FR3058508A1 - METHOD FOR CRYOGENIC SEPARATION OF NATURAL GAS CURRENT - Google Patents

Abstract

A process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile feed stream compounds and a liquid product containing the heavier compounds of the feed stream, comprising at least steps following: Step a): at least partial condensation of a feed stream of natural gas in a first heat exchange system; Step b): introduction of the at least partially condensed stream resulting from step a) into an absorption column at an introduction level located in the lower part of said absorption column, said absorption column producing, at the top, a gaseous stream containing the most volatile compounds and in the tank, a liquid product; - Step c): introduction of the liquid product from step b) in a fractionation column to obtain, in the bottom of a fractionation vessel, a liquid product containing the heavier compounds of the feed stream and, at the head of the fractionating column, a distillate, at least partially condensed in a second heat exchange system; - Step d): introducing, at a level located in the upper part of the absorption column, the gas phase of the condensed distillate from step c) as a supply stream of the absorption column; characterized in that the gas stream produced at the top of the absorption column from step b) is used to condense, in the second heat exchange system, the distillate from the head of the fractionation column. .

Description

Holder (s): AIR LIQUIDE, ANONYMOUS COMPANY FOR THE STUDY AND EXPLOITATION OF GEORGES CLAUDE PROCESSES Société anonyme.

Extension request (s)

Agent (s): AIR LIQUIDE.

PROCESS FOR CRYOGENIC SEPARATION OF A NATURAL GAS STREAM.

FR 3 058 508 - A1 (5 /) Process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds in the feed stream and into a liquid product containing the heaviest compounds in the supply current, comprising at least the following steps:

- Step a): at least partial condensation of a natural gas supply stream in a first heat exchange system;

Step b): introduction of the at least partially condensed current from step a) into an absorption column at an introduction level located in the lower part of said absorption column, said absorption column producing, at the head, a gas stream containing the most volatile compounds and in the tank, a liquid product;

- Step c): introduction of the liquid product from step b) into a fractionation column to obtain, in the fractionation column tank, a liquid product containing the heaviest compounds in the feed stream and, at the top of fractionation column, a distillate, at least partially condensed in a second heat exchange system;

- Step d): introduction, at a level located in the upper part of the absorption column, of the gas phase of the condensed distillate from step c) as a feed stream to the absorption column;

characterized in that the gas stream produced at the top of the absorption column from step b) is used to condense, in the second heat exchange system, the distillate from the head of the fractionation column .

The present invention relates to a method for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds in the feed stream and into a liquid product containing the heaviest compounds in the feed stream.

When exploiting natural gas fields, many steps can be planned. A step after drying and removing relatively conventional impurities is the separation of liquids associated with natural gas (NGL).

It is often desirable to separate heavy hydrocarbons, or more generally NGLs (Natural Gas Liquids) from natural gas, for example such as ethane, butane, propane, C5 + and C6 + hydrocarbons (i.e. - say having at least five carbon atoms and having more than six carbon atoms).

The interest of this stage can be multiple but often it is a question of valorizing various products (ethane, propane ...) which are generally sold considerably more expensive than the natural gas product. It is in particular frequent to market hydrocarbons having at least three carbon atoms as propane, butane and condensate products.

Numerous industrial installations have been described allowing the fractionation of gaseous charges into a waste gas containing the most volatile compounds of the charge and into a liquid product containing the heaviest compounds of the charge, this in order to obtain in said produces a given component of the filler with a high recovery rate.

In this regard, there may be mentioned for example the recovery of liquefied petroleum gas (hydrocarbons having three or four carbon atoms) from natural gas or refinery, the recovery of ethane intended in particular for supplying steam cracking units , or the desulfurization and degassing of natural gases by recovery of sulfur compounds such as carbon oxysulfide and mercaptans.

Several technologies exist to produce hydrocarbons having at least three carbon atoms from natural gas.

One of the most effective is a process using a two-column turboexpander in which the first column is an absorber dedicated to pushing the recovery of propane as much as possible and the second column is a de-ethanizer.

