Effect of Different Aromatic Fractions on Colloidal Property of Tahe Atmospheric Residue

Transcription

1 Scientific Research 2012,Vol. 14, No. 3, pp September 30, 2012 Effect of Different Aromatic Fractions on Colloidal Property of Tahe Atmospheric Residue Yu Shuanglin 1,2 ; Zhang Jiling 3 ; Cheng Tao 2 ; Rong Lili 2 ; Xue Peng 2 ; Shan Honghong 1 (1. State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Qingdao ; 2. Petrochemical Research Institute, PetroChina Corporation, Beijing ; 3 CPECC East-China Design Branch, Qingdao ) Abstract: Different amounts of FCC slurry oil and HVGO were added to Tahe atmospheric residue respectively. The colloidal stability and asphaltene agglomeration of atmospheric residue and mixed oils were characterized by means of the mass fraction normalized conductivity and the small-angle X-ray scattering technology (SAXS). The results indicated that the stability of Tahe atmospheric residue decreased with an increasing amount of these oil fractions. It was found that the decline of the colloidal stability was attributed to the component polarity difference between oil fractions and the atmospheric residue. Though the aromaticity of FCC slurry oil was higher than that of HVGO, the polarity of aromatics and resins of FCC slurry oil was lower than those of HVGO. So the degree of the colloidal stability was more seriously destroyed by FCC slurry oil. The dispersion of asphaltenes in Tahe atmospheric residue was changed by adding FCC slurry oil and HVGO. The particle size of asphaltenes increased along with the decline of the colloidal stability. Key words: residue; colloidal stability; polarity; SAXS; particle size 1 Introduction Asphaltenes, the heaviest and the most polar fraction in petroleum, play a key role in the utilization of petroleum resources while causing severe problems in petroleum exploration, storage and processing. In petroleum processing, asphaltenes can deposit on the surface and block the pores or channels of the catalyst, leading to many problems such as deactivation of the catalyst, equipment fouling and the increase of operating cost. The abovementioned problems are resulted from the poor stability of asphaltenes that vary among different crude resources. The critical micelle concentration of asphaltenes originated from unstable crude oil is lower than that from stable crude oil in the same solvent used [1-2]. Asphaltenes from unstable crude oil are characterized by high content of aromatics with more condensed rings and low hydrogen content. Piyarat Wattana, et al. [3] separated asphaltenes into some subfractions based on their polarity and found out that the stability of asphaltenes decreased with an increasing polarity of asphaltenes. These studies improved the understanding on the relationship between the chemical structure and stability of asphaltenes. Many studies were carried out to prevent or minimize asphaltene precipitation. Researchers found out that aromatic hydrocarbons [4-5] and amphiphiles [6] could inhibit asphaltene precipitation and improve its flow property and reaction performance, but the performance of aromatic hydrocarbons and amphiphiles was influenced by the chemical structure of asphaltenes and the composition of petroleum. Vegetable oils could also be used to stabilize asphaltenes [7]. In the relevant literature, few studies have evaluated the influence of heavy oil fractions, like FCC slurry oil (FCCS) and coker gas oil, on preventing asphaltene precipitation. The FCCS, for example, a by-product of refinery, has good compatibility with crude oil and is rich in condensed aromatic hydrocarbons, which may peptize asphaltenes to change its reaction performance [8]. Although the colloidal stability of heavy oil determines the coking characteristics of asphaltenes during processing [9], there are few studies focused on the colloidal stability of heavy oil that changes with adding fractions. So in this study, different heavy oil fractions were assessed on Corresponding Author: Yu Shuanglin, yushuanglin@ petrochina.com.cn. 17

