2024
Ind. Eng. Chem. Res. 1990,29, 2024-2027
Lanthanum-NaY Zeolite Ion Exchange. 2. Kinetics Ting-Yueh Lee,+Tsan-Sheng Lu,+Shiann-Horng Chen,f and Kuei-Jung Chao*J Departments of Chemical Engineering and Chemistry, National Tsinghua University, Hsinchu, Taiwan, Republic of China
La-NaY ion exchange breakthrough curves were obtained experimentally at 27 and 60 "C. A mathematical model of a n ion exchanger was formulated and employed to calculate the ion exchange cm2/s was obtained. T h e effects of coefficients. An ionic diffusion coefficient of the order of zeolite particle size, temperature, and column packing conditions on the kinetics of the exchange were investigated also. Crystalline aluminosilicate zeolites are widely used as adsorbents, shape-selective catalysts, and cation exchangers due to their specific crystal structure and negatively charged tetrahedral framework. The rare-earth (RE) forms of zeolite Y obtained by exchange of synthetic NaY with RE ions are important catalysts in petroleum-refining processing (Dwyer and Dyer, 1984; Breck, 1974). However, little is published on the dynamics of RE-NaY ion exchange reactions. Normally La3+ions are the predominant cation species in the commercially available RE solution. Sherry (1976) and Moscou and Lakeman (1970) reported -that, because of its large size, a hydrated La3+ion cannot migrate from a supercage to a small sodalite cage to replace the residing Na+ ions, and the La-NaY ion exchange reaction terminates at an exchange level of 0.69 f 0.01 at 25 "C. Unless the temperature of exchange is raised to 100 "C or more, the La3+ions can only replace Na+ ions in the supercages (Corma and Wojciechowski, 1980; Nickashina et al., 1986). Moreover, the ion exchange of La3+-Na+ is expected to be reversible at low reaction temperatures. Hsu and Chen (1980) studied the La-NaY ion exchange reaction a t elevated temperatures and found that at 180 "C the exchange rate was controlled by the steps of stripping off the water molecules from the hydrated La3+ ion, which facilitates the La3+ions going into the sodalite cages and double hexagonal prisms. For the determination of the various dynamic properties of the ion exchange process, it is useful to simulate the process based on a mathematical model. Nickashina et al. (1986) reported the ion exchange dynamics of the recovery of strontium from natural freshwater and copper from seawater by natural clinoptilolite. In their study, the concentrations of strontium and copper ions were extremely low; therefore, linear ion exchange isotherms and "thin-layer" approximation methods were employed to obtain the kinetics of the ion exchange process. In this work, an ion exchange column was established and the exchange breakthrough curves of La-NaY were obtained experimentally. Moreover, a mathematical model of an ion exchanger was formulated. The ion exchange rate was calculated based on the assumption that the exchange reaction was relatively fast and the ion migration in the zeolitic pores, mainly in the supercages, played a dominant role in the process. This study was limited in the lowtemperature range, in which the ion exchange was reversible and the cation sites in the supercage were assumed to be uniform and homogeneous. It is analogous to the monolayer adsorption phenomenon on a solid surface. On the basis of this similarity, the formality of a modified Langmuir isotherm and adsorption column dynamics (Chen et al., 1990) were employed to simulate and deter-
* To whom the correspondence should be addressed 'Department of Chemical Engineering. *Department of Chemistry.
mine the kinetics of the ion exchanger and the ion exchange coefficients.
