Ind. Eng. Chem. Process Des. Dev. 1986, 25, 17-21
Subscripts A, B = component A and B, respectively, in feedstock
cIo,czO,Cz',
C3O, c3== CHI, CzH6,CzH4, C3H8,C3H6, respectively i = component j = component m = mixture or simultaneous pyrolysis P = product R = reactant T = tracer, nitrogen in this experiment Registry No. CzH4,74-85-1; C3H6,115-07-1;CzHs,74-84-0; CSHS, 74-98-6. Literature Cited Dunkbman. J. J.; Albright, L. F. ACS Symp. Ser. 1974, 3 2 , 241. Froment, G. F.; Van de Steene, 8. 0.; Van Damme, P. S.; Narayanan, S.; Ooossens, A. G. Ind. Eng. Chem. Process Des. Dev 1078, 15, 495.
17
Froment, G. F.; Van de Steene, B. 0.;Sumedha, 0. Oil Gas J . 1970, 77(16), 87. Goossens, A. G. Kinetics Technology International, B. V., The Hague, The Netherlands, personal communication, Aug 14, 1979. Hoffmann, H. University of Erlangen, 8520 Erlangen, West Germany, personal communlcation. Nov 25, 1980. Minet. R. G.; Hammond, J. D. 011Gas J . 1075, 73(31), 80. Mol, A. Hydrocarbon Process. 1981. 60(2), 129. Ross, L. L.; Shu, W. R. Oil Gas J . 1977, 75(43), 58. Sundaram, K. M.; Froment, G. F. Ind. Eng. Chem. Fundam. 1978, 17, 174. Zou Renjun Sci. Sin. 1979, 22(1), 53. Zou Renjun Sci. Sin. 1079, 22(6), 637. Zou Renjun "Principles & Techniques of Pyrolysis in Petrochemical Industry", 1st ed.; Chemical Industry Press: Beijing, 1981.
Received for review September 21, 1983 Revised manuscript received August 17, 1984 Accepted August 28, 1984
Adsorption of Water from Aqueous Ethanol Using 3-A Molecular Sieves Wah Koon Teo and Douglas M. Ruthven' Department of Chemical Engineering, National University of Singapore. Kent Ridge, Singapore 05 11
The adsorptive dehydration of aqueous ethanol using 3-A molecular sieve adsorbent has been studied experimentally by following the uptake curves for a closed batch system and by measuring breakthrough curves for a packed column. This system is potentially attractive for the dehydration of rectified spirit in the production of fuel alcohol. The equilibrium isotherm is almost rectangular, and the kinetic data for both systems can be satisfactorily Correlated in terms of simple kinetic models. The resuits of experiments in which particle size and fluid velocity were varied show that intraparticle diffusion is the main resistance to mass transfer with some contribution from external film resistance at low fluid velocities and/or water concentrations.
The production of fuel alcohol from rectified spirit requires almost complete removal of the residual moisture. This has traditionally been accomplished by azeotropic or extractive distillation, but the high energy costa of these processes have stimulated a search for a more efficient method of separation. Dehydration by adsorption on a 3-A molecular sieve has been suggested as a promising alternative to the conventional processes (Hartline, 1979). The 3-A sieve has the advantage that the micropores are too small to be penetrated by alcohol molecules so the water is adsorbed without competition from the liquid phase. In order to obtain the basic data required to assess the economic viability of such a process, we have studied the kinetics and equilibrium of dehydration of rectified spirit on a 3-A molecular sieve in both batch and column experiments. Since adsorption is noncompetitive and the isotherm for water is highly favorable, the data can be accurately correlated in terms of a very simple mathematical model. Experimental Section Material. A type 3-A molecular sieve in 1/16-in.cylindrical pellet form (Sigma Chemical Co.) was used exclusively in the experimental work. Smaller particle sizes of molecular sieve were obtained by crushing and screening
* Permanent address: Department of Chemical Engineering, University of New Brunswick, Fredericton, N.B., Canada. 0196-4305/86l1125-0017$01.50l0
the 1/16-in.pellets. The molecular sieves were preconditioned by thermal activation in a furnace at 300 "C for 24 h and then stored in a vacuum desiccator for use in adsorption studies. Mixtures of ethanol and water were prepared from deionized water and analytical grade absolute alcohol (0.79 g/L, minimum purity 99,9 wt %) supplied by Merck. Concentrations of ethanol-water mixtures were determined by gas chromatography by using a Perkin-Elmer Model F-17 gas chromatograph with a Chromosorb-102 column and a hot-wire detector. Equilibrium Data. The equilibrium isotherm was determined at 24 "C over a range of water concentrations in the fluid phase from 1.3 to 7.3 w t %. The solid-liquid mixtures were allowed to equilibrate for 3 days with periodic gentle swirling. The concentration in the adsorbed phase was found by mass balance from the initial and final concentrations in the fluid. Batch Kinetic Studies. Batch uptake experiments were carried out in closed circulating system sketched in Figure 1. The ethanol-water mixture containing 5.915 wt % H,O from a reservoir (42 mL) maintained at constant temperature was circulated continuously through a small packed bed of molecular sieve particles (8 g). Such a system proved superior to a stirred vessel used in preliminary experiments, since it gave better solid-liquid contact and eliminated the solid-solid attrition problem encountered in the latter system. Rates of sorbate uptake by the adsorbent from the ethanol-water mixtures were moni0 1985 American Chemical Society
18
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
Adsorbent Somple Valve
F \z r w
0
Circulating Pump
0
Figure 1. Schematic diagram of experimental system for batch uptake rate measurements.
