System and method for online detection of membrane infiltration degree in membrane distillation

Abstract

The invention relates to the technical field of membrane separation, and discloses a system for on-line detection of membrane infiltration degree in membrane distillation, comprising: a membrane distillation subsystem for providing a membrane distillation separation process and forming flow of feed liquid and distillate; the electrochemical monitoring subsystem is used for continuously measuring the change of a current signal in the membrane distillation process and outputting an electric signal, and the invention also discloses a method for online detection of the membrane infiltration degree in membrane distillation, and the method comprises the following steps: S1, installing the membrane distillation subsystem device, and placing a microporous hydrophobic membrane between two stainless steel electrodes. The electrochemical monitoring technology and a data processing system are combined, current signal changes on the two sides of a membrane in the membrane distillation process are measured in real time, online monitoring is achieved through the mathematical relation between an equivalent circuit model and normalized impedance, the problem that membrane wetting cannot be accurately detected and the membrane state cannot be monitored in real time in the traditional technology is solved, and meanwhile, the measurement accuracy is improved. The system can automatically run around the clock without shutdown or sampling analysis.

Description

Technical Field

[0001] The invention relates to the technical field of membrane separation, in particular to a system and method for online detection of membrane wetting degree in membrane distillation. Background Art

[0002] Membrane distillation is a new type of membrane separation technology based on the principle of gas-liquid phase change. Its core mechanism is to use the dielectric barrier properties of hydrophobic microporous membranes to achieve selective separation of water vapor and solutes. This process drives mass transfer by maintaining a vapor pressure difference on both sides of the membrane. The volatile components (such as water molecules) in the hot side feed liquid are vaporized on the membrane surface, and the formed steam molecules diffuse through the membrane pores to the cold side and condense, thereby achieving separation of solutes and solvents. Although this technology has the advantage of being able to treat high-salinity wastewater, the membrane wetting phenomenon that is prevalent in actual operation seriously restricts its separation efficiency, which manifests as engineering bottlenecks such as membrane flux attenuation, decreased retention rate, and reduced operating stability.

[0003] Membrane wetting is essentially the process of surface property degradation of hydrophobic membrane materials in a complex feed liquid environment, which directly causes liquid substances to penetrate the membrane pore structure, resulting in deterioration of water quality on the osmotic side and loss of system desalination efficiency. It is an important indicator for measuring membrane distillation performance.

[0004] At present, the methods for monitoring membrane wetting mainly include conductivity measurement, flux measurement and visualization measurement, but these methods have certain limitations. For example, conductivity and flux measurement cannot provide sufficient sensitivity in the early stage of wetting, while visualization measurement is limited by the transparency of the feed liquid. In addition, existing monitoring methods often require shutdown or sampling analysis, and cannot achieve online real-time monitoring.

[0005] In order to solve the deficiencies of the prior art, this patent proposes a system and method for online detection of membrane wetting degree in membrane distillation. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a system and method for online detection of membrane wetting degree in membrane distillation, which solves the problem of online real-time monitoring of membrane wetting during membrane distillation.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions: A system for online detection of membrane wetting degree in membrane distillation, comprising: A membrane distillation subsystem, used to provide a membrane distillation separation process and form the flow of feed liquid and distillate; Electrochemical monitoring subsystem, used to continuously measure the changes in transmembrane current during membrane distillation and output electrical signals; A data processing subsystem, connected to the electrochemical workstation and receiving the electrical signal, for processing the electrical signal and calculating the membrane wetting degree in real time; The membrane wetting degree is calculated by real-time processing of electrical signals, using an equivalent circuit model to derive the relationship between the normalized impedance of membrane wetting and the dynamic wetting index, and the membrane wetting degree is calculated based on this relationship.

[0008] Preferably, the electrochemical monitoring subsystem comprises: An electrochemical workstation, used to measure the transmembrane current on both sides of the membrane during the membrane distillation process and convert it into an electrical signal, wherein the electrochemical workstation is connected to the electrodes through a wire; Electrodes, including a working electrode and a counter electrode, which are respectively arranged on both sides of the membrane for current measurement, and the electrodes are made of stainless steel or metal alloy materials to ensure stable electrochemical performance; The signal converter is used to convert the current signal obtained by the electrochemical workstation into data and transmit it to the data processing subsystem for analysis.

[0009] Preferably, the membrane distillation subsystem comprises: A feed liquid loop, used for circulating the feed liquid and controlling the flow of the feed liquid; A membrane distillation membrane assembly, comprising a microporous hydrophobic membrane, and connected to the electrodes of the electrochemical workstation for measuring transmembrane current; The distillate loop is used to circulate the distillate and maintain the temperature difference across the membrane.

[0010] Preferably, the feed liquid loop includes a water inlet pump, one end of which is connected to the membrane distillation membrane assembly via a pipeline, the other end of which is connected to the agitator via a pipeline, the agitator is connected to the heater via a pipeline, and the heater is connected to the membrane distillation membrane assembly via a pipeline.

