Ind. Eng. Chem. Res. 1996,34, 4071-4077
4071
Transport Properties of Carbon Dioxide through Amine Functionalized Carrier Membranes Takeo YamaguchiJ Lars M. Boetje,?Carl k Koval,’ Richard D. Noble,*pt and Christopher N. Bowman? Department of Chemical Engineering and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309-0424
COz facilitated transport was studied using a membrane with amine sites covalently bound to the polymer backbone and ion exchange membranes which have amine complexing agents (carrier) as counterions. The two types of membranes were compared, and several amine functionalities for the ion-exchange membrane were studied. Although the covalently bonded membranes had a larger absorption capacity than amine functionalized Nafion ion-exchange membranes, the facilitation effect of C02 through the covalently bonded membrane was very small (less than 1.5 at 10 Wa C02 partial pressure) compared with the ion-exchange membrane (8.0 a t 11kPa C02 pressure with mono protonated ethylenediamine carrier form in Nafionll7). A simulation model for the covalently bound membranes was developed to understand the results. The facilitation effect depends largely on carrier d f i s i v i t y , and reactive diffusion through the covalently bonded membrane is much slower than counterion W s i v i t y through the ion-exchange membrane. The ethylenediamine carrier exhibited a large facilitation effect in a Nafionll7 membrane. Other secondary or hindered diamines exhibited a small facilitation effect. For a swollen Nafion membrane, both ethylenediamine and ethylenediamine derivatives showed a high facilitation effect, and a secondary diamine produced a facilitation factor of 4.09 and a COZ mol/cm2 s at 22.5 kPa C02 pressure in the feed. flux of 1.02 x
Introduction Several researchers reported that ion-exchanged amine carriers promote C02 transport across ion-exchange membranes (LeBlanc et al., 1980; Way et al., 1987; Way and Noble, 1989; Pellegrino et al., 1988; Langevin et al., 1993; Matsuyama et al., 1994). This membrane has a much higher carrier stability than conventional immobilized liquid membranes because the carrier is retained by strong electrostatic forces. LeBlanc et al. (1980) evaluated several types of ionic carriers, such as carbonate, arsenite, glycinate, and mono protonated ethylenediamine (EDA). The EDA carrier showed the highest facilitation effect among them, thus, EDA has been chosen as the carrier for this study. The amine carrier exists as a counterion in the ion-exchange membrane, and it can react with C 0 2 through the carbamate reaction. The counterions are mobile, and the diffisivity was measured by electrical conductivity (LeBlanc et al., 1980; Langevin et al., 1993). Although many kinds of secondary, tertiary, and hindered amines are used as an absorbent for acid gas in industry, EDA has been primarily used t o study the carrier effect in ion-exchange membranes. On the other hand, a fixed site carrier membrane is also an attractive candidate for facilitated transport. Nishide et al. have reported that a membrane containing a cobalt porphyrin complex as a fixed carrier facilitated oxygen transport (Nishide et al., 1986; Tsuchida et al., 1988). The fixed site carrier membrane has a different type of transport mechanism than the ionexchange carrier membrane. The carrier is bonded to the polymer backbone in a fixed site carrier membrane, and the carrier itself cannot diffise through the membrane. Thus, one diffusion path in these membranes is hopping (reactive diffusion). Yoshikawa et al. (1988) studied a covalently bonded membrane with a pyridine + 4
Department of Chemical Engineering. Department of Chemistry and Biochemistry.
