Adsorption Equilibrium of Ethylene-Carbon Dioxide Mixture on Zeolite ZSM5

Adsorption Equilibrium of Ethylene-Carbon Dioxide Mixture on Zeolite ZSM5 Jin-Gu Wang and Yang-chun Chang Department of Chemical Engineering, Dalian Institute of Technology, Dalian, 116024, People's Republic of China Yi Hua Ma, Hai-qing Li* and T.D. Tang Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, USA Binary gas mixture data for the ethylene-carbon dioxide system were obtained for cation exchanged forms of ZSMS (Li+, Na+, K+, Rb+, Mg+2, Ca+2, Sr+ 2 and Ba+2)for the gas phase C02 mole fraction of 0.766 at 308K and 101.3KPa. The experimental adsorption phase diagrams were obtained for C02-C2H4 on NaZSMS and MgZSMS. Single component adsorption isotherms for C02 and C2H4 were also obtained for these two zeolites. The single component data were used to obtain parameters derived in the vacancy solution model and the statistical thermodynamic model. These parameters were, in turn, used to predict binary mixture isotherms for these two zeolites. The agreement between experimental data and predicted values is generally good. INTRODUCTION Much work has been done to study the rate of sorption of gases in molecular sieves due to their potential applications in sorption processes and in catalysis. However, a large portion of the work is done to study the sorption of single pure gases in zeoli tic materials while multi-component sorption is always involved in practical applications. Earlier work on adsorption of gaseous mixtures on solids has been summarized by Brunaur [1] and Barrer [2,3]. The two commonly used techniques for the determination of equilibrium adsorption isotherms for gaseous mixtures are volumetric and chromatographic techniques. The chromatographic techniques were employed by van der Vlist and van der Meijden [4] to study the adsorption of N2-02 mixtures on SA, by Shah and Ruthven [5] to study the diffusion of CH4-N2 mixtures in 4A, by Danner et al [6] to study the adsorption of ethane-ethylene on 13X, by Rolniak and Kobayashi [7] to study the adsorption of CH4-C02 mixtures on 13X and SA at elevated pressures, and by Ruthven and Kumar [8] to study the adsorption equilibria of several gaseous mixtures on 4A and SA. A flow system was used by Danner and Wenzel [9] to study the adsorption of CO-02 and 02-N2 mixtures on SA and lOX. A volumetric method was employed by Dorfman and Danner [10] to measure the adsorption equilibria of N2-02-CO mixtures on lOX and by Danner and Choi [11] to determine the mixture adsorption equilibria of C2H2 and C2H4 on 13X, by BUlow et a1 [12] to study CH4-Kr mixture adsorption on CaA, by Hyun and Danner [13] to study the adsorption of i-C4H10-C2H4, i-C4H10-C2H6 and C2H4-C02 on 13X and by Sorial et al [14] for 02-N2 mixtures on SA pellets. A volumetric flow system was employed by Vansant and Voets [15] to determine the equilibrium adsorption isotherms of CO-C02' C02-N2 and C02-CH4 on mordenite, by Nolan et al [16] to study the adsorption of 02-N2, N2-CO, and 02-CO on lOX.

*

On leave from South China Institute of Technology, Canton, People's Republic of China

555

556 (AD-6-3) The present study deals with the equilibrium adsorption of ethylene and carbon dioxide mixtures on cation exchanged forms of ZSM5 (Li+, Na+, K+, RB+, Mg+2, Ca+2, Sr+ 2 and Ba+2) to investigate the effect of cations on the adsorption of binary gas mixtures. Moreover,single component equilibrium isotherms for carbon dioxide and ethylene were determined for NaZSMS and MgZSMS and used to predict the mixture iGotherms. EXPERIMENTAL The equilibrium adsorption measurements for the single components, C02 and C2H4' on NaZSMS and MgZSMS were done by the gravimetric method in a constant volume, constant pressure system. The equipment and experimental procedure are identical to those previously reported [15]. It should be pointed out that a specially designed sample pan equipped with a thermocouple was used to simultaneously monitor the temperature and weight change during sorption. The sample temperature, system pressure and sample weight were continuously monitored during both activation and adsorption runs. The activation temperature was 823K at a vacuum of less than 5 ~m Hg. Detai~ed description of the equipment and procedure can be found in Wu et al [17] . A flow system shown schematically in Figure 1 was employed to measure the adsorption equilibrium of ethylene-carbon dioxide mixtures at 1 atm and 308K. Mixtures of ethylene-carbon dioxide of known concentrations were fed to the bed packed with zeolite powder and the effluent concentration was automatically recorded. Adsorption equilibrium was achieved when the bed effluent concentration was the same as that of the feed. The amount adsorbed and its composition were determined by desorption at 623K purged with N2 by the integration of the monitored effluent composition with an on-line computer. Further, the total amount adsorbed was checked by weighing the bed prior to desorption. The agreement between the weight increases and the results from integration of the effluent composition data was generally good. 2

4

3

7

Figure 1.

