Abstract:
A method for reducing the salinity, as well as the hydrocarbon concentration of produced water to levels sufficient to meet surface water discharge standards. Pressure vessel and coflow injection technology developed at the Oak Ridge National Laboratory is used to mix produced water and a gas hydrate forming fluid to form a solid or semi-solid gas hydrate mixture. Salts and solids are excluded from the water that becomes a part of the hydrate cage. A three-step process of dissociation of the hydrate results in purified water suitable for irrigation.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for forming solid and semi-solid clathrate hydrate structures useful for removing salt and other soluble materials from water. In particular, a series of completion steps involving pressurization and clathrate formation, depressurization, ice formation and desalting is used to recover the desalted water from the clathrate hydrate structures. 
     2. Description of Prior Art 
     Produced water is the raw water that is a byproduct of natural gas and oil production. Produced water contains organic matter, suspended silts and clays, and formation brine. It is commonly re-injected below ground due to the high cost of purification for above ground disposal. In particular, the cost of separating the dissolved formation salts from the water is expensive. An economical method to purify produced water would be of great benefit to oil and natural gas producers, particularly in regions where oil and gas are produced and surface irrigation or stream water is in demand. 
     Common techniques for the desalination of water include distillation, reverse osmosis and freezing. Distillation involves the evaporation of saline water and collection of the freshwater condensate. Reverse osmosis requires high-pressure pumps to force saltwater through a semi-permeable membrane to produce freshwater. This technique is more expensive due to the costs of membranes and pumps. 
     Various techniques are used to produce freshwater using a freezing process. Fundamentally, the freezing process relies on the fact that salt is rejected from the ice. In typical freezing processes, cooling elements are located in saltwater. When ice forms on the cooling elements, they are separated from the saltwater, melted, and freshwater is produced. 
     Hydrate formation is being developed as a desalination technique for use in oceans (McCormack, Ref. 2), (Max, Refs. 3, 5). Such techniques involve the use of a long tube (˜100 meters), or a desalination fraction column that is situated to transect the hydrate stability zone, thereby allowing hydrates to form and dissociate in the column. The hydrate forms at the bottom of the column and rises through the column due to its buoyant properties. As the hydrate rises, it crosses into its zone of instability and dissociates to produce freshwater. The freshwater collects at the top portion of the column due to density differences between freshwater and saltwater. 
     It is also known that solid or semi-solid clathrate hydrate structures can be formed by mixing water and certain gases (gas hydrates) such as carbon dioxide and methane. As the clathrate hydrate structures form, they tend to exclude salt or any dissolved species from the clathrate structure (Donath, Ref. 1), (Knox, Ref. 6). The present invention takes advantage of this feature in a series of completion steps that recover the desalted water. 
     A technology called coflow injection has been developed at the Oak Ridge National Laboratory for continuous hydrate production. It involves high energy mixing of a hydrate forming fluid and water in a coflow injector, and then ejecting the mixture into a pressurized vessel (West, Refs. 4, 7), (Lee, Ref. 8). It has been observed that when the injected water has solid particles suspended or dissolved in it, the hydrate formation process appears to exclude the solids from the solid hydrate composite. 
     An advantage of the coflow injection method over conventional batch reactor type hydrate producers is that the injector may be used for continuous hydrate production, and the shape of the produced solid material allows for easier transfer of excluded solids and dissolved species. A further advantage of the coflow injector is the demonstrated ability to precisely control the density. 
     REFERENCES 
     1) W. E. Donath, “Method and Apparatus for Producing Purified Water from Aqueous Saline Solutions”, U.S. Pat. No. 2,904,511, issued Sep. 15, 1959. 
     2) R. A. McCormack, “Clathrate Freeze Desalination Apparatus and Method”, U.S. Pat. No. 5,553,456, issued Sep. 10, 1996. 
     3) M. D. Max, R. E. Pellenbarg, “Desalination through Methane Hydrate”, U.S. Pat. No. 5,873,262, Feb. 23, 1999. 
     4) O. R. West, C. Tsouris, L. Liang, “Method and Apparatus for Efficient Injection of CO 2  in Oceans,” U.S. Pat. No. 6,598,407, Jul. 29, 2003. 
     5) M. D. Max, “Desalination using Positively Buoyant or negatively Buoyant/Assisted Buoyancy Hydrate”, U.S. Pat. No. 6,733,667, May 11, 2004. 
     6) W. G. Knox, M. Hess, G. E. Jones, Jr., H. B. Smith, Jr., “The Hydrate Process”, Chem. Eng. Prog., 1961, Vol. 57(2) pp. 66-71. 
     7) O. R. West, C. Tsouris, L. Liang, S. Y. Lee, S. McCallum, “Negatively Buoyant CO 2 -Hydrate Composite for Ocean Carbon Sequestration”, AIChE J, 2003, Vol. 49, p.p. 283-285. 
     8) S. Y. Lee, L. Liang, D. E. Riestenberg, O. R. West, C. Tsouris, “CO 2  Hydrate Composite for Ocean Carbon Sequestration”, Environ. Sci. Tech., 2003, Vol. 37, pp. 3701-3708. 
     BRIEF SUMMARY OF THE INVENTION 
     In a method wherein solid or semi-solid clathrate hydrate structures are formed in a pressure vessel from water containing dissolved materials and a hydrate-forming gas, the additional steps of: depressurizing the pressure vessel to turn the solid or semi-solid clathrate hydrate structures into an ice-like clathrate hydrate mass; washing the ice-like clathrate hydrate mass with unreacted water containing dissolved materials to partially melt the ice-like clathrate hydrate mass and wash out dissolved materials; removing the melt water containing the dissolved materials from the pressure vessel; and melting the remaining ice-like clathrate hydrate mass to recover the purified water therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the formation of solid and semi-solid clathrate hydrate in a coflow hydrate formation process using produced water. 
         FIG. 2  illustrates the formation of solid and semi-solid clathrate hydrate in an alternate coflow hydrate formation process using produced water. 
         FIG. 3  illustrates the formation of solid and semi-solid clathrate hydrate in a stirred tank hydrate formation process using produced water. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a series of completion steps for incorporation into any of several desalting, dewatering or soluble material removal/recovery processes that involve the formation of solid or semi-solid clathrate hydrate structures. 
