WO2017053281A1

WO2017053281A1 - Osmotically driven membrane processes and systems and methods for draw solute recovery

Info

Publication number: WO2017053281A1
Application number: PCT/US2016/052644
Authority: WO
Inventors: Zachary W. GOODMAN; Brian Gallagher; Eric Segal
Original assignee: Oasys Watere, Inc.
Priority date: 2015-09-21
Filing date: 2016-09-20
Publication date: 2017-03-30
Also published as: WO2017053281A8

Abstract

The invention relates to osmotically driven membrane processes and systems and methods for recovering draw solutes in the osmotically driven membrane processes. Osmotically driven membrane processes involve the extraction of a solvent from a first solution by using a second concentrated solution to draw the solvent from the first solution across a semi-permeable membrane. Draw solute recovery may be carried out by various means to recover and recycle draw solutes contained within a diluted second solution and obtain a product solvent.

Description

OSMOTICALLY DRIVEN MEMBRANE PROCESSES AND SYSTEMS AND

METHODS FOR DRAW SOLUTE RECOVERY

FIELD OF THE TECHNOLOGY

[0001] Generally, the invention relates to osmotically driven membrane processes and more particularly to draw solute recovery techniques for osmotically driven membrane processes.

BACKGROUND

[0002] In general, osmotically driven membrane processes involve two solutions separated by a semi-permeable membrane. One solution may be, for example, seawater, while the other solution is a concentrated solution that generates a concentration gradient between the seawater and the concentrated solution. This gradient draws water from the seawater across the membrane, which selectively permits water to pass, but not salts, into the concentrated solution. Gradually, the water entering the concentrated solution dilutes the solution. The solutes then need to be removed from the dilute solution to generate potable water. Traditionally, the potable water was obtained, for example, via distillation; however, the solutes were typically not recovered and recycled.

[0003] In certain prior art systems that use distillation and low grade heat to recover draw solutes, it is necessary to perform condensation and absorption steps under vacuum in an attempt to maximize draw solute recovery. For example, a knock-out pot and an eductor (using air as a driving medium) are disposed downstream of the condensation and/or absorption processes in an attempt to improve draw solute recovery. However, this arrangement requires the venting of the non-condensable gases, which can also result in a loss of draw solutes and possible

environmental issues.

SUMMARY

[0004] The invention generally relates to osmotically driven membrane systems and methods, for example, forward osmosis (FO), pressure retarded osmosis (PRO), osmotic dilution (OD), direct osmotic concentration (DOC), and the like, and to systems and methods for draw solute recovery in the osmotically driven membrane systems/processes.

[0005] In various embodiments, the separation operation includes using an absorber configured to condense the draw solutes into the concentrated draw solution. The solvent stream, dilute draw solution, or concentrated draw solution may be used as an absorbent in the absorber. Cooling may be used with the absorber. In some embodiments, the process may further include the step of compressing a gas stream resulting from separation of the draw solutes from the dilute draw solution using a gas compressor or a steam eductor driven by hydraulic pressure on an absorbing liquid stream to promote reabsorption of draw solutes into the concentrated draw solution. In various embodiments, the recovery system uses a distillation apparatus (e.g., a packed column or a membrane device) to thermally separate the draw solutes out of a dilute draw solution prior to re-concentrating the separated draw solutes in to the concentrated draw solution.

[0006] In one aspect, the invention relates to an apparatus and related method for recovering a product solvent from a feed solution and draw solution solutes from a dilute draw solution. The apparatus includes an osmotically driven membrane system having a plurality of forward osmosis membranes, wherein the osmotically driven membrane system is configured to introduce the feed solution to a first side of the membranes and a concentrated draw solution to a second side of the membrane and output a concentrated feed solution from the first sides of the membranes and the dilute draw solution from the second sides of the membranes, and a separation system in fluid communication with the osmotically driven membrane system and configured to receive the concentrated feed solution and the dilute draw solution. The separation system includes a first distillation apparatus in fluid communication with the second sides of the membranes and configured for receiving the dilute draw solution and outputting a first vaporized draw solute stream and the product solvent in response to the introduction of a source of thermal energy, a first condenser disposed downstream of and in fluid communication with the first distillation apparatus, the first condenser configured for receiving the first vaporized draw solute stream and a cooling fluid and outputting an at least partially condensed draw solute stream, a second distillation apparatus in fluid communication with the first sides of the membranes and configured for receiving the concentrated feed solution and outputting a second vaporized draw solute stream and a further concentrated feed solution in response to the introduction of a source of thermal energy, a second condenser disposed downstream of and in fluid communication with the second distillation apparatus, the second condenser configured for receiving the second vaporized draw solute stream and a cooling fluid and outputting a condensed, or at least substantially condensed, draw solute stream, a liquid vapor separator disposed downstream of and in fluid communication with the first condenser and configured for receiving the at least partially condensed draw solute stream and outputting vaporized draw solutes and a first re- concentrated draw solution stream, and an absorber disposed downstream of and in fluid communication with the first and second condensers and configured for receiving the vaporized draw solutes from the liquid-vapor separator and the condensed draw solute stream from the second condenser and outputting a second re-concentrated draw solution stream.

