Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates in general to the tubes
used in hydrocarbon reforming processe~ and particularly to
processes wherein a hydrocarbon is reformed to obtain hydrogen.
The production of hydrogen from natural gas and other
hydrocarbons is well known in the art. Generally, natural gas,
such as methane, or other hydrocarbons, and water in the form of
steam, are combined in a series of chemical reactions to produce
hydrogen in a catalyst-filled tube. The following two chemical
reactions are the principal reactions involved in the process:
Reforming Reaction
CH4 + H20 -. CO ~ 3~2 ~ 97,000 Btu/mole
Shift Reaction
CO + ~2 ,~ C2 + H2 + 16,500 Btu/mole
The field of the present invention relates gPn~r~lly to the
heating surface of catalyst tubes in ~team reforming heaters
commonly u~ed in ammonia and hydrogen plants to produce
hydrogen.
The endothermic reforming reaction takes place by
reacting ~ome portion of hydrocarbon feed with steam to produce
hydrogen and carbon monoxide in cataly~t-filled tubes ~n steam
reforming furnace~. Previously, all of the required heat
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imparted to the steam reformer tubes took place in the radiant
section of the furnace where the entire tube was exposed to
radiant heat from the burner flame.
The total fired heat liberation is proportional to the
amount of radiant heat required. Typically less than half of the
heat released is imparted to the catalyst-filled tube by thermal
radiation. The balance of the heat is carried by the flue gases
leaving the radiant section and is recovered in various coils
located in the convection section where flue gas flow~ transverse
to the horizontal tubes that make up the convection coils of the
steam reforming furnace. The various convection coils used to
recover heat from the flue gases include: the combined hydro-
carbon feed plus steam preheat coil, other process preheat coils,
boiler feedwater coil, fuel preheat coil, combustion air prehéat
coil, and the superheated steam coil. The alternative to using
the above coil~ for cooling the convection flue gases is to pass
the hot flue gases directly to the atmosphere, thereby losing the
energy contained in the hot flue gas.
To reduce the heat load in the radiant section, the
combined hydrocarbon feed plus steam is typically preheated to
very high temperatures in the convection coils before passing to
the radiant section of the furnace, thereby requiring
construction from expensive alloy material for the combined
hydrocarbon feed plus steam preheat coil and crossover piping
interconnecting with the catalyst-filled tubes.
For primary reformers using high temperature gas turbine
exhaust for combustion air, combustion air preheaters cannot be
used to recover heat from the convection flue gases. Instead,
coils for boiler feedwater preheating, steam generation, and
steam superheating are the only viable means of recovering
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maximum heat ~rom flue gases. This heat recovery may require
either using steam drivers in the plant for equipment which could
otherwise be operated at lower capital cost with electric motors
or exporting excess steam production to unfavorable local
markets.
Fig. 1 illustrates a conventional reformer with a
furnace 10 having burners 12 located therein. Tube 40 is filled
with catalyst 45 and runs the height of the furnace 10. A
process fluid mixture enters through process inlet 5 and is
preheated by flue gas 92 within a convection section 20 before
being injected into tube 40. The fluid mixture travels through
the catalyst-filled tube 40 and exits to a manifold 80 and
process outlet 85. The fluid mixture within tube 40 is heated
almost completely by radiant heat transfer within furnace 10.
The flue gas 92 exiting through stack convection section 20 is at
a very hiqh temperature. Some of the heat value within flue gas
92 is recovered by pre-heating the fluid mixture in exchanger
tube 70 in stack convection section 20. Other heat is recovered
by making steam by running fluid through exchanger tubes 90 also
positioned in stack convection section 20.
An object of this invention is to reduce the fired duty
and consequently the heat load of the convection flue gases to
suit overall,plant requirements. This invention may also
increase the heat absorbed in the catalyst tubes as a percentage
of the total heat fired in the furnace. The reduced fired duty
and increased heat absorption also allow the hydrocarbon feed
plus steam preheat temperature to be reduced, which permits more
economical alloys to be used for the preheater exchanger.
