Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEAT EXCI~UNGER FOR THE COOLING OF REACTION GAS
The invention relates to a heat exchanger for the cooling of reaction
gas. In particular, the invention relates to heat exchangers used for
quickly cooling reaction gases from, for example, cracking furnaces and
reactors in industrial operations.
To attain the highest possible cracking yield, hot cracking ~as which
exits a tube furnace (the type of furnace mostly used in cracking
operations) must be cooled as quickly as possible to an intermediate
temperature at which the chemical reactions taking place in the cracking gas
are inhibited. Further cooling of the cracking gas to an appropriate end
temperature may be carried out more gradually in consideration of other
technical process and economic criteria. Small total pressure loss in the
gas has a large impact on the yield of the reaction. Thus, for economic
reasons, a physically short overall construction is desirable.
:.;
In prior art apparatus such as disclosed in British patent GB 10 87
512, cracking gas is cooled to the end temperature in a single tube which is
directly connected to an exit of the furnace. Such a construction
guarantees fast cooling of the gas, but in consequence, considerable
pressure loss must be accepted. That reference teaches the method of
carrying out the cooling in two steps. In the first step the gas is cooled
as ~uickly as possible to the desired intermediate temperature within the
single cooled tube. ~ubsequently, the cracking gas is transported through
interconnecting conduits to a second separate apparatus wherein the second
step of the cooling ls realized. The construction costs of such an
arrangement which requires two separate individual pieces of apparatus are
very high. The interconnecting conduits between the two pieces cause a high
pressure loss,,which lowe,rs the reaCtion yield.
It is further known in the art to connect a plurality of furnace
exits to a heat exchanger entry chamber and to distribute the cracking gas
from the entry chamber into a plurality of cooling tubes. A disadvantage of
such an arrangement is that the velocity of the reaction gas becomes slower
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PAT 159~0-1
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at the entry chaMber by reason of its larger volume. As a result, cooling
of the gas after leaving the furnace is delayed, which reduces the reaction
yield. Cooling of the gas is in fact even further delayed, since the entry
chamber is not cooled.
s
It is an ob~ect of the disclosure to reduce the construction costs of
such a heat exchanger while providing favourable processing and cooling
condltions.
Here described in a heat exchanger including first and second cooling
arrangements which are integrally enclosed for containment of the cooling
medium. Thus, both cooling arrangements are combined into a single
apparatus, which reduces construction costs. The fast cooling of the
reaction gas to the intermediate temperature in the first cooling
arrangement commences immediately after the furnace exit and without
reduction in the gas velocity. The final cooling in the directly integrated
second cooling arrangement is achieved at low mass speed and, therefore,
with small pressure loss. As a result, the second cooling arrangement may ~ i
be much shorter than a single cooled tube. The distribution chamber between
20 the first and the second cooling arrangement may also be cooled and thus ~
contribute to the heat exchange. In a preferred embodiment, conical -
construction of the distribution chamber provides for efficient pressure
recovery and, therefore, a smaller total pressure loss.
Embodiments of the invention will now be described by way of example
only with reference to the accompanying drawings, wherein, ~ -
Fig. 1 is a schematic axial cross-section through a heat exchanger embodying
the invention for the cooling of reaction gas; and
Fig. 2 is a schematic axial cross-section through a preferred embodiment of
the heat exchanger shown in Fig. 1.
The illustrated heat exchangers are used for the quick cooling of
cracking gas or any other reaction gas, which is produced in a furnace
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constructed as a tube furnace or in a reactor of a chemical plant. A tube
furnace (not illustrated) generally includes a plurality of separately
heated tubes through which the reaction gas streams. The heat exchanger as
illustrated in Fig. 1, includes two cooling arrangements, the first being
constructed as a single tube heat exchanger 20 having a single tube 1, and
the second being constructed as a tube bundle heat exchanger 30 having tubes
2. Single tube 1 is surrounded by an outer tube 3 and is, at its gas entry
end, sealingly connected with that outer tube 3 through an annular flange
4. Single tube 1 directly communicates with a heated tube of the tube
furnace through a thermal stress free connection (not illustrated). The
inner dimensions of the heated tube and of single tube 1 are substantially
the same.
