Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR STEAM CRACKING
HEAVY HYDROCARBON FEEDSTOCKS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the cracking of hydrocarbons that
contain relatively non-volatile hydrocarbons and other contaminants.
Description of Background and Related Art
Steam cracking has long been used to crack various hydrocarbon
feedstocks into olefins. Conventional steam cracking utilizes a pyrolysis
furnace
which has two main sections: a convection section and a radiant section. The
hydrocarbon feedstock typically enters the convection section of the furnace
as a
liquid (except for light feedstocks which enter as a vapor) wherein it is
typically
heated and vaporized by indirect contact with hot flue gas from the radiant
section
and by direct contact with steam. The vaporized feedstock and steam mixture is
then introduced into the radiant section where the cracking takes place. The
resulting products including olefins leave the pyrolysis furnace for further
downstream processing, such as quenching.
Conventional steam cracking systems have been effective for
cracking high-quality feedstock which contain a large fraction of light
volatile
hydrocarbons, such as gas oil and naphtha. However, steam cracking economics
sometimes favor cracking lower cost heavy feedstocks such as, by way of non-
limiting examples, crude oil and atmospheric resid. Crude oil and atmospheric
resid contain high molecular weight, non-volatile components with boiling
points
in excess of 1100 F (590 C). The non-volatile, components of these feedstocks
lay down as coke in the convection section of conventional pyrolysis furnaces.
Only very low levels of non-volatile components can be tolerated in the
convection section downstream of the point where the lighter components have
fully vaporized. Additionally, during transport some naphthas are contaminated
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with heavy crude oil containing non-volatile components. Conventional
pyrolysis furnaces
do not have the flexibility to process resids, crudes, or many resid or crude
contaminated
gas oils or naphthas which are contaminated with non-volatile components
hydrocarbons.
To solve such coking problems, U. S. Patent 3,617,493 discloses the use of an
external
vaporization drum for the crude oil feed and discloses the use of a first
flash to remove
naphtha as vapor and a second flash to remove vapors with a boiling point
between 450
and 1100 F (230 and 600 C). The vapors are cracked in the pyrolysis furnace
into olefins
and the separated liquids from the two flash tanks are removed, stripped with
steam, and
used as fuel.
U. S. Patent 3,718,709 discloses a process to minimize coke deposition. It
describes
preheating of heavy feedstock inside or outside a pyrolysis furnace to
vaporize about 50%
of the heavy feedstock with superheated steam and the removal of the residual,
separated
liquid. The vaporized hydrocarbons, which contain mostly light volatile
hydrocarbons, are
subjected to cracking.
U. S. Patent 5,190,634 discloses a process for inhibiting coke formation in a
furnace by
preheating the feedstock in the presence of a small, critical amount of
hydrogen in the
convection section. The presence of hydrogen in the convection section
inhibits the
polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U. S. Patent 5,580,443 discloses a process wherein the feedstock is first
preheated and
then withdrawn from a preheater in the convection section of the pyrolysis
furnace. This
preheated feedstock is then mixed with a predetermined amount of steam (the
dilution
steam) and is then introduced into a gas-liquid separator to separate and
remove a required
proportion of the non-volatiles as liquid from the separator.
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The separated vapor from the gas-liquid separator is returned to the pyrolysis
furnace for heating and cracking.
The present inventors have recognized that in using a flash to
separate heavy liquid hydrocarbon fractions from the lighter fractions which
can
be processed in the pyrolysis furnace, it is important to effect the
separation so
that most of the non-volatile components will be in the liquid phase.
Otherwise,
heavy, coke-forming non-volatile components in the vapor are carried into the
furnace causing coking problems.
The present inventors have also recognized that in using a flash to
separate non-volatile components from the lighter fractions of the hydrocarbon
feedstock, which can be processed in the pyrolysis furnace without causing
coking
problems, it is important to carefully control the ratio of vapor to liquid
leaving
the flash. Otherwise, valuable lighter fractions of the hydrocarbon feedstock
could be lost in the liquid hydrocarbon bottoms or heavy, coke-forming
components could be vaporized and carried as overhead into the furnace causing
coking problems.
