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Review Article
Polymeric Gas-Separation Membranes for Petroleum Refining
Yousef Alqaheem, Abdulaziz Alomair, Mari Vinoba, and Andrés Pérez
Petroleum Research Centre, Kuwait Institute for Scientic Research, Ahmadi, Kuwait
Correspondence should be addressed to Yousef Alqaheem; yqaheem@kisr.edu.kw
Received November ; Accepted January ; Published February
Academic Editor: Eliane Espuche
Copyright © Yousef Alqaheem et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Polymeric gas-separation membranes were commercialized years ago. e interest on these systems is increasing because of the
simplicity of concept and low-energy consumption. In the renery, gas separation is needed in many processes such as natural gas
treatment, carbon dioxide capture, hydrogen purication, and hydrocarbons separations. In these processes, the membranes have
proven to be a potential candidate to replace the current conventional methods of amine scrubbing, pressure swing adsorption,
and cryogenic distillation. In this paper, applications of polymeric membranes in the renery are discussed by reviewing current
materials and commercialized units. Economical evaluation of these membranes in comparison to traditional processes is also
indicated.
1. Introduction
Implementation of membrane systems is growing in the
industry because of the unique features that the membrane
can provide []. Compared to other separation processes, the
membrane is simple to install and requires minimum super-
vision []. Furthermore, it occupies less space and does not
have moving parts; thus it needs almost no maintenance [].
In addition, it operates with low energy and is considered as
an environmentally friendly technology because it does not
emit gases nor work with solvents []. e membrane is also
easy to scale up for better commercialization [].
Based on the material, the membranes are categorized
into metallic, inorganic, and polymeric []. Metallic mem-
branes made of platinum or palladium have excellent per-
formance but the cost of precious metals greatly inuences
the membrane selection. Inorganic membranes are good
alternatives and they have better chemical stability with lower
fabrication cost []. Nevertheless, high temperature of to
∘C is needed to operate inorganic membranes []. Nowa-
days, polymeric membranes dominate the industry because
of the outstanding economy and competitive performance
[]. e membranes can be operated at ambient temperature
and they have good mechanical and chemical properties [].
Revolution of polymeric membranes started in s
when Loeb and Sourirajan developed a membrane made
from cellulose acetate for water desalination by reverse
osmosis []. e thin membrane of . mwassupported
on a porous substrate and it was capable of converting
seawater to potable water. ey found later that cellulose
acetate membrane can be used for gas separation as well [–
]. Aerwards, Stern et al. in studied the diusion of
dierent gases such as helium and nitrogen in polyethylene
membrane at high temperatures and this opened the oppor-
tunity for more research in this area [].
e rst large scale membrane was developed by Permea
(Air Products) in for separation of hydrogen. e
hollow ber membrane was made of polysulfone and it
was designed to separate hydrogen from methane [, ].
In , Cynara and Separex also manufactured a cellulose
acetate membrane but for carbon dioxide separation from
methane []. A few years later, nitrogen production from
air using membranes was introduced []. e applications
of membrane were expanded hereaer to cover removal of
hydrogen sulde from methane, removal of volatile organic
compounds (VOCs) from air, oxygen enrichment, and air
dehydration []. Today, the membrane is used in the ren-
ery to purify natural gas by removing acid gases such as
hydrogen sulde and carbon dioxide from methane []. It
is also implemented in many hydrotreatment processes to
recover hydrogen from hydrogen sulde []. Adjustment
of hydrogen-to-carbon monoxide ratio in syngas to meet
Hindawi
International Journal of Polymer Science
Volume 2017, Article ID 4250927, 19 pages
https://doi.org/10.1155/2017/4250927
International Journal of Polymer Science
T : Processes where membrane technology is implemented
[–].
Process Gas to be separated from
Natural gas purication
H2S/CH4
CO2/CH4
H2O/CH4
C3+/CH4
Hydrocracker H2/light hydrocarbons
Hydrotreatment H2/H2S
Steam-methane reforming H2/CO
Ammonia plant H2/N2
Polyolen plant VOCs/N2
Renery waste-gases
VOCs/Air
H2from other gases
CH4from other gases
CO2from other gases
the requirement of petrochemical feedstock can be done
using the membranes. Oxygen enrichment in furnaces for
better oxidation is also practiced in many processes [].
Applications of the membrane for petroleum industry and the
corresponding separation gases are presented in Table . In
this review, uses of these membranes are discussed in detail
including the membrane materials, commercialized systems,
and comparison with traditional separation methods. In the
following section, transport mechanism of these membranes
is given.
