Note: Descriptions are shown in the official language in which they were submitted.
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IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
TWO STEP NITROGEN AND METHANE SEPARATION PROCESS
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] This invention relates to a system and method for separating nitrogen
from methane and other components from natural gas streams. The invention also
relates to a system and method for integrating natural gas liquids (NGL)
extraction with
nitrogen removal. The system and method of the invention are particularly
suitable for
use in recovering and processing feed streams typically in excess of 50
MMSCFD.
2. Description of Related Art
[0002] Nitrogen contamination is a frequently encountered problem in the
production of natural gas from underground reservoirs. The nitrogen may be
naturally
occurring or may have been injected into the reservoir as part of an enhanced
recovery
operation. Transporting pipelines typically do not accept natural gas
containing more
than 4 mole percent inerts, such as nitrogen. As a result, the natural gas
feed stream is
generally processed to remove such inerts for sale and transportation of the
processed
natural gas.
[0003] One method for removing nitrogen from natural gas is to process the
nitrogen and methane containing stream through a nitrogen rejection unit or
NRU. The
NRU may be comprised of two cryogenic fractionating columns, such as that
described
in U.S. Pat. Nos. 4,451,275 and 4,609,390. These two column systems have the
advantage of achieving high nitrogen purity in the nitrogen vent stream, but
require
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higher capital expenditures for additional plant equipment, including the
second column,
and may require higher operating expenditures for refrigeration horsepower and
for
compression horsepower for the resulting methane stream.
[0004] The NRU may also be comprised of a single fractionating column, such
as that described in U.S. Pat. Nos. 5,141,544, 5,257,505, and 5,375,422. These
single
column systems have the advantage of reduced capital expenditures on
equipment,
including elimination of the second column, and reduced operating expenditures
because no external refrigeration equipment is necessary. In addition to
capital and
operating expenditures, many prior NRU systems have limitations associated
with
processing NRU feed streams containing high concentrations of carbon dioxide.
Nitrogen rejection processes involve cryogenic temperatures, which may result
in
carbon dioxide freezing in certain stages of the process causing blockage of
process
flow and process disruption. Carbon dioxide is typically removed by
conventional
methods from the NRU feed stream, to a maximum of approximately 35 parts per
million
(ppm) carbon dioxide, to avoid these issues.
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SUMMARY OF THE INVENTION
[0005] The system and method disclosed herein facilitate the economically
efficient removal of nitrogen from methane in a two step process. The system
and
method are particularly suitable for NRU feed gas flow rates in excess of 50
MMSCFD
and are capable of processing NRU feed gas flow rates of up to around 750
MMSCFD.
The system and method are also capable of processing NRU feed gas containing
concentrations of carbon dioxide up to approximately 75 ppm for typical
nitrogen levels
between 20-50%
[0006] According to one embodiment of the invention, a system and method are
disclosed for processing an NRU feed gas stream containing primarily nitrogen
and
methane through two fractionating columns to produce a processed natural gas
stream
suitable for sale to a transporting pipeline. The first stage column is
designed to remove
methane and heavier hydrocarbon components from nitrogen, while the second
stage
column is designed to remove nitrogen from the remaining methane. The overhead
stream from the first stage column feeds the second stage column. The NRU feed
gas
and the first stage overhead stream are not cooled to traditional targeted
temperatures
of -200 to -245 degrees F. The bottoms streams from the first and second
fractionating
columns are at varying pressures after further processing and are separately
fed to a
series of compressors to achieve a processed gas product stream of sufficient
pressure
for sale, typically at least 615 psia. The higher temperatures in the feeds to
the
fractionating columns allows the bulk of the methane to be separated from the
NRU
feed stream while reducing the overall compression required for the process by
up to
40% when compared to traditional NRU processes.
[0007] According to another embodiment of the invention, a system and method
is disclosed for NGL extraction integrated into the two column NRU process
downstream from the first stage column. In traditional nitrogen separation
systems, the
separation of NGL components is more difficult in streams containing more than
5%
nitrogen because nitrogen has a stripping effect, absorbing ethane and heavier
components. According to this embodiment of the invention, the bulk methane
and
heavier components are removed from the nitrogen in the first column, allowing
the
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bottoms stream containing less than 4% nitrogen, to be further processed for
extraction
of NGL.
[0008] There are several advantages to the system and method disclosed herein
not previously achievable by those of ordinary skill in the art using existing
technologies.
These advantages include, for example, an ability to process higher flow rate
NRU feed
streams from around 50 MMSCFD up to around 750 MMSCFD, NRU feed streams
containing up to 75 ppm carbon dioxide, reduction in overall compression
requirements,
and integration of NGL extraction. Although the present system and method has
the
disadvantage of higher capital costs associated with additional equipment,
compared to
prior single column NRU processes, the costs of such are sufficiently offset
by the
savings in operating expenses, such as those from the reduced compression
requirements, and the ability to efficiently produce a suitable processed
natural gas
stream and valuable NGL stream.
[0009] It will be appreciated by those of ordinary skill in the art upon
reading this
disclosure that references to separation of nitrogen and methane used herein
refer to
processing NRU feed gas to produce various multi-component product streams
containing large amounts of the particular desired component, but not pure
streams of
any particular component. One of those product streams is a nitrogen vent
stream,
which is primarily comprised of nitrogen but may have small amounts of other
components, such as methane and ethane. Another product stream is a processed
gas
stream, which is primarily comprised of methane but may have small of other
components, such as nitrogen, ethane, and propane. A third product stream,
according
to one embodiment of the invention, is an NGL product stream, which is
primarily
comprised of ethane, propane, and butane but may contain amounts of other
components, such as hexane and pentane.
