Quality of Distillates from Repeated Recycle of Residue - Energy & Fuels

The quality of the distillates obtained at each stage deteriorated based on the concentrations of nitrogen and polyaromatic hydrocarbons. Although cok...
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Energy & Fuels 2002, 16, 477-484

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Quality of Distillates from Repeated Recycle of Residue Shakir Japanwala,† Keng H. Chung,‡ Heather D. Dettman,§ and Murray R. Gray*,§ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada, Syncrude Canada Ltd., Edmonton Research Centre, 9421 17th Avenue, Edmonton, Alberta, Canada, and National Centre for Upgrading Technology, 1 Oil Patch Drive, Devon, Alberta, Canada Received September 24, 2001. Revised Manuscript Received December 17, 2001

The yields of gas, liquid, and coke from the coking of Athabasca vacuum residue were determined as a function of conversion of the residue. High conversion was achieved by subjecting the unconverted residue to repeated coking steps. The cumulative yield of liquid increased by 12% from stage 1 of coking, where 80% residue conversion was obtained, to stage 3 where over 97% cumulative residue conversion was obtained. The quality of the distillates obtained at each stage deteriorated based on the concentrations of nitrogen and polyaromatic hydrocarbons. Although coking reactions resulted in a significant decrease in the number of paraffinic chains, the average chain length remained constant. A mass balance showed that the aromatic carbon increased in the first coking step, then remained constant.

Introduction Approximately 80% of the oil sands bitumen produced in Canada (ca. 350 000 BBL/D) is processed by two coking technologies: fluid coking and delayed coking. A characteristic feature of these processes is a recycle stream in which the unconverted residue fraction (524 °C+) is returned to the reactor. The recycle stream makes it possible to eventually achieve 100% conversion of residue, but limited research has been done to determine the varying characteristics of this stream with increasing conversion. Recycling also results in an increased liquid product yield, but again the changing quality of the liquid product has not been comprehensively quantified. This information is particularly important for understanding how the coker recycle stream will impact hydrotreater performance downstream. Distillate quality has been correlated to catalyst deactivation in hydrotreaters. In comprehensive surveys on hydrotreating catalysts by Halabi et al.1 and Thakur and Thomas,2 polycyclic aromatic hydrocarbons (PAHs), asphaltenes, metals, and heteroatoms were identified as the major sources of catalyst deactivation in the hydrotreaters. Stohl and Stephens3 performed various experiments to study the impact of the chemical constituents of coal liquids on hydrotreating catalysts. They found that the catalyst activity decreased by 95% after a 2-h reaction with * Author to whom correspondence should be addressed. Telephone: (780) 492-7965. Fax: (780) 492-2881. E-mail: [email protected]. † University of Alberta. ‡ Syncrude Canada Ltd., Edmonton Research Centre. § National Centre for Upgrading Technology. (1) Absi Halabi, M.; Stanilaus, A.; Trimm, D. L. Appl. Catal. 1991, 72, 193. (2) Thakur, D. S.; Thomas, M. G. Appl. Catal. 1985, 15, 197. (3) Stohl, F. V.; Stephens, H. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1986, 251.

nitrogenous extract and by 41% after reaction with PAHs. Vital information about the deactivation process also comes from the composition of the carbonaceous deposits on the hydrotreating catalyst. The deposits from gas oil and naphtha hydrotreating catalysts are dramatically enriched in nitrogen and oxygen relative to feed concentrations. Obviously, adsorption of these compounds is an important factor in catalyst deactivation4-6 and thus any increase in concentration of these compounds will result in more rapid catalyst deactivation. A structural analysis of coker recycle material was done by Kirchen et al.7 and Gray et al.8 A combined coker recycle stream, consisting of condensed recycled residue from the coker and unconverted residue, was compared to feed bitumen, atmospheric residue (343 °C+), and vacuum residue (427 °C+). An analysis of the coker recycle showed that there was a significant decrease in the total concentration of paraffinic groups compared to topped fresh bitumen. As expected, the aromatic content of the recycle stream increased in comparison to topped fresh bitumen. Considering that the coker recycle was composed of both topped fresh bitumen and actual recycle material, these trends would be more pronounced for the recycle stream alone. Zhao et al.9 analyzed feed and product residue fractions from single pass coking, and observed a systematic increase in aromatic carbon content in all fractions separated by supercritical solvent extraction, consistent with a gen(4) Choi, J. H. K.; Gray, M. R. Ind. Eng. Chem. Res. 1988, 27, 1587. (5) Furimsky, E. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 329. (6) Furimsky, E. Fuel Process. Technol. 1982, 6, 1. (7) Kirchen, R. P.; Sanford, E. C.; Gray, M. R.; George, Z. M. AOSTRA J. Res. 1989, 5, 225. (8) Gray, M. R.; Choi, J. H. K.; Egiebor, N. O.; Kirchen, R. P.; Sanford, E. C. Fuel Sci. Technol. Int. 1989, 7, 599. (9) Zhao, S.; Kotlyar, L. S.; Woods, J. R.; Sparks, B. D.; Chung, K. H. Energy Fuels 2001, 15, 113.

