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Energy & Fuels 2001, 15, 756-763
Determination of Wax Precipitation Temperature and Amount of Precipitated Solid Wax versus Temperature for Crude Oils Using FT-IR Spectroscopy R. M. Roehner*,† and F. V. Hanson University of Utah, Department of Chemical & Fuels Engineering, Merrill Engineering Building, Room 3290, Salt Lake City, Utah 84112 Received January 29, 2001. Revised Manuscript Received January 29, 2001
Wax precipitation in crude oils can produce problems in production and transportation operations. A novel FT-IR spectroscopy method is described for the determination of the wax precipitation temperature (WPT), and the estimation of the amount of precipitated solid wax material (both crystalline and amorphous) present in petroleum crude oils. A reference model oil system is analyzed using the described method. Comparisons are provided between FT-IR generated data and data generated using conventional analyses for Alaska North Slope, Utah, and Gulf of Mexico crude oils. The FT-IR method is shown to provide comparative results with conventional analysis methods, and offers several advantages over existing test methods.
Introduction Determination of the wax precipitation temperature (WPT), and the amount of solid wax precipitated at temperatures below the WPT are critical for understanding crude oil rheology and solids deposition. Many descriptions can be found in the literature for measurement of crude oil WPT, including: viscometry,1-6 cross polarized microscopy (CPM),1,4,7-10 differential scanning calorimetry (DSC),1,3,4,11,12 densitometry,3,13 near-IR * Corresponding Author. † Current address: Alyeska Pipeline Service Company, 8670 South Snow Mountain Drive, Sandy, Utah 84093. E-mail: roehnerrm@ alyeska-pipeline.com. (1) Ronningsen, H. P.; Bjorndal, B.; Hansen, A. B.; Pedersen, W. B. Wax Precipitation from North Sea Oils. 1. Crystallization and Dissolution Temperatures and Newtonian and Non-Newtonian Flow Properties. Energy Fuels 1991, 5 (6), 895-908. (2) Deo, M.; Wavrek, D. A. Wax Precipitation: Compositional Study and Cloud Point Measurements. 2nd International Symposium on Colloid Chemistry in Oil Production, SPE, Rio de Janeiro, 1997, paper 29. (3) Kruka, V. R.; Cadena, E. R.; Long, T. E. Cloud-Point Determination for Crude Oils. J. Pet. Technol., August 1995, 681-687. (4) Kok, M.; Letoffe, J.; Claudy, P.; Martin, D.; Garcin, M.; Volle, J. Comparison of Wax Appearance Temperatures of Crude Oils By Differential Scanning Calorimetry, Thermomicroscopy, and Viscometry. Fuel 1996, 75 (7), 787-790. (5) Adewusi, V. A. Waxing Tendencies And Rheological Evaluation of Crude-Condensate Blends For An Offshore Pipeline Transportation. Pet. Sci. Technol. 1998, 16 (6&8), 697-717. (6) Fogler, H.; Singh, P.; Nagarajan, N. Prediction of the Wax Content of the Incipient Wax-Oil Gel In a Pipeline: An Application of the Controlled-Stress Rheometer. J. Rheol. 1999, 43 (6), 1437-1459. (7) Erickson, D. D.; Niesen, V. G.; Brown, T. S. Thermodynamic Measurement and Prediction of Paraffin Precipitation in Crude Oil. SPE 26604 1993, 933-948. (8) Hammami, A.; Raines, M. Paraffin Deposition From Crude Oils: Comparison of Laboratory Results to Field Data. SPE 38776 1997, 273-287. (9) Ferworn, K.; Hammami, A. Control of Wax Deposition: An Experimental Investigation of Crystal Morphology and an Evaluation of Various Chemical Solvents. SPE 37240 1997, 291-310. (10) Garcia, M.; Orea, M.; Carbognani, L. The Effect of Paraffinic Fractions On Crude Oil Wax Crystallization. 3rd International Symposium on Colloid Chemistry in Oil Production (ISCOP), Huatulco, Oaxaca, Mexico, November 14-17, 1999.
