1018
Ind. Eng. Chem. Res. 1993,32,1018-1023
Effect of Coke on Catalysts in Distillate FCC Unit Performance Sukumar Mandal, Asit K. Das, and Sobhan Ghosh' Research and Development Centre, Indian Oil Corporation Ltd., Sector 13,Faridabad, Haryana 121 007,India
The effect of coke on regenerated catalyst (CRC) in a fluid catalytic cracking (FCC) unit performance has been studied in a continuous riser pilot plant a t low-conversion distillate mode operation. The objective was to establish the quantitative effects of CRC on the catalyst activity and selectivity. In the distillate FCC operation, the maximization of middle distillate is of primary concern. Due emphasis was therefore given to investigate the effect of CRC on the distillate yield. It was found that there lies an optimum coke level around 0.2-0.3 wt % on catalyst at which the distillate yield is maximum. The existence of such an optimum CRC level is established from continuous circulating pilot plant data. Controlling the CRC level on catalyst for maximizing middle-distillate yield will open up another direction for improving the plant performance.
Introduction Distillate mode FCC units normally operate at low severity for maximization of the middle-distillate yield. These units are designed with partial combustion regenerators and operate at lower temperature. Consequently, the coke of the spent catalyst does not burn completely in the regenerator. The effect of this residual coke on FCC performance has been a major subject of research in the past. The previous works (e.g., Ritter, 1975;Venuto and Habib, 1979) indicated that the zeolite catalyst performance is more sensitive to residual coke as compared to the amorphous catalyst. Therefore, it is desirable to burn the total residual coke of the catalyst for better utilization of the zeolite potential. This has been possible by operating the regenerator at higher temperature (>700 "C) in total combustion operation. However, most of these units are intended to maximize the gasoline yield. In distillate mode operation, the unit conditions are widely different from those in gasoline mode. The objective of the present study is to establish the quantitative effect of CRC, particularly at low-severityoperation, and explore the possibility of the existence of an optimum region of coke on catalyst.
Background The effect of CRC on FCC performance has been studied by many authors. In general, it was found that, for zeolite catalysts, there is significant activity and selectivity loss as compared to the amorphouscatalyst (Venuto and Habib, 1979). Ritter (1975,1988)studied the residual coke on zeolite catalyst and concluded that the active sites of the zeolite were first attacked by coke precursor resulting in reduced conversion and gasoline yield and increase in gas and coke yield. He also observed that increase in CRC produced gasoline with a higher octane number. Venuto and Habib (1979)also reported a similarobservation. Ritter and Creighton (1984)and Ritter (1988)reported that light cycle oil (LCO; 216-370 "C) could be increased with increasing CRC level,but no quantitative effect was shown. From the figures given by Ritter (1975))it appears that some of the selectivity curves intersect around 50% conversion level for low and high CRC. For example, the selectivity plots of LPG intersect each other at 45% conversion for the low and high coke catalyst. Thus, from the existing literature, the following could be concluded: (i) The effect of CRC was studied mainly in the gasoline mode, i.e., at high reaction severity, where maximization of gasoline is of prime interest.
(ii) The selectivity plots at different CRC levels are not linearly extrapolable from high to low conversion. (iii)The effect of residual coke on LCO was not properly established. For low-conversiondistillate mode operation, sufficient information is not available by which the effect of CRC on FCC performance can be predicted. This paper attempts to highlight the effect of CRC for low-severity operation.
Experimental Methodology Since the objective of the present work is to collect data at different CRC levels mainly at low-conversionoperation, the reactor conditions were properly tuned to simulate a typical distillate mode commercial FCC unit. The present study was conducted in a FCC pilot plant equipped with a shortcontact time riser reactor. The plant was originally designed by ARC0 Inc. and subsequently modified to simulate the commercial distillate cracker. The operating conditions of the pilot plant are listed in Table I. The feed used in this work is a mixture of paraffinic Bombay High and Imported HVGO,the characteristics of which are given in Table 11. The feed includes heavy cycle oil (HCO; 37&430 "C) along with the raw oil in the same proportion as that of commercial cracker. The catalyst was collected from the plant, and the characteristics of this equilibrium catalyst are summarized in Table 111. In order to generate data at different CRC levels, the regenerator air rate was varied in the pilot plant, keeping other conditions approximately at the base level. To establish the selectivity effect, it is required to generate data at equal conversion and at different catalyst coke levels. This was done by taking runs at various catalyst/ oil (catloil) ratios. In all these runs, the reactor temperature and the feed rate (thus residence time) were maintained constant.
