Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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POWER RECOVERY PROCESS
FIELD OF THE INVENTION
The field of this invention relates to use of multiple expansion
turbines for efficient recovery of power from a plurality of very high
pressure streams of superheated vapor. More particularly, these
power recovery processes employ at least two classes of expansion
turbines. Processes according to this invention are particularly useful
for recovery of power from a plurality of very high pressure streams of
superheated vapor generated in manufacturing petrochemicals. High
pressure streams of superheated steam are typically produced during
thermal cracking or pyrolysis of suitable petroleum derived feed
stocks. For example, superheated steam is generated where olefins,
typically ethylene and/or propylene, are produced.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
This invention was made with government support under
United States Department of Energy Cooperative Agreement No. DE-
FC07-01ID 14090.
BACKGROUND OF THE INVENTION
As is well known, olefins, or alkenes, are a homologous series of
hydrocarbon compounds characterized by having a double bond of
four shared electrons between two carbon atoms. The simplest
member of the series, ethylene, is the largest volume organic chemical
produced today. Olefins including, importantly, ethylene, propylene
and smaller amounts of butadiene, are converted to a multitude of
intermediate and end products on a large scale, mainly polymeric
materials.
Commercial production of olefins is almost exclusively
accomplished by pyrolysis of hydrocarbons in tubular reactor coils
installed in externally fired heaters. Thermal cracking feed stocks
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include streams of ethane, propane or hydrocarbon liquids ranging in
boiling point from light straight-run gasoline through gas oil.
This endothermic process is carried out in a plurality of large
pyrolysis furnaces with the expenditure of large quantities of heat
which is provided in part by burning the methane produced in the
cracking process. After cracking, the reactor effluent is cooled and
put through a series of separation steps involving cryogenic
separation of products such as ethylene and propylene. The total
energy requirements for the process are thus very large and ways to
reduce the net energy use of olefins manufacturing facilities are of
substantial commercial interest.
Each of the plurality of pyrolysis furnaces produces a byproduct
stream of very high pressure superheated steam. These streams are
typically combined and directed to one or more multi-stage expansion
turbines which produce power for use in the cryogenic separation
system. The efficient conversion of the multiple very high pressure
superheated steam streams into mechanical energy is crucial for the
economic production of olefins. Processes which allow more efficient
conversion of very high pressure steam into mechanical energy or
electricity, such as he process of the present invention, beneficially
reduce the net energy use of the olefins manufacturing facility.
It is therefore a general object of the present invention to
provide an improved process which overcomes the aforesaid problem
of prior art methods for recovery of power from a plurality of very high
25' pressure vapor streams, and generate turbine exhaust streams at a
plurality of pressures
An improved method for recovery expansion power should
exhibit higher efficiency thereby providing lower net energy use and
therefore lower variable costs of operation.
Other objects and advantages of the invention will become
apparent upon reading the following detailed description and
appended claims.
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SUMMARY OF THE INVENTION
Economical processes are disclosed for the use of multiple
expansion turbines for efficient recovery of power from a plurality of
very high pressure streams of superheated vapor. More particularly
processes are disclosed for recovery of power using at least two
classes of expansion turbines. Processes according to this invention
are particularly useful for recovery of power from very high pressure
streams of superheated steam generated in the manufacture of light
olefins by the pyrolysis of hydrocarbons in a plurality of furnaces.
Heat is removed from the furnaces and/or reactor effluent streams at
least in part by the formation and removal therefrom of a plurality of
very high pressure steam streams.
Processes of the invention comprising a power generation
system employing a plurality of steam expansion turbines wherein
high-pressure steam is expanded to produce power and generate
turbine exhaust streams at a plurality of pressures. More
particularly, this invention comprises power recovery processes
employing at least two classes of expansion turbines, which comprise:
(a) expanding a first stream of superheated vapor at first inlet
conditions, including temperature and pressure, to obtain at least one
first expanded stream of superheated vapor at first intermediate
conditions using at least one primary class expansion turbine to
thereby recover a first amount of power; (b) combining two or more
vapor streams into a single very high-pressure superheated vapor
stream; (c) cooling the resulting single very high-pressure stream from
step (b) by indirect heat exchange with at least a portion of the first
expanded stream from step (a) to provide all or a portion of the first
stream of superheated vapor for expansion in step (a), and a resulting
heated first expanded stream at second intermediate conditions
including a second intermediate temperature; (d) expanding at least a
portion of the resulting heated stream from step (c) at second
expansion inlet conditions to obtain at least one second expanded
stream of superheated vapor at third intermediate conditions using at
least one secondary class expansion turbine to thereby recover a
second amount of power.
