CN115193262A - A flat membrane module for directly cooling permeate and its application in membrane distillation - Google Patents

Abstract

The invention belongs to the technical field of membrane distillation, and particularly relates to a flat-plate membrane module and a system for directly cooling penetrating fluid; the flat plate type membrane component mainly comprises: the device comprises a hydrophobic microporous membrane, a hot side cavity, a cold side cavity, a semiconductor heat pump assembly and an external heating unit; a hot side cavity and a cold side cavity are respectively arranged at two sides of the hydrophobic microporous membrane, and a feeding channel and a discharging channel are respectively arranged at two ends of the two side cavities; the heat absorbing surface of the semiconductor heat pump component is attached to the cold side cavity, and the heat radiating surface of the semiconductor heat pump is closely attached to the external heating unit; the distance between the heat absorbing surface of the semiconductor heat pump component and the hydrophobic microporous membrane is 1-5mm. The heat absorption unit of the semiconductor heat pump is integrated in the cold side cavity of the membrane component, so that the penetrating fluid is directly cooled and kept at a lower temperature to improve the average temperature difference of two sides of the membrane, and the energy consumption and the cost of the membrane distillation method in the evaporation concentration processes of seawater desalination, sewage treatment, food concentration and the like are reduced.

Description

A flat membrane module for directly cooling permeate and its application in membrane distillation

technical field

The invention belongs to the technical field of membrane distillation, and in particular relates to a flat membrane module for directly cooling permeate and its application in membrane distillation. Through the invention, a compact and efficient flat membrane distillation unit can be obtained, and the cost of the membrane distillation method is reduced. Energy consumption and cost of evaporative concentration processes such as desalination, sewage treatment and food concentration.

Background technique

Membrane distillation (MD) is a new type of membrane separation technology in which the vapor pressure difference on both sides of a microporous hydrophobic membrane drives steam permeation. It is widely used in the fields of seawater desalination, sewage treatment and food concentration. Different from traditional thermal evaporation and concentration methods (such as multi-effect evaporation, multi-stage flash evaporation, etc.) and other membrane separation technologies (such as reverse osmosis, nanofiltration, electrodialysis, etc.), membrane distillation has the advantages of high rejection rate, mild operating conditions and easy operation. advantages of scale.

According to the capture method on the permeate side, membrane distillation can generally be divided into direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) and Vacuum type (vacuum membranedistillation, VMD). Compared to other MD methods, DCMD is the most widely studied MD process because of the simplest process configuration.

A typical DCMD system needs to both heat the feed liquid and cool the permeate liquid, so the operation of the system requires both a heat source and a cold trap to drive. The heat source required for heating can generally use the waste heat resources in the process industry (such as low-temperature steam, hot water, etc.), and the feed liquid is heated by an external heat exchange device and then enters the hot side of the membrane module; while the permeate is cooled by a cold trap. Then enter the cold side of the membrane module. Due to the evaporation of the feed liquid on the hot side of the membrane module and the loss of heat dissipation, the temperature of the hot side of the membrane module will decrease along the flow direction of the feed liquid, while the temperature of the cold side will increase along the flow direction of the permeate temperature, resulting in the average temperature difference of the membrane surface. (The effective driving force of membrane separation) is less than the inlet temperature difference between the cold and hot sides of the membrane module (ie the driving force for the process system to improve). As a result, the separation efficiency of the DCMD membrane module is low; at the same time, there is polarization (ie, boundary layer phenomenon) on both the cold and hot sides of the membrane module, so that the temperature of the membrane surface fluid on the hot side is lower than that of the mainstream, while the surface fluid on the cold side of the membrane is polarized. At temperatures higher than the mainstream, this inevitable polarization further reduces the DCMD separation efficiency.

