EFFECT OF IONOL EXTRACTION TEMPERATURE ONTO ITS GAS CHROMATOGRAPHIC DETECTION AT TRANSFORMER OIL

Introduction. Content of ionol antioxidant additive in the transformer oil (the oil) maintained within the normalized values limits enables preserving oils’ operational properties and ensure reliable operation of oil-filled electrical equipment. The oil content in ionol may be assessed by gas chromatography (GC) method using the ionol isothermal liquid extraction with extractant and absolute calibration of the gas chromatograph with linear calibration curves. Provided the ionol distribution coefficient i K for the system “analyzed oil — ionol — extractant” differs from the distribution coefficient

Introduction.Content of ionol antioxidant additive in the transformer oil (the oil) maintained within the normalized values limits enables preserving oils' operational properties and ensure reliable operation of oil-filled electrical equipment.The oil content in ionol may be assessed by gas chromatography (GC) method using the ionol isothermal liquid extraction with extractant and absolute calibration of the gas chromatograph with linear calibration curves.Provided the ionol distribution coefficient i K for the system "analyzed oil -ionol -extractant" differs from the distribution coefficient c i K in the "calibration oil -ionol -extractant" system then GC assessment includes calculation using the single solvent extraction equation and the value of ionol distribution coefficient i K within equilibrated system "analyzed oil -ionol -extractant" followed by GC analysis of the extract containing ionol.
At that, to use a single solvent extraction equation, the analyzed oil distribution coefficient i K must be found experimentally beforehand [1,2].When using ethanol as extractant within narrow extraction temperature interval from t1 to t2 the ionol concentration C TM in the oil is calculated by the formula , , 2 wherе С ext,t1 , С ext,t2 -ionol concentration at extracts; K i,t1 , K i,t2 -distribution coefficients; V ТМ,t1 , V ext,t1 , V ТМ,t2 , V ext,t2 -oil and extractant volumes at respective extraction temperatures t1 and t2.
Analysis of recent researches and publications.It is known that the extraction temperature raise results in: changing the extractables' distribution coefficients in the "analyzed liquid -analyzed component -liquid extractant" system; improved GC sensitivity in detecting the analyzed component the liquid sample of analyzed substance; reduced duration of the analyzed component extraction procedure as compared, e.g. to the example room temperature [3].At temperature level t=20 °C the duration τ of ionol extraction from oil using ethanol to reveal contents through GC is long enough: τ(20 °C) ≈ 2 h [2].While ionol extraction from oil using ethanol at elevated temperatures that can lead to increased ionol concentrations at the extract reducing the extraction process duration, that has a practical significance when oils GC analysis.Studying the effect of temperature changes in extractants-aided ionol extraction from oil on the distribution coefficients values i K , and accordingly, on the result of ionol-in-oil presence assessment allows to set the desired temperature range establishing the ionol extraction optimum range of temperature control for the extracting device thermostating.In this connection studying the temperature effect on the ionol distribution coefficients i K at the "oil -ionolextractant" system within the extraction temperature range of 15...75 °С (288...348 K) is of practical importance when optimizing the conditions of ionol content GC-detection in oils.
Aim of the research is to study the temperature effect on the value of extraction degree and dis-tribution coefficients for ionol in the "oil -ionol -ethanol" system in the ionol extraction temperature range of 15...75 °C (288...348 K) for optimizing the conditions of GC implementation to determine ionol contents in the transformer oil by the method of ionol single extraction with ethanol and further calculation with estimated equation using the ionol distribution coefficient.Main Body.The measurements are performed as described in [1] using petroleum oil of GK-type (ionol content C TM =0,30 % mass fraction) and gas chromatograph with a thermal conductivity detector.Extractant agent: ethanol.The ext TM N V V = relation is equal to 0,2 or 1,0.i K value at a given temperature t of ionol ethanolic extraction is calculated according to the formula The concentration value С ext , corresponding to the ionol chromatographic peak area ext S is found by linear calibration curves obtained through absolute calibration chromatography using calibration ionol ethanolic solutions.The distribution coefficients' i K dependencies upon extraction temperature are established within temperature range 15...75 °C (temperature control accuracy Δt=±0,5 °C) for each given value of fixed extraction temperatures.Reaching the phase equilibrium when ionol extraction under isothermal conditions at given temperatures t is assessed studying the extracts concentration in ionol dependence upon ionol extraction duration of.The relative ionol extraction degrees R i , %, are calculated by the formula 1 100 1 V V or TM ext V V values.The Table 1 exposes the relationship between temperature t, °C ionol distribution coefficients K i , ionol concentration at extracts С ext , % mass fraction, relative ionol extraction degrees а R i at N = 0,2 and N = 1,0.0,076 0,081 0,085 0,089 0,096 0,108 0,119 0,126 0,133 0,139 0,143 From Table 1 we clearly observe that: -the extraction temperature approaching to the ethanol boiling point t bp =78,37 °C correlates with the distribution coefficient K i approaching to 1; -when extraction temperature increase at N=const the concurrent effects of K i value decrease, ionol extraction degree R i increase and ionol concentration at extract С ext increase take place; -under ionol extraction from oil isothermal conditions the extraction degree R i depends upon the distribution coefficient K i for ionol and volumes' ratio N, e.g. at K i =const the augmented N increases the ionol extraction degree R i thus involving the resulting extract's ionol concentration lowering.
The obtained K i =2,5 at t=20 °C is fairly close to the mean value K i =2,2 (relative divergence making 13,6 %), as the source [4] specifies.The dependency between distribution coefficients K i and the absolute temperature Т having shape of K i =f(Т) can be represented with an equation used to describe the ideal solutions' extraction equilibrium [3] ln Fig. 1 represents the dependency between distribution coefficients K i and temperature values t.
The measurements' and calculations' results are given at the Table 2, including: -0,5( ) -mean values of ionol distribution coefficient at b i K (given the temperature t b at thermic range beginning) and e i K (given the temperature t e at thermic range end- ing) within the given temperatures range; -  Referring to the Table 2 we observe that there exist three temperature ranges for investigated oils , where the coefficients "А" and "В" of equa- tion (4) differ from the corresponding values for each of these temperature ranges.Since the oils operated at oil-immersed electric machinery have different physical and chemical properties, including the structural group composition (e.g., differences in content of aromatic hydrocarbons) affecting oil susceptibility to ionol, for each oil brand or different grades oils mixture coefficients "A" and "B" of the equation (4) are different.Under isothermal conditions, the extraction ratio "A" in equation (4) represents the difference between the standard values chemical potentials for ionol in oil and in ethanol [3] and can be associated with the function ( , ) where Q TM , Q ext -heat of ionol dissolution in oil and ethanol, respectively.In the studied ionol extraction temperature range (15...75 °C) the ionol melting range is t m =69,6…70,0 °C (ionol category "А" as by TU38.5901237-90), indicating that the ionol dissolution in oil concurs to a thermal effect of dissolving the solid ionol in oil at t<69,6 °C, dissolving the solid and liquid ionol mixture in the temperature range of t=69,8…70,0 °C, and dissolving the liquid ionol at t>70,0 °C.Considering that the ionol solubility in oil and ethanol under isothermal conditions is limited to respective saturated concentrations of ionol, the afore exposed can determine the presence of being also characteristic when other liquid extractants, for example, 2-propanol, or acetonitrile, for which the evaporation temperature t ev is higher than the melting point t m .
Results.The experimental series results showed that: -Increasing the extraction temperature from t=20 to 75 °C at optimum values V TM =50 сm 3 , V ext =10 сm 3 , N=0,2 gives rise to 2,1 times increase of extract content in ionol and to reduce the GC threshold value as to ionol oil content approx.about 2 times; -At t = 65°C duration of ionol ethanolic extraction maxes approximately τ(65 °C)≈55 min, i.e. average 2-fold decreases comparing to t=20 °C, that can be due to an increased rate of mass transfer in a heterogeneous extraction system "oil -ionol -ethanol"; -Temperature change in the range of 15...32 °C results in a relative change in the distribution coefficient value K i no more than ±40%, so the ionol extraction in this temperature range is recommended to perform at t=20 °C and Δt≤±1 °C; -Ionol extraction temperature change in the range of 32…40 °C results in a relative change in the distribution coefficient value K i no more that ±14,1 %, so the ionol extraction in this temperature range is recommended to perform at t=36 °C and Δt≤±2 °C; -Ionol extraction temperature change in the range of 40…75 °C results in a relative change in the distribution coefficient value K i no more that ±27,6 %, so the ionol extraction in this temperature range is recommended to perform at t=65 °C and Δt≤±5 °C; -When ionol concentration at oil makes С TM ≥0,1wt % the value K i , t2 for temperature t2 when knowing the K i , t1 value for temperature t1,can be calculated from the formula (1).

