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A New Method for Investigation of Sorption Kinetics of Volatile Multi-Component Mixtures on Porous Solids
Fundamentals of Adsorption Proc. IVth Int. Conf. on Fundamentals of Adsorption, Kyoto, May 11-22, 1992 Copyright Q 1993 International Adsorption Society
A New Method for Investigation of Sorption Kinetics of Volatile Multi-Component Mixtures on Porous Solids
J m e n m e , Martin Billow and Andr6 Micke Centre of Heterogeneous Catalysis, KAI e.V., Rudower Chaussee 5, Berlin, D-1199,GERMANY
ABSTRACT
An experimental technique to follow sorption kinetics under constant volume-variable concentration conditions is described. The system consists of a sampling and a sorption vessel both separated by an electro-pneumatically driven valve with known charackaistics. Analyzing the gaseous phase above the sorbent by mass spectroscopic multiple ion detection, uptake curves of singte components and binary mixture sorption are registered. Valve effects, conductance of the whole system and wall adsorption must be c m t e d by blank experiments. Equilibrium and non-equilibrium data for benzene and ethylbenzene and for their binary mixtures on silicalite-I are reported.
INTRODUCTION A large number of technological processes which take place on solid sorbents proceed under
non-eqdbium conditions. Therefore,both theoretical work and development of reliable experimental methods which allow for the investigation of multi component sorption kinetics becomes highly important [l]. Fast pressure measurement tools, e.g. membrane manometers, were used to monitor Sorption kinetics of single components on molecular sieves (MS). To observe the sorption kinetics of multi component mixtures, partial pressure measurement methods have to be used. Applying mass spectrometersto the investigation of sorption kinetics, the indication of total and partial pressures, of residual gaseous components, of catalytic reactions and their kinetics, of leaks as well as the control of sorbent acthion become possible. Quadrupole mass spectrometers (QMS) allow for Multiple Ion Detection (MID) with very short measurement cycles, i.e. in the millisecond range on up to 16 channels. Recently, sorption kinetics of binary benzene-ethylbenzene mixtures on a MS of ZSM-5-type were investigakd by a new FTIR method [2]. For a possible comparison, thie work deals with a sitttitar sorption system. Single component sorption kinetics of aromatics on MF'I-type MS has been considered in numerous papers, e.g. [3 81.
-
EXPERIMENTAL
The principle realized works under constant volume-variable concentration conditions. The main feaiures of apparatus and problems inherent are as follows. The coupling of a sorption vessel to the QMS must be worked out carefblly. In order to reduce inlet pressures amounting to more than 10 mbar, a two-phase pressure reduction might be recommended. That should render mixed flow processes (latllinary gas transport and molecular flow, respectively, on the high pressure and the low pressure sides). But such pressure reduction would strongly affect the mass balance (up to 20 ml min-' for one sampling). To avoid this situation a one-phase pressure reduction was chosen. In the case considered, a needle valve is used to reduce the pressure over a range of ca five M ~ WofSmagnitudes.
286
J. Hille, M. Biilow and A. Micke
In case of high inlet pressures, the measured composition of mixture can be corrected via the molecular weights of the mixture components.
