EP2659211A2

EP2659211A2 - Method for producing a methane-rich stream and a c2+ hydrocarbon-rich stream, and associated equipment

Info

Publication number: EP2659211A2
Application number: EP11802438.9A
Authority: EP
Inventors: Vanessa Gahier; Julie Gouriou; Sandra Thiebault; Loïc BARTHE
Original assignee: Technip France SAS
Current assignee: Technip Energies France SAS
Priority date: 2010-12-27
Filing date: 2011-12-26
Publication date: 2013-11-06
Anticipated expiration: 2031-12-26
Also published as: CA2822766C; US20140290307A1; WO2012089709A2; US10619919B2; FR2969745A1; CA2822766A1; WO2012089709A3; MX362997B; EP2659211B1; FR2969745B1; US20200208911A1; AR084608A1; MX2013007552A

Abstract

The invention relates to a method comprising separating a feed stream (16) into a first fraction (41 A) and a second fraction (41 B). The method comprises feeding the first cooled feed fraction (42) into a first disengager (22) in order to produce a light head stream (44). The method comprises expanding a turbine feed fraction (48) resulting from the light head stream (44) in a first turbine (26) up to a first pressure, and feeding the first expanded fraction (54) into a distillation column (30). The method comprises expanding the second fraction of the feed stream (41 B) in a second turbine (40) up to a second pressure substantially equal to the first pressure. The second expanded fraction (91A) from the second dynamic expansion turbine (40) is used to form a cooled reflux stream (91 B) to be fed into the column (30).

Description

Process for producing a methane-rich stream and a C ₂ ⁺ hydrocarbon-rich stream and associated plant

The present invention relates to a process for producing a methane-rich stream and a C ₂ ⁺ hydrocarbon-rich stream from a hydrocarbon-containing feed stream, of the type comprising the steps of:

separating the feed stream into a first fraction of the feed stream and at least a second fraction of the feed stream;

cooling the first fraction of the feed stream in a first heat exchanger;

introducing the first fraction of the cooled feed stream into a first separator flask to produce a light head stream and a heavy bottom stream;

- Relaxing a turbine feed fraction formed from the light head stream in a first dynamic expansion turbine to a first pressure and introducing at least a portion of the first fraction expanded from the first turbine in a first distillation column;

- relaxing at least a portion of the heavy foot current to form a relaxed foot stream and introduction of the expanded foot stream into the first distillation column without passing through the first heat exchanger between the first separator balloon and the first column of distillation;

recovering a bottom stream from the bottom of the first distillation column, the C ₂ ⁺ hydrocarbon rich stream being formed from the bottom stream;

recovering and reheating a methane-rich overhead stream; compressing at least a fraction of the overhead stream in at least one first compressor coupled to the first dynamic expansion turbine and in at least one second compressor;

formation of the methane-rich stream from the heated and compressed column head stream.

Such a process is intended for extracting C ₂ ⁺ hydrocarbons, such as methylene, ethane, propylene, propane and heavier hydrocarbons, especially from natural gas, refinery gas or synthetic gas obtained from from other hydrocarbon sources such as coal, crude oil, naphtha.

Natural gas generally contains a majority of methane and ethane constituting at least 50 mol% of the gas. It also contains in a more negligible quantity heavier hydrocarbons, such as propane, butane, pentane. In some cases, it also contains helium, hydrogen, nitrogen and carbon dioxide.

It is necessary to separate heavy hydrocarbons from natural gas to meet at least two requirements.

First of all, economically, C ₂ ⁺ hydrocarbons, and in particular ethane, propane and butane, can be recovered. In addition, the demand for natural gas liquids as a burden for the petrochemical industry is growing steadily and is expected to continue to grow in the coming years.

In addition, for process reasons, it is desirable to separate the heavy hydrocarbons to prevent them from condensing during transport and / or handling of the gases. This avoids incidents such as the arrival of liquid plugs in transport or treatment facilities designed for gaseous effluents.

In order to separate the C ₂ ⁺ hydrocarbons from natural gas, it is known to use an oil absorption process which makes it possible to recover up to 90% of the propane and up to approximately 40% of the ethane.

To achieve higher recovery rates, cryogenic expansion methods are used.

In a known cryogenic expansion process, a portion of the hydrocarbon feed stream is used for the secondary reboilers of a methane separation column.

Then, the different effluents, after partial condensation, are combined to feed a gas-liquid separator.

As described in US Pat. No. 5,555,748, the light stream obtained at the top of the separator is divided into a first column feed fraction, which is condensed before being sent to the overhead feed of the distillation column and in a second fraction. which is sent to a dynamic expansion turbine before being introduced into the distillation column.

This method has the advantage of being easy to start and offer significant operational flexibility, combined with good efficiency and good safety.

However, the economic constraints require to further increase the efficiency of the process while maintaining a very high ethane extraction yield. It is also necessary to minimize the size of the installations and to reduce or even eliminate the supply of external refrigerants such as propane, especially for the implementation of the process on floating installations or in sensitive areas in terms of security. An object of the invention is therefore to obtain a production process which makes it possible to separate a feed stream containing hydrocarbons in a stream rich in C ₂ ⁺ hydrocarbons and in a stream rich in methane, very economically, compact and very efficient.

For this purpose, the object of the invention is a process of the aforementioned type, characterized in that the method comprises the following steps:

- relaxing at least a portion of the second fraction of the feed stream in a second dynamic expansion turbine, separate from the first dynamic expansion turbine to a second pressure, to form a second fraction relaxed from the second dynamic expansion turbine, the second pressure being substantially equal to the first pressure; and

cooling and at least partially liquefying at least a portion of the second expanded fraction from the second dynamic expansion turbine to form a cooled reflux stream and introducing the cooled reflux stream into the first distillation column.

