Energy & Fuels 1991,5, 908-913
908
WPT
wax precipitation temperature
A a b Ea
Nomenclature constant in Arrhenius equation linear regression constant linear regression coefficient activation energy of viscous flow, J mol-'
R T Tg ll
P V
P
gas constant (=8.3144 J K-' mol-') temperature, K glass transition temperature, K apparent (shear rate dependent) viscosity, Pa s dynamic viscosity, Pa s kinematic viscosity, cSt mm2 s-l) mass density, kg m-3
Wax Precipitation from North Sea Crude Oils. 2. Solid-Phase Content as Function of Temperature Determined by Pulsed NMR Walther B. Pedersen,* Asger B. Hansen, Elfinn Larsen, and Anne B. Nielsen R i s National ~ Laboratory, DK-4000 Roskilde, Denmark
Hans Petter Rranningsen S T A T O I L as., Forus, N-4001 Stavanger, Norway Received April 29, 1991. Revised Manuscript Received September 9, 1991
A simple, low-resolution pulsed NMR method was applied for determining the amount of solid phase at different temperatures in 17 crude oils of different origins. Protons present in the oils were excited by a pulse of radio frequency radiation. After the pulse, the decay in magnetization was characterized by its signal amplitude obtained 10 and 70 ps after the pulse. The first signal was proportional to the number of protons in the solid and liquid phases, and the latter signal was proportional to the number of protons in the liquid phase. The NMR signals were calibrated against samples of polyethylene dispersed in a wax-free oil. For most of the oils investigated, except for some biodegraded and asphaltenic oils, the NMR estimated solid-phase content at -40 "C correlated well with the amount of wax determined by acetone precipitation a t -25 "C. This pulsed NMR method was demonstrated to be capable of determining the temperature dependence of the solid-phase content of crude oils during a single working day. Introduction Wax precipitation temperatures and the amount of wax precipitated at a given temperature are important characteristics of crude oils as discussed in part 1of this series of Experimental data on wax precipitation are normally limited to determining the cloud point temperature and total wax content of each oil. A detailed characterization of the wax content as function of temperature is very important for establishing thermodynamic models of wax formation in crude oils with good predictive capability. The correct measurement of the amount of solid wax in partially frozen oils, especially black oils, by filtration is not easy due to considerable contribution from entrapped The same problem applies to centrifugation. The objective of this work was by some alternative method to provide reasonably accurate solid content vs temperature data required for the estimation of various parameters in the thermodynamic model described in part 43 of this series of papers. The present paper describes the determination of the solid content of crude oils at different temperatures using a low-resolution pulsed nuclear magnetic resonance (NMR) technique. NMR is well documented as a powerful technique for chemical analysis, whereas its use for characterizing physical properties and distinguishing among different phases of crude oils is rare.5 However, pulsed NMR is
* To whom correspondence should be addressed.
Table I. Brief Description of Crude Oil Samples Studied in This Work oil no. descriution 1 biodegraded, aromatic oil 2 paraffinic oil 3 waxy oil 4 waxy condensate 5 waxy oil 6 waxy oil 7 paraffinic oil 8 paraffinic oil 9 waxy oil 10 light, paraffinic oil 11 heavy, biodegraded, naphthenic oil 12 paraffinic condensate 13 very light, paraffinic condensate 14 waxy oil 15 paraffinic oil 16 paraffinic, asphaltenic oil 17 paraffinic oil
used routinely in the food industry to determine the solid content in fats of different origins and develop new fat (1) Rsnningsen, H. P.; Bjsmdal, B.; Hansen, A. B.; Pedersen, W. B. Energy Fuels, this issue. (P).Hansen, A. B.; Larsen, E.; Pedersen, W. B.; Nielsen, A. B.; Rsnningsen, H. P. Energy Fuels, this issue. (3) Pedersen, K. S.; Skovborg, P.; Rsnningsen, H. P. Energy .. Fuels,
this issue. (4) Van Winkle, T. L.; Affens, W. A,; Beal, E. J.; Mushrush, G. W.; Hazlett, R. N.; DeCuzman, J. Fuel 1987, 66, 890-896.
