Simulated Distillation of Heavy Oils Using an Evaporative Light

Sep 19, 1996 - A rapid method for measuring intermediate and heavy fractions in fossil fuels has been developed using HPLC equipment and an evaporativ...
0 downloads 0 Views 145KB Size
Download PDF
+

+

VOLUME 10, NUMBER 5

SEPTEMBER/OCTOBER 1996

© Copyright 1996 American Chemical Society

Articles Simulated Distillation of Heavy Oils Using an Evaporative Light Scattering Detector David M. Padlo and Edwin L. Kugler* Chemical Engineering Department, West Virginia University, P.O. Box 6102, Morgantown, West Virgina 26506-6102 Received January 2, 1996. Revised Manuscript Received June 10, 1996X

A rapid method for measuring intermediate and heavy fractions in fossil fuels has been developed using HPLC equipment and an evaporative light scattering detector (ELSD). Measurements at two detector temperatures are sufficient to determine the concentration of vacuum gas oil and residua in heavy oils. Results are comparable to those obtained by GC-simulated distillation (GC-SD). Analysis time is short, and the sample size requirement is small. Good analyses may be obtained with only one or two drops of sample. Since the ELSD detector measures nondistillable molecules directly and GC-SD measures nondistillables indirectly by using internal standards, the ELSD measurements may be the method of choice for measuring residua in heavy oils.

Introduction The most widely used separation technique in the petroleum industry is distillation. In the laboratory, the boiling point distribution of fossil fuels is routinely determined using simulated distillation by gas chromatography. ASTM methods exist for analyzing gasoline,1 distillates and vacuum gas oils,2 and crude oils.3 The analysis uses small quantities of sample, takes less than 1 h to run, and is usually automated for unattended operation. Whereas simulated distillation by gas chromatography works well for samples that are completely distillable, we have had difficulty with samples containing * Author to whom correspondence should be addressed [telephone (304) 293-2111, ext. 414; fax (304) 293-4139; e-mail kugler@cemr. wvu.edu]. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) ASTM Designation: D3710-88. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1992; Vol. 05.03. (2) ASTM Designation: D2887-89. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1991; Vol. 05.02. (3) ASTM Designation: D5307-92. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1993; Vol. 05.02.

S0887-0624(96)00001-1 CCC: $12.00

residua which do not elute from a GC column. The ASTM method for crude oils3 addresses this problem by using internal standards. The internal standard is added to calculate the amount of residua left behind on the GC column. However, we have found large uncertainties in our measurements with small quantities of coal liquids containing residua. This motivated us to seek alternative techniques to directly measure nondistillable hydrocarbons in small samples. The evaporative light scattering detector (ELSD) or evaporative mass detector for HPLC has received considerable attention in recent years for quantification of high-boiling compounds ranging from vacuum gas oils4 to triglycerides.5 Within the light scattering detector, the HPLC effluent containing analyte and mobile phase is nebulized with an inert gas into a heated vaporization tube. Within this tube, solvent is evaporated to leave behind high-boiling components in micrometer-sized (4) Hsu, C. S.; McLean, M. A.; Qian, K.; Aczel, T.; Blum, S. C.; Olmstead, W. N.; Kaplan, L. H.; Robbins W. K.; Schulz, W. W. Energy Fuels 1991, 5, 395. (5) Letter, W. S. J. Liq. Chromatogr. 1993, 16, 225-239.

