Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR CRACHING HYDROCARBON
FEED WITH WATER SUBSTITUTION
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the cracking of hydrocarbon feed
using water as a supplement to or substitute for dilution steam.
Description of Background
Steam cracking has long been used to crack various hydrocarbon
feeds into olefins. Conventional steam cracking utilizes a pyrolysis furnace,
which has two main sections: a convection section and a radiant section
reaction
zone. The hydrocarbon feed typically enters the convection section of the
furnace
as a liquid (except for light feeds 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 mixing with steam. The vaporized feed 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.
By way of non-limiting illustration, in a typical pyrolysis reactor
furnace for the production of ethylene from naphtha feed, the hydrocarbon feed
enters the convection section of the furnace where it is preheated in first
heat
exchange tubes by indirect contact with furnace flue gas from the radiant
section.
A dilution steam stream can enter the convection section wherein it is
superheated, also in heat exchange tubes by indirect contact with furnace flue
gas
from the radiant section. The superheated dilution steam is then mixed with
the
hydrocarbon feed to reduce the hydrocarbon partial pressure in the radiant
section
reaction zone of the furnace. It is well known in the art that reducing the
hydrocarbon partial pressure in the reaction zone (1) increases the
selectivity of
the reactor to desired olefinic products such as ethylene, and (2) reduces the
rate at
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which undesirable coke is formed and deposited on the interior surfaces of
radiant
section tubes. The superheated steam is mixed with the preheated hydrocarbon
feed producing a vapor hydrocarbon/steam mixture which is further preheated to
a
temperature suitable for conveying the mixture to the radiant section of the
furnace. The cracking reactions which produce the desired ethylene product and
other byproducts take place predominantly in the radiant section of the
furnace.
After leaving the radiant section, the reactor effluent is rapidly quenched in
a
quench system to stop the cracking reactions.
For well-known energy efficiency purposes, it is desirable to
recover as much heat as possible from the flue gas leaving the radiant section
and
flowing through the convection section of the furnace to the furnace flue gas
exhaust. Thus, hydrocarbon feed and dilution steam are heated in the
convection
section, typically by indirect contact with flue gas from the radiant section.
Other
recovery services may also be included in the convection section such as a
boiler
feed water preheater andlor a steam superheater used to superheat high
pressure
steam which may be generated in the quench system of the furnace.
In some furnace designs, boiler feed water preheat and/or high
pressure steam superheat services may not be available to absorb heat from the
flue-gas stream flowing through the convection section. In such cases, the
flue
gas may exit the furnace at unacceptably high temperatures, for example, as
high
as 600 to 700°F (315 to 370°C). This represents a substantial
energy inefficiency,
as some designs provide for flue-gas discharge temperatures as low as, for
example, 250 to 300°F (120 to 150°C).
In other instances, it may be desirable to provide additional dilution
steam to further decrease the hydrocarbon feed partial pressure. But such
steam
may not be available at reasonable cost.
The present invention provides an advantage of providing for
additional dilution steam when it is otherwise unavailable at a reasonable
cost.
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The present invention also provides another advantage of
improving furnace energy efficiency. These and other features and advantages
of
the present invention will become apparent from the following description and
claims.
SUMMARY OF THE INVENTION
The present invention provides a process for treating hydrocarbon
feed in a furnace, the process comprising: (a) heating hydrocarbon feed, (b)
adding water and dilution steam to the heated feed to form a mixture, (c)
heating
the mixture and (d) feeding the heated mixture from (c) to the furnace,
wherein
the water in (b) is added in an amount of from at least about 1 % to 100%
based on
water and dilution steam by weight. In one embodiment, the water is added in
an
amount of at least about 3% based on water and dilution steam by weight (i.e.,
from at least about 3% to 100% water). In another embodiment, the water is
added in an amount of at least about 10% based on water and dilution steam by
weight. In a further embodiment, the water is added in an amount of at least
about
30% based on water and dilution steam by weight. In accordance with the
present
invention, water can be a total substitute for dilution steam (i.e., no
addition of
steam). It is preferred, however, that both dilution steam and water are added
to
the hydrocarbon feed.
According to a preferred embodiment, the water is added prior to
the addition of dilution steam, if any.
According to another embodiment, the ratio of water to steam
added to the heated feed is varied according to fluctuations in at least one
process
variable. In a preferred embodiment, the process variable is process
temperature.
In this regard, the process temperature can be the temperature of the flue gas
exiting the furnace, the temperature of process in the convection section of
the
furnace and/or the temperature of process to the radiant section (reaction
zone) of
the furnace.
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According to a further embodiment, the water is added to the
hydrocarbon feed in a sparger and dilution steam, if any, is added to the feed
in
another sparger. In a preferred embodiment, a first and a second sparger are
part
of a sparger assembly in which the first and second spargers are connected in
fluid
flow communication in series.
