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Permeation and separation behaviour of a silicalite (MFI) membrane
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
215
Permeation and separation behaviour of a silicalite (MFI) membrane F. Kaptei~, W.J.W. Bakker, G. Zheng, J.A. Moulijn and H. van Bekkum "~ Department of Chemical Engineering and Organic Chemistry .) Delft University of Technology Julianalaan 136, 2628 BL Delft, The Netherlands
Introduction Combining catalytic conversion processes with membrane permeation in-situ, i.e. in the reactor configuration, offers in principle many new opportunities such as increased yields of equilibrium limited reactions, increased selectivities in complex reaction networks and coupling of catalytic reactions by mass and/or heat exchange. This requires controlled addition of reactants or separation of products under reaction conditions. Hence, knowledge of permeation and separation characteristics are indispensable for the design and process control of this emerging new type of reactors. The behaviour of membranes operating in the molecular- and Knudsen type diffusion region can be predicted on the basis of established theories. If the membrane pores approach the size of molecular dimensions, however, and the socalled configurational diffusion and molecular sieving are operative, hardly any theory and data are available to predict permeation and separation properties. This is mainly due to the fact that up to now these zeolite type of membranes are hardly available. Recently, we succeeded to prepare a silicalite (MFl-type) membrane [1], which turned out to possess high permeabilities and interesting and surprising separation properties [2], on which we report here further with new insights and results.
Experimental The RVS supported membrane, of thickness 40#m and area 3 crn:, was tested in a WickeKallenbach type cel, using helium as a purge gas and mass spectrometric gas analysis. The total pressure could be varied between 0 and 10 bar and the temperature between 200 to 700 K. In general the permeation and separation behaviour was measured at constant gas phase conditions while cycling the temperature at 1-2 K/min. Additionally, gas adsorption measurements with silicalite crystals have been performed in a thermobalance at corresponding conditions. Results and Discussion The membrane turned out too be very thermostable. After one year of operation it did not show any sign of changing properties, which is promising for reactor applications at elevated pressures and temperatures. Figure 1 shows the permeation behaviour as a function of the temperature of a 1:1 mixture of H~ and n-butane at 1 bar total pressure. At low temperatures nbutane completely blocks the hydrogen permeation, which only becomes appreciable above 400 K, when the permeation flux goes through its maximum. This occurs around the critical temperature
, E
f
20
/
is
i
o
300
400
500
600
T e m p e r a t u r e (K)
Figure 1 Separation behaviour of a H: / n-butPne mixture (1:1) as a function of temperature by a silicalite membrane at 100 kPa
216 of n-butane. Around 600 K the n-butane flux starts to increase again, while hydrogen increased monotonically over the whole range. As reference the n-butane uptake by silicalite at 0.5 bar is given in figure 2 as a function of the temperature. From this it is evident that the amount of adsorbed n-butane changes markedly just in the region where the permeation flux goes through its maximum. From this adsorption data values of -95 J/mol.K and -40.4 kJ/mol were calculated for the entropy and enthalpy of adsorption of nbutane, in agreement with literature. These type of results can be observed for all lower alkanes and alkenes. The temperature dependency can be completely ascribed to the occupancy and mobility of the molecules that dominates the flux, resulting in the maximum occurring around the critical temperature and where the coverage changes most. If corrected for this occupancy a monotonically increasing curve is obtained with an apparent activation energy for n-butane diffusion of 34.6 kJ/mol. In general weakly adsorbing species do not hinder each other and separation is proportional to the ratio of the adsorption equilibrium constants. Strongly adsorbing species completely block weakly adsorbing ones, while the separation behaviour of two strongly adsorbing species is not yet clear. Multicomponent adsorption data are needed to this purpose.
'~
2
t,.. 0 "10 <
0
m
" 300
400
500
600
Temperature (K)
Figure 2 n-Butane adsorption at silicalite at 50 kPa as a function of temperature.
5.0 "(~ 4.0
_~3.0 ~2.0
i-Butane
x ~ 1.0
, -'.~---
0.0 300
400
500
600
Temperature (K)
Figure 3 Single component permeation of isobutane (50 kPa) as a function of temperature.
Figure 3 shows the activated permeation of iso-butane at 0.5 bar, which clearly indicates that for this molecule the molecular sieving effect becomes manifest. An apparent activation energy for permeation of 25 kJ/mol was observed in this case, resulting in a diffusion activation energy estimation of 64 kJ/mol after correction for the surface coverage variation. At the workshop more detailed results for alkanes and alkenes will be presented. Further developments that will be undertaken are, among other things, the incorporation of catalytic activities in the membrane and the preparation of other zeolitic type of membranes. References .
2.
E.R. Geus, Ph.D. Thesis, Delft University of Technology, 1993. W.J.W. Bakker, G. Zheng, F. Kapteijn. M. Makkee, J.A. Moulijn, E.R. Geus and H. van Bekkum. in M.P.C. Weijnen and A.H.H. Drinkenburg (eds.) Precision Process Technology Kluwer: Dordrecht. 1993, p.425-436.
