Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions

Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions

Fuel Processing Technology 146 (2016) 20–28 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/...

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Fuel Processing Technology 146 (2016) 20–28

Contents lists available at ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions Yun Lei, Shanpeng Han, Jinjun Zhang ⁎ National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing 102249, China

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 10 March 2015 Accepted 2 February 2016 Available online xxxx Keywords: Waxy crude oil Asphaltene Wax deposition Dispersion degree of asphaltene Coldfinger experiments DSC measurements

a b s t r a c t This paper summarizes the experimental work done on the wax deposition in a waxy crude oil, considering the impact of the dispersion degree of asphaltene. Asphaltene from Venezuela residue was added into the waxy crude oil at several concentrations, and the dispersion degree of asphaltene was determined by the size distribution of asphaltene measured with microscopic observation and software analysis. Based on the size distribution of asphaltene, it shows two types of asphaltenes: dispersed asphaltenes (b2 μm) and aggregated asphaltenes (N 2 μm). By using a differential scanning calorimeter (DSC) measurements and coldfinger experiments, it is shown that the dispersed asphaltenes can inhibit the wax precipitation, and the concentration gradient of wax molecules between the bulk and deposit surface is increased, favoring wax deposition. The aggregated asphaltenes can provide crystal cores, promote wax molecules to precipitate in advance, and decrease the concentration gradient of wax molecules, depressing the wax deposition. Additionally, it is found that the dispersed asphaltenes play little role in hindering wax molecule diffusion towards the deposition surface. Furthermore, the idea that the n-paraffin component in deposits is a function of the dispersion degree of asphaltene is supported by the high-temperature gas chromatography (HTGC) measurements. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In the petroleum industry, wax deposition is a major engineering problem, which is usually faced in cold environments and for highly waxy crude oils. Often it will result in some consequences in which the transfer pressure is increased and the transportation capacity of the pipeline is dropped. In some worse cases, it may block the whole pipeline, and cause the whole pipeline to be stopped or the plugged section to be scraped [1,2]. With the increased development and utilization of deeper reservoirs which generally have asphaltenic oil, as one component of crude oil, whether the asphaltene can affect the process of wax deposition for flow assurance will be important and crucial [3,4]. Asphaltenes are usually defined as the heaviest component in crude oil, which can be precipitated by n-heptane but dissolved by toluene [5–8]. In crude oils, asphaltenes often exist as the dispersed state rather than dissolved form. When the equilibrium between the dispersed asphaltenes and other components is disruptive, dispersed asphaltenes can aggregate each other and form larger aggregated asphaltenes, resulting in forming some different dispersion degrees of asphaltene [9–13]. Anisimov et al. [14] found that the size of dispersed asphaltenes in hydrocarbon solutions was about in the level of 1 μm, and the ultimate aggregated asphaltenes can have sizes of 4–5 μm. Priyanto et al. [15] studied the relationship between the asphaltene aggregation and ⁎ Corresponding author. E-mail address: [email protected] (J. Zhang).

http://dx.doi.org/10.1016/j.fuproc.2016.02.005 0378-3820/© 2016 Elsevier B.V. All rights reserved.

the critical micelle concentration, and found that dispersed asphaltenes will be self-associated in pure polar solvents when the asphaltene concentration is above the critical micellization concentration. Fogler et al. [16] observed three kinds of asphaltenes based on the size: stable asphaltene (b0.1 μm), colloidal asphaltene (0.1–1 μm), and flocculated asphaltene (N1 μm), when they use asphaltene dispersants to study the size distribution of asphaltene and growth. Additionally, few exploratory researches about the effect of asphaltene on wax deposition have also been carried out. Tinsley et al. [17] introduced a small indoor apparatus and studied the deposit components, the deposition thickness, and the structure and morphology of deposits. However, they finally failed to clarify the effects of asphaltene. Semenov et al. [18] proposed that the asphaltene concentration in waxy oil can affect the wax deposition rate, but the theoretical explanation is rare. Few literatures have further reported this aspect. Based on the practice of the petroleum industry and the lack of the theoretical research, this paper discussed two aspects to preliminarily explore the influences of the dispersion degree of asphaltene on the process of wax deposition in waxy crude oil. The first aspect is paying attention to the influence of the dispersion degree of asphaltene on the concentration gradient of wax molecules in waxy crude oil, which can ultimately affect the process of wax deposition. According to the existing researches [19–22], the researchers did not deeply focus on the relationship between the asphaltene and the whole process of wax precipitation, especially when the temperatures of the studied systems are lower than the wax appearance temperature

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Fig. 1. Viscosity versus temperature for waxy crude oil at a shear rate of 400 s−1.

