Impellers, agitation sizing example

The size of the bubbles produced in the reactor and the gas volume fraction will depend on the agitation conditions, and the rate at which fresh hydrogen is fed to the impeller, as shown in Fig. 4.20. (Some hydrogenation reactors use gas-inducing impellers to recirculate gas from the head-space above the liquid while others use an external compressor.) For the purposes of the present example, the typical values, db = 0.8 mm and eg m 0.20, will be taken. Also dependent to some extent on the agitation conditions are the values of the transfer coefficients kL and k, although these will depend mainly on the physical properties of the system such as the viscosity of the liquid and the diffusivity of the dissolved gas. The values taken here will be kL = 1.23 x 10 5 m/s and k, = 0.54x 10"3 m/s. 

The power number provides important design information about the correct motor size necessary to operate an impeller at a given speed. However, these calculations do not give any indication of whether or not the agitation produced is adequate for process requirements. The following example shows a method for determining the horsepower and speed required to achieve a given process result. 

The choice of agitator is dictated mainly by the type of mixing required, for example heat transfer, gas absorption, or emulsification, the suspension of solid particles in a liquid and the type of material being mixed, for example its viscosity, specific gravity, heat conductivity or size of the suspended particles. Generally, paddle, impeller, and anchor agitators are rotated at lower speeds (up to 150 r.p.m.) than the propeller or turbine type (up to 2900 r.p.m. in smaller vessels). The efficiency of agitation is considerably increased by the use of... 

Agitated vessels represent yet another example of an unusual and not easily quantifiable geometry. The prominent irregularity here is the shape and size of the impeller and the geometry of its blades. Internal baffles, which are frequently used to enhance transport rates, are an additional unusual feature. 

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