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Flocculation of Particulates in Fluid Catalytic Cracking Slurry Oil: Characterization of the Particulates and the Effect of Thermal Treatment on Their Flocculation Cunhui Lin, Jingqi Wang, Zongxian Wang, He Liu, Kun Chen, Aijun Guo, and Tianping Xia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02219 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017
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Flocculation of Particulates in Fluid Catalytic Cracking Slurry Oil: Characterization of the Particulates and the Effect of Thermal Treatment on Their Flocculation Cunhui Lin, Jingqi Wang, Zongxian Wang,* He Liu, Kun Chen, Aijun Guo, and Tianping Xia State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China
ABSTRACT Removal of particulates in slurry oil (SO) from fluid catalytic cracking (FCC) process is a challenging task in refining. In this study, we investigate the intrinsic characteristics of particulates in SO and the effect of thermal treatment on their flocculation and further separation. The particulates in SO were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and Fourier transform infrared spectroscopy (FTIR), etc. Results show that the particulates in SO are composed of coke powders, and catalysts powders coated with coke species, with diameters approximately 1 µm dominating the particle size distribution. The solid contents of middle fractions depend on thermal treating severities. The particulates are originally dispersed in SO, whereas flocculation of particulates is observed after thermal treatment. A negative linear correlation is found between the solid contents of middle fractions and the asphaltene contents of thermally treated SOs under different thermal treating severities. The mechanism of particle growth due to flocculation of particulates with asphaltenes which favors efficient removal of particulates is proposed. 1
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KEYWORDS: slurry oil; thermal treatment; particulate; flocculation 1. INTRODUTION Slurry oil (SO) derived from fluid catalytic cracking (FCC) is an important by-product in FCC process, containing predominantly aromatic hydrocarbons, which makes it a potential versatile feedstock for producing high value-added products such as needle coke1−5, carbon fiber6, etc. However, the fact that SO contains varying contents of particulates restricts the high value-added utilization of SO. In the production of carbon materials, solid particles in pitch materials have been proved to affect the structure of mesophase and therefore the structure of cokes derived from mesophase7, 8. It is the micron-sized or even submicron-sized particulates that may be particularly troublesome. Such particulates cannot be removed readily using conventional separation methods such as filtration9, sedimentation10, electrostatic separation11, centrifugation12, etc. Therefore, particle growth by flocculation is believed to permit effective separation of the particulates combining conventional separation methods. Asphaltene precipitation by treating with paraffinic solvents to flocculate solid particles in heavy oils is a widely used method to enhance the removal of fine particles together with asphaltene rejection13−16. Asphaltenes are the most polar and surface-active components in heavy oils and are defined as a solubility class of heavy oils that are insoluble in paraffinic solvents such as n-pentane or n-heptane but soluble in aromatic solvents such as benzene or toluene. Asphaltenes can be adsorbed on a variety of surfaces, including mineral-based sorbents, silica and alumina, glass, metals, metal oxides, carbon, and polymers17. Researches show that resins and asphaltenes likely interact with the hydroxyl groups on the dispersed silica or clays in oil phase18, and asphaltenes are more readily adsorbed on silica compared to resins19. Upon addition of a paraffinic solvent, the asphaltene molecules aggregate and their adsorption on silica particles increases. At the same time, bridges
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between silica particles may be formed by the coated asphaltenes on particles and the precipitating asphaltenes, leading to the formation of asphaltene−silica flocs, which may be beneficial for separating particulates in oil streams containing asphaltenes13. However, solvent deasphalting usually consumes large volumes of paraffinic or naphthenic solvents in order to precipitate acceptable amount of asphaltenes and asphaltene-coated particles, and can inevitably reject too much valuable non-asphaltenic polar-type components which are entrapped within and coprecipitate with the undesirable asphaltenes17. Apart from the precipitation caused by treating with paraffinic solvents, asphaltenes can also be destabilized by thermal effect. According to Wiehe’s phase separation model20, the asphaltene cores precipitated to form a new phase due to solubility limit during thermal cracking. If there are solid particles present in the thermal reaction system, the asphaltenes or newly formed coke from asphaltene precipitation may adsorb on the solid particles, and the adsorption is influenced by the surface wettability of particles. Liu21 investigated the effect of solids on coke formation from Athabasca bitumen and vacuum residue, and found that the newly formed liquid coke could wet and coat the surface of oleophilic solids such as carbon black and asphaltene-treated kaolin, or tended to form droplets by itself without fine solids or with oleophobic solids. Similar results were observed by Wang et al.22 and Sanaie et al.23 Wang et al.24 studied the effect of fine carbonous particle additives on coke formation of residue oil during thermal reaction. They reported that the ability of particles adsorbing asphaltenes is higher when the wettability of particles on polar components is better. SO usually contains much lower amount of asphaltenes compared to bitumen or petroleum residues, but the amount is much higher than the solid content level of SO, which indicates a possibility of flocculation of particulates with asphaltenes to promote particulates removal. Instead of the costly solvent deasphalting 3