[0011] Preferably, the distillate loop includes an inlet pump, one end of the inlet pump is connected to the membrane distillation membrane assembly through a pipeline, and the other end of the inlet pump is connected to a feed tank through a pipeline, a feed tank balance is provided at the bottom, a conductivity probe is provided in the feed tank, the membrane distillation membrane assembly is connected to a cooler through a pipeline, and the cooler and the feed tank are connected through a pipeline.

[0012] Preferably, the data processing subsystem includes: The data acquisition module is used to receive the electrical signals transmitted by the electrochemical monitoring subsystem and perform preliminary data filtering and preprocessing; The signal processing module is used to calculate the real-time impedance data of the membrane according to the electrochemical signal and perform fitting processing through the equivalent circuit model; A calculation module is used to calculate the membrane wetting index based on the impedance data and the mathematical model to obtain the real-time monitoring result of the membrane wetting state; The display and feedback module is used to display the calculated membrane wetting index to the operator and provide operation prompts or operating condition adjustments through the automation system.

[0013] A method for online detection of membrane wetting degree in membrane distillation comprises the following steps: S1. Install the membrane distillation subsystem device, place the microporous hydrophobic membrane between two stainless steel electrodes, and ensure that both sides of the membrane are contacted by the electrodes respectively, and measure the transmembrane current; S2, regulating the temperature, concentration and flow rate of the feed liquid and the distillate to make the membrane distillation system enter a stable operation state, the feed liquid temperature is 55-85°C, the concentration is 25-40% NaCl solution, and the distillate is pure water at a temperature of 5-25°C; S3, measuring the transmembrane current change in real time through the electrochemical monitoring system, and transmitting the electrical signal to the data processing subsystem; S4, the data processing subsystem processes the electrical signals in real time and uses an equivalent circuit model to derive the degree of membrane wetting; S5. Based on the membrane wetting index calculated in real time, the membrane wetting degree is monitored and the membrane distillation process is adjusted.

[0014] Preferably, the equivalent circuit model in step S4 includes: S4.1. In the membrane distillation process, the electrical characteristics of the membrane can be described by an equivalent layered RC circuit, where the expressions for resistance and capacitance are: ; ; in, is the wetted area resistance, is the solution resistivity, is the thickness of the wetting layer, is the porosity, is the effective area of ​​the membrane, is the wetted area capacitance, is the dielectric constant of vacuum, is the relative dielectric constant of water; S4.2, the capacitance expression of the unwetted area is: ; in, is the capacitance of the unwetted area, is the relative dielectric constant of air, is the total thickness of the film; S4.3, the total capacitance expression is: ; Arranged: ; in, is the total capacitance; S4.4, high frequency impedance expression is approximately: ; in, is the impedance in the high frequency region, is the electrical frequency; S4.5. Define normalized impedance: ; in, Normalized impedance, is the initial impedance; S4.6, solve the equation to get the wetting layer thickness ratio: ; in, is the wetting layer thickness ratio; S4.7. Define the dynamic wetting index in combination with the salt retention rate: ; in, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt retention rate.

[0015] Preferably, the step S4.6 and the step S4.7 together constitute a quantitative model of the degree of infiltration, which is: when <0.05 and When <0.06, it is judged as non-wetting state; When 0.05≤ <0.3 and 0.06≤ When <0.4, it is judged as surface wetting; When 0.3≤ <0.95 and 0.4≤ When <0.9, it is judged as partial wetting; when ≥0.95 and When ≥0.9, it is judged as complete wetting.

[0016] The present invention provides a system and method for online detection of membrane wetting degree in membrane distillation, which has the following beneficial effects: 1. The present invention adopts a solution combining electrochemical monitoring technology with a data processing system. By measuring the changes in transmembrane current on both sides of the membrane during membrane distillation in real time, it can dynamically detect the degree of membrane wetting during the membrane distillation process. Compared with the existing conductivity measurement and flux measurement methods, this technical solution can provide highly sensitive detection results at the initial stage of membrane wetting, and timely warn the membrane wetting state, avoiding the deficiency of traditional methods that cannot accurately detect membrane wetting in the early stage. In addition, the system provides accurate membrane status data by real-time calculation of the dynamic wetting index, providing a reliable basis for the optimization of the membrane distillation process and the maintenance of the membrane.

[0017] 2. The present invention combines an equivalent circuit model with an electrochemical workstation. By constructing a mathematical relationship between the degree of membrane wetting and the normalized impedance, it can accurately calculate the dynamic wetting index, solving the problem that traditional monitoring methods cannot monitor the membrane status online and in real time. Unlike the prior art that requires shutdown or sampling analysis, the present invention can perform continuous monitoring through real-time processing of current signals without interrupting the membrane distillation process, thereby greatly improving the stability of the system operation and the service life of the membrane.

