Ind. Eng. Chem. Res. 1989, 28, 542-546
542
PROCESS ENGINEERING AND DESIGN Effects of Recycle on Heavy Oil Cracker Fu-Ming Lee Phillips Research Center, Phillips Petroleum Company, Bartlesville, Oklahoma 74004
In the operation of a heavy oil cracker (HOC), it is possible to recycle various combinations of light cycle oil, heavy cycle oil, and slurry recycle oil with the fresh feed. To develop a more effective recycle operation, bench-scale cracking tests were conducted on HOC fresh feed, cycle oils, and mixtures of cycle oils and fresh feed. T h e conversion for cracking the pure components was found in the following order: fresh feed > slurry recycle oil > heavy cycle oil >> light cycle oil. This order is consistent with the trends in aromatic content and average molecular weight of the feeds. Blending the cycle oils with fresh feed gave a product distribution upon cracking very close to the linear combination of the percentages of products from the individual components. Thus, based on conversion alone, the rating of cycle oils as blending stocks also went in the order slurry recycle oil > heavy cycle oil >> light cycle oil. Finally, blending light cycle oil with fresh feed increases the octane number of the catalytically cracked gasoline. In the operation a heavy oil cracker, the slurry recycle oil (SRO), as well as portions of light cycle oil (LCO) and heavy cycle oil (HCO), can be recycled and cracked along with fresh feed (FF). There is, however, limited information in the open literature on the effects of recycling these various streams. Since the most easily cracked molecules react in their first pass through a catalytic cracker, it would be expected that all recycle streams would be more difficult to crack than fresh feed. However, there are specific refinery and market conditions that can make a particular recycle condition most desirable. For example, Ritter and Creighton (1984) presented an overview of recycling LCO in fluid catalytic cracker operation. Earlier, Ritter (1982) reported that the crackability of LCO was very low, and it would generally not be worthwhile to replace a portion of fresh feed with LCO on a continuous basis. However, for a refinery where there was both excessive amounts of LCO available and the throughput to the fluidized catalytic cracker could be increased, it may be beneficial to recycle LCO to produce more gasoline and C3-C4 unsaturates. Pohlenz (1970) noted that, by use of a combination of recycle and low activity catalyst, a LCO yield of 60% (based on FF) could be achieved. Also, Nace et al. (1971) reported that addition of aromates to n-hexadecane retarded its cracking rate. Recently, Ritter (1988) pointed out that, in a recycle operation, LCO yield will be maximized by minimizing the first pass severity for FF and by providing additional cracking (increased severity) to that portion of FF that is not easily converted (uncracked recycle). There is obviously some difficulty in relating bench-scale results to plant operation. This study was designed to answer two very specific questions about recycle operation: (1) the effect of recycling LCO, and (2) the difference between HCO and SRO as recycle feeds. In particular, the performance of FF, LCO, HCO, and SRO when cracked over an equilibrium catalyst is correlated with the oil feed properties. The cracking results of the blended feeds were then compared with those calculated from the individual 0888-5885/89/2628-0542$01.50/0
Table I. Physical Properties of Equilibrium GX-30 Catalyst from a Heavy Oil Cracker surface area, m2/g 92.5 pore vol, cm3/g 0.36 Ni, wt % 0.17 v, wt % 0.29 Sb, wt % 0.06
components to determine the effects of blending recycle streams with fresh feed.
