Fluid Motion in Vessels

Relationship between Fluid Motion and Process Performance Several phenomena which can be used to promote various processing objectives occur during fluid motion in a vessel. 

Empirical analysis of wave motion, fluid velocities. and stress can be made to enable vessel size, configuration and baffle system to be optimized. Analysis of the data Is made by direct visual observation of the fluid motion In the vessel, and computer plotting of wave profiles, fluid velocities, etc. 

Heat transfer is an important consideration when the fluid motion in the vessel is in the laminar flow regime. It influences the design and operation of agitated process vessels such as reactors, evaporators, and crystallizers. For a review of working relationships, see Dream [76]. 

Concentration and temperature differences are reduced by bulk flow or circulation in a vessel. Fluid regions of different composition or temperature are reduced in thickness by bulk motion in which velocity gradients exist. This process is called bulk diffusion or Taylor diffusion (Brodkey, in Uhl and Gray, op. cit., vol. 1, p. 48). The turbulent and molecular diffusion reduces the difference between these regions. In laminar flow, Taylor diffusion and molecular diffusion are the mechanisms of concentration- and temperature-difference reduction. 

Theoretical representation of the behaviour of a hydrocyclone requires adequate analysis of three distinct physical phenomenon taking place in these devices, viz. the understanding of fluid flow, its interactions with the dispersed solid phase and the quantification of shear induced attrition of crystals. Simplified analytical solutions to conservation of mass and momentum equations derived from the Navier-Stokes equation can be used to quantify fluid flow in the hydrocyclone. For dilute slurries, once bulk flow has been quantified in terms of spatial components of velocity, crystal motion can then be traced by balancing forces on the crystals themselves to map out their trajectories. The trajectories for different sizes can then be used to develop a separation efficiency curve, which quantifies performance of the vessel (Bloor and Ingham, 1987). In principle, population balances can be included for crystal attrition in the above description for developing a thorough mathematical model. 

The pattern of the fluid motion is a function of the fluid system, impeller, vessel configuration, and location of the impeller in the fluid system relative to the vessel walls and/or bottom. The patterns illustrated in Figures 5-23A-5-23K indicate that almost any pattern can be established provided the particular impeller type is located in the proper position. This is easier to accomplish in some s) stems than others. 

Anchors, helical ribbons and screws, are generally used for high viscosity liquids. The anchor and ribbon are arranged with a close clearance at the vessel wall, whereas the helical screw has a smaller diameter and is often used inside a draft tube to promote fluid motion throughout the vessel. Helical ribbons or interrupted ribbons are often used in horizontally mounted cylindrical vessels. 

If the fluid enters the vessel at one central point, as indicated in Figure 23.3, rather than at many points spaced across a circular distributor, as in Figure 23.1, the action is different as us increases a spouted bed results rather than a fluidized bed. A spouted bed is characterized by a high-velocity spout of gas moving up the center of the bed, carrying particles to the top. This action induces particle circulation, with particle motion toward the wall and downward around the spout and toward the center. The particles in a spouted bed are relatively large and uniformly sized. 

In Effects of Motion on Design of Process Facilities for Floating Systems" Rice provides a theoretical analysis coupled with motion-simulation work in an effort to understand fluid motion inside process equipment on floating vessels. This work has led to the establishment of process-equipment sizing criteria for all types of vessel motion transmitted from the marine vessel. Design of internal baffling and other internals to dampen and control the fluid motion is reviewed. 

For mold pellets and other suspended particles with densities close to that of the continuous phase, the agitation in a stirred mixing vessel creates the dominant force for relative fluid motion between the two phases. The intrinsic gas-liquid mass-transfer coefficient under these conditions is given by Calderbank (1967) as... 

Close-Clearance Stirrers For some pseudoplastic fluid systems stagnant fluid may be found next to the vessel walls in parts remote from propeller or turbine impellers. In such cases, an anchor impeller may be used (Fig. 18-6). The fluid flow is principally circular or helical (see Fig. 18-7) in the direction of rotation of the anchor. Whether substantial axial or radial fluid motion also occurs depends on the fluid viscosity and the design of the upper blade-supporting spokes. Anchor agitators are used particularly to obtain improved heat transfer in high-consistency fluids. 

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