Article pubs.acs.org/IECR
Nonconventional Vacuum Residue Upgrading Blended with Coal Tar by Solvent Deasphalting and Fluid Catalytic Cracking Jian Long, Benxian Shen,* Hao Ling, and Jigang Zhao State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China 200237 ABSTRACT: To optimize the ratio of vacuum residua/coal tar (VR/CT) in VR solvent deasphalting processing and catalytic cracking processing, their compatibility and properties are investigated in the present work. The investigation reveals two competitive processes of dissolution and adsorption during the blend process. At low CT blending ratio, the molecule interactions in the VR colloidal system are unchanged. The dissolution is nearly balanced with flocculation, and VR is almost compatible with CT. At high CT blending ratio, the flocculation predominates over dissolution and VR is incompatible with CT. Solvent deasphalting experiments revealed that the supercritical fluid extraction and fractionation process of VR has been improved by blending with CT. Fluid catalytic cracking (FCC) results of this deasphalted oil (DAO) indicated that more light oil can be produced with no obvious increase in the diesel/gasoline ratio. The VR can be upgraded by blending with an appropriate amount of CT (120 μm recycling agent microactivity (% mass)
CHZ-4
LANK98
48.9
46.4
0.12
0.29
0.31
0.28
112
115
0.18
0.14
22.3 56.0 21.7 61
17.6 57.7 24.7 64
operating conditions
value
temp of reactor (°C) temp of preheater (°C) temp of incubator (°C) catalyst loading (g) residence time of oil gas (s) catalyst-to-oil weight ratio water inflow (g/min)
500 350 90 80 90 8
Figure 4. Relationship between CT ratio and group composition. 3.5
handle samples up to about 540 °C.23 It can separate VR, CT, and model residua into many narrow fractions, and the ratio of CT/VR reflects the change in the composition. The experimental and the calculated yields of narrow fractions are listed Table 3. As Table 3 shows, when the ratio of CT/VR is low (540
0.95 0.95 2.27 4.00 6.61 10.37 15.50 22.98 77.02
69.54 80.03 87.57 89.11 90.67 92.95 93.73 95.07 4.93
4.40 4.93 6.61 8.43 11.41 15.31 20.48 28.09 71.91
4.41 4.93 6.62 8.41 11.09 14.94 20.09 27.60 72.40
5.40 5.69 6.00 11.85 14.82 18.88 23.91 31.23 68.77
7.19 8.14 10.03 11.73 14.26 18.08 22.90 29.96 70.04
8.32 10.89 14.44 15.71 19.39 22.99 27.80 36.29 63.71
12.38 14.13 16.49 18.18 20.62 24.32 28.80 35.39 64.61
R, ratio of CT (wt %); IBP, initial boiling point; CY, [experiment yield of VR + (experiment yield of CT)R]/(1 + R).
When blending CT with VR, the CT is the solvent. The aromatics from CT are miscible in different proportions with maltenes. At low CT blend ratio, their limited solvation did not change the molecules’ interactions in the VR colloidal system, dissolution was almost balanced with flocculation, the number of blended CT was in the tolerance limit of the VR colloidal system, and VR was almost compatible with CT. When the blending ratio of CT/VR increased, the content of aromatics became high. The peptizing limit of resins with asphaltenes in VR was sufficiently broken through by the solvation power of aromatics. Then the asphaltenes were naked and separated increasingly from the dispersed phase. As a result, the colloidal system of VR was transformed into an emulsion. Some small compounds, which are adsorbed onto the surface or even in the gaps of asphaltenes by the dispersion force, polar interactions, and hydrogen bonding, will have an opportunity to enter the dispersed phase. Meanwhile, asphaltene−asphaltene interactions are preferred over asphaltene−resin interactions and flocculation predominates over dissolution, which leads to phase separation in the colloidal system and deposition of the asphaltenes. Thus, under this condition, VR is incompatible with CT. Effect of Addition Amount of CT on C4 Solvent Deasphalting. DAO Yield and Composition. The yield of DAO is influenced by the properties of feedstocks, extraction temperature, extraction pressure, solvent/oil ratio, solvent types, etc. In order to produce more light oil available for subsequent processing by this solvent deasphalting process, many studies have been carried out on the solvent deasphalting
Figure 5. Viscosity of feedstock variation in the concentration of coal tar.