[0010] It will also be appreciated by those of ordinary skill in the art upon
reading
this disclosure that additional processing sections for removing carbon
dioxide, water
vapor, and possibly other components or contaminants that are present in the
NRU feed
stream, can also be included in the system and method of the invention,
depending
upon factors such as, for example, the origin and intended disposition of the
product
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streams and the amounts of such other gases, impurities or contaminants as are
present in the NRU feed stream.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The system and method of the invention are further described and
explained in relation to the following drawings wherein:
FIG. 1 is a simplified process flow diagram illustrating principal processing
stages
of one embodiment of a system and method for separating nitrogen and methane;
FIG. 2 is a simplified process flow diagram illustrating principal processing
stages
of another embodiment of a system and method for separating nitrogen and
methane
including NGL extraction;
FIG. 3 is a more detailed process flow diagram illustrating the nitrogen-
methane
separation portion of the simplified process flow diagram of FIG. 1;
FIG. 4 is a more detailed process flow diagram illustrating the compression
portion of the simplified process flow diagram of FIG. 1;
FIG.5 is a more detailed process flow diagram illustrating the nitrogen-
methane
separation portion of the simplified process flow diagram of FIG. 2;
FIG. 6 is a more detailed process flow diagram illustrating the NGL extraction
portion of the simplified process flow diagram of FIG. 2; and
FIG. 7 is a more detailed process flow diagram illustrating the compression
portion of the simplified process flow diagram of FIG. 2.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Referring to FIG. 1, system 10 comprises processing equipment useful
for separating nitrogen from methane according to one embodiment of the
invention is
depicted. System 10 of the invention includes processing stages 12, 14, and 18
for
processing NRU feed gas 11 to produce a nitrogen vent stream 16 and a
processed gas
stream 20. Processing stage 12 includes a first stage fractionating column,
the
overhead stream from which serves as the feed for processing stage 14, which
includes
a second stage fractionating column. The overhead stream from processing stage
14 is
a nitrogen vent stream 16. The bottoms streams from processing stages 12 and
14
feed a series of compressors in processing stage 18 to produce processed gas
20 of
sufficient pressure and methane composition to be suitable for sale.
[0013] Referring to FIG. 2, system 210 comprises processing equipment useful
for separating nitrogen and methane, as well as extracting NGL, according to
another
embodiment of the invention is depicted. System 210 of the invention includes
processing stages 212, 214, and 218 for processing NRU feed gas 211 to produce
a
nitrogen vent stream 216 and a processed gas stream 220, similar to system 10.
Processing stage 212 includes a first stage fractionating column, the overhead
stream
from which serves as the feed for processing stage 214, which includes a
second stage
fractionating column. The overhead stream from processing stage 214 is a
nitrogen
vent stream 216. The bottoms streams from processing stages 212 and 214 feed a
series of compressors in processing stage 218 to produce processed gas 220 of
sufficient pressure and methane composition to be suitable for sale. The
bottoms
stream from processing stage 212 also feeds processing stage 410, which
includes an
NGL fractionating column, the overhead stream from which serves as additional
feed for
processing stage 218. The bottoms stream from processing stage 410 is the NGL
product stream 412.
[0014] Referring to both FIGS. 1 and 2, the source of NRU feed gas 11 or 211
is
not critical to the system and method of the invention; however, natural gas
drilling and
processing sites with flow rates of 50 MMSCFD or greater are particularly
suitable. The
NRU feed gas 11 or 211 used as the inlet gas stream for system 10 or 210 will
typically
contain a substantial amount of nitrogen and methane, as well as other
hydrocarbons,
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such as ethane and propane, and may contain other components, such as water
vapor
and carbon dioxide.
[0015] Where present, it is generally preferable for purposes of the present
invention to remove as much of the water vapor and other contaminants from the
NRU
feed gas 11 or 211 as is reasonably possible prior to separating the nitrogen
and
methane. It may also be desirable to remove excess amounts of carbon dioxide
prior to
separating the nitrogen and methane; however, the method and system are
capable of
processing NRU feed streams containing up to around 75 ppm carbon dioxide
without
encountering the freeze-out problems associated with prior systems and
methods.
Methods for removing water vapor, carbon dioxide, and other contaminants are
generally known to those of ordinary skill in the art and are not described
herein.
[0016] System 10 is depicted in greater detail in FIGS. 3 and 4, with
processing
stages 12 and 14 depicted in FIG. 3 and processing stage 18 depicted in FIG.
4.
Referring to FIG. 3, a 250 MMSCFD NRU feed stream 11 containing approximately
25% nitrogen and 70% methane at 115 F and 865 psia passes through heat
exchanger
22 from which it emerges as stream 24, having been cooled to -132.5 F. This
cooling
is the result of heat exchange with other process streams, 60, 82, 102, 128,
and 136.
Stream 24 passes through expansion valve 26 to produce stream 28 having cooled
slightly and having a reduction in pressure of around 250 psia (to 615 psia)
before
entering as the feed stream for the first stage fractionating column 13.
Column 13
operates at approximately -122 F to -147 F and 615 psia, which is at a
higher
temperature and pressure than targeted values in traditional double-column NRU
systems.
