Ind. Eng. Chem. Res. 1993,32, 2595-2601
2595
Gas Holdups and Bubble Characteristics in a Bubble Column Operated at High Temperature Julie Chabot and Hugo I. de Lasa' Chemical Reactor Engineering Center, The University of Western Ontario, London, Ontario, Canada N6A 5B9
The local gas-phase characteristics in a bubble column were investigated using refractive spherical bulb optical sensors. The local gas holdup, bubble rise velocity, bubble chord length, and bubble chord length distribution were measured a t several radial and axial locations in a 0.2-m-diameter bubble column in the presence of a paraffinic oil and nitrogen. The gas superficial velocity was varied between 2.2 and 14.7 cm/s, while two operating temperatures were studied: T = 100 "C and T = 175 "C. The radial gas holdup profile was found t o be a function of the axial position (prior to stabilization),the operating temperature, and the gas superficial velocity. The bubble rise velocity and the bubble chord length were found t o increase with both an increase in the gas superficial velocity and a decrease in the operating temperature, the effect of the gas superficial velocity being maximum in the central region of the column. The bubble chord length distribution was found to be well represented by the inverse Gaussian probability distribution function, and the standard deviation of the bubble chord length distribution was found to be a function of the radial position, the gas superficial velocity, the axial position, and the operation temperature. Table I. Properties of the Paraffinic Oil (LP-100) (Witco
Introduction
Cora.)
Bubble column and slurry bubble column reactors are used in many areas of the chemical industry, and many more promising applications are presently being investigated. However, the design, scale-up, and optimum operation of these reactors are still not fully mastered, mainly due to the complexity of the hydrodynamic phenomena encountered in these systems. Despite the fact that many investigators have addressed this issue, as reviewed by Shah et al. (1982), Deckwer and Schumpe (1987),and Yoshida (1988),there are still a lack of reliable hydrodynamic data for these types of contactors. This is particularly true for systemsof practical interest, involving hydrocarbon fluids at high temperature. The hydrodynamic properties of bubble columns are intimately linked to the motion, characteristics (size and size distribution), and distribution of gas bubbles. The study of these parameters (gas holdup, bubble size, bubble size distribution, bubble velocity) is then crucial for a more thorough understanding of bubble columns. However, the precise local measurement of these parameters requires the availability of reliable instrumental tools. In this respect, a spherical bulb optical fiber probe was recently proposed and successfully tested (Chabot, 1993; Chabot et al., 1992). The principle of operation of this sensor is based on the difference in refractive indices between the gas phase and the liquid phase, which allows the local observation of gas bubbles. In the present study, the spherical bulb dual fiber optic sensor was used in conjunction with a newly designed single-window periscopic insertion system, allowing flexible radial and axial displacement of the optical sensors. The objective of this work is to investigate the radial and axial variations of important hydrodynamic parameters, including the gas holdup, bubble rise velocity, bubble chord length, and bubble chord length distribution, and the effect of the operating conditions (temperature and gas superficial velocity) on these gas-phase parameters.
