One of the more difficult decisions facing the pollution engineer about to purchase a wet scrubber is selecting the correct materials. In wet scrubbing equipment, almost all types of corrosion, erosion, and temperature problems are encountered at one time or another. Often, several of these conditions may exist simultaneously. Therefore, careful selection of the scrubber material which will perform satisfactorily for a specific installation is extremely important. Moreover, the cost of scrubbing equipment must be held as low as possible since equipment may offer no economic return.
The following is a survey of materials being used in the construction of commercially available wet scrubbers, materials most commonly recommended for certain scrubbing applications, and relative material costs.
Metallic Scrubber Materials
Carbon Steel is probably the material most commonly specified for scrubber construction because it is easily fabricated and versatile. However, because of its limited corrosion resistance, which depends upon an oxide film, carbon steel is not recommended for scrubbing applications where dilute acids are present. It is recommended for dust collection systems or in scrubbers operating with basic solutions.
Cast Iron, where available is probably the most economical material for wet scrubbers. Units with few or no moving parts may be fabricated from it. Most cast iron scrubbers use stainless steel trim or other corrosion-resistant alloys for spray nozzles and components where close dimensional tolerances must be maintained.
Cast Iron may also be used in pilot plant scrubbing applications or where corrosion problems are not severe. Normal water supplies and air will not cause problems. However, S02 or similar gases in the fluid being scrubbed will cause the formation of dilute acids which will attack cast iron.
Ni Resist Cast Iron Alloys containing 13.5 to 36 percent nickel are generally recommended for scrubbing applications where corrosion, erosion, high temperature, or abrasion exist simultaneously. These alloys are particularly well suited to the scrubbing of incinerator fumes or other fumes where high temperatures and flash are present. Ni Resist alloys have a much higher resistance to corrosion than plain cast iron or steel but are not recommended where oxidizing agents are encountered.
Stainless Steels most commonly used in wet scrubber fabrication include types 304 and 316, either in their standard or low carbon forms. Both of these materials form an oxide film and perform best under oxidizing conditions. They are not recommended, however, for use with hydrochloric acid or other reducing acid environments. Both have good temperature resistant properties and are often recommended for incinerator applications where relatively high temperatures are present. Extra-low carbon types are recommended in both types 304 and 316 to prevent intergranular corrosion from scrubber fabrication.
Medium Alloys such as Alloy 20 are used in many scrubber applications, particularly for sulfuric acid service. Materials such as Hastelloy C can also be used, but these alloys are not recommended unless absolutely necessary.
Typical Materials Of Construction For Common Scrubbing Problems
|SCRUBBING LIQUID Water
Water or Caustic
Caustic/or Lime Slurry
|SCRUBBER MATERIAL Cast Iron, Steel, FRP, PVC, Ni-Resist
FRP, PVC, Kynar
Cast Iron, Steel, Ni-Resist
FRP (with Dynel Shield), Rubber Lined Steel, Graphite lined, Kynar
FRP, 316 SS, PVC
FRP, PVC, Kynar, Teflon
FRP, 316 SS, 304 SS
FRP, 316 SS (tends to pit)
FRP, Alloy 20
Plastic Scrubber Materials
Fiberglass Reinforced Plastic (FRP) is used in wet scrubber construction because the material is economical, easily fabricated, lightweight, and has good resistance to both alkaline and acid environments. The particular FRP resin used for scrubber construction, however, should be carefully selected on the basis of its resistance to the environment. In general, FRP can be used for handling most strong acids or alkalis. Where necessary, an FRP scrubber can be equipped with a Dynel shield to prevent attack of the glass fibers in fluoride scrubbing. Limitations on the use of fiberglass include temperature (which generally must be below 220 F) and the presence of certain organic compounds that will attack the resin. FRP is used both as the basic material for scrubber fabrication, or as liner material for steel equipment which is subject to corrosion.
Polyvinyl Chloride (PVC) has limited use as a material for wet scrubber construction because of its low-temperature limitations (generally 140-160 F). However, this material can be used for low-temperature applications involving acids, alkalis, or dust. Most often, PVC is used for components such as spray nozzles or other standardized parts, as well as for packed tower grids and redistributors.
Furan (Furfuryl Alcohol Resin) is particularly well suited to scrubber applications where the acid and alkali resistance of FRP is required, but where organic solvents such as Xylene or Trichlorobenzene are present. Strength and temperature properties are similar to FRP which cannot be used with these organic compounds.
Resins of the phenolic type are normally recommended for scrubber applications where chlorine or hydrogen chloride are encountered.
Generally speaking, linings can be used in many applications to hold down the cost of construction. While a rubber lining may be more expensive than FRP, in some applications, it may be better suited. If the particular scrubbing problem involves high abrasion, for example, a rubber lining may be the ideal answer. The rubber tends to give and wear less rapidly than other lining materials. Carbon graphite linings are often recommended for small-scale scrubbing operations because they are inert to most fluids except those containing strong oxidizing agents.
Carbon Graphite Is not recommended for moving parts, for large units, or for use with designs involving complicated baffles.
Teflon Linings are suitable for very severe conditions involving HCI or similar reducing agents, bromine, or where temperature limitations will not allow the use of less expensive materials such as FRP. However, since Teflon cannot be bonded to a metal surface, a Teflon-lined steel unit requires multiple flanges to hold the Teflon sheet in pace.
Kynar, a material with properties similar to those of Teflon, Is now available in sheet form bonded to a glass backing. This material can be used with FRP to produce a relatively inexpensive, inert material where FRP would be otherwise suitable except for the contaminants involved. Kynar-lined FRP units are recommended for scrubbing of chlorinated hydrocarbons and similar fluids.
Creative Approaches To Cleaning And Purifying Air
The Clean Air Group offers a variety of wet scrubbing systems for cleaning and purifying the air and other gases as well as for reclaiming product instead of exhausting it into the atmosphere.
The Clean Air Group’s equipment is backed by more than 30 years of experience in the air pollution control field, with successful installations all over the world.
The majority of Clean Air Group units are custom designed for the individual application. In addition, smaller units are available from stock.
Jet-Venturi Scrubbers: The Clean Air Group’s Jet-Venturi Fume Scrubber is a dependable device designed to entrain and scrub large volumes of gases without the use of complicated internals or moving parts. Standard and custom designs are available in a variety of materials, including metals and fiberglass-reinforced polyester. Capacities to 50,000 CFM.
Packed Towers: Channels the gas flow upward through a packed bed while the scrubbing liquid flows downward by gravity over the packing. This method is well suited to high-efficiency gas absorption, but is not recommended for use in dust collection applications. Diameters from 1 to 12 feet. Capacities from 100 to 60,000 CFM.
Hi-Energy Venturi Scrubbers: Use energy from a high-velocity gas stream to atomize a liquid stream. Designed without moving parts for long term service and little or no maintenance. Available in a wide range of construction materials. For use with recirculating or nonrecirculating systems.
Spiral-Pac™ Tower Packing: Molded plastic integral spiral loops present maximum area for mass transfer. Prevents channeling or partial wetting. Provides dramatic increases in capacity over more conventional types of packing. Available in a wide variety of plastics.