Note: Descriptions are shown in the official language in which they were submitted.
CA 02764014 2012-01-16
1 "A JACKETED FIRETUBE SYSTEM FOR A PROCESS VESSEL"
2
3 FIELD OF THE INVENTION
4 Embodiments of the invention relate to firetubes for use in process
vessels, such as heaters or heater/treaters used in the oil and gas industry,
and
6 more particularly, to apparatus and systems for maximizing heat recovery
from the
7 firetube and for minimizing fouling and localized overheating.
8
9 BACKGROUND OF THE INVENTION
It is known to heat process fluids in a variety of vessels, such as
11 ASME code process vessels, atmospheric bath heaters and tanks. Generally a
"U"
12 tube heat exchanger is fit within the process vessel. Heat is transferred
from the
13 heat exchanger to the process fluids therein for heating the process fluids
as part of
14 the handling and refining operation.
In the oilfield, U-tube heat exchangers or U-shaped "firetubes" are
16 common for use in separation vessels such as heater-treaters and free-water
17 knockout vessels and in line heaters and tanks. Traditionally, the
firetube, generally
18 made of one or more sections of round, steel pipe, is heated by a burner
which is
19 connected to an inlet end of the firetube and discharges flame and exhaust
gases to
the firetube for passage therethrough. A vent or exhaust stack at an outlet
end of
21 the firetube discharges heat-depleted exhaust gases therefrom. Any heat
remaining
22 in the exhaust gases is typically lost to the exhaust stack.
1
CA 02764014 2012-01-16
1 In a direct-fired vessel, the firetube is immersed in process fluid
2 contained in the vessel. An external surface of the firetube is in direct
contact with
3 the process fluid for effecting the heat transfer thereto. Heat is
transferred from the
4 hot gases passing through in the firetube by transfer through the firetube's
wall and
directly to the process fluid.
6 Conventional firetubes are built in a "U"- shape, having the burner end
7 and the exhaust stack end fit at a common wall. The "U"-shape extends into
the
8 vessel from the common wall. The common walls of one or more firetubes are
9 installed inside the vessel through one or more obround or oval shaped
manways in
a front wall of the vessel.
11 It is known in the oil and gas industry that hydrocarbon-based process
12 fluids, such as hydrocarbon emulsions, contain not only hydrocarbon and
water, but
13 may also contain contaminants such as polymers, solids, sand and the like.
14 Polymer additives are often used in operations on subterranean hydrocarbon-
bearing formations including water flood recovery. Accordingly, fluids
returned from
16 the formation contain not only oil and water, but also contain significant
amounts of
17 polymer. In fluid recovery vessels, clean, dry product oil is recovered
from
18 contaminant-rich, hydrocarbon returned fluids including emulsions. The
returned
19 fluids are directed to a conventional heater/treater vessel for separation
therein. In
the case of a direct-fired heater vessel, contaminants and particularly the
polymers,
21 are susceptible to attaching to the very hot external surfaces of the
firetube,
22 resulting in a coating or fouling of the firetube's surfaces and reducing
the ability to
23 effectively exchange heat to the process fluids.
2
CA 02764014 2012-01-16
1 Further, in automated operations, as the monitored temperature of the
2 process fluid does not increase to desired operational temperatures or even
lowers,
3 the control system increases the burner input in response. Increased burner
input
4 results in a larger flame which can cause flame impingement at inside walls
of the
firetube, impingement creating localized hot spots. Hot spots can ultimately
result in
6 burn-through of the firetube walls. Applicant is aware that in some cases,
firetubes
7 in polymer-contaminated process vessels can be changed out as frequently as
8 every two weeks.
9 Clearly there is a need for means to extend the service life of the
firetube, particularly when the process fluid to be heated is a contaminant or
11 polymer-rich fluid emulsion.
12
13
3
CA 02764014 2012-01-16
1 SUMMARY OF THE INVENTION
2 Embodiments of a jacketed firetube system efficiently utilize and
3 conserve heat within a direct-fired heater and protect the firetube from
developing
4 hotspots, by circulating a heat transfer fluid, externally or internally,
along at least
the portion of the firetube which is at greatest temperature.
