analysis of a single-stage fluidized bed reactor should be useful in a rational design of a continuous multistage reactor of a large capacity.
B I.(
= bed voidage =
p =
viscosity, centipoise, g/cm sec gas density, g/lit, g/cm3
Acknowledgment
Literature Cited
Thanks are due to the Director, University Department of Chemical Technology, Bombay-19, for his keen encouragement during this work.
Athavale, A. S., Altekar, V. A,, Indian J . Technol., 7 (2), 52-54 (1969). Danckwerts, P. V., Jenkins, J. W., Place, G., Chem. Eng. Sci., 3, 26 (1954). Dunn, W. E., Trans. 1Met. SOC. AZME, 218, 6 (1960). Feinman, J., Anner, W. D., AZChE J., 10 ( 5 ) , 652 (1964). Feinman, J., Dexter, T., zbid., 7 (4), 584 (1961). Hougen, 0. A., Watson, K., Znd. Eng. Chem., 53, 529 (1943;I. A I M E , 188, 863 (1950). Kellogg, H. H., Trans. M e t . SOC. Ketteridge, I. B., Brzt. Chem. Eng., 10 ( 7 ) , 452 (1965). Krishnamurthi, RI. S . , CSIR Symposium, on “Fluidization and Related Processes,J’ Kharagpur, India, 1964, p 248. Lakshmanan, C. AI., Hoelscher, 11. E., Chennakesavan, B.1 Chem. Eng. Sci., 20, 1107 (1965). Leva, Max, “Fluidization,” McGraw-Hill, New York, N. y., 19.59, PD 80-98. Levenspid, O., “Chemical Reartion E n_ r i n --.... w.r i~ n_, r ”~ 2nd pd Wiley, . New York. N. ?t‘., 1962, pp 338. LU, w., TriAs. M e t . SOC. A I M E , McKewan, W. M., ibid., 212, 791 I l Y J o j . bleissner, H. P., Schora, 1F. C., ibid., 218. 12 11960) Nordin, A. W.. Brit. Chem. Ena.. 8 (4). 2:i7 i1963).’ bok. W. Or.’ ATPhE’ J . , 12
Nomenclature
a, = do = E‘ = G = k,? = PHCl =
n Q
= =
s =
V
= = zo = XB=
TI’
y
=
Ye I’,
=
=
YT = 0
=
@ =
initial analysis of iron oxide, g mol/g inerts initial density of solids, g/cm3, g mol/cm3 feed rate of solids, g inerts/hr cmZ,kg/hr m2 gas feed velocity, g mol/hr cm2 rate constant’, weight loss of iron oxide/cm2, sec a t m partial pressure of HC1, a t m number of the stage average kinetic and geometric constant, g mol/g inerts hr a,(l - X B ) , g mol of iron oxide/g of inerts bed inventory, g inerts/cm2 weight, grams initial dimensions of t’he particle, em percentage conversion of iron oxide gas efficiency, g mol/g mol mole fraction of the gaseous component a t time 8 mole fraction of the gaseous component a t the bottom of the reactor volume mole fraction of the gaseous component at the top of the reactor volume time, see, hr shape factor, area of same volume of sphere/act’ual area of particle
~
---
Ostberg, G., Jerhontorets Ann., 140 ( l ) , 46 (1960). Overholt, J. L., Vaux, G., Rodda, J. L., Amer. Mineral., 35, 117 (1930). Themelis, N. J., Gauvin, W. H., Trans. Met. Soc. AZME, 227, 290 (1963). !enz. F. A,. Othmer. D. F.. “Fluidization and Fluid Particle
RECEIVED for review September 21, 1970 ACCEPTEDApril 12, 1971
Application of a Kinetic Model for Catalytic Cracking Effects of Charge Stocks Donald M. Nace, Sterling E. Voltz,l and Vern W. Weekman, Jr. J . 08066
Research Department, Illobi1 Research and Development Corp., Paulsboro,
A kinetic model for catalytic cracking has been applied to gas oils of widely different molecular compositions and properties. The model adequately describes conversion and gasoline selectivities over wide ranges of reaction conditions. Charge stocks with high concentrations of paraffins and/or naphthenes have the highest rate constants for gas oil cracking and gasoline formation. Recycle and coker feedstocks have lower rate constants than virgin gas oils. Catalyst deactivation increases with increasing concentration of aromatics in the charge stock. Coke formation i s related to the concentrations of high-molecular-weight aromatics.
