- Email: [email protected]
Butane isomer separation with composite zeolite MFI mebranes
Desalination 245 (2009) 437–443
Butane isomer separation with composite zeolite MFI mebranes P. Hrabáneka,*, A. Zikánováa, B. Bernauerb, V. Fílab, M. Kočiříka a
J. Heyrovský Institute of Physical Chemistry of the ASCR, v.v.i.; Dolejškova 2155/3, 18223 Prague 8, Czech Republic Email: [email protected] b Institute of Chemical Technology Prague, Technická 5, 16628 Prague 6 Dejvice, Czech Republic Received 30 June 2008; revised 16 December 2008; accepted 09 February 2009
Abstract Zeolite MFI crystal layers were grown on different α-alumina supports. Permeation and separation characteristics of selected zeolite MFI composite membranes were measured for the system of butane isomers. Flux, permeation and separation factors were measured as a function of feed pressure, temperature and feeding amount of butane isomers. The experiments, based on Wicke–Kallenbach method, were carried out using steady-state membrane apparatus MEMFIS. The observed fluxes and permeances of n-butane were higher in comparison with isobutane for all membranes. The separation factors exhibited the maxima in the dependence on temperature and decreased with increasing feed pressure. It follows from the comparison of single component and binary mixture experimental data that n-butane permeance was strongly reduced by the presence of isobutane as a co-permeating component in the binary mixture. However, isobutane was practically not influenced by the presence of n-butane. Long-term continuous membrane separation resulted in gradual decrease of butane isomers flux and corresponding separation factor. The flux decrease was more pronounced for isobutane. Reactivation of the membranes showed a slight increase of butane isomers flux but continuous decrease of separation factor. Keywords: Zeolite MFI; Membrane; Separation; Butane isomers; Stability
1. Introduction Zeolite membranes are promising candidates for high temperature separation of close boiling compounds, azeotropes and isomers [1,2]. Zeolites have *Corresponding author.
precisely defined pore structure and high thermal and chemical stability in comparison with other membrane materials. Since the zeolites are widely used as catalysts, ion-exchangers, adsorbents and molecular sieves, the zeolite membranes have been also researched for potential applications in the field of catalytic membrane reactors [3,4].
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.02.006
P. Hrabánek et al. / Desalination 245 (2009) 437–443
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Table 1 Ceramic supports used for the synthesis of zeolite MFI crystal layers
Membrane M1 M2 M3
Support material
Geometry
Porosity (%)
α-Aluminab α-Aluminab α-Aluminac
Disc (∅ 20 mm, δ = 3 mm) Disc (∅ 20 mm, δ = 3 mm) Disc (∅ 20 mm, δ = 3 mm)
44 44 38
Maximal defect size (μm)a 1.6–2 1.6–2 0.6–0.8
a
Diameter measured by bubble point method. Modified asymmetric support manufactured by Carborundum Electrite [5]. c Symmetric support developed at laboratory in cooperation with TECERAM. b
In the present study, zeolite MFI (silicalite-1) membranes were tested for the separation of butane isomers, which is currently very expensive in terms of energy cost using conventional separation techniques. The permeation and separation characteristics of the membranes and their longterm stability were measured in the dependence on feed pressure, temperature and feeding amount of butane isomers. 2. Experimental Zeolite MFI (silicalite-1) crystal layers were hydrothermally synthesised on the surface of αalumina planar supports using Teflon-lined autoclaves. The characteristics of the ceramic supports used for the synthesis are summarized in Table 1. The synthesis conditions for membrane preparation are summarized in Table 2. The first zeolite layer was formed using the synthesis mixture with the molar composition of 1SiO2:0.43TPAOH:22.3H2O [6], with Aerosil
(silicon dioxide) used as silica source and tetrapropylammonium hydroxide (TPAOH) as structure directing agent. For membrane M2, the second zeolite layer was synthesized using the synthesis mixture with the molar composition of 1SiO2:0.32TPAOH:165H2O [7], with tetraethylorthosilicate (TEOS) used as silica source. Membranes M1, M3 contained single and membrane M2 two zeolite layers prepared in two subsequent synthesis procedures. The second zeolite layer of membrane M2 was synthesized following the procedure for the preparation of b-oriented silicalite-1 crystal layer [7]. During the measurements of long-term stability, membrane M2 was reactivated at 300°C for 48 h (ACT 300°C) in the flow of dried air (600 ml min–1). Permeation and separation characteristics of calcined membranes M1, M2 and M3 were measured using steady-state apparatus MEMFIS (Fig. 1) [8,9].
