Shear stirring systems

Due to their different traits, the bioreactors initially designed for microbial cultures have to be adapted to plant cell cultures. As plant cells are very sensitive to shear stress, the first type of modification in the stirred tanks was to change the shape and disposition of the blades of the bioreactor. Big blades present in bioreactors for bacterial and yeast cultures are removed and replaced by a marine propeller, which is less harmful for plant cells. It is also possible to change the big blade at the bottom of the bioreactor for small blades lined up along the entire axis of the stirred system. Similarly, in the paddle system, the large lower blades of the classical bioreactors are replaced by a completely perforated central blade, which moves the culture medium by the peripheral side, or the helix system where the blades are replaced by an axis shaped like a corkscrew (Fig. 89.12). 

Here the different steps of the preparation of the nanocomposites are briefly summarized. First, PPO (a low molar mass polymer powder supplied by Sabic-IP, the Netherlands] was end-capped by acetylation in order to avoid inhibition of the styrene polymerization by phenolic OH groups. It was subsequently mixed with styrene and hexadecane. Then the mixture was added to an aqueous solution of 4-dodecylbenzenesulfonic acid (SDBS). This mixture of SDBS/ hexadecane was chosen as a stabilization system since it is suitable to achieve and guarantee efficient stabilization of polymer particles of sizes smaller than 1 ymP The emulsification process was split up into two steps, namely, a first pre-emulsification step performed by ultra-high shear stirring, followed by ultrasonication in order to obtain submicron particles. Finally, the polymerization was initiated and carried out at 80°C under inert atmosphere (final monomer conversion of 90 %]. The latex obtained had a solid content of 23.6 wt% and contained 10 wt% of PPO dissolved in PS and had a particle size of 100 nm. At the end of the polymerization, the PS molar mass was about... 

Preparation of Emulsions. An emulsion is a system ia which one Hquid is coUoidaHy dispersed ia another (see Emulsions). The general method for preparing an oil-ia-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosia acid, and separately mix a proportionate quantity of an alkah, such as potassium hydroxide, with the water. The alkah solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use ia emulsions, as it is readily miscible with most oils. 

Thus in a mixed system, as e.g. in a stirred tank, the rate of agglomeration additionally depends on the shear field and therefore on the energy dissipation e in the vessel. Furthermore, in precipitation systems solution supersaturation plays an important role, as the higher the supersaturation, the stickier the particles and the easier they agglomerate (Mullin, 2001). This leads to a general formulation of the agglomeration rate... 

Figure 11 shows the reference floe diameter for viscometers as a function of shear stress and also the comparison with the results for stirred tanks. The stress was determined in the case of viscosimeters from Eq. (13) and impeller systems from Eqs. (2) and (4) using the maximum energy density according to Eq. (20). For r > 1 N/m (Ta > 2000), the disintegration performance produced by the flow in the viscosimeter with laminar flow of Taylor eddies is less than that in the turbulent flow of stirred tanks. Whereas in the stirred tank according to Eq. (4) and (16b) the particle diameter is inversely affected by the turbulent stress dp l/T, in viscosimeters it was found for r > 1.5 N/m, independently of the type (Searle or Couette), the dependency dp l/ pi (see Fig. 11). 

Much higher shear forces than in stirred vessels can arise if the particles move into the gas-liquid boundary layer. For the roughly estimation of stress in bubble columns the Eq. (29) with the compression power, Eq. (10), can be used. The constant G is dependent on the particle system. The comparison of results of bubble columns with those from stirred vessel leads to G = > 1.35 for the floccular particle systems (see Sect. 6.3.6, Fig. 17) and for a water/kerosene emulsion (see Yoshida and Yamada [73]) to G =2.3. The value for the floe system was found mainly for hole gas distributors with hole diameters of dL = 0.2-2 mm, opening area AJA = dJ DY = (0.9... 80) 10 and filled heights of H = 0.4-2.1 m (see Fig. 15). 

Viscosimeter flow produces less stress than technical reactors (see Sect. 6.3.3). From the results with the floccular particle system it can be derived the following relationship (30). It estimates the turbulent stress of a technical, fully baffled stirred reactor which leads to the same damage of particles as the viscosimeter flow with the shear stress r. 

For reactors with free turbulent flow without dominant boundary layer flows or gas/hquid interfaces (due to rising gas bubbles) such as stirred reactors with bafQes, all used model particle systems and also many biological systems produce similar results, and it may therefore be assumed that these results are also applicable to other particle systems. For stirred tanks in particular, the stress produced by impellers of various types can be predicted with the aid of a geometrical function (Eq. (20)) derived from the results of the measurements. Impellers with a large blade area in relation to the tank dimensions produce less shear, because of their uniform power input, in contrast to small and especially axial-flow impellers, such as propellers, and all kinds of inclined-blade impellers. 

Quantities useful for predicting phase continuity and inversion in a stirred, sheared, or mechanically blended two-phased system include the viscosities of phases 1 and 2, and and the volume fractions of phases 1 and 2, and ij. (Note These are phase characteristics, not necessarily polymer characteristics.) A theory was developed predicated on the assumption that the phase with the lower viscosity or higher volume fraction will tend to be the continuous phase and vice versa (23,27). An idealized line or region of dual phase continuity must be crossed if phase inversion occurs. Omitted from this theory are interfacial tension and shear rate. Actually, low shear rates are implicitly assumed. 

---