T h e cat.alyt’ic cracking of wide boiling gas oils over acidic catalysts produces a broad spectrum of products as well as carbon deposits which cause rapid decay of the catalytic activity. The chemical composition of the gas oil has a profound effect, on the rates of cracking and rat,e of catalyst TGwhom correspondence should be addressed. 530 Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
decay. I n t,his paper, the effects of gas oil composition on the rate constants of important cracking reactions as well as cat,al\-st decay are presented. Most of the previous studies on the kinetics of catalytic cracking were concerned only with the overall conversion of gas oil to products (Aindrews,1959; Blanding, 1953; Voorhies, 1945; Waterinan et al., 1960). The coupling of catalyst
Table I. Properties of Charge Stocks
Charge stock
P1 P2 P3 N1 32 N3 PN33 PA31 PA32 PA33 PA331 PA34 PA37 PA38 *\A45 PC32
ASTM distillation 5% ' F 95%
451 642 467 443 657 457 46 1 483 482 473 477 48 1 460 465 723 467
650 939 901 687 908 877 836 913 882 908 886 911 855 959 892 911
F
Av mol wt
Sulfur,wt%
206 402 276 228 354 27 2 27 1 250 28 1 320 332 295 278 325 366 298
decay kinetics with the conversion kinetics was report'ed by Weekniaii (1968) using a time-depeiideiit decay term. One of the earliest treatnieiits of decaying reactor systems was a t,lieoretical study by Frotnent aiid I3isclioff (1961, 1962). Subwqueiitly, AIasaniune and Sniit'li (1966) and Sagara et, al. (1967) studied tlie cotisequeiices of diffusion on cat,alyst decay. Ozawa and 13ischoff (1968) employed a linear decay law based 011 carlion formation in describing the kinet,ics of crarkiiig 011 hilien-alniniiia catalyst's. 13y regression t~ec4iniques, Kberly et al. (1966) had previously developed a relationship for cc~keformation with respect to catalyst residence time and sl~nc'e velocity. Again, using time-dependent decay t,ernis Weeknian (1969) and Weekman and S a c e (1970) studied the cotisequeiices of decay 011 the selectivity of fixed, moving, anti fluid t,ed reactors for the catalytic cracking of gas oils. Szepe a n d Levenhpiel (1968) ill a general study of catalyst de:icti\-at,ioti showed that, a simple n t h order decay law deof catalyst fouling. Kojcieclioivski (1968), Campbell and Wojciechorvski (1969), aiid X o j ciechoivski et al. (1969) reported a kinet8icmodel with a tinie-depeiident decay rat'e which mas applied t,o catalytic tracking. I n inost, of the earlier ~ ~ o dealing rk ivit'li tlie kinetics of catalytic cracking, 110 attenipts were made t.0 show the effects of charge stock composition. The "n-d-W' method of ideiitifyiiig c w b o i i atoms in l~araffiiiic,iiaphtheiiic, and aromatic d Rrif et' al. (1961) to develop polyst'ructures was u ~ 11). nomial correlatioiis of yields. X o r e rereiitly White (1968a,b) correlated catalytic cracking yields wit'll various classes of parafhns, iiaphthenes, aiid aroniat,ics based on a inass spect'rogral)hic tecliiiique of Fitzgerald et al. (1970). Shiiaider (1969) and Slinaider et' al. (1969) developed a kinetic scheme for catalytic cracking and studied t'he effect's of chemical compositions of charge stocks on crackitig rates in fluidized reactors. hkhmetshina et. al. (1969) studied the effects of and chemical composition of some petropure hydroca~~bons leum fractions 011 coke formation on an aluminosilicate catalyst. ITsiiig smreral techniques to ideiitify paraffins, naphtlieiiex, atid aromatics, the present paper discusses the effect of these classes of rompounds O I L the rate coiistaiits of a kitietic model for rat'alytic cracking. -1series of charge st.ocks with widely differeiit molecular compositions and other properties has been ciiaracterized, and detailed kinetic cracking data have beeti obtained for each charge st,ock.
0.09 0.01 0.10 0.03 0.13 0.10 0.11 1.37 0.23 0.28 0.42 0.80 0.87 1.78 3.77 0.59
Nitrogen, wt %
Hydrogen, wt %
0.01 0.05 0.03 0.001 0.03 0.02 0.02 0.02 0.03 0.06 0.09 0.06 0.06 0.09 0.14 0.09
13.85 12.17 13.48 12.83 12,61 12.65 13.28 13.28 12.67 12.65 11.44 12.60 11.08 12,51 10.87 13.02
Conrodson carbon no.
0.01 0.03 0.02 0.01 0.04 0.04 0.03 0.02 0.09 0.10 0.22 0.15 0.07 0.73 0.19 0.10
Bromine no.
