Title of Invention	"A PROCESS FOR SEPARATING LIQUEFIED NATURAL GAS(LNG) INTO LIQUEFIED PETROLEUM GAS (LPG) AND HIGHER MOLECULARWEIGHT COMPONENT"
Abstract	The present invention discloses a process and an appratus for for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components According to the invention, LNG under a pressure that is less than its critical pressure is caused to split into two unequal parts. Major part of the LNG is heated and thereafter separated into vapour and liquid fractions. The pressure of the vapour fraction is reduced and fed to a power plant. The liquid fraction is also heated and further divided into a vapor and liquid fractions. The pressure of vapour fraction is again reduced before feeding it to the power plant. The liquid fraction is pressurised, heated and introduced into the upper sections of a first distillation column. The original minor portion of the feed LNG is also introduced into the first distillation column as sub-cooled reflux. The first distillation column is provided with a reboiler. The liquid stream drawn from the reboiler is fed to the second distillation column after pressure reduction. The product LPG is drawn from a total condenser located at the top of the second distillation column. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn from reboiler located at the bottom of the second distillation column.

Title of Invention

"A PROCESS FOR SEPARATING LIQUEFIED NATURAL GAS(LNG) INTO LIQUEFIED PETROLEUM GAS (LPG) AND HIGHER MOLECULARWEIGHT COMPONENT"

Abstract

The present invention discloses a process and an appratus for for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components According to the invention, LNG under a pressure that is less than its critical pressure is caused to split into two unequal parts. Major part of the LNG is heated and thereafter separated into vapour and liquid fractions. The pressure of the vapour fraction is reduced and fed to a power plant. The liquid fraction is also heated and further divided into a vapor and liquid fractions. The pressure of vapour fraction is again reduced before feeding it to the power plant. The liquid fraction is pressurised, heated and introduced into the upper sections of a first distillation column. The original minor portion of the feed LNG is also introduced into the first distillation column as sub-cooled reflux. The first distillation column is provided with a reboiler. The liquid stream drawn from the reboiler is fed to the second distillation column after pressure reduction. The product LPG is drawn from a total condenser located at the top of the second distillation column. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn from reboiler located at the bottom of the second distillation column.

