A system and method for online detecting the membrane wetting degree in membrane distillation

Abstract

The present invention relates to the field of membrane separation technology, and discloses a system for on-line detecting the degree of membrane wetting in membrane distillation, comprising: a membrane distillation subsystem for providing a membrane distillation separation process to form the flow of a feed liquid and a distillate; an electrochemical monitoring subsystem for continuously measuring the change of current signals during the membrane distillation process and outputting electrical signals. The present invention also discloses a method for on-line detecting the degree of membrane wetting in membrane distillation, comprising the following steps: S1. Install the device of the membrane distillation subsystem, and place a microporous hydrophobic membrane between two stainless steel electrodes. By combining the electrochemical monitoring technology with a data processing system, the change of current signals on both sides of the membrane during the membrane distillation process is measured in real time, and on-line monitoring is realized through the mathematical relationship between an equivalent circuit model and a normalized impedance, solving the problems that the traditional technology cannot accurately detect membrane wetting and monitor the membrane state in real time. At the same time, the system can operate automatically all day long without shutdown or sampling analysis.

Description

Technical Field

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

[0002] Membrane distillation is a novel membrane separation technology based on the gas-liquid phase change principle. Its core mechanism is to achieve the selective separation of water vapor and solutes by utilizing the medium barrier characteristics of a hydrophobic microporous membrane. This process drives mass transfer by maintaining a vapor pressure difference across the membrane. The volatile components (such as water molecules) in the feed liquid on the hot side vaporize on the membrane surface, and the formed vapor molecules diffuse through the membrane pores to the cold side for condensation, thereby achieving the separation of solutes and solvents. Although this technology has the advantage of being able to treat high-salt wastewater, the common membrane wetting phenomenon in actual operation seriously restricts its separation efficiency, manifested as engineering bottleneck problems such as membrane flux decay, rejection rate decline, and reduced operation stability.

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

[0004] Currently, the methods for monitoring membrane wetting mainly include conductivity measurement, flux measurement, and visualization measurement, etc. However, these methods have certain limitations. For example, conductivity and flux measurements cannot provide sufficient sensitivity in the initial 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] To solve the deficiencies of the existing technology, this patent proposes a system and method for online detecting the membrane wetting degree in membrane distillation. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the present invention provides a system and method for online detecting the membrane wetting degree in membrane distillation, which solves the problem of online real-time monitoring of membrane wetting during the membrane distillation process.

[0007] To achieve the above objectives, the present invention is realized through the following technical solutions: A system for online detecting the membrane wetting degree in membrane distillation, comprising:

[0008] A membrane distillation subsystem for providing a membrane distillation separation process to form the flow of the feed liquid and the distillate;

[0009] An electrochemical monitoring subsystem for continuously measuring the change in the transmembrane current during the membrane distillation process and outputting an electrical signal;

[0010] A data processing subsystem, which is connected to an electrochemical workstation and receives electrical signals, is used to process the electrical signals and calculate the degree of membrane wetting in real time;

[0011] Among them, the calculation of the degree of membrane wetting is carried out by processing electrical signals in real time, using an equivalent circuit model to deduce the relationship between the normalized impedance of membrane wetting and the dynamic wetting index, and calculating the degree of membrane wetting through this relationship.

[0012] Preferably, the electrochemical monitoring subsystem includes:

[0013] An electrochemical workstation, which is used to measure the transmembrane current on both sides of the membrane during the membrane distillation process and convert it into an electrical signal. The electrochemical workstation is connected to the electrode through a wire;

[0014] Electrodes, including a working electrode and a counter electrode, 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;

[0015] A signal converter, which is used to convert the current signal obtained by the electrochemical workstation into data and transmit it to the data processing subsystem for analysis.

[0016] Preferably, the membrane distillation subsystem includes:

[0017] A feed liquid loop, which is used to circulate the feed liquid and control the flow of the feed liquid;

[0018] A membrane distillation membrane module, which includes a microporous hydrophobic membrane and is connected to the electrodes of the electrochemical workstation, is used to measure the transmembrane current;

[0019] A distillate loop, which is used to circulate the distillate and maintain the temperature difference on both sides of the membrane.

[0020] Preferably, the feed liquid loop includes a feed water pump. One end of the feed water pump is connected to the membrane distillation membrane module through a pipeline. The other end of the membrane distillation membrane module is connected to a stirrer through a pipeline. The stirrer is connected to a heater through a pipeline. The heater is connected to the membrane distillation membrane module through a pipeline.

[0021] Preferably, the distillate loop includes a feed water pump. One end of the feed water pump is connected to the membrane distillation membrane module through a pipeline. The other end of the feed water pump is connected to a feed tank through a pipeline. A feed tank balance is arranged at the bottom of the feed tank. A conductivity probe is arranged in the feed tank. The membrane distillation membrane module is connected to a cooler through a pipeline. The cooler is connected to the feed tank through a pipeline.

