Pitch viscosity pattern

Figure 13.7 Viscosity pattern on heating a coal tar pitch, with mesophase formation...

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

The Ekato intermig impeller has reverse pitch on the inner and outer blades and they are almost always used with multiple impellers. They are used at high D/T and promote a more uniform axial flow pattern than other turbine impellers. They are advertised to be very effective for solids suspension, blending, and heat transfer in the medium viscosity range. Lower Nrc limit not given by Ekato (9), perhaps 5. 

Once the swirling is stopped, the specific flow pattern in the vessel depends on the type of impeller. Propeller agitators usually drive the liquid down to the bottom of the tank, where the stream spreads radially in all directions toward the wall, flows upward along the wall, and returns to the suction of the propeller from the top. Propellers are used when strong vertical currents are desired, e.g., when heavy solid particles are to be kept in suspension. They are not ordinarily used when the viscosity of the liquid is greater than about 50 P. Pitched-blade turbines with 45° down-thrusting blades are also used to provide strong axial flow for suspension of solids. 

As-spun mesophase pitch filaments exhibit a high degree of preferred orientation of the discotic molecules along the fiber axis, because the mesogen molecules are oriented under low shear stresses and because of the flow pattern of the melt. The degree of preferred orientation depends on the viscosity of the melt and the final diameter of the fiber. For 100% anisotropic mesophase coal tar pitch, the best results are obtained with a low viscosity melt and small, i.e., 10 ym diameter, fibers [25]. 

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