Note: Descriptions are shown in the official language in which they were submitted.
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PYROLYSIS HEATER
BACKGROUND OF THE INVENTION
The production of light olefins (ethylene, propylene,
butadiene and butylenes) and associated aromatics (benzene,
toluene, ethylbenzene, xylenes and styrene) is usually carried
out by the thermal cracking of hydrocarbon feedstocks in the
presence of steam. This process is known as the steam
pyrolysis of hydrocarbons for the production of olefins.
The hydrocarbon feedstocks used for the production of
olefins range from essentially pure ethane to vacuum gas oils
and any combination thereof. Hydrogen and methane are
impurities found in the feed. The process consists of a
pyrolysis section and a recovery section. The feedstock
preheating system, the steam pyrolysis coils and the exchangers
to cool the coil effluent are included in the pyrolysis section
of the plant. The majority oF the feed preheating system and
the pyrolysis coils are contained in the pyrolysis furnace or
reactor. The chemical reactions of this process take place in
the pyrolysis coils in the absence of catalyst.
Approximately 30 to 40 percent of the total plant
capital investment is required in the pyrolysis section.
Furthermore, the economics of the process, i.e. feedstock
consumption and byproducts produced for a fixed ethylene
production, are determined by the design of the pyrolysis
section. Thus, traditionally, improvements in the design of
the pyrolysis section of the plant have resulted in dramatic
impact on the economics of the steam pyrolysis process.
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The pyrolysis furnaces consist of a convection
section and a radiant section or any combination thereof. The
hydrocarbon ~eed is first preheated in the convection section
of the furnace. Dilution steam is then added and the
steam-hydrocarbon mixture is further preheated in the mixed
preheat coil of the convection section. In some designs, the
dilution steam is also preheated prior to addition to the
hydrocarbon stream. The mixture is preheated up to the
required transition temperature for pyrolysis in the radiant
section. This temperature is identified as the crossover
temperature between the convection and the radiant sections.
This temperature varies with the type of feedstock and with the
specific coil design.
With liquid hydrocarbon feedstocks, vaporization of
the feed takes place in the mixed preheat coil and/or at the
point where the dilution steam is injected. In some designs,
the vaporization of the feedstock is e~ternal to the convection
section coils to avoid potential coke laydown. Furthermore,
boiler feedwater, saturated steam and dilution steam ~ay also
be heated in the convection section. It should be noted that
this description is only typical. The requirements for the
heating services described above, as well as their locations
and sizes in the convection section of a pyrolysis furnace,
depend upon the specifications of each plant's requirements.
The pyrolysis coils, where the hydrocarbon feed in
the presence of dilution steam is pyrolyzed, are contained in
the radiant section of the pyrolysis furnace or reactor. The
number of pyrolysis coils per radiant section is a function of
the re~uired ethylene capacity per pyrolysis furnace, the
desired pyrolysis yields, the coil configuration and
dimensions, the feedstock type and the terminal operating
conditions such as coil outlet pressure. Transferline
exchangers, followed by direct quenching with oil, are used to
cool the ef~luent coming out ~rom the coils. For a fixed
3S ethylene capacity per furnace, pyrolysis yields~ feedstock type
and terminal operating conditions, the pyrolysis coils based on
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small diameter tubes have less capacity per co;l than those
based on large diameter tubes. Therefore, the number of
pyrolysis coils with small d;ameter tubes required to meet the
specified ethylene production per furnace is larger than the
number required with coils of large diameter tubes.
The current practice in the design of pyrolysis coils
includes three basic types. One type employs small to moderate
tube diameters ( 1 to 4 inches) with a single tube per pas~ and
one or more passes per pyrolysis coil ~1 to 8). The second
type employs large tube diameters (4 to 7 inches) also with a
single tube per pass and several passes per coil (2 to 12).
The third type uses a combination of small and large tube
diameters (1 to 7 inches) and multiple tubes per pass toward
the front end of the coil and single tube per pass toward the -
back end of the pyrolysis coil, and several passes per coil ~2
to 12).
