1132
Ind. Eng. Chem. Res. 1988, 27, 1132-1135
Colorimetric Evaluation of the Efficiency of Liquid-Solid Contacting in Trickle Flow C6sar L. Lazzaroni, Hugo R. Keselman, and Nora S. Figoli* Znstituto de Investigaciones en Catdlisis y Petroqulmica, INCAPE, Santiago del Estero 2654, 3000 Santa Fe, Argentina
A colorimetric method to evaluate directly the efficiency of liquid-solid contacting under trickle-flow conditions by adding a coloring reactant t o the liquid current wa8 developed. Radial distribution of liquid and the efficiency of liquid-solid contacting for different liquid and gas flow rates were determined. Results were compared with those obtained by using a collector radially divided into three sections. T h e air-water system was used. Trickle-bed reactors are nowadays widely used in oil industry for the hydrotreatment of several cuts and also in other chemical processes. In these reactors, the liquid and gas phases flow concurrently downward through a fixed bed of catalyst particles while reaction takes place. From many studies carried out during the last few years, evidence arises that it is necessary to know what fraction of the catalyst is really wetted (contacting or wetting efficiency) to predict the performance of a trickle-bed reador and to carry out its design. Until 1975, most of the studies about wetting efficiency and the correlations obtained were for packed beds, absorption towers, etc., using particles of large diameters and liquid mass velocities considerably greater than in trickle-bed reactors (Shulman et al., 1955; Yoshida and Koyanagi, 1958, 1962; Puranik and Vogelpohl, 1974). Mears (1974) and Satterfield and Ozel (1975) demonstrated the importance of the problem, but they questioned the validity of the correlations obtained by the previous authors when applied to trickle-bed reactors. Satterfield and Ozel(1975), based on data from Bondi (1971),defined the wetting efficiency as the ratio between the reaction rate constants obtained in trickle-bed and stirred tank reactors. A similar definition was proposed by Koros (1976), Montagna et al. (1977), and Van Klinken and Van Dongen (1980), among others. Schwartz et al. (1976) is the first author who measured the wetting efficiency by using a two-tracer method (one adsorbable and the other nonadsorbable). Mills and Dudukovic (1981) found that the method suggested by Schwartz et al. (1976) corresponded to internal wetting efficiency, and they obtained additional data by using the Schwartz et al. method. Mills and Dudukovic (1981) calculated the external wetting efficiency following the method of Colombo e t al. (1976)) who determined the external wetting efficiency by the analysis of the response to the addition of a tracer and defined the external wetting efficiency as the ratio between the effective diffusivity coefficients in trickle-bed and stirred tank reactors. Specchia and Baldi (1978) used a different method. They measured the dissolution of phthalic anhydride in aqueous potassium carbonatebicarbonate solutions. From solutibility measurements and the physicochemical constants of the system, they calculated the wetting efficiency as the ratio of the effective area for liquid-solid mass transfer to the total external area of the particles. This method is strongly dependent on the selected hydrodynamic model. In this paper, a technique is developed to determine directly the external efficiency of liquid-solid contacting in trickle flow, thus letting us know not only overall but also radial wetting efficiencies, as well as radial liquid distribution. Moreover, the formation of vicious channels 0888-5885/88/2627-1132$01.50/0
and the irregular manner of the particles wetting of each radial fraction can be visualized. Wetting efficiency is defined as the fraction of the external area of the packing that is wetted by the liquid under dynamic conditions. The technique is based on the coloring of particles of the bed with a coloring reactant introduced in the liquid current; in this way, a colored zone, representative of the liquid-solid contacting area, results. The colored portion is evaluated by using a densitometer registering the color intensity.
