Note: Descriptions are shown in the official language in which they were submitted.
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MIXED REFRIGERANT LIQUEFACTION PROCESS
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional Application
60/565,589, filed June 23, 2004.
BACKGROUND
Technical Field
[0002] Embodiments of the present inventions generally relate to methods for
refrigerating gas streains, such as natural gas, using mixed component
refrigerants.
Description of Related Art
[0003] Natural gas is commonly liquefied and transported to supply major
energy-
consuming nations. To liquefy natural gas, the feed gas is first processed to
remove
contaminants and hydrocarbons heavier than at least pentane. This purified
gas,
typically at an elevated pressure, is then chilled through indirect heat
exchange by one
or more refrigeration cycles. Such refrigeration cycles are costly in terms of
both
capital expenditure and operation due to the complexity of the required
equipment and
the efficiency performance of the refrigerant. There is a need, therefore, for
a method
to improve refrigeration efficiency, reduce equipment size, and reduce
operating
expenses.
SUMMARY
[0004] Methods for liquefying a natural gas stream are provided. In one
embodiment, the method includes placing a mixed component refrigerant in a
heat
exchange area with a process stream; separating the mixed component
refrigerant at
one or more pressure levels to produce a refrigerant vapor and a refrigerant
liquid;
bypassing the refrigerant vapor around the heat exchange area to a compression
unit;
and passing the refrigerant liquid to the heat exchange area.
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[0005] In another embodiment, the method includes placing a mixed component
refrigerant in a heat exchange area with a process stream; withdrawing two or
more
side streams of the mixed coinponent refrigerant from the heat exchange area;
separating the side streams of mixed coinponent refrigerant at one or more
pressure
levels to produce refrigerant vapors and refrigerant liquids; bypassing the
refrigerant
vapors around the heat exchange area to a compression unit; and passing the
refrigerant liquids to the heat exchange area.
[0006] In another embodiment, the method includes placing a mixed component
refrigerant in a heat exchange area with a process stream; separating the
mixed
component refrigerant at one or more pressure levels to produce a refrigerant
vapor
stream and a refrigerant liquid stream; bypassing the refrigerant vapor stream
around
the heat exchange area to a compression unit; passing the refrigerant liquid
stream to
the heat exchange area; and partially evaporating the refrigerant liquid
stream within
the heat exchange area to retain a liquid fraction of at least 1% by weight.
[0007] In yet another embodiment, the method includes placing a first mixed
component refrigerant in a first heat exchange area with a process stream;
separating
the first mixed component refrigerant at one or more pressure levels to
produce a
refrigerant vapor stream and a refrigerant liquid stream; bypassing the
refrigerant
vapor stream around the first heat excllange area to a compression unit;
passing the
refrigerant liquid stream to the first heat exchange area to cool the process
stream; and
placing a second mixed component refrigerant in a second heat exchange area
with
the cooled process stream to liquefy the process stream.
[0008] In yet another embodiment, the method includes placing a first mixed
component refrigerant in a first heat exchange area with a process stream;
separating
the mixed component refrigerant at one or more pressure levels to produce a
refrigerant vapor stream and a refrigerant liquid stream; bypassing the
refrigerant
vapor stream around the first heat exchange area to a compression unit;
returning the
refrigerant liquid stream to the first heat exchange area to cool the gas
stream; placing
a second mixed component refrigerant in a second heat exchange area with the
cooled
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process stream; and evaporating the second mixed component refrigerant at a
single
pressure level to liquefy the gas stream.
[0009] In still yet another embodiment, the method includes placing a mixed
component refrigerant stream in heat exchange with a process stream, the
refrigerant
stream comprising liquid refrigerant; and discontinuing the heat exchange
before the
liquid refrigerant stream is completely vaporized.
[0010] In still other embodiments, the method includes liquefying a natural
gas
stream by placing a mixed component refrigerant in a heat exchange area with a
process stream; separating the mixed component refrigerant at one or more
pressure
levels to produce a refrigerant vapor and a refrigerant liquid; passing at
least the
refrigerant liquid to the heat exchange area; and partially evaporating the
refrigerant
liquid within the heat exchange area to retain a liquid phase. In an
alternative
embodiment, the method includes placing a mixed component refrigerant in a
heat
exchange area with a process stream; withdrawing two or more side streams of
the
mixed component refrigerant from the heat exchange area; separating the side
streams
of mixed component refrigerant at one or more pressure levels to produce
refrigerant
vapors and refrigerant liquids; passing at least the refrigerant liquids to
the heat
exchange area; and partially evaporating the refrigerant liquids within the
heat
exchange area to retain a liquid phase.
DETAILED DESCRIPTION
Introduction and Definitions
[0011] A detailed description will now be provided. Each of the appended
claims
defines a separate invention, which for infringement purposes is recognized as
including equivalents to the various elements or limitations specified in the
claims.
Depending on the context, all references below to the "invention" may in some
cases
refer to certain specific embodiments only. In other cases it will be
recognized that
references to the "invention" will refer to subject matter recited in one or
more, but
not necessarily all, of the claims. Each of the inventions will now be
described in
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greater detail below, including specific embodiments, versions and examples,
but the
inventions are not limited to these embodiments, versions or examples, which
are
included to enable a person having ordinary skill in the art to make and use
the
inventions, when the information in this patent is combined with available
information and technology. Various terms as used herein are defined below. To
the
extent a term used in a claim is not defined below, it should be given the
broadest
definition persons in the pertinent art have given that term as reflected in
printed
publications and issued patents.