Full Text	A Method for Recovering Liquefied Petroleum Gases from Liquefied Natural Gas Field of invention The present invention relates to recovery of Liquefied Petroleum Gases (hereinafter referred to as LPG) from Liquefied Natural Gas (hereinafter referred to as LNG). In particular, the present invention relates to recovery of Liquefied Petroleum Gases and higher molecular weight components from Liquefied Natural Gas in a LNG gasifying terminal. More particularly, the present invention relates to a method for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components. The high volatile components, which forms the major part of the LNG, can be fed to pipelines with prior vaporisation in case of liquid mixture. The minor part, which is less volatile, is recovered as liquids. The major part can be used for power generation, production of fertilizers or petrochemicals. The minor part, which can be transported in bottles, can be used as domestic fuel and other applications where pipelines are not available. Background of the invention LNG normally produced near the large gas fields is transported at cryogenic temperatures of about -160°C and at atmospheric pressures. Depending on the source of natural gas, its pretreatment and liquefaction conditions, LNG composition varies from place to place. Further, the gross calorific value of the fuel gas generated by complete evaporation of the supplied LNG should meet a value agreed by the buyer and seller. This value is normally between 1100 to 1150 BTU/SCF, which is comparatively higher than that of pure methane i.e. 1030 BTU/SCF. In order to maintain the agreed heating value, the concentrations of high heating value components like ethane, propane, butane etc. are adjusted during liquefaction. In most of the cases, certain amounts of propane and butane are retained in the LNG. Presence of other heavier components like pentane, hexane, heptane etc. is not desirable as they cause solids formation during liquefaction process. On the other hand, an advantage of retaining the heavier components is liquefaction of LNG can be carried out at higher temperatures. This will result in increased through put of LNG production plant for given refrigeration duty. With the above restrictions and advantages, the composition of the most of the LNGs shipped out from the production plants has the ranges as given in Table 1. Table-1 - Variation of Compositions of Modern LNG Plants (Table Removed) From the above table, it can be seen that the concentrations propane and butanes which are the predominant components of LPG are high. These can be economically recovered with the method described by the present invention. Description of the Prior Art A number of patents describe the separation of LNG into its components. US Patent 2,952,984 granted on September 20, 1960, describes use of a pressurised distillation column to separate LNG. Part of the vapours from the top of the column is condensed with the help of cool feed and is used as reflux. The feed after further heating enters the column at the middle portion. Remaining vapours are further heated and expanded to recover work before pipelining. The liquefied heavier components are withdrawn from the reboiler of the column. US Patent No. 3,261,169 (1966), describes a method for separating LNG into a lean gas and rich liquefied gas with the help of a distillation column. In this method, the feed is partially vapourised and resulting liquid fraction is fed to the distillation column after heat exchange with intercooler of the evaporated gas compressors and for partially condensing overhead vapours of the column. The condensed liquid is refluxed and uncondensed gas is mixed with the lean gas from partial vapourisation step and fed to the pipeline. Rich LNG, which is enriched with heavier components, is withdrawn from the reboiler. The same patent also suggested the use of ethane as heating medium during partial vaporisation of LNG feed and to extract work with organic Rankine cycle from evaporated ethane. US Patent 3,253,418 teaches a method of separating LNG into a lean gas and rich gas by partial vaporisation of LNG feed using ethane as heating media. Residual liquid is evaporated using propane as heating medium at higher pressure to obtain rich gas. Work is extracted from the heated ethane and propane gases. A method of separation of LNG into its constituents during regassification is described in US Patent 3,405,530. LNG feed is pressurised and divided into two or more streams. The first stream is fed to the top of the demethaniser and the other stream(s) are fed to the middle of the same column after heat exchange with the condensers of deethaniser, depropaniser and debutaniser columns. Methane product is recovered from the top of demethaniser column. Product liquid withdrawn from the bottom of the column is fed to de-ethaniser that separates it to ethane and bottom liquid containing of propane and butane. These are separated in similar fashion in depropaniser and debutaniser. US Patent 3,446, 029 (1969) details a method of gasifying LNG while using heat from a reformer unit operating with heavier hydrocarbon fractions separated from LNG. The separation is carried out in a series of three flash drums and the first one is fed with pressurised and heated LNG feed. Separated liquid from the last drum is fed to a distillation column, which receives reflux from a partial condenser cooled with LNG feed. US Patent No. 3,656,312 (1969) discusses the use of LNG feed as a coolant for the in-built top heat exchangers of methane and ethane columns. Heated LNG is fed to the methane column. Liquid methane generated in the top heat exchanger of the methane column can be pressurised to a desired level and vapourised before feeding to the pipeline. Bottom liquid from methane column is fed to ethane separation column. Bottom liquid from ethane column is separated into propane and other fractions in a separate column. On a similar theme, another US Patent No. 3,420,068 describes a method of producing a liquid fraction or gas fraction at higher pressure from a low-pressure LNG feed. LNG is subjected to first partial vaporisation providing first methane rich gas stream and a residual liquid fraction, which is partially vaporised to provide second methane rich gas fraction. The first methane rich gas fraction is re-liquefied in heat exchange with feed LNG undergoing a warming up. The second methane-rich gas stream is also re-liquefied in heat exchange with the feed LNG undergoing first partial vaporisation. In US Patent No. 3,837,172 a method is described for separating pressurised LNG by dividing into two unequal streams. The major stream is partly or fully vapourised before introducing into a rectification column in the middle portion. The minor portion is fed to the top of the column to serve as a sub-cooled reflux after pressure reduction through a valve. Another variation covered in the patent was recovery of mechanical work to be used for compressing the vapour leaving the column from fully-vapourised major stream before it is fed to the column. The rectification column provided with a reboiler separates the LNG into methane enriched vapour fraction and a bottom liquid containing ethane and heavier fraction. US Patent No. 5,114,451(1992) deals with recovery of ethane, ethylene, propane, propylene etc. from LNG. It is achieved by directing a portion of LNG feed stream to heat exchange with compressed recycle portion of the column overhead. Resulting warm LNG is fed to the fractionating column at the mid-column position. Different types of heating and pressurising arrangements for column overhead are also suggested in the same patent. Other part of the LNG feed is further heated and/or heat exchanged with bottom liquid product before introducing into the column which is provided with a reboiler. While, it is clear from the above that considerable research has been going on in the field of processing mixtures of liquefied gasses in the past four decades, these processes still leave a lot to be desired. In most prior art processes, the entire LNG has to be fed into rectification column for the separation to affect. The column required for such separation is large causing the cost of the process to increase tremendously. A few others used partial condensers to provide reflux for the distillation column which are expensive and not very easy to control and operate. Therefore, there is an urgent need for a commercially viable alternative for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components which obviates the disadvantages of the prior art. Objects of the invention Accordingly, it an object of the present invention to provide a process for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components which overcomes the disadvantages associated with the prior art processes. It is another object of the present invention to provide a process for the recovery of LPG and higher molecular weight components from LNG by an