Catalytic Cracking over C rysta Iline Alu minosilicates Microreactor Study of Gas Oil Cracking Donald M. Nace Applied Research and Development Division, Mobil Research and Development Corp., Paulsboro, N . J . 08066
Instantaneous rate data are obtained in a microreactor for the cracking of a 500" to 600"F gas oil fraction over a crystalline aluminosilicate catalyst a t on-stream times ranging from 10 seconds to 2 minutes. Aromatic and nonaromatic extracts from this gas oil are separately cracked and their contributions to catalyst activity decay, coke formation, and product distribution evaluated. From a comparison of rate data for a series of fused-ring naphthenes, evidence of molecular size limitation on cracking rate i s given. Abundance of five-member ring products i s indicative of scission and recyclization reactions of the naphthenes.
APPLICATIONS
of a microreactor technique to kinetic and reaction mechanism studies of catalytic cracking of pure paraffins, olefins, and aromatics have been discussed (Nace, 1969a,b). Relationships between molecular structure of reactants and their reactivity are of considerable value in understanding the behavior of commercial feedstocks in refinery catalytic cracking processes. Interaction among hydrocarbon species and the inhibiting effects of some components on the reactions of others are bound to be necessary considerations. This paper describes microreactor experiments using a 500" to 600" F gas oil fraction in which instantaneous analysis of the product stream allows changes in cracking rate to be observed as a function of on-stream time. Extracted aromatic and nonaromatic components of this gas oil fraction are also used as reactants and results compared to those obtained from pure hydrocarbons. Experimental
The microreactor previously described (Nace, 1969a) was utilized in studying gas oil cracking by the continuous flow technique, in which the reactor effluent stream is sampled and analyzed after a given time on-stream. Results provide instantaneous rate data indicating the activity of the catalyst and product selectivity a t the time the stream was sampled. A gas chromatographic technique was used which simulates distillation of the product into weight fractions representing carbon number distribution. Details of the microreactor and analytical technique are given by Nace (1969a). Reaction rate constants for gas oil cracking are calculated from the same integrated first-order flow reactor equation used for the pure hydrocarbon reactions (Nace, 196913). The rate equation was derived for the reaction A + iB where the
dilution term i (moles of product per mole of reactant cracked) can be calculated from the product and reactant distributions following each run. The R E H X catalyst (Nace, 1969a) was also used for the gas oil experiments. This 20140-mesh size catalyst was diluted with Vycor chips of the same size a t a 4 to 1 Vycor-catalyst ratio in the glass reactor. An East Texas gas oil was distilled a t 10-mm pressure in a %foot packed column using a reflux ratio of 15 to 1 to collect a 500" to 600°F fraction. A sample of the distilled gas oil was separated into an aromatic and a nonaromatic portion by liquid elution chromatography using a silica gel column. Saturates were eluted with n-pentane; aromatics were eluted with benzene, a benzene-methanol mixture, and finally methanol. Solvents were stripped and a 99+% recovery was achieved. The nonaromatic portion was further reduced in complexity by extracting the normal paraffins using Linde 5A molecular sieve and cyclohexane as the eluting solvent. The adsorbed normal paraffins were recovered from the 5A sieve by extraction with n-heptane in a Soxhlet extractor. Gas chromatographic analysis of the straight-chain paraffin extract showed it to be 97.8% normal paraffins. Analyses of the gas oil extracts by mass spectroscopy and gas chromatography are summarized in Tables I and 11. Acid extraction of aromatics provided an analysis which agreed to &1% with the results from silica gel extraction. A sample of the gas oil was also hydrogenated in a glasslined pressure bomb for 4 hours under 2500 psig of hydrogen a t 450" F in the presence of a supported nickel catalyst. It was shown by ultraviolet absorption spectroscopy that only 0.3% polynuclear aromatics remained in the hydrogenated gas oil. The composition of the hydrogenated oil in Table I indicates a slight shift in the paraffin carbon Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970 203
Table I. Composition of light East Texas Gas Oil
(Wt. % by mass spectroscopy) Using Silica Gel and Molecular Sieve Separation n-
Par a ffin s CI1 C12
CI? CI4 CIS
C16
c1;
CIR C19
Using Acid Extraction
After Hvdrogenatian
Is0
Total
...
