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Effect of Asphaltenes on Crude Oil Wax Crystallization Pavel Kriz and Simon I. Andersen* IVC-SEP, Department of Chemical Engineering, Building 229, Technical University of Denmark,DK-2800 Kgs. Lyngby, Denmark Received July 27, 2004. Revised Manuscript Received December 6, 2004
The paper summarizes the experimental work done on asphaltene influenced wax crystallization. Three different asphaltenes (from stable oil, instable oil, and deposit) were mixed at several concentrations or dispersions into the waxy crude oil. These blends were evaluated by viscometry and yield stress measurement and compared with the original crude oil. A complex asphaltenewax interaction as a function of asphaltene concentration and degree of asphaltene dispersion under dynamic and static condition was observed. The crystallization and the wax network strength was strongly dependent on the degree of asphaltene dispersion. The effect of asphaltenes on the wax appearance temperature (WAT) was examined by polarized light microscopy. The idea that the WAT is a function of asphaltene surface area was introduced and supported by experiment. It was observed that well-dispersed asphaltenes influence the wax crystallization at static condition more significantly than the more flocculated.
Introduction Solid deposition is a common problem in crude oil production, transportation, and storage operations and causes huge economic losses for the petroleum industry. From a compositional point of view, the high molecular weight n-alkanes (n-paraffins) are the main components in wax deposits, however, long iso- and cycloalkanes and high molecular weight polyaromates (asphaltenes/ resins) are also often found ibid.1 The solubility of this high molecular weight alkanes-waxes is decreasing alongside with decreasing temperature;2 at thermodynamically suitable temperature (wax appearance temperature (WAT)), the paraffins crystallize out of the solution and start to build a 3D network with a complex morphology. The portion of solids needed to build a stable structure is very low, in the range of 2% w/w.3 Generated network entraps the rest of the fluid inside and so-called wax-oil gel is made.4 The gelling significantly increases the fluid viscosity and changes the rheological behavior to non-Newtonian.5 During regular pipeline flow, the paraffin structure is usually weak and might be disturbed by an ordinary flow itself. More problems occur during the pipeline shutdown. In such a case, solids build a compact network across the whole pipeline cross section area and additional energy (yield pressure) is needed to restart the flow satisfactorily.3 In addition to this, the gel deposits all over the insides of the production equipment over the entire operating * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Garcı´a, M. C. Energy Fuels 2000, 14, 1043-1048. (2) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. AIChE J. 2000, 46, 1059-1074. (3) Kane´, M.; Djabourov, M.; Volle, J.-L.; Lechaire J.-P.; Frebourg, G. Fuel 2003, 82, 127-135. (4) Singh, P.; Youyen, A.; Fogler, H. S. AIChE J. 2001, 47, 21112124. (5) Singh, P.; Fogler, H. S.; Nagarajan, N. J. Rheol. 1999, 43, 14371459.
period, especially on applicable places where a large thermal gradient exists.2 The gels become harder with time,6,7 cannot be disturbed by the flow itself, and a mechanical removal is needed.4 The role of asphaltenes during the wax crystallization has not been satisfactorily explained yet. In thermodynamic terms, asphaltenes are dispersed rather than dissolved in the crude oil matrix,8 and as shown the association with resin maintains the asphaltenes as a single phase. The fragile equilibrium among the high molecular weight crude oil species (asphaltenesaromatics/resins-saturates/paraffins) is the crucial parameter in the view of crude oil stability.9 Once the equilibrium is broken, the asphaltenes do not interact sufficiently with the resins and start to associate/ flocculate among themselves. The flocculated asphaltenes may provide the crystallization sites for paraffins.1 Another recent study showed that asphaltenes and waxes build complex agglomerates conformable to those found in the storage tank bottom sediments.10 Asphaltenes separated from the oil tank deposit contained a high portion of waxes.11 The laboratory deasphaltened crude oil exhibits even worse rheological behavior/higher crystallization rate than the original one and the asphaltenes were assumed as a “natural flow improver”.7 The existence of asphaltene-wax complexes has already been discussed in the literature. The objective of this study was to shed some light on the (6) Lakshmi, D. S.; Krishna, M. R.; Rao, M. V.; Rao, M. B.; Purohit, R. C.; Srivastava, S. P.; Nautiyal, S. P. Pet. Sci. Technol. 1997, 15, 495-502. (7) Chanda, D.; Sarmah, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam, B.; Das, H. C. Fuel 1998, 77, 1163-1167. (8) Lira-Galeana, C.; Hammami, A. Wax Precipitation from Petroleum Fluids: A Review. In Asphaltenes and Asphalt, 2; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science B. V.: Amsterdam, 2000; Chapter 21, pp 557-608. (9) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19, 1-34. (10) Carbognani, L.; Orea, M. Pet. Sci. Technol. 1999, 17, 165-187. (11) Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Pet. Sci. Technol. 2000, 18, 607-643.