Condensation of the flow at the top of the de-ethanizer is often carried out in part with the fluid coming from the absorber tank. This fluid leaves partially vaporized to enter the main exchange line requiring a two-phase introduction.

This makes the process very complex to ensure good distribution in this heat exchanger.

Such a process is described in documents US 4,690,702 and US 5,114,450.

The inventors of the present invention then developed a solution which makes it possible to solve the problems raised above.

The subject of the present invention is a process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds in the feed stream and into a liquid product containing the heaviest compounds in the feed stream. feeding, comprising at least the following steps:

Step a): at least partial condensation of a natural gas feed stream in a first heat exchange system;

Step b): introduction of the at least partially condensed current from step a) into an absorption column at an introduction level located in the lower part of said absorption column, said absorption column producing, in head, a gas stream containing the most volatile compounds and in the tank, a liquid product;

Step c): introduction of the liquid product from step b) into a fractionation column to obtain, in the fractionation column tank, a liquid product containing the heaviest compounds in the feed stream and, at the top of the column fractionation, a distillate, at least partially condensed in a second heat exchange system;

Stage d): introduction, at a level situated in the upper part of the absorption column, of the gas phase of the condensed distillate resulting from stage c) as a feed stream for the absorption column;

characterized in that the gas stream produced at the top of the absorption column from step b) is used to condense, in the second heat exchange system, the distillate from the head of the fractionation column .

According to other embodiments, the invention also relates to:

A process as defined above, characterized in that it comprises a step prior to step d) of condensing the distillate from the head of the fractionation column in a third heat exchanger system.

A process as defined above, characterized in that the entire gas stream produced at the top of the absorption column from step b) is used to condense, in the second heat exchange system , the distillate from the head of the fractionation column.

A process as defined above, characterized in that the gas stream produced at the top of the absorption column from step b) is separated into several streams, at least one of which is used to condense, in the second heat exchange system, the distillate from the head of the fractionation column.

A process as defined above, characterized in that the liquid phase of the condensed distillate from step c) is used as reflux at the top of the fractionation column.

Thus, the solutions of the method which is the subject of the present invention make it possible to eliminate the two-phase input, or at least limit to a very high L / V (liquid / vapor) ratio, of the current taken from the absorption column tank before l '' introduce into a main heat exchange system prior to its introduction into the fractionation column.

Solution A - The unique use of the top flow from the absorption column to condense the top flow from the fractionation column in a dedicated exchanger has the following advantages in particular: the elimination of the two-phase input of the current drawn from the absorption column in the main exchange line: and limitation of the discharge pressure of the pump leaving the tank of the absorption column.

Solution B - Separation of the fluid at the head of the absorption column into several streams, at least one of which ensures condensation in the condenser at the head of the fractionation column: this results in better regulation of the condenser at the head of the fractionation column.

Solution C - Condensation of the reflux fluid from the absorption column in a dedicated exchanger thanks to the head of the absorption column only: this allows a simplification of the main exchange line.

The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from natural gas fields, oil reservoirs or a domestic gas network distributed via pipelines.

Usually the flow of natural gas is made up mostly of methane. Preferably, the feed stream comprises at least 80 mol% of methane. Depending on the source, natural gas contains quantities of heavier hydrocarbons than methane, such as for example ethane, propane, butane and pentane as well as certain aromatic hydrocarbons. The natural gas stream also contains non-hydrocarbon products such as H ₂ O, N _2i CO ₂ , H ₂ S and other sulfur compounds, mercury and others.

The feed stream containing natural gas is therefore pretreated before being introduced into the heat exchanger allowing the first cooling step of the process which is the subject of the present invention. This pretreatment includes the reduction and / or elimination of undesirable components such as CO ₂ and H ₂ S, or other steps such as pre-cooling and / or pressurization. Since these measures are well known to those skilled in the art, they are not further detailed here.

