U.S. patent application number 12/852832 was filed with the patent office on 2012-02-09 for low grade heat recovery from process streams for power generation.
This patent application is currently assigned to UOP LLC. Invention is credited to Richard K. HOEHN, Saadet ULAS ACIKGOZ, Xin X. ZHU.
Application Number | 20120031096 12/852832 |
Document ID | / |
Family ID | 45555050 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120031096 |
Kind Code |
A1 |
ULAS ACIKGOZ; Saadet ; et
al. |
February 9, 2012 |
Low Grade Heat Recovery from Process Streams for Power
Generation
Abstract
Methods are described for generating electrical power from low
grade heat sources from refining and petrochemical processes,
including overhead vapors from vapor-liquid contacting apparatuses
such as distillation columns, absorbers, strippers, quenching
towers, scrubbers, etc. In many cases, these overhead vapors exit
the apparatuses at a temperature from about 90.degree. C.
(194.degree. F.) to about 175.degree. C. (347.degree. F.). Rather
than rejecting the low temperature heat contained in these vapors
to cooling air and/or cooling water, the vapors may instead be used
to evaporate an organic working fluid. The vapors of the working
fluid may then be sent to a turbine to drive a generator or other
load.
Inventors: |
ULAS ACIKGOZ; Saadet; (Des
Plaines, IL) ; HOEHN; Richard K.; (Mt. Prospect,
IL) ; ZHU; Xin X.; (Long Grove, IL) |
Assignee: |
UOP LLC
Des Plaines
IL
|
Family ID: |
45555050 |
Appl. No.: |
12/852832 |
Filed: |
August 9, 2010 |
Current U.S.
Class: |
60/651 ; 60/671;
60/692 |
Current CPC
Class: |
F01K 25/08 20130101 |
Class at
Publication: |
60/651 ; 60/692;
60/671 |
International
Class: |
F01K 25/08 20060101
F01K025/08; F01K 9/00 20060101 F01K009/00 |
Claims
1. A method for generating electrical power from a low grade heat
source, the method comprising: (a) passing a hydrocarbon-containing
feed to a vapor-liquid contacting apparatus to provide an overhead
product and a bottoms liquid; (b) indirectly exchanging heat
between the overhead product and an organic fluid to provide a
cooled overhead product and a vapor-enriched fluid; and (c) passing
the vapor-enriched fluid to a turbine to generate electricity and
provide a turbine exhaust.
2. The method of claim 1, further comprising, (d) condensing the
turbine exhaust to regenerate the organic fluid, and (e) pumping
the organic fluid to an evaporator for indirect heat exchange with
the overhead vapor in step (b).
3. The method of claim 1, wherein the hydrocarbon-containing feed
stream comprises at least about 90% hydrocarbons by weight.
4. The method of claim 1, wherein the vapor-liquid contacting
apparatus is a distillation column.
5. The method of claim 4, wherein the distillation column is a
product fractionation column of a hydrocracking process.
6. The method of claim 1, wherein, in step (b), the overhead
product has a temperature from about 90.degree. C. (194.degree. F.)
to about 150.degree. C. (302.degree. F.) immediately prior to
exchanging heat and the cooled overhead product has a temperature
from about 65.degree. C. (149.degree. F.) to about 100.degree. C.
(212.degree. F.) immediately subsequent to exchanging heat.
7. The method of claim 1, wherein the organic fluid comprises a
fluorocarbon or a chlorofluorocarbon.
8. The method of claim 2, wherein step (d) is carried out using an
air cooled exchanger.
9. The method of claim 1, wherein indirect heat exchange in step
(b) comprises (i) indirectly exchanging heat between the overhead
vapor and an intermediate heat transfer medium and (ii) indirectly
exchanging heat between the intermediate heat transfer medium and
the organic fluid.
10. The method of claim 9, wherein the intermediate heat transfer
medium is water.
11. The method of claim 9, wherein, immediately prior to step
(b)(ii), the heat transfer medium comprises heat that is
transferred from the overhead vapor and from at least one
additional source of low grade heat.
12. The method of claim 11, wherein the additional source of low
grade heat is a refinery or petrochemical process stream having a
temperature from about 90.degree. C. (194.degree. F.) to about
150.degree. C. (302.degree. F.).
13. The method of claim 1, wherein, a net benefit of electricity
generated in step (c) is from about 0.005 to about 0.02 watts per
BTU/hr of waste heat in the hydrocarbon-containing feed stream.
15. A method for generating electrical power from a low grade heat
source, the method comprising: (a) passing liquid and vapor feeds
to upper and lower sections, respectively, of a vapor-liquid
contacting apparatus to provide an overhead vapor and a bottoms
liquid; (b) indirectly exchanging heat between the overhead vapor
and an organic fluid to provide a cooled overhead product and a
vapor-enriched fluid; and (c) passing the vapor-enriched fluid to a
turbine to generate electricity.
16. An apparatus for generating electrical power a from low grade
heat source, the apparatus comprising: (a) an evaporator for
indirectly exchanging heat between the low grade heat source and an
organic fluid; (b) a turbine in communication with a vapor-enriched
fluid conduit from the evaporator; (c) a condenser in communication
with an exhaust conduit from the turbine; and (d) a cooler in
communication with a low grade heat source outlet from the
evaporator.
17. The apparatus of claim 16, wherein the condenser is an air
cooled exchanger.
18. The apparatus of claim 16, wherein both the evaporator and
condenser are tubular heat exchangers.
19. The apparatus of claim 18, wherein the evaporator, the
condenser, or both have disposed therein a plurality of tubes
having fins or recessions on their surfaces.
