The Design of High Flux Nanoporous Carbon Membranes and Their Application in Small Gas Molecule Separations
Open Access
- Author:
- Merritt, Anna Roberta
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 29, 2007
- Committee Members:
- Henry C Foley, Committee Chair/Co-Chair
Elizabeth C Dickey, Committee Member
Janna Kay Maranas, Committee Member
Andrew Zydney, Committee Member - Keywords:
- carbon
molecular sieving
gas separation
membrane
nanoporous carbon
inorganic membrane
microporous
carbon membrane - Abstract:
- Size selective carbon membranes have begun to emerge as a potentially viable and promising technology for efficient and environmentally sound gas separations. The small pore size (4-5Å) and its narrow distribution in these nanoporous carbons (NPC) gives rise to a molecular sieving property which allows them to discriminate gas molecules on the basis of their size and shape. Relatively high selectivities can be obtained between gas molecules with sufficiently dissimilar features. In this work, the design of supported carbon membranes is engineered to maximize the properties of permeance and selectivity. High throughput, asymmetric carbon membranes are fabricated on a novel support composed of porous stainless steel filled with nanoparticles. The incorporation of nanofiller inside the steel support is shown to increase the single gas permeance by as much as 3 orders of magnitude over membranes fabricated on empty porous stainless steel supports. Importantly, this increase in gas permeance is achieved without loss of ideal selectivity. The increase in gas flux is attributed to a ~3 orders of magnitude decrease in the effective membrane thickness resulting from the smoother and more refined pore structure of the nanofiller modified support. The design of the selective layer is also examined in terms of the effect of the polymeric precursor solution properties. It is found that the properties of the coating solution such as the viscosity and surface tension can significantly influence the permeation properties of NPC membranes. Efforts to increase viscosity and decrease surface tension resulted in membranes which had higher permeance values due to the reduction of carbon membrane penetration inside the porous stainless steel support. Other design aspects of the separation layer such as the pyrolysis temperature and the effect of post-treatments and dopants are also explored. Finally, the application of these carbon membranes in a number of mixture separations is examined. Frequently, ideal (single) gas selectivities are reported for carbon membranes, however, the behavior of gas mixtures can significantly diverge from ideal values based on competitive adsorption effects and pore blocking. The permeation characteristics of high performance NPC membranes in the separation of gas mixtures significant to the air separation and the hydrogen economy are studied.