[0018] 3. The present invention also realizes the precise collection and efficient processing of transmembrane current signals during the membrane distillation process through the coordinated work of the signal converter and the data processing subsystem. It can continuously obtain changes in the electrical characteristics of the membrane without interrupting the membrane distillation process and provide highly timely data feedback, which greatly reduces the monitoring pressure of operators and improves the automation level of the entire membrane distillation system. Compared with traditional technologies that require manual operation or intermittent sampling, the system can operate automatically around the clock to ensure continuous tracking of the membrane status. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the experimental device diagram of the membrane distillation subsystem of the present invention; Figure 2 This is a test result diagram of the method for monitoring the membrane wetting degree in membrane distillation according to Example 1 of the present invention, wherein the feed liquid is a 30% NaCl solution at 50° C.; Figure 3 This is a test result diagram of the method for monitoring the degree of membrane wetting in membrane distillation according to Example 2 of the present invention, wherein the feed liquid is a 35% NaCl solution at 60° C.; Figure 4 is an equivalent circuit of the present invention; Figure 5 It is a framework diagram of the system of the present invention; Figure 6 It is a framework diagram of the electrochemical monitoring subsystem of the present invention; Figure 7 It is a framework diagram of the data processing subsystem of the present invention; Figure 8Flow chart of the method of the present invention.

[0020] Among them, 1. membrane distillation membrane assembly; 2. water inlet pump; 3. agitator; 4. feed tank balance; 5. heater; 6. cooler; 7. conductivity probe; 8. electrochemical workstation. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the specification of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] Please refer to the attached Figure 1 , Attachment Figure 4 and attached Figure 5 The embodiment of the present invention provides a system for online detection of membrane wetting degree in membrane distillation, comprising: A membrane distillation subsystem, used to provide a membrane distillation separation process and form the flow of feed liquid and distillate; The membrane distillation subsystem includes: A feed liquid loop, used for circulating the feed liquid and controlling the flow of the feed liquid; The membrane distillation membrane assembly 1 includes a microporous hydrophobic membrane and is connected to the electrodes of the electrochemical workstation 8 for measuring the transmembrane current; The distillate loop is used to circulate the distillate and maintain the temperature difference across the membrane.

[0023] The feed liquid loop includes an inlet pump 2, one end of the inlet pump 2 is connected to the membrane distillation membrane assembly 1 through a pipeline, the other end of the membrane distillation membrane assembly 1 is connected to the agitator 3 through a pipeline, the agitator 3 is connected to the heater 5 through a pipeline, and the heater 5 is connected to the membrane distillation membrane assembly 1 through a pipeline.

[0024] The distillate loop includes an inlet pump 2, one end of which is connected to the membrane distillation membrane assembly 1 through a pipeline, and the other end of the inlet pump 2 is connected to the feed tank through a pipeline, a feed tank balance 4 is arranged at the bottom, a conductivity probe 7 is arranged in the feed tank, the membrane distillation membrane assembly 1 is connected to the cooler 6 through a pipeline, and the cooler 6 is connected to the feed tank through a pipeline.

[0025] Electrochemical monitoring subsystem, used to continuously measure the changes in transmembrane current during membrane distillation and output electrical signals; Please refer to the attached Figure 6 , the electrochemical monitoring subsystem includes: An electrochemical workstation 8, used to measure the transmembrane current on both sides of the membrane during the membrane distillation process and convert it into an electrical signal, and the electrochemical workstation is connected to the electrode through a wire; Electrodes, including working electrodes and counter electrodes, which are respectively arranged on both sides of the membrane for current measurement. The electrodes are made of stainless steel or metal alloy materials to ensure stable electrochemical performance; The signal converter is used to convert the current signal obtained by the electrochemical workstation 8 into data and transmit it to the data processing subsystem for analysis.

[0026] In this embodiment, the electrochemical monitoring subsystem is used to continuously measure the changes in the transmembrane current during the membrane distillation process and output an electrical signal, which is transmitted to the data processing subsystem for analysis to achieve online monitoring of the membrane wettability. The key to this system is to monitor the changes in the electrical properties of the membrane during the membrane distillation process in real time through an electrochemical method, and then deduce the membrane wettability.

[0027] In this embodiment, the electrochemical workstation 8 is the core component of the electrochemical monitoring subsystem. The function of the electrochemical workstation is to measure the transmembrane current on both sides of the membrane during the membrane distillation process and convert the measurement results into electrical signals. These electrical signals will be transmitted as input to the subsequent data processing subsystem.

[0028] Function of electrochemical workstation: The electrochemical workstation continuously monitors the current changes on both sides of the membrane through connection with the electrodes to obtain transmembrane current data. During the membrane distillation process, the feed liquid and the distillate are separated by the microporous hydrophobic membrane of the membrane distillation membrane assembly 1, forming a potential difference. The electrochemical workstation reflects the electrical characteristics of the membrane by measuring the potential difference (i.e., transmembrane current).

[0029] Signal conversion: The measured current signal will be connected to the electrochemical workstation through a wire and converted into a standardized electrical signal, which will then be transmitted to the signal converter for further processing. The electrochemical workstation can not only provide real-time data of transmembrane current, but also record and store relevant data to support subsequent analysis.