20. The apparatus of claim 19, wherein the plurality of tubes of
the evaporator, the condenser, or both have circumferentially
extending fins having outer edges that include a plurality of
notches.
Description
FIELD OF THE INVENTION
[0001] The invention relates to apparatuses and methods for
generating power from low grade heat sources including refinery and
petrochemical process streams. A representative stream is an
overhead vapor from a distillation column, such as a product
fractionation column of a hydrocracking process.
DESCRIPTION OF RELATED ART
[0002] Sources of low grade heat are prevalent in refining,
petrochemical, and other industrial applications. For example, low
grade heat is estimated to account for 20-30% of the overall energy
in a refinery. Low grade heat is normally considered waste heat and
treated accordingly. This is due to the relatively low (although
often well above ambient) temperatures of low grade heat sources
that render them unsuitable for feed preheating, generation of
pressurized steam, distillation column reboiling, and generally any
other practical use involving heat exchange. As a result, the heat
content of such low grade heat sources, though significant, is
rejected, for example to the atmosphere using air cooling.
[0003] Low grade heat often results, for example, from the
generation of overhead vapors in vapor-liquid contacting
apparatuses such as distillation columns, in which upwardly moving
vapor and downwardly moving liquid fractions contact each other for
heat and mass exchange, with the contacting normally aided by the
use of vapor-liquid contacting devices such as trays or packing
materials. Often, a single feed (e.g., containing predominantly
hydrocarbons) enters the distillation column and an overhead vapor,
enriched in one or more components of relatively higher volatility,
as well as a liquid bottoms, enriched in one or more components of
relatively lower volatility, exit the distillation column.
Distillation can also involve the use of two or more feeds that
enter a distillation column at differing axial heights,
corresponding to different contacting stages of the column. Whether
a single feed or multiple feeds are used, in many cases other
product fractions may be desired, in addition to the overhead vapor
and liquid bottoms, with each product fraction containing
components (e.g., hydrocarbons) having boiling points within
different ranges. In addition to distillation columns, examples of
vapor-liquid contacting apparatuses include columns used for the
contacting of, and exchange of components between, at least two
streams fed to the column, for example liquid and vapor streams
that are fed in a countercurrent manner. Absorption, quenching,
scrubbing, stripping, etc. are representative of operations that
occur in such columns.
[0004] Generally, overhead product such as overhead vapors from
distillation columns or other types of vapor-liquid contacting
apparatuses are cooled by indirect heat exchange with air (e.g.,
using an air cooled exchanger) and/or cooling water (e.g., using a
trim condenser), since the residual heat in the overhead streams is
not considered of sufficient value for recovery of energy in an
economical manner.
[0005] A Rankine cycle is useful for generating power from a low or
medium temperature heat source. A working fluid is evaporated, for
example in an evaporator or boiler, upon exchanging heat with the
source, and the vaporized fluid is then used in a turbine that
drives an electrical generator or other load. Exhaust vapors from
the turbine are condensed, and the resulting fluid may be recycled
for heat exchange. An Organic Rankine Cycle (ORC) uses an organic
fluid as a working fluid. Known applications of ORC systems include
generating power from geothermal heat sources, as described in U.S.
Pat. No. 5,497,624 and U.S. Pat. No. 6,539,718. The use of an ORC
in combination with fuel cell products and other forms of waste
heat is described in US 2006/0010872, WO 2006/104490, and WO
2006/014609. Applications of an ORC involving solar energy and
biomass are described in CN11055121, JP2003227315A2, INTERNATIONAL
JOURNAL OF ENERGY RESEARCH, 28(11): 1003-1021 (2004), and ENERGY,
32(4): 371-377 (2007). The use of an ORC is also taught in U.S.
Pat. No. 7,049,465, for improving energy recovery from exothermic
reactions and particularly the liquid phase oxidation of paraxylene
to terephthalic acid.
[0006] There is an ongoing need in the art for methods for
recovering energy such as electricity from sources providing low
grade heat that is otherwise wasted, especially those sources
resulting from refinery and petrochemical plant operations. This
need is particularly significant in view of the large quantities of
low grade heat generated in these operations and the high energy
and cost savings potentially realized from recovering even a
fraction of this heat. The net generation of electricity is of
significant benefit to refiners and petrochemical producers, due to
reduced emissions of the combustion product CO.sub.2 from materials
(e.g., coal) used as a raw material in power plants.
SUMMARY OF THE INVENTION
[0007] Aspects of the present invention are associated with the
discovery of methods for generating electrical power from low grade
heat sources from refining and petrochemical processes, including
overhead products such as vapors from vapor-liquid contacting
apparatuses. These apparatuses include distillation columns,
absorbers, strippers, quenching towers, scrubbers, etc., from which
the overhead product exits, in many cases, at a temperature from
about 90.degree. C. (194.degree. F.) to about 150.degree. C.
(302.degree. F.). Rather than rejecting the low temperature heat
contained in these overhead products to cooling air and/or cooling
water, the products may instead be used, in an evaporator, to
evaporate an organic working fluid. The vapors of the working fluid
may then be sent to a turbine to drive a generator or other load.
Exhaust vapors from the turbine may be condensed and recycled via a
pump to the evaporator. The power generated using such an Organic
Rankine Cycle (ORC) can be used within the process to supply at
least a portion of its power requirement (e.g., to operate pumps
and/or compressors) or otherwise sent to a central power
station.
[0008] The methods described herein are therefore suitable for
power recovery from overhead products, and especially overhead
vapors, generated in vapor-liquid contacting operations and
particularly those associated with oil refining and petrochemical
production. The temperature of the overhead product, although
characteristic of a low grade heat source, is nevertheless
sufficient to vaporize the organic working fluid used in the ORC.
