SPECIFICATION
THERMAL CRACKING TUBE
TECHNICAL FIELD
[0001]
The present invention relates to thermal cracking tubes
for use in thermal cracking reactors for producing ethylene
or the like, and more particularly to hydrocarbon thermal
cracking tubes having projections formed on the inner surface
of the tube for producing an enhanced effect to agitate a fluid
flowing inside the tube, the tube being adapted to reduce
pressure losses to the greatest possible extent while ensuring
a high thermal transfer efficiency for the fluid inside the
tube.
BACKGROUND ART
[0002]
Olefins such as ethylene and propylene are produced by
causing a material fluid of hydrocarbon (such as naphtha,
natural gas or ethane) to flow at a high velocity through a
thermal cracking tube heated from outside and heating the
material fluid to a reaction temperature range for thermal
cracking.
To effect the thermal cracking reaction efficiently, it
is important that the material fluid to be passed at a high
velocity through the tube be heated to the thermal cracking
temperature range throughout the tube channel to the radially
central portion thereof, with overheating avoided to the
greatest possible extent. If the material fluid is overheated,
hydrocarbons are excessively converted to light fractions
(formation of methane, free carbon, etc.) or cracked products
will be subjected to polycondensation reaction to result in
a greater reduction in the yield of the desired product.
Overheating also results in promoted coking (deposition of
free carbon on the tube inner surface) to give a lower heat
transfer coefficient to the tube, entailing a need for frequent
decoking work.
[0003]
Accordingly, it is practice to provide on the inner
surface of the thermal cracking tube projections serving to
agitate the fluid flow, such that the agitation by the
projections ensures improved heat transfer to the fluid
flowing at a high velocity for a rise in the temperature by
rapid heating (see, for sample, Patent Literature 1).
Consequently, the reaction terminates within a shorter period
of time to avoid the conversion to light fractions due to
overheating. Furthermore, the improved heat transfer
efficiency of the thermal cracking tube makes it possible to
heat the thermal cracking tube at a lower temperature to entail
the advantage of extending the service life of the cracking
tube.
[0004]
Patent Literature 1 discloses a thermal cracking tube
wherein a region having no projections extends over the entire
length of the tube in parallel to the axis thereof to assure
a reduced pressure loss without impairing the heat transfer
efficiency of the tube.
[0005]
Patent Literature 1: Republished Patent WO2004-046277
DISCLOSURE OF THE INVENTION
The Problem to be Solved
[0006]
It is required that the thermal cracking tube of Patent
Literature 1 be further improved in thermal transfer
efficiency and reduced in pressure losses.
We have found that the thermal cracking tube can be
further improved in heat transfer efficiency and reduced in
pressure losses by adjusting the height of projections, length
of the projections, and the area range wherein the projections
act on the fluid inside the tube in accordance with the inside
diameter of the tube. This finding has matured to the present
invention.
[0007]
The present invention provides a thermal cracking tube
which is adapted to suppress pressure losses to the greatest
possible extent while maintaining a heat transfer promoting
effect on the fluid inside the tube.
Means for Solving the Problem
[0008]
To solve the foregoing problem, the present invention
provides a thermal cracking tube for passing a fluid
therethrough from one end thereof toward the other end, the
tube having a plurality of rows of projections extending
circumferentially along an inner surface of the tube
orthogonal to or as inclined with respect to the axis of tube,
each of the projection rows including a plurality of
projections provided on the tube inner surface, the tube being
defined by:
S/10nD=0.2-0.7,
h/D=0.02-0.05, and
L/nD=0.04-0.5
wherein D is the inside diameter of the tube, h is the height
of an optional projection A included in a projection row a,
L is the length of circular arc of the projection A in a
direction orthogonal to the tube axis, and S is the area of
a region R surrounded by the projection A, phantom lines L1
and L2 extending respectively from circumferentially opposite
ends of the projection A in parallel to the tube axis and a
projection row ß downstream from the projection row a.
