JP2014100692A - Water treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water treatment method by which salt content can be inexpensively and easily removed from sea water or the like with small energy.SOLUTION: The water treatment method includes: a forward osmosis step in which water to be treated is brought into contact with draw solution obtained by dissolving thermosensitive substance having lower limit critical temperature in water through a semipermeable membrane, water in the water to be treated is made to move to the draw solution through the semipermeable membrane, the diluted draw solution diluted by water and membrane concentrated water are thereby obtained; a heating step in which the diluted draw solution is heated to temperature that is less than the lower limit critical temperature and allows the thermosensitive substance to be made hydrophobic; a solid liquid separation step in which the thermosensitive substance that is heated and made hydrophobic in the heating step is subjected to solid liquid separation to be thereby separated from the diluted draw solution as a thermosensitive substance concentrated liquid; and a cooling step in which the thermosensitive substance concentrated liquid that is separated in the solid liquid separation step is cooled to temperature at which the thermosensitive substance is made hydrophilic to regenerate the draw solution.

Description

  The present invention relates to a water treatment method for producing fresh water from water to be treated such as seawater and brine.

  Various methods for producing fresh water from seawater using a semipermeable membrane are known, but a reverse osmosis method for forcibly permeating water by applying a pressure higher than the osmotic pressure to seawater has been mainly developed. However, since this method needs to be pressurized to a high pressure, there is a problem that the equipment cost and the operation cost are high. Therefore, seawater and salt solution with a higher concentration than seawater are brought into contact with each other through a semipermeable membrane, and water in seawater is transferred to this salt solution by osmotic pressure without being pressurized, and separated and recovered to produce fresh water. A method has been developed. (Patent Documents 1 and 2).

  In the method of Patent Document 1, the water to be treated is brought into contact with one surface of the first semipermeable membrane, and the other surface of the first semipermeable membrane is kept at a relatively high temperature in an intermediate solution whose solubility depends on temperature. A moisture absorption step for allowing the intermediary solution to absorb the moisture of the water to be treated through the first semipermeable membrane, and the intermediary solution after the moisture absorption step to be at a relatively low temperature, And a moisture releasing step of bringing the mediator solution after the depositing step into contact with the second semipermeable membrane at a fluid pressure that allows the moisture of the mediator solution to be released through the second semipermeable membrane. Fresh water production method.

  In the method of Patent Document 2, a salt solution obtained by dissolving ammonia and carbon dioxide is passed through a semipermeable membrane on the side opposite to seawater, and water in the seawater is passed through the semipermeable membrane into the salt solution. The resulting diluted salt solution is separated into ammonium ions and carbonate ions individually using an ion exchange membrane or distillation tower to obtain purified water, and the separated ammonium ions and carbonate ions are dissolved in the salt solution. This is the method of returning to the original room of the semipermeable membrane.

JP 2010-162527 A JP 2011-83663 A

  In the method of Patent Document 1, an alum aqueous solution is used as a mediator, that is, a hyperosmotic solution. However, since alum has a moderate increase in solubility with respect to a temperature rise, the alum aqueous solution is heated to 60 ° C. in the moisture absorption step and precipitated. In the process, there is a problem that the liquid temperature adjustment range is large and the equipment operation cost is high, such as cooling to 30 ° C. Further, for example, in the case of potash alum, there is a problem that the pH is close to 3 and the apparatus is easily corroded.

In the method of Patent Document 2, the induction substance (for example, ammonium carbonate) is separated and recovered by an evaporation method. At that time, the latent heat of vaporization of ammonia and accompanying water is enormous, requiring enormous energy and cost. high. Furthermore, the size of the evaporation facility is extremely large, and is not suitable for producing a large amount (for example, 100,000 m ³ / day) of drinking water. Moreover, since the input energy is large, the size of the heat exchanger is also large, which is not suitable for mass processing. Further, when ammonium carbonate is used, an inducer that leaks into the environment through the membrane concentrated water due to backflow from the FO membrane contains nitrogen, which causes eutrophication.

  The present invention has been made to solve these problems, and an object of the present invention is to provide a water treatment method that can easily produce fresh water from seawater or the like with less energy and at low cost.

  The present invention has been made in order to solve the above-mentioned problems, and performs forward osmosis membrane treatment using a high osmotic pressure induction solution prepared by dissolving a thermosensitive substance having a lower critical temperature in water. The resulting dilution-inducing solution is heated to near the lower critical temperature, and part of the temperature-sensitive substance is hydrophobized and precipitated, and separated from this, so that fresh water can be easily obtained at low cost from seawater. As a result, the present invention has been completed.

