Matrix Effects Affecting the Indirect Calibration of the Static Headspace

Anal. Chem. 2003, 75, 5230-5239

Matrix Effects Affecting the Indirect Calibration of the Static Headspace-Gas Chromatographic Method Used for Dissolved Gas Analysis in Dielectric Liquids J. Jalbert,†,‡ R. Gilbert,*,† P. Te´treault,† and M. A. El Khakani‡

Institut de Recherche d’Hydro-Que´ bec (IREQ), 1800, boulevard Lionel-Boulet, Varennes, Que´ bec, Canada J3X 1S1, and Institut National de la Recherche Scientifique (INRS), 1650, boulevard Lionel-Boulet, Varennes, Que´ bec, Canada J3X 1S2

To evaluate the effects of the density and polarity of the matrix on the dissolution of H2, O2, N2, CH4, CO, CO2, C2H2, C2H4, C2H6, C3H6, and C3H8 in various dielectric liquids, the gas-liquid partitioning coefficients (Ostwald coefficients) have been measured under the conditions of method C of ASTM D 3612-01 using a vapor-liquidphase equilibrium technique. Altogether, 286 gas-liquid systems distributed throughout samples of mineral oils, vegetable oils, and synthetic oils were investigated with the aim of extending the scope of applicability of method C for dissolved gas analysis. The possibility of applying the method to a range of high-viscosity liquids (up to 120 cSt at 40 °C) was first demonstrated by measuring the equilibration of the species in the coexisting vapor phase of the samples. The effect of oil aging upon gas solubility was then addressed by assessing samples of a given mineral oil collected at different ages of in-service equipment. The oxidation of a weak polar dielectric was found to exert a negligible influence on the solubility of the nonpolar gas solutes, while for the only species with a permanent dipole moment (CO), an important reduction is seen in the solubility with the building up of oxidation products in the matrix. The Ostwald coefficients determined with samples of mineral oils (N ) 13) obtained from naphthenic and paraffinic petroleum crudes showed that in the absence of strong intermolecular interactions, the solubility of the individual gases decreased with the matrix density as expected from the literature. Other measurements carried out with samples of vegetable oils (N ) 7) revealed that solutes with highly polarizable π electrons (e.g., C2H2 and CO2) are susceptible to strong intermolecular interactions with some matrix polar components, which increase their ability to dissolve in this type of dielectrics. Last, the results collected with samples of synthetic oils (N ) 6) were conclusive regarding the role played by some functional groups of the matrix components (carbonyl groups, conjugated double bonds, SiO bonds) in the intermolecular forces acting on the gas solutes. In light of these direct measurements of solubility data, the possibility of using a typical set of Ostwald 5230 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

coefficients for assessing mineral oils of various origins and as they age in the electrical apparatus was confirmed. Recently, a fully automated method was included in the Annual Book of ASTM Standards, Volume 10.03, of the American Society for Testing and Materials for the extraction of light hydrocarbons and permanent gases (H2, O2, N2, CH4, CO, CO2, C2H2, C2H4, C2H6, C3H6, C3H8) from insulating oils and the identification and determination of the individual gas components (method C of ASTM D 3612-01).1 The concentrations of the gas components and their ratios are used by the electrical utilities to diagnose an incipient fault in a power transformer and take preventive action before the advent of a catastrophic failure.2 In this method, the gas solutes are partially extracted from the oil by creating a space over an oil sample, according to a principle governed by Henry’s law. When the gas-liquid partitioning coefficients (Ostwald coefficients) (the solubility of a gas expressed as the ratio of the concentration of a gas in the liquid-phase solution to that in the coexisting vapor-phase solution at a given temperature and total pressure.) are known at the temperature and pressure of the method determinations, the quantification of the individual gas components can be achieved by calibrating the chromatographic signal with gas mixtures rather than using gas-in-oil standards. However, because of a possible effect upon gas solubility of varying amounts of polar compounds, such as those produced by field oxidation, and oil components, this approach of using the coefficients for the determination of the gas components became a source of concern. In an attempt to demonstrate the viability of this concept for dissolved gas analysis (DGA), comparisons were done with the proposed method and method A of ASTM D 3612 (gas extracted by the introduction of a sample into a preevacuated known volume) on a given oil collected at different ages of inservice equipment (variable amounts of polar compounds) and on mineral oils of differing sources (variable chemical compositions).3,4 These comparisons carried out with a limited number of * Corresponding author. E-mail: [email protected]. † Institut de Recherche d’Hydro-Que ´ bec. ‡ Institut National de la Recherche Scientifique. (1) Annual Book of ASTM Standards. American Society for Testing and Materials: Philadelphia, PA, 2001; Vol. 10.03. ASTM D 3612-01: Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography. (2) Rogers, R. R. IEEE Trans. Electron. Insul. 1978, EI-13 (5), 349-354. 10.1021/ac0343634 CCC: $25.00

