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Investigation on the Compatibility and Incompatibility of Vacuum Residua with Catalytic Cracking Bottom Oil Wang Yanfei,*,†,§ Cheng Jian,‡ Jian Shengsheng,‡ and Shen Benxian§ Department of Chemical Engineering, Central-South Institute of Technology, Hengyang 421001, Hunan, China, National Laboratory of Heavy Oil Processing, Petroleum University of China, Dongying 257062, Shandong, China, East China University of Science and Technology, Shanghai 200237, China Received December 10, 2001
VR (vacuum residua) and CCB oil (catalytic cracking bottom oil) are complex mixtures of hydrocarbons. The investigation of the compatibility and the incompatibility of them is helpful to optimize the ratio of CCB oil/VR in FCC processing and VR solvent deasphalting processing to prevent troubles. In the present work, the mixture property of VR with CCB oil was studied from the standpoint of colloidal dispersion in several different experiments. The investigation shows that in the course of CCB oil being blended with VR, there are two competitive processes of dissolution and flocculation, and the CCB oil’s function is to act as both solvent and dispersant. At a low blending ratio of CCB oil, flocculation is almost balanced with dissolution; VR is almost compatible with CCB oil. When the blending ratio of CCB oil is increased, the solvation and dispersal powers of the CCB oil break through the tolerance limit of VR colloidal system, and flocculation predominates over dissolution, which leads to phase separation in the colloidal system and to deposition of the asphaltenes. Under this condition, VR is incompatible with CCB oil.
Introduction Vacuum residua (VR), which are obtained by distillation of atmospheric residua under vacuum, are widely used for road application as the paving bitumen, while their properties enable them to reach the qualifications of paving materials. At present, VR are upgraded to produce lighter fuel oil to meet the worldwide demand. It consists of complex mixture of hydrocarbons of various chemical structures and molecular weights, which contain oxygen, sulfur, and nitrogen. Nitrogen and oxygen are involved in various acid and basic functional groups of chain or aromatics, There are many different procedures for separating residua,1 for example, gel permeation chromatography (GPC), ion exchange chromatography (IEC), HPLC, and others.2 The most commonly employed is probably the SARA separation, which divides residua into four generic groups: saturates (S), aromatics (Ar), resins (R), and asphaltenes (As).3 Asphaltenes are defined as n-heptane insoluble and toluene, and resins are n-heptane soluble: the two types differ in polarities and structures. CCB oil, which is from the fluid catalytic cracking (FCC) bottom of the tower, is widely used in, among †
Department of Chemical Engineering. National Laboratory of Heavy Oil Processing. East China University of Science and Technology. (1) Salu, M.; Siffert, B.; Jada, A. Fuel 1998, 77, 343-346. (2) Branthaver, J. F.; Petersen, J. C.; Robertson, R. E.; Duvall, J. J.; Kim, S. S.; Harnsberger, P. M.; Mill, T.; Ensley, E. K.; Barbour, F. A.; Scharbron, J. F. Binder Characterization and Evaluation, Vol. 2, Chemistry; Strategic Highway Research Program SHRP-A-368; Strategic Highway Research Program: Washington, DC, 1993 . (3) Altgelt, K. H.; Jewell, D. M.; Latham, D. R.; Selucky, M. L. In Chromatographic Science Series; Altgelt, K. H., Ed.; Marcel Dekker: New York, 1979; pp 194-196. ‡ §
others, electric and ceramic factories, while mixed with other fuel oils. As residua, it is composed of saturated compounds, aromatic compounds, and resins, but it contains little asphaltenes, besides it contains trace quantities of catalyst materials. In general, resins of CCB oil, which consists of three or more aromatic rings condensed by short alkyl chains, is strong aromaticity and strong polarity, because it contains nitrogen, oxygen, and sulfur atoms, such as VR. With worldwide increasing demand of light fuels oil, VR and CCB oil are used as the feed food of deep processing, residua fluid catalytic cracking, and residua solvent deasphalting, These have become the main heavy oil processing options for different blends of CCB oil in Chinese refineries.4 The blending ratio of CCB oil has a complicated effect on the downstream processing, for example, the rapid deactivation of catalysts and coke formation during processing. Much progress have been made lately on the compatibility of the processed oils with oils or solvent; thus, for example Dickakian found that visbreaker tars and other processed oils could deposit asphaltenes when blended with paraffinic oils.5 Also Wiehe and Kennedy had built the oil compatibility models on the basis of the solubility parameter to predict which mixtures of crude’s are incompatible, and found that when the volume ratio of Forties crude was more than 67%, Souedie and Forties crude are incompatible.6 However, the compatibility predictions are rather empirical or based upon the basic hypothesis of the oil compatibility (4) Shengsheng, J.; Jian, C.; Yunhua, L. Pet. Refin. Eng. 1995, 25 (4), 8. (5) Dickakian, G. U.S. Patent 4,853, 337, 1989.