[0012] The terms "mixed component refrigerant" and "MCR" are used
interchangeably and mean a mixture that contains two or more refrigerant
components. Examples of the MCRs described herein are a "first MCR" and a
"second MCR."
[0013] The term "refrigerant component" means a substance used for heat
transfer
which absorbs heat at a lower temperature and rejects heat at a higher
temperature.
For example, a "refrigerant component," in a coinpression refrigeration
system, will
absorb heat at a lower temperature and pressure through evaporation and will
reject
heat at a higher temperature and pressure through condensation. Illustrative
refrigerant coinponents may include, but are not limited to, alkanes, alkenes,
and
alkynes having one to 5 carbon atoms, nitrogen, chlorinated hydrocarbons,
fluorinated
hydrocarbons, other halogenated hydrocarbons, and mixtures or combinations
thereof.
[0014] The term "natural gas" means a light hydrocarbon gas or a mixture of
two
or more light hydrocarbon gases. Illustrative light hydrocarbon gases may
include,
but are not limited to, methane, ethane, propane, butane, pentane, hexane,
isomers
thereof, unsaturates thereof, and mixtures thereof. The term "natural gas" may
further
include some level of impurities, such as nitrogen, hydrogen sulfide, carbon
dioxide,
carbonyl sulfide, mercaptans and water. The exact percentage composition of
the
natural gas varies depending upon the reservoir source and any pre-processing
steps,
such as amine extraction or desiccation via molecular sieves, for example. At
least
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one example of a "natural gas" composition is a gas containing about 55 mole%
of
methane or more.
[0015] The terms "gas" and "vapor" are used interchangeably and mean a
substance or inixture of substances in the gaseous state as distinguished from
the
liquid or solid state.
[0016] The term "partially evaporated" describes a substance which may include
a
mixture of substances that is not 100% vapor. A "partially evaporated" stream
may
have both a vapor phase and a liquid phase. At least one example of a
"partially
evaporated" streain includes a stream having a liquid phase of at least 1% by
weight,
or at least 2% by weight, or at least 3% by weight, or at least 4% by weight,
or at least
5% by weight, and the balance being the vapor phase. In one or more specific
embodiments, a "partially evaporated" stream has a liquid phase ranging from a
low
of 1% by weight, or 3% by weight, or 10% by weight to a high of 90% by weight,
or
97% by weiglit, or 99% by weight.
[0017] The tenn "heat exchange area" means any one type or combination of
similar or different types of equipment known in the art for facilitating heat
transfer.
For example, a "heat exchange area" may be contained or at least partially
contained
within one or more spiral wound type excllanger, plate-fin type exchanger,
shell and
tube type exchanger, or any other type of heat exchanger known in the art that
is
capable of witlistanding the process conditions described herein in more
detail below.
[0018] The term "compression unit" means any one type or combination of
similar or different types of compression equipment, and may include auxiliary
equipment, known in the art for compressing a substance or mixture of
substances. A
"compression unit" may utilize one or more compression stages. Illustrative
compressors may include, but are not limited to, positive displacement types,
such as
reciprocating and rotary compressors for example, and dynamic types, such as
centrifugal and axial flow compressors, for example. Illustrative auxiliary
equipment
may include, but are not limited to, suction knock-out vessels, discharge
coolers or
chillers, recycle coolers or chillers, and any combination thereof.
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Specific Embodiments
[0019] Various specific embodiments are described below, at least some of
which
are also recited in the claims. For example, at least one embodiment is
directed to a
method for liquefying a natural gas stream by placing a mixed coinponent
refrigerant
in a heat exchange area with a process stream and separating the mixed
component
refrigerant at one or more pressure levels to produce a refrigerant vapor and
a
refrigerant liquid. The refrigerant vapor bypasses around the heat exchange
area to a
compression unit, and the refrigerant liquid passes to the heat exchange area.
[0020] At least one other specific embodiment is directed to liquefying a
natural
gas stream by placing a mixed component refrigerant in a heat exchange area
with a
process stream and withdrawing two or more side streams of the mixed component
refrigerant from the heat exchange area. The side streams of mixed component
refrigerant are then separated at one or more pressure levels to produce
refrigerant
vapors and refrigerant liquids. The refrigerant vapors are bypassed around the
heat
exchange area to a coinpression unit, and the refrigerant liquids are passed
to the heat
exchange area.
[0021] Yet another specific embodiment is directed to liquefying a natural gas
stream by placing a mixed component refrigerant in a heat exchange area with a
process stream and separating the mixed component refrigerant at one or more
pressure levels to produce a refrigerant vapor stream and a refrigerant liquid
stream.
The refrigerant vapor stream bypasses around the heat exchange area to a
compression unit. The refrigerant liquid stream is passed to the heat exchange
area,
and at least partially evaporated within the heat exchange area to retain a
liquid
fraction of at least 1% by weight.
[0022] Yet another specific embodiment is directed to a method for liquefying
a
natural gas stream by placing a first mixed component refrigerant in a first
heat
exchange area with a process stream and separating the first mixed component
refrigerant at one or more pressure levels to produce a refrigerant vapor
stream and a
refrigerant liquid stream. The refrigerant vapor stream is bypassed around the
first
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heat exchange area to a compression unit, and the refrigerant liquid stream is
passed
to the first heat exchange area to cool the process stream. A second mixed
component
refrigerant is then placed in a second heat exchange area with the cooled
process
streain to liquefy the process stream.
