Note: Descriptions are shown in the official language in which they were submitted.
CA 02754378 2011-10-12
PROCESS AND DEVICE FOR THE GENERATION OF A CONSTANT AMOUNT
OF STEAM FROM THE WASTE HEAT OF AN ALKANE DEHYDROGENATION
PROCESS
The invention relates to one or more burners used as additional heating in
the flue gas duct of a reactor for the performance of an endothermic reaction
by
which it is possible to produce a nearly constant amount of flue gas, the
reactor
being equipped with a steam generator located in the outlet of the flue gas
duct of
the heating chamber, and the burners being used as auxiliary burners for
balancing the flue gas reduction in the flue gas duct of the heating chamber
which
normally occurs during the regeneration phase of the exothermic regeneration
of
the catalyst in use. The invention also refers to a device comprising one or
more
auxiliary burners installed in a reactor for the performance of an endothermic
reaction including the necessary equipment by which it is possible to control
the
amount of flue gas in the outlet of the flue gas duct housing the steam
generator.
In this way, the steam flow obtainable from the steam generator is equalised
to a
substantial degree, which is of advantage in the operation of turbines or
compressors. This device is especially suited for reactors typically used for
alkane
dehydrogenation processes.
A common process for performing an alkane dehydrogenation is to pass an
alkane-containing hydrocarbon mixture across a dehydrogenation catalyst which
causes the alkane contained in the gas mixture to react to the corresponding
alkene. The catalyst is provided in a typical configuration in downward
reaction
tubes which the reaction gas enters through an inlet duct so that the product
gas
which contains, as a component, the required alkene can be obtained at the
outlet
of the reaction tubes. The reaction is endothermic so that it is required to
heat the
reaction tubes from the outside. This is normally accomplished by means of a
reactor with a heating chamber with integrated reaction tubes, which can be
heated with fuel gas. The reaction tubes are sealed towards the heating
chamber.
The heating chamber ends in a flue gas duct where the hot flue gas is
thermally
utilised and finally discharged into a stack.
A typical embodiment of the alkane hydrogenation process with an
appurtenant device is described in WO 2004/039920 A2. A selection of various
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dehydrogenation processes and the corresponding catalysts involved is given in
the publication by F. Buonomo, D. Sonfillipo, F. TrifirO, Handbook of
Heterogeneous Catalysis, 1st Edition, VCH, Weinheim, 1997 p. 2140 if. and the
literature references made therein.
The flue gas from the heating is discharged from the heating chamber via
flue gas ducts. Its temperature is around 1000 C depending on the respective
constructional design. To further utilise the heat from the heating of the
reaction
tubes, a steam generator is typically installed for the flue gas in or
downstream of
the outlet of the flue gas duct.
An alkane dehydrogenation is typically accompanied by the formation of
carbonaceous by-products which deposit on the catalyst after a certain
reaction
period. This will reduce the reaction yield and the production of required
alkene.
The reaction is, for this reason, interrupted after a certain period of time
and the
reaction gas flow across the catalyst stopped. In a typical embodiment an
oxygen-
containing gas is subsequently passed across the catalyst. It serves to remove
the carbonaceous deposits and to regenerate the catalyst. After the
regeneration,
the alkane dehydrogenation in the respective reaction tube or reactor is
resumed.
According to this procedure the process is carried out in a cyclic operating
mode.
As the dehydrogenation of alkanes is endothermic and the regeneration of
the catalyst exothermic, the reactor requires a significantly lower amount of
heat
during the regeneration period than during normal operation. For this purpose,
the
burners are normally operated with less fuel gas during the regeneration phase
resulting in an analogously lower production of flue gas.
WO 2007/118825 Al describes a process for the production of olefins from
hydrocarbons and a device for running the process. By switching off the
burners
during the regeneration phase the heat supply into the catalyst bed is
interrupted
so that no more heat is fed to the catalyst bed when oxygen-containing gas is
passed across it during regeneration and the catalyst is prevented from
overheating and degradation. For running the process the burners are provided
with a switch-off device and are re-ignited for re-start by means of pilot
burners
after the regeneration. Both the heating burners and the pilot burners may be
equipped with a monitoring device. No indications are made with reference to
the
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generation of steam by the use of a steam generator and a compensation of the
interrupted heating.
