Note: Descriptions are shown in the official language in which they were submitted.
1062644
The field of art to which this invention pertains
is hydrocarbon processing. It particularly relates to the
recovery of desired liquid and vaporous constituents from a
hydrocarbonaceous liquid-vapor feed stream. Specifically,
this invention relates to the recovery of LPG and stabilized
gasoline in an improved separation process. ; -
Hydrocarbons which are vaporous at normal condi- ;-
; tions are employed in the synthesis of many organic com-
pounds. Ethylene, for example, is in demand as a starting
: ! lo material in the synthesis of alcohols and synthetic rubber.
~- Propylene and butylenes are in particularly great demand
for plastics manufacture and for conversion to high octane
:1 . . .
motor fuel blending components by polymerization and alkyl-
' ation processes.
Sburces of these vaporous hydrocarbons are petro-
j~ leum cracking and conversion processes such as thermal crack-
; ~ ing, catalytic cracking, reforming, hydrocracking, etc. The
l chemical reactions occurring in these processes produce com-
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mercially desirable quantities of vaporous hydrocarbons,
~20 and because of their utility it is desirable to recovex them.~ ~ .
in as high a concentration as possible. For this reason,
separation processes are commonly used to concentrate and
recover these hydrocarbons.
~;~ Separation processes currently in use are com-
.
~; 25 prised of three major sections or zones: absorption, strip-
ping and fractionation. In essencer these zones serve, re-
- spectively, to absorb all but the lightest gaseous compo-
~¦ ~ nents, strip dissolved gases from the absorbed components,
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;i3l and fractionate the absorbed components into various product
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106;2644
streams. ~n outstanding feature of these processes is that
usually the heavier components in the feed to the process
are suitable for use as an absorber oil in the ab~oxption
zone. A portion of the product stream containing these com-
ponents is recycled back to the absorption zone. I have
discovered an improvement which significantly improves the
efficiency of such hydrocarbon separation processes.
It is an object of this invention to provide an
improved method for the separation of hydrocarbons.
It is another object of this invention to provide
a method for separating a liquid-vapor hydrocarbonaceous ,
mixture into normally vaporous products and normally liq-
uid products in a more efficacious manner. In one embodi-
ment, my invention affords an improvement in a process for
the recovery of selected hydrocarbon liquid and vapor con-
`, stituents from a feed stream containing those constituents
wherein (i) the feed stream contacts a lean oil in an ab~
sorption zone; (ii) a rich oil from the absorption zone
passes to a stripping zone; (iii) a stripped oil from the
stripping zone passes to a fractionation zone; (iv) a por-
tion of fractionation zone bottoms is returned to the ab-
sorption zone as said lean oil, which improvement comprises
returning individually to said absorption zone a portion of
,
; said stripped oil and a portion of said fractionation zone
bottoms as said lean oil.
The present invention involves a process for the
recovery of normally vaporous and normally liquid hydro-
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carbonaceous components from a composite stream containing
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these components, such as an effluent stream from a hydro-
carbon conversion zone.
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Lean oils absorb normally vaporous components
from the composite stream in an absorption zone. Resulting
rich oil passes to a stripping zone where light qases dis-
solved in the rich oiL are removed and a stripped oil is
S produced. A first portion of the stripped oil is returned
to the absorption zone as a lower lean oil; the remaining
portion of stripped oil passes to a fractionation zone.
The fractionation zone provides a liquefied hydrocarbon
vapor product and a hydrocarbon liquid product. A portion
of the hydrocarbon liquid product is returned to the ab-
sorption zone as an upper lean oil. The provision and jux-
taposition of the upper and lower lean oils improves re-
covery of light feed components.
An embodiment of the present invention is illus-
trated in the attached drawing. Only such details are in-
r cluded as are necessary for a clear understanding of my in-
vention, and no intention is thereby made to unduly limit
its scope. Unnecessary items such as certain process streams,
valves, pumps, instrumentation and other equipment have been
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omitted for the sake of simplicity.