[0023] Yet anotller specific embodiment is directed to liquefying a natural
gas
stream by placing a first mixed component refrigerant in a first heat exchange
area
with a process stream, and separating the mixed component refrigerant at one
or more
pressure levels to produce a refrigerant vapor stream and a refrigerant liquid
stream.
The refrigerant vapor stream is bypassed around the first heat exchange area
to a
compression unit, and the refrigerant liquid stream is passed to the first
heat exchange
area to cool the gas stream. A second mixed component refrigerant is placed in
a
second heat exchange area with the cooled process stream, and evaporated at a
single
pressure level to liquefy the gas stream.
[0024] Yet another specific embodiment is directed to cooling a process
streain of
natural gas by placing a mixed component refrigerant stream in heat exchange
with a
process stream. The refrigerant stream comprises liquid refrigerant, and the
heat
exchange is discontinued before the liquid refrigerant streain is completely
vaporized.
[0025] In still other embodiments, the refrigerant vapor stream or streams
need
not bypass the heat exchanger or exchangers and/or need not be sent directly
to a
compression unit. In such embodiments, the vapor stream or streams may, for
example, be returned to the heat exchanger or exchangers, or they may bypass
the
heat exchanger or exchangers and be sent to equipment other than a compression
unit.
Thus, embodiments of the present method include modifications of any
embodiment
described herein wherein the refrigerant vapor stream or streams do not bypass
the
heat exchanger or exchangers and/or are not sent directly to a compression
unit.
Such embodiments, include, for example, liquefying a natural gas stream by
placing a
mixed component refrigerant in a heat exchange area with a process stream;
separating the mixed component refrigerant at one or more pressure levels to
produce
a refrigerant vapor and a refrigerant liquid; passing at least the refrigerant
liquid to the
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heat exchange area; and partially evaporating the refrigerant liquid within
the heat
exchange area to retain a liquid phase. Such embodiments also include placing
a
mixed component refrigerant in a heat exchange area with a process stream;
withdrawing two or more side streams of the mixed coinponent refrigerant from
the
heat exchange area; separating the side streains of mixed component
refrigerant at one
or more pressure levels to produce refrigerant vapors and refrigerant liquids;
passing
at least the refrigerant liquids to the heat exchange area; and partially
evaporating the
refrigerant liquids within the heat exchange area to retain a liquid phase.
Specific Embodiments In Drawings
[0026] Specific embodiments shown in the drawings will now be described. It is
emphasized that the claims should not be read to be limited to aspects of the
drawings.
Figure 1 schematically depicts a refrigeration process utilizing an at least
partially
evaporated mixed component refrigerant to cool or liquefy a process stream or
feed
gas. Figure 2 schematically depicts a refrigeration process utilizing a heat
exchanger
having two or more heat exchange areas contained therein to cool or liquefy a
process
stream or feed gas. Figure 3 schematically depicts a refrigeration process
utilizing
two mixed component refrigerants to cool or liquefy a process stream or feed
gas.
Figure 4 schematically depicts another method for refrigerating a process
stream or
feed gas that utilizes a liquid refrigerant collection system. For simplicity
and ease of
description, these refrigeration processes will be further described herein as
they
relate to a process stream or feed gas of natural gas that is sub-cooled to
produce
liquefied natural gas ("LNG").
FIGURE 1
[0027] Figure 1 schematically depicts a refrigeration process 5 utilizing an
at least
partially evaporated mixed component refrigerant to at least cool a process
stream or
feed gas. The feed gas stream 12 is placed in heat exchange with a mixed
component
refrigerant ("MCR") stream 30 within a heat exchanger 10. As explained in more
detail below, the MCR stream 30 is expanded and cooled to remove heat from the
feed gas stream 12 within the heat exchanger 10. Although not shown,
additional
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process streams that require refrigeration can enter the heat exchanger 10.
Non-
limiting examples of such additional streams include other refrigerant
streams, other
hydrocarbon streams to be blended with the gas of stream 12 at a later
processing
stage, and streams that are integrated witli one or more fractionation
processing steps.
[0028] The heat exchanger 10, as shown in Figure 1, is a single unit
containing at
least one heat exchange area. Although not shown, but described below, the
heat
exchanger 10 may include two or more heat exchange areas, such as two, three,
four,
or five, for exainple, that may be contained within a single unit, or each
area may be
contained in a separate unit.
[0029] The feed gas streain 12 is preferably natural gas and may contain at
least
55 mole%, or at least 65 mole%, or at least 75 mole% of methane. The MCR
stream
30 may include one or more of alkanes, alkenes, and alkynes having one to 5
carbon
atoms, nitrogen, chlorinated hydrocarbons, fluorinated hydrocarbons, other
halogenated hydrocarbons, and mixtures or combinations thereof. In one or more
specific enibodiments, the MCR stream 30 is a mixture of ethane and propane.
In one
or more specific embodiments, the MCR stream 30 is a mixture of ethane,
propane
and isobutane. In one or more specific embodiments, the MCR stream 30 is a
mixture
of methane, ethane, and nitrogen.
[0030] The MCR stream 30 is cooled in the heat exchange area 10 and exits the
heat exchange area 10 as stream 40. Stream 40 is expanded using an expansion
device 45, producing a two-phase stream 50 (i.e. a stream having a vapor phase
and a
liquid phase). Illustrative expansion devices include, but are not limited to
valves,
control valves, Joule Thompson valves, Venturi devices, liquid expanders,
hydraulic
turbines, and the like. Preferably, the expansion device 45 is an
automatically
actuated expansion valve or Joule Thompson-type valve. The two-phase stream 50
is
then separated within a separator 55 to produce a vapor stream 60 and a liquid
stream
65. Preferably, the two-phase stream 50 is subjected to a flash separation.
The vapor
stream 60 bypasses the heat exchange area 10 and is sent directly to the
compression
unit 75.