The present invention also provides a process for cracking
hydrocarbon feed in a furnace, the furnace comprising a radiant section
comprising burners that generate radiant heat and hot flue gas, and a
convection
section comprising heat exchange tubes, the process comprising:
(a) preheating the hydrocarbon feed in heat exchange tubes in
the convection section by indirect heat exchange with the hot flue
gas from the radiant section to provide preheated feed;
(b) adding water to the preheated feed in a first sparger and
adding dilution steam to the preheated feed in a second sparger to
form a feed mixture;
(c) heating the feed mixture in heat exchange tubes in the
convection section by indirect heat transfer with hot flue gas from
the radiant section to form a heated feed mixture; and
(d) feeding the heated feed mixture to the radiant section
wherein the hydrocarbon in the heated feed mixture is thermally
cracked to form products;
wherein the water in (b) is added in an amount of from at least about 1 % to
100%
based on water and dilution steam by weight.
In a preferred embodiment, the first sparger comprises an inner
perforated conduit surrounded by an outer conduit so as to form an annular
flow
space between the inner and outer conduits. Preferably, the preheated
hydrocarbon
flows through the annular flow space and the water flows through the inner
conduit and is injected into the preheated hydrocarbon feed through the
openings
(perforations) in the inner conduit.
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In yet another preferred embodiment, the second sparger comprises
an inner perforated conduit surrounded by an outer conduit so as to form an
annular flow space between the inner and outer conduits. Preferably, the feed
from the first sparger flows through the annular flow space and the dilution
steam
5 flows through the inner conduit and is injected into the first feed mixture
through
the openings (perforations) in the inner conduit.
In a further preferred embodiment, the first and second spargers are
part of a sparger assembly in which the first and second spargers are
connected in
fluid flow communication in series.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig.l illustrates a schematic flow diagram of a process in
1 S accordance with the present invention employed with a pyrolysis furnace,
with
particular emphasis on the convection section of the furnace. This figure also
shows a control schematic for varying the ratio of water to dilution steam
according to a process variable, namely, the temperature of process gas to the
radiant section of the furnace. Fig. 2 illustrates a schematic diagram of a
control
system for use in varying the ratio of water to dilution steam in connection
with a
process parameter, specifically, the temperature of the flue gas exiting the
furnace.
Fig. 3 illustrates a schematic diagram of the same control system, but for
varying
the ratio of water to dilution steam in connection with the temperature of
process
gas in the convection section of the furnace.
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.
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Further, when an amount, concentration, or other value or
parameters 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.
The present invention relates to a process for treating hydrocarbon
feed in a furnace. According to one embodiment, the process comprises (a)
heating hydrocarbon feed, (b) adding water and dilution steam to the heated
feed
to form a mixture, (c) heating the mixture, and (d) feeding the heated mixture
to
the furnace, wherein the water in (b) is added in an amount of from at least
about
1 % to 100% based on water and dilution steam by weight.
With particular reference to Fig. 1, 1 generally refers to a pyrolysis
furnace comprised of a lower radiant section 2, an intermediate convection
section
3 and an upper flue gas exhaust section 4. In the radiant section, radiant
burners
provide radiant heat to hydrocarbon feed to produce the desired products by
thermal cracking of the feed. The burners generate hot gas that flows upwardly
through convection section 3 and then out of the furnace through flue gas
exhaust
section 4. As shown in Fig. l, hydrocarbon feed 33 enters an upper portion of
the
convection section 3 where it is preheated. The preheating of the hydrocarbon
feed 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 feed 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 feed through heat exchange tubes 17 located within the convection
section 3 of the furnace 1. The preheated feed has a temperature between 200
and
600°F (95 and 31 S°C). Preferably the temperature of the heated
feed is about 300
to 500°F (150 to 260°C) and more preferably between 350 and
500°F (175 and
260°C).
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After the preheated hydrocarbon feed exits the convection section
at 41, water 5 and dilution steam 6 are added thereto to form a mixture. Water
is
added to the preheated feed in an amount of from at least about 1 % to 100%
based
on the total amount of water and dilution steam added by weight. Preferably,
the
water is added in an amount of at least about 3% (i.e., about 3% to about 100%
water) based on water and dilution steam by weight. More preferably, the water
is
added in an amount of at least about 10%, most preferably at least about 30%,
based on water and dilution steam by weight. It is understood that, in
accordance
with an embodiment of the invention, 100% water could be added to the
hydrocarbon feed such that no dilution steam is added. The sum of the water
flow
and dilution steam flow provides the total desired reaction zone H~CI required
to
achieve the desired hydrocarbon partial pressure.
