Addition of olefins to cat cracker feed to modify product selectivity and quality

A process for increasing the octane rating of gasoline and decreasing the quantity of coke produced by a catalytic cracking process by adding C.sub.2 to C.sub.6 linear olefins to the feed to the reactor with a zeolite containing cracking catalyst. The olefins may be added separately or mixed with the gas oil feed just before the oil preheat section ahead of the reactor.

BACKGROUND OF THE INVENTION 
In the catalytic cracking of gas oil, gasoline quality is currently 
controlled by primarily varying the catalyst activity and reactor 
operating conditions. Gasoline of relatively good octane quality, i.e. 
having a research clear octane rating of 91 to 93 can be made using these 
techniques. 
The use of varying catalytic systems to improve the octane quality of the 
gasoline recovered from the cracking of hydrocarbons is disclosed in U.S. 
Pat. Nos. 3,788,977 to Dolbear et al., 3,830,725 to Dolbear et al. and 
3,835,032 to Dolbear et al. These patents disclose the use of zeolite 
metals impregnated into or exchanged into the zeolite component of 
zeolite-promoted cracking catalysts to improve the octane content of the 
gasoline recovered from the cracking process. It is generally known that 
hydrocarbon cracking catalysts which are promoted with stabilized 
zeolites, particularly ion exchanged synthetic faujasite, are capable of 
producing high yields of gasoline fractions from petroleum feedstocks such 
as gas oil. These cracked gasoline fractions are subsequently combined 
with octane enhancing additives such as tetraethyl lead to produce high 
octane motor fuel. 
Recent emphasis on air pollution control has dictated a need for removing 
metal-type octane enhancing additives from commercial gasolines. It is 
generally necessary for the refiner to use a blend of petroleum gasoline 
fractions which have an inherently high octane rating to produce non-lead 
gasoline of sufficient octane rating for use in modern automobile engines. 
Highly aromatic gasoline fractions are of particular use to the refiner. 
Unfortunately, however, the gasoline fractions produced by the catalytic 
cracking of gas oil using normal amorphous and crystalline zeolite type 
cracking catalysts are of relatively low aromatic content. 
It has been reported in the literature [Van Hook, W. A. and Emmett, P. H.: 
Journal of Am. Chem. Society, 84, p. 4410 (1962)] that olefins react in a 
cracking atmosphere to polymerize, alkylate and/or cyclize to yield higher 
molecular weight hydrocarbons and even aromatics through a dehydrogenation 
step. 
BRIEF DESCRIPTION OF THE INVENTION 
We have found that if linear olefins in the C.sub.2 to C.sub.6 range are 
added to the feedstock of catalytic cracking units these olefins act to 
produce hydrocarbons in the gasoline range which are of high octane 
quality and improve the quality of the gasoline produced, and in addition 
the addition of olefins reduces the quantity of coke produced during the 
cracking reaction. 
DETAILED DESCRIPTION OF THE INVENTION 
The C.sub.2 to C.sub.6 olefins such as ethylene, propylene, butene and 
isobutene are added to the gas oil feed. The olefins are added separately 
and mixed with the gas oil feed just before the oil preheat section ahead 
of the reactor. The olefins are added in a concentration of about 5 to 35 
weight percent of the total feed. The feed to these units is a 
conventional gas oil having a boiling point of from 600.degree. to 
975.degree. F. Improved aromatic content, as evidenced by both the lower 
aniline point numbers (aniline point measurements decrease as aromatic 
content increases) and generally higher aromatic concentrations as 
measured by ASTM D-1319, result from operating the units at temperatures 
from 920.degree. to about 950.degree. F. and with catalyst to oil ratios 
of between 2 and 10 and preferably of between 2 to 4.5 pounds of catalyst 
per pound of oil, and a weight hourly space velocity of 20 to 60. The 
catalysts used are typically zeolite promoted cracking catalysts on a 
silica-alumina base. The bases may also contain substantial quantites of 
clay. 
In performing a particularly preferred embodiment of our invention, the 
olefin is added to the gas oil feedstock in amounts which decrease the 
coke yield to a desireable level. In many commercial catalytic cracking 
operations the catalytic cracking process produces 6 to 8% by weight coke 
based on fresh feed. We have found that by adding olefins in amounts of 
about 5 to 35 weight percent of fresh feed, the coke yield may be reduced 
by up to about 50%, i.e. coke yields are decreased from a non-olefin 
addition level of about 6 to 8% down to a level of about 3 to 4%. 