Figure 3. Schematic diagram showing concentration profile for adsorption of an irreversibly adsorbed (rectangular isotherm) sorbate in a porous adsorbent particle.
co at the surface (R = R,) to 0 at Rf. The sorbate profile through the saturated layer is given by c
and, at the particle surface, 01
0
1
1
2 Fluid Phase
I
3
4
Concentration, C
Figure 2. Experimental equilibrium isotherm for adsorption of water on 3-A sieve.
tored over a wide range of circulation flow rate (from 1.2 to 190 mL/min) and particle size (R, varies from 0.0225 to 0.122 cm) at 24 "C to study the mechanism of mass transfer and verify the rate controlling step(s) of the adsorption process. Fixed-Bed Breakthrough Experiments. Breakthrough experiments were carried out in jacketed glass columns (2.54-cm diameter X 76.1-cm height) packed with preconditional molecular sieve pellets. The column was maintained at 24 OC, and the feed, an ethanol-water mixture containing about 4.7 wt % water, was introduced at the bottom. The operating flow rates ranging from 0.789 to 1.654 cm/min were well below the terminal settling velocity of the adsorbent particles in the bed. Samples of the effluent from the column were collected at suitable time intervals and analyzed chromatographically for water and ethanol contents to establish the breakthrough curves. Results and Discussion Batch Uptake Kinetics. Kinetic Models. The experimental isotherm, as shown in Figure 2, may be seen to be highly favorable, approaching rectangular or irreversible form. The simplified model of an irreversible equilibrium system (c = 0, q* = 0; c > 0, q* = qs) is therefore adopted for the analysis of the kinetic data. Consider a set of identical porous spherical adsorbent particles immersed in a well-mixed reservoir of fluid and subjected at time zero to a step increase in the sorbate concentration in the liquid phase from 0 to CP In a well-mixed system, the fluid side mass-transfer resistance is considered negligible, and equilibrium is therefore established immediately a t the external surface of the particles. Since the equilibrium isotherm is essentially rectangular, the uptake of sorbate may be described in terms of the "shrinking coren model (Dedrick and Beekman, 1967; Brausch and Schlunder, 1975). The general form of the sorbate concentration profile is sketched in Figure 3. All adsorption occurs at the front (radius Rf < R,) which divides the sorbate-free core from the saturated external layer. Over the region, R, > R > Rf, the total flux of sorbate is constant and the sorbate concentration falls from
The total flux may be equated to the net uptake rate:
dRf dt
-4i~Rf"q,- (3)
This equation may be rearranged to give ds 4, co -= (T2 - 77) RP2 4s
(4)
where q = Rf/R,. The sorbate concentration in the external fluid phase, co, varies with time and is related to q by an overall mass balance = Co[l - A(q/qJl = Co[1 - a3)l (5) where Co is the initial sorbate concentration in the external fluid phase, A is the fraction of the sorbate, initially present in the system, which is eventually taken up by the adsorbant, and ( q / q B )= Q is the fractional uptake by the adsorbent at time t. Combining eq 4 and 5 and integrating, we obtain
where
'
I2
c(:
s d ~ 1 - A - Aq3 V2dV
1 - A - A$
Equations 7 and 8 can be integrated to give
(7)
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 19
and 1 Iz = - In [l - A - Aq3] (10) 3A where X = [(l- ~ i ) / A ] l / ~Since . p3 = 1 - (q/q,) = 1- Q, it is evident that both Il and Iz may be expressed as functions of A and Q, so that eq 6 may be written in the form
-Det = - - EPp R,2 R,2
cot
= 12(Q,A) - Il(Q,A)
(11)
48
Equation 11 suggests that a plot of ( I z - 11)vs. t should give a straight line through the origin with slope De/R,2. If the reservoir of fluid is effectively infinite, that is A 0 and co = Co = constant, then eq 7 and 8 become
-
I1 =
J‘7 dq =
Iz =
O’t
- 1)
(12)
q2 dq = 1/(q3- 1)
(13)
f/2(v2
and eq 11reduces to the familiar form derived by Dedrick and Beekman (1967)
A family of theoretical uptake curves, calculated according to eq 11, is shown in Figure 4. It is evident that in the intermediate and long-time region, the effect of finite system volume is quite significant. If external mass-transfer resistance cannot be neglected, the relationship between the concentration at the particle surface, c,, and the bulk concentration, co, may be derived from a mass balance at the particle surface y3rR:(
2)
= 4aR,2kf(co- c,) = 47rR2tP, dc = RrJ dR
01
I
0
I
0.1
I
I
0.2 DR~IR~‘
0.3
Figure 4. Theoretical uptake curves calculated according to eq 11 showing effect of A.
