Patent Application
20230408191
The present invention relates to a method for extracting ethane from a stream of initial natural gas, comprising the following steps:
Such a method is intended in particular for extracting ethane and C3+ hydrocarbons from an initial natural gas, while producing a pressurized treated natural gas, which is then liquefied before being expanded for storage.
Ethylene, ethane, propylene, propane and heavier hydrocarbons can be extracted from gases such as natural gas, refinery gas and synthetic gases obtained from other hydrocarbon sources such as coal, crude oil, naphtha.
Natural gas usually contains a majority of methane and ethane (e.g., methane and ethane make up at least 50 mol % of the gas). Natural gas can also contain more negligible amounts of heavier hydrocarbons such as propane, butanes, pentanes and also hydrogen, nitrogen and carbon dioxide.
The invention described herein relates more particularly to the recovery, from natural gas, of ethane, propane and heavier hydrocarbons. In addition to the fact that heavy hydrocarbons in natural gas, such as ethane, propane and butane, can be highly valued by marketing same separately with a high purity, said hydrocarbons might condensate during transport or freeze in liquefaction exchangers (for the heaviest of hydrocarbons).
The above can lead to incidents, such as the arrival of liquid plugs in transport facilities or shut-downs of the liquefaction plant to unclog frozen exchangers.
U.S. Pat. No. 6,578,379 describes a very efficient method for recovering ethane and propane from a stream of natural gas. Such a method generally operates in a very efficient manner, in particular for obtaining a very high extraction (e.g. greater than 99 mol %) of the ethane contained in the natural gas feed, while minimizing energy consumption.
In order to obtain such extraction yields, it is known how to use a flow very depleted in ethane at the main reflux, namely for the highest reflux of the methane and ethane separation column.
To this end, a recirculation stream is taken from the recompressed gas coming from the head of the separation column for methane and ethane. The recirculation stream is cooled in countercurrent with regard to the gas coming from the column head and is then expanded to form the main reflux introduced at the column head.
However, under certain operating conditions, the quality of the main reflux is likely to deteriorate in terms of temperature and/or composition.
E.g., if the main reflux becomes depleted in methane, the ethane separation rate in the column decreases, and the quality of the head stream produced at the head of the column deteriorates further, aggravating the methane depletion of the main reflux. A “snowball” effect occurs, leading to a significant decrease in the ethane extraction rate. Such can be the case in particular if liquid is entrained on the upper plates of the column.
One aim of the invention is to provide a flexible and very efficient method for extracting ethane and C3+ hydrocarbons from an initial stream of natural gas, wherein the ethane extraction rate is not at all or only slightly affected when fluctuations occur in the quality of the head of the separation column.
To this end, the subject matter of the invention is a method of the aforementioned type, characterized by the following steps:
The method according to the invention can comprise one or a plurality of the following features, taken individually or according to any technically possible combination:
The invention also relates to a plant for the extraction of ethane from a stream of initial natural gas, comprising:
The invention will be better understood upon reading the following description, given only as an example and made in reference to the enclosed drawings, wherein:
Throughout hereinafter, the same reference will be used for identifying a flow of liquid and the pipe which carries the fluid, the pressures considered are absolute pressures, and the percentages considered are molar percentages.
The methods described were modeled on a process simulator. 82% polytropic efficiencies for compressors and 86% adiabatic efficiencies for turbines, were defined.
A first installation 10 for extracting ethane according to the invention, is illustrated in
The installation 10 is intended for the simultaneous production, from a stream initial natural gas 12, of an ethane-rich flow 14, of a bottom stream 16 rich in C3+ hydrocarbons, of an expanded liquefied natural gas 18, and of a fuel stream 20, advantageously intended for being reused in the installation 10.
With reference to
The extraction unit 22 includes first and second upstream heat exchangers 28, 30, a separating drum 32, and a column 34 for separating methane and C2+ hydrocarbons. The column 34 is herein provided with a bottom reboiler 35.
The unit 22 further includes a dynamic expansion turbine 36 coupled to a first compressor 38, a second compressor 40, each compressor 38, 40 being provided downstream with a cooler 42, 44.
The unit 22 further includes a bottom pump 46, a fractionation column 48 provided with a bottom reboiler 50 and a reflux system 52, the reflux system 52 including a cooler 54, a reflux drum 56, and a reflux pump 58.
