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Advanced Materials Research Vol. 983 (2014) pp 246-250
Online available since 2014/Jun/30 at www.scientific.net
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.983.246
Gas Permeation Properties and Characterization of Polymer Based
Carbon Membrane
N. Sazali1,a, W.N.W. Salleh2,b, Zawati Harun1,c and A.F. Ismail2,d
1
Advanced Materials and Manufacturing Centre (AMMC Department of materials and Design
Engineering, Faculty of Maechanical and Manufacturing Engineering, Universiti Tun Hussein Onn
Malaysia, 86400 Parit Raja, Batu Pahat, Johor Darul Takzim, Malaysia.
2
Advanced Membrane Technology Research Centre (AMTEC), Faculty of Petroleum and
Renewable Energy Engineering (FPREE), Universiti Teknologi Malaysia, 81310 Skudai, Johor
Darul Takzim, Malaysia.
a
[email protected], [email protected], [email protected], [email protected]
Keywords: Polymeric precursor, heat treatment process, Permeation, carbonization, carbon
membrane, gas separation.
Abstract. Membrane gas separation is a forthcoming technology that advertised a great commercial
potential in diverse industrial applications. Consequently, membrane-based natural gas processing
has been among the fastest growing segments of the economic growth. The turbostratic structure of
carbon membranes has been affirmed to accommodate with good separation selectivity for
permanent gases. With that, the most auspicious technique acquired is by controlling the
carbonization temperature during the carbon membrane fabrication. In this study, polymer-based
carbon tubular membranes have been fabricated and characterized in terms of its structural
morphology and gas permeation properties. Polyimide (Matrimid 5218) was used as a precursor for
carbon tubular membrane preparation to produce high quality of carbon membrane via
carbonization process. The polymer solution was coated on TiO2 –ZrO2 tubular tubes (Tami) by
using dip-coating method. The polymer tubular membrane was then carbonized under Nitrogen
atmosphere at 600, 750, and 850 ◦C. The structural morphology of the resultant carbon membranes
was analyzed by means of scanning electron microscope (SEM). Pure gas permeation tests were
performed using CO2 and N2 gases at 8 bars and room temperature. Based on the results, the highest
CO2/ N2 selectivity of 79.53 was obtained for carbon membrane prepared at 850 oC.
Introduction
In the past recent years, many efforts have been made to develop effective ways to separate
the impurities in natural gases. Consequently, natural gas must designate cleansed to raise its fuel
heating cost, decrease transport expenses, pipeline erosion besides atmospheric contamination [1, 2].
High permeation flux and high selectivity are essential requirements for a successful membrane [3].
Several methods assumed towards development of polyimide membranes involve tailoring
molecular structure to achieve an innovative materials plus altering current polyimide materials
through cross-linking method, grafting side groups on polymer backbone also heat treatment
process [4].Various structures of polyimide have remained established in literatures thru changing
the monomer structures [5,6].
It is well known that the membrane performance appears to be a tradeoff between selectivity
and permeability, i.e. a highly selective membrane tends to have a low permeability [7]. Previous
researcher stated that when the driving force (pressure ratio) was lower, the selectivity results was
higher and the separation process said to be more appropriated; in fact, the operating costs for the
separation system also lower [8]. Previous membrane researchers mention that membranes which
have the potential to exceed such upper bound were inorganic membranes. Therefore,
ultramicroporous (0.3–0.5 nm) membranes such as zeolite and carbon membranes have shown their
performance [9]. The use of carbon membrane technology for the separation of CO2 from light
gases such as N2 is still in the research stage. In fact, the carbon membranes are seemly for CO2/N2
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Advanced Materials Research Vol. 983
247
separation [10]. Furthermore, former reports comprising the use of polymide precursor for carbon
membrane synthesis were also stated by Tanihara et al. [11], and Tin et al. [10].