10.1021/ef049819e CCC: $30.25 © 2005 American Chemical Society Published on Web 04/28/2005
Effect of Asphaltenes on Wax Crystallization
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Table 1. Basic Properties of the Reference Waxy Oil Used in This Study oil
nature
origin
typea
oil A
condensate
North Sea
II
density at 60 °C [g/cm3]
viscosity at 60 °C [cP]
pour pointb [°C]
WAT/DSCc [°C]
0.8352
4.389
30
51
saturatesd
aromatics
resins
86.6
10.6
2.8
asphaltenes
a
See ref 14. b The maximum (upper) pour point was measured according to ASTM D 5853-95 standard.15 c Wax appearance temperature measured by differential scanning calorimetry. d Normalized wt %, see ref 16.
effect of asphaltenes during the wax crystallization, mainly on the interaction of asphaltenes and paraffins at and below WAT. There were two basic techniques involved in characterization: viscometry to evaluate the wax network strength and polarized light microscopy to reveal the change in the WAT. The present study addresses the situation where a waxy oil is contacted with an asphaltene containing oil, in other words, the commingling of different oil streams during pipelining contrary to the presence of naturally occurring asphaltenes in a single waxy oil. However, it is assumed that similar effects could be expected. Experimental Section Waxy Crude Oil and Asphaltenes. The reference waxy crude oil was an asphaltene-free dead oil condensate from a North Sea reservoir. The asphaltenes used were separated from different oils and deposit using the modified IP 143 procedure.12 There were three different asphaltenes involved in this study. The LM2 asphaltenes separated from a stable oil, the OMV asphaltenes separated from an oil deposit, and the Lagrave (LG) asphaltenes separated from an unstable oil. The purpose of using various asphaltenes was to reveal whether the asphaltene type is a key parameter during the asphaltene-paraffin interaction or if it is the presence of partially solidified bulky aggregates that leads to changes in the crystal lattice. Some crude oil characteristics are reviewed in Table 1. Asphaltene-Toluene-Oil (ATO) Mixture Preparation. The procedure was divided into the two following steps. First of all, the asphaltene-in-toluene blends were prepared and the homogeneous dispersion was ensured in the ultrasonic bath. The second step was the blending of asphaltene-in-toluene mixtures with the oil to achieve the same asphaltene concentration in all samples (0.1 wt %). The blending temperature was found to be an important parameter. If the temperature is too low, the wax network is partly built-up and asphaltenes cannot be fully implemented within. This resulted in a high degree of uncertainty throughout the experiment. After several trial and errors, the optimal temperature was found at 90 °C. At this temperature, the paraffins were completely melted and the asphaltenes could enter the structure. For each asphaltene-toluene-oil blend, a blind sample was prepared to eliminate the influence of solvent in subsequent experiments. The blind sample was the crude oil diluted with the corresponding amount of toluene. The second set of samples was prepared using a similar procedure; however, in this case the asphaltene concentration was changed while the amount of toluene was fixed. The samples were prepared at a concentra(12) Standards for Petroleum and its Products; IP 143/90; Institute of Petroleum: London, 1985.
