Ind. Eng. Chem. Res. 1987, 26, 1407-1412 Engel, T.; Ertl, G. Adu. Catal. 1979,28, 1. Espinoza, R. L.; Mandersloot, W. G. B. J. Mol. Catal. 1984,24, 127. Frost, A. A.; Pearson, R. G. Kinetics and Mechanism; Wiley: New York, 1953. Gates, B. C.; Katzer, J. R.; Schuit, G . C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. Gault, F. G. Adu. Catal. 1981, 30, 1. Grasselli, R. K.; Burrington, J. D. Adu. Catal. 1981, 30, 133. Kaeding, W. W.; Butter, S. A. J . Catal. 1980, 61, 155. Laidler, K. J. Chemical Kinetics; McGraw Hill: New York, 1965. Machiels, C. J.; Anderson, R. B. J. Catal. 1979, 58, 268. Madix, R. J. Adu. Catal. 1980, 29, 1. Paal, Z. Ado. Catal. 1980, 29, 273. Ponec, V. Adu. Catal. 1983,23, 149. Rudd, D. F.; Fathi-Afshar, S.; Trevino, A. A.; Stradtherr, M. A. Petrochemical Technology Assessment; Wiley: New York, 1981.
1407
Santacessaria, E.; Gelosa, D.; Cara, S.; Adami, I. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 68. Shacham, M. Comp. Chem. Eng. 1985,9, 2. Sinfelt, T. Adu. Catal. 1973, 23, 92. Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornel1 University Press: Ithaca, NY, 1981. Temkin, M. I. Adu. Catal. 1979, 28, 173. van den Berg, J. P.; Wolthuizen, J. P.; Van Hooff, J. H. C. Proceedings of the 5th International Conference on Zeolites, Naples; Rees, L. V. Ed.; Heyden: London, 1980; p 649. van Schaik, J. R. H.; Dressing, R. P.; Ponec, V. J . Catal. 1975, 38, 273. Venuto, P. B.; Landis, P. S. Adu. Catal. 1968, 18, 259. Received for review July 3, 1986 Accepted April 13, 1987
A Comparative Study of Zeolite and Resin Adsorbents for the Separation of Fructose-Glucose Mixtures Cecilia Ho, Chi Bun Ching, and Douglas M. Ruthven* Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 0511
T h e kinetics and equilibria of sorption of fructose and glucose on Ca2+ion-exchange resins and Ca2+-exchangedzeolite adsorbents have been studied experimentally by pulse and step chromatographic methods. The best of the resin adsorbents shows a higher equilibrium separation factor than Cay zeolite, but this advantage is largely offset by the greater resistance to mass transfer. Under practical operating conditions in a simulated countercurrent chromatographic separation unit, the resin and zeolite adsorbents show little difference in performance, although the flow conditions required for the two adsorbents are significantly different. The separation of fructose-glucose mixtures in the production of high fructose syrup is generally carried out by simulated countercurrent adsorption using as the adsorbent either a cation-exchange resin in Ca2+ form (Mitsubishi and Illinois Water Treatment Processes) or a CaY synthetic zeolite (UOP process). Both of these adsorbents are fructose selective, but at least from the open literature, it is not clear whether one or the other has any intrinsic advantage, either in terms of fundamental factors such as capacity, selectivity, or adsorption kinetics or in terms of less obvious but equally important factors such as cost and service life. In order to provide comparative data on the intrinsic properties of these adsorbents, a series of experimental studies was undertaken in which breakthrough curves were measured, over a range of liquid flow rates, for two different resin adsorbents and samples of CaY and CaX zeolites. The CaX zeolite showed virtually no selectivity between fructose and glucose, but the selectivity and capacity of the CaY zeolite were found to be similar to the Ca2+resins. The kinetic properties of the CaY zeolite are superior although the equilibrium selectivity is somewhat less favorable. The comparative performance of these two adsorbents is therefore very sensitive to the extent to which the adsorber is affected by masstransfer resistance.
