Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TITLE
[0001] Method of recovery of natural gas liquids from natural gas at NGLs
recovery
plants.
FIELD
[0002] The present invention relates to methods for recovery of natural
gas liquids
(NGLs) from methane rich gases using liquid natural gas (LNG). More
particularly, the
present invention provides methods to efficiently and economically achieve
higher recoveries
of natural gas liquids at NGL recovery plants.
BACKGROUND
10003] Natural gas from producing wells contain natural gas liquids
(NGLs) that are
commonly recovered. While some of the needed processing can be accomplished at
or
near the wellhead (field processing), the complete processing of natural gas
takes place at
gas processing plants, usually located in a natural gas producing region. In
addition to
processing done at the wellhead and at centralized processing plants, some
final
processing is also sometimes accomplished at 'straddle plants'. These plants
are located on
major pipeline systems. Although the natural gas that arrives at these
straddle plants is
already of pipeline quality, there still exists quantities of NGLs, which are
recovered at
these straddle plants.
100041 The straddle plants essentially recover all the propane and a
large fraction of the
ethane available from the gas before distribution to consumers. To remove
NGLs, there are
three common processes; Refrigeration, Lean Oil Absorption and Cryogenic.
[00051 The cryogenic processes are generally more economical to operate
and more
environmentally friendly, cun-ent technology generally favors the use of
cryogenic processes
over refrigeration and oil absorption processes. The first generation
cryogenic plants were
able to extract up to 70% of the ethane from the gas, modifications and
improvements to these
cryogenic processes overtime have allowed for much higher ethane recoveries
>90%. This
increase in recovery comes with consumption of relatively large quantities of
energy due to
their compression requirements. Prior art has taught that use of lean reflux
streams reduce
energy consumption and achieves high ethane recoveries. Moreover, methane gas
has been
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proven to be a superior stripping gas to control carbon dioxide concentrations
in NGL
product Many patents exist disclosing improved designs for generation of lean
reflux to
recover ethane and heavier components in NGL plants, they typically involve
significant
capital expenditures and increased operational costs. A need exists for an
efficient ethane and
NGL recovery process that is capable of achieving very high ethane recoveries
at a lower
energy consumption and a lower capital cost when compared to prior art.
SUMMARY
[0006] The present invention provides a method for recovery of natural gas
liquids from
natural gas streams in a NGL recovery plant. The method involves the use of
LNG as a reflux
stream, a feed mixer and a stripping gas in the operation of a LNG recovery
plant. The use of
LNG as stored cold energy to control a NGL distillation column temperature
profile and
operation, increases the efficiency and recovery of NGLs in natural gas
streams. Moreover,
LNG, primarily methane, is an ideal stripping gas to control carbon dioxide
concentration in
the NGL product stream.
[0007] As will hereinafter be further described, the interacting step can
be either direct or
indirect. Direct interaction is achieved by injecting LNG as a liquid reflux
to the distillation
column to control overhead temperature, by direct mix with expanded gas stream
to control
distillation column pressure and as a stripping gas for carbon dioxide control
in NGL product
stream. Indirect interaction is achieved by, first cooling the distillation
column overhead
stream in a heat exchanger and then used as a reflux in the distillation
column. The
condensate generated from overhead stream is used as a second reflux stream
for a dual reflux
operation, increasing NGLs recovery.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other features will become more apparent from the
following
description in which reference is made to the appended drawings, the drawings
are for the
purpose of illustration only and are not intended to be in any way limiting,
wherein:
FIG. 1 is a schematic diagram of a facility equipped with LNG storage and
supply for direct cooling in accordance with the teachings of the present
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invention.
FIG. 2 is a schematic diagram of a facility equipped with LNG storage and
supply for indirect cooling in a heat exchanger to generate a second reflux
stream.
DETAILED DESCRIPTION
[0009] The method will now be described with reference to FIG. I.
[0010] Referring to FIG. I, a pressurized natural gas stream 1 is routed
to heat exchanger
2 where the temperature of the feed gas stream is reduced by indirect heat
exchange with
counter-current cool streams 24, 19, 6 and 21. The cooled stream 101 enters
feed separator 3
where it is separated into vapour and liquid phases. The liquid phase stream 4
is expanded
through valve 5 and pre-heated in heat exchanger 2 prior to introduction into
distillation
column 20 through line 6. The gaseous stream 7 is routed to gas expander 8.