Figure 1. Poor data repeatability without temperature pretreatment (a). Improved repeatability after temperature pretreatment (b). (0) PURE 200 1st, (×) PURE 200 2nd, (2) LM2 200 1st, (O) LM2 200 2nd measurement. tion range 0.01-0.5 wt % of asphaltenes and the amount of toluene was fixed to 28 wt %. Around 6 g of every single blend was prepared. Sample Pretreatment. To achieve repeatability and reproducibility of measured viscosities and yield stresses, several steps to optimize measurement were carried out. First of all, the sample pretreatment was found to be important. Different thermal and shear history of a particular sample could strongly affect the experiment. The structure character and strength depends on the temperature and cooling rate. With decreasing temperature, there are more stable waxes crystallizing and participating in the growing network, thus making it stronger. The effect of cooling rate is not simple to explain, but in general at a higher cooling rate a less organized and weaker structure is usually produced.5 To ensure the same initial thermal history for all samples, the high-temperature pretreatment was found as the only possible solution. The sample, closed in a vial, was for 10 min exposed to high temperature (90 °C) which was sufficient to melt all wax crystals in the solution; thus, the thermal history was deleted. Subsequent cooling (20 °C/h) to the first experiment temperature (31 °C) was controlled by a programmable thermostat. The temperature pretreatment was uniform for all samples and all measurements carried out in viscometer. Poor repeatability of viscosity data for samples without the temperature pretreatment is shown in Figure 1a. The improved repeatability after the temperature pretreatment is presented in Figure 1b. Viscosity Measurement. The coaxial rotational viscometer Rheology International Series 2 was used for this measurement equipped with programmable Julabo HP thermostat and A/D converter. The “small sample adapter” was applied. The measurement started at 31 °C, and the shearing was applied through the entire experiment. The viscosity was recorded for
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Figure 2. Evaluation routine for viscometric data. Data series refers to three different temperatures in the non-Newtonian region. 60 min at each temperature. Subsequently, the temperature was decreased by 2 °C and the viscosity at the new temperature was recorded. To maintain the same shear environment for each sample, the shear time (60 min) and shear rate (80 s-1) were constant during the experiment. The constant shear time is needed because viscosity of non-Newtonian fluids is shear time dependent and become independent only at infinite time.13 The solid structure is disturbed by flow continuously and the viscosity is decreasing all over the shearing period. To obtain a set of “time-independent data”, a simple evaluation routine was applied (Figure 2). All previously suggested routines ensure the same thermal and shear history for all samples and thus the data can be compared on the basis of sample material property (i.e., composition), which was the most important in this study. Yield Stress Measurement. The experiment was carried out using the same equipment. The shear rate was 13.2 s-1. The sample was heated to 80 °C directly inside the cell. Subsequent cooling was controlled at the rate of 20 °C per hour and was followed by 1 h isothermal aging at experiment temperature (25, 19, 17, 15, 13 °C). The viscometer output was recorded to the computer and the yield stress was found as the structure breaking point. Wax Appearance Temperature (WAT) Measurement. The WAT was measured using a polarized light microscopy. The instrument used was a Zeiss Axionskop equipped with a Mettler FP90 controlled hot-stage unit. The sample was heated to 80 °C and cooled at a rate of 1 °C/min to 0 °C. The measurement was repeated three times for each sample.
Results and Discussion Viscosity of Asphaltene “Enriched” Crude Oil. The issue of the experiment was to shed some light on the influence of the asphaltene dispersion on the wax crystallization under continuous shearing. The test was supposed to simulate normal operating condition where the oil is sheared continuously and where it is not possible to build the proper paraffin structure. First of all, the influence of asphaltene dispersion on viscosity (13) Morrison, F. A. Understanding Rheology; Oxford University Press: New York, 2001; pp 131-138. (14) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Pet. Sci. Technol. 1998, 16, 1001-1021. (15) Standard Test Method for Pour Point of Crude Oils; ASTM D 5853-95; Annual Book of ASTM Standards: 1995. (16) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; M. Dekker: New York, 1999; pp 272-278.
