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Results of Processing VGO-LCO Blends in a Fluid Catalytic Cracking Commercial Unit J. Ancheyta*,†,‡ and S. Rodrı´guez† Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Me´ xico 07730 D. F., Mexico, and Instituto Polite´ cnico Nacional, ESIQIE, Me´ xico 07738 D. F., Mexico Received September 5, 2001. Revised Manuscript Received December 13, 2001
We report commercial results about the incorporation of light cycle oil together with vacuum gas oil as FCC (Fluid Catalytic Cracking) feedstock. Four stages were considered in the commercial evaluation: (1) operation with 37000 BPD of VGO (Vacuum Gas Oil), (2) operation with 35000 BPD of VGO, (3) operation with a mixture of 35000 BPD of VGO and 2000 BPD of LCO (Light Cycle Oil), and (4) operation with a mixture of 35000 BPD of VGO and 4000 BPD of LCO. Reductions in conversion, and gasoline, LPG, dry gas, and coke yields were found in those stages using LCO. Similar trends about the increase or decrease in product production were observed in commercial plant compared to MAT (Microactivity Test) unit. The optimum amount of LCO in the feed was defined to be between 5 and 7 vol %, which maximizes the valuable product production.
1. Introduction The composition of the feed to a fluid catalytic cracking unit (FCC) has a very important influence on the resulting yields and products quality. In fact, FCC feed composition is considered more relevant than operating variables or catalyst selection.1 A typical FCC feedstock is usually integrated by mixture of heavy straight-run gas oil, light vacuum gas oil, and heavy vacuum gas oil. In most refineries these streams are commonly mixed in the following amounts: 30-40, 10-20, and 40-50 vol %, respectively. These volumetric proportions depend mainly on the type and quality of the crude oil processed. Sometimes the amount of these streams is limited, which may be due to operational problems of crude oil distillation columns or to changes in crude oil quality. Of course, in those refineries operating with constant crude oil quality this problem almost never is present. However, when this happens, FCC feed flowrate is reduced, and refiners face a conflict, because production of some important products, such as gasoline, is also affected. For this reason, it is common to employ other nonconventional streams to increase FCC feed flowrate. Light straight-run gas oil (LSRGO) is the most used distillate for this purpose, with the consequent decrease in conversion and product yields.2 In fact, worldwide FCC units operate almost always with certain amount of LSRGO. * To whom correspondence should be addressed. FAX (+52-5) 3338429. E-mail:
[email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Instituto Polite ´ cnico Nacional. (1) Letzsch, W. S.; Ashton, A. G. Stud. Surf. Sci. Catal. 1993, 76, 441-498. (2) Engelhard Co. Feedstock Crackability. Presentation at the FCC Technical Seminar, Mexico, 1991.
Light cycle oil (LCO) is another possibility to be considered for increasing FCC feed flowrate. The main features of this stream are3,4 •distillate boiling range product of an FCC unit, •highly unsaturated, mainly di- and tri-aromatics compounds, •lower API gravity and higher sulfur content than LSRGO, •more aromatic and resistant to cracking than fresh FCC feeds. As it is well-known, the non-easy-to-crack nature of the LCO causes a decrease in conversion and in the more valuable products yields, and an increase in coke and dry gas yields. Then, if LCO is not a good stream to be processed in FCC units, why to think in feeding it to this plant? This question could be answered since the point of view of refinery production policies based on demand of products. The common use of LCO as diesel fuel component is being reduced due to new diesel specifications and to its adverse effect when it is incorporated together with LSRGO as hydrotreating feedstock.5 Instead of this, LCO can be cracked in a catalytic cracker with the main objective of increasing gasoline production as a consequence of operation with more FCC feed flowrate when the amount of traditional feedstocks is not enough to achieve normal FCC operation capacity. In a previous work,6 we have reported a microactivity (MAT) experimental study about the effect of LCO in a (3) Sadeghbeigi, R. Fluid Catalytic Cracking Handbook: Design, Operation and Troubleshooting of FCC Facilities; Gulf Publishing Co.: Houston, TX, 1995. (4) Depauw, G. A.; Froment, G. F. J. Chromatogr. A 1997, 761, 231247. (5) Ancheyta, J.; Aguilar, E.; Salazar, D.; Betancourt, G.; Leiva, M. Appl. Catal. A 1999, 180, 195-205. (6) Ancheyta, J.; Rodrı´guez, S.; Valenzuela, M. A. Energy Fuels 2001, 15, 675-696.