10.1021/ef010234j CCC: $22.00 © 2002 American Chemical Society Published on Web 02/05/2002

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Figure 1. Flowsheet of the experimental plan.

eral increase in aromatic carbon content. Average chemical structures, mainly from 1H NMR and elemental analysis, suggested a significant reduction in the length of paraffinic side chains in the coked residue material relative to the fresh feed. Chung and Xu10 found that the supercritical solvent extraction fractions of coked residue had a very different distribution of molecular weight than the feed. Rather than a uniform distribution, the coked residue material contained predominantly low molecular weight components in the range 300-600 Da, mixed with 20 wt % of material of molecular weight over 2400 Da. This research aimed to achieve a better understanding of how exactly the incremental yield will affect the overall product quality and to determine the yields of gas, liquid, and coke as a function of conversion of the residue fraction of bitumen. The flowsheet of Figure 1 summarizes the experimental framework of the study. The residue fraction was subjected to repeated coking steps and the conversion and composition of the distillate and unconverted residue were determined at each step. The product material obtained from the first pass in the quartz tube reactor was vacuum distilled using a technique similar to ASTM D-1160 into a distillate and residue fraction. The residue fraction was then fed into the reactor as second pass feed material and the sequence of steps repeated until complete residue conversion was achieved. The products at each of the stages were then subjected to a thorough chemical analysis to determine the concentration of the different carbon types, the boiling point distribution, the heteroatom content, and the molecular weight. Experimental Section Materials. Athabasca vacuum residue provided by Syncrude Canada Limited was used as the first stage feed to the coking reactor. Table 1 lists some of the important properties of the feed material. Carbon disulfide (99.9%) was supplied by Fisher Scientific. Reactor System. The reactor system was designed as an open system for high temperature thermal cracking reactions. The main reactor was a 2.5 cm quartz tube with an internal volume of approximately 60 mL. It was dipped in a molten salt bath maintained at the reaction temperature of 530 °C. The salt used was provided by APCO Industries Co. Ltd. and (10) Chung, K. H.; Xu, C. Fuel 2001, 80, 1165.

Table 1. Properties of Athabasca Vacuum Residue carbon, wt % hydrogen, wt % sulfur, wt % nitrogen, wt % toluene-insoluble, wt % average molecular weight asphaltene content, wt %

81.4 9.6 5.8 0.7 1.8 1012 24.7

was a mixture of sodium and potassium nitrate. The temperature reached 95% of the final set temperature within 60 s. Nitrogen was used as the sweep gas at a flow rate of 0.17 L/min, which corresponds to a residence time of 7.9 s at 530 °C. After exactly 10 min the tube was quenched in water at room temperature and the nitrogen purge stream was shut off. The evolved products accumulated in the downstream collection assembly that was maintained at dry ice-acetone temperature (-78 °C). The weight of the feed and liquid products was determined gravimetrically. The collected liquid was filtered to separate the CS2-insoluble material. It was found that CS2 gave results similar to those obtained using toluene (data not shown). Millipore filter papers (0.22 µm) were used to filter the carbon disulfide-insoluble material from the reaction products.

% Coke yield )

mCS2I - mfCS2I mfeed - mfCS2I

× 100

(1)

where mCS2I represents the weight of CS2-insoluble, mfCS2I represents the weight of CS2-insoluble in the feed, and mfeed is the weight of the feed. Distillate yields were determined as the fraction of liquid product collected on condensation while the amount that did not volatilize during vacuum distillation was called residue. The loss of weight during product handling, i.e., the difference in weight of liquid after the thermal cracking experiment and before vacuum distillation was assumed to represent yield of naphtha. For a typical experiment, the gas produced was collected in a gas bag and the gases analyzed by gas chromatography. Since the amount of nitrogen was known the gas yields were determined by calibration with the concentration of nitrogen determined by the gas chromatograph.