(WAT). However, if the precipitation process of wax molecules under WAT can be influenced by asphaltenes, subsequently the concentration gradient of wax molecules will be altered. According to the mechanism of molecular diffusion, it will influence the amount of wax molecules which can diffuse towards the deposition surface, ultimately affecting the wax deposition rate. However, this aspect is unclear. The second aspect is focused on the diffusion path of wax molecules due to the effect of asphaltenes as the spatial barriers. As the asphaltene size or the number per unit volume is increased, one possibility is that these suspended asphaltene particles in waxy crude oil may potentially form more spatial barriers which may prevent wax molecules from diffusing toward the deposition surface. If this were indeed the case, one would expect both the whole diffusion time and the diffusion distance of wax molecules to be increased. According to the mechanism of molecular diffusion, the wax deposition rate will be reduced. Therefore, it should also be interesting to argue whether asphaltenes can serve as spatial obstacles for wax diffusion. However, few literatures have reported this aspect. Consequently, the object of this paper was to explore the influences of the dispersion degree of asphaltene on the process of wax deposition for waxy crude oil, mainly on wax deposition rate and deposit components. The dispersion degree of asphaltene is changed by adding asphaltenes into a waxy crude oil and varying the stirring speeds at which the oil samples were prepared. In this work, there are four basic techniques involved in characterization: namely, the combinations in the microscopic observation and software analysis to obtain the size distribution of asphaltene and character of the dispersion degree of asphaltene, the DSC measurements to investigate the phase behaviors of wax molecules, the coldfinger experiments to reveal the change rules of the wax deposition rate, and the HTGC technology to analyze the distinction in the n-paraffin component in deposits.

Fig. 2. Brief schematic diagram of the coldfinger equipment used in this work.

Fig. 3. Comparison on the size distribution of asphaltene for waxy crude oils with asphaltene concentrations of 0.10 wt.% and 0.80 wt.% and stirring speed of 300 rpm.

2. Material and methods 2.1. Materials The properties of the waxy crude oil used in this work are shown below: the wax and asphaltene contents are 15.36 wt.% and 0.10 wt.%, respectively; the density at 20 °C is 818.7 kg/m3; the content of the mechanical impurities is less than 0.028 wt.%; the WAT is 40 °C. As Fig. 1 shows the viscosity–temperature curve, the experimental error is controlled within ± 5%. Additionally, to minimize changes in the properties of waxy crude oil with time, it was stored in a metal barrel. For better repeatability and reproducibility of measurements, the waxy crude oil was firstly heated at 80 °C for 2 h with a stirrer speed of 900 rpm in an open glass bottle to remove the memory and light hydrocarbons, then cooled quiescently at ambient temperature, and maintained at this temperature for 48 h before used [23]. The asphaltene used in this study is obtained from the Venezuela crude oil using the modified IP 143 procedure [22]. The major procedures are shown as in the following: Heptane (HPLC grade, 99.4%) was added into the vacuum residue at a residue–heptane proportion of 1 g:40 mL. Then the mixture was ultrasonically stirred for 45 min, and kept for 24 h in stationary condition. After that, the mixture was stirred for another 15 min, filtered using a glass filter with a pore size of 10–15 μm, and subsequently dissolved the filter cake into the toluene. After the toluene was evaporated in a vacuum oven, the hot n-heptane was used to wash the asphaltene cake for removing the non-asphaltene components. Then the asphaltenes were dried and stored in the dark.

Fig. 4. WATs obtained by DSC for waxy crude oils at various asphaltene concentrations.

