Single-phase liquids agitated

FIG. 15-23 Power for agitation impellers immersed in single-phase liquids, baffled vessels with a gas-liquid surface [except curves (c) and (g)]. Curves correspond to (a) marine impellers, (h) flat-blade turbines, w = dj/5, (c) disk flat-blade turbines witb and without a gas-liquid surface, (d) curved-blade turbines, (e) pitcbed-blade turbines, (g) flat-blade turbines, no baffles, no gas-liquid interface, no vortex. 

Consider a stirred tank vessel having a Newtonian liquid of density p and viseosity p, is agitated by an impeller of diameter D, rotating at a rotational speed N. Let the tank diameter be D, the impeller width W, and the liquid depth H. The power P required for agitation of a single-phase liquid ean be expressed as ... 

The basic principles employed in the preparation of parenteral products do not vary from those widely used in other sterile and nonsterile liquid preparations. However, it is imperative that all calculations be accurate and precise. Therefore, the issue of parenteral solution scale-up essentially becomes a liquid scale-up task, which requires a high degree of accuracy. A practical yet scientifically sound means of performing this scale-up analysis of liquid parenteral systems is presented in this chapter. The approach is based on the scale-of-agitation method. For single-phase liquid systems, the primary scale-up criterion is equal liquid motion when comparing pilot-size batches to a larger, production-size batches. 

The power dissipated by an impeller rotating in a homogenous, single-phase liquid in a baffled vessel is a function of the type of impeller, the flow regime in which the impeller operates (laminar vs. turbulent), which is, in turn, a function of the impeller Reynolds number. Re = plAD /p, and a number of geometric ratios. For an agitated vessel, the impeller power dissipation, P, and the power dissipation per unit liquid mass, , can be calculated, respectively, from ... 

The flow patterns of single-phase liquids in tanks agitated by various types of impeller have been reported in the literature. The experimental techniques used include the use of coloured tracer liquid, mutually-buoyant particles, hydrogen bubble generation and mean velocity measurements using pitot probes, hot-rdm devices and lasers. 

Typical equipment for low viscosity liquids consists of a vertical cylindrical tank, with a height to diameter ratio of l.5 to 2, fitted with an agitator. For low viscosity liquids, high-speed propellers of diameter about one-third that of the vessel are suitable, running at 10-25 Hz. Although work on single-phase mixing of low viscosity liquids is of limited... 

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

In a single-phase system there are usually less transport limitations, resulting in a higher productivity per unit volume. This is particularly true when the liquid has a relatively high viscosity, since the dispersion of a gas in a viscous liquid is mostly not very effective. However, reaction in viscous liquids requires powerful agitation, in order to minimize transport limitations. 

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