Ind. Eng, Chem. Process Des. Dev. 1983, 22, 329-334 Kwtln, K.; Taub, 1. A,; Weinstock, E. Inorg. Chem. 1966, 5 , 1079. Nljslng, R. A. T. 0.; Hendrlksz, R. H.; Kramers, H. Chem. €ng. Scl. 1969, 10, 88. Obuchl, A.; Hanel, T.; Okugekl, A.; Okabe, T. Nbpon Kagaku KaMl 1974, 1425. onde,K.; Sada,E.; KobayasM, T.; KRo, S.; Ito, K. J . Chem. €ng. Jpn. 1970, 3, 18. Sada, E.; Kumazawa, H.; Tsubol, N.; Kudo, I.: Kondo, T. Chem. Eng. Scl. 1977, 32,1171. Sad, E.; Kumazawa, H.; Kudo, I.; Kondo, T. Chem. €ng. Scl. 1976a, 33, 315. Sada, E.; Kumazawa, H.; Tsubol, N.; Kudo, I.; Kondo, T. Ind. Eng. Chem. Rocwss Des. Dev. 1978b, 17, 321. Sab, S. Tokkyo Koho (Japan), S.51-83883, 1976.
329
Seklell, A.; Lineke, W. F. "Solubllltles of Inorganic and Metal Organic Compounds", 4th ed.; American Chemical Society: Washington, DC, 1965. Stefan, A. 2.; Whitemore, R. C. Chem. €ng. Scl. 1971, 26, 509. Teramoto, M.; Ikeda, M.; Teranlshl, H. Kagaku Kogaku Ronbunshu 1976a. 2 . 88. Teramoto, M.; Ikeda, M.; Teranlshl, H. Kagaku Kogaku Ronbunshu 1976b, 2. 837. Yoshida, F.; Akka. K. A I C N J . 1965, 1 1 , 9. Yoshida, F.; Miura. H. A I C M J . 1963, 9 . 331.
Received for review August 21, 1981 Accepted September 15, 1982
Increase of the Gas Conversion in a Fluidized Bed by Enlarging the Cross Section of the Upper Zone of the Bed J d Corelia' and Rafael Bilbao Departamento de Qdmhx Thnlca, Facultad de Clenclas, Universkled de Zaragoza, Zaragoza. Spaln
To improve the gas-solid contact and in order to increase the gas conversion in a fluidized bed, a new contactor Is proposed. Thls contactor consists of two dlfferent beds contained, without any separating device, in the same vessel. These two beds are produced by a considerable increase of the cross section of the vessel on the upper zone. The gas conversion at the outlet has been experimentally determined for three different types of reactors: an isothermal flxed bed with gas piston flow, a small cylindrical fluidized bed, and the present fluidized bed with varying geometry. The gas conversions for this new type of contactor are greater than those obtained wlth a cylindrical fluidized bed and somewhat lower than those corresponding to a gas piston flow. Correlations between the efficiency of the contact and the parameters of the system (soiM weight fraction in the various zones and gas velocity at the inlet) are obtained.