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In a particularly useful aspect of the present invention, three or
more vapor streams are combined in step (b) into a single very high-
pressure superheated vapor stream. Advantageously, three or more of
the vapor streams combined in step (b) into a single very high-
pressure superheated vapor stream are derived from a petrochemical
process.
In another aspect of the present invention, the vapor comprises
a light organic compound component containing from about 2 to
about 4 carbon atoms, for example propane. The temperature
differential between the second intermediate temperature and the first
inlet temperature is advantageously no more than 100 Fahrenheit
degrees. More advantageously, the temperature differential between
the second intermediate temperature and the first inlet temperature is
no more than 70 Fahrenheit degrees.
In another particularly useful aspect, this invention comprises
power recovery processes employing at least two classes of expansion
turbines, which comprise: (a) expanding a first stream of superheated
steam at first inlet conditions of temperature and pressure to obtain
at least one first expanded stream of superheated steam at first
intermediate conditions using at least one primary class expansion
turbine to thereby recover a first amount of power; (b) combining
three or more streams of very high pressure steam into a single very
high-pressure superheated stream; (c) cooling the resulting single very
high-pressure stream from step (b) by indirect heat exchange with at
least a portion of the first expanded stream from step (a) to provide all
or a portion of the first stream of superheated vapor for expansion in
step (a), and a resulting heated first expanded stream at second
intermediate conditions including a second intermediate temperature;
(d) expanding at least a portion of the resulting heated stream from
step (c) at second expansion inlet conditions to obtain at least one
second expanded stream of superheated steam at third intermediate
conditions using at least one secondary class expansion turbine to
thereby recover a second amount of power.
A particularly useful aspect of the present invention further
comprises treating at least a portion of one or more second expanded
stream of superheated steam from step (d) to thereby provide at least
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a portion of the resulting single very high-pressure stream of step (b).
Beneficially, three or more of the vapor streams combined in step (b)
into a single very high-pressure superheated vapor stream are
generated in a process for thermal cracking of suitable petroleum
derived feed stocks to produce olefins, and advantageously the olefins
being produced are ethylene and/or propylene.
In another particularly useful aspect of this invention three or
more of the vapor streams combined in step (b) into a single very
high-pressure superheated vapor stream are generated in a process
for the manufacture of light olefins by the pyrolysis of hydrocarbons
in a plurality of furnaces from which heat is removed in a plurality of
very high pressure streams of superheated steam. The first inlet
pressure is at least 900 psig, and/or the first inlet temperature is at
least 800 F. In yet another useful aspect of this invention, the second
intermediate temperature is no more than 100 Fahrenheit degrees
below the first inlet conditions temperature.
Yet another particularly useful aspect of the invention further
comprises partially desuperheating the second expanded stream of
superheated steam from step (d) by indirect heat exchange with a cooling
medium to provide a supply of superheated low-pressure steam. More
advantageously, the cooling medium is boiler feed water, and at least a
portion of the heated boiler feed water is used to produce at least a portion
of the very high pressure steam streams of step (b).
Efficiency is improved in accordance with this invention for any
power recovery steam cycle in which there are a relatively large
number of steam generation and superheating units and where there
are multiple pressure levels from which steam is expanded to produce
power. The reheating of an intermediate-pressure steam, which has
been extracted from an expansion turbine, in a single location by
desuperheating a higher-pressure steam that has been superheated
in the multiple steam superheating units is critical for best results.
This invention avoids the complex and expensive system that would
be needed to distribute and then re-collect the intermediate pressure
steam if it were reheated in the multiple superheating units.
