Effect of thermal treatment temperature on the flowability and wax

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Effect of thermal treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil Haoran Zhu, Chuanxian Li, Fei Yang, Hongye Liu, Dinghong Liu, Guangyu Sun, Bo Yao, Gang Liu, and Yansong Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02552 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Energy & Fuels

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Effect of thermal treatment temperature on the flowability and wax deposition

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characteristics of Changqing waxy crude oil

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Haoran Zhu1, Chuanxian Li1, 2, Fei Yang1, 2*, Hongye Liu1, Dinghong Liu1, Guangyu Sun1, 2, Bo Yao1,

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Gang Liu1, 2, Yansong Zhao3

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1

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People’s Republic of China

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Shandong 266580, People’s Republic of China

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College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266580,

Shandong Provincial key laboratory of Oil & Gas Storage and Transportation Safety, Qingdao,

Department of Biomedical Laboratory Sciences and Chemical Engineering, Western Norway

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University of Applied Sciences, NO-5063 Bergen, Norway

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*Corresponding Author: Fei Yang E-mail: [email protected] or [email protected]

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ABSTRACT: The influence of thermal treatment temperature on the flowability and wax deposition

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characteristics of Changqing waxy crude oil was detailedly researched through pour point/rheological

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tests, a cylindrical Couette wax deposition experimental device, DSC analyses, asphaltenes stability tests

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and microscopic observations. It is found that the flowability of the crude oil can be greatly improved

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through increasing the thermal treatment temperature. Meanwhile, the wax deposition rate of the crude

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oil can be outstandingly inhibited with the increase of thermal treatment temperature from 50 °C to

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70 °C. Moreover, the flow regime can also influence the wax deposition characteristics. Under cold flow

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regime (22 °C/12 °C), the structure of formed wax deposits is homogeneous while the aging of the wax

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deposits is not obvious. Under hot flow regime (30 °C/20 °C), the aging of the wax deposits is obvious

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but a heterogeneous two-layer structure exists in the formed wax deposits at the thermal treatment

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temperatures 50 °C and 60 °C, with a hard/thin bottom layer and a relatively soft/thick surface layer.

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With the increase of thermal treatment temperature to 70 °C, the two-layer wax deposit structure can not

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be identified due to the extremely thin wax deposit thickness. Increasing the thermal treatment

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temperature can disperse and activate the asphaltenes in the crude oil; the activated asphaltenes have

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stronger interactions with waxes, and then decrease the WAT of the oil and change the precipitated wax

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crystals’ morphology further, hence dramatically improving the crude oil flowability and inhibiting wax

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deposition. Under hot flow regime, the two-layer wax deposit structure is mainly triggered through the

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diffusion of wax molecules and asphaltenes from the bulk oil into the wax deposits. Under cold flow

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regime, the asphaltenes have already co-precipitated with wax molecules into big wax flocs, which is

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difficult to diffuse from the bulk oil into the already existed wax deposits. Meanwhile, the aggravated

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flowability of bulk oil under cold flow regime further hinders the diffusion of wax molecules into the

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interior of the wax deposit. Therefore, the two-layer wax deposit structure cannot be found under cold

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flow regime.

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1. INTRODUCTION

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With the exploration and development of offshore oilfields, wax deposition problems during the crude

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oil transportation in the submarine pipelines are becoming more and more serious due to the low

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ambient temperature 1. Waxy crude oil is a complicated mixture including large quantities of paraffin

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waxes. Normally, the hot waxy crude oil may experience two different flow regimes 2-4 (the cold and hot

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flow regimes) when it flows along a pipeline in cold surroundings. When the crude oil temperature is

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higher than the wax appearance temperature (WAT) and pipe wall temperature is lower than the WAT,

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the condition is called hot flow regime. Under hot flow regime, wax molecules are dissolved in the bulk

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oil and the wax deposition is mainly occurred on the surface of inner pipe wall. With the continuous

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flowing of waxy crude oil along the pipeline, the crude oil will dissipate heat to the surrounding causing

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the decrease of oil temperature. When the oil temperature decreases below WAT and pipe wall

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temperature is lower than the WAT, the condition is called cold flow regime. Under cold flow regime, a


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certain number of wax molecules will precipitate and suspend in the bulk oil and another part of wax

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molecules will deposit on the inner wall of pipeline 5. The existence of wax deposition not only

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decreases the efficiency of crude oil pipelines but also increases the risk of the entire transportation

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system. A simple and effective method to deal with wax deposition problems is pigging, that is, using a

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special instrument to pass through the pipeline and scrape off the wax deposits 1, 5. In order to effectively

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guide the pigging operation of waxy crude oil pipelines, many scholars have carried out lots of

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theoretical and experimental studies on the wax deposition phenomenon in pipelines. In terms of wax

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deposition mechanism, molecular diffusion, shear dispersion, Brown motion, gravitational settling, heat

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transfer and other mechanisms were proposed 6-8, but most of the researchers pointed out that molecular

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diffusion mechanism played a leading role in the process of wax deposition. In terms of wax deposition

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test systems, cold finger

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deposition system

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influencing factors on wax deposition were well evaluated such as oil temperature

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temperature 16,17, temperature difference 18,19, flow rate 20-23 and deposition time 16-23. The aging property

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of wax deposits with time was also explained by the molecular diffusion/counter diffusion mechanism 20,

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24

14,15

9,10

, flow loop

10,11

, Taylor-Couette device

12,13

and visual parallel plate wax

are developed to simulate the wax deposition characteristics. Numerous 16,17

, pipe wall

and Ostwald ripening mechanism 25.

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Asphaltenes are the largest molecular weight nonhydrocarbon with the strongest polarity in crude oil.

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Asphaltenes can disperse in crude oil in the form of associating particles by hydrogen bond, π-π bond

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and charge transfer

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of wax molecules will be influenced by the associated asphaltene particles and the crystallization habit

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of wax will be changed, thus improving the low temperature flowability of waxy crude oil 28-32. In other

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words, the asphaltenes can act as natural flow improvers. However, how the asphaltenes influence wax

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deposition process is insufficient and controversial. Some researchers

26, 27

. It has been widely confirmed that the precipitation and crystallization process

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considered that the asphaltenes


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34-37

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do not participate in the process of wax deposition. However, some researchers

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asphaltenes do affect the wax deposition behavior. Zhang et al.

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asphaltenes dispersion state on the wax deposition characteristics of crude oil through cold finger

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experiments. It was found that the dispersed asphaltenes favor wax deposition, but the aggregated

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asphaltenes depress wax deposition. Yang et al.

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asphaltenes on the wax deposition characteristics of model waxy oil. The results showed (a) the wax

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deposition rate greatly decreased after adding a small number of asphaltenes (0.1~3 wt%), indicating the

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asphaltenes can act as wax inhibitors; (b) the wax deposit structure is also changed from the original

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homogeneous structure into the two-layer wax deposit structure (including an inner hard layer and an

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outer soft layer) after adding the asphaltenes. The authors also speculated that the diffusion of wax

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molecules and asphaltenes from the bulk oil to the wax deposits are crucial for the formation of the two-

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layer wax deposit structure.

35, 37

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found that the

investigated the influence of

studied the influence of n-pentane precipitated

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It is believed that suitable thermal treatment of waxy crude oil can significantly improve crude oil low

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temperature flowability 37, 38. When waxy crude oil is thermally treated ideally, the paraffin waxes in the

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oil is fully dissolved while the associated asphaltenes aggregates in the oil could be dissociated and fully

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dispersed; then, during the cooling process of the waxy crude oil, the dissociated asphaltenes have much

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stronger interactions with wax molecules, thus improving the low temperature flowability of the oil.