Introduction One of the main objections to fluidized beds is their deficient gas-solid contact owing to the bubbling. This deficient contact means that, when they are used as chemical reactors, the gas conversion obtained is smaller than that corresponding to a gas piston flow. Several methods are known for improving the solid-gas contact and for achieving the increase of the gas conversion in a fluidized bed for a given space time and without eliminating any of its advantages. Some of these are aa follows: (a) the installation of devices inside the fluidized bed for breaking the bubbles; these devices can consist of horizontal or vertical internals (screens, tubes, rods, perforated plates) (Jodra et al., 1979a, 1979b; Harrison and Grace, 1971; Furusaki, 1973; Rooney and Harrison, 1976; Fujikawa et al., 1976; Claus et al., 1976; Guigon et al., 1978), or floating bubble breakers (Keillor and Bergougnou 1976); (b) the introduction of a tube with fixed packing in the fluidized bed with which Laguerie et al. (1973) increased the efficiency of butane oxidation in a fluidized bed; (c) the use of gas velocities very close to the minimum fluidizing velocity or, on the other hand, sufficiently high for the existence of parallel entrainment of solids, with which one may enable the gas flow to be close to the piston type, ~ F Iin the fluid catalytic cracking in which the reactor first of all consists of a riser in which the greater part of the gas conversion takes place (Venuto and Habib, 1978). In a previous work, the improvement of solid-gas contact in a fluidized bed was achieved with internals consisting of arrangements of horizontal tubes. Methods for quantitatively correlating the design parameters of the internals 01964305/83/1122-0329$01.50/0
with the effects produced, breakage of the bubbles (Jodra et al., 1979a), or increase of the conversion (Jodra et al., 1979b), have been previously described. To increase the efficiency of solid-gas contact maintaining the advantages of a fluidized bed, a new type of contactor, which may be designated fluidized/ (fixed or fluidized bed), is hereby proposed. It is basically constituted by two beds in the same vessel, without any separating device, produced by a considerable increase of the cross section in its upper zone, Figure 1. According to the fluidodynamic study of this contactor (Corella and Bilbao, 1982),when the system is discontinuous for the solid, the contador can operate: (a) as a fixed bed, when u1 < u, (u, = bed breaking velocity); (b) as a fluidized/ fixed bed with intermediate gas chamber, when u, < u1 < u, (u, = velocity at which discontinuity disappears); (c) as a fluidizedffixed bed without intermediate gas chamber, when u, < u1 < ud,= (umf,== minimum fluidizing velocity in the upper zone); or (d) as a fluidized/(fluidized bed with different bubbling), when u1 > Umf,m*
From the gas pressure drop aspect, the contactor shows its greatest advantages for u1 > u,. But if u1 >> ude the bubbling in the upper zone of the contador is very vigorous and the gas flow in this zone also deviates from the piston type. Therefore, in this work, the experiments with chemical reaction have been carried out at 1'5umf,, > u1
> u,.
The gas conversions obtained here with this type of reactor are compared with those obtained in an isothermal fixed bed with gas piston flow and in a conventional 0 1983 American Chemical Society
330
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983
Figure 1. Diagram of the new type of contactor. XA
1.0
-
piston flow
b a c k m i x flow
Figure 3. Experimental assembly: 1, pressure supply; 2, feed supply; 3, capillary flowmeter; 4, preheater; 5, reactor; 6, external fluidized bed; 7, condensers; 8, liquid product collector.
I I I I
I
W / FA
Figure 2. Curves of X, against WlF, for piston flow, backmix flow, fluidized bed, and the new type of contactor.
fluidized bed under the same working conditions (gas superficial velocity at the inlet, particle diameter, and space time).
