Hold-up in Hot Sparged Reactors

When gas is sparged into a hot liquid, there is an immediate change in the thermodynamic status as the liquid vaporizes into the bubbles. As noted above, this process continues until the latent heat required removes all that is available. In continuous operation the liquid will settle at a temperature below its nominal boiling at a value determined by the rate of supply of sparge gas and heat. Any sparged or evolved gas will produce this effect. The vapor dilutes the sparged gas, so reduces the driving force for mass transfer. 

Extensive work with configurations involving up-pumping hydrofoUs, which have become a generally favored arrangement for large gas-liquid reactors, has led to a correlation for overall gas retention that is a function of the absolute temperature. Specifically, in an air-water system with multiple impellers, the 

Example 11-4 Void Fraction in a Gas-Liquid Reactor. A void fraction of 7% is measured in an aerated reactor containing water at 20°C. What will be the void fraction if the reactor is operated at the same specific power input and superficial gas velocity (after allowing for the contribution of water vapor) at 90°C  

SOLUTION Void fraction varies with absolute temperature, 0. For this example 

The mean residence time of gas passing through a reactor at 20°C (when the partial pressure of water is negligible) is 1 min [i.e., the sparge rate is 1 vvm (1 volume of gas per volume of liquid per minute)], at which flow rate the void 

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