Carbonized PVDC Membrane for Hydrogen Separation
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Solution Overview
Problem
Current carbon molecular sieves (CMS) membranes formed from polyvinylidene chloride copolymers suffer from low hydrogen selectivity and large micropore sizes, making them ineffective for separating hydrogen from gas mixtures, particularly in syngas and olefin cracker gas streams.
Innovation Solution
A process involving the pyrolysis of polyvinylidene chloride copolymer membranes at specific temperatures and with controlled pretreatment to create a CMS membrane with high hydrogen permeance and selectivity, achieving a combination of at least 30 GPU hydrogen permeance and 200 hydrogen/methane selectivity, despite having average pore sizes larger than hydrogen and methane.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If polyvinylidene chloride copolymer membranes are pyrolyzed to form CMS membranes, then micropore structures are created for gas separation, but the micropore sizes become too large to achieve high hydrogen selectivity
Solution Approach 1:
The patent applies parameter changes by precisely controlling pyrolysis temperature (350-750°C), pretreatment temperature (100-180°C), and treatment time to achieve optimal micropore size distribution. By adjusting these parameters, the invention creates a narrow pore size distribution centered around 3-5 Å that enables high hydrogen selectivity while maintaining reasonable permeance.
Solution Approach 2:
The invention uses polyvinylidene chloride copolymer as a precursor material that transforms into a composite carbon structure during pyrolysis. The copolymer composition (containing 3-40 wt% comonomer) creates a composite microstructure with both micropores and an asymmetric barrier layer, combining the benefits of porosity and selective rejection.
2Productivity
If higher hydrogen permeance is achieved in CMS membranes, then hydrogen flux increases, but hydrogen selectivity decreases
Solution Approach 1:
The patent applies local quality by creating an asymmetric microstructure with different regions serving different functions: a microporous bulk structure for high hydrogen permeance and a dense asymmetric barrier layer at the retentate side for high hydrogen selectivity. This local differentiation allows simultaneous optimization of both permeance and selectivity.
Solution Approach 2:
The invention utilizes pressure-driven gas flow through the asymmetric membrane structure. The asymmetric configuration creates different flow paths for hydrogen versus larger molecules, with hydrogen preferentially passing through the micropores while larger molecules are rejected by the asymmetric barrier, achieving separation under pressure gradient conditions.
3Ease of manufacture
If conventional pyrolysis methods are used on PVDC copolymers, then carbon membranes are formed, but the membranes lack the narrow pore size distribution needed for effective hydrogen separation
Solution Approach 1:
The patent applies preliminary action by performing pretreatment at 100-180°C for 1-24 hours before pyrolysis. This preliminary step modifies the polymer structure and stabilizes it, creating a more uniform precursor that pyrolyzes into a membrane with narrower pore size distribution. The pretreatment ensures consistent micropore formation during subsequent pyrolysis.
Solution Approach 2:
The invention employs continuous heating at controlled rates (1-10°C/min) during pyrolysis to maintain uniform temperature distribution and consistent micropore formation throughout the membrane. This continuous, controlled process ensures uniform pore size distribution across the entire membrane structure.
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 resulting CMS membrane effectively separates hydrogen from gas mixtures with high selectivity, producing a permeate stream enriched in hydrogen and a retentate stream enriched in larger gas molecules, such as methane, through an asymmetric microstructure formation.
Implementation Method 1
flowing the gas mixture through the carbonized polyvinylidene chloride copolymer membrane to produce a first permeate stream having an increased concentration of hydrogen and a second retentate stream having an increased concentration of the larger gas molecule
Implementation Method 2
A process involving the pyrolysis of polyvinylidene chloride copolymer membranes at specific temperatures and with controlled pretreatment to create a CMS membrane
Implementation Method 3
The micropore size determines which gas in a gas mixture is adsorbed and which is not. Adsorption and desorption techniques may be alternated to carry out the separation
Data Source
AI summary
A process for separating hydrogen from a gas mixture having hydrogen and a larger gas molecule is comprised of flowing the gas mixture through a carbonized polyvinylidene chloride (PVDC) copolymer membrane having a hydrogen permeance in combination with a hydrogen/methane selectivity, wherein the combination of hydrogen permeance and hydrogen/methane selectivity is (i) at least 30 GPU hydrogen permeance and at least 200 hydrogen/methane selectivity or (ii) at least 10 GPU hydrogen permeance and at least 700 hydrogen/methane selectivity. The carbonized PVDC copolymer may be made by heating and restraining a polyvinylidene chloride copolymer film or hollow fiber having a thickness of 1 micrometer to 250 micrometers to a pretreatment temperature of 100° C. to 180° C. to form a pretreated polyvinylidene chloride copolymer film and then heating and restraining the pretreated polyvinylidene chloride copolymer film to a maximum pyrolysis temperature from 350° C. to 750° C.