Schematic diagram for the flow system for the measurement of mixture isotherm. 1. gas cylinder 2. pressure regulator 3. needle valve 4. flow meter 5. adsorption column 6. sampling valve 7. constant temperature bath 8. furnace 9. sample loop

The purity of the gases used was better than 99.9%.The zeolite samples were synthesized according to the procedure reported by Argauer and Landolt [18] and Chang et al [19] with Si02/A1203 ratio equal to 79.9. Ion exchanges were performed by contacting the samples with respective nitrates for an extended period of time. Calcination was done at 623K for 6 hours.

J.-G·. Wang et a1.

557

RESULTS AND DISCUSSION Equilibrium adsorption data for the binary gas mixtures of ethylene-carbon dioxide were obtained for cation exchanged forms of ZSM5 (Li+, Na+, K+, Rb+, Mg+2, Ca+2, Sr+ 2 and Ba+2) for the gas phase C02 mole fraction of 0.766 at 308 K and 1 atm. The effects of cations on adsorption is shown in Figure 2 for the univalent cations Li+, Na+, K+ and Rb+ at the vapor ·composition of Yl= 0.766 and Y2=0.234, where subscripts 1 and 2 indicate C02 and ethylene respectively.

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The effect of cation on adsorption, T=308K p 101.3 KPa and Yl = 0.766

The slight increase in adsorption capacity with an increase in ionic radius is due, in part, to the higher polarizability associated with larger cations. As the pores of ZSM5 are readily accessible to the ethylene and carbon dioxide molecules due to their relatively small sizes, one would probably expect that interaction between the cations and the molecules possessing olefinic-bond and quadrupole would be more important than the relative size between the adsorbing molecules and the cations. A similar trend was also observed for the ZSM5 samples with exchanged divalent cations, Mg+2, Ca+2, Sr+ 2 and Ba+2, as shown Ln Table 1.

Table 1.

Adsorbent

Mg Ca Sr Ba

ZSM5 ZSM5 ZSM5 ZSM5

Adsorption Equilibrium on Di-valent Cation Exchanged ZSM5 at T=308K, Pc101.3KPa Yl=0.784 Y2=0.216

Total adsorption Component adsorption Capacity mmol g-l Capacity mmo1 g-l

1.486 1.478 1.479 1.631

Mole Fraction

CO2

C2H4

Xl

X2

1.063 1.079 1.091 1.146

0.403 0.399 0.389 0.484

0.725 0.730 0.738 0.703

0.275 0.270 0.262 0.297

Separation Factor Ct

1. 38 1.33 1. 30 1.54

558 (AD-6-3) The adsorption capacities of COZ-CZH4 mixtures on NaZSM5 at 308K and 1 atm are shown in Figure 3. Similar results for MgZSM5 are shown in Table Z along with calculated separation factor which is defined as

where Y and X are the mole fractions in the vapor and adsorbed phases, respectively. It can be seen that the separation factor is relatively constant indicating

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Adsorption capacity of COZ-CZH4 on NaZSM5 at T = 308K, P = 101. 3KPa

the ideality of the system. A similar result is obtained for NaZSM5 with an average a value equal to 1.67. The X-Y diagrams for the adsorption of ethylene-carbon dioxide on NaZSM5 and MgZSM5 is shown in Figures 4 and 5 respectively. Table Z. Adsorption. Equilibrium of COZ - CZH4 on MgZSM5 at T = 308K and P = lOl,3 KPa Total Adsorption Capacity mmol g-l

Component Adsorption Capacity mmol g-l

COZ 1. 501 1.468 1.388 1.534 1.588 1.711

1. 333 1.065 0.845 0.674 0.450 0.109

CZ H4 0.l68 0.403 0.543 0.860 1.138 1.60Z

Mole Fraction Separation Adsorbed Phase Gas Phase Factor Xl Xz Y1 YZ COZ a CZ H4 COZ CZ H4 0.888 0.725 0.609 0.439 0.Z83 0.064

0.11Z 0.Z75 0.391 0.561 0.717 0.936

0.914 0.785 0.678 0.509 0.3Z9 0.106

0.086 0.Z15 O.32Z 0.491 0.671 0.894

1.34 1.38 1.35 1.33 1.Z4 1. 73

It is generally recognized that the collection of mixture adsorption data is tedious and requires substantial efforts. The prediction of mixture equilibrium isotherms from single component adsorption isotherm data is normally preferred. However, the application of various models for the prediction of mixture equilib-

J.-G. Wang et al.

559

rium isotherms is not always successful. In order to test the applicability of the vacancy solution model [20] and the statistical thermodynamic model [21] to the present system, pure component equilibrium adsorption isotherms were determined for C02 and C2H4 on NaZSM5 and MgZSM5. The vacancy solution model was developed by Suwanayuen and Danner [20] based on treating the adsorption equilibrium as an osmotic equilibrium between two vacancy solutions having different compositions. Ruthven [21] extended his statistical thermodynamic model for single component to the prediction of binary adsorption equilibria. The model is based on the assumption that the adsorbate molecule is confined to a particular cavity but not adsorbed at specific localized sites within the cavity. Except for the limiting value nS'oo in the vacancy solution model, which was determined from the experimental saturation capacity, all other parameters in both models were obtained by fitting the single component adsorption data using the quasi-Newton optimization algorithm in the International Mathematical and Statistical Library (IMSL).

