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Ind. Eng. Chem. Res. 1998, 37, 4828-4834
Effect of Pulsed Electromagnetic Field on Crude Oil Rheology Radmila S ˇ ec´ erov Sokolovic´ ,† Slobodan Sokolovic´ ,† ^or]e Mihajlovic´ ,‡ Tanja Gelei,† Neda Pekaric´ ,§ and Snezˇ ana S ˇ evic´ ‡ Department of Food and Chemical Engineering, Faculty of Technology, and Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Yugoslavia, and Serbia Oil Company, Novi Sad, Yugoslavia
The effect of low-frequency pulsed electromagnetic field (PEMF) on crude oil rheology was studied on 11 samples of domestic crude oil with a high pour point. Experiments were carried out by using homogeneous single unipolar 70 µs-wide quasi-square pulses with peak intensities of 1 mT and 5 mT and frequency varying from 8 to 60 Hz. The rheotests were performed at 20, 25, 30, and 35 °C, using shear rates in the interval from 16.2 to 438 s-1. PEMF influenced the rheological behavior of samples by increasing/decreasing the shear stress, but not changing the shape of the flow curves. The PEMF frequency exhibited crucial effects on shear stress, and a certain periodicity in the relation between these two quantities, similar for both PEMF intensities, was observed for all the samples. Introduction In the petroleum technology one deals with crude oils of diverse characteristics, the structural composition and sulfur content being the most important from a commercial point of view. The presence of crystallized waxes in crude oil and petroleum products can convert a simple Newtonian fluid into a very complex nonNewtonian fluid whose flow properties are time- and history-dependent (Wardhaugh and Boger, 1991), causing pipelining difficulties. At temperatures below the pour point, such crude oils behave as a Bingham plastic, with a yield stress that must be exceeded to initiate the flow. The transportation costs of such oils are markedly increased because of the necessity of adding large quantities of special additives or working at higher temperatures. In some instances, even these measures do not allow the transport of such oils (Ford and Russell, 1965). Crude oils in the Vojvodina region (northern part of Yugoslavia) are usually characterized by a high pour point, which makes them unsuitable for exploitation. The majority of conventional chemical processing operations apply mechanical or thermal energy in combination with pressure or gravity forces. The capability of electric and magnetic fields to improve certain separation processes has been well known and widely used for many years (Minenko, 1981; Kul’skii and colleagues, 1984, 1987; Bailes, 1992; Mori et al., 1994; Chang et al., 1995; Tsouris et al., 1995; Yiacoumi et al., 1996). Some previous investigations showed that the properties of many liquids may be altered by different types of electric and magnetic fields (Chang and Watson, 1994). KulÅskii and Duskin (1987) found that a magnetic field increased viscosity and caused changes in the infrared spectrum of water in the course of 5 h. They also found that a magnetic field increases the probability of formation of crystal centers, affecting the rate of crystallization. * Corresponding author’s address: Cara Lazara 1, 21000 Novi Sad, Yugoslavia. Phone: +38121450288. Fax: +38121450413. E-mail:
[email protected]. † Faculty of Technology, University of Novi Sad. ‡ Serbian Oil Company. § Faculty of Technical Sciences, University of Novi Sad.
Electromagnetic (EM) fields can be either ionizing or nonionizing. The ionizing fields, by their high frequencies, influence molecular ionization and hence are harmful to man. They cause oscillation of molecules, the consequence being an increase in temperature of the fluid exposed to their action. To this group also belong microwaves, the frequency of which exceeds 300 MHz. The nonionizing EM fields yield strong orientation of particles, which may result in formation of additional ion flow in the fluid. Molecules vibrate under the influence of the geomagnetic field and ambient temperature. A steady magnetic-geomagnetic field causes uniform vibrations in a corresponding direction. An additional magnetic field causes bunching of the vibration axes in one direction, in accordance with the additional magnetic field direction (Polk and Postow, 1986). Pulsed electromagnetic fields (PEMFs) are specific fields consisting of a series of low-frequency packages of high-frequency pulses. The signal is not of a regular sinusoidal form but of a squared one (Ross, 1990); hence the frequency spectrum is very wide. Such signals are composed of many sinusoidal subcomponents. The first principle of frequency composition, therefore, is that a periodic signal is composed of the sum of a set of discrete sinusoidal subcomonents. The fundamental frequency is determined by the repetition rate 1/Tr. Such fields combine the useful effects of both low-frequency and high-frequency fields (Polk and Postow, 1986). PEMFs of nonsinusoidal type are commonly applied in the domain of healing (Blackman et al., 1988; Serpesu et al., 1983). PEMFs also have some positive effects on cultured growth (Okihana and Shimomura, 1988). The rheology of suspensions is a complex and insufficiently known matter. Suspensions are heterogeneous systems that can contain solid particles of different size, concentration, and structure. As a consequence, they exhibit time dependence of the yield stress. Chen et al. (1994) in their study of the rheological consequences of microstructural transitions in polycrystalline colloidal systems found that suspensions show a distinct dynamic yield stress and two regions in which the stress is a decreasing function of shear rate. Bonnecaze and Brady (1992) described and determined the static and dynamic
10.1021/ie980082o CCC: $15.00 © 1998 American Chemical Society Published on Web 10/31/1998
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4829
Figure 1. Schematic of the apparatus for exposing samples to PEMF: (1) microprocessor generator; (2) beaker with sample; (3) stripline antenna; (4) Helmholtz rings; (5) Plexiglas chamber; (6) beaker.