Experimental Section The 0.16-cm NaY zeolite pellet with about 20% binder was purchased from Strem Chemical Company. The NaY pellets were subsequently screened and crushed and screened again and collected in three average diameter sizes: 0.0346, 0.0216, and 0.0160 cm. The zeolite was washed with 1 N NaCl solution and with deionized water. The washed samples were dried at room temperature and stored for at least 2 days in a desiccator containing saturated NH,Cl solution. The reagent-grade LaC13 and NaCl were purchased from E. Merck. Deionized water was used to prepare a pure 0.1 N LaC1, solution. Initially, a constant-volume batch reactor with stirring was found to be employed for the ion exchange kinetic study. The exchange was essentially completed within a few minutes. In this study, a Pyrex column of inner diameter of 0.95 cm and length of 45 cm, packed tightly with the zeolite particles, was employed for the transient ion exchange study at 27 and 60 "C. Before the run, the column was purged with deionized water, and at time zero the LaCl, solution was fed into the column at a constant flow rate of 11.5 cm3/min. Samples of the effluent were taken at certain time intervals, and the concentrations of La3+ ion were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) (Chen, 1987). The schematic diagram of the experimental setup is shown in Figure 1,where the container of the LaCl, solution was kept in a constant-temperature bath; all the delivery line was heat traced to maintain the desired temperature, the Pyrex column was wrapped with heating tape and connected to a temperature controller, and the column was well insulated. Ion Exchanger Mathematical Model A complete ion mass balance in the column is formulated that takes into account the ions inside the zeolite and the fluid phase of the column. There are several assumptions involved in the model: (1) Cation sites in the zeolite are uniform. (2) Ion diffusion in the zeolite cages is the rate-controlling step; hence, resistance in the macropore of the particle is neglected. (3) Particles are uniform and spherical. (4)Fluid flow in the column is plug dispersion flow. ( 5 ) A film resistance exists in the interface between the solid and the fluid. (6) The ion migration inside the zeolitic pores can be characterized by Fick's law. ( 7 ) Ionic equilibrium between the zeolite surface and the solution is attained instantaneously and can be expressed by a form of the modified Langmuir isotherm (Chen et al., 1990).
0888-5885/90/2629-2024$02.50/0 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2025 Thermal couple
---A---
I
1
i
D,=0.0346
cm
I
Lac13 b I N )
!
;;1 TemDerature controller
1 c
I 0
u
' 0.5. U
-- --1-
--
z -_
constant temperature bath
I .' 'i
Figure 1. Schematic representation of a laboratory-scale flow-type
0.oi
unit for continuous-flow exchanging experiments.
0
(8) Interference of countercurrent ionic flow is neglected. The ionic mass balance in the zeolite is
30
60
90 TIME (MIN!
I
' 20
150
180
Figure 2. Experimental
(w) and calculated (-) breakthrough curve of La-NaY ion exchange using NaY particles of 0.0346-cm diameter at 27 "C.
=...-
with initial condition t=0, and boundary conditions
qi=o
T=27'G
(la)
0,=0.0216
cm
0
u
' 0.5u
r = R,,
aqi
Di ar = kfi(Ci - Cis)
(IC)
The ionic mass balance in the solution phase of the column is U.U
1 'I
0
30
80
Dl
120
150
180
TIME ( M I N I
with initial condition t=0, and boundary conditions
z=L,
Figure 3. Experimental (m)and calculated (-) breakthrough curve
ci=o
(24
aci
-az= o
The ionic equilibrium isotherm is
where the ionic dispersion coefficient Dgi is estimated (Bischoff, 1961). The k, coefficient is calculated from the empirical correlation (Wilson and Geankoplis, 1966) 2kfiRP - -Re0.33SC0.33 1.09 -
Dim where
, for 0.0015 < Re < 55
CB
SC = p/PfDim
Re = ~R,P+B/P and Dim,the ionic diffusion coefficient in the solution, is estimated by the Stokes-Einstein equation (Daniels and Alberty, 1955), which gives a value of 3.66 X lo* cm2/s at 27 "C. The La-Nay ion exchange equilibrium isotherms are treated as in our previous paper (Chen et al., 1990). The system of partial differential equations, (1)-(3), is nonlinear, of which there is no analytical solution. Con-
of La-NaY ion exchange using NaY particles of 0.0216-cm diameter at 27 "C.
sequently, orthogonal collocation method was used to solve this system of equations numerically. The modified method of Lee and Tan (1987) was adopted. Because in that report artificial values of the parameters were used for the adsorber simulation, divergence and oscillation instabilities were encountered, once the real values of the ion exchange kinetics were introduced in the program. Fortunately, difficulties were ironed out and calculations were speeded up. The computation time on a CDC Cyber 840 for a run with the number of internal collocation points in the adsorber NB = 4 and the number of particle internal collocation points NP = 2 was 70 s. Correspondingly, for NB = 4, NP = 2 and NB = 5, NP = 3 runs, computer times of 70 and 680 s were needed, respectively. The calculation results of NB = 4, NP = 2 and NB = 5, NP = 3 were extremely close.