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[0031] After being reduced in pressure and thus cooled, the liquid stream 65
returns to the heat exchange area 10 where it is completely evaporated or
partially
evaporated due to the heat exchange with the process gas stream 12 and the MCR
stream 30. This completely evaporated or partially evaporated stream exits the
heat
exchange area 10 as stream 70. In one or more specific embodiments, the stream
70
has a vapor fraction of at least 85% by weight, or at least 90% by weight, or
at least
99% by weiglit, and the balance is the liquid phase fraction. In one or more
specific
embodiments, the stream 70 is a vapor stream having no liquid phase. Stream 70
then
flows to the coinpression unit 75.
[0032] The compression unit 75 may utilize one or more compression stages
depending on the process conditions and requirements. Preferably, the
compression
unit 75 utilizes two or more compression stages where each stage utilizes an
inter-
stage cooler to reinove the heat of compression. The compressed stream is then
sent
to the heat exchange area 10 as streain 30. An exemplary compression unit is
discussed in more detail below.
[0033] By sending the vapor stream 60 around the heat exchange area 10
directly
to the compression unit 75 (i.e. bypassing the refrigerant vapor around the
heat
exchange area to the compression unit), certain distribution problems
associated with
two-phase refrigerants may be avoided. The term "two-phase refrigerant" refers
to a
refrigerant having at least some of the refrigerant in the liquid phase and at
least 10%
by volume in the vapor phase. Two-phase distribution may result in reduced
liquefied
gas production and lost revenue because of the inadequate distribution of the
two-
phase refrigerant within the heat exchange area. The inadequate distribution
of the
two-phase refrigerant within the heat exchange area results in inefficient
heat transfer
because the vapor phase of the two-phase refrigerant occupies more volume
within
the heat exchange area compared to the liquid phase. Since the vapor phase
contributes very little to the heat exchange in comparison to the evaporating
liquid
phase, the cooling capacity of the refrigerant is compromised.
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[0034] Furthermore, the hydraulic design of a system that can effectively
distribute the two-phase refrigerant to the heat exchanger or exchangers can
be
expensive in both engineering time and purchased equipment. The behavior of
such
designs are more difficult to predict in situations that stray too far from
the design
conditions in terms of temperature, pressure, and/or flow rate. The benefits
achieved
according to the one or more embodiments described herein are particularly
applicable to arrays of heat exchangers in a parallel arrangement that are fed
refrigerant from a common source because the vapor phase has been removed
eliminating this distribution consideration.
FIGURE 2
[0035] Figure 2 schematically depicts a refrigeration process 100 utilizing a
heat
exchanger having more than one heat exchange area contained therein to cool or
liquefy a process stream or feed gas. The refrigeration process 100 utilizes a
heat
exchanger 200 having two or more heat exchange areas contained therein, such
as
three areas as shown in Figure 2, and a MCR compression unit 300. A feed gas
stream 102 is cooled against a mixed component refrigerant ("MCR") within the
heat
exchanger 200. Although not shown, additional process streams that require
refrigeration can enter the heat exclianger 200. Non-limiting exainples of
such
additional streams include other refrigerant streams, other hydrocarbon
streains to be
blended with the gas of stream 102 at a later processing stage, and streams
that are
integrated with one or more fractionation processing steps.
[0036] The composition of the feed gas stream 102 depends on its source
reservoir, but can include up to 99 mole% of methane, up to 15 mole% of
ethane, up
to 10 mole% of propane, and up to 30 mole% of nitrogen, for example. In one
specific embodiment, the feed gas stream 102 may contain at least 55 mole%, or
at
least 65 mole%, or at least 75 mole% by volume of inethane. In another
specific
embodiment, the feed gas stream 102 may also contain up to 1 mole%, or up to 2
inole%, or up to 5 mole% of non-hydrocarbon compounds, such as water, carbon
dioxide, sulfur-containing compounds, mercury, and combinations thereof. In
one or
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more specific embodiments, the feed gas stream 102 may be subjected to a
purification process (not shown) to strip or otherwise remove a majority, if
not all, of
these non-hydrocarbon compounds from the feed gas stream 102 prior to entering
the
heat exchanger 200.
[0037] In certain embodiments, the feed gas stream 102 enters the heat
exchanger
200 at a temperature within a range of from a low of 15 C, or 25 C, or 35 C to
a high
of 40 C, or 45 C, or 55 C, and at a pressure within a range of from a low of
4,000
kPa, or 6,000 kPa, or 7,000 kPa to a high of 8,500 kPa, or 10,000 kPa, or
12,000 kPa.
The feed gas stream 102 exits the heat exchanger 200 as a chilled streain 104.
The
chilled streain 104 exits the heat exchanger 200 at a temperature within a
range of
from a low of -70 C, or -80 C, or -100 C to a high of -60 C, or -50 C, or -35
C. For
example, the chilled stream 104 can exit the heat exchanger 200 at a
teinperature of
about -70 C to about -75 C.
