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Separation of hydrocarbon gas mixtures using phenolic resin-based carbon membranes
Separation and Purification Technology 28 (2002) 29 – 41 www.elsevier.com/locate/seppur
Separation of hydrocarbon gas mixtures using phenolic resin-based carbon membranes Antonio B. Fuertes *, Ivan Menendez Instituto Nacional del Carbo´n (CSIC), Apartado 73, 33080, O6iedo, Spain Accepted 18 January 2002
Abstract Carbon membranes are prepared by the carbonisation of a thin film of phenolic resin deposited on the inner face of an alumina tube. Air oxidative treatments at temperatures in the range of 75 – 350 °C, prior to carbonisation (pre-oxidation) or after carbonisation (post-oxidation) were tested in order to improve the separation characteristics of carbon membranes when used with hydrocarbon mixtures such as olefin/paraffin and n-butane/iso-butane. The range of selectivities obtained for the systems studied are: ethylene/ethane, 2 – 11; propylene/propane, 10 – 50; n-butane/iso-butane, 10–40. A trade-off between selectivity and permeability or permeance was observed for all systems. The composition of the hydrocarbon mixture affects the selectivity of separation and permeance. However, feed pressure has hardly any influence on separation. The modification of permeance with temperature reveals that separation takes place according to an activated mechanism. The separation of hydrocarbon molecules by the membrane seems to occur by means of a combination of molecular sieving and adsorption mechanisms. The storage of carbon membranes under a hydrocarbon environment (i.e. propylene or n-butane) does not cause any significant change in membrane performance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenolic resin; Carbon membrane; Hydrocarbon separation; Porous carbon; Adsorption
1. Introduction The separation of mixtures formed by hydrocarbons with close boiling points, such as ethane (184.5 K) –ethylene (169.4 K), propane (231.1 K) – propylene (225.4 K) and n/iso-paraffins (i.e. n-butane (272.7 K) –iso-butane (261.3 K)) is commonly carried out in the petrochemical industry
* Corresponding author. Tel.: +34-985-28-0800; fax: + 34985-29-7662. E-mail address: [email protected] (A.B. Fuertes).
by means of cryogenic distillation. This technology requires an enormous capital layout and entails heavy operational costs associated with high energy consumption [1]. There is, therefore, great interest in the development of new separation technologies that may result in substantial energy savings. Recently, various alternative technologies for separating hydrocarbons have been investigated: (a) reversible chemical complexation [2,3]; (b) adsorption by molecular sieve sorbents [4,5]; and (c) membranes. Membrane-based technologies, entail both low capital costs and high energy efficiency compared
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to conventional separation methods and so constitute a reliable option for separating hydrocarbon mixtures (olefin/paraffin and n/iso-paraffins). Many studies of facilitated transport membranes containing Ag+ ions have already been published [6 – 8]. These membranes show good combinations of flux-selectivity for separating olefin/paraffin mixtures. However, the process must be carried out with saturated water vapour, which involves a considerable degree of complexity. A variety of conventional polymeric membranes have been studied for separating olefin/paraffin mixtures: cellulosic membranes [9], polyphenyleneoxide (PPO) [10], polyimides [11– 14]. So far, polyimides have proved to be the most promising polymeric membranes for the separation of olefin/paraffin mixtures. Olefins, on the other hand, induce a plasticization of the polymer, resulting in a loss of selectivity. Recently, growing interest has centred on zeolite-based membranes [15], which have been successfully used in the separation of mixtures formed by high hydrocarbons, such as n-butane/ iso-butane, C4 and C6 isomers, etc. [16– 23]. Carbon membranes are prepared by the carbonisation of polymeric films. Precursors of carbon membranes include synthetic polymers such as cellulose acetate [24], poly(vinylidene chloride) [25], polyimides [26,27], poly(furfuryl alcohol) [28] and phenolic resins [29]. Taking into account the mechanism of gas transport through the microporous carbon film, two types of carbon membrane have been developed: (a) Molecular sie6e carbon membranes (MSCM). The separation of gas molecules by means of MSCM takes place via a molecular sieving mechanism. (b) Adsorption-selecti6e carbon membranes (ASCM). The separation of gas molecules by means of ASCM can be achieved because of their different adsorption properties. The more easily condensable components are preferentially adsorbed into the micropores of the membrane, reducing open porosity and limiting the diffusion of the less adsorbable gases into the micropores. Recently, a new type of ASCM has been developed in our laboratory [30,31].
Work in the field of carbon membranes has been focused on the separation of gas mixtures consisting of permanent gases such as air, CO2/ CH4, CO2/N2, etc. In contrast only a few works on hydrocarbon mixture separation are described in the literature. Hayashi et al. [32,33] prepared carbon membranes by the carbonisation of a BPDA-pp%ODA polyimide and reported good permeances and selectivities for the separation of ethylene/ethane and propylene/propane mixtures. Okamoto et al. [34] analysed olefin/paraffin separation by means of carbon membranes derived from an asymmetric hollow fibre polyimide-based carbon membrane. Soffer et al. [35] patented the preparation of a carbon membrane obtained from the carbonisation of a cellulosic polymer and described the separation of linear from branched hydrocarbons. The work presented here analyses the separation of olefin/paraffin mixtures (ethane/ethylene and propane/propylene) and the n-butane/iso-butane mixture by means of carbon membranes prepared by coating a phenolic resin on a porous ceramic tube. In order to improve the separation characteristics, some of the carbonised membranes were oxidised with air under different conditions. The main objectives were: (a) to optimise carbon membrane performance for hydrocarbon separation; (b) to analyse the effect of composition, feed pressure and temperature on separation; and (c) to evaluate membrane aging under hydrocarbon environments.
2. Experimental section
2.1. Preparation of the carbon membranes Ceramic tubular membranes manufactured by U.S. Filter for ultrafiltration processes were used as carbon membrane supports. The dimensions of the substrates are 10 mm o.d., 7 mm i.d. and 20 mm length. The tubes used consist of four layers: the inner layer is made of g-alumina film (Pore size: 20 nm); the other layers are made of a-alumina. The selective carbon film was prepared from a commercial novolac-type phenolic resin. A thin phenolic resin film was deposited on the inner
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face of the porous substrate. The supported phenolic resin film was cured in air at 150 °C for 2 h. Carbonisation of the polymeric film was carried out in a vertical tubular furnace (Carbolite) at a temperature of 700 °C, under vacuum ( B 0.01 mbar). The obtained carbon membranes have a thickness of around 2 mm. Pre-oxidation treatments with air were carried out on deposited and cured phenolic resin films. Post-oxidation treatments on the carbonised samples were performed with air at temperatures between 75 and 350 °C, for 30 min. In some cases, the carbon membranes were modified by chemical vapour deposition (CVD), before or after the oxidation step. In CVD experiments a stream of nitrogen saturated with trychloroethane (TCE) was brought into contact with the carbon membrane at a temperature of 500 °C for times ranging from 1 to 10 min.
Analysis of the separation of gas mixtures was carried out at ambient temperature (209 1 °C) by means of a system described elsewhere [29]. A gas mixture was injected into the inner side of the tubular membrane (in contact with the carbon microporous film). Commonly the feed pressure was 1 bar, but some experiments were carried out at feed pressures of up to 2.5 bar. Helium was used as carrier gas and was made to flow through the permeate side (Pressure: 1 bar). The gas concentration on the permeate side was measured by means of a TCD-GC (Hewlett-Packard, Mod. 5890) equipped with a Porapak Q column. To ensure operation under quasi-differential conditions, the stage cut ((Permeate flow rate/feed flow rate)× 100) was maintained in the range of 5– 10%. By modifying the flow rate of the carrier gas, concentrations of permeating gases in the permeate stream were maintained below 5%.