S.V. Zaitsev. Effect of ionol extraction temperature onto its gas chromatographic detection at transformer oil.
The gas chromatography used for detecting antioxidizing additive ionol at transformer oil presence helps to ensure reliable operation of oil-filled electrical equipment.Changes in the ionol preliminary extraction temperature do affect the reliability of measurement result.This study aim consisted in investigating the temperature effect on the value of extraction degree and distribution coefficients for ionol in the system "oil -ionol -ethanol" at extraction temperatures 15...75 °C.The experiment included optimization of gas chromatographic ionol in transformer oil detection conditions using method of ionol ethanol extraction and an estimated equation

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When ethanol densities 15 ext ρ =0,806 g/cm 3 at 15 °C and 75 ext ρ =0,740 g/cm 3 at 75 °C, as well as at oil densities 15 TM ρ =0,890 g/cm 3 at 15 °C and 75 TM ρ =0,850 g/cm 3 at 75 °C, the difference between values never exceed 4,4 % relatively.That provides us possibility to apply formulae (1)…(3) without account of ionol extraction temperature influence onto ext TM -relative change in K i coefficient at extraction temperature change by 1°C within the given extraction temperatures range; -Δt -recommended accuracy of ionol extraction temperature control when the relative deviation of distribution coefficient K i from the mean value mean i K does not exceed 6 % for the established temperature within the given ionol extraction temperatures range.

Fig. 1 .
Fig. 1.Dependency between the distribution coefficients K iand temperature t for a single extraction.Found is that the ionol extraction temperature increase in the range of 15...75 °C reduces the values of the ionol distribution coefficients and increases the value of ionol extraction degree and its concentration in the extract reducing the extraction duration, lowering the value of the detection threshold and the total duration of ionol in transformer oil detection.The recommended values for ionol extraction in the temperature range of 15...32 °C at a temperature of extraction 20 °C with precision temperature Δt ≤ ±1 °C, in the range of 32…40 °C, with a temperature of extraction 36 °C and Δt ≤ ±2 °C, in the range of 40...75 °C, with a temperature of extraction 65 °C and Δt ≤ ±5 °C.Keywords: gas chromatography, transformer oil, ionol, distribution factor, extraction, ethanol, temperature.Received August 4, 2014

Table 1
Dependencies between temperature t and the distribution coefficients i K , ionol extraction grades R i , ionol concentrations at extracts С ext