0887-0624/91/2505-0908$02.50/00 1991 American Chemical Society
Wax Precipitation from North Sea Crude Oils mixtures with controlled melting The simplicity of the method, as used in the fat industry, as well as the moderate capital investment in low-resolution pulsed NMR equipment, prompted the investigation of a method for characterizing wax precipitation as a function of temperature in the range of +50 to -40 O C for 17 different North Sea crude oils. A short description of these oils is given in Table I. More detailed descriptions are given in Table I1 of part 1' in this series of papers. Further details with regard to applications of the obtained NMR data in combination with differential scanning calorimetry and for thermodynamic modeling of wax precipitation are given in parts 32 and 43, respectively. In the proton pulsed NMR method, the protons in all phases are exited by a pulse of radio frequency radiation polarized perpendicular to the static magnetic field. After the pulse, the 'HNMR relaxation signal is observed. The signal decay is a superposition of components arising from nuclei in different phases. The initial decay is dominated by protons present in the solid state, whereas the signal amplitude at a later time is proportional to the number of protons in the liquid phase. A quantitative determination of the % solid can be obtained from eq 1, where
S1 is the signal obtained 10 ps after the 90' pulse and S2 the signal obtained 70 ps after the pulse. S2is proportional to the number of protons in the liquid state and F(S1- S2) to the number of protons in the solid state, where F is a correction factor used tn compensate for the decay taking place during the rf pulse and the dead time of the instrument. The F factor is estimated by measuring standard samples with known solid contents. As discussed in the paper, the method has some obvious shortcomings which are presumably related to the variations in composition, crystallinity, etc. among waxes of different origin and with temperature, which in turn give rise to a spectrum of relaxation times. Nevertheless, by careful calibration with appropriate standards, the method is capable of providing quite reasonable estimates of the solid-phase content a t different temperatures.
Experimental Section Measurements were performed with a Bruker Minispec p20i pulsed NMR instrument operating at 20 MHz. During operation the sample compartment temperature was kept constant at 34 "C. The correction factor F was set on the instrument (see below) at a value of 1.45, and for all measurements, the times for measuring the signal amplitudes SIand S2were fixed at 10 and 70 ps, respectively. Crude Oil Measurements. Oil samples to be measured were initially heated to 80 "C for 1 h in order to dissolve any previously formed wax. After the oil samples were cooled to room temperature and shaken thoroughly, representative samples were taken out and poured into NMR glass tubes (dimensions 18 X 0.85 cm i.d.) to a height of at least 3 cm, i.e., ca. 2 mL. For solid content measurements, tubes containing crude oil samples were sealed permanently, placed in a thermostated bath, and equilibrated normally for 24 h. Immediately before the NMR measurement, the tube was quickly removed from the thermostated bath, wiped off, and placed in the NMR sample compartment. (5) Aranjo, M.; Hunger, M.; Martin, R. Fuel 1989, 68, 107+1081. (6) IUPAC Method 2.323. Solid content determination in fats by NMR. &re Appl. Chem. 1982,54, 2766-2774. (7) Van den Enden, J. C.; Haighton, A. J.; van Putte, K.; Vermaas, L. F.; Waddington, D. Fette Seifen Anstrichm. 1978,80, 180-186. (8) Shukla, V. K. S . Fette Seifen Anstrichm. 1983,85, 467-471.
