BRINE SATURATOR

Abstract

A brine saturation process is disclosed. The process comprises increasing the salinity of an unsaturated saline stream (15) by passage through a brine saturator (5) in which salt is dissolved into the unsaturated saline stream (15) to produce a high salinity stream (11); and then converting latent osmotic energy present in said high salinity stream (11) into power by passage through an osmotic power unit (20). The process further comprises using an output stream derived from the high salinity stream (11) following passage through the osmotic power unit (12) as the unsaturated saline stream (15).

Claims

1-17. (canceled)

18. A brine saturation process comprising: increasing the salinity of an unsaturated saline stream by passage through a brine saturator in which salt is dissolved into the unsaturated saline stream to produce a high salinity stream; converting latent osmotic energy present in said high salinity stream into power by passage through an osmotic power unit comprising a membrane in which said high salinity stream is passed over one side the membrane, a low salinity stream being passed over a second side of said membrane; using an output stream derived from the high salinity stream following passage through the osmotic power unit as the unsaturated saline stream.

19. The process according to claim 18, wherein the latent osmotic energy is converted into electricity by passage through the osmotic power unit.

20. The process according to claim 18, wherein the process comprises passing a first part of said high salinity stream to the osmotic power unit and outputting a second part of said high salinity stream.

21. The process according to claim 20, wherein the second part of the high salinity stream in provided to a chlorine production process.

22. The process according to claim 21, wherein the second part of the high salinity stream is used as the electrolyte in an electrolytic cell configured to produce chlorine by electrolysis.

23. The process according to claim 18, further comprising both (i) increasing the pressure of the high salinity stream prior to passage through the osmotic power unit and (ii) decreasing the pressure of the output stream by passage through a pressure exchanger in which pressure is transferred from the output stream to the high salinity stream.

24. The process according to claim 23, further comprising passing a first part of the output stream through the pressure exchanger and passing a second part of the output stream through a turbine in which electricity is generated by expansion of the output stream.

25. The process according to claim 24, further recombining the first and second parts of the output stream after passage through the pressure exchanger and the turbine, respectively, for use as the unsaturated saline stream.

26. The process according to claim 24, wherein a flow rate of said first part of the output stream passed to the pressure exchanger is equal to a flow rate of the high salinity stream on entry to the osmotic power unit.

27. The process according to claim 18 wherein the membrane of the osmotic power unit comprises a semi-permeable membrane which permits passage of water but not passage of salts, and wherein said high salinity stream is passed over one side of the semi-permeable membrane, the low salinity stream being passed over a second side of said semi-permeable membrane.

28. The process according to claim 18 wherein the osmotic power unit comprises a cation exchange membrane and an anion exchange membrane and wherein the high salinity stream is passed over one side of the cation exchange membrane and one side of the anion exchange membrane, the low salinity stream being passed over a second side of the cation exchange membrane and a second side of the anion exchange membrane.

29. A brine saturation system, comprising: a brine saturator configured to increase the salinity of an unsaturated saline stream to produce a high salinity stream; and an osmotic power unit configured to generate power using a difference in salinity between a low salinity stream and the high salinity stream; and wherein the system is arranged such that an output stream from the osmotic power unit is passed to the brine saturator for use as the unsaturated saline stream, said output stream being derived from the high salinity stream following passage through the osmotic power unit.

30. The brine saturation system according to claim 29, wherein the osmotic power unit is configured to generate electricity using the difference in salinity between the low salinity stream and the high salinity stream.

31. The brine saturation system according to claim 30, wherein the osmotic power unit is arranged to generate electricity through Pressure Retarded Osmosis (PRO).

32. The brine saturation system according to claim 30, wherein the osmotic power unit is arranged to generate electricity through Reverse Electrodialysis (RED).

33. The brine saturation system according to claim 29, wherein the brine saturator is configured to produce the high salinity stream being a saturated saline stream.