6 In one broad aspect, a method for recovering heat from a firetube
7 immersed in a process fluid in a process vessel, for heating the process
fluid,
8 comprises circulating a cool heat transfer fluid in a circulation circuit in
thermal
9 communication with and extending along at least a portion of the firetube
and
therewith. Heat is recovered from the firetube for heating the heat transfer
fluid
11 therein and thereafter heat is transferred from the heated, heat transfer
fluid to the
12 process fluid.
13 In another broad aspect, a jacketed firetube system is used to transfer
14 heat from a firetube to a process fluid in a process vessel where the
firetube is
adapted for connection to a burner at a first burner end and a vent stack at a
16 second outlet end and having a passageway therebetween for directing hot
gases
17 from the burner to the vent stack. The firetube is immersed in the process
fluid. The
18 jacketed firetube system comprises a circulation circuit extending along at
least a
19 portion of the firetube for circulating a heat transfer fluid therein. The
circulation
circuit comprises a tubular jacket, extending along the firetube from the
first burner
21 end along at least a portion thereof, the jacket in thermal communication
therewith
22 and defining a circulation space therebetween. An inlet to the jacket
delivers cool,
23 heat transfer fluid to the circulation space. An outlet from the jacket
discharges
4
CA 02764014 2012-01-16
1 heated, heat transfer fluid therefrom. The heat transfer fluid is circulated
through the
2 circulation space for recovering at least a portion of the heat from the
firetube.
3 Process vessels can be manufactured using embodiments of the
4 jacketed firetube system and existing process vessels can be retrofit using
embodiments which can be inserting internally into the conventional firetube
or the
6 conventional firetube can be removed and an external jacketed firetube
7 embodiment inserted into the vessel.
8 Using embodiment of the jacketed firetube system, Applicant believes
9 the efficiency of the firetube increases from a conventional efficiency in
the range of
about 60% to about 90%.
11
12 BRIEF DESCRIPTION OF THE DRAWINGS
13 Figure 1 is a side view of a prior firetube installed in a vessel and
14 immersed in fluid therein, a portion of the vessel having been removed for
clarity;
Figure 2 is a side view of a jacketed firetube system according to an
16 embodiment, having a jacket external to the firetube and enclosing the
entirety of
17 the firetube therein, a portion of the vessel having been removed for
clarity;
18 Figure 3 is a side view according to Fig. 2, a divider wall being
19 positioned in the jacket between a first and second leg of the U-tube
firetube for
directing fluid circulated therein;
21 Figure 4 is a side view of a jacketed firetube system according to
22 another embodiment, the external jacket extending along only a portion of
the
5
CA 02764014 2012-01-16
1 firetube and enclosing a portion of the first and second legs therein, a
portion of the
2 vessel having been removed for clarity;
3 Figure 5A is a side view of a jacketed firetube system according to
4 another embodiment, the external jacket extending along the first leg of the
firetube,
a portion of the vessel having been removed for clarity;
6 Figure 5B is a cross-sectional view along section lines B-B according
7 to Fig. 5A;
8 Figure 6 is a side view of a jacketed firetube system according to
9 another embodiment, an internal jacket insert extending within and along the
first
leg of the firetube, a portion of the vessel having been removed for clarity;
11 Figure 7A is a cross-sectional view of the internal jacket insert
12 according to Fig. 6;
13 Figure 7B is a perspective view of the internal jacket according to Fig.
14 6;
Figure 8 is a side view of a jacketed firetube system according to an
16 embodiment installed in a heater or heater/treater vessel where the heat
transfer
17 fluid is a vessel product fluid, illustrating fluid connections between the
vessel and
18 the jacketed firetube;
19 Figure 9 is a side view of a jacketed firetube system according to an
embodiment installed in a heater or heater/treater vessel where the heat
transfer
21 fluid is a fluid incompatible with the process fluid, illustrating fluid
connections
22 between the vessel, a heat exchanger and the jacketed firetube;
6
i
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1 Figure 10 is a partial side view of a jacketed firetube having thermal
2 conducting passageways in the firetube; and
3 Figure 11 is a partial side view of a jacketed firetube having thermal
4 conducting passageways in the jacket.