Figure 2. ASTM distillation curves for crude oils: samples A, B, E, G, and J (a) and samples K, L, M, N, O, and S (b).
yield stresses in an electrorheological fluid from a microstructural model. The static yield stress they determined from nonlinear elasticity strain-energy theory applied to an electrorheological fluid for a variety of volume fractions and particle-to-fluid dielectric constant ratio. In view of the above facts it was of interest to study the effect of PEMFs of different frequency and intensity on the rheology of crude oils, especially of those with high pour points, as positive findings on possible lowering of their viscosity/yield stress could be of remarkable practical importance. Equipment and Operating Procedure The homogeneous PEMF was generated with a suitable microprocessor generator emitting single unipolar 70 µs-wide quasi-square pulses of a defined intensity, with frequencies varying from 8 to 60 Hz (Figure 1). The
intensity of 1 mT was realized by using a stripline antenna (a), and 5 mT by using the Helmholtz rings (b). The generator was developed at the Faculty of Technical Sciences, University of Novi Sad (Yugoslavia). The rheotests were carried out on a WEB MLW Prufgerate rheometer with an inner rotating cylinder, using shear rates from 16.2 to 437.4 s-1. The working temperature for sample manipulation and field exposure was 50 °C, ensuring complete liquefaction (checked by microscopy) of the sample with the maximum pour point of 38 °C. All samples were heated to this temperature at a rate of 1°/min. For the rheological measurements, samples were cooled at the same rate down to 35, 30, 25, and 20 °C, and thermostated at each temperature under shear for 1 h. Steady readings were obtained in ∼15 min after changing the shear rate. This procedure was in agreement with that used by Wardaugh and Boger (1991) for waxy crude oils.
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Figure 3. Flow curves for sample A (a) and sample G (b).
Figure 4. Flow curves for sample K (a) and sample M (b). Table 1. Crude Oil Properties
The heated samples were placed in a Plexiglas chamber thermostated at 50 °C and exposed to the PEMF only once for 60 min. No temperature change was observed. Immediately after, sample rheology was measured. To avoid the rheometer effect on PEMF, it had to be held at a distance of at least 0.5 m from the chamber. Crude Oil Properties. The experiments were carried out on 11 domestic crude oil samples, marked A, B, E, G, J, K, L, M, N, O, and S. All of them were characterized by determining the distillation curve (Figure 2), pour point, mean molecular mass, neutralization number, contents of solid paraffins and asphaltenes, and the density-temperature relation (Table 1). As can be seen from the Table, the chosen samples cover a wide range of pour points, the majority being > 30 °C, causing marked manipulation problems. Sample A, with a pour point of -30 °C, was chosen as a reference. The choice of working temperatures (20, 25, 30, and 35 °C) was made having in mind highest ambient temperatures in the region of oil exploitation, and the objective was to examine what would be a potential additional effect of the PEMF.
pour mean mol. neutr. paraffins asphaltpoint mass no. mg solid enes sample (°C) (kg/kmol) KOH/L (% w/w) (% w/w) A
-30
374
0.979
2.25
1.40
B
15
217
0.554
7.82
0.49
E
12
201
0.384
9.92
1.28
G
24
250
0.491
14.32
1.68
J
38
279
25.22
0.54
K
33
299
0.779
8.85
7.18
L
38
252
2.475
16.61
2.12
M
30
301
0.787
11.00
6.2
N
36
235
4.426
17.63
4.64
O
33
284
1.059
15.10
1.33
S
24
228
1.577
9.42
0.30
F ) f(t) (kg/m3) F ) 934.03 - 0.66t R ) 0.999 F ) 906.78 - 0.94t R ) 0.997 F ) 943.75 - 1.05t R ) 0.999 F ) 947.18 - 1.01t R ) 0.999 F ) 886.95 - 0.12t R ) 0.999 F ) 945.07 - 0.82t R ) 0.999 F ) 885.70 - 0.11t R ) 0.999 F ) 959.10 - 0.54t R ) 0.982 F ) 896.67 - 0.11t R ) 0.993 F ) 933.28 - 0.47t R ) 0.966 F ) 898.95 - 1.99t R ) 0.973
Results and Discussion The flow and viscosity curves were recorded for all 11 samples in the absence and presence of PEMF. The
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Figure 5. Dependence of shear stress at 30 °C and shear rate of 437.4 s-1 (a) and pour point (b) on the sum of paraffins and asphaltenes.