spectroscopy (NIR),14 and acoustic resonance technology (ART).15 Filtration and centrifugation are the methods typically used for determining solid wax content versus temperature for crude oil systems,16,17 even though the results are influenced by occluded oil and difficult for high-pressure applications. Pulsed 1H NMR and DSC have also been used,12,18 but are problematic and ineffective for low wax crude oils. Background Infrared (IR) spectroscopy has been previously used to identify solid-solid and solid-liquid-phase transitions for alkanes19,20 and petroleum waxes,21 to measure crystallinity and its temperature dependence for poly(11) Hansen, A.; Pedersen, W.; Larsen, E.; Nielsen, A.; Ronningsen, H. Wax Precipitation from North Sea Oils. 3. Precipitation and Dissolution of Wax Studied by Differential Scanning Calorimetry. Energy Fuels 1991, 5 (6), 914-923. (12) Calange, S.; Ruffier-Meray, V.; Behar, E. Onset Crystallization Temperature and Deposit Amount for Waxy Crudes: Experimental Determination and Thermodynamic Modeling. SPE 37239 1997, 283290. (13) Davidsen, S.; Hamouda, A. Innovative Method to Determine the Wax Content and the Wax Precipitation Temperature Simultaneously for Crude Oils at Pipeline Pressures. SPE 50747 1999, 459473. (14) Alex, R. F.; Fuhr, B. J.; Klein, L. L. Determination of Cloud Point for Waxy Crudes Using a Near-Infrared/Fiber Optic Technique. Energy Fuels 1991, 5 (6), 866-868. (15) Sivaraman, A.; Hu, Y.; Jamaludin, A.; Thomas, F.; Bennion, D. Asphaltene Onset, Effects of Inhibitors and EOS Modeling of Solids Precipitation In Live Oil Using Acoustic Resonance Technology. Third International Symposium on the Thermodynamics of Asphaltenes and Heavy Oils, 1999 AICHE Spring Meeting, Houston, Texas. (16) Burger, E.; Perkins, T.; Striegler, J. Studies of Wax Deposition in the Trans Alaska Pipeline. J. Pet. Technol. 1981, 1075-1086. (17) Weingarten, J. S.; Euchner, J. Methods for Predicting Wax Precipitation and Deposition. SPE 15654, 1986. (18) Pedersen, W. B.; Hansen, A. B.; Larsen, E.; Nielsen, A. B.; Ronningsen, H. P. Wax Precipitation from North Sea Oils. 2. SolidPhase Content as Function of Temperature Determined by Pulsed NMR. Energy Fuels 1991, 5 (6), 908-913. (19) Zerbi, G.; et al. Molecular Mechanics for Phase Transition and Melting of n-Alkanes: A Spectroscopic Study of Molecular Mobility of Solid n-Nonadecane. J. Chem. Phys. 1981, 75 (7), 3175-3194.
10.1021/ef010016q CCC: $20.00 © 2001 American Chemical Society Published on Web 03/14/2001
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Energy & Fuels, Vol. 15, No. 3, 2001 757
ethylene (PE),22 and as an indicator of methylene crystallinity in petroleum-derived asphalts.23 IR spectroscopy measures the absorbance associated with molecular vibrations present in the sample analyzed. The infrared bands at approximately 720 cm-1 have been associated with the existence of long chain methylene (LCM) carbon in hydrocarbon systems, specifically CH2 rocking vibrations.24,25 LCM carbon is defined as having more than four CH2 groups in a row.25 Snyder et al.26 documented that the infrared band intensities of the 735-715 cm-1 CH2 rocking mode bands of crystalline pure n-alkanes and PE increase with decreasing temperature at a magnitude much larger than predicted from temperature-associated changes in density and refractive index (less than 5%, and 2%, respectively, over the range of 0 to 300 K). They defined intensity, I, of a band as
I)
∫
1 Aν dν bF
(1)