6 DESCRIPTION OF THE PREFERRED EMBODIMENT
7 Having reference to Fig. 1, a prior art heater system for a process
8 vessel V for heating a process fluid F is shown. An example of a process
fluid F
9 treated in such a vessel V is a contaminant-rich hydrocarbon process fluid
emulsion. The heater system comprise a firetube 12 having a first burner end
14
11 adapted for connection to a burner B, a second outlet end 16 adapted for
12 connection to an exhaust stack X and a passageway A therebetween for
conducting
13 hot gases G from the inlet 14 to the outlet 16. The prior art firetube 12
is immersed
14 directly in the process fluid F to be heated in the vessel V. Heat which is
not
transferred from exhaust gases G flowing through the firetube 12 to the
process
16 fluid F is lost to the exhaust stack X. Further, at least a portion of the
firetube 12,
17 particularly at the first burner end 14 adjacent the burner B, is exposed
to the flame
18 and is at an elevated temperature. This high temperature portion of the
firetube is
19 at risk, to itself and to the process fluid. The high temperatures are
detrimental to
process fluid F and to the firetube 12 as a result, particularly when the
process fluid
21 F contains polymers, causing fouling of the firetube 12. Fouling of the
firetube 12
22 causes a reduction in heat transfer efficiency and the encouragement of the
23 conditions for formation of hotspots.
7
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1 Applicant understands that, in a conventional U-tube firetube 12, this
2 high temperature portion corresponds, at least in part, to the flame portion
from the
3 burner B, typically extending from the first burner end 14 of the firetube
12 about 1/3
4 of the outgoing length of the firetube 12. Longer firetubes 12 receive
greater input
with greater flame lengths, still being in the order of 1/3 of the outgoing
length.
6 Temperatures, in the first 1/3 of the firetube 12, can typically reach about
1300 F,
7 depending upon system parameters.
8 As shown in Figs. 2 - 9, embodiments of a jacketed firetube system 10
9 comprise a U-shaped firetube 12, having the first burner end 14 adapted for
connection to the burner B and the second outlet end 14 adapted for connection
to
11 the exhaust stack X. The burner B discharges flame and exhaust gas G along
a
12 firetube bore 11.
13 The system further comprises a circulation circuit 17 for recovering
14 heat from the firetube 12, extending from about the first burner end 12. A
heat
transfer fluid P, such as an unheated or cool heat transfer fluid P', is
circulated
16 along at least a portion of the firetube 12, particularly along a high
temperature
17 portion TH, extracting heat therefrom and producing a heated heat transfer
fluid Ph.
18 As a result, the temperature of the firetube 12 along the high temperature
portion
19 TH is reduced, recovering maximal heat therefrom, and reducing the
temperature
below that temperature which is detrimental to the heat transfer fluid P and
21 ultimately detrimental to the firetube 12. While shown in the context of a
U-shaped
22 firetube 12, conveniently supported from a common front wall 20, the heat
recovery
8
CA 02764014 2012-01-16
1 and embodiments herein can also be applied to other straight and arcuate
firetubes
2 having different interfaces with the vessel V.
3 The heat transfer fluid P may be a product fluid produced by a
4 treatment of the process fluid, or may be another fluid, such as gycol.
The circulation circuit 17 comprises structure extending along at least
6 the high temperature portion TH and forming a fluid passageway for placing
the
7 heat transfer fluid P in thermal contact with the firetube 12. The
circulation structure
8 comprises a tubular jacket 18 which extends from the firetube's first burner
end 14,
9 adjacent the burner B, and along the at least a portion of the firetube 12
where the
temperature of the firetube 12 is highest. The jacket forms a circulation
space C
11 between itself and the firetube 12. The jacket 18 is closed, such as by an
annular
12 front wall 21 and an end wall 22. The circulation circuit 17 further
comprises a
13 jacket inlet 30 which receives and delivers the received, cool heat
transfer fluid Pc to
14 the circulation space C and a jacket outlet 32 which discharges heated heat
transfer
fluid Ph therefrom.
