Gas holdup, measurement

Table I. Experimental conditions for average gas holdup measurements. (Gas phase is nitrogen)...

The gas holdup measurement based on Eq. (4.11) completely neglects the effects of wall shear stress and has been identified by Tang and Heindel (2006a) as Method... 

Hills, J.H. (1976), The operation of a bubble column at high throughputs. I - Gas holdup measurements, The Chemical Engineering Journal, 12 89-99. 

Thatte, A.R., Ghadge, R.S., Patwardhan, A.W., Joshi, J.B., and Singh, G. (2004), Local gas holdup measurement in sparged and aerated tanks by y-ray attenuation technique, Industrial Engineering Chemistry Research, 43(17) 5389-5399. 

This correlation reqnires information on u, which can be estimated using Equation 10.7. This latter equation requires data on as a function of superficial gas velocity to evaluate the terminal rise velocity of the bubble, These data can be obtained throngh simple gas holdup measurements. The drift flux model of Zuber and Findlay (1965) can be used to obtain as per Equation 10.10 ... 

Kumar SB, Moslemian D, Dudukovic MP Gas-holdup measurements in bubble columns using computed tomography, AIChE J 43 1414—1425, 1997. http //dx.doi.org/ 10.1002/aic.690430605. 

Equation (10) is valid for a column with an inner diameter of 100 mm and a clear liquid height greater than 1200 mm. In a further step we therefore examined, wether gas holdup is influenced by the column dimensions. In figure 3 gas holdup measurements are plotted versus gas linear velocity. The experiments were carried out in columns with inner dimensions larger than 150 mm and clear liquid heights higher than 1000 mm. Furthermore, the employed gas distributors caused a churn turbulent flow already at low gas throughputs. 

The shape of successively rising bubbles in molten metal baths is closely related to the horizontal distribution of gas holdup, as shown in Fig. 1.3. Thus the shape can be predicted from the results of gas holdup measurements. 

Only a few investigations concerned with the measurement of gas holdup and residence-time distribution have been reported. The information regarding liquid holdup, which will be discussed in the following section, is considerably more abundant in some cases, values of gas holdup can be deduced from the reported data on liquid holdup and total voidage. 

Weber (Wl) has reported measurements of gas holdup for the experimental system described in Section V,B,4. The following empirical relationships can be derived from the graphical correlations ... 

Measurements for water containing 0.2% ethanol, the addition of which was found to influence markedly the gas holdup (see Section V,B,3), indicate that variation of surface tension has no significant effect upon axial mixing. The results for 2-mm spheres could not be correlated by a similar expression. It is proposed in that work that the flow mechanism in this case is significantly different because of the higher ratio between bubble size and particle size. 

Various methods may be used for the determination of gas holdup—for example, displacement measurements and tracer experiments. Farley and Ray (F2) have described the use of gamma-radiation absorption measurement for the determination of gas holdup in a slurry reactor for the Fischer-Tropsch synthesis. 

Foust et al. (F4) measured gas holdup in mechanically stirred gas-liquid contactors of various diameters (from 1 to 8 ft) and various liquid contents (from 5 to 2250 gal). The nominal gas velocity varied from 1 to 5 ft/min and the power input from 0.01 to 6.5 hp. The contact time (sec/ft) could be correlated by the following expression ... 

Viswanathan et al. (V6) measured gas holdup in fluidized beds of quartz particles of 0.649- and 0.928-mm mean diameter and glass beads of 4-mm diameter. The fluid media were air and water. Holdup measurements were also carried out for air-water systems free of solids in order to evaluate the influence of the solid particles. It was found that the gas holdup of a bed of 4-mm particles was higher than that of a solids-free system, whereas the gas holdup in a bed of 0.649- or 0.928-mm particles was lower than that of a solids-free system. An attempt was made to correlate the gas holdup data for gas-liquid fluidized beds using a mathematical model for two-phase gas-liquid systems proposed by Bankoff (B4). 

Measurement of the expansion of a gas-liquid fluidized bed provides a measure of the holdup of solids or of the corresponding combined holdup of gas and liquid. From such measurements, the holdup of liquid may be calculated if the gas holdup has been determined independently. 

In 1944, Foust et al. (F2) studied air holdup in water in baffled vessels agitated with a special impeller developed for gas dispersion. The impeller consisted of arrowhead-shaped blades mounted on a flat disk. The gas holdup was determined by measuring the liquid level before the air was introduced and while the air was fed at a point underneath the impeller. They found that the gas holdup ranged from 2% to 10% of the air-free-liquid volume. 

According to their measurements, the gas holdup increases with the gas velocity but the average contact time drops. This is not surprising, as will be shown. The volumetric gas flow rate is... 

Clark and Vermeulen (C8) measured gas holdup in three different liquids —isopropyl alcohol, ethylene glycol, and water. They measured the increase in holdup with agitation as compared to no agitation, and correlated their results as a function of the volumetric gas velocity, Weber number, P/P0, and a geometric factor. Typical volumetric gas holdup values reported in the literature vary from about 2% to 40% of the total dispersion volume (Cl, C2, C8, F2, G10). 