[0007] In another aspect, the invention relates to an apparatus and related method for recovering a product solvent from a feed solution and draw solution solutes from a dilute draw solution. The apparatus includes an osmotically driven membrane system having a plurality of forward osmosis membranes, wherein the osmotically driven membrane system is configured to introduce the feed solution to a first side of the membranes and a concentrated draw solution to a second side of the membrane and output a concentrated feed solution from the first sides of the membranes and the dilute draw solution from the second sides of the membranes, a separation system in fluid communication with the osmotically driven membrane system and configured to receive the concentrated feed solution and the dilute draw solution and outputting a partially re- concentrated draw solution stream including a portion of vaporized draw solutes and a portion of condensed draw solutes, and an absorber disposed downstream of and in fluid communication with the separation system, the absorber configured for separately receiving the vaporized portion of draw solutes and the condensed portion of draw solutes and outputting a re- concentrated draw solution stream for return to the osmotically driven membrane system.

[0008] In various embodiments of the foregoing aspects, the first and second distillation apparatus can be oriented in parallel and the source of thermal energy for each can be a single source, for example, the thermal energy can pass through the apparatus in series or be portioned therebetween. The first and second re-concentrated draw solution streams can be combined and returned to the osmotically driven membrane system as the source of concentrated draw solution. In one embodiment, the condensed draw solute stream is introduced to the absorber as an absorbent. Additionally, the absorber(s) can be oriented vertically or horizontally and the vaporized draw solutes and the condensed draw solute stream can be introduced to the absorber in either a co-current or counter-current manner. For example, the absorber can be oriented vertically with the vaporized draw solutes introduced to the absorber at or proximate the top of the absorber and the condensed draw solute stream can be introduced at or proximate the bottom of the absorber for a relative counter-current flow. In some embodiments, the absorber can be a packed column. The absorber can also be an integral part of a condensation drum, where the drum is elevated relative to the first and second distillation apparatus and configured to create a vacuum on the first distillation apparatus. In some embodiments, where the absorber is elevated, the second condenser may be eliminated and the second vaporized draw solute stream can be introduced directly to the first distillation apparatus. In one embodiment of this configuration, the absorber/condensation drum drains into a vented holding tank.

[0009] In another aspect, the invention relates to an apparatus and related method for recovering a product solvent from a feed solution and draw solution solutes from a dilute draw solution. The apparatus includes an osmotically driven membrane system having a plurality of forward osmosis membranes, wherein the osmotically driven membrane system is configured to introduce the feed solution to a first side of the membranes and a concentrated draw solution to a second side of the membrane and output a concentrated feed solution from the first sides of the membranes and the dilute draw solution from the second sides of the membranes, and a separation system in fluid communication with the osmotically driven membrane system and configured to receive the concentrated feed solution and the dilute draw solution. The separation system includes a first distillation apparatus in fluid communication with the second sides of the membranes and configured for receiving the dilute draw solution and outputting a first vaporized draw solute stream and the product solvent in response to the introduction of a source of thermal energy, a second distillation apparatus in fluid communication with the first sides of the membranes and configured for receiving the concentrated feed solution and outputting a second vaporized draw solute stream and a further concentrated feed solution in response to the introduction of a source of thermal energy, and a refrigeration system disposed downstream of and in fluid communication with the first and second distillation apparatus, the refrigeration system configured for receiving the first and second vaporized draw solute streams and outputting a re-concentrated draw solution and a vaporized draw solute stream. The re- concentrated draw solution can be returned to the osmotically driven membrane system and the vaporized draw solute stream can be recycled back to the refrigeration system. The refrigeration system includes one or more of a compressor, a condenser, an evaporator, and an expansion valve. [0010] These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention and are not intended as a definition of the limits of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

[0012] FIG. 1 is a schematic representation of an exemplary osmotically driven membrane system/process using a solute recovery system in accordance with one or more embodiments of the invention;

[0013] FIG. 2 is a simplified schematic representation of an osmotically driven membrane system/process including an alternative draw solute recovery system/process in accordance with one or more embodiments of the invention;

[0014] FIG. 3 is a simplified schematic representation of an osmotically driven membrane system/process including an alternative draw solute recovery system/process in accordance with one or more embodiments of the invention;

[0015] FIGS. 4A-4D are simplified schematic representations of osmotically driven membrane systems/processes including alternative draw solute recovery systems/processes in accordance with one or more embodiments of the invention; and

[0016] FIG. 5 is a simplified schematic representation of an osmotically driven process/system including an alternative draw solute recovery system/process in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

[0017] Various embodiments of the invention may be used in any osmotically driven membrane process, such as FO, PRO, OD, DOC, etc. An osmotically driven membrane process for extracting a solvent from solution may generally involve exposing the solution to a first surface of a forward osmosis membrane. In some embodiments, the first solution (known as a process or feed solution) may be seawater, brackish water, wastewater, contaminated water, a process stream, or other aqueous solution. In at least one embodiment, the solvent is water; however, other embodiments may use non-aqueous solvents. A second solution (known as a draw solution) with an increased concentration of solute(s) relative to that of the first solution may be exposed to a second, opposed surface of the forward osmosis membrane. Solvent, for example water, may then be drawn from the first solution through the forward osmosis membrane and into the second solution generating a solvent-enriched solution via forward osmosis.

