1. Technical Field
The present invention relates to the field of desalination, and more particularly, to forward osmosis.
2. Discussion of Related Art
Desalination by osmosis is carried out according to two main principles: reverse osmosis—extracting water through a semi-permeable membrane from feed water by applying on the feed water a gauge pressure that is higher than the osmotic pressure of the feed water; and forward osmosis—drawing water through a semi-permeable membrane from feed water by a draw solution having a higher osmotic pressure than the feed water.
Forward osmosis has been implemented using a draw solution of a very high osmotic pressure (e.g. NH3—CO2 in water) to draw water from sea water. Forward osmosis has also been implemented to utilize the expanding draw solution to generate power, such as by contacting sea water and river water through a semi-permeable membrane, and allowing the expanding sea water to move a turbine.
Embodiments of the present invention provide a desalination system comprising: a forward osmosis (FO) unit, arranged to expand draw solution under high gauge (G-) and high osmotic (O-) pressures with water drawn from the feed water through a semi-permeable membrane, to yield an increase in a throughput and a decrease in the osmotic pressure of the draw solution; a gauge pressure generating module arranged to introduce draw solution of high osmotic pressure into the FO unit at a high gauge pressure; a power producing work exchanger arranged to receive the expanded draw solution from the FO unit and to generate mechanical power from the expansion of the draw solution against the high gauge pressure, to yield G-de-pressurized draw solution; and an extraction module arranged to receive G-de-pressurized draw solution of decreased osmotic pressure and to the extract product water therefrom to re-concentrate the draw solution, wherein the desalination system is arranged to simultaneously produce product water and mechanical power from forward osmosis at high osmotic and high gauge pressures.
These, additional, and/or other aspects and/or advantages of the present invention are: set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention.
The present invention will be more readily understood from the detailed description of embodiments thereof made in conjunction with the accompanying drawings of which:
Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.
For a better understanding of the invention, the usages of the following terms in the present disclosure are defined in a non-limiting manner:
The term “gauge (G-) pressure” as used herein in this application, is defined as an applied mechanical pressure in respect to atmospheric pressure.
The term “osmotic (O-) pressure” as used herein in this application, is defined as the pressure that must be applied on a solution to prevent solvent from moving through a semi-permeable membrane into the solution, due to solute that is impermeable through the membrane.
The term “feed water” as used herein in this application, is defined as saline water that is fed into a desalination system, such as sea water.
The term “draw solution” as used herein in this application, is defined as a solution of a high osmotic pressure used to draw water through a semi-permeable membrane from feed water.
The term “forward osmosis” as used herein in this application, is defined as a process of drawing water through a semi-permeable membrane from feed water by a draw solution having a higher osmotic pressure.
The term “reverse osmosis” as used herein in this application, is defined as a process of extracting water through a semi-permeable membrane from feed water by applying on the feed water a gauge pressure that is higher than the osmotic pressure of the feed water.
Gauge pressure generating module 121 is arranged to introduce the draw solution (at high osmotic pressure, or O-pressurized) into pressurized membrane module 133 at a high gauge pressure (G-pressurized), which may reach 70-150 bars. The high gauge pressure balances the high osmotic pressure of the draw solution to allow the operation of the membrane in pressurized membrane module 133. Furthermore, the expansion of the (G- and O-pressurized) draw solution against the high gauge pressure allows utilizing the expansion to generate power by work exchanger 141 (see below).
Pressurized membrane module 133 is arranged to utilize the draw solution of high osmotic pressure (and high gauge pressure) to draw water from feed water through a membrane, to yield expanded draw solution of low osmotic pressure. The osmotic pressure decreases due to the addition of drawn water. The addition of drawn water expands the volume and increases the throughput of the draw solution.
Work exchanger 141 is arranged to generate mechanical power from the expansion of the draw solution against the high gauge pressure, e.g. by means of a piston pushed by the draw solution. Leaving work exchanger 141 the draw solution is at low gauge and low osmotic pressures (G- and O-depressurized).
Extraction module 150 is arranged to extract product water from the draw solution of low osmotic pressure to re-concentrate the draw solution to the original high osmotic pressure.
In this constellation, by using the G- and O-pressurized draw solution, FO unit 100 utilizes the forward osmosis process to simultaneously produce product water and energy.
FO unit 130 may comprise a pressurized FO module 133 and a non-pressurized FO module 136.
Pressurized FO module 133 utilizes the G- and O-pressurized draw solution to draw water from the feed water through a membrane, to yield expanded G-pressurized draw solution of lower osmotic pressure. The osmotic pressure decreases due to the addition of drawn water. The addition of drawn water expands the volume and increases the throughput of the G-pressurized draw solution. The feed water leaves the membrane of pressurized FO module 133 as an intermediately concentrated feed.
