System and method for recycling byproduct concentrated seawater from hot-process seawater desalination

Abstract

The invention discloses a hot-process seawater desalination byproduct concentrated seawater recycling system and method, and belongs to the technical field of chemical engineering. The system comprises a pretreatment subsystem and a bipolar membrane electrodialysis unit, the pretreatment subsystem comprises a front ultrafiltration unit, a nanofiltration unit, a softening and hardness removing unit, a rear ultrafiltration unit, a high-pressure reverse osmosis unit and an ion exchange resin unit which are connected in sequence; the softening and hardness-removing unit comprises an aerogel adsorption softening and hardness-removing unit or / and a dosing softening and hardness-removing unit; and the ion exchange resin unit is connected with the bipolar membrane electrodialysis unit. The invention also discloses a method for recycling the by-product concentrated seawater from hot-process seawater desalination by adopting the system. By coupling the pretreatment subsystem and the bipolar membrane electrodialysis system, the byproduct concentrated seawater of the multi-effect evaporation equipment (MED) is converted into an acid-base product which can be used for an industrial process, so that the resource utilization of the MED byproduct concentrated seawater is realized, the medicament consumption is reduced, and the membrane blockage is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical engineering, and particularly relates to a system and method for resource utilization of concentrated seawater by - products in thermal seawater desalination. Background Art

[0002] At present, thermal seawater desalination technologies mainly include multi - effect evaporation (MED) and multi - stage flash evaporation (MSF). Among them, the multi - effect evaporation technology (MED) has the characteristics of being easy to achieve large - scale, low water quality requirements, low maintenance cost, high thermal efficiency, etc., and is particularly suitable for application scenarios with waste heat to be consumed. However, whether it is MED or other thermal seawater desalination technologies, concentrated seawater will be produced as a by - product during the desalination process. Compared with the original seawater, the salt content in the concentrated seawater is higher, and it may contain components such as scale inhibitors and chemical cleaning agents. If the untreated concentrated seawater is directly discharged into the ocean, it will cause the salinity of the sea area near the discharge port to increase and gradually accumulate to the open - sea area, resulting in the ecological imbalance of the marine environment. In severe cases, it may even form a large - area "dead sea" phenomenon. In addition, different from membrane - based seawater desalination (such as reverse osmosis) technologies, the concentrated seawater by - produced by MED may carry scaling substances and impurities such as sediment due to contact with heat exchange equipment, resulting in a higher turbidity. Therefore, the resource utilization process of MED - by - produced concentrated seawater should pay more attention to the removal of turbidity in water by adopting a suitable process flow in the pretreatment stage.

[0003] At the present stage, the resource utilization ways of concentrated seawater mainly include the preparation of inorganic salts, the extraction of high - economic - value chemical elements (such as bromine, lithium, etc.) and the preparation of chemical raw materials (such as inorganic acids and bases). Compared with the preparation of inorganic salts and the extraction of valuable elements, the reuse channels of inorganic acids and bases produced in the factory are wider (such as the cleaning of resin and membrane systems, pH adjustment, etc.). In addition, the higher the concentration of the acids and bases produced, the lower the subsequent treatment cost (such as evaporation and concentration), and the higher their economic value. But before that, it is necessary to deeply remove the turbidity, hardness ions such as Ca 2+ , Mg 2+ etc. to prevent damage to the resource utilization equipment. Therefore, it is particularly important to develop a concentrated seawater deep pretreatment and resource utilization system with a simple process, easy to scale up and good economy, which can realize the resource utilization of seawater and convert it into concentrated acid and base products.

[0004] As a clean production equipment based on the ion separation process, bipolar membrane electrodialysis realizes the migration and separation of anions and cations in an inorganic salt aqueous solution through the drive of an electric field, and combines with hydrogen ions (H + ) and hydroxide ions (OH -)The combined formation of acid-base solutions has broad technical development space and market promotion potential in the field of realizing the resource treatment of high-salt wastewater. The integrated process for the resource treatment of high-salt wastewater with bipolar membrane electrodialysis technology as the core can become a new strategy for the high-value resource treatment of the by-product concentrated seawater from MED seawater desalination. However, bipolar membrane electrodialysis has strict requirements for the influent water quality. Divalent cations such as Ca 2 + and Mg2+ are likely to form scale on the membrane surface, resulting in the attenuation of membrane efficiency. In current equipment, it is usually required that the total concentration of Ca 2+ and Mg 2+ does not exceed 0.3 mg / L. Therefore, when using bipolar membrane electrodialysis equipment to carry out resource treatment on concentrated seawater, it is necessary to support an efficient pretreatment system to achieve the softening and hardness removal of concentrated seawater.

[0005] At present, there are relevant reports on the resource treatment of concentrated seawater using bipolar membrane electrodialysis technology. The patent "A Reverse Osmosis Concentrated Seawater Treatment System and Method" with the publication number CN107265734A proposed a combined process of "nanofiltration + resin softening + membrane distillation + bipolar membrane" to treat the by-product concentrated seawater from membrane method seawater desalination. However, the membrane distillation unit in this method requires a heat source and will cause membrane fouling and membrane wetting problems, resulting in poor sustainability of the process route. The patent "A Reverse Osmosis Seawater Desalination System" with the publication number CN204918258U proposed to directly pass the concentrated seawater produced by the reverse osmosis unit through ultrafiltration treatment and then to the bipolar membrane electrodialysis system to produce acid and base. However, this method does not set up a softening unit, and the divalent cations in the concentrated seawater will damage the bipolar membrane, resulting in the decline of the quality of acid and base. In the existing technologies for converting concentrated seawater into acid and base using bipolar membrane electrodialysis technology, only the method of adding chemicals for softening is adopted in the pretreatment stage to remove Ca 2+ and Mg 2+ , and a large amount of chemicals (including PAC, PAM, NaOH, and Na2CO3) need to be used, and the problem of pollution and blockage caused by the relatively high turbidity in the by-product concentrated seawater from MED to the fine membrane separation units (such as nanofiltration and reverse osmosis) is not fully considered.