MCR
[0038] The mixed component refrigerant ("MCR") is preferably a mixture of
ethane, propane and isobutane. The MCR may contain between about 20 mole% and
80 mole% of ethane, between about 10 mole% and 90 mole% of propane, and
between about 5 mole% and 30 mole% of isobutane. In one or more specific
embodiments, the concentration of ethane within the first MCR ranges from a
low of
20 mole%, or 30 mole%, or 40 mole% to a high of 60 mole%, or 70 mole%, or 80
mole%. In one or more specific embodiments, the concentration of propane
within
the MCR ranges from a low of 10 mole%, or 20 mole%, or 30 mole% to a high of
70
mole%, or 80 mole%, or 90 mole%. In one or more specific embodiments, the
concentration of isobutane within the MCR ranges from a low of 3 mole%, or 5
mole%, or 10 mole% to a high of 20 mole%, or 25 mole%, or 30 mole%.
[0039] In one or more specific embodiments, the MCR has a molecular weight of
about 32 to about 45. More preferably, the molecular weight of the MCR ranges
from
a low of 32, or 34, or 35 to a high of 42, or 43, or 45. Further, the molar
ratio of the
MCR to the feed gas stream 102 ranges from a low of 1.0, or 1.2, or 1.5 to a
high of
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1.8, or 2.0, or 2.2. In one or more specific embodiments, the molar ratio of
the MCR
to the feed gas stream 102 is at least 1.0 , or at least 1.2, or at least 1.5.
HEAT EXCHANGER
[0040] Considering the heat exchanger 200 in more detail, the MCR enters the
heat exchanger 200 as stream 202. At least a portion of stream 202 is
withdrawn from
a first heat exchange area of the heat exchanger 200 as a side stream 203. The
side
streain 203 is expanded to a first pressure using an expansion device 205,
producing a
two-phase stream 207 (i.e. a stream having a vapor phase and a liquid phase).
In one
or more specific embodiments, this first pressure ranges from a low of 800
kPa, or
1,200 kPa, or 1,500 kPa to a high of 1,900 kPa, or 2,200 kPa, or 2,600 kPa.
Accordingly, the temperature of the expanded stream 207 ranges from a low of 0
C,
or 3 C, or 4 C to a high of 6 C, or 10 C, or 15 C. Preferably, the side stream
203 is
expanded to a pressure of from 1,600 kPa to 1,800 kPa and a temperature of
from 4 C
to 6 C.
[0041] The two-phase stream 207 is then separated within a separator 210 to
produce a vapor stream 214 and a liquid stream 212. Preferably, the two-phase
stream 207 is subjected to a flash separation. The vapor stream 214 bypasses
the heat
exchanger 200 and is sent directly to the compression unit 300. By sending the
vapor
stream 214 around the heat exchanger 200 directly to the coinpression unit 300
(i.e.
bypassing the refrigerant vapor around the heat exchange area to the
compression
unit), the certain distribution problems associated with two-phase
refrigerants noted
above may be avoided. .
[0042] After being reduced in pressure and thus cooled, the liquid stream 212
returns to the heat exchanger 200 where it is completely evaporated or
partially
evaporated due to the heat exchange within the heat exchanger 200. This
completely
evaporated or partially evaporated stream exits the heat exchanger 200 as
stream 216.
In one or more specific embodiments, the stream 216 has a vapor fraction of at
least
85% by weight, or at least 90 % by weight, or at least 99% by weight, and the
balance
is the liquid phase fraction. In one or more specific embodiments, the stream
216 is a
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vapor stream having no liquid phase (i.e. completely evaporated). Stream 216
may be
combined as shown in Figure 1 with the vapor stream 214 from the separator 210
to
form a recycle strea.in 218 that flows to the compression unit 300.
[0043] At least another portion of stream 202 is withdrawn from a second heat
exchange area of the heat exchanger 200 as a side streain 213. The side stream
213 is
expanded to a second pressure using an expansion device 215, producing stream
217.
The stream 217 has a vapor phase and a liquid phase. In one or more specific
embodiments, this second pressure ranges from a low of 250 kPa, or 400 kPa, or
500
kPa to a high of 600 kPa, or 700 kPa, or 850 kPa. Accordingly, the temperature
of the
expanded stream 217 ranges from a low of -60 C, or -50 C, or -40 C to a high
of -
30 C, or -20 C, or -10 C. Preferably, the side stream 213 is expanded to a
pressure of
from 550 kPa to 570 kPa and a temperature of from -35 C to -45 C.
[0044] The two-phase stream 217 is then separated within a separator 220 to
produce a vapor stream 224 and a liquid stream 222. Preferably, the two-phase
stream 217 is subjected to a flash separation. The vapor stream 224 bypasses
the heat
exchanger 200 and is sent directly to the compression unit 300. The liquid
stream
222, having been reduced in pressure and thus cooled, returns to the heat
exchanger
200 where it is completely evaporated or partially evaporated due to the heat
exchange within the heat exchanger 200. This completely evaporated or
partially
evaporated streain exits the heat excllanger 200 as stream 226. In one or more
specific embodiments, stream 226 has a vapor fraction of at least 85% by
weight, or at
least 90% by weight, or at least 99% by weight, and the balance is the liquid
phase
fraction. Stream 226 may be combined as shown in Figure 1 with the vapor
stream
224 from the separator 220 to form a recycle stream 228 that flows to the
compression
unit 300.
[0045] Yet another portion of stream 202 is withdrawn from a third heat
exchange
area of the heat exchanger 200 as a side stream 223. The side stream 223 is
expanded
to a third pressure using an expansion device 225, producing stream 227 that
has a
vapor phase and a liquid phase. In one or more specific embodiments, this
third
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pressure ranges from a low of 80 kPa, or 120 kPa, or 150 kPa to a high of 180
kPa, or
200 kPa, or 250 kPa. Accordingly, the temperature of the expanded stream 227
ranges from a low of -110 C, or -90 C, or -80 C to a high of -60 C, or -50 C,
or -
30 C. Preferably, the side stream 223 is expanded to a pressure of from 160
kPa to
180 kPa and a temperature of from -65 C to -75 C.