EP 179322 B1 describes a process for the exothermic regeneration of a
catalyst which has been deactivated during an endothermic catalytic conversion
in
the course of a discontinuous process. By reducing the burner capacity to
below
50 percent of the original capacity, preferably below 10 percent of the
original
burner capacity, which is accomplished by reducing the supply of heating fuel,
savings in heating medium and combustion air can be achieved. As suitable
application processes, special reference is made to the dehydrogenation of /-
butane, n-butane or mixtures thereof. If several reactors are used, it is
possible to
operate them alternately so that there is in total no change in the supply of
heating
media, combustion streams and no load change in the waste heat system. This
teaching as well does not give any indications with reference to the
generation of
steam by the use of a steam generator or to the compensation of the
interrupted
heating.
When the heating process output is decreased for the regeneration of the
catalyst, the flue gas feed to the steam generators installed in the flue gas
duct is
reduced. This is problematic as in an important embodiment the steam generated
by the steam generators is used for driving a compressor by means of a steam
turbine. The dehydrogenation process works such that steam is supplied to
serve
as turbine steam during normal operation and also during regeneration. During
the regeneration phase the unit itself produces less steam. At the same time,
however, the steam consumption level in the regeneration mode is almost as
high
as during normal operation. The amount of steam supplied during the
regeneration phase therefore determines the amount of steam supplied to the
dehydrogenation unit.
It is therefore the aim to provide a process for the production of alkene by
endothermic catalytic reaction making available as high an amount of flue gas
as
possible for the operation of a steam generator during the regeneration phase
so
that a constant amount of steam is provided throughout the whole "Production-
Regeneration" cycle. It is another aim of the invention to provide a device
for this
purpose. The latter is also expected to allow monitoring and remote control of
the
process.
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The invention achieves this aim by a process for the constant or controlled
supply of flue gas from an endothermic catalytic reaction, by which it is
possible to
use this flue gas to produce a steam flow which is as high as possible in
quantity
by means of a steam generator, in which at least one auxiliary burner is
installed
in the outlet of the flue gas duct, the burner being used to generate a flue
gas
stream which does not get into contact with the reaction tubes to be heated
and
by which the flue gas stream is increased in quantity on the heat exchange
surfaces of the steam generator during the regeneration. The invention
achieves
this aim also by a device made up by one or more auxiliary burners at the
inlet of
the flue gas duct of a reactor for the performance of an endothermic catalytic
reaction, the auxiliary burners being arranged in the flue gas stream behind
the
reaction tubes. The device also includes a monitoring and control device for
the
auxiliary burners.
By the present invention it is possible to reduce the quantity of steam
supplied. In this way, no additional costs are incurred by heat-exchange
devices
as equipment and internals dimensioned for normal operation are used. The
auxiliary burners, which are mainly required for the regeneration mode, are
comparatively inexpensive.
Typical endothermic catalytic processes suitable for the application of the
invention are alkane dehydrogenation processes. These are, in any case,
reactions which are performed in reaction tubes loadable with catalyst, the
reaction tubes being arranged in a heatable reaction chamber and the reaction
chamber being heated with fuel gas from burners. A typical process for the
dehydrogenation of alkanes suited for the application of the invention is
described
in WO 2004/039920 A2. This document also describes a reactor in which the
hydrogen generated in the alkane dehydrogenation is incinerated in a separate
process step.
Especially claimed is a process for the regeneration of a fixed-bed catalyst
with temporally constant generation of steam from the heating of the reactor,
in
which
a fixed-bed catalyst is arranged in one or several reaction tubes
where an endothermic reaction is carried out by passing through a reaction gas
mixture, and
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CA 02754378 2011-10-12
the reaction tube or tubes are heated from the outside by
combustion of a fuel gas in a heating chamber housing the reaction tubes for
carrying out the endothermic reaction, and
the reaction in the reaction tube or tubes is performed in a cyclic
operating mode over a limited period of time, the period not used for the
reaction
being used for the regeneration of the catalyst by passing an oxygen-
containing or
water vapour-containing gas or a mixture of both across the catalyst, and
the flue gas stream produced by the heating of the reaction tubes is
discharged from the heating chamber and utilised for steam generation by a
steam generator,
and characterised in that
at least one auxiliary burner is installed in the outlet of the flue gas
duct, this burner producing a flue gas stream not getting into contact with
the
reaction tubes to be heated and increasing the flue gas stream in quantity on
the
heat exchange surfaces of the steam generator during the regeneration.
