Fiber beds velocities

Venturi scmbbers can be operated at 2.5 kPa (19 mm Hg) to coUect many particles coarser than 1 p.m efficiently. Smaller particles often require a pressure drop of 7.5—10 kPa (56—75 mm Hg). When most of the particulates are smaller than 0.5 p.m and are hydrophobic, venturis have been operated at pressure drops from 25 to 32.5 kPa (187—244 mm Hg). Water injection rate is typicaUy 0.67—1.4 m of Hquid per 1000 m of gas, although rates as high as 2.7 are used. Increasing water rates improves coUection efficiency. Many venturis contain louvers to vary throat cross section and pressure drop with changes in system gas flow. Venturi scmbbers can be made in various shapes with reasonably similar characteristics. Any device that causes contact of Hquid and gas at high velocity and pressure drop across an accelerating orifice wiU act much like a venturi scmbber. A flooded-disk scmbber in which the annular orifice created by the disc is equivalent to a venturi throat has been described (296). An irrigated packed fiber bed with performance similar to a... 

To solve the preceding set of equations, Equation 5.62 is plugged into Equation 5.60. By separately determining the compaction properties of the fiber bed [32] an evolution equation for the pressure can be obtained. Because this is a moving boundary problem the derivative in the thickness direction can be rewritten [32] in terms of an instantaneous thickness. The pressure field can then be solved for by finite difference or finite element techniques. Once the pressure is obtained and the velocity computed, the energy and cured species conservation equations can be solved using the methodology outlined in Section 5.4.1. 

Pressure drop in these devices is only about 1 in. H2O which is a big advantage for very small particle removal in comparison to fiber bed or venturi separators. The low gas velocity and tow pressure drop, however, require a unit with a large volume that has high capital cost. 

FICU RE 4.4(f) Mass transfer coefficients for a vane module. The coefficients for vane module can be correlated in terms of overall Reynolds number based on the incident velocity Oo or in terms of an estimated local velocity in the fiber bed The latter choice gives rough agreement with the correlation for modules built one fiber at a time (the solid line). (From S. R. Wick-ramasinghe, M. J. Semmens, and E. L. Cussler, J. Membr. ScL, 84,1,1993. With permission.)... 

Mascosko concluded that because the cavity is filled with many small fibers, the content of the mold can be treated as a porous medium. Because the pores act as many tiny capillary tubes, the fluid moves like a plug in the mold. For a Newtonian liquid, the average velocity (based on the empty channel) in any direction (x) through the fiber bed is proportional to the pressure gradient and the permeability in that direction is... 

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