Note: Descriptions are shown in the official language in which they were submitted.
CA 02743089 2011-05-09
Production of process gas by heat recovery from low-temperature waste heat
The invention relates to a process for the steam reforming of
hydrocarbonaceous feedstocks, focussing in particular on the production of
process gas
by heat recovery from low-temperature waste heat. The invention aims at a
greater
efficiency in exploiting the energy of a hydrogen- and steam-containing
process gas
produced in a steam reforming process. In addition, the invention relates to
an apparatus
for running the process according to the invention.
The steam reforming process serves to convert a reaction mixture of steam
and hydrocarbonaceous feedstocks into a hydrogen-enriched process gas. This
process
gas is obtained from the steam reforming process at a temperature above 100 T.
In
most cases, this temperature ranges between 700 and 1000 C.
To allow subsequent processing of the process gas, which, for example, may
consist in a treatment and/or an increase of the hydrogen portion by pressure-
swing
adsorption or a membrane process, it is to be cooled. In most cases, the
temperature
required for subsequent processing ranges between 20 and 50 C. Between the
individual cooling steps, further reaction steps may be provided which, for
example, may
include the reaction of carbon monoxide with water to give carbon dioxide and
hydrogen.
From the patent literature various different approaches are known to exploit
the amount of heat contained in the process gas for heating substances
involved in
and/or outside the process. Frequently the amount of heat contained is
specifically used
to preheat the boiler feed water for the steam reforming process by way of
heat
exchange.
In a typical conventional heat recovery process integrated into a syngas
production plant, the amount of heat in the process gas is normally used by
generating a
high-pressure steam in a waste-heat boiler in a first step and converting the
process gas
in a CO conversion unit into carbon dioxide and hydrogen. Subsequently passage
through various different heat exchangers is provided in order to heat, for
example, the
hydrocarbonaceous feedstock, the boiler feed water and/or the make-up water.
In most
cases, the residual heat of the process gas is dissipated to the atmosphere
via a cooling
section. The condensate obtained in the cooling section is fed to a water
treatment unit,
where the make-up water is added, and then directed to the boiler feed water
preheater,
from where the heated flow is conveyed to the steam generation system.
The disadvantage involved in this conventional heat recovery method is that
the major part of the heat of the process gas which leaves the CO conversion
unit is the
heat from a moist condensation. This condensation is subject to a pinch effect
resulting
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CA 02743089 2011-05-09
from further cooling, which makes it very difficult to recover the contained
heat and a
significant portion is dissipated to the atmosphere via the cooling section.
Here, the pinch
effect is defined by the approximation of the temperatures of two streams, by
which the
temperature difference between the two streams is reduced, thus also
minimising the
driving force for the heat exchange. In this way, a lot of energy from the
process gas gets
lost unexploited.
A proposal to avoid this problem is disclosed in US 2006/0231463 Al. Here,
water is heated and fed to a water treatment unit. A first water stream from
this unit is
directed to a low-pressure steam generator and a second water stream to a
first boiler
feed water preheater. Process gas for heat exchange is passed through both
units. The
water stream obtained from the first boiler feed water preheater is subdivided
into two
partial streams and sent to two additional boiler feed water preheaters, the
first of the two,
hereinafter referred to as boiler feed water preheater 1, also being passed by
process gas
for heat exchange, and the second, hereinafter referred to as boiler feed
water preheater
2, being passed by flue gas for heat exchange. The two water streams obtained
from the
two before-mentioned boiler feed water preheaters are then conveyed to the
steam
generation unit.
The disadvantage involved in this system is that the heat exchange in boiler
feed water preheater 1 through which process gas is passed is subject to a
pinch effect,
thus restricting the desired heat transfer to a very limited degree. The
general rule applies
that the larger the amount of boiler feed water passing through this unit, the
higher the
useful heat yield. The subdivision of the water stream before passing through
boiler feed
water preheater 1, however, results in a limited amount of boiler feed water
passing
through the unit so that a notable portion of the heat contained in the
process gas is
dissipated to the atmosphere via the cooling section - usually in the form of
air coolers -
and thus gets lost unexploited. In addition, part of the heat of the flue gas
is used to heat
the boiler feed water. This heat portion of the flue gas is thus no longer
available for the
actual steam generation.
