U.S. patent application number 12/086380 was filed with the patent office on 2009-06-25 for power recovery process.
This patent application is currently assigned to INEOS USA LLC. Invention is credited to Wayne W.Y. Eng, Michael J. Foral, Rik Klavers, Guang-Chung Lee, Jeffery S. Logsdon, Christos G. Papadopoulos, Rian Reyneke, Iain Sinclair.
Application Number | 20090158737 12/086380 |
Document ID | / |
Family ID | 38228637 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090158737 |
Kind Code |
A1 |
Klavers; Rik ; et
al. |
June 25, 2009 |
Power Recovery Process
Abstract
Processes using multiple expansion turbines for efficient
recovery of power from a plurality of very high pressure streams of
superheated vapor are disclosed. Beneficially, processes of the
invention use 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 in an
olefins manufacturing process. Such streams are typically produced
by thermal cracking of suitable petroleum derived feed stocks, and
the olefins being produced and purified are typically ethylene
and/or propylene.
Inventors: |
Klavers; Rik; (Houston,
TX) ; Reyneke; Rian; (Katy, TX) ; Lee;
Guang-Chung; (Houston, TX) ; Sinclair; Iain;
(Warrington, GB) ; Eng; Wayne W.Y.; (Calgary,
CA) ; Logsdon; Jeffery S.; (Naperville, IL) ;
Papadopoulos; Christos G.; (Naperville, IL) ; Foral;
Michael J.; (Aurora, IL) |
Correspondence
Address: |
INEOS USA LLC
3030 WARRENVILLE RD, S/650
LISLE
IL
60532
US
|
Assignee: |
INEOS USA LLC
Lisle
IL
|
Family ID: |
38228637 |
Appl. No.: |
12/086380 |
Filed: |
December 15, 2005 |
PCT Filed: |
December 15, 2005 |
PCT NO: |
PCT/US2005/045139 |
371 Date: |
June 10, 2008 |
Current U.S.
Class: |
60/645 |
Current CPC
Class: |
F22B 31/04 20130101;
F01K 7/22 20130101; F01K 7/025 20130101 |
Class at
Publication: |
60/645 |
International
Class: |
F01K 13/00 20060101
F01K013/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made with government support under United
States Department of Energy Cooperative Agreement No. DE-FC07-O1ID
14090.
Claims
1. A power recovery process employing at least two classes of
expansion turbines, which process comprises: (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; (d) expanding at least a portion of the resulting
heated stream from step (c) at second 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.
2. The process of claim 1 wherein three or more vapor streams are
combined in step (b) into a single very high-pressure superheated
vapor stream.
3. The process of claim 1 wherein 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.
4. The process of claim 1 wherein the vapor comprises a light
organic component containing from about 2 to about 4 carbon
atoms.
5. The process of claim 1 wherein the second intermediate
temperature is no more than 100 Fahrenheit degrees below the first
inlet conditions temperature.
6. A power recovery process employing at least two classes of
expansion turbines, which process comprises: (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 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.
7. The process of claim 6 which 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 a portion of the
resulting single very high-pressure stream of step (b).
8. The process of claim 7 wherein 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.
9. The process of claim 8 wherein the olefins being produced are
ethylene and/or propylene.
10. The process of claim 6 wherein 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.
11. The process of claim 6 wherein the first inlet pressure is at
least 900 psig.
12. The process of claim 6 wherein the first inlet temperature is
at least 800.degree. F.
13. The process of claim 6 which 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.
14. The process of claim 13 wherein the cooling medium is boiler
feed water, and wherein 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).
15. The process of claim 6 wherein the second intermediate
temperature is no more than 100 Fahrenheit degrees below the first
inlet conditions temperature.
Description
FIELD OF THE INVENTION
[0002] 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.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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
include streams of ethane, propane or hydrocarbon liquids ranging
in boiling point from light straight-run gasoline through gas
oil.
[0005] 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.
[0006] 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
the process of the present invention, beneficially reduce the net
energy use of the olefins manufacturing facility.
[0007] 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 pressure vapor streams, and generate turbine exhaust streams
at a plurality of pressures
[0008] An improved method for recovery expansion power should
exhibit higher efficiency thereby providing lower net energy use
and therefore lower variable costs of operation.
[0009] Other objects and advantages of the invention will become
apparent upon reading the following detailed description and
appended claims.
SUMMARY OF THE INVENTION
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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 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.
[0016] 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.degree. 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.
[0017] 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).
[0018] 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.
[0019] 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
[0020] 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:
[0021] FIG. 1 is a schematic diagram of a comparative power
recovery process for the steam system in an olefins manufacturing
thermal cracking unit.
[0022] FIG. 2 is a schematic diagram of an embodiment of this
invention in which intermediate-pressure steam is heated and
low-pressure steam is desuperheated.
[0023] FIG. 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.
[0024] It should be noted that only essential expansions and
heating/cooling steps are shown in these schematic diagrams.
BRIEF DESCRIPTION OF THE INVENTION
[0025] 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.
[0026] 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.
[0027] 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.
[0028] The literature is replete with disclosures of suitable
pyrolytic furnaces for the thermal cracking of hydrocarbons. For
example, U.S. Pat. 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.