[0018] Forward osmosis generally utilizes fluid transfer properties involving movement of solvent from a less concentrated solution to a more concentrated solution. Osmotic pressure generally promotes transport of solvent across a forward osmosis membrane from feed to draw solutions. The solvent-enriched solution, also referred to as a dilute draw solution, may be collected at a first outlet and undergo a further separation process. In some non-limiting embodiments, purified water may be produced as a product from the solvent-enriched solution. A second product stream, i.e., a depleted or concentrated process solution, may be collected at a second outlet for discharge or further treatment. The concentrated process solution may contain one or more target compounds which it may be desirable to concentrate or otherwise isolate for downstream use.

[0019] FIG. 1 depicts one exemplary osmotically driven membrane system/process 10 utilizing a draw solute recovery system 22 in accordance with one or more embodiments of the invention. As shown in FIG. 1, the system/process 10 includes a forward osmosis module 12, such as those described in U.S. Patent Nos. 6,391,205 and 7,560,029; 9,039,899; 9,248,405; 9,266,065; and 9,352,281; and U.S. Patent Publication No. 2014/0224716; the disclosures of which are hereby incorporated by reference herein in their entireties. The module 12 is in fluid communication with a feed solution source or stream 14 and a draw solution source or stream 16. The draw solution source 16 can include, for example, a saline stream, such as sea water, or another solution as described herein that can act as an osmotic agent to dewater the feed source 14 by osmosis through a forward osmosis membrane within the module 12. Examples of draw solutions and draw solute recovery schemes are described in U.S. Patent Publication No.

2015/0273396, the disclosure of which is hereby incorporated by reference herein in its entirety. The module 12 outputs a stream of concentrated solution 18 from the feed stream 14 that can be further processed. The module 12 also outputs a dilute draw solution 20 that can be further processed via the recovery system 22, examples of which are described and/or incorporated herein, where draw solutes and a target solvent can be recovered. In accordance with one or more embodiments of the invention, the draw solutes are recovered for reuse.

[0020] In accordance with one or more embodiments, a portion of the dilute draw solution may be used to absorb draw solute gases from, for example, a distillation column. In at least one embodiment, both cooling and mixing with an absorbent may occur in an absorption column or membrane module. The mixing of the gases with a portion of the dilute draw solution acting as an absorbent (to then become the concentrated draw solution) may occur in a vessel. The vessel may generally be sized to provide an area large enough to facilitate interaction between the absorbent and the gases. In some embodiments, a packed column may be used as an absorber. In one or more embodiments, a stripping distillation column and an absorbing column may be used in conjunction. Heating may occur in the distillation column, while cooling and contact with the dilute draw solution absorbent may occur in the absorbing column. In one embodiment, approximately 25% of the dilute draw solution stream may be directed to an absorber to serve as an absorbent fluid, with the remaining approximately 75% of the dilute stream being directed to the stripper as its feed stream. The balance between these two streams will dictate the concentration of the draw solution returned to the membrane system 12, as well as the size of the absorber and/or stripper, and the quantity of heating required in the stripper and cooling required before, after, and/or within the absorber or stages of the absorber.

[0021] Additionally, the first solution may be any solution containing solvent and one or more solutes for which separation, purification or other treatment is desired. A process stream to be treated may include salts and other ionic species such as chloride, sulfate, bromide, silicate, iodide, phosphate, sodium, magnesium, calcium, potassium, nitrate, arsenic, lithium, boron, strontium, molybdenum, manganese, aluminum, cadmium, chromium, cobalt, copper, iron, lead, nickel, selenium, silver, and zinc. Such streams may be from an industrial process such as a pharmaceutical or food grade application. Target species may include pharmaceuticals, salts, enzymes, proteins, catalysts, microorganisms, organic compounds, inorganic compounds, chemical precursors, chemical products, colloids, food products, or contaminants. The first solution may be delivered to a forward osmosis membrane treatment system from an upstream unit operation such as industrial facility, or any other source such as the ocean.

