Note: Descriptions are shown in the official language in which they were submitted.
CA 02614414 2011-07-19
52900-73
NGL RECOVERY METHODS AND CONFIGURATIONS
Field of The Invention
The field of the invention is gas processing, and especially gas
processing for ethane recovery and/or propane recovery from various natural
gas
sources.
Background of The Invention
Expansion of gas is often used as a source of refrigeration in various
processes for recovery of hydrocarbon liquids from feed gases, and
particularly for
recovery of ethane and propane from high pressure feed gas. However, and
especially where feed gas pressure is relatively low or contains significant
quantities
of propane and/or heavier components, extra refrigeration (e.g., propane
refrigeration) may also be required.
In most known expander-based plants for natural gas liquids (NGL)
recovery, the feed gas is cooled and partially condensed, typically by heat
exchange
with demethanizer overhead vapor, side reboilers, and/or external propane
refrigeration. The so formed liquid portion (that contains less volatile
components) is
then separated, while the vapor portion is usually split in two portions. One
portion is
then chilled and fed to the upper section of a demethanizer, while the other
portion is
letdown in pressure in a turbo-expander and fed to the mid section. These
configurations are often used for feed gas with low CO2 (e.g., less than 2%)
and
relatively high C3+ (e.g., greater than 5%) content, and are generally not
practical nor
economically viable for feed gas with high CO2 content (greater than 2%) and
low
C3+ content (less than 2%, and more typically less than 1 %).
However, the residue gas from the fractionation column in commonly
known plants often still contains significant amounts of ethane and propane
that could
be further recovered if chilled to an even lower temperature, or subjected to
another
1
CA 02614414 2011-07-19
52900-73
rectification stage. Lower temperature is often achieved using a higher
expansion
ratio across the turbo-expander, which lowers the column pressure and
temperature.
However, even with significantly lowered temperatures in commonly known
plants,
high ethane recovery in excess of 90% is often not achievable due to CO2
freezing in
the demethanizer. Moreover, such operation is also typically economically not
justifiable due to the high capital cost of the compression equipment and
energy
costs. Thus, based on most known configurations, ethane recovery is
la
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
typically limited to the 70% to 80% range due to CO2 freezing problems and
economic
constraints.
Exemplary NGL recovery plants with a turbo-expander, feed gas chiller,
separators,
and a refluxed fractionation column are described, for example, in U.S. Pat.
No. 4,854,955 to
Campbell et al. Here, a configuration is employed for moderate ethane recovery
with turbo-
expansion, in which the demethanizer column overhead vapor is cooled and
condensed by an
overhead exchanger using refrigeration generated from feed gas chilling. Such
an additional
cooling step condenses most of the NGL components (especially propane and
heavier) from
the column overhead gas, which is later recovered in a separator, and returned
to the column
as reflux. Unfortunately, while high propane recovery can be achieved with
such a process,
ethane recovery is often moderate (typically at about 70% to 80%). In another
example, as
taught in U.S. Pat. No. 6,453,698 by Jain et al., the inventors describe a
configuration that
withdraws an intermediate vapor stream from the column followed by
compression, chilling
with cold residue gas to produce a lean vapor that is further cooled with the
residue gas and
eventually condensed to generate a lean reflux to the column. Although high
ethane recovery
can be achieved with such a process, the cost of additional equipment such as
additional
compressors, separators, and heat exchangers is usually difficult to justify.
Further examples include two-column configurations, with the first column
acting as a
reflux absorber while the second column is operated as a demethanizer or
deethanizer. Such
plants generally employ an additional fractionation column that receives
multiple vapor
streams in distinct positions, which allows the column to produce top reflux
to the reflux
absorber. For example, U.S. Pat. Nos. 5,953,935 to Sorensen describes the use
of overhead
vapor from the second column to chill the overhead gas from the first column
generating a
lean reflux. Similarly, as disclosed in U.S. Pat. No. 5,771,712 to Campell,
the overhead liquid
from the first distillation column is employed as a lean reflux to the second
column.
In yet another approach, as described in U.S. Pat. No. 6,363,744 to Finn et
al., residue
gas is recycled from the residue gas compressor, subsequently chilled with the
column
overhead vapor, and used as a lean reflux to the demethanizer. Alternatively,
residue gas
recycling as described in U.S. Pat No. 4,687,499 to Aghili, is commonly used
to generate
refrigeration and a methane rich lean reflux when high ethane recovery is
required.
