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Co(SalPr) complex membranes
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Journal of Membrane Science 135 (1997) 9-18
Oxygen/nitrogen separation by polycarbonate/Co(SalPr) complex membranes Ruoh-Chyu Ruaan*, Shih-Hsiung Chen, Juin-Yih Lai Department of Chemical Engineering, Chung Yuan University, Chung Li, Taiwan 32023, ROC Received 18 June 1996; received in revised form 8 October 1996; accepted 7 May 1997
Abstract An oxygen carrier, cobalt di-(salicylal)-3,Y-diimino-di-n-propylamine (Co(SalPr)), was added into a polycarbonate membrane for improving its oxygen/nitrogen selectivity. Both the oxygen permeability and oxygen/nitrogen selectivity increased when only 3 wt% of Co(SalPr) was added. The permeability kept increasing but the selectivity decreased when more than 3 wt% of Co(SalPr) was added. The oxygen to nitrogen solubility ratio decreased when 3 wt% of Co(SalPr) was added. Further increase in Co(SalPr) content led to an increase in oxygen/nitrogen solubility ratio. It was astonishing to know that the effect of Co(SalPr) content on the oxygen/nitrogen solubility ratio was totally opposite to that on the oxygen/nitrogen selectivity. A membrane gas transport model which combines the dual mobility model with pore model was adopted to explain the above phenomenon. The specific volume measurement implied that the pore diffusion was responsible for this behavior. The contribution of sorption--diffusion type transport was also investigated by examining the transport behavior of the 3 wt% Co(SalPr) containing membrane through which the pore diffusion is relatively low. The effect of upstream pressure on the oxygen permeability and solubility implied that the diffusivity of Henry's mode was much higher than that of Langmuir's mode. It was also found that the effects of upstream pressure and operating temperature on the oxygen/nitrogen selectivity were both in accordance with those on the Henry's mode solubility ratio. The above information suggested that in addition to the pore diffusion the ratio of Henry's mode diffusion dominated the O2/N2 separation instead of the overall O2 to N2 solubility ratio. K e y w o r d s : Polycarbonate; Cobalt di-(salicylal)-3,3'-diimino-di-n-propyl-amine;Gas permeability; Gas selectivity
1. Introduction In recent years, the method of oxygen carrier addition to improve the oxygen/nitrogen selectivity of a gas separation membrane has attracted tremendous attention [1-5]. In our previous paper [6] N, Nt-diali cylidene ethylene diamine cobalt(II) (Cosalen) was *Corresponding author. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
P I I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 12 9 - 4
added in a polycarbonate (PC) membrane and the PC/ CoSalen membrane showed a high oxygen selectivity at low temperature (5°C). On the other hand Nishide et al. [7] reported a 1-methylimidazole membrane containing another oxygen carder, [ a , a l , a n , c J n - m e s o tetrakis(o-pivalamidophenyl)porphinato] cobalt(II) (CoPIm), which showed exceptional high selectivity. The oxygen/nitrogen selectivity was greater than 10 at room temperature. However, the high oxygen/nitrogen
10
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
selectivity only happened at low pressure (below atmosphere pressure). All the above investigations showed that the high oxygen selectivity could only be obtained at either low pressure or low operating temperature. In order to improve the performance of gas separation membranes, another cobalt containing oxygen carrier was considered. Cobalt di-(salicylal)-3,3'-diimino-di-n-propylamine (Co(SalPr)), of which the cobalt chelating component was formed by reacting salicylaldehyde with 7,7'-diaminodipropylamine, showed excellent oxygen absorbing capability at atmosphere pressure and room temperature [7]. According to the solution-diffusion model [8], the oxygen permeability is proportional to its solubility in membrane. Therefore Co(SalPr) containing membranes should exhibit an improved oxygen/nitrogen selectivity at normal range of operating conditions. Co(SalPr) was then added to a polycarbonate membrane for oxygen enrichment. Surprisingly it was found that the addition of Co(SalPr) did not necessarily increase the oxygen to nitrogen solubility ratio. It was even more disturbing that the oxygen/nitrogen selectivity sometimes increased as the solubility ratio decreased. Apparently the effect caused by Co(SalPr) addition on the gas transport behavior cannot be simply explained by the traditional solution-diffusion type mechanism. In this study the oxygen and nitrogen sorption isotherms were measured and modeled by the dual sorption model which comprised both Lagmuir's and Henry's modes of sorption [9]. The mathematical form of the overall gas sorption (c) can be described by CHIbP c = kDP + - 1 +bP
(1)
where kD is the Henry's law dissolution constant, and CH' and b represent the maximum capacity and equilibrium constant, respectively, in Langmuir isotherm. The solubility can therefore be described as S = SH ÷ SL ~-- kD ÷
CHtb -
-
1 +bP
the pore diffusion transports [10]. When the downstream pressure is controlled near zero, the gas permeability (Pm) can be expressed as P m = D L S L ' + DHSH I + Dp
(3)
where DL represents the diffusivity through Langmuir adsorption layer, DH represents the diffusivity through the region of Henry's sorption and Dp denotes the diffusivity of pore diffusion that relates the part of gas transport independent of gas-polymer interaction. The solubility SLf and SH' represent the Langmuir's and Henry's solubilities at the upstream pressure. The effective diffusivity of a gas across a glassy polymer membrane can therefore be expressed as Deft
Pm -
-
St - -
1 DL(1 -- Xn') + DHXH' + DP-S7
(4)
where XH' denotes the fraction of Henry's sorption SH'/S'. The selectivity of oxygen to nitrogen can therefore be expressed as Selectivity _ DL(O2)SL'(O2) + DH(O2) + SH'(O2) + Ov(O2) DL(N2)SL'(N2) + DH(N2)SH'(N2) + De(N2)
(5) The effects of Co(SalPr) content, upstream pressure and operating temperature on the membrane permeability and selectivity are described by the above equations.
2. Experimental 2.1. Materials
Polycarbonate (Uplion S-2000) (Mw=28 000) was supplied by Mitsubishi Gas Chemical. Dichloromethane, salicylaldehyde, cobalt acetate, and "7,7'diaminodipropylamine were supplied by Merck. The above mentioned chemicals are all of reagent grade and were used without further purification.
(2)
where SL denotes the solubility caused by Langmuir sorption and SH denotes the Henry's mode solubility. The gas permeation through the membrane is presumably determined by both the sorption-diffusion and
2.2. Membrane preparation
The PC membranes were prepared from a casting solution of polycarbonate in dichloromethane. The membranes were formed by casting the solution onto
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
a glass plate to a predetermined thickness using a Gardner knife at room temperature. The membranes were dried in vacuum for 24 h before gas sorption and permeation measurement. 2.3. Gas permeability measurements
The apparatus (Yanaco Gas permeability Analyzer, model GTR- 10) for measuring the gas permeability was shown in our previous report [6]. The gas permeability was measured by the following equation: 1 q/t Pm - - P 1 - P2 A w h e r e Pm is the gas permeability [cm3(STP)cm/
cm2s cmHg], q/t is the volume flow rate of gas permeate [cm3(STP)/s], l is the thickness [cm], P1 and P2 are the pressures(cmHg) on the high pressure and low pressure side of the membrane, respectively. A is the effective membrane area [cmZ]. The gas selectivity is calculated by the following expression: Pm(O2)
Selectivity - Pro(N2)
2.4. Gas sorption measurements
The experimental setup for gas sorption measurement was shown in our previous report [6]. The amount of gas absorbed was measured by a microbalance. The microbalance (Cahn Model D-202 Electrobalance) was enclosed in a stainless chamber and the chamber was enclosed in a constant temperature box. The system pressure was then reduced to about 4× 10 - 3 torr before the gas sorption measurement.
