Facilitated transport of CO2 through various ion exchange membranes prepared by plasma graft polymerization

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 117 (1996) 251-260

Facilitated transport of C O 2 through various ion exchange membranes prepared by plasma graft polymerization Hideto Matsuyama *, Masaaki Teramoto, Hiroshi Sakakura, Kiyoshi Iwai Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Received 25 September 1995; accepted 20 February 1996

Abstract Ion exchange membranes were prepared by grafting acrylic acid (AA) and methacrylic acid to the substrates such as microporous polyethylene (PE) and polytetrafluoroethylene (PTFE), and homogeneous poly[1-(trimethylsilyl)-l-propyne] (PTMSP) by the use of the plasma graft polymerization technique. Experiments on the facilitated transport of CO 2 through the grafted membrane were carried out by using various diamines, diethylenetriamine and triethylenetetramine as the carrier. Among all the membranes tested, the AA grafted membrane containing ethylenediarnine as the cartier showed the highest selectivity of CO 2 over N 2. For example, when the CO 2 partial pressure in the feed phase was 0.047 atm, the selectivity reached more than 4000 with 1.0 × 10 -4 cm3/(cm 2 s cmHg) of CO 2 permeation rate. The use of PTFE and PTMSP substrates were effective for increasing the permeation rate of CO 2. The effects of CO 2 partial pressure in the sweep gas and temperature on the permeation rates and selectivities were also investigated. The CO 2 partial pressure in the sweep gas hardly influenced the membrane efficiencies up to 0.023 atm. Keywords: Facilitated transportofCO2; Plasma graft polymerization; Separation of CO2 over N2; Preparation of ion exchange membrane; Acrylic acid

1. Introduction Facilitated transport membrane involves a carrier mediated transport in addition to a permeant physical dissolution and diffusion. The existence of a carrier which can react reversibly with the permeant brings about high selectivity and also, usually, high permeability. This high selectivity and permeability makes the facilitated transport membrane very attractive for gas separation [1]. The facilitated transport membrane for gas separation was prepared at an early stage by impregnating

* Corresponding author.

the pores of a microporous support with the carrier solution. This type of membrane is known as a supported liquid membrane (SLM). Although remarkably high selectivities for gas separations have been reported for SLMs [2,3], the disadvantage of the membrane degradation due to the evaporation of the membrane solution a n d / o r the " w a s h o u t " of the carrier is well recognized. One of the methods for preventing this degradation is the use of an ion exchange membrane as the support for SLMs. LeBlanc et al. first reported the facilitated transport of CO 2 and C2H 4 using ion exchange membranes [4]. In CO z transport, a cationic carrier, monoprotonated ethylenediamine ( E D A H + ) , was immobilized

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H. Matsuyama et aL / Journal of Membrane Science 117 (1996) 251-260

in a cation exchange membrane by electrostatic force, which was considered to lead to the prevention of the washout of carrier to some extent. This work has generated considerable research interest in the facilitated transport using ion exchange membranes, and a number of studies were carried out with similar constructions [5-10]. The use of the gel-type swollen support membrane is another way to prevent the degradation of SLMs. Neplenbroek et al. reported the removal of nitrate ion from water using a poly(vinyl chloride) gelled membrane containing an organic solution [11]. They indicated that by applying the homogeneous gel network in the pores of the support, the mechanical stability against liquid displacement increased substantially. Bromberg et al. reported the transport of metals through a similar gelled supported liquid membranes [12]. Also for the facilitated transports of gases, studies using gel-type supports were reported [13,14]. In this case, the membrane solutions were aqueous solutions, and therefore hydrophilic gels such as poly(vinyl alcohol) were used. There are several important points in designing efficient mobile carrier transport membrane using an ion exchange substrate. The first is the mobility of the permeant in the membranes. The transport mechanism of the complex with the carrier through the ion exchange membrane is different from that through the usual mobile carrier membrane because in the former membrane the complex moves under electrostatic interaction with the ion sites on the membrane matrix. This interaction of the complex with ion sites usually leads to the lower mobility of the complex [5]. The mobility is related to the water content of the membrane, and the usage of the swollen membrane with a high water content is favorable to obtain a high mobility [7]. Some attempts have been done to improve the mobilities in Nation ® membrane by swelling them in hot glycerin [15,16]. The morphology of the ion exchange membrane such as ion exchange capacity and the distribution of the ion sites also affects the mobilities. The second is the enhancement of the permeation rate by the chemical reaction between the permeant and cartier. Usually, the high cartier concentration is favorable to obtain both high permeability and selectivity. When the ionic carrier is used just as in the case of ethylenediamine-mediated transport of CO 2,