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For a more complete understanding of the present invention,
reference should now be made to the embodiments illustrated in greater
detail in the accompanying drawing and described below by way of
examples of the invention.
BRIEF DESCRIPTION OF THE FIGURES
The appended claims set forth those novel features which
characterize the present invention. The present invention itself, as well
as advantages thereof, may best be understood, however, by reference to
the following brief description of preferred embodiments taken in
conjunction with the annexed drawings, in which:
FIGURE 1 is a scheniatic diagram of a comparative power recovery
process for the steam system in an olefins manufacturing thermal
cracking unit.
FIGURE 2 is a schematic diagram of an embodiment of this
invention in which intermediate-pressure steam is heated and low-
pressure steam is desuperheated.
FIGURE 3 is a schematic diagram of another embodiment of this
invention in which reheated intermediate-pressure steam is reheated and
distributed to multiple expansion turbines.
It should be noted that only essential expansions and
heating/cooling steps are shown in these schematic diagrams.
BRIEF DESCRIPTION OF THE INVENTION
Hydrocarbon cracking processes have been commonly employed
in the petroleum and allied industries for several decades, and many
commercial cracking processes have been the subject of much
academic and commercial interest. Cracking consists of breaking
down the hydrocarbon molecules into smaller molecules, usually at a
higher temperature. There are generally two types of cracking,
thermal cracking and catalytic cracking, which utilize either the effect
of temperature alone or in combination with the active sites of a
catalyst.
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In a conventional thermal cracking unit, the hydrocarbon
feedstock is gradually heated in a tubular furnace. The thermal
cracking reaction takes place mainly in the portion of the tubes
receiving the maximum heat flow, and the desired temperature is
determined by the nature of the hydrocarbons to be cracked.
In general, a cracking unit includes a plurality of pyrolysis
furnaces. Each furnace includes a tubular or plug-flow reactor
through which feedstock flows and in which the feedstock is thermally
decomposed. A pyrolysis furnace is designed to transfer heat to
internal reactor tubes which are conventionally arranged in three
sections: a convection section, in which the hydrocarbon feedstock is
preheated and very high pressure steam is superheated; a radiant
section, in which the preheated hydrocarbon feedstock is thermally
decomposed to olefins, diolefins, and aromatics; and a quench section
where the cracked gas furnace effluent from the radiant section is
cooled through the generation of very high pressure steam.
The literature is replete with disclosures of suitable pyrolytic
furnaces for the thermal cracking of hydrocarbons. For example, U.S.
Patent No. 5,271,809 in the name of Hans-Joachim Holzhausen.
Pyrolytic furnaces advantageously comprise a radiation zone including
burners and cracking tubes in the radiation zone consisting of
parallel, vertically extending linear tube sections joined to one another
by tube elbows located in the bottom region of the radiation zone. At
least four cracking tubes are combined into groups uniformly
arranged in the radiation zone, each group of cracking tubes being
united in an outlet tube via manifold, tube sections wherein the linear
tube sections and the manifold tube sections of the individual groups
are arranged in one row in the transverse direction of the pyrolytic
furnace.
When light olefins and monoaromatic compounds are to be
produced, the necessary temperature is and generally ranges from
about 1,440 F to about 1,600 F, depending on the type of feedstock to
be cracked, but is limited by the operating conditions of the process
and by the operating complexity of the furnaces, which use
supplementary heating energy.
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Suitable light hydrocarbon fraction or fractions may be
advantageously chosen from the group consisting of light paraffins,
such as ethane, propane and the butanes, and heavier hydrocarbons
such as gasolines, naphthas and gas oils, and even certain higher-
boiling but strongly paraffinic or naphthenic fractions, such as the
paraffins or slack wax or the hydrocarbon recycles. These
hydrocarbon fractions may come from different units of the refinery,
for example the atmospheric distillation, visbreaking, hydrocracking,
oil manufacturing or olefin oligomerization units, or from the effluents
of the conversion unit itself. Additionaly, the various fractions may be
injected either alone or in combination with steam and optionally
other gases such as hydrogen or light gases.
