U.S. patent number 3,671,419 [Application Number 05/014,908] was granted by the patent office on 1972-06-20 for upgrading of crude oil by combination processing.
This patent grant is currently assigned to Mobil Oil Corporation. Invention is credited to Henry R. Ireland, Paul W. Snyder, Jr..
United States Patent |
3,671,419 |
Ireland , et al. |
June 20, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
UPGRADING OF CRUDE OIL BY COMBINATION PROCESSING
Abstract
Efficient utilization of hydrogen and catalyst compositions is
achieved in a petroleum refinery operation by a specific
combination of unit operations which tend to optimization of
premium products. A significant feature of this combination is
catalytic hydrogenation of a fraction boiling above the gasoline
range up to 1,100.degree. F. followed by cutting or separating the
hydrogenated product to form a high boiling fraction boiling above
about 700.degree. F. as charge to a crystalline aluminosilicate
catalytic cracking operation with the low boiling fraction thereof
boiling below about 700.degree. F. being charge to catalytic
hydrocracking.
Inventors: |
Ireland; Henry R. (West
Deptford Township, Gloucester County, NJ), Snyder, Jr.; Paul
W. (Pitman, NJ) |
Assignee: |
Mobil Oil Corporation
(N/A)
|
Family
ID: |
21768489 |
Appl.
No.: |
05/014,908 |
Filed: |
February 27, 1970 |
Current U.S.
Class: |
208/57; 208/93;
208/92 |
Current CPC
Class: |
C10G
69/00 (20130101); C10G 69/14 (20130101) |
Current International
Class: |
C10G
69/14 (20060101); C10G 69/00 (20060101); C10g
037/00 () |
Field of
Search: |
;208/57,92,93,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Levine; Herbert
Claims
We claim:
1. A method for optimizing a processing combination for upgrading a
hydrocarbon charge material which comprises selecting from a crude
oil source by distillation a hydrocarbon fraction boiling
throughout the range of from about 650.degree. F. to about
1,100.degree. F.,
hydrogenating said hydrocarbon fraction under conditions to reduce
nitrogen therein below about 500 ppm and hydrogenate polycyclic
condensed ring compounds,
separating the hydrogenated product of said hydrocarbon fraction to
obtain a fraction boiling up to about 700.degree. F. from a
hydrogenated fraction boiling above 700.degree. F.,
catalytically cracking said hydrogenated fraction boiling above
700.degree. F. by contact with a crystalline aluminosilicate
cracking catalyst under conditions to produce gasoline, a light
fuel oil product and an incompletely converted higher boiling
fraction, recycling the high boiling fraction to the process,
catalytically hydrocracking said hydrogenated fraction boiling
below about 700.degree. F. under conditions to obtain a naphtha
fraction, a jet fuel boiling range fraction and a higher boiling
insufficiently converted hydrocarbon fraction, and
recycling the insufficiently converted hydrocarbon fraction to the
process.
2. A method for refining a crude oil to useful products which
comprises
a. separating and recovering from said crude oil, a straight run
naphtha fraction, a light fuel oil fraction, a gas oil fraction
boiling throughout the range 650.degree. F. to about 1,100.degree.