[0017] Stream 62 from the bottom of the first stage fractionating column 13 is
desirably directed to virtual heat exchanger 64 that receives heat (designated
by energy
stream Q-10) from heat exchanger 22. Stream 62 is at approximately -123 F and
617
psia and contains approximately 4.6% nitrogen and 85% methane. Vapor stream
66, at
approximately -117 F, is returned to the first stage fractionating column 13
as the
ascending stripping vapor that strips nitrogen from the hydrocarbon flowing
downward
through the column. The
first stage fractionating column also receives heat
(designated by energy stream Q-14) from heat exchanger 22.
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[0018] In this example, the NRU feed stream 11 contains no carbon dioxide.
However, system 10 is capable of processing NRU feed streams containing up to
75
ppm carbon dioxide. The physical separation characteristics of carbon dioxide
are
similar to an average of ethane and propane. With these parameters, the carbon
dioxide would be separated in the first stage fractionating column 13 into the
bottoms
stream, along with methane, ethane, propane, and other hydrocarbons. The
bottoms
stream 62 (and subsequent process streams) of the first stage fractionating
column 13
does not feed the second stage fractionating column 15 so the carbon dioxide
containing stream does not enter the cryogenic section of the process
(processing stage
14). This eliminates freeze-out problems with prior systems and increases the
carbon
dioxide tolerance of system 10 according to the invention from approximately
35 ppm in
prior systems to 75 ppm.
[0019] Overhead stream 30, containing approximately 37% nitrogen and 63%
methane at -147.5 F, exits the first stage fractionating column 13. It is not
necessary to
use a reflux stream in the first stage fractionating column 13 according to
the invention.
The operating parameters allow sufficient separation of nitrogen, methane, and
carbon
dioxide without reflux; however, a reflux stream and related equipment could
be used
with the first stage column of system 10 if desired. Overhead stream 30 then
passes
through heat exchanger 32 and exits as stream 34 at -215 F. This cooling is
the result
of heat exchange with other process streams 54, 80, 100, and 126. Stream 34
passes
through primary JT valve 36 and exits the valve as stream 38 having the same
temperature as stream 34 but having a reduction in pressure of almost half.
The
primary JT valve is capable of cooling by the well known Joule-Thomson effect,
but in
post-start up, steady state operation the valve provides less actual thermal
cooling, but
does provide the necessary pressure reduction for stream 38, which feeds the
second
stage fractionating column 15 at -215 F and 325 psia.
[0020] Stream 86 from the bottom of the second stage fractionating column 15
is
directed to virtual reboiler 88 that receives heat (designated as energy
stream Q-16)
from heat exchanger 32. Stream 86 is at approximately -169 F and 315 psia and
contains approximately 5.4% nitrogen and 94% methane. Vapor stream 90, at
approximately -163
F, is returned to the second stage fractionating column 15. The
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second stage fractionating column also receives heat from heat exchanger 32,
designated by energy streams Q-18 and Q-20.
[0021] Overhead stream 40, containing approximately 98% nitrogen and 1.7%
methane at -247 F, internally feeds a reflux condenser depicted by separator
42 and
heat exchanger 118 and then exits the second stage fractionating column 15.
Internal
stream 40 passes through internal condenser 118 and then on to separation
chamber
42. Liquid stream 44 exits the separation chamber 42 and to provide reflux to
the
second stage fractionating column 15. Vapor stream 46 exits condenser 42
containing
approximately 99.2% nitrogen and 0.8% methane and passes through expansion
valve
48 to drop the pressure and temperature of exiting stream 50 to approximately
30 psia
and -306.5 F. Stream 50 then passes through subcooler 52, exiting as stream
54 at
approximately -187 F and 25 psia. Stream 54 passes through heat exchanger 32
and
exits as stream 56, warmed to -152 F. Valve 58 controls stream 56, but
exiting stream
60 is at substantially the same temperature and pressure as stream 56. Valve
58 is
strategically placed so as to provide another level of refrigeration and made
available in
the heat exchanger 22. This valve and the associated Joule-Thomson effect
allows for
further cooling of the process stream 24. Stream 60 then passes through heat
exchanger 22 and exits the system as nitrogen vent stream 16. Vent stream 16
contains approximately 99.2% nitrogen, 0.8% methane and a trace amount of
ethane at
a temperature and pressure of approximately 105 F and 15 psia. Vent stream 16
may
be recycled for supplying enhanced oil and gas recovery efforts.
[0022] There are several methane enriched streams produced in processing
stages 12 and 14. One such stream is stream 138, which contains approximately
3%
nitrogen, 84% methane, and 8% ethane. Stream 138 is essentially the bottoms
stream
from the first stage fractionating column 13, after being further processed as
described
below. Bottoms stream 62 enters virtual heat exchanger 64 to produce vapor
stream 66
and liquid stream 68. Liquid stream 68 is split in splitter 70 into streams 72
and 132.
Under the parameters of the specific example and operating conditions
described
herein, splitter 70 is set so that 100% of stream 68 is directed to stream
132. However,
under other operating conditions and parameters, some of the flow from stream
68 may
be directed to stream 72. Stream 132 is pumped by the first stage bottom pump
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(powered by energy stream Q-12), with stream 136 exiting pump 134 at
approximately
865 psia. Stream 136 then passes through heat exchanger 22 and exits as stream
138.