[0046] The two-phase stream 227 is then separated within a separator 230 to
produce a flash vapor stream 234 and a saturated liquid stream 232.
Preferably, the
two-phase stream 227 is subjected to a flash separation. The vapor stream 234
bypasses the heat exchanger 200 and is sent directly to the compression unit
300. The
saturated liquid stream 232, having been reduced in pressure and thus cooled,
returns
to the heat exchanger 200 where it is completely evaporated or partially
evaporated
due to the heat exchange within the heat exchanger 200. This completely
evaporated
or partially evaporated refrigerant exits the heat exchanger 200 as stream
236. In one
or more specific embodiments, stream 236 has a vapor fraction of at least 85%
by
weiglit, or at least 90% by weight, or at least 99% by weight, and the balance
is the
liquid phase fraction. Stream 236 may be combined as shown in Figure 2 with
the
vapor stream 234 from the separator 230 to form a recycle stream 238 that
flows to
the compression unit 300.
[0047] In the one or more specific embodiments described above, the expansion
device may be any pressure reducing device. Illustrative expansion devices
include,
but are not limited to valves, control valves, Joule Thompson valves, Venturi
devices,
liquid expanders, hydraulic turbines, and the like. Preferably, the expansion
devices
205, 215, 225 are automatically actuated expansion valves or Joule Thompson-
type
valves.
[0048] As described above, the vapor streams 214, 224, 234 bypass the heat
exchanger 200 and are sent directly to the compression unit 300. This bypass
configuration avoids the distribution problems associated with two-phase
refrigerants
as explained above. Furthermore, the partially evaporated refrigerant exiting
the heat
exchange area with two phases has been configured to reduce mechanical stress
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16
within the heat exchange area. Mechanical stress may be a product of a rapid
temperature transition across the volume occupied by a liquid phase and the
volume
occupied by a vapor phase. The temperature transition from the volume of the
liquid
or two-phase fluid portion to the volume of the vapor portion may result in
stress
fracture during startups, shutdowns, or upsets, or may result in fatigue
failure of the
exchanger. Therefore, configuring the refrigerant flow conditions allows for
incomplete vaporization of the refrigerant liquid streams 212, 222 and 232
without the
inherent effects of mechanical stress caused by a rapid temperature gradient.
To
transition from a system in which the refrigerant is fully evaporated to a
system in
which the refrigerant is partially evaporated, the flow rate may be increased,
the
evaporation pressure may be changed, the refrigerant composition may be
changed to
include more components with higher boiling points, or a coinbination of any
of these
design parameters.
MCR COMPRESSION UNIT 300
[0049] The MCR compression unit 300 includes one or more different pressure
levels. Preferably, the suction of each compression stage corresponds to the
pressure
levels of the recycle streams 218, 228, 238. In at least one specific
embodiment, the
first compression stage includes a suction knock-out vessel 310 and a
compressor
320. In at least one specific embodiment, the second compression stage
includes a
suction knock-out vessel 330, a compressor 340, and a discharge cooler or
condenser
350. In at least one specific embodiment, the third compression stage includes
a
suction knock-out vessel 360, a compressor 370, and a discharge cooler 380. In
at
least one specific embodiment, the compression unit 300 further includes a
final
cooler or condenser 390.
[0050] The coolers 350, 380, and 390 may be any type of heat exchanger
suitable
for the process conditions described herein. Illustrative heat exchangers
include, but
are not limited to, shell-and-tube heat exchangers, core-in-kettle exchangers
and
brazed aluininum plate-fin heat exchangers. In one or more specific
embodiments,
plant cooling water is used as the heat transfer medium to cool the process
fluid
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17
within the coolers 350, 380, and 390. In one or more specific embodiments, air
is
used as the heat transfer medium to cool the process fluid within the coolers
350, 380,
and 390. Furthermore, in one or more of the embodiments described above, the
bypassed flash vapor streams 214, 224, 234, cool the at least partially
evaporated
refrigerant streams 216, 226, 236 exiting the heat exchanger 200. As such, the
combined streams 218, 228, 238, which recycle to the suction to the
compression unit
300, are lower in temperature thereby reducing the duty requirements of the
discharge
coolers 350, 380, and 390.
[0051] Referring to the first compression stage in more detail, stream 322
exits
the first stage 320. In one or more specific embodiments, the pressure of
stream 322
ranges from a low of 200 kPa, or 300 kPa, or 400 kPa to a high of 600 kPa, or
700
kPa, or 800 kPa. The temperature of stream 322 ranges from a low of 5 C, or 10
C,
or 15 C to a high of 20 C, or 25 C, or 30 C.
[0052] Referring to the second coinpression stage, stream 342 exits the second
stage 340 and is cooled within the discharge cooler 350 to produce streain
352. In
one or more specific embodiments, the pressure of stream 342 ranges from a low
of
800 kPa, or 1,200 kPa, or 1,400 kPa to a high of 1,800 kPa, or 2,000 kPa, or
2,500
kPa. In one or more specific embodiments temperature of stream 352 ranges from
a
low of 15 C, or 25 C, or 35 C to a high of 40 C, or 45 C, or 55 C.