dependent on their matching properties of saturates, aromatics, resins, and asphaltenes. There is an equilibrium process, dissolution−flocculation equilibrium, in the VR colloidal system. These interactions operate like a seesaw in that one predominates at the expense of the other. Different asphaltene entities are in metastable equilibrium with the surrounding maltene environment. The gradual addition of light paraffin or heavy constituent ultimately breaks up this equilibrium with the precipitation of the asphalt phase. CT, similar to VR, is a complex mixture of hydrocarbons which mainly consists of aromatics and asphaltenes (as seen in Table 1). In general, aromatics comprise two or three aromatic rings bounded by short alkyl chain and they are potent solvents. Table 4. Structure Parameters of VR, CT, and Mixture Residue
CT (wt %) Mw C (wt %) H (wt %) H/C (mol/mol) fa CI HT CT CA RA RN RT
0
5
10
15
20
30
100
980 87.23 11.50 1.58 0.25 0.17 112.70 71 17.53 3.88 3.24 7.12
935 87.36 11.29 1.55 0.26 0.19 105.56 68 17.94 3.98 3.44 7.42
901 87.57 11.09 1.52 0.28 0.20 99.92 66 18.47 4.12 3.54 7.66
878 87.81 10.90 1.49 0.30 0.21 95.70 64 19.14 4.28 3.55 7.84
840 87.96 10.71 1.46 0.31 0.23 89.96 61 19.32 4.33 3.65 7.98
796 88.15 10.34 1.41 0.34 0.25 82.31 58 20.10 4.52 3.63 8.16
159 91.34 6.65 0.87 0.64 0.48 10.57 12 7.78 1.44 2.48 3.93
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process.24−27 Though solvent deasphalting processes of VR, CT, and VR blended with 5, 10, and 20% CT were investigated under the same experimental conditions here, less energy was put into the details of the progress. The effect of the amount of CT added on the DAO yield of the isobutane solvent deasphalting process was explored carefully, and the results are shown in Figure 6. The DAO yield obtained by the solvent deasphalting process of pure VR was 47.6 wt %, while that of
Table 5. Effect of CT Addition on DAO Properties at the Same Extraction Condition CT (wt %) CCR (wt %) metal content (μg·g−1) Ni V elementary composition (wt %) C H S N O SARA (wt %) saturates aromatics resins asphaltenes oil fractions H/C atomic ratio mol wt fa CI HT CT CA RA RN RT
Figure 6. Relationship between yields from experiment and calculated. Calculated yield CY = [experimental DAO yield of VR + (experimental DAO yield of CT)R]/(1 + R).