The method according to the invention may comprise one or more of the following characteristics, taken alone or in any combination (s) technically possible (s):

the pressure difference between the first pressure and the second pressure is less than 8 bar;

the temperature of the portion of the second fraction of the feed stream introduced into the second dynamic expansion turbine is greater than the temperature of the turbine feed fraction introduced into the first dynamic expansion turbine;

the method comprises introducing the first expanded fraction from the first dynamic expansion turbine into a second heat exchanger to be cooled and partially liquefied, the cooled first cooled fraction forming an additional cooled reflux flow, the process comprising introducing the additional cooled reflux stream into the first distillation column;

the method comprises the following steps:

Introducing the second fraction expanded from the second dynamic expansion turbine in a downstream separator tank to form a second gaseous head stream and a second liquid bottom stream,

Cooling the second gaseous head stream to form a cooled reflux stream. Introducing at least a portion of the second expanded fraction from the second dynamic expansion turbine into an auxiliary column, and

• forming the cooled reflux current from the bottom stream of the auxiliary column;

· Optionally, partial condensation of the second fraction of the feed stream;

Introducing the second fraction of the feed stream into an upstream separator tank to form a second gaseous fraction and a second liquid fraction;

Introduction of the second gaseous fraction into the second dynamic expansion turbine;

Introduction of the second liquid fraction, after expansion, into a lower part of the first distillation column;

the whole of the second fraction of the feed stream is introduced into the second dynamic expansion turbine, possibly without cooling between the step of separating the feed stream and the step of introducing the second fraction of the feed stream; supply in the second dynamic expansion turbine;

the method comprises the following steps:

Taking a secondary compression fraction from the methane-rich column stream before passing a fraction of the methane-rich column head stream into the first compressor,

• passage of the secondary fraction in a third compressor coupled to the second dynamic expansion turbine;

Introducing the compressed secondary fraction from the third compressor into the fraction of the compressed overhead stream downstream of the first compressor;

the second compressor comprises a first compression stage, at least a second compression stage and a refrigerant interposed between the first compression stage and the second compression stage, the method comprising a step of passing the compressed column head stream from the first compressor successively in the first compression stage, in the refrigerant, then in the second compression stage;

at least a portion of the second expanded fraction from the second dynamic expansion turbine, at least a fraction of the column top stream and optionally the first expanded fraction derived from the first dynamic expansion turbine, are placed in a controlled relationship; heat exchange; the method comprises the following steps:

• division of the overhead light stream into the turbine feed fraction and into a column feed fraction;

At least partial cooling and condensation of the column feed fraction in a second heat exchanger,

• expansion and at least partial introduction of the cooled column feed fraction into the first distillation column,

at least a portion of the second expanded fraction from the second dynamic expansion turbine and the column feed fraction being advantageously placed in heat exchange relationship;

- at least a fraction of the overhead stream and at least a portion of the second expanded fraction from the second dynamic expansion turbine are placed in heat exchange relationship in a downstream heat exchanger separate from the second heat exchanger;

the method comprises the following steps:

• withdrawal of a withdrawal stream in the column top stream;

• cooling of the withdrawal stream at least in the first heat exchanger and introduction of the cooled withdrawal stream into the first distillation column; and

· Optionally, heat exchange of the withdrawal stream with at least a portion of the second fraction relaxed from the second turbine.

the method comprises the following steps:

• collecting a reboiling stream in the first distillation column at a sampling level;

· Placing in heat exchange relationship the reboiling current with at least a portion of the second expanded fraction from the second dynamic expansion turbine for cooling and at least partially liquefying the portion of the second expanded fraction from the second turbine of dynamic relaxation; and

Possibly, putting in heat exchange relation with the first fraction relaxed from the first turbine;

Reintroduction of the reboiling stream into the first distillation column at a level below the sampling level.

the method comprises the following steps:

• taking a makeup cooling stream from the methane-rich overhead stream or a stream formed from the methane-rich overhead stream; • expansion and introduction of the additional cooling stream expanded in a current flowing upstream of the first expansion turbine, preferably in the first fraction of the cooled feed stream or in the turbine feed fraction;

the method comprises the following steps:

• passage of the methane-rich overhead stream in the first heat exchanger;

• drawing of an auxiliary expansion stream in the methane-rich column head stream, after passing through the first heat exchanger;

Dynamic expansion of the auxiliary expansion current in a dynamic auxiliary expansion turbine; and

Introduction of the expanded stream from the dynamic expansion auxiliary turbine into the methane-rich column head stream before it passes through the first heat exchanger.

The invention further relates to a plant for producing a methane-rich stream and a stream rich in C ₂ ⁺ hydrocarbons from a feed stream containing hydrocarbons, of the type comprising:

means for separating the feed stream at a first fraction of the feed stream and at least a second fraction of the feed stream;

a first heat exchanger for cooling the first fraction of the feed stream;

means for introducing the first cooled feed fraction into a first separator flask to produce a light head stream and a heavy bottom stream;

a first dynamic expansion turbine and means for introducing a turbine feed fraction formed from the light head stream into the first dynamic expansion turbine in order to relax the turbine feed fraction up to a first pressure;

a first distillation column;

- means for introducing at least a portion of the first fraction expanded in the first turbine in the first distillation column;

means for relaxing at least a portion of the heavy foot current to form a relaxed foot stream and means for introducing at least a portion of the foot stream expanded in the first distillation column; introduction of the relaxed foot stream being configured so that the foot current does not does not pass through the first heat exchanger between the first separator flask and the first distillation column;

means for recovering a bottom stream from the bottom of the first distillation column, the C ₂ ⁺ hydrocarbon-rich stream being formed from the bottom stream;

means for recovering and reheating a methane-rich overhead stream,

at least one first compressor coupled to the first dynamic expansion turbine and at least one second compressor for compressing at least a fraction of the overhead stream;

means for forming the methane-rich stream from the heated and compressed column head stream issuing from the second compressor;

characterized in that the installation comprises:

A second dynamic expansion turbine, distinct from the first dynamic expansion turbine,

Means for introducing at least a portion of the second fraction of the feed stream into the second dynamic expansion turbine to form a second expanded fraction from the second dynamic expansion turbine at a second pressure, the second turbine dynamic expansion device being arranged so that the first pressure is substantially equal to the second pressure; and

Means for cooling and at least partially liquefying at least a portion of the second fraction derived from the second dynamic expansion turbine to form a cooled reflux flow and means for introducing the cooled reflux flow into the first distillation column.

The installation according to the invention may comprise the following characteristic:

- the installation comprises:

an auxiliary column;

- Means for introducing at least a portion of the second expanded fraction from the second dynamic expansion turbine in the auxiliary column; and

means for forming the cooled reflux current from the foot stream of the auxiliary column.