% Distillate yield ) % Residue yield )

md × 100 mfeed - mfCS2I

mr × 100 mfeed - mfCS2I

(2)

(3)

Distillates from Repeated Recycle of Residue where md represents the weight of liquid condensed on the entire assembly, and mr represents the weight of liquid not volatilized during distillation. Analytical Techniques. A microscale vacuum distillation apparatus based on ASTM D-1160 was used to distill the small quantities of product from the reactor. Product samples were placed in a round-bottom flask, then the headspace was packed with glass wool to prevent entrainment and connected to a riser and condenser assembly. The temperature of the boiling liquid was measured by thermocouple. A cold trap kept in liquid nitrogen was used to collect any light product not condensed in the collection flask. After the pressure was reduced to 0.5 mmHg using a rotary-vane vacuum pump, the heating mantle was switched on. The temperature was increased rapidly to 100 °C and then ramped up at a rate of 5 °C/min to 325 °C. This maximum temperature minimized thermal degradation. The flask was weighed before and after distillation to obtain the weight fraction of residue. The distillate was measured by weighing the entire assembly before and after distillation. The total recovery was approximately 100% for all samples used. Separation of a reference sample of Athabasca bitumen using this apparatus showed that the cut point between distillate and residue was 450 °C (i.e., 95% of the distillate boiled below 450 °C and 5% of the residue boiled above this temperature). The samples for 1H NMR spectroscopy were prepared by mixing approximately 20 mg of the sample with 700 µL of deuteriochloroform (CDCl3). The distillate 13C NMR samples used approximately 100 mg of material in 600 µL of CDCl3 and the residue 13C NMR spectroscopy used the same amount of material in 700 µL of CDCl3. The NMR spectroscopic analyses was performed at room temperature (20 ( 1 °C) on a Varian XL-300 NMR spectrometer, operating at 299.943 MHz for proton and 75.429 MHz for carbon. The proton spectra were collected with an acquisition time of 2.1 s, a sweep width of 7000 Hz, a pulse flip angle of 30.8° (8.2 µs), and a 1 s recycle delay. These pulse recycle conditions permit the collection of quantitative spectra for all protonated molecular species in the petroleum samples. The spectra, resulting from 128 scans and using 0.3-Hz line broadening, were referenced to the residual chloroform resonance at 7.24 ppm. The quantitative carbon spectra were acquired using an acquisition time of 0.9 s and a sweep width of 16 500 Hz. A flip angle of 26.2° (4.6 µs) and a recycle delay of 10 s were used for the distillate, while for the residue samples, a flip angle of 31.9° (5.7 µs) and a 4 s delay were used. These parameters are quantitative for carbons with spin lattice relaxation times (T1) of the order of 100 s in distillate and 30 s in the residue. Reverse-gated waltz proton decoupling was used to avoid nuclear Overhauser effect enhancements of the carbon signals. The spectra were the result of 5000 scans for the distillate. The distillate spectra used 5-Hz line broadening to improve the signal-to-noise ratio of the spectra. The residue spectra resulted from 15 000 scans and used 10 Hz line broadening. All spectra were referenced to the CDCl3 resonance at 77.0 ppm. The nomenclature used to describe the various carbon types is shown in Figure 2. The chemical shift assignments for the 1H and 13C NMR spectra are shown in Tables 2 and 3, respectively. These assignments are comparable to previous studies, and were based on model compound assignments,11,12 two-dimensional NMR spectroscopic techniques such as HETCOR (heteronuclear chemical shift correlation) and COSY (homonuclear correlation spectroscopy),13 as well as the onedimensional technique DEPT (distortionless enhanced polarization transfer).14,15 (11) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Anal. Chem. 1979, 51, 2189. (12) Thiel, J.; Gray, M. R. AOSTRA J. Res. 1988, 4, 63.

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Figure 2. Hypothetical molecule to represent different carbon types. Table 2. Chemical Shifts of Proton Spectral Regions region

chemical shifts (ppm)

structural type

HA1 HA2 HO1 HO2 HO3 HP1 HP2 HP3 HP4

10.7 to 7.4 7.4 to 6.2 6.2 to 5.1 5.1 to 4.8 4.8 to 4.3 4.3 to 2.4 2.4 to 2.0 2.0 to 1.09 1.09 to -0.5

polyaromatic monoaromatic olefinic CH olefinic CH2 olefinic CH2 R to aromatic CH2 R to aromatic CH3 paraffinic CH2 paraffinic CH3

Table 3. Chemical Shifts of Carbon Spectral Regions region

chemical shifts (ppm)

structural type

CA1 CA2 CA3 CA4 CA5 CP1 CP2 CP3

190 to 170 170 to 129 129 to 115.5 115.5 to 113.5 113.5 to 100 70 to 45 45 to 32.7 32.7 to 30.8