Experimental Section Breakthrough curves were measured in a small column packed with the test adsorbent and surrounded by a water jacket through which water from a thermostat was circulated to maintain a constant temperature. Effluent con-
centrations were monitored with the aid of an on-line refractive index detector. At the start of an experiment, a controlled flow rate of pure water was passed through the column to establish the detector base line, and at time zero the inlet stream was switched to a solution containing either fructose or glucose at the desired concentration level. The flow was continued until the breakthrough was complete and the steady detector signal corresponding to the inlet stream composition was established. Most of the measurements were carried out at low concentrations (- 1%wt) to ensure linearity of the equilibrium. However, a series of measurements was also performed with concentration up to 30% wt in order to establish the form of the equilibrium relationship at higher concentration levels. The moments of the response were calculated from the experimentalbreakthrough curves by integration according to the expressions (see, for example, Ammons et al., 1977) / . m i .
I =J, (:-l)dt
u2=
lm(l-d)tdt-P
In the earlier experiments, measurements were made also by the pulse response method. The first and second moments of the pulse response were calculated according to
* Author t o whom correspondence should be addressed. Permanent address: Department of Chemical Engineering, University of New Brunswick, Fredericton, N.B., Canada E3B 5A3. 0888-5885/87/2626-1407$01.50/0
\
0 1987 American Chemical Society
1408 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987
Table I. Details of Adsorbents and Columns adsorbent Ca2+Zerolit 225 SRC14 Ca2+Duolite C-204 CaY Zeolite Si/Al = 2.2 CaX Zeolite Si/Al = 1.3
av particle diam. mm 0.25 0.30 1.5 0.7 1.0
column i.d.. cm 1.15 2.5 1.1 5.1 1.1 1.1 1.1
column length. m voidage 0.65 0.4 0.64 0.38 0.48 0.38 1.0 0.4 0.48 0.41 0.48 0.41 0.48 0.41
(c)
i/i.
100
Within the linear region of the equilibrium isotherms, both methods yielded consistent results. However, the data obtained by the step response method proved to be more reliable and reproducible than the pulse data so the step method was adopted in later experiments. Details of the adsorbents and columns are given in Table I. The CaY adsorbent was prepared from a standard NaY sample (&/A1 = 2.2) by repeated ion exchange with CaC1,. After completion of the experiments with the larger particles, the sample was crushed and screened to obtain the smaller particle size sample.
Equilibrium Data The first moment of the response yields directly the slope of the equilibrium curve (dq*/dc) or, a t concentrations within the linear region, the Henry's law constant:
++(F)g] u
=t[l+(+)K]
(3)
It follows from this equation that a plot o f t vs. l / t u should be linear with slope E (1 - t)K and the equilibrium constant may be calculated, provided that the bed voidage (E) is known. Representative plots of f vs. l / w are shown in Figure 1, and it is evident that these plots are linear, in conformity with eq 3. Bed voidages were determined from the retention times measured for a pulse of starch, a large molecule which cannot significantly penetrate the adsorbent pores on the time scale of the measurement. The CaX adsorbent clearly shows virtually no selectivity,
+
COX Zeohte-29?
80
2 60
F 60
E
,: 40
I=
@ ', e
e
LO
20
+*
@ ,
&
E
20
20
40
60
80
1W 120
O
20
40 €0 I/'"
I / n Imln)
ea 1w
120 160
imn1
Figure 1. Representative plots showing variation of mean retention time ( f ) with l / a . Slope gives [ e + (1 - t)K]-see eq 3. (a) CaY zeolite a t 29 "C; (b) CaY zeolite at 60 "C; (c) Duolite resin a t 29 "C; (d) CaX zeolite a t 29 "C.