[0022] Preferably, the data processing subsystem includes:

[0023] A data acquisition module, which is used to receive the electrical signals transmitted by the electrochemical monitoring subsystem and perform preliminary data filtering and preprocessing;

[0024] A signal processing module, configured to calculate real-time impedance data of the membrane based on the electrochemical signal and perform fitting processing through an equivalent circuit model;

[0025] A calculation module, configured to calculate the membrane wetting index based on the impedance data and a mathematical model to obtain a real-time monitoring result of the membrane wetting state;

[0026] A display and feedback module, configured to display the calculated membrane wetting index to the operator and perform operation prompts or process condition adjustments through an automated system.

[0027] A method for on-line detecting the wetting degree of a membrane in membrane distillation, comprising the following steps:

[0028] S1. Install a membrane distillation subsystem device, place a microporous hydrophobic membrane between two stainless steel electrodes, and ensure that both sides of the membrane are in contact with the electrodes, and measure the transmembrane current;

[0029] S2. Regulate the temperatures, concentrations and flow rates of the feed liquid and the distillate to make the membrane distillation system enter a stable operation state. The temperature of the feed liquid is 55 - 85 °C, the concentration is a 25 - 40% NaCl solution, and the distillate is pure water at a temperature of 5 - 25 °C;

[0030] S3. Real-time measure the change of the transmembrane current through an electrochemical monitoring system and transmit the electrical signal to the data processing subsystem;

[0031] S4. The data processing subsystem processes the electrical signal in real time and uses an equivalent circuit model to deduce the wetting degree of the membrane;

[0032] S5. Monitor the wetting degree of the membrane according to the real-time calculated membrane wetting index and adjust the membrane distillation process.

[0033] Preferably, the equivalent circuit model in the step S4 includes:

[0034] S4.1. During the membrane distillation process, the electrical characteristics of the membrane can be described by an equivalent layered RC circuit, where the expressions of the resistance and capacitance are respectively:

[0035]

[0036] ;

[0037] Wherein, is the resistance of the wetting zone, is the solution resistivity, is the thickness of the wetting layer, is the porosity, is the effective area of the membrane, is the capacitance of the wetting zone, is the vacuum permittivity, is the relative permittivity of water;

[0038] S4.2. The capacitance expression for the unwetted region is:

[0039] ;

[0040] where, is the capacitance of the unwetted region, is the relative permittivity of air, is the total thickness of the membrane;

[0041] S4.3. The total capacitance expression is:

[0042]

[0043] After rearrangement:

[0044] ;

[0045] where, is the total capacitance;

[0046] S4.4. The impedance expression in the high-frequency region is approximately:

[0047]

[0048] where, is the impedance in the high-frequency region, is the electrical frequency;

[0049] S4.5. Define the normalized impedance:

[0050]

[0051] where, is the normalized impedance, is the initial impedance;

[0052] S4.6. Solve the equation to obtain the wetting layer thickness ratio:

[0053]

[0054] where, is the wetting layer thickness ratio;

[0055] S4.7. Combine the salt rejection rate to define the dynamic wetting index:

[0056] ;

[0057] where, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt rejection rate.

[0058] Preferably, the steps S4.6 and S4.7 together constitute a wetting degree quantification model, which is:

[0059] When < 0.05 and < 0.06, it is determined as the non-wetting state;

[0060] When 0.05 ≤ < 0.3 and 0.06 ≤ < 0.4, it is determined as surface wetting;

[0061] When 0.3 ≤ < 0.95 and 0.4 ≤ < 0.9, it is determined as partial wetting;

[0062] When ≥ 0.95 and ≥ 0.9, it is determined as complete wetting.

[0063] The present invention provides a system and method for on-line detecting the wetting degree of a membrane in membrane distillation. It has the following beneficial effects:

[0064] 1. By adopting a scheme combining electrochemical monitoring technology and a data processing system, the present invention can dynamically detect the wetting degree of the membrane during the membrane distillation process by measuring the change of the transmembrane current on both sides of the membrane in real time. Compared with the existing conductivity measurement and flux measurement methods, this technical scheme can provide highly sensitive detection results at the initial stage of membrane wetting, timely warning the wetting state of the membrane, avoiding the deficiency of the traditional method that cannot accurately detect membrane wetting in the early stage. In addition, the system provides accurate membrane state data by calculating the dynamic wetting index in real time, providing a reliable basis for the optimization of the membrane distillation process and the maintenance of the membrane.

[0065] 2. By combining an equivalent circuit model and an electrochemical workstation, the present invention can accurately calculate the dynamic wetting index by constructing a mathematical relationship between the wetting degree of the membrane and the normalized impedance, solving the problem that the traditional monitoring method cannot monitor the membrane state online and in real time. Different from the existing technology that requires shutdown or sampling analysis, the present invention can continuously monitor without interrupting the membrane distillation process through the real-time processing of the current signal, greatly improving the stability of the system operation and the service life of the membrane.