10.1021/ef0102869 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/01/2003
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Table 1. Properties of Raw Materiala VR CCB oil a
H/C, mol/mol
CCR, wt %
η cSt, 90 °C
F g/cm3, 20 °C
S, wt %
Ar, wt %
R, wt %
At, wt %
1.67 1.38
12.43 3.3
386.2 16.78
0.9620 0.9928
28.27 44.34
31.47 40.29
39.97 15.37
0.29
CCR: Conradson carbon residue. S: saturates. Ar: aromatics. R: resins. At: asphaltenes.
Figure 1. The separation of SARA.
model that states that the asphaltenes/resins dispersion has the same flocculation solubility parameter, while the property and composition of the oil are neglected. Since asphaltenes are defined as n-heptane insoluble and toluene soluble, and resins as n-heptane soluble fractions, they have different polarities and structures; therefore, it is not too difficult to think that the application of models is limited. Both VR and CCB oil are the processed oils. To the best of our knowledge, few studies of compatibility of their mixture appear to be in the scientific literature. The present work is the first study to report the result of study of the compatibility and incompatibility of VR with CCB oil, when CCB oil is poured slowly into VR. However the blending order’s influence on the compatibility, which is not discussed, is more thoroughly in an extention of the present work. The present investigation of the compatibility and the incompatibility of these oil was intended to optimize the ratio of CCB oil in FCC processing and in VR solvent deasphalting processing. Experimental Section Raw Material. VR and CCB oil used in the present study are supplied by NanYan refinery; they are from waxy crude oil in China. Their properties are summarized in Table 1. It is shown that saturates and aromatics are rich in CCB oil and are poor in VR; relative to them, resins are rich in VR and poor in CCB oil. However, the content of n-heptane asphaltenes is 0.25%very low, and CCB oil has little asphaltenes. Mixture Residua. To investigate the compatibility of CCB oil with VR, mixture residua in a 8 L stainless steel container is prepared by slowly pouring CCB oil into VR at 150 °C, and the resulting solution is stirred at 1000 rpm with a stirrer for complete mixing. The temperature is controlled with a controller connected to a thermocouple immersed in the VR. Elemental Analysis. A Carlo Erba automatic analyzer determines carbon, hydrogen, and nitrogen. Sulfur is detected by the microCoulometer method. SARA Separation. The asphaltenes of VR, CCB oil, and model residua, defined as n-heptane insoluble in the present work, is determined according to the separation scheme shown in Figure 1. About 1 g of VR or CCB oil is precipitated by n-heptane into asphaltene (As) and maltene. Then the maltene is fractionated on an alumina (40 g, 100-200 mesh, with 1 wt %H2O) column into saturates (S), aromatics (A), and resins (R). The corresponding solvent was composed of mixing n(6) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 56-59.
Figure 2. Relationship between CCB oil ratio and group composites. pentane (40 mL), benzene (40 mL), and benzene/ethanol (1:1,v/v, 20 mL), followed by the addition of 20 mL of benzene and 20 mL of ethanol. Molecular Weight. The average number molecular weights are determined by a Kaauer vapor phase osmometer (VPO) in benzene at 45 °C. Simulated Distillation (SIMDIS). SIMDIS is carried out in a Varian 3400 gas chromatograph using the ASTMD-5307 procedure to determine the yields of 545 °C- products.. Propane Deasphalting. VR, CCB oil, and model residua are deasphalted out in the pilot’s continuous solvent deasphalting unit. Propane is used as the solvent. The temperature in the bottom and top of tower are respectively controlled at 60 and 70 °C with the controller connected to thermocouple in the bottom and top of tower. The operation pressure is 5.0MPa. Once the system has stabilized, usually within 30 min, deasphalted oil is collected from the sample line and weighted to determine the yield after the solvent is removed.