2. Transport Mechanism in
Polymeric Membranes
For gas applications, the polymeric membranes are usually
made from a thin, dense layer []. To enhance the mechan-
ical properties, the dense layer is supported on a porous
substrate []. e widely accepted theory for the transport
mechanism is based on solution diusion model []. is
model consists mainly of three steps: (1) absorption of
molecules on the polymer surface, (2) diusion of molecules
inside the polymer, and (3) desorption of molecules on
the low-pressure side []. e driving force is the pressure
gradient across the membranes, and each compound has
dierent absorption and diusion rate. e membrane per-
formancecanbeevaluatedbythemeasuringthepermeability
and selectivity of gases. e permeability is the product of
absorption and diusion coecients as follows:
=
𝑖𝑖,()
where 𝑖is the sorption coecient and 𝑖is the diusion
coecient. e unit of permeability is Barrer that equals −10
(cm3/cm⋅s⋅cmHg). Experimentally, the permeability can be
calculatedbasedontheux[]:
=
,()
where is the ux (volume ow rate per unit area), is the
membrane thickness, and is the pressure dierence across
the membrane. On the other hand, selectivity (𝑖𝑗) refers to
permeability ratio of two gases:
𝑖𝑗 =𝑖
𝑗.()
e polymers are classied based on the structure to rubbery
and glassy. Rubbery polymers have the ability to return to
their original shape once stretched while glassy ones do not
[]. Furthermore, rubbery polymers tend to have higher
permeation but lower selectivity and this is because the
transport mechanism is controlled by absorption rather than
diusion []. Conversely, glassy membranes have higher
selectivity but low permeation because they are diusion
limited. is indicates that there is a trade-o between per-
meability and selectivity and it is dicult to have a polymer
having both characteristics. In the following section, uses of
membranes for hydrogen sulde separation, carbon dioxide
recovery, hydrogen purication, air separation, gas dehydra-
tion, organic vapors recovery, and liqueed petroleum gas are
discussed in detail.
3. Removal of Hydrogen Sulfide
Hydrogen sulde is well known for its rotten-egg smell even
in low concertation of parts per billion (ppb) []. e gas is
emitted naturally from volcanoes and can be formed during
the decomposition of organic matters []. e gas is also
found in natural gas and it is called sour gas if hydrogen
sulde concertation is above ppm []. Because the gas is
corrosive and can cause damage to pipelines, the sale gas
shouldnothavemorethanppmofhydrogensuldeand
mol% of carbon dioxide []. Hydrogen sulde is a man-
made gas too, and dehydrosulfurization process (to remove
sulfur compounds from fuel) is considered as the main source
[].
3.1. Current Technologies. ere are three methods for hydro-
gen sulde removal: (1) physical/chemical absorption, (2)
adsorption, and (3) membranes. Chemical absorption by
amine scrubbing is the dominant process for hydrogen sulde
separation from natural gas []. e process can remove
carbon dioxide as well and the treated stream can have lower
than ppm of hydrogen sulde. e technology is based on
the absorption of hydrogen sulde and then the reaction with
amine by []:
2RNH2+H2S←→ RNH32S()
RNH32S+H2S←→ 2RNH3HS ()
e solvent (mainly monoethanolamine, MEA) can be regen-
erated by increasing the temperature or reducing the pres-
sure. In spite of the high eciency of amine scrubbing, there
are some drawbacks, which are (1) high capital investment,
(2) massive energy required to regenerate the solvent, (3)
oxidation of amines which can cause foaming or ooding,
and (4) requirement of special alloys to withstand the solvent
corrosivity [–].
International Journal of Polymer Science
Physical absorption by methanol is another way to rem-
ove hydrogen sulde from natural gas. e process is called
Rectisol (licensed by Linde Group and Air Liquide) and it
can remove carbon dioxide, carbonyl sulde, and mercap-
tans []. At lower temperature, the absorption capacity of
methanol increases and that is why the process operates at
− to −∘C []. It should be mentioned that methanol
can be replaced with other solvents like polyethylene glycol
(Selexol process) or potassium carbonate, but methanol
has better absorption capacity and higher regeneration rate
[, , , ]. Compared to amine scrubber, methanol
absorption has better removal eciency but at the expense
of capital and operating costs [].
Adsorption by carbon molecular sieve (CMS) is another
technique to separate hydrogen sulde from methane. e
concept is based on adsorption of hydrogen sulde on the
carbon surface at high pressure []. Activated carbon has
large surface area with high porosity, and the capacity can
reach mg of hydrogen sulde to one gram of carbon
[]. e desorption (regeneration) step can be performed by
reducing the pressure or increasing the temperature to –
∘C[].Unfortunately,CMScannotbeusedtoremove
high content of hydrogen sulde of more than . mol% due
to the lower adsorption capacity compared to amine scrubber
[]. Furthermore, carbon suers from low mechanical prop-
erties making it unstable at high content of hydrogen sulde
[].
e membrane technology can provide an alternative
solution for removal of hydrogen sulde. Unlike amine
scrubbing or methanol absorption, the membrane does not
require a solvent to operate, and this will cut down the cost
of purchasing and disposing of the solvent. e membrane
has also an advantage over CMS as it can operate with
feeds containing up to mol% of hydrogen sulde []. In
the following section, performances of dierent membrane
materials are reviewed for removal of hydrogen sulde from
natural gas.