It should be noted that, for the first two types, the
tube diameter could be constant throughout the coils or could
be increasing from the first pass to the last pass of the
pyrolysis coils.
The pyrolys;s coils are located in a longitudinal
plane in the radiant section of the pyrolysis furnaces. The
pyrolysis coils could be staggered or located in a single row
or multiple rows. The radiant heat source is provided by
firing either burners from the lateral walls of the radiant
section, or burners from the floor (hearth) of the radiant
section or a combination thereof.
For designs ~ith a single diameter tube throughout
the coil, it is obvious that the ratio of the metal surface to
the coil volume per pass remains constant from the beginning to
the end of the pyroloysis coils. In these designs, the axial
temperature profile of the gases reacting in the pyrolysis coil
approaches a straight line with a positive slope.
Pyrolysis co;ls w;th small diameter tubes, although
having better heat transfer characteristics, result in smaller
capacity per co;l when compared to the other two design types
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because of the faster coking rate observed during the cycle,
and the increase in coil pressure drop due to the coke
deposited on the coil inner walls during the run. This
increase has a detrimental effect on the pyrolysis yield
(decreasing olefins production and increasing fuel oil
byproduct at constant feedstock conversion with cycle time)
produced by the first design mentioned above.
By enlarging the diameters of the tubes from the
beginning to the end of the pyrolysis coil, the surface to
volume ratio is also reduced along the direction of the flow in
the pyrolysis coil. The larger tube diameters in the second
half of the pyrolysis coil reduce the coking rate and, thus,
the effect of the deposited coke on the coil pressure drop and
the concomitant detrimental effect on the pyrolysis yields.
Also, the larger tubes ultimately result in a larger capacity
coil. However, the axial temperature profile of the reacting
gases still approaches a straight line w;th a positive slope.
The drawback of the larger d;ameter tubes is the lower heat
transfer coefficient resulting in h;gher metal temperatures.
Since the surface to volume ratio of a coil with
enlarged tube diameter toward the outlet is smaller than that
of a coil with constant diameter, the coil must be longer to
achieve a higher aver2ge ethylene production per coilO Both
coils can be designed to achieve essentially identical yields
by trading increments in residence time against reductions in
hydrocarbon partial pressure. An obvious limitation with the
enlargement of the tube diameter toward the outlet scction of
the pyrolysis coil is the poorer heat transfer coefficient
since, for a given throughput, the coeffic;ent ;s ;nversely
proportional to D1-3 where D is the diameter.
To significantly increase the ethylene production per
pyrolysis coil, thus reducing the required number of coils per
pyrolysis furnace, the ultimate objective is to develop an
axial gas tPmperature profile that maximizes the utilization of
the metal surface available in the pyrolysis coil. In general,
the target temperature profile is concave down and as close as
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possible to an isothermal profile instead of the almost
straight line with positive slope or concave up profile
achieved with the first two coil design types mentioned
earlier. The isothermal axial gas temperature profile
represents the best heat ut;lization of the metal in the
pyrolysis coil, i.e., for a given yield and run length, the
maximum capacity per unit weight of pyrolysis coil metal and~
thus, the least expensive pyrolysis coil.
One design approach is to use zone firing which
requires the partitioning of the firebox into several
compartments. In addition, the firing system has to be
properly controlled to achieve the zone firing effect. The
operating principle behind this design approach is to initiate
the cycle with a straight line or a concave up temperature
profile by firing uniformly throughout the pyrolysis coil or
shifting the intensity of the firing more toward the outlet
section of the pyrolysis coil. Gradually, during the progress
of the run or as coking of the coil takes place, the firing is
shifted from more intensity toward the outlet section of the
coil to more intensity toward the inlet section of the coil.
Ultimately, toward the end of the cycle, an isothermal or
concave down axial temperatu~e profile is used to operate the
coil.
