Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Partial Upgrading of Bitumen by Thermal Conversion at 150−300 °C Lina Maria Yañez Jaramillo and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, 9211 116th Street, Edmonton, Alberta T6G 1H9, Canada S Supporting Information *
ABSTRACT: Bitumen produced from oilsands deposits has a high viscosity, which presents a challenge to pipeline transport. Ways to reduce the viscosity at low incremental production cost are desirable. Thermal conversion of oilsands-derived bitumen at temperatures in the range of 150−300 °C was explored as a potential strategy for viscosity reduction. Viscosity increased compared to the bitumen feed following thermal treatment of bitumen at 150 and 200 °C but decreased following thermal treatment at 250 and 300 °C. At all temperatures studied, changes in the chemical and physical nature of the product were observed within 1 h of reaction time, and changes continued as reaction time was extended to a period of 8 h. Hydrogen transfer and methyl transfer were important reactions. These transfer reactions appeared to be concerted in nature and did not involve cracking to release free hydrogen or methyl radicals. In fact, thermal cracking was a minor reaction. The olefin content of the liquid was low, there was little gas-make, and the H2S concentration in the gaseous product was low. The n-pentane insoluble (asphaltenes) content of the liquid, with a few exceptions, increased during thermal conversion, but it was poorly correlated to viscosity. It is unlikely that the change in viscosity can be attributed to a single factor. The two most important factors appeared to be (i) the formation of heavier molecules that caused an increase in “excluded volume” with a concomitant increase in viscosity and slight decrease in density and (ii) a change in the phase behavior of the product due to chemical changes in the product.
1. INTRODUCTION Pipeline transport of bitumen from oilsands production sites to the market requires the bitumen to meet pipeline specifications. Both the viscosity and the density of raw bitumen exceed pipeline specifications. Typical North American pipeline specifications are viscosity ≤350 cSt at 7.5 °C (winter), density ≤940 kg/m3 at 15.6 °C (≥19°API), total olefin content ≤1 wt % expressed as 1-decene equivalent, and ≤0.5 vol % basic sediment and water. Two strategies that are currently employed are diluting the bitumen with light naphtha and upgrading the bitumen by thermal conversion (cracking) and hydroprocessing. A different strategy that is emerging is partial upgrading. The aim of partial upgrading is to reduce the intensity and cost of upgrading so that the partially upgraded bitumen will just meet pipeline specifications. One potential approach is performing thermal conversion at milder conditions. Over the years, a number of studies indicated that the viscosity of heavy oil and bitumen could be decreased by 1−2 orders of magnitude by thermal conversion below 400 °C.1−8 These are temperatures below the typical operating range of mild thermal cracking processes, such as visbreaking. The prospect of decreasing the product viscosity at temperature conditions where cracking is limited is alluring, because at such conditions the olefin content of the product might be low and the liquid yield high. Although such partially upgraded bitumen would not meet pipeline specifications without dilution,9 there are several benefits: (i) The amount of naphtha required for dilution is less. (ii) Liquid yield is higher. (iii) No hydroprocessing is required. This investigation explored thermal conversion of Cold Lake bitumen over the temperature range 150−300 °C at reaction times up to 8 h. The temperature range was chosen to also © XXXX American Chemical Society
provide information on an operating region where little information is available but where the slow evolution of gaseous products10 and hydrogen-transfer reactions11 were observed. Slow reactions do not noticeably affect normal oil processing operation, because the duration of oil exposure to temperature is short, but it nevertheless causes a change in the oil composition. However, during subsurface recovery of bitumen where the reservoir is kept at elevated temperature for a prolonged period, these changes may affect the nature of the produced bitumen. The study also straddles the temperature region where free-radical conversion changes from addition to cracking dominant.12 The impact of this change on the product properties and viscosity in particular is an aspect of the thermal conversion of bitumen that was not highlighted before. The work will also show that orders of magnitude permanent decrease in bitumen viscosity can be achieved at temperatures below 300 °C within a 1−2 h reaction time. This is not anticipated from the application of the equivalent residence time concept,13 which is often used to relate temperature and time for constant visbreaking severity. Some of the initial results obtained at 300 °C were previously reported.14 Analysis of the products by infrared spectroscopy provided data sets for mathematical analysis. 15,16 The spectroscopic data and analysis are not repeated in this work, but they were used to assist in the interpretation.