[0030] Electrodes are used to measure current in electrochemical monitoring subsystems and are an indispensable component of the system. Electrodes consist of working electrodes and counter electrodes, which are placed on both sides of the membrane to measure the transmembrane current.

[0031] Functions of working electrode and counter electrode: During membrane distillation, the working electrode and counter electrode contact the feed liquid and distillate on both sides of the membrane respectively, and the electrical characteristics of the membrane are reflected by measuring the current signal. The materials of the working electrode and counter electrode should be selected with stable electrochemical properties, generally stainless steel or metal alloy materials. This is because these materials have good corrosion resistance in the membrane distillation process and can provide stable electrochemical response.

[0032] Electrode configuration: The electrode configuration design needs to ensure uniform distribution on both sides of the membrane and be able to effectively measure the transmembrane current. The spacing and size of the electrodes should be designed based on parameters such as membrane thickness and membrane porosity to ensure accurate measurement and reduce noise and interference.

[0033] The function of the signal converter is to convert the current signal measured by the electrochemical workstation into digital data and transmit the data to the data processing subsystem for further analysis.

[0034] Function of signal converter: The signal converter not only amplifies the current signal, but also has the function of noise filtering to ensure the clarity and accuracy of the signal. By converting the current signal, the signal converter can convert the analog current signal into data that can be processed in the digital system and transmit the data to the data processing subsystem through the appropriate transmission channel.

[0035] Data transmission and analysis: After converting the current signal into digital data, the signal converter transmits the data to the data processing subsystem. The data will be used to deduce the degree of membrane wetting through real-time calculation. The signal converter and the data processing subsystem are connected through a high-speed data channel to ensure that the signal can be transmitted and analyzed in real time.

[0036] The close cooperation between the electrochemical monitoring subsystem and the membrane distillation subsystem is the key to the real-time monitoring of the membrane wetting degree of the present invention. The membrane distillation subsystem feeds the liquid into the membrane distillation membrane assembly 1 through the feed liquid loop, while the electrochemical monitoring subsystem monitors the current signal on both sides of the membrane through the electrodes. The current change reflects the electrical characteristics of the membrane, and thus the membrane wetting degree can be deduced.

[0037] Connection between membrane distillation module 1 and electrochemical workstation: The membrane distillation module 1 and the electrochemical workstation are connected via electrodes, which reflect the state of the membrane by measuring the current signal. As the degree of membrane wetting changes, the current signal will change, and the electrochemical workstation can accurately capture these changes and provide real-time current signals.

[0038] Based on the current signal collected by the electrochemical monitoring subsystem, the data processing subsystem calculates the membrane wetting degree through a mathematical model. This model can accurately characterize the membrane wetting state by normalizing the impedance signal.

[0039] Relationship between impedance normalization and membrane wetting index: According to the formula in the briefing, there is a clear mathematical relationship between normalized impedance and dynamic wetting index. Specifically, the dynamic wetting index can be calculated by the following formula: ; in, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt retention rate, Normalized impedance, is the initial impedance.

[0040] Please refer to the attached Figure 7 , a data processing subsystem, connected to the electrochemical workstation 8 and receiving the electrical signal, for processing the electrical signal and calculating the membrane wetting degree in real time; The data processing subsystem includes: The data acquisition module is used to receive the electrical signals transmitted by the electrochemical monitoring subsystem and perform preliminary data filtering and preprocessing; The signal processing module is used to calculate the real-time impedance data of the membrane according to the electrochemical signal and perform fitting processing through the equivalent circuit model; A calculation module is used to calculate the membrane wetting index based on the impedance data and the mathematical model to obtain the real-time monitoring result of the membrane wetting state; The display and feedback module is used to display the calculated membrane wetting index to the operator and provide operation prompts or operating condition adjustments through the automation system.

[0041] In this embodiment, the data processing subsystem receives and processes the electrical signals from the electrochemical monitoring subsystem by working closely with the electrochemical workstation 8, and calculates the degree of membrane wetting in real time. The subsystem is composed of a plurality of key modules, each of which has different functions, ensuring real-time monitoring and feedback of the membrane wetting state. These modules include a data acquisition module, a signal processing module, a calculation module, and a display and feedback module. By accurately calculating the dynamic wetting index, the present invention can achieve accurate control of the membrane distillation process, ensuring that the membrane assembly will not have problems such as excessive wetting or drying up during operation.

[0042] The main function of the data acquisition module is to receive electrical signals from the electrochemical monitoring subsystem. Since the electrochemical signals will be affected by noise and external interference during the membrane distillation process, preliminary signal filtering and preprocessing must be performed. The data acquisition module will filter out noise, enhance and process the electrical signals to ensure the accuracy of subsequent steps. The preprocessed electrical signals will be sent to the signal processing module to provide high-quality input for subsequent calculations.

