Energy & Fuels 1991,5,895-908 tually be generated during thermal treatment whilst the a,@-isomercan decrease in relative concentration. Such changes would influence P,B/a,P-homohopane maturity ratios if they were to occur in source rock kerogen formation or analysis.
895
Acknowledgment. M.L. acknowledges the award of a University of Melbourne Postgraduate Scholarship and P.W. study leave from the Jianghan Petroleum Institute, Shashi, P. R. China. We thank the referees for helpful comments.
Wax Precipitation from North Sea Crude Oils. 1. Crystallization and Dissolution Temperatures, and Newtonian and Non-Newtonian Flow Properties Hans Petter Ranningsen* and Brit Bjamdal STATOIL as., Production Laboratories, Forus, N-4001 Stauanger, Norway
Asger Baltzer Hansen and Walther Batsberg Pedersen R i s National ~ Laboratory, DK-4000 Roskilde, Denmark Received March 28, 1991. Revised Manuscript Receiued June 11, 1991 ~
This paper is the first in a series which summarizes experimental work on wax precipitation in North Sea crude oils, leading to an improved thermodynamic model for prediction of wax formation. Seventeen crude oils and condensates were characterized analytically and rheologically with emphasis on properties related to the content of wax. In particular, three different methods for determination of wax precipitation temperature (WPT), namely polarization microscopy, differential scanning calorimetry (DSC), and viscometry, are discussed and results obtained by the three methods are compared. Microscopy invariably gave the highest WPTs and probably the most relevant values for predicting the onset of wax deposition on cold surfaces. The WPTs from microscopy were found to depend on factors such as thickness of the sample film and cooling rate. DSC and viscometry are likely to underestimate the onset temperature of initial wax deposition. Activation energies of viscous flow in the Newtonian temperature range (usually above 30-35 "C), calculated by fitting viscosity data to a simple Arrhenius type exponential equation, are presented and found to be closely correlated with type of petroleum fluid. Light condensates had activation energies approaching those of pure n-alkanes (110kJ mol-') while a highly biodegraded heavy oil had an activation energy of nearly 40 kJ mol-'. Finally, compositional analyses of wax precipitated from one of the oils indicated that wax crystallizing just below the WPT was richer in condensed naphthenes and poorer in isoalkanes than wax formed at lower temperatures. Isoalkanes appeared to be the most abundant class at all temperatures.
Introduction The majority of crude oils and crude oil products contain substantial amounts of petroleum wax. Depending on the kind of oil, the composition of the wax may range from predominantly low molecular weight n-alkanes (C20-C40) to high proportions of high molecular weight isoalkanes and cyclic alkanes. The former type of wax is termed paraffin or distillate wax and generally crystallizes as large needles and plates while the latter type is termed microcrystalline or amorphous wax. The precipitation of wax from petroleum fluids may give rise to a variety of problems well-known within the petroleum industry. Numerous papers have addressed problems associated with deposition of wax in pipelines and production equipment%' as well as rheological behavior,&16which is largely de(1) Edwards, R. T. Ind. Eng. Chem. 1957,49(4), 750-757. (2) McClaflin, C. G.; Whitfill, D. L. SPE Paper 1983, No. 12204. (3) Bott, T. R.; Gudmundsaon, J. S. Inst. Pet. Tech. 1977, Tech. Paper
-- . . -.
TP-77-1-177 ..
(4) Bott, T. R.; Gudmundsson, J. S. Can. J. Chem. Eng. 1977, 55, 381-385.
( 5 ) Agrawal, K. M.; Khan, H. U.; Surianarayanan, M.; Joshi, G.C. Fuel
1990,69, 794-796. (6) Bern, P. A.; Withers, V. R.; Cairns, J. R. Proc. Eur. Offshore Pet. Conf.,London 1980,571-578. (7) Carnahan, N. F. J. Pet. Technol. 1989, Oct., 1024-1025. (8) Sifferman, T. R. J. Pet. Technol. 1979, Aug., 1042-1050.