11
3. Results and discussion 3.1. Effect o f Co(SalPr) content on gas selectivity and permeability
The effects of Co(SalPr) content in the membranes on the gas permeability and selectivity are shown in Fig. 1. Both the oxygen and nitrogen permeability increased with increasing Co(SalPr) content in the polycarbonate membrane. According to our previous experience, the oxygen/nitrogen selectivity usually decreases as the permeability increases. However, the oxygen/nitrogen selectivity of these Co(SalPr) containing membranes first increased with increasing Co(SalPr) content and then decreased when more than 3 wt% of Co(SalPr) was added. The original polycarbonate membrane exhibited an O2/N2 selectivity of 5.00. The oxygen/nitrogen selectivity of the 3 wt% Co(SalPr) containing membrane reached 6.92 at 35°C. The oxygen permeability of the same membrane was 1.65 barrer. Usually the more oxygen carrier was added in the membrane, the higher oxygen/nitrogen selectivity would be obtained [11]. But in this PC/Co(SalPr) membrane system, when more than 3 wt% of oxygen carrier was added, the oxygen/nitrogen selectivity gradually decreased. It was interesting to explore why both the permeability and selectivity increased
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2.01 2.5. Specific volume measurements
The specific volume was estimated by dividing the membrane volume by the mass of membrane. The membrane volume was calculated by multiplying the average membrane thickness by the area. The membrane thickness was measured by the thickness gauge (Teclock MS1201). The average membrane thickness was obtained from the average of ten measurements.
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i 4
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Co(~iPr) content(wt %) Fig. 1. Effect of Co(SalPr) content in membranes on gas permeabilities and gas selectivity. (/k): oxygen permeability, (O): nitrogen permeability, ( I ) : P(O2)/P(N2).
12
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18 8
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Preuure(stm) Fig. 2. Oxygen sorption isotherm of PC/Co(SalPr) membranes with various Co(SalPr) content at 35°C. ( i ) : 10 wt%, (A): 5 wt%, (O): 3 wt%, (O) PC.
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Fig. 3. Nitrogen sorption isotherm of PC/Co(SalPr) membranes with various Co(SalPr) content at 35°C. ([]): 10 wt%, (/X) 5 wt%, (C)): 3 wt%, (~): PC.
I.O
when the carrier content was lower than 3 wt% and why the selectivity decreased with increasing carrier content when more than 3 wt% of Co(SalPr) was added.
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J 3.2. Effect of Co(SaIPr) content on the gas solubility In order to understand the gas transport behavior in the Co(SalPr) complexed membranes, the oxygen and nitrogen sorption isotherms were measured. The sorption isotherms of oxygen and nitrogen at 35°C are shown in Figs. 2 and 3. Both the amounts of oxygen sorption and nitrogen sorption increased with increasing Co(SalPr) content. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure. As shown in Fig. 4, both the oxygen and nitrogen solubilities at 1 atm increased with the increase of Co(SalPr) content, but the solubility ratio of oxygen to nitrogen first decreased at low Co(SalPr) content and then increased as the amount of Co(SalPr) raised above 3 wt%. To explain the above results sorption isotherms of both oxygen and nitrogen were fitted by the dual sorption model shown in Eq. (1). The dual sorption model describes the part of gas sorption in the polymer nodules in Henry's mode and the part of gas sorption between polymer
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Fig. 4. Effect of Co(SalPr) content in membranes on gas solubility and gas solubility selectivity. (A): oxygen solubility, (O): nitrogen solubility, (11): solubility ratio (S(02)/S(N2)).
nodules in Langmuir's mode [12]. The fitted parameters are shown in Table 1 and the Henry's and Langmuir's solubilities are listed in Table 2. It was found that the Henry's mode of sorption of nitrogen increased rapidly as the Co(SalPr) content increased
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
13
Table 1 Dual sorption parameters of PC/Co(SalPr) membrane at 35°C Membrane
Oxygen
Nitrogen
Co(SalPr)
KD
CH I
[cm3(STP)/cm 3 atm]
[cm3(STP)/cm 3]
b [atm l]
KD [cm3(STP)/cm 3 atm]
CH !
content [wt%]
[cm3(STP)/cm 3]
b [atm -1]
0 3 5 10
0.162 0.22 0.24 0.276
0.076 0.100 0.190 0.235
1000 1000 1000 1000
0.107 0.189 0.188 0.192
0.048 0.04 0.08 0.10
10 100 100 100
Table 2 Langmuir mode sorption and Henry mode sorption of PC/Co(SalPr) membrane at 35°C Membrane
Oxygen
Nitrogen
Co(SalPr) content [wt%]
SH [cm3(STP)/cm 3 atm]
SL [cm3(STP)/cm 3 atm]
$8 [cm3(STP)/cm 3 atm]
SL
0 3 5 10
0.162 0.220 0.240 0.276
0.075 0.099 0.189 0.245
0.107 0.189 0.188 0.192
0.043 0.039 0.079 0.099
from 0 to 3 wt% but that of oxygen increased only slightly. This result might explain why the oxygen to nitrogen solubility ratio slightly decreased after 3 wt% of Co(SalPr) was added. As more Co(SalPr) was added, the Langmuir sorption of oxygen increased more rapidly than that of nitrogen, which resulted in an increase in oxygen/nitrogen solubility ratio.
Sn( O2)/SH(N2)
[cm3(STP)/cm 3 atm] 1.51 1.16 1.27 i .60
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3.3. Effect of Co(SalPr) content on the effective diffusivities of oxygen and nitrogen The dependence of effective diffusivity on Co(SalPr) content is shown in Fig. 5. According to the solution-diffusion model the effective diffusivity was defined as the ratio of permeability to solubility. The effective diffusivity of oxygen increased slightly as the Co(SalPr) content increased to 3 wt% and then decreased when more Co(SalPr) was added. However, the effective diffusivity of nitrogen behaved opposite to that of oxygen. To understand the behavior of nitrogen diffusivity, the specific volume of the membranes were measured. As shown in Fig. 6, the Co(SalPr) content had the same effect on the effective diffusivities of nitrogen as on the specific volume of membranes. This result indicated that the transport of nitrogen might be dominated by pore diffusion. The
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Co(SmlPr) eonteut(wt %) Fig. 5. Effect of Co(SalPr) content in membranes on effective diffusivity and diffusivity ratio. (A): oxygen solubility, (O): nitrogen solubility, ( I ) : effective diffusivity ratio of oxygen to nitrogen.
effect of Co(SalPr) on the effective diffusivity of oxygen was rather difficult to understand. According to the specific volume measurement, pore diffusivity Dp decreased when only 3 wt% of Co(SalPr) was
14
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
1.0
2.0
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0,0
0
0
Pressul'e(itm)
0.6 0
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Co(Sal~Xwt% )
8
12
Fig. 6. Effect of Co(SalPr) content in membranes on membrane specific volume.