Table 2 Synthesis conditions for membrane preparation
Membrane M1 M2 M3 a
Number of zeolite layer
Ageinga (day)
Crystallization (h)
Temperature (°C)
1 2 1
7 14/4 h 14
24 10/7 10
180 180/165 180
Ageing of synthesis mixture prior hydrothermal synthesis. Heating and cooling rate of 0.5°C min–1 in the flow of dried air (600 ml min–1).
b
Template removal (°C/h)b 480/48 480/48 430/40
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Gas chromatography
Mass flow controllers
Panel of valves
M E M B R A N E
Feed
Pressure control unit Mixing unit 6-way valve thermostat
Main control unit
Permeate Ssilicalite-1 layer
(n-C4H10/iso-C4H10)
N
N
n-butane ienatub-os Nhmuile
Retentate
Membrane module Wicke-Kallenbach cell
Retentate
Sweep (He)
Permeate
(B)
(A)
Fig. 1. (A) Scheme of permeation apparatus MEMFIS. (B) Wicke–Kallenbach permeation cell.
120 30°C 30°C 60°C 60°C
100 80 60 40 20 0
75
100
125
150
175
200
225
250
Feed pressure (kPa)
Fig. 2. Permeance of n-butane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: single component, open symbols: (1:1) binary mixture.
3. Results and discussion Flux and permeance of butane isomers with separation factor to n-butane were measured for membranes M1, M2 and M3 as a function of feed pressure, temperature and feeding amount for single component and equimolar (1:1) binary mixture. Permeanceisobutane (×10–10 mol·m–2·s–1·Pa–1)
Permeancen-butane (×10–9 mol·m–2s–1·Pa–1)
The membrane mounted into Wicke–Kallenbach cell was fed with butane isomers (99.95%, 100 ml min–1) using helium (99.96%) as sweeping and balancing gas (100 ml min–1). Gas chromatograph (Agilent Technologies 6890N) equipped with FID detector was used for the analysis of feed, retentate, and permeate streams.
70
30°C 30°C 60°C 60°C
60 50 40 30 20 10 0
75
100
125 150 175 200 Feed pressure (kPa)
225
250
Fig. 3. Permeance of isobutane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: single component, open symbols: (1:1) binary mixture.
440
P. Hrabánek et al. / Desalination 245 (2009) 437–443 30
30
20
10
0 75
125 150 175 200 Feed pressure (kPa)
225
250
3.1. Single component versus mixture permeation The single component and binary mixture permeation was studied for membrane M1. Figs. 2 and 3 show the permeance of butane isomers as a function of feed pressure measured at 30°C and 60°C. The permeance of n-butane was approximately one order of magnitude higher in comparison with that of isobutane. Higher permeance of n-butane is
Flux J (×10–4mol·m–2·s–1)
100
10
M1 n-butane M1 isobutane M2 n-butane M2 isobutane M3 n-butane M3 isobutane
1
0
40
80 120 160 Temperature (°C)
200
Fig. 5. Flux of butane isomers (1:1 mixture) as a function of temperature at Δp = 0 for membranes M1, M2, M3. Closed symbols: n-butane, open symbols: isobutane.
M1 M2 M3
25 20 15 10 5 0
100
Fig. 4. Separation factor to n-butane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: ideal separation factor, open symbols: separation factor for (1:1) binary mixture.
