METHOD TO PRODUCE NATURAL GAS LIQUIDS (NGL'S) AT GAS PRESSURE REDUCTION STATIONS

METHODE DE PRODUCTION DE LIQUIDES DE GAZ NATUREL (LGN) AUX STATIONS DE REDUCTION DE PRESSION DES GAZODUCS

English Abstract

A method to recover NGL's at gas Pressure Reducing Stations. A first step involve providing at least one heat exchanger having a flow path for passage of high pressure natural gas with a counter current depressurized lean cold gas. A second step involves passing the high pressure natural gas stream in a counter current flow with the lean cold gas and cooling it before de-pressurization. A third step involves the expansion of the high pressure cooled gas in a gas expander. The expansion of the gas generates shaft work which is converted into electrical power by the power generator and the expanded low pressure and cold gas enters a separator where NGL's are recovered. This process results in the recovery NGL's, electricity and the displacement of a slipstream of natural that is presently used to pre- heat gas at Pressure Reduction Stations.

French Abstract

Divulgation d'une méthode pour récupérer du GNL dans des postes de réduction de la pression gazeuse. Cette méthode comprend les étapes suivantes : fourniture à au moins un échangeur de chaleur comportant un circuit pour le passage de gaz naturel sous haute pression avec un gaz pauvre froid dépressurisé par un contre-courant: le passage du circuit de gaz naturel sous haute pression dans un flux à contre-courant avec le gaz pauvre froid et son refroidissement avant la dépressurisation: l'expansion du gaz refroidi sous haute pression dans un dispositif d'expansion. L'expansion du gaz produit du travail mécanique qui est converti en puissance électrique dans une génératice, et le gaz froid à faible pression détendu passe dans un séparateur dans lequel le GNL est récupéré. Ce procédé permet la récupération de GNL, d'électricité et un écoulement de gaz naturel qui est utilisé pour préchauffer le gaz dans des postes de réduction de la pression.

Claims

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What is Claimed is:
1. A method to recover natural gas liquids (NGL's) at pressure reduction
stations,
comprising the steps of:
providing at least one heat exchanger, each heat exchanger having a flow path
for
passage of a high pressure natural gas stream and a counter current passage
for a
depressurised cold lean gas stream;
passing the high pressure natural gas stream along the heat exchanger in order
to
cool the high pressure natural gas stream through a heat exchange with the
depressurized
cold lean gas stream before pressure reduction, such that hydrates are
condensed out of the
high pressure natural gas stream;
removing hydrates from the high pressure natural gas stream;
passing the high pressure natural gas stream, which has had hydrates removed,
through a gas expander to reduce pressure of the natural gas stream;
passing the natural gas stream, which has been reduced in pressure through a
separator to produce a first stream of depressurized cold lean natural gas and
a second
stream of NGL's.
2. The method of Claim 1, including a step of heating a portion of the first
stream and then
blending selected quantities of the heated portion of the first stream with
selected quantities
of an unheated portion of the first stream.
3. The method of Claim 1, including a step of heating at least a portion of
the first stream by
passing the portion of the first stream through a heat exchanger.
4. The method of Claim 3, including a step of heating at least a portion of
the first stream by
passing the portion of the first stream through a heat exchanger to effect a
heat exchange
with ambient outdoor air.
5. The method of Claim 3, including a step of heating at least a portion of
the first stream by
passing the portion of the first stream through a heat exchanger having a
counter current

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waste heat stream.
6. The method of Claim 1, including a step of connecting the gas expander to a
power
generator and using the power generated to run heaters and heating at least a
portion of the
first stream with the heaters.