Non-pressurized FO module 136 utilizes G-de-pressurized draw solution (coming from power producing work exchanger 140, see below) to draw additional water from the intermediately concentrated feed to produce the G- and O-de-pressurized draw solution, which is a dilute draw solution.
FO unit 130 further comprises an extraction unit 150 arranged to draw product water from the G- and O-de-pressurized draw solution, and re-concentrate the draw solution.
RO unit 110 is arranged to receive feed water which are gauge pressurized by power producing work exchanger 140 utilizing the expansion of the G- and O-pressurized draw solution (see below). RO unit 110 is arranged to produce product water and pressurized brine from the pressurized feed water through a regular reverse osmosis process.
Power producing work exchanger 140 connects FO unit 130 to RO unit 110 and is arranged to receive the G-pressurized draw solution from FO unit 130 and to utilize the increased throughput (through expansion) of the G-pressurized draw solution to drive feed water to RO unit 110, thereby G-de-pressurizing the G-pressurized draw fluid. Power producing work exchanger 140 is thus an embodiment of work exchanger 141, in which the expansion power is utilized directly to G-pressurize feed water to RO unit 110.
Brine work exchanger 120 connects RO unit 110 to FO unit 110, and is arranged to receive the G-pressurized brine from RO unit 110 and drive the O-pressurized draw solution to FO unit 130 as G- and O-pressurized draw solution. The de-pressurized brine is removed from desalination system 101. Brine work exchanger 120 receives the regenerated O-pressurized draw solution from extraction unit 150.
Substantially all feed water throughput to RO unit 110 is supplied by power producing work exchanger 140 utilizing the increase in throughput of the expanded G-pressurized draw solution. In this constellation, by using the G- and O-pressurized draw solution, FO unit 100 utilizes the forward osmosis process to simultaneously produce product water and energy that is directly utilized to produce additional product water at a low energetic cost through RO unit 110.
In embodiments, the throughput of the pressurized brine may be about a half of the throughput of the feed water to RO unit 110, and pressurized FO module 133 may double the throughput of the G-pressurized draw solution.
An intake pump 111 pushes feed water, e.g. sea water, to power producing work exchanger 140, pressurized FO module 133 and high pressure pump 112. Most feed water is supplied to pressurized FO module 133 and to power producing work exchanger 140. For example, in the illustrated example, intake pump 111 may push 4 m3/sec to pressurized FO module 133 (for FO desalination) and 2 m3/sec to power producing work exchanger 140 (for RO desalination). High pressure pump 112 may increase the gauge pressure of feed water from 2-3 bars up to 70-83 bars. Feed water pumped directly to RO unit 110 via high pressure pump 112 may be of a marginal throughput (e.g. 0.02 m3/sec at 83 bar) selected to compensate for losses. The route via high pressure pump 112 may also be used for initialization of RO unit 110 and as reserve capability. Intake pump 111 may push feed water at a low pressure, e.g. 5 bar.
A circulation pump 113 may be used to compensate for pressure losses in RO unit 110, pressurized FO module 133, power producing work exchanger 140 and pipes. Circulation pump 113 may add 6 bar to a throughput of 2 m3/sec coming from power producing work exchanger 140.
Power producing work exchanger 140 and brine work exchanger 120 may be positive displacement devices, for example of DWEER (Dual Work Exchanger Energy Recovery) type, or ERI type (Energy Recovery Inc. PX energy recovery device that uses the principle of positive displacement and isobaric chambers) or any other pressure or energy exchange device which transfer pressure from one liquid to other liquid. DWEER includes two cylindrical modules with pistons hermetically separated different liquids. The device includes link valves and check valves that allow switching two cylindrical modules between a high pressure working cycle and a low pressure filling cycle. ERI includes a rotor in a bearing that alternately couples the high pressure input and output, and the low pressure input and output.
In the illustrated example, the 2 m3/sec entering RO unit 110 are used to generate 1 m3/sec product water and 1 m3/sec pressurized brine (e.g. at 70 bar), which is used to drive the 1 m3/sec draw solution through brine work exchanger 120 to pressurized FO membrane module 133. The expansion of the draw solution may be twofold to 2 m3/sec entering power producing work exchanger 140 and utilized to drive pressurized feed water to RO unit 110. The flow of 2 m3/sec through non-pressurized FO module 136 may result in 2 m3/sec brine and 3 m3/sec G- and O-depressurized dilute draw solution entering extraction unit 150.