Referring now to the separation process shown in
the drawing, a feed stream comprising hydrogen, normally
~ vaporous hydrocarbons such as methane, ethane, propane,
: etc., and normally liquid hydrocarbons such as pentane, hex-
ane, etc. enters absorption zone 2 through conduit 1. A
first lean oil enters the top of absorption zone 2 in con-
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duit 9. A second lean oil enters absorption zone 2 below
the entry point of the first lean oil. These lean oils
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I pass downwardly through absorption zone 2, absorbing at
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106Z64~
least a portion of the vaporous hydrocarbon components of
the feed. Gases not absorbed by the lean oil exit absorp-
tion zone 2 in conduit 3. A rich oil stream, containing
liquid feed components, lean oils and absorbed gaseou and
vaporous feed components, exits absorption zone 2 in con-
duit 4 and passes to stripping zone 5. Absorbed gases are
removed from the rich oil in stripping zone 5 and pass in
conduit 6 to conduit 1, intermixing therein with the feed.
Astripped oil stream is withdrawn from stripping zone 5
in conduit 7. A portion of stripped oil exits conduit 7
in conduit 8 and passes to absorption zone 2 as the afore-
said second lean oil. The remaining portion of stripped
oil exits conduit 7 in conduit 10 and passes to fractiona-
tion zone 11 where this portion of stripped oil is separat-
ed into a liquefied hydrocarbon vapor product and a hydro-
carbon liquid. The liquefied hydrocarbon vapor product is
withdrawn ~rom fractionation zone 11 and exits the process
in conduit 14. The hydrocarbon liquid exits fractionation
zone 11 in conduit 12. A portion of the hydrocarbon liquid
: 20 exits conduit 12 in conduit 9, passing as aforesaid first
lean oil to absorption zone 2. The remaining portion of
the hydrocarbon liquid is withdrawn from conduit 12 in con-
duit 13 and exits the process as a hydrocarbon liquid product.
~; The refining of petroleum involves numerous pro-cesses such as crude oil distillation, catalytic reforming
of naphtha, catalytic cracking o~ residual oils, etc. These
. processes are well known ~o those skilled in the art andneed not be discussed in great detail herein. However, a
characteristic of these processes and many others to be
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106Z644 b
fouhd in petroleum refineries and hydrocarbon processing
plants is the production of components which are broadly re-
ferred to herein as "normally vaporous hydrocarbons" and ~ `
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"normally liquid hydrocarbons." Normally vaporous hydro-
carbons are hydrocarbons which at "normal" conditions of
, temperature and pressure exist in the vapor state. Condi-
; tions referred to in the art as normal conditions are a
pressùre of 1 atmosphere and a temperature of 60F. When
~, normally vaporous hydrocarbons are transformed from the
vapor state to the liquid state, they are said to become
liquefied hydrocarbon vapors. Normally liquid hydrocarbons
are those which exist in the liquid state at normal condi- ~
-~ tions. The present invention broadly provides a method for ; ;
separating the effluent from any petroleum or hydrocarbon
~; 15 re~ining or conversion process which contains the types of
Z components which are referred to as normally vaporous and
normally liquid hydrocarbons.
~j~ ; For illustrative purposes, the present invention
Z will be described with reference to the effluent from a
fluid catalytic cracking process. Normally liquid and nor-
mally vaporous hydrocarbons from a fluid catalytic cracking
"
unit conventionally leave in the liquid and vapor streams
from the fractionation zone of the unit. These streams are
Z sent to a separation process for recovery of a stabilized
liquid hydrocarbon product, or stabilized gasoline, lique- ;
fied hydrocarbon vapors and light gases. The stabilized -~
'~ ~ gasoliné product principally comprises hydrocarbons having
` 5 or more carbon atoms per molecule. This gasoline is re-
ferred to as stabilized because it does not contain light
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106Z6~
material~ such as ethane or propane or hydrogen which would
contribute effervescence to the gasoline. The liquefied
hydrocarbon vapor from the separation process typically com-
prises hydrocarbons having from 2 to 4 carbon atoms per
molecule and includes such compounds as propane, butylenes
and the like. Liquefied hydrocarbon vapor is commonly re-
ferred to in the art as liquefied petroleum gas, or LPG.
