Abstract:
CO 2  absorption and desorption affords differing osmotic pressure metal salt osmolyte draw solutions from a common solution. These draw solutions serve a staged forward osmosis membrane process. First stage draw solution is the lowest osmotic pressure osmolyte. First stage concentrate is fed to the second stage and fresh water is externally extracted from the first stage diluted osmolyte. Concentrated first stage osmolyte returns from fresh water extraction, blends and is heated with solid precipitates of the lower osmotic pressure solute. CO 2  desorbs from the lower osmotic pressure osmolyte converting to a higher osmotic pressure osmolyte. The higher osmotic pressure osmolyte serves as second stage draw solution to further dewatering the first stage concentrate. Second stage concentrate conveys to external processing or discharge. CO 2  absorption converts the dilute high osmotic pressure osmolyte from the second stage to the lower osmotic pressure osmolyte serving as draw solution in the first stage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority based upon prior U.S. Provisional Patent Application Ser. No. 62/033,946 filed Aug. 6, 2014, in the name of James Jeffrey Harris and Upen Jayant Bharwada, entitled “Alternating Osmotic Pressure from a Metal Salt Osmolyte to Enhance Forward Osmosis Processes,” the disclosure of which is incorporated herein in its entirety by reference as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The art of the present invention relates generally to providing a thermally responding osmolyte draw solution for a forward osmosis assisted freshwater generation process wherein the osmolyte draw solution is thermally shifted between existing as a lower and a higher osmotic pressure osmolyte. The effect parlays both an advantageous efficiency toward freshwater recovery as well as providing a means for freshwater recovery from otherwise intractable very high total dissolved solids (TDS) feed waters. Embodiments of the present invention apply particularly well to increasing the recovery of freshwater from reverse osmosis processes, although the present invention is amenable for use with other freshwater extraction processes. 
     U.S. Pat. No. 3,721,621 presents a method wherein a forward osmosis process dewaters a feed water, such as seawater, employing a pH sensitive, high TDS, high osmotic pressure draw solution. A higher volume, dilute draw solution results. The pH of this dilute draw solution is thereafter shifted facilitating precipitation of solute, resulting in a lower TDS product than the feed water serviced. This prior art suffers from the consumable expense of the pH sensitive solute as well as pH shifting chemicals. Further the final product effluent remains relatively high in TDS, reducing its applicable value. 
     The prior art has attempted achieving increased recovery from reverse osmosis processes by improvements of commercial membrane rejection ratios as well as employing higher pressure amenable membranes and associated strengthened support structures. The burden is structural limitations due to the high osmotic pressures associated with increased TDS and recovery ratio. The present invention, in particular, resolves the high pressure limitations and performance frailties associated with the prior art. When combined with reverse osmosis processes as well as other freshwater extraction processes, the present invention affords a novel means to increase the freshwater recovery factor and concentrate TDS with the employ of low grade thermal energy. 
     BRIEF SUMMARY OF THE INVENTION 
     A process is provided to facilitate improved freshwater recovery and increased concentrate effluent TDS from a conventional freshwater extraction process (hereinafter, for simplicity, referenced as reverse osmosis or RO regardless of the freshwater extraction technology by the employ of thermal energy. The process also provides a means to extract freshwater from very high TDS feed waters which are otherwise intractable by the prior art. 
     The process employs two or more forward osmosis processes wherein the employed osmotic draw solutions contain an aqueous solution of a metallic salt. This salt has a thermally dependent affinity for carbon dioxide (CO 2 ). At lower temperatures the salt absorbs CO 2  while at higher temperatures the salt liberates CO 2 . The CO 2  entrained metal salt has a moderately high solubility, TDS and associated osmotic pressure. In contrast, the metal salt lacking CO 2  has a very high solubility, TDS and associated osmotic pressure. 
     For ease of discussion, a draw solution containing solutes of the metal salt that is primarily entrained with CO 2  will be referred to as Osmolyte A. A draw solution containing solutes that primarily do not contain CO 2  will be referred to as Osmolyte B. 
     As a consequence of solutes, Osmolyte A preferentially has a lower solubility, TDS, and osmotic pressure while Osmolyte B has a much higher solubility, TDS, and osmotic pressure. This variance of osmotic pressures is relevant to the present invention. 
