Shear-Sensitive Mixing

For the laminar flow region of Newtonian fluid, shear stress r is equal to the viscosity /t times the velocity gradient du/dy as 

---

Bowen, R. L., "Unraveling the Mysteries of Shear-sensitive Mixing Systems," Chem. Eng. 9 (1986) 55-63. 

In this chapter, we study various correlations for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficient, which are vital tools for the design and operation of fermenter systems. Criteria for the scale-up and shear sensitive mixing are also presented. First of all, let s review basic mass-transfer concepts important in understanding gas-liquid mass transfer in a fermentation system. 

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. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

Two-stage rotors are also available for continuous mixers (Fig. 22), which incorporate a second screw section to enable down-stream feeding of additives such as heat or shear sensitive fillers, or extraction of volatiles from the mixture [147]. For a fixed rotor geometry, rotor speed, fiow rate, barrel temperature and orifice opening are the principal operational variables which control mixing intensity. 

Stirred- tank 1. Flexible and adaptable 2. Wide range of mixing intensity 3. Can handle high viscosity media 1. High power consumption 2. Damage shear sensitive cells 3. High equipment costs... 

The design and operation of a bioreactor are mainly determined by biological needs and engineering requirements, which often include a number of factors efficient oxygen transfer and mixing, low shear and hydrodynamic forces, effective control of physico-chemical environment, easy scale-up, and so on. Because some of these factors can be mutually contradictory, it is difficult to directly employ a conventional microbial reactor to shear-sensitive plant tissue cultures. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

The optimal configuration depends on numerous factors, including the required gas transfer and mixing rates, and an acceptable range of shear rates. Some factors may be mutually contradictory as in a crystallizer where the required gas-liquid transfer rate is best achieved by increasing turbulence and shear yet the crystals are shear sensitive. In these cases, successful design carefully balances the contradictory factors. 

Wide range of mixing intensity Damage shear sensitive cells... 

---