3.2. Membrane Materials. Cellulose acetate is widely used
for hydrogen sulde removal from natural gas []. is
material is extracted from wood pulp and it has a hydrogen
sulde permeability of . Barrer with hydrogen sulde to
methane selectivity (H2S/CH4)of.[].ematerialwas
tested with natural gas containing heavy hydrocarbons, and,
unexpectedly, the selectivity dropped signicantly due to the
penetration of sorption sites [, ].
Polydimethylsiloxane (PDMS) gives a superior hydrogen
sulde permeability of Barrer, and this high permeabil-
ity is related to the rubbery structure but at the expense of
selectivity of . []. To enhance the membrane durability
under the harsh environment of hydrogen sulde, cross-
linking was introduced []. It helps in reducing the poly-
meric chain mobility and this increases the glass transition
temperature. As a result, resistance to plasticization and
aging is improved []. In addition, cross-linking generally
aects the segmental mobility of the polymer making the
diusion process rely more on the size and shape of the
molecule to be separated and this improves the selectivity but
reduces permeability []. In , Chatterjee et al. developed
a copolymer consisting of ether, urethane, and urea and it
was prepared by the two-step polycondensation technique.
In the rst step, methylene bis-(-phenyl isocyanate) (MDI)
is added to polyethylene glycol (PEG) with the use of
dimethyl sulfoxide (DMSO) as a solvent. In the second
step, a chain extender (,-diaminoethane) was added to
the solution to form poly(ether-urethane-urea) (PUU) [].
Unlike membranes made of single polymer, PUU consists of
two segments: so and hard. e hard segment has a glassy
state and acts as a ller while the so segment is rubbery
giving the membrane elasticity and exibility []. PUU was
tested for hydrogen sulde separation from methane, and the
permeability was Barrer with outstanding selectivity of
[].
Pebax is another copolymer made of polyether and poly-
amide. e term “Pebax” stands for polyether-block-amide
and it was manufactured by Arkema []. ere are many
grades of Pebax and each grade depends on the concentration
of polyether and polyamide. For example, the popular Pebax
is made from wt% polyether and wt% polyamide
[]. Hydrogen sulde permeability of this material reached
Barrer with selectivity of []. Permeability and
selectivity of various membrane materials is given in Table .
e choice of material depends strongly on the composition
of the feed gas and whether permeability or selectivity is the
rst priority.
3.3. Case Studies and Economical Evaluation. Membrane
Technology and Research (MTR) is one of the companies
for manufacturing gas-separation membranes. SourSep (by
MTR) is a membrane system to convert sour gas to sweet
gas by the removal of hydrogen sulde and it is expected to
bebasedonPebax.eunitwasinstalledinanoilwellina
remote area to treat wellhead gas so it can be used as a fuel
[]. Indeed, the system reduced hydrogen sulde content
from ,ppm to less than ppm. e feed pressure was
at bar and volume ow rate was Nm3/h. Compared to
amine scrubber, the membrane system achieved lower capital
and operating costs. e capital cost covers the membrane
material, frame, heat exchanger, and vacuum pump while
operating cost refers to energy used by compressors and
pumps []. FuelSep is another system developed by MTR and
designed to meet the quality of fuel gas by removing hydrogen
sulde and other impurities such as carbon dioxide, nitrogen,
and heavy hydrocarbons [].
Universal Oil Products (UOP) Separex membrane is
basedoncelluloseacetateanddesignedtotreatgasescon-
taining hydrogen sulde up to mol% []. e system was
commercialized for nearly years. e system was installed
in an o-shore gas reservoir containing mol% of hydrogen
sulde. e feed volume was , Nm3/h of gas at
bar. e membrane was capable of reducing hydrogen sulde
content to ppm in the treated gas.
An economical study was conducted by Bhide and Stern
for natural gas treatment using membranes and amine
scrubbing []. e membranes were based on cellulose
acetate, and content of hydrogen sulde varied from .
to mol%. e feed also contained carbon dioxide of to
mol%. Feed ow rate was , Nm3/h at bar. e
International Journal of Polymer Science
T : Permeability and selectivity of dierent polymeric membranes for removal of hydrogen sulde from natural gas.
Material H2S(Barrer) H2S/CH4T(∘C) P(bar) Ref.
