UNC-Chapel Hill ENVR 754 - Scrubbers for Particle Collection (12 pages)

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Scrubbers for Particle Collection



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Scrubbers for Particle Collection

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Pages:
12
School:
University of North Carolina at Chapel Hill
Course:
Envr 754 - Air Pollution Control

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SCRUBBERS FOR PARTICLE COLLECTION Background To understand how scrubbers work we must first define some terms Single droplet efficiency d is similar to single fiber efficiency It is the fraction of particles in the gas upstream of a spherical droplet that collect on that droplet as the droplet moves through the gas Although the same mechanisms contribute to single droplet efficiency that contribute to single fiber efficiency and include impaction interception and diffusion impaction alone usually dominates For particle collection due to impaction on a droplet the following equation is often used Stk d Stk 0 35 2 1 where Stk is the Stokes number for the drop moving through gas that contains particles The particles are assumed to be embedded in the gas that is they do not move relative to the gas except to cross streamlines by impaction Inspection of Eq 1 shows that d is zero if Stk is zero and approaches a value of unity as Stk becomes large Stk d 2 p Vd g C C 18 d d 2 Here d p Vd g CC dd is particle diameter is particle density is the velocity of the droplet relative to the gas see below is the Cunningham slip correction factor is gas viscosity and is droplet diameter Because droplets are liquid particle collection on liquid droplets will depend on some mechanisms that do not operate for collection on dry spheres or on dry fibers For example diffusiophoresis and Stefan flow can be important If the droplets are water Stefan flow will enhance particle collection under conditions when water vapor condenses on the droplets but will reduce particle collection for evaporating droplets Diffusiophoresis for condensing water vapor will tend to drive particles away from the droplet and toward an evaporating droplet however this effect is usually less important than Stefan flow Liquid holdup Hd is the fraction of the scrubber volume that is filled with droplets It is analogous to solidity for a filter If for example all the droplets in a scrubber were made of ice then the ice



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