The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: - Figure 1 is a functional block diagram of a first production facility for the implementation of a first method according to the invention;

FIG. 2 is a functional block diagram of a second production installation intended for the implementation of a second method according to the invention;

- Figure 3 is a functional block diagram of a third production facility for the implementation of a fifth method according to the invention;

- Figure 4 is a functional block diagram of a fourth production facility for the implementation of a sixth method according to the invention;

- Figure 5 is a functional block diagram of a fifth production facility for the implementation of a seventh method according to the invention;

- Figure 6 is a functional block diagram of a sixth production facility for the implementation of an eighth method according to the invention.

In all that follows, we will designate by the same references a current flowing in a pipe and the pipe that carries it.

In addition, unless otherwise indicated, the percentages mentioned are molar percentages and the pressures are given in absolute bar.

In the numerically simulated examples, the efficiency of each compressor is selected to be 82% polytropic and the efficiency of each turbine is 85% adiabatic.

Similarly, the described distillation columns use trays but they can also use loose packing or structured. A combination of trays and packing is also possible. The additional turbines described involve compressors but they can also cause variable frequency electric generators whose electricity produced can be used in the network via a frequency converter. Currents with a temperature above ambient are described as being cooled by aero-refrigerants. Alternatively, it is possible to use water exchangers for example freshwater or seawater.

FIG. 1 illustrates a first installation 10 for producing a stream 12 rich in methane and a section 14 rich in C ₂ ⁺ hydrocarbons according to the invention, from a gas stream 16 for supply.

The gaseous stream 16 is a stream of natural gas, a stream of refinery gas, or a stream of synthetic gas obtained from a hydrocarbon source such as coal, crude oil, naphtha. In the example shown in the figures, stream 16 is a stream of dehydrated natural gas. The method and the installation 10 are advantageously applied to the construction of a new unit for recovering methane and ethane.

The plant 10 comprises, from upstream to downstream, a first heat exchanger 20, a first separator tank 22 and a first dynamic expansion turbine 26, capable of producing work during the expansion of a current passing through the turbine .

According to the invention, the installation 10 further comprises a second heat exchanger 28, a first distillation column 30, a first compressor 32 coupled to the first dynamic expansion turbine 26, a first refrigerant 34, a second compressor 36, a second refrigerant 38, and a bottom pump 39.

According to the invention, the installation 10 further comprises a second dynamic expansion turbine 40 and a third compressor 41 coupled to the second dynamic expansion turbine 40.

A first production method according to the invention, implemented in the installation 10 will now be described.

By way of example, the feed stream 16 is formed of a dehydrated natural gas which comprises, in moles, 2.06% of nitrogen, 83.97% of methane, 6.31% of ethane, , 66% propane, 0.70% isobutane, 1.50% n-butane, 0.45% isopentane, 0.83% n-pentane and 0.51% carbon dioxide.

The feed stream 16 is more generally in moles between 5% and 15%.

% C ₂ ⁺ hydrocarbons to extract and between 75% and 90% methane.

The term "dehydrated gas" means a gas whose water content is as low as possible and is in particular less than 1 ppm.

The feed stream 16 has a pressure greater than 35 bar, in particular greater than 50 bar and a temperature close to ambient temperature and in particular substantially equal to 30 ° C. In this example, the flow rate of the feed stream is 15,000 kmol / hour.

The feed stream 16 is first divided into a first feed stream fraction 41A and a second feed stream fraction 41B.

The ratio of the molar flow rate of the first fraction 41 A to the second fraction 41 B is, for example, greater than 2 and is in particular between 2 and 15.

In the example shown, the first fraction 41 A is introduced into the first heat exchanger 20 where it is cooled and partially condensed to form a fraction 42 of cooled feed stream.

The temperature of the fraction 42 is below -10 ° C. and is in particular equal to

- 26.7 ° C. Then, the cooled fraction 42 is introduced into the first separating flask 22. The liquid content of the cooled fraction 42 is less than 50 mol%.

A light head stream 44 and a heavy liquid bottom stream 45 are removed from the first separator tank 22.

In this example, the gas stream 44 is divided into a minor column feed fraction 46 and a major turbine feed fraction 48. The ratio of the molar flow rate of the majority fraction 48 to the minor fraction 46 is greater than 2.

The column feed fraction 46 is introduced into the second heat exchanger 28 to be fully liquefied and subcooled. It forms a cooled column feed fraction 49. This fraction 49 is expanded in a first static expansion valve 50 to form a expanded fraction 52 introduced into reflux in the first distillation column 30.

The temperature of the relaxed fraction 52 obtained after passing through the valve

50 is below -70 ° C and is in particular equal to -1 1 1 ° C.

The pressure of the expanded fraction 52 is also substantially equal to the operating pressure of the column 30 which is less than 40 bar and in particular between 10 bar and 30 bar, advantageously equal to 17 bar.

Fraction 52 is introduced into an upper part of column 30 at a level

N1, located on the first floor from the top of column 30.

The turbine feed fraction 48 is introduced into the first dynamic expansion turbine 26. It is dynamically expanded to a pressure P1 close to the operating pressure of the column 30 to form a first relaxed feed fraction. 54 which has a temperature below -50 ° C, in particular equal to -79 ° C.

The expansion of the feed fraction 48 in the first turbine 26 makes it possible to recover 3574 kW of energy which cool the fraction 48.

The first expanded fraction 54, which is the effluent from the first dynamic expansion turbine 26, constitutes a first cooled reflux stream 56.

The liquid content of the cooled reflux stream 56 is greater than 5 mol%. The cooled reflux stream 56 is introduced into an average part of the column 30 located under the upper part, at a level N2 below the level N1, and corresponding in this example to the sixth stage starting from the top of the column 30.

The heavy liquid stream 45 recovered at the bottom of the first separator tank 22 is expanded in a second static expansion valve 58 to form a relaxed heavy stream 60. The pressure of the expanded heavy stream 60 is less than 50 bars and is notably substantially equal to the pressure of the column 30. The temperature of the expanded heavy stream 60 is less than -30 ° C. and is notably substantially equal to -48 ° C.

