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N2 mixtures through PDMS membrane
Journal of Membrane Science 198 (2002) 129–143
Vapor permeations of a series of VOCs/N2 mixtures through PDMS membrane C.K. Yeom a,∗ , S.H. Lee b , H.Y. Song c , J.M. Lee b a
b
PETROSEP Membrane Technologies Inc., 2270 Speers Road, Oakville, Ont., Canada L6L 2X8 Chemical Process and Engineering Center, Applied and Engineering Chemistry Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-606, South Korea c Department of Polymer Science and Engineering, Chung-Nam National University, Yusong, Taejon 305-335, South Korea Received 2 January 2001; accepted 3 September 2001
Abstract The permeation performances of volatile organic compounds (VOCs)/nitrogen mixtures through poly(dimethylsiloxane) have been measured at various operating conditions by a newly developed permeation apparatus which could measure its permeation transient rapidly, precisely and directly. In this study, a series of chlorinated hydrocarbons, that is, methylene chloride, chloroform, 1,2-dichloroethane and 1,1,2-trichloroethane were adopted as organic vapor. The permeability, diffusion, and solubility coefficients of the VOC mixtures were evaluated from the determined permeation transients. The permeation performance of the mixture as well as the selectivity towards VOC component showed a strong dependence of both VOC content in feed and the condensability of VOC. In the permeation of the VOC mixture, the nitrogen permeability was depressed below pure nitrogen permeability by the preferential sorption of VOC into PDMS membrane. This effect was more noticeable for the more condensable VOCs. Also, it is observed that the permeation and separation of the mixtures were more controlled by sorption process rather than by diffusion process. Through a quantitative analysis of the permeation parameter and apparent activation energies, the transport of VOC component in the permeation of the mixtures was discussed, in compared with pure nitrogen permeation. © 2002 Published by Elsevier Science B.V. Keywords: Poly(dimethylsiloxane); Volatile organic compounds; Chlorinated hydrocarbon; Permeation transient; Permeability; Diffusion coefficient; Solubility coefficient
1. Introduction In order to protect environment from contaminating as well as to reuse expensive materials, it is necessary to separate volatile organic compounds (VOCs) from waste gas stream in industrial processes. If the waste stream is to be released to the environment, the VOCs have to be removed substantially to control environmental pollution. There are a number of ∗ Corresponding author. E-mail address: [email protected] (C.K. Yeom).
conventional separation processes for the removal of VOCs from waste stream but vapor permeation through membranes offers significant opportunities of energy savings and reuse of the VOCs, compared to the conventional VOC control processes, particularly if the VOC concentration is high [1,2]. For the recovery of VOCs, two kinds of membranes, that is, glassy and rubbery polymers, may be selected. In most cases, organophilic rubbery membranes are preferred because they are much permeable to VOCs. Especially, when VOCs are minor components in a waste stream, the rubbery membranes could be used
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 6 5 7 - 3
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Nomenclature D Ed , Ep Hs l L P R S t ts x, y
diffusion coefficient of permeant (cm2 /s) diffusion and permeation activation energies of water, respectively (kcal/mole) heat of sorption (kcal/mole) membrane thickness (m) membrane strip length (m) permeability coefficient (cm3 (STP) cm/(cm2 s cmHg)) swelling ratio solubility coefficient (cm3 (STP)/ (cm3 cmHg)) permeating time (s) response times (s) feed and permeate compositions (vol.%)
favorably because less feed gas has to permeate the membrane to remove the bulk of the vapor, requiring relatively small membrane areas. Among the rubbery membranes, poly(dimethylsilixane) (PDMS) exhibits an excellent membrane performance for the removal of VOCs from non-condensable gases; high permeability and high selectivity for VOCs. The PDMS membranes are used for such a process in a form of composite, wherein a thin layer of nonporous silicone rubber is coated on an appropriate porous polymeric substrate [2–5]. In the removal and recovery of VOCs from air/N2 streams at a certain pressure by membrane separation, a large volume of air/N2 is allowed to flow on one side of the membrane at the feed pressure. When a vacuum is applied to the other side of create a partial pressure driving force, the VOCs selectively permeate through the membrane in preference to air/N2 , and the permeate is highly enriched in the VOCs. The VOCs are continuously recovered/recycled by condensing the small permeated gas stream while the non-condensables are recycled to the contaminated feed air/N2 stream inlet. A more fundamental understanding of the sorption, diffusion and permeation effects will help to gain further insight into the transport of VOCs through rubbery polymers. There have been very few studies on diffusion, sorption and also permeation, of VOC/gas mix-
tures through rubbery membranes so far although numerous researches [6–10] have been focused on pure organic components. Singh et al. [10] reported that the nitrogen permeability coefficient in the permeation of acetone/nitrogen mixture through PDMS membrane is equal to the pure nitrogen permeability, and VOC permeability also has the same value in pure gas permeation experiments, indicating that nitrogen and VOC sorb, diffuse and permeate as independent molecules. Pinnau and Toy [11] found in the permeation of chlorofluorocarbon/nitrogen mixtures that the nitrogen permeability is reduced dramatically by the presence of the organic vapor, showing coupled permeation. Often, the permeation behaviors of individual components in a gas mixture through rubbery membrane are found to be different from those of their pure gases due to some coupling effects in sorption [12]. Therefore, for the design of membrane process and module for the separation of a gas mixture, the fundamental permeation parameters of the mixture will be as important as those of their pure individual gases through the membrane. In this study, the permeations of chlorinated hydrocarbon/N2 mixtures through a homogeneous PDMS membrane were carried out at various operating conditions by the newly developed permeation apparatus that was designed to be able to generate a certain composition of VOC/nitrogen mixture and measure its permeation transient. From the permeation transient measured, the permeability, diffusion and solubility coefficients of the respective VOC/N2 mixture and pure N2 , and respective activation energies were evaluated. In particular, this paper concentrated on the chlorinated hydrocarbons, which are part of the homologous series of the chloromethanes and chloroethanes. The permeation and separation behaviors of the mixtures through the rubbery membrane were characterized through the analysis of the membrane performances determined. Especially, individual component permeations were characterized to analyze the coupled permeation in compared with pure nitrogen permeation. 2. Experimental 2.1. Materials Poly(dimethylsiloxane) (GE655, General Electrics) was generously provided by Dongyang Silicone
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(Seoul, Korea). It was composed of two parts; part A is mainly PDMS oligomers terminated with vinyl groups and part B a mixture of Pt catalyst and PDMS oligomer with active hydrogens. Methylene chloride, chloroform, 1,2-dichloroethane, and 1,1,2-trichloroethane, which were used as VOCs vapor, all were obtained from Junsei Chemical Co. (Tokyo, Japan) and all of them were an extra pure grade. All chemicals were used without any further purification. 2.2. Membrane preparation A casting solution was prepared by dissolving part A and part B with a ratio of 9:1 in n-hexane. The casting solution was poured into a glass petri dish and then allowed to dry in a fume hood. After curing at room temperature over night, the membranes were placed in an oven at 110 ◦ C for 2 h to ensure complete cross-linking. The cross-linking reaction took place by an addition reaction in which the active hydrogens attack the vinyl groups under Pt catalyst. The prepared membranes were 130–160 m in thickness. 2.3. Swelling measurements In order to evaluate the affinity of the VOCs toward the PDMS membrane, the swelling measurement of the membrane [13] was performed in each liquid VOC, and the amount of the liquid VOC absorbed in the membrane was determined. Dry membrane strips were immersed in the liquid VOC thermostated at 25 ◦ C for 48 h to allow the strips to reach equilibrium sorption. The dimension of a strip was 7 cm × 1.5 cm. After measuring the swollen length, L of the strip at equilibrium sorption, the strip was dried for 30 h at room temperature under vacuum and then the dry length, Lo was measured. The swelling ratio, R for an isotropic material is defined as L − Lo R= (1) Lo All measurements were repeated four or five times and the resulting data had standard deviation of ±6%. 2.4. Permeation apparatus For vapor permeation, the gas permeation apparatus that had been built for gas permeation in the pre-
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vious work [14,15] was modified to be suitable for generating a certain composition of vapor/gas mixture and measuring its permeation transient. The schematic modified apparatus is illustrated in Fig. 1, which is composed of three constituents; (a) feeding system, (b) membrane cell, and (c) measuring and data acquisition system. 2.4.1. Feeding system The feeding system is to feed a gas to the membrane cell or to generate a certain composition of vapor/N2 mixture and feed it to the membrane cell. Therefore, gas permeation as well as vapor permeation experiment could be performed with the dual feeding system. The first N2 stream was introduced to the vapor generator filled with a liquid VOC to be vaporized. To enhance the contact of the N2 with the liquid, an air diffuser was used to make fine gas bubbles in the generator. This stream was then mixed with the second stream of pure N2 to produce a mixture stream of a desired composition and flow rate. The vapor generator was equipped with a chiller, an electrical heater, a mass flow controller and a pressure regulator so that the mixture composition was able to be controlled precisely by adjusting the flow rates of the first and second N2 stream, temperature and pressure. The composition of the produced mixture was measured in on-line mode by the gas chromatograph (GC). The 1.5 l of the buffer tank was installed to keep feed pressure in the membrane cell constant. 2.4.2. Membrane cell The membrane cell was placed in the heating oven to control its temperature at a desired value. The membrane cell was made of stainless steel. The feed mixture entered the cell through the center opening, flew radially through the thin channel and left the cell through the side opening. The effective membrane area in the membrane cell was 14.7 cm2 . The membrane was supported on a filter paper and a stainless steel porous disk. The membrane cell was sealed by double O-rings to prevent vacuum leak. Before measuring permeation transient, the produced vapor mixture was by-passed to vent by opening the solenoid vale SV1 and closing SV2, simultaneously. At the same time, with the electrical actuated valve EVA opened, both the upstream and downstream sides of PDMS
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Fig. 1. A schematic representation of permeation apparatus.