economically viable process. It is yet another object of the present invention to provide a process for the recovery of LPG and higher molecular weight components from LNG by a process, which is simple to operate and employs a simple apparatus. It is yet another object of the present invention to provide a process for the recovery of propane and butane in an economical manner. It is another important object of the present invention to provide a process for separating LNG into a methane rich gas, LPG consisting predominantly of propane and butane and NGL consisting predominantly of pentane, hexane and higher molecular weight hydrocarbons. Summary of the invention The above and other objects of the present invention are achieved by the process of the present invention, which inter alia, aims at reducing the column size significantly while eliminating the use of partial condenser without compromising on the recovery of LPG components. According to the invention, LNG under a pressure that is less than its critical pressure is caused to split into two unequal parts. Major part of the LNG is heated and thereafter separated into vapour and liquid fractions. The pressure of the vapour fraction is reduced and fed to a power plant. The liquid fraction is also heated and further divided into a vapor and liquid fractions. The pressure of vapour fraction is again reduced before feeding it to the power plant. The liquid fraction is pressurised, heated and introduced into the upper sections of a first distillation column. The original minor portion of the feed LNG is also introduced into the first distillation column as sub-cooled reflux. The first distillation column is provided with a reboiler. The liquid stream drawn from the reboiler is fed to the second distillation column after pressure reduction. The product LPG is drawn from a total condenser located at the top of the second distillation column. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn from reboiler located at the bottom of the second distillation column. Most of the new LNG terminals are located close to power generation stations wherein evaporated gas is used as fuel for which the method described above provides most economical path for the recovery of LPG. Under normal circumstances gas pressure required for the power generation is about 25 Kg/cm2. Therefore, it not necessary to pressurise the methane rich gas streams using compressor and it can directly be used to feed the power plants. In case the vapourised gas can not be completely consumed, then part of the gas needs to be transported to consumers located in far away places in pipelines. In such an event, a portion of the methane-rich gas destined for far way customers needs to be pressurised with a compressor. The costs involved in compression are fairly high. This may however, be avoided by an alternative embodiment of the present invention described below. In this embodiment, the portion of the methane rich gas destined for consumers is reliquefied using the cold of the total feed. Reliquefield methane rich gas is pressurised with a cryogenic liquid pump to the desired level and evaporated. The gas at desired pressure is fed to the pipelines. The methane rich gas stream obtained is into two parts. The part meant for pipelines is diverted for cooling and thereafter is liquefied with a pressurised feed. Liquid stream is stored in a high pressure tank and is further pressurised. The pressurised reliquefied methane-rich gas is evaported in a high pressure heat exchanger before introducing into the pipeline. Accordingly, the present invention provides a process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components which comprises: a) pressurising LNG to below its critical pressure and dividing into a major and a minor portions; b) subjecting the major portion of LNG to partial vapourisation by raising its temperature in any conventional manner followed by separation of gas and liquid; c) subjecting said liquid to step (b) at least once; d) pressurising, heating and fractionating the separated liquid in step (c) in a column fitted with a reboiler; e) feeding minor portion of the pressurised LNG of step (a) at the top of the said column in step (d) as sub cooled reflux. f) mixing methane-rich vapour fractions from separators of step (b), (c) and from the top of the column in step (d) to form one or more methane-rich gas streams; g) fractionating bottom liquid rich in LPG from the reboiler of column in step (d) in another column after pressure reduction and subjecting column top stream to total condensation; i) withdrawing product LPG from the total condensation step ; j) withdrawing product NGL rich in pentane, hexane and higher components from step (g). In an alternative embodiment, LNG is separated into methane-rich gas at high pressure, another methane-rich gas stream at moderate pressure, LPG consisting predominantly propane and butane and NGL consisting predominantly pentane, hexane and higher molecular weight hydrocarbons without employing any expensive compression systems. This is achieved by the following steps: k) drawing a portion of the gas from any or all the three or more said streams in step (f) such that it does not exceed 70% of the total methane rich gas. 1) directing the said gas stream in step (k) to heat exchange with all or a portion of either pressurised feed of step (a) before dividing or with the major fraction of feed LNG divided in step (a) to cool it sufficiently and condense it substantially, m) pressurising the condensed liquid to the desired high pressure and vapourising completely with the help of a heating means. Preferably, the feed gas is under a pressure of about 26 Kg/cm2, although, the actual pressure may vary depending upon the exact composition of the LNG and its critical pressure. Ideally, it is preferred that the pressure of the feed gas is to below its critical pressure. In another preferred embodiment, the pressurised feed gas is divided into two parts, a major portion and a minor portion in a ratio of 9:1. In yet another preferred embodiment said major portion is vapourised by raising its temperature to about -90°C. Preferably, the liquid separated in step (C) is heated -70°C. In another preferred embodiment, the bottom liquid rich in LPG is subjected to fractionation after reducing its pressure to about 13 Kg/cm2. The present invention also provides an apparatus for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components comprising a feed means for supplying pressurised LNG, splitter means connected to said feed means for dividing said pressurised LNG into a major portion and a minor portion, a heat exchange means connected to said splitter means for heating said major portion, flash means connected to said heat exchange means for separating said heating major portion into a vapour fraction and liquid fraction, a first distillation column connected to said heat exchanger for receiving said liquid fraction, said heat exchanger also being connected to said splitter means for receiving said minor portion as a sub-cooled reflux stream, the vapour fraction from said first distillation column being diverted to a power plant, a reboiler means connected to said distillation column for heating the liquid stream to a high temperature and a second distillation column connected to said reboiler to separate said liquid fraction into liquefied petroleum gas (LPG) and higher molecular weight components. Preferably, a first valve means is connected to said flash means for reducing the pressure of the separated vapour fraction. In another preferred embodiment, a header means is connected to said valve means for directing said vapour fraction to a power plant. In yet another preferred feature, a further heat exchanger is connected to said flash means to further heat the liquid fraction released from said flash means. Preferably, a second flash means is connected to said further heat exchanger to further separate said liquid fraction to a further liquid fraction and a further vapour fraction. A further valve means may be connected to said second flash means to reduce the pressure of said vapour fraction prior to diverting it said power plant. Preferably, a third valve means may be located between said first and second distillation columns. Detailed description The present invention will now be described in greater detail with reference to the accompanying drawings wherein: Fig 1 shows a schematic representation of the apparatus of the present invention for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components and; Fig. 2 depicts an alternative embodiment of the apparatus of the present invention separated into methane-rich gas at high pressure, another methane-rich gas stream at moderate pressure, LPG consisting predominantly propane and butane and NGL consisting predominantly pentane, hexane and higher molecular weight hydrocarbons without employing any expensive compressors. The invention is schematically represented in Fig. 