The heavy liquid stream 45 is introduced entirely into the column 30 after expansion in the valve 58, without passing through the first heat exchanger 20. Thus, the heavy liquid stream 45 before it passes through the valve 58 and the expanded heavy stream 60 do not enter into heat exchange relation with the feed stream 16, nor with the fractions 41 A, 41 B of this feed stream 16.

In particular, the heavy current 45 does not pass into the heat exchanger 20 between the outlet of the balloon 22 and the inlet of the column 30.

A first reboiling stream 74 is taken near the bottom of the column 30 at a temperature greater than -3 ° C. and in particular substantially equal to 9.6 ° C., at a level N6 situated below the level N3, advantageously at the and a first stage starting from the top of the column 30.

The first stream 74 is brought to the first heat exchanger 20 where it is heated up to a temperature greater than 3ΐ and in particular equal to 16.3 ^ before being returned to a level N7 corresponding to the twenty-second stage on leaving from the top of column 30.

A second reboil stream 76 is drawn at a level N8 above the N6 level and below the N3 level, preferably at the seventeenth stage from the top of the column. The second reboiling stream 76 is introduced into the first heat exchanger 20 to be heated to a temperature above -8 ° C and in particular equal to -4.1 ° C. It is then returned to the column 30 at a level N9 located below the level N8 and above the level N6, advantageously at the eighteenth stage from the top of the column 30.

A third reboiling current 78 is taken at a level N10 located below the level N3 and above the level N8, advantageously at the thirteenth stage starting from the top of the column 30. The third reboiling current 78 is then brought to the first heat exchanger 20 where it is heated to a temperature above -30 ° C and in particular equal to -19 ° C before being returned to a level N1 1 of the column 30 located under the N10 level and located at above the N8 level, advantageously at the fourteenth stage from the top of the column 30.

Thus, the stream 52 is introduced into the upper part of the column 30 which extends from a height greater than 35% of the height of the column 30, whereas the stream 60 is introduced into an average part which is extends under the upper part. The column 30 produces at the bottom a liquid stream 82 of the bottom of the column. The stream 82 of the bottom of the column has a temperature greater than 4 ° C. and in particular equal to 16.3 ° C.

Thus, the bottom stream 82 contains in mol 1, 17% of carbon dioxide, 0.00% of nitrogen, 0.43% of methane, 42.89% of ethane, 28.40% of propane, 51% i-butane, 11%, 66% n-butane, 3.47% i-pentane, 6.46% n-pentane.

More generally, the stream 82 has a C ₁ / C ₂ ratio of less than 3 mol%, for example equal to 1%.

The stream 82 contains more than 80%, advantageously more than 87 mol% of the ethane contained in the feed stream 16 and contains substantially 100 mol% of the C ₃ ⁺ hydrocarbons contained in the feed stream 16 .

The column bottom stream 82 is pumped into the pump 39 to form the C ₂ ⁺ hydrocarbon rich section 14.

It can be advantageously heated by placing in heat exchange relation with at least a fraction of the feed stream 16 to a temperature below its bubble temperature, to maintain it in liquid form.

The column 30 produces at the top a gaseous stream 84 of column head rich in methane. Current 84 has a temperature below -70 ° C and in particular substantially equal to -Ι Οδ'Ό. It has a pressure substantially equal to the pressure of the column 30, for example equal to 17.0 bar.

The overhead stream 84 is successively introduced into the second heat exchanger 28, then into the first heat exchanger 20 to be reheated and form a heated head stream 86 rich in methane. Current 86 has a temperature above -10 ° C and in particular equal to 22.9 ° C.

At the outlet of the first exchanger 20, the stream 86 is divided into a first fraction of the heated overhead stream 87A and a second fraction of the heated overhead stream 87B.

The ratio of the molar flow rate of the first fraction 87A to the molar flow rate of the second fraction 87B is greater than 2 and is for example between 2 and 5, for example.

The first fraction 87A is introduced into the first compressor 32 driven by the main turbine 26 to be compressed at a pressure greater than 20 bar.

The second fraction 87B is introduced into the third compressor 41 to be compressed at a pressure greater than 20 bar and substantially equal to the pressure at which the first fraction 87A is compressed in the first compressor 32. Then, the compressed fractions 87A, 87B respectively from the compressors 32, 41 are combined before being introduced into the first air cooler 34. The combined fractions 87A, 87B are cooled to a temperature below 60 ° C, in particular Room temperature.

The compressed stream 88 thus obtained is introduced into the second compressor

36 and then in the second refrigerant 38 to form a compressed head stream 90.

The current 90 thus has a pressure greater than 40 bars and in particular substantially equal to 63.1 bars.

The compressed overhead stream 90 forms the methane-rich stream 12 produced by the process of the invention.

Its composition is advantageously 96.28 mol% of methane, 2.37 mol% of nitrogen and 0.92 mol% of ethane. It comprises more than 99.93% of the methane contained in the feed stream 16 and less than 5% of the C ₂ ⁺ hydrocarbons contained in the feed stream 16.

The second fraction 41 B of the feed stream 16 is introduced into the second dynamic expansion turbine 40 to be expanded at a second pressure P2 substantially equal to the pressure of the column 30 and thus form a second relaxed feed fraction 91A.

The temperature of the second fraction 41 B feeding the second dynamic expansion turbine 40 is greater than the temperature of the turbine feed fraction 48 supplying the first dynamic expansion turbine 26, for example at least 30 ° C.

Moreover, the second pressure P2 is substantially equal to the first pressure P1. The difference between the pressure P1 and the pressure P2 is in particular less than 8 bar, advantageously less than 5 bar and in particular less than 2 bar.

The second fraction 91 A relaxed thus has a temperature below 0 ° C and in particular of the order of - 25 ° C.

Then, the second fraction 91A is introduced into the second heat exchanger 28 to be cooled to a temperature below -70 ° C and in particular equal to -102.5 ° C and to be partially condensed, by heat exchange with the current 84 and optionally, with the column feed fraction 46, when present.

The second expanded fraction 91 B from the second heat exchanger 28 forms a second reflux stream which is conveyed to the column 30 to be introduced into the upper part at a level N12 situated for example between the level N1 and the level N2 on the fourth floor from the top of the column. Examples of temperatures, pressures, and molar flow rates of the different streams are given in Table 1 below.

