Fluid shearing

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

The particle size of typical natural mbber latex ranges from slightly higher than 1 )Tm to as small as 20 nm, and can be destabilized by mechanical or chemical action. Machinery used to stir or maintain latex circulation should be designed to minimize fluid shear, particularly where that may be locally intensified, eg, at the junction of blade with shaft. 

Anodic deposition is controlled by either fluid shear (cross-flow filtration) (48), similar to gel-polarization control, or by continual anode replacement (electrodeposited paints) (46). High fluid shear rates can cause deviations from theory when E > (49). The EUF efficiency drops rapidly... 

Methods of Gas Dispersion The problem of dispersing a gas in a liqmd may be attacked in several ways (1) The gas nubbles of the desired size or which grow to the desired size may be introduced directly into the liqmd (2) a volatile hquid may be vaporized by either decreasing the system pressure or increasing its temperature (3) a chemical reaction may produce a gas or (4) a massive bubble or stream of gas is disintegrated by fluid shear and/or turbulence in the liquid. 

For any given process, one takes a qualitative look at the possible role of fluid shear stresses. Then one tries to consider pathways related to fluid shear stress that may affect the process. If there are none, then this extremely complex phenomenon can be dismissed and the process design can be based on such things as uniformity, circulation time, blend time, or velocity specifications. This is often the case in the blending of miscible fluids and the suspension of sohds. 

If fluid shear stresses are likely to be involved in obtaining a process result, then one must qualitatively look at the scale at which the shear stresses influence the resiilt. If the particles, bubbles, droplets, or fluid clumps are on the order of 1000 [Lm or larger, the variables are macro scale and average velocities at a point are the predominant variable. 

If the blending process is between two or more fluids with relatively low viscosity such that the blending is not affected by fluid shear rates, then the difference in blend time and circulation between small and large tanks is the only factor involved. However, if the blending involves wide disparities in the density of viscosity and surface tension between the various phases, then a certain level of shear rate may be required before blending can proceed to the required degree of uniformity. 

One potential difficulty with CF-EF is the electrodeposition of the particles at the electrode away from the filtration medium. This phenomenon, if allowed to persist, will result in performance decay of CF-EF with respect to maintenance of the electric field. Several approaches such as momentaiy reverses in polarity, protection of the electrode with a porous membrane or filter medium, and/or utilization of a high fluid shear rate can minimize electrodeposition. 

For non-New tonian fluids, viscosity data are very important. Every impeller has an average fluid shear rate related to speed. For example, foi a flat blade turbine impeller, the average impeller zone fluid shear rate is 11 times the operating speed. The most exact method to obtain the viscosity is by using a standard mixing tank and impeller as a viscosimeter. By measuring the pow er response on a small scale mixer, the viscosity at shear rates similar to that in the full scale unit is obtained. 

According to Smoluchowski s theory (equation 6.53), the agglomeration rate increases proportional with the fluid shear rate 7... 

Camp and Stein (1943) originally proposed for flocculation that the mean fluid shear rate is proportional to the square root of the mean energy dissipation... 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

In the stirred tank, the final mean size of particles was reduced by the increase of stirring rate, being consistent with increased fluid shear induced particle disruption relative to aggregation. Use of three different gas velocities in the bubble column, however, results in no significant difference in agglomerate size but since the size is relatively small, it may simply reflect an asymptotic value. 

The fluid shear stress actually brings about the mixing process, and is the multiplication of fluid shear rate and viscosity of the fluid [29]. 

Equation (15) indicates that the wall shear stress changes with position on the cell surface and at each position the stress changes with time. Figures 9 and 10 show the distributions of the fluid shear rate in the eddy and the cell surface shear stress as a function of the dimensionless radius, R, and time, yt, respectively. 

The maximum fluid shear rate in the eddy occurs at the radial position R= 1.793 [86] (Fig. 10). Thus, letting cos yt= 1.0, in Eq. (19) gives the maximum shear stress acting on the cell as ... 

Fig. 8. Sustained damage in Daucus carota suspensions, as a function of total energy expended, under laminar flow conditions in a Couette viscometer. Redrawn from Dunlop et al. (1994) Effect of fluid shear forces on plant cell suspensions. Chem Eng Sci 49 2263 - 2276, with permission of Elsevier Science...

Taba, Y., et al. (2000). Fluid shear stress induces lipocalin-type prostaglandin D2 synthase expression in vascular endothelial cells. Circul. Res. 86, 967-73. 

Haidekker MA, L Heureux N, Frangos JA (2000) Fluid shear stress increases membrane fluidity in endothelial cells a study with DCVJ fluorescence. Am J Physiol Heart Circ Physiol 278(4) H1401-H1406... 

Chachisvilis, M., Zhang, Y. L. and Frangos, J. A. (2006). G protein-coupled receptors sense fluid shear stress in endothelial cells. Proc. Natl. Acad. Sci. USA 103, 15463-8. 

Confer, D. R. and Logan, B. E. (1991). Increased bacterial uptake of macromol-ecular substrates with fluid shear, Appl. Environ. Microbiol., 57, 3093-3100. 

If colloids are sufficiently large or the fluid shear rate high, the relative motion from velocity gradients exceeds that caused by Brownian (thermal) effects (orthokinetic agglomeration). 

Heterodisperse Suspensions. The rate laws given above apply to monodisperse colloids. In polydisperse systems the particle size and the distribution of particle sizes have pronounced effects on the kinetics of agglomeration (O Melia, 1978). For the various transport mechanisms (Brownian diffusion, fluid shear, and differential settling), the rates at which particles come into contact are given in Table 7.2. 

In concluding it must be emphasized that fluid shear can and does control the foulant film thickness. For excunple, higher velocities reduce the film thickness as shown in Table I. 

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