2. EXPERIMENTAL SECTION 2.1. Materials. The oilsands bitumen used as feed material in this study came from the Cold Lake region in Alberta, Canada. The batch Received: December 29, 2017 Revised: February 22, 2018
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DOI: 10.1021/acs.energyfuels.7b04145 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels of bitumen used for this work was characterized (Tables 1 and 2) as part of the study.
chromatography. The liquid and solid products in the reactor were removed with the aid of methylene chloride as solvent. The solids were separated by vacuum filtration using a 0.22 μm filter. All solids obtained were dried and weighed at room temperature. Liquid products were kept at 45 °C under atmospheric pressure inside a fumehood for a period of 1 week to evaporate the methylene chloride. The liquid products were then characterized. The mass of all materials was recorded throughout the procedure. Material balance closure was typically within the range of 98−102% for most experiments, and the material balance for each set of experiments is reported. Despite care taken during the experimental work, for those not familiar with high-viscosity heterogeneous materials (bitumen and mineral matter mixture, see Table 1), it should be noted that reproducibility is always a concern. All experiments and analyses were performed in triplicate at least. However, some properties that are influenced by, or are sensitive to the formation of agglomerates or new phase domains, such as viscosity and n-pentane insoluble content, exhibited considerable variation. These were experiment-to-experiment variations, i.e., caused by reaction, and not measurement-tomeasurement variations (as discussed in the Supporting Information). Material balance and properties such as density were less sensitive to the aforementioned changes and exhibited less variation. However, it was nevertheless clear from the variation recorded for refractive index measurements (Supporting Information) that the composition of the reaction products exhibited differences and that experiment-toexperiment variations were due to reaction. However, the recorded variations did not undermine the ability to interpret the data. 2.3. Analyses. Viscosity measurements were carried out in an Anton Paar RheolabQC viscometer. A concentric cylinder CC17/QCLTC measuring cup was used; its internal diameter is 16.664 mm, and its length is 24.970 mm. Approximately 4 g of the sample was required for the analysis. Viscosity measurements were performed at four different temperatures: 20, 30, 40, and 60 °C. The temperature was controlled by a Julabo F25-EH circulating heater/chiller whose controller is capable of maintaining the temperature constant to within ±0.2 °C. Viscosity measurements were performed at a constant shear rate in the range of 1.2−10 s−1 depending on the viscosity being measured. Density measurements were performed in an Anton Paar DMA 4500M. The instrument was calibrated with ultra pure water, and the temperature accuracy was 0.01 °C. Results were accurate to 0.02 kg/ m3. A calibration was carried out with a standard (ultra pure water) provided by Anton Paar at 20 °C, and it was confirmed that the instrument performed within specification. Because of the sample volume required for this measurement and the difficulty of performing this analysis with viscous products, this measurement was performed last, only once, and on a composite sample of the individual samples. Refractive index was determined by an Anton Paar Abbemat 200 using the sodium D-line (589 nm). Measurements were performed at four different temperatures (20, 30, 40, and 60 °C), and the accuracy of the temperature control was 0.05 °C. Proton nuclear magnetic resonance (1H NMR) spectra were obtained using a Nanalysis 60 MHz NMReady-60 spectrometer. The equipment was precalibrated with deuterated chloroform. For analysis, 0.15 g of the sample was dissolved in 0.7 mL of deuterated chloroform and placed in NMR tubes. The analysis was performed as an average of 32 scans per sample. The area in the shift range δ > 6.3 ppm was assigned to aromatic hydrogen; such demarcation for classification can be justified only in the absence of olefins or when the olefinic hydrogen fraction is low. A limited number of samples were submitted for olefin-content analysis. Olefin analysis was performed using 1H NMR (400 MHz Varian), and the olefin-content was expressed in terms of the 1-decene equivalent. In this method all hydrogen in the range 4.2−6.3 ppm was considered olefinic, in accordance with the test procedure for olefins in oil.17 The method detection limit was reported to be 0.5 wt % 1decene equivalent. The fractional olefinic area of the sample was compared with the fractional olefinic area of the same sample after spiking it with a known amount of 1-decene. The fractional olefinic
Table 1. Characterization of Cold Lake Bitumen (See Table 2 for the Temperature-Dependent Properties)a Cold Lake bitumen x
s
82.6 10.3 0.6 4.7
0.1 0.1 0.1 0.1
85.9 14.1