F. and a heavy fraction boiling above 1,100.degree. F.,
b. coking said heavy fraction and recovering a naphtha fraction
separately from a heavier fraction boiling in the range of
400.degree. F. to about 900.degree. F.,
c. passing said gas oil fraction boiling throughout the range
650.degree. F. to 1,100.degree. F. in combination with said product
of coking boiling in the range of 400.degree. F. to about
900.degree. F. in combination with a liquid recycle fraction from
catalytic cracking to a catalytic hydrogenation step,
d. reducing nitrogen contaminates in the hydrocarbon charge in said
catalytic hydrogenation step to less than 500 ppm concurrently with
hydrogenating polycyclic condensed ring compounds existing in the
charge,
e. separating the effluent of said catalytic hydrogenation step
into a hydrogen rich fraction for recycle thereto, a hydrocarbon
fraction boiling up to about 700.degree. F. and a hydrocarbon
fraction boiling above about 700.degree. F.,
f. catalytically cracking said fraction of catalytic hydrogenation
boiling above about 700.degree. F. and separating the cracked
product effluent into a light products fraction, a gasoline
fraction, a light fuel oil and a heavier fraction boiling above
about 650.degree. F.,
g. catalytically hydrocracking said fraction of catalytic
hydrogenation boiling up to about 700.degree. F. and separating the
product effluent thereof into a light products fraction, a naphtha
fraction, a jet fuel fraction and a heavier recycle fraction
boiling above about 550.degree. F.,
h. reforming the light products fraction of catalytic cracking and
hydrocracking to form hydrogen for use in said catalytic
hydrogenation step and said hydrocracking step,
i. removing by hydrogenation sulfur and nitrogen contaminates from
said straight run naphtha fraction before passage to a catalytic
reforming step in admixture with naphtha obtained from the effluent
of said catalytic hydrocracking,
j. separating hydrogen rich gases from reformed naphtha obtained
from said catalytic reforming and employing hydrogen rich gases
thus obtained in said reforming step, in said straight run naphtha
hydrogenation step and as make up hydrogen passed to said catalytic
hydrogenation and hydrocracking steps.
3. A method for upgrading fractions of a crude oil boiling above
about 650.degree. F. which comprises
a. recovering from crude oil a hydrocarbon fraction boiling
throughout the range of 650.degree. F. to about 1,100.degree. F.
from a higher boiling fraction boiling above 1,100.degree. F.,
b. thermally converting said higher boiling fraction under
conditions to produce naphtha and a fuel oil fraction boiling from
about 400.degree. F. up to about 900.degree. F.,
c. combining said thermally produced fuel oil fraction with said
hydrocarbon fraction boiling throughout the range of 650.degree. F.
to about 1,100.degree. F.,
d. catalytically hydrogenating said combined fractions under
conditions to hydrogenate polycyclic aromatic compounds therein and
reduce nitrogen therein to less than 500 ppm,
e. separating the product effluent of said catalytic hydrogenation
into hydrocarbon fractions boiling up to about 700.degree. F. and a
fraction boiling above 700.degree. F.,
f. passing the hydrocarbon fraction boiling above about 700.degree.
F. to catalytic cracking and passing the fraction boiling up to
about 700.degree. F. to hydrocracking and
g. recovering from said cracking steps, light product material,
hydrocracked naphtha, catalytic gasoline, jet fuels, light fuel oil
and insufficiently converted recycle hydrocarbon materials.
4. The method of claim 3 wherein insufficiently converted recycle
hydrocarbons recovered from said catalytic cracking are combined
with the hydrocarbon charge to the catalytic hydrogenation
step.
5. The method of claim 3 wherein crystalline zeolites are a part of
the catalysts employed in each of the hydrocracking and catalytic
cracking steps.
6. The method of claim 3 wherein hydrogen required for said
catalytic hydrogenation and hydrocracking steps is obtained from
reforming straight run naphtha in combination with the naphtha
product of hydrocracking and conversion of light products recovered
from the cracking steps with hydrogen rich gases.
7. The method of claim 3 wherein light fuel oil product of the
catalytic cracking step is passed as a part of the charge to the
hydrocracking step.
Description
BACKGROUND OF THE INVENTION
The invention is concerned with the catalytic refining of crude
petroleum and fractions thereof for the production of liquid fuels.
It is adaptable to refining for both fuels and lubricants, in which
case portions of certain fractions may be diverted to lubricant
processing in the manner well known in the art; the principles of
this invention being applied to that portion of the refinery input
intended for fuels production.
The input to a petroleum refinery is crude petroleum constituted by
a complex mixture of literally thousands of diverse compounds of
carbon and hydrogen with minor amounts of sulfur, nitrogen and
metals. These compounds vary from very simple aliphatic compounds
such as short chain paraffins through naphthenes, aromatics,
heterocyclic and open chain compounds of sulfur and nitrogen to
fused ring compounds such as pyrene and the metal bearing
porphyrins. Each individual compound or group of like compounds
could be treated alone with considerable ease to produce the
products demanded by modern industrial and consumer equipment. The
processing of complex mixtures, however, is made necessary by the
economics of an industry in which current prices cannot be met by
expensive separation and individual processing of chemical
types.