Apparatus The two-vessel inlet system constructed is situated on the QMS SX 300, VG ( m e 1). Both vessels are separated by a valve driven electro-pneumatically. The sorption vessel is connected to the ion so&e of the QMS by a needle valve. The sample vessel is equipped with a sample lock and the whole inlet system can be evacuated towards high vacuum by opening the evacuation valve. In both the sample and the sorption vessels, the total pressure can be measured by Baratron systems. Furthermore, pressures can be registered by the QMS. The sorbent sample, e.g. MS ..u u 3 . 3 1 1 u*= lms "I crystals, inside the sorption vessel (cf. Fig 1. Principal Scheme of the Apparatus figure 2) is packed into a mantle situated between a stainless steel sieve and a cylindric quartz tube. Inside the tube, heating PT 100 there is placed a heater helix controlled by a thermic resistance of the type Pt 100. To prevent concentration gradients within the sorption vessel and rateitainleos steel limiting extemal Nm transport, a second sorption Unit with a stirred MS package was constructed. It will be used in further experiments. The volumes of the individual parts of the system are given in figure 1. As visible from the scheme, opening or closing the v&e situated between the main vessels in the actual setup causes a volume change amounting to 7.1 ml. This disadvantage can either be met by modelling the Fig. 2. Scheme of the Sorption Vessel processes in the apparatus by the software ZEUS [9] or by appropriate exchange. A large-sizedsilicdte-I sample (crYs$l size: 150 x 20 x 20 p3 ; sample weight: 44.3 mg) was used in the experiments. For data evaluation, the crystal geometIy was approximated by a two dimensional parallelepiped with extensions amounting to 20 p n in both x and y directions. U b C
,
Experiments and Data Evaluation Sorption equilibria and adsorption rates of benzene and ethylbenzene were measured on a silicalite crystal package in a differential concentration mode. It was ensured that no intercrystalline transport effects iduence on uptake curves. The ranges of temperature and pressure were 323K ... 363K and 10 pbar ... 10 mbar, respechly, for both single components and binary mixtures. In the mixhue experiments, there was used stepwise dosing which followed the condition to keep the gaseous phase composition as near as possible to a molar ffaction of x(') * 0.5 (co-diffusion). Besides, after g presorption of lo00 pbar benzene increasing amounts of ethylbenzene were dosed within a pressure range of ca 10 pbar ... 10 mbar. The reverse case, the sorption uptake of benzene after presorption of ethylbenzene has also been investigated (counter-diffusion).
Sorption Kinetics of Volatile Mixtures
287
Single components or bmry mixtures of benzene and ethylbenzene were dosed into the sorption vessel using an absolute pressure measurement head of the Baratron type. After starting the QMS MID analysis, at time t = 0 s the electro-pneumatical valve was opened to allow for expansion of the substances into the sorption vessel. Their mass detection to obtain time-dependent concentration courses fll118 up to the equilibrium state between gaseous and sorption phases. For the system considered, the time required to reach equilibrium amounts to cu 20 minutes. Blank experiments were wried out before or after sorption experiments, using the same sorption unit but without a MS package. Thereat, series of loading steps were simulated for both compounds within the same pressure range as in sorption experiments. Their evaluation leads to an 1.n approximated v& characteristic shown in the small ..so window of figure 3. This i! ..a. v h e characteristic takes into account any time delay caused by the introduction -1.a of the adsorbate as well as -1.the adsorption already -1.a. taking place during this penod [lo, 111. These infor- a . a. mation have to be used for firrther treatment of primary -a.r pressure vs time curves in order to estimate rate conFig. 3. Measured @) and Fitted (-) Blank Curves and the Cmspondmg stants. In general the difference between time depenFitted Valve Characteristic dences of concentration in soption and in blank experiments gives the time dependence of sorbed amount, i.e. the uptake curve of either single component or mixture sorption. To evaluate uptake curves, the software package ZEUS was used. This method is based upon the solution of a non-linear Volterra integral equation system to transfer the variable boundary condition problem to such one for constant boundary condition. Inter alias, by this way it becomes also possible to account for the interaction between the intrinsic sorption kinetics with the dynamics of the apparatusincluding the effect of finite rate of sorbate supply through vahres and tubmgs of the apparatus. Thus, kinetic data of high accuracy can be provided. Appropriate methods to evaluate mixture sorption kinetics data by means of the Voltema integral equation approach have already been published [111. For reasons to be mentioned below, their application to the actual uptake curves of binary benzene-ethylbenzene mixtures became meaningless.
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I
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RESULTS AND DIS-
CUSSION
Single Component Adsorption Equilibria
o
Fig.4. Sorption Uptake Curves for the Benzene / Silicalite-I System at 323K
From data for the final stages of kinetic measurements, isothenm were obtained. From the benzene isotherms which obey the Imgmuir equation, the
288
J. Hille, M. Biilow and A. Micke
0 lPW
n. imn
0.me.m
values of both isosteric enthalpies of adsorption and Darken factor, dbn(p> d In (cy
were estimated. The isotherms for the benzene / silidte-I system corresa . 969s pond well to those which were published pmiously for both small- and largea. Sp.0 sized MS samples of the oipe MFI-lyp [7, 81. The adsorption enthalpy values scatter within the range 55 ... 60 k~ moP a conFig. 5. Example of Uptake Curve F m by ZEUS for Sorption of Ben- camtion resionover of o.oo5 zene on Silicalite-I at 323K (D = 6.5 m2 s-' ; Equilibrium .,. o.2 mmol g-l.For the Concentration: 0.027 mmol g-') ethylbenzene / silicalite-I 0.0840 0. nlpo
..
system, the adsorption heat amounts to cu 70 ... 80 kJ mol-' within a concentration region of 0.03 ... 0.4mmol 8'. These values approach literaaue &ta [ll]. The isotherms of this system were modelled by spline approximations. They also lit literature data [2]sufficiently well.