2
CA 02614414 2010-02-03
52900-73
While such plants improve ethane and propane recovery to at least some degree,
they
also require very low temperatures (-100 F or lower) in the dernethanizer to
ensure high
ethane recovery. However, due to the very low temperatures, the methane
content in the tray
liquid is also very high, which invariably causes significant internal recycle
of the methane
component in flue lower section. Consequently, such configurations are
inefficient as they
require high reboiler duties and refrigeration requirements. Moreover, due to
the relatively
low temperatures in the lower section, CO2 freezing is frequently encountered,
which
presents a significant obstacle for continuous operation. Alternatively, CO2
concentration
must be reduced in the feed gas to a tolerable limit, which typically adds
significant expense.
Thus, numerous attempts have been made to improve the efficiency and economy
of
processes for separating and recovering ethane and heavier natural gas liquids
from natural
gas and other sources. However, all or almost all of them fail to achieve
economic operation
for high ethane recovery. Therefore, there is still a need to provide improved
methods and
configurations for flexible natural gas liquids recovery.
Summary of the Invention
Aspects of the present invention are directed to configurations and methods
for
recovery of NGL from feed gases with a CO2 content of about >_ 2%, and
especially to those
configurations in which the recovery of ethane, propane, and heavier
components can be
variable. Especially contemplated configurations are those that allow high
recovery
(i.e., greater than 80%) of ethane while preventing at the same time freezing
of C02-
In one aspect of the inventive subject matter, a method of operating a plant
for NGL
recovery from a feed gas includes a step of separating the feed gas in a
refluxed column to
thereby produce a residue gas, and using a portion of the compressed residue
gas after
cooling as a first reflux. Ina further step, a portion of the feed gas is
expanded upstream. of
<<s the column to thereby form a second reflux, and in yet another step, the
temperature of the
column is controlled by using a control circuit that controls a temperature of
an expander
inlet stream that is fed to the column after expansion and/or by using an
intermediate reflux
condenser disposed. between an upper section and a lower section of the column
that
maintains temperature of the column above a temperature sufficient to prevent
carbon dioxide
freezing.
3
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
In especially preferred methods, the feed gas comprises at least 2% C02, and
the
column is operated such that at least 90% of propane and heavier components,
and variable
amounts of ethane up to 90% are recovered from the feed gas. In some aspects
of
contemplated methods, the column temperature is controlled using an
intermediate reflux
condenser, and the upper section of the column generates a liquid intermediate
product that is
used to cool the feed gas. Where desirable, the feed gas is cooled and
separated in a separator
into a vapor portion and a liquid portion, wherein a portion of the vapor is
further cooled and
expanded to form the second reflux.
Alternatively, the column temperature is controlled using a control circuit,
wherein
the column is a demethanizer that produces a bottom product, which is
preferably fed to a
second column that is operated at a lower pressure. The second column will
typically
produce a NGL bottom product and a methane and ethane rich overhead product.
In such
methods, it is generally preferred to route at least part of the methane and
ethane rich
overhead product back to the demethanizer (after appropriate compression).
Moreover, such
methods will also typically include a step of splitting the feed gas into
three streams, wherein
the first stream is sub-cooled (below the bubble point temperature of the gas)
to a first
temperature before entering the column, wherein the second stream is cooled
and expanded in
a turbo expander before entering the column at a second temperature (typically
a higher
temperature than the first temperature), and wherein the third stream bypasses
the feed
exchanger and feeds the turbo expander suction for temperature control by the
control circuit.
Therefore, in another aspect of the inventive subject matter, contemplated
plants will
include a column having an intermediate reflux condenser (located between a
upper
fractionation section and a lower rectification section of the column) that is
configured to
operate at a temperature of between about -20 F and about -40 F. The column
is further
configured to receive a first and a second reflux stream and to produce an
overhead product.
A conduit is preferably coupled to the column such that a liquid stream is fed
from the upper
section to the lower section via a heat exchanger that is configured such that
the liquid stream
is heated in the heat exchanger. Additionally, contemplated plants will
include a recycle
circuit that is configured to provide a portion of the overhead product as the
first reflux
30. stream to the column. In most cases, the column is configured to provide
an ethane side
product stream and a propane plus liquids bottom product stream.
4
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
Most preferably, the heat exchanger in such plants is a feed gas exchanger
that is
configured to heat the liquid stream to a temperature that is suitable for at
least partial
removal of methane and ethane from C3+ components in the lower section, and/or
to cool the
portion of the overhead product. A second heat exchanger may also be included
that is
configured to cool a vapor portion of a feed gas using refrigeration cold of
the overhead
product to thereby produce a cooled vapor portion. Where desirable, an
expansion device
may be added that is configured to reduce the temperature of the cooled vapor
portion to
thereby form the second reflux stream.