Suitable working fluids therefore include non-flammable, low
toxicity chemical compounds having a boiling point that is
typically from about 5.degree. C. to about 14.degree. C. (about
10.degree. F. to about 25.degree. F.) lower than the temperature of
an overhead product used in a particular ORC application.
[0009] Also, if two or more sources of low grade heat are available
(e.g., from different vapor-liquid contacting apparatuses, from
heat exchangers operating in series or in parallel, or from other
processing equipment), their low grade heat may be consolidated by
transfer to a common, intermediate heat transfer medium (e.g.,
water). The intermediate heat transfer medium may then be used to
evaporate the working fluid of the ORC to generate power, for
example, from a single turbine. In this manner of consolidating
sources of low grade heat, the ORC can be sized to realize greater
efficiencies and economies of scale. Moreover, such a consolidated
operation improves flexibility in design and plot space planning,
since the requirement for close proximity of the turbine to the low
grade heat source(s) is relaxed.
[0010] Embodiments of the invention are therefore directed to
methods for generating electrical power from a low grade heat
source (e.g., a refining or petrochemical process stream having a
temperature from about 90.degree. C. (194.degree. F.) to about
175.degree. C. (347.degree. F.)). Representative methods comprise
passing a hydrocarbon containing feed (e.g., comprising at least a
portion of a refining process reactor effluent) to a vapor-liquid
contacting apparatus, such as a distillation column, to provide an
overhead product (e.g., an overhead vapor), as well as a bottoms
liquid. The methods also comprise indirectly exchanging heat
between the overhead product and an organic fluid (e.g., a working
fluid as described above) to provide a cooled and/or condensed
overhead product and a vapor-enriched fluid, as a result of the
indirect heat exchange. The methods further comprise passing the
vapor-enriched fluid to a turbine to generate electricity. In order
to complete the ORC, methods described herein may further comprise
condensing (e.g., using an air cooled exchanger) the vapor-enriched
fluid to regenerate the organic fluid and then pumping this fluid
to an evaporator for re-use in indirect heat exchange with the
overhead product.
[0011] According to other embodiments for generating electrical
power from a low grade heat source, liquid and vapor feeds (i.e.,
feeds that are predominantly in the liquid and vapor phases,
respectively) are passed to respective upper and lower sections of
a vapor-liquid contacting apparatus to provide an overhead product
(e.g., overhead vapor) and a bottoms liquid, having compositions
that differ from those of the vapor and liquid feeds, respectively,
as a result of contacting. The methods further comprise indirectly
exchanging heat between the overhead product and an organic fluid
to provide a cooled or condensed overhead product and a
vapor-enriched fluid, and passing the vapor-enriched fluid to a
turbine to generate electricity.
[0012] In any of the methods described herein, a net benefit of
electricity generated from passing the vapor-enriched fluid to the
turbine is generally from about 0.005 to about 0.05, and often from
about 0.01 to about 0.03, watts per BTU/hr of waste heat in the low
grade heat source (e.g., an overhead vapor of a vapor-liquid
contacting apparatus). The latter range corresponds to a cycle
efficiency of the ORC from about 3% to about 10%.
[0013] Further embodiments of the invention are directed to
apparatuses for generating electrical power from at least one low
grade heat source. Representative apparatuses comprise an
evaporator for indirectly exchanging heat between the low grade
heat source and an organic fluid; a turbine in communication with a
vapor-enriched fluid conduit from the evaporator; and a condenser
in communication with an exhaust conduit from the turbine.
According to more specific embodiments, the evaporator and/or
condenser may be tubular heat exchangers. To improve the heat
transfer coefficient and reduce exchanger area, the external
surfaces of the tubes may have surface enhancements such as fins
and/or recessions.
[0014] These and other aspects and embodiments associated with the
present invention are apparent from the following Detailed
Description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 depicts a representative refining process in which an
upgraded hydrocarbon product, such as a product obtained from a
hydrocracking reaction zone, is passed to a distillation column to
provide an overhead product as a low grade heat source, from which
electricity is generated.
[0016] FIG. 1 is not necessarily drawn to scale and should be
understood to present an illustration of the invention and/or
principles involved. Some features depicted have been enlarged or
distorted relative to others, in order to facilitate explanation
and understanding. As is readily apparent to one of skill in the
art having knowledge of the present disclosure, methods for
generating electrical power from low grade heat sources, according
to various other embodiments of the invention, will have
configurations and components determined, in part, by their
specific use.
[0017] FIG. 2 is a bar graph illustrating the estimated annual
benefit of generating electricity, using an Organic Rankine Cycle
(ORC), from low grade heat that is normally rejected from a main
fractionation column overhead product of a commercial hydrocracking
process. Estimates are shown at varying electrical power costs and
using two average air inlet temperatures to the condenser, namely
20.degree. C. (68.degree. F.) and 25.degree. C. (77.degree.
F.).
[0018] FIG. 3 is a bar graph illustrating the investment payback,
at varying electrical power costs, associated with generating
electricity from the overhead product as described with respect to
FIG. 2
DETAILED DESCRIPTION
[0019] The invention is associated with methods for generating
electrical power from sources of low grade heat, particularly
refining and petrochemical process streams, through the use of an
Organic Rankine Cycle (ORC). Sources of low grade heat can include
any process stream from which recovery of at least part of its heat
content in the form of electricity is desired. These streams are
often conventionally subjected to cooling with air and/or water,
since they are not at a sufficiently high temperature for
economically useful heat integration applications (e.g.,
preheating, medium- or high-pressure steam generation, or
distillation column reboiling). The temperature of these sources of
low grade heat is generally from about 75.degree. C. (167.degree.
