Note: Descriptions are shown in the official language in which they were submitted.
CA 02552865 2006-07-14
TITLE OF THE INVENTION:
Method for selective extraction of natural gas liquids from "rich" natural gas
FIELD OF THE INVENTION
The present invention relates to a method for selective extraction of natural
gas liquids
from "rich" natural gas
BACKGROUND OF THE INVENTION
Natural gas corning from a producing well contains many natural gas liquids
(NGLs) that are commonly removed. The removal of natural NGLs usually takes
place in
a relatively centralized processing plant. The objective is to reduce the
hydrocarbon dew
point to prevent problems in the pipelines from liquid fallout. To remove
NGLs, there are
three common processes; Refrigeration, Lean Oil Absorption and Cryogenic.
With Refrigeration, a refrigeration plant is employed to provide cold to lower
the
temperature of the natural gas. Refrigeration is able to extract a large
percentage of
propane and most of the butane and heavier components.
With Lean Oil Absorption, an absorbing oil with an affinity for NGLs is
brought
2 0 into contact with natural gas in a contact tower where it soaks up a
high proportion of
NGLs. The "rich" absorption oil, now containing NGLs exits the absorption
tower. This
"rich" mixture of absorbing oil and NGLs is chilled to ¨30 F to separate the
NGLs and
absorbing oil. This process can extract 90% of the propane and heavier
hydrocarbons and
about 30% of the ethane.
The cryogenic process enables higher recoveries of ethane. The first
generation
cryogenic plants were able to extract up to 70% of the ethane from the gas,
since the early
1990s, modifications to the cryogenic process have allowed ethane recoveries
up to 99%
extraction level. This increase in recovery comes with higher operating costs.
There are a
3 0 number of different ways to chill the gas the one most commonly used is
the turbo
expander process. In this process external refrigerants are used to cool the
natural gas
stream, then an expansion turbine is used to rapidly expand the chilled gases,
which
causes the temperature to drop significantly. This rapid temperature drop
condenses
CA 02552865 2006-07-14
2
ethane and other hydrocarbons in the gas stream while maintaining methane in a
gaseous
form.
Operations of gas processing plants in reduced recovery modes is difficult,
the plants are
typically designed to achieve high recoveries of all the NGLs and are not
designed to
recover only pentanes and heavier or only butanes.
SUMMARY OF THE INVENTION
There is provided a method for selective extraction of natural gas liquids
from "rich"
natural gas. The method involves the step of effecting a heat exchange between
a rich natural
gas stream and a refrigerant fluid to lower a temperature of the rich natural
gas stream. The
heat exchange is controlled to lower the temperature of the rich natural gas
stream to a
selected hydrocarbon dew point in order to condense at least one selected
hydrocarbon liquids
carried in the rich natural gas stream.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention 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 in any way limit
the scope of the
2 0 invention to the particular embodiment or embodiments shown, wherein:
FIG. 1 is a schematic diagram of a facility equipped with indirect cooling in
accordance with the teachings of the present invention.
FIG. 2 is a schematic diagram showing a variation of the indirect cooling
illustrated
in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred method will now be described with reference to FIG. 1.
Referring to FIG. 1, "rich" natural gas stream 20 is straddled into stream 30
for
indirect pre-cooling in heat exchanger 1. The cooling is provided by the
countercurrent flow
of stream 40. The now colder stream 31 then enters separator 2 where water and
heavy
hydrocarbons are condensed and separated from lighter fractions (hydrocarbons,
carbon
CA 02552865 2011-12-20
3
dioxide, nitrogen, etc..). The separated heavier fractions exit separator
vessel 2 through line 60.
The lighter fractions exit through line 32 which then enters heat exchanger
cold box section 6 for
further cooling. The colder stream 33 now enters separator vessel 3. The
condensed and
separated propane exits separator vessel 3 through line 61. The separated
lighter fractions leave
separator 3 through line 34 for further cooling in heat exchanger cold box
section 7. The colder
stream 35 now enters separator vessel 4. The condensed and separated carbon
dioxide exits
separator vessel 4 through line 62. The separated lighter fractions leave
separator 4 through line
36 for further cooling in heat exchanger cold box section 8. .The colder
stream 37 now enters
separator vessel 5. The condensed and separated ethane exits separator vessel
5 through line
63. The now "lean" gas leaves separator 5 through line 38 for pre-heating in
cold box section 7
and cold box section 6. The now warmer "lean" gas exits cold box through
stream 39 and mixes
with the vaporized LNG stream 52 to form the mixture stream 40. The mixed
"lean" gas now
enters heat exchanger 1 for pre-heating, it then exits it at or near
transmission line temperature
through stream 41. The refrigerant is LNG supplied from tank 9 and pressurized
by pump 10 into
stream 51. The pressurized LNG flows first into heat exchanger cold box
section 8 where it
begins to pick up heat, then into heat exchanger cold box section section 7
and finally into heat
exchanger cold box section 6. This vaporized stream exits the cold box as
stream 52 and is mixed
with the "lean" gas stream 39 to form a "lean" mixture as stream 40.