The problems of refining are intensified to a considerable degree
by (1) the necessity for input of several different crude types to
each refinery in order to meet the output demands; and (2) the
unfortunate disparity between the hydrogen/carbon ratio of the
total liquid output (the "premium product slate") and that of the
composite of crude inputs. A typical fuels refinery will face a
slate of premium products (gasoline, kerosene, jet fuel, diesel
fuel and No. 2 burning oil) in which the hydrogen content is 11.0 -
15.0 wt. %. Typical crudes available in large supply have the
following hydrogen content. For example:
Crude Hydrogen (wt. %)
__________________________________________________________________________
Alaskan Crude (Cook Inlet) 12.89 California San Ardo 11.11 San
Joaquin Valley 11.75 11.50 av. Wilmington 11.64 Libyan 13.41
Mid-Continent 13.44 Kuwait 12.87 Oficina 12.67 West Texas 12.55
__________________________________________________________________________
The petroleum refiner faces a clear-cut alternative: reject carbon,
add and reject hydrogen in the refining system. Typically he does
some of each and this invention is concerned with a combination
operation in which some carbon is rejected as by formation of
petroleum coke, asphalt or heavy fuel very poor in hydrogen and
commanding a low price. Despite flexibility to accommodate some
carbon rejection, the invention also concerns itself with that
portion of a fuels refinery characterized by hydrogen addition.
Advantages are provided in management of the hydrogen in such
manner that this element essential to premium liquid fuels shall be
husbanded for the desired purpose. This conservation of assets
applies to both sources of hydrogen, viz., that present in the
refinery input and that manufactured from any suitable source,
including refinery by-products.
Efforts have been made to apportion the tasks of petroleum refining
among different types of equipment according to their respective
capabilities. Of interest in this respect are U.S. Pat. Nos.
3,172,842 and 3,245,900 to Paterson.
Many unit operations are known for input of hydrogen to the total
refinery system. Catalytic hydrodesulfurization, denitrogenation,
conversion of aromatics to saturated compounds and demetallization
all result in some input of hydrogen. Some is also rejected from
the system in removing impurities, e.g. sulfur and nitrogen are
withdrawn as hydrogen sulfide and ammonia, respectively.
Hydrocracking also introduces hydrogen to the system. A recently
introduced technique involves hydrogenation of charge to catalytic
cracking for purposes described in U.S. Pat. No. 3,413,212, P.B.
Weisz.
SUMMARY OF THE INVENTION
This invention relates to the cracking of hydrocarbons to produce
conversion products such as gasoline, jet fuels, light fuel oils
and hydrogen. More particularly the present invention is concerned
with the method for optimizing upon the combination of gasoline
producing or refining steps comprising particularly catalytic
cracking and hydrocracking operations. In accordance with the
method of this invention heavy hydrocarbon charge stock boiling
above about 700.degree. F. of acceptable nitrogen and polycyclic
aromatic levels are cracked with a crystalline alumino-silicate
cracking catalyst under catalyst cracking conditions with that
portion of the hydrocarbon charge stocks boiling below about
700.degree. F. being preferably subjected to elevated pressure
hydrocracking conditions in the presence of a catalyst composition
comprising a crystalline aluminosilicate cracking component. The
separation steps and cracking combination above identified is
enhanced to a considerable degree by hydrogenating the charge so as
to improve upon the hydrogen concentration of the heavy charge and
reduce the level of polycyclic aromatics therein as well as reduce
its nitrogen content below about 500 ppm of nitrogen. Thus we have
found that the hydrogen available from the crude input and from
generated hydrogen may be more selectively directed to premium
products by the specific combination of processing steps herein
described.
Despite the name "coke" applied to carbonaceous solids produced in
a refinery, these materials contain a small amount of hydrogen. The
solid product of delayed coking typically contains 4 wt. %
hydrogen, unimportant to its use as solid fuel. The "Coke"
deposited on catalysts in the refinery may contain 6-8 wt. %
hydrogen, a typical value for catalytic cracking operation. The
coke on catalyst is burned for regeneration of the catalyst,
converting its hydrogen to water which is discharged from the
process.