As shown in Fig. 1, water 5 is preferably added to the preheated
feed 4I prior to addition of dilution steam. It is believed that this order of
addition
will reduce undesirable pressure fluctuations in the process stream
originating '
from mixing the hydrocarbon, water and dilution steam. Such fluctuations are
commonly referred to as water-hammer or steam-hammer. While the addition of
water and dilution steam to the preheated hydrocarbon feed could be
accomplished using any known mixing device, it is preferred to use a sparger
assembly 7 as illustrated in the drawings. Water is preferably added in a
first
sparger 8. As shown, first sparger 8 comprises an inner perforated conduit 9
surrounded by an outer conduit 10 so as to form an annular flow space 11
between
the inner and outer conduits. Preferably, the preheated hydrocarbon feed 41
flows
through the annular flow space 11. Also preferably, water S flows through the
inner perfoxated conduit 9 and is injected into the preheated hydrocarbon feed
through the openings (perforations) shown in inner conduit ~.
Dilution steam 6 is preferably added to the preheated hydrocarbon
feed in a second sparger 12. As shown, second sparger 12 comprises an inner
perforated conduit 13 surrounded by an outer conduit 14 so as to form an
annular
flow space I S between the inner and outer conduits. Preferably, the preheated
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hydrocarbon feed 41 to which the water has been added flows through the
annular
flow space I5. Also preferably, dilution steam flows through the inner
perforated
conduit 13 and is injected into the preheated hydrocarbon feed through the
openings (perforations) shown in inner conduit.
Preferably, the first and second spargers are part of a sparger
assembly as shown in which the spaxgers are connected in fluid flow
communication in series. As shown in the drawings, the spargers 8 and I2 are
interconnected in fluid flow communication in series by fluid flow
interconnector
16.
As further illustrated in the drawings, upon exiting the sparger
assembly 7, the mixture (of hydrocarbon feed, water and dilution steam) flows
back into furnace 1 wherein the mixture is further heated in a lower portion
of
convection section 3. The further heating of the hydrocarbon feed 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 feed in the lower convection section
3 of
the furnace 1 with not flue gases from the radiant section of the furnace.
This can
be accomplished, by way of non-limiting example, by passing the feed through
heat exchange tubes 18 located within the convection section 3 of the furnace
1.
Following the additional heating of the mixture at I 8, the resulting heated
mixture
exits the convection section at 19 and thCnr flows to ~the~ xadiaiit section
of the
furnace for thermal cracking of the hydrocarbon. The heated feed to the
radiant
section preferably has a temperature between 800 and 1400°F (425 and
760°C).
Preferably the temperature of the heated feed is about 1050 to 1350°F
(560 to
730°C).
Figure 1 further illustrates using the invention to control the
process temperature to the radiant section 2. The process temperature is an
input
to a controller 26 which controls the flow rate of water via a flow meter 28
and a
control valve 29. The water then enters the sparger 7. When the process
temperature is too high, controller 26 increases the flow of water 5.
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Controller 26 also sends the flow rate signal to a computer control
application schematically shown at 31, which determines the dilution steam
flow
rate as detailed below. A pre-set flow rate of the hydrocarbon feed 33 is
measured
by flow meter 34, which is an input to controller 35, which in turn sends a
signal
to feed control valve 36. Controller 35 also sends the feed rate signal to a
computer control application 37, which determines the total HZO to the radiant
section by multiplying the feed rate by ~a pre-set total H2Q to feed rate
ratio. The
total H20 rate signal is the second input to computer application 31. Computer
application 31 subtracts the water. flow rate from the total HBO rate; the
difference
is the set point for the dilution steam controller 38. Flow meter 39 measures
the
dilution steam rate, which is also an input to the controller 38~. When water
flo~.v
rate increases, as discussed above, the set point inputted to the dilution
steam
controller 38 decreases. Controller 38 then instructs control valve 40 to
reduce the
dilution steam rate 32 to the new set point. When the process temperature 25
is
too low the control scheme instructs control valve 29 to reduce water rate and
instructs control valve 40 to increase the steam rate while maintaining
constant
total Hz~ rate.
Alternatively, this control scheme works the same way to control
the discharge temperature of the flue gas 42 as illustrated in Figure 2, and
to
control the process temperature in the convection section of the furnace,
illustrated
in Figure 3. in connection with controlling the temperature of the flue gas
discharge, it is preferred that flue gas exits at a temperature of less than
about
650°F (345°C), preferably less than about 450°F
(230°C), more preferably less
than about 350°F (175°C).
Processes in accordance with the present invention make it possible
to maintain a desired hydrocarbon partial pressure in the radiant section
reaction
zone of a furnace, while increasing the convection section heat recovery
requirement due to the heat of vaporization of the water stream. Such a system
can result in a lower flue-gas discharge temperature and, thus, a more energy
efficient furnace.
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Similarly, processes in accordance with the present invention
enable the desired reaction zone hydrocarbon partial pressure to be maintained
in
a facility where the available supply of dilution steam is limited and/or is
insufficient for the desired furnace operating conditions.
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