but the other adsorbents all adsorb fructose preferentially ( a > 1.0). Values of K , calculated according to eq 3, are summarized in Table I1 which includes also the values reported for similar adsorbents by other investigators. The values obtained in replicate experiments with a given system were quite consistent. Our values for CaY sieve agree reasonably well with the data of Hashimoto et al. (1983b),but for the Zerolit resin there are evidently significant differences between the distribution coefficients derived for the same adsorbent by different investigators. In a composite adsorbent, the distribution coefficient or particle-based adsorption equilibrium constant ( K ) is related to the true (crystal or microparticle) adsorption equilibrium constant ( K ' ) by K = Ep + (1- tp)K' (4) The data show quite consistently that, for all adsorbents,
Table 11. Comparison o f Adsorption Equilibrium Constants, Separation Factors, and Heats of Adsorption adsorbent Zerolit SCR-14 in Ca2+form
Dowex 50W X8 (Ca2+) X8 (Sr2*) X8 (Zn2+) Amberlite IRlZOB (CaZ+) IRl2OB (Mgz+) S-07 (Ca? S-07 (Mg2+) Duolite (Ca2+)
CaX sieve CaY sieve (exchanged from Linde NaY) CaY sieve
R , mm
T , "C
KF
0.125
0.125
25 52 30 60 25
0.047 0.047 0.047
0.125
0.15
0.5 0.75 0.35 0.75 0.35
a=
-mF,
0.724 0.61 0.67 0.49 0.38
KG 0.28 0.28 0.20 0.20 0.13
KF/KG 2.59 2.18 3.35 2.45 3.02
KF' 0.62 0.46 0.59 0.36 0.29
30 30 30
0.60 0.60 0.23
0.23 0.23 0.15
2.60 2.60 1.53
0.48 0.48 0.09
Ghim and Chhng, 1982)
0.10 0.047 0.231 0.223 0.35 0.31 0.36 0.29 0.51 0.38 0.512 0.436 0.515 0.586 0.518
1.6 2.0 2.0 1.17 1.81 1.48 1.83
0.07 0.05 0.30 0.05 0.44 0.22 0.47 0.18 0.14 0.62 0.59 0.44 0.52 0.24 0.48
Hashimoto et al., 1983a
29 60 28 60 29 29 29 60 60 50 50
0.164 0.095 0.464 0.26 0.634 0.46 0.66 0.41 0.58 0.78 0.79 0.683 0.767 0.686 0.743
1.41
1.14 2.02 1.59 1.57 1.49
1.17 1.43
kcal/mol 2.8
5.1
notes column i.d. 1.15 cm column i.d. 2.5 cm column i.d. 2.5 cm
column i.d. 1.1 cm column i.d. 5.5 cm
ref Ching, 1978 Ching, 1978 Barker and Thawait, 1984
this work this work
1.4
column i.d. 1.1 cm 2 different samples of CaY
this work Hashimoto et al., 1983b
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1409
- 10
29
30
32 33 103/T I K ' I
31
34
Figure 2. Temperature dependence of adsorption equilibrium constant (&'I for fructose on CaY zeolite, Zerolit resin, and Duolite resin, plotted according to the van't Hoff equation (K = KO X exp (-AH/Rn).
the distribution coefficient of glucose is essentially independent of temperature. This implies that the glucose is not really adsorbed, in the formal sense, but merely occupies the intraparticle pore space in direct proportion to its concentration in the fluid (Le., KG i= e,). Values of KF', calculated according to eq 4, are also included in Table 11. An estimate of the heat of sorption of fructose may be derived from the temperature dependence of KF' (see Figure 2). I t appears that the heat of sorption is somewhat lower for CaY zeolite than for the resin adsorbents, but because of the limited temperature range, the values so obtained are subject to considerable uncertainty. Data for the Duolite resin were obtained in both largediameter (5.5 cm) and small-diameter (1.1cm) columns. It is evident that the data are self-consistent, and at both temperatures there is good agreement between the values for both columns and both sorbates. The data presented above are limited to the low-concentration region in which Henry's Law is obeyed. However, for the CaY zeolite it has been shown (Hashimoto et al., 1983b) that the equilibrium relationship remains essentially linear even up to relatively high concentrations (-50%
Liquid Concentration
(g/ml )
Figure 3. Equilibrium data for adsorption of fructose and glucose on Duolite Ca2+ resin a t 60 "C showing agreement between data derived from low-concentration pulse measurements (- - - -) and from breakthrough curves (-). Note the slight curvature of the isotherm a t high sorbate concentration (data for 5.1-cm diameter column).
desirable to minimize the macroporositywhich determines the glucose holdup as well as to maximize the affinity for fructose. The adsorbent with the highest overall separation factor is therefore not necessarily the adsorbent with the highest affinity for fructose. This point is illustrated by the data at 60 "C which show that the Zerolit adsorbent has a higher overall separation factor but a lower affinity for fructose than the Cay adsorbent (asmeasured by KF').