  That is, the present invention is to contact the water to be treated with a derivative solution in which a temperature-sensitive substance having a lower critical temperature is dissolved in water through a semipermeable membrane, and the water in the treated water passes through the semipermeable membrane. A forward osmosis step of transferring to the induction solution to obtain a diluted induction solution and membrane concentrated water diluted with water, and a part of the thermosensitive substance is made hydrophobic at a temperature below the lower critical temperature. A heating step for heating to a temperature to be heated, a solid-liquid separation step for solid-liquid separation of the thermosensitive material that has been heated in the heating step and made hydrophobic, as a temperature-sensitive substance concentrated liquid from the dilution-inducing solution, And a cooling step of regenerating the induction solution by cooling the temperature-sensitive substance concentrate separated in the solid-liquid separation step to a temperature at which the temperature-sensitive substance becomes hydrophilic. To do.

  For example, when polyoxyethylene alkyl ether having a lower critical temperature of 40 ° C. is used as an inducer, the thermosensitive substance exhibits hydrophilicity and dissolves in the solution in a normal temperature (5 to 25 ° C.) dilution induction solution. ing. When this is heated to 40 ° C., the thermosensitive substance becomes hydrophobic and aggregates. Since the solution becomes cloudy at this time, it is generally known as a cloud point phenomenon. The cloud point is determined visually as the temperature at which the solution becomes cloudy. However, even if the size of the agglomerated particles cannot be visually confirmed as cloudiness at a temperature slightly below the cloud point, the agglomeration reaction proceeds and the number of molecules apparently decreases due to the aggregation of molecules together. To drop. When the solution in which the temperature sensitive substance is aggregated is subjected to UF membrane filtration, the temperature sensitive substance is easily removed by the membrane, and pure water is obtained as a filtrate. The membrane concentrate is an induction solution in which the hydrophobized thermosensitive substance is aggregated. When this is cooled to 30 ° C., the temperature sensitive material is redissolved and the induction solution is regenerated. In the regeneration-inducing solution in which the temperature-sensitive substance is redissolved, the concentration of the inducing substance is increased and a predetermined high osmotic pressure is obtained. By introducing the regenerated induction solution into the semipermeable membrane device and bringing it into contact with the water to be treated through the membrane, pure water can be repeatedly obtained from the water to be treated. Unlike the evaporation method, this method does not require latent heat for phase change, so the input heat amount is extremely small. In addition, since the inducer is agglomerated in the solid-liquid separation membrane, it can be filtered with a high permeation flux even when using a membrane having a coarser eye than RO and NF such as UF and MF. . At this time, since the osmotic pressure is reduced, the filtration power is extremely small.

  In the present invention, by using the induction solution in which the thermosensitive substance is dissolved, the temperature adjustment range during the separation of the fresh water and the regeneration of the induction solution in the forward osmosis membrane treatment is reduced. Fresh water can be obtained efficiently.

It is the figure which showed one embodiment of this invention typically. It is the figure which showed typically the method of desalting by the conventional forward osmosis method.

FIG. 1 schematically shows an embodiment of the present invention.

The water to be treated to be treated by the method of the present invention is a solution using water as a solvent, such as seawater or brine. Brine is
Also included are associated water from wells that mine shale gas, oil sands, CBM (coal bed methane), oil and the like.

Accompanying water is water that is discharged along with the object to be mined from the well, and includes salt, organic matter, suspended matter, and the like. Concentrations of pollutants include, for example, evaporation residues (mainly Na ⁺ , K ⁺ , Ca ²⁺ , Cl ⁻ , SO ₄ ^2−, etc.) of 1,000 to 100,000 mg / L, organic substances (such as oil and added chemicals) Is contained in a range of 10 to 1,000 mg / L as TOC and suspended material in a range of 100 to 10,000 mg / L.

  There is no limitation on the means for separating oil and associated water, but oil-water separation is performed, for example, by sedimentation.

