1432
Energy & Fuels 2007, 21, 1432-1441
Fluid Catalytic Cracking Quality Improvement of Bitumen after Paraffinic Froth Treatment Siauw H. Ng,*,† Tadek Dabros,‡ and Adrian Humphries§ National Centre for Upgrading Technology, 1 Oil Patch DriVe, Suite A202, DeVon, Alberta, Canada T9G 1A8, AdVanced Separation Technologies, 1 Oil Patch DriVe, Suite A202, DeVon, Alberta, Canada T9G 1A8, and Albemarle Catalysts Company LP, 2625 Bay Area BlVd., Suite 250, Houston, Texas 77058 ReceiVed October 4, 2006. ReVised Manuscript ReceiVed January 22, 2007
This study dealt with a systematic investigation on the fluid catalytic cracking (FCC) performances of two bitumen samplessa partially deasphalted bitumen from a froth treatment process using a paraffinic solvent and a dry bitumen extracted using an aromatic solvent (the regular bitumen). Each bitumen sample was characterized and diluted with a heavy gas oil (an FCC feed) derived from Alberta conventional crude Rainbow Zama to produce two series of blends in 0, 10, 25, 50, 75, and 100 wt % bitumen concentrations. All samples were catalytically cracked in a microactivity test (MAT) reactor loaded with a wide-pore FCC catalyst at 540 °C. Liquid products from selected runs were characterized for hydrocarbon type and distribution of sulfur by boiling point. The improvement in cracking yields and product quality of bitumen after froth treatment with paraffinic solvent was assessed. The economic level of bitumen addition to heavy gas oil in FCC operation was also established.
1. Introduction The fluid catalytic cracking (FCC) unit for converting heavy gas oils (HGOs) to gasoline and other valuable products is the heart of a refinery. The composition of an HGO, having the most important influence on the yields and product quality, depends on the crude used and the processes involved. It is known that bitumen-derived crude (BDC), including synthetic crude oil (SCO), is less superior to produce FCC feed than stocks from conventional sources. For this reason, refiners in North America limit the use of BDC in their conventional FCCbased operations. With advancements in FCC technologyssuch as better catalysts with larger pores and higher metals-tolerance and improved hardware such as the catalyst-regeneration systems that allow higher coke-burning capacitysrefiners can now accept more dirty feed in their resid FCC (RFCC) operations. It is not surprising that bitumen can be added directly to HGO, without prior upgrading in a coker or hydrocracker, and that the mixture can be catalytically cracked. This option is similar to the case where atmospheric tower bottoms, containing both vacuum gas oil and vacuum tower bottoms, is cracked for conversion of residua to transportation fuels. After being processed with paraffinic solvent in a froth treatment, some asphaltenes, Conradson Carbon Residue (CCR) precursors, and metals in the bitumen are removed. The investigation on the improved catalytic cracking performance of this treated bitumen is the primary objective of this study. 2. Experimental Section Two bitumen samples were prepared from a feed to an AST (Advanced Separation Technologies) froth treatment pilot plant. * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 1-780-987-5349. † National Centre for Upgrading Technology. ‡ Advanced Separation Technologies. § Albemarle Catalysts Company LP.
The froth feed was composed of approximately 12, 25, and 63 wt % solids, water, and bitumen, respectively. Either paraffinic solvent (n-heptane) or aromatic solvent (toluene) was added to the froth feed in 3:1 (w/w) ratio. After mixing and settling overnight, each mixture was centrifuged to remove solids and water. The remaining diluted bitumen was further treated in a rotavap (Bu¨chi Rotavapor R-124, ∼100 rpm, 60-120 °C water or oil bath temperature, 5060 mbar vacuum) to evaporate the solvent. The resulting “dry” bitumen is called “P-” or “A-” bitumen, depending on if it has been treated with paraffinic or aromatic solvent. The recoveries were roughly 90 and 98 wt % for P- and A-bitumen, respectively (about 50 wt % rejection of pentane-asphaltenes and +98 wt % maltenesrecovery for P-bitumen). Each bitumen was successively added, in 0, 10, 25, 50, 75, and 100 wt % concentrations, to a heavy gas oil from the conventional sweet crude Rainbow Zama obtained from Syncrude Research. A microactivity test (MAT) unit (Zeton Automat IV), equipped with a fluid-bed reactor and collection systems for gas and liquid products, was used for cracking experiments. For each oil blend, a minimum of four runs were conducted at 540 °C at nominal catalyst/oil (C/O) ratios of 4, 6, 8, and 10, respectively. The reactor was loaded with 9 g of equilibrium catalyst HRO-610 from Albemarle Catalysts Company LP with 30 s of catalyst contact time for all runs. HRO-610 is known to have large pores suitable for cracking BDC-derived HGOs as was reported previously.1 A specially designed liquid receiver with extra large volume (300 mL) was used to collect over 99 wt % of liquid products that were free of contamination by wash solvents (e.g., CS2). The total liquid products (TLPs) were characterized (without prior separation) for the following: (1) simulated distillation (ASTM D-2887); (2) hydrocarbon types of gasoline (using a PIONA analyzer, a specially configured gas chromatograph (GC) with a prefractionator); and (3) boiling-point distributions of sulfur (using a GC with a Sievers sulfur chemiluminescence detectorsGC-SCD). Coke deposited on the catalyst after cracking was determined by (1) Ng, S. H.; Zhu, Y.; Humphries, A.; Zheng, L.; Ding, F.; Gentzis, T.; Charland, J-P.; Yui, S. Energy Fuels 2002, 16, 1196-1208.