The natural gas liquefaction unit 24 is a known unit, in particular a C3MR or a DMR unit.
In the example shown in
The storage 66 is e.g. a thermally insulated storage tank.
In the present example, the flash and storage unit 26 further includes a downstream heat exchanger 68, if appropriate a suction drum 70, and a compression apparatus 72 including a plurality of compressors 74 mounted in series, separated therebetween by coolers 76.
A first method according to the invention, implemented in the installation 10, will now be described.
The initial natural gas forming the stream 12 is advantageously a dry, at least partially decarbonated, desulphurized natural gas.
The term “at least partially decarbonated” means that the concentration of carbon dioxide in the stream of initial natural gas 13 is advantageously less than or equal to 50 ppmv.
Similarly, the concentration of water is less than 1 ppmv, advantageously less than ppmv.
The concentration of sulfur-containing elements, including hydrogen sulfide, is less than 10 ppmv and advantageously less than or equal to 4 ppmv.
An example of the molar composition of the stream of initial natural gas 12 is given in the table below.
More generally, the molar fraction of methane in the stream of initial natural gas 12 is comprised between 75 mol % and 95 mol %, the molar fraction of C2 hydrocarbons is comprised between 3 mol % and 12 mol %, and the molar fraction of C3+ hydrocarbons is comprised between 1 mol % and 8 mol %.
The flow rate of the stream of initial natural gas 12 is e.g. greater than 2,000 kmol/h and is e.g. comprised between 2,000 kmol/h and 70,000 kmol/h, in particular equal to kmol/h.
The stream of initial natural gas 12 has a temperature close to ambient temperature, in particular between 0° C. and 40° C., herein equal to 21.5° C., and a pressure advantageously greater than 35 bar, in particular greater than 70 bar, in the present example equal to 81 bar.
The initial natural gas 12 is introduced into the first heat exchanger 28 to be cooled therein. It forms a stream 80 of cooled natural gas. The initial natural gas 12 is herein supercritical, so it is simply cooled. In a variant, the initial natural gas is not supercritical and is at least partially condensed in the first heat exchanger 28.
It has a temperature below −20° C., and in particular between −25° C. and −45° C., in particular equal to −37° C.
The stream 80 is then introduced into the separator drum 32, to be separated therein into a liquid flow 82, recovered at the bottom of the separator drum 32, and a gas flow 84 recovered at the head of the separator drum 32. The flow rate of the liquid flow 82 can be zero, in particular when the stream of cooled natural gas 80 is supercritical.
The liquid flow 82 flows through a static expansion valve 86 to form a mixed expanded phase 88. The pressure of the expanded mixed phase 88 is less than 50 bar, in particular less than 30 bar, and is e.g. equal to 28.7 bar. The expanded mixed phase 88 is introduced at a bottom level N1 of the separation column 34.
The gas flow 84 is divided into a main turbine feed stream 90 and a secondary reflux stream 92.
The molar flow rate of the turbine feed stream 90 is greater than the molar flow rate of the reflux stream 92, and in particular comprised between 5% and 25% of the molar flow rate of the reflux stream 92.
The turbine feed stream 90 is introduced into the dynamic expansion turbine 36 to be expanded therein to a pressure of less than 50 bar, in particular less than 30 bar, e.g. equal to 28.7 bar.
The dynamic expansion of the current 90 makes it possible to recover more than kW of energy, e.g. 10,865 kW of energy.
The temperature of the cooled and expanded stream 94 coming from the dynamic expansion turbine 36 is e.g. less than −70° C., in particular less than −80° C., e.g. equal to −80.8° C.
The cooled and expanded stream 94 is then introduced into the separation column 34 at a level N2 situated above level N1.
The reflux stream 92 is introduced into a static expansion valve 96 in order to be expanded therein to a pressure of less than 50 bar, in particular less than 30 bar, in particular equal to 28.7 bar. It is cooled in the second upstream heat exchanger 30 to a temperature below −80° C., in particular below −90° C., in particular equal to −95.8° C.
The expanded and cooled reflux stream is introduced into the separation column 34 at a level N3 situated above the level N2 at the head of the column 34.