[0006] Therefore, providing a resource system for the by-product concentrated seawater from thermal method seawater desalination, which can avoid the pollution and blockage of the membrane separation unit, and has the advantages of convenient and flexible process, simple operation, no secondary pollution generation, and good economy, has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0007] One of the purposes of the present invention is to provide a resource system for the by-product concentrated seawater from thermal method seawater desalination, which can convert the by-product concentrated seawater from MED equipment into HCl and NaOH with different concentrations. The Na + content in HCl is not higher than 500 mg / L, and the Cl -The content is not higher than 2000 mg / L. It has the characteristics of convenient and flexible process, simple operation, no secondary pollution generation, and good economy.

[0008] The second object of the present invention is to provide a method for resource utilization of concentrated seawater by-product from thermal seawater desalination using this system.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] The present invention discloses a system for resource utilization of concentrated seawater by-product from thermal seawater desalination in the first aspect, including a pretreatment subsystem and a bipolar membrane electrodialysis unit; wherein the pretreatment subsystem includes a front ultrafiltration unit, a nanofiltration unit, a softening and hardness removal unit, a rear ultrafiltration unit, a high-pressure reverse osmosis unit, and an ion exchange resin unit connected in sequence;

[0011] The softening and hardness removal unit includes an aerogel adsorption softening and hardness removal unit or / and a chemical addition softening and hardness removal unit;

[0012] The ion exchange resin unit is connected to the bipolar membrane electrodialysis unit; the front ultrafiltration unit is connected with an MED by-product concentrated seawater input pipe, and the bipolar membrane electrodialysis unit is connected with a bipolar membrane makeup water input pipe, and is connected with a hydrochloric acid output pipe and a sodium hydroxide solution output pipe.

[0013] In some embodiments of the present invention, when the softening and hardness removal unit includes an aerogel adsorption softening and hardness removal unit and a chemical addition softening and hardness removal unit, the two are arranged in parallel.

[0014] In some embodiments of the present invention, the fresh brine outlet of the bipolar membrane electrodialysis unit is connected to the high-pressure reverse osmosis unit through a pipeline.

[0015] The present invention discloses a method for resource utilization of concentrated seawater by-product from thermal seawater desalination in the second aspect, which is carried out using the above system, and includes the following steps:

[0016] S1. Front ultrafiltration: Send the MED by-product concentrated seawater into the front ultrafiltration unit for ultrafiltration to intercept suspended solids and dissolved solids;

[0017] S2. Nanofiltration: The water produced by the front ultrafiltration unit enters the nanofiltration unit to remove Ca 2+ , Mg 2+ ;

[0018] S3. Softening and hardness removal: The water produced by the nanofiltration unit enters the softening and hardness removal unit to further remove Ca 2+ and Mg 2+ ;

[0019] S4. Rear ultrafiltration: The water produced by the softening and hardness removal unit enters the rear ultrafiltration unit to remove the suspended solids existing in the water;

[0020] S5. Concentration: Feed the post-ultrafiltration product water into the high-pressure reverse osmosis unit for concentration;

[0021] S6. Ion exchange adsorption for impurity removal: Feed the concentrated water from the high-pressure reverse osmosis unit into the ion exchange resin unit to adsorb and remove the residual high-valent cations in the concentrated water of the high-pressure reverse osmosis unit;

[0022] S7. Bipolar membrane electrodialysis: Feed the water produced by the ion exchange resin into the bipolar membrane electrodialysis unit for electrodialysis to generate hydrochloric acid and sodium hydroxide solutions.

[0023] In some embodiments of the present invention, the pore size of the ultrafiltration membrane in the pre-ultrafiltration unit is 30 - 50 nm, the pore size of the nanofiltration membrane in the nanofiltration unit is 1 - 2 nm, and the pore size of the ultrafiltration membrane in the post-ultrafiltration unit is 30 - 50 nm.

[0024] In some embodiments of the present invention, the operating pressure of the nanofiltration unit is 5 - 18 bar.

[0025] In some embodiments of the present invention, in step S3, for the airgel adsorption softening and hardness removal unit, airgel adsorption is used for hardness removal, or while using airgel adsorption for hardness removal in the airgel adsorption softening and hardness removal unit, a chemical dosing softening and hardness removal unit is also used for chemical dosing softening and hardness removal;

[0026] Preferably, for airgel adsorption for hardness removal, the airgel is loaded into the absorption tower as a filler, and the nanofiltration product water stays therein for 1 - 3 h to fully adsorb Ca 2+ and Mg 2+ ;

[0027] Preferably, the chemical dosing softening and hardness removal uses the double-alkali method: Sodium hydroxide solution and sodium carbonate solution are added to the water produced by the nanofiltration unit to convert the calcium ions Ca 2+ and magnesium ions Mg 2+ into calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) precipitates, and then polyaluminum chloride (PAC) solution and polyacrylamide (PAM) solution are added to agglomerate and sediment the precipitates; More preferably, the mass concentration of the sodium hydroxide (NaOH) solution is 10 - 30%; the mass concentration of the sodium carbonate (Na2CO3) solution is 10 - 15%, the mass concentration of the PAC solution is 10 - 30%, and the mass concentration of the PAM solution is 0.2 - 0.8%.