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method, thermal treatment as the oldest and in principle the simplest refinery conversion process25, is believed to be able to achieve the purpose of asphaltene precipitation accompanied with the flocculation of particulates. The asphaltene rejection and flocculated particulates removal can be achieved by distillation process, which is often used to obtain desired SO fractions for further utilization. The distillation process itself can clarify the fractions, leaving most of the solids in the bottoms. Still, some of the smaller particulates will be inevitably entrained into fractions during distillation. Hence, thermal treatment aiming at particle growth is necessary for SO before distillation. Thermal treatment also serves to convert the thermally unstable components into condensed macro molecules which are expected to be removed in the following distillation process, leaving the fractions with higher thermal stability. The thermal stability issue of SO is not in the discussion scope of this paper. In the present paper, the particulates in SO are characterized in detail to provide a basis for the subsequent separation study. Thermal treatments are carried out to investigate their effect on the efficiency of particulates removal. Moreover, the mechanism of particle growth is further studied with the expectation to shed light on the feasibility of employing this method to separate any solid particulates in heavy oils containing asphaltenes. 2. EXPERIMENTAL SECTION 2.1. Materials. The SO used in this investigation was derived from a commercial FCC unit of a refinery in China. Table 1 shows the analysis of the raw SO. The main characteristic of the SO for the purpose of this study is the high solid content which is hardly qualified for the high value-added utilization of SO. Analytical reagent grade toluene was purchased from Sinopharm Chemical Reagent Co., Ltd and used as received.
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2.2. Determination of the Solid Content and Accumulation of Particulates. According to our previous study26, filtration method using 0.22 µm membrane filter is of both high accuracy and precision in determining the solid content of SO, and the ash content of the filtrate was determined to be zero. Therefore, the solid content was determined by dissolving SO sample in 40 parts of toluene followed by filtration using a 0.22 µm membrane filter. The resultant filter cake was washed with toluene several times until the toluene passed through the filter cake without color. The filter cake was then vacuum dried at 70 °C for 2 hours. The dried filter cake was weighed and the solid content was calculated. The accumulation of particulates was accomplished by performing the same filtration process several times until the amount of particulates was enough for characterization. 2.3. Scanning Electron Microscopy (SEM)/Energy-Dispersive X-ray (EDX) Analysis of Particulates. A scanning electron microscope (S-4800, Hitachi Ltd, Japan) equipped with EDX detector was used for imaging and elemental analysis of particulates. SEM samples were prepared by mounting particulates in an electron probe mount followed by polishing and then carbon coating before SEM analysis. 2.4. Fourier Transform Infrared Spectroscopy (FTIR) analysis of particulates. Infrared spectra of particulates were obtained using a Nicolet Nexus FTIR spectrometer from Thermal Nicolet, USA. Samples were prepared for FTIR spectroscopy by mixing particulates with potassium bromide (KBr) to prevent scattering effects from large crystals. The spectra were obtained at a spectral resolution of 4 cm-1 using 32 scans per spectrum. 2.5. Light Microscopy analysis of particulates. Images of particulates in SO before and after thermal treatment were obtained using a light microscope (SGO-PH200) coupled with a camera (SGO-1000E) at a total magnification of 1200×. Apparent particle size distributions of particulates before and after thermal treatment were further obtained from the images using ImageJ2x, a software developed by National Institutes of Health, 5