Polyamide (Torlon) . . . []
Cellulose acetate . . []
Polyamide (F-PAI-) . . []
Polyamide (F-PAI-) . . []
Polyamide (F-PAI-) . . . []
Polyether-block-amide (Pebax ) . []
Polyether-urethane-urea (PUU) []
Polyether-urethane-urea (PUU) []
Polyether-block-amide (Pebax ) . []
Polyether-urethane-urea (PUU) []
Polyvinylthrimethilsilane (PVTMS) . = 1 []
Polyether-urethane-urea (PUU) []
Polyether-block-amide (Pebax ) []
Dimethyl silicone rubber . []
Polydimethylsiloxane (PDMS) . []
processing cost (dened as the capital and operating costs
over production volume) was calculated to achieve less than
ppm of hydrogen sulde and mol% of carbon dioxide. It
was found that the processing cost in a membrane system
is a function of the concentration of hydrogen sulde and
carbon dioxide; the more the content, the higher the cost,
but in amine scrubbing the processing cost was dependent on
carbon dioxide content only. For a stream containing mol%
hydrogen sulde and mol% carbon dioxide, the processing
cost for a membrane system was . ×−7 /(Nm3/h) com-
pared to . ×−7 /(Nm3/h) for amine absorption.
erefore, the use of membrane resulted in % reduction
in processing cost. However, if the feed was changed to
mol% carbon dioxide and ppm hydrogen sulde,
both systems had a processing cost of . ×−7 /(Nm3/h).
Amine scrubbing showed lower processing cost of . ×
−7 /(m3/h) if the feed contained % carbon dioxide and
ppm hydrogen sulde while the membrane gives . ×
−7 /(Nm3/h).
4. Carbon Dioxide Capture
e atmosphere consists before of ppm carbon dioxide
( reading), but because of the transportation and indus-
trial activities, the content is increased to ppm causing
global warming and increase in the climate temperature [].
e petroleum industry accounts for % of carbon dioxide
emissionandinordertoreducetheimpact,carboncapture
from ue gases is necessary [].
In the renery, separation of carbon dioxide is required
in the following processes: natural gas treatment, syngas pro-
duction (hydrogen and carbon monoxide), and combustion.
Today, wells are injected with a high-pressure carbon dioxide
stream to enhance the oil recovery, and this results in pro-
duction of natural gas with high amounts of carbon dioxide
[]. Removal of this carbon dioxide is essential because the
gasiscorrosiveandcandamagepipelines[].emaximum
content of carbon dioxide in commercial natural gas should
notexceedmol%[].Furthermore,theuegasesofmost
combustion processes (furnaces) have amounts of carbon
dioxide and nitrogen. Carbon dioxide capture is necessarily
before releasing this gas to the atmosphere [].
4.1. Current Technologies. Most of the techniques for hydro-
gen sulde removal work as well for carbon dioxide because
both gases are polar. e dominant method for carbon diox-
ide removal from natural gas is still amine scrubbing [].
e process can remove bulk quantities of carbon dioxide
and the nal stream can have as low as ppm of carbon
dioxide []. Physical absorption by water, polyethylene
glycol,methanol,andpotassiumcarbonateispossibleto
separate carbon dioxide. In water absorption, the gas enters
a packed tower where carbon dioxide dissolves in water
and the concentrated stream is stripped by air to generate
carbon dioxide back and water is recycled. e process is
cost eective because water is readily available; however the
recirculated water can cause fouling; therefore special piping
is needed []. Polyethylene glycol (PEG) on the other hand
has better selectivity compared to water and is considered as a
noncorrosive solvent []. e drawback of using PEG is the
low regeneration rate [].
Hot potassium carbonate is ecient for removing large
amounts of carbon dioxide. e process can also remove
small amounts of hydrogen sulde. e mechanism is based
on the reaction of carbon dioxide with potassium carbonate
solution []:
K2CO3+CO2+H2O←→ 2KHCO3()
e carbon dioxide-enriched stream enters an absorber
whereitowsinacounter-currentwithahotpotassium
carbonate solution at ∘C []. e solution is then sent to
aashdrumwheremostoftheacidgaswillberemoved
due to the reduction of the pressure. To regenerate the
solvent, it is sent to a stripper that operates at ∘Cand
International Journal of Polymer Science
T : Current technologies for carbon dioxide separation.
Technology Advantages Disadvantages
Chemical and
physical
absorption
(i) No need for pretreatment.
(ii) Can treat wider range of CO2.
(iii) High removal eciency.
(i)Highcapitalandoperatingcosts.
(ii) Regeneration of solvent.
PSA
(i) Does not involve a solvent.
(ii) Better stability toward impurities in the
feed.
(i) Low solid-to-gas capacity.
(ii) Low regeneration rate.
(iii) Pressure cycle is energy-intensive.
Cryogenic
distillation
(i) Achieves >% of CO2capture.
(ii) Produces liqueed CO2for easier storage.
(i) Economical only if the feed contains –% CO2.
(ii) Higher pressure is required to avoid CO2sublimation.
Membranes
(i) Requires minimum supervision.
(ii) Can remove H2SandH
2Oaswell.
(iii) Long-operating life (>years).
(i) High capital cost.
(ii) Pretreatment is required to remove particulates and some inhibitors.
atmospheric pressure. Unfortunately, potassium carbonate
has lower sorption properties compared to amine and it is
highly corrosive [, ].
