Читать книгу Agitator Design for Gas-Liquid Fermenters and Bioreactors - Gregory T. Benz - Страница 59

It is not the purpose of this book to be a treatise on rheology. Indeed, many books have been written on the subject. However, due to the extreme difficulty of dispersing gases into viscous liquids, highly viscous liquids are not used or produced in industrially significant fermenters. (I know, some will object, saying they have viscous fermentations. But it is a matter of perspective. While a Gellan gum fermentation may seem viscous to someone operating a fermenter, it doesn’t seem so viscous to those in the polymer industry. But they do not disperse gas into plastic or synthetic rubber melts. The difference is that a very high fermentation viscosity may be 5000 cP at the impeller, whereas in the polymer industry, they may see hundreds of thousands or even several million cP.)

Therefore, we will focus on a few simple models that do an adequate job of describing the behavior of fermentation broths. We will look at shear stress versus shear rate and assume that normal (i.e. at right angles to the shear plane) stresses, such as found in viscoelastic fluids, do not apply. We will also confine ourselves to time‐independent models, because the fluids in fermenters are always under shear and in a quasi‐steady state.