34. A system for the production of chlorine, comprising: a brine saturation system configured to: increase the salinity of an unsaturated saline stream by passage through a brine saturator in which salt is dissolved into the unsaturated saline stream to produce a high salinity stream; convert latent osmotic energy present in said high salinity stream into power by passage through an osmotic power unit comprising a membrane in which said high salinity stream is passed over one side the membrane, a low salinity stream being passed over a second side of said membrane; and use an output stream derived from the high salinity stream following passage through the osmotic power unit as the unsaturated saline streaming accordance with any of claims 12 to 16; and at least one electrolytic cell configured to produce chlorine from electrolysis of brine, wherein the system is arranged such that the electrolytic cell receives at least part of the high salinity stream from the brine saturation system.

Description

DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0060] FIG. 1 shows an example process according to the invention;

[0061] FIG. 2 shows a variation on the process of FIG. 1;

[0062] FIG. 3 shows a variation on the process of FIG. 1;

[0063] FIG. 4 shows an osmotic power unit suitable for use in the process of FIG. 1;

[0064] FIG. 5 shows (a) a downflow brine saturator and (b) an upflow brine saturator suitable for use in the process of FIG. 1 or 7;

[0065] FIG. 6 shows an electrolysis cell suitable for use in the process of FIG. 1 or 7;

[0066] FIG. 7 shows a second example process according to the invention; and

[0067] FIG. 8 shows an osmotic power unit suitable for use in the process of FIG. 7.

DETAILED DESCRIPTION

[0068] In many cases it is necessary to dissolve solids into solution. In this process, atoms or molecules locked in the solid will interact with the solvent and move out into the solution as dissolved species. The change from a system consisting of two relatively pure phases, a solid and a solvent, to a mixed solution will give an increase in entropy.

ΔS=S.sub.A(aq).sup.θ−S.sub.A(s).sup.θ

[0069] Depending on the relative difference in energy between solid-solid, solvent-solvent and solid-solvent interactions, enthalpy may either increase, decrease or stay unchanged, which can lead to further changes in entropy as heat is either added or removed from the system. As long as the net change in entropy is increasing, the dissolution process will be spontaneous.

[0070] The entropy generation is available as Gibbs free energy, in this case called mixing energy as described by the following equation:

−ΔG.sub.mix=RT([Σx.sub.i ln(a.sub.i)].sub.M−ϕ.sub.A[Σx.sub.i ln(a.sub.i)].sub.A−ϕ.sub.B[Σx.sub.i ln(a.sub.i)].sub.B)

where G.sub.mix is the Gibbs free energy of mixing, R is the gas constant, T the absolute temperature, x.sub.i the mole fraction of species “i”, a.sub.i the activity of species “i”, ϕ.sub.A and ϕ.sub.B are the ratios of moles in solutions A and B respectively to the total moles in the system and solution M is a mix of solutions A and B.

[0071] One method that can be used to extract mixing energy is pressure retarded osmosis, but any method that can harness mixing energy can be used.

[0072] FIG. 1 shows an example brine saturation process in accordance with an embodiment of the invention. Unsaturated brine 15 enters a salt saturator 5 where it mixes with solid salt 10 to produce a stream of saturated solution 11 (hereafter the saturated stream 11). In other embodiments, the solution leaving the salt saturator 5 may be at less than saturation. In some embodiments the salt saturator 5 is an upflow saturator or a downflow saturator, or combines elements of upflow and downflow saturators. An output stream 13 is split off from saturated stream 11 and sent for further processing while the remainder of the saturated stream 11 is sent to an osmotic power unit 20 (denoted by a dashed box in FIG. 1), in this embodiment, an osmotic power unit 20 that generates electricity using PRO.

