WO2025244584A1

WO2025244584A1 - Method and apparatus for concentrating waste brine for a chlor-alkali process

Info

Publication number: WO2025244584A1
Application number: PCT/SG2025/050350
Authority: WO
Inventors: Siva Kumar Kota; Saba PALANIAPPA; Seng Yong GOH
Original assignee: Gradiant International Holdings Pte Ltd
Current assignee: Gradiant International Holdings Pte Ltd
Priority date: 2024-05-24
Filing date: 2025-05-23
Publication date: 2025-11-27
Anticipated expiration: 2026-11-24

Abstract

Disclosed are apparatuses and methods for concentrating brine to a high concentration level for a chlor-alkali process, hence enabling energy-efficient recovery of useful chemical products including caustic soda and chlorine from waste brine from sea desalination. The method involves pretreating the waste brine to remove at least one bivalent solute impurities and removing at least a portion of liquid in the brine in a plurality of osmotic stages using osmosis membranes that are more permeable and/or salt rejecting than conventional reverse osmosis membrane under hydraulic pressure of not more than preferably 1200 psi. Passive forward osmosis using a counter solution may be further used to concentrate the waste brine to a concentration level suitable for the chlor-alkali process.

Description

METHOD AND APPARATUS FOR CONCENTRATING WASTE BRINE FOR A CHLOR-ALKALI PROCESS

TECHNICAL FIELD

The present invention generally relates to a method and apparatus for treating wastewater, and more particularly to a method and apparatus for concentrating waste brine for a chloralkali process.

BACKGROUND

Seawater desalination is an increasingly common method for obtaining potable water in many parts of the world. Waste brine that is generated by the desalination process is typically returned to the sea or injected into deep aquifers and poses significant environmental issues owing to the high salt concentration. Hence, there is intense interest in processing the waste brine for safer disposal.

One method is to utilise the waste brine in a chlor-alkali process that is an electrolysis process for industrial production of chlorine gas, caustic soda (sodium hydroxide) and hydrogen. In addition to improving water recovery, the method generates useful products from the waste brine. Chlorine, for instance, is used in a wide variety of industries like textile, paper, general cleaning and disinfection. However, the waste brine from seawater desalination is required to be concentrated to near saturation limits to be suitable for the chlor-alkali process.

Presently, evaporative techniques like multi-stage flash distillation, multi-effect distillation and membrane distillation are used for concentrating brine to very high concentration levels. These evaporative techniques are high in energy consumption and unsustainable in the long term On the other hand, while non-evaporative membrane-based techniques like reverse osmosis have been widely used for water separation and volume reduction, such techniques are not suitable for concentrating waste brine as high hydraulic pressure (more than 70 bar), beyond the mechanical limits of existing reverse membranes, would be required to overcome the osmotic pressure. The current limit of reverse osmosis process is approximately 70,000 ppm (7%) which is considerably lower than the minimum 30% concentration level required for the chlor-alkali process.

Therefore, there is need for concentrating waste brine from seawater desalination to a high concentration level that is suitable for the chlor-alkali process to efficiently close the water loop for desalination.

SUMMARY

In a first aspect, the present disclosure provides a method for concentrating brine to a high concentration level that is suitable for the chlor-alkali process. The method involves pretreating an input brine by passing the input brine through a filter membrane to produce a pretreated brine where a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine. The pretreated brine is then transported to a retentate side of a first osmotic stage where at least a portion of liquid in the pretreated brine is passed through a first osmosis membrane to a permeate side of the first osmotic stage to obtain a first concentrated brine having a higher sodium chloride concentration than the pretreated brine at the retentate side of the first osmotic stage. The first concentrated brine is then transported to a retentate side of a second osmotic stage where at least a portion of liquid in the first concentrated brine is passed through a second osmosis membrane to a permeate side of the second osmotic stage to obtain a second concentrated brine having a higher sodium chloride concentration than the first concentrated brine at the retentate side of the second osmotic stage that is used to form a first brine product

In some embodiments, the input brine is obtained from a seawater desalination process

In some embodiments, a hydraulic pressure of not more than 1200 psi is applied to pass the portion of liquid in the pretreated brine through the first osmosis membrane.

In some embodiments, a hydraulic pressure of not more than 1200 psi is applied to pass the portion of liquid in the first concentrated brine through the second osmosis membrane. Tn some embodiments, the second osmosis membrane has a higher permeability and/or salt rejection than the first osmosis membrane.

In some embodiments, the method further involves returning at least a portion of the first concentrated brine to the retentate side of the first osmotic stage.

In some embodiments, the method further involves returning at least a portion of the second concentrated brine to the retentate side of the second osmotic stage.

In some embodiments, the method further involves collecting a permeate from the permeate side of the first and/or second osmotic stage.

In some embodiments, the method further involves transporting the permeate to the retentate side of the first or second osmotic stage and passing a portion of liquid from the permeate through the osmosis membrane to the permeate side of the first or second osmotic stage

In some embodiments, the method further involves processing the second concentrated brine in one or more further osmotic stages to form the first brine product. The processing includes transporting the brine from a retentate side of an upstream osmotic stage to a retentate side of a downstream osmotic stage, passing a portion of liquid from the brine through an osmosis membrane in the downstream osmotic stage to a permeate side of the downstream osmotic stage, and obtaining the first brine product having a higher sodium chloride concentration than the second concentrated brine from the retentate side of a most downstream osmotic stage.

In some embodiments, the method further includes processing the brine in a forward osmotic stage. This includes transporting the first brine product to a retentate side of a forward osmotic stage, transporting a counter solution having a higher osmolarity than the first brine product to a permeate side of the forward osmotic stage, such that a portion of liquid from the first brine product is transferred across a semipermeable membrane that separates the retentate side and permeate side of the forward osmotic stage. Hence, a second brine product having a higher sodium chloride concentration than the first brine product is obtained at the retentate side of the forward osmotic stage. Tn some embodiments, the counter solution on the permeate side of the forward osmotic stage and the first brine product on the retentate side of the forward osmotic stage are flowed in opposite directions.