where F is the density of the sample, b is the sample path length, and Aν is the absorbance at wavenumber ν. Hagemann et al.22 documented a method to measure crystallinity in PE using these integrated intensities (with curve fitting procedures) and established assignments of bands at 730 and 722 cm-1 to represent the crystalline fraction, and at 723 cm-1 to represent the amorphous fraction. Integrated intensities for quantitative measurements were used rather than values based on peak heights alone, since the integrated intensities were not affected by band shapes (which are impacted by the temperature effects of interest).22 The assigned bands at 730 and 722 cm-1 occur due to crystal field splitting from out-of-phase rocking in the closely packed LCM carbons in the orthorhombic crystals.25 Further, Hagemann et al.22 identified that through Beer’s law the observed intensity, Iiobs, of a given ith component (or assigned band) was related to its concentration, xi, and intrinsic intensity, Iiintr; or
Iiobs ∝ xi Iiintr
[ (bF1 ) /∂T] + [∂ ln(∫
∂ ln I735-715/∂T ) ∂ ln
735
715
Aνdν)/∂T] (4)
(2)
As such, the temperature dependence of the observed intensity has a contribution from the change in concentration with temperature and the change in intrinsic intensity with temperature; expressed by Hagemann et al.22 as
∂ ln Iiobs/∂T ) ∂ ln xi/∂T + ∂ ln Iiintr/∂T
From these referenced studies,22,26 the observed increase in intensity with decreasing temperature for the rocking mode vibrations of LCM carbon can be viewed primarily as a complicated interplay between increases in solidphase crystallinity, increases in conformational order in the liquid, and changes in the intrinsic intensity of the LCM solid phase (both amorphous and crystalline). It is recognized that crude oil multicomponent systems are even more complicated (i.e., mix of various nalkanes, changing peak locations) than pure component n-alkane or PE systems for which previous measurements of the temperature dependence of IR absorbance of LCM have been made. Nonetheless, LCM carbon is the primary functional group present in precipitated crude oil solids, and therefore it was hypothesized that measuring changes in integrated absorbance from 735 to 715 cm-1 would provide a measurement of crude oil WPT, and an estimation of weight percent precipitated solid wax versus temperature. This hypothesis is based on indications from the previously discussed studies22,26 of n-alkanes and PE that the primary change in the integrated absorbance with decreasing temperature is coming from the creation of a crystalline (orthorhombic) phase and a more conformationally ordered amorphous phase from the disordered liquid phase. The work reported here does not represent a comprehensive study of all the conformational changes taking place in crude oil as solid wax precipitates with decreasing temperature. Rather, this work demonstrates what is possible using the methods developed to quickly describe wax precipitation in a given crude oil in terms of WPT and estimation of weight percent precipitated solid versus temperature. WPT Measurement. Above the WPT, the LCM carbon exists only as a liquid, and therefore the change in the intensity of the 735 to 715 cm-1 bands should be as given in eq 4:
(3)
(20) Snyder, R.; et al. Nonplanar Conformers and the Phase Behavior of Solid n-Alkanes. J. Am. Chem. Soc. 1982, 104 (23), 62376247. (21) Gupta, A.; Brouwer, L.; Severin, D. Phase Transitions In Petroleum Waxes Determined By Infrared Spectroscopy. Pet. Sci. Technol. 1998, 16 (1&2), 59-69. (22) Hagemann, H.; Snyder, R.; Peacock, A.; Mandelkern, L. Quantitative Infrared Methods for the Measurement of Crystallinity and Its Temperature Dependence: Polyethylene. Macromolecules 1989, 22, 3600-3606. (23) Smith, C.; Schuetz, C.; Hodgson, R. Relationship Between Chemical Structures and Weatherability of Coating Asphalts as Shown by Infrared Absorption Spectroscopy. I & EC Prod. Res. Dev. 1966, 5 (2), 153-161. (24) Streitwieser, A.; Heathcock, C. Introduction To Organic Chemistry; Macmillan Publishing: New York, 1976; p 330. (25) Smith, B. Infrared Spectral Interpretation, A Systematic Approach; CRC Press: New York, 1999; p 36.