The control of the ratio of vapor to liquid leaving flash has been
found to be difficult because many variables are involved. The ratio of vapor
to
liquid is a function of the hydrocarbon partial pressure in the flash and also
a
function of the temperature of the stream entering the flash. The temperature
of
the stream entering the flash varies as the furnace load changes. The
temperature
is higher when the furnace is at full load and is lower when the furnace is at
partial
load. The temperature of the stream entering the flash also varies according
to the
flue gas temperature in the furnace that heats the feedstock. The flue-gas
temperature in turn varies according to the extent of coking that has occurred
in
the furnace. When the furnace is clean or very lightly coked, the flue-gas
temperature is lower than when the furnace is heavily coked. The flue-gas
temperature is also a function of the combustion control exercised on the
burners
of the furnace. When the furnace is operated with low levels of excess oxygen
in
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the flue gas, the flue gas temperature in the mid to upper zones of the
convection
section will be lower than that when the furnace is operated with higher
levels of
excess oxygen in the flue-gas. With all these variables, it is difficult to
control a
constant ratio of vapor to liquid leaving the flash.
The present invention offers an advantageously controlled process
to optimize the cracking of volatile hydrocarbons contained in the heavy
hydrocarbon feedstocks and to reduce and avoid the coking problems. The
present invention provides a method to maintain a relatively constant ratio of
vapor to liquid leaving the flash by maintaining a relatively constant
temperature
of the stream entering the flash. More specifically, the constant temperature
of the
flash stream is maintained by automatically adjusting the amount of a fluid
stream
mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid
optionally is water.
The present invention also provides a method to maintain a
relatively constant hydrocarbon partial pressure of the flash stream. The
constant
hydrocarbon partial pressure is maintained by controlling the flash pressure
and
the ratio of fluid and steam to the hydrocarbon feedstock.
SUMMARY OF THE INVENTION
The present invention provides a process for heating heavy
hydrocarbon feedstock which comprises heating a heavy hydrocarbon, mixing the
heavy hydrocarbon with fluid to form a mixture, flashing the mixture to form a
vapor phase and a liquid phase, and varying the amount of fluid mixed with the
heavy hydrocarbon in accordance with at least one selected operating parameter
of
the process and feeding the vapor phase to a furnace. The fluid can be a
liquid
hydrocarbon or water.
According to one embodiment, at least one operating parameter
may be the temperature of the heated heavy hydrocarbon before it is flashed.
At
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least one operating parameter may also be at least one of the flash pressure,
temperature of the flash stream, flow rate of the flash stream, and excess
oxygen
in the flue gas.
5 In a preferred embodiment, the heavy hydrocarbon is mixed with a
primary dilution steam stream before the flash. Furthermore, a secondary
dilution
steam can be superheated in the furnace and then mixed with the heavy
hydrocarbon.
The present invention also provides a process for cracking a heavy
hydrocarbon feedstock in a furnace which is comprised of radiant section
burners
which provide radiant heat and hot flue gas and a convection section comprised
of
multiple banks of heat exchange tubes comprising:
(a) preheating the heavy hydrocarbon feedstock to form
a preheated heavy hydrocarbon feedstock;
(b) mixing the preheated heavy hydrocarbon feedstock
with water to form a water heavy hydrocarbon mixture;
(c) injecting primary dilution steam into the water
heavy hydrocarbon mixture to form a mixture stream;
(d) heating the mixture stream in a bank of heat
exchange tubes by indirect heat transfer with the hot flue gas to
form a hot mixture stream;
(e) controlling the temperature of the hot mixture
stream and controlling the ratio of steam to hydrocarbon by
varying the flow rate of the water and the flow rate of the primary
dilution steam;
(f) flashing the hot mixture stream in a flash drum to
form a vapor phase and liquid phase and separating the vapor
phase from the liquid phase;
(g) feeding the vapor phase into the convection section
of the furnace to be further heated by the hot flue gas from the
radiant section of the furnace to form a heated vapor phase; and
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(h) feeding the heated vapor phase to the radiant section
tubes of the furnace wherein the hydrocarbons in the vapor phase
thermally crack to form products due to the radiant heat.
