Interfacial bubble columns

Region II, 0.02 < P < 2. Most of the reaction occurs in the bulk of the liquid. Both interfacial area and holdup of liquid should be high. Stirred tanks or bubble columns will be suitable. 

Region III, P < 0.02. Reaction is slow and occurs in the bulk hquid. Interfacial area and liquid holdup should be high, especially the latter. Bubble columns will be suitable. 

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. 

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,... 

Bubble columns rely on nozzles, mixing plates, and impellers within the reactor to control the bubble size, which determines the interfacial area between gas and liquid phases. Clearly, the interfacial area can be varied over a wide range by suitable design of the mixer and flow pattern. 

The mass transfer coefficients considered so far - namely, kQ,kj, KQ,andKj - are defined with respect to known interfacial areas. However, the interfacial areas in equipment such as the packed column and bubble column are indefinite, and vary with operating conditions such as fluid velocities. It is for this reason that the volumetric coefficients defined with respect to the unit volume of the equipment are used, or more strictly, the unit packed volume in the packed column or the unit volume of liquid containing bubbles in the bubble column. Corresponding to /cg, Kq, and we define k a, k, a, K, /i, and K a, all of which have units of (kmol h m )/(kmol m ) - that is, (h ). Although the volumetric coefficients are often regarded as single coefficients, it is more reasonable to consider a separately from the Ar-terms, because the effective interfacial area per unit packed volume or unit volume of liquid-gas mixture a (m m ) varies not only with operating conditions such as fluid velocities but also with the types of operation, such as physical absorption, chemical absorption, and vaporization. 

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. 

The correlations detailed in Sections 7.6.2.1-7.6.2.5 [17,18] are based on data for the turbulent regime with 4 bubble columns, up to 60 cm in diameter, and for 11 liquid-gas systems with varying physical properties. Unless otherwise stated, the gas holdup, interfacial area, and volumetric mass transfer coefficients in the correlations are defined per unit volume of aerated liquid, that is, for the liquid-gas mixture. 

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. 

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. 

The reaction is slow and occurs in the bulk of the liquid. Mass transfer serves to keep the bulk concentration CAL of A, close to the saturation value CAi (Fig. 4.2e) and sufficient interfacial area should be provided for this purpose. However, a high liquid hold-up is the more important requirement, and a bubble column is likely to be suitable. If CAL > CAt, then ... 

One of the purposes of giving Example 4.4 (on the chlorination of toluene) is to demonstrate the effect of different gas flowrates on the performance of a bubble column. The higher the gas flowrate, the larger the interfacial area a per unit volume of dispersion gas-liquid mass transfer will take place more readily and the concentration of the dissolved gas in the liquid will rise. Although the rate of reaction will increase, this is offset, as will be seen, by the disadvantage of a lower... 

Fig. 4.8. Alternative bubble columns each using the same volume of liquid, and the same total gas flowrate, (a) Low aspect-ratio column low superficial gas velocity ue small interfacial area a low rate of reaction, (b) High aspect-ratio column high superficial gas velocity u0 large interfacial area <2 higher rate of reaction. Note For case (b) it must not be assumed that both gas and liquid phases are well-mixed...

Akita, K. and F. Yoshida, "Bubble Size, Interfacial Area, and Liquid-Phase Mass Transfer Coefficient in Bubble Columns," l EC Proc. Des. Dev. 13 (1974) 84-91. 

G. Vazquez, M.A. Cancela, C. Riverol, E. Alvarez, J.M. Navaza, Application of the Danckwerts method in a bubble column. Effects of surfactants on mass transfer coefficient and interfacial area, Chem. Eng. J. 78 (2000) 13-19. 

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. 

M. Bouaifi, G. Hebrard, D. Bostoul, M. Roustan, A comparative study of gas hold-up, bubble size, interfacial area and mass transfer coefficients in gas—liquid reactors and bubble columns, Chem. Eng. Proc. 40 (2001) 97-111. 

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