© 1996 American Chemical Society

+

+

1032 Energy & Fuels, Vol. 10, No. 5, 1996

packets. At the bottom of the vaporization tube, these packets scatter light from a laser beam to produce the detector signal. Bartle and co-workers6 studied the response of the ELSD detector as a function of sample molecular weight. They showed that a plot of peak-integrator counts versus molecular weight approximates a step function for a wide variety of alkane, aromatic, and polymer standards. Low molecular weight compounds (300) did not evaporate within the vaporization tube; hence, all of the sample was measured. Compounds between 200 and 300 molecular weight varied from no response to full response. All compounds with molecular weight above 300 including alkanes, polystyrenes, and polyacenaphthylenes had very similar responses (within 10%). Similar response for a variety of compounds shows that the detector responds to the mass of the nonvolatile analyte. The operating temperature for the ELSD determines the detection limit at which response becomes independent of boiling point or molecular weight. Ordinarily, operating temperature is optimized for a particular solvent either to maximize detector response or to minimize the lower boiling point detection limit so that a maximum range of compounds will be seen by the detector. We have chosen to vary the detector temperature so that the lower boiling point detection limit can be set to correspond to various distillation cuts used in the petroleum industry. For example, the detector operating temperature can be set to measure analytes with boiling points >350 °C. Under these conditions, the ELSD can measure the amount of sample that corresponds to refinery atmospheric distillation bottoms. Or the detector might be set to measure analytes with boiling points >550 °C. This would be a direct measure of refinery vacuum distillation bottoms. The response of the ELSD is nonlinear and is sensitive to many operating parameters including composition and flow rate of the mobile phase, flow rate of the nebulizing gas, and temperature of the vaporization tube.7,8 For good precision and reproducible results, the ELSD was operated with a single solvent at set flow rates for solvent and nebulizing gas. Since the detector response is normally nonlinear, calibration curves were determined with standards for each operating temperature. Experimental Section Sample Preparation. Oil samples and alkane standards for ELSD analysis were weighed and then dissolved in HPLC grade hexane (Fisher Scientific) using volumetric flasks for precise volume determination. The samples were filtered and stored at 0 °C for later use. Oil samples for analysis were prepared with concentrations of approximately 3 mg/mL except for the naptha stabilizer bottom (NSB) samples, which were prepared at concentrations of about 10 mg/mL. Oil samples for gas chromatography were dissolved in reagent grade carbon disulfide obtained from Fisher Scientific. Alkane standards (6) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson, C. Fuel 1983, 62, 1181-1185. (7) Stolyhwo, A.; Colin, H.; Martin, M.; Guichon, G. J. Chromatogr. 1984, 16, 253-275. (8) Oppenheimer, L. E.; Mourey, T. H. J. Chromatogr. 1984, 323, 297-304.

Padlo and Kugler

Figure 1. (Top) ELSD peak-integrator area as a function of boiling point for a series of n-alkanes (0.6 mg/mL), detector at 40 °C, N2 flow of 5.0 L/min, pentane mobile phase at 1.5 mL/ min. (Bottom) GC-SD chromatogram of ASTM D2887 reference gas oil. Line at 12.5 min marks 315 °C boiling point. were obtained from Fluka. The ASTM D2887 calibration test mix and reference oil were obtained from Supelco. HPLC Instrumentation. HPLC instrumentation was used without an analytical column. Instead, 10 ft of 0.007 mm i.d. PEEK tubing (Supelco) was used to provide backpressure for the pump. Oil samples and standards were injected using a Varian 9095 autosampler equipped with a 30 µL sampling loop. An 80 µL sample was withdrawn from sample vials to ensure complete filling of the loop. Mobile phase was pumped at 1.5 mL/min using a Varian 9010 solvent delivery system. The mobile phases used were either HPLC grade pentane (for all measurements at detector temperatures of 40 and 80 °C) or HPLC grade hexane (for all measurements at detector temperatures of 115 and 150 °C). The ELSD was a VAREX MARK IIA. The nebulizing gas used was standard grade nitrogen with a flow rate of 5.0 L/min. All instrumentation was automated using Varian Star Workstation software and computer interface boards. GC Instrumentation. GC-SD was performed using a Varian 8100 autosampler and a Varian 3400 gas chromatograph equipped with a flame ionization detector. The column used was a 15 m, thin film (0.15 µm), 0.53 mm i.d., DB-1 capillary column (J&W Scientific). Simulated distillation analysis was performed on a Varian 654 data station. The GC oven temperature program was from 0 to 350 °C at 15 °C/ min. The injector and detector temperatures were both set to 350 °C. The nitrogen carrier gas flow rate was 5 mL/min. The ASTM D2887 simulated distillation procedure was followed.2 Since all samples analyzed were completely distillable, no internal standard was needed for GC-SD.

Results The boiling point of a compound is the primary factor in determining the response of the ELSD. Figure 1 shows the response of the ELSD detector versus boiling point for an alkane series. When the detector temperature was set to 40 °C, lower boiling alkanes up to n-C16 showed zero response and higher boiling alkanes begin-

+

+

Simulated Distillation of Heavy Oils

Energy & Fuels, Vol. 10, No. 5, 1996 1033 Table 1. Response of Alkane Series to Detector Temperature detector temp (°C)

N flow rate (L/min)

mobile phase at 1.5 mL/min

bp detection limit (°C)

alkane with full response

40 80 115 150

5.0 5.0 5.0 5.0

pentane pentane hexane hexane

315 380 435 482

n-C22 n-C30 n-C32 n-C40

Figure 2. ELSD peak area as a function of boiling point for a series of n-alkanes (0.6 mg/mL) with detector temperatures of 40, 80, 115, and 150 °C. N2 flow was 5.0 L/min.