10.1021/ef060496r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/23/2007
FCC Quality ImproVement of Bitumen
Energy & Fuels, Vol. 21, No. 3, 2007 1433 Table 1. Feedstock Properties
feed name
series A (RZ + A-bitumen)
RZ HGO
bitumen addition, wt % density at 15 °C, g/mL H, wt % C, wt % H/C atomic ratio total nitrogen, wppm total sulfur, wt % CCR, wt % Ni, wppm V, wppm Ni + V, wppm 343 °C- by simdist, wt % 524 °C+ by simdist, wt % saturates, wt % aromatics, wt % polar compounds, wt % asphaltenes (C5-insol.), wt %
0 0.8935 13 86.3 1.795 600 0.59 0.05 0.0 0.0 0.0 18.0 2.9 61.7 35.6 2.7 0.0
series P (RZ + P-bitumen)
10
25
50
75
100
10
25
50
75
100
0.9041 12.70 86.05 1.759 900 1.04 1.42 6.7 17.1 23.8 17.68 7.75 56.54 37.61 4.02 1.83
0.9204 12.26 85.67 1.705 1350 1.72 3.47 16.8 42.8 59.5 17.19 15.02 48.80 40.63 5.99 4.58
0.9491 11.52 85.03 1.614 2100 2.86 6.89 33.5 85.5 119.0 16.38 27.12 35.90 45.66 9.29 9.17
0.9795 10.78 84.40 1.522 2850 3.99 10.31 50.3 128.3 178.5 15.58 39.23 22.99 50.68 12.58 13.75
1.0120 10.04 83.76 1.428 3600 5.12 13.73 67.0 171.0 238.0 14.8 51.3 10.1 55.7 15.9 18.3
0.9024 12.77 86.11 1.767 850 0.96 0.99 4.6 12.0 16.6 18.08 7.20 57.93 36.91 4.17 0.98
0.9161 12.42 85.83 1.724 1225 1.50 2.41 11.6 30.0 41.6 18.21 13.63 52.28 38.87 6.39 2.46
0.9399 11.84 85.36 1.653 1850 2.42 4.77 23.1 60.0 83.1 18.42 24.34 42.87 42.15 10.07 4.92
0.9649 11.26 84.88 1.581 2475 3.33 7.13 34.7 90.0 124.7 18.63 35.06 33.45 45.42 13.76 7.38
0.9913 10.68 84.41 1.508 3100 4.24 9.49 46.2 120 166.2 18.8 45.8 24.0 48.7 17.4 9.8
Table 1 shows the properties of the three parent feeds (RZ HGO, A-bitumen, and P-bitumen) and their blends. One can observe the good quality of the conventional gas oil, with no metals and asphaltenes, low in density, CCR, and aromatics, high in H/C atomic ratio, and moderate in nitrogen and sulfur due to its virgin nature. In contrast, the two bitumen samples were rather poor in quality, especially A-bitumen. It is obvious that the paraffinic solvent removed a significant amount of asphaltenes originally in P-bitumen (46 wt % relative to that of A-bitumen), resulting in a considerable quality improvement. This is reflected by reductions of CCR, Ni, and V (by 30 wt % each) which are the most important elements to be considered in resid FCC. 3.1. Cracking Characteristics. Considering the heavy nature of bitumen, the original work plan called for the determination of the cracking behavior of the bitumen by extrapolation from the RZ-bitumen blends. However, as will be shown later, the relationship between yield and bitumen addition in a blend was not necessarily linear. This introduced some uncertainties to the extrapolated yields. Significant efforts were then made to overcome the difficulty in delivery of bitumen at 90 °C into the reactor using a heat-resistant syringe. Eventually, cracking experiments could be performed on the bitumen sample without dilution. Results are discussed below. 3.1.1. Conversion. In this study, since the reaction temperature, the catalyst weight, and the catalyst contact time are all fixed for a given feed, the only variable is the C/O ratio, which affects the weight hourly space velocity (WHSV, in h-1) through the relationship WHSV ) 3600/[(C/O)×t], where t is the catalyst contact time in seconds. Figure 1 illustrates the increases in conversion with the C/O ratio for all blends. Conversion is defined as the portion of the feed converted to gas, liquid product with a boiling point below 221 °C, and coke. At a given C/O ratio, the conversion of a blend increased with its improved quality, with RZ HGO being the highest. However, two observations were somewhat unexpected: (1) at a low concentration of bitumen, the increase in conversion was rather minor,