One primary benefit of this design configuration is that all vaporized product
in stream
138 can be routed directly to sales gas pipeline without typical sales gas
compression.
The result is a dramatic reduction in the overall compression requirement as
compared
to other typical processes.
[0023] The remaining methane enriched streams 84, 104, and 130 are
essentially the bottoms stream from the second stage fractionating column 15,
after
being further processed as described below. Bottoms stream 86 enters reboiler
88 to
produce vapor stream 90 and liquid stream 92. Liquid stream 92 is split by
splitter 94
into streams 96 (approximately 15% of the flow), 108 (approximately 26% of the
flow),
and 126 (approximately 59% of the flow). Streams 92, 96, 108, and 126 are all
at
approximately -163 F and 315 psia. Stream 96 is controlled by valve 98, with
stream
100 exiting the valve at -200 F and 125 psia. Stream 100 then passes through
heat
exchanger 32 to stream 102, then through heat exchanger 22 to stream 104.
Stream
104 is approximately 3% nitrogen and 96% methane at 105 F and 116 psia.
[0024] Stream 108 passes through subcooler 52, exiting as stream 110 at
approximately -290 F and 310 psia. Stream 110 passes through secondary JT
valve
112, with stream 114 exiting the valve. Stream 114 is approximately the same
temperature as stream 110, but the pressure has been reduced to approximately
37
psia. Further pressure drop is achieved as stream 114 flows through a vertical
(up)
length of pipe, becoming stream 116 at 22 psia. Stream 115 passes through heat
exchanger 118, supplied with energy stream Q-22 from condenser 42, and exits
as
stream 120 warmed to -249 F. Stream 120 flows through a vertical (down)
length of
pipe, becoming stream 122, although there is a negligible change in
temperature and
pressure between streams 120 and 122 in this example. Stream 122 then passes
through subcooler 52, exiting as stream 124 with a slight drop in temperature
and
pressure. Stream 124 then passes through mixer 78 where it is combined with
stream
76 to form stream 80. Stream 72 from splitter 70 is controlled by valve 74,
from which
stream 76 exits. In this example, no flow is directed to streams 72 or 76, so
stream 80
is the same composition as stream 124. Stream 80 then passes through heat
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exchanger 32, with stream 82 warmed to -152 F exiting and passing through
heat
exchanger 22. Stream 84, containing 3% nitrogen and 96% methane at 105 F and
17
psia, exits heat exchanger 22 from stream 82.
[0025] Stream 126 passes through heat exchanger 32, with stream 128 warmed
slightly exiting and passing through heat exchanger 22. Stream 130, containing
3%
nitrogen and 96% methane at 95 F and 307 psia, exits heat exchanger 22 from
stream
128. Three of the four methane enriched streams, 84, 104, and 130, are each at
different pressures, increasing from the low pressure stream 84 (at 17 psia)
to the high
pressure stream 130 (at 307 psia). These streams all feed into processing
stage 18,
where they pass through a series of compressors (described below) to achieve a
processed gas stream of sufficient pressure for sale.
[0026] Referring to FIG. 4, stream 84 is compressed by compressor 140
(supplied by energy stream Q-140) emerging as stream 142. Stream 142 is at 285
F
and 45 psia, but decreases in temperature (and slightly in pressure) after
passing
through combination heat exchanger/vessel 144 to emerge as stream 146 at 120
F and
40 psia. Stream 146 is compressed by compressor 148 (supplied by energy stream
Q-
148) emerging as stream 150 at 320 F and 115 psia. Stream 104 is combined
with
stream 150, both having substantially equal pressures, and the combined stream
passes through the next combination heat exchanger/vessel 152 to emerge as
stream
154 at 120 F and 110 psia. Stream 154 is then compressed by compressor 156
(supplied by energy stream Q-156) emerging as stream 158 at 314.5 F and 305
psia.
Stream 130 is combined with stream 158, both having substantially equal
pressures,
and the combined stream passes through the next combination heat
exchanger/vessel
160 to emerge as stream 162 at 120 F and 300 psia. Stream 162 is compressed
by
compressor 164 (supplied by energy stream Q-164) emerging as stream 166.
Stream
166 passes through the next vessel 168 to emerge as stream 170 at 120 F and
865
psia. Stream 138 is then mixed with stream 170 in mixer 172, resulting in
processed
gas stream 20. The processed gas stream 20 is at 111 F and 860 psia,
containing 3%
nitrogen and 90% methane, suitable for sale. As the temperature of the streams
passing through vessels 144, 152, 160, and 168 drops, energy streams
represented by
Q-144, Q-152, Q-160, and Q-168 are created by commercially available heat
exchange
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cooling equipment and may be used to supply energy to other components of the
system 10 or other process systems. The power requirements for successively
compressing the streams, represented by Q-140, Q-148, Q-156, and Q-164 (see
Table
3 below), are substantially lower than the overall power requirements for
traditional NRU
systems.
[0027] Acceptable inlet compositions in which this invention may operate
satisfactorily are listed in the following Table 1:
TABLE 1
INLET STREAM COMPOSITIONS
Inlet Component
Acceptable Inlet Composition Ranges
Methane 20-90% 20-95%
Ethane and Heavier Components 5-10% 0-20%
Carbon Dioxide 0-75 ppm
Nitrogen 5-80%
[0028] The flow rates, temperatures and pressures of various flow streams
referred to in connection with the discussion of the system and method of the
invention
in relation to FIGS. 3 and 4, for an NRU feed gas flow rate of 250 MMSCFD
containing
25% nitrogen and 70% methane and no carbon dioxide, appear in Table 2 below.
