Study on the method for determining the solid content of catalytic cracking slurry

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Study on the Determination Method of Solid Content in Catalytic Cracking Slurry Oil
Lin Cunhui, Tang Yiqiang (State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China)
Abstract: The solid contents of five different catalytic slurry oils were determined by membrane filtration, centrifugation, ash analysis, and burning methods. The composition of the solids obtained through these four methods was also analyzed. The results show that the solids obtained by membrane filtration can represent the inherent solids in the slurry oil, and the accuracy of this method is very high. The solid content determined by the centrifugation method is greatly influenced by the particle size as well as the asphaltenes content in the slurry oil, which reduces its accuracy. On the other hand, both ash analysis and burning methods can provide high accuracy, but they cannot reflect the content of coke fines in the slurry oil.
Keywords: slurry oil; filtration; centrifugation; ash analysis; burning method; solid content
Introduction: Catalytic cracking is one of the important methods for converting heavy oil into lighter fractions. As global oil resources become increasingly heavy, more and more catalytic cracking units are being used to process such heavy feedstocks as atmospheric residue, vacuum residue, vacuum gas oil, coker gas oil, or mixtures of various heavy oils. As a result, catalytic cracking of heavy oil has emerged and been widely applied. In addition, the growing demand in the market for light fuels such as gasoline and diesel has led to an increasing processing volume of catalytic cracking units. As a by-product of catalytic cracking, the output of oil slurry is also rising year by year; in recent years, China’s production of catalytic cracking oil slurry has reached 7.5 Mt/a. At present, in our country, oil slurry is mainly used as a component for blending fuel oils. Although this approach solves the problem of what to do with large quantities of oil slurry, it results in a low efficiency of utilizing it. Catalytic cracking slurry contains a high level of aromatic compounds, which makes it a potential raw material for producing high-value chemical products such as activated carbon, needle coke, and carbon fibers. However, the slurry typically contains more than 2 g/L of catalysts and coke dust particles, and these solid particles severely limit the efficient utilization of the slurry. Currently, both domestically and internationally, sedimentation, filtration, centrifugation, and electrostatic methods are generally used to remove solids from oil slurry. A prerequisite for the efficient removal of these solids is the accurate determination of the solid content in the oil slurry. Currently, the main methods for determining the solid content in catalytic cracking slurry both domestically and internationally include filtration, centrifugation, ash analysis, and carbonization-burning methods. In this study, the filter membrane suction filtration method was employed, based on the principles of conventional filtration, to determine the solid content of five catalytic cracking slurries with different properties. The results obtained were compared with those obtained using centrifugation, ash analysis, and carbonization-burning methods, and the appropriate application ranges of each method were analyzed. 1 Experiment 1.1: The raw materials used in this experiment were five types of catalytic cracking sludges with different properties, sourced from Jinzhou Petrochemical, Jilin Petrochemical (first, second, and third catalyst stages), and Daqing Petrochemical. Their basic properties are shown in Table 1. As can be seen from Table 1, these five catalytic cracking sludges exhibit significant differences in kinematic viscosity, hydrogen-to-carbon atom ratio, and group composition. Table 1 Basic properties of five different catalytic cracking slurry oils. Table 1: Basic properties of five various slurry oils. 1.2 Test methods 1.2.1 Determination of solid content by filter membrane filtration: A certain mass of the slurry oil is taken, diluted with a solvent, and then filtered under vacuum using a sand-core filtration device; the filtering material used is an organic filter membrane with a pore size of 0.22 μm. After suction filtration, hot toluene is used for reflux extraction of the filter membrane and the solid particles remaining on it, in order to remove the oil slurry components adsorbed by those solid particles. Once the extraction is complete, the filter membrane and solid particles are dried under vacuum; after weighing, the solid content (mass fraction) is calculated. 1.2.2 Determination of solid content by centrifugation: This method is primarily based on the techniques used by oil refining companies to determine the solid content in catalytic cracking slurry. The method involves diluting the slurry with kerosene, heating it to ensure uniform mixing, followed by centrifugation. The volume (mL) of the solid precipitate in the centrifuge tube is measured, and using a table, the solid content of the slurry (g·L-1) is determined. For ease of comparison, the obtained solid content (g·L-1) is finally converted into a mass fraction based on the density of the slurry. 