At present, in order to improve the separation efficiency of DCMD, methods such as countercurrent operation, increasing the fluid flow rate on both sides of the membrane module, and strengthening the fluid disturbance in the flow channel are mainly adopted to increase the average temperature difference of the membrane surface and alleviate the polarization effect. However, these methods improve the membrane separation efficiency at the expense of increasing the power consumption of fluid transport, so it is difficult to significantly improve the overall energy efficiency of the DCMD system. In addition, cold traps commonly used in the process industry are generally realized by refrigeration cycles, which include processes such as compression, throttling, and heat exchange, which are complex and expensive.

Therefore, in order to reduce the process energy consumption of the DCMD system, MD integrated systems using heat pumps have appeared in recent years. A heat pump is a device that transfers heat energy from a low-temperature material system to a heating object. By simultaneously achieving high-efficiency energy conversion between cooling and heating, the overall energy efficiency of the MD system can be significantly improved. Chinese invention patent application CN108622983b discloses a membrane distillation device and method using a heat pump, which integrates the traditional heat pump cycle into the DCMD membrane module, and uses the heat pump principle to simultaneously realize the cooling of the permeate and the heating of the feed liquid. In addition, the membrane distillation method utilizing the heat pump also includes: a two-effect membrane distillation system with integrated heat pump (CN105709601A), a DCMD system with integrated solar preheating and heat pump cooling (CN105749752A), and a method for improving the thermal efficiency of a heat pump membrane distillation system by optimizing hollow fiber membranes (CN106582292A) and so on.

Compared with traditional compression heat pumps (including compressors, throttle valves, etc.) that use steam as the working fluid, semiconductor refrigeration chips using thermoelectric refrigeration technology have the advantages of small size, low cost, and easy system miniaturization. The energy conversion aspect of refrigeration has a higher simplicity. Semiconductor heat pumps use the Peltier effect to move heat from a low-temperature endothermic surface to a high-temperature exothermic surface through the action of an electric current. Chinese invention patent CN113716785A discloses a membrane module that uses thermoelectric refrigeration technology to realize VMD, but in this technology, the heat dissipation surface of the semiconductor refrigeration sheet needs to be adapted to a large area of metal heat dissipation fins, which increases the cost of equipment materials, and it is difficult to The size of the membrane module is further reduced; at the same time, the patented technology requires an additional heating device to provide the heat required for the operation of the system, which also increases the implementation cost of the solution.

In summary, in order to promote the application of membrane distillation technology in the fields of seawater desalination, sewage treatment and food concentration, it is urgent to solve the following problems: (1) To reduce the complexity of the system configuration of high-efficiency heat and cold source utilization methods such as heat pump membrane distillation systems (2) Optimizing the design of membrane modules to alleviate polarization, enhance heat and mass transfer processes, and improve energy utilization in membrane modules.

Invention content:

The purpose of the present invention is to provide a flat membrane module for directly cooling permeate which improves the separation efficiency of membrane distillation and reduces the cost of the DCMD method at the same time, and its application in membrane distillation, so as to realize evaporation and concentration processes such as seawater desalination, sewage treatment and food concentration. Significant reduction in energy consumption and cost.

The invention adopts the semiconductor refrigerating sheet to replace the traditional heat pump circulation system, so as to reduce the system configuration complexity and cost of the cooling and heat source utilization method; The technical scheme that the semiconductor refrigeration sheet absorbs heat from the air gap on the permeate side through the metal partition wall, the present invention adopts the method of directly cooling the permeate liquid, and uses the characteristic that the thermal conductivity of the liquid is significantly higher than that of the gas to improve the heat transfer efficiency and effectively increase the two sides of the membrane. The average temperature difference can significantly improve the membrane separation efficiency.

The object of the invention is to realize through the following technical solutions:

A flat membrane module for directly cooling permeate, mainly comprising a membrane separation unit, a semiconductor heat pump module, and an external heating unit;

The membrane separation unit includes a hydrophobic microporous membrane, a hot side chamber, and a cold side chamber; a hot side chamber and a cold side chamber are respectively provided on both sides of the hydrophobic microporous membrane; the heat absorption surface of the semiconductor heat pump assembly It is attached to the cold side cavity of the membrane separation unit, and the heat dissipation surface of the semiconductor heat pump assembly is closely attached to the external heating unit;

Preferably, at least one material liquid inlet channel and material liquid outlet channel are respectively provided at both ends of the hot side chamber, and the axes of any material liquid inlet channel and any material liquid outlet channel are not collinear.

