Ind. Eng. Chem. Res. 2000, 39, 2447-2458
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Selective CO2 Separation from CO2-N2 Mixtures by Immobilized Glycine-Na-Glycerol Membranes H. Chen, A. S. Kovvali, and K. K. Sirkar* Department of Chemical Engineering, Chemistry and Environmental Sciences, New Jersey Institute of Technology, Newark, New Jersey 07102
This paper reports the results of our continuing efforts to develop glycerol-based immobilized liquid membranes (ILMs) for the selective separation of CO2 from a mixed-gas (CO2, N2) feed having low CO2 concentrations in space-walk and space-cabin atmospheres. The items of specific interest are replacement of the carrier sodium carbonate (studied by Chen et al. (Ind. Eng. Chem. Res. 1999, 38, 3489-3498) by glycine-Na in glycerol, ILM thickness reduction, performance of environmentally benign carriers, e.g., glycine-Na vis-a`-vis toxic and volatile carriers, e.g., ethylenediamine. The effects of glycine-Na concentration (range 0-5.0 mol/dm3), CO2 partial pressure (between 0.006 and 0.8 atm), and feed relative humidity (RH; range 40-100%) have been investigated. The sweep gas was always dry helium. As the glycine-Na concentration was increased, N2 permeability decreased, while the CO2 permeability increased drastically at lower glycinate concentrations, leveling off at higher glycinate concentrations. Lower feed stream RHs yielded lower species permeances but greater CO2/N2 selectivities. For a feed RH of 70%, pCO2,f ) 0.006 atm, and a glycine-Na concentration of 2.5 mol/dm3, the CO2/N2 separation factor was found to be a very high 5000 in an ILM spanning the whole thickness of a hydrophilized poly(vinylidene fluoride) flat film. ILMs containing both carbonate and glycinate demonstrated high CO2 permeances and high CO2/N2 selectivity. The ILM stability was also tested by a 25-daylong run. Permeances of N2 through glycerol-based membranes and of CO2 through pure glycerol membrane have been estimated and compared with experimentally obtained values. I. Introduction Gas separation using facilitated transport membranes (FTMs) has been the subject of considerable research for many years.1-3 The types of FTMs investigated generally fall into the following three categories:3 (1) immobilized liquid membrane (ILM), (2) solvent-swollen polymer membrane, and (3) fixed-carrier membranes. Major advantages of FTM over conventional polymeric membrane include enhanced selectivity and permeability for the target species because of reversible reactions between the carriers in FTM and the target species. This unique characteristic makes FTM especially attractive when the target species in the feed gas mixture exist in low concentrations because, to accomplish the separation/purification task, the limited transmembrane driving force would be too low for conventional polymeric membranes. Though generally considered to be the least stable configuration of FTMs, ILM has been widely investigated for the facilitated transport of carbon dioxide using various carriers. Ward and Robb4 made the pioneering study on CO2 permeation through a thin layer of carbonate/bicarbonate aqueous solution. Otto and Quinn5 and later Suchdeo and Schultz6 made theoretical analysis of CO2 transport through carbonate/ bicarbonate ILMs. Other investigators used amines as the carriers and/or ion-exchange membranes as the substrates. LeBlanc et al.7 and later Way et al. 8 studied the facilitated transport of CO2 in ion-exchange membranes using various organic amine counterions. Teramoto et al.9 used monoethanolamine (MEA) solutions, * Corresponding author. Tel.: +1-973-596-8447. Fax: +1973-642-4854. E-mail:
[email protected].
while Guha et al.10 and Davis and Sandall11 used diethanolamine (DEA) solutions immobilized in porous substrates as ILMs to study CO2 transport. Matsuyama et al.12 studied CO2 transport through a plasma-polymerized ion-exchange substrate employing ethylenediamine (EDA) as the carrier. Despite the attractive features of and intensive studies on ILMs for gas separation, commercial gas separation applications have been limited. Major work is still needed to improve the membrane permeances and demonstrate much longer operating life. In ILMs, the liquid solution is physically trapped in but not chemically bonded to the support matrix. The low stability can be a result of liquid washout and/or the evaporation of the liquids into the gas phases during operation. Various strategies have been employed to alleviate the problems of carrier loss and ILM drying out. Hughes et al.13 tried to circumvent the stability problem of a Ag+containing ILM for olefin-paraffin separation by periodically regenerating it. A more common practice when aqueous solutions are used as the ILM fluid is to humidify both the feed and sweep gas streams simultaneously. Another alternative is to use low volatility and hygroscopic liquids such as poly(ethylene glycol) (PEG) 11,14,15 or glycerol1 as the major component in the ILM fluid. We have recently reported1 that hydrophilic poly(vinylidene fluoride) (PVDF)-based Na2CO3-glycerol ILMs are stable when challenged with feed streams of very low relative humidities (RH). Because of the relatively low carrier concentrations and high viscosity of the glycerol-based ILM fluid, the Na2CO3-glycerol ILM showed high CO2/N2 selectivities but relatively low CO2 permeances.1 The glycerol-based ILM could be useful for CO2 removal from gas streams containing low
10.1021/ie9908736 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000