[0073] On entry into osmotic power unit 20, a pressure exchanger 1 pressurizes the saturated stream 11 to a pressure below the osmotic pressure of the saturated stream 11. The saturated stream 11 is then passed to one side of a semipermeable membrane 2. A feed pump 4 is used to pump a feed solution 12, having lower salinity than the saturated stream 11, to the other side of the membrane. Optionally, the feed solution 12 may go through various pre-treatment steps before being passed to the semipermeable membrane 2. Due to the osmotic pressure gradient, water from the feed solution 12 permeates the semipermeable membrane, 2 and mixes with the saturated stream 11 to produce a diluted outlet stream 18 which may be said to be derived from the saturated stream 11. The feed solution 12 that has not permeated the semipermeable membrane 2 leaves the osmotic power unit as concentrated feed solution 14. The diluted outlet stream 18 is split into two parts; a first stream 18a is passed to the pressure exchanger 1 and a second stream 18b is passed to an electricity generating device 3. In some embodiments, electricity generating device 3 is a turbine, in other embodiments other devices may be used. After passage through the pressure exchanger 1 and electricity generating device 3 the first and second streams 18a, 18b are recombined and passed to the salt saturator 5 as unsaturated brine 15. Concentrated output stream 14 is disposed of as appropriate. As a consequence of the flow of water from the feed solution 12 to the saturated stream 11 the flow rate of outlet stream 18 from the membrane 2 is larger than the flow rate of the saturated stream 11 at entry to the membrane. The flow rate of the second part 18a going back to the pressure exchanger 1 will be equal to the flow rate of the saturated stream 11 at entry to the pressure retarded osmosis unit 20 while the stream going to the turbine 3 will have a flow rate equal to the permeate flow rate through the membrane, which is equal to the difference in flow rate between the feed solution 12 and concentrated feed solution 14.

[0074] In some embodiments, stream 13 is sent for use in an electrolysis cell 16 of a chlor alkali process to produce chlorine 17. In other embodiments, stream 13 may be used in any other industrial process requiring a brine stream, for example in the preparation of brine for road deicing.

[0075] In some embodiments the concentrated feed solution 14 may be mixed with the unsaturated brine 15 downstream of the turbine 3, thereby increasing the volume of brine produced as stream 13.

[0076] Processes and systems in accordance with the present example embodiment add water to the brine saturator through the pressure retarded osmosis unit. As a result part of the entropy released during the dissolution process may be captured and used for energy generation. Since the water is added via a semi-permeable membrane, any impurities may be rejected to the same extent as would be achieved using reverse osmosis thereby ensuring that the water added to the brine saturator will have a very high purity. The filtering of the water inherent to this process may facilitate omission of a water purification plant like reverse osmosis, ion exchange or evaporators as pretreatment of the feed stream. The inherent filtering can allow different wastewaters, such as process condensate and cooling water, to be used and recycled into the process, thereby saving resources. Processes in accordance with the present example embodiment may also allow the use of a wider variety of sources for the feed stream, provided the feed stream has a lower salinity than the saturated stream and the diluted outlet stream. For example, seawater may be used.

[0077] In this embodiment the pressure retarded osmosis system is based on the use of a pressure exchanger 1 to pressurize the stream 11 which gives an overall increase in system efficiency and therefore larger net energy generation. In other embodiments the pressure exchanger may be absent or the stream 11 may be pressurized in other ways.

