 |
Claims  |
|
|
I claim:
1. A fractionation method, comprising passing a feedstream to be
fractionated to a first fractionator and
(a) withdrawing a first relatively heavy product and a second relatively
light product from said first fractionator;
(b) introducing said second relatively light product into a light product
stripping zone;
(c) introducing said first relatively heavy product into a stripping zone
of a second fractionator for stripping relatively lighter components from
said first relatively heavy product, said second fractionator operating in
a predetermined moderate pressure range sufficient to provide integration
of said second fractionator with said first fractionator and said light
product stripping zone; and
(d) introducing a quench stream comprising relatively light stripped
product from said light product stripping zone into a rectifying zone of
said second fractionator to control an end point of said overhead product
exiting said second fractionator.
2. The method as in claim 1, wherein said overhead product exiting said
second fractionator is introduced into said light product stripping zone.
3. The method as in claim 1, wherein said overhead product exiting said
second fractionator is introduced into said first fractionator.
4. The method as in claim 1, wherein said first relatively heavy product is
withdrawn from a bottom draw of said first fractionator.
5. The method as in claim 1, wherein said first relatively heavy product is
withdrawn from a side draw of said first fractionator.
6. A fractionation method, comprising passing a feedstream to be
fractionated to a first fractionator and
(a) withdrawing bottoms product from said first fractionator and
introducing said withdrawn bottoms product into a second fractionator
operating in a predetermined moderate pressure range sufficient to provide
integration of said second fractionator with said first fractionator and
to allow transfer of overhead product from said second fractionator into a
stripper zone and thereafter into said first fractionator;
(b) separating said withdrawn bottoms product into relatively light ends
and relatively heavy ends by introducing stripping vapor into said second
fractionator;
(c) introducing a controlled stream of light product quench comprising
bottoms product from said stripper zone at a predetermined low temperature
and variable flow rate into said second fractionator to adjust an end
point of overhead products exiting said second fractionator;
(d) passing said overhead products exiting said second fractionator into
said stripper and introducing relatively lighter product components from
said first fractionator into said stripper zone, said overhead products
from said second fractionator stripping light ends from said relatively
lighter product components; and
(e) separating overhead product from said stripper zone into relatively
light ends and relatively heavy ends by introducing overhead product from
said stripper zone into said first fractionator, such that said stripper
overhead product is further fractionated in said first fractionator.
7. The method as in claim 6, wherein said first fractionator is a main
column fractionator of a fluid catalytic conversion system.
8. The method as in claim 7, wherein said stripping zone comprises a light
cycle oil stripper.
9. The method as in claim 8, wherein said second fractionator comprises a
main column bottoms/light cycle oil fractionator.
10. The method as in claim 9, wherein said stripping vapor and said bottoms
product from said main column fractionator are mixed prior to being
introduced into said main column bottoms/light cycle oil fractionator.
11. The method as in claim 10, wherein said controlled stream of light
cycle oil quench is passed through cooling means prior to being introduced
into said main column bottoms/light cycle oil fractionator.
12. The method as in claim 9, wherein said stripping vapor comprises steam.
13. The method as in claim 9, wherein said overhead product from said main
column bottoms/light cycle oil fractionator introduced into said light
cycle oil stripper comprises stripping vapor for said cycle oil introduced
into said light cycle oil stripper.
14. The method as in claim 9, wherein said light cycle oil stripper and
said main column fractionator fractionate light ends overhead product
introduced from said main column bottoms/light cycle oil fractionator into
said light cycle oil stripper.
15. The method as in claim 9, wherein said controlled stream of light cycle
oil quench is introduced into a top tray in said main column bottoms/light
cycle oil fractionator to control an ASTM End Point of light ends
separated from said main column bottoms product introduced into said main
column bottoms/light cycle oil fractionator.
16. The method as in claim 15, wherein said light cycle oil quench is taken
directly from said light cycle oil stripper bottoms.
