Membrane distillation device and process for the treatment of saline wastewater

Abstract

This invention discloses a membrane distillation device and process for treating saline wastewater, belonging to the field of wastewater treatment. It includes a membrane distillation unit, a heat exchanger built into a purification tank, a steam compressor, and a temperature control system. Water vapor generated on the permeate side preheats the saline wastewater entering the system in the heat exchanger, achieving the first stage of heat recovery. The waste heat steam, whose temperature decreases after heating the saline wastewater, is compressed again by the steam compressor and then recycled (this steam is referred to as recycled waste heat steam to distinguish it from external waste heat steam), achieving the second stage of heat recovery. The temperature control system intelligently replenishes external waste heat steam based on the wastewater outlet temperature on the feed side of the membrane distillation unit to maintain system stability. This invention, through dual closed-loop heat recovery and utilization, significantly reduces the energy consumption and operating costs of the membrane distillation process, achieving automated and stable operation, and demonstrating significant economic and environmental benefits.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment, and more specifically, relates to a membrane distillation device and process for treating saline wastewater. Background Technology

[0002] Membrane distillation is a novel separation technology that couples membrane separation with evaporation. Membrane distillation equipment is simple, can operate at atmospheric pressure, has undemanding requirements on membrane mechanical properties, is insensitive to feed water concentration, and is highly adaptable to fluctuations in water quality. In treating saline wastewater, water molecules can permeate through the membrane as vapor, resulting in high-quality effluent with a high salt removal rate. Therefore, membrane distillation technology has great application potential in the treatment of saline wastewater. The membrane distillation unit is the core of membrane distillation technology. In this unit, the feed liquid on the feed side is in direct contact with the membrane surface. Driven by the vapor pressure difference across the membrane, the vapor enters the permeate side and is condensed, while the salt is retained by the membrane, thus achieving salt and water separation. Membrane distillation technology involves evaporation and condensation, resulting in high energy consumption. Therefore, improving the heat of vaporization, fully utilizing heat, and reducing energy consumption have become hot topics in the research and development of membrane distillation units.

[0003] To fully utilize the heat, it is necessary to maximize the use of steam generated on the permeate side of membrane distillation to preheat saline wastewater, while simultaneously using the wastewater to cool the steam and produce purified water. Furthermore, combining this with the recompression of recycled waste heat steam can effectively address the problem of excessive energy consumption during membrane distillation. Summary of the Invention

[0004] To address the above deficiencies, the present invention provides a membrane distillation device for treating saline wastewater. The membrane distillation device has a hydrophobic membrane installed inside, which divides the membrane distillation device into a feed side at the bottom and a permeate side at the top. An internal coil is installed at the bottom of the feed side for receiving waste heat steam (including circulating waste heat steam and external waste heat steam) to generate steam on the feed side. A steam compressor is installed at the outlet of the internal coil for recompressing and circulating the circulating waste heat steam.

[0005] A membrane cleaning device connected to a membrane distillation apparatus, the membrane cleaning device being used for backwashing and cleaning the surface of a hydrophobic membrane;

[0006] A water purification tank used to collect purified water produced from the osmotic side;

[0007] The interior of the water purification tank is equipped with a heat exchanger, which is used to connect the saline wastewater source with the raw material side. A security filter and a booster pump are also installed between the heat exchanger and the saline wastewater source.

[0008] The system includes a temperature control system, comprising an online temperature sensor installed at the wastewater outlet on the raw material side, a temperature interlock valve for controlling the entry of external waste heat steam, and a PLC control system. The PLC control system is used to control the opening and closing of the temperature interlock valve based on the detection result of the online temperature sensor, so as to supplement external waste heat steam to the inlet of the built-in coil.

[0009] Furthermore, the exterior of the water purification tank is also equipped with a vacuum pump, a pressure interlock valve, and an online pressure gauge for controlling the negative pressure inside the tank.

[0010] It is also equipped with an online conductivity meter for detecting the conductivity of purified water in the water purification tank.

[0011] Furthermore, the built-in coil is a serpentine hollow stainless steel tube, and the vertical distance between the surface of the hydrophobic membrane and the built-in coil is 0.5 to 1 mm;

[0012] The hydrophobic membrane is a hollow fiber membrane or a flat sheet membrane.

[0013] Furthermore, the cleaning solution used in the membrane cleaning device includes clean water, alkaline cleaning solution, and acidic cleaning solution.

[0014] Furthermore, the security filter is a manganese sand filter, a diamond sand filter, an activated carbon filter, an ultrafiltration device, or a combination thereof.