The zone firing approach permits the utilization of
~5 higher capacity per coil at constant running time. However,
due to the complexities in the construction of the firebox of
the pyrolysis furnace and in the firing control system, this
approach has not been too widely practiced in the industrial
production of ethylene. Furthermore, it should be noted that
the metal in the pyrolysis coil is fully utilized only when the
temperature profile approaches isothermal conditions which, in
this type of design, occurs only during a fraction of the
running time.
The coil type three mentioned above which uses
multiple parallel tubes of small d;ameter in the passes of the
inlet section of the coil and large diameter single tubes in
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the passes of the outlet section of the coil is discussed next.
This design is commonly referred to as the swage coil and that
term will be used herein.
The swage coil design approach has been utilized in a
large number of worldwide ethylene plants since the seventies.
Instead of using a firebox of complex construction and a very
sophisticated and expensive firing control system, it relies on
the coil configuration to achieve the concave down axial gas
temperature prsfile during the entire running time. Because of
this efficient utilization of the metal in the pyrolysis coil,
the coil is characterized by larger production capaclty at
equal average yields and constant running time. The swage coil
has a higher capacity and a lower coking rate resulting in a
longer running time per cycle~
The technical advantages of the large diameter
pyrolysis tubes in the outlet section outweigh its poor heat
transfer characteristics. Designers have tried to compensate
for this drawback by installing inserts inside the outlet tubes
and/or installing studs or longitudinal fins on the outer walls
of the outlet tubes with the objective to impro~e the heat
transfer rate in that section of the pyrolysis coil. However,
the pyrolys;s conditions are more intense in the last half of
the coil. The coke forms predominantly in this location of the
coil during the pyrolysis of the feedstock and the coke
deposits on the inner walls of the pyrolysis tubes. The coke
deposition is responsible for the increase in metal temperature
with days on stream. Due to the mild pyrolysis conditions in
the ~irst half of the pyrolysis coil, the coke formation in
this inlet region is significantly less than in the second half
of the coil. In this inlet region of the coil, the increases
in metal temperature due to coke deposition on the walls are
only moderate.
Because of the above characteristics of pyrolysis
coils, inserts located inside the outlet tubes are expected to
act as nucleus for the growth of the coke formed during
pyrolysis. Thus, the utilization of inserts in this reg;on
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would result in shorter than desirable run lengths, higher than
desirable pressure drops, poor operating reproducibility of
conditions and significant losses in olefins yields.
In principle, because the equivalent outside heat
transfer coefficients of the outlet tubes are lower than the
inside heat transfer coefficients, it appears attractive to
utilize extended surfaces in the form of studs or fins in the
outlet portion of the coil. However, the utilization of the
extended surfaces in the outlet position of the coil is not
1n effective because the temperature of the stud or fin tip will
limit the run length as a result of the coke deposition on the
inner walls of this section of the pyrolysis coils.
SUMMARY OF THE INVENTION
The present invention relates to the incorporation of -
extended surfaces on the inlet portion of a pyrolysis co;l in
order to make the axial gas temperature profile even closer to
an isothermal profile than it has been possible to achieve with
uniform firing in the pryolysis coils currently used in the
olefins production industry. This permits higher production
capacity per unit weight of pyrolysis coil while preserving the
desired pyrolysis yields and on-stream time in between decoking
cycles. Conversely, this invention, at constant ethylene
production per pyrolysis coil, permits longer on-stream time
and/or somewhat higher ethylene yields. More specifically, the
invention involves the placing of the extended surface in the
first half and preferahly the first quarter of the coil an~
preferably involves the use of s~uds or longitudinal straight
fins or ribs on either the outside or the inside of the tubes
or both locations.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified schematic representation of
a pyrolysis furnace which can employ the present invention;
Figure 2 is a schematic presentat;on of an
arrangement of the tubes in one coil of a pyrolysis furnace
employing the present invention; and
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Figure 3 shows a short section of a tube with the
studs of the present invention thereon.
Figure 4 illustrates a cross-section of a tube with
longitudinally extending fins or ribs around the inside
circumference.