By the increased amount of flue gas in the flue gas duct it is possible to use
the heat exchange surfaces in the flue gas duct during the regeneration period
of
the catalyst more efficiently. In this way, the amount of produced steam can
be
kept nearly constant throughout the whole duration of the process. For this
purpose, the auxiliary burners are equipped with a control device by which it
is
possible to control the amount of combustion gas. This can be achieved by
controlling the auxiliary burners by means of the flue gas temperature in the
flue
gas stream downstream of the auxiliary burners. The control device adjusts the
supply of fuel gas or combustion air into the auxiliary burners.
To further increase the steam amount generated during the regeneration,
the auxiliary burners are used advantageously such that the temperature of the
flue gas stream at the inlet of the flue gas duct is increased on the heat
exchange
surfaces of the steam generator. The temperature in the flue gas duct and on
the
heat exchange surfaces can be controlled via the fresh air supply into the
flue gas
duct if this is required.
The auxiliary burners are advantageously equipped with a control device so
as to control the fuel gas supply and thus the production of flue gas. The
control
device is governed by a temperature sensor provided near the heat exchange
CA 02754378 2011-10-12
= ,
surfaces of the steam generator so that it is possible to control the
auxiliary
burners by means of the temperature in the flue gas duct. In a simpler
embodiment it is also possible to provide for a manual control of the
auxiliary
burners.
The control device of the auxiliary burners can also be controlled by the
amount of produced steam. In such case, a steam flow meter for the amount of
steam produced is installed in a suitable position of the steam generator so
that
the auxiliary burners can be controlled by the amount of steam produced.
Processes which are qualified for the process according to the invention
are in particular alkane dehydrogenation processes used to convert an alkane
into
an alkene by releasing hydrogen. This may be carried out in a single process.
It
is, however, also common practice to carry out the alkane dehydrogenation
converting an alkane into an alkene by releasing hydrogen and oxidising the
hydrogen in a subsequent separate process step in which a further
dehydrogenation of not yet converted alkane is achieved. The auxiliary burners
may then be installed in one or more reactors. In this way the whole
endothermic
process is supported by the regeneration of the catalyst.
Also claimed is a device by which it is possible to run the process
according to the invention. Especially claimed is a device for the generation
of a
constant amount of steam from the waste heat of an alkane dehydrogenation
process, comprising
a reactor with heating chamber for carrying out an endothermic
reaction with integrated reaction tubes which can be loaded with a catalyst,
an
inlet for the reaction gas and an outlet for the product gas in the reaction
tubes,
one or more main burners which do not get into contact with the catalyst or
the
reaction gas and heat the reaction tubes in the heating chamber from the
outside,
and a flue gas duct with outlet for the flue gas at the end of the heating
chamber,
and one or more steam generator/s in the flue gas duct,
and characterised in that
one or more auxiliary burners are installed at the outlet of the flue
gas duct behind the reaction tubes and upstream of the entry into the steam
generator or generators.
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. .
Reactors for the performance of endothermic processes which are
equipped with auxiliary burners for bridging the start-up procedure are known
according to the state of the art. US 2003/0101651 Al describes a device for
an
endothermic catalytic reaction by which a hydrocarbonaceous gas is passed
through tubes which can be loaded with catalyst and are heated from the
outside,
the reaction gas being heated by convection in counter-current flow. By the
reaction arrangement it is possible to significantly reduce the size of the
reactor
and to design the complete assembly as a mobile unit. The device describes
auxiliary burners used to start the reaction by installing them in the heating
chamber for combustion of the fuel gas. Not described is a regeneration of the
catalyst or interruption of the heating process. Neither mentioned is a
control
device for the auxiliary burners.