© 2003 American Chemical Society Published on Web 08/23/2003

samples (mostly mineral oils derived from naphthenic crudes) provided the initial data required for supporting the use of an indirect calibration of the method. Notwithstanding the fact that there was a real lack of published solubility data under the method C temperature and pressure conditions for the gases of interest in such multicomponent mixtures (for other conditions, see refs 5-9), only a few direct measurements of the gas-liquid partitioning coefficients had been carried out since the inclusion of the method in the test standard. At the time the above-mentioned method comparisons were done, the efficiency of the vapor-liquid-phase equilibrium techniques used for such determinations was not yet established for gas-tooil systems. However, the recent development of agriculturally based dielectric liquids10-12 and the increasing use of synthetic dielectric liquids and high-fire-point fluids of petroleum origin make us reconsider such measurements given that most of these liquids have a density and viscosity exceeding the maximum values specified in the test standard. These dielectrics are known to differ in their chemical compositions, and, even in a given category such as mineral oils, the molecular structure and the degree of unsaturation may vary greatly from one source to another. Strictly from the standpoint of solvents, they could be considered as ill-defined mixtures. In a recent work reported by our laboratory,13 the vapor-phase calibration method (VPC method) developed originally by Kolb et al.14,15 was shown to be the most efficient vapor-liquid-phase equilibrium technique for simultaneously measuring all the °C values). Excellent acOstwald coefficients of interest (K70 i curacy with a precision better than 2% (RSD) for most of the coefficients (11 species) was achieved under the temperature and total pressure conditions of method C. In the present paper, this method was applied in an attempt to evaluate the effect upon the coefficients of an accumulation of polar compounds in the oil. The method was also applied to measure the coefficients in hydrocarbon oils of various chemical compositions, including naphthenic and paraffinic mineral oils, and nonpetroleum dielectrics such as vegetable oils and synthetic oils. Prior to proceeding with the measurements, the conditions retained by the method for equili(3) Jalbert, J.; Gilbert, R. IEEE Trans. Power Delivery 1997, 12 (2), 754-760. (4) Gilbert, R.; Jalbert, J. Dissolved Gas Analysis in Insulating Oils by Controlled Headspace Sampling Coupled with Capillary Gas Chromatography. Proceedings of the 8th BEAMA International Electrical Insulation Conference and Exhibition, 1998; pp 444-452. (5) Beerbower, A. ASLE Trans. 1979, 23 (4), 335-342. (6) Peterson, R. E. Fed. Proc. 1970, 29 (5), 1714-1716. (7) Logvinyuk, V. P.; Makarenkov, V. V.; Malyshev, V. V.; Panchenkov, G. M. Khim. Tekhnol. Topliv .Masel 1970, 5, 27-29 (Solubility of Gases in Petroleum Products. Translation). (8) Nanda, J. R.; Krishnaswamy, K. R. Pet. Hydrocarbons 1968, 3 (2), 49-52. (9) Baldwin, R. R.; Daniel, S. G. J. Inst. Pet. 1953, 39, 105-123. (10) Oommen, T. V.; Clairborne, C. C. Electrical Transformers Containing Electrical Insulation Fluids Comprising High Oleic Acid Oil Compositions. U.S. Patent 5,949,017, Sept. 7, 1999. (11) Cannon, G. S.; Honary, L. A. T. Soybean Based Transformer Oil and Transmission Line Fluid. U.S. Patent 5,958,851, Sept. 28, 1999. (12) McShane, C. P.; Corkran, J. L.; Harthun, R. A.; Gauger, G. A.; Rapp, K. J.; Howells, E. Vegetable Oil Based Dielectric Coolant. U.S. Patent 6,037,537, March 14, 2000. (13) Jalbert, J.; Gilbert, R.; El Khakani, M. A. Chromatographia 2002, 56 (9/ 10), 623-630. (14) Kolb, B.; Welter, C.; Bichler, C. Chromatographia 1992, 34 (5/8), 235240. (15) Kolb, B.; Ettre, L. S. Static Headspace-Gas Chromatography: Theory and Practice; Wiley-VCH Inc.: New York, 1997.