Energy & Fuels, Vol. 5, No. 6, 1991 909 Two to five measurements, each taking about 10 s, were performed
successively on each sample. After the measurements, the tube was replaced in the thermostated bath. The temperature of the bath was decreased in steps of 5 O C in the temperature range of +50 to -40 O C . Temperaturechanges taking place in a sample during simulated NMR measurements were monitored by placing a thermocouple as a probe in the middle of an open NMR tube containing oil 5. The tube containing the thermocouple was equilibrated as described above and placed in the sample compartment where its temperature was monitored as a function of time; time zero corresponded to the time when the tube was removed from the bath. Tubes equilibrated at nine different initial temperatures in the range of +27.6 to -50 "C were measured. To study the effect of cooling rate, NMR measurements also were carried out on samples not maintained for 24 h at constant temperature but subjected to a controlled mling rate of 12.5 "C/h in the thermostated bath. In this experiment, sample tubes also were removed from the bath and measured at approximately 5 " C intervals as described above. Calibration. Standard samples with known amounts of solids were used for calibration of the NMR measurements. As acetone precipitation of total wax from oil 13 revealed none or only negligible wax content, this oil was used as solvent for preparing standard samples. Low molecular weight polyethylenes (PE) were chosen as reference materials as they are linear paraffin waxes that presumably behave chemically quite similar to wax precipitated from crude oils. Dispersions of polyethylenes with mean molecular weights of 2000 (PE 2000), lo00 (PE 1000),and 700 (PE 700) were prepared at 80 O C in oil 13 in the concentration range of 0.5-20%; pure (Le. 100%) PE 700 and PE lo00 were also included to obtain the 100% fix point. The solid contents of standard samples were measured at room temperature and the correction factor F was adjusted until the measured values showed a good correlation with the nominal values; in this way a correction factor of 1.45 was obtained. Furthermore, by measurement of polyethylene samples along with the oil samples at all temperatures, the temperature dependency of F could be compensated for by correcting the crude oil results according to the solid content of the standard samples. Samples containing total wax obtained by acetone precipitation from the waxy oils 5, 6, 9, and 14 were included to check the validity of the PE calibration procedure as the behavior of polyethylenes as model compounds might deviate from that of "real" wax. The isolation of total wax from crude oils by acetone precipitation is described in part 1' in this series of papers. In order to examine the effect of morphology and chemical structure of wax, two samples containing mainly crystalline or amorphous wax fractions, respectively, were redissolved in oil 13 and characterized by pulsed NMR. The two samples were obtained by fractionation of total wax from a waxy condensate, oil 4,by distillation.' The crystalline was fraction contained predominantly n-alkanes (C15-C3,,) while the amorphous fraction contained predominantly isoparaffins and naphthenes (C,+).
Results and Discussion Reproducibility of the Measurement of Solids Content. Preliminary investigations of the reproducibility of the pulsed NMR measurements were carried out with oils 3,5,6,9,14, and 15. Five independently prepared and sealed NMR tubes of each oil were measured a t different temperatures. No difference was observed between pulsed NMR values obtained during decreasing and increasing temperatures. Table I1 shows the mean values and the standard deviations of the measurements for oil 5 at five different temperatures. Similar results were obtained with the other five oils. Generally, for all six oils at all temperatures, the absolute standard deviation of the NMR measurements for the five individual tubes was approximately 0.2%, which was the same for the individual measurements as for the measurements on different tubes. This means that the relative standard deviation was about 5% at the level of 4 % solid and about 20% at the level of 1% solid. The sample temperatures during simulated
Pedersen et al.
910 Energy & Fuels, Vol. 5, No. 6, 1991 40
- 1 II 0
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++
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Figure 3. Pulsed NMR measurements of oils 1-4. Mean values, corrected. (+) Oil 1; (+) oil 2; (0)oil 3; (0) oil 4.
z a 9
o !
-10 Temp. ' C
Figure 1. Temperature of an oil sample (oil 5) plotted versus time during simulated pulsed NMR measurements. Nine initial temperatures.
12-1
-30
0
-40 - 2 0
Figure 2. Pulsed NMR measurements of standard samples. (a) PE-700: 2.04% ('3) and 10.2% (*) PE-700 dispersed in oil 13. (b) PE-1000: 2.14% (W) and 10.7% (+) PE-1000 dispersed in oil 13. (c) PE-2000: 2.06% ('3) and 9.29% (+) PE-2000 dispersed in oil 13.
