Agitators critical speed

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

The studies of Juvekar (1976) and Ramanarayanan and Sharma (1982) with a 0.2 m i.d. multistage contactor with d,/dx = 0.5 showed that both aL and kLaL are independent of superficial gas velocity above the critical speed of agitation, and they vary linearly with impeller speed. Both aL and kLaL were independent of the number of stages under otherwise identical conditions of dfdj, N, ug, and gas-liquid system. Thus, the results for a single stage are applicable to multistage systems. This conclusion needs to be verified for larger-diameter columns. 

A typical formula for calculating the first natural frequency (critical speed) of an agitator shaft considers the shaft stiffness, the shaft length, the weights of impellers and shaft, and the rigidity of the shaft mounting ... 

The critical-speed problem may in some cases be solved hand in hand with a common problem related to dynamic loads. One source of dynamic loads on an agitator shaft is the waves and vortices that occur when an impeller operates near the liquid surface, such as when a tank fills or empties. 

Adding stabilizer fins to the impeller blades will help reduce some of these loads. Such fins also permit the agitator to operate closer to critical speed, perhaps at 80 percent rather than 65 percent. 

Critical speeds of agitator Critical impeller speed for solid suspension The speed shonld be adequate to keep the particles suspended. The critical speed for snspension in the presence of gas, Nj q, is calcnlated from the following correlation (Rewatkar and Joshi 1991d) ... 

Agitator designs in dead-end converters are critical because of the need to suck or splash the hydrogen from the head space of the vessel into the oil (Patterson, 1983). The usual design consists of a shaft with two or three turbine-type impellers, the top one being sited just below the oil level, such that a vortex is produced which sucks the head space gas into the oil. It is therefore important that the charge size should be reasonably accurately measured. Agitator shaft speed is usually between 75 and 150 rev./min. 

Critical speed of agitation for complete suspension of solids (rev/s) Critical speed of agitation for just suspension of solids in solid-liquid... 

In aU these cases, there is an increase in the critical speed for suspension of soUds when a gas is introduced. Most investigators have accounted for this increase through an incremental speed of agitation, Thus, the minimum speed for just suspension,... 

The shaft must not operate at speeds higher then 70% of the first critical speed if the impeller is not dynamic y balanced, or higher than 85% of the first critical speed if the impeller is dynamically balanced. The deflection of the shaft should be limited, particularly in the case of anchor agitators, so that the blades do not hit the walls or baffles. 

One factor to be aware of when making this decision is the critical speed of the shaft and impeller assembly. Running at a higher speed may reduce the torque, but the agitator may require a larger shaft anyway because the operating speed is... 

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