     The process operates in a fashion which dramatically improves RO recovery and associated generation of high TDS concentrate. 
     Moderately high TDS feed water (such as seawater) is dewatered by a forward osmosis process employing Osmolyte A. The resulting diluted Osmolyte A effluent is conveyed to an external RO system for dewatering and generation of a fresh permeate as well as concentrated Osmolyte A. 
     The concentrated Osmolyte A is returned from the external RO, heated, and converted by CO 2  emission to Osmolyte B. Since Osmolyte B has a much higher solubility than Osmolyte A, additional Osmolyte A solute is blended and converted to Osmolyte B during the heating process serving to drive the generated Osmolyte B TDS and osmotic pressure very high. This high osmotic pressure Osmolyte B is employed as draw solution in a second forward osmosis process. 
     This second forward osmosis system employs the very high osmotic pressure Osmolyte B draw solution to dewater the concentrate effluent from the first forward osmosis process. The very high osmotic pressure of Osmolyte B efficiently dewaters this concentrate, generating a very high osmotic pressure concentrate effluent from the second forward osmosis process. The diluted from the Osmolyte B effluent from the second forward osmosis process is cooled while in absorbing contact with CO 2  resulting in conversion of Osmolyte B back into the much lower solubility Osmolyte A. 
     The conversion of the much higher solubility Osmolyte B into Osmolyte A generates a saturated Osmolyte A solution as well as a solute precipitate of Osmolyte A. The saturated Osmolyte A solution is used as the draw solution in the first forward osmosis process to initially dewater the feed water. 
     The solute precipitate of Osmolyte A is conveyed as the Osmolyte A solute to the previously discussed heating process, in which the Osmolyte B is generated from the external RO return concentrate. The cycle is closed with a very high TDS and osmotic pressure concentrate discharged from the second forward osmosis process affirming to a very high recovery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying figures, wherein: 
         FIG. 1  depicts a process diagram of an embodiment of the present invention wherein the highest freshwater recovery is engendered. This embodiment assists cooling of Osmolyte B by pre-cooling CO 2  emitted during the heating process to convert Osmolyte A into Osmolyte B. This embodiment also employs mixing of returned freshwater extraction process concentrate with Osmolyte A solids precipitate before said heating process; 
         FIG. 2  depicts a process diagram of the preferred embodiment of the present invention employing two forward osmosis processes wherein the concentrate from the first is dewatered from the second wherein the highest freshwater recovery is engendered; 
         FIG. 3  depicts a diagram of an embodiment of the sent invention employing two forward osmosis processes wherein a very high TDS feed water, such as, but not limited to offsite RO or other freshwater extraction concentrate and a lower TDS feed water are sourced for each forward osmosis process; 
         FIG. 4  depicts a diagram of an embodiment of the present invention wherein the highest freshwater recovery is engendered. This embodiment employs multiple recycle of thermal energy to imbue thermal efficiency; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Information relating to the application, usage, and benefits of the presently preferred embodiment is discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the present invention, and do not limit its scope. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely as a process for improving the recovery performance of freshwater extraction processes and a means to increase the concentration (TDS) level and associated osmotic pressure of the concentrate effluent normally associated with freshwater extraction processes. The present invention may also be relevant, however, to other situations where osmotic process functions are affected. 
     With reference now to  FIG. 1  a feed water stream  3 , from which fresher water is to be extracted, conveys into a first forward osmosis process  2 . A moderately high TDS and osmotic pressure solution (Osmolyte A)  1  conveys into and services dewatering of feed water  3  in the first forward osmosis process  2 . 
     Moderately high TDS and osmotic pressure concentrate  4  egresses the first forward osmosis process  2  and conveys as feed water into a second forward osmosis process  12 . Dilute Osmolyte A effluent  5 , from the first forward osmosis process  2 , conveys as feed fluid  6  to an external freshwater extraction process. Concentrated Osmolyte A  7  returns from the external freshwater extraction process as a high TDS, high osmotic pressure concentrate. 
     The returning concentrated Osmolyte A stream  7  combines and is heated  9  with precipitated solids of Osmolyte A  8 . CO 2    10  is expelled and conveyed for further use  15 . Combined addition of Osmolyte A solute  8 , heating, and CO 2  emission  10 , converts the concentrated Osmolyte A stream  7  into a very much higher TDS and osmotic pressure Osmolyte B  11 . Emitted CO 2    10  conveys for cooling  14  and other process application. 