0.1
Separation factor to n-butane (-)
30°C 30°C 60°C 60°C
0
40
80 120 Temperature (°C)
160
200
Fig. 6. Separation factor to n-butane (1:1 mixture) as a function of temperature at Δp = 0 for membranes M1, M2, M3.
explained by the preferential adsorption of n-butane that hinders transport of isobutane through zeolite MFI pores [10]. The permeance increased for nbutane with the increasing temperature but was essentially invariant of temperature for isobutane. The single component permeance of n-butane was considerably higher compared with n-butane permeance observed in binary mixture. The permeance of isobutane was similar for single component and binary mixture. The results showed
Permeance (×10–8 mol·m–2·s–1·Pa–1)
Separation factor to n-butane (-)
40
10
1
M1 n-butane M1 isobutane M2 n-butane M2 isobutane M3 n-butane M3 isobutane
0.1
0.01 75
100
125 150 175 200 Feed pressure (kPa)
225
250
Fig. 7. Permeance of butane isomers (1:1 mixture) as a function of feed pressure at T = 30°C for membranes M1, M2, M3. Closed symbols: n-butane, open symbols: isobutane.
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Flux Jisobutane (×10–5 mol·m–2·s–1)
Separation factor to n-butane (-)
25 M1 M2 M3
20 15 10 5 0
18
14 12 10 8 6 4 2 0
75
100
125 150 175 200 Feed pressure (kPa)
225
M1 M2 M2 ACT 300°C
16
250
0
5
10 15 20 nisobutane (mol)
25
30
35
Fig. 8. Separation factor to n-butane (1:1 mixture) as a function of feed pressure at T = 30°C for membranes M1, M2, M3.
Fig. 10. Flux of isobutane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
that the permeation of n-butane was significantly reduced in the presence of isobutane (Fig. 2) while the permeance of isobutane was practically not influenced by the presence of n-butane (Fig. 3). It has been suggested that the decrease of n-butane permeance in the presence of isobutane can be caused by the occupation of zeolite MFI pore mouth openings and intersections of straight and sinusoidal channels with isobutane [11]. The decreasing permeance of n-butane with the
increasing feed pressure (Fig. 2) is characteristic for microporous diffusion. However, the increased permeance of isobutane at higher feed pressure is typical for transport through intercrystalline voids [12]. Fig. 4 represents separation factor to nbutane as a function of feed pressure. Separation factor is lower compared with the ideal separation factor, which was calculated from the ratio of single component permeances of butane isomers. Both separation factors increased with temperature but decreased at
M1Δp = 0 M2 T = 30°C M2 ACT 300°C
16 12 8 4 0
0
5
10 15 20 nn-butane (mol)
25
30
35
Fig. 9. Flux of n-butane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
25 Separation factor to n-butane (-)
Flux Jn-butane (×10–4 mol·m–2.s–1)
20
20 15 10 5 0
M1 M2 M2 ACT 300°C
0
5
10 15 20 25 nn-butane/isobutane (mol)
30
35
Fig. 11. Separation factor to n-butane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
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higher feed pressure. The reduction of separation factors at higher feed pressure is probably caused by the nonselective permeation through intercrystalline voids. 3.2. Comparison of membranes The comparison between permeation and separation characteristics of membranes M1, M2 and M3 as a function of temperature and feed pressure is shown in Figs. 5–8. The increasing flux of butane isomers with temperature is shown in Fig. 5. For all membranes, the maximum of nbutane flux was observed for temperature around 150°C. The position of flux maximum on the temperature dependence is explained by the cooperation of temperature dependence of adsorbed amount and that of species mobility. Fig. 6 shows the comparison of temperature dependence of separation factors to n-butane for membranes M1, M2 and M3. The maxima were observed at 60°C (M1) and 90°C (M2, M3) as the result of a monotonic increase of isobutane flux with increasing temperature and of gradual decrease of n-butane flux at high temperature. Similar dependences already observed for membrane M1 are shown in Figs. 7 and 8. For all membranes, the permeance of n-butane decreased with increasing feed pressure. This was contrast to the permeance of isobutane. As shown in Fig. 8, the highest separation factors were found for all membranes at low feed pressure. 3.3. Long-term stability of separation efficiency The long-term stability of separation efficiency was tested for membranes M1 and M2. Figs. 9 and 10 show the evolution of long-term butane isomer fluxes. The long-term flux of butane isomers was lower for membrane M2 except for the initial stage of flux evolution for reactivated membrane M2. The reactivation of membrane M2 increased flux for butane isomers in long-term separation.