2 2012,14(3): their peptizing ability. The stability of asphasltenes could be evaluated through determining the flocculation onset. Joel Escobedo [10] and Denis Fenistein [11] used viscosity measurements to study the onset of asphaltene aggregation. The interfacial tension method was used to investigate the critical micelle concentration of asphaltenes [3]. By using optical methods, such as the near-infrared spectroscopy [5], the UV-viscosity spectrometer [12], the refractive index measurements [13], and the fluorescence measurements [14], researchers detected the onset of asphaltene agglomeration in solvent. By measuring signal changes of NMR technique, F. P. Miknis [15] and Alexandre [16] determined the onset of asphaltene flocculation in toluene. Potland [17-19] presented electrical conductivity method to investigate the asphaltene precipitation in different crude oils in the presence of precipitating solvents. Other methods, like the calorimetry measurements, the ultrasonic method and microscopy observation, could also be used to test the onset of asphaltene flocculation. The discipline of these methods is based on the mutation of the physical or chemical properties at the point of asphaltene flocculation or precipitation. Most methods intend to extract asphaltenes from heavy oil prior to dilution with different solvents, or employ diluting heavy oil with the solvent coupled with addition of precipitating agents. These treatments may change the natural dispersed morphology of asphaltenes and require much more time. The electrical conductivity method, compared with other methods, which can add precipitating agents to heavy oil directly, is more convenient in operation and can save time. Thus, the electrical conductivity method was chosen to evaluate the change of colloidal stability of heavy oil with added fractions. The SAXS technique is a useful tool to investigate asphaltene particles in solvents [20-21] or in their natural state [22]. The synchrotron-based X-ray method is employed to study asphaltene aggregate in the residue and in a mixture. Synchrotron radiation light has the advantage of strong energy, good collimation, wide spectral range, small beam divergence, high stability and short time scanning for samples. The SAXS data can be analyzed by using a monodisperse model, assuming that the colloidal aggregates are spherical. The scattering data in this work indicated that the asphaltene aggregate size increased with the addition of oil fractions corresponding to the variation tendency of colloidal stability. 2 Experimental 2.1 Materials The feed oils used in this study were Tahe atmospheric residue (THAR), FCCS and HVGO, with their properties listed in Table 1. The mass ratio of atmospheric residue to oil fraction was 10:1, 10:3, 1:1, and 3:10, respectively. Table 1 The properties of feedstock Sample THAR FCCS HVGO SARA, m% Saturates Aromatics Resins C 7 -asphatenes Elemental analysis, m% C H S N Metal content, μg/g Ni V Fe Ca Na Asphaltenes were separated by blending 40 parts of n- heptane to one part of atmospheric residue or formulated oils. The mixture was refluxed, and was then kept in darkness until cooling down prior to filtration. The precipitate was extracted by n-heptane to remove the heptane-soluble components. Then heptane was removed to obtain maltenes. Maltenes were separated into three fractions by liquid chromatography to give saturates, aromatics and resins. The number average molecular weight of fractions in toluene separated from the atmospheric residue was determined by a Knauer vapor pressure osmometer at 80. Viscosities of these fractions were measured at 100 using a Cannon-Fenske viscometer. 2.2 Small angle X-ray scattering technique Small angle X-ray scattering measurements were per- 18

3 Yu Shuanglin, et al. Effect of Different Aromatic Fractions on Colloidal Property of Tahe Atmospheric Residue formed at the Beijing Synchrotron Radiation Laboratory. All the X-ray scattering measurements were carried out at room temperature. The sample-to-detector distance was mm with the used wavelength of X-ray equating to nm. The X-rays that were scattered by the sample were collected using an image plate. The wave vector q is defined as q=4π sin (θ/2)/λ, where λ is the wavelength of the X-ray and θ is the scattering angle. The X-ray scattering experiments were performed on the Tahe atmospheric residue and formulated oils. The scattering intensity of the deasphalted oil from the samples was also measured. The net intensity was the scattering from the asphaltene aggregate particles. 2.3 Colloidal stability analysis Colloidal stability of the atmospheric residue and its mixtures were characterized quantitatively by the mass fraction normalized conductivity method. A Hewlett-Packard impedance analyzer was used to measure the changes in the conductivity of samples upon addition of n-heptane. The conductivity was measured at 35±0.1. At a frequency of 1 khz, the mass fraction normalized conductivity of samples could be calculated by the dilution ratio and the conductivity of samples. The calculated formula is Λ=κ(1+νρ/m), where Λ is the mass fraction normalized conductivity of residuum (S/m), κ the measured conductivity of atmospheric residue (S/m), m the mass of the atmospheric residue sample (g), v the volume of solvent added (cm 3 ), and ρ is the density of solvent (0.685g/cm 3 for n-heptane). The colloidal stability parameter (CSP) is defined as the mass ratio of solvent to sample at the highest value of the mass fraction normalized conductivity, namely at the asphaltene precipitation onset. using a HP4194A analyzer. The refractive index of solutions was measured by using a refractometer. The dielectric permittivity and refractive index were measured at 25± Results and Discussions 3.1 Colloidal stability change with addition of fractions The mass fraction normalized conductivity of Tahe atmospheric residue and its mixtures with HVGO or FCCS are shown in Figures 1 & 2. Table 2 compares the colloidal stability parameter (CSP) of the Tahe atmospheric residue in admixture with FCCS with that of its mixture after adding HVGO. It can be seen from Table 2 that the value of CSP decreased with an increasing concen- Figure 1 Mass fraction normalized conductivity of Tahe atmospheric residue and its mixtures with HVGO THAR; T:H=10:1; T:H=10:3; T:H=1:1; T:H=3: Measurement of mean dipole moment Petroleum components contain condensed aromatic hydrocarbons and heteroatoms along with metal species. The presence of heteroatoms and metal elements leads to charge imbalances and can form permanent electrical dipoles. By measuring the dielectric constant and refractive index of the solution, the dipole moment of fractions in a non-polar solvent could be calculated [23-24]. The fractions could be diluted with benzene to different concentrations. The dielectric permittivity of solutions was measured by Figure 2 Mass fraction normalized conductivity of Tahe atmospheric residue and its mixtures with FCCS THAR; T:F=10:1; T:F=10:3; T:F=1:1; T:F=3:10 19