brating the species in the coexisting vapor phase of the samples were validated for liquids with a viscosity as high as 120 cSt at 40 °C. Finally, the possibility of using a typical set of coefficients for performing DGA in mineral oils as they age in the electrical apparatus as well as for the new generation of liquids has been evaluated. EXPERIMENTAL SECTION Instrumentation. All the measurements were performed on an Agilent 7694 automatic headspace sampler equipped with a 3-mL sample loop (Agilent Technologies). The sampler is connected to an Agilent chromatograph, model 5890A Series II, equipped with a thermal conductivity detector, a nickel catalyst system (NICAT II system), and a flame ionization detector. Light hydrocarbons are separated on a 30-m × 0.53-mm-i.d. Carboxen1006 PLOT column (Supelco/Sigma-Aldrich), while a 25-m × 0.53mm-i.d. MolSieve 5-Å PLOT column (Chrompack) is used for the permanent gases. The connection of the columns to the bypass valve, the valve operating sequence for column isolation, and the instrumental conditions are identical to those given in method C of ASTM D 3612-01 (see Figures 5 and 6 and Table 2 in ref 1). These conditions were rigorously applied to the samples of the three categories of dielectric liquids investigated. The density of the liquids was measured at 15 °C with a Parr density meter, model DMA 58 (Anton Paar), while the kinematic viscosity and the acid number of the samples were measured according to ASTM D 445-9716 and ASTM D 974,17 respectively. The intensity of the carbonyl band at 1750 cm-1 for the liquids containing ester functions was measured on a Nicolet FT-IR spectrometer, model 520, using a 6-µm-path length cell mounted with CaF2 windows. Chemicals. Representative samples of dielectric liquids regularly used or investigated for use in electrical apparatus were chosen from the following. (i) Mineral oils obtained by refining a fraction of the hydrocarbons collected during the distillation of a naphthenic crude: Voltesso 35 (Imperial Oil, Canada), Nytro 10CX (Nynas, Sweden), Shell Diala B (Shell, Australia), Univolt N61 (Exxon, USA), Penneco type I (Penzoil, USA), Hyvolt type II (Ergon, USA), and TC2288/84 (cable fluid from Dussek Campbell, Canada). A paraffinic crude: Univolt 52 (Esso, France), Univolt 54 (Esso, France), Diekan R2613p (Fina, Belgium), Diekan 1500P (Fina, Belgium), Drakeol 35 (Penreco, USA), and R-Temp (Cooper Power Systems, USA). (ii) Vegetable oils based on agricultural sources such as grains and seeds: Envirotemp FR3 (Cooper Power Systems, USA), Biotemp (ABB Inc., USA), BioTrans 1000 (Cargill Industrial Oils and Lubricants, USA), peanut oil, soya oil, sunflower oil and olive oil (the last four are edible oils from Fluka, Supelco/Sigma-Aldrich, USA, Catalog No. 45420, 85471, 85485, and 75348, respectively). (iii) Synthetic oils produced by a chemical synthesis or a special treatment applied to a petroleum product: Dussek T3788 (cable fluid from Dussek Campbell, Canada), dioctyl phthalate (Supelco/Sigma-Aldrich, USA), Envirotemp 200 (Cooper Power Systems, USA), DC 561 (Dow Corning, USA), SF97-50 (GE Silicones, USA), and Luminol TR-i (Petro(16) Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1995; Vol. 10.03. ASTM D 445-94: Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. (17) Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1995; Vol. 10.03. ASTM D 974-93: Standard Test Method for Acid and Base Number by Color-Indicator Titration.

Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

5231

Table 1. Some Physical Properties of Interest of the Dielectric Liquids Investigated As Measured in Our Laboratory on Received Samples type of insulating liquid

viscosity cSt (mm2 s-1) at 40 °C

density (g cm-3) at 15 °C

area under the CO band at 1750 cm-1 (absorb cm-1)

Mineral Oils from Naphthenic Crudes Voltesso 35 7.9 0.871 10 Nytro 10CX 7.1 0.877 25 Shell Diala B 11.7 0.873 71 Univolt N61 9.2 0.872 73 Penneco type I 10.0 0.884 84 Hyvolt type II 9.3 0.886 11 TC2288/84 7.8 0.879 15 Univolt 52 Univolt 54 Diekan R2613p Diekan 1500P Drakeol 35 R-Temp

Mineral Oils from Paraffinic Crudes 7.7 0.851 30 7.1 0.844 32 7.7 0.853 47 7.8 0.854 40 68.8 0.877 90 119.1 0.883 18

nda nd nd nd nd nd nd nd nd nd nd nd nd

Envirotemp FR3 Biotemp BioTrans 1000 peanut oil soya oil sunflower oil olive oil

Vegetable Base Oils 33.4 0.923 41 40.7 0.917 96 40.6 0.921 12 37.9 0.918 67 31.6 0.922 98 32.1 0.922 50 43.2 0.916 54

30 39 30 27 23 28 33

Dussek T3788 dioctyl phthalate Envirotemp 200 DC 561 SF97-50 Luminol TR-i

Synthetic Base Oils 4.2 0.861 19 26.9 0.987 83 29.0 0.974 53 37.3 0.969 11 50 0.968 77 8.7 0.836 68

nd 41 50 nd nd nd

a

the glovebox where 400 µL of the certified gas mixture is added just before its introduction into the inner compartment of the headspace sampler (corresponding to time zero of an equilibration test). Samples for Measuring the Gas-Liquid Partitioning Coefficients. The dispenser is used to discharge 15 mL of the liquid to 15 open 20-mL headspace vials (two-phase vials). These vials and an equivalent number of empty vials (one-phase vials used for calibration) are sealed and then removed from the glovebox where they are spiked with 400 µL of the certified gas mixture. A vortex mixing of 20 s is applied to the two-phase vials before proceeding with equilibration in the headspace sampler. Given the volume of the calibrating vials, the oil volume, and the headspace volume of the two-phase vials, the GC measurements could then be used to compute the gas-oil partitioning coefficients. Details on the data treatment, the equation used for computing the coefficients, and the performance of the VPC method are provided elsewhere.13 Finally, the volume of the vials used for these computations was estimated at 20.61 ( 0.08 mL, according to a procedure in ASTM D 3612-01.1 Safety Considerations. Most of the dielectric liquids evaluated in this study are rated in the Hazardous Materials Identification System (HMIS) as one or zero for health, one for flammability, and zero for reactivity. Inhalation of vapors of these finely misted materials may irritate the mucous membranes and cause irritation, dizziness, and nausea. Dioctyl phthalate is a toxic material that may cause cancer and irritation to the eyes, respiratory system, and skin. Possible risks of impaired fertility and harm to the unborn child are also associated with this product. Proper precautions should then be taken in the laboratory to minimize exposure and to comply with the disposal regulations.

Not detected.

