METHOD FOR PRODUCING A FLOW WHICH IS RICH IN METHANE AND A CUT WHICH IS RICH IN C2+ HYDROCARBONS FROM A FLOW OF FEED NATURAL GAS AND AN ASSOCIATED INSTALLATION

• US 20100263407A1
• Filed: 04/20/2010
• Published: 10/21/2010
• Est. Priority Date: 04/21/2009
• Status: Active Grant

• Current Assignee: Technip France (Technip)
• Original Assignee: Technip France (Technip)
• Inventors: THIEBAULT, Sandra, BARTHE, Loïc, PARADOWSKI, Henri
• Granted Patent: US 8,752,401 B2
• Time in Patent Office: Days
• US Class Current: 62/621
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2200/76 Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/04 Mixing or blending of fluids with the feed stream
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2230/24 Multiple compressors or compressor stages in parallel
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2270/04 Internal refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 2270/06 with multiple gas expansion loops
• CPC Class Codes: F25J 2270/88 Quasi-closed internal refrigeration or heat pump cycle, if not otherwise provided
• CPC Class Codes: F25J 2290/80 Retrofitting, revamping or debottlenecking of existing plant
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more

Abstract

This method comprises cooling the feed natural gas in a first heat exchanger and introducing the cooled, feed natural gas into a first separation flask.

It comprises the dynamic expansion of a turbine supply flow in a first expansion turbine and introducing the expanded flow into a separation column. This method comprises removing, at the head of the separation column, a head flow rich in methane and removing a first recirculation flow from the compressed head flow rich in methane.

The method comprises forming at least a second recirculation flow obtained from the head flow rich in methane downstream of the separation column and forming a dynamic expansion flow from the second recirculation flow.

Claims

1. Method for producing a flow which is rich in methane and a cut which is rich in C₂⁺ hydrocarbons from a flow of dehydrated feed natural gas, which is composed of hydrocarbons, nitrogen and CO₂ and which advantageously has a molar content of C₂⁺ hydrocarbons greater than 10%, the method being of the type comprising the following steps of:

cooling the feed natural gas flow advantageously at a pressure greater than 40 bar in a first heat exchanger and introducing the cooled, feed natural gas flow into a first separation flask;

separating the cooled natural gas flow in the first separation flask and recovering a light fraction which is substantially gaseous and a heavy fraction which is substantially liquid;

dividing the light fraction into a flow for supplying to a turbine and a secondary flow;

dynamic expansion of the turbine supply flow in a first expansion turbine and introducing the expanded flow into an intermediate portion of a separation column;

cooling the secondary flow in a second heat exchanger and introducing the cooled secondary flow into an upper portion of the separation column;

expanding the heavy fraction, vaporisation in the first heat exchanger and introduction into a second separation flask in order to form a head fraction and a bottom fraction;

introducing the head fraction, after cooling in the second heat exchanger, in the upper portion of the separation column;

introducing the bottom fraction into an intermediate portion of the separation column;

recovering, at the bottom of the separation column, a bottom flow which is rich in C₂⁺ hydrocarbons and which is intended to form the cut rich in C₂⁺ hydrocarbons;

removing, at the head of the separation column, a head flow rich in methane;

reheating the head flow rich in methane in the second heat exchanger and in the first heat exchanger and compressing that flow in at least a first compressor which is connected to the first expansion turbine and in a second compressor in order to form a flow rich in methane from the compressed head flow rich in methane;

removing a first recirculation flow from the head rich in methane;

passing the first recirculation flow into the first heat exchanger and into the second heat exchanger in order to cool it, then introducing at least a first portion of the first cooled recirculation flow into the upper portion of the separation column;

wherein the method comprises the following steps of;

forming at least a second recirculation flow obtained from the head flow rich in methane downstream of the separation column;

forming a dynamic expansion flow from the second recirculation flow and introducing the dynamic expansion flow into an expansion turbine in order to produce frigories.

2. Method according to claim 1, wherein the second recirculation flow is introduced into a flow downstream of the first heat exchanger and upstream of the first expansion turbine in order to form the dynamic expansion flow.

3. Method according to claim 2, wherein the second recirculation flow is mixed with the turbine supply flow from the first separation flask in order to form the dynamic expansion flow, the dynamic expansion turbine receiving the dynamic expansion flow being formed by the first expansion turbine.

4. Method according to claim 2, wherein the second recirculation flow is mixed with the cooled natural gas flow before it is introduced into the first separation flask, the dynamic expansion flow being formed by the turbine supply flow from the first separation flask.

5. Method according to claim 2, wherein the second recirculation flow is removed from the first recirculation flow.

6. Method according to claim 2, wherein it comprises the following steps of:

removing a removal flow from the head flow rich in methane, before it is introduced into the first compressor and the second compressor;

compressing the removal flow in a third compressor;

forming the second recirculation flow from the compressed removal flow from the third compressor, after cooling.

7. Method according to claim 6, wherein it comprises passing the removal flow into a third heat exchanger and into a fourth heat exchanger before it is introduced into the third compressor, then passing the compressed removal flow into the fourth heat exchanger, then into the third heat exchanger in order to supply the head of the separation column, the second recirculation flow being removed from the cooled, compressed removal flow, between the fourth heat exchanger and the third heat exchanger.

8. Method according to claim 6, wherein the removal flow is introduced into a fourth compressor, the method comprising the following steps of:

removing a secondary branch flow from the cooled, compressed removal flow from the third compressor and the fourth compressor;

dynamic expansion of the secondary branch flow in a second expansion turbine which is connected to the fourth compressor;

introducing the expanded secondary branch flow into the removal flow before it is passed into the third compressor and into the fourth compressor.

9. Method according to claim 1, wherein the second recirculation flow is removed from the compressed head flow rich in methane, the method comprising the following steps of:

introducing the second recirculation flow into a third heat exchanger;

separating the feed natural gas flow into a first feed flow and a second feed flow;

placing the second feed flow in a heat exchange ratio with the second recirculation flow in the third heat exchanger;

mixing the second feed flow after cooling in the third heat exchanger with the first feed flow, downstream of the first exchanger and upstream of the first separation flask.

10. Method according to claim 9, wherein it comprises the following steps of:

removing a secondary cooling flow from the compressed head flow rich in methane, downstream of the first compressor and downstream of the second compressor;

dynamic expansion of the secondary cooling flow in a second expansion turbine and introduction of the expanded secondary cooling flow into the third heat exchanger in order to place it in a heat exchange ratio with the second feed flow and the second recirculation flow;

reintroducing the expanded secondary cooling flow into the flow rich in methane before it is introduced into the first compressor and into the second compressor;

removing a recompression fraction from the cooled flow rich in methane downstream of the introduction of the expanded secondary cooling flow and upstream of the first compressor and the second compressor;

compressing the recompression fraction in at least one compressor connected to the second expansion turbine and reintroducing the compressed recompression fraction into the compressed flow rich in methane from the first compressor and the second compressor.

11. Method according to claim 1, wherein the second recirculation flow is branched off from the first recirculation flow in order to form the dynamic expansion flow, the dynamic expansion flow being introduced into a second expansion turbine separate from the first expansion turbine, the dynamic expansion flow from the second expansion turbine being reintroduced into the flow rich in methane before it is introduced into the first heat exchanger.

12. Method according to claim 11, wherein it comprises the following steps of:

removing a recompression fraction from the reheated head flow rich in methane from the first heat exchanger and the second heat exchanger;

compressing the recompression fraction in a third compressor which is connected to the second expansion turbine;

introducing the compressed recompression fraction into the compressed flow rich in methane from the first compressor.

13. Method according to claim 1, wherein it comprises the branching-off of a third recirculation flow, advantageously at ambient temperature, from the at least partially compressed flow rich in methane, advantageously between two of the second compressor, the third recirculation flow being cooled successively in the first heat exchanger and in the second heat exchanger before being mixed with the first recirculation flow in order to be introduced into the separation column.

14. Installation for producing a flow rich in methane and a cut rich in C₂⁺ hydrocarbons from a dehydrated feed natural gas flow which is composed of hydrocarbons, nitrogen and CO₂ and which advantageously has a molar content of C₂⁺ hydrocarbons greater than 10%, the installation being of the type comprising:

a first heat exchanger for cooling the feed natural gas flow which advantageously flows at a pressure greater than 40 bar;

a first separation flask;

means for introducing the cooled feed natural gas flow into the first separation flask, the flow of cooled natural gas being separated in the first separation flask in order to recover a light, substantially gaseous fraction and a heavy, substantially liquid fraction;

means for dividing the light fraction into a flow for supplying a turbine and a secondary flow;

a first dynamic expansion turbine for the turbine supply flow;

a separation column;

means for introducing the expanded flow into the first dynamic expansion turbine in an intermediate portion of the separation column;

a second heat exchanger for cooling the secondary flow and means for introducing the cooled secondary flow in an upper portion of the separation column;

means for expanding the heavy fraction and means for passing the heavy fraction through the first heat exchanger;

a second separation flask;

means for introducing the heavy fraction from the first heat exchanger into the second separation flask in order to form a head fraction and a bottom fraction;

means for introducing the head fraction, after it has been introduced into the second exchanger to cool it, into the upper portion of the separation column;

means for introducing the bottom fraction into an intermediate portion of the separation column;

means for recovering, at the bottom of the separation column, a bottom flow which is rich in C₂⁺ hydrocarbons and which is intended to form the cut rich in C₂⁺ hydrocarbons;

means for removing, at the head of the separation column, a head flow rich in methane;

means for introducing the head flow rich in methane into the second heat exchanger and into the first heat exchanger in order to reheat it;

means for compressing the head flow rich in methane comprising at least a first compressor which is connected to the first turbine and a second compressor in order to form the flow rich in methane from the compressed head flow rich in methane;

means for removing a first recirculation flow from the head flow rich in methane;

means for introducing the first recirculation flow into the first heat exchanger then into the second heat exchanger in order to cool it;

means for introducing at least a portion of the first cooled recirculation flow into the upper portion of the separation column;

wherein the installation comprises;

means for forming at least a second recirculation obtained from the head flow rich in methane downstream of the separation column;

means for forming a dynamic expansion flow from the second recirculation flow;

means for passing the dynamic expansion flow through an expansion turbine in order to produce frigories.

15. Installation according to claim 14, wherein the means for forming a dynamic expansion flow from the second recirculation flow comprise means for introducing the second recirculation flow into a flow which flows downstream of the first heat exchanger and upstream of the first expansion turbine in order to form the dynamic expansion flow.