Experimental Section Experiments were carried out in a glass column of 30.5-mm internal diameter using spherical particles (mesh 6-8) of y-alumina calcined a t 1100 OC, supported on a perforated metallic disk. The air-water system in a range of flow rates from 0 to 0.08 kg m-2 s-l for the gas, and 1.90 to 5.70 kg m-2 s-l for the liquid, was used operating at atmospheric pressure and room temperature. Water and air were introduced in the column by PVC flexible tubes; the liquid passed through a tube going to the center of the transversal area of the bed, and the gas passed through four holes distributed in a ringlike pattern. Measurement of both flows was carried out by using rotameters previously calibrated. In order to assure a reproducible and uniform distribution of particles, the different charges were made with the column soaked. The column was then completely dried by passing a warm air stream through it during 10 h. Since the liquid enters through the center of the bed, all the experiments were carried out with a previous bed (pre-bed) of 50 cm containing particles of the same size and shape in order to obtain a radial liquid distribution of equilibrium (as indicated by previous results of Keselman et al. (1988)). During the experiments, two operating modes (both with recirculation of the liquid), according to the following procedure, were used. (a) Initially Dried Bed (D.B.). Once the gas flow rate has been set a t the desired value, liquid is introduced a t an established flow rate and recirculated until the system is stabilized (stationary state); the coloring reactant is then added to the liquid and recirculated for a time t. A standard is simultaneously obtained, introducing a sample of y-alumina particles in a fraction of the colored liquid that is recirculating;this is performed during the same time ( t ) that the previous operation lasted. Such standard is used like a reference to evaluate the colored area percentage. (b) Initially Flooded Bed (F.B.). At the start of the experiment, the bed is liquid filled after the liquid flow rate is set a t the working value, the bed is gradually drained, and the gas is then introduced a t the desired flow 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1133 100
,I
1
4
v)
c
‘g
7
e
I
I
PO
40
,b
Y
I
J,400
T i m e , min
Figure 1. Percent color intensity (I,) as a function of the time of contact between particles and coloring reactant, for different reactant concentrations (C,). C,, = 0.20 g L-l; C,, = 0.60 g L-l; C,, = 12.5 g L-1.
rate. Once the system is stabilized, the procedure follows like in the previous case (D.B.). By adding the coloring reactant (Chrome-Azurol-S) to the liquid current and once a contact with the bed is established, Chrome-Azurol-S reacts with the aluminum of the alumina leaving the contacted area with a reddishviolet color. The reaction is irreversible, showing no change in the color of the particles when the feed in switched again to clear water. When the passage of the washing liquid is stopped, the bed is drained and dried with an air stream. Later on, the bottom of the tube is closed and the bed is soaked with a 3% agar-agar solution; after this solution is solidified, the bed with motionless particles is removed. To determine the radial liquid-solid containing efficiency, the bed is divided into three concentric sections, central, medium, and external (Sc, Sm, and Se), of 11-, 21-, and 30.5-mm external diameter, respectively. Particles to be analyzed are released from the adhered agar using hot water and then are dried a t 120 OC. The same procedure was made with the standard sample. The area of each particle that was in contact with the liquid shows an homogeneous reddish-violet color, while the part that was not in contact with the liquid is colorless. The color intensity, related to the wetted and dried areas, determined in a Chromoscan MK I1 recording and integrating densitometer, is a measure of the wetting efficiency. Optimal results will be obtained if the appropriate filter (complementary color of the sample) is selected. Under these conditions, a satisfactory linearization may be achieved subject to the limits of the particular application. In order to set the time ( t )during which the coloring reactant must remain in contact with the bed, as well as its concentration, some experiments were performed to find out color intensity as a function of time for different coloring reactant concentrations. Particles were immersed in coloring reactant solutions of several concentrations during different periods of time. Results are shown in Figure 1 where color intensity (I,) vs contact time of the solution of coloring reactant with particles can be visualized. As can be seen, if operated during a time longer than 15 min, the error of a possible variation in the coloring of particles after interrupting the reaction is minimized. In addition, there is an indication that a coloring reactant concentration of 0.2 g 1-1 gives a color intense enough to permit evaluation of the wetted area. This concentration was used in our experiments, during 20 min. A volume of solution large enough to avoid coloring reactant concentration variations with time was used. By use of fixed coloring reactant concentration and time of contact, all the particles show the same color intensity. A calibration curve was previously made. Noncolored particles represented nonwetted particles ( e , = 0). Completely colored particles (at fixed coloring reactant con-
e, %
Figure 2. Calibration curve of densitometer versus wetting efficiency. Coloring reactant concentration: 0.20 g L-l. Time of contact: 20 min.
centration and time) represented 100% wetting efficiency (e, = 100%). Intermediate points were obtained, mixing known amounts of noncolored and completely colored particles. The calibration curve is shown in Figure 2. Densitometer readings are normalized and expressed in arbitrary units; Figure 2 shows the linearity of the readings. The densitometer data for each studied sample are transformed into wetting efficiency values as follows:
A-C 100 = e, (%) B-C where A = reading for partially wetted sample, B = reading for totally wetted sample (standard), and C = reading for nonwetted sample. Each determination was made with a representative sample (15 cm3) of each section (external, central, and medium). The wetting efficiency is homogeneous along the bed because a pre-bed of 50 cm was used, as previously mentioned. The sample was placed in a rectangular sample holder of 20 cm X 1 cm X 1 cm. Each given value is the average of three determinations of the same sample. For each determination, the position of the particles in the sample holder was modified.