In some embodiments, the draw solution is a solution of ammonia gas NH3 and carbon dioxide gas CO2 in water. The gases transform in solution to ammonium carbamate (NH2COONH4), ammonium bicarbonate (NH4HCO3) and ammonium carbonate ((NH4)2CO3) Ammonium carbamate (NH2COONH4) results from the reaction CO2+2NH3→NH2COONH4, its solubility is about 423 gr/lit of water, starting to decompose at 35° C., and completely decomposes above 60° C. to NH3 and CO2. Ammonium bicarbonate (NH4HCO3) results from the reaction CO2+NH3+H2O→NH4HCO3 its solubility is about 178 gr/lit of water, starting to decompose at 30° C., and completely decomposes above 60° C. to NH3 and CO2. Ammonium carbonate ((NH4)2CO3) and ammonium carbamate (NH2COONH4) result from the reaction 2CO2+4NH3+H2O→(NH4)2CO3+NH2COONH4 at a large excess of NH3.
In these embodiments, extraction unit 150 may comprise a crystallizer 151, resolvent chamber 152, a CO2 desorber 155 with a heat exchanger 153 at the entrance and a vacuum pump 157 for exiting gaseous CO2, a NH3 desorber 156 with a heat exchanger 154 at the entrance and a vacuum pump 158 for exiting gaseous CO2 and NH3.
Crystallizer 151 is a conical tank for crystallizing ammonium carbamate, ammonium bicarbonate and ammonium carbonate. The tank may be constructed from stainless steel, glass-reinforced plastic, steel rubber covered etc. Resolvent chamber 152 is a smaller conical tank for dissolution of chemicals ammonium carbamate, ammonium bicarbonate and ammonium carbonate, that is connected to crystallizer 151. The materials of construction are similar to crystallizer 151.
CO2 desorber 155 is a tower which filled with packing. The duty of the tower is to remove CO2 from draw solution from crystallizer 151, that is heated by heat exchanger 153 (e.g. to 35° C., which together with the low pressure induces CO2 release from the solution). Vacuum pump 157 pumps gaseous CO2 from CO2 desorber 155 to crystallizer 151.
NH3 desorber 156 is a tower which filled with packing. The duty of the tower is to remove NH3 and CO2 residuals from the draw solution from CO2 desorber 155, that is heated by heat exchanger 154 (e.g. to 60° C. for decomposing ammonium bicarbonate to NH3 and CO2 gases and water). Vacuum pump 158 pumps gaseous CO2 and NH3 from NH3 desorber 156 to resolvent chamber 152.
Extraction unit 150 further comprises a product pump 163 for delivering product water from NH3 desorber 156, a CO2 desorber pump 161 (e.g. a centrifugal pump) for pumping draw solution from CO2 desorber 155 to crystallizer 151.
The increased level of CO2 in respect to NH3 (e.g. NH3/CO2<1) resulting from the CO2 input from CO2 desorber 155, causes ammonium carbamate to transform to the less soluble ammonium bicarbonate Ammonium bicarbonate crystallizes because it reaches its solubility limit. The conical shape of crystallizer 151 slows down the solution with the increase of the diameter. The decrease in velocity causes the precipitated ammonium bicarbonate to float at a certain level in unit crystallizer 151. Above the layer of floating crystallized ammonium bicarbonate, the solution is saturated with dissolved ammonium bicarbonate, leaves crystallizer 151 and continues via heat exchanger 153 to CO2 desorber 155. The crystallized ammonium bicarbonate particles are sucked into resolvent chamber 152.
Draw solution that reenters crystallizer 151 from CO2 desorber pump 161 comprises a large proportion of NH3 and causes the ammonium bicarbonate to transform in resolvent chamber 152 (e.g., at NH3/CO2 level exceeding 1.75) to the more soluble ammonium carbamate which thus completes the regeneration of the draw solution.
To summarize: crystallizer 151 is arranged to receive the G-de-pressurized draw solution of decreased osmotic pressure from FO unit 130 (e.g. non-pressurized membrane module 136) and a CO2 rich gas from CO2 desorber 155, to yield crystallized ammonium bicarbonate and an ammonium bicarbonate saturated solution thereabove in the conical tank. CO2 desorber 155 is arranged to extract the CO2 rich gas from the ammonium bicarbonate saturated solution, to yield a NH3 rich solution. NH3 desorber 156 is arranged to receive the NH3 rich solution from CO2 desorber 155 and to extract a NH3 rich gas therefrom to yield the product water. Resolvent chamber 152 is arranged to receive a part of the NH3 rich solution, the NH3 rich gas and a part of the ammonium bicarbonate saturated solution, to yield the regenerated draw solution.
Extraction unit 150 finally comprises a resolvent chamber pump 159, e.g., a centrifugal pump, that is arranged to pump the re-concentrated draw solution from resolvent chamber 152 out of extraction unit 150 to brine work exchanger 120.