Alternatively, in a further aspect of the inventive subject matter,
contemplated plants
will include a first column configured to receive a first and a second reflux
stream and further
configured to receive an expanded feed gas stream. A temperature control unit
is typically
thermally coupled to the first column and configured to control a temperature
in the first
column, and a heat exchanger is configured to cool a first and a second
portion of the feed
gas to thereby form the second reflux stream and a cooled second portion of
the feed gas,
respectively. A bypass valve is generally configured to control flow volume of
a third portion
of the feed gas to the cooled second portion of the feed gas, while a control
system is
provided to adjust the flow volume of the third portion of the feed gas as a
function of the
temperature in the first column.
Most preferably, a recycle circuit is added that is configured to provide a
portion of a
compressed overhead product of the first column back to the first column as
the first reflux
stream. A second column and an expansion device are preferably fluidly coupled
with the
recycle circuit, wherein the first column is configured to produce a bottom
product, wherein
the expansion device is configured to receive expanded first column bottom
product, and
wherein the second column is configured to produce a propane plus liquids
bottom product
and an ethane overhead product. In such plants, it is generally preferred that
the expansion
device is configured to reduce pressure of the first column bottom product by
at least 50-400
psi. Therefore, the columns are typically operated at a pressure differential
of between about
50 psi and about 400 psi, wherein the first column is typically operated at a
pressure between
about 450 - 700 psig, and the second column is typically operated at a
pressure between
3o about 300 - 450 psig. It is further preferred that a further recycle
circuit provides at least a
portion of the methane and ethane rich overhead product back to the first
column.
5
CA 02614414 2010-02-03
52900-73
According to one aspect of the present invention, there is provided a
method of operating a plant for NGL recovery from a feed gas, comprising:
separating the feed gas in a refluxed column to thereby produce a residue gas,
and using a portion of the residue gas after cooling as a first reflux;
expanding a
portion of the feed gas upstream of the column to thereby form a second
reflux;
and controlling temperature of the column by using at least one of (1) a
control
circuit that is configured to control a temperature of an expander discharge
stream
that is fed to the column in response to a temperature in the column, and (2)
an
intermediate reflux condenser disposed between an upper section and a lower
section of the column that maintains temperature of the lower section above a
temperature sufficient to prevent carbon dioxide freezing.
According to another aspect of the present invention, there is provided
a plant comprising: a column having an intermediate reflux condenser that is
configured to operate at a temperature of between about -20 OF and about -40
OF
and that is located between a fractionation section and a rectification
section of the
column, and wherein the column is further configured to receive a first and a
second
reflux stream and to produce an overhead product; a circuit coupled to the
column
such that a liquid stream is fed from the fractionation section to the
rectification
section via a heat exchanger that is configured such that the liquid stream is
heated
in the heat exchanger; and a recycle circuit that is configured to provide a
portion of
the overhead product as the first reflux stream to the column.
According to still another aspect of the present invention, there is
provided a plant comprising: a first column configured to receive a first and
a
second reflux stream and further configured to receive an expanded feed gas
stream; a temperature control unit thermally coupled to the first column and
configured to determine a temperature in the first column; a heat exchanger
that is
configured to cool a first and a second portion of a feed gas to thereby form
the
second reflux stream and a cooled second portion of the feed gas,
respectively; a
bypass valve configured to control flow volume of a third portion of the feed
gas to
the cooled second portion of the feed gas; and a control system that is
configured to
adjust the flow volume of the third portion of the feed gas as a function of
the
temperature in the first column.
5a
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
Various objects, features, aspects and advantages of the present invention
will become
more apparent from the following detailed description of preferred embodiments
of the
invention.
Brief Description of the Drawings
Figure 1 is a schematic of an exemplary ethane recovery plant with integral
intermediate reflux condenser according to the inventive subject matter.
Figure 2 is a schematic of another exemplary ethane recovery plant with two-
column
configuration according to the inventive subject matter.
Figure 3 is a schematic of a further exemplary ethane recovery plant with two-
column configuration according to the inventive subject matter.
Figure 4 is a graph depicting the mole fraction of methane and ethane in the
column
liquids of a plant according to the inventive subject matter.
Figure 5 is a graph depicting the mole fraction of methane and ethane in the
column
liquids of a typical known plant.
Figure 6 is a composite curve of the ethane recovery process according to the
inventive subject matter.