the high ion exchange capacity is desirable to retain the ionic carrier in the membrane at high concentration. The equilibrium constant of the complex between the permeant and the carrier is also very important. When the carrier having an extremely large equilibrium constant is used, the sorption of the permeant at the feed side interface occurs easily, while the desorption at the sweep side interface becomes difficult, which results in a low permeation rate. A carrier having fast association and dissociation rate constants which are similar in magnitude is desirable. The third is the stability of SLMs. When the aqueous carrier solution is used, the hydrophilic water swollen membrane is suitable to retain the membrane solution. Further, the high ion site concentration is desirable to prevent the loss of the ionic carrier by the washout. In designing the membranes, these points must be considered. However, there have been very few studies focused on the development of new ion exchange membranes which can be used as the supports of the facilitated transport membranes for gas separations. In a previous work, we demonstrated that a highly swollen ion exchange membrane obtained by grafting acrylic acid onto a microporous polyethylene substrate using a plasma graft polymerization technique is useful as the support for the facilitated transport of CO 2 with monoprotonated ethylenediamine as the carrier [17]. The schematic mechanism of this facilitated transport of CO 2 is shown in Fig. 1. In the membrane, CO 2 reacts with EDAH + to form carbamate (NH~-(CH2)2NHCOO-), and C O 2

Membrane phase Feed side

NH3+(CH2) 2 NHCOO-

Sweep side

CO 2

2NHa+(CH2) 2NH2

Fig. 1. Schematic mechanism of facilitated transport of CO2 by monoprotonatedethylenediamineused as carrier.

253

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is transported in the carbamate form in addition to its physical diffusion. The obtained permeation rate of CO 2 and selectivity of CO 2 over N 2 were as high as 1.0 X 10 -4 c m 3 / ( c m 2 s cmHg) and 4700, respectively, when the CO 2 partial pressure in the feed gas was 0.047 atm. These results were attributable to the high ion exchange capacity (13.1 m e q / g - d r y grafted layer) and the high water content (80% in grafted layer) of the membrane. The newly prepared membrane was found to be stable as far as the feed and sweep gas streams were saturated with water. In this work, highly swollen ion exchange membranes having carboxyl groups were prepared by grafting methacrylic acid (MAA) as well as acrylic acid (AA) on to various microporous substrates such as polyethylene (PE) or polytetrafluoroethylene (PTFE) membrane, and homogeneous poly[1-(trimethylsilyl)-l-propyne] (PTMSP) substrate for the further improvement in the membrane efficiencies. These supports obtained were tested in the separation of CO 2 over N 2 using various amines as the carrier of CO 2. The purpose of this work is to clarify the effects of these experimental factors on the membrane efficiencies.

less than 0.13 Pa before plasma graft polymerization. Amines used as the carriers were ethylenediamine (guaranteed reagent grade), trimethylenediamine (extra pure reagent grade), tetramethylenediamine (guaranteed reagent grade), diethylenetriamine (extra pure reagent grade) and triethylenetetramine (reagent grade). All amines were used without further purification. Microporous membranes used as the substrates for the plasma graft polymerization are listed in Table 1. Microporous PE membranes with various diameters and porosities, and microporous PTFE membrane were used. Besides these porous substrates, homogeneous poly[1-(trimethylsilyl)- 1-propyne], which has the highest gas permeability among the polymers, was used as the substrate. Poly[1-(trimethylsilyl)-l-propyne] was synthesized from 1-(trimethylsilyl)-l-propyne with TaC15 catalyst. The synthesis method was the same as reported previously [18]. The polymer was identified by IR spectrum. Films were prepared by casting polymers from toluene solutions. The respective substrate membranes were abbreviated as described in Table 1. 2.2. M e m b r a n e preparation

2.