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Does the thermal treatment influence the wax deposition behavior of waxy crude oil? If so, what’s the

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influencing mechanism? Up to now, people have no answer to the questions.

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With the purpose to answer above questions, in this work, the influence of thermal treatment

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temperature on the flowability of Changqing waxy crude oil was first researched. Then, the wax

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deposition characteristics of the crude oil thermally-treated at different temperatures was studied by an

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in-house Couette wax deposition experimental device; the composition and structure of the obtained


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wax deposit samples were analyzed through the differential scanning calorimeter (DSC) and direct observation, respectively. The results showed that increasing the thermal treatment temperature not only improves the crude oil flowability but also greatly inhibits the wax deposition rate. Meanwhile, the wax deposit structure was also influenced. The influencing mechanisms were well discussed based on the asphaltenes stability test and wax precipitation test of the crude oil. 2. EXPERIMENTAL SECTION 2.1 Crude oil sample. The crude oil sample investigated in this paper is collected from Changqing oilfield in China. As illustrated in Fig. 1, the crude oil has a wide carbon number distribution range from C8 to C36. As listed in Table 1, the contents of resins and asphaltenes are small (no more than 5 wt%); both the high wax content (12.5 wt%) and the low density (0.860 g/cm3) indicate that Changqing crude oil belongs to light waxy crude oil. 2.2 Experiments. 2.2.1 Flowability of Changqing waxy crude oil thermally-treated at different temperatures. Pour Point Tests The method given by Chinese Standard SY/T 0541-2009 39 was chosen to determine the pour points of Changqing crude oil thermally-treated at different temperatures (40 °C, 50 °C, 60 °C, 70 °C and 80 °C). The pour point test of each crude oil sample was repeated 3 times to make sure the reproducibility. Rheological Tests All rheological tests were carried out through a DHR-1 rheometer (TA Instruments Co., America). Before the rheological tests, the crude oil samples were thermally-treated at specific thermal treatment temperatures (50 °C, 60 °C, and 70 °C) for 1 hour in sealed glass bottles. In order to minimize the evaporation of the crude oil samples during testing, a matching cover is placed on the test cylinder to inhibit the evaporation of the oil.


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The rheological tests were carried out under two different modes. In the flow mode, a fixed shear rate (50 s−1) was applied to the oil sample during the cooling process from initial thermal treatment temperature to 15 °C at a cooling rate of 0.5 °C/min. The variation of the oil viscosity/apparent viscosity with test temperature can be obtained. In the oscillation mode, the storage modulus G′, loss modulus G″, and loss angle δ can be measured during the cooling process of the oil sample from initial thermal treatment temperature to 10 °C at a cooling rate of 0.5 °C/min. The oscillatory amplitude and frequency were kept constant at 0.0005 and 1 Hz, respectively. The gelation point at which G′ = G″ or δ = 45°, can also be recorded. The rheological tests of each crude oil sample were repeated 3 times to make sure the reproducibility. 2.2.2 Wax deposition characteristics of Changqing waxy crude oil thermally-treated at different temperatures. All the wax deposition experiments were carried out via an in-house Couette device. The device mainly consists of inner cylinder, outer cylinder, high/low temperature water baths, motor, and so on. More information about the device can be available in formal published works 36, 37, 40. The crude oil was first thermally-treated in sealed glass bottles at different temperatures (50 °C, 60 °C, and 70 °C) for 1 hour. After the thermal treatment, the crude oil was cooled to 30 °C (hot flow regime) or 22 °C (cold flow regime), and then the crude oil was poured into the outer cylinder and the wax deposition experiments were conducted. All the wax deposition experiments were conducted using 1.5 kg of crude oil with a 150 r/min rotation speed. The deposition time was fixed at 3 h, 6 h, 12 h and 24 h, respectively. The wax deposition experiments were conducted under both cold flow regime and hot flow regime. As evident from Fig. 11, the WATs of the crude oils are in the range of 24~27 °C. Under cold flow regime, the hot bath temperature and cold bath temperature were kept constant at 22 °C and 12 °C, respectively, both of which are lower than the WAT. Under hot flow regime, the hot bath temperature


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was fixed at 30 °C, which is slightly higher than the WAT; the cold bath temperature was fixed at 20 °C, which is slightly lower than the WAT. Immediately after each wax deposition experiment, the wax deposit barrel is lifted out of the rotating sample barrel. The macroscopic morphology of the formed wax deposits was observed and photographed. Both the wax content and WAT of the wax deposits can also be gained through DSC thermal analysis. Meanwhile, the total volume of each wax deposit sample (Vt) was tested, and the average thickness of wax deposit could be calculated as Vt/Se, where Se is the effective surface area of the wax deposition barrel. To identify the structure of wax deposit, the innermost layer and outermost layer wax deposits were carefully scraped off and analyzed. The twolayer structure can be identified from two aspects: on the one hand, the two-layer structure could be clearly detected by touch because the outer-layer is easy to be removed from the wax deposition barrel surface while the inner-layer is very difficult to be scraped off; on the other hand, we could test and compare the WAT and wax content of the outermost layer and innermost layer. 2.2.3 Asphaltenes stability of Changqing waxy crude oil thermally-treated at different temperatures. Electronic conductivity tests The electronic conductivity of the crude oil thermally-treated at different temperatures (50 °C, 60 °C, and 70 °C) were measured via a DDS-11A digital conductivity meter (Leici Co., China). A DJS-0.01 titanium alloy electrode was installed on the conductivity meter. The crude oil was first thermally-treated at a specific temperature for 1 h; subsequently, gradually change the crude oil temperature to test temperature at a cooling or heating rate of 0.5 °C/min; finally, hold the oil temperature constant at test temperature (30 °C, 40 °C, 50 °C, and 60 °C) for 2 min, and then the electronic conductivity could be tested. Light transmittance tests The crude oil was first thermally-treated at different temperatures (50 °C, 60 °C, and 70 °C) for 1 h; subsequently, n-heptane was added to the crude oil at a crude oil/n-heptane mass ratio of 1:40. After mixing evenly, the supernatant liquid was collected after a 30 min centrifugal


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operation in which the centrifuge speed was fixed at 10000 r/min. Finally, a UV-Vis Spectrophotometer (Shanghai Spectrometer Co., China) was used to measure the light transmittance of the supernatant liquid. The light transmittance of n-heptane measured at 600 nm wavelength was selected as the reference sample. 2.2.4 Wax precipitation characteristics of Changqing waxy crude oil thermally-treated at different temperatures. DSC test The exothermic characteristic of the oil sample was determined via a DSC821e differential scanning calorimeter (Mettler-Toledo Co., Switzerland). The oil sample was statically cooled from its initial thermal treatment temperature (50 °C, 60 °C, and 70 °C) to -20 °C with a cooling rate of 10 °C/min. Based on the DSC thermal analysis, the wax content and WAT of the oil sample can be obtained. The DSC test of each wax deposit sample was repeated 3 times to make sure the reproducibility. The deviations of WAT and wax content are around 0.1 °C and 0.3 wt%, respecitively. Microscopic observation of wax crystals The morphology of wax crystals in the oil sample thermallytreated at different temperatures (50 °C, 60 °C, and 70 °C) was obtained employing an Olympus BX51 polarized microscope. In order to control the oil sample temperature, a thermal stage (Motic China Group Co., China) was equipped. Firstly, a glass slide was placed in the thermal stage and a small amount of crude oil was loaded on the slide; subsequently, a cover glass is placed on the slide to prevent the evaporation of the crude oil; then the oil sample in the thermal stage was thermally-treated at a specific temperature for 1 h; after that, the oil sample was statically cooled to 15 °C at 0.5 °C /min; finally, the morphology of precipitated wax crystals in the oil sample can be observed and recorded. 3. RESULTS AND DISCUSSION 3.1 Flowability of Changqing waxy crude oil thermally-treated at different temperatures. 3.1.1 Pour point test.