Detailed Description
The inventors have discovered that flexible and high ethane and/or propane
recovery
(e.g., at least about 90% C2 and at least about 99% C3) can be achieved for a
feed gas with
relatively high CO2 content (greater than 2%) using optimum temperature
control in the
separation column(s). In most configurations, optimum temperature control is
achieved with
an intermediate reflux condenser in a single-column configuration or by
control of the
expander discharge in a dual-column configuration. Such temperature control
advantageously
reduces, if not even entirely eliminates carbon dioxide freezing in the
column. Most typically,
the fractionator in contemplated configurations will receive at least two lean
reflux streams.
In a particularly preferred configuration as illustrated in Figure 1, an
exemplary plant
includes a fractionation column 58 that is fluidly coupled to an intermediate
reflux condenser
61 that is used to provide cooling to a lower rectification section. It should
be appreciated that
6
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
high ethane recovery (e.g., over 95%) is achieved in such configurations at
least in part by
recycling, cooling, and JT expanding a portion of the lean residue gas, turbo
expansion of a
vapor portion of the feed gas, and/or refrigeration at the intermediate reflux
condenser and at
the feed exchanger (the refrigerant streams can be either internally generated
or externally
supplied). As used herein in the following examples, the term "about" in
conjunction with a
numeral refers to a range of that numeral starting from 10% below the absolute
of the
numeral to 20% above the absolute of the numeral, inclusive. For example, the
term "about -
100 F" refers to a range of -80 F to -120 F, and the term "about 1000 psig"
refers to a range
of 800 psig to 1200 psig.
A typical feed gas composition in mole percent is as follows: 2.2 % C02, 83.6%
C1,
4.9% C2, 4.0% C3, 3.1% C4 and 2.2% C5+. The feed gas stream 1, at about 110 F
and about
1200 psig, is cooled in heat exchanger 51 with residue gas stream 17,
separator liquid stream
8, stream 18 drawn from a chimney tray (typically above intermediate reflux
condenser), and
refrigerant stream 41 (optional). Feed gas is cooled to about -25 F to about -
45 F forming a
two phase stream 2 that is separated in the separator 52 into a vapor stream
3, and a liquid
stream 4 that is further split into stream 5 and stream 6. It should be noted
that the now ratio
(ratio of stream 5 to stream 4) is adjusted as necessary to provide an optimum
internal reflux
(i.e. stream 6) and stripping of the lighter components (C1 , C2, and C02) in
the liquids (i.e.
stream 5) in the upper section of the column for a specific feed gas
composition.
For example, when processing a relatively lean feed gas, the flow ratio (that
is, stream
5 to stream 4) is reduced, resulting in an increased flow in stream 6 that is
letdown in
pressure via JT valve 54 forming a semi-lean reflux stream 7, which is fed to
the upper
rectification section of column 58. When operated with a relatively rich feed
gas, the flow
ratio is increased, resulting in an increased flow in stream 5. Stream 5 is
letdown in pressure
via JT valve 53 forming stream 8, which is used to provide cooling to the feed
gas in
exchanger 51. The heated stream 9, typically at about -10 , is then routed to
the upper mid
section just above the intermediate reflux condenser, stripping at least a
portion of the lighter
components, thereby reducing the reboiler duty in the column. It should be
especially
recognized that the fractionation column produces at least one liquid stream
in the upper
section that is heat exchanged with the feed gas, and that is fed to the lower
rectification
section. This stream then supplies at least a portion of feed gas chilling
duty and stripping
7
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
requirement by the reboiler, thereby advantageously improving removal of
undesirable light
components and preventing of CO2 freezing.
Further feed gas chilling is achieved by chilling vapor stream 3 from the high-
pressure separator 52 using sub-cooling, JT and turbo-expansion. Here, vapor
stream 3 is split
into two portions, stream 11 and stream 10. The first portion, stream 11, is
expanded in a
turbo-expander 55 forming an expanded stream 14, typically at about -95 F to
about -115 F,
which is introduced to the upper section of column. The second portion, stream
10, is cooled
in heat exchanger 56 to stream 12 by overhead vapor stream 16 to typically
about -120 F to
about -140 F, and further reduced in pressure and temperature via JT valve 57
forming a sub-
cooled reflux stream 13, typically at about -125 F to about -145 F. The so
formed stream 13
is then fed to the column as the second reflux stream. It should be
appreciated that where
relatively rich gas is processed, the vapor split of the vapor stream 3 will
be at a flow ratio
(i.e., stream 10 to stream 3) ranging from about 0.1 to about 0.3. Leaner gas
processing
typically will increase the ratio of stream 10 to stream 3. An increase of the
compressed
residue gas recycle flow (stream 42) generally requires a corresponding change
in the flow
ratio to maintain high ethane recovery.