In some embodiments, the first brine product and/or second brine product are used in a chloralkali process. The method involves transporting the first brine product and/or the second brine product to an anode side chamber of an electrolytic cell. The electrolytic cell contains water in a cathode side chamber that is separated from the anode side chamber by a sodium ion permeable diaphragm. An electric potential is applied across an anode and a cathode of the electrolytic cell to electrolyse the first brine product and/or the second brine product.

In a second aspect, the present disclosure provides an apparatus for concentrating brine to a high concentration level that is suitable for the chlor-alkali process. The apparatus includes a pretreatment stage, a first osmotic stage and a second osmotic stage. In the pretreatment stage, an input brine can be passed through at least one filter membrane to produce a pretreated brine such that a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine. The first osmotic stage includes at least one first osmosis membrane that separates the first osmotic stage into a retentate side and a permeate side. The pretreated brine is received at the retentate side of the first osmotic stage and at least a portion of liquid in the pretreated brine can be passed through the first osmosis membrane to the permeate side, leaving a first concentrated brine having a higher sodium chloride concentration than the pretreated brine at the retentate side of the first osmotic stage. The second osmotic stage includes at least one second osmosis membrane that separates the second osmotic stage into a retentate side and a permeate side. The first concentrated brine can be received at the retentate side of the second osmotic stage and at least a portion of liquid in the first concentrated brine can be passed through the second osmosis membrane to the permeate side to leave a second concentrated brine having a higher sodium chloride concentration than the first concentrated brine at the retentate side of the second osmotic stage.

In some embodiments, the apparatus includes a pump that applies a hydraulic pressure of not more than 1200 psi to pass the portion of liquid in the pretreated brine or the first concentrated brine through the first osmosis membrane or second osmosis membrane respectively.

In some embodiments, the second osmosis membrane has a higher permeability and/or salt rejection than the first osmosis membrane.

In some embodiments, the first or second osmotic stage is configured to return at least a portion of the first concentrated brine or second concentrated brine to the retentate side respectively.

In some embodiments, the first and/or second osmotic stage is of a spiral wound membrane configuration.

In some embodiments, the apparatus further includes a forward osmotic stage. The forward osmotic stage has at least one semipermeable membrane that separates the forward osmotic stage into a retentate side and a permeate side. The second concentrated brine can be received at the retentate side and a counter solution having a higher osmolarity than the second concentrated brine can be received at the permeate side. At least a portion of liquid in the second concentrated brine can pass through the semipermeable membrane to the permeate side, hence a forward concentrated brine obtained at the retentate side has a higher sodium chloride concentration than the second concentrated brine. Preferably, the forward osmotic stage is designed to flow the second concentrated brine at its retentate side and the counter solution at its permeate side in opposite directions.

In some embodiments, the apparatus further includes an electrolytic cell for electrolysing the forward concentrated brine. The electrolytic cell includes a sodium ion permeable diaphragm that separates the electrolytic cell into an anode side chamber and cathode side chamber, an anode provided in the anode side chamber and a cathode provided in the cathode side chamber. The forward concentrated brine can be received in the anode side chamber while the cathode side chamber receives water. An electric potential can be applied across the anode and cathode to electrolyse the forward concentrated brine to produce chlorine gas, sodium hydroxide and hydrogen. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described, by way of example only, with reference to the accompanying drawings in which:

FIG. I shows an exemplary apparatus for concentrating an input brine for a chlor-alkali process.

FIG. 2 shows another exemplary apparatus incorporating a forward osmosis stage for processing an input brine.

FIG. 3 shows an exemplary apparatus for a chlor-alkali process to produce caustic soda and chlorine from brine

FIG. 4 depicts a flow chart of an exemplary method for concentrating an input brine.

FIG. 5 is a graph of total dissolved solids (TDS) vs time for Test 1.

FIG. 6 is a graph of total dissolved solids (TDS) vs time for Test 2.

FIG. 7 is a graph of total dissolved solids (TDS) vs time for Test 3.

DETAILED DESCRIPTION

Exemplary apparatuses and methods for concentrating waste brine for a chlor-alkali process will be described with reference to FIGS. 1 to 7. The presently disclosed methods and apparatuses generally concentrate an aqueous solution to a high osmolarity level in an energy-efficient manner without requiring heating.

In this disclosure, seawater desalination is taken to be any process that converts seawater into potable water, including but not limited to distillation, reverse osmosis and the like. Waste brine includes but is not limited to discharge from a seawater desalination process and comprises salt (sodium chloride), although the waste brine may also comprise other solute impurities. The term “about” can mean within ±10% of the value recited. In addition, where a range of values is provided, each subrange and each individual value encompassed within the range inclusive of the upper and lower ends is contemplated and therefore disclosed. The term “brine” is taken to mean any solution containing sodium chloride. Particularly, when the term is used alone in the following description, the term refers to any intermediate form of a concentrated brine product that is obtained by the presently disclosed apparatuses or methods. Further, for the sake of brevity and clarity, similar components as shown in the drawings are labelled with the same reference number.

FIG. 1 shows a schematic illustration of an exemplary apparatus 100 for producing a concentrated brine product that can be used in a chlor-alkali process. The apparatus 100 includes a pretreatment stage 104, and a concentration stage including a first osmotic stage 106 and a second osmotic stage 108.