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Further, this amount of reduction may be varied to allow balancing the steam
production to the plant consumption.
Toward the fillfilment of these and other objectives, the reforming
furnace tubes of the present invention allow additional heat to be imparted to the
catalyst tubes by convection from the heat bearing flue gases leaving the radiant
section. Thus by extracting more heat from the flue gases leaving the reforming
section, the reforming section efflciency is increased and the fired liberation is
reduced. In addition the heat input to other services in the stack convection
section is reduced.
- Accordingly, in one of its aspects, the present invention provides a
reformer furnace comprising a radiant section containing burners, a tube
convection portion through which hot flue gas from said radiant section exhausts,
and at least one tube therein adapted to contain reforming catalyst, said tube
having a first end and a second end, said ffrst end positioned in said tube
convection portion and said second end positioned within said radiant section,
said first end of said tube having an extended suri:àce to enhance convection heat
tr~n ~fer
In another of its aspects, the present invention provides a method for the
production of hydrogen from a hydrocarbon stream in a steam reforming furnace
for a hydrogen or ammonia plant, said furnace having a radiant section and a
convection section, the width of the furnace in the convection section being
substantially narrower than the width of the radiant section to provide enhanced
velocity to the flue gas, the furnace contain ng a plurality of reforming catalyst-
con~ining single pass tubes, the portion of each of said tubes within said furnace
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being filled with reforming catalyst, comprising the steps of preheating the
hydrocarbon stream, introducing, heating and reacting the hydrocarbon stream
and steam in said tubes within said convection section of the furnace wherein a
portion of the catalyst-filled tubes have an extended surface integral with or
attached to an outer surface of said tubes within the convection section to
enhance convection heat transfer to the hydrocarbon stream within the
catalyst-filled tube and thereafter heating the hydrocarbon stream in said radiant
section of the furnace in another portion of said tubes having a substantially bare
outer surface within the radiant section thereby ca.using the hydrocarbon and
steam to flow through the tubes in a direction countercurrent to the flow of flue
gas through the furnace.
In yet another of its aspects, the present invention provides a method for
the production of hydrogen which comprises reacting a vaporized hydrocarbon
with steam in a vertical tube containing steam reforming catalyst, said tube
having two sections, one section of said tube has a substantially bare outer wall
and receives heat largely by radiation from a radiant section of the furnace, and
the other section of tube has extended surface and receives heat largely by
convection from flue gases leaving the radiant section of the furnace.
In yet another of its aspects, the present invention provides a method for
the production of hydrogen which comprises (a) reacting a vaporized
hydrocarbon with steam in a steam reforming furnace for a hydrogen or
ammonia plant, said furnace containing a pl~lrality of steam reforming
catalyst-cont~ining single pass vertical tubes each having two sections, the
portion of each of said tubes within said furnace being filled with steam
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~rolllflng catalyst, wherein the step of reacting comprises (1) introducing and
heating the hydrocarbon and steam largely by convection in a first section of said
tubes filled with steam reforming catalyst, the first section having an extended
surface integr~l with or attached to an outer surface of said tubes, and
(2) heating the hydrocarbon and steam in a second steam reforming catalyst filled
section of said tubes largely by radiation from a radiant section of the furnace,
the second section of said tubes having substantially bare outer walls thereby
causing the hydrocarbon and steam to flow through the tubes in a direction
countercurrent to the flow of flue gas along the first section of said tubes by
providing a substantially narrowed furnace ~vidth.