Material and Method
In this study, a commercially available Matrimid 5218 was selected as a main precursor
polymer. It was dried overnight at 80 oC prior to be used. N-methyl-2-pyrrolidone (NMP) was
purchased by Merck (Germany) and used as solvent polymers. Methanol was used as a solvent
exchange during post treatment step for polymeric flat- sheet membranes. Matrimid 5218 was
prepared by dissolving 15 wt. % of Matrimid 5218 in N-methyl-2-pyrrolidone (NMP) for 7 hours
with mechanical stirring. The mixture was maintained under a controlled vacuum to remove all
bubbles from the solution. Polymer supported membranes were prepared by dip-coating a uniform
layer of the polymeric solution over the external surface of a tubular ceramic supported (1kD
membrane of 6cm in length x 13mm outer radii; Tami). The ceramic tube consisted of TiO2
structure that supported a ZrO2 membrane located on the inner part of the tube. The support was
dip-coating horizontally during the deposition of polymer solution. After 15 minutes coated, the
membranes were then aged at 80 oC for 24 hours. The membranes next immersed with methanol for
2 hours and then placed at 100 oC for 24 hours inside oven to allow slow removal of the solvent.
The same procedure was used to fabricate flat sheet membranes for characterization purposes.
The supported carbon membranes were prepared after the polymer supported membranes
had been placed inside Carbolite horizontal tubular furnace, where polymeric membranes were
placed in the center of the ceramic tube. The polymer carbonization was performed following a
temperature program up to 600, 750 and 850 oC. Normally, the final carbonization temperature will
be reached in several steps; the polymeric membranes were heated at 300 oC from room temperature
at a heating rate of 3 oC/min under Nitrogen gas (200 ml/min) flow. Subsequently the temperature
was raised to final carbonization temperature with the same heating rate. At 300 oC and final
carbonization temperature, the membrane were held for 30 minutes before proceed to the next step.
After completing each heating cycle, membranes were cooled naturally to room temperature. The
detailed carbonization protocol is illustrated in Fig 1. The nomenclature of the resultant carbon
tubular membranes is given in the form of CM-Carbonization Temperature.
Membranes Characterization
The cross section morphologies of the precursor membrane were observed under JEOL
JSM-5610LV scanning electron microscopy (SEM). The performance of the membrane can be
characterized into two important parameters which are permeance and selectivity.
Temperature, T (°C)
Temperature, T (°C)
700
600
500
400
300
200
100
0
0
50
100
150
Time, t (min)
Figure 1: Carbonization protocol
200
250
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Advanced Materials and Engineering
The carbon tubular membranes were tested in pure gas system. The 8 cm carbon tubular was
placed inside the tubular module. Pure CO2 and N2 were fed into the module at a trans membrane
pressure of 8 bars. A tubular stainless steel module of 14 cm in length was used to contain the
tubular ceramic membrane. The membrane was fitted with rubber O-rings that allowed the
membrane to be housed in the module without leakages. The permeance, P/l (GPU) and selectivity,
α of the membranes were calculated using equations below:
Permeance, P:
(P / l )i =
Qi
Q
=
∆p. A ηπDl∆P
(1)
Selectivity, α:
α A/ B =
PA ( P / l ) A
=
PB ( P / l ) B
(2)
Where P/l is the permeance of the tubular membrane, Qi is the volumetric flow rate of gas i
at standard temperature and pressure (cm3 (STP/s), p is the pressure difference between the feed
side and the permeation side of the membrane (cmHg), A is the membrane surface area (cm2), n is
the number of fibers in the module, D is an outer diameter of carbon tubular membrane (cm) and l is
an effective length of carbon tubular membrane (cm).
Result and Discussion
Gas Permeation properties.
Carbonization process plays an important role in tuning the gas permeation performance of
carbon membranes, while the carbonization temperature is one of the process parameters that
significantly affect the gas separation performance of carbon membranes. The carbonization process
was performed by heating the Matrimid-based polymeric membrane under Nitrogen flow from
room temperature to final carbonization temperature of 600, 750 and 850oC. The permeance was
measured using gas permeation test apparatus at 8 bars and room temperature. The different final
carbonization temperatures would result in different structure and permeation properties. The results
of the permeation performance of the carbon membrane prepared from different final carbonization
temperature shown in Table 1.
