Simulated Distillation Yield Curves in Heavy Crude Oils: A

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Simulated Distillation Yield Curves in Heavy Crude Oils: A Comparison of Precision between ASTM D-5307 and ASTM D-2892 Physical Distillation Marcela Espinosa-Pen˜a,* Yolanda Figueroa-Go´mez, and Federico Jime´nez-Cruz* Programa de Tratamiento de Crudo Maya, Instituto Mexicano del Petro´ leo (IMP), Eje Central La´ zaro Ca´ rdenas No. 152 Col. San Bartolo Atepehuacan C.P. 07750 Me´ xico D.F., Me´ xico Received April 2, 2004. Revised Manuscript Received August 5, 2004

To develop an accurate and rapid evaluation of distillation yield curves of heavy crude oils, simulated distillation by gas chromatography (SIMDIS GC) curves were performed for the first time in Mexican Istmo and Maya crude oil samples. In this study, we found large uncertainties using ASTM D-5307, and some improvements to the technique were applied as alternatives to measure the assay distillation. These results were compared with the curves obtained from the physical distillation method outlined in ASTM Method D-2892, showing good agreement. The statistical analysis of the SIMDIS data is addressed as a good alternative for measuring these heavy crude oils.

Introduction Physical distillation is the major separation process used in the petroleum industry. To ensure uniform quality control in the refinery processes and in the final petroleum products, it is essential to ascertain the interval of the boiling point of crude petroleum and the products thereof. Currently, the method used most widely to determine the true boiling point (tBP) curves is ASTM Method D-2892,1 which is a physical distillation with 15 theoretical plates with a relationship of reflux of 5:1, followed by the application of ASTM Method D-52362 for the heavier portion. The methods based in the physical distillation, such as ASTM D-11603 and ASTM D-86,4 are described for one theoretical plate. However, the inconveniences of these methods are the excessively time-consuming analysis (∼48 h) and the high level of operator intervention.5,6 In striking contrast, the simulated distillation by gas chromatography (GC SIMDIS) method is a technique in which the simulation of a distillation is involved.6-8 Simulated distillation is the term used to designate the results * Authors to whom correspondence should be addressed. E-mail addresses: [email protected], [email protected]. (1) ASTM Designation D-2892-03, 2003 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2003; Vol. 05.02. (2) ASTM Designation D-5236-03. 2003 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2003; Vol. 05.02. (3) ASTM Designation D-1160. 2002 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2002; Vol. 05.02. (4) ASTM Designation D-86. 2002 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2002; Vol. 05.01. (5) Previous work has been reported. For example, see: EspinosaPen˜a, M.; Figueroa Go´mez, Y.; Cano-Dominguez, J. L. Analytical Validation of Test Standard Simdis Method ASTM D 5307-97 for Determination of Range Boiling Point of Crude Oil by Gas Chromatography. Presented at ACHEAMERICA 2002 in the 1st International Congress on Process Technologies, Mexico City, March 18-20, 2002. (6) Ceballo, C.; Murguia, E. Rev. Tec. INTEVEP 1983, 3, 35-45. (7) Eggertsen, F. T.; Groennings, S.; Holst, J. J. Anal. Chem. 1960, 32, 904-909. (8) Green, L. E.; Schmauch, L. J.; Worman, J. C. Anal. Chem. 1964, 36, 1512-1516.