The data presented by Snyder et al.26 for pure n-alkanes and PE indicated this quantity to be approximately linearly increasing with decreasing temperature for the narrow temperature range of 0 to 60 °C. In addition, density data for crude oil systems is known to be linearly increasing with decreasing temperature for temperatures above the WPT, and measurements of integrated absorbance for n-alkanes made in the course of this study were found to be linearly increasing with decreasing temperature. Once below the WPT, a change in the slope of integrated absorbance versus temperature would be expected owing to the creation of crystalline and amorphous and solid as anticipated from eq 3. Specifically, since the integrated intensity for LCM carbon rocking vibrations in the solid phase is more than 50% larger than for the liquid phase as shown by Snyder et al.,26 a large change in slope would indicate the creation of a solid phase in the sample. For these reasons, the (26) Snyder, R.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. Temperature and Phase Behavior of Infrared Intensities: The Poly(methylene) Chain. J. Phys. Chem. 1986, 90, 5623-5630.
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Figure 1. Model Oil, aliquot B, integrated absorbance (peak area 735-715 cm-1) versus temperature depicting identification of WPT, estimation of wt % S versus temperature, with insets of general IR band shape.
temperature where a significant change in slope of integrated absorbance from 735 to 715 cm-1 versus temperature is taken to be the WPT of the crude oil tested. Estimation of Weight Percent Solid versus Temperature. The amount of solid LCM carbon present corresponds to the amount of solid present in the crude oil, since the solid usually consists of alkanes with a carbon number in excess of 25. Since Snyder et al.26 found the integrated intensity of LCM carbon rocking vibrations in the solid phase to be upward of 50% larger than for the liquid phase, it can be assumed that the change in absorbance (735-715 cm-1 band) due to temperature observed for crude oils samples below the WPT is due to the creation of a solid phase. From this assumption, it can easily be shown27 that eq 5 provides an estimate of the weight percent precipitated solid wax, wt % S, at temperatures below the WPT.
wt % S ) C * [(Atotal - Aext. liq.)/Atotal] × 100% (5) In eq 5, C is a constant assumed equal to 1 in this initial work, Atotal is the integrated “observed” absorbance, Aνobs, at a given temperature (below the WPT) obtained using eq 6, and Aext. liq. is the “extrapolated” integrated liquid-phase absorbance as shown in Figure 1.
Atotal )
735 Aνobsdν ∫715
(6)
Figure 1 is a plot of integrated absorbance (735-715 cm-1) versus temperature for a Model Oil which was analyzed as described in the following sections. Figure 1 depicts the identification of the WPT, indicates the relationship of Atotal to Aext liq., and provides an indication of IR band changes through insets of observed spectra for the initial and final test temperatures. (27) Roehner, R. M. Measurement and Prediction of Wax Precipitation for Alaska North Slope Crude Oil Transported in the Trans Alaska Pipeline System. Ph.D. Dissertation, University of Utah, Salt Lake City, Utah, August 2000.
Figure 2. FT-IR spectroscopic analyzer.
Experimental Section FT-IR analysis was conducted using a Perkin Elmer 16PC Spectrometer, with Perkin Elmer Spectrum v2.0 software.28 Spectra were collected from 4000-650 cm-1. The spectra were an average of 16 scans, with a spectral resolution of 4.0 cm-1. A modified SpectraTech HC-32 liquid cell holder was used to control sample temperature. The sample temperature was measured with a calibrated Type-K thermocouple and an Omega HH-22 Digital Thermometer ((1.0 °C), and controlled ((0.1 °C) using a Brookfield HT-105 controller with a Brookfield TC-500 bath. NaCl windows (32 mm), with a 0.1 mm lead spacer, were used in the cell holder. No mixing is provided in the small volume liquid cell. It should be noted that the cell assembly used allowed for density changes in the sample, as contraction of the liquid in fill and exit ports of the cell could occur as the sample was cooled. A sketch of the experimental equipment is presented in Figure 2. The HC-32 liquid cell holder thermocouple was always within 0.1 °C of the sample temperature for the cooling rates employed (0.05-0.20 °C per minute) and the experimental temperature range (-5.0-95 °C) used. The comparison was generated by placing a micro-thermocouple in the liquid cell, and comparing the micro-thermocouple temperature readings with the HC-32 liquid cell thermocouple readings on the digital thermometer for coolant temperature settings covering the anticipated operating range of experiments. Since the observed differences in temperature readings were less than the accuracy attributed to the thermocouples ((1.0 °C), the HC-32 liquid cell thermocouple temperature was used to represent the sample temperature.
