2. Nitrogen removal for chemicals
production
A
s natural gas is widely used
as feedstock for the manufac-
ture of many chemicals, the
current low gas price is a major
benefit to chemicals producers in
improving their profit margins. It is
not surprising that many plant
construction projects, based on
utilising low cost natural gas feed-
stock, are in the planning stage or
are already in the engineering
design phase.
Methane is the starting point for
chemical synthesis gas (syngas)
production. Steam reforming or
catalytic partial oxidation of meth-
ane can be used to produce a
hydrogen and carbon monoxide
rich synthesis gas, which acts as the
initial ‘building block’ for down-
stream chemicals manufacture.
Carbon dioxide in the natural gas
feed will normally need to be
removed to low levels using well-
known acid gas removal
technology.1
Lighter paraffinic
hydrocarbons in the feed gas can be
tolerated. If the natural gas
proposed as plant feedstock is rela-
tively rich in ethane and heavier
hydrocarbons, these can be
removed as natural gas liquids
(NGL) using well-established
process technology2
and fraction-
ated for use as either petrochemical
plant feedstock or liquid fuel
(propane and butane) or for gaso-
line production (butane and natural
gas condensate).
A significant proportion of world-
wide natural gas reserves contains
nitrogen. Nitrogen is inert and
needs to be removed for natural gas
to be used as fuel. With greater use
of natural gas for fuel and/or chem-
icals production, the future
Increasing nitrogen levels in gas fields drive an increased need for efficient
cryogenic nitrogen removal plants
JORGE ARIZMENDI-SÁNCHEZ and ADRIAN FINN
Costain
development of ‘marginal’ gas fields
will often mean relatively high
nitrogen content gas being used, if it
can be processed profitably.
The need for low nitrogen
content gas
The nitrogen content of natural gas
is typically reduced to about 5% for
the gas to be used as fuel. For use
as chemical plant feedstock, the
nitrogen content must normally be
much less than this, of the order of
1% or less. As nitrogen is inert, it
passes through the reaction section
and remains in the synthesis gas
with hydrogen and carbon monox-
ide. For ammonia production,
nitrogen does not need to be
removed and its presence can be
beneficial by reducing the need for
partial oxidation with air. However,
for most chemicals manufacture,
nitrogen leads to dilution of reac-
tants and higher volumes of
processed gas, increases equipment
size, heat transfer duties, capital
and operating costs, and requires
purification of products.
Carbon monoxide purity for
chemicals production has normally
to be well over 99%, to promote
high reaction yields and avoid the
need for purification of products.
Cryogenic distillation is the most
economical way to remove nitro-
gen, but the volatilities of nitrogen
and carbon monoxide are very simi-
lar (the difference in their respective
boiling points is only 4ºC), so to
achieve high purity product is
energy intensive and expensive.
Failure to fully consider the
importance of feed gas nitrogen
content can have major ramifica-
tions for chemical plant
performance and can seriously
reduce plant capacity and eventual
product quality. Removal of nitro-
gen from the natural gas feedstock
upstream of the chemicals manu-
facturing plant should therefore be
considered as a preferred alterna-
tive to downstream processing. If
some natural gas is intended for
use as fuel (and the balance for
chemicals manufacture) then nitro-
gen removal may be needed in any
event to ensure fuel quality natural
gas. Well-proven and reliable cryo-
genic technology is available for
nitrogen removal from natural gas.
The difference in the boiling points
of nitrogen and methane is 34ºC,
which leads to a much easier and
less energy-intensive separation
compared to nitrogen and carbon
monoxide distillation.
Process technology selection for
nitrogen removal
The typical feed gas capacity of
syngas plants using natural gas
feedstock is of the order of 30
million standard cubic feet per day
www.eptq.com Gas 2016 29
Failure to fully
consider the
importance of
feed gas nitrogen
content can have
major ramifications
for chemical plant
performance
3. 30 Gas 2016 www.eptq.com
consumption required to provide
low level refrigeration and mini-
mises the capital and operating
costs of machinery.
Cryogenic nitrogen removal
plants (for a moderate nitrogen
content of about 5-20%) usually
have at least two stages of separa-
tion based on distillation (see Figure
1). The first stage (pre-separation)
performs a bulk separation of nitro-
gen and methane. Nitrogen is
concentrated in a stream that is
further processed in a second sepa-
ration stage (nitrogen rejection) to
produce pure nitrogen and meth-
ane. The pre-separation column
operates at elevated pressure,
allowing about half of the methane
in the feed gas to be recovered and
then pumped to high pressure so
minimising expensive downstream
compression.6
Indeed, based on
feed gas being available at about 40
bar, the cryogenic nitrogen removal
plant can usually produce much of
the product methane at a high
enough pressure (of the order of 35
bar) for gas reforming, without any
further methane compression. The
second column system operates at
reduced pressure to produce a
methane stream with low nitrogen
content (for compression) and a
pure nitrogen stream, which can be
safely vented to atmosphere.
