Determination of Oxygen Solubility in Refinery Streams with a

Ind. Eng. Chem. Res. 2006, 45, 3707-3710

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Determination of Oxygen Solubility in Refinery Streams with a Membrane-Covered Polarographic Sensor Xin A. Wu* and Keng H. Chung Syncrude Canada Ltd., Edmonton Research Center, Edmonton, Alberta, Canada T6N 1H4

A membrane-covered polarographic sensor is commonly used to measure dissolved oxygen activities in test fluids. However, it is incapable of determining the oxygen concentration or solubility in fluids directly. To convert the activity to its corresponding concentration, the oxygen solubility data are needed. For complex samples such as refinery hydrocarbon streams, a complementary technique for oxygen solubility measurement is often employed to aid the polarographic determination of the dissolved oxygen concentration. In this work, an indirect polarographic method has been developed to determine the oxygen solubility in a refinery hydrocarbon, thus eliminating the need of any complementary techniques. The method involves mixing an air-saturated hydrocarbon sample with an oxygen-free reference solvent such as toluene at various blending ratios. The oxygen solubility in the sample can be obtained by regression of the measured activities versus the volume fractions of the sample. The determined oxygen solubilities for several common hydrocarbons are consistent with the literature data. Introduction Molecular oxygen in hydrocarbon feedstocks plays a key role in fouling and corrosion of refinery units.1-3 In one case, oxidative polymerization by air leaking into heavy gas oil streams caused fouling and plugging of hydrotreater feed filter units. Dissolved oxygen also contributes to shortened shelf life of refinery fuel products due to deposit and sediment formation.4,5 Among many techniques to determine and monitor the dissolved oxygen concentrations in hydrocarbons, the polarographic (voltammetric) method is the most convenient one. Early work on petroleum fractions using this method dates back to the 1950s.6 Over the past several years, reliable and user-friendly polarographic sensors, based on the principles of membranecovered Clark cells7 and specifically designed for hydrocarbon media, have become commercially available. Theoretically, a polarographic sensor measures the oxygen activity rather than the concentration, which is more useful in practical applications. Converting activities to concentrations requires the knowledge of the oxygen solubility in the hydrocarbon of interest since the measured activity is essentially a ratio of the oxygen concentration to its solubility (or its equilibrium concentration in a standard state). While the oxygen solubilities in common hydrocarbons are available in the literature, the data for refinery feedstocks and fuels usually require individual measurements. The earliest technique for oxygen solubility measurement is the Winkler method, which is still used for aqueous systems. It is a chemical method, based on stoichiometric conversion of oxygen to iodine and subsequent titration with thiosulfate.7 A similar method was attempted for hydrocarbons with limited success.8 So far, the most precise measurement of gas solubilities in hydrocarbons has been achieved by physical means using manometers and burets, etc., to determine pressure or volume changes of the overhead gas after a degassing/reabsorption process. Numerous measuring devices have been developed for this purpose.9-11 Reviews on the techniques can be found in the literature.12,13 These measuring devices, however, require * To whom correspondence should be addressed. Tel.: (780) 9706945. Fax: (780) 970-6805. E-mail: [email protected].

great effort to construct and operate in order to achieve high precision. An alternative would be the gas chromatography (GC) technique.14,15 The accuracy of the GC method for dissolved oxygen measurement has been significantly improved in the past decade.16-19 Although some studies showed that this method is applicable to most petroleum fractions,15 highly volatile hydrocarbons such as naphtha and n-heptane tend to interfere with the oxygen detection.19 Viscous hydrocarbons, such as heavy gas oils, also present a challenge in the sample injection into a GC. In this study, an indirect polarographic method was adopted to determine oxygen solubilities in hydrocarbons. This technique was developed on the basis of a fast-working method proposed by Bub and Hillebrand20 for aqueous systems. The study shows that it is applicable to both volatile and viscous hydrocarbons. Hence, for polarographic sensor users, complementary techniques such as physical, chemical, or GC methods are no longer needed in determination of dissolved oxygen concentrations in refinery streams. Principle of Measurement The O2 activity, a, in a test fluid determined with a polarographic sensor is related to its concentration, C, by

a)

