Agitated reactors hydrodynamics

If the impeller is operated below a minimum tip speed (2.25 m/s for RT), the reactor hydrodynamics are dominated by the gas flow and the reactor acts as a bubble column. At this point, gas-liquid mass transfer has an exclusive dependence on the superficial gas velocity (Charpentier, 1981). Since the intent of the STR is to provide agitation that would be superior to gas sparging alone, STRs are operated such that impeller agitation dominates the hydrodynamics (Nishikawa et al., 1981). 

Lemoine R, Morsi BI. Hydrodynamic and mass transfer parameters in agitated reactors. Part II gas-holdup, Sauter mean bubble diameters, volumetric mass transfer coefficients, gas-liquid interfa-dal areas, and liquid-side mass transfer coefficients. InL J. Chem. React. Eng. 2005 3 A20. 

Shaker tube reactors are commonly used for the evaluation of catalysts at elevated pressure. The liquid reactant and powdered catalyst are introduced into a metal or glass ampoule, which is sealed and pressurized to a predetermined level with the gaseous reactant. The ampoule is immersed into a thermostatted liquid and maintained at this temperature for a certain period of time while shaking. Then the reactor is opened and the reaction mixture analysed. Ampoules of ca. 10-100 cm are typically used. The usefulness of data obtained using such reactors for process scale-up is nearly zero due to poor agitation and unknown hydrodynamics in the ampoule. These reactors are, however, very useful for fast screening of catalysts. 

The simplest device providing rudimentary control of hydrodynamics is the Lewis cell [ 19 ], of which a variation by the present author is easier to construct. It consists of a reactor containing two contra-rotated agitators positioned on either side of a liquid-liquid interface, Fig. 5.16. 

Fig. 37. Production diagram of antifoaming silicone emulsion 1 - reactor agitator 2, 3 - batch boxes 4 - reactor emulsifier 5 - hydrodynamic changer 6 -pump 7- collector...

Gas holdup is an important hydrodynamic parameter in stirred reactors, because it determines the gas-liquid interfacial area and hence the mass transfer rate. Several studies on gas holdup in agitated gas-liquid systems have been reported, and a number of correlations have been proposed. These are summarized in Table VIII. For a slurry system, only a few studies have been reported (Kurten and Zehner, 1979 Wiedmann et al, 1980). In general, the gas holdup depends on superficial gas velocity, power consumption, surface tension and viscosity of liquids, and the solid concentration. The dependence of gas holdup on gas velocity, power consumption, and surface tension of the liquid can be described as... 

The deactivation of the diromia-alumina catalyst for the oxidation of gas streams A and B was smdied in the "fluid-bed" reactors under similar hydrodynamic conditions. The gas flow was just suHldcnt to keep the bed agitated at the reaction temperature axtd thus the hydrodynamic regime ofihcsc reactors corresponded to conditions of incipient fluidization. The initial temperature <360 for the oxidation of stream A and 385... 

Midoux N., Charpentier J.-C., Mechanically agitated gas-liquid reactors. Part 1 Hydrodynamics Part 2 Interfacial area, Int. Chem. Engng. 24 (1984) 2, p. 249-287 und 3,... 

Pericleous KA, Patel MK (1987) The Modelhng of Tangential and Axial Agitators in Chemical Reactors. PhysicoChemical hydrodynamics (PCH), 8(2) 105-123, 1987. 

Gogate [151] also compares the energy efficiency of acoustic and hydrodynamic techniques with that of conventional reactors in the case of trans-esterification of vegetable oils. Cavitation can be successfully applied to trans-esterification reactions to afford a more than 90% yield of the product as per stoichiometry in a reaction time as low as 15 min. The technique hence appears to be very effective compared to the conventional approach. Hydrodynamic cavitation is about 40 times more energy efficient than acoustic cavitation and 160-400 times more efficient than the conventional agitation/heating/refluxing method. 

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