2.2. Permeation measurements
3. Results and discussion
In order to analyse the permeation characteristics of the membranes, different gases were selected: He (2.6 A, ), CO2 (3.3 A, ), O2 (3.46 A, ), N2 (3.64 A, ), CH4 (3.8 A, ), C2H4 (3.9 A, ), C2H6 (4.0 A, ), C3H6 (4.5 A, ), C3H8 (4.3 A, ), n-C4H10 (4.3 A, ), i-C4H10 (5.0 A, ). The values in brackets correspond to the kinetic diameters of the gases [36]. The permeance of pure gases was measured at ambient temperature (209 1 °C). The carbon membrane (effective area in the range 2×10 − 4 – 3 × 10 − 4 m2) was attached to a permeation cell. To measure the permeation of pure gases, highpressure high-purity gases supplied from compressed gas cylinders were placed in contact with the membrane layer. A manometer was used to measure the pressure. A vacuum was maintained in the low-pressure side of the membrane, and the permeate was pulled through a calibrated volume. The variation in pressure was determined by using a pressure transducer (Leybold CM 1000) and a digital unit connected to a computer. The permeance and permeability of pure gases through the membrane was estimated from the variation of pressure with time at the low-pressure side of the device up to a pressure of 100 mbar.
3.1. Permeation and separation of olefin/paraffin and n-butane/iso-butane mixtures Fig. 1(a and b) illustrate the effect that different treatments have on the permeance of pure gas through carbonised membranes. As a reference, Fig. 1b shows the change of permeance with the kinetic diameter of a carbon membrane that was post-oxidised (350 °C) and treated in a stream of N2 + TCE for 10 min (CVD). Pure gas permeance through this membrane exhibits an abrupt change with the kinetic diameter at around 3.5 A, . The membrane is impervious to gases with kinetic diameters greater than 4 A, . This suggests that separation occurs by means of a molecular sieve mechanism, as a result of which high permselectivities (Ratio of pure gas permeances) of gas pairs formed by permanent gases are obtained (Table 1). In contrast, for untreated carbon membranes or carbon membranes treated in air (pre-oxidation, post-oxidation and mild CVD+ post-oxidation), pure gas permeance does not exhibit any correlation with the kinetic diameter. This suggests that not only molecular size, but also the adsorption characteristics of the perme-
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ant gases play an important role in gas transport. Thus, it can be seen that some hydrocarbons exhibit higher permeances than those obtained for permanent gases with a lower molecular size (O2, N2 and in some cases He). Furthermore, in the
case of permanent gases these membranes show a clear molecular sieve effect, as can be deduced from the value of permselectivity calculated for the O2 –N2 system. Table 1 shows this value together with those of other gas pairs. Unoxidised
Fig. 1. Modification of pure gas permeance with kinetic diameter for carbon membranes prepared using to different methods.
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Table 1 Permselectivity values (ratio of pure gas permeances) for carbon membranes prepared according to different protocols Treatment
h(O2/N2)
h(CO2/N2)
Fig. 1a Unoxidised Unoxidised+regenerated Post-oxidised (75 °C) Post-oxidised (100 °C) CVD+post-oxidised (300 °C) Pre-oxidised (150 °C)
2.6 4.3 3.2 3.9 3.9 3.1
13 2.4 22 22 14 13
3.4 6.2 4.5 6.5 6.0 3.3
Fig. 1b Post-oxidised (125 °C) Post-oxidised (200 °C) Post-oxidised (350 °C) Pre-oxidised (200 °C) Post-oxidised (350 °C)+CVD
2.1 1.8 1.5 1.5 9.2
7.3 5.4 5.7 3.7 39
2.2 1.5 1.0 0.9 42
carbon membranes or those subjected to mild oxidation (Fig. 1a) exhibit permeances lower than 10 − 7 (mol m − 2 s − 1 Pa − 1). These membranes have high permselectivity values for the separation of permanent gas pairs and hydrocarbons (olefin/paraffin and n-butane/iso-butane) (Table 1). In contrast, the carbon membranes prepared under more severe oxidation conditions (shown in Fig. 1b) exhibit high permeances, which are in many cases above 10 − 7 (mol m − 2 s − 1 Pa − 1), but lower permselectivity values, as indicated in Table 1. Particulary interesting is the effect of post-oxidation temperature on permeance. From Fig. 1 it can be seen that as post-oxidation temperature increases from 75 to 350 °C an increase of permeances of all gases takes place. This change is consequence of widening of micropores as postoxidation temperature increases. This fact has been discussed elsewhere [31]. Figs. 2 and 3 show a comparative analysis of carbon membranes for the separation of ethylene – ethane and propylene– propane systems, respectively. The results obtained in this study and reported by other authors for carbon and polymeric membranes are plotted in a h– P diagram (Selectivity vs. Permeability). The carbon membranes prepared in this work by CVD+post-oxidation (300 °C), show the best performances in terms of selectivity– permeability for both systems (ethylene –ethane and propylene– propane). The
h(He/N2)
h(C2H4/C2H6)
h(C3H6/C3H8)
h(n-C4/i-C4)
2.3 2.0 1.6 1.8 4.9 1.9
9.6 17 4.2 11 54 13
50 37 12 32 – 31
2.0 1.4 1.0 1.2 11
2.5 3.4 1.7 2.5 –
8.1 33 3.0 8.3 –
other carbon membranes, prepared by simple carbonisation or by mild pre-/post-oxidation in air at temperatures below 150 °C, show h– P values that are far below the boundary line. From Fig. 2 it can be seen that the phenolic resin-based carbon membranes described here show a similar ethylene/ethane separation performance with respect to the polyimide-based carbon membranes reported by other authors [32,33]. The data in Fig. 3, however, confirm that the carbon membranes in this work have, in some cases, better separation capabilities towards the propylene/propane system than the polyimide-based carbon membranes reported in the literature [32–34]. In both figures the h–P boundary lines deduced from data reported by Hayashi et al. [32] are plotted. For the ethylene–ethane system the boundary line (h= 14×P − 1/5) is still valid and can be used as a guide for assessing the capability of the carbon membrane to separate ethylene–ethane mixtures. As for the propylene–propane system, some of the carbon membranes reported in this work show h–P values which are over Hayashi’s boundary line (h= 200× P − 1/2.5). In general, carbon membranes show better performances than polymeric membranes. Polymeric membranes prepared by Illinicht et al. [10] from PPO and some PPO-based copolymers possess high h–P values in the case of the separation of ethylene–ethane and propylene –propane mixtures (see Figs. 2 and 3). How-
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ever, these data for PPO membranes were not confirmed by the experiments realised by Tanaka et al. [12] and Okamoto et al. [13]. Thus, for a PPO membrane, Tanaka et al. [12] obtained: h(propylene/propane) =9.1 and P(propylene) = 2.3 Barrer. These results are very different to the data reported by Illinich et al. [10] and represented in Fig. 3. The performance of carbon membranes in the separation of the n-butane/iso-butane system is illustrated in Fig. 4. Those prepared by post-oxidation (100–200 °C) or simple carbonisation show the best performances, in terms of selectivity –permeability, for n/iso-butane separation. Fig. 4 reveals the trade-off relationship between n-butane/iso-butane selectivity (h) and n-butane permeance (F). Cellulosic-based carbon membranes produced by activating a carbon membrane with oxygen and then with hydrogen have recently been patented by Soffer et al. [35] for the separation of linear from branched hydrocarbons. The data plotted in Fig. 4 indicate that the carbon membranes developed in the present study and those obtained by Soffer et al. [35] show a similar performance for the separation of n-butane/iso-
butane mixtures. Fig. 4 shows the h–F boundary line deduced from the best results obtained for carbon membranes: h= 6.5× 10 − 5 × F − 1/1.25. The permeation and separation of n-butane/ iso-butane is currently being used to characterise zeolite membranes [15]. Fig. 4 contains the h–F values obtained by several authors from different kinds of zeolite membranes [16–23]. This graph allows the performance of carbon and zeolite membranes to be compared. From the data plotted, the carbon membranes obtained here and by Soffer et al. [35] exhibit n-butane/iso-butane separation performances similar to those of zeolite membranes. It can be observed that the upper h– F limit obtained for carbon membranes is also valid for zeolite membranes. The results of the permeation of pure gases or the separation of hydrocarbon mixtures shown in Figs. 2–4 correspond to the best performing carbon membranes prepared in this study. For membranes prepared according to the same procedure the results show some dispersion. In consequence, it is difficult to ascertain the effect of small changes in treatment conditions on membrane performance. This uncertainty is typical of carbon
Fig. 2. Relationship between C2H4/C2H6 selectivity and C2H4 permeability for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; (), Hayashi et al. [32]; ( ), Yamamoto et al. [33]; ( +), Polymeric membranes (PPO). Illinitch et al. [10]; (), Polymeric membranes (Polyimide) Tanaka et al. [12]. (1 Barrer =10 − 10 cm3 cm cm − 2 s − 1 cmHg − 1).
