Eskom mainly uses uninhibited mineral oil in its older transformers and the effects of PCB dechlorination on the oil's natural inhibitor content are uncertain. The study confirmed the reduction of oxidation stability and therefore the natural inhibitor content of mineral oil after sodium dechlorination.
Background
This is done in accordance with the conditions set out in Article 6 of the Convention (Stockholm Convention, 2001). Complying with the permitting requirements for a hazardous waste treatment facility is understandably exponentially more stringent, comprehensive and therefore more expensive compared to that of a general waste treatment facility or site.
The PCB situation in Eskom
The acid present in the oil will initiate a chemical breakdown reaction of the entire insulation systems (paper, pressure plate & oil). Lombard (1996) mentions the presence of two groups of polyaromatic hydrocarbons (PAHs) which are formed during the oil refining process of the mineral insulating oil. Oxidation of the oil through breakdown during optimal and loaded transformer operation results in acid formation.
The difference between the network output y and the target output t (i.e. the output from the actual process) or the error is found using equation (10). The neutralization value of the oil is a measure of the presence of acidic compounds in the oil. The oxidation stability test was performed by the author with the help of a laboratory technician.
Potential alternative uses for dechlorinated oils
Expected benefits
Improved economic benefit by promoting the reuse of dechlorinated oil in other areas within or outside the company, offsetting the costs of PCB dechlorination. Will support Eskom and the government's PCB phase-out objectives in terms of its 2025 Stockholm Convention commitments.
Hypothesis
This can cause accelerated aging due to increased oil and cellulose oxidation rates and ultimately reduce the life expectancy of the transformers.
Research design
Research methodology
Evaluation and interpretation of the test results and the derivation of a preliminary algorithm to predict the reduction in oxidation stability after sodium dechlorination, as a function of PCB concentration before dechlorination. This will provide an indirect indication of natural inhibitor depletion of the oil based on its exposure to the sodium dechlorination process conditions.
Outline of dissertation chapters
A mineral oil can be classified as paraffinic, naphthenic or aromatic depending on the relative proportions of the molecular types present in the oil. Uninhibited naphtha-based mineral oil contains natural inhibitors such as certain of the sulfur, aromatic, and nitrogen-based compounds present in the oil. The warm-up time is dependent on the moisture concentration of the PCB-contaminated oil entering the system.
The dechlorination reaction of the PCB molecule is limited by the amount of active sodium present, resulting in the formation of sodium chloride. Oil maintenance is essential to maintain the dielectric strength properties of the oil. From most of the results obtained in the PCB range >500 ppm it can be concluded that the natural oil inhibitor content is reduced through dechlorination.
Introduction to literature review
Mineral insulating oils
The oxidation stability of the oil is directly related to the inhibitor content of the oil and is inversely proportional to the rate of oxidation. It is hypothesized that the additional dechlorination cycle will have a greater negative impact on the natural inhibitor content of the oil. THE ACTUAL EXECUTION OF THE MATLAB / NETLAB CODE USED FOR THE DEVELOPMENT OF THE NEURAL NETWORK.
Role of mineral insulating oil in transformers…
The effects of oxidation on mineral oil characteristics
Oxidation stability is one of the most vital properties of insulating oil and gives an indication of the oil's resistance to oxidation processes. The "bad" PAHs have an oxidizing effect on the mineral oil and a negative effect on the oil's impulse breakdown.
Characteristics of PCBs
A number of PCB congeners are combined with quantities of organic solvents (eg trichlorobenzene) to produce complex commercial mixtures commonly referred to as Aroclors. Where "12" represents the number of chlorine atoms in the biphenyl structure followed by the number representing the percentage of Aroclor chlorination by weight (Erickson, 1997).
The history of PCBs
However, by 1972 sufficient scientific evidence existed to indicate that the toxic, persistent and bioaccumulative properties of PCBs represented a serious hazard to human health and the environment, discussed further in section 2.7. PCB production ceased in the late 1970s after its toxic and bioaccumulative properties became known.
Health & environmental impacts of PCB
Until the 1960s, knowledge regarding the toxicity of PCBs within the industry was limited and not made public. Epidemiological studies have shown no significant increase in the incidence of cancer among individuals exposed to PCBs.
PCB regulation in South Africa
The South African Department of Water Affairs classifies PCBs as hazardous in the national specification entitled "Minimum Requirements for the Treatment, Classification and Disposal of Hazardous Wastes" (1998). Based on this classification, all national laws regarding the treatment and disposal of hazardous waste are relevant to the management and disposal of PCBs.
Base catalysed dechlorination for PCB destruction
The results obtained from the reactions shown in Figure 2-8 indicate a directly proportional relationship between the particle size of the sodium dispersion and the time required to complete the reaction. A study conducted by Pahlavanpour et al. (1992) mentions two major issues related to sodium-based PCB dechlorination processes.