Results and Discussion Following the methodology as mentioned in the previous section, conversion and product yield data were generated at different CRC levels (between 0.00and 0.70 wt % ) and catloil ratios (4-8on totalfeed). Since, for distillate mode FCC operation, hydrocarbons in the range of light cycle oil (216-370 "C) are of interest, conversion has been redefined as material boiling below 370 "C. However, for the sake of comparison, 216 "C conversion is also reported. The effects of CRC on activity and product yields from
0888-5885/93/2632-1018$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1019 Table I. Pilot Plant Conditions
CONVERSION(WT%I
reactor temp, OC regenerator temp, OC feed residence time, s reactor press. diluent stripping stream cat/oil ratio feed rate, glmin
75 I
490 660 3 atmospheric nitrogen nitrogen 4-8 13.0
65 60
Table 11. Feed Characteristics feed type feed source combined feedlfresh feed feed density, g/cm3 Ramsbottom carbon residue, wt %
raw oikHCOCL0 (80.5618.93:0.51) Bombay High & Imported (6040) 1.30 0.8906 0.38
TBP Distillation of FCC Feed temp, OC 262 333 356 370 383 403 418
wt%
IBP 5 10 15 20 30 40
temp, OC 433 445 460 477 500 519 561
wt%
50
60 70 80 90 95 FBP
&os,
wt % Sios, w t % NazO, wt % rare earth, wt % (Renos) Fe, w t %
surface area, m2/g % crystallinity pore volume, cm3/g size,@m wt%
APS,fim
v, PPm
Ni, P P ~
840 570
Metal Level Fe, w t % Na, w t %
40 3
20 0
45 40
-
35
\ 0.2
0.6
0.4
0.8
CRC(WT%) Figure 1. CRC level vs conversion at constant cat/oil. At constant severity, 370 "C conversion is constant up to about 0.3 wt % CRC and then decreases but 216 OC conversion drops continuously. (*) 370OC conversion;(0) 216 OC conversion. Distillate mode operation.
zeolite-based FCC catalyst 29.80 67.60 0.31 1.60 0.69 80.00 9.70 0.22
Particle Size Distribution 120 104 80 60 85 72 45 10 85
t
0
Table 111. Equilibrium Catalyst Properties catalyst
55
CUNVERSION(WT%)
80 I 75 70 1 0
0.81 0.22
the low-severity experimental data are summarized in subsequent sections. Catalyst Activity. The change of 370 and 216 "C conversion with CRC level is plotted in Figure 1. It can be seen that 216 "C conversion continuously drops from 50 to 40 wt % while CRC level changes from 0.03 to 0.67 wt %. On the other hand, 370 "C conversion remains constant in the range 0.03-0.30wt % CRC and then drops sharply beyond 0.5 wt % CRC. Thus, at low CRC level, its effect on 216 and 370 "C conversions is somewhat different from that given with high-conversion data. This means that the product yield in the boiling range of 216370 OC, Le., LCO, does not follow a linear trend with CRC level. In the subsequent plots, this selectivity difference in LCO is clearly demonstrated. In literature, the catalyst activity is reported to drop with CRC level (Ritter, 1975;Wallendorf, 1979). However, it is quite interesting to note from Figure 1 that there is no net gain in 370"C conversion when CRC level is reduced beyond 0.30 wt %. This implies that for distillate operation there lies an optimum CRC level beyond which there is no gain in actual 370 "C conversion by reducing the CRC level further. In Figure 2,the effects of catalystloil ratio on 216 OC and 370"C conversions at different CRC levels are plotted. This plot shows the catloil sensitivity of these two
65 '
6o
t
55 50
-
.4
4.5
5
55
6
6.5
CAT/OIL Figure 2. &/oil vs conversion at different CRC levels. At constant CRC, 370 OC conversion and 216 OC conversion increase with cat/oil. At low CRC, conversion is more sensitive to cat/oil. 370OC conversion at (0) 0.03 wt % CRC, ( 0 ) 0.3 w t % CRC; 216 OC conversion at (*) 0.03 w t % CRC, (X) 0.3 w t % CRC. Distillate mode operation.