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USA. At least ten images and up to one thousand particulates were analyzed for each sample in order to get reliable results. 2.6. Thermal Treatment of SO. Thermal treatment of raw SO was carried out in a 500 mL batch autoclave. For each experiment, the reactor was loaded with approximately 200 g of SO, pressure tested at 20 MPa, purged of air with nitrogen at 20 MPa for three times, and closed at 2 MPa nitrogen pressure at room temperature to ensure an inert reaction atmosphere. After the desired reaction time had elapsed at certain reaction temperature, the reactor was quenched in cold water to terminate the reaction. 2.7. Distillation of SO. SO samples before and after thermal treatment were distilled into front fraction (500 °C) using a heavy oil distillation apparatus (VDS, Oilpro Distillation Technology, Beijing, China) which follows ASTM D2892−03a. 3. RESULTS AND DISCUSSION 3.1. Morphology of Particulates. In order to have a macroscopical cognition of particulates in SO, the particulates were separated from SO both by filtration and ash analysis method (Figure 1). It is shown that the particulates derived from filtration are dark colored and finely dispersed powders without agglomeration. In comparison, the particulates derived from ash analysis present white color, which is the original color of FCC catalysts. The difference in color implies that the particulates in SO are possibly coated with organic materials which are burnt off during ash analysis. It is further proved by the lower ash content than solid content as shown in Table 1. Therefore, the particulates derived from filtration can better represent the solids in SO. The particulates accumulated by filtration were further subjected to SEM analysis which is a powerful means for characterizing micromorphology of solid materials. SEM micrographs of the particulates are shown in Figure 2. The micrographs show that the solids are granular particles with sizes smaller than 20 µm on the 6
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whole. The solids are found to be composed of large particulates in the range of 5−20 µm and small particulates of about 1 µm. It is worth noting that the number of small particulates is far more than that of large particulates, and that even smaller particulates are attached to large as well as small particulates, which makes the particulates look very irregular. 3.2. Composition of Particulates. Lin et al.27 studied the composition of solids in SO using X-ray fluorescence (XRF) and found that more than 90 wt% of the components are derived from FCC catalysts. However, they used burning method to accumulate the solids, rendering that the resulting solids are mainly oxides, and that the burning method can burn off possibly existing coke species in the solids. Therefore, filtration using a 0.22 µm membrane filter was employed as the solid-liquid separation method in this study in order to obtain solids that can better reflect the intrinsic property of the solids in SO. Table 2 shows the elemental analysis of the particulates obtained by filtration. The elemental analysis reveals that the particulates contain considerable amount of organic materials, with 17.22 wt% of carbon as the predominant element. The H/C mass ratio of the particulates is calculated to be 0.082, falling into the range of 0.06−0.10, which was reported to be the H/C mass ratio range of coke species on FCC catalysts28, 29. Thus, the organic materials present in the solids are toluene insoluble cokes. Assuming the coke species is mainly composed of carbon, hydrogen, sulfur, and nitrogen, the amount of coke species can be estimated to be about 19 wt% of the total particulates in SO. Now we know that the particulates in SO contain coke species in addition to catalysts. With the purpose of investigating which species do the various sized particulates as shown in Figure 2 belong to, the particulates derived from ash analysis as well as the particulates derived from filtration of SO after electrostatic treatment using a home-made electrostatic separator were characterized by SEM. These two kinds of particulates were 7
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selected in consideration of the fact that they can represent catalysts and coke powders, respectively. The particulates derived from ash analysis (Figure 3a) show spherical appearances with smooth surfaces, and these particulates correspond to catalyst powders due to loss of coke species during ash analysis. In contrast, the particulates derived from filtration of SO after electrostatic treatment (Figure 3b) are mainly irregular particles smaller than 1 µm, which correspond to coke powders since no elements derived from catalysts were detected by EDX. Therefore, based on the morphology characteristics of catalyst powders and coke powders, it can be speculated that the spherical main body parts of the large and small particulates as shown in Figure 2 are catalyst powders, and that the smaller irregular particulates attached to spherical catalyst powders or existing independently are mainly coke powders. To verify the above speculation, selected areas of interests as shown in Figure 4 were analyzed by EDX. Area (a) corresponds to large particulates, and area (b) corresponds to small particulates. The results (Table 3) show that both areas contain the following five elements: C, O, Al, Si, and Sb. Among these detected elements, O, Al, and Si are derived from FCC catalysts which are composed of active components (zeolites) and supporters, both containing aluminum silicates. The detection of Sb indicates that the solids in SO contain antimonial metal passivators. Although both areas contain the same elements, the relative contents are different. Compared to area (b), area (a) has relatively lower carbon content, but relatively higher contents of oxygen, aluminum, silicon, and antimony. That is, area (a) has relatively lower content of coke powders, but relatively higher contents of catalyst powders and antimonial metal passivators. Area (b), on the contrary, has relatively higher content of coke powders, but relatively lower contents of catalyst powders and antimonial metal passivators. Therefore, it is verified that the spherical main body parts of the large particulates as shown in area (a) are