Light gaseous hydrocarbons are useful fuel but command a low price
in the United States. They require an inordinate share of the
available hydrogen. Methane is 25 wt. % hydrogen. Ethane contains
20 percent hydrogen by weight.
The invention is concerned with processing a 650.degree. to
1,100.degree. F. hydrocarbon fraction which has been separated from
lower and higher boiling portions of a crude charge and a
400.degree. to 900.degree. F. petroleum fraction obtained from
thermal and catalytic cracking. Processing of the 400.degree. to
1,100.degree. F. fraction is achieved by a combination of steps
which includes the catalytic hydrogenation thereof to remove
nitrogen constituents to an acceptable level below 500 ppm and more
usually to below 200 ppm. In addition the polycyclic aromatic found
in the charge are hydrogenated thereby reducing their deactivating
effect upon the catalyst subsequently contacted as by cracking; and
permit conversion of an otherwise very refractory stock. Following
hydrogenation, the hydrogenated hydrocarbon fraction is separated,
into at least two fractions, one boiling above and one below a
characteristic temperature selected from within the range of
650.degree. to about 700.degree. F. This separation will depend
upon specific factors peculiar to the nature of the crude, the
product slate and idiosyncracies of catalyst and equipment
employed. The heavier fraction that has its crackability improved
and nitrogen level reduced to a tolerable level by hydrogenation is
transferred to catalytic cracking which consumes no additional
hydrogen. The lighter fraction goes to catalytic hydrocracking,
requiring hydrogen, wherein the catalyst aging rate is improved by
processing the light fraction in the absence of the heavier
fraction. There are further benefits realized when the crystalline
aluminosilicate zeolite cracking catalysts are used by more
selectively converting the hydrogenated heavy fraction. This result
is attributed to their hydrogen transfer capability.
DESCRIPTION OF DRAWING
The concepts of the present invention are more conveniently
considered with reference to a particular embodiment in which those
concepts are utilized in a complete fuels refinery. The drawing
attached hereto is a diagrammatic representation (simplified flow
diagram) of the fuels refinery of this invention. It will be
understood by those skilled in the art that portions of certain
streams may be diverted for special purposes such as lube refining
without material effect on the fuels manufacture specifically
shown. For example, a portion of the vacuum tower bottoms may be
processed for bright stock. Suitable portions of other fractions
may be processed to manufacture neutral oils.
The processing flow arrangement represented by the drawing
illustrates one scheme for practising the method and concepts of
this invention. Some alternative flow arrangements are provided for
depending on the crude source, market demand, variations in crude,
specific characteristics of catalyst at the time in question and
other like factors.
Referring more particularly to the exemplary showing in the
drawing; a crude petroleum is supplied to fractionating equipment
11 by a line 10, as from crude storage adjacent the refinery. Crude
fractionation may be of any style suited to the purposes discussed
and is here shown as a two-stage operation including an atmospheric
tower 11 operating at enough pressure above atmospheric to overcome
downstream pressure drop prior to pumps on subsequent units, and a
vacuum tower 12 operating at a top pressure of about 2 pounds per
square inch absolute.
Overhead product from atmospheric tower 11 is constituted by light
products of the crude, if any be present, withdrawn by line 13 and
transferred to a gas plant, not shown, for recovery of components
desired for pressuring gasoline (butanes), liquified petroleum gas,
feed to gas reforming, or furnace fuel.
A naphtha stream is taken from tower 11 by line 14 to be processed
for motor gasoline in a catalytic reformer described hereinafter or
for other purposes such as manufacture of solvents or basic
chemicals, e.g. ethylene, aromatics. The end point of the naphtha
may vary between about 350.degree. F. and 400.degree. F. depending
on specific demands of the refinery. That naphtha end point
effectively determines the initial boiling point of the next higher
fraction such as a choice No. 2 fuel oil boiling up to about
650.degree. F. and removed from tower 11 by line 15. Generally this
fraction in line 15 will boil in the range of 400.degree. F. up to
650.degree. F.