Figure 4. Calibration curves for ELSD operating at 40 and 80 °C. Pentane mobile phase was at 1.5 mL/min. N2 flow was 5.0 L/min.

Figure 3. Boiling point detection limits as a function of drift tube temperature.

ning at n-C20 showed full response. Between these lowand high-boiling regions is a transition region from n-C16 to n-C20 which ramps upward from no response to full response. We have chosen to define the lower boiling point detection limit as the boiling point that is halfway up the transition region ramp. The top panel of Figure 1 shows that this lower boiling point detection limit for the ELSD is approximately 315 °C (600 °F) when operated at 40 °C. The bottom panel of Figure 1 shows a GC-SD plot for the ASTM D2887 reference gas oil.2 The line shown at 12.5 min is the retention time for n-octadecane with a boiling point of 315 °C. The fraction of oil to the left of the line is lighter than 315 °C boiling point, and the fraction of oil to the right of the line is heavier than 315 °C boiling point. The top panel of Figure 1 predicts that only the fraction heavier than 315 °C boiling point will be visible to the ELSD detector when operated at 40 °C. The goal of simulated distillation using the ELSD detector is to measure the amount of oil that has a boiling point greater than the boiling point detection limit. The effect of detector operating temperature on the response of an alkane series is shown in Figure 2. Operating parameters and detection limits are listed in Table 1. When the detector temperature is 80 °C, sample components with normal boiling points >380 °C (715 °F) are detected. Operating at 115 °C, sample components with normal boiling points >435 °C (816

°F) are seen. When the ELSD is operated at 150 °C, sample components with normal boiling points >482 °C (900 °F) are detected. Figure 3 shows a plot of the lower boiling point detection limit versus detector temperature. Notice that the plot is linear, allowing one to determine boiling point detection limits at detector temperatures not studied. The highest detector temperature used in this study is 150 °C. The evaporative mass detector9 manufactured by Polymer Laboratories has a 220 °C maximum operating temperature, which should raise the detection limit for this type of measurement to a normal boiling point over 550 °C. This would bring ELSD data to the same range as the heaviest vacuum distillation cuts separated in the petroleum industry. Direct measurements of components in an oil sample above the boiling point detection limit can be made with calibration curves. The calibration curves used for 40 and 80 °C operating temperatures (pentane mobile phase) are shown in Figure 4. Calibration curves used for 115 and 150 °C operating temperatures (hexane mobile phase) are shown in Figure 5. The alkane standards chosen for calibration at each measurement temperature were those that gave a full response at each detector temperature and are listed in Table 1. These curves show the typical nonlinear response expected for the ELSD. The calibration curves for 40 and 80 °C were fit to a third-order regression. The curves for 115 and 150 °C were fit to a second-order regression. Notice that the sensitivity changes as a function of detector temperature. This is shown for both the response of the alkane series (Figure 2) and the calibra(9) Laboratory evaluation of evaporative mass detector, Polymer Laboratories, Amherst, MA, June 1995.

+

+

1034 Energy & Fuels, Vol. 10, No. 5, 1996

Figure 5. Calibration curves for ELSD operating at 115 and 150 °C. Hexane mobile phase was at 1.5 mL/min. N2 flow was 5.0 L/min.