membrane loaded in the cell were degassed simultaneously until the pressure in the cell reached below 0.1 Torr. After then, when the valve SV1 was close, SV2 open, and EVA close simultaneously, the by-passed vapor mixture was introduced into the upstream side of the cell and then permeation started. The very important point to obtain an accurate permeation transient curve is that feed pressure in the cell must jump from the vacuum to the desired feed pressure as soon as the produced mixture is introduced, minimizing a pressure drop occurring temporarily in the membrane cell due to volume extension. If the pressure drop happens seriously in the upstream side of the cell, the response of the permeation will be retarded, resulting in slower diffusion than real diffusion. The equipment was specially designed to obtain a step change in feed pressure from vacuum to the controlled pressure in introducing the vapor mixture.
2.4.3. Measuring and data acquisition system The measuring system which was installed along with the permeate line consists of two mass flow meters (MFM), a precise pressure transducer (PT) and gas chromatography (GC). The whole permeate line was wrapped with heating band to control the temperature in it and thereby prevent any condensation of permeate vapor on the inner wall of the line. The permeate pressure in permeate line could be controlled by a digital pressure controller. The pressure transducer (LABOVAC PIZA 2000, SASKIA, Germany) equipped with micropirani and piezo dual sensors could measure permeate pressure ranging from 1 × 10−5 to 1999.9 mbar. The mass flow meters (MFM) (Brooks, Japan) used in this study have a capacity of 100 and 1000 cm3 (STP)/min, respectively. To minimize the pressure drop developed across the MFM during permeation, the pressure difference between the inlet and outlet of the MFM was adjusted
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to be as low as 4.5 psi so that permeate pressure could be maintained as low as possible in the downstream side of the membrane cell. When permeation happened through the membrane the PT and MFM produced voltages corresponding with the permeation, respectively. Since both the PT and MFM hooked up with a computer through a 21 bit interface module for an analog to digital conversion at a sampling rate, the produced voltages could be monitored and displayed in real time by the computer, performing data acquisition. Thus, two kinds of permeation transients could be measured in on-line simultaneously; one was from permeate rate flow determined by the MFM and the other one from permeate pressure measured with time by the PT. The transient determined by the MFM has a slightly retarded response of permeation because of an occurrence of some friction in flowing permeant through the MFM, so that it could give somewhat larger value of response time and smaller diffusion coefficient than its real value, but permeability at steady-state could be measured accurately. The permeation transient measured by the PT has a real response of permeation without any retardation so it could give a correct diffusion coefficient, but since the transient curve presented the ratio of permeability at a permeating time to a steady-state permeability, the absolute value of permeability could not be obtained. For the reason mentioned above, these two permeation transients were used complementarily to make up for the weak point of each determination of permeation properties. After the data acquisition, a computer program calculated permeability coefficient and diffusion coefficient automatically from the transient curves measured by the MFM and PT, respectively. The maximum resolution of the interface module was 1 V, which was sensitive enough to detect a very small instantaneous flux in comparing with full output voltages of 5 and 10 V produced in the MFM and PT, respectively. When the MFM is operated on a gas other than air it is calibrated with, a scale shift will occur in the relationship between the output signal and the mass flow rate. This is due to the difference in heat capacities between the two gases. Using the ratio of the molar specific heat of the two gases can approximate this scale shift. The relative scale shift of a gas to air is called “a gas conversion factor”. To calculate the mass flow rate of a gas, the output reading was multiplied by the gas conversion factor of the gas.
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The gas chromatography (5890 Series II Plus HP GC) was equipped with a sample injector (6-port valve) actuated by air, a TCD detector and a packed column. The column was 6 ft long with 1/8 in. inside diameter having a Porapak Q. Thus, permeate could be injected directly into the column by the injector and thereby the composition of permeate was determined directly and accurately. The GC was also connected with the computer by the 21 bit of interface module and the permeate concentration could be displayed at the computer monitor. The evacuation of permeate was provided by the vacuum pump and the cold trap (JeioTech Ltd., Seoul, South Korea) was installed before the vacuum pump for condensing the permeate vapor. 2.5. Vapor permeations In this study the permeations of various chlorinated hydrocarbon/nitrogen mixtures through the poly(dimethylsiloxane) membrane have been carried out at different operating conditions by using the novel permeation apparatus. VOC content in feed mixture used ranged from 0 to 1.5 vol.%, and operating temperatures were 35–65 ◦ C. In all of the vapor permeation experiments, feed pressure was kept 1.5 kgf/cm2 , and feed flew along with membrane surface at a rate of 600 cm3 (STP)/min which was fast enough to minimize concentration polarization effects in feed adjacent to the membrane surface, if any. 2.6. Determination of permeation parameters from transient curves The basic principle and procedure of determining permeation parameters from the permeation transient are well described elsewhere [14]. Fig. 2 shows two transient curves monitored by MFM and PT in the new permeation apparatus, respectively, in the permeation of C2 H4 Cl2 /N4 mixture with 1.2 vol.% of VOC content at 45 ◦ C. They are the typical permeation transients that can be obtained by the permeation apparatus. The permeability coefficient P could be calculated from the voltage difference DV between the initial value and the steady-state value in permeation transient measured by the MFM. The diffusion coefficient
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Fig. 2. Permeation transient curves monitored by the MFM and PT in the permeation of C2 H4 Cl2 /N2 mixture with 1.2 vol.% of VOC content at 45 ◦ C and 600 cm3 /min of feed flow rate.
D could be evaluated from response times ts , respectively, by the expressions [8]: D=
l2 5.91ts
(2) 3. Results and discussion
The response time ts is a time for the flux to rise from its initial value to its final along the time of maximum slope as described in Fig. 2b. The solubility coefficient S could be calculated: P S= D
(3)
The following relationship was used to calculate the selectivity α: αi/j =
yi /yj xi /xj
where x is the feed composition, y the permeate composition, and component i is the preferentially permeating component.
(4)
3.1. Physical properties of PDMS membrane in various VOCs Generally, the permeation of permeants through the rubbery membrane is characterized as a sorptioncontrolled process, so the sorption behavior of a permeant could determine significantly permeation and separation performance. The sorption behavior of permeant in a polymeric membrane follows the rule of thumb; “like dissolves like”; the greater the affinity of a permeant towards a membrane is, the higher the
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Table 1 Physical parameters of each chlorinated hydrocarbon and its swelling ratios in the PDMS membrane at 25 ◦ C
CH2 Cl2 CHCl3 1,2-C2 H4 Cl2 1,1,2-C2 H3 Cl3 N2 PDMS a
Solubility parameter [18] (MPa1/2 )
Swelling ratioa
Critical temperature [19] Tc (K)
20.3 19.0 20.9 19.6 – 14.9
0.279 0.330 0.179 0.207 – –
510.0 536.4 561.2 602.0 33.2 –
Swelling ratio: (swollen length − dry length)/(dry length).
solubility of the permeant in the membrane is. In the permeation of an organic vapor/gas mixture through a polymeric membrane, the transport of gas component is characterized as zero or negligible interacted permeation with the membrane while organic vapor component shows an interacted transport behav-
ior through the membrane, especially, organophilic membranes such as PDMS. Thus, the investigation of interaction between organic vapor and membrane material is very important to analyze the permeation behavior of the permeant. The solubility parameter of permeant can be taken as a measure of the interaction
Fig. 3. Permeability coefficients and selectivity of various VOC/N2 mixtures through PDMS membrane with VOC content in feed at 45 ◦ C.