[0028] Further preferably, in 1 m 3 of the nanofiltration product water, 6.5 - 7 L of the NaOH solution with a mass concentration of 10 - 30%, 6.5 - 7 L of the Na2CO3 solution with a mass concentration of 10 - 15%, 4 - 6 L of the PAC solution with a mass concentration of 10 - 30%, and 4 - 6 L of the PAM solution with a mass concentration of 0.2 - 0.8% are added.

[0029] In some embodiments of the present invention, the water produced by the ion exchange resin enters the bipolar membrane electrodialysis unit for electrodialysis to generate hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions; the fresh brine produced by the bipolar membrane electrodialysis unit is recycled to the high-pressure reverse osmosis unit in the pretreatment system and concentrated together with the newly incoming concentrated seawater.

[0030] In some embodiments of the present invention, in step S1, the turbidity of the water produced by the pre-ultrafiltration unit is less than or equal to 0.15 NTU;

[0031] In step S2, the total hardness of the water produced by the nanofiltration unit is 1500 - 2000 mg / L in terms of calcium carbonate;

[0032] In step S3, the total hardness of the concentrated seawater passing through the aerogel adsorption softening and hardness removal unit is less than or equal to 650 mg / L in terms of calcium carbonate; the total hardness of the concentrated seawater treated by the chemical softening and hardness removal unit is less than or equal to 250 mg / L in terms of calcium carbonate;

[0033] In step S5, after the water produced by the post-ultrafiltration is concentrated by the high-pressure reverse osmosis unit, the salt concentration therein is not less than 8 wt%;

[0034] In step S6, after the concentrated water of the high-pressure reverse osmosis unit is subjected to ion exchange adsorption for impurity removal, the total concentration of Ca 2+ and Mg 2+ is less than 0.3 mg / L, and the sodium chloride concentration is 9 - 10 wt%;

[0035] In step S7, the water produced by the ion exchange resin enters the bipolar membrane electrodialysis unit for electrodialysis to generate hydrochloric acid and sodium hydroxide solutions with a concentration of 1 - 3 mol / L.

[0036] In some embodiments of the present invention, in step S7, the concentration of the acid and base is changed by regulating the stable voltage, stable current, current efficiency, and the flow rate of the supplementary pure water;

[0037] Preferably, the stable voltage varies in the range of 150 - 190 V, the stable current varies in the range of 150 - 180 A, the current efficiency varies in the range of 50% - 90%, and the pure water flow rate varies in the range of 70% - 140% of the feed liquid flow rate.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The design of the present invention is scientific and ingenious. By coupling a pretreatment subsystem with a bipolar membrane electrodialysis system, the concentrated seawater by-produced from MED is converted into acid and base products that can be used in industrial processes, realizing the resource utilization of the concentrated seawater by-produced from MED. The present invention fully considers the problems of high turbidity of the concentrated seawater by-produced from MED and large consumption of chemicals in the traditional double-alkali method for chemical softening process. An ultrafiltration unit and an aerogel adsorption softening unit that does not require chemical consumption are set in the pretreatment unit to remove Ca 2+ and Mg 2+ from the concentrated seawater.

[0040] Fully considering the characteristic of high turbidity of the concentrated seawater by-produced from MED due to entrainment of scale deposits and sediment in the MED system, etc., the present invention uses ultrafiltration as the first pretreatment unit to remove the turbidity of the water to a reasonable range, with an efficiency of over 99.5%, avoiding the problems of pollution and blockage of the subsequent membrane separation unit caused by suspended solids in the water.

[0041] Considering the problem of large chemical consumption in the traditional double-alkali method for chemical softening, an aerogel adsorption softening and hardness removal unit is set in the softening and hardness removal unit. Without chemical dosing, a removal rate of not less than 65% of Ca 2+ and Mg 2+ in the concentrated seawater can be achieved. The operating cost of the aerogel adsorption softening and hardness removal is reduced by 20% - 30% compared with the operating cost of the double-alkali method for chemical softening.

[0042] The aerogel adsorption process adopted in the softening and hardness removal unit of the present invention relies on electrostatic adsorption and mesoporous adsorption, rather than the chemical adsorption process based on acidic / basic groups used in conventional ion exchange resins, and desorption and regeneration are easier.

[0043] The bipolar membrane electrodialysis system of the present invention can continuously and stably prepare 1 - 3 mol / L of HCl and NaOH by regulating the stable voltage, stable current, current efficiency, and supplementing the pure water flow rate. The Na + content in HCl is not higher than 500 mg / L, and the Cl - content in NaOH is not higher than 2000 mg / L. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. Figure 1 is a schematic structural diagram of the resource utilization system for concentrated seawater by-produced from thermal seawater desalination of the present invention, where the names corresponding to the reference numerals are: 101 - concentrated seawater by-produced from MED input pipe, 102 - bipolar membrane makeup water input pipe, 103 - hydrochloric acid output pipe, 104 - sodium hydroxide solution output pipe;

[0045] FIG. Figure 2 is a graph showing the removal effect and stability investigation results of the turbidity in the concentrated seawater by the pre-ultrafiltration unit;

[0046] FIG. Figure 3Graph showing the separation performance of the nanofiltration unit for monovalent and divalent ions in water under different operating pressures;

[0047] Appendix Figure 4 Graph showing the effect of long-term operation on the nanofiltration product water;

[0048] Appendix Figure 5 Scanning electron micrograph of the aerogel;

[0049] Appendix Figure 6 Graph showing the removal ability of the aerogel for Ca 2+ and Mg 2+ in concentrated seawater, where Figure 6 a is the removal ability of the aerogel for Ca 2+ and Mg 2+ in concentrated seawater, Figure 6 b is the stability of the removal ability of the aerogel for Ca 2+ and Mg 2+ in concentrated seawater.