Methanol can also be used for physical absorption of
carbon dioxide and it has the highest selectivity compared to
other solvents []. e solvent can be regenerated by either
reducing the pressure or increasing the temperature []. e
nal stream can have very low amounts of carbon dioxide
of ppm, which is more ecient than amine scrubbing.
e only disadvantage of this process is the high capital
investment [].
Pressure swing adsorption (PSA) is another technique
for carbon dioxide separation. Unlike previous methods,
PSA does not require a solvent. e gas passes at a high
pressure through a bed of activated carbon (also known as
carbon molecular sieve), and due to the dierence in polarity
adsorption of carbon dioxide will take place []. e bed
can be regenerated by reducing the pressure to vacuum.
e technique has an excellent separation performance, and
the gas can have more than % methane purity and it is
expected to run for three years []. Other PSA materials
are zeolite and alumina. Disadvantages of this system are
the extensive energy for pressure cycle and low adsorption
capacity compared to amine scrubbing [].
Cryogenic distillation at very low temperature of −∘C
is ecient for carbon dioxide removal. Because of the low
triple point of carbon dioxide of −∘Catatmospheric
pressure,carbondioxidewillnothavealiquidstateandwill
solidify directly []. erefore, the distillation should take
placeatapressureabovebartoovercomethetriplepoint
limitation; otherwise, carbon dioxide will cause blockage.
e technology is used to liquify and produce high quality
streams of carbon dioxide. For the process to be economical,
the feed should contain to % carbon dioxide, and this is
because of the high capital and operating costs of cryogenic
distillation []. Unfortunately, most of the renery streams
do not have that concentration of carbon dioxide [].
In comparison with the above-mentioned, the mem-
branes have a unique feature as they can remove carbon
dioxide along hydrogen sulde and water with one step [,
]. In addition to low operating energy, the membrane has a
longlifeanditcanbeoperatedcontinuouslyforatleastyears
[]. However, the operating life is greatly aected if partic-
ulates were presented in the feed; therefore pretreatment is
needed. Table shows the advantages and disadvantages of
dierent methods for carbon dioxide capture.
4.2. Membrane Materials. Removal of carbon dioxide started
whenRobbstudiedinthediusionofgasesinPDMS
membrane []. e work was expanded in when Stern
determined the permeability coecient of gases at higher
temperature []. CO2-permeable membranes are similar to
thosethatpermeatehydrogensulde,butthepermeabil-
ity diers due to the dierence in sorption and diusion
coecients between carbon dioxide and hydrogen sulde.
e state-of-the-art materials for carbon dioxide separation
are cellulose acetate, polyamide, polyimide, and Pebax. As
showninTable,celluloseacetatehasthelowestpermeability
of.Barrerbutyettheselectivityofcarbondioxideto
methane (CO2/CH4) reached [, ]. Unfortunately, pre-
sence of heavy hydrocarbons in the feed caused a sig-
nicant drop in the selectivity; therefore cellulose acetate
was not suitable for fuel gas separation []. Polyimides
on the other hand show better thermal and chemical sta-
bilities compared to cellulose acetate []. ese polymers
are made from diacid with diamine in amic acid inter-
mediate []. Matrimid is a polyimide containing
phenylindane group and it gives carbon dioxide permeabil-
ityof.Barrer[,].ispolymershowsoutstanding
selectivity of and . for carbon dioxide to methane
(CO2/CH4) and carbon dioxide to nitrogen (CO2/N2), respec-
tively [, ]. Carbon dioxide permeability of polyimide
canbefurtherenhancedbytheintroductionofuo-
ride. Fluorinated polyimides are made using ,-bis(,-di-
carboxyphenyl)hexauoropropanedianhydride (FDA), and
the permeability can be boosted to Barrer [, ].
Copolymers like PUU and Pebax show also high permeability
of and Barrer, respectively [, ]. e rubbery
polymer PDMS has an excellent permeability of Barrer
but the lowest carbon dioxide selectivity of ., as given in
Table .
4.3. Commercial Units and Economical Evaluation. e
largest CO2-removal unit is manufactured by Cynara
International Journal of Polymer Science
T : Permeability and selectivity of dierent polymers for carbon dioxide removal.
Material CO2(Barrer) CO2/CH4CO2/N2T(∘C) P(bar) Ref.
Cellulose acetate . . – [, ]
Polyamide (Nylon ) . . . – []
Polyimide (Matrimid ) . . – - [, ]
Polysulfone (PSF) . . . []
Polycarbonate . . . []
Polyimide (FDA-TBAPB) . . []
Poly(,-dimethylphenylene oxide) (PPO) . . — []
Polyethylene glycol (PEG) . . []
Polyether-urethane-urea (PUU) . . []
Polyether-block-amide (Pebax ) . []
Dimethyl silicone rubber . . []
Polyimide (FDA-durene) . []
Polytetrauoroethylene (Teon AF ) . . . []
Polydimethylsiloxane (PDMS) . . – []
(NATCO Group) for natural gas sweeting in an o-shore
area in ailand. e hollow ber membrane is based on
cellulose triacetate and capable of handling , Nm3/h
[]. Another system was installed to treat , Nm3/h of
gas and it reduced carbon dioxide content from % to less
than % [, ].
