4.4 Chemical Processes
Processes based on a specialized chemical reaction to destroy PCBs are fundamentally different from incineration processes. The foregoing processes are quite properly chemical processes but a distinction has been made based largely on the need for high temperature for the incineration to take place. Conversely, these processes are driven by the potential for the chemicals to react rather than the need for high temperature to cause the chemicals to react. Some of the processes produce products at high temperature but that is an effect of the process not a requirement.
The types of process described are:
- Chemical dechlorination;
- Radiant energy processes;
- and (Low) temperature oxidations.
4.4.1 Chemical Dechlorination
All chemical dechlorination processes use chemical reagents to break apart the extremely stable PCB molecule, rearranging it to form other chemical compounds that are considered harmless and environmentally safe. These processes destroy the PCB molecule but do not break down the biphenyl structure of the molecule. Only the chlorine atoms which give the PCB molecule chemical and biological stability are removed.
Most chemical dechlorination processes use a sodium reagent to strip away the chlorine atoms from the PCB molecule. The wastes generated from the process are sodium chloride and non-halogenated polyphenyls. The exact constitutents of the polyphenyls are often not known but indications show that the sodium chloride and polyphenyls can be disposed of safely.
Most applications involve destruction of PCBs that contaminate otherwise valuable oil. The sodium dechlorination processes can be run at ambient or moderate temperature and although they chemically destroy the PCBs contained in oil they do not destroy the oil itself, therefore, the oil can be recycled for reuse. Sodium dechlorination is limited in that it is only capable of economically dechlorinating PCBs in otherwise valuable oil.
Dechlorination of PCBs by sodium reagents must be conducted in a nitrogen or similar inert atmosphere to prevent excessive reagent consumption and fire hazard due to hydrogen generation on contact with any water or moisture present in the oil.
4.4.1.1 The Goodyear Sodium Naphthalide Process
Head Office: Goodyear Tire and Rubber Co.
1144 East Market St.
Akron, Ohio 44316-0001
The sodium naphthalide process uses a reagent made from sodium and naphthalene to dechlorinate PCBs at ambient temperature. The sodium naphthalide reagent is prepared in a two-step, one-pot procedure. In the first step, metallic sodium is heated to 150-170°C in either heat-transfer or transformer fluid under an inert atmosphere. Cooling the mixture to ambient temperature with stirring forms a finely divided sodium sand. In the second step, a tetrahydrofuran solution of naphthalene is added to the sodium sand in the fluid at ambient temperature forming the soluble greenish-black sodium naphthalide reagent. The reagent is added to the PCB-contaminated oil in an amount sufficient to give a reagent/chlorine ratio of at least 6:1 to 100:1 depending on the type of fluid and its contamination level. The reaction proceeds rapidly at room temperature.
After one hour any excess reagent is slowly water quenched under an inert atmosphere and the quenched fluid is vacuum stripped to recover tetrahydrofuran and naphthalene for recycle. The residue is subjected to further vacuum distillation, if the treated oil is to be recycled.
The sodium naphthalide dechlorination yields the following products:
- treated oil;
- non-halogenated polyphenyls; and
- sodium chloride.
This dechlorination process can only be used to degrade PCBs in waste oil. It cannot be used to dechlorinate PCBs in water.
The Goodyear process has been tested both in the laboratory and on a commercial scale. In laboratory tests the process was shown to reduce PCB levels from 20 000 ppm to less than 2 ppm. On a commercial scale the treatment was run in carbon steel reactors under an inert nitrogen atmosphere and the process was able to reduce PCB levels from 130 ppm to less than 2 ppm.
Goodyear is not interested in commercially developing the sodium naphthalide technology by itself, so the company has made its information public. Several adaptations of the process have been recently commercialized by other companies.
Although the technology has been tested, further testing is required to determine compliance with PCB regulations and composition of waste by-products.
This batch process is quite sophisticated in design and serves as a good template to those companies modifying the system to obtain EPA approvals.
Based on a flow rate of 910 L/h, the cost of operating this system would be about $0.30/kg of contaminated oil. This cost could be increased to at least $3.00/kg when destroying 10% PCB in oil similar to what the conventional incinerators can handle. The potential for recycle of the treated oil offers additional economic benefits to users of this process.
The sodium naphthalide process is run at ambient temperature in an inert atmosphere with only very small temperature rise occurring during the reaction with lower levels of PCB. Therefore the risks to workers treating PCB-contaminated oil by the sodium naphthalide process would be minimal. One possible worker hazard could come from the generation of hydrogen when unreacted sodium contacts quenching water. This hazard can be controlled by careful, slow addition of quenching water and by maintaining an inert atmosphere in the vessel at all times.
Risk to workers handling the flammable chemicals, such as tetrahydrofuran, used to manufacture the sodium naphthalide reagent would be minimal given that adequate care is taken in handling these chemicals.
The composition of the polyphenyl residue created as a waste by-product of this reaction has not been documented. Therefore incineration of this compound may or may not be hazardous to the public and environment. Further testing must be conducted before this can be determined.
References: (Goodyear Tire and Rubber Co., 1980).
4.4.1.2 Sunohio PCBX
Head Office: Sunohio
1700 Gateway Blvd. S.E.
Canton, Ohio 44707
Contact: Carl Sorenson
(216) 452-0837
The PCBX process is a continuous process that uses a proprietary reagent to strip chlorine atoms from PCB molecules and convert them to metal chlorides and polyphenyl compounds. The PCBX unit is housed on a large tractor/trailer rig. Oil reclaiming equipment and a mobile laboratory are housed in a smaller trailer rig. The twin units are self-contained and can operate from any 460 or 230 volt, 60 hertz power source. The units are so constructed that all process work takes place within spill pans leading to emergency reservoirs capable of holding one hour of full process flow.
The unit is designed to process 2.3 m3/h of transformer oil at a nominal 2 000 ppm PCB kill. The more PCB the oil contains, the higher the kill. A typical procedure on oil containing 3 000 ppm of PCB would follow this pattern:
- First pass reduction to below 1 000 ppm.
- Second pass reduction to 100 ppm, more or less.
- Third pass reduction to below 2 ppm.
The procedure runs as follows:
- A gas chromatographic analysis of the oil is made to determine PCB content.
- The oil is heated, then goes through a preliminary treatment (data not available) to remove moisture and contaminants.
- The heated oil is admitted to the reactor section and, as it enters, reagent is added consistent with the PCB content of the oil. The mixture reacts as it passes through the reactor section.
- The mixture is centrifuged, filtered three times, and vacuum-degassed.
- The treated oil is returned to the transformer or held in a retention tank. The polyphenyl and inorganic salts residue is solidified and sent to a landfill.
The PCBX process is capable of treating pure PCBs or PCB-contaminated oil. The reagent cannot be used to treat aqueous solutions such as PCB-contaminated ground water. In two tests conducted for the US EPA, the PCBX unit treated a 255 ppm transformer oil and askarel. After treatment (one pass), the transformer oil was tested and contained 1 ppm PCB. Askarel was treated at a rate of 150 mL/min, and a gas chromatograph scan showed that the processing had destroyed the PCBs to a content of 7 ppm in the working fluid. In this test, the working fluid was entering the reactor with 4 400 ppm and emerging at 7 ppm. A test conducted for Region V, US EPA in May, 1982, resulted in transformer oil containing 1 760 ppm PCBs being reduced to the limits of detection in three passes. |