and (2) between the two blend series, the one with A-bitumen had a higher conversion than its counterpart at the same bitumen addition. These phenomena were due to the synergy between feeds and the conflicting effects of the two types of converted productssdry gas plus coke for one and liquefied petroleum gas plus gasoline for the other. This will be discussed in more detail later. Also, the conversion profiles of the blends tended to converge at high C/O ratios. This suggests that the poisoning effect on the catalyst by some deleterious components (e.g., nitrogen compounds) in feeds could be reduced or compensated for by higher C/O ratios (more catalyst per unit weight of feed). The compensation effect was much more pronounced for the two pure bitumens due to their higher nitrogen contents and more refractory natures in terms of having more large aromatic molecules (aromatics with two or more rings). 3.1.2. Dry Gas. Dry gas (H2, H2S, and C1-C2 hydrocarbons) is a low-value product that should be kept to a minimum. Excessive dry gas production may cause limitations in the plant operation in terms of gas compression. Figure 2 shows that yields of dry gas increased with conversion for all blends. Components in the dry gas are secondary products from thermal cracking and catalytic cracking of gasoline and butenes.4 These cracking reactions are mostly nonselective, resulting in yield profiles parallel to one another. Being an end product, similar to coke, dry gas should exhibit an exponential increase in yield at higher conversions. At a given conversion, dry gas increased with bitumen addition. At the same bitumen level, the feed containing A-bitumen gave a higher dry gas yield than its counterpart, except at low addition levels. This was mainly due to differences in both CCR and sulfur in the feeds. In FCC, CCR might block the catalyst pores, promoting nonselective cracking to produce dry gas, whereas about 40% feed sulfur might contribute to dry gas as H2S (i.e., 1 wt % feed sulfur might produce 0.43 wt % H2S).5 On the basis of this assumption, A-bitumen with 5.12 wt % S and higher CCR would produce more dry gas than P-btumen with 4.24 wt % S by at least (5.12 - 4.24) × 0.43 ) 0.38 wt %, compared with an actual difference of ∼0.5 wt % observed in Figure 2. Careful examination of dry gas components revealed that the H2S yield decreased with conversion. This implies that, at a higher conversion, either H2S reacted with olefins to form mercaptans
(2) Ng, S. H.; Zhu, Y.; Humphries, A.; Nakajima, N.; Tsai, T. Y. R.; Ding, F.; Ling, H.; Yui, S. Fuel Process. Technol. 2006, 87, 475-485. (3) Ng, S. H.; Zhu, Y.; Humphries, A.; Zheng, L.; Ding, F.; Yang, L.; Yui, S. Energy Fuels 2002, 16 (5), 1209-1221.
(4) John, T. M.; Wojciechowski, B. W. J. Catal. 1975, 37, 240-250. (5) Keyworth, D. A.; Reid, T. A.; Asim, M. Y.; Gilman, R. H. Presented at the 1992 National Petroleum Refiners Association (NPRA) Annual Meeting, New Orleans, LA, March 22-24, 1992; Paper No. AM-92-17.
in situ combustion through the use of a CO2 absorber. Experimental details can be found elsewhere.2,3
3. Results and Discussion
1434 Energy & Fuels, Vol. 21, No. 3, 2007
Ng et al.
Figure 1. Relationship between conversion and C/O ratio.
Figure 2. Correlation of dry gas yield with conversion.