C S

(1)

where S is the oxygen solubility in the test fluid. A common standard state chosen for S is a fluid in equilibrium with pure O2 at 100 kPa (or 1 bar) and 25 °C. A common unit for S and C is milligrams per liter. The activity coefficient is omitted in eq 1 by assuming an ideal system. In hydrocarbon systems, S varies significantly from sample to sample.12,21 Therefore, the polarographic meter cannot be calibrated to show C (or S if the sample is saturated with O2) explicitly. To determine S of an unknown sample, it is necessary to mix this sample, saturated with air or oxygen, with an oxygenfree reference fluid, i.e. a hydrocarbon of known S. From a material balance, the oxygen concentration, Cm, in the mixture consisting of a reference fluid and an unknown

10.1021/ie051383i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006

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sample can be expressed as

Cm ) C1(1 - φ) + C2φ

(2)

where subscripts 1 and 2 represent the reference fluid and the unknown sample, respectively, and φ is the volume fraction of the unknown sample. In the Bub-Hillebrand method, the reference fluid was pure water and the unknown sample was a water-soluble organic compound. The value of φ was kept at 0.01.20 Since the mixture was quite “watery”, its oxygen solubility equaled that of pure water. Hence, Cm was directly measured with a polarographic sensor, and C2, equivalent to S2 in an oxygen-saturated sample, was calculated from eq 2. Success of the Bub-Hillebrand method is mainly attributed to the low S value for the reference fluid (water) and the high S value for the sample (organics). In hydrocarbon mixtures, the opposite is often true, especially when the reference fluid is a solvent, e.g. toluene, and the unknown sample is a heavy hydrocarbon. Therefore, a much larger φ is needed for the dissolved oxygen, carried by the sample to the mixture, to be detected. The obvious problem is that the oxygen solubility for the mixture, Sm, significantly deviates from S1 for a large φ. Therefore, Cm cannot be directly determined without the prior knowledge of Sm. To solve this problem, it was assumed that the mixing is ideal and the oxygen solubility in the mixture follows a linear relationship,

Sm ) S1(1 - φ) + S2φ

(3)

Experimental results from the present work indicated that this simplified relationship is satisfactory for mixing toluene with n-heptane, isooctane, n-tetradecane, or a heavy gas oil. In the present technique, toluene, used as the reference fluid, is free of O2; i.e. C1 ) 0. Combining eqs 1-3 yields,

am )

C2 1-φ S2 + S φ 1

(4)

where am is the activity of the mixture that can be determined with a polarographic sensor. Since the sample is saturated with air prior to mixing, on the basis of Henry’s law,

C2 ) rS2

(5)

where r ) 0.2095Patm (bar)/1 (bar), Patm being the local atmospheric pressure. Substituting C2 in eq 4 yields,

S2 am (1 - φ) ) φ r - am S1

(6)

A series of (1 - φ)am/(r - am) can be obtained if the sample is mixed with toluene (the reference fluid) at various φ’s. Plotting (1 - φ)am/(r - am) against φ should result in a straight line, and the slope of the line gives the oxygen solubility of the sample, S2, with the prior knowledge of S1. Experimental Section Materials. Toluene, n-heptane, isooctane (2,2,4-trimethyl pentane), and n-tetradecane were supplied by Fisher Scientific. The first two are HPLC grade reagents and the last two are

Figure 1. Experimental setup for oxygen solubility measurement. Four major steps of the experiment were marked with number signs: step 1, bubble argon through toluene and air through the sample (dotted arrows); step 2, fill the Welker cylinder with the O2-free toluene (solid arrows); step 3, fill the Welker cylinder with the air-saturated sample (solid arrows); step 4, measure the O2 activity in the mixture.

certified reagents. A heavy vacuum gas oil (HVGO) was sampled from the Syncrude bitumen upgrading plant in northern Alberta, Canada. All hydrocarbon samples were used without further purification. A 500 mL piston sample cylinder was made by Welker Engineering Co. (model CP2-GMPPA). Two 1 L sample cylinders (without pistons), 6.35 mm diameter Teflon tubing, and stainless steel valves and fittings were supplied by Swagelok. Procedure. The experimental setup is shown in Figure 1. Toluene and the hydrocarbon sample were stored in the 1 L sample cylinders shown on the left. Argon was continuously bubbled through the toluene for at least 2 h to remove dissolved air. The hydrocarbon sample was bubbled with compressed air and then allowed to equilibrate at the local atmospheric pressure to ensure air saturation. After the preparative steps, these two hydrocarbons were pushed by argon at a pressure of approximately 300 kPa, mixed in the 500 mL Welker piston cylinder. The Teflon tubing leading toward the cylinder was purged with argon prior to the transfer. Precautions were taken to avoid any gas pockets trapped in the Welker cylinder. The weights of the toluene and the sample in the Welker cylinder were measured with a 0.1 g resolution balance. The volume fraction of the sample, φ, was calculated on the basis of their weights and densities from the literature. The density of HVGO was determined to be 968 kg/m3 at 20 °C using a density meter (Anton Paar DMA 5000). Volume shrinkage after mixing was negligible. After thorough mixing using the built-in gravity mixer in the Welker cylinder, the mixture was pushed out of the cylinder into an oxygen analyzer by applying pressure on the other side of the piston. Dissolved oxygen activity was determined using an Orbisphere 3650EX analyzer equipped with a flow-through cell and a 29522A membrane sensor. Small amounts of the test fluid were transferred to the flow-through cell and stayed there for 10 min to precondition the sensor. Subsequently, the bulk of the test fluid was allowed to trickle through the cell for 30 min to further condition the sensor. The flow rate was increased to approximately 50 mL/min at the end for actual measurement. The value shown on the Orbisphere analyzer was normalized by the reading for an air-saturated sample or air-saturated toluene

Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3709 Table 2. Oxygen Solubilities in Hydrocarbons at 100 KPa (or 1 bar) of Pure O2 hydrocarbon toluene

n-heptane

isooctane

Figure 2. Oxygen activity plots for toluene/toluene, n-tretradecane/toluene, isooctane/toluene, and n-heptane/toluene mixtures. The filled squares are experimental data. The dotted lines are results of linear regression. The solid lines are the theoretical predictions based on the literature data. Table 1. Results for Linear Regression of Experimental Data hydrocarbon

slope

intercept

linear correlation coefficient, r2

toluene n-heptane isooctane n-tetradecane HVGO

0.9876 1.3753 1.5267 0.7992 0.5792

0.0568 0.0794 0.1212 0.1205 -0.0168

0.9949 0.9329 0.9526 0.9698 0.9651

and then multiplied by the r value (cf. eq 5) to give the dimensionless activity, am. All measurements were carried out at 22 °C. Local atmospheric pressure was determined with a mercury barometer during the experimental runs. Results and Discussion Comparison with Literature Data. The samples for oxygen solubility measurement are toluene, n-heptane, isooctane, and n-tetradecane. In the first case, air-saturated toluene was mixed with oxygen-free toluene as a test of the method. The plots of (1 - φ)am/(r - am) vs φ are shown in Figure 2. Reproducibility of the measurement is reasonably good as testified by the data points of similar θ’s (three points at θ ≈ 0.5 for isooctane and two points at θ ≈ 0.18 for n-tetradecane). The solid line passing through the origin is the theoretical prediction using solubility data in the literature.21 The experimental data, shown as filled squares, were fitted to a dotted line by linear regression (see Table 1 for regression data). The determined oxygen solubilities based on the slopes of the fitted lines are listed in Table 2. Literature data are also summarized for comparison. Note that the temperatures in the literature data vary from 10 to 25 °C. Comparing these results is, nevertheless, valid since the temperature effect on the oxygen solubility is relatively small in this range.12 The literature data from different sources vary significantly with a standard deviation up to 8%. The results from the present work are within the range of variation in most cases. In all figures, the fitted lines were not forced to pass through the origin, resulting in nonzero intercepts. In addition to random error, a slow drift of the oxygen analyzer reading toward a lower value might contribute to these nonzero intercepts. The drift is slightly more pronounced at a lower oxygen activity and is likely caused by slow diffusion of oxygen through the sensor membrane to reach equilibrium. A preconditioning step, recommended by the sensor manufacturer, was incorporated into our procedure to minimize the drift. The remaining error, which is

n-tetradecane

O2 solubility (mg/L)

method

temp (°C)

ref

314 318 317 274 261 437 430 416 465 371 485 492 481 495 538 440 254 220 219

polarographic NAb physical physical GC polarographic NA physical physical GC polarographic NA physical physical physical chemical polarographic NA physical

22 25 18 10 25 22 25 25 25 25 22 25 25 20 25 20 22 25 25

present worka 21 22 23 19 present worka 21 24 25 19 present worka 21 26 27 28 8 present worka 21 24

a Results from the present work are based on the solubility value of 318 mg/L for toluene. b NA: not available, likely a physical method.