F.) to about 180.degree. C. (356.degree. F.) and often from about
90.degree. C. (194.degree. F.) to about 175.degree. C. (347.degree.
F.). Representative streams as sources of low grade heat include
refining and petrochemical process streams having temperatures
within these ranges, with particular examples being overhead
vapors, and more generally overhead products, from vapor-liquid
contacting apparatuses such as distillation columns and other
columns (e.g., absorbers, strippers, quenching towers, scrubbers,
etc) as described above.
[0020] Distillation columns refer to those used in separation
processes based on differences in the relative volatility of
components present in an impure mixture. Distillation involves the
purification of components having differing relative volatilities
by achieving multiple theoretical stages of vapor-liquid
equilibrium along the length of a vertical column. Rising vapor,
enriched in a lower boiling component relative to the liquid from
which it is vaporized in a lower stage in the column, is contacted
with falling liquid, enriched in a higher boiling component
relative to the vapor from which it is condensed in a higher stage
in the column.
[0021] Distillation columns, which also include fractionation
columns that provide number of product fractions, each having
components within certain boiling point ranges, are widely used,
for example, in separating effluents of reaction zones. Reaction
zones generally comprise one or more reactors that are used to
convert (e.g., catalytically) a feedstock to the more valuable
reactor effluent, containing the product fractions that are
subsequently resolved through fractionation. In refining,
representative hydrocarbon conversions occurring in catalytic
reaction zones include hydrocracking, fluid catalytic cracking
(FCC), catalytic reforming, isomerization, dehydrogenation,
alkylation, disproportionation, and others. Hydrocarbon-containing
feeds to vapor-liquid contacting apparatuses such as distillation
columns therefore include reactor effluents or otherwise upgraded
hydrocarbon products comprising at least a portion of a reactor
effluent, optionally after initial treatments or separations (e.g.,
in single stage high-, medium-, and/or low-pressure separators to
remove hydrogen and/or light hydrocarbons such as methane). These
hydrocarbon-containing feeds generally comprise at least about 80%
by weight, and often comprise at least about 90% by weight,
hydrocarbons.
[0022] In the case of a catalytic hydrocracking process, for
example, the feedstock to the reaction zone is generally a
distillate hydrocarbon, namely a distillable petroleum derived
fraction having a boiling point range which is above that of
naphtha. Suitable distillate feedstocks that may be obtained from
refinery fractionation and conversion operations include middle
distillate hydrocarbon streams, such as highly aromatic hydrocarbon
streams. Distillate feedstocks include distillate hydrocarbons
boiling at a temperature greater than about 149.degree. C.
(300.degree. F.), typically boiling in the range from about
149.degree. C. (300.degree. F.) to about 399.degree. C.
(750.degree. F.), and often boiling in the range from about
204.degree. C. (400.degree. F.) to about 371.degree. C.
(700.degree. F.).
[0023] Representative distillate feedstocks to hydrocracking
processes can therefore include various hydrocarbon mixtures, such
as straight-run fractions, or blends of fractions, recovered by
fractional distillation of crude petroleum. Such fractions produced
in refineries include coker gas oil and other coker distillates,
straight run gas oil, deasphalted gas oil, and vacuum gas oil.
These fractions or blends of fractions can be a mixture of
hydrocarbons boiling in range from about 343.degree. C.
(650.degree. F.) about 566.degree. C. (1050.degree. F.), with
boiling end points in other embodiments being below about
538.degree. C. (1000.degree. F.) and below about 482.degree. C.
(900.degree. F.). Thus, distillate feedstocks are often recovered
from crude oil fractionation or distillation operations, and
optionally following one or more hydrocarbon conversion reactions.
However, distillate feedstocks may be utilized from any convenient
source such as tar sand extract (bitumen) and gas to liquids
conversion products, as well as synthetic hydrocarbon mixtures such
as recovered from shale oil or coal. Suitable conditions and
catalysts for hydrocracking distillate feedstocks are described,
for example, in co-pending U.S. application Ser. No. 12/268,048,
hereby incorporated by reference with respect to its description of
hydrocracking feedstocks, process conditions, and catalysts.
[0024] The effluent from the hydrocracking reaction zone, usually
configured with one or more hydrocracking reactors in series, has a
higher value relative to the distillate feedstock, due to the
overall decrease in the average molecular weight of the products of
hydrocracking. For example, the effluent often comprises a
significant proportion of hydrocarbons suitable for blending into
either gasoline or diesel fuel. Fractionation of the hydrocracking
reaction zone effluent, or at least a portion of this effluent such
as an upgraded hydrocarbon product obtained after separation of
hydrogen for recycle and possibly after other stages of light ends
or heavy ends removal, therefore desirably yields naphtha and
diesel product fractions. These product fractions are typically
obtained from a main fractionation section or main fractionation
column used to distill a large proportion of the hydrocracking zone
reactor effluent.
[0025] The overhead product of this fractionation column generally
comprises C.sub.4 (e.g., butane and butenes) and lighter
hydrocarbons (e.g., propane, propylene, ethane, and ethylene) and
can also contain H.sub.2S and NH.sub.3, which are the hydrogenation
reaction products of sulfur- and nitrogen-containing components in
the feedstock to the hydrocracking reaction zone. Generally, this
overhead product is removed from the column completely or
substantially in the vapor phase, at a temperature from about
90.degree. C. (194.degree. F.) to about 150.degree. C. (302.degree.