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concentrations of CO2 if its CO2 permeance is significantly increased. This paper presents the results of our continuing efforts to develop an ILM with improved permeation performances for separating CO2 from N2. Specifically, the goal was to develop a membrane for CO2 separation from breathing mixtures for space-walk applications. Conventionally, this separation is done by adsorption/ reaction using adsorbents/reagents discarded when saturated. This work investigates the suitability of the glycine-Na-glycerol and glycine-Na-carbonate-glycerol ILMs for CO2 separation for spacesuit applications. The background of this particular application imposes important limitations on the selection of ILM liquid and the carrier species. Because the feed gas is normally not completely humidified (i.e., RH < 100%), the ILM must be stable when the feed stream RH is relatively low. Also, to conserve oxygen, the membrane should have very high CO2/O2 selectivity (e.g., >2000) at the low RHs. Moreover, the ILM components should be completely environmentally benign. Therefore, the most studied amines in the literature (e.g., MEA, DEA, and EDA) are not suitable because of their relatively high volatilities and irritative nature. Glycine has been used as an additive in carbonate/ bicarbonate solutions for the selective removal of CO2 from industrial gas streams.16,17 LeBlanc et al.7 demonstrated that glycine salt can be a better carrier species for CO2 than carbonate in ion-exchange substrate-based ILMs. Recently, glycine salts have been incorporated into polymeric membranes for enhanced CO2 separation from gas streams containing CO2 and H2. Ho18 disclosed a CO2 separating polymeric membrane fabricated from poly(vinyl alcohol) and glycine salts (e.g., glycine-K and glycine-Li). In the present work, glycine-Na is used as the carrier species for the facilitated transport of CO2 in glycerol-based ILMs. As an amino acid salt, glycine-Na is environmentally friendly. Like other amines, glycinate ion forms labile complexes with carbon dioxide, but not with oxygen or nitrogen. Because glycine-Na is significantly more soluble than sodium carbonate in glycerol, it allows achievement of considerably higher carrier concentrations in the ILM. This also helps to decrease N2 or O2 solubility in the ILM because of the salting-out effect. Therefore, the glycine-Na-glycerol ILMs have potential for significantly higher CO2 permeance and CO2/N2 selectivity than carbonate-glycerol ILMs.1 Extendedterm runs have been carried out using low-RH gas streams to explore the usefulness of such an ILM in environments having a wide variation of RH. Some studies have also been carried out using EDA as a carrier in glycerol for comparative purposes. Further, efforts have been made to study the utility of a thin substrate as well as the effect of a reduction in the ILM thickness in a given substrate. 2. Background 2.1. Glycerol Membrane. Glycerol is a highly viscous liquid with extremely low volatility and high hygroscopicity. The viscosity of pure glycerol at 20 °C is 1412 cP. The viscosity of its aqueous solution, however, decreases rapidly with an increase of the water content. For example, at 20 °C, the viscosity of an aqueous glycerol solution containing 30 wt % water is about 22.5 cP.19 Because of its highly hygroscopic nature, glycerol strongly absorbs moisture from the
surroundings. This characteristic is useful for maintaining the water content in the ILM. As experimentally determined earlier by us,1 the equilibrium water content in glycerol decreases monotonically with a decrease in its environmental RH. We have recently studied Na2CO3-glycerol solutionbased ILMs for CO2 separation from CO2/N2 mixtures.1 Experimental results showed that the ILMs exhibit highly selective, facilitated transport of CO2 over N2/ O2. Unlike water-based ILMs reported in the literature, the glycerol-based ILMs are stable even when dry helium is used as the sweep gas and the feed stream has very low RH. Because the equilibrium water content in the ILM decreases with a decrease in the feed stream RH, the viscosity of the ILM fluid is expected to increase rapidly. Therefore, the permeances of both CO2 and N2 decrease with a decrease in the feed stream RH. Earlier results1 also showed that CO2 permeability increases with an increase in the carrier concentration. Because carbonate solubility in glycerol is limited, it is likely that the CO2 permeance through the ILMs can be further improved by employing more soluble carrier species to increase the carrier concentration. Another approach to increase the CO2 permeance is to employ thinner substrates to reduce the ILM thickness. The underlying relations between the facilitated CO2 flux and the glycine-Na concentration as well as the ILM thickness are briefly discussed below. 2.2. CO2 Transport through Glycine-Na ILMs. Glycine is the lowest molecular weight amino acid. It has long been used as an additive in carbonate solutions for promoting the CO2 absorption rate in industrial applications.16,17 In alkaline solutions, glycine exists in anionic form.20 In this work, glycine sodium salt is used as the carrier species that dissociates in the presence of water as follows:
H2NCH2COONa S H2NCH2COO- + Na+
(1)
Although Jansen and Feron21 argued that glycine could be treated as a sterically hindered amino acid, they did not give any information about the stability of the corresponding carbamate. Like other primary amines (e.g., ethanolamine), glycinate ion (hereafter designated as RNH2) reacts with CO2 to form carbamate:22
CO2 + RNH2 S RHNCOO- + H+
(2)
RNH2 + H+ S RNH3+
(3)
with the overall reaction being
CO2 + 2RNH2 S RHNCOO- + RHNH2+
(4)
Under the conditions of chemical equilibrium,
Keq )
[RHNCOO-][RHNH2+] [CO2][RNH2]2
(5)
There are three nonvolatile species: B1 ) RNH2, B2 ) RHNCOO-, and B3 ) RHNH2+. The molarity
m ) [RNH2] + [RHNCOO- ] + [RHNH2+]
(6)
where m is the initial glycinate molar concentration. The saturation factor y is defined as