[0022] Like the first solution, the second solution may be an aqueous solution, i.e., the solvent is water. In other embodiments, non-aqueous solutions such as organic solvents may be used for the second solution. The second solution may be a draw solution containing a higher concentration of solute relative to the first solution. The draw solution may generally be capable of generating osmotic pressure within an osmotically driven membrane system. The osmotic pressure may be used for a variety of purposes, including desalination, water treatment, solute concentration, power generation, and other applications. A wide variety of draw solutions may be used. In some embodiments, the draw solution may include one or more removable solutes. In at least some embodiments, thermally removable (thermolytic) solutes may be used. For example, the draw solution may comprise a thermolytic salt solution. In some embodiments, an ammonia and carbon dioxide draw solution may be used, such as those disclosed in U.S. Patent No. 7,560,029; however, electrolytic draw solutions are also contemplated and considered within the scope of the invention. Generally, the draw solution should create osmotic pressure and be removable, such as for regeneration and recycling. In some embodiments, the draw solution may be characterized by an ability to undergo a catalyzed phase change in which a draw solute is changed to a gas or solid that can be precipitated from an aqueous solution using a catalyst. In some embodiments, the mechanism may be coupled with some other means, such as heating, cooling, addition of a reactant, or introduction of an electrical or magnetic field.

[0023] FIG. 2 depicts an osmotically driven membrane system/process 300 similar to those previously described, but with an alternative system/process 322 for recovering draw solutes. Generally, the system 300 includes one or more FO modules 312 in fluid

communication with one or more draw solute recovery/separation subsystems 322. The subsystem 322 depicted includes at least two distillation apparatus 324a, 324b, such as distillation columns or membrane distillation apparatus, in fluid communication with a separate absorber apparatus 325, such as a packed column. The system 300 also includes a liquid-vapor separator 334 in fluid communication with at least one of the distillation apparatus 324a and the absorber 325. The system 300 also includes any necessary condensers, pumps, valves, plumbing, etc., along with the other system features (e.g., reboilers, compressors, eductors, etc.) as described or incorporated elsewhere herein. The system 300 also operates similar to those described or incorporated above; however, with a slightly different arrangement of the components and corresponding change in operation.

[0024] As shown in FIG. 2, a feed stream 314 and a concentrated draw solution 316 are introduced to the FO membrane module(s) 312, which in turn outputs a concentrated feed stream 318 and a dilute draw solution 320. The concentrated feed stream 318 is directed to the second distillation apparatus 324b, either directly or via a holding tank 331. The concentrated feed 318 is heated (e.g., through the introduction of thermal energy, such as steam or low grade heat, via a reboiler) and the draw solutes that reverse fluxed through the membrane are vaporized (typically along with a small amount of solvent, such as water) and exit the apparatus 324b. The vaporized solutes and solvent mixture 321b is directed to a condenser 338b, which can use an independent source of a cooling fluid 337b (e.g., plant water or other cooling medium, such as an existing fluid stream within the system 300, for example, a portion of the concentrated feed stream 318) introduced to the condenser 338b with a heated fluid 339b exiting the condenser 338b, which can be recycled within the system 300 (e.g., used to preheat the feed 314 or the dilute draw solution 320 prior to introduction to the distillation apparatus 324a. In most, but not all embodiments, the condensed draw solute/solvent mixture 321b' will be a nitrogen rich liquid. The condensed mixture is directed to the absorber 325 to be used as an absorbent therein. A further concentrated feed stream 318' is discharged from the second distillation apparatus 324b and can be used as is, discarded or sent for further processing (e.g., to a crystallizer).

[0025] Similarly, the dilute draw solution 320 is directed to the first distillation apparatus

324a, either directly or via a holding tank 329. The first and second distillation apparatus 324 can be provided with thermal energy via any of the schemes described or incorporated herein. The dilute draw solution 320 is heated vaporizing the draw solutes out of the solution along with a small amount of solvent (e.g., water). The draw solutes and solvent mixture 321a exits the distillation apparatus 324a and is directed to a condenser 338a, which can also use an

independent source of cooling fluid 337a or another fluid stream within the system 300. The condensed mixture 321a', which includes liquid and vapor portions of the draw solution, is directed to the liquid-vapor separator 334, where the liquid portion can be removed as concentrated draw solution 316 and the remaining vaporized draw solutes 327 can be directed to the absorber 325. Generally, the separator 334 can be any conventionally known device for liquid-vapor separation, such as a knock-out pot or other gravity-based device; however, it could also be as simple as a vent line in the plumbing to bleed off at least a portion of the gaseous portion of the mixture 321a' . This gaseous portion is typically a carbon rich vapor. The first distillation apparatus 324a also discharges the recovered solvent (e.g., water) 342 that can be used as is or sent for further processing.