tion plots of the alkane standards (Figures 4 and 5). The highest sensitivity is observed for measurements with pentane at a detector temperature of 80 °C. This pattern of response as a function of vaporization tube temperature has been observed for triglycerides,10,11 steroids,12 and carbohydrates.13 However, these observations have not been satisfactorily explained. The decrease in response with increasing temperature occurring after 80 °C is due to a decrease in particle size through volatilization of the solute.14 If the analyte is partially volatile, high detector temperatures will cause partial evaporation of the analyte. These curves also show that measurements using pentane solvent (detector temperatures of 40 and 80 °C) were much more sensitive than measurements using hexane solvent (detector temperatures of 115 and 150 °C). The two solvents used have different densities, viscosities, and surface tensions, which causes changes in the size of the droplets produced by the nebulizer.15 An illustration of using the ELSD for direct measurement of heavy components is provided in the following example. A no-column chromatogram for the ASTM D2887 reference gas oil is shown in Figure 6. This is the same sample used for GC-SD in the bottom panel of Figure 1. The peak is symmetric, and the run time is very short since no column was used other than a short section of small-diameter PEEK tubing to provide backpressure for the pump. The detector temperature was 40 °C so that sample components with normal boiling points >315 °C were detected. The peakintegrator area obtained from this oil was compared to the areas obtained from an n-C22 calibration plot (Figure 4) and was equivalent to 2.14 mg/mL of n-C22. The concentration of oil injected was 4.27 mg/mL. Hence, 50.2% of this sample had a normal boiling point >315 °C. GC-SD measurements indicate that 48% of the sample has a boiling point >315 °C. (10) Stolyhwo, A.; Colin, H.; Guiochon, G. J. Chromatogr. 1983, 265, 1-18. (11) Robinson, J. L.; Tsimidou, M.; Macrae, R. J. Chromatogr. 1985, 324, 35-51. (12) Asmus, P. A.; Landis, J. B. J. Chromatogr. 1984, 316, 461472. (13) Macrae, R.; Dick, J. J. Chromatogr. 1981, 210, 138-145. (14) Charlesworth, J. M. Anal. Chem. 1978, 50, 1414-1420. (15) Nukiyama, S.; Tanasawa, Y. Trans. Soc. Mech. Eng. Jpn. 1939, 5, 68-73.

Padlo and Kugler

Results for 10 oil samples are shown in Table 2. The table shows that the percentage of high-boiling compounds measured by the ELSD at each lower boiling point detection limit corresponds closely with the percentage of high-boiling compounds determined by GCSD. The average difference between the ELSD data and the GC-SD data was 3.2% for the 315 °C-plus measurement, 1.9% for the 380 °C-plus measurement, 1.2% for the 435 °C-plus measurement, and 2.1% for the 482 °Cplus measurement. The precision on repeat runs was generally within 1 or 2%. When sample ASB 7/25 was injected 20 times, the average integrator-area counts was 3053K with a standard deviation of 16K (0.5%). The petroleum samples listed in Table 2 are the ASTM D2887 reference gas oil and three vacuum gas oils typical of fluidized-catalytic-cracking feedstocks. Sample RM-8590 obtained from the National Institute for Standards and Technology is the reference feedstock used for the ASTM D3907 Microactivity Test. All of the coal liquefaction samples were obtained from Hydrocarbon Technology Inc. (HTI), Lawrenceville, NJ, from pilot plant runs made in 1994. The samples are distillation cuts: NSB, naptha stabilizer bottoms; ASB, atmospheric still bottoms. The rest of the designation shows the date that samples were withdrawn from the pilot plant. Data for the HTI samples have been published by other laboratories.16,17 Since oils are composed of a mixture of many compounds, the response of the ELSD toward alkane mixtures was investigated. These results are shown in Table 3 for the ELSD when operated at 40 °C. The integrator-area counts of each mixture were compared to the area counts from an n-C22 calibration plot. A mixture of n-C20 and n-C22 (alkanes with full response) showed that the signals are essentially additive. A mixture of n-C14 (bp ) 254 °C) and n-C22 showed that only 2% of the n-C14 was measured by the ELSD detector. A mixture of n-C18 (bp ) 315 °C) and n-C22 showed that 83% of the n-C18 was measured by the ELSD detector. A mixture of n-C16 (bp ) 287 °C) and n-C22 showed that 17% of the n-C16 was measured by the ELSD detector. From these data we can conclude that a boiling point of 315 °C approximates the dividing line between compounds that give a full response to the ELSD detector and those which give essentially no response to the ELSD detector. Discussion The evaporative light scattering detector provides a rapid method for measuring high-boiling distillation fractions in heavy oils. Direct measurement of nondistillables in small samples is difficult. Vacuum distillation is a straightforward measurement, but usually requires 300-500 g samples. Thermal gravimetric analysis in vacuum18-20 or inert gas21 has been used to estimate nondistillables in small samples. This last (16) Robbins, G. A.; Winschel, R. A.; Burke, F. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 92-96. (17) Pradhan, V. R.; Comolli, A. G.; Lee, L. K.; Stalzer, R. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 82-86. (18) Mondragon, F.; Ouchi, K. Fuel 1984, 63, 61-65. (19) Altgelt, K. L.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Dekker: New York, 1994; pp 53-56. (20) Huang, H.; Wang, K.; Wang, S.; Klein, M. T.; Calkins, W. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 485-491. (21) Brandes, S. D. CONSOL Inc., Library, PA, personal communication, May 1994.

