Membrane Net Charge Density Measurement via Pressure-Flow Equilibrium
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Solution Overview
Problem
Current methods for measuring net charge density in nanofiltration membranes are unreliable due to overlapping electric double layers and varying results from different estimation methods, especially for membranes with small pore diameters, leading to inaccuracies in filtration performance analysis.
Innovation Solution
A method and apparatus utilizing the equilibrium condition between applied mechanical pressure difference and electroosmotic flow, or the relation between mechanical pressure difference and streaming potential, to accurately measure net charge density per unit volume of membranes, including microfiltration, ultrafiltration, and nanofiltration membranes, by calculating the product of porosity and average electrical conductivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the streaming potential method or electroosmosis method is used to measure zeta potential, then the charge parameters can be obtained, but the net charge density inside the membrane remains uncertain due to overlapping electric double layers in small pore diameter membranes
Solution Approach 1:
The patent replaces the electrical measurement method (streaming potential/electroosmosis) with a mechanical measurement approach. By applying a mechanical pressure difference across the membrane and measuring the resulting volumetric flow rate, the net charge density is calculated through a mechanical-equilibrium-based formula that avoids the electric field measurement problems in small pores.
Solution Approach 2:
The patent introduces an intermediary parameter (volumetric flow rate under pressure difference) to indirectly determine the net charge density. Instead of directly measuring the difficult-to-obtain zeta potential or electrical parameters, the method uses the measurable volumetric flow rate as an intermediary to calculate the charge density through the relationship: net charge density = (volumetric flow rate × membrane resistance) / (pressure difference × porosity).
2Measurement precision
If estimation methods using theoretical simulation accompany experimental results are used, then net charge density per unit volume can be acquired, but the results vary significantly depending on the model used (e.g., Speigler-Kedem model, Donnan Steric Pore Model)
Solution Approach 1:
The patent makes the measurement method self-service by directly obtaining the net charge density from the membrane's own response to a applied pressure difference. The volumetric flow rate measured under pressure difference, combined with the membrane's resistance and porosity, provides a direct calculation of net charge density without requiring external theoretical models or assumptions about ion transport mechanisms.
Solution Approach 2:
The patent changes the measurement parameter from electrical parameters (zeta potential, streaming potential) to mechanical parameters (volumetric flow rate, pressure difference). This parameter change eliminates the dependency on theoretical models because the mechanical measurement directly reflects the membrane's charge properties without requiring interpretation through different theoretical frameworks.
3Manufacturing precision
If the pore diameter is very small in nanofiltration membranes, then the filtration performance can be improved, but the overlapping of electric double layers becomes serious and causes difficulty in measuring the real zeta potential
Solution Approach 1:
The patent substitutes the electrical measurement system with a mechanical measurement system. By using pressure difference and volumetric flow rate measurements, the method avoids the electric field measurement problems that arise from electric double layer overlapping in small pores, while still accurately determining net charge density.
Solution Approach 2:
The patent segments the measurement process into independent measurable quantities: pressure difference, volumetric flow rate, membrane resistance, and porosity. By breaking down the complex electrical measurement problem into these separate mechanical parameters, the method can accurately determine net charge density without being affected by electric double layer effects in small pores.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides a more reliable measurement of net charge density, reducing deviations and improving the accuracy of filtration performance analysis by directly measuring the net charge density per unit volume, even in membranes with small pore diameters.
Implementation Method 1
a mechanical pressure difference ΔP is applied across the membrane to generate permeate flow as well as streaming potential
Implementation Method 2
the key of the species rejection mechanism of a nanofiltration (NF) membrane is the electrostatic repulsion between species and the NF membrane
Implementation Method 3
an electric field is applied across the membrane to generate electroosmotic flow
Data Source
AI summary
The present invention provides a method for measuring net charge density of membrane and apparatus thereof. The method measures the net charge density of a membrane by utilizing the relation between the mechanical pressure difference applied across the membrane and the generated streaming potential or the relation between the applied electric field and the generated electroosmotic flow. The present invention also provides a method and an apparatus for measuring the resistance of a membrane.