A feed tank 102 stores an input brine for feeding to the pretreatment stage 104. The input brine can be waste brine obtained from a seawater desalination process and contain between 4 to 6 % by mass of sodium chloride and other impurities. However, this should not be seen as limiting to the scope of the present disclosure as various sources of brine known by the skilled person in the art are also contemplated. In some embodiments, seawater is used directly as the input brine.

The input brine flows from the feed tank 102 to the pretreatment stage 104. In some embodiments, a pump 116 is used to transport the input brine to the pretreatment stage 104 and to pressurize the input brine for pretreatment The pretreatment stage 104 includes at least one filter membrane that hinders passage of bivalent ions, sulphates and other impurities through the filter membrane. The input brine is passed through the filter membrane to produce a pretreated brine such that a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine.

In some embodiments, the filter membrane is a nanofiltration membrane with typical pore size in the range of 0.2 to 2 nm with molecular weight cut-off (MCWO) from 200 to 1000 Da, although other membranes, as familiar to the skilled person in the art, that enables sodium chloride to pass while hindering passage of impurities, can also be used. In some embodiments, the pretreatment stage 104 comprises more than one filter membrane that can be the same or different. The pretreatment stage 104 can assume various configurations that are known in the art such as, but not limited to, a spiral wound configuration or a hollow fibre configuration.

In some embodiments (not shown), more than one pretreatment stages 104 are connected serially, in parallel or in combination thereof. For instance, the input brine may be split into multiple branches, with each branch of the input brine fed to one of the pretreatment stages that are connected parallel to one another. Permeate (i.e. brine that has passed through the filter membrane) obtained from each of the pretreatment stages 104 are then combined to produce the pretreated brine. Alternatively, the input brine may be fed through a plurality of the pretreatment stages that are serially connected, where the brine is passed serially through each filter membrane of the pretreatment stages to produce the pretreated brine.

In some embodiments, the permeate that has passed through the filter membrane of the pretreatment stage 104 is returned to the input brine and entered into the pretreatment stage 104 again for pretreatment, i.e. cycled through the pretreatment stage 104.

In the exemplary embodiment as shown in FIG. 1, the portion of the input brine that has not passed through the filter membrane is returned to the feed tank 102.

In all embodiments, the pretreated brine from the pretreatment stage 104 is transported to the first osmotic stage 106. Optionally, anti-scalants or dispersants are added to the pretreated brine prior to feeding the pretreated brine into the first osmotic stage 106 to prevent scaling. In some embodiments, the anti-scalants or dispersants may be added to the input brine in the feed tank 102 prior to pretreatment. Optionally, the pH of the pretreated brine is also adjusted. In some embodiments, the pH may be adjusted to be slightly acidic such as about pH 6. In some embodiments, the pH of the input brine in the feed tank 102 may be adjusted prior to pretreatment

In some embodiments, a pump 118 may be used to transport and pressurize the pretreated brine. Preferably, the pump 118 is a high-pressure pump.

The first osmotic stage 106 includes at least one first osmosis membrane that separates the first osmotic stage 106 into a retentate side and a penneate side and hinders the passage of sodium chloride from the retentate side to the permeate side. The pretreated brine enters the first osmotic stage 106 from the retentate side. At least a portion of liquid, for instance at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90%, in the pretreated brine is passed under a hydraulic pressure through the first osmosis membrane from the retentate side to the permeate side, while the dissolved salt in the pretreated brine is largely retained. Hence, the portion of the pretreated brine that remains on the retentate side has a higher sodium chloride concentration than the pretreated brine stream that is fed into the retentate side, and exits the first osmotic stage 106 as a first concentrated brine.

The hydraulic pressure in the first osmotic stage 106 is required to overcome the osmotic pressure that is higher on the retentate side of the first osmosis membrane for passing the at least a portion of liquid in the pretreated brine through the first osmosis membrane. Tn some embodiments, the hydraulic pressure that is applied on the retentate side of the first osmotic stage 106 is not more than (nmt) 2000 psi, nmt 1800 psi, nmt 1600 psi, nmt 1400 psi, preferably nmt 1200 psi and preferably nmt 1000 psi.

The first osmosis membrane is relatively permeable and salt rejecting to allow the liquid in brine to pass through without requiring exceedingly high hydraulic pressures, while largely retaining the salt in the brine.

In some embodiments (not shown) similar to the pretreatment stage 104 described earlier, there may be more than one first osmotic stages 106 that are connected serially, in parallel or in combination thereof. For instance, the pretreated brine may be split into multiple branches, with each branch of the pretreated brine fed to one of the first osmotic stages that are connected parallel to one another Retentate (i.e. the portion of brine that is retained on the retentate side) from each of the first osmotic stages are then combined to produce the first concentrated brine. Alternatively, the pretreated brine may be fed through the first osmotic stages that are serially connected to produce the first concentrated brine, where the retentate of an upstream first osmotic stage is fed to a retentate side of a downstream first osmotic stage.

In some embodiments (not shown), a portion of the first concentrated brine may be returned to the pretreated brine that is fed to the retentate side of the first osmotic stage 106, i.e. recycled through the first osmotic stage 106. The first concentrated brine from the first osmotic stage 106 is then transported to the second osmotic stage 108. In some embodiments, a pump 120 may be used to transport and pressurize the first concentrated brine. Preferably, the pump 120 is a high-pressure pump.