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Fig. 3. Relationship between C3H6/C3H8 selectivity and C3H6 permeability for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; (), Hayashi et al. [32]; ( ), Yamamoto et al. [33]; ( ), Okamoto et al. [34]; ( +), Polymeric membranes (PPO). Illinitch et al. [10]; (), Polymeric membranes (Polyimide) Tanaka et al. [12]. (2), Polymeric membranes (Polyimide). Okamoto et al. [13]; ( × ), Polymeric membranes with facilitated transport. Bai et al. [8]. (1 Barrer =10 − 10 cm3 cm cm − 2 s − 1 cmHg − 1).
membranes, as observed by other authors [32– 34]. The carbon membranes reported here show molecular sieve effects, as was deduced from the change in permeance with kinetic diameter (Fig. 1) and permselectivity values obtained from permanent gas pairs (Table 1). Thus, the h(O2/N2) values are in the range of 2– 4. However, as indicated previously, the fact that more strongly adsorbable gases (i.e. CO2 and C2 – C4 hydrocarbons) exhibit permeances which in some cases turn out to be higher than those measured for permanent gases with a lower molecular size, clearly indicates that adsorption affects hydrocarbon permeation strongly. Moreover, taking into account the similarity of the adsorption characteristics of the components of the hydrocarbon mixtures studied, it seems clear that separation selectivity (in the case of hydrocarbon mixtures) is mainly dependent on the size of the micropore in the carbon membranes.
3.2. Effect of composition and pressure on the separation of olefin/paraffin and n-butane/iso-butane mixtures The effect of gas mixture composition and operational pressure on the separation of C2H4/ C2H6, C3H6/C3H8 and n-C4H10/i-C4H10 mixtures is illustrated in Figs. 5–7. It can be observed in all cases that as the concentration of the most permeable component (ethylene, propylene or n-butane) increases, the separation factor and permeance decrease (Fig. 5a, Fig. 6a and Fig. 7a). Moreover, as the feed gas pressure increases, there is a decline in the permeance of the most permeable component (Fig. 5b, Fig. 6b and Fig. 7b). This behaviour is a consequence of the effect of adsorption on hydrocarbon permeance. For moderately adsorbable gases, such as those studied here, gas permeance (F) is the sum of two contributions, surface diffusion (adsorption), Fa, and gas
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diffusion, Fg. The following equation can be deduced for permeance [37], F= Fa +Fg =
m 1 csat(1 − q)DaK + Dg , ~l RT
(1)
where m is membrane porosity, ~ the tortuosity factor, d membrane thickness, csat the maximum amount adsorbed at a given temperature, q the surface coverage, K the Langmuir constant, Da is the diffusion coefficient for the adsorbed phase and Dg is the diffusivity in gas phase. A more complete discussion in relation with this subject is given elsewhere [37]. From Eq. (1), gas permeance will decrease with surface coverage (q), which grows with adsorbable gas pressure (isotherm). This behaviour is coherent with the modification of permeance in the more strongly adsorbable components (ethylene, propylene and n-butane), as the molar fraction (Fig. 5a, Fig. 6a and Fig. 7a) and feed pressure change (Fig. 5b, Fig. 6b and Fig. 7b). The modification in feed pressure between 1 and 2.5 bar hardly affects the separation factor of equimolar mixtures (Fig. 5b, Fig. 6b and Fig. 7b).
However, the separation factor clearly diminishes, as the partial pressure of the most adsorbable components increases (Fig. 5a, Fig. 6a and Fig. 7a). This change can be interpreted as a consequence of the hindering effect of the most strongly adsorbed gas (i.e. ethylene, propylene, n-butane) on the permeation of the least adsorbable gas (i.e. ethane, propane, iso-butane). This effect occurs because a part of the open porosity is occupied by the adsorbable species and it has already been theoretically analysed by Fuertes [37] following the approach proposed by Yang et al. [38].
3.3. Influence of temperature on the separation of olefin/paraffin and n-butane/iso-butane mixtures Figs. 8–10 show the dependence of the separation of the equimolar mixtures of ethylene–ethane (Fig. 8), propylene–propane (Fig. 9) and n-butane/iso-butane (Fig. 10) on temperature. In the temperature range studied (25–125 °C), hydrocarbon transport throughout the carbon membranes occurs according to an activated mechanism. This is clearly shown in Figs. 8–10.
Fig. 4. Relationship between n-C4H10/i-C4H10 selectivity and n-C4H10 permeance for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; ( gas mixtures; pure gases), Soffer et al. [35]. ( +), Zeolite membranes (Data taken from Ref. [16–23]).
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not reversible. Initially the permeances and separation factor change with temperature as shown in Fig. 10. After this experiment, the membrane was cooled down to room temperature and the permeances were again measured at 25 °C. It was observed that both the n-butane and iso-butane permeances and the separation factor show values which are below those initially measured. Afterwards, the membrane was reheated up to 100 °C and separation experiment with an n-butane/isobutane mixture was again carried out. The permeance values at this temperature were: 100× 10 − 10 (n-butane) and 0.13×10 − 10 mol m − 2 s − 1 Pa − 1
Fig. 5. Modification of ethylene permeance with composition (a) and pressure (b) for the separation of ethylene –ethane mixtures. The carbon membrane was prepared by CVD and post-oxidation at 300 °C (T =20 °C).
