Note: Descriptions are shown in the official language in which they were submitted.
2176617
This invention relates to a process of cooling a
combustion gas coming from a combustion chamber in which an
H2S-containing gas is partially combusted at a maximum
temperature in the range from about looo to 1800C to form a
combustion gas that contains SO2, H2S, H2, H2O, and elementary
sulfur, wherein the combustion gas immediately after it has
left the combustion chamber is conducted through an indirect
heat exchanger having a cooling space supplied with a cooling
fluid for dissipating the heat, and to an apparatus for
carrying out that process.
Such a process and the associated apparatus are
known from EP-B-0 455 285. But in that case it is not desired
to cool as rapidly as possible the combustion gas that has
been produced in the combustion chamber for the Claus process.
In U.S. Patent 4,481,181 it is proposed that a gas mixture
which has been produced by a combustion and contains hydrogen
and elementary sulfur is cooled so rapidly by an admixing of
cold gas that the sulfur can no longer combine or can combine
only in part with the hydrogen to form H2S. For that purpose
it is recommended to effect a sudden temperature drop from
about 1100C to about 950C within less than 1 second.
It is an object of the invention that the combustion
gas which is produced, e.g., in the combustion chamber of a
Claus process plant, should rapidly be cooled in a succeeding
indirect heat exchanger. That cooling should result in a
cooled gas mixture which contains molecular hydrogen that has
not combined with existing elementary sulfur to form H2S. In
the process described first hereinbefore this is accomplished
in accordance with the invention in that the combustion gas
coming from the combustion chamber is first conducted in the
first section of the indirect heat exchanger through numerous
~ 2176617
tubes which have a small diameter d from 5 to 50 mm and is
thus cooled in the tubes of the first section to a temperature
not in excess of 800C within a residence time not in excess
of 0.05 second and the combustion gas coming from the narrow
tubes is conducted in a second section of the indirect heat
exchanger in tubes having a diameter D which is at least 2d.
In the process in accordance with the invention a
single heat exchanger is preferably used, which is divided
into two sections so that a most intense cooling can be
effected in the first section. This permits the use of an
inexpensive apparatus. Besides, the cooling conditions in the
first section can be controlled substantially independently
of the conditions in the second section.
In the first section of the heat exchanger the gas
mixture is cooled to such a degree that the hydrogen can no
longer combine with the elementary sulfur to form H2S. But
a formation of condensate is avoided because it might partly
clog the narrow tubes. Only in the second section, in which
the tubes are much larger in diameter, is it permissible to
cool to such a degree that condensate and particularly liquid
sulfur is formed. It will be understood that the tubes of a
section need not be perfectly uniform in diameter.
Water is usually employed as a cooling fluid in both
sections of the heat exchanger and steam is produced, which
in most cases is under a pressure in the range from 5 to 30
bars. On principle, other cooling fluids, such as air, may
be used in the process in accordance with the invention. To
still more effectively suppress the recombination of H2 and
elementary sulfur to form H2S in the combustion gas being
cooled, it is recommendable to cool in the tubes of the first
section of the heat exchanger to a temperature not in excess
of 750C within a residence time of the gas not in excess of
0.03 second. A cooling to a temperature in the range from 500
to 700C is preferably effected within that residence time.
The invention also provides a combustion chamber for
a partial combustion of an H2S-containing gas at maximum
` 2176~17
.~
temperatures in the range from about 1000 to 1800C to produce
a combustion gas which contains S02, H2S, H2, H2O, and
elementary sulfur. An indirect heat exchanger is directly
connected to the combustion chamber and comprises numerous
tubes, which are flown through by the combustion gas. The
heat exchanger comprises a first section and a second section,
which communicates with the first section. The tubes of the
first section have a small diameter d from 5 to 50 mm, and the
tubes of the second section have a larger diameter D, which
lo is at least 2d. The outlet ends of the tubes of the first
section are disposed in the inlet region of the tubes of the
- second section, and each section is provided with at least one
æupply line for cooling fluid.
In the process in accordance with the invention it
is possible to withdraw from the second section of the heat
exchanger a gas mixture which on a dry basis contains 2 to 30%
by volume free hydrogen. In a Claus process plant that
hydrogen is useful mainly for hydrogenating, and the Claus
process plant is relieved from a part of the H2S which is to
be reacted. This results in a considerable saving of
operating costs of a Claus process plant such as is known from
EP-B-0 455 285.