CP4

30.8 to 28.5

CP5 CP6

28.5 to 25 25 to 21.9

CP7 CP8 CP9 CP10

21.9 to 17.6 17.6 to 14.7 14.7 to 12.3 12.3 to 0

oxygenated quaternary aromatic aromatic CH olefinic CH2 olefinic CH2 paraffinic CH paraffinic CH & CH2 chain γ -CH2 β to aromatic CH2 chain δ -CH2 R to aromatic naphthenes aromatic-attached ethyl CH2 cycloparaffin CH2 chain β -CH2 R to ring CH3 R to ring CH3 aromatic-attached ethyl CH3 chain R -CH3 branched-chain CH3

The final calculations performed to obtain the different carbon types are shown in Table 4. The aromatic CH’s, terminal methyls, and the methyl of aromatic ethyl groups (13) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Fuel 1996, 75, 483. (14) Netzel, D. A. Anal. Chem. 1987, 59, 1775. (15) Kotlyar, L. S.; Morat, C.; Ripmeester, J. A. Fuel 1991, 70, 90.

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Table 4. Group Classifications of Chemical Species region CHar E-CH3 C-CH3 Qar-S Qar-P Ar-CH3 Cy-CH3 CH CHAIN NAPH

chemical species aromatic CH methyl of an aromatic ethyl terminal methyl of a paraffinic chain paraffin-substituted Qar

spectral region CA3 CP8 CP9

HP1 + HP2 converted to C wt % polyaromatic quaternary CA2 - Qar-S R to aromatic ring methyl HP2 converted to C wt % R to cycloparaffin ring methyl (CP7 + CP6/2) - Ar-CH3 paraffinic CH CP1 + CP2/2 >C5 chains CP4 - CP8 - CP5/2 cycloparaffin CH2 CP5

were determined directly from the carbon spectra. The other groups listed in Table 4 were calculated using data from both proton and carbon spectra or had to be estimated due to spectral overlap and are therefore subject to larger margins of error. The average molecular weights were obtained using vapor pressure osmometry (Westcan Corona Model 232A) at the Micro Analytical Laboratory at the University of Alberta. The samples were dissolved at a concentration of ca. 2 mg/mL in o-dichlorobenzene and analyzed at a temperature of 130 °C. Elemental analysis was performed on a Carlo Erba Elemental Analyzer 1108 at the Micro Analytical Laboratory, University of Alberta. HPLC (high-performance liquid chromatography) was performed by the Analytical Laboratory at Syncrude Canada Limited, in Edmonton. The HPLC method used two detectors; an Alltech model 500 evaporative light scattering detector run at 35 °C and 1.2 L/min of nitrogen and a Polychrom 9065 diode array detector. The two columns used were the PAC (Whatman Partisil 5 4.6 mm × 25 cm) and the RingSep (ES Industries, 4.6 mm × 25 cm) column. The solvent program was hexane at 1.5 mL/min, for 12.5 min, followed by mixtures of methylene chloride and hexane stepping at intervals from 5% methylene chloride to 2%, 25%, 20%, and 90% over a period of 15 min. Mixtures of 10% and 25% isopropyl alcohol in methylene chloride were then used for 8 and 1.4 min, respectively, all at a flow of 1.5 mL/min. Sample volumes were 20 µL and a calibration mixture of hexadecane, hexadecyl benzene, naphthalene, fluorene, dibenzothiophene, phenanthrene, and pyrene was used to check the column separations and UV detector response.

Results Yield of Product Fractions. The cumulative product yields are shown in Figure 3 and residue conversions are shown in Table 5. Mass balance closures in excess of 98% were obtained for all three stages of coking. Stage 1 of coking achieved almost 80% conversion of the residue fraction. Recycling the residue to achieve a cumulative residue conversion of over 97% gave a 12% increase in liquid yield and an increase of 5% in the yield of coke plus gases. A substantial increase in liquid yields was thus obtained by recycling the unconverted residue. Analytical Data. The results from the elemental analysis are shown in Table 6. The hydrogen content of the distillate and residue fractions decreased from stage to stage, although hydrogen content of the coke did not change. Since the carbon content for the liquid fractions was almost constant, the H/C ratio also showed a trend similar to the hydrogen content. Nitrogen content more than doubled in the distillate fraction from stage 1 to stage 3. The sulfur content of the liquid products did not show any distinct trends. The quality of the coke from the later stages of the experiment showed a

Figure 3. Cumulative yields of gas, naphtha, distillate, and coke products from cracking of Athabasca residue through three reaction stages. Table 5. Cumulative Residue and MCR Content sample

cumulative residue conversion

MCR content

feed Stage 1 residue Stage 2 residue Stage 3 residue

79.8 93.9 97.1

24.9a 16.43 21.87 33.92

a Reduced by the amount of toluene-insolubles in the feed, from Table 1.