The long-term separation factors to n-butane measured for membrane M1 and M2 are shown in Fig. 11. In comparison with membrane M1, membrane M2 showed higher separation factor continuously decreasing with the increasing feeding amount of butane isomers.
4. Conclusions The good quality zeolite MFI membranes were synthesised and studied for the separation of butane isomers. The membranes were effective to separate butane isomers at low feed pressure and in the temperature range between 60°C and 90°C. The longterm permeation experiments showed continuous decrease of flux and separation factor showing the necessity of periodical membrane reactivation.
Acknowledgments The financial support by the Czech Science Foundation via grant no. 203/07/1443, the Grant Agency of ASCR via grant KAN 400720701 and ASCR via grant 1QS401250509 is gratefully acknowledged.
References [1] B. Smitha, D. Suhanya, S. Sridhar and M. Ramakrishna, J. Membr. Sci., 241 (2004) 1–21. [2] J. Caro and M. Noack, Micropor. Mesopor. Mater., 115 (2008) 215–233. [3] E.E. McLeary, J.C. Jansen and F. Kapteijn, Micropor. Mesopor. Mater., 90 (2006) 198–220. [4] T. Masuda, T. Asanuma, M. Shouji, S.R. Mukai, M. Kawase and K. Hashimoto, Chem. Eng. Sci., 58 (2003) 649–656. [5] J. Pavlů, J. Kudová, Z. Zikánová, M. Kočiřík, P. Uchytil, O. Šolcová, J. Roček, V. Fíla, B. Bernauer, V. Krystl and P. Hrabánek, Chem. Listy, 98 (2004) 29–32. [6] A. Giroir-Fendler, A. Julbe, J.D.F. Ramsay and J.A. Dalmon, Patent no. WO 95/29751.
P. Hrabánek et al. / Desalination 245 (2009) 437–443 [7] Z. Wang and Y. Yan, Chem. Mater., 13 (2001) 1101– 1107. [8] P. Hrabánek, A. Zikánová, B. Bernauer, V. Fíla and M. Kočiřík, Desalination, 224 (2008) 76–80. [9] P. Hrabánek, A. Zikánová, B. Bernauer, V. Fíla, L. Brabec and M. Kočiřík, Stud. Surf. Sci. Catal., 174 (2008) 673–676. [10] J. Coronas, J.L. Falconer and R.D. Noble, AIChE J., 43 (1997) 1797–1812.
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[11] H. Takaba, R. Koshita, K. Mizukami, Y. Oumi, N. Ito, M. Kubo, A. Fahmi and A. Miyamoto, J. Membr. Sci., 134 (1997) 127–139. [12] N. Nishiyama, L. Gora, V. Teplyakov, F. Kapteijn and J.A. Moulijn, Sep. Purif. Tech., 22–23 (2001) 295–307.
Butane isomer separation with composite zeolite MFI mebranes P. Hrabáneka,*, A. Zikánováa, B. Bernauerb, V. Fílab, M. Kočiříka a
J. Heyrovský Institute of Physical Chemistry of the ASCR, v.v.i.; Dolejškova 2155/3, 18223 Prague 8, Czech Republic Email: [email protected] b Institute of Chemical Technology Prague, Technická 5, 16628 Prague 6 Dejvice, Czech Republic Received 30 June 2008; revised 16 December 2008; accepted 09 February 2009
Abstract Zeolite MFI crystal layers were grown on different α-alumina supports. Permeation and separation characteristics of selected zeolite MFI composite membranes were measured for the system of butane isomers. Flux, permeation and separation factors were measured as a function of feed pressure, temperature and feeding amount of butane isomers. The experiments, based on Wicke–Kallenbach method, were carried out using steady-state membrane apparatus MEMFIS. The observed fluxes and permeances of n-butane were higher in comparison with isobutane for all membranes. The separation factors exhibited the maxima in the dependence on temperature and decreased with increasing feed pressure. It follows from the comparison of single component and binary mixture experimental data that n-butane permeance was strongly reduced by the presence of isobutane as a co-permeating component in the binary mixture. However, isobutane was practically not influenced by the presence of n-butane. Long-term continuous membrane separation resulted in gradual decrease of butane isomers flux and corresponding separation factor. The flux decrease was more pronounced for isobutane. Reactivation of the membranes showed a slight increase of butane isomers flux but continuous decrease of separation factor. Keywords: Zeolite MFI; Membrane; Separation; Butane isomers; Stability
1. Introduction Zeolite membranes are promising candidates for high temperature separation of close boiling compounds, azeotropes and isomers [1,2]. Zeolites have *Corresponding author.