Sin& Comnent Adsorption Kinetics The Benzene / Silicalite-I System Typical examples of sorption kinetic curves for the benzene / silicalite-I system are shown in figure 4 as pressure vs sqrt(the) plots. For this set of uptake curves measured at 323K,the equilibrium concentration ranges fiom 0.01 mmol g-' (lower curve) to 0.5 mmol g-' (upper curve). Corresponding curve sets were obtained for both 343K and 363K. Results derived therefrom will be shown and discussed elsewhere [131. Assuming intracrystalIine diffusion as the rate-limiting process influenced by finite gas flow through valves and tubings of the apparatus, uptake curves were treated to yield diffusion coefficients. -10 The general behaviour of the kinetic system is exemplified in w e 5. The curves (1 -3)therein denote, respectively, the calculated pressure vs sqrt(time) -1. courses in the dosing vessel (l), in the valve (2) and in the sorption vessel (3)- all calculated by the model via using both the valve characteristic known and an -16 diffusion " appropriate coefficient. The symbols p Fig. 6. Comparison of Intracrystalhe Diffusion Coefficients Dofor Ben- indicate the experimental c m . Analogous imalysk zene on Silicalite-I (p) with Literature Data (+) [7]at 323K of all uptake curves with subsequent treatment of diffuson coefficienta obtained by the Darken equation gave. the dependence Do vs sorbate concentration shown in figure 6 @). The mtracrystake diffusion data derived agree well with those published in [7]for a silicalite-I sample with size dimensions 190 x 56 x 35 pm3
Sorption Kinetics of Volatile Mixtures
289
(plotted as +). One can conclude, therefore, that the apparatus works reliably in case of single gas adsorption
The Ethylbenzene/ Sibcabte-I System
Measurements and data evaluation analogous to those for benzene were I, perfomed for ethylbenzene in the above mentioned parameter regions. Although Fig. 7. Sorption Uptake Cutves for the Ethylbenzene / Silicalite-I System the primary pressure vs at 323K (Equilibrium Concentrations:0.004 ... 0.02 mmol g-') sqr(time) curves do not as e&tit irregularities visible from figure 7 for tnm experiments cariied out at 0.16. 323K difEcdties arose when the model fitting by 0.140 ZEUS was executed. The 0 1.9 is exemplified in situation m 1.0 figure 8 which shows D. M O insufiicient fitting quality 0.060 being weal of all uptake curves. In this particular case of "best fit", the only ..OW conclusion to be drawn is that the diffusion coeflicient Fig. 8. Example of Uptake Curve Fitting by ZEUS for the Ethylbenzene I may to 1.2 10-~4 Silicalite-I System at 323K (Equilibrium Concentration: 0.017 mmol 6'; upper curve pressure in the doser vessel; middle curve pressure in the m2 s-l* Although a valve and lower cutve pressure in the sorption vessel; dotted line value as well as smilar data derived from the curves of measured data in the sorption vessel) figwe 8 do agree surprim aim singly well with the asion data for a similar MFI system [2], we hesitate to confide in these data of ours due to insuf3icient quality of complex data fitting. The main reason for this beham.wum viour of ethylbenzene .m a 0 should be due to its strong .-a. adsorption on the inner 0 Dell0 stainless steel walls of the u apparatus. This additional mocess affects the eauilibria which cannot be modelled Fig. 9. Co-diffusion of Benzene-EthylbenzeneMixtures at Pressures by any two-paramettic iso0.7 ... 1.5 pbar (Benzene) and 0.05 ... 0.09 pbar (Ethyibenzme) therm equation with suffi-
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-
-
.