215
Permeation and separation behaviour of a silicalite (MFI) membrane F. Kaptei~, W.J.W. Bakker, G. Zheng, J.A. Moulijn and H. van Bekkum "~ Department of Chemical Engineering and Organic Chemistry .) Delft University of Technology Julianalaan 136, 2628 BL Delft, The Netherlands
Introduction Combining catalytic conversion processes with membrane permeation in-situ, i.e. in the reactor configuration, offers in principle many new opportunities such as increased yields of equilibrium limited reactions, increased selectivities in complex reaction networks and coupling of catalytic reactions by mass and/or heat exchange. This requires controlled addition of reactants or separation of products under reaction conditions. Hence, knowledge of permeation and separation characteristics are indispensable for the design and process control of this emerging new type of reactors. The behaviour of membranes operating in the molecular- and Knudsen type diffusion region can be predicted on the basis of established theories. If the membrane pores approach the size of molecular dimensions, however, and the socalled configurational diffusion and molecular sieving are operative, hardly any theory and data are available to predict permeation and separation properties. This is mainly due to the fact that up to now these zeolite type of membranes are hardly available. Recently, we succeeded to prepare a silicalite (MFl-type) membrane [1], which turned out to possess high permeabilities and interesting and surprising separation properties [2], on which we report here further with new insights and results.
Experimental The RVS supported membrane, of thickness 40#m and area 3 crn:, was tested in a WickeKallenbach type cel, using helium as a purge gas and mass spectrometric gas analysis. The total pressure could be varied between 0 and 10 bar and the temperature between 200 to 700 K. In general the permeation and separation behaviour was measured at constant gas phase conditions while cycling the temperature at 1-2 K/min. Additionally, gas adsorption measurements with silicalite crystals have been performed in a thermobalance at corresponding conditions. Results and Discussion The membrane turned out too be very thermostable. After one year of operation it did not show any sign of changing properties, which is promising for reactor applications at elevated pressures and temperatures. Figure 1 shows the permeation behaviour as a function of the temperature of a 1:1 mixture of H~ and n-butane at 1 bar total pressure. At low temperatures nbutane completely blocks the hydrogen permeation, which only becomes appreciable above 400 K, when the permeation flux goes through its maximum. This occurs around the critical temperature
, E
f
20
/
is
i
o
300
400
500
600
T e m p e r a t u r e (K)
Figure 1 Separation behaviour of a H: / n-butPne mixture (1:1) as a function of temperature by a silicalite membrane at 100 kPa
216 of n-butane. Around 600 K the n-butane flux starts to increase again, while hydrogen increased monotonically over the whole range. As reference the n-butane uptake by silicalite at 0.5 bar is given in figure 2 as a function of the temperature. From this it is evident that the amount of adsorbed n-butane changes markedly just in the region where the permeation flux goes through its maximum. From this adsorption data values of -95 J/mol.K and -40.4 kJ/mol were calculated for the entropy and enthalpy of adsorption of nbutane, in agreement with literature. These type of results can be observed for all lower alkanes and alkenes. The temperature dependency can be completely ascribed to the occupancy and mobility of the molecules that dominates the flux, resulting in the maximum occurring around the critical temperature and where the coverage changes most. If corrected for this occupancy a monotonically increasing curve is obtained with an apparent activation energy for n-butane diffusion of 34.6 kJ/mol. In general weakly adsorbing species do not hinder each other and separation is proportional to the ratio of the adsorption equilibrium constants. Strongly adsorbing species completely block weakly adsorbing ones, while the separation behaviour of two strongly adsorbing species is not yet clear. Multicomponent adsorption data are needed to this purpose.
'~
2
t,.. 0 "10 <
0
m
" 300
400
500
600
Temperature (K)
Figure 2 n-Butane adsorption at silicalite at 50 kPa as a function of temperature.
5.0 "(~ 4.0
_~3.0 ~2.0
i-Butane
x ~ 1.0
, -'.~---
0.0 300
400
500
600
Temperature (K)
Figure 3 Single component permeation of isobutane (50 kPa) as a function of temperature.
Figure 3 shows the activated permeation of iso-butane at 0.5 bar, which clearly indicates that for this molecule the molecular sieving effect becomes manifest. An apparent activation energy for permeation of 25 kJ/mol was observed in this case, resulting in a diffusion activation energy estimation of 64 kJ/mol after correction for the surface coverage variation. At the workshop more detailed results for alkanes and alkenes will be presented. Further developments that will be undertaken are, among other things, the incorporation of catalytic activities in the membrane and the preparation of other zeolitic type of membranes. References .
2.
E.R. Geus, Ph.D. Thesis, Delft University of Technology, 1993. W.J.W. Bakker, G. Zheng, F. Kapteijn. M. Makkee, J.A. Moulijn, E.R. Geus and H. van Bekkum. in M.P.C. Weijnen and A.H.H. Drinkenburg (eds.) Precision Process Technology Kluwer: Dordrecht. 1993, p.425-436.