Efficiency of the Contactor The efficiency of the enlargement of the cross section of the contactor can be defined in several ways. In this work, two different definitions have been used. First, it is defined as the ratio between the increment of gas conversion which is obtained related to a fluidized bed and the increment of gas conversion which would be obtained if the gas flow was a piston type
these values of X are shown with precision in Figure 2. According to this definition, the maximum efficiency (E, = 1)would be obtained when the gas flow in the contactor is of a piston type (X,,r. = Xp.f.);the minimum value of efficiency (E, = 0) would be when the gas flow is similar to that of a cylindrical fluidized bed (Xo.r.= xf.b.). This definition of efficiency has the disadvantage that, for a given u/umf,Xf.b. is not unique and it is dependent on the geometry of the fluidized bed. Therefore, xf.b.must be taken as a comparative value, with respect to it the increase in gas conversion is quantified. To remove this limitation, the efficiency of the contactor can also be defined as the ratio between the gas conversion obtained in the contactor and the gas conversion which would be obtained if the gas flow was a piston type E2 = [X,r./Xp.f.l
W/F*o,t = 0
(2)
This definition of the efficiency shows the advantage, with respect to the first definition, that Xp.f.is a unique value, but it has the disadvantage that it does not quantify the increase in gas conversion related to a fluidized bed. In this work, both definitions of efficiency have been used as complementary values. For the calculation of the efficiency, the X, vs. W/F& curves must be previously known, both for an ideal piston flow and for a cylindrical fluidized bed. The same gas velocity must be kept at the inlet of the new contactor and of the fluidized bed. Equipment and Reactors To study the effect of the variation of the cross section of the bed on the solid-gas contact, the gas conversion at the outlet has been experimentally determined for three types of reactors: an isothermal fiied bed with gas piston flow, a small cylindrical fluidized bed, and the present fluidized bed contactor with varying geometry. The standard reaction chosen is the gas-phase dehydration of 2-ethylhexanol to olefins and water, with a silica-alumina catalyst. All the experiments have been carried out at 240 "C.The feed of the reactor is 100% pure alcohol at a total pressure of 740 mmHg. Under these conditions, the formation of the aldehyde has not been detected with 2,4-dinitrophenylhydrazine.In addition, analyzing the products of the reaction by gas chromatography, only traces of ether formed by intermolecular dehydration have been observed. Therefore, the reaction responds to the model: A + R + S. This reaction can also be considered irreversible as, at 240 OC,ita equilibrium constant has the value of 2700. Its heat of reaction is small, AHo513K = 13.8 kcal/mol, so that, in the experimental fmed bed, the isothermal condition was easily reached. The installation used for determining the gas conversion in the three types of reactors under different conditions is shown in Figure 3. Briefly, it consists of a device which allows one to introduce the reactant with a constant and predetermined flow for each experiment, of the preheater-reactor, and of a system for condensation of the products and for a periodical sample collection. The liquid product of the reaction was periodically analyzed for determining the gas conversion, X,. This was done by
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983 331 I
0.2
I
\
.
"
Isothermal fixed bed - p i s t o n f l o w
1
2
3
4
5
6 W/F A,
0 0
15
30
45
60
75 t (min)
Figure 4. Gas conversion as a function of the reaction time in an isothermal fixed bed with piston flow, for different W/Fb,at 240 OC.
measuring the concentration of water with Karl Fischer reagent which offered good reproductivity (Olmo and Banda, 1967). The product of the reaction was previously homogenized with absolute ethanol. All the reactors used were constructed of Pyrex glass. In order to guarantee the isothermal condition inside the reactors, their walls were maintained at a constant temperature. For this purpose, the preheater-reactor was submerged in an 80 mm diameter external fluidized bed with corundum particles of -250 +lo0 pm (Figure 3). The catalyst of silica-alumina was supplied by Akzo Chemie, L.A. 300 type. A sample of this catalyst of -300 +200 pm, fluidized with air during 10 h at a temperature of 240 OC and a t a gas velocity of u = 6umf,did not show any noticeable diminution of size by erosion. It can, therefore, be used in experiments of short duration in a fluidized bed without changing the particle size and, therefore, its umf. On the other hand, this catalyst shows a small deactivation, and the gas conversion diminishes linearly with the reaction time, as can be seen in Figure 4 with the experiments carried out in the fixed bed. This allows one to calculate, easily and with precision, the gas conversion at zero time, (X&, by extrapolation to the origin.