In still yet another of its aspects, the present invention provides a
reforming furnace for producing hydrogen, comprising a radiant section
cont~ining burners, a tube convection section connected to and aligned vertically
with respect to said radiant section such that hot flue gas from the radiant section
exhausts through said tube convection section, a plurality of reaction tubes
vertically disposed within said furnace, each of said reaction tubes extending
from the radiant section through the tube convection section, each of said
reaction tubes having a first and a second tube portion, said first tube portion
being the portion of the reaction tube posi~ioned in the tube convection section
and said second tube portion being the portion of the reaction tube positioned
within the radiant section, wherein both the first tube yortion and the second tube
portion contain reforming catalyst, the first tube portion including extended
surface means integral with or attached to an outer surface thereof for enh~ncing
convection heat transfer within the tube convection section and means for
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separating said radiant section from said tube convection section whereby during
furnace operation, heat transfer in the tube convection section is predomin~ntly
convective and heat transfer in the radiant section is predominantly radiant, said
separating means including parallel furnace side walls in the tube convection
section having a width between the side walls substantially narrower than the
width between side walls of the furnace in the radiant section.
In another aspect of its aspects the present invention provides a reforming
furnace for producing hydrogen, comprising a lower radiant section cont~ining
burners positioned along furnace side walls, an upper tube convection section
connected to and aligned vertically above said radiant section, said tube
convection section constructed and arranged such that hot flue gas from said
radiant section exhausts through said tube convection section, a plurality of
reaction tubes vertically disposed within said furnace, each of said reaction tubes
PYt~nding from the radiant section through the tube convection section, each of
said reaction tubes having a first and a second tub,~ portion, said first tube
portion being the portion of the reaction tube positioned in the tube convection
section and said second tube portion being the portion of the reaction tube
positioned within the radiant section, wherein both the first tube portion and the
second tube portion contain reforming catalyst, means for enhancing convection
heat transfer within the tube convection section comprising radially outwardly
eYten(ling heat transfer elements integral with or attached to an outer surface of
said first tube portion, means for isolating the tube convection section from
radiant heat produced in the radiant section and means for providing enhanced
velocity to the flue gas in said tube convection section, comprising parallel
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vertical furnace side walls in the tube convection section with a distance between
the parallel side walls in the tube convection section being less than that between
the furnace side walls in the radiant section.
Embodiments of the invention will now be described by way of example
only with reference to the accompanying drawings in which:
Fig. 1 schematically illustrates a refi~rmer furnace of the prior art;
Fig. 2 schem~tic~lly illllstr~tes a reformer furnace according to the
present invention having a tube convection portion;
Fig. 3 is a cross sectional view of Fig. 2 taken along the line 3-3;
Figs. 4a and 4b are detailed views of a studded extended surface section
of the catalyst-filled tube of Fig. 2; and
Fig. 5 is a perspective view of an alternative extended surface section of
Fig. 4 comprised of fins.
The pl~relled embodiments will now be described with reference to the
drawings. Figure 2 illustrates a reformer furnace 110 having a radiant sectio
122 and a tube convection portion 125.
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The integral radiant-convection reformer tubes of the
present invention are vertical catalyst tube~ 140 with top inlet
107 and bottom outlet 182. The bottom of tube 140 which is in
the radiant section 122 is substantially bare and located
adjacent the burners 112. The top of the tube 140 contains the
extended outer surface 150, and is located between two parallel
walls 130, extending from the top of the radiant section 122. A
~eries of vertical catalyst-filled tubes are arranged in a
straight line and since Fig. 1 illustrates a side view of the
furnace, only one tube 140 is shown. Within the radiant section
122 are the burners 112, which supply the heat input for the
furnace 110. The radiant heat from burners 112 impacts upon the
bare walls of catalyst tubes 140. Tube 140 is filled with
catalyst 145 which is supported in the tube on a catalyst support
plate 142.
The tube convection portion 125 of the reformer furnace
110 has a reduced width through which exiting flue gas 192 must
pass out of furnace 110. The tube convection portion 125 has two
parallel walls 130, 130 which are much closer to the tube 140
than the walls of the radiant section 122. As such, the velocity
of flue gas exiting through the tube convection portion 125 is
much higher because of ~he reduced area through which flue gas
192 must travel, thereby increasing the convection heat transfer
from the flue gas 192 to the fluid in the catalyst-filled tube
140.
