Ind. Eng. Chem. Res. 2007, 46, 3841-3848
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RESEARCH NOTES Experimental Investigation into the Radial Distribution of Local Phase Holdups in a Gas-Liquid-Solid Fluidized Bed Changqing Cao,† Mingyan Liu,‡ and Qingjie Guo*,† College of Chemical Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China, and School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China
Experiments were conducted in a gas-liquid-solid fluidized bed (GLSFB), using two types of particles. The light particles were styrene-blend spheres 1.45 mm in diameter, with a density of 1264 kg/m3, whereas the heavy particles were spherical glass beads 0.48 and 1.25 mm in diameter, with a density of 2460 kg/m3. In both cases, compressed air and tap water and carboxymethyl cellulose sodium with concentrations of 0.05 and 0.20 wt % (CMCS) were used as the gas phase and liquid phase, respectively. Local holdups in the GLSFBs were measured simultaneously using an improved double-sensor micro-electric conductivity probe under different liquid viscosities, superficial gas velocity, superficial liquid velocity, and particle sizes. It was determined that local solid holdup decreased with increasing liquid viscosity, superficial gas velocity, and superficial liquid velocity, whereas the local solid holdup at the center increased as the particle size increased. A maximum value of local solid holdup existed at r/R ) 0.75-0.85, whereas the minimum value of the local solid holdup was located at the wall region. Dimensionless correlations of local holdups were developed using measurement results. Calculation values using correlations are in reasonable agreement with the measurement data. Introduction Gas-liquid-solid three-phase fluidized beds (GLSFBs) are an important class of contacting devices in chemical industry and biotechnology. Their simple setup makes them ideal reactors for three-phase operations, such as fermentations and heterogeneous catalytic processes. However, the design and operation of these reactors are applied in mostly empirical correlations, giving only approximate scale-up criteria.1 The mechanically simple setups of such reactors are contrary to the complex flow structures that are developing inside these pneumatically agitated vessels. GLSFB reactors are very difficult to commercialize, because of their complex flow patterns.2 Local phase holdup is one of their most important design parameters. The crosssectional average holdup was investigated using various methods.3-7 Note that the detailed investigations into the local phase holdups raise a wealth of questions, in regard to physical reasons. Therefore, an accurate measurement technique is necessary to predict the local phase holdup distribution in a GLSFB. Hu and Yu8 detected local gas and solid holdups, using a combination of a double-sensor electrical conductivity probe and an optic fiber probe. The accuracy of this detecting technique suffered when used in a three-phase flow system, because of reflected light on the bubble surface. Lang et al.9 reported radial variation of the cross-sectional average gas holdup by the electrical conductivity measurement technique. * To whom correspondence should be addressed. Tel.: 0086-53284022506. E-mail address:
[email protected]. † College of Chemical Engineering, Qingdao University of Science and Technology. ‡ School of Chemical Engineering & Technology, Tianjin University.
The cross-sectional average solid holdup was presented using differential pressure and a unitary relationship of phase holdups. However, local solid holdup failed to detect variations using this measurement technique. A time series of bubble and particle frequencies were measured by Ryuji et al.,10 who described the voltage variation as the detecting time in gas-liquid and solidliquid systems, using a novel optical transmittance probe. Dziallas and co-workers11,12 developed a measurement technique that used a combination of differential pressure and conductivity or time domain reflectometry (TDR) measurement in a single probe, which allowed for the determination of local gas and solid holdups in a three-phase pilot-plant-sized bubble column. The detailed investigations into the influence of superficial gas velocity, distributor geometry, and solid loading on local gas, solid holdups, fluidization, and mixing phenomena were performed by such techniques. The double-sensor electrical conductivity probe method is attractive for three-phase flow measurements, because of its relative simplicity and wide applicability. Kocamustafaogullari and Wang13 determined the void fraction, bubble size, and velocity by detecting the passage of interfaces at the tip of each sensor for bubbles. The problem with the double-sensor electrical conductivity probe is the difficulty involved in matching the signals from the two needles corresponding to the same bubble, which can be attributed to bubbles striking the probe with a glancing blow. One attempt to improve the measurement accuracy is to use a resistance probe in a novel configuration in a slurry bubble column.14-16 Until now, few researchers have investigated the distributions of local gas and solid holdups using a double-sensor electric conductivity probe in a GLSFB.17-19
10.1021/ie060798g CCC: $37.00 © 2007 American Chemical Society Published on Web 04/28/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007
Figure 2. Time series of voltage signals obtained under Ug ) 0.023 m/s and Ul ) 0.031 m/s in the gas-liquid-solid three-phase system.
