Article pubs.acs.org/EF
The Automated Asphaltene Determinator Coupled with Saturates, Aromatics, and Resins Separation for Petroleum Residua Characterization Ryan B. Boysen and John F. Schabron* Western Research Institute, 365 North Ninth Street, Laramie, Wyoming, 82072, United States ABSTRACT: A new fully automated saturates, aromatics, resins, and asphaltenes (SARA) separation for asphalt bitumen and petroleum residua has been developed and optimized. This system performs separations on 2 mg sample portions utilizing four columns packed with different stationary phases. The direction of solvent flow is controlled by automated four-port and six-port switching valves. The asphaltenes precipitate out of solution within a ground polytetrafluoroethylene (PTFE) packed column in an excess of heptane. The heptane-soluble maltenes pass through glass beads, aminopropyl bonded silica, and activated silica packed columns where chromatographic separation of the maltenes occurs. The saturates material is not retained, and it passes through all three adsorption columns. The more highly polar and pericondensed aromatic resins are retained by the glass bead column, and the remaining resins are adsorbed on the aminopropyl silica column. The glass bead column minimizes irreversible adsorption of resins by preventing the more polar resins from reaching the aminopropyl silica column. The aromatics are adsorbed on the activated silica column. In the next step of the method, the asphaltenes are selectively redissolved from the PTFE column with cyclohexane, toluene, and methylene chloride:methanol (98:2 v/v), yielding highly alkyl substituted asphaltene components, less alkyl substituted pericondensed aromatic asphaltenes, and precoke pericondensed aromatic asphaltenes, respectively. In the final steps, the aromatics and resins are eluted from their respective columns. An evaporative light scattering detector is used to quantify the amounts of each fraction, and an optical absorbance detector set at 500 nm records the relative amounts of pericondensed aromatic material with extended π systems that absorb visible light contained within the eluting fractions. The entire system is regenerated to the original column activity with a toluene and heptane solvent flush sequence, prior to the next injection. An automated separation is performed every four hours compared to several days for a corresponding gravimetric manual method.
1. INTRODUCTION 1.1. Background. Separating a material into its constituent parts is necessary in defining its composition. Normal phase chromatographic separations of oils have been in existence for several decades. Early versions of this type of analysis divided heavy oils or asphalts into saturate, aromatic, resin, and asphaltene (SARA) fractions.1,2 SARA separations mainly employ chromatography using polar stationary phases such as activated silica gel or activated aluminum oxide. Prior to the chromatography, the oils are typically divided into two solubility classes by a gravimetric separation utilizing a low polarity hydrocarbon solvent such as isooctane, pentane, or heptane. The soluble material is by definition called the maltenes, and the insoluble material is called the asphaltenes. The gravimetric asphaltenes and maltenes separation typically takes 24 h. The chromatographic separation of maltenes takes another day. If the asphaltenes are to be further subdivided gravimetrically into two solubility fractions such as cyclohexane soluble and cyclohexane insoluble or additional solubility subfractions, additional time is required. The maltenes are often divided into three fractions by normal-phase liquid chromatography called saturates, aromatics, and resins/polars (SAR). The saturates fraction consists of both linear and branched fully saturated organic molecules of low polarity containing carbon and hydrogen with essentially no hetero-atoms. A molecule in the aromatics fraction contains mainly carbon and hydrogen, possibly some thiophenic sulfur, © 2013 American Chemical Society
and few to no heteroatoms and is distinct from the saturates fraction by containing one or more aromatic carbon rings. The resins and asphaltenes fractions both contain many aromatic rings, some containing extended π systems that absorb visible light at 500 nm, with polar substituents.3 When discussing SARA separations, it should be noted that any two methods are not directly comparable. Different stationary phases possess differing activities and will adsorb a different quantity of the heterogeneous mixtures comprising the individual SARA fractions. Additionally, solvents with differing polarities and solubility parameters will desorb a different quantity of the heterogeneous fractions from the same stationary phase. Further, the type, quantity, and temperature of solvent used to separate the asphaltenes from the maltenes significantly impacts the cut point between the resins/polars and asphaltene fractions. This results in little congruity between the various SARA separation methods. 1.2. Rod Thin-Layer Chromatography. The methods for SARA separation can be divided into two groups. The first technique uses thin-layer chromatography (TLC) which can be combined with flame ionization detection (FID) to become semiautomated. One such method is a commercial technique called Iatroscan in which capillary thin layer chromatography is Received: May 21, 2013 Revised: July 8, 2013 Published: July 16, 2013 4654