li
1
I
““‘I
Recirculation Rate Exptx (ml/minl Exptl 0 (21 48 (121 1.20 0 (1) 51 (11) A 3.30
0
C.:5.9%,
D
24’C ,1/16” pellets
Q
0
40
20
60 TIME ( m i n )
80
100
Q
P.
I.:
L
which may be rearranged to give co
.
E P P 6
Clrc Rate 9.5mlhntn. C.:59% 1
0
Following the same analysis as shown earlier for eq 11,the expression for the uptake curve becomes
This equation suggests that a plot of t / I z vs. I1/Izshould be linear with intercept (1+ epDp/kfRp) and slope (De/R,2). Analysis o f Batch Uptake Experimental Results. Representative uptake curves are shown in Figure 5 , and the conditions of the ten successful runs are presented in Table I. In most of the experiments, the final uptake of sorbate was about 0.16 g/g, but slightly lower values (0.13 to -0.14 g/g) were obtained in runs 9-12 probably due to imperfect dehydration of the adsorbent during preconditioning. Figure 6 shows the results plotted in the form suggested by eq 11 (i.e., [Iz(Q,A)- I1(Q,A)]vs. t ) . It
1
10
1
1
20
I
I
30 TIME (min)
I
I
40
,24‘C I
I
50
Figure 5. Experimental uptake curves showing (a, top) effect of circulation rate and (b, bottom) effect of particle size.
is evident that the plots for all runs are essentially linear and pass through the origin, as required by the mathematical model. Furthermore, the uptake curves for runs 1-4 and 9-12, which were carried out over a wide range of circulation flow rates are essentially coincident, indicating that external mass-transfer resistance cannot be significant. This conclusion was confirmed by plotting I l / I z ,as suggested by eq 17. The intercepts of such plots were in all cases close to unity, indicating that the term $ I p / k f R pmust be small, even at the lowest flow rate. Results from experimental runs 5 and 7, which were carried out with crushed adsorbent particles, further confirm the dominance of intraparticle pore diffusional resistance since the time constants show the expected dependence on R:. There is some slight difference be-
20
Ind. Eng.
Chem. Process Des. Dev., Vol. 25, No. 1,
1986
Table I. Summary of Experimental Uptake Rate Measurements h R,, cm run no. flow rate, mL/min qs, g/g 51.0 0.16 0.66 1 48.0 0.157 0.64 0.122 2 190.0 0.163 3 0.66 141.0 0.154 0.63 4 0.57 9.3 0.14 9 0.57 0.122 10 9.5 0.14 3.3 0.135 0.56 11 1.2 0.55 0.13 12 0.66 0.076 10.0 0.16 5" 0.66 0.044 10.0 0.16 70 "Run with crushed adsorbent particles:
p
- 0.2 -
.r
0.1
20
40
60
80
100
1200 10 TIME Lmtnl
20
30
40
50
60
Figure 6. Uptake curves plotted according to eq 11. (See Table I for details.) (a) Shows plot for runs 1-4 in which circulation rate is varied. (b) Shows plots for runs 5-11 in which particle size is varied.