BRIEF DESCRIPTION OF THE FIGURE
Figure 1 illustrates a schematic flow diagram of a process in
accordance with the present invention employed with a steam cracking furnace,
specifically the convection section.
DETAILED DESCRIPTION OF THE INVENTION
Unless otherwise stated, all percentages, parts, ratios, etc., are by
weight. Unless otherwise stated, a reference to a compound or component
includes the compound or component by itself, as well as in combination with
other compounds or components, such as mixtures of compounds.
Further, when an amount, concentration, or other value or
parameter is given as a list of upper preferable values and lower preferable
values,
this is to be understood as specifically disclosing all ranges formed from any
pair
of an upper preferred value and a lower preferred value, regardless whether
ranges
are separately disclosed.
Also as used herein: Non-volatile components can be measured as
follows: The boiling point distribution of the hydrocarbon feed is measured by
Gas Chromatograph Distillation (GCD) by ASTM D-6352-98 or another suitable
method. The Non-volatile components are the fraction of the hydrocarbon with a
nominal boiling point above 1100 F (590 C) as measured by ASTM D-6352-98.
This invention works very well with non-volatiles having a nominal boiling
point
above 1400 F (760 C).
The present invention relates to a process for heating and steam
cracking heavy hydrocarbon feedstock. The process comprises heating a heavy
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hydrocarbon, mixing the heavy hydrocarbon with a fluid to form a mixture,
flashing the mixture to form a vapor phase and a liquid phase, and varying the
amount of fluid mixed with the heavy hydrocarbon in accordance with at least
one
selected operating parameter of the process.
As noted, the feedstock comprises a large portion, about 5 to 50%,
of heavy non-volatile components. Such feedstock could comprise, by way of
non-limiting examples, one or more of steam cracked gas oil and residues, gas
oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam
cracked
naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate
reformate, Fischer-Tropsch liquids, Fischer-Tropsch gases, natural gasoline,
distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum
pipestill streams including bottoms, wide boiling range naphtha to gas oil
condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas
oils, heavy gas oil, naphtha contaminated with crude, atmospheric resid, heavy
residium, C4's/residue admixture, and naphtha residue admixture.
The heavy hydrocarbon feedstock has a nominal end boiling point
of at least 600 F (315 C). The preferred feedstocks are low sulfur waxy
resids,
atmospheric resids, and naphthas contaminated with crude. The most preferred
is
resid comprising 60-80% components having boiling points below 1100 F
(590 C), for example, low sulfur waxy resids.
The heavy hydrocarbon feedstock is first preheated in the upper
convection section 3. The heating of the heavy hydrocarbon feedstock can take
any form known by those of ordinary skill in the art. However, it is preferred
that
the heating comprises indirect contact of the feedstock in the upper
convection
section 3 of the furnace 1 with hot flue gases from the radiant section of the
furnace. This can be accomplished, by way of non-limiting example, by passing
the feedstock through a bank of heat exchange tubes 2 located within the
convection section 3 of the furnace 1. The preheated feedstock has a
temperature
between 300 and 500 F (150 and 260 C). Preferably the temperature of the
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heated feed is about 325 to 450 F (160 to 230 C) and more preferably between
340 and 425 F (170 and 220 C).
The preheated heavy hydrocarbon feedstock is mixed with a fluid.
The fluid can be a liquid hydrocarbon, water, steam, or mixture thereof. The
preferred fluid is water. The temperature of the fluid can be below, equal to
or
above the temperature of the preheated feedstock.
