Field of the Invention
5
[OOOI] The present invention relates to a method for
peocessing Liquefied Natural Gas (LNG) . More
particularly, the present invention relates to a method
for recovering heavier hydrocarbons from an LNG stream.
10
Background of the Invention
[0002] Liquefied Natural Gas (LNG) is a natural gas
predominantly comprising methane (CH4 or CI) in liquid
15 form at a very low temperature of approximately -161"~.
The liquid state of methane is easy to store and
transport. The LNG is produced in natural gas rich
regions and transported in liquid state in cryogenic
containers to one or more LNG receiving terminals and LNG
20 re-gasification terminals. The regasification terminals
convert the Liquefied Natural Gas (LNG) back to its
gaseous state at atmospheric conditions.
[0003] Although LNG is predominantly liquefied methane,
25 it also contains small proportions of ethane and other
heavier hydrocarbons including propane, butane, pentane
components. These heavier hydrocarbon components are also
known as value added products and have great demand in
various industries and domestic households. For example,
30 ethane and propane are significant feed stocks for the
petrochemical industry. On the other hand, propane-butane
mixture commonly known as LPG, has great demand as
domestic fuel. Thus, LNG provides an important
alternative source of value added products, which can be
extracted at the LNG receiving terminals. The extracted
value added products have commercial value that
ultimately enhance the economics of the LNG receiving and
5 regasification terminals.
[0004] However, the recovery of the hydrocarbon compounds
from an LNG stream is a complex thermodynamic process
that involves a series of conventional distillation
10 columns for fractionating the compounds. It further
involves various provisions for optimally providing cold
energy to ensure maximum recovery of the hydrocarbons.
[0005] Further, the hydrocarbon extraction process
15 requires compatibility with the existing LNG processing
systems so that it can be implemented effortlessly using
the existing LNG systems. Furthermore, the hydrocarbon
extraction process and systems should be flexible enough
to accommodate new modules or future terminals/facilities
20 of the LNG systems or mechanisms.
[0006] In light of the above, there is a need for a
method for processing Liquefied Natural Gas (LNG) for
recovering heavier hydrocarbons from LNG. Further, there
25 is a need for a method for utilizing cold energy of LNG
feed itself, to ensure maximum recovery of the
hydrocarbons while using minimum equipment to implement
the method. Furthermore, there is a need for a method
that is compatible with the existing LNG systems and is
30 flexible to be accommodated in terminals of future LNG
systems for hydrocarbon recovery.
Summary
[0007] A method for processing Liquefied Natural Gas
(LNG) for recovering hydrocarbons is disclosed. The
5 method comprises the steps of: heating cold liquid LNG
feed in a first heat exchanger to obtain a proportion of
the liquid LNG feed as hot vapour; condensing the hot
vapour by utilizing cold energy of remaining liquid
proportion of the LNG feed; pumping the remaining liquid
10 proportion of the LNG feed into top of a first
hydrocarbon recovery column for recovering overhead
vapours of methane; directing downstream of the first
hydrocarbon recovery column into a second hydrocarbon
recovery column for recovering ethane vapour and propane
15 vapour; directing downstream of the second hydrocarbon
recovery column into a third hydrocarbon recovery column
for recovering LPG (Liquefied Petroleum Gas) component,
the LPG component containing negligible proportions of
pentane and pentane+ hydrocarbons; and providing a side
20 stripper for separating the LPG component, when the LNG
feed contains non-negligible proportions of pentane and
pentane+ hydrocarbons. The processing of the liquid LNG
feed for the recovery of the heavier hydrocarbon
components subsequently provides a return LNG rich in
25 methane component in liquid state.
[0008] In one embodiment of the present invention, the
first hydrocarbon recovery column, the second hydrocarbon
recovery column, and the third hydrocarbon recovery
30 column, are thermodynamically integrated with each other.
The first hydrocarbon recovery column is a de-methanizer
column used for separating methane component that is a
predominant hydrocarbon component of the LNG feed. The
second hydrocarbon recovery column is a de-ethanizer/depropanizer
column (C2/C3 column) . The third hydrocarbon
recovery column is a de-pentanizer column. The
hydrocarbon components recovered from the second
5 hydrocarbon recovery column and the third hydrocarbon
recovery column, are free of methane component.
[OOOS] A compressor is provided for compressing the hot
vapour of the LNG feed, and subsequently feeding the
10 compressed hot vapour to the first heat exchanger along
with the overhead vapours of the first hydrocarbon
recovery column. The compressed hot vapour is therefore
mixed with the overhead vapours of the first hydrocarbon
recovery column for continuously preheating the LNG feed
15 in the first heat exchanger and for obtaining a vapourliquid
mixture containing the predominant methane
component. Further, the vapour-liquid mixture is flashed
into a separator for separating the methane component
from the liquid LNG feed.
20
[OOl 01 In various embodiments of the present invention,
the heavier hydrocarbon components of the LNG feed
provides cold energy to condense the overhead hot vapours
of the second hydrocarbon recovery column. Also, the LNG
25 feed is fed to the hydrocarbon recovery columns at low
pressures. The hydrocarbon components including ethane,
propane, LPG, pentane, and pentane+, are recovered in
liquid state. The LPG is a mixture of propane and butane
that are mixed in proportions according to the Indian
30 Standard (IS) specification.
Brief Description of the Accompanying Drawings
[OOII] The present invention is described by way of
embodiments illustrated in the accompanying drawings
5 wherein:
[0012] FIG. 1 is a schematic flow diagram illustrating
the method for recovering heavier hydrocarbons from LNG,
in accordance with an embodiment of the present
10 invention.
Detailed description of the invention
[0013] A method for recovering heavier hydrocarbons is
15 disclosed. The invention provides a method for recovering
heavier hydrocarbons by processing Liquefied Natural Gas
(LNG). Further, the invention provides a method for
facilitating optimum utilization of cold energy of the
LNG for obtaining maximum recovery of the hydrocarbons
20 while using minimum equipment to implement the process.
In addition, the invention provides a method that is
compatible with the existing LNG systems and is flexible
to accommodate future LNG terminals for hydrocarbon
recovery.
25
[0014] The following disclosure is provided in order to
enable a person having ordinary skill in the art to
practice the invention. Exemplary embodiments are
provided only for illustrative purposes and various
30 modifications will be readily apparent to persons skilled
in the art. The general principles defined herein may be
applied to other embodiments and applications without
departing from the spirit and scope of the invention.
Also, the terminology and phraseology used is for the
purpose of describing exemplary embodiments and should
not be considered limiting. Thus, the present invention
is to be accorded with the widest scope encompassing
5 numerous alternatives, modifications and equivalents
consistent with the principles and features disclosed.
For purpose of clarity, details relating to technical
material that is known in the technical fields related to
the invention have not been described in detail so as not
10 to unnecessarily obscure the present invention.
[0015] The present invention would now be discussed in
context of embodiments as illustrated in the accompanying
drawings.
15
[0016] FIG. 1 is a schematic flow diagram illustrating
the method for recovering heavier hydrocarbons from LNG,
in accordance with an embodiment of the present
invention. The method comprises the steps of receiving
20 LNG feed from one or more sources and processing the
received LNG feed for recovering heavier hydrocarbon
components. The Liquefied Natural Gas (LNG) is generally
used as a refrigerant due to its cold state. The
refrigeration property of the LNG may be utilized for
25 cooling vapours of various component gases, and for
converting the vapours into liquid form. In one or more
embodiments of the present invention, one or more
fractionating columns are used to recover the heavier
hydrocarbons by utilizing the cold energy of the
30 liquefied LNG feed. The one or more fractionating columns
are also known as distillation columns or hydrocarbonrecovery
columns. The lighter hydrocarbons are withdrawn
as 'top products' of the one or more fractionating
7
columns, and are subsequently condensed in one or more
condensers associated with the one or more fractionating
columns. The heavier hydrocarbons on the other hand, are
withdrawn as 'bottom products' from the lower end of the
5 respective fractionating columns. For example, majority
of methane (C1) and a small amount of ethane (C2) are
withdrawn as 'top productsf or 'overhead productsf from
the de-methanizer column because methane and ethane are
lighter hydrocarbons.
10
[0017] The LNG feed may originate from any source of
natural gas or hydrocarbon gas including gas pipelines
and oil production facilities. For example, the LNG feed
can be pumped from an LNG storage tank to the one or more
15 hydrocarbon recovery columns. The 'LNG feed' is a
pressurized feedstock of LNG which is rich in methane (Cl)
component. Other components that are mixed with the
methane in the LNG feed are ethane (C2) , propane (C3) ,
butane (C4) , pentane (C5) , and C5+ components.
20
[0018] In one embodiment of the present invention, and as
shown in Fig. 1, the LNG feed is initially fed into a
heat exchanger 104a, also referred to as a re-condenser
104a, through a pump 102a under a pressure of 2.00kg/cm2.
25 The heat exchanger or the re-condenser 104a exchanges
cold energy of feed LNG with the methane rich vapour from
the system and liquefies the methane rich vapour stream.
Thus, the liquefied LNG feed is heated by the heat
exchanger 104a to convert the cold liquid LNG 'Stream 1'
30 into partial vapour stream. The heat exchanger 104a heats
the LNG feed to the extent of liberating a large portion
of methane (Cl) component in the form of vapour from the
LNG feed. Moreover, the cold energy of the LNG feed due
to its low temperature, is utilized to condense the
liberated methane rich vapour as Stream 2, as shown in
Fig 1. Thus, the LNG feed gets partially vapourized by
exchange of heat with methane rich vapours from
5 downstream equipment. The partial vapour liquid mixture
is flashed in a separator 108 so that the vapour and the
liquid can be separated. The separated vapour is a
stream, predominantly containing methane, and is fed to a
compressor 106. The compressor 106 compresses the methane
10 enriched vapour stream. The compressed methane enriched
stream is then fed to the heat exchanger 104a along with
the vapours from the top of a de-methanizer column 110.
This process facilitates in removing a large portion of
the methane component from the LNG feed. Further, the
15 separated liquid component from the separator 108 may be
pumped via a pump 102c to another heat exchanger 104b
also called as C2/C3 column condenser 104b. The chilled
liquid from pump 102c facilitates in exchanging cold
energy with that of the hot top vapour of C2/C3 column
20 114. The hot top vapour contains a large portion of
ethane (CZ), and propane (C3) components of the C2/C3
column 114. The exchange of energies between the chilled
liquid and the hot vapour facilitates in condensation at
a reflux drum 118a. The C2/C3 column condenser 104b cools
25 the top vapours of a second or C2/C3 column 114. The
cooled stream is routed to the reflux drum 118a and the
preheated hot vapour stream 3 is routed back into the demethanizer
column 110 as a feed. The de-methanizer column
110 thus receives methane rich vapour from the heat
30 exchanger 104b, and subsequently removes a majority of
methane component from the LNG feed as the top product.
In one embodiment of the present invention, the demethanizer
column 110 separates approximately 98.85% of
methane from the LNG feed. The separated methane can
either be collected in a methane storage tank or can be
sent to an existing re-gasification terminal after
liquefaction in the re-condenser 104a. The re-
5 liquefaction of the methane rich vapours of LNG,
facilitates the re-gasification terminal to boost the
pressure of methane rich liquid LNG to any desired level
with minimum energy consumption. This also facilitates in
saving energy to a large extent as compared to the direct
10 compression of hot gas which needs large quantity of
water for cooling, before being routed to the consumers
units.
[0019] As explained above, the process of separating
15 methane CI from the LNG feed utilizes the cold energy of
the LNG for condensation of methane rich gas. In various
embodiments of the present invention, the cold energy of
the LNG feed is utilized to condense the remaining
heavier components including ethane (C2) and heavier
20 components, in the separator 108. This facilitates in
reducing the vapour load on the compressor 106
substantially. Subsequently, the compressor capacity and
associated cost can be reduced in the process of
compressing the methane rich vapour.
25
[0020] The liquid from the separator 108 is enriched in
heavier hydrocarbon components and is used to condense
the overhead hot vapours of the second column or C2/C3
column 114. The preheated liquid is then fed at the top
30 of the de-methanizer column 110. This arrangement enables
the de-methanizer column 110 to operate as a stripper. In
an embodiment of the present invention, the abovementioned
arrangement facilitates in omitting the need
for any external cooling medium as additional reflux
system or mechanism either through condensation of
overhead vapours, or by using a side stream of feed LNG.
Also, since the total LNG feed is preheated, and the
5 major portion is separated as top vapour in the separator
108 instead of directly feeding it to the de-methanizer
column 110 (as in many of the conventional techniques),
the reboiler duty of the demethanizer column 110 gets
minimized. Further, since no external physical reflux
10 system is used for the de-methanizer column 110, the cost
of associated facilities, such as installation of reflux
condensers, ' reflux drums, reflux pumps etcetera can be
prevented. According to the embodiments of the present
invention, the configuration consisting of the LNG feed
15 pump 102a, the heat exchanger 104a, the separator 108
along ,with the de-methanizer column 110, economically
fractionates the LNG feed into methane CI rich overhead
vapours and a mixture of heavier hydrocarbons as the
bottom products.
20
[0021] Once the methane (Cl) is separated from the LNG
feed, ethane C2 and heavier hydrocarbon components are
left to be recovered. The heavier hydrocarbons are
recovered from Stream 4, which is the output or the
25 bottom product of the de-methanizer column 110. The major
components that are present in Stream 4 include ethane
(Cz), propane (C3), and butane (Cg) . Stream 4 may also
contain minor quantity of pentane (Cs), and Cs+
components. A second column or C2/C3 column 114 is
30 provided that receives the feed as Stream 4 to separate
the components of ethane (C2) and propane (C3). The bottom
liquid of the first column 110 is boiled in demethaniser
reboiler 112a, before it is fed to the second column 114.
The C2/C3 column 114 is associated with the condenser
104b, a reflux drum 118a, reflux pump 102d, a side
stripper 116, a reboiler 112b and a side stripper
reboiler 112c. The condensed C2C3 liquid from the reflux
5 drum 118a is partially pumped back as reflux to C2/C3
column 114 and the rest of the liquid is extracted as
ethane (C2) and Propane C3 product as Stream 5. The bottom
liquid from C2/C3 column 114 is a mixture of Propane (C3)
and Butane (Cq) conforming to Indian Standard (IS)
10 specifications of LPG.
[0022] In one embodiment of the present invention, the
second column or the C2/C3 column 114 is provided with a
side stripper 116. The side stripper 116 is used to
15 remove pentane (C5) and pentane+ (C5+) components present
in the LNG feed. Since the LNG feed comprises varying
composition of hydrocarbons, the heavier components,
particularly the C5+ content may or may not be present in
significant proportions in the LNG feed. When pentane (C5)
20 content in LNG feed increases, or when there is a
possibility of contaminates like C5+ components, the side
stripper 116 is used for extracting LPG as shown in
Stream 6. The provision of the side stripper 116
therefore facilitates in the separation of propane (C3)
25 and butane (C4) as LPG components when pentane and heavier
hydrocarbons are present. The pentane Cs rich mixture is
extracted as the bottom product of C2/C3 column 114.
[0023] In one embodiment of the present invention, if the
30 feed LNG contains substantial quantities of pentane and
pentane plus components, then the bottom product of C2/C3
column 114 will be routed to the de-pentanizer column
120. The de-pentanizer column 120, is associated with the
condenser 104c also known as de-Pentanizer condenser
104c, a reflux drum 118b, reflux pump 102f and a reboiler
112d. The de-pentanizer condenser 104c is used to
exchange cold energy of the chilled liquid from the pump
5 102d with the hot top vapour (containing predominantly
LPG and C3C4 components) of the de-pentanizer column 120
for condensation at the reflux drum 118 b.
[0024] In the scenarios of the LNG feed containing the
10 substantial components of LPG and components of pentane
(Cs) or heavier hydrocarbons, the de-pentanizer 120
extracts the LPG as Stream 6. Pentane (Cs) and heavier
hydrocarbons (C5+) remain in the LPG as minor components
within the acceptable limits of the Indian Standard (IS)
15 specification i.e. IS:4576 of LPG. The condenser duty of
the de-pentanizer column 120 is met by the cold energy
available within the C2/C3 mixture produced from the C2/C3
column 114, before it is sent to the storages. In an
embodiment of the present invention, Pentane (C5) and C5+
20 are separated from LPG and extracted as Stream 7 by using
the de-pentanizer column 120, if the feed LNG contains
substantial amount of pentane (C5) and pentane plus (C5+)
components which cannot be separated along with the LPG
stream of the side stripper 116. Finally, the propane-
25 butane mixture from the side stripper 116 of the C2/C3
column 114 and the predominantly butane (C4) component
from the top of the de-pentanizer column 120, are mixed
to produce LPG conforming to the IS specifications. As
explained earlier, the side stripper 116 is used to
30 produce LPG only when the heavier hydrocarbons including
pentane (C5) and C5+ are found in substantial amount in
the LNG feed. The provision of the side stripper 116
reduces the energy consumption to a large extent because
fractionating the entire bottom product of the C2/C3
column 114 which will consume huge energy. The use of the
side stripper 116 facilitates in separating LPG
components. The side stripper 116 also facilitates in
5 reducing the load and the size of the de-pentanizer
column 120 and its associated facilities.
100251 The side stripper 116 is thus an optional column
that facilitates pentane C5 and heavier hydrocarbons Cs+to
10 either remain with the LPG component in Stream 6 as minor
components, or to be extracted out as the bottom product
of the de-pentanizer column 120.
[0026] The state of the liquid LNG feed is changed into
15 vapour state and is processed for the separation of
hydrocarbons. However, the return LNG stream i. e. Stream
8 is converted to liquid state by heat transfer with the
cold feed LNG that comprises the methane rich components.
The conversion of the return LNG into liquid state is the
20 salient feature of the present invention. Further, the
embodiments of the present invention are compatible with
the existing LNG processing systems. Furthermore, the
embodiments of the present invention can easily be
implemented in any new module or future terminals and of
25 future LNG systems or mechanisms.
[0027] Tables la and lb are provided below for
illustrating the principles of the present invention. A
set of data is included in the tables that respectively
30 represent stream summary and mole fraction of exemplary
embodiments of the invention, and is not intended to
limit the disclosure in any way.
Table la: Stream Summary
Table (lb): Mole Fraction
[0028] While the exemplary embodiments of the present
invention are described and illustrated herein, it will
be appreciated that they are merely illustrative. It will
be understood by those skilled in the art that various
10 modifications in form and detail may be made therein
without departing from or offending the spirit and scope
of the invention as defined by the appended claims.

We claim:
1. A method for processing Liquefied Natural Gas
(LNG) for recovering hydrocarbons, the method
5 comprising the steps of:
(a) heating cold liquid LNG feed in a first heat
exchanger to obtain hot vapour from a
proportion of the liquid LNG feed;
(b) condensing the hot vapour by utilizing cold
10 energy of remaining liquid proportion of the
LNG feed;
(c) pumping the remaining liquid proportion of the
LNG feed into top of a first hydrocarbon
recovery column for recovering overhead
15 vapours of methane;
(d) directing downstream of the first hydrocarbon
recovery column into a second hydrocarbon
recovery column for recovering ethane and
propane components;
20 (e) directing downstream of the second hydrocarbon
recovery column into a third hydrocarbon
recovery column for recovering LPG (Liquefied
Petroleum Gas)component, the LPG component
containing negligible proportions of pentane
25 and pentane+ hydrocarbons;
(f) providing a side stripper for separating the
LPG component, when the LNG feed contains nonnegligible
proportions of pentane and pentane+
hydrocarbons.
2. The method as claimed in claim 1, wherein the
first hydrocarbon recovery column, the second
hydrocarbon recovery column, and the third
hydrocarbon recovery column, are thermodynamically
integrated with each other.
3. The method as claimed in claim 1, wherein the
5 first hydrocarbon recovery column is a de-methanizer
column and wherein methane is a predominant
hydrocarbon component of the LNG feed.
4. The method as claimed in claim 1, wherein the
10 second hydrocarbon recovery column is a deethanizer/
de-propanizer column (C2/C3 column) .
5. The method as claimed in claim 1, wherein the
third hydrocarbon recovery column is a de-pentanizer
15 column.
6. The method as claimed in claim 1, wherein a
compressor is provided for compressing the hot vapour
and feeding the compressed hot vapour to the first
20 heat exchanger along with the overhead vapours of the
first hydrocarbon recovery column.
7. The method as claimed in claim 6, wherein the
compressed .hot vapour is mixed with the overhead
vapours of the first hydrocarbon recovery column for
continuously preheating the LNG feed in the first
heat exchanger and for obtaining a vapour-liquid
mixture containing the predominant component of
methane.
8. The method as claimed in claim 7, wherein the
vapour-liquid mixture is flashed into a separator for
separating the methane component from the liquid LNG
feed.
5
9. The method as claimed in claim 1, wherein heavier
hydrocarbon components of the LNG feed provides cold
energy to condense the overhead hot vapours of the
second hydrocarbon recovery column.
10
10. The method as claimed in claim 1, wherein the LPG
is a mixture of propane and butane in proportions
according to the Indian Standard (IS) specification.
15 11. The method as claimed in claim 1, wherein the LNG
feed is fed to the hydrocarbon recovery columns at
low pressures.
12. The method as claimed in claim 1, wherein the
20 hydrocarbon components including ethane, propane,
LPG, pentane, and pentane+, are recovered in liquid
state.
13. The method as claimed in claim 1, wherein the
processing of the LNG feed for the' recovery of the
hydrocarbon components subsequently provides a return
LNG in liquid state.
14. The method as claimed in claim 1, wherein the
30 hydrocarbon components recovered from the second