Preferably, at least one permeate inlet channel and permeate outlet channel are respectively provided at both ends of the cold side chamber, and the axes of any permeate inlet channel and any permeate outlet channel are not collinear.

The distance from the endothermic surface of the semiconductor heat pump to the hydrophobic microporous membrane is 1-5mm.

Preferably, the semiconductor heat pump assembly includes a mounting frame and a semiconductor cooling chip. More preferably, the semiconductor cooling chip is TEC2-19006 with a size of 40ⅹ40ⅹ6.3mm, and is embedded in the heat-resistant epoxy resin mounting frame.

Preferably, the hydrophobic microporous membrane is a flat polyvinylidene fluoride membrane with an average pore size of 0.22um and an average membrane thickness of 0.012mm.

Preferably, the external heating unit is made of high thermal conductivity material, with a size of 40ⅹ40ⅹ7mm, an inlet channel and an outlet channel are provided at one end, and the internal structure is an M-shaped flow channel. More preferably, the high thermal conductivity material is selected from aluminum material, copper material, aluminum alloy or copper alloy.

A direct-contact membrane distillation system includes a material-liquid storage tank, a material-liquid circulation pump, a flat-plate membrane module for directly cooling permeate, a permeate storage tank, and a permeate circulation pump;

The material-liquid storage tank is connected with the material-liquid circulation pump through the pipeline, the material-liquid circulation pump is connected with the inlet channel of the external heating unit of the flat-plate membrane module which directly cools the permeate through the pipeline, and the outlet channel of the external heating unit is connected with the pipeline They are respectively connected with the feed liquid inlet channels of the hot side chamber of the flat membrane module for directly cooling the permeate, and the material and liquid outlet channels of the hot side chamber of the flat membrane module for directly cooling the permeate are connected with the material and liquid storage tanks through pipes; The permeate storage tank is connected to the permeate circulation pump through pipes, and the permeate circulation pump is respectively connected to the permeate inlet channel of the cold side chamber of the flat membrane module that directly cools the permeate through the pipeline, and the flat membrane module that directly cools the permeate. The permeate outlet channel of the cold side chamber is connected to the permeate storage tank through a pipeline.

In the direct contact membrane distillation system of the present invention, the feed liquid to be concentrated in the feed liquid storage tank is sent to the external heating unit of the flat membrane module for directly cooling the permeate through the feed liquid circulation pump and then enters the membrane separation unit module after heating. Hot side volume chamber; the permeate in the permeate storage tank is sent to the cold side volume chamber of the flat membrane module directly cooling the permeate through the permeate circulation pump for cooling; in the membrane separation unit of the flat membrane module directly cooling the permeate , Driven by the fluid temperature difference on both sides of the membrane, the water vapor migrates from the hot side to the cold side through the membrane driven by the vapor pressure difference on both sides of the membrane, thereby realizing the concentration of the feed liquid and the enrichment of the permeate.

In the present invention, the heating and cooling process of the feed liquid and the permeate in the flat membrane module that directly cools the permeate is specifically described as follows: the permeate in the cold side chamber is directly cooled by the semiconductor heat pump module, and the absorbed heat is heated to the outside The unit transfers to heat the feed liquid, which then enters the hot side chamber in the membrane separation unit and transfers heat to the permeate side.

Compared with the prior art, the present invention has the following advantages and effects:

(1) The present invention can develop a heat pump membrane distillation system at a lower cost by using an economical semiconductor refrigeration sheet to replace the traditional heat pump refrigeration.