tween the time constants obtained in runs 1-4 (high-circulation flow rate) and 9-12 (low-circulationflow rate), in which the equivalent particle size was the same; but, this difference appears to be due to the small difference in the final uptake values. The final values of e,,Dp/R,2 and epDp for these runs are almost the same despite the very large range of flow rates. The effective diffusivities, epDp, evaluated from the slopes of the plots in Figure 6 are summarized in Table I which gives an average value of 2.05 X lo4 cm2s-l. With a porosity of 0.33, the mean pore diffusivity is calculated to be about 6 X lo4 cm2s-l. The molecular diffusivity of the water-ethanol mixture (containing more than 96 wt % ethanol) at 298 K is about 1.24 X cm2 s-l (Reid et al., 1977). So these results suggest that the tortuosity factor is about 2 (Dp= D J 7 ) which is very similar to the value obtained by Lee and Ruthven (1977) in their study of the liquid-phase adsorption of n-heptane in a somewhat similar 5-A zeolite adsorbent. Breakthrough Curves in Fixed-Bed Adsorption. Breakthrough Curves from Pore Models. The expression for the breakthrough curve for a rectangular isotherm system in which the mass-transfer rate is controlled by the combined effects of external film and internal pore diffusional resistances has been given by Weber and Chakravorti (1974). For the constant pattern regime (8 2 5 / 2 + Np/Nf),the relevant expression is 8-N,,= 15 tan-' 3112 (1- Q)'l3]
-
epDP/R,2, s-'
cPp,cm2 s-l
3.63 x 10-5
1.36 x 10-4
2.02 x 104
4.3 x 10-5
1.39 x 10-4
2.06
X
lo4
2.07 2.09
X X
lo4 lo4
9.6 x 10-5 29 x 10-5
3.6 x 10-4 10.8 x 10-4
= 1.1 g/cm3; C, = 0.047 g/cm3; cp = 0.33.
0.3
n 0
D,/R,2, s-l
E In [l + (1- Q)1/3 +
+ 2.5 - 5*/2(3)'12 +
where Q is the fractional uptake and 8, Np,and Nf are
Table 11. Summary of Experimental Conditions and Parameters for Calculation of Breakthrough Curves run no. 1 2 3 4 76.1 76.1 76.1 bed length, cm 152.2 1.16 0.79 1.65 cV, cm min-' 1.65 18.4 26.3 38.6 t/v,min 36.8 0.176 0.176 0.176 0.176 qwb g/cm3 0.0386 0.0386 0.0386 0.0386 C O P g/cm3 0.1224 0.1224 0.1224 0.1224 15cpD,"/RP2,min-' 3.42 4.87 7.08 6.84 N P 0.03 0.025 0.022 0.03 kr,cm/min-' 19.5 16.5 23.00 46.00 Nf 0.15 0.25 0.43 0.15 N,/Nf nFrom Table I, cP,/R,2 = 1.36 X s-'. bCapacity = 0.16 g/g = 0.176 g/cm3. Bed voidage c = 0.4.
dimensionless parameters with respect to contact time, pore resistance, and film resistance, respectively. These parameters are defined as
While within the constant pattern regime, the fractional uptake Q is 4
c
46
CO
Q = - = -
Since the equilibrium capacity, qs,and the diffusional time constant, e,,Dp/R,2, are known from the batch uptake rate measurements while the external mass-transfer coefficient may be estimated from established correlations, eq 18 provides an a priori prediction of the form of the breakthrough curve once the fluid composition, the velocity, the bed length, and the bed voidage are specified. If external mass-transfer resistance is negligible (Np/Nf 0), eq 18 reduces to the expression for a pore diffusion-controlled system given earlier by Cooper and Liberman (1970). Analysis of Experimental Breakthrough Curves. Breakthrough curves for water from ethanol-water mixtures were measured at several different flow rates and for two bed heights. The experimental conditions for these runs are summarized in Table 11; and the experimental breakthrough curves are shown in Figures 7 and 8 together with the theoretical curves calculated according to eq 18 with the parameters derived from the uptake rate measurements. External mass-transfer coefficients were es-
-
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
0.81 0.6
1
f
!O
d
0.21 0
'
n
o
;
0
I
0 0
7
0
,
O
'I
100
200
, 300
400
TIME (min)
Figure 7. Comparison of experimental and theoretical breakthrough curves for runs 1 and 2 showing effect of bed lengths: (-) curve calculated from eq 18; (--) as above with no film resistance. Details of parameters are given in Table 11.