Table 6. Elemental Analysis and Ash Content carbon hydrogen nitrogen sulfur H/C ash (wt %) (wt %) (wt %) (wt %) ratio (wt %) feed Stage 1 residue Stage 2 residue Stage 3 residue Stage 1 distillate Stage 2 distillate Stage 3 distillate Stage 1 coke Stage 2 coke Stage 3 coke

81.4 83.7 84.2 84.5 83.7 83.3 83.9 79.4 86.9 86.4

9.6 9.5 8.2 6.6 10.6 9.9 9.0 3.1 3.5 3.4

0.7 0.7 0.9 1.3 0.3 0.5 0.8 1.8 1.4 1.4

5.8 5.0 5.8 6.6 4.9 5.6 5.3 6.6 2.6 2.7

1.42 1.36 1.17 0.94 1.52 1.42 1.29 0.47 0.48 0.47

5.5 1.6 0.5

Table 7. Molecular Weight and Average Molecular Formula

feed Stage 1 residue Stage 2 residue Stage 3 residue Stage 1 distillate Stage 2 distillate Stage 3 distillate

MW (dalton)

average molecular formula

1013 543 457 428 350 393 306

C68.6H96.9N0.3S1.8 C37.8H50.9N0.1S0.8 C32.0H37.2N0.1S0.8 C30.1H27.9N0.2S0.9 C24.4H36.7N0.0S0.5 C27.3H38.4N0.1S0.7 C21.3H27.4N0.1S0.5

significant improvement because the sulfur and ash content dropped significantly from stage 1 to stage 3. The molecular weight of the residue sharply dropped almost 50% after the initial feed material was first cracked (Table 7). Subsequently, there was a gradual decrease in molecular weight with every reaction stage. The variation of molecular weight for the distillate fraction was within the normal error associated with vapor pressure osmometry. From the elemental analysis and molecular weights, the average molecular formulas were calculated and are shown in Table 7. The average molecular formula showed that the residue and distillate materials after the first stage of coking were quite similar and the removal of five to 10 carbon atoms on average can convert the residue to distillate.

Distillates from Repeated Recycle of Residue

Figure 4. Carbon structural groups in residue fractions from NMR spectroscopy (definitions of carbon groups are given in Table 4).

In Figure 4, the total aromatic carbon content increased from 40% in the virgin feed to 78% in the stage 3 residue fraction. The mole percentage of both quaternary carbon (Qar) and protonated carbon (CHar) species increased. The content of substituted quaternary carbon (Qar-S) showed little or no variation and thus the increase in quaternary carbon is almost entirely due to quaternary bridgehead carbon (Qar-P). The increase in total aromatic carbon can indicate either that the number of rings per aromatic cluster is increasing (polyaromatic ring clusters are getting larger) or that the number of aromatic clusters per unit volume is increasing. The ratio of bridgehead carbon to total aromatic carbon (Qar-P/total aromatic carbon) suggests that the latter is occurring (see Figure 6 and the discussion below). The contents of all aliphatic carbon types in the residue fraction decreased with coking cycle. Interestingly the contents of the NAPH and CH species, indicative of cycloparaffinic rings, decreased significantly but still remained after the third coking cycle. Similarly, the contents of methyl groups on cycloparaffins (Cy-CH3) decreased. As well, despite the increase in aromatic carbon contents, the contents of methyl (Ar-CH3) and ethyl (E-CH3) species on aromatic rings decreased. Finally, the species that indicate long chain paraffins and their terminal methyls (CHAIN and C-CH 3, respectively) decreased but, similar to the cycloparaffins, remained after the third cycle. The implications of these results are presented in the discussion. The contents of aromatic and aliphatic carbon types in the distillate fractions followed similar trends to those found for the residue fractions (Figure 5). The only exception was the content of the aliphatic CH carbon type that did not decrease from the second stage distillate fraction to the third stage distillate fraction.

Energy & Fuels, Vol. 16, No. 2, 2002 481

Figure 5. Carbon structural groups in distillate fractions from NMR spectroscopy (definitions of carbon groups are given in Table 4).

Figure 6. Chain length and cluster size of distillate and residue fractions.

The NMR data were also used to calculate other parameters. An indicator of chain length can be calculated from the ratio of CHAIN/C-CH3.16 As illustrated in Figure 6, the variation in this ratio was small and remained similar to that of the original feed. Another important parameter calculated from the NMR data was the ratio of bridgehead to aromatic carbon, which indicates the degree of condensation.17,18 The variation of this ratio was minimal, indicating that the average number of rings per aromatic cluster was not increasing. The NMR data, elemental analysis, and the yields of the product fractions enabled the calculation of an (16) Netzel, N. A.; McKay, D. R.; Heppner, R. A.; Guffey, F. D.; Cooke, S. D.; Varie, D. L.; Linn, D. E. Fuel 1981, 60, 307. (17) Snape, C. E.; Bartle, K. D. Fuel 1984, 63, 883. (18) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.