The tubes 140 may have an extended surface 150 in the
tube convection portion 125 to further enhance heat transfer.
Over a length "D" between the parallel walls 130, the extended
surface 150 may be comprised of a series or a plurality of studs
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152 attached-to the outer surface of tube 140 and extending
radially outward therefrom.
Combustion gases from the radiant section 122 pass
between the parallel walls 130, which contain the extended
surface portion 150 of the catalyst-filled tubes 140. Convection
heat from the flue gas 192 is efficiently imparted to the tubes
140 via the extended surface 150. Some additional heat is also
transferred to the tubes 140 by radiation from the flue gas 192
and radiation from the parallel walls 130. After passing through
~7 the catalyst-tube convection section 125, the flue gas 192 goes
to a conventional horizontal tube convection section 120 (which
may include preheater exchanger tubes 170 and recovery exchanger
tubes 190 for example) and up stack 121 as shown in Figure 2.
The basic process has a fluid mixture entering the
furnace 110 at a process inlet 105 and passing through a series
of heat exchanger tubes 170 located within stack 120. The fluid
mixture is preheated by the flue gas 192 before the mixture
enters the catalyst-filled tube 140. As the fluid mixture
travel~ through the catalyst-filled tube 140, it is first heated
by convection within the tube convection portion 125 where high
velocity combustion gases from the radiant section 122 impact the
extended surface 150 on tube 140. The fluid mixture within tube
140 thereby under goes substantial heating in the presence of
catalyst even before entering the radiant section 122. Within
the radiant section 122, tube 140 has a substantially bare outer
wall and the fluid mixture within tube 140 is heated primarily
through radiant heat transfer. Once the fluid mixture has passed
through the radiant section 122, it leaves the furnace 110
through esit line 182 and enters manifold 180 in which the fluid
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mixture from all the catalyst-filled tubes 140 is combined and
exits through process outlet 185.
Figure 3 is a cross-sectional top view of the furnace
110 of figure 2. Figure 3 illustrates that furnace 110 has many
catalyst-filled tubes 140, running the length thereof. Each of
the catalyst-filled tubes 140 has an extended surface 150 within
the radiant section 122 (see also Figure 2). A typical reformer
furnace may have 150 or more reformer tubes. The catalyst-filled
tubes 140 are positioned between the two parallel walls 130,
130. Fluid from tubes 140 exit through exit line 182 into
manifold 180, combining and exiting out the process outlet 185.
The catalyst-tube convection portion 125 of tube 140 has
an extended surface 150. Figs. 4a and 4b illustrate details of
extended surface lS0. Extended surface lS0 is comprised of a
series or plurality of studs 152 attached to the outer surface of
the tube 140 and extending radially outward therefrom. The studs
152 are arranged in planes 155 which are spaced a distant of "d"
apart. Each plane 155 has approximately 30 studs 152 positioned
around the circumference of tube 140.
The combination of stud size, quantity, shape, and
spacing of the extended surface section 150 exposed to flue gases
leaving the radiant section 122 and their enclosure may be varied
to achieve the desired heat absorption characteristic within the
catalyst-tube convection portion 125.
Fig. 5 illustrates an alternative embodiment of extended
surface 160 comprised of a plurality of fins 165 longitudinally
attached alonq the outer surface of tube 140 and extending
radially outward therefrom. Again the size, orientation, and
spacing of fins 165 are chosen to achieve the desired heat
absorption characteristics. Though two particular designs for
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the extended-surface 150 have been described, other designs may
be selected by those skilled in the art to achieve the desired
heat trans~er characteristics, given the description and
disclosure set forth herein.
The catalyst-tube convection portion 125 may also
include baffles (not shown) to enhance convection heat transfer
to tube 140.