The expression natural gas as used in the present application relates to any composition containing hydrocarbons including at least methane. This includes a "crude" composition (prior to any treatment or washing), as well as any composition that has been partially, substantially or entirely treated for the reduction and / or elimination of one or more compounds, including, but not limited to limit, sulfur, carbon dioxide, water, mercury and some heavy and aromatic hydrocarbons.

The heat exchanger can be any heat exchanger, any unit or other arrangement adapted to allow the passage of a certain number of flows, and thus allow at least one direct or indirect heat exchange system between one or more lines of refrigerant, and one or more feed streams.

The invention will be described in more detail with reference to Figures 1 to 3.

FIG. 1 illustrates the diagram of a process according to the prior art as described in the preamble to this description.

FIG. 2 illustrates a diagram of an embodiment of an implementation of a method according to the invention.

FIG. 3 illustrates a diagram of a particular embodiment of an implementation of a method according to the invention.

In FIG. 1, a natural gas feed stream 1 is introduced into a main heat exchanger 2 in order to be cooled. The gas thus cooled 3 is partially condensed and introduced into a phase separator pot 4. The gas phase 5 at the head of phase separator pot 4 is introduced into a turbine 6 in order to recover the expansion energy and to condense part of the stream 5, then is introduced into an absorption column 7 comprising a lower part 7 'and an upper part 7 ”. The liquid phase 8 in the tank of phase separator pot 4 is introduced after expansion 9 into the absorption column 7. The absorption column produces in the column tank a liquid and at the head of the column, a waste gas 11. The liquid 10 is heated in a heat exchanger 12 in which it is partially vaporized. The stream thus heated 13 is then introduced into the main exchanger 2, this introduction 13 is then strongly two-phase.

At the top of the absorption column 7, the waste gas 11 which contains only the products more volatile than ethane is heated in the main heat exchanger 2, the resulting current 14, is then compressed and sent to a processing unit A.

The stream 13 ′ at the outlet of the heat exchanger 2 coming from the tank of the absorption column 7 is introduced into a fractionation column 15. This column 15 produces in tank 16, a reboiled liquid product 18 using a reboiler 17 in order to obtain a liquid rich in propane and depleted in ethane. At the head 19 of the fractionation column 15, a gas 20 is produced. This gas 15 is condensed in the heat exchanger 12 and the product 21 which leaves this exchanger 12 is introduced into a phase separator pot 22. The gas phase 23 at the head of the phase separator pot 22 serves as reflux for the column absorption 7. The liquid 25 in the tank of the phase separator pot 22 serves as reflux 26 at the head of the fractionation column 15. A pump 30 is necessary to pump the liquid 25.

The problem linked to the two-phase introduction of the current 13 into the main heat exchanger 2 is resolved by the process which is the subject of the present invention.

In fact, in FIG. 2, the waste gas 11 at the top of the absorption column 7 which contains only the products more volatile than ethane is heated in a heat exchanger 27 located just downstream from the head of said column 7. Then, the gas thus heated 28 at the outlet of the heat exchanger 27 is introduced into the heat exchanger 12 at the head of the fractionation column before being introduced into the main exchanger 2 to constitute the stream 14 then compressed and sent to a processing unit A.

Unlike what is illustrated in FIG. 1, the liquid stream 10 in the tank of the absorption column 7 is pumped using a pump 29, then directly introduced 13 into the main exchanger 2 to form the current 13 'which is sent to the fractionation column 15.

The advantages of such a process are as follows:

Energy efficiency: the pressure of the absorption column 7 is thus maximized.

Simplicity of the exchangers: none of the three heat exchangers 2, 27, 12 has a two-phase introduction, the temperature differences in cold fluids and hot fluids are reasonable (ie: less than 25-30 ° C, differences beyond from which brazed aluminum heat exchangers could be damaged).

Alternatively, other configurations are possible such as the following for example: the heat exchanger 12 can be mounted inside the fractionation column 15. The heat exchanger 27 can itself be mounted directly to the above the absorption column 7. The advantage compared to installing it on the ground, is to avoid a pump for raising the reflux.