Gas Conversions Reached in the Three Reactors Experiments were carried out in an isothermal fixed bed in order to use, as a reference, the gas conversion obtained for gas piston flow. In these experiments special care was taken to reach such ideal flow and isothermicity. The isothermal condition was achieved with a reactor having a small diameter, 11.8 mm, in which the catalyst was diluted to 50% wt with inert Pyrex glass particles of the same size. The temperatures were measured at the center of the bed and at the wall. On the other hand, when L = 10 cm, according to Levenspiel (1972) the modulus of dispersion (D,/uL) was 0.004 and the ratio N , length of the reactor/particle diameter, was 290. With these values the gas piston flow can be assumed. The results obtained for the fixed bed with gas piston flow are shown as the upper curves of Figures 5,6, and 7, and they will be used as a reference. Gas conversion in a small cylindrical fluidized bed has also been obtained. The experiments have been carried out varying the space time, the linear velocity of the gas, and the particle size or the minimum fluidizing velocity, ud, of the solid. The reactor was made of glass, 35 mm in diameter, and the bed support was a porous plate. In Figures 5,6, and 7 the gas conversion obtained at zero time is shown as a function of the space time. Each figure
Figure 5. Gas conversions,at the outlet of the reactor as a function of the space time in a fluidized bed with d, = -300 +200 pm; parameter: u/u&
0
1
2
3
4
5
6 W / FA
Figure 6. (Xb)o vs. W/Fb, in a fluidized bed for d, = -200 +160 pm, parameter: u/u&
I
I
Ix. \
Qy, v
1
Key
,
I
,
2
3
4
5
u/u
6 W / FA 0
Figure 7. (X& vs. W/Fb, in a fluidized bed for d, = -160 +lo0 pm,parameter: u/ud.
is obtained with a different size of the solid. The sizes vary between -300 +200 pm (umt= 3.0 cm/s), Figure 5; -200 +160 pm (umt= 1.8 cm/s), Figure 6; and -160 +lo0 pm (ud = 1.0 cm/s), Figure 7. Each curve corresponds to a linear velocity of the gas a t the inlet, given as u/umr. Similar curves are obtained taking as parameter (u - u d ) or the average bubble size. The experimental points have been obtained in the region of high gas conversions, with high space times. In this region, bubbling in the fluidized bed has more important effects on the deviation of the ideal piston flow. To obtain the gas conversion and to calculate the efficiency of the new type of contactor, different Pyrex glass reactors have been used. These reactors differ in their characteristic dimensions (Figure 1). All the reactors have
332
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983
Table I. Gas Conversions and Values of the Efficiency for d, = -300 5 g.h/mol of A (X, f = 0.985)
+ 200 pm (umf = 3.0 cmls) and WIFA,=
~
u/Umf 4
reactor R-121214 R-61214 R-61212 R-61416.25 R-101213 R-121214 R-61214 R-61212 R-61416.25 R-101213
XOJ. 0.954 0.962 0.946 0.966 0.949 0.923 0.931 0.909 0.946 0.915
Xf.b. 0.887 0.887 0.887 0.887 0.887 0.813 0.813 0.813 0.813 0.813
a diameter of 16 mm in their lower zone. Their specification includes three parameters: length of the lower zone (ITl4, in cm), length of the tronconical zone (HH, in cm), and the ratio between sections (S/s) (Figure 1). Thus, the reactors designated R-12/2/4, R-6/2/4, R-6/4/6.25, R6/2/2, and R-10/2/3 have been used. In the different experiments carried out with this type of reactors the variables were the particle diameter of the catalyst, the gas velocity, given by u/ud, and the space time. HH will depend on the values of these last two variables. The experiments have been carried out at relatively high space times to obtain high gas conversions, a zone in which the type of gas flow has the most influence on the gas conversion*(Figure2). The (u/ud) values used in these experiments have been 4 and 6. The u/ud = 8 value haa not been used because with this value u1 ?> u-, the bubbling in the upper zones of the contactor is great and the gas-solid contact in these zones is not very good. The gas conversions obtained with the new type of contactor, in experiments carried out with d, -300 +200 pm (ud = 3.0 cm/s) and W/F4= 5 gh/mol of A, are shown in the third column of Table I. With this space time, the gas conversion obtained in the isothermal fixed bed with gas piston flow was X,, = 0.985. The gas conversions obtained in the fluidized bed under similar conditions are shown in the fourth column of Table I. In all the experiments, the gas conversions obtained with this new type of contactor are greater than those obtained in a fluidized bed which works at the same gas velocity at the inlet of the reactor and with the same space time, but they are smaller than those corresponding to a isothermal fixed bed with gas piston flow. With the different gas conversion values and from eq 1 and 2 the efficiency of the contactor has been calculated. The efficiency values are shown in the fifth and sixth columns of Table I.