Fig. 7. Effect of operating pressure on gas permeability and gas selectivity with 3 wt% Co(SalPr) content. (A): oxygen permeability, (O): nitrogen permeability, (11): P(O2)/P(N2).
0.6
added, but the effective diffusivity of oxygen slightly increased. It was obvious that the effective diffusivity of oxygen at Co(SalPr) content below 3 wt% was not dominated by pore diffusion but dominated by the first two terms in Eq. (3), namely, the Henry's and Langmuir's sorption--diffusion transports.
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3.4. Effect of operating pressure on gas separation performance The effect of operating pressure on the gas separation performance of 3 wt% Co(SalPr) membrane at 35°C is shown in Fig. 7. The gas permeabilities of both oxygen and nitrogen were independent of the operating pressure, and the oxygen/nitrogen selectivity remained constant. In order to further understand the effect of operating pressure on the gas transport behavior, the gas solubilities at various pressures were measured. Fig. 8 shows the effect of pressure on the solubility of oxygen and nitrogen. The solubility of oxygen decreased with the increase of pressure but that of nitrogen remained almost constant. This result was in good agreement with the dual sorption model. Although the solubility of oxygen decreased with the increase of pressure, the oxygen permeability was not affected by the operating upstream pressure. Accord-
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~ 6
Prmure(ttm)
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Fig. 8. Effect of operating pressure on gas solubility and gas solubility selectivitywith 3 wt% Co(SalPr) content. (A): oxygen solubility, (O): nitrogen solubility, (11): selectivity ratio (S(O2)/ S(N2)).
ing to Eq. (2) there were only two possibilities for oxygen permeability to be independent of pressure: one possibility was that the transport of oxygen was dominated by pore diffusion and the pore diffusivity Dp is independent of pressure. The other is that the DH of oxygen is much higher than DL(O2) so that the permeability remains constant regardless of the
R.- C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
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Co(SaIPr)co.teal(wt%) Fig. 9. Effect of Co(SalPr) content on the Henry's mode sorption of oxygen in the membranes.
decrease in solubility. It was shown in the previous section that the oxygen transport through the membrane containing 3 wt% of Co(SalPr) was not dominated by pore diffusion but dominated by the first two terms in Eq. (2). Therefore, the DH of oxygen should be much higher than DL(O2). If it were the case, the fraction of Henry's mode sorption played a much more important role than did the Langmuir sorption mode. Fig. 9 shows the fraction of Henry's sorption(XH) at various Co(SalPr) content. This figure might explain the dependence of Deff(O2) o n the Co(SalPr) contents shown in Fig. 5. It was found that the XH of oxygen varied in the same direction as the effective diffusivity did, which indicated that the transport of oxygen through the PC/Co(SaIPr) membranes was dominated by the diffusion through the regions where the Henry's sorption occurred. The effect of operating pressure on the effective gas diffusivity is shown in Fig. 10. The effective diffusivity of oxygen increased with increasing operating pressure, but that of nitrogen remained almost constant. It was surprising to find that the diffusivity ratio of oxygen to nitrogen was higher than 5.0 and increased as the operating pressure increased. It indicated that the gas separation was determined by the high oxygen diffusivity of the membrane rather than its high oxygen solubility. As discussed previously the
Fig. 10. Effect of operating pressure on gas effective diffusivity and gas diffusivity ratio of oxygen to nitrogen of membrane with 3 wt% Co(SalPr) content. (A): oxygen diffusivity, (©): nitrogen diffusivity, (11): effective diffusivity ratio of oxygen to nitrogen.
transport of oxygen was dominated by the diffusion through the Henry's sorption area. Because the O2/N2 solubility ratio was only slightly higher than one, it suggested that the Drt(OE)/DH(N2) had a value much higher than one. All the above discussions actually suggested that the separation of oxygen from nitrogen through the PC/Co(SalPr) complexed membrane was facilitated by a high DH(O2)/DH(N2) value. In addition, the increase of oxygen effective diffusivity with the increase in pressure could be explained by Eq. (3). As shown in Fig. 11 the fraction of Henry's sorption increased with the increase of pressure. Since DR was much larger than DE the increase of XH led to an increase in effective diffusivity.
3.5. Effect of operating temperature The effects of operating temperature on the gas separation performance are shown in Fig. 12. The oxygen and nitrogen permeability increased but the oxygen/nitrogen selectivity decreased with the increase of temperature. In order to understand the effect of operating temperature on gas transport behavior, the oxygen and nitrogen sorption isotherms at various temperatures were measured. The results are shown in Figs. 13 and 14. The solubility of oxygen
16
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18 1.0
8
i
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t ~ygen 0.8
6
@
0.6
36 °C 460C
@
T
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0.44
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0.0
i
t
0
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I
10
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0
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20
o
30
i
L
10
20
Fig. 11. Effect of oxygen pressure on the Henry's sorption in the membrane with 3 wt% Co(SalPr) content. ,
30
Presnure(ttm)
Pressure(arm)
10.0
~
@o eO~ O~
•
Fig. 13. Oxygen sorption isotherm of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (@): 25°C, (~): 35°C, (<>): 45°C.
8
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Nitrogen
|
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es o om
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A 480C
A A
0.1 3.00
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1.20 1/T X I0 3 (l/K)
I 3.40
0
Fig. 12. Effect of operating temperature on gas permeability and gas selectivity of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen permeability, (O): nitrogen permeability, ( l l ) P(O2)IP(N2).
and nitrogen could therefore be calculated. The effect of operating temperature on gas solubility is shown in Fig. 15. It was found that the oxygen and nitrogen solubilities decreased with increasing temperature and the solubility ratio also decreased with the increase of temperature. The effect of temperature on the effective
& 0
10
2o
3o
Pressure(am) Fig. 14. Nitrogen sorption isotherm of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (&): 25°C, (ZX): 35 °C, (A): 45°C.
diffusivity is shown in Fig. 16. Both the effective diffusivities of oxygen and nitrogen increased with the operating temperature as did the O2/N2 diffusivity ratios. The above information indicated that the rate of oxygen and nitrogen transporting through the membrane was controlled by the effective diffusivities
17
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
3
10.0
/
k~ 02
/,
6
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1.0
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°m
/ ~ ~
0.1
Nitroges ~en
0.1 3.00
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I
n
I
0
3.40
3.20
0.0
I/T X 10 a (l/K)
.
3.00
Fig. 15. Effect of operating temperature on gas solubility and solubility ratio of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen solubility, (©): nitrogen solubility, (11): solubility ratio. 100
! , 3.20 I / T X 10 3 ( l / K )
I :3.40
0
Fig. 17. Effect of operating temperature on Henry's sorption and Henry's sorption ratio of oxygen and nitrogen selectivity with 3 wt% Co(SalPr) content. (A): oxygen solubility, (C)): nitrogen solubility, (11) Srt(O2)/SH(N2).