17. The method as in claim 15, further comprising cooling said light cycle
oil stripper bottoms product and taking said light cycle oil quench from
said cooled light cycle oil stripper bottoms product.
18. The method as in claim 9, wherein said main column bottoms/light cycle
oil fractionator operates in a pressure range of approximately 40-50 psi.
19. The method as in claim 9, wherein step (b) comprises a flash separation
process.
20. The method as in claim 9, further comprising introducing overhead
product from said first fractionator into a gas-liquid separator and
recovering unstabilized gasoline from said separator.
21. A fractionation apparatus, comprising:
(a) a first fractionator having a first relatively heavy product draw and a
second relatively light product draw;
(b) a light product stripper receiving relatively light product from said
second relatively light product draw; and
(c) a second fractionator operating in a predetermined moderate pressure
range sufficient to provide integration of said second fractionator with
said first fractionator and said light product stripping zone, said second
fractionator receiving said relatively heavy product draw and having a
stripping zone for stripping lighter components from relatively heavier
components introduced therein from said first relatively heavy product
draw and a rectifying zone receiving a quench stream comprising relatively
light stripped product from said light product stripper to control an end
point of an overhead vapor stream exiting said second fractionator.
22. The apparatus as in claim 21, wherein the overhead vapor stream exiting
the second fractionator is introduced into said light product stripper.
23. The apparatus as in claim 21, wherein the overhead vapor stream exiting
said second fractionator is introduced into said first fractionator.
24. The apparatus as in claim 21, wherein said relatively heavy product
draw is a bottoms product draw.
25. The apparatus as in claim 21, wherein said relatively heavy product
draw is a side product draw.
26. A fractionation apparatus, comprising:
(a) a first fractionator including a bottoms outlet at a bottom section
thereof for removal of bottoms product, an intermediate inlet at an
intermediate section thereof and a light product outlet below said
intermediate inlet;
(b) a stripper including a bottoms outlet, an inlet at a lower section
thereof, an inlet at an upper section thereof connected to said light
product outlet of said first fractionator, and an overhead outlet
connected to said intermediate inlet of said first fractionator;
(c) a second fractionator operating in a predetermined moderate pressure
range sufficient to provide integration of said second fractionator with
said first fractionator and to allow transfer of overhead product from
said second fractionator into said stripper and thereafter into said first
fractionator, said second fractionator having a bottoms product inlet
connected to said bottoms outlet of said first fractionator, means for
admitting stripping vapor into said second fractionator below said bottoms
product inlet and an overhead products outlet connected to said inlet at
said lower section of said stripper; and
(d) means for introducing a controlled stream of bottoms product from said
bottoms outlet of said stripper into said second fractionator, said stream
having a predetermined low temperature and variable flow rate to control
an end point of overhead products exiting said overhead products outlet of
said second fractionator, whereby bottoms product from said first
fractionator flashes in said second fractionator from contact with
stripping vapor introduced therein, light ends from said second
fractionator pass into said stripper to strip light ends introduced into
said stripper from said first fractionator and light ends from said
stripper pass into said first fractionator for further fractionation.
27. The apparatus as in claim 26, wherein said first fractionator comprises
a main column fractionator of a fluid catalytic conversion system.
28. The apparatus as in claim 27, wherein said stripper comprises a light
cycle oil stripper.
29. The apparatus as in claim 28, wherein said second fractionator
comprises a main column bottoms/light cycle oil fractionator.
30. The apparatus as in claim 29, further comprising means for mixing said
stripping vapor with said bottoms product and subsequently introducing
said mixture into said main column bottoms/light cycle oil fractionator.
31. The apparatus as in claim 30, further comprising a condenser
interconnecting said bottoms outlet of said light cycle oil stripper and
said main column bottoms/light cycle oil fractionator.
32. The apparatus as in claim 29, wherein said stripping vapor comprises
steam.