0.8 0.4 0.6 0.0 0.7 0.4 0.4 1.9 1.3 1.3 3.0 1 .o 4.0 ... 4.0 4 2
Experimental Methods
Preparation a n d Characterization of Charge Stocks. A series of 16 catalytic cracking charge st'ocks was prepared which covered wide ranges of properties and molecular compositions. Particular emphasis was placed on boiling poilit ranges and relative concentrations of paraffins, tiapht~lieiies, and aromatics. The desired range of charge stocks was obt:iiiied by bletidiny together selected fract,ioiis of different refinery gas oil rtocks. Some of their properties and niolecular compositioiis are given in Tables I aiid 11. P I and P2 were highly paraffinic fractions with boiling point ranges representative of light and heavy fuel oil fract'ioiis (430-650" F aiid 650" F+, respectively). 1'3 was a misture of equal volumes of PI and P 2 and was equivalent to a highly paraffinic charge stock of wide boiliiig point range. Corresponding charge stocks of highly naphthenic conceiitrations (Le., S I , K2, 1 3 ) were also prepared. 5 3 was composed of equal volumes of ?;I and S 2 . T o cover a range of paraffin/' iiaphthene coiicentratioiis;, P S 3 3 was prepared froni equal volumes of P 3 and X3. These latter three charge stocks represented wide cut gas oils of relatively low aromat,ics coiiceiitratioiis and similar boiling point ranges, but with coiiceiitratiotis of paraffiiis and naplithenes of 10-46 wt Yo and 35-64 wt o/G, respectively. Refinery aromatic extracts were used to 1)rei)are most of the blended charge stocks with high coiiceiitrations of aromat,ics. The properties and niolecular cotiipositioiis of the extracts are given in Table 111. They all contained more than 70 wt 70aromatics as determined by silica gel separation and m a s spect,rometry. Most of the aromatic blended stocks were irepared by co~nbinitigP3 with the extracts listed in Table 111. P.131, PA32, P*133, PX34, I'A37, PA38, and I T 3 2 consisted of mixtures of 30 vol 70of the respective aromatic ext'ract added to P3. PA331 contained 60 vol yo of 1 1 3added to 1'3. ;3.145 was a mixture of equal volumes of A4 and A5. Hydrocarbon type analysis of the charge stocks involved distillation to separate boiling poilit, ranges;, liquid chromat'opraphy over silica gel, and mass spertroinetric analysis on each fractioii. The 1)erceiitagesof paraffiiis (c/c l'), tiaphtheiies (70I), and aromatics (70A) were obtained from these data. Additional information based 011 a correlative technique (n-d-JI method) which uses refractive index, density, rnolecular weight', aiid sulfur content provided the percentages of Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
531
Table II. Molecular Compositions of Charge Stocks Charge stock
Paraffins, wt
51.9 40.9 46.2 11.3 8.6 9.8 27.8 33.8 32.1 31.3 17.7 34.9 30.2 32.5 11.o 37.3
P1 P2 P3
N1 x2 N3 PN33 PA31 PA32 PA33 PA331 PA34 PA37 PA38 AA45 PC32a a
Mass spectroscopy Naphthenes, wt %
%
n-d-M method Aromatics, wt
%
cr, wt %
14.4 22.6 18.6 19.9 32.4 26.3 22.5 40.1 36.0 38.3 56.1 36 5 46.1 41.0 74.8 30.0
33.7 36.5 35.1 68.8 59.4 64.0 49.9 26.1 31.9 30.4 26.2 28,6 23.7 26.5 14.2 29.5
c s , wt
66.5 69.9 66.7 40.1 48.4 43.5 54.3 56.1 56.7 57.9 47.9 59.6 58.8 64.4 53.0 60.6
____
%
CA Wt
24.7 22.8 25.0 53.8 40.7 47.5 35.9 25.9 25.4 26.1 29 5 23.6 6.1 18.1 15.7 24.4
%
8 8 7 4 8 2 6 1 10 9 9 1 9 7 18 0 17 9 15 9 22 6 16 8 35 1 17 5 31 3 15 0
Contains 3.2YGolefins as determined by hydrogenation.
Table Ill. Properties a n d Molecular Compositions of Aromatic Gas Oils Aromatics from virgin stocks ~
Code ASTM 570, O F Dist. 95%, O F Mol wt, esptl RIolecular analysis, wt Aromatics Xaphthenes Paraffins n-&M method, wt
A2
495 610 202
637 778 285
A3 i54 909 363
x5
h8
A7
c2
740 8 i7 359
868 994 406
7 14 832 257
626 924 330
85.4 7.1 7.5
71.4 25.5 3.3
77.5 21.1 1.4
73 . O 15.2 11.9
76 7 13 2 10 1
85 6 9 4 5 0
95 6 3.2 1 3
53 8 27.4b 18 7
37.8 20.9 41.3
31.3 36.4 32.2
31.4 33.5 35.1
32.7 16.6 50.7
30 7 13 7 55 6
34 6 11 2 54 2
88.0 ...
30.6 13.9 55.6
10 36 3 74 0 011 0 02 2 7
10 0 0 0 4
10 0 0 0 4
11 1 0 0 3
10 4 0 0 4
%
CP Analyses, wt % H content
%S %X % C (Conradson) Bromine no.
72
51 047 22 3
71 53 13 29 6
03 89 12 21 4
90 34 15 08 5
9 54 4 94 0 20 1 6
...