[0043] The signal processing module calculates the real-time impedance of the membrane by analyzing the electrochemical signal. This process is based on the electrical properties of the membrane and uses an equivalent circuit model to describe the behavior of the membrane. The changes in the electrical properties of the membrane during the wetting process can be represented by the following circuit model: The electrical properties of the membrane are divided into a wetted area and a non-wetted area, and these two areas are represented by different resistances and capacitances.

[0044] The calculation formula for the wetted area resistance is: ; The parameters in the formula are defined as follows: is the wetted area resistance (Ω), which characterizes the resistance of the liquid-permeable part of the membrane pores.

[0045] is the solution resistivity (Ω·m), which is used to describe the electrical conductivity of the solution.

[0046] is the thickness of the wetting layer (m), which refers to the thickness of the portion of the membrane pores that is penetrated by the liquid.

[0047] is the porosity.

[0048] is the effective area of ​​the membrane (m 2 ), which is the area of ​​the membrane surface in contact with the liquid.

[0049] The calculation formula for the wetted area capacitance is: ; The parameters in the formula are defined as follows: is the wetted area capacitance (F), which represents the capacitance of the liquid-permeable part of the membrane pores.

[0050] is the dielectric constant of vacuum, which is 8.854×10 −12 F / m.

[0051] is the relative dielectric constant of water (80), which is used to describe the response of water to electric fields.

[0052] is the effective area of ​​the membrane (m 2 ).

[0053] is the thickness of the wetting layer (m).

[0054] The calculation formula for the capacitance of the unwetted area is: ; in, is the capacitance of the unwetted area, is the relative dielectric constant of air, is the total thickness of the film.

[0055] Through these calculation formulas, the signal processing module can obtain the resistance, capacitance and impedance data of the membrane, providing necessary data support for the subsequent calculation of the membrane wetting index.

[0056] The calculation module calculates the dynamic wetting index by analyzing the impedance data provided by the signal processing module and combining it with the mathematical model. There is a clear mathematical relationship between the dynamic wetting index and the impedance of the membrane, and the specific calculation formula is: ; in, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt retention rate, Normalized impedance, is the initial impedance.

[0057] The formula accurately calculates the degree of membrane wetting by normalizing the relationship between impedance and dynamic wetting index. The calculation module provides the real-time calculated wetting index to the display and feedback module.

[0058] The display and feedback module displays the membrane wetting index output by the calculation module to the operator in real time, ensuring that the operator can understand the working status of the membrane in a timely manner. In addition, the module is also integrated with the automatic control system. When the membrane wetting index reaches the set threshold, the system can automatically adjust, such as adjusting the flow rate and temperature of the feed liquid, to ensure that the membrane infiltration state is always maintained within the ideal range.

[0059] Derivation of equivalent circuit parameters and further relationships In this embodiment, the derivation process of the equivalent circuit is crucial to understanding the electrical properties of the membrane. In order to accurately describe the wetting state of the membrane, especially the electrical properties of the wetting layer and the non-wetting layer, we adopted a layered RC circuit model. This model simulates the different states (wetting and non-wetting) of the membrane pore area during the membrane distillation process through the series and parallel structures of resistors and capacitors. Through this model, we can accurately calculate the electrical properties of the membrane and further derive the dynamic wetting index. The following is a detailed derivation process: 1. Circuit model of wetted and non-wetted areas In the membrane distillation process, the degree of wetting on the membrane surface directly affects the electrical properties of the membrane. For the electrical behavior of the membrane, we can describe the resistance and capacitance characteristics of its different regions through a layered RC circuit. Specifically, the electrical properties of the membrane can be divided into two regions: Wetted area: The area of ​​the membrane pores that the liquid penetrates. The resistance and capacitance of this area are mainly determined by factors such as the resistivity of the solution, the pore structure of the membrane, and the electrical constant of the membrane.

[0060] Unwetted Area: The area of ​​membrane pores that is not penetrated by liquid. This area is primarily filled with air and has significantly different electrical properties than the wetted area, manifesting as higher resistance and corresponding capacitance.

[0061] 2. Derivation of total capacitance The electrical properties of the membrane can be determined by its total capacitance The total capacitance is a comprehensive reflection of the capacitance of the wetted and unwetted areas, so we need to derive the capacitance of each area separately and then combine them. First, the capacitance of the wetted and unwetted areas is derived from their resistance and capacitance models (RC circuits).

[0062] Wetted Area Capacitance Determined by the wetted portion of the membrane, it can be expressed as: ; in, is the wetted area resistance, is the solution resistivity, is the thickness of the wetting layer, is the porosity, is the effective area of ​​the membrane, is the wetted area capacitance, is the dielectric constant of vacuum, is the relative dielectric constant of water.

[0063] Unwetted Area Capacitance Determined by the air filling of the unpermeated portion, it can be expressed as: ; in, is the capacitance of the unwetted area, is the relative dielectric constant of air, is the total thickness of the film.

[0064] Total Capacitance is the parallel and series combination of the capacitance of the wetted and unwetted layers. By analyzing the relationship between these two capacitances, we derive an expression for the total capacitance.