Experimental Section Equipment and Procedure. Cracking experiments were carried out in a microconfined fluid bed (MCBU) reactor system, shown in Figure 1. About 35-40 g of catalyst was charged to this quartz reactor and was fluidized with water-saturated nitrogen during the cracking and stripping cycles. The use of nitrogen as a fluidizing medium may result in low partial pressures of the hydrocarbons. Air was used as the fluidizing gas during the regeneration cycle. The oil feed was injected about 1 in. above the catalyst bed through a syringe pump over a 30-s time period. The oil feed tube can be raised or lowered by an air-operated top piston shown in Figure 1. Cracked products were collected in an ice trap followed by a gas receiver at room temperature. Liquid and gas products were analyzed by gas-liquid chromatography. The gasoline end point was set at 430 OF. Coke was determined by weighing the reactor plus catalyst before and after regeneration. Cracking temperature was 950 OF,and the catalyst regeneration temperature was 1300 O F . For the runs to be acceptable, the material balance had to be 100% f 5%, and all experimental results reported were normalized to 100% material balances. When only fresh feed was used, the conversion was calculated based on the total oil feed. For cases where cycle oil was mixed with fresh feed, two conversions were calculated; one was based on the total oil feed, while the other was based on the fresh feed only. Catalyst and Oil Feed Properties. The catalyst used in this study was equilibrium Davison GX-30 samples from 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 543 Table 11. Physical Properties of Fresh Feed and Cycle Oils from a Heavy Oil Cracker wopertv FFl" FF2" LCO HCO 325 194 520 MW 12.9 17.2 API 20.2 0.32 4.59 0.45 4.20 carbon residue, w t 90 89.0 89.9 86.9 c, w t 90 10.4 9.7 12.3 H, wt % 0.19 0.06 N, wt '70 0.20 0.20 0.10 0.14 0.16 0 , wt '70 0.46 0.44 0.29 s, w t '70 6.9 Ni, ppm 13.0 v, PPm 42.8 48.7 saturates, wt 90 49.6 50.9 38.7 36.6 aromatics, wt % 11.4 7.3 13.9 resins, wt '70 0.3 3.3 3.5 asphaltenes, wt % 19.9 saturates, wt % 80.1 aromatics, wt 90 21.7 one ring two rings 50.6 7.0 three rings
" FF1 and FF2 are fresh feeds sampled on different dates from the same unit. from the same unit.
SROlb 438 12.0 4.93 88.5 10.5 0.16 0.24 0.48 2.1 2.6 36.9 48.1 13.5 1.3
SR02b
5.60
0.19
34.4 46.6 13.6 6.2
SROl and SR02 are slurry oils sampled on different dates
P I S T O N FOR FEED I N J E C T I O N TUBE
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a commercial HOC. The analysis of the catalyst is presented in Table I. The coke on regenerated catalyst was completely burned off before testing. Most of the oil feeds used in this study were also sampled from the same commercial HOC. The physical properties of FF, LCO, HCO, and SRO analyzed were gravity, elemental composition, and hydrocarbon type. These analyses are tabulated in Table 11. The simulated distillation curves for these feeds are presented in Figure 2.
Results and Discussion This section includes the bench-scale results for cracking pure FF, LCO, HCO, and SRO at a constant catalyst-to-oil
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Figure 2. Simulated distillation curves for the fresh feed and cycle oils.
ratio. These results are then related to the physical properties of the feed. The effects of combining LCO, HCO, or SRO with the fresh feed are then discussed. Finally, the results are presented, which indicate the effects of recycling on the octane number of the catalytically cracked gasoline. Crackability of Individual Oil Feeds. The results of cracking the six individual oil feeds over GX-30 at 950 O F with a catalyst-to-oil ratio of about 7 are summarized in Table 111. Not surprisingly, fresh feed showed the highest conversion (86 vol 90)and gasoline yield (69 vol YO) among the oil feeds. Light cycle oil showed the lowest crackability. As shown in Table 111, the cracking conversion and gasoline yield of the LCO were 44 vol % and 24.7 vol 70,respectively. These results are similar to the values reported by Ritter (19821, which showed that, from a MAT test, about 34 vol 70 LCO was converted to gasoline and lighter (the gasoline yield was 17.4 vol %). Table I11 also reveals that the conversion and gasoline yield of SRO were much higher than those of HCO. In relating the cracking results with the feed properties, we first examined the simulated distillation curve (SDC) of the oil feeds (Figure 2) and found that the easier cracked oils (FF and SRO) have similar SDC's. This is probably an indication that SRO contains a portion of FF which has never been converted during the previous cracking cycles. Indeed, not only does noncracked FF yield SRO but also hydrogen-transfer occurs, as is clear from the hydrogen
544
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989
Table 111. Cracking Fresh Feed and Cycle Oils from a Heavy Oil Cracker" conv, vol % gasoline, vol % LCO, vol % HCO, vol % oil feed catalyst/oil 86.0 69.0 9.5 4.5 FF1 6.8 67.5 9.5 4.9 FF2 6.8 85.6 6.7 44.0 24.7 47.9 8.2 LCO 34.7 12.4 28.6 HCO 6.9 59.2 18.2 43.3 11.0 SROl 6.9 70.8 41.2 11.7 21.2 SR02 7.0 67.2 57.5 15.5 9.2 FF1 4.8 75.3 21.3 58.6 7.0 LCO 4.8 34.4 a
Cracking temperature = 950
OF;
regeneration temperature = 1300 O
It gases, wt % 16.2 16.6 8.0 10.1 12.2 11.6 14.9 6.0
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Figure 3. Effect of hydrogen-to-carbon ratio on cracking product distribution.