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of the permeant with membrane material by a comparison with membrane’s solubility parameter. Table 1 exhibits the solubility parameter of respective VOC used in this study and its swelling ratios in the PDMS membrane at 25 ◦ C. The membrane was more swollen in the liquid VOC that has a lower solubility parameter, being closer to PDMS value and characterizing a better affinity towards PDMS. Another parameter that determines the solubility behavior is the ease of condensation [16,17], with molecules becoming more condensable with increasing molecular size. The critical temperature Tc is used as a measure of the ease of condensation. Both the critical temperature and the solubility coefficient of VOCs in polymer increase as the molecular dimensions increase. From Table 1, it is found that the chlorinated hydrocarbon having more chlorine atoms in molecule has a lower solubility parameter and a greater solubility, and the chloroethane has a higher Tc than the chloromethane for a given number of chlorines in molecule. These sorption parameters affect permeation and separation performance, which will be discussed in next section. 3.2. Effect of feed composition on permeation performance of vapor mixtures through PDMS Fig. 3 presents the permeability coefficients and selectivities of various VOC/N2 mixtures through
the PDMS membrane as a function of feed composition at 45 ◦ C. In literature [10,11,16,18], the permeability coefficient of pure nitrogen through a PDMS membrane is reported to be (220–280) × 10−10 cm3 (STP) cm/(cm2 s cmHg) in a temperature range of 20–40 ◦ C, and the value evaluated in this study was 210 × 10−10 cm3 (STP) cm/(cm2 s cmHg), showing a good agreement with the literature values. It is observed that the VOC permeability coefficient is dependent on both VOC size and VOC content in feed. The permeability of the mixture increased with increasing VOC content in feed, and when the critical temperature of VOC Tc was higher or VOC component in the mixture was bigger, the mixture permeability was not only higher but also the permeability increased with VOC content more significantly. As a result, the permeability increased linearly for the mixture having small VOC, such as, CH2 Cl2 or CHCl3 , while the permeability curve of the mixture having larger VOC, such as, C2 H4 Cl2 or C2 H3 Cl3 was more convex to the VOC content axis, and the slope of the curve increased as the VOC content increased. The larger permeability for the mixture with larger VOC could be only be explained realizing that the larger VOC has a larger solubility in the PDMS membrane, leading to a higher overall permeability, as will be discussed later. The selectivity to VOC component was observed to be very high, ranging from 135 to 370
Fig. 4. Permeability coefficient of individual component in the permeation of various VOC/N2 mixtures through PDMS membrane with VOC content in feed at 45 ◦ C.
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Table 2 A comparison of the permeability coefficient of pure nitrogen with that of nitrogen component in various VOC/nitrogen mixtures through the PDMS membrane at 45 ◦ C VOC content in mixture (vol.%)
Nitrogen permeability coefficient (×1010 ), cm3 (STP) cm/(cm2 s cmHg) Pure nitrogen
CH2 Cl2 /N2
CHCl3 /N2
C2 H4 Cl2 /N2
C2 H3 Cl3 /N2
0 0.3 1.5
210 – –
– 160 159
– 151 147
– 148 141
– 143 139
depending on the critical temperature of VOC component in feed. Most of the permeability coefficients of the VOC mixtures increased above pure nitrogen permeability coefficient with increasing VOC content in feed but selectivity did not change so much. More details on these behaviors will be discussed through the analysis of individual component permeations. The permeability coefficients of individual components are plotted against VOC content in feed in Fig. 4. The individual permeability coefficients were determined from the data of mixture permeability coefficients and permeate concentrations. The permeability coefficient of VOC component in the VOC mixture was very high as compared to that of nitrogen component, and was independent of VOC content in the given range of feed composition. Singh et al. [10] report that pure nitrogen permeability is almost identi-
cal with that of nitrogen component in the permeation of acetone/nitrogen mixtures through PDMS membrane. However, in this study, it is unexpectedly found that the nitrogen permeability coefficient in the mixtures is reduced from the pure nitrogen permeability, depending on the VOC critical temperature or VOC content in feed. The VOC mixtures contained one of the following chlorinated hydrocarbons, ranked in order of increasing condensability: CH2 Cl2 < CHCl3 < C2 H4 Cl2 < C2 H3 Cl3 . The VOC/nitrogen mixture data, listed in Table 2, show that the nitrogen permeability is reduced more dramatically as VOC content increases or VOC condensability increases. The same observations had also been found in the previous work [18] in which a series of alcohols were employed as VOC vapor in the permeation of VOCs/N2 mixtures.
Fig. 5. Membrane selectivity with VOC critical temperature in the permeation of VOC/N2 mixtures at 45 ◦ C and 0.9 vol.% of VOC content in feed.
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In another report, Pinnau and Toy [11] suggested that a non-condensable gas such as nitrogen is dramatically by the presence of a condensable organic vapor in a high-free-volume glassy polymer poly(1-trimethylsilyl-1-propyne) (PTMSP). According to the permeation mechanism, the large amount of condensable organic vapor sorbed into the membrane causes partial blocking of the small free volume elements, reducing the nitrogen permeation. The permeation principle could be also applied to explain the transport behavior of the PDMS membrane with the chlorinated hydrocarbon/nitrogen mixtures. The unique transport through PDMS might be simply rationalized by postulating that the nitrogen permeability is depressed below the pure nitrogen permeability because of diluting nitrogen component with VOC
component selectively absorbed into the membrane matrix (Herny sites) and thereby blocking nitrogen permeation. Thus, more reducing the nitrogen permeability in the permeation of VOC mixture having larger VOC could be explained by absorbing VOC component more selectively in the membrane and thereby more diluting nitrogen component. Thus, as can be seen in Fig. 5, membrane selectivity increased with increasing the condensability of VOC component because the nitrogen permeability was depressed more. Analysis of the diffusion and solubility coefficient data could help to understand the permeation behavior of the VOC mixtures. Fig. 6 presents the diffusion and solubility coefficients of the mixtures with VOC content in feed at 45 ◦ C. Generally, in a polymeric
Fig. 6. Diffusion and solubility coefficients of various VOC/N2 mixtures with VOC content in feed at 45 ◦ C.
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membrane, the diffusion coefficient of a permeant decreases with increasing its molecular size because large molecules interact with more segments of the polymer chains than do small molecules, favoring the passage of small molecules such as nitrogen over larger ones such as VOCs, as shown in Fig. 6. The diffusion coefficient of the mixture decreases with increasing VOC content in feed. As increasing VOC content in feed, more VOC molecules could be absorbed in to membrane to such an extent that the diffusing permeant size could increase, resulting in slowing the diffusion of the permeants through the membrane. The solubility coefficients of the mixtures were determined from Eq. (3). In literature [16], the solubility coefficient of pure nitrogen in PDMS determined at 25 ◦ C and 1 MPa
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is reported to be 25 × 10−4 cm3 (STP)/(cm3 cmHg) while the value evaluated in this study ranges (19.66–22.82) × 10−4 cm3 (STP)/(cm3 cmHg), depending on temperature, which is in a good agreement with the literature value. All the solubility coefficients of the mixtures were well above pure nitrogen solubility because of the larger sorption of VOC components. As shown in Fig. 6, the solubility coefficient of the VOC mixture was a function of VOC size as well as its activity in feed. The solubility coefficient of the mixture increased with increasing VOC content in feed, and when the critical temperature of VOC Tc was higher, the mixture solubility coefficient was higher. As a result, the permeability increased linearly for the mixture having small VOC such as CH2 Cl2 or CHCl3 , while the solubility coefficient curve of
Fig. 7. Permeability coefficients and selectivity of various VOC/N2 mixtures through PDMS membrane with operating temperature at 0.9 vol.% VOC content in feed.
