Thesis - Oztek_Muzaffer_T_200508_MA

ecause of the difference in partial pressure of gas components on the two sides of the
membrane [3]. Similarly, in absorption processes the difference in solubility is the basis
of successful separation where a solvent preferentially dissolves one of the components
[4].
The recovery of H 2 or He from their mixtures is not a common task and has not
been investigated in detail. Existing studies that most commonly employ membrane
technologies, involve mixtures of hydrogen with gases other than helium. Successful
systems for separating hydrogen from methane or carbon dioxide at high temperatures
(450-900 ºC) in La 2 NiO 4 -zeolite membrane reactors are reported [5, 6]. Metals, alloys
and intermetallics are also used as H 2 selective membranes due to the high solubility of
H 2 in these materials that arises from the smaller size of H 2 . However, because H 2 and He
are so similar in size, most membranes have very close permeances for this gas pair [7].
For H 2 and He mixtures, the reported separation factor using a polymeric membrane is
4.4 at 308 K, where the separation factor is the ratio of the compositions in the enriched
stream relative to the ratio in the depleted stream [8]. A separation factor of 4.4 is a
relatively low ratio compared to other gas pairs of hydrogen. Therefore, membrane
separation of H 2 -He mixtures is not an efficient solution.
Materials such as graphite nanofibers, fullerenes, nanotubes, and hydrides that are
utilized for hydrogen storage and reversible sorption of hydrogen have potential for
application to the H/He separation processes. Nanofibers are reported to absorb hydrogen
to 0.6% w/w at most. Fullerenes have high reversible sorption capacities of 7.7% but
their use requires large engineering difficulties. These difficulties include high pressure
of activation (500-800 atm) and high operation temperatures of around 500 ºC [9].
3