-
-
-
290
J. Hille, M. Billow and A. Micke
cient accuracy as well as the uptake kinetics. Furthermore, it cannot be excluded entirely that the system considered shows pecuiiarities described for both equilibria and kinetics of pxylene on gallosilicate of MFI- type [9, 141. Without having ZEUS at the disposal, the model inadequacy might not become visible. 7a
Adsorption Kinetics of Binary Mixtures Fig. 10. Co-diffusion of Benzene-Ethylbenzene Mixtures at Pressures 14 Since a quantitative ... 20 pbar (Benzene) and 0.3 ... 0.5 pbar (Ethylbenzene)
analysis of uptake curves for binary mixtures of ethylbermme and benzene which should have been done by means of the theoretical approach [ll] becomes impossible because of the specific behaviour of ethylbenzene, the efficiency of the new method will be shown qualitatively, only. Due to lack of space, Oa mixture equilibrium data are omitted In all cases given Fig. 11. Counter-diffusion of Benzene and Ethylbenzene after Presorp- below, the Plots Pressure Vs tion of Benzene (1 mbar) at Pressures 73 ... 94 pbar (Benzene) and 1.3 P a sqrt(time) dependences.
Co-difusion and Counter-diffusron
Figures 9 and 10 exemplify simultaneous sorption uptake of both components in different concentration ranges (Henry region and higher loading). Unhindered codiffusion is demonstrated. In the isotherm region with *a displacement of benzene by ethylbenzene, the latter Fig. 12. Counter-diffusion of Benzene and Ethylbenzene after Presorp- process also occurs in cortion of Benzene (1 mbar) at Pressures 94 ... 132 pbar (Benzene) and 5.7 msponding plots of mixture ... 21 pbar (Ethylbenzene) kinetic curves.
Sorption Kinetics of Volatile Mixtures
291
Examples of counter-diffusion c m s are given for presorption of either benzene or ethylbenzene. The finst case is visible fiom figures 11 and 12 for wious concentrations of ethylbenzene. After presorption of ca 1 mbar of benzene, increasing amounts of ethylbenzene within the pressure range of about 10 pbar to 10 mbar were dosed onto the MS. AAer a fast desorption peak at the beginning of the process maybe due to desorption Erom either the intercrystalline pore system or fiom apparatus walls slow benzene &sorption fiom the intracrystalline space with simultaneous adsorption of ethylbenzene occw. The reverse case, i.e. adsorption of benzene after presorption of ca 1 mbar of ethylbenzene, is illustrated by figures 13 and 0 szse 14. In the concentration e DLPS ranges considered, benzene is unable to replace ethylD.9178 benzene Erom the MS to an e 915. observable extend. The D 9 1 . 3 fiuther step in the development of the technique proposed will be the compari...om son of appropriate mixture kinetics results with those 0.m5 published in ref. [ 141.
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D.poop
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!J?
CONCLUSIONS
The work shows the adequacy between the experimental technique proposed and the problem of non-equilibrium sorption processes of mixtures on microporous sorbents. To verify experimental results, a complex analysis of information gained is needed, especially, to eliminate the external influences on intrinsic transport behaviour. 37 The method is highly sensitive with respect to time, Fig. 14. Counter-diffusion of Benzene and Ethylbenzene after Presorp- concentration and phase composition. For the bention of Ethylbenzene (1 mbar) at Pressures 12 ... 29 pbar (Benzene) and zene I silicalite-I system, 1.05 ... 1.07 pbar (Ethylbenzene) intracrystalline diffusion coefficients obtained correspond to literature data, quantitatively. In case of ethylbenzene sorbed by siticalite-I, wall adsorptionviolates the non-equilibrium data. The latter disadvantage can be avoided by appr-te change of the apparatus construction. For bmuy mixtures, both co-diffusion and counterdiffusion curves were obtained. Their shapes are typical of those for mixture kinetics. The adsorbability of ethylbenzene is stronger than that of benzene. Its diffusivity for both single component and mixture kinetics seems to be lower than that of benzene. Fig. 13. Counter-diffusion of Benzene and Ethylbenzene after Presorp-