0887-0624/91/2505-0895$02.50/0
pendent on the waxy constituents of the oils. Fundamental '-~ issues related to the wax crystallization p r o ~ e s s , ~crystal m0rphology,'7~~and physical properties of petroleum wax25i26have been studied in detail. A parameter of (9) Barrv. E. G. J. Inst. Pet. 1971.57(554). 74-85. (10) Agrkwal, K. M.; Purohit, R. C.';S&ian&ayanan, M.; Joshi, G. C.; Krishna, R. Fuel 1989,68,937-939. (11)Economides, M. J.; Chaney, G.T. SOC.Pet. Eng. J. 1983, June, 408-416. (12) Seitzer, W. H.; Lovell, P. F. hoc. Oil Shale Symp., Colorado School of Mines 1979, 213-220. (13) Irani, C. A.; Zajac, J. J. Pet. Technol. 1982, Feb., 289-298. (14) Perkins, T. K.; Turner, J. B. J. Pet. Technol. 1971, March; 301-308. - - - - - -. (15) Smith, P. B.; Ramsden, R. M. J. Proc. Eur. Offshore Pet. Conf., London 1978,283-290. (16) Van Enaelen, G. P.: Kaul, C. L.: Vos. B.: Aranha. H. P. Proc. Offshore TechGl. Conf., Houston 1979,'138$1390. (17) Pudgett, F. W.; Hefley, D. G.; Henriksen, A. Ind. Eng. Chem. 1926,18, 832-835. (18) Koolvoort, E. C. H.; Moser, F. R.; Verver, C. G. J.Pet. Technol. 1937,23, 734-745. (19) Holder, G . A.; Winkler, J. J. Inst. Pet. 1965, 51(499), 228-252. (20) Brownawell, D. W.; Hollyday, W. C. J. Inst. Pet. 1962, 48(463), 209-216. (21) Ackroyd, G. C.; Cawley, C. M. J. Inst. Pet. 1953, 39, 82-111. (22) Affens, W. A.; Hall, J. M.; Holt, S.;Hazlett, R. N. Fuel 1984,63, 543-547. (23) Halter, E.; Kanel, J.; Schruber, D. Liq. Fuel Technol. 1985,3(4), 477-487. (24) Ferris, S.W.; Cowles, H. C. Ind. Eng. Chem. 1945,37,1054-1062.
0 1991 American Chemical Society
896 Energy & Fuels, Vol. 5, No. 6,1991
particular practical importance is the wax appearance or precipitation temperature (WPT), traditionally known as the cloud point, at which visible crystallizationcommences, and below which deposition problems are likely to arise. Furthermore, the amount of wax suspended in the hydrocarbon solvent matrix at any given temperature determines the potential for wax deposition, and therefore is essential input to models for prediction of amounts actually deposited on cold surfaces, such as pipeline walls. A number of thermodynamic models for predicting the onset of crystallization as well as equilibrium amounts of wax precipitated, based on either regular solution theory27128or polymer solution theory,29have been proposed. The main common deficiency of the models appears to be their inability to predict the wax amounts correctly, which rests, at least in part, on the lack of appropriate experimental data for tuning of the models. Therefore, a research program was initiated with the following main objectives: (1)provide experimental wax precipitation data (i.e., precipitation temperatures and amount precipitated as a function of temperature), (2) provide experimental enthalpy data for the liquid-solid transition associated with wax formation, and (3) incorporate the experimental results into an improved thermodynamic model. The present paper is the first in a series of papers summarizing the experimental work and the model development. Two of these, by Hansen et alewand Pedersen et al.,31respectively, will describe the differential scanning calorimetry (DSC)and pulsed NMR techniques applied on a series of North Sea crude oils, and one paper by Pedersen et al.32 will describe in detail the thermodynamic model. The present paper covers the following main topics: (1) chemical and physical characterization of the oils investigated, (2) Newtonian and non-Newtonian rheological properties, (3) the practical use of polarization microscopy, DSC,and viscometry for determination of wax precipitation temperatures, (4) measurement of wax dissolution temperatures, and ( 5 ) comparison of results obtained by the different methods for determination of wax crystallization and dissolution temperatures.
Experimental Section F l u i d s Investigated. Seventeen North Sea crude oils and condensates from various fields on the Norwegian continental shelf were used in this investigation. They covered a wide range of fluid compositions and physical properties including "normal" paraffinic oils, heavy biodegraded oils, waxy oils, light paraffinic condensates, and waxy condensates. For simplicity, the common designation "oil" will be used for all fluids throughout the text. A brief description of the oils investigated, is given in Table I. Oil no. 13 was in fact a very light condensate, containing a negligible amount of wax. The oils were either stock tank oil stored in Jerry or Salzkottner cans or oil obtained by flashing the pressurized separator oil to ambient conditions. These are referred to as "dead" or stabilized oils, meaning that the content of dissolved gas is only that soluble at ambient pressure and temperature. Some of the oils had been stored for several years and may have been altered by aging processes. This did, however, not affect the general conclusions drawn from the investigation. All cans and bottles were heated to 40 "C and shaken thoroughly before (25) Mazee, W. M. J. Inst. Pet. 1958, 44(419), 401-405. (26) Templin, P. R. Ind. Eng. Chem. 1956,48, 154-161. (27) Won, K. W. Fluid Phase Equilib. 1989,53, 377-396. (28) Weingarten, J. S.; Euchner, J. A. SPE Paper 1986, No. 15654. (29) Hansen, J. H.; Fredenslund, Aa.; Pedersen, K. S.; Rsnningsen, H. P. AIChe J. 1988, 34(12), 1937-1942.