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the mixture having larger VOC such as C2 H4 Cl2 or C2 H3 Cl3 was more convex to the VOC content axis, increasing the slope of the curve with VOC content. In compared with the change in the solubility coefficient with VOC content, the change in the diffusivities of the mixtures is not significant, and also the difference in diffusivities between the VOC mixtures and pure nitrogen is not distinguished in the given range of feed composition. In addition, diffusivity selectivity, which is predicted to be less than one because VOC is larger than nitrogen, decreases with increasing VOC content because mixture diffusion coefficients decrease as much. Thus, the high selectivity of PDMS for VOC in VOC/nitrogen mixtures is due to enormously larger solubility of VOC in the polymer. As a result, it could be confirmed that the separation as well as the permeation of the VOC mixtures through the PDMS membrane was determined mainly by the sorption behavior of permeant. 3.3. Temperature effect on permeation performance of VOC/N2 mixtures through PDMS membrane Fig. 7 shows the Arrhenius plots of mixture permeability coefficient and selectivity against temperature
at 0.9 vol.% VOC content in feed. It is interesting to note that the permeability coefficient of the VOC mixture decreased while the pure nitrogen permeability coefficient increased with increasing operating temperature. Details on the observation will be discussed with individual permeability. The selectivity shows a decrease with increasing operating temperature. Looking at the permeability coefficients of the individual components of the mixture shown in Fig. 8, with increasing operating temperature, the permeation of VOC component decreases but the permeation of non-condensable nitrogen component increases. The activity of the interactive component VOC tends to decrease with increasing temperature so that the activity difference between both membrane surfaces, which acts as a driving force for the VOC permeation, can be reduced, resulting in decreasing VOC permeability. Since the non-interactive gas, nitrogen has a negligible interaction with the membrane material, the nitrogen permeation could be determined mainly by membrane structure. As temperature increases, the PDMS membrane has an increase in free volumes through which the nitrogen molecules permeates, resulting in increasing nitrogen permeability. Thus, the opposite changes in the permeation of these two components with
Fig. 8. Permeability coefficient of individual component in the permeation of various VOC/N2 mixtures through PDMS membrane with operating temperature at 0.9 vol.% VOC content in feed.
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Fig. 9. Diffusion and solubility coefficients of various VOC/N2 mixtures with operating temperature at 0.9 vol.% VOC content in feed.
operating temperature can presumably cause higher selectivity at lower temperature. The diffusion and solubility coefficient data could provide good clues to explain the opposite changes in the permeation of the components described above. Fig. 9 demonstrates the diffusion and solubility coefficients of the mixtures with operating temperature, respectively, as compared with those of pure nitrogen. The diffusivities of the mixtures and pure nitrogen all had an increase with increasing operating temperature, resulting from creating larger free volumes in the membrane by more vigorous motion of polymeric chains and thereby enhancing membrane mobility. Also they were not much different from each other, especially at higher temperature. The diffusion behaviors could be explained that the intrinsic characteristic
of rubbery polymeric membrane, such as, large free volume fraction in it might weaken the contribution of permeant size to the diffusion property, and the permeant size effect would be less important at higher operating temperature due to larger free volumes created in the membrane [20]. On the other hand, the solubility coefficient of pure nitrogen had a slight increase with increasing temperature, whereas those of the VOC mixtures had a decrease. Since nitrogen is an inert to the PDMS membrane, its solubility is likely to be dependent on the membrane structure rather than interaction between nitrogen and the membrane. Thus, it could be postulated that the membrane has a looser structure owing to larger free volume at higher temperature, accommodating more nitrogen molecules in it. However, for VOC component, its
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Fig. 10. Apparent activation energies of various VOC/N2 mixtures and nitrogen through PDMS membrane determined at 0.9 vol.% of VOC content in feed.
solubility is already explained to be affected by the interaction of the VOC and membrane material. With increasing permeation temperature, VOC solubility in the membrane might be declined because the interaction as well as the activity of VOC in feed is reduced as observed in previous work [20]. Therefore, now one can see that the decreasing of both the permeability coefficient and selectivity in Fig. 7 must be attributed to the sorption behaviors of the VOC components. Three kinds of apparent activation energies, that is, permeation activation energy Ep , diffusional activation energy Ed , and heat of sorption Hs could be determined from the slope of respective Arrhenius plots in Figs. 7 and 9. The result is presented in Fig. 10. According to the solution-diffusion model, the permeation activation energy is defined as the sum of diffusional activation energy and the heat of sorption; Ep = Ed + Hs
(5)
It could be seen from Fig. 10 that the relationship between the activation energies determined for each mixture follows well the above equation. A peculiarity could be found in the figure that the diffusional activation energies between the VOC mixtures and nitrogen are not much different but the heat of sorption goes reversely in the order of permeant size. So, the
overall permeational activation energy could change in the same way as the sorptional activation energy does with permeant size, confirming that sorption process has a more contribution to the permeation of the VOC mixture rather than diffusion process has.
4. Conclusions The permeation parameters of various VOC/nitrogen mixtures through PDMS membrane were precisely measured at various operating conditions by the newly developed permeation apparatus which had been built to be able to generate a certain composition of VOC/gas mixture and measure its permeation transient from which permeability, diffusion, solubility coefficients, and respective activation energy were evaluated. The permeations of the mixtures presented very high selectivity towards VOC component by diluting nitrogen component with the selective sorption of alcohol component in the membrane and thereby blocking nitrogen permeation. The solubility coefficient of the mixture and the selectivity towards VOC component were found to increase with increasing VOC molecular size or VOC critical temperature. The
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diffusion coefficients between the VOC mixtures and pure nitrogen were not much different and did not change so much with VOC content in feed, while the solubility coefficient of the mixture increased with VOC content in feed above that of pure N2 and the change was observed to be more significant for VOC with higher critical temperature. With increasing operating temperature, all of the diffusion coefficients of the mixtures and pure N2 increase, but the solubility coefficients changed in opposite way; the solubility of pure nitrogen increased and that of the VOC mixture decreased. The change in the solubility coefficient of the mixture is found more remarkable for VOC with larger molecular size. From the analysis of the permeation parameter data and the activation energy data, it was found that sorption process affects the permeation of VOC component more that diffusion process. References [1] R.W. Baker, C.M. Bell, J. Wimans, On membrane vapor separation versus carbon adsorption, AIChE Annual Meeting, San Francisco, CA, 1989, p. 174d. [2] J.S. Cha, V. Mailik, D. Bhaumik, R. Li, K.K. Sirkar, Removal of VOCs from waste gas stream by permeation in a hollow fiber permeator, J. Membr. Sci. 128 (1997) 195–211. [3] U. Beucher, C.H. Gooding, The influence of the porous support layer of composite membranes on the separation of binary gas mixtures, J. Membr. Sci. 12 (1999) 99–116. [4] K. Kimmerle, C.M. Bell, W. Gundernatsch, H. Chemiel, Solvent recovery from air, J. Membr. Sci. 36 (1988) 363. [5] I. Pinnau, J.G. Wijmans, I. Blume, T. Kuroda, K.-V. Peinemann, Gas permeation through composite membranes, J. Membr. Sci. 37 (1988) 81. [6] M.V. Candak, Y.S. Lin, W. Ji, R.J. Higgins, Sorption and diffusion of volatile organic compounds in polydimethylsiloxane membranes, J. Appl. Polym. Sci. 67 (1998) 165–175. [7] I. Blume, P.J.F. Schwering, M.H.V. Mulder, C.A. Smolders, Vapour sorption and permeation properties of
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
[18]
[19] [20]
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poly(dimethlysiloxane) films, J. Membr. Sci. 61 (1991) 85– 97. J.M. Watson, P.A. Payne, A study of organic compound pervaporation through silicone rubber, J. Membr. Sci. 49 (1990) 171–205. M.S. Suwandi, S.A. Stern, Transport of heavy organic vapors through silicone rubber, J. Polym. Sci.: Polym. Phys. Ed. 11 (1973) 663–681. A. Singh, B.D. Freeman, I. Pinnau, Pure and mixed gas acetone:nitrogen permeation properties of polydimethylsiloxane (PDMS), J. Membr. Sci. 36 (1988) 289– 301. I. Pinnau, L.G. Toy, Transport of organic vapors through poly(1-trimethylsilyl-1-propyne), J. Membr. Sci. 116 (1996) 199–209. C.K. Yeom, S.H. Lee, J.M. Lee, Study of transport pure and mixed CO2 /N2 gases through polymeric membranes, J. Appl. Polym. Sci. 78 (2000) 179–189. C.K. Yeom, K.-H. Lee, Vapor permeation of ethanol–water mixtures using sodium alginate membranes with cross-linking gradient structure, J. Membr. Sci. 135 (1997) 225–235. C.K. Yeom, J.M. Lee, Y.T. Hong, K.Y. Choi, S.C. Kim, Analysis of permeation transients of pure gases through dense polymeric membranes measure by a new permeation apparatus, J. Membr. Sci. 166 (2000) 71–83. C.K. Yeom, C.U. Kim, B.S. Kim, K.J. Kim, J.M. Lee, Permeation apparatus for on-line measurement of the permeation characteristics of permeants through dense polymeric membranes, Korean Patent 267507, 2000 and USA Patent pending 09/329492. M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1996, Chapter 4. R.W. Baker, J.G. Wijmans, J.H. Kaschemekat, The design of membrane vapor–gas separation systems, J. Membr. Sci. 151 (1998) 55–62. A.F.M. Barton, CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, Boca Raton, FL, 1985, Chapter 8. J.A. Dean, Lange’s Handbook of Chemistry, 14th Edition, McGraw-Hill, New York, pp. 6.134–6.147. C.K. Yeom, S.H. Lee, J.M. Lee, H.Y. Song, Analysis of the permeation transients of VOCs/N2 mixtures through PDMS membrane: I. Permeation of alcohol/N2 mixtures, J. Membr. Sci., submitted for publication.