Figure 3. Reaction pathway involving sulfur species in gasoline and light diesel fractions.
or thiophenes, or the precursors of H2S (e.g., alkyl-tetrahydrothiophenes from thiophene and/or alkyl-thiophenes) were reduced in concentration (possibly through coke formation) as shown in the reaction pathway (Figure 3) reported by Corma et al.6 3.1.3. Liquefied Petroleum Gas (LPG). In FCC operation, LPG (C3 + C4 hydrocarbons) is considered a valuable product (6) Corma, A.; Martinez, C.; Ketley, G.; Blair, G. Appl. Catal., A: Gen. 2001, 208, 135-152.
since it consists of components that can be used as alkylation and petrochemical feedstocks. In this study, similar to the dry gas, LPG also increased with conversion (Figure 4) in a slightly concave shape. This indicates that as conversion increased the combined cracking rate of feed and gasoline to form LPG was marginally higher than its decomposing rate to form dry gas and coke. The yield curves were spread wider at high conversion due to overcracking for some lighter feeds (with no or low bitumen addition), as will be discussed later. At a given
FCC Quality ImproVement of Bitumen
Energy & Fuels, Vol. 21, No. 3, 2007 1435
Figure 4. LPG yield versus conversion.
Figure 5. Correlation of gasoline yield with conversion.
conversion, LPG yield increased as the feed quality improved. Between a pair of feeds with the same bitumen addition, the one containing P-bitumen gave higher LPG than its counterpart (except at low bitumen concentration) due to better feed quality. 3.1.4. Gasoline. Gasoline (C5-221 °C boiling point) is the major and most desirable product in FCC operation. Figure 5 shows the general increase in gasoline yield with conversion prior to overcracking for some lighter feeds at ∼74 wt % conversion. At the same conversion, gasoline yield decreased with the deteriorated feed quality that resulted from bitumen addition. This was particularly pronounced at high levels of bitumen addition. For example, a significant drop in gasoline yield by ∼4.2 wt % at 74 wt % conversion was noticed when A- or P-bitumen was added to RZ from 50 to 75 wt %. This was caused by both a gradual depletion of the feed gasolineprecursors (saturates and monoaromatics), crackable at FCC conditions to produce gasoline, and serious catalyst poisoning by feed nitrogen and CCR, which can neutralize the acid sites and block the pores of the catalyst, respectively. Beyond 50 wt
% bitumen addition, a blend with P-bitumen gave 1.5-2.0 wt % higher gasoline than its counterpart. At 25 wt % bitumen addition, which is usually considered an acceptable level by refiners, the blend produced about 47 wt % gasoline. 3.1.5. Coke. In FCC operation, coke is necessary to supply heat for feed preheating and cracking. However, too much coke can seriously poison the catalyst and overload the air blower during catalyst regeneration, causing excessively high temperature in the regenerator. Figure 6a shows the exponential increase in coke yield with conversion. The yield profiles appear to be similar to those of dry gas (Figure 2) except the gap between a pair of coke yield curves of blends at the same bitumen level is wider. The similarity between the two figures will be closer if dry gas and coke yields are corrected for H2S and CCR, respectively, as was reported by Ng et al.7 At a given conversion, coke yield increased with bitumen addition, reaching a mild level ∼6.9 wt % at 74 wt % conversion, for a blend (7) Ng, S. H.; Wang, J.; Fairbridge, C.; Zhu, Y. X.; Zhu, Y. J.; Yang, L.; Ding, F.; Yui, S. Energy Fuels 2004, 18 (1), 172-187.
1436 Energy & Fuels, Vol. 21, No. 3, 2007
Ng et al.
Figure 6. (a) Coke yield versus conversion. (b) Relationship between coke yield and catalyst/oil ratio.
containing 25 wt % bitumen. Between the two blends at the same bitumen addition, the one containing A-bitumen gave 0.52.0 wt % higher coke yield at 74 wt % conversion, depending on the bitumen concentration, than its counterpart. Coke yield can be represented in an alternative format that gives a linear correlation with the C/O ratio (Figure 6b). The linear relationship can be seen below:
coke (wt %) ) constant × C/O ratio + constant
derives from catalytic dehydrogenation of metal poisons such as Ni, V, Fe, and Cu; A and n are empirical constants which are catalyst-and-feedstock dependent in Voorhie’s equation;10 and t is the catalyst contact time, which is 30 s in this study. When C/O ) 0, at which no catalytic coke is formed, the y-intercept of a straight line represents the feed coke (assuming the contaminant coke is negligible since the metals content is low in this study), which should be close to CCR in the feed. This is confirmed in Table 2 with an average absolute difference of 0.50 wt % based on 11 feeds. Some feeds showed greater deviations mainly due to insufficient data points, especially those in the low C/O ratio (
