Mist eliminator pressure drop

Figure 4-21. Typical wire mesh mist eliminator pressure drop curves for one style of mesh at three different liquid loadings. Others follow similar pressure drop patterns. By permission, Otto H. York Co., Inc.

Packed fiber bed mist eliminators can be designed to operate at almost any desired particle collection efficiencies, depending on the allowable pressure drop and cost. A good discussion of sulfuric acid mist generation, control, and mist eliminator design is available (109,110). 

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

Centrifugal mist eliminators separate liquid dropi by centrifugal force. These can be more efficient than either wire mesh or mist eliminators, and are the least susceptible to plugging. However, they are not in common use in production operations because removal efficiencies are sensitive to small changes in flow. In addition, they require relatively large pressure drops to create the centrifugal force. 

In general, the power industry desires the following of a scrubbing device in a flue gas-cleaning system operational reliability, a minimum of four-to-one turndown ratio, low pressure drop characteristics, high absorption efficiency potential, good alkali utilization, acceptable mist elimination, and future process adaptability. 

In an actual plant good contact must be made between the large volumes of flue gas and the relatively small melt stream. However, power plant integration requirements make it important to impose as small a pressure drop as possible on the gas stream. Because of this, a spray contactor has been selected as the molten salt scrubber concept. The spray contactor uses spray nozzles to break up the melt into small droplets for good gas-liquid contact and a very efficient mist eliminator to prevent the gas stream from carrying melt mist out of the scrubber. 

The power required to move flue gas through the scrubber, from the absorber inlet to the mist eliminator outlet, was calculated from pressure drop and gas flow-rate data reported in the literature for a range of superficial velocities, liquid/gas ratios, and internal scrubber packings. A fan of the wet induced-draft type was assumed for each case, operating on saturated flue gas at 125°F. The gas-side power input was added to the power delivered through the slurry recirculation pumps which was calculated from the volumetric flow rates and the minimum discharge pressures required for the given scrubbers. The total power input for SO2 absorption was thus determined as ... 

AP is the gas pressure drop across the absorber (including the mist eliminator), lb/sq ft ... 

Packed Tower.—A conventional countercurrent packed tower is shown Tor comparison. When used Tor flue gas scrubbing, a short section of low-pressure-drop packing (e.g., 3 in. Intalox saddles) is employed with a mist eliminator above the liquid feed point. 

The deposition of solids onto adsorbents not only causes deactivation but can substantially increase the pressure drop, Thus, if there is any possibility of solids or mist, it is good design pmclfce to provide for their removal jus) upstream of tbe ndsorbers by installing filters and/or mist eliminators. 

Pressure. The location of pressure sensors in evaporative crystallizers also affect their reliability. If the operating pressure measurement is made in the vacuum line or condenser, it will be affected by the evaporation rate of the solvent and pressure drop changes due to any incrustation in the lines or mist eliminator. Measurement of the pressure in the vessel headspace circumvents this interaction. The sensor should be equipped with a solvent flush line to ensure that incrustation does not distort the measurement. 

Break in pressure after dry mist eliminator represents control valve drop. 

Pressure drop across mist eliminator shall be assumed as 0.2 PSI for support design. 

There are other comments to be made about this type of application. The liquid spray nozzle should supply liquid to the face of the mixing elements without appreciably wetting the vessel wall. A 30° full cone nozzle is typically used. Ideally, the spray should consist of droplets in the range 1000 to 2000 p-m range. A quick review of spay nozzle literature indicates that the appropriate spray nozzle for 44 gal/min of alkaline water would operate at about 100 psi pressure drop. Multiple nozzles could be used if a lower liquid-side pressure drop is desired. Fine atomization spray nozzles should be avoided since fine spray drops are difficult to separate in downstream mist eliminator equipment. A Alter or strainer should be installed on the liquid feed to prevent plugging the feed nozzle, especially if the nozzle orifice size is small. 

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