Advantage of the Invention
[0009]
The thermal cracking tube is adapted to suppress pressure
losses to the greatest possible extent while maintaining a high
heat transfer efficiency by defining the height h of the
projection A on the inner surface of the tube, the length L
of the circular arc thereof in a direction orthogonal to the
tube axis and the area S, based on the inside diameter D of
the tube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
FIG. 1 is a perspective view in section of a thermal
cracking tube of the invention;
FIG. 2 is a fragmentary view in section taken along the
line X-X in FIG. 1;
FIG. 3 is a development along the axial direction of the
tube of FIG. 1;
FIGS. 4(A) to (C) are developments showing projections
in different arrangements according to the invention; and
FIGS. 5(D) and (E) are developments showing projections
in still different arrangements according to the invention.
BEST MODE OF CARRYING OUT THE INVENTION
[0011]
A thermal cracking tube 10 generally comprises a
plurality of straight tubes interconnected by bends into a
zigzag form and is provided in a thermal cracking reactor and
heated from outside the tube.
[0012]
FIG. 1 is a sectional perspective view showing an
embodiment of thermal cracking tube 10 (straight tube) of the
invention, FIG. 2 is a fragmentary view in section taken along
the line X-X in FIG. 1, and FIG. 3 is a development along the
axial direction of the tube of FIG. 1. In FIGS. 1 and 3, the
left side of the plane of the drawing is the upstream side,
and the right side thereof is the downstream side.
[0013]
With reference to the drawings, the thermal cracking tube
10 is provided with a plurality of rows a, ß of projections
circumferentially of the tube. The projection rows a, ß
include projections A and projections B, respectively.
According to the present invention, the term projection
row means a group of projections provided on the inner surface
of the thermal cracking tube 10 in a row circumferentially
thereof. When the projection row is defined, for example, with
respect to the length or area thereof, the row is defined as
a line through the projections centrally thereof (the
projection row a or ß shown in FIGS. 1 and 3 and indicated in
a chain line).
The projection row a and the projection row ß can be made
to extend in parallel helically at an angle of inclination ?
with respect to a plane orthogonal to the tube axis (see FIGS.
1 and 3) . In this case the angle of inclination 9 is preferably
up to 20 deg, more preferably up to 15 deg. The angle of
inclination 9 can be altered in one row, or can be different
from row to row.
Each projection row can be annular independently of
another, and orthogonal to the tube axis or positioned at an
angle of inclination with respect to the tube axis. In the
case where the projection row is helical, the row is not limited
to a single row but a plurality of helical rows may be provided.
[0014]
The projections can be formed efficiently in the shape
of overlay beads, for example, by powder plasma welding (PTA
welding) or like overlay welding method. The projections can
be formed in parallel to or as inclined from the row.
The projections can be made from a heat-resistant alloy
material of the same kind as the thermal cracking tube 10.
Examples of such materials are 25Cr-Ni(SCH22),
25Cr-35Ni(SCH24) and Incoloy (trademark). Also suitably
usable are various other heat-resistant alloy materials which
are capable of withstanding the environment in which the
thermal cracking tube 10 is to be used.
[0015]
Now, with reference to FIGS. 1 to 3, suppose the inside
diameter of the thermal cracking tube 10 is D (mm) , the height
of an optional projection A included in the projection row a
is h (mm), the length of circular arc of the projection A in
a direction orthogonal to the tube axis is L (mm) , the
center-to-center distance between the projection row a and the
projection row ß downstream therefrom is P (mm), the area of
a region R surrounded by the projection A, phantom lines L1
and L2 extending respectively from circumferentially opposite
ends of the projection A in parallel to the tube axis and the
projection row ß downstream from the projection row a is S
(mm2) , the length of a circular arc as measured in a direction
orthogonal to the tube axis and corresponding to the end-to-end
circular arc distance between the projection A and another
projection A' adjacent thereto in the projection row a is s
(mm) and the angle of inclination of the projection row a with
respect to a plane orthogonal to the tube axis is ?.