6
4
2
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r~ 0 3.00
2
I
,
3.20 I/T X 103 (I/K)
I
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3.40
Fig. 16. Effect of operating temperature on the effective diffusivity and diffusivity ratio of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen solubility, (C)): nitrogen solubility, (11): effective diffusivity ratio of oxygen to nitrogen.
because the effect of temperature on the permeability is similar to that on the effective diffusivity. But, in spite of the fact that the effective diffusivity ratio of oxygen to nitrogen was much higher than the solubility ratio, it was obvious that the solubility played an important role in the separation of oxygen and nitrogen since the effect of temperature on the O2/N2 selectivity followed the same trend as that on the
solubility ratio. However, the solubility ratio increased by two fold as the operating temperature decreased from 45 to 25°C, but the increase in oxygen/nitrogen selectivity was relatively small (from 6 to 7.6). This could be explained by the decrease of the effective diffusivity ratio which balanced part of the increase in solubility. But the effective diffusivity, defined as Deff=Pm/S, is actually a complexed function which also varies with solubilities. A better explanation is needed. Compared with the huge increase in SL(O2)/SL(N2), the solubility ratio of Henry's mode, SH(O2)/SH(N2), shown in Fig. 17, increased only slightly from 1 to 1.3 when the operating temperature was reduced from 45 to 25°C. As mentioned before, the oxygen transport at 35°C was dominated by the diffusion through the Henry's sorption area. If this was also true at 25 and 45°C, the effect of temperature on O2/N2 selectivity was mostly reflected by the effect on the solubility ratio of Henry's mode, i.e. SH(O2)/SH(N2)
4. C o n c l u s i o n s
The gas permeability and O 2 f N 2 selectivity of the polycarbonate membrane were both increased by adding 3 wt% of Co(SalPr). The obtained oxygen perme-
18
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
ability and oxygen/nitrogen selectivity were 1.65 barrer and 6.92 at 35°C, respectively. Further increase in Co(SalPr) content led to an increase in gas permeability but a decrease in oxygen selectivity. It was found that the O2/N2 solubility ratio decreased in spite of the increase in Oe/N2 selectivity after 3 wt% of Co(SalPr) was added. Specific volume measurement implied that the pore diffusion was responsible for the above behavior. The contribution of diffusion-solution type transport was also investigated by examining the transport behavior of the 3 wt% Co(SalPr) containing membrane through which the pore diffusion is relatively low. The pressure dependence of oxygen permeability and solubility of the 3 wt% Co(SalPr) membrane revealed the importance of Henry's mode diffusion in O2/N2 separation. It suggested that it was the ratio of Henry's mode diffusion dominating the O2/ N2 separation instead of the overall 02 to N2 solubility ratio. The temperature dependence of O2/N2 selectivity indirectly supported this assumption. It was also found that the O2/N2 selectivity was offered more by the 02 to N2 diffusivity ratio than by the Henry's mode solubility ratio. However, the increase in Henry's mode solubility ratio amplified the O2/N2 selectivity.
References [1] B.M. Johnson, R.W. Baker, S.L. Matson, K.L. Smith, I.C. Roman, M.E. Tuttle and H.K. Lonsdale, Liquid membranes for the production of oxygen-enriched air. II. Facilitatedtransport membranes, J. Membrane Sci., 31 (1987) 31~57. [2] E. Tsuchida, H. Nishide, M. Ohyanagi and H. Kawakami, Facilitated transport of molecular oxygen in the membrane of polymer-coordinated cobalt Schiff base complex, Macromolecules, 20 (1987) 1907-1912.
[3] H. Nishide, H. Kawakami, T. Suzuki, Y. Azechi and E. Tsuchida, Effect of polymer matrix on the oxygen diffusion via a cobalt porphrin fixed in a membrane, Macromolecules, 24 (1991) 6306--6309. [4] J.M. Yang and G.H. Hsiue, Modified styrene-butadienestyrene block copolymer membranes complexed with (N,N~diasalicylideneethylene diamine) cobalt(H) for oxygen permeation: polymeric axial ligand effect, J. Membrane Sci., 87 (1994) 233-244. [5] M.J. Choi, C.K. Park and Y.M. Lee, Chelate membrane from poly(vinyl alcohol)/poly(salicylidene ally amine) blend. I1. Effect of Co(II) content on oxygen/nitrogen separation, J. Appl. Polym. Sci., 58 (1995) 2373-2379. [6] S.H. Cheng and J.Y. Lai, Polycarbonate/N,N'-dialicylidene ethylene diamine) cobalt(ll) complex membrane for gas separation, J. Appl. Polym. Sci., accepted for publication (1995). [7] H. Nishide, M. Ohyanagi, O. Okada and E. Tsuchida, Dualmodel transport of molecular oxygen in a membrane containing a cobalt porphyrin complex as a fixed carrier, Macromolecules, 20 (1987) 417-422. [8] O.L. Harle and M. Calvin, The oxygen-carrying synthetic chelate compounds. VI. Equilibrium in solution, J. Am. Chem. Soc., 68 (1946) 2612-2618. [9] W.R. Vieth, J.M. Howell and J.H. Hsieh, Dual sorption theory, J. Membrane Sci., 1 (1976) 177-220. [10] R. Rangarajan, M.A. Mazld, T Matsuura and S. Sourirajan, Permeation of pure gases under pressure through asymmetric porous membranes. Membrane characterization and prediction of performance, Ind. Eng. Chem. Process Des. Dev., 23 (1984) 79-87. [11] E. Tsuchida, H. Nishide, M. Ohyanogi and H. Kawakami, Facilitated transport of molecular oxygen in the membranes of polymer-coordinated cobalt Schiff base complexes, Macromolecules, 20 (1987) 1907-1912. [12] R.E. Kesting, A.K. Fritzsche, M.K. Murphy, C.A. Cruse, A.C. Handermann, R.E Malon and M.D. Moore, The secondgeneration polysulfone gas separation membrane. I. The use of Lewis acid: Base complexed as transient templates to increase free volume, J. Appl. Polym. Sci., 40(9/10) (1990) 1557.
