52
D. Nedeljkovic
dioxide became one of the main research topics in modern chemical and environmental engineering [3, 4]. Current conventional processes are based on various techniques for the carbon dioxide separation that have significant disadvantages, either
due to the high energy demand (cryo-processes) or because of demand for costly
equipment and chemicals (chemical adsorption). Possible approach to the carbon
dioxide separation may be the separation membrane. The applicable membrane
should exhibit high permeability for the carbon dioxide and as low permeability as
possible for all other gasses commonly present in the flue gases (nitrogen, oxygen,
unburnt fuel, hydrogen). As this exclusion must be obtained on the molecular size,
standard porous membrane (the one that acts as a sieve) would not be suitable for this
application [5]. With the current technology, it is not possible to obtain industrially
feasible porous membrane that would separate the gas mixtures based on the size of
their molecules [6]. Additional problem is in the size of the molecules of interest as
the carbon dioxide molecule is larger and bulkier in comparison with hydrogen or
nitrogen molecule [7].
A good alternative is so-called non-porous, dense membranes whose separation
mechanism is based on the different solubility and diffusivity of the components of
the mixture in the bulk of the membrane. Mechanism of the separation is adsorption—
solution—diffusion. One component of the mixture is adsorbed on the surface of the
membrane; it is then dissolved in the bulk of the membrane and diffuses through
the membrane to the permeate side [8–10]. It was reported that the presence of the
ethylene-oxide as the repeating units of the polymer material improves the solubility
of the carbon dioxide in comparison to nitrogen and oxygen [11]. On the other hand,
polymer that contains only poly(ethylene oxide) (PEO) as the repeating unit, has a
strong affinity to crystallization, and therefore decrease permeation properties of the
membrane [12]. Therefore co-polymers of PEO with other polymers is a good choice
for this application.
One of the possible choices may be PEBAX, commercially available co-polymer
(supplier: Arkema) which is poly(amide-b-ether) by composition [13]. PEBAX
belongs to the group of the thermoplastic elastomers where polyamide block can be
composed of nylon-6 or nylon-12 (this block gives the stiffness to the membrane and
serves as the mechanical carrier) and polyether is soft, rubbery poly(tetramethylene
oxide), or standard poly(ethylene oxide) (this block is responsible for the diffusion
[14, 15]. Variation in composition and fraction between two blocks can tailor the
required properties of the membrane (both chemical and physical) [16]. Based on
the given facts, PEBAX appeared as a good candidate for the construction of the
membrane for the carbon dioxide separation [17–20].
As a second possibility, the co-polymer of poly(ethylene glycol) and poly(butylene
terephthalate) was taken. This polymer is produced and supplied by IsoTis OrthoBiologics under the commercial name of Polyactive. The molar mass of the copolymer
was 1500 g/mol, and it contained 77 mass% of PEG.
As the size and the mass of the carbon dioxide molecule is significantly higher than
the mass and size of the hydrogen molecule, and that hydrogen molecule is non-polar,
it is expected that the latter will have higher diffusion coefficient if temperature and
pressure of both the gases are same (as it is the case in the gas mixture). Therefore,
D. Nedeljkovic
dioxide became one of the main research topics in modern chemical and environmental engineering [3, 4]. Current conventional processes are based on various techniques for the carbon dioxide separation that have significant disadvantages, either
due to the high energy demand (cryo-processes) or because of demand for costly
equipment and chemicals (chemical adsorption). Possible approach to the carbon
dioxide separation may be the separation membrane. The applicable membrane
should exhibit high permeability for the carbon dioxide and as low permeability as
possible for all other gasses commonly present in the flue gases (nitrogen, oxygen,
unburnt fuel, hydrogen). As this exclusion must be obtained on the molecular size,
standard porous membrane (the one that acts as a sieve) would not be suitable for this
application [5]. With the current technology, it is not possible to obtain industrially
feasible porous membrane that would separate the gas mixtures based on the size of
their molecules [6]. Additional problem is in the size of the molecules of interest as
the carbon dioxide molecule is larger and bulkier in comparison with hydrogen or
nitrogen molecule [7].
A good alternative is so-called non-porous, dense membranes whose separation
mechanism is based on the different solubility and diffusivity of the components of
the mixture in the bulk of the membrane. Mechanism of the separation is adsorption—
solution—diffusion. One component of the mixture is adsorbed on the surface of the
membrane; it is then dissolved in the bulk of the membrane and diffuses through
the membrane to the permeate side [8–10]. It was reported that the presence of the
ethylene-oxide as the repeating units of the polymer material improves the solubility
of the carbon dioxide in comparison to nitrogen and oxygen [11]. On the other hand,
polymer that contains only poly(ethylene oxide) (PEO) as the repeating unit, has a
strong affinity to crystallization, and therefore decrease permeation properties of the
membrane [12]. Therefore co-polymers of PEO with other polymers is a good choice
for this application.
One of the possible choices may be PEBAX, commercially available co-polymer
(supplier: Arkema) which is poly(amide-b-ether) by composition [13]. PEBAX
belongs to the group of the thermoplastic elastomers where polyamide block can be
composed of nylon-6 or nylon-12 (this block gives the stiffness to the membrane and
serves as the mechanical carrier) and polyether is soft, rubbery poly(tetramethylene
oxide), or standard poly(ethylene oxide) (this block is responsible for the diffusion
[14, 15]. Variation in composition and fraction between two blocks can tailor the
required properties of the membrane (both chemical and physical) [16]. Based on
the given facts, PEBAX appeared as a good candidate for the construction of the
membrane for the carbon dioxide separation [17–20].
As a second possibility, the co-polymer of poly(ethylene glycol) and poly(butylene
terephthalate) was taken. This polymer is produced and supplied by IsoTis OrthoBiologics under the commercial name of Polyactive. The molar mass of the copolymer
was 1500 g/mol, and it contained 77 mass% of PEG.
As the size and the mass of the carbon dioxide molecule is significantly higher than
the mass and size of the hydrogen molecule, and that hydrogen molecule is non-polar,
it is expected that the latter will have higher diffusion coefficient if temperature and
pressure of both the gases are same (as it is the case in the gas mixture). Therefore,