Supercritical water oxidation of a heavily PCB-contaminated mineral transformer oil: Laboratory-scale data and economic assessment

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Abstract

Although a great deal of research has been devoted to PCBs destruction by supercritical water oxidation, PCB-contaminated oils, which represent a high percentage of existing PCB wastes in South American countries, have been barely studied. In this paper we present experimental results on the supercritical water oxidation of a real-world mineral transformer oil highly contaminated with PCBs. Process conditions of 539 °C, 350% oxygen excess and 241 bar allow one to obtain a 99.6% organic matter destruction of the complex mixture of hydrocarbons and PCBs. The PCB concentration in the aqueous effluent was below the detection limit of the chromatographic method used in the analysis, and a toxicity test for Daphnia pulex showed that the aqueous effluent of the process is not ecotoxic. Based on these results, a process flowsheet of a supercritical water oxidation mobile pilot plant was proposed as an alternative to incineration. A preliminary economic assessment of the proposed flowsheet shows that the process is feasible when compared to treatment by incineration.

Introduction

Supercritical water oxidation (SCWO), a process carried out at temperatures and pressures above the critical point of water (647 K and 22.1 MPa) has proved to be effective for destroying a wide spectra of industrial wastewaters and highly stable persistent organic pollutants that cannot be treated in a cost-effective manner by conventional technologies. In fact, a great deal of research has been directed to the destruction by SCWO of nitrogenated compounds such as pyridine, aniline, nitrobenzene and ammonia [1], [2], [3], [4], phenolic compounds [5], [6], and less conventional wastes such as obsolete chemical weapons agents and radioactive wastes [7], [8], [9]. Perhaps the highest interest has been given to the destruction by SCWO of the extremely toxic and possibly carcinogenic polychlorinated biphenyls (PCBs) [10], [11], [12], which were widely used in electrical transformers due to their insulating properties and to their resistance to thermal, chemical and biological degradation. SCWO of PCBs has been proposed as an alternative to incineration, which faces an increasing public resentment because it is perceived to produce harmful byproducts such as polychlorinated dibenzofurans/dioxins (PCDFs/Ds) [13].

The Stockholm Convention on persistent organic pollutants (POPs), signed in 2001 by more than 200 countries, requires actions to reduce the utilization and eliminate in an environmentally sound manner all of the equipment and wastes contaminated with PCBs as soon as possible but before 2028 [14]. An initial inventory of PCBs in Colombia, according to the guidelines of the Stockholm Convention, accounts for the existence of approximately 13,000 tons of PCB wastes. Similar figures are reported for other South American countries with similar degree of economic development [15]. In order to meet the 2028 deadline, PCB destruction in South American countries could be carried out by incineration as is being done up to now. However, there are not available state-of-the-art incineration facilities that can deal with PCBs in the region, and thus the local PCB wastes must be exported to industrialized countries such as France, Canada and the United States at prices that are prohibitively high for many industries.

Another possibility for meeting the 2028 deadline is co-processing the PCBs in cement kilns. Although this process has for decades been thought to cause increased emissions of PCDD/PCDFs, Karstensen [16] reports a great deal of experimental data of PCDD/PCDFs measurements by the cement industry that do not support this perception, and indicates that most modern cement kilns co-processing waste today can meet the most stringent emission standards when well managed and operated. For example, Karstensen et al. [17] carried out a test burn with PCB oil in a local cement kiln in Sri Lanka. A 3-day test burn showed that the cement kiln was able to destroy PCB in an environmentally sound manner without causing any new formation of PCDD/PCDFs. The destruction and removal efficiency was better than 99.9999% at the highest PCB feeding. Unfortunately, in Colombia no cement factory currently process PCB containing wastes, because the existing facilities are located in very populated areas and such activity would face strong public opposition, besides the requirement of a comprehensive infrastructure for waste management and PCDD/PCDFs monitoring, which at this moment is not available.

