Energy & Fuels 2003, 17, 321-328
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Influence of Wax Inhibitors on Wax Appearance Temperature, Pour Point, and Viscosity of Waxy Crude Oils Karen S. Pedersen* Calsep, Gl. Lundtoftevej 1C, DK-2800 Kgs. Lyngby, Denmark
Hans P. Rønningsen STATOIL, N-4035 Stavanger, Norway Received June 24, 2002
Experimental data are presented for the viscosity, pour point, and wax appearance temperature of a stabilized, waxy North Sea crude oil treated by 12 different commercial wax crystal modifiers, all of which may potentially act both as wax deposition inhibitors and pour point depressants. The viscosity data cover the temperature range from 40 to 5 °C. In general the studied chemicals only marginally influence the wax appearance temperatures whereas the majority has a pronounced effect on pour points and apparent viscosity. The viscosity data suggest that the inhibitors, probably by some kind of steric hindrance, “inactivate” wax components within a certain range of molecular weight by preventing them from building of network structures. It is shown that this effect can be modeled by assuming a lowering of the melting temperatures of the affected range of wax molecules.
Introduction Above the wax appearance temperature (WAT) the rheological behavior of crude oils is generally Newtonian (viscosity is independent of shear rate), while oil mixtures containing solid wax particles exhibit non-Newtonian behavior. For very waxy oils the deviation from Newtonian behavior can be quite pronounced. Already a few degrees below the WAT a deviation from Newtonian behavior is normally observed. When the temperature approaches the pour point, a sharp increase in viscosity may be seen in offshore pipelines transporting waxy crude oils. The non-Newtonian behavior can give rise to very high pressure drops and cause problems in connection with shutdown situations, because the oil may gel completely and develop a significant gel strength, characterized by a yield stress. If the yield stress is high enough, it may not be possible to restart the line at all. Pedersen and Rønningsen1 have presented viscosity data for 18 different North Sea crude oils above and below the wax appearance temperature. At conditions where these oils contained precipitated wax particles, a non-Newtonian rheological behavior that can be classified as either pseudo-plastic or Bingham plastic, was observed. For a shear rate of 100 s-1 viscosities as high as 1840 mPas are reported for a temperature of 1 °C. The viscosity of the same oil at 30 °C is only 20 mPas. The non-Newtonian behavior of the oils could be represented using a shear rate dependent model for the (1) Pedersen, K. S.; Rønningsen, H. P. Effect of Precipitated Wax on Viscosity-A Model for Predicting Non-Newtonian Viscosity of Crude Oils. Energy Fuels 2000, 14, 43-51.
apparent viscosity η of the oil phase with suspended wax particles:
[
η ) ηliq exp(DΦwax) 1 +
EΦwax
x
dvx dy
+
]
FΦ4wax dvx dy
(1)
In this equation ηliq is the viscosity of the oil phase not considering wax particles; Φwax is the volume fraction of wax particles in the combined oil plus wax phase; dvx/ dy is the shear rate; and D, E and F empirical constants. For D ) 37.82, E ) 83.96, and F ) 8.559 × 106 , the viscosities (in mPas) of the 18 oils studied were represented with an average deviation of 47%. D, E, and F may be used as tuning parameters in order to match viscosity data for particular oils. The viscosity of the oil not considering wax particles is found using the corresponding states model of Pedersen et al.2,3 In certain cases it may be necessary to lower the apparent viscosity (and the pour point) of waxy crude oils being transported in offshore pipelines. During normal operation the frictional pressure drop in the pipeline increases with viscosity. During shutdown the temperature may drop to below the pour point, and it may be difficult to restart the pipeline. The pour point is the lowest temperature at which an oil will flow freely under its own weight under specific test conditions (2) Pedersen, K. S.; Fredenslund, Aa.; Christensen, P. L.; Thomassen, P. Viscosity of Crude Oils. Chem. Eng. Sci. 1984, 39, 1011-1016. (3) Pedersen, K. S.; Fredenslund, Aa. An Improved Corresponding States Model for the Prediction of Oil and Gas Viscosities and Thermal Conductivities. Chem. Eng. Sci. 1987, 42, 182-186.
