Development Of Hollow Fiber Carbon Membranes From Poly (P-Phenylene Oxide) For Gas Separation
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Date
2018-04-01
Authors
Tamar Jaya, Muhammad Azan
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
Gas separation based on membrane technology has become more attractive in
chemical industry compared to conventional approaches such as cryogenic process and
pressure swing adsorption since membrane separations are energy-effective,
mechanically simpler system, and can be operated under lower pressures and
temperatures. Due to limited chemical resistance and physically unstability of current
polymeric membranes, carbon membranes were introduced. However, the current
development of carbon membranes have been too focus on novel material
development to achieve state-of-the-art separation efficiency. Another important and
critical factor is the continuity to further develop the membrane up to commercially
valuable which includes the membrane configuration, performance and synthesis
optimization and realistic separation tests, unfortunately, appeared very lacking in the
literature. This study attempts to fill in the gaps by attempting to continue the
development on a hollow fiber carbon membrane synthesized from poly(p-phynylene
oxide) (PPO) for H2/N2, O2/N2 and CO2/CH4 gas separations. The PPO membrane
(PPOM) was found to be acceptably thermostabilized at optimum temperature of 240°C. The thermostabilized PPOM was then subjected to various pyrolysis conditions,
which were pyrolysis temperature, heating rate, and thermal soak time to produce
carbon membranes. The carbon membrane performances were determined and
optimized based on their single gas permeabilities (H2, N2, O2, CH4, and CO2) and
ideal selectivities (H2/N2, O2/N2, and CO2/CH4) with Robeson’s 2008 upperbounds as
references. The permeabilities and ideal selectivity changed significantly when
different pyrolysis temperatures was applied. Based on the order of kinetic diameter
of the gases, the transport of the inert gases through the carbon membrane pyrolyzed
at 600 °C were dominated by molecular sieving mechanism and the presence of CO2
surface diffusion was detected through the carbon membrane pyrolyzed at 500 and 700
°C. This properties are directly related to microporous pore structure. Increasing the
heating rate increased the permeabilities of the gases and O2/N2 ideal selectivity, while
decreased the H2/N2 and CO2/CH4 ideal selectivities. Increasing the thermal soak time
slightly increased the H2 and CO2 permeabilities and H2/N2 and CO2/CH4 ideal
selectivities. However, it decreased the O2 permeability and O2/N2 ideal selectivity
indicating that every separation has different preference of membrane structure which
was changed through different pyrolysis formulation. Through the optimization using
one-factor-at-time and Robeson’s 2008 upperbound, the performances of H2-H2/N2,
O2-O2/N2 and CO2-CO2/CH4 were respectively found to be optimized at 1 °C/min (4
hours), 4 °C/min (0 hours) and 1 °C/min (4 hours). These optimized samples gave
averaged H2, O2 and CO2 permeabilities of 2868, 222 and 1205 Barrer, respectively
with corresponding H2/N2, O2/N2, and CO2/CH4 ideal selectivities of 586, 40 and 195,
respectively. The improvement on the carbon membranes after the optimization were
recorded as 80, 9 and 43 times of increment for H2, CO2 and O2 permeabilities and 13,
1.3 and 7 times of increment for H2/N2, CO2/CH4 and O2/N2 ideal selectivities,
respectively. The H2 permeability (268 Barrer) and H2/N2 permselectivity (13) from
binary gas test were lower than their single H2 permeability and H2/N2 ideal selectivity
due to high resistance by N2 and concentration polarization. The O2/N2 permselectivity
(13) was 70% lower than the O2/N2 ideal selectivity due to competitive gas transport
and concentration polarization. The O2 permeability (243 Barrer) from binary gas test
was almost equal to its O2 single permeability. The CO2 permeability (1320 Barrer)
and CO2/CH4 permselectivity from binary gas test were almost equal to their CO2
permeability and CO2/CH4 ideal selectivity obtained from the single gas test due to
strong CO2 affinity towards the carbon membrane pore walls and pore blocking effect.
The purities and recoveries of the H2, O2 and CO2 were 94% and 9%, 57% and 24%,
and 96% and 8%, respectively. This study has shown that, the carbon membrane has
been successfully synthesized and studied. The optimization implemented has
successfully multiplied the carbon membrane separation performance without the need
of complicated modifications or additional materials. The realistic separation
performance verified that the carbon membrane can indeed deliver high separation
efficiency for O2/N2 and CO2/CH4 based on ideal separation characteristics but very
poorly for H2/N2.