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Figure 7. Mass balance for aromatic carbon in all three coking stages (assumed 35% aromatic carbon in naphtha).

aromatic carbon mass balance. The calculation was based on the assumption that the percentage of aromatic carbon of the coke was 91%, the measured value for delayed coke from Athabasca.19 The H/C ratio for the coke obtained from commercial delayed coking of oilsands bitumen20 was similar to that of the coke produced from the coking experiments in this study. The carbon content of the liquid fractions and the coke was obtained from the elemental analysis. The gas composition allowed the determination of the carbon content of the gas, which contained no aromatic carbon. The total aromatic carbon (TAC) in a given fraction was determined as:

Japanwala et al.

Figure 8. MCR versus aromatic carbon content for residue fractions.

TACfraction ) Weight ×

% Aromaticity Weight % Carbon × (4) 100 100

The main uncertainty in the aromatic carbon balance was the concentration of aromatic carbon in the volatile liquid product, reported as naphtha. Variable contents of aromatic carbon have been reported for coker naphthas. The aromatic balance illustrated in Figure 7 was calculated for a naphtha fraction with 35% aromatic carbon. If the naphtha contained no aromatic carbon, then the top section of each bar would be eliminated. The data in Figure 7 give, therefore, the bounds on the contribution of the naphtha. Regardless of the aromatic carbon in the naphtha, the data of Figure 7 show that the total aromatic carbon content increased significantly in the first stage, then remained constant, within experimental error. The MCR data for the residue fractions (Table 5) showed that the residue after the first stage of coking had a lower MCR content than the fresh feed. The subsequent residue samples had significantly higher MCR content, increasing in proportion to the aromatic carbon content (Figure 8). The fresh feed did not follow the same trend as the residues from coking, giving a higher MCR content than the first and second stage residues despite its lower aromatic carbon content. The fresh feed had more high-boiling, high-molecular-weight components in comparison to the processed residues (Table 7). Volatilization of the residue components of molecular weight of less that 700 tends to reduce the yield of coke.21 The minimum in the MCR content in (19) Egiebor, N. O.; Gray, M. R.; Cyr, N. Chem. Eng. Commun. 1989, 77, 125. (20) Hepler, L. G. AOSTRA Technical Handbook on Oil Sands, Bitumens and Heavy Oils; AOSTRA: Edmonton, AB, 1989.

Figure 9. MCR content of distillate fractions (bars: stagewise product properties; points: average cumulative properties).

Table 5 may, therefore, be due to the balance between increasing aromatic carbon content, which increased coke yield, and decreasing average molecular weight which increased volatilization. The bars in Figure 9 indicate the variation of the distillate MCR over the various stages of coking. The cumulative MCR after each stage is also shown in the figure as a solid line. The distillate feed to the last stage had a very high MCR (6.3%) for a volatile “non-MCR” distillate fraction, which was consistent with its high aromatics content. On a cumulative basis, however, this material did not give a significant change in the average MCR of the distillates. HPLC was used to separate the fractions into saturates, monoaromatics, diaromatics, triaromatics, and polyaromatics, with detection by UV absorption at several wavelengths. The data of Figure 10 at 210 nm show a general shift from mono- and di-aromatics in the distillate from Stage 1 to tri- and polyaromatics in Stage 3. The step in the baseline at 20 min was due to the change in solvent. The UV absorption at 282 nm clearly showed the buildup of the polyaromatic fraction from stage 1 to stage 3. Discussion The analysis of the distillates obtained from repeated recycling of the residue showed that the quality of the distillates deteriorated with each coking stage. The (21) Wiehe, I. A. Energy Fuels 1994, 8, 536.

Distillates from Repeated Recycle of Residue

Figure 10. HPLC chromatograms of distillate fractions at 210 nm (all aromatics) and 282 nm (polyaromatics).