now small and consistent, could be further diminished if the fitted line is not forced through the origin and the intercept is ignored. This treatment, which moves the error component to the intercept term, can be safely carried out since the intercepts are usually small (see Table 1). The pure hydrocarbons selected for the present study (C7 to C14) resemble refinery naphtha and light gas oil streams. The high volatility of the samples has no adverse effects on the measurement. To further test the applicability of this method in refinery streams, a heavy vacuum gas oil was chosen for the study. Like most heavy gas oils, the HVGO sample has a high viscosity, 777 mPa‚s at 25 °C. This poses a challenge to almost all types of oxygen solubility measurements. Viscous samples cannot be readily degassed and do not flow freely into a GC syringe or a polarographic flow-through cell. In contrast, the present method is quite tolerant toward high sample viscosity since toluene dilution is an inherent part of the measurement. The only limitation of this method is the maximum temperature of 60 °C as specified by the oxygen sensor manufacturer. In practice, this is not a severe limitation. As mentioned above, the main purpose of determining oxygen solubility in refinery streams is to provide a database to convert the dissolved oxygen activities into more useful concentrations. The concentration results are valid if both measurements (oxygen solubility and activity) are performed at the same temperature, which could be significantly lower than the sample temperature in the refining process. Figure 3 shows the plot of (1 - φ)am/(r - am) vs φ for the HVGO. Linearity and intercept of the fitted line are both acceptable. The chemical complexity of this industrial sample has no apparent effect on the measurement. From the slope of the fitted line, the oxygen solubility in the HVGO at 100 kPa of O2 and 22 °C was determined to be 184 mg/L. Further reason to study this specific sample is to be given in the next section. Industrial Application. Oxygen solubility measurement in an HVGO sample was one of the key research efforts that helped solve a fouling problem in a commercial bitumen upgrading plant. The fouling occurred in hydrotreater feed filter units after a flow sheet change, which included blending a relatively small stream of HVGO with a coker heavy gas oil stream prior to filtration. The cause of fouling was identified as oxidative polymerization by dissolved oxygen leaking into the streams.

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Figure 3. Oxygen activity plot for HVGO/toluene mixture. Definitions of the symbols are identical to the previous graph.

Two hypotheses were proposed: the HVGO acting as a dissolved oxygen carrier and the HVGO acting as a catalyst carrier for the fouling reaction. The suspected catalyst was iron naphthenate, a corrosion product from the vacuum distillation unit. To identify the true cause, a sealed HVGO sample was taken from the plant and the oxygen activity in the sample at 22 °C was determined to be 5.43 × 10-3 using a polarographic sensor and a toluene-dilution device similar to that in Figure 1. The determined oxygen activity was interpreted on the basis of the aforementioned oxygen solubility in the HVGO (184 mg/ L). The dissolved oxygen concentration in the sealed HVGO sample was, therefore, 5.43 × 10-3 × 184 ) 1.0 mg/L, which would become 0.2 mg/L (or ppm) after blending with the coker gas oil stream at a ratio of 1:4 if the HVGO stream were the sole source of ingress oxygen. The value of 0.2 ppm is below the general threshold of 0.5-5 ppm oxygen for oxidative fouling to occur in refinery units.1 Hence, oxygen may leak in from multiple locations, and the HVGO stream is unlikely the main source of ingress oxygen. This conclusion prompted us to focus on the second hypothesis about corrosion and fouling catalysis. After eliminating the corrosion in the vacuum distillation unit, the filter fouling problem abruptly disappeared indeed. Conclusions Oxygen solubilities in four hydrocarbon samples have been determined with a newly developed polarographic method for hydrocarbon media. The results are in agreement with the literature data. This method was then successfully applied to the oxygen solubility measurement in a complex sample of heavy vacuum gas oil. Literature Cited (1) Vadekar, M. Oxygen Contamination of Hydrocarbon Feedstocks. Petr. Technol. Q. 2002/2003 (winter), 87. (2) Watkinson, A. P.; Wilson, D. I. Chemical Reaction Fouling: A Review. Exp. Therm. Fluid Sci. 1997, 14, 361. (3) Watkinson, A. P. Chemical Reaction Fouling of Organic Fluids. Chem. Eng. Technol. 1992, 15, 82.