The
values for the energy streams referred to in connection with the discussions
of the
system and method of the invention in relation to FIGS. 3 and 4 appear in
Table 3
below. The values discussed herein and in the tables below are approximate
values.
TABLE 2
FLOW STREAM PROPERTIES ¨ Minimum Recompression Case
Stream % % Flow Temperature Pressure
Reference N2 C114 Rate (deg. F) (psia)
Numeral (Ibmol/h)
11 25 70 27450 115 865
16 99.2 0.8 6277 105 15
20 3 90.5 21172 112 860
24 25 70 27450 -132.5 860
28 25 70 27450 -148 615
30 36.6 62.8 17985 -147.5 615
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Stream % % Flow Temperature Pressure
Reference N2 CH4 Rate (deg. F) (psia)
Numeral , (lbmol/h)
34 36.6 62.8 17985 -215 613
38 36.6 62.8 17985 -216 325
40 98.3 1.7 14816 -247 315
44 97.7 2.3 8539 -248 315
46 , 99.2 0.8 6277 -248 315
50 99.2 0.8 6277 -306.5 30
54 99.2 0.8 6277 -187 25
56 99.2 0.8 6277 -153 _ 21
60 99.2 0.8 6277 -153 20
62 4.6 85.3 13168 -122.9 617
66 8.8 89.6 3704 -117 617
68 3 86.7 9464 -117 617
80 3 96 3000 -250 19
82 3 96 3000 -152.5 18
84 3 _ 96 3000 105 17
86 5.4 93.9 16570 -169 315
90 11.2 88.7 4863 -163 315
92 3 96 117-08 -163 315
96 3 96 1750 , -163 315
100 3 , 96 1750 -200 125
102 3 96 1750 -152.5 121
_
104 3 96 1750 105 116
108 3 96 3000 -163 315
110 3 96 3000 -290 310
114 3 96 3000 -288.5 37
116 3 96 3000 -289 22
120 3 96 3000 -249 20
122 3 96 3000 -249 20
124 , 3 96 3000 -250 19
126 3 96 6958 -163 315
128 3 96 6958 -160 310
130 3 96 6958 95 307
132 3 83.7 9464 -117 617
136 3 83.7 9464 -112 865
138 3 83.7 9464 105 860
142 3 96 3000 285 45
146 3 96 3000 120 40
150 3 96 3000 321 115
154 3 96 4750 120 110
158 _ 3 96 4750 314.5 305
162 3 96 11708 120 300
_
166 3 96 11708 326 870
14
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Stream % % Flow Temperature Pressure
Reference N2 CH4 Rate (deg. F) (psia)
Numeral (Ibmol/h)
170 3 96 11708 120 865
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TABLE 3
ENERGY STREAM REPORT ¨ Minimum Recompression Case
Eneregy Energy Power From To
Stream Rate
Reference (Btu/h) (hp)
Numeral
Q-10 6.24E+06 2451 Heat Virtual Heat
Exchanger Exchanger
22 64
Q-12 552334 217 Stg 1 Btm
Pump 134
Q-14 6E+06 2358 Heat Fractionator
Exchanger 13
22
Q-16 1.2E+07 4718 Heat Reboiler 88
Exchanger
32
Q-18 1E+07 3930 Heat Fractionator
Exchanger 15
32
Q-20 3.75E+06 1474 Heat Fractionator
Exchanger 15
32
Q-22 1.11E+07 4366 Condenser Heat
42 Exchanger
118
Q-140 4.99E+06 1960 Compressor
140
Q-144 4.63E+06 1819 Vessel
144
Q-148 5.64E+06 2219 Compressor
148
Q-152 5.51E+06 2165 - Vessel
152
Q-156 8.55E+06 3360 Compressor
156
Q-160 7.36E+06 2892 Vessel
160
Q-164 2.19E+07 8613 Compressor
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Eneregy Energy Power From To
Stream Rate
Reference (Btu/h) (hp)
Numeral
164
Q-168 2.50E+07 9839 Vessel
168
[0029] It will be appreciated by those of ordinary skill in the art that these
values
are based on the particular parameters and composition of the feed stream in
the above
example. The values will differ depending on the parameters and composition of
the
NRU Feed stream 11.
[0030] System 210 is depicted in greater detail in FIGS. 5, 6, and 7, with
processing stages 212 and 214 depicted in FIG. 5; processing stage 410
depicted in
FIG. 6; and processing stage 218 depicted in FIG. 7. Many of the process steps
depicted in FIGS. 5 and 7 are the same as those in FIGS. 3 and 4.
[0031] Referring to FIG. 5, a 250 MMSCFD NRU feed stream 211 containing
25% nitrogen, 70% methane, 3% ethane, 25 ppm of carbon dioxide at 115 F and
865
psia passes through heat exchanger 222 from which it emerges as stream 224,
having
been cooled to -162.5 F. Stream 224 passes through expansion valve 226 to
produce
stream 228 having substantially the same temperature but having a reduction in
pressure of around 250 psia (to 615 psia) before entering as the feed stream
for the first
stage fractionating column 213. Column 213 operates at approximately -126 F
to -163
F and 615 psia, and causes the nitrogen gas to separate from the methane and
flow
upwardly through the tower as a vapor.