NMR measurements over a period of 180 s are shown in Figure 1 for samples with nine different initial equilibrium temperatures. A significant temperature variation was
-30
-10 10 Temp.'C
30
50
Figure 4. Pulsed NMR measurements of oils 5-8. Mean values, corrected. (+) Oil 5; (+) oil 6; ( 0 )oil 7; ( 0 )oil 8. observed only for initial sample temperatures below room temperature. Therefore, only two measurements could be performed on each tube at "constant" temperature below room temperature whereas up to five measurements could be carried out above. Calibration of the NMR Signal for Quantitative Determinations of 5% Solid. The determinations of 9% solids in the crude oils at different temperatures were based upon calibration curves obtained for polyethylenes dispersed in oil 13. The correlations for 2% and 10% PE samples are shown in Figure 2a-c. A good correlation was observed between 5% PE and the % solid obtained by NMR for all PE samples in the temperature range of +30 to -10 "C. A deviation from the total amount was observed in the NMR % solid content at higher and lower temperatures. At higher temperatures some degree of melting of PE 700 and P E lo00 apparently takes place. Therefore, P E 2000 was used as standard above room temperature. A t lower temperatures, all P E standards showed some decrease in the NMR % solid content; this probably was due to changes in the 'HNMR relaxation behaviors of the solid and liquid components. PE 700 and P E 1000, presumably showing a greater similarity than P E 2000 to crystalline wax with respect to both chemical structure and molecular weight, were chosen as standards for measurements below room temperature. Measurements on Crude Oils. The results of the pulsed NMR measurements of all the crude oils studied, except oil 13, are shown in Figures 3, 4, 5, and 6. The results are corrected in accordance with the calibration curves obtained with the standard P E samples. For oil 13 the noncorrected results are shown in Figure 7. This particular oil is supposed to be free of wax in the temperature range investigated, an assumption which is in good agreement with the noncorrected NMR data. No systematic errors were observed during the measurements; hence, mean values of several independent series of NMR measurements were calculated and used for comparing the
Energy & Fuels, Vol. 5, No. 6,1991 911
Wax Precipitation from North Sea Crude Oils Table 11. Reproducibility Testn temp, "C ___ 34
NMR ~~
20
4
-10
-28
tube no, 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 6
n
5
5
2
2
2
meanl, % 0.86 0.92 0.90 0.90 0.92 3.72 3.82 3.72 3.78 3.56 8.25 8.20 8.35 8.05 7.80 9.50 10.40 9.85 9.85 9.55 9.00 9.30 9.05 9.25 8.65
81, % I
0.23 0.22 0.26 0.22 0.16 0.26 0.15 0.08 0.24 0.11
meanz,
sz,
%
%
0.90
0.02
+
12 '-/
++
0
I
e
3.72
0.10 -50
8.13
0.21
9.83
0.36
9.05
0.26
-30
30
-10 10 Temp. C '
50
Figure 5. Pulsed NMR measurementa of oils 9-12, Mean values, corrected. (+) Oil 9; (e)oil 10; (0) oil 11; (0)oil 12.
Five independently prepared and sealed tubes containing oil 5. n, number of consecutive measurements performed on each tube. meanl, average of n measurements on each tube; sl, absolute standard deviation in the individual measurements on one single tube; meanz, average of the five mean values; s2,absolute standard deviation in the mean values. a
NMR values are noncorrected weight percent solid phase.
