Article pubs.acs.org/EF
Molecular Weight Distributions and Average Molecular Weights of Pyrolysis Oils from Oil Shales: Literature Data and Measurements by Size Exclusion Chromatography (SEC) and Atmospheric Solids Analysis Probe Mass Spectroscopy (ASAP MS) for Oils from Four Different Deposits Oliver Jar̈ vik* and Vahur Oja Department of Chemical Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia S Supporting Information *
ABSTRACT: Molecular weight (MW) data for shale oils, including molecular weight distributions (MWDs) as well as average MWs, is difficult to find. A thorough literature review of MW data for shale oils from different pyrolysis processes, found scattered across a wide range of topics (or papers on various topics) and analyzed here, was carried out as part of the study. However, because the data are for oils produced under different conditions, and the exact pyrolysis conditions are sometimes not fully described, then the MW values generally are not directly comparable. Therefore, in the experimental part of the current study, average MWs and MWDs were measured for shale oils from four different oil shales (Green River in the western United States, Attarat Umm Ghudran in Jordan, Kukersite in Estonia, and Dictyonema in Estonia) obtained by retorting under identical pyrolysis conditions, using a standardized Fischer assay method. The main goal of the study, to measure the MWDs of the Fischer assay oils, was accomplished using size exclusion chromatography (SEC). The MWD data obtained was also compared to that from atmospheric solids analysis probe mass spectroscopy (ASAP MS). In addition, MWDs of industrial Kukersite shale oil from Kiviter and Galoter processes were evaluated for comparison. The MWDs of Fischer assay shale oils obtained in the current study ranged from 100 g mol−1 to 600 g mol−1. While the oils show comparable average MW values, the parameters describing the width and shape of MWDs were more dependent on the parent oil shale.
1. INTRODUCTION The molecular weight distribution (MWD) and average molecular weight (MW) of an unconventional oil are important fundamental properties for product characterization and process development/design; however, these parameters are often not reported. Similarly, data on the MWs and MWDs of shale oils (unconventional oils from oil shale retorting, or lowtemperature pyrolysis products) seem to be rare and scattered among different studies, with more data on MWs than on MWDs. To our knowledge, number-average molecular weight (MWn) values have been published for Fischer assay oils obtained from 18 different deposits (Table 1), and for shale oils produced by industrial or experimental retorts (other than a Fischer retort) from 7 deposits (Table S1 in Supporting Information). MWD data obtained under various experimental conditions, on the other hand, are available only for shale oils from four different oil shale deposits (Table S2 in the Supporting Information). The most studied shale oils, with regard to the MWD, are those produced (by different methods) from Green River oil shale. For these oils, the MWD has been characterized using field ionization mass spectroscopy (FIMS),1−4 Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS),5,6 and size exclusion chromatography (SEC).7 There are also data on the MWDs of pyrolysis oils from Israeli oil shale,3 Kukersite oil shale,5,8 and Dictyonema oil shale.8 However, any comparison of MWDs of different pyrolysis oils from different studies is questionable © XXXX American Chemical Society