Experimental

2.1. Materials

Acrylic acid (Wako Pure Chemical Industries Ltd.) and methacrylic acid (Nacalai Tesque Inc.) used as the monomers were purified by distillation under vacuum, and their aqueous solutions were degassed by repeated freezing and thawing under a vacuum of

Table 1 Membranes used as substrate Materials Polyethylene Polyethylene Polyethylene Polyethylene Polytetrafluoroethylene a Tonen Chemical Co., Ltd. b Sumitomo Electric Co., Ltd.

Plasma graft polymerization was performed according to the literature [17,19,20]. First, the substrate membrane was treated by a glow-discharge argon plasma of the inductive coupling type, generated at a frequency of 13.56 MHz for 1 min at 67 Pa. Then the membrane was soaked under a vacuum in the aqueous monomer solution to graft the monomer onto the substrate membrane. The graft polymerization was carried out in a shaking bath for a pre-

Thickness (l~m)

Pore diameter (Ixm)

Porosity (%)

Abbreviation

13 30 30 25 60

0.03 0.02 0.02 0.02 0.10

60 40 37 30 57

PE1 a PE2 a PE3 a PE4 ~ PTFE b

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H. Matsuyama et al. / Journal of Membrane Science 117 (1996) 251-260

scribed time at 303 K. After graft polymerization, the membrane was washed and soaked in distilled water overnight to remove the residual monomer and the homopolymers. Then the membrane was dried in an oven at about 323 K. The amount of the graft polymer was determined by the weight change of the dry membrane before and after the polymerization. The degree of grafting was controlled by changing the grafting conditions such as grafting time and the monomer concentration. The water content of the grafted membrane were obtained by measuring the difference between the membrane weight in the water swollen condition and that in the dry condition. The grafted membrane was soaked overnight in the aqueous amine solutions of various concentrations adjusted to pH 11 in order to introduce the carriers into the membrane. For the purpose of obtaining the ion exchange capacity of the grafted membrane, the membrane soaked in KOH solutions with various concentrations was re-soaked in 0.1 m o l / d m 3 HC1 solution, and the K + concentrations released into HC1 solution were measured by an inductive coupled plasma emission spectrophotometer (Shimadzu Co., Ltd., ICPS 1000). Similarly, to obtain the cartier amine concentrations in the membrane, the membrane containing the amines was soaked in 0.1 m o l / d m 3 KOH solution, and the amine concentration released into the KOH solution was measured spectrophotometrically [21]. As described below, in the comparison of the membrane efficiencies with various kinds of amines, the experiments were carried out in conditions that the concentrations of the carriers in the membranes were the same as those of the ion sites. The thicknesses of wet and dry membranes were measured by a micrometer (Japan Micrometer MFG Co., Ltd., DMII). For the direct observation of the grafted membranes, the membranes were soaked in 1 g / d m 3 aqueous Methylene Blue solution [22] and the dried membranes were cut by microtome (Yamato Koki Co., Ltd., TU-213). The cross section of the grafted membranes were observed by a microscope (Olympas Co., Ltd., BH).

in the previous work [17]. The permeation cell consisted of two compartments (depth: 3.7 mm) for a feed and a sweep gas. The feed gas, a mixture of CO 2 and N 2 (N 2 flow rate:, 260 cm3/min) and the sweep gas, helium (flow rate: usually 112 cm3/min) were supplied to the cell at atmospheric pressure. Both gas streams were saturated with water upstream of the cells. The permeation rates for CO 2 and N 2 were calculated from the outlet sweep gas composition, which was analyzed by a gas chromatograph equipped with a thermal conductivity detector (Shimadzu Co., Ltd., GC-8APT), and the sweep gas flow rate. The measurements of the permeation rates were carried out in a constant temperature environment maintained at the desired temperature. Even when the sweep gas flow rate decreased to 50 cm3/min, the permeation rates of CO 2 and N 2 hardly changed. This means that the external mass transfer resistance in this system is negligibly small.