A further disadvantage involved in the interconnection of the individual units
in
US 2006/0231463 Al is that the water to be heated and then sent to the water
treatment
unit is to be heated via the amount of heat contained in the process gas. The
water
treatment unit usually consists in a degasifier, which is mostly operated at
approximately
atmospheric pressure or slight overpressure, typically below 5 bar (abs.), in
order to
remove as much oxygen and other gases from the water as possible.
Conceptually, the
temperature of the water supply stream of this water treatment unit is
typically limited to
between 80 and 95 C. Technically, the water supply stream could, however, be
heated
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by the heat contained in the process gas to a temperature above 100 C.
Therefore an
additional control device must be provided to ensure that the temperature of
the supply
stream to the water treatment unit does not exceed the limit of 95 C. In this
way, the heat
of the process gas cannot be exploited completely and the contained residual
heat is
finally dissipated unexploited into the atmosphere.
The present invention has been developed against the background of the
above-described state of the art, with the aim to make available a process for
the
production of process gas which does not involve the afore-mentioned problems
related
to the heat recovery from the amount of heat contained in the process gas and
in which
the heat recovery is designed even more efficiently. It is also the subject
matter of the
invention to disclose an apparatus to run the process according to the
invention.
This is achieved by employing a heat recovery process in the steam reforming
of hydrocarbonaceous feedstocks by means of steam, in which a steam reformer
generates a process gas which contains a first amount of heat, and a flue gas
which
contains a second amount of heat, comprising at least six heat exchangers, a
water
treatment unit, a cooling section, a high-temperature conversion unit, at
least two
pressure boosting units, at least one consumer and at least one unit for
subsequent
processing of the resulting process gas. The generated process gas containing
the first
amount of heat passes the high-temperature conversion unit, where it is, for
the most
part, converted into carbon dioxide and hydrogen, after which the resulting
heat-
containing process gas is directed into a first heat exchanger for subsequent
heat
transfer, and afterwards into at least two more heat exchangers which are
operated as
boiler feed water preheaters, product condensate heat exchangers or low-
pressure
evaporators, and are connected in series in any sequence desired, the process
gas
resulting from the low-pressure evaporator being first fed into a further
boiler feed water
preheater, where heat energy is transferred to a partial stream of the boiler
feed water
from the water treatment unit, after which the process gas obtained passes the
cooling
section, where it is further cooled generating a condensate flow, and finally
fed into at
least one unit for subsequent processing of the resulting process gas.
Furthermore, a deionised water stream is sent to a second heat exchanger for
being heated. The deionised water stream from the second heat exchanger is
directed for
degassing into the water treatment unit, the boiler feed water stream from the
water
treatment unit passes a pressure boosting unit and is subdivided, a first part
of the boiler
feed water stream being sent to the low-pressure evaporator, where a low-
pressure
steam is generated, and the generated low-pressure steam is subdivided and a
first
partial low-pressure stream is directed for heat transfer to the water
treatment unit and a
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CA 02743089 2011-05-09
second partial low-pressure stream is sent to at least one consumer. This
second partial
stream of low-pressure steam may also be used for preheating other process
media such
as liquid feedstock or may be transferred for a use outside battery limit. A
second part of
the boiler feed water stream is passed through the second heat exchanger for
the
purpose of energy transfer and subsequently through one or more boiler feed
water
preheaters for being heated by the heat amount contained in the process gas
and finally
conveyed to the steam generation unit.
In the deaerator of the water treatment unit, the deionised water is degassed
from a major part of the oxygen. Subsequently other dosing fluids may be added
as, for
example, ammonia for adjusting the pH value. The product resulting from this
treatment is
referred to as boiler feed water.
Via a pressure boosting unit, the condensate flow from the cooling section is
passed to the product condensate heat exchanger for being heated by the heat
amount
contained in the process gas, after which the condensate flow is heated again.
The process gas from the first heat exchanger first runs preferably through a
first boiler feed water preheater, in which heat energy is transferred to a
boiler feed water
stream, subsequently a product condensate heat exchanger, where heat energy is
transferred to a condensate flow, and from there the resulting process gas is
directed to
the low-pressure evaporator, in which low-pressure steam is generated from a
boiler feed
water stream by means of the heat amount contained, from where it is sent to
the
subsequent steps of the defined process chain.