Kinetic Data For an axial dispersed plug flow system, the height equivalent to a theoretical plate in a chromatographic column (H) is related to the dispersion and mass-transfer coefficients by (see for example Ruthven, 1984)
Wt).
A limited series of measurements was therefore carried out at 60 "C using the large-diameter column to determine the extent to which the equilibrium isotherms for the Duolite resin depart from linearity at higher concentrations. Capacities were determined directly by integration of breakthrough curves measured with sugar solutions containing 10.5% wt and 25% wt glucose and fructose. For each solution, three breakthrough curves were measured at several different flow rates, and the results, which are summarized in Figure 3, represent the average capacity values. It is clear that the isotherms for both fructose and glucose show some upward curvature, but at least up to concentrations of 25% wt the deviations from the limiting Henry's law line, derived from the low concentration measurements, are small. In order to reduce pressure drop to an acceptable level when operating at high concentrations, it is necessary to run the separation process at elevated temperatures, generally 55-60 "C. (At temperatures higher than 65 "C, charring occurs, thus reducing the quality of the product.) For an efficient separation, we therefore require an adsorbent with high equilibrium selectivity and acceptable capacity at 60 "C. From the data presented in Table 11, it is evident that, under these conditions, the capacity and selectivity of the CaY zeolite and the Duolite resins are similar. The Zerolit resin has a somewhat higher selectivity but a slightly lower capacity. Since it is the overall (pellet based) separation factor which is important, it is evidently
where k K is the overall mass-transfer coefficient defined in terms of a fluid-phase concentration driving force, aq _ - kK(c - c*); at
C*
= B/K
For a composite adsorbent, the overall mass-transfer coefficient is related to the external film resistance and the macropore and micropore diffusional resistances by
For a liquid system, D, = Dm/r,and in the low Reynolds number region, we may assume Sh = 2.0 or k f = D,/R so that eq 7 becomes
If intracrystalline (micropore) diffusion is sufficiently rapid, we have the macrocontrolled situation in which 1 7 Dm/kKR2 = - + (9) 3 15cp which with typical values for 7 and cp is of the order of unity. In the other limit where macropore and film re-
1410 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 Table 111. Summary of Kinetic Parameters sorbent T,"C R , cm
CaX
29
0.075
29
0.035
60
0.075
60 29
0.035 0.05
29
0.015
60
0.015
25
0.0125
43
0.0125
G F G F G F G F G F G F G F G F G F
Duolite
Zerolit
"D, = 6.7
X
lo4 cm2/s a t 25
OC,
1.45 X
k K , s-' 1.7 x 10-3 1.8 x 10-3 8.9 x 10-3 io x 10-3 3.2 x 10-3 3.0 x 10-3 0.015 0.014 4.1 x 10-3 2.9 x 10-3 4.4 x 10-3 7.0 x 10-3 7.1 x 10-3 0.015 2.8 x 10-3 4.8 x 10-3 7.2 x 10-3 0.0164
sorbate
CaY
> 1.0
(10)
(a1 Duolite
Resin
4L
Values of H were calculated from the first and second moments of the response arrives according to
2
H=-1
f2