moiety for COz transport. They used dry gas, and the pyridine site exhibited a weak acid-base interaction for COz. In this study, we investigated the reactive diffusion effect in a fixed site carrier membrane using the carbamate reaction to compare with the ion-exchange membrane. Other amine carriers for the ion-exchange membrane were also studied. We employed poly(ally1amine)as the covalently bonded amine site material. This polymer is the only simple primary amine polymer that we could obtain commercially. The polymer can be easily cross-linked (Kobayashi et al., 1985) or modified (Seo et al., 19911, and the polymer can be used to fabricate a reverse osmosis membrane (Oikawa et al., 1990). For the ion-type amine carrier, primary-primary type EDA, secondary-secondary type Nj”-dimethylethylenediamine (SS-EDA), and hindered-primary type dimethylaminopropane (HP-EDA) were used as the carrier. Commercial Nafion (Nafionll7) and a heat treated Nafion membrane were used as the ion-exchange membrane. Heat treatment swells the Nafion membrane, and the treatment changes the cluster size. Heat treated Nafion with an EDA carrier exhibited a higher facilitation effect and flux than Nafionll7 with EDA because heat treated Nafion has a larger cluster size which affects EDA carrier mobility (Pellegrino et al., 1988; Heaney and Pellegrino, 1989). This morphology change of the Nafion membrane will also affect the facilitation effect of the other types of amine carriers.
Simulation Model Reaction Mechanism of COZwith Amine. Carbonate or bicarbonate ion concentrations are negligible in the membrane (Langevin et al., 1993). Thus, we should consider the aminelcarbon dioxide reaction. The amine site can react with carbon dioxide to form a zwitterion which reacts with base (Danckwerts, 1979). So, the zwitterion can react with another amine site in the membrane because most base in the membrane is
0888-5885/95/2634-4071$09.00l00 1995 American Chemical Society
4072 Ind. Eng. Chem. Res., Vol. 34,No. 11,1995
present as amine sites.
RjNH+COO
I
+
'
kAM
RjNH
R'
I
RiNCOO
=
+
R'
k-AM
R2
RjNHz+
(2)
I
fraction of water content. Solute and carriers are existing in the water region in the membranes, and the reaction and diffusion take place in the region. The diffusion coefficients of C02 include tortuosity and viscosity effects in the water region of the membrane. For this case, the film model was used t o solve the transport equations. The mass balance of each species is given as follows.
R'
(11)
Thus, the total reaction equilibrium constant, Keq, is as follows.
Assuming a steady state concentration of the zwitterion, the overall reaction rate of eq 1 and 2, R1,.is described as follows.
External mass transport resistance is negligible (Noble et al., 1986) for this case. Boundary conditions are as follows. (l)atx=O
Rl = -d[CO,ydt -
kl
(1+ k-i/kM[-NHI)
(13) X
[CO,], = 0
(15)
d[NCOOlldx = 0
(16)
The total C02 flux, Jtotal, is
Keq[-NHl In the case of monoethanolamine, we can assume k m [-"I > > k-1 (Danckwerts, 19791, and 121 = to k1'. Transport Model for the Covalently Bonded Membrane. For this case, carrier diffusion is given by reactive diffusion (Noble, 1990, 1993). Thus, the physical meaning of the diffusion coefficient is different from the ionic carrier diffusion coefficient in the ion-exchanged membrane. The total amine concentration is constant at any point in the membrane, and, from the electrical neutrality condition, the following equation is obtained. [-NH2+l = [-NCOO-l
(6)
The total fixed carrier concentration, C,, is constant throughout the membrane.
+ [-"I
(14)
(2) a t x = L
[-NCOO-l[-NH,fl
[-",+I
d[NCOOYdx = 0
+ [-NCOO-l=
C,
(7)
The reactive diffusion coefficients, D", of the two species are the same because the reactive diffusion is based on zwitterion mechanisms described above.
Thus, the electrical field effect across the membrane is negligible. The flux expressions for each mobile species are: (9)
where QI is membrane porosity which corresponds to the
Jtotal
(17)
= JCO, -k JNCOO
The Facilitation factor, F, is defined as follows.
F = Jtotal/JC02 (without carrier a t the same CO, feed pressure) (18) To calculate the model, a numerical method was employed instead of an analytical model (Noble et al., 1986)because the carbamate reactions (eqs 1and 2) are not a simple first order reaction (A B = AB). Simulation conditions and specific values of some amine components in aqueous solution taken from the literature are shown in Tables l and 2, respectively.