     The very high osmotic pressure Osmolyte B solution  11  conveys to and serves as a draw solution in a second forward osmosis process  12 . This second forward osmosis process  12  exploits the very high osmotic pressure of the Osmolyte B solution  11  to dewater the concentrate  4  from the first forward osmosis process  2 . 
     A highly concentrated effluent  13  conveys from the second forward osmosis process  12  to external discharge or other external processes. A dilute Osmolyte B draw solution  16  conveys from the second forward osmosis process  12  to cooled contacting  20  with cooled, previously emitted CO 2    15 . The dilute Osmolyte B solution  16  absorbs the CO 2    15  converting to a much lower solubility but saturated Osmolyte A solution  1  and precipitated Osmolyte A solids  8 . The saturated, lower solubility Osmolyte A solution  1  conveys to and serves as a draw solution for the first forward osmosis process  2  which completes the process cycle. 
     In another embodiment of the present invention, reference  FIG. 2 , a feed water stream, from which fresher water is to be extracted  203 , conveys into a first forward osmosis process  202 . A moderately high TDS and osmotic pressure solution (Osmolyte A)  201  conveys into and services dewatering of feed water  203  in the first forward osmosis process  202 . Moderately high TDS and osmotic pressure concentrate  204  egresses the first forward osmosis process and conveys as feed water into a second forward osmosis process  212 . 
     A dilute Osmolyte A effluent  205  from the first forward osmosis process  202  conveys as feed fluid  206  to an external freshwater extraction process. Concentrated Osmolyte A  207  returns from the external freshwater extraction process as a high TDS, high osmotic pressure concentrate. 
     The returning concentrated Osmolyte A stream  207  combines and is heated  209  with precipitated solids of Osmolyte A  208 . CO 2  is expelled  210  and conveyed for further use  215 . Combined addition of Osmolyte A solute  208 , heating, and CO 2  emission  210  converts the concentrated Osmolyte A stream  207  into a very much higher TDS and osmotic pressure Osmolyte B  211 . 
     The very high osmotic pressure Osmolyte B solution  211  conveys to and serves as a draw solution in a second forward osmosis process  212 . This second forward osmosis process exploits the very high osmotic pressure of the Osmolyte B solution  211  to dewater the concentrate  204  from the first forward osmosis process  202 . 
     A highly concentrated effluent  213  conveys from the second forward osmosis process  212  to external discharge or other external processes. A dilute Osmolyte B draw solution  216  conveys from the second forward osmosis process  212  to cooled contacting  220  with the previously expelled CO 2    215 . The dilute Osmolyte B solution  216  absorbs the CO 2    215  converting to a much lower solubility but saturated Osmolyte A solution and precipitated Osmolyte A solids  208 . The saturated, lower solubility Osmolyte A solution  201  conveys to and serves as a draw solution for the first forward osmosis process  202  which completes the process cycle. 
     In another useful embodiment of the invention, reference  FIG. 3 , a feed water stream, from which fresher water is to be extracted  303 , conveys into a first forward osmosis process  302 . A moderately high TDS and osmotic pressure solution (Osmolyte A)  301  conveys into and services dewatering of feed water  303  in a first forward osmosis process  302 . 
     Moderately high TDS and osmotic pressure concentrate  304  egresses the first forward osmosis process for external discharge or further external processes. A dilute Osmolyte A effluent  305  from the first forward osmosis process  302  conveys as feed fluid  306  to an external freshwater extraction process. 
     Concentrated Osmolyte A  307  returns from the external freshwater extraction process as a high TDS, high osmotic pressure concentrate. The returning concentrated Osmolyte A stream  307  combines and is heated  309  with precipitated solids of Osmolyte A  308 . CO 2  is expelled  310  and conveyed for further use  315 . Combined addition of Osmolyte A solute  308 , heating, and CO 2  emission  310  converts the concentrated Osmolyte A stream  307  into a very much higher TDS and osmotic pressure Osmolyte B  311 . 