Cryogenic nitrogen removal uses
the evaporation of liquid methane
process streams at appropriate
temperatures to provide the neces-
sary refrigeration for the process, so
there is effectively no external
refrigeration system (see Figure 1).
Liquid methane products are
pumped to the required evaporat-
ing pressure, which minimises the
power required for re-compression.
Part of the methane product is
delivered at a lower pressure than
the feed gas, and a conventional
compression system is used for gas
pressure boosting and transporta-
tion. Energy integration reduces
overall product gas compression so
that modern plants offer gas
processing with low operating cost.
The plant design is optimised to
minimise overall cost by considera-
tion of the compression system in
parallel with design of the cryo-
genic plant.
(mmscfd), for instance at Clear
Lake, La Porte and Texas City
syngas plants.3
The capacity of
syngas plants is increasing with the
use of multiple reactor units. The
Bintulu Gas to Liquids (GTL) plant
has a feed gas capacity of 100
mmscfd, whilst the world’s largest
GTL plant (Pearl) can process in
excess of 1000 mmscfd.3
For very small gas processing
plants, it may be possible to employ
pressure swing adsorption (PSA) or
membrane technology for nitrogen
removal from natural gas feedstock.
However, these processes operate
at low pressure and incur high
power consumption per unit of gas
processed, as they need relatively
expensive compression equipment
to increase the product gas pres-
sure to the operating pressure
required for syngas production. No
PSA plant has been built and oper-
ated to remove nitrogen at a
capacity over about 20 mmscfd and
membrane plants are only economi-
cal at much smaller capacity.
Cryogenic distillation is almost
always the only technically and
commercially feasible process solu-
tion for nitrogen removal from
natural gas of moderate to high
nitrogen content as it consumes far
less power per unit of gas processed,
requires less costly machinery, can
use a relatively simple machinery
configuration (based on conven-
tional gas processing compressors),
is reliable, ensures specification
products even with varying feed
composition and, importantly, is
environmentally by far the best
approach, as discussed below.
Benefits of cryogenic distillation
Distillation is energy efficient and
the most economical solution for
large scale processing of binary
mixtures to give relatively pure
products, especially for feeds where
both components are present in
significant quantities.4
For nitrogen
rich natural gas, all these require-
ments are met with cryogenic
distillation, and plants for process-
ing high nitrogen content (over
30%) have been operational for
over 30 years. It is important to
appreciate that even though cryo-
genic distillation is more energy
intensive at lower feed gas nitrogen
levels, plants based on thermody-
namically efficient process designs,
with relatively low energy
consumption and operating costs,
have been operational for well over
20 years.5
High thermodynamic
efficiency reduces the power
Feed gas
Compression
system
Pre-separation
system
Nitrogen
rejection
system
HP methane LP methane
Nitrogen
Product
Figure 1 Nitrogen removal from natural gas of up to about 20% nitrogen content
Distillation is energy
efficient and the
most economical
solution for large
scale processing of
binary mixtures
4. If the nitrogen content of the
natural gas feedstock is higher than
approximately 3%, then nitrogen
would probably be best removed
upstream of the chemical plant by
cryogenic distillation. At lower
nitrogen levels in the natural gas
feedstock, it may be better to
consider separation of carbon
monoxide and nitrogen down-
stream of the syngas production
step. Whichever of these two sepa-
ration approaches is used,
cryogenic distillation is required as
no other technology can be cost
competitive at the feed gas flows
(much higher than 20 mmscfd) and
product purities required for chem-
icals production.
A key point in terms of environ-
mental protection and legislation
compliance (and not always appre-
ciated) is that the nitrogen removed
from natural gas must be disposed
of, with the obvious choice being
for it to be vented directly to the
atmosphere. For this to be accept-
able, effective separation is needed
and essentially total removal of
hydrocarbons from the vented
nitrogen is required. Methane has a
high global warming potential so
emissions to atmosphere should be
minimised. Technologies other than
distillation have severe limitations
in producing almost pure nitrogen
for venting at a competitive cost
and without being both machinery
intensive and complex. This is
another key reason why very few
non-cryogenic plants have been
built, and those plants that have
been built rely on the nitrogen rich
gas being disposed of in alternative
ways to atmospheric venting.
Cryogenic distillation can easily
achieve minimal hydrocarbon
levels in the nitrogen stream (for
instance, less than 1%) so venting
to atmosphere is acceptable in
terms of environmental compliance.