Polaris membrane (made by MTR) was installed aer
methane-reforming unit and it successfully increased carbon
dioxide concentration from mol% in the tail gas to more
than mol% []. e stream was used aerwards for well
injection to enhance oil recovery. Polaris membrane can also
treat ue gases with excellent selectivity (CO2/N2)of[].
UOP membranes are based on cellulose acetate and were
installedinPakistanin.esystemworkedcontinuously
for years to cut down carbon dioxide concentration from
. to mol% []. e system was designed to process
, Nm3/h of gas at bar.
UBE on the other hand developed a robust membrane for
better stability under feed impurities. e system is based on
polyimide membrane and it can work without any drop in
performance under the presence of mol% hydrogen sulde,
full water saturation, and heavy hydrogen carbons of C5+
[].
An economical study was done by Peters et al. to compare
themembranesystemwithaminescrubbingfornaturalgas
purication []. e feed gas contained . mol% CO2,
ppm H2S, ppm H2O and . mol% CH4and the
remaining for C2to C6. e operating conditions were ∘C
and bar. Results show that both technologies achieved
the sale gas specication of ppm H2Sandmol%CO
2;
however, the treated gas by amine has better carbon dioxide
purity compared to the membrane but this was at the expense
of the capital investment. It was concluded that the membrane
technology was still a better choice due to the environmental
issue related to solvent disposal.
Another economical evaluation was performed by He et
al., and it conrmed that the membrane can replace amine
scrubbing for natural gas treatment containing mol%
carbon dioxide and lower []. Natural gas processing cost by
the membrane system was . /Nm3, which was .%
less than amine scrubbing.
5. Hydrogen Recovery
Hydrogen is a key element for many processes in the renery
such as hydrocracking and hydrotreating. In hydrocracking,
hydrogen is used to convert large hydrocarbons into smaller
ones in presence of a catalyst, while in hydrotreating hydro-
genisusedtoremovesulfurcompoundsfromfuelsinthe
form of hydrogen sulde []. Furthermore, hydrogen is a
feedstock for many industries like ammonia synthesis and
methanol production [].
Hydrogen is produced in the renery by steam-methane
reforming (SMR) where methane reacts with water to pro-
duce hydrogen and carbon monoxide. e produced gas is
called syngas, and hydrogen yield can be further increased
by the reaction of carbon monoxide with water to form
hydrogen and carbon dioxide [].
In petroleum industry, hydrogen separation can be prac-
ticed in the following processes: (1)to recover some hydrogen
during natural gas production, (2) to adjust hydrogen-to-
carbon monoxide ratio (H2/CO) in syngas, (3) to recycle
part of hydrogen from hydrocracker and hydrotreatment tail
gases, (4) to separate hydrogen from nitrogen in ammonia
plant, and (5) to purify hydrogen so it can be used as a
feedstock for other industries [–]. Content of hydrogen
in renery o-gases is given in Table .
5.1. Current Technologies. Mainly, there are three methods
to separate hydrogen from gas mixtures: (1) cryogenic dis-
tillation, (2) PSA, and (3) membrane system. e selection
of technology depends on feed composition, product purity,
product ow rate, reliability, turndown, and last but not least
capital and operating costs. Comparison between the three
technologies is given in Table . As indicated, the membrane
has a better capability to treat a wider range of hydrogen from
International Journal of Polymer Science
T : Hydrogen composition in renery o-gases [].
Process Hydrogen content (vol%)
Catalytic reforming –
ermal hydrodealkylation –
Hydrocracking –
Hydrotreating –
Catalytic cracking –
to mol%. PSA comes rst for the product purity of
over mol% and cryogenic distillation is favorable to handle
large volumes of , Nm3/h and above. Furthermore, the
membrane provides the best reliability where unexpected
shutdown occurs. is is because the membrane does not
have mechanical parts whereas cryogenic distillation has the
lowest reliability. Turndown refers to a small change in the
operating condition and the membrane system is proven to
be the most stable. For example, a change in the feed pressure
can reduce the product purity in the membrane system by
%, while PSA and cryogenic can be aected by and %,
respectively.