1.2.3 Determination of solid content by the ash method: This method is primarily based on the standard method for determining the ash content in petroleum products [GB/T 508-1985, Method for determining the ash content in petroleum products. Beijing: Petroleum Chemical Research Institute, 1986]. The basic steps involve weighing a certain amount of oil slurry in a crucible, igniting it using ash-free filter paper; once the oil slurry in the crucible stops burning, the crucible is transferred to a muffle furnace where the carbonaceous residues continue to burn at high temperatures (775°C) until they are completely converted into ash. After weighing, the solid content of the oil slurry (as a mass fraction) is calculated. 1.2.4 Determination of solid content by carbonization and burning: The key steps of this method involve drying and carbonizing the oil slurry at a temperature of 500–600°C in a specific heating furnace under nitrogen purge; thereafter, the remaining carbon is completely burned at 600°C using air. Finally, the remaining powder is weighed to calculate the solid content (mass fraction) of the oil slurry. 2 Results and Discussion 2.1 Solid content determined by different methods 2.1.1 Filtration membrane method Using toluene as a diluent, three parallel determination experiments were conducted on the five aforementioned catalytic cracking sludges, and the results are shown in Table 2. As can be seen from Table 2, the Jinzhou oil slurry has the highest solid content, at 0.26%. The order of solid content among the five types of oil slurries is: Jinzhou > Daqing > First Catalyst > Second Catalyst > Third Catalyst. Regarding the particle size range of the solid contents in the slurry, some studies report it as 1–70 μm, while others indicate it as 0.6–100 μm. The pore size of the filter membrane used in this method is 0.22 μm, which is below the lower limit of the reported particle size range for the solid contents; therefore, the amount of particles that pass through the filter membrane can be considered negligible. The diluent toluene is fully miscible with the oil slurry, offering excellent diluting properties; it enables the dissolution of the heavy components adsorbed on the surface of the solid particles in the oil slurry, such as resins and asphaltenes. This reduces the residue of these heavy components on the filter membrane. After filtration, back-extraction of the filter membrane with toluene further removes the oil slurry components adsorbed on the surface of the solid particles, thereby enabling more accurate determination of the solid content. Table 2 Solid content determined by membrane filtration. Table 2: Solid content measured using membrane filtration methods. https://w8.sanwen8.cn/mmbiz/WIMuJBMHP3D2w187vKOAlu4GYgxtlckbQriarxMOmh0Ahn0d5ryaYsA6YticyLxw2icibQuibIpbk6Qx2jQ64Bl7Lag/0?wx_fmt=jpeg 2.1.2 Centrifugation method: In this experiment, 50 mL petroleum centrifuge tubes were used; 25 mL of each of the slurry and the diluent, kerosene, was added. The centrifuge was operated at a speed of 2500 r·min-1 for a duration of 2 minutes. Three parallel determination tests were conducted on the aforementioned five types of catalytic cracking sludges, and the results are shown in Table 3. As can be seen from Table 3, compared with the filter membrane filtration method, the solid content of the five types of oil slurry determined by the centrifugation method is higher; in particular, the solid content of Jinzhou oil slurry, Sancui oil slurry, and Daqing oil slurry is even over 1%. As can be seen from Table 1, among these three oil sludges with a solid content of over 1%, the Jinzhou and Daqing oil sludges have higher asphaltenes contents. Although the Sancuai oil sludge has a lower asphaltenes content, it has the highest gum content. Since the diluent kerosene has poor solubility for the heavier components in the slurry, and the three aforementioned slurries contain high levels of asphaltenes or resins, some of these resins and asphaltenes settle at the bottom of the centrifuge tube along with the solid particles in the slurry during the centrifugation process, thereby causing the measured solid content to be significantly higher than actual. For the second-stage catalyst-treated slurry and the first-stage catalyst-treated slurry, the former contains no asphaltenes while the latter has a low asphaltenes content; therefore, the degree of overestimation in their measurement results is significantly lower than that of the other three types of slurries. The measurement result for the second-stage catalyst-treated slurry is only 0.02% higher than the value obtained using the filter membrane filtration method. 