7 2 0 0 14
48 25 13 14 2
10 68 1 78 0 22 0.22 10.9
From coking or recycle stocks. * Including 10.0% olefins as determined bv hydrogenation.
carbon atoms in the tot'al charge st'ock in ternis of paraffitiic carbons (% C,), napht'henic carbons (% CN),and aromatic h with 70 C A , the percarbons (yoCA), By combining centage of aromatic molecules can be classified into aromatic ring carbon atoms and substituent carbon groups attached to aromatic rings (Le., yGA - 7, CA = aromatic substituent' groups). The ranges of t,he properties and molecular conipositioiis of the 16 blended charge stocks are summarized as follo\ys: Average molecular weight Sulfur Kitrogen Hydrogen Conradson carbon Bromine number 532
A4 756 877 364
%
(2.4
a
,41
Pre-processedu aromatics
___
206-402 0.01-3.77 wt 7c 0.001-0.14 W t yo 10.87-13.85 \\-t 7 0 0.01-0.73 wt ye 0.0-5.3
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
Molecular coinpositio~i Masq yiectrometiy Paraffiiif Saphtheiies lroniatics
8 6-51 9 n t 7 0 14 2-68 8 l y t 7 0 14 4-74 8 lyt 70
n-d-N CP
40 1-69 9 Iyt 70 6 1-53 8 15-t 7 6 6 1-35 1 Iyt 7 0
CN CA
Description of Fluidized Dense Bed Reactor and Experimental Procedures. bench scale cracking unit with a stainlesh steel fluidized deiise bed (coiitained -110 replacement) react,or was used in this study. Gas oil wa.: vaporized ill a separatc preheater; t,he oil vapor.: were bypassed A\
around the reactor while the catalyst was fluidized with nitrogen. T h e switch from nitrogen fluidization to gas oil cracking in the reactor was practically instantaneous. Gas oil vapors were diluted with 10 mole 70iiit'rogen in the preheater. Temperat.ures in the fluidized dense bed reactor were measured b y t,hermocouples a t t,hree positions in the reactor and automatically recorded every 25 sec. Stripping of catalysts was i n i h t e d immediately a t t'he end of the cracking run with 900" F steam aiid cont,inued for 10 min. X commercial FCC zeolite c a h l y s t mas used. The on-stream times (catalyst residence times) of oil through the reactor were 1.25 aiid 5.0 min. The weight rat,io of catalyst/total oil charged was varied from 0.5 to 7 by changing catalyst volume in t,he dense bed andlor the feed rate of gas oil. Since 110 catalyst was added during a run, the activity, aiid thus t,he cracking rates, decreased with onstream time. Gaseous products included a light gas which passed through traps a t -40' and -110' F and a heavy gas produced by weathering all the condensed liquid products at 130" F. Distillations of the liquid products were conducted in a microstill with 10 in. of Cannon packing operated a t a constant reflux ratio of 3 : I . T h e gasoline end point was 430" F. Cat8alystcoke was determined by burning a sample of 15-25 grams of coked cat'alyst in oxygen and analyzing the combustion gas for carbon dioxide by Orsat gas analysis. T h e products were collected over each entire experiment aiid represented time-averaged products.
The rate constants calculated from the model are: a1
72
K O= K 1 K1 Kt K3 KiIKo
=
catalyst decay constants
=
rate constant for gasoline formation rate constant for gasoline cracking rate constant for gas oil cracking to A, initial select,ivity ratio for gasoline format,ioii ratio of gasoline cracking to gas oil cracking
+ K 3 = overall rate constant for gas oil cracking
KtIKo
= = = =
Weekman (1968) and Weekmaii and S a c e (1970) derived integrated rate equations for both catalyst tleca~-fuinctioiis in different types of reactors. In this work, the equations for a fixed bed were used to calculate the rat'? const'aiits from the cracking data of t'he contained fluidized dense bed reactor. The integrated rate equatioiir: which were derived for fiserl beds, are equally applicable to this type of fluidized reactor assuming plug flow iii the gas phase. The catalyst, activity in the latter reactor decays uniformly throughout the elitire catalyst bed during a particular run hecause of the high rate of catalyst mixing. The equations for conversion are suimmarized below: First-Order Decay
Kinetic Model
A kinetic model for the catalytic cracking of gas oils was developed b y Weekman (1968) and Weekman and Xace (1970). This model can be represented schematically as: t," Decay
KI
-4,+ A t Kz\
(time-averaged)
dKa
A3 iil is the weight fraction of unreacted gas oil, A z is the weight fraction of gasoline, aiitl -42represents the weight fraction of all other products (i.e., C1-C4, coke). Gas oil aiid gasoline cracking are assumed to be second and first order, respectively. The justifications for these reaction orders have been discussed previously b y Weekman (1968, 1969). The differential rate equations can be w i t t e n as,
-43
=
1 - A1 - A ?