33. The apparatus as in claim 29, wherein said overhead product from said
main column bottoms/light cycle oil fractionator introduced into said
lower section of said light cycle oil stripper comprises stripping vapor
for said cycle oil introduced into said upper section of said light cycle
oil stripper.
34. The apparatus as in claim 29, wherein said light cycle oil stripper and
said main column fractionator fractionate light ends overhead product
introduced from said main column bottoms/light cycle oil fractionator into
said lower section of said light cycle oil stripper.
35. The apparatus as in claim 29, wherein said controlled stream of light
cycle oil quench is introduced into a top tray in said main column
bottoms/light cycle oil fractionator to control an ASTM End Point of light
ends separated from said main column bottoms product introduced into said
main column bottoms/light cycle oil fractionator.
36. The apparatus as in claim 35, wherein said light cycle oil quench is
taken directly from said light cycle oil stripper.
37. The apparatus as in claim 35, further comprising means for cooling said
light cycle oil stripper bottoms product and wherein said light cycle oil
quench comprises light cycle oil stripper bottoms product.
38. The apparatus as in claim 29, wherein said main column bottoms/light
cycle oil fractionator operates in a pressure range of approximately 40-50
psi.
39. The apparatus as in claim 29, further comprising a gas-liquid separator
receiving overhead product from said first fractionator and means for
recovering unstabilized gasoline from said separator.
40. A method of recovering overhead liquid and light product from bottoms
product in a fractionation system which includes a first fractionator,
stripper means, and a second fractionator, comprising the steps of:
operating said second fractionator in a predetermined moderate pressure
range sufficient to provide integration of said second fractionator with
said first fractionator and to allow transfer of overhead product from
said second fractionator into said stripper and thereafter into said first
fractionator;
introducing a controlled stream of bottoms product from a bottoms outlet of
said stripper means into said second fractionator, said stream having a
predetermined low temperature and flow rate to control and end point of
overhead product exiting an overhead product outlet of said second
fractionator;
flash separating bottoms product from said first fractionator in said
second fractionator from contact with stripping vapor introduced therein;
passing light ends from said second fractionator into said stripper means
to strip relatively light ends introduced into said stripper means from
said first fractionator and to recover relatively heavy ends from said
stripper means; and
passing light ends from said stripper means into said first fractionator
for further fractionation. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus and method for fractionation, for
example, for recovering gasoline and light cycle oil from bottoms product
of a main column associated with a fluid catalytic cracking system.
2. Description of the Prior Art
In fluid catalytic cracking operations, because gasoline and light cycle
oil (LCO) are economically more valuable than main column bottoms (MCB)
product, it is desirable to recover light ends from MCB product. The light
ends content of MCB product depends on the operating conditions of the
fluid catalytic cracking (FCC) main column fractionator unit, and
particularly, on the flash zone temperature which is limited to a maximum
value because of increased coking tendency of heavy hydrocarbons at
elevated temperatures. The maximum flash zone temperature requirement
limits the separation obtainable between LCO and MCB product. Typically,
approximately 10% of the MCB product comprises LCO and lighter components.
FIG. 1 illustrates a conventional system using a single side stripper 67
associated with main column fractionator 115. MCB product is withdrawn
along line 101 as residuals product. Side draw 123 from main column 115 is
passed to stripper 67, with overhead product from stripper 67 being
recycled to main column 115. Stripping steam is introduced via line 127
into stripper 67 and LCO is withdrawn from stripper 67 along line 119.
Also, receiver/separator 117 receives main column 115 overhead, after
partial condensing, to provide recovered products via lines 49 and 53.
This system is disadvantageous in that the MCB product contains a
significant quantity of light components.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system for
integrating plural side strippers of a fractionator which lessens the main
fractionator loadings and for reducing stripping steam usage.
