PROCESS AND APPARATUS FOR ENHANCING RECOVERY OF LIOUEFIED PETROLEUM GAS (LPG) FROM NATURAL GAS ATLOWER CONDENSERDUTY
FIELD OF INVENTION
The present invention relates to the field of recovery of hydrocarbon gas
constituents from natural gas. The present invention particularly relates to an improved process and apparatus for separating hydrocarbon gas constituents such as methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane and heavier hydrocarbons using fractionators into different constituents like Residue Gas, Liquefied Petroleum Gas (hereinafter referred to as "LPG") and Natural Gas Liquids (hereinafter referred to as "NGL"). The present invention more particularly relates to a process and an apparatus which enhances recovery of hydrocarbon gas constituents from natural gas without increasing condenser refrigeration duty of deethanizer fractionator. BACKGROUND AND PRIOR ART OF THE INVENTION
The composition of natural gas varies considerably between gas fields. Typically natural gas contains methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane and heavier hydrocarbon constituents in varying amount. The primary component of natural gas is methane. The history of separating hydrocarbon gas constituents from natural gas can be traced back to 1940s'.
Natural gas containing methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane and heavier hydrocarbons are separated into its different constituents using fractionators in cryogenic process. Cryogenic process conditions for such separation can be generated by using turbo-expander, refrigerant or a combination of both. Separation of natural gas into Residue Gas (mixture of methane and ethane), Liquefied Petroleum Gases or LPG (mixture of propane & i-butane, n-butane) and Natural Gas Liquids or NGL (mixture of pentanes and heavier hydrocarbons) are normally carried out using two fractionators system.
US Patent 4,698,081 (Hafez K. Aghili: Process for separating hydrocarbon gas constituents utilizing a fractionater) discloses cryogenic turbo-expander process for separation of natural gas constituents into methane, ethane and propane by supplying deethanized product to the top of demethanizer column. The addition of said product

alters the chemical equilibrium existing in the top of said demethanizer column thereby enhancing the recovery of ethane constituents at the expense of a small reduction in recovery of propane and heavier constituents. US Patent 4,698,081 does not discuss any saving in refrigeration duty achieved by the process disclosed therein.
US Patent 5,687,584 (Yuv R. Mehra: Absorption Process with Solvent Pre-saturation) discloses a process unit that can be retrofitted to an existing simple refrigerated natural gas treatment plant to increase recovery of propane, butanes and natural gasoline components and optionally ethane from natural gas. The said process contains an absorber where the desired component for recovery and all heavy components heavier than that is absorbed by a lean solvent stream comprised of the heavier components present in the feed stream. The process also contains a solvent regenerator. US Patent 5,687,584 does not discuss about any saving in refrigeration duty of condenser to be achieved in the existing scheme for increase in recovery of some component by retrofitting of the invention. In both cases the separating column is converted to an absorber. Use of such process for meeting present objective is not cost effective as undesired ethane would also be removed along with LPG which is to be further separated in a separate column. Need for the Present Invention
There is a large gap between commercial demand and supply of LPG, Residue Gas and NGL obtained from natural gas; therefore there exists a need to bridge this gap.
Natural gas is a non-renewable resource, the formation of which takes millions of years. As natural gas is essentially irreplaceable, there is a continuing need to improve the existing processes for separating useful gas constituents from natural gas so as to maximize the utilization of this limited resource of energy.
There is also a long felt need in this area of technology to develop more economical processes and apparatus for separation and recovery of various hydrocarbon gas constituents contained in natural gas so as to reduce processing cost and provide commercially significant alternatives. OBJECTS OF THE INVENTION
It is an important object of the present invention to provide for an improved process and apparatus for separating Residue Gas, LPG and NGL from natural gas