dx.doi.org/10.1021/ef400952b | Energy Fuels 2013, 27, 4654−4661
Energy & Fuels
Article
asphaltenes are then selectively dissolved with stronger, more polar solvents to elute three fractions of asphaltenes.17,18 Using the AD system coupled on the front end to remove the asphaltenes, the maltenes can then be separated into SAR fractions giving a fully automated SAR-AD separation reported herein.19 For the AD method, volatile oils can be injected and analyzed. Volatiles losses in the ELSD occur in the maltenes. By comparing total ELSD peak areas with the total ELSD peak areas for a quality control vacuum residuum, a mathematical correction can be made to calculate the amount of volatiles lost from the maltenes fraction and correct the results accordingly. With the SAR-AD separation, volatile losses can occur from both saturates and aromatic fractions, and possibly some form the polars fraction. There is no easy way to mathematically determine from which of the maltenes subfractions the losses occurred. We currently are investigating novel alternative approaches for dealing with this issue. In the meantime, the SAR-AD separation, similar to manual gravimetric SARA separations, is somewhat restricted to the separation of relatively nonvolatile oils.
conducted with whole oils on silica or alumina rods as a stationary phase, followed by evaporating the elution solvents and then slowly passing the rods through a FID to provide information on the relative amounts of the fractional zones on the rod.4,5 The Iatroscan instrument typically elutes the fractions in a sequence of solvents consisting of a linear alkane, cyclohexane, toluene, and methylene chloride:methanol mixtures. However, the Iatrocsan method has severe drawbacks including variable response factors for the polar fractions, relatively high amounts of polar compounds retained near the spot location on the TLC rod, and aromatics grouping together to act like resins during separation.6,7 The separation is reported to have a large amount of error on repeat analysis, and there is a problem with the strongly adsorbed, asphaltene material which does not migrate up the rod and thus is not characterized.7 1.3. Gravimetric Adsorption Chromatography. The second type of method requires gravimetric precipitation of the asphaltenes by dissolving the sample in an excess of an alkane solvent before further separation of the maltenes into the SAR fractions by liquid chromatography. Typical methods for asphaltene separations are described in ASTM D4124,8 ASTM D3279,9 ASTM D6560,10 and ASTM D2007.11 These are all different from each other and thus provide different results. 1.4. High Performance Liquid Chromatography. Many variations of the SAR separation have been developed using amino, cyano, or alumina columns including several automated or semiautomated methods utilizing high performance liquid chromatography (HPLC).8,9,12−16 The variations for automated separation of the maltenes typically use silica gel derivatized with aminopropyl or cyano functional groups. These generally do not provide fully resolved separations of saturates and aromatics, and irreversible adsorption occurs on the columns due to resins and soluble asphaltene-type component molecules. Detection is typically by refractive index (RI), and the different component molecules of oil have highly different refractive indexes, so even with known fraction calibrations these methods are semiquantitative at best. In addition, use of an RI detector precludes solvent switching since the detector would have to be rezeroed between each solvent change. A version of an HPLC SARA method that uses a chemically bonded aminosilane stationary phase for an automated SAR separation of crude oil maltenes was unable to be applied to paving grade asphalt material as the most polar fraction of asphalt was not desorbed from the system, resulting in poor recovery and fouling of the column.12 Fan and Buckley developed a similar method that utilizes two aminosilane columns.13 While their system appears to work well for crude oils, the most polar components of the resins fraction of asphalt became irreversibly bonded to the column. It was evident that a new system was needed for asphalt that performs the SAR separation without fouling the separation column by resins and that allows full resolution of the fractions as well as near full recovery of all the fractions. 