contents. This reaction transfers hydrogen from naphthenes to olefins and produces aromatics and paraffins. The least crackable oil (LCO) has already gone through some severe cracking cycles and contains mainly mono- and multiring aromatics, as shown in Table 11. Figure 3 indicates that the cracking conversion, gasoline selectivity, and light gas yields were proportional to the ratio of hydrogen to carbon (H/C) of the feeds. The net coke selectivity (net coke = total coke - carbon residue in feed) was inversely proportional to H/C of the feeds. These results are not surprising, because the H/C of cycle oils is an indicator for the concentration of polyaromatics which are to be precursers for the coke. This fact is also mentioned by Nace (1970). Similar relationships were found when the cracking conversion and product selectivity were plotted against the ratio of (saturates f resins)/ aromatics of the feeds (Figure 4). This, of course, agrees
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with the fact that hydrotreating residual oil not only removes metals, sulfur, and nitrogen from the oil feed but greatly enhances its crackability and selectivity. From a theoretical view, it seems likely that, better than the conversion, some rate constant will also show a relation with feed properties. So Nace et al. (1971) applied a Langmuir-type relation on rate constants of the cracking of n-hexadecane. Size exclusion chromatography (SEC) was used to analyze the size of the oil feed molecules. The SEC results are plotted in Figure 5. Fresh feed had the largest average molecule, followed by SRO, HCO, and LCO. The size exclusion chromatograms of the oil feeds, along with the MCBU tests, showed that the larger molecules are more easily cracked, giving higher conversion and selectivity (FF > SRO > HCO > LCO). The size of the molecules is roughly related to the average molecular weight (AMW), although some variations may occur when comparing pa-
Ind. Eng. Chem. Res., Vol. 28, No. 5 , 1989 545 Table IV. Cracking the Mixtures of Fresh Feed and Cycle Oils from a Heavy Oil Cracker"' catalyst / conv, conv,d gasoline, LCO, oil %FF1 %FF2 %LCO %HCO %SRO1 %SR02 vol 70 vol % vol % vol % 6.8 80 0 20 0 0 0 78.5 98.1 61.0 16.9 (77.6)' (97.0) (60.1) (17.2) 3.9 80 0 0 20 0 0 71.9 89.9 59.1 15.7 4.2 80 0 0 0 20 0 74.7 93.4 60.5 14.3 7.6 0 80 0 0 0 20 80.8 100.9 58.8 10.9 (81.9) (102.5) (9.9) (62.2)
HCO, It gas, coke, voi % 4.6 (5.2) 12.4 11.0 8.4 (8.2)
wt %
wt %
15.0 (14.6) 12.3 12.6 15.5 (15.6)
10.5 (11.8) 9.0
10.8 13.8 (13.6)
Hz,scf/ bbl conv 264 (303) 219 217 363 355
"Cracking temperature = 950 O F ; regeneration temperature = 1300 "F; catalyst, equilibrium GX-30. *The cracking results were the average of several acceptable runs. The cracking results were normalized for material balance = 100%. Conversion based on fresh feed only: [ (FF - net cycle oil yield)/FF]lOO. eThe values in parentheses are the yields calculated from results for individual components shown in Table 111.