F.), prior to being cooled via indirect heat exchange with an air
(e.g., using an air cooled exchanger) and/or a water (e.g., using a
trim condenser). This overhead product of the main fractionation
column of the hydrocracking process therefore serves as a favorable
low grade heat source for electricity generation according to
methods described herein.
[0026] Other product fractions may similarly be used for
electricity generation, although these are typically removed from
the main fractionation column of a hydrocracking process at higher
temperatures that are more suitable for conventional heat
integration in a refinery. Product fractions typically recovered
from this column of a hydrocracking process include light naphtha
having a distillation end point temperature of about 149.degree. C.
(300.degree. F.) (e.g., from about 138.degree. C. (280.degree. F.)
to about 160.degree. C. (320.degree. F.)) and heavy naphtha having
a distillation end point temperature of about 204.degree. C.
(400.degree. F.) (e.g., from about 193.degree. C. (380.degree. F.)
to about 216.degree. C. (420.degree. F.)). Otherwise, the naphtha
in the upgraded hydrocarbon product of the hydrocracking reaction
zone may be fractionated into light naphtha, gasoline, and heavy
naphtha, with representative distillation end points being in the
ranges from about 138.degree. C. (280.degree. F.) to about
160.degree. C. (320.degree. F.), from about 168.degree. C.
(335.degree. F.) to about 191.degree. C. (375.degree. F.), and from
about 193.degree. C. (380.degree. F.) to about 216.degree. C.
(420.degree. F.), respectively. In any naphtha or naphtha fraction
characterized as discussed above with respect to its distillation
end point temperature, a representative "front end" or initial
boiling point temperature is about 85.degree. C. (185.degree. F.)
(e.g., from about 70.degree. C. (158.degree. F.) to about
100.degree. C. (212.degree. F.)).
[0027] Methods for generating electrical power, according to
representative embodiments of the invention, are illustrated in the
FIG. 1. As shown, fractionation column 100 fractionates hydrocarbon
feed 1, which may, for example, be an upgraded hydrocarbon product
from the reaction zone of a refining or petrochemical process
(e.g., a portion of a hydrocracking reaction zone effluent after
removal of hydrogen and light ends). Fractionation column 100, in
addition to providing other product fractions 12, 14, 16 for
example light naphtha, heavy naphtha, and relatively unconverted
bottoms liquid (all or a portion of which may be recycled to a
hydrocracking reaction zone), respectively, also provides overhead
product 10, for example comprising predominantly (e.g., greater
than about 50% by volume) C.sub.4 and heavier hydrocarbons. As
discussed above, overhead product 10, which may be completely or
substantially in the vapor phase, is generally removed from
fractionation column 100 at a temperature that renders it a low
grade heat source suitable for power generation according to
methods described herein. Overhead product 10 may also be cooled
somewhat, after exiting fractionation column 100, to a suitable
temperature for this application. Overhead product 10 may be all or
a portion of the net overhead withdrawn from fractionation column
100.
[0028] According to the embodiment shown in the FIG. 1, overhead
product 10 is passed to evaporator 200 for indirect heat exchange
with organic fluid 18, generally having a boiling point that is
from about 5.degree. C. to about 14.degree. C. (about 10.degree. F.
to about 25.degree. F.) lower than the temperature of an overhead
product 10. Suitable organic fluids include fluorocarbons and
chlorofluorocarbons (CFCs) that are used commercially as
refrigerants. Representative fluorocarbons include
1,1,1,3,3-pentafluoropropane (HFC-245fa), 1,1,1,2-tetrafluoroethane
(HFC-134a), 1,1,2,2-tetrafluoroethane (HFC-134),
1,1,1,3,3-pentafluorobutane (HFC-365mfc),
1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), and mixtures thereof.
Representative CFCs include CFC-113
(1,1,2-trichloro-1,2,2-trifluoroethane), CFC-11
(trichlorofluoromethane), CFC-12 (dichlorodifluoromethane), CFC-22
(chlorodifluoromethane), and mixtures thereof. Mixtures of
fluorocarbons and chlorofluorocarbons may also be used in an
organic fluid.
[0029] Indirect heat exchange, in evaporator 200, between overhead
product 10 and organic fluid 18, provides cooled overhead product
20, having a temperature that is generally from about 15.degree. C.
(27.degree. F.) to about 75.degree. C. (135.degree. F.) less than
that of overhead product 10 immediately prior to the heat exchange.
In representative embodiments, cooled overhead product 20 has a
temperature generally from about 50.degree. C. (122.degree. F.) to
about 125.degree. C. (257.degree. F.), and often from about
65.degree. C. (149.degree. F.) to about 100.degree. C. (212.degree.
F.), immediately after exchanging heat. Some or all of the heat
removed from overhead product 20, as a result of indirect heat
exchange, may be latent heat that causes condensation of at least a
portion of cooled overhead product 20 (and an overall increased
liquid fraction of this product), as opposed to sensible heat that
causes a temperature decrease of this product.
[0030] Also exiting evaporator 200, as a result of indirect heat
exchange, is vapor-enriched fluid 22, having an increased vapor
fraction relative to organic fluid 18. Preferably, vapor-enriched
fluid 22 is completely in the vapor phase after exiting evaporator
200. Vapor-enriched fluid 22 is then utilized in turbine 300 to
drive an electrical generator (for electricity generation) or other
type of load. To establish a complete ORC, turbine exhaust 24 from
turbine 300 may be condensed, for example using air cooled
exchanger 400, to regenerate organic fluid 18, which is generally
completely in the liquid phase. Organic fluid 18 may then be pumped
via pump 500 for indirect heat exchange, as discussed above, in
evaporator 200.