This product may be further processed in downstream frac-
tionation facilities if it is desired to produce further
purified hydrocarbon product streams such as, for example,
a stream substantially comprising hydrocarbons of only 3
carbon atoms. The light gas stream from the separation
process commonly contains compounds such as methane and
hydrogen. These liqht components are valuable primarily
as fuel and are conducted from the separation process to
a fuel system.
In the embodiment of the separation process of
my invention shown in the attached drawing an admixture of
normally liquid hydrocarbons and normally vaporous hydxo-
carbons and hydrogen enters near the bottom of absorption
,' zone 2 through conduit 1. It should be noted that this is
only one preferred embodiment. Another preferred embodiment
would be to introduce the vaporous portion of the feed as
shown and to introduce the liquid portion of the feed near
the top of the absorption zone. The considerations involved
here are: (a) the vaporous portion of the feed should tra-
verse upwardly as many contact stages as possible within
the absorption zone such that soluble components of the va-
por are dissolved into liquid within the absorption zone to
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the greatest extent possible; (b) if the liquid portion of
the feed comes to the separation process substantially un-
saturated in soluble vaporous components,then it should
enter the absorption zone nearer the top in order to pass
as many absorption stages as necessary to absorb as much
as possible of these vaporous components. Therefore, this
second preferred embodiment of points of introduction of
the feed to the absorption zone can be desirable when the
configuration of the upstream fluid catalytic cracking unit
is such that liquid and vapor portions of the feed are de-
livered to the separation process in separate conduits and
when the liquid portion arrives at the separation process
unsaturated in soluble vaporous feed components.
Absorption zone 2 may be one or more vertically
disposed plate or packed absorption towers, having a total
of 20 or more contact stages. The absorption zone is main-
tained at conditions selected to absorb at least a portion
; Of the soluble vaporous components of the feed into the
liquid within the absorption zone. These conditions include
a pressure of from about 150 to 500 psig and a temperature
of from about 80 to 150F. since absorption is normally
exothermic, it may be necessary to provide one or more heat
removal means to prevent the temperature within the absorp-
tion zones from exceeding these limits. A preferred range
of temperature of the effluent streams from absorption zone
2 is 100 to 140F. Tbe heat removal means can be a system,
such as those well known in the art, which removes liquid
from an otherwise overheated stage within the absorption
zone, pumps this liquid through a cooling device and re-
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~062644
turns the cooled liquid to the stage immediately below.
Liquid flowing downwa~d within the absorption zone, counter-
current to upward-flowiny vapors, is provided by streams of
lean oil which enter near the top of absorption zone 2 in
conduits 8and 9. These streams are referred to as being
lean because they are substantially less than saturated
with vaporous feed components. In flowing downwardly through
the absorption zone, the lean oils absorb vaporous feed com-
ponents and a portion of gaseous feed components. Com-
; 10 bined lean oils, absorbed vaporous feed components and
liquid feed components exiting absorption zone 2 in conduit
4 are collectively referred to as rich oil. Gases which
have not been absorbed by the lean oil are withdrawn from
the top of absorption zone 2 in conduit 3. Rich oil with-
drawn from absorption zone 2 in conduit 4 is introduced
near the top of stripping zone 5.
As was indicated above, it is desired that the
hydrogen and methane exit the separation process as gases
; in conduit 3 and that the ethane, propane, propylene, butanes
: zo and butylenes exit the process as liquefied hydrocarbon va-
por in conduit 14. Ideally, thereforer the rich oil enter-
ing stripping zone 5 would contain no hydrogen or methane.