pure CT was 48.5 wt %. However, when they were mixed together, the yield of the mixture was 64.1 wt % with a blend ratio of CT of 5%, which was increased by almost 16 wt %. Within the investigated blend ratio, the yield of DAO kept on increasing with the concentration of CT in VR growing, even if the increase was slight. By comparing the DAO yields of experiment and calculation, it shows that the experimental value obtained from VR that blended with CT is higher than the calculated value. In similar experimental conditions, the solvent extraction process of VR that blended with CT distinctly improved during the solvent deasphalting process and the yield of DAO increased. Comparing the details of the physical and chemical structural properties of the obtained DAO (Table 5) reveals that not all the properties of the DAO have worsened with the yield of the DAO increasing. The contents of carbon residual, S, N, aromaticity, and condensation index increased as the yield increased. The H/C ratio, the content of oil (saturates and aromatics), and the weight-average molecular weight declined with that increase. Also, at the higher DAO yield, by blending with CT, the contents of metals and asphaltenes decreased obviously instead of increasing as expected. When the ratio of the blended CT increased from 0 to 20%, the metal content (Ni + V) in the DAO declined from 3.41 to 1.62 μg·g−1 and the asphaltene content reduced from 1.60 to 0.80 wt %, which illustrates that the addition of CT can distinctly affect the isobutane solvent deasphalting process. Relationship between Quality and Yield of DAO. From a cracking point of view, the most desirable FCC feeds should contain low nitrogen, metals (nickel and vanadium in particular), and Conradson carbon, as these are poisons to the cracking catalyst. Though the sulfur in FCC feeds appears to be harmless to the catalyst, it does affect the product quality and impose process problems. It is thus important to further examine the specific properties of DAOs for avoiding effects on the following process.28−30 The solvent deasphalting process of VR blended with CT has been investigated here to gain a larger
0
5
10
20
100
2.80
3.19
3.51
3.96
5.13
0.63 2.78
0.57 2.61
0.49 2.20
0.44 2.02
0.19 1.43
86.33 11.83 0.06 0.06 1.72
86.24 11.31 0.07 0.08 2.30
86.42 11.18 0.09 0.09 2.22
86.45 11.02 0.11 0.10 2.32
89.34 8.11 0.19 0.16 2.20
68.70 19.77 9.93 1.60 88.47 1.64 793 0.21 0.14 93.81 57.05 12.05 2.51 2.61 5.12
67.43 20.72 10.96 0.90 88.14 1.57 810 0.25 0.18 91.61 58.21 14.59 3.15 2.96 6.11
65.81 21.29 12.21 0.70 87.09 1.55 851 0.26 0.18 95.14 61.29 16.10 3.52 3.14 6.67
63.21 22.16 13.83 0.80 85.37 1.53 896 0.28 0.19 98.74 64.55 17.78 3.94 3.35 7.29
65.88 19.69 12.36 2.07 85.57 1.09 703 0.52 0.39 57.01 52.34 27.32 6.33 4.84 11.17
quantity of DAO without decline or even to enhance in quality at the same yield, which would avoid bringing new problems for the further processing (FCC). Therefore, though it was clear that the blend of CT with VR can enhance the DAO yield of the VR, it was still necessary to study the properties of the DAO that was obtained by blending with CT. Then the effect of the CT on the solvent deasphalting process could be revealed. The solvent deasphalting progress of pure VR and the solvent deasphalting progress of VR that was blended with various ratios of CT (5, 10, and 20%) were investigated. Several groups of samples with different DAO yields were obtained, and their properties were analyzed when their yields were similar. The effect of blended CT on the properties of DAO was investigated, and the results that reveal the variation of the contents of metals (Ni + V), Conradson carbon residue (CCR), S, and N are shown in Figure 7. The properties of DAO changed distinctly after blending with CT. Within the same yield, a higher CT blending amount can induce a decline in the metals content of DAO, while the contents of CCR, S, and N increase. The contents of CCR, S, and N in the DAO obtained from VR blending with CT are higher than that of pure VR when the DAO yield was low. However, with the yield increasing, the differences between them get close and even overturned, by which these contents in the pure VR are higher. These are mainly dependent on the lower metal content and higher S, N, and CCR contents in the DAO extracted from the CT. As well, the extraction was fractionated by supercritical fluid extraction and the VR was detached by the size of the molecule. Molecules with small size would be detached first 3063
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Figure 7. Relationship between the yield and quality of DAO at different CT additions.
