Method for improving crystallization rate of nanofiltration concentrated water by using waste heat

Abstract

The invention belongs to the field of resource utilization of industrial byproduct heavy salt water, and particularly relates to a method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat. The method comprises the following steps: taking hot-process concentrated seawater, and pretreating the hot-process concentrated seawater as a waste heat utilization resource; the method comprises the following steps: mixing hot-process concentrated seawater and membrane-process desalination byproduct concentrated seawater, and carrying out nanofiltration on the formed mixed concentrated seawater to produce nanofiltration concentrated water; carrying out heat exchange on the hot-process concentrated seawater and the nanofiltration concentrated water in the previous step in a coil heat exchanger, and feeding into a calcium sulfate crystallizer; calcium sulfate seed crystals are added into a calcium sulfate crystallizer, induced crystallization is carried out through stirring, and supernate is conveyed to an inlet pipeline of a nanofiltration process through a clear liquid pump. Compared with the prior art, the method has the advantages that the temperature of the nanofiltration concentrated water is increased, so that the crystallization rate of calcium sulfate induced crystallization is increased; waste heat of an upstream process is fully utilized, and heat is recycled.

Description

Technical Field

[0001] The invention belongs to the field of resource utilization of industrial by-product high-salt water, and specifically relates to a method for utilizing waste heat to increase the crystallization rate of nanofiltration concentrated water. Background Art

[0002] In the field of water treatment, many brines containing high concentrations of calcium and magnesium sulfate are extremely difficult to treat or have high treatment costs due to scaling during salt concentration or salt separation. In particular, there is a lack of efficient, low-cost and low-cost methods for the treatment of calcium sulfate. Most of the traditional chemical hardness removal methods will cause problems such as high costs and more solid waste. The crystallization rate of induced crystallization is usually low under normal temperature conditions. Increasing the temperature can accelerate crystallization, but the energy consumption required is high and uneconomical. For example: 1. The patent name applied by Huadian Water Engineering Co., Ltd.: A high-salt wastewater calcium sulfate crystallization system and a method for treating high-salt wastewater, publication number: CN110790351A, which discloses a method of treating calcium sulfate by adding sodium carbonate, which is costly and will produce by-products; 2. The patent name applied by Yantai Jinzheng Environmental Protection Technology Co., Ltd.: A system and method for treating wastewater containing high-concentration calcium sulfate, publication number: CN111039453A; although the principle of induced crystallization is also adopted, coagulation precipitation, dilution, filtration and membrane concentration steps are added, and the economic benefits are obviously lower. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art, thereby providing a method for utilizing waste heat to increase the crystallization rate of nanofiltration concentrated water.

[0004] The present invention solves the above technical problems by adopting the following technical solutions:

[0005] A method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat is carried out according to the following steps:

[0006] S1, taking concentrated seawater by thermal method and using it as waste heat resource after pretreatment;

[0007] S2, taking concentrated seawater from the thermal process and concentrated seawater produced as a byproduct of membrane desalination, mixing the concentrated seawater from the thermal process and the concentrated seawater produced as a byproduct of membrane desalination, and subjecting the resulting mixed concentrated seawater to a nanofiltration process to produce nanofiltration concentrated water;

[0008] S3, after heat exchange in a coil heat exchanger, the concentrated seawater in step 1 and the concentrated nanofiltration water in step 2 enter a calcium sulfate crystallizer;

[0009] S4. Add calcium sulfate seeds into the calcium sulfate crystallizer, induce crystallization by stirring, and transport the supernatant to the inlet pipeline of the nanofiltration process through a supernatant pump.

[0010] The present invention adopting the above technical solution has the following advantages:

[0011] (1) Increase the temperature of the nanofiltration concentrate to increase the crystallization rate of calcium sulfate induced crystallization.

[0012] (2) Make full use of waste heat from upstream processes to recover heat.

[0013] The preferred embodiment of the present invention is:

[0014] Preferred solution 1: The temperature of the concentrated seawater obtained in step S1 is 40-45°C.

[0015] Preferred solution 2: The volume ratio of the thermal concentrated seawater and the membrane desalination byproduct concentrated seawater taken in step S2 is 10-15:1, and the temperature of the nanofiltration concentrated water generated after nanofiltration is 20-30°C.

[0016] Preferred solution 3: The nanofiltration concentrated water obtained in step S2, Ca 2+ Concentration 2-3g / L, Mg 2+ Concentration 6.5-9g / L, SO4 2- Concentration 17-20g / L.

[0017] Preferred solution 4: The temperature range of the concentrated seawater after heat exchange in step S3 is 35-40°C.

[0018] Preferred solution five: The nanofiltration concentrated water obtained in step S2 enters the coil heat exchanger for heat exchange with thermal concentrated seawater. The temperature range before heat exchange is 20-30°C, and the temperature range after heat exchange is 30-40°C.