REFERENCES [l] R.M. Maruutovsky and M. Biilow, Gas Sep. M.1 (1987) 66 H.G. Karge and W. NieBen, Stud. Suf Sci. Ca*is 65 (1991) 213 [2] K. Beschmann, G.T.Kokotailo and L. Rieckert, Chem. Eng. Process. 22 (1987) 223 [3] D. B. Shah,D.T. Hayhurst, G. Evanina and C.J. Guo, AIChE J. 34 (1988) 1713 [4]
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J. Hille. M. BUlow and A. Micke
S.D.pickett, A.K. Now& J.M. Thomas and A.K. Cheetham, Zeolites 11 (1989)123 D.M. Ruthven, M. Eic and E. Richard, Zeolites 11 (1991) 647 M. Billow, J. Caro, B. R6hl-Kuhn and B. Zibrowius, Stud. Surf Sci. Catalysis 46 (1989)505 A. Zikanova, M. Billow and H. Schlodder, Zeolites 7 (1987)11 5 A. Micke and M. Billow, Poster presented at this Conference P. S w e , M. Kocirik, M. Billow, A. Zikhova and A.G. Bezus, 2. Phys. Chem.[Leipzig] 264 (1983)49 A. Micke andM. Billow, Gas Sep. W.4 (1990)158;165 W.NieDen, Dissertation, Fritz-Haber-Jnstitut der MPG, Berhn, 1991 J. Hille, M. BOlow and A. Micke, in preparation for Zeolites P. Struve, A. Bergmann, A. Brenner,M. Biilow and K.K. Unger, Lecture presented at tbis Conference
A New Method for Investigation of Sorption Kinetics of Volatile Multi-Component Mixtures on Porous Solids
J m e n m e , Martin Billow and Andr6 Micke Centre of Heterogeneous Catalysis, KAI e.V., Rudower Chaussee 5, Berlin, D-1199,GERMANY
ABSTRACT
An experimental technique to follow sorption kinetics under constant volume-variable concentration conditions is described. The system consists of a sampling and a sorption vessel both separated by an electro-pneumatically driven valve with known charackaistics. Analyzing the gaseous phase above the sorbent by mass spectroscopic multiple ion detection, uptake curves of singte components and binary mixture sorption are registered. Valve effects, conductance of the whole system and wall adsorption must be c m t e d by blank experiments. Equilibrium and non-equilibrium data for benzene and ethylbenzene and for their binary mixtures on silicalite-I are reported.
INTRODUCTION A large number of technological processes which take place on solid sorbents proceed under
non-eqdbium conditions. Therefore,both theoretical work and development of reliable experimental methods which allow for the investigation of multi component sorption kinetics becomes highly important [l]. Fast pressure measurement tools, e.g. membrane manometers, were used to monitor Sorption kinetics of single components on molecular sieves (MS). To observe the sorption kinetics of multi component mixtures, partial pressure measurement methods have to be used. Applying mass spectrometersto the investigation of sorption kinetics, the indication of total and partial pressures, of residual gaseous components, of catalytic reactions and their kinetics, of leaks as well as the control of sorbent acthion become possible. Quadrupole mass spectrometers (QMS) allow for Multiple Ion Detection (MID) with very short measurement cycles, i.e. in the millisecond range on up to 16 channels. Recently, sorption kinetics of binary benzene-ethylbenzene mixtures on a MS of ZSM-5-type were investigakd by a new FTIR method [2]. For a possible comparison, thie work deals with a sitttitar sorption system. Single component sorption kinetics of aromatics on MF'I-type MS has been considered in numerous papers, e.g. [3 81.
-
EXPERIMENTAL
The principle realized works under constant volume-variable concentration conditions. The main feaiures of apparatus and problems inherent are as follows. The coupling of a sorption vessel to the QMS must be worked out carefblly. In order to reduce inlet pressures amounting to more than 10 mbar, a two-phase pressure reduction might be recommended. That should render mixed flow processes (latllinary gas transport and molecular flow, respectively, on the high pressure and the low pressure sides). But such pressure reduction would strongly affect the mass balance (up to 20 ml min-' for one sampling). To avoid this situation a one-phase pressure reduction was chosen. In the case considered, a needle valve is used to reduce the pressure over a range of ca five M ~ WofSmagnitudes.
286
J. Hille, M. Biilow and A. Micke
In case of high inlet pressures, the measured composition of mixture can be corrected via the molecular weights of the mixture components.