[0066] 3. The present invention also realizes the accurate acquisition and efficient processing of transmembrane current signals during the membrane distillation process through the collaborative work of the signal converter and the data processing subsystem. It can continuously obtain the changes in the electrical characteristics of the membrane without interrupting the membrane distillation process, provide high-timeliness data feedback, greatly reduce the monitoring pressure of operators, and improve the automation level of the entire membrane distillation system. Compared with the traditional technology that requires manual operation or intermittent sampling, the system can operate automatically all day long to ensure continuous tracking of the membrane state. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 It is an experimental device diagram of the membrane distillation subsystem of the present invention;

[0068] Figure 2 It is a test result diagram of the method for monitoring the degree of membrane wetting in membrane distillation in Example 1 of the present invention, where the feed liquid is a 30% NaCl solution at 50°C;

[0069] Figure 3 It is a test result diagram of the method for monitoring the degree of membrane wetting in membrane distillation in Example 2 of the present invention, where the feed liquid is a 35% NaCl solution at 60°C;

[0070] Figure 4 It is the equivalent circuit of the present invention;

[0071] Figure 5 It is the framework diagram of the system of the present invention;

[0072] Figure 6 It is the framework diagram of the electrochemical monitoring subsystem of the present invention;

[0073] Figure 7 It is the framework diagram of the data processing subsystem of the present invention;

[0074] Figure 8 It is the flowchart of the method of the present invention.

[0075] Among them, 1. Membrane distillation membrane module; 2. Feed water pump; 3. Stirrer; 4. Feed tank balance; 5. Heater; 6. Cooler; 7. Conductivity probe; 8. Electrochemical workstation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0076] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0077] Please refer to the attached Figure 1 and the attached Figure 4and the attached Figure 5 , an embodiment of the present invention provides a system for on-line detecting the membrane wetting degree in membrane distillation, including:

[0078] A membrane distillation subsystem for providing a membrane distillation separation process to form the flow of the feed liquid and the distillate;

[0079] The membrane distillation subsystem includes:

[0080] A feed liquid loop for circulating the feed liquid and controlling the flow of the feed liquid;

[0081] A membrane distillation membrane module 1, including a microporous hydrophobic membrane, and connected to the electrode of an electrochemical workstation 8 for measuring the transmembrane current;

[0082] A distillate loop for circulating the distillate and maintaining the temperature difference across the membrane.

[0083] The feed liquid loop includes a feed water pump 2. One end of the feed water pump 2 is connected to the membrane distillation membrane module 1 through a pipeline. The other end of the membrane distillation membrane module 1 is connected to a stirrer 3 through a pipeline. The stirrer 3 is connected to a heater 5 through a pipeline. The heater 5 is connected to the membrane distillation membrane module 1 through a pipeline.

[0084] The distillate loop includes a feed water pump 2. One end of the feed water pump 2 is connected to the membrane distillation membrane module 1 through a pipeline. The other end of the feed water pump 2 is connected to a feed tank. A feed tank balance 4 is arranged at the bottom of the feed tank. A conductivity probe 7 is arranged in the feed tank. The membrane distillation membrane module 1 is connected to a cooler 6 through a pipeline. The cooler 6 is connected to the feed tank through a pipeline.

[0085] An electrochemical monitoring subsystem for continuously measuring the change of the transmembrane current during the membrane distillation process and outputting an electrical signal;

[0086] Please refer to the attached Figure 6 , the electrochemical monitoring subsystem includes:

[0087] An electrochemical workstation 8 for measuring the transmembrane current across the membrane during the membrane distillation process and converting it into an electrical signal. The electrochemical workstation is connected to the electrode through a wire;

[0088] The electrode includes a working electrode and a counter electrode. The working electrode and the counter electrode are respectively arranged on both sides of the membrane for current measurement. The electrode is made of stainless steel or metal alloy material to ensure stable electrochemical performance;

[0089] A signal converter for converting the current signal obtained by the electrochemical workstation 8 into data and transmitting it to the data processing subsystem for analysis.

[0090] In this embodiment, the electrochemical monitoring subsystem is used to continuously measure the change in transmembrane current during the membrane distillation process and output an electrical signal, which will be transmitted to the data processing subsystem for analysis to achieve online monitoring of the degree of membrane wetting. The key of this system is to monitor the change in the electrical characteristics of the membrane during the membrane distillation process by electrochemical methods, and then deduce the degree of membrane wetting.

[0091] 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 inputs to the subsequent data processing subsystem.

[0092] Function of the electrochemical workstation: The electrochemical workstation continuously monitors the change in current on both sides of the membrane by connecting 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 module 1, forming a potential difference. The electrochemical workstation reflects the electrical characteristics of the membrane by measuring this potential difference (i.e., transmembrane current).

[0093] Signal conversion: The measured current signal will be connected to the electrochemical workstation through wires and converted into a standardized electrical signal, and then transmitted to the signal converter for further processing. The electrochemical workstation can not only provide real-time data of the transmembrane current, but also record and store relevant data to support the subsequent analysis process.