[0019] Preferred solution six: In step S5, the added calcium sulfate seed crystals are calcium sulfate dihydrate crystals with a snowflake-like appearance and a particle size of 2-3 mm.

[0020] Preferred solution seven: In step S5, the stirring rate of the calcium sulfate crystallizer is 10-500r / min, and the liquid flow rate of the calcium sulfate crystallizer is 0.5-3000m 3 / h, calcium sulfate crystallizer circulating liquid flow rate 0.1-300m 3 / h.

[0021] Preferred solution eight: The mixed concentrated seawater in step 2 is adjusted to pH 6-6.5 using hydrochloric acid, a scale inhibitor is added at 2-3 ppm, and the Ca is enriched by a nanofiltration membrane device at a pressure of 3 MPa. 2+ Mg 2+ 、SO4 2- Then nanofiltration concentrated water is produced.

[0022] Preferred solution nine: In step S5, a metering pump is used to continuously add the polyacrylamide solution in an amount of 3 ppm into the central barrel of the calcium sulfate crystallizer to promote the flocculation and sedimentation of the calcium sulfate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0024] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments:

[0025] A method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat is carried out according to the following steps:

[0026] S1. The concentrated seawater is taken as a heat source for waste heat utilization after pretreatment to remove various impurities, including but not limited to colloids, particulates, macromolecular substances and other impurities.

[0027] S2, take concentrated seawater from thermal desalination and membrane desalination to form mixed concentrated seawater in the pipeline, use hydrochloric acid to adjust the pH to 6-6.5, add scale inhibitor at 2-3ppm, and enrich Ca in the nanofiltration membrane device in the nanofiltration process at a pressure of 3MPa. 2+ Mg 2+ 、SO4 2- The nanofiltration concentrated water produced later: Ca 2+ Concentration 2-3g / L, Mg 2+ Concentration 6.5-9g / L, SO4 2- Concentration 17-20g / L.

[0028] S3, the concentrated seawater from the thermal process in step 1 is transported from bottom to top to the shell side of the coil heat exchanger through a pipeline, and heat exchange is performed in the coil heat exchanger with the nanofiltration concentrated water (the temperature of the nanofiltration concentrated water is about 20-30° C. at this time) entering the tube side of the coil heat exchanger from top to bottom in the aforementioned step 2, and the temperature of the nanofiltration concentrated water is increased to 30-40° C. At this time, the temperature of the concentrated seawater from the thermal process is reduced to 35-40° C. after heat exchange, and it enters the nanofiltration concentrated water buffer tank by self-pressure; and enters the calcium sulfate crystallizer through the nanofiltration concentrated water delivery pump.

[0029] S4. Add calcium sulfate seeds into the calcium sulfate crystallizer: dihydrate calcium sulfate crystals, which are snowflake-shaped and have a diameter of about 2-3 mm; by controlling the stirring rate at 10-500 r / min and the liquid flow rate at 0.5-3000 m 3 / h, circulating fluid flow rate 0.1-300m 3 / h and other parameters, inducing crystallization through stirring, using a metering pump to continuously add polyacrylamide solution at a rate of 3ppm into the central barrel of the calcium sulfate crystallizer to promote the flocculation and sedimentation of calcium sulfate; the supernatant is transported to the inlet pipeline of the nanofiltration process through a clear liquid pump.

[0030] In the specific application embodiment, the main equipment includes a coil heat exchanger, a calcium sulfate crystallizer, a dosing device, a nanofiltration concentrate delivery pump, a clear liquid pump, a metering pump, a clear liquid barrel, a thickener, a centrifuge, a metering instrument, etc.

[0031] The 40-45℃ hot concentrated seawater is transported from bottom to top through the pipeline to the shell side of the coil heat exchanger, and its flow is controlled by the automatic regulating valve and the temperature gauge. The 20-30℃ nanofiltration concentrated water comes from the nanofiltration process: the nanofiltration concentrated water is obtained by mixing the hot concentrated seawater with the membrane desalination byproduct concentrated seawater and entering the nanofiltration inlet pipeline through the nanofiltration process. The nanofiltration concentrated water is transported from top to bottom to the coil heat exchanger tube side through the feed water pump, and its flow is controlled by the automatic regulating valve. The hot concentrated seawater and the nanofiltration concentrated water exchange heat in the coil heat exchanger. After the heat exchange, the temperature of the hot concentrated seawater is reduced to 35-40℃, and then mixed with the membrane desalination byproduct concentrated seawater and enter the nanofiltration process inlet pipe; the temperature of the nanofiltration concentrated water rises to 30-40℃, and enters the nanofiltration concentrated water buffer tank by self-pressure. There is a liquid level meter in the buffer tank to control the liquid level to be stable at 85%.