3. Results and discussion

3.1. Effects of substrates and amounts grafted Fig. 2 shows the relation between the acrylic acid (AA) amounts grafted and the grafting time in the cases using PE1 and PTFE substrates. The amount grafted was defined as the mass of the grafted layer per unit projected surface area of the substrate with the surface porosity included. The data using PE1 substrate were those already reported in our previous paper [23]. For PTFE substrate, the amounts grafted increased with the increase in both the grafting time

0.003

The apparatus used in the gas permeation experiments and the procedure were the same as described

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H. Matsuyama et al. / Journal of Membrane Science 117 (1996) 251-260

and the monomer concentration. The amounts grafted for PTFE substrate were lower than those for PE1 substrate. The effects of the amounts grafted on the membrane thicknesses and the water content are shown in Fig. 3. The thicknesses of the grafted membranes for both PE1 and PTFE substrates were larger than those of the respective substrates alone, and increased with increasing amounts grafted. The water contents of the A A grafted PTFE membrane were very high and were about 90% in the grafted layer. The water content of Nation ® 117, which was usually used as the support of the facilitated transport membrane, was reported as 11% [24]. It should be noted that the water content obtained here is much higher than that of Nation ® 117. The water contents of the AA grafted PTFE membrane were also higher than those of the PE1 membrane (80%). As described below on the direct observation of the grafted membranes, at the same amounts grafted, acrylic acid was grafted more loosely inside the pores of the porous PTFE substrate than inside the pores of PE1. It is considered that poly(acrylic acid) grafted loosely in the pores of PTFE substrate is sufficiently swollen by water. This leads to the higher water content of the g

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Fig. 4. Effect of amount grafted on permeation rate and selectivity. The line in this figure is a fit of the PE1 data only. Monomer: acrylic acid; carrier, ethylenediamine; [EDA] = 25% (v/v); feed CO 2 pressure = 0.047 atm; 298 K; • , PTFE; O, PE1; ~ , PE2; zx, PE3; v , PE4; [3, PE1 (one side grafted); m, PTMSP.

AA grafted PTFE membrane. Further, the pore size is bigger and less confined to swell in the PTFE substrate than in the PE1 substrate. This is probably another reason for the higher water content in the PTFE membrane. Fig. 4 shows the effects of the amounts grafted on the permeation rate of CO 2, Rco 2 and the selectivity a of CO 2 over N 2 when various A A grafted membranes were used as the supports. In these cases, ethylenediamine was used as the carder. The driving force normalized flux Rco 2 and cx are given by

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llllll

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Fig. 3. Effects of amount grafted on membrane thickness and water content: O • [] n , acrylic acid; zx A, methacrylic acid. The data using PE1 were those already reported in our previous work [17].

Rco 2 = N c o J A P

(1)

et = RcoJRN2

(2)

where Nc% is the molar flux of CO 2 and A p is the CO 2 partial pressure difference between the upstream and downstream side of the membrane. In the case of the AA grafted PE1 membrane, when the amount grafted was low, the grafted layer was formed mainly on the substrate surface as shown in Fig. 5, and all the pores were not covered with the grafted layer. This leads to the high permeation rate and low selectivity in the low grafting amount region in Fig. 4. As the grafting degree increased, both Rco: and c~

256

H. Matsuyama et al. / Journal of Membrane Science 117 (1996) 251-260

approached constant values. The obtained constant value of oL was as high as 4000 and Rco ~ obtained was 1.0 × 10 -4 cm3/(cm 2 s cmHg), which agreed with those already reported in our previous paper [17]. The constant a value suggests that the pores of the substrate are nearly filled with poly(acrylic acid). The membrane thickness increased with increasing the amounts grafted as shown in Fig. 3. The constant Rco 2 value with increasing the amounts grafted suggests that the mass transfer resistance through the substrate pores filled with the grafted polymer is predominant, and that through the grafted layer on the substrate surface, which brings about the increase in the total membrane thickness, is negligibly small [20]. This may be due to the very highly swollen