[0065] 3. Derivation of total capacitance formula According to the series and parallel relationship of capacitors, the total capacitance of the membrane It can be expressed by the following formula: ; This equation represents a weighted combination of the capacitance of the wetted and unwetted regions, where the capacitance of the wetted region is proportional to the thickness of the membrane’s wetting layer, and the capacitance of the unwetted region is proportional to the total thickness of the membrane minus the thickness of the wetting layer.

[0066] By rearranging the formula, we get the final expression for the total capacitance of the membrane: ; This formula provides us with a method to calculate the total capacitance of the membrane's electrical characteristics, which takes into account both the degree of membrane wetting and the electrical characteristics of the membrane material.

[0067] 4. Derivation of high frequency impedance and normalized impedance Under high frequency conditions, the impedance of the membrane shows a certain frequency dependence. Under high frequency conditions, the impedance of the membrane can be expressed by the following formula: ; in, is the impedance in the high frequency region, The electrical frequency.

[0068] Next, we define the normalized impedance To express the relative change of membrane impedance, it is helpful to quantitatively analyze the state of the membrane. The expression of normalized impedance is: ; in, is the initial impedance, which reflects the impedance of the membrane in the unwetted state.

[0069] 5. Derivation of the wetting layer thickness ratio By normalizing the impedance, we can further deduce the wetting layer thickness ratio , this ratio is an important indicator of the degree of membrane wetting. It can be expressed as: ; in, It represents the ratio of the wetting layer to the total thickness of the membrane. is the impedance at the current moment, is the initial impedance, and are the relative dielectric constants of water and air, respectively.

[0070] 6. Derivation of the dynamic wetting index In order to further quantitatively describe the degree of membrane wetting, we defined the dynamic wetting index in combination with the salt rejection rate: , which combines the impedance change and salt retention capacity of the membrane. The formula for the dynamic wetting index is: ; in, is the initial impedance, is the weight coefficient (usually between 0.1 and 0.3, obtained through experimental calibration), is the dynamic salt retention rate, is the initial salt rejection.

[0071] 7. Infiltration degree quantification model Through the above formula, we constructed a quantitative model of membrane wetting degree. and dynamic wetting index , the membrane wetting state is divided into the following levels: when <0.05 and When <0.06, it is judged as non-wetting state; When 0.05≤ <0.3 and 0.06≤ When <0.4, it is judged as surface wetting; When 0.3≤ <0.95 and 0.4≤ When <0.9, it is judged as partial wetting; when ≥0.95 and When ≥0.9, it is judged as complete wetting.

[0072] The method for online detection of membrane wetting degree in membrane distillation described below and the system for online detection of membrane wetting degree in membrane distillation described above can be referred to each other.

[0073] Please refer to the attached Figure 8 , a method for online detection of membrane wetting degree in membrane distillation, comprising the following steps: S1. Install the membrane distillation subsystem device, place the microporous hydrophobic membrane between two stainless steel electrodes, and ensure that both sides of the membrane are contacted by the electrodes respectively, and measure the current signal; S2, regulating the temperature, concentration and flow rate of the feed liquid and the distillate to make the membrane distillation system enter a stable operation state, the feed liquid temperature is 55-85°C, the concentration is 25-40% NaCl solution, and the distillate is pure water at a temperature of 5-25°C; S3, measuring the transmembrane current change in real time through the electrochemical monitoring system, and transmitting the electrical signal to the data processing subsystem; S4, the data processing subsystem processes the electrical signals in real time and uses an equivalent circuit model to derive the degree of membrane wetting; S5. Based on the membrane wetting index calculated in real time, the membrane wetting degree is monitored and the membrane distillation process is adjusted.

[0074] The equivalent circuit model in step S4 includes: S4.1. In the membrane distillation process, the electrical characteristics of the membrane can be described by an equivalent layered RC circuit, where the expressions for resistance and capacitance are: ; ; in, is the wetted area resistance, is the solution resistivity, is the thickness of the wetting layer, is the porosity, is the effective area of ​​the membrane, is the wetted area capacitance, is the dielectric constant of vacuum, is the relative dielectric constant of water; S4.2, the capacitance expression of the unwetted area is: ; in, is the capacitance of the unwetted area, is the relative dielectric constant of air, is the total thickness of the film; S4.3, the total capacitance expression is: ; Arranged: ; in, is the total capacitance; S4.4, high frequency impedance expression is approximately: ; in, is the impedance in the high frequency region, is the electrical frequency; S4.5. Define normalized impedance: ; in, Normalized impedance, is the initial impedance; S4.6, solve the equation to get the wetting layer thickness ratio: ; in, is the wetting layer thickness ratio; S4.7. Define the dynamic wetting index in combination with the salt retention rate: ; in, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt retention rate.

[0075] The steps S4.6 and S4.7 together constitute a quantitative model of the degree of infiltration, which is: when <0.05 and When <0.06, it is judged as non-wetting state; When 0.05≤ <0.3 and 0.06≤ When <0.4, it is judged as surface wetting; When 0.3≤ <0.95 and 0.4≤ When <0.9, it is judged as partial wetting; when ≥0.95 and When ≥0.9, it is judged as complete wetting.