TABLE 1

Table 2 below illustrates the power consumed by the compressor 36 as a function of the flow rate of the second fraction 41B sent to the second turbine 40.

TABLE 2

The energy consumption of the method according to the invention, constituted by the drive energy of the second compressor 36, is 12244 kW, compared with 141 1 1 kW with a method of the state of the art according to US 4,157,904 or US 4,278,457, wherein the same charge rate to be processed is used and the same recovery achieved. Compared with the state of the art, the method according to the invention thus makes it possible to obtain a significant reduction in the power consumed, while maintaining a high selectivity for the extraction of ethane.

A second installation 1 10 according to the invention is shown in Figure 2. This installation 1 10 is intended for the implementation of a second method according to the invention.

The second method differs from the first method in that a withdrawal stream 92 is taken from the compressed top stream 90.

The withdrawal stream 92 has a non-zero molar flow rate between 0% and 35% of the molar flow rate of the compressed head stream 90 upstream of the sample, the remainder of the compressed head stream 90 forming the stream 12.

The withdrawal stream 92 is successively cooled in the first exchanger 20, then in the second exchanger 28, before being expanded in a third static expansion valve 94.

The current 96, which before expansion in the valve 94, is essentially liquid, has after expansion a liquid fraction greater than 0.8.

The expanded withdrawal stream 96 coming from the third valve 94 is then introduced in reflux in the vicinity of the head of the column 30 at a level N14 situated above the level N1 and advantageously corresponding to the first stage of the column 30.

The temperature of the expanded draw stream 96 prior to introduction into the column 30 is below -70 ° C and is preferably -1.13.5 μ.

Examples of temperatures, pressures and molar flow rates of the different streams are given in Table 3 below.

TABLE 3

In a variant (not shown), the second compressor 36 may comprise two compression stages separated by an air cooler.

The power consumed by the compressor 36 (single-stage) as a function of the flow rate of the second fraction of the supply current 41 B is given in Table 4 below.

TABLE 4

The second method according to the invention thus makes it possible to obtain extremely high ethane recovery rates, greater than 90%, and especially greater than 99%. This almost total recovery of the ethane contained in the feed stream 16 can be obtained as in the method described in US 5,568,737, but with a saving in terms of power consumption that can be greater than 8%, of the order of 1300 kW.

A third installation 170 according to the invention is shown in FIG. 3. The third installation 170 is intended for the implementation of a third method according to the invention.

The third method according to the invention differs from the first method according to the invention in that the relaxed feed fraction 54 intended for the column 30 is introduced at least partially into the second heat exchanger 28 to be connected to it. heat exchange with the gaseous stream 84 of methane-rich column head, with the second expanded feed fraction 91 A from the second dynamic expansion turbine 40, and advantageously with the column feed fraction 46, when this this is present.

The fraction 54 is thus cooled to a temperature below -60 ° C, and in particular substantially equal to -84 ° C. It is at least partially condensed to form the cooled first reflux stream 56.

The cooled reflux stream 56 is then introduced into the middle portion of the column 30 at the N2 level, as previously described.

A bypass may be provided to introduce a portion of the expanded fraction 54 into the column 30 without passing through the exchanger 28.

Examples of temperatures, pressures and molar flow rates of the different streams are given in Table 5 below.

TABLE 5

A fourth installation 180 according to the invention is shown in FIG. 4. The fourth installation 180 is intended for the implementation of a fourth method according to the invention.

The fourth method according to the invention differs from the third method according to the invention, shown in FIG. 3, in that a draw-off stream 92 is taken from the compressed top stream 90, then passed successively into the first heat exchanger. 20, then in the second heat exchanger 28, as described in the second method according to the invention.

The fourth method according to the invention is moreover analogous to the third method according to the invention.

A fifth installation 210 according to the invention is shown in FIG. 5. This fifth installation 210 is intended for the implementation of a fifth method according to the invention.

The fifth facility 210 is intended to advantageously increase the recovery of C ₂ ⁺ in an existing installation including the type described in US Patents 4,157,904 and US 4,278,457. The existing plant comprises the first heat exchanger 20, the first separator tank 22, the distillation column 30, the first compressor 32 coupled to the first expansion turbine 26 and the second compressor 36.

The fifth installation 210 according to the invention further comprises a second dynamic expansion turbine 40, a third compressor 41, and a downstream flask 152 for collecting the effluent from the second dynamic expansion turbine 40.

The plant 210 further comprises an upstream heat exchanger 212, a downstream heat exchanger 214, and an auxiliary distillation column 216 provided with a bottom auxiliary pump 218.

The fifth facility 210 also includes a fourth compressor

220 interposed between two refrigerant 222A, 222B.

The fifth installation 210 further comprises a downstream flask 152, disposed downstream of the second turbine 40.

The fifth method according to the invention differs from the first method according to the invention in that the feed stream 16 is further separated into a third fraction 224 of the feed stream which is introduced into the upstream heat exchanger 212, before to be mixed with the first fraction 41 A from the exchanger 20 to form the first fraction 42 cooled.

The ratio of the molar flow rate of the third fraction 224 to the molar flow rate of the feed stream 16 is greater than 5%.

Thus, the fifth method according to the invention differs from the first method according to the invention in that the second cooled and partially liquefied feed fraction 91A is introduced into the downstream flask 152.

This fraction 91 A is separated in the downstream flask 152 into a second liquid foot stream 154 and into a second gaseous head stream 156.

The second liquid foot stream 154 is introduced into a fourth static expansion valve 157 to be substantially expanded under the pressure of the column 30 and form a second relaxed foot stream 158.

Unlike the first method according to the invention described above, the second head stream 156 coming from the downstream flask 152 is introduced into the downstream heat exchanger 214 to be cooled to a temperature below -70.degree. second cooled head stream 225.

The second cooled overhead stream 225 is introduced into the auxiliary column 216 at a lower stage E1.

Column 216 has a theoretical number of stages less than the number of theoretical stages of column 30. This number of stages is advantageously understood. between 1 and 7. The auxiliary column 216 operates at a pressure substantially equal to that of the column 30.