It is a further object to provide a fractionation method and apparatus
wherein a fractionator has two product draws, one heavier, the other
lighter, the lighter product being stripped in a light product stripper,
and wherein it is desirable to recover lighter products from the
relatively heavy product draw, with a second fractionator receiving the
relatively heavy product, stripping the relatively lighter components from
the relatively heavier ones therein, and rectifying the vapor stream
exiting the second fractionator with a quench stream from the light
product stripper, such that overall economy of the system is improved.
It is also an object to provide a method and apparatus for recovery of
gasoline and light cycle oil from the main column bottoms product of a
fluid catalytic conversion system.
It is another object to provide such method and apparatus wherein steam is
utilized as the only medium for separating light components from main
column bottoms product.
It is yet another object to provide such method and apparatus wherein the
overhead vapor of a main column bottoms/light cycle oil fractionator is
used as the stripping medium for a light cycle oil stripper.
It is still another object to provide such method and apparatus wherein the
light cycle oil stripper and main column system are used to fractionate
the light ends recovered in the main column bottoms/light cycle oil
fractionator, and further without significantly affecting the equipment
loadings and normal operations of the various units.
Another object is to provide such method and apparatus wherein the end
point of the light ends separated from main column bottoms product is
controlled by adjusting a flow and temperature of light cycle oil quench
passed to an upper section of the main column bottoms/light cycle oil
fractionator.
According to the present invention, a fractionation method is provided
which comprises the steps of (a) withdrawing a first relatively heavy
product and a second relatively light product from a first fractionator;
(b) introducing the second relatively light product into a light product
stripping zone; (c) introducing the first relatively heavy product into a
stripping zone of a second fractionator, this latter stripping zone for
stripping relatively lighter components from relatively heavier components
of the first relatively heavy product, the second fractionator operating
in a predetermined moderate pressure range sufficient to provide
integration of the second fractionator with the the first fractionator and
the light product stripping zone; and (d) introducing a quench stream
comprising relatively light stripped product from the light product
stripping zone into a rectifying zone of the second fractionator to
control an end point of the overhead product exiting the second
fractionator. The overhead product exiting the second fractionator can be
introduced into the light product stripping zone or, alternatively, can be
introduced into the first fractionator. The first relatively heavy product
can be a bottoms product draw of the first fractionator or alternatively,
can be a side draw of the first fractionator.
An an alternative to step (c) above, the first relatively heavy product can
be introduced into a flash drum operating in a predetermined moderate
pressure range sufficient to provide integration of the flash drum with
the first fractionator, and the overhead product exiting the flash drum
can be introduced into the first fractionator for further fractionation of
this overhead product. The overhead product from the flash drum can be
introduced into the first fractionator at a point above a quench nozzle of
the first fractionator.
Also according to the present invention, a fractionation method is
provided, which includes the steps of withdrawing bottoms product from a
first fractionator and introducing the withdrawn bottoms product into a
second fractionator, operating in a predetermined moderate pressure range
sufficient to provide integration of the second fractionator with the
first fractionator and to allow transfer of overhead product from the
second fractionator into a stripper zone and thereafter into the first
fractionator. The method also includes separating the withdrawn bottoms
product into relatively light ends and relatively heavy ends by
introducing stripping vapor into a lower section of the second
fractionator, and introducing a controlled stream of light cycle oil
quench comprising bottoms product from the stripper zone at a
predetermined low temperature and flow rate into an upper section of the
second fractionator to adjust an end point of overhead products exiting
the second fractionator. The method further includes passing the overhead
products exiting the second fractionator into a lower section of the
stripper and introducing cycle oil from an intermediate section of the
first fractionator into an upper section of the stripper zone, with the
overhead products from the second fractionator stripping light ends from
the cycle oil. Additionally, the method includes separating overhead
product from the stripper zone into relatively light ends and relatively
heavy ends by introducing the overhead product from the stripper zone into
an upper section of the first fractionator, such that the overhead
products from the stripper zone are further fractionated in the first
fractionator.
