Ind. Eng. Chem. Res. 2006, 45, 5239-5245
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Impacts of Sonochemical Process Variables on Number Average Molecular Weight Reduction of Asphaltene Namgoo Kang, Inez Hua,* and Changhe Xiao School of CiVil Engineering, Purdue UniVersity, 1284 CiVil Engineering Building, 550 Stadium Mall DriVe, West Lafayette, Indiana 47907-2051
Athabasca asphaltene was ultrasonically processed under a variety of conditions. Five discrete ultrasonic frequencies and four saturating gases were explored. The kinetics of the reduction in the number average molecular weight (NAMW) were determined by vapor pressure osmometry measurements and UV/visible spectrophotometric measurements. The NAMWs decreased significantly after short treatment times (from ∼1200 to ∼470 g/mol in 15 min). The observed reduction in the NAMWs of asphaltene was greatest at 358 kHz. At a frequency of 205 kHz, the observed reduction in NAMW was fastest with a mixture of Ar/H2 (50%/50% v/v), whereas saturating with hydrogen, air, and oxygen gases exhibited minimal effect on the kinetics. Visible light absorption spectra of asphaltene samples demonstrate that heptane soluble fractions increase by >50% after sonochemical treatment as an upgrading process. Introduction Asphaltenes consist of brown to black amorphous solids containing carbon, hydrogen, nitrogen, oxygen, sulfur, and some metals.1 The refractory characteristics of asphaltenes are closely associated with the high molecular weight, high viscosity, adhesive, and cohesive properties.2 Due to these characteristics, asphaltenes often cause wellbore plugging, pipeline deposition, coking, and other problems that hinder the petroleum refining processes. Therefore, one of the most challenging and important goals in petroleum research is the upgrading of asphaltenes to less refractory and lighter (i.e., lower molecular weight) fractions. Conventional processes for upgrading petroleum asphaltenes usually require elevated temperatures (above 380 °C) via cracking reactions such as thermal cracking, hydrocracking, and catalytic cracking processes with amendments of acids, metals (e.g., platinum, palladium, molybdenum), and organic/inorganic chemicals.3-6 Despite the effectiveness of catalytic cracking processes, disadvantages include intensive energy consumption, significant quantities of chemicals, relatively longer treatment time, and hazardous work environments. Over the past two decades, ultrasonic treatment has been investigated as a potential alternative to traditional cracking processes. For example, ultrasonic processing has demonstrated great potential in pipeline descaling and cleaning, improved transportation, and refining and production processes such as upgrading (i.e., the rate and efficiency of molecular weight reduction) under ambient conditions.7-15 However, detailed information is currently unavailable on the impacts of primary ultrasonic process variables such as discrete frequencies and sparge gases on the molecular weight reduction of asphaltene samples. Moreover, this is the first investigation that evaluates sonochemical upgrading of asphaltene in terms of number average molecular weight, a direct and quantitative measure. The effectiveness of ultrasound is based on acoustic cavitation: the formation, growth, and sudden collapse of gas bubbles in liquids. High temperatures and pressures occur within these * To whom correspondence should be addressed. Tel.: (765) 4942409. Fax: (765) 496-1988. E-mail:
[email protected].
bubbles during the implosion; however, the overall solution remains at ambient conditions (∼25 °C, 1 atm). A range of temperatures during bubble implosion has been observed: 2000-4000 K in aqueous solution and ∼5000 K in organic solvents.16-20 The bubble interiors are under such extreme conditions that they emit light, a phenomenon known as sonoluminescence.21-24 It was recently discovered that sonoluminescence originates from a hot plasma core formed during bubble cavitation.25 The resulting microenvironments facilitate a variety of reactions. Organic compounds in cavitating solutions rapidly transform by thermolysis (bond rupture at high temperatures) and attack by free radicals. The primary goal of this study was to identify the conditions under which sonication transforms high molecular weight compounds into smaller, lower molecular weight species and thereby enhances asphaltene solubility in heptane (C7). In particular, this study presented for the first time some experimental results regarding the effects of acoustic frequency and saturating gases on number average molecular weights (NAMWs) of asphaltene during sonochemical treatment. Five discrete frequencies (20, 205, 358, 618, and 1071 kHz) and four sparge gases (argon, hydrogen, oxygen, and air) were explored as the reaction variables in this study. Changes to NAMWs as a function of sonication time (0, 5, 15, 30, 60, and 120 min) were investigated under selected conditions. Vapor pressure osmometry (VPO) was employed to determine NAMWs of untreated and treated asphaltene. Experimental Methods and Materials Chemicals. Toluene (99.5+%, spectrophotometric grade), heptane (99+%, HPLC grade), Span20 (sodium monolaurate, mol wt 346.46 g/mol), and sodium borohydride (NaBH4) were obtained from Aldrich. All chemicals were used as received without further purification. All aqueous solutions were prepared with reagent grade water purified from distilled water by reverse osmosis and Barnstead NANOpure II deionizing system (resistivity > 18.0 mΩ-cm). Preparation and Purification of Athabasca C7-Asphaltene. The asphaltene originated from Athabasca, Canada (Oil sands A-6 asphaltene) and was comprised of the very heavy Athabasca bitumen, residual inorganic solids, and water. The original