10.1021/ef0102263 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002
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Table 1. Stages of Commercial Evaluation days of operation VGO, MBPD vol % LCO, MBPD vol % total, MBPD
Table 2. Properties of Feedstocks
stage 1
stage 2
stage 3
stage 4
6 37 100 0 0 37
4 35 100 0 0 35
7 35 94.6 2 5.4 37
7 35 89.7 4 10.3 39
typical FCC feed. It was verified that a decrease in conversion and gasoline and LPG yields is observed when LCO is added to the conventional FCC feedstock, while dry gas and coke yields exhibited the opposite behavior. It was also found that gasoline RON increases as the LCO in the feed was also increased, which was attributed to the high aromatic content in feed containing LCO. In that study, when MAT results were projected to a commercial FCC plant, the following behavior was found in product production (in ton/h) when LCO was added to the feed: • LPG and dry gas were very similar. • Coke and LCO exhibited an increase. • Decant oil (DO) presented a decrease. • Gasoline increased and then decreased. This projection considered an increase in feed flowrate of which the base VGO was constant (41000 BPD) but that of LCO in the feed increased (0, 1094, 3000, and 6000 BPD for 0, 2.6, 6.8, and 12.8 vol % LCO, respectively), and C/O ratio of 6. The most important finding was the maximum gasoline production observed at about 7 vol % of LCO in the feed. As it was mentioned, this commercial behavior was estimated by projection of MAT experiments. To confirm these results, in this work we present industrial information about processing of vacuum gas oil-light cycle oil blends in a FCC unit. 2. Commercial Test 2.1. Periods of Evaluation. Incorporation of LCO in the traditional FCC feed was carried out in a catalytic cracking commercial unit. Evaluation was divided into four stages with different plant capacities and with or without LCO in the feed. In the initial period of evaluation (stage 1) the FCC unit was operated with 37 MBPD (millebarrels per day). This flowrate was reduced to 35 MBPD (stage 2), and then it was increased again to 37 MBPD but integrating 2 MBPD of LCO (stage 3), and finally 4 MBPD were incorporated to the FCC feed (stage 4) to achieve 39 MBPD (Table 1). 2.2. Materials and Operating Conditions. Properties of vacuum gas oil (VGO), light cycle oil (LCO), and two blends prepared with these two streams, which were used in commercial evaluation, are presented in Table 2. VGO was employed for stages 1 and 2, while B-3 and B-4 feeds were utilized for stages 3 and 4, respectively. Properties of equilibrium catalyst (REUSY-based sample) were reported elsewhere,6 which was the same used in MAT experiments. Operating conditions during commercial test are shown in Table 3. Regenerator temperature in dense phase was kept almost constant during the test (variation of 2 °C), while for the diluted phase the variation in temperature was higher (8 °C), specially when the flowrate was reduced from 37 to 35 MBPD.
vol % of LCO API gravity sulfur, wt % basic nitrogen, wppm carbon distribution, wt % paraffinics naphthenics aromatics
VGO
LCO
B-3
B-4
25.1 2.05 336
18.0 2.91 51
5.4 24.7 2.10 320
10.3 24.3 2.14 306
62.94 12.54 24.52
36.04 10.80 53.16
61.42 12.44 26.14
60.07 12.35 27.58
Table 3. Operating Conditions during Commercial Evaluation stage 1 stage 2 stage 3 stage 4 exit reactor temperature, °C dense phase regenerator temperature, °C diluted phase regenerator temperature, °C feed preheating temperature, °C CCR, ton/min C/O ratio
515 684
518 683
519 682
520 684
705
697
701
701
306
284
302
319
25.4 6.9
26.7 7.6
25.9 7.0
26.2 6.7
Reactor temperature was higher in stages 2, 3, and 4 compared to that reported in stage 1, which was mainly due to the increase in catalyst circulation rate (CCR). Catalyst-to-oil (C/O) ratio was the highest for stage 2 due to reduction in feed flowrate and the increase in CCR because of low feed preheating temperature. Other C/O ratios (stages 1, 3 and 4) were quite similar.
