The smell from the fermenter exhaust at our plant was horrible. The gas chromatograph specified the exhaust composition to the nearest ppmv but no words could describe its stench to mortal man. Not surprisingly, our neighbors in the industrial park in Bakersfield, Calif., turned their noses up at the aroma wafting over our fence. We paid the fines; I couldn’t justify a treatment project. That was quite a while ago, though. Today, a reeking plant usually creates a public stink that may jeopardize its ongoing operations.
Fume control depends upon the nature of the problem vapor: inorganic or organic (e.g., hydrogen sulfide or toluene), corrosive or mildly corrosive, flammable or nonflammable, and, of course, toxic or merely unpleasant. Let’s look at a few choices to consider for handling fumes.
One of the most common fumes is hydrogen sulfide, which smells like rotten eggs. Although chlorine and caustic soda once were sound economical choices to handle it, they have issues: chlorine is a regulatory problem; caustic soda has a high heat of mixing and liberates odors downstream; strong agitation, adding the caustic to the water — not the other way around — and not using hot or cold water during dilution are critical for avoiding accidents. Bleach or an iron chloride (FeCl3) solution are the best choices to use in a scrubber. Of the two options, I recommend FeCl3 because bleach can release chlorine if mixed with acids. However, FeCl3 is not without its problems. Heat tracing is required because of freezing. In addition, a small amount of hydrochloric acid usually is needed to keep the FeCl3 in solution; fiberglass, therefore, is the construction material of choice.
Another common source of odor problems is ammonia (NH3), which has a disagreeably pungent smell. Although water sparging or caustic (pH>11) will eliminate the vapor, you’re stuck with mulch for algae. The best option for water-bound wastewater with >100 mg/L NH3 concentration is an aerobic spray tower followed by anaerobic beds (settling ponds). The NH3 is converted to nitrogen and water. BP applied this approach successfully at its refinery in Lima, Ohio, in the 1990s.
Now, let’s talk about odors from organics, i.e., volatile organic compounds (VOCs). Treating these almost always requires an expensive step-wise, hybrid system: solvent recovery, then drying to remove water, decanting, refrigerated condensation, decanting again (sometimes of three phases), and finally adsorption. Adsorption generally involves activated carbon beds (ACBs) for non-polar compounds or silica gel, alumina or molecular sieves for polar ones, or a combination non-polar/polar bed; water competition is a problem with polar media. Vegetable-based (e.g., coconut) carbons can be used for polar compounds. VOC system post-treatment usually involves steam stripping (distillation), vacuum, and thermal oxidation. ACBs are more economical if regenerated on-site.
ACBs work best with gas streams with ≤10 ppmv effluent. Use a safety factor of two for bed life. Based on benzene, the basis most bed manufacturers use, carbon consumption typically is 0.015 lb/d/scfm at 5 ppmv and 0.006 lb/d/scfm for 1.5 ppmv.
Making ACBs economical generally demands a lot of pre-treatment: for a 10,000-ppmv (1%-toluene) stream, lowering the temperature to -17°F will leave 1,300 ppmv; achieving 100 ppmv, the upper range economically for ACBs, by refrigeration alone will require -112°F, which is within the range of fluorinated aliphatic refrigerants like Freons.
Selecting the right carbon is crucial to bed performance.
Pore size and the source of carbon, which affects surface area, are important in optimizing selectivity: carbons with a large pore size (>30 Å) provide more bed capacity and are easily regenerated but have poor capture efficiency. High pressure is best to get the most of large pores. A bed with a mix of 5–50-Å carbons usually provides good performance.
The performance of an ACB also depends upon the effluent. Less volatile organics, e.g., heavy oils with boiling points above 200°C, will adhere to the pores, making regeneration almost impossible even with steam. Extreme temperature can burn the carbon.
Fire always is a potential problem with ACBs, especially with new beds. Trace water left from regeneration helps avoid hot spots, so regenerate a new bed before use. Small pores are more difficult to regenerate and tend to be converted to large pores by the process.
DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at email@example.com