Vapor permeations of a series of VOCs/N2 mixtures through PDMS membrane C.K. Yeom a,∗ , S.H. Lee b , H.Y. Song c , J.M. Lee b a
b
PETROSEP Membrane Technologies Inc., 2270 Speers Road, Oakville, Ont., Canada L6L 2X8 Chemical Process and Engineering Center, Applied and Engineering Chemistry Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-606, South Korea c Department of Polymer Science and Engineering, Chung-Nam National University, Yusong, Taejon 305-335, South Korea Received 2 January 2001; accepted 3 September 2001
Abstract The permeation performances of volatile organic compounds (VOCs)/nitrogen mixtures through poly(dimethylsiloxane) have been measured at various operating conditions by a newly developed permeation apparatus which could measure its permeation transient rapidly, precisely and directly. In this study, a series of chlorinated hydrocarbons, that is, methylene chloride, chloroform, 1,2-dichloroethane and 1,1,2-trichloroethane were adopted as organic vapor. The permeability, diffusion, and solubility coefficients of the VOC mixtures were evaluated from the determined permeation transients. The permeation performance of the mixture as well as the selectivity towards VOC component showed a strong dependence of both VOC content in feed and the condensability of VOC. In the permeation of the VOC mixture, the nitrogen permeability was depressed below pure nitrogen permeability by the preferential sorption of VOC into PDMS membrane. This effect was more noticeable for the more condensable VOCs. Also, it is observed that the permeation and separation of the mixtures were more controlled by sorption process rather than by diffusion process. Through a quantitative analysis of the permeation parameter and apparent activation energies, the transport of VOC component in the permeation of the mixtures was discussed, in compared with pure nitrogen permeation. © 2002 Published by Elsevier Science B.V. Keywords: Poly(dimethylsiloxane); Volatile organic compounds; Chlorinated hydrocarbon; Permeation transient; Permeability; Diffusion coefficient; Solubility coefficient
1. Introduction In order to protect environment from contaminating as well as to reuse expensive materials, it is necessary to separate volatile organic compounds (VOCs) from waste gas stream in industrial processes. If the waste stream is to be released to the environment, the VOCs have to be removed substantially to control environmental pollution. There are a number of ∗ Corresponding author. E-mail address: [email protected] (C.K. Yeom).
conventional separation processes for the removal of VOCs from waste stream but vapor permeation through membranes offers significant opportunities of energy savings and reuse of the VOCs, compared to the conventional VOC control processes, particularly if the VOC concentration is high [1,2]. For the recovery of VOCs, two kinds of membranes, that is, glassy and rubbery polymers, may be selected. In most cases, organophilic rubbery membranes are preferred because they are much permeable to VOCs. Especially, when VOCs are minor components in a waste stream, the rubbery membranes could be used
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 6 5 7 - 3
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Nomenclature D Ed , Ep Hs l L P R S t ts x, y
diffusion coefficient of permeant (cm2 /s) diffusion and permeation activation energies of water, respectively (kcal/mole) heat of sorption (kcal/mole) membrane thickness (m) membrane strip length (m) permeability coefficient (cm3 (STP) cm/(cm2 s cmHg)) swelling ratio solubility coefficient (cm3 (STP)/ (cm3 cmHg)) permeating time (s) response times (s) feed and permeate compositions (vol.%)
favorably because less feed gas has to permeate the membrane to remove the bulk of the vapor, requiring relatively small membrane areas. Among the rubbery membranes, poly(dimethylsilixane) (PDMS) exhibits an excellent membrane performance for the removal of VOCs from non-condensable gases; high permeability and high selectivity for VOCs. The PDMS membranes are used for such a process in a form of composite, wherein a thin layer of nonporous silicone rubber is coated on an appropriate porous polymeric substrate [2–5]. In the removal and recovery of VOCs from air/N2 streams at a certain pressure by membrane separation, a large volume of air/N2 is allowed to flow on one side of the membrane at the feed pressure. When a vacuum is applied to the other side of create a partial pressure driving force, the VOCs selectively permeate through the membrane in preference to air/N2 , and the permeate is highly enriched in the VOCs. The VOCs are continuously recovered/recycled by condensing the small permeated gas stream while the non-condensables are recycled to the contaminated feed air/N2 stream inlet. A more fundamental understanding of the sorption, diffusion and permeation effects will help to gain further insight into the transport of VOCs through rubbery polymers. There have been very few studies on diffusion, sorption and also permeation, of VOC/gas mix-
tures through rubbery membranes so far although numerous researches [6–10] have been focused on pure organic components. Singh et al. [10] reported that the nitrogen permeability coefficient in the permeation of acetone/nitrogen mixture through PDMS membrane is equal to the pure nitrogen permeability, and VOC permeability also has the same value in pure gas permeation experiments, indicating that nitrogen and VOC sorb, diffuse and permeate as independent molecules. Pinnau and Toy [11] found in the permeation of chlorofluorocarbon/nitrogen mixtures that the nitrogen permeability is reduced dramatically by the presence of the organic vapor, showing coupled permeation. Often, the permeation behaviors of individual components in a gas mixture through rubbery membrane are found to be different from those of their pure gases due to some coupling effects in sorption [12]. Therefore, for the design of membrane process and module for the separation of a gas mixture, the fundamental permeation parameters of the mixture will be as important as those of their pure individual gases through the membrane. In this study, the permeations of chlorinated hydrocarbon/N2 mixtures through a homogeneous PDMS membrane were carried out at various operating conditions by the newly developed permeation apparatus that was designed to be able to generate a certain composition of VOC/nitrogen mixture and measure its permeation transient. From the permeation transient measured, the permeability, diffusion and solubility coefficients of the respective VOC/N2 mixture and pure N2 , and respective activation energies were evaluated. In particular, this paper concentrated on the chlorinated hydrocarbons, which are part of the homologous series of the chloromethanes and chloroethanes. The permeation and separation behaviors of the mixtures through the rubbery membrane were characterized through the analysis of the membrane performances determined. Especially, individual component permeations were characterized to analyze the coupled permeation in compared with pure nitrogen permeation. 2. Experimental 2.1. Materials Poly(dimethylsiloxane) (GE655, General Electrics) was generously provided by Dongyang Silicone
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(Seoul, Korea). It was composed of two parts; part A is mainly PDMS oligomers terminated with vinyl groups and part B a mixture of Pt catalyst and PDMS oligomer with active hydrogens. Methylene chloride, chloroform, 1,2-dichloroethane, and 1,1,2-trichloroethane, which were used as VOCs vapor, all were obtained from Junsei Chemical Co. (Tokyo, Japan) and all of them were an extra pure grade. All chemicals were used without any further purification. 2.2. Membrane preparation A casting solution was prepared by dissolving part A and part B with a ratio of 9:1 in n-hexane. The casting solution was poured into a glass petri dish and then allowed to dry in a fume hood. After curing at room temperature over night, the membranes were placed in an oven at 110 ◦ C for 2 h to ensure complete cross-linking. The cross-linking reaction took place by an addition reaction in which the active hydrogens attack the vinyl groups under Pt catalyst. The prepared membranes were 130–160 m in thickness. 2.3. Swelling measurements In order to evaluate the affinity of the VOCs toward the PDMS membrane, the swelling measurement of the membrane [13] was performed in each liquid VOC, and the amount of the liquid VOC absorbed in the membrane was determined. Dry membrane strips were immersed in the liquid VOC thermostated at 25 ◦ C for 48 h to allow the strips to reach equilibrium sorption. The dimension of a strip was 7 cm × 1.5 cm. After measuring the swollen length, L of the strip at equilibrium sorption, the strip was dried for 30 h at room temperature under vacuum and then the dry length, Lo was measured. The swelling ratio, R for an isotropic material is defined as L − Lo R= (1) Lo All measurements were repeated four or five times and the resulting data had standard deviation of ±6%. 2.4. Permeation apparatus For vapor permeation, the gas permeation apparatus that had been built for gas permeation in the pre-