The mixing of the preheated heavy hydrocarbon feedstock and the
fluid can occur inside or outside the pyrolysis furnace 1, but preferably it
occurs
outside the furnace. The mixing can be accomplished using any mixing device
known within the art. However it is preferred to use a first sparger 4 of a
double
sparger assembly 9 for the mixing. The first sparger 4 preferably comprises an
inside perforated conduit 31 surrounded by an outside conduit 32 so as to form
an
annular flow space 33 between the inside and outside conduit. Preferably, the
preheated heavy hydrocarbon feedstock flows in the annular flow space and the
fluid flows through the inside conduit and is injected into the feedstock
through
the openings in the inside conduit, preferably small circular holes. The first
sparger 4 is provided to avoid or to reduce hammering, caused by sudden
vaporization of the fluid, upon introduction of the fluid into the preheated
heavy
hydrocarbon feedstock.
The present invention uses steam streams in various parts of the
process. The primary dilution steam stream 17 is mixed with the preheated
heavy
25- hydrocarbon feedstock as detailed below. In a preferred embodiment, a
secondary
dilution steam stream 18 is treated in the convection section and mixed with
the
heavy hydrocarbon fluid primary dilution steam mixture before the flash. The
secondary dilution steam 18 is optionally split into a bypass steam 21 and a
flash
steam 19.
In a preferred embodiment in accordance with the present
invention, in addition to the fluid mixed with the preheated heavy feedstock,
the
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primary dilution steam 17 is also mixed with the feedstock. The primary
dilution
steam stream can be preferably injected into a second sparger 8. It is
preferred
that the primary dilution steam stream is injected into the heavy hydrocarbon
fluid
mixture before the resulting stream mixture enters the convection section at I
I for
additional heating by radiant section flue gas. Even more preferably, the
primary
dilution steam is injected directly into the second sparger 8 so that the
primary
dilution steam passes through the sparger and is injected through small
circular
flow distribution holes 34 into the hydrocarbon feedstock fluid mixture.
The primary dilution steam can have a temperature greater, lower
or about the same as heavy hydrocarbon feedstock fluid mixture but preferably
greater than that of the mixture and serves to partially vaporize the
feedstock/fluid
mixture. Preferably, the primary dilution steam is superheated before being
injected into the second sparger 8.
The mixture of the fluid, the preheated heavy hydrocarbon
feedstock, and the primary dilution steam stream leaving the second sparger 8
is
heated again in the pyrolysis furnace I before the flash. The heating can be
accomplished, by way of non-limiting example, by passing the feedstock mixture
through a bank of heat exchange tubes 6 located within the convection section
of
the furnace and thus heated by the hot flue gas from the radiant section of
the
furnace. The thus-heated mixture leaves the convection section as a mixture
stream 12 to be further mixed with an additional steam stream.
Optionally, the secondary dilution steam stream 18 can be further
split into a flash steam stream 19 which is mixed with the heavy hydrocarbon
mixture 12 before the flash and a bypass steam stream 21 which bypasses the
flash
of the heavy hydrocarbon mixture and, instead is mixed with the vapor phase
from
the flash before the vapor phase is cracked in the radiant section of the
furnace.
The present invention can operate with all secondary dilution steam 18 used as
flash steam 19 with no bypass steam 21. Alternatively, the present invention
can
be operated with secondary dilution steam 18 directed to bypass steam 21 with
no
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flash steam 19. In a preferred embodiment in accordance with the present
invention, the ratio of the flash steam stream 19 to bypass steam stream 21
should
be preferably 1:20 to 20:1, and most preferably 1:2 to 2:1. The flash steam 19
is
mixed with the heavy hydrocarbon mixture stream 12 to form a flash stream 20
5 before the flash in flash drum 5. Preferably, the secondary dilution steam
stream
is superheated in a superheater section 16 in the furnace convection before
splitting and mixing with the heavy hydrocarbon mixture. The addition of the
flash steam stream 19 to the heavy hydrocarbon mixture stream 12 ensures the
vaporization of nearly all volatile components of the mixture before the flash
10 stream 20 enters the flash drum 5.
The mixture of fluid, feedstock and primary dilution steam stream
(the flash stream 20) is then introduced into a flash drum 5 for separation
into two
phases: a vapor phase comprising predominantly volatile hydrocarbons and a
liquid phase comprising predominantly non-volatile hydrocarbons. The vapor
phase is preferably removed from the flash drum as an overhead vapor stream
13.
