Physical Description.

Plate scrubbers are counter-flow devices in which liquid moves downward and gas moves upward. Liquid-gas contact is obtained in a mixing zone consisting of a plate with some type of openings on it.

Openings can be perforations, valves, or slots. Often multiple plates are used. Liquid flows downward from plate to plate. The simplest plate is the perforated plate or slotted plate. Liquid flows over the perforations. The velocity of the air stream is sufficient to prevent weeping (liquid flow) through the openings. The action creates a frothing column above the plate. Liquid flows across the plate to a downcomer and then downward to the next plate. Figure III-1 illustrates a scrubber of this design.

Bubble cap towers utilize the principle just described, except that the perforations are covered with caps. Air bubbles out of slots or notches in the cap. This arrangement causes effective gas dispersion and prevents liquid from weeping if the flow is momentarily reduced.

Some plate towers do not utilize downcomers and are therefore truly counter-flow. Rod decks are examples of this type. Plate towers have been widely used for mass transfer but also have been used for particulate collection.

For mass transfer applications, many of the towers have been individually designed. However, a number of companies are quite active in the sale of plate towers for absorption purposes.

Absorption Efficiency.

Plate towers are effective for absorption. Efficiency can be increased by the addition of plates and tower height: Contact takes place in bubbles and droplets. Since the leanest gas contacts the fresh slurry at the top of a tower after passing through the lower stages where absorption takes place, low outlet emissions can be achieved. The pressure drop across the plate effects the efficiency. Greater contact is achieved with higher pressure drop per plate. Liquid flows can also be increased to enhance absorption efficiency.

Particulate Collection Efficiency.

Multiple plate towers are effective in removing particles above approximately one micron in size. They are not nearly as effective on submicron particulate as are venturi scrubbers. Increasing the velocity of the gas through the plates will increase particulate removal efficiency. Adding additional trays is not likely to have much effect. A three-tray unit at 2" pressure drop per tray or 6" total pressure drop might not be as efficient as one tray at 3" pressure drop. The axiom is that for particulate collection, the energy should be consumed in one place for maximum efficiency. Increasing L/G ratios can have some effect on particulate removal but very minor. Doubling the liquid rate might not be more beneficial than a 10 percent increase in pressure drop across the plate.

Maintenance Characteristics.

Tray towers are nearly always designed so that there is access to each tray section from outside the unit. Thus, the units can be cleaned much more easily than packed towers if build-up occurs. The underside of the bottom tray section can be sprayed with the scrubbing slurry to eliminate wet-dry interfaces which cause plugging. In some special cases, the trays have been designed for easy removal from the unit. The purpose was to enable cleaning of the trays outside the unit. If trays are not level, the liquid-gas contact will be reduced. Care must be taken on installation to achieve good level conditions. If plates are not sealed to the side wall, gas can bypass the contact areas reducing efficiency. Flooding will occur in plate towers with either an excessive liquid rate or gas velocity. This will cause increased pressure drop across the towers and decreased gas flow. Plate towers do not resist plugging and scaling to the extent of venturis, centrifugals and other open design scrubbers. Their application should be limited to applications where plugging potential is not severe.

Size.

Plate towers are found more frequently in larger mass transfer applications as opposed to packed towers. The plates themselves can be designed to any diameter tower. Moisture eliminator designs offer a more complicated sizing problem.

Materials of Construction.

Plate towers can be designed out of many materials of construction. However, in comparison with packed towers, the cost of internals rises sharply with need for corrosion resistance. The plates in particular present a problem. In units with liquid under-spray, the nozzles and piping are expensive. The problem is accentuated if the inlet temperatures are high. The total weight of plate towers is generally less than packed towers.

Liquid-to-Gas Ratios.

Rates as low as 2 gal./1000 cfm are used for particulate collection. For absorption, high liquid rates can be used. Plate towers can usually handle higher liquid rates than packed towers.

Pressure Drop.

The pressure drop depending on design can range from 1/4" to several inches water gauge per plate.

Design Advantages
(a)        High mass transfer rates with multiple plates
(b)        Both particulate removal and absorption can be accomplished in one tower
(c)        Can handle high liquid rates
(d)        Can be built in large sizes
(e)        Can handle volume fluctuations
(f)         Can handle temperature fluctuations

Design Disadvantages.
(a)        Cannot handle applications with high scaling potential such as a limestone SO2 scrubbing
(b)        Cannot handle foamy liquids
(c)        Corrosion resistant design is expensive
(d)        Cannot remove fine particulate