     Solid and semi-solid clathrate hydrate structures can form when water is mixed with certain gases that can form gas hydrates such as carbon dioxide and methane. In the invention, after the hydrate structure is formed, the more concentrated solute solution can be discharged. As a secondary purification step, a subsequent rapid depressurization below the hydrate stability zone converts the hydrate to ice. The ice may rest on an elevated screen that allows saline water to drain out of the ice. The ice is then allowed to warm. As the ice warms, the saline water or water containing other dissolved or miscible species drains out first. After sufficient saline water has drained, the remaining mass of ice contains fresh water having a much-reduced volume of salt trapped in the pore space of the ice mass. The ice or fresh water is then recovered. 
     A first preferred embodiment of the invention pertains to a coflow hydrate formation process using produced water. In  FIG. 1 , a pressure vessel  15  is initially empty or partially full of produced water  16 . Hydrate is formed by injecting produced water  17  and a hydrate-forming gas  18  into the pressure vessel  15 . A coflow injector  19  is used for this purpose. The hydrate that is formed may or may not have the same density as the produced water in the system. The pressure vessel  15  and coflow injector  19  may be such as described in Refs  4 ,  7  and  8 , for example. 
     Natural gas is the preferred hydrate-forming gas because it is available at the wellhead. However, CO 2 , methane or propane mixed gas hydrates could also be used. Hydrate formation is continued until the pressure vessel  15  contains a considerable mass  20  of a mixture of ice and clathrate hydrate. Production of the ice and clathrate hydrate mixture  20  is then stopped by turning off the flow of produced water  17  and hydrate-forming gas  18  to the coflow injector  19 . A screen such as shown at  21  or  22  may be used in conjunction with a drain to remove the drain water that contains the runoff salts and other materials from the ice-like clathrate hydrate mass in the pressure vessel. A secondary separation effect can be carried out by rapid depressurization with subsequent water ice formation wherein the particular dissolved species is far less favored in the water ice and the solute becomes more concentrated in the free water. 
     An additional, optional step may be incorporated into the above method. It is also shown in  FIG. 1 . In the particular case where the pressure vessel  15  is partially full of produced water, rapid buildup of pressure in the pressure vessel may be avoided by recycling the unreacted produced water  16  from the pressure vessel  15  back through the coflow injector  19 . This additional step also aids the hydrate formation kinetics in the coflow injector  19  because of the “memory effect” of water. The memory effect takes place when water that has been in coexistence with clathrate hydrate is reused to form hydrate. 
     Another preferred embodiment of the invention, shown in  FIG. 2 , uses a different coflow hydrate formation process. In  FIG. 2 , a pressure vessel  30  is filled with produced water  31 . A hydrate forming gas  32  such as natural gas, CO2, etc in the form of bubbles or drops is injected into the produced water  31  to form the solid or solid-like clathrate hydrate  33 . Injection of the hydrate-forming gas  32  is continued until a predetermined pressure limit is reached in the pressure vessel  30 . The injection is then stopped. 
     A still different embodiment of the invention is based on a stirred tank hydrate formation process. In  FIG. 3 , a Parr pressure vessel  40  is used. Inside the vessel  40  are a baffle  41 , impeller  42 , drain tube  43 , and screen  44 . Produced water  45  nearly fills the pressure vessel  40 . The baffle  41  and impeller  42  are used to promote mixing of a hydrate forming gas such as natural gas or CO 2  throughout the water  45 . The drain tube  43  serves to remove excess water that has been excluded after hydrate formation. The screen  44  separates the ice/clathrate hydrate mass  46  from the excluded water and allows the excess water to drain. In this embodiment, the stirring impeller  42  may also be replaced by a vibrating, sonicating source or other instrument used to disperse the gas phase in the liquid phase to increase the gas-liquid interfacial area where hydrate formation occurs. 
     In the stirred tank apparatus of  FIG. 3 , hydrates are formed by first pressurizing the vessel  40  with CO 2 , for example, below the hydrate stability point for approximately 15 minutes. This allows the CO 2  to dissolve into the water  45 . The pressure is then ramped up to ˜1000 psi, well above the equilibrium pressure, which induces hydrate formation in the water  45 . The high pressure is not necessary, but does increase the rate of hydrate formation. The hydrates may be allowed to form for approximately one hour. 
     Turning now to the invention, a set of completion steps involving depressurization, ice formation and desalting is provided. These steps may be utilized with any of the above described hydrate formation methods. The completion steps comprise a method to remove salt and other soluble species from the ice-like clathrate hydrate structures produced in the pressure vessel embodiments of  FIGS. 1-3 . 
     In the case of the coflow injection formation methods illustrated by  FIGS. 1 and 2 , the starting point for the completion steps is that the pressure vessel contains a considerable fraction of clathrate hydrate and unreacted produced water. By “considerable fraction” it is meant that an upper pressure limit has been reached in the pressure vessel. Unreacted produced water is water that was excluded from the clathrate structure during formation. 
     The steps of the completion method for  FIGS. 1 and 2  are as follows:
         1) Drain the unreacted, more concentrated water, i.e., the water that was excluded from the hydrate structure.   2) Depressurize the pressure vessel rapidly to a low pressure. Rapid depressurization cools the water that is trapped in the clathrate hydrate cage and forms ice. Salts and other constituents of the produced water are rejected from the ice that forms. Some fraction of the salts are physically associated, i.e., sorbed onto the surface of the ice and clathrate hydrate. This cooling below the freezing point keeps the hydrate stable and unable to dissociate at this step.   3) Allow part of the ice to melt by exchanging heat with room temperature produced water. The rinsing/melting step removes a large part of the salts and other materials that migrated to the ice surface as the ice formed.   4) Remove the drain water that contains the runoff salts and other materials from the pressure vessel.   5) Allow the hydrate to dissociate by increasing the temperature of the pressure vessel. The fresh water may then be recovered from the pressure vessel. The last two steps may be carried out repeatedly to recover different portions of water having varying concentrations of salts and other materials. In the dewatering embodiment of the invention, the drain water would contain a concentration of a desired product, as could be the case for recovering acids or alcohols for subsequent purification (i.e., distillation).       