[0016]
[Inside Diameter D of Thermal Cracking Tube]
It is desired that the inside diameter D of the thermal
cracking tube 10 be 30 mm to 200 mm. The lower limit of the
inside diameter D is intended to ensure space for providing
the rows of projections, and the upper limit can be determined
in view of the agitation of the fluid by the projections and
heating efficiency.
[0017]
[Center-to-Center Distance P between Projection Rows a, ß]
In the case where the projection rows are helical, the
center-to-center distance P between the rows a,ß is determined
according to the inside diameter D of the tube 10 and the
above-mentioned angle of inclination ?, and is preferably 20
to 100 mm. In the case where a plurality of projection rows
are provided, the center-to-center distance P between the
adjacent rows is preferably 20 to 100 mm.
In either of the case where a plurality of projection
rows are provided and the case where each projection row is
annular independently of another, and orthogonal to the tube
axis or positioned at an angle of inclination with respect to
the axis, the center-to-center distance P between the adjacent
rows is preferably 20 to 100 mm.
[0018]
[Area S of Region R]
The area S of the region R is defined by S/10?D=0. 2-0. 7,
preferably by S/10?D=0.3-0.6.
A fluid of high temperature remains on the inner surface
layer of the thermal cracking tube 10, forming a laminar film
of high temperature and producing a phenomenon that full heat
transfer is not effected between the tube 10 and the fluid
inside the tube. In the case where projections A are formed
on the inner surface of the tube 10, the projections A agitate
the fluid, presumably breaking the laminar film to thereby
effect appropriate heat transfer. When a large number of
projections are provided on the inner surface of the tube 10,
the projections will individually agitate the fluid. When the
downstream side of each projection where the projection acts
to agitate the fluid is given a specified area, we have found
that the projection acts to agitate the fluid uniformly over
the specified area, whereby heat is transferred from the tube
to the fluid inside the tube within the specified area without
variations in temperature. More specifically, the area
surrounded by a projection and the projection row downstream
from the projection, i.e., the area S of a region R (see FIG.
1) provided between the projection A and the projection row
ß downstream therefrom, is compared with the interior area of
the tube having a diameter D over a unit tube length of 10 mm.
We have found that if this value is in the foregoing range of
0.2 to 0.7 (preferably in the range of 0.3 to 0.6), the heat
transfer can be effected efficiently.
[0019]
The region R provided downstream from the projection A
is a region surrounded by the projection A, the phantom lines
L1 and L2 extending respectively from circumferentially
opposite ends of the projection A in parallel to the tube axis
and the projection row ß downstream from the projection A. As
shown in FIG. 1, when the projection A and the projection row
ß are parallel, the area of the region R is that of a rectangular
parallelogram. If the projection row a and the projection row
ß are both orthogonal to the tube axis, the region R is
rectangular.
In the case where the angle of inclination ? of the
projection row ß is different from the angle of inclination
of the projection B in the row ß, the region R is an area
surrounded by the projection A, the projection B and phantom
lines L1, L2 as shown in FIGS. 5, (D) and (E).
[0020]
The ratio of the area S of the region R wherein the
projection A acts on the fluid to the tube interior area 10?D
having a unit tube length of 10 mm is limited to 0.7 (preferably
0.6). This gives improved heat transfer characteristics to
the fluid and reduces pressure losses as will be understood
with reference to Examples. The lower limit of the ratio is
0.2 (preferably 0.3) for the following reason. To make this
ratio smaller than 0.2 (preferably 0.3), there arises a need
to diminish the length of the projection A and/or the
center-to-center distance between the projection A and the
projection row (3, with the result that a larger number of
projections than is necessary are provided on the inner surface
of the tube 10. These projections create inappropriate
turbulence on the inner surface layer of the tube 10, entailing
increased pressure losses and failing to produce the
contemplated fluid agitating effect.
[0021]
The total sum SS of the areas S for all the projections
to be formed on the inner surface of the thermal cracking tube
10 is preferably at least 80% of the inner surface area of the
tube 10. This results in improved heat transfer
characteristics and reduced pressure losses as already
described generally over the entire length of the tube 10.