wax content of the oils studied (Table 111). Generally, all oils except the highly biodegraded oil 11 showed the same pattern with a plateau in the temperature range of +5 to -15 "C. Below -15 "C, wax precipitation increases strongly. For oil 11, values are given only down to -20 "C; below this temperature, the pulsed NMR data show a very steep increase in the 9% solid; these high 9% solid values were omitted as no calibration was performed above 119% solid. Measurements on W a x Dissolved in Oil 13. The total amounts of wax obtained by acetone precipitation from the waxy oils 5 , 6,9, and 14 were redissolved in oil 13 and measured by pulsed NMR. The NMR measurements were corrected according to the PE standards. In Table IV only the results for wax from oil 5 are given as they were typical for the other wax samples. It was necessary to cool the samples below -30 "C to obtain complete temp, O C 45 40 35 30 25 20 15 10
5 0
-5 -10 -15 -20 -25 -30 -35 -40 (I
oil 1 0 0.3 0.1 0.3 0.4 0.6 0.6 0.5 0.9 1.2 1.1 1.2 1.2
2.0 1.9 3.4 4.3 6.2
oil 2 0.1 0.3 0.1 0.2 0.2 0.4 0.5 1.4 1.7 2.2 1.9 2.2 2.1 3.0 2.7 4.5 5.0 7.0
oil 3 0 0.6 0.2 0.5 0.9 2.3 2.8 3.9 3.8 4.1 3.9 3.9 3.7 4.5 4.5 7.3 7.0 10.5
oil 4 0 0.2 0 0.4 0.5 0.9 1.3 2.1 3.2 4.4 4.8 4.6 3.9 4.8 3.9 5.8 6.0 8.5
Table 111. Corrected oil 5 oil 6 oil 7 0 0.3 0.2 0.4 0.5 0.4 0.1 0.4 0.1 1.1 0.5 0.4 0.9 0.6 2.1 2.3 1.7 4.4 1.8 5.9 3.2 8.2 4.9 2.5 4.9 2.4 8.6 9.8 5.5 2.8 2.6 10.0 5.7 10.4 5.8 2.8 9.9 5.8 2.8 11.8 6.9 3.6 10.6 6.6 3.5 13.1 9.5 5.7 13.2 8.8 5.9 14.6 12.5 8.3
Mean values in weight percent solid phase.
-30
-50
30
-10 10 Temp. "C
50
Figure 6. Pulsed NMR measurements of oils 14-17. Mean
oil 17. values, corrected. (+) Oil 14; (e)oil 15; ( 0 )oil 16; (0)
,
15
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,
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.
30
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Figure 7. Pulsed NMR measurements of oil 13. Noncorrected values.
precipitation of the wax, which is not surprising since the acetone precipitation had been performed a t -25 O C . l An
Pulsed NMR oil 8 oil 9 0.1 0.3 0.3 0.5 0.3 0.1 0.2 0.4 0.4 0.7 0.9 1.7 2.2 1.4 2.1 3.0 2.8 3.2 2.9 3.7 2.7 3.8 2.7 4.0 2.7 4.1 3.5 5.1 3.4 5.0 5.3 7.8 5.7 7.8 8.0 11.6
Measurementsn oil 10 oil 11 oil 12 0 0 0.8 0.1 0.3 0.8 0.2 0.5 0.1 0 0.3 0.9 1.0 0.1 0.5 0.2 0.9 1.7 0.5 1.6 1.8 0.6 2.8 2.2 1.5 3.5 2.4 3.1 2.2 4.6 3.7 2.6 4.0 3.7 4.9 2.5 3.1 6.9 2.3 3.8 12.7 3.0 3.4 2.7 5.1 4.1 5.3 4.8 7.1 5.9
oil 14 0.1 0.1 0.1 0.2 0.2 1.0 2.2 3.8 4.2 4.6 4.5 4.4 4.3 5.1 4.8 7.3 7.1 10.4
oil 15 0.1 0.3 0 0.1 0.1 0.1 0.3 0.8 1.3 2.3 2.4 2.6 2.3 2.7 2.3 3.8 4.3 5.4
oil 16 0.1 0.5 0.4 0.6 0.9 1.5 2.3 2.9 3.3 3.5 3.8 4.0 4.1 4.9 5.1 7.3 7.3 10.5
oil 17 0.9 1.1 0.9 1.1 1.3 1.8 2.2 2.9 3.0 3.4 3.6 3.9 4.2 5.3 5.6 8.5 8.5 12.7
Pedersen et al.
