A review of existing literature has shown that bubble column reactor research is typically conducted in the superficial gas velocity range of 1 - 40 cmls. Correlation with superficial gas velocity and diameter Correlation with Peclet number and Froude number Effect of liquid velocity on axial dispersion. 8.5) Comparison of experimental results with literature correlations Page Prediction of EL via superficial gas velocity and eolumn diameter Pages 146-149.
Comparison of literature distribution coefficients with predicted EL values via the correlation of Deckwer et al. Schematic to show variation of gas holdup measured with Sparger 8 and Spargcr 9 for BC2 compared to Hughmark's (1967) gas holdup prediction. Schematic to show mass of injected traccr and injection volume for a given batch fluid volume.
LIST OF TABLES
NOMENCLATURE
C FO CSTR
CHAPTER ONE
INTRODUCTION
An extensive review of the literature showed that there is a lack of information in the literature on the hydrodynamic behavior of bubble column reactors in the surface gas velocity range of interest to Sasol (ug < 0.8 cm/s). Furthermore, knowledge about the behavior of bubble reactors in South Africa is limited. This dissertation serves as an introductory study on the hydrodynamic behavior of two-phase gas-liquid co-flow reactors with bubbles.
CHAPTER TWO
BUBBLE COLUMN REACTORS - AN OVERVIEW
As the transition occurs, significant changes in the hydrodynamic behavior of the system are observed. At gas velocities ranging from 0.5 to 10 cmls, the bubble diameter becomes a strong function of the gas velocity in the orifice. It is common to use the axial dispersion model (ADM) to describe liquid phase mixing in bubble column reactors.
CHAPTER THREE
HYDRODYNAMICS OF BUBBLE COLUMN REACTORS
Co is given by the concentration of the mass of tracer M in the bulk liquid volume VL. The distribution of the tracer is measured either directly or by sampling as shown in Figure 3-8. The steady-state method also reveals information about the constancy of the dispersion coefficient in the axial direction.
CHAPTER ONE
The choice and design of the gas distributor affects the properties of the gas distribution within the reactor. The pressure drop is composed of the drop exerted by the gas nozzle and the hydrostatic head of the liquid. At gas velocities ranging from 0.5 to 10 cm/s, the bubble diameter becomes a strong function of the gas velocity in the orifice.
The gas storage provides an indication of the residence time and the effective interface area of the gas. The fluid dynamics of the gas and liquid phases are often characterized by their respective residence time distributions (Chapter 3). 3-4 serves as a check of the tracer mass balance to determine that there are no errors in the tracer measurement.
The difference between the tracer input with respect to the top of the reactor is that it is a test for back mixing. Input from the bottom of the reactor, as in Figure 3-6(a), assumes that back mixing occurs. The longitudinal distribution coefficient EL is used to express the properties of the liquid mixing in bubble columns.
EL can be calculated from the slope of the straight line gradient of a graph of ln(c) versus z. A major disadvantage of this method is that the curvature of cylindrical columns affects the size of the bubble in the photograph. Essentially, the key design parameters for bubble columns (phase hold-ups, mixing, RTD, mass and heat transfer coefficients) are all determined by the velocity field and turbulence properties of the fluid phase.
CHAPTER FOUR
HYDRODYNAMICS: PARAMETER DEPENDENCY AND ESTIMATION
The input of energy into a bubble column is due to the introduction of the dispersed gas phase. 4-1 and 4-2 that the level of axial distribution must depend on the gas flow rate to the column and the diameter of the column. It is assumed that the distribution coefficient is proportional to the scale and to the velocity of the eddies in the column.
1993) report that the effect of liquid velocity is negligible when the actual column liquid velocity is less than 0.1 of the bubble rise rate (ULaCIl/al ~O_l Ub). A requirement for the validity of the isotropic turbulence theory is that the Reynolds number of the fluid must be sufficiently high. The separation point between the two mixing zones was found to be approximately in the middle of the bubble column.
The net effect of going from one liquid to another is seen in gas hold-up measurements rather than the effect of a change in just one physical property. 1996) attempted to conduct a systematic study of the effect of the nature of the gas nebulizer on bubble hydrodynamics. The most important criterion in the design of the dispenser is the possibility of the liquid flowing out through the holes of the dispenser.
Sekizawa and Kubota (I974) observed that the reflux rate was independent of the separator plate spacing and the column diameter.