The vapor phase, preferably, is fed back to the lower convection section 23 of
the
furnace for optional heating and through crossover pipes to the radiant
section of
the pyrolysis furnace for cracking. The liquid phase of the separation is
removed
from the flash drum 5 as a bottoms stream 27.
It is preferred to maintain a predetermined constant ratio of vapor
to liquid in the flash drum 5. But such ratio is difficult to measure and
control.
As an alternative, temperature of the mixture stream 12 before the flash drum
5 is
used as an indirect parameter to measure, control, and maintain the constant
vapor
to liquid ratio in the flash drum 5. Ideally, when the mixture stream
temperature
is higher, more volatile hydrocarbons will be vaporized and become available,
as a
vapor phase, for cracking. However, when the mixture stream temperature is too
high, more heavy hydrocarbons will be present in the vapor phase and carried
over
to the convection furnace tubes, eventually coking the tubes. If the mixture
stream 12 temperature is too low, hence a low ratio of vapor to liquid in the
flash
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drum 5, more volatile hydrocarbons will remain in liquid phase and thus will
not
be available for cracking.
The mixture stream temperature is limited by highest
recovery/vaporization of volatiles in the feedstock while avoiding coking in
the
furnace tubes or coking in piping and vessels conveying the mixture from the
flash drum to the furnace 13. The pressure drop across the piping and vessels
conveying the mixture to the lower convection section 13, and the crossover
piping 24, and the temperature rise across the lower convection section 23 may
be
monitored to detect the onset of coking problems. For instance, when the
crossover pressure and process inlet pressure to the lower convection section
23
begins to increase rapidly due to coking, the temperature in the flash drum 5
and
the mixture stream 12 should be reduced. If coking occurs in the lower
convection section, the temperature of the flue gas to the superheater 16
increases,
requiring more desuperheater water 26.
The selection of the mixture stream 12 temperature is also
determined by the composition of the feedstock materials. When the feedstock
contains higher amounts of lighter, hydrocarbons, the temperature of the
mixture
stream 12 can be set lower. As a result, the amount of fluid used in the first
sparger 4 is increased and/or the amount of primary dilution steam used in the
second sparger 8 is decreased since these amounts directly impact the
temperature
of the mixture stream 12. When the feedstock contains a higher amount of non-
volatile hydrocarbons, the temperature of the mixture stream 12 should be set
higher. As a result, the amount of fluid used in the first sparger 4 is
decreased
while the amount of primary dilution steam used in the second sparger 8 is
increased. By carefully selecting a mixture stream temperature, the present
invention can find applications in a wide variety of feedstock materials.
Typically, the temperature of the mixture stream 12 is set and
controlled at between 600 and 950 F (315 and 510 C), preferably between 700
and 920 F (370 and 490 C), more preferably between 750 and 900 F (400 and
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480 C), and most preferably between 810 and 890 F (430 and 475 C). These
values will change with the concentrating volatiles in the feedstock as
discussed
above.
The temperature of mixture stream 12 is controlled by a control
system 7 which comprises at least a temperature sensor and any known control
device, such as a computer application. Preferably, the temperature sensors
are
thermocouples. The control system 7 communicates with the fluid valve 14 and
the primary dilution steam valve 15 so that the amount of the fluid and the
primary dilution steam entering the two spargers is controlled.
In order to maintain a constant temperature for the mixture stream
12 mixing with flash steam 19 and entering the flash drum to achieve a
constant
ratio of vapor to liquid in the flash drum 5, and to avoid substantial
temperature
and flash vapor to liquid ratio variations, the present invention operates as
follows: When a temperature for the mixture stream 12 before the flash drum 5
is
set, the control system 7 automatically controls the fluid valve 14 and
primary
dilution steam valve 15 on the two spargers. When the control system 7 detects
a
drop of temperature of the mixture stream, it will cause the fluid valve 14 to
reduce the injection of the fluid into the first sparger 4. If the temperature
of the
mixture stream starts to rise, the fluid valve will be opened wider to
increase the
injection of the fluid into the first sparger 4. In the preferred embodiment,
the
fluid latent heat of vaporization controls mixture stream temperature.
