PROCESS AND APPARATUS FOR ENHANCING RECOVERY OF PROPANE AND LIQUEFIED PETROLEUM GAS (LPG) FROM NATURAL GAS
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 enhancing recovery of propane and Liquefied Petroleum Gas (hereinafter referred to as "LPG") from Natural Gas without recovering undesired ethane component. The present invention more particularly relates to a process comprising a combination of fractionation and absorption steps for said recovery of LPG product. 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.
In India there is a large gap between demand and supply of LPG. As demand for these gases increased many processes were developed for separating it from the natural gas. Many technologies with improved processes and apparatus design were developed for separating methane, ethane, propane, butane and other hydrocarbon components contained in the natural gas. Further recovery of propane and LPG from natural gas is more economical in an existing gas processing unit.

In LPG certain ratio of propane and butane is maintained as per specification stipulated by BIS. Therefore, while processing natural gas of typical composition given in Table-1 above, processing is carried by maintaining the propane and butane ratio as per BIS specification. It is observed that the natural gas has excess of propane which is not recovered, because its recovery would skew propane and butane ratio. Alternatively, additional propane can be recovered by modifying process operation and spike butane in the end product such that ratio of propane and butane is maintained in the final product. This method calls for elaborate mixing arrangement between pure butane liquid and propane rich LPG product. Since the ratio of butane and recovered LPG is small, it is expensive to carry out proper mixing. Such process is also hazardous.
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 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 5,462,583 (Wood, C.G. and Y.R. Mehra: Absorption Process without external solvent) relates to enhancement of recovery of ethane/propane from Natural Gas using additional refrigeration or using absorption process.
Need for the Present Invention
There is a large gap between commercial demand and supply of LPG, 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.
Implementations of prior art processes require massive modification of recovery plant. Therefore, need for an economical process for enhanced recovery of propane from natural gas was felt.
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 natural gas into sales gas, LPG and condensate which overcomes the disadvantage of the prior art processes.
Another object of the invention is to provide for an improved process for the separation of natural gas into sales gas, LPG and condensate.
Yet another object of the present invention is to provide for a process for enhancing recovery of LPG production from natural gas in an economical way.
Still another object of the present invention is to provide for a process for enhancing recovery of LPG production from natural gas by an environmentally friendly process. SUMMARY OF THE INVENTION
Accordingly, the present invention relates to a process for separating the constituents of natural gas comprising the steps of:
(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) introducing and thereby mixing liquid butane to the said feed streams of step
(c);
(e) feeding the said mixed liquefied natural gas feed streams to a deethanizer
fractionator to obtain different fractions;
(f) partially condensing the top vapour fraction of a deethanizer fractionator and
refluxing the vapour product and condensed liquid back to the deethanizer
fractionator column;
(g) collecting the remaining condensate of the top vapour fraction of deethanizer
fractionator as Residue Gas;
(h) feeding the bottom product fraction of the deethanizer fractionator to
debutanizer fractionator; (i) completely condensing the top vapour fraction of debutanizer fractionators to
obtain condensed liquid;
(j) returning a part of the said condensed liquid of step (i) as reflux to the debutanizer fractionator column and collecting another part of it as top product fraction; (k) compressing the top product fraction of step (j) and recovering it as
Liquefied Petroleum Gas (LPG);
(1) withdrawing the bottom product fraction from debutanizer fractionator column as condensate product, further compressing and collecting is as Natural Gas Liquids (NGL).
In accordance with the process of the invention there is one or more natural gas feed streams to feed deethanizer fractionator column.
In a preferred embodiment the cooled gas stream(s) of steps (b) or (c) are compressed, separated, further compressed and then fed to the deethanizer fractionator.
In another preferred embodiment of the process of the invention the recovery of propane and making of LPG is enhanced by introducing butane into propane rich stream comprising:
pressurising liquid butane and mixing it with feed vapour and liquid;

subjecting liquid butane to mix with internal liquid coming from above inside column;
subjecting the liquid butane to participate as absorption liquid such that propane is preferentially absorbed compared to ethane;
recovering the liquid butane injected into the system as top product of debutaniser column.
In yet another embodiment, expander separate liquid and cold separate liquid are mixed with the said liquid butane.
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) one or more feed lines (13, 6) connected to deethanizer fractionator (B7) to
feed compressed and cooled natural gas; (iii) at least one supply line (18) connected to feed lines (13) and (6) to introduce
liquid butane into the feed streams;
(iv) pumping means (B15) to deliver liquid butane to supply line (18); (v) condensing means for partially condensing and refluxing the top vapour
fraction of deethanizer fractionator (B7); (vi) at least one recovery line (16) connected to the top of deethanizer
fractionator (B7) for collecting Residue Gas; (vii) connector line (7) for feeding the bottom product of the deethanizer
fractionator (B7) into debutanizer fractionator (B8); (viii) condensing means for completely condensing and refluxing the top vapour
fraction of debutanizer fractionator (B8); (ix) at least one recovery line (10) connected to the top of debutanizer
fractionator column (B8) for recovering condensed top product; (x) compressing means (B9) for compressing condensed top product;