The apparent activation energies are higher for less permeable gases such as ethane, propane and iso-butane than for more permeable ones such as ethylene, propylene and n-butane. In consequence, the separation factor decreases markedly with temperature. Thus, when the temperature increases, the separation selectivity diminishes from 25 (25 °C) to 10 (100 °C) for the n-butane/iso-butane (50/50) system, from 22 (22 °C) to 8 (125 °C) for propylene/propane (50/50) and from 3.3 (25 °C) to 2.4 (125 °C) for ethylene/ethane (50/50). The results indicate that optimal separation of olefin/paraffin mixtures by means of carbon membranes will be attained at low operation temperatures. It has been observed that the permeance changes in n-butane and iso-butane with temperature are
Fig. 6. Modification of propylene permeance with composition (a) and pressure (b) for the separation of propylene – propane mixtures. The carbon membrane was prepared by simple carbonisation (Unoxidised) (T=20 °C).
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3.4. Membrane aging under a hydrocarbon en6ironment Carbon membranes may undergo alterations of their properties during storage or application because of interaction with surrounding gases. For example, a significant decrease in gas permeance for carbon membranes stored under air at room temperature has been observed [39]. This would constitute a serious obstacle to the application of carbon membranes for the separation of gas mixtures containing oxygen, such as air. Therefore, an analysis of the stability of carbon membranes for the long-term separation of hydrocarbon gas mixtures was performed in this work. Fig. 11 shows the modification, with time, of the hydrocarbon pure gas permeances (Fig. 11a) and permselectivities (Fig. 11b) of a carbon membrane stored under propylene for 220 days. After this period of storage the permeances of ethane and ethylene are more
Fig. 7. Modification of n-butane permeance with composition (a) and pressure (b) for the separation of n-butane/iso-butane mixtures. The carbon membrane was prepared by simple carbonisation (Unoxidised) (T= 20 °C).
(iso-butane). This means that whereas n-butane permeance decreased by around 65%, the drop in iso-butane permeance was more than 99%. After the second cycle, the carbon membrane was practically impervious to iso-butane and, in consequence, the resulting separation factor, measured at 100 °C, was very high ( 750). In order to regenerate the membrane, this was treated at 150 °C under vacuum for several hours. However, no increase in permeance was achieved. These results suggest that some kind of irreversible adsorption must have taken place during the heating step of the carbon membrane under an n-butane/iso-butane environment.
Fig. 8. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar ethylene – ethane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
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4. Conclusions Supported carbon membranes were prepared by the carbonisation of a thin phenolic resin film deposited on the inner face of a porous ceramic tube. Carbon membrane performance towards the separation of hydrocarbon mixtures was enhanced by applying different treatments: air oxidation of either the phenolic resin (pre-oxidation) or the carbonised membrane (post-oxidation) and CVD treatment of the carbonised membrane. The carbon membranes prepared show good properties for separating the hydrocarbon mixtures of olefin/paraffin (ethylene/ethane and propylene/propane) and n-butane/iso-butane. Carbon membranes prepared by mild CVD and air oxidation at 300 °C show the best perfor-mance for the separation of olefin/paraffin mixtures. Optimal separation of n-butane/iso-butane is achieved by carbon membranes that are simply
Fig. 9. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar propylene– propane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
than five times higher with respect to the fresh membrane. However, for the same period of time the ethylene/ethane selectivity did not change. The permeances of propylene and propane and the permseletivity of this system hardly varied during storage. It can be concluded therefore that keeping the carbon membrane under a hydrocarbon atmosphere for long periods of time does not cause any damage to its performance. By contrast the increase in the permeances of ethylene and ethane is somewhat surprising. A possible explanation might be the small expansion of the carbon micropores as a consequence of hydrocarbon adsorption [39]. The stability of a carbon membrane stored under n-butane was also analysed. The membrane was maintained for more than 100 days and no significant changes in permeances and permselectivities were detected for the hydrocarbons tested (ethylene, ethane, propylene, propane, n-butane and iso-butane).
Fig. 10. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar n-butane/iso-butane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
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permeance after long storage times (up to 220 days). This proves that carbon membranes can be used to separate hydrocarbons for long-term applications. Acknowledgements The authors would like to acknowledge the financial support received from Comisio´ n Interministerial de Ciencia y Tecnologı´a (CICYT: MAT99-1130; Accio´ n Especial: MAT1997-2049CE) and FICYT (PB-MAT99-01). References
Fig. 11. Modification with time of: (a) the normalised permeance (P/Po); and (b) the normalised permselectivity (h/ho) of a carbon membrane stored under propylene. Permeances of fresh membrane (Po in, mol m − 2 s − 1 Pa − 1 × 10 − 10) at 20 °C are: C2H6, 29; C2H4, 64; C3H8, 23; C3H6, 283; n-C4H10, 27; i-C4H10, 8. Permselectivities of fresh membrane: ho(C2H4/ C2H6)= 2.2; ho(C3H6/C3H8)= 11.4; ho(n-C4H10/i-C4H10)= 3.5.
carbonised or post-oxidised with air at temperatures between 100–200 °C. The separation of hydrocarbon mixtures by means of carbon membranes is influenced by the composition of the mixture, feed pressure and temperature of separation. The modification of permeance with temperature reveals that the permeation of hydrocarbon through the membrane takes place in accordance with an activated mechanism. The transport of hydrocarbons through the carbon membranes probably occurs according to a mechanism that combines molecular sieving and adsorption effects. Under a hydrocarbon environment carbon membranes do not show any loss in selectivity or
[1] R.B. Eldridge, Ind. Eng. Chem. Res. 32 (1993) 2208. [2] D.J. Safarik, R.B. Eldridge, Ind. Eng. Chem. Res. 37 (1998) 2571. [3] J. Padin, R.T. Yang, C.L. Munson, Ind. Eng. Chem. Res. 38 (1999) 3614. [4] W. Zhu, F. Kapteijn, J.A. Moulijn, Chem. Commun. (1999) 2453. [5] J. Padin, S.U. Rege, R.T. Yang, L.S. Cheng, Chem. Eng. Sci. 55 (2000) 4525. [6] H.H. Funke, R.D. Noble, C.A. Koval, J. Memb. Sci. 82 (1993) 229. [7] D.T. Tsou, M.W. Blachman, J.C. Davis, Ind. Eng. Chem. Res. 33 (1994) 3209. [8] S. Bai, S. Sridar, A.A. Khan, J. Memb. Sci. 147 (1998) 131. [9] A. Ito, S.T. Hwang, J. Appl. Polym. Sci. 38 (1989) 483. [10] O.M. Illinitch, G.L. Semin, M.V. Chertova, K.I. Zamaraev, J. Memb. Sci. 66 (1992) 1. [11] K.R. Lee, S.T. Hwang, J. Memb. Sci. 73 (1992) 37. [12] K. Tanaka, A. Taguchi, J. Hao, H. Kita, K. Okamoto, J. Memb. Sci. 121 (1996) 197. [13] K. Okamoto, K. Noborio, J. Hao, K. Tanaka, H. Kita, J. Memb. Sci. 134 (1997) 171. [14] C. Staudt-Bickel, W.J. Koros, J. Memb. Sci. 170 (2000) 205. [15] A. Tavolaro, E. Drioli, Adv. Mater. 11 (1999) 975. [16] K. Kusakabe, A. Murata, T. Kuroda, S. Morooka, J. Chem. Eng. Japan 30 (1997) 72. [17] M. Nomura, T. Yamaguchi, S. Nakao, Ind. Eng. Chem. Res. 36 (1997) 4217. [18] V. Tuan, J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res. 38 (1999) 3635. [19] Y. Yan, M. Tsapatsis, G. Gavalas, M. Davis, Chem. Commun. (1995) 227. [20] J. Coronas, R. Noble, J.L. Falconer, Ind. Eng. Chem. Res. 37 (1998) 166. [21] Y. Yan, M.E. Davis, G.R. Gavalas, J. Memb. Sci. 126 (1997) 53.