Bottoms from atmospheric tower 11 is passed by line 16 to vacuum
tower 12 for distillation under reduced pressure imposed, for
example, by a barometric condenser, not shown, on the overhead
product withdrawn by line 17. The overhead product in line 17
generally will boil in the range of from about 650.degree. F. up to
about 1,100.degree. F. The overhead fraction in line 17 is normally
passed by line 18 to hydrogenation in zone 19. However, if the
hydrocarbon fraction in line 17 is sufficiently saturated or low in
metal it may be passed directly to catalytic cracking in zone 34
thus by-passing hydrogenation zone 19 and subsequent separation in
zones 28 and 31.
Bottoms from vacuum tower 12 and boiling above about 1,100.degree.
F. are transferred by line 20 to suitable processing equipment. In
a refinery processing for lubricants as well as fuels, a portion of
this stream may be diverted to deasphalting, solvent extraction and
dewaxing stages as known for such products. The stream in line 20
may also be used, in whole or part, for heavy fuel or charge to
asphalt manufacture. In the preferred embodiment shown here, the
heavy fraction from the vacuum tower 12 is passed to a coking unit
21 which is conveniently of the type known as delayed coking of
fluid coking. Products from the coker are solid fuel, withdrawn at
22 for use in furnaces, as electrode coke, etc., and a vaporized
fraction passing by line 23 to a fractionator 24 where light
products for the gas plant and naphtha for further processing are
withdrawn by lines 25 and 26, respectively. This coker naphtha may
be passed to preheat step 57. Bottoms from fractionator 24 pass by
line 27 to be blended with the stream in line 18 as feed to
catalytic hydrogenation, it being understood that portions thereof
may be diverted to other purposes, as is true of other streams
heretofore mentioned.
As the art well knows, severity of hydrotreating varies with
boiling point to achieve a desired degree of desulfurization,
denitrogenation and conversion of fused ring aromatics to partially
or fully hydrogenated compounds more amenable to cracking and
capable of acting as hydrogen donors. The higher the boiling point
of the charge, the greater the severity required for any desired
degree of effect by catalytic hydrogenation. This is explicable
from knowledge that the higher boiling fractions of petroleum
contain greater amounts of polycyclics and contain polycyclics of
greater complexity. Sulfur, nitrogen, metals and unsaturated carbon
atoms secreted within the web of fused rings are understandably
more resistant to effective attack by hydrogen, activation by
catalysts or both. The range of effective severity can be a full
order of magnitude. Naphthas and light gas oils can be effectively
hydrotreated at space velocities of 4-5 liquid volumes per volume
of catalyst per hour (LHSV) over a catalyst and at conditions of
temperature, pressure and hydrogen concentration such that a
residuum will require a space velocity of 0.5 LHSV. It has
therefore been customary to hydrotreat relatively narrow fractions
in order that conditions may be adjusted to the needs of a
relatively small variety of chemical groups.
We have now found that, for the purposes of our combined process,
that advantages may be achieved by hydrotreating the total
hydrocarbon charge (boiling 400.degree. F. to 1,100.degree. F.) and
particularly that portion to be supplied to catalytic cracking
(either or of Fluid) and hydrocracking. Hydrogenation and nitrogen
removal to specific levels (about 200 ppm) improve the catalytic
crackability of heavy gas oils and the lighter fraction is made
more suitable for charging to hydrocracking to reduce the catalyst
aging rate. The severity of treatment is dictated by the most
refractory, (most difficult to crack) portion of the charge as
might be expected. This effect may be due in part to hydrogen
donors and to the diluent effect of the lighter components in the
composite. Effective hydrotreating for purposes of catalytic
cracking and catalytic hydrocracking (not wholly coincident) are
served and improvement in economical overall use of hydrogen is
attained by practising the present combination of processing
steps.