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catalyst powders, which are attached with smaller coke powders, and that the irregular small particulates as shown in area (b) are mainly coke powders apart from some smaller sized catalyst powders. FTIR analysis of particulates was carried out as the supplementary evidence for the composition of particulates revealed by EDX. The FTIR spectrogram is shown in Figure 5. The peaks at 1094, 807, and 472 cm-1 are characteristic peaks of TO4 (T=Si, Al) tetrahedrons in zeolites, corresponding to asymmetrical stretching vibration, symmetrical stretching vibration, and bending vibration of T-O-T (T=Si, Al) bonds, respectively30. The peak at 3430 cm-1 is ascribed to stretching vibration of silicon hydroxyl and hydroxyl in water, while the peak at 1618 cm-1 is ascribed to bending vibration of hydroxyl in water, indicating that the aluminum silicates are in hydrated forms31, 32. Coke fines present several relevant characteristic peaks in the FTIR spectrogram. The peaks at 2921 and 2858 cm-1 correspond to asymmetrical and symmetrical stretching vibration of C-H bond in methylene, respectively. The peak at 1457 cm-1 is attributed to in-plane bending vibration of C-H bond in methyl or methylene, and the peak at 1375 cm-1 is attributed to in-plane bending vibration of C-H bond in methyl. The peak at 873 cm-1 corresponds to out-of-plane bending vibration of C-H bond in multi-substituted aromatic rings. Besides, the spectrum shows a smooth appearance in the range of 4000−2000 cm-1 on the whole, which is analogous to the characteristic absorptions of coke species33. The characteristic of particulates in SO as illustrated above makes it very difficult to efficiently separate these particulates using traditional methods. Removal of very fine particulates with a settler, centrifuge or filtrator is not feasible. Even the fine particulates-oriented electrostatic separation is not well qualified due to the existence of coke powders. Though difficult, the fact that the particulates are coated with coke do implicate these particulates may interact with asphaltenes during thermal treatment. Therefore, the effectiveness of thermal treatment on particulates removal was investigated as shown in the following section. 9
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3.3. Effect of Thermal Treatment on Solid Contents. Considering that the thermally treated SO will be subjected to vacuum distillation process, the thermal treating temperature lower than 380 °C would be unnecessary for that the temperature of heated feedstock before introduced into the vacuum column is about 380 °C or higher in order to obtain more distillate. Besides, lower thermal treating temperatures would be ineffective due to the refractory nature of slurry oil. On the other side, the upper thermal treating temperature limit depends on the coking propensity of the feedstock, which was investigated starting from 380 °C and the result is shown in Figure 6. According to the variation trend of coke yield over thermal treating time, the subsequent thermal treating temperatures were set as 400, 420, and 430 °C successively. The operable thermal treating time (i.e. maximum thermal treating time without coke formation) decreases as thermal treating temperature increases, and the upper thermal treating temperature limit is considered to be about 430 °C, at which temperature the rate of coke formation increases markedly. The onset times of coke formation were determined to be at least 120 min at 380 °C, 60 min at 400 °C, 30 min at 420 °C, and 5 min at 430 °C. Therefore, the thermal treating times were selected within the operable thermal treating times under different temperatures (380 °C-30 min, 60 min, 90 min, 400 °C-30 min, 60 min, 420 °C-30 min, and 430 °C-5 min) to investigate the effect of thermal treating severities on particulates removal. Solid contents of fractions as well as distillation bottoms of thermally treated SO were measured and the results are shown in Figure 7. Compared with the original solid content of 0.26 wt% in SO, the solid contents of distillation bottoms (Figure 7a) increase markedly, all higher than 3.6 wt%, meaning that the particulates in SO can concentrate in the bottoms efficiently after distillation. In detail, the solid content of bottoms varies from one to another as thermal treating conditions changes, indicating that thermal treatment do play a certain role in particulates removal. On the whole, the solid content of bottoms does not change much at lower thermal treating 10
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severities (380 °C, 30 min; 380 °C, 60 min; 380 °C, 90 min; and 400 °C, 30 min). Only slight increases in solid content of bottoms can be observed at higher thermal treating severities (400 °C, 60 min; 420 °C, 30 min; and 430 °C, 5 min). It is found that thermal treating temperatures and times both have impacts on the particulates in SO, and the particulates will concentrate in distillation bottoms more readily after thermally treated at higher temperatures or longer times. Since no coke formation takes place in thermal treating processes, the enrichment of particulates in distillation bottoms will necessarily reduce the solid content of fractions greatly. Figure 7b shows the solid content of fractions (
