Polyimide Hollow Fiber CMS Membrane Structural Collapse Prevention
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Solution Overview
Problem
Existing methods for producing carbon molecular sieve (CMS) membranes from polyimides often result in structural collapse during pyrolysis, leading to poor permeance and selectivity for gas separation, making them commercially impractical.
Innovation Solution
A method involving a dope solution of polyimide with a storage modulus minimum above 250°C, formed into hollow fibers and pyrolyzed in a non-oxidizing atmosphere, where the polyimide is a reaction product of specific dianhydrides and diamines with controlled rotational freedom, avoiding heat treatments prior to pyrolysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polyimides are pyrolyzed to form CMS membranes, then gas separation capability is improved, but structural collapse occurs leading to poor permeance
Solution Approach 1:
The patent applies parameter changes by carefully controlling pyrolysis temperature (500-800°C), atmospheric composition (inert gases like N2, Ar, or vacuum), and heating rate to prevent structural collapse while achieving the desired carbonization for gas separation. This resolves the contradiction by optimizing thermal processing parameters to maintain structural integrity during the transformation to CMS membrane.
Solution Approach 2:
The patent uses composite materials by combining polyimide precursors with specific additives or blending different polyimide compositions to enhance structural stability during pyrolysis. The resulting CMS membrane represents a composite structure that maintains the porous support framework while forming the dense separation layer, thereby preventing collapse and improving both strength and gas separation capability.
2Strength
If pyrolysis is performed in air atmosphere, then structural collapse is reduced, but oxidation occurs degrading membrane performance
Solution Approach 1:
The patent applies inert atmosphere by conducting pyrolysis in nitrogen, argon, or vacuum environments instead of air. This prevents oxidation of the polyimide precursor and the resulting CMS membrane structure, eliminating the harmful oxidative effects while maintaining structural stability through controlled thermal processing in the oxygen-free environment.
3Shape
If complicated pre-treatment methods are applied to prevent structural collapse, then membrane structure is preserved, but process complexity increases
Solution Approach 1:
The patent extracts or eliminates the need for complicated pre-treatment steps by directly pyrolyzing the polyimide hollow fiber under optimized conditions. Instead of applying multiple preparatory treatments, the invention removes unnecessary process steps while achieving structural preservation through careful control of pyrolysis parameters alone, thereby simplifying the overall manufacturing process.
4Reliability
If thin dense separating layer is formed, then selectivity is improved, but structural collapse during pyrolysis increases
Solution Approach 1:
The patent applies parameter changes by optimizing pyrolysis temperature, time, and atmospheric conditions to prevent the thin dense separating layer from collapsing during the thermal transformation. By carefully controlling these parameters, the invention maintains the integrity of the thin separation layer while achieving the carbonized structure necessary for high selectivity in gas separation.
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 prevents structural collapse, enhancing the selectivity and permeability of CMS membranes for gas pairs like hydrogen/ethylene, ethylene/ethane, and propylene/propane, resulting in improved gas separation performance.
Implementation Method 1
heating the polyimide hollow fiber in an atmosphere that is non-oxidizing to form the asymmetric hollow fiber carbon molecular sieve
Implementation Method 2
carbon molecular sieve (CMS) membranes for use in gas separation
Implementation Method 3
Gas transport through such membranes is commonly modeled by the sorption-diffusion mechanism
Data Source
Figure 1
Figure 2a~2b
Figure 3a~3b
AI summary
An asymmetric hollow fiber (CMS) carbon molecular sieve is made by providing a dope solution comprised of a polyimide and a solvent, at a temperature greater than 250°C that is less than the storage modulus at a temperature of 250°C, but no more than ten times less as measured using dynamic mechanical thermal analysis from 250°C to a temperature where the polyimide carbonizes. The polyimide is shaped into a hollow polyimide fiber, the solvent removed and the polyimide hollow fiber is heated to pyrolyze the polyimide and form the asymmetric hollow carbon molecular sieve. The asymmetric hollow fiber carbon molecular sieve has a wall that is defined by an inner surface and outer surface of said fiber and the wall has an inner porous support region extending from the inner surface to an outer microporous separation region that extends from the inner porous support region to the outer surface. Surprisingly, when the polyimide has the particular storage modulus characteristics, the method allows for the hollow fiber CMS to be made without any pretreatments or additives to inhibit structural collapse of the inner microporous region.