Apparatus The two-vessel inlet system constructed is situated on the QMS SX 300, VG ( m e 1). Both vessels are separated by a valve driven electro-pneumatically. The sorption vessel is connected to the ion so&e of the QMS by a needle valve. The sample vessel is equipped with a sample lock and the whole inlet system can be evacuated towards high vacuum by opening the evacuation valve. In both the sample and the sorption vessels, the total pressure can be measured by Baratron systems. Furthermore, pressures can be registered by the QMS. The sorbent sample, e.g. MS ..u u 3 . 3 1 1 u*= lms "I crystals, inside the sorption vessel (cf. Fig 1. Principal Scheme of the Apparatus figure 2) is packed into a mantle situated between a stainless steel sieve and a cylindric quartz tube. Inside the tube, heating PT 100 there is placed a heater helix controlled by a thermic resistance of the type Pt 100. To prevent concentration gradients within the sorption vessel and rateitainleos steel limiting extemal Nm transport, a second sorption Unit with a stirred MS package was constructed. It will be used in further experiments. The volumes of the individual parts of the system are given in figure 1. As visible from the scheme, opening or closing the v&e situated between the main vessels in the actual setup causes a volume change amounting to 7.1 ml. This disadvantage can either be met by modelling the Fig. 2. Scheme of the Sorption Vessel processes in the apparatus by the software ZEUS [9] or by appropriate exchange. A large-sizedsilicdte-I sample (crYs$l size: 150 x 20 x 20 p3 ; sample weight: 44.3 mg) was used in the experiments. For data evaluation, the crystal geometIy was approximated by a two dimensional parallelepiped with extensions amounting to 20 p n in both x and y directions. U b C
,
Experiments and Data Evaluation Sorption equilibria and adsorption rates of benzene and ethylbenzene were measured on a silicalite crystal package in a differential concentration mode. It was ensured that no intercrystalline transport effects iduence on uptake curves. The ranges of temperature and pressure were 323K ... 363K and 10 pbar ... 10 mbar, respechly, for both single components and binary mixtures. In the mixhue experiments, there was used stepwise dosing which followed the condition to keep the gaseous phase composition as near as possible to a molar ffaction of x(') * 0.5 (co-diffusion). Besides, after g presorption of lo00 pbar benzene increasing amounts of ethylbenzene were dosed within a pressure range of ca 10 pbar ... 10 mbar. The reverse case, the sorption uptake of benzene after presorption of ethylbenzene has also been investigated (counter-diffusion).
Sorption Kinetics of Volatile Mixtures
287
Single components or bmry mixtures of benzene and ethylbenzene were dosed into the sorption vessel using an absolute pressure measurement head of the Baratron type. After starting the QMS MID analysis, at time t = 0 s the electro-pneumatical valve was opened to allow for expansion of the substances into the sorption vessel. Their mass detection to obtain time-dependent concentration courses fll118 up to the equilibrium state between gaseous and sorption phases. For the system considered, the time required to reach equilibrium amounts to cu 20 minutes. Blank experiments were wried out before or after sorption experiments, using the same sorption unit but without a MS package. Thereat, series of loading steps were simulated for both compounds within the same pressure range as in sorption experiments. Their evaluation leads to an 1.n approximated v& characteristic shown in the small ..so window of figure 3. This i! ..a. v h e characteristic takes into account any time delay caused by the introduction -1.a of the adsorbate as well as -1.the adsorption already -1.a. taking place during this penod [lo, 111. These infor- a . a. mation have to be used for firrther treatment of primary -a.r pressure vs time curves in order to estimate rate conFig. 3. Measured @) and Fitted (-) Blank Curves and the Cmspondmg stants. In general the difference between time depenFitted Valve Characteristic dences of concentration in soption and in blank experiments gives the time dependence of sorbed amount, i.e. the uptake curve of either single component or mixture sorption. To evaluate uptake curves, the software package ZEUS was used. This method is based upon the solution of a non-linear Volterra integral equation system to transfer the variable boundary condition problem to such one for constant boundary condition. Inter alias, by this way it becomes also possible to account for the interaction between the intrinsic sorption kinetics with the dynamics of the apparatusincluding the effect of finite rate of sorbate supply through vahres and tubmgs of the apparatus. Thus, kinetic data of high accuracy can be provided. Appropriate methods to evaluate mixture sorption kinetics data by means of the Voltema integral equation approach have already been published [111. For reasons to be mentioned below, their application to the actual uptake curves of binary benzene-ethylbenzene mixtures became meaningless.