(3)
e w
where t aiid :are, respectively, iiistantaneouq and time-averaged conversions and S = space velocity. Equations 6 mid 7 give the conversioiis for @ = e and Equations 8 and 9 give those for @ = t , - n . Equation 9 is normalized with respect to catalyst residence time and t , represents t'he total on-stream t,ime. Instaiitaneous gasoline yields are calculated h y the follon-ing equation which can be derived froin Equatioiis 1 and 2:
where to = oil resideiice time aiid @ iq a catalyst decay function. Various function. have been used to describe catalyst deactivation during cracking reactions. Weekman has uqed the following two: @
= e-a''
(-2)
where in which t , = catalyd residence time and constants for catalyst decay.
cy
a i d n are rate
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
533
~~
Table IV. Kinetic Rate Constants at 900" Charge stock
a
KO
cy
Ki
KP
P1 24.8 31.8 26.3 1.83 29.9 32.7 P2 26.2 1.09 P3 30.5 34.0 28.0 1.86 N1 18.5 39.2 33.5 1.54 28.7 34.2 N2 29.4 2.35 N3 25.5 36.2 31 . O 2.02 PN33 27.4 36.4 30.9 1.94 PA31 24.7 31.4 21.0 2.87 PA32 31.6 22.9 18.6 1.95 PA33 33.9 22.1 17.6 1.48 15.5 12.6 2.66 PA331 36.4 PA34 34.4 22.1 17.8 1.78 PA37 37.7 10.3 7.71 2.18 PA38 34.7 21.1 16.6 1.66 Ail45 40.1 12.3 9.30 2.28 PC32 31.6 19.3 15.0 1.15 Rate constant for gas oil cracking associated with n decay constant.
EXPERIMENTAL CATALYST RESIDENCE TIMES
*\*
z
1
-
MINUTES MINUTES
, !
n .
1 *%*J
: 5
'o-,
\
0.8
.I
Y b
- 1.25 - 5.0
0
0.8
s
KdKo
KdKa
n
KO"
0.83 0.80 0.82 0.85 0.86 0.85 0.85 0.85 0.81 0.80 0.81 0.81 0.75 0 79 0.76 0.78
0.06 0.03 0.05 0.04 0.07 0.06 0.05 0.12 0.09 0.07 0.17 0.08 0.21 0.08 0.19 0.06
0.58 0.71 0.72 0.42 0.68 0.60 0.64 0.72 0.71 0.77 0.78 0.78 0.77 0.78 0.83 0.71
1.52
0.82 0.79 0.42 0.99 1.57 1.26 0.56 0.53 0.38 0.23 0.37 0.16 0.34 0.14 0.44
Results and Discussion
1.0
s Y
F
>Lo-\--
Kinetic Rate Constants. R a t e constants were calculated for each charge stock from the cracking data obtained in the fluidized dense bed reactor Equations 7 and 13 were used to calculate a , KO, K1,K 2 .The values of n and KO (associated with n) were calculated from Equation 9. i i n improved version of the technique for least-squares estimation of nonlinear parameters, developed by Marquardt (1963), was used to obtain the best fit of the rate constants to the data. The rate constants a t 900' F are given in Table IV and their ranges are shown below:
1 p--,
0.4
,_L I 1
0.2
0 V
I
0.0 I 0
I
I
5
IO
I
I
15 20 SPACE V E L O C I T Y ,
I
I
I
35
40
I
25 30 WT/(WT)(HR)
Figure 1. Conversion vs. space velocity for charge stock
P3
-
Equation 7, rate constants Table IV
Equations 6 and 8 can be used with Equation 10 to obtaiii the iiistmantaiieous gasoline yields since, E=l-Al
(12)
Similarly the time-averaged gasoline yields are calculated with Equations 6 anti 8 with Equation 10 as, 1
Xp =
A2de
(13)
E X P E R I M E N T A L CATALYST RESIDENCE T I M E S 0
-
1.25 M I N U T E S 5.0 MINUTES
L
CONVERSION, WT. FRACTION
Figure 2. Gasoline selectivity for charge stock
-
P3
Equations 10, 13, rote constants Table IV
534 Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
Rate constant
Range at 900' F
18.5-40.1 10.3-39.2 7,7-33.5 0.97-2.87 0.75-0,86 0.03-0.21 These ranges represent the kiiietic behavior of charge stocks of widely different properties and molecular compositions. Some correlations between the rate constants and charge stock compositions are immediately obvious. Catalyst deactivation (a) is highest for charge stocks with the highest concentrations of aromatics. The rate constants for gas oil cracking (KO) and gasoline formation (Kl) increase with increasing concentrations of paraffins and naphthenes. It is somewhat surprising that the ratio of K1/Ko which is a measure of the initial selectivity for gasoline formation varied only from 0.750.86 over the entire range of charge stocks. The ratio of gasoline cracking to gas oil cracking, K 2 / K o ,changed by a factor of seven over the same range of charge stocks. The values of n. (from tCn decay law) and associated KO are also given in Table IV. As expected, the correspondence between CY and n is reasonably good. The general trends of the relative values of K Ofrom both decay functions are similar; some discrepancies may exist for specific charge stocks. Paraffinic Charge Stocks. P1 has a lower value of CY than eit,her P2 or P 3 which is probably relat,ed to its lower boiling range. P2 aiid P 3 have similar decay rates. The small differences in K O and K l for these three charge stocks are probably not significant. h plot of time-averaged conversioll vs. space velocity for P 3 is shown in Figure 1. The time-averaged conversion a t the
1.0
,_
I
1
I
J I 1
E X P E R I M E N T A L CATALYST RESIDENCE T I M E S
EXP ER I M ENTAL CATALYST RESIDENCE TIMES
--
0 0
\*
?