precisely defined pore structure and high thermal and chemical stability in comparison with other membrane materials. Since the zeolites are widely used as catalysts, ion-exchangers, adsorbents and molecular sieves, the zeolite membranes have been also researched for potential applications in the field of catalytic membrane reactors [3,4].
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.02.006
P. Hrabánek et al. / Desalination 245 (2009) 437–443
438
Table 1 Ceramic supports used for the synthesis of zeolite MFI crystal layers
Membrane M1 M2 M3
Support material
Geometry
Porosity (%)
α-Aluminab α-Aluminab α-Aluminac
Disc (∅ 20 mm, δ = 3 mm) Disc (∅ 20 mm, δ = 3 mm) Disc (∅ 20 mm, δ = 3 mm)
44 44 38
Maximal defect size (μm)a 1.6–2 1.6–2 0.6–0.8
a
Diameter measured by bubble point method. Modified asymmetric support manufactured by Carborundum Electrite [5]. c Symmetric support developed at laboratory in cooperation with TECERAM. b
In the present study, zeolite MFI (silicalite-1) membranes were tested for the separation of butane isomers, which is currently very expensive in terms of energy cost using conventional separation techniques. The permeation and separation characteristics of the membranes and their longterm stability were measured in the dependence on feed pressure, temperature and feeding amount of butane isomers. 2. Experimental Zeolite MFI (silicalite-1) crystal layers were hydrothermally synthesised on the surface of αalumina planar supports using Teflon-lined autoclaves. The characteristics of the ceramic supports used for the synthesis are summarized in Table 1. The synthesis conditions for membrane preparation are summarized in Table 2. The first zeolite layer was formed using the synthesis mixture with the molar composition of 1SiO2:0.43TPAOH:22.3H2O [6], with Aerosil
(silicon dioxide) used as silica source and tetrapropylammonium hydroxide (TPAOH) as structure directing agent. For membrane M2, the second zeolite layer was synthesized using the synthesis mixture with the molar composition of 1SiO2:0.32TPAOH:165H2O [7], with tetraethylorthosilicate (TEOS) used as silica source. Membranes M1, M3 contained single and membrane M2 two zeolite layers prepared in two subsequent synthesis procedures. The second zeolite layer of membrane M2 was synthesized following the procedure for the preparation of b-oriented silicalite-1 crystal layer [7]. During the measurements of long-term stability, membrane M2 was reactivated at 300°C for 48 h (ACT 300°C) in the flow of dried air (600 ml min–1). Permeation and separation characteristics of calcined membranes M1, M2 and M3 were measured using steady-state apparatus MEMFIS (Fig. 1) [8,9].
Table 2 Synthesis conditions for membrane preparation
Membrane M1 M2 M3 a
Number of zeolite layer
Ageinga (day)
Crystallization (h)
Temperature (°C)
1 2 1
7 14/4 h 14
24 10/7 10
180 180/165 180
Ageing of synthesis mixture prior hydrothermal synthesis. Heating and cooling rate of 0.5°C min–1 in the flow of dried air (600 ml min–1).
b
Template removal (°C/h)b 480/48 480/48 430/40
439
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Gas chromatography
Mass flow controllers
Panel of valves
M E M B R A N E
Feed
Pressure control unit Mixing unit 6-way valve thermostat
Main control unit
Permeate Ssilicalite-1 layer
(n-C4H10/iso-C4H10)
N
N
n-butane ienatub-os Nhmuile
Retentate
Membrane module Wicke-Kallenbach cell
Retentate
Sweep (He)
Permeate
(B)
(A)
Fig. 1. (A) Scheme of permeation apparatus MEMFIS. (B) Wicke–Kallenbach permeation cell.