Representative plots showing the variation of H with fluid velocity are shown in Figure 4. In all cases we find that H increases approximately linearly with fluid velocity and is higher for fructose than for glucose. H evidently decreases with increasing temperature and in the zeolite adsorbents, with decreasing particle size. For liquid systems, the Peclet number is approximately independent of velocity (DL/vd constant), so the increasing trend of H with velocity implies that, for all adsorbents tested, the mass-transfer resistance term in eq 5 is dominant. Detailed analysis, however, reveals that macropore resistance is dominant in the zeolite adsorbents, whereas micropore resistance is dominant in the ion-exchange resins. The experimental kinetic data (Table 111) were analyzed in the following way. Assuming Pe = 1.0 (or DL/u = 2R), values of the effective mass-transfer coefficient ( k ) were calculated from the HETP data according to eq 5, using the values of K derived from the first moments. These values are compared with values of Dm/R2estimated from the known liquid-phase molecular diffusivity. For the zeolite adsorbents, the ratio Dm/kR2 is seen to be approximately constant, independent of particle size or temperature, essentially the same for fructose and glucose in both the CaX and CaY adsorbents and always in the range 0.55-0.9. The mean value of T / t , calculated according to eq 9 (i.e., assuming macropore and film resistances to be dominant) is about 6, which is typical of such adsorbents ( 7 = 2.0, t = 0.33). (See, for example, Lee and Ruthven (1977).) In Pigure 4 the theoretical lines are calculated according to eq 5 and 8 (assuming macropore and film resistances to be dominant with r/cp = 6.0 and DL/u = 2R (Pe = 1.0). It is apparent that these lines provide a good representation of the experimental data for both sorbates over the entire range of experimental conditions. The experimental evidence is thus seen to be entirely consistent with the hypothesis that in these adsorbents intracrystalline diffusional resistance is negligible and the sorption kinetics are controlled entirely by the
(cmlmln)
cv
(b)
CaY Zeollte
cv
'
(crn/mm)
Figure 4. Representative plots showing variation of HETP with fluid velocity for (a) Duolite Ca2+resin and (b) CaY zeolite.
combined effects of macropore diffusion and external film resistances. By contrast, for the resin adsorbents, we find Dm/kKR2 >> 1.0, indicating micropore control. For these adsorbents
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1411 Table IV. Comparison of Overall Mass-Transfer Resistances based on Particles of Diameter 0.03 cm sorbate sorbent temp, O C 1/Kk, s 1/K, s 9.5 G 21 29 CaY F 20 16 60 G 12.4 5.8 F 13.2 8.3 Duolite 29 G 221 77 F 143 91 60 G 141 43 F 67 30 Zerolite 25 G 360 100 F 208 140 60 G 139 39 F 61 39
the values of k (the rate coefficient based on adsorbedphase concentration as the driving force) are about the same for both sorbates in both the Zerolit and Duolite resins, corresponding to micropore diffusional time con-2 X stants (D,/r2) within the range 7 X s-l. In the case of the zeolite adsorbents, we were able to confirm directly the dominance of macrodiffusional resistance by varying the particle size. Such a direct test was not possible for the resins since we had only a single particle size available, and unlike the zeolite adsorbents, the resin beads cannot be crushed to reduce their size. Nevertheless, the evidence presented above based on the magnitude of the ratio D,/kKR2 provides convincing proof that, in the resin adsorbents, the intraparticle mass-transfer resistance is much higher so that film and macropore resistances are not significant. To illustrate the difference in mass-transfer resistance between the zeolite and resin absorbents, in Table IV the overall mass-transfer resistances (based on adsorbed-phase concentration as the driving force) are compared on the basis of a standard size of particle. On this basis, the mass-transfer resistance of the resin adsorbents is seen to be between 5 and 10 times higher than that of the zeolite adsorbents.