Results Figure 2 shows that saturates and aromatics contents increase with the increasing of CCB oil, and contents composed of the resins and the asphaltenes are reduced after blending CCB oil. It is interesting to find that the changing tendency of various compounds content is almost linear. In view of these results, the possible explanation for this result is that CCB oil is rich in saturates and aromatics and poor in resins. In general, SARA separation appears to be coarse, stimulated distillation (SIMDIS) can greatly save on sample and operation time, and it has the additional advantage of being more precise and covering even
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Table 2. The Experimental Yield (EY, wt %) and the Calculated Yield (CY, wt %) of Fractiona R ) 90
R ) 70
R ) 50
R ) 30
fractions, °C
VR, wt %
CCB oil, wt %
EY
CY
EY
CY
EY
CY
EY
CY
IBP-449 449-502 502-522 522-531 531-539 539-545 IBP-545 545+
1.0 2.0 2.0 1.0 1.0 1.0 8.0 92.0
51.0 26.5 7.5 2.5 1.5 1.5 90.5 9.5
44.0 23.3 6.7 2.0 1.4 1.4 78.8 21.2
46.0 24.1 6.9 2.4 1.4 1.5 82.3 17.7
34.5 18.5 5.8 2.0 1.4 1.1 63.3 36.7
36.0 19.2 5.8 2.1 1.3 1.4 65.8 34.2
25.7 13.8 4.9 1.8 1.3 1.3 48.8 51.2
26.0 14.3 4.7 1.8 1.2 1.3 49.3 50.7
16.0 9.4 3.6 1.5 1.2 1.3 33.0 67.0
16.0 9.4 3.6 1.5 1.1 1.2 32.8 67.2
a R: ratio of CCB oil/VR, wt %. IBP: initial boiling point. CY: the experiment yield of VR fraction × (1 - R) + (the experiment yield CCB oil fraction × R).
Figure 3. Relationship between the experimental yields and the calculated yields of deasphalted oil.
Figure 4. Relationship between viscosity and ratio of CCB oil.
higher temperature. SIMDIS is ordinarily performed by gas chromatography(GC) which can handle samples up to about 540 °C,7 and it can separate VR, CCB oil, and model residua into many narrow fractions, which can reflect the change of compositions as the ratio of CCB oil/VR, The experimental and the calculated yields of narrow fractions are listed Table 2. Table 2 shows that, when ratio of CCB oil/VR is low (e30%), the experimental yield of various fractions is almost equal to the calculated yield. It means that the dissolution is balanced with adsorption in blending. On the other hand, when the ratio of CCB oil/VR is high, the situation is opposite. However, the experimental yield of the 545 °C+ fraction is more than its calculated yield, which indicates the adsorption prevails in the course of blending. Solvent deasphating process, which consists of reducing the asphaltene of residua, is usually affected with the aid of a paraffinic solvent which flocculates the asphaltenes as well as a variable proportion of resins. The method involves the use of propane, butane(s), or pentane(s) either singly or as mixtures. The liquid hydrocarbon causes the separation of the asphaltenes and/or the resins from the feedstock, leaving deasphalted oil. Propane is used as the solvent in the present work. Figure 3 compares the experimental yield and calculated yield of the deasphalted oil. It can be seen that the experimental yield of the deasphalted oil is less than the calculated yield, when the ratio of CCB oil/VR is low (e30%). However, when the ratio of CCB oil/VR is high, the situation is opposite. The root reason is that
blending less CCB oil is less able to change the compatibility of asphaltenes with maltene, and consequently, the system is unstable. Although supercritical propane has strong solvation power, it is difficult to overcome the resistance from maltene. That is to say, the interaction of compounds can prevent the dispersion from entering into the propane phase to some degree. On the other hand, when the CCB oil ratio is high, the compatibility of asphaltenes with maltene is poor, and propane can break through of obstacles from maltene; the dispersed matter then has the opportunity to enter into the propane phase, which then leads to the experimental yield of the deasphalting oil becoming more than the calculated. Figure 4 shows that the effect of ratio of CCB oil/VR on the viscosity of model residua. It is shown that the viscosity of model residua decreases as an increase in the ratio of CCB oil/VR, and the changing tendency is not linear. It is well-known that the viscosity of system depends on the molecules weight and the interaction between the molecules: the greater the energy, the higher the viscosity. Adding CCB oil decreases the dispersion of the colloid system and changes its composition shown in Table 2 and Figures 2 and 3, which reduces the viscosity of model residua. Structure parameters of heavy oil may predict some complicated structural information. Structure parameters of VR, CCB oil and model residua, which are calculated by E-d-M methods, are listed in Table 3. It can be shown that fa and CI increase with the increasing with CCB oil ratio. The larger fa and CI, the stronger the aromatization and condensation polymerization, meaning that the tendency of flocculation increases with CCB oil and the stability of the colloid system decreases. When the ratio of CCB oil/VR rise from zero to 100%,
(7) Butler, R. D. Simulated Distillation by Gas chromatography, Chromatography in Petroleum Analysis; Algelt, K. H., Gouw, T. H., Eds.; Marcel Dekker: New York, 1979. (8) Corbett, L. W. Anal. Chem. 1964, 36, 1967.