Each olefins-producing pyrolysis furnace typically produces one
or more streams of high-pressure superheated steam as a byproduct
of the furnace operation. The steam is typically generated through
the quenching of hot furnace effluent gases, and then superheated in
the convective section of the pyrolysis furnace. The maximum
temperature to which the steam is superheated is typically limited by
the maximum inlet temperature of the expansion turbine to which the
superheated steam is fed. The maximum inlet temperature of the
expansion turbine is in turn a function of the design and metallurgy
employed in the turbine. The maximum inlet temperature of steam
turbines is typically in the range of 980 to 1000 F.
The superheated steam streams are beneficially combined and
enter one or more multi-stage expansion turbines. These turbines
produce mechanical and/or electrical power which is beneficially used
in the recovery and purification of olefins.
The recovery and purification of light olefins such as ethylene
and propylene from the furnace effluent is an enegy-intensive process.
A typical ethylene recovery and purification section comprises a
cracked gas compressor to compress the quenched furnace effluent
stream to a relatively high pressure, typically between 200-500 psig.
At least a portion of the mechanical energy required for cracked gas
compression is produced through the expansion of the very-high
pressure steam generated in the pyrolysis furnaces.
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The ethylene contained in the compressed cracked gas is then
typically recovered and purified through cryogenic distillation. While
the design of the ethylene recovery and purification section admits of
many variations, it typically contains a deethanizer tower to separate
C3 and heavier material from the ethylene-containing stream, a
demethanizer tower to separate methane and lighter material from the
ethylene-containing stream, and a C2 splitter tower to separate
ethylene from ethane. Such distillation steps are typically cryogenic
in nature, that is they are carried out at temperatures below ambient
temperature. They therefore demand significant amounts of process
refrigeration. At least a portion of the mechanical energy required for
providing the process refrigeration is produced through the expansion
of steam generated in the pyrolysis furnaces.
In the course of its extensive work in this field, the applicants
have found that use of multiple expansion turbines increases the
efficiency in the recovery of power from a plurality of very high
pressure streams of superheated vapor generated and superheated
among a relatively large number of furnaces. More particularly, power
recovery processes in accordance with the invention employ at least
two classes of expansion turbines. Reheating of intermediate-
pressure exhaust or extraction vapor from the primary class of
expansion turbines is critical for improving the efficiency of the overall
power recovery system.
This invention is useful for power generation systems from a
plurality of streams at very high pressure that use any working fluid,
though steam is by far the most common. Processes of this invention
are particularly suitable for use in the thermal cracking of
hydrocarbons, for example, using steam streams from a plurality of
thermal cracking furnaces.
BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF
THE INVENTION
While this invention is susceptible of embodiment in many different
forms, this specification and accompanying drawings disclose only some
specific forms as an example of the use of the invention. In particular, a
preferred embodiment of the invention for recovery of mechanical and/or
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electrical power from a plurality of high pressure superheated vapor
streams is illustrated and described. The invention is not intended to be
limited to the embodiment so described, and the scope of the invention
will be pointed out in the appended claims.
The apparatus of this iiivention is used with certain conventional
components the details of which, although not fully illustrated or
described, will be apparent to those having skill in the art and an
understanding of the necessary function of such components. Various
values of compositions, flow rates, temperatures, and pressures are given
in association with a specific example described below; those conditions
are approximate and merely illustrative, and are not meant to limit the
invention.
Detailed Description of the Invention
This invention represents an improved, more energy-efficient method
for utilizing high-pressure superheated vapor generated from multiple
sources to generate mechanical energy through the use of expansion
turbines. It can be utilized with any high-pressure superheated vapor, but
a common use would be in a steam system where the vapor is water vapor.
For ease of understanding the invention will be described in terms of an
improved steam system for the generation of power within an olefins
manufacturing complex. It should be noted that the concept and methods of
this invention are not limited to this application.
In olefins manufacture, high-pressure steam is generated in a
number of cracking furnaces. The number of cracking furnaces in a
particular olefins unit wiIl depend on many factors, including the capacity
of the olefins manufacturing unit, the capacity of the furnaces, and the
design of the furnaces. Typically between four and 12 furnaces are utilized
within an olefins manufacturing complex. Each of these furnaces produces
a stream of superheated steam as a byproduct of the olefin-producing
process. These streams are typically combined and then directed to steam
turbines to produce mechanical energy. The mechanical eiiergy thus
produced is typically used to compress the olefin-containing gas and to drive
machinery designed to provide refrigeration to the olefins process.