Experimental Section The experiments were conducted in a 0.2-m-diameter and 2.4-m-height carbon steel column, equipped with a 0888-588519312632-2595$04.00/0
surface ternD ( O C ) 100 175 ~~
density
viscosity
(ka/mg) 827 783
(kdm.8)
0.003 0.00086
tension (kidsa) 0.0265 0.0213
steel perforated plate consisting of 40 holes, 1.59 mm in diameter, arranged in a triangular pitch of 2.8 cm. The column was operated in the batch mode with respect to the liquid phase (no liquid flow rate), and the static liquid height was kept constant throughout the experiments at 96 cm (HID = 5) above the gas distributor. It has to be pointed out that the above conditions were safely within the recommended column dimensions and sparger layout configurations criteria for laboratory column design, as recently emphasized by Wilkinson et al. (1992). The column was operated at atmospheric pressure and high temperature (100-175 "C). The temperature of the column was kept constant using four band heaters (MoldersSupply, 500 Watts), regulated by a temperature controller (Omega CN5001K2). Several thermocouples (OmegaTJ36-CASS-116G-8)were implementedat various strategic locations to ensure isothermal operation of the column. Nitrogen was used as the gas phase. The superficial gas velocity was varied between 2.2 and 14.7 cm/s, based on the pressure at the top of the column. The gas flow rate was measured using two gas flow meters (Brooks 1307DOSFlAlA) at both the entry and the exit of the column. The gas was preheated to the temperature of the bed (b5"C)to minimize heat-transfer effects and possible variations in the gas volumetric flow rate. For all the conditions studied, a minimum of 30 min was allowed for the flow to stabilize before measurements were performed. A commercial paraffinic oil (LP-100)was used as the liquid phase. The physical properties of the oil are presented in Table I. Two spherical bulb optical fiber sensors, made of 400pm silica fiber optic, were mounted and sealed, using a synthetic porcelain cement (Sauereisen),into two separate tubes of HTX-13 stainless steel, separated by a vertical distance of 0.5 cm. The vertical spacing between both sensors' detecting tips was selected to provide an optimum 0 1993 American Chemical Society
2596 Ind. Eng. Chem. Res., Vol. 32, No. 11,1993
1
10
To Fume Hood
9 8 7
Control
Distributor
ter
2 Valves
Nitrogen Cylinder
1
Figure 1. Process flow sheet.
Results and Discussion Gas Holdup. The gas holdup was measured at six different radial positions (8= 0.193, 0.419, 0.621, 0.782, 0.890,0.922) and at various axial positions, going from 10 cm above the gas distributor to 76 cm above the gas distributor. The gas holdup is defined as the summation of the number of times of gas-sensor contact over the total sampling time, as discussed in more detail in Chabot et al. (1992). The local gas holdup measurements reported in the present study were derived from the readings obtained from the lower optical sensor (upstream sensor), less susceptible to flow disturbance. However, the difference between the lower and upper sensor's gas holdup measurements was found to be relatively small, the
Equation (1)
-: t
0.0
compromise between maximum bubble velocity measurement resolution at the data acquisition frequency used (1000 data/s), minimum fiber optic sensors' reciprocal disturbance, and maximum correspondence between the measurements of the two sensors. The two fiber optic sensors' protective tubes, pointing downward at an angle of 45", were soldered to a metallic block that was attached to a specially designed insertion system. This system consisted of a 2.75 m long, 0.0191-m-diameter stainless steel tube. This tube was inserted into the column through the top flange of the unit, in a median point between the center of the column and the wall of the column. The system was designed in such a way that when performing a 180" rotation of the long vertical tube the detecting tips of the optical sensors were displaced from the vicinity of the center to the vicinity of the column wall. For safety reasons, a small gap was allowed between the wall of the column and the delicate fiber optic sensors tips (and consequently a similar gap was present between the center of the column and the sensors' tips). The sensors' detecting tips could also be displaced axially by moving the insertion tube up or down. The long vertical tube then served the purpose of controlling the axial and radial positions of the sensors' detecting tips and of protecting the fiber optics from the bubble column's severe environment. A stabilizer was implemented, 1.2 m below the point of insertion of the vertical tube, to prevent any excessive vibration of the optical sensors. More details concerning the principle of operation and design of the optical sensors, as well as the related optical equipment and related signals data acquisition procedure, were discussed in previous publications (Chabot, 1993;Chabot et al., 1992). Figure 1presents aschematic diagram of the column and the optical sensors' insertion system.