To ensure high ethane recovery, absorber 58 also receives a first reflux
stream 43 that
is formed from cooling (e.g., via JT valve 70, and exchangers 56 and 51, via
streams 15 and
19) a portion of the compressed vapor stream 42 from residue compressor 71. It
should be
noted that about 5% to about 60%, and more typically 10% to about 25% of the
total residue
gas flow is used as a recycle stream, preferably after the residue gas is
cooled at high pressure
using ambient cooler 72 (forming stream 31), feed gas exchanger 51, and/or
reflux exchanger
56 to about -125 F to about -145 F. The recycle vapor is thus totally
condensed and/or sub-
cooled that is letdown in pressure in JT valve 70 to about 450 psig to about
700 psig to the
column.
It should be particularly pointed out that the fractionation column is self-
sufficient in
heating requirement and typically does not require external heating for ethane
recovery:
Stripping of methane from the NGL stream 25 is achieved with vapor stream 7 in
the upper
section, stream 23 in the lower section, and finally with stream 42 at the
bottom of the
column. To supply bottom reboiler duty, stream 30 at about 40 F is withdrawn
from the
bottom tray, pumped by pump 66 fonning stream 32 that is heated in the feed
exchanger 51
to about 85 F. The two phase stream 32' is then flashed at the bottom of the
column forming
8
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
the NGL product with suitable methane content (typically 0.02 to 0.6% by
volume).
Optionally, the high pressure residue gas compressor discharge (stream 38) can
also be used
to supplement the reboiler duty.
Residue gas from feed exchanger 51 at a temperature of about 80 F to about 100
F
(stream 20) is compressed by expander compressor 55, forming residue gas
stream 21 that is
further compressed by residue gas compressor 71 forming stream 38 at about
1200 psig or
other suitable pipeline or delivery pressure. The compressor discharge 38 is
typically cooled
by an ambient air cooler 72, and about 10% to 25% is diverted as stream 42 and
recycled
back to column 58 forming the first reflux stream 43 that is required for high
ethane recovery
(over 90%). The remaining portion of the discharge vapor forms pipeline sales
gas stream 39.
It should be noted that recycling of the compressed residue gas stream is
typically not
required when ethane recovery drops below 80%, as the chilling requirements
can be satisfied
with the feed gas turboexpansion alone.
In especially preferred aspects, the expander discharge is fed to the mid
section of the
column, at a location below the second reflux that effectively improves the
fractionation
efficiency over known configurations. With respect to the liquid portion from
the feed gas
separator, and especially when processing a rich gas, it is preferred that the
liquid is split into
two portions with one portion being letdown and fed to the column as a cold
semi-lean reflux
and the second portion being heated in the feed gas exchanger to thereby form
a heated vapor
that is used for removal of the light components (e.g., methane) in the upper
section. Thus, it
should be appreciated that in especially preferred aspects of the inventive
subject matter, an
intermediate reflux condenser coupled to the fractionator operation allows
rectification of the
vapors from the lower section with refrigeration cooling, subsequently
improving stripping of
methane and recovery of ethane in the lower section of the column.
The intermediate reflux condenser 61 preferably has a plurality of exchanger
surfaces
that are cooled with refrigerant stream 40 (e.g., internally generated or
externally supplied
with propane refrigeration, or with column overhead gas). Ascending vapor
stream 83 from
the lower section is cooled to about -20 F to -40 F in the intermediate reflux
exchanger 61,
thereby condensing most of the C2 and heavier components, and a portion of the
condensate
(i.e. stream 80) is refluxed to the lower section via a downcomer for
rectification and
recovery of the propane and heavier components. The remaining portion is
withdrawn as the
ethane product stream 29. The vapor stream 82 ascending from the intermediate
reflux
9
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
exchanger 61 is redistributed in a chimney tray to the upper section of the
column.
Consequently, a C2 plus NGL product is produced with a low CO2 content and low
energy
consumption while eliminating CO2 freezing in the' column.
Moreover, it should be appreciated that an intennediate reflux condenser is
especially
advantageous, as it maintains the lower section of the column at temperatures
at typically
above -40 F, thereby minimizing methane and maximizing ethane content in the
mid to lower
section of the column. Thus, where the fractionation column is optionally
fluidly coupled to
an intermediate condenser (integrated internally or externally), cooling to
the rectification
section is provided and fractionation efficiency is improved. In contrast,
heretofore known
plants typically operate the mid section at cryogenic temperatures (-100 F or
lower), which
increase the vapor liquid traffics and energy consumption (refrigeration and
reboiler duties).