...
0.8 2.2 2.9 4.0 4.8 3.5 3.2 2.7
0.1 1.7 4.9 7.0 9.0 9.8 6.7 3.2 2.7 -
2.3 6.9 9.2 10.8 10.1 6.3 1.5
21.1
24.1
45.1
47.1
47.1 Increase
19.4 9.4 4.3
20.3 9.4 3.9
25.4 21.2 6.4
33.1
33.6
53.0
1.o
...
5.9 3.0 2.4) 5.9 1.0
...
0.1 0.9 2.7 4.1 5.0 5.1 3.2
...
Cycloparaffins 1 ring 2 rings 3 and 4 rings Olefins Aromatics Alkyl benzenes Tetralins and indanes Indenes, etc. Naphthalenes Acenaphthenes, etc. Fluorenes, acenaphthalenes, etc.
7.1 4.7 1.4 6.0 1.1
...
0.6 -
0.1
20.9
18.3
1.6 3.9 6.6 8.3 9.6 8.3 6.5 1.5 0.8
Gas Oil Cracking Studies
5.1 11.8 2.5
... 11.3
...
...
Table II. Composition of light East Texas Gas Oil
(Wt.% by gas chromatography) Boiling Point Range Denoted by Paraffin Carbon Numbers
CII C 12 CI? c 1 4
c c
15
CIS li
CL8 19
Before Hydrogenation
After Hydrogenation
0.2 3.6 12.3 18.2 19.5 23.1 18.4 4.1
0.1 5.4 14.5 19.5 20.3 20.2 16.0 4.0
number distribution. Light gases in the bomb accounted for only 1.1% conversion to C1 to Cs hydrocarbons. Increases in single-ring cycloparaffins (+5.1%) and doublering cycloparaffins (+11.8%) during hydrogenation agree well with the initial content of alkyl benzenes (5.9%) and fused double-ring aromatics (11.3%). The shift in boiling point distribution on hydrogenation as determined by gas chromatography (Table 11) results from conversion of aromatics to the lower boiling perhydro compounds. Reactions of several fused-ring naphthenes were also studied as pure hydrocarbon reactants. Several of these were made by hydrogenating the following aromatic compounds: 1,3,5-triethylbenzene from Eastman; 2,3,6trimethylnaphthalene from Aldrich; and fluorene from Matheson, Coleman, and Bell. Hydrogenation was conducted a t 2500 psig over reduced Harshaw Ni0-107 catalyst in the glass-lined bomb a t 400" to 500"F, depending on reactant boiling point, for at least 3 hours. Cyclohexane was used as solvent when necessary, and 204
reduced pressure distillation was used to remove solvent and lower boiling impurities. After silica gel percolation, ultraviolet absorption spectroscopy indicated no more than trace amounts of unsaturated products and mass spectroscopic analysis showed less than 2 mole 'Z impurities with lower carbon number than the starting material. The saturated fused-ring compound perhydropyrene, purchased from the Aldrich Chemical Co., was also silica gel-treated.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970
Cracking experiments were conducted with the 500" to 600°F distillation cut of East Texas gas oil over the REHX catalyst [a rare earth chloride and ammonium chloride exchanged type X zeolite as described by Nace (1969a) 1. Gas chromatographic analysis of the reactor effluent stream gave a weight distribution as a function of boiling point. The boiling ranges are designated in this paper by the carbon number of paraffins having the same boiling range. While this grouping according to paraffin carbon number does not necessarily denote the exact carbon number for aromatic and cycloparaffin components, the assignment of paraffinic carbon numbers provides a relative basis for comparing distribution changes which occur during cracking. An advantage of the chromatographic scanning of the whole C1 to Cls range of the reactor stream is that changes in distribution of the unconverted components (C12 to C18) indicate which carbon number groups have been cracked as well as what products (C2 to C12) are formed. T o calculate total conversion, it is assumed that the small amounts of components in the initial Cl1 and CI? range of the gas oil (3.8% of total range) were not cracked; therefore, any increase in these components is due to cracking of higher boiling components. The distribution of product hydrocarbons changes very little when conversion of the gas oil is increased from