[0050] Appendix Figure 7 Graph showing the ability of the chemical softening and hardness removal unit to remove Ca 2+ and Mg 2+ from water;

[0051] Appendix Figure 8 Graph showing the concentration performance and stability of the high-pressure reverse osmosis unit for total dissolved solids (TDS) in concentrated seawater;

[0052] Appendix Figure 9 Graph showing the removal ability and stability of the ion exchange resin unit for Ca 2+ and Mg 2+ in water;

[0053] Appendix Figure 10 Schematic diagram showing the principle of producing acids and bases from concentrated seawater by bipolar membrane electrodialysis;

[0054] Appendix Figure 11 Graph showing the corresponding relationship between different concentrations of acids and bases produced by bipolar membrane electrodialysis and conductivity.

[0055] Appendix Figure 2 -Appendix Figure 11 The Chinese meanings corresponding to the English names are as follows:

[0056] feed - raw water, permeate - product water (membrane system), turbidity - turbidity, operating time - operating time, concentration - concentration, operating pressure - operating pressure, ratio - ratio, parameter - index, type - type, product - product water, cycle - number of cycles, removal rate - removal rate, repeating arrangement - repeating arrangement, mass fraction - mass fraction, conductivity - conductivity, acid - acid, alkali - alkali. Detailed implementation manners

[0057] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0058] The aerogel described in the embodiments of the present invention is a prior art. The aerogel used in the embodiments of the present invention is the H-U series aerogel powder produced by Youte Sen New Materials Group.

[0059] Embodiment 1

[0060] As shown in the appendix Figure 1 As shown, this embodiment discloses a resource utilization system for by-product concentrated seawater in thermal seawater desalination, including a pretreatment subsystem and a bipolar membrane electrodialysis unit.

[0061] The pretreatment subsystem includes a pre-ultrafiltration unit, a nanofiltration unit, a softening and hardness removal unit, a post-ultrafiltration unit, a high-pressure reverse osmosis unit, and an ion exchange resin unit connected in sequence, and each unit is an integrated device.

[0062] The ion exchange resin unit is connected to the bipolar membrane electrodialysis unit; the pre-ultrafiltration unit is connected with an MED by-product concentrated seawater input pipe 101, the bipolar membrane electrodialysis unit is connected with a bipolar membrane makeup water input pipe 102, and a hydrochloric acid output pipe 103 and a sodium hydroxide solution output pipe 104 are led out.

[0063] The softening and hardness removal unit is an aerogel adsorption softening and hardness removal unit or a chemical addition softening and hardness removal unit.

[0064] Embodiment 2

[0065] As shown in the appendix Figure 1 As shown, this embodiment discloses a resource utilization system for by-product concentrated seawater in thermal seawater desalination, including a pretreatment subsystem and a bipolar membrane electrodialysis unit.

[0066] The pretreatment subsystem includes a pre-ultrafiltration unit, a nanofiltration unit, a softening and hardness removal unit, a post-ultrafiltration unit, a high-pressure reverse osmosis unit, and an ion exchange resin unit that are connected in sequence, and each unit is an integrated device.

[0067] The ion exchange resin unit is connected to the bipolar membrane electrodialysis unit; the pre-ultrafiltration unit is connected to the MED by-product concentrated seawater input pipe 101, the bipolar membrane electrodialysis unit is connected to the bipolar membrane makeup water input pipe 102, and a hydrochloric acid output pipe 103 and a sodium hydroxide solution output pipe 104 are led out.

[0068] The softening and hardness removal unit includes an aerogel adsorption softening and hardness removal unit and a chemical addition softening and hardness removal unit; the two are arranged in parallel. The water outlet of the nanofiltration unit is connected to the aerogel adsorption softening and hardness removal unit and the chemical addition softening and hardness removal unit through pipelines respectively, and the water outlets of the aerogel adsorption softening and hardness removal unit and the chemical addition softening and hardness removal unit are connected to the post-ultrafiltration unit through pipelines respectively.

[0069] In Example 2, a more preferable technical solution is given on the basis of Example 1. Specifically, the softening and hardness removal unit includes an aerogel adsorption softening and hardness removal unit and a chemical addition softening and hardness removal unit; the two are arranged in parallel; and the connection relationships with the nanofiltration unit and the post-ultrafiltration unit are defined.

[0070] Example 3

[0071] As shown in the attached Figure 1 As shown, this example discloses a system for recycling by-product concentrated seawater from thermal seawater desalination, including a pretreatment subsystem and a bipolar membrane electrodialysis unit, and each unit is an integrated device.

[0072] The pretreatment subsystem includes a pre-ultrafiltration unit, a nanofiltration unit, a softening and hardness removal unit, a post-ultrafiltration unit, a high-pressure reverse osmosis unit, and an ion exchange resin unit that are connected in sequence,

[0073] The ion exchange resin unit is connected to the bipolar membrane electrodialysis unit; the pre-ultrafiltration unit is connected to the MED by-product concentrated seawater input pipe 101, the bipolar membrane electrodialysis unit is connected to the bipolar membrane makeup water input pipe 102, and a hydrochloric acid output pipe 103 and a sodium hydroxide solution output pipe 104 are led out.

[0074] The softening and hardness removal unit includes an aerogel adsorption softening and hardness removal unit and a chemical addition softening and hardness removal unit; the two are arranged in parallel. The water outlet of the nanofiltration unit is connected to the aerogel adsorption softening and hardness removal unit and the chemical addition softening and hardness removal unit through pipelines respectively, and the water outlets of the aerogel adsorption softening and hardness removal unit and the chemical addition softening and hardness removal unit are connected to the post-ultrafiltration unit through pipelines respectively.