Calculations Total absorbance, Atotal, for LCM rocking vibrations was calculated by integrating all spectra between 735 and 715 cm-1 using the Spectrum 2.0 software peak area/height routine.28 The corrected peak area obtained for each temperature from this routine was added to the base area of the peak determined from the first temperature tested to obtain total absorbance at each (28) IR Spectroscopy Software, Spectrum v2.0; Perkin-Elmer: Norwalk, CT, 1998.
Determination of Wax Precipitation Temperature
test temperature. The base of the peak analyzed at the first test temperature was obtained by subtracting the corrected peak area obtained from the area/height routine, from the uncorrected peak area obtained from the area/height routine. This was done to eliminate the effect of baseline shifts, which might occur as the sample temperature was reduced. The WPT determination was accomplished by fitting linear relations to the integrated absorbance data from both the start and end of the temperature range evaluated. The intersection of these two best-fit lines was taken to be the WPT, which was easily identified graphically within the accuracy of temperature measurement employed ((1.0 °C). Equation 5 was used to estimate a weight percent precipitated solid wax for temperatures below the observed WPT. Values for the integrated extrapolated liquid absorbance, Aext. liq., below the WPT were calculated using a linear regression fit to integrated absorbance data above the WPT. Comparative Testing Methods. A comparison between FT-IR data and data from conventional methods including viscometry, CPM, DSC, and centrifugation was undertaken. Details of these comparative methods have been previously described.27 Viscometry. In this investigation, a capillary viscometer was used to minimize evaporative loss of crude oil light ends during analysis. The pressure differential across a coiled tube under conditions of steady-state flow and constant temperature was determined, and the apparent dynamic viscosity was calculated using the Hagen-Poiseuille equation. The temperature and shear histories of the crude oil tested were carefully controlled. The deviation of experimental viscosity data from Arrhenius behavior was interpreted as the presence of solid wax particles in the crude oil sample, and graphically identified as the WPT. Cross-Polarized Microscopy. Cross-polarized microscopy (CPM) is the current preferred method used by industry for identification of crude oil WPT.8 It involves viewing an oil sample under polarized light, where only light rotated by any crystalline material present would be visible. As the sample temperature is reduced, the first appearance of solid crystals is taken to be the WPT. A Nikon Optophot-PDL microscope with total magnification of 100× was used to perform this test, which allowed identification of crystals down to 1 micron in size. Differential Scanning Calorimetry. Measurement of crude oil WPT was undertaken using a Dupont Instruments Thermal Analyst 2000 System, coupled with a Dupont Instruments 910 Differential Scanning Calorimeter, and a T/A Instruments Liquid Nitrogen Cooling Assembly (LNCA). The T/A Instruments LNCA was used to obtain test temperatures below the anticipated glass transition temperature of the crude oil tested (approximately -100 °C), to allow for baseline interpretation, and integration for total latent heat effects. Problems related to the lack of stable baselines produced in a cooling mode, and the low wax content of the ANS crude oil, precluded identification of an initial exotherm (indicating the WPT) on the DSC thermograms produced for the ANS crude oil tested. DSC analysis of waxy crude oils did show distinct exotherms
Energy & Fuels, Vol. 15, No. 3, 2001 759
Figure 3. Model Oil, aliquot B, spectra at 720 cm-1.