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Figure 1 depicts a schematic diagram of a portion of a conventional
steam system for an olefins manufacturing unit. This schematic contains
only the major heat transfer and power production steps that are required
to understand the basic operation of such a steam system, and to allow
comparison with the current invention. Those skilled in the art will
recognize that olefins unit steam systems admit to many variations in
design, but most contain the steps outlined in Figure 1.
Very high pressure superheated steam is generated by the multiple
furnaces and combined into a single very high-pressure steam header line
depicted as stream 1. The temperature and pressure of stream 1 can vary
significantly between units. Stream 1 is typically at a pressure of at least
900 psig and a temperature of at least 900 F.
The entirety of this very high pressure steam is typically directed to
steam turbine 2. This steam turbine expands the very high-pressure steam
to produce power for other parts of the process. Typically, the power
derived from steam turbine 2 would be used to drive a cracked gas
compressor to compress the cooled olefin-containing furnace effluent gas to
a higher pressure. Turbine 2 is shown as an extracting turbine, with two
stages (stage 2a and stage 2b) which are typically mechanically coupled.
High-pressure steam (typically at about 600 psig) is recovered from stage
2a as stream 3.
A portion of stream 3 is directed as stream 4 to stage 2b of the
turbine and withdrawn as stream 5. Stream 5 is typically recovered at as
low a pressure as feasible (typically under vacuum) and condensed against
a near-ambient cooling medium.
Another portion of stream 3 is directed as stream 6 to the high-
pressure steam header. Portions of the high-pressure steam from the
header, depicted as streams 7 and 8, can be directed to other steam
turbines, depicted as 9 and 10. It is understood that more or fewer turbines
can be fed by the high-pressure steam header, depending on the needs of
the olefins process. In order to simplify the Figure 1, only two turbines are
depicted.
A further portion of the high pressure steam can be directed as
stream 11 to one or more heat exchangers to provide heating to one or more
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units in the olefins process. While a single exchanger 12 is shown in Figure
1, it is understood that it may represent multiple heat exchangers in a
commercial olefins facility. The condensate stream 13 from exchanger 12 is
withdrawn as shown and at least a portion is typically re-used as boiler
feed water for the process. A final portion of the high-pressure steam can
be exported as stream 14 to another process or otherwise used within the
olefins unit.
In Figure 1 steam turbine 10 is shown as an extracting turbine, with
two stages (stage 10a and stage lOb) which are typically mechanically
coupled. Low-pressure steam (typically at about 65 psig) is recovered from
stage l0a as stream 15.
A portion of stream 15 directed as stream 16 to stage 10b of the
turbine and withdrawn as stream 17. Stream 17 is typically recovered at
as low a pressure as feasible (typically under vacuum) and condensed
against a near-ambient cooling medium. It should be noted that this
turbine could produce more than two expanded steam streams, each at
different pressure levels. In practice, turbine 10 could, for example, provide
power to drive a refrigeration compressor in a commercial olefins unit.
Another portion of stream 15 is directed as stream 18 to the low-
pressure steam header, along with stream 19, the expanded high-pressure
steam from turbine 9. The majority of the low-pressure steam is typically
withdrawn as stream 20 and used for process heating needs in exchanger
21 as shown. The single exchanger 21 in Figure 1 would typically
represents a number of separate exchangers in the commercial unit. The
condensate stream 22 from exchanger 21 is withdrawn as shown and at
least a portion is typically re-used as boiler feed water for the process. A
further portion of the low-pressure steam can be exported as stream 23 to
another process or otherwise used within the olefins unit.