[0094] The electrodes are used for current measurement in the electrochemical monitoring subsystem and are indispensable components in the system. The electrodes consist of a working electrode and a counter electrode, which are respectively arranged on both sides of the membrane to measure the transmembrane current.

[0095] Functions of the working electrode and the counter electrode: During the membrane distillation process, the working electrode and the counter electrode are respectively in contact with the feed liquid and the distillate on both sides of the membrane, and reflect the electrical characteristics of the membrane by measuring the current signal. The materials of the working electrode and the counter electrode should be selected as materials with stable electrochemical properties, generally stainless steel or metal alloy materials. This is because these materials have good corrosion resistance during the membrane distillation process and can provide a stable electrochemical response.

[0096] Configuration of the electrodes: The configuration design of the electrodes needs to ensure uniform distribution on both sides of the membrane and can effectively measure the transmembrane current. The spacing and size of the electrodes should be designed according to parameters such as the thickness of the membrane and the membrane porosity to ensure accurate measurement and reduce noise and interference.

[0097] The function of the signal converter is to convert the current signal measured by the electrochemical workstation into digital data and transmit these data to the data processing subsystem for further analysis.

[0098] Function of the signal converter: The signal converter not only amplifies the current signal but also has a function of filtering the noise of the signal 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 a digital system and transmit the data to the data processing subsystem through an appropriate transmission channel.

[0099] Data transmission and analysis: After converting the current signal into digital data, the signal converter transmits the data to the data processing subsystem. Through real-time calculation, the degree of membrane wetting can be deduced from this data. 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.

[0100] The close cooperation between the electrochemical monitoring subsystem and the membrane distillation subsystem is the key for the present invention to be able to monitor the degree of membrane wetting in real time. The membrane distillation subsystem sends the liquid into the membrane distillation membrane module 1 through the feed liquid loop, while the electrochemical monitoring subsystem monitors the current signal on both sides of the membrane through the electrode. The change in this current reflects the electrical characteristics of the membrane, and thus the degree of membrane wetting can be deduced.

[0101] Connection between the membrane distillation membrane module 1 and the electrochemical workstation: The membrane distillation membrane module 1 and the electrochemical workstation are connected through the electrode. The electrode reflects 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.

[0102] Based on the current signal collected by the electrochemical monitoring subsystem, the data processing subsystem calculates the degree of membrane wetting through a mathematical model. This model can accurately characterize the wetting state of the membrane through the normalization process of the impedance signal.

[0103] Relationship between impedance normalization and membrane wetting index: According to the formula in the disclosure, there is a clear mathematical relationship between the normalized impedance and the dynamic wetting index. Specifically, the dynamic wetting index can be calculated by the following formula:

[0104] ;

[0105] where is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt rejection rate, is the normalized impedance, is the initial impedance.

[0106] Please refer to the appendix Figure 7, a data processing subsystem, is connected to the electrochemical workstation 8 and receives electrical signals, for processing the electrical signals and calculating the membrane wetting degree in real time;

[0107] The data processing subsystem includes:

[0108] A data acquisition module, for receiving the electrical signals transmitted by the electrochemical monitoring subsystem, and performing preliminary data filtering and preprocessing;

[0109] A signal processing module, for calculating the real-time impedance data of the membrane according to the electrochemical signal, and performing fitting processing through an equivalent circuit model;

[0110] A calculation module, for calculating the membrane wetting index according to the impedance data and a mathematical model, and obtaining the real-time monitoring result of the membrane wetting state;

[0111] A display and feedback module, for displaying the calculated membrane wetting index to the operator, and performing operation prompts or working condition adjustments through an automated system.

[0112] In this embodiment, the data processing subsystem closely cooperates with the electrochemical workstation 8, receives and processes the electrical signals from the electrochemical monitoring subsystem, and calculates the wetting degree of the membrane in real time. This subsystem consists of multiple key modules, each module undertaking different functions, ensuring the 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 precise control of the membrane distillation process, ensuring that problems such as over-wetting or drying up do not occur during the operation of the membrane module.

[0113] The main function of the data acquisition module is to receive the electrical signals from the electrochemical monitoring subsystem. Since during the membrane distillation process, the electrochemical signals will be affected by noise and external interference, preliminary signal filtering and preprocessing must be carried out. The data acquisition module will filter out noise, enhance the signal, 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.

[0114] The signal processing module calculates the real-time impedance of the membrane by analyzing the electrochemical signal. This process is based on the electrical characteristics of the membrane, and an equivalent circuit model is used to describe the behavior of the membrane. The change of the electrical characteristics of the membrane during the wetting process can be represented by the following circuit model: The electrical characteristics of the membrane are divided into a wetting area and an unwetted area, and these two areas are represented by different resistors and capacitors respectively.

[0115] The calculation formula for the resistance of the wetting area is:

[0116]

[0117] Among them, the parameters in the formula are defined as follows:

[0118] is the resistance of the wetting zone (Ω), representing the resistance of the liquid-permeated part in the membrane pores.

[0119] is the solution resistivity (Ω·m), used to describe the conductivity of the solution.