Figure 1. Schematic illustration of the double-sensor microelectric conductivity probe with an inclination angle of 120°.
In this investigation, a double-sensor microelectric conductivity probe with a 120° inclination angle was developed to measure the local gas and solid holdups in a GLSFB simultaneously. The solid materials and the liquid phase were, respectively, simulated particles with a bio-film growing on them and a non-Newtonian fluid that was used in biotechnological applications. Furthermore, the effects of the liquid viscosity, superficial gas and liquid velocities, particle size, and density on the variation of radial distribution of local phase holdups were studied. In addition, the dimensionless correlations of local holdups were established based on the measurement data. Measurement Techniques Probe Design. A schematic illustration of the double-sensor microelectric conductivity probe with an inclination angle of 120° is given in Figure 1. It consists of two identical stainless steel wires, with an inner diameter (ID) of 0.15 mm, whose tips are aligned vertically with a distance of 5.0 mm between them. In theory, a large separation can introduce errors in the detected signals. The multibubble and particles contact may occur between two signals originating from the same bubble and particle. It should be noted that a small separation leads to errors in the estimation of velocity. The two sensors are electrically insulated from the probe body, except their tips, which are made by simply cutting the ends off the insulated wires. The other ends of wires are connected with two coaxial cables. The opposite ends of the coaxial cables are connected to conductance meters (LDD Model 501). The signal processor is dependent primarily on the comparison the resistance between the probe tip and the ground. A constant potential of 5 V (AC) is applied across each needle, and the potential across a series resistor is amplified as the output signal. Data Processing. The data acquisition system consists of a 12-bit AD personal computer (LDD Model 501). The output from the meters is sent to a computer after AD processing. The sampling frequency used is 200 Hz, with a sampling duration of 100 s. It is acceptable for the phase holdups to use a sampling frequency of 200 Hz. In the measurements, the raw signals are not square waves, because of the relatively slow drainage of liquid film formed around the sensor tip, which leads to a slow rise time, in comparison to the sharp fall time when the sensor
re-enters the water. To obtain the bubble properties, it is necessary to have data in terms of perfect square waves when the needle enters and leaves the gas and solid phases. The data is then analyzed via a discrimination program drafted by Visual Basic and Visual C++ language. The program checks to confirm that the rise and fall signals on each sensor alternate, rejecting two successive rise or fall signals, and then attempts to match corresponding rise and fall signals from the two sensors. Because the upstream probe disturbs the flow around the downstream needle, the downstream signals are less reliable, and results for local phase holdups are derived from the data of the upstream probe. The signals obtained from measurements have minimal unbiased noise associated with fluctuations in the supply voltage and is amplified by a computer. Because the probe output is not the recommended square wave, the peak width varies with the height at which it is measured. A trigger level of 20% of the peak height is used throughout in this work. After the comparator, the signals are converted to digital form by an analog/digital (A/D) converter with a sampling period of 250 µs. The signals generated here are featured in square-wave form. To permit any bubbles moving nonvertically to be disregarded, we compare the signal lengths of the lower tip and upper tip, corresponding to the passage of time of a bubble at each tip. When the bubble signal lengths differed by >10%, the bubble signals were rejected in the calculation. The bubble impact rate is defined as the number of bubbles arriving at the measuring point for a given duration and measured by counting the number of the modified squarewave signals from the upper tip. Figure 2 shows that the local gas holdup can be calculated by eq 1 at each sampling time:
g )
∑ ∆ti ∑ ∆tiU0 )
T
TU0
(1)