10.1021/ef020142+ CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003
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(ASTM D-92). Rønningsen et al.4 distinguish between the maximum and the minimum pour point. The latter is the pour point of a sample that has been thermally preconditioned at a high temperature (at least 80 °C) and the former is the pour point of an unconditioned sample. Wax inhibitors are sometimes added to waxy oils to minimize transport problems. Basically three main groups of chemicals are used: ‚wax crystal modifiers ‚detergents ‚dispersants The last two groups are primarily surface-active agents as for example polyesters and amine ethoxylates. These may act partly by modifying the surface of the pipe wall, but primarily by keeping the crystals dispersed as separate particles, thereby reducing their tendency to interact and adhere to solid surfaces. The chemicals considered in this paper are crystal modifiers. These are substances capable of building into wax crystals and alter the growth and surface characteristics of the crystals. One effect utilized in oil production is the reduced tendency of the crystals to stick to metal surfaces such as pipe walls. Besides, the crystal modifiers will have the effect of reducing the tendency to form a three-dimensional network, thereby lowering the pour point as well as the viscosity. Hence, the name pour point depressants is also used for this class of chemicals. Although the exact way in which wax crystal modifiers operate is not absolutely clear, they all basically modify the crystal morphology and the way the crystals interact. They thereby reduce the tendency of crystals to interlock and form three-dimensional networks. According to Gilby5 there is certainly a combination of different mechanisms involving nucleation, cocrystallization, and adsorption. Different observations have been reported6-11 as to whether the additives cause larger or smaller crystals, aggregate structures, etc. Knox et al.7 reported that the wax crystal modifiers lowered the pour point by preventing needle star (spherulite) formation, while Lorensen6 claimed that alkylated aromatics lower pour points by creating spherulites. Lorensen further concluded that although size reduction of single crystals is often observed, it is more important that the total surface area is reduced by the formation of (4) Rønningsen, H. P.; Bjørndal, B.; Hansen, A. B.; Pedersen, W. B. Wax Precipitation from North Sea Crude Oils. 1. Crystallization and Dissolution Temperatures, and Newtonian and Non-Newtonian Flow Properties. Energy Fuels 1991, 5, 895-907. (5) Gilby, G. The Use of Ethylene-Vinyl Acetate Copolymers as Flow Improvers in Waxy Crude Oil. Chem. Oils Ind. 1983. Special publication, Proceedings, Manchester.. (6) Lorensen, L. E. Pour Point Depression I: Mechanism Studies. Proc. Symp. Polym. Lubr. Oils 1962. (Division of Petroleum Chemistry, American Chemical Society, Atlantic City, Sept 1962.) (7) Knox, J.; Waters, A. B.; Arnold, B. B. Checking Paraffin Deposition by Crystal Growth Inhibition. Presented at the SPE 37th Annual Fall Meeting, Los Angeles, Oct 1962; SPE paper 443. (8) Holder, G. A.; Winkler, J. Crystal Growth Poisoning of n-Paraffin Wax by Polymeric Additives and Its Relevance to Polymer Crystallization Mechanism. Nature 1965, Aug, 719. (9) Holder, G. A.; Winkler, J. Wax Crystallization from Distillate Fuels. J. Inst. Pet. 1965, 51, 228. (10) Irani, C.; Zajac, J. Handling of High Pour Point West African Crude Oils. J. Pet. Technol. 1982, Feb, 289. (11) van Engelen, G. P.; Kaul, C. L.; Vos, B.; Aranha, H. P. Study on Flow Improvers for Transportation of Bombay High Crude through Submarine Pipeline. Presented at the 11th Annual OTC in Houston, Texas, April/May 1979; OTC paper 3518.