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2449
ym ) [RHNCOO-] ) [RHNH2+]
(7)
Nitrogen is an inert species in the system. Its flux through the ILM can be written as
Then, eq 5 can be written as
y2 Keq ) [CO2](1 - 2y)2
NN2 ) (8)
In the presence of water, there are other reactions that could also lead to the facilitated transport of CO2. For example,
CO2 + OH- S HCO3-
(9)
Because [OH-] is low, its contribution is negligible compared to that of eq 4. The total CO2 flux, which is equal to the total flux of carbon at any point in the ILM, is
NCT ) NCO2 + NB2
(10)
where N is the molar flux (mol/s‚cm2). The first term on the right-hand side of eq 10 represents the contribution to the CO2 flux via solution diffusion. It can be written as
NCO2 )
DCO2 (C0CO2 - CLCO2) L
(11)
where D is the diffusion coefficient, C is the molar concentration, and L is the ILM thickness. Superscripts 0 and L refer to feed and permeate sides, respectively. For the present ILM, the contribution to the CO2 flux via solution diffusion is much less than that via facilitation. Under the condition of equilibrium, the flux due to facilitation, NB2, can also be written as
NB2 )
DB2 0 (C B2 - CLB2) L
(12)
Therefore, the total CO2 transport rate across the ILM is
NCT )
DCO2 DB2 0 (C0CO2 - CLCO2) + (C B2 - CLB2) L L
(13)
From eq 7, if saturation factor y is constant (this is equivalent to [CO2] being constant), an increase in the carrier concentration (m) should lead to a proportional increase in C0B2. That, in turn, increases the CO2 flux (from eq 13). For the glycerol-water-CO2 system,1 the addition of 1.0 M Na2CO3 in glycerol-water ILM increased the CO2 permeability by more than 1 order of magnitude. Equation 13 also shows that the CO2 flux can be increased by decreasing L, the ILM thickness. If diffusion is the controlling step, CO2 and N2 fluxes should increase proportionally with the reciprocal of L. If the ILM phase is homogeneous, the species diffusivity D can be experimentally determined or estimated. As discussed in a previous paper,1 there exists a sharp water concentration gradient both along the membrane and across the ILM thickness. That causes drastic viscosity changes in the ILM phase. As a result, the diffusivity of any species is drastically affected by the RH and the water content, making the estimation of D difficult.
DN 2 (C0N2 - CLN2) L
(14)
As will be experimentally confirmed later, an increase of the glycinate concentration m can significantly influence the ILM performance. First, C0B2 increases with an increase in m. Second, because of a salting-out effect, increasing the carrier concentration can decrease CO2 and N2 solubility (C0CO2 and C0N2). Therefore, the carrier concentration increase can increase the relative contribution of facilitated transport. The effects of ILM thickness on its performances are more complicated than they appear to be. As in the case of polymeric membranes, gas permeances are inversely proportional to the membrane thickness. This seems to suggest that membrane selectivity will not be affected by decreasing L. However, in the case of ILMs, membrane integrity may deteriorate as L decreases, as will be seen later for particular membranes. On the other hand, ILM selectivity will also decrease if the reaction resistance is not negligible compared to the diffusion resistance. There are two reasons why glycine-Na can be a better choice than sodium carbonate. First, reaction 2 is fairly rapid at room temperature.22 Second, glycine-Na is considerably more soluble in glycerol than sodium carbonate. The greater solubility of glycine-Na in the ILM not only provides more carriers for CO2 transport but also decreases N2 permeance through the ILM by decreasing N2 solubility in the ILM due to the saltingout effect.1 Therefore, greater CO2/N2 selectivity can be expected when glycine-Na is used as the carrier species. 3. Experimental Section 3.1. Materials. Flat sheets of hydrophilic PVDF microporous membranes (Millipore, Bedford, MA) and hydrophilized Celgard 2500 microporous membranes (Celgard, Charlotte, NC) were used to support the glycerol-based ILMs. Selected parameters of the substrates are listed in Table 1. Glycine-Na salt and glycerol of 99% purity (Sigma, St. Louis, MO) were used without further purification. Sodium carbonate was obtained from Mallinckrodt Chemical Co. (St. Louis, MO) and EDA from Fisher Scientific (Pittsburgh, PA). Glycine-Na-glycerol, glycine-Na-carbonate-glycerol, and EDA-glycerol solutions were prepared by dissolving the desired amount of the solutes in known volumes of glycerol. Because glycine-Na is significantly more soluble than Na2CO3 in glycerol, preparing a concentrated glycine-Na-glycerol solution is straightforward and much easier than preparing concentrated sodium carbonate-glycerol solutions,1 which entails the addition of some amount of water. Deionized water was obtained using a Barnstead E-Pure system (model D4641, Dubuque, IA). All gas mixtures and pure gases used were obtained from Matheson (East Rutherford, NJ). 3.2. Preparation of ILMs. Detailed procedures for preparing the glycerol-based ILMs using immersion or coating techniques were provided earlier1 and are only briefly described here. In the immersion technique, the hydrophilic substrate is immersed in the glycine-Naglycerol solution for a predetermined period and then removed from the liquid. After the extra fluid is wiped
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Table 1. Physical Parameters of the Substrates Used in This Work
a
substrate
nominal pore size, µm
porosity
nominal thickness, µm
tortuosity
PVDF hydrophilized Celgard 2500
0.1 0.04
0.7 0.45
100 25
2.58a 2.62b
Determined in Chen et al.1
b
Determined in this work.