1.25 M I N U T E S 5.0 MINUTES
U
4
0.6
0-$
0.4
1
'\a
\.. I
I
X
~
?
O
-
\
b-
0.2
I
0.0
1
I
I
5
IO
,
I
20
15
SPACE VELOCITY,
I
I
25
30
I 35
i
I
U
0
-
I
W d
0.0
0 1.25 M I N U T E S 0 -5.0 MINUTES
1 0 .
n
g >
-
1
I
I 40
WT/(WT)(HR)
Figure 3. Conversion vs. space velocity for charge stock N3
-
Equation 7, rate constants Table IV
cat,alyst residence time of 1.25 min is considerably higher than that, a t 5.0 min, as mould be expected. The points are the experimental data, and the solid lines are the values computed .with the kinetic model with first-order decay from the rate constants in Table IV. The agreement between the experimental and computed results is very good. The corresponding plots for P I and P2 are not shown, but they are quite similar to Figure 1 except t h a t the conversion for P2 is slight.ly higher, particularly a t 1.25 min. The values of K ffor P1 and P3 are about the same. P2 has a relat,ively low value which is somewhat unexpected. h plot of time-averaged gasoline yield vs. time-averaged conversion is shown in Figure 2 for P3. Again the points are the experimental dat.a, and the solid lines are the computed values. The time-averaged gasoline selectivity a t 1.25 min is higher than that at, 5.0 min. The agreement between the experimental and computed result,s a t 5.0 min is quite good. The computer curve for 1.25 mill is slightly higher than the experimental points. The corresponding selectivity plots for P1 are about the same as those in Figure 2. P2 has slightly higher gasoline selectivities which are largely owing to the low rate of gasoline cracking (low K 2 ) . Naphthenic Charge Stocks. T h e values of CY for t h e naphthenic charge stocks are in the order N1 < N3 < S 2 ; K O is the reverse order. These results suggest that the conversions would be in the order of 1\;1 > K3 > X2. h plot of time-averaged conversion vs. space velocity for N3 is showi in Figure 3. The agreement between the esperimental points and the solid lines, which were computed from the rate con-
stant,s in Table IV, is quit,e good. Similar plots for S1 and 9 2 show the same agreement between the esperimental and computed results and verify that the coilversions are in the order N 1 > N3 > N2. The select,ivity plot of time-averaged gasoline yield vs. time-averaged conversion is shown in Figure 4 for 5 3 . From similar plot,s for SI and N2, it can be observed that the m a l l differences in select'ivity are in the same order as coiiveriioii, Le., X l > .V3 > 5 2 . The values of K 1are i i i the sanie ordcr and those of K 2are the reverse order. iis mentioned previously, P S 3 3 was a ldeiid of equal ' volumes of P3 and K3. Plots of time-ayeraged conver.s1011 vs. space velocity and time-averaged gasoline yield \-s. timeaveraged conversion are shown in Figures 5 and 6. P3, 5 3 , and PN33 have similar boiling ranges and about the same concentrations of aromatics, nitrogen, and sulfur. The ratios of naphthenes/paraffins are significantly different in the t'hree charge stmocksComparison of t'he plots of time-averaged conversion vs. space velocit'y indicates slight differences in cracking rates (Le., 5 3 > P S 3 3 > P3) which is the same order as the ratio of naphtheiies/paraffiiiF. These slight differences are also reflected in the rate constants in Table ITT. P3 has the highest, catalyst decay constant ( a ) and t'he lowest rate constant for gas oil cracking (&). 5 3 and P S 3 3 have similar values of KO,but S 3 has a lower a. The gasoline selectivity plots of K3 and PK33 are about the same which agrees with the closeness of their values of K l and K?.The gasoline selectivity of P 3 is slightly lower. Aromatic Charge Stocks. Efects of droniatic Concentra-
1
1.0,
1.0, EX PER IM E NTA L CATALYST RESIDENCE T I M E S
,
f
; 0.8
4
Y L
$
0
LL L
:
0.6
I
E X P E R I M E N T A L CATALYST RESIDENCE T I M E S
7
-
1.25 M I N U T E S 5.0 M I N U T E S
,
I
I
f"
1
1
I
I
o.6
n
i n
_1
! 0.4
! 0.4
W
**
5 0
n
0.2
z 0
-J
W
2 0.2 0
1
0 0
0.2
0.4
0.6
0.8
1.0
0
5
C O N V E R S I O N , WT FRACTION
Figure 8. Comparison of gasoline selectivities for different charge stocks
536
Equations 10, 13, rate constants Table IV
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1 9 7 1
10 I5 20 SPACE V E L O C I T Y ,
I
I
25 30 WT/(WT)(HR)
1
I
35
40
Figure 9. Conversion vs. space velocity for charge stock
PA37
-
Equation 7, rate constants Table IV
1.01
Comparison of the data for P3, PX33, Pd331, and ,$A45 shows that the constant a increases sharply with iucreasing concentration of aromatics in the charge stocks. Coilstant, b is relatively insensitive to differences iii aromatics conceiitrations. The value of a increases with increasing boiling range of the aromatic extract for charge stocks PA31, l'A32, 1'2133, PA34, and PA-i38.The value of b is about the same for these five charge stocks. T h e recycle and coker stocks, PA37 and PC32, have relatively high values of a,as would be expected. Again b is relatively unaffected by the cornposit,ion of the charge stocks.