The pressure of the separation column 34 is preferentially between 10 bar and 40 bar, in particular between 20 bar and 40 bar, e.g. substantially equal to 28.5 bar.
The separation column 34 produces a head stream 98. The head stream 98 is heated in the second upstream heat exchanger 30, then in the first upstream heat exchanger 28 in countercurrent with the initial natural gas 12, for forming a heated head stream 100.
The temperature of the heated head stream 100 is greater than 0° C., in particular greater than 15° C., and is e.g. equal to 17.6° C.
The heated head stream 100 is then compressed in the compressor 38 coupled to the turbine 36, then cooled in the cooler 42, for obtaining a stream at a pressure greater than 30 bar, in particular equal to 34.6 bar.
It is then recompressed in the compressor 40, then cooled in the cooler 44 in order to produce a stream of compressed purified natural gas 102 intended for the liquefaction unit 24.
The stream compressed purified natural gas 102 has a pressure greater than 60 bar, in particular greater than 80 bar, e.g. equal to 91 bar. It has a temperature greater than in particular greater than 10° C., more particularly equal to 21.5° C.
The coolers 42, 44 are herein fed by a cooling flow with a temperature of less than in particular equal to 7° C. The cooling flow can be, in particular, air or water.
The compressed purified natural gas stream 102 is rich in methane. It has a concentration of methane greater than 99.0 mol %, in particular equal to 99.1 mol %. It has a low concentration of nitrogen, in particular less than 1.0 mol %, and a low concentration of C2+ hydrocarbons, more particularly a concentration of ethane of less than 0.5 mol %, substantially equal to 0.2 mol % ethane.
The separation column 34 produces at the bottom, a bottom stream 106 rich in C2+ hydrocarbons. The stream 106 contains e.g. more than 95 mol % of the ethane contained in the initial natural gas 10, and 100 mol % of the C3+ hydrocarbons contained in said stream.
The bottom current 106 has a temperature greater than 10° C., in particular comprised between 20° C. and 30° C., e.g. equal to 23.2° C. It contains less than 1000 ppmv of carbon dioxide, preferentially between 200 ppmv and 500 ppmv of carbon dioxide, e.g. 313 ppmv of carbon dioxide. It has a concentration of methane of less than 5 mol %, e.g. between 0 mol % and 3 mol %, in particular less than 1 mol %.
The table below illustrates an example of the composition of the bottom current 106.
A first lateral reboiling stream 108 is extracted from the separation column 34, at a level N5 lower than level N1, e.g. located at the 20th stage from the top of the separation column 34.
The first liquid reboiling stream 108 is brought to the first heat exchanger 28, to be heated therein in the heat exchanger 28 by heat exchange, in particular with the initial natural gas 12, to a temperature greater than 0° C., in particular equal to 8.25° C. The reboiling stream 108 is then reintroduced into the separation column 34 at a level N7 situated below the level N5, e.g. at the 21st stage starting from the top of the column 34.
Similarly, a second liquid reboiling stream 110 is extracted from the separation column 34 at a level N7 lower than the level N6, e.g. from the 22th stage from the top of the separation column 34, to be brought to the bottom reboiler 35 in order to be heated therein to a temperature above 0° C., e.g. equal to 10.7° C. An energy greater than 1 MW, e.g. equal to 4 MW, is supplied to the second liquid reboiling stream 110.
The second liquid reboiling stream 110 is then returned to the separation column 34 at a level N8 located below level N7. The level N8 is e.g. located on the 23th stage starting from the top.
The bottom stream 106 is pumped into the pump 46 to be introduced at an intermediate level P1 of the fractionation column 48.
The fractionation column 48 produces, at the head, a head flow 112 containing less than 1 mol % of C3+ hydrocarbons, more particularly less than 1 mol % of propane.
The head flow 112 is partially condensed in the cooler 54, then separated in the reflux drum 56 so as to form at the top the ethane-rich stream 14, and at the bottom, a liquid reflux stream 114 reintroduced at the top of the fractionation column 48, after pumping by the reflux pump 58.
The ethane-rich flow 14 contains more than 96 mol % of the ethane contained in the initial natural gas 12. It contains more than 97 mol % of ethane.
The ethane-rich flow 14 is herein gaseous. In a variant (not shown), the ethane-rich flow 14 is a liquid taken from the liquid stream 114.