Figure 2 depicts a preferred embodiment of the present invention,
wherein reheat of the high-pressure steam and desuperheating of the low-
pressure steam is accomplished. Very high-pressure superheated steam
from each of the olefins cracking furnaces is combined as shown and
directed to the very high-pressure steam header stream 30. It is a
characteristic of the current invention that stream 30 is superheated in the
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furnaces to a significantly higher temperature than the corresponding
stream 1 of Figure 1. Stream 30 is partially de-superheated in the reheat
exchanger 31. The resulting very high-pressure steam stream 32 exits
exchanger 31 at a temperature roughly similar to that of stream 1 of Figure
1. The maximum temperature of stream 32 is typically limited by the
design and metallurgy of the downstream expansion turbine 33.
Stream 32 is directed to steam turbine 33, which provides similar
functionality as turbine 2 of Figure 1. Turbine 33 is shown as an extracting
turbine, with two stages (stage 33a and stage 33b) which are typically
mechanically coupled. High-pressure steam (typically at about 600 psig) is
recovered from stage 33a as stream 34. A portion of stream 34 is directed
as stream 35 to stage 33b of the turbine and withdrawn as stream 36.
Stream 36 is typically recovered at as low a pressure as feasible (typically
under vacuum) and condensed against a near-ambient cooling medium.
Another portion of stream 34 is directed as stream 37 to the reheat
exchanger 31 where it is reheated against the desuperheating very high-
pressure steam stream 30. The reheated high-pressure stream 38 is
directed to the high-pressure steam header as shown. It is a characteristic
of this invention that the high-pressure steam stream 38 entering the high-
pressure steam header of Figure 2 is at a higher temperature than the
corresponding high-pressure steam stream 6 in the conventional steam
system of Figure 1.
Portions of the high-pressure steam from the high-pressure steam
header, depicted as streams 39 and 40, can be directed to other steam
turbines, depicted as 41 and 42. It is understood that more or fewer
turbines can be fed by the high-pressure steam header, depending on the
needs of the olefins process. In order to simplify the Figure 2, only two
turbines are depicted.
A further portion of the high-pressure steam can be directed as
stream 43 to one or more heat exchangers to provide heating to one or more
units in the olefins process. While a single exchanger 44 is shown in Figure
2, it is understood that it may represent multiple heat exchangers in a
commercial olefins facility. The condensate stream 45 from exchanger 44 is
withdrawn as shown and at least a portion is typically. re-used as boiler
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feed water for the process. A final portion of the high-pressure steam can
be exported as stream 46 to another process or otherwise used within the
olefins unit.
Alternatively, the high-pressure steam export can be taken as stream
47 at a point before the high-pressure steam reheater. This could be
advantageous if the external high-pressure steam users are not equipped to
utilize the hotter high pressure steam represented by stream 46.
In Figure 2 steam turbine 42 is shown as an extracting turbine, with
two stages (stage 42a and stage 42b) which are typically mechanically
coupled. Superheated low-pressure steam (typically at about 65 psig) is
recovered from stage 42a as stream 48.
A portion of streani 48 is directed as stream 49 to stage 42b of the
turbine and withdrawn as stream 50. Stream 50 is typically recovered at
as low a pressure as feasible (typically under vacuum) and condensed
against a near-ambient cooling medium. It should be noted that this
turbine could produce more than two expanded steam streams, each at
different pressure levels. In practice, turbine 42 could, for example, provide
power to drive a refrigeration compressor in a commercial olefins unit.
Another portion of stream 48, stream 51, is combined with the
expanded superheated steam stream 52 from turbine 41 and the combined
stream 53 enters the desuperheater exchanger 54. It is a characteristic of
this invention that the low-pressure steam streams 51 and 52 are at a
higher temperature than the corresponding low-pressure steam streams 18
and 19 in the conventional steam system of Figure 1.
Stream 53 is at least partially desuperheated in exchanger 54 to
produce the low-pressure steam stream 55. Cooling for the desuperheater
exchanger 54 can be supplied by any suitable cooling medium. For
example, a relatively cool boiler feed water stream 56 could be used as the
cooling medium to produce a relatively warmer boiler feed water stream 57,
thereby recovering heat within the steam system and improving the overall
efficiency of the process of the present invention.