In some embodiments, the throughput into crystallizer 151 from non-pressurized FO module 136 may comprise 3 m3/sec of which 2.5 m3/sec is directed to CO2 desorber 155 and 0.5 m3/sec is directed to resolvent chamber 152. Additional 0.5 m3/sec is directed to resolvent chamber 152 from the solution exiting CO2 desorber 155 (enriched with gaseous NH3 from NH3 desorber 156) and the other 2 m3/sec are directed from CO2 desorber 155 to NH3 desorber 156 and are eventually turned into product water. The 1 m3/sec leaving resolvent chamber 152 is the regenerated draw solution.
Pressurized FO module 133 and non-pressurized FO module 136 are illustrated such that feed water flow from top to bottom and exit non-pressurized FO module 136 as brine. The draw solution flows in the opposite direction to the feed water. The concentrated O- and G-pressurized draw solution (from brine work exchanger 120) flows through pressurized FO module 133 first, such that gauge pressure somewhat balances the high osmotic pressure and allows the operation of the membrane. In pressurized FO module 133 the draw solution expands due to water extracted from the feed water and its expansion against the gauge pressure is utilized to recover energy by power producing work exchanger 140.
The draw solution enters non-pressurized FO module 136 after exiting power producing work exchanger 140 in a low gauge pressure and an intermediate osmotic pressure (after extracting water from the feed water in pressurized FO module 133), and is utilized again to extract more water from the intermediately concentrated feed water exiting pressurized FO module 133. In non-pressurized FO module 136 the draw solution is diluted further, before being re-concentrated by extraction unit 150.
The method may further comprise utilizing the generated mechanical power to desalinate additional feed water (stage 212), e.g. through a direct pressure exchange between the expanded G-pressurized draw solution and the additional feed water, such as via a reverse osmosis process.
The method may further comprise configuring the membrane to operate under the high osmotic and high gauge pressures (stage 204).
The method may further comprise sequentially contacting the G- and O-pressurized draw solution with the feed water through at least two membrane modules (stage 202), to exhaust the osmotic pressure of the draw solution for desalinating the feed water. The sequentially contacting (stage 202) may comprise: utilizing the gauge pressure to counter a high osmotic pressure of the draw solution such as to allow using the draw solution to draw a first water throughput from the feed water in through a first membrane (stage 207), to yield a draw solution of intermediate osmotic pressure and feed water of intermediate concentration; and drawing a second water throughput from the feed water of intermediate concentration (stage 208) by contacting it through a second membrane with the draw solution of intermediate osmotic pressure (stage 209), to yield O-de-pressurized dilute draw solution and concentrated brine. Re-concentrating the G- and O-depressurized draw solution (stage 220) thus yields substantially a sum of the first and second water throughput as product water. The draw solution of intermediate osmotic pressure may be used to draw water from other water feeds, such as sea or brackish water.
Re-concentrating the G- and O-depressurized draw solution to regenerate the O-pressurized draw solution and to yield product water (stage 220) may comprise the following stages, as illustrated in
Advantageously, FO unit 100 operates at a very high osmotic pressure of the draw solution to achieve a very effective desalination of feed water by forward osmosis. The high osmotic pressure is countered by a high gauge pressure that is applied in order to enhance the functionality of the membrane and FO unit 100 as a whole in face of the high osmotic pressure.
The high gauge pressure is utilized to a further end, by using it to counter the expansion of the draw solution to transform the expansion to mechanical work. For example, in desalination system 101, the mechanical work is used directly to generate additional desalinated feed water through RO unit 110 at an essentially zero energy cost (except for some compensation for pressure losses).
FO unit 100 and desalination system 101 have the following decisive advantages over known FO systems for power generation that use the drawing pressure of sea water in respect to river water: (i) The use of high O- and G-pressures of the draw solution allows a higher energetic yield as well as a more effective use of feed water resulting in a smaller footprint. (ii) The use of a single source of feed water, enabled by using draw solution which is regenerated in a closed loop, relieves the need of having sea and river water sources in close proximity, and furthermore requires a single filtration system instead of two. (iii) The direct conversion of power to desalinated product water is both energy efficient and provides a solution to areas lacking potable water. In contrast, prior art power generating FO systems turns a large amount of non-saline river water to brackish water as byproduct of the process.
In the above description, an embodiment is an example or implementation of the invention. The various appearances of “one embodiment”, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments.
Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.
Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above.
The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.
Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.
While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.
This application claims the benefit of U.S. Provisional Patent Application 61/224,082 filed on Jul. 9, 2009, which is incorporated herein by reference.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB10/53048 | 7/2/2010 | WO | 00 | 1/9/2012 |
Number | Date | Country | |
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61224082 | Jul 2009 | US |