Table 6. Effect of CT Addition on Quality of DAO at the Same Yields of 40 and 50% DAO yield (wt %) 40 CT (wt %) elementary composition (wt %) C H H/C SARA (wt %) S A R As oil Mw fa CI HT CT CA RA RN RT
50
0
5
10
20
0
5
10
20
85.03 12.01 1.69
85.21 11.89 1.67
85.37 11.81 1.66
85.54 11.43 1.60
85.27 11.29 1.59
85.31 11.53 1.62
85.42 11.41 1.60
86.90 10.92 1.51
70.74 18.77 9.66 0.83 89.51 625 0.18 0.12 75.06 44.29 8.10 1.52 2.18 3.71
69.19 19.94 10.16 0.72 89.13 609 0.19 0.13 72.41 43.24 8.40 1.60 2.24 3.84
68.43 20.72 10.22 0.64 88.84 591 0.20 0.14 69.80 42.04 8.51 1.63 2.26 3.89
66.07 21.06 12.32 0.55 87.13 586 0.23 0.16 66.98 41.77 9.78 1.94 2.45 4.39
69.14 19.08 10.75 1.03 88.22 798 0.24 0.17 90.09 56.70 13.74 2.93 2.85 5.79
68.19 20.34 10.66 0.82 88.53 782 0.22 0.15 90.16 55.59 12.44 2.61 2.68 5.29
66.58 20.82 11.91 0.70 87.39 762 0.23 0.16 86.94 54.24 12.71 2.68 2.73 5.41
64.67 21.96 12.68 0.69 86.63 753 0.29 0.20 82.23 54.53 15.68 3.42 3.16 6.58
from the mixed VR. As the extraction went on, the property of the DAO was mainly dependent on the property of the distillations from the VR. Two groups of samples whose extracted yields were 40 and 50% were chosen for further analysis (Table 6). Within the same yield, the H/C ratio of the DAO remained the same or even increased as the CT blending amount increased. The content of oil (saturates and aromatics) remained unchanged. The content of saturates in the DAO decreased slightly as the blend amount of CT increased, though the amount of saturates existing in the CT was very small. This illustrates that a vast
amount of asphaltenes in the mixed VR has not entered the obtained DAO. The average molecular weight declines gradually. The decline is attributed to the entrance of not only the small molecules in the CT into the DAO, but also to part of the alkanes and low condensation aromatic hydrocarbons in the VR substituted by the highly condensed aromatic hydrocarbons in the CT. The released small molecules are extracted into the DAO and lower its average molecular weight. There is no distinct increase in the aromaticity and condensation index. Except for the structural parameters 3064
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remaining almost unchanged, the other parameters decrease when the CT blend amount is low. Known from above, the addition of CT can improve the extraction efficiency of the solvent deasphalting progress and enhance the yield of the residuum. As well, the inferior property of the CT has no effect on that of the obtained DAO, except for a slight worsening when the blend ratio of CT is as high as 20%. This predicts that the extracted yield of the DAO in the same condition is mainly dependent on the extraction of the oil in the VR. GPC Distribution of DAOs. Because the average molecular weight is a statistical value, it cannot reveal the polydispersity of the oil as exactly as the distribution of the relative molecular weight. Therefore, the distribution of the relative molecular weight of DAO obtained from VR blended with various amounts of CT and with the same yield, 50%, was investigated here (Figure 8). The accumulation mass fraction increased as
Figure 9. Relationship between interfacial tension and mass fraction of DAO in oil phase.
declined when the mass fractions of DAO in the oil phase increased (from 0.01 to 0.5%). Just the decline of the DAO obtained from CT was more distinct than that from VR blended with or without CT. In addition, the oil−water interfacial tension of DAO obtained from the pure VR and mixed VR remained unchanged when the mass fraction of DAO in the oil phase was larger than 0.2, while that of the CT declined obviously. This illustrates that the interfacial activity of the DAO obtained from CT was greatly stronger than that from VR blended with or without CT. Further investigation and comparison of the oil−water interfacial tensions of DAO obtained from VR blended with or without CT revealed that the tension decreased when the blend amount of CT increased. When the mass fraction reached 0.5%, all the tensions were approached gradually. The interfacial activity of the DAO is affected variously by the transmutation of its parameters, such as average relative molecular weight, element content (S and N), content of conjugate aromatics, and so on. For example, the polar groups were enhanced when the contents of S and N increased. This enhancement induces the polarity of the DAO to be amplified and the interfacial activity increased finally. Otherwise, the content of conjugate aromatics will increase and the polarity of the DAO willl decline. Then, the interfacial activity decreases. However, the multiple effects of these factors on the oil−water interfacial tension are nearly approached. Also, the Gibbs adsorption function reveals that35 the interfacial tension declines as the mass fraction of the fractions in the oil phase increases. It has been identified with the experimental results very well.