[0029] When light olefins and monoaromatic compounds are to be
produced, the necessary temperature is and generally ranges from
about 1,440.degree. F. to about 1,600.degree. 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.
[0030] 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. Additionally,
the various fractions may be injected either alone or in
combination with steam and optionally other gases such as hydrogen
or light gases.
[0031] 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.degree. F.
[0032] 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.
[0033] The recovery and purification of light olefins such as
ethylene and propylene from the furnace effluent is an
energy-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.
[0034] 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.
[0035] 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.
[0036] 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
[0037] 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 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.
[0038] The apparatus of this invention 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
[0039] 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.
[0040] In olefins manufacture, high-pressure steam is generated in
a number of cracking furnaces. The number of cracking furnaces in a
particular olefins unit will 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 energy thus produced is
typically used to compress the olefin-containing gas and to drive
machinery designed to provide refrigeration to the olefins
process.
[0041] FIG. 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 FIG. 1.
[0042] 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.degree. F.
[0043] 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.
[0044] 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.
[0045] 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 FIG. 1, only two turbines are depicted.
[0046] 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 units in the olefins process. While a single exchanger
12 is shown in FIG. 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.
[0047] In FIG. 1 steam turbine 10 is shown as an extracting
turbine, with two stages (stage 10a and stage 10b) which are
typically mechanically coupled. Low-pressure steam (typically at
about 65 psig) is recovered from stage 10a as stream 15.
[0048] 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.
[0049] 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 FIG. 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.
[0050] FIG. 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 furnaces to
a significantly higher temperature than the corresponding stream 1
of FIG. 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 FIG. 1. The maximum temperature of stream 32 is
typically limited by the design and metallurgy of the downstream
expansion turbine 33.
[0051] Stream 32 is directed to steam turbine 33, which provides
similar functionality as turbine 2 of FIG. 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.
[0052] 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 FIG. 2
is at a higher temperature than the corresponding high-pressure
steam stream 6 in the conventional steam system of FIG. 1.
[0053] 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 FIG. 2, only two turbines are depicted.
[0054] 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 FIG. 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 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.
[0055] 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.
[0056] In FIG. 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.
[0057] A portion of stream 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.
[0058] 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 FIG. 1.
[0059] 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.
[0060] The low-pressure steam stream 55 from the desuperheater
exchanger enters a low-pressure steam header as shown. The majority
of the low-pressure steam is typically withdrawn as stream 58 and
used for process heating needs in exchanger 59 as shown. The single
exchanger 59 in FIG. 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 stream 61 to
another process or otherwise used within the olefins unit.
[0061] FIG. 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 FIG. 2, but the high
pressure steam to the second stage of the first turbine is reheated
before entering the second stage.
[0062] 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 FIG. 1 and stream 32 of FIG.
2.
[0063] 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 steam
is directed as stream 76 to the reheat exchanger 71 where it is
reheated against the desuperheating very high pressure steam.
[0064] 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.
[0065] 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 FIG. 3, but it is
understood that it can be similar in nature to that of FIG. 2
(where stream 80 of FIG. 3 corresponds to stream 38 of FIG. 2), or
it can be of a different configuration.
[0066] FIGS. 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
[0067] 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
[0068] To demonstrate several beneficial aspects of the present
invention, both the comparative process depicted in FIG. 1 and the
embodiment of FIG. 2 were simulated using commercially available
process simulation software.
Comparative Example
[0069] 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 FIG. 1, and
all stream and unit numbers in this example refer to those in FIG.
1. Very high-pressure steam at a temperature of 980.degree. 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.
[0070] Turbine 2 generates approximately 112,000 HP. Turbine 10
represents the combination of two separate refrigeration turbines
which generate a total of 33,800 HP. Turbine 9 represents a number
of smaller turbines which combined generate approximately 4,900
HP.
[0071] Stream flows and conditions for this example are given in
Table 1. Stream numbers correspond to those of FIG. 1. A total of
941,500 lb/hr of very high pressure steam is used.
Example of the Present Invention
[0072] 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 FIG. 2, and all stream and unit numbers in
this example refer to those in FIG. 2. Very high-pressure steam at
a temperature of 1090.degree. 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.
[0073] 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.degree. F. in stream 38. In addition,
the amount of low-pressure steam produced by both the current and
previous examples was kept constant.
[0074] 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.degree.
F.
[0075] 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 FIG. 2. Therefore,
compared with the comparative 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.
[0076] Stream flows and conditions for this example are given in
Table 2. Stream numbers correspond to those of FIG. 2.
[0077] 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 (FIG. 2)
produces over 4000 HP more power than a conventional steam system,
from similar furnace steam duty.
[0078] 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.
[0079] 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.
[0080] 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 percent 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 macroscopic qualities and final outcome are permitted,
typically up to about one percent.
TABLE-US-00001 TABLE 1 Comparative Example Steam Flow Temperature
Pressure Stream (lb/hr) (Deg F.) (psia) 1 941,475 980 1,815 5
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-US-00002 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-US-00003 TABLE 3 Turbine Power Comparative Example Example of
This Invention (FIG. 1) (FIG. 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 Total 149612 Total 154039
* * * * *