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raffins and aromatics. Since FF and the cycle oils consist of mixed molecules, the variations are minimized. We plotted the cracking yields against AMW of the oil feeds determined by freezing point (Figure 6). Again, the cracking conversion, gasoline selectivity, and light gas yield were proportional to AMW, while the net coke selectivity was inversely proportional to AMW. However, the relationship between the molecular size (or AMW) and crackability is complicated by the fact that the cycle oils have already been chemically converted through previous cracking cycles. Effect of Mixing Oil Feeds. One of the main objectives of this study was to determine the effect of combining LCO, HCO, or SRO with FF. Another objective was to determine whether experimental yields from cracking mixed feeds could be calculated from cracking results of individual components. In order to detect an observable difference in the experimental lab runs, it was necessary to mix 20% cycle oils with FF (higher than actual commercial operation) to overcome the uncertainties of the lab data. However, it is reasonable to assume that relative crackability will be the same at the lower concentration of cycle oils in the feed. The mixed feed results are presented in Table IV. The mixture of LCO and FF gave yields very close to that obtained from weighted linear combinations of the yields of the individual components. Also shown in Table IV, the cracking yields from a mixture of 80% FF2 and 20% SR02, except for lower gasoline yield, agree fairly well with the calculated values (FF1 and FF2 are fresh feeds and SROl and SR02 are slurry oils sampled on different dates, respectively). From these limited number of experiments, it can be concluded that mixing cycle oils with fresh feed gives yields close to or slightly worse than those calculated from individual components. Therefore, it is not surprising that the conversion of mixed feeds goes in
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Figure 6. Effect of average molecular weight of the feeds on cracking product distribution. Table V. Octane Numbers of the Cracked Gasoline Produced from the Fresh Feed and Cycle Oils octane nosa oil feed I I1 FF 93.6 93.6 LCO 95.0 SRO 94.4 94.4 80% FF/20% LCO 94.0 94.1
'I and I1 represent duplicate determinations.
the order SRO > HCO > LCO. Effect of Recycles on Gasoline Octane Ratings. The analysis of oil feeds (Table 11)showed that LCO, HCO, and SRO had much higher aromatic contents than FF. Although the multiring aromatics are more difficult to crack, the catalytically cracked gasoline produced from these feeds should have a higher octane rating. The order of the feeds tested for the highest gasoline octane rating is LCO > SRO > 80% FF + 20% LCO > FF. The test results are presented in Table V. This octane rating was determined by a gas chromatographic method originally developed by
546
I n d . E n g . Chem. Res. 1989, 28, 546-552
Anderson et al. (1972) and modified by Shiblom et al. (1983) for catalytically cracked products. The reproducibility of the octane number for each gasoline was fO.l. The method requires prefractionation of the total sample (containing gasoline, light cycle oil, and heavier materials) in order to remove the heavier-than-gasoline materials by back-flush from the GC column. The gasoline fraction is then separated by boiling point on a nonpolar column and the peak data divided into 32 groups of roughly similar components. Analysis of a large number of catalytically cracked gasoline samples with known research octane values allowed the use of multiple linear regression techniques to determine relative octane contributions of each group of components. Analysis of an unknown sample is then carried out by determining the relative contribution of each group and summing the product. It should be noted that the method is only applicable to samples similar to those for which the regression coefficients were determined, i.e., catalytically cracked gasolines, and cannot be valid for blended gasolines or those derived from coal or shale oil. The gasoline octane produced from the mixture of LCO and FF is close to a linear combination of the octanes obtained from the individual feeds. This result indicates that removing the recycle oil, especially LCO from the HOC feed stream, may decrease the octane rating of the cracked gasoline in the refinery. However, the octane effect may not be observable in a commercial unit since the inclusion of LCO in a riser reduces the overall cracking severity.
Conclusions It is obviously difficult to generate data from bench-scale tests that can be directly translated to predict the performance of a commercial catalytic cracker. Nevertheless, it is believed that the information presented in this article can help refiners choose between various recycle operations. Listed below are the major conclusions from this study: 1. Fresh feed had the best crackability and gasoline selectivity of the feeds tested. The conversion for cracking single feeds was in the order FF > SRO > HCO >> LCO.