The hydrocarbon charge passed to catalytic hydrogenation, boiling
as herein defined and vaporizable at about 2 psia is contacted in a
hydrogenation zone 19 with a suitable hydrogenation catalyst known
in the art in the presence of hydrogen under sufficient pressure to
achieve a limited desired degree of conversion. Conversion is
controlled during hydrogenation so that less than about 500 but
preferably less than 200 ppm of nitrogen remains in the
hydrogenated effluent. Molecular weight reduction up to about 40
percent of material boiling below 400.degree. F. can be sustained
with visible and with ultraviolet light absorbance of the
700.degree. F. fraction to maintain indication of less than 5 ppm
polycyclic aromatics therein.
In general, we may use any of the hydrotreating catalyst known to
the art for this purpose. These usually have moderately strong
hydrogenation/dehydrogenation activity functions in the form of
metals, oxides and sulfides of metals in Groups VI and VIII of the
Periodic Table. Cobalt molybdate on alumina and cobalt molybdate
silica zirconia are typical. The hydrotreating will be conducted at
a pressure in the range of 1,500 to 2,000 psig hydrogen partial
pressure at 700.degree. F. to 770.degree. F. and LHSV suitable for
reducing nitrogen in the hydrogenated effluent to less than 500 ppm
and preferably to at least 200 ppm.
We find that conjoint hydrogenation of the process fractions
boiling above naphtha, that is boiling between about 400.degree. F.
and preferably above 650.degree. F. and capable of vaporization
without severe cracking produced significant advantages for the
overall operation. The hydrogenation severity is substantially less
than that normally required for optimum hydrotreating of the
heavier portion, boiling upwards of 650.degree. F. The severity
will vary with differing crude sources and is best defined with
respect to the product flowing from catalytic hydrogenation. In
general, cracking is held to a low level -- less than about 20
percent conversion to materials boiling in the naphtha range or
lower; that is, below about 400.degree. F. Condensed ring aromatics
are desirably hydrogenated to such an extent that no more than one
aromatic ring remains, the final ring being the most difficult to
saturate and being desirably retained, if possible. The content of
multiple ring aromatics, as determined by absorption of
ultra-violet light and defined by Theory and Application of
Ultra-Violet Spectroscopy of H. H. Jaffe and Milton Orchin, John
Wiley & Sons, Published 1962, should not exceed about 5 ppm by
weight.
The effluent of catalytic hydrogenation unit 19 passes to a
relatively high pressure separator 28 from which hydrogen is taken
off for recycle through line 29 to mix with fresh make-up hydrogen
from line 30 as hydrogen feed to unit 19. The liquid hydrocarbons
from separator 28 are transferred to a fractionator 31 wherein they
are split into a lower boiling overhead fraction withdrawn as vapor
by line 32 and a higher boiling liquid bottoms fraction withdrawn
by line 33.
The "cut point" or temperature representing division between the
fractions taken from fractionator 31 vary within a relatively
narrow range and will generally be in the neighborhood of about
700.degree. F. Depending on such circumstances as the desired
product slate, character of the crude utilized, and aging rate of
the hydrocracking catalyst, the cut point may be as low as
600.degree. F, for example to maximize catalytic gasoline and save
reformer capacity or it may be raised to about 750.degree. F. so
that among other results, an increase in the jet fuel production
will be realized.
The higher boiling bottoms from fractionator 31 is transferred by
line 33 to catalytic cracking unit 34 which may be any of several
types known to the art. Good results are obtained using a rare
earth exchanged crystalline alumino-silicate in combination with a
silica-alumina matrix as the catalyst such as that described in
U.S. Pat. Nos. 3,140,249 and 3,140,253 for catalytic operation in
either a Thermofor Catalytic Cracking Unit (TCC) or a Fluid
Catalytic Cracking Unit (FCC). The FCC style cracking unit is
particularly advantageous and preferred for processing the very
heavy high end point fractions herein discussed.
Conditions of cracking will generally be set for obtaining
relatively high levels of conversion consistent with that which can
be readily achieved at relatively high gasoline make and quality
when the charge stock to the cracker is the high boiling
hydrocarbon fraction obtained from hydrogenation as defined in
accordance with this invention. In localities and at times of high
distillate fuel demand, the conversion level may be adjusted to
meet a different desired product slate and one which will provide a
high quality fuel in the combination to which the invention is
directed.