[0078] FIG. 2 shows a portion of a variant of the process of FIG. 1 in which the osmotic power unit 5 comprises multiple osmosis units 19a, 19b and 19c connected in series. Like elements are denoted with like reference numerals. Only those elements of the FIG. 2 embodiments which differ from the FIG. 1 embodiment will be discussed here. Each osmosis unit 19a, 19b and 19c contains a semi-permeable membrane (not shown) which permits passage of water but not of salts. Feed stream 12 branches into three streams, 12a, 12b, 12c, each going to a different one of the osmotic units 19a, 19b, 19c. Original high saline stream 11 flows at one side of the semipermeable membrane of the first unit 19a, while lower salinity stream 12a obtained from original lower salinity stream 12 flows at the other side. Output stream 14a from osmosis unit 19a, which is derived from lower salinity stream 12a is disposed of as appropriate. Output stream 11a from osmosis unit 19a, which has a salt content lower than that of original input stream 11, is fed to a second osmosis unit 19b where it is passed over one side of a semi-permeable membrane. A second input stream 12b of relatively low salinity water obtained from stream 12 flows at the other side. Although the difference in salinity between streams 11a and 12b is lower than the difference in salinity between streams 11 and 12a, there is still a difference in salinity, and electricity can be generated by osmosis. Output stream 14b from osmosis unit 19b, which is derived from lower salinity stream 12b is disposed of as appropriate. Output stream 11b from osmosis unit 19b, which has a salt content lower than that of original input stream 11, and also lower than stream 11a, is fed to a third osmosis unit 19c where it is passed over the other side of a semi-permeable membrane from a further input stream 12c of relatively low salinity water. Although the difference in salinity between streams 11b and 12c is lower than the difference in salinity between streams 11 and 12a, or between streams 11a and 12b, there is still a difference in salinity, and electricity can be generated by osmosis. Output stream 14c from osmosis unit 19c, which is derived from lower salinity stream 12c is disposed of as appropriate. Output streams from the process of FIG. 2 are aqueous exit streams 14a, 14b, 14c which are derived from the feed solution 12 and diluted output stream 11c which is derived from the saturated solution 11. Output stream 11c is recycled to the brine saturator (not shown in FIG. 2), optionally via a pressure exchanger as discussed above in connection with FIG. 1.

[0079] FIG. 3 shows a variant of FIG. 2 in which input streams 12a, 12b and 12c of relatively low salinity water are provided as separate input streams, each undergoing one or more pre-treatments steps (not shown). Only those elements of the FIG. 3 embodiments which differ from the FIG. 2 embodiment will be discussed here.

[0080] In the system of FIG. 3 the output streams are also handled in a different way. Outlet streams 14a and 11a from osmosis unit 19a are merged, and at least part of the merged stream is provided as input stream 22 to osmosis unit 19b. The merged stream 22 will have a salt content lower than that of original input stream 11, and although the difference in salinity between stream 22 and stream 12b is lower than the difference in salinity between streams 11 and 12a, there is still a difference in salinity, and electricity can be generated by osmosis. Similarly, outlet streams 14b and 11b from osmosis unit 19b are merged, and at least part of the merged stream is provided as input stream 23 to osmosis unit 19c.

[0081] It will be understood that FIGS. 2 and 3 show an osmosis power unit 20 consisting of three osmosis units 19 each containing a semi-permeable membrane, but that any suitable number of units can be used, the choice being determined by a combination of technical and economic factors. In general, the higher the initial salinity of the saline stream 1, the higher the number of osmosis units which may be used.

[0082] FIG. 4 shows more details of an osmotic power unit 20, for example of the type used in FIG. 1. A saline stream 31 (which may for example be saturated stream 11 of FIG. 1) is passed to an osmosis unit 29 containing a semi-permeable membrane 30 which permits passage of water but not of salts, and flows at one side of membrane 30. An aqueous stream 33 which is of lower salinity than stream 31 (for example feed stream 12 in FIG. 1) enters osmosis unit 29 and flows at the other side of membrane 30. Arrows show the direction of water transport by osmosis across membrane 30. An output stream 35 (for example stream 14 in FIG. 1) derived from original input stream 33 and now containing a higher concentration of salt, leaves osmosis unit 29. An output stream 36 consisting of original input stream 31 now containing a lower concentration of salt (for example output stream 18 in FIG. 1), leaves osmosis unit 29 via a turbine 37 which drives a generator 38 thus producing electricity.