[0053] Referring to the third compression stage, stream 372 exits the third
stage
370 and is cooled within the discharge cooler 380 to produce stream 382. In
one or
more specific embodiments, the pressure of stream 372 ranges from a low of
1,600
kPa, or 2,400 kPa, or 2,900 kPa to a high of 3,500 kPa, or 4,000 kPa, or 5,000
kPa.
The temperature of stream 372 ranges from a low of 40 C, or 50 C, or 60 C to a
high
of 100 C, or 120 C, or 150 C. In one or more specific embodiments, the
temperature
of stream 382 ranges from a low of 0 C, or 10 C, or 20 C to a high of 40 C, or
50 C,
or 60 C.
[0054] In one or more certain embodiments, stream 382 flows to the condenser
390 to produce streain 392. The temperature of stream 392 ranges from a low of
0 C,
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18
or 10 C, or 20 C to a high of 40 C, or 45 C, or 55 C. In one or more certain
embodiments, stream 392 flows to a surge vessel 295 to provide residence time
for
operability considerations as the high pressure liquid refrigerant enters heat
exchanger
200 as stream 202.
FIGURE 3
[0055] The refrigeration or liquefaction process 100 may further utilize a
second
heat exchanger 400 and a second MCR compression unit 500 as shown in Figure 3.
Figure 3 schematically depicts a refrigeration process that utilizes two mixed
component refrigerants in separate heat exchangers to cool or liquefy a
process stream
or feed gas. However, the first heat exchanger 200 and the second heat
excllanger
400 may be contained within a common unit. In either case, the first heat
exchanger
200 and the second heat exchanger 400 are preferably arranged in series as
shown.
[0056] The chilled stream 104 leaving the first heat exchanger 200 is sub-
cooled
against a second mixed component refrigerant ("second MCR") within the second
heat exchanger 400. The chilled stream 104 exits the second heat exchanger 400
as a
liquefied stream 106. In certain embodiments, the liquefied stream 106 exits
the heat
exchanger 400 at a temperature within a range of from a low of -220 C, or -180
C, or
-160 C to a high of -130 C, or -110 C, or -70 C. In one specific embodiment,
the
liquefied stream 106 exits the heat exchanger 400 at a temperature of about -
145 C to
about -155 C. In certain embodiments, the liquefied stream 106 exits the heat
exchanger 400 at a pressure within a range of from a low of 3,900 kPa, or
5,800 kPa,
or 6,900 kPa to a high of 9,000 kPa, or 10,000 kPa, or 12,000 kPa.
SECOND MCR
[0057] In one or more specific embodiments, the second mixed component
refrigerant ("second MCR") may be the same as the first mixed component
refrigerant
("first MCR"). In one or more specific embodiments, the second MCR may be
different. For example, the second MCR may be a mixture of nitrogen, methane,
and
ethane. In one or more specific embodiments, the second MCR may contain
between
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19
about 5 mole% and 20 mole% of nitrogen, between about 20 mole% and 80 mole% of
methane, and between about 10 mole% and 60 mole% of ethane. In one or more
specific embodiments, the concentration of nitrogen within the second MCR
ranges
from a low of 5 mole%, or 6 mole%, or 7 mole% to a high of 15 mole%, or 18
mole%, or 20 mole%. In one or more specific embodiments, the concentration of
methane within the second MCR ranges from a low of 20 mole%, or 30 mole%, or
40
mole% to a high of 60 mole%, or 70 mole%, or 80 mole%. In one or more specific
embodiments, the concentration of ethane within the second MCR ranges from a
low
of 10 mole%, or 15 mole%, or 20 mole% to a high of 45 mole%, or 55 mole%, or
60
mole%.
[0058] The molecular weight of the second MCR ranges from a low of 18, or 19,
or 20 to a high of 25, or 26, or 27. In one or more specific embodiments, the
second
MCR has a molecular weight of about 18 to about 27. Further, the molar ratio
of the
second MCR to the chilled stream 104 ranges from a low of 0.5, or 0.6, or 0.7
to a
high of 0.8, or 0.9, or 1Ø In one or more specific embodiments, the molar
ratio of
the second MCR to the chilled stream 104 is at least 0.5, or at least 0.6, or
at least 0.7.
[0059] The second MCR may be fed to the first heat exchanger 200 via stream
402 to pre-cool or condense the second MCR prior to entering the second heat
exchanger 400. The stream 402 is cooled within the first heat exchanger 200 by
indirect heat transfer with the first MCR. The stream 402 has a pressure
within the
range of from a low of 2900 kPa, or 4300 kPa, or 5500 kPa to a high of 6400
kPa, or
7500 kPa, or 9000 kPa. The stream 402 has a temperature within the range of
from a
low of 0 C, or 10 C, or 20 C to a high of 40 C, or 50 C, or 70 C.
[0060] The second MCR exits the first heat exchanger 200 as streain 404. In
one
or more specific embodiments, the stream 402 is completely condensed within
the
first heat exchanger 200 to a liquid stream 404 having no vapor fraction. In
one or
more specific embodiments, the stream 402 is partially condensed by indirect
heat
transfer with the first MCR such that the stream 404 has a liquid fraction of
at least
85% by weight, or at least 90% by weight, or at least 95% by weight, or at
least 99%
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by weight. In one or more specific embodiments, the stream 404 has a pressure
within the range of from a low of 2,500 kPa, or 4,000 kPa, or 5,000 kPa to a
high of
6,000 kPa, or 7,000 kPa, or 9,000 kPa. In one or more specific embodiments,
the
stream 404 has a temperature within the range of from a low of -110 C, or -90
C, or -
80 C to a high of -60 C, or -50 C, or -30 C.
[0061] In one or more specific embodiments, additional process streams that
require refrigeration can enter the heat exchanger 400. Non-limiting examples
of
such additional streams include other refrigerant streams, other hydrocarbon
streams
to be blended with the gas of stream 102 at a later processing stage, and
streams that
are integrated witli one or more fractionation processing steps.