However, a portion of the hydrogen and methane in the feed -
~is unavoidably absorbed in the rich oil, and the primary
function of the stripping zone is to remove these absorbed
~; gases. Stripping zone 5 may be a conventional, vertically
disposed plate or packed tower provided with heat input
m~ans to furnish the heat for stripping. These heat input
means may ke a conventional kettle reboiler or other such
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1~6Z644
device. The stripping tower should preferably have 20 or
more contact stages. Feed to the stripping tower prefer-
ably enters at or near the top in order -that rich oil pro-
vides liquid for rectification in all contact stages. Va-
por generated through boiling of liquid in the heat input
means flows upwardly in the stripping zone and counter-
currently contacts the rich oil. The rectification which
ensues enriches the vapor in the lighter components such
that vapor withdrawn from stripping zone 5 in conduit 6,
referred to as stripped vapors, contains substantially all
~, of the hydrogen and methane which entered stripping zone 5
with the rich oil. Because the stripped vapor also contains
some heavier materials it is returned in conduit 6 to con~
, duit 1 and enters absorption zone 2 with the feed. In this
manner the heavier materials which have unavoidably been
stripped from the rich oil may be recovered, and the gaseous
material in the stripped vapor may leave the absorption
zone in conduit 3. Rich oil, after being stripped of absorbed
gases in stripping zone 5, is referred to as stripped oil
and exits stripping zone 5 in conduit 7. This stripped oil
' is suitable for use as lean oil and is divided into two ,
, fractions. One fraction of stripped oil exits conduit 7 in ''
conduit 10 and passes to fractionation zone 11. The re~,
maining fraction of stripped oil exits conduit 7 in conduit
~, ~ 25 8 and passes as a lean oil to absorption zone 2.
Stripped oil entering fractionation zone 11 in
~' conduit 10 is fractionated into an overhead liquid material
and a bottoms liquid material. The bottoms liquid material, - -~
which is suitable,for use as lean oil, exits fractionation
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1~626~4
zone 11 in conduit 12. A portion of bottoms liquid material
exits conduit 12 in conduit 9, and passes to absorption
zone 2. The portion of bottoms material remaining on con-
duit 12 exits in ccnduit 13, leaving the process as a sta-
bilized hydrocarbon liquid product. This product princi-
pally comprises hydrocarbons having 5 or more carbon atoms.
Fractionation zone 11 may be a conventional, vertically
oriented, plate or packed fractionation tower, having 25 or
more contact stages, and furnished with overhead vapor con-
~ 10 densing and reboiler heat input means. Fractionation zone
; feed preferably enters at or near the middle o the frac-
tionation tower. The fractionation zone also has means for
returning a portion of the overhead liquid material (con-
densed overhead vapor) to the fractionation tower as reflux.
The remaining portion of overhead liquid material exits
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,! fractionation zone 11 in conduit 14 as a liquefied hydro-
carbon vapor product. The liquefied hydrocarbon vapor pro-
duct comprises hydrocarbons having 2 to 4 carbon atoms per
~; molecule.
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The separation zone of my invention is particùlar- -
ly useful and novel by virtue of its manner of provlsion Of
lean oil to the absorption zone. Stripped oil, while suit-
able for use as lean oil, is not as effective as is frac-
tionation zone bottoms material for absorption of the light-
~, 25 est feed components. I have discovered that the recovery of
light feed components can be maximized by a certain preset
geometry of the upper section of ~he absorption zone. It
is necessary that fractionation zone bottoms used as a lean
oil enter the absorption zone above the top contact stage.
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It is also necessary that strlpped oil used as a lean oil
enter the absorption zone below the entry of fractionation
zone bottoms. In a preferred embodiment of my invention the
two entry points of the lean oils should be separated by 5
to 10 of the absorption zone contact stages.
:: Reasonable variation and modification are possible
within the scope of the foregoing d1sclosure, the drawing
and the claims to the invention withou-t departing from the
spirit thereofO
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