In the last few decades, a direct correlation between industrial greenhouse gas emissions and global temperature rise has been established. At the same time global energy demand continues to rise. A way to sustainably transport energy is to use hydrogen gas, which has a high energy storage capacity of 119 MJ/kg and produces only water upon combustion. Approximately 8.3×1011 m3 of hydrogen—carrying 6×1012 MJ of energy—is produced annually, with over 90% from fossil fuels (mainly methane and coal) or derivatives such as biomass. A much smaller fraction is produced using water electrolysis.
During steam cracking of natural gas (steam-methane reforming, SMR), methane is first reacted with water at ˜800° C. to produce CO and H2. Then the H2/CO feed is converted at about 350° C. into a mixture of H2 and CO2. Composition of output streams can vary depending on the specific method employed. A typical SMR plant produces a 75/20 H2/CO2 ratio with 5% methane and <1% other impurities. Integrated Gasification Combined Cycle (IGCC) plants can produce H2/CO2 ratios of 50/50.
Currently about 50% of hydrogen is used for the production of ammonia for use as fertilizer by the Haber process, while the remaining is employed in hydrocracking i.e. breaking large hydrocarbons into smaller ones for use as fuel. Smaller proportions are used for production of methanol, plastics, pharmaceuticals, hydrogenation of oils, desulfurization of fuels, etc. Hydrogen production is growing at 10% annually, but it is estimated that availability of lower-cost could immediately boost its use by 500 to 1000%.
The state-of-the-art technologies for H2 purification, i.e. cryogenic distillation and pressure swing adsorption, are energy intensive. This adds a significant cost for synthesized hydrogen estimated around 30% of total plant capital and operating cost. Estimates show that membrane-based H2/CO2 separation can reduce process costs up to 80% compared to distillation. Such debottlenecking of H2 production processes could enable the dream of a hydrogen-driven economy.
The USDOE lists membrane performance targets for hydrogen purification from syngas mixtures. See Table 1. A number of materials are being considered, including inorganics such as carbon molecular sieve, zeolite, and metal membranes, and glassy polymers such as polybenzimidazole and polyimides. The latter have been explored both in pristine form and with nanoparticles. The economic and environmental benefits of using membranes for H2/CO2 separations have been discussed by others arguing that use of membranes with high H2/CO2 selectivities (>10) can significantly reduce hydrogen production cost. Proteus™ by Membrane Technology & Research Inc. is a commercial membrane offering H2/CO2 selectivity of approximately 11 with H2 permeance of 500 GPU (1 GPU=10−6 cm3(STP) cm−2 s−1 cmHg) during 150° C. mixed-gas operation.
TABLE 1USDOE specified requirements for H2/CO2 membranesLow fabrication cost: approximately 100 USD/ft2 or lowerAbility to manufacture large membrane areas and modulesHigh operating temperature: 130-150° C. and aboveHigh pressure operability: 7 bar and aboveHigh hydrogen purity and recoveryHigh durability: around 5 yearsPerformance: H2 permeance > 200 GPUMixed-gas H2/CO2 selectivity @ 150° C. > 12 (IGCC operation)
Interfacial polymerization (IP) is a commercial method for fabricating thin-film composite (TFC) membranes. Pioneered by Cadotte (U.S. Pat. No. 4,277,344), IP has been employed in industry for decades to fabricate TFCs with polyamide active layers used for desalination by reverse osmosis (RO). These TFCs have a structure of partially cross-linked polyamide fabricated by reacting m-phenylenediamine (MPD) and trimesoyl chloride (TMC) on a microporous polysulfone support. The original membrane of this chemistry was named “FT-30”. This membrane and derivatives thereof are currently employed in more than 15,000 desalination plants, accounting for 90% of the global market.
In commercial settings, the FT-30-type TFC membranes are produced by impregnating (via dipping or spraying) a highly porous support material (usually polysulfone) with MPD dissolved in water. The support roll passes through a roller and is very briefly (less than 60 seconds) exposed to TMC dissolved in a hydrocarbon solvent (n-hexane or Isopar®). All solutions are at room temperature (20-25° C.). The membrane is then immediately exposed to high temperatures (≈20-100° C.) for drying and curing of the polyamide. All such membranes have been laboriously studied and reported in the literature with no useable gas separation properties for commercial separation processes.
Accordingly, it would be desirable to form a thin-film composite membrane with properties suitable for gas separations using fabrication methods that are energy efficient and low cost.