Other PCB destruction technologies
These concerns relate to the safety of the system, due to the use of highly reactive sodium metal dispersions and the risk of explosion in the presence of excess oxygen and moisture. SCWO is a high temperature and pressure technology that incorporates the properties of supercritical water in the destruction of organic compounds and toxic waste.
Insulating oil regeneration
The process involves the purification and subsequent percolation of the oil through a set of columns containing the Fuller's earth adsorption medium. This is followed by forced percolation of the oil through the adsorbent clay bed consisting of a series of columns containing Fuller's earth clay, at a temperature of 65°C.
Artificial neural networks
Where y = f(W, x), M is the number of training samples, N is the number of outputs and f represents the activation function of a layer in the neural network. The first step in training the neural network is to initialize the weights and biases with random values.
Summary of literature reviewed
This error is used to find the next set of values of weights and biases from equation (9). The number of layers is determined by the complexity of the process being modeled, but for most applications 2 layers are sufficient.
Comparative analysis of the benefits of dechlorination versus local PCB licensed
The average time taken for reaction and regeneration of oils contaminated with PCBs in the range from 12 to 144 ppm is 3 hours. The cost of dechlorination is directly proportional to the PCB concentration of the oil and typically ranges from R8.00 to R12.00 per litre.
Introduction to the experimental investigation
Experimental methodology
Description of process & equipment used in experimental work
The percent sludge formation and neutralization number were measured to determine the oxidation stability of the uninhibited mineral oil. Tables 4-2 and 4-3 below contain the results of mud analysis and titrametric acidity analysis, respectively. Of particular interest are the results of oxidation stability and how they relate to the content of natural inhibitors in the oil during each phase of the experimental process, i.e.
Tables 4-4 through 4-8 will be discussed in relation to the oxidation stability results in Tables 4-2 and 4-3 for each of the three PCB concentration ranges. This indicates a lower oxidation stability and therefore a lower content of natural inhibitors of the oil after dechlorination and regeneration compared to pre-dechlorination. After regeneration, the rate of acid and sludge formation of the oils is improved over the entire sample range.
Chemical analyses of uninhibited dechlorinated mineral oil
Introduction to evaluation & interpretation of results
Results obtained
The amount of current used to generate iodine to reach the end point of the titration is used to calculate the concentration of water in the sample. Dielectric strength analysis was carried out by the author at an Eskom SABS accredited laboratory and was carried out in accordance with IEC 60156 test method for determination of breakdown voltage at power frequency of batches of oil samples.
Evaluation & interpretation of results used
Sample/batch 4 with a concentration range of 50-500 ppm indicates a decrease in both sludge and acid formation after regeneration, indicating that oils have higher oxidation stability after the initial pre-dechlorination sample. The high acid content was removed through a clay regeneration process, resulting in lower acid and sludge formation and therefore higher oxidation stability of the post-regeneration samples for batch 4.
Trend evaluation of results across the PCB test range
A sample of the data obtained during the experimental study was used as input and output to establish the algorithm. Based on the results obtained from the experimental tests carried out during this study, it can be concluded that the sodium dechlorination process negatively affects the natural inhibitor content of the uninhibited naphthenic based mineral oil. The reports described in subparagraph (g) shall, as appropriate, be considered by the Conference of the Parties in its reviews on Polychlorinated Biphenyls.
MATLAB/NETLAB SOURCE CODE FOR TRAINING AND TESTING NEURAL NETWORK MODELS OF THE OXIDATION PROCESS AND TEST 164 HOURS.
Introduction to algorithm application and the application of neural networks to
The results obtained from the established algorithm
The error seen in the table is calculated by first subtracting the values in column C from column B and taking the absolute value of the result. Based on the findings of this study, it can be concluded that for uninhibited PCB contaminated oil above 100 ppm, the natural inhibitor of the oil may not be at a level that would allow its reuse in transformers.
Materials safety data sheet for PCBs in oil
Stockholm Convention on persistent organic pollutants – Annex A, part II
In lieu of Note (ii) of Part 1 of this Annex, endeavor to identify other articles containing more than 0.005 per cent polychlorinated biphenyls (eg cable sheaths, cured sealant and painted articles) and deal with them in accordance with para 1 of Article 6;. (g) Submit every five years a report on progress in eliminating polychlorinated biphenyls and submit it to the Conference of the Parties in accordance with Article 15. (h). The Conference of the Parties shall review progress towards the elimination of polychlorinated biphenyls at five-year intervals or other period, as appropriate, taking into account such reports.
The actual execution of the Matlab / Netlab code used for the development
Result of the Matlab / Netlab code execution from Appendix 3