Figure 5 is a graph illustrating the temperature
profile through a coil of the prior art as compared to a coil
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1, there is provided a vertical
tube type pyrolysis heater supported on structural steel
framework generally indicated as 10. The heater is comprised
of outer walls 11 and 12, inner walls 13 and 14, end walls 15
and floors 16 and 17. The outer walls 11 and 12 are
su~stantially parallel to inner walls 13 and 14 with the height
of outer walls 11 and 12 extending above the height of inner
walls 13 and 14. Mounted in outer walls 11 and 12 and inner
walls 13 and 14 are a plurality of vertical rows of high
intensity radiant type burners, generally indicated at 18. The
floors 16 and 17 extend between the outer walls 11 and 12 and
inner walls 13 and 14, respectively. The floors 16 and 17 are
provided with floor burners, generally indicated as 19 which
are preferably of the flame type.
The outer wall 11, inner wall 13 and floor 16
together with end walls 15 form a radiant heating zone,
generally indicated as 20, while outer wall 12, inner wall 14
and floor 17 together w;th end walls 15 form a second radiant
heating zone, generally indicated as 21. End walls 15 are in
the shape of an inverted U thereby forming an open area 22
permitting axis to the burners 18 mounted in the inner walls 13
and 14.
Horizontally positioned and mounted on inner walls 13
and 14 is inner roof 25. Horizontally positioned and extending
inwardly from outer wall 11 is upper roof 26 mounted on outer
wall 11 and end walls 15. Similarly, upper roof 27 is
horizontally positioned and extends inwardly from outer wall 12
and is mounted on outer wall 12 and end walls 15. Mounked on
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upper walls 26 and 27 are upper walls 28 and 29 which form with
the upper extending portions of end walls 15, a convection zone
generally ;ndicated as 3~. All of the walls, floors and roofs
are provided with suitable refractory material.
In the radiant heating zones 20 and 21, there is
provided a plurality of vertical tubes forming prGcess coils 31
and 32 suitably mounted from supportin~ structure 10 by hangers
33. The process coils 31 and 32 are positioned intermediate
the outer and inner walls 11 and 13 and 12 and 14,
respectively. The configuration of these process coils will be
described in more detail hereinafter. Mounted within the
convection zone 30 are horizontally disposed conduits,
schematically illustrated and generally indicated as 35. The
conduits 35 are in fluid communication with the process coils -
31 and 32 through crossovers 36. Also positioned within the
convection section 30 is a second section of horizontally
disposed conduits generally ind;cated as 38. Inlet and outlet
manifolds 38A and 38B are in fluid communication with the
conduits 38.
The burners 18 are supplied with the fuel through
lines 40 from a plurality of manifolds 39. The fuel is
introduced into manifolds 39 through a manifold 41 under
control of valves 42. The flow of fuel to burners 18 may be
varied in vertical rows depending on the described severity of
firing of the process co;ls 31 and 32. Individual burners may
be further adjusted by ~alves 44 in lines 40 with the total
flow of fuel to the heater being controlled by valve 45. It is
understood that the burners mounted in outer walls 11 and 12
and inner walls 13 and 14 have sim;lar manifold means wh;ch is
not shown. Similarly, lines 46 carry the fuel to the floor
burners.
Referring now to Figure 2, there is schematically
illustrated a layout of the process coil 31 and it is to be
understood that the process coil 32 would be similar. This
general type of pyrolysis heater is described in U.S. Patent
3,274,97~3. ~lowever, the present invention is also applicable
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to pyrolysis coils that can be installed in other types of
heaters currently used in industry.
Referring now to Figure 2, there is illustrated a
schematic arrangement of the process coil 31 of the present
invention and it is to be understood that the process coil 32
would be similar. This process coil 31 is generally of the
swage type previously discussed and consists of a first pass
46, a second pass 47, a third pass 48, a fourth pass 49, a
fifth pass 50 and a sixth pass 51. As can be seen, the first
pass 46 comprises four tubes, the second pass ~7 and the third
pass 48 each comprise two tubes, and the passes ~9, 50 and 51
each comprise one tube. However, this coil should be
considered typical only and not limiting the present invention.