(2) When the high temperature feed liquid and the low temperature permeate in the MD enter the membrane unit respectively, the temperature difference formed on both sides of the membrane is the driving force of the membrane distillation process. Due to the evaporation and heat transfer of the feed liquid on the hot side of the membrane module, the membrane module The temperature of the hot side decreases along the membrane surface, while the temperature of the cold side rises along the membrane surface, resulting in a decrease in the separation efficiency of the membrane. The invention integrates the heat-absorbing surface of the semiconductor heat pump into the cold-side cavity of the membrane separation unit, removes the heat transferred from the hot-side trans-membrane in time, increases the effective temperature difference between the cold and hot sides of the fluid on the membrane surface, and effectively improves the DCMD Process thermal efficiency.

(3) The present invention directly contacts the heat-absorbing surface of the semiconductor heat pump assembly with the permeate, avoiding high heat transfer resistance caused by heat transfer through the air gap, and increasing the effective temperature difference between the hot and cold sides of the fluid on the membrane surface.

(4) In the present invention, the thickness of the chambers on both sides of the membrane separation unit including the external heating unit does not exceed 10mm, and neither contains internal components such as metal fins, which not only effectively reduces the size of the membrane module but also fully adopts The membrane modules are made of light materials such as plastics, which realizes the compactness and weight reduction of the DCMD system, and significantly reduces the configuration cost of the MD system.

(5) By optimizing the flow channels on both sides of the membrane, the present invention effectively avoids phenomena such as "short circuit" and "dead zone" without significantly increasing the energy consumption of the flow channel, and further improves the separation efficiency of the membrane module.

Description of drawings

1 is a schematic structural diagram of a flat-plate membrane module for directly cooling permeate of the present invention;

FIG. 2 is a schematic structural diagram of the two sides of the chambers in the membrane separation unit in the flat membrane module for directly cooling the permeate;

3 is a schematic structural diagram of an external heating unit in a flat membrane module for directly cooling permeate;

FIG. 4 is a schematic structural diagram of the flat membrane module for directly cooling the permeate described in the embodiment;

FIG. 5 is a schematic diagram of the DCMD system structure established by the application of the flat membrane module directly cooling the permeate in the embodiment;

6 is a schematic structural diagram of a DCMD system using a single cold and heat source;

FIG. 7 is a schematic structural diagram of a DCMD system in which a semiconductor heat pump is provided outside the membrane module.

Detailed ways

For better understanding of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments.

As shown in Figure 1, the flat membrane module for directly cooling the permeate consists of a membrane separation unit, a semiconductor heat pump module 6, and an external heating unit 12, wherein the membrane separation unit includes a hydrophobic microporous membrane 1, a hot side chamber 7, a cooling Side chamber 10 , material liquid inlet channel 8 , material liquid outlet channel 2 , permeate liquid inlet channel 9 , permeate liquid outlet channel 3 .

Two sides 1 of the hydrophobic microporous membrane are respectively provided with a hot side chamber 7 and a cold side chamber 10; one end of the hot side chamber 7 is provided with a material liquid inlet channel 8, and the other end is provided with a material liquid outlet channel 2; the cold side chamber 10 One end is provided with a permeate inlet channel 9, and the other end is provided with a permeate outlet channel 3.

The semiconductor heat pump assembly 6 includes a mounting frame and a semiconductor cooling sheet, wherein the semiconductor cooling sheet is selected as TEC2-19006, the size is 40ⅹ40ⅹ6.3mm, and is embedded in the heat-resistant epoxy resin mounting frame. The heat-absorbing surface 4 of the semiconductor heat pump assembly 6 is close to the cold side chamber 10 of the membrane separation unit, the heat dissipation surface 5 of the semiconductor heat pump assembly 6 is close to the external heating unit 12; the heat-absorbing surface 4 of the semiconductor heat pump assembly 6 is away from the hydrophobic microporous membrane. The distance of 1 is 1 to 5 mm.