Figure 6. Flow curves in the absence (empty symbols) and presence of PEMF, at 5 mT and 8 Hz for sample K (a) and sample M (b) at 25, 30, and 35 °C.
representative flow curves obtained at different temperatures are presented in Figures 3 and 4. As can be seen, the samples exhibit a most diverse behavior. Sample A is Newtonian in the whole temperature range; samples B, E, G, and S are Newtonian at higher and pseudoplastic at lower temperatures. Samples K, O, and L are Newtonian at highest, pseudoplastic at the middle, and plastic at lower temperatures. Finally, samples J, M, and N are plastic in the whole range of working temperatures. Some of our samples are characterized by a high pour point and high contents of paraffins and asphaltenes, making them unsuitable for exploitation. We can establish a correlation between the sum of paraffins and asphaltenes on shear stress and pour point for all 11 samples in the absence of PEMF (Figure 5). As can be seen from Figure 5 (a), shear stress shows a drastic increase with the paraffins + asphaltenes sum in the range of 15-20% (w/w), whereas pour point (b) shows
an increase to saturation. Unfortunately, analogous (or any) correlation could not be established in the presence of PEMF. Analogous flow curves were also obtained in the presence of PEMF, for both field intensities and all applied frequencies. In Figure 6 are shown the flow curves for samples K (a) and M (b) at 5 mT and 50 Hz. For the sake of comparison we superimposed the corresponding curves obtained in the absence of PEMF (empty symbols). Similar curves were obtained for all the samples under all experimental conditions. On the basis of the results obtained a general conclusion to be drawn is that the PEMF does influence crude oil rheology. However, the presence of PEMF changed only the values of shear stress, causing either its increase or decrease, which resulted in a shift of the flow curves in the diagram. The field of a defined intensity and frequency did not affect rheological behavior of the
4832 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 7. Dependence of shear stress on PEMF frequency for all crude oil samples (a) and samples with shear stress < 100 N/m2 (b) at 30 °C, shear rate 437.4 s-1, and 1 mT.
Figure 8. Dependence of shear stress on PEMF frequency at 1 mT for sample M at all shear rates at 25 °C (a) and all temperatures at 48.6 s-1 (b).
samples, which means that the Newtonian fluids remained Newtonian and the plastic ones remained plastic. In view of the many variables involved, the effect of PEMF can be analyzed best on the basis of the diagrams obtained at a constant temperature and field intensity. Thus we obtain changes in shear stress independent of field frequency at a constant shear rate, whereby zero frequency means the absence of the field. In Figure 7 (a) is presented the dependence of shear stress on frequency for all the samples recorded at 30 °C, for a shear rate of 437.4 s-1 and PEMF intensity of 1 mT. Samples A, B, E, G, and S have shear stress values 20 °C are solidified. Solidification of such a mixture takes place over several degrees centrigrade. If the oil pour point is high, then there is an increased content of solid paraffins, long-chain paraffins, and asphaltenes whose structure is very complex and still unresolved. In that case we deal with suspensions, so that PEMF can influence the kinetics of crystallization and orientation of crystals. These samples can be thought of as belonging to both liquids and solids, which can build up an interparticle network involving polar, van der Waals, and other binding forces. In such circumstances, an external force, such as PEMF, can produce changes in the interparticle network.