Canada, Canada). The density and the viscosity of these samples as determined in our laboratory are given in Table 1. The argon (ultrahigh-purity/zero grade, 99.999%) for the glovebox and the GC, the hydrogen (UHP/zero grade) for the NICAT II system, and the air (UHP/zero grade) for the FID were from Air Products and Chemicals, Canada. A certified gas mixture in argon of ∼1000 ppm H2, CH4, CO, C2H2, C2H4, C2H6, C3H6, and C3H8, 1% CO2, 10% O2, and 20% N2 (Matheson, Canada) was used for the sample preparation. Sample Preparation. A volume of ∼500 mL of a given dielectric liquid is poured into a five-valve 1-L mobile-phase reservoir system (Kontes from Supelco/Sigma-Aldrich) located in an argon glovebox. The reservoir is first used for 2 h under sparging conditions with an argon flow rate of ∼170 mL/min. Argon bubbles are then used to drag the gases from the liquid to the reservoir headspace where a vacuum of 1/2 atm is applied. After extraction of the dissolved gases, the reservoir is used under a delivery mode to transfer the liquid into a bottle equipped with a dispenser head of 25-mL nominal volume (Dispensette III, Brand, Wertheim/Main, Germany). Samples for Measuring the Effect of Viscosity on Species Equilibration. The dispenser is used to discharge 15 mL of the liquid to an open 20-mL headspace vial. The vial is sealed by crimping an open-center aluminum cap containing a Teflon-faced butyl molded septum (Supelco/Sigma-Aldrich) and then removed from 5232


RESULTS AND DISCUSSION In method C of ASTM D 3612-01, the analysis of gases dissolved in electrical insulating oil is achieved by introducing a volume of oil into a closed vial filled with argon in order to create a gas-oil system.1 As a result, a portion of the dissolved gas in the oil is transferred to the vial headspace according to a principle governed by Henry’s law. At equilibrium, the relationship between the remaining concentration of gas i in the oil (Cli), its concentration in the headspace (Cgi), and its initial concentration in the oil (Cli0 ) is given by the following equation: °C Cli0 ) Cgi(K70 + Vg/Vl) i

(1)

°C where, K70 is the gas-oil partitioning coefficient of gas i, Vl, i the volume of the oil sample, and Vg, the volume of the coexisting vapor phase. Under the conditions stipulated in method C (15 mL of oil in a vial with a 20-mL nominal volume, T ) 70 °C, and mechanical shaking), it takes ∼15 min for all the species to reach 90% of their equilibrium concentration when a conventional oil such as Voltesso 35 (density of 0.871 10 g cm-3 at 15 °C with a viscosity of 7.9 cSt at 40 °C) is used.3 As the chromatography of the extracted species is the limiting step with a 30-min run, it was then decided to equilibrate the samples during the full time required to complete the analysis. Under these conditions, the chromatographic signal was shown to be linear over the range of concentrations of interest, e.g., 0-10 000 ppm (v/v) for H2 and

Figure 1. Effect of equilibration time on gas solutes in the coexisting vapor phase of dielectrics of variable viscosities: (O) Voltesso 35, ([) Biotemp, (4) Biotrans 1000, (b) peanut oil, (3) DC 561, and (1) R-Temp.

most of the hydrocarbons, with a detection limit (S/N ) 3) better than 1 ppm (v/v) for all the species except for O2 and N2, where the value is ∼10 ppm (v/v). Effect of Viscosity on Species Equilibration. The rate of evolution or solution of a gas is dependent upon the viscosity of a matrix (Szebehely in ref 8). As seen in Table 1, most of the nonpetroleum dielectric liquids show a viscosity much greater than the Voltesso 35, and in many cases, the value is well over the limit accepted by the scope of the ASTM method (restricted to viscosities of e20 cSt at 40 °C). Before applying the VPC method, it was important to verify the system’s capacity to equilibrate the species in the coexisting vapor phase of the samples regardless of the viscosity of the liquid investigated. The evolution with time of some of the species of interest is depicted in Figure 1 for the Voltesso 35 (low-viscosity liquid used as reference), Biotemp, BioTrans 1000, peanut oil, and DC 561 (all four with intermediate viscosities) and R-Temp (liquid with the highest viscosity). Because each test was started immediately after spiking the coexisting vapor phase of an oil with 400 µL of gas mixture (new vial used for each data point), the highest concentrations are then observed in the headspace just after the gas addition, while with