[0026] Generally, the absorber 325 receives the typically nitrogen rich solution 321b'

(this solution will typically be nitrogen rich if using an NH₃-CO₂ based draw solution, but will vary depending on the type of draw solution used, for example, a two-part draw solution where one element is more likely to diffuse across the membrane and be recovered via apparatus 324b) near the top of the absorber 325, while receiving the carbon rich vapor 327 (this will typically be carbon rich if using the NH₃-CO₂ based draw solution, but will also vary depending on the type of draw solution used) is introduced to the bottom of the absorber 325. Generally, the nitrogen rich solution 321b will absorb the carbon rich vapor, thereby reforming concentrated draw solution 316. The absorber 325 outputs the concentrated draw solution stream 316, which can be combined with the concentrated draw solution 316 exiting the liquid-vapor separator 334. The concentrated draw solution 316 is directed back to the FO membrane module(s) 312, either directly or via a holding tank or additional process (e.g., chemical addition or additional cooling). The absorber 325 receives a source of cooling fluid, similar to those previously disclosed, via its heat exchanger 368. Typically, the reformed concentrated draw solution collects in the bottom of the absorber 325 and can be circulated through the heat exchanger 368 to be cooled before exiting the absorber 325 and being returned to the FO module 312. This arrangement is generally desirable due to the exothermic nature of the process. Again, a portion of the concentrated feed stream 318 can be used for cooling, which also acts to preheat the concentrated feed stream (e.g., prior to introduction to the second distillation apparatus), thereby lessening the thermal requirements for additional concentration thereof and removal of the draw solutes.

[0027] Generally, sufficient mixing of the carbon rich vapor 327 and the nitrogen rich solution 321b will occur within the absorber 325; however, in some embodiments, the resulting mixture (i.e., at least partially re-concentrated draw solution 316) may be directed to an external mixing device (e.g., a static mixer) to ensure that the vapors 327 are well mixed within the solution 316 prior to being sent back to the forward osmosis module(s) 312. For example, in some embodiments, the mixture may be condensed within the distillation apparatus 324a reboiler to capture additional waste heat and further cool the re-concentrated draw solution 316.

[0028] FIG. 3 depicts an osmotically driven membrane system/process 400 similar to those previously described, but with an alternative system/process 422 for recovering draw solutes. Generally, the system 400 includes one or more FO modules 412 in fluid

communication with one or more draw solute recovery/separation subsystems 422. The subsystem 422 depicted includes at least two distillation apparatus 424a, 424b, such as distillation columns or membrane distillation apparatus, in fluid communication with a separate condenser/absorber apparatus 425, as described in greater detail below. The system 400 also includes any necessary condensers, pumps, valves, plumbing, etc., along with the other system features (e.g., reboilers, compressors, eductors, etc.) as described elsewhere herein. The system 400 also operates similar to those described above; however, with a slightly different

arrangement of the components and corresponding change in operation.

[0029] As shown in FIG. 3, a feed stream 414 and a concentrated draw solution 416 are introduced to the FO membrane module(s) 412, which in turn outputs a concentrated feed stream 418 and a dilute draw solution 420. The concentrated feed stream 418 is directed to the second distillation apparatus 424b, either directly or via a holding tank 431. The concentrated feed 418 is heated and the draw solutes that reverse fluxed through the membrane are vaporized (typically along with a small amount of solvent, such as water) and exit the apparatus 424b. The vaporized solutes and solvent mixture 421b is introduced (either directly or via, for example, a compressor) to the first distillation apparatus 424a, as opposed to a condenser as shown in FIG. 2. Similarly, the dilute draw solution 420 is also directed to the first distillation apparatus 424a, either directly or via a holding tank 429 and combined with the mixture 421b from the second distillation apparatus 424b. The first and second distillation apparatus 424 can be provided with thermal energy via any of the schemes disclosed herein. The dilute draw solution 420 is heated vaporizing the draw solutes out of the solution along with a small amount of solvent (e.g., water). The draw solutes and solvent mixture 421a exits the distillation apparatus 424a and is directed to the condenser/absorber apparatus 425 (e.g., a condensation drum). In some embodiments, an optional condenser 438 is provided before the condenser/absorber apparatus 425 to assist in the condensation of the concentrated draw solution vapors 421a. The optional condenser 438 can use an independent source of cooling water 437 or another fluid stream within the system 400 for cooling. Typically, the bottoms product of the first distillation apparatus 424a is a product solvent 442 (e.g., water) that can be used as is or subjected to further processing, and the bottoms product of the second distillation apparatus 424b is a further concentrated feed 418' (e.g., brine) that can be discarded or sent for further concentration (e.g., via a crystallizer).

[0030] The system 400 uses a condensation drum (condenser/absorber apparatus 425) to aid in the condensation of the concentrated draw solution vapors 421a and allow the distillation apparatus 424 to operate under vacuum, which will vastly decrease the energy required to separate the draw solutes from the dilute draw solution 420 and the concentrated feed 418.

Generally, the first and second distillation apparatus 424 are located at substantially the same level (e.g., ground level), with the condenser/absorber apparatus 425 located at a higher level (e.g., at or substantially above the height of the distillation apparatus), which enables the vacuum to be generated as described below. Generally, the height of the drum 425 will be dictated by the required level of vacuum for the particular application and may range from about 20 feet to about 80 feet.

[0031] During operation, the concentrated draw solution vapors 421a, separated from the product solvent 442 in the first distillation apparatus 424a, are directed into the bottom or proximate the bottom of the drum 425, which is partially filled with dilute draw solution 420 via, for example, a by-pass (stream 420') from the FO membrane module(s) 412 or holding tank 429 as previously described herein. In one or more embodiments, a portion of dilute draw solution 420' is also sprayed in at or proximate the top of the drum 425 as a secondary condensing process. In some embodiments, the drum 425 includes a condensing aid, such as a packing material, in order to enhance interfacial contact and increase mixing.