HPLC data indicated a buildup of polyaromatic hydrocarbons and the weight % of nitrogen in the distillate fraction more than doubled from stage 1 to stage 3 (Table 6). The increase in the coke-forming potential of the distillate fraction, shown by an increase in MCR content from 1.0% for stage 1 to 6.3% for the last stage (Figure 9), was consistent with the increasing concentration of polyaromatics. Nitrogen and polyaromatic hydrocarbons are catalyst poisons,1,2 therefore, the distillate fractions from the recycled residues would tend to increase catalyst deactivation in subsequent hydrotreating relative to distillate from fresh feed on the first pass through the coker. Recycling the residue fraction resulted in an increase in yield of distillate of 8 wt % but by every quantitative measure of distillate product quality the distillate fraction became progressively more refractory at every stage (Table 6, Figures 5, 9, and 10). The HPLC data pointed to a buildup of polyaromatics in the distillate fractions. On the other hand, the NMR data showed that the ratio of bridgehead to total aromatic carbon remained constant. This result indicates that there were more polyaromatic species in the distillate after each coking cycle but that the average number of aromatic rings per polyaromatic group remained constant. The mono aromatics are likely to migrate into the naphtha fraction by cracking and the pentacyclic and higher aromatics form coke. This disproportionation would lead to a build up of tri- and tetra-aromatic structures in the distillate products while maintaining a relatively constant ratio of bridgehead to total aromatic carbon.

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The sharp drop in molecular weight in the first stage was consistent with the cracking of the thermally labile bonds (for example sulfur linkages) linking the aromatic groups, especially in the asphaltenes.9 The MW for the processed residue fractions generated after the first stage of coking remained almost constant. The constant ratio of bridgehead to aromatic carbon as well as the constant average molecular weight (Table 7) for the residue after the first coking stage indicated that the number of rings per polyaromatic species did not increase in the recycled residue fraction. The aromatic structures in the molecules in the residue fraction likely did not change appreciably after stage 1. The structural differences from stage 2 onward could simply be accounted for as the gradual disappearance of the paraffinic groups. The elemental analysis of the coke (Table 6) also showed that its composition was almost constant, thus indicating an origin from similar aromatic groups. The majority of carbon types identified by NMR data and the elemental analysis showed no significant difference between recycled residue and the corresponding distillate. Structurally there was a shift at each stage toward a more aromatic species due to the loss of naphthenic and paraffinic species, however, the data do not suggest the formation of a large, condensed naphtheno-aromatic structure in the recycled residue as suggested in the literature.22 Although the NMR data showed that the aromatic carbon content of the distillate and residue fractions increased with each coking stage, a mass balance on the aromatic carbon showed that after the first stage of cracking there was no overall increase in the net amount of aromatic carbon (Figure 7). The net increase in total aromatic carbon of the first stage was most likely due to the dehydrogenation of the hydroaromatics. The second and third stage residues were unlikely to have significant concentrations of these easily dehydrogenable compounds, and therefore no further increase in aromatic carbon was observed. The remaining NAPH and CH carbon species detected by NMR in the subsequent stages were probably due to cyclic naphthenes such as steranes and terpanes, as well as aromatic steroids. These compounds do not easily dehydrogenate under reaction conditions.23,24 Fivemembered naphthenic rings are also difficult to dehydrogenate.25 The naphthenic compounds lost from stage 2 onward cracked to give lighter products or underwent condensation reactions along with the aromatics to form coke. The major reactions that occurred during thermal cracking included dealkylation of aromatic rings and ring dehydrogenation. Our analysis indicated that although the number of alkyl side chains on aromatic rings decreased, the average chain length remained constant (Figure 6). This observation was consistent with previous 13C NMR analysis of coked residues,8 but it contradicts the data of Zhao et al.,9 who presented an index that suggested a significant drop in the length of side chains with a single stage of coking. Model compound studies on dealkylation of alkyl-pyrenes and (22) Sanford, E. C. Ind. Eng. Chem. Res. 1994, 33, 109. (23) Peters, K. E.; Scheuerman, G. L.; Lee, C. Y.; Moldowan, J. M.; Reynolds, R. N.; Pea, M. M. Energy Fuels 1992, 6, 560. (24) Sullivan, R. F.; Boduszynski, M. M.; Fetzer, J. C. Energy Fuels 1989, 3, 603. (25) Fu, P. P.; Harvey, P. G. Chem. Rev. 1978, 78, 317.