The present invention is applicable to pyrolysis coils of any
configuration and tube dimensions.
The followin~ table sets forth the details of the
coil configuration:
Pass No. No. of Tubes Inside Diameter, Inches
~0 1 4 3.5
2 2 5
3 2 5
4 1 7.5
1 7.5
6 1 7.5
As depicted in Figure 2, extended heating surface 52
is located on the four tubes of first pass 46. Th;s extended
heating surface can be in the form of studs or straight
longitudinal fins or ribs. The studs may be of any desired
shape but they are preferably cylindrical. The size and number
of studs or fins per unit length of pyrolysis tubing are
selected according to the process parameters of any particular
installation. As an example, the studs may be 0.5 inches in
diameter with a length ranging from 0.5 to 0.75 inches. There
may be 8 to 12 studs around the circumference of the tube at
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any one plane. Figure 3 illustrates a short section of tube
with studs. Studs are applicable to the outside of the tubes.
Straight longitudinal fins or ribs are preferred for the inside
of the tubes. For example, the fins may be 0.'2 inches in
height having 6 to 10 fins around the c;rcumference of the
tubes. F;gure ~ ;llustrates a cross-sect;on of a tube with
straight longitudinal fins or ribs around the inside
circumference thereof. Also, the extended heating surface is
installed in the f;rst half of the pyrolysis coil and
preferably in the first quarter. As indicated, the embodiment
illustrated in Figure 2 has the studs only in the first pass.
The effect of the extended heating surface on the
first pass can be seen in Figure 5 which compares the
temperature profile for a conventional pyrolysis co,l and the
same coil with extended heating surface. In this Figure 5, it
can be seen that the temperature in the first part o~ the coil
is significantly increased over the temperature in a
conventional coil while the temperature in the outlet portion
is only slightly affected. With this h;gher temperature at the
inlet portion, pyrolysis severity and coil capacity are
increased without increasing the maximum outlet temperature or
greatly increasing the temperature in the outlet portion where
coking would otherwise take place.
Following is a comparison of the calculated process
characteristics o~ a conventional swage coil design with two
different designs incorporating the present invention. In each
case, the coil configuration is four tubes in the first pass,
two tubes in each of the second and third passes, and one tube
in each of the fourth, fifth and sixth passes:
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Conventional
Swaqe Coil Coil A Coil B
Length/pass, ft. 31 33 31
Capacity per coil
tons HC/hr. 5.756- 7.212 6.577
Capacity increase, % basis 24.4 13.5
Heat duty, MMBTU/hr. 16.07 20.1 18.41 -
Run length, days 60 60 60
Lthylene yield, wt %
Once through 28.9 28.7 28.7
Ultimate 32.9 32.9 32.9
Operating Coils 30 24 26.3
Stud Addition, % effective
surface area increase
Pass No. 1 None 50 100
Other Passes None None None
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To make the most effective US2 of the metal in the
first half of the coil, an isothermal gas temperature profile
would be desired. The use of zone firing and the prior art
swage coil design both bring the temperature profile closer to
the isothermal. The use of the internal and/or external
extended heating surface of the present invention in the first
half or quarter of the coil brings the temperature profile even
closer to the isothermal. Use of extended heating surface in
the last part of the coil would tend to take the temperature
profile further away ~rom an isothermal profile as well as
create the coking previously mentioned. The use of the
extended surface in the first part of the coil maintains or
enhances the run length or cycle time, maintains or enhances
pyrolysis selectively toward olefins and enhances ethylene
capacity per unit weight of tube metal and any combination
thereof.
Although the temperature profiles in Figure 5 appear
to be very close together, the temperature difference in favor
of the coil with extended surface results in an increase in the
capacity of the coil of approximately 10%. Since the kinetic
reaction velocities vary exponentially with changes in
temperature, small differences in gas temperature have a
pronounced effect on the pyrolysis reactions.
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