MUNICIPAL & INDUSTRIAL ODOR CONTROL
ODOR CONTROL SYSTEMS
D. R. Technology offers complete inexpensive, highly efficient odor control systems for low, medium and high sulfur odor ranges emanating from various sewage plant locations.
D. R. Technology dry and wet odor scrubbers can bring objectionable odor levels down to undetectable ranges. Our units have been tested by both in stack and ASTM odor panel methods. Although a combination of many intricate organo-sulfur compounds, the predominant odor generated at sewage facilities is caused by Hydrogen Sulfide. Present to a lower extent are mercaptans, organic alcohol compounds where oxygen atoms are replaced by sulfur. If the key odor generating points are properly ventilated, air emissions to be treated will have odor levels at 1 to 100 parts per million. As can be seen from Table One, Hydrogen Sulfide and Ethyl Mercaptan are easily detectable at low concentrations. It is imperative to prevent their release to the surrounding community. If odor levels in vented gas are at or above 5 parts per million, we recommend a wet absorption system such as depicted in Figure 1. The basis for this system is gas contact with a dilute solution of sodium hypochlorite at alkaline pH’s. This occurs in a counterflow gas/liquid contactor known as a packed tower. This employs:
- Either caustic stabilized sodium hypochlorite solution fed to the absorber tower recycle tank, or separatly controlled feeding of the hypochlorite and caustic solution if cost will justify this.
- Chemical feed based on pH control in an on/off mode if the gas flow is low. More sophisticated modulating proportionating systems are maintenance problems and costly, but can be justified if higher gas flows are to be treated. If hypochlorite solution is fed separately, this is done to maintain a constant oxidation level with “ORP” control, a technique similar to pH control. Caustic is added to this absorber based on pH control.
- The most efficient method of gas/liquid contact for this service, is countercurrent packed bed contact. Furthermore, we strongly recommend a vertical upflow gas design. The only time a cross flow design should be used is when physical constraints prevent the upflow unit’s placement. We propose a minimum tower bed depth of 10’-0”. This is based upon previous D. R. Technology’s and other’s installations. Ninety percent of the year, odor is not a serious problem. The units we propose will work all of the time, even on hot summer days.
- Choosing correct materials of construction will maximize corrosion and wear resistance while minimizing cost. We recommend the use of Fiberglass Reinforced Polyester (FRP) for scrubber, fan and ductwork. Less expensive PVC, although also corrosion resistant, will a) be attacked adversely by ultra violet radiation (e.g. sunlight), and b) not be capable of taking continued pumps’ stresses that arise over long periods of time. The packing used in odor control systems is typically of Polypropylene construction. Because the solution seriously attacks this material, replacements after a year of operation are typical. If glass filled Polypropylene is used, much longer (5 year) service life will result. Because it does not see the liquid loading encountered by the packing, the mist eliminator construction in Polypropylene will be satisfactory.
- A double flushed mechanical seal centrifugal recycle pump is advisable. This unit’s operation will prevent clogging buildup and avoid shutdowns.
- A continuous blowdown and makeup to account for removal of neutralization products and evaporation is necessary. Although this can be done automatically, we recommend the use of a small continuous makeup water and blowdown water stream. This is a simple, sure way of eliminating pluggage and avoiding the risk of instrumental failure. A sight level should be included on the recycle tank to assure flooded pump suction. Chemical reactions with hypochlorite are depicted on the table enclosed.
If odor levels are below the 5 PPM level, we recommend the use of a D. R. Technology dry scrubbing system. This employs one or more beds of the D. R. Technology COH potassium hydroxide impregnated virgin activated charcoal.
Table Three gives a description of a suitable carbon for this application. The granular carbon pellets, suitably supported on a screen/grating plates are housed in a cylindrical Fiberglass Reinforced Polyester vessel. Vertical up or downflow is the method of choice.
The carbon technique can bring odor levels to the undetectable levels. The carbon will eventually become saturated with the sulfur compounds. Odor breakthrough will occur at this time. The carbon can be regenerated in place by the introduction of a 50% Potassium Hydroxide solution.
D. R. Technology provides a full installation, operation and maintenance manual for all its units. Regeneration procedures are included in these manuals.
CHEMISTRY OF OXIDATION OF SULFUR COMPOUNDS BASIS: SODIUM HYPOCHLORITE, NaOCl
CHEMICAL REACTIONS (IN ALKALINE SOLUTIONS)
HYDROGEN SULFIDE:
H2S + 3 NaOCl + 2 NaOH Na2SO3 + 3 NaCl + 2 H2O
Or
6.5 pounds of NaOCl and 2.35 pounds of NaOH are required to remove 1 pound of H2S.
Also expressed as 4.44 gallons of 15% NaOCl solution and 0.3 gallons of 50% NaOH solution.
METHYL MERCAPTAN
CH3SH + 6 NaOCl + 4 NaoH Na2SO3 + Na2CO3 + 6 NaCl + 4 H2O
or
9.25 pounds (6.3 gallons @ 15%) NaOCl and 3.33 pounds (0.4 gallons @ 50%) NaOH solution.
BUTYL MERCAPTAN
C4H2SH + 15 NaOCl + 10 NaOH 4 Na4CO3 + Na2SO3 + 15 NaCl + 10 H2O
or
12.33 pounds (8.4 gallons @ 15%) NaOCl and 4.44 pounds (0.53 gallons at 50%) NaOH solution.
DIMETHYL SULFIDE, C 2HS
C2H6S + 9 NaOCl + 6 NaOHà 2 Na2CO3 + Na2SO3 + 9 NaCl
10.74 pounds (7.3 gallons @ 15%) NaOCl and 3.87 pounds (0.69 gallons @ 50%) NaOH solution.
Whereas Sulfur based odors are relatively simple to control by using HypoChlorite there are other odorous organic compounds that require other types of chemicals for control.
Cosmetic manufacturing typically uses chemicals that are either fragrant or odorous. One category of odor are Acrylates. These are used as solvents and the vapor is pungent in even small concentrations. These can be scrubbed using special organic solvent water solutions that also contain alkali such as Sodium Hydroxide. Acrylates are used in plastics manufacturing and various other chemical processes.
Odor is also generate by Amine based organic chemicals. These are used in paint, foundry, adhesives, and numerous other operations. Amines are organic compounds that have been reacted with Ammonia to form a combination compound. Scrubbing process exhausts with a dilute solution of Sulfuric Acid (H2SO4) will remove these vapors so well that the odor is undetectable in the scrubber exhaust.
Both odor and toxicity exhausts are generated in the semiconductor business. Some of the fugitive emissions are Phosphine (PH3 )and Arsine gas (AsH3). DR Technology inc has installations scrubbing these fumes with a very strong oxidant solution Potassium Permanganate( KMnO4). Because the byproduct of this oxidation is insoluble Manganese Dioxide (MnO2).ing. The scrubbers must be tailored to avoid pluggage and yet be efficient.