I

condition of the grafted layer on the substrate surface. The AA grafted PTFE membrane showed a higher Rco 2 compared with that of the PE1 membrane at the same amount grafted. This is because the pores of PTFE substrate were more loosely filled with the grafted polymer, as shown in Fig. 5, and the mass transfer resistance through the pores is smaller. The value of Rco 2 for the PTFE membrane reached to about 2.0 × 10 -4 cm3/(cm 2 s cmHg) with a = 100. When other PE substrates were used, the values of Rco 2 were smaller than Rco ~ of the grafted PE1 membrane. The values of Rco 2 for PE substrates decreased in order of PE1, PE2, PE3 and PE4, and this order is in agreement with that of the porosity of

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Fig. 5. Gross-sectional view of AA grafted membranes.

257

H. Matsuyama et aL / Journal of Membrane Science 117 (1996) 251-260

the used substrates shown in Table 1. The data of the membrane grafted only on the one side by sealing other side with aluminum foil in the plasma treatment is also included in Fig. 4. It was hardly distinguishable from data of the usual grafted membrane. Fig. 5 shows the cross sections of the AA grafted membranes. It was confirmed that PE1 and PTFE substrate themselves were not dyed. The dyed black portions show the grafted layer. In PE1 substrate membrane, the grafted layer was found to be formed mainly on the substrate surface in the low amount grafted [Fig. 5(a)]. With increasing the amount grafted, the grafted layer was formed inside the pores of the substrate [Fig. 5(b)], and the further graft brought about the increase in the total membrane thickness [Fig. 5(c)]. Yamaguchi et al. reported that the methyl acrylate grafted layer was formed inside the porous substrate even with a low grafting amount [20]. The difference between our results and theirs is probably attributable to the difference in the apparatus and procedures for the plasma graft polymerization a n d / o r that in the monomers used. Plasma is well-known to treat only the surface of the substrate. Therefore, the plasma-grafted layer is not formed inside the substrate when the solid nonporous substrate is used. On the other hand, when the porous substrate like PE1 is used, plasma can penetrate into the pores, and the graft layer is formed inside the pores. By comparing the AA grafted PTFE membrane with the PE1 membrane with a similar grafting amount condition [Fig. 5(b) and (d)], it was shown that the grafted layer was more loosely formed inside the PTFE substrate. This is probably attributable to the larger PTFE substrate thickness and also to the hydrophobic property of PTFE. The results using poly[1-(trimethylsilyl)-l-propyne] (PTMSP) membrane are listed in Table 2. Although the permeation rate of the substrate alone was as high as 1.51 × 10 -3 cm3/(cm 2 s cmHg), the selectivity was only about 3. The permeabilities of CO 2 and N 2 were 27 800 and 9 600 Barrer, respectively, and were roughly in agreement with the respective values of 40500 and 9 170 Barrer at 303 K reported previously [25]. When the AA grafted PTMSP membrane was impregnated with water, the selectivity increased to 49. The selectivity based on the solution-diffusion mechanism is given by the

Table 2 Results using PTMSP membranes Rco~ (cm3-/(cm2 s emHg)) PTMSP alone a AA grafted PTMSP b (water) AA grafted PTMSP b (EDA 25% (v/v))

1.51 × 10 -3 1.00× 10- 4 2.22× 10 -4

ct(COz/N 2)

(-) 2.88 49 310

a Membranethickness: 20 txm. b Amountgrafted: 3.61× 10-5 g / c m 2. Feed CO2 pressure= 0.047 atm; 298 K. product of the ratio of the solubilities and that of the diffusivities. By evaluating these values into the water, for CO 2 and N 2, the selectivity of the water containing membrane was estimated as 60 [17]. This value approximately agrees with the experimental data. When the grafted membrane was soaked in 25% ( v / v ) EDA solution, Rco 2 increased compared with that of the water containing membrane due to the facilitated transport of CO 2 by the carrier. At the same time, there was about a three fold decrease in Rr%. The transport of N 2 is a simple physical transport. The viscosity of the EDA solution was found to be about 2.5 times higher than that of water. The decrease in the diffusivity due to the increase in the membrane solution viscosity and also the decrease in the solubility due to the salt-out effect [26] probably leads to the decrease in Rr%. The increase in Rco ~ and the decrease in Rr% brought about the remarkable increase in the selectivity. In this case Rco 2 reached 2.2 × 10 - 4 cm3/(cm 2 s cmHg) with a = 310. The membrane efficiencies are also shown in Fig. 4. 3.2. Effect o f grafted m o n o m e r