In another embodiment of the invention the process gas from the first heat
exchanger first runs through a first boiler feed water preheater, in which
heat energy is
transferred to a boiler feed water stream, subsequently it is sent to a low-
pressure
evaporator, in which low-pressure steam is generated from a boiler feed water
stream by
means of the heat amount contained, and from there the resulting process gas
is directed
into the product condensate heat exchanger, where heat energy is transferred
to a
condensate flow, from where it is sent to the subsequent steps of the defined
process
chain.
Advantageously the process gas from the first heat exchanger first runs
through a product condensate heat exchanger, in which the heat energy is
transferred to
a condensate flow, from there it runs through the first boiler feed water
preheater, in
which heat energy is transferred to a boiler feed water stream, and then it is
passed to a
low-pressure evaporator, where low-pressure steam is generated from a boiler
feed water
stream by means of the amount of heat contained, and subsequently the
resulting
process gas is passed to the subsequent steps of the defined process chain as
described
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above. In a further embodiment of the invention the process gas from the
product
condensate heat exchanger runs first through the first boiler feed water
preheater, where
heat energy is transferred to a boiler feed water stream, and then through
another
product condensate heat exchanger, before it is directed into the low-pressure
evaporator, from where it is passed through the subsequent steps of the
defined process
chain.
Another possible embodiment of the invention is that the process gas from the
first heat exchanger runs first through a product condensate heat exchanger,
where heat
energy is transferred to a condensate flow and to a partial stream of the
boiler feed water
stream, from where it is passed to the low-pressure evaporator, where low-
pressure
steam is generated from a boiler feed water stream by means of the heat energy
contained, and the resulting process gas is then passed to the subsequent
steps of the
defined process chain.
Optionally, the process gas leaving the first heat exchanger is sent for
subsequent heat transfer to a further boiler feed water preheater, which is
fed with
another partial stream resulting from a further subdivision of the second part
of the boiler
feed water stream which has passed the water treatment unit, the pressure
boosting unit
and the second boiler feed water preheater, and is thus further heated.
The process gas leaving the first heat exchanger and/or the further boiler
feed
water preheater is preferably fed into a low-temperature conversion unit, in
which carbon
dioxide and hydrogen are formed, from where it is passed to one of the
downstream heat
exchangers of the defined process chain.
In a further embodiment of the invention the process gas which has run
through a heat exchanger is subsequently passed to a separator, and a
resulting liquid
stream is separated from the heat-containing process gas and united with the
condensate
flow from the cooling section and from other separators, and this mixture is
passed via
the pressure boosting unit and afterwards through a product condensate heat
exchanger
for being heated by the heat contained in the process gas.
Optionally it is further advisable to pass the process gas for subsequent heat
transfer through additional heat exchangers which are integrated into the
process
upstream and downstream of the low-pressure evaporator.
The related apparatus for steam reforming of hydrocarbonaceous feedstocks
by means of steam is suited to run a process according to claim 1, consisting
of a
sequence of equipment items for the passage of process gas, comprising a high-
temperature conversion unit, at least four heat exchangers, a cooling section
and at least
one unit for subsequent processing of the resulting process gas, wherein
conveying lines
CA 02743089 2011-05-09
are provided to interconnect the individual devices via their gas outlets and
gas inlets to
convey the process gas.
The apparatus for steam reforming further comprises another heat exchanger,
a water treatment unit, at least two pressure boosting units, at least one
consumer, a
device for the inlet of a deionised water stream into the subsequent heat
exchanger, a
device for transferring the deionised water stream from the before-mentioned
heat
exchanger into the water treatment unit, a device for transferring the boiler
feed water
stream leaving the water treatment unit into the pressure boosting unit, a
device for
subdividing the boiler feed water stream leaving the pressure boosting unit, a
first feed
line being provided to transport a first part of the boiler feed water stream
to the low-
pressure evaporator and a discharge line for removing the generated low-
pressure steam
from the low-pressure evaporator, comprising a device for transferring a first
partial
stream of the generated low-pressure steam to the water treatment unit and a
further
device for transferring a second partial stream of the generated low-pressure
steam into
the subsequent consumers, and providing a second feed line for the transport
of the
second part of the boiler feed water stream to a subsequent heat exchanger,
and from
there discharging a feed to the second boiler feed water preheater and from
there
providing a discharge line to the first boiler feed water preheater or to a
product
condensate heat exchanger and/or directly to the subsequent steam generation,
and
providing a device for transferring the condensate flow from the cooling
section via a
pressure boosting unit into one or more product condensate heat exchangers.