Though Figs. 2 and 3 have been described to have a
substantially vertically oriented tube 140, other orientations
may be employed. Figs. 2 and 3 illustrates the inlet to the tube
140 on the top of furnace 110, alternatively the inlet to the
catalyst tube may be at the bottom and the outlet at the top. In
such a case, it would be more suitable to have the radiant
portion of the tube at the top and convection portion of the tube
at the bottom. In some cases, it may be desirable to have the
inlet at the top, outlet at the bottom, and the convection
portion at the bottom. A feature of the invention is the
combination of a radiant ~ection and a convection portion in a
single catalyst tube.
The temperature of the flue gases leaving the catalyst
tube portion of this invention can be reduced to approximately
1200 to 1500F versus 1700 to 1900F in current state-of-the-art
furnaces, without substantially increasing the catalyst volume or
bare tube surface. As a result, the quantity of fuel required
per unit of production may be reduced by up to approximately 25
percent.
The overall cost of the reforming furnace using integral
radiant-convection catalyst-filled tubes can be significantly
less than for those that do not employ this invention. Lower
material cost is achieved by the lower flue gas temperature.
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Typically the preheat exchanger tubes (such as tubes 70 in
Fig. 1) are exposed to flue gas at a temperature of 1700 to
1900F. Such a temperature requires more expensive alloy tube
construction as compared to tubes (such as tubes 170 in Fig. 2)
of the present invention which are exposed to a lower temperature
of 1200 to 1500F.
Examples will now be described comparing the present
invention to processes of the prior art. The examples compare
processes of reformers for a typical 1500 short tons per day
ammonia plant. The examples are summarized in Table 1.
TABLE 1
Hydrocarbon
Feed Furnace Turbine ~otal
Plu~ Steam Burner Fuel Fuel
Preheat Input Consumption Consumption Consumption
Q Q Q Q
Example MMBtu/hr MMBtu/hr MMBtu/hr MMBtu/hr
1 44 302 191 493
2 44 230 191 421
3 61 274 191 465
Energy
Ab~orbed Other
Feed Flue Ga3 Radiant ~ Stack Recovery
Inlet Exit Temp. Temp. Flue Convection Gas Coil~
Temp.From Rad. GaY entry Q Exit Q
to TubeSection to Stack MMBtu/hr Temp MMBtu/hr
1025F 1850F 1850F 154 350F 203
1025F 1850F 1470F 154 350F 131
1150F 1850F 1850F 137 350F t79
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EXAMPLE 1
This example is for a typical 1500 short tons per day
ammonia plant according to current technology as in Fig. 1. The
reforming furnace contains 152 catalyst-filled tubes of 5.75
inches ID by 39.49 feet high. About 36.5 feet of the catalyst
tube height is in the radiant zone. Hydrocarbon feed enters the
process inlet 5 and is preheated in exchanger tubes 70 at a heat
input rate of 44 MMBtu/hr. The feed enters the catalyst-filled
tubes 140 at a temperature of 1025F. The feed i8 then heated in
furnace 10 at an absorption rate of 154 MMBtu/hr which is
entirely in a radiant section since this example has no
convection section. Fuel consumption is 302 MMBtu/hr for the
burners 12 and 191 MMBtu/hr. for the gas turbine (not shown)
which supplies air for the combustion process. Total fuel
consumption is 493 MMBtu/hr. The combustion gases leave the
combustion zone and enter the convection section 20 at a
temperature of 1850F. Within convection ~ection 20, the gases
preheat the hydroca~bon feed in exchanger tubes 70. Further heat
is recovered in eYrh~n~er tubes 90 at a rate of 203 MMBtu/hr.
The flue ga~ to the ~tack then exits at 350F.