[0120] is the thickness of the wetting layer (m), referring to the thickness of the part in the membrane pores permeated by the liquid.

[0121] is the porosity.

[0122] is the effective area of the membrane (m 2 ), that is, the area of the membrane surface in contact with the liquid.

[0123] The calculation formula for the wetting zone capacitance is:

[0124] ;

[0125] Among them, the parameters in the formula are defined as follows:

[0126] is the wetting zone capacitance (F), representing the capacitance of the liquid-permeated part in the membrane pores.

[0127] is the permittivity of vacuum, with a value of 8.854×10 −12 F / m.

[0128] is the relative permittivity of water (80), used to describe the response ability of water to the electric field.

[0129] is the effective area of the membrane (m 2 ).

[0130] is the thickness of the wetting layer (m).

[0131] The calculation formula for the non-wetting zone capacitance is:

[0132] ;

[0133] Among them, is the non-wetting zone capacitance, is the relative permittivity of air, is the total thickness of the membrane.

[0134] Through these calculation formulas, the signal processing module can obtain the resistance, capacitance, and impedance data of the membrane, providing the necessary data support for the subsequent calculation of the membrane wetting index.

[0135] The calculation module calculates the dynamic wetting index by analyzing the impedance data provided by the signal processing module and combining it with a mathematical model. . There is a clear mathematical relationship between the dynamic wetting index and the impedance of the membrane. The specific calculation formula is:

[0136] ;

[0137] Where, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt rejection rate, Normalized impedance, is the initial impedance.

[0138] This formula accurately calculates the degree of membrane wetting through the relationship between the normalized impedance and the dynamic wetting index. The calculation module provides the wetting index calculated in real time to the display and feedback module.

[0139] 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 timely understand the working state of the membrane. In addition, this module is also integrated with the automated control system. When the membrane wetting index reaches the set threshold, the system can automatically adjust, such as adjusting the flow rate, temperature, etc. of the feed liquid, so as to ensure that the wetting state of the membrane is always maintained within the ideal range.

[0140] Derivation of equivalent circuit parameters and further relationships

[0141] In this embodiment, the derivation process of the equivalent circuit is crucial for understanding the electrical characteristics of the membrane. To accurately describe the wetting state of the membrane, especially the electrical characteristics of the wetting layer and the non-wetting layer, we adopted a hierarchical RC circuit model. This model simulates the different states (wetting and non-wetting) of the membrane pore region during the membrane distillation process through the series and parallel structures of resistors and capacitors. Through this model, we can accurately calculate the electrical characteristics of the membrane and further obtain the dynamic wetting index. The following is the detailed derivation process:

[0142] 1. Circuit models of the wetting area and the non-wetting area

[0143] During the membrane distillation process, the wetting degree of the membrane surface directly affects the electrical characteristics of the membrane. For the electrical behavior of the membrane, we can describe the resistance and capacitance characteristics of its different regions through a hierarchical RC circuit. Specifically, the electrical characteristics of the membrane can be divided into two regions:

[0144] Wetting region: The region of the membrane pores where liquid penetrates. The resistance and capacitance of this region are mainly determined by factors such as the resistivity of the solution, the pore structure of the membrane, and the electrical constants of the membrane.

[0145] Non-wetting region: The region of the membrane pores that is not penetrated by liquid. This region is mainly filled with air, and its electrical properties are significantly different from those of the wetting region, showing higher resistance and corresponding capacitance.

[0146] 2. Derivation of the total capacitance

[0147] The electrical properties of the membrane can be described by its total capacitance We need to derive the capacitance of each region separately and then combine them. First, the capacitances of the wetting and non-wetting regions are derived from their resistance and capacitance models (RC circuits).

[0148] Capacitance of the wetting region Determined by the wetting part of the membrane and can be expressed as:

[0149] ;

[0150] Where is the resistance of the wetting region, is the resistivity of the solution, is the thickness of the wetting layer, is the porosity, is the effective area of the membrane, is the capacitance of the wetting region, is the vacuum permittivity, is the relative permittivity of water.

[0151] Capacitance of the non-wetting region Determined by the air filling in the non-penetrated part and can be expressed as:

[0152] ;

[0153] Where is the capacitance of the non-wetting region, is the relative permittivity of air, is the total thickness of the membrane.

[0154] Total capacitance Is a parallel and series combination of the capacitances of the wetting and non-wetting layers. By analyzing the relationship between these two capacitances, we obtain the expression for the total capacitance.

[0155] 3. Derivation of the total capacitance formula

[0156] According to the series and parallel relationships of capacitances, the total capacitance It can be expressed by the following formula:

[0157]

[0158] This formula represents the weighted combination of the capacitances of the wetting area and the non - wetting area. Among them, the capacitance of the wetting area is proportional to the thickness of the wetting layer of the membrane, while the capacitance of the non - wetting area is proportional to the total thickness of the membrane minus the thickness of the wetting layer.