constituent that employs cryogenic process with at least two fractionator system at a lower refrigeration duty of condenser of deethanizer fractionator.
Another object of the invention is to provide for a process for separating the constituents of natural gas using at least two fractionators system, wherein the separation of Liquefied Petroleum Gas (LPG) is enhanced and the separation of ethane is controlled at a lower refrigeration duty of condenser of deethanizer fractionator.
Yet another object of the present invention is to provide for a process for separating the constituents of natural gas wherein the separation of LPG is enhanced and that of the ethane is controlled by recycling part of the condensate generated in the process to appropriate location in the preceding separation column.
Still another object of the present invention is to provide for a process and apparatus for separating the constituents of natural gas wherein the separating device partially act as fractionating column and partially as absorption column. The dual purpose column of the apparatus is of critical design as both fractionation and absorption processes are carried out simultaneously. SUMMARY OF THE INVENTION
Accordingly, the present invention relates to a process for separating the constituents of natural gas comprising:
(a) compressing the feed natural gas to obtain compressed natural gas;
(b) cooling the compressed natural gas to obtain cooled gas stream(s);
(c) further compressing, cooling and separating the said cooled gas streams of
step (b) to obtain liquefied natural gas feed streams;
(d) feeding the said liquefied natural gas feed streams to a deethanizer
fractionator to obtain different fractions;
(e) partially condensing the top vapour fraction of the deethanizer fractionator
and refluxing the vapour product and condensed liquid back to the
deethanizer fractionator column;
(f) collecting the remaining condensate of the top vapour fraction of deethanizer
fractionator as Residue Gas;

(g) feeding the bottom product fraction of the deethanizer fractionators to
debutanizer fractionator; (h) completely condensing the top vapour fraction of debutanizer fractionator to
obtain condensed liquid;
(i) returning a part of the said condensed liquid of step (h) as reflux to the debutanizer fractionator column and collecting another part of it as top product fraction; (j) compressing the top product fraction of step (i) and recovering it as
Liquefied Petroleum Gas (LPG);
(k) withdrawing a part of the bottom product obtained from debutanizer fractionator column as condensate product, further compressing this condensate product and collecting it as Natural Gas Liquids (NGL); (1) recycling another part of the bottom product fraction obtained from debutanizer fractionator column to deethanizer fractionator column as a recycled product.
In a preferred embodiment of the invention there are one or more natural gas feed streams which are liquefied at different temperatures to feed deethanizer fractionator column.
In another embodiment of the invention cooled gas stream(s) of steps (b) or (c) are compressed, separated, further compressed and then fed to the deethanizer fractionator.
In yet another embodiment of the invention the recycling step (1) is carried out at a controlled rate.
In still other embodiment of the invention the said recycling is done by a pumping means.
In a preferred embodiment of the invention the said recycled product is cooled before recycling it back to the deethanizer fractionator column.
In another embodiment of the invention the said top product LPG is a mixture of propane and butanes of ratio in the range of 65:35 to 45:55.

In yet another embodiment of the invention the refrigeration duty of condenser consumed by the deethanizer fractionation column is lowered by 15-20%.
The present invention also relates to an apparatus for separating the constituents of natural gas comprising in combination:
(i) at least two fractionator system consisting of deethanizer system (B7) and
debutanizer system (B8) connected to each other through at least one
connector line (7);
(ii) at least one main feed line (1) to feed natural gas to the said system; (iii) compressing means (Bl) connected to line (1) for receiving and
compressing the feed natural gas and delivering it to line (2); (iv) cooling means (B2) connected to line (2) for receiving and cooling the
compressed feed natural gas and delivering it to supply lines (3, 5); (v) supply lines (3) and (5) connected to one or more feed lines (13) and (6)
respectively for supplying liquefied natural gas to deethanizer fractionator
column (B7); (vi) feed lines (13, 6) connected to deethanizer fractionator (B7) to feed
compressed and cooled natural gas; (vii) at least one recycle line (21) connecting deethanizer fractionator (B7) to
debutanizer fractionator (B8) to recycle bottom product of debutanizer
fractionator through line (19); (viii) pumping means (B12) to recycle debutanizer fractionator (B8) bottom
product to deethanizer fractionator (B7) through recycle lines (19, 21); (ix) condensing means (B13) for partially condensing and refluxing the top
vapour fraction of deethanizer fractionator (B7); (x) at least one recovery line (16) connected to the top of deethanizer
fractionator (B7) for collecting Residue Gas; (xi) connector line (7) connecting the bottom section of deethanizer fractionator
(B7) to debutanizer fractionator (B8) for feeding the bottom product of the
deethanizer fractionator (B7) into debutanizer fractionator (B8);