[0032] Stream 262 from the bottom of the first stage fractionating column 213
is
desirably directed to virtual heat exchanger 264 that receives heat
(designated by
energy stream Q-210) from heat exchanger 222. Stream 262 is at approximately -
127
F and 617 psia and contains 5.6% nitrogen and 90% methane. Vapor stream 266,
at
-119 F, is returned to the first stage fractionating column 213 as the
ascending
stripping vapor that strips nitrogen from the hydrocarbon flowing downward
through the
column.
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[0033] In this example, the NRU feed stream 211 contains 25 ppm carbon
dioxide. However, system 210 is capable of processing NRU feed streams
containing
up to 75 ppm carbon dioxide as previously discussed. The bottoms stream 262
(and
subsequent process streams) of the first stage fractionating column 213, which
contains
29 ppm, does not feed the second stage fractionating column 215 so the carbon
dioxide
containing stream does not enter the cryogenic section of the process
(processing stage
214). The overhead stream 230 (and subsequent process streams 234 and 238),
which
contains only 4.9 ppm carbon dioxide, feeds the second stage fractionating
column;
however, this small amount of carbon dioxide does not create significant
freeze-out
problems. The carbon dioxide tolerance of system 210 according to the
invention is
increased from a maximum of around 35 ppm in prior systems to a maximum of
around
75 ppm for typical nitrogen levels in the NRU feed stream.
[0034] Overhead stream 230 exits the first stage fractionating column 213
containing approximately 50% nitrogen and 49.6% methane at -164 F. It is not
necessary to use a reflux stream in the first stage fractionating column 213
according to
the invention. The operating parameters allow sufficient separation of
nitrogen,
methane, NGL components, and carbon dioxide without reflux; however, a reflux
stream
and related equipment could be used with the first stage column of system 210
if
desired. Overhead stream 230 then passes through heat exchanger 232 and exits
as
stream 234 at -225 F. Stream 234 passes through primary JT valve 236 and
exits the
valve as stream 238 having substantially the same temperature as stream 234
but
having a pressure reduction of almost half. The primary JT valve is capable of
cooling
by the well known Joule-Thomson effect, but in post-start up, steady state
operation the
valve provides less actual thermal cooling, but does provide the necessary
pressure
reduction for stream 238, which feeds the second stage fractionating column
215 at -
225 F and 325 psia. Stream 238 enters fractionating column 215 at an
intermediate
stage of the column.
[0035] Stream 286 from the bottom of the second stage fractionating column 215
is directed to virtual reboiler 288 that receives heat (designated as energy
stream Q-
216) from heat exchanger 232. Stream 286 is at -168 F and 315 psia and
contains 5%
nitrogen and 94% methane. Vapor stream 290, at approximately -164 F, is
returned to
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the second stage fractionating column 215. The second stage fractionating
column also
receives heat from heat exchanger 232, designated by energy streams Q-218 and
Q-
220.
[0036] Overhead stream 240, containing approximately 98% nitrogen and 1.7%
methane at -247 F, internally feeds a reflux condenser depicted by separator
242 and
heat exchanger 318 and then exits the second stage fractionating column 215.
Internal
stream 240 passes through internal condenser 318 and then on to separation
chamber
242. Liquid stream 244 exits the separation chamber 242 and to provide reflux
to the
second stage fractionating column 215. Vapor stream 246 exits condenser 242
containing approximately 99.2% nitrogen and 0.8% methane and passes through
valve
248 to drop the pressure and temperature of exiting stream 250 to
approximately 30
psia and -306.5 F. Stream 250 then passes through subcooler 252, exiting as
stream
254 at -258 F and 25 psia. Stream 254 passes through heat exchanger 232 and
exits
as stream 256, warmed to -172 F. Stream 256 then passes through heat
exchanger
222 and exits the system as nitrogen vent stream 216. Vent stream 216 contains
approximately 99% nitrogen, 0.8% methane and a trace amount of ethane at a
temperature and pressure of approximately 105 F and 16 psia. Vent stream 216
may
be recycled for supplying enhanced oil and gas recovery efforts.
[0037] There are several methane enriched streams produced in processing
stages 212 and 214. One such stream is stream 338, which contains
approximately 3%
nitrogen, 88% methane, and 5% ethane, and 4.3 ppm carbon dioxide. Stream 338
is
essentially the bottoms stream from the first stage.fractionating column 213,
after being
further processed as described below. Bottoms stream 262 enters virtual heat
exchanger 264 to produce vapor stream 266 and liquid stream 268. Liquid stream
268
is split in splitter 270 into streams 272 and 332. Under the parameters of the
specific
example and operating conditions described herein, splitter 270 is set so that
100% of
stream 268 is directed to stream 332. However, under other operating
conditions and
parameters, some of the flow from stream 268 may be directed to stream 272.
Stream
332 at -119 F and 617 psia passes through expansion valve 334 exiting as
stream 336
at -154 F and 315 psia. Stream 336 then passes through heat exchanger 222 and
exits as stream 338.
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[0038] The remaining methane enriched streams 284, 304, and 230 are
essentially the bottoms stream from the second stage fractionating column 215,
after
being further processed as described below. Bottoms stream 286 enters reboiler
288 to
produce vapor stream 290 and liquid stream 292. Liquid stream 292 is split by
splitter
294 into streams 296 (approximately 42% of the flow), 308 (approximately 37%
of the
flow), and 326 (approximately 21% of the flow). Streams 292, 296, 308, and 326
are all
at -164 F and 315 psia. Stream 296 passes through expansion valve 298, with
stream
300 exiting the valve at -200 F and 125 psia. Stream 300 then passes through
heat
exchanger 232 to stream 302, then through heat exchanger 222 to stream 304.
