8.C.8 Fume Hood Exhaust Treatment

Until now, treatment of fume hood exhausts has been limited. Because effluent quantities and concentrations are relatively low compared to those of other industrial air emissions sources, their removal is technologically challenging. And the chemistry for a given fume hood effluent can be difficult to predict and may change over time.

Nevertheless, legislation and regulations increasingly recognize that certain materials exhausted by a fume hood may be sufficiently hazardous that they can no longer be expelled directly into the air. Therefore, the practice of removing these materials from hood exhaust streams will become increasingly more prevalent.

8.C.8.1 Fume Hood Scrubbers and Contaminant Removal Systems

A number of technologies are evolving for treating fume hood exhaust by means of fume hood scrubbers and containment removal systems. Whenever possible, experiments involving such materials should be designed so that the toxic materials are collected in traps or scrubbers rather than being released into the hood. If for some reason this is impossible, then HEPA filters are recommended for highly toxic particulates. Liquid scrubbers may also be used to remove particulates, vapors, and gases from the exhaust system. None of these methods, however, is completely effective, and all trade an air pollution problem for a solid or liquid waste disposal problem. Incineration may be the ultimate method for destroying combustible compounds in exhaust air, but adequate temperature and dwell time are required to ensure complete combustion (see section 8.C.9).

Incinerators require considerable capital to build and energy to operate; hence, other methods should be studied before resorting to their use. The optimal system for collecting or destroying toxic materials in exhaust air must be determined on a case-by-case basis. In all cases, such treatment of exhaust air should be considered only if it is not practical to pass the gases or vapors through a scrubber or adsorption train before they enter the exhaust airstream. Also, if an exhaust system treatment device is added to an existing fume hood, the impact on the fan and other exhaust system components must be carefully evaluated. These devices require significant additional energy to overcome the pressure drop they add to the system. (See also Chapter 7, section 7.B.6.1.)

8.C.8.2 Liquid Scrubbers

A fume hood scrubber is a laboratory-scale version of a typical packed-bed liquid scrubber used for industrial air pollution control. Figure 8.7 shows a schematic of a typical fume hood scrubber.

Contaminated air from the fume hood enters the unit and passes through the packed bed, liquid spray section, and mist eliminator, and then into the exhaust system for release up the stack. The air and the scrubbing liquor pass in a countercurrent fashion for efficient gas-liquid contact. The scrubbing liquor is recirculated from the sump and back to the top of the system using a pump. Water-soluble gases, vapors, and aerosols are dissolved into the scrubbing liquor. Particulates are also captured quite effectively by this type of scrubber. Removal efficiencies for most water-soluble acid- and base-laden airstreams are usually between 95 and 98%.

Scrubber units are typically configured vertically and are located next to the fume hood as shown in Figure 8.7. They are also produced in a top mount version, in which the packing, spray manifold, and mist eliminator sections are located on top of the hood and the sump and liquid handling portion are underneath the hood for a compact arrangement taking up no more floor area than the hood itself.

8.C.8.3 Other Gas-Phase Filters

There is another basic type of gas-phase filtration available for fume hoods in addition to liquid scrubbers. These are "inert" adsorbents and chemically active adsorbents. The "inert" variety includes activated carbon, activated alumina, and Molecular Sieves®. These substances typically come in bulk form for use in a deep bed and are available also as cartridges and as panels for use in housings similar to particulate filter housings. They are usually manufactured in the form of beads, but they may take many forms. The beads are porous and have extremely large surface areas with sites onto which gas and vapor molecules are trapped or adsorbed as they pass through. Chemically active adsorbents are simply inert adsorbents impregnated with a strong oxidizer, such as potassium permanganate (purple media), which reacts with and destroys the organic vapors. Although there are other oxidizers targeted to specific compounds, the permanganates are the most popular. Adsorbents can handle hundreds of different compounds, including most volatile organic components (VOCs), but also have an affinity for harmless species such as water vapor.

As the air passes through the adsorbent bed, gases are removed in a section of the bed. (For this discussion, "gas" means gases and vapors.) As the bed loads with gases, and if the adsorbent is not regenerated or replaced, eventually contaminants will break through the end of the bed. After breakthrough occurs, gases will pass through the bed at higher and higher concen-

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