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,iouruano~ MEMBRANE SCIENCE
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I
ELSEVIER
Journal of Membrane Science 135 (1997) 9-18
Oxygen/nitrogen separation by polycarbonate/Co(SalPr) complex membranes Ruoh-Chyu Ruaan*, Shih-Hsiung Chen, Juin-Yih Lai Department of Chemical Engineering, Chung Yuan University, Chung Li, Taiwan 32023, ROC Received 18 June 1996; received in revised form 8 October 1996; accepted 7 May 1997
Abstract An oxygen carrier, cobalt di-(salicylal)-3,Y-diimino-di-n-propylamine (Co(SalPr)), was added into a polycarbonate membrane for improving its oxygen/nitrogen selectivity. Both the oxygen permeability and oxygen/nitrogen selectivity increased when only 3 wt% of Co(SalPr) was added. The permeability kept increasing but the selectivity decreased when more than 3 wt% of Co(SalPr) was added. The oxygen to nitrogen solubility ratio decreased when 3 wt% of Co(SalPr) was added. Further increase in Co(SalPr) content led to an increase in oxygen/nitrogen solubility ratio. It was astonishing to know that the effect of Co(SalPr) content on the oxygen/nitrogen solubility ratio was totally opposite to that on the oxygen/nitrogen selectivity. A membrane gas transport model which combines the dual mobility model with pore model was adopted to explain the above phenomenon. The specific volume measurement implied that the pore diffusion was responsible for this behavior. The contribution of sorption--diffusion type transport was also investigated by examining the transport behavior of the 3 wt% Co(SalPr) containing membrane through which the pore diffusion is relatively low. The effect of upstream pressure on the oxygen permeability and solubility implied that the diffusivity of Henry's mode was much higher than that of Langmuir's mode. It was also found that the effects of upstream pressure and operating temperature on the oxygen/nitrogen selectivity were both in accordance with those on the Henry's mode solubility ratio. The above information suggested that in addition to the pore diffusion the ratio of Henry's mode diffusion dominated the O2/N2 separation instead of the overall O2 to N2 solubility ratio. K e y w o r d s : Polycarbonate; Cobalt di-(salicylal)-3,3'-diimino-di-n-propyl-amine;Gas permeability; Gas selectivity
1. Introduction In recent years, the method of oxygen carrier addition to improve the oxygen/nitrogen selectivity of a gas separation membrane has attracted tremendous attention [1-5]. In our previous paper [6] N, Nt-diali cylidene ethylene diamine cobalt(II) (Cosalen) was *Corresponding author. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
P I I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 12 9 - 4
added in a polycarbonate (PC) membrane and the PC/ CoSalen membrane showed a high oxygen selectivity at low temperature (5°C). On the other hand Nishide et al. [7] reported a 1-methylimidazole membrane containing another oxygen carder, [ a , a l , a n , c J n - m e s o tetrakis(o-pivalamidophenyl)porphinato] cobalt(II) (CoPIm), which showed exceptional high selectivity. The oxygen/nitrogen selectivity was greater than 10 at room temperature. However, the high oxygen/nitrogen
10
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
selectivity only happened at low pressure (below atmosphere pressure). All the above investigations showed that the high oxygen selectivity could only be obtained at either low pressure or low operating temperature. In order to improve the performance of gas separation membranes, another cobalt containing oxygen carrier was considered. Cobalt di-(salicylal)-3,3'-diimino-di-n-propylamine (Co(SalPr)), of which the cobalt chelating component was formed by reacting salicylaldehyde with 7,7'-diaminodipropylamine, showed excellent oxygen absorbing capability at atmosphere pressure and room temperature [7]. According to the solution-diffusion model [8], the oxygen permeability is proportional to its solubility in membrane. Therefore Co(SalPr) containing membranes should exhibit an improved oxygen/nitrogen selectivity at normal range of operating conditions. Co(SalPr) was then added to a polycarbonate membrane for oxygen enrichment. Surprisingly it was found that the addition of Co(SalPr) did not necessarily increase the oxygen to nitrogen solubility ratio. It was even more disturbing that the oxygen/nitrogen selectivity sometimes increased as the solubility ratio decreased. Apparently the effect caused by Co(SalPr) addition on the gas transport behavior cannot be simply explained by the traditional solution-diffusion type mechanism. In this study the oxygen and nitrogen sorption isotherms were measured and modeled by the dual sorption model which comprised both Lagmuir's and Henry's modes of sorption [9]. The mathematical form of the overall gas sorption (c) can be described by CHIbP c = kDP + - 1 +bP
(1)
where kD is the Henry's law dissolution constant, and CH' and b represent the maximum capacity and equilibrium constant, respectively, in Langmuir isotherm. The solubility can therefore be described as S = SH ÷ SL ~-- kD ÷
CHtb -
-
1 +bP
the pore diffusion transports [10]. When the downstream pressure is controlled near zero, the gas permeability (Pm) can be expressed as P m = D L S L ' + DHSH I + Dp
(3)
where DL represents the diffusivity through Langmuir adsorption layer, DH represents the diffusivity through the region of Henry's sorption and Dp denotes the diffusivity of pore diffusion that relates the part of gas transport independent of gas-polymer interaction. The solubility SLf and SH' represent the Langmuir's and Henry's solubilities at the upstream pressure. The effective diffusivity of a gas across a glassy polymer membrane can therefore be expressed as Deft
Pm -
-
St - -
1 DL(1 -- Xn') + DHXH' + DP-S7
(4)
where XH' denotes the fraction of Henry's sorption SH'/S'. The selectivity of oxygen to nitrogen can therefore be expressed as Selectivity _ DL(O2)SL'(O2) + DH(O2) + SH'(O2) + Ov(O2) DL(N2)SL'(N2) + DH(N2)SH'(N2) + De(N2)
(5) The effects of Co(SalPr) content, upstream pressure and operating temperature on the membrane permeability and selectivity are described by the above equations.
2. Experimental 2.1. Materials
Polycarbonate (Uplion S-2000) (Mw=28 000) was supplied by Mitsubishi Gas Chemical. Dichloromethane, salicylaldehyde, cobalt acetate, and "7,7'diaminodipropylamine were supplied by Merck. The above mentioned chemicals are all of reagent grade and were used without further purification.
(2)
where SL denotes the solubility caused by Langmuir sorption and SH denotes the Henry's mode solubility. The gas permeation through the membrane is presumably determined by both the sorption-diffusion and
2.2. Membrane preparation
The PC membranes were prepared from a casting solution of polycarbonate in dichloromethane. The membranes were formed by casting the solution onto
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
a glass plate to a predetermined thickness using a Gardner knife at room temperature. The membranes were dried in vacuum for 24 h before gas sorption and permeation measurement. 2.3. Gas permeability measurements
The apparatus (Yanaco Gas permeability Analyzer, model GTR- 10) for measuring the gas permeability was shown in our previous report [6]. The gas permeability was measured by the following equation: 1 q/t Pm - - P 1 - P2 A w h e r e Pm is the gas permeability [cm3(STP)cm/
cm2s cmHg], q/t is the volume flow rate of gas permeate [cm3(STP)/s], l is the thickness [cm], P1 and P2 are the pressures(cmHg) on the high pressure and low pressure side of the membrane, respectively. A is the effective membrane area [cmZ]. The gas selectivity is calculated by the following expression: Pm(O2)
Selectivity - Pro(N2)
2.4. Gas sorption measurements
The experimental setup for gas sorption measurement was shown in our previous report [6]. The amount of gas absorbed was measured by a microbalance. The microbalance (Cahn Model D-202 Electrobalance) was enclosed in a stainless chamber and the chamber was enclosed in a constant temperature box. The system pressure was then reduced to about 4× 10 - 3 torr before the gas sorption measurement.