Dependency of the Efficiency with the Contactor Geometry and with the Gas Velocity Efficiency values of the contactor have been correlated with the contactor geometry and with the gas velocity at the inlet. The increase of gas conversion obtained in our reactor, compared with a conventional cylindrical fluidized bed, is due to the fact that the catalyst placed in the tronconical and in the upper zones is under different conditions than those which would exist if the bed were a cylindrical one. In both zones indicated above and due to the increase in cross section, the gas velocity is smaller than it is in a cylindrical bed, with the result of a better solid-gas contact and a consequent increase in gas conversion. Therefore, the bigger the solid fraction placed in the zones where the gas velocity is smaller, the bigger the gas conversion will be.
El
0.684 0.765 0.602 0.806 0.632 0.640 0.686 0.558 0.773 0.593
E2 0.969 0.977 0.960 0.981 0.963 0.937 0.945 0.923 0.960 0.929
X0.L0 0.550 0.763 0.425 0.920 0.482 0.493 0.576 0.306 0.937 0.394
In this work, it is considered that the variable which is going to determine the increase in gas conversion is the ratio between the solid weight fraction in the zone i-j and the average reduced gas velocity in the same zone
where yi-, = Wi-j/ W and u R , = The gas pressure drop through the bed and the change of gas volume due to chemical reaction make the linear gas velocity vary throughout the reactor. However, as the reactors used are small, the first cause can be neglected. The second cause affects the different reactors studied here in the same way. Therefore, the calculation of the two terms of eq 3 will be carried out without considering these effects in order not to complicate the calculation. Taking as the gas velocity of reference the one at the inlet of the reactor, uR1= ul/umf,the terms implicated in eq 3 will be as follows. (a) Our Reactor (0.r.). It is considered divided into three zones: 1-4, 4-5, and 5-6 (Figure 1)
where
(7)
which substituted in eq 4 gives
The values of this parameter are shown in the seventh column of Table I. (b) Cylindrical Fluidized Bed (f.b.). Without considering the effect of the gas pressure drop and of the change of gas volume due to chemical reaction, the superficial velocity of the gas throughout the fluidized bed
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 2, 1983
0.r.
333
The values of these parameters are shown in the third and seventh columns of Table I. From these values, the following equations are obtained for ul/umt = 4
f.b.
0.20
Xo.1. =
0.2
01
0.406
10
for ul/umf = 6
L*$L5.M y5-6 - 1
Xn.r. =
"l/Umt
Figure 8. Increase of gas conversion in our reador with respect to a fluidized bed, as a function of the parameters of the syetem.
will be constant and equal to the one at the inlet of the reactor
From the definition of efficiency of the contactor, eq 2, and with Xp.f.= 0.985, it is obtained that for ul/umf = 4 E2
= 1.00
[(
y1-r
2M
+M + 1y4-5
0.028
Substituting eq 8 and 9 in eq 3 (X0.r. - Xf.b.)
for ul/umf= 6
a
0.036
The values of the parameters of eq 10 are represented in Figure 8. From this figure the following equations are obtained for ul/umf= 4 (X0.r. .. - X f d =
2M
for u1/u& = 6 (xo.~.- xf.b.) =
[
0.148 (Y14
2M
114
+ m y 4 - 5 + MY54 - 1 ) / ( U l / U m f ) ] (12)
On the other hand, from experimental values, it is obtained that for u1/umf = 4 (X,f, - Xf.b.) = 0.098 (13) for
U1/Umf
=6
(X,L - Xf.b.) = 0.173
(14)
With eq 11,12,13, and 14 and from the first definition of the efficiency eq 1,there is obtained for both gas velocities
(15) For correlating the second definition of efficiency, eq 2, with the system parameters, it is also assumed that
(20) Notice that with this definition of efficiency, eq 19 and 20 are different depending on the gas velocity at the inlet.