The first fractionator can be a main column fractionator of a fluid
catalytic conversion system. The stripper zone can comprise a light cycle
oil stripper, and the second fractionator can comprise a main column
bottoms/light cycle oil fractionator. The aforementioned stripping vapor
and the bottoms product from the main column fractionator can be mixed
prior to being introduced into the main column bottoms/light cycle oil
fractionator. The controlled stream of light cycle oil quench can be
passed through a cooler prior to being introduced into the upper section
of the main column bottoms/light cycle oil fractionator. The
aforementioned stripping vapor can comprise steam. The overhead product
from the main column bottoms/light cycle oil fractionator, which is
introduced into the lower section of the light cycle oil stripper, can
constitute the only stripping vapor for the cycle oil introduced into the
upper section of the light cycle oil stripper. The light cycle oil
stripper and main column fractionator can fractionate light ends overhead
product introduced from the main column bottoms/light cycle oil
fractionator into the lower section of the light cycle oil stripper. The
controlled stream of light cycle oil quench can be introduced into the top
tray in the upper section of the main column bottoms/light cycle oil
fractionator to control and ASTM End Point of light ends separated from
the main column bottoms product introduced into the main column
bottoms/light cycle oil fractionator. The light cycle oil quench can be
taken directly from the light cycle oil stripper bottoms or,
alternatively, the light cycle oil bottoms product can be cooled and the
light cycle oil quench can be taken after such cooling. The main column
bottoms/light cycle oil fractionator can operate in a pressure range of
approximately 40-50 psi. The method of the present invention can further
include introducing overhead product from the first fractionator into a
gas-liquid separator and recovering unstabilized gasoline from this
separator.
Also according to the present invention, a fractionation apparatus is
provided which comprises (a) a first fractionator having a first
relatively heavy product draw and a second relatively light product draw;
(b) a light product stripper receiving relatively light product from said
second relatively light product draw; and (c) a second fractionator
operating in a predetermined moderate pressure range sufficient to provide
integration of the second fractionator with the first fractionator and the
light product stripping zone, the second fractionator receiving the
relatively heavy product from the relatively heavy product draw and having
a stripping zone for stripping relatively lighter components from
relatively heavier components introduced therein from the first relatively
heavy product draw, and a rectifying zone receiving a quench stream
comprising relatively light stripped product from the light product
stripper to control an end point of an overhead vapor stream exiting the
second fractionator. The overhead vapor stream exiting the second
fractionator can be introduced into the light product stripper or,
alternatively, into the first fractionator. The relatively heavy product
draw can be a bottoms product draw or, alternatively, a side product draw.
As an alternative to the second fractionator, a flash drum can be provided,
which is operated in a predetermined moderate pressure range sufficient to
provide integration of the second fractionator with the first
fractionator, and further including means for introducing overhead product
exiting the flash drum into the first fractionator for further
fractionation of the overhead product. The overhead product from the flash
drum can be introduced into the first fractionator at a point above a
quench nozzle of the first fractionator.
Also according to the present invention, a fractionation apparatus is
provided, which includes a first fractionator having a bottoms outlet at a
bottoms section thereof for removal of bottoms product, an intermediate
inlet at an intermediate section thereof, and a cycle oil outlet below the
intermediate inlet. The apparatus also includes a stripper having a
bottoms outlet, an inlet at a lower section thereof, an inlet at an upper
section thereof connected to the cycle oil outlet of the first
fractionator, and an overhead outlet connected to the intermediate inlet
of the first fractionator. A second fractionator is provided, which
operates in a predetermined moderate pressure range sufficient to provide
integration of the second fractionator with the first fractionator, and to
allow transfer of overhead product from the second fractionator into the
stripper and thereafter into the first fractionator. The second
fractionator has a bottoms product inlet connected to the bottoms outlet
of the first fractionator, means for admitting stripping vapor into the
second fractionator below the bottoms product inlet thereof, and an
overhead product outlet connected to the inlet at the lower section of the
stripper. The apparatus further includes means for introducing a
controlled stream of bottoms product from the bottoms outlet of the
stripper into an upper section of the second fractionator, with the
aforementioned stream having a predetermined low temperature and flow rate
to control an end point of overhead products exiting the overhead products
outlet of the second fractionator, whereby bottoms product from the first
fractionator flashes in the second fractionator from contact with
stripping vapor introduced therein, light ends from the second
fractionator pass into the stripper to strip light ends introduced into
the stripper from the first fractionator and light ends from the stripper
pass into the first fractionator for further fractionation.