Hydrocarbon effluent obtained from the catalytic cracking step 34
passes by line 35 to a fractionator 36, from which several streams
are withdrawn. A light overhead portion, C.sub.3 and lighter
together with some butanes and butenes, is taken by line 37 to
blend with a similar product from the hydrocracker for utilization
in a manner presently to be described. Gasoline is withdrawn as an
intermediate product by line 38 for processing in the manner known
to the art by reforming, treating, stabilization and blending with
other components to manufacture finished motor fuel. A second
intermediate fraction or distillate fuel oil is removed from the
fractionator 36 by line 39. This fraction normally has a boiling
range in the interval 400.degree. F. to about 650.degree. F. and up
to about 700.degree. F., and provides domestic heating oils, diesel
fuels and the like after further contact in finishing steps well
known in the art. Depending on the relative demand for different
products, i.e. domestic heating oil versus jet fuel, a portion or
all of the light fuel at line 39 may be transferred to the
hydrocracking step 43 as indicated by broken line 40. Under some
circumstances a portion of the light oil fraction in line 39 may be
recycled with the 650.degree. to 900.degree. F. bottom fraction by
line 41 to the hydrogenation step 19.
The bottoms fraction from fractionator 36, which may include a part
or all of the fraction designated "light fuel oil" (as by suitable
adjustment of fractionator operation) may be returned or recycled
by line 41 and 42 as charge for the catalytic cracker 39. For some
types of operation, the overall efficiency of the system is
considerably improved by hydrogenation of this cycle stock before
charging to catalytic cracking. It is particularly amenable to
effect joint hydrogenation of this recycle charge with fresh feed
in reactor 19 as shown in the drawing as an alternative to direct
recycle to the catalytic cracker by line 42. The preferred
catalytic cracking operation of this invention processes recycle
hydrocarbon material which has passed through hydrogenation reactor
19. This provides a further opportunity for interchange between
components for cracking with and without hydrogenation, relatively
lighter compounds being thereafter diverted to hydrocracking
relatively free of fused ring aromatics and multiple ring compounds
which generally have a high boiling point.
Turning now to the hydrocracking unit indicated generally at 43,
the light fraction removed from splitter 31 by line 32 and boiling
below about 700.degree. F. is converted in the presence of hydrogen
under elevated pressure hydrocracking conditions in the presence of
a catalyst characterized by an active crystalline aluminosilicate
cracking component and by a hydrogenation component which will
include a Group VIII metal as metal, oxide or sulfide. Preferably,
the crystalline cracking component is embedded in a matrix such as
silica-alumina which also has cracking activity. The hydrogenation
component may be incorporated in either the crystalline or the
amorphous cracking component, but preferably it is in both.
Typical operating conditions for the hydrocracking step discussed
herein include a temperature selected from within the range of
500.degree. F. up to about 800.degree. F., a pressure selected from
within the range of 1,000 to 2,000 and preferably from 1,200 psig
up to about 1,600 psig hydrogen pressure, a LHSV in the range of
from about 0.5 up to about 1.0 (volumes of fresh feed per volume of
catalyst per hour). Unconverted liquid product and boiling
generally in the range of from about 550.degree. F. up to about 700
F. is recycled to the hydrocracking step. The recycle to fresh feed
ratio is generally maintained at about 0.60. Hydrogen recycle ratio
may be in the range of from about 1,000 to about 10,000 SCF/B, it
being preferred to maintain hydrogen recycle rates in the range of
3,000 to 7,000 SCF/B.
In order that one may more fully appreciate the improved method and
combination of processing steps herein defined, the following table
of data is presented. ##SPC1##
It will be observed that high yields of C.sub.5 .sup.+ gasoline are
produced by the combination to which the present invention is
particularly directed and significant improvements in product
yields are realized from the hydrogenated charge passed to
catalytic cracking over that obtained with raw charge not
previously hydrogenated. It is to be further observed that the D9Z
catalyst provided a better product distribution than either D7 type
catalyst or an amorphous silica-alumina cracking catalyst.