Figure 4. Model Oil, aliquot B, peak area (735-715 cm-1) versus temperature. Table 1. Model Oil Compositon component
wt %
n-C10 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30 total
64.73 10.30 7.40 5.29 3.79 2.70 1.93 1.37 0.97 0.69 0.49 0.35 100.00
using sample-cooling rates of 10 °C per minute for 5-10 mg samples contained in hermetically sealed sample pans. Centrifugation. A quantitative estimation of crude oil solid wax content at temperatures below the crude oil WPT was obtained by centrifugation with correction for occluded oil using HTGC analysis of the solid. A Beckman Instruments Model J-6M induction drive centrifuge operating at 1500 rpm with a temperaturecontrolled centrifuge chamber was used to centrifuge a set of five duplicate 50-mL crude oil samples for 40 h. Centrifugation of the samples at 1500 rpm produced an approximate gravity force of 450g. The temperature of the centrifuge chamber was recorded with a digital thermometer certified to (0.3 °C from -40 °C to 100 °C using NIST traceable standards. Correction of cen-
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Table 2. Model Oil Wax Precipitation Data aliquot:
A
B
C
D
20
20
19
20
E
F
av
SD
RSD (%)
20
20
0.76
3.92
1.50 7.84 19.00 21.69
1.56 0.75 1.20 1.70
103.51 9.59 6.34 7.83
referencea WPT (°C) 20.35 T (°C)
measured WPT (°C)
referencea
estimated wt % solid based on peak (735-715 cm-1) area
wt % solid 18.0 14.5 5.1 0.0 a
2.46 6.80 22.16 23.71
18
1.95 6.81 16.74 18.88
3.25 8.85 19.36 22.38
-0.07 7.32 19.10
7.96 18.89 21.50
-0.18 7.65 19.61 22.38
2.57 8.48 20.28 23.33
Ref Pauly et al.29
trifuge solid weight percentages for occluded oil content was obtained by performing high-temperature gas chromatography (HTGC) on the centrifuge solid. To accomplish this correction, the HTGC chromatograms of the centrifuge solids were used to determine weight percents attributed to single carbon number (SCN) groups to C60, and ratios of n-alkane to non n-alkane components in each SCN group. By comparing these data for the centrifuge solids with the same data for the parent crude oil, precipitated solids were identified as those SCN groups with an n-alkane to non n-alkane ratio in excess of the same SCN group in the parent crude oil. Model Oil Analysis A Model Oil was analyzed using the FT-IR method to demonstrate the capability of the method. The Model Oil comprised of n-alkanes was purchased from Supelco as a custom mix to match the system previously analyzed by Pauly et al.29 and identified in the reference as Mixture A. The composition of the Model Oil is reported in Table 1. Chemicals used to make the mix were of 98 to 99% purity. Prior to sampling the mixture, the sample was heated to 40 °C (approximately 20 °C above the WPT) for 1 h. Aliquots were removed from the heated sample using a preheated syringe (approximately 50 °C), and introduced into the preheated FT-IR liquid cell (approximately 42 °C). Six aliquots were taken from the same sealed 10-mL vial to make six replicate analyses to establish repeatability. Each analysis involved lowering the sample to the desired test temperature, and obtaining a spectrum at the desired test temperature. The Model Oil was analyzed at 18, 14.5, 5.1, and 0 °C to match the literature reference data points for direct comparison.29 The spectra at 720 cm-1 obtained for Model Oil Aliquot B at test temperatures are presented in Figure 3. It is important to note that a doublet results at the lower temperatures, indicating the presence of orthorhombic crystals. A plot of peak area for the spectra in Figure 3, obtained by integration from 735 to 715 cm-1, versus temperature, is presented in Figure 4. The WPT of 20 °C ((1.0 °C) indicated in Figure 4 matches the 20.35 °C value for Mixture A in the literature reference.29 A summary of the Model Oil wax precipitation data obtained is provided in Table 2. The average WPT observed for the aliquots tested was 20 °C, with a calculated relative standard deviation of 3.92%. It also (29) Pauly, J.; Dauphin, C.; Daridon, J. Liquid-Solid Equilibria in a Decane + Multi-Paraffins System. FPE 1998, 149, 191-207.