Pedersen and Rønningsen
aggregates. This view was supported by Birdwell12 who found that most cases of pour point reduction was accompanied by formation of relatively large crystal aggregates. Rønningsen et al.13,14 also observed a correlation between the tendency to form ordered spherulitic aggregates and the ability to lower the pour point. The spherulitic structures very closely resembled those observed by Lovell and Seitzer15,16 in US shale oils containing natural pour point depressants. There were, however, also examples of efficient pour point depressants not giving spherulitic aggregates. Some of the confusion that exists about mechanisms of operation, certainly has to do with the fact that different mechanisms may be active at the same time and especially when different chemistries are considered. The prevailing theories and observations, with a few exceptions, suggest that the lowering of pour points in the presence of effective crystal modifiers, is often accompanied by the transition from single needle or platelike crystals to spherulitic or dendritic crystal aggregates. This study presents data for twelve different wax crystal modifiers. The data comprise wax appearance temperatures (WAT’s), pour point data, and viscosity data for a waxy North Sea crude oil treated with the actual chemicals in three different concentrations. It is not the intention of this paper to present or discuss detailed mechanisms of action, but rather propose a way of modeling the effect of these chemicals, namely, a significant reduction of viscosity at low temperatures. Experimental Methods Apparent viscosity as a function of temperature was measured with a Haake RV12 concentric cylinder viscometer equipped with double gap geometry (details in Rønningsen et al.11). The cooling rate was 12 °C/h. Prior to viscosity and pour point measurements, the oil was pretreated (conditioned) for at least 2 h at 80 °C in a gastight steel cell. Viscosity was measured at different shear rates in the range 20 to 500 s-1. The oil was treated with 12 different commercial wax inhibitors. The inhibitors were all added at a temperature of 50 °C. Pour points were measured by slightly modified ASTM D-97 method.13 Wax appearance temperatures were measured by crossed polar microscopy. Experimental Data. The composition of the oil studied is given in Table 1. It is a 33° API gas-free North Sea crude oil with about 15 wt % wax content (acetone precipitation). The viscosity of the oil for shear rates between 20 and 500 s-1 and temperatures between 40 (12) Birdwell, B. F. Effects of Various Additives on Crystal Habit and Other Properties of Petroleum Wax Solutions. Ph. D. dissertation, Petroleum Engineering, University of Texas, Austin, Jan 1964. (13) Rønningsen, H. P.; Bjørndal, B.; Hansen, A. B.; Pedersen, W. B. Wax Precipitation in North Sea Crude Oils. 1. Crystallization and Dissolution Temperatures, Newtonian and non-Newtonian Flow Properties. Energy Fuels 1991, 5, 895-908. (14) Rønningsen, H. P.; Karan, K. Gelling and Restart Behaviour of Waxy Crude Oils from a North Sea Field: A Study on the Effect of Solution Gas, Mixing with Other Fluids and Pour Point Depressants. In Proceedings of the 10th BHR International Conference on Multiphase ’01, Cannes, France, June 2001; pp 439-458. (15) Lovell, P. F.; Seitzer, W. H. Some Flow Characteristics of Utah Shale Oils. In Proceedings of the Oil Shale Symposium, Colorado School of Mines, 1979; pp 213-220. (16) Lovell, P. F.; Seitzer, W. H. Effect of Retorting on Wax Crystallization in Utah Shale Oils. Am. Chem. Soc., Div. Fuel Chem. 1978, 23, 38-45.
Waxy Crude Oils
Energy & Fuels, Vol. 17, No. 2, 2003 323
Table 1. Composition of Oil component
mol %
C3 iC4 nC4 iC5 nC5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20+
0.010 0.040 0.331 0.601 1.212 3.466 8.484 11.139 8.504 7.212 5.930 5.009 5.139 4.167 4.057 3.236 3.306 2.524 2.685 22.769
molecular weight
density (g/cm3)
crude
shear rate 20 30 70 100 300 500
92.5 105 119 134 148 161 175 189 203 216 233 248 260 466
1 2 3 4 5 6 7 8 9 10 11 12
0.735 0.763 0.788 0.790 0.793 0.806 0.821 0.833 0.838 0.844 0.839 0.842 0.852 0.928
+13 +7 +19 +23 +25 +23 +25 +21 +23 +22 +12 +17
1000 ppm
max
min
max
-3