The relaxed foot stream 158 obtained after expansion of the second foot stream 154 in the valve 157 is introduced into the column 30 at a level N1 advantageously corresponding to the first stage from the top of the column 30.

A first portion 226 of the fraction 52 expanded in the valve 50 is introduced into the auxiliary column 216 at a stage E3 located above the level E1. A second portion 228 of the fraction 52 is introduced directly into the column 30 at the level N1, after mixing with the stream 158.

Auxiliary column 216 produces a methane-rich head auxiliary stream 230 and a foot auxiliary current 232.

The auxiliary head stream 230 is mixed with the methane-rich head stream 84 produced by the distillation column 30.

Foot stream 232 is pumped by auxiliary pump 218 to form a cooled reflux stream 234 which is introduced into column 30 after mixing with stream 158.

The stream 234 thus constitutes a cooled reflux stream which is obtained from a portion of the expanded fraction 91 A resulting from the second dynamic expansion turbine 40, after separation of this effluent.

The mixture 235 of the overhead streams 84 and 230 is separated into a first major fraction 236 of the overhead stream and a second minor fraction 238 of the overhead stream.

The ratio of the molar flow rate of the majority fraction 236 to the minor fraction 238 is greater than 1.5.

The majority fraction 236 is introduced successively into the second heat exchanger 28, then into the first heat exchanger 20, in order to form the heated overhead stream 86.

The second overhead stream fraction 238 is passed through the downstream heat exchanger 214 countercurrently to the second overhead stream 156 to heat to a temperature above -50 ° C and form a second heated fraction.

240.

The second heated fraction 240 is then separated into a return stream 242, and a compression stream 244.

The return current 242 is reintroduced into the first head stream fraction 236, downstream of the second heat exchanger 28 and upstream of the first heat exchanger 20 to partially form the heated head stream 86. The recompression stream 244 is then introduced into the upstream exchanger 212 to cool the third fraction of the feed stream 224. The stream 244 warms to a temperature above -10 ° C to form a warmed recompression stream 246 .

A first portion 248 of the recompression stream 246 is mixed with the first fraction of the overhead stream 86, downstream of the first heat exchanger 20 to form the heated overhead stream 87A.

A second portion 250 of the recompression stream 246 is introduced into the third compressor 41, then into the refrigerant 222A, before being recompressed in the fourth compressor 220 and introduced into the refrigerant 222B.

The second compressed portion 252 from the refrigerant 222B has a temperature below 60 ° C and in particular substantially equal to 40 ° C and a pressure greater than 35 bar and in particular equal to 63.1 bar.

This first compressed portion 252 is mixed with the compressed overhead stream 90 to form the methane-rich stream 12.

The fifth installation 210 and the fifth method according to the invention therefore make it possible to increase the C ₂ ⁺ hydrocarbon recovery rate in an existing state of the art installation, without having to modify the existing equipment of the plant. installation, and in particular by keeping the heat exchangers 20 and 28, the column 30, the compressors 32, 36 and the turbine 26 identical and using the entries already present on the column 30.

To preserve the existing equipment intact and improve the C ₂ ⁺ recovery, the pressure of the column 30 has been slightly decreased. Without countermeasure, this decrease would have resulted in the increase of the power of the compressor 36.

However, the addition of the compressor 220 overcomes this problem. In addition, the flow through the existing turbine 26 and its power have not been increased compared to the existing unit.

This installation nevertheless makes it possible to obtain, with an excellent yield, a recovery of ethane much higher than that observed in the state of the art.

A sixth installation 270 according to the invention is shown in FIG. 6.

This sixth installation 270 is intended for the implementation of a sixth method according to the invention.

The sixth method according to the invention differs from the fifth process according to the invention in that a withdrawal stream 92 is taken from the compressed methane-rich top stream 90, advantageously upstream of the point of introduction of the second compressed part. 252 in the current 90. The withdrawal stream 92 is reintroduced into the column 30 at a head level N14. Unlike the fifth method according to the invention, the second portion 228 of the fraction 52 and the relaxed foot stream 158 are introduced into the column at a level N5 located below the head level N14 and above the level N2.

The implementation of the sixth method according to the invention is moreover analogous to that of the fifth method according to the invention.

To maintain the C ₂ ⁺ recovery of the existing unit, the pressure of the column 30 is slightly decreased. The presence of the new compressor 220 makes it possible to keep the power of the second compressor 36 the same despite the increase in the flow rate of the feed stream 16.

In addition, the capacity of the first dynamic expansion turbine 26 has been kept constant. The second dynamic expansion turbine 40 is used to process the addition of capacity.

The presence of an auxiliary column 216 also prevents clogging of the column 30 during the flow increase.

The sixth installation according to the invention makes it possible to maintain an ethane recovery greater than or equal to 99%, a temperature and a pressure of the feed stream 16 that are substantially identical. Similarly, the losses of charges allocated in the equipment, the efficiency of the trays in the column 30 and the position of the withdrawals, the maximum methane specification of the bottom stream 82 of the column 30, the efficiencies of the turbines and the compressors, the power of the second compressor 36 and the existing turbine 26 and the heat exchange coefficients of the existing exchangers 20 and 28 are kept identical.

In a variant (shown in dashed lines in FIG. 1), which can be applied to each of the embodiments of FIGS. 1 to 6, the second fraction 41 B of the feed stream is taken from the first exchanger 20 and not upstream of this one. The second fraction 41 B is thus partially cooled and is partially liquefied in the first heat exchanger 20.

The second fraction 41 B from the first heat exchanger 20 is then optionally introduced into an upstream separator tank 250. It is then separated in the upstream separator tank 250 into a second bottom liquid fraction 252 and into a second top gas fraction 254. The second bottom fraction 252 is expanded in a static expansion valve 256 to a pressure of less than 40 bar and substantially equal to the pressure of the column 30.

The second fraction of relaxed foot 258 is then introduced into the column

30, advantageously between the level N1 1 and the level N8. The second head fraction 254 is introduced into the second dynamic expansion turbine 40 to form the second expanded feed fraction 91 A.

This arrangement with an upstream separator tank is also applicable in the case where the feed stream 16 contains a liquid fraction.