It is of advantage to arrange the sequence of equipment items for the
passage of process gas in a series connection of a high-temperature conversion
unit, a
first heat exchanger, a first boiler feed water preheater, a product
condensate heat
exchanger, a low-pressure evaporator, a second boiler feed water preheater, a
cooling
section and at least one unit for processing the resulting process gas, in the
given
sequence.
In a further advantageous embodiment of the apparatus, the sequence of
equipment items for the passage of process gas consists in a series connection
of a high-
temperature conversion unit, a first heat exchanger, a first boiler feed water
preheater, a
low-pressure evaporator, a product condensate preheater, a second boiler feed
water
preheater, a cooling section and at least one unit for processing the
resulting process
gas, in the given sequence.
Optionally the sequence of equipment items for the passage of process gas
consists in a series connection of a high-temperature conversion unit, a first
heat
exchanger, a product condensate heat exchanger, a first boiler feed water
preheater, a
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low-pressure evaporator, a second boiler feed water preheater, a cooling
section and at
least one unit for processing the resulting process gas, in the given
sequence.
Preferably the sequence of equipment items for the passage of process gas
consists in a series connection of a high-temperature conversion unit, a first
heat
exchanger, a product condensate heat exchanger, a low-pressure evaporator, a
second
boiler feed water preheater, a cooling section and at least one unit for
processing the
resulting process gas, in the given sequence, wherein a device for
transferring a first
partial stream of boiler feed water stream from the second boiler feed water
preheater
into a product condensate heat exchanger is provided as well as a further
device for
transferring the second partial stream of boiler feed water stream from the
second boiler
feed water preheater directly to the steam generation.
Another possible embodiment of the invention provides for an additional third
boiler feed water preheater in the sequence of equipment items for the passage
of
process gas, the gas inlet of which is connected to the gas outlet of the
first heat
exchanger and the gas outlet of which is connected to the gas inlet of an
optional low-
temperature conversion unit or a subsequent heat exchanger, and where a device
for
transferring another partial stream of boiler feed water from the water
treatment unit and
the second boiler feed water preheater ends.
In a further embodiment of the apparatus, a low-temperature conversion unit
is integrated into the sequence of equipment items for the passage of process
gas, the
gas inlet of which is connected to the gas outlet of the first heat exchanger
or the
additional third boiler feed water preheater and the gas outlet of which is
connected to a
subsequent heat exchanger.
It is of advantage that additional separators are integrated into the sequence
of equipment items for the passage of process gas, the gas inlets of which are
connected
to the gas outlets of the respective upstream heat exchanger and the gas
outlets of which
are connected to the respective heat exchanger downstream in the process
chain, and
which are each provided with a discharge line for the produced liquid, which
ends into the
device for transferring the condensate flow from the cooling section into a
product
condensate heat exchanger and is passed via a pressure boosting unit.
In a further embodiment of the invention a second boiler feed water preheater
is integrated into a separator which is optionally equipped with additional
internals and/or
packings and which is provided with a discharge line for conveying the
obtained process
condensate into the device for transferring the condensate flow from the
cooling section
into a product condensate heat exchanger.
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A further embodiment of the apparatus according to the invention is to
integrate further additional heat exchangers into the sequence of equipment
items for the
passage of process gas.
It is of advantage to use an air preheating unit as a consumer which is
designed for the passage of low-pressure steam in order to preheat ambient
air.
In addition, it is recommended to provide a pressure-swing adsorption unit or
a cooling box as a unit for subsequent processing of the resulting process
gas.
Optionally, another device for subdividing the second stream of low-pressure
steam may be provided in addition so to establish a feed line for air-
preheating and a feed
line to further consumers.
The invention is illustrated below in more detail in an exemplary fashion by
means of seven figures, i.e.:
Fig. 1: shows a process diagram of the process according to the invention for
the
recovery of heat from the steam reforming of hydrocarbonaceous
feedstocks by means of steam.