3. Results and Discussion 3.1. MAT Experiments. Before doing the commercial test various experiments were conducted at microactivity scale.6 It was found that as the LCO content in the feed is increased, the total conversion decreased. Figure 1 summarizes these results, which were obtained at reaction temperature of 516 °C, space-velocity (WHSV) of 16 h-1, and catalyst-to-oil ratio of 6. Other MAT results obtained at catalyst-to-oil ratios of 5 and 7 were presented elsewhere.6 It is very clear from this figure that the inclusion of LCO in the FCC feed adversely affects the most valuable product yields, since reductions in gasoline and LPG yields, and increase in dry gas and coke yields were observed. The effect of incorporating LCO in the FCC feedstock was experimentally studied in the MAT unit up to 12.8 vol %. These MAT data were projected to a commercial FCC plant operating with 35 MBPD VGO as feed flowrate with 0, 2.6, 6.8, and 12.8 vol % LCO for C/O ratio of 6. A maximum point of gasoline production (in ton/h) was found at 6.8 vol % of LCO in the feed, which also corresponds to the point where gasoline RON value remains almost unchanged (Figure 2). Table 4 shows the complete projected data from MAT unit to a commercial FCC plant run at C/O ratio of 6 and at a base VGO flowrate of 35 MBPD with addition of 0 and 6.8 vol % LCO in the feed. Some product properties are also presented in this table. Sulfur content, research octane (RON), and motor octane (MON) values were determined by ASTM D-4294, D-2699, and D-2700 methods, respectively. Sulfur contents in gasoline, LCO and DO increased when light cycle oil was used together with VGO as FCC feed. This result is quite obvious because of the higher
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Figure 1. Experimental results in MAT unit. Reaction temperature of 516 °C, WHSV of 16 h-1, and C/O ratio of 6. (O) 0% LCO, (b) 6.8% LCO.
sulfur concentration exhibited by feed containing LCO. Gasoline RON and MON also increase when adding LCO to the FCC feed. Most of the sulfur in LCO is contained in aromatic components,4 which are very difficult to crack. Hence, when LCO is cracked, these aromatic sulfur-containing components remain almost unchanged, and they concentrate in heavier FCC products. That is the reason for having higher increases in sulfur content for both light cycle and decant oils compared to gasoline. 3.2. Commercial Results. Most refineries produce sufficient gas oil to meet the catalytic cracking plants
demand. However, in those refineries in which the gas oil produced does not meet the FCC capacity, it may be convenient to supplement feed by using other nontraditional streams, such as LCO. It is well-known that the nature of LCO is mostly aromatic, and its properties makes it very hard to cracking. This does not mean that LCO cannot be cracked, what it means is that more severe operating conditions than traditional feeds are needed to convert this refractory stream. The inclusion of LCO in FCC feed should be considered only when traditional feed is not enough, and it is
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Energy & Fuels, Vol. 16, No. 3, 2002 721
Figure 2. Estimated gasoline production and octane for a 37 MBPD FCC commercial unit based on MAT results (Gasoline production was calculated by multiplying gasoline mass yield by feed flowrate in ton/h for each amount of LCO in feed. Gasoline RONs were considered to be the same than those reported in MAT experiments). Table 4. Projected Data from MAT to Commercial FCC Unit composition, vol % VGO LCO total feed flowrate, MBPD VGO, MBPD LCO, MBPD dry gas, ton/day LP gas, MBPD gasoline, MBPD RON MON S, wt % LCO, MBPD S, wt % DO, MBPD S, wt % coke, ton/day
feed 1
feed 2
100 0 35 35 0 136.6 10.64 19.46 90.8 80.0 0.19 7.23 3.35 2.34 4.0 156.82
93.2 6.8 37.55 35 2.55 160.6 11.80 20.45 92.2 80.8 0.20 8.81 3.40 2.27 4.5 191.94
required to operate the industrial units very close to their maximum capacity. On the basis of this, we conducted a commercial test with the main purpose of increasing the productions of the most valuable FCC products in those refineries which have problems with vacuum gas oil production. Figure 3 shows the results obtained in the four stages of evaluation for dry gas, LPG, and gasoline volumetric flows. Table 5 presents the average values of products production and the corresponding values of product yields, as well as conversion level for each stage of evaluation. It is seen that the reduction in feed flowrate from 37 to 35 MBPD (stages 1 and 2), without LCO in the feed, increases all product yields, which is mainly due to the
most severe operating conditions employed in stage 2, such as higher reaction temperature and C/O ratio (Table 3). Despite this more severe operation, conversion in stage 2 was about 0.5 vol % lesser than that of stage 1. This may be due to differences in liquid flowrate measuring, since mass balances during all tests were in the range of 100 ( 2%. Frequently, small changes in catalyst and feed properties are not visible during commercial operation. It means that the slight decrease in conversion with increasing severity is probably due to a slight change in feed properties to a more refractory feed and/or a slight shift in catalyst properties to a catalyst with a lower activity. Feed flowrate in stages 1 and 3 was the same (37 MBPD), the only difference is that, for the third stage, 2 MBPD of LCO were employed. Reductions in gasoline, LPG, dry gas and coke yields, and total conversion, with the consequent increase in LCO and DO yields, were observed in these two stages. If we make a differential analysis between stage 2 and stages 3 and 4, we can have an approximate calculation of how much amount of product is converted from LCO. For instance, in the case of gasoline 21.23 and 22.13 MBPD were obtained in stages 2 and 3, which correspond to FCC feed flowrates of 35 MBPD (100% VGO) and 37 MBPD (35 MBPD VGO and 2 MBPD LCO), respectively. This means in principle that the difference in gasoline production in both stages (0.9 MBPD) is the net contribution by the LCO in the feed. These calculations for all liquid FCC products are shown in Table 6.