off from the surfaces with a tissue paper, the ILM is ready for permeation tests. If the immersion time is long enough, the ILMs thus prepared can be completely wetted and translucent, showing that all of the pores are filled with the glycine-Na-glycerol solution. Generally, ILMs prepared by the immersion technique have thicknesses comparable to those of the substrates. Using the coating technique, ILMs with reduced thicknesses can be prepared. In this technique, the porous substrate is carefully laid on top of the glycineNa-glycerol solution. Because only one side of the substrate is in contact with the coating liquid, the fluid goes into the pores via wicking. The wicking rate of the liquid into the substrate pores is dependent on the solution viscosity, coating time, and surface properties of the substrate.23 After a predetermined period of coating time, the substrate is removed from the solution and the extra liquid on the coating surface is carefully wiped out with a glass rod and tissue paper. The average thickness of the ILM, δ, can be estimated gravimetrically:
δ)
W1 - W0 AtFm
(15)
Here W0 and W1 refer to the weights of the substrate and the prepared ILM, respectively, At is the total substrate area, F is the density of the glycine-Naglycerol solution, and m is the porosity of the substrate. Because the sodium glycinate-glycerol solution is quite viscous, it normally took up to 1 h or longer to completely wet a hydrophilic PVDF or hydrophilized Celgard film by the coating technique. As a result, ILMs with average thicknesses significantly less than that of the substrate can be readily prepared. 3.3. Apparatus. The CO2/N2 permeation performances through the glycine-Na-glycerol membranes were measured by the sweep gas flow cell technique.24 The experimental setup of the present work was similar to that reported earlier.1 Dry helium, used as the sweep gas, lowered the partial pressures of the permeating species in the sweep chamber of the permeation cell. Only a small fraction of the feed gas diffused across the membrane and was carried away by the sweep stream whose flow rate and compositions were continuously recorded. CO2 and N2 concentration changes as well as the feed side pressure drop along the feed flow direction were minimal and negligible. Freshly prepared ILMs were used for all permeation runs. For a typical run, a circular piece of PVDF substrate or hydrophilized Celgard substrate (nominal diameter 47 mm) impregnated with the glycine-Naglycerol solution was housed in a cell designed to allow a feed stream and a sweep stream to pass on opposite sides of the film. The film was sealed by a 4.0 cm diameter O-ring and supported by a porous 152-µmthick fine circular stainless steel frit (Pall Trinity Micro, Cortland, NY). The total exposed membrane area on each surface was 12.6 cm2. The feed gas stream was humidified via water bubblers before passing into the cell. Feed streams with
RHs less than 100% were obtained by the blending of dry and humidified gas streams. The RH of the resulting stream was determined using a humidity probe (Model HMP 31UT, Vaisala, Woburn, MA). Unless otherwise stated, the sweep gas was dry helium. The feed and sweep flow rates were controlled using electronic mass flow transducers and a flow controller (model 8227; Matheson, Horsham, PA). The permeate side pressure was essentially atmospheric. The feed side pressure was controlled using a back pressure regulator (model 10182 BP, Fairchild, Towaco, NJ) and measured with a pressure gauge (Matheson, East Rutherford, NJ). All connecting lines were 1/8-in. soft copper or stainless steel tubing. The sweep gas line was connected to a Hewlett-Packard 5890 gas chromatograph (GC) for analysis of the exiting sweep gas samples containing the permeating species. A Porapak column (Alltech, Deerfield, IL) was used for the entire analysis. All experiments were carried out at room temperature (23 ( 2 °C). In a typical permeation run, the feed gas flow rate was set such that the partial pressure of CO2 remained essentially constant along the feed side of the membrane. The sweep flow rate was set to keep the partial pressure of CO2 on the permeate side relatively low, yet high enough for accurate determination by the GC. Data gathered over 6-24 h of steady-state operation were used to calculate the permeances of the respective gases. CO2 and N2 permeances were calculated from their concentrations in the sweep gas and the sweep gas flow rate. Water flux through the membrane was obtained from feed and retentate RHs as well as feed flow rate. 4. Analysis of the Experimental Data For a homogeneous membrane of thickness tm, under the process conditions imposed, the effective permeance of a permeating species i, (Qi/tm)eff, is calculated as follows:
() Qi tm
eff
)
Vi At∆pi
(16)
Here Vi is the volumetric permeation rate, and At and ∆pi are the membrane area and the partial pressure differential of species i across the membrane, respectively. For the current ILM consisting of a microporous polymer support and a liquid phase constrained in the pores of the support, eq 16 should be adapted accordingly. If the substrate porosity and tortuosity are designated by m and τm, respectively, the true permeance of species i through the liquid membrane can be written as1
()
()( )
Viτm Qi Qi ) ) tm Atm∆pi tm
eff
τm m
(17)
Because m < 1 and τm > 1, the true permeance value calculated from eq 17 is always greater than the effective permeance from eq 16. The permeability of species i is given by
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2451
Qi )
Vitmτm Atm∆pi
(18)
The separation factor Ri-j for two gas species i and j is defined by
Ri-j ) Qi/Qj
(19)
5. Results and Discussion Hydrophilized PVDF and Celgard substrate based ILMs having average thicknesses ranging from 25 to 100 µm were prepared from glycine-Na-glycerol solutions using the immersion or coating techniques. The concentration of glycine-Na examined ranged from 0.3 to 5.0 mol/dm3. On a weight basis this represents 0.0240.60 g of glycine-Na/g of glycerol. Because higher glycinate concentrations in glycerol lead to turbid and highly viscous solutions at room temperature, most of the ILMs tested in this work were prepared using glycine-Na-glycerol solutions with glycine-Na concentrations of less than 4.0 mol/dm3. Membranes examined visually after permeation tests showed no sign of salt crystallization. Other glycerol-based membranes studied were 1.0-1.5 M Na2CO3 with glycine-Na, EDA, and EDA-glycine-Na. 5.1. Tortuosity Factor of the Support Membrane. The tortuosity factor, τm, which accounts for the effects of both the varying direction of diffusion and the varying pore cross section of a porous substrate, can be determined from gas permeation experiments.24 The τm of the PVDF substrate was measured and reported to be 2.58.1 The tortuosity of the hydrophilized Celgard 2500 film was determined here from the permeation rates of N2 and CO2 through a pure water ILM. For these experiments, both feed and sweep streams were saturated with water vapor. The calculation algorithm and relevant equations can also be found in the literature.1,24 The experimental value of DN2 in water available from the literature25 was used. Henry’s constant for N2 in water can also be found in the literature.26 At 25 °C, DN2 ) 3.0 × 10-5 cm2/s25 and SN2 ) 1.86 × 10-4 cm3(STP)/cm3(water)‚cmHg. Therefore, QN2 ) DN2SN2 ) 5.58 × 10-9 cm3(STP)‚cm/cm2‚s‚cmHg. Carbon dioxide permeability through pure water QCO2 is 210 × 10-9 cm3(STP)‚cm/cm2‚s‚cmHg.4 As noted in the previous section, the porosity m and thickness of the Celgard film tm were taken to be 0.45 and 25 µm, respectively (as specified by the manufacturer). From the above data and the permeation results, the average value of τm calculated was 2.62; it is close to the tortuosity value for hydrophobic Celgard 2500 film reported by Bhave and Sirkar.24 5.2. Effects of the Initial Carrier Concentration in the ILM. Figure 1 shows the effects of changing the glycine-Na concentration in glycine-Na-glycerol ILM on CO2 /N2 permeabilities and CO2/N2 selectivity. The data points in this figure were obtained under similar CO2 partial pressures with a feed gas containing 0.46% CO2 in N2. The PVDF-based ILMs had thicknesses of around 100 µm as determined gravimetrically. Like the carbonate-glycerol ILM permeation behavior reported in a previous paper,1 the glycinate-glycerol ILM has very high CO2/N2 selectivities even when the carrier concentration is low. In the lower carrier concentration ranges, CO2 permeance increases with an increase in the carrier concentration. However, at higher carrier concentration ranges, CO2 permeance did not increase proportionally with an increase in the glycine-Na concentration. This
Figure 1. Effect of the carrier concentration in PVDF based ILMs ([, glycine-Na; b, 1.5 M Na2CO3 + 1.0 M glycine-Na; O, 1.0 M Na2CO3 + 1.0 M glycine-Na; feed inlet RH ) 98-100%, feed outlet RH ) 50-60%; retentate pressure ) 1.40 atm; CO2 partial pressure differential ) 0.36-0.44 cmHg).