In another variant (not shown) of the embodiments of the Figures

2, 4 and 6, the installation comprises a bypass valve of a portion of the withdrawal stream 92 to divert this portion upstream of the first dynamic expansion turbine 26.

In this variant of the process, a supplementary cooling stream is taken from the withdrawal stream obtained after passing through the first heat exchanger 20. The additional cooling stream is reintroduced upstream of the turbine 26, ie in the head stream 44, upstream of the balloon 22 in the cooled supply stream 42.

In another variant (not shown) of the embodiments of Figures 1 to 8, the installation comprises a plurality of first exchangers 28, each being intended to receive a fraction of the overhead stream 84 and another stream.

The overhead stream 84 is then divided into a plurality of fractions corresponding to the number of second exchangers 28.

Each second heat exchanger 28 can then put in heat exchange only two flows each including a fraction of the overhead stream 84 and respectively, the first relaxed feed fraction 54, the second relaxed feed fraction 91A, and if necessary, the fraction column supply 46 and / or the sampling stream 92.

In another variant (not shown) of the embodiments of Figures 1 to 6, a reboiling stream is withdrawn from the distillation column at a sampling level. The reboiling current is then put in heat exchange relation with at least a part of the second expanded fraction 91A resulting from the dynamic expansion turbine 40 and optionally with the first expanded fraction 54 coming from the first turbine 26.

This heat exchange connection can be performed within the second heat exchanger 28.

In yet another variant (not shown), an auxiliary expansion current is taken from the methane rich column head stream 86 from the first heat exchanger 20. This auxiliary expansion stream is introduced into a dynamic auxiliary expansion turbine, distinct from the first dynamic expansion turbine 26 and the second dynamic expansion turbine 40. The relaxed current from the auxiliary turbine is reintroduced into the methane-rich column head stream, before it passes through the first heat exchanger 20 to constitute a supplementary cooling stream of the first heat exchanger 20.

More generally, the entire head stream 44 from the first flask 22 can form the turbine feed fraction 48. The method according to the invention is then devoid of separation of the overhead stream 44.

Claims

1 .- A process for producing a stream (12) rich in methane and a stream (14) rich in C ₂ ⁺ hydrocarbons from a feed stream (16) containing hydrocarbons, the process comprising the following steps:

separating the feed stream (16) into a first fraction (41 A) of the feed stream and at least a second portion (41 B) of the feed stream;

cooling the first fraction (41 A) of the feed stream in a first heat exchanger (20);

- introducing the first fraction of the cooled feed stream (42) into a first separator tank (22) to produce a light head stream (44) and a bottom heavy stream (45);

- Relaxing a turbine feed fraction (48) formed from the light head stream (44) in a first dynamic expansion turbine (26) to a first pressure (P1) and introducing at least a portion (56) of the first expanded fraction (54) from the first turbine (26) in a first distillation column (30);

- relaxing at least a portion of the heavy foot current (45) to form a relaxed foot stream (60) and introducing the expanded foot stream (60) into the first distillation column (30) without passing through the first heat exchanger (20) between the first separator flask (22) and the first distillation column (30);

recovering a bottom stream (82) from the base of the first distillation column (30), the C ₂ ⁺ hydrocarbon rich stream (14) being formed from the bottom stream (82) ;

recovering and reheating a methane-rich column head stream (84),

compressing at least a fraction of the overhead stream (84) in at least one first compressor (32) coupled to the first dynamic expansion turbine (26) and in at least one second compressor (36);

- formation of the methane-rich stream (12) from the warmed and compressed column head stream (90);

characterized in that the method comprises the following steps:

- relaxing at least a portion of the second fraction of the feed stream (41 B) in a second dynamic expansion turbine (40), distinct from the first dynamic expansion turbine (26) to a second pressure, to form a second relaxed fraction (91A) from the second dynamic expansion turbine (40), the second pressure (P2) being substantially equal to the first pressure (P1);

at least partial cooling and liquefaction of at least a portion of the second expanded fraction (91 A) from the second dynamic expansion turbine (40) to form a cooled reflux flow (91 B; 160; 232) and introduction cooled reflux stream (91B; 160; 232) in the first distillation column (30).

2. - Method according to claim 1, characterized in that it comprises the introduction of the first expanded fraction (54) from the first dynamic expansion turbine (26) in a second heat exchanger (28) to be cooled and partially liquefied, the cooled first cooled fraction forming an additional cooled reflux stream (56), the process comprising introducing the additional cooled reflux stream (56) into the first distillation column (30).

3. - Method according to any one of the preceding claims, characterized in that it comprises the following steps:

- introducing the second expanded fraction (91 A) from the second dynamic expansion turbine (40) into a downstream separator tank (152) to form a second gaseous head stream (156) and a second liquid bottom stream (154); )

cooling the second gaseous head stream (156) to form a cooled reflux stream.

4. Method according to any one of the preceding claims, characterized in that it comprises the following steps:

introducing at least a portion of the second expanded fraction (91 A) from the second dynamic expansion turbine (40) into an auxiliary column (216), and

- forming the cooled reflux current from the foot stream (232) of the auxiliary column (216).

5. Method according to any one of the preceding claims, characterized in that it comprises the following steps:

optionally, partial condensation of the second fraction of the feed stream (41 B);

- introducing the second fraction of the feed stream (41 B) into an upstream separator tank (250) to form a second gaseous fraction (254) and a second liquid fraction (256);

introducing the second gaseous fraction (254) into the second dynamic expansion turbine (40);

- Introducing the second liquid fraction (256), after expansion, in a lower portion of the first distillation column (30).

6. - Method according to any one of claims 1 to 4, characterized in that the whole of the second fraction of the feed stream (41 B) is introduced into the second dynamic expansion turbine (40), possibly without cooling between the step of separating the feed stream (16) and the step of introducing the second fraction (41 B) of the feed stream into the second dynamic expansion turbine (40).

7. - Method according to any one of the preceding claims, characterized in that it comprises the following steps:

taking a compression secondary fraction (87B) from the methane-rich column head stream (86) before a fraction (87A) of the methane-rich column head stream passes through the first compressor ( 32)

passing the secondary fraction (87B) into a third compressor (41) coupled to the second dynamic expansion turbine (40);

introducing the compressed secondary fraction from the third compressor (41) into the fraction of the compressed overhead stream downstream of the first compressor (32).