[0076] The method of this embodiment can be used to execute the above system embodiment, and its principles and technical effects are similar, which will not be repeated here.

[0077] Embodiment 1: Install the membrane distillation subsystem: Place a PVDF microporous hydrophobic membrane with a pore size between two stainless steel electrodes to measure the current signal. Figure 1 As shown, a plastic gasket is placed between the electrode and the membrane to provide insulation. The entire assembly is wrapped in a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. A 30% NaCl solution at 50°C is injected on the feed liquid side, and pure water at 20°C is injected on the distillate side. The system is started to form a cross-flow filtration between the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation model CS315 with an operating frequency of 1000kHz and an impedance test using an operating voltage of 5mV. After 24 hours of stable operation, the data at that moment was extracted and calculated, and it was measured and , the test results are as follows Figure 2 Substitute the above data into the formula: ; ; The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as follows: =0.04, =0.0402, which is judged as surface wetting.

[0078] Embodiment 2: Install the membrane distillation subsystem: Place a PVDF microporous hydrophobic membrane with a pore size between two stainless steel electrodes to measure the current signal. Figure 1As shown, a plastic gasket is placed between the electrode and the membrane to provide insulation. The entire assembly is wrapped in a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. A 35% NaCl solution at 60°C is injected on the feed liquid side, and pure water at 20°C is injected on the distillate side. The system is started to form a cross-flow filtration between the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation model CS315 with an operating frequency of 1000kHz and an impedance test using a working voltage of 5mV. After 24 hours of stable operation, the data at that moment was extracted and calculated, and it was measured and , the test results are as follows Figure 3 Substitute the above data into the formula: ; ; The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as follows: =0.665, =0.67, judged as partial wetting.

[0079] Compared with Example 1, it can be seen that the membrane is more easily wetted when the feed liquid temperature and concentration increase.

[0080] Embodiment 3: Install the membrane distillation subsystem device: Place the PVDF microporous hydrophobic membrane with a pore size between the carbon cloth electrodes. The two electrodes are used as the working electrode and the counter electrode respectively to measure the current signal. The device is as follows: Figure 1 As shown, a plastic gasket is placed between the electrode and the membrane to provide insulation. The entire assembly is wrapped in a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. A 30% NaCl solution at 50°C is injected on the feed liquid side, and pure water at 20°C is injected on the distillate side. The system is started to form a cross-flow filtration between the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation model CS315 with an operating frequency of 1000kHz and an impedance test using an operating voltage of 5mV. After 24 hours of stable operation, the data at that moment was extracted and calculated, and it was measured and , put the above data into the formula: ; ; The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as follows: =0.04, =0.397, judged as non-wetting.

[0081] Compared with Example 1, after the stainless steel electrode was replaced with a carbon cloth electrode in the experiment, the dynamic response of the membrane wetting process did not change significantly. However, the carbon cloth electrode absorbed water and swelled in the liquid environment due to its porous structure, had low mechanical strength, and the impedance signal fluctuation amplitude in the high-frequency region exceeded ±15%, and its stability was lower than that of the stainless steel electrode.

[0082] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A system for online detection of membrane wetting degree in membrane distillation, characterized in that: include: A membrane distillation subsystem, used to provide a membrane distillation separation process and form the flow of feed liquid and distillate; Electrochemical monitoring subsystem, used to continuously measure the change of current signal during membrane distillation and output electrical signal; A data processing subsystem, connected to the electrochemical workstation (8) and receiving the electrical signal, for processing the electrical signal and calculating the membrane wetting degree in real time; The calculation of the membrane wetting degree is carried out by real-time processing of electrical signals, using an equivalent circuit model to derive the relationship between the normalized impedance of membrane wetting and the dynamic wetting index, and the membrane wetting degree is calculated by the relationship, wherein the equivalent circuit is a hierarchical structure including a wetted area and a non-wetted area, wherein: The equivalent circuit of the wetted area consists of the first capacitor With the first resistor The parallel structure characterizes the membrane pore area that is penetrated by the liquid; The equivalent circuit of the unwetted area consists of the second capacitor Independently constituted, characterizing the non-permeated membrane pore area containing air.

2. A system for online detection of membrane wetting degree in membrane distillation according to claim 1, characterized in that: The electrochemical monitoring subsystem comprises: An electrochemical workstation (8), used to measure the transmembrane current on both sides of the membrane during the membrane distillation process and convert it into an electrical signal, wherein the electrochemical workstation is connected to the electrodes via a wire; Electrodes, including a working electrode and a counter electrode, which are respectively arranged on both sides of the membrane for current measurement, and the electrodes are made of stainless steel or metal alloy materials to ensure stable electrochemical performance; The signal converter is used to convert the current signal obtained by the electrochemical workstation (8) into data and transmit it to the data processing subsystem for analysis.

