Salt Rejection Additives for Thin Film Composite Membranes
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Solution Overview
Problem
Reverse osmosis membranes, particularly thin film composite (TFC) membranes, face challenges in achieving high salt rejection due to hydrolysis of polyfunctional acyl halides like trimesoyl chloride, which reduces their effectiveness in desalinating seawater and brackish water.
Innovation Solution
Incorporating a salt rejection-enhancing additive, such as biguanide or dicarbonate compounds, during the interfacial polymerization process to form a complex with hydrolyzed polyfunctional acyl halides, thereby reducing their concentration and increasing crosslinking, resulting in membranes with enhanced salt rejection and lower permeability coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If polyfunctional acyl halide is used in interfacial polymerization to form TFC membranes, then membrane formation proceeds effectively, but hydrolysis of the acyl halide occurs reducing salt rejection
Solution Approach 1:
The patent converts the harmful hydrolysis products into beneficial components by adding polyol compounds that react with the hydrolyzed acyl halide to form crosslinks. This transforms the degradation pathway into a useful crosslinking mechanism that enhances membrane performance.
Solution Approach 2:
The polyol compound acts as an intermediary substance that mediates between the hydrolyzed acyl halide and the polymerization process. It captures the hydrolysis products and converts them into crosslinking agents, preventing them from negatively affecting salt rejection.
2Reliability
If stringent humidity controls are implemented to prevent hydrolysis, then salt rejection is maintained, but manufacturing complexity and cost increase
Solution Approach 1:
The membrane system becomes self-correcting by incorporating polyols that automatically react with hydrolysis products under normal manufacturing conditions. The system handles its own degradation issue without requiring external humidity control interventions.
Solution Approach 2:
The patent changes the chemical parameters of the polymerization system by adding polyol compounds with specific functional groups. This chemical modification allows the system to tolerate hydrolysis conditions that would otherwise be problematic, eliminating the need for strict humidity parameter control.
3Reliability
If TMC is purified prior to use to prevent hydrolysis, then salt rejection improves, but manufacturing time and cost increase
Solution Approach 1:
Instead of purifying TMC beforehand, the patent takes preliminary action by adding polyol compounds to the polymerization mixture that will neutralize hydrolysis products as they form. This prevents the need for time-consuming pre-purification steps.
Solution Approach 2:
The patent skips the purification step entirely by using a formulation approach where impurities (hydrolysis products) are converted into useful crosslinks in situ, allowing the process to move directly from mixing to polymerization without intermediate purification.
4Reliability
If crosslinking is increased to improve salt rejection, then membrane density increases but permeability decreases
Solution Approach 1:
The patent applies crosslinking locally at specific sites where hydrolysis products are generated, rather than uniform crosslinking throughout the membrane. This localized approach maintains overall membrane porosity while providing enhanced salt rejection at critical interfaces.
Solution Approach 2:
The patent creates a composite polymer structure combining polyamide from interfacial polymerization with polyol-derived crosslinks. This composite architecture provides both the dense barrier needed for salt rejection and the porous structure required for water permeability.
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
The membranes exhibit improved salt rejection characteristics, with rejection rates exceeding 99.8% and permeability coefficients less than 3×10−8, effectively purifying seawater and brackish water at pressures of 800 psi or less, while maintaining permeability.
Implementation Method 1
Incorporating a salt rejection-enhancing additive, such as biguanide or dicarbonate compounds, during the interfacial polymerization process to form a complex with hydrolyzed polyfunctional acyl halides
Implementation Method 2
the polyamide layer is formed by interfacial polymerization of a polyfunctional amine and a polyfunctional acid halide, creating a dense barrier layer in the polymer matrix
Implementation Method 3
hydrolysis of the polyfunctional acid halide readily occurs. Trimesoyl chloride (TMC) is a polyfunctional acid halide commonly used in the formation of thin film composite (TFC) membranes that has three acyl halide groups that readily hydrolyze in air
Implementation Method 4
Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure
Implementation Method 5
The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute
Data Source
AI summary
Provided is an interfacial polymerization process for preparation of a thin film composite membrane, which can be used for nanofiltration, forward osmosis, or reverse osmosis, particularly for use with brackish water or seawater. The process includes contacting a porous support membrane with an aqueous phase containing a polyamine to form a coated support membrane, and applying an organic phase containing a polyfunctional acyl halide to the coated support membrane to interfacially polymerize the polyamine and the polyfunctional acyl halide to form a discrimination layer of a thin film composite membrane, where during formation of the membrane, the polyfunctional acyl halide is purified in situ by removal of hydrolyzed acyl halide through addition of a salt rejection-enhancing additive that includes a biguanide compound, dicarbonate compound, pentathiodicarbonate compound, or salt thereof. Also provided are the membranes prepared by the methods and reverse osmosis modules containing the membranes.