Particle re-entrainment

This equation shows that for a given collection efficiency, the precipitator size is inversely proportional to particle drift velocity and directly proportional to gas flow rate. Increasing the gas density (migration velocity is a function of gas viscosity) by reducing its temperature or increasing the pressure will reduce the precipitator size. However, theory does not account for gas velocity. This is a variable that influences particle re-entrainment and the drift velocity. This typically requires an ESP design at lower velocities than predicted in theory. 

Generally the co-current supercritical mode of operation is superior to both the subcritical mode and counter-current flow. It is an inherently more stable system and this reduces the problems of particle re-entrainment so that maximum achievable overflow... 

The retention of the capmred blood cells in the nonwoven filter mostly depends on the following mechanisms sieving, direct interception, direct adhesion (including inertial impaction, diffusion, interception and electrokinetic attraction), and indirect adhesion (including bridging and particle re-entrainment). 

The particle re-entrainment occurs when deposited particles on a surface by adhesion and gravity forces can be detached and reentered into the liquid flow if there is... 

Ching HK. Studies of particle re-entrainment/detachment from flat surface [PhD Dissertation]. University of Minnesota 1971. 

The unit shown in Figure 4-49 has been used in many process applications with a variety of modifications [18,19,20]. It is effective in liquid entrainment separation, but is not recommended for solid particles due to the arrangement of the bottom and outlet. The flat bottom plate serves as a protection to the developing liquid surface below. This prevents re-entrainment. In place of the plate a vortex breaker type using vertical cross plates of 4-inch to 12-inch depth also is used, (Also see Reference [58].) The inlet gas connection is placed above the outlet dip pipe by maintaining dimension of only a few inches at point 4. In this type unit some liquid will creep up the walls as the inlet velocity increases. 

Wetting—of particle to help agglomerate and prevent re-entrainment. 

Various modifications may be made to improve the operation of the cyclone separator in special cases. If there is a large proportion of fine material present, a bag filter may be attached to the clean gas outlet. Alternatively, the smaller particles may be removed by means of a spray of water which is injected into the separator. In some cases, the removal of the solid material is facilitated by running a stream of water down the walls and this also reduces the risk of the particles becoming re-entrained in the gas stream. The main difficulty lies in wetting the particles with the liquid. 

Waste solids Waste or complex mixtures consisting of <502 water and having relative firmness and coherence of particles or persistence of form as matter that is not liquid or gaseous at 25 °C. This category includes tarry material that is sticky or viscous, such as coal tar, adhesive waste, sludge, airborne particulates (e.g., re-entrained particles blown from a waste site), and solids partitioned from waste water. 

Void [52] developed a variety of ballistic deposition models to simulate sedimentation processes. Void used ballistic models to determine deposition densities for spherical particles which traveled via vertical paths and were deposited on horizontal surfaces. Recently, Schmitz et al. [53] used a ballistic aggregation model to describe particle aggregation at the surface of a crossflow microfiltration membrane. Schmitz and co-workers were able to account for interfacial forces empirically, and demonstrated the influence of physical and chemical variables on the resulting morphology of the fouling deposits (such as aggregate density variation with depth, and influence of shear flow and re-entrainment properties on fouling deposit density and porosity). 

M. W. Hahn, Deposition and Re-entrainment of Brownian particles under unfavorable chemical conditions, Ph.D. Dissertation, The Johns Hopkins University, Baltimore, Maryland, 1995. 

Another major factor is the difference in engine parameters and exhaust system configuration. Exhaust pipe diameter and length from the engine to observation plane effect the important mechanisms of coagulation, deposition, and re-entrainment of particles thereby altering the size distributions. 

Straining capture refers to the condition that results when a particle lodges in a pore throat of size smaller than its own. The rate of capture is directly proportional to the velocity. Re-entrainment of strained droplets occurs either by squeezing of droplets through pore constrictions due to locally high pressures or by breakup of the oil droplets. 

Air filters operate under conditions of streamline flow as indicated by the streamlines drawn around a cylindrical fiber. It was assumed that capture of a particle takes place if any contact is made during its movement around the fiber. Once captured, the particle is not re-entrained in the air stream and deposited deeper in the bed. Support for this assumption has been found by using an atomized suspension of Staphylococcus albus and spores of Bacillus subtilisP Nevertheless, some fiber filters are treated with viscous oils, presumably to make capture more positive and to reduce re-entrainment. 

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