Kriz and Andersen
Figure 3. Effect of asphaltene dispersion on viscosity. (0) PURE 400, (2) OMV 400, (O) PURE 20, (9) OMV 20. Table 2. Composition and Complete Viscosity Data for Blind and Enriched Samples P20 P100 P200 P400
LM2 LM2 LM2 20 100 200
LM2 400
asphaltene wt % 0 0 0 0 0.1 0.1 0.1 0.1 toluene wt % 1.96 9.09 16.66 28.57 1.96 9.09 16.66 28.57 temperature [°C] 25 23 21 19 17 15 13 11 9
viscosity [cP] 7.78 12.47 14.42 15.52 17.46 20.25 25.07 30.79 n/a
5.45 8.03 11.54 12.59 13.6 15.07 18.04 22.52 29.16
4.11 3.08 5.72 5.09 3.31 7.30 7.84 3.96 13.51 9.97 5.32 16.58 10.66 8.17 19.32 11.69 8.44 23.64 13.34 8.90 30.20 15.26 10.13 38.21 18.91 11.49 50.52
OMV OMV OMV OMV LG 20 100 200 400 20 asphaltene wt % 0.1 toluene wt % 1.96
LG 100
3.52 3.60 5.22 9.52 13.76 14.26 15.87 18.50 22.32
2.83 3.00 3.44 4.55 6.77 11.65 13.71 14.06 15.07
LG 200
LG 400
0.1 0.1 0.1 0.1 0.1 0.1 0.1 9.09 16.66 28.57 1.96 9.09 16.66 28.57
temperature [°C] 25 23 21 19 17 15 13 11 9
4.93 5.05 8.61 13.49 14.74 17.21 19.79 24.85 30.19
viscosity [cP] 5.95 8.05 16.52 19.81 22.44 26.52 33.54 44.00 n/a
4.59 4.85 10.22 15.19 16.58 18.84 23.14 29.12 36.00
3.70 4.23 5.38 8.59 11.81 13.20 15.00 16.96 20.47
3.11 3.30 3.67 4.54 6.70 10.16 11.39 11.85 14.56
5.91 8.91 19.56 20.96 22.86 27.41 34.05 44.63 n/a
4.41 4.86 10.33 17.25 18.51 20.72 24.20 29.46 36.74
3.77 4.28 5.76 9.66 14.38 14.61 15.90 18.28 22.39
3.26 3.30 3.45 4.56 7.18 11.89 13.55 13.67 15.94
was observed. Waxy crude oil condensate was enriched with asphaltene-in-toluene blends at concentration 0.05, 0.01, 0.005, and 0.0025 g/gsolvent (marked as 20, 100, 200, and 400 which refers to the toluene/asphaltene ratio). The total portion of asphaltenes was fixed at 0.1 wt %. These four samples were compared with the reference blind samples (P20-P400) on the basis of viscosity. Figure 3 shows selected results for the OMV asphaltenes. The complete set of measured data are presented in Table 2. To affect viscosity significantly and change the rheological behavior to non-Newtonian, a stable wax network is needed within the oil bulk. The transition to the non-Newtonian region (onset on the viscosity curve) was observed at temperatures some 15 °C lower than WAT. This indicates that there are already enough solids in the solution to build a stable network even under shearing and eventually to turn the oil to gel. The
Effect of Asphaltenes on Wax Crystallization
Figure 4. Effect of asphaltene concentration on viscosity. (0) LM2 0.01 wt %, (2) LM2 0.02 wt %, (O) LM2 0.2 wt %, (9) LM2 0.5 wt %.