Besides, if the field affects the crystallization kinetics, it can influence the number, nature, and the shape of the crystals (waxy and nonwaxy, as well as their crosslinking), causing a decrease/increase of shear stress. We can speculate that the frequency influences the polycrystallinity and particle-to-fluid dielectric constant ratio as the two dominant quantities determining the sample rheology. This is in concordance with previous findings on the rheology of suspensions (Chen et al., 1994; Bonnecaze and Brady, 1992). The relative changes of shear stress caused by PEMF differ from sample to sample, ranging from 5% to 170% [Figure 9 (a)]. When we deal with a homogeneous system (samples A, B, E, G, J, and L), these changes are < 45%, whereas for heterogeneous systems they are 45-170%. Such observations are in agreement with the above discussion. If we consider the PEMF effect in relation to the mean molecular mass, we can conclude that this is more significant for samples of higher molecular weight. As can be seen from Figure 9 (b), this is only a general trend, as the correlation is rather weak. As for the effect of PEMF intensity, the fact that we could technically realize only two PEMF intensities is a limiting factor for a detailed analysis. However, on
4834 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
the basis of our measurements (Figure 10), the form of the dependence of the shear stress on frequency is generally similar for both field intensities, the values of shear stress being only somewhat different and shifted along the frequency axis. Thus, for example, the position of the minima for sample M are at 8 and 60 Hz at 1 mT (a), whereas the corresponding values for 5 mT are observed at 20 and 60 Hz (b). Conclusion The PEMF of defined characteristics can cause changes in shear stress, not affecting the rheological nature of the crude oils. Depending on the frequency the shear stress can either increase or decrease, causing only changes in the positions of flow curves and not of their shapes. These changes of shear stress show a periodical dependence on frequency, exhibiting a minimum, the obtaining of which was in fact the objective of this study. The magnitude of the change of shear stress depends primarily on the pour point and molecular mass of the sample, and is most significant for samples that are suspensions at working temperatures. Literature Cited Bailes, P. J. Electrically Augmented Settlers and Coalescers for Solvent Extraction. Hydrometallurgy 1992, 30, 417. Blackman, C.; Benane, S.; Elliott, D.; House, D.; Pollock, M. Influence of Electromagnetic Field on the Efflux of Calcium Ions from Brain in Vitro: A Three-Model Analysis Consistent with the Frequency Response up to 510 Hz. Bioelectromagnetics 1988, 9, 215. Bonnecaze, R. T.; Brady, J. F. Yield Stresses in Electrorheological Fluids. J. Rheol. 1992, 36, 73. Chang, J.-S.; Watson, A. Electromagnetic Hydrodynamics. IEEE Trans. Dielectr. Elec. Insul. 1994, 1, 871. Chang, J.-S.; Kelly, A. J.; Crowley, J. M. Handbook of Electrostatic Processes; Marcel Dekker: New York, 1995.
Chen, L. B.; Ackerson, B. J.; Zukoski, C. F. Rheological Consequences of Microstructural Transitions in Colloidal Crystals. J. Rheol. 1994, 38, 193. Ford, P. E.; Russell, R. J. Pipelining High Pour Point Crude, Part III. Oil Gas J. 1965, 10, 183. Kul’skii, L. A.; Kochmarskii, V. Z. Vodoochistka v Elektromagnitnom Pole, Znanie, Kiev, 1984. Kul’skii, L. A.; Duskin, S. S. Magn. Pole Proc. Vodoobrabotki, Kiev 1987. Minenko, V. I. Elektromagn Obrabotka Vody Teploenerg, Harkov 1981. Mori, Y.; Tanigaki, M.; Maehara, N.; Eguchi, W., Effect of Frequency in A. C. High Voltage on Coalescence of Water-inOil Emulsion Stabilized with Sorbtan Monooleare-Type Nonionic Surfactant, J. Chem. Eng. Jpn. 1994, 27, 340. Okihana, H.; Shimomura, Y. Effect of Direct Current on Cultured Growth Cartilage Cells in vitro. J. Orthop. Res. 1988, 6, 690. Polk, C.; Postow, E. CRC Handbook of Biological Effects of Electromagnetic Fields; CRC Press: Boca Raton, FL, 1986. Ross, S. M. Principles of the Frequency Composition of Pulsed Electromagnetic Fields. J. Bioelectr. 1990, 9, 67. Serpersu, E.; Tsong, T. Y.; Schrot, J. Stimulation of a OuabainSensitive Rb+ Uptake in Human Erythrocytes with an External Field. J. Membr. Biol. 1983, 74, 191. Tsouris, C.; Scott, T. C. Flocculation of Paramagnetic Particles in a Magnetic Field. J. Colloid Interface Sci. 1995, 171, 319. Tsouris, C.; Yiacoumi, S.; Scott, T. C. Kinetics of Heterogeneous Magnetic Flocculation Using a Bivariate Population-Balance Equation. Chem. Eng. Commun. 1995, 137, 147. Wardhaugh, L. T.; Boger, D. V. Flow Characteristics of Waxy Crude Oils: Application to Pipeline Design. AIChE J. 1991, 37, 871. Yiacoumi, S.; Rountree, D.; Tsouris, C. Mechanism of Particle Flocculation by Magnetic Seeding. J. Colloid Interface Sci. 1996, 184, 477.
Received for review February 17, 1998 Revised manuscript received August 10, 1998 Accepted August 13, 1998 IE980082O
