Agitation of Fluids

Various types of vessels and tanks of differing geometrical shapes and sizes are used for mixing fluids. The top of the vessel may be open or sealed. A typical batch reactor, as discussed in Chapter 4, is applicable in many operations. The vessel bottom is normally not flat. 

Dispersing a gas m a liquid as fine bubbles (e.g., oxygen from air m a suspension of microorganism for fermentation or for activated sludge treatment). 

Agitation of the fluid to increase heat transfer between the fluid and a coil or jacket. 

Suspension of fine solid particles m a liquid, such as m the catalytic hydrogenation of a liquid where solid catalyst and hydrogen bubbles are dispersed m the liquid. 

Dispersion of droplets of one immiscible liquid m another (e.g., in some heterogeneous reaction process or liquid-liquid extraction). 

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One of the most ubiquitous operations in the chemical process industries is the agitation of fluid systems. Such agitation is undertaken to bring about the mixing (i.e., blending, homogenizing) of such systems. 

The pour point of crude oils is measured to give an approximate indication as to their pumpability . In fact, the agitation of the fluid brought on by pumping can stop, slow down or destroy the formation of crystals, conferring on the crude additional fluidity beyond that of the measured pour point temperature. 

Agitation of the Fluid. Agitation of the solvent increases local turbulence and the rate of transfer of material from the surface of the particles to the bulk of the solution. Agitation should prevent settling of the soHds, to enable most effective use of the interfacial area. 

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

Heating or cooling of process fluids in a batch-operated vessel is common in the chemical process industries. The process is unsteady state in nature because the heat flow and/or the temperature vary with time at a fixed point. The time required for the heat transfer can be modified, by increasing the agitation of the batch fluid, the rate of circulation of the heat transfer medium in a jacket and/or coil, or the heat transfer area. Bondy and Lippa [45] and Dream [46] have compiled a collection of correlations of heat transfer coefficients in agitated vessels. Batch processes are sometimes disadvantageous because ... 

When an agitated bateh eontaining M of fluid with speeifie heat e and initial temperature t is heated using an isothermal eondensing heating medium Tj, the bateh temperature tj at any time 6 ean be derived by the differential heat balanee. For an unsteady state operation as shown in Figure 7-27, the total number of heat transferred is q, and per unit time 6 is ... 

Tables 7-21 and 7-22 summarize the rules-of-thumb involving mixing, agitation, and reactors, respectively [48]. The following considerations are essential during mixing of fluids in a reactor [49] ...

Mixing of fluids is necessary in many chemical processes. It may include mixing of liquid vith liquid, gas with liquid, or solids with liquid. Agitation of these fluid masses does not necessarily imply any significant amount of actual intimate and homogeneous distribution of the fluids or particles, and for this reason mixing requires a definition of degree and/or purpose to properly define the desired state of the system. 

Sg = specific gravity of fluid N = agitator impeller speed, rpm... 

Convection is heat transfer between portions of a fluid existing under a thermal gradient. The rate of convection heat transfer is often slow for natural or free convection to rapid for forced convection when artificial means are used to mix or agitate the fluid. The basic equation for designing heat exchangers is... 

Da = diameter of agitator, ft k = thermal conductivity of fluid processed,... 

Thus, the energy per unit mass increases with radius r and is independent of depth In the absence of an agitator or mechanical means of rotation energy transfer will take place to equalise j/ between all elements of fluid. Thus the forced vortex tends to decay into a free vortex (where energy per unit mass is independent of radius). 

This procedure can be repeated for different values of N. A compilation of the experimental values of ks for a variety of impellers, turbine, propeller, paddle, anchor, and so on, has been given by Skelland(16), and an examination of Table 7.1 suggesLs that for pseudo-plastic liquids, ks lies approximately in the range of 10-13 for most configurations of practical interest/22 231 SKELLAND l6) has also correlated much of the data on the agitation of purely viscous non-Newtonian fluids, and this is shown in Figure 7.8. 

The prediction of power consumption for agitation of a given non-Newtonian fluid in a particular mixer, at a desired impeller speed, may be evaluated by the following procedure. 

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