16 In the embodiment of Fig. 2, the tubular jacket 18 is external to the
17 firetube 12, enclosing the entirety of the U-shaped firetube 12 therein. As
shown, a
18 conventional U-shaped firetube 12 has a first outgoing leg 13 extending
from the
19 first burner end 14 to a U-shaped bend 36 and a second return leg 15
extending
from the U-shaped bend 36 to terminate at the second outlet end 14. The first
21 burner and second outlet ends 14, 16 extend through the front wall 21 of
the jacket
22 18 for connection to the burner B and exhaust stack X. The jacket's front
wall 21
23 can also act or form the common front wall 21 for connection of the
firetube 12 to
9
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1 the vessel V. Hot exhaust gases G flow through the firetube 12 from the
first burner
2 end 14 to the second outlet end 14. The jacket inlet 30 and jacket outlet 32
are
3 connected to the jacket 18 at the jacket's front wall 21.
4 In an embodiment, best seen in Fig. 3, a divider wall 34 is positioned
within the jacket 18 and extends from the front wall 20 toward the end wall 22
for
6 fully dispersing the fluid flow over the entirety of the firetube 12 for
maximal heat
7 transfer. The divider wall 34 blocks any direct short circuiting of fluid
flow from the
8 jacket inlet 30 to the jacket outlet 32. The divider wall 34 extends between
the
9 firetube's first outgoing and second return legs 13, 15 and only partially
toward the
end wall 22 so as to accommodate the "U"-shaped bend 36 of the firetube 12
11 therebetween. The divider wall 34 forms a U-shaped circulation space C and
flow
12 path through the jacket 18, the heat transfer fluid P flowing from the
jacket inlet 30
13 and all along the firetube 12 to the jacket outlet 32. In embodiments, the
jacket inlet
14 30 can be positioned in the jacket's front wall 21 or a side wall 24
accessible outside
the vessel V.
16 Having reference to Fig. 4, in another embodiment, the tubular jacket
17 18, is again external to the firetube 12 however the jacket 18 only extends
outwardly
18 toward the U-shaped bend 36 from the first and second ends 14, 16 and along
a
19 portion of both the first and second legs 13, 15 for forming the
circulation space C.
The first and second legs 13,15 extend through the jacket's end wall 22, while
21 containing the circulation space C within the jacket 18. Again, a divider
wall 34 can
22 extend between the first and second legs 13, 15 only partially toward the
jacket's
23 end wall 22 for forming a U-shaped circulation space C through the jacket
18.
CA 02764014 2012-01-16
1 In yet another embodiment, shown in Figs. 5A and 5B, the tubular
2 jacket 18 is external to only the first leg 13 of the firetube 12 and
extends outwardly
3 therealong from the firetube's first burner end 14 towards the U-shaped bend
36.
4 The jacket 18 can extend along a portion of the firetube 12, such as just
the high
temperature portion TH, or up to substantially the entire length of the first
leg 13.
6 The first leg 13 extends through the jacket's end wall 22 while containing
the
7 circulation space C within the jacket 18. The front wall 21 and end wall 22
are in the
8 form of annular rings. One of skill would appreciate baffles, flow diverters
or other
9 forms of dividers 34 may be placed in the circulation space C to guide the
heat
transfer fluid P along the entirely the firetube 12 from the jacket inlet 30
to adjacent
11 the end wall 22 and back to the jacket outlet 32.
12 Having reference to Figs. 6, 7A and 7B, the tubular jacket 18 is a
13 tubular insert, installable internal to at least the first leg 13 of the
firetube 12 and
14 extending therein from the first burner end 14 towards the U-shaped bend 36
for
forming the circulation space C therebetween. The tubular insert form of
jacket 18
16 can extend along a portion of the firetube 12, such as just the high
temperature
17 portion TH, or up to substantially the entire length of the first leg 13.
