium is recovered as either agricultural gypsum or byproduct calcium chloride.

(61) Additional Benefits of the Calcium and Magnesium Purge Embodiment

(62) 1) Produces a salt brine with a lower CaCl.sub.2 and MgCl.sub.2 content which reduces the boiling point elevation in the salt crystallizer. This reduces the capital cost and power consumption of the salt crystallizer. The salt brine can also be cost effectively polished before evaporation with cation exchange resins to essentially eliminate the residual Ca and Mg from the salt brine. This enables the production of high value, high purity, chemical grade salt (>99.995% purity).
2) Uses a mono and divalent anion permselective ED membranes and monovalent cation permselective ED membranes which reduces membrane resistance by 50% versus the monoselective anion and cation permselective ED membranes typically used for salt brine production. This also reduces gypsum and calcium carbonate fouling since sulfate, bicarbonate and carbonate is directed to the concentrate and calcium is directed to the diluate, preventing calcium sulfate, or calcium carbonate concentration increase and gypsum or carbonate scaling in the ED unit.
3) Enables economical use of boric acid ion exchange resin at optimal pH of 9-10 on a small stream (<5% of feed) with elevated boric acid content (20 times feed seawater). This concentrates the boric acid into a high concentration (2 wt % boric acid) small 0.04 MGD purge stream suitable for boric acid recovery.

(63) While certain features and embodiments of the invention have been described in detail herein it should be understood that the invention encompasses all modifications and enhancements within the scope and spirit of the attached claims.