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2.2. Experimental apparatus As shown in Fig. 2, the coldfinger apparatus consisted of a coldfinger (i.e., a small cylindrical metal tube), a hot water bath maintaining the temperature of oil samples, a cold water bath controlling the temperature of the coldfinger surface, and a stainless-steel vessel which is used to contain the oil sample. In this work, the stainless-steel vessel is then placed into the hot water bath to maintain a constant oil temperature for different experimental conditions. The coldfinger is used to provide the deposition surface for wax molecules when it is placed into the oil sample. Additionally, the type of the baths is HAAKE F3, and its temperature-controlled accuracy is within ±0.1 °C. 2.3. Preparation of oil sample During the preparations of oil sample, the asphaltene was added into the waxy crude oil at different concentrations by three stirring speeds of 300, 600, and 900 rpm, respectively. The temperature of the waxy crude oil was at 55 °C. The concentrations of the added asphaltene are 0.00%, 0.10%, 0.20%, 0.30%, 0.50%, 0.80% and 1.00% by weight. The type of the stirrer is IKA RW20, with a range of 200–2100 rpm. The stirred temperature was maintained at 55 °C to eliminate the precipitated waxes. The stirring time was kept for 30 min. The type of the heat bath is HH-1, and its accuracy is controlled within ±0.1 °C. The purpose of this work is to explore the effect of the dispersion degree of asphaltene on the wax deposition characteristics. The premise must ensure that the asphaltene is stable during the coldfinger experiments in waxy crude oils. In order to avoid the situation, two oil samples with added asphaltene concentrations of 0.10 wt.% and 0.80 wt.% are selected and discussed from the perspective of the size distribution of asphaltene. If the size distribution of asphaltene is slightly changed, it means that the asphaltenes are stable during the experiments. In view of this, the two oil samples were firstly placed into two glass bottles, and the size distributions of asphaltene in the two oil samples were measured, respectively. Then, the two glass bottles were put into the hot water bath and kept in static state for 48 h. After that, at the same vertical location, the two oil samples were also obtained to measure the size distribution of asphaltene. According to the results in Fig. 3, the size distribution of asphaltene slightly changes after 48 h. Hence, it can be concluded that the oil samples are stable during the wax deposition tests, and that 48 h can be determined as the deposition time. 2.4. Experimental procedure and method 2.4.1. Coldfinger experiments In wax deposition tests, the oil sample was firstly heated for 30 min at 55 °C to dissolve any precipitated wax completely, and then poured

Fig. 5. Average wax deposition rate within 48 h versus the asphaltene concentration with the coldfinger temperatures of 35 °C and 33.5 °C and a stirring speed of 300 rpm.

Fig. 6. Asphaltene particle size distribution and cumulative frequency curves for waxy crude oils at a stirring speed of 300 rpm.

into the stainless-steel vessel which had been preheated to 55 °C. The coldfinger was also maintained at 55 °C in advance, and then placed into the oil sample. After 30 min, the temperature of the cold water bath was adjusted from 55 °C to the experimental temperature. Once the temperature arrived at the set value, the time was recorded and the coldfinger deposition experiment was started. Note that in this work, the temperature of the hot water bath was maintained at 55 °C in all the coldfinger experiments. To form temperature differential between the bulk and coldfinger surface, the coldfinger temperatures (i.e., the experimental temperature) were controlled at 35 °C and 33.5 °C. All the coldfinger experiments were going on for 48 h. Then the residual oil sample was slowly and carefully drained through the bottom valve of the stainless-steel vessel. After waiting for another 2 h until no liquid oil was dropped under the same environments, the coldfinger was pulled out. The deposit was then scraped from the coldfinger surface, weighed and saved for potential analysis. In this work, for the reliability of the experimental results, another two coldfinger experiments were also done for each oil sample at the same conditions. Three experimental results are averaged as the final result.