(xi) recovery line (11) connected to compressing means (B9) for collecting Liquefied Petroleum Gas (LPG);
(xii) at least one recovery line (8) connected to the bottom of debutanizer fractionator column (B8) for recovering condensed bottom product;
(xiii) compressing means (BIO) for compressing condensed bottom product;
(xiv) recovery line (9) connected to compressing means (BIO) for collecting Natural Gas Liquids (NGL) through recovery line (9).
In an embodiment of the invention a supplementary line (12) is connected to the said feed line (13) for providing expander separate liquid, wherein the expander separate liquid is compressed by compressing means (B5).
In yet another embodiment another supplementary line (5) is connected to the said feed line (6) for providing cold separate liquid, wherein the cold separate liquid is compressed by compressing means (B6).
In a preferred embodiment the said pumping means (B15) is connected to storage vessel (B14).
In still another embodiment the said supply line (18) bifurcates into two separate lines (19) and (20).
In a preferred embodiment the said lines (19) and (20) meet supplementary lines (5) and (12) respectively, before finally meeting the feed lines (6) and (13).
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. DETAILED DESCRIPTION OF THE INVENTION
Liquefied Petroleum Gas (LPG) which consists of mainly butane/propane of desired ratio is recovered from natural gas by subjecting it to pressurization and depressurization and fractionating in a deethaniser column and recovering LPG from bottom of the column along with condensate, which is further separated in debutaniser column and LPG is drawn as top product. However, if propane is present in feed gas in

excess of the desired quantity required for butane, it is to be released to sales gas at an economic penalty.
The present invention discloses that by mixing butane with liquid feed to the deethaniser column, LPG of desired ratio of propane and butane is obtained. This is effectively achieved when butane is spiked in the final propane rich product. The improved process and apparatus of the invention also allows additional recovery of propane with marginal increase in energy consumption.
The present invention is hereinafter explained with reference to the accompanying drawings (Figures 1 and 2).
The process and apparatus set up for separating the constituents of natural gas using at least two fractionators system (B7 and B8) is depicted in Figure 1, wherein the separation of Liquefied Petroleum Gas (LPG) is enhanced and the separation of ethane is controlled, comprising the steps of:
(A) compressing the feed natural gas (1) using a compressor (Bl) to obtain
compressed natural gas (2);
(B) cooling the compressed natural gas (2) in a heat exchanger (B2) to obtain
cooled gas stream(s) (3 and/or 5), simultaneously or selectively;
(C) further compressing, cooling and separating the said cooled gas streams
(3 and/or 5) to obtain liquefied natural gas feed streams (13) and/or (6);
(D) feeding the said liquefied natural gas (13 and/or 6) into the deethanizer
fractionator (B7) to obtain different fractions;
(E) partially condensing the top vapour fraction of deethanizer fractionator (B7)
in a condenser and refluxing the vapour product and condensed liquid back
to the deethanizer fractionator column (B7);
(F) collecting the remaining condensate (14) of the top vapour fraction of
deethanizer fractionator (B7) as Residue Gas (16);
(G) feeding the bottom product (7) of the deethanizer fractionator (B7) into
debutanizer fractionator (B8);
(H) completely condensing the top vapour fraction of debutanizer fractionator (B8) in a condenser (B15);

(I) returning part of the said condensed liquid of step (H) as reflux to the debutanizer fractionator column (B8) and collecting part of it as top product (lO)orLPG; (J) withdrawing part of the bottom product from debutanizer fractionator
column (B8) as condensate product (9) or NGL;
In present invention, additional butane is introduced into the system in such a way that the cost of mixing is minimized and recovery of propane from natural gas is increases, thereby increasing LPG production.
Figure 2 depicts the schematic illustration of the improved process and apparatus of the invention, wherein in a preferred embodiment liquid butane from storage vessel (B14) is pumped by a pump (B15) and mixed with feed streams (13) and/or (6). Feed streams (13) and/or (6) are fed to deethaniser fractionator column (B7). Bottom product (7) from column (B7) is then fed to debutaniser fractionator column (B8). Top product (10) from column (B8) is drawn and collected as LPG (11).
The present invention provides an improved process and apparatus for the enhancement of LPG production by externally introducing additional butane in the fractionating column. Liquid butane is fed into the fractionating column at a point depending on the composition of the feed gas and permissible operating conditions of the existing column. The apparatus is designed in such a way so that minimum ethane is carried over by bottom liquid from the column. Main Advantages of the Present Invention are:
1. The scheme is standardized for the LPG production.
2. Introduction of additional butane in the fractionating column increases
recovery of propane than is normally achieved. Thus increasing overall LPG
production.
3. The process does not use additional energy as butane is not vaporized.
4. The process is optimally balanced such that only extra propane is trapped with
butane and lighter hydrocarbon ethane is not allowed to slip into bottom.