(30) Hansen, A. B.; Pedersen, W. B.; Nielsen, A. B.; Larsen, E.; Rsnningsen, H. P. Energy Fuels, this issue. (31) Pedersen, W. B.; Hansen, A. B.; Nielsen, A. B.; Larsen, E.; Rsnningsen, H. P. Energy Fuels, this issue. (32) Pedersen, K. S.; Skovborg, P.; Rsnningsen, H. P. Energy Fuels, this issue.
Rsnningsen et al. Table I. Brief Description of Oil Samples Used in This Investigation oil no. oil characterization 1 biodegraded, aromatic oil paraffinic oil" 2 waxy oil 3 4 waxy condensate waxy oil 5 waxy oil 6 paraffinic oil" 7 paraffinic oil" 8 waxy oil 9 light, paraffinic oil" 10 heavy, biodegraded, naphthenic oil 11 paraffinic condensate" 12 very light, paraffinic condensate 13 waxy oil 14 paraffinic oila 15 paraffinic, asphaltenic oil 16 paraffinic oil" 17 This is the kind of oil which sometimes in the text is referred to as 'normal" oils. the oil was transferred to gas-tight 1-L stainless steel pretreatment cells. In the cells, they were heated to 80 "C for 2 h (termed beneficiation) to erase any memory of previous wax formation, and cooled at a rate of 9 "C/h to 30 or 35 "C. At this temperature, samples for the different analyses were taken from the cell and transferred to the actual instrument. Samples which were not given a thermal pretreatment (i.e., were only heated to 40 "C) are called "as received" oils. The detailed compositions of the oils investigated will be reported together with the thermodynamic model in a separate work.= Table I1 presents some of the essential bulk properties only. Microscopy. A drop of thermally pretreated oil (Le., heated to 80 "C and cooled to 30 or 35 "C) was placed on a glass slide between adhesive tape spacers with a cover glass on top. The sample thickness normally used was approximately 50 r m (one layer of tape). The sample was viewed through a Leitz Laborlux 12 microscope set up for transmitted illumination with crossed Nicols. The microscope was equipped with a 1OX magnifying objective with numerical aperture 0.25 and binocular eyepieces with 12.5X magnification. Hence, the total magnification was 125X and the limit of resolution was 1.10 pm. Temperature regulation was provided by a Mettler hot stage coupled to a Mettler FPSO control unit. The sample was heated from abient temperature to 70 "C. After 10 min a t 70 "C, no wax could be observed; the sample was then cooled a t a rate of 0.5 "C/min (if not otherwise specified) until wax crystals appeared. The wax precipitation temperature is reported as the first temperature where wax crystals could be observed. Normally three parallels were run and averaged. Equivalently, wax dissolution temperatures were measured by heating the same samples from ambient temperature a t a rate of 0.5 OC/min until the last crystals disappeared. Differential Scanning Calorimetry. Crystallization onset temperatures on cooling and the dissolution temperatures on heating were measured by using a Perkin Elmer System 4 microprocessor controlled thermal analyzer interfaced to a Model 3600 data station. Liquid N2 cooling was applied. The sample compartment was purged with He. Crude oil samples were heated to 80 "C in closed containers and shaken thoroughly before being transferred to tarred sample capsules and weighed. Stainless steel capsules of 75 r L capacity were used for sample and reference. The filled and closed capsules were kept at 80 "C for 10-15 min. Under cooling conditions, the samples were cooled a t a rate of 10 "C/min from 80 to -145 "C. In the heating experiments the samples were heated from -145 "C at a rate of 10 "C/min. Five parallel runs were and averaged to improve precision. Blank thermograms were run regularly to compensate for instrument drifting. A series of n-alkanes with known melting points and melting enthalpies were used to calibrate the temperature and energy scales (see discussion below). The crystallization onset temperature was determined manually as the onset of the exothermic peak corresponding to the liquid-solid transition. Similarly, the dissolution temperature was taken as the temperature where the solid-liquid exotherm reached the baseline. T h e ac-
Wax Precipitation from North Sea Crude Oils
Energy & Fuels, Vol. 5, No. 6, 1991 897
Table 11. Analytical and Rheological Data of Oils Investigated oil no. 1 2 3 4 5 6 7 8 9 10 11 12 13" 14 15 16 17 243.9 210.3 226.1 176.9 262.7 241.0 216.6 205.5 252.9 179.2 311.4 167.2 123.1 233.0 169.1 273.9 229.7
~~
~