The gas exit end of single tube 1 opens into a distribution chamber 5
which is defined at its far axial end by a first tube end plate 6. The gas
entry ends of tubes 2 of tube bundle heat exchanger 30 are welded to the
first tube end plate 6 to produce a gas tight connection. A second tube end
plate 7 is affixed to the gas exit ends of tubes 2, the connection also
being gas tight. A gas exit chamber ô is positioned downstream and adjacent
the second tube end plate 7 for removal of cold cracking gas. An outer
mantle 9 surrounds tubes 2 thereby defining an inner chamber 10, In the
heat exchanger illustrated in Fig. 1, distribution chamber 5 has a conical
cross-section and its diameter increases in the direction of flow of the
cracking gases from that of single tube 1 to the diameter of first tube end
plate 6. This end plate 6, which partly defines distribution cbamber 5, has
a smaller diameter than the inner diameter of outer mantle 9. Outer tube 3
is connected with outer mantle 9 through a conical intermediate member 11 in
the region of distribution chamber 5. Intermediate member 11 provides for
connection between an annular space 12, which is radially defined by and
located between single tube 1 and outer tube 3, and chamber 10 which is
radially defined by outer mantle 9 and single tube 1, so that coolant may
continuously flow through the zone which comprises both space 10 and then
chamber 12. As a result, intermediately located distribution chamber 5 is
also located within the coolant stream and may thus be used for the cooling
of the cracking gas.
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The preferred coolant used is highly pressurized water which is
supplied to the heat exchanger through one or more coolant supply unions
13. The water is partially vapourized by the transfer of heat from the
cracking gas, which streams through single tube 1 distribution chamber 5 and
tubes 2, and exits through one or more coolant discharge unions 14 as a
water/steam mixture. Supply union 13 is mounted to outer tube 3 and
discharge union 14 to outer mantle 9.
In the heat exchanger shown in Fig. 2, single tube 1' and tubes 2'
are mounted in a parallel arrangement within outer mantle 9', with tube 1
mantle 9' preferably co-axial. Distribution chamber 5' is defined by first ;
tube end plate 6' and a hood 15 for redirecting the reaction gas stream
which are both mounted on and sealingly close one end of outer mantle 9'.
Second tube end plate 7' is positioned at the gas exit end of tubes 2' and
secured to outer mantle 9' and to outer tube 3' at its end remote from first
tube end plate 6'. Outer tube 3', in this embodiment, surrounds single tube
1' for only part of its length. Outer tube 3' is over at least part of its
axial extent surrounded by an annular gas collecting chamber 16, which
includes a gas exit spigot 17 and is defined in the axial direction of outer
tube 3', by second tube end plate 7' separating chamber 16 from chamber 10'
and by an end wall 22 remote from second tube end plate 7', and in the
radial direction by outer mantle 9'.
In contrast to the heat exchanger shown in Fig. 1, wherein the
cracking gas to be cooled flows through single tube 1 and tubes 2 with no
change in the flow direction, in the heat exchanger as shown in Fig. 2, the
gas flows through single tube 1' and tubes 2' in a counterflow arrangement.
In a heat exchanger as shown in Fig. 2, annular space 12' and chamber 10'
are, like the embodiment shown in Fig. 1 for space 12 and chamber 10,
connected so that the same coolant may continuously flow through the coolant
zone which they define. Supply union 13' for the coolant is mounted to
outer tube 3' close to the gas entry end of the heat exchanger, and
discharge union 14' is secured to outer mantle 9' adjacent first tube end
plate 6'.
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The cracking gas G produced in the tube furnace flows into gas entry
end 21, 21' of single ~ube 1, 1' of single tube heat exchanger 20, 20' as
indicated by the solid arrow, which single tube represents the first cooling
arrangement, for cooling at constant cross section and without significant
velocity reduction. Immediately as the cracking gas exits the tube furnace,
its heat starts to be transferred to the coolant C which enters the heat
exchanger through coolant supply union 13 and which then flows to annular
space 12, 12' which surrounds single tube 1, 1'. The cracking gas can thus
be cooled quickly to the required intermediate temperature at a high mass
speed. The gas exits from single tube 1, 1' directly into distribution
chamber 5, 5', which by allowing velocity reduction permits conversion of
dynamic energy to pressure energy from the Bernoulli effect and provides for
regaining of pressure and thus reduction of the total pressure loss. From
distribution chamber 5, 5', the intermediate temperature cracking gas enters
15 directly into tubes 2, 2' of tube bundle heat exchanger 30, 30' which
represents the second cooling arrangement. The cracking gas flows through
these tubes 2, 2' with a lower mass speed and therefore with a desired
reduced pressure loss. The remaining heat in the cracking gas is
transferred to the surrounding coolant within chamber 10, 10', which allows
cooling to a selected end temperature. The heated coolant exits the heat
exchanger through coolant discharge union 14, 14' and the gas at the
selected end temperature leaves through gas exit chamber 8 of the embodiment
shown in Fig. 1 or through gas exit spigot 17 of the embodiment shown in - -
Fig. 2.
PAT 15970-1