Also according to the present invention, a fractionation method is provided
in a fractionation system, which includes a first fractionator, stripper
means, and a second fractionator. An improvement is provided comprising
(a) operating the second fractionator in a predetermined moderate pressure
range sufficient to provide integration of the second fractionator with
the first fractionator, and to allow transfer of overhead product from the
second fractionator into the stripper and thereafter into the first
fractionator; (b) introducing a controlled stream of bottoms product from
the bottoms outlet of the stripper into an upper section of the second
fractionator, with the stream having a predetermined low temperature and
flow rate to control an end point of overhead products exiting the
overhead products outlet of the second fractionator; (c) flash separating
relatively heavy ends from relatively light ends of the bottoms product
from the first fractionator in the second fractionator from contact with
stripping vapor introduced therein; (d) passing light ends from the second
fractionator into the stripper means to strip relatively light ends
introduced into the stripper means from the first fractionator and to
recover relatively heavier ends from said stripper means; and (e) passing
light ends from the stripper means into the first fractionator for further
fractionation.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present
invention will be more fully understood when considered in conjunction
with the following figures, of which:
FIG. 1 illustrates a conventional main column fractionation system with a
single side stripper;
FIG. 2 illustrates a system having plural unintegrated side strippers;
FIG. 3 illustrates a first embodiment of a system according to the present
invention;
FIG. 4 illustrates a second embodiment of the present invention;
FIG. 5 illustrates a third embodiment of a system according to the present
invention; and
FIG. 6 illustrates a fourth embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 illustrates a light cycle oil recovery system which uses a low
pressure flash-down design, including main column fractionator 115,
flash-down tower 105, stripper 67 and receiver/separator 117 receiving
main column overhead product, wherein the MCB product stream in line 101
is mixed with steam injected via line 103 before flashing in the bottom of
a flashdown tower 105. The vapor phase is rectified by the reflux created
by a LCO pumparound, including pump 107 and cooler 109. A LCO side draw
111 from the pumparound is used to recover the condensable LCO components.
The overhead vapor line 113, containing steam and uncondensable
hydrocarbons, i.e., C.sub.5 -gasoline and some LCO, is tied into the flare
line. The total MCB product approximately consists of the liquid feed to
the tower, plus the liquid from the lowest fractionator tray in the tower.
The system of FIG. 2 is disadvantageous, in that it requires numerous
pieces of large equipment with high energy consumption, and moreover, as a
result of the above considerations, valuable gasoline and some LCO
components are generally required to be flared. In this system, as an
alternative to flaring, an expensive recovery system may be used which
employs, e.g., condensers, separators and pumps.
In FIG. 3, reference numeral 10 refers to a main column (MC) fractionator
tower, in which reactor effluent is introduced along line 81. The reactor
effluent is fractionated by MC fractionator 10, MCB/LCO fractionator 20
and LCO stripper 30 to recover desired end products. For example, in the
case where MC tower 10 receives FCC effluent, the desired recovered
products are light cycle oil, gasoline, liquid petroleum gas and fuel gas.
MC fractionator tower 10 produces a heavy bottoms product fraction which
is withdrawn through bottoms draw 11 and passed into MCB/LCO fractionator
tower 20. A lighter fraction, e.g., a cycle oil, is withdrawn from section
61 of tower 10 through side draw 13 and passed to LCO stripper 30 for
further fractionation, as described below in greater detail. FIG. 4
illustrates another embodiment, wherein a heavy product side draw 201
replaces bottoms product draw 11, in FIG. 3, to produce a heavy product
which is passed to second fractionator 20. Line 37 carries a light product
output from stripper 30.