In FIG. 3, another embodiment is shown diagrammatically. Compared to the diagram in Figure 2, the modification consists of a separation of the fluid 11 at the top of the absorption column 7 into several streams 11 ’and 11”. Current 11 ’provides condensation in condenser 12 at the top of fractionation column 15.

The stream 11 ′ is introduced into the condenser 12 at the top of the fractionation column 15 and is then introduced into the main exchanger 2. The stream 11 ”is directly introduced into the main exchanger 2.

The liquid 10 from the tank of the absorption column 7 is pumped and then directly introduced into the main exchanger 2.

A control valve can precisely control the fraction sent to the top of the fractionation column 15, allowing precise and efficient control of the unit.

Advantage:

Better regulation of the condenser 12 at the head of the fractionation column 15.

Minimization of the number of devices while maintaining a single-phase introduction of the liquid 13 from the absorption column into the heat exchanger 2.

Alternatively, other configurations are possible, such as the following, for example: the heat exchanger 12 can be mounted directly above the absorption column 7. The advantage with respect to installing it on the ground is avoid a reflux lift pump.

In addition to this, the invention can be advantageously combined with integration between the columns and the exchangers. In the case corresponding to FIGS. 2 and 3, a configuration allowing the column 15 and the exchanger 12 to be integrated in the same module, avoiding the use of the pump 25 for example and avoiding a shell dedicated to the exchanger 12 is proposed. In this case, said module is characterized in that, directly above the fractionation column 15 is installed a separator pot, above which is installed a condenser. The condenser is connected to the head of the fractionation column and to the separator pot on the condensation side. The tank of the separator pot is connected to the fractionation column (indirectly with a valve between the two typically). By module is therefore meant a single structure comprising the column 15, the separator pot and the heat exchanger 12.

Claims (5)

1. Process for the cryogenic separation of a feed stream (1) of natural gas into a gas containing the most volatile compounds (14) from the feed stream (1) and into a liquid product (18) containing the compounds the heaviest of the supply current (1), comprising at least the following steps: Step a): at least partial condensation of a feed stream (1) of natural gas in a first heat exchange system (2); Step b): introduction of the at least partially condensed current (3) from step a) into an absorption column (7) at an introduction level located in the lower part of said absorption column, said column absorption producing, at the top, a gas stream (11) containing the most volatile compounds and in the tank, a liquid product (10); Step c): introduction of the liquid product from step b) into a fractionation column (15) to obtain, in the tank (16) of the fractionation column (15), a liquid product (18) containing the most heavy with the feed stream and, at the top (19) of the fractionation column, a distillate (21), at least partially condensed in a second heat exchange system (12); Stage d): introduction, at a level situated in the upper part of the absorption column (7), of the gas phase (23) of the condensed distillate (21) resulting from stage c) as a feed stream ( 24) of the absorption column (7); characterized in that the gas stream (11) produced at the top of the absorption column (7) from step b) is used to condense, in the second heat exchange system (12), the distillate (21) from the head (19) of the fractionation column (15). 2. Method according to one of the preceding claims, characterized in that it comprises a step prior to step d) of condensing the distillate (21) from the head (19) of the fractionation column (15) in a third heat exchanger system (27). 3. Method according to one of the preceding claims, characterized in that all of the gas stream (11) produced at the top of the absorption column (7) from step b) is used to condense, in the second heat exchange system (12), the distillate (21) from the head (19) of the column 5 fractionation (15). 4. Method according to claim 1, characterized in that the gas stream (11) produced at the top of the absorption column (7) from step b) is separated into several streams (11 ', 11 ”) including at at least one (11 ') is used 10 to condense, in the second heat exchange system (12), the distillate (21) from the head (19) of the fractionation column (15). 5. Method according to any one of the preceding claims, characterized in that the liquid phase (25) of the condensed distillate (21) from 15 step c) is used as reflux (26) at the top of the fractionation column (15).