unless the oil preparation conditions are known to be identical, as both heating rate and pressure affect the composition (and, thereby, also the MWD) of the resulting shale oil. The current study is a continuation of the previous investigation from our laboratory, where MWDs of Kukersite, Green River, and Dictyonema oils from direct pyrolysis into the inlet of a pyrolysis FIMS (Py-FIMS) were compared8 (note that, in this reference, the correct x-axis title in Figures 3−5 is molecular weight (in daltons)). The study indicated that the MWD of shale oil from higher-oil-yield oil shale (Green River with Type I kerogen; Kukersite with kerogen on the Type I/II border) had a wider MWD, compared to that of oil from lowoil-yield oil shale (Dictyonema with Type II kerogen). However, this previous investigation was performed for pyrolysis modeling purposes8 and was aimed at studying the MWDs of primary pyrolysis oils; therefore, the MWDs obtained could not be considered to be representative of retort oils from ex-situ retorts. Thus, the objective of the current paper is characterizing retort oils. The oils in this study were either produced using the laboratory-scale Fischer assay method9 or obtained from commercial industrial retorts (Galoter and Kiviter processes). The laboratory-scale Fischer assay retorting technique (actually, a modified Fischer assay, since the method Received: September 23, 2016 Revised: November 16, 2016 Published: November 22, 2016 A
DOI: 10.1021/acs.energyfuels.6b02452 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Table 1. Average Molecular Weights, Densities, and Atomic H/C Ratios of the Shale Oils from Different Deposits Obtained by Fischer Assaya Reaction Conditions deposit
ref
density (kg m−3)
Alaska, USA Anvil Points, USA Baisun, Uzbekistan Boltysh, Ukraine Carpathian, unknown Ef’e, Israel Fushun, China Glen Davis, Australia Green River, USA Green River, USA Green River, USA Green River, USA Göynük, Turkey Irati, Brazil Irati, Brazil Irati, Brazil Kukersite, Estonia Kukersite, Estonia Maoming, China Seyitömer, Turkey Stuart, Australia Sunbury, USA Timahdit, Morocco Volga, Russia
43 43 42 42 42 3 43 48 49 18 43 2 43 43 42 18 42 18 43 43 43 43 43 42
909.3 908.3 953.3 885.6 979.5 ns 886.5 881.2 917.1 927 ns
temperature (°C)
20 20 20
15.6 15.6 20
929.2 914.2 920.6 918 976 978 899.4 881.5 889.4 976.1 981 1035
20 20 20 20
20
MWn (g mol−1)
MWn technique
H/C of oil
310 310 231 230 206 361 300 252c 290 285b 290 441c 300 300 226 210 233 233b 300 280 280 280 270 188
VPO VPO ns ns ns FIMS VPO ns ns ns VPO FIMS VPO VPO ns ns ns ns VPO VPO VPO VPO VPO ns
1.7 1.75 1.51 1.67 1.41 1.5 1.74 N.S. 1.6 1.7 1.66 N.S. 1.74 1.61 1.4 1.56 1.37 1.47 1.71 1.75 1.81 1.44 1.52 1.36
VPO = vapor pressure osmometry; FIMS = field ionization mass spectrometry; ns = not specified. bThe molecular weights of Kukersite and Green River given by Yefimov and Rooks18 are interchanged in the table. cWeight-average molecular weight.
a
Table 2. Characteristics of Dry Oil Shale Samples Including Expanded Uncertainties with 95% Confidence Levels constituent (wt %) moisture ash content, Ad mineral content, (CO2)dM organic content, Orgd a C (wt %) H (wt %) atomic H/C oil pyrogenous water gas + loss semicoke a
Kukersite
Dictyonema argillite
± ± ± ±
Green River
0.05 0.51 ± 0.23 0.14 ± 0.13 5.3 77.1 ± 1.1 52.7 ± 2.3 1.8 0.5 ± 2.6 8.2 ± 1.0 5.6 22.0 ± 2.9 39.0 ± 2.5 Elemental Analysis of the Organic Matter (Kerogen) 75.42 ± 3.50 88.85 ± 0.27 74.52 ± 3.65 9.36 ± 0.74 8.59 ± 0.13 9.87 ± 1.26 1.49 ± 0.07 1.16 ± 0.02 1.59 ± 0.09 Yield, %OMb 59.1 ± 8.7 17.3 ± 6.8 45.9 ± 5.9 15.1 ± 2.5 20.2 ± 4.9 7.1 ± 4.0 24.6 ± 7.8 21.2 ± 11.7 19.0 ± 7.7 1.2 ± 13.1 41.2 ± 0.1 28.0 ± 5.3
0.14 57.2 24.9 17.7
Attarat Umm Ghudran 0.74 64.6 17.1 17.5
± ± ± ±
0.12 2.6 1.4 3.0
70.76 ± 0.70 8.37 ± 0.39 1.42 ± 0.07 38.6 ± 6.0 19.2 ± 3.3 32.7 ± 6.6 9.5 ± 4.6
Orgd = 100 − [Ad − (CO2)dM]. bOM = organic matter.
used to characterize coals was adopted to characterize oil shales) is a widely applied technique to evaluate the oil production potentials of oil shale deposits and the characteristics of the oils produced.10 The MWDs of these oils were measured using SEC, which is readily available in many laboratories and enables detection of components in a sample under mild conditions (no heating),11 and by ASAP MS. The latter is a relatively new soft ionization technique that utilizes heated desolvation gas for sample volatilization and has been proven useful in the analysis of polymers with molecular weights of up to 2000 g mol−1.12 It is a convenient method, since samples can be studied without any preparation and without the use of specialized instruments or techniques.