I EXPER I M E N T A L CATALYST RESIDENCE T I M E S
; 0.8
0 #
d
l
l
- 1.25 - 5.0
MINUTES MINUTES
l
l
l
0.0 0.0
0.2 0.4 0.6 0.8 C O N V E R S I O N , W T . FRACTION
1.0
Summary and Conclusions
Figure 10. Gasoline selectivity for charge stock PA37
-
Equations 10, 13, rate constants Table IV
centrat'ion of aromatics. PA37 has a relatively high a aiid a very low KO.The gasoline selectivity of 1'-137 is also lower than A.145. Gasoline cracking appears to be relatively high with PA37; the ratio of KliZ P 3 > PI and K2 > N3 > N1 which is the order of increased concentrations of highmolecular-weight aromatics. The values of a are in the same order as constaiit, a for these charge stocks. Interestingly, the constant b increases in the reverse order of constant a.
A kiiietic model for catalytic cracking has heel1 applied to charge stocks of extremely widely different inolecular compositions and properties. The rnodel quantitatively descrilies the conversions and gasoline selectivities. Charge st,oc high concentrations of parafins and/or Iiaphthciies have the largest rate constants for gas oil crackiiig a i d gasoline forrnation. Catalyst deactivation increases with increasing conceiitrat.ioii of aromatics in the charge st.ocks. The molecular compositions of recycle and coker st,ocks are significantly different from those of virgin pas oils. T h e formcr cotitailis higher conceiitrations of refractory aromatic compouiids which have lower cracking rates. The kiiietic model can also be used to describe the catalytic crackiiig of these charge stocks. Some correlations have been observed for thr ~ 0 0 1 . h i w cwist8antsfor coke tleposit,ion and charge stock cornpositiou. Acknowledgment
The authors are indebted t,o Benjamiii Gross, Solomon 11. Jacob, and ITooyouiig Lee for helpful comnients and suggcst,ions, ant1 to Xleksarider 1 2 . Topoliiicki for assirt.aiice in the computer calculations. Nomenclature =
Aa A
~. 1
Table V. Voorhies Constants for Charge Stocksa Charge stack
PI P2 1'3 N1 x2 ?i3 PN33 PA31 PA32 PA33 PA34 PA38 P.1331 ,u45 PA37 PC32
a
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
22 46 41 23 46 36 32 42 47 54 51 64 69 75 78 52
Note t , in mirnite5 for coking data
a b C
c = at?
Ki b
0.30 0.12 0.16 0.27 0.18 0.23 0.24 0.21 0.21 0 24 0.24 0.21 0.17 0.20 0.21 0.22
wt fraction unreacted gas oil wt fractiou gasoline = wt fraction C1-C4 and coke = time averaged yield of A = coking constant, (niin-b (wt fraction c) = coking exponent = coke-on-catalyst, wt yo = gasoline formatioii rate constaiit, (hr-l) (wt fract'ion gas oil in charge)-' = gasoline cracking rate constant, (hr-l) (wt fract'ioii gas oil iii charge)-' = -1,formation from A1rate coilstant, (hr-l) (wt fraction gas oil in charge) -l K1 + KR= overall gas oil cracking rate constaiit, (hr-l) (wt fraction gas oil in charge)-' = esponent in t," decay, dimensioiiless = Weight hourly space velocity, (wt gas oil)/ (wt cat) (hr) = catalyst residence t'ime, hr = catalyst residence time a t end of fixed bed run, hr =
K2
K, K O n
S
=
GREEKLICTTIZRS = decay velocity constant, hr a = instantaneous wt fraction converted E = time averaged wt fraction converted E 0 = normalized time-oil-stream, t , / t , @? = catalyst decay fuiiction Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
537
literature Cited
Akhmetshina, 11. N., Levinter, hI. E., Tanatarov, M. A., Nochalov, Y. D., Seftepererab. Neftekhim. (hIoscow), 1969 (81, 10. Andrew, J. AI., Ind. Eng. Chem., 51, 507 (1959). Blanding, F. H., ibid., 45, 1186 (1953). Campbell, D. It., Wojciechowski, B. W., Can. J . Chem. Eng., 47, 413 (1969). Eberly, P. E., Kimberlin, C. N., Miller, W.H., Drushel, H. V., I n d . Eng. Cheni. Process Des. Develop., 5 , 193 (1966). Fitzgerald, 31. E., hIoirano, J. L., Morgan, H., Cirillo, V. A,, Appl. Spectrosc., 24, 106 (1970). Froment, G. F., Bischoff, K. B., Chem. Eng. Sci., 16, 189 (1961). Froment, G. F., Bischoff, K. B., ibid., 17, 105 (1962). Narquardt, D. W., J . SOC.Appl. Math., 11,431 (1963). Masamune, S., Smith, J. AI., AIChE J., 12, 384 (1966). Ozawa, Y., Bischoff, K. B., Ind. Eng. Chem. Process Des. Develop., 7, 73 (1968). Reif. H. E.. Kress. R. F.. Smith, J. S..Petrol. Refiner, 40 ( 5 ) 237 (1961). ’ Sagara, Y., LIasamune, S., Smith, J. 11.,AIChE J., 13, 1226 (1967).