[0083] FIG. 5 (a) shows more detail of a downflow brine saturator 5, of a type suitable for use as the brine saturator of FIG. 1 or FIG. 7. A tank 40 comprises an inlet 41 in an upper region of the tank 40 and an outlet 42 in a lower region of the tank. A bed of gravel 43 extends across the width of the tank 40 at a location between the inlet 41 and outlet 42. A layer of salt 44 extends across the top of the bed of gravel 43. In use an unsaturated saline stream 45 (for example unsaturated saline stream 15 of FIG. 1) enters the top of the tank 40 via the inlet 41 and flows downward under gravity through the layer of salt 44 and bed of gravel 43 before leaving the tank 40 via the outlet 42 as a saturated saline stream 46 (for example high salinity stream 11). Salt from the layer of salt 44 dissolves into the unsaturated saline stream 45 as it passes and thereby increases the salinity of the stream. Provided that the layer of salt 44 has an appropriate thickness the saline stream is saturated by the time it reaches the outlet 42.

[0084] FIG. 5(b) shows more detail of an upflow brine saturator, of a type suitable for use as the brine saturator of FIG. 1 or 7. A tank 40 comprises an inlet 41 in a lower region of the tank 40 and an outlet 42 in an upper region of the tank. A plurality of flow generators 47, for example nozzles and/or jets connected to a pump (not shown) are located at the bottom of the tank 40. In use, salt and an unsaturated saline stream 45 (for example unsaturated saline stream 15) enter the bottom of the tank 40 via the inlet 41 and forms a fluidized bed with the salt in a lower region of the tank. As salt is dissolved into the stream 45, the buoyancy of the solution and the action of the flow generators 46 causes more highly saturated solution to rise upwards forming a saturated saline stream 46 that leaves the tank 40 via the outlet 42.

[0085] FIG. 6 shows more detail of an electrolysis cell 50, of a type suitable for use as the electrolysis cell of FIG. 1 or FIG. 7. The cell 50 is divided into a first compartment 51 and a second compartment 52 by an ion-selective membrane 53. A cathode 54 is located in the first compartment 51 and an anode 55 is located in the second compartment 52. Each compartment 51, 52 has a fluid inlet 56, a fluid outlet 57 and a gas outlet 58. In use, brine 63 (predominantly comprising sodium chloride as the salt) enters and exits (with reduced salinity) via the fluid inlet 56 and fluid outlet 57 respectively of the first compartment. Water 59 enters the second compartment 52 via the fluid inlet 56. The ion-selective membrane 53 allows positively charged sodium ions to pass, but prevents other negatively charged ions (including hydroxide and chloride) from passing. Applying a voltage across the cathode 54 and anode 55 causes sodium ions to pass across the membrane and the production of chlorine gas 30 in the first chamber 51 which exits via the gas outlet 58. Simultaneously, hydrogen gas 61 (leaving via the gas outlet 58) and sodium hydroxide 62 (leaving via the fluid outlet 56) are produced in the second chamber 52. In addition to the membrane cell process for chlor alkali production as described here, the diaphragm cell (where the anode is separated from the cathode by a permeable diaphragm) and mercury cell (in which sodium forms an amalgam with mercury at the cathode, and is then separated in a decomposer to produce hydrogen gas and caustic soda solution) processes may also be used.

[0086] In the embodiment a FIG. 1 a PRO osmotic power unit is used. In other embodiments a RED osmotic power unit may be used to generate electricity from the difference in salinity between a saturated or high salinity stream and the feed stream by reverse electrodialysis. FIG. 7 shows an example of such a system. Only those elements of the FIG. 7 embodiments which differ from the FIG. 1 embodiment will be discussed here. In FIG. 7 the semipermeable membrane 2 is replaced with an cation exchange membrane 2a and an anion exchange membrane 2b. Turbine 3 is absent in FIG. 7. As for FIG. 1, the salinity of output stream 18 is reduced compared to saturated stream 11 and the salinity of waste stream 14 is increased compared to feed stream 12. However, in FIG. 7 this is because positive and negatively charged ions (for example sodium ions and chlorine ions) have passed from the saturated stream 11 to the feed stream 12. This movement across the cation exchange membrane 2a and anion exchange membrane 2b generates an electric charge. The pressure exchanger is absent in the system of FIG. 7.

