Stirrer heat generation

Homogenisation with respect to temperature This effectively means that if conditions are to be adiabatic (or nearly so), the outer layers of the mixture which are in contact with the vessel and the gas phase must be mixed with the bulk so effectively that no important temperature difference persists. If a reaction mixture is to be kept isothermal, the stirring must be even more effective, and there is a danger that the stirrer mechanism may dissipate so much energy that the heat generated in the solution becomes appreciable. 

The mass transfer rate (solubility, diffusion, and erosion) was lowest for degraded starch with a magnetic stirrer (washing machine action) in cold water (Table VII). The highest mass transfer rate was for degraded starches when used with ultrasonics at the ambient temperature (Table VII). The temperature of 40 C was a result of a small amount of heat generated by the transducer and water agitatlm. The ultrasonic power used in these laboratory tests was 75 W. 

Numerous types of stirrers are used in practice, and Fig. 3.2 shows the most commonly used ones. The stirrers for low-viscosity media are typically marine propeller and pitched-blade turbine stirrers, which cause axial fluid motion, and flat-blade turbines and impellers, which generate radial fluid motion. The former stirrers are suitable for uniformly suspending solids. The latter type are the preferred ones for carrying out exothermic reactions, like autooxidation reactions (see Section 8.3), where the heat generated dining the process has to be removed effectively through the reactor walls. 

Merzhanov et al. stated in the latter case that the heat transfer in molten tetryl was performed by convection only, because molten tetryl was stirred vigorously by means of a piston stirrer, so that, while the temperature distribution in molten tetryl was uniform, the temperature gradient was present only in the vessel wall [40]. On the other hand, the balance between the rate of heat generation in a liquid and the rate of heat transfer from the liquid to the atmosphere at the critical state for the thermal explosion which exists at the end of the early stages of the selfheating process, is dealt with in the approach adopted herein with regard to the thermal explosion of the liquid. The latter orthodox approach to the thermal explosion of every liquid has thus no concern with any convective flow in liquids. 

Heat generation (kW/m ) Power input (kWm-3) Scale of operation (m3) Installed stirrer power (kW) Process temperature (°C)... 

It is important to maintain a uniform reaction environment within the vessel chamber by using a stirrer to agitate and mix the reactants. Gaseous discharge is vented through filters to the outside environment. As the reactions may generate substantial heat (exothermic) and pressure, or may even be potentially explosive, special precautionary features are designed into the vessel. 

In a 5-I. round-bottom flask equipped with a mechanical stirrer are mixed 385 g. (335 cc.) (3.8 moles) of 95 per cent furfural (Org. Syn. 1, 49) (Note 1) and 3 1. of water. Then 500 g. (630 cc.) (8.6 moles) of acetone is added. The mixture is stirred and cooled to io° and to it is added 75 cc. of 33 per cent sodium hydroxide solution (Note i), whereupon some heat is generated. Without cooling, the stirring is continued for four hours. At the end of this time 10 per cent sulfuric acid is added until the mixture is acid to litmus (about 350 cc.) (Note 2). The two layers which have formed are separated and the upper aqueous layer is distilled (Note 3) under ordinary pressure until the distillate no longer forms two layers. The bottom layer of this distillate is added to the original lower layer and distilled under reduced pressure from a i-l. modified Claiscn flask (Org. Syn. 1, 40) provided with an air condenser and heated in an oil bath. The... 

Poly-tetrafluoroethylene or -hexafluoropropylene sealing tapes burned vigorously in contact with the alloy in a helium atmosphere [1], A teflon-coated magnetic stirrer bar used to stir the alloy under propane atmosphere ignited when the speed was increased and generated enough heat to melt the glass. Triboelectric initiation was postulated [2],... 

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