-
-
I
i--
RESULTS AND DIS-
CUSSION
Single Component Adsorption Equilibria
o
Fig.4. Sorption Uptake Curves for the Benzene / Silicalite-I System at 323K
From data for the final stages of kinetic measurements, isothenm were obtained. From the benzene isotherms which obey the Imgmuir equation, the
288
J. Hille, M. Biilow and A. Micke
0 lPW
n. imn
0.me.m
values of both isosteric enthalpies of adsorption and Darken factor, dbn(p> d In (cy
were estimated. The isotherms for the benzene / silidte-I system corresa . 969s pond well to those which were published pmiously for both small- and largea. Sp.0 sized MS samples of the oipe MFI-lyp [7, 81. The adsorption enthalpy values scatter within the range 55 ... 60 k~ moP a conFig. 5. Example of Uptake Curve F m by ZEUS for Sorption of Ben- camtion resionover of o.oo5 zene on Silicalite-I at 323K (D = 6.5 m2 s-' ; Equilibrium .,. o.2 mmol g-l.For the Concentration: 0.027 mmol g-') ethylbenzene / silicalite-I 0.0840 0. nlpo
..
system, the adsorption heat amounts to cu 70 ... 80 kJ mol-' within a concentration region of 0.03 ... 0.4mmol 8'. These values approach literaaue &ta [ll]. The isotherms of this system were modelled by spline approximations. They also lit literature data [2]sufficiently well.
Sin& Comnent Adsorption Kinetics The Benzene / Silicalite-I System Typical examples of sorption kinetic curves for the benzene / silicalite-I system are shown in figure 4 as pressure vs sqrt(the) plots. For this set of uptake curves measured at 323K,the equilibrium concentration ranges fiom 0.01 mmol g-' (lower curve) to 0.5 mmol g-' (upper curve). Corresponding curve sets were obtained for both 343K and 363K. Results derived therefrom will be shown and discussed elsewhere [131. Assuming intracrystalIine diffusion as the rate-limiting process influenced by finite gas flow through valves and tubings of the apparatus, uptake curves were treated to yield diffusion coefficients. -10 The general behaviour of the kinetic system is exemplified in w e 5. The curves (1 -3)therein denote, respectively, the calculated pressure vs sqrt(time) -1. courses in the dosing vessel (l), in the valve (2) and in the sorption vessel (3)- all calculated by the model via using both the valve characteristic known and an -16 diffusion " appropriate coefficient. The symbols p Fig. 6. Comparison of Intracrystalhe Diffusion Coefficients Dofor Ben- indicate the experimental c m . Analogous imalysk zene on Silicalite-I (p) with Literature Data (+) [7]at 323K of all uptake curves with subsequent treatment of diffuson coefficienta obtained by the Darken equation gave. the dependence Do vs sorbate concentration shown in figure 6 @). The mtracrystake diffusion data derived agree well with those published in [7]for a silicalite-I sample with size dimensions 190 x 56 x 35 pm3
Sorption Kinetics of Volatile Mixtures
289
(plotted as +). One can conclude, therefore, that the apparatus works reliably in case of single gas adsorption
The Ethylbenzene/ Sibcabte-I System
Measurements and data evaluation analogous to those for benzene were I, perfomed for ethylbenzene in the above mentioned parameter regions. Although Fig. 7. Sorption Uptake Cutves for the Ethylbenzene / Silicalite-I System the primary pressure vs at 323K (Equilibrium Concentrations:0.004 ... 0.02 mmol g-') sqr(time) curves do not as e&tit irregularities visible from figure 7 for tnm experiments cariied out at 0.16. 323K difEcdties arose when the model fitting by 0.140 ZEUS was executed. The 0 1.9 is exemplified in situation m 1.0 figure 8 which shows D. M O insufiicient fitting quality 0.060 being weal of all uptake curves. In this particular case of "best fit", the only ..OW conclusion to be drawn is that the diffusion coeflicient Fig. 8. Example of Uptake Curve Fitting by ZEUS for the Ethylbenzene I may to 1.2 10-~4 Silicalite-I System at 323K (Equilibrium Concentration: 0.017 mmol 6'; upper curve pressure in the doser vessel; middle curve pressure in the m2 s-l* Although a valve and lower cutve pressure in the sorption vessel; dotted line value as well as smilar data derived from the curves of measured data in the sorption vessel) figwe 8 do agree surprim aim singly well with the asion data for a similar MFI system [2], we hesitate to confide in these data of ours due to insuf3icient quality of complex data fitting. The main reason for this beham.wum viour of ethylbenzene .m a 0 should be due to its strong .-a. adsorption on the inner 0 Dell0 stainless steel walls of the u apparatus. This additional mocess affects the eauilibria which cannot be modelled Fig. 9. Co-diffusion of Benzene-EthylbenzeneMixtures at Pressures by any two-paramettic iso0.7 ... 1.5 pbar (Benzene) and 0.05 ... 0.09 pbar (Ethyibenzme) therm equation with suffi-
-
-
-
.