Suggestions for the Design of the Contactor If this contactor is going to be used as a chemical reactor, it should be designed so that the velocity of the gas in the upper zone will be equal to or slightly higher than that of the minimum fluidization in that zone, At lower velocities the gas pressure drop can be very high and the upper zone will be a fixed bed, it being possible, therefore, to produce high-temperature gradients in that zone. On the other hand, the bigger the gas velocity in the upper level of the bed, 246 with respect to u & , ~the ~ , bigger the bubbling will be in the upper zone; the gas flow in that zone separates itself from that of the piston type, and the contactor quickly loses efficiency. The equations proposed here correlate the efficiency of the contactor with the geometry of the bed but without minding the scale, something which should be kept in mind in the design of a large contactor. In this, the ratios yi-j and M can be the same as in our small contactor. However, if the difference in diameters between the upper zone and the lower one is big, there could appear a dead zone in the tronconical zone and thus the efficiency can be less than that given by eq 15. For avoiding these dead zones in large size contactors, the angle of the tronconical zone should be at least 70° and the introduction of internals in the said zone is recommended (Corella et al., 1981, 1983). Regarding the geometry of the contactor, the bigger the weight fraction of solid in the lower fluidized zone, the bigger the gas conversion will be at the outlet of the lower zone and the dissipation of heat to the exterior will be better facilitated due to its being fluidized, but however less gas conversion at the outlet of the contactor, there will be obtained eq 15, 19, and 20. If the weight fraction of solid in the lower zone diminishes, greater gas conversion at the outlet of the reactor will be obtained, eq 15, but the amount of heat to be dissipated in the upper zones will be very large and radial gradients of temperature can be
334
Ind. Eng. Chem. process Des. Dev. 1003, 22, 334-339
produced in that zone. Therefore, to determine the contactor dimensions, a compromise between temperature control and high efficiency must be found. In the particular instance of this work, it is suggested that the height of the lower zone must be the required one to obtain a gas conversion a t the range of 0.50.60. Nomenclature A, R, S = 2-ethylhexanol,olefins, and water De = axial dispersion coefficient d = particle diameter I$ = efficiency of the contactor, eq 1 E2= efficiency of the contactor, eq 2 FA0 = molar flow of A at the reactor entrance HiTj= height of the i-j zone i, J = points 1, 4, 5, or 6 in Figure 1 L = length of reactor M = ratio between sections of the upper and lower zones S = cross section of the upper zone s = cross section of the lower zone t = reaction time, min u = linear gas velocity u, = velocity at which the discontinuity in the bed disappears ui = gas velocity at point i u d = minimum fluidizing velocity ud,= = minimum fluidizing velocity in the upper zone u, = bed breaking velocity U R = reduced gas velocity (u/umf) W = weight of the catalyst Wi-j = catalyst weight in the i-j zone WlF, = space time, g-hlmol of A (X& = gas conversion at the outlet of the reactor at zero time
X
= gas conversion with gas piston flow 4 : = gas conversion with cylindrical fluidized bed X,,.= gas conversion with the new type of contactor
yi-, = weight fraction of the solid in the i-j zone