8. - Process according to any one of the preceding claims, characterized in that the second compressor (36) comprises a first compression stage, at least a second compression stage and a refrigerant interposed between the first compression stage and the second compression stage, the method comprising a step of passing the compressed column head stream (88) from the first compressor successively into the first compression stage, into the refrigerant, and then into the second compression stage.

9. - Method according to any one of the preceding claims, characterized in that at least a portion of the second expanded fraction (91 A) from the second dynamic expansion turbine (40), at least a fraction of the current of column head (84) and optionally the first expanded fraction (54) from the first dynamic expansion turbine (26) are placed in a heat exchange relationship.

10. - Method according to any one of the preceding claims, characterized in that it comprises the following steps:

dividing the leading light stream (44) into the turbine feed fraction (48) and a column feed fraction (46);

cooling and at least partial condensation of the column feed fraction (46) in a second heat exchanger (28),

- Relaxing and at least partial introduction of the cooled column feed fraction into the first distillation column (30), at least a portion of the second expanded fraction (91 A) from the second dynamic expansion turbine (40) and the column feed fraction (46) being advantageously placed in heat exchange relationship.

1 1. - Method according to claim 10, characterized in that at least a fraction (238) of the column top stream (84) and at least a portion of the second expanded fraction (91 A) from the second dynamic expansion turbine are placed in heat exchange relationship in a downstream heat exchanger (214) separate from the second heat exchanger (28).

12. - Method according to any one of the preceding claims, characterized in that it comprises the following steps:

- withdrawal of a withdrawal stream (92) in the overhead stream

(90);

- cooling the withdrawal stream at least in the first heat exchanger (20) and introducing the cooled withdrawal stream (96) into the first distillation column (30);

- Possibly, heat exchange of the withdrawal stream with at least a portion of the second expanded fraction (91 A) from the second turbine (40).

13. - Method according to any one of the preceding claims, characterized in that it comprises the following steps:

- withdrawing a stream (80) of reboiling in the first distillation column (30) at a sampling level;

putting in heat exchange relation the reboiling current (80) with at least a part of the second expanded fraction (91A) resulting from the second dynamic expansion turbine (40) for cooling and at least partially liquefying the part of the second expanded fraction (91A) from the second dynamic expansion turbine; and

possibly, putting in heat exchange relation with the first relaxed fraction resulting from the first turbine (26);

reintroduction of the reboiling stream (80) into the first distillation column (30) at a level below the sampling level.

14.- Method according to any one of the preceding claims, characterized in that it comprises the following steps:

- taking a makeup cooling stream in the methane-rich overhead stream (84, 86, 88, 90) or a stream (92) formed from the methane-rich overhead stream ( 84, 86, 88, 90);

- Expansion and introduction of the additional cooling stream expanded in a current (42, 48) flowing upstream of the first expansion turbine (26), advantageously in the first fraction of the cooled feed stream (42) or in the turbine feed fraction (48).

15.- Method according to any one of the preceding claims, characterized in that it comprises the following steps:

- passing the methane-rich overhead stream (84) into the first heat exchanger (20);

- withdrawing an auxiliary expansion stream in the methane-rich column head stream (84) after passing through the first heat exchanger (20);

dynamic expansion of the auxiliary expansion current in a dynamic auxiliary expansion turbine;

- Introduction of the expanded stream from the auxiliary dynamic expansion turbine in the methane-rich column head stream, before its passage through the first heat exchanger (20).

16.- Installation for producing a current (12) rich in methane and a current

(14) rich in C ₂ ⁺ hydrocarbons from a feed stream (16) containing hydrocarbons, of the type comprising:

means for separating the feed stream (16) into a first fraction (41 A) of the feed stream and at least a second fraction (41 B) of the feed stream;

a first heat exchanger (20) for cooling the first fraction (41 A) of the feed stream;

means for introducing the first cooled feed fraction (42) into a first separator flask (22) to produce a light head stream (44) and a heavy bottom stream (45);

a first dynamic expansion turbine (26) and means for introducing a turbine feed fraction (48) formed from the light head stream into the first dynamic expansion turbine (26) to relax the turbine feed fraction (48) to a first pressure;

a first distillation column (30);

means for introducing at least a portion (56) of the first expanded fraction (54) into the first turbine (26) in the first distillation column (30);

means for expanding at least a portion of the heavy foot current (45) to form a relaxed foot stream and means for introducing at least a portion of the expanded foot stream (60) into the first column for distillation (30), the means for introducing the relaxed foot stream being configured so that the foot current 45) does not pass through the first heat exchanger (20) between the first separator flask and the first distillation column (30);

means for recovering a bottoms stream (82) at the bottom of the first distillation column (30), the stream (14) rich in C ₂ ⁺ hydrocarbons being formed from the bottom stream of the column (82);

means for recovering and reheating a current (84) of column head rich in methane,

at least one first compressor (32) coupled to the first dynamic expansion turbine (26) and at least one second compressor (36) for compressing at least a fraction of the overhead stream (84);

- Methane-rich stream forming means (12) from the heated and compressed column head stream (90) from the second compressor (36); characterized in that the installation comprises:

a second dynamic expansion turbine (40), distinct from the first dynamic expansion turbine (26),

means for introducing at least a portion of the second fraction of the feed stream into the second dynamic expansion turbine (40) to form a second expanded fraction (91A) issuing from the second turbine of dynamic expansion (40) at a second pressure, the second dynamic expansion turbine being arranged so that the first pressure is substantially equal to the second pressure;

means for cooling and at least partially liquefying at least a portion of the second fraction (91 A) issuing from the second dynamic expansion turbine (40) to form a cooled reflux flow (91 B; 160; 232); and means for introducing the cooled reflux stream (91B; 160; 232) into the first distillation column (30).

17.- Installation according to claim 16, characterized in that it comprises:

an auxiliary column (216);

means for introducing at least a portion of the second expanded fraction (91 A) issuing from the second dynamic expansion turbine (40) into the auxiliary column (216); and

means for forming the cooled reflux current from the foot stream (232) of the auxiliary column (216).