     The very high osmotic pressure Osmolyte B solution  311  conveys to and serves as a draw solution in a second forward osmosis process  312 . This second forward osmosis process exploits the very high osmotic pressure of the Osmolyte B solution  311  to dewater an externally supplied, high TDS and osmotic pressure feed water  332 . 
     A highly concentrated effluent  313  conveys from the second forward osmosis process  312  to external discharge or other external processes. A dilute Osmolyte B draw solution  316  conveys from the second forward osmosis process  312  to cooled contacting  320  with the previously expelled CO 2    315 . The dilute Osmolyte B solution  316  absorbs the CO 2    315  converting to a much lower solubility but saturated Osmolyte A solution and precipitated Osmolyte A solids  308 . 
     The saturated, lower solubility Osmolyte A solution  301  conveys to and serves as a draw solution for the first forward osmosis process  302  which completes the process cycle. 
     In another useful potentially energy recycling embodiment of the invention, reference  FIG. 4 , a feed water stream  403 , from which fresher water is to be extracted, conveys into a first forward osmosis process  402 . A moderately high TDS and osmotic pressure solution (Osmolyte A)  401  conveys into and services dewatering of feed water  403  in a first forward osmosis process  402 . 
     Moderately high TDS and osmotic pressure concentrate  404  egresses the first forward osmosis process and conveys as feed water into a second forward osmosis process  412 . A dilute Osmolyte A effluent  405  from the first forward osmosis process  402  conveys as feed fluid  406  to an external freshwater extraction process. Concentrated Osmolyte A  407  returns from the external freshwater extraction process as a high TDS, high osmotic pressure concentrate. 
     The returning concentrated Osmolyte A stream  407  splits to provide cooling to other processes in this embodiment, being itself warmed by this service. The warmed concentrated Osmolyte A split stream  472  and  471  recombines  473  and is further combined and heated as necessary  409 , with precipitated solids of Osmolyte A  408 . CO 2  is expelled  410  for further process use. The combined addition of Osmolyte A solute  408 , heating, and CO 2  emission  410  converts the concentrated Osmolyte A stream  473  into a very much higher TDS and osmotic pressure Osmolyte B  411 . 
     The very high osmotic pressure Osmolyte B solution  411  conveys to and serves as a draw solution in a second forward osmosis process  412 . This second forward osmosis process exploits the very high osmotic pressure of the Osmolyte B solution  411  to dewater the concentrate  404  from the first forward osmosis process  402 . 
     A highly concentrated effluent  413  conveys from the second forward osmosis process  412  to external discharge or other external processes. A dilute Osmolyte B draw solution  416  conveys from the second forward osmosis process  412  for cooling by heat exchange  460  with a split stream of the Osmolyte A concentrate  407 . The cooled dilute Osmolyte B draw solution  417  conveys to a cooled CO 2  contactor  420 . The emitted CO 2    410  is cooled by heat exchange  450  with a split stream of Osmolyte A concentrate  407  providing cooled CO 2    415  to the contactor  420 . During cooled CO 2  contact the cooled, dilute Osmolyte B solution  416  absorbs the cooled CO 2    415  converting to a much lower solubility but saturated Osmolyte A solution  401  and precipitated Osmolyte A solids  408 . 
     The saturated, lower solubility Osmolyte A solution  401  conveys to and serves as a draw solution for the first forward osmosis process  402  which completes the process cycle. 
     A novel and beneficial feature of the preferred embodiment of the invention is the high concentration which can be achieved from the very high TDS and osmotic pressure draw solution servicing the second forward osmosis process. This feature engenders the process invention with much higher water recovery than demonstrated by the prior art, thereby enhancing performance, efficiency and environmental stewardship. 
     A novel aspect of the present invention is the use of a salt which, through thermally activated addition or emission of CO 2 , can be converted from a moderately high solubility solute, with associated moderately high osmotic pressure, to a very high solubility solute, with associated very high osmotic pressure in a completely reversible manner. The engagement of this effect while in the employ of two or more forward osmosis processes proffers the ability to assist conventional freshwater recovery processes to both substantially improve their freshwater recovery efficiency and additionally to enable said freshwater extraction processes to successfully generate freshwater from much higher osmotic pressure water sources than previously possible. 
     Although the present invention and its advantages and benefits have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the present invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. Finally, in the foregoing discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”.