912 Energy & Fuels, Vol. 5, No. 6,1991 Table IV. Corrected Pulsed NMR Measurements (wt % Solid Phase) of Wax" from Oil 5 Dissolved in Oil 13 at Five Different Concentrations (wt 70) waxo in oil 13 temD. "C 0.570% 1.12% 2.03% 5.46% 9.88% 0.3 0 0.2 0 40 0 0.3 0.1 0 0.3 0 30 0 0 0 0.2 0 20 2.5 0 0 0 0.2 10 5.7 0 0.1 0.5 2.3 0.2 4.4 7.6 0.6 1.4 -10 0.7 9.4 0.5 1.o 1.9 5.3 -20 8.4 1.7 2.8 4.9 -30 0.5 2.1 9.0 2.4 6.0 -40 0.4 a
Wax that was obtained by acetone precipitation at -25 "C.
Table V. Crystalline and Amorphous Wax from Oil 4 Dissolved in Oil 13: Corrected Pulsed NMR Measurements (wt % Solid Phase) crystalline amorphous temp, O C wax, 9.7% wax, 10.1% 47 0 0 0 0 39 34 0 0 0 30 0.6 0 25 1.2 19 0.2 1.9 0 2.4 15 0.6 2.7 10 2.6 3.5 4 3.5 0 3.4 5.3 -6 3.9 -11 6.2 4.3 7.1 -17 4.6 7.8 -20 4.8 -25 7.5 3.8 -30 7.3 3.5 5.9 -33 3.2 -41 4.9 3.3
acceptable agreement between the nominal and measured values of 70solid was observed. Thus, the correction obtained from the PE calibration measurements seemed to give reasonable corrections for "real" crude oils. Only about 80% and 5070,respectively, of the crystalline and amorphous wax fractions obtained from oil 4 and redissolved in oil 13 were detectable by NMR on cooling below -30 "C (Table V). This single experiment could indicate that the PE calibration standards may only be of marginal value for highly amorphous wax fractions. However, as the wax was initially isolated by acetone precipitation, a smaller amount of solids found by redissolution in a hydrocarbon mixture, e.g., oil 13, is to be expected. It can be noted in Table V that the precipitation of the crystalline wax fraction appeared to be delayed considerably (about 20 "C) compared with the amorphous wax residue (C30+). Obviously, this is due in part to the different boiling point ranges of the fractions. In accordance with the wax composition reported for oil 4,' it was indicated that the first material precipitating from an oil would be dominated by isoparaffins and naphthenes, which constitute a major part of the wax residue. Comparison of Total Wax Content Determined by Acetone Precipitation and Pulsed NMR Measurements. Table VI summarizes the experimental determinations of the solid-phase content for all crude oils, except oil 13, as found by acetone precipitation (cf. part 1,Table 11)' and by pulsed NMR measurements. For the majority of the oils a satisfactory agreement is observed. For oils 1,11, and 16, however, substantial differences between the two experimental methods are noticed. Two of these three oils, 1 and 11, were biodegraded, whereas oil 16 was an
Table VI. Total Wax Determined by Acetone Precipitation at -25 "C and by Pulsed NMR Measurements at -40 "C acetone precipitated, pulsed NMR oil no. wt % (cor), wt % 1 2.2 6.2 7.0 2 6.7 10.5 14.3 3 4 8.5 7.5 14.6 5 15.6 14.8 12.5 6 7 8.3 8.0 8 8.0 7.0 11.6 14.1 9 7.1 10 7.0 12.7" 11 2.7 12 5.9 6.3 13 0.0 14 10.4 11.1 5.4 4.7 15 10.5 16 18.3 11.6 12.7 17 (I
Determined at -20 "C. -" 1
.60
-40
-20
0 Temp.'%
20
40
60
F i g u r e 8. Noncorrected NMR signals of oil 5 plotted versus temperature: (H) in thermostated equilibrium (24 h); (X) during temperature changes (12.5 "C/h).