1. LNG from the storage tank 1 is pressurised using pump 2. LNG feed pressurised to 26 Kg/cm2 is divided into two parts in splitter 3 approximately in the ratio of 9:1. Major part of the LNG, 4 is further heated with heat exchanger 6 to about -90°C and separated into vapour and liquid fractions in a flash drum 7. The pressure of the vapour fraction 8 is reduced through a valve 10 and fed to the header 25 which supplies gas to the power plant. Separated liquid 9 is heated in heat exchanger 11 to -70°C and fed to the second flash drum 12. The pressure of vapour fraction 13 from 12 is reduced through the valve 14 before feeding it into the header 25. Separated liquid 15 from 12 is pressurised to about 25 Kg/cm2 with pump 16 and heated further to 15°C in heat exchanger 17 and introduced into the upper sections of the distillation column 18. Minor portion of the feed LNG, 5 is introduced into the distillation column as sub-cooled reflux. The vapour fraction from the top of the column 19 is fed to the header 25 after pressure reduction through a valve 20. The distillation column is provided with a reboiler which is provided with a heating means. The liquid stream 21 drawn from the reboiler at 64°C is fed to the distillation column 23 after pressure reduction to 13 Kg/cm2 through a valve 22. LPG 24 is drawn from the total condenser located at the top of the fraction column 23. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn as stream 26 from reboiler located at the bottom of the distillation column. Most of the new LNG terminals are located close to power generation stations wherein evaporated gas is used as fuel for which the scheme given in Fig 1 provides most economical path for the recovery of LPG. Under normal circumstances gas pressure required for the power generation is about 25 Kg/cm2. Therefore the methane rich gas streams need not be pressurised using compressor and can directly be used to feed the power plants. In case the vapourised gas can not be completely consumed in the near-by power plant, then part of the gas needs to be transported to consumers located in far away in pipelines. In this, a portion of the methane-rich gas destined for far way customers needs to be pressurised with a compressor. The costs involved in compression are fairly high. The compressor can be avoided by modifying the flow scheme given in Fig. 1. In this part of the invention the portion of the methane rich gas destined for consumers is reliquefied using the cold of the total feed. Reliquefield methane rich gas is pressurised with a cryogenic pump to the desired level and evaporated. The gas at desired pressure is fed to the pipelines. In Fig.2, methane rich gas stream from header 25 is into two parts. Pipeline part 28 is diverted to the heat exchanger 6 and liquefied with pressurised feed 4. Liquid stream 29 is stored in a high pressure tank 35 and pressurised by means of the pump 30. The pressurised reliquefied methane-rich gas 31 is evaported in a high pressure heat exchanger 32 before introducing into the pipeline. The process of the invention is now described with reference to the following non-limiting Examples. It will be apparent to a person skilled in the art that the preferred embodiments described herein are for illustrative purposes only and that various modifications and embodiments of the invention may be envisaged without departing from the scope and spirit of the invention. Example The process schemes described in Fig. 1 and 2 can be simulated with Industry standard Softwares like ChemShare, PRO-II, HYSIS and Aspen plus which give similar results. The results for a typical LNG composition given in Table 2 are presented in Table 3. Table -2 : Composition of Example LNG (Table Removed) claim: 1. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components which comprises: a) pressurising LNG to below its critical pressure and dividing into a major and a minor portions; b) subjecting the major portion of LNG to partial vapourisation by raising its temperature in any conventional manner followed by separation of gas and liquid; c) subjecting said liquid to step (b) at least once; d) pressurising, heating and fractionating the separated liquid in step (c) in a column fitted with a reboiler; e) feeding minor portion of the pressurised LNG of step (a) at the top of the said column in step (d) as sub cooled reflux. f) mixing methane-rich vapour fractions from separators of step (b), (c) and from the top of the column in step (d) to form one or more methane-rich gas streams; g) fractionating bottom liquid rich in LPG from the reboiler of column in step (d) in another column after pressure reduction and subjecting it to partial condensation; i) withdrawing product LPG from the total condensation step ; j) withdrawing product NGL rich in pentane, hexane and higher components from step (g). 2. A process as claimed in claim 1 wherein LNG is separated into methane-rich gas at high pressure, another methane-rich gas stream at moderate pressure, LPG consisting predominantly propane and butane and NGL consisting predominantly pentane, hexane and higher molecular weight hydrocarbons without employing any compressor by: k) drawing a portion of the gas from any or all the three said streams in step (f) such that it does not exceed 70% of the total methane rich gas. m) directing the said gas stream in step (k) to heat exchange with all or a portion of either pressurised feed of step (a) before dividing or with the major fraction of feed LNG divided in step (a) to cool it sufficiently and condense it substantially, m) pressurising the condensed liquid to the desired high pressure and vapourising completely with the help of a heating means. 3. A process as claimed in claim 1 or 2, wherein said feed gas is under a pressure of about 26 Kg/cm2. 4. A process as claimed in claim 1 or 2, wherein the pressure of the feed gas is below its critical pressure. 5. A process as claimed in any preceding claim, wherein the pressurised feed gas is divided into two parts, a major portion and a minor portion in a ratio 9:1. 6. A process as claimed in claim 5 wherein said major portion is vapourised by raising its temperature to about -90°C. 7. A process as claimed in any preceding claim wherein the liquid fraction separated in step (C) is heated to -70°C. 8. A process as claimed in any preceding claim wherein the bottom liquid rich in LPG is subjected to fractionation after reducing its pressure to about 13 Kg/cm2. 9. An apparatus for carrying out the process as claimed in any preceding claim comprising a feed means for supplying pressurised LNG, splitter means connected to said feed means for dividing said pressurised LNG into a major portion and a minor portion, a heat exchanger means connected to said splitter means for heating said major portion, flash means connected to said heat exchanger means for separating said heating major portion into a vapour fraction and liquid fraction, a first distillation column connected to said heat exchanger for receiving said liquid fraction, said heat exchanger also being connected to said splitter means for receiving said minor portion as a sub-cooled reflux stream, the vapour fraction from said first distillation column being diverted to a power plant, a reboiler means connected to said distillation column for heating the liquid stream to a high temperature and a second distillation column connected to said reboiler for to separate said liquid fraction into liquefied petroleum gas (LPG) and higher molecular weight components. 10. An apparatus as claimed in claim 9 wherein a first valve means is connected to said flash means for reducing the pressure of the separated vapour fraction. 11. An apparatus as claimed in claim 10, wherein a header means is connected to said valve means for directing said vapour fraction to a power plant. 12 An apparatus as claimed in any one of claims 9 to 11 wherein a further heat exchanger is connected to said flash means to further heat the liquid fraction released from said flash means. 13 An apparatus as claimed in any one of claims 9 to 12 wherein a second flash means is connected to said further heat exchanger to separate said liquid fraction to a further liquid fraction and a further vapour fraction. 14 An apparatus as claimed in any one of claims 10 to 13 wherein a further valve means is connected to said second flash means to reduce the pressure of said vapour fraction prior to diverting it said power plant. 15 An apparatus as claimed in claims 14 wherein a third valve means is located between said first and second distillation columns. 16. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components substantially as herein described with reference to the accompanying drawings. 17. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components with reference to the forgoing Example. 18. An apparatus for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components substantially as herein described with reference to the accompanying drawings