A.B. Fuertes, I. Menendez / Separation and Purification Technology 28 (2002) 29–41 [22] J. Coronas, R.D. Noble, J.L. Falconer, Am. Inst. Chem. Eng. J. 43 (1997) 1797. [23] R. Lai, G.R. Gavalas, Ind. Eng. Chem. Res. 37 (1998) 4275. [24] A. Soffer, J.E. Koresh, S. Saggy, U.S. Patent 4,685,940, 1987. [25] T.A. Centeno, A.B. Fuertes, Carbon 38 (2000) 1067. [26] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1419. [27] A.B. Fuertes, T.A. Centeno, J. Memb. Sci. 144 (1998) 105. [28] M.B. Shiflett, H.C. Foley, Science 285 (1999) 1902. [29] T.A. Centeno, A.B. Fuertes, J. Memb. Sci. 160 (1999) 201. [30] A.B. Fuertes, J. Memb. Sci. 177 (2000) 9. [31] A.B. Fuertes, Effect of air oxidation on gas separation properties of adsorption-selective carbon membranes, Carbon 39 (2001) 697.
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[32] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka, Ind. Eng. Chem. Res. 35 (1996) 4176. [33] M. Yamamoto, K. Kusakabe, J. Hayashi, S. Morooka, J. Memb. Sci. 133 (1997) 195. [34] K. Okamoto, S. Kawamura, M. Yoshino, H. Kita, T. Hirayama, N. Tanihara, Y. Kusuki, Ind. Eng. Chem. Res. 38 (1999) 4424. [35] A. Soffer, J. Gilron, H. Cohen, U.S. Patent 5,914,434, June 22, 1999. [36] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. [37] A.B. Fuertes, Adsorption 7 (2001) 117. [38] M. Yang, B.D. Crittenden, S.P. Perera, H. Moueddeb, J.A. Dalmon, J. Memb. Sci. 156 (1999) 1. [39] I. Menendez, A.B. Fuertes, Aging of carbon membranes under different environments, Carbon 39 (2001) 733.
Separation of hydrocarbon gas mixtures using phenolic resin-based carbon membranes Antonio B. Fuertes *, Ivan Menendez Instituto Nacional del Carbo´n (CSIC), Apartado 73, 33080, O6iedo, Spain Accepted 18 January 2002
Abstract Carbon membranes are prepared by the carbonisation of a thin film of phenolic resin deposited on the inner face of an alumina tube. Air oxidative treatments at temperatures in the range of 75 – 350 °C, prior to carbonisation (pre-oxidation) or after carbonisation (post-oxidation) were tested in order to improve the separation characteristics of carbon membranes when used with hydrocarbon mixtures such as olefin/paraffin and n-butane/iso-butane. The range of selectivities obtained for the systems studied are: ethylene/ethane, 2 – 11; propylene/propane, 10 – 50; n-butane/iso-butane, 10–40. A trade-off between selectivity and permeability or permeance was observed for all systems. The composition of the hydrocarbon mixture affects the selectivity of separation and permeance. However, feed pressure has hardly any influence on separation. The modification of permeance with temperature reveals that separation takes place according to an activated mechanism. The separation of hydrocarbon molecules by the membrane seems to occur by means of a combination of molecular sieving and adsorption mechanisms. The storage of carbon membranes under a hydrocarbon environment (i.e. propylene or n-butane) does not cause any significant change in membrane performance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenolic resin; Carbon membrane; Hydrocarbon separation; Porous carbon; Adsorption
1. Introduction The separation of mixtures formed by hydrocarbons with close boiling points, such as ethane (184.5 K) –ethylene (169.4 K), propane (231.1 K) – propylene (225.4 K) and n/iso-paraffins (i.e. n-butane (272.7 K) –iso-butane (261.3 K)) is commonly carried out in the petrochemical industry
* Corresponding author. Tel.: +34-985-28-0800; fax: + 34985-29-7662. E-mail address: [email protected] (A.B. Fuertes).
by means of cryogenic distillation. This technology requires an enormous capital layout and entails heavy operational costs associated with high energy consumption [1]. There is, therefore, great interest in the development of new separation technologies that may result in substantial energy savings. Recently, various alternative technologies for separating hydrocarbons have been investigated: (a) reversible chemical complexation [2,3]; (b) adsorption by molecular sieve sorbents [4,5]; and (c) membranes. Membrane-based technologies, entail both low capital costs and high energy efficiency compared
1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 0 6 - 0
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to conventional separation methods and so constitute a reliable option for separating hydrocarbon mixtures (olefin/paraffin and n/iso-paraffins). Many studies of facilitated transport membranes containing Ag+ ions have already been published [6 – 8]. These membranes show good combinations of flux-selectivity for separating olefin/paraffin mixtures. However, the process must be carried out with saturated water vapour, which involves a considerable degree of complexity. A variety of conventional polymeric membranes have been studied for separating olefin/paraffin mixtures: cellulosic membranes [9], polyphenyleneoxide (PPO) [10], polyimides [11– 14]. So far, polyimides have proved to be the most promising polymeric membranes for the separation of olefin/paraffin mixtures. Olefins, on the other hand, induce a plasticization of the polymer, resulting in a loss of selectivity. Recently, growing interest has centred on zeolite-based membranes [15], which have been successfully used in the separation of mixtures formed by high hydrocarbons, such as n-butane/ iso-butane, C4 and C6 isomers, etc. [16– 23]. Carbon membranes are prepared by the carbonisation of polymeric films. Precursors of carbon membranes include synthetic polymers such as cellulose acetate [24], poly(vinylidene chloride) [25], polyimides [26,27], poly(furfuryl alcohol) [28] and phenolic resins [29]. Taking into account the mechanism of gas transport through the microporous carbon film, two types of carbon membrane have been developed: (a) Molecular sie6e carbon membranes (MSCM). The separation of gas molecules by means of MSCM takes place via a molecular sieving mechanism. (b) Adsorption-selecti6e carbon membranes (ASCM). The separation of gas molecules by means of ASCM can be achieved because of their different adsorption properties. The more easily condensable components are preferentially adsorbed into the micropores of the membrane, reducing open porosity and limiting the diffusion of the less adsorbable gases into the micropores. Recently, a new type of ASCM has been developed in our laboratory [30,31].
Work in the field of carbon membranes has been focused on the separation of gas mixtures consisting of permanent gases such as air, CO2/ CH4, CO2/N2, etc. In contrast only a few works on hydrocarbon mixture separation are described in the literature. Hayashi et al. [32,33] prepared carbon membranes by the carbonisation of a BPDA-pp%ODA polyimide and reported good permeances and selectivities for the separation of ethylene/ethane and propylene/propane mixtures. Okamoto et al. [34] analysed olefin/paraffin separation by means of carbon membranes derived from an asymmetric hollow fibre polyimide-based carbon membrane. Soffer et al. [35] patented the preparation of a carbon membrane obtained from the carbonisation of a cellulosic polymer and described the separation of linear from branched hydrocarbons. The work presented here analyses the separation of olefin/paraffin mixtures (ethane/ethylene and propane/propylene) and the n-butane/iso-butane mixture by means of carbon membranes prepared by coating a phenolic resin on a porous ceramic tube. In order to improve the separation characteristics, some of the carbonised membranes were oxidised with air under different conditions. The main objectives were: (a) to optimise carbon membrane performance for hydrocarbon separation; (b) to analyse the effect of composition, feed pressure and temperature on separation; and (c) to evaluate membrane aging under hydrocarbon environments.