molecular weight density, kg m-3 at 883.3 842.4 848.7 824.7 865.7 856.3 852.2 840.9 15 "C 873.1 831.6 837.7 813.3 841.2 830.1 30 "Cb 866.1 824.2 830.3 805.8 847.7 838.5 833.9 822.7 40 "Cb 859.0 816.8 823.0 798.3 840.5 831.3 50 "Cb 852.0 809.4 815.6 790.7 833.3 824.0 819.3 807.8 60 "Cb 844.9 802.0 808.2 783.0 826.1 816.6 70 OCb 837.8 794.5 800.8 754.4 818.8 809.3 804.5 792.9 80 "Cb 91.20 82.54 81.61 75.10 90.15 89.12 84.23 80.28 CIO+tew t % 42.4 29.8 33.8 26.7 28.8 29.0 35.0 35.0 ClW, aromatics? wt % 15.6 14.8 8.0 7.0 total wax?' wt % 2.2 6.7 14.3 7.5 7.5 10.4 11.3 5.7 purified wax,'J 1.3 5.0 8.1 5.0 wt % 0.4 0.5 0.2 1.2 1.3 0.3 0.2 0.1 asphaltenes! w t % pour point, "C -26 -12 +16 -8 +26 +25 +2 +8 as received -30 99% purity). The observed melting point, determined as the intersection of the processing baseline and the tangent to the leading edge of the melting endotherm at a scan rate of +10 OC/min, was on average 6 "C higher than the actual melting points. The error introduced by not correcting the slope of the tangent for impurities in the n-alkane standards was found to be marginal, in the case of n-C18(melting point 28.24 "C) less than -1 "C. The +6 "C error was initially suspected to include a certain temperature lag due to the heating rate in addition to the actual instrument offset. As in the microscopic measurements, slow heating/cooling scans would be desirable from an equilibrium point of view. And the work
--
(49) Claudy, P.; Letoffe, J.-M.; Chague, B.; Orrit, J. Fuel 1988, 67,
by Faust51 on waxes and petrolatum indicated a slight temperature lag due to nonequilibrium at scan rate 10 OC/min. However, small sample volumes and the fact that the signal is the time derivative of the enthalpy change generally restricted the use of scan rates lower than 10 "C/min. The n-CI8standard was therefore used to examine the influence of scan rates in the range 2.5-10 "C/min on observed temperatures. It turned out that the heating/cooling rate did not have a significant effect on the observed melting and crystallization temperaturestl which was in agreement with observations made by he in^^^ for middle distillates. This, in turn, implied that the temperature lag due to instrument offset alone, was +6 "C. Consequently, WPTs and WDTs reported in this paper were obtained by correcting the raw data by -6 "C. A source of uncertainty in the reported temperatures was the baseline determination. It was sometimes rather difficult to decide exactly where to draw the baseline in the high-temperature region, and consequently to establish the exact onset temperature (i.e., where the curve first deviated from the baseline). Drawing of the baseline also inevitably introduced some operator dependence in the procedure. In conclusion, the overall accuracy of the reported WPTs and WDTs was estimated to be on the order of *2 "C. Replicate measurements generally agreed within f1 "C. 4. Viscometry. The potential of viscosity measurements as a method for determining the onset temperature of wax crystallization lies in the gradual change of rheological properties of an oil that occurs as wax precipitates. Above the wax precipitation temperature, almost any crude oil, with exception of extremely asphaltene-rich oils, is a Newtonian fluid; i.e., the viscosity is a single-valued function of temperature in laminar flow. The temperature dependence in the Newtonian range is most often adequately expressed by a simple exponential Arrhenius or G~zman-Andrade~~ type equation p =
AeEaJRT
(2)
where p is the Newtonian dynamic viscosity, E, is viewed as the activation energy of viscous flow, A is a constant largely dependent on the entropy of activation of flow, R is the universal gas constant, and T is the absolute temperature. A and E, can generally be considered constant over limited temperature ranges." Below the non-Newtonian temperature limit, p has to be replaced by the apparent or shear rate dependent viscosity 7. The constant A and E, will in general be functions of shear stress or rate of shear. Utilizing the fact that the crude oil viscosity obeys eq 2 until wax starts to precipitate, a wax precipitation temperature can be estimated in two ways: 1. The first makes use of the linear relationship between In p and 1/T. The slope of a plot of In p against 1 / T is equal to E J R . As wax becomes suspended in the oil, E, is likely to increase, resulting in a larger slope. This is illustrated for oil no. 5 in Figure 5a. The WPT is identified as the temperature corresponding to the break in the curve. 2. The second approach is first to fit the Newtonian viscosities over a sufficiently broad temperature range, at least 40 "C, to the Arrhenius equation. The lowest temperature included in the fit should be one where the viscosity still lies on a straight line in a semilog plot. The viscosity-temperature curve predicted by the Arrhenius
58-61.