Full Text

A Method for Recovering Liquefied Petroleum Gases from Liquefied Natural Gas Field of invention
The present invention relates to recovery of Liquefied Petroleum Gases (hereinafter referred to as LPG) from Liquefied Natural Gas (hereinafter referred to as LNG). In particular, the present invention relates to recovery of Liquefied Petroleum Gases and higher molecular weight components from Liquefied Natural Gas in a LNG gasifying terminal. More particularly, the present invention relates to a method for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components. The high volatile components, which forms the major part of the LNG, can be fed to pipelines with prior vaporisation in case of liquid mixture. The minor part, which is less volatile, is recovered as liquids. The major part can be used for power generation, production of fertilizers or petrochemicals. The minor part, which can be transported in bottles, can be used as domestic fuel and other applications where pipelines are not available. Background of the invention
LNG normally produced near the large gas fields is transported at cryogenic temperatures of about -160°C and at atmospheric pressures. Depending on the source of natural gas, its pretreatment and liquefaction conditions, LNG composition varies from place to place. Further, the gross calorific value of the fuel gas generated by complete evaporation of the supplied LNG should meet a value agreed by the buyer and seller. This value is normally between 1100 to 1150 BTU/SCF, which is comparatively higher than that of pure methane i.e. 1030 BTU/SCF. In order to maintain the agreed heating value, the concentrations of high heating value components like ethane, propane, butane etc. are adjusted during liquefaction. In most of the cases, certain amounts of propane and butane are retained in the LNG. Presence of other heavier components like pentane,
hexane, heptane etc. is not desirable as they cause solids formation during liquefaction process. On the other hand, an advantage of retaining the heavier components is liquefaction of LNG can be carried out at higher temperatures. This will result in increased through put of LNG production plant for given refrigeration duty. With the above restrictions and advantages, the composition of the most of the LNGs shipped out from the production plants has the ranges as given in Table 1.
Table-1 - Variation of Compositions of Modern LNG Plants
(Table Removed)
From the above table, it can be seen that the concentrations propane and butanes which are the predominant components of LPG are high. These can be economically recovered with the method described by the present invention. Description of the Prior Art
A number of patents describe the separation of LNG into its components. US Patent 2,952,984 granted on September 20, 1960, describes use of a pressurised distillation column to separate LNG. Part of the vapours from the top of the column is condensed with the help of cool feed and is used as reflux. The feed after further heating enters the column at the middle portion. Remaining vapours are further heated and expanded to recover work before pipelining. The liquefied heavier components are withdrawn from the reboiler of the column.
US Patent No. 3,261,169 (1966), describes a method for separating LNG into a lean gas and rich liquefied gas with the help of a distillation column. In this method, the feed is partially vapourised and resulting liquid fraction is fed to the distillation column after heat exchange with intercooler of the evaporated gas compressors and for partially condensing overhead vapours of the column. The condensed liquid is refluxed and uncondensed gas is mixed with the lean gas from partial vapourisation step and fed to the pipeline. Rich LNG, which is enriched with heavier components, is withdrawn from the reboiler. The same patent also suggested the use of ethane as heating medium during partial vaporisation of LNG feed and to extract work with organic Rankine cycle from evaporated ethane.
US Patent 3,253,418 teaches a method of separating LNG into a lean gas and rich gas by partial vaporisation of LNG feed using ethane as heating media. Residual liquid is evaporated using propane as heating medium at higher pressure to obtain rich gas. Work is extracted from the heated ethane and propane gases.
A method of separation of LNG into its constituents during regassification is described in US Patent 3,405,530. LNG feed is pressurised and divided into two or more streams. The first stream is fed to the top of the demethaniser and the other stream(s) are fed to the middle of the same column after heat exchange with the condensers of deethaniser, depropaniser and debutaniser columns. Methane product is recovered from the top of demethaniser column. Product liquid withdrawn from the bottom of the column is fed to de-ethaniser that separates it to ethane and bottom liquid containing of propane and butane. These are separated in similar fashion in depropaniser and debutaniser.
US Patent 3,446, 029 (1969) details a method of gasifying LNG while using heat from a reformer unit operating with heavier hydrocarbon fractions separated from LNG. The separation is carried out in a series of three flash drums and the first one is fed with pressurised and heated LNG feed. Separated liquid from the last drum is fed to a distillation column, which receives reflux from a partial condenser cooled with LNG feed. US Patent No. 3,656,312 (1969) discusses the use of LNG feed as a coolant for the in-built top heat exchangers of methane and ethane columns. Heated LNG is fed to the methane column. Liquid methane generated in the top heat exchanger of the methane column can be pressurised to a desired level and vapourised before feeding to the pipeline. Bottom liquid from methane column is fed to ethane separation column. Bottom liquid from ethane column is separated into propane and other fractions in a separate column.
On a similar theme, another US Patent No. 3,420,068 describes a method of producing a liquid fraction or gas fraction at higher pressure from a low-pressure LNG feed. LNG is subjected to first partial vaporisation providing first methane rich gas stream and a residual liquid fraction, which is partially vaporised to provide second methane rich gas fraction. The first methane rich gas fraction is re-liquefied in heat exchange with feed LNG undergoing a warming up. The second methane-rich gas stream is also re-liquefied in heat exchange with the feed LNG undergoing first partial vaporisation.