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vious work [14,15] was modified to be suitable for generating a certain composition of vapor/gas mixture and measuring its permeation transient. The schematic modified apparatus is illustrated in Fig. 1, which is composed of three constituents; (a) feeding system, (b) membrane cell, and (c) measuring and data acquisition system. 2.4.1. Feeding system The feeding system is to feed a gas to the membrane cell or to generate a certain composition of vapor/N2 mixture and feed it to the membrane cell. Therefore, gas permeation as well as vapor permeation experiment could be performed with the dual feeding system. The first N2 stream was introduced to the vapor generator filled with a liquid VOC to be vaporized. To enhance the contact of the N2 with the liquid, an air diffuser was used to make fine gas bubbles in the generator. This stream was then mixed with the second stream of pure N2 to produce a mixture stream of a desired composition and flow rate. The vapor generator was equipped with a chiller, an electrical heater, a mass flow controller and a pressure regulator so that the mixture composition was able to be controlled precisely by adjusting the flow rates of the first and second N2 stream, temperature and pressure. The composition of the produced mixture was measured in on-line mode by the gas chromatograph (GC). The 1.5 l of the buffer tank was installed to keep feed pressure in the membrane cell constant. 2.4.2. Membrane cell The membrane cell was placed in the heating oven to control its temperature at a desired value. The membrane cell was made of stainless steel. The feed mixture entered the cell through the center opening, flew radially through the thin channel and left the cell through the side opening. The effective membrane area in the membrane cell was 14.7 cm2 . The membrane was supported on a filter paper and a stainless steel porous disk. The membrane cell was sealed by double O-rings to prevent vacuum leak. Before measuring permeation transient, the produced vapor mixture was by-passed to vent by opening the solenoid vale SV1 and closing SV2, simultaneously. At the same time, with the electrical actuated valve EVA opened, both the upstream and downstream sides of PDMS
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Fig. 1. A schematic representation of permeation apparatus.
membrane loaded in the cell were degassed simultaneously until the pressure in the cell reached below 0.1 Torr. After then, when the valve SV1 was close, SV2 open, and EVA close simultaneously, the by-passed vapor mixture was introduced into the upstream side of the cell and then permeation started. The very important point to obtain an accurate permeation transient curve is that feed pressure in the cell must jump from the vacuum to the desired feed pressure as soon as the produced mixture is introduced, minimizing a pressure drop occurring temporarily in the membrane cell due to volume extension. If the pressure drop happens seriously in the upstream side of the cell, the response of the permeation will be retarded, resulting in slower diffusion than real diffusion. The equipment was specially designed to obtain a step change in feed pressure from vacuum to the controlled pressure in introducing the vapor mixture.
2.4.3. Measuring and data acquisition system The measuring system which was installed along with the permeate line consists of two mass flow meters (MFM), a precise pressure transducer (PT) and gas chromatography (GC). The whole permeate line was wrapped with heating band to control the temperature in it and thereby prevent any condensation of permeate vapor on the inner wall of the line. The permeate pressure in permeate line could be controlled by a digital pressure controller. The pressure transducer (LABOVAC PIZA 2000, SASKIA, Germany) equipped with micropirani and piezo dual sensors could measure permeate pressure ranging from 1 × 10−5 to 1999.9 mbar. The mass flow meters (MFM) (Brooks, Japan) used in this study have a capacity of 100 and 1000 cm3 (STP)/min, respectively. To minimize the pressure drop developed across the MFM during permeation, the pressure difference between the inlet and outlet of the MFM was adjusted
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to be as low as 4.5 psi so that permeate pressure could be maintained as low as possible in the downstream side of the membrane cell. When permeation happened through the membrane the PT and MFM produced voltages corresponding with the permeation, respectively. Since both the PT and MFM hooked up with a computer through a 21 bit interface module for an analog to digital conversion at a sampling rate, the produced voltages could be monitored and displayed in real time by the computer, performing data acquisition. Thus, two kinds of permeation transients could be measured in on-line simultaneously; one was from permeate rate flow determined by the MFM and the other one from permeate pressure measured with time by the PT. The transient determined by the MFM has a slightly retarded response of permeation because of an occurrence of some friction in flowing permeant through the MFM, so that it could give somewhat larger value of response time and smaller diffusion coefficient than its real value, but permeability at steady-state could be measured accurately. The permeation transient measured by the PT has a real response of permeation without any retardation so it could give a correct diffusion coefficient, but since the transient curve presented the ratio of permeability at a permeating time to a steady-state permeability, the absolute value of permeability could not be obtained. For the reason mentioned above, these two permeation transients were used complementarily to make up for the weak point of each determination of permeation properties. After the data acquisition, a computer program calculated permeability coefficient and diffusion coefficient automatically from the transient curves measured by the MFM and PT, respectively. The maximum resolution of the interface module was 1 V, which was sensitive enough to detect a very small instantaneous flux in comparing with full output voltages of 5 and 10 V produced in the MFM and PT, respectively. When the MFM is operated on a gas other than air it is calibrated with, a scale shift will occur in the relationship between the output signal and the mass flow rate. This is due to the difference in heat capacities between the two gases. Using the ratio of the molar specific heat of the two gases can approximate this scale shift. The relative scale shift of a gas to air is called “a gas conversion factor”. To calculate the mass flow rate of a gas, the output reading was multiplied by the gas conversion factor of the gas.
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The gas chromatography (5890 Series II Plus HP GC) was equipped with a sample injector (6-port valve) actuated by air, a TCD detector and a packed column. The column was 6 ft long with 1/8 in. inside diameter having a Porapak Q. Thus, permeate could be injected directly into the column by the injector and thereby the composition of permeate was determined directly and accurately. The GC was also connected with the computer by the 21 bit of interface module and the permeate concentration could be displayed at the computer monitor. The evacuation of permeate was provided by the vacuum pump and the cold trap (JeioTech Ltd., Seoul, South Korea) was installed before the vacuum pump for condensing the permeate vapor. 2.5. Vapor permeations In this study the permeations of various chlorinated hydrocarbon/nitrogen mixtures through the poly(dimethylsiloxane) membrane have been carried out at different operating conditions by using the novel permeation apparatus. VOC content in feed mixture used ranged from 0 to 1.5 vol.%, and operating temperatures were 35–65 ◦ C. In all of the vapor permeation experiments, feed pressure was kept 1.5 kgf/cm2 , and feed flew along with membrane surface at a rate of 600 cm3 (STP)/min which was fast enough to minimize concentration polarization effects in feed adjacent to the membrane surface, if any. 2.6. Determination of permeation parameters from transient curves The basic principle and procedure of determining permeation parameters from the permeation transient are well described elsewhere [14]. Fig. 2 shows two transient curves monitored by MFM and PT in the new permeation apparatus, respectively, in the permeation of C2 H4 Cl2 /N4 mixture with 1.2 vol.% of VOC content at 45 ◦ C. They are the typical permeation transients that can be obtained by the permeation apparatus. The permeability coefficient P could be calculated from the voltage difference DV between the initial value and the steady-state value in permeation transient measured by the MFM. The diffusion coefficient
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Fig. 2. Permeation transient curves monitored by the MFM and PT in the permeation of C2 H4 Cl2 /N2 mixture with 1.2 vol.% of VOC content at 45 ◦ C and 600 cm3 /min of feed flow rate.
D could be evaluated from response times ts , respectively, by the expressions [8]: D=
l2 5.91ts
(2) 3. Results and discussion
The response time ts is a time for the flux to rise from its initial value to its final along the time of maximum slope as described in Fig. 2b. The solubility coefficient S could be calculated: P S= D
(3)
The following relationship was used to calculate the selectivity α: αi/j =
yi /yj xi /xj
where x is the feed composition, y the permeate composition, and component i is the preferentially permeating component.