The present work compares the MWDs of shale oils obtained via low-temperature pyrolysis of four oil shales under identical conditions. Three of the oil shales contain high oil yield and thermally softening kerogens13 (Green River in the western United States, Attarat Umm Ghudran in Jordan, and Kukersite in Estonia) and one oil shale contains low oil yield and thermally nonsoftening kerogen14 (Dictyonema shale in Estonia). The Attarat Umm Ghudran deposit in Jordan is located ∼50 km away from the extensively studied El-Lajjun deposit.15 As a result, the properties of both oil shales are very similar, as shown by Hamarneh.16 The MWDs of the Fischer assay oils were compared to those of industrial Kukersite shale oil from the Galoter17 and Kiviter18 processes. Here, it is worth B
DOI: 10.1021/acs.energyfuels.6b02452 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
influence the elution time;25 and third, solute−solvent and solute−solute interactions (for example, as a result of agglomeration) may also influence elution time.11 In addition, band broadening (BB), which is a common problem in chromatographic systems that is mainly related to diffusion, should be taken into account. In the present study, the apparatus and procedure used was as follows: 5 μL of each shale oil sample was dissolved in dichloromethane (Chromasolv, ≥99.8%, Sigma−Aldrich; note that dichloromethane is considered potentially carcinogenic) at concentrations of ∼3.2 mg mL−1 and was injected into the Waters 2965 separation module, where it was separated in a 7.8 mm × 300 mm Styragel HR 0.5 column (Waters Corporation, USA). Separation was carried out at 25 °C using dichloromethane (HPLC grade, Sigma−Aldrich) as the eluent at a flow rate of 0.600 mL/min. The MWD was detected at 250−700 nm, using a Waters 996PDA detector (Waters Corporation, USA). A PDA detector may be considered suitable for the analysis of pyrolysis oils, since most compounds in these oils contain chromophores.11 The SEC column was calibrated using Kukersite oil narrow fractions obtained from technical middle oil fractions by Engler distillation (ASTM D8626) or vacuum distillation, technical heavy oil fractions by vacuum distillation, and technical gasoline fractions by rectification. Some middle oil vacuum distillation fractions were separated by extraction into dephenolated and phenols fractions, as described by Baird et al.27 The SEC calibration curve of the elution time (top of the peak) vs ln(MW) is shown in Figure 1, together with the calibration
noting that, although industrial production of oil from Kukersite oil shale has nearly a 100 year history in Estonia,19 MWDs of industrially produced shale oils are unavailable, at least publicly. Pyrolysis conditions differ, depending on the process used, and, as a result, the MWD of oil from industrial retorting technology is also assumed to differ from that of Fischer assay oil or oils from other types of industrial technologies. For example, in the early days of shale oil production in Estonia tunnel ovens, Davidson horizontal rotary retorts and chamber ovens were used. The MWn values for shale oils, produced using Kukersite oil shale in these retorts, were reported to be 235, 220,20 and 242 g mol−1,21 respectively. Shale oil obtained from the Kiviter process, which is utilized in Estonia for shale oil production, has an average MW ranging from 285 g mol−1 to 290 g mol−1, as reported by Yefimov,19 and by Qian and Liang,22 respectively. For the Galoter (solid heat carrier) process (or its modifications), which is currently the main shale oil production process in Estonia, the average MW has been determined to be ∼275 g mol−1.19
2. MATERIALS, METHODS, AND UNCERTAINTY ANALYSIS 2.1. Samples. Fischer assay oils were obtained according to ISO-6479 using the oil shale samples shown in Table 2. The samples of industrially produced Kukersite shale oil used in the study, including wide technical fractions, were obtained from the Galoter process23 of Eesti Energia Oil Industry and the Kiviter process23 of Viru Keemia Grupp AS. (Before handling shale oils, it is advisible to consult the corresponding Material Safety Data Sheets, if available, as it may cause cancer.) Some of the technical fractions from the Galoter process were fractionated by distillation to narrower fractions for which MWn values were then measured via a cryoscopic method or via vapor pressure osmometry (VPO). These fractions were used to evaluate the applicability of SEC (and also to calibrate SEC) and ASAP MS for MWD measurements. Because of the design of the industrial process, the entire shale oil could not be obtained from the Galoter process. Therefore, an approximation of the entire oil was prepared by mixing gasoline, middle oil, and heavy oil technical fractions (mass ratio of 20:60:20, which matches the ratio given in the plant’s design documents). The reason for this is that vapors escaping the pyrolysis reactor are directed, after purification (dust/ash removal), to the rectification column without prior condensation.17,23 The heavy oil fraction is mostly directed back to the pyrolysis reactor. Note that a fraction of the lower MW compounds (retort gas) is not condensed in the condensation system. 2.2. SEC Setup and Calibration. A Waters 2965 separation module with a Styragel HR 0.5 (Waters) SEC column was used to obtain MWD data, as will be described in greater detail later. The main advantages of using SEC for MWD measurement are mild conditions during analysis (low temperature, usually