the equilibration time, these concentrations declined due to species diffusion into the oil until an equilibrium is reached, corresponding to a plateau in the curves. The effect of the matrix viscosity is clearly seen with the C2H6 species, which showed a net decrease of the initial solution rate when moving to a higher viscosity liquid. Figure 1 shows that, under the conditions of method C (same as those used with the VPC method), it is still possible to achieve more than 95% of species equilibrium in the two-phase vials even for a matrix with a viscosity as high as 120 cSt. It is also interesting to note a constant evolution of H2 in the coexisting vapor phase of the BioTrans 1000 and peanut oil samples (also seen for Envirotemp FR3 and the three remaining nonelectrical grade vegetable oils). The hydrogen forming in such matrixes could result from a slow reaction between the dissolved oxygen and some constituents present in the vegetable oils (no plateau found in the curves). As reported in a review of the lipid oxidation mechanisms,18 the susceptibility to oxidation increases with the presence of conjugated double bonds in the fatty acids of the triglyceride components (such as linoleic and linolenic acids). Peanut oil contains a greater amount of these highly unsaturated acids,19 which could explain the variation noted in the H2 evolution rates of the liquids (∼0.1 µL L-1 min-1 for the BioTrans 1000 and Envirotemp FR3 vs ∼0.3 µL L-1 min-1 for the peanut oil). Effect of Varying Amounts of Polar Compounds on the Ostwald Coefficients. To slow the oxidation reactions that occur on normal operation of open-breathing equipment, a compound containing a hindered phenol group, 2,6-di-tert-butyl-p-cresol (DBPC), is generally added to mineral oils. Commonly used oxidation inhibitors such as DBPC, 3-tert-butyl-4-hydroxyanisole, tert-butylhydroquinone, and R-, β-, or γ-tocopherol (natural antioxidants known as vitamin E) are also found alone or in combination in the vegetable oils investigated for electrical applications. However, even with the use of DBPC, the acidity number (indicative of the content of polar compounds in an oil) of a transformer mineral oil could reach 0.1 mg of KOH/g while a value as low as 0.002 mg of KOH/g is normally associated with a new sample. To establish the extent to which oil oxidation could influence gas solubility, the gas-liquid partitioning coefficients were determined in Voltesso 35 samples of various in-service ages collected from Hydro-Quebec’s power system. The changes in the Ostwald coefficients with the acidity of the field samples are shown in Figure 2. The unexpected result for O2 at 0.088 mg of KOH/g of oil could be explained by the fact that the DBPC was totally depleted from this sample. A slight increase is noted in the solubility of the nonpolar solutes (H2, O2, N2, CH4, CO2, C2H2, C2H4, C2H6, C3H6, C3H8) once the oxidation products begin to accumulate in the matrix, which is then attenuated with the abundance of these products in the oil. To be fully confident with the trend shown by the second portion of the curves, i.e., a decrease of solubility with an increase in acidity, these species, additional tests would have to be performed with samples prepared under controlled oxidation conditions rather than using field samples. On the other hand, a monotonic decrease of the coefficients is observed with sample acidity for carbon monoxide, which is the only solute showing a permanent dipole moment. A (18) Labuza, T. P. CRC Crit. Rev. Food Technol. 1971, 3, 355-395. (19) Oommen, T. V. IEEE Electr. Insul. Mag. 2002, 18 (1), 6-11.


5233

Table 3. Ostwald Coefficients at 70 °C in Mineral Oils Originating from Distillations of Paraffinic Petroleum Crudes As Determined by the VPC Method gas

Univolt 52

Univolt 54

Diekan R2613p

Diekan 1500P

Drakeol 35

R-Temp

H2 O2 N2 CH4 CO CO2 C2H2 C2H4 C2H6 C3H6 C3H8

0.111 0.263 0.152 0.427 0.157 0.878 0.981 1.372 1.918 5.169 6.669

0.099 0.238 0.141 0.454 0.155 0.927 0.933 1.459 2.077 5.775 7.834

0.093 0.248 0.147 0.452 0.141 0.950 1.083 1.483 2.079 5.845 7.653

0.105 0.221 0.121 0.425 0.135 0.893 1.018 1.413 2.053 4.915 5.803

0.070 0.147 0.082 0.319 0.094 0.724 0.707 1.116 1.620 4.069 4.777

0.105 0.207 0.120 0.336 0.120 0.740 0.809 1.121 1.595 4.015 4.593

Table 4. Ostwald Coefficients at 70 °C in Vegetable Oils of Electrical and Edible Grades As Determined by the VPC Method gas

Figure 2. Effect of oil aging on the Ostwald coefficients measured at 70 °C in the Voltesso 35 mineral oil: ([) C3H8, (f) C3H6, (9) C2H6, (0) C2H4, (2) C2H2, (4) CO2, (b) CH4, (O) O2, (3) N2, (]) H2, and (1) CO. Table 2. Ostwald Coefficients at 70 °C in Mineral Oils Originating from Distillations of Naphthenic Petroleum Crudes As Determined by the VPC Method gas H2 O2 N2 CH4 CO CO2 C2H2 C2H4 C2H6 C3H6 C3H8

Voltesso Nytro Shell Univolt Penneco Hyvolt TC2288/ 35 10CX Diala B N61 type I type II 84 0.093 0.206 0.121 0.446 0.146 0.936 1.076 1.456 2.068 5.131 5.827

0.096 0.216 0.128 0.451 0.152 0.945 1.027 1.484 2.113 6.170 9.070

0.085 0.205 0.133 0.409 0.132 0.865 0.898 1.346 1.947 5.132 6.657

0.097 0.218 0.138 0.440 0.150 0.926 1.017 1.440 2.026 5.482 6.573

0.092 0.200 0.126 0.427 0.140 0.892 0.974 1.406 2.008 5.541 7.241

0.090 0.206 0.128 0.423 0.140 0.892 0.998 1.401 1.979 5.417 7.008

0.090 0.204 0.118 0.427 0.139 0.891 0.953 1.398 2.014 4.712 5.530

reduction of 50.7% of the initial value is calculated for this species when moving from a new oil to an oxidized oil. Based on these results, the formation of polar compounds substantially reduces the capability of a weak polar medium to dissolve polar solutes. The impact that such coefficient variations could have on the accuracy of method C will be discussed in the last section of this paper. Effect of Varying Oil Compositions on the Ostwald Coefficients. The gas-liquid partitioning coefficients measured at 70 °C under a vial pressure ranging from 114 to 117 kPa (depending on the nature of the liquid tested) are compiled by product category in Tables 2-5. Among these products, the petroleumbased dielectrics are by far the most widely used for electrical applications (see Tables 2 and 3). A rapid screening of the data obtained for these products reveals that the solubility generally increases with the critical temperature (TC) of the gas solutes, 5234 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003