obtained by chromatography of gases equivalent to those calculated by physical distillation tBP curves; however, SIMDIS reduces analysis time by offering the possibility of automating the process.6 It also improves the accuracy and precision of the results.6,9 SIMDIS has substantial advantages, in comparison to the methods of physical distillation, not only for the simplicity of the technique and management of the samples in small quantity (0.2-0.5 µL), but for a minimum of contamination of the samples and accidents in the laboratory. Both techniques are good to determine the distribution of boiling-point intervals, for example, in gasoline and distilled fractions of crude oils.9 In this context, ASTM Method D-371010 corresponds to the simulated distillation for gasoline and ASTM Method D-288711 method is used for fractions and products of petroleum such as jet fuel, kerosene and diesel, light oils, and heavy gas oils from coking and hydrotreatment. The methods used for crude oils include ASTM Method D-530712 and ASTM Method D-6352;13 both are currently used in refining industry laboratories as a good alternative to physical distillation. The difference between ASTM Method D-5307 and ASTM Method D-6352 is that the former has the possibility of calculating the residue content (as a weight percentage) for samples with nonvolatile material up to 538 °C. In addition, SIMDIS techniques also have been developed in combination with gas chromatography (GC)-mass spectroscopy,14 (9) Ceballo, C. D.; Bellet, A.; Aranguren, S.; Herrera, M. Rev. Tec. INTEVEP 1987, 7, 81-83. (10) ASTM Designation D-3710-95. 1995 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1995; Vol. 05.02. (11) ASTM Designation D-2887-02. 2002 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2002; Vol. 05.02. (12) ASTM Designation D-5307-97. 2002 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2002; Vol. 05.02. (Reapproved in 2002.) (13) ASTM Designation D-6352-02. 2002 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2002; Vol. 05.02. (14) Roussis S. G.; Fitzgerald, P. Anal. Chem. 2000, 72, 1400-1409.

10.1021/ef049919k CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004

Distillation Yield Curves in Heavy Crude Oils

GC-vacuum thermogravimetry,15 or high-performance liquid chromatography (HPLC)-evaporative light scattering detector.16 The method of high temperature for simulated distillation (HT SIMDIS), as described in ASTM Method D-5307, have not been extensively proven by the oil industry, because of the existence of many problems in the preparation and nonhomogenization of the sample. These problems must be solved, because, to obtain reliable results, a quantitative method requires a rigorous and exact mass balance. In this paper, we wish to describe a strategy for developing and optimizing an appropriate methodology for the rapid evaluation of distillation yield curves in Mexican crude oils using GC SIMDIS described in ASTM Method D-5307. In addition, an expeditious and improved methodology for preparing heavy oil samples is described. Also, a comparison with the results of physical distillation (ASTM Method D2892) tests was performed.5 Experimental Section The samples of Istmo and Maya crude oil were supplied from Pemex. Carbon disulfide (99.00%) was purchased from Aldrich Chemical Company. A mixture of n-alkanes C5 through C98 (Polywax 655) was purchased from Analytical Control U.S.A. A mixture of n-alkanes C14, C15, C16, and C17, which were used as internal standards traceable to the National Institute for Standards and Technology (NIST), was purchased from Hewlett-Packard. A mixture of n-alkanes C5 through C44 was used as a reference in the SIMDIS analysis, for comparison and validation of the test. The analyses were performed with a Hewlett-Packard model HP 6890-II gas chromatograph that was equipped with a flame ionization detector (FID) with an outlet diameter of 0.30 mm, a liquid nitrogen cryogenic system, automatic injection for eight vials, and software for calculation of the SIMDIS methods. The column used was 5-6 m long, comprised of stainless-steel capillary columns 0.53 mm in diameter, and filled with methyl silicon, and the film thickness was 0.090.15 µm, respectively. The carrier gas was helium (99.999% purity), with a flow of 20 mL/min and an inlet pressure of 80 psi. The injector and detector temperatures were each 430 °C. The combustible gas for feeding the detector was hydrogen (99.999% purity), with an inlet pressure of 60 psi and a flow rate of 35 mL/min and air (99.999%, chromatographic grade) with an inlet pressure of 80 psi and a flow rate of 350 mL/ min. The FID is highly sensitive (5 pg C/s) to hydrocarbons, which allows for detection of the high-boiling-point components, such as heavy crude oil and their fractions. The detector has a low sensitivity to carbon disulfide, which allows injection of the dissolved crude oils into the chromatograph. Although the FID is a mass-dependent detector, the analytic results are comparable to the results of a physical distillation expressed in terms of a volume percentage. Conditioning of the Column. The column was conditioned without being connected to the detector, by purging with helium at low temperature. The column outlet was covered immediately with an appropriate seal. The flow of helium was continued for 2-3 h at 430 °C. The oven was cooled at 100 °C, and the seal was removed, to re-establish the helium flow in the column. The equipment was programmed 2-3 times at the operation conditions and the column was connected to the detector. The equipment was stabilized at the required conditions. The oven was programmed to operate from 40 °C to 430 (15) Southern, T. G.; Iacchelli, A.; Cuthiell, D.; Selucky, M. L. Anal. Chem. 1985, 57, 303-308. (16) Padlo, D. M.; Kugler, E. L. Energy Fuels 1996, 10, 1031-1035.