[0032] The nanofiltration concentrate pump is used to transport the nanofiltration concentrate in the buffer tank to the calcium sulfate crystallizer, and enters from the center barrel of the crystallizer. A certain amount of calcium sulfate seeds are added before operation. After induced crystallization and gravity sedimentation, the supernatant is collected in the overflow tank outside the top of the crystallizer, and then flows into the clear liquid barrel by gravity, and is transported to the nanofiltration process inlet pipeline through the clear liquid pump. The liquid at the bottom of the crystallizer that reaches a suitable concentration enters the thickener through the discharge valve for solid-liquid separation, and the clear liquid returns to the center barrel of the crystallizer. The solid part enters the bipolar pusher centrifuge from the bottom of the thickener through the discharge valve, and produces qualified calcium sulfate products after centrifugal separation.

[0033] The dosing device uses a metering pump to continuously add polyacrylamide solution at a dosage of 3ppm into the central barrel of the calcium sulfate crystallizer to promote the flocculation and sedimentation of calcium sulfate.

[0034] Embodiment 1:

[0035] 8-hour average operating index

[0036]

[0037] Table 1

[0038] 43℃ thermal concentrated seawater is transported from bottom to top through pipelines to the shell side of the coil heat exchanger, and its flow rate is controlled at 10 cubic meters per hour by an automatic regulating valve. 20℃ nanofiltration concentrated water comes from the nanofiltration process and is transported from top to bottom to the tube side of the coil heat exchanger through a water feed pump, and its flow rate is controlled at 1 cubic meter per hour by an automatic regulating valve. After heat exchange, the temperature of thermal concentrated seawater drops to 40℃, and the temperature of nanofiltration concentrated water rises to 30℃. The nanofiltration concentrated water buffer tank controls the liquid level to be stable at 85%, and a metering pump is used to continuously add 3ppm of polyacrylamide. 1.5 kg of calcium sulfate seeds are added before operation. After induced crystallization and centrifugal separation, calcium sulfate products are produced.

[0039] The device was operated continuously for 7 days. The inlet and outlet water temperatures of the heat exchanger, the calcium ion concentrations of the inlet and outlet liquids of the crystallizer, and the output of the centrifuge were measured every 8 hours, and the crystallization rate was calculated, as shown in Table 1 above.

[0040] Embodiment 2:

[0041] 8-hour average operating index

[0042]

[0043] Table 2

[0044] 43℃ thermal concentrated seawater is transported from bottom to top through pipelines to the shell side of the coil heat exchanger, and its flow rate is controlled at 1 cubic meter per hour by an automatic regulating valve. 20℃ nanofiltration concentrated water comes from the nanofiltration process and is transported from top to bottom to the tube side of the coil heat exchanger by a water feed pump, and its flow rate is controlled at 1 cubic meter per hour by an automatic regulating valve. After heat exchange, the temperature of thermal concentrated seawater drops to 38℃, and the temperature of nanofiltration concentrated water rises to 32℃. The nanofiltration concentrated water buffer tank controls the liquid level to be stable at 85%, and a metering pump is used to continuously add 3ppm of polyacrylamide. Before operation, 1.75 kg of calcium sulfate seeds are added, and calcium sulfate products are produced after induced crystallization and centrifugal separation.

[0045] The device was operated continuously for 7 days. The inlet and outlet water temperatures of the heat exchanger, the calcium ion concentrations of the inlet and outlet liquids of the crystallizer, and the output of the centrifuge were measured every 8 hours, and the crystallization rate was calculated, as shown in Table 2 above.

[0046] Embodiment 3:

[0047] 8-hour average operating index

[0048]

[0049] Table 3

[0050] 42℃ thermal concentrated seawater is transported from bottom to top through pipelines to the shell side of the coil heat exchanger, and its flow rate is controlled at 15 cubic meters per hour by an automatic regulating valve. 20℃ nanofiltration concentrated water comes from the nanofiltration process and is transported from top to bottom to the tube side of the coil heat exchanger through a water feed pump, and its flow rate is controlled at 1 cubic meter per hour by an automatic regulating valve. After heat exchange, the temperature of thermal concentrated seawater drops to 38℃, and the temperature of nanofiltration concentrated water rises to 34℃. The nanofiltration concentrated water buffer tank controls the liquid level to be stable at 85%, and a metering pump is used to continuously add 3ppm of polyacrylamide. Before operation, 2 kg of calcium sulfate seeds are added, and calcium sulfate products are produced after induced crystallization and centrifugal separation.

[0051] After the device was put into operation, the inlet and outlet water temperatures of the heat exchanger, the calcium ion concentration of the inlet and outlet liquids of the crystallizer, and the output of the centrifuge were measured every 8 hours, and the crystallization rate was calculated, as shown in Table 3 above.