Shnaider, G. S., Khim. Tekhnol. Topk. Masel, 1969 (5), 5. Shnaider, G. S., Mukhin, I. I., Chueva, M. A., Kogan, Y. S., ibid., 1969 ( l ) ,10. Szepe, S., Levenspiel, O., European Federation of Chemical Engineers, Symposium on Reaction Engineering, Brussels, Se tember 1968. Vooriies, A., Ind. Eng. Chem., 37, 318 (1945). Waterman, H. I., Boelhowuer, C., Huibers, D. T. A., “Process Characterization,” Elsevier, Amsterdam, Netherlands, 1960. Weekman, V. W.,Ind. Eng. Chem. Process Des. Develop., 7, 90 (1968). Weekman. 5’. W.. ibid.. 8- z. 38R .- . . . . .- - 11969). I
\ -
Weekman: 1’.W.: ru’ace, D . M., AIChE J., 16,397 (1970). White, P. J., Hydrocarbon Process. Petrol. Refiner, 47,103 (1968a). White. P. J.. PreDrint No. 24-68, API Division of Refining, - 33rd Mid-Year Megting, Philadelphia, May 1968b. Wojciechowski, B. W., Can. J . Chem. Eng., 46, 48 (1968). Wojciechowski, B. W., Juusola, J. A,, Downie, J., ibid., 47, 338 ( 1969). RECEIVED for review October 29, 1970 ACCEPTEDMay 3, 1971
Application of a Kinetic Model for Catalytic Cracking Some Correlations of Rate Constants Sterling E. Voltz,l Donald M. Nace, and Vern W. Weekman, Jr. Research Department, X o b i l Research and Development Corp., Paulsboro, N . J . 08066
The rate constants from a kinetic model for catalytic cracking have been correlated with the molecular compositions of a wide variety of gas oils. The catalyst decay constant and rate constants for gas oil cracking and gasoline formation can b e quantitatively related to the ratio of aromatics/naphthenes in virgin gas oils. The corresponding rate constants for recycle and coker feedstocks deviate from the correlations. Some relationships between the rate constants and coke formation have also been observed.
N u m e r o u s studies have been made of the reaction mechmisins and kinetics of pure hydrocarbons over acidic cracking RIost of the results are consistent wit,h the concept that catalytic cracking occurs primarily through carbonjuni ioii ~nechaiiisnis.The catalyt,ic cracking of gas oils is complicated by t’he lireseiice of thousands of different molecules; eac811 ttype has specific rates of adsorption and reaction. Fort~unatrly,many of the catalytic reactions of hydrocarbons can lie described both mechanist’ically and kinetically in terms of the behavior of cert,ain classes of hydrocarbons. During the past decade, there has been considerable progress reported 011 analytical methods to determine various classes of hydrocarbons in gas oils and development of quantit#ativerelationslii~isbetweell molecular composit,ion of charge stocks and product yields in catalytic cracking. (’roroll and Jaquay (1960) pointed out the necessity to considrr the differences in the cracking behavior of major hydror a r l m i tylies in charge stocks to accurately predict product’ yields iii catalyt,ic cracking. They described the relat,ive cracking rates of saturates (paraffins and naphthenes) and To whom correspondence should be addressed. 538
Ind. Eng. Chern. Process Des. Develop., Vol. 10, No. 4, 1971
unsaturates. In the same year, Service (1960) discussed t h e detailed charact,erization of feedstocks for catalytic cracking including the various correlation methods to determine carbon atom distributions. He also recognized the utility of mass spect,ronietry to elucidate classes of hydrocarbons in cracking feedstocks. Reif et al. (1961) developed correlation equations to predict, yields and properties based on the det.ailed characterization of a wide variety of cracking feedstocks and cracking data from a pilot plant unit. Their equations were based to a large degree on the carbon atom distributions in t.he feedstocks. One of the most comprehensive studies relating feed composit,ion with product yields in catalytic cracking was made by White (1968). He developed a regression correlation model to predict product yields based on nine different hydrocarbon types in the feedstocks. The detailed analytical methods for the gas oil charact~erizationwere recently reported by Fit’zgerald et al. (1970). Their analysis included distillation, separations by molecular sieves, mass spect,rometry, and ultraviolet spectroscopy. Shnaider et al. (1969) recently developed a kinetic model for cat,alytic cracking and utilized it to analyze the catalytic