7.3 % CONVERSION 20
I
14.9 % CONVERSION
2oh / \ I 10
0
I
I
I
I
'
l
I
I
I
I
1 ' 1
C4 C, Ce CIO C12 Cia CIS Cie-ie PRODUCT AND UNCONVERTED REACTANT DISTRIBUTION
Figure 1. Carbon number distribution from cracking to 600°F gas oil
500"
range from 10 to 20% lower than those for cracking n-hexadecane (b.p. 550°F) a t the same time on-stream, but coke left on the catalyst is higher. For example, at the end of a 60-second on-stream period the average rate constant calculated for n-hexadecane runs was 1150 molesikg cat.-hr-atm compared to 1035 for the gas oil. Coke concentration on catalyst was 1.4% for hexadecane compared to 1.6% for the gas oil. Moles of product per mole cracked averages about 2.5 for gas oil cracking compared to 3.1 for n-hexadecane. Because the catalyst was diluted 4 to 1 with Vycor chips and the catalyst bed was only 0.25 inch in diameter, temperature changes in the bed during reaction were small. At 35% conversion, for example, a temperature profile measurement indicated a maximum temperature spread of 15" F. Space velocity has no effect on calculated rate constants for either previously reported pure hydrocarbon cracking experiments or the gas oil cracking runs made in the 325 to 1300 LHSV range. The change in the reaction rate constant k with on-stream time (10 to 120 seconds) is plotted in Figure 2 a t three different space velocities. As indicated in Table IV, coke concentration formed on the catalyst from 10 to 120 seconds increased from 1.2 to 1.9%. The activity of the catalyst for cracking drops during this time period by a factor of nearly 3. While cracking activity is known to decline with time on-stream, Elanding's work (1953) using fluidized catalyst is the only detailed published investigation of the extent of change which occurs during the first few minutes of gas oil reaction. Using a light East Texas gas oil and cracking over silica-alumina catalyst, Blanding measured an instantaneous rate constant k a t 10 seconds on-stream of 350-mole conversion per kg of catalyst-hr-atm (recalculated to corre-
7 to 28% by decreasing space velocity a t constant temperature. The distribution of components in the unconverted gas oil does change. Figure 1 illustrates the molecular weight distribution in the product and in the unconverted gas oil at four different conversion levels. The increase in cracking rate as molecular weight increases, demonstrated in an earlier paper (Nace, 1969a), explains the change in composition of unconverted gas oil as conversion is increased. The change in composition of unconverted gas oil can be expressed as the number of moles that disappear from 100 moles originally present in each carbon number range. Comparisons shown in Table I11 for three conversion levels indicate that about 50% of the C17-~8components are converted before a net decrease in the C13 boiling range components begins to occur. Gas oil cracking data a t six reaction conditions are summarized in Table IV. The calculated reaction rate constants for conversion of the 500" to 600°F gas oil Table 111. Net Changes in Individual Boiling Range Portions of 500' to 600' F Gas Oil during Cracking Change in Moles per 100 Moles in Each Carbon No. Conversion, Wt.
Yo
7.3
21.5
51.9
Paraffin carbon no. equivalent boiling range"
Cl? C 14
+7 +2 +4 -12 Cl5 0 -19 C16 -14 -34 c 1 7 18 -11 -42 "Boiling range of components defined as range in and n-paraffins of given carbon number occur.
-20 -43 -56 -63 -77 which iso-
Table IV. Cracking of 500' to 600' F Gas Oil over REHX at 900' F LHSV No Catalyst
1300
650
...
120
120
325 Time On-Stream, Sec 120 Wt.
0
7.28
650
650
650
60
30
10
18.1
21.5
28.2
1035
1275
1830
Yo Converted
15.0
24.7
k , [Moles/(Kg Cot-Hr-Atm)]
...
750
825 Wt.
Product, mole 4c CS C4
C5
CS
C7
1.9
1.8
1.9
1.6
1.3
1.2
... ... ... ... ...
...