The second osmotic stage 108 includes at least one second osmosis membrane that separates the second osmotic stage 108 into a retentate side and a permeate side, and hinders the passage of sodium chloride from the retentate side to the permeate side. The first concentrated brine enters the second osmotic stage 108 from the retentate side. At least a portion of liquid, for instance at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90%, in the first concentrated brine is passed under a hydraulic pressure through the second osmosis membrane from the retentate side to the permeate side while dissolved salt is largely retained. Hence, the portion of the first concentrated brine remaining on the retentate side has a higher sodium chloride concentration than the first concentrated brine that is fed into the retentate side of the second osmotic stage 108, and exits the second osmotic stage 108 as a second concentrated brine

The hydraulic pressure in the second osmotic stage 108 is required to overcome the osmotic pressure that is higher on the retentate side of the second osmosis membrane for passing the at least a portion of liquid in the first concentrated brine through the second osmosis membrane. In some embodiments, the hydraulic pressure that is applied on the retentate side of the second osmotic stage 108 is not more than (nmt) 2000 psi, nmt 1800 psi, nmt 1600 psi, nmt 1400 psi, preferably nmt 1200 psi and preferably nmt 1000 psi.

The second osmosis membrane is relatively permeable and salt rejecting to allow the liquid in brine to pass through without requiring exceedingly high hydraulic pressures, while largely retaining the salt in the brine.

In some embodiments (not shown) similar to the pretreatment 104 and first osmotic 106 stages described earlier, there may be more than one second osmotic stages 108 that are connected serially, in parallel or in combination thereof. For instance, the first concentrated brine can be split into multiple branches, with each branch of the first concentrated brine fed to one of the second osmotic stages that are connected parallel to one another Retentate from each of the second osmotic stages are then combined to produce the second concentrated brine. Alternatively, the first concentrated brine may be fed through the second stages that are serially connected to produce the second concentrated brine, where the retentate of an upstream second osmotic stage is fed to a retentate side of a downstream second osmotic stage.

In some embodiments, a portion of the second concentrated brine may be returned to the first concentrated brine that is fed to the retentate side of the second osmotic stage 108, i.e. recycled through the second osmotic stage 108.

In some embodiments, the second osmosis membrane may be of the same type as the first osmosis membrane.

In other embodiments, the second osmosis membrane may be different from the first osmosis membrane due to the higher osmolarity of the first concentrated brine in comparison to the pretreated brine. Particularly, the second osmosis membrane possesses higher permeability and/or salt rejection than the first osmosis membrane.

In some embodiments, the first and second osmosis membranes in the first and second osmotic stages 106, 108 can be obtained by modifying conventional reverse osmosis membranes. Without wishing to be bound to theory, a typical reverse osmosis membrane includes a thin crosslinked polyamide active layer that is about 100 to 200 nm thick deposited on a polyethersulfone or polysulfone porous layer that is about 50 pm thick that is in turn coated on a non-woven fabric support sheet. The polyamide active layer is orientated towards the retentate side and serves as the main selective barrier to passage of dissolved salt through the membrane. The skilled person will recognise that materials other than polyamide may also be used as the active layer.

Various methods may be used to modify the permeability and salt rejection of the first and second osmosis membranes. As an example, the osmosis membrane may be modified by oxidising the polyamide active layer by cleaving C-N bonds to form COOH- groups. This reduces the extent of crosslinking within the active layer which in turn increases pore size and hence permeability. The skilled person will be aware of other modifications for adjusting the permeability and/or salt rejection, as common in the art. Within each osmotic stage 106, 108, the osmosis membrane may be the same or different The first and second osmotic stages 106, 108 may assume various configurations that are known in the art such as, but not limited to, a spiral wound configuration or a hollow fibre configuration. Preferably, the first and second osmotic stages 106, 108 are of the spiral wound configuration.

Optionally, prior to feeding the brine into the first and second osmotic stages 106, 108, the brine may be passed through a filtration cartridge to remove slit, algae, precipitation, or other solid impurities.

In some embodiments, permeate that is the portion of the brine that has passed through the first or second osmosis membrane to the permeate side of the first or second osmotic stage 106, 108 respectively may be collected in a permeate tank (not shown). The permeate can be returned to the feed tank 102 for processing.

In some embodiments, the second concentrated brine is of sufficient sodium chloride concentration and is collected as a first brine product for use in the chlor-alkali process.

In other embodiments, further concentration of the second concentrated brine may be required to reach the desired sodium chloride concentration. For instance, the second concentrated brine can be transported to additional osmotic stages (e.g. a third osmotic stage, fourth osmotic stage, etc) if there is sufficient volume of the second concentrated brine. In this case, brine remaining on a retentate side of an upstream osmotic stage is transported to the retentate side of a downstream osmotic stage where a portion of liquid in the brine is passed through an osmosis membrane of the downstream osmotic stage to a permeate side of the downstream osmotic stage. In some embodiments, the osmotic membrane of the downstream osmotic stage possesses a higher permeability and/or salt rejection than the osmotic membrane of the upstream osmotic stage. The first brine product is obtained from the retentate side of a most downstream osmotic stage, with the first brine product having a higher sodium chloride concentration than the second concentrated brine. In other embodiments, the second concentrated brine stream is re-run through the second osmotic stage 108 by feeding the second concentrated brine back to the retentate side of the second osmotic stage 108, provided there can be sufficient permeate flow through the second osmosis membrane. The skilled person in the art would recognise that such re-run may also be performed for the first osmotic stage 106 to further concentrate the first concentrated brine.

For a small volume of the second concentrated brine that is below the sodium chloride concentration required for the chlor-alkali process, the second concentrated brine may be transported to a forward osmotic stage 204 as shown in FIG. 2.

In some embodiments, the second concentrated brine may be transported and/or pressurized by a pump 202.

The forward osmotic stage 204 includes at least one semipermeable membrane that separates the forward osmotic stage 204 into a retentate side and permeate side. The semipermeable membrane is permeable to water but impermeable to sodium chloride. The second concentrated brine is transported to the retentate side of the forward osmotic stage 204 while a counter solution is transported to the permeate side. The counter solution has a higher osmolarity than the second concentrated brine. Hence, a portion of liquid in the second concentrated brine diffuses across the semipermeable membrane from the retentate side to the permeate side. A forward concentrated brine that exits the forward osmotic stage 204 on the retentate side has a higher sodium chloride concentration than the second concentrated brine stream that enters the forward osmotic stage 204.