[0031] Often, cooling of overhead product 10 using evaporator 200
replaces at least part of the cooling using conventional air and/or
water indirect heat exchangers to reject the low grade heat to the
environment. Cooled overhead product 20 may, in some cases
depending on its temperature, be further cooled, for example using
a cooler 600, such as a water cooled exchanger or trim condenser,
prior to passing to overhead receiver 700. In overhead receiver
700, the vapor phase is removed as net column overhead 26 and the
liquid phase is returned as reflux 28 back to fractionator 100.
[0032] According to other embodiments of the invention, a low grade
heat source, other than overhead product 10, may be an intermediate
heat transfer medium (e.g., water) having been initially subjected
to heat exchange against overhead product 10, and optionally at
least one additional source of low grade heat. Representative
methods according to the invention therefore comprise (i)
indirectly exchanging heat between the overhead product, such as an
overhead vapor, and an intermediate heat transfer medium and (ii)
indirectly exchanging heat between the intermediate heat transfer
medium and the organic fluid as discussed above. In embodiments in
which at least one additional source of low grade heat is used to
provide heat, via indirect heat exchange, to the intermediate heat
transfer medium, this additional source may be any refinery or
petrochemical process stream having a temperature as discussed
above with respect to sources of low grade heat (e.g., generally
from about 75.degree. C. (167.degree. F.) to about 180.degree. C.
(356.degree. F.) and often from about 90.degree. C. (194.degree.
F.) to about 175.degree. C. (347.degree. F.)).
[0033] In yet further embodiments, the low grade heat source,
rather than overhead product 10 of fractionation column 100, may be
an overhead product, such as an overhead vapor, obtained from a
countercurrent vapor-liquid contacting apparatus (e.g., an
absorber, stripper, quenching tower, scrubber, etc). Liquid and
vapor feeds, which may be predominantly in the liquid and vapor
phases, respectively, are generally passed to upper and lower
sections, respectively, of such an apparatus to provide the
overhead product, in addition to a bottoms liquid. These overhead
and bottoms products have compositions that differ from those of
the vapor and liquid feeds as a result of the contacting that
occurs in the vapor-liquid contacting apparatus. Regardless of
whether the vapor-liquid contacting apparatus is a distillation
column or any other apparatus as described above, it will generally
contain packing material and/or contacting trays, as conventionally
used in the art to improve contacting efficiency.
[0034] Still other embodiments of the invention are directed to
apparatuses for performing the methods described herein for
generating electrical power from one or more low grade heat
sources. Representative apparatuses comprise the equipment shown
and described with respect to FIG. 1, including an evaporator 200
for indirectly exchanging heat between the low grade heat source(s)
and an organic fluid; a turbine 300 in fluid communication with a
vapor-enriched fluid conduit from the evaporator 200; and a
condenser 400 (e.g., an air cooled exchanger such as an exchanger
referred to in the art as a "fin fan" cooler) in fluid
communication with an exhaust conduit from the turbine. To complete
an ORC, the apparatuses generally also comprise a pump 500 in fluid
communication with both a condensed fluid outlet of the condenser
400 and an inlet for the organic fluid to the evaporator 200.
Additionally, a cooler 600, such as a water cooled exchanger or
trim condenser as discussed above, may be in fluid communication
with a low grade heat source outlet (e.g., a cooled overhead
product conduit) from the evaporator 200, and an overhead receiver
700 may be in fluid communication with an outlet of cooler 600.
Otherwise, in embodiments in which cooler 600 is not used, overhead
received may be directly in fluid communication with a low grade
heat source outlet (e.g., a cooled overhead product conduit) from
the evaporator 200.
[0035] If an air cooled exchanger is used for condenser 400, the
ambient air temperature significantly affects the condenser size,
or surface area for heat exchange. Directionally, cooler ambient
air temperatures result in a smaller condenser requirement. Higher
ambient air temperatures and lower prices of cooling water
directionally favor the use of cooling water over air as a medium
of heat exchange in condenser 400. In many cases, both evaporator
200 and condenser 400 (whether using air or cooling water), or at
least one of these, is a tubular heat exchanger with either of the
heat exchange medium or the process fluid flowing through a
plurality of tubes, having surfaces exposed to the other of the
heat exchange medium or the process fluid. To improve the heat
transfer coefficient and reduce exchanger area, the external
surfaces of the tubes may have surface enhancements such as fins
and/or recessions. For example, if tubular exchangers are used, the
plurality of tubes of the evaporator 200, condenser 400, or both
may have circumferentially extending fins having outer edges that
include a plurality of notches. Various surface enhancements of
heat exchanger tubes are known for improving heat transfer. For
example various enhancements are described in U.S. Pat. No.
4,219,078; U.S. Pat. No. 4,288,897; U.S. Pat. No. 4,191,181; U.S.
Pat. No. 4,136,427; U.S. Pat. No. 4,136,428; and U.S. Pat. No.
3,847,212, including features characteristic of tubes referred to
in the art as "High Flux" or "High Cond" tubes. Surface
enhancements and other features of evaporator and/or condenser
tubes may also beneficially reduce the temperature approach to
equilibrium of the exchanging streams, increase cycle efficiency of
the ORC, and/or reduce plot space.