[0159] By rearranging the formula, we obtain the final expression for the total capacitance of the membrane:

[0160] ;

[0161] This formula provides us with a method for calculating the total capacitance of the electrical properties of the membrane, which takes into account both the degree of wetting of the membrane and the electrical properties of the membrane material.

[0162] 4. Derivation of the Impedance and Normalized Impedance in the High - Frequency Region

[0163] Under high - frequency conditions, the impedance of the membrane exhibits a certain frequency dependence. At higher frequencies, the impedance of the membrane can be expressed by the following formula:

[0164]

[0165] Among them, is the impedance in the high - frequency region, is the electrical frequency.

[0166] Next, we define the normalized impedance to represent the relative change in the impedance of the membrane, which helps to quantitatively analyze the state of the membrane. The expression for the normalized impedance is:

[0167]

[0168] Among them, is the initial impedance, which reflects the impedance of the membrane in the non - wetting state.

[0169] 5. Derivation of the Wetting - Layer Thickness Ratio

[0170] Through the normalized impedance, we can further derive the wetting - layer thickness ratio , which is an important index of the degree of membrane wetting. The wetting - layer thickness ratio can be expressed as:

[0171]

[0172] Among them, represents the ratio of the wetting layer of the membrane to the total thickness, is the impedance at the current moment, is the initial impedance, and are the relative permittivities of water and air, respectively.

[0173] 6. Derivation of the dynamic wetting index

[0174] To further quantitatively describe the degree of membrane wetting, we defined a dynamic wetting index in combination with the salt rejection rate , which comprehensively considers the impedance change and salt rejection ability of the membrane. The formula for the dynamic wetting index is:

[0175] ;

[0176] where, is the initial impedance, is the weight coefficient (usually between 0.1 and 0.3, obtained through experimental calibration), is the dynamic salt rejection rate, is the initial salt rejection rate.

[0177] 7. Quantification model of wetting degree

[0178] Based on the above formula, we constructed a quantification model for the membrane wetting degree. According to the wetting layer thickness ratio and the dynamic wetting index , the wetting state of the membrane is divided into the following levels:

[0179] When < 0.05 and < 0.06, it is determined as the non-wetting state;

[0180] When 0.05 ≤ < 0.3 and 0.06 ≤ < 0.4, it is determined as surface wetting;

[0181] When 0.3 ≤ < 0.95 and 0.4 ≤ < 0.9, it is determined as partial wetting;

[0182] When ≥ 0.95 and ≥ 0.9, it is determined as complete wetting.

[0183] A method for on-line detecting the wetting degree of the membrane in membrane distillation described below and a system for on-line detecting the wetting degree of the membrane in membrane distillation described above can be referred to each other correspondingly.

[0184] Please refer to Appendix Figure 8 , a method for on-line detecting the wetting degree of the membrane in membrane distillation, comprising the following steps:

[0185] 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 in contact with the electrodes to measure the current signal;

[0186] S2. Regulate the temperature, concentration, and flow rate of the feed liquid and the distillate to bring the membrane distillation system into a stable operating state. The temperature of the feed liquid is 55 - 85 °C, the concentration is a 25 - 40% NaCl solution, and the distillate is pure water at a temperature of 5 - 25 °C;

[0187] S3. Use the electrochemical monitoring system to measure the transmembrane current change in real time and transmit the electrical signal to the data processing subsystem;

[0188] S4. The data processing subsystem processes the electrical signal in real time and uses an equivalent circuit model to deduce the wetting degree of the membrane;

[0189] S5. Monitor the wetting degree of the membrane according to the membrane wetting index calculated in real time and adjust the membrane distillation process.

[0190] The equivalent circuit model in step S4 includes:

[0191] S4.1. During 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 respectively:

[0192]

[0193] ;

[0194] Among them, is the resistance of the wetting zone, is the resistivity of the solution, is the thickness of the wetting layer, is the porosity, is the effective area of the membrane, is the capacitance of the wetting zone, is the vacuum permittivity, is the relative permittivity of water;

[0195] S4.2. The expression for the capacitance of the non-wetting zone is:

[0196] ;

[0197] Among them, is the capacitance of the non-wetting zone, is the relative permittivity of air, is the total thickness of the membrane;

[0198] S4.3. The expression for the total capacitance is:

[0199]

[0200] After arranging, we get:

[0201] ;

[0202] Among them, is the total capacitance;

[0203] S4.4. The impedance expression in the high-frequency region is approximately:

[0204]

[0205] Among them, is the impedance in the high-frequency region, is the electrical frequency;

[0206] S4.5. Define the normalized impedance:

[0207]

[0208] Among them, is the normalized impedance, is the initial impedance;

[0209] S4.6. Solve the equation to obtain the wetting layer thickness ratio:

[0210]

[0211] Among them, is the wetting layer thickness ratio;

[0212] S4.7. Combine the salt rejection rate to define the dynamic wetting index:

[0213] ;

[0214] Among them, is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt rejection rate.