Envirotemp BioTrans peanut soya sunflower olive FR3 Biotemp 1000 oil oil oil oil 0.097 0.255 0.141 0.387 0.148 1.187 1.763 1.389 1.677 4.078 4.041

0.069 0.129 0.061 0.324 0.097 1.055 1.495 1.261 1.550 4.189 4.278

0.052 0.157 0.096 0.305 0.088 1.040 1.465 1.213 1.480 4.210 4.174

0.083 0.166 0.100 0.339 0.101 1.072 1.677 1.282 1.571 4.331 4.248

0.104 0.233 0.120 0.383 0.153 1.168 1.820 1.373 1.658 4.432 4.161

0.079 0.238 0.141 0.377 0.128 1.169 1.835 1.376 1.666 4.451 4.255

0.083 0.216 0.090 0.327 0.087 1.019 1.580 1.264 1.542 4.222 3.538

Table 5. Ostwald Coefficients at 70 °C in Synthetic Oils As Determined by the VPC Method gas

Dussek T3788

dioctyl phthalate

Envirotemp 200

DC 561

SF9750

Luminol TR-i


0.090 0.247 0.167 0.490 0.148 1.127 1.590 1.700 2.249 6.050 6.195

0.129 0.225 0.150 0.428 0.180 1.436 2.367 1.567 1.828 6.698 7.499

0.103 0.193 0.137 0.355 0.143 1.365 2.459 1.310 1.504 4.281 4.087

0.114 0.264 0.165 0.519 0.199 1.078 1.448 1.518 1.997 6.440 8.539

0.132 0.256 0.160 0.540 0.206 1.217 1.381 1.602 2.188 4.131 5.082

0.109 0.223 0.132 0.473 0.167 0.943 0.947 1.507 2.154 6.242 9.125

which is indicative of the relative tendency of various gases to exist in the condensate state: H2 (33 K) < N2 (126.2 K) < CO (132.9 K) < O2 (154.6 K) < CH4 (190.5 K) < CO2 (304.1 K) < C2H2 (308.3 K) < C2H4 (282.3 K) < C2H6 (305.4 K) < C3H6 (364.9 K) < C3H8 (369.8 K). A similar trend was reported by De Angelis et al.20 for the solubility of permanent gases and hydrocarbons in polymers in the absence of strong intermolecular forces between solute and polymer matter. It is also known, under such conditions, that the free volume per mole of a solvent that could be occupied by a gas solute will be proportional to the molar volume of the solvent and hence inversely proportional to the matrix (20) De Angelis, M. G.; Merkel, T. C.; Bondar, V. I.; Freeman, B. D.; Doghieri, F.; Sarti, G. C. J. Polym. Sci. B: Polym. Phys. 1999, 37, 3011-3026.

Figure 3. Influence of the density of the dielectric liquids on the Ostwald coefficients measured at 70 °C for H2, O2, CO, and CH4 with the VPC method.

density.21,22 The effect of the density, which is a parameter taken into account in ASTM D 2779-9223 for estimating gas solubilities in petroleum liquids, was investigated in this study by plotting in °C Figures 3-5 the individual K70 (y-axis) against the density of i the liquids (x-axis). It appears from the clusters of points relevant to mineral oils that the solubility of a given gas is effectively reduced with the matrix density as expected from the literature. Table 6 gives the values of the regression parameters for the linear °C fit of K70 versus the density of the mineral oils (density i ranging from 0.844 32 to 0.886 11 g cm-3 at 15 °C) where the systematic negative slopes measured confirm the presence of a (21) Shaw, J. M. Can. J. Chem. Eng. 1987, 65, 293-298. (22) Wilhelm, E. J. Therm. Anal. 1997, 48, 545-555. (23) Annual Book of ASTM Standards; American Society for Testing and Materials, Philadelphia, PA, 1993; Vol. 05.02. ASTM D 2779-92: Standard Test Method for Estimation of Solubility of Gases in Petroleum Liquids.

density effect. Note that Drakeol 35, R-Temp, and Nytro 10CX, °C values were found to be for which more than half of the K70 i outside of the 95% confidence limits of the regression lines, were not included in the final results presented in Table 6. The worst case was observed with Drakeol (all coefficients found outside of these limits), which is a light paraffin oil that contains a volume fraction of polar and unsaturated hydrocarbons lower than 0.1%.24 In this respect, it is probably the least polar product of all the mineral oils investigated in this study. However, even with the rejection of these three oils in the fits, the fact that some r-values are well removed from unity is indicative of the presence of some intermolecular forces acting between the gas molecules and some functional groups of oil components. A good example of the sensitivity to such intermolecular interactions is given by C2H2 (24) Margolis, S. A.; Mele, T. Anal. Chem. 2001, 73, 4787-4792.