(4) Wallace, T. J. Chemistry of Fuel Instability. AdVances in Petroleum Chemistry and Refining; Wiley-Interscience: New York, 1964; p 353. (5) Mushrush, G. W.; Speight, J. G. Instability and Incompatibility of Petroleum Products. In Petroleum Chemistry and Refining; Speight, J. G., Ed.; Taylor & Francis: Washington, D.C., 1998; p 175. (6) Hall, M. E. Polarographic Determination of Dissolved Oxygen in Petroleum Fractions. Anal. Chem. 1951, 23, 1382. (7) Hitchman, M. L. Measurement of DissolVed Oxygen; John Wiley & Sons: New York, 1978. (8) Schlapfer, P.; Audykowski, T.; Bukowiecki, A. Schweiz. Arch. Angew. Wiss. Tech. 1949, 15, 299. (9) Lannung, A. The Solubilities of Helium, Neon and Argon in Water and Some Organic Solvents. J. Am. Chem. Soc. 1930, 52, 68. (10) Morrison, T. J.; Billett, F. The Measurement of Gas Solubilities. J. Chem. Soc. 1948, 2033. (11) Bodor, E.; Bor, G.; Mohai, B.; Siposs, G. Veszpremi Vegyip. Egy. Kozl. 1957, 1, 55. (12) Battino, R., Ed. Oxygen and Ozone; IUPAC Solubility Data Series; Pergamon Press: Oxford, U.K., 1981; Vol. 7. (13) Battino, R.; Rettich, T. R.; Tominaga, T. The Solubility of Oxygen and Ozone in Liquids. J. Phys. Chem. Ref. Data 1983, 12, 163. (14) Elsey, P. G. Gas Chromatographic Determination of Dissolved Oxygen in Lubricating Oil. Anal. Chem. 1959, 31, 869. (15) Petrocelli, J. A.; Lichtenfels, D. H. Determination of Dissolved Gases in Petroleum Fractions by Gas Chromatography. Anal. Chem. 1959, 31, 2017. (16) Rubey, W. A.; Striebich, R. C.; Tissandier, M. D.; Tirey, D. A.; Anderson, S. D. Gas Chromatographic Measurement of Trace Oxygen and Other Dissolved Gases in Thermally Stressed Jet Fuel. J. Chromatogr. Sci. 1995, 33, 433. (17) Jones, E. G.; Balster, W. J. Phenomenological Study of the Formation of Insolubles in a Jet-A Fuel. Energy Fuels 1993, 7, 968. (18) Heneghan, S. P.; Martel, C. R.; Williams, T. F.; Ballal, D. R. Effects of Oxygen and Additives on the Thermal Stability of Jet Fuels. ASME J. Eng. Gas Turbines Power 1995, 117, 120. (19) Sundaram, B. N. The Effects of Oxygen on Synthetic Crude Oil Fouling. MSc. Thesis, University of British Columbia, 1998. (20) Bub, G. K.; Hillebrand, W. A. Solubility of O2 in 2-Propanone, 2-Butanone, 2-Pentanone, and 2-Hexanone. J. Chem. Eng. Data 1979, 24, 315. (21) Hale, J. M. Oxygen Measurements in Non-Aqueous Process Fluids. Obisphere Tech Bull.; 2002, No. 6 (June). (22) Naumenko, N. K.; Mukhin, N. N.; Aleskovskii, V. B. Investigation of the Solubility of Oxygen in Liquid Derivatives of the Aromatic Series. J. Appl. Chem. 1969, 42, 2376. (23) Field, L. R.; Wilhelm, E.; Battino, R. The Solubility of Gases in Liquids. 6. Solubility of N2, O2, CO, CO2, CH4, and CF4 in Methylcyclohexane and Toluene at 283 to 313 K. J. Chem. Thermodyn. 1974, 6, 237. (24) Makranczy, J.; Megyery-Balog, K.; Rusz, L.; Patyi, L. Solubility of Gases in Normal-Alkanes. Hung. J. Ind. Chem. 1976, 4, 269. (25) Thomsen, E. S.; Gjaldbaek, J. C. The Solubility of Hydrogen, Nitrogen, Oxygen and Ethane in Normal Hydrocarbons. Acta Chem. Scand. 1963, 17, 127. (26) Kretschmer, C. B.; Nowakowska, J.; Wiebe, R. Solubility of Oxygen and Nitrogen in Organic Solvents from -25 to 50 °C. Ind. Eng. Chem. 1946, 38, 506. (27) Baldwin, R. R.; Daniel, S. G. A Method for the Determination of the Solubility of Gases in Liquids with Particular Reference to Viscous Liquids. J. Appl. Chem. 1952, 2, 161. (28) Kobatake, Y.; Hildebrand, J. H. Solubility and Entropy of Solution of He, N2, Ar, O2, CH4, C2H6, CO2, and SF6 in Various Solvents; Regularity of Gas Solubilities. J. Phys. Chem. 1961, 65, 331.

ReceiVed for reView December 12, 2005 ReVised manuscript receiVed March 2, 2006 Accepted March 7, 2006 IE051383I