J. M. ANDREWS, Humble Oil & Refining Co., Baytown, Tex.
Cracking Characteristics of Catalytic Cracking Units Want a procedure for evaluating cracking characteristics of various fluid units? Here is a mathematical model based on variations in oil and catalyst contact conditions which can be adapted for computer use
THE
x
greatest differences between cracking units are observed in comparing the continuous and cyclic types, if the data are not properly interpreted. Cyclic cracking units are exemplified by Humble's 610-gram cyclic fluid unit. This contains a mechanically agitated, fluidized bed with excellent catalyst-oil contacting properties, which is restrained at the bottom and uses filters to separate the catalyst fines from the products. The commercial continuous units are divided into three general categories.
Conventional dense phase.
One type is the low gas velocity dense phase catalytic cracking unit. The second is the recently developed transfer line reactor. Transfer line reactors employ high gas velocity with low density, disperse-phase catalyst operating conditions. The third type of unit is the dual reactor most generally found in commercial fluid catalytic cracking installations. This type is the perfectly mixed conventional dense bed, which is preceded by a small increment of high velocity disperse-phase transfer line. Mathematical models show the carbon conversion relations for these units. The basic relations were developed employing Voorhies' ( 7 ) equation for the catalytic carbon and conversion from light gas oils on a cyclic unit. However, it became apparent that a different type of carbon, designated additive carbon, was present in most heavy gas oils, and additional relations were developed to characterize this factor. Equations 1 to 13 relate operating conditions and carbon yield or conversion.
Additive Carbon Yield
xf = d i n
(1 1
bVjm
(2)
yf
=
Transfer line unit. =
(2)" Irn
x; = u
[m + mnn- 1
bVtm
vp
=
[(" +:
reactor*
__ Ht
a)!
-
m
)!
..)!Irn
b Vpm
>
xd
{ (n,
(2)'eHpu
m)!
-
(n,z)!f
Transfer line unit.
Conventional dense phase unit and dw reactor. (10
Yap = Yao
Conversion Transfer line unit.
(" +:
V; = 1
-
n-1
n
' ) (F ~ ) " ( ~ )Htm " (11) b
Conventional dense phase unit.
I AVAILABLE FOR ONE DOLLAR The complete manuscript, containing all derivations and additional data, and equivalent to 14 published pages in I/EC. Address: EDITOR, I/EC, 1155 Sixteenth St., N.W., Washington 6, D. C., sending cash, money order, or check payable to American Chemicol Society.
(3)
(4)
(n,
(n,
Catalytic Carbon Yield
Cyclic unit.
(2)"
xp = a
I VOL. 51, NO. 4
APRIL 1959
507
Dual reactor.
Va = Ht -
(
+m a)!
1)(5 ;&{:(E&);
%)!I [
- (7, n - 1 Ht
Equations 1 and 2, proposed by Voorhies, show the relationship between the catalytic carbon on a feed and catalyst basis as a function of conversion and cycle time in a cyclic unit. T h e catalytic carbon on a catalyst basis is independent of oil feed rate (Equation l ) , and this relation has been verified for a large variety of light gas oil feed stocks. Coefficients a and b and exponents m and n, which are used in subsequent comparisons of the various types of units, should be evaluated from cyclic unit data by plotting the indicated quantities on logarithmic paper. A mathematical analysis relating the cyclic unit to the transfer line unit resulted in Equations 3 and 4, wherein the relationship among carbon yield, conversion, and operating conditions is defined in terms of the previously described coefficients and exponents. T h e same type of relations was derived from Equations 1 and 2 for the conventional dense phase unit under the assumption that the dense bed was perfectly mixed with the piston displacement flow of the oil vapors. Although oil vapors d o not proceed in this manner, the agreement between predicted and observed data verify the validity of the assumption. T h e dual reactor is represented by Equations 7 and 8, and in this particular reactor it was again assumed that the dense bed was perfectly mixed with piston displacement of the oil vapors. T h e additive carbon at present can be evaluated only from cyclic unit operations. The additive carbon precursor content of a feed stock, y,,, is evaluated by plotting the total carbon on a catalyst basis as a function of total feed with constant cycle time. This operation necessitates varying feed rates while holding cycle time constant and recharging catalyst for each cycle in sequential cyclic unit operations. T h e slope of the relationship between total carbon on a catalyst basis and total feed
(1
+
+ 2)' 1
(z)'])
(13)
is the additive carbon precursor content, I t is possible to evaluate the additive carbon in this manner, because the catalytic carbon is constant for a constant cycle time, as indicated in Equation 1. If a curvilinear relationship results from such a plot, shorter cycle times and lower feed rates must be used to obtain the desired slope for the evaluation ofy,,. Once this value has been obtained, the additive carbon content on a catalyst basis for continuous reactors can be evaluated by Equations 9 and 10 as a function of operating conditions. T h e relationship between conversion and operating conditions for the continuous units as presented in Equations 11, 12, and 13 is again expressed in terms of the coefficients and exponents determined previously. Total carbon is the sum of both additive and catalytic carbons. This should be considered in evaluating the coefficients and exponents from Equations 1 and 2. I t is first necessary in cyclic unit evaluations to determine the extent of additive carbon present by the suggested plot, and then subtract the additive carbon from the total carbon to obtain the relationships presented in the first two equations.