+
Experimental Section Membrane Preparation. (1) Covalently Bound Membrane. Poly(ally1amine) hydrochloride was purchased from Poly Sciences Inc. 2-bromopropane and ethylene glycol diglycidyl ether were used as an amine modifier and cross-linker, respectively. Poly(ally1amine) hydrochloride was dissolved in water, and then the polymer was precipitated in methanol and dried in air. The polymer was poured into a methanol solution which contained 1equiv of KOH and agitated overnight. The poly(ally1amine)hydrochloride changed to free poly(allylamine), and the polymer was dissolved into methanol. To make a partially secondary amine polymer, 2-bromopropane was added to the methanol polymer solution, and the solution was stirred at 50 "C for 48 h. A 1equiv amount of 2-bromopropane was added to the amine polymer solution. Some of the free polymer was readted with 2-bromopropane in methanol. The polymer solution with the cross-linker was cast onto both sides of a porous membrane filter. The
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4073 Table 1. Reference Data of Amine COZReaction Kinetics in Aaueous Solutiona MEA (primary) 5.92b 58.6
DEA DIPA EDA AMP (secondary) (secondary) (diamine) (hindered) kl [m3/(molS)I 3.17c 2.70d 12.5e 1of Kes [m3/moll 1.43f 1.72f 1.13h 0.12f k - ~ / ImoVm31 k ~ 458Oe 13600d 7900of a MEA, monoethanolamine; DEA, diethanolamine; DIPA, diisopropanolamine; EDA, ethylenediamine; AMP,2-amino-2-methylpopanol. Blauwhoff et al., 1984. Versteea and Ovevaar, 1989. Versteee and van Swaaij, 1988. e Hikita et al., 1977. f Bosch et al., 1990. g Chan and Danckwerts,'l981. Langevg et a1.,-1993. amine
I
'
Table 2. Simulation Conditions for the Covalently Bonded Membrane temp [ T I
25
C02 pressure in feed [kPal Kes [m3/moll
amine concna 17 [mmoV(gof water)] membrane 320 kl' [m3/(molS)I thicknessa Cum] DCO; [m2/sl 8.4 x 10-lo D"/Dco2 porosity 0.42
-(CHz-CH),I
-(CHz-CH),-
-(CHz-CH),-
I
0-50
CHZ
CH2
I
I
0.1-100
"2
NH I
1-100 0-0.005
1) urimarv amine
CH3-CH-CH3 (poly(alli1arnine)) 2) secondary amine
1
CH2
CH2 I
I
NH OH I
HO
I
I
NH 1
CH-CH-...........-CH-CH 3) crosslinking points
Figure 1. Chemical structure of amine polymers,
a Data were based on primary type cross-linked poly(ally1amine) membrane.