is presumably because, in the high carrier concentration range, the ILM viscosity increases and the CO2 solubility decreases. Such changes can compromise the CO2 facilitation rate through the ILM. However, the CO2/ N2 selectivity can be very high, as much as 7000 at the glycine-Na concentration of 3.5 M. Figure 1 also shows that the permeance of N2, which is transported via the simple solution-diffusion mechanism, decreased with an increase in the glycine-Na concentration in the ILM over the concentration range investigated. Theoretically, the ILM viscosity and the ionic strength of the membrane solution should increase with the increase in the glycine-Na concentration. As a result, both the solubility and the diffusivity of N2 in the ILM would decrease. Therefore, the observed decreasing trend of (Q/tm)N2 with the increase in the glycine-Na concentration is expected. From the above discussion, it is apparent that, at similar CO2 partial pressure differentials, CO2/N2 selectivity increases with an increase in the glycine-Na concentration. Under similar CO2 partial pressures, the CO2/N2 selectivities obtained are significantly higher than those observed with the carbonate-glycerol ILMs.1 For example, under the same feed inlet RH of 100%, similar CO2 partial pressure differentials, and a carrier concentration of 1 M, glycine-Na-glycerol ILM gave a CO2/N2 selectivity of 2200 while Na2CO3-glycerol ILM gave a CO2/N2 selectivity of around 800.1 Also, the CO2 permeance with the glycine-Na-glycerol ILM was 100% greater than that under the same conditions. Also shown in Figure 1 are a few data points obtained using a mixture of carrier species, i.e., glycine-Na + Na2CO3. The mixed carriers based ILMs gave CO2/N2 permeation performances comparable to those of the glycine-Na-based ILMs of corresponding concentration. 5.3. Effect of Feed CO2 Partial Pressure. The effects of CO2 partial pressure across the ILM, pCO2,f,
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Table 2. Effect of the Feed Stream RH on ILM Permeation Performancesa QCO2,c barrer
QN2, barrer
CO2/N2 selectivity
water vapor permeation rate, cm3(STP)/min‚cm2
ILM: 2.25 M Glycine-Na-Glycerol 2.71 997 3.86 1420 4.53 1670 7.09 2610 9.37 3450 11.4 4190 13.0 4780 15.7 5780 26.4 9710 27.5 10100
n.a. n.a. n.a. n.a. 0.405 0.643 0.831 0.971 1.89 2.54
n.a. n.a. n.a. n.a. 8520 6510 5750 5950 5140 3980
0.0031 0.0033 0.0089 0.0074 0.0112 0.0108 0.0132 0.0138 0.0139 0.0144
ILM: (1.0 M Glycine-Na + 1.0 M Na2CO3)-Glycerol 5.61 2060 0.805 6.77 2490 0.992 15.8 5810 1.83 12.3 4530 1.38 11.9 4380 1.68 14.9 5480 2.73
2560 2510 3170 3280 2600 2003
0.0105 0.0141 0.0257 0.0167 0.0179 0.0279
feed-in RH, %
feed-out RH, %
(Q/tm)N2,eff × 109,b cm3(STP)/cm2‚s‚cmHg
42.5 47.0 58.5 59.3 69.4 77.0 86.4 88.3 98.0 99.9
33.3 37.2 31.3 38.0 36.6 43.7 44.4 44.7 50.3 56.5
n.a.d n.a. n.a. n.a. 1.10 1.75 2.26 2.64 5.14 6.89
59.5 72.1 86.3 89.2 96.5 97.1
34.2 40.5 53.6 47.8 50.7 56.1
2.19 2.70 4.98 3.75 4.57 7.44
(Q/tm)CO2,eff × 106, cm3(STP)/cm2‚s‚cmHg
a PVDF-based ILM; t ) 100:m, A ) 25.1 cm2; feed ) 0.46% CO -N ; feed pressure ) 1.41-1.51 atm. Sweep gas: dry helium. b Effective m m 2 2 permeances as defined by eq 16. c 1 barrer ) 10-10 cm3(STP)‚cm/cm2‚s‚cmHg; values are based on true permeances. d The nitrogen peak area was too small to be determined with reasonable accuracy.
Figure 2. Effect of the CO2 partial pressure differential on glycine-Na-glycerol ILM permeation performances (PVDF-based ILM thickness, 100 µm; feed inlet RH ) 100%; feed outlet RH ) 50-60%; retentate pressure ) 1.45 atm; glycine-Na concentration ) 2.25 M).