The C3+ hydrocarbon stream contains less than 500 ppmv of ethane, more particularly less than 100 ppmv of ethane.
The stream of compressed purified natural gas 102 is brought into the liquefaction unit 24 which produces, in a known manner, a stream of pressurized liquefied natural gas 120.
The stream of pressurized natural gas has a pressure greater than 20 bar, in particular comprised between 20 bar and 90 bar, advantageously equal to 73 bar. It has a temperature below −120° C., in particular below −130° C., and advantageously equal to −136.8° C.
The compressed liquefied natural gas 120 is introduced into the expansion component 60, herein into a dynamic expansion turbine. It is expanded to a pressure of less than 5 bar, in particular less than 2 bar, e.g. equal to 1.25 bar, to form a stream of flashed liquefied natural gas 122.
The stream of flashed liquefied natural gas 122 is introduced into a flash drum 62 to be separated therein into a stream of expanded liquefied natural gas 124 and a first flow of flash gas 126.
The expanded liquefied natural gas stream 124 is pumped into the storage tank 66 by means of the pump 64, to form the expanded liquefied natural gas 18.
The first flow of flash gas 126 is recovered at the head of the flash drum 62. It is introduced into the downstream heat exchanger 68 in order to be heated there in countercurrent with a portion of the compressed purified natural gas 102, which is reintroduced into the stream of flashed liquefied natural gas 122, upstream of the flash drum 62.
After heat exchange in the downstream heat exchanger 68, the flow of heated flash gas 130 thus formed has a temperature greater than −60° C., and in particular substantially equal to 5° C. It has a very high concentration of methane, e.g. greater than 80 mol %, e.g. greater than 85 mol %, in particular greater than 90 mol %. Such concentration is advantageously greater than 95 mol % of methane, in particular greater than 96 mol % of methane, e.g. equal to 96.46 mol % of methane.
It has a concentration of nitrogen of less than 20 mol %, e.g. less than 15 mol %, in particular less than 10 mol %. Said concentration is advantageously less than 5 mol %, in particular less than 4 mol %, e.g. substantially equal to 3.54 mol % of nitrogen.
The flow of heated flash gas 130 has a concentration of ethane of less than 50 ppmv, in particular less than 10 ppmv, e.g. equal to 5 ppmv.
After flowing through a suction flask 70, the flow of heated flash gas 130 is compressed in the compression apparatus 72 to a pressure greater than 25 bar, in particular greater than 30 bar, and e.g. equal to 60 bar in order to produce a flow of compressed flash gas 132.
The flow of compressed flash gas 132 is separated into the stream of fuel 20 and a recycle stream 134.
The fuel stream 20 is intended to be sent to the fuel gas network of the installation to supply e.g. gas turbines of the natural gas liquefaction unit 24 or the gas turbines of an electric current generation unit intended e.g. for supplying the compressor 40 or other equipment of the installation 10.
The recycle stream 134 has a pressure greater than 30 bar, in particular greater than 50 bar, e.g. equal to 58.5 bar.
It is conveyed successively into the first heat exchanger 28 and then into the second heat exchanger 30 in order to be cooled therein to a temperature below −80° C., in particular below −90° C., e.g. equal to −95.5° C.
The recycle stream 134 is then expanded in a static expansion valve 136 to a pressure of less than 50 bar, in particular less than 30 bar, e.g. equal to 28.7 bar, to be introduced into the separation column 34 at a head level N9 of the column 34, e.g. at the first stage starting from the top of the column 34. The level N9 is situated above the level N3 for introducing the expanded and cooled reflux stream.
As indicated hereinabove, the recycle stream 134 coming from the flow of flash gas 126 is very rich in methane, since the ethane remains in the liquefied natural gas 18, or is extracted successively in the separation column 34 and then in the fractionation column 48.
Thus, the composition of the reflux introduced at the head of the separation column 34 stays very rich in methane, whatever the fluctuations in the quality of the head stream 98 of the separation column 34.
The presence of the new reflux also provides operational flexibility during the implementation of the method, and also during the design phase.
