Plate Scrubbers

Plate scrubbers are a highly efficient means of particulate removal, absorption of contaminants and gas cooling. In this type of scrubber (see photo) the gas passes upwards through a number of small holes in one or more plates. Liquor enters on the top plate, flows across it and through a down-comer to the plate below. Intimate gas liquid contact is obtained as the gas bubbles through the liquid on the plate.

In the target plate design a "target baffle" is located over each hole in the plate. As the gas passes through the perforations the dust particles impinge on the "targets" and they are collected by the liquor. For increased dust removal performance a venturi scrubber can be incorporated into a tray design in the form of a venturi slot plate.

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Venturi Scrubbers

Venturi scrubbers are an effective means of removing sub micron particulates. The contaminated gas is accelerated in the converging section before entering the throat (see photo). Scrubbing liquor enters around the top of the converging section and completely flushes the wall. In addition, liquor enters through a spray to fill the throat with droplets.

In the venturi throat, the high velocity gas atomizes the liquid droplets with trap the solid particles. The scrubbed gas and liquid droplets leave the venturi throat and pass through the diverging section where further agglomeration takes place to produce larger droplets. The liquor droplets are then separated from the gas stream in a cyclonic entrainment separator.

To ensure high scrubbing efficiencies, even with varying gas flows, the throat can be made adjustable, thus maintaining the required scrubber pressure drop, even at reduced throughputs.

The liquor from the separator is recycled via pumps to the venturi scrubber with a small bleed being discharged to control the build up of solids in the circulating liquor.

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Packed Towers

Wetted packed towers are the simplest and most commonly used approaches to gas scrubbing. The principle of this type of scrubber is to remove contaminants from the gas stream by passing the stream through a packed structure which provides a large wetted surface area to induce intimate contact between the gas and the scrubbing liquor. the contaminant is absorbed into or reacted with the scrubbing liquor.

The packing of the tower is normally a proprietary loose fill random packing designed to encourage dispersion of the liquid flow without tracking, to provide maximum contact area for the 'mass transfer' interaction and to offer minimal back pressure to the gas flow. The reactivity between the contaminant and the scrubbing liquor influences the system designer's determination of gas and liquor flow and the height and diameter of the packed bed.

A demister is fitted at the top of the tower to prevent entrainment of droplets of the scrubbing liquor into the extraction system or stack.

Wetted packed towers can be designed for very high efficiencies with relatively low capital and running costs. The low pressure drop associated with packed bed scrubbers permits the use of smaller more economical fans. Although efficiency may be affected, a packed tower will usually function when gas or liquor flows vary from its original design parameters.

Air Strippers

Dissolved gases such as hydrocarbons ( VOC's), carbon dioxide, ammonia and hydrogen sulfide can be removed from water by stripping (desorbing) by contacting the water with ambient air in a packed column. The water to be treated enters the top of a packed tower and is distributed over the top of a packed bed and flows down through the packing. Fans blow ambient air into the base of the tower and this flows up through the packing where it contacts the water stripping out the VOC contaminants. The air containing the stripped VOC's passes through a demister to remove water droplets and is vented to atmosphere. The stripped water collects in the base of the stripping tower from where it is discharged. The efficiency of the VOC removal depends on the vapor pressure of the specific VOC compounds, the temperature of operation and the packed height of the tower. The materials of construction of the stripping towers would be either stainless steel or glass reinforced polypropylene. The internals would be manufactured from polypropylene.

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