conversions a t various CRC levels. Whereas the effect on 216 "C conversion of cat/oil matches quite closely the earlier reported values (Ritter, 19751,the effect on 370 "C conversion appears to be less pronounced. Since 216 "C conversion excludes the LCO-range material in the definition, any LCO overcracking results in more 216 "C conversion for constant 370 "C conversion, whereas for 370 OC conversion,the extent of LCO cracking is not reflected. In a practical sense, this means that real
1020 Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 CN YIELO(WT%)
10
Sji 32
I
28
0.3
8
__ 65
70
7s
ao
6 65
370 "C conversion is less sensitive to cat/oil ratio as compared to the 216 "C conversion (normally reported). Light Cycle Oil Yield. The LCO (216-370 "C) yield vs 370 "C conversion at different CRC levels is plotted in Figure 3. It can be seen that clean catalyst produces minimum LCO, and when the CRC level is increased, the LCO yield is significantly improved. Thus when the CRC level is increased from 0.03 to 0.5 wt % ,there is a net gain in maximum LCO yield by about 4.1 wt % a t same 370 "C conversion level. The above observation is in line with the findings by Ritter and Creighton (1984) and Ritter (1988). Heavy Naphtha (HCN; 150-216 "C)Yield. In Figure 4, the yield for heavy naphtha (HCN) at different CRC levels is ploted. The overcracking of HCN is clear with reduction in CRC level. Consequently, when the CRC level is reduced from 0.50 to 0.03 wt % , there is loss in HCN yield by about 1.0 wt %. Total Cycle Oil (TCO;150-370 "C)Yield. Total cycle oil (TCO) is the middle distillate which includes HCN and LCO. The TCO selectivity is shown in Figure 5. It is seen from this plot that CRC has a significant role in enhancing TCO yield. Thus for a clean catalyst (0.03 wt % CRC), there is no optimum in the TCO curve at all. It sharply drops as 370 "C conversion increases, whereas in the case of higher CRC level,the TCO yield passes through a maximum. There is also a net gain of about 6.0 wt % in TCO yield when the CRC level is increased from 0.03 to 0.5 wt % . The above observation implies significant LCO overcracking when CRC is reduced beyond a certain level. Also it is important to note that the TCO gain between 0.30 and 0.50 wt % CRC is marginal as compared to the gain at 0.03-0.3 wt % CRC. This indicates that the TCO yield can be optimized by keeping CRC a t certain level (around 0.30 wt 5%). Gasoline (LCN;CS-150 "C)Yield. Gasoline vs 370 "C conversion is plotted at different CRC levels in Figure 6. It is seen that there is again an increase in gasoline yield
7s
80
370 CONVERSlON(WT%)
370 MNVERSlON(WT%I Figure 3. LCO selectivity at different residual carbon levels. At 0.03 wt % CRC,LCO yield decreases with conversion, and at 0.3 wt % CRC and above, LCO yield passes through a maxima. As CRC increases, LCO selectivity also increases and it passes through a maximum. But there is no such maximum where CRC is very close to zero. CRC: (+) 0.03, (*) 0.3, and (0) 0.50 wt %. Distillate mode operation.
70
Figure 4. HCN selectivity at different residual carbon levels. HCN yield increases with conversion. HCN selectivity increases with increasing CRC. CRC: (+) 0.03,(*) 0.3, and ( 0 )0.5 wt %, Distillate mode operation. TCO YIELD(WTB)
3 4
-;: /
0.5
0.05
30
+
t
66
70
76
80
370 CONVERSION(WT%) Figure 5. TCO selectivity at different residual carbon levels. At 0.03 wt % CRC,TCO drops monotonically with conversion, but beyond 0.03 wt % CRC,TCO passes througha maximum. CRC: (+) 0.03, (*) 0.3, and (0) 0.50 wt %. Distillate mode operation.
when CRC is reduced. In other words, for a clean catalyst the gasoline yield increases. This is in line with the reported data (Ritter, 1975;Venuto and Habib, 1979).The increase in gasoline yield is attributed to the significant overcracking of TCO range material as shown in earlier plots. Liquefied Petroleum Gas (LPG) Yield. LPG vs 370 "C conversion is plotted in Figure 7 for various CRC levels. The increase in LPG is significant when CRC is reduced. Thus when LPG and gasoline-range products are of interest, we should make the catalyst as clean as possible.
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1021 YIELDMT%)
3D/mTm
1.2
1.2
1.1
1.G
'
18 65
I
I
70
75
a9 80
0.2
0
a4
370 CDNVERSlON(WT%)
a8
0.6
CRC (WT %I
Figure 6. LCN selectivity at different residual carbon levels. Gasoline yield increases with conversion, but gasoline selectivity decreases with increase in CRC. CRC: (+) 0.03, (*) 0.3, and (0) 0.5 w t %. Distillate mode operation.
Figure 8. TCO/bottom vs CRC at constant &/oil. TCO/bottom passes through a maximum a t about 0.3 wt % CRC. Distillate mode operation.
LFG YIELD(WT%) 19 18
-
17
-
16
-
I
15 14
-
13
-
12
-
11
0
65
70
75
80
370 mNVERSlON(WT%) Figure 7. LPG selectivity at different residual carbon levels. LPG yield increases with conversion, but LPG selectivity increases with 0.5wt % Distillate decrease in CRC. CRC: (+) 0.03, (*) 0.3, and (0) mode operation.
.
Effect of CRC at Constant Reaction Severity From Figures 1 and 5,it is clear that in the lower range of CRC (