In most embodiments of the invention the auxiliary burners are provided
with control devices by which it is possible to control the capacity of the
burners.
Suitable control devices are, for instance, valves, gate valves, flaps or
stems
which serve to control the supply of fuel gas into the auxiliary burners. The
control
device may also be used to control the supply of combustion air into the
auxiliary
burners.
The auxiliary burners may also be controlled by parameters measured in
the outlet of the flue gas duct. Devices required according to the related
process
claims are especially sensors measuring the flow rate of the combustion gas or
the temperature of the combustion gas. To serve this purpose, these are
installed
in the outlet of the flue gas duct. If the aim is to control the auxiliary
burners by the
combustion gas flow rate, the flue gas ducts are equipped with a measuring
device which serves to measure the combustion gas flow rate of the flue gas in
the flue gas duct, by which it is possible to control the auxiliary burner.
If the aim is to control the auxiliary burner/s by the temperature of the flue
gas stream, the flue gas ducts are provided with a device for measuring the
temperature of the flue gas in the flue gas duct by which it is possible to
control
the auxiliary burner. If a comparison measurement in comparison to the overall
flow rate or temperature of the flue gas is required, appropriate sensors can
be
installed in the flue gas duct itself or on the heat exchange surfaces of the
steam
generator. In another embodiment it is also possible to use Lambda probes for
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.
measuring the oxygen content in the flue gas duct, should the auxiliary
burners be
controlled by the oxygen content in the flue gas duct.
The reactor for integrating the device according to the invention is typically
configured as is common practice according to the state of the art. For
carrying
out the invention this includes a reactor for carrying out an endothermic
reaction
with integrated reaction tubes which can be loaded with a catalyst, burners
which
do not get into contact with the catalyst or the reaction gas and heat the
reaction
tubes from the outside, an inlet for the reaction gas and an outlet for the
product
gas in the reaction tubes, an inlet for the fuel gas and a flue gas duct, and
a steam
generator with heat exchange surfaces in or downstream of the outlet of the
flue
gas duct. The main burners and the auxiliary burners according to the
invention
may be installed in any position in the heating chamber or in the flue gas
duct.
This applies analogously to the heat exchange surfaces to be heated. The
burners, reaction tubes or steam generators can be provided as single or as
multiple units. The auxiliary burners are, in any case, arranged such that the
escaping flue gases do not get into contact with the reaction tubes and the
enclosed catalyst.
To control the auxiliary burners especially such devices may be used that
serve the purpose of burner control according to the state of the art. These
are
typically valves, gate valves, flaps or stems which serve to control the
supply of
fuel gas or combustion air into the auxiliary burners. To measure control
data,
especially thermocouples, pressure gauges, gas flow meters and oxygen probes
may be used.
Used as auxiliary burners may be gas burners, liquid-fuel burners, rocket
burners or solid-fuel blower burners. The type is determined by the size of
the flue
gas duct and of the heat exchange surfaces. The auxiliary burner device
according to the invention also includes suitable ignition devices as there
are, for
example, electric or electronic igniters, pilot burner or flint stones. The
auxiliary
burners are preferably equipped with a control device by which the capacity of
the
auxiliary burner/s can be controlled. This can be implemented such that the
flue
gas duct, for example, is equipped with a device for measuring the flue gas
temperature in the flue gas duct and by which the capacity of the auxiliary
burner/s can be controlled.
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To additionally utilise the waste heat, steam generators are used which
may be arranged as desired and provided in any number desired. Typically these
are steam generators which are arranged as indirect heat exchangers with heat
exchange surfaces. These may be of optional design. These may also include
measuring devices for measuring the amount of steam produced. The steam
generators which are heated by means of the auxiliary burner/s may be equipped
with a steam flow meter by which it is possible to control the capacity of the
auxiliary burner/s. Furthermore included in the device according to the
invention is
ancillary equipment such as pipelines, for example. These may be of optional
design and number.
The device according to the invention involves the advantage that an
amount of steam which is as constant as possible throughout the duration of
the
process can be generated from the waste heat of an alkane dehydrogenation. By
the device and the process according to the invention the generation of steam
from the waste heat of the before-mentioned processes can be optimised and
used to recover mechanical energy.
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