membrane was dried overnight at room temperature; then the membrane was annealed at 80 "C for 1 h to complete the cross-linking reaction. The cross-linking reaction also made secondary amine sites. A Gelman Science membrane filter was used as the porous substrate. The substrate has a thickness of 178 pm and a 0.2-pm pore size. The cast membrane is transparent, implying that polymer penetrates into the pores of the substrate. The structures of poly(ally1amine) and modified poly(ally1amine) are illustrated in Figure 1. The cross-linker concentration is defined as a molar ratio of cross-linker to amine sites in the casting solution. To make primary type poly(ally1amine) membrane, the polymer solution which did not react with 2-bromopropane was used as the casting solution, and the crosslinker ratio was controlled at 0.1. For a secondary type membrane, partially secondary amine solution was used, and the cross-linker rate was set at 0.2 or 0.4. The primary type poly(ally1amine) membrane has 42% water and a thickness of 320 pm, and the secondary type poly(allylamine) membrane which has a cross-linker ratio of 0.2 has 27% water content and a thickness of 200 pm. The membranes were analyzed by FT-IR. (2) Ion-Exchange Membrane. Nafionl17 and heat treated Nafionll7 were used as ion-exchange membranes. The dry membrane has 1100 g/equiv molecular weight, and a thickness of 170 pm. All Nafion membranes were washed in boiling 1 M nitric acid aqueous solution for 1 h. After the membranes were dried in air, the membranes were kept in distilled water at room temperature. To obtain a swollen Nafion membrane, the same method described by Pellegrino et al. (1988) was used. The acid form membranes were converted to the Na salt form by soaking them in 1M NaOH solution overnight with stirring. Some of the membranes were heated to 210 "C in a glycerin bath. After heating, the membrane was washed in 1 M hydrochloric acid solution, then the membrane was converted to the Na salt form again. After the COS transport measurements through a Na+ type membrane were made, the membrane was converted to mono protonated EDA or the mono protonated ethylenediamine derivative form. SSEDA and HP-EDA were used as the ethylenediamine derivatives. The Na+ form nonswollen membrane and the swollen membrane had 16 and 45%water contents, respectively. The carrier concentration, which corresponds t o ion exchange capacity, in the normal Nafion
-(CHz-CH),-
I
c
lm
Flow
GC
Flow meter
He
ConstantTemprature Box
Figure 2. Gas transport apparatus.
and the heat treated Nafion were 4.8 and 1.1mmol/(g of water), respectively. Absorption. C02 absorption into the membranes was measured a t room temperature (about 23 "C). Water swollen membranes were placed into a bottle containing a nitrogen atmosphere, and then C02 was introduced into the bottle. The C02 concentration change was measured by gas chromatography. C02 Transport. The transport apparatus is shown in Figure 2. Two sizes of effective membrane area were used to reduce the permeate side CO2 pressure effect. The areas were 4.5 or 10.7 cm2. The feed gas mixture was composed of C02 and N2. He flow rate at the permeate side was controlled between 0.1 and 1.0 mL/ s, and the C02 concentration in the He gas was measured by gas chromatography. The flow rate of the feed gas was kept above 2 mUs. Both feed and permeate gases were humidified by bubbling through deionized water before contacting the membrane. All of measurements were made at 25 "C, and the total feed gas pressure was 84 kPa.
Results and Discussion FT-IR Measurement. FT-IR spectrum of poly(allylamine)and cross-linked polfiallylamine)are shown in Figure 3. The primary amine group has a specific peak at 1580 cm-l (Kobayashi et al., 1985). Although the poly(ally1amine) polymer has an intense peak at this wavenumber, the secondary type polymer does not. The cross-linking reaction also changes the primary amine site to a secondary site. The spectrum showed most of the primary amine sites changed to secondary amine sites in the cross-linked secondary poly(ally1amine) membrane. Absorption. C02 absorption results are shown in Table 3. A Na+ form Nafionll7 membrane was used as a reference. The Na+ form Nafionll7 has less than 0.1 mmol/(g of swollen membrane) absorption. Commercial Nafionll7 has 0.76 mmol/(g of swollen membrane) of ion-exchange sites, and the EDA concentration
4074 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
- - k, =IO [rnJirnols]
k
1800
1
,
I
I
1600 1400 Wavenumber[cm"l
"0
1200
Figure 3. FT-IR spectra of cross-linked poly(ally1amine) membranes.
0 0
10
20
30
40
50
CO,Pressure [kPal
Figure 4. COz partial pressure dependence on COz flux through cross-linked poly(ally1amine) membranes: (A) PAAM (primary); (B) cross-linked PAAM (secondary) (cross-linker concentration, 0.2); (C) cross-linked PAAM (secondary) (cross-linker concentration, 0.4). Table 3. COz Absorption Results of Amine Functionalized Membranes amine concn co2 amt absorbed [mmol/(gof [mmoV(g of pressure membrane [kPal swollen membrane)] swollen membrane)] Nafion 71