Figure 8. Molecular weight distribution of DAO obtained by different CT additions.
molecules in the samples effused sequentially with molecular weight decreasing from large to small. The distribution ranges of all the samples were similar, which was mainly arranged from 100 to 3000. When VR was blended with CT, the molecular distribution of DAO shifted to a lower range. The content of large molecules decreased obviously. This meant that when the VR was blended with CT, large molecules that were highly condensed, with high molecular weight and complex structure, were extracted less into the extract phase. With the same yield, this decrease resulted in an increase of the light components content in the solvent deasphalting process, while the content of the heavy components decreased. Oil−Water Interfacial Tension of DAO. Oil−water interfacial tension of the DAO, which is the interfacial energy, is the chief parameter to determine its property.31 The adsorption rate and amount, as well as the interfacial structural, are impacted directly by its value. For the various content amounts of resins and asphaltenes, also for the different compositions and structures of the oil, the oil samples exhibit interfacial properties with their own features.32 As reported,33,34 these polar materials, such as resins, asphaltenes, and so on, which contain large amounts of aromatic rings, and with large average molecular weights and complex molecular structures as well, are mainly absorbed on the oil−water interfacial surface. Thus the amounts of the active substances on these natural interfacial surfaces are reflected by the interfacial tension. It is revealed by Figure 9 that the oil− water interfacial tension variation tendencies are similar (restricted to the investigated samples). All the tensions
Γ = ( −1/RT )(dγ /d(ln c));
dγ = −ΓRT d(ln c)
where Γ is is the interfacial adsorption quantity, R is the gas constant, T is temperature, γ is the interfacial tension, and c is the equilibrium concentration. IR Spectra of DAOs. Figure 10 reveals that the IR spectra of the three DAO samples obtained from VR and VR blended with different ratios of CT (5, 10, and 20%) are similar. In the stretching vibration region (wavelength range 3000−2800 cm−1) of C−H in straight-chain alkanes and cyclanes, two strong absorption peaks around 2920 cm−1 (contribution of group −CH2−) and 2850 cm−1 (cocontribution of groups −CH2− and −CH3) emerged separately. Though there are some differences in the absorption strength among there four components in this region, it is always the strongest peak in the same spectra. The stretching vibration peak of CC is around 1600 cm−1. There are two relatively strong absorption peaks 3065
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Table 7. Effect of CT Addition on FCC Performance of DAO DAO of the Same Yield CT (wt %) 0
5
10
20
dry gas 8.32 5.91 3.38 3.49 liquefied petroleum gas 14.98 13.79 14.02 13.26 gas 23.30 19.70 17.40 16.75 gasoline 30.60 33.38 35.13 30.49 diesel 17.44 20.69 21.43 21.34 residua oil 1.26 1.53 2.14 2.87 liquid oil 49.30 55.60 58.70 54.70 coke 27.40 24.70 23.90 28.55 light oil 48.04 54.07 56.56 51.83 diesel/gasoline 0.57 0.62 0.61 0.70 DAO Obtained at the Same Extraction Condition (Yield Given in Figure 6)
Figure 10. IR spectra of some DAO samples. (1) DAO obtained by VR; (2) DAO obtained by VR blending 5% CT; (3) DAO obtained by VR blending 10% CT; (4) DAO obtained by VR blending 20% CT; (5) DAO obtained CT.