The relation between the membrane efficiencies and EDA concentration in aqueous solution in which the grafted membranes were soaked is shown in Fig. 6 when acrylic acid (AA) and methacrylic acid (MAA) were used as the monomer. The data of the AA grafted membrane were those already reported in our previous paper [17]. Both Rco 2 and oL for the MAA grafted membrane were lower than those for the AA grafted membrane. As shown in Fig. 3, the

H. Matsuyama et a l . / Journal of Membrane Science 117 (1996) 251-260

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10-2 10-1 10 0 [KOH] [mol/dm 3]

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Fig. 7. Relation between [K + ] incorporated in the membrane and [KOH] in the solutions. Amount grafted: 1.07 × 10 3 g / c m 2 (AA grafted membrane); 7 . 8 8 × 1 0 4 g / c m 2 ( M A A grafted membrane); substrate: PEI.

_.10 -5 6POAcrylic acid grafted membrane o 10- ~ - • Methacrylic acid grafted membrane FI , I , I , 0 10 20 30 [EDA] [ %(v/v)]

methacrylic acid (86.1). The lower ion exchange capacity of the MAA grafted membrane means a lower cartier concentration in the membrane. The lower carrier concentration leads to a lower complex concentration in the membrane. This is the reason for the lower Rco 2 and o~ for MAA grafted membrane shown in Fig. 6. Further, the lower ion exchange capacity means that the distance between the ion sites is larger. As described below, it is considered that the complex between CO 2 and the carrier moves electrostatically interacting with the ion sites in the membrane matrix. The decrease in the diffusivity of the complex due to the larger distance between the ion sites may be another reason for the lower Rco 2 and a for the MAA grafted membrane.

Fig. 6. Comparison between membrane efficiencies of A A grafted membrane and those of M A A grafted membrane. Amount grafted: 1.18 X 10 -3 g / c m 2 (AA grafted membrane)', 1.05 X 10 -3 g / c m 2 ( M A A grafted membrane); substrate: PE1.

thicknesses and the water contents of the MAA grafted membrane are similar to those of the AA grafted membrane. Fig. 7 shows potassium concentrations incorporated in the membrane by ion exchange when the MAA grafted membrane was soaked in KOH solutions with various concentrations. In this figure, the data of the AA grafted membrane reported previously [17] are also included. The almost constant K + concentrations in the membranes correspond to their ion exchange capacities. The ion exchange capacities obtained for the AA and MAA grafted membranes were 13.1 meq/g-dry grafted layer and 10.5 m e q / g dry grafted layer, respectively, and are nearly in accordance with the respective values of 13.9 m e q / g and 11.6 m e q / g , which are evaluated from the molecular weight of acrylic acid (72.1) and

3.3. Effects of other experimental conditions In addition to EDA, some other amines were tested as the carrier of CO 2. Typical results are shown in Table 3. Although the amounts grafted were somewhat different for each case, Rco 2 and o~ hardly depend on the amounts grafted so far as the

Table 3 Effect of carrier on Rco 2 and for the A A grafted PE1 membrane a

HzN(CH2)2NH 2 H2N(CH2)3NH2 H2N(CH2)4NH 2 H2N(CH2)2NH(CH2)2NH 2 H2N(CH2)2NH(CH2)2NH(CH2)2NH2

Rco ~ (cm:r/(cm 2 s cmHg))

~.(CO2/N 2) (-)

Amount grafted ( g / c m 2)

8.26 1.88 1.38 2.86 0.93

1900 560 320 1160 680

1.33 1.67 1.91 1.67 1.91

× X X X X

10 -5 10 - 5 10 -5 10 -5 10 5

X X X × X

10 -3 10 -3 10 - 3 10 -3 10 -3

a The concentrations of the carriers in the membranes were the same as those of ion sites: feed CO 2 pressure = 0.047 atm; 298 K.

