Shear-sensitive materials

Many reactions involve shear-sensitive materials, which severely limit the maximum mixing rate and make impeller and reactor design important. Mixing becomes the limiting factor. 

Traditional Apparatus. The above reference need only be expanded to Include the common use of cylindrical culture vessels (1), in place of the spherical reaction flasks, for shear sensitive materials and simple boiling water type baths (2) as their heating medium. 

Note This section is intended to deal with the transfer of shear sensitive materials, including product or partially manufactured product. A single piece of equipment can be placed in either a low or high shear class, depending on its operating parameters. If a single piece of equipment is capable of performing multiple discrete unit operations, the unit has been evaluated solely for its ability to transfer materials. 

This indicates that a higher cross-flow velocity under turbulent conditions can result in more than proportional increase in the pressure drop requiring larger pump discharge pressure to maintain a specified recirculation rate. This limits the number of modules that may be placed in series to minimize capital costs. Typical range of cross-flow velocity values is 2 to 7 m/s. The choice of pump is critical to obtain efficient fluid recirculation. It is critical to understand the shear sensitivity of the fluid/particle to be processed to determine the optimal cross-flow velocity in situations where shear-sensitive materials are involved. 

Evaluation of Mechanical Properties Viscosity of Shear-Sensitive Materials Dispersed Systems Polymer Solutions Viscoelasticity... 

Thus shear sensitive materials are less likely to be damaged on scale-up. 

The low head pressure generated by twin-screw extruders initially limited their use to processing of shear-sensitive material, such as PVC, and to compounding. 

The mixing in OBRs is very uniform, as the radial and axial velocities are of the same magnitude (Baird et al., 2003). This has been shown to be an advantage when handling shear-sensitive materials, such as certain pharmaceutical crystals, and in... 

The rheological and thermal conductivity properties of the polymer matrix determine the heat necessary to melt the material. Relatively shear-sensitive materials become less viscous as they pass through the nozzle. For example, nylons form low viscous fluids when melted while polyethylene can undergo considerable mechanical working, producing heat necessary for plastica-tion. 

Parallel twin-screw extruders have more surface area compared to conical twin screws. However, the flight flanks in a conical have more surface area than in a parallel extruder and can transfer more heat to the material in the channel. The screw surface area is 40% larger compared to a parallel extruder. Due to the larger channel volumes, conical extruders generate less shear heat and more conductive heat compared to parallel twin-screw extruders, making them better for processing shear-sensitive materials such as PVC. 

When the jump is made from P/pVtank to s, an assumption that the turbulence is homogeneous is implied. This assumption is clearly a poor one in a stirred tank, where the levels of turbulence can vary by a factor of 100 from the impeller to the bulk. Generation and dissipation are vastly different between the impeller region and the regions away from the impeller. For shear-sensitive materials such as cells, their survival depends more on the maximum shear they see than on the average. In such cases, using P/pVtank as some kind of an average dissipation is... 

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