[0087] Processes and systems in accordance with the present example embodiment provide recirculation of the high salinity stream/unsaturated saline stream between the brine saturator and the RED osmosis unit, using the osmosis unit to reduce the salinity of the stream before it is passed back to the brine saturator. As a result part of the entropy released during the dissolution process may be captured and used for energy generation. This recirculation (i.e. the closed nature of the feed to the brine saturator) may maintain the purity of the unsaturated saline stream, thereby reducing and/or removing the need for filtering and/or may allow the use of a wider variety of sources for the feed stream, provided the feed stream has a lower salinity than the saturated stream and the diluted outlet stream (particularly as the feed stream does not mix into the brine saturator feed). For example, seawater may be used.

[0088] FIG. 8 shows more details of an osmotic power unit 20, for example of the type used in FIG. 7. The osmotic power unit 20 comprises a stack 70 of cation exchange membranes 75 alternating with an anion exchange membranes 76. The stack 70 is located between a cathode 79 (on the left of FIG. 8) and an anode 80 (on the right of FIG. 8). A saline stream 71 (which may for example be saturated stream 11) flows between each cation exchange membrane 75 (on the left of stream 71 in FIG. 7) which permits the passage of cations (e.g. sodium) but not anions (e.g. chlorine) and an anion exchange membrane 76 (on the right of stream 71 in FIG. 8). An aqueous stream 73 which is of lower salinity than stream 71 (for example feed stream 12) flows on the other side of each cation exchange membrane 75 and the anion exchange membrane 76. Thus, there is an alternating series of saline streams 71 and aqueous streams 73 flowing through the stack 70. For the sake of clarity only four membranes are shown in FIG. 8, but the stack may include many more membranes. Arrows show the direction of sodium transport across cation exchange membrane 75 and chloride transport across anion exchange membrane 76. This movement of cations and anions across the membranes generates an electric current. An output stream 77 (for example stream 14 in FIG. 7) derived from original input stream 73 and now containing a higher concentration of salt, leaves osmotic power unit 70. An output stream 78 consisting of original input stream 71 now containing a lower concentration of salt (for example output stream 18 in FIG. 7), leaves osmotic power unit 70.

[0089] The energy output of a brine saturator combined with an osmotic power system depends on the specific configuration. Using the equation for mixing energy, it can be shown that the energy potential can be as high as 9 kWh per cubic meter brine produced, equal to 29 kWh per ton of NaCl consumed. Not all this energy can be extracted as components efficiencies will limit the energy available for extraction. For a PRO system, it is known that the optimum operational pressure is 200 bar. Table 1 below shows a balance for the system of FIG. 1, which allows a comparison with the performance of a traditional brine saturator.

[0090] Assuming efficiencies of 0.7 for the feed pump (4), 0.84 for the energy generating device (3) and 0.95 for the energy recovery device (1), a system as specified in FIG. 1 operating at 80 bar would generate 1.3 kWh per cubic meter brine produced, or 4.2 kWh per ton of NaCl consumed, while a system operating at 200 bar would generate 3.5 kWh per cubic meter brine, or 11.4 kWh per ton NaCl. For a chlor alkali plant with a yearly consumption of 1,000,000 ton NaCl this is equal to an energy potential of 4.2-11.4 GWh per year.

[0091] In comparison a standard brine saturator would have an energy potential of 0 GWh per year.

TABLE-US-00001 1 3 4 11 12 13 14 15 18a 18b Operating at 80 bar Flow (m3/h) 55 100 125 55 125 100 25 155 55 100 Concentration 310 155 0 310 0 310 0 110 110 110 (g/L) Pressure (bar) 80 80 10 0/80 0/10 0 0 0 80/0 80/0 Operating at 200 bar Flow (m3/h) 209 100 125 209 125 100 25 309 209 100 Concentration 310 210 0 0 0 310 0 210 210 210 (g/L) Pressure (bar) 200 200 10 0/200 0/10 0 0 0 200/0 200/0

[0092] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

[0093] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.