CT (wt %)
around 1480 and 1350 cm−1. The former is the cocontributed result of −CH2− and −CH3, while the latter is the characteristic peak of the group −CH3. The wavelength range from 800 to 700 cm−1 can be attributed to the rocking vibration of groups (−CH2)n. When the value of n increases, the wavelength can shift to a lower wavenumber. The absorption peak around 750 cm−1 is attributed to the absorption of groups (−CH2)n with the value n > 4. Results above reveal that, for DAO obtained by the CT solvent deasphalting process, there are more absorption peaks in the IR spectrogram when the wavelength is 3500−2800 and 2000−500 cm−1. It is obviously different from the IR spectra obtained from the others. However, the absorption peaks of DAO obtained by VR blended with CT are similar to that of pure VR. FCC Performance of DAOs. The catalytic cracking of DAO obtained from the same extraction progress and with the same extraction yield (50%) after blending with CT was investigated here. These cracking properties are compared, and the results are shown in Table 7. The results indicate that the cracking performances of DAOs obtained from pure VR and mixed VR and the cracking product distribution are different. Dry gas is formed by thermal cracking and secondary catalytic cracking of butanes and gasoline. An abnormally high dry gas yield can be attributed to excessive coke formation that blocks the active sites of the catalyst, rendering it nonselective.36 High dry gas yield may impose process problems due to the limited capacity of the wet compressor in the FCC unit. In FCC experiments of DAO, at the same yield, with increasing CT, the dry gas yield decreased. When CT addition was from 0 to 20%, the dry gas yield reduced from 8.32 to 3.49%. Liquefied petroleum gas is valuable as the majority of it is isobutene and C3 + C4 olefins which can be used as alkylation feeds or for production of oxygenates as octane enhancers. The FCC experiments of DAO at the same yield indicated that a similar liquefied petroleum gas yield can be obtained as CT addition increases. Gasoline is the most desired and the most abundant product in FCC operations. Saturates (paraffins and naphthenes) and monoaromatics are gasoline precursors.37 There is some discrimination in gasoline production between the DAO obtained from pure VR and the DAO obtained from the mixed VR. When the addition of CT is 10%, 5 wt % more gasoline can be produced in FCC of DAO at the same yield. Compared with pure VR, the oil content of the DAO obtained
dry gas liquefied petroleum gas gas gasoline diesel residua oil liquid oil coke light oil diesel/gasoline
0
5
10
20
7.62 22.98 30.60 32.46 16.88 1.27 50.61 20.40 49.34 0.52
6.18 18.52 24.70 29.95 20.67 2.78 53.40 21.90 50.62 0.69
5.75 15.95 21.70 31.62 23.08 4.69 59.39 18.90 54.71 0.73
3.28 11.82 15.10 27.91 21.49 12.90 62.30 22.70 49.40 0.77
from the mixed VR is not increased obviously. However, the resin content, in which the lighter resins are another source of gasoline precursors, is increased in this study. Diesel in general is not a most desired product because of its high aromatics content which results in a low cetane number. With increasing CT, the diesel yield increases, too. Coke is important because it provides heat required to maintain a heat balance between the cracker (endothermic) and the regenerator (exothermic). However, excessive coke represents a loss of high-value products and an additional heat load on the burning capacity of the regenerator. Too much coke on the catalyst will hinder its selectivity and result in excessive amounts of dry gas. Coke precursors are known to be polynuclear aromatics, olefins, CCR, and compounds containing basic nitrogen.38 The coke yield does not increase with the increase of CT addition in the FCC experiment. Residua oil is a low-value unconverted product. The yield of residua oil also increases with the additional CT increase. In general, FCC experiment of DAO at the same yield indicates that the light oil yield increases from 48.04 to 58.70 wt % when the blending amount of CT increases from 0 to 10 wt %. This is because the cracking reaction is promoted by lower contents of metals and asphaltenes in DAO when VR is blended with CT. The coke yield can prove this point. When the blending amount of CT is 0−10%, the coke yield of DAO obtained by mixed VR is lower than that of pure VR. With an even larger amount of CT (20%), gasoline yield drops, coke yield increases, unconverted oil content rises, and the diesel/gasoline ratio increases. This is mainly because CT contains a large number of highly condensed polycyclic aromatic hydrocarbons, which will inevitably be extracted into DAO while the blending amount of CT in the mixed VR 3066
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tons per year, accounting for 1/3 of the CT annual output in the world. The main disadvantages of conventional CT processing in China are the small scale and serious pollution. Most CT has been burned directly. Nowadays, the processing capacity of Chinese built and being built solvent deasphalting devices is over 7 Mt/year and continues to improve. When VR is blended with CT (