+

+

Simulated Distillation of Heavy Oils

Energy & Fuels, Vol. 10, No. 5, 1996 1035

Table 2. Boiling Point Distributions of Oils Determined from GC-SD and the ELSD Detector sample name

fuel source

supplier

reference Sigho VGO FCC 1313 RM 8590 NSB 7/18 NSB 7/24 NSB 7/28 ASB 7/18 ASB 7/25 ASB 7/28

petroleum petroleum petroleum petroleum coal coal coal coal coal coal

Supelco Davison AMOCO NIST HTI HTI HTI HTI HTI HTI

% > 315 °C ELSD GC-SD 50.2 84.7 93.8 80.6 1.8 22.8 21.9 71.2 75.9 78.7

48.0 88.0 98.0 86.0 2.0 20.0 18.0 68.0 79.5 82.3

% > 380 °C ELSD GC-SD 22.0 63.7 84.4 60.7 0 1.5 0.6 26.4 29.3 32.4

18.7 64.0 82.5 62.5 0 1.5 0.5 23.2 31.5 35.0

% > 435 °C ELSD GC-SD 3.4 38.2 58.2 38.8 0 0 0 5.4 3.3 4.0

3.5 37.0 55.0 39.0 0 0 0 5.2 4.5 5.1

% > 482 °C ELSD GC-SD 0 15.3 28.7 19.0 0 0 0 0 0 0

0 17.0 26.0 21.0 0 0 0 0 0 0

Table 3. Response of ELSD to Mixtures of Alkanes (ELSD at 40 °C and 5.0 L/min N2 Flow)

mixture

equiv n-C22 concn counts (mg/mL)

n-C22 0.6 mg/mL + 630K n-C14 0.6 mg/mL 884K n-C22 0.6 mg/mL n-C16 0.6 mg/mL n-C22 0.6 mg/mL + 2080K n-C18 0.6 mg/mL n-C22 0.6 mg/mL + 2370K n-C20 0.6 mg/mL

comments

0.615

2% of n-C14 responds to ELSD

0.705

17% of n-C16 responds to ELSD

1.100

83% of n-C18 responds to ELSD

1.185

n-C20 and n-C22 are additive with 1.2% error

measurement of every fraction in the sample. For example, a measurement at a detector temperature of 150 °C will determine the amount of 482 °C-plus material, and the GC data will provide a detailed description of the 482 °C-minus material. Conclusions Figure 6. ELSD chromatogram with no column for ASTM D2887 reference gas oil. Detector was at 40 °C. Pentane mobile phase was at 1.5 mL/min. N2 flow was 5.0 L/min.

method has been used to measure residua in hexanesoluble coal liquids.16,17,21 The ELSD approach offers an alternative method for measuring distillation fractions in heavy oils. By performing measurements at detector temperatures of 40 and 150 °C, one can determine the mass of oil with boiling point 482 °C. This provides a two-measurement method for obtaining ELSD simulated distillation data. When a sample is completely distillable, there is no compelling reason to run both GC simulated distillation and ELSD simulated distillation. However, when a sample contains nondistillable components such as asphaltenes, running both GC and ELSD analyses provides direct

Simulated distillation with the evaporative light scattering detector offers a direct measurement of intermediate- and high-boiling fractions in heavy oils. Results are comparable to those obtained by GC-SD. Analysis times are short and sample size requirements small. Good analyses may be run with 1 or 2 drops of sample. Since ELSD simulated distillation measures high-boiling fractions directly and GC-SD can only measure nondistillables indirectly using internal standards, the ELSD may be the method of choice for measuring residua in heavy oils. Acknowledgment. We thank Vivek Pradhan (HTI) and Gary Robbins (CONSOL) for providing coal liquefaction product samples. We gratefully acknowledge financial support for this research through the Consortium for Fossil Fuel Liquefaction Science, U.S. DOE Contract DE-FC-93PC93053. EF9600016