In US Patent No. 3,837,172 a method is described for separating pressurised LNG by dividing into two unequal streams. The major stream is partly or fully vapourised before introducing into a rectification column in the middle portion. The minor portion is fed to the top of the column to serve as a sub-cooled reflux after pressure reduction through a valve. Another variation covered in the patent was recovery of mechanical work to be used for compressing the vapour leaving the column from fully-vapourised major stream before it is fed to the column. The rectification column provided with a reboiler
separates the LNG into methane enriched vapour fraction and a bottom liquid containing ethane and heavier fraction.
US Patent No. 5,114,451(1992) deals with recovery of ethane, ethylene, propane, propylene etc. from LNG. It is achieved by directing a portion of LNG feed stream to heat exchange with compressed recycle portion of the column overhead. Resulting warm LNG is fed to the fractionating column at the mid-column position. Different types of heating and pressurising arrangements for column overhead are also suggested in the same patent. Other part of the LNG feed is further heated and/or heat exchanged with bottom liquid product before introducing into the column which is provided with a reboiler.
While, it is clear from the above that considerable research has been going on in the field of processing mixtures of liquefied gasses in the past four decades, these processes still leave a lot to be desired. In most prior art processes, the entire LNG has to be fed into rectification column for the separation to affect. The column required for such separation is large causing the cost of the process to increase tremendously. A few others used partial condensers to provide reflux for the distillation column which are expensive and not very easy to control and operate. Therefore, there is an urgent need for a commercially viable alternative for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components which obviates the disadvantages of the prior art. Objects of the invention
Accordingly, it an object of the present invention to provide a process for separating LNG into a mixture of gases or liquids consisting of high volatile components and less volatile components which overcomes the disadvantages associated with the prior art processes.
It is another object of the present invention to provide a process for the recovery of
LPG and higher molecular weight components from LNG by an economically viable process.
It is yet another object of the present invention to provide a process for the recovery of LPG and higher molecular weight components from LNG by a process, which is simple to operate and employs a simple apparatus.
It is yet another object of the present invention to provide a process for the recovery of propane and butane in an economical manner.
It is another important object of the present invention to provide a process for separating LNG into a methane rich gas, LPG consisting predominantly of propane and butane and NGL consisting predominantly of pentane, hexane and higher molecular weight hydrocarbons. Summary of the invention
The above and other objects of the present invention are achieved by the process of the present invention, which inter alia, aims at reducing the column size significantly while eliminating the use of partial condenser without compromising on the recovery of LPG components.
According to the invention, LNG under a pressure that is less than its critical pressure is caused to split into two unequal parts. Major part of the LNG is heated and thereafter separated into vapour and liquid fractions. The pressure of the vapour fraction is reduced and fed to a power plant. The liquid fraction is also heated and further divided into a vapor and liquid fractions. The pressure of vapour fraction is again reduced before feeding it to the power plant. The liquid fraction is pressurised, heated and introduced into the upper sections of a first distillation column. The original minor portion of the feed LNG is also introduced into the first distillation column as sub-cooled reflux. The first distillation column is provided with a reboiler. The liquid stream drawn from the reboiler
is fed to the second distillation column after pressure reduction. The product LPG is drawn from a total condenser located at the top of the second distillation column. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn from reboiler located at the bottom of the second distillation column.
Most of the new LNG terminals are located close to power generation stations wherein evaporated gas is used as fuel for which the method described above provides most economical path for the recovery of LPG. Under normal circumstances gas pressure required for the power generation is about 25 Kg/cm2. Therefore, it not necessary to pressurise the methane rich gas streams using compressor and it can directly be used to feed the power plants. In case the vapourised gas can not be completely consumed, then part of the gas needs to be transported to consumers located in far away places in pipelines. In such an event, a portion of the methane-rich gas destined for far way customers needs to be pressurised with a compressor. The costs involved in compression are fairly high. This may however, be avoided by an alternative embodiment of the present invention described below.
In this embodiment, the portion of the methane rich gas destined for consumers is reliquefied using the cold of the total feed. Reliquefield methane rich gas is pressurised with a cryogenic liquid pump to the desired level and evaporated. The gas at desired pressure is fed to the pipelines.
The methane rich gas stream obtained is into two parts. The part meant for pipelines is diverted for cooling and thereafter is liquefied with a pressurised feed. Liquid stream is stored in a high pressure tank and is further pressurised. The pressurised reliquefied methane-rich gas is evaported in a high pressure heat exchanger before introducing into the pipeline.
Accordingly, the present invention provides a process for separating liquefied
natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components which comprises:
a) pressurising LNG to below its critical pressure and dividing into a major and a minor portions;
b) subjecting the major portion of LNG to partial vapourisation by raising its temperature in any conventional manner followed by separation of gas and liquid;
c) subjecting said liquid to step (b) at least once;
d) pressurising, heating and fractionating the separated liquid in step (c) in a column fitted with a reboiler;
e) feeding minor portion of the pressurised LNG of step (a) at the top of the said column in step (d) as sub cooled reflux.