Further features of the process and of the apparatus
will be explained with reference to the drawing, in which
Figure 1 is a schematic longitudinal sectional view
showing a combustion chamber and a heat e~ch~nger connected
thereto,
Figure 2 is a longitudinal sectional view showing
the inlet portion of a tube of the first section of the heat
exchanger,
Figure 3 is a longitudinal sectional view showing
a modified arrangement in which a plurality of tubes of the
first section open into a tube of the second section of the
heat exchanger,
Figure 4 is an enlarged sectional view taken on line
IV-IV in Figure 3,
2176617
Figure 5 is an enlarged sectional view taken on line
V-V in Figure 1,
Figure 6 shows a heat exchanger provided with a by-
pass line, and
Figure 7 shows a further modification of a heat
exchanger.
In accordance with Figure 1, a burner 1 of a
combustion chamber 2 is supplied through line 3 with an H2S-
containing gas and through line 4 with an oxygen-containing
gas, such as air, oxygen-enriched air, or technically pure
oxygen. In practice, a plurality of burners may be associated
with a combustion chamber. The combustion space 5 in the
combustion chamber 2 is defined by refractory walls because
maximum combustion temperatures from about 1000 to 1800C and
usually of at least 1200C will be adjusted.
Owing to the high temperatures and the simultaneous
hypo-stoichiometric supply of oxygen, the combustion gas
formed in the combustion space 5 contains SO2, H2O, and
residual H2S and, as a result of a thermal cracking, also H2
and elementary sulfur. By a rapid cooling in the succeeding
heat exchanger 6 it is ensured that the hydrogen content is
not entirely eliminated by a recombination with sulfur. For
that purpose the hot gases must rapidly be cooled to a
temperature not in excess of 800C. That rapid cooling is
effected in a first section 10 of the heat exchanger 6 in
numerous narrow tubes 11, which are disposed in a first
cooling space 12. A cooling fluid, particularly water, is
supplied to the first cooling space 12 through line 13. Steam
which has been formed is discharged through line 14. The
outlet ends of the tubes 11 are held by a first tube plate 15.
Figure 2 shows the inlet end of a single tube 11,
which receives in the direction indicated by the arrow 8 the
hot combustion gas coming from the combustion chamber 5. A
ceramic sleeve 9 is provided for protection from the high
temperatures because experience has shown that the cooling
liquid which surrounds the tube 11 cannot sufficiently
217~617
effectively be cooled in that inlet region. The refractory
lining defining the combustion space 5 is designated 5a.
A æecond section 20 of the heat exchanger 6
comprises a smaller number of gas-conducting tubes 21. Two
or more tubes 11 of the first section are associated with each
tube 21 of the second æection 20. The ratio of the diameter
d of the tube 11 to the diameter D of the tubes 21 is in most
cases in the range from 1:2 to 1:5. The tubes 21 of the
second section extend between a second tube plate 22 and a
lo third tube plate 23. A second cooling space 24 is supplied
through line 25 with a cooling fluid, such as water, and steam
is discharged through line 26.
The two tube plates 15 and 22 are gas-tightly
interconnected by a ring 30. Besides, the exchange of liquid
between the first and second cooling chambers is restricted
by a perforated separating disk 31 provided between the ring
30 and the housing 7 of the heat exchanger. Because the disk
31 is permeable to the steam which has been formed, the same
pressure is maintained in the two cooling spaces 12 and 24.
Furthermore, the disk 31 supports the two tube plates 15 and
22 against the housing 7.
Care is taken that the gas flowing through the tubes
11 in the first section 10 of the heat exchanger 6 will be
cooled to a temperature in the range from 500 to 800C within
a residence time which is not in excess of 0.05 second and
preferably not in excess of 0.03 second. As is shown in the
simplified Figure 1 the gas which has thus been cooled flows
from two tubes 11 into one larger tube 21 of the second
cooling section and is cooled further therein. The gas
mixture leaving the tubes 21 first flows into a collecting
chamber 28, from which any condensate which has been formed
is withdrawn through line 29. Such condensate may contain,
e.g., elementary sulfur. The gas which in addition to SO2,
H2O, and H2S contains also molecular hydrogen finally leaves
the heat exchanger 6 through a discharge line 33. The gas
conducted through the discharge line 33 is usually at
~ 2176617
temperatures in the range from 200 to 400C and may be
subjected to a catalystic further processing in a Claus
process plant.