[0032] The condensation drum 425 also includes a down pipe 435 with a diameter "y" to facilitate gravity draining of the condensed concentrated draw solution 416 at a sufficient rate and will generally be based on the volume of condensed draw solution exiting the drum 425. The down pipe 435 will have a length "z" (e.g., about 20 feet to about 80 feet), such that a vacuum will be drawn on the distillation apparatus 424 via the apparatus 425. The condensed concentrated draw solution 416 will be drained into a holding tank 433 from where it can be held before returning to the FO membrane module(s) 412. In some embodiments, the tank 433 will include conservation vents to prevent pressure build-up within the tank 433. Additionally, the recovered draw solution 416 can also be subjected to further processing (e.g., additional cooling or other temperature conditioning) prior to being returned to the FO membrane module(s) 412.

[0033] FIGS. 4A-4D generally depict alternative draw solute recovery schemes that incorporate an absorber 525, 625, 725, 825 after the condenser 538, 638, 738, 838 in various orientations and flow patterns to increase residence time of the concentrated draw solution vapor 521 ', 621 ', 721 ', 82 and thereby improve reformation of the concentrated draw solution 516, 616, 716, 816. Specifically, and similar to the previously described systems, the systems 500, 600, 700, 800 all include one or more forward osmosis modules 512, 612, 712, 812 configured for receiving a feed stream 514, 614, 714, 814 and a concentrated draw solution 516, 616, 716, 816 and outputting a concentrated feed (e.g., brine) 518, 618, 718, 818 and a dilute draw solution 520, 620, 720, 820. One or both of the concentrated feed and dilute draw solution streams can be directed to a separation system 522, 622, 722, 822 as previously described. As part of or in addition to the separation system, the systems 500, 600, 700, 800 include the aforementioned condenser(s) 538, 638, 738, 838 configured for receiving a vaporized draw solute stream 521, 621, 721, 821 and outputting an at least partially condensed concentrated draw solution stream 521', 621', 721', 821' to the absorber(s) or absorption system(s) 525, 625, 725, 825.

[0034] Generally, the absorber 525, 625, 725, 825 is sized based, in part, on the bubble rise rate of the vaporized draw solutes entering the absorber in order to provide sufficient residence time to as fully as possible absorb the vaporized draw solutes into the

concentrated/condensed draw solution 516. The overall flow rates and volumes of the various draw solution based streams will vary to suit a particular application and also dictate the size and orientation of the absorber 525, 625, 725, 825.

[0035] In the system 500 shown in FIG. 4A, the absorber 525 has a vertical configuration and the system uses a counter-current flow of absorbent 516', which in the embodiment shown is a portion of the condensed concentrated draw solution from the condenser 538. Specifically, as shown in FIG. 4A, the mixture of condensed draw solution and draw solute vapors 52 exit the condenser 538 and are apportioned (e.g., via a liquid-vapor separator or natural separation via the plumbing) with the vapor portion directed to the bottom of the absorber 525 and the liquid portion being further apportioned (e.g., manually or automatically via a three-way valve) with one portion being returned to the forward osmosis module(s) 512 as concentrated draw solution 516 and the second portion 516" being directed to the absorber 525 (e.g., via a pump 530) at or near the top thereof to absorb the draw solute vapors rising in the absorber 525. In some embodiments, the entire liquid portion is directed to the absorber as an absorbent, in others, the entire liquid portion is directed to the forward osmosis module(s) 512 with an alternative absorbent being directed to the absorber. In some embodiments, the absorber is a packed column; however, it is also possible to use other vessels, with or without packing, to suit a particular application taking into consideration, for example, flow rates, temperatures, volumes, types and number of different draw solutes used, and ambient conditions. Concentrated draw solution 516" will exit from at or near the top of the absorber 525 and be returned to the forward osmosis module(s) 512. Generally, the re-concentrated draw solution 516" can be returned to the forward osmosis module(s) 512 either directly or via a holding tank to suit a particular application and/or layout of equipment. As previously disclosed, the re-concentrated draw solution 516 can be subjected to further processing prior to reintroduction into the forward osmosis module(s) 512.

[0036] The system 600 depicted in FIG. 4B is substantially similar to the system 500 of

FIG. 4A except the draw solute vapors 62 and absorbent 616' flow co-currently within the absorber 625 (e.g., condensed/concentrated draw solution 616' being introduced to the absorber 625 at or near the bottom thereof along with the vaporized portion of the draw solutes 621 '). In some embodiments, the absorber can include a coil of hose or tubes, with or without packing, to suit a particular application, as opposed to, for example, a packed column.

[0037] The system 700 shown in FIG. 4C is a variation of the systems 500, 600 of FIGS.