ya0, on a total feed basis.
Application of Carbon and Conversion Equations
Constants a, b, m, n, and yaa must be evaluated with the same feed stock, regenerated catalyst, temperature, and pressure that are to be utilized in the continuous units where the carbon and conversions are to be estimated from the applicable constants. All the constants in the present article were evaluated in Humble's 610-gram cyclic, fluid unit (FFBU) employing feed rates of 5, IO, 15, and 20 grams per minute with cycle times of IO, 15, and 20 minutes. T o compare the catalytic carbon yieldconversion relationships for the various
4 Figure 1 . Catalytic carbon yields show there was no additive carbon in this feed
types of continuous reactors, average values of 3 for m and 0.5 for n are employed. Under these conditions, the coefficient in Equation 4 for piston displacement is 0.297 and the coefficient in Equation 6 for perfect mixing is 1.07. I t may be concluded that, for a given conversion, the catalytic carbon yields calculated for an idealized piston displacement reactor will be only 0.297 + 1.07 or about 27.8y0 of the value calculated for a perfectly mixed reactor. Less dramatic carbon reductions are predicted for other feedstock-catalyst combinations. Limited experimental data comparing the dense phase and transfer line operation in pilot unit and commercial equipment have verified the predicted carbon yield advantage for this progressive flow system. Although the coefficient for a dual reactor is not evaluated here, the carbon yield for the dual reactor is always intermediate between the piston displacement reactor and the perfectly mixed bed operation. The greater the ratio of H,/H,, the lower the carbon yield will be, and as H , / H , becomes large, the carbon yield for the dual reactor approaches that for the piston displacement reactor. However, in commercial practice, the transfer lines preceding perfectly mixed vessels are relatively inefficient because of the extremely high velocities employed. These high velocities cause separation of the catalyst and oil vapors a t bends, and unless the line is fairly long, g,ood contact between the catalyst and oil vapors is not realized, so that the conclusions derived from Equation 6 may be somewhat misleading. These carbon yield ratios do not provide for additive carbon, which will aIso be lower in a piston displacement reactor than in either a perfectly mixed bed or a dual reactor. T o verify the correlations, operations were conducted in both the 610-gram cyclic unit and a continuous 5-barrel catalytic cracking unit charging a heavy catalytic naphtha. This naphtha had been produced in a commercial cracking unit and has a boiling range of 320' to 430' F. T h e carbon yields with this feedstock and catalyst combination in the cyclic unit are shown by the open points in Figure 1. As there was no variation in catalytic carbon on the catalyst with feed rate, it has been concluded that there was no additive carbon
I
ADDtIIIL
CLISDN
rn
1.1.
CVCLIC UNIT CHARGE R A T E .
GOAIN.
b Figure 2. BO feed stock contains additive carbon
I-
508
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
0
100
200
TOTAL
300
FEED, GMS.
400
CATALYTIC CRACKING L
G
5
4 o
-
z
'
3.0
CYCLIC U N I T I C C B U ) CHARGE RATE, GNIN. 0-20 0-15
A-IO
-
4 Figure 3. Catalytic carbon is independent of feed rate
I I
z
2rn
,Am
2.0
P '
Figure 4. The agreement b between predicted and observed carbon yields is excellent
u 5
i 1.0
in this particular type of feed. Therefore, the catalytic carbon yield on catalyst-reaction time relationship can be expressed by the following equations: XI X/
= =
0.0637P.6'5, atmospheric pressure (14) (1.55)(0.0637)00.646,10 p.s.i.g.