This also ensures minimal methane
losses and very high hydrocarbon
recovery levels. There are many
cryogenic nitrogen removal plants
around the world operating reliably
to produce specification hydrocar-
bon product with minimal methane
loss to the nitrogen vent, even with
fluctuations in feed gas nitrogen
content. During start-up, natural
www.eptq.com Gas 2016 31
gas leaving the cryogenic section is
recycled to the plant inlet using the
compression system. The separated
nitrogen rich stream is recombined
and compressed with the recycled
gas until normal operating temper-
atures are reached (indicating the
purity of the nitrogen stream will
be acceptable for atmospheric
venting).
Operating temperatures in cryo-
genic nitrogen removal plants
(possibly below -180ºC) are well
below the freezing temperature of
carbon dioxide (-56.6ºC). For this
reason, carbon dioxide in the natural
gas feedstock must be removed, to
as low as 0.5% or lower depending
on the process flowsheet, to avoid
the risk of solids formation and
potential blockages. This can be
achieved by acid gas removal
upstream of the nitrogen removal
plant.1
For natural gas that will be
used as fuel, this acid gas removal
can be a cost burden as the maxi-
mum carbon dioxide content
allowed in the feed gas to the
Delivering whole life,
end-to-end asset
lifecycle solutions
Costain is one of the UK’s
leading engineering solutions
providers, with a focus on the
infrastructure, energy and
water markets. We provide our
customers with integrated
solutions to their complex
business challenges.
Using our extensive knowledge
and experience, we deliver value
to customers’ assets across all
phases of the project lifecycle;
from consultancy and advanced
engineering design, through
complex programme delivery to
operations and maintenance.
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5. 32 Gas 2016 www.eptq.com
gas conditions and possible fluctua-
tions, acceptable nitrogen level in
the product hydrocarbon stream,
optimisation of nitrogen vent
conditions, gas pre-treatment
requirements and choice of gas
compression type and configura-
tion. These plants have proven to
be highly stable in operation and
very reliable, thus meeting the
investment criteria used in first
selecting the process technology.
A typical nitrogen removal plant
is shown in Figure 2. This plant was
designed to process the surplus gas
received at an onshore terminal
that was not used by a local power
plant. Whilst the nitrogen content
(8-11%) was acceptable for use as
fuel gas in power generation, nitro-
gen removal was required to meet
the national transmission system
specifications. The plant was
designed for a normal feed gas
flow of 70 mmscfd, with capacity to
process an increased feed gas flow
of up to 200 mmscfd when the
power station was operated at
reduced capacity.5
Therefore, the
plant was designed with a large
degree of flexibility to allow rapid
operator response to demand
variation.
Natural gas liquids
In an NGL recovery plant, the
removal of propane and heavier
hydrocarbon components will
require operating temperatures of
typically -40°C, with ethane extrac-
tion needing temperatures below
-70°C. If extraction of NGL is
needed as well as the removal of
nitrogen from the natural gas feed-
stock, the two separation duties can
be combined in a single cryogenic
plant. This minimises energy
requirements by making the best
use of cold evaporating process
streams to provide refrigeration to
cool and condense warmer streams.
By reducing plant energy require-
ments, the need for methane
evaporation at low pressure and
thus gas compression, such process
integration can reduce capital cost
and gas processing costs as
compared to having separate
processing plants for NGL extrac-
tion and nitrogen removal. Modern
plant designs and plant control
for their individual needs.
Understanding of these economies
of scale, and that they only apply to
cryogenic distillation, could influ-
ence strategic decisions at national
energy companies and other major
gas producers to ensure an optimal
nitrogen removal strategy and
minimum gas costs for consumers.
Nitrogen removal experience
Design and construction of cryo-
genic distillation units is well
understood and ample experience
is available in the safe design,
supply and operation of plants.
Costain has designed and built
major cryogenic nitrogen removal
plants in the United Kingdom,
Pakistan, Mexico and Tunisia, with
Costain process technology being
used on other large scale nitrogen
removal plants.7
Each plant is
customised to minimise processing
cost, considering the specific feed
cryogenic nitrogen removal plant is
much lower than the maximum
carbon dioxide content for use of the
gas as fuel (typically around 5%).
However, for chemicals production
the carbon dioxide content of the
natural gas feedstock normally
needs to be reduced to low levels
anyway so the additional cost to
provide gas suitable for cryogenic
nitrogen removal is minimal.