If it is desired to omit the two tube plates 15 and
22 shown in Figure 1 it will be possible to connect two or
more tubes 11 to a larger tube 21 in the manner illustrated
in Figure 3. In that case the tube 21 is provided at its
inlet end with a cover 21a, through which the tubes 11 tightly
extend. Figure 4 is associated with Figure 3 and a sectional
view taken on line IV-IV in Figure 3 shows three tubes 11
associated with one larger tube 21. For mechanical stability
a plurality of tubes 21 are supported in the housing 7 of the
heat exchanger close to their covers 2la on a perforate
separating wall 31, which is indicated in Figure 3.
If cooling water is used to indirectly dissipate
heat from the tubes 11 of the first section 10 and a large
number of tubes are provided, a problem may arise because the
steam which has been formed substantially restricts the
contact between the tubes and the cooling fluid so that a
sufficient dissipation of heat is not effected. This is
avoided in that, as is shown in Figure 5, rooflike guide walls
17 are provided in the first cooling space 12 and in different
regions guide the steam into a central region 18 the steam
which has been formed. That central region 18 flares in wedge
shape upwardly to the steam outlet 14 and does not contain
tubes 11. Broken lines 18a and 18b indicate the boundaries
of the central region 18. In that case the cooling water
which has been supplied through line 13 can freely rise along
the inside surface of the housing 7 in the directions
indicated by arrows 13a and 13b and can flow in partial
streams through the channels defined by the guide plates 17
in order to contact each of the tubes 11 as effectively as
possible. At the same time, the steam formed on the outside
surface of each tube 11 can flow along the undersurface of the
next upper guide wall 17 into the central region 18 without
a disturbing flow past other tubes 11. Three narrow tubes 11
- 217~17
of the first section open into each of the wider tubes 21 of
the second section, which are indicated by broken circular
lines, see also Figures 1 and 4.
Figure 6 shows a heat exchanger, which is provided
with a by-pass line 35, which extend continuously from the
combustion chamber 2 to the collecting chamber 28. The rate
at which the gas mixture flows through line 35 can be adjusted
by a control valve, which is not shown in the drawing. It is
thus possible to control the temperature of the gas mixture
which is processed further after it has been treated in the
heat exchanger. The remaining reference characters have the
meanings explained hereinbefore.
In the heat exchanger shown in Figure 7 the two
sections 10 and 20 may be disposed one over the other or one
beside the other. A wall 37 which is permeable to gas and
liquid extends between the cooling chambers 12 and 24. The
gas-vapor mixture coming from the narrow tubes 11 flows into
the intermediate chamber 38 and from the latter through the
wide tubes 21 of the second section 20. If the second section
is disposed under the first section, any condensate which has
been formed will drain through a line 29. The lines for
supplying the cooling fluid have been omitted in Figure 7.
~,XaM~;~E
In an arrangement as shown in Figures 1 and 5 but
having no guide walls 17, the heat exchanger in its first
section 10 comprises 279 tubes made of steel and having the
dimensions stated in Column A.
A B
Length 1.6 m 6.4 m
Outside diameter 20 mm 52.5 mm
Wall thickness 1.5 mm 3.9 mm
The dimensions of the 93 steel tubes of the second section are
2176617
. ~
stated above in column B. Water is used as a cooling fluid
and æaturated steam at 15 bare is produced.
The combustion chamber 2 of a Claus process plant
is supplied with a gas as defined in column C of the flowing
Table and with air at a rate of 4900 sm3/h ~sm3 = standard
cubic metre) and technically pure oxygen at a rate of 644
sm3/h:
C D E F
Rate ~sm3/h) 3560 7930 7460 6950
H2S (vol-%) 77.1 5.2 8.8 7.3
H2O (vol-%) 12.3 32.5 33.9 38.6
NH3 (vol-%) 5.6 1.1 - ~
CO (vol.%) 4.5 - - -
S2 (vol.%) _ 4.0 4.6 3.8
H2 (vol-%) ~ 7.2 4.9 5.2
C2 (vol-%) 0 5 1.4
COS (vol.%) - 0.03 <0.01 <0.01
Temperature (C) 40 1416 675 353
The maximum temperature in the combustion chamber is 1416C.
A gas mixture which is at that temperature and composed as
stated in column D of the above Table flows through the 279
tubes of the first section. The N content of the mixture and
traces of other gases are not taken into account in the Table.
During a residence time of 24 milliseconds in the tubes 11 the
gas mixture is cooled to a temperature of 675C. At that
temperatures and with the composition stated in column E of
the above Table the gas mixture enters the second section 21.
The gas mixture which has been cooled to 353C and has the
composition stated in column F of the above Table leaves the
second section 21 of the heat exchanger. Liquid elementary
sulfur is simultaneously withdrawn at a rate of 2040 kg/h.
The data stated in columnæ D and E have been calculated.