The low-pressure steam stream 55 from the desuperheater exchanger
enters a low-pressure steam header as shown. The majority of the low-
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pressure steam is typically withdrawn as stream 58 and used for process
heating needs in exchanger 59 as shown. The single exchanger 59 in
Figure 2 would typically represents a number of separate exchangers in a
commercial unit. The condensate stream 60 from exchanger 59 is
withdrawn as shown and at least a portion is typically re-used as boiler
feed water for the process. A further portion of the low-pressure steam can
be exported as streani 61 to another process or otherwise used within the
olefins unit.
Figure 3 depicts an alternate configuration of the high-pressure
steam reheat section of the present invention. It is similar in function to
the reheat section of Figure 2, but the high pressure steam to the second
stage of the first turbine is reheated before entering the second stage.
Very high-pressure superheated steam from each of the olefins
cracking furnaces is combined as shown and directed to the very high-
pressure steam header stream 70. Stream 70 is partially de-superheated in
the reheat exchanger 71. The resulting very high-pressure steam stream
72 exits exchanger 71 at a temperature roughly similar to that of stream 1
of Figure 1 and stream 32 of Figure 2.
Stream 72 is directed to steam turbine 73. Turbine 73 is shown as
an extracting turbine, with two stages (stage 73a and stage 73b) which are
typically mechanically coupled. High-pressure steam (typically at about
600 psig) is recovered from stage 73a as stream 74. If desired, a portion of
the high pressure steam can be exported from the process a stream 75. The
remainder of the steani is directed as stream 76 to the reheat exchanger 71
where it is reheated against the desuperheating very high pressure steam.
A portion of the reheated stream 77 is directed as stream 78 to stage
73b of the turbine and withdrawn as stream 79. Stream 79 is typically
recovered at as low a pressure as feasible (typically under vacuum) and
condensed against a near-ambient cooling medium.
Another portion of stream 77 is directed as stream 80 to the high-
pressure steam header as shown. For simplicity the remainder of the steam
process is not depicted in Figure 3, but it is understood that it can be
similar in nature to that of Figure 2(where stream 80 of Figure 3
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corresponds to stream 38 of Figure 2), or it can be of a different
configuration.
Figures 2 and 3 depict two configurations which utilize the concept of
recovering superheat from a combined vapor stream in order to re-heat at
least a portion of a lower-pressure vapor stream which has been extracted
from an expansion turbine. Those skilled in the art will recognize that,
once the basic concept is grasped, other configurations can be developed,
and all such configurations are covered within the scope of this invention.
EXAMPLE OF THE INVENTION
The following Example will serve to illustrate a certain specific
embodiment of the herein disclosed invention. This Example should not,
however, be construed as limiting the scope of the novel invention as there
are many variations which may be made thereon without departing from
the spirit of the disclosed invention, as those of skill in the art will
recognize.
General
To demonstrate several beneficial aspects of the present invention,
both the comparative process depicted in FIGURE 1 and the embodiment of
FIGURE 2 were simulated using commercially available process simulation
software.
Comparative Example
Following is an example of a conventional steam system
configuration for an olefins manufacturing unit. The design of this
conventional steam system is similar to that shown in Figure 1, and all
stream and unit numbers in this example refer to those in Figure 1. Very
high-pressure steam at a temperature of 980 F and a pressure of 1800 psig
is generated from multiple furnaces. High-pressure steam is extracted from
turbine 2 at 600 psig, while the low-pressure header operates at 50 psig.
No high-pressure steam is exported in this case.
Turbine 2 generates approximately 112,000 HP. Turbine 10
represents the combination of two separate refrigeration turbines which
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generate a total of 33,800 HP. Turbine 9 represents a number of smaller
turbines which combined generate approximately 4,900 HP.
Stream flows and conditions for this example are given in Table 1.
Stream numbers correspond to those of Figure 1. A total of 941,500 lb/hr of
very high pressure steam is used.
Example of the Present Invention
Following is an example of a steam system configuration of the
present invention for an olefins manufacturing unit. This novel steam
system incorporates the high-pressure steam reheat and low-pressure
steam desuperheating functions contained within the process of this
invention. The steam system of this example is similar to that shown in
Figure 2, and all stream and unit numbers in this example refer to those in
Figure 2. Very high-pressure steam at a temperature of 1090 F and a
pressure of 1800 psig is generated from multiple furnaces. High-pressure
steam is extracted from turbine 33a at 605 psig, and experiences a 5 psi
pressure drop across the reheat exchanger so that the high-pressure header
operates at 600 psig. The low-pressure header operates at 50 psig.