H. Matsuyama et al./ Journal of Membrane Science 117 (1996) 251-260

Table 4 Effect of temperature on the AA grafted PE1 membrane efficiencies a Temperature (K)

Rco ~

288 298 323

2.58 × 10-5 4.04× 10-5 9.07 × 10-5

(cm:r/(cm 2 s

~(CO2/N 2) (-)

cmHg))

850 850 770

a Amount grafted: 1.15 X 10-3 g / c m 2 ; [EDA] = 5% ( v / v ) ; feed CO 2 pressure = 0.15 atm.

amount is higher than 1.0 × 1 0 - 3 g / c m 2, as shown in Fig. 4. As the chain lengths of the diamine (ethylenediamine, trimethylenediamine, tetramethylenediamine) increase, both the permeation rates and the selectivities decreased. Even in the carriers having more amino groups such as diethylenetriamine and triethylenetetramine, the values of Rco 2 and et were lower than those of EDA. Therefore, E D A was the best carrier of CO 2. The effect of temperature are shown in Table 4. As the temperature increases, the diffusivity of the complex increases, which leads to higher Rco ~. Even at the high temperature of 323 K, the selectivity hardly decreases. In membrane separation used in the practical removal of CO 2 from flue gases, the membrane must be operated at more than 323 K [27]. 10000

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259

Under such conditions, our membrane is considered to be useful. CO 2 concentrations in the flue gases are usually 10-15 mol%. The results shown in Table 4 were obtained when the CO 2 partial pressure in the feed gas was 0.15 atm. Fig. 8 shows the effect of the CO 2 partial pressure in the sweep gas. If the equilibrium constant of the complex is extremely large, the desorption of CO 2 at the sweep side interface becomes difficult as the CO 2 partial pressure in the sweep gas increases, which leads to the lower permeation rate. As shown in the figure, Rco 2 and e~ decreased slightly with CO 2 partial pressure. However, because the changes were not so much, it was confirmed that the CO 2 partial pressure in the sweep gas influenced to a lesser extent both Rco 2 and cx up to 0.023 atm.

4. Conclusion The facilitated transport of CO 2 through ion exchange membranes prepared by the plasma graft polymerization was investigated using amines as the carrier. The effects of various experimental factors such as amounts grafted, substrates, monomers, cartiers and so on were investigated. When the amount grafted was low it was found by direct observation of the membrane cross section that the grafted layer was formed mainly on the microporous substrate surface, and all the pores were not covered with the grafted layer. This leads to high Rco 2 and low eL in the low amount grafted region. As the amount grafted increased and the pores were nearly filled with the grafted layer, both Rco 2 and et approached constant values. The acrylic acid grafted PTFE membrane showed a higher permeation rate of CO 2 compared with the grafted polyethylene membrane. This was because the pores of the more hydrophobic PTFE substrate were more loosely filled with the grafted layer. The use of the poly[1-(trimethylsilyl)-l-propyne] substrate was also favorable in obtaining a higher permeation rate. When methacrylic acid was used as the monomer, the ion exchange capacity of the obtained membrane was lower than that of the acrylic acid grafted membrane. This low ion exchange capacity led to lower Rco 2 and o~. Various amines were tested as the carrier of CO 2.

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H. Matsuyama et al. / Journal of Membrane Science 117 (1996) 251-260

As the chain lengths of diamines increases, both Rco 2 and c~ decreased. Ethylenediamine was the best among these amines. Our future work with this system will be directed at the investigation of the stability of the grafted membrane.

[13]

[14]

[15]

Acknowledgements [16]

We wish to thank Tonen Chemical Co., Ltd. for supplying the porous polyethylene membranes. We also thank Sumitomo Electric Co., Ltd. for supplying the porous polytetrafluoroethylene membrane.

[17]

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