Performance of a Simulated Countercurrent Separation Unit In order to compare the adsorbents in a practically meaningful way, we have simulated mathematically the performance of a standard four-section (Sarex-type) separation unit (eight columns, two columns per section), using the equivalent countercurrent model (see Figure 5) and assuming the equilibria for both glucose and fructose to be linear. Details of the model have been given elsewhere (Ching and Ruthven, 1985;Ching et al., 1985). The feed is assumed to consist of a solution containing 25% fructose and 25% glucose, and the system is assumed to operate isothermally at 60 "C. The extract product (fructose) is required to contain less than 5% (dry basis) glucose, and the raffinate product (glucose) should contain less than 5% fructose (dry basis). In addition, we require 95% recovery of both products and we wish to minimize product dilution. We have arbitrarily limited the maximum interstitial fluid velocity to 10 cm/min for the resin and 12 cm/min for the Zeolite adsorbent. We consider two adsorbents, the Zerolit adsorbent which is representative of the more selective resins, and the CaY zeolite adsorbent, both with the same particle size (0.03 cm) and with the corresponding kinetic and equilibrium parameters as summarized in Tables I1 and IV. The flow rate ratios required in each section of the bed are derived from the overall flow constraint inequalities, assuming 5% margin, in the manner described by Ching et al. (1985). For each adsorbent, we consider two cases with different
v! z 0
3 z L
U
I-
z m w
B Q
s
i
c W
I
6 >-
r
MAKE-UP -WATER (E*R-Fl
Figure 5. Schematic diagram of the separation system considered as an equivalent countercurrent flow system. Table V. Comparison of Performance of Zerolit Resin and CaY Zeolite based on Numerical Simulation of a Continuous Countercurrent Separation Unit" R , cm
KF Kr.
DL (_u
Zerolit 0.03 0.49 0.20
+1
H',cm DIF EIF
RIF SIF I , cm
CaY 0.03 0.73 0.42 a O.O3(u 10 3.8 2.0 1.24 1.05 3.9 40
kF, mi& kG, m i d
Zerolit 1.5 1.5
CaY 7.2 10.3
b 5(u u ) 10 10 2.0 1.19 1.08 3.9 160
a O.O3(u + u ) 12.3 0.7 3.0 1.32 1.12 3.94 9.0
b 5(u + u ) 12.3 10 3.0 1.28 1.16 3.94 155
19.1 0.99
20 1.1
18 0.92
18.5 0.95
1.36 22.7 368
1.2 22 1470
1.0 20.5 83
1.04 21.2 1425
+ u)
+
extract
%F %G raffinate
%F %G total ads. vol./feed rate, min a
Comparison is based on same feed rate in all cases.
degrees of axial mixing. In case (a), we assume DL/vd -1.0 as for a packed bed under carefully controlled flow conditions. In case (b), we consider the axial dispersion coefficient to be given by DL = 5(u + v) (where u and v are in cm/min and DL is in cm2/min). This corresponds approximately to the degree of dispersion observed experimentally in our pilot unit which is much higher than for an ideal packed bed. The results of these calculations are summarized in Table V, and the simulated concentration profiles through the unit are shown in Figure 6. In order to fulfill the product specifications, quite different flow conditions are required for the two adsorbents. The calculated profiles for all four cases are, however, almost the same. The assumption concerning the extent of axial mixing has only a minor effect on the required flow conditions, but of course it has a large effect on the required column length. The comparison in Table V is based on the same throughput for both adsorbents, but to achieve this we require a somewhat higher adsorbent circulation rate (lower switch time) and correspondingly higher fluid velocities with the zeolite adsorbent in order to compensate for the lower selectivity.
1412 Ind. Eng. Chem. Res., Vol. 26,No. 7, 1987
I
0 0
--+
tD
1
--r
/
,
,
,
2 3 L 5 tE +F Column Number
6 tR
7
8
Figure 6. Theoretical steady-state concentration profile for the countercurrent system operated under the conditions given in Table
V. (The profiles calculated for CaY and Zerolit resin adsorbents, each for the appropriate flow conditions given in Table V, are almost identical.)
As a result of the lower mass-transfer resistance, we find that, when axial mixing effects are small, the bed length (or adsorbent volume) requirement is much smaller for the zeolite adsorbent than for the resin. However, if axial mixing is high, we reach a situation in which the height equivalent of a theoretical plate (HETP) is determined mainly by axial mixing and mass-transfer resistance has little effect. Under these conditions, the performance of the two adsorbents becomes almost identical. The final figure in Table V gives the total volume of adsorbent required per unit feed rate. This figure provides a useful overall comparison of the relative performance of the two adsorbents for the specified duty.