10.1021/ie051413h CCC: $33.50 © 2006 American Chemical Society Published on Web 06/27/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006
Table 1. Physical Dimensions and Acoustic Characteristics of the Reactor Systems ultrasonic frequency (kHz)
diameter (cm)
ultrasound emitting surface area (cm2)
total reaction volume (mL)
total output power (W)
power density (W/cm3)
power intensity (W/cm2)
20 205, 358, 618, 1071
1.25 6.1
1.23 29
150 300
71 128
0.47 0.43
58 4.4
asphaltene was dissolved in warm heptane, and the heptanesoluble fractions and inorganic solids were removed. The residual heptane was steam-stripped from the heptane-insoluble fractions. The remaining asphaltene fraction was then dried in a slight vacuum at ∼100 °C. The ultimately processed product (Athabasca C7-asphaltene) was received from TTS Inc. and further processed before sonochemical treatment. A further purification of the Athabasca C7-asphaltene was necessary to formulate an analytical standard with which to quantify asphaltene transformation during sonolysis. Residual non-asphaltenic solids were separated by dissolving about 1.0 g of the asphaltene in 30 mL of toluene. The sample was vortexed for 1 min, mixed on a rotator at 50 rpm for 3 days, and then centrifuged for 30 min at 3175 rpm in a clinical centrifuge (International Equipment Company). The supernatant for each sample was carefully decanted, and the remaining fractions were dried under a nitrogen atmosphere and weighed. The above procedures were repeated three times. To obtain “solids-free” asphaltene, toluene was evaporated from the decanted supernatant using nitrogen gas in a fume hood. The purified sample was then transferred to a desiccator under vacuum. The solidsfree asphaltene was used for developing the analytical protocols. An asphaltene stock solution (50 g/L) was prepared by dissolving 5.0 g of the solids-free asphaltene in 100 mL of toluene. Sonication in the 20-kHz Ultrasonic Probe Reactor. Sonications were performed in a 20-kHz ultrasonic system (Sonics & Materials, Inc.). The system consists of an ultrasonic power generator (Model VCX-400), a transducer (Model CV26), and an ultrasonic probe. The probe has a replaceable tip (diameter, 1.25 cm) of high-grade titanium alloy (Ti-6Al-4V). The ultrasonic reactor system was operated for time periods of 5-120 min at an output power of 71.1 W (determined by calorimetry).26 To prevent overheating of the transducer during sonication, the transducer was cooled with a steady stream of air. A cooling unit was utilized to pump water through the reactor jacket, and the sonicated solution was maintained at 25 °C. The asphaltene stock solution (10 mL) was placed in a 250 mL flask. To form an oil-in-water emulsion, 150 mL of reagent grade water was added gradually to the flask while mixing with a glass magnetic stir bar at ∼300 rpm. Span20 (0.1 g), a nonionic surfactant, was added to the flask while stirring, because it demonstrated the best performance in enhancing both asphaltene particle suspension and hydrogenation through ultrasonic irradiation.12,13 The emulsion was preflushed with the gas and placed in the stainless steel reactor with an effective capacity of 150 mL. A continual sparge of gases was introduced and maintained at 100 mL/min through a small stainless steel insert through a port of the reactor during sonication. A fine adjustment valve and a regulator were used for constant gas flow. The sample in the reactor was stirred with a glass magnetic bar at ∼200 rpm during sonication. Sonication in the Multifrequency Ultrasonic Reactor. Sonochemical experiments were performed with an Allied Signal URS Ultrasonic Transducer powered by an Allied Signal R/F generator equipped with a water-jacketed glass reactor. The ultrasonic frequencies employed in the experiments were 205,
358, 618, and 1017 kHz. The stock solution (20 mL) was placed in a 500 mL flask. Reagent grade water (280 mL) was added slowly to the flask while the solution was being stirred with a glass magnetic stirring bar at ∼300 rpm. For most experiments, Span20 (0.2 g) as a surfactant was added while the emulsion was being stirred. The emulsion (300 mL) was transferred to the Pyrex glass reactor (AlliedSignal, Germany) and sonicated for 5-120 min. The solution temperature was maintained at 25 °C, and the sonicated emulsion was sparged with gas via a glass diffuser during ultrasonic irradiation. Table 1 lists the physical dimensions and acoustic characteristics of the two reactor systems. Collection of Sonicated and Surfactant Treated Samples. After processing, the emulsion was transferred to a 500 mL flask, and the asphaltene constituents were extracted into toluene (100 mL for 20-kHz probe system; 200 mL for the multifrequency reactor system) by vigorous shaking for 10 min. The sample was then allowed to equilibrate for 5-6 h; two layers appeared. The toluene layer (supernatant) containing asphaltene was collected for analysis. UV/Visible Spectrophotometry. To determine the concentration of asphaltene in toluene, UV/visible light absorption spectra of asphaltene in toluene were measured (λ ) 190-800 nm) with a Perkin-Elmer Lambda 20 UV/visible spectrophotometer. A wavelength of 450 nm was selected for quantification. Beer’s law was applied when the absorbance reading of the asphaltene samples was between 0.25 and 1.0. A 50-fold dilution of samples was usually required to obtain an absorbance in the correct range. Based on concentrations measured by UV/ visible spectrophotometry, four asphaltene samples were prepared in toluene (concentration