[0022]
[Height h of Projection A]
The height h of the projection A is defined by
h/D=0.02-0.05.
The height h of the projection A exerts a great influence
on the agitation of the fluid.
As the height h of the projection A increases, the
projection A offers resistance, producing an increased
pressure loss and presenting difficulty in diminishing the
pressure losses of the thermal cracking tube 10. Accordingly
the upper limit of the ratio of the height h of the projection
A to the inside diameter D of the thermal cracking tube 10 is
0.05.
With a decrease in the height h of the projection A, the
effect of the projection A to agitate the fluid becomes
seriously impaired, so that the lower limit of the ratio of
the height h of the projection A to the inside diameter D of
the tube 10 should be 0.02.
[0023]
[Circular Arc Length L of Projection A in a Direction Orthogonal
to Tube Axis]
The length L of circular arc of the projection A in a
direction orthogonal to the tube axis is defined by
L/?D=0.04-0.5, preferably by L/?D=0.04-0.2.
The circular arc length L of the projection A
orthogonal to the tube axis relates to the number of projections
included in the projection row, i.e., to the number of the
lengths s of circular arcs in a direction orthogonal to the
tube axis and corresponding to the end-to-end circular arc
distance between projections, and exerts a great influence on
pressure losses.
With an increase in the circular arc length L of the
projection A orthogonal to the tube axis, the number of the
end-to-end spaces between the projections for passing the
fluid decreases to result in increased pressure losses.
Accordingly the upper limit of the ratio of the length L of
circular arc of the projection A in a direction orthogonal to
the tube axis to the length of circumference nD of the thermal
cracking tube 10 should be 0.5 (preferably 0.2).
With a decrease in the circular arc length L of the
projection A orthogonal to the tube axis, the area S on which
the projection exerts an influence becomes too small to provide
the structure, and suitably agitated turbulence becomes no
longer available. Accordingly, the lower limit of the ratio
of the length L of circular arc of the projection A in a
direction orthogonal to the tube axis to the length of
circumference nD of the thermal cracking tube 10 should be 0.04.
[0024]
[Circular Arc Length s Orthogonal to Tube Axis and
Corresponding to the End-to-End Circular Arc Distance between
the Projection A and Projection A']
The length s of a circular arc as measured in a direction
orthogonal to the tube axis and corresponding to the end-to-end
circular arc distance between the projection A and another
projection A' adjacent thereto in the projection row a, i.e.,
the spacing between the projection A and the projection A'
should preferably be s/?D=0.05.
The circular arc length s orthogonal to the tube axis
and corresponding to the end-to-end circular arc distance
between the projections exerts influence on the range of
passage of the fluid and on pressure losses. Accordingly the
ratio of the circular arc length s orthogonal to the tube axis
and corresponding to the end-to-end circular arc distance
between the projections to the length nD of the circumference
of the tube 10 should preferably be up to 0.05.
[0025]
The number of projections in the projection row a is
determined by specifying the circular arc length L of the
projection A in a direction orthogonal to the tube axis, and
the circular arc length s orthogonal to the tube axis and
corresponding to the end-to-end circular arc distance. The
number of projections is 2 to 25 per projection row.
[0026]
[Projection Row ß]
With respect to the projection row ß positioned
downstream from the projection A, the region between the
phantom lines L1 and L2 downstream from the projection A axially
of the tube is preferably provided with one or a plurality of
projections B so as to be opposed to the projection A. The
reason is that if no projection B of the row ß is provided in
this region, it is difficult to maintain an agitating action
inside the tube because there is an upper limit for the range
of influence to be exerted by the fluid agitating projection
A, and it is impossible to maintain a high heat transfer
efficiency. It is further desired that the projection row ß
extend over at least 50% of the circular arc length of the
above-mentioned region.
[0027]
The projection rows a, ß, projections A, B, etc. thus
defined reduce pressure losses to the greatest possible extent
while maintaining a high heat transfer efficiency for the fluid
inside the tube as will be apparent from Examples to follow.
This makes it possible to avoid overheating while rapidly
heating the material fluid to the thermal cracking temperature
range thoroughly to the radially central portion of the tube
channel to produce the desired product in an improved yield.