2. Experimental section
2.1. Preparation of the carbon membranes Ceramic tubular membranes manufactured by U.S. Filter for ultrafiltration processes were used as carbon membrane supports. The dimensions of the substrates are 10 mm o.d., 7 mm i.d. and 20 mm length. The tubes used consist of four layers: the inner layer is made of g-alumina film (Pore size: 20 nm); the other layers are made of a-alumina. The selective carbon film was prepared from a commercial novolac-type phenolic resin. A thin phenolic resin film was deposited on the inner
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face of the porous substrate. The supported phenolic resin film was cured in air at 150 °C for 2 h. Carbonisation of the polymeric film was carried out in a vertical tubular furnace (Carbolite) at a temperature of 700 °C, under vacuum ( B 0.01 mbar). The obtained carbon membranes have a thickness of around 2 mm. Pre-oxidation treatments with air were carried out on deposited and cured phenolic resin films. Post-oxidation treatments on the carbonised samples were performed with air at temperatures between 75 and 350 °C, for 30 min. In some cases, the carbon membranes were modified by chemical vapour deposition (CVD), before or after the oxidation step. In CVD experiments a stream of nitrogen saturated with trychloroethane (TCE) was brought into contact with the carbon membrane at a temperature of 500 °C for times ranging from 1 to 10 min.
Analysis of the separation of gas mixtures was carried out at ambient temperature (209 1 °C) by means of a system described elsewhere [29]. A gas mixture was injected into the inner side of the tubular membrane (in contact with the carbon microporous film). Commonly the feed pressure was 1 bar, but some experiments were carried out at feed pressures of up to 2.5 bar. Helium was used as carrier gas and was made to flow through the permeate side (Pressure: 1 bar). The gas concentration on the permeate side was measured by means of a TCD-GC (Hewlett-Packard, Mod. 5890) equipped with a Porapak Q column. To ensure operation under quasi-differential conditions, the stage cut ((Permeate flow rate/feed flow rate)× 100) was maintained in the range of 5– 10%. By modifying the flow rate of the carrier gas, concentrations of permeating gases in the permeate stream were maintained below 5%.
2.2. Permeation measurements
3. Results and discussion
In order to analyse the permeation characteristics of the membranes, different gases were selected: He (2.6 A, ), CO2 (3.3 A, ), O2 (3.46 A, ), N2 (3.64 A, ), CH4 (3.8 A, ), C2H4 (3.9 A, ), C2H6 (4.0 A, ), C3H6 (4.5 A, ), C3H8 (4.3 A, ), n-C4H10 (4.3 A, ), i-C4H10 (5.0 A, ). The values in brackets correspond to the kinetic diameters of the gases [36]. The permeance of pure gases was measured at ambient temperature (209 1 °C). The carbon membrane (effective area in the range 2×10 − 4 – 3 × 10 − 4 m2) was attached to a permeation cell. To measure the permeation of pure gases, highpressure high-purity gases supplied from compressed gas cylinders were placed in contact with the membrane layer. A manometer was used to measure the pressure. A vacuum was maintained in the low-pressure side of the membrane, and the permeate was pulled through a calibrated volume. The variation in pressure was determined by using a pressure transducer (Leybold CM 1000) and a digital unit connected to a computer. The permeance and permeability of pure gases through the membrane was estimated from the variation of pressure with time at the low-pressure side of the device up to a pressure of 100 mbar.
3.1. Permeation and separation of olefin/paraffin and n-butane/iso-butane mixtures Fig. 1(a and b) illustrate the effect that different treatments have on the permeance of pure gas through carbonised membranes. As a reference, Fig. 1b shows the change of permeance with the kinetic diameter of a carbon membrane that was post-oxidised (350 °C) and treated in a stream of N2 + TCE for 10 min (CVD). Pure gas permeance through this membrane exhibits an abrupt change with the kinetic diameter at around 3.5 A, . The membrane is impervious to gases with kinetic diameters greater than 4 A, . This suggests that separation occurs by means of a molecular sieve mechanism, as a result of which high permselectivities (Ratio of pure gas permeances) of gas pairs formed by permanent gases are obtained (Table 1). In contrast, for untreated carbon membranes or carbon membranes treated in air (pre-oxidation, post-oxidation and mild CVD+ post-oxidation), pure gas permeance does not exhibit any correlation with the kinetic diameter. This suggests that not only molecular size, but also the adsorption characteristics of the perme-
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ant gases play an important role in gas transport. Thus, it can be seen that some hydrocarbons exhibit higher permeances than those obtained for permanent gases with a lower molecular size (O2, N2 and in some cases He). Furthermore, in the
case of permanent gases these membranes show a clear molecular sieve effect, as can be deduced from the value of permselectivity calculated for the O2 –N2 system. Table 1 shows this value together with those of other gas pairs. Unoxidised
Fig. 1. Modification of pure gas permeance with kinetic diameter for carbon membranes prepared using to different methods.
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Table 1 Permselectivity values (ratio of pure gas permeances) for carbon membranes prepared according to different protocols Treatment
h(O2/N2)
h(CO2/N2)
Fig. 1a Unoxidised Unoxidised+regenerated Post-oxidised (75 °C) Post-oxidised (100 °C) CVD+post-oxidised (300 °C) Pre-oxidised (150 °C)
2.6 4.3 3.2 3.9 3.9 3.1
13 2.4 22 22 14 13
3.4 6.2 4.5 6.5 6.0 3.3
Fig. 1b Post-oxidised (125 °C) Post-oxidised (200 °C) Post-oxidised (350 °C) Pre-oxidised (200 °C) Post-oxidised (350 °C)+CVD
2.1 1.8 1.5 1.5 9.2
7.3 5.4 5.7 3.7 39
2.2 1.5 1.0 0.9 42
carbon membranes or those subjected to mild oxidation (Fig. 1a) exhibit permeances lower than 10 − 7 (mol m − 2 s − 1 Pa − 1). These membranes have high permselectivity values for the separation of permanent gas pairs and hydrocarbons (olefin/paraffin and n-butane/iso-butane) (Table 1). In contrast, the carbon membranes prepared under more severe oxidation conditions (shown in Fig. 1b) exhibit high permeances, which are in many cases above 10 − 7 (mol m − 2 s − 1 Pa − 1), but lower permselectivity values, as indicated in Table 1. Particulary interesting is the effect of post-oxidation temperature on permeance. From Fig. 1 it can be seen that as post-oxidation temperature increases from 75 to 350 °C an increase of permeances of all gases takes place. This change is consequence of widening of micropores as postoxidation temperature increases. This fact has been discussed elsewhere [31]. Figs. 2 and 3 show a comparative analysis of carbon membranes for the separation of ethylene – ethane and propylene– propane systems, respectively. The results obtained in this study and reported by other authors for carbon and polymeric membranes are plotted in a h– P diagram (Selectivity vs. Permeability). The carbon membranes prepared in this work by CVD+post-oxidation (300 °C), show the best performances in terms of selectivity– permeability for both systems (ethylene –ethane and propylene– propane). The
h(He/N2)
h(C2H4/C2H6)
h(C3H6/C3H8)
h(n-C4/i-C4)
2.3 2.0 1.6 1.8 4.9 1.9
9.6 17 4.2 11 54 13
50 37 12 32 – 31
2.0 1.4 1.0 1.2 11
2.5 3.4 1.7 2.5 –
8.1 33 3.0 8.3 –