0.8
6
06
0.4
1
0.2
" I
0 0
100
,
I
I
0 0
I
I
Q
200
I
300
400
TIME lmm)
Figure 8. Comparison of experimental and theoretical breakthrough curves for runs 3 and 4 showing effect of fluid velocity: (-) curve calculated from eq 18; (--) as above with no film resistance. Details of parameters are given in Table 11.
timated from the correlation of Wakao and Funazkri (1978):
21
of the breakthrough curve for this region from the full expression for the developing region (given also by Weber and Chakravorti (1974))gives only an inperceptible change in the theoretical curve in this region. Comparison of the experimental breakthrough curves for runs 1and 2, which were obtained with two different bed lengths under otherwise identical conditions, confirms constant pattern behavior. Conclusions The feasibility of dehydrating rectified spirit by adsorbing the water on 3-A type molecular sieve has been demonstrated. It has been shown that the uptake rate is controlled primarily by intraparticle pore diffusion resistance with some additional contribution from external film resistance, depending on the hydrodynamic conditions. The breakthrough curves for this system are well predicted by the model of Weber and Chakravorti which therefore provides useful guidance for the design and economic evaluation of a larger scale unit. Acknowledgment We thank Larry Tan for carrying out part of the experimental work. The financial support from the ASEAN Working Group on Management and Utilization of Foodwaste Materials under the ASEAN-Australia Economic Cooperation Programme is gratefully acknowledged. Nomenclature c = liquid-phase concentration of sorbate (water) c, = concentrations of sorbate in liquid phase at surface of adsorbent particle c o ( t ) = concentration of sorbate in bulk liquid phase Co = initial value of co D, = molecular diffusivity D, = pore diffusion coefficient (defied on pore sectional area basis) De = effective diffusivity = e$ Co/qs I,, I 2 = integrals defined in eq f and 8, respectively k = constant kf = external fluid film mass-transfer coefficient N = dimensionless parameter = (15t$,/R,2)(z/u)(l - e)/€ I$= dimensionless f i resistance parameter = [3kfz/(uRP)](1
- ,)/e
qs = saturation adsorbed-phase concentrations (particle
volume basis) It is evident from Figures 7 and 8 that in general the theoretical curves give a good prediction of the experimentally observed behavior. The fit is especially good for runs 1 and 4 but runs 2 and 3 show some deviation in the initial region. It is possible that such deviation may arise from axial dispersion which is neglected in the theoretical model. However, in view of the excellent agreement which is observed in runs 1and 4, it seems more likely that these deviations arise simply from errors in the analysis. Such errors tend to be most pronounced at low water concentrations. Also shown in Figures 7 and 8 are the theoretical curves, calculated according to the model of Cooper and Liberman assuming no external mass-transfer resistance (i.e., eq 18 with N /Nf = 0). These theoretical curves provide a reasonabre representation of the later portions of the breakthrough curves, but they do not fit in the initial region in which the water concentration is very low and the breakthrough curve is very sensitive to even a modest contribution from external resistance. In a practical system, it would therefore be desirable to use a higher fluid velocity in order to minimize external resistance and avoid premature breakthrough. The constant pattern condition (A > 5 / 2 + Np/Nf)is fulfilled except in the initial region of run 1. Recalculation
q = adsorbed-phase concentration averaged over a particle q; = equilibrium value of q 6 = fractional approach to equilibrium = q/qs R = radial coordinate
R, = particle radius Rf = radius of adsorption front t = time 6' = dimensionless time variable = (15r$,./R,2)(c0/q8)(t 7z/u) A = fraction of sorbate initially present in solution which is eventually taken up by adsorbent 7 = Rf/Rp cp = porosity of particle t = voidage of adsorbent bed Registry No. Ethanol, 64-17-5. Literature Cited Brausch, V.; Schlunder, E. V. Chem. Eng. Sci. 1975, 3 0 , 540. Cooper, R. S.; Llberman, D. A. Ind. Eng. Chem. Fundum. 1970, 9 , 620.
Dedrick, R. L.; Beekman, R. B. Chem. Eng. Pfog., Symp. Ser. 1967. 63 (74), 68.
Hartline, F. F. Science (Washington, O.C.)1979, 6 , 41. Lee, L.-K.;Ruthven, D. M. Ind. Eng. Chem. Fundum. 1977, 16, 290. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. "Properties of Gases and Liquids", 3rd ed.; McGraw-Hill: New York, 1977; p 570. Wakao, N.; Funazkrl, T. Chem. Eng. Sci. 1978. 3 3 , 1375. Weber, T. W.; Chakravorti. R. K . AIChE J . 1974, 20, 228. Received for review August 21, 1984 Accepted December 26, 1984