Conclusions CaX zeolite shows virtually no selectivity between glucose and fructose, but the selectivity of CaY is comparable with that of the Ca2+ion-exchange resins. The best of the resin adsorbents have somewhat higher equilibrium selectivities than the CaY zeolite, but the zeolite adsorbent has the advantage that, for a given size of particle, masstransfer resistance is considerably smaller. When compared on the basis of a separation unit operating under typical conditions, we find that the difference in the equilibrium constants requires somewhat different flow conditions for the two adsorbents. If axial mixing can be kept to a minimum, the superior mass-transfer properties of the zeolite adsorbent lead to a considerable reduction in adsorbent volume requirements for a specified duty. With less optimistic and possibly more realistic assumptions about axial mixing, this advantage is lost and the performance of both adsorbents becomes almost identical. However, with properly selected flow conditions, the performance of the zeolite adsorbent should be at least as good as that of the resin. In the final analysis, therefore, the choice of adsorbent will probably depend more on factors such as cost and durability rather than on the fundamental kinetic and equilibrium properties which have been considered here. Nomenclature c = sorbate concentration in liquid phase co = sorbate concentration in feed stream
d = particle diameter D = hypothetical desorbent flow rate in countercurrent separation system (see Figure 5) D, = intracrystalline or micropore diffusivity in zeolite adsorbent (based on adsorbed-phase concentration as driving force) DL = axial dispersion coefficient D, = molecular diffusivity of sorbate in liquid phase D, = macropore diffusivity in adsorebent particle (based on fluid-phase concentration as driving force) E = extract rate in equivalent countercurrent system (Figure 5) F = feed rate in equivalent countercurrent separation system (Figure 5) H = height equivalent to chromatographic theoretical plate H' = height equivalent to theoretical plate in simulated countercurrent adsorption system k = overall mass-transfer coefficient defined by eq 6 k f = external fluid film mass-transfer coefficient K = adsorption equilibrium constant based on particle volume K' = adsorption equilibirium constant based on solid volume (see eq 4) 1 = length of adsorbent bed or length of section in simulated countercurrent adsorption system Pe = ud/DL q = adsorbed-phase concentration qo = adsorbed-phase concentration at equilibrium with feed r = radius of zeolite crystal R = radius of adsorbent pellet (also raffinate flow rate in countercurrent unit) Sh = k&/D, t = time E = mean retention time u = hypothetical solid velocity in simulated countercurrent system (Figure 5) u = interstitial fluid velocity
Greek Symbols a = separation factor (ratio of equilibrium constants) AH = heat of adsorption u* = variance of chromatographic response peak t = voidage of adsorbent bed c, = porosity of adsorbent particles r = tortuosity factor (defined by D, = Dm/T) Subscripts
G, F = glucose, fructose Registry No. Duolite C204,107657-94-3; Zerolit 225 SRC14, 91728-46-0; fructose, 57-48-7; glucose, 50-99-7.
Literature Cited Ammons, R. D.; Dougharty, N. A.; Smith, J. M. Ind. Eng. Chem. Fundam. 1977, 16, 363. Barker, P. E.; Thawait, S. J . Chromatogr. 1984, 295,479. Ching, C. B. Ph. D. Thesis, University of Aston, England, 1978. Ching, C. B.;Ruthven, D. M.; Hidajat, K. Chem. Eng. Sci. 1985,40, 1411. Ching, C. B.; Ruthven, D. M. AIChE Symp. Ser. 1985,81 (242), 1. Ghim, Y. S.;Chang, H. N. Ind. Eng. Chem. Fundam. 1982,21,369. Hashimoto, K.;Adachi, S.; Noujima, H.; Veda, Y. Biotechnol. Bioeng. 1983a, 25, 2317. Hashimoto, K.; Adahi, S.; Noujima, H.; Marayama, H. J.Chem. Eng. Jpn. 1983b,16, 400. Lee, L.-K.; Ruthven, D. M. Ind. Eng. Chem. Fundam. 1977,16,290. Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984; p 249.
Received for review August 4, 1986 Accepted March 16, 1987