120 30°C 30°C 60°C 60°C
100 80 60 40 20 0
75
100
125
150
175
200
225
250
Feed pressure (kPa)
Fig. 2. Permeance of n-butane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: single component, open symbols: (1:1) binary mixture.
3. Results and discussion Flux and permeance of butane isomers with separation factor to n-butane were measured for membranes M1, M2 and M3 as a function of feed pressure, temperature and feeding amount for single component and equimolar (1:1) binary mixture. Permeanceisobutane (×10–10 mol·m–2·s–1·Pa–1)
Permeancen-butane (×10–9 mol·m–2s–1·Pa–1)
The membrane mounted into Wicke–Kallenbach cell was fed with butane isomers (99.95%, 100 ml min–1) using helium (99.96%) as sweeping and balancing gas (100 ml min–1). Gas chromatograph (Agilent Technologies 6890N) equipped with FID detector was used for the analysis of feed, retentate, and permeate streams.
70
30°C 30°C 60°C 60°C
60 50 40 30 20 10 0
75
100
125 150 175 200 Feed pressure (kPa)
225
250
Fig. 3. Permeance of isobutane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: single component, open symbols: (1:1) binary mixture.
440
P. Hrabánek et al. / Desalination 245 (2009) 437–443 30
30
20
10
0 75
125 150 175 200 Feed pressure (kPa)
225
250
3.1. Single component versus mixture permeation The single component and binary mixture permeation was studied for membrane M1. Figs. 2 and 3 show the permeance of butane isomers as a function of feed pressure measured at 30°C and 60°C. The permeance of n-butane was approximately one order of magnitude higher in comparison with that of isobutane. Higher permeance of n-butane is
Flux J (×10–4mol·m–2·s–1)
100
10
M1 n-butane M1 isobutane M2 n-butane M2 isobutane M3 n-butane M3 isobutane
1
0
40
80 120 160 Temperature (°C)
200
Fig. 5. Flux of butane isomers (1:1 mixture) as a function of temperature at Δp = 0 for membranes M1, M2, M3. Closed symbols: n-butane, open symbols: isobutane.
M1 M2 M3
25 20 15 10 5 0
100
Fig. 4. Separation factor to n-butane (single, 1:1 mixture) as a function of feed pressure at T = 30°C and 60°C for membrane M1. Closed symbols: ideal separation factor, open symbols: separation factor for (1:1) binary mixture.
0.1
Separation factor to n-butane (-)
30°C 30°C 60°C 60°C
0
40
80 120 Temperature (°C)
160
200
Fig. 6. Separation factor to n-butane (1:1 mixture) as a function of temperature at Δp = 0 for membranes M1, M2, M3.
explained by the preferential adsorption of n-butane that hinders transport of isobutane through zeolite MFI pores [10]. The permeance increased for nbutane with the increasing temperature but was essentially invariant of temperature for isobutane. The single component permeance of n-butane was considerably higher compared with n-butane permeance observed in binary mixture. The permeance of isobutane was similar for single component and binary mixture. The results showed
Permeance (×10–8 mol·m–2·s–1·Pa–1)
Separation factor to n-butane (-)
40
10
1
M1 n-butane M1 isobutane M2 n-butane M2 isobutane M3 n-butane M3 isobutane
0.1
0.01 75
100
125 150 175 200 Feed pressure (kPa)
225
250
Fig. 7. Permeance of butane isomers (1:1 mixture) as a function of feed pressure at T = 30°C for membranes M1, M2, M3. Closed symbols: n-butane, open symbols: isobutane.