3. A system for online detection of membrane wetting degree in membrane distillation according to claim 2, characterized in that: The membrane distillation subsystem comprises: A feed liquid loop, used for circulating the feed liquid and controlling the flow of the feed liquid; A membrane distillation membrane assembly (1), comprising a microporous hydrophobic membrane, and connected to the electrodes of the electrochemical workstation (8) for measuring current signals; The distillate loop is used to circulate the distillate and maintain the temperature difference across the membrane.

4. A system for online detection of membrane wetting degree in membrane distillation according to claim 1, characterized in that: The feed liquid loop comprises a water inlet pump (2), one end of the water inlet pump (2) is connected to a membrane distillation membrane assembly (1) via a pipeline, the other end of the membrane distillation membrane assembly (1) is connected to an agitator (3) via a pipeline, the agitator (3) is connected to a heater (5) via a pipeline, and the heater (5) is connected to the membrane distillation membrane assembly (1) via a pipeline.

5. The system for online detection of membrane wetting degree in membrane distillation according to claim 1, characterized in that: The distillate loop comprises a water inlet pump (2), one end of the water inlet pump (2) is connected to a membrane distillation membrane assembly (1) via a pipeline, the other end of the water inlet pump (2) is connected to a feed tank via a pipeline, a feed tank balance (4) is arranged at the bottom of the water inlet pump (2), a conductivity probe (7) is arranged in the feed tank, the membrane distillation membrane assembly (1) is connected to a cooler (6) via a pipeline, and the cooler (6) and the feed tank are connected via a pipeline.

6. The system for online detection of membrane wetting degree in membrane distillation according to claim 1, characterized in that: The data processing subsystem comprises: The data acquisition module is used to receive the electrical signals transmitted by the electrochemical monitoring subsystem and perform preliminary data filtering and preprocessing; The signal processing module is used to calculate the real-time impedance data of the membrane according to the electrochemical signal and perform fitting processing through the equivalent circuit model; A calculation module is used to calculate the membrane wetting index based on the impedance data and the mathematical model to obtain the real-time monitoring result of the membrane wetting state; The display and feedback module is used to display the calculated membrane wetting index to the operator and provide operation prompts or operating condition adjustments through the automation system.

7. A method for online detection of membrane wetting degree in membrane distillation, characterized in that: The system for assisting in the online detection of membrane wetting degree in membrane distillation according to any one of claims 1 to 6 comprises the following steps: S1. Install the membrane distillation subsystem device, place the microporous hydrophobic membrane between two stainless steel electrodes, and ensure that both sides of the membrane are contacted by the electrodes respectively, and measure the transmembrane current; S2, regulating the temperature, concentration and flow rate of the feed liquid and the distillate to make the membrane distillation system enter a stable operation state, the feed liquid temperature is 55-85°C, the concentration is 25-40% NaCl solution, and the distillate is pure water at a temperature of 5-25°C; S3, measuring the transmembrane current change in real time through the electrochemical monitoring system, and transmitting the electrical signal to the data processing subsystem; S4, the data processing subsystem processes the electrical signals in real time and uses an equivalent circuit model to derive the degree of membrane wetting; S5. Based on the membrane wetting index calculated in real time, the membrane wetting degree is monitored and the membrane distillation process is adjusted.

8. The method for online detection of membrane wetting degree in membrane distillation according to claim 7, characterized in that: The equivalent circuit model in step S4 includes: S4.

1. In the membrane distillation process, the electrical characteristics of the membrane can be described by an equivalent layered RC circuit, where the expressions for resistance and capacitance are: ； ； in, is the wetted area resistance, is the solution resistivity, is the thickness of the wetting layer, is the porosity, is the effective area of ​​the membrane, is the wetted area capacitance, is the dielectric constant of vacuum, is the relative dielectric constant of water; S4.2, the capacitance expression of the unwetted area is: ； in, is the capacitance of the unwetted area, is the relative dielectric constant of air, is the total thickness of the film; S4.3, the total capacitance expression is: ； Arranged: ； in, is the total capacitance; S4.4, high frequency impedance expression is approximately: ； in, is the impedance in the high frequency region, is the electrical frequency; S4.

5. Define normalized impedance: ； in, Normalized impedance, is the initial impedance; S4.6, solve the equation to get the wetting layer thickness ratio: ； in, is the wetting layer thickness ratio; S4.

7. Define the dynamic wetting index in combination with the salt retention rate: ； in, is the initial salt retention rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt retention rate.

9. A method for online detection of membrane wetting degree in membrane distillation according to claim 8, characterized in that: The steps S4.6 and S4.7 together constitute a quantitative model of the degree of infiltration, which is: when <0.05 and When <0.06, it is judged as non-wetting state; When 0.05≤ <0.3 and 0.06≤ When <0.4, it is judged as surface wetting; When 0.3≤ <0.95 and 0.4≤ When <0.9, it is judged as partial wetting; when ≥0.95 and When ≥0.9, it is judged as complete wetting.

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