The D9Z type of catalyst referred to in the above table of data was
a cracking catalyst having a composition comprising 5 percent REY
dispersed in a cracking base comprising 58 percent silica, 2
percent zirconia and 40 percent clay. The D7 type of catalyst, on
the other hand, comprised 5 percent REY dispersed in a
silica-alumina matrix comprising 13% Al.sub.2 O.sub.3 and 87%
SiO.sub.2.
Hydrocracking step 43 may employ one or more reactor vessels known
in the prior art and comprising a plurality of catalyst beds which
will be suitable for handling the exothermic reactions to be
encountered therein. The reaction products are removed and passed
by line 44 to separator 45 wherein a hydrogen rich recycle gas is
separated from hydrocarbon product material. The separated hydrogen
rich gas is recycled to the hydrocracking step by line 46. Hydrogen
rich make up gas obtained from reforming may be added to the gas in
line 46 by line 47. The hydrocarbon product separated in separator
45 is then passed by line 48 to a fractionator 49. In fractionator
49, the product of hydrocracking is separated to permit recovery of
an overhead light product fraction withdrawn by line 50, a naphtha
fraction withdrawn by line 51 and a jet fuel fraction boiling in
the range of from about 400.degree. F. to about 550.degree. F.
withdrawn by line 52. Insufficiently converted hydrocarbon material
boiling above about 550.degree. F. is withdrawn from the lower
portion of fractionator 49 by line 53 for recycle to the
hydrocracking step. Under some circumstances depending on the
source of the hydrocarbon materials being processed, the liquid
recycle fraction in line 53 may be passed to the hydrogenation step
19 by a line not shown.
In the processing combination shown, light product gases in line 37
obtained from catalytic cracking and light product gases in line 50
obtained from hydrocracking are combined and passed to a gas
reforming step 55 by line 54. In gas reforming step 55, conditions
are selected to convert the light gases passed thereto into
hydrogen or at least a hydrogen rich gaseous stream. Such an
operation is known in the prior art. Hydrogen thus produced is
recovered and passed by line 30 and line 47 to catalytic
hydrogenation and hydrocracking discussed before.
The naphtha product of hydrocracking removed from fractionator 49
by line 51 is thereafter passed to a catalytic reforming step 56
wherein it is subjected to reforming conditions to improve upon its
octane rating in a manner well known in the prior art. In addition
to the above, straight run naphtha separated from fractionator 11
is passed by line 14 to hydrogen pretreating unit 57 for removal of
sulfur and nitrogen therefrom. The thus treated naphtha is then
passed by line 58 to separator 59 wherein a recycle gas stream is
separated from the naphtha pretreated with hydrogen in step 57. The
recycle gas is passed by lines 60 and 61 as recycle to step 57.
Pretreated naphtha is passed by line 62 to catalytic reforming step
56. The reformed naphtha is passed by line 64 to separator 65
wherein a reformate or gasoline product is separated from gaseous
components comprising hydrogen. Separated hydrogen is recovered and
passed by lines 66, 63 and 67 to pretreat step 57 or by line 66 and
68 to be mixed with hydrogen rich gas in line 30 or by line 66, 63
and 69 as gas recycle to catalytic reforming step 56.
Under some circumstances when processing a paraffin rich charge low
in polycyclic aromatics and containing less than about 500 ppm
N.sub.2, the material separated in tower 12 and boiling in the
range of 650.degree. F. up to 1,100.degree. F. may be passed by
lines 17 and 18a directly to catalytic cracking. However, if the
charge is low in nitrogen but high in polycyclic aromatics, then it
needs to pass through the hydrogenation step 19. Also a charge
containing nitrogen in an amount which is usually more than about
200 ppm of nitrogen will be passed through the catalytic
hydrogenation step 19 described above before further
processing.
The combination of processing steps thus described and the example
of data presented in table provide support for the concept of this
invention and attest to the unexpected results obtained by such a
processing arrangement.
Having thus provided a general discussion of the concept of this
invention and presented specific examples in support thereof, it is
to be understood that no undue restrictions are to be imposed by
reasons thereof except as defined by the appended claims.
* * * * *