[0075] The fresh brine outlet of the bipolar membrane electrodialysis unit is connected to the high-pressure reverse osmosis unit through a pipeline.

[0076] Example 4 gives a more preferred technical solution on the basis of Example 3. Specifically, the fresh brine outlet of the bipolar membrane electrodialysis unit is connected to the high-pressure reverse osmosis unit through a pipeline, so as to return the fresh brine generated by the bipolar membrane electrodialysis unit to the high-pressure reverse osmosis unit in the pretreatment system and concentrate it together with the newly incoming concentrated seawater.

[0077] Example 5

[0078] This example discloses a method for recycling the concentrated seawater by-product of the thermal seawater desalination of the present invention, which is carried out by using the system for recycling the concentrated seawater by-product of the thermal seawater desalination in Example 4.

[0079] In this example, the TDS of the MED by-product concentrated seawater is 50000-65000 mg / L, and the main ions are sodium ion Na + , calcium ion Ca 2+ , magnesium ion Mg 2+ , potassium ion K + , chloride ion Cl - , sulfate ion SO4 2- and bicarbonate ion HCO3 - . Taking Na + and Cl - as the target retained ions, the total concentration is about 40000-50000 mg / L; Ca 2+ , Mg 2+ are the target removed ions, and the total hardness is about 6000-9000 mg / L (calculated as CaCO3). In addition, because the concentrated seawater will entrain scale and sediment in the MED system, its turbidity is relatively high, which is 100-200 NTU.

[0080] The method of this example specifically includes the following steps:

[0081] S1. Pre-ultrafiltration

[0082] Send the MED by-product concentrated seawater into the pre-ultrafiltration unit for ultrafiltration to achieve the interception of suspended solids and dissolved solids; the pore size of the ultrafiltration membrane in the pre-ultrafiltration unit is 30-50 nm, and the turbidity of the water produced by the pre-ultrafiltration unit is less than or equal to 0.15 NTU;

[0083] S2. Nanofiltration

[0084] The water produced by the pre-ultrafiltration unit enters the nanofiltration unit to remove Ca 2+ , Mg 2+ in the water; the pore size of the nanofiltration membrane in the nanofiltration unit is 1-2 nm, and the total hardness of the water produced by the nanofiltration unit is 1500-2000 mg / L in terms of calcium carbonate; the concentrated water is discharged;

[0085] S3. Softening and hardness removal

[0086] The water produced by the nanofiltration unit enters the aerogel adsorption softening and hardness removal unit and the chemical dosing softening and hardness removal unit respectively for hardness removal.

[0087] Among them, for aerogel adsorption and hardness removal, the aerogel is loaded into the absorption tower as a filler, and the nanofiltration-produced water stays in it for 1 - 3 h to fully adsorb Ca 2+ and Mg 2+ in the water. During this period, no other reactants or catalysts are added. The total hardness of the concentrated seawater passing through the aerogel adsorption softening and hardness removal unit, calculated as calcium carbonate, is less than or equal to 650 mg / L. The removal efficiency of Ca 2+ and Mg 2+ in the concentrated seawater by the aerogel adsorption softening unit is not less than 70%.

[0088] For chemical dosing softening and hardness removal, the double - alkali method is adopted: a sodium hydroxide solution with a mass concentration of 10 - 30% and a sodium carbonate solution with a mass concentration of 10 - 15% are added to the water produced by the nanofiltration unit to convert Ca 2+ and Mg 2+ in the water into CaCO3 and Mg(OH)2 precipitates. Then, a PAC solution with a mass concentration of 10 - 30% and a PAM solution with a mass concentration of 0.2 - 0.8% are added to agglomerate and sediment the precipitates. After the above process, depending on the actual pH value of the produced water, HCl with a concentration of 10% - 30% is selectively added for neutralization. The total hardness of the concentrated seawater treated by the chemical dosing softening and hardness removal unit, calculated as calcium carbonate, is less than or equal to 250 mg / L. The removal efficiency of Ca 2+ and Mg 2+ in the concentrated seawater by the chemical dosing softening and hardness removal unit is not less than 90%.

[0089] S4. Post - ultrafiltration

[0090] The water produced by the softening and hardness removal unit enters the post - ultrafiltration unit to remove the suspended solids present in the water; the pore size of the ultrafiltration membrane in the post - ultrafiltration unit is 30 - 50 nm;

[0091] S5. Concentration

[0092] The water produced by the post - ultrafiltration is fed into the high - pressure reverse osmosis unit for concentration. Under normal operating conditions, the salt mass concentration of the feed water to the bipolar membrane electrodialysis system is 8% - 20%. However, the salt content of the concentrated seawater treated by the post - ultrafiltration unit is generally below 5 wt%. Therefore, before the concentrated seawater enters the bipolar membrane electrodialysis system, it first enters the high - pressure reverse osmosis unit to concentrate it to more than 8 wt% (i.e., the concentrated water of the high - pressure reverse osmosis unit).

[0093] S6. Ion exchange adsorption for impurity removal

[0094] The concentrated water of the high - pressure reverse osmosis unit enters the ion exchange resin unit to adsorb and remove the residual high - valence cations in the concentrated water of the high - pressure reverse osmosis unit; the Ca 2+and Mg 2+ The total concentration of the two is less than 0.3 mg / L.

[0095] S7. Bipolar membrane electrodialysis:

[0096] After being treated by steps S1 - S6, the main component of the concentrated seawater is sodium chloride, with a concentration of about 9 wt% - 10 wt%. It enters the bipolar membrane electrodialysis system and is converted into hydrochloric acid and sodium hydroxide solutions with a concentration of about 1 - 3 mol / L and a purity greater than 97%. The reaction equation is as follows.