The elevated price of PCBs destruction by incineration overseas might be justified in the case of bulk PCB wastes. However, most of the PCB wastes in South American countries correspond to mineral dielectric oils contaminated with PCBs in the range from 50 to 1000 ppm. This cross-contamination [18] of mineral dielectric oils was originated in a common practice in the electrical power sector during the transformer maintenance that consisted in retrofilling the transformer, previously containing pure PCBs, with fresh mineral oil. In fact, cross-contamination has been identified as the main mechanism responsible for dispersion of PCBs in the environment [18]. According to the Stockholm Convention guidelines, dielectric oils contaminated with PCBs must be considered as pure PCBs even at concentrations as low as 50 ppm. As a result, even these diluted mixtures must be handled and treated as pure PCBs [14].

Although extensive research has been conducted on SCWO of pure PCB congeners, either as model compounds [12], [19], [20], [21], [22], [23] or commercial PCB mixtures [10], [11], [23], [26], not much efforts have been oriented towards the SCWO of PCB-contaminated mineral transformer oils. In fact, to the best of our knowledge, there exists only one work that reports on the SCWO of a mineral oil slightly contaminated with PCB [25].

Previous work by other researchers on SCWO of monochlorobiphenyls has been conducted to elucidate the kinetics of the dechlorination reactions and to determine optimal operating conditions to attain a destruction higher than 99.99% of the PCB congeners [12], [20]. However, the complexity of a real-world mineral transformer oil heavily contaminated with PCB might require a different set of operating conditions for complete oxidation. Thus, in this work we report on our results regarding the optimal operating conditions of a continuous SCWO process applied to a real-world mineral transformer oil heavily contaminated with PCB.

Due to the large amount of contaminated oils in South America and the lack of local incineration facilities for appropriate disposal of PCB wastes in the region, a process flowsheet for SCWO of PCB-contaminated dielectric oils in a mobile pilot plant is suggested, and the feasibility of such a process as an alternative to incineration overseas is assessed by means of an economic analysis.

Section snippets

Chemicals

The PCB-contaminated mineral transformer oil that was used in this study was obtained from an electrical transformer in operation in the city of Cali, Colombia. An 8 wt% hydrogen peroxide aqueous solution was prepared from 50 wt% industrial grade solution of the same substance, and was used as oxidant reagent in our experiments. All water was distilled and deionized using a laboratory apparatus. No additional purification of the reactants was performed.

Apparatus and procedure

The SCWO experiments were carried out in the

Laboratory SCWO data

Table 1 shows the process conditions and organic matter conversions for selected SCWO experiments at 24.1 MPa. The effect of pressure was assessed in preliminary runs (not shown in Table 1), which indicated a negative effect of pressures higher than 24.0 MPa on the organic matter disappearance. The reaction temperature was calculated as the arithmetic average of the temperatures at the reactor inlet and outlet. Under the selected operating conditions the flow regime was turbulent. The

A mobile plant for SCWO of PCB-contaminated mineral transformer oils

Incineration of heavily PCB-contaminated transformer oils does not appear as a viable treatment technology for PCB wastes in South America in the near future. Based on the experimental results described above, we thus propose a process flowsheet of a SCWO mobile pilot plant for treating PCB-contaminated oils. A mobile plant is advantageous because it allows one to treat the waste directly in the site where they are located, avoiding cumbersome transportation procedures and security precautions

Conclusion

The supercritical water oxidation of a real-world heavily PCB-contaminated mineral transformer oil was studied in the temperature range from 520 to 560 °C and oxidant excess in the range 300–400% at 24.1 MPa. At 539 °C, 24.1 MPa and 350% oxygen excess a 99.6% conversion of the organic matter and a destruction of PCBs under the detection limit of the chromatographic technique was achieved. Due to the lack of incineration facilities in South American countries the treatment of PCB-contaminated

Acknowledgments

The authors wish to thank Departamento Administrativo de Ciencia, Tecnología e Investigación (COLCIENCIAS), for financial support of this research, and for V. Marulandás fellowship for graduate studies.

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