In the exemplary embodiment as shown in FIG. 2, the forward osmotic stage 204 is operated in a counterflow configuration whereby the second concentrated brine on the retentate side is flowed along the semipermeable membrane in a direction that is opposite to the flow of the counter solution on the permeate side. A pump 208 may be used to transport the counter solution from storage tank 206 to the permeate side of the forward osmotic stage 204. The counter solution that has flowed through the forward osmotic stage 204 may be collected in tank 210.

Preferably, the forward osmotic stage 204 is of a hollow core membrane configuration with the second concentrated brine flowing in the lumen (retentate side) of the hollow core membranes that are arranged parallel to one another in the counter solution (permeate side). Tn some embodiments, the brine exiting the forward osmotic stage 204 on the retentate side may be returned to the forward osmotic stage 204 for recycling to further concentrate the brine.

The forward concentrated brine that is of sufficient sodium chloride concentration is collected as a second brine product for use in the chlor-alkali process.

It should be noted that while the apparatus 200 of FIG. 2 shows two osmotic stages 106, 108, with the second concentrated brine from the second osmotic stage fed to the forward osmotic stage 204, the invention should not be limited as such. As described in the previous paragraph, some embodiments can comprise additional osmotic stages. In these embodiments, the brine on the retentate side of the most downstream osmotic stage can be fed into the forward osmotic stage 204 for further concentration.

In some embodiments, salt is recovered from the first brine product and/or second brine product by crystallisation or other known methods in the art to be used in various applications such as de-icing.

In the above manner, the input brine is concentrated in steps through a cascade of osmotic stages 106, 108 and, if required, also through a forward osmotic stage 204 to obtain the concentrated brine product having a sodium chloride concentration (e.g. at least 30% w/v) that is suitable for use in the chlor-alkali process. Advantageously, due to the permeability and salt rejection properties of the osmosis membranes in the osmotic stages 106, 108, a consistent hydraulic pressure that is not exceedingly high is sufficient to concentrate brine to beyond the concentration levels achievable by conventional reverse osmosis. A passive forward osmotic stage 204 can be added to further concentrate the brine if required. Hence, the apparatus and method of the present disclosure achieves energy efficiency without requiring evaporative techniques or exceedingly high pressures.

FIG. 3 shows a schematic diagram of an exemplary apparatus 300 comprising a setup for the chlor-alkali process. The first and/or second brine product that is of sufficient sodium chloride concentration is transported to an electrolytic cell 301 for electrolysis by passing a direct electric current that converts chloride ions (Cl’) to chlorine. The overall process reaction is given by Equation 1 below: 2 NaCl + 2 H₂0 - Cl₂ I I b I 2 NaOH Equation 7)

In the electrolytic cell 301, an anode 302 and cathode 304 are provided in an anode-side chamber and an cathode-side chamber respectively, the two chambers being separated by a permeable diaphragm 306. The first and/or second brine product is fed via a brine inlet 308 to the anode-side chamber while water is fed via a water inlet 310 to the cathode-side chamber. In some embodiments, it may be desirable to pretreat the brine product to remove impurities before feeding to the electrolytic cell 301. The diaphragm 306 is impermeable to chloride ions (C1‘) while being permeable to sodium ions (Na⁺) that move from the anodeside chamber to the cathode-side chamber as indicated by arrow 312 in FIG. 3 when an electric potential 314 is applied across the electrodes 302, 304. This process produces chlorine and hydrogen at the anode 302 and cathode 304 respectively. Sodium hydroxide is indirectly formed at the cathode-side chamber and removed from the electrolytic cell 301 via an alkali outlet 316. Depleted brine may be drained from the anode-side chamber via a brine outlet 318.

FIG. 4 outlines an exemplary method of the present disclosure that is performed by the above-described apparatuses. The method 400 includes step 402: pretreating an input brine by passing the input brine through a filter membrane to produce a pretreated brine such that a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine, processing the pretreated brine by step 404: transporting the pretreated brine to a retentate side of a first osmotic stage, passing at least a portion of liquid for instance at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% in the pretreated brine under a hydraulic pressure through a first osmosis membrane to a permeate side of the first osmotic stage while retaining dissolved salt on the retentate side, and obtaining a first concentrated brine from the retentate side of the first osmotic stage, wherein the first concentrated brine has a higher sodium chloride concentration than the pretreated brine, processing the first concentrated brine by step 406: transporting the first concentrated brine to a retentate side of a second osmotic stage, passing at least a portion of liquid for instance at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% in the first concentrated brine under a hydraulic pressure through a second osmosis membrane to a permeate side of the second osmotic stage while retaining dissolved salt on the retentate side, and obtaining a second concentrated brine from the retentate side of the second osmotic stage as a first brine product, wherein the second concentrated brine has a higher sodium chloride concentration than the first concentrated brine.

The hydraulic pressure that is applied for passing at least a portion of liquid in the pretreated brine / first concentrated brine through the first osmosis membrane / second osmosis membrane is not more than (nmt) 2000 psi, nmt 1800 psi, nmt 1600 psi, nmt 1400 psi, preferably nmt 1200 psi and preferably nmt 1000 psi.

The first and second osmosis membrane are relatively permeable and salt rejecting to allow the liquid in brine to pass through without requiring exceedingly high hydraulic pressures, while largely retaining the salt in the brine.

Depending on factors such as sodium chloride concentration and volume of the second concentrated brine, the method 400 may further comprise processing the second concentrated brine via one of the following steps.