f) mixing methane-rich vapour fractions from separators of step (b), (c) and from the top of the column in step (d) to form one or more methane-rich gas streams;
g) fractionating bottom liquid rich in LPG from the reboiler of column in step (d) in another column after pressure reduction and subjecting column top stream to total condensation;
i) withdrawing product LPG from the total condensation step ;
j) withdrawing product NGL rich in pentane, hexane and higher components
from step (g).
In an alternative embodiment, LNG is separated into methane-rich gas at high pressure, another methane-rich gas stream at moderate pressure, LPG consisting predominantly propane and butane and NGL consisting predominantly pentane, hexane and higher molecular weight hydrocarbons without employing any expensive compression
systems. This is achieved by the following steps:
k) drawing a portion of the gas from any or all the three or more said streams
in step (f) such that it does not exceed 70% of the total methane rich gas.
1) directing the said gas stream in step (k) to heat exchange with all or a
portion of either pressurised feed of step (a) before dividing or with the major fraction of feed LNG divided in step (a) to cool it sufficiently and condense it substantially, m) pressurising the condensed liquid to the desired high pressure and
vapourising completely with the help of a heating means. Preferably, the feed gas is under a pressure of about 26 Kg/cm2, although, the actual pressure may vary depending upon the exact composition of the LNG and its critical pressure. Ideally, it is preferred that the pressure of the feed gas is to below its critical pressure.
In another preferred embodiment, the pressurised feed gas is divided into two parts, a major portion and a minor portion in a ratio of 9:1.
In yet another preferred embodiment said major portion is vapourised by raising its temperature to about -90°C.
Preferably, the liquid separated in step (C) is heated -70°C.
In another preferred embodiment, the bottom liquid rich in LPG is subjected to fractionation after reducing its pressure to about 13 Kg/cm2.
The present invention also provides an apparatus for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components comprising a feed means for supplying pressurised LNG, splitter means connected to said feed means for dividing said pressurised LNG into a major portion and a minor portion, a heat exchange means connected to said splitter means for heating said major portion,
flash means connected to said heat exchange means for separating said heating major portion into a vapour fraction and liquid fraction, a first distillation column connected to said heat exchanger for receiving said liquid fraction, said heat exchanger also being connected to said splitter means for receiving said minor portion as a sub-cooled reflux stream, the vapour fraction from said first distillation column being diverted to a power plant, a reboiler means connected to said distillation column for heating the liquid stream to a high temperature and a second distillation column connected to said reboiler to separate said liquid fraction into liquefied petroleum gas (LPG) and higher molecular weight components.
Preferably, a first valve means is connected to said flash means for reducing the pressure of the separated vapour fraction.
In another preferred embodiment, a header means is connected to said valve means for directing said vapour fraction to a power plant.
In yet another preferred feature, a further heat exchanger is connected to said flash means to further heat the liquid fraction released from said flash means. Preferably, a second flash means is connected to said further heat exchanger to further separate said liquid fraction to a further liquid fraction and a further vapour fraction. A further valve means may be connected to said second flash means to reduce the pressure of said vapour fraction prior to diverting it said power plant. Preferably, a third valve means may be located between said first and second distillation columns. Detailed description
The present invention will now be described in greater detail with reference to the accompanying drawings wherein:
Fig 1 shows a schematic representation of the apparatus of the present invention for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher
molecular weight components and;
Fig. 2 depicts an alternative embodiment of the apparatus of the present invention separated into methane-rich gas at high pressure, another methane-rich gas stream at moderate pressure, LPG consisting predominantly propane and butane and NGL consisting predominantly pentane, hexane and higher molecular weight hydrocarbons without employing any expensive compressors.
The invention is schematically represented in Fig. 1. LNG from the storage tank 1 is pressurised using pump 2. LNG feed pressurised to 26 Kg/cm2 is divided into two parts in splitter 3 approximately in the ratio of 9:1. Major part of the LNG, 4 is further heated with heat exchanger 6 to about -90°C and separated into vapour and liquid fractions in a flash drum 7. The pressure of the vapour fraction 8 is reduced through a valve 10 and fed to the header 25 which supplies gas to the power plant. Separated liquid 9 is heated in heat exchanger 11 to -70°C and fed to the second flash drum 12. The pressure of vapour fraction 13 from 12 is reduced through the valve 14 before feeding it into the header 25. Separated liquid 15 from 12 is pressurised to about 25 Kg/cm2 with pump 16 and heated further to 15°C in heat exchanger 17 and introduced into the upper sections of the distillation column 18. Minor portion of the feed LNG, 5 is introduced into the distillation column as sub-cooled reflux. The vapour fraction from the top of the column 19 is fed to the header 25 after pressure reduction through a valve 20. The distillation column is provided with a reboiler which is provided with a heating means. The liquid stream 21 drawn from the reboiler at 64°C is fed to the distillation column 23 after pressure reduction to 13 Kg/cm2 through a valve 22. LPG 24 is drawn from the total condenser located at the top of the fraction column 23. Heavier components consisting of predominantly pentane and higher molecular weight compounds are withdrawn as stream 26 from reboiler located at the bottom of the distillation column.
Most of the new LNG terminals are located close to power generation stations wherein evaporated gas is used as fuel for which the scheme given in Fig 1 provides most economical path for the recovery of LPG. Under normal circumstances gas pressure required for the power generation is about 25 Kg/cm2. Therefore the methane rich gas streams need not be pressurised using compressor and can directly be used to feed the power plants. In case the vapourised gas can not be completely consumed in the near-by power plant, then part of the gas needs to be transported to consumers located in far away in pipelines. In this, a portion of the methane-rich gas destined for far way customers needs to be pressurised with a compressor. The costs involved in compression are fairly high. The compressor can be avoided by modifying the flow scheme given in Fig. 1.
In this part of the invention the portion of the methane rich gas destined for consumers is reliquefied using the cold of the total feed. Reliquefield methane rich gas is pressurised with a cryogenic pump to the desired level and evaporated. The gas at desired pressure is fed to the pipelines.
In Fig.2, methane rich gas stream from header 25 is into two parts. Pipeline part
28 is diverted to the heat exchanger 6 and liquefied with pressurised feed 4. Liquid
stream 29 is stored in a high pressure tank 35 and pressurised by means of the pump 30.
The pressurised reliquefied methane-rich gas 31 is evaported in a high pressure heat
exchanger 32 before introducing into the pipeline.
The process of the invention is now described with reference to the following non-limiting Examples. It will be apparent to a person skilled in the art that the preferred embodiments described herein are for illustrative purposes only and that various modifications and embodiments of the invention may be envisaged without departing from the scope and spirit of the invention.
Example
The process schemes described in Fig. 1 and 2 can be simulated with Industry standard Softwares like ChemShare, PRO-II, HYSIS and Aspen plus which give similar results. The results for a typical LNG composition given in Table 2 are presented in Table 3.
Table -2 : Composition of Example LNG
(Table Removed)