4A and 4B, but with the absorber 725 oriented horizontally with a portion of the re-concentrated draw solution 716' exiting the absorber 725 being recycled back to the absorber 725 for use as an absorbent. The mixture of vaporized draw solutes and condensed draw solution 72 exiting the condenser flow co-currently with the absorbent through the absorber 725. As previously described, the absorber could be a packed column, a pressure vessel, or a coil. In alternative embodiments applicable to all of the systems of FIGS. 4A-4C, a portion of dilute draw solution 720 can be used in place of or in addition to the concentrated draw solution 716 as an absorbent, and the absorbent can be introduced to the absorber 725 at various locations to suit a particular application (e.g., counter or co-current flow at various points in the absorber).

[0038] FIG. 4D depicts yet another variation of the systems of FIGS. 4A-4C. Generally, the system 800 uses a packed column as the absorber 825, where the absorber is coupled to and in fluid communication with a concentrated draw solution tank 831 to keep the absorber 825 flooded with concentrated draw solution 816. The mixture of vaporized draw solutes and condensed draw solution 82 are introduced (e.g., bubbled through) to the top of the absorber 825. Generally, the concentrated draw solution 816 is circulated through the absorber 825 by virtue of the head of the solution 816 in the tank 831 (i.e., atmospheric pressure working on the surface of the concentrated draw solution contained within the tank 831) such that it flows through the absorber 825, as the vaporized draw solutes are absorbed by the concentrated draw solution 816 resident therein.

[0039] FIG. 5 generally depicts an alternative draw solute recovery scheme that does not require an absorber; however, it can be included to further enhance the recovery process. Similar to the previously described systems, the system 900 includes one or more forward osmosis modules 912 configured for receiving a feed stream 914 and a concentrated draw solution 916 and outputting a concentrated feed (e.g., brine) 918 and a dilute draw solution 920. One or both of the concentrated feed and dilute draw solution streams can be directed to a separation system 922 similar to those previously described insofar as the system 922 includes two or more distillation apparatus 924 for thermally recovering draw solutes. As part of or in addition to the separation system 922, the system 900 includes a refrigeration system/process 969 in fluid communication with the distillation apparatus 924.

[0040] The refrigeration system 969 includes a compressor 970, a condenser 938, an evaporator 967 and one or more valves 928, 930, with the vaporized draw solutes 921 from the distillation apparatus 924 acting as the "refrigerant." Typical refrigeration systems require the use of a boiler or generator to provide the refrigerant (e.g., NH₃ gas), which can now be supplied from the draw solute separation process 922, specifically the distillation apparatus 924.

Generally, the refrigeration system 969 assists in the condensation/absorption of the vaporized draw solutes 921 to re-concentrate the draw solution 916. For example, for a system 900 using an NH₃-C0₂ draw solution, the system 969 will aid in the condensation of the draw solute vapors (e.g., NH₃), absorption of certain draw solutes (e.g., C0₂), and cooling via the exothermic process of carbon dioxide dissolution into ammonia.

[0041] As shown in FIG. 5, the refrigeration medium is taken from the draw solute separation/recovery portion of the osmotically driven membrane system/process and run in a closed-loop, where the refrigerant is recycled continuously. Specifically, ammonia vapor 921b is generated in the second distillation apparatus by stripping it out of the concentrated feed 918, with at least a portion thereof directed to the compressor 970 in the refrigeration system 969. In some embodiments, a portion of the draw solute vapor 921b is directed to the first distillation apparatus 924a, either directly or after compression via valve 928. The vapor 921b is compressed, condensed via condenser 938, and sent through an expansion valve 930 resulting in an ammonia vapor/liquid stream 921b". This stream 921b" is then directed to the evaporator 967 (or other enclosed space), where the liquid ammonia is evaporated to provide cooling to the vaporized draw solutes 921a exiting the top of the first distillation apparatus 924a for the purposes of condensing and absorbing the draw solutes and re-concentrating the draw solution 916, which can then be directed back to the FO module(s) 912 either directly or via a holding tank 929. Any remaining draw solute vapors 921b exiting the evaporator can be returned to the compressor 970.

[0042] In accordance with one or more embodiments, the devices, systems and methods described herein may generally include a controller for adjusting or regulating at least one operating parameter of a device or a component of the systems, such as, but not limited to, actuating valves and pumps, as well as adjusting a property or characteristic of one or more fluid flow streams through an osmotically driven membrane module, or other module in a particular system. A controller may be in electronic communication with at least one sensor configured to detect at least one operational parameter of the system, such as a concentration, flow rate, pressure, pH level, or temperature. The controller may be generally configured to generate a control signal to adjust one or more operational parameters in response to a signal generated by a sensor. For example, the controller can be configured to receive a representation of a condition, property, or state of any stream, component, or subsystem of the osmotically driven membrane systems and associated recovery systems. The controller typically includes an algorithm that facilitates generation of at least one output signal that is typically based on one or more of any of the representation and a target or desired value such as a set point. In accordance with one or more particular aspects, the controller can be configured to receive a representation of any measured property of any stream, and generate a control, drive or output signal to any of the system components, to reduce any deviation of the measured property from a target value.