4 2012,14(3): tration of HVGO or FCCS, indicating that the two oil fractions were not effective additives for stabilization of asphaltenes. Although the FCCS was rich in aromatics, the CSP of atmospheric residue after adding HVGO was greater than its mixture with FCCS. That is to say, the negative influence of HVGO was less than FCCS. These phenomena may be explained through digging into the SARA composition, viscosity and polarity of oil fractions and Tahe atmospheric residue, and will be elucidated in the following part. Table 2 Mass ratios of Tahe atmospheric residue to FCCS or HVGO at the onset of asphaltene precipitation Sample 10:1 10:3 1:1 3:10 T:F T:H Fraction composition and viscosity variation Among the SARA components, resins and aromatics are in favor of asphaltene stability, while saturates can accelerate the aggregation of asphaltenes. The colloidal stability of atmospheric residue is in dynamic equilibrium, and the variation in asphaltenes and maltenes ratio has an intrinsic relationship to the colloidal stability [9, 25]. For this purpose, the atmospheric residue and its mixtures were separated into SARA components for further study. The variation in SARA components with the addition of FCCS or HVGO are shown in Table 3. It can be seen from the data listed in Table 3 that the asphaltenes and resins contents decreased gradually, while the contents of saturates and aromatics increased after adding FCCS. When HVGO was added to THAR, the aromatics and resins contents also decreased. Mixing of THAR with petroleum fractions is a physical dilution procedure, so the tendency of SARA composition variation is related to the feedstock property. The mass ratio of saturates and asphaltenes to aromatics and resins could be used to analyze the variation of colloidal stability, because the saturates and asphaltenes contents would reflect the asphaltenes aggregation tendency, while the aromatics and resins contents could reflect their dispersing capability. Upon addition of HVGO to THAR, an increasing HVGO ratio in the mixture would lead to a decreasing peptizing and dispersing capability of aromatics and resins to enable more ready aggregation of asphaltene particles. Thus, the colloidal stability of the mixture would trend down. But for the case of adding FCCS, the ratio of slurry oil would not be relevant to variation in the colloidal stability of the mixture. An increased ratio of FCCS would result in a better peptization of asphaltenes. In contrast, a decreased colloidal stability would indicate that the SARA composition is not a key factor influencing the asphaltene stability, as reported by O. Leon [2]. In addition, the effect of petroleum fractions on the viscosity of Tahe atmospheric residue was also investigated, as shown in Figure 3. The curves of the kinematic viscosity versus the amount of petroleum fractions were not linear. The kinematic viscosity decreased sharply at first, then changed slightly after the mass ratio of petroleum fractions exceeded 60%. The decrease in kinematic viscosity indicates the decline of intra-molecular friction, making the asphaltene molecules prevented from the enhanced aggregation of colloidal particles. The association or adsorption velocity of asphaltenes intermolecular interactions was increased, so the colloidal stability would decrease. But the variation in kinematic viscosity of the mixture could not explain why the CSP of Tahe AR after addition of HVGO was larger than that after addition of FCC slurry oil. Table 3 Influence of different petroleum fractions on the SARA composition of THAR Items THAR THAR:FCCS THAR:HVGO 10:1 10:3 1:1 3:10 10:1 10:3 1:1 3:10 SARA, m% Saturates Aromatics Resins C 7 -asphaltenes w(sat.+asp.)/w(arom.+res.)

