Shear damage

To avoid this damage great efforts were made to improve the cultivation systems. Feder and Tolbert [5] developed special stirrers with low shear damage and Lehmann et al. [6] replaced gas bubbling through indirect aeration by diffusion of oxygen through porous polypropylene membranes. 

Many alternative fermenters have been proposed and tested. These fermenters were designed to improve either the disadvantages of the stirred tank fermenter-high power consumption and shear damage, or to meet a specific requirement of a certain fermentation process, such as better aeration, effective heat removal, cell separation or retention, immobilization of cells, the reduction of equipment and operating costs for inexpensive bulk products, and unusually large designs. 

Animal cells do not have a cell wall and are, thus, highly susceptible to the effects of shear stress. Cells respond to hydrodynamic stress within minutes, altering their metabolism and the gene expression pattern (Nol-lert et al., 1991). Under sub-lethal levels of shear stress, there is initially an increase in passive transmembrane transport, simultaneously with damage to surface receptors. The plasma membrane is generally the main site for shear damage, and it may lose its capacity to mediate the transport of ions and molecules, so that the cell loses its viability. It has been demonstrated... 

Constant impeller tip speed (often used to limit shear damage to sensitive fluids) ND D2 Industrial rule-of-thumb keep tip speed below 10 m s 1 for shear sensitive products... 

Micro-mixing is important in such stirring operations where the result depends upon the size of the smallest eddies, for example dispersion processes in liquid/liq-uid systems and shear damage to microorganisms. Therefore, it is not surprising that in such tasks it is the volume related power, P/V, that counts it is an intensively formulated process quantity. 

Mechanically agitated reactors have been used for cultivation of plant cells (Kato et al., 1972 Tanaka, 1981). Ajar fermenter with a six fiat-blade turbine and a modified paddle has been used by Tanaka (1981) and a similar jar fermenter with two disk turbine impellers has been used by Kato et al. (1972) at about 50 rpm with no significant shear damage to plant cells. Paddle-type impellers were found to be more appropriate (less shear damage) than flat-blade turbine type impellers (Kato et al., 1972). The only production-scale reactor used for shikonin production in Japan is also an agitated vessel. 

Fig. 1.36 Schematic indicating the two modes of shear damage (a) in-plane, and (b) interlaminar.

Figure 9.17. Successive images taken by the image-converter camera at the speed of 2x10 frames/s. The nylon projectile was impacted at the velocity of 3.6 km/s. The notations A, B, and C labeled on the image at 5.5 ps mean precursor wave, reflection wave from side, and shear damage zone expansion, respectively. (Figure from Arakawa etal. [28].)...

In summary, the effect of dextrans can be positive or negative depending on the cell type, bioreactor or even dextran grade and make used, but all experimental evidence so far suggests that dextran offers no mechanical protection. Dextrans and modified starches should not be used as protectants against shear damage. 

Low-shear pumps can be effective in limiting shear damage and include lobe and diaphragm pumps. The reader is referred to an analysis of pump and transfer energy by Mersmann (2001, pp. 454ff.) to aid in design of slurry transfer and control of flow. 

Shear damage on transfer can be eliminated by gravity transfer when equipment layout includes the necessary vertical clearances. 

Issues Slow growth rate, shear damage during long approach to equilibrium. Loss of yield because actual equilibrium solubility was not achieved. 

Increase impeller speed (reduce him thickness) or reduce it (less shear damage). 

As with any crystallization process, reactive crystallization will, in general, produce fine particles unless the entire operation is run within the metastable region. This condition can be realized by provision of heavy seeding and by slow addition to control supersaturation at a low level. Adequate mixing is necessai-y, but shear damage must be avoided by selection of the correct impeller speed and type. 

Fine powder PTFE is susceptible to shear damage, particularly above its transition point (19°C). Handling and transportation of the containers could easily subject the powder to sufficient shear rate to spoil it if the resin temperature is above its transition point. The phenomenon caWedfibrillation (Fig. 5.26) occurs when particles rub against each other, in which fibrils are pulled out of the surface of PTFE particles. Uncontrolled fibrillation must be prevented to insure good quality production from the powder. Premature fibrillation leads to the formation of lumps which cannot be broken up easily. 

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