Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
1991-03-15
2002-04-30
Griffin, Steven P. (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C588S253000, C588S253000, C588S249000, C423S659000, C208S262500
Reexamination Certificate
active
06380454
ABSTRACT:
the present invention relates to the destruction of polychlorinated biphenyls (PCBs). Such biphenyls have been used in the past on a large scale in electrical equipment as fire resistant, heat stable dielectric material and as a coolant. However, these compounds have been identified as being environmentally hazardous, and manufacture and sale of them is now prohibited in most countries. Fluids contaminated with more than 2 ppm PCBs may in some jurisdictions be considered to require special handling and may not be disposed of in ordinary disposal sites.
A number of methods are known for removal or destruction of PCBs and like polyhalogenated biphenyls. For example, known methods include absorption in swellable solid polymers, destruction of PBBs (polybrominated biphenyls) in a thermite reaction, reaction with a mixture of polyalkylene glycol alkyl ether and alkali metal hydroxide, extraction into methanol, combustion in a diesel engine or similar combustor, decomposition in molten salts, and dehalogenation employing finely divided molten sodium dispersion.
The latter method, namely dehalogenation with molten sodium dispersion, offers the advantages that it is relatively easily controllable and under carefully maintained conditions can be operated without substantial risk of escape of PCBs or similar materials from the system, has relatively low operating and energy consumption costs, and is capable of substantially complete destruction of the PCBs or similar materials.
The present applicants have, however, determined that known processes of dehalogenation using sodium dispersion consume quantities of sodium that are greater than are desirable, are less energy efficient than are desirable and produce undesirable byproducts which may tend to be emitted from the system unless special precautions are taken. These problems arise because typically insulating fluids based on PCB and similar materials-contain substantial quantities of chlorobenzenes, which are normally added to reduce the viscosity of the mixture. For example askarel, usually a PCB based electrical insulating fluid, usually contains about 30 to about 70% by weight of mixture of different PCBs and the balance a mixture of different di-, tri- and tetrachlorinated benzenes. On reaction with the sodium dispersion, the halogenated benzene species are reduced to benzene, with concomitant oxidation of the sodium to sodium halide and hence their presence adds to the consumption of sodium in the system. In addition, the presence of halogenated benzene in the reaction mixture increases the total volume of material to be reacted and hence increases the energy consumption and operating costs of the dehalogenation process. Moreover, the dehalogenated product, namely benzene, is toxic and is a volatile liquid which tends to be evolved in vapour form from the reaction mixture under the elevated temperatures normally reached in the exothermic dehalogenation process. Accordingly, with known processes, special absorptive filters or other arrangements should be employed to avoid releases of toxic benzene vapour.
The present invention in one aspect relates to a procedure for removing chlorobenzenes from askarels and like mixtures of PCBs and chlorobenzene in order to obtain a mixture with reduced chlorobenzene content which can be more efficiently subjected to PCB destruction employing reaction with sodium dispersion. Further, the invention relates to improvements in the PCB destruction processes whereby the mixture impoverished in chlorobenzenes can be rapidly and efficiently reacted with the sodium dispersion, without the need to add reaction catalysts, to achieve a product substantially free from PCB. Further, the invention relates to improvements in the techniques employed for dispensing a measured quantity of sodium dispersion.
In one aspect, the present invention is based on the finding by the inventors that by conducting fractional distillation of askarels and like mixtures of PCBs and chlorobenzenes, the quantities of chlorobenzenes present in such mixtures can be significantly reduced, in that substantial quantities of the more volatile chlorobenzenes can be efficiently distilled out, and that it is readily possible to separate off, in substantial quantities, a distillate which contains less than 2 ppm PCBs and is thus safely eligible for disposal without needing to take the stringent precautions necessary for disposal of PCBS. Such separation can be effected at relatively high rates of throughput without requiring the use of large or expensive distillation apparatus.
Accordingly, the present invention provides a method for the destruction of polychlorinated biphenyls (PCBs) wherein said PCBs are present in the form of a mixture with chlorinated benzenes, comprising subjecting said mixture to fractional distillation, separating a distillate rich in chlorinated benzenes and containing less than 2 ppm PCBs, collecting a bottoms poor in chlorinated benzenes and rich in PCBs, and reacting said bottoms with a dispersion of sodium particles to reduce substantially all said PCBs to biphenyl.
As is, of course, well understood by those skilled in the art, in the course of fractional distillation, separation of more volatile from less volatile components takes place within a fractionating column through which vapours rise, and a certain amount of liquid, termed reflux, descends. The vapours usually originate from a heated reboiler at the bottom of the column and the reflux liquid usually originates from condensation of vapours at a condenser at the upper end of the column. As the hot vapours from the reboiler come into contact with cooler descending reflux liquid, there is a progressive enrichment of the more volatile constituents upwardly through the column and progressive enrichment of the less volatile constituents downwardly through the column. The column may be, for example, a packed or differential stage contactor column or may be a plate, tray or finite stage contactor column, and the distillation may be conducted continuously or in batch mode. Preferably, the distillation is conducted continuously by reason of greater efficiency of operation.
Askarel and like mixtures currently stored and requiring disposal and destruction contain a wide variety of components. Some of these mixtures contain substantial quantities of mono-, di- and trichlorinated biphenyls. Such mixtures normally exist as neat PCBs, since the PCBs having lower degrees of chlorination, namely having up to about three chlorines atoms per molecule, tend to have adequate low temperature flow characteristics or viscosities without requiring addition of chlorobenzenes or like diluents in order to thin the mixture. Other askarels, however, contain substantial quantities of tetra- to nonachlorinated or more highly chlorinated biphenyl species and these normally exist in the form of a mixture with chlorobenzenes, the total concentration of the chlorobenzenes varying somewhat depending on the nature of the PCBs and on the application for which the askarel was intended. Further variability in the composition of the askarels is added by the fact that the chlorobenzenes employed as viscosity-reducing diluents range from monochlorobenzene to hexachlorobenzene making a total of twelve congeners including the various isomers of di-, tri- and tetrachlorobenzene.
Advantageously, in the present invention the fractional distillation is applied selectively to mixtures having relatively low contents of mono, di- and trichlorobiphenyls, relatively high contents of chlorinated benzenes and relatively low contents of tetra- or more highly chlorinated benzenes. If the content of lower chlorinated biphenyls is too high, there tends to be greater difficulty in significant reduction of the content of chlorobenzenes without carry over of any substantial quantity of PCBs in the distillate. If the total content of chlorinated benzenes is excessively low, significant reduction in the quantity of PCB mixture cannot be achieved, and if the content of tetra- or more highly chlorinated benzenes is exc
Ferrie James S.
Gonzalez Luciano A.
Janis W. John
Mullins Dennis F.
Griffin Steven P.
Shlesinger & Arkwright & Garvey LLP
Vanoy Timothy C
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