CHAPTER FIVE
The validity of extrapolation from literature correlations (Chapter 4) to low superficial gas velocities was determined using the literature data points shown in Table 5-3. The results of the comparison between literature distribution coefficients and predicted EL values via the correlation of Deckwer et al. The correlation of Ityokumbul et al. 1994) tend to underestimate the reported literature distribution coefficients.
As was the case for the correlation of Ityokumbul et al. 1994), the correlation between Ohki and Inoue (1970) is observed to greatly underestimate the literature dispersion coefficients. It is clear that there is a wide spread in the prediction of the literature dispersion coefficients. It must be emphasized that the error in literature data is assumed to be negligible.
The literature data at low superficial gas velocities show that no correlation from a similar analysis as Reith et al. The prediction of distribution coefficients for Ulbrecht and Baykara's (1981) data failed Cf. 4. 16 tend to overestimate the reported literature data of the other authors. The correlation of Baird and Rice (1975) does not provide an adequate estimate of the reported literature distribution coefficients. 5.1.4) Prediction of EL by recirculation and centerline velocities.
It is evident that there is a large discrepancy in the measured dispersion coefficients reported in the literature.
CHAPTER SIX
APPARATUS AND EXPERIMENTAL PROCEDURE
The design of the column was such that the splitter plate could not be changed or modified as the gas chamber and PVC plate were welded together as a single unit. The design of the column was such that the liquid and gas entered a PVC gas chamber and then a Plexiglas section. When the aspect ratio of the column is greater than the development zone, the circulation rate in the column will be independent of the aspect ratio (Joshi, 2001).
The dynamic response of the probe and meter was found to be on the order of nanoseconds. For BCl, a scissor jack was used to provide additional support to the bottom of the gas chamber (Figure 6-10). The following description applies to both BC1 and BC2 after installation of the desired sparger.
The probe was inserted from the top of the column to the desired height from the gas distributor plate. The probe was placed from the top of the column at a level near the liquid outlet. The conductivity of the water in the column was compared to the conductivity of the water in the tank.
Conductivity was recorded until the conductivity in the column was within five percent of the conductivity of the base fluid.
CHAPTER SEVEN
MODEL IMPLEMENTATION AND DATA REDUCTION
Their method based on the variance of the curve requires only the area over the time-concentration curve. To determine the quality of the prediction, the relative percentage deviation between the experimental concentration data Cexp and the predicted ADM concentrations C model as defined by Eq. Levenspiel (1962) reported that the tail of the RTD curve is heavily weighted for the variance method.
It is most often the tail of the RTD curve that is subject to error as it is the region of low concentration where there is great uncertainty in the calibration. A common practice by researchers is to use the last part of a CSTR model to predict the ADM exceedance so that the variance method can be used (Figure 7-5). Clements stated that the most attractive feature of the least squares procedure was that it weighted the tail of an experimental RTD curve no more than the rest of the curve.
The only significant downfall of analytical solutions is that they are based on idealized tracer inputs (Dirac delta pulse). Numerical solutions allow tracer input functions to be used, which are more representative of the actual tracer input. However, it is well known (Levenspiel, 1962) that the tracer input can be assumed to be a pulse if the tracer injection time is linear and the mean residence time of the phase of interest 7 .
The method of variance and the graphical method were used to demonstrate that the implementation of the models was carried out accurately.
CHAPTER EIGHT
RESULTS AND DISCUSSION
The measured gas storage in BC2 as a function of the superficial gas velocity, shown in Figure 8-1 below. However, the experimental error III must be taken into account when obtaining the gas hold measurements. Given the small scale of gas retention fractions, this small amount of liquid greatly exaggerates the gas accumulation.
These measurements are subject to measurement precision error, and even a small amount of liquid flowing through the SP9 would greatly exaggerate the gas holdup, as the HL measurement would be lower than if no weeping occurred. For these reasons, gas hold-up was not measured with the other spreaders, as a procedure to properly account for the weep rate could not be established. The measurements using a single 1 mm orifice spreader would provide the most representative overall gas hold-up values.
This is also seen in Figure 8-1, as the Hughmark (1967) correlation provides an excellent prediction of the total gas holdup in BC2 measured with a single aperture. For the purposes of this study, only an assessment of overall gas holdup was required and as such a comprehensive gas holdup study was not warranted. The Hughmark (1967) correlation is recommended for predicting gas holdup at low superficial gas velocities. Chapter 8 8.2) Longitudinal axial dispersion coefficients with batch mixing tests.
Batch mixing tests were performed with the probe in a vertical and horizontal orientation, as shown in Figure 8-7.