In order to alleviate the influence of "dead zone" and "short circuit" caused when the flow enters and exit the cavity, and realize the optimization of the flow channel, as shown in Fig. 2, preferably, a plurality of feed liquids are respectively provided at both ends of the hot side cavity 7 The inlet channel 8 and the material liquid outlet channel 2, the axes of any material liquid inlet channel 8 and any material liquid outlet channel 2 are not collinear; the two ends of the cold side chamber 10 are respectively provided with a plurality of permeate liquid inlet channels 9 and 2 The axes of the permeate outlet channel 3, any permeate inlet channel 9 and any permeate outlet channel 3 are not collinear.

The external heating unit 12 is made of high thermal conductivity material with a size of 40ⅹ40ⅹ10mm. In order to strengthen the heat transfer efficiency of the external heating unit 12, as shown in FIG. 3, an inlet channel 111 and an outlet channel 112 are provided at one end of the external heating unit 12. , the internal structure is an M-type flow channel.

When the flat membrane module of the present invention that directly cools the permeate is in operation, the feed liquid enters the external heating unit 12 through the inlet channel 11, and then leaves the outlet channel 112 after being heated and raised, and then enters the hot side chamber through the feed liquid inlet channel 8. 7; The permeate enters the cold side chamber 10 through the permeate inlet channel 9, and is directly cooled by the semiconductor heat pump assembly 6. A temperature difference is formed on both sides of the hydrophobic microporous membrane 1, that is, the water vapor pressure of the feed liquid is higher than that of the permeation side, and the water vapor in the membrane pores migrates from the feed liquid side to the permeation side under the impetus of the vapor pressure difference, thereby realizing the material liquid Evaporative concentration on the side and water enrichment on the permeate side.

Example 1

According to the structure of Figures 1-3, a flat-plate membrane module for direct cooling of permeate as shown in Figure 4 is established, including: epoxy resin frame 13, plexiglass cover plate 14, membrane separation unit 15, semiconductor heat pump module 16, external heating unit 17. The plexiglass cover plate 14 has a size of 80ⅹ80ⅹ3mm, and an epoxy resin frame 13 with a size of 80ⅹ80ⅹ4mm is fixed outside.

The membrane separation unit 15 is made of plexiglass, and the dimensions of the hot side chamber and the cold side chamber are both 40ⅹ40ⅹ5mm, and the effective volume is 8ml. In order to improve the "dead zone" and "short circuit" of the fluid flow in and out of the cavity, three feed channels with a diameter of 4mm are evenly distributed on the feed side of the hot side cavity, and three outlets with a diameter of 4mm are evenly distributed on the discharge side. There are three feeding channels with a diameter of 4 mm evenly distributed on the feeding side of the cold chamber, and three discharging channels with a diameter of 4 mm are evenly distributed on the discharging side. And any inlet channel does not coincide with the axis of the outlet channel; in order to better observe the membrane separation state, both the hot side chamber and the cold side chamber are equipped with PT100 thermal resistance temperature sensors connected to the computer acquisition system; The pore membrane adopts Millipore's PVDF flat membrane, the effective size after cutting is 40ⅹ40mm, the average pore size is 0.22um, and the average membrane thickness is 0.012mm.

The semiconductor heat pump assembly 16 includes a mounting frame and a semiconductor cooling chip, wherein the semiconductor cooling chip is selected as TEC2-19006, with a size of 40ⅹ40ⅹ6.3mm, and is embedded in a heat-resistant epoxy resin frame with a size of 80ⅹ80ⅹ4mm.

The external heating unit 17 is made of aluminum alloy, the size is 40ⅹ40ⅹ11mm, the interior is an M-shaped flow channel, and a feeding channel and a discharging channel are arranged on the bottom surface.

Applying the present invention, the energy consumption measurement experiment is carried out according to the DCMD system shown in FIG. 5 .