Second fractionator tower 20 receives a stripping vapor, e.g., steam, in a
lower section 65. This stripping vapor constitutes the only medium
necessary for separating light components from MCB product in second
fractionator tower 20. Tower 20 includes six stages, with the lower two
stages serving as MCB stripping stages 67, 69 and the upper four stages
serving as light end rectification stages 71, 73, 75 and 77.
Top tray 77 of second fractionation tower 20 receives LCO quench along line
39, which is taken along line 31 from the bottom of LCO stripper 30. In
the embodiment illustrated in FIGS. 3 and 4, line 31 feeds into LCO cooler
33 which controls the temperature of the quench stream. Output 35 of
cooler 33 is divided into two lines 37 and 135. Line 37 carries LCO
product. The LCO provided by line 39 is provided to flow control valve
means 91, which may be controlled by LCO end point analyzer 90 which
provides a control signal to valve means 91. Condenser 33 and valve means
91 together control the ASTM End Point of the overhead vapor exiting
second fractionating tower 20 via overhead line 21, by adjusting the flow
rate and temperature of the LCO quench stream provided to top tray 75 of
second fractonation tower 20. As seen from the above, the LCO quench
comprises bottoms product from LCO stripper 30 and is provided at a
predetermined low temperature and a predetermined variable flow rate.
Vaporized LCO quench and recovered LCO pass via overhead line 21 into a
lower section of LCO stripper 30. The vapor input to stripper 30, provided
by line 21, provides the stripping medium for stripper 30. Because the
MCB/LCO fractionator 20 overhead vapor primarily comprises steam, it can
totally replace LCO stripping steam, which would be required by a
conventional unit, such as that shown in FIG. 1. Also, condensation of
quenched LCO and recovered LCO in LCO stripper 30 act as a heating source
to improve fractionation between naptha and LCO in LCO stripper 30. LCO
boiling range components are recovered in the bottoms product of LCO
stripper 30 along line 37.
It should be noted that MCB/LCO fractionator tower 20 is operated at a
sufficiently high pressure to provide integration of towers 10 and 20 and
to allow transfer of overhead vapors to LCO stripper 30 and main column
fractionation tower 10, thus enabling separation of recovered light ends
into LCO, gasoline, LPG and fuel gas. It should also be noted that LCO
stripper 30 and main column fractionator tower 10 fractionate the light
ends recovered from MCB/LCO fractionator tower 20 without significantly
affecting the equipment loadings and normal operations. It should be noted
further that, as an alternative embodiment, LCO stripper 30 and MCB/LCO
fractionator tower 20 can be combined into one single tower.
LCO stripper overhead line 41 carries lighter components from stripper 30
into main column fractionator tower 10. Thereafter, the lighter components
pass via main column overhead line 43 to condenser 45, and then to
gas/liquid separator 81 which provides gas exit line 49 and liquid exit
line 53, both of which provide inputs to an FCC unsaturated gas plant (not
shown), where these lighter components are further fractionated. Also, a
portion of the product carried by liquid line 51 from separator 81 is
diverted to a top section of main column fractionator 10, along line 55,
to control the end point of the main column overhead. Overhead line 43 of
main column fractionator tower 10 carries main column overhead vapor to
condenser 45, which provides an output along line 47 to gas/liquid
separator 81. Output 49 from separator 81 provides a gas exit line, while
liquid line 51 is separated into liquid exit line 53, and line 55 which is
passed into upper section 57 of main column fractionator tower 10.