5235

Figure 4. Influence of the density of the dielectric liquids on the Ostwald coefficients measured at 70 °C for CO, CO2, C2H2, and C2H4 with the VPC method.

with an r-value of -0.0456, while an r-value of -0.6594, -0.8283, and -0.5473, for H2, O2, and N2, respectively, indicates that these latter species are less affected than acetylene (see Table 6). As for the mineral oils, the vegetable oils can also be considered as having a ill-defined composition. For instance, Envirotemp FR3 is made of a blend of soybean, vegetable compounds, and food-grade additives, Biotemp, a high oleic oil with over 80% oleic content from selective breeding or genetically modified sunflower seeds, and BioTrans 1000, a soybean oil chemically modified by at least partial hydrogenation. These products are all found at higher densities than the mineral oils (ranging from 0.916 54 to 0.923 41 g cm-3 at 15 °C). In this respect, reduced solubility of the gas solutes is expected even if the highly polar double bonds and carbonyl groups present in the oil triglyceride components are expected to modify the intermolecular interactions with the solutes. It is interesting to note from the clusters of points relevant to these products that the data are 5236 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

generally well grouped in the x-axis as expected from the low variability seen in the density values (see Figures 3-5). With the exception of CO2 and C2H2, a normal trend is observed for the gas solubilities considering the higher density of the liquids °C (K70 values lower than those found for the mineral oils). The i fact that acetylene has twice the solubility experienced in mineral oils is certainly indicative of strong intermolecular forces acting between the solute and some functional groups of the solvent components. As proposed by Prausnitz and Shair25,26 in discussing solvent selectivity, solutes with unsaturated bonds could serve as electron donors and therefore may form loose charge-transfer complexes with acidic polar components of the solvent, which can act as electron acceptors (Lewis concept of acids-bases). Such complexing behavior may be expected of any solute possessing highly polarizable π electrons as seen in the C2H2 molecule with (25) Prausnitz, J. M.; Shair, F. H. AIChE J. 1961, 7 (4), 682-687. (26) Prausnitz, J. M. J. Phys. Chem. 1962, 66, 640-645.

Figure 5. Influence of the density of the dielectric liquids on the Ostwald coefficients measured at 70 °C for C2H6, C3H6, and C3H8 with the VPC method.

Table 6. Regression Parameters of the Linear Fit of Ki70 °C vs Mineral Oil Density (Ki70 °C ) m × Density + b) (N ) 10) gas

m

b

corr coeff r


-0.34 -1.20 -0.40 -0.48 -0.26 -0.54 -0.18 -0.94 -1.17 -2.47 -7.87

0.39 1.26 0.48 0.85 0.37 1.38 1.15 2.23 3.03 7.52 13.62

-0.6594 -0.8283 -0.5473 -0.4854 -0.4601 -0.2875 -0.0456 -0.3240 -0.3051 -0.1199 -0.1688

four electrons in π orbitals. This could explain the highest solubility seen in our results for CO2, which also has four electrons in π orbitals, and the fact that C3H6 is now more soluble than C3H8, contrary to what is seen in a slightly polar solvent such as a mineral oil. This effect was not as great on the solubility of C2H4, which has only two electrons in the π orbitals. Some of the synthetic oils used in the electrical apparatus have their chemical composition well defined, while for others, they consist of more precisely characterized and standardized mixtures of polymers and isomers than the hydrocarbon mixtures making

up the mineral oils. For instance, Dussek T3788 is made of a blend of linear C11-, C12-alkylbenzenes, Envirotemp 200, a mixture of pentaerythritol esters of heptanoic and isononanoic acids, DC 561 and SF97-50, different mixtures of dimethylpolysiloxanes, and Luminol TR-i, an isoparaffin synthesized fluid obtained by applying a special refining process to a crude oil. It is likely that component homogeneity is much more easily assured in each of these synthetic oils than for the products of the other dielectric categories. As indicated by the data in Table 5, a very large °C dispersion in the K70 values is seen when these solvents are i pooled together. This variability in the coefficients is certainly due to the diversity found in the product composition and density (density ranging from 0.836 68 to 0.987 83 g cm-3 at 15 °C). It is revealing to note from these results that, out of all the dielectric liquids investigated, dioctyl phthalate and Envirotemp 200 show the highest ability to dissolve acetylene and carbon dioxide. These two liquids were found to possess the highest intensity for the carbonyl groups of the ester components (see Table 1). As demonstrated in Figure 6 for C2H2, it is likely that the solutes with highly polarizable π orbitals will be strongly influenced by some intermolecular interactions with the carbonyl groups. Conversely, Drakeol 35 and R-Temp components (the latter belonging to the high molecular weight hydrotreated heavy paraffinic hydrocarbons) were seen to produce very weak intermolecular interactions with solutes. The figure also shows that Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

5237

°C Table 7. Ranges of the Percent Errors Found in the C0li When Using an Average K70 Value by Category of Products i Rather Than the Specific Values Determined for Each Dielectric Liquid

mineral oils (N ) 10)a

vegetable oils (N ) 7)

synthetic oils (N ) 6)

gas

mean K values

% RSD

range found for % deviations

mean K values

% RSD


mean K values

% RSD


uncertainties of the VPC methodb


0.096 0.223 0.134 0.434 0.144 0.907 0.998 1.420 2.017 5.379 6.807

7.8 9.4 7.8 3.3 5.7 3.0 5.7 2.9 2.7 5.5 9.9

-2.4 to +0.2 -4.0 to +0.8 -3.3 to +0.8 -3.2 to +1.5 -2.4 to +1.2 -3.4 to +2.3 -7.9 to +5.4 -4.3 to +2.0 -3.0 to +2.1 -13.1 to +2.8 -21.6 to +5.7