Accordingly, by use of our process the refiner may add olefins in the 
amount required to obtain the coke yield necessary to maintain proper heat 
balance for the operation while at the same time minimizing the yield of 
non-productive coke. In addition, the added olefin results in the 
production of gasoline fractions of increased octane rating.

Our invention is illustrated by the following specific, but non-limiting, 
examples. 
EXAMPLE 1 
In this example, propylene was added to the gas oil feed. Propylene was 
mixed with the gas oil feed just before the oil preheat section ahead of 
the reactor. The catalyst was a synthetic SiO.sub.2 /Al.sub.2 O.sub.3 gel 
matrix (30% Al.sub.2 O.sub.3) promoted with 35 weight percent of a rare 
earth exchanged Y-type zeolite. The catalyst was deactivated by steaming 
at 1520.degree. F. in a 20 weight percent steam in the air mixture for 12 
hours prior to use in the reactor. 
The reactor was operated at a temperature of 920.degree. F., a catalyst to 
oil ratio of 2 and a weight hourly space velocity of 60. A typical West 
Texas heavy gas oil catalytic cracker feed was used as the base feedstock 
for these tests. The yield from the mixture of West Texas Gas Oil 
containing propylene and West Texas Gas Oil alone were compared. The data 
is set out in Table I below. 
TABLE I 
______________________________________ 
Conversion V % 73.0(68.5).sup.1 
69.5 
Hydrogen W % FF 0.009 0.014 
C.sub.1 + C.sub.2 
fresh 0.69 0.77 
feed 
Total C.sub.3 's 
V % 17.6 5.5 
C.sub.3.sup..dbd. 
" 15.7 4.2 
Total C.sub.4 
V % FF 8.2 7.7 
C.sub.4.sup..dbd. 
" 2.2 2.8 
iC.sub.4 
" 4.6 3.9 
C.sub.5.sup.+ Gasoline 
V % FF 55.5(63.5).sup.1 
66.0 
C.sub.5.sup.+ Gaso./Conv. 
0.93.sup.1 0.94 
Octane No. 
F-1 82.8 82.4 
F-1 + 3cc TEL 
92.9 92.5 
F-2 72.8 72.5 
F-2 + 3cc TEL 82.2 82.0 
Gravity .degree. API 55.6 56.3 
Aniline Pt. 
.degree. F. 95 102 
Bromine No. 38 38 
Paraffin V % 58.8 60.7 
Olefin V % 8.3 11.1 
Aromatic V % 32.9 28.3 
Light Cycle Oil 
V % FF 10.3 11.2 
Gravity .degree. API 21.8 22.0 
Aniline Pt. 
.degree. F. 75 93 
640.degree. F..sup.+ Residue 
Gravity .degree. API 13.9 13.4 
Aniline Pt. 
.degree. F. 155 165 
Coke W % FF 1.2 2.4 
Total C.sub.3.sup..dbd. Added 
W % 9.0 -- 
gms 70 -- 
V % of 15.5 -- 
total feed 
______________________________________ 
.sup.1 Based on conversion of only the West Texas Gas Oil portion of tota 
feed. 
It is apparent from the aniline point of the gasoline and the increase in 
aromatic content, based on ASTM D-1319 analysis, that the octane quality 
of the gasoline was improved. The increase in gasoline aromatic content is 
interpreted to be the result of propylene polymerization and cyclization 
into a C.sub.6 naphthene followed by dehydrogenation to benzene or heavier 
aromatics. To some extent the lower coke yields, higher C.sub.4 olefin 
yields, and higher aromatic content of the cycle oils (light cycle oil 
plus 640.degree. F. residue) may be the result of the increased velocity 
through the reactor effected by increasing the feed volume with the 
propylene added. However, the increased velocity does not fully account 
for the 50% reduction in coke yield, i.e. 1.2 vs. 2.4%. 
EXAMPLE 2 
In this example the catalyst oil ratio was increased to 4.5 and the weight 
hourly space velocity was decreased to 35. The catalyst was the same as 
the catalyst used in Example 1 and it was pretreated in the same manner 
prior to use. The data collected in this run is set out in Table 2 below. 
TABLE II 
______________________________________ 
Conversion V % 84.5(80.0).sup. 1 
81.0 
Hydrogen W % FF 0.012 0.017 
C.sub.1 + C.sub.2 
fresh 0.95 1.16 
feed 
Total C.sub.3 's 
V % 22.9 8.9 
C.sub.3.sup..dbd. 