Gas holdup and liquid circulation velocity are the most important parameters to determinate the conversion and selectivity of airlift reactors. Most of the reported works are focused on the global hydrodynamic behavior, while studies on the measurements of local parameters are much more limited [20]. In recent years, studies on the hydrodynamic behavior in ALRs have focused on local behaviors [20-23], such as the gas holdup, bubble size and bubble rise velocity. These studies give us a much better understanding on ALRs. 

Although the values of kba dr in the literature are reasonable and comparable each other, the different trend mentioned above may be due to the different operating conditions. The gas-liquid interfacial area(a) and liquid side mass transfer coefHcient(ki) have been determined from the knowledge of measured values of gas holdup and kcacir [11]- The values of a and ki increase almost linearly with increasing Ug or Ul- The values of h cir and ktacir in circulating beds can be predicted by Eqs.(ll) and (12) with a correlation coefBcient of 0.92 and 0.93,... 

Fig. 4. Effect of solid holdup on gas holdup at Fig. 5. Comparison of calculated and measured different superficial gas velocities gas holdup...

In this model, energy balances are set up for the reactor and the separator tube separately, and two equations are obtained. The gas holdup can then be obtained from combining these two equations. Details can be found in Zhang et al. [7]. The comparison between the measured and calculated cross-sectional mean gas holdups is shown in Fig. 5. It can be seen that there is a satisfactory agreement between the experimental and calculated gas holdup in the different operating conditions. Therefore, it is reasonable to conclude that the energy balance model used in this work can describe the circulation flow behavior in the novle internal-loop airlift reactor proposed in this work. 

Methane is commonly used as a marker for measuring the gas holdup time (tm), which was done on a capillary column 25 m long by 0.25 mm ID by 0.25 pm film thickness. A retention time for methane of 1.76 min was obtained. Determine the average linear gas velocity (v) and the average volumetric flow rate (Fc). Explain how these values differ from the actual velocity and flows at the column inlet and outlet. 

The fractional gas holdup can be easily measured from the height of the expanded column height Zf and the settled sluny height Zs, i.e. the height of the column before aeration (liquid volume plus solids volume) (DOE, 1985 NTIS, 1983) ... 

The gas holdups can be obtained by measuring Zp and Zp, or by measuring the corresponding hydrostatic heads. Evidently, the following relationship holds. 

Gas-liquid mass transfer in fermentors is discussed in detail in Section 12.4. In dealing with in gas-sparged stirred tanks, it is more rational to separate and a, because both are affected by different factors. It is possible to measure a by using either a light scattering technique [9] or a chemical method [4]. Ihe average bubble size can be estimated by Equation 7.26 from measured values of a and the gas holdup e. Correlations for have been obtained in this way [10, 11], but in order to use them it is necessary that a and d are known. 

The gas-liquid interfacial area per unit volume of gas-liquid mixture a (L 1. or L ), calculated by Equation 7.26 from the measured values of the fractional gas holdup and the volume-surface mean bubble diameter d, were correlated... 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

Injector and detector temperatures were maintained at 150 and 200°C, respectively. Nitrogen carrier flow rates were measured with a Gasmet flow meter and were maintained between 22 and 24 ml min-1. Gas holdup times were measured with 20 1 injections of methane. 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

The local gas holdup and bubble behavior were measured by a reflective optic fiber probe developed by Wang and co-workers [21,22]. It can be known whether the probe is im-merging in the gas. The rate of the time that probe immerg-ing in the gas and the total sample time is gas holdup. Gas velocity can be got by the time difference that one bubble touch two probes and the distance between two probes. Chord length can be obtained from one bubble velocity and the time that the probe stays in the bubble. Bubble size distribution is got from the probability density of the chord length based on some numerical method. The local liquid velocity in the riser was measured by a backward scattering LDA system (system 9100-8, model TSI). Details have been given by Lin et al. [23]. 

The upper surface of the internal is defined as the zero of the axial position. Axial positions are positive above this and negative below this. The radial profiles of the gas holdup at five axial positions were measured, as shown in Fig. 3. The radial profile of the gas holdup becomes much flatter after flowing through the internal, with an increase in the gas holdup near the wall and a decrease in the center region as compared with the gas holdup below the internal. As the distance above and away from the internal increases, the profile becomes more and more similar to that before contact with the internal. The local radial profiles of the gas holdup at axial positions 144 and 209 cm show almost no difference. This shows that the influence of the internal on the radial profile of the gas holdup becomes weaker and weaker with increasing distance away from the internal and is no longer felt beyond a certain distance. The analysis of the local gas holdup profiles shows that the influence of the internal persists to a distance of about 1.4 m. In order to show the effect of the internal on the radial profile of... 

---