Results and Discussion The experimental La-NaY ion exchange breakthrough data on zeolite particles of 0.0346-, 0.0216-, and 0.0160-cm diameters a t 27 "C are shown as the square points in Figures 2-4, respectively. The results a t 60 "C on 0.0346-cm-diameter particles are shown in Figure 5. The solid lines in these figures are obtained from the mathematical model by choosing the best ion diffusion coefficient in the zeolite (DJ.I t should be noted that there are no other adjustable parameters in the model. The best Di was selected by the least-squares method to give the smallest
2026 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 I .o
T=27'C 0,-0.0160
cm
I
I
m
I
0.01
0
I . 3J. :
TIME (MINI
Figure 4. Experimental (D) and calculated (-1 breakthrough curve of La-NaY ion exchange using NaY particles of 0.0160-cm diameter at 27 "C.
i
T=60°C 0,=0.0346
0.0'
,
0
cm
60
90
120
153
i
,
80 TIME ( M I N )
120
150
I
180
Figure 6. Calculated breakthrough curves of La-NaY ion exchange with a fixed u value of 0.608 cm/s: (1)tB = 0.2, L = 31.2 cm, u = 0.122 cm/s; (2) tg = 0.3, L = 35.7 cm, u = 0.182 cm/s; (3) tg = 0.4, L = 41.6 cm, u = 0.243 cm/s; (4) tg = 0.5, L = 50.0 cm, u = 0.304 cm/s.
,
1
5
30
Bo
1
180
TIME ( M I N )
Figure 5. Experimental (D) and calculated (-1 breakthrough curve of La-NaY ion exchange using NaY particles of 0.0346-cm diameter at 86 "C. Table I. Parameters Used for Calculation of Theoretical Breakthrough Curves 27 27 27 60 T, "C L, cm 45.0 44.0 43.5 45.0 0.425 0.445 0.445 0.432 CB 0.0346 0.0216 0.0160 0.0346 DP,cm 9 5 mol/g 2.0 x 10-4 2.0 x 10-4 2.0 x 10-4 2.2 x 10-4 5.6 X lo4 5.6 X lo4 5.6 X lo4 9!, mol/g 5.6 X lo4 Kb, L/mol 3.9 x 105 3.9 x 105 3.9 x 105 6.0 x 105 Kh,L/mol 3.2 x 103 3.2 x 103 3.2 x 103 6.2 x 103 6.76 x 10-3 9.57 x 10-3 1.19 x 10-2 6.78 x 10-3 kfi, cm/s 3.39 x 10-2 2.21 x 10-2 D,, cm2/s 2.63 X 3.38 X 1.3 X 1.5 X lo* 3.0 X D,, cm2/s 1.5 X
sum of squares of the differences between the model and the experimental data. The experimental conditions and the parameters used in the model are listed in Table I. The 20% binder effect has been considered in the formulation and reflected in the physical properties of porosity, density, etc. The agreement between the experimental data and the model calculation is fairly good as shown in Figures 2-5. It is worthwhile to note that the ion migration rate does not change significantly by increasing the particle size from 0.0160 cm (Di = 1.5 X cm2/s) to 0.0346 cm (Di = 1.3 X cm2/s). This indicates the validity of the assumption that the migration of cations in the macropores is not the controlling step of the ion exchange process. The rate of ion exchange was found to increase with the tempera-
60
90
120
I
5c
'80
(WA:".i)
Figure 7. Calculated breakthrough curves of La-NaY ion exchange with a fixed u value of 0.122 cm/s: (1)tB = 0.5, L = 50.0 cm, u = 0.243 cm/s; (2) eg = 0.4, L = 41.6 cm, u = 0.304 cm/s; (3) tg = 0.3, L = 35.7 cm, u = 0.405 cm/s; (4) tg = 0.2, L = 31.2 cm, L' = 0.608 cm/s.
ture. An Arrhenius activation energy of 13.6 kJ/mol was obtained. This rather low activation energy is probably another justification that the ion exchange process is controlled by ion mass transport instead of by reaction. It was important to investigate the packing effect on the ion exchange. With this in mind, four simulated runs with different compact tightness were carried out. The corresponding void fraction tg and the length of the column L are given in Figure 6. It is conceivable that the time of breakthrough is much shorter on the loosely packed column even though the column is actually longer. In this respect, the fluid linear velocity u in the column can be kept constant, and the correspondingly superficial velocity u = tBv is proportional to tB. With a fixed amount of packing adsorbent in the column, the smaller the tB, the longer the breakthrough time observed. Conversely, if the superficial velocity u in the column was maintained constant, the fluid linear velocity u became inversely proportional to eg; consequently, the looser the packing, the longer the column length and the breakthrough time, as shown in Figure I. Other variables or process parameters such as the solution flow rate, length and diameter of the column, etc., can also be simulated by this model. Therefore, the model can be employed for process simulation, control optimi-
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2027 zation, and parametric variables studies, but unlike adsorption, the ion exchange reaction is endothermic instead of exothermic.
