Method of processing a feed natural gas to obtain a processed natural gas and a cut of C5+ hydrocarbons, and associated installation

An installation for processing a dried and decarbonated feed natural gas to obtain a processed natural gas intended to be liquefied and a cut of C5+ hydrocarbons, the installation comprising:

• a cooler for cooling the feed natural gas to form a pre-cooled feed flow, comprising a first heat exchanger;
• a first separator flask for forming a pre-cooled gaseous flow;
• a device for introducing the pre-cooled feed flow into the first separator flask;
• a dynamic expansion turbine for dynamic expansion of the pre-cooled gaseous flow;
• a first purification column;
• a device for introducing the expanded flow issuing from the dynamic expansion turbine into the first column;
• a device for recovery at the head of the first column of a purified head natural gas;
• a device for recovery at the bottom of the first column of a liquefied bottom natural gas;
• a second column for elimination of the C5+ hydrocarbons;
• a device for introducing the liquefied bottom natural gas into the second column;
• a device for recovery at the bottom of the second column of the cut of C5+ hydrocarbons;
• a device for recovery at the head of the second column of a gaseous column head flow;
• a second separator flask;
• a device for introducing the gaseous column head flow into the second separator flask to form a liquid bottom flux and a gaseous head flux;
• a device for introducing a first part of the liquid bottom flux in reflux into the second column and a device for introducing a second part of the liquid bottom flux in reflux into the first column;
• an injector for injecting at least a part of the gaseous head flux issuing from the second separator flask into the purified head natural gas to form a flow of the processed natural gas;
• a device for introducing the purified head natural gas issuing from the first column into the first heat exchanger to heat the purified head natural gas issuing from the first column in the first heat exchanger by heat exchange with the feed natural gas;
• an expansion valve for expansion of the liquefied bottom natural gas issuing from the first column before its introduction into the second column;
• and
• a first compressor for compressing the gaseous column head flow before its introduction into the second separator flask,
• wherein the dynamic expansion turbine is able to expand the pre-cooled gaseous flow to a pressure of more than 35 bars, the pressure prevailing in the second column is below 25 bars, the installation is configured to introduce the purified head natural gas into the first heat exchanger:
• (1) without the purified head natural gas entering into heat exchange with a flow introduced into the first column and (2) without the purified head natural gas entering into heat exchange in a heat exchanger with a flow introduced into the second column, the first heat exchanger exclusively exchanges heat between the purified head natural gas and the feed natural gas to exclusively cool the feed natural gas, the feed natural gas being exclusively cooled by the purified head natural gas, without being cooled by an external refrigeration cycle, and the cut of C5+ hydrocarbons recovered at the bottom of the second column has a molar content of C3− hydrocarbons of less than 1%.

A method of processing a dried and decarbonated feed natural gas in order to obtain a processed natural gas intended to be liquefied and a cut of C5+ hydrocarbons, the method comprising the following steps:

• cooling of the feed natural gas in a first heat exchanger to form a pre-cooled feed flow;
• introduction of the pre-cooled feed flow into a first separator flask to form a pre-cooled gaseous flow;
• dynamic expansion of the pre-cooled gaseous flow in an expansion turbine and introduction of the expanded flow issuing from the expansion turbine into a first purification column;
• production at the head of the first column of a purified head natural gas;
• recovery at the bottom of the first column of a liquefied bottom natural gas;
• introduction of the liquefied bottom natural gas into a second column for elimination of the C5+ hydrocarbons;
• production, at the bottom of the second column, of the cut of C5+ hydrocarbons;
• production, at the head of the second column, of a gaseous column head flow and introduction of the gaseous column head flow into a second separator flask to form a liquid bottom flux and a gaseous head flux;
• introduction of a first part of the liquid bottom flux in reflux into the second column and introduction of a second part of the liquid bottom flux in reflux into the first column;
• injection of at least a part of the gaseous head flux issuing from the second separator flask into the purified head natural gas to form the processed natural gas,
• expansion of the liquefied bottom natural gas issuing from the first column before its introduction into the second column, and
• compression of the gaseous column head flow in a first compressor and cooling before its introduction into the second separator flask,
• wherein during the step of dynamic expansion, the pre-cooled gaseous flow is expanded to a pressure of more than 35 bar,
• wherein the step of cooling of the feed natural gas in the first heat exchanger is performed by exchanging heat in the first heat exchanger exclusively with the purified head natural gas issuing from the first column, whereby the purified head natural gas issuing from the first column is heated in the first heat exchanger by heat exchange with the feed natural gas,
• wherein the pressure prevailing in the second column is below 25 bar, the purified head natural gas is introduced into the first heat exchanger without the purified head natural gas entering into heat exchange in a heat exchanger with a flow introduced into the first column, the purified head natural gas enters into heat exchange in a heat exchanger exclusively with the feed natural gas in the first heat exchanger, the feed natural gas is cooled down exclusively by the purified head natural gas, without being cooled down by an external refrigeration cycle, and the molar content of C3− hydrocarbons in the cut of C5+ hydrocarbons is less than 1%, and
• wherein the method further comprises a step of tapping-off a secondary flux in the second part of the liquid bottom flux, before its introduction into the first column, and the introduction of the secondary flux into the flow of purified head natural gas issuing from the first column.