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As illustrated in Table 2, the pour point of the crude oil depends heavily on the thermal treatment temperature. When the thermal treatment temperature is ≤ 50 °C, the pour point is relatively high (15 °C) and does not change with the thermal treatment temperature. When the thermal treatment temperature increases from 60 °C to 70 °C, the pour point of the crude oil decreases from 9 °C to 2 °C. With the further increase of the thermal treatment temperature to 80 °C, the pour point is still 2 °C. Obviously, for Changqing crude oil investigated in this paper, the optimum thermal treatment temperature is 70 °C. 3.1.2 Viscosity-temperature curve test. The effect of thermal treatment temperature on the viscosity/apparent viscosity of the crude oil can be observed in Fig. 2. When the test temperature is higher than the WAT (27 °C), the viscosities of the crude oil thermally-treated at different temperatures are nearly the same and at a lower level. But at the test temperatures < WAT, the crude oil apparent viscosity increases with the decrease of test temperature and the increasing rate is directly related to the thermal treatment temperature. Increasing the thermal treatment temperature greatly decreases the viscosity increasing rate. For example, at the test temperature 15 °C, the apparent viscosity of the crude oil thermally-treated at 50 °C is 470 mPa·s, while the apparent viscosity of the crude oil thermally-treated at 60 °C and 70 °C decreases to 338 mPa·s and 134 mPa·s, respectively. 3.1.3 Viscoelastic test. As illustrated in Fig. 3, when the test temperature is above or around the WAT, the G′ and G″ are very small but the G″ is greatly bigger than G′, resulting in the high value of δ (approaching 90 º). Obviously, the crude oil shows Newtonian fluid behavior in the temperature range. With the further decrease of the test temperature, the wax will precipitate in the form of wax crystal particles causing the rapid increase of both the G′ and G″, and the crude oil exhibits viscoelastic fluid behavior. The increasing trend of G′ is more pronounced than that of G″, resulting in the crossing of G′ and G″ at the gelation point. When the


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test temperature is below the gelation point, the crude oil elastic response becomes superior to the viscous response. With the increase of thermal treatment temperature, the gelation point of the crude oil gradually decreases from 24 °C under thermal treatment temperature 50 °C to 21.5 °C under thermal treatment temperature 70 °C. Meanwhile, both G′ and G″ decrease obviously with the increase of thermal treatment temperature, indicating the viscoelasticity of the crude oil is weakened by increasing the thermal treatment temperature. For example, at 10 °C, the G′/G″ of the crude oil thermally-treated at 50 °C, 60 °C, 70 °C are 87100 Pa/11700 Pa, 41000 Pa/6870 Pa and 2670 Pa/559 Pa, respectively. According to the results in 3.1.1~3.1.3, it can be concluded that increasing the thermal treatment temperature impedes the formation of wax crystal network structure, consequently decreasing the pour point, viscosity, gelation point and G′/G″ of the crude oil. 3.2 Wax deposition characteristics of Changqing waxy crude oil thermally-treated at different temperatures. 3.2.1 Wax deposition characteristics under cold flow regime. Under cold flow regime, the bulk oil temperature is 22 °C while the wax deposition barrel temperature is 12 °C. As we can see from Fig. 4a and b, the formed wax deposit of the crude oil thermally-treated at 50 °C is a thick deposit layer with bright black color. According to Fig. 4c and d, both the wax content and WAT of the surface layer and bottom layer wax deposits are highly similar, indicating the structure of the formed wax deposit is homogeneous. As detailed in Table 3, the wax deposit thickness develops with deposition time from 3.5 mm at 3 h to 5.5 mm at 24 h. As listed in Table 2, the pour point of the crude oil thermally-treated at 50 °C is 15 °C, which is higher than the wax deposition barrel temperature (12 °C). We think this is the main reason to generate such a thick wax deposit. Nevertheless, the wax content and WAT of the wax deposit increases very slowly with the


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increase of experimental duration (deposition time), indicating the aging of the wax deposit is not obvious. For example, as the experimental duration delayed from 3 h to 24 h, the wax content and WAT of the wax deposit only increase from 13.8 wt%/27 °C to 14.6 wt%/27.5 °C. As illustrated by Fig. 5 and Table 3, increasing the thermal treatment temperature to 60 °C greatly inhibited the wax deposition. When the experimental duration is 24 h, the wax deposit thickness is only 0.5 mm. Nevertheless, compared with the formed wax deposit in Fig. 4, the wax content and WAT of the formed wax deposit increase outstandingly, meaning that the thin wax deposit is very hard. As the experimental duration delayed from 6 h to 24 h, the wax content and WAT of the wax deposit only increase from 36.7 wt%/41 °C to 38.2 wt%/42.5 °C, indicating the aging rate of the formed wax deposit is very slow. When the thermal treatment temperature further increases to 70 °C, the wax deposition is further inhibited and the formed wax deposit becomes harder (see Fig. 6 and Table 3). The wax deposit thickness is only 0.4 mm at 24 h, but the wax content and WAT of the wax deposit increase further to 49 wt%/47 °C. The aging rate of the formed wax deposit is also very low. 3.2.2 Wax deposition characteristics under hot flow regime. Under hot flow regime, the bulk oil temperature is 30 °C while the wax deposition barrel temperature is 20 °C. As indicated in Fig. 7a and b, the formed wax deposit of the crude oil thermally-treated at 50 °C shows a heterogeneous two-layer wax deposit structure, with a hard/thin bottom layer and a relatively soft/thick surface layer. According to Fig. 7c and d, the wax content and WAT of the bottom layer wax deposit is greatly higher than those of the surface layer wax deposit, thus confirming a twolayer structure exists in the formed wax deposit. As listed in Table 4, the wax deposit thickness develops from 0.6 mm to 1.8 mm as the experimental duration delayed from 3 h to 24 h. The wax content and WAT of the surface/bottom layer wax deposits also develop clearly with deposition time, proving that


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the aging of the wax deposit is obvious. For example, the wax content and WAT of the surface layer wax deposit increase from 12.8 wt%/28 °C at 3 h to 16.8 wt%/35 °C at 24 h; meanwhile the wax content and WAT of the bottom layer wax deposit increase from 20 wt%/37 °C at 3 h to 27.8 wt%/44 °C at 24 h. Obviously, the wax content and WAT of the bottom layer wax deposit are great higher than those of the surface layer wax deposit, thus imparting a harder structure of the bottom layer wax deposit. As indicated in Fig. 8, the formed wax deposit of the crude oil thermally-treated at 60 °C also shows a clear two-layer wax deposit structure. As detailed in Table 4, increasing the thermal treatment temperature to 60 °C inhibited the wax deposition. The wax deposit thickness is 1.2 mm at 24 h. The aging of the wax deposit is obvious: the wax content and WAT of the surface layer wax deposit increase from 12.9 wt%/28 °C at 3 h to 16.9 wt%/35 °C at 24 h; the wax content and WAT of the bottom layer wax deposit increase from 22.4 wt%/40 °C at 3 h to 28.8 wt%/45 °C at 24 h. By comparison, the wax content and WAT of the surface layer wax deposit formed under thermal treatment 50 °C and 60 °C are very similar; but the wax content and WAT of the bottom layer wax deposit formed under thermal treatment temperature 60 °C are higher, indicating a harder structure of the bottom layer wax deposit formed under thermal treatment temperature 60 °C. When the thermal treatment temperature increases to 70 °C (see Fig. 9 and Table 4), the wax deposition is further inhibited, but the wax deposit is too thin (0.4 mm at 24 h) to identify the two-layer structure. The aging of the formed wax deposit is also obvious: the wax content and WAT of the wax deposit increase from 40.1 wt%/49 °C at 6 h to 50.6 wt%/52 °C at 24 h. Obviously, the formed wax deposit becomes harder resulted from the increased WAT and wax content. It should also be noticed that compared with the wax deposits formed under cold flow regime, the wax deposits formed under hot flow regime have much higher WAT at the similar wax content. This is perhaps because the wax deposits formed under hot flow regime contain more high carbon number wax