The permeance of CO2 will increase while N2 permeance significantly decreased with the
increase of final carbonization temperature. This indicates that pore and carbon structure of the
carbon membrane become rigid, compact and some of the pores might change into closed pores
during the carbonization process. Moreover, the CO2 permeance is consistently higher than that of
N2. With the expected of high temperature, it will induce the higher porosity. Polyimides are known
to exhibit high permselectivity for various gas pairs, especially for CO2 /N2 [13], and high chemical
resistance, thermal stability and mechanical strength [12]. Many researchers reported polyimide of
Matrimid 5218 as one of the best material choices for membrane based CO2 /N2 separation, due to
its attractive combination of gas permselectivity and high Tg [13, 14].
Advanced Materials Research Vol. 983
249
Table 1
Gas Permeation Properties of the Matrimid/NMP –Based Carbon Tubular Membrane.
Permeability (GPU)
Carbon Membrane
CM-600
CM-750
CM-850
CO2
81.29
364.30
400.06
Selectivity
CO2/ N2
10.09
64.41
79.53
N2
8.06
5.66
5.03
450
400
350
300
250
200
150
100
50
0
CO2 Permeance
N2 Permeance
600
750
9
8
7
6
5
4
3
2
1
0
N2 Permeance (GPU)
CO2 Permeance (GPU)
According to gas permeation results, it is indicates that the transport mechanisms of the prepared
carbon membrane are not dominant by Knudsen diffusion. The trend is indicative of molecular
sieving, as the kinetic diameter of CO2 is substantially smaller than N2. A continuous increase of
selectivity is observed with the rise of the carbonizations temperature from 600- 850 oC. The
selectivity of CO2/ N2 increased approximately 6 times from the carbon membrane carbonized from
600 to 750oC. The highest selectivity of 79.53 was achieved of carbon membrane carbonized at
850oC. The result shows that Matrimid–based carbon tubular membrane with more selective
behaviors’ can be obtained at high carbonization temperature. Figure 3 shows comparison between
CO2 permeance readings with N2 permeance readings. It was known that membranes that
carbonized with 600 oC will have big pore size; therefore N2 can go through more compared to the
carbon membranes with higher temperature. Therefore, when the porosity was low, it will limit the
N2 permeance. In the nutshell, to gain the highest separation efficiency of carbon tubular membrane
derived from Matrimid, the final carbonization temperature of around 750 to 850 oC are the best
conditions for CO2/ N2 separation at room temperature.
850
Temperature (°C)
Figure 3: Comparison between CO2 permeance readings with N2 permeance readings
Summary
From this study, the result shows that Matrimid 5218 is a good candidate precursor for
carbon membranes applied in gas separation system. The morphological structure properties
together with the gas permeation of the Matrimid-based polymeric and carbon membrane are
reported. It is indicated that the gas separation properties of Matrimid carbon membranes are
depends on the carbonization temperature during the heat treatment process. The results reveal that
an excellent CO2/ N2 separation of 79.53 was obtained for carbon membranes carbonized at 850oC.
It is because a high compactness of carbon membrane structure was produced at high temperature
and it leads to the increase in selectivity.
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Advanced Materials and Engineering
Acknowledgment
The authors gratefully acknowledge the financial support of the Research University Grant
Scheme (GUP) (Vot No: Q.J130000.2542.05H08) from The Ministry of Higher Education (MOHE)
to University Technology Malaysia (UTM).
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Advanced Materials and Engineering
10.4028/www.scientific.net/AMR.983
Gas Permeation Properties and Characterization of Polymer Based Carbon Membrane
10.4028/www.scientific.net/AMR.983.246
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