Bubble reactors, mechanically agitated

Contactors in which gas is dispersed into the liquid phase Plate columns (including control cycle reactors) Mechanically agitated reactors (principally stirred tanks) Bubble columns Packed bubble columns Sectionalized bubble columns Two-phase horizontal contactors Cocurrent pipeline reactors Coiled reactors Plunging jet reactors, ejectors Vortex reactors... 

Bubble Reactors In bubble columns the gas is dispersed by nozzles or spargers without mechanical agitation. In order to improve the operation, redispersion at intei vals may be effected by static mixers, such as perforated plates. The liquid may be clear or be a slurry. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

In a bubble-column reactor for a gas-liquid reaction, Figure 24.1(e), gas enters the bottom of the vessel, is dispersed as bubbles, and flows upward, countercurrent to the flow of liquid. We assume the gas bubbles are in PF and the liquid is in BMF, although nonideal flow models (Chapter 19) may be used as required. The fluids are not mechanically agitated. The design of the reactor for a specified performance requires, among other things, determination of the height and diameter. 

In the absence of mechanical agitation and for bubbles with diameter less than 2.5 mm (the usual size range for sluny reactors), the following conelation of Calderbank is available (Smith, 1981) ... 

In the following we will describe mechanically agitated slurry reactors and slurry bubble columns. 

The choice of a bubble column or an agitated vessel depends primarily on the solubility of the gas in the liquid, the corrosiveness of the liquid (often a gas compressor can be made of inexpensive material, whereas a mechanical agitator may have to be made of exotic, expensive materials), and the rate of chemical reaction as compared with the mass-transfer rate. Bubble columns and agitated vessels are seldom used for gas absorption except in chemical reactors. As a general rule, if the overall reaction rate is five times greater than the mass-transfer rate in a simple bubble column, a mechanical agitator will be most economical unless the mechanical agitator would have to be made from considerably more expensive material than the gas compressor. 

Conventional reactor designs have been optimized to create a basis for comparison. The mechanically agitated vessel achieves the highest yield of 74.4%, followed by the bubble column reactor with a yield of 72.9%, and the co-as well as the countercurrent reactors both achieving a yield of 69.5%. 

Conventional mechanically agitated gas-liquid reactors, wherein gas and liquid make contact in batch, semibatch, or continuous mode, are widely used in processes involving chlorination, sulfonation, hydrogenation, selective absorptions in amine solutions, etc. (Doraiswamy and Sharma, 1984). These reactors are popular for laboratory studies because of their simplicity in construction and low cost. As a rule of thumb with noncorrosive liquids, the mechanically agitated reactor is most economical when the overall reaction rate is five times greater than the mass transfer rate in a bubble column. If a... 

Three phase, upflow, bubble column reactors are used in the process industry because of their simplicity and good liquid mixing characteristics, without the need for mechanical agitation. 

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