[0097]

[0098] During the preparation of acids and bases, the concentrations of acids and bases can be changed by regulating the stable voltage, stable current, current efficiency, and supplementing the pure water flow rate. In this embodiment, the stable voltage variation range is 150 - 190 V, the stable current variation range is 150 - 180 A, the current efficiency variation range is 50% - 90%, and the pure water flow rate variation range is 70% - 140% of the feed liquid flow rate. While generating acids and bases, fresh brine with a salt concentration of about 1% - 5% will also be produced. The fresh brine generated by the bipolar membrane electrodialysis unit will flow back to the high - pressure reverse osmosis unit in the pretreatment system and be concentrated together with the newly incoming concentrated seawater. In addition, the water produced by the high - pressure reverse osmosis unit treated by the low - pressure reverse osmosis unit or external industrial water is provided as the supplementary pure water for the bipolar membrane electrodialysis system. The conductivity of the pure water is not higher than 10 μs / cm.

[0099] Test Example 1

[0100] This test example investigated the removal effect and stability of the turbidity in concentrated seawater by the pre - ultrafiltration unit.

[0101] The turbidity of the water produced by the pre - ultrafiltration unit at different time periods was measured using a turbidity meter (Shanghai Boqu Instrument Co., Ltd., model TBG - 2088S), and the results are as shown in the appendix Figure 2 as follows.

[0102] As can be seen from the appendix Figure 2 : The turbidity of the water produced by the pre - ultrafiltration unit is about 0.1 - 0.15 NTU, and there is no obvious fluctuation during the 90 - day operation.

[0103] Test Example 2

[0104] This test example investigated the separation performance of the nanofiltration unit for divalent and monovalent ions in water.

[0105] 1. Investigation of the separation performance of the nanofiltration unit for divalent and monovalent ions in water under different operating pressures

[0106] The same batch of pretreated ultrafiltration unit product water was subjected to nanofiltration at different operating pressures (5, 10, 15, 18 bar), and an ion chromatography detection device (Metrohm China Co., Ltd., model 940 Professional IC Vario) was used to measure the chloride ion Cl - 、magnesium ion Mg 2+ 、calcium ion Ca 2+ and sulfate ion SO4 2- concentrations in the product water at different operating pressures, and calculate the ratio of chloride ion Cl - to divalent ions (magnesium ion Mg 2+ 、calcium ion Ca 2+ 、sulfate ion SO4 2- ). The results are shown in the appendix Figure 3 .

[0107] As can be seen from the appendix Figure 3 : As the nanofiltration operating pressure increases, the chloride ion Cl - concentration in the nanofiltration product water rises from about 17000 mg / L to 21000 mg / L, and the magnesium ion Mg 2+ concentration drops from about 460 mg / L to about 220 mg / L. The calcium ion Ca 2+ concentration drops from about 280 mg / L to about 160 mg / L. The sulfate ion SO4 2- concentration drops from about 380 mg / L to about 80 mg / L. Based on the above trends, the ratio of chloride ion Cl - to divalent ions (magnesium ion Mg 2+ 、calcium ion Ca 2+ 、sulfate ion SO4 2- ) also increases with the increase of the operating pressure.

[0108] 2. Investigation of the influence of long-term operation on nanofiltration product water

[0109] Under the operating pressure of 15 - 18 bar, the nanofiltration unit was operated for a long time, and a conductivity meter (Shanghai Yidian Analytical Instrument Co., Ltd., model DDSJ-319L) was used to measure the total dissolved solids (TDS) concentration in the nanofiltration product water at different time periods. An ion chromatography detection device (Metrohm China Co., Ltd., model 940 Professional IC Vario) was used to measure the calcium ion Ca 2+ 、magnesium ion Mg 2+ and chloride ion Cl - concentrations in the nanofiltration product water at different time periods. The results are shown in the appendix Figure 4 .

[0110] As can be seen from the appendix Figure 4 : In the nanofiltration product water, the total dissolved solids TDS, calcium ion Ca 2+, magnesium ion Mg 2+ and chloride ion Cl - concentrations are both stable. The total dissolved solids (TDS) content is about 33000 - 35000 mg / L, calcium ion Ca 2+ content is about 155 - 170 mg / L, magnesium ion Mg 2+ content is about 220 - 280 mg / L, chloride ion Cl - content is about 20000 - 21000 mg / L.

[0111] Test Example 3

[0112] This test example investigated the performance of the aerogel adsorption softening and hardness removal unit in removing Ca 2+ and Mg 2+ .

[0113] The investigation method is as follows: An ion chromatography detection device (Metrohm China Co., Ltd., model 940 Professional IC Vario) was used to measure the calcium ion Ca 2+ and magnesium ion Mg 2+ concentrations in the influent and effluent of the aerogel adsorption softening and hardness removal unit.

[0114] The scanning electron microscope image of the aerogel is as shown in Figure 5 .

[0115] The removal ability of the aerogel for calcium ion Ca 2+ and magnesium ion Mg 2+ in concentrated seawater is as shown in Figure 6 a. As can be seen from Figure 6 a: The calcium ion Ca 2+ and magnesium ion Mg 2+ in the influent of the aerogel adsorption softening and hardness removal unit are about 270 mg / L and 457 mg / L, and the calcium ion Ca 2+ and magnesium ion Mg 2+ concentrations in the effluent are about 66 mg / L and 111 mg / L.

[0116] The stability of the aerogel in removing calcium ion Ca 2+ and magnesium ion Mg 2+ is as shown in Figure 6 b. As can be seen from Figure 6 b: After two cleaning and regeneration processes, the removal rates of the aerogel for calcium ion Ca 2+ and magnesium ion Mg 2+ are maintained at 65% - 75%, showing good stability.