It should be especially appreciated that the intermediate reflux condenser
significantly
improves the fractionation efficiency relative to known processes, which is
evident when
methane and ethane compositions in the tray liquids are compared. Figures 4
and 5 depict
the methane and ethane contents in the tray liquids starting from the top
(here: Tray 1) to the
bottom (here: Tray 28) of the column. Figure 4 is a plot for methane and
ethane compositions
in the tray liquids in configurations presently contemplated herein, wherein
Figure 5 is the
same plot for heretofore known configurations. For example, in configurations
according to
the inventive subject matter, the methane content in tray 8 has been reduced
to about 0.1 mol
fraction (Figure 4) as compared to about 0.6 mol fraction of previously known
configurations. These composition curves show that contemplated configurations
are about
five times more effective in stripping methane content from C2 plus NGL
product.
Additionally, the temperature profile of the upper column is significantly
higher than that of
all known configurations, which plays a significant role in eliminating the
potential CO2
freezing problems. Similarly, in terms of ethane recovery, presently
contemplated
configurations are also more efficient as can be taken from the plots. For
example, ethane
content in tray 9 is at about 0.5 mol fraction of contemplated configuration
(Figure 4), as
compared to 0.15 mol fraction of previously kriown configurations (Figure 5).
These
composition curves show that contemplated configurations are at least three
times more
effective than previously known configurations in the ethane recovery. Such
comparative
composition profiles are indicative of the more efficient stripping of methane
and recovery of
ethane with the intermediate reflux condenser of contemplated configurations.
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
It is generally preferred that at least a portion of the residue gas
compressor discharge
is cooled and recycled to the column overhead as a first lean reflux to the
column in the
recovery of the ethane and heavier components when ethane recovery higher than
90% is
desired. Furthermore, with respect to the vapor portions from the feed gas
separator, it should
be recognized that the reflux vapor portion is fed into an exchanger that is
cooled and
condensed by the column overhead vapor prior being used as reflux in the
column. It is
therefore preferred that the column overhead product may act as a refrigerant
in at least one,
and preferably at two additional heat exchangers, wherein the first column
overhead product
typically cools at least a portion the feed gas and/or separated vapor
portion, and may also
provide the second column reflux condensing duty. After heat exchange, the
warmed gas
may then be recompressed to residue gas pressure of which a portion is then
recycled to the
column as first lean reflux. Similarly, with respect to the high pressure
separator liquid and
the liquid withdrawn from the upper section it is generally preferred that
such liquids are
employed as a refrigerant to cool the feed gas stream before entering the
column as column
feed. Suitable columns may vary depending on the particular configurations,
however, it is
generally preferred that the column is a tray or packed bed type column.
Where the feed gas has a relatively high level of CO2 (e.g., %) and is
substantially
depleted of C3+ components (e.g., :!d % C3+), a two-column configuration may
be employed.
An exemplary two-column configuration is depicted in Figure 2. It should be
noted that
substantially the same configuration can be used for high pressure feed gas
(e.g., 1200 psig
and above) using a two-column configuration as shown in Figure 3, with the
first column
operating at high pressure (e.g., 450 psig to 700 psig), the second column
operating at lower
pressures (e.g., 300 psig to 450 psig), and with a methane rich vapor recycled
from the
second column to the first column with the use of a compressor. This enhanced
configuration
is more energy efficient when compared to known configurations with single
column
operating at lower pressure. The contemplated configuration requires less
overall power
(compression plus external refrigeration) for the comparable ethane recovery.
In addition, it should be appreciated that such high first column pressure is
particularly advantageous for higher ethane recovery with lower compression
horsepower
than known processes, and the corresponding high column temperatures also
helps avoid CO2
freezing problems. It should be recognized that the fractionation column in
such
configurations is also fed by at least two reflux streams, wherein the first
reflux stream is
11
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
generated by JT expansion of a portion of the chilled compressed residue gas,
and wherein
the second reflux stream is generated by expansion of a chilled portion of the
feed gas. The
expander discharge temperature in such configurations is controlled
(preferably using a
bypass stream from the feed gas) to avoid CO2 freezing in the column.
Moreover, as the
second fractionation column operates at a lower pressure than the first column
(e.g., by 50 psi
to 400 psi lower) and separates the first column bottoms into desirable
products, the first
column overhead vapor can advantageously be used for refrigeration in reflux
condensation
for the second column, thus eliminating the need for additional external
refrigeration.