Where there is sufficient volume of the second concentrated brine, the second concentrated brine can be processed by step 408 which involves processing the second concentrated brine in one or more further osmotic stages. Similar to steps 404 and 406 that are previously described, step 408 involves transporting the second concentrated brine to a retentate side of the further osmotic stage, passing at least a portion of liquid in the second concentrated brine under a hydraulic pressure through a osmosis membrane to a permeate side of the further osmotic stage, obtaining a further concentrated brine from the retentate side of the further osmotic stage as the first brine product.

In some embodiments where there are more than one further osmotic stages, the further osmotic stages can be connected serially, and step 408 involves transporting retentate from a retentate side of an upstream osmotic stage to a retentate side of a downstream osmotic stage, passing a portion of liquid in the retentate through an osmosis membrane in the downstream osmotic stage to a permeate side of the downstream osmotic stage, and obtaining the first brine product from the retentate side of a most downstream osmotic stage, wherein the first brine product has a higher sodium chloride concentration than the second concentrated brine.

In some embodiments, the further osmotic stages include one or more osmosis membrane having higher permeability and/or salt rejection than the second osmosis membrane. In some embodiments, the osmosis membrane of a downstream osmotic stage has higher permeability and/or salt rejection than the osmosis membrane in an upstream osmotic stage.

In some embodiments, the hydraulic pressure applied on the retentate side of the further osmotic stages is nmt 2000 psi, nmt 1800 psi, nmt 1600 psi, nmt 1400 psi, preferably nmt 1200 psi and preferably nmt 1000 psi. In some embodiments, the hydraulic pressure applied in the further osmotic stages corresponds to the hydraulic pressure applied in the first or second osmotic stage.

Alternatively, the second concentrated brine stream can be processed under step 410, which includes returning at least a portion of the second concentrated brine into the first concentrated brine that is fed to the retentate side of the second osmotic stage, i.e. recycling through the second osmotic unit, to form the first brine product.

In some embodiments, a combination of steps 408 and 410 is carried out, whereby the brine is concentrated in more than two osmotic stages and the processed brine is recycled in one or more of the osmotic stages.

Where there is insufficient volume of the second concentrated brine, step 412 can be performed. Step 412 involves transporting the second concentrated brine to a retentate side of a forward osmotic stage, transporting a counter solution to a permeate side of the forward osmotic stage. A semipermeable membrane that is permeable to water but impermeable to dissolved sodium chloride separates the retentate side and permeate side of the forward osmosis stage. The counter solution stream has a higher osmolarity than the first brine product such that a portion of liquid from the second concentrated brine is transferred across the semipermeable membrane. A second brine product is obtained from the retentate side of the forward osmotic stage, wherein the second brine product has a higher sodium chloride concentration than the second concentrated brine. In some embodiments, step 412 includes flowing the counter solution on the permeate side of the forward osmotic stage and the first brine product on the retentate side of the forward osmotic stage in opposite directions.

In some embodiments, the concentrated brine that flows out from the retentate side of the forward osmotic stage is returned to the retentate side for recycling to further increase sodium chloride concentration.

In some embodiments where the first brine product that is produced in step 408 or step 410 is not at a sodium chloride concentration that is sufficient for the chlor-alkali process, step 412 as previously described can be performed to further concentrate the first brine product.

The first and/or second brine product that is of sufficient sodium chloride concentration is electrolysed in the chlor-alkali process in step 414 which includes transporting the first and/or second brine product to an anode side chamber of an electrolytic cell, the electrolytic cell comprising water in a cathode side chamber that is separated from the anode side chamber by a sodium ion permeable diaphragm, and applying an electric potential across an anode and a cathode of the electrolytic cell to electrolyse the first and/or second brine product.

In some embodiments, the second concentrated brine from step 406, or the first brine product from step 408 or step 410 is of sufficient sodium chloride concentration and is directly electrolysed in step 414.

In some embodiments, the method further comprises collecting a permeate from the permeate side of one or more of the first, second and further osmotic stages The permeate can be processed according to any of the earlier described steps to increase the sodium chloride concentration For instance, the method can further include transporting the permeate to the retentate side of the second osmotic stage, and passing a portion of liquid in the permeate across the osmosis membrane to the permeate side of the second osmotic stage. The portion of permeate remaining on the permeate side of the second osmotic can be recycled through the second osmotic stage or be transported to the forward osmotic stage to further remove liquid.

The present disclosed apparatuses and methods are further demonstrated in the following examples.

Examples

Example 1. Characterisation of waste brine

Waste brine samples from a seawater reverse osmosis (SWRO) process were obtained and characterised, with the average values shown in Table 1.

The SWRO waste brine samples contained elevated levels of calcium, magnesium, potassium, sodium, chloride, bromide and sulfate. On average, the SWRO waste brine had total dissolved solid (TDS) of about 59,063 mg/L and total hardness of about 8,121 mg/L.

Table 1 : Characterisation of SWRO waste brine. Example 2. Pilot Testing

The SWRO waste brine was stored as input brine in a feed tank. An antiscalant was added every 2 hours to the feed tank and properly mixed. pH of the input brine was adjusted to about pH 6.

A pilot plant was set up on a reduced scale for testing. 2.5-inch membrane elements are used for the osmotic stages. Each stage was run separately and parameters like flow rates, pressure, conductivity, pH, salinity and refractive index were recorded every 30 to 60 mins, with samples collected for performing gravimetry analysis to determine actual TDS.

2. 1 Test I— 3 osmotic stages and a forward osmotic stage (without pretreatment stage)

In Test 1, 500L of the input brine was concentrated by three osmotic stages and a forward osmotic stage. The pretreatment stage was absent.

FIG. 5 plots the TDS (by gravimetry) and TDS (by gravimetry based on sodium chloride) with time. Table 2 summarises the salinity and TDS at various stages of Test 1. A TDS of 278,700 mg/L was achieved after the osmotic stages which utilize spiral wound osmosis membranes.