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Adsorption phase diagram of C02-C2H4 on NaZSM5 at T = 308K and P = 101.3KPa

The measured and calculated adsorption isotherms are shown in Figure 5 and 7 and the parameters determined from the experimental data are summarized in Table 3. It appears that both models fit the single component data quite well. The parameters determined from the single component system were used to predict the binary adsorption equilibrium for the C02-C2H4 mixtures. The computed curves are shown in Figures 4 and 5. Again it appears that both models predict the binary equilibrium adsorption for both ion-exchanged zeolites quite well although the agreement between the experimental data and the statistical thermodynamic model is somewhat better for MgZSM5. Inclusion of the adsorbate-adsorbate interactions in the vacancy solution model may improve the agreement between the experimental data and the model.

560 (AD-6-3) Table 3.

Parameters for Vacancy Solution Model (VSM) and Statistical Thermodynamic Model (STM) NaZSM5

Model

Parameters

0.586xI0- 4 1.947 0.164xI0- 1 0.897xI0- 2 0.275

bl 11 13 11 3 1 K B/v

USM STM bl: lI i j • K: B/v:

~lgZSM5

C2H4

C02

C02

0.133xlO- 3 2.189 0.836xlO- 2 0.173xI0- 1 0.288

C2H4

0.860xI0- 4 0.141xlO- 4 l. 875 2.372 0.732xlO- 1 0.505xlO- 1 0.288xlO- 1 0.401xlO- 1 0.236 0.261

Henry's law constant in VSM (kmol/kg.KPa) IIj i : Wilson's parameters for interaction between i and j in VSM Henry's law constant in STM expressed in molecules/cavity torr reciprocal of the number of molecules per cavity

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Adsorption phase diagram of COZ-CZH4on MgZSM5 at T = 308K and P = 101.3KPa

CONCLUSION Binary gas mixture data for the ethylene-carbon dioxide system were obtained for the cation-exchanged forms of ZSM5 at a gas phase C02 mole fraction of 0.766. The total adsorption capacity increases slightly with an increase in ionic radius due, in part, to the higher polarizability associated with larger ionic radius. Both the vacancy solution model and the statistical thermodynamic model give satisfactory predictions of adsorption phase diagrams for the binary mixture of COZ and CZH4 on NaZSM5 and MgZSM5. The system appears to show ideal behavior with a relatively constant separation factor.

J.-G. Wang et al.

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Single component equilibrium adsorption isotherm for NaZSM5 at T = 308K

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561

562 (AD-6-3) ACKNOWLEDGEMENT The assistance provided by Chun-hua Li. Yu-kuo Sun, Feng-zhen Chen and Shu-hua Gao of the Dalian Institute of Technology is gratefully acknowledged. REFERENCES 1. S. Brunaur. The Adsorption of Gases and Vapors, 1 (1945). 2. R.M. Barrer and A.B. Robins. Trans. Faraday Soc.~ 49. 804 (1953). 3. Ibid .• 929 (1953). 4. S. van der Vlist and J. van der Meijden, J. Chromatography, 79, 1 (1973). 5. D.B. Shah and D.M. Ruthven. AIChE J .• 23, 804 (1977). 6. R.P. Danner, M.P. Nicoletti and R.S. AI-Arneeni, Chern. Eng. Sci., 35. 2129 (1980) • 7. P.D. Rolniak and R. Kobayashi. AIChE J .• 24. 616 (1980). 8. D.M. Ruthven and R. Kumar. Can. J. Chern. Eng.• 57, 342 (1979). 9. R.P. Danner and L.A. Wenzel. AIChE J •• 15. 515 (1969). 10. L.R. Dorfman and R.P. Danner. AIChE Syrn~ Series. 71. No. 152, 30 (1975). 11. R.P. Danner and E.C.F. Choi. I & EC Fund .• 17. 248-c1978). 12. M. BUlow. H.J. Wappler. M. Jaroniec and J. Piotrowska. J. ColI. Interface ser., 85, 457 (1982). 13. S:H. Hyun and R.P. Danner. J. Chern. & Eng. Data. ~. 196 (1982). 14. G.A. Sorial, W.H. Granville and W.O. Daly. Chern. Eng. Sci •• 38. 1517 (1983). 15. E.F. Vansant and R. Voets. J. Chern. Soc .• Faraday Trans. I.,-r7. 1371 (1981). 16. J.T. Nolan. T.W. McKeehan and R.P. Danner. J. Chern. & Eng. Data. ~. 112 (1981). 17. P.D. Wu. A. Debebe and Y.H. Ma. ZEOLITES. 3. 118 (1983). 18. R.J. Argauer and G.R. Landolt, U.S. Patent 3.702.886. 19. C.D. Chang and A.J. Silvestri. J. Cat •• 47. 249 (1977). 20. S. Suwanayuen and R.P. Danner. AIChE J •• -Z6. 76 (1980). 21. D.M. Ruthven. AIChE J., 22. 753 (1976). --