The present invention includes introducing steam via line 93 into a bottom
stripping section of LCO/MCB fractionator 20. Alternatively, steam via
line 95 can be mixed with MCB coming from main column tower 10 along line
11. The steam mixes with MCB product, which results in flashing at the
bottom of fractionator 20. Vapor which ascends through tower 20 is
rectified by a cold stream of LCO quench entering top tray 75 of
fractionator 20. A LCO quench stream provided via line 39 controls the
recovered LCO ASTM End Point. This quench stream is preferably taken from
the cooled LCO going to storage along line 37. The recovered LCO and
gasoline components, plus the LCO quench, are carried by the steam
injected via line 93 and/or 95 into tower 20, from the overhead of LCO/MCB
fractionator 20 via line 21 to the bottom of LCO stripper 30. This
arrangement eliminates the need for LCO stripping steam which would
otherwise be introduced through line 99, which is required by prior art
units (i.e., line 127 in FIGS. 1 and 2). These recovered hydrocarbons from
the MCB product are then separated in LCO stripper 30 and main column
system 10.
It should be noted that, according to the present invention, the operating
pressure of LCO/MCB fractionator 20 falls within a moderate pressure
range, e.g., approximately 40-50 psi, to make it possible to integrate
fractionator 20 with main column fractionator tower 10. It has been found
in computer simulations that the total light hydrocarbons recovery from
MCB product is about 7%, which can be increased by using a steam stripping
section at a bottom section of LCO/MCB fractionator 20, as discussed
above. In such case, the total steam mixed with the MCB coming from main
column tower 10 is preferably used as the stripping steam.
FIGS. 5 and 6 illustrate alternative embodiments, wherein LCO/MCB
fractionator 20 (FIG. 5) or flash drum 197 (FIG. 6), have their overhead
vapor taken to the main column system 10 via line 131 to a point above the
MCB quench nozzle 133, for further fractionation of light components and
MCB product. The arrangement of FIG. 6 results in an increase in the steam
consumption and main column tray loadings, as compared with the FIGS. 3
and 4 embodiments. In this case, the liquid phase of the flash drum is the
MCB product. Because the FIGS. 3 and 4 embodiments reuse the MCB stripping
steam as LCO stripping steam and also reduce MC loadings, they are
preferred over that of FIGS. 5 and 6.
The above-described embodiments of FIGS. 3-6 provide substantially improved
results over those of the FIG. 2 system, which recovers LCO from MCB
product using MCB flash-down in which the MCB is mixed with steam and
flashed at low pressures, i.e., atmospheric or vacuum pressures. As noted
above, the present invention provides for operating fractionator 20 or
flash drum 197 at moderate pressures, which allows integration of the
fractionator with main column 20. This reduces steam consumption,
equipment sizing and the number of pieces of equipment required. Also,
light product recovery and overhead liquid product recovery are improved
significantly. By providing a system wherein fractionator 20 or flash drum
197 is integrated with main column 10 and stripper 30, the FIG. 2 liquid
side draw 111 in fractionator 111 can be eliminated. Further, introduction
of a light product quench stream from the rundown cooler-condenser 33
allows the FIG. 2 side pumparound in fractionator 20 to be eliminated.
Also, replacement of stripper 30 stripping steam by fractionator 20
overhead vapor allows reduction of steam consumption. Additionally, the
total light hydrocarbons recovery from the heavy product drawn from the
main column can be increased by introduction of a stripping section to the
bottom of fractionator 20, as discussed above. Furthermore, in the
embodiment shown in FIGS. 5 and 6, the overhead of LCO/MCB fractionator 20
or flash drum 197 can be taken directly to the main column fractionator 10
for further fractionation.
Table 1 below represents data from a computer simulation of a conventional
main column system, as in FIG. 1, and Table 2 includes data from a
computer simulation of a system in accordance with the present claimed
invention, with both tables being based on maximum gasoline operation at
55,000 barrels per stream day (BPSD) of FCC fresh feed rate. These
simulations are based on the assumption that 99% ASTM distillation is
equivalent to ASTM End Point. These tables provide material balanc | | |
