FR3047422A3 - PROCESS AND APPARATUS FOR THE PRODUCTION OF HELIUM WITH CO-PRODUCTION OF LIQUID NITROGEN - Google Patents

Abstract

In a method of separating a gaseous mixture containing nitrogen, helium and at least one heavier impurity than nitrogen, the mixture is cooled and separated in several partial condensation steps and at least one part of a nitrogen-enriched liquid and / or a heavier-impurity enriched liquid to provide at least a portion of the process frigories.

Description

The disclosed invention provides an innovative process for cryogenic helium extraction with co-production of liquid nitrogen. Helium is usually extracted from a gas stream from an underground reservoir. Some sources are very rich in nitrogen (greater than 50 mol%, more particularly greater than 80 mol%) and contain helium at a variable content (for example between 1 mol% and 15 mol%). Helium being a product of high value, it is interesting to extract it from this stream, to purify it and possibly to liquefy it.

In addition, there is usually a need for liquid nitrogen at liquid helium production sites to: • Screen liquid helium storage • Screen the liquid helium containers that remove the product • Pre-cool the product helium gas in the helium liquefier

This nitrogen must be pure (purity greater than 99.999 mol%) to be as cold as possible in the liquid phase. In addition, its hydrocarbon content and in particular methane must be low (typically less than 10 ppm, more particularly less than 1 ppm) to avoid any risk of explosion where it could accumulate. It is known from US4659351 to separate a flow containing nitrogen and helium through a series of partial condensation steps to produce liquid nitrogen. The invention provides a method for extracting a helium enriched stream from a feed stream while producing liquid nitrogen with good purity, without using frigories from an associated helium liquefier.

According to one object of the invention, there is provided a method for separating a gaseous mixture containing at least 50 mol. %, or even at least 80 mol% of nitrogen and between 1 mol% and 15 mol% of helium and at least one heavier impurity than nitrogen in which: i) the gaseous mixture is cooled in a line of exchange and partially condensed to produce a liquid enriched in the at least one heavier impurity than nitrogen and a gas depleted in this at least one heavier impurity ii) The gas depleted in the heavier impurity is cooled in the exchange line and partially condensed to form a nitrogen-enriched liquid and a helium-enriched gas iii) At least a portion of the nitrogen-enriched liquid is withdrawn as product and the helium enriched gas is heated in the exchange line characterized in that at least a portion of the nitrogen-enriched liquid and / or enriched liquid is expanded to at least one heavier impurity to provide at least a portion of the process refrigerants.

According to other optional aspects: at least partially the enriched liquid is vaporized in at least one heavier impurity and the gas thus produced is expanded in a turbine. the at least one heavier impurity is chosen from the group: methane, hydrogen, argon, carbon monoxide, oxygen, neon. - The gas depleted in the at least one heavy impurity is cooled in the exchange line to a temperature below -150 ° C, more particularly below -180 ° C. the gaseous mixture is cooled in the exchange line to a temperature of between 0 ° C. and -180 ° C., more particularly between -100 ° C. and -150 ° C.

According to another object of the invention, there is provided an apparatus for separating a gaseous mixture containing at least 50 mol. %, or even at least 80 mol% of nitrogen and between 1 mol% and 15 mol% of helium and at least one heavier impurity than nitrogen comprising an exchange line, means for sending the gaseous mixture cool in the exchange line, a first separator pot for separating the partially condensed gas mixture into the exchange line to produce a liquid enriched in the at least one heavier impurity than the nitrogen and a depleted gas in this at least one heavier impurity, means for sending the depleted gas to the at least one heavier impurity to cool in the exchange line, a second separator pot for separating the partially condensed depleted gas to form a nitrogen-enriched liquid and a helium-enriched gas, means for withdrawing at least a portion of the nitrogen-enriched liquid as a product, means for sending the helium enriched gas to heat in the exchange line and means for detecting at least a portion of the nitrogen enriched liquid and / or the enriched liquid in the at least one heavier impurity to provide at least a portion of the process refrigerants.

According to other optional objects: at least part of the liquid enriched in the at least one heavier impurity is vaporized in the exchange line and then relaxed. the vaporized liquid is expanded in a turbine.

The feed gas is a mixture containing mainly nitrogen (more than 50 mol%, more particularly than 80 mol%) and helium (for example between 1 mol% and 15 mol%). It can also contain impurities such as methane, hydrogen, argon, carbon monoxide, oxygen, neon.

An existing solution for extracting helium from a feed gas is, after pretreatment (drying and removal of CO2), to treat it in a cold box. A helium-rich gas can be easily obtained by partial condensation: • By cooling and partially condensing the feed gas to a cryogenic temperature (<-150 ° C, more particularly <-180 ° C) • Separating the phase liquid and the gas phase; the gas phase, rich in helium, is then reheated in the exchanger

An existing solution for producing liquid nitrogen is to install an air separation unit to produce liquid nitrogen by cryogenic distillation. This solution has the disadvantage of significantly increasing the cost of production of helium in terms of investment and operating cost.

Another existing solution for producing liquid nitrogen is to recover a portion of condensed liquid during the helium extraction process. This solution has the disadvantage of producing liquid nitrogen which still contains a lot of methane. The invention consists in using a fluid derived from the feed gas to produce cold by relaxing it in a turbine.

Firstly, the feed gas F is cooled and partially condensed in a 1.2 exchange line to an intermediate temperature (between 0.degree. C. and -180.degree. C., more particularly between -100.degree. 150 ° C). Then the liquid phase and the gas phase are separated in a first condensation pot 4. The liquid phase of the first pot is expanded in a valve and sent to an auxiliary condensation pot 6. The liquid of the auxiliary pot 6 is vaporized in the exchange line against the feed gas and in this example then expanded in a turbine 7. The gas turbine is warmed in the heat exchange line to the hot end to form the waste gas W. The gas auxiliary pot is also heated in the exchange line 1,2.

The resulting gas phase of the first pot 4 is very depleted in hydrocarbons. It is then again condensed partially in the same exchange line or in another exchange line 3 to a cryogenic temperature lower than the intermediate temperature (<-150 ° C., more particularly <-180 ° C.) . The resulting liquid and gaseous phases are separated again in a second pot 5. The gas phase, rich in helium, is reheated and constitutes the product rich in helium He. The liquid phase is nitrogen which contains only traces of hydrocarbons. A part or all is taken out of the cold box as LIN liquid and is used for the different needs of liquid nitrogen of the unit or sent to a storage.

Generally, all or part of the liquid phases of the first and second partial condensation pots 4,5 is expanded and then vaporized and heated in the countercurrent flow line or lines of the feed gas.

Optionally, depending on the amount of liquid nitrogen to be produced, a turbine 7 may be added as shown in the figure to provide the power necessary for the production of liquid. The advantage of this solution is to produce liquid nitrogen with a purity satisfactory for the uses directly from the helium extraction unit, thus to limit the investment of the unit (no other necessary units ) or operating costs (no import of liquid nitrogen required).

Claims (8)

claims 1. Process for separating a gaseous mixture containing at least 50 mol. %, or even at least 80 mol% of nitrogen and between 1 mol% and 15 mol% of helium and at least one heavier impurity than nitrogen in which: i) the gaseous mixture (F) is cooled in an exchange line (1,2) and partially condensed to produce a liquid enriched in the at least one impurity heavier than nitrogen and a gas depleted in this at least one heavier impurity; The heavier impurity is cooled in the exchange line and partially condensed to form a nitrogen enriched liquid and a helium-enriched gas. iii) At least a portion of the nitrogen-enriched liquid is withdrawn as product (LIN) and the helium-enriched gas in the exchange line characterized in that at least a portion of the nitrogen-enriched liquid and / or enriched liquid is expanded to the heavier impurity to provide at least a portion of the process refrigerants. 2. Method according to claim 1 wherein the enriched liquid is vaporized at least partially in the heavier impurity and the gas thus produced is expanded in a turbine (7). 3. Method according to one of the preceding claims wherein the at least one heavier impurity is selected from the group: methane, hydrogen, argon, carbon monoxide, oxygen, neon. 4. Method according to one of the preceding claims wherein the gas depleted in the at least one heavy impurity is cooled in the exchange line (1,2,3) to a temperature below -150 ° C, more particularly below -180 ° C. 5. Method according to one of the preceding claims wherein the gaseous mixture is cooled in the exchange line (1,2) to a temperature between 0 ° C and -180 ° C, more particularly between -100 ° C and -150 ° C. 6. Apparatus for separating a gaseous mixture containing at least 50 mol. %, or even at least 80 mol% of nitrogen and between 1 mol% and 15 mol% of helium and at least one heavier impurity than nitrogen comprising an exchange line (1,2,3), means for sending the gaseous mixture to cool in the exchange line, a first separator pot (4) for separating the partially condensed gas mixture into the exchange line to produce a liquid enriched in the at least one heavier impurity that nitrogen and a gas depleted in this heavier impurity, means for sending the depleted gas in 1 'the at least one heavier impurity to cool in the exchange line, a second separator pot (5) for separating the gas depleted partially condensed to form a nitrogen-enriched liquid (LEST) and a helium-enriched gas (He), means for withdrawing at least a portion of the nitrogen-enriched liquid as a product, means for sending the helium-enriched gas to heat up in the exchange line and means for relaxing at least a portion of the nitrogen-enriched liquid and / or enriched liquid in the at least one heavier impurity to provide at least a portion of the process refrigerants. Apparatus according to claim 6 wherein at least a portion of the liquid enriched in the at least one heavier impurity is vaporized in the exchange line and then expanded. 8. Apparatus according to claim 7 wherein the vaporized liquid is expanded in a turbine (7).