(4)
3.1. Physical properties of PDMS membrane in various VOCs Generally, the permeation of permeants through the rubbery membrane is characterized as a sorptioncontrolled process, so the sorption behavior of a permeant could determine significantly permeation and separation performance. The sorption behavior of permeant in a polymeric membrane follows the rule of thumb; “like dissolves like”; the greater the affinity of a permeant towards a membrane is, the higher the
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Table 1 Physical parameters of each chlorinated hydrocarbon and its swelling ratios in the PDMS membrane at 25 ◦ C
CH2 Cl2 CHCl3 1,2-C2 H4 Cl2 1,1,2-C2 H3 Cl3 N2 PDMS a
Solubility parameter [18] (MPa1/2 )
Swelling ratioa
Critical temperature [19] Tc (K)
20.3 19.0 20.9 19.6 – 14.9
0.279 0.330 0.179 0.207 – –
510.0 536.4 561.2 602.0 33.2 –
Swelling ratio: (swollen length − dry length)/(dry length).
solubility of the permeant in the membrane is. In the permeation of an organic vapor/gas mixture through a polymeric membrane, the transport of gas component is characterized as zero or negligible interacted permeation with the membrane while organic vapor component shows an interacted transport behav-
ior through the membrane, especially, organophilic membranes such as PDMS. Thus, the investigation of interaction between organic vapor and membrane material is very important to analyze the permeation behavior of the permeant. The solubility parameter of permeant can be taken as a measure of the interaction
Fig. 3. Permeability coefficients and selectivity of various VOC/N2 mixtures through PDMS membrane with VOC content in feed at 45 ◦ C.
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of the permeant with membrane material by a comparison with membrane’s solubility parameter. Table 1 exhibits the solubility parameter of respective VOC used in this study and its swelling ratios in the PDMS membrane at 25 ◦ C. The membrane was more swollen in the liquid VOC that has a lower solubility parameter, being closer to PDMS value and characterizing a better affinity towards PDMS. Another parameter that determines the solubility behavior is the ease of condensation [16,17], with molecules becoming more condensable with increasing molecular size. The critical temperature Tc is used as a measure of the ease of condensation. Both the critical temperature and the solubility coefficient of VOCs in polymer increase as the molecular dimensions increase. From Table 1, it is found that the chlorinated hydrocarbon having more chlorine atoms in molecule has a lower solubility parameter and a greater solubility, and the chloroethane has a higher Tc than the chloromethane for a given number of chlorines in molecule. These sorption parameters affect permeation and separation performance, which will be discussed in next section. 3.2. Effect of feed composition on permeation performance of vapor mixtures through PDMS Fig. 3 presents the permeability coefficients and selectivities of various VOC/N2 mixtures through
the PDMS membrane as a function of feed composition at 45 ◦ C. In literature [10,11,16,18], the permeability coefficient of pure nitrogen through a PDMS membrane is reported to be (220–280) × 10−10 cm3 (STP) cm/(cm2 s cmHg) in a temperature range of 20–40 ◦ C, and the value evaluated in this study was 210 × 10−10 cm3 (STP) cm/(cm2 s cmHg), showing a good agreement with the literature values. It is observed that the VOC permeability coefficient is dependent on both VOC size and VOC content in feed. The permeability of the mixture increased with increasing VOC content in feed, and when the critical temperature of VOC Tc was higher or VOC component in the mixture was bigger, the mixture permeability was not only higher but also the permeability increased with VOC content more significantly. As a result, the permeability increased linearly for the mixture having small VOC, such as, CH2 Cl2 or CHCl3 , while the permeability curve of the mixture having larger VOC, such as, C2 H4 Cl2 or C2 H3 Cl3 was more convex to the VOC content axis, and the slope of the curve increased as the VOC content increased. The larger permeability for the mixture with larger VOC could be only be explained realizing that the larger VOC has a larger solubility in the PDMS membrane, leading to a higher overall permeability, as will be discussed later. The selectivity to VOC component was observed to be very high, ranging from 135 to 370
Fig. 4. Permeability coefficient of individual component in the permeation of various VOC/N2 mixtures through PDMS membrane with VOC content in feed at 45 ◦ C.
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Table 2 A comparison of the permeability coefficient of pure nitrogen with that of nitrogen component in various VOC/nitrogen mixtures through the PDMS membrane at 45 ◦ C VOC content in mixture (vol.%)
Nitrogen permeability coefficient (×1010 ), cm3 (STP) cm/(cm2 s cmHg) Pure nitrogen
CH2 Cl2 /N2
CHCl3 /N2
C2 H4 Cl2 /N2
C2 H3 Cl3 /N2
0 0.3 1.5
210 – –
– 160 159
– 151 147
– 148 141
– 143 139
depending on the critical temperature of VOC component in feed. Most of the permeability coefficients of the VOC mixtures increased above pure nitrogen permeability coefficient with increasing VOC content in feed but selectivity did not change so much. More details on these behaviors will be discussed through the analysis of individual component permeations. The permeability coefficients of individual components are plotted against VOC content in feed in Fig. 4. The individual permeability coefficients were determined from the data of mixture permeability coefficients and permeate concentrations. The permeability coefficient of VOC component in the VOC mixture was very high as compared to that of nitrogen component, and was independent of VOC content in the given range of feed composition. Singh et al. [10] report that pure nitrogen permeability is almost identi-
cal with that of nitrogen component in the permeation of acetone/nitrogen mixtures through PDMS membrane. However, in this study, it is unexpectedly found that the nitrogen permeability coefficient in the mixtures is reduced from the pure nitrogen permeability, depending on the VOC critical temperature or VOC content in feed. The VOC mixtures contained one of the following chlorinated hydrocarbons, ranked in order of increasing condensability: CH2 Cl2 < CHCl3 < C2 H4 Cl2 < C2 H3 Cl3 . The VOC/nitrogen mixture data, listed in Table 2, show that the nitrogen permeability is reduced more dramatically as VOC content increases or VOC condensability increases. The same observations had also been found in the previous work [18] in which a series of alcohols were employed as VOC vapor in the permeation of VOCs/N2 mixtures.
Fig. 5. Membrane selectivity with VOC critical temperature in the permeation of VOC/N2 mixtures at 45 ◦ C and 0.9 vol.% of VOC content in feed.
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In another report, Pinnau and Toy [11] suggested that a non-condensable gas such as nitrogen is dramatically by the presence of a condensable organic vapor in a high-free-volume glassy polymer poly(1-trimethylsilyl-1-propyne) (PTMSP). According to the permeation mechanism, the large amount of condensable organic vapor sorbed into the membrane causes partial blocking of the small free volume elements, reducing the nitrogen permeation. The permeation principle could be also applied to explain the transport behavior of the PDMS membrane with the chlorinated hydrocarbon/nitrogen mixtures. The unique transport through PDMS might be simply rationalized by postulating that the nitrogen permeability is depressed below the pure nitrogen permeability because of diluting nitrogen component with VOC
component selectively absorbed into the membrane matrix (Herny sites) and thereby blocking nitrogen permeation. Thus, more reducing the nitrogen permeability in the permeation of VOC mixture having larger VOC could be explained by absorbing VOC component more selectively in the membrane and thereby more diluting nitrogen component. Thus, as can be seen in Fig. 5, membrane selectivity increased with increasing the condensability of VOC component because the nitrogen permeability was depressed more. Analysis of the diffusion and solubility coefficient data could help to understand the permeation behavior of the VOC mixtures. Fig. 6 presents the diffusion and solubility coefficients of the mixtures with VOC content in feed at 45 ◦ C. Generally, in a polymeric
Fig. 6. Diffusion and solubility coefficients of various VOC/N2 mixtures with VOC content in feed at 45 ◦ C.