x P = (1.55)(0.0637)80.6'5
r
(15)
(1.645) (16)
x p = 0.088880.6'5
(17)
Equation 14 is the carbon yieldresidence time relationship for atmospheric pressure operations. Equation 15 used a pressure correction at constant reaction time (developed from previous studies) to elevate the carbon yield to the 10-p.s.i.g. total pressure which was used in the continuous unit. I n Equation 16, the carbon yield-residence time relationship for a perfectly mixed bed is derived in terms of constants a and n and the pressure correction. Equation 17 summarizes the carbon yield-conversion relationship predicted from the cyclic unit constants for the continuous unit. The line for Equation 17 is reproduced in Figure 1, with the observed carbon yield-reaction time points from the continuous pilot unit's operations; the agreement between the predicted curve and the observed data points for the continuous unit is excellent. I t would not be possible to determine the 430 conversion for a catalytic naphtha with a n end point of 430' F., and no conversion prediction for the continuous unit can be made. T h e effect of additive carbon was demonstrated by operations employing BO feedstock in both the fixed bed and continuous units over the same catalyst. BO feedstock is a very heavy boiling gas oil which does contain additive carbon,
FFBU Data ?n n 5.24
0.58
2/00
2/01
4.33
3.78
If the slope is then multiplied by the catalyst charge of 610 grams, the resulting change in carbon yield will be expressed on a fresh feed basis. The additive carbon yo,, thus evaluated is 1.18 weight %. T o obtain the catalytic carbonreaction time relation, it was necessary to calculate the additive carbon content of the catalyst for each of the individual cyclic unit operations. This value was then subtracted from the total carbon to determine the catalytic carbon yields for the individual operations, and the latter were plotted as a function of reaction time in Figure 3. After the additive carbon content of each particular yield period is subtracted, the catalytic carbon is independent of feed rate. I n Figure 3, the value of the constant coefficient, a, was determined to be 0.358, and the value for exponent n was estimated to be 0.667. T h e same pressure correction coefficient factor of 1.55 is used for both the additive and catalytic carbon in Equations 5 and 10. The sum of the two carbon yields is the total carbon yield, and the values predicted for the continuous unit operation from the constants evaluated in the cyclic unit are presented in Figure 4 os. the reciprocal of catalyst to oil ratio. The actual values from the continuous unit operation are also presented in Figure 4; the agreement between the predicted and observed carbon yields is excellent. Predicted conversion-operating conditions showed good agreement with the observed data. I t is probably more important to compare the cyclic unit with commercial operations. The following data were obtained from test runs at Humble's FCCU No. 1, which is a transfer line reactor.
Transfer Line Predicted Values Observed Values Tot. car. V1 Tot. car. 0.24
as indicated in Figure 2. With a reaction time of 15 minutes, the feed rate was varied from 10 to 20 grams per minute. If the slope of total carbon yield is evaluated, it will give the rate of change of total carbon with respect to the total feed.
4.02
60.9
3.95
No pressure corrections were used, as the cyclic unit operations were conducted under commercial unit pressures. T h e results predicted from Equations 4 and 9 show excellent agreement \rith the observed data.
Although such correlations are not vet fully developed, it is anticipated that the same type of relations can be derived for cracking products other than carbon. However, the present developments clearly demonstrate the advantage for transfer line operation in both theory and practice, while the theory provides a better understanding of the variables affecting carbon production. Nomenclature a
b
F H rn n
V
x
y 0
constant depending only on feedstock, catalyst, temperature, and pressure, defined by XI = aOfn = constant depending only on feedstock, catalyst, temperature, and pressure, defined by y J = b T'Jn = mass feed rate, grams per minute = total catalyst holdup, grams = constant depending only on feedstock, catalyst, temperature, and pressure, defined by y r = b V f n = constant depending only on feedstock, catalyst, temperature, and pressure, defined by x , = aOIn in feed bed = corrected 430 conversion, vol. 70 on fresh feed = 100 - (% product based on feed above 430' F.,) 100 100 - yo below 430" F.; in feed = catalytic carbon yield, wt. yo on fresh feed = catalytic carbon yield, \vt. 7, on fresh feed = catalyst residence time in fixed bed, minutes = fluid bed cycle time, minutes =
SUBSCRIPTS a = instantaneous additive carbon pre-
c
=
d = f = o =
p
=
t =
cursor content of oil or additive carbon yield on catalyst or fresh feed basis catalyst dual reactor fixed bed yields or fluid catalyst densities oil perfectly mixed bed transferline
literature Cited
(1) Voorhies, Alex, IND.ENG.CHEM.37, 318 (1945). RECEIVED for review January 23, 1958 ACCEPTEDFebruary 24, 1959 VOL. 51 NO. 4
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APRIL 1959
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