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Figure 3. Commercial results for dry gas, LPG, and gasoline. Table 5. Product Production during Commercial Test total conversion, vol % dry gas, m3/B Mm3/D LP gas, vol % MBPD gasoline, vol % MBPD LCO, vol % MBPD DO, vol % MBPD coke, wt % ton/D
stage 1
stage 2
stage 3
stage 4
75.05 7.58 282.98 21.03 7.85 60.23 22.49 19.88 7.42 5.08 1.90 4.19 224.8
74.51 7.87 275.45 22.14 7.75 60.66 21.23 20.34 7.12 5.14 1.80 4.60 231.4
72.71 6.93 256.41 20.69 7.66 59.81 22.13 21.59 7.99 5.71 2.11 4.10 218.0
68.45 6.98 272.22 20.40 7.96 56.25 21.94 25.99 10.14 5.56 2.16 3.85 215.8
In the case of MAT data, this differential analysis is more complicated since values given in Table 4 are based on the same C/O ratio of 6. Thus, the base VGO in Feed 2 was not cracked by the same amount of catalysts as in Feed 1. Anyway, in Table 6 we have included these results only for observing trends in product production.
Table 6. Product Production from LCO stage 3 stage 4 MAT data 2000 BPD LCO 4000 BPD LCO 2550 BPD LCO LPG, BPD gasoline, BPD LCO, BPD DO, BPD total LCO conversion, %
-90 900 870 310 1990 56.5
210 710 3020 360 4300 24.5
1160 990 1580 -70 3660 38.0
It should be noted that more LCO was converted in stage 3 compared to stage 4 (56.5 vs 24.5%). These values were determined as (LCO in feed - LCO in products)/LCO in feed X 100. One very important point that needs to be highlighted is that gasoline formation was higher in stage 3 compared to stage 4 (900 vs 710 BPD). In the case of MAT experiments, they were conducted with 6.8 vol % of LCO in the feed, which is higher than that used in stage 3 (5.4 vol %). Despite this slight difference and the different C/O ratio used for feeds 1 and 2, explained above, some conclusions can be made by comparing these data.
Processing VGO-LCO Blends
It should be mentioned that results of stage 3 were obtained with 2000 BPD of LCO in the feed, while 2550 BPD were employed in MAT results. LCO conversion was lesser in MAT unit in 18.5%. The trends observed in MAT unit about the increase or decrease in product production was the same than that of commercial unit, except for LPG and decant oil. Absolute values were different, which is mainly due to differences in operating conditions and reactor mode of operation. In addition, during commercial test various additives were utilized (CO promoter, octane, and SOx additives), which affected conversion and product distribution. Although all these differences in results found in MAT and commercial units, the same conclusion about the optimal amount of LCO in the feed was obtained (5-7 vol %), which is the point where maximum gasoline production was achieved. Finally, we highlight that LCO addition to FCC feed reduces valuable products yields in each case (stages 3 and 4) compared to those cases without LCO addition (stages 1 and 2). However, the main objective of this study was to show that LCO, which is commonly considered as a refractory stream, can be used to complete FCC feed only when traditional feedstocks are not enough. Of course, it is always better to feed conventional streams to FCC plants, but if these traditional FCC feed components are limited and LCO is available the recommended amount of this stream in the feed is 5-7
Energy & Fuels, Vol. 16, No. 3, 2002 723
vol %. This does not mean that LCO addition between 5 and 7% will maximize the formation of valuable products. On the contrary, product yield and total conversion will reduce when incorporating LCO in the FCC feed. What it means is that production of some product (in BPD) will increase because of operation of FCC plant with higher feed flowrate. Conclusions On the basis of commercial results about the effect of incorporating LCO in a typical FCC feed, the following conclusions can be raised: • Conversion decreased as result of increase in light cycle and decant oils yields, and hence gasoline, LPG, dry gas, and coke yields also decreased, when LCO was used in mixture with VGO as FCC feedstock. • Very similar trends in MAT and commercial results about the increase or decrease in product production was observed. • LCO conversion was higher in stage 3, in which light cycle oil concentration was of 5.4 vol %. • The optimum content of LCO in the FCC was found to be around 5-7 vol %, which maximizes gasoline production. This result agrees very well with MAT data. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. EF0102263