Figure 5. Averaged FT-IR data for wt % solid versus temperature compared with reference data for the Model Oil.
should be noted that all but the highest temperature (where the solids content is lowest) show repeatability within 10% relative standard deviation (RSD) for weight percent solid. Figure 5 provides a bar chart comparison of the averaged weight percent solids determined by FTIR with the weight percent precipitated solids reported by Pauly et al.29 The FT-IR data compare well with the literature reference data. Crude Oil Analysis Sample preparation of crude oils with high light ends (CO2 through C6) content were handled differently in order to prevent loss of light ends, which adversely impacts accurate WPT measurement. For these oils an aliquot of approximately 25-mL was taken from a well mixed sample container at room temperature (approximately 20 °C) and placed in a wide mouth jar with a Teflon coated lid. A room temperature syringe was then used to transfer the sample from the jar into the FT-IR liquid cell. The temperature of the FT-IR cell was then raised to 50 °C and held for 1 h to dissolve any solids present, after which the sample was lowered to the test temperature for collection of FT-IR spectra. Set point temperatures were manually entered into the Brookfield HT-105 controller, which controlled the Brookfield TC-500 bath to produce coolant temperatures leaving the HC-32 liquid cell assembly within 0.1 °C of the desired set point. Test temperatures were typically 5 °C apart. In general, the sample cooling rates were always less than 0.2 °C per minute, and for the majority
Determination of Wax Precipitation Temperature
Energy & Fuels, Vol. 15, No. 3, 2001 761
Table 3. Waxy Crude Oil-Wax Precipitation Temperature Measurements Summary WPT (°C) crude oil Utah-Grand County Gulf of Mexico
CP microscopy FT-IR DSC viscometry no data 34
41 33
34 27
no data no data
Figure 8. Estimated Utah-Grand County Crude Oil weight percent solid versus temperature by FT-IR analysis.
Figure 6. Utah-Grand County Crude Oil FT-IR spectra at 720 cm-1.
Figure 9. Gulf of Mexico Crude Oil FT-IR spectra at 720 cm-1.
Figure 7. Utah-Grand County Crude Oil FT-IR absorbance peak area (735-715 cm-1) versus temperature.
of testing the cooling rate was less than 0.1 °C per minute. These cooling rates produced minimal change in WPT values obtained by Ronningsen et al.1 using CPM. Several minutes were spent at a constant temperature prior to collection of spectra to ensure equilibrium conditions. Crude Oil Results
Figure 10. Gulf of Mexico Crude Oil FT-IR absorbance peak area (735-715 cm-1) versus temperature.
WPT values measured using FT-IR for Gulf of Mexico (proprietary source), and Utah-Grand County crude oils are compared with WPT data generated by DSC and CPM in Table 3. These oils can be considered to be waxy crude oils. No aromatic carbon was found in the UtahGrand County crude oil by 13C NMR analysis. The high paraffinicity of the Utah-Grand County crude oil is also demonstrated in Figure 6, where it is observed that a doublet develops in the spectra around 720.7 cm-1 at the lowest test temperatures indicating the presence of orthorhombic crystals. The identification of the WPT for the Utah-Grand County crude oil is shown in Figure 7.