delayed onset on the viscosity curve was observed for the asphaltene-enriched samples. This effect was ascribed to the interference of the large asphalteneparaffin agglomerates and the rest of the waxes during a structure spatial arrangement. Thus, the wax crystal cannot be built properly during an early stage of the process and the structure buildup is delayed. This effect is stronger if there are more asphaltenes (“obstacles”) per volume unit (more concentrated sample). Subsequently, the crystallization continues with decreasing temperature and there are more and more short-chain paraffins appearing in the solid phase. These shorter molecules start to inlay “holes” between the separated wax clusters and asphaltenes-paraffin agglomerates. This is the turning point. The structure buildup starts now in the enriched oil too. The slope of the viscosity curve shows that the progress to the non-Newtonian region is much faster. In other words, the crystallization rate is much higher than in the asphaltene-free oil. This can be explained in two possible ways. Either the paraffins may exist in asphaltene-enriched oil in a socalled subcooled state because they are not thermodynamically suitable to participate in crystal and subsequently crystallize even more rapidly once the temperature is low enough to overcome asphaltene spatial interference, or the asphaltene-paraffin cocrystallization provides much larger solid particles which affect the viscosity more significantly. We believe that both described actions are proceeding simultaneously; however, not all of the asphaltenes participate in the paraffin structure (see the yield stress experiment). The asphaltenes which are not incorporated in the paraffin structure increase the actual viscosity as any other solid particles in the media; thus, the fact that the enrichment initially increased the amount of solids should be also taken in account. The effect of asphaltene concentration on viscosity at the same condition was studied too. Alongside with increasing asphaltene concentration, the onset on the viscosity curve was delayed more and more and followed with higher viscosity in the non-Newtonian region (Figure 4). Figure 5 shows a typical viscosity curve, where the transition to non-Newtonian behavior was observed. In the non-Newtonian region, the viscosity grows more rapidly with decreasing temperature until the “slip point”, where the gel is separated into layers
Energy & Fuels, Vol. 19, No. 3, 2005 951
Figure 5. Determination of the slope of the viscosity curve.
Figure 6. Rate of crystallization for oil enriched with LM2 asphaltenes.
and a slip is happening. The dashed curve estimates the real viscosity progress. The viscosity in the nonNewtonian region refers to the portion of solids in the continuous phase, and consequently the slope of the viscosity curve refers to the rate of crystallization. The slope (A) as a function of asphaltene concentration is presented in Figure 6. The rate of crystallization is increasing with increasing asphaltene concentration; however, the major shift was observed at low concentration. The previous experiment studied the influence of asphaltenes on the crystallization under shear. The dynamic experiment reflects the real pipeline situation, where the continuous shear is applied. Solids exist in clusters, the solid network cannot be built properly, and the oil exhibits behavior similar to a suspension rather than to a gel. So, it is the magnitude and size of solid particles that affect viscosity the most. The key parameter is the amount (concentration) and size (dispersion) of asphaltene particles per volume unit. The Effect of Asphaltenes on the Network Strength. To describe the asphaltene influence on the solid network strength, the static condition is necessary. The solids build the network properly and the strength can be evaluated by yield stress measurement. Figure 7 shows yield stress versus asphaltene concentration. The highest yield stresses exhibit the oil with 0.01 wt % of asphaltenes and the values are decreasing with increasing asphaltene concentration subsequently. To explain such a behavior, the term of “critical asphaltene concentration in the wax network” should be introduced.
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Figure 7. Effect of LM2 asphaltene concentration on yield stress at (×) 13 °C, (0) 15 °C, (2) 17 °C, (O) 19 °C, (9) 25 °C. Note the logarithmic scale.