Fig. 2: shows an alternative integration of the heat exchangers represented in
Fig.
1 into the process for the recovery of heat from the steam reforming of
hydrocarbonaceous feedstocks by means of steam.
Fig. 3: shows another advantageous process variant for the recovery of heat in
the steam reforming of hydrocarbonaceous feedstocks by means of steam,
in which process gas passes through the product condensate heat
exchanger upstream of the first boiler feed water preheater.
Fig. 4: shows another exemplary variant of an interconnection of the heat
exchangers used. Here, the major difference as compared to Figs. 1 to 3 is
that there is no first boiler feed water preheater.
Fig. 5: supplements the representation of Fig. 1, in which various optional
elements are integrated into the process such as a third boiler feed water
preheater, a low-temperature conversion unit, an additional optional
separator and a heat exchanger.
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Fig. 6: shows the additional integration of a further product condensate heat
exchanger into the process chain according to Fig. 1.
Figs. 7A to D: show the graphic representation of the temperature decrease of
the
process gas (dashed line) and the heating behaviour of the individual
media (solid line) by the energy transfer involved in the process according
to the invention.
Fig. 1 shows a process diagram for the recovery of heat from the steam
reforming of hydrocarbonaceous feedstocks by means of steam, in which the
produced
heat-containing process gas la first passes through a high-temperature
conversion unit 2
where part of the carbon monoxide is converted to give carbon dioxide and
hydrogen.
The resulting heat-containing process gas 1 b is then directed to a first heat
exchanger 3
for subsequent heat transfer. Subsequently, heat-containing process gas 1c
passes
through a first boiler feed water preheater 4, where the heat contained in the
process gas
is transferred to preheated boiler feed water 14e which is discharged from
water
treatment unit 13 and has passed through pressure boosting unit 25, heat
exchanger 16
and boiler feed water preheater 8. Deionised water 12a is heated in heat
exchanger 16
and the heated deionised water 12b is sent to water treatment unit 13 for
degassing. If
the deionised water is preheated, this involves the advantage that one side of
the heat
exchanger need to be designed for low pressures only and part of the heat
exchanger
may therefore be fabricated of low-alloy steel, which will save cost. This
results in boiler
feed water 14a which is then preheated as described above. The resulting
boiler feed
water stream 14f is then conveyed to the steam generation for subsequent
processing.
The heat-containing process gas 1 d discharged from boiler feed water
preheater 4 is subsequently passed to product condensate heat exchanger 5
where it
transfers heat to process condensate 15a which has passed pressure boosting
unit 27
and been obtained from cooling section 10. Preheated process condensate 15b is
then
used for subsequent heating.
Process condensate 15a is collected by separators of cooling section 10,
which, in this example, comprises an air cooler and a water cooler, and is
reheated in a
product condensate heat exchanger 5. This process could be carried out in a
reaction
vessel supplied with water, in which at least part of the steam to be
separated from the
process gas condenses by direct cooling and is discharged with the water used
for
cooling. If such a vessel is used, it will be possible to preheat the process
condensate
even further, which would be an advantage, as the higher the preheating degree
of the
process condensate the higher the amount of heat in the flue gas used for
other media
and the steam generation.
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Heat-containing process gas 1 e resulting from product condensate heat
exchanger 5 is subsequently passed to low-pressure evaporator 6 where the heat
is
transferred to part of the boiler feed water stream 14c generated in water
treatment unit
13 and has been pressurised. Low-pressure steam 19a thus obtained is recycled
in a first
partial stream 19b to water treatment unit 13, whereas a second partial stream
of heated
boiler feed water 19c is fed into a consumer, in this example an air preheater
18 which
serves to heat ambient air 17 which is subsequently used as combustion air 20.