[0036] Representative surface enhancements include shaped
recessions, circumferentially extending fins, axially extending
fins, or combinations of these, as described in co-pending U.S.
application Ser. No. 12/433,064, hereby incorporated by reference
with respect to its description of these surface enhancements and
other features of heat exchanger tubes. Circumferentially extending
fins may be characteristic of those used for "low finned" tubes,
with the fins having a height from about 0.76 mm (0.03 inches) to
about 3.8 mm (0.15 inches). Circumferentially extending fins
generally refer to a plurality of "plates" that are spaced apart
(e.g., uniformly or at regular intervals) along the axial direction
of the tube. The plates of circumferentially extending fins, in an
alternative embodiment, may be provided by a single, continuously
wound, helical spiral rather than discreet extensions. In either
case, the plates often each have an outer edge (or outer
perimeter), with a single tube extending through central sections
of a plurality of plates. The outer edges of the plates may be
circular or may have some other geometry, such as rectangular or
elliptical. In the case of circumferentially extending fins,
further tube surface enhancements can include one or more notches
on the outer edges of all or a portion of these fins or plates,
where the notches may be spaced apart radially about the edges, for
example, in a uniform manner or at a constant radial spacing. In
other embodiments, non-uniform radial spacing may be used. Also, it
may be desirable to align the notches axially with respect to the
immediately adjacent fins. The axial alignment of these notches,
such that they may be superimposed when viewed axially, can improve
condensate drainage.
[0037] In the case of shaped recessions on the tube surface, all or
at least a portion of the recessions may extend axially (e.g., in
the form of one or more elongated troughs) or otherwise be aligned
in one or more axially extending rows (e.g., in the form of a
plurality of discreet, smaller recessions). One or more axially
extending fins may also be used as a tube surface enhancement to
improve the heat transfer coefficient of the tubes. Combinations of
any of the surface enhancements described herein are generally all
located in the same region of the tubes used for heat transfer, for
example a region extending over a section of the length of the
evaporator and/or condenser. The surface enhancements may also be
combined with other features such as a twisted tube geometry in
this region. In a particular embodiment, for example, tubes having
a twisted tube geometry may also have circumferential fins as
surface enhancements. In a more specific embodiment, these
circumferential fins can have outer edges that include a plurality
notches. In yet more specific embodiments, the notches may be
aligned axially with respect to adjacent circumferentially
extending fins and/or they may be bent at their respective corners
outside of the plane of the circumferentially extending fins.
[0038] Alone or in combination with surface enhancements, the tubes
themselves, while extending in a generally linear direction, may
have, in at least one region of the tubes used for heat transfer as
described above, a non-linear central axis, which can provide a
non-linear internal flow path for fluid flow through the tubes. For
example, the tubes, as well as their internal central axes, may
have a wave, jagged, or helical (coiled) shape to increase pressure
drop and/or fluid mixing. Otherwise, an overall helical fluid flow
path can be provided, for example, in the case of a flattened or
eccentric profile tube (e.g., having a rectangular cross-section or
otherwise an oval-shaped or elliptical cross section) that has a
twisted tube geometry (i.e., such that a major axis of the
cross-sectional shape, for example the major axis of an ellipse,
rotates clockwise or counterclockwise along the linear direction of
the tube). In the case of a twisted tube geometry, the central axis
of fluid flow may be linear or non-linear (e.g., helical). Adjacent
tubes extending generally linearly, for example in a region of the
tubes where heat transfer takes place, but having a wave, jagged,
or helical shape or a twisted tube geometry may have a plurality of
external contact points with adjacent tubes, with these contact
points possibly being evenly spaced apart by regions where the
adjacent tubes are not in contact. Such spaced apart contact points
with one or more adjacent tubes can physically stabilize the
positions of the tubes and even avoid the need for baffles or tube
supports.
[0039] Alternatively, an enhanced condensing layer (ECL) may be
applied to the outside or external surfaces of the evaporator
and/or condenser tubes as another type of surface enhancement.
Examples of ECLs include textured surfaces, chemical coatings that
improve drop-wise condensation, nano-coatings, etc.
[0040] In addition to their exterior surfaces, the tube internal
surfaces may be modified to improve heat transfer capability. For
example, all, a majority, or at least a portion of the tubes in a
tube bundle may have internal surfaces, at least in a region of the
tubes that extends (e.g., vertically or horizontally) over a
section of the evaporator and/or condenser, onto which a coating is
bonded. If a coating is used, it is generally bonded to at least a
region of the tubes (e.g., where condensation occurs on the
external tube surfaces) having the surface enhancement(s), as
discussed above, on outer or external surfaces. A representative
internal tube surface coating comprises a porous metallic matrix
that can improve the internal heat transfer coefficient of the tube
and consequently the overall heat exchange capacity of an
evaporator and/or condenser using the tubes. Some suitable coatings
are referred to as enhanced boiling layers (EBLs), which are known
in the art for their applicability to heat transfer surfaces on
which boiling occurs, and particularly for their ability to achieve
a high degree of heat transfer at relatively low temperature
differences. An EBL often has a structure comprising a multitude of
pores that provide boiling nucleation sites to facilitate
boiling.
[0041] An EBL or other coating may be applied to the inside or
internal surfaces of tubular evaporator and/or condenser tubes. A
representative metal coating is applied as described, for example,
in U.S. Pat. No. 3,384,154. The coated metal is subjected to a
reducing atmosphere and heated to a temperature for sufficient time
so that the metal particles sinter or braze together and to the
base metal surface. An EBL may also have mechanically or chemically
formed reentrant grooves as described, for example, in U.S. Pat.
No. 3,457,990. Other known methods of applying coatings and EBLs in
particular to metal surfaces, such as the internal surfaces of
metal tubes, that may be used include those described in GB 2 034
355, U.S. Pat. No. 4,258,783, GB 2 062 207, EP 303 493, U.S. Pat.
No. 4,767,497, U.S. Pat. No. 4,846,267, and EP 112 782.