In
18 embodiments, as shown in Fig. 7A and 7B, the internal tubular jacket 18
comprises
19 an inner tubular wall 26 and an outer tubular wall 27, an annulus 28 being
formed
therebetween and defining the circulation space C. The circulation space C is
21 enclosed by the front and end walls 20, 22. The front wall 21 and end wall
22 are in
22 the form of annular rings. The inner tubular wall 26 defines a bore 29
formed in the
11
CA 02764014 2012-01-16
1 jacket 18, the bore 29 being the passageway A for the flame and exhaust
gases G
2 passing through the firetube 12.
3 The embodiment of Figs. 6 - 7B, being insertable to a conventional
4 firetube without modification of the firetube, enables convenient and rapid
retrofitting
of existing process vessels V. The insert jacket 18 can be fit with a flange
20F
6 compatible with and for securing to the burner end 14.
7 In one mode of operation, and having reference to Fig. 8, a jacketed
8 firetube system 10 comprising a firetube 12, jacket 18 and circulation
circuit 17,
9 according to an embodiment, is shown fit into a heater or heater/treater
process
vessel 40. Any of the various jacket embodiments are equally applicable in the
11 implementation of this system 10. The illustrated process vessel 40
comprises one
12 or more manways 42 in a front wall 44 through which one or more of the
jacketed
13 firetube systems 10 extend. A weir 44, positioned within the process vessel
40,
14 separates the process vessel 40 into a first containment area 46 and a
second
containment area 48. The first containment area 46 is adjacent the front wall
44 of
16 the vessel 40 for receiving the contaminant-rich feedstream F, such as a
polymer-
17 rich oil/gas/water emulsion, through a feed inlet 50. The jacketed firetube
system 10
18 protrudes into the first containment area 46 and is in contact therein with
the
19 emulsion feedstream F for heat transfer thereto. The feedstream F is
exposed to
heat from the firetube 12 for treatment. The second containment area 48 acts
to
21 hold the treated product fluid, such as a clean, dry product oil produced
in the
22 process vessel 40.
12
CA 02764014 2012-01-16
1 In an embodiment, the heat transfer fluid P is the product oil from the
2 vessel 40. A pump 54 is fluidly connected between the vessel's second
3 containment area 48 and the jacket inlet 30 for providing a slipstream of
clean
4 product oil as the cool heat transfer fluid PC to be circulated through the
circulation
space C. In an embodiment, the pump 54 is a variable speed pump which is
6 controlled by operational parameters of the system, such as the temperature
of the
7 firetube 12. As the temperature of the heated fluid PH rises, the pump 54
increases
8 the amount of the clean product oil P provided to the jacket 18 for
regulating the
9 temperature of the fluid P and the firetube 12. Maximum heat is removed from
the
firetube 12 and high temperatures are avoided for minimizing fouling and
11 substantially preventing hotspots and firetube damage.
12 The cool product oil Pc is circulated within the circulation space C
13 from the jacket inlet 30 to the jacket outlet 32, heating the product fluid
PH therein.
14 As the product fluid P is compatible with the process fluid feedstream F,
heat
recovered from the firetube 12 by the heated product fluid PH can be
transferred to
16 the process fluid F in the vessel 40 by reintroducing the heated product
fluid PH to
17 the feedstream F.
18 In an embodiment, the jacket outlet 32 is fluidly connected to the first
19 containment area 46, such as through the vessel's feed inlet 50. Thus, the
heated
product oil PH is mixed with the contaminant-rich feedstream F within the
first
21 containment area 46 in the process vessel 40. Heat in the heated product
oil PH is
22 transferred to the incoming contaminant-rich feedstream F, thus conserving
heat in
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CA 02764014 2012-01-16
1 the system and reducing energy consumption required to heat the contaminant-
rich
2 feedstream F in the vessel 40.