asphaltenic oil. The higher solid content obtained with acetone precipitation for oil 16 was probably caused by extensive coprecipitation of asphaltenes. The higher solid content obtained by pulsed NMR for oils 1and 11is likely to be related to the pronounced aromatic/naphthenic character of these oils relative to the rest of the oils. This may reduce the efficacy of the acetone precipitation and influence the relaxation behavior of the samples and hence preclude a correct determination of solids by measuring signals amplitudes at the two fixed times after the rf pulse. The NMR instrument used did not directly allow a determination of relaxation behavior as function of temperature. Hence, a t present, we have no satisfactory explanation of the special behavior of these two oils. It should be emphasized, however, that these low-wax oils normally are the least interesting from a wax deposition point of view. It could also be mentioned that the pulsed NMR method gave reasonable agreement with fitration experiments with oil 4 described in part 1.' At 23 and 6 "C filtration gave 1.5% and 4.5%,respectively, while NMR measurements gave about 0.8% and 3.0%. The somewhat lower NMR values can probably be explained by some contribution from entrapped oil in the filtration experiments as pointed out in the introduction. Effect of Cooling Rate. The experimental data for oil 5 shown in Figure 4 were obtained after a long equilibration time (ca. 24 h) at each temperature. About 1 week of experiments was required for the determination of the solid content in this broad temperature range. A shorter/faster analysis time is possible only by increasing the
Wax Precipitation from North Sea Crude Oils cooling rate of the NMR tube. Figure 8 shows the % solid in oil 5 determined after long equilibration time a t each temperature (m) and in an experiment where the temperature of the NMR tube was changed a t a rate of 12.5 OC/h ( X ) . Within experimental uncertainty, the same solid content was obtained in the two experiments for a given temperature. Therefore, the pulsed NMR method typically allows the determination of the solid content in crude oil samples during a single working day. Summary a n d Conclusions Pulsed NMR measurements in combination with a calibration procedure using crystalline polyethylene in a very light paraffinic condensate turned out to be a simple and fast method that provided reasonable estimates of the solid-phase content in most crude oils of different origins. During a normal working day, it is possible to determine the solid-phase content as a function of temperature for several oils over a temperature range of 80 “C. In order to achieve a better characterization of the solid/liquid phase behavior by pulsed NMR, a better understanding of how the relaxations times of the liquid and solid components depend on temperature as well as chemical structure and the degree of crystallinity of the solid phase is needed. This knowledge might explain the major discrepancies observed between solids content for a few oils, particularly oils 1 and 11, as determined by different methods. This would probably require experiments using more advanced (and expensive) instrumentation. However, the present NMR method has given reasonable estimates of solid wax content as a function of temperature,
Energy & Fuels, Vol. 5, No. 6, 1991 913 results that were needed in combination with differential scanning calorimetry measurements2 for the development of an improved thermodynamic model for wax precipitationa3 Acknowledgment. We are grateful to the STATOILGroup for financing this study and for granting permission to publish this paper. The STATOIL-Group consists of STATOIL Efterforskning og Produktion A/S, BHP Petroleum (Denmark), Inc., Total Marine Danmark, LD Energi A/S, EAC Energy A/S, DENERCO K/S, and Dansk Olie og Gas Produktion A/S (DOPAS). We also thank STATOIL as., Norway, for providing the crude oil samples for the study. Karen Schou Pedersen and Per Skovborg, CALSEP A/S, as participants of the project, are greatly acknowledged for valuable discussions and suggestions. Finally, the reviewers’ advices and suggestions are warmly welcomed.
NMR PE rf
C“
cn+
F
SI
SZ
% PE % solid
Abbreviations nuclear magnetic resonance polyethylene radio frequency Nomenclature carbon number carbon number “plus fraction” pulsed NMR correction factor decay signal after 10 p s decay signal after 70 ps weight percent polyethylene solid weight percent