[0042] In addition to EBLs, another internal enhancement for
evaporator and/or condenser tubes involves the use of one or a
plurality of ridges, which may, for example, be in the form of a
spiral or multiple spirals. Such ridges may be used to further
improve the transfer of heat, and particularly sensible heat,
across the internal tube surface. Internal ridges may be used alone
or in combination with other features of evaporator and/or
condenser tubes as described herein. Further internal enhancements
include twisted tape, wire matrix inserts (e.g., from Cal-Gavin
Limited, Warwickshire, UK), and other in-tube heat transfer devices
that can enhance the tubeside heat transfer coefficient.
[0043] Overall, aspects of the invention are directed to methods
for generating electrical power from low grade heat sources from
refining and petrochemical processes, including overhead products
such as overhead vapors from vapor-liquid contacting apparatuses,
including distillation columns, absorbers, strippers, quenching
towers, scrubbers, etc. Rather than rejecting the low temperature
heat contained in these vapors to cooling air and/or cooling water,
the vapors may instead be used to evaporate an organic working
fluid. The vapors of the working fluid may then be sent to a
turbine to drive a generator or other load, thereby reducing
overall utility requirements and emissions, such as CO.sub.2,
otherwise generated in electricity production.
[0044] In view of the present disclosure, it will be seen that
several advantages may be achieved and other advantageous results
may be obtained. Those having skill in the art will recognize the
applicability of the methods disclosed herein to any of a number of
refining, petrochemical, and other processes. Those having skill in
the art, with the knowledge gained from the present disclosure,
will recognize that various changes could be made in the above
processes without departing from the scope of the present
disclosure. Mechanisms used to explain theoretical or observed
phenomena or results, shall be interpreted as illustrative only and
not limiting in any way the scope of the appended claims.
[0045] The following example is set forth as representative of the
present invention. The example is not to be construed as limiting
the scope of the invention as other equivalent embodiments will be
apparent in view of the present disclosure and appended claims.
Example 1
[0046] The economics of generating electricity were investigated,
using an Organic Rankine Cycle (ORC) and a low grade heat source
from a commercial refining process stream. In particular, this
stream was the main fractionation column overhead product of a
hydrocracking process, exiting the column at a temperature of
126.degree. C. (259.degree. F.). Normally, this stream is cooled
using an air cooled exchanger to reject 231 GJ/hr (55.1 Gcal/hr) of
low grade heat and achieve a cooler outlet temperature of
91.degree. C. (196.degree. F.).
[0047] Using Unisim.RTM. Design Suite R390 (Honeywell
International, Inc., Morristown, N.J.) an ORC process was
simulated, in which the overhead product was exchanged against the
organic fluid 1,1,1,3,3-pentafluoropropane (HFC-245fa) for
evaporation of this fluid and utilization of the evaporated fluid
in a turbine and generator to obtain electricity. The condensation
of this fluid in an air condenser downstream of the turbine and
pumping of the condensed fluid back to the evaporator were also
included in the simulation, according to the flowscheme shown in
FIG. 1.
[0048] Performance was evaluated for two cases, based on air inlet
temperatures to the condenser of 20.degree. C. (68.degree. F.)
(CASE I) and 25.degree. C. (77.degree. F.) (CASE II). As expected,
the simulation results showed that ambient air temperature affects
the temperature at the outlet of the turbine, the size of the air
condenser, the cycle efficiency, and the overall power recovery.
The air condenser sizes and other parameters associated with the
air condenser, as estimated for the two average air temperatures,
are shown in Table 1 below.
TABLE-US-00001 TABLE 1 Comparison of Air Condenser Parameters CASE
I CASE II Process Inlet 51.degree. C. (123.degree. F.) 54.degree.
C. (129.degree. F.) Temperature Process Outlet 30.degree. C.
(86.degree. F.) 35.degree. C. (95.degree. F.) Temperature Duty 201
GJ/hr (48.1 Gcal/hr) 203 GJ/hr (48.4 Gcal/hr) Air Inlet 20.degree.
C. (68.degree. F.) 25.degree. C. (77.degree. F.) Temperature Plot
Size 1650 m.sup.2 (17,800 ft.sup.2) 1730 m.sup.2 (18,600 ft.sup.2)
Finned Area 170,000 m.sup.2 (1,830,000 ft.sup.2) 178,000 m.sup.2
(1,920,000 ft.sup.2)
[0049] The results of the process simulation showed that, using an
ORC and the low grade heat contained in the main fractionation
column overhead product of the commercial hydrocracking process,
4.0-4.5 megawatts (MW) of electricity could be generated, depending
on the average ambient air temperature, which is the air inlet
temperature to the condenser. Moreover, since electricity is
normally consumed in the air cooled exchanger conventionally used
to reject waste heat from the overhead product, the net power
benefit of recovering this low grade heat in an ORC increases to
4.3-4.8 MW. This translates to a cost benefit of $2.5 million-$4.5
million annually (US dollars), assuming a cost of electrical power
of $0.07-$0.12 per kilowatt-hour. The relationship between the
annual benefit of the electrical power generation (as determined
using CASE I and CASE II simulations) and the cost of power is
illustrated in the bar graph of FIG. 2.
[0050] The estimated investment cost for the ORC system evaluated
in these simulations is $18 million-$20 million Subtracting the
cost of the conventional air cooled exchanger used to reject heat
from the main fractionation column overhead product (which is now
replaced by the ORC system), the net investment is reduced to $15
million-$17 million The payback period for this investment is
estimated to vary between 3.5-6.5 years, based on the cost of
electricity, as illustrated in the bar graph of FIG. 3.
* * * * *