other carbon membranes, prepared by simple carbonisation or by mild pre-/post-oxidation in air at temperatures below 150 °C, show h– P values that are far below the boundary line. From Fig. 2 it can be seen that the phenolic resin-based carbon membranes described here show a similar ethylene/ethane separation performance with respect to the polyimide-based carbon membranes reported by other authors [32,33]. The data in Fig. 3, however, confirm that the carbon membranes in this work have, in some cases, better separation capabilities towards the propylene/propane system than the polyimide-based carbon membranes reported in the literature [32–34]. In both figures the h–P boundary lines deduced from data reported by Hayashi et al. [32] are plotted. For the ethylene–ethane system the boundary line (h= 14×P − 1/5) is still valid and can be used as a guide for assessing the capability of the carbon membrane to separate ethylene–ethane mixtures. As for the propylene–propane system, some of the carbon membranes reported in this work show h–P values which are over Hayashi’s boundary line (h= 200× P − 1/2.5). In general, carbon membranes show better performances than polymeric membranes. Polymeric membranes prepared by Illinicht et al. [10] from PPO and some PPO-based copolymers possess high h–P values in the case of the separation of ethylene–ethane and propylene –propane mixtures (see Figs. 2 and 3). How-
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ever, these data for PPO membranes were not confirmed by the experiments realised by Tanaka et al. [12] and Okamoto et al. [13]. Thus, for a PPO membrane, Tanaka et al. [12] obtained: h(propylene/propane) =9.1 and P(propylene) = 2.3 Barrer. These results are very different to the data reported by Illinich et al. [10] and represented in Fig. 3. The performance of carbon membranes in the separation of the n-butane/iso-butane system is illustrated in Fig. 4. Those prepared by post-oxidation (100–200 °C) or simple carbonisation show the best performances, in terms of selectivity –permeability, for n/iso-butane separation. Fig. 4 reveals the trade-off relationship between n-butane/iso-butane selectivity (h) and n-butane permeance (F). Cellulosic-based carbon membranes produced by activating a carbon membrane with oxygen and then with hydrogen have recently been patented by Soffer et al. [35] for the separation of linear from branched hydrocarbons. The data plotted in Fig. 4 indicate that the carbon membranes developed in the present study and those obtained by Soffer et al. [35] show a similar performance for the separation of n-butane/iso-
butane mixtures. Fig. 4 shows the h–F boundary line deduced from the best results obtained for carbon membranes: h= 6.5× 10 − 5 × F − 1/1.25. The permeation and separation of n-butane/ iso-butane is currently being used to characterise zeolite membranes [15]. Fig. 4 contains the h–F values obtained by several authors from different kinds of zeolite membranes [16–23]. This graph allows the performance of carbon and zeolite membranes to be compared. From the data plotted, the carbon membranes obtained here and by Soffer et al. [35] exhibit n-butane/iso-butane separation performances similar to those of zeolite membranes. It can be observed that the upper h– F limit obtained for carbon membranes is also valid for zeolite membranes. The results of the permeation of pure gases or the separation of hydrocarbon mixtures shown in Figs. 2–4 correspond to the best performing carbon membranes prepared in this study. For membranes prepared according to the same procedure the results show some dispersion. In consequence, it is difficult to ascertain the effect of small changes in treatment conditions on membrane performance. This uncertainty is typical of carbon
Fig. 2. Relationship between C2H4/C2H6 selectivity and C2H4 permeability for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; (), Hayashi et al. [32]; ( ), Yamamoto et al. [33]; ( +), Polymeric membranes (PPO). Illinitch et al. [10]; (), Polymeric membranes (Polyimide) Tanaka et al. [12]. (1 Barrer =10 − 10 cm3 cm cm − 2 s − 1 cmHg − 1).
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Fig. 3. Relationship between C3H6/C3H8 selectivity and C3H6 permeability for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; (), Hayashi et al. [32]; ( ), Yamamoto et al. [33]; ( ), Okamoto et al. [34]; ( +), Polymeric membranes (PPO). Illinitch et al. [10]; (), Polymeric membranes (Polyimide) Tanaka et al. [12]. (2), Polymeric membranes (Polyimide). Okamoto et al. [13]; ( × ), Polymeric membranes with facilitated transport. Bai et al. [8]. (1 Barrer =10 − 10 cm3 cm cm − 2 s − 1 cmHg − 1).
membranes, as observed by other authors [32– 34]. The carbon membranes reported here show molecular sieve effects, as was deduced from the change in permeance with kinetic diameter (Fig. 1) and permselectivity values obtained from permanent gas pairs (Table 1). Thus, the h(O2/N2) values are in the range of 2– 4. However, as indicated previously, the fact that more strongly adsorbable gases (i.e. CO2 and C2 – C4 hydrocarbons) exhibit permeances which in some cases turn out to be higher than those measured for permanent gases with a lower molecular size, clearly indicates that adsorption affects hydrocarbon permeation strongly. Moreover, taking into account the similarity of the adsorption characteristics of the components of the hydrocarbon mixtures studied, it seems clear that separation selectivity (in the case of hydrocarbon mixtures) is mainly dependent on the size of the micropore in the carbon membranes.
3.2. Effect of composition and pressure on the separation of olefin/paraffin and n-butane/iso-butane mixtures The effect of gas mixture composition and operational pressure on the separation of C2H4/ C2H6, C3H6/C3H8 and n-C4H10/i-C4H10 mixtures is illustrated in Figs. 5–7. It can be observed in all cases that as the concentration of the most permeable component (ethylene, propylene or n-butane) increases, the separation factor and permeance decrease (Fig. 5a, Fig. 6a and Fig. 7a). Moreover, as the feed gas pressure increases, there is a decline in the permeance of the most permeable component (Fig. 5b, Fig. 6b and Fig. 7b). This behaviour is a consequence of the effect of adsorption on hydrocarbon permeance. For moderately adsorbable gases, such as those studied here, gas permeance (F) is the sum of two contributions, surface diffusion (adsorption), Fa, and gas
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diffusion, Fg. The following equation can be deduced for permeance [37], F= Fa +Fg =
m 1 csat(1 − q)DaK + Dg , ~l RT
(1)
where m is membrane porosity, ~ the tortuosity factor, d membrane thickness, csat the maximum amount adsorbed at a given temperature, q the surface coverage, K the Langmuir constant, Da is the diffusion coefficient for the adsorbed phase and Dg is the diffusivity in gas phase. A more complete discussion in relation with this subject is given elsewhere [37]. From Eq. (1), gas permeance will decrease with surface coverage (q), which grows with adsorbable gas pressure (isotherm). This behaviour is coherent with the modification of permeance in the more strongly adsorbable components (ethylene, propylene and n-butane), as the molar fraction (Fig. 5a, Fig. 6a and Fig. 7a) and feed pressure change (Fig. 5b, Fig. 6b and Fig. 7b). The modification in feed pressure between 1 and 2.5 bar hardly affects the separation factor of equimolar mixtures (Fig. 5b, Fig. 6b and Fig. 7b).