Cryogenic nitrogen removal by
distillation benefits from economies
of scale (unlike the other technolo-
gies noted), so costs per unit of gas
processed are lower for larger plant
capacities. The maximum single
train feed gas capacity is about 300
mmscfd for most processing scenar-
ios. The maximum plant size is
generally dictated by the maximum
feasible diameter of distillation
columns due to fabrication and
transport constraints. The gas
processing cost savings with
increased plant capacity are large
enough to possibly encourage
co-operation between companies to
build jointly owned nitrogen
removal plants to minimise nitro-
gen removal costs where the
individual chemical feedstock
needs are much less than about 100
mmscfd. Alternatively, gas produc-
ers could install cryogenic nitrogen
removal facilities and sell the nitro-
gen depleted gas at a premium as
this would be cost-effective for
downstream gas users compared to
each of them installing a relatively
small nitrogen removal plant
Figure 2 Connah’s Quay nitrogen removal plant
Cryogenic nitrogen
removal by
distillation benefits
from economies of
scale, so costs per
unit of gas processed
are lower for larger
plant capacities
6. www.eptq.com Gas 2016 33
with increasing content of nitrogen
need to be profitably exploited, the
need for efficient cryogenic nitro-
gen removal plants will increase.
Historically, nitrogen removal from
natural gas has been applied to
achieve sales gas specifications.
This technology can also be applied
to remove nitrogen to lower levels
upstream of chemical plants to
ensure product quality specifica-
tions can be met economically.
Acknowledgement
The authors thank Terry Tomlinson for his
valuable comments in the preparation of this
article.
References
1 Hydrocarbon Treating, Section 21 of GPSA
EngineeringDataBook, 13th Ed, Gas Processors
and Suppliers Association (GPSA), Tulsa, OK,
2012.
2 Hydrocarbon Recovery, Section 16 of GPSA
EngineeringDataBook, 13th Ed, Gas Processors
and Suppliers Association (GPSA), Tulsa, OK,
2012.
3 World Gasification Database, Gasification &
Syngas Technologies Council (GSTC) Arlington,
VA, 2016. www.gasification-syngas.org/what-
is-gasification/world-database
4 King C J, Separation Processes, 2nd Ed,
McGraw-Hill, NewYork, 1980.
5 Healy M J, Finn A J, Halford L, UK nitrogen-
removal plant starts up,Oil&GasJournal, 1 Feb
1999, 36.
6 Wilkinson D, Johnson G L,AnAbu Dhabi case
study, Hydrocarbon Engineering, Feb 2012, 22.
7 Finn AJ, Rejection strategies, Hydrocarbon
Engineering, Oct 2007, 49.
Jorge Arizmendi-Sánchez is a Principal
Process Engineer with Costain, Manchester,
UK, with process consultancy and engineering
design experience in the oil and gas industry.
He has worked in conceptual design, FEED
and EPC projects in natural gas processing
and liquefaction and holds a MSc from the
University of Ferrara, Italy, and a PhD in
chemical engineering from the University of
Manchester, UK.
Email:jorge.arizmendi-sanchez@costain.com
Adrian Finn is Process Technology Manager
with Costain, Manchester, UK, with
responsibility for process technology selection,
development and commercialisation, proposal
management and supervision of feasibility
studies, pre-FEEDs and FEEDs. He has 33 years
with Costain, focused mainly on cryogenic
gas processing. He holds a bachelor’s degree
in chemical engineering and fuel technology,
a master’s degree in integrated design of
chemical plant, has authored 50 technical
papers and holds nearly 20 granted patents.
Email:adrian.finn@costain.com
systems offer very good perfor-
mance, simplicity, operability and
reliability for such integrated
plants.
In cases where not all of the
available gas needs processing to
remove nitrogen, there may be a
need to remove heavier hydrocar-
bons to meet a gas transmission
hydrocarbon dew point specifica-
tion. This will require chilling of
the gas to maybe -20 to -30°C and
this duty can be easily and effec-
tively incorporated into the
cryogenic nitrogen removal plant.
Helium
Cryogenic nitrogen removal may
also include helium production
where the helium content of the
feed gas is sufficiently high, typi-
cally in excess of 0.2%. Helium
concentrates in the cryogenic distil-
lation system to a level of up to
90% in a crude helium stream prior
to purification and liquefaction.
Helium is produced commercially
in this way in the United States,
Qatar, Russia, Poland, Australia
and Algeria, though in recent years
a lower cost source of helium has
been to process nitrogen rich flash
gas from large scale liquefied natu-
ral gas (LNG) production. This
helium rich stream is available at
-150°C. This process route has been
exploited on LNG plants, in Qatar
in particular, to produce pure
helium at relatively low cost.
Enhanced oil recovery
Production of a pure nitrogen
stream by cryogenic distillation
means the nitrogen can potentially
be used for enhanced oil recovery
(EOR). Nitrogen is normally
produced from a cryogenic nitro-
gen removal facility at low pressure
(for venting), whereas EOR requires
high pressure nitrogen. In this situ-
ation, nitrogen compression costs
can dictate the nitrogen removal
plant design and promote flow-
sheet options, which reliably and
cost-effectively produce nitrogen at
elevated pressure.
Conclusion
Cryogenic nitrogen removal tech-
nology has been successfully used
for many years. As new gas fields