It should further be noted that the two column configurations contemplated
herein can
typically operate without a feed gas separator, due to the feed gas bypass
that maintains the
chilled gas in superheated state (i.e., without liquid formation), thus
avoiding liquid dropout
in the expander. Known configurations typically chill the feed gas to lower
temperatures
requiring a separator for removal of the liquids prior to the expander. It
should also be
appreciated that the feed gas inlet is split into two streams that are chilled
separately and to
different temperatures, which is particularly energy efficient as illustrated
in Figure 6 by the
close temperature approaches of the heat composite curves. In contrast,
conventional
configurations typically chill the feed gas to a common temperature which is
not energy
efficient when processing a lean feed gas (with less than 1 % C3 plus
components).
A typical lean feed gas composition in mole percent for a two-column
configuration is
as follows: 0.58% N2, 3.0 % C02, 89% C1, 7.0% C2, 0.6% C3, and 0.07% C4+. An
exemplary
two-column configuration typically includes a first fractionation column
(demethanizer) that
is fluidly coupled to a second fractionation column (deethanizer). Here, a
feed gas bypass is
used to control the demethanizer tray temperature to thereby avoid CO2
freezing. It should be
appreciated that the residue gas is once more used to provide refrigeration
cold to the reflux
condenser of the deethanizer, thereby eliminating the need for external
propane refrigeration.
In Figure 2, feed gas stream 1, at about 40 F and about 1250 psig, is split
into three
streams, 2, 3, and 4. Streams 2 and 3 are separately cooled by the residual
gas stream 16
(thereby forming stream 20) in exchanger 51 to different temperatures. Stream
2 (typically at
a flow rate of 10% to 30% of stream 1) is cooled, condensed, and subcooled to
about -120 F
forming stream 12 while stream 3 (at a flow rate of 70% to 90% of stream 1) is
cooled to
about -10 F forming stream 5. It should be noted that cooling is achieved by
self-generated
refrigerant (e.g., the demethanizer overhead gas stream 16, column side-draw
streams 18 and
12
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
stream 30), thereby eliminating external refrigeration requirement. The third
portion, stream
4, (at a flow rate of about 0% to 5% of stream 1) bypasses exchanger 51 via
control valve 60,
and is combined with stream 5 forming stream 11, thus maintaining the suction
temperature
to expander 53 at a desired/predetermined level.
It is generally preferred that the expander suction temperature is controlled
using a
control unit (not shown) and feedback from temperature sensing elements
located in the
demethanizer trays. Most typically, the control unit is a microprocessor
controlled device that
controls operation of the control valve 60 in dependence of a temperature
measurement in the
first column. Alternatively, or additionally, the control unit may also
receive temperature
information from the expander outlet, sensors thermally coupled to streams 1,
2, 3, and/or 13,
or other streams that directly or indirectly affect column temperature. In
less preferred
aspects, the control unit may also be replaced (at least temporarily) with
manual operator
intervention. Increasing the bypass flow of stream 4 will increase the
expander discharge
temperature, subsequently increasing tray temperatures, and thereby eliminate
CO2 freezing.
Furthermore, the higher expander suction temperature has the side benefit of
an increase in
power output from the expander 55, thus reducing the overall energy
consumption. The
mixed stream 11 is preferably maintained in a superheated state. Thus, a feed
gas separator
(commonly used in known processes) is not required in the feed gas circuit.
Stream 11 is then
expanded via expander 55 to a pressure of about 510 psig, forming stream 14 at
about -90 F
that is fed to the mid section of demethanizer 58.
Similar to the previous configuration of Figure 1, the demethanizer column is
refluxed
with two reflux streams. The first reflux (stream 23) is generated by chilling
a portion of the
residue gas compressor discharge in exchanger 51 to about -120 F, and then by
expansion in
JT valve 70. The second reflux (stream 13) is produced by chilling by a
portion of the feed
gas to about -120 F and then by JT expansion in valve 57.
The demethanizer column is reboiled with heat content from feed gas streams 2
and/or 3, and residue gas streams 38 and/or 42, thereby controlling the
methane content in the
bottom product of demethanizer 58 at about 2 wt% or less. An upper side draw
stream 18 at
about -5 F, and a lower side draw stream 30 at about 20 F, coupled with the
bottom reboiler
(65, heated by stream 38, which then forms stream 31) supply the demethanizer
column
reboiler duties. Heated upper and lower side draw streams 9 and 23 are
returned to the
column. The demethanizer produces an overhead vapor stream 16 at about -125 F
and about
13
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
510 psig, and a bottom stream 24 at about 50 F and about 515 psig. The
overhead vapor is
first used to supply cooling in exchanger 51 and then in the deethanizer
reflux condenser 62.