[0015] Furthermore, the negative pressure in the purified water tank is maintained between -0.04MPa and -0.08MPa. The start and stop of the vacuum pump and pressure interlock valve are also controlled by the PLC control system, and the start and stop thresholds are respectively the online pressure detector detecting that the current negative pressure in the purified water tank is below -0.04MPa and above -0.08MPa.

[0016] Furthermore, the wastewater temperature at the raw material side wastewater outlet is between approximately 70°C and 90°C, and the start / stop thresholds of the temperature interlock valve are respectively the online temperature sensor detecting a current temperature below 75°C and above 85°C.

[0017] The present invention also discloses a membrane distillation process for treating saline wastewater, comprising the above-mentioned membrane distillation apparatus for treating saline wastewater, and further comprising the following steps:

[0018] S1. The saline wastewater is fed into the security filter through the booster pump, and then into the heat exchanger for preheating. The preheated wastewater enters the feed side of the membrane distillation equipment and is heated by the waste heat steam (including circulating waste heat steam and external waste heat steam) in the built-in coil.

[0019] S2. The heated wastewater generates steam, which permeates through the hydrophobic membrane and enters the permeate side of the membrane distillation equipment. Under vacuum, the steam enters the water purification tank and condenses on the surface of the heat exchanger, while preheating the wastewater inside the heat exchanger. The concentrate on the feed side enters the saline wastewater recycling treatment.

[0020] S3. The circulating waste heat steam at the outlet of the built-in coil is compressed and recycled again by the steam compressor. The amount of external waste heat steam is controlled by an online temperature sensor and a temperature interlock valve.

[0021] S4. Detect the water quality in the water purification tank using an online conductivity meter, and determine whether to activate the membrane cleaning device based on the changes in water quality.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] The device of this invention makes maximum use of the steam generated on the permeate side of membrane distillation to preheat saline wastewater, reducing the external waste heat steam required for subsequent heating of saline wastewater. At the same time, it uses wastewater cooling steam to generate purified water. In addition, by recompressing the circulating waste heat steam and controlling the amount of external waste heat steam through temperature interlocking, the heating efficiency of waste heat steam is maintained. This effectively solves the problem of excessive energy consumption caused by the need for a large amount of coolant and heat source in the membrane distillation process, which is of great significance for achieving energy conservation, emission reduction and cost reduction and efficiency improvement.

[0024] Among them, saline wastewater is conventional wastewater in this field, but it is not limited to this. High-mineralization wastewater from oil and gas fields and other wastewater containing non-volatile substances are also suitable for this invention and have good effects. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the membrane distillation apparatus for treating saline wastewater in this invention.

[0026] Figure 2 This is a schematic diagram showing the connection between the hydrophobic membrane and the built-in coil in this invention.

[0027] In the diagram: 1. Membrane distillation equipment; 2. Hydrophobic membrane; 3. Built-in coil; 4. Membrane cleaning device; 5. Steam compressor; 6. Online temperature sensor; 7. Temperature interlock valve; 8. Online pressure sensor; 9. Pressure interlock valve; 10. Vacuum pump; 11. Booster pump; 12. Security filter; 13. Heat exchanger; 14. Clean water tank; 15. Online conductivity meter; 16. Valve. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] like Figures 1 to 2 As shown, this embodiment provides a membrane distillation apparatus for treating saline wastewater, including the following devices:

[0031] Membrane distillation equipment 1, with a hydrophobic membrane 2 installed inside. The hydrophobic membrane 2 can be a hollow fiber membrane or a flat sheet membrane, and the material is polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene (PP) and polyethylene (PE), etc. The built-in coil is a serpentine hollow stainless steel tube. The vertical distance between the surface of the hydrophobic membrane 2 and the built-in coil 3 is 0.5-1 mm.

[0032] The hydrophobic membrane 2 divides the membrane distillation equipment 1 into a feed side at the bottom and a permeate side at the top. The bottom of the feed side is equipped with an internal coil 3 for receiving waste heat steam (including circulating waste heat steam and external waste heat steam) to generate steam on the feed side. A steam compressor 5 is installed at the outlet of the internal coil 3 for re-compressing and circulating the circulating waste heat steam. The circulating waste heat steam, after being heated, continues to enter the steam inlet of the internal coil 3 for recycling, reducing the amount of external waste heat steam used. It should be noted that the steam compressor 5 can be a centrifugal steam compressor, a Roots steam compressor, a screw steam compressor, a reciprocating steam compressor, etc.

[0033] It should be noted that the waste heat steam described in this embodiment is divided into external waste heat steam and circulating waste heat steam. External waste heat steam refers to waste heat steam introduced from the outside (which has not yet participated in the circulation work in the built-in coil 3), while circulating waste heat steam refers to the waste heat steam that is recycled after being compressed again by the steam compressor at the outlet of the built-in coil 3.