2.4.2. Optical microscopy Microscopic observation technology was applied to study the dispersion degree of asphaltene. During the measurements, the details of the oil sample preparation, the experimental apparatus and test steps can be seen in our previous work [24]. Note that the asphaltene micrographs were obtained at a temperature of 55 °C which is 15 °C higher than the WAT of the oil sample. Using the processing software “Nano Measurer 1.2”, the size distribution of asphaltene in oil samples can be extracted in number from asphaltene images. The details about

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where φn and n are the mass fraction and carbon number of n-paraffin, respectively. 3. Results and discussion In this work, all the oil samples with various dispersion degrees of asphaltene were kept static during the coldfinger experiments. Note that the asphaltene concentration used in this work represents the concentration of added asphaltene. Additionally, the wax deposition rate is an average in the 48 h, i.e., the total mass of the deposit is divided by the depositional time (48 h) and the depositional area (1.256 × 10−3 m2). The WATs of all the oil samples were measured firstly, to inspect the wax depletion after asphaltenes were added. As shown in Fig. 4, approximately 0.5 °C difference of WATs was observed for all the oil samples. Therefore, it can be concluded that the extent of wax depletion in the experiment can be ignored and did not affect the study purposes. Additionally, one assumption was given that the temperature of deposit interface is assumed to be as a constant and equal to the coldfinger temperature, not considering the insulation effect of the deposits. For its rationality, it is mainly because the temperature of the hot water bath and coldfinger temperature are fixed and the thickness of the deposits is very thin. Therefore, the radial temperature gradient in crude oil can be approximated and regarded as a constant. 3.1. Effect of the dispersion degree of asphaltene on wax deposition due to the change in the asphaltene concentration

Fig. 7. (a) Wax solubility curves of waxy crude oils with various asphaltene concentrations measured by DSC; (b) concentration of dissolved wax at 35 °C in waxy crude oils.

obtaining the asphaltene size by the “Nano Measurer 1.2” can be seen in the “Supporting information” section. In this study, for the accuracy of experimental data, 15 micrographs of high quality were saved for each oil sample, and each experiment was repeated two times. Finally, 30 micrographs were used to count the size distribution of asphaltene to characterize the dispersion degree of asphaltene. 2.4.3. DSC measurements The phase behaviors of wax molecules in waxy crude oil were measured by DSC equipment, and the type of DSC apparatus is TA Q20. In this work, the WAT and the concentration of precipitated wax of oil samples were discussed to study the effect of the dispersion degree of asphaltene. During the measurements, the details of the preparation of the oil samples and the experimental steps can be seen in our previous work [24]. Note that these DSC measurements were repeated three times for each oil sample. The accuracy of the WAT measurements is within 1 °C. The repeatability of the concentration of the precipitated wax is within 4%. Three experimental results for each oil sample are averaged as the final result. 2.4.4. High-temperature gas chromatography The n-paraffin component in deposits was subjected to HTGC technology for their simulated distillation analysis according to the ASTM D2887 method. An analyzer (model: AC high-temperature SIMDIS) equipped with an Agilent 6890 N gas chromatographer was used in the process of analysis. In this work, the average carbon number (n) in the deposit was calculated using the following formula:



X

nφn

As shown in Fig. 5, it can be seen clearly that within a deposition time of 48 h, the average wax deposition rate firstly increases almost linearly with increasing asphaltene concentration. The highest wax deposition rate appears in the oil sample with an asphaltene concentration of 0.30 wt.%, and subsequently the wax deposition rate decreases with the further increase in asphaltene concentration. In other words, a critical asphaltene concentration exists, and its value is 0.30 wt.%. Additionally, at the same asphaltene concentrations, there is an expected result in which the average wax deposition rate within a deposition time of 48 h increases with the decreasing in the temperature of the coldfinger surface. To reveal the links between the dispersion degree of asphaltene and the wax deposition rate, firstly we have to find out the relationship between the asphaltene concentration and the dispersion degree of asphaltene. As shown in Fig. 6, a phenomenon is very obvious in which the asphaltene size also corresponds to a “critical size of 2 μm”. When the asphaltene concentration is below a critical value of 0.30 wt.%, more than 95% asphaltenes in oil samples are detected with sizes less than 2 μm. As the asphaltene concentration further increases and exceeds the critical value, the asphaltenes with sizes smaller than 2 μm significantly decrease, but the asphaltenes with sizes larger than 2 μm significantly increase. Based on the phenomenon, asphaltenes with sizes less than 2 μm are called the “dispersed asphaltene” in this work, and the “aggregated asphaltene” is referred to the asphaltenes with sizes larger than 2 μm. In other words, the change in the asphaltene concentration actually reflects the change in the dispersion degree of asphaltene. However, note that the boundary of “2 μm” is not completely absolute for any other systems, and the dispersion degree of asphaltene mainly depends on the dispersion ability of the liquid oil. Additionally, as suggested in the “Introduction” section, the influence of the dispersion degree of asphaltene on the wax deposition may reflect on the effect of behaviors of wax molecules. According to the mechanism of molecular diffusion, the wax deposition rate is mainly associated with the radial temperature gradient and the wax solubility in waxy crude oil. In this work, the radial temperature gradient can be approximated and regarded as a constant. So our research should focus on the wax solubility in crude oil. For the convenience of analysis, the cases with the coldfinger temperature of 35 °C are taken as examples. Note that the concentration of dissolved wax in waxy crude

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Fig. 8. Micrographs of the asphaltenes (i.e., black points) for bulk crude oils at various asphaltene concentrations: (a) 0.00 wt.%; (b) 0.20 wt.%; (c) 0.30 wt.%; (d) 0.80 wt.%; and (e) 1.00 wt.%.

oils was indirectly obtained by the total wax amount (15.36 wt.%) minus the amount of the precipitated wax between 40 °C (WAT) and 35 °C. In Fig. 7(a), it can be seen that the solubility curve of wax from 40 °C to 35 °C is associated with the asphaltene concentration or the dispersion degree of asphaltene. When the asphaltene concentration is below the critical value (i.e., solid line shown), the amount of the dissolved wax at each temperature is increased as the asphaltene concentration increases. However, once the asphaltene concentration is above the critical value (i.e., dotted line shown), the amount of the dissolved wax at each temperature is decreased with the increase in asphaltene concentration. Additionally, as shown in Fig. 7(b), obviously, the change trend of the dissolved wax in waxy crude oils is the same as that of the solubility curve. Consequently, combining the dispersion degree of asphaltene and the solubility curve of wax in waxy crude oils, it means that the dispersed asphaltenes can inhibit wax precipitation, make more waxes exist in the supersaturated state, and then increase the concentration

gradient of wax molecules between the bulk and deposition interface, resulting in a certain increase in the average wax deposition rate. As the asphaltene concentration in bulk oils exceeds the critical value, the dispersed asphaltenes aggregate each other, and the aggregated asphaltenes appear gradually. In this case, the aggregated asphaltenes can act as crystal nucleus for wax molecules, promote wax precipitation, and then decrease the concentration of dissolved wax in bulk crude oils. Therefore, the concentration gradient of wax between the bulk and deposition interface decreases, leading to depressed wax deposition rates.

3.2. Effect of the dispersion degree of asphaltene on wax deposition due to the change in the stirring speed at which the oil samples were prepared In order to further verify the conclusions proposed in Section “3.1”, oil samples at each asphaltene concentration were created with stirring speeds of 300, 600, and 900 rpm, respectively. The purpose is to form different dispersion degrees of asphaltene in bulk crude oils by shearing.

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Fig. 9. Asphaltene size distribution frequency curves for waxy crude oils.

Similarly, asphaltene particles in all oil samples were observed using the same procedures, as shown in Fig. 8. As shown in Fig. 9, it can be seen that for the oil samples with asphaltene concentrations less than the critical value of 0.30 wt.%, the curves of the size distribution of asphaltene only slightly shift as the stirring speed increases during the oil sample preparations. In other words, the dispersed asphaltenes cannot be further crushed by the applied shearing. However, when the asphaltene concentration is above 0.30 wt.%, with the increase in the stirring speed, the size distribution of asphaltene shifts to the left; i.e., the amount of the dispersed asphaltenes increases, and on the contrary, the amount of aggregated asphaltenes decreases. Additionally, as shown in Fig. 10, when the asphaltene concentration is lower than the critical value, the average wax deposition rate within 48 h is nearly identical with the increase in the stirring speed. However, when the asphaltene concentration is higher than the critical value, the average wax deposition rate within 48 h increases with increase in the stirring speed. In these cases, it can be concluded that the smaller asphaltene size and the more dispersed asphaltenes both favor the wax deposition happening and increase the wax deposition rate, which further verify the conclusion in Section “3.1”.