With this arrangement, it should be appreciated that the NGL plant is self
sufficient in
refrigeration, thus significantly reducing capital and operating costs.
The residue gas stream 32 from exchanger 62 is compressed by compressor 53
driven
by expander 55 forming stream 21 at about 45 F and about 600 psig, which is
further
compressed by residue gas compressor 59, forming stream 38 at about 1260 psig
and about
150 F. At least a portion of the high pressure residue gas is used to supply
the demethanizer
reboiler duty (supra). Furthermore, at least a portion of the chilled residue
gas, stream 42, is
recycled, cooled to form stream 19 and JT expanded to the demethanizer, while
the remaining
portion of the residue gas 39 is delivered to a gas pipeline or downstream
processing facility.
The demethanized bottom product is letdown in pressure to about 350 to 450
psig, let
down in pressure via valve 67 forming stream 26 that is fed to deethanizer 61.
The
deethanizer overhead product 27 is condensed in exchanger 62 using
refrigeration from
residue gas stream 20. The so obtained two phase stream 28 is separated in
reflux drum 63,
producing a deethanizer reflux stream 30 that is pumped to the column via pump
64 as stream
31, and an ethane product stream 29. The deethanizer is reboiled in reboiler
61 with external
heat, producing a propane plus NGL product 25.
It may also be desirable to reduce the CO2 content in the NGL product even
further to
the maximum extent that is economically feasible. Among other advantages, a
reduced CO2
content has significant economic benefits, including lower transmission cost,
reduced treating
requirements, and/or CO2 emissions. Figure 3 shows an exemplary configuration
for the
removal of CO2 from the ethane product using a plant configuration
substantially as
described above for Figure 2. In the configuration of Figure 3, a compressor
77 is used to
compress the CO2 rich stream from the deethanizer overhead to the
demethanizer, while
producing an ethane liquid that is even lower in CO2 content. Although
economically
attractive, it should be noted that the additional heat from the compressor
discharge must be
removed. Most typically, this can be achieved using increased residue gas
recycle volume.
Therefore, it should be appreciated that preferred configurations will avoid
CO2 freezing
commonly encountered in conventional processes. Viewed from another
perspective,
contemplated configurations may also be used to remove CO2 from the NGL to low
levels
without impacting ethane recovery. Operation of the components of the process
of Figure 3 is
14
CA 02614414 2008-01-04
WO 2007/008254 PCT/US2006/004346
similar to the configuration of Figure 2, and with respect to the components
and numbering,
the same considerations as described for Figure 2 above apply.
With respect to suitable feed gas streams, it is contemplated that various
feed gas
streams are appropriate, and especially suitable feed gas streams typically
include various
hydrocarbons of different molecular weight. With respect to the molecular
weight of
contemplated hydrocarbons, it is generally preferred that the feed gas stream
predominantly
includes C1-C6 hydrocarbons, with Cl components being the dominant component.
Suitable
feed gas streams may additionally comprise acid gases (e.g., carbon dioxide,
hydrogen
sulfide) and other gaseous components (e.g., hydrogen). Consequently,
particularly preferred
feed gas streams are processed and unprocessed natural gas and natural gas
liquids.
With respect to the C2 recovery, it is contemplated that configurations
according to
the inventive subject matter provide at least 90%, more typically at least
92%, and most
typically at least 95% recovery, while it is contemplated that C3 recovery
will be at least 90%,
more typically at least 98%, and most typically at least 99%. Further aspects
and
considerations related to this application are presented in our International
patent applications
with the publication numbers WO 2005/045338 and WO 03/100334, both of which
are
incorporated by reference herein.
Thus, specific embodiments and applications of NGL recovery have been
disclosed. It
should be apparent, however, to those skilled in the art that many more
modifications besides
those already described are possible without departing from the inventive
concepts herein.
The inventive subject matter, therefore, is not to be restricted except in the
spirit of the
appended claims. Moreover, in interpreting both the specification and the
claims, all terms
should be interpreted in the broadest possible manner consistent with the
context. In
particular, the terms "comprises" and "comprising" should be interpreted as
referring to
elements, components, or steps in a non-exclusive manner, indicating that the
referenced
elements, components, or steps may be present, or utilized, or combined with
other elements,
components, or steps that are not expressly referenced. Furthermore, where a
definition or
use of a term in a reference, which is incorporated by reference herein is
inconsistent or
contrary to the definition of that term provided herein, the definition of
that term provided
herein applies and the definition of that term in the reference does not
apply.