[0052] Comparative Example

[0053] 8-hour average operating index

[0054]

[0055] Table 4

[0056] 20℃ nanofiltration concentrated water comes from the nanofiltration process and is transported from top to bottom to the coil heat exchanger tube by a water feed pump. The flow rate is controlled at 1 cubic meter per hour by an automatic regulating valve. The nanofiltration concentrated water buffer tank controls the liquid level to be stable at 85%. A metering pump is used to continuously add 3ppm of polyacrylamide. Before operation, 1.5kg of calcium sulfate seeds are added. After induced crystallization and centrifugal separation, calcium sulfate products are produced.

[0057] The device was operated continuously for 7 days. The crystallizer inlet temperature, inlet and outlet calcium ion concentrations and centrifuge output were measured every 8 hours, and the crystallization rate was calculated, as shown in Table 4 above.

[0058] From the data in the above embodiments and comparative examples, it is obvious that the crystallization rate of calcium sulfate is affected by temperature. The higher the temperature, the faster the crystallization rate. The above relevant experimental data show that the average deposition rate of calcium ions at 40° C. is 1.5 times that at 20° C. Using the hot concentrated seawater in the nanofiltration process influent as a heat source, heating the nanofiltration concentrated water to increase its average temperature from 20-30° C. to 30-40° C. can accelerate the precipitation rate of calcium sulfate in the induced crystallization process, thereby improving the crystallization rate of calcium sulfate.

[0059] Advantages and positive effects of the implementation of the present invention:

[0060] (1) Increase the temperature of the nanofiltration concentrate to increase the crystallization rate of calcium sulfate induced crystallization.

[0061] (2) Make full use of waste heat from upstream processes to recover heat.

[0062] The present invention is not limited to the data and index ranges listed in the above embodiments. Any method or process that uses thermal concentrated seawater as a heat source to increase the crystallization rate of calcium sulfate in nanofiltration concentrated water by heat exchange is considered to be within the protection scope of the present invention.

Claims

1. A method for increasing the crystallization rate of nanofiltration concentrated water by utilizing waste heat, comprising the following steps: S1, taking concentrated seawater by thermal method and using it as waste heat resource after pretreatment; S2, taking concentrated seawater from the thermal process and concentrated seawater from the membrane desalination process as a byproduct, mixing the concentrated seawater from the thermal process and the concentrated seawater from the membrane desalination process, and subjecting the resulting mixed concentrated seawater to a nanofiltration process to produce nanofiltration concentrated water; S3, after heat exchange in a coil heat exchanger, the concentrated seawater in step 1 and the concentrated nanofiltration water in step 2 enter a calcium sulfate crystallizer; S4. Add calcium sulfate seeds into the calcium sulfate crystallizer, induce crystallization by stirring, and transport the supernatant to the inlet pipeline of the nanofiltration process through a supernatant pump.

2. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The temperature of the concentrated seawater obtained in step S1 is 40-45°C.

3. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The volume ratio of the thermal concentrated seawater and the membrane desalination byproduct concentrated seawater taken in step S2 is 10-15:

1. The temperature of the nanofiltration concentrated water generated after nanofiltration is 20-30°C.

4. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The nanofiltration concentrated water obtained in step S2, Ca 2+ Concentration 2-3g / L, Mg 2+ Concentration 6.5-9g / L, SO4 2- Concentration 17-20g / L.

5. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The temperature of the concentrated seawater after heat exchange in step S3 is in the range of 35-40°C.

6. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The nanofiltration concentrated water obtained in step S2 enters the coil heat exchanger to exchange heat with the thermal concentrated seawater. The temperature range before the heat exchange is 20-30°C, and the temperature range after the heat exchange is 30-40°C.

7. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: In step S4, the added calcium sulfate seed crystals are calcium sulfate dihydrate crystals, which are snowflake-shaped and have a particle size of 2-3 mm.

8. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The calcium sulfate crystallizer in step S4 has a stirring rate of 10-500 r / min and a liquid inlet flow rate of 0.5-3000 m 3 / h, calcium sulfate crystallizer circulating liquid flow rate 0.1-300m 3 / h.

9. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 1, characterized in that: The mixed concentrated seawater in step 2 is adjusted to pH 6-6.5 using hydrochloric acid, and a scale inhibitor is added at 2-3 ppm. The Ca is enriched by a nanofiltration membrane device at a pressure of 3 MPa. 2+ Mg 2+ 、SO4 2- Then nanofiltration concentrated water is produced.

10. The method for improving the crystallization rate of nanofiltration concentrated water by utilizing waste heat according to claim 7, characterized in that: In step S4, a metering pump is used to continuously add the polyacrylamide solution in an amount of 3 ppm into the central barrel of the calcium sulfate crystallizer to promote the flocculation and sedimentation of the calcium sulfate.

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