The expanded
and cooler vapor stream 9 is mixed with LNG for temperature control and routed
through
stream 17 into the upper section of distillation column 20. A LNG storage drum
10, supplies
LNG through line 11 to LNG pump 12. The pressurized LNG stream 13 is routed
through
temperature control valve 14 providing the reflux stream to distillation
column 20. A
slipstream from the pressurized LNG stream 13 provides temperature control to
stream 9
through temperature control valve 16, temperature controlled stream 17 enters
the upper
section of distillation column 20. The controlled temperature of stream 17 by
addition of LNG
enables operation of the distillation column at higher pressures to compensate
for the loss of
coolth energy generated by the expander at higher backpressures. A second
slipstream from
pressurized LNG stream 13 provides methane for carbon dioxide stripping
through flow
control valve 18, the LNG is pre-heated in heat exchanger 2 before
introduction into the lower
section of the distillation column 20 as a stripping gas. The distilled stream
21, primarily
methane, is pre-heated in heat exchanger 2 and routed to compressor 22 for
distribution and or
recompression through line 23. The liquid fraction stream 24 is reboiled in
heat exchanger 2
and routed back to the bottom section of distillation column 20, to control
NGL product
stream 25.
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[0011] Referring to FIG. 2, the coolth energy of LNG is used to first
condense the
overhead stream of the distillation column generating a second reflux stream
before its use as
the primary reflux stream, allowing for an increase in efficiency in plant
operations. A
pressurized natural gas stream 1 is routed to heat exchanger 2 where the
temperature of the
feed gas stream is reduced by indirect heat exchange with counter-current cool
streams 24, 19,
6 and 125. The cooled stream 101 enters feed separator 3 where it is separated
into vapour
and liquid phases. The liquid phase stream 4 is expanded through valve 5 and
pre-heated in
heat exchanger 2 prior to introduction into distillation column 20 through
line 6. The gaseous
stream 7 is routed to gas expander 8, the expanded and cooler vapor stream 9
is routed
through stream 17 into the upper section of distillation column 20. A LNG
storage drum 10,
supplies LNG through line 11 to LNG pump 12. The pressurized LNG stream 13
enters heat
exchanger 114 and is routed through temperature control valve 115 as reflux
stream 116 to
distillation colutrut 20. A slipstream from pressurized LNG stream 13 provides
methane for
carbon dioxide stripping through flow control valve 18, the LNG is pre-heated
in heat
exchanger 2 before introduction into the lower section of the distillation
column 20 as a
stripping gas. The distilled stream 120, primarily methane, is cooled in heat
exchanger 114
and discharged into overhead separator 121. The condensed stream 122 feeds
reflux pump
123, and the pressurized reflux stream 124 enters distillation column 20 as a
second reflux
stream for a dual reflux distillation column operation. The vapour stream 125
is pre-heated in
heat exchanger 2 and routed to compressor 22 for distribution and/or
recompression through
line 23. The liquid fraction stream 24 is reboiled in heat exchanger 2 and
routed back to the
bottom section of distillation column 20, to control NGL product stream 25.
[0012] In the preferred method, LNG provides stored cold energy that
improves the
operation and efficiency of NGL distillation columns. The above described
method uses this
stored cold energy to condense natural gas liquids from natural gas streams by
direct mixing.
This direct mixing provides better heat transfer and reduces the energy
requirements to
condense NGLs. It also reduces the energy required for recompression of gas
for distribution.
[0013] In this patent document, the word "comprising" is used in its non-
limiting sense to
mean that items following the word are included, but items not specifically
mentioned are not
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excluded. A reference to an element by the indefinite article "a" does not
exclude the
possibility that more than one of the element is present, unless the context
clearly requires that
there be one and only one of the elements.
5 [0014] The following claims are to be understood to include what is
specifically
illustrated and described above, what is conceptually equivalent, and what can
be obviously
substituted. Those skilled in the art will appreciate that various adaptations
and modifications
of the described embodiments can be configured without departing from the
scope of the
claims. The illustrated embodiments have been set forth only as examples and
should not be
taken as limiting the invention. It is to be understood that, within the scope
of the following
claims, the invention may be practiced other than as specifically illustrated
and described.