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molecules due to the higher wax deposition temperature and the more obvious aging phenomenon. 3.3 Influencing mechanism of thermal treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil. For the sake of discovering the influencing mechanism of thermal treatment temperature on the flowability and wax deposition characteristics of the crude oil, the influences of thermal treatment temperature on asphaltenes stability and wax precipitation characteristics were studied here. 3.3.1 Asphaltenes stability test. The electrical conductivity of colloidal dispersion (dispersed in aqueous phase or organic phase) is an important parameter of the dispersion. It has been verified that the test temperature 41, the size 42, 43 and concentration 42-45 of the colloid particles are three key factors affecting the electrical conductivity of the dispersion. At a fixed test temperature, increasing the particle concentration (with the size unchanged) or decreasing the particle size (with the concentration unchanged) leads to the increase of the dispersion electrical conductivity. Similar to the colloidal dispersion, crude oil also has the electrical conductance property due to the existence of some polar impurities such as resins and asphaltenes in the oil,

46

and

the electrical conductivity of the crude oil is greatly influenced by the content of polar impurities (resins and asphaltenes) and the size of asphaltenes particles. Recently, Yang et al. 26 investigated the effect of two amphiphiles on the stability of three heavy oils. They found that at a fixed test temperature, a small dosage of dodecyl benzene sulfonic acid (DBSA) obviously increases the electrical conductivity of the heavy oils, but a small dosage of lauric amine (LA) decreases the electrical conductivity of the heavy oils. The DBSA is a typical asphaltenes dispersant or stabilizer 26-49, which can disperse the asphaltenes into smaller size with larger amount and then increase the electrical conductivity. The LA is a typical asphaltenes flocculants or distabilizer

26, 48

, which can flocculate the asphaltenes into bigger size with


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Page 14 of 42

smaller amount and then decrease the electrical conductivity. Therefore, using the electrical conductivity test to analyze the asphaltenes stability of crude oil is feasible and convenient. In this paper, the electronic conductivity of Changqing waxy crude oil thermally-treated at different temperatures (50 °C, 60 °C, and 70 °C, above the WAT of the oil) were measured via a DDS-11A digital conductivity meter (Leici Co., China) to discover the effect of thermal treatment temperature on the asphaltenes stability in the crude oil. As shown in Fig. 10, at a fixed thermal treatment temperature, the oil electronic conductivity increases with increasing the test temperature. Increasing test temperature facilitates Brownian motion of asphaltenes particles, thus increasing the electronic conductivity. At a fixed test temperature, the oil electronic conductivity also increases clearly with the increase of thermal treatment temperature proving that increasing the thermal treatment temperature could disperse asphaltenes particles into smaller ones and then improves the asphaltenes stability in the oil phase. Table 5 shows the effect of thermal treatment temperature on the light transmittance of the supernatant liquid of the crude oil/n-heptane mixture (with the mixing mass ratio 1:40) after centrifuge. It is clear that the light transmittance decreases gradually from 56.3 % at thermal treatment temperature 50 °C to 55.1 % at thermal treatment temperature 60 °C, then to 52.7 % at thermal treatment temperature 70 °C. The decrease of light transmittance indicates that more smaller asphaltenes particles suspend in the supernatant liquid, which cannot be centrifuged into sediment. That is, increasing the thermal treatment temperature improves the asphaltenes stability in the oil phase. In conclusion, the increase of thermal treatment temperature promotes the dispersion of asphaltenes, therefore improving the asphaltenes stability in the crude oil. 3.3.2 Wax precipitation test. Based on Fig. 11a, the WAT of the crude oil decreases from 26.9 °C to 26.4 °C with the increase of thermal treatment temperature from 50 °C to 60 °C. With the further increase of thermal treatment


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Energy & Fuels

temperature to 70 °C, the WAT decreases to 24.8 °C. However, as seen in Fig. 11b, increasing the thermal treatment temperature almost has no impact on the precipitated wax content at low temperatures. From Fig. 12a, we can see that the wax crystals of the oil thermally-treated at 50 °C are a large number of small needle-like particles, which favors the establishment of three-dimensional network structure due to the large oil/wax crystals interface. With the increase of thermal treatment temperature, the wax crystal tends to become large and aggregated wax flocs with a relatively smaller amount (Fig. 12b and c), which are adverse for the formation of three-dimensional network structures. 3.3.3 Influencing mechanism of thermal treatment temperature on the flowability of Changqing waxy crude oil. In the crude oil industry, polymeric flow improvers or wax inhibitors have been widely used to improve the crude oil flowability and inhibit wax deposition

39,50-53

. In fact, asphaltenes are the natural

flow improves/wax inhibitors of waxy crude oil 31, 32. With the increase of thermal treatment temperature, the asphaltenes in the crude oil are well dispersed and activated (see Fig. 10 and Table 5). The dispersed and activated asphaltenes have stronger interactions with wax molecules, thus greatly improving the flowability of the crude oil. During the cooling process of waxy crude oil, the interactions of asphaltenes and wax molecules are very complex. Many scholars

27, 31

have found the aggregation state of

asphaltenes has an important effect on the interaction between asphaltenes and wax molecules. Based on many experimental phenomena, many mechanisms (nucleation and cocrystallization) were presented to explain the interaction of asphaltenes and wax molecules. In this paper, the more aggregated asphaltenes in the crude oil thermally-treated at 50 °C can act as nucleation site of wax molecules and have stronger nucleation interaction with wax molecules than the more dispersed asphaltenes in the crude oil thermally-treated at 70 °C. Thus, the wax molecules in the crude oil thermally-treated at 50 °C can precipitated at highest temperature and the WAT of the crude oil decreased with the increase of thermal


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Page 16 of 42

treatment temperature (see Fig. 11a). Moreover, the more dispersed asphaltenes in the crude oil can serve as connecting point between wax crystals so the wax crystals in the crude oil thermally-treated at 70 °C are more aggregated than the crude oil thermally-treated at 50 °C and 60 °C (see Fig. 12). The great change of the wax crystals morphology among the crude oil thermally-treated at different temperatures are responsible for the difference of low temperature flowability of the crude oil thermallytreated at different temperatures. 3.3.4 Influencing mechanism of thermal treatment temperature on the wax deposition characteristics of Changqing waxy crude oil. 3.3.4.1 Formation mechanism of different thickness and wax content in wax deposit layer of Changqing waxy crude oil thermally-treated at different temperatures. The wax deposit layer formed by Changqing waxy crude oil thermally-treated at 50 °C is thicker than those of the crude oil thermally-treated at 60 °C and 70 °C whether under cold flow regime or hot flow regime. The main reasons are as follows: the low temperature gelation structure of Changqing crude oil thermally-treated at 50 °C is stronger than those of the crude oil thermally-treated at 60 °C and 70 °C therefore the initial wax deposit layer of the crude oil thermally-treated at 50 °C is easier to exist stably under the same oil/wall temperature and rotation speed. Compared with the initial wax deposit layer of the crude oil thermally-treated at 70 °C, the initial wax deposit layer of the crude oil thermally-treated at 50 °C has stronger ability to resist shearing and peeling. Secondly, the wax crystal particles of the crude oil thermally-treated at 50 °C are dispersed which is not conducive to the diffusion of wax molecules in the wax deposit layer. The higher viscosity of the crude oil thermally-treated at 50 °C also hinders the diffusion of wax molecules into formed wax deposit layer. Therefore, only a small number of wax molecules diffused into the interior of the wax deposit layer. A large number of wax molecules migrated from the bulk oil deposit on the surface of the formed wax deposit layer, leading to a rapid increase in