Conclusion Experimental results of breakthrough curves of La-NaY ion exchange at 27 and 60 “C on various particle sizes were obtained. A theoretical model for ion exchange kinetics has been formulated and used to obtain the ion exchange coefficient. The simulation curves based on the model are in good agreement with experimental measurements; therefore, they can be applied for design, simulation, control, optimization, and parametric variable studies in the ion exchange process.
Acknowledgment T.Y.L. thanks his colleague Professor C. S. Tan for his advice on the numerical solution of the breakthrough curve.
Nomenclature Ci = ionic concentration, mol/L Cif = ionic concentration at inlet, mol/L Cis = ionic concentration at zeolite surface, mol/L Dgi = ionic axial dispersion coefficient, cm2/s Di = ionic diffusion coefficient in zeolite, cm2/s Dim= ionic diffusion coefficient in solution, cm2/s k, = ionic mass-transfer coefficient, cm/s K = ion exchange isotherm constant, L/mol L = length of ion exchange column, cm NB = number of internal collocation points in bed NP = number of internal collocation points in particle q. - ionic concentration in exchanged phase, mol/g qt]-= ion exchange capacity on SU sites of zeolite, mol/g q! = ion exchange capacity on SI1 sites of zeolite, mol/g r = radial distance from the center of the particle, cm Re = Reynolds number, Re = 2Rpp$cB/1 R, = particle radius, cm Sc = Schmidt number, Sc = p/pfDim Sh = Sherwood number, Sh = 2kfiRp/Dim t = time, s u = linear interstitial velocity, cm/s t = distance measured from bed inlet, cm
Greek Symbols tg = voidage of ion exchange column p = fluid viscosity pf = fluid density Registry No. La, 7439-91-0.
Literature Cited Bischoff, K. B. Backmixing in the Design of Chemical Reactors. Ph.D. Thesis, Illinois Institute of Technology, Chicago, 1961. Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; Chapters 2 and 7. Chen, S. H. The Study of La-NaY Ion Exchange. M.S. Thesis, National Tsinghua University, Taiwan, ROC, 1987. Chen, S. H.; Chao, K. J.; Lee, T. Y. Lanthanum-NaY Zeolite Ion Exchange. 1. Thermodynamics and Thermochemistry. Znd. Eng. Chem. Res. 1990, preceding paper in this issue. Corma, A.; Wojciechowski, B. W. The Nature of the Active Sites in the Reactions of Cumene on HY and LaY Catalysts. Can. J . Chem. Eng. 1980,58,620-625. Daniels, F.; Alberty, R. A. Physical Chemistry; Wiley: New York, 1955; p 650. Dwyer, J.; Dyer, A. Zeolites for Industry. Chem. Ind. 1984, 2, 237-269. Hsu, R. R.; Chen, Y. G. The Mechanism of the La-NaY Ion-Exchange Reaction a t Elevated Temperatures. In 5th Znt’l Zeolite Conf.;Rees, L. V. C., Ed.; Heyden & Son: Naples, FL, 1980; pp 321-326. Jacobs, P. A. Carboniogenic Activity of Zeolites; Elsevier: New York, 1977; Chapter 1. Lee, T. Y.; Tan, C. S. Simulation of a Fixed-Bed Adsorber Consisting of Bidisperse Pellets. J. Chinese Znst. Chem. Eng. 1987, 18 (3), 165-171. Moscou, L.; Lakeman, M. Acid Sites in Rare-Earth Exchanged YZeolites. J. Catul. 1970, 16, 173-180. Nickashina, V. A.; Senyavia, M. M; Mironova, L. I.; Tyurina, V. A. Modelling and Calculating Ion-Exchange Processes of Metal Sorption by Natural Clinoptilolite. In 7th Znt’l Zeolite Conf.; Murakani, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Tokyo, 1986; pp 283-288. Sherry, H. S. Irreversibility in Rare Earth Ion-Exchange of the Synthetic Zeolites X and Y. Colloid Interface Sci. (Proc. 50th Int’l Conf.) 1976, 5, 321-332. Wilson, E. J.; Geankoplis, C. J. Liquid Mass Transfer a t Very Low Reynolds Numbers in Packed Beds. Znd. Eng. Chem. Fundam. 1966, 5, 9-14.
Received for review July 28, 1989 Revised manuscript received March 19, 1990 Accepted April 11, 1990