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membrane, the diffusion coefficient of a permeant decreases with increasing its molecular size because large molecules interact with more segments of the polymer chains than do small molecules, favoring the passage of small molecules such as nitrogen over larger ones such as VOCs, as shown in Fig. 6. The diffusion coefficient of the mixture decreases with increasing VOC content in feed. As increasing VOC content in feed, more VOC molecules could be absorbed in to membrane to such an extent that the diffusing permeant size could increase, resulting in slowing the diffusion of the permeants through the membrane. The solubility coefficients of the mixtures were determined from Eq. (3). In literature [16], the solubility coefficient of pure nitrogen in PDMS determined at 25 ◦ C and 1 MPa
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is reported to be 25 × 10−4 cm3 (STP)/(cm3 cmHg) while the value evaluated in this study ranges (19.66–22.82) × 10−4 cm3 (STP)/(cm3 cmHg), depending on temperature, which is in a good agreement with the literature value. All the solubility coefficients of the mixtures were well above pure nitrogen solubility because of the larger sorption of VOC components. As shown in Fig. 6, the solubility coefficient of the VOC mixture was a function of VOC size as well as its activity in feed. The solubility coefficient of the mixture increased with increasing VOC content in feed, and when the critical temperature of VOC Tc was higher, the mixture solubility coefficient was higher. As a result, the permeability increased linearly for the mixture having small VOC such as CH2 Cl2 or CHCl3 , while the solubility coefficient curve of
Fig. 7. Permeability coefficients and selectivity of various VOC/N2 mixtures through PDMS membrane with operating temperature at 0.9 vol.% VOC content in feed.
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the mixture having larger VOC such as C2 H4 Cl2 or C2 H3 Cl3 was more convex to the VOC content axis, increasing the slope of the curve with VOC content. In compared with the change in the solubility coefficient with VOC content, the change in the diffusivities of the mixtures is not significant, and also the difference in diffusivities between the VOC mixtures and pure nitrogen is not distinguished in the given range of feed composition. In addition, diffusivity selectivity, which is predicted to be less than one because VOC is larger than nitrogen, decreases with increasing VOC content because mixture diffusion coefficients decrease as much. Thus, the high selectivity of PDMS for VOC in VOC/nitrogen mixtures is due to enormously larger solubility of VOC in the polymer. As a result, it could be confirmed that the separation as well as the permeation of the VOC mixtures through the PDMS membrane was determined mainly by the sorption behavior of permeant. 3.3. Temperature effect on permeation performance of VOC/N2 mixtures through PDMS membrane Fig. 7 shows the Arrhenius plots of mixture permeability coefficient and selectivity against temperature
at 0.9 vol.% VOC content in feed. It is interesting to note that the permeability coefficient of the VOC mixture decreased while the pure nitrogen permeability coefficient increased with increasing operating temperature. Details on the observation will be discussed with individual permeability. The selectivity shows a decrease with increasing operating temperature. Looking at the permeability coefficients of the individual components of the mixture shown in Fig. 8, with increasing operating temperature, the permeation of VOC component decreases but the permeation of non-condensable nitrogen component increases. The activity of the interactive component VOC tends to decrease with increasing temperature so that the activity difference between both membrane surfaces, which acts as a driving force for the VOC permeation, can be reduced, resulting in decreasing VOC permeability. Since the non-interactive gas, nitrogen has a negligible interaction with the membrane material, the nitrogen permeation could be determined mainly by membrane structure. As temperature increases, the PDMS membrane has an increase in free volumes through which the nitrogen molecules permeates, resulting in increasing nitrogen permeability. Thus, the opposite changes in the permeation of these two components with
Fig. 8. Permeability coefficient of individual component in the permeation of various VOC/N2 mixtures through PDMS membrane with operating temperature at 0.9 vol.% VOC content in feed.
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Fig. 9. Diffusion and solubility coefficients of various VOC/N2 mixtures with operating temperature at 0.9 vol.% VOC content in feed.
operating temperature can presumably cause higher selectivity at lower temperature. The diffusion and solubility coefficient data could provide good clues to explain the opposite changes in the permeation of the components described above. Fig. 9 demonstrates the diffusion and solubility coefficients of the mixtures with operating temperature, respectively, as compared with those of pure nitrogen. The diffusivities of the mixtures and pure nitrogen all had an increase with increasing operating temperature, resulting from creating larger free volumes in the membrane by more vigorous motion of polymeric chains and thereby enhancing membrane mobility. Also they were not much different from each other, especially at higher temperature. The diffusion behaviors could be explained that the intrinsic characteristic
of rubbery polymeric membrane, such as, large free volume fraction in it might weaken the contribution of permeant size to the diffusion property, and the permeant size effect would be less important at higher operating temperature due to larger free volumes created in the membrane [20]. On the other hand, the solubility coefficient of pure nitrogen had a slight increase with increasing temperature, whereas those of the VOC mixtures had a decrease. Since nitrogen is an inert to the PDMS membrane, its solubility is likely to be dependent on the membrane structure rather than interaction between nitrogen and the membrane. Thus, it could be postulated that the membrane has a looser structure owing to larger free volume at higher temperature, accommodating more nitrogen molecules in it. However, for VOC component, its
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Fig. 10. Apparent activation energies of various VOC/N2 mixtures and nitrogen through PDMS membrane determined at 0.9 vol.% of VOC content in feed.
solubility is already explained to be affected by the interaction of the VOC and membrane material. With increasing permeation temperature, VOC solubility in the membrane might be declined because the interaction as well as the activity of VOC in feed is reduced as observed in previous work [20]. Therefore, now one can see that the decreasing of both the permeability coefficient and selectivity in Fig. 7 must be attributed to the sorption behaviors of the VOC components. Three kinds of apparent activation energies, that is, permeation activation energy Ep , diffusional activation energy Ed , and heat of sorption Hs could be determined from the slope of respective Arrhenius plots in Figs. 7 and 9. The result is presented in Fig. 10. According to the solution-diffusion model, the permeation activation energy is defined as the sum of diffusional activation energy and the heat of sorption; Ep = Ed + Hs
(5)
It could be seen from Fig. 10 that the relationship between the activation energies determined for each mixture follows well the above equation. A peculiarity could be found in the figure that the diffusional activation energies between the VOC mixtures and nitrogen are not much different but the heat of sorption goes reversely in the order of permeant size. So, the
overall permeational activation energy could change in the same way as the sorptional activation energy does with permeant size, confirming that sorption process has a more contribution to the permeation of the VOC mixture rather than diffusion process has.
4. Conclusions The permeation parameters of various VOC/nitrogen mixtures through PDMS membrane were precisely measured at various operating conditions by the newly developed permeation apparatus which had been built to be able to generate a certain composition of VOC/gas mixture and measure its permeation transient from which permeability, diffusion, solubility coefficients, and respective activation energy were evaluated. The permeations of the mixtures presented very high selectivity towards VOC component by diluting nitrogen component with the selective sorption of alcohol component in the membrane and thereby blocking nitrogen permeation. The solubility coefficient of the mixture and the selectivity towards VOC component were found to increase with increasing VOC molecular size or VOC critical temperature. The
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diffusion coefficients between the VOC mixtures and pure nitrogen were not much different and did not change so much with VOC content in feed, while the solubility coefficient of the mixture increased with VOC content in feed above that of pure N2 and the change was observed to be more significant for VOC with higher critical temperature. With increasing operating temperature, all of the diffusion coefficients of the mixtures and pure N2 increase, but the solubility coefficients changed in opposite way; the solubility of pure nitrogen increased and that of the VOC mixture decreased. The change in the solubility coefficient of the mixture is found more remarkable for VOC with larger molecular size. From the analysis of the permeation parameter data and the activation energy data, it was found that sorption process affects the permeation of VOC component more that diffusion process. References [1] R.W. Baker, C.M. Bell, J. Wimans, On membrane vapor separation versus carbon adsorption, AIChE Annual Meeting, San Francisco, CA, 1989, p. 174d. [2] J.S. Cha, V. Mailik, D. Bhaumik, R. Li, K.K. Sirkar, Removal of VOCs from waste gas stream by permeation in a hollow fiber permeator, J. Membr. Sci. 128 (1997) 195–211. [3] U. Beucher, C.H. Gooding, The influence of the porous support layer of composite membranes on the separation of binary gas mixtures, J. Membr. Sci. 12 (1999) 99–116. [4] K. Kimmerle, C.M. Bell, W. Gundernatsch, H. Chemiel, Solvent recovery from air, J. Membr. Sci. 36 (1988) 363. [5] I. Pinnau, J.G. Wijmans, I. Blume, T. Kuroda, K.-V. Peinemann, Gas permeation through composite membranes, J. Membr. Sci. 37 (1988) 81. [6] M.V. Candak, Y.S. Lin, W. Ji, R.J. Higgins, Sorption and diffusion of volatile organic compounds in polydimethylsiloxane membranes, J. Appl. Polym. Sci. 67 (1998) 165–175. [7] I. Blume, P.J.F. Schwering, M.H.V. Mulder, C.A. Smolders, Vapour sorption and permeation properties of
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