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Flux Jisobutane (×10–5 mol·m–2·s–1)
Separation factor to n-butane (-)
25 M1 M2 M3
20 15 10 5 0
18
14 12 10 8 6 4 2 0
75
100
125 150 175 200 Feed pressure (kPa)
225
M1 M2 M2 ACT 300°C
16
250
0
5
10 15 20 nisobutane (mol)
25
30
35
Fig. 8. Separation factor to n-butane (1:1 mixture) as a function of feed pressure at T = 30°C for membranes M1, M2, M3.
Fig. 10. Flux of isobutane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
that the permeation of n-butane was significantly reduced in the presence of isobutane (Fig. 2) while the permeance of isobutane was practically not influenced by the presence of n-butane (Fig. 3). It has been suggested that the decrease of n-butane permeance in the presence of isobutane can be caused by the occupation of zeolite MFI pore mouth openings and intersections of straight and sinusoidal channels with isobutane [11]. The decreasing permeance of n-butane with the
increasing feed pressure (Fig. 2) is characteristic for microporous diffusion. However, the increased permeance of isobutane at higher feed pressure is typical for transport through intercrystalline voids [12]. Fig. 4 represents separation factor to nbutane as a function of feed pressure. Separation factor is lower compared with the ideal separation factor, which was calculated from the ratio of single component permeances of butane isomers. Both separation factors increased with temperature but decreased at
M1Δp = 0 M2 T = 30°C M2 ACT 300°C
16 12 8 4 0
0
5
10 15 20 nn-butane (mol)
25
30
35
Fig. 9. Flux of n-butane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
25 Separation factor to n-butane (-)
Flux Jn-butane (×10–4 mol·m–2.s–1)
20
20 15 10 5 0
M1 M2 M2 ACT 300°C
0
5
10 15 20 25 nn-butane/isobutane (mol)
30
35
Fig. 11. Separation factor to n-butane (1:1 mixture) as a function of feeding amount at Δp = 0 and T = 30°C for membrane M1 and M2.
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higher feed pressure. The reduction of separation factors at higher feed pressure is probably caused by the nonselective permeation through intercrystalline voids. 3.2. Comparison of membranes The comparison between permeation and separation characteristics of membranes M1, M2 and M3 as a function of temperature and feed pressure is shown in Figs. 5–8. The increasing flux of butane isomers with temperature is shown in Fig. 5. For all membranes, the maximum of nbutane flux was observed for temperature around 150°C. The position of flux maximum on the temperature dependence is explained by the cooperation of temperature dependence of adsorbed amount and that of species mobility. Fig. 6 shows the comparison of temperature dependence of separation factors to n-butane for membranes M1, M2 and M3. The maxima were observed at 60°C (M1) and 90°C (M2, M3) as the result of a monotonic increase of isobutane flux with increasing temperature and of gradual decrease of n-butane flux at high temperature. Similar dependences already observed for membrane M1 are shown in Figs. 7 and 8. For all membranes, the permeance of n-butane decreased with increasing feed pressure. This was contrast to the permeance of isobutane. As shown in Fig. 8, the highest separation factors were found for all membranes at low feed pressure. 3.3. Long-term stability of separation efficiency The long-term stability of separation efficiency was tested for membranes M1 and M2. Figs. 9 and 10 show the evolution of long-term butane isomer fluxes. The long-term flux of butane isomers was lower for membrane M2 except for the initial stage of flux evolution for reactivated membrane M2. The reactivation of membrane M2 increased flux for butane isomers in long-term separation.
The long-term separation factors to n-butane measured for membrane M1 and M2 are shown in Fig. 11. In comparison with membrane M1, membrane M2 showed higher separation factor continuously decreasing with the increasing feeding amount of butane isomers.
4. Conclusions The good quality zeolite MFI membranes were synthesised and studied for the separation of butane isomers. The membranes were effective to separate butane isomers at low feed pressure and in the temperature range between 60°C and 90°C. The longterm permeation experiments showed continuous decrease of flux and separation factor showing the necessity of periodical membrane reactivation.
Acknowledgments The financial support by the Czech Science Foundation via grant no. 203/07/1443, the Grant Agency of ASCR via grant KAN 400720701 and ASCR via grant 1QS401250509 is gratefully acknowledged.
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