When the primary dilution steam stream 17 is injected to the
second sparger 8, the temperature control system 7 can also be used to control
the
primary dilution steam valve 15 to adjust the amount of primary dilution steam
stream injected to the second sparger 8. This further reduces the sharp
variation
of temperature changes in the flash 5. When the control system 7 detects a
drop
of temperature of the mixture stream 12, it will instruct the primary dilution
steam
valve 15 to increase the injection of the primary dilution steam stream into
the
second sparger 8 while valve 14 is closed more. If the temperature starts to
rise,
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the primary dilution steam valve will automatically close more to reduce the
primary dilution steam stream injected into the second sparger 8 while valve
14 is
opened wider.
In a preferred embodiment in accordance with the present
invention, the control system 7 can be used to control both the amount of the
fluid
and the amount of the primary dilution steam stream to be injected into both
spargers.
In the preferred case where the fluid is water, the controller varies
the amount of water and primary dilution steam to maintain a constant mixture
stream temperature 12, while maintaining a constant ratio of water-to-
feedstock in
the mixture 11. To further avoid sharp variation of the flash temperature, the
present invention also preferably utilizes an intermediate desuperheater 25 in
the
superheating section of the secondary dilution steam in the furnace. This
allows
the superheater 16 outlet temperature to be controlled at a constant value,
independent of furnace load changes, coking extent changes, excess oxygen
level
changes. Normally, this desuperheater 25 ensures that the temperature of the
secondary dilution steam is between 800 and 1100 F (430 and 590 ), preferably
between 850 and 1000 F (450 and 540 ), more preferably between 850 and 950 F
(450 and 510 C), and most preferably between 875 and 925 F (470 and 500 C).
The desuperheater preferably is a control valve and water atomizer nozzle.
After
partial preheating, the secondary dilution steam exits the convection section
and a
fine mist of water 26 is added which rapidly vaporizes and reduces the
temperature. The steam is then further heated in the convection section. The
amount of water added to the superheater controls the temperature of the steam
which is mixed with mixture stream 12.
Although it is preferred to adjust the amounts of the fluid and the
primary dilution steam streams injected into the heavy hydrocarbon feedstock
in
the two spargers 4 and 8, according to the predetermined temperature of the
mixture stream 12 before the flash drum 5, the same control mechanisms can be
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applied to other parameters at other locations. For instance, the flash
pressure and
the temperature and the flow rate of the flash steam 19 can be changed to
effect a
change in the vapor to liquid ratio in the flash. Also, excess oxygen in the
flue gas
can also be a control variable, albeit a slow one.
In addition to maintaining a constant temperature of the mixture
stream 12 entering the flash drum, it is also desirable to maintain a constant
hydrocarbon partial pressure of the flash stream 20 in order to maintain a
constant
ratio of vapor to liquid in the flash. By way of examples, the constant
hydrocarbon partial pressure can be maintained by maintaining constant flash
drum pressure through the use of control valves 36 on the vapor phase line 13,
and
by controlling the ratio of steam to hydrocarbon feedstock in stream 20.
Typically, the hydrocarbon partial pressure of the flash stream in
the present invention is set and controlled at between 4 and 25 psia (25 and
175
kPa), preferably between 5 and 15 psia (35 and 100 kPa), most preferably
between
6 and 11 psia (40 and 75 kPa).
The flash is conducted in at least one flash drum vessel.