9.5 16.8 13.1 12.6 13.2 12.1 9.3 6.0 3.3 4.1
9.6 18.2 14.5 13.6 13.3 11.5 9.0 5.2 2.6 2.4
9.4 18.7 14.7 14.5 13.3 11.0 8.3 5.5 2.5 2.1
9.3 18.3 14.5 14.1 13.8 11.5 8.9 5.1 2.3 2.2
9.2 18.5 14.9 14.6 13.3 11.2 8.1 5.3 2.5 2.5
8.8 17.7 13.9 13.5 13.3 11.2 9.2 6.0 3.5 3.1
0.2 4.6 14.3 19.6 19.6 22.4 19.3
0.2 5.1 16.6 22.2 21.1 20.9 18.6
0.3 5.3 16.6 21.0 20.2 19.3 17.2
0.3 5.9 19.2 21.7 19.9 19.0 14.0
0.3 5.5 17.9 21.7 20.1 19.3 15.3
0.3 5.7 18.5 21.8 20.2 18.8 14.7
0.3 6.2 19.8 22.6 19.7 18.0 13.4
... ...
Cll
CIL
780 C on Catalvst
...
... ...
C8 CS CI"
Yo
Unconverted gas oil, mole :5 c 1 1 C1L
CIS
c c
I4 15
CIS
c1:
18
Moles product/ mole cracked
...
2.55
2.53
2.56
2.58
2.43
2.46
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
205
201 5a
I
1600-
Q
0
1400
ii
-
\
; 1200i 0
I
1
!
+ m
position that includes 33% cycloparaffins and 24% isoparaffins, the gas oil might crack a t a much faster rate, but the rate is undoubtedly inhibited by the 20% aromatics in the gas oil. T o compare the cracking rate of the different types of hydrocarbons present in the gas oil and to determine the specific products which, each of these types produces, the aromatics were separated from the gas oil by liquid chromatography over silica gel and the normal paraffins removed by Linde 5A molecular sieve absorption. The cracking characteristics of these individual fractions removed and those remaining were examined and compared to data obtained with the composite gas oil. A sample of gas oil which had been hydrogenated to saturate all aromatic compounds completely was also used as a reactant in the micro unit.
1
0
1300 L H S V
A
650 LHSV
0
325 L H S V
1000 -
v
*oat
Y
6 0
Cracking of Fractions Extracted from Gas Oil 0 0 20 40 60 80 100 120 TIME ON STREAM (SECONDS)
The distributions of carbon number groupings in each of the extracted fractions and in the hydrogenated gas oil are compared in Table V, which also shows the product composition differences with the various fractions as reactants. Carbon number distributions in product and reactant are illustrated in Figure 3. Each fraction has a characteristic product distribution which changes only slightly with conversion in the 15 to 40% range. The nonaromatic fractions, including the hydrogenated gas oil, have product molecules more concentrated in the C4 to Ci carbon number group, similar to the distribution occurring when n-hexadecane is cracked (Nace, 1969a). The normal paraffin fraction gives an almost identical pattern to that from n-hexadecane. The fraction containing cycloparaffins
6 I40
Figure 2. Catalyst activity decline during cracking of to 600" F gas oil
500"
spond to units used in this report) a t 850°F. The value of k fell t o 80 a t 1 minute, 20 a t 2 minutes, and 10 a t 60 minutes on-stream. Although conversion of the 500" to 600" F gas oil fraction involves cracking of many hydrocarbon species, mainly in the CI4 to Cli range, the rate constant is not greatly different from that of n-hexadecane cracking. With a com-
Table V. Distribution of Products and Unconverted Charge for REHX Cracking at 900' F Charge Stock
n-Paraffin Froction
Nonaromatic Extract
Cyclo- and Isoparaffin Froction
Hydrogenated Gas Oil
Aromatic Extract
LHSV
...
1300
325
..,
1300
,,
.
1300
,
.,
1300
325
...
1300
10
10
...
120
15.5
33.4
0
14.0
28.9
12.3 12.5 6.8 8.3 11.3 13.1 13.3 7.3 6.0 9.0
14.5 16.5 8.2 8.8 8.9 10.5 8.6 9.0 6.8 8.2
6.3 17.2 14.0 13.5 17.6 12.8 6.3 4.4 2.8 5.0
7.1 18.0 14.3 14.6 15.4 11.6 7.8 5.9 3.0 2.2
0.1 2.1 15.0 24.3 14.7 25.6 13.1 5.2 2.30
0.1 2.6 20.8 24.8 12.2 22.3 9.5 7.9 3.10
0.2 7.7 17.7 20.6 20.4 17.7 13.2 2.4 3.14
0.2 9.3 18.7 20.7 21.4 16.1 11.4 2.3 1.4
1300
Time on Stream, Sec.