At low concentration, the asphaltenes are well dispersed; they are not extensively flocculated and the particle size is very small so they can be easily incorporated in the structure. Because of previously described asphaltene-wax spatial interference, the n-alkane molecules have more time (sample is subcooled, crystallization is slower) to find an optimal position in the crystal, so the structure is more compact (organized) and stronger. The strongest structure is built once the asphaltene concentration in the solution reaches its critical value. Beyond this point, there are no more spaces in the structure and the rest of the asphaltenes remain outside. With increasing asphaltene concentration, there are more and more asphaltenes outside the structure and they tend to flocculate among themselves. This provides large asphaltene particles or even asphaltene layers inside the network. These highly flocculated asphaltenes are the weak point of the structure, because they are not connected to it so tightly. Once the shear is applied, the structure breaks in this weak point. With increasing asphaltene concentration, the number of “free” solids is increasing, part of the waxes are participating in asphaltene-wax composite instead of building a proper wax network, and the gel is becoming weaker and weaker. This results in low yield stress at higher asphaltene concentration. The Effect of Asphaltenes on WAT. Measurement of WAT is a static procedure. The value itself has no relation to structure strength or to viscosity, except for indicating the temperature where the first wax crystal appears in solution. Below WAT the wax crystallization starts. There is a strong disagreement between the WAT values obtained from different measuring techniques. The polarized light microscopy (PLM) provides one of the highest WAT estimate and it can indicate the very first crystal in the solution.8 For that reason, the PLM was employed in the experiment. Figure 8 shows the results. It was already shown that the flocculated asphaltenes provide crystal sites for waxes.1 This fact was more or less confirmed for the asphaltene concentration 0.05 wt % and higher. However, at very low concentration (0.01%), WAT was surprisingly high at almost 10 °C higher than the WAT for asphaltene-free oil. The explanation could be that the asphaltenes are perfectly dispersed or almost dissolved and that they are affecting paraffins at the molecular level. There are
Kriz and Andersen
Figure 8. The effect of LM2 asphaltene concentration on wax appearance temperature.
several possible crystal sites and all of them are easily accessible for paraffins since there are very few asphaltenes per volume unit and there is almost no risk of spatial interference. With increasing asphaltene concentration, the surface area (number of possible sites) is increasing more and very possibly the WAT is increasing as well. This ends when the “critical asphaltene concentration in the solution” is reached. At this point, the highest WAT is reached and the asphaltene flocculation starts. Once the flocculation starts, the surface area is decreased drastically. The lowest value at the WAT versus asphaltene concentration curve (Figure 8) refers to the lowest asphaltene surface area. It is the lowest concentration where the asphaltenes are flocculated. With increasing concentration, the asphaltene surface area depends on how many asphaltenes are in the solution (increase in surface area) and how big the particles are that they tend to build (decrease in surface). The experiment shows that both actions are possible; however, the influence of concentration is prevailing, even though one can expect a high degree of the spatial interference at high concentration as well. Conclusions The asphaltenes play an important role during the crude oil wax crystallization. The fragile equilibrium among asphaltenes-resins-paraffins was a very important factor. The effect of asphaltenes in the view of wax crystallization strongly depends on the degree of asphaltene dispersion or flocculation more than on the asphaltene type or origin. At very low concentration, the asphaltenes are well dispersed or even dissolved in the crude oil matrix. They are easily accessible for any kind of interaction with the paraffins. They can be fully incorporated in the wax structure and play an important role within. To build the asphaltene molecule in the wax structure, a higher driving force is needed because of asphaltene-paraffin spatial interference, and it was observed during the viscosity measurement as a certain delay in crystallization. Once the asphaltene concentration reaches the critical value, where the complete dispersion cannot be maintained by the rest of the oil, the fragile equilibrium is broken and asphaltene molecules tend to flocculate among themselves. The asphaltenes are packed together and the amount of possible interacting sites for paraffins is considerably
Effect of Asphaltenes on Wax Crystallization
decreased. Flocculated asphaltenes might coprecipitate with the waxes as well (part of the sites are still accessible), but they cannot be fully and tightly incorporated in the wax network. Beyond this point, the asphaltenes flocculate into large condensed particles or layers. For waxes, it is very difficult to build a proper wax network within this highly flocculated system because of spatial interference, and they crystallize on the asphaltene particle and produce an unorganized asphaltene-paraffin composite rather than a proper wax network. This leads to depressed yield stress and wax appearance temperature. The dynamic viscosity is increased at this point because of a higher amount of
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solids in the system. As it was shown, the role of asphaltenes during the wax crystallization is very complex, many parameters are involved, and some of them remain unknown. Further research is needed to describe the asphaltene-wax interaction during the crude oil wax crystallization. Acknowledgment. The authors thank Mr. Povl V. Andersen for help in the laboratory. The financial support of the Czech Ministry of Education and of IVCSEP is gratefully acknowledged. EF049819E