Stream
304 is approximately 3% nitrogen and 96% methane at 107.5 F and 116 psia.
[0039] Stream 308 passes through subcooler 252, exiting as stream 310 at
approximately -285 F and 310 psia. Stream 310 passes through secondary JT
valve
312, with stream 314 exiting the valve. Stream 314 is approximately the same
temperature as stream 310, but the pressure has been reduced to approximately
36
psia. Further pressure drop is achieved as stream 314 flows through a vertical
(up)
length of pipe, becoming stream 316 at 21 psia. Stream 316 passes through
condenser
or heat exchanger 318, supplied with energy stream Q-222 from condenser 242,
and
exits as stream 320 warmed to -252 F. Stream 320 flows through a vertical
(down)
length of pipe, becoming stream 322, although there is a negligible change in
temperature and pressure between streams 320 and 322 in this example. Stream
322
then passes through subcooler 252, exiting as stream 324 warmed to -200 F and
with
a slight drop pressure. Stream 324 then passes through mixer 278 where it is
combined with stream 276 to form stream 280. Stream 272 from splitter 270 is
controlled by valve 274, from which stream 276 exits. In this example, no flow
is
directed to streams 272 or 276, so stream 280 is the same composition as
stream 324.
However, under other operating conditions and parameters, some of the flow
from
stream 268 may be directed to stream 272 through slitter 270. Stream 280 then
passes
through heat exchanger 232, with stream 282 warmed to -169 F exiting and
passing
through heat exchanger 222. Stream 284, containing 3% nitrogen and 96% methane
at
107.5 F and 16 psia, exits heat exchanger 222 from stream 282.
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[0040] Stream 326 is mixed with stream 414 (from FIG. 6) in mixer 416
resulting
in stream 328. Stream 328 passes through heat exchanger 322, with stream 330
warmed to 109 F and at 307 psia exiting the heat exchanger. Stream 330
contains 3%
nitrogen and 94% methane. Three of the four methane enriched streams, 284,
304,
and 330, are each at different pressures, increasing from the low pressure
stream 284
(at 16 psia) to the high pressure stream 330 (at 307 psia). These streams all
feed into
processing stage 218 (Fig 7), where they pass through a series of compressors
(described below) to achieve a processed gas stream of sufficient pressure for
sale.
[0041] Referring to FIG. 6, the NGL extraction processing stage 410 of system
210 is depicted. Stream 338 containing 3% nitrogen, 88% methane, 5% ethane,
and
1.9% propane at -115 F and 312 psia feeds NGL fractionating column 411. This
fractionating column 411 produces an overhead stream 414, containing 3.2%
nitrogen
and 94% methane, that is mixed with stream 326 (see FIG. 5) and a bottoms
stream
418 primarily containing NGL, such as ethane and propane. Fractionating column
411
is supplied with heat (designated as energy stream Q-214) from heat exchanger
222.
Bottoms stream 418 enters virtual reboiler 420 to produce vapor stream 422 and
liquid
stream 412. The liquid stream 412 is the NGL product stream containing 42.5%
ethane,
27% propane, 0.53% methane, 138 ppm carbon dioxide and a trace amount of
nitrogen
at 90 F and 314 psia. Virtual reboiler is supplied with heat (designated as
energy
stream Q-212) from heat exchanger 222.
[0042] Referring to FIG. 7, stream 284 is compressed by compressor 340
(supplied by energy stream Q-340) emerging as stream 342. Stream 342 is at 299
F
and 45 psia, but decreases in temperature (and slightly in pressure) after
passing
through combination heat exchanger/vessel 344 to emerge as stream 346 at 120
F and
40 psia. Stream 346 is compressed by compressor 348 (supplied by energy stream
Q-
348) emerging as stream 350 at 321 F and 115 psia. Stream 304 is combined
with
stream 350, both having substantially equal pressures, and the combined stream
passes through the next combination heat exchanger/vessel 352 to emerge as
stream
354 at 120 F and 110 psia. Stream 354 is then compressed by compressor 356
(supplied by energy stream Q-356) emerging as stream 358 at 315 F and 305
psia.
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Stream 330 is combined with stream 358, both having substantially equal
pressures,
and the combined stream passes through the next combination heat
exchanger/vessel
360 to emerge as stream 362 at 120 F and 300 psia. Stream 362 is compressed
by
compressor 364 (supplied by energy stream Q-364) emerging as stream 366.
Stream
366 passes through the next vessel 368 to emerge as processed gas stream 220.
The
processed gas stream 220 is at 120 F and 825 psia, containing 3% nitrogen
and
94.5% methane, suitable for sale. As the temperature of the streams passing
through
vessels 344, 352, 360, and 368 drops, energy streams represented by Q-344, Q-
352,
Q-360, and Q-368 are created by commercially available heat exchange cooling
equipment and may be used to supply energy to other components of the system
10 or
other process systems. The power requirements for successively compressing the
streams, represented by Q-340, Q-348, Q-356, and Q-364 (see Table 7 below),
are
substantially lower than the overall power requirements for traditional NRU
systems.