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Energy & Fuels

the thickness of the wax deposit layer. In summary, compared with the crude oil thermally-treated at 60 °C and 70 °C, the wax deposit layer formed by the crude oil thermally-treated at 50 °C is thicker. As can be seen from Table 4 and 5, with the increase of thermal treatment temperature, the wax deposition rate was inhibited while the wax content in the wax deposit layer was also increased. The main reasons why the wax content of the wax deposit layer formed by the crude oil thermally-treated at 70 °C is higher than those of the crude oil thermally-treated at 50 °C and 60 °C are that the low temperature gelation structure of the crude oil thermally-treated at 70 °C is weaker than those of the crude oil thermally-treated at 50 °C and 60 °C. Therefore, it is necessary to have a higher wax content in order to exist stably on the surface of wax deposition barrel. Moreover, the viscosity of the crude oil thermally-treated at 70 °C is lower than those of the crude oil thermally-treated at 50 °C and 60 °C, which is conducive to the diffusion of wax molecules both in the bulk oil and in the wax deposit layer. The wax crystal particles in the crude oil thermally-treated at 70 °C are more aggregated and the shape of wax crystal particles is closer to the spherical one. This kind of wax crystal morphology is favorable to the diffusion of wax molecules in the wax deposit layer. Therefore, the wax content of the wax deposit layer formed by the crude oil thermally-treated at 70 °C is higher than those of the crude oil thermally-treated at 50 °C and 60 °C. 3.3.4.2 Formation mechanism of heterogeneous wax deposit structure of Changqing waxy crude oil thermally-treated at different thermal treatment temperatures. As mentioned in the recently published paper of our research group

36

, under hot flow regime, the

two-layer stratification phenomenon was triggered by the diffusion of wax molecules and asphaltenes and the formation process was speculated to include four steps. (a) Right after the wax deposition test, an outer-layer wax deposit is formed. The inner-layer wax deposit cannot be found within the deposition time 1 h, indicating that the inner-layer stems from the outer-layer. That is, a part of outer-layer (the part


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Page 18 of 42

approaching the wax deposition barrel surface) will gradually transform into the inner-layer. (b) Because the part of outer-layer wax deposit close to the deposition barrel surface has the lowest temperature, the wax molecules and asphaltenes begin to co-precipitate out of the oil phase and then generate the largest concentration gradient at this part, thus facilitating the wax molecules/asphaltenes diffusion from bulk oil to this part. With the continuous co-precipitation of wax molecules and asphaltenes at this part, the structural strength of this part enhances progressively. In the end, this part will transform into an initial inner-layer wax deposit, which can be indentified at deposition time 1 h. (c) With the further increase of wax deposition time, the associated asphaltene particles cannot diffuse into the initial inner-layer wax deposit any more due to the increased compactness of this layer. Therefore, the asphaltenes content in the inner-layer wax deposit is almost unchanged after deposition time 1 h. Nevertheless, the smaller wax molecules could still diffuse into the initial inner-layer wax deposit and then cause aging of this layer with time. Meanwhile, the new inner-layer wax deposit will generate on the surface of the existed innerlayer, leading to the slowly development of the inner-layer wax deposit. (d) The outer-layer wax deposit could be considered as a medium between the bulk oil and the inner-layer wax deposit. Most of the wax molecules and asphaltenes diffuse across the outer-layer wax deposit into the inner-layer wax deposit. Therefore, although the outer-layer wax deposit is much thicker than the inner-layer wax deposit, the WAT, wax and asphaltenes content of the outer-layer wax deposit are lower and develops gradually with the increase of deposition time. Under hot flow regime, obvious heterogeneous structures exist in the wax deposition layer of Changqing crude oil thermally-treated at 50 °C and 60 °C. In this paper, we considered that the twolayer wax deposit structure formed under hot flow regime is also triggered by the mechanism mentioned in (a)~(d). However, the heterogeneous structure disappears when the thermal treatment temperature rises to 70 °C. The reason of this phenomenon is that the low temperature gelation structure of


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Energy & Fuels

Changqing crude oil thermally-treated at 70 °C is so weak that it is difficult to adhere to a layer with low wax content on the formed wax deposit layer. Moreover, the overall wax content of the wax deposit layer is extremely high and the wax deposit layer is so thin that can not identify the two-layer structure. When the flow regime changed from hot flow to cold flow, the heterogeneous structure disappeared. There are mainly two reasons. Firstly, under cold flow regime, the asphaltenes have already coprecipitated with wax molecules into big wax flocs, which is difficult to diffuse from the bulk oil into the already existed wax deposit. Secondly, the decrease of oil temperature and wall temperature under cold flow regime leads to the decrease of diffusion coefficient of wax molecules. Meanwhile, the large increase of viscosity in cold flow regime greatly hinders the diffusion of wax molecules. Thus the outermost-layer and innermost-layer wax deposition shows the same appearance. And, the two-layer wax deposit structure cannot be found under cold flow regime. 4. CONCLUSIONS The influence of thermal treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil was researched in detail based on the pour point test, rheological measurement, wax deposition test, DSC analysis, asphaltenes stability test and microscopic observation. The conclusions are as follows: (1) Through increasing the thermal treatment temperature, the crude oil flowability can be greatly improved by decreasing the pour point, viscosity, gelation point, and G’/G’’. Meanwhile, the wax deposition rate of the crude oil can be outstandingly inhibited with the increase of thermal treatment temperature from 50 °C to 70 °C. (2) The wax deposition characteristics is also affected by the flow regime. Under cold flow regime (22 °C/12 °C), the structure of formed wax deposits is homogeneous while the aging of the wax deposits is not obvious. Under hot flow regime (30 °C/20 °C), the aging of the wax deposits is obvious while the


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Page 20 of 42

formed wax deposits at the thermal treatment temperatures 50 °C and 60 °C exhibit a two-layer structure, with a hard/thin bottom layer and a relatively soft/thick surface layer. With the increase of thermal treatment temperature to 70 °C, the formed wax deposit is too thin to identify the two-layer structure. (3) Based on the asphaltenes stability test and wax precipitation test, we consider that increasing the thermal treatment temperature can disperse and activate the asphaltenes in the crude oil; the activated asphaltenes have stronger interactions with waxes. The WAT of the crude oil is further decreased and the precipitated wax crystals’ morphology is further changed, hence dramatically improving the crude oil flowability and inhibiting wax deposition. (4) Under hot flow regime, the two-layer wax deposit structure is mainly triggered through the diffusion of wax molecules and asphaltenes from the bulk oil into the wax deposit. Under cold flow regime, the asphaltenes have already co-precipitated with wax molecules into big wax flocs, which is difficult to diffuse from the bulk oil into the already existed wax deposits. Meanwhile, the aggravated flowability of bulk oil under cold flow regime further hinders the diffusion of wax molecules into the interior of the wax deposit. Therefore, the two-layer wax deposit structure cannot be found under cold flow regime. Acknowledgement The authors thank the financial support from the National Natural Science Foundation of China (51774311),