-
-
-
290
J. Hille, M. Billow and A. Micke
cient accuracy as well as the uptake kinetics. Furthermore, it cannot be excluded entirely that the system considered shows pecuiiarities described for both equilibria and kinetics of pxylene on gallosilicate of MFI- type [9, 141. Without having ZEUS at the disposal, the model inadequacy might not become visible. 7a
Adsorption Kinetics of Binary Mixtures Fig. 10. Co-diffusion of Benzene-Ethylbenzene Mixtures at Pressures 14 Since a quantitative ... 20 pbar (Benzene) and 0.3 ... 0.5 pbar (Ethylbenzene)
analysis of uptake curves for binary mixtures of ethylbermme and benzene which should have been done by means of the theoretical approach [ll] becomes impossible because of the specific behaviour of ethylbenzene, the efficiency of the new method will be shown qualitatively, only. Due to lack of space, Oa mixture equilibrium data are omitted In all cases given Fig. 11. Counter-diffusion of Benzene and Ethylbenzene after Presorp- below, the Plots Pressure Vs tion of Benzene (1 mbar) at Pressures 73 ... 94 pbar (Benzene) and 1.3 P a sqrt(time) dependences.
Co-difusion and Counter-diffusron
Figures 9 and 10 exemplify simultaneous sorption uptake of both components in different concentration ranges (Henry region and higher loading). Unhindered codiffusion is demonstrated. In the isotherm region with *a displacement of benzene by ethylbenzene, the latter Fig. 12. Counter-diffusion of Benzene and Ethylbenzene after Presorp- process also occurs in cortion of Benzene (1 mbar) at Pressures 94 ... 132 pbar (Benzene) and 5.7 msponding plots of mixture ... 21 pbar (Ethylbenzene) kinetic curves.
Sorption Kinetics of Volatile Mixtures
291
Examples of counter-diffusion c m s are given for presorption of either benzene or ethylbenzene. The finst case is visible fiom figures 11 and 12 for wious concentrations of ethylbenzene. After presorption of ca 1 mbar of benzene, increasing amounts of ethylbenzene within the pressure range of about 10 pbar to 10 mbar were dosed onto the MS. AAer a fast desorption peak at the beginning of the process maybe due to desorption Erom either the intercrystalline pore system or fiom apparatus walls slow benzene &sorption fiom the intracrystalline space with simultaneous adsorption of ethylbenzene occw. The reverse case, i.e. adsorption of benzene after presorption of ca 1 mbar of ethylbenzene, is illustrated by figures 13 and 0 szse 14. In the concentration e DLPS ranges considered, benzene is unable to replace ethylD.9178 benzene Erom the MS to an e 915. observable extend. The D 9 1 . 3 fiuther step in the development of the technique proposed will be the compari...om son of appropriate mixture kinetics results with those 0.m5 published in ref. [ 141.
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D.poop
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!J?
CONCLUSIONS
The work shows the adequacy between the experimental technique proposed and the problem of non-equilibrium sorption processes of mixtures on microporous sorbents. To verify experimental results, a complex analysis of information gained is needed, especially, to eliminate the external influences on intrinsic transport behaviour. 37 The method is highly sensitive with respect to time, Fig. 14. Counter-diffusion of Benzene and Ethylbenzene after Presorp- concentration and phase composition. For the bention of Ethylbenzene (1 mbar) at Pressures 12 ... 29 pbar (Benzene) and zene I silicalite-I system, 1.05 ... 1.07 pbar (Ethylbenzene) intracrystalline diffusion coefficients obtained correspond to literature data, quantitatively. In case of ethylbenzene sorbed by siticalite-I, wall adsorptionviolates the non-equilibrium data. The latter disadvantage can be avoided by appr-te change of the apparatus construction. For bmuy mixtures, both co-diffusion and counterdiffusion curves were obtained. Their shapes are typical of those for mixture kinetics. The adsorbability of ethylbenzene is stronger than that of benzene. Its diffusivity for both single component and mixture kinetics seems to be lower than that of benzene. Fig. 13. Counter-diffusion of Benzene and Ethylbenzene after Presorp-
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