Data Collection

112

For a pilot scale treatment, we newly designed an irradiation reactor having a declined stainless steel plane to flow the oil into the irradiation area of the electron beam accelerator. The irradiation reactor was consistently cooled by water during the irradiation to prevent a rise in temperature. The irradiation dose for one passage of oil on the declined plane was measured as 15 kGy at a fixed electron beam power of 50 kW. The absorbed dose was measured with a dichromate solution.

The electron beam was generated with an ELV-4 (EB-tech Co., LTD, Daejeon, Korea) with energy of 1.5 MeV, a 50 mA current, and 75 kW power of a commercial scale accelerator. All experiments were performed at room temperature and ambient pressure without any additives.

The concentration of PCBs was measured by both a GC/ECD and a HRGC/MS. The molecular structure and chemical element were determined by FT-NMR.

11.1.3. Results and Discussion 11.1.3.1. Molecular weight changes

To observe the molecular weight changes between pre- and post-irradiation, the transformer oil was irradiated with 200 kGy. Comparing the data in Fig. 11.1. between the pre- and post-irradiation, the peak intensity of the C17 compound was increased by 5% against pre-irradiation, but the C19 and C22 carbon compounds were decreased as much as 5%. Based on this, electron beam irradiation seems to transform large molecular weight compounds into lower ones. In this experiment, the magnitude of molecular weight transformation was less than 5%, which was considered too small to impact the physical properties of the oil.

FIG. 11.1. Molecular weight before (left) and after irradiation (right) of transformer oil (50ppm of PCBs)

11.1.3.2. Physical and electrical property changes

Table 11.1 shows that the physical and electrical property changes both pre- and post-irradiation being considered important characteristics for transformer oil. Kinematic viscosity was increased a little; this seems that molecular weight is transformed into lower ones by electron beam irradiation. The increase of the dielectric breakdown voltage related to the concentrations of the impurities, and the total acid number related to the amount of moisture in the oil, seem to be due to the fact that the oil was used for more than several tens of years and held outside for a long time. No other properties were changed remarkably.

113 A decrease of the dielectric breakdown voltage can be protected by preventing oxidation of impurities during irradiation.

11.1.3.3. Decomposition of PCBs

To determine the proper irradiation intensity according to the concentrations of PCBs, additional experiments for 16 different kinds of samples were performed, and their residue PCBs concentration we measured are as shown in Table 11.2.

Fig. 11.2. represents the FT-NMR results after irradiation. The results indicate that aliphatic chloride compounds are produced with an increasing dose of irradiation. The NMR data show that the newly produced materials are mono chloro-alkane from the peak between 3-4 ppm, and di-chloro-alkane from the peak between 5-6 ppm. Therefore, aliphatic chloride compounds are produced at higher doses of irradiation.

11.1.3.4. By-Products

Table 11.3 shows the concentrations of the residual PCBs isomers in ppb after irradiation at 0 kGy, 180 kGy, 240 kGy, and 300 kGy for a 37 ppm sample. The higher dose in the sample, the more the low chlorinated isomers increased. This means that the hepta-chlorinated biphenyl decomposed into hexa-chlorinated biphenyl, and then into penta-, tetra-, tri-, di-, and mono-, and finally into bi-phenyl, which is no longer a toxic compound. Compared to the concentrations of PCBs between GC/ECD and HRGC/MS, the concentrations from the HRGC were always higher than the results with GC/ECD. Therefore, the analysis from HRGC to confirm the residual PCBs is very important

TABLE 11.1. PHYSICAL AND ELECTRICAL PROPERTIES OF TRANSFORMER OIL BEFORE AND AFTER IRRADIATION.

Property Before

irradiation

After irradiation Kinematic viscosity at 40℃, mm²/s 10.82 11.86 Kinematic viscosity at 100℃, mm²/s 2.589 2.763 Sludgy (%), Oxidation stability at 120℃ for 75h 0.01 0.01 Total acid number (mg·KOH/g), Oxidation stability at

120℃ for 75h 0.38 0.55

Dielectric breakdown voltage, kV, 2.5mm 28.61 16.09

Total acid number,mg·KOH/g 0.01 0.05

Volume resistivity, Ω·cm, 80℃ 1.1x10¹⁴ 2.1x10¹²

114

TABLE 11.2. RESIDUAL CONCENTRATION OF PCBS ACCORDING TO IRRADIATION DOSE IN A BATCH SYSTEM.

Sample ID

Dose, kGy

Residual Concentration,

ppm

Sample ID

Dose, kGy

Residual Concentration,

ppm

0601 0 111.5 0708-2 0 27

〃 250 ND 600 ND

〃 300 ND 0708-3 0 49

〃 350 ND 600 ND

0602 0 13.39 0708-4 0 98

〃 50 4.78 600 ND

〃 100 1.90 0708-5 0 498

〃 150 ND 600 ND

〃 200 ND 0709 0 73.58

0606-1 0 1.83 20 36.32

〃 50 ND 50 23.87

0606-2 0 10.85 100 8.84

〃 150 ND 200 0.19

0606-3 0 27.3 0806 0 81.571

〃 150 ND 〃 200 ND

0606-4 0 98.91 0809-1 0 19.8

〃 300 ND 〃 200 ND

0606-5 0 853 〃 200 ND

〃 600 1.10 〃 200 ND

〃 650 ND 〃 200 ND

〃 700 ND 0809-2 0 47

〃 750 ND 〃 150 ND

0708-1 0 10 〃 150 ND

〃 600 ND *ND: no PCBs detected, i.e. <0.05 ppm

115 FIG. 11.2. FT-NMR results after irradiation according to the dose of the electron

11.1.4. Conclusion

From an investigation of molecular weight changes post-irradiation, electron beam irradiation seemed to transform a large molecular weight compound into a lower one; however, the impact was considered too small on the physical properties of the oil. Comparison results of the physical and electrical properties between pre- and post-irradiation revealed that the properties were almost maintained after irradiation. The residual concentrations of the PCBs after irradiation depended on the absorption dose of the electron beam energy, but aliphatic chloride compounds were produced at higher doses of irradiation. The difference between GC/ECD and HRGC/MS for the residual concentration of PCBs indicated that the analysis by HRGC to confirm the residual PCBs is very important.

Recently, the electron beam accelerator has come into the spotlight in the field of environmental conservation. Its advantages compared to other methods such as chemical destruction, biodegradation, IR, UV, and so on for the decomposition of PCBs in transformer

Absorbance

ppm (¹H)

(a)

(b)

(c)

(d)

(e)

(f)

116

oil are its economic benefits, massive, and lack of needed additives. It is also a simple process due to its normal temperature and ambient pressure operation.

TABLE 11.3. CONCENTRATION OF PCBS AND PCBS ISOMERS

AFTER IRRADIATION WITH AN ELECTRON BEAM

ACCELERATOR.

Sample ID 090306-0-7-0-V01

090306-0-7-0-V14

090306-0-7-0-V14

090306-0-7-0-V14

Dose, kGy 0 180 240 300

PCBs by

GC/ECD 37ppm <0.5ppm <0.5ppm <0.5ppm

PCBs isomers (unit: ng/g)

MoCBs 91 4000 3100 2000

DiCBs 3900 7400 4000 2700

TrCBs 13000 690 240 160

TeCBs 9700 170 40 40

PeCBs 11000 16 0 7

HxCBs 13000 0 0 0

HpCBs 8400 0 0 0

OcCBs 2300 0 0 0

NoCBs 210 0 0 0

DeCB 4 0 0 0

Total PCBs 62000 12000 7400 4900

PART 2. TREATMENT OF SEWAGE SLUDGE