Furthermore with coking inhibited, decoking work can be
dispensed with to produce an increased quantity of product due
to an increase in the operation period.
[0028]
[Other Arrangements of Projections]
FIGS. 4, (A) to (C) and FIGS. 5, (D) and (E) show other
different arrangements of projections A, B. These drawings
show projection rows a, ß which are annular and independent
of each other and are positioned orthogonal to the tube axis,
whereas these rows may be inclined with respect to the tube
axis as seen in FIG. 3.
FIG. 4, (A) shows an arrangement comprising a projection
A and a projection B downstream therefrom and parallel thereto.
The space defined by the projection A and phantom lines L1,
L2 is completely closed with the projection B.
FIGS. 4, (B) and (C) show arrangements wherein the space
defined by a projection A and phantom lines L1, L2 is at least
50% (less than 100%) closed with a projection or projections
B. In FIG. 4, (B), the phantom line L1 or L2 intersects with
the projection row (3 at a portion thereof between its opposite
ends. FIG. 4, (C) show that the space between the phantom lines
L1, L2 includes the entire length of the space between opposed
ends of projections of the row ß.
FIG. 5, (D) shows an arrangement wherein projections A,
B are inclined toward the same direction with respect to
projection rows a, ß, and FIG. 5, (E) shows an arrangement
wherein projections A, B are inclined toward opposite
directions with respect to projection rows a, ß. In the case
of FIG. 5, (D), a region R has a parallelogrammatic area S,
while FIG.. 5, (E) shows a region R having a trapezoidal area
S.
[0029]
In any of the cases shown in FIGS. 4 and 5, it becomes
possible to ensure reduced pressure losses to the greatest
possible extent while achieving a high heat transfer
efficiency for the fluid inside the tube, by satisfying the
definitions as to the projection lows a, ß, projections A, B,
etc.
Examples
[0030]
Specimen tubes were prepared in Examples 1 to 6 of the
invention and Comparative Examples 1 to 18, and checked for
heat transfer characteristics and pressure losses. Table 1
shows fabrication conditions for the specimen tubes. In Table
1, the lengths are expressed in mm, areas in mm2 and projection
numbers are numbers of projections.
[0031]
[0032]
Given below are the conditions of experiments.
Test fluid: air
Fluid temperature: heated air of 250 to 350°C for measuring
heat transfer characteristics, room temperature for
pressure loss measurement
Reynolds number: 25,000-120,000 for measuring heat transfer
characteristics, 40,000 to 120,000 for measuring pressure
losses
Pressure loss measuring section: 1000 mm-1500 mm
[0033]
For the measurement of heat transfer efficiency, air
heated to about 300°C by a hot air generator was fed to the
thermal cracking tube, and temperature variations in the fluid
and temperature variations in the tube wall were measured to
calculate an average Nusselt number. The fluid temperature was
measured by an anemomaster.
[0034]
Pressure losses were measured by using a blower for
sending room temperature air to the tube and a digital manometer
for measuring pressure differences. An anemomaster was used
for measuring the fluid temperature and the flow velocity
thereof.
[0035]
Table 1 shows the results of measurement. Each
of the measurements as expressed by the symbol O or x in Table
1 was evaluated as compared with the corresponding
characteristic value, serving as a standard, of a conventional
product of thermal cracking tube having projections extending
continuously circumferentially of the tube without
interruption. When showing an improvement not smaller than
a predetermined ratio, the measurement was evaluated as O,
whereas if otherwise, the measurement was evaluated as x.
Table 1 shows that the Invention Examples 1 to 6 are
excellent in heat transfer characteristics and are reduced tin
pressure losses.
On the other hand, Comparative Examples 1 to 18 are
inferior in heat transfer characteristics or pressure losses,
or in both of these characteristics.