Results and Discussion Results obtained from the evaluation of the percentage of wetting efficiency, starting from F.B., along the radius for different gas and liquid flow rates are shown in Figure 3. Without gas flow, a uniform radial wetting efficiency is obtained, while if the gas flow rate is increased an increase in the wetting efficiency of the central section can be observed simultaneously with a decrease in that of the external one; that is to say, gas flow causes a distortion in the uniformity of the radial wetting distribution. Overall wetting efficiency (e,) as a function of the liquid flow rate for different gas flow rates is shown in Figure 4 where it can be clearly observed that wetting efficiency increases with the liquid flow rate whereas it decreases with gas flow rate. A remarkable difference in the wetting efficiency depending on the operating mode used (F.B. or D.B.) can be observed in the same figure. e , is less dependent on the liquid flow rate when starting from D.B. Little information is available in the literature to compare with our results, since most of studies have been made with particles of different size and shape and without gas flow. Some authors, as reviewed by Herskowitz and Smith (1983), have reported the effect of liquid flow rate on the external wetting efficiency. They found that normally the external wetting efficiency ranges from 0.75 to 1.0, for liquid flow rates similar to those used in this work.
1134 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
50 I
t
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90
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0
100 r
01 00
I
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I
3 80
5 70
/
Considering a thickness of liquid film remaining constant for the whole transversal section of the bed, radial distribution of the liquid was calculated with the following expression:
20
e, (%) = C ew,siSi(%) i=c,m,e
I
(2)
Results were compared with those obtained with an annular collector. The collector employed, similar to that used by Herskowitz and Smith (1978),consisted of annular bronze rings dividing the cross section of the column into the same three sections (Sc, Sm, and Se) mentioned above. The upper edge of the rings were beveled to a fine line in order to minimize the metal area at the top of the collector and to reduce the disturbance to the flow distribution caused by the collector rings. The bed was supported in the rings with pins of spherical head of similar diameter to the particles to permit the liquid discharge of each section without obstructions, in order to prevent overflow of liquid from one annular section to an adjacent one. The same flow rates and pre-bed height were used. Resulb are shown in Figures 5,6, and 7 where it can be observed that,
08
I
I
I
I
c
00
(1)
where
I
P /
1
1
i = c, m, e
I
04
s-l
40
ewsi (% ) Si (%) e, (%)
I
Figure 5. Percent radial liquid distribution versus gas flow rate starting from F.B. Dashed line: data from collector. Full line: data from colorimetric method. A = central section (Sc); B = medium section (Sm); C = external section (Se). Liquid flow rate: 1.90 kg
L , kgW2 s-' Figure 4. Percent wetting efficiency (e,) versus liquid flow rate ( L ) , for different gas flow rates. Dashed line: starting from D.B. Full lines: starting from F.B. Go = 0; G1= 0.04;Gz = 0.06; G3= 0.08 (kg m-2 8-l).
LSi (%) =
1
1
G kgm-2s-
m-2
I90
1
t@
Figure 3. Percent wetting efficiency (ewJ) starting from F.B. versus radial position for different gas flow rates (G). Go = 0; G1= 0.04; Gz = 0.06;G3= 0.08 (kg m-z 8-l). (A) Liquid flow rate = 1.90 kg m-l s-'; (B) liquid flow rate = 3.80 kg mT2s-'; (C) liquid flow rate = 5.70 kg m-2 s-l.
01 0
I
1
15
10
r ,mm
I
I
0.4
1
G kgm-2s-i
08
Figure 6. Percent radial liquid distribution versus gas flow rate starting from F.B. Dashed line: data from collector. Full line: data from colorimetric method. A = central section (Sc); B = medium section (Sm); C = external section (Se). Liquid flow rate: 3.80 kg m-2
s-l
in case of liquid flow rates of 1.90 and 3.80 kg m-2 s-l, there is a great similarity in the liquid distribution values of each section obtained by both techniques for the whole range of gas flow rates tested. When working with a liquid flow rate of 5.70 kg m-2 s-l, very different values are obtained by both methods in the presence of gas flow in the medium and external sections. If the wetting efficiency of each section, for the last liquid flow rate, is calculated using radial liquid distribution values obtained with the collector, and taking into account the same overall efficiency, values higher than 100% will be obtained for medium and central sections. This result is observed in the central section only when the gas flow rate is 0.08 kg m-2 s-l. But this does not really take place because when making visual evaluation,
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1135 60 I
I
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1I
L
1
I
I
I
1
0
190
3 80
5 70
L ,kg r f Z s'' Figure 7. Percent radial liquid distribution versus gas flow rate starting from F.B. Dashed line: data from collector. Full line: data from colorimetric method. A = central section (Sc); B = medium section (Sm); C = external section (Se). Liquid flow rate: 5.70 kg m-2 s-l.