Preferably, the flash is a one-stage process with or without reflux. The flash
drum
5 is normally operated at 40 to 200 psia (275 to 1400 kPa) pressure and its
temperature is usually the same or slightly lower than the temperature of the
flash
stream 20 before entering the flash drum 5. Typically, the pressure of the
flash
drum vessel is about 40 to 200 psia (275 to 1400 kPa) and the temperature is
about
600 to 950 F (310 to 510 C). Preferably, the pressure of the flash drum vessel
is
about 85 to 155 psia (600 to 1100 kPa) and the temperature is about 700 to 920
F
(370 to 490 C). More preferably, the pressure of the flash drum vessel is
about
105 to 145 psia (700 to 1000 kPa) and the temperature is about 750 to 900 F
(400
to 480 C). Most preferably, the pressure of the flash drum vessel is about 105
to
125 psia (700 to 760 kPa) and the temperature is about 810 to 890 F (430 to
480 C). Depending on the temperature of the flash stream, usually 50 to 95% of
the mixture entering the flash drum 5 is vaporized to the upper portion of the
flash
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drum, preferably 60 to 90%, more preferably 65 to 85%, and most preferably 70
to 85%.
The flash drum 5 is operated, in one aspect, to minimize the
5 temperature of the liquid phase at the bottom of the vessel because too much
heat
may cause coking of the non-volatiles in the liquid phase. Use of the
secondary
dilution steam stream 18 in the flash stream entering the flash drum lowers
the
vaporization temperature because it reduces the partial pressure of the
hydrocarbons (i.e., larger mole fraction of the vapor is steam) and thus
lowers the
10 required liquid phase temperature. It may also be helpful to recycle a
portion of
the externally cooled flash drum bottoms liquid 30 back to the flash drum
vessel
to help cool the newly separated liquid phase at the bottom of the flash drum
5.
Stream 27 is conveyed from the bottom of the flash drum 5 to the cooler 28 via
pump 37. The cooled stream 29 is split into a recycle stream 30 and export
stream
15 22. The temperature of the recycled stream is ideally 500 to 600 F (260 to
320 C), preferably 505 to 575 F (263 to 302 C), more preferably 515 to 565 F
(268 to 296 C), and most preferably 520 to 550 F (270 to 288 C). The amount of
recycled stream should be about 80 to 250% of the amount of the newly
separated
bottom liquid inside the flash drum, preferably 90 to 225%, more preferably 95
to
2 10%, and most preferably 100 to 200%.
The flash drum is also operated, in another aspect, to minimize the
liquid retention/holding time in the flash drum. Preferably, the liquid phase
is
discharged from the vessel through a small diameter "boot" or cylinder 35 on
the
bottom of the flash drum. Typically, the liquid phase retention time in the
drum is
less than 75 seconds, preferably less than 60 seconds, more preferably less
than 30
seconds, and most preferably less than 15 seconds. The shorter the liquid
phase
retention/holding time in the flash drum, the less coking occurs in the bottom
of
the flash drum.
In the flash, the vapor phase 13 usually contains less than 400 ppm
of non-volatiles, preferably less than 100 ppm, more preferably less than 80
ppm,
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and most preferably less than 50 ppm. The vapor phase is very rich in volatile
hydrocarbons (for example, 55-70%) and steam (for example, 30-45%). The
boiling end point of the vapor phase is normally below 1400 F (760 C),
preferably below l I00 F (600 C), more preferably below 1050 F (570 C), and
most preferably below 1000 F (540 C). The vapor phase is continuously
removed from the flash drum 5 through an overhead pipe which optionally
conveys the vapor to a centrifugal separator 38 which removes trace amounts of
entrained liquid. The vapor then flows into a manifold that distributes the
flow to
the convection section of the furnace.
to
The vapor phase stream 13 continuously removed from the flash
drum is preferably superheated in the pyrolysis furnace lower convection
section
23 to a temperature of, for example, about 800 to 1200 F (430 to 650 C) by the
flue gas from the radiant section of the furnace. The vapor is then introduced
to
the radiant section of the pyrolysis furnace to be cracked.
The vapor phase stream 13 removed from the flash drum can
optionally be mixed with a bypass steam stream 21 before being introduced into
the furnace lower convection section 23.