Filtration step Although not shown in FIG. 1, the water to be treated is first filtered. This filtration treatment is performed with a filter using a microfiltration membrane, and a normal membrane used as a microfiltration membrane can be used as the filtration membrane. For example, in addition to cellulose acetate, polytetrafluoroethylene, polysulfone, polyvinyl chloride, etc., ceramic membranes and porous glass membranes can also be used. In the micromembrane filtration treatment, membrane filtrate water that has passed through the microfiltration membrane and membrane concentrated water remaining without passing through the membrane are obtained.
In addition to precision membrane filtration, filtration treatment such as ultramembrane filtration and sand filtration is used. The material for the ultrafiltration is the same as that for precision membrane filtration.

Forward osmosis step The forward osmosis step is a step of bringing the treated water to be filtered into contact with a high osmotic pressure induction solution obtained by dissolving a thermosensitive substance having a lower critical temperature in water through a semipermeable membrane. Is transferred to the induction solution through the semipermeable membrane to obtain a diluted induction solution and membrane concentrated water diluted with water.

  A thermosensitive substance is a substance that is hydrophilic and well soluble in water at low temperatures, but becomes hydrophobic and decreases in solubility at a temperature above a certain temperature, and the temperature at which it changes from water-soluble to water-insoluble is the lower critical temperature or cloud point. be called. When this temperature is reached, the temperature-sensitive material that has become hydrophobic is deposited, resulting in white turbidity.

  This thermosensitive substance is used as various surfactants, dispersants, emulsifiers, and the like. For example, alcohol or fatty acid and ethylene oxide compound, alcohol or fatty acid and propylene oxide compound, acrylamide and alkyl group Compounds, glycerin fatty acid esters, sorbitan fatty acid ester ethylene oxide adducts, amino acids and derivatives thereof. As the temperature-sensitive substance used in the present invention, those having a lower critical temperature in the range of 30 ° C. to 80 ° C., particularly in the range of 40 ° C. to 60 ° C. are preferable. The concentration of the temperature-sensitive substance contained in the induction solution is preferably as high as possible so that a high osmotic pressure can be obtained, and a concentration of about 0.1 to 10 mol / L, particularly about 2 to 5 mol / L is preferable.

  The semipermeable membrane is preferably one that can selectively permeate water, and is preferably a forward osmosis membrane, but a reverse osmosis membrane can also be used. The material is not particularly limited, and examples thereof include cellulose acetate-based, polyamide-based, polyethyleneimine-based, polysulfone-based, and polybenzimidazole-based materials. The form of the semipermeable membrane is not particularly limited and may be any of a flat membrane, a tubular membrane, a hollow fiber, and the like.

  A device for mounting this semipermeable membrane is usually a semi-permeable membrane installed in a cylindrical or box-shaped container, and membrane filtered water flows into one chamber partitioned by this semipermeable membrane, and the other chamber is filled with water. The induction solution can be flowed, and a known semipermeable membrane device can be used, and a commercially available product can be used.

  When the water to be treated is brought into contact with the induction solution through the semipermeable membrane in the forward osmosis step, the water in the water to be treated moves to the induction solution through the semipermeable membrane due to the difference in osmotic pressure.

Heating step In the forward osmosis step, the dilution-inducing solution diluted by moving water from the water to be treated is heated to hydrophobize and deposit at least a part of the temperature-sensitive substance. This precipitation is a phase separation of a concentrated solution of the temperature sensitive substance. The temperature to be heated is preferably a temperature at which a part of the temperature-sensitive substance becomes hydrophobic because it is less than the lower critical temperature of the temperature-sensitive substance.
A range up to a temperature lower by -10 ° C, particularly a temperature lower by 2-5 ° C is preferable. However, the heating temperature may be the lower critical temperature or higher, and may be heated to about 10 ° C. higher than the lower critical temperature.

  It is preferable to use the sensible heat of the separated liquid separated in the next solid-liquid separation process as the heat source for this heating process.

Solid-Liquid Separation Step The dilution-inducing solution heated in the heating step and depositing the thermosensitive substance separates the deposited thermosensitive substance by solid-liquid separation. The solid-liquid separation means is preferably a coarse one, and an ultrafiltration membrane having a molecular weight cut off of 3,000 or more, a microfiltration membrane, sand filtration, filter cloth filtration, centrifugation, sedimentation separation, etc. can be used. . Preferred are ultrafiltration membranes, microfiltration membranes, and centrifugal separations because relatively low power and good separation efficiency can be obtained.

  The water separated by the solid-liquid separation means can usually be used as fresh water as it is, but it is further purified and used if necessary.