2.1.3 Ash method Three parallel determination experiments were conducted using the ash method on the aforementioned five types of catalytic cracking sludges, and the results are shown in Table 4. By comparing Table 4 and Table 2, it can be seen that the solid content of the five sludges determined by the ash method has decreased to varying degrees. During the experiment, large amounts of black smoke were observed when the slurry was burned; this smoke carried some solid particles out of the slurry system, resulting in lower values for the solid content measurement. Furthermore, the carbonaceous residues remaining after combustion burn off the carbon deposits on the catalyst surface as well as the coke particles in the solid contents within a high-temperature muffle furnace; this is the main reason for the low results obtained by the ash method. To verify this claim, taking Jinzhou oil slurry as an example, elemental analysis was conducted on the solid particles enriched by the filter membrane suction filtration method, while energy dispersive spectroscopy (EDS) analysis was performed on the solid particles enriched by the ashing method. The mass fractions of hydrogen and carbon in the solid particles enriched by the filter membrane filtration method are 1.42% and 17.30%, respectively, with a hydrogen-to-carbon mass ratio of 0.082, which falls within the range of 0.06–0.10. Therefore, the hydrogen and carbon elements in these solid particles originate from the coke on the catalyst; in other words, the solid particles obtained through this method contain coke. The energy spectrum analysis results of the solid particles enriched by the ash method are shown in Figure 5. As can be seen from Figure 5, no carbon element was detected in the solid particles enriched by this method; the element with the highest concentration, oxygen, originates from the catalytic cracking catalyst and the oxides formed during combustion. Elements aluminum and silicon come from the catalytic cracking catalyst, while antimony originates from the metal passivators added during the catalytic cracking process. Calcium and iron originate from the salt crystals formed during the combustion of the oil slurry. Therefore, the main components of the solid particles enriched by the ash method are the catalytic cracking catalyst and metal passivators. Table 3 Solid content determined by centrifugation Table 3 Solid content determined by centrifugation Table 4 Solid content determined by ash analysis Table 4 Solid content determined by ash analysis https://w8.sanwen8.cn/mmbiz/WIMuJBMHP3D2w187vKOAlu4GYgxtlckbicLNJL4pFyjddtickSg3dOLq4AHOkCWXpLz6DEqhhkchic8KF2x6iaAJEQ/0?wx_fmt=jpeg Fig. 1 EDS analysis of the solid particles from Jinzhou oil slurry enriched by ash analysis Fig. 1 EDS analysis of the solids enriched through ash analysis 2.1.4 Carbonization and calcination methods: Three parallel experiments were conducted on the five aforementioned catalytic cracking oil slurries, and the results are shown in Table 5. By comparing Table 5 and Table 2, it can be seen that the solid content determined by the carbonization and burning method is much lower. This may be due, on the one hand, to the burning away of the carbon deposits on the catalyst during combustion with air, and on the other hand, to the fact that the nitrogen and air streams used in the testing process blew away some of the particles, resulting in lower test values. To verify that the coke powder in the solid contents is burned off during carbonization and calcination, an elemental analysis was conducted on the solid contents obtained by this method, and the results are shown in Table 6. As can be seen from Table 6, since the presence of C, S, and N elements was not detected, it can be concluded that both the carbonization and burning method and the ashing method burn away the coke particles contained in the solid material, resulting in a lower solid content. The detection of a small amount of H element may be due to the sample absorbing moisture from the air. Table 5 Solid content determined by burning method. Table 6 Element analysis of the solid enriched by burning method. 2.2 Comparison of the four methods for determining solid content and selection of the best method. 2.2.1 Precision analysis of the four methods for determining solid content: Based on the results of three parallel tests conducted using each of the four methods on five different oil sludges, the corresponding standard deviations were calculated, and error bar charts for each method were prepared, as shown in Figures 2–5. As can be seen from Figures 2 to 5, the standard deviations of the solid content determined by the filter membrane filtration method, the ash method, and the carbonization and burning method are relatively small, whereas the standard deviation of the solid content determined by the centrifugation method is very large. Among the solid content values of the five types of sludges determined by centrifugation, the sludge from Cuiwei had the smallest standard deviation, as its asphaltenes content was zero; the Daqing sludge had the next lowest standard deviation, due to its low gum content. Conversely, the Jinzhou sludge, which had the highest asphaltenes content, exhibited the largest standard deviation. Therefore, it can be inferred that there is a certain relationship between the solid content of sludges measured by centrifugation and their gum and asphaltenes contents. To this end, a binary linear regression was performed on the standard deviations of the solid content in the five types of oil slurry determined by the centrifugation method, along with the corresponding levels