[0117] Test Example 4

[0118] This test example investigated the calcium ion Ca removal performance of the chemical dosing softening and hardness removal unit2+ and magnesium ion Mg 2+ removal effect.

[0119] The investigation method is as follows: An ion chromatography detection device (Metrohm China Co., Ltd., model 940 Professional IC Vario) was used to measure the calcium ion Ca 2+ and magnesium ion Mg 2+ concentrations in the influent and effluent of the chemical softening and hardness removal unit. A conductivity meter (Shanghai INESA Analytical Instrument Co., Ltd., model DDSJ-319L) was used to measure the total dissolved solids (TDS) in the influent and effluent of the chemical softening and hardness removal unit.

[0120] The results are as attached Figure 7 shown. From the attached Figure 7 it can be seen that: The calcium ion Ca 2+ and magnesium ion Mg 2+ concentrations in the effluent of the chemical softening and hardness removal unit are approximately 63 mg / L and 18 mg / L respectively, and the removal rates are approximately 77% and 96% respectively.

[0121] Test Example 5

[0122] This test example investigated the concentration performance and stability of the total dissolved solids (TDS) in the concentrated seawater by the high-pressure reverse osmosis unit.

[0123] The investigation method is as follows: A conductivity meter (Shanghai INESA Analytical Instrument Co., Ltd., model DDSJ-319L) was used to measure the total dissolved solids (TDS) in the concentrate of the high-pressure reverse osmosis unit.

[0124] The results are as attached Figure 8 shown. From the attached Figure 8 it can be seen that: The TDS content in the concentrate of the high-pressure reverse osmosis unit is approximately 90000 - 100000 mg / L and remains stable during the 90-day operation period.

[0125] Test Example 6

[0126] This test example investigated the removal effect and stability of calcium ion Ca 2+ and magnesium ion Mg 2+ in water by the ion exchange resin unit.

[0127] The investigation method is as follows: An ion chromatography detection device (Metrohm China Co., Ltd., model 940 Professional IC Vario) was used to measure the calcium ion Ca 2+ and magnesium ion Mg 2+ concentrations in the influent and effluent of the ion exchange resin unit.

[0128] The results are as attached Figure 9 shown. From the attachedFigure 9 It can be known that the sum of the concentrations of calcium ions Ca 2+ and magnesium ions Mg 2+ in the product water of the ion exchange resin unit is not greater than 0.2 mg / L and remains stable during the 90-day operation period.

[0129] Test Example 7

[0130] This test example investigated the conductivity of acids and bases with different mass fractions produced by the bipolar membrane electrodialysis unit. The schematic diagram of producing acids and bases from concentrated seawater by the bipolar membrane electrodialysis system is as shown in the appendix Figure 10 as follows.

[0131] The investigation method is as follows: Use a conductivity meter (Shanghai Yidian Analytical Instrument Co., Ltd., model DDSJ-319L) to measure the conductivity of acids and bases with different mass fractions.

[0132] The results are as shown in the appendix Figure 11 as follows. It can be known from the appendix Figure 11 that: as the mass fraction of the acids and bases produced by the bipolar membrane electrodialysis increases, the conductivity also increases accordingly. The conductivity change range of the acid is 210 - 450 mS / cm, and the conductivity change range of the base is 170 - 440 mS / cm.

[0133] Finally, it should be noted that: the above embodiments are only relatively preferred embodiments of the present invention to illustrate the technical solutions of the present invention, rather than limiting it, and certainly not limiting the patent scope of the present invention. Any modification or polishing made without substantial significance in the main design concept and spirit of the present invention, as long as the technical problems solved are still the same as those of the present invention, should be included within the protection scope of the present invention; in addition, directly or indirectly applying the technical solutions of the present invention to other related technical fields should also be included within the patent protection scope of the present invention by the same token.

Claims

1. A system for resource utilization of concentrated seawater produced by thermal desalination, characterized in that: It includes a pretreatment subsystem and a bipolar membrane electrodialysis unit; wherein the pretreatment subsystem includes a pre-ultrafiltration unit, a nanofiltration unit, a softening and hardness removal unit, a post-ultrafiltration unit, a high-pressure reverse osmosis unit and an ion exchange resin unit connected in sequence; The softening and hardening removal unit includes an aerogel adsorption softening and hardening removal unit and / or a drug addition softening and hardening removal unit; The ion exchange resin unit is connected to the bipolar membrane electrodialysis unit; the pre-ultrafiltration unit is connected to a MED byproduct concentrated seawater input pipe (101); the bipolar membrane electrodialysis unit is connected to a bipolar membrane water replenishment input pipe (102), and is connected to a hydrochloric acid output pipe (103) and a sodium hydroxide solution output pipe (104).

2. A system for utilizing concentrated seawater as a byproduct of thermal desalination according to claim 1, characterized in that: When the softening and hardening removal unit comprises an aerogel adsorption softening and hardening removal unit and a drug addition softening and hardening removal unit, the two are arranged in parallel.

3. A system for resource utilization of concentrated seawater byproduct from thermal desalination according to claim 1 or 2, characterized in that: The brine outlet of the bipolar membrane electrodialysis unit is connected to the high-pressure reverse osmosis unit through a pipeline.