As shown in Fig. 5, the DCMD system specifically includes a material liquid storage tank 18 with a volume of 250ml and a material of polypropylene, a set of Masterflex L/S type material liquid circulation pump 19 from Cole-Parmer Company in the United States, a set of direct cooling infiltration The liquid flat membrane module 21 (the specific structure is shown in Figure 4), a permeate storage tank 24 with a volume of 250mL and a polypropylene with an overflow outlet, a set of Masterflex L/S type of Cole-Parmer Company in the United States Permeate circulating pump 23; in addition, it also includes a DC adjustable power supply 20 with a rated power of 300W, a computer 22 for data acquisition and monitoring, and an analytical balance 25 for measuring permeate overflow.

The specific operation method and process parameters of the DCMD system are as follows:

At room temperature, a sufficient amount of normal temperature pure water is respectively added to the feed liquid storage tank 18 and the permeate liquid storage tank 24, and the feed liquid is input into the external heating unit through the hot side circulating pump 19 for heating and then enters the flat membrane module 21 that directly cools the permeate liquid. The permeate is input into the cold side volume of the flat membrane module 21 which directly cools the permeate through the cold side circulation pump 23, and the output power of the DC adjustable power supply 20 driven by the semiconductor heat pump module is set to be 23.63W. The flow rates of the feed liquid circulating pump 19 and the permeate circulating pump 23 are both 2.5kg/h, and the temperature of the cold and hot side chambers of the flat membrane module that directly cools the permeate is collected and recorded by the computer 22 (the temperature between the computer temperature acquisition module and the membrane module) All levels of thermal resistance connection), driven by the steam pressure difference between the hot side and the cold side, the feed liquid evaporates on the membrane surface of the hot side chamber and transfers heat and mass to the cold side, thereby increasing the permeate, resulting in The permeate flows out through the overflow pipe of the permeate storage tank 24, and the amount of permeate produced per unit time is measured by the balance 25.

Comparative Example 1

In order to better illustrate the performance difference between the present invention and the traditional DCMD system, a DCMD system using a single cold and heat source as shown in FIG. 6 is established.

The traditional DCMD system specifically includes a material-liquid storage tank 35 with a volume of 250 mL and a material of polypropylene, a material-liquid circulation pump 27 of the Masterflex L/S type of Cole-Parmer Company in the United States, a 50W electric heating system 28, a DCMD membrane module 34, and a volume of 250mL permeate storage tank 33 made of polypropylene with overflow outlet, permeate circulation pump 31 of Masterflex L/S type from Cole-Parmer, USA, heat pump refrigeration cycle system 30, computer 36 for data acquisition and monitoring Analytical balance 32 for permeate overflow measurement. Among them, the traditional DCMD membrane module is made of colorless and transparent plexiglass, including the hot-side chamber 3411 and the cold-side chamber 3412 with a size of 40x40x5mm; the hydrophobic microporous membrane is made of Millipore's PVDF flat membrane, with an effective size of 40x40mm , the average film thickness is 0.012mm, and the average pore size is 0.22um; both the cold and hot sides of the cavity are provided with a fluid outlet channel 3414 and a fluid inlet channel 3415.

The traditional DCMD system shown in Figure 6 is used to measure the energy consumption, and the specific operation method and process parameters are as follows:

At room temperature, a sufficient amount of normal temperature pure water is added to the material liquid storage tank 35 and the permeate liquid storage tank 33 respectively, and the material liquid is sent to the electric heating system 28 through the material liquid circulation pump 27 and heated, and then input to the hot side chamber of the membrane module 34 The pure water placed in the permeate storage tank 33 is sent to the heat pump refrigeration cycle system 30 through the permeate circulation pump 31, and then enters the cold side volume of the membrane module 34 after cooling. The flow rates of the feed liquid circulating pump 27 and the permeate circulating pump 31 are both set to 2.5kg/h, the input power of the electric heater 28 is set to 11.55W and the input power of the heat pump refrigeration cycle is set to 23.63W. The inlet and outlet temperatures of the membrane module 34 are collected and recorded by the computer 36 . In the membrane separation unit, driven by the steam pressure difference between the hot side and the cold side, the feed liquid evaporates on the membrane surface of the hot side chamber and transfers heat and mass to the cold side, thereby increasing the permeate and the resulting permeate. The liquid flows out through the overflow pipe of the permeate storage tank 33, and the amount of permeate produced per unit time is measured by the balance 32.