5.2. Membrane Materials. e rst application of gas-separa-
tion membranes was for hydrogen removal. It was developed
in s by Monsanto (Air Products) to recover hydrogen
from purge gas in ammonia plant [–]. e spiral-
woundmembranewasbasedonpolysulfoneandithas
a permeability of Barrer. Cellulose acetate membranes
were introduced then by Separex and they showed a better
permeability and stability; therefore they were employed for
removal of hydrogen from natural gas []. e permeability
was greatly improved from to Barrer when cellulose
acetate was used instead of polysulfone. For adjustment of
H2/CO ratio in syngas, polyimide (made by UBE) gave a
better permeation of Barrer with superior selectivity of
H2/CH4(), H2/CO (), and H2/N2() []. ough
PDMS gives maximum hydrogen permeability of Barrer,
ithasalowH
2/CH4selectivity of unity making it unsuitable
for hydrogen separation from natural gas. Furthermore, it is
reported that performance of PDMS membrane signicantly
drops if carbon monoxide was presented in the feedstock
[]. Table shows hydrogen permeability and selectivity of
dierent membrane materials.
5.3. Commercial Units and Economical Evaluation. e
world-leading companies for hydrogen-permeable mem-
branes are Air Products, MTR, UOP, GENERON, Praxair,
and UBE. PRISM membrane (based on polysulfone and
developed by Air Products) is able to recover to mol%
of hydrogen from purge gas in ammonia plant []. e
membrane can also upgrade hydrocracker o-gas stream
containing – mol% hydrogen to – mol% in a single
stageortomol%bytwostages[].esystemisexpected
to run for years without any interruption.
VaporSep membrane manufactured by MTR can recover
hydrogen from renery waste gases. e system can also
be used to adjust H2/CO ratio in syngas to meet the feed
requirement for dierent industries. e system can handle
a feed pressure up to bar with dierent concentrations
of – mol% of hydrogen with a maximum volume of
, Nm3/h.epermeateisestimatedtohaveahydrogen
purity of – mol% []. e system was installed in a
Korean renery to recover hydrogen from a hydrocracker o-
gas,andtheunitimprovedtheprocesseconomyandpaid
itself aer only one month of operation.
UOP PolySep is another membrane for hydrogen produc-
tion that can treat renery o-gases. e membrane operates
at temperatures of to ∘C with feed pressures of to
bar. Compared to VaporSep, PolySep can handle larger
volume of , Nm3/h. e permeate pressure ranges from
to bar with hydrogen recovery of –% [].
Hydrogen recovery is considered economical if the waste
gas contains mol% hydrogen or more []. Other wise,
production of hydrogen by SMR will be a better choice
rather than separation. A study was performed by Mivechian
and Pakizeh to evaluate the feasibility of using a membrane
system to separate hydrogen from renery o-gas containing
mol% hydrogen with light hydrocarbons (C1–C6). e
membrane was based on polyimide and it showed a better
recovery of % compared to % using PSA. e membrane
also achieved a hydrogen purity of . mol%, which is close
to PSA of . mol%. e capital cost was almost the same for
both the membrane system and PSA [].
6. Air Separation
Air contains . mol% of oxygen and . mol% of nitrogen,
and the remaining is for other gases such as argon and
carbon dioxide. An increase in oxygen content (> mol%)
in the feed can improve the oxidation process due to the
higher ame temperature. is raise in temperature is directly
related to the reduction in nitrogen content in the feed [].
Idea of using enriched oxygen for Claus process was initiated
in s and then fully commercialized in in Lake
Charles Renery (US) by Goar Allison and Air Products [].
Aer hydrotreatment, the sulfur-enriched gas is sent to Claus
process to recover hydrogen sulde in the form of solid sulfur.
e concept of Claus process is based on oxidizing hydrogen
sulde to sulfur and water:
H2S+1
2O2→ S+H2O()
Because air is used to oxidize hydrogen sulde, presence of
nitrogenlowerstheametemperatureandthiscouldresult
in the formation of ammonia salts too. ese salts cause a
pressure drop in the system. Use of enriched oxygen instead
of air can greatly improve the capacity of sulfur removal and
prevent salt formation. For example, use of mol% oxygen
can increase sulfur capacity up to % []. Furthermore, use
of % oxygen nearly doubles the sulfur capacity.
Oxygen enrichment can be benecial for uid catalytic
cracking (FCC) unit as well. is unit is used to break
down large hydrocarbons (usually vacuum gas oil) to useful
products such as gasoline and diesel. e feed is rst heated to
–∘C and then enters a reactor where it gets in contact
with a catalyst []. e catalyst is then regenerated thermally
(to remove coke) by burning it with air. However, studies
International Journal of Polymer Science
T : Comparison between current technologies for hydrogen recovery [].
Category Cryogenic distillation PSA Membrane
Feed composition (H2mol%) – – –
Product purity (H2mol%) – > –
Product volume (Nm3/h) >, –, <,
Reliability (%) Poor
Turnd ow n ( % ) –
T : Hydrogen permeability and selectivity of various membrane materials.
Material H2(Barrer) H2/CH4H2/CO H2/N2T(∘C) P(bar) Ref.
