Energy Fuels 2009, 23, 4988–4994 Published on Web 09/03/2009
: DOI:10.1021/ef900173b
Measurement of Wax Appearance Temperature Using Near-Infrared (NIR) Scattering K. Paso,*,† H. Kallevik,‡ and J. Sj€ oblom† †
Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway, and ‡StatoilHydro ASA, Rotvoll, N-7005 Trondheim, Norway Received February 27, 2009. Revised Manuscript Received July 29, 2009
A near-infrared (NIR) scattering technique is used to measure the wax appearance temperature of several petroleum fluids under nonquiescent conditions. Within the Rayleigh scattering limit, NIR attenuation measurements at a wavelength of 1100 nm can theoretically detect wax crystallites 0.05), the wavelength dependence of the scattering cross section is reduced. The total particle cross section is related to radiation (21) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: London, 1969. (22) Aske, N.; Kallevik, H.; Johnsen, E. E.; Sjoblom, J. Energy Fuels 2002, 16, 1287–1295.
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Energy Fuels 2009, 23, 4988–4994
: DOI:10.1021/ef900173b
Paso et al.
Linkam LTS120 temperature control stage, which was mounted onto the microscope. Fluid samples were first heated to ∼20-30 °C above the wax precipitation temperature to completely dissolve the wax crystals. Cross-polarized observation was subsequently performed while cooling at 0.2-4 °C/ min. Wax precipitation temperatures were recorded when the first paraffin wax crystals were identified at a magnification of 10 or 20. NIR. Near-infrared (NIR) spectra were recorded using a Bruker Optik MPA FT-NIR spectrometer that was equipped with a fiber-optic liquid transflectance probe and a TE-InGaAs detector. The fluid gap of the probe is 1 mm, for a total optical path length of 2 mm. The combination of relatively low fluid viscosities and a probe path length of 1 mm provides adequate fluid flowability through the probe during the nonquiescent measurements. Identical spectrometer settings were used for all fluids, with a recorded wavelength range of 780-2500 nm and a spectral resolution of 0.3 nm. Thirty two scans were recorded for each spectrum. Because an appropriate wax-free reference fluid was unavailable for the crude oils and gas condensate fluid, background scanning was performed in air for all fluid measurements. During measurement, the NIR measuring probe was inserted into a continuously stirred fluid sample that was contained within a jacketed vessel in fluid communication with a circulating controlled-temperature water bath. WAT measurements were performed by first heating the fluid sample to ∼20-30 °C above the WAT to dissolve the wax crystals completely. After melting, the fluid was cooled to a specified temperature above the WAT, and NIR spectra were recorded at sequential decreasing temperature intervals of ∼1 °C or 2 °C. At each interval step, the temperature was first allowed to equilibrate (a process that required ∼13 min), and subsequently the fluid was maintained isothermally for 5 or 10 min before a spectrum was recorded. Spectrum measurement continued at sequential decreasing temperature intervals until a significant and continuous rise was evident in the optical density baseline. Melting of paraffin wax solids was followed using a similar NIR protocol. A fluid sample was maintained in the controlledtemperature jacketed vessel under continuous stirring conditions. Subsequently, an aliquot of cold waxy fluid was added to the solution under continued stirring. NIR attenuation spectra were recorded at 30 or 60 s intervals following the addition of the cold waxy fluid. Temperature measurements of the fluid mixture were recorded at identical intervals.
Figure 1. Wax precipitation temperatures for the waxy gas condensate fluid, as observed by cross-polarized microscopy (CPM). The extrapolated WAT value is 41.0 °C. Table 2. Wax Precipitation Temperatures Observed by Cross-Polarized Microscopy (CPM) for Oil A and Oil B at a Cooling Rate of 0.5 °C/min oil Oil A Oil B
wax precipitation temperatures (°C) 33.5, 35.5, 35.0, 35.5, 33.0 32.5, 33.5, 31.5, 34.5, 35.0, 34.0
Results and Discussion Microscopy. Figure 1 shows average wax precipitation temperatures measured by CPM for the waxy gas condensate fluid at cooling rates in the range of 0.2-4 °C/min. The observed wax precipitation points are strongly dependent on cooling rate, demonstrating the effects of nucleation and crystallization kinetics.20 At cooling rates of