Shandong

Provincial

Natural

Science

Foundation

of

China

(ZR2016EEM22,

ZR2017MEE022), and Shandong Provincial Key Research and Development Program of China (2017GSF216003). References (1) Huang, Z.; Zheng, S.; Fogler, H. S., Wax deposition: experimental characterizations, theoretical modeling, and field practices. CRC Press: 2016. (2) Bidmus, H. O.; Mehrotra, A. K. Solids deposition during “Cold Flow” of wax-solvent mixtures in a flow-loop


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Energy & Fuels

apparatus with heat transfer. Energy Fuels 2009, 23 (3), 184-94. (3) Arumugam, S.; Kasumu, A. S.; Mehrotra, A. K. Modeling of solids deposition from waxy mixtures in hot flow and cold flow regimes in a pipeline operating under turbulent flow. Energy Fuels 2013, 27 (11), 6477-6490. (4) Merino-Garcia, D.; Correra, S. Cold flow: a review of a technology to avoid wax deposition. Petrol. Sci. Technol. 2008, 26 (4), 446-459. (5) Aiyejina, A.; Chakrabarti, D. P.; Pilgrim, A.; Sastry, M. K. S., Wax formation in oil pipelines: A critical review. International Journal of Multiphase Flow 2011, 37 (7), 671-694. (6) Azevedo, L. F. A.; Teixeira, A. M., A critical review of the Modeling of wax deposition mechanisms. Petrol Sci Technol 2003, 21 (3-4), 393-408. (7) Haj-Shafiei, S.; Serafini, D.; Mehrotra, A. K., A steady-state heat-transfer model for solids deposition from waxy mixtures in a pipeline. Fuel 2014, 137, 346-359. (8) Van der Geest, C.; Guersoni, V. C. B.; Merino-Garcia, D.; Bannwart, A. C., Wax Deposition Experiment with Highly Paraffinic Crude Oil in Laminar Single-Phase Flow Unpredictable by Molecular Diffusion Mechanism. Energy & Fuels 2018, 32 (3), 3406-3419. (9) Kasumu, A. S.; Mehrotra, A. K., Solids Deposition from Wax-Solvent-Water "Waxy" Mixtures Using a Cold Finger Apparatus. Energy & Fuels 2015, 29 (2), 501-511. (10) Chi, Y. D.; Daraboina, N.; Sarica, C., Effect of the Flow Field on the Wax Deposition and Performance of Wax Inhibitors: Cold Finger and Flow Loop Testing. Energy & Fuels 2017, 31 (5), 4915-4924. (11) Valinejad, R.; Nazar, A. R. S., An experimental design approach for investigating the effects of operating factors on the wax deposition in pipelines. Fuel 2013, 106, 843-850. (12) Eskin, D.; Ratulowski, J.; Akbarzadeh, K., A model of wax deposit layer formation. Chem Eng Sci 2013, 97, 311319. (13) Ji, Z. Y.; Li, C. X.; Yang, F.; Cai, J. Y.; Cheng, L.; Shi, Y. A., An experimental design approach for investigating and modeling wax deposition based on a new cylindrical Couette apparatus. Petrol Sci Technol 2016, 34 (6), 570-577. (14) Soedarmo, A. A.; Daraboina, N.; Sarica, C., Microscopic Study of Wax Deposition: Mass Transfer Boundary Layer and Deposit Morphology. Energy & Fuels 2016, 30 (4), 2674-2686. (15) Cabanillas, J. P.; Leiroz, A. T.; Azevedo, L. F. A., Wax Deposition in the Presence of Suspended Crystals. Energy


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& Fuels 2016, 30 (1), 1-11. (16) Huang, Z. Y.; Lu, Y. D.; Hoffmann, R.; Amundsen, L.; Fogler, H. S., The Effect of Operating Temperatures on Wax Deposition. Energy & Fuels 2011, 25 (11), 5180-5188. (17) Tian, Z.; Jin, W.; Wang, L.; Jin, Z., The study of temperature profile inside wax deposition layer of waxy crude oil in pipeline. Frontiers in Heat and Mass Transfer (FHMT) 2014, 5 (1). (18) Mehrotra, A. K.; Bhat, N. V., Deposition from "Waxy" Mixtures under Turbulent Flow in Pipelines: Inclusion of a Viscoplastic Deformation Model for Deposit Aging. Energy & Fuels 2010, 24 (4), 2240-2248. (19) Lashkarbolooki, M.; Seyfaee, A.; Esmaeilzadeh, F.; Mowla, D., Experimental Investigation of Wax Deposition in Kermanshah Crude Oil through a Monitored Flow Loop Apparatus. Energy & Fuels 2010, 24 (2), 1234-1241. (20) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R., Morphological evolution of thick wax deposits during aging. Aiche J 2001, 47 (1), 6-18. (21) Wu, C. H.; Wang, K. S.; Shuler, P. J.; Tang, Y. C.; Creek, J. L.; Carlson, R. M.; Cheung, S., Measurement of wax deposition in paraffin solutions. Aiche J 2002, 48 (9), 2107-2110. (22) Fong, N.; Mehrotra, A. K., Deposition under turbulent flow of wax-solvent mixtures in a bench-scale flow-loop apparatus with heat transfer. Energy & Fuels 2007, 21 (3), 1263-1276. (23) Lu, Y. D.; Huang, Z. Y.; Hoffmann, R.; Amundsen, L.; Fogler, H. S., Counterintuitive Effects of the Oil Flow Rate on Wax Deposition. Energy & Fuels 2012, 26 (7), 4091-4097. (24) Zheng, S.; Zhang, F.; Huang, Z. Y.; Fogler, H. S., Effects of Operating Conditions on Wax Deposit Carbon Number Distribution: Theory and Experiment. Energy & Fuels 2013, 27 (12), 7379-7388. (25) Coutinho, J. A. P.; da Silva, J. A. L.; Ferreira, A.; Soares, M. R.; Daridon, J. L., Evidence for the aging of wax deposits in crude oils by Ostwald Ripening. Petrol Sci Technol 2003, 21 (3-4), 381-391. (26) Yang, F.; Li, C.; Yang, S.; Zhang, Q.; Xu, J., Effect of dodecyl benzene sulfonic acid (DBSA) and lauric amine (LA) on the associating state and rheology of heavy oils. J Petrol Sci Eng 2014, 124, 19-26. (27) Li, Y. Z.; Han, S. P.; Lu, Y. D.; Zhang, J. J., Influence of Asphaltene Polarity on Crystallization and Gelation of Waxy Oils. Energy & Fuels 2018, 32 (2), 1491-1497. (28) Oh, K.; Deo, M., Characteristics of Wax Gel Formation in the Presence of Asphaltenes. Energy & Fuels 2009, 23 (3-4), 1289-1293.