...
10
30
...
10
...
0
20.3
42.6
8.0 20.4 17.1 15.8 13.8 11.0 7.2 4.5 1.7 0.4
8.6 20.9 17.4 16.2 12.8 9.8 6.7 4.9 2.1 0.6
0.3 6.3 17.2 20.5 22.6 17.3 13.4 2.5 0.74
0.3 8.6 19.8 22.7 22.1 15.8 9.1 1.7 1.24
0
15.2
0
...
10 Wt.
Ya
10
Conv.
24.6
0
Product" mole 70 C3
C4 C5 c 6
C, C8 CS
C!O ClI
c
12
14.1 26.8 21.7 16.6 10.9 4.8 2.7 1.5 0.8
...
Unconverted charge", mole 70
C,, C!2 c 1 3 c 1 4
c
I5
c 1 6 c 1 7 (218-19
Wt. % C on cat.
0.2 5.1 14.9 19.1 22.1 19.7 16.1 3.0
...
0.5 5.5 14.8 20.1 22.7 22.1 13.4 0.9
...
0.6 6.3 16.0 21.6 23.5 20.3 11.2 0.5 0.76
Carbon number of paraffins in each boiling range.
206
6.4 19.7 15.4 14.8 14.0 11.8 8.1 5.9 2.4 1.5
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
... 1.9 10.8 18.4 23.9 20.3 20.2 4.5
...
2.5 13.1 19.9 24.5 18.6 17.8 3.6 1.10
0.1 1.7 12.3 21.6 15.3 28.4 19.2 1.5
...
0.2 6.7 16.5 20.6 20.1 18.7 14.0 3.3
...
L
also lead to the high coke formation by the aromatic extract.
50O-60O0F GAS O I L
Activity Decline with lime On-Stream HYDROGENATED GAS O I L
I S 0 A N D CYCLO-PARAFFIN FRACTION OF GAS O I L
0
l
~
l
l
~
,
,
I
l
l
l
l
NON-AROMATIC EXTRACT FROM GAS O I L
c4 c, C8 CIO c12 c14 CI. CIS+ PRODUCT AND UNCONVERTED REACTANT DISTRIBUTION
Figure 3. Carbon number distributions from cracking gas oil and extracted fractions and isoparaffins cracks to a product containing much less CB to Ca's and appreciably more C7 to Cl0's than does a-hexadecane. This product distribution is very similar to that from n-hexadecene cracking (Nace, 1969b), which was shown to involve isomerized and cyclized intermediates. For each of the six reactants illustrated in Figure 3, disappearance of components in the CI6 to Cl8 boiling point range is faster than that of components in the Clr to C14range. The aromatic extract produces a much more balanced distribution of hydrocarbons from C3 to C1?.The products in the C7, Cs, and C9 paraffin carbon number boiling range correspond to benzene, toluene, and xylenes. Naphthalene is represented in the C12 paraffin equivalent boiling range. However, as shown in Figure 3, increasing conversion affects product distribution more with the aromatic extract than with the other fractions as a result of continued dealkylation of substituent groups from the aromatic structures. Coke deposited on the catalyst during 10-second on-stream time is approximately three times as high with the aromatic fraction as with the nonaromatic fraction under similar conditions. The composition of the unconverted aromatic fraction, as shown in Figure 3, changes in a manner similar to the other fractions and is consistent with an increasing rate constant as molecular weight increases. The aromatic extract is unique, however, in that the CIS to C19 components increase from 1.5 to 7.9% during a 325-LHSV run, while for all other reactants these components diminish in concentration on passing through the reactor. This increase in high molecular weight hydrocarbons is indicative of condensation reactions which
l
Table VI contains data obtained from a 10-second and a 30-second run made with each of the various oil fractions a t 900°F and 1300 LHSV over the steamed R E H X catalyst. The instantaneous conversions at these two times on-stream and the accumulated coke on the catalyst during these two time intervals are recorded. As individual fractions, the nonaromatic fraction cracks at a faster rate than the aromatic fraction, and the activity declines less with increasing time on-stream than for either the aromatic fraction or the composite gas oil. The cyclo- and isoparaffin portion of the nonaromatics is more reactive for cracking than the normal paraffin portion. The hydrogenated gas oil is more reactive than the nonaromatic fraction, indicating that the structures of the aromatic components when hydrogenated to naphthenes are very reactive. Coke formation is highest with the aromatic