0.081 0.199 0.107 0.349 0.115 1.101 1.662 1.308 1.592 4.273 4.099

21.3 24.1 27.1 9.5 24.5 6.4 9.1 5.3 4.7 3.2 6.3


0.113 0.235 0.152 0.468 0.174 1.194 1.699 1.534 1.987 5.640 6.755

14.1 11.2 9.7 14.4 14.9 15.4 35.0 8.5 14.2 20.1 29.3


4.9 2.8 9.7 0.7 2.8 1.2 1.5 0.7 0.8 1.3 1.9

a

Drakeol 35, R-Temp, and Nytro 10CX not included in the statistics. b Percent RSD based on three replicates; data from ref 13.

Figure 6. Relation between solute solubility and the intensity of the FT-IR carbonyl band measured for the ester components of the dielectric liquids.

other types of component characteristics (see the data point distribution obtained for the liquids with no carbonyl groups), such as those found in Dussek T3788 (conjugated double bonds in aromatic rings), DC 561, and SF97-50 (SiO bonds), could be at the origin of strong intermolecular interactions with solutes. It is not surprising that the difficulty of supporting the CO2 in such ester- and silicone-based liquids when applying a modified form of ASTM D 2779 method for estimating gas solubilities in nonpetroleum liquids has been reported in the literature.5 Considering the overall gas solutes of interest, Luminol TR-i appears to behave more like a mineral oil, the solubility of which is principally influenced by the matrix density, while for the remaining products (Dussek T3788, DC 561, SF-97-50, dioctyl phthalate, Envirotemp 200), the solubility characteristics of the liquids are dominated by the intermolecular interactions between solutes and matrix components. Effect of the Ostwald Coefficient Variations on the Accuracy of the Determinations. Inaccurate DGA results may lead 5238 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

to incorrect fault diagnosis, especially if gas ratios are close to a fault zone boundary, or to inappropriate actions on the equipment if concentration values are close to the typical or alarm values. To comply with an application in this field, the IEC 567 International Standard recommends an accuracy of 13% at medium gas concentration levels, i.e., in the range of 10-100 ppm for H2 and the C1-, C2-hydrocarbons.27 With the Vg/Vl ratio set at 0.371 by the method’s procedure (see eq 1), the accuracy of the individual determinations will then be unevenly affected by the effects of the matrix on gas solubility. Variations in the low Ostwald °C coefficients (K70 < 0.371) will have a negligible effect on the i °C accuracy, while for the higher coefficients (K70 > 1), they i could make the method unacceptable for DGA. This aspect is °C easily understood by looking at the variations found in the K70 CO values under a buildup of oxidation products in the matrix (rather insignificant variations were found under such circumstances for all the other gas solutes of interest). Because of a relatively low solubility of the species, a reduction of 50.7% in the K value causes the CO to be underestimated by only 14% of its true value for an oil at the very limit of oxidation acceptability (0.088 g of KOH/ g). On the other hand, the ranges in percent errors found in the °C Cli0 when an averaged K70 value is used by category of i dielectrics rather than using the specific value measured for each liquid are shown in Table 7. Note that these deviations also reflect the error associated with the VPC coefficient determinations (see the uncertainties given in the last column of the table). In the °C case of mineral oils, the data show that a typical set of K70 i values could be used in eq 1 providing that the oils are comprised in a narrow range of density (such as those found in Table 2). However, this practice could hardly be generalized for all the mineral oils found in dielectric applications (e.g., R-Temp) without incurring a significant loss of accuracy in the determination of the more soluble species. The Table 7 data also indicate that a typical set of coefficients could be applied to vegetable oils, such as those averaged over the seven liquids tested, although to be fully conclusive in this respect, more information would be needed concerning the effect of field oxidation. Any mixing of these new (27) Guide for the Sampling of Gases and of Oil from Oil-Filled Electrical Equipment and for the Analysis of Free and Dissolved Gases. International Standard, International Electrotechnical Commission, 1992; EI IEC 567.

dielectrics with mineral oils, as commonly done in the field, may complicate the DGA determinations unless someone is using a specific set of Ostwald coefficients for each mixture. It is also °C evident from these results that specific K70 values should be i used for the synthetic oils rather than averaged values as proposed for mineral oils and vegetable oils. These liquids are found in a wide range of densities, and their characteristics as solvents were also shown to be strongly influenced by specific intermolecular forces. ACKNOWLEDGMENT The authors are particularly grateful to Dr. G. Be´langer from IREQ, Hydro-Que´bec, for the encouragement and substantial

funding received for this project. Thanks also go to Petro Canada Lubricants and Nyna¨s Naphthenics AB for their assistance in the funding. The authors are also grateful to Dr. T. V. Oommen, who kindly agreed to comment on the first version of the manuscript. Finally, many thanks go to the ASTM D27 committee members for their support and to Pirelli, Cooper Power Systems, ABB Inc., and Cargill Industrial Oils and Lubricants, who kindly furnished samples of the new generation of dielectric liquids.

Received for review April 9, 2003. Accepted July 18, 2003. AC0343634


5239

Matrix Effects Affecting the Indirect Calibration of the Static Headspace

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