Heat-containing process gas if resulting from low-pressure evaporator 6 is
subsequently fed into boiler feed water preheater 8 where partial stream 14d
of the boiler
feed water generated in water treatment unit 13 is further preheated before it
is
transferred to boiler feed water preheater 4. Process gas 1 g resulting from
boiler feed
water preheater 8 then passes cooling section 10 where the process gas is
further cooled
and a condensate flow produced, and condensate flow 15a is passed into product
condensate heat exchanger 5. Finally the condensed heat-containing process gas
1 h
passes through the unit for subsequent processing of the resulting process gas
11, which
may, for example, be a pressure-swing adsorption unit, where the generated
hydrogen is
separated from the process gas.
fig.. .2 represents a process variant of Fig. 1. The difference between Fig. 2
and Fig. 1 is that heat-containing process gas 1d which leaves boiler feed
water
preheater 4 passes first through low-pressure evaporator 6 and subsequently
product
condensate heat exchanger 5. The interconnection of the individual equipment
items
remains unaffected. The energy recovery of the variant shown in Fig. 1,
however, should
be expected to be higher.
A further embodiment is shown in Fig. 3. The difference to Fig. 1 is that heat-
containing process gas 1c resulting from heat exchanger 3 passes through
product
condensate heat exchanger 5 first and then through boiler feed water preheater
4. The
interconnection of the individual equipment items remains unaffected and is
analogous to
the sequence of equipment shown in Fig. 1.
In fig., _4 boiler feed water preheater 4 is completely omitted. Here, heat-
containing process gas 1 c obtained from heat exchanger 3 is directed to
product
condensate heat exchanger 5, from where the resulting heat-containing process
gas 1 d
passes through low-pressure evaporator 6 and subsequently boiler feed water
preheater
8. Preheated boiler feed water stream 14e generated in boiler feed water
preheater 8 is
subdivided in this example and a partial stream 14f is passed via product
condensate
heat exchanger 5 together with product condensate 15a, where it is submitted
to further
CA 02743089 2011-05-09
preheating. The second partial stream 14g of the preheated boiler feed water
is conveyed
to the steam generation.
The interconnection shown in Fig. 5 includes further optional equipment items
of positive effect on the process. Basis for the description and the
specification of
differences is Fig. 1. Heat-containing process gas 1 c from heat exchanger 3
is passed to
an additional boiler feed water preheater 21 which is fed from a further
partial stream 14g
of the boiler feed water stream, which has been preheated in boiler feed water
preheater
8. The resulting heated boiler feed water 14h is also conveyed to the steam
generation
and hence further used. According to the embodiment shown in this figure, the
process
water resulting from boiler feed water preheater 21 is subsequently passed to
a low-
temperature conversion unit 22 where carbon dioxide and hydrogen are formed.
The
resulting heat-containing process gas 1 e subsequently passes boiler feed
water
preheater 4 and product condensate preheater 5 as shown in Fig. 1. Process gas
1g
resulting from product condensate preheater 5 is subsequently passed into
separator 23,
where the obtained process condensate 15c is separated from the process gas
and - with
the other process condensate flows - directed as process condensate 15d via a
pressure
boosting unit 27 to product condensate heat exchanger 5. Furthermore, the
resulting
heat-containing process gas 1 h passes through low-pressure evaporator 6 and
separator
7. Condensate flow 15e from separator 7 is also sent to product condensate
heat
exchanger 5 together with the other condensate flows 15d resulting from the
overall
process. Low-pressure steam 19a resulting from low-pressure evaporator 6 is
subdivided
into three partial streams. Partial stream 19b of the low-pressure steam is
directed to
water treatment unit 13, 19c to air preheater 18 and 19d to subsequent
consumers 26.
Downstream of separator 7 a further heat exchanger 24 is connected and serves
for
additional energy transfer. The process continues according to the process
chain as
shown in Fig. 1, consisting of boiler feed water preheater 8, cooling section
10 and
pressure-swing adsorption unit 11. In this embodiment, however, an additional
heat
exchanger 9 is provided between boiler feed water preheater 8 and cooling
section 10.
Fig. 6 shows another variant of Fig. 1. Process condensate flow 15a from
cooling section 10 is sent via a pressure boosting unit 27 and a further
additional product
condensate heat exchanger 28 before it is passed through product condensate
heat
exchanger 5. This involves the advantage that the product condensate absorbs
even
more heat, which can be used for heating other media in the subsequent course
of the
process.
The equipment items additionally integrated in Fig. 5 may be used in a
combination as shown in Fig. 5 but may also be integrated as individual
components into
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the respective process chains. In addition, not only Fig. 1 may serve as a
basis for such
equipment integration but all figures can be used as a basis for the
integration. This
shows that the process involves many options to adapt the respective process
to the
individual requirements of a plant operator and also to integrate the
appurtenant plant
sections into existing plants. Furthermore, it is possible to implement these
process
variants in new plants.