FIG.2 shows another arrangement for indirect contact method for extraction of
natural
gas liquids where the cold energy supply fluid is one other than LNG, the main
difference
being that when LNG is used it can be injected into the transmission pipeline
while an
optional fluid may or may not be injected into the pipeline. "Rich" natural
gas stream 20 is
straddled into stream 30 for indirect pre-cooling in heat exchanger 1. The
cooling is provided
by the countercurrent flow of stream 39. The now colder stream 31 then enters
separator 2
where water and heavy hydrocarbons are condensed and separated from lighter
fractions
(hydrocarbons, carbon dioxide, nitrogen, etc..). The separated heavier ft-
actions exit separator
vessel 2 through line 60. The lighter fractions exit through line 32 which
then enters heat
exchanger cold box section 6 for further cooling. The colder stream 33 now
enters separator
vessel 3. The condensed and separated propane exits separator vessel 3 through
line 61. The
separated lighter fractions leave separator 3 through line 34 for further
cooling in heat
CA 02552865 2006-07-14
4
exchanger cold box section 7. The colder stream 35 now enters separator vessel
4. The
condensed and separated carbon dioxide exits separator vessel 4 through line
62. The
separated lighter fractions leave separator 4 through line 36 for further
cooling in heat
exchanger cold box section 8. The colder stream 37 now enters separator vessel
5. The
condensed and separated ethane exits separator vessel 5 through line 63. The
now "lean" gas
leaves separator 5 through line 38 for pre-heating in cold box section 7 and
cold box section
6. The now warmer "lean" gas exits cold box through stream 39. The "lean" gas
now enters
heat exchanger 1 for pre-heating, it then exits it at a near transmission line
temperature
through stream 40. The refrigerant is supplied from tank 9 into stream 51.
This refrigerant
1 0 flows first into heat exchanger cold box section 8 where it begins to
pick up heat, then into
heat exchanger cold box section section 7 and finally into heat exchanger cold
box section 6.
This vaporized stream exits the cold box as stream 52. For those skilled in
the art, the
number of heat exchangers and separators can be re-arranged to achieve the
desired
separation of hydrocarbons and other components present in the "rich" gas
stream.
Liquid Natural Gas (LNG) has been selected for the purpose of illustration. It
will be
appreciated that other refrigerants such as liquid nitrogen, liquid carbon
dioxide, liquid
oxygen and the like can be used to condense the "rich" gas stream. It is
preferred that the
refrigerant fluid be within the cryogenic temperature range, merely because
colder
temperatures are required in order to condense some of the natural gas
liquids, such as ethane.
Other hydrocarbon refrigerants can be used such as ethane and propane. For
example, liquid
carbon dioxide could be used as a refrigerant to condense a number of natural
gas liquids, but
would not be effective in condensing ethane. There are drawbacks to the use of
some
refrigerant fluids, such as liquid oxygen. Liquid oxygen could be used, but is
not preferred
2 5 due to safety concerns. Liquid Natural Gas and Liquid Nitrogen are two
of the more viable
refrigerants which could be used.
In the preferred method, refrigerant fluids provide the "cold energy" required
to
condense and extract the NGLs . A typical straddle plant is designed to
achieve high
recoveries of all NGLs and the "turndown" to lower recoveries are difficult to
obtain. The
above method allows for ease of "turndown" by simply changing the temperature
set point
controller which then changes the LNG flow rate. As the LNG gives up its cold
energy to
CA 02552865 2011-12-08
condense the NGLs in the "rich" stream it becomes a "lean" gas ready for
distribution.
Existing plants operate in a mode that recovers at least some percentage of
all
components, it is not generally possible to operate the plants to achieve a
specific
5 hydrocarbon dew point. Control of hydrocarbon dew point for gas
transportation is
critical due to the influence of ambient temperatures and pressure reductions
during
transportation that can cause liquid fallout. To reach higher extraction
levels more
expensive metallurgy, more compression, and more capital investment is
required.
According to the present invention there is provided a method for liquefaction
and extraction
of NGLs from natural gas. A pre-cooling takes place in a heat exchanger of the
incoming
"rich" natural gas stream, containing methane, ethane, propane, butanes,
pentanes, other
heavier hydrocarbons, water and carbon dioxide with a countercurrent flow of
"lean" natural
gas. Separation of water and heavier hydrocarbons from lighter hydrocarbons
then takes
place in a series of separators by controlling the temperature at each
separator through a heat
exchange with refrigerant fluids. Cooling upstream of each separator through a
heat exchange
with refrigerant fluids permits selective control of the extraction of NGL's.
The method
provides for ease of "turndown" to achieve high or low recoveries ratios
between and
Hydrocarbon Dew Point (HDC) control. The use of the above described method at
a straddle
plant facility provides a distinct advantage over methods currently in use.
Existing systems
bring the pressure of the natural gas down to remove the natural gas liquids
and then
increasing the pressure of the natural gas back up in order to return the
natural gas to the
pipeline after processing. With the present method, the natural gas can be
freed of the natural
gas liquids without a change in pressure.
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
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.
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.
CA 02552865 2011-12-08
6
The scope of the claims should not be limited by the preferred embodiments set
forth in the
examples, but should be given the broadest interpretation consistent with the
description as a
whole.