Polyimide (Matrimid ) . [, ]
Polysulfone – — [, ]
Polyethylene . — . []
Polystyrene — []
Cellulose acetate – — []
Polyetherimide .–. []
Polyimide (BPDA-based) — []
Dimethyl silicone rubber . . . []
Poly(,-dimethylphenylene oxide) (PPO) — — []
Polydimethylsiloxane (PDMS) — . – []
show that when mol% of oxygen is used, the capacity of
regenerating the catalyst increases by to %. In addition,
useofenrichedoxygeninfurnacescanreducenitrogen
compounds (NO𝑥) and this will reduce the emissions [].
6.1. Current Technologies. Idea of using enriched oxygen in
furnaces was practiced since s for iron production
by cryogenic distillation []. e process gives ultra-pure
oxygen (>. mol%) by compressing air and then cooling it
to a very low temperature below −∘Cusingarefrigeration
cycletoliquifyair.Aerthat,itissenttoadistillationtower
whereoxygenleavesintheformofliquidandnitrogeninthe
form of gas due to the dierence in boiling point [].
PSA by zeolite can produce enriched oxygen within the
rangeoftomol%oxygen[].Actually,bothoxygen
andnitrogenwillbeadsorbedonzeolitebutnitrogenhasa
higher adsorption rate; thus the gas passing through zeolite
will have a higher content of oxygen. Unfortunately, due to
low adsorption rate of .–. mol oxygen per one mol of
sorbent, the process is not widely used [].
Polymericmembraneisanalternativetechnologyfor
air separation. e technology has an advantage over cryo-
genic distillation as it does not require cold temperatures.
Furthermore, the membrane does not need a regeneration
step same as PSA. It is worthwhile to mention that ceramic
membranes made of ionic-electronic conducting materials
are capable of producing oxygen with % purity []. e
mechanism is based on oxygen vacancies that are created
at temperature of ∘Candabove[].Unfortunately,the
technology is not yet commercialized due to many issues
relatedtosealingandinstabilityduetopresenceofimpurities
in the feedstock making the polymeric membrane a solid
choice at the moment [–].
6.2. Membrane Materials. Use of polymeric membranes for
oxygenenrichmentstartedinsanditshowedpromising
results compared to cryogenic distillation and PSA [].
e selection of membrane material relies on the selectivity
toward nitrogen (O2/N2). It is stated that a selectivity of
at least is needed for the membrane to compete with
other technologies []. List of materials meeting these cri-
teria is cellulose acetate, polysulfone, polyamide, polyimide,
polyetherimide, and poly(-methyl--pentene) (TPX) [, ,
,,].AsgiveninTable,polyetherimideshowsthe
highest selectivity of . yet lowest oxygen permeability of
. Barrer. Polysulfone (PSF) has a better permeability of
. Barrer with very good selectivity of . and it is used in
fabrication of many commercial units []. Poly(-methyl-
-pentene) (TPX) is also used commercially and it has a
permeability of Barrer and good selectivity of [].
6.3. Commercial Units and Economical Evaluation. UOP dev-
eloped a membrane called SPIRAGAS that produces a stream
containing mol% of oxygen from air []. e membrane
is based on a porous polysulfone coated with silicone and
it has a spiral-wound module. It operates at ∘C, and the
product ow rate can reach up to . Nm3/h with feed
pressure varying from to . bar. GENERON on the other
hand fabricated a membrane based on TPX and it gives a
higher oxygen content up to mol% [].
Moreover, AVIR membrane (manufactured by A/G Tech-
nology Corporation) can produce to mol% of oxygen-
enriched air []. It should be mentioned that the mem-
branes in Table also produce a nitrogen-enriched stream
in the retentate. For example, PRISM hollow ber membrane
(based on PDMS and made by Air Products) produces not
International Journal of Polymer Science
T : Oxygen and nitrogen permeabilities of dierent polymeric materials.
Material O2(Barrer) N2(Barrer) O2/N2T(∘C) P(bar) Ref.
Polyetherimide . . . — []
Polysulfone (PSF) . . . — []
Polycarbonate . . . — []
Cellulose acetate . . . — []
Polystyrene . . . []
Polyimide (Matrimid ) . . . []
Polyvinyl acetate (PVA) . . . []
Polyamide . . . []
Polyimide (FDA-based) . []
Polyphenylene oxide (PPO) . . . — — []
Natural rubber . . — []
Poly(-methyl--pentene) (TPX) . . — — []
Dimethyl silicone rubber . []
Polydimethylsiloxane (PDMS) . – []
Poly(-trimethylsilyl--propyne) (PTMSP) . — — []
T : Economical study for the production of tons of enriched oxygen (mol%) with dierent technologies [].
Technology Power requirement
(kWh/tons O2)
Capital cost
(/tons O2)
Operating cost
(/tons O2)
Cryogenic distillation >,
Pressure swing
adsorption (PSA) ,–,
Membrane ,–,
only enriched oxygen but also nitrogen with purity of –
mol%.emembraneoperatesatfeedpressureof.to
bar with volume ow rate up to Nm3/h [].
An economical analysis