Cooling step On the other hand, the separated concentrated solution of the temperature sensitive substance is cooled to a temperature at which the temperature sensitive substance becomes hydrophilic, specifically, a temperature at which the white turbidity disappears, and is regenerated into an induction solution. This temperature can be used in a wide range, but considering the economical efficiency, a temperature lower by 10 to 20 ° C. than the lower critical temperature is suitable, and a temperature of room temperature or higher is preferable. As this cooling heat source, it is preferable to use the water to be treated or the dilution induction solution obtained in the forward osmosis step from the viewpoint of efficient use of energy.

  The regenerated induction solution can be recycled as it is.

  On the other hand, since the membrane concentrated water obtained in the forward osmosis step contains a high concentration of salt, it can be concentrated and precipitated to separate and be effectively used.

  This method of the present invention is shown schematically in FIG. The treatment may be a batch method, but in principle, the treatment is continuous, that is, the water to be treated and the induction solution are continuously flowed, and fresh water is continuously taken out.

Example 1
A cellulose acetate FO membrane was used as the forward osmosis membrane, and a polyoxyethylene alkyl ether dissolved in water at a concentration of 400 g / L was used as the induction solution. The osmotic pressure of the induction solution can be measured based on whether fresh water can be sucked from a predetermined concentration of salt water using an FO membrane, and was 5 MPa in this solution. The cloud point can be specified by visually confirming the cloudiness state, and the cloud point of the induction solution was 38 ° C.

  The inflow amount of the induction solution inlet of the forward osmosis membrane device was 5 L / min. As the liquid to be treated, a 3.5% sodium chloride aqueous solution simulating seawater was used. The amount of water transferred to the induction solution through the membrane filtration device was 0.3 L / min, the amount of diluted induction solution flowing out from the outlet with the induction solution was 5.3 L / min, and the temperature was 25 ° C. .

When this dilution induction solution was heated to 35 ° C. and held for 1 minute, the dissolved polyoxyethylene alkyl ether aggregated and the osmotic pressure decreased to 0.1 MPa.
This solution was subjected to precision membrane filtration to obtain 0.94 kg of membrane concentrated water containing polyoxyethylene alkyl ether at a concentration of 400 g / L, and 0.06 kg of fresh water per kg of the solution.

The concentration of polyoxyethylene alkyl ether contained in the fresh water was 100 ppm or less.
The membrane concentrated water containing polyoxyethylene alkyl ether at a concentration of 400 g / L could be returned to the original transparent induction solution by cooling to 30 ° C.

  The method of the present invention can be widely used for production of fresh water from seawater, treatment of associated water from a well, and the like.

Claims (6)

  The water to be treated and the induction solution in which the thermosensitive substance having the lower critical temperature is dissolved in water are brought into contact with each other through the semipermeable membrane, and the water in the water to be treated is moved to the induction solution through the semipermeable membrane, A forward osmosis step for obtaining a dilution-inducing solution diluted with water and membrane concentrated water, and a heating step for heating the dilution-inducing solution at a temperature below the lower critical temperature to a temperature at which the thermosensitive substance becomes hydrophobic. And the solid-liquid separation step of separating the temperature-sensitive material heated and hydrophobized in the heating step as a temperature-sensitive material concentrate from the dilution-inducing solution, and the solid-liquid separation step. A water treatment method comprising: cooling a temperature-sensitive substance concentrate to a temperature at which the temperature-sensitive substance becomes hydrophilic, and regenerating the induction solution.   The solid-liquid separation means in the solid-liquid separation step is a means by any of an ultrafiltration membrane having a fractional molecular weight of 3,000 or more, a microfiltration membrane, sand filtration, filter cloth filtration, centrifugation, and sedimentation separation. The water treatment method according to claim 1, wherein the water treatment method is characterized.   The temperature-sensitive substance is selected from alcohol or fatty acid and ethylene oxide compounds, alcohol or fatty acid and propylene oxide compounds, acrylamide and alkyl group compounds, glycerin fatty acid esters, sorbitan fatty acid ester ethylene oxide adducts, amino acids and derivatives thereof. The water treatment method according to claim 1, wherein the water treatment method comprises:   The water treatment method according to any one of claims 1 to 3, wherein the lower critical temperature is in the range of 30C to 80C.   The water treatment method according to any one of claims 1 to 4, wherein the sensible heat of the separated liquid separated in the solid-liquid separation step is used as a heat source in the heating step.   The water treatment method according to any one of claims 1 to 5, wherein the dilution induction solution obtained in the forward osmosis step is used as a cold heat source in the cooling step.

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