No export steam was taken through either streams 45 or 47. The
amount of steam withdrawn as stream 37 was set so as to maintain a
temperature of 980 F in stream 38. In addition, the amount of low-
pressure steam produced by both the current and previous examples was
kept constant.
Further, in recognition that the olefins furnaces can provide a finite
duty for steam generation, the furnace convective bank duty required to
generate the very high pressure steam stream of the present invention
(stream 30) was approximately equal to that required for the previous
example (stream 1). Stream 30 contains 917,000 lb/hr of steam at 1800
psig and 1090 F.
Note that although the total convective bank furnace duties in these
two examples are approximately equal, there are differences in how the
duty is utilized to generate steam. In the invention of the present
invention, boiler feed water is preheated by superheated expanded steam in
exchanger 54 of Figure 2. Therefore, compared with the comparative
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example, in the process of the present invention the furnace convection
section provides relatively less preheating of the boiler feed water and
relatively more superheating of the very high pressure steam. The result is
that slightly less very high pressure steam is generated, but it is at a
higher final temperature.
Stream flows and conditions for this example are given in Table 2.
Stream numbers correspond to those of Figure 2.
The improved efficiency of the present invention is manifested in
increased power production in turbines 33 and 42 as compared to turbines 2
and 10. Table 3 compares the turbine power generation results from the
above examples. It is clear that the higher efficiency of the system of the
present invention (Figure 2) produces over 4000 HP more power than a
conventional steam system, from similar furnace steam duty.
It should be noted that the improved efficiency of the process of this
invention can be manifested in a number of ways. One is the ability to
produce more usable power from the same furnace steam duty, as
demonstrated above. It may be desirable instead to produce similar power
as the conventional system, but with reduced furnace steam duty or
through the export of high-pressure steam. These and other methods of
taking advantage of the increased efficiency of the present invention will be
apparent to those skilled in the art.
An Example has been presented and hypotheses advanced herein in
order to better communicate certain facets of the invention. The scope of
the invention is determined solely by the scope of the appended claims.
For the purposes of the present invention, "predominantly" is defined
as more than about fifty percent. "Substantially" is defined as occurring
with sufficient frequency or being present in such proportions as to
measurably affect macroscopic properties of an associated compound or
system. Where the frequency or proportion for such impact is not clear,
substantially is to be regarded as about twenty per cent or more. The term
"a feedstock consisting essentially of' is defined as at least 95 percent of
the
feedstock by volume. The term "essentially free of' is defined as absolutely
except that small variations which have no more than a negligible effect on
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macroscopic qualities and final outcome are permitted, typically up to about
one percent.
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Table 1: Comparative Example
Steam Flow Temperature Pressure
Stream (lb/hr) (Deg F) (psia)
1 941,475 980 1,815
511,524 126 2
6 429,951 710 615
7 97,090 710 615
8 332,861 710 615
11 0 N/A N/A
14 0 N/A N/A
17 158,385 126 2
18 174,476 310 65
19 97,090 392 65
23 0 N/A N/A
Table 2: Example of This Invention
Steam Flow Temperature Pressure
Stream (lb/hr) (Deg F) (psia)
30 917,000 1,089 1,815
32 917,000 980 1,810
36 505,714 126 2
37 411,286 712 620
38 411,286 980 615
39 76,086 980 615
40 335,200 980 615
43 0 N/A N/A
46 0 N/A N/A
47 0 N/A N/A
50 139,720 126 2
51 195,480 511 65
52 76,086 622 65
55 271,566 308 62
Table 3: Turbine Power
Example of This Invention
Comparative Example (Figure 1) (Figure 2)
Turbine Number Power (HP) Turbine Number Power (HP)
2a + 2b 110923 33a + 33b 108882
9 4889 41 4888
10a + 10b 33800 42a + 42b 40269
5 Total 149612 Total 154039