0 : z=15 cm A : z=30 cm 0 : z=46 cm V : z=61 cm 0 : z=76 cm
0.2
I
I
I
0.4 0.6 0.8 RADIAL POSITION (r/R)
1 .o
Figure 2. Gas holdup vs radial position ( T =100 O C , V, = 2.2 cm/s).
majority of the readings from both sensors being safely within 10% of each other. The gas holdup radial profiles obtained at 100 "C and Vg = 2.2 cm/s, for the various axial positions investigated, are presented in Figure 2. Each local gas holdup value reported (cg,loc) in this figure represents the average of a minimum of three experimental local gas holdup measurements (tg,repeat).The average relative percentage error (RPE = ((cg,ioc - cg,repeat)/cg,loc)X 100) observed for all the conditions investigated in the present study was found to be lower than 10%. As can be observed in Figure 2, the gas holdup radial profile wm relatively flat in the proximity of the gas distributor (z = 15 cm). This was essentially due to the uniform gas distribution resulting from the perforated plate sparger, as confirmed by the measurementa of the pressure drop through the distributor, which were found to be safely in excess of the recommended minimum pressure drop. However, as the axial position (distance above the gas distributor) was increased, the typical parabolic radial profile emerged: low gas holdup values near the column wall and maximum gas holdup values in the central region of the bed. This behavior was also noticed for all the other conditions studied (T= 100 "C; T = 175 "C; Vg = 2.2, 4.1, 9.0, and 14.7 cm/s). The analysis of the gas holdup radial profiles observed suggested that the following expression, where the local gas holdup is expressed in terms of the dimensionless radial distance 8 = r/R, and the average gas holdup tg:avg should be suitable in representing the gas holdup radial profile:
Actually, the radial gas holdup profiles obtained for all the conditions presented in this study were successfully depicted by eq 1, with regression coefficients in excess of 0.964, an average regression coefficient equal to 0.987, and standard deviations on the parameter m within 1476.The Marquardt optimization technique (Marquardt, 1963)was used in obtaining the parameter m in the above equation. The distinct evolution of the gas holdup radial profile and the axial position was corroborated by the value of the parameter m found. As shown in Figure 3, a marked decrease of the parameter m with the axial position was observed for the lower axial positions, corresponding to the emergence and/or the development of the parabolic gas holdup radial profile. For the higher axial positions
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2597 PARAMETER m vs AXIAL POSITION v
24
I
I
I
I
o : Vg-2.2 cm/r v : Vg-4.1 cm/r o : Vg-0.0 cm/a A : Vg-14.7 cm/r 0 : Vg-2.2 cm/s v : Vg=4.1 cm/s : Vg-9.0 cm/s A : Vg-14.7 cm/s
T=10o4C:
22
I
I
20 T-176.C
4
-a
2 10
-
-0-0
0
-
-
I
I
I
1
I
I
20
30
40
50
60
70
obtained from the fiber optic local gas holdup measurementa was validated by performing static pressure profile measurements along the column. Seven pressure taps,
locatedat2.5,15.2,25.4,40.6,55.9,71.1,and91.4cmabove the gas distributor, were connected to a pressure transducer (Data Instrument, Model AB) via a mechanical switch wafer (Scanivalve Corp.). The wafer was turned by a solenoid drive that was activated by a time controller, synchronized with the HP-1000computer (Model A600+) data acquisition system. All the static pressure profiles recorded in the present study were successfully fitted to a straight line with coefficients of linear regression in excess of 0.991 and an average coefficient of linear regression equal to 0.998.The averagegas holdup was calculated from the pressure profile measurements via the two following equations:
80
f l l h = g(cgPg+ € 1 ~ 1 )
AXIAL POSITION (distance from gas distributor, cm) Figure 3. Parameter m versus axial position.
(161cm or HID L 3.051,the gas holdup radial profile was found to have reached stabilization, since the parameter m became a weak function of the axial position, showing a constant value or only a slight decrease with further increase in the axial position. Furthermore, as a general trend for all the axial positions investigated, the parameter m was found to diminish with an increase in the gas superficial velocity. The influence of the gas superficial velocity on the parameter m was found to be marked for the lower gas superficial velocities (