The local solid holdup is evaluated by the linear relationship between the conductance measurement and the solid holdup measurement, as well as the voltage differences of a pure liquid phase and a liquid-solid system. The model for a liquid-solid system is equivalent to a cube with edge L, which contains a spherical particle with radius Rs. The solid holdup is expressed as
s )
(4/3)πRs3 L3
(2)
As two respective electrodes are installed on the relative planes, the conductance for model is given by
Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007 3843
S)
L3 - (4/3)πRs3 L3 - L3s L 1 1 ) ) (1 ) ) R F(L/A) F FL2 FL2 s) ) Q(1 - s) (3)
It can be observed that the relationship between the conductance and the solid holdup is linear. Among solid holdup, conductance and voltage signals measurements could be transferred linearly, using a conductivity probe and a conductivity meter in the gasliquid-solid three-phase system. The relationship between voltage signals and solid holdup is expressed by
U ) Ks
(4)
where K is a constant and is dependent on the physical properties of the three phases and the detection system. The average voltage (U) in T - ∑∆ti is given by
U)
∫0T U(t) dt T-
∑
(5)
∆ti
where U(t) is the voltage signal at time t. Based on the relationship of U, Ul, and U0, we can obtain eq 5:
Ks - Ksl sl - s U - Ul ) ) U0 - Ul Ks0 - Ksl sl
(6)
where Ul and U0 are the voltage of liquid-solid system and pure liquid, respectively; s, sl, and s0 are the local solid holdup of the three-phase system, liquid-solid system, and pure liquid system, respectively. Thus, eq 6 becomes
(
s ) sl 1 -
)
U - Ul U 0 - Ul
g + s + l ) 1
(7a) (7b)
Probe Calibration. When applying the conductivity method for phase holdup measurements, it is essential to identify a valid equation that can be applied to both liquid-solid and gasliquid systems, so that the method can be consistently extended to three-phase systems. Figure 3 summarizes the calibration results for various spherical particles in a liquid-solid fluidized bed and gas-liquid system. The figure shows that the measured effective conductivity-solids holdup relationship of 1.45-mm styrene particles follows the Bruggeman equation closely, whereas the Buyevich equation best fits for a solids holdup of 1.0 mm. Figure 3 also shows the calibration results for the gas holdup in a gas-liquid system at various gas flow rates. It is observed that the Buyevich equation accurately describe the effective conductivity-gas volume fraction relationship, up to a gas holdup of 0.3. The applicability of the Buyevich equation to both liquidsolid and gas-liquid systems provides a basis for its use for gas-liquid-solid systems with spherical particles with sizes of >1.0 mm. In addition, integrating the axial solids holdups obtained from differential pressure measurement gives the overall solids mass within ( 10% of the solids weight,
Figure 3. Probe calibration for liquid-solid fluidized beds and for a gasliquid system.
suggesting that the axial solids holdup can be determined accurately using conductivity probe and differential pressure measurement. Good agreement was observed between the conductivity probe and the differential pressure measurement. Experimental Apparatus A schematic diagram of the experimental apparatus is shown in Figure 4. The column consisted of four parts: a gas distributor, a column, a segregating section, and a liquid distributor. The column was made of Plexiglas, with an ID column diameter of 150 mm and a height of 4.35 m. A segregator was located at the top of the column. There were seven sampling ports 200 mm apart. Port No. 1 was located at the bottom of the column and above the gas distributor at a height of 350 mm. The liquid distributor was composed of 58 pipes with ID ) 10 mm and a height of 300 mm. The compressed air was supplied to the column through a pressure regulator and a filter, which was measured by a rotameter. The gas was admitted to the column through a gas distributor with seven 45-mm-ID perforated pipes. The pipes were evenly spaced across a grid that had 252 holes whose diameter was 1 mm. All particles were regenerated and rinsed in the laboratory to ensure uniform wettability by the identical treatment course before use. The regenerate solution was 1 M HCl for cationic resin. The regenerated resin was backwashed with deionized water until the sodium concentration in the effluent was