5 Yu Shuanglin, et al. Effect of Different Aromatic Fractions on Colloidal Property of Tahe Atmospheric Residue Figure 3 Effect of petroleum fractions on the kinematic viscosity of THAR HVGO; FCC Slurry Oil 3.3 Dipole moment of atmospheric residue and oil fractions In this work, the asphaltene property changed little with the addition of oil fractions. The effect of polarity of other components on the stability of asphaltenes was studied. The number average molecular weight ( ) of SARA components and the composition polarities of atmospheric residue and oil fractions are shown in Tables 4 & 5. Table 4 Number average molecular weight of SARA components in AR and petroleum fractions Feed (saturates) (aromatics) (resins) (C 7 -asphalthenes) THAR FCCS HVGO dependent mainly on their good match with the polarity and the molecular weight of the aromatic fraction. As the amount of aromatic fraction increased, the low-polarity component also increased. When the balance between asphaltenes and maltenes was broken, the colloidal stability would decrease. Simultaneously, the small molecules with short aliphatic side-chains and higher polarity in FCCS could substitute for the big molecules with long aliphatic side-chains and lower polarity in colloidal particles of asphaltenes. So the solvent layer became thinner and the steric hindrance effect decreased, and then the small particles could aggregate into big ones. Because the polarity of resins and the number average molecular weight of FCCS were less than those of HVGO, the degree of the colloidal stability of THAR was more seriously destroyed upon addition of FCCS. 3.4 Analysis of colloidal particles size Although petroleum is an opaque system, the SAXS technique can be used to study the colloidal state of asphaltene molecules thanks to the difference in electron density between the colloidal particles and the solvent. By using the SAXS technique, researchers detected that larger asphaltenes particles could dissociate when the atmospheric residue was diluted with maltenes. The impact of temperature and pressure on the aggregate size was also studied. At higher temperatures, asphaltene aggregate turned to small entities [22]. Pressure had a minor impact on the aggregate size. The scattering data of THAR and its mixtures are shown in Figures 4 & 5. Table 5 Mean dipole moment of fraction composition Fraction THAR FCCS HVGO Saturates Aromatics Resins C 7 -asphaltenes The polarity of aromatics and resins of THAR is higher than that of FCCS and HVGO. The polarity of resins in HVGO is higher than that of FCCS. J. Ph. Pfeiffer considered that the peptizing capability of maltenes was Figure 4 Scattering curves of THAR and its mixtures with FCC slurry oil THAR; T:F=10:3; T:F=3:10 21

6 2012,14(3): Figure 5 Scattering curves of THAR and its mixtures with HVGO THAR; T:H=10:3; T:H=3:10 Table 6 Effect of oil fraction on the colloidal particle size of asphaltenes Samples D, nm THAR 6.7 THAR:FCCS=10:3 6.9 THAR:FCCS=3: THAR:HVGO=10:3 7.0 THAR:HVGO=3: The diameters of gyration of asphaltenes shown in Table 6 were obtained with Guinier plots. It can be seen from Table 6 that the aggregate size of asphaltenes increased. When the mass ratio of Tahe atmospheric residue to oil fraction was 3:10, the aggregate size of asphaltene increased apparently. Fractions with low polarity added to the atmospheric residue could accelerate the flocculation process, and the asphaltene aggregate size increased. 4 Conclusions In this work, two oil fractions, FCC slurry oil and HVGO, were added to the Tahe atmospheric residue, respectively, and the results showed that both of them had negative impact on the stability of Tahe atmospheric residue. The addition of fractions into the atmospheric residue changed the composition of the atmospheric residue, resulted in asphaltene agglomeration to influence the colloidal stability of asphaltenes, but the difference in polarity of components between oil fractions and the atmospheric residue was the main factor responsible for the decrease in colloidal stability. Thus appropriate oil fractions with their polarity well matched with the atmospheric residue should be selected to peptize asphaltenes. The aggregate size of asphaltenes was monitored using the small-angle X-ray scattering technique. The addition of oil fractions changed the aggregate state of asphaltenes. Small asphaltene entities could combine together to form larger aggregate upon addition of oil fractions. When the amount of added oil fraction exceeded 50 m% in its mixture with THAR, the aggregate size of asphaltenes could increase greatly. The size of asphaltene aggregate became larger due to the decline of colloidal stability, which indicated that the colloidal stability could determine the asphaltene aggregated properties. Acknowledgement: Financial support was provided by the Ministry of Science and Technology of China through the National Basic Research Program (Grant No. 2010CB217807). References [1] Andersen S I, Speight J G. Observation on the critical micelle concentration of asphaltenes[j]. Fuel, 1993, 72(9): [2] Rogel E, Leon O, Torres G, et al. Aggregation of asphaltenes in organic solvents using surface tension measurements[j]. Fuel, 2000, 79(11): [3] Wattana P, Fogler H S, Yen A, et al. Characterization of polarity-based asphaltene subfractions[j]. Energy & Fuels, 2005, 19: [4] Clarke P F, Pruden B B. Asphaltene precipitation: Detection using heat transfer analysis, and inhibition using chemical additives[j]. Fuel, 1997, 76(7): [5] Kyeongseok O H, Ring T A, Deo M D. Asphaltene aggregation in organic solvents[j]. Journal of Colloidal and Interface Science, 2004, 27(1): [6] 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[j]. Fuel, 1994, 19(2): [7] Luiz C R J, Ferreira M S, da Silva Ramos A C. Inhibition of asphaltene precipitation in Brazilian crude oils using new oil soluble amphiphiles[j]. Journal of Petroleum Science and Engineering, 2006, 51(1/2): [8] Niu Chuanfeng, Zhang Ruichi, Dai Lishun, et al. A new integration process of residue hydrotreating combined with 22

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