" 16.9 5.9 
Total C.sub.4 
V % FF 14.6 14.4 
C.sub.4.sup..dbd. 
" 5.6 4.4 
iC.sub.4 
" 7.3 8.4 
C.sub.5.sup.+ Gasoline 
V % FF55.0(68.5).sup.1 
67.5 
C.sub.5.sup.+ Gaso./Conv. 
0.85.sup.1 0.83 
Octane No. 
F-1 85.0 84.3 
F-1 + 3cc TEL 93.0 93.6 
F-2 75.0 74.2 
F-2 + 3cc TEL 84.4 83.6 
Gravity .degree. API 54.9 57.6 
Aniline Pt. 
.degree. F. 89 99 
Bromine No. 28 25 
Paraffin V % 62.8 64.8 
Olefin V % 8.4 2.8 
Aromatic V % 28.8 32.4 
Light Cycle Oil 
V% FF 7.1 8.4 
Gravity .degree. API 14.8 15.9 
Aniline Pt. 
.degree. F. 30 42 
640.degree. F..sup.+ Residue 
Gravity .degree. API 3.8 5.5 
Aniline Pt. 
.degree. F. opaque opaque 
Coke W % FF 3.0 6.1 
Total C.sub.3.sup..dbd. Added 
W % 14.5 -- 
ams 75 -- 
V % of 24.9 -- 
total feed 
______________________________________ 
.sup.1 Based on conversion of only the West Texas Gas Oil portion of tota 
feed. 
It is apparent that increasing the catalyst oil ratio and decreasing the 
space velocity caused a 10 point decrease in the aniline point of the 
gasoline product and in the light cycle oil product. As in Example 1 the 
coke was decreased substantially in this run. 
EXAMPLE 3 
In this example a catalyst consisting of 30% clay, 15.6% rare earth 
exchanged faujasite and 54.4% synthetic amorphous silica alumina cracking 
catalyst having an alumina content of about 30% was used as the catalyst. 
The unit was operated at a temperature of 920.degree. F., a catalyst oil 
ratio of 4 and a weight hourly space velocity of 20. The cracking 
characteristics of a West Texas Gas Oil containing 25.1% of propylene were 
compared with the cracking characteristics of a West Texas Gas Oil without 
the addition of propylene. The data collected is set out in Table III 
below. 
TABLE III 
______________________________________ 
Conversion V % 86.0(81.0).sup.1% 
82.0 
Hydrogen W % FF 0.019 0.02 
C.sub.1 + C.sub.2 
fresh 1.36 1.51 
feed 
Total C.sub.3 's 
V % 26.6 9.3 
C.sub.3.sup..dbd. 
41 18.4 7.5 
Total C.sub.4 
V % FF 13.2 12.0 
C.sub.4.sup..dbd. 
" 5.0 4.0 
iC.sub.4 
" 6.9 6.8 
C.sub.5.sup.+ Gasoline 
V % FF 53.0(71.0).sup.1 
70.5 
C.sub.5.sup.+ Gaso./Conv. 
0.87.sup.1 0.85 
Octane No. 
F-1 89.6 88.2 
F-1 + 3cc TEL 96.5 95.9 
F-2 77.6 76.8 
F-2 + 3cc TEL 84.6 84.0 
Gravity .degree. API 53.6 55.9 
Aniline Pt. 
.degree. F. 80 90 
Bromine No. 34 30 
Paraffin V % 51.8 59.6 
Olefin V % 9.3 6.6 
Aromatic V % 38.9 33.8 
Light Cycle Oil 
V % FF 5.8 7.1 
Gravity .degree. API 16.7 12.3 
Aniline Pt. 
.degree. F. 46 34 
640.degree. F..sup.+ Residue 
Gravity .degree. API 4.2 5.9 
Aniline Pt. 
.degree. F. 125 135 
Coke W % FF 2.4 4.9 
Total C.sub.3.sup..dbd. Added 
W % 14.7 -- 
gms 125 -- 
V % of 25.1 -- 
total feed 
______________________________________ 
.sup.1 Based on conversion of only the West Texas Gas Oil portion of tota 
feed. 
It is apparent from these data that the use of a different catalyst gave 
satisfactory results. The aniline point of the gasoline decreased by 10 
points and the coke make decreased by about 50% as in the runs described 
in Example 1 and 2. Additionally, gasoline octanes (Research and Motor 
clear (no lead) were up substantially (1.4 and 0.8, respectively) as a 
result of increased aromatic content due to propylene, cyclization and 
dehydrogenation.