on CO2/N2 permeances and the separation factor are shown in Figure 2. As can be seen in the figure, at a glycine-Na concentration of 2.25 M and feed RH of 100%, the CO2 permeance decreased significantly with an increase of pCO2,f although N2 permeance did not change appreciably. This characteristic, known as “carrier saturation”, is intrinsic to facilitated transport systems.3 The carrier saturation phenomenon shown in Figure 2 is similar to that observed with carbonate-glycerol
ILMs.1 As can be noted from eq 13, the total flux of CO2 is composed of two parts, i.e., that of physically dissolved carbon dioxide and that due to facilitated transport. The relative proportion of CO2 transport in the solutiondiffusion mode and in the facilitation mode is dependent on, among others, the concentration of carriers available for the facilitation reaction. At low CO2 partial pressures, the CO2 concentration in the liquid is low and the proportion of carriers utilized by facilitation is relatively small. As pCO2,f increases, the proportion of carriers used by the facilitation reaction also increases. At a sufficiently great pCO2,f, nearly all of the carriers are used for the facilitation reaction and a further increase in pCO2,f will not lead to a proportional increase in CO2 permeance. In Figure 2, the lowest value of CO2/ N2 selectivity was about 120 when the CO2 partial pressure differential approached 50 cmHg. Because the ILMs display pressure-dependent CO2 permeances, very high separation factors were obtained at lower CO2 partial pressures and at higher glycineNa concentrations. CO2/N2 selectivity observed at the glycine-Na concentration of 3.5 M, pCO2,f ) 0.40 cmHg, and feed RH ) 100% was around 7000 (Figure 1). 5.4. Effect of Feed Relative Humidity. Because glycerol is highly hygroscopic, it tends to absorb moisture from its surroundings. In the previous paper,1 it was shown that the equilibrium water content in glycerol is strongly dependent on the gas-phase RH. For example, at an environmental RH of 50-60% (typical RH of the exiting sweep/permeate stream in this work), the equilibrium water content is approximately in the range of 0.20-0.25 g of water/g of glycerol. This corresponds to a water mole fraction of 0.51-0.56. Because the viscosity of the water-glycerol mixture is strongly dependent on the water content,19 any change in the gas-phase RH will lead to changes in the viscosity of the ILMs. Such water concentration changes will also affect the gas solubilities in the ILM. Therefore, feed gas RH is an important factor affecting the glycerolbased ILM performances. Table 2 lists the CO2/N2 permeation performances of 2.25 M glycine-Na-glycerol ILM as well as (1.0 M glycine-Na + 1.0 M Na2CO3)-glycerol ILM at different
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2453 Table 3. Performancesa of PVDF-Based 2.25 M Glycine-Na-Glycerol ILMs as a Function of the ILM Thickness ∆pCO2, mHg
average ILM thickness, µm
inlet RH, %
outlet RH, %
(Q/tm)N2,eff, scc/cm2‚s‚cmHg
QN2,b barrer
(Q/tm)CO2,eff, scc/cm2‚s‚cmHg
QCO2,b barrer
CO2/N2 selectivity
0.354 0.365 0.400 0.423 0.430 0.467 0.467 0.383
35 39 51 60 67 67 79 100
85.0 86.7 86.4 86.0 86.0 85.6 87.0 86.4
56.9 46.7 42.5 49.5 48.5 40.9 37.5 44.4
1.70 × 10-6 8.57 × 10-8 6.17 × 10-8 7.27 × 10-8 4.86 × 10-8 3.08 × 10-8 4.05 × 10-9 2.26 × 10-9
219 12.3 11.6 16.1 12.0 7.60 1.18 0.830
4.89 × 10-5 2.95 × 10-5 1.68 × 10-5 2.68 × 10-5 2.53 × 10-5 1.96 × 10-5 1.25 × 10-5 1.30 × 10-5
6350 4230 3160 5970 6250 4840 3650 4770
29 344 272 371 520 636 3090 5750
a
Feed-in RH: 86 ( 0.8%. b 1 barrer ) 10-10 cm3(STP)‚cm/cm2‚s‚cmHg; values are based on true permeances.
feed stream RHs. The feed-in RH range of the feed stream investigated was varied between 42.5% and 99.9%. In this set of experiments, two permeation cells (total membrane area 25.1 cm2) were used in series to ensure that the nitrogen peak areas were well above the GC detection limit. Therefore, a significant RH decrease was observed between feed-in and feed-out streams. As shown in Table 2, with an increase in the feed stream RH, permeances of N2 and CO2 increased and CO2/N2 selectivity decreased. At relatively low feed-in RHs, both CO2 and N2 permeances were low and the N2 permeance decreased below the GC detection limit. Such changing trends can be explained by the increased liquid viscosity and salting-out effect in the ILM at low RHs.1 The decreased water concentration in the ILM can drastically increase the liquid-phase viscosity and decrease the diffusivities of the species in the ILM, which can be responsible for the observed decreases in CO2 and N2 permeances. Also, at low water concentrations, the increased “salting-out” effect would substantially reduce the solubilities of the gas species in the ILM. This helped to further decrease the permeances of the species through the ILM. Also from Table 2, it can be noted that the feed stream RH significantly affects the CO2/N2 selectivity of the ILM. For the feed RH range 69.4-99.9%, the observed selectivity for 2.25 M glycine-Na-glycerol for the feed gas of 0.46% CO2-balance N2 was between 8520 and 3980. These selectivities are higher than anything reported so far. These values are 3.5-1.7 times greater than 2400, the selectivity observed for 1.0 M glycineNa-glycerol ILM under a similar CO2 partial pressure differential and 100% feed RH as shown in Figure 1. The changing trends of the ILM performances with the feed side RH is seen more clearly in Figure 3 in which feed average RH, the x-coordinate, is the arithmatic mean of feed-in and feed-out RHs. 5.5. Thinner ILMs. 5.5.1. Thinner ILMs Prepared Using the PVDF Substrates. To explore the potential of increasing CO2 permeances by decreasing the ILM thickness, PVDF-based ILMs of varying thicknesses were prepared by the coating technique and evaluated at similar feed RHs and CO2 partial pressures. Table 3 lists the results obtained for a feed of 0.46% CO2balance N2. In these experiments, the feed-in RH was kept relatively constant at around 86%. It can be noted from the table that CO2 permeance through the ILM gradually increases with the decrease of the ILM thickness. Despite some fluctuations, (Q/tm)CO2,eff increased by a factor of about 2 as the ILM thickness decreased from 100 to 39 µm. However, corresponding to the same ILM thickness decrease, N2 permeance increased much more significantly. (Q/tm)N2,eff increased
Figure 3. Effect of the average relative humidity on the permeation performances of 2.25 M sodium glycinate-glycerol ILM (PVDF substrate; feed gas, 0.46% CO2-balance N2; feed pressure ) 1.5 atm).