[0215] The steps S4.6 and S4.7 together constitute a wetting degree quantification model, which is:

[0216] When < 0.05 and < 0.06, it is determined to be in the non-wetting state;

[0217] When 0.05 ≤ < 0.3 and 0.06 ≤ < 0.4, it is determined to be surface wetting;

[0218] When 0.3 ≤ <0.95 and 0.4 ≤ <0.9, it is determined as partial wetting;

[0219] When ≥0.95 and ≥0.9, it is determined as complete wetting.

[0220] The method of this embodiment can be used to implement the above system embodiment, and their principles and technical effects are similar, so they will not be elaborated here.

[0221] Example 1:

[0222] Install the membrane distillation subsystem device: Place the PVDF microporous hydrophobic membrane with a pore size between two stainless-steel electrodes for measuring the current signal. The device is as shown in Figure 1 , where plastic gaskets are placed between the electrodes and the membrane to provide insulation. The entire assembly is wrapped with a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. Inject a 30% NaCl solution at 50 °C on the feed liquid side and pure water at 20 °C on the distillate side. Start the system to form cross-flow filtration of the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation of model CS315, with a working frequency of 1000 kHz, and uses a working voltage of 5 mV for impedance testing. After stable operation for 24 hours, extract and calculate the data at this moment, The test results are as shown in Figure 2 . Substitute the above data into the formula:

[0223]

[0224] The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as = 0.04, = 0.0402, and it is determined as surface wetting.

[0225] Example 2:

[0226] Install the membrane distillation subsystem device: Place the PVDF microporous hydrophobic membrane with a pore size between two stainless-steel electrodes for measuring the current signal. The device is as shown in Figure 1 , where plastic gaskets are placed between the electrodes and the membrane to provide insulation. The entire assembly is wrapped with a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. Inject a 35% NaCl solution at 60 °C on the feed liquid side and pure water at 20 °C on the distillate side. Start the system to form cross-flow filtration of the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation of model CS315, with a working frequency of 1000 kHz, and uses a working voltage of 5 mV for impedance testing. After stable operation for 24 hours, extract and calculate the data at this moment, The test results are asFigure 3 as shown. Substitute the above data into the formula:

[0227]

[0228] The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as =0.665, =0.67, and it is determined to be partially wetted.

[0229] Comparing with Example 1, it can be obtained that after the temperature and concentration of the feed liquid increase, the membrane is more likely to be wetted.

[0230] Example 3:

[0231] 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 Figure 1 shown, where plastic gaskets are placed between the electrodes and the membrane to provide insulation. The entire assembly is wrapped by a cylindrical insulating tube and fixed with clamping bolts to ensure electrical insulation and structural stability. Inject a 30% NaCl solution at 50 °C on the feed liquid side and pure water at 20 °C on the distillate side. Start the system to form cross-flow filtration of the feed liquid and the distillate. The electrochemical monitoring subsystem uses an electrochemical workstation of model CS315 with a working frequency of 1000 kHz and an impedance test is carried out using a working voltage of 5 mV. After stable operation for 24 hours, extract and calculate the data at this moment, Substitute the above data into the formula:

[0232]

[0233] The wetting layer thickness ratio and dynamic wetting index at this moment are obtained as =0.04, =0.397, and it is determined to be non-wetted.

[0234] Comparing with Example 1, it can be obtained that after replacing the stainless steel electrode with a carbon cloth electrode in the experiment, the dynamic response of the membrane wetting process does not change significantly. However, due to the porous structure of the carbon cloth electrode, water absorption and swelling occur in the liquid phase environment, and the mechanical strength is low. The impedance signal fluctuation amplitude in the high-frequency region exceeds ±15%, and the stability is lower than that of the stainless steel electrode.

[0235] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made in these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An online system for detecting the degree of membrane wetting in membrane distillation, characterized in that Comprising: A membrane distillation subsystem for providing a membrane distillation separation process to form the flow of the feed liquid and the distillate; An electrochemical monitoring subsystem for continuously measuring the change of the current signal during the membrane distillation process and outputting an 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 degree of membrane wetting in real time; The data processing subsystem includes a display and feedback module for displaying the calculated membrane wetting index to the operator, and performing operation prompts or condition adjustments through an automation system, which is integrated with the automation control system. When the membrane wetting index reaches the set threshold, the system can automatically adjust; Wherein, the calculation of the degree of membrane wetting is carried out by real-time processing of the electrical signal, using an equivalent circuit model to deduce the relationship between the normalized impedance of membrane wetting and the dynamic wetting index, and calculating the degree of membrane wetting through this relationship. The equivalent circuit is a layered structure including a wetting area and an unwetted area, where: The equivalent circuit of the wetting area consists of a first capacitor in parallel with a first resistor to represent the membrane pore area penetrated by the liquid; The equivalent circuit of the unwetted area is composed of the second capacitor independently, representing the pore area of the unpenetrated membrane containing air; The equivalent circuit model includes: S4.