2. The crackability of the single feeds is consistent with their aromaticities and average molecular weights. The more closely the cycle oils resembled fresh feed, the more easily they were cracked. 3. Mixing cycle oils with fresh feed gave yields close to or slightly worse than those calculated from weighted linear combinations of the yields from the cycle oils and the fresh feed. 4. Combining LCO with FF increased the octane number of the catalytically cracked gasoline.
Acknowledgment The author thanks Phillips Petroleum Company for the opportunity to perform this work and for permission to publish the results. Registry No. C, 7440-44-0.
Literature Cited Anderson, P. C.; Sharkey, J. M.; Walsh, R. P. Calculation of the Research Octane Number of Motor Gasolines from Gas Chromatographic Data and a New Approach to Motor Gasoline quality Control. J . Inst. Pet. 1972, 58(560), 83. Nace, D. M. Catalytic Cracking over Crystalline Aluminosilicates: Microreactor Study of Gas Oil Cracking. Znd. Eng. Chem. Prod. Res. Deu. 1970, 9(2), 203. Nace, D. M.; Voltz, S. E.; Weekman, V. W., Jr. Application of a Kinetic Model for Catalytic Cracking: Effects of Charge Stocks. Znd. Eng. Chem. Process Des. Dev. 1971, 10(4), 530. Pohlenz, J. B. New Development Boosts Production of Middle Distillate from FCC. Oil Gas J. 1970, 68(32), 158. Ritter, R. E. Is it Worthwhile to Recycle Light Cycle Oil on My Fluid Cat Cracker? Davison Catalagram 1982, 64, 5. Ritter, R. E. Light Cycle Oil from the FCC Unit. Presented a t the NPRA Annual Meeting, San Antonio, TX 1988; paper AM-88-57. Ritter, R. E.; Creighton, J. E. Cat Cracker LCO Yield Can Be Increased. Oil Gas J . 1984 (May 281, 71. Shiblom, C. M.; Fu, C. M.; White, B. J. Determination of Octane Number of Catalytically Cracked Liquid Product. Unpublished R&D Report 9551-83, 1983; Phillips Petroleum Co., Bartlesville, OK.
Received for review July 18, 1988 Revised manuscript received December 16, 1988 Accepted January 19, 1989
Approximate Dynamic Models for Chemical Process Systems Antonis Papadourakis,+Michael F. Doherty,* and James M. Douglas* Department of Chemical Engineering, University of Massachusetts, A m h e r s t , Massachusetts 01003
Modern integrated chemical plants can be viewed as consisting of a number of smaller interconnected subsystems. Approximate dynamic models for both individual process units and subsystems are the necessary tools for preliminary dynamic studies of plantwide operability and control. A variable-order method is presented that can be used to develop simple dynamic models for such systems. T h e method is successful in retaining the dominant dynamic modes of the process, and examples are given to demonstrate this point. Most of the previous work on process dynamics and control has focused on the behavior of single processing units. The general philosophy has been that, if each unit is properly controlled, then the control of the complete process will be satisfactory. However, the current trend (Haggin, 1984a-c) is to look for ways of controlling groups of units in order to operate at the most profitable conditions. Present address: Engineering Technology Group, Rohm and Haas Co., P.O. Box 584, Bristol, PA 19007.
0888-5885/89/2628-0546$01.50/0
Since most processes contain both material and energy recycle loops, we must consider the effects of recycle dynamics if we are to obtain a realistic assessment of the dynamics and control of groups of interconnected units. The studies of Gilliland et al. (1964), Attir and Den (19781, Silverstein and Shinnar (1982), Denn and Lavie (1982),and Rinard and Benjamin (1982) indicate that recycle streams introduce positive feedback loops into the flow sheet. The recycle loops make the plant less stable, increase the gain, and make the effective time constant of the process significantly greater than the largest of the time constants Ci 1989 American Chemical Society