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Table 3: Structure Parameters of VR, CCB Oil, and Mixture Residuaa,b CCB oil ratio, wt %
0
30
50
65
90
100
Mw (VPO) 960 768 640 544 384 320 C, wt % 86.6 87.40 87.94 87.96 88.46 89.29 H, wt % 12.10 11.57 11.22 10.93 10.52 10.34 H/C, mol/mol 1.67 1.58 1.52 1.48 1.42 1.38 fa 0.16 0.22 0.27 0.30 0.34 0.37 C.I. 0.17 0.20 0.21 0.22 0.24 0.25 #C 69 56 47 40 28 24 #C 11.0 12.3 12.7 12.0 9.5 8.9 a a Densicimetric method.8 b C%: carbon mass fraction. H%: hydrogen mass fraction. fa: fraction aromatic. C.I.: condensation index. #C: average number of carbon atoms. #Ca: number of carbon in aromatic rings. Mw: average molecular weight.
CT is reduced from 69 to 24, meaning that some of the small molecules adsorbed on the asphaltene are dissolved. Discussion about Compatibility and Incompatibility It is well-known that VR has a colloidal dispersion. The dispersed phase (as referred as the asphalt phase) is made of the asphaltenes, which may be present in different forms, i.e., isolated molecules solvated by resins and per condensed aromatic hydrocarbons, clusters of a few molecules, or colloidal micelles resulting from the agglomeration of asphaltene cluster and their procession of resins; and the dispersing phase (as referred as the oil phase) is composed of saturates and aromatics. The farther they are from the micelles, the less polar the compounds are. There is no distinct interface between the dispersed and the dispersing phase. The close relationships of the various hydrocarbon series gives rise to much overlapping of fraction into neighboring series, both in molecular weight and in H/C ratio.Thus, the physicochemical behavior of VR colloidal system depends on their matching property of saturates, aromatics, resins, and asphaltenes. There are two equilibrium processes: (1) dissolutionflocculation in VR colloidal system, these interactions operate like teeter-totters; one predominates at the expense of the other. The different asphaltene entities are in metastable equilibrium with the surrounding maltene environment. The gradual addition of light paraffin ultimately breaks up this equilibrium with the precipitation of an asphalt phase. CCB oil, like VR, is a complex mixture of hydrocarbons which mainly consist of saturates and aromatics (as seen in Table 1). In general, saturates contain paraffins and naphthenes; aromatics comprise two or three aromatic rings bounded by short alkyl chain. Aromatics are the potent solvents followed by naphthenes and paraffins
When CCB oil is blended with VR, the CCB oil’s function is to act as both solvent and dispersant. The aromatics from CCB oil are miscible in different proportions with maltenes; the role of saturates as dispersants is to decrease the effective interaction between resins and asphaltenes. At a low blending ratio of CCB oil, their limited solvation and dispersal powers do not change the molecules’ interactions in the VR colloidal system; dissolution is almost balanced with flocculation; the number of blended CCB oils is in the tolerance limit of VR colloidal system; and VR is almost compatible with CCB oil. When the blending ratio of CCB oil/VR increases, the contents of aromatics and saturates are high. The solvation power of aromatics is sufficient to break through the peptizing limit of resins with asphaltenes in VR; on the other hand, saturates can break through the disperse limit of the dispersing phase, which gives rise to that asphaltenes is separeated from the dispersed phase with being naked. As a result, colloidal system of VR changes into emulsion. Some small compounds, which are adsorbed onto the surface of asphaltenes and between the leaves of asphaltenes by the dispersion force, polar interactions, and hydrogen bonding, will have an opportunity to enter into the dispersing phase. On the other hand, asphaltenes-asphaltenes interactions are preferred over asphaltenes-resins interactions, in which flocculation predominates over dissolution, which leads to phase separation in the colloidal system and to the deposition of the asphaltenes. Under this condition, VR is incompatible with CCB oil. Conclusion It is proved that, in the course of CCB oil being blended with VR, there are two competitive processes of dissolution and adsorption, and that the CCB oil’s function is to act as both solvent and dispersant. At a low blending ratio of CCB oil, flocculation is almost balanced with dissolution; VR is almost compatible with CCB oil. When the blending ratio of CCB oil is increased, the solvation and dispersal power of CCB oil break through the tolerance limit of VR colloidal system, flocculation predominates over dissolution; VR is incompatible with CCB oil. Acknowledgment. The author thanks the National Laboratory of Heavy Oil Processing, Petroleum University of China for its technical assistance on simulated distillation (SIMDIS) experiments. The author also acknowledges the financial support of the Nanyan refinery cooperative. EF0102869