In the case of favourable dimensions, the low-pressure evaporator could be
provided with a safety reserve and, in the event of a shutdown, cool the
process gas by
generating and blowing off low-pressure steam. In addition to air-preheating
and water
treatment as described above, the generated low-pressure steam may just as
well be
used to boil out CO2 in a CO2 process-gas scrubbing process. The maximum
temperature
of the generated low-pressure steam in such case is 200 C.
Some calculation examples below are used to illustrate the improvement of
energy recovery represented as a total from low-pressure steam, boiler feed
water and
condensate flows. They are based on a typical interconnection according to the
state of
the art, using a minimum number of equipment items employed in conventional
processes according to the state of the art. Based on Fig. 1, low-pressure
evaporator 6 is
omitted as well as boiler feed water preheater 8 so that boiler feed water
stream 14d is
directly sent into boiler feed water preheater 4. The below tables serve to
show how
drastically the present invention positively influences the energy recovery in
comparison
to this typical interconnection. Some of the before-described figures have
been used as a
basis for the calculation. It is assumed that there is a separator downstream
of the first
four series-connected heat exchangers of the sequence of equipment items for
the
passage of process gas. The exemplary calculations are based on a plant
capacity of
33,455 Nm3/h of hydrogen.
Interconnection variant Energy recovery:
Total from:
steam + boiler feed water +
product condensate
[kW]
Typical interconnection 10,480
Fig. 2 12,670
Fig. 3 13,760
Fig. 6 13,760
As Fig. 3; + integral boiler feed 13,760
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water preheater in separator
From this results that the interconnection variant of the invention reflected
by
Fig. 3 and Fig. 6 involves a very high level of energy recovery as compared to
the typical
interconnection according to the state of the art. Consequently, an increase
in the total of
heat recovery of approx. 3270 kW can be expected, which would be unused and
lost in
the typical interconnection variant according to the state of the art.
The basic conditions for the calculations are shown in Figs. 7A to D as a
graphic function of temperature and energy recovery. The dashed line
represents the
temperature decrease of the process gas depending on the energy contained,
whereas
the solid line represents the heating behaviour of the individual media used
in the
process. The individual process steps represented in the graphs are reflected
by the
inserted reference numbers which are also used in the other figures 1 to 6.
Advantages resulting from the invention:
Improved energy recovery from the amount of heat contained in the process
gas.
- Additional preheating of the process condensate in a product condensate
heat exchanger effects that more energy from the flue gas is available for
heating other media and can be used for steam generation.
According to the state of the art the process condensate in the flue gas duct
is
preheated until boiling. By preheating the process condensate by process gas
as provided by the invention, it is possible to do without conventional
heating
in the flue gas duct, which will contribute to a simplification of the process
concept.
- The process according to the invention involves the advantage that it can be
integrated into already existing plants which are without access to low-
pressure steam and must generate it from valuable high-pressure steam.
- The temperature and pressure conditions in heat exchanger 16 exclude the
risk of steam hammers which contribute to the improvement of the operational
safety.
13
CA 02743089 2011-05-09
List of references used
la, 1b, 1c, 1d, le, 1f, 1g, Heat-containing process gas
1h, 1i, 1j, 1k, 11, 1m, 1n
2 High-temperature conversion unit
3 Heat exchanger
4 Boiler feed water preheater
Product condensate heat exchanger
6 Low-pressure evaporator
7 Separator
8 Boiler feed water preheater
9 Heat exchanger
Cooling section
11 Pressure-swing adsorption unit
12a, 12b Deionised water
13 Water treatment unit
14a, 14b, 14c, 14d, 14e, Boiler feed water stream
14f, 14g, 14h, 14i
15a, 15b, 15c, 15d, 15e Process condensate
16 Heat exchanger
17 Ambient air
18 Air preheater
19a, 19b, 19c, 19d Low-pressure steam
Combustion air
21 Boiler feed water preheater
22 Low-temperature conversion unit
23 Separator
24 Heat exchanger
Pressure boosting unit
26 Subsequent consumers
27 Pressure boosting unit
28 Product condensate heat exchanger
14