Description

CA 02588664 2007-05-09
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TITLE OF THE INVENTION:
Method to produce Natural Gas Liquids (NGL's) at Gas Pressure Reduction
Stations.
FIELD OF THE INVENTION
The present invention relates to a method of producing NGL's at gas Pressure
Reduction Stations when the pressure is letdown from gas main transmission
lines to local gas
distribution lines.
BACKGROUND OF THE INVENTION
In gas Pressure Reduction Stations, the gas is pre-heated before the pressure
is
dropped to prevent the formation of hydrates which can cause damage to the
pipeline and
associated equipment. The typical pressure reduction varies between 400 to 900
PSIG
(pounds per square inch gage) for main transmission gas lines to local
distribution lines and
from 50 to 95 PSIG from local distribution lines to consumers. When gas is
depressurised the
temperature drops. The rule of thumb is that for every 100 pounds of pressure
drop across a
pressure reducing valve the gas temperature will drop by 7 F. When the
pressure is reduced
by the use of an expander, the temperature drop is greater because it produces
work. The heat
required to prevent formation of hydrates is normally provided by hot water
boilers, gas fired
line heaters or waste heat from; gas turbines, gas engines or fuel cells. In
some stations, due
to its large volumetric flows and pressure drops, energy can be and is
recovered, by a
combination of gas expander and boiler. For a more efficient recovery,
combinations of gas
expanders with CHP processes (Combined Heat and Power) or CCHP (Combined
Cooling
Heat and Power) processes are possible. The limitation in these applications
are the
economics which are driven by flow volumes, pressure delta, seasonal
volumetric flows and
24 hour volumetric flows. Because of so many variables that impact on the
economics of
adding a gas expander be it with: a boiler, CHP or CCHP the current gas
pipeline operators
choose to pre-heat the gas by the use of boilers and or heaters. In all of the
above practices,
there is no attempt made to recover NGL's present in the natural gas stream at
Metering
and Pressure Reduction Stations. The typical practice is to have large
facilities upstream
in the transmission line known as Straddle Plants which recover a percentage
of the
NGL's for feedstock to the petrochemical industry.

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SUMMARY OF THE INVENTION
According to the present invention there is provided a method to remove water
present in the gas stream, produce NGL's and then pre-heat the gas to meet
pipeline
specifications. This method recovers NGL's, removes water and eliminates the
present
practice of using natural gas as a fuel for; boilers, heaters, gas turbines,
gas engines or fuel
cells to pre-heat the natural gas before pressure reduction. Moreover, the
present invention
provides the ability to recover most of the energy available for recovery at
pressure reduction
stations. A first step has at least one heat exchanger, with a first flow path
for passage of
incoming high pressure gas that indirectly exchanges heat with a counter
current lower
pressure cold gas stream. The low pressure cold gas stream flow can be
controlled to meet
desired temperatures in the high pressure gas stream through the use of a by-
pass around the
heat exchanger. The now cold high pressure gas enters a vessel separator,
where water is
removed. A second step involves passing the high pressure cold and water free
gas stream
through a gas expander, dropping the pressure to local distribution pipeline
spec generating
shaft work and a further drop in temperature. The shaft rotates a power
generator producing
electricity and the lower pressure colder gas enters a separator where NGL's
are recovered.
The objective being to control the temperature upstream of the gas expander to
meet the
desired NGL's recovery. The third step involves the use of the generated
electricity as an heat
source to the heat exchanger that controls the gas supply temperature to the
local distribution
pipeline. This eliminates the existing practice of combusting natural gas to
pre-heat the gas to
prevent the formation of hydrates. The fourth step involves the use of air
exchangers to
release part or all of the cold energy to the surroundings, this provides the
ability to export
electricity at warm atmospheric conditions.
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
invention to the particular embodiment or embodiments shown, wherein:
FIG. 1 is a schematic diagram of a typical method to pre-heat gas at gas
Pressure

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Reduction Stations (PRS) in the prior art.
FIG. 2 is a schematic diagram that depicts the embodiment of the invention.
FIG. 3 is a variation on the embodiment of the invention.
FIG. 4 is another variation on the embodiment of the invention.
FIG. 5 is another variation of the embodiment of the invention to liquefy
gases.
DETAILED DESCRIPTIONS OF A TYPYCAL PRS AND THE PREFERRED
EMBODIMENT
The typical method that presently is used to pre-heat natural gas at Pressure
Reduction
Stations will now be described with reference to FIG. 1.
In this typical gas pre-heating process, gas enters a station via gas supply
line 1.
The gas stream enters filter 20 to remove any debris in the stream. The
filtered gas exits
the filter through line 2 and enters heat exchanger 21 for pre-heating. The
heated gas exits
through line 3 and the pressure is reduced at Pressure Reducing Valve (PRV)
22. A by-
pass with PRV 23 is provided for service reliability, for scheduled and
unscheduled
maintenance. The PRV pressure is controlled by Pressure Transmitter (PT) 27 at
a pre-set
pressure. The low pressure controlled gas stream 4 feeds a gas slipstream 5
for
combustion in a heater/boiler 24. The gas slipstream flow 5 is controlled by
Temperature
Controller (TC) 26 at a pre-set temperature. The gas stream 6 is metered at
Flow Meter
(FM) 25 and delivered to consumers.
The preferred embodiment will now be described with reference to FIG. 2. In
the
preferred embodiment, the gas enters a station through supply line 1. The high
pressure
gas stream enters filter 50 to remove any debris in the stream. The filtered
gas exits filter
50 through gas line 2 and passes through heater exchanger 51. At heater
exchanger 51 the
high pressure gas is cooled by the counter current depressurized gas stream to
condense
any water present in the high pressure gas stream. The cooled high pressure
gas stream in
line 5 is discharged into separator 52. The water exits through line 7 and the
dried gas
exits through line 6. The high pressure gas is routed through line 9 to gas
expander 54,
producing shaft work and a drop in gas temperature. The shaft rotates power
generator 55,
producing electricity. The produced electricity is carried by electrical wires
23 to

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electrical heater 58. A by pass JT valve 53, supplied by line 8 is provided
for startup and
emergency services.
The low pressure cold gas in line 10 flows into separator 56 where NGL's are
separated and recovered. The NGL's exit through line 11. The lean cold gas
exits the
separator through line 12 and can be routed through line 13 and line 15 to
meet desired
operations temperatures. The lean gas stream in line 13 enters an air
exchanger 57 where
the cold energy is dissipated into the atmosphere by natural draft, the amount
of cold
energy dissipated to the atmosphere is dependent on the choice and objectives
of the local
plant. The lean stream exits air exchanger 57 through line 14 at near
atmospheric
temperatures. The warmer lean gas stream 14 can be blended through line 16 or
line 18 to
meet desired operations temperatures. The lean and cold gas stream in line 15
can be sent
directly or blended with stream 16 and sent to heat exchanger 51 to cool in a
counter
current flow the incoming high pressure rich gas stream. The lean
depressurized gas exits
heat exchanger 51 through line 19 and blends with stream 18 into stream 20.
The blended
stream 20 enters line 4 and is routed to heater 58 to increase the lean gas
temperature to
local distribution pipeline specifications. The heat is supplied by the power
generator 55
and transmitted through electrical wires 23 to the heating elements in heater
58. The
heated lean gas in line 21 is measured in meter 59. A temperature controller
60 controls
the heat supplied to heater 58. A pressure controller 61 controls the pressure
to the local
distribution pipeline 22.
A variation is depicted in FIG. 3, which shows stream 6 passing through a JT
valve
rather than through a gas expander as shown in FIG. 2. There is no power
generation and no
air/heat exchangers just NGL's recovery. Moreover, the cold temperatures
generated by
dropping the pressure through a JT valve will not be as cold as through the
expander since
no work is done.
A further variation is depicted in FIG. 4, which shows stream 3 going straight
into
separator 51, no pre-cooling heat exchange upstream of this separator as in
FIG. 2 and FIG.
3. The NGL's are recovered and separated in vessel 55 and removed through line
9. The lean
gas flow 10 is pre-heated in a atmospheric air/heat exchanger.

CA 02588664 2007-05-09
A further variation is depicted in FIG. 5, which shows the pre-heating
exchanger 56
being through a waste heat stream 14. This stream could be hot water, steam,
flue gases, etc.
5 The preferred embodiment in FIG. 2 has the advantage over the present
practice in
that it substantially reduces and or eliminates the use of a gas slipstream to
pre-heat the
gas prior to de-pressurization and recovers NGL's, a feedstock to the
petrochemical
industry. This is significant when one considers that it can replace existing
PRV's (known
in the industry as JT valves) and line heaters. Associated with it is the
reduction or
elimination of emissions presently generated in these line heaters. Moreover,
the energy
used to replace the slipstream gas is recovered energy (no new emissions
generated) which
presently is dissipated across a PRV.

Representative Drawing