The bypass steam stream 21 is a split steam stream from the
secondary dilution steam 18. Preferably, the secondary dilution steam is first
heated in the pyrolysis furnace I before splitting and mixing with the vapor
phase
stream removed from the flash 5. In some applications, it may be possible to
superheat the bypass steam again after the splitting from the secondary
dilution
steam but before mixing with the vapor phase. The superheating after the
mixing
of the bypass steam 21 with the vapor phase stream 13 ensures that all but the
heaviest components of the mixture in this section of the furnace are
vaporized
before entering the radiant section. Raising the temperature of vapor phase to
800
to 1200 F (430 to 650 C) in the lower convection section 23 also helps the
operation in the radiant section since radiant tube metal temperature can be
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17
reduced. This results in less coking potential in the radiant section. The
superheated vapor is then cracked in the radiant section of the pyrolysis
furnace.
From the foregoing description, one skilled in the art can easily
ascertain the essential characteristics of this invention, and without
departing from
the spirit and scope thereof, can make various changes and modifications of
the
invention to adapt it to various usages and conditions. For instance, although
the
preferred embodiment calls for the use of water to mix with the preheated
feedstock in a sparger, other fluids such as naphtha can also be used.
The invention is illustrated by the following Examples which are
provided for the purpose of representation and are not to be construed as
limiting
the scope of the invention. Unless stated otherwise, all percentages, parts,
etc. are
by weight.
Example 1
Engineering calculations which simulate processing atmospheric
pipestill bottoms (APS) and crude oil by this invention have been conducted.
The
attached Table 1 summarizes the simulation results for cracking Tapis APS
bottoms and Tapis crude oil in a commercial size furnace with a flash drum.
The
very light components in crudes act like steam reducing the partial pressure
of the
heavy components. Hence, at a nominal 950 F (510 C) cut point, the flash drum
can operate 100 F (50 C) lower temperature than for atmospheric resids.
TABLE 1
Summary of Atmospheric Pipestill (APS) Bottoms
And Crude Oil Flash Drum Simulations
APS Fig 1
Bottoms Crude Ref. #
Convection feed rate, klb/hr (t/h) 126 (57) 100 (45) n/a
950 F minus (510 C), wt% 70 93 n/a
Temperature before sparger, F ( C) 400 (205) 352 (178) 4
Sparger water rate, klb/h (t/h) 12 (5) 43 (20) 14
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Primary dilution steam rate, klb/h (t/h) 18 (8) 8 (4) 17
Secondary dilution steam rate, klb/h (t/h) 17 (8) 19 (9) 18
Desuperheater water rate, klb/h (t/h) 6 (3) 6 (3) 26
Flash Drum Temperature, F ( C) 847 (453) 750 (400) 5
Flash Drum Pressure, psig (kPag) 107 (740) 101 (694) 5
Feed vaporized in flash drum, wt% 74 93 5
Residue exported, klb/h (t/h) 33 (15) 7 (3) 22
Example 2
Table 2 summarizes the simulated performance of the flash for
residue admixed with two concentrations of C4's. At a given flash temperature,
pressure and steam rate, each percent of C4's admixed with the residue
increases
the residue vaporized in the flash by about 1/4%. Therefore, the addition of
C4's
to feed will result in more hydrocarbon from the residue being vaporized.
TABLE 2
C4's/Residue Admixture Flash Performance
Pure Mix 1: Mix 2:
Residue Residue+C4's Residue+C4's
Wt% residue in convection feed 100 94 89
Wt% C4's in convection feed 0 6 11
Bubble point, F 991 327 244
@112 psig
Wt% of residue vaporized in flash 65.0% 68.2% 70.8%
Overall wt% vaporized in flash 65.0% 69.9% 74.0%
Temperature, F 819 819 819
Wt% of residue vaporized in flash 70.0% 72.8% 75.1%
Overall wt% vaporized in flash 70.0% 74.3% 77.8%
Temperature, F 835 835 835
Wt% of residue vaporized in flash 75.0% 77.4% 79.4%
Overall wt% vaporized in flash 75.0% 78.6% 81.7%
Temperature, F 855 855 855