Similarly, as demonstrated in Fig. 11, for the oil samples with lower asphaltene concentrations (i.e., b 0.30 wt.%), because the change in the amount of the dispersed asphaltenes is smaller as the stirring speed increases, the effect of dispersed asphaltenes on the wax precipitation in crude oils is less distinct. As shown in Fig. 11(a), the concentration of dissolved wax in crude oils is nearly identical at different stirring speeds, then resulting in the constant concentration gradient of wax molecules. Obviously, the wax deposition rate will remain the same. For the oil samples which contain asphaltene concentrations higher than 0.30 wt.%, the vast majority of asphaltene in crude oils exist in aggregation state. As some aggregated asphaltenes are crushed by increased shearing, many dispersed asphaltenes appear, and the action as crystal nucleus is weakened. As shown in Fig. 11(b), the concentration of dissolved wax in bulk oil increases with the increase in the stirring speed and dispersed asphaltenes. In this case, the concentration gradient of wax molecules between the bulk and deposition interface is increased. Therefore, the wax deposition rate is increased. Consequently, the process of wax precipitation under WAT in bulk crude oils is indeed influenced by the dispersed and aggregated asphaltenes, then resulting in the change in the concentration gradient of wax molecules between the bulk and deposition interface.

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Fig. 12. Comparison on the average area of wax crystals between bulk crude oils with the asphaltene concentration of 0.10 wt.% and 0.50 wt.%.

Eventually, according to the mechanism of molecular diffusion, the wax deposition rate is affected. 3.3. Effect of the “spatial obstacle effects” for asphaltene on wax deposition

Fig. 10. Average wax deposition rate within 48 h versus the stirring speeds for waxy crude oils with the asphaltene concentrations: 0.20 wt.%, 0.30 wt.%, 0.80 wt.%, and 1.00 wt.%.

Fig. 11. Concentration of dissolved wax in bulk crude oils with various asphaltene concentrations: (a) 0.20 wt.%, 0.30 wt.%; (b) 0.80 wt.%, and 1.00 wt.%.

As discussed in the “Introduction” section, another possibility when conducting the high stirring experiments or adding more asphaltene into a waxy crude oil is that more asphaltene particles will be generated per unit volume, thereby potentially forming much more spatial obstacles, which may hinder wax molecules to diffuse from the bulk oil to the coldfinger surface. If this were indeed the case, the wax deposition rate will also decrease in theory. Therefore, it is difficult to determine whether the change in phase behaviors of wax molecules due to the effect of the dispersion degree of asphaltene is the real and only reason on the change in the wax deposition rate. Based on the above-mentioned problem, two cases in the coldfinger deposition experiments were carefully discussed. First, when the asphaltene concentration is lower than the critical value, it can be found that as the asphaltene concentration increases, the average wax deposition rate also increases; i.e., the increase in the amount of the dispersed asphaltenes is favored for the process of wax deposition. Compared with the influence on the phase behavior of wax molecules, the role of the spatial obstacles can be neglected. Second, when the asphaltene concentration is higher than the critical value, it can be seen that the average wax deposition rate at a fixed stirring speed decreases as the asphaltene concentration increases. In this case, the presence of the aggregated asphaltenes may play two functions: one is that the crystal nucleus promoteswax precipitation, which decreases the wax deposition rate; the other is to play the role

Fig. 13. Cumulative mass fraction obtained by HTGC measurements for six deposits.