4. A method for resource utilization of concentrated seawater produced as a byproduct of thermal seawater desalination, characterized in that: The method is carried out using the system described in any one of claims 1 to 3, comprising the following steps: S1. Pre-ultrafiltration: The concentrated seawater produced as a byproduct of MED is sent to the pre-ultrafiltration unit for ultrafiltration to intercept suspended solids and dissolved solids; S2. Nanofiltration: The water produced by the front ultrafiltration unit enters the nanofiltration unit to remove calcium ions Ca in the water 2+ and magnesium ion Mg 2+ ; S3. Softening and hardness removal: The water produced by the nanofiltration unit enters the softening and hardness removal unit to further remove calcium ions Ca in the water. 2+ and magnesium ion Mg 2+ ; S4. Post-ultrafiltration: The water produced by the softening and hardness removal unit enters the post-ultrafiltration unit to remove suspended solids in the water; S5. Concentration: The post-ultrafiltration water enters the high-pressure reverse osmosis unit for concentration; S6. Ion exchange adsorption and impurity removal: The concentrated water from the high-pressure reverse osmosis unit enters the ion exchange resin unit, and the high-valent cations remaining in the concentrated water from the high-pressure reverse osmosis unit are adsorbed and removed; S7. Bipolar membrane electrodialysis: The water produced by the ion exchange resin enters the bipolar membrane electrodialysis unit for electrodialysis to generate hydrochloric acid and sodium hydroxide solution.

5. The method for resource utilization of concentrated seawater produced as a byproduct of thermal seawater desalination according to claim 4, characterized in that: The pore size of the ultrafiltration membrane in the front ultrafiltration unit is 30-50 nm, the pore size of the nanofiltration membrane in the nanofiltration unit is 1-2 nm, and the pore size of the ultrafiltration membrane in the rear ultrafiltration unit is 30-50 nm.

6. The method for resource utilization of concentrated seawater produced as a byproduct of thermal seawater desalination according to claim 4, characterized in that: The operating pressure of the nanofiltration unit is 5 to 18 bar.

7. The method for resource utilization of concentrated seawater produced as a byproduct of thermal desalination according to claim 5, characterized in that: In the step S3, an aerogel adsorption softening and hardness removal unit is used to remove hardness by aerogel adsorption, or an aerogel adsorption softening and hardness removal unit is used to remove hardness by aerogel adsorption and hardness removal, and a drug addition softening and hardness removal unit is used to remove hardness by drug addition; Preferably, the aerogel adsorption and hardness removal is to load the aerogel into an absorption tower as a filler, and the nanofiltration produced water stays therein for 1 to 3 hours to fully absorb the calcium ions Ca in the water. 2+ and magnesium ion Mg 2+ ; Preferably, the double alkali method is used for softening and removing hardness by adding chemicals: sodium hydroxide solution and sodium carbonate solution are added to the water produced by the nanofiltration unit to remove calcium ions Ca 2+ and magnesium ion Mg 2+ Converted into calcium carbonate and magnesium hydroxide precipitates, and then added with polyaluminium chloride solution and polyacrylamide solution to aggregate and settle the precipitates; More preferably, the mass concentration of the sodium hydroxide solution is 10-30%; the mass concentration of the sodium carbonate solution is 10-15%, the mass concentration of the PAC solution is 10-30%, and the mass concentration of the PAM solution is 0.2-0.8%; More preferably, 1m 3 In the nanofiltration produced water, add 6.5-7 L of NaOH solution with a mass concentration of 10-30%, 6.5-7 L of Na2CO3 solution with a mass concentration of 10-15%, 4-6 L of PAC solution with a mass concentration of 10-30%, and 4-6 L of PAM solution with a mass concentration of 0.2-0.8%.

8. The method for resource utilization of concentrated seawater produced as a byproduct of thermal desalination according to claim 5, characterized in that: The water produced by the ion exchange resin enters the bipolar membrane electrodialysis unit for electrodialysis to generate hydrochloric acid and sodium hydroxide solution; the brine produced by the bipolar membrane electrodialysis unit flows back to the high-pressure reverse osmosis unit in the pretreatment system to be concentrated together with the newly introduced concentrated seawater.

9. The method for resource utilization of concentrated seawater produced as a byproduct of thermal seawater desalination according to claim 5, characterized in that: In step S1, the turbidity of the water produced by the front ultrafiltration unit is less than or equal to 0.15 NTU; In step S2, the total hardness of the water produced by the nanofiltration unit is 1500-2000 mg / L in terms of calcium carbonate; In step S3, the total hardness of concentrated seawater after the aerogel adsorption softening and hardness removal unit is less than or equal to 650 mg / L in terms of calcium carbonate; the total hardness of concentrated seawater after the treatment by the dosing softening and hardness removal unit is less than or equal to 250 mg / L in terms of calcium carbonate; In step S5, the post-ultrafiltration produced water is concentrated by a high-pressure reverse osmosis unit, wherein the salt concentration is not less than 8 wt %; In step S6, the concentrated water from the high pressure reverse osmosis unit is subjected to ion exchange adsorption to remove impurities, and its calcium ions Ca 2+ and magnesium ion Mg 2+ The total concentration of the two is less than 0.3 mg / L, and the concentration of sodium chloride is 9-10 wt%; In step S7, the water produced by the ion exchange resin enters the bipolar membrane electrodialysis unit for electrodialysis to generate 1-3 mol / L hydrochloric acid and sodium hydroxide solutions.

10. The method for resource utilization of concentrated seawater produced as a byproduct of thermal seawater desalination according to claim 5, characterized in that: In step S7, the concentrations of the hydrochloric acid and sodium hydroxide solutions are changed by regulating the stable voltage, the stable current, the current efficiency and the flow rate of pure water; Preferably, the stable voltage varies in the range of 150 to 190 V, the stable current varies in the range of 150 to 180 A, the current efficiency varies in the range of 50% to 90%, and the pure water flow varies in the range of 70% to 140% of the feed liquid flow.

Images