of gum and asphaltenes, resulting in the following regression equation: S = -0.4530 + 5.4210Re + 17.8195Rs, where S represents the standard deviation ； Re represents the gel content, (ω), % ； As represents the asphaltenic content, (ω), in %. The correlation coefficient of the regression equation is 0.9591, which is close to 1; thus the regression effect is significant, indicating that the regression model is valid. As can be seen from Equation (1), the higher the content of resins and asphaltenes in the oil slurry, the greater the standard deviation of the solid content determined by centrifugation; moreover, the effect of asphaltenes on the standard deviation is greater than that of resins. For ease of comparison, taking Jinzhou oil slurry as an example, the standard deviations of the four methods are plotted in Figure 6. As can be clearly seen from Figure 6, the order of solid content values determined by the four methods is: centrifugation method > filter membrane filtration method > ash method > carbonization and burning method ； Compared to the centrifugation method, the filter membrane filtration method, the ash method, and the carbonization and ignition method exhibit very good repeatability in determining the solid content. Fig. 2 Error bar of the membrane filtration method. Fig. 2 Error bars for the membrane filtration approach. Fig. 3 Error bar of the centrifugation method. Fig. 3 Error bars for the centrifugation technique. Fig. 4 Error bar of ash analysis. Fig. 4 Error bars related to ash analysis. Fig. 5 Error bar of the burning method. Fig. 5 Error bars for the burning procedure. Fig. 6 Error bars of the four methods for determining solid content (using Jinzhou oil slurry as an example). Fig. 6 Error bars for the four methods used to determine solid content. 2.2.2 Accuracy analysis and applicable ranges of the four methods for determining solid content: The organic filter membrane used in the membrane filtration method has a pore size of 0.22 μm; this method actually measures the amount of solid matter with particle sizes greater than 0.22 μm. To determine the content of particles that pass through the filter membrane, namely the amount of solid matter with a particle diameter of less than 0.22 μm, the solid content of five oil slurry filtrates (after solvent evaporation) was measured using the ash method; the results were all zero, indicating that the catalyst remained on the filter paper. The solid particles enriched by the filter membrane filtration method contain both catalyst components and coke components on the catalyst, and the determination of the solid content is not affected by the levels of gums and asphaltenes in the oil slurry; therefore, the solid content determined by this method is highly accurate and suitable for all types of oil slurries. During the centrifugation method for determination, the readings taken from the centrifuge tubes are greatly affected by human factors, resulting in certain deviations. Additionally, if the oil slurry contains high levels of gums and asphaltenes, this will cause the measured solid content to be significantly higher than the actual value. Therefore, the centrifugation method is suitable for determining solid content in oil slurries with low levels of gums and asphaltenes, especially those with low asphaltene content. The ash method and the carbonization and ignition method actually measure only the content of the catalyst in the oil slurry, and do not include the amount of coke present in the solid components; as a result, the values obtained for the solid content are lower than the actual solid content. Therefore, these two methods are suitable for oil slurries with low amounts of coke formation. In terms of the time required for the four solid content determination methods, the filter membrane filtration method takes the longest time due to the extraction step; the ash method and carbonization and combustion methods come next, while the centrifugation method takes the shortest time. Therefore, the centrifugation method is often used for the timely monitoring of the solid content in refinery sludges. Based on the comparative analysis of the four solid content determination methods mentioned above, the analysis results are presented in Table 7. Table 7 Comparison of the four methods for determining solid content. Table 7: Comparison of the four methods. 3 Conclusions: (1) The solid content of the slurry determined by the filter membrane filtration method reflects the amount of coke powder and catalyst present in the slurry; it offers high accuracy and precision, making it suitable for all types of slurries. (2) The solid content of oil slurry determined by the centrifugation method is greatly affected by the levels of gums and asphaltenes in the oil slurry, resulting in low precision and accuracy. It is suitable for oil slurries with low levels of gums and asphaltenes, as well as for the timely monitoring of the solid content in refinery oil slurries. (3) The solid content of the slurry determined by the ash method and the carbonization and calcination methods reflects only the catalyst content in the slurry; it has high precision but low accuracy, and is suitable for slurries with little coking.