Comparative Example 2

To better illustrate the performance difference between the present invention and a DCMD system with a semiconductor heat pump provided outside the membrane module, an integrated heat pump DCMD system with a heat pump set outside the membrane module as shown in FIG. 7 is established.

The DCMD system with a semiconductor heat pump set outside the membrane module specifically includes a material-liquid storage tank 37 with a volume of 250 mL and a material of polypropylene, a material-liquid circulation pump 45 of the Masterflex L/S type of Cole-Parmer Company in the United States, and an external semiconductor heat pump assembly 42 , DCMD membrane module 38, 250mL volume, polypropylene permeate storage tank 46 with overflow outlet, US Cole-Parmer company Masterflex L/S type permeate circulating pump 40, a DC rated power of 300W An adjustable power supply 47, a computer 43 for data acquisition and monitoring, and an analytical balance 39 for permeate overflow measurement. Among them, the DCMD membrane module 38 is made of colorless and transparent plexiglass, including a hot-side cavity and a cold-side cavity (the same as in Comparative Example 1) with a size of 40x40x5mm; the hydrophobic microporous membrane is made of Millipore's PVDF flat Mask, its effective size is 40x40mm, the average film thickness is 0.012mm, and the average pore size is 0.22um. In addition, the external semiconductor heat pump assembly includes a mounting frame and a semiconductor cooling chip. The semiconductor cooling chip is selected as TEC2-19006, with a size of 40ⅹ40ⅹ6.3mm, and is embedded in a heat-resistant epoxy resin frame with a size of 80ⅹ80ⅹ4mm. An external cooling unit 41 is attached to the heat absorbing surface of the refrigerating sheet, and an external heating unit 44 is attached to the heat dissipation surface of the semiconductor refrigeration sheet; The M-type flow channel is provided with a feeding channel and a discharging channel on the bottom surface.

The energy consumption measurement experiment was carried out using the DCMD system with a semiconductor heat pump outside the membrane module shown in Figure 7. The specific operation method and process parameters are as follows:

At room temperature, a sufficient amount of normal temperature pure water is respectively added to the material liquid storage tank 37 and the permeate liquid storage tank 46, and the material liquid is sent to the external heating unit 44 of the semiconductor heat pump assembly 42 through the material liquid circulation pump 45 and heated and then input to the membrane module. 38; the pure water placed in the permeate storage tank 46 is sent to the external cooling unit of the semiconductor heat pump assembly 42 through the permeate circulating pump 40 and then enters the cold side cavity of the membrane module 38 for cooling. The flow rates of the feed liquid 45 and the permeate circulating pump 40 are both set at 2.5kg/h, and the input power of the DC power supply is set at 23.63W. The inlet and outlet temperatures of the membrane modules are collected and recorded by the computer 43 . Driven by the steam pressure difference between the hot side and the cold side, the feed liquid evaporates on the membrane surface of the hot side chamber and transfers heat and mass to the cold side, thereby increasing the permeate, and the generated permeate is stored through the permeate. The overflow pipe of the tank 46 flows out, and the amount of permeate produced per unit time is measured by the balance 39 .