11
3. Results and discussion 3.1. Effect o f Co(SalPr) content on gas selectivity and permeability
The effects of Co(SalPr) content in the membranes on the gas permeability and selectivity are shown in Fig. 1. Both the oxygen and nitrogen permeability increased with increasing Co(SalPr) content in the polycarbonate membrane. According to our previous experience, the oxygen/nitrogen selectivity usually decreases as the permeability increases. However, the oxygen/nitrogen selectivity of these Co(SalPr) containing membranes first increased with increasing Co(SalPr) content and then decreased when more than 3 wt% of Co(SalPr) was added. The original polycarbonate membrane exhibited an O2/N2 selectivity of 5.00. The oxygen/nitrogen selectivity of the 3 wt% Co(SalPr) containing membrane reached 6.92 at 35°C. The oxygen permeability of the same membrane was 1.65 barrer. Usually the more oxygen carrier was added in the membrane, the higher oxygen/nitrogen selectivity would be obtained [11]. But in this PC/Co(SalPr) membrane system, when more than 3 wt% of oxygen carrier was added, the oxygen/nitrogen selectivity gradually decreased. It was interesting to explore why both the permeability and selectivity increased
S.O
I0
4.0 t.
i
i 3.0 i
2.01 2.5. Specific volume measurements
The specific volume was estimated by dividing the membrane volume by the mass of membrane. The membrane volume was calculated by multiplying the average membrane thickness by the area. The membrane thickness was measured by the thickness gauge (Teclock MS1201). The average membrane thickness was obtained from the average of ten measurements.
E 2
O.
, 0
i 4
,
i 8
,
0
12
Co(~iPr) content(wt %) Fig. 1. Effect of Co(SalPr) content in membranes on gas permeabilities and gas selectivity. (/k): oxygen permeability, (O): nitrogen permeability, ( I ) : P(O2)/P(N2).
12
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18 8
10
!
•
•
i
!
:
|
H
! 0
4)
4,
I 0
!°
•
A
I
I
10
20
J
<>
o m <> 30
l
0
30
2O
Pm~re(..cm)
Preuure(stm) Fig. 2. Oxygen sorption isotherm of PC/Co(SalPr) membranes with various Co(SalPr) content at 35°C. ( i ) : 10 wt%, (A): 5 wt%, (O): 3 wt%, (O) PC.
I
J
10
Fig. 3. Nitrogen sorption isotherm of PC/Co(SalPr) membranes with various Co(SalPr) content at 35°C. ([]): 10 wt%, (/X) 5 wt%, (C)): 3 wt%, (~): PC.
I.O
when the carrier content was lower than 3 wt% and why the selectivity decreased with increasing carrier content when more than 3 wt% of Co(SalPr) was added.
~
,
-'
3
0.8
J 3.2. Effect of Co(SaIPr) content on the gas solubility In order to understand the gas transport behavior in the Co(SalPr) complexed membranes, the oxygen and nitrogen sorption isotherms were measured. The sorption isotherms of oxygen and nitrogen at 35°C are shown in Figs. 2 and 3. Both the amounts of oxygen sorption and nitrogen sorption increased with increasing Co(SalPr) content. The gas solubility was calculated by dividing the amount of gas sorption by the corresponding pressure. As shown in Fig. 4, both the oxygen and nitrogen solubilities at 1 atm increased with the increase of Co(SalPr) content, but the solubility ratio of oxygen to nitrogen first decreased at low Co(SalPr) content and then increased as the amount of Co(SalPr) raised above 3 wt%. To explain the above results sorption isotherms of both oxygen and nitrogen were fitted by the dual sorption model shown in Eq. (1). The dual sorption model describes the part of gas sorption in the polymer nodules in Henry's mode and the part of gas sorption between polymer
_~ o., J
©
J,H
0.2
o.~
*
I
4
n
I
8
Co(S.~Pr) conteut(wt
a
%)
o 12
Fig. 4. Effect of Co(SalPr) content in membranes on gas solubility and gas solubility selectivity. (A): oxygen solubility, (O): nitrogen solubility, (11): solubility ratio (S(02)/S(N2)).
nodules in Langmuir's mode [12]. The fitted parameters are shown in Table 1 and the Henry's and Langmuir's solubilities are listed in Table 2. It was found that the Henry's mode of sorption of nitrogen increased rapidly as the Co(SalPr) content increased
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
13
Table 1 Dual sorption parameters of PC/Co(SalPr) membrane at 35°C Membrane
Oxygen
Nitrogen
Co(SalPr)
KD
CH I
[cm3(STP)/cm 3 atm]
[cm3(STP)/cm 3]
b [atm l]
KD [cm3(STP)/cm 3 atm]
CH !
content [wt%]
[cm3(STP)/cm 3]
b [atm -1]
0 3 5 10
0.162 0.22 0.24 0.276
0.076 0.100 0.190 0.235
1000 1000 1000 1000
0.107 0.189 0.188 0.192
0.048 0.04 0.08 0.10
10 100 100 100
Table 2 Langmuir mode sorption and Henry mode sorption of PC/Co(SalPr) membrane at 35°C Membrane
Oxygen
Nitrogen
Co(SalPr) content [wt%]
SH [cm3(STP)/cm 3 atm]
SL [cm3(STP)/cm 3 atm]
$8 [cm3(STP)/cm 3 atm]
SL
0 3 5 10
0.162 0.220 0.240 0.276
0.075 0.099 0.189 0.245
0.107 0.189 0.188 0.192
0.043 0.039 0.079 0.099
from 0 to 3 wt% but that of oxygen increased only slightly. This result might explain why the oxygen to nitrogen solubility ratio slightly decreased after 3 wt% of Co(SalPr) was added. As more Co(SalPr) was added, the Langmuir sorption of oxygen increased more rapidly than that of nitrogen, which resulted in an increase in oxygen/nitrogen solubility ratio.
Sn( O2)/SH(N2)
[cm3(STP)/cm 3 atm] 1.51 1.16 1.27 i .60
$
10
4 m q~
jm
3.3. Effect of Co(SalPr) content on the effective diffusivities of oxygen and nitrogen The dependence of effective diffusivity on Co(SalPr) content is shown in Fig. 5. According to the solution-diffusion model the effective diffusivity was defined as the ratio of permeability to solubility. The effective diffusivity of oxygen increased slightly as the Co(SalPr) content increased to 3 wt% and then decreased when more Co(SalPr) was added. However, the effective diffusivity of nitrogen behaved opposite to that of oxygen. To understand the behavior of nitrogen diffusivity, the specific volume of the membranes were measured. As shown in Fig. 6, the Co(SalPr) content had the same effect on the effective diffusivities of nitrogen as on the specific volume of membranes. This result indicated that the transport of nitrogen might be dominated by pore diffusion. The
~
z~
i
l~J&
J i
4
|
$
i
o 12
Co(SmlPr) eonteut(wt %) Fig. 5. Effect of Co(SalPr) content in membranes on effective diffusivity and diffusivity ratio. (A): oxygen solubility, (O): nitrogen solubility, ( I ) : effective diffusivity ratio of oxygen to nitrogen.
effect of Co(SalPr) on the effective diffusivity of oxygen was rather difficult to understand. According to the specific volume measurement, pore diffusivity Dp decreased when only 3 wt% of Co(SalPr) was
14
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
1.0
2.0
[
±,
1.6
J.