[0036]
The tubes of Comparative Examples are inferior to those
of the invention because they are outside the specified range
with respect to one of the ratios of: area S/tube interior area
10nD over unit tube length of 10 mm, projection height h/inside
diameter D, circular arc length L of projection orthogonal to
tube axis/circumferential length nD, and circular arc length
s orthogonal to tube axis and corresponding to end-to-end
distance/circumferential length nD, thus failing to fully meet
the projection forming conditions or projection row forming
conditions. Unsatisfactory heat transfer characteristics lead
to the problem of coking in addition to a lower yield.
INDUSTRIAL APPLICABILITY
[0037]
The invention provides a useful thermal cracking tube
adapted to suppress pressure losses to the greatest possible
extent while maintaining a heat transfer promoting effect on
the fluid inside the tube.
CLAIMS
1. A thermal cracking tube for passing a fluid
therethrough from one end thereof toward the other end, the
tube having a plurality of rows of projections extending
circumferentially along an inner surface of the tube
orthogonal to or as inclined with respect to the axis of tube,
each of the projection rows including a plurality of
projections provided on the tube inner surface, the tube being
a hydrocarbon thermal cracking tube characterized by:
S/10?D=0.2-0.7,
h/D=0.02-0.05 and
L/?D=0.04-0.5
wherein D is the inside diameter of the tube, h is the height
of an optional projection A included in a projection row a,
L is the length of circular arc of the projection A in a
direction orthogonal to the tube axis, and S is the area of
a region R surrounded by the projection A, phantom lines L1
and L2 extending respectively from circumferentially opposite
ends of the projection A in parallel to the tube axis and a
projection row ß downstream from the projection row a.
2. The hydrocarbon thermal cracking tube according to
claim 1 which is defined by s/?D=0.05 wherein s is the length
of a circular arc orthogonal to the tube axis and corresponding
to the end-to-end circular arc distance between the projection
A and another projection A' adjacent thereto in the projection
row a.
3. The hydrocarbon thermal cracking tube according to
claim 1 or 2 wherein between the phantom lines L1 and L2 at
one side of the projection A of the projection row a downstream
therefrom axially of the tube, the projection row ß has one
or a plurality of projections B as opposed to the projection
A.
4. The hydrocarbon thermal cracking tube according to
claim 3 wherein the projection or projections B extend over
at least 50% of the circular arc length of a space defined by
the phantom lines L1 and L2.
5. The hydrocarbon thermal cracking tube according to
any one of claims 1 to 4 wherein the projection row a and the
projection row ß are provided generally in parallel, and the
region R is approximately parallelogrammatic.
6. The hydrocarbon thermal cracking tube according to
any one of claims 1 to 5 wherein the total sum SS of the areas
S for all the projections formed on the inner surface of the
tube is at least 80% of the total inner surface area of the
tube.
7. The hydrocarbon thermal cracking tube according to
any one of claims 1 to 6 wherein the projection row a and the
projection row ß extend helically.
8. The hydrocarbon thermal cracking tube according to
any one of claims 1 to 7 wherein the projection row a and the
projection row ß are each provided orthogonal to the tube axis.
9. The hydrocarbon thermal cracking tube according to
any one of claims 1 to 8 wherein the tube is 30 to 200 mm in
inside diameter D.

A thermal cracking tube is provided which is adapted to
suppress pressure losses to the greatest possible extent while
maintaining a heat transfer promoting effect on the fluid
inside the tube. The tube passes a fluid therethrough from
one end thereof toward the other end and has a plurality of
rows of projections extending circumferentially along an inner
surface of the tube orthogonal to or as inclined with respect
to the axis of tube, each of the projection rows including a
plurality of projections provided on the tube inner surface.
The tube is defined by S/10ΠD=0.2-0.7, h/D=0.02-0.05 and
L/ΠD=0.04-0.5 wherein D is the inside diameter of the tube,
h is the height of an optional projection A included in a
projection row α, L is the length of circular arc of the
projection A in a direction orthogonal to the tube axis, and
S is the area of a region R surrounded by the projection A,
phantom lines L1 and L2 extending respectively from
circumferentially opposite ends of the projection A in
parallel to the tube axis and a projection row β downstream
from the projection row α.