Table 1 Performance comparison of the DCMD system with a single cold and heat source, the DCMD system with an external semiconductor heat pump, and the DCMD system with a flat membrane module using direct cooling of the permeate

Under the same operating conditions, the comparison of Example (DCMD system of flat membrane module with direct cooling permeate), Comparative Example 1 (DCMD system with single cooling and heat source) and Comparative Example 2 (DCMD system of external semiconductor heat pump) The results are shown in Table 1. It can be seen that: (1) The use of thermoelectric refrigeration technology can effectively reduce the unit energy consumption of the DCMD system, and the unit energy consumption of Comparative Example 2 and Example 1 is reduced by 34% and 55%, respectively. %; (2) Compared with Comparative Example 2, the present invention has higher membrane separation efficiency, the water production is increased by 47%, and the unit energy consumption is reduced by 32%. This is because the heat-absorbing surface of the semiconductor heat pump is attached to the membrane. The cold side chamber of the separation unit directly cools the permeate, keeping the temperature of the permeate lower, increasing the effective temperature difference between the cold and hot sides of the fluid on the membrane surface. In addition, an optimized flow channel design is applied in the invented flat membrane module. The scheme improves the heat and mass transfer process of the membrane distillation process.

Those skilled in the art should understand that the present invention is not limited by the embodiments. All modifications, equivalent replacements and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention.

Claims (6)

1. a flat-plate membrane module for directly cooling permeate, characterized in that: mainly by comprising membrane separation unit, semiconductor heat pump assembly, external heating unit; The membrane separation unit includes a hydrophobic microporous membrane, a hot side chamber, and a cold side chamber; both sides of the hydrophobic microporous membrane are respectively provided with a hot side chamber and a cold side chamber; the heat absorption of the semiconductor heat pump assembly The surface is attached to the cold side chamber of the membrane separation unit, and the heat dissipation surface of the semiconductor heat pump assembly is closely attached to the external heating unit. 2. The flat membrane module according to claim 1, wherein: At least one material liquid inlet channel and material liquid outlet channel are respectively provided at both ends of the hot side chamber, and the axes of any material liquid inlet channel and any material liquid outlet channel are not collinear; At least one permeate inlet channel and permeate outlet channel are respectively provided at both ends of the cold side chamber, and the axes of any permeate inlet channel and any permeate outlet channel are not collinear. 3 . The flat membrane module according to claim 1 , wherein the distance from the heat-absorbing surface of the semiconductor heat pump to the hydrophobic microporous membrane is 1-5 mm; the semiconductor heat pump module comprises a mounting frame and a semiconductor refrigeration sheet. 4 . 4. The flat membrane module according to claim 1, wherein the microporous hydrophobic membrane is selected from PVDF microporous hydrophobic membrane, the average pore size is 0.22um, and the average membrane thickness is 0.012mm; The model of the semiconductor refrigeration chip is TEC2-19006, the size is 40ⅹ40ⅹ6.3mm, and it is embedded in the heat-resistant epoxy resin mounting frame. 5. The flat membrane module according to claim 1, wherein: The external heating unit is made of high thermal conductivity material, the size is 40ⅹ40ⅹ7mm, an inlet channel and an outlet channel are provided at one end, and the internal structure is an M-shaped flow channel. 6. A direct-contact membrane distillation system, characterized in that: comprising a feed-liquid storage tank, a feed-liquid circulation pump, a flat-plate membrane module for directly cooling permeate, a permeate storage tank, and a permeate circulation pump; The material-liquid storage tank is connected with the material-liquid circulation pump through the pipeline, the material-liquid circulation pump is connected with the inlet channel of the external heating unit of the flat-plate membrane module which directly cools the permeate through the pipeline, and the outlet channel of the external heating unit is connected with the pipeline They are respectively connected with the feed liquid inlet channels of the hot side chamber of the flat membrane module for directly cooling the permeate, and the material and liquid outlet channels of the hot side chamber of the flat membrane module for directly cooling the permeate are connected with the material and liquid storage tanks through pipes; The permeate storage tank is connected to the permeate circulation pump through pipes, and the permeate circulation pump is respectively connected to the permeate inlet channel of the cold side chamber of the flat membrane module that directly cools the permeate through the pipeline, and the flat membrane module that directly cools the permeate. The permeate outlet channel of the cold side chamber is connected to the permeate storage tank through a pipeline.

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