The hydraulic pressure that was applied in the osmotic stages was approximately 1000 psi, although higher pressure such as 1200 psi can be applied instead. After the osmotic stages, there was insufficient feed volume to concentrate further in the osmotic stage.

Further concentration was conducted for the remaining volume in a forward osmotic stage using a micromodule membrane and counter solution in a counterflow configuration. The final TDS achieved was 296,000 mg/L, that is close to the target brine TDS for chlor-alkali process (300,000 mg/L).

Table 2: Summary of results of Test 1.

An analysis of the brine at various stages is shown in Table 3 below.

A counter solution of higher osmolarity than the third concentrated brine was prepared and used in the forward osmotic stage to further concentrate the brine to 25.3 %.

The final IDS achieved in Test 1 was 296,000 mg/L, that is close to the target brine TDS for chlor-alkali process (300,000 mg/L).

Table 3: Analysis of brine at various stages of Test 1. 2.2 Test 2— Pretreatment stage with 2 osmotic stages

In Test 2, 500L of the input brine was pretreated in a pretreatment stage prior to concentration in two osmotic stages.

The hydraulic pressure that was applied in the osmotic stages was approximately 1000 psi, although higher pressure such as 1200 psi can be applied instead.

FIG. 6 plots the TDS (by gravimetry) and TDS (by gravimetry based on sodium chloride) with time. Table 4 summarises the salinity and TDS at various stages of Test 2.

A TDS of 343,000 mg/L, exceeding the target brine TDS was achieved, although it should be noted that the brine was replaced with synthetic brine at the end of the first osmotic stage due to insufficient volume.

Tabic 4: Summary of Test 2.

Pretreatment was conducted in two batches to remove bivalents in the brine.

The brine from various stages of Test 2 were analyzed, and results are shown in Table 5. It can be seen that calcium, magnesium and sulfate levels were reduced 71%, 81% and 98% respectively by pretreatment.

Table 5: Analysis of brine at various stages of Test 2. As the brine volume after the first stage is reduced to about 20L which was insufficient for the second stage, synthetic brine that mimics the first concentrated brine was used for the second stage.

The synthetic brine comprised 14.5 w/w% sodium chloride with other added impurities in accordance with Table 6.

Table 6: Composition of synthetic brine.

Salinity of the synthetic brine was increased from 14.5% to 25.2% salinity by running the brine three times in the second osmotic stage.

It is hence observed that overall salt recovery from the input brine was better than in Test 1. Hence, the pretreatment stage improves the performance of the reverse osmosis process by removal of bivalents. A third osmotic stage nor a forward osmosis stage was not required since the target TDS was reached in the second stage.

2.3 Test 3— Pretreatment stage, 3 osmotic stages and a forward osmotic stage

In Test 3, 3000L of input brine was fed into a pretreatment stage in four batches to make sufficient feed volume for the subsequent osmotic stages. TDS of 288,000 mg/L was achieved after two osmotic stages. The hydraulic pressure that was applied in the osmotic stages was approximately 1000 psi, although higher pressure such as 1200 psi can be applied instead. A further osmotic stage was not conducted since permeate flux was still high enough to continue with the second stage reverse osmosis and there was insufficient volume left (~1L) for a further stage.

Hence, concentration was conducted for the remaining volume using a forward osmotic stage using a micromodule membrane. The final TDS achieved was 330,400 mg/L, which met the target brine TDS of 300,000 mg/L. FIG. 7 plots the TDS (by gravimetry) and TDS (by gravimetry based on sodium chloride) with time for the osmotic stages. Table 7 summarises the salinity and TDS at various stages of Test 3.

Tabic 7: Summary of Test 3. The pretreatment stage was conducted without recirculation of reject stream into the feed tank since the sample volume of 3,000L was presumed to be enough to generate the required feed volume for subsequent osmotic stages.

Due to insufficient volume, the second concentrated brine was not further concentrated.

The permeate of the second osmotic stage was collected and fed to a third osmotic stage comprising the same osmosis membrane as the second stage.

Table 8 shows the refractive index, salinity and TDS (based on sodium chloride) of the permeate from the second stage during the third osmotic stage. The salinity is increased from 7.7% to 9.7% in the third osmotic stage.

Table 8: Monitoring of refractive index, salinity and TDS in the third osmotic stage of Test 3.

Forward osmosis was conducted on the 9.7% concentrate using a micromodule membrane to further concentrate to 26.7% final salinity. Final TDS measured by gravimetry was 330,400 mg/L. The brine from various stages of Test 3 are analyzed, and results are shown in Table 9.

Table 9: Analysis of brine at various stages of Test 3.

Hence, it is shown that further salt can be recovered from the permeate of the osmotic stages to produce the brine product. Hence, an efficient apparatus and method of concentrating waste brine to a concentration level suitable for the chlor-alkali process is demonstrated. It should be understood that the present invention is not limited to the described embodiments as the skilled person in the art may modify or combine features from various embodiments to obtain variations that do not depart from the scope of the invention as set forth in the following claims.

Claims

1. A method compri sing : pretreating an input brine, wherein the pretreating comprises passing the input brine through a filter membrane to produce a pretreated brine such that a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine; transporting the pretreated brine to a retentate side of a first osmotic stage; passing at least a portion of liquid in the pretreated brine through a first osmosis membrane to a permeate side of the first osmotic stage; obtaining a first concentrated brine from the retentate side of the first osmotic stage, wherein the first concentrated brine has a higher sodium chloride concentration than the pretreated brine, transporting the first concentrated brine to a retentate side of a second osmotic stage; passing at least a portion of liquid in the first concentrated brine through a second osmosis membrane to a permeate side of the second osmotic stage; obtaining a second concentrated brine from the retentate side of the second osmotic stage to form a first brine product, wherein the second concentrated brine has a higher sodium chloride concentration than the first concentrated brine

2. The method of claim 1, wherein the input brine is obtained from a seawater desalination process.

3 The method of claim 1, further comprising applying a hydraulic pressure of not more than 1200 psi to pass the portion of liquid in the pretreated brine through the first osmosis membrane.