extract and least with the nonaromatic fraction. The change's in the rate constant, k , for cracking during increasing time on-stream are compared in Figure 4 for the nonaromatic fraction, the aromatic fraction, and the composite gas oil. Rate constants are plotted as a function of time on log-log paper to show that decay in the rate constant fits reasonably well the much used exponential decay equation, k = k&". For the original gas oil, n = 0.3. The nonaromatics decay with n = 0.2 and the aromatics with n = 0.4. However, the calculated rate constants are dependent on space velocify for the aromatic concentrate. When the aromatics are cracked alone, the rate constant decreases as space velocity is lowered (increasing catalyst-oil ratio a t a given length of time on-stream). The condensation reaction of the aromatics Table VI. Reactivity and Coke Deposition from Gas Oil Fractions
(900" F, 1300 LHSV) Wt. Yo Conversion Charge
50P to 600" F gas oil Aromatic extract Nonaromatic fraction n-Paraffin extract Iso- and cycloparaffins Hydrogenated gas oil
% Carbon on Catalyst
10 sec
30 sec
10 sec
30 sec
18.4 15.5 20.3 15.2 24.6 28.9
10.8 10.1 16.6
1.2 2.3 0.74 0.76 1.1 1.4
1.6 3.0 1.2
...
19.2 16.9
...
... 2.0
LHSV I300
650
325
0
-
0
0
a
o
NON-AROMATICS AROMATICS GAS OIL
-
Y
6 00 IO
20
30 40 60 80 TIME ON STREAM (SECONDS)
100
I
Figure 4. f-" decay of catalyst activity Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 2, 1970 207
observed when passed in high concentration over the catalyst is influenced by space velocity; and, as a result, the extent of poisoning by condensation products is a function of space velocity only for the concentrated aromatic reactant. Coke concentration on catalyst a t the end of 10 seconds on-stream is 2.3% for the 1300-LHSV run compared to 3.1% for the 325-LHSV run. For less aromatic reactants, coke formation is essentially independent of space velocity, as shown for the composite gas oil in Table IV. Reactions of Fused-Ring Naphthenes
The product distribution from gas oil cracking is a combination of the three types of reactions occurring individually by the 45% paraffins, 33% naphthenes, and 21% aromatics composing the oil. Reactions involving individual paraffins, single-ring cycloparaffins, and alkyl benzenes were discussed in the previous paper (Nace, 1969b). While the dealkylation reactions of fused-ring aromatics would be like those of the alkyl benzenes, fused-ring naphthenes should be very reactive and produce a complex mixture of cracked products. Since very little has been published concerning reactions of fused-ring naphthenes, five specific pure hydrocarbons were investigated in regard to their cracking reactions over the steamed R E H X catalyst. These hydrocarbons are the single-ring 1,3,5triethylcyclohexane, the double-ring trimethyl Decalin, two triple-ring compounds, perhydrophenanthrene and perhydrofluorene, and perhydropyrene with four rings. Boiling points of these five hydrocarbons are within the range of a 500" to 600" F gas oil cut. Rate data and product distributions from the cracking of these five naphthenes are given in Table VII. I n the chromatographic analysis, Cj, C6, C;, and some Cs cyclic products can be distinguished from noncyclic components. The Cs+ noncyclic and cyclics could not be separately identified. In the cracking of the triethylcyclohexane and trimethyl Decalin, noncyclic products exceed cyclic products because of scission of the ring structure. The distribution of noncyclic components is very similar to the distribution of products from paraffin cracking-Le., largest concentrations of C3 and C4products. The tri-ring naphthenes perhydrophenanthrene and perhydrofluorene produce