     The completion steps for the stirred tank embodiment of  FIG. 3  are alike in substance to those for the coflow injection embodiments.
         1) After allowing the hydrates to form for approximately one hour, the pressure is decreased to a pressure still well above the hydrate equilibrium pressure and the excluded water is drained from the pressure vessel using the drain tube, which allows the hydrate mass to rest on the screen.   2) The vessel is then cooled to 1.5° C. using an ethylene glycol circulator. The additional cooling provided by the ethylene glycol circulator may be unnecessary if cooling provided by rapid depressurization is sufficient or if the hydrate forming gas has a low equilibrium pressure.   3) Once the vessel reaches 1.5° C., the pressure is rapidly decreased to considerably below the equilibrium pressure. The rapid decrease in pressure serves to cool the vessel to approximately −4° C. The hydrate is now no longer stable, and the cold temperature of −4° C. serves to convert the hydrate mass to ice.   4) At this point, the vessel is allowed to warm, allowing the ice mass resting on the screen to slowly melt. The vessel is kept at ˜100 psi and the melt water is drained from the vessel.   5) The vessel is then depressurized. The ice mass resting on the screen represents the freshest water, along with a small amount of liquid that remains at the bottom of the vessel.       

     EXAMPLES 
     Below is a summary of desalination experiments that have been performed in a 450 mL Parr pressure vessel. All the experiments started with NaCl water at a concentration of 35 ppt. Experiments always started with 370 mL of saltwater in the Parr Vessel (with the exception of experiments using the coflow injector). It was found that the primary controls on the final salinity of the water were the amount of hydrate that formed and the amount of washing allowed. 
     The table below summarizes the results of the desalination experiments. The most important column to note is the one titled “Ice Mass on Screen (Freshest H2O)”. The ice mass on the screen (once melted) was the freshest water, or the final result of the desalination experiment. The “Excluded H 2 O” is the water that is drained after hydrate formation is complete. The “Aliquots” are collected as the ice mass on the screen melts, and represent the washing cycle. It is also important to note that the wash cycle did not add any fresh water to the system, but melting of the ice mass was considered a wash step. The “Fluid Below Screen” represents the fluid that could not be collected using the sampling dipstick, and is essentially a third aliquot. In the early experiments the fluid below the screen and ice above the screen were combined to define the “Fluid below and ice mass above screen”. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Summary of desalination experiment results. 
               