The weight percent precipitated solids versus temperature values as estimated by FT-IR analysis for the Utah-Grand County Oil are given in Figure 8; however, no comparative data is available. The spectra at 720 cm-1 obtained for the Gulf of Mexico crude oil at various test temperatures is given in Figure 9. The graphical indication of the WPT for the Gulf of Mexico crude oil is shown in Figure 10. The weight percent precipitated solids versus temperature as estimated by FT-IR analysis for the Gulf of Mexico oil is given in Figure 11. No comparative data for the Gulf of Mexico crude oil weight percent solid versus
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Figure 11. Estimated Gulf of Mexico Crude Oil weight percent solid versus temperature by FT-IR analysis. Table 4. Summary of ANS Crude Oil WPT Measurements WPT (°C) CP microscopy
FT-IR
DSC
viscometry
25
25
no data
19
temperature is available. However, it is observed that the 1 to 2 wt % solids content estimated to exist at room temperature in the Gulf of Mexico sample corresponds well with the observed gelling of the oil around room temperature (a solid content of 2 wt % has been observed to induce gel behavior in crude oil30). Experimental values for WPT for the ANS crude oil tested are summarized in Table 4. No values are provided for DSC measurement as it was not possible to identify a distinct exotherm due to the changing baseline of the calorimeter used for analysis. The values for WPT by CPM and FT-IR match within stated precision estimates. The value for WPT obtained by viscometry is 6 °C lower than the value obtained by CPM. This difference is slightly larger than typical results in other comparative studies.3,4 A possible explanation for this, is that it was observed by CPM and videomicroscopy that the wax formed in ANS crude oil is comprised of rounded (1-2 µm) crystals in comparison with larger platelet and needle-shaped crystals present in more waxy oils. Because of these rounded crystals, the ANS crude oil would be expected to require a higher solids content to affect sample viscosity. The FT-IR spectra obtained for the ANS crude oil are provided in Figure 12. It is important to note that the increased absorbance of the 720 cm-1 band attributed to rocking of long chain methylene groups only produces an enlarged shoulder on the peak without splitting into a doublet over the range of temperatures tested. This could indicate reduced crystallinity of the solid formed. The plot of FT-IR absorbance (in terms of peak area from 735 to 715 cm-1) versus temperature used to identify the WPT for ANS crude oil is shown in Figure 13. Results for experimental estimation of precipitated solid wax associated with decreasing crude oil sample temperature using centrifuge testing and FT-IR analysis were found to be complimentary for ANS crude oil as shown in Figure 14. Limitations in the cooling apparatus used to control FT-IR liquid cell temperature (30) Fogler, H.; Singh, P.; Nagarajan, N. Prediction of the Wax Content of the Incipient Wax-Oil Gel in a Pipeline: An Application of the Controlled-Stress Rheometer. J. Rheol. 1999, 43, 1437-1459.
Figure 12. ANS Crude Oil FT-IR spectra.
Figure 13. ANS Crude Oil FT-IR peak area (735 to 715 cm-1) versus temperature.
Figure 14. ANS Crude Oil weight percent solid versus temperature by centrifuge and FT-IR analyses.
produced a minimum FT-IR analysis temperature of approximately 0 °C. Collection of centrifuge data was limited by time, cost, and sample volumes to the three data points shown. These data were corrected to account for occluded oil using HTGC analysis of the centrifuge solid samples as described in the following section. The centrifuge data, which has been corrected for occluded oil brackets the FT-IR data as shown in Figure 14. The ANS crude oil was found to precipitate 0.2, 2.6, and 6.2 wt % solid wax at 21, -1, and -18 °C, respectively, when centrifuge data were corrected by HTGC analysis of centrifuge solids for occluded oil.27
Determination of Wax Precipitation Temperature
Conclusion Even with simplifying assumptions, the use of FT-IR spectroscopy for determination of WPT and estimation of weight percent solid wax versus temperature compares well with other experimental data for both a model oil, and petroleum crude oils. The use of FT-IR to determine WPT and estimate solid wax content versus temperature for petroleum crude oil systems offers many advantages over existing analytical methods, including the potential for in-situ applications and on-line monitoring of process streams. Nomenclature Aext. liq. Atotal ANS b C CPM DSC FT-IR
extrapolated liquid absorbance total absorbance Alaska North Slope FT-IR liquid cell path length (cm) constant, dimensionless cross polarized microscopy differential scanning calorimetery Fourier transform infrared spectroscopy
Energy & Fuels, Vol. 15, No. 3, 2001 763 HTGC I LCM PE RSD SD T WPT F ν
high-temperature gas chromatography intensity long chain methylene carbon (-CH2-)N, N > 4 polyethylene relative standard deviation standard deviation temperature wax precipitation temperature density wavenumber (cm-1)
Acknowledgment. The authors acknowledge the support of Alyeska Pipeline Service Company for this research, which was conducted as part of the Trans Alaska Pipeline SystemsCrude Oil Studies Project (TAPS-COS). Westport Technology Center International in Houston, Texas, performed the CPM, viscometry, and centrifuge analysis of the ANS crude oil. HTGC analyses of centrifuge solids were performed by the University of Utah, Energy & Geosciences Institute. EF010016Q