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(29) Tinsley, J. F.; Jahnke, J. P.; Dettman, H. D.; Prud'home, R. K., Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology. Energy & Fuels 2009, 23 (3-4), 2056-2064. (30) Alcazar-Vara, L. A.; Garcia-Martinez, J. A.; Buenrostro-Gonzalez, E., Effect of asphaltenes on equilibrium and rheological properties of waxy model systems. Fuel 2012, 93 (1), 200-212. (31) Yao, B.; Li, C.; Yang, F.; Zhang, X.; Mu, Z.; Sun, G.; Zhao, Y., Ethylene–Vinyl Acetate Copolymer and ResinStabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 1. Effect of Wax Content and the Synergistic Mechanism. Energy & Fuels 2018, 32 (2), 1567-1578. (32) Yao, B.; Li, C.; Yang, F.; Zhang, X.; Mu, Z.; Sun, G.; Liu, G.; Zhao, Y., Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 2. Effect of Asphaltene Content. Energy & Fuels 2018, 32 (5), 5834-5845. (33) Yang, X.; Kilpatrick, P., Asphaltenes and waxes do not interact synergistically and coprecipitate in solid organic deposits. Energy & Fuels 2005, 19 (4), 1360-1375. (34) Lei, Y.; Han, S. P.; Zhang, J. J., Effect of the dispersion degree of asphaltene on wax deposition in crude oil under static conditions. Fuel Process Technol 2016, 146, 20-28. (35) Li, C. X.; Cai, J. Y.; Yang, F.; Zhang, Y.; Bai, F.; Ma, Y. Y.; Yao, B., Effect of asphaltenes on the stratification phenomenon of wax-oil gel deposits formed in a new cylindrical Couette device. J Petrol Sci Eng 2016, 140, 73-84. (36) Yang, F.; Cai, J. Y.; Cheng, L.; Li, C. X.; Ji, Z. Y.; Yao, B.; Zhao, Y. S.; Zhang, G. Z., Development of Asphaltenes-Triggered Two-Layer Waxy Oil Gel Deposit under Laminar Flow: An Experimental Study. Energy & Fuels 2016, 30 (11), 9922-9932. (37) Cheng, C.; Boger, D.; Nguyen, Q., Influence of thermal history on the waxy structure of statically cooled waxy crude oil. SPE Journal 2000, 5 (02), 148-157. (38) Li, C., Crude oil rheology. China University of Petroleum Press: 2007. (39) Yang, F.; Yao, B.; Li, C. X.; Shi, X.; Sun, G. Y.; Ma, X. B., Performance improvement of the ethylene-vinyl acetate copolymer (EVA) pour point depressant by small dosages of the polymethylsilsesquioxane (PMSQ) microsphere: An experimental study. Fuel 2017, 207, 204-213. (40) Yang, F.; Cheng, L.; Liu, H. Y.; Yao, B.; Li, C. X.; Sun, G. Y.; Zhao, Y. S., Comb-like Polyoctadecyl Acrylate (POA) Wax Inhibitor Triggers the Formation of Heterogeneous Waxy Oil Gel Deposits in a Cylindrical Couette Device.


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Energy & Fuels 2018, 32 (1), 373-383. (41) Ganguly, S.; Sikdar, S.; Basu, S. Experimental investigation of the effective electrical conductivity of aluminum oxide nanofluids. Powder Technol. 2008, 196, 326-330. (42) White, S. B. Enhancement of Boiling Surfaces Using Nanofluid Particle Deposition (Ph.D. Thesis). University of Michigan, 2010. (43) Sarojini, K. G. K.; Manoj, S. V.; Singh, P. K.; Pradeep, T.; Das, S. K. Electrical conductivity of ceramic and metallic nanofluids. Colloid Surf. A 2013, 417, 39-46. (44) Banisi, S.; Finch, J. A.; Laplante, A. R. Electrical conductivity of dispersions: a review. Miner. Eng. 1993, 6 (4), 369-385. (45) Fang, F.; Zhang, Y. F. DC electrical conductivity of Au nanoparticle/chloroform and toluene suspensions. J. Mater. Sci. 2005 (11), 40, 2979-2980. (46) Fotland, P.; Anfindsen, H.; Fadnes, F.H. Detection of asphaltene precipitation and amounts precipitated by measurement of electrical conductivity. Fluid Phase Equilibr. 1993, 82, 157-164. (47) Lian, H.; Lin, J.-R.; Yen, T. F. Peptization studies of asphaltene and solubility parameter spectra. Fuel 1994, 74 (3), 423-428. (48) Chang, C.-L.; Fogler, H. S. Stabilization of asphaltenes in aliphatic solvents using alkylbenzene-derived amphiphiles. 1. Effect of the chemical structure of amphiphiles on asphaltene stabilization. Langmuir 1994, 10 (6), 1749-1757. (49) León, O.; Contreras, E.; Rogel, E. The influence of the adsorption of amphiphiles and resins in controlling asphaltene flocculation. Energy Fuels 2001, 15, 1028-1032. (50) Yang, F.; Zhao, Y.; Sjöblom, J.; Li, C.; Paso, K. G., Polymeric wax inhibitors and pour point depressants for waxy crude oils: a critical review. Journal of Dispersion Science and Technology 2015, 36 (2), 213-225. (51) Yao, B.; Li, C.; Yang, F.; Sjöblom, J.; Zhang, Y.; Norrman, J.; Paso, K.; Xiao, Z., Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate. Fuel 2016, 166, 96-105. (52) Yao, B.; Li, C.; Yang, F.; Zhang, Y.; Xiao, Z.; Sun, G., Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant. Fuel 2016, 184, 544-554. (53) Yao, B.; Yang, F.; Li, C. X.; Shi, X.; Sun, G. Y.; Ma, X. B., Performance improvement of the ethylene-vinyl


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acetate

Energy & Fuels

copolymer

(EVA)

pour

point

depressant

by

small

dosages

of

the

Amino-functionalized

Polymethylsilsesquioxane (PAMSQ) microsphere: An experimental study. Fuel 2017, 207, 204-213.


Energy & Fuels

Figures:

8 Mass fraction / wt%

1 2 3 4 572 5 6 7 573 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28574 29575 30 31576 32 33577 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 42

6 4 2 0

8

12

16 20 24 28 Carbon number

32

36

Fig. 1 Carbon number distribution of Changqing waxy crude oil.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26578 27 579 28 29580 30 31581 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Viscosity(apparent viscosity) / mPa·s

Page 27 of 42

500 50 °C 60 °C 70 °C

400

shear rate: 50 s-1

300 200 100 0

15

20 25 Temperature / °C

30

Fig. 2 Viscosity-temperature curves of Changqing crude oil thermally-treated at different temperatures.


Energy & Fuels

105

60

24°C

40

G' G'' / Pa

10-1

G'10°C=87100Pa G''10°C=11700Pa

101

10-1

80

G' G'' δ

103 δ/°

101

80

G' G'' δ

103

G'10°C=41000Pa G''10°C=6870Pa

60

24°C

40

10-3

20

10-3

20

10-5

0

10-5

0

10

15

20 25 30 35 Temperature / °C

40

10

15

(a)


δ/°

105

G' G'' / Pa

40 (b)

104

10-2

G'10°C=2670Pa G''10°C=559Pa

60 40

21.5°C

20

10-4 10-6

δ/°

100

80

G' G'' δ

102 G' G'' / Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18582 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34583 35584 36 37585 38 39586 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

0 10

15


40 (c)

Fig. 3 Viscoelastic tests of Changqing crude oil thermally-treated at (a) 50 °C (b) 60 °C (c) 70 °C.


Page 29 of 42

0.65

outer layer wax deposit inner layer wax deposit

Heat flow / W·g-1

0.60 0.55 27 °C

0.50

27 °C

0.45

-20

0 20 40 Temperature / °C

(a)

Precipitated wax amount / wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21587 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41588 42 43589 44590 45 46591 47 48592 49 593 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


15 10

60 (c)

14.2 wt%

5 0 -20

-10 0 10 20 Temperature / °C

30 (d)

(b) Fig. 4 Macroscopic appearances of the outermost layer (a)/innermost layer (b) wax deposits formed by the crude oil thermally-treated at 50 °C under cold flow regime at 6 h; and the exothermic characteristics (c) and precipitated wax amount (d) of the wax deposition sample at different test temperatures.