1. During the membrane distillation process, the electrical characteristics of the membrane can be described by an equivalent layered RC circuit, where the expressions of the resistance and capacitance are respectively: ； Among them, is the resistance of the wetting area, is the resistivity of the solution, is the thickness of the wetting layer, is the porosity, is the effective area of the membrane, is the capacitance of the wetting area, is the vacuum permittivity, is the relative permittivity of water; S4.

2. The capacitance expression of the unwetted area is: ； Among them, is the capacitance of the unwetted area, is the relative permittivity of air, is the total thickness of the film; S4.

3. The total capacitance expression is: After arrangement: ； Among them, is the total capacitance; S4.

4. The impedance expression in the high-frequency region is: Among them, is the impedance in the high-frequency region, is the electrical frequency; S4.

5. Define the normalized impedance: Among them, Normalized impedance, is the initial impedance; S4.

6. Solve the equation to obtain the wetting layer thickness ratio: Among them, is the wetting layer thickness ratio; S4.

7. Combine the salt rejection rate to define the dynamic wetting index: , where is the initial salt rejection rate, is the dynamic wetting index, is the weight coefficient, is the dynamic salt rejection rate.

2. The system for on-line detecting the membrane wetting degree in membrane distillation according to claim 1, wherein, The electrochemical monitoring subsystem includes: An electrochemical workstation (8) for measuring the transmembrane current on both sides of the membrane during the membrane distillation process and converting it into an electrical signal. The electrochemical workstation is connected to the electrode through a wire; Electrodes, including a working electrode and a counter electrode. The working electrode and the counter electrode 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; A signal converter for converting the current signal obtained by the electrochemical workstation (8) into data and transmitting it to the data processing subsystem for analysis.

3. The system for on-line detecting the membrane wetting degree in membrane distillation according to claim 2, wherein, The membrane distillation subsystem includes: A feed liquid loop for circulating the feed liquid and controlling the flow of the feed liquid; A membrane distillation membrane module (1) including a microporous hydrophobic membrane and connected to the electrodes of the electrochemical workstation (8) for measuring the current signal; A distillate loop for circulating the distillate and maintaining the temperature difference on both sides of the membrane.

4. An on-line detection system for the degree of membrane wetting in membrane distillation according to claim 3, characterized in that, The feed liquid loop includes a feed water pump (2). One end of the feed water pump (2) is connected to the membrane distillation membrane module (1) through a pipeline. The other end of the membrane distillation membrane module (1) is connected to a stirrer (3) through a pipeline. The stirrer (3) is connected to a heater (5) through a pipeline. The heater (5) is connected to the membrane distillation membrane module (1) through a pipeline.

5. An on-line detection system for the degree of membrane wetting in membrane distillation according to claim 3, characterized in that, The distillate loop includes a feed pump (2). One end of the feed pump (2) is connected to the membrane distillation membrane module (1) through a pipeline, and the other end of the feed pump (2) is connected to the feed tank through a pipeline. A feed tank scale (4) is provided at the bottom of the feed tank, and a conductivity probe (7) is arranged in the feed tank. The membrane distillation membrane module (1) is connected to a cooler (6) through a pipeline, and the cooler (6) is connected to the feed tank through a pipeline.

6. The system for on-line detecting the membrane wetting degree in membrane distillation according to claim 1, characterized in that The data processing subsystem includes: A data acquisition module, which is used to receive the electrical signals transmitted by the electrochemical monitoring subsystem and perform preliminary data filtering and preprocessing; A signal processing module, which is used to calculate the real-time impedance data of the membrane according to the electrochemical signals and perform fitting processing through an equivalent circuit model; A calculation module, which is used to calculate the membrane wetting index according to the impedance data and a mathematical model to obtain the real-time monitoring result of the membrane wetting state.

7. A method for online detecting the degree of membrane wetting in membrane distillation, characterized in that, It is used to assist the use of a system for on-line detecting the degree of membrane wetting in membrane distillation according to any one of the above claims 1-6, and includes the following steps: S1. Install the membrane distillation subsystem device, place the microporous hydrophobic membrane between two stainless steel electrodes, ensure that both sides of the membrane are in contact with the electrodes respectively, and measure the transmembrane current; S2. Regulate 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 temperature of the feed liquid 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. Real-time measure the change of the transmembrane current through the electrochemical monitoring system and transmit the electrical signals to the data processing subsystem; S4. The data processing subsystem processes the electrical signals in real time and uses an equivalent circuit model to deduce the degree of membrane wetting; S5. Monitor the degree of membrane wetting according to the real-time calculated membrane wetting index and adjust the membrane distillation process.

8. A method for on-line detecting the degree of membrane wetting in membrane distillation according to claim 7, characterized in that, The wetting layer thickness ratio and the dynamic wetting index jointly constitute a wetting degree quantification model, which is: When <0.05 and <0.06, it is determined as the unwetted state; When 0.05 ≤ <0.3 and 0.06 ≤ <0.4, it is determined as surface wetting; When 0.3 ≤ <0.95 and 0.4 ≤ <0.9, it is determined as partial wetting; When ≥ 0.95 and ≥ 0.9, it is determined as complete wetting.

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