1.2
!
,.a
o.a
4 ~
0.8
_=
£ 0.4
0
0
0
0
i
i
!
l
0
2
4
6
8
10
0,0
0
0
Pressul'e(itm)
0.6 0
i 4
I
Co(Sal~Xwt% )
8
12
Fig. 6. Effect of Co(SalPr) content in membranes on membrane specific volume.
Fig. 7. Effect of operating pressure on gas permeability and gas selectivity with 3 wt% Co(SalPr) content. (A): oxygen permeability, (O): nitrogen permeability, (11): P(O2)/P(N2).
0.6
added, but the effective diffusivity of oxygen slightly increased. It was obvious that the effective diffusivity of oxygen at Co(SalPr) content below 3 wt% was not dominated by pore diffusion but dominated by the first two terms in Eq. (3), namely, the Henry's and Langmuir's sorption--diffusion transports.
~
~
-i
-
I i
i lift 1.2 I
i
u[
3.4. Effect of operating pressure on gas separation performance The effect of operating pressure on the gas separation performance of 3 wt% Co(SalPr) membrane at 35°C is shown in Fig. 7. The gas permeabilities of both oxygen and nitrogen were independent of the operating pressure, and the oxygen/nitrogen selectivity remained constant. In order to further understand the effect of operating pressure on the gas transport behavior, the gas solubilities at various pressures were measured. Fig. 8 shows the effect of pressure on the solubility of oxygen and nitrogen. The solubility of oxygen decreased with the increase of pressure but that of nitrogen remained almost constant. This result was in good agreement with the dual sorption model. Although the solubility of oxygen decreased with the increase of pressure, the oxygen permeability was not affected by the operating upstream pressure. Accord-
~
0.0
t
0
o.4
'
L
2
,
_
L 4
~ 6
Prmure(ttm)
J II
,.a
0.0 10
Fig. 8. Effect of operating pressure on gas solubility and gas solubility selectivitywith 3 wt% Co(SalPr) content. (A): oxygen solubility, (O): nitrogen solubility, (11): selectivity ratio (S(O2)/ S(N2)).
ing to Eq. (2) there were only two possibilities for oxygen permeability to be independent of pressure: one possibility was that the transport of oxygen was dominated by pore diffusion and the pore diffusivity Dp is independent of pressure. The other is that the DH of oxygen is much higher than DL(O2) so that the permeability remains constant regardless of the
R.- C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
15
7
10,
1.0
0"8t 0.6~
3'
6
,g
J \
~
N
4
0.4
i
0.2 I
2
0.0
i
0
I
4
i
d
i
i
4
6
8
2
10
Prm,,~(,,u)
I
8
_
12
Co(SaIPr)co.teal(wt%) Fig. 9. Effect of Co(SalPr) content on the Henry's mode sorption of oxygen in the membranes.
decrease in solubility. It was shown in the previous section that the oxygen transport through the membrane containing 3 wt% of Co(SalPr) was not dominated by pore diffusion but dominated by the first two terms in Eq. (2). Therefore, the DH of oxygen should be much higher than DL(O2). If it were the case, the fraction of Henry's mode sorption played a much more important role than did the Langmuir sorption mode. Fig. 9 shows the fraction of Henry's sorption(XH) at various Co(SalPr) content. This figure might explain the dependence of Deff(O2) o n the Co(SalPr) contents shown in Fig. 5. It was found that the XH of oxygen varied in the same direction as the effective diffusivity did, which indicated that the transport of oxygen through the PC/Co(SaIPr) membranes was dominated by the diffusion through the regions where the Henry's sorption occurred. The effect of operating pressure on the effective gas diffusivity is shown in Fig. 10. The effective diffusivity of oxygen increased with increasing operating pressure, but that of nitrogen remained almost constant. It was surprising to find that the diffusivity ratio of oxygen to nitrogen was higher than 5.0 and increased as the operating pressure increased. It indicated that the gas separation was determined by the high oxygen diffusivity of the membrane rather than its high oxygen solubility. As discussed previously the
Fig. 10. Effect of operating pressure on gas effective diffusivity and gas diffusivity ratio of oxygen to nitrogen of membrane with 3 wt% Co(SalPr) content. (A): oxygen diffusivity, (©): nitrogen diffusivity, (11): effective diffusivity ratio of oxygen to nitrogen.
transport of oxygen was dominated by the diffusion through the Henry's sorption area. Because the O2/N2 solubility ratio was only slightly higher than one, it suggested that the Drt(OE)/DH(N2) had a value much higher than one. All the above discussions actually suggested that the separation of oxygen from nitrogen through the PC/Co(SalPr) complexed membrane was facilitated by a high DH(O2)/DH(N2) value. In addition, the increase of oxygen effective diffusivity with the increase in pressure could be explained by Eq. (3). As shown in Fig. 11 the fraction of Henry's sorption increased with the increase of pressure. Since DR was much larger than DE the increase of XH led to an increase in effective diffusivity.
3.5. Effect of operating temperature The effects of operating temperature on the gas separation performance are shown in Fig. 12. The oxygen and nitrogen permeability increased but the oxygen/nitrogen selectivity decreased with the increase of temperature. In order to understand the effect of operating temperature on gas transport behavior, the oxygen and nitrogen sorption isotherms at various temperatures were measured. The results are shown in Figs. 13 and 14. The solubility of oxygen
16
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18 1.0
8
i
• 2¢c
t ~ygen 0.8
6
@
0.6
36 °C 460C
@
T
E II 0
0 4
0
0.44
@ m o
2
0.2,
0.0
i
t
0
,
I
10
;+
0
J
20
o
30
i
L
10
20
Fig. 11. Effect of oxygen pressure on the Henry's sorption in the membrane with 3 wt% Co(SalPr) content. ,
30
Presnure(ttm)
Pressure(arm)
10.0
~
@o eO~ O~
•
Fig. 13. Oxygen sorption isotherm of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (@): 25°C, (~): 35°C, (<>): 45°C.
8
i
Nitrogen
|
E
26°¢
1.0
A 36°C-
Jm
& A
es o om
E 2
e
A 480C
A A
0.1 3.00
'
I
1.20 1/T X I0 3 (l/K)
I 3.40
0
Fig. 12. Effect of operating temperature on gas permeability and gas selectivity of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen permeability, (O): nitrogen permeability, ( l l ) P(O2)IP(N2).
and nitrogen could therefore be calculated. The effect of operating temperature on gas solubility is shown in Fig. 15. It was found that the oxygen and nitrogen solubilities decreased with increasing temperature and the solubility ratio also decreased with the increase of temperature. The effect of temperature on the effective
& 0
10
2o
3o
Pressure(am) Fig. 14. Nitrogen sorption isotherm of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (&): 25°C, (ZX): 35 °C, (A): 45°C.
diffusivity is shown in Fig. 16. Both the effective diffusivities of oxygen and nitrogen increased with the operating temperature as did the O2/N2 diffusivity ratios. The above information indicated that the rate of oxygen and nitrogen transporting through the membrane was controlled by the effective diffusivities
17
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
3
10.0
/
k~ 02
/,
6
2 ~
|
,i
1.0
,J
°m
/ ~ ~
0.1
Nitroges ~en
0.1 3.00
'
I
n
I
0
3.40
3.20
0.0
I/T X 10 a (l/K)
.
