Bubble specific interfacial area

The two micro bubble columns comprising the smaller micro channels reached nearly 100% conversion [5], The micro bubble column with the largest hydraulic diameter reached at best 75% conversion. The curve obtained displays the typical shape, passing through a maximum due to the antagonistic interplay between residence time and specific interfacial area. 

P 28/The Hquid feed was introduced by a pump and the gas feed using a mass-flow controller [10], The reaction was carried out using liquid flows of 20.7-51.8 ml h and gas flows of 1.7-173 mlrnin . The gas and liquid velocities amounted to 0.02-1.2 and 0.03-3.0 m s , respectively. The reaction was performed in mixed flow regimes, including bubbly, slug and annular patterns. The specific interfacial areas amoimted to about 5000-15 000 m m . The reaction was conducted at room temperature. 

Figure 5.32 Calculated values for conversion of butyraldehyde as a function of specific interfacial area for a micro bubble column [10],...

GL 27] [R 3] [P 29] By means of sulfite oxidation, the specific interfacial areas of the fluid system nitrogen/2-propanol were determined for different flow regimes [5]. For two types of micro bubble columns differing in micro-channel diameter, interfaces of 9800 and 14 800 m m , respectively, were determined (gas and liquid flow rates 270 and 22 ml h in both cases). Here, the smaller channels yield the multi-phase system with the largest interface. 

Characterization of a gas/liquid microreactor, the micro bubble column Determination of specific interfacial area, in Matlosz, M., Ehreeld, W., Baselt,... 

Finally, Bakker and Van den Akker calculated local values for the specific mass transfer rate kfl, by estimating local Ay-values from local values of the Kolmogorov time scale /(v/e) and by deriving local values of the specific interfacial area a from local values for bubble size and bubble hold-up. 

In these equations, a is the specific interfacial area for a significant degree of surface aeration (m2/m3), I is the agitator power per unit volume of vessel (W/m3), pL is the liquid density, o is the surface tension (N/m), us is the superficial gas velocity (m/s), u0 is the terminal bubble-rise velocity (m/s), N is the impeller speed (Hz), d, is the impeller diameter (m), dt is the tank diameter (m), pi is the liquid viscosity (Ns/m2) and d0 is the Sauter mean bubble diameter defined in Chapter 1, Section 1.2.4. 

With regards to handling data on industrial apparatus for gas-liquid mass transfer (such as packed columns, bubble columns, and stirred tanks), it is more practical to use volumetric mass transfer coefficients, such as KqU and K a, because the interfacial area a cannot be well defined and will vary with operating conditions. As noted in Section 6.7.2, the volumetric mass transfer coefficients for packed columns are defined with respect to the packed volume - that is, the sum of the volumes of gas, liquid, and packings. In contrast, volumetric mass transfer coefficients, which involve the specific gas-liquid interfacial area a (L L 5), for liquid-gas bubble systems (such as gassed stirred tanks and bubble columns) are defined with respect to the unit volume of gas-liquid mixture or of clear liquid volume, excluding the gas bubbles. In this book, we shall use a for the specific interfacial area with respect to the clear liquid volume, and a for the specific interfacial area with respect to the total volume of gas-liquid mixture. 

In the group with positive spreading coefficients (e.g., toluene-in-water and oleic acid-in-water emulsions), the values ofkj a in both stirred tanks and bubble columns decrease upon the addition of a very small amount of oil, and then increase with increasing oil fraction. In such systems, the oils tend to spread over the gas-liquid interface as thin films, providing additional mass transfer resistance and consequently lower k values. Any increase in value upon the further addition of oils could be explained by an increased specific interfacial area a due to a lowered surface tension and consequent smaller bubble sizes. 

A. Schumpe, W.D. Deckwer, Gas holdups, specific interfacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, I EC Process Des. Develop. 21 (1982) 706-711. 

When two phases are mixed together (gas-liquid, immiscible liquid-liquid), a fine dispersion of bubbles or drops and a high specific interfacial area are produced because of the intensive turbulence and shear. For this reason, resistance to interphase mass transfer is considerably smaller than in conventional equipment. In addition, a wide range of gas-liquid flow ratios can be handled, whereas in stirred tanks the gas-flow rate is often limited by the onset of flooding. Mass transfer coefficients (kLa) can be 10-100 times higher than in a stirred tank. 