However, the separation factor clearly diminishes, as the partial pressure of the most adsorbable components increases (Fig. 5a, Fig. 6a and Fig. 7a). This change can be interpreted as a consequence of the hindering effect of the most strongly adsorbed gas (i.e. ethylene, propylene, n-butane) on the permeation of the least adsorbable gas (i.e. ethane, propane, iso-butane). This effect occurs because a part of the open porosity is occupied by the adsorbable species and it has already been theoretically analysed by Fuertes [37] following the approach proposed by Yang et al. [38].
3.3. Influence of temperature on the separation of olefin/paraffin and n-butane/iso-butane mixtures Figs. 8–10 show the dependence of the separation of the equimolar mixtures of ethylene–ethane (Fig. 8), propylene–propane (Fig. 9) and n-butane/iso-butane (Fig. 10) on temperature. In the temperature range studied (25–125 °C), hydrocarbon transport throughout the carbon membranes occurs according to an activated mechanism. This is clearly shown in Figs. 8–10.
Fig. 4. Relationship between n-C4H10/i-C4H10 selectivity and n-C4H10 permeance for the carbon membranes. ( 1/1 gas mixtures; pure gases), this work; ( gas mixtures; pure gases), Soffer et al. [35]. ( +), Zeolite membranes (Data taken from Ref. [16–23]).
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not reversible. Initially the permeances and separation factor change with temperature as shown in Fig. 10. After this experiment, the membrane was cooled down to room temperature and the permeances were again measured at 25 °C. It was observed that both the n-butane and iso-butane permeances and the separation factor show values which are below those initially measured. Afterwards, the membrane was reheated up to 100 °C and separation experiment with an n-butane/isobutane mixture was again carried out. The permeance values at this temperature were: 100× 10 − 10 (n-butane) and 0.13×10 − 10 mol m − 2 s − 1 Pa − 1
Fig. 5. Modification of ethylene permeance with composition (a) and pressure (b) for the separation of ethylene –ethane mixtures. The carbon membrane was prepared by CVD and post-oxidation at 300 °C (T =20 °C).
The apparent activation energies are higher for less permeable gases such as ethane, propane and iso-butane than for more permeable ones such as ethylene, propylene and n-butane. In consequence, the separation factor decreases markedly with temperature. Thus, when the temperature increases, the separation selectivity diminishes from 25 (25 °C) to 10 (100 °C) for the n-butane/iso-butane (50/50) system, from 22 (22 °C) to 8 (125 °C) for propylene/propane (50/50) and from 3.3 (25 °C) to 2.4 (125 °C) for ethylene/ethane (50/50). The results indicate that optimal separation of olefin/paraffin mixtures by means of carbon membranes will be attained at low operation temperatures. It has been observed that the permeance changes in n-butane and iso-butane with temperature are
Fig. 6. Modification of propylene permeance with composition (a) and pressure (b) for the separation of propylene – propane mixtures. The carbon membrane was prepared by simple carbonisation (Unoxidised) (T=20 °C).
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3.4. Membrane aging under a hydrocarbon en6ironment Carbon membranes may undergo alterations of their properties during storage or application because of interaction with surrounding gases. For example, a significant decrease in gas permeance for carbon membranes stored under air at room temperature has been observed [39]. This would constitute a serious obstacle to the application of carbon membranes for the separation of gas mixtures containing oxygen, such as air. Therefore, an analysis of the stability of carbon membranes for the long-term separation of hydrocarbon gas mixtures was performed in this work. Fig. 11 shows the modification, with time, of the hydrocarbon pure gas permeances (Fig. 11a) and permselectivities (Fig. 11b) of a carbon membrane stored under propylene for 220 days. After this period of storage the permeances of ethane and ethylene are more
Fig. 7. Modification of n-butane permeance with composition (a) and pressure (b) for the separation of n-butane/iso-butane mixtures. The carbon membrane was prepared by simple carbonisation (Unoxidised) (T= 20 °C).
(iso-butane). This means that whereas n-butane permeance decreased by around 65%, the drop in iso-butane permeance was more than 99%. After the second cycle, the carbon membrane was practically impervious to iso-butane and, in consequence, the resulting separation factor, measured at 100 °C, was very high ( 750). In order to regenerate the membrane, this was treated at 150 °C under vacuum for several hours. However, no increase in permeance was achieved. These results suggest that some kind of irreversible adsorption must have taken place during the heating step of the carbon membrane under an n-butane/iso-butane environment.
Fig. 8. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar ethylene – ethane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
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4. Conclusions Supported carbon membranes were prepared by the carbonisation of a thin phenolic resin film deposited on the inner face of a porous ceramic tube. Carbon membrane performance towards the separation of hydrocarbon mixtures was enhanced by applying different treatments: air oxidation of either the phenolic resin (pre-oxidation) or the carbonised membrane (post-oxidation) and CVD treatment of the carbonised membrane. The carbon membranes prepared show good properties for separating the hydrocarbon mixtures of olefin/paraffin (ethylene/ethane and propylene/propane) and n-butane/iso-butane. Carbon membranes prepared by mild CVD and air oxidation at 300 °C show the best perfor-mance for the separation of olefin/paraffin mixtures. Optimal separation of n-butane/iso-butane is achieved by carbon membranes that are simply
Fig. 9. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar propylene– propane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
than five times higher with respect to the fresh membrane. However, for the same period of time the ethylene/ethane selectivity did not change. The permeances of propylene and propane and the permseletivity of this system hardly varied during storage. It can be concluded therefore that keeping the carbon membrane under a hydrocarbon atmosphere for long periods of time does not cause any damage to its performance. By contrast the increase in the permeances of ethylene and ethane is somewhat surprising. A possible explanation might be the small expansion of the carbon micropores as a consequence of hydrocarbon adsorption [39]. The stability of a carbon membrane stored under n-butane was also analysed. The membrane was maintained for more than 100 days and no significant changes in permeances and permselectivities were detected for the hydrocarbons tested (ethylene, ethane, propylene, propane, n-butane and iso-butane).
Fig. 10. Modification of separation factor (a) and permeance (b) with temperature for the separation of an equimolar n-butane/iso-butane gas mixture by means of a carbon membrane prepared by post-oxidation at 200 °C.
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permeance after long storage times (up to 220 days). This proves that carbon membranes can be used to separate hydrocarbons for long-term applications. Acknowledgements The authors would like to acknowledge the financial support received from Comisio´ n Interministerial de Ciencia y Tecnologı´a (CICYT: MAT99-1130; Accio´ n Especial: MAT1997-2049CE) and FICYT (PB-MAT99-01). References
Fig. 11. Modification with time of: (a) the normalised permeance (P/Po); and (b) the normalised permselectivity (h/ho) of a carbon membrane stored under propylene. Permeances of fresh membrane (Po in, mol m − 2 s − 1 Pa − 1 × 10 − 10) at 20 °C are: C2H6, 29; C2H4, 64; C3H8, 23; C3H6, 283; n-C4H10, 27; i-C4H10, 8. Permselectivities of fresh membrane: ho(C2H4/ C2H6)= 2.2; ho(C3H6/C3H8)= 11.4; ho(n-C4H10/i-C4H10)= 3.5.
carbonised or post-oxidised with air at temperatures between 100–200 °C. The separation of hydrocarbon mixtures by means of carbon membranes is influenced by the composition of the mixture, feed pressure and temperature of separation. The modification of permeance with temperature reveals that the permeation of hydrocarbon through the membrane takes place in accordance with an activated mechanism. The transport of hydrocarbons through the carbon membranes probably occurs according to a mechanism that combines molecular sieving and adsorption effects. Under a hydrocarbon environment carbon membranes do not show any loss in selectivity or
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