3 Having reference to Fig. 9, in an embodiment a heat transfer fluid P
4 that is not compatible with the process fluid F, such as glycol, can be
circulated
though the circulation space C. Heat from the heated glycol PH is thereafter
6 recovered, such as through a conventional heat exchanger 58 and the heat
7 recovered thereby reused in the system 10, as is understood by those of
skill on the
8 art. In an embodiment, a feed stream of process fluid F is flowed through
the heat
9 exchanger 58 to recover the heat from the heated glycol PH. The cooled
glycol Pc is
then recirculated through the circulation space C.
11 Process vessels 40, such as conventional heater/treater vessels, are
12 retrofit using embodiments of the jacketed firetube system 10.
13 With reference again to Figs. 2-5, where the jacketed firetube system
14 10 has the jacket 18 external to the firetube, the conventional firetube is
removed
and the jacketed firetube system 10 inserted therein. In order to fit the
externally
16 jacketed firetube 12 within the existing manway 42, the firetube 12 within
the jacket
17 18 is made smaller in diameter than the conventional firetube 12 removed
from the
18 vessel 40. For example, a conventional 24" diameter firetube 12 is replaced
by a
19 20" diameter firetube 12 housed within a jacket 18 sized to fit the opening
of the
conventionally sized firetube 12 removed therefrom.
21 With reference again to Figs. 6 to 7B, in the case where the jacket 18
22 is an insert to be fit internal to the firetube 12, the jacket 18 is
dimensioned to fit
14
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1 within at least the first leg 13 of the firetube 12 and the jacket 18 is
merely inserted
2 therein.
3 As is understood by one of skill in the art, additional retrofit is required
4 to provide a fluid connection 31 between the jacket outlet 32 and the
vessel's feed
inlet 50 and to provide a fluid connection 33 between the vessel 40 and the
pump
6 54 for removing the slipstream of clean product oil P to be pumped to the
jacket
7 inlet 30, if required.
8 As will be appreciated by one of skill in the art, reduction in the size of
9 the firetube 12 and removal of the slipstream of clean product oil P from
the vessel
40, for circulation through the jacket 18, with subsequent reintroduction with
the
11 feedstream F, will result in only a small percentage loss of the overall
capacity of
12 the heater/treater vessel 40.
13 As shown in Fig. 10, in embodiments having the jacket 18 external to
14 the firetube 12, a plurality of thermal conducting passageways 60, such as
taught in
Applicant's co-pending US application 13/324,938, filed December 13, 2011 and
16 incorporated herein in its entirety, extend through the U-shaped firetube
12. Each
17 thermal conducting passageway 60 directs cool heat transfer fluid Pc to be
heated
18 therethrough. Each of the passageways 60 has a thermal conductive wall 62
19 extending through the firetube 12 which adds to the external surface area
of the
firetube 12, for heat transfer to the fluid flowing therethrough.
21 The cool heat transfer fluid Pc in the jacket 18, circulated therein, is
22 caused to pass through the thermal conducting passageways 60 enhancing heat
23 transfer thereto. Applicant believes the cool heat transfer fluid Pc flows
generally
CA 02764014 2012-01-16
1 upwardly through the thermal conducting passageways 60 as a result of a
2 thermosiphon effect caused by a temperature differential between the heated
heat
3 transfer fluid PH above the firetube 12 and the cooler heat transfer fluid
Pc below
4 the firetube 12. The cool heat transfer fluid Pc flows from a fluid inlet 64
at a portion
66 of the circulation space C in the jacket 18 below the firetube 12 to a
fluid outlet
6 68 at a portion 70 of the circulation space C in the jacket 18 above the
firetube 12.
7 As one of skill will appreciate flow diverters may be positioned within
8 the jacket 18 to enhance the thermosiphon effect urging the cooler heat
transfer
9 fluid Pc to flow through the thermal conductive passageways 60.
In another embodiment, as shown in Fig. 11, the thermal conducting
11 passageways 60 are formed in the jacket 18 and the process fluid F flows
12 therethrough as a result of the thermosiphon effect caused by a temperature
13 differential in the process fluid F in the vessel V between heated process
fluid F
14 above the jacket 18 and cooler process fluid F below the jacket 18.
16