Literature Cited Claus, G.; Vergnes, V.; Le Goff, P. In "Fluldlzation Technology"; Keakns, D. L., Ed.; Hemisphere Publishing Corporation: Washington, 1976; Voi. 11, p 87. Corella, J.; Ellbao, R. Id.Eng. Chem. process Des. Dev. 1982, 21, 545. Cordla. J.; Bllbao, R.; Aznar, M. P. Proceedings of the 2nd World Congress of Chemical Englneerlng, Montreal, Canada, oct 1981; Voi. 111, p 36. Coreila, J.; Ellbao, R.; Lezaun, J.; Monzh, A. Proceedings Fowth IntemaUonal Conference on Fluidkatlon, KashlkoJlm, Japan, May 1983. Fujlkawa, M.; Kugo, M.; Sa@, K. I n "Fluldlzatkm Technology"; Kealrns, D. L., Ed.; Hemisphere Publishing Corporation: Washington, 1978; Vol. I. p 41. Furusaki, S. A I C N J . 1973, 19, 1009. Gulgon, P.; Large, J. F.; Eergougnou, M. A.; Eaker. C. G. J. In "Fluldlzetbn"; Davldson, J. F.; Keakns, D. L., Eds.; Cambridge University Press: London, 1978; p 134. HanJSOn, D.; Grace, J. R. In "Fluldhtbn"; Davideon, J. F.; Harrison, D. Eds.; Acedemlc Press Inc.: London, 1971; Chapter 13. Jodra, L. G.; Corella, J.; Aragon, J. M. Int. Chem. Erg. 1#7#r, 19, 654. Jodra, L. G.; Corella, J.; Aragon, J. M. Int. Chem. Eng. 1979b, 19, 664. Kelllor, S. A.; Bergougnou, M. A. In "Fluldkatkn Technology"; Keakns, D. L., Ed.; Hemisphere Publlshlng Corporation: Washington, 1978; Vd. 11, p 45. Laguede, C.; Mdinler, J.; Angellno, H. In "Fluldlzation and Its AppNcetions", Congress of Touiouse, oct 1973 Vol. 11, p 237. Levenspiel, 0. "Chemical Reaction Enginwrlng", 2nd ed.; Wiley: New York, 1972. Olmo, A. B.; Qarcla de la Eanda, J. F. An. Quim. (Spin) 1967, 63, 179. Rooney, N. M.; Harrison, D. I n "Fluldkatkn Technology", Keakns, D. L., Ed.; Hemisphere Publlshlng Corporatlon: Washington, 1976, Voi. 11, p 7. Venuto, P. E.; Hablb, E. T. Catel. Rev. Scl. Eng. 1978, 18, 15.
Received for review February 6, 1980 Revised manuscript received May 26, 1982 Accepted September 8, 1982
Model for the Gas Flow in a Fluidized Bed with Increase of the Cross Section in Its Upper Zone JOG Corella" and Rafael Bllbao Departemento de Qdmica TBCnlca, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain
A gas flow model is developed to calculate the gas conversion at the exit of a reactor consisting of different beds in the same vessel, without any separating device, originated by a noticeable increase of the cross section in its upper zone. The calculation of the gas conversion is carrled out by adapting the known models for fluidized and flxed beds according to the different statm in which the contactor can operate depending on the gas velocity. The model Is tested by means of experiments in which the contactor dimensions, the particle diameter of the catalyst, and the gas velocity are varied. In these experhnents gocd agreement between the experimental and the theoreticai values predlcted by the model was obtained.
Introduction An enlargement of the cross section in the upper zone of a fluidized bed results in a reduction of the gas velocity in ita upper zone. The gas velocity in the lower zone of the contactor, Figure 1, is greater than ud, but in the upper zone it is less, equal to, or greater than umf. This contactor offers several advantages for the catalytic and noncatalytic gas-solid reactions and for drying processes. According to its fluidodynamic behavior (Corella and Bilbao, 1982a), when the system is discontinuous for the solid, the contactor can operate: (a) as a fixed bed, when u1 < u, (u, = bed breaking velocity), (b) as a fluidized/fiied bed with intermediate gas chamber, when u,
umf,=. For gas-solid reactions, the gas conversion at the contactor exit must be calculated in a different manner depending on the state of the contactor. If it is a fixed bed (u, < u,), the corresponding design equations are applied. If it operates as a fluidized/fiied bed with an intermediate gas chamber (u, < u1 < u,), the gas conversion can be calculated as two reactors in series: the f i i t one a fluidized bed and the second one a fixed bed.
Q19&43Q5/83/1122-Q334$Q1.50/Q 0 1983 American Chemical Society