[0043] In accordance with one or more embodiments, process control systems and methods may monitor various concentration levels, such as may be based on detected parameters including pH and conductivity. Process stream flow rates and tank levels may also be controlled. Temperature and pressure may be monitored, along with other operational parameters and maintenance issues. Various process efficiencies may be monitored, such as by measuring product water flow rate and quality, heat flow and electrical energy consumption. Cleaning protocols for biological fouling mitigation may be controlled such as by measuring flux decline as determined by flow rates of feed and draw solutions at specific points in a membrane system. A sensor on a brine stream may indicate when treatment is needed, such as with distillation, ion exchange, breakpoint chlorination or like protocols. This may be done with pH, ion selective probes, Fourier Transform Infrared Spectrometry (FTIR), or other means of sensing draw solute concentrations. A draw solution condition may be monitored and tracked for makeup addition and/or replacement of solutes. Likewise, product water quality may be monitored by conventional means or with a probe such as an ammonium or ammonia probe. FTIR may be implemented to detect species present providing information which may be useful to, for example, ensure proper plant operation, and for identifying behavior such as membrane ion exchange effects.

[0044] Having now described some illustrative embodiments of the invention, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other

embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention. In particular, although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives.

Claims

What is claimed is: CLAIMS

1. An apparatus for recovering a product solvent from a feed solution and draw solution solutes from a dilute draw solution, the apparatus comprising:

an osmotically driven membrane system comprising a plurality of forward osmosis membranes, wherein the osmotically driven membrane system is configured to introduce the feed solution to a first side of the membranes and a concentrated draw solution to a second side of the membranes and output a concentrated feed solution from the first sides of the membranes and the dilute draw solution from the second sides of the membranes; and

a separation system in fluid communication with the osmotically driven membrane system and configured to receive the concentrated feed solution and the dilute draw solution, the separation system comprising:

a first distillation apparatus in fluid communication with the second sides of the membranes and configured for receiving the dilute draw solution and outputting a first vaporized draw solute stream and the product solvent in response to the introduction of a source of thermal energy;

a first condenser disposed downstream of and in fluid communication with the first distillation apparatus, the first condenser configured for receiving the first vaporized draw solute stream and a cooling fluid and outputting an at least partially condensed draw solute stream; a second distillation apparatus in fluid communication with the first sides of the membranes and configured for receiving the concentrated feed solution and outputting a second vaporized draw solute stream and a further concentrated feed solution in response to the introduction of a source of thermal energy;

a second condenser disposed downstream of and in fluid communication with the second distillation apparatus, the second condenser configured for receiving the second vaporized draw solute stream and a cooling fluid and outputting a condensed draw solute stream;

a liquid vapor separator disposed downstream of and in fluid communication with the first condenser and configured for receiving the at least partially condensed draw solute stream and outputting vaporized draw solutes and a first re-concentrated draw solution stream; and an absorber disposed downstream of and in fluid communication with the first and second condensers and configured for receiving the vaporized draw solutes from the liquid-vapor separator and the condensed draw solute stream from the second condenser and outputting a second re-concentrated draw solution stream.

2. An apparatus for recovering a product solvent from a feed solution and draw solution solutes from a dilute draw solution, the apparatus comprising:

an osmotically driven membrane system comprising a plurality of forward osmosis membranes, wherein the osmotically driven membrane system is configured to introduce the feed solution to a first side of the membranes and a concentrated draw solution to a second side of the membrane and output a concentrated feed solution from the first sides of the membranes and the dilute draw solution from the second sides of the membranes;

a separation system in fluid communication with the osmotically driven membrane system and configured for receiving the concentrated feed solution and the dilute draw solution and outputting a partially re-concentrated draw solution stream comprising a portion of vaporized draw solutes and a portion of condensed draw solutes; and

an absorber disposed downstream of and in fluid communication with the separation system, the absorber configured for separately receiving the vaporized portion of draw solutes and the condensed portion of draw solutes and outputting a re-concentrated draw solution stream for return to the osmotically driven membrane system.

3. The apparatus any one of the preceding claims, wherein the first and second distillation apparatus are oriented in parallel and the source of thermal energy for each can be a single source.

4. The apparatus of claim 1, wherein the first and second re-concentrated draw solution streams can be combined and returned to the osmotically driven membrane system as the source of concentrated draw solution.

5. The apparatus of claim 1 or 2, wherein the condensed draw solute stream is introduced to the absorber as an absorbent.

6. The apparatus of claim 1 or 2, wherein the absorber can be oriented horizontally and the vaporized draw solutes and the condensed draw solute stream can be introduced to the absorber in either a co-current or counter-current manner.

7. The apparatus of claim 1 or 2, wherein the absorber is a packed column.

8. The apparatus of claim 1 or 2, wherein the absorber is an integral part of a condensation drum that is elevated relative to the first and second distillation apparatus and configured to create a vacuum on the first distillation apparatus.