it can be observed that none of the particles is completely wetted. For this reason, it can be inferred that, in the case of high liquid flow rates in the presence of gas, radial distribution of liquid undergoes a distortion when entering the collector used in our experiments. When values of liquid distribution obtained from the radial wetting efficiency (Figure 8) are taken into account, it can be deduced that the variation in the liquid flow rate does not practically affect the liquid distribution values. However, the variation of the gas flow rate modifies the liquid distribution. When the gas rate is increased, the radial liquid distribution is increased in the central and medium sections, while it is decreased in the external section. Radial liquid distribution is homogeneous when the gas is absent. Similar observations were made by Herskowitz and Smith (1978), although they concluded that the variations of both liquid and gas rates did not measurably influence the liquid distribution. When the Herskowitz and Smith (1978) model was applied to our system, a similar radial liquid distribution (without gas) was obtained. Results of the application of the model are shown in Figure 8.
Conclusions From the results obtained, it can be concluded that the colorimetric technique proposed offers important advantages when compared to other techniques in use. On one side, it allows us to directly, by experiments with a low dispersion of results (lo%),make longitudinal and radial evaluations of the wetting efficiency and, a t the same time, obtain information about the overall wetting efficiency. From these data, radial distribution of the liquid can be obtained without errors due to distortion in the flow. It is very important to know the usefulness of different collectors, limiting the working range adequate to them. Moreover, when a macroscopic evaluation of particles tested is carried out, the morphology of the wetting can be visualized, thus helping us to understand the fluidodynamics of the system.
Figure 8. Percent radial liquid distribution as a function of the liquid flow rate for different gas flow rates starting from F.B. Go = 0; GI= 0.04;G2= 0.06;G3= 0.08 (kg m-2 s-l). Dashed line indicates homogeneous distribution. A = central section (Sc); B = medium section (Sm); C = external section (Se). (---) Following Herskowitz and Smith (1978)model: S = 1; f = 0.5.
Nomenclature C, = coloring reactant concentrations D.B. = dried bed e, = overall wetting efficiency = radial wetting efficiency e,,si = wetting efficiency of each section F.B. = flooded bed G = gas flow rate I , = color intensity L = liquid flow rate LSc = liquid flow rate of central section LSe = liquid flow rate of external section LSm = liquid flow rate of medium section r = bed radius Sc = bed central section, volume of section/total volume Se = bed external section, volume of section/total volume Sm = bed medium section, volume of section/total volume Literature Cited Bondi, A. Chem. Technol. 1971,3,185. Colombo, A. J.; Baldi, G.; Sicardi, S. Chem. Eng. Sci. 1976,31,1101. Herskowitz, M.; Smith, J. M. AZChE J. 1978,24, 439. Herskowitz, M.; Smith, J. M. AZChE J. 1983,29, 1. Keselman, H.R.;Lazzaroni, C. L.; Figoli, N. S., to be submitted to Latin Am. Res. J. 1988. Koros, R. M. Chem. Eng. Sci. 1976,1. Mears, D.E. Adv. Chem. Ser. 1974,133,218. Mills, P. L.; Dudukovic, M. P. AIChE J. 1981,27, 893. Montagna, A. A.; Shah, Y. T.; Paraskos, J. A. Ind. Eng. Chem. Process Des. Deu. 1977,16, 152. Puranik, S. S.; Vogelpohl, A. Chem. Eng. Sci. 1974,29, 501. Satterfield, C. N.;Ozel, F. AZChE J. 1975,21, 209. Schwartz, J. G.;Weger, E.; Dudukovic, M. P. AIChE J. 1976,22,953. Shulman, H. L.;Ullrich, C. F.; Wells, N. AZChE J. 1955, 1, 247. Specchia, V.; Baldi, G. Znd. Eng. Chem. Process Des. Deu. 1978,17, 362. Van Klinken, J.; Van Dongen, R. H. Chem. Eng. Sci. 1980,35,59. Yoshida, F.; Koyanagi, T. Znd. Eng. Chem. 1958,50,365. Yoshida, F.; Koyanagi, T. AZChE J. 1962,8,309. Received for review May 6, 1987 Revised manuscript received December 8, 1987 Accepted December 21, 1987