(50) Noel, F. Thermochim. Acta 1972,4, 377-392. (51) Faust, H. R. Thermochim. Acta 1978,26, 383-398. (52) Heino,E.-L. Thermochim. Acta 1987, 114, 125-130.
(53) Andrade, E. N. da C. Nature 1930,125, 309-310. (54) Bestul, A. B.; Belcher, H. V. J.Appl. Phys. 1953,24(6), 696-702.
Wax Precipitation from North Sea Crude Oils 0908 A
OD
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Energy & Fuels, Vol. 5, No. 6, 1991 903
I
I
I
OIL5
0,07
OIL 9
E 0.06 -I
TEMPERATURE(
e
100
1
10 O A
OD
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4
2,9
i S
I
i
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3,l 3,3 1 / T ( K ) x lo00
3,5
4
3,7
Figure 5. Determination of wax precipitation temperature from viscosity-temperature curve of oil no. 5; cooling rate 12.5 OC/h; shear rate 100 s-l. (a, top) Linear viscosity-temperature plot; WPT identified as the first temperature where the experimental curve deviates from the extrapolated Arrhenius curve. (b, bottom) Arrhenius plot; WPT identified as the temperature where the slope of the curve starts to deviate from linear.
equation is extrapolated to low temperatures and plotted in a linear graph together with the experimental curve. The WPT is identified as the point where the experimental and predicted uNewtonianncurves separate. This is illustrated for oil no. 5 in Figure 5b. The choice of approach is largely a matter of convenience, as they are fully equivalent. The results presented in this work were obtained by the second approach. Because the accuracy of the determination will largely depend on how distinct the deviation from the Arrhenius curve is, it is desirable not to use too high shear rate, since the oils are generally shear-thinning. Because the cooling rate and the thermal history has been seen to highly influence the apparent viscosity, these factors had to be assessed as well. A typical example of the effects is illustrated in Figure 6. Neither cooling rate nor thermal history, i.e., whether the oil was thermally beneficiated or not, was found to influence the WPT significantly, meaning that the effects were within the repeatability of the method, which was approximately f1.5 "C. The results presented here refer to thermally beneficiated oils which were cooled at a rate of 12.5 "C/h at a shear rate of 100 s-l. The apparent viscosity vs temperature curves in Figures 5-7 were obtained by continuously monitoring the shear stress while cooling the oil at the specified constant shear rate. Somewhat different viscosity values probably would have been obtained if the sample had been cooled statically (Le., without shear) between subsequent measurements at progressively lower temperatures with, say, 5 or 10 "C
2,7
2,9
3,l 3,3 1 / T (K) x lo00
3,5
3,7
Figure 6. Effect of thermal beneficiation, shear rate and cooling rate on the apparent viscosity of oil 9. (a, top) Linear plot, (b, bottom) Arrhenius plot. Notice that the curves converge toward a single point. The viscometric WPTs were thus essentially unaffected by the condition of the oil. Table VI. Wax Precipitation Temperatures (WPT) Determined by Microscopy, Differential Scanning Calorimetry (DSC), and Viscometry WPT from oil no. microscopp DSCb viscositrC 1 30.5 11.0 23 2 38.5 17.0 28 3 41.0 33.5 35 4 48.0 32.5 31 5 39.5 39.5 40 6 39.0 39.5 39 7 34.5 32.0 28 8 38.0 32.0 31 9 35.5 31.5 34 10 41.0 31.5 29 11 22.0 ndd 32 12 32.0 25.5 30 13 c5 -26.0