Energy & Fuels, Vol. 18, No. 6, 2004 1833 °C and the injector was programmed to operate from 100 °C to 430 °C, at a rate of 15 °C/min, and the detector was programmed to operate from 50 °C to 430 °C, a rate of 10 °C /min. The chromatogram was integrated over a period of 44 min, and the final temperature of the oven continued for 10 min to clean the system. The injector temperature was maintained for 2 min. Preparation of the Sample. The samples were previously stored at a temperature of 0-5 °C for at least 4 h before exposing them to the environment. To ensure homogeneity, the heavy and viscous crude may require warming, as well as stirring. The samples should be dried with anhydrous calcium chloride or sodium sulfate by shaking the mixture of sample and drying agent vigorously. According to ASTM Method D-5307, the low-viscosity liquid samples can be analyzed directly. The dried sample is removed from the desiccant using a pipet and accurately weighted (0.15-0.20 g) in a 25 mL volumetric flask and dissolved with 10 g of carbon disulfide. The solution was weighed, and it had a concentration of ∼2%. Two milliliters of this solution was withdrawn and poured into a vial that had been weighed; this sample should be considered as the sample without internal standard. The remainder of the solution was weighed after this process and then a weighed quantity of internal standard (20 µL, measured in a tared 100 µL syringe) was added to the remaining solution and mixed by shaking. As done previously, 2 mL of this solution was poured into a vial and should be considered as the sample with internal standard. Physical Distillation. This procedure was performed according to ASTM Method D-2892. The water-free crude was tested using columns of 15 theoretical plates under a relationship of reflux of 5:1; this method is known as the true boiling point (tBP) assay. The distillation started under barometric pressure (760 mm Hg) and is continued later under vacuum conditions (5-100 mm Hg) at 370 °C. The boiling points are converted to their equivalent value at 1 atm (AEBP). At this point, the method was changed to the corresponding ASTM Method D-5236 procedure, in which the distillation continued at a pressure of 0.5 mm Hg until reaching a limit near the AEBP (538 °C). Conversion charts for the conditions of AEBP are included in the method.

Results and Discussion For the purposes of this study, four analyses were performed with two different crude oils from the two following categories: Istmo crude (20-30 °API, 0.8932 g/cm3) and Maya crude (538 °C, is conducted at 430 °C. Under these conditions,

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Figure 1. Chromatogram (top) and calibration curve (bottom) for the mixture C5-C44.

no evidence of cracking has been visualized. In the selected chromatographic conditions, the separation of

the corresponding peaks is good, which avoids saturation of the column. For the heavy crude oil, the sample

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Table 1. Boiling Point Distribution in Reference Gas Oila mass %

boiling point, BP (°C)

mass %

boiling point, BP (°C)

mass %

boiling point, BP (°C)

mass %

boiling point, BP (°C)

IBPb 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

199.0 334.5 374.5 389.0 397.5 403.5 408.0 412.0 415.0 418.0 420.5 422.5 424.5 426.5 428.0 429.5 431.0 432.5 434.0 435.0 436.5

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

437.5 439.0 440.0 441.0 442.0 443.0 444.0 445.0 446.0 446.5 447.5 448.5 449.5 450.5 451.0 452.0 452.5 453.5 454.0 455.0 456.0

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

456.5 457.5 458.5 459.0 460.0 461.0 461.5 462.5 463.0 463.5 464.5 465.0 466.0 467.0 467.5 468.5 469.0 469.5 470.5 471.0 472.0

63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

473.0 473.5 474.5 475.0 476.0 477.0 477.5 478.5 479.0 480.0 481.0 481.5 482.5 483.5 484.5 485.5 486.5 487.0 488.0 489.5 490.5

mass %

boiling point, BP (°C)

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 FBPc

491.5 492.5 494.0 495.0 496.5 497.5 499.0 500.5 502.5 504.0 506.5 509.0 511.5 515.5 520.0 527.5 534.0

a Pertinent sample data: calibration method, external standard method; sample mass, 0.2184 g; solvent mass, 10.0860 g; mass of internal standard, 0.0000 g; elution at start, 0.00; elution at end,