FR3110223A1 - Process for extracting nitrogen from a stream of natural gas or bio-methane - Google Patents

Abstract

Title: Process for extracting nitrogen from a stream of natural gas or bio-methane Process for extracting nitrogen from a feed stream (1) of natural gas comprising at least nitrogen and methane by cryogenic distillation using a column system with recycling of compressed nitrogen (37) from the third column (K3) to the second column (K2). Single figure

Description

Process for extracting nitrogen from a stream of natural gas or bio-methane

The subject of the invention is a process for extracting nitrogen from a natural gas or bio-methane feed stream (potentially resulting from bio-gas) comprising at least nitrogen and methane. It uses cryogenic distillation to separate nitrogen from methane.

Raw natural gas or bio-methane can contain a large number of troublesome impurities to remove. An example is nitrogen. From a certain concentration of nitrogen in natural gas or in bio-methane, it is typically not salable because of its low calorific value or simply because of a limitation of the quantity of inert gases, including nitrogen, in natural gas or in bio-methane. To remove nitrogen, a cryogenic process is most often used, carried out in a unit called a Nitrogen Rejection Unit (NRU).

The nitrogen content entering the NRU is of great importance in the overall performance of the unit, a low nitrogen content reducing the flexibility or even the feasibility of certain schemes. However, it turns out that in the exploitation of certain fields, the nitrogen content tends to change over time.

Similarly, the flow rate variation can be a limit to the operation of these nitrogen separation units. Indeed, it is necessary to manage to maintain in the columns to be distilled a sufficient liquid / vapor circulation to maintain the distillation.

Several schema families exist, including:

• Single column process
• Double column process
• Two-column process
• Three-column process

The present invention is particularly concerned with the three-column process which proves to be particularly effective for gas fields containing between 5 and 35% mol of nitrogen. The problem is to manage to reconcile this type of efficient scheme with operator requests related to flexibility (variations in composition, variations in speed).

A stage often carried out first in an NRU is a purification of heavy components called "dew-pointing" of the natural gas, in order to avoid any risk that the heavy hydrocarbons contained in the natural gas (ethane and other alkanes more heavy, up to C ₂₀ , plus BTX (benzene, toluene, xylene) can cause condensation (rich in heavy components) of the latter when it is transported in gaseous form through long pipes.

A three column scheme is known from US4664686, WO14/167283 and "Removing Nitrogen", Hydrocarbon Engineering, November 2002.

Three-column schemes typically allow gases with a relatively low nitrogen content to be treated. An example of this type of schema contains at least the following steps:

• Feed gas cooling
• Cooling of the gas from the separator pot then introduction into the first column operating at a first pressure, typically at the top
• Reboiling of the liquid from the first column operating at a first pressure
• Vaporization of the liquid from the first column to produce a medium pressure methane-rich product ‘MP CH4’
• Cooling of the gas from the first column and at least partial condensation of this gas before expansion and introduction into the second column operating at a second pressure lower than the first pressure
• Sub-cooling and expansion of the liquid from the second column then introduction of this stream as reflux into the third column operating at a third pressure lower than the second pressure
• Extraction of a liquid rich in nitrogen from the condenser of the second column to be subcooled and then used as reflux of the third column
• Reboiling of the third column liquid thermally linked to the second column condenser
• Pumping liquid from the third column to a fourth pressure then vaporizing against the feed stream to produce a low pressure methane rich product ‘LP CH4’.

In addition to this, the following steps are typically required to produce methane-rich gas at sufficient pressure to be exported to a grid or transported to a liquefaction unit:

• Compression of the ‘LP CH4’
• Compression of ‘MP CH4’

These two compressions can for example be carried out in a first section to bring the 'LP CH4' to the same pressure as the 'MP CH4' then in a second section to compress the whole until the final pressure.

The vaporization of the heavy-rich liquid condensed in the first pot is done in the exchange line at lower pressure compared to the natural gas at the NRU inlet, to exploit the cooling effect coming from the expansion of the liquids at the pot outlet. This liquid after expansion is separated in a second gas/liquid separator pot before being introduced into the exchange line.

The default of such a diagram is the minimum operation of the unit which cannot wait for a reduction in flow rate of less than 60% of the nominal supply gas flow rate.

According to one object of the invention, there is provided a process for extracting nitrogen from a natural gas or bio-methane feed stream, possibly resulting from bio-gas, comprising at least nitrogen and methane by cryogenic distillation comprising the following steps:

1. Feed stream cooling in a first heat exchanger
2. Sending a first portion of the feed stream to a first distillation column operating at a first pressure
3. Reboiling of the liquid from the first column operating at a first pressure
4. Vaporization of bottom liquid from the first column in the first heat exchanger to produce a first methane-rich product
5. Cooling of the top gas from the first column and at least partial condensation of this gas before expansion and introduction into a second distillation column operating at a second pressure lower than the first pressure and having a top condenser
6. Subcooling and expansion of the bottom liquid of the second column then introduction of this stream into a third distillation column operating at a third pressure lower than the second pressure
7. Extraction of a liquid rich in nitrogen from the condenser of the second column to be subcooled and then used as reflux of the third column
8. Reboiling of the bottom liquid of the third column thermally linked to the condenser of the second column
9. Pumping liquid from the third column to a fourth pressure then vaporizing against the feed stream in the first heat exchanger to produce a second methane-rich product
10. Withdrawal of a nitrogen-enriched gas at the top of the third column, heating of the gas in the first heat exchanger and compression of at least part of the nitrogen-enriched gas in a compressor
11. At least part of the nitrogen-enriched gas compressed in the compressor is sent to the second column as a recycling flow to be separated there.

According to other optional aspects:

• the at least part of the recycle flow is mixed with the feed stream upstream of the second column
• no part of the recycling flow is sent to the second column if the flow of the supply current constitutes at least 60% of the nominal flow
• the recycling flow is sent to the second column if, preferably only if, the flow of the supply current constitutes less than 60% of the nominal flow
• the recycling flow is sent to the second column if, preferably only if, the flow of the supply current constitutes more than 30% of the nominal flow
• the recycling flow is sent to the second column if, preferably only if, the nitrogen content of the feed stream flow is less than a threshold, possibly 7% mol if the nominal flow contains more than 7% mol d 'nitrogen
• the feed stream contains at least one constituent heavier than methane and part of the stream is partially condensed in the first heat exchanger and sent to a phase separator whose gaseous part is mixed with the feed stream and optionally the liquid part is heated in the heat exchanger

• the nominal flow rate of the feed stream contains between 4 and 80% nitrogen, preferably between 4 and 35% nitrogen

• the recycling flow is sent to the second column if at least of these conditions are met:

i) the nitrogen content of the feed stream is below a fraction of the rated flow content,

ii) the supply current flow rate is less than 60% of the nominal flow rate

• the recycling flow is not sent to the second column if the two conditions below are not met:

i) the nitrogen content of the feed stream is above the fraction of the nominal flow content, and

ii) the supply current rate is at least 60% of the nominal rate

• the feed stream contains between 20 and 96% methane, preferably between 65 and 96% methane.

According to one object of the invention, there is provided an apparatus for extracting nitrogen from a natural gas or bio-methane feed stream, possibly resulting from bio-gas, comprising at least nitrogen and methane by cryogenic distillation comprising a first heat exchanger, a first column operating at a first pressure, a second distillation column operating at a second pressure lower than the first pressure and having an overhead condenser, a third distillation column operating at a third pressure lower than the second pressure, a bottom liquid reboiler of the third column thermally linked to the condenser of the second column, means for cooling the feed stream in the first heat exchanger, means for sending a first portion of the feed stream to the first distillation column operating at a first pressure, a reboiler for reboiling liquid from the first column e operating at a first pressure, means for sending bottom liquid from the first column to vaporize in the first heat exchanger to produce a first methane-rich product, means for cooling and at least partially condensing overhead gas from the first column before expansion and introduction into the second distillation column, a sub-cooler, means for sending bottom liquid from the second column into the sub-cooler, means for expanding the subcooled bottom liquid, means for introducing this expanded stream into the third distillation column, means for extracting a liquid rich in nitrogen from the condenser of the second column, for sub-cooling it in the sub-cooler and means for sending it as reflux from the third column, a pump for pressurizing liquid from the third column to a fourth pressure, means for sending the pressurized liquid to vaporize co between the feed stream in the first heat exchanger to produce a second product rich in methane, means for withdrawing a gas enriched in nitrogen at the top of the third column, means for sending the withdrawn gas to heat up in the first exchanger heat, a compressor for compressing at least a portion of the nitrogen-enriched gas and means for sending at least a portion of the nitrogen-enriched gas compressed in the compressor to the second column as recycling flow to be separated there.

Preferably, the means for sending the at least part of the nitrogen-enriched gas to the second column are regulated by means for detecting the flow rate and/or by means for detecting the nitrogen or methane content of the flow of feed.

The proposed invention relates to a three-column scheme for separating nitrogen from methane using the first column, then a second column, then a third column. In the proposed invention, the nitrogen returned to the atmosphere from the third column (in English ''rejected nitrogen'') is compressed and returned to the inlet of the second column, so that in reduced operation a minimum flow rate (composed mainly of nitrogen) is maintained in the second and third columns.

The device thus designed can be operated with said nitrogen compression stopped for all operations between 100% and 60% of the nominal load, and with said nitrogen compression on for loads below 60% which can go up to at 30% of rated load.

The invention will be described in more detail with reference to the .

illustrates a particular embodiment of a method according to the invention implemented by an installation as shown schematically.

The same reference designates a liquid stream and the pipe which conveys it, the pressures considered are absolute pressures and the percentages considered are molar percentages.

A flow 1 is cooled in the passages of a heat exchanger E1 by flows produced by the distillation Flow 1 contains nitrogen and methane and may also include impurities present in natural gas, such as carbon dioxide carbon and hydrocarbons. Flow 1 has often been treated to reduce its content of hydrocarbons and other impurities containing sulfur and mercury. Flow 1 contains between 5 and 80 mol% nitrogen, preferably between 5 and 35 mol% nitrogen.

Part 3 of flow 1 is taken from an intermediate point of exchanger E1 in partially condensed form and is sent to a phase separator S1 in order to remove the heavier components of flow 1 with the liquid. The gaseous part 5 joins flow 1. The liquid in this pot is optionally totally or partially vaporized and produced directly as condensate. Here the liquid part 7 is expanded in a valve and heated in the exchanger E1. This step avoids any risk that the heavy hydrocarbons contained in flow 1 (ethane and other heavier alkanes, up to C ₂₀ , plus BTX (benzene, toluene, xylene) may cause condensation (rich in heavy) purified methane when transported in gaseous form through long pipelines.

If flow 1 does not contain very heavy components, this step is optional.

Another part 9 of flow 1, in liquid form, is expanded then sent to the first column K1, preferably at the top of the first column K1, operating at a first pressure, as feed flow. The K1 column is a distillation column, which can operate as a stripping column. The stream splits in column K1 to produce a methane-depleted stream 11 and a methane-enriched liquid 13. The stream 13 is split in two. Part 15 is heated in exchanger E2 and part 17 is expanded then vaporized in exchanger E1 to form a gas rich in methane. The methane-rich gas MP CH4 is compressed by a compressor C1 to serve as product 39.

Gas 11 is returned to flow 11 at an intermediate level of exchanger 11 and flow 11 cooled in exchanger E1 and mixed with flow 37 to form flow 19 which is expanded in a valve then sent to the second column K2 operating at a second pressure lower than the first pressure.

The bottom liquid 21 from column K2 is sent to the third column K3 at an intermediate level after expansion and subcooling in exchanger E3. The overhead gas 23 condenses in the bottom vaporizer E4 of the third column K3 to form a liquid 25 which is returned to the top of the second column K2 and a liquid 27 is sent to the top of the third column K3 after expansion and under - cooling in the E3 exchanger. The bottom liquid 29 of the third column K3 partly vaporizes in the exchanger K4 to form a gas 31.

The rest 33 of the bottom liquid of the third column K3 is pressurized by a pump P up to a fourth pressure, reheated in the sub-cooler E3 then vaporized in the exchanger E1 to form a gas rich in methane. This gas is pressurized in a compressor C2 then sent to an intermediate level of the compressor C1.

Nitrogen gas 35 is withdrawn from the top of the third column K3, reheated in the subcooler E3 and then in the heat exchanger E1. The gas heated to the hot end of exchanger E1 is compressed in a compressor C3.

If flow 1 reduces to a flow constituting less than 60% of the nominal flow, part 37 of the compressed nitrogen is cooled in exchanger E1 and mixed with flow 1 downstream of exchanger E1 to form part of the K2 column feed rate.

The amount of nitrogen recycled in this way can be chosen to compensate for the lack of Flow 1, so the more Flow 1 is reduced, the greater the Recycled Flow 37.

This recycling can allow the device to operate until flow 1 reduces to 30% of the nominal flow. In this case the device will include means to trigger recycling depending on the value of flow 1, typically an FIC will be used to measure flow 1 and a valve for flow 37 will be opened if the flow goes below a threshold.

The same recycling can also be used to compensate for a drop in the nitrogen content of flow 1. Thus if flow 1 becomes less rich in nitrogen, gaseous nitrogen 37 can be recycled to the second column K2. The less the flow 1 is rich in nitrogen, the greater the flow 37 is. For example, gaseous nitrogen can be recycled if the nitrogen content of flow 1 falls below a threshold, for example 7% mol if nominal flow 1 contains more than 7% mol of nitrogen. In this case, the apparatus will include means for triggering recycling by opening a valve as a function of the nitrogen content of flow 1. In the case of a binary mixture, either the nitrogen content is detected directly or the nitrogen content methane is detected and is representative of the nitrogen content.

By combining the two techniques, it is possible to compensate for a flow 1 which becomes smaller and at the same time less rich in nitrogen.

Claims (10)

Process for extracting nitrogen from a feed stream (1) of natural gas or bio-methane, possibly resulting from bio-gas, comprising at least nitrogen and methane by cryogenic distillation comprising the following steps :

1. Cooling of the feed stream in a first heat exchanger (E1)
2. Sending a first portion (9) of the feed stream to a first distillation column (K1) operating at a first pressure
3. Reboiling (E2) of the liquid from the first column operating at a first pressure
4. Vaporization of bottom liquid (17) from the first column in the first heat exchanger to produce a first methane-rich product
5. Cooling of the overhead gas (11) from the first column and at least partial condensation of this gas before expansion and introduction into a second distillation column (K2) operating at a second pressure lower than the first pressure and having an overhead condenser (E4)
6. Sub-cooling and expansion of the bottom liquid (21) of the second column then introduction of this liquid into a third distillation column (K3) operating at a third pressure lower than the second pressure
7. Extraction of a liquid rich in nitrogen (27) from the condenser of the second column to be subcooled and then used as reflux of the third column
8. Reboiling of the bottom liquid of the third column thermally linked to the condenser of the second column
9. Pumping (P) liquid from the third column to a fourth pressure then vaporizing against the feed stream in the first heat exchanger to produce a second methane-rich product (39)
10. Withdrawal of a gas enriched in nitrogen at the top (35) of the third column, heating of the gas in the first heat exchanger and compression of at least part of the gas enriched in nitrogen in a compressor (C3)
11. Sending at least a portion (37) of the nitrogen-enriched gas compressed in the compressor to the second column as a recycling flow to be separated there.

Process according to Claim 1, in which the at least part of the recycle flow (37) is mixed with the feed stream (1) upstream of the second column (K2). Process according to claim 1 or 2, in which no part of the recycling flow (37) is sent to the second column (K2) if the flow rate of the supply current constitutes at least 60% of the nominal flow rate. Method according to one of the preceding claims, in which the recycling flow (37) is sent to the second column (K2) if, preferably only if, the flow rate of the supply current constitutes less than 60% of the nominal flow rate. Method according to one of the preceding claims, in which the recycling flow (37) is sent to the second column if, preferably only if, the flow of the feed stream (1) constitutes more than 30% of the nominal flow. Process according to one of the preceding claims, in which the recycling flow (37) is sent to the second column (K2) if, preferably only if, the nitrogen content of the feed stream flow is less than a threshold , possibly 7% mol if the nominal flow rate (1) of the current contains more than 7% mol of nitrogen. Process according to one of the preceding claims, in which the feed stream (1) contains at least one constituent heavier than methane and a part (3) of the stream is partially condensed in the first heat exchanger (E1) and sent to a phase separator (S1) whose gaseous part (5) is mixed with the feed stream and optionally the liquid part (7) is heated in the heat exchanger. Process according to one of the preceding claims, in which the nominal flow rate of the feed stream (1) contains between 4 and 80% nitrogen, preferably between 4 and 35% nitrogen. Apparatus for extracting nitrogen from a natural gas or bio-methane feed stream, optionally derived from bio-gas, comprising at least nitrogen and methane by cryogenic distillation comprising a first heat exchanger ( E1), a first column operating at a first pressure (K1), a second distillation column (K2) operating at a second pressure lower than the first pressure and having an overhead condenser, a third distillation column (K3) operating at a third pressure lower than the second pressure, a bottom liquid reboiler (E4) of the third column thermally linked to the condenser of the second column, means for cooling the feed stream in the first heat exchanger, means for sending a first portion of the feed stream to the first distillation column operating at a first pressure, a reboiler (E2) for reboiling the liquid from the first column operating at a p first pressure, means for sending bottom liquid from the first column to vaporize in the first heat exchanger to produce a first product rich in methane, means for cooling and at least partially condensing overhead gas from the first column before expansion and introduction into the second distillation column, a sub-cooler (E3), means for sending bottom liquid (21) from the second column into the sub-cooler, means for expanding the sub-cooled bottom liquid , means for introducing this expanded stream into the third distillation column, means for extracting a liquid rich in nitrogen (27) from the condenser of the second column, for sub-cooling it in the sub-cooler and means for sending as reflux from the third column, a pump (P) for pressurizing liquid from the third column up to a fourth pressure, means for sending the pressurized liquid to vaporize c against the feed stream in the first heat exchanger to produce a second product rich in methane, means for withdrawing a nitrogen-enriched gas (35) at the top of the third column, means for sending the withdrawn gas to be heated in the first heat exchanger, a compressor (C3) for compressing at least a part of the nitrogen-enriched gas and means for sending at least a part (37) of the compressed nitrogen-enriched gas in the compressor to the second column as recycling flow to be separated there. Apparatus according to claim 9 wherein the means for supplying the at least part of the nitrogen-enriched gas (37) to the second column (K2) is regulated by flow detection means and/or by flow detection means. the nitrogen or methane content of the feed stream.