EXAMPLE 2
This example illustrates a 1500 short tons per day
ammonia plant according to the present invention as illustrated
in Fig. 2. The number and diameter of the cataly-Qt tubes i5 the
same as in Example 1. The length of the tubes is increased to
42.06 feet with 29.94 feet of the tube length in the radiant
section 122. Tubes 140 have 7.27 feet of extended surface 150
which comprises the convection section 125 of the catalyst-filled
tubes 140. The extended surface 150 ~refer to Figs. 4a ~ b) is
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comprised of studs 152 of 3/8 inches diameter by 3/4 inches high
with 30 studs 152 per plane 155 around the circumference of the
tube 140. The planes 155 are spaced 1/2 inch apart "d" for the
full 7.27 foot height ~D" of the convection section 125. The
same results can be o~tained with other types of extended
surface.
Referring to Table 1, the hydrocarbon feed is preheated
at a rate of 44 MM Btu/hr in exchanger tubes 170. The feed
enters the catalyst-filled tubes 140 at 1025F. The feed is
heated in furnace 110 at an absorption rate of 154 MM Btu/hr,
some of which occurs in the radiant section 122 and the remainder
in the catalyst-tube convection portion 125. Fuel consumption is
230 MM Btu/hr at the burner~ 112 and 191 MM Btu/hr for the gas
turbine Inot shown). Total fuel consumption is 421 MM Btu/hr.
The combustion gases exit the radiant section 122 and
enter the catalyst-tube convection portion 125 at 1850F. The
flue ga~ 192 enters convection section 20 at 1470F. Within
convection section 20, the flue gas 192 preheats the hydrocarbon
stream in exchanger tubes 170. Further heat is recovered in
exchanqer tubes 190 at a rate of 131 MM Btu/hr. The flue gas to
the stack exits at 350F.
In this example in which the temperature entering the
catalyst tube is 1025F (the same as in Example 1), the fuel to
the reformer is reduced by about 24 percent. The total fuel to
the reformer plus gas turbine is reduced by about 15 percent.
EXAMPLE 3
This example is for a conventional reformer similar to
that of Example 1 except that the inlet temperature is raised
from 1025F to 1150F to reduce the overall fired duty. In this
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case, there are 124 catalyst tubes of 6.0 inc~e~ ID by 39 feet
high.
Referring to Fig. 1 and Table 1, feed enters inlet 5 and
is preheated in exchanger tubes 70 at a heat input rate of 61
MMBtu/hr. The feed enters the catalyst-filled tube 40 at 1150F
and i8 then heated in furnace 10 at an absorption rate of 137
MMBtu/hr which is entirely in a radiant section since this
example has no catalyst-tube convection portion. Fuel
consumption is 274 MMBtu/hr for the burners 12 and 191 for the
gas turbine (not shown) for a total fuel consumption of 465
MMBtu/hr.
The combustion gases leave the combustion zone and enter
convection section 20 ~the flue gas 92) at a temperature of
1850F. The flue gas 92 preheats the hydrocarbon feed in
exchanger tubes 70. Further heat is recovered at a rate of 179
MMBtu/hr in exchanger tubes 90. Flue gas to the stack exits at
350F.
Comparing the process conditions for the integral
radiant-convection catalyst-filled tube of Example 2 with
e 3, the present invention as shown in Example 2 reduces
the fuel to the reformer by 16 percent and reduces the~ total fuel
required by 9 percent over Example 3.
In this example, the hydrDcarbon feed plus steam
temperature entering the catalyst-filled tubes is 125F less for
the inteqral radiant-convection catalyst-filled tube of
Example 2, thus achieving two objectives simultaneously: tl) a
substantially lower cost for the hydrocarbon feed plus steam coil
and (2) reducing the fuel firing required for the reforming
reaction.
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ThuS, a furnace and process are disclosed which reform
hydrocarbons to obtain a gas containing substantial amounts of
hydrogen. While embodiments and applications of this invention
have been shown and described, it would be apparent to those
skilled in the art that other modifications are possible without
departing from the inventive concepts herein. The invention,
therefore, it not to be restricted except as in the appended
claims.