3.00
Fig. 15. Effect of operating temperature on gas solubility and solubility ratio of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen solubility, (©): nitrogen solubility, (11): solubility ratio. 100
! , 3.20 I / T X 10 3 ( l / K )
I :3.40
0
Fig. 17. Effect of operating temperature on Henry's sorption and Henry's sorption ratio of oxygen and nitrogen selectivity with 3 wt% Co(SalPr) content. (A): oxygen solubility, (C)): nitrogen solubility, (11) Srt(O2)/SH(N2).
6
4
2
J
r~ 0 3.00
2
I
,
3.20 I/T X 103 (I/K)
I
,
0
3.40
Fig. 16. Effect of operating temperature on the effective diffusivity and diffusivity ratio of PC/Co(SalPr) membranes with 3 wt% Co(SalPr) content. (A): oxygen solubility, (C)): nitrogen solubility, (11): effective diffusivity ratio of oxygen to nitrogen.
because the effect of temperature on the permeability is similar to that on the effective diffusivity. But, in spite of the fact that the effective diffusivity ratio of oxygen to nitrogen was much higher than the solubility ratio, it was obvious that the solubility played an important role in the separation of oxygen and nitrogen since the effect of temperature on the O2/N2 selectivity followed the same trend as that on the
solubility ratio. However, the solubility ratio increased by two fold as the operating temperature decreased from 45 to 25°C, but the increase in oxygen/nitrogen selectivity was relatively small (from 6 to 7.6). This could be explained by the decrease of the effective diffusivity ratio which balanced part of the increase in solubility. But the effective diffusivity, defined as Deff=Pm/S, is actually a complexed function which also varies with solubilities. A better explanation is needed. Compared with the huge increase in SL(O2)/SL(N2), the solubility ratio of Henry's mode, SH(O2)/SH(N2), shown in Fig. 17, increased only slightly from 1 to 1.3 when the operating temperature was reduced from 45 to 25°C. As mentioned before, the oxygen transport at 35°C was dominated by the diffusion through the Henry's sorption area. If this was also true at 25 and 45°C, the effect of temperature on O2/N2 selectivity was mostly reflected by the effect on the solubility ratio of Henry's mode, i.e. SH(O2)/SH(N2)
4. C o n c l u s i o n s
The gas permeability and O 2 f N 2 selectivity of the polycarbonate membrane were both increased by adding 3 wt% of Co(SalPr). The obtained oxygen perme-
18
R.-C. Ruaan et al./Journal of Membrane Science 135 (1997) 9-18
ability and oxygen/nitrogen selectivity were 1.65 barrer and 6.92 at 35°C, respectively. Further increase in Co(SalPr) content led to an increase in gas permeability but a decrease in oxygen selectivity. It was found that the O2/N2 solubility ratio decreased in spite of the increase in Oe/N2 selectivity after 3 wt% of Co(SalPr) was added. Specific volume measurement implied that the pore diffusion was responsible for the above behavior. The contribution of diffusion-solution type transport was also investigated by examining the transport behavior of the 3 wt% Co(SalPr) containing membrane through which the pore diffusion is relatively low. The pressure dependence of oxygen permeability and solubility of the 3 wt% Co(SalPr) membrane revealed the importance of Henry's mode diffusion in O2/N2 separation. It suggested that it was the ratio of Henry's mode diffusion dominating the O2/ N2 separation instead of the overall 02 to N2 solubility ratio. The temperature dependence of O2/N2 selectivity indirectly supported this assumption. It was also found that the O2/N2 selectivity was offered more by the 02 to N2 diffusivity ratio than by the Henry's mode solubility ratio. However, the increase in Henry's mode solubility ratio amplified the O2/N2 selectivity.
References [1] B.M. Johnson, R.W. Baker, S.L. Matson, K.L. Smith, I.C. Roman, M.E. Tuttle and H.K. Lonsdale, Liquid membranes for the production of oxygen-enriched air. II. Facilitatedtransport membranes, J. Membrane Sci., 31 (1987) 31~57. [2] E. Tsuchida, H. Nishide, M. Ohyanagi and H. Kawakami, Facilitated transport of molecular oxygen in the membrane of polymer-coordinated cobalt Schiff base complex, Macromolecules, 20 (1987) 1907-1912.
[3] H. Nishide, H. Kawakami, T. Suzuki, Y. Azechi and E. Tsuchida, Effect of polymer matrix on the oxygen diffusion via a cobalt porphrin fixed in a membrane, Macromolecules, 24 (1991) 6306--6309. [4] J.M. Yang and G.H. Hsiue, Modified styrene-butadienestyrene block copolymer membranes complexed with (N,N~diasalicylideneethylene diamine) cobalt(H) for oxygen permeation: polymeric axial ligand effect, J. Membrane Sci., 87 (1994) 233-244. [5] M.J. Choi, C.K. Park and Y.M. Lee, Chelate membrane from poly(vinyl alcohol)/poly(salicylidene ally amine) blend. I1. Effect of Co(II) content on oxygen/nitrogen separation, J. Appl. Polym. Sci., 58 (1995) 2373-2379. [6] S.H. Cheng and J.Y. Lai, Polycarbonate/N,N'-dialicylidene ethylene diamine) cobalt(ll) complex membrane for gas separation, J. Appl. Polym. Sci., accepted for publication (1995). [7] H. Nishide, M. Ohyanagi, O. Okada and E. Tsuchida, Dualmodel transport of molecular oxygen in a membrane containing a cobalt porphyrin complex as a fixed carrier, Macromolecules, 20 (1987) 417-422. [8] O.L. Harle and M. Calvin, The oxygen-carrying synthetic chelate compounds. VI. Equilibrium in solution, J. Am. Chem. Soc., 68 (1946) 2612-2618. [9] W.R. Vieth, J.M. Howell and J.H. Hsieh, Dual sorption theory, J. Membrane Sci., 1 (1976) 177-220. [10] R. Rangarajan, M.A. Mazld, T Matsuura and S. Sourirajan, Permeation of pure gases under pressure through asymmetric porous membranes. Membrane characterization and prediction of performance, Ind. Eng. Chem. Process Des. Dev., 23 (1984) 79-87. [11] E. Tsuchida, H. Nishide, M. Ohyanogi and H. Kawakami, Facilitated transport of molecular oxygen in the membranes of polymer-coordinated cobalt Schiff base complexes, Macromolecules, 20 (1987) 1907-1912. [12] R.E. Kesting, A.K. Fritzsche, M.K. Murphy, C.A. Cruse, A.C. Handermann, R.E Malon and M.D. Moore, The secondgeneration polysulfone gas separation membrane. I. The use of Lewis acid: Base complexed as transient templates to increase free volume, J. Appl. Polym. Sci., 40(9/10) (1990) 1557.