               
                 C = Conductivity (mS); S = Salinity (ppt); and V = Volume (mL) 
               
             
          
           
               
                   
                   
                   
                   
                 Ice Mass 
                   
                   
               
               
                   
                   
                   
                   
                 on Screen 
                 Fluid 
                 Fluid below 
               
               
                   
                   
                   
                   
                 (Freshest 
                 Below 
                 and ice mass 
               
               
                   
                 Excluded H 2 O 
                 Aliquot 1 
                 Aliquot 2 
                 H 2 O) 
                 Screen 
                 above screen 
               
               
                   
                   
               
             
          
           
               
                   
                 C 
                 S 
                 V 
                 C 
                 S 
                 V 
                 C 
                 S 
                 V 
                 C 
                 S 
                 V 
                 C 
                 S 
                 V 
                 C 
                 S 
                 V 
               
             
          
           
               
                 Expt 1 
                 62 
                 30 
                 211 
                 56 
                 25 
                 21 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 28 
                 15 
                 138  
               
               
                 Expt 2 
                 60 
                 40 
                 212 
                 62 
                 42 
                 52 
                 61 
                 41 
                 46 
                 15 
                 9 
                 38 
                 47 
                 31 
                 22 
                 28 
                 17 
                 56 
               
               
                 Expt 3 
                 60 
                 40 
                 223 
                 61 
                 40 
                 48 
                 60 
                 41 
                 18 
                 38 
                 24 
                 69 
                 55 
                 36 
                 29 
                 41 
                 26 
                 98 
               
               
                 Expt 4 
                 58 
                 39 
                 237 
                 56 
                 37 
                 46 
                 57 
                 38 
                 41 
                 38 
                 24 
                 23 
                 48 
                 32 
                 25 
                 44 
                 28 
                 44 
               
               
                   a Expt 5 
                 64 
                 43 
                 168 
                 65 
                 44 
                 29 
                 32 
                 20 
                 17 
                 20 
                 12 
                 36 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   b Expt 6 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 20 
                 12 
                 11 
                 55 
                 37 
                 61 
                 — 
                 — 
                 — 
               
               
                   c Expt 7 
                 52 
                 34 
                 270 
                 60 
                 41 
                 69 
                 — 
                 — 
                 — 
                 9 
                 5 
                 27 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   c Expt 8 
                 55 
                 37 
                 242 
                 59 
                 39 
                 47 
                 45 
                 29 
                 63 
                 3 
                 2 
                 19 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                   a = Four aliquots were actually taken, only the first and last are shown 
               
               
                   b = Experiment 6 used the coflow injector. Multiple coflow injection arrangements were tested. Cofiow experiments are described below. 
               
               
                   c = Experiments using Snomax 
               
             
          
         
       