(xii) condensing means (B15) connected to the top section of debutanizer
fractionator (B8) for completely condensing and refluxing the top vapour
fraction of debutanizer fractionator (B8); (xiii) at least one recovery line (10) connected to the top of debutanizer
fractionator column (B8) for recovering condensed top product; (xiv) compressing means (B9) connected to recovery line (10) for compressing
condensed top product; (xv) recovery line (11) connected to compressing means (B9) for collecting
Liquefied Petroleum Gas (LPG); (xvi) at least one recovery line (8) connected to the bottom of debutanizer
fractionator column (B8) for recovering condensed bottom product; (xvii) compressing means (B10) connected to recovery line (8) for compressing
condensed bottom product; (xviii) recovery line (9) connected to compressing means (BIO) for collecting
Natural Gas Liquids (NGL) through recovery line (9); (xix) at least one recovery line (18) connected to the bottom section of
debutanizer fractionator column (B8) for recycling a fraction of the bottom
product to deethanizer fractionator column (B7); (xx) recycle line (19) connected to recovery lines (17) and (18) for recycling of
the recycle product.
In a preferred embodiment of the invention the liquefied natural gas in the feed lines (13) and (6) are liquefied at different temperatures.
In another preferred embodiment of the invention the supply line (3) is connected to said feed line (13) through compressing means (B3), intermediated line (4) and cooling means (B4) for providing liquefied and compressed natural gas to the feed line.
In yet another preferred embodiment of the invention apparatus the supply line (5) is connected to said feed line (6) through compressing means (B6) for providing liquefied and compressed natural gas to the feed line.

In another embodiment of the invention a recycle storage tank (Bll) and recycling pump (B12) are connected to said recycle line (19) at one end and said recovery lines (17) and (18) at another end.
In yet another embodiment of the invention the said recycled product is cooled by exchanging heat with connector line (7), through an exchanger (B17) and further cooled by water cooled exchanger (B18).
In still another embodiment of the invention recovery line (17) connected to the bottom section of debutaniser column (B8) is used as recycle product. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of the operation of the two fractionators system
[deethanizer fractionators (B7) and debutanizer fractionators (B8)]. Figure 2 illustrates the process and apparatus of the invention, showing recycle of debutanizer fractionator bottom product, wherein the recycle stream is cooled by a chiller.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes a process for enhancing separation of LPG from natural gas containing methane, ethane, propane, butanes, pentanes and heavier hydrocarbons wherein the separation of LPG is enhanced and that of the ethane is controlled by recycling part of the condensate generated in the process to appropriate location in the preceding separation column such that the separating device partially act as fractionating column and partially as absorption column. The dual purpose column is of critical design as both fractionation and absorption processes are carried out simultaneously. The process of the invention is more efficient and economical as energy consumption is minimized and is markedly less from the prior art processes. In prior art the same objective is achieved by increasing condenser refrigeration duty substantially resulting in higher capital and operating cost.
Accordingly, the present invention discloses an improved, efficient and more economical process for separation of Residue Gas, LPG and NGL from natural gas that consumes 15-20% lower refrigeration duty of condenser of the deethanizer fractionator compared to the consumption of condenser refrigeration duty in prior art processes.

The process of the invention is described in detail hereinafter with reference to the accompanying drawings (Figures 1 and 2). Natural gas containing methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane and other heavier hydrocarbon constituents are separated into different constituents using fractionators as shown in the Figure 1. The fractionation is required to liquefy the natural gas partially and feed into deethanizer fractionator (B7). Feed streams (13) and (6) carrying natural gas liquefied at different temperatures are fed into the deethanizer fractionator (B7). The top vapour of deethanizer fractionator is partially condensed in a condenser (B13) and the vapour product and condensed liquid is refluxed to deethanizer fractionator column (B7). The top vapour product from deethanizer fractionator (B7) contains mainly methane and most of the ethane, collectively known as Residue Gas. The bottom product from deethanizer fractionator (B7) is fed into debutanizer fractionator (B8). The top vapour from debutanizer is fully condensed in the condenser (B15) and part of the liquid condensed is returned as reflux to the debutanizer fractionator column (B8) and part is withdrawn as top product (10). The top product from debutanizer contains a mixture of propane and butane known as LPG. The bottom product from debutanizer contains a mixture of pentane and heavier hydrocarbons known as NGL.
A part of debutanizer fractionator (B8) bottom product (18) is returned to deethanizer column as a recycled product (19) to deethanizer fractionation column (B7) at an appropriate stage. A recycle storage tank (Bl 1) and recycling pump (B12) are used to control the recycle rate. For recovery of similar amount of propane the recycling step reduces the refrigeration duty of condenser as compared to the condenser duty required when the process is carried out without the recycling step. Recycle rate of debutanizer bottom product vis-a-vis the amount of propane recovered and condenser duty of deethanizer fractionator is reported in Table-1 below.
Table-1. Comparison of Condenser Duty of Deethanizer Fractionators
(Table Removed)

The typical composition of feed (natural gas) processed, LPG and NGL (debutaniser bottom product) obtained by the process of the invention is given in Table-2.
Table-2 . Typical composition (mole%) of feed (natural gas). LPG and NGL
(Table Removed)

The typical material balance of the LPG recovery plant is shown in Table-3
below.
Table-3. Typical material balance of the plant
(Table Removed)

The results show that for the same amount of propane recovery approximately 17% reduction in condenser duty is reached with recycle step. This reduction in
condenser duty is achieved at a marginal reduction of ethane recovery in deethanizer fractionator.
In one embodiment of the process of the invention the debutaniser fractionator bottom recycled product (19) is cooled by exchanging heat with stream (7), through an exchanger (B17) and further cooled by water cooled exchanger (B18).
In yet another embodiment of the process of the invention a stream (17) from bottom section of debutaniser column (B8) is used as recycle product to column (B7). This may be independent or in association with recycle product (18). The preferred arrangement would depend on feed composition and requirement for the enhancement of LPG recovered.

We Claim:
1. A process for separating the constituents of natural gas comprising:
(a) compressing the feed natural gas to obtain compressed natural gas;
(b) cooling the compressed natural gas to obtain cooled gas stream(s);
(c) further compressing, cooling and separating the said cooled gas streams of
step (b) to obtain liquefied natural gas feed streams;
(d) feeding the said liquefied natural gas feed streams to a deethanizer
fractionator such as hereinbefore described to obtain different fractions;
(e) partially condensing the top vapour fraction of the deethanizer fractionator
and refluxing the vapour product and condensed liquid back to the
deethanizer fractionator column;
(f) collecting the remaining condensate of the top vapour fraction of deethanizer
fractionator as Residue Gas;
(g) feeding the bottom product fraction of the deethanizer fractionators to
debutanizer fractionator such as hereinbefore described;
(h) completely condensing the top vapour fraction of debutanizer fractionator to
obtain condensed liquid; (i) returning a part of the said condensed liquid of step (h) as reflux to the
debutanizer fractionator column and collecting another part of it as top
product fraction; (j) compressing the top product fraction of step (i) and recovering it as
Liquefied Petroleum Gas (LPG); (k) withdrawing a part of the bottom product obtained from debutanizer
fractionator column as condensate product, further compressing this
condensate product and collecting it as Natural Gas Liquids (NGL); (1) recycling another part of the bottom product fraction obtained from
debutanizer fractionator column to deethanizer fractionator column as a
recycled product.

2. The process as claimed in claim 1, wherein there are one or more natural gas feed
streams which are liquefied at different temperatures to feed deethanizer
fractionator column.
3. The process as claimed in claims 1 and/or 2, wherein the cooled gas stream(s) of
steps (b) or (c) are compressed, separated, further compressed and then fed to the
deethanizer fractionator.
4. The process as claimed in claim 1, wherein the recycling step (1) is carried out at a
controlled rate.
5. The process as claimed in any preceding claim, wherein said recycling is done by
a pumping means.
6. The process as claimed in any preceding claim, wherein the said recycled product
is cooled before recycling it back to the deethanizer fractionator column.
7. The process as claimed in any preceding claim, wherein the said top product LPG
is a mixture of propane and butanes of ratio in the range of 65:35 to 45:55.
8. The process as claimed in any preceding claim, wherein the refrigeration duty
of condenser consumed by the deethanizer fractionation column is lowered by
15-20%.
9. An apparatus for separating the constituents of natural gas comprising in
combination:
(i) at least two fractionator system consisting of deethanizer system (B7) and debutanizer system (B8) connected to each other through at least one connector line (7);
(ii) at least one main feed line (1) to feed natural gas to the said system;
(iii) compressing means (Bl) connected to line (1) for receiving and compressing the feed natural gas and delivering it to line (2);

(iv) cooling means (B2) connected to line (2) for receiving and cooling the
compressed feed natural gas and delivering it to supply lines (3, 5); (v) supply lines (3) and (5) connected to one or more feed lines (13) and (6)
respectively for supplying liquefied natural gas to deethanizer fractionator
column (B7); (vi) feed lines (13, 6) connected to deethanizer fractionator (B7) to feed
compressed and cooled natural gas; (vii) at least one recycle line (21) connecting deethanizer fractionator (B7) to
debutanizer fractionator (B8) to recycle bottom product of debutanizer
fractionator through line (19); (viii) pumping means (B12) to recycle debutanizer fractionator (B8) bottom
product to deethanizer fractionator (B7) through recycle lines (19, 21); (ix) condensing means (B13) for partially condensing and refluxing the top
vapour fraction of deethanizer fractionator (B7); (x) at least one recovery line (16) connected to the top of deethanizer
fractionator (B7) for collecting Residue Gas; (xi) connector line (7) connecting the bottom section of deethanizer
fractionator (B7) to debutanizer fractionator (B8) for feeding the bottom
product of the deethanizer fractionator (B7) into debutanizer fractionator
(B8); (xii) condensing means (B15) connected to the top section of debutanizer
fractionator (B8) for completely condensing and refluxing the top vapour
fraction of debutanizer fractionator (B8); (xiii) at least one recovery line (10) connected to the top of debutanizer
fractionator column (B8) for recovering condensed top product; (xiv) compressing means (B9) connected to recovery line (10) for compressing
condensed top product; (xv) recovery line (11) connected to compressing means (B9) for collecting
Liquefied Petroleum Gas (LPG);

(xvi) at least one recovery line (8) connected to the bottom of debutanizer fractionator column (B8) for recovering condensed bottom product;
(xvii) compressing means (BIO) connected to recovery line (8) for compressing condensed bottom product;
(xviii) recovery line (9) connected to compressing means (BIO) for collecting Natural Gas Liquids (NGL) through recovery line (9).
(xix) at least one recovery line (18) connected to the bottom section of debutanizer fractionator column (B8) for recycling a fraction of the bottom product to deethanizer fractionator column (B7);
(xx) recycle line (19) connected to recovery lines (17) and (18) for recycling of the recycle product.
10. The apparatus as claimed in claim 9, wherein the liquefied natural gas in the feed
lines (13) and (6) are liquefied at different temperatures.
11. The apparatus as claimed in claim 10, wherein the supply line (3) is connected to
said feed line (13) through compressing means (B3), intermediated line (4) and
cooling means (B4) for providing liquefied and compressed natural gas to the feed
line.
12. The apparatus as claimed in claim 10, wherein the supply line (5) is connected to
said feed line (6) through compressing means (B6) for providing liquefied and
compressed natural gas to the feed line.
13. The apparatus as claimed in claims 9 to 12, wherein a recycle storage tank (Bl 1)
and recycling pump (B12) are connected to said recycle line (19) at one end and
said recovery lines (17) and (18) at another end.

14. The apparatus as claimed in claims 9 to 13, wherein the said recycled product is
cooled by exchanging heat with connector line (7), through an exchanger (B17)
and further cooled by water cooled exchanger (B18).
15. The apparatus as claimed in claims 9 to 14, wherein recovery line (17) connected
to the bottom section of debutaniser column (B8) is used to recycle bottom
product of debutaniser column (B8).
16. A process for separating the constituents of natural gas such as hereinbefore
described with reference to the accompanying drawings.
17. An apparatus for separating the constituents of natural gas such as hereinbefore
described with reference to the accompanying drawings.