1.5. Automated Solubility Separation of Asphaltenes. The Asphaltene Determinator (AD) is a novel automated solubility-based asphaltene separation developed by Schabron and Rovani.17 Although HPLC equipment is used, the separation does not involve chromatography, and it is strictly solubility-based. Following an injection of 2 mg of oil, asphaltenes precipitate within a polytetrafluoroethylene (PTFE) packed column in an excess of heptane. The
2. EXPERIMENTAL SECTION 2.1. Rolling Thin Film Oxidation (RTFO). The RTFO was performed according to AASHTO T-240 on a portion of the original binder.20 The apparatus used was a James Cox and Sons Model CS 325-B RTFO oven with an RTF controller. The test was run at 163C for 85 min and the air flow reading was 4.5 cc/min. 2.2. Pressure Aging Vessel (PAV). A portion of the RTFO sample was poured into a single 50 g PAV pan for the PAV test. The PAV was run according to ASTM D 6521 in an ATS oven for 20 h, at 100 °C under 2.10 MPa of air.21 After aging, the sample was degassed in a vacuum oven at 150 °C for 10 min without a vacuum and 20 min at 15 kPa of a mercury vacuum. 2.3. Asphalt Core Slicing. The asphalt cores were sliced using a standard 12-in.-diameter, water cooled masonry saw, fitted with a standard 12 in. notched masonry diamond blade. The diagram for the slices is provided in Figure 3. Each slice is a nominal 13-mm (0.5-in.)thick, and the saw blade cut loss is about 2 mm (0.08 in.) for each cut. 2.4. Asphalt Core Extraction. Each core slice was broken into smaller pieces, and extraction was performed by mixing the pieces with toluene:95% ethanol (85:15 v:v). All solvents were reagent grade. Extractions were repeated until the extract solution was near clear. The extract portions from repeat extractions were each centrifuged at 2200 rpm for 30 min, and these were then combined to provide a sample extract solution. The solution was initially rotary evaporated at 70 °C. Remaining solvent was removed by rotary evaporation in a silicone oil bath by slowly increasing the rotovap bath temperature to 150 °C, which takes about 30 min to avoid foaming under a 15 kPa vacuum, with continued rotovapping at 150−170 °C for 90 min, with a slight argon bleed above 15 kPa of vacuum. The samples were tested to ensure complete solvent removal using a CS2 liquid cell FTIR (1.0 mm path length, NaCl windows, 50 mg/mL concentration, 32 scans). The peak at 692 cm−1 was used to qualitatively determine the absence of toluene. Toluene removal was deemed complete when there was only a very small or nondetectable peak at 692 cm−1. The peaks at 1700 cm−1 and 1032 cm−1 were also evaluated for a loss or gain of CO and SO, respectively. There was no significant increase for the C O and SO functional group absorbances under these conditions. The samples were stored under nitrogen in amber wide-mouth precleaned I-Chem bottles with polytetrafluoroethylene (PTFE)-lined lids. 2.5. Automated Separation Instrumentation. Waters brand HPLC equipment was used for the SAR-AD separation. Components include a model 600 pump, 717 plus autosampler, 2489 variable wavelength absorbance detector set at 500 nm, and 2424 evaporative light scattering detector (ELSD). The 2707 autosampler is set to provide 20 μL injections containing a partial loop in needle overflow 4655
dx.doi.org/10.1021/ef400952b | Energy Fuels 2013, 27, 4654−4661
Energy & Fuels
Article
Figure 1. Column switching, solvent flow, time scheme for automated SAR-AD separations. mode. The 2424 ELSD has a 241 kPa nitrogen flow and 60 °C drift tube, and the nebulizer is set to cooling (about 12 °C). The separation is performed with four stainless steel 250 mm × 7 mm i.d. columns packed with 40−60 mesh polytetrafluoroethylene (PTFE) ground from Chemware Ultra-Pure boiling stones, 150−212 μm glass beads from Sigma-Aldrich, 15−35 μm 9 mm pore size aminopropyl bonded silica gel from Fluka, and 37−70 μm 150 Å silica gel from Sigma-Aldrich activated at 120 °C overnight. All columns are thermostatted to 30 °C. Automated four-port and six-port high pressure electronic actuated switching valves control the flow path of the solvents, and these switches are activated by the pump and UV detector electronics. Step gradient solvent changes are heptane, cyclohexane, toluene, and methylene chloride:methanol (98:2 v:v), all at 2 mL/min. The cyclohexane is HPLC grade, and all other solvents are reagent grade or better. Typical pump back pressure is