     
     Brief Description of Experiments 1 through 8 
     The experiments used a technique that is being called the Hydrate to Ice Technique. Essentially the experiment begins with 370 mL of 35 ppt NaCl water in the Parr Vessel. Hydrates are then allowed to form. After hydrate formation, the excluded water is drained from the bottom of the vessel. As the water drains, the hydrate mass accumulates on an elevated screen situated in the Parr vessel. The vessel is kept cool in a cold room and a chilling circulator is sometimes used to keep the vessel cool. However, a rapid depressurization will also serve to cool the vessel. It is believed that water trapped inside the hydrate is converted to ice during the rapid depressurization to ˜100 psi. Once the vessel is depressurized to 100 psi, it is removed from the chilling units and allowed to warm. As the ice (and hydrate) warm, the salt is washed off and accumulates as saltwater below the screen. The amount of water that is allowed to melt/wash helps to control the final salinity of the desalinated water.  FIG. 3  shows the setup of the Parr vessel. 
     Experiment 1 
     In Experiment 1, the conductivity of the water above the screen was not measured because the screen in the Parr vessel was not installed yet. This was also the only experiment performed using a conductivity meter that had questionable accuracy. However, the final freshwater concentration was approximately 15-20 ppt. Hydrate formation persisted for 63 minutes and 21 mL of water were allowed to melt/wash. Out of 370 mL, the final volume of partially desalinated water was 138 mL. 
     Experiment 2 
     In Experiment 2, the produced desalinated water had a final concentration of 8.7 ppt. Hydrates were allowed to form for 66 minutes, and 120 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 38 mL 
     Experiment 3 
     In Experiment 3, produced desalinated water with a final concentration of 23.9 ppt was produced. Hydrates were allowed to form for 53 minutes and 95 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 69 mL. It appears from these results that a shorter time period for hydrate formation and smaller wash/melt volume do not reduce the salinity as strongly. 
     Experiment 4 
     In Experiment 4, produced desalinated water with a final concentration of 24 ppt was produced. Hydrates were allowed to form for 60 minutes, and 112 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 23 mL. One possible reason for the high final concentration (24 ppt) could be due to the warmer temperatures that existed in the cold room. The cold room was not functioning correctly due to freezing. These warmer temperatures could have meant that hydrate was not forming at as rapid a rate as previous experiments. 
     Experiment 5 
     In Experiment 5, produced desalinated water with a final concentration of 12.2 ppt was produced. Hydrates were allowed to form for 112 minutes, and 165 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 36 mL. This experiment attempted to repeat experiment 2, except a longer time span was allowed for hydrate formation. However, this time span did not seem to greatly increase the amount of hydrate that formed. 
     Experiment 6 
     A total of five coflow experiments were performed, during which five different coflow injection arrangements were tested ( FIGS. 1 ,  2 ). In all experiments where a composite formed, it was very loosely consolidated and difficult to separate from the excluded water. To make a more consolidated composite, higher pressures would be required. These pressures are attainable in the Parr vessel, but constant venting is required to prevent from approaching the 2000 psi pressure limit. These experiments could be repeated in the 72 l Seafloor Process Simulator, but we would have to look for a conductivity/salinity increase in the vessel water as we inject through the coflow instead of a direct measurement of the injected composite. We could also attempt using liquid propane in the coflow injector, which would require only ˜70 psi at 4° C., and the experiments could be conducted in the Parr vessel. The pressure problem is due to the Parr vessel having a small volume. 
     One coflow experiment did produce a fairly low desalinated concentration of 11.6 ppt. This final concentration was measured immediately after the composite was formed. Therefore, almost no time was allowed for washing. The composite that formed was loosely consolidated, but formed a reasonably solid mass upon depressurization. 
     Experiment 7 
     Experiment 7 was performed using the same general procedure as the first five experiments, except Snomax was added. Snomax is a crystal formation seed for the hydrate, and is available from York Snow, Inc. A surfactant may give the same beneficial effect as Snomax. Snomax was added at a concentration of 10 ppm to a 35 ppt NaCl water solution. Before the experiment, a standard curve was generated using Snomax and saltwater (and also with distilled water). It was found that Snomax does not affect the conductivity/salinity measurements at a concentration of 10 ppm. The final concentration of the desalinated water was 4.8 ppt. Hydrates were allowed to form for ˜60 minutes, and 69 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 27 mL. The addition of Snomax appears to be very effective at helping to reduce the salinity. This could be partially due to the hydrate morphology that resulted from this approach. The hydrate mass had much fewer CO 2  droplets coated by hydrate, and much more small flakes of hydrate. 
     Experiment 8 
     Snomax was again used to attempt to repeat the results observed in Experiment 7. The final concentration of the desalinated water was 1.6 ppt. Hydrates were allowed to form for 257 minutes and 110 mL of water was allowed to melt/wash. The final volume of partially desalinated water was 19 mL. The final volume of desalinated water was slightly lower than expected. 
     The use of a nucleation agent such as Snomax does appear to improve the desalination process. Primary controlling factors of the final salinity of the water are likely the amount of hydrate formed and the amount of water that is allowed to wash/melt. 
     The purification process described herein could be repeated to achieve higher purity water. If this technology were to be scaled up, it could provide desalination solutions in coastal environments or potential product concentration from aqueous solutions as in bio-alcohol formation. 
     The methods described herein are carried out in only a single pressure vessel. All earlier approaches have used multiple pressure vessels, i.e. a hydrate formation vessel and a dissociation vessel, or long tubes between vessels.