For example, it is found that the mass transfer coefficient, Kca, for gas-liquid processes, is mostly a function of the linear superficial gas velocity and the power per unit volume with the constant D/T ratio for various size tanks. This is because the integrated volumetric mass transfer coefficient over the entire tank can be quite similar in large and small tanks even though the individual bubble size, interfacial area, and mass transfer coefficient can vary at specific points within the small and large tanks. 

Simple transmission measurements with inexpensive components were made to estimate the local specific interfacial area of a suspended phase (i. e. of gas bubbles) in a bioreactor [473]. 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

This result also proved, that through suppressing the bubble coalescence and hence by increasing ktct, the specific interfacial area a increased much more strongly, than k] was reduced at the same time. 

Sometimes the foam dispersity is characterized by the specific interfacial area e equal to the total surface of gaseous bubbles per unit volume of foam,... 

After about half an hour, the bubble velocity dropped to the original value, which indicates that the antifoam had disappeared from the cultivation medium. However, after several antifoam additions, the base line and the maxima of the bubble velocity gradually increased. The cultivation medium became more and more coalescence promoting. Monitoring the intensity of the reflected ultrasound allowed the specific gas/liquid interfacial area a to be measured in situ (Fig. lb). The specific interfacial area a... 

Flg.1. Measurements during the cultivation of recombinant E. coli K-12 MF cells in a 60-1 working volume air lift tower loop reactor without gene expression using SE9 antifoam agent (AFA) [50,51]. a Variation in the mean bubble velocity. Bubble velocity measured in situ by an ultrasound Doppler technique. AFA added to the medium, b Variation in the specific interfacial area (m ) measured in situ by an ultrasound technique... 

In Fig. 2 the key parameters are presented for recombinant E. coli batch cultivation in a 60-1 working volume airUft tower loop reactor at constant aeration rate up to 16 h, whereupon the temperature was increased from 30 to 42 °C and gene expression was induced. At the same time concentrated Luxia-Bertani (LB) medium was added to the reactor. To avoid oxygen limitation, the aeration rate was increased (Fig. 2 a). At 12 h the foaming increased and SE9 was added to the medium. The bubble velocities (Fig. 2b) and the specific gas/liquid interfacial area (Fig. 2 c) quickly increased and passed a narrow maximum, but kLa dropped and the OTR was not influenced (Fig. 2d). After the induction of the gene expression by a temperature increase and medium supplement the dissolved oxygen concentration with respect to the saturation increased due to the elevation of the aeration rate (Fig. 2 a) the mean bubble velocity (Fig. 2 b) and specific interfacial area (Fig. 2 c) decreased, OTR increased and kLa remained at low values (Fig. 2d). The mass transfer coefficient with respect to the liquid phase kL dropped from about 1.67 to 0.67 ms after the addition of SE9 to the medium [51]. 

In the two-phase model, the mass-transfer parameter needs to be determined using actual polymerization reactor data because the specific interfacial area is strongly dependent upon the geometry of the reactor internals. Moreover, the formation of bubbles of volatile species contributes to the total vapor-liquid contact area, and the liquid holdup on a rotating disk can change with the melt viscosity or polymer molecular weight, affecting the mass-transfer coefficient. 

The bubbly flow in a single channel at low gas hold-up (Figure 7.4a) shows well-defined specific interfacial area, which is, however, a way below to other flow regimes. At high gas hold-up, the small size of the bubble provides very high... 

The mass transfer resistance through the caps of the bubble and the liquid phase are particularly difficult to estimate as assured models are missing. The transfer from the gas bubble through the wall film can be estimated. Ignoring the thickness of the film, the specific interfacial area Is given by... 

The previous assumptions together with the classical relationships of the different specific absorption rates, depending on the chemical reaction regime used to determine interfacial parameters, lead to the theoretical mean reduced values of the gaseous reactant in the gas exit stream and to absorption efficiency for a gas-liquid dispersion, where a is the geometrical specific interfacial area and Rl is the mean true liquid-side mass transfer coefficient defined for a bubble of diameter dg. ... 

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