claim:
1. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas
(LPG) and higher molecular weight components which comprises:
a) pressurising LNG to below its critical pressure and dividing into a major and a minor portions;
b) subjecting the major portion of LNG to partial vapourisation by raising its temperature in any conventional manner followed by separation of gas and liquid;
c) subjecting said liquid to step (b) at least once;
d) pressurising, heating and fractionating the separated liquid in step (c) in a column fitted with a reboiler;
e) feeding minor portion of the pressurised LNG of step (a) at the top of the said column in step (d) as sub cooled reflux.

f) mixing methane-rich vapour fractions from separators of step (b), (c) and from the top of the column in step (d) to form one or more methane-rich gas streams;
g) fractionating bottom liquid rich in LPG from the reboiler of column in step (d) in another column after pressure reduction and subjecting it to partial condensation;
i) withdrawing product LPG from the total condensation step ;
j) withdrawing product NGL rich in pentane, hexane and higher components
from step (g).
2. A process as claimed in claim 1 wherein LNG is separated into methane-rich gas
at high pressure, another methane-rich gas stream at moderate pressure, LPG
consisting predominantly propane and butane and NGL consisting predominantly
pentane, hexane and higher molecular weight hydrocarbons without employing any
compressor by:
k) drawing a portion of the gas from any or all the three said streams in step
(f) such that it does not exceed 70% of the total methane rich gas. m) directing the said gas stream in step (k) to heat exchange with all or a
portion of either pressurised feed of step (a) before dividing or with the major
fraction of feed LNG divided in step (a) to cool it sufficiently and condense it
substantially, m) pressurising the condensed liquid to the desired high pressure and
vapourising completely with the help of a heating means.
3. A process as claimed in claim 1 or 2, wherein said feed gas is under a pressure of about 26 Kg/cm2.
4. A process as claimed in claim 1 or 2, wherein the pressure of the feed gas is below its critical pressure.
5. A process as claimed in any preceding claim, wherein the pressurised feed gas is divided into two parts, a major portion and a minor portion in a ratio 9:1.
6. A process as claimed in claim 5 wherein said major portion is vapourised by raising its temperature to about -90°C.

7. A process as claimed in any preceding claim wherein the liquid fraction separated in step (C) is heated to -70°C.
8. A process as claimed in any preceding claim wherein the bottom liquid rich in LPG is subjected to fractionation after reducing its pressure to about 13 Kg/cm2.
9. An apparatus for carrying out the process as claimed in any preceding claim comprising a feed means for supplying pressurised LNG, splitter means connected to said feed means for dividing said pressurised LNG into a major portion and a
minor portion, a heat exchanger means connected to said splitter means for heating said major portion, flash means connected to said heat exchanger means for separating said heating major portion into a vapour fraction and liquid fraction, a first distillation column connected to said heat exchanger for receiving said liquid fraction, said heat exchanger also being connected to said splitter means for receiving said minor portion as a sub-cooled reflux stream, the vapour fraction from said first distillation column being diverted to a power plant, a reboiler means connected to said distillation column for heating the liquid stream to a high temperature and a second distillation column connected to said reboiler for to separate said liquid fraction into liquefied petroleum gas (LPG) and higher molecular weight components.
10. An apparatus as claimed in claim 9 wherein a first valve means is connected to said flash means for reducing the pressure of the separated vapour fraction.
11. An apparatus as claimed in claim 10, wherein a header means is connected to said valve means for directing said vapour fraction to a power plant.

12 An apparatus as claimed in any one of claims 9 to 11 wherein a further heat exchanger is connected to said flash means to further heat the liquid fraction released from said flash means.
13 An apparatus as claimed in any one of claims 9 to 12 wherein a second flash means is connected to said further heat exchanger to separate said liquid fraction to a further liquid fraction and a further vapour fraction.
14 An apparatus as claimed in any one of claims 10 to 13 wherein a further valve means is connected to said second flash means to reduce the pressure of said vapour fraction prior to diverting it said power plant.
15 An apparatus as claimed in claims 14 wherein a third valve means is located
between said first and second distillation columns.
16. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components substantially as herein described with reference to the accompanying drawings.
17. A process for separating liquefied natural gas (LNG) into liquefied petroleum gas (LPG) and higher molecular weight components with reference to the forgoing Example.
18. An apparatus for separating liquefied natural gas (LNG) into liquefied petroleum
gas (LPG) and higher molecular weight components substantially as herein
described with reference to the accompanying drawings

Documents:

935-del-2001-abstract.pdf

935-del-2001-claims.pdf

935-del-2001-correspondence-others.pdf

935-del-2001-correspondence-po.pdf

935-del-2001-description (complete).pdf

935-del-2001-drawings.pdf

935-DEL-2001-Form-1.pdf

935-del-2001-form-19.pdf

935-del-2001-form-2.pdf

935-del-2001-form-3.pdf

935-del-2001-gpa.pdf

935-del-2001-petition-others.pdf

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Patent Number

232471

Indian Patent Application Number

935/DEL/2001

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

17-Mar-2009

Date of Filing

07-Sep-2001

Name of Patentee

ENGINEERS INDIA LIMITED,

Applicant Address

ENGINEERS INDIA BHAWAN, 1, BHIKAJI CAMA PLACE, R K PURAM, RING ROAD, NEW DELHI-110066, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	NANDA, RAVEEV	ENGINEERS INDIA LTD, PT BUILDING, 4 SANSAD MARG, NEW DELHI-110 001, INDIA.
2	JOSHI, MANOJ	ENGINEERS INDIA LTD, PT BUILDING, 4 SANSAD MARG, NEW DELHI-110 001, INDIA
3	SONI, ADARSH	ENGINEERS INDIA LTD, PT BUILDING, 4 SANSAD MARG, NEW DELHI-110 001, INDIA
4	BABU, DASARI RAM	R&D CENTER, EIL, SECTOR 16, GURGAON-122001, HARYANA, INDIA.
5	MAITI, RABINDRANATH	R&D CENTER, EIL, SECTOR 16, GURGAON-122001, HARYANA, INDIA.

PCT International Classification Number

F25J 3/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA