Title: Choosing the right nitrogen rejection scheme
Authors: Nicolas Chantant, Paul Terrien, Sylvain Gérard (Air Liquide Global E&C Solutions)
Abstract:
Nitrogen Rejection Units (NRU) are used to extract nitrogen from natural gas, either from naturally occurring sources
or from associated gas from Enhanced Oil Recovery (EOR) using nitrogen. The final product is generally gas but it can
also be LNG. Many possible process schemes to reject nitrogen from natural gas exist but choosing the best one
necessitate the correct understanding of the drivers.
First, the main categories of Cryogenic Nitrogen Rejection Unit will be presented as well as their potential variants.
The paper will then present some general trends helping to decide which type of unit to select. The paper will
specially focus on some critical factors such as:




Feed conditions and nitrogen concentration
Product methane and rejected nitrogen specifications
Composition variation of the feed gas
Co-products impact such as NGL, Helium or LNG
The paper will finally highlight some common misconceptions regarding the nitrogen rejection units. In particular,
the impacts of various parameters (such as the nitrogen content in the raw natural gas or the type of scheme
selected) on the overall economics of a cryogenic unit solution, compared to alternate solutions, will be further
explored.
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1. Introduction
The nitrogen as an inert gas has no added value in the natural gas. In small quantities it can be tolerated on most of
the gas, however above a certain value it is necessary to extract it for different reasons. Most of the time the
nitrogen rejection is necessary to match natural gas specification such as Higher Heating Value, maximum inert
composition. In such case, the target value is close to few percent depending on the downstream application. For
some applications, the rejection shall be deeper to avoid the pollution of a catalyst for instance or because of very
stringent environmental requirements. In such cases, a purification of the natural gas down to ppm level is
necessary. Nitrogen rejection is also used for Natural Gas Liquefaction process in order to avoid Nitrogen
Stratification in the LNG tanks.
Nitrogen Rejection Units (NRU) are used to extract nitrogen from natural gas, either from naturally occurring sources
or from associated gas from Enhanced Oil Recovery (EOR) using nitrogen. The final product is generally gas but it can
also be LNG.
NRU is one of the typical cleaning steps in a gas treatment plant like amine wash unit or dehydration although not
frequently used.
2. Main processes for nitrogen rejection
Although it is possible to use other technologies to remove nitrogen from natural gas (membranes, Pressure Swing
Adsorption), the vast majority of the nitrogen rejection is performed with cryogenic technologies because it is a
highly efficient process and cost effective solution. This paper focuses on cryogenic NRU and here the word “NRU”
means specifically cryogenic NRU.
At least five main categories of NRU exist:





Simple flash systems (typically used in some LNG plants)
Single column schemes
Double column schemes
Two column schemes
Three column schemes
Each of those takes advantage of the volatility difference between methane and nitrogen to separate the two
components by distillation. Nitrogen has a much higher volatility than methane making distillation process easy and
efficient. Distillation temperatures range between -190°C and -100°C depending of the selected process scheme.
The simple flash system:
This scheme is the simplest process possible. The separation of the methane and nitrogen is done in a separation
drum without any reboiler or condenser. As the volatility of the nitrogen is higher than the methane, liquid will be
enriched in methane and vapor will be enriched in nitrogen. As the difference of volatility increases when the
pressure decreases this type of separation is well suited at low pressure. This is typically the case on LNG plant just
before the flat bottom storage at low pressure, producing LNG with very low nitrogen content.
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Figure 1: Simple flash System
This simple process will be used mostly for low nitrogen content and on processes consuming fuel gas as it produce
enriched rich- nitrogen gas stream that cannot be vented to the atmosphere. Indeed regulation often requests a
nitrogen purity of less than 1%mol of methane for rejection to the atmosphere which is not achievable by simple
flash.
Many alternatives of this scheme exist varying the number of separation drum, adding condenser, external cycle for
refrigeration...
The Single Column Process:
In this process the cryogenic distillation is performed in one column under pressure and purity of both methane and
nitrogen are controlled respectively using a reboiler and a condenser. The reboiler and the condenser are most
frequently connected together using a process cycle to bring heat at the bottom of the column and cold at the top of
the column.
Figure 2: Simple Column Process
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The main advantage of this scheme is to theoretically be able to treat all the possible range of nitrogen contained in
the feed gas and to purify products at any purity. Indeed when the nitrogen has to be rejected to the atmosphere,
the nitrogen shall have a very high purity of methane to comply with regulation.
The column on this process operates in a range of 20-30bar. At this pressure the distillation is not as efficient as a low
pressure process however the advantage is to directly produce high pressure methane and high pressure nitrogen.
Also, at this pressure, the use of low pressure methane to condensate pure nitrogen is possible due to high pressure
difference.
On some cases, it can be interesting to decrease the pressure of the distillation column. To avoid sub atmospheric
methane cycle pressure a possible alternate is to use a nitrogen cycle or a mix refrigerant.
Many possible alternates exist on this scheme. One of the most frequent is to expand methane produced at the
bottom of the column and the expanded stream is used in the overhead condenser. The methane gas shall then be
recompressed from low pressure to product pressure specification but will require only one compressor.
The Double Column Process:
Figure 3: Double Column Process
This process scheme is similar to air separation process. Two distillation columns are used; one high pressure (HP)
column and one low pressure (LP) column thermally integrated with a condenser-vaporizer heat exchanger. This
equipment is the key element of the process as it has the function of condenser of the HP column and reboiler of the
LP column. This double function is possible only because the pressures in the two columns are different. Indeed the
boiling point of nitrogen is lower than the boiling point of the methane at the same pressure. In other words at
equivalent pressure it would not be possible to use the condensation of nitrogen to vaporize the methane. The
pressure of the LP column and the temperature pinch in the vaporizer-condenser fixes the pressure of the HP
column. In this process scheme, a Joule-Thompson expansion provides the refrigeration of the unit.
Additionally, the use of a condenser-reboiler requires careful design of the heat exchangers as the temperature pinch
shall be as low as possible. Air Liquide expertise and extensive knowledge of such equipments (design, manufacture
and operation) are key to properly design such a plant.
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The nitrogen is usually produced at very low pressure and directly vented to the atmosphere. This process scheme is
perfectly adapted to reach very low methane content in the nitrogen vent.
The methane is pumped from the sump of the LP column and routed to the product line after vaporization in the
exchanger. Depending on the required pressure of the methane product, a compressor can be necessary.
The advantage of this scheme is the very good energy efficiency to separate the components. The drawback is that
the methane recovery will be limited if the nitrogen content of the feed is low. At very low nitrogen content in the
feed, the methane will slip in the nitrogen-rich stream and methane recovery will decrease. The limit is around 2530% mol of nitrogen in the feed in order to achieve a reasonable nitrogen content in methane (e.g. <1%mol of CH4 in
N2).
The Three Columns Process:
Figure 4: Three Columns Process
This process scheme is similar to a double column process but with an additional column upstream the high pressure
column. This column produces a high pressure methane and also concentrate the nitrogen at the top. This enriched
nitrogen stream is sent directly to a double column scheme. By enriching the nitrogen content of the stream going to
the double column part, this first column increases the possible range of application of the double column scheme.
Also the first step of production of methane at high pressure before the cold section of the process makes this
scheme more tolerant to impurities subject to freezing (e.g. CO2, heavy hydrocarbons…).
The two Columns Process:
This process is similar to the three columns process, however the HP column is replaced by a separation drum and
the vaporizer-condenser is integrated to the main exchange line. By doing so, the pressure of the LP column can be
increased.
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Comparative analysis:
Principle
Pros
Cons
Flash System
Flash
separation

Simplicity


Low recovery
Limited range
Single Column
Heat
pump
cycle scheme



Higher energy consumption
Refrigeration cycle required



High
flexibility
on
Nitrogen content in the
feed
CO2 tolerant *
Nitrogen under pressure
Very high HC recovery


Energy efficient
Very high HC recovery

Low CO2 tolerance and low
tolerance to presence of
impurities in general
Limited range of Nitrogen
content in the feed.
Not suitable for low
pressure feed
Double Column
Joule-Thomson
effect


Three Columns
Joule-Thomson
effect with preconcentration





Energy efficient
CO2 tolerant *
Tolerance to impurities
Wide range of application
Very high HC recovery

Not suitable
pressure feed
for
low
Two Column
Joule-Thomson
effect with preconcentration




Energy efficient
CO2 tolerant *
Tolerance to impurities
High
flexibility
on
Nitrogen in the feed

Not suitable
pressure feed
for
low
* Each of these schemes will at minimum tolerate more CO2 than a double column scheme. Simple additional
features can even enhance the CO2 tolerance (see next chapter)
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3. Cartography of NRU processes
Selection criteria of the NRU scheme are numerous and none of the process scheme presented in the previous
chapter is better than the other on every possible range of application. It is then very important to carefully define
the condition of use of the NRU to select the most suitable scheme. Furthermore, additional features can be
integrated to each scheme, which might in some cases change significantly the general conclusions or trends that will
be presented in this chapter.
The main NRU selection criteria are the following:
Feed





Methane Product



Nitrogen Content
Pressure
Impurities
(CO2,
Heavy hydrocarbons)
Variability
Flowrate

Nitrogen Content
Purity
HHV,
Wobbe
specification,
Pressure
Nitrogen Product


Methane content
Pressure
index
As an NRU is almost always linked to other processing units, the selection of the right scheme also involves
consideration of direct environment (i.e. Amine wash unit, existing compressor, NGL extraction, helium production…)
In this chapter the impact of some parameters on the scheme selection are illustrated.
Nitrogen content in the feed:
A case study is presented to give a trend of the energy consumption function of the nitrogen content in the feed.
Feed



40 bar a
Variable Nitrogen content
No CO2 content
Methane product


40 bar a
3%mol N2
Nitrogen Product


Vented
1%mol CH4
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NRU Power Trend
5000
4500
4000
Power (kW)
3500
3000
2500
One Column
2000
Double Column
1500
Three Columns
1000
500
0
0
20
40
60
80
Percentage Mole of N2 contained in the Feed
Figure 5: Energy Consumption vs Nitrogen Content
As can be seen from Figure 5: Energy Consumption vs Nitrogen Content, at low nitrogen content in the feed, the
three columns process has a reduced energy consumption compared to one column scheme. This is mostly due to
the fact that the single column process requires energy to produce high purity nitrogen to be rejected to the
atmosphere.
At medium nitrogen content in the feed (around 30%mol) the double column process scheme can be used as the
methane specification in the nitrogen vent stream can be achieved. In this case, the advantages of this scheme are
twofold: being the most simple and the most efficient scheme.
The three columns scheme is more power consuming as it is necessary to expand the feed prior entering the first
distillation column; in addition, this stream has a higher nitrogen content. These two effects will reduce the
condensation temperature of the feed, it is then necessary do reduce the vaporization pressure of the countercurrent methane flow to be produced, increasing the compression power.
At very high nitrogen content, the energy consumption is reduced as it is possible to use the nitrogen expansion to
compress the methane product (a turbo-expander is used to valorize the pressure of the vented nitrogen).
This curve gives a good overview of scheme energy consumption depending on the nitrogen content. However even
a change in one parameter can drastically change the profile of these curves, for example the presence of CO2.
Moreover a scheme selection is rarely only dependent of the energy consumption, other parameters shall be taken
into account.
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Nitrogen Rejected to the atmosphere:
Methane in rejected Nitrogen (%mol)
0.02
0.018
0.016
0.014
0.012
0.01
Double Column
0.008
Three Columns
0.006
0.004
0.002
0
0
10
20
30
40
50
60
Nitrogen in feed (%mol)
Figure 6: Methane content in Nitrogen
Figure 6 shows the methane content in the vented nitrogen for the double and three columns scheme. In both cases
reducing the nitrogen content in the feed will increase the quantity of methane present in the vented nitrogen, other
things being equal. This is due to a lack of refluxed nitrogen in the low pressure column. The consequence is both a
lower recovery of methane and an increased rejection of methane to the atmosphere. As CH4 is one of the gas
involved in the global warming effect (each molecule of methane having a contribution to global warming more than
50 times higher than carbon dioxide), regulation are becoming more and more stringent regarding its emission.
However it can be observed that this effect is significantly reduced by the use of a three columns scheme because
the first column is used as pre-concentration column, increasing the Nitrogen content feeding the double column
system and therefore shifting the curves as shown above.
CO2 content in the feed:
CO2 is almost always present in raw Natural Gas sources. However the partial pressure of CO2 varies greatly
depending on the source. CO2 is an inert gas with a very high triple point temperature (approximately -56.6°C); at
cryogenic temperatures the main issue is the risk of freezing.
One of the easiest solutions is to remove CO2 upstream the NRU, however it requires a dedicated unit that will
impact the economics of the treatment solution. Having a NRU tolerant to CO2 has an evident cost interest. The idea
of tolerant CO2 NRU is to let CO2 enter the unit while operating above the freezing condition. CO2 then simply
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passes through the NRU and ends up in the methane product. Indeed, although having a high triple point
temperature, CO2 is quite soluble in methane at medium or high pressure.
The tolerance of the different schemes is the following:
The double column scheme has a very low tolerance to CO2 as the impurities will be concentrated in a low pressure
(2 bar a) and cold temperature (-185°C) stream. At these conditions the freezing point of CO2 is of ppmv order.
The simple flash process tolerance simply depends on the pressure and temperature of the end-flash separation.
Tolerances of the other schemes are not so trivial. For the single column scheme, the amount of CO2 tolerated is
dependant on the feed pressure, the column pressure and the composition of the gas to be treated. Indeed at very
high nitrogen content, CO2 will concentrate in the methane. For instance, having a nitrogen content of 70% at NRU
inlet and methane product purity of 97% mol will concentrate the CO2 by a factor 3 in the product. The freezing
point of this stream is then dependant on the vaporizing curve in the exchange line. In other words, in case of lean
feed gas, it may be impossible to vaporize methane without freezing.
On a three columns and two columns, the amount of CO2 that can be accepted in the feed depends on both:


tolerance of the pre concentration column.
tolerance of the downstream process.
In some way the tolerance of pre-concentration column is similar to single column scheme. An additional constraint
is the ability of the first column to remove the CO2 from the feed as this stream will continue downstream to much
colder part of the process. Additional reflux at the top of the column can reduce the CO2 content down to few ppm;
however theses reflux are power consuming and it is sometime preferred to directly remove CO2 upstream the NRU.
CO2 is then a real challenge for the scheme selection as it can eliminate a process solution that would be otherwise
selected. The knowledge of freezing point of CO2 in pure methane and mixture of methane and nitrogen is essential.
Air Liquide, as leader in cryogenic separations and specifically in CO2 production, has an extensive know how and
operational feedback of the freezing behavior of such components, allowing to design safe and reliable NRU unit. As
explained above, a CO2 tolerant NRU can either consist in a scheme naturally tolerant to the level of CO2 required
with the information of composition and pressure of the feed, or sometimes in a specifically designed unit that will
include one or several of the following: additional condenser and reflux, additional refrigeration cycles…
Feed pressure:
As describe in the first chapter, in some solution, the refrigeration is provided by Joule-Thompson effect. In other
words, the thermal balance of the unit is only dependant of the expansion of the feed gas. In some cases this
refrigeration may be insufficient.
The worst scenario will be a low pressure feed and low nitrogen content. For such case, a dedicated cycle and / or a
feed compressor will be required to ensure the correct frigorific production in the unit. However adding rotating
equipment to three columns or two columns schemes highly increases the total cost of the solution as well as the
power consumption. This configuration is more in favor of a single column scheme with external process cycle.
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The size of the unit is an additional factor to take into account for the thermal balance of the unit. Indeed the heat
leaks are not directly proportional to the flow to be treated: lower the feed flow, higher the relative heat leaks.
Accurate calculation of frigorific balance is then essential. In some extreme / limit cases, heat leaks alone might force
the use of additional machines (turbo-expander or compressor).
Nitrogen use:
During the NRU scheme selection, the definition of the rejected nitrogen is as important as the produced methane. If
nitrogen is not vented it can be used as fuel gas or re injected in a reservoir. In these two cases, the pressure and the
maximum nitrogen content has be defined.
Process scheme such as three columns and double columns are disadvantaged in such configuration as the nitrogen
will require an additional compression from low pressure column to product pressure.
Schemes having a distillation under pressure such as single column are better suited for such application. The
nitrogen will be directly produced at the specified pressure and condenser duty will be reduced.
Feed variation:
Feed to be treated by the NRU can have a great variability in terms of flowrate but also in terms of composition. This
is especially true in case of NRU for Enhanced Oil Recovery (EOR) where the nitrogen content in the feed may vary
from 5% mol at the beginning of the production and finish with 70% mol or above. For such application a process
scheme tolerant to fluctuation is essential. Three columns and single column are often then preferred schemes for
their intrinsic flexibility.
Co-products NGL, Helium, LNG:
As natural gas almost always contains other components such as NGL or Helium, it can be attractive from an
economical point of view to extract them to produce highly valuable co-product.
Helium has a much higher volatility than nitrogen; cryogenic separation is perfectly adapted for this production. By
principle, the cryogenic separation will concentrate all the most volatile components in the same stream. On a
double column scheme for instance the top of high pressure column will be a mixture of nitrogen and concentrated
Helium. The valorization of such product can be very valuable. Single column with nitrogen open cycle will not be
preferred as the mixture of helium and nitrogen will be produced at low pressure.
NGL (C2+) will be easily separated in the NRU process and can be produced with few additional equipment. As per
helium stream, if the recovery is specified at the design stage, the optimum process scheme can be directly selected.
By principle, NRU liquefies methane. The frigorific balance of the unit imposes the vaporization of this LNG to cool
down the feed gas. However during the design phase, it is possible to select a scheme that will increase the cold
production allowing a co-production of LNG; for instance, single column scheme with dedicated cycle, or double
column scheme with additional cycle.
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Additional
itional productions are then possible during engineering phase and can considerably steer the scheme selection.
As described in this chapter, the numbers of parameters impacting the design of an NRU are numerous and in many
cases, only a specifically developed design will maximize the profit for the final customer.
customer Only macro trends about
process scheme can be provided here as specificity of project, integration of the NRU in an existing plan can give
different optimization.
4. Misconceptions about nitrogen rejection
Due to the complexity to select an optimized process for an NRU, different misconceptions exist about Nitrogen
Rejection Units. In this part some common misconceptions will be highlighted. These results are extracted from
detailed sensitivity analysis of recent NRU projects
project and FEED experiences of Air Liquide.
Misconception 1: “A process using less columns is cheaper”.
cheaper
This misconception is based on the logical idea that multiplying the number of equipment will increase the cost of
the solution.
For an NRU, the Total Cost of Ownership (TCO),
(TCO) per MMBTU of sales gas produced, for a gas field operator,
operator is defined
as followed:
Équation 1: Total Cost of Ownership
To compare two different solutions with similar recovery,
recovery only CAPEX and OPEX of the solution shall be compared.
OPEX criterion:
As shown in Figure 5:: Energy Consumption vs Nitrogen Content,
Content the energy consumption of a NRU can be much
lower for the three columns scheme than single column. For instance, on the studied case with a Nitrogen
Nitrog content in
the feed of 25%mol,
%mol, the energy consumption is 50% higher for the one column scheme than for the three columns.
Cryogenic distillation is a very reliable way to separate components. Distillation columns, heat exchangers are static
equipments that do not require regular maintenance. OPEX cost of an NRU is then mostly linked to the energy price.
Having an energy consumption reduced by 30% then gives a huge saving.
CAPEX criterion:
A distillation column is static equipment with quite limited cost and only few additional types of equipment are
required between a single column and a three column scheme (exchanger, separation drums). On the cold part of
the process, the three column scheme will then be slightly more expensive
exp nsive than a single column scheme.
sch
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However the major impact is on the warm part of the process regarding rotating equipments. In this case study, the
three columns scheme requires a product compressor with different stages to compress from 10 bar abs to 40 bar
abs. In comparison the single column scheme requires a product compressor to compress from 3 bar to 40 bar and a
second compressor for the cycle. Compressors are expensive pieces of equipment in regards to the price of an NRU.
Adding a compressor will then increase the price of the solution in a larger amount than adding columns.
In this case, an alternative exist with a single compressor but only with multiple intermediate stages and a larger flow
rate making this equipment more expensive.
In this example, both OPEX and CAPEX of the three columns solution are lower, leading to an obvious reduction of
the TCO.
Misconception 2: “Less nitrogen in the feed gas will be more economical for the operator of the field”.
This misconception is based on the idea that having more nitrogen in the natural gas is further away from the desired
product specifications and therefore makes the separation more expensive.
A case study has been performed by Air Liquide showing that for the same amount of sales gas being produced, the
total cost of ownership is lower for a nitrogen rich gas (50-60% N2) than for a nitrogen lean gas (20-30% N2).
The main reason is that in the first case, the plant is able to meet the final specifications taking advantage of the
refrigeration available by expansion of the nitrogen. At high nitrogen content in the feed, the methane is directly
produced at high pressure thanks to a cryogenic pump. The power consumption is then quite small. On the opposite,
at low nitrogen content in the feed, it is not possible to directly produce a high pressure methane, a final
compression is then required.
Figure 7: NRU Solutions with different N2 content in Feed
The power consumption on the low nitrogen case can be 10 to 30 times higher leading to a huge increase of the
plant OPEX. On large plant, the power reduction can reach megawatts, what drastically reduce the OPEX of the unit.
On the other hand, the increase of nitrogen content in the feed to be treated means larger equipment for equivalent
sales gas to be produced, especially distillation column, separation drum, valves... This effect is offset by the
reduction of the rotating equipment cost.
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On large plant, saving on OPEX easily offset the increase of CAPEX, leading to a lower TCO of the unit. Having a high
amount of nitrogen content in the feed is then potentially an opportunity to reduce the TCO of the plant.
Misconception 3: “Separating nitrogen with cryogenic technology is more expensive than with membranes”.
This misconception is based on the fact that cryogenic separation requires pre treatment and that the price of such
NRU unit is often higher than membranes NRU for instance. However cryogenic NRU and membranes NRU are suited
for very different applications.
Membranes solutions can be interesting on processes requiring bulk nitrogen removal and rather on small scale unit.
Typical application is the increase of higher heating value of fuel gas to feed gas turbines. Membranes are also very
attractive solution on biogas sources with small amounts of nitrogen.
In this case membrane solution will be very simple with no pre treatment unit. With current membranes NRU
technology, the enriched methane stream will be produced at low pressure and the nitrogen enriched stream at high
pressure. Also, membrane separation, intrinsically, is not suited for large nitrogen content reduction (for instance
from more than 20% down to 1 or 2%), or to produce relatively pure nitrogen to be vented to the atmosphere
(below 1%mol methane).
Typically, in the following cases, multiple stages membranes shall be installed requiring several major compression
steps that would overshadow CAPEX of everything else for small plants:


Large nitrogen rejection from Feed to product (example: from 20% down to 2%)
Very high purity of any of the products (below % levels)
Also, in case of large feed flow rate to be treated (> 20 000Nm3/h), membranes cannot compete with cryogenic
nitrogen removal unit because its costs generally become prohibitive.
The scenarios described above show that, irrespective of efficiency and energy consumption, membrane NRU
solution will be more expensive than cryogenic NRU.
Based on its own state-of-the-art membrane technology, Air Liquide is in position to select the most appropriate
solution between membranes and cryogenic, on a case by case basis.
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5. Conclusion
Cryogenic Nitrogen Removal Unit is a simple, cost effective and efficient way to remove nitrogen from natural gas.
However the number of parameters to take into account for the selection of the unit requires an extensive
knowledge of all the problematic. Only a customized solution taking into account all the parameters of the project,
the technical specifications and local regulation will provide the most suitable solution.
Through its various references and unique cryogenic expertise, Air Liquide is able to offer any of the most advanced
designs for nitrogen rejection technologies while ensuring the highest standards of safety and reliability, including:



One-column, two-column, three-column, and double column processes and their variants
A variety of flexible NGL / NRU integration concepts ranging from moderate to very high levels of integration
to fulfill project-specific requirements
A variety of refrigeration schemes ranging from simple JT expansion to external refrigeration cycles using
nitrogen, methane, or mixtures of refrigerants.
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