by a factor of about 37. As a result, CO2/N2 selectivity decreased very substantially from 5750 to 344. Apparently, the low-thickness ILMs were developing high N2 transport channels (observe the increased N2 permeability in Table 3). The corresponding increases in CO2 transport through the high N2 transport channels are of much lower significance (see Table 3 for CO2 permeability values) because CO2 transport rates through the membrane are orders of magnitude greater. The hydrophilic PVDF film used in this work has a pore structure with a nominal pore size of 0.1 µm and a water bubble-point pressure of about 4.8 atm.27 When the coating time is controlled, ILMs with average thicknesses below 40 µm can be readily obtained. Unfortunately, the thin ILMs were showing much lower CO2/N2 selectivities than full-thickness ILMs when tested. Possible reasons for such high N2 transport are the following. The PVDF films have quite open (porosities of ca. 70%) and interconnected pore networks which may have a significant pore-size distribution. Moreover, unlike the essentially uniform pore-size distribution of polycarbonate membrane filters27 achieved by the tracketching technique, the diameters of the pores or path channels in the PVDF film may change appreciably
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Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000
Table 4. Performances of Hydrophilized Celgard Film Based 2.25 M Glycine-Na-Glycerol ILMa ∆pCO2, cmHg
inlet RH, %
outlet RH, %
(Q/tm)N2,eff × 109, scm3/cm2‚s‚cmHg
(Q/tm)CO2,eff × 105, scm3/cm2‚s‚cmHg
QCO2,b barrer
QN2, barrer
CO2/N2 selectivity
water vapor flux cm3/(min‚cm2)
0.420 0.422 0.364 0.427 0.403 0.406 0.458 0.407 0.416 0.414
100 100 89.6 75.9 74.5 72.8 70.2 69.6 64.5 63.7
56.4 54.7 55.7 45.8 48.5 50.1 41.2 43.4 42.2 45.4
17.0 23.8 15.8 8.77 7.39 6.79 6.35 9.51 3.67 9.44
3.56 3.94 2.78 1.92 1.57 1.48 1.43 1.35 0.946 1.18
5180 5730 4050 2790 2280 2150 2080 1960 1380 1720
2.48 3.47 2.29 1.27 1.08 0.986 0.924 1.38 0.535 1.38
2090 1650 1770 2190 2120 2180 2250 1420 2580 1250
0.0285 0.0289 0.0190 0.0243 0.0158 0.0134 0.0217 0.0162 0.0110 0.0111
a Feed gas, 0.50% CO -balance N ; feed gas pressure, 1.28 atm; sweep gas, dry helium. b 1 barrer ) 10-10 cm3(STP)‚cm/cm2‚s‚cmHg; 2 2 values are based on true permeances.
across the film thickness. Right after coating, the thin ILM fluid layer is held by surface tension in a section of the pores on one side of the film which is facing the feed stream in subsequent permeation tests. When a transmembrane pressure (TMP) differential is applied against the ILM, the liquid may go to deeper sections of the pores if such nonwetted sections are immediately available along the path channels due to larger diameters. When the initial liquid thickness is small, such movements of part of the ILM are likely to create some interconnections or channels between the two sides of the ILM, leading to high transport corridors, defects, or leaks. Because such leakages are not normally observed with relatively thick ILMs (e.g., >70 µm), we suspect that in thicker ILMs the liquid movements under transmembrane pressure differentials, if any, are not likely to produce the above-mentioned defects. The somewhat large decrease in the observed N2 permeance between two separate ILMs having thicknesses of 79 and 100 µm in Table 3 should not detract from the main effect of reduced ILM thickness in the PVDF substrate; namely, the leaking tendency increases. From this point of view, the ideal substrates for preparing thin ILMs should have rather uniform pore diameters throughout the thickness (e.g., in Celgard membrane). The above analysis is confirmed by the following observation: full-thickness PVDF-based ILMs were able to hold TMPs above 20 psig (1.36 atm) and showed consistent gas permeances, while the thinner ILMs (