It is thus possible to optimize overall the energy consumption between the ethane extraction unit 22 and the liquefaction unit 24 in order to adjust the parameters of the two units 22, 24, in order to select, as well as possible, the compressors and the driving mode thereof, as required in the two units 22, 24. Thereby, the investment costs are significantly reduced, and also the operating costs as will be seen in the example described hereinbelow.
In a variant (not shown), the heated head stream 100 is compressed at the outlet of the compressor 38 coupled to the turbine 36 in a compression machine comprising two compression stages of the same power, the total power being equal to the power of the compressor 40. The compression machine includes an intermediate cooler which cools the gas between the compression stages. The arrangement thereby obtained provides a power saving of 5.8 MW.
A second installation 140 intended for implementing a second method according to the invention is shown in
The second method according to the invention is analogous to the first method according to the invention. It differs from the first method according to the invention in that the second method comprises a step of removal of a recirculation stream 142 from the stream of compressed purified natural gas 102.
The molar flow rate of the recirculation stream 142 is advantageously less than the molar flow rate of the residual stream of compressed purified natural gas 102, after removal of the recirculation stream 142, at the introduction thereof into the liquefaction unit 22.
The recirculation stream 142 has a pressure greater than 50 bar, in particular greater than 80 bar, e.g. equal to 90 bar. It is introduced successively into the first heat exchanger 28 and then into the second heat exchanger 30 in order to be cooled therein to a temperature below −90° C., preferentially below −95° C. and e.g. substantially equal to −95.4° C.
Then, the recirculation stream 142 is expanded to a pressure of less than 50 bar, in particular less than 30 bar, in particular equal to 28.7 bar, and is introduced into the separation column 34 between the recirculation stream 134 and the reflux stream 92.
A third installation 150 intended for implementing a third method according to the invention is shown in
The installation 150 differs from the first installation 10 in that it comprises a system 152 for collecting and recompressing the evaporation gases formed in the storage 66.
The collection system 152 comprises a protective drum 154 and a compression apparatus 156 including a plurality of compression stages 158 spaced two-by-two by a cooler 160.
A second flow of flash gas 162 resulting from the evaporation of the liquefied natural gas in the storage 66 is collected at the head of the storage 66 and is then introduced into the compression apparatus 156 to be compressed therein to a pressure greater than 25 bar, in particular between 26 bar and 70 bar, e.g. equal to 60 bar.
The second flow of compressed flash gas 164 thus produced is separated into the fuel stream 20 and the recycle stream 134, which is reintroduced into the separation column 34, after cooling in the heat exchangers 28, 30 and expansion in the expansion valve 136.
Advantageously, in the example shown in
A fourth installation 170 intended for implementing a fourth method according to the invention is shown in
The fourth installation 170 differs from the first installation 10 in that the storage units 66 are equipped, like the third installation 150, with a system 152 for collecting the evaporation gases.
During the implementation of the fourth method according to the invention, the first flow of compressed flash gas 132 and the second stream of compressed flash gas 164 are mixed, before the mixture is separated into the fuel stream 20 and the recycle stream 134.
Like before, the recycle stream 134 is reintroduced into the separation column 34 after flowing through the heat exchangers 28, 30, then being expanded in the static expansion valve 136.
A fifth installation 200 for implementing a fifth method according to the invention is illustrated in
The fifth method differs from the second method shown in
By means of the invention which has just been described, it is possible to maintain a substantially constant composition of the reflux of the separation column 34, which prevents the snowball effect which occurs during fluctuations in the composition of the head stream 98 extracted from the separation column 34, in the absence of the supply of the recycle stream 134.
Said method is thus both simple and effective in maintaining a constant concentration of extracted ethane, without increasing the investment costs or the operating costs.
The energy consumption of the method is detailed in the following table.
As indicated in the above-table, the total power consumed in the presence of a reflux generated from the recycle stream 134 represents a significant reduction of the power consumed and the specified power divided by the flow-rate of the liquefied natural gas produced by the installation.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2011521 | Nov 2020 | FR | national |
This application is a U.S. National Stage filed under 35 U.S.C. § 371, based on International PCT Application No. PCT/EP2021/081135, filed on Nov. 9, 2021, which claims priority to French Application FR 2011521 filed on Nov. 10, 2020 in the French Patent Office. The entire contents of these applications are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/081135 | 11/9/2021 | WO |