5. The process requires suitable distributor in the fractionator column for achieving proper mixing. Design of such distributor is subject of another invention.
In the process of the invention, natural gas of the typical composition as given in Table-1 below is separated into sales gas containing mostly methane, LPG and condensate, in this turbo expander process is used and no external refrigeration is required. Figure 1 and 2, illustrates flow scheme of the said process.
Table-1. Typical composition (mole%) of feed (natural gas), LPG and Condensate (debutanizer bottom product)
(Table Removed)

The data given in Table-1 shows that by mixing butane in the feed column net recovery of propane increases.

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

2. The process as claimed in claim 1, wherein there are one or more natural gas feed
streams 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 any preceding claim, wherein the recovery of propane
and making of LPG is enhanced by introducing butane into propane rich stream
comprising:
pressurising liquid butane and mixing it with feed vapour and liquid;
subjecting liquid butane to mix with internal liquid coming from above inside
column;
subjecting the liquid butane to participate as absorption liquid such that
propane is preferentially absorbed compared to ethane;
recovering the liquid butane injected into the system as top product of
debutaniser column.
5. The process as claimed in any preceding claim, wherein expander separate liquid
and cold separate liquid are mixed with the said liquid butane.
6. 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) one or more feed lines (13,6) connected to deethanizer fractionator (B7) to
feed compressed and cooled natural gas; (iii) at least one supply line (18) connected to feed lines (13) and (6) to
introduce liquid butane into the feed streams;

(iv) pumping means (B15) to deliver liquid butane to supply line (18);
(v) condensing means for partially condensing and refluxing the top vapour
fraction of deethanizer fractionator (B7); (vi) at least one recovery line (16) connected to the top of deethanizer
fractionator (B7) for collecting Residue Gas; (vii) connector line (7) for feeding the bottom product of the deethanizer
fractionator (B7) into debutanizer fractionator (B8); (viii) condensing means for completely condensing and refluxing the top vapour
fraction of debutanizer fractionator (B8); (ix) at least one recovery line (10) connected to the top of debutanizer
fractionator column (B8) for recovering condensed top product; (x) compressing means (B9) for compressing condensed top product; (xi) recovery line (11) connected to compressing means (B9) for collecting
Liquefied Petroleum Gas (LPG); (xii) at least one recovery line (8) connected to the bottom of debutanizer
fractionator column (B8) for recovering condensed bottom product; (xiii) compressing means (BIO) for compressing condensed bottom product; (xiv) recovery line (9) connected to compressing means (BIO) for collecting
Natural Gas Liquids (NGL) through recovery line (9).
7. The apparatus as claimed in claim 6, wherein a supplementary line (12) is
connected to the said feed line (13) for providing expander separate liquid,
wherein the expander separate liquid is compressed by compressing means (B5).
8. The apparatus as claimed in claims 6 or 7, wherein a supplementary line (5) is
connected to the said feed line (6) for providing cold separate liquid, wherein the
cold separate liquid is compressed by compressing means (B6).

9. The apparatus as claimed in claims 6 to 8, wherein the said pumping means (B15)
is connected in between lines (19) and (18) for pumping liquid butane from line
(18) to line (19).
10. The apparatus as claimed in claims 6 to 9, wherein the said pumping means (B15)
is connected to storage vessel (B14).
11. The apparatus as claimed in claims 6 to 9, wherein the said supply line (18)
bifurcates into two separate lines (19) and (20).
12. The apparatus as claimed in claim 11, wherein the said lines (19) and (20) meet
supplementary lines (5) and (12) respectively, before finally meeting the feed
lines (6) and (13).
13. A process for separating the constituents of natural gas such as hereinbefore
described with reference to the accompanying drawings.
14. An apparatus for separating the constituents of natural gas such as hereinbefore
described with reference to the accompanying drawings.