4. The method of claim 1, further comprising applying a hydraulic pressure of not more than 1200 psi to pass the portion of liquid in the first concentrated brine through the second osmosis membrane.

5. The method of claim 1, wherein the second osmosis membrane has a higher permeability and/or salt rejection than the first osmosis membrane.

6. The method of claim 1, further comprising returning at least a portion of the first concentrated brine to the retentate side of the first osmotic stage.

7. The method of claim 1, further comprising returning at least a portion of the second concentrated brine to the retentate side of the second osmotic stage.

8. The method of claim 1, further comprising collecting a permeate from the permeate side of the first and/or second osmotic stage.

9. The method of claim 8, further comprising transporting the permeate to the retentate side of the first or second osmotic stage and passing a portion of liquid from the permeate through the osmosis membrane to the permeate side of the first or second osmotic stage.

10. The method of claim 1, further comprising processing the second concentrated brine in one or more further osmotic stages to form the first brine product, wherein the processing comprises transporting brine from a retentate side of an upstream osmotic stage to a retentate side of a downstream osmotic stage; passing a portion of liquid from the brine through an osmosis membrane in the downstream osmotic stage to a permeate side of the downstream osmotic stage; and obtaining the first brine product from the retentate side of a most downstream osmotic stage, wherein the first brine product has a higher sodium chloride concentration than the second concentrated brine.

11. The method of claim 1 or 10, further comprising transporting the first brine product to a retentate side of a forward osmotic stage; transporting a counter solution to a permeate side of the forward osmotic stage, wherein the counter solution has a higher osmolarity than the first brine product such that a portion of liquid from the first brine product is transferred across a semipenneable membrane that separates the retentate side and permeate side of the forward osmotic stage, obtaining a second brine product from the retentate side of the forward osmotic stage, wherein the second brine product has a higher sodium chloride concentration than the first brine product.

12. The method of claim 11, further comprising flowing the counter solution on the permeate side of the forward osmotic stage and the first brine product on the retentate side of the forward osmotic stage in opposite directions.

13. The method of claim 1, 10 or 11, further comprising transporting the first brine product and/or the second brine product to an anode side chamber of an electrolytic cell, the electrolytic cell comprising water in a cathode side chamber that is separated from the anode side chamber by a sodium ion permeable diaphragm; and applying an electric potential across an anode and a cathode of the electrolytic cell to electrolyse the first brine product and/or the second brine product.

14. An apparatus comprising: a pretreatment stage configured to pass an input brine through at least one filter membrane to produce a pretreated brine, wherein a concentration of at least one bivalent solute impurity in the pretreated brine is lower than the input brine, a first osmotic stage comprising at least one first osmosis membrane that separates the first osmotic stage into a retentate side and a permeate side, the first osmotic stage configured to receive the pretreated brine at its retentate side and pass at least a portion of liquid in the pretreated brine through the first osmosis membrane to its permeate side to leave a first concentrated brine at its retentate side, wherein the first concentrated brine has a higher sodium chloride concentration than the pretreated brine; a second osmotic stage comprising at least one second osmosis membrane that separates the second osmotic stage into a retentate side and a permeate side, the second osmotic stage configured to receive the first concentrated brine at its retentate side and pass at least a portion of liquid in the first concentrated brine through the second osmosis membrane to its permeate side to leave a second concentrated brine at its retentate side, wherein the second concentrated brine has a higher sodium chloride concentration than the first concentrated brine

15. The apparatus of claim 14, further comprising a pump configured to apply a hydraulic pressure of not more than 1200 psi to pass the portion of liquid in the pretreated brine or the first concentrated brine through the first osmosis membrane or second osmosis membrane respectively.

16. The apparatus of claim 14, wherein the second osmosis membrane has a higher permeability and/or salt rejection than the first osmosis membrane.

17. The apparatus of claim 14, wherein the first or second osmotic stage is configured to return at least a portion of the first concentrated brine or second concentration brine to the retentate side respectively.

18. The apparatus of claim 14, wherein the first and/or second osmotic stage is of a spiral wound membrane configuration.

19. The apparatus of claim 14, further comprising a forward osmotic stage comprising at least one semipermeable membrane that separates the forward osmotic stage into a retentate side and a permeate side, the forward osmotic stage configured to receive the second concentrated brine at its retentate side and receive a counter solution at its permeate side, wherein the counter solution has a higher osmolarity than the second concentrated brine and the semipermeable membrane is configured to pass at least a portion of liquid in the second concentrated brine through the semipermeable membrane to its permeate side to leave a forward concentrated brine at its retentate side, the forward concentrated brine having a higher sodium chloride concentration than the second concentrated brine, wherein the forward osmotic stage is configured to flow the second concentrated brine at its retentate side and the counter solution at its permeate side in opposite directions.

20. The apparatus of claim 19, further comprising an electrolytic cell comprising a sodium ion permeable diaphragm that separates the electrolytic cell into an anode side chamber and cathode side chamber, an anode provided in the anode side chamber and a cathode provided in the cathode side chamber, wherein the anode side chamber is configured to receive the forward concentrated brine and the cathode side chamber is configured to receive water, and the anode and cathode are configured to receive an electric potential for electrolysing the forward concentrated brine to produce chlorine gas, sodium hydroxide and hydrogen.