much greater amounts of single-ring cyclic products than noncyclics, because of ring opening of the central ring. Since large concentrations of cyclopentanes are also identified from these reactants, the mechanism of ring scission and recyclization suggested (Nace, 1969b) for single-ring cycloparaffin cracking is again presumed to be taking place. Indications are that indanes and indenes are produced in abundance, even from perhydrophenanthrene. Perhydrofluorene, because of the presence of a five-member ring in the reactant, forms greater amounts of cyclopentane derivatives than perhydrophenanthrene. Perhydropyrene also converts to appreciable quantities of cyclopentanes. The rates of cracking of these fused-ring naphthenes are higher than those of paraffins of the same molecular weight, except for perhydropyrene. The cracking rate decreases, however, when the number of rings is increased frqm 2 to 4. This suggests that the larger molecules are more hindered in diffusing t o all of the active sites in the aluminosilicate catalyst. T o verify this assumption, four of the naphthenes were cracked over a noncrystalline silica-alumina catalyst. This catalyst (Nace, 1969a), cracks n-hexadecane with a rate constant 5/17 of that of the aluminosilicate rate constant after 2 minutes of catalyst decay a t 900°F. Table VI11 compares rate constants with the two catalysts for hydrocarbons with increasing number of rings per molecule. For silicaalumina, the rate constant increases with molecular weight for the four naphthenes. For the aluminosilicate catalyst, there is a sudden drop off in the rate of cracking when the number of rings increases from 2 to 3 and again from 3 to 4. The cracking rate of the triethylcyclohexane over the R E H X catalyst is 17 times greater than the rate over the SiAl catalyst, while for perhydropyrene the ratio of rates is only 2.4. Molecular size restrictions definitely limit rates of cracking in the case of the aluminosilicate catalyst. Inhibiting Effects of Fused-Ring Aromatic Components on Cracking Rates
Although fused-ring aromatics account for less than 10% of the 500" to 600°F gas oil fraction, their inhibiting effect on the catalyst was anticipated because of strong
Table VII. Cracking of Fused-Ring Naphthenes over REHX Catalyst
(900" F , 2 min on-stream) Triethylcyclohexane Hydrocarbon
LHSV Cracking conv., % Cracking rate const., k Wt. % C on catalyst Cracked product distribution Aromatic C; 8 Cyclo 5-member ring C5 Cyclo 6-member ring Cs Noncyclic CS L a
cs CS Ci Noncyclic + cyclic C r l l (including indane) Noncyclic + cyclic C12-16 (Decalins, Tetralins, and naphthalenes)
208
0,3, 5 )
Trimethyl Decalin
Perhydrophenanthrene
Perhydrofluorene
Perhydropyrene
1300 17.7 2370 2.0
1300 18.3 2420 0.7
650 14.6 953 1.o
1300 12.3 1615 1.1
650 9.2 513 1.6
5.8
g'8\ 9.8 9.7 21.7 16.9
19.6
60.1
2.4
10.6 12"\ 13.4
4.7 15.4 14.6
/
5.8
25.5
22.7 23'5\
44.6
4.1 8.9 5.2
1.3
46.2
4.5 24.0 3O.41
54.4
:::!\
5.5
5.8 6.1
0.5
1.1
20.8
0.6
5.0 37.1
/
14.5
19.5
22.0
13.8
30.1
0.0
0.0
5.1
4.3
8.2
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
adsorption on active sites. To determine the extent of inhibition, experiments were conducted whereby n-hexadecane and various concentrations of aromatics were cracked over the REHX catalyst. At a 10 mole % concentration, xylene present in the n-hexadecane had no inhibiting effect on n-hexadecane cracking, while naphthalene a t the same concentration reduced the rate constant for hexadecane cracking 28% a t 1 minute on-stream. Acenaphthene inhibited cracking activity to a much greater degree than naphthalene, although the amount of coke remaining on the catalyst after nitrogen purging following the cracking run was not much greater than when no aromatic was present in the hexadecane (Table IX). The rate constants for cracking of the n-hexadecane were calculated on the basis of the partial pressure of hexadecane in the reactant. Dilution of hexadecane with either Decalin or xylene produced rate constants calculated on the