40 1.4

Heat flow / W·g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21594 22 23595 24 25596 26 597 27 28598 29 30599 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

1.2 1.0

30

36.7 wt% Heat flow

20

Precipitated wax amount

0.8

41 °C

0.6 -20


40

10

0 60 (b)


Energy & Fuels

(a) Fig. 5 (a) Macroscopic appearance of the wax deposit formed by the crude oil thermally-treated at 60 °C under cold flow regime at 6 h; and (b) the exothermic characteristics and precipitated wax amount of the wax deposition sample at different test temperatures.


50

1.8

Heat flow / W·g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21600 22 23601 24 25602 26 603 27 28604 29 30605 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

1.5 43.8 wt%

1.2 0.9

30 Heat flow Precipitated wax amount

20 45 °C

0.6 0.3

-20


40

10

0 60 (b)


Page 31 of 42

(a) Fig. 6 (a) Macroscopic appearance of the wax deposit formed by the crude oil thermally-treated at 70 °C under cold flow regime at 6 h; and (b) the exothermic characteristics and precipitated wax amount of the wax deposition sample at different test temperatures.


Energy & Fuels

0.9

Heat flow / W·g-1


0.8 0.7 0.6

38 °C

0.5 30 °C

0.4

-20

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21606 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41607 42 43608 44 609 45 46610 47 48611 49 50612 51 52 53 54 55 56 57 58 59 60

Page 32 of 42


60 (c)

25 outer layer wax deposit inner layer wax deposit

20 15

20.5 wt%

10 13.0 wt%

5 0 -20 -10


40

50 (d)

(b) Fig. 7 Macroscopic appearances of the outermost layer (a)/innermost layer (b) wax deposits formed by the crude oil thermally-treated at 50 °C under hot flow regime at 6 h; and the exothermic characteristics (c) and precipitated wax amount (d) of the wax deposition sample at different test temperatures.


Page 33 of 42

1.0

Heat flow / W·g-1


0.9 0.8 0.7 0.6

42 °C

0.5

31 °C

-20 (a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21613 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41614 42 43615 44 616 45 46617 47 48618 49 50619 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


60 (c)

25 outer layer wax deposit inner layer wax deposit

20 24.1 wt%

15 10

14.9 wt%

5 0 -20 -10


40

50 (d)

(b) Fig. 8 Macroscopic appearances of the outermost layer (a)/innermost layer (b) wax deposits formed by the crude oil thermally-treated at 60 °C under hot flow regime at 6 h; and the exothermic characteristics (c) and precipitated wax amount (d) of the wax deposition sample at different test temperatures.


1.4 40

Heat flow / W·g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21620 22 23621 24 25622 26 623 27 28624 29 30625 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

1.2 40.1 wt%

1.0 0.8

30

Heat flow

20

Precipitated wax amount 47 °C

0.6 0.4

-20


40

10

0 60 (b)


Energy & Fuels

(a) Fig. 9 (a) Macroscopic appearance of the wax deposit formed by the crude oil thermally-treated at 70 °C under hot flow regime at 6 h; and (b) the exothermic characteristics and precipitated wax amount of the wax deposition sample at different test temperatures.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 626 23 24627 25 26628 27629 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Electronic conductivity /10 -3 µ S/cm

Page 35 of 42

0.45 0.40 0.35 0.30 0.25 50 °C 60 °C 70 °C

0.20 0.15 30

35


55

60

Fig. 10 Electronic conductivity of Changqing crude oil thermally-treated at different temperatures.


Energy & Fuels

Heat flow / W·g-1

70 °C 60 °C 50 °C 24.8 °C

26.4 °C

26.9 °C

-20

Precipitated wax crystals' amount/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22630 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 631 43 44632 45 46633 47 634 48 49635 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42


60

80 (a)

15 70 °C 60 °C 50 °C

10

5

0 -20

-10


30 (b)

Fig. 11 DSC curves (a) and precipitated wax crystals’ amount (b) of Changqing crude oil thermallytreated at different temperatures.


Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16636 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31637 32638 33 34639 35 36640 37 641 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

(b)

(c) Fig. 12 Wax crystal morphology of Changqing crude oil thermally-treated at (a) 50 °C (b) 60 °C (c) 70 °C.


Energy & Fuels 1 2 3 642 4 5 643 6 7 644 8 9 10 11 12 13 14645 15 16646 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

Tables:

Table 1. Basic physical properties of Changqing waxy crude oil. Saturates

Aromatics

Resins

83.37 wt%

11.71 wt%

4.11 wt%

Asphaltenes Wax content 0.81 wt%

12.5 wt%

ρ420 0.860

Initial boiling point 62 °C


Page 39 of 42 1 2 3 647 4 5 648 6 7 8 9 10 11 12649 13 14650 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2. Pour points of Changqing crude oil thermally-treated at different temperatures. Thermal treatment temperature /°C Pour point /°C

40

50

60

70

80

15

15

9

2

2


Energy & Fuels 1 2 3 651 4 5 652 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24653 25 26654 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

Table 3. Wax deposition characteristics of Changqing waxy crude oil under cold flow regime. Thermal treatment

50 °C

60 °C

70 °C

temperature Wax

Wax

deposition

deposit

time

thickness

3h

3.5 mm

13.8%/27 °C

0 mm

-

0 mm

-

6h

4 mm

14.2%/27 °C

0.2 mm

36.7%/41 °C

0.1 mm

43.8%/45 °C

12h

5 mm

14.5%/27.5°C

0.3 mm

37.3%/42 °C

0.2 mm

47.3%/46 °C

24h

5.5 mm

14.6%/27.5°C

0.5 mm

38.2%/42.5 °C

0.4 mm

49.0%/47 °C

Wax content and WAT

Wax deposit thickness

Wax content and WAT


Wax content and WAT


Page 41 of 42 1 2 3 655 4 5 656 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31657 32 33658 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 4. Wax deposition characteristics of Changqing waxy crude oil under hot flow regime. Thermal treatment

50 °C

60 °C

70 °C

temperature Wax

Wax

deposition

deposit

time

thickness

3h

0.6 mm

6h

1.0 mm

12h

1.5 mm

24h

1.8 mm

Wax content and WAT 12.8%/20.0% 28 °C /37 °C 13.0%/20.5% 30 °C /38 °C 16.0%/24.0% 32 °C /41 °C 16.8%/27.8% 35 °C /44 °C

Wax deposit thickness 0.2 mm

0.5 mm

1.0 mm

1.2 mm

Wax content and WAT 12.9%/22.4% 28 °C /40 °C 14.9%/24.1% 31 °C /42 °C 16.3%/26.3% 32 °C /43 °C 16.9%/28.8% 35 °C /45°C


Wax content and WAT

0 mm

-

0.1 mm

40.1%/47 °C

0.2 mm

44.4%/50 °C

0.4 mm

50.6%/52 °C


Energy & Fuels 1 2 3 659 4 5 660 6 7 661 8 9 10 11 12 13 14662 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 42

Table 5. Light transmittance of supernatant liquid of Changqing crude oil thermally-treated at different temperatures. Thermal treatment temperature Light transmittance

50 °C

60 °C

70 °C

56.3 %

55.1 %

52.7 %


Effect of thermal treatment temperature on the flowability and wax

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