[0043] Acceptable inlet compositions in which this invention may operate
satisfactorily are listed in the following Table 4:
TABLE 4
INLET STREAM COMPOSITIONS - NGL Recovery
Inlet Component
Acceptable Inlet Composition Ranges
Methane 20-90% 20-95%
Ethane and Heavier Components 5-10% 0-20%
Carbon Dioxide 0-75 ppm
Nitrogen 5-80%
[0044] The flow rates, temperatures and pressures of various flow streams
referred to in connection with the discussion of the system and method of the
invention
in relation to FIGS. 5, 6, and 7, for an NRU feed gas flow rate of 250 MMSCFD
containing 25% nitrogen, 70% methane, 3% ethane, and 25 ppm carbon dioxide,
appear in Tables 5 and 6 below. The values for the energy streams referred to
in
connection with the discussions of the system and method of the invention in
relation to
FIGS. 5, 6, and 7 appear in Table 7 below. The values discussed herein and in
the
tables below are approximate values.
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TABLE 5
FLOW STREAM PROPERTIES - NGL Recovery
Stream % % Flow Temperature Pressure
Reference N2 CI-14 Rate (deg. F) (psia)
Numeral (lbmol/h) ,
211 25 70 27450 _ 115 865 ,
216 99.2 0.8 6277 105 16
'
220 3 94.5 20277 120 825 ,
224 25 70 27450 -162.5 860
228 25 70 27450 -164 615
230 50 49.6 12837 -164 615
234 50 49.6 12837 -225 613 .
238 50 49.6 12837 -226 325
240 98.4 1.6 12754 -247 315
244 97.7 2.3 6477 -248 315
246 99.2 , 0.8 6277 -248 315
250 99.2 0.8 6277 -306.5 30
254 99.2 0.8 6277 -258 25
256 99.2 , 0.8 6277 -172.5 21
262 5.6 89.6 31962 -127 617
266 7.7 91 17350 - -119 617
268 3 87.9 14613 -119 617
'
280 3 96.4 2400 -200 18.3
282 3 96.4 2400 -169 17
284 3 96.4 2400 107.5 16
286 5.1 94.4 8862 -168 315
290 11.1 88.8 2302 -164 315
292 3 96.4 6560 -164 315
296 3 96.4 2750 -164 315
300 3 96.4 2750 -200 , 125
302 3 96.4 2750 -169 121
304 3 96.4 2750 107.5 116
308 3 96.4 2400 -164 315
310 3 96.4 2400 -285 310
314 3 96.4 2400 -284 36
316 3 96.4 2400 -284 21
320 3 96.4 2400 -252 19
322 3 96.4 2400 -252 19
324 3 96.4 2400 -200 18
326 3 96.4 1410 -164 315
328 3.2 93.8 15127 -134 312
330 3.2 93.8 15127 109.5 307
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Stream % % Flow Temperature Pressure
Reference N2 CH4 Rate (deg. F) (psia)
Numeral (lbmol/h)
332 3 87.9 14613 -119 617
336 3 87.9 14613 -154 315
338 3 87.9 14613 -115 312
342 3 96.4 2400 298.5 45
346 3 96.4 2400 120 40
350 3 96.4 2400 321 115
354 3 96.4 5150 120 110
358 3 96.4 5150 315 305
362 3.1 94.5 20277 120 300
366 3.1 94.5 20277 315 830
414 3.2 93.6 13717 -115 312
TABLE 6
FLOW STREAM PROPERTIES - NGL Recovery
Stream % N2 % % % Flow Temp. Pressure
Reference CH4 C2H3 C3H8 Rate (deg. F) (psia)
Numeral (Ibmol/h)
338 3 87.9 5.4 1.9 14613 -115 312
412 trace .53 42.5 26.8 895 90 314
414 3.2 93.6 2.95 0.23 13717 -115 312
418 trace 1.15 48.5 24.9 1118.8 74.3 314
422 trace 3.6 72.8 17.5 223.5 90 314
TABLE 7
ENERGY STREAM REPORT - NGL Recovery
Eneregy Energy Power From To
Stream Rate
)
Reference (Btu/h) (hp
Numeral
Q-210 2.54E+07 9980 Heat Virt.
Exchanger Reboiler
222 264
Q-212 1.43E+06 563 Heat Viii.
Exchanger Reboiler
222 420
Q-214 5E+06 1965 Heat Fractionator
Exchanger 411
222
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Q-216 5.67E+06 2229 Heat Virt.
Exchanger Reboiler
222 288
Q-218 1.1E+07 4323 Heat Fractionator
Exchanger 215
232
Q-220 4E+06 1572 Heat Fractionator
Exchanger 215
232
Q-222 8.43E+06 3311 Condenser Heat
242 Exchanger
318
Q-340 4.25E+06 1669 Compressor
340
Q-344 4.01E+06 1577 Vessel
344
Q-348 4.52E+06 1775 Compressor
_____________________________________________ 348
Q-352 4.29E+06 1685 Vessel
352
Q-356 9.27E+06 3644 Compressor
356
Q-360 8.21E+06 3228 Vessel
360
Q-364 3.59E+07 14114 Compressor
364
Q-368 4.11E+07 16142.5 Vessel
368
[0045] Other alterations and modifications of the invention will likewise
become
apparent to those of ordinary skill in the art upon reading this specification
in view of the
accompanying drawings, and it is intended that the scope of the invention
disclosed
herein be limited only by the broadest interpretation of the appended claims
to which the
inventor is legally entitled.