basis of the hexadecane present that were slightly higher than the rate constant for undiluted hexadecane. Using various concentrations of naphthalene in n-hexadecane, different levels of activity were reached a t 60 seconds on-stream. However, a t each concentration, the activity decay slope attained after 60 seconds was no greater than that of pure n-hexadecane. Carbon residues on the helium purged catalyst following a 60-second cracking run with 16% naphthlene in the charge are also no larger than with 10 or 5% naphthalene in the charge and only slightly greater than the coke produced by n-hexadecane alone during 60 seconds. If the inhibition effect depends on competitive adsorption equilibrium between hexadecane and aromatic molecules on active sites, the following equation derived from Langmuir isotherms of the two hydrocarbons applies: Table VIII. Cracking Rate Constants at 900’ F
(2 min on-stream instantaneous value) Ratio Reactant Hydrocarbon
/
~REHX
Si-AI
60
‘zHs
1000 (0.1W )
“C16H?4
‘ZH5
&
@
17
2370 (0.45X)
CH3
17 (1.4%)
140
‘ZH5 CH3 CH3
ks,,,
REHX
(2.onc)
190
2420 (0.27cC)
13 (0.7%)
205
953 (0.2%C)
210
4.7 (l.O%C)
513 (0.4‘7,C)
2.4 (1.670C)
Table IX. n-Hexadecane Cracking in Presence of Aromatics over REHX a t 900’ F, 60 Seconds On-Stream
(325 LHSV)
Charge
Pure n-hexadecane nClh + 10 mole 7 xylene + 10 mole ‘% Decalin + 5 mole % naphthalene + 10 mole 7 naphthalene + 16 mole “0 naphthalene + 2 mole % acenaphthene a
Wt. % Conv. n-C16
k,“ Moles/ Kg-Hr
25.1 24.0 24.1 19.1 14.3 12.3 11.0
Wt.
Yo C
on Catalyst
1.12 811 835 553 404 354 267
1.28 1.02 1.28 1.31 1.29 1.30
Calculated on basis of partial pressure of hexadecane charged.
t I
3.5
0
2 X
1.0
I
1
I
I
1
I
I
I
I
where k” is the cracking rate constant when pure hexadecane is used, k is the rate constant when an aromatic impurity reduces site availability for hexadecane, K and KA are the adsorption equilibrium constants for adsorption of n-hexadecane and aromatic hydrocarbon, respectively, and p and p A are the corresponding partial pressures of n-hexadecane and aromatic. Data for the three concentrations of naphthalene in n-hexadecane are plotted according to this linear equation in Figure 5. Attempts to demonstrate reversible adsorption of the aromatic inhibitor by helium purging of the catalyst followed by a pure n-hexadecane cracking run were unsuccessful. The activity level a t the end of a 10 mole 5% naphthalene-inhibited n-hexadecane run was raised 20% by a 2-minute helium purge and 60 seconds of pure hexadecane cracking. Even this activity gain may not come from desorption of naphthalene from active sites, since a similar increase in activity was obtained when a pure n-hexadecane run was stopped, the catalyst purged with helium, and pure n-hexadecane again charged to the reactor. From these experiments it must be concluded that much of the naphthalene inhibiting the catalyst’s activity is not easily desorbed from the surface. Perhaps naphthalene competes as a “coke former” with other cokeforming molecules-e.g., olefin products of cracking. If the naphthalene coked site were less active as a cracking site than a site coked by a nonaromatic coke precursor, activity differences for equally coked catalyst with pure and aromatic contaminated n-hexadecane could be explained. The ratio of “aromatic coke” to “nonaromatic coke” would depend on the partial pressure of naphthalene present in the n-hexadecane reactant and the Figure 4 plot would be justified. literature Cited
Blanding, F . H., Ind. Eng. Chem. 45, 1186 (1953). Nace, D. M., IND. ENG. CHEM. PROD.RES. DEVELOP. 8, 24 (1969a). Nace, D. M., IND. ENG. CHEM. PROD.RES. DEVELOP. 8, 31 (196913). RECEIVED for review September 30, 1969 ACCEPTED January 29, 1970 Division of Petroleum Chemistry, 159th Meeting, ACS, Houston, Tex., February 1970. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970 209