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4. Conclusions

Fig. 14. Average carbon number obtained by HTGC measurements for six deposits.

of the spatial obstacles, which also depresses the wax deposition rate. In this study, the former has been proved using the DSC measurements. For the latter, it is difficult to verify because the two effects cannot be clearly distinguished. However, further study found that as the stirring speed further increases, after many aggregated asphaltenes are broken into more and smaller dispersed asphaltene particles, the wax deposition rate increases as well, which further confirms that the role of dispersed asphaltenes as spatial obstacles can be neglected. However, whether the aggregated asphaltenes can be the spatial obstacles still needs further study. Consequently, it can be inferred that for the dispersed asphaltenes, their role as the spatial obstacles is minimal and do not significantly affect the diffusion process of wax molecules. In such case, they are more inclined to play the roles of inhibiting wax precipitation. As their sizes grow to a certain extent (N2 μm) and form the aggregated asphaltenes, wax molecules will directly precipitate easily in the form of asphaltene–paraffin co-crystallization to form larger wax crystals, as shown in Fig. 12. In other words, for the aggregated asphaltenes, their role as the spatial obstacles perhaps can also be neglected.

Currently, whether there are some interactions between the asphaltene and wax in waxy crude oils that would influence the process of wax deposition is an unresolved issue. To address this, by adding asphaltene into a waxy crude oil and varying the stirring speeds at which the oil samples were prepared, this paper demonstrates that the dispersion degree of asphaltene can influence the process of wax deposition. This work found that when the asphaltene concentration is below the critical asphaltene concentration (0.30 wt.%), the dispersion degree of asphaltene is larger, and the asphaltenes in waxy crude oils existed as the dispersed asphaltenes (b 2 μm). The dispersed asphaltenes can suppress the process of wax precipitation under WAT and increase the concentration gradient of wax molecules between the bulk and deposition interface. During the coldfinger experiments, the wax deposition rate increases with the increase in the dispersed asphaltenes. However, once the asphaltene concentration is below the critical asphaltene concentration, the dispersed asphaltenes aggregate each other and form the aggregated asphaltenes (N 2 μm), which made the dispersion degree of asphaltene smaller. The aggregated asphaltenes will promote wax precipitation and decrease the amount of dissolved wax or the concentration gradient of wax molecules between the bulk and deposition interface, leading to a depressed wax deposition rate. The role of asphaltenes as spatial obstacles on wax diffusion is also discussed. The results in the coldfinger experiments indicate that for the diffusion of wax molecules in crude oil, the spatial obstacles of the dispersed asphaltenes can be ignored. We also demonstrate that the component of n-paraffin in deposits is associated with the dispersion degree of asphaltene. Acknowledgments Financial support from the National Natural Science Foundation of China (Nos. 51134006 and 51204194) and the Science Foundation of China University of Petroleum Beijing (Nos. LLYJ201155 and KYJJ0406) is greatly appreciated.

3.4. Effect of the dispersion degree of asphaltene on the n-paraffin component in deposits

References

As demonstrated in Figs. 13 and 14, it can be seen that the cumulative mass fraction of n-paraffin is associated with the asphaltene concentration, and mainly includes two ranges: the low carbon number (≤ 29) and the high carbon number (≥ 40). Note that the n-paraffin content in the range of carbon numbers 29–40 was very few and was not detected through the HTGC measurements, so it will not be considered in this study. From the results in Figs. 13 and 14, it is observed that when the asphaltene concentration is lower than the critical value (0.30 wt.%), the cumulative mass fractions of n-paraffin with the low carbon number and the high carbon number both increase as the asphaltene concentration increases, and the average carbon number increases as well. Above the critical value, as the asphaltene concentration further increases, the cumulative mass fraction of n-paraffin with the low carbon number in deposits still increases, while the cumulative mass fractions at high carbon number and the average carbon number both decrease. For this phenomenon, as discussed in Sections “3.1” and “3.2”, it can be inferred that because of the differences in the dispersion degree of asphaltene, the precipitation behavior of wax molecules is influenced, resulting in some waxes to directly precipitate from the bulk crude oil, and lose the chance to diffuse toward the deposition surface and then deposit. That is to say, the composition of n-paraffin in deposit is associated with the dispersion degree of asphaltene, which will be significantly interesting to incorporate into a prediction model of wax deposition.

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