SECOND HEAT EXCHANGER
[0062] Considering the second heat exchanger 400 in more detail, the second
MCR that has been cooled and at least partially condensed, if not completely
condensed, within the first heat exchanger 200, is collected in a surge vessel
406 and
fed to the second heat exchanger 400 as stream 410. The second MCR exits the
second heat exchanger 400 as stream 415. In one or more specific embodiments,
the
stream 415 has a pressure within the range of from a low of 2,800 kPa, or
4,200 kPa,
or 5,500 kPa to a high of 6,200 kPa, or 7,000 kPa, or 8,500 kPa. In one or
more
specific embodiments, the stream 415 has a temperature within the range of
from a
low of -230 C, or -190 C, or -170 C to a high of -140 C, or -120 C, or -70 C.
[0063] In one or more specific embodiments, the stream 415 exiting the second
heat exchanger 400 is reduced in pressure (i.e. expanded) using an expansion
device
450. The stream 415 is then further reduced in pressure (i.e. expanded) using
an
expansion device 420 to produce stream 425. As mentioned above, the expansion
devices 420, 450 may be any pressure reducing device including, but not
limited to
valves, control valves, Joule Thompson valves, Venturi devices, liquid
expanders,
hydraulic turbines, and the like. Preferably, the expansion device 420 is an
automatically actuated expansion valve or Joule Thompson-type valve.
Preferably,
the expansion device 450 is a liquid expander or a hydraulic turbine. In one
or more
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21
specific embodiments, stream 425 has a pressure within the range of from a low
of
200 kPa, or 300 kPa, or 400 kPa to a high of 500 kPa, or 600 kPa, or 700 kPa;
a
temperature within the range of from a low of -250 C, or -200 C, or -170 C to
a high
of -140 C, or -110 C, or -70 C. Preferably, stream 425 is expanded to a
pressure of
from 435 kPa to 445 kPa and a temperature of from -150 C to -160 C.
[0064] After isenthalpic expansion within the expansion device 420, the stream
425 is completely evaporated or partially evaporated within the second heat
excllanger 400 and exits the second heat excllanger 400 as stream 430. In one
or
more specific embodiinents, the stream 425 is completely evaporated or
partially
evaporated at a single pressure level within the second heat exchanger 400. In
one or
more specific embodiments, the stream 425 is completely evaporated (i.e. all
vapor
phase) at a single pressure level within the second heat exchanger 400. In one
or
more specific einbodiments, the single pressure level within the second heat
exchanger 400 is maintained within the range of from a low of 150 kPa, or 250
kPa,
or 350 kPa to a high of 400 kPa, or 500 kPa, or 600 kPa. Preferably, the
single
pressure level within the second heat exchanger 400 is between about 350 kPa
and
about 450 kPa.
SECOND MCR COMPRESSION UNIT
[0065] The stream 430 is then sent to a second compression unit 500. The
compression unit 500 may include one or more compression stages depending on
the
process requirements. In one or more specific embodiments, the compression
unit
500 includes two coinpression stages as shown in Figure 3. For example, the
compression unit 500 has a first compression stage 510 and a second
compression
stage 520.
[0066] In operation, the stream 430 flows through a suction knock-out vessel
510A where a vapor stream continues to the first compression stage 510 and is
cooled
in after-cooler 515 to produce stream 512. In one or more specific
embodiments,
stream 512 has a pressure within the range of from a low of 1,900 kPa, or
2,800 kPa,
or 3,500 kPa to a high of 4,000 kPa, or 4,800 kPa, or 5,800 kPa; and a
temperature
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within the range of from a low of 15 C, or 25 C, or 30 C to a high of 40 C, or
50 C,
or 60 C.
[0067] Stream 512 flows through a suction knock-out vessel 520A where a vapor
stream continues to the second compression stage 520 and is cooled. In one or
more
specific embodiments, the vapor stream 522 leaving the second compression
stage
520 has a pressure within the range of from a low of 2,900 kPa, or 4,300 kPa,
or 5,200
kPa to a high of 6,400 kPa, or 7,500 kPa, or 9,000 kPa; and a temperature
within the
range of from a low of 15 C, or 25 C, or 35 C to a high of 40 C, or 45 C, or
60 C.
The vapor stream 522 is then cooled within the after-cool 525 and recycled to
the first
heat exchanger 200 as stream 402.
FIGURE 4
[0068] Figure 4 schematically depicts another method for refrigerating a
process
stream or feed gas that utilizes a liquid refrigerant collection system. As
shown in
Figure 4, liquid refrigerant collected from the separators 510A and 520B may
be in
fluid communication with a pump 530. The pump 530 returns this liquid
refrigerant
to the process via stream 532. This allows an effective and efficient way to
deal with
the mixed component refrigerant that partially evaporates within the heat
exchange
area. Alternatively, the collected liquid refrigerant from the separators 510A
and
520B may be drained and disposed. Similarly, although not shown, the knock-out
drums of the compression unit 300 (e.g. drums 310, 330, and 360) may be
equipped
with a similar liquid refrigerant collection system.