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alkyl-benzenes showed that dealkylation was the preferred pathway, with less cracking of the side chain to give shorter chains,26-29 consistent with the present results. In alkyl benzenes, the preferred cleavage is β or γ to the aromatic ring, leaving toluene or styrene.29 The data of Figure 4 show no evidence for an increase in aromatic methyl or ethyl groups due to such cleavage. One explanation is that that cleavage was at the strong alkyl-aryl bond, giving an alkyl chain and an unsubstituted aromatic. Although this pathway would appear energetically less favorable, it is consistent with the model compound studies by Freund et al.26 and Smith and Savage28 who observed preferential dealkylation of alkyl-pyrenes by breaking the bond R to the aromatic ring. The discrepancy in the observations on length of paraffin chains with Zhao et al.9 is difficult to reconcile, given that the use of the band at ca. 29 ppm for the long-chain CH2 groups in their study, following Suzuki et al.,30 was consistent with the present work (Table 2) and previous studies.7,12,16 On the basis of the present data and model compound work, however, we conclude that the NMR data are consistent with polycyclic aromatic compounds (3-5 rings) undergoing dealkylation reactions to remove side chains as a predominant reaction, along with coke formation at each stage. The average molecular formulas of the distillates and residues (Table 7) showed that for each stage the difference was 9-13 carbons. By the last stage of cracking the residue molecules were highly aromatic, but almost half of the aliphatic carbon remained in the form of chains and methyl groups. The analysis of narrow fractions of bitumen and coked residue by Chung and Xu10 showed that 80% of the residue from once-through coking had a molecular weight in the range of 300-600, but that the remaining 20% had an average molecular weight of ca. 2500. Both the averaged data from Table 7, and the data of Chung and Xu10 indicated that the removal of a 10-carbon side group, such as an alkyl chain, would be sufficient to convert a portion of the residue material to distillate. Process Implications. In the context of heavy oil upgrading, the purpose of the coker is to eliminate the undesirable constituents such as metals, the heteroatoms, and most of the polyaromatics in the form of coke, while at the same time maximizing the distillate yield for a given feed. In the present study, the coking reactions were most effective in removing these components in the first stage. The distillate product carried greater amounts of these undesirable components in the second and third stages, therefore, the process was less effective on recycled feed than fresh feed. While the most common approach is to recycle all of the unconverted residue in coking processes, other approaches include recycling less residue by passing more 524 °C+ material into the products31 or by passing the unconverted residue from one coker into a second unit. The latter option is feasible in complexes with more than one coking unit, (26) Freund, H.; Matturro, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuels 1991, 5, 840. (27) Greinke, R. A. Carbon 1992, 30 (3), 407. (28) Smith, C. M.; Savage, P. E. AIChE J. 1991, 37, 1613. (29) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 488. (30) Suzuki, T.; Itoh, M.; Takegami, Y.; Watanabe, Y. Fuel 1982, 61, 402. (31) Elliott, J. D. Delayed coking innovations and new design trend, NPRA 1999 Annual Meeting, San Antonio, TX, 1999.

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and it allows the possibility of tuning the conditions in the second unit to achieve more selective conversion. For example, more of the undesirable material would be eliminated from the recycle stream if the reaction conditions in a second stage unit were changed to increase the yield of coke, by increasing the residence time for the recycle residue stream or by reducing the temperature to suppress volatilization. The experimental results showed that the quality of the distillate produced on recycle coking approached that of the residue feed. This observation, along with the results of Chung and Xu,10 suggests that a portion of the recycle residue could be processed along with the distillate in hydrotreaters, particularly from the first pass through the reactor. The distillate and residue from the third stage of repeated coking had similar average molecular weights. The molecular weights would affect diffusion in the catalyst pores and thus the catalyst performance, but the diffusivity is approximately proportional to the square of the MW32 and thus diffusion is not highly sensitive. A larger concern is the effects of this third-stage residue versus the distillate on catalyst deactivation in hydrotreaters due to high nitrogen and aromatic content. In contrast to the decrease in distillate quality found after each coking cycle, the quality of the coke increased (Table 6). In particular, the sulfur and ash contents of the coke from Stages 2 and 3 were significantly lower than those found after Stage 1. This observation raises the possibility of producing a higher-value coke product by separating first-stage coke from recycle coke, as in the series-coking approach mentioned above. Conclusions The cumulative yield of liquid increased by 12 wt % from stage 1 of coking where 80% residue conversion was obtained, to stage 3 where over 97% residue conversion was obtained. On the other hand, the total yield of coke plus gases increased by only 5 wt %. The coke quality showed significant improvement in terms of lower ash and heteroatom content. The analysis of the distillates obtained from repeated recycling of the residue showed that the quality of the distillates deteriorated with every coking stage. The distillate fractions showed a trend toward increased aromatic carbon and nitrogen content. The percentage of aromatic carbon of the residue doubled from 40% to 78% from feed residue to stage 3 residue. Initially there was a sharp drop in molecular weight of the residue fraction due to the breaking up of the labile bonds that link the bitumen macromolecule. As the reaction progressed in the second and third stages, residue conversion involved mainly the removal of side chains from relatively unchanging aromatic groups. Acknowledgment. The authors are grateful to Jean Cooley and Wilhemena Rip for analytical assistance, and to Syncrude Canada and the Natural Sciences and Engineering Research Council for support under the Industrial Research Chair in Advanced Upgrading of Bitumen. EF010234J (32) Wilke, C. R.; Chang, P. Am. Inst. Chem. Eng. J. 1955, 1, 264.