[0034] The membrane cleaning device 4, which is connected to the membrane distillation equipment 1, uses cleaning solutions including water, alkaline cleaning solutions, and acidic cleaning solutions.

[0035] Purification tank 14 is used to collect purified water produced from the osmotic side;

[0036] The interior of the water purification tank 14 is equipped with a heat exchanger 13. The heat exchanger 13 is used to connect the saline wastewater source to the raw material side. It is used to receive the saline wastewater filtered by the security filter 12 and preheat it. The heat exchanger 13 condenses the steam produced by the permeate side of the membrane distillation equipment 1 into purified water. At the same time, the saline wastewater after steam preheating enters the raw material side of the membrane distillation equipment 1. The security filter 12 and the booster pump 11 are also installed between the heat exchanger 13 and the saline wastewater source. The security filter is placed at the front end of the membrane distillation equipment 1 and is used to filter impurities in the saline wastewater. The security filter 12 is a manganese sand filter, a diamond sand filter, an activated carbon filter, an ultrafiltration device, or a combination thereof. The booster pump 11 is used to pressurize the saline wastewater.

[0037] The temperature control system includes an online temperature sensor 6 installed at the wastewater outlet on the raw material side, a temperature interlock valve 7 that controls the entry of external waste heat steam, and a PLC control system. The wastewater temperature is between 70°C and 90°C, with about 80°C being the normal threshold. When the online temperature sensor 6 detects that the current temperature is below 75°C, the PLC control system activates the temperature interlock valve 7 to supplement external waste heat steam to the steam inlet of the built-in coil 3. When the temperature is above 85°C, the temperature interlock valve 7 is closed, and no external waste heat steam needs to be supplemented.

[0038] The pressure control system is installed outside the water purification tank 14 and includes a vacuum pump 10, a pressure interlock valve 9, and an online pressure gauge 8 for controlling the negative pressure inside the water purification tank 14. It is used to maintain the negative pressure in the water purification tank 14 between -0.04MPa and -0.08MPa. When the negative pressure displayed by the online pressure gauge 8 is lower than -0.04MPa, the pressure interlock valve 9 and the vacuum pump 10 are activated through the PLC control system. When the negative pressure displayed is greater than -0.08MPa, the pressure interlock valve 9 and the vacuum pump 10 are closed through the pressure interlock to maintain the negative pressure at approximately -0.06MPa.

[0039] An online conductivity meter 15 is also installed to detect the conductivity of the purified water in the water purification tank 14. When the conductivity of the effluent is greater than that of tap water, the PLC control system will start the membrane cleaning device 4 to backwash the surface of the hydrophobic membrane 2 and restore the permeation and separation performance of the hydrophobic membrane 2.

[0040] It should also be noted that valves 16 for controlling discharge are installed between the security filter 12 and the booster pump 11, between the membrane distillation equipment 1 and the membrane cleaning device 4, and at the bottom of the clean water tank 14.

[0041] Example 2

[0042] The process flow of the membrane distillation device for treating saline wastewater described above is as follows:

[0043] After being pressurized by booster pump 11, the saline wastewater first passes through security filter 12 to remove suspended particulate matter, protecting the subsequent membrane elements. The filtered saline wastewater then enters heat exchanger 13 inside purification tank 14 for preheating. The preheated saline wastewater enters the feed side of membrane distillation equipment 1, where it is heated by waste heat steam (including circulating waste heat steam and external waste heat steam) input into built-in coil 3. (During this process, the circulating waste heat steam at the outlet of built-in coil 3 is compressed again by steam compressor 5 and then circulates in built-in coil 3. The amount of external waste heat steam is automatically adjusted by temperature control system, and online temperature detection is used.) (The temperature of the device 6 can be controlled at around 80℃). The water vapor generated by heating and evaporating the saline wastewater permeates through the hydrophobic membrane 2 and enters the permeate side of the membrane distillation equipment 1. The water vapor generated by heating and evaporating the saline wastewater enters the purified water tank 14 with a negative pressure of about -0.06MPa under vacuum (the negative pressure is automatically adjusted by the pressure control system). It condenses on the surface of the heat exchanger 13, and at the same time preheats the saline wastewater in the heat exchanger 13, reducing the energy consumption of heating steam. The concentrated brine on the raw material side (i.e., the treated saline wastewater, which will be concentrated after treatment) is recycled to treat the saline wastewater until the purified water produced contains Ca. 2+ Mg 2+ K + Na + The removal rate is greater than 99.99%, and the conductivity is less than 10 μs / cm, so it can be used as production water in oil and gas fields. During this process, the online conductivity meter 15 monitors the water quality in the water purification tank 14 in real time, and determines whether to start the membrane cleaning device 4 based on the changes in water quality.

[0044] It should be noted that the structure described in this invention can be implemented in many different forms and is not limited to the embodiments described. Any equivalent transformations made by those skilled in the art based on the description and drawings of this invention, or direct or indirect applications in other related technical fields, such as the loading and unloading of other items, are included within the protection scope of this invention.

Claims

1. A membrane distillation apparatus for treating saline wastewater, characterized in that, include: A membrane distillation device, wherein a hydrophobic membrane is installed inside the membrane distillation device, the hydrophobic membrane dividing the membrane distillation device into a feed side at the bottom and a permeate side at the top, wherein an internal coil is installed at the bottom of the feed side for receiving circulating waste heat steam and external waste heat steam to generate steam on the feed side, and a steam compressor is installed at the outlet of the internal coil for recompressing and circulating the circulating waste heat steam. A membrane cleaning device connected to a membrane distillation apparatus, the membrane cleaning device being used for backwashing and cleaning the surface of a hydrophobic membrane; A water purification tank used to collect purified water produced from the osmotic side; The interior of the water purification tank is equipped with a heat exchanger, which is used to connect the saline wastewater source with the raw material side. A security filter and a booster pump are also installed between the heat exchanger and the saline wastewater source. The system includes a temperature control system, comprising an online temperature sensor installed at the wastewater outlet on the raw material side, a temperature interlock valve for controlling the entry of waste heat steam, and a PLC control system. The PLC control system is used to control the opening and closing of the temperature interlock valve based on the detection result of the online temperature sensor, so as to supplement external waste heat steam to the inlet of the built-in coil.

2. The membrane distillation apparatus for treating saline wastewater as described in claim 1, characterized in that: The outside of the water purification tank is also equipped with a pressure control system, including a vacuum pump, a pressure interlock valve and an online pressure gauge for controlling the negative pressure inside the water purification tank; It is also equipped with an online conductivity meter for detecting the conductivity of purified water in the water purification tank.

3. The membrane distillation apparatus for treating saline wastewater as described in claim 1, characterized in that: The built-in coil is a serpentine hollow stainless steel tube, and the vertical distance between the surface of the hydrophobic membrane and the built-in coil is 0.5 to 1 mm. The hydrophobic membrane is a hollow fiber membrane or a flat sheet membrane.

4. The membrane distillation apparatus for treating saline wastewater as described in claim 1, characterized in that: The cleaning solution used in the membrane cleaning device includes any one of water, alkaline cleaning solution, and acidic cleaning solution.

5. A membrane distillation apparatus for treating saline wastewater as described in claim 1, characterized in that: The security filter is a manganese sand filter, a diamond sand filter, an activated carbon filter, an ultrafiltration device, or a combination thereof.

6. A membrane distillation apparatus for treating saline wastewater as described in claim 2, characterized in that: The negative pressure in the purified water tank is maintained between -0.04MPa and -0.08MPa. The start and stop of the vacuum pump and pressure interlock valve are also controlled by the PLC control system, and the start and stop thresholds are respectively the online pressure detector detecting that the negative pressure in the purified water tank is below -0.04MPa and above -0.08MPa.

7. A membrane distillation apparatus for treating saline wastewater as described in claim 1, characterized in that: The wastewater temperature at the raw material side wastewater outlet is between approximately 70°C and 90°C. The start and stop thresholds of the temperature interlock valve are respectively when the online temperature sensor detects a current temperature below 75°C and above 85°C.

8. A membrane distillation process for treating saline wastewater, comprising the membrane distillation apparatus for treating saline wastewater according to any one of claims 1-7, characterized in that, It also includes the following steps: S1. The saline wastewater is fed into the security filter through the booster pump, and then into the heat exchanger for preheating. The preheated wastewater enters the feed side of the membrane distillation equipment and is heated by the circulating waste heat steam in the built-in coil and the external waste heat steam. S2. The heated wastewater generates steam, which permeates through the hydrophobic membrane and enters the permeate side of the membrane distillation equipment. Under vacuum, the steam enters the water purification tank and condenses on the surface of the heat exchanger, while preheating the wastewater inside the heat exchanger. The concentrate on the feed side enters the saline wastewater recycling treatment. S3. The circulating waste heat steam at the outlet of the built-in coil is compressed and recycled again by the steam compressor. The amount of external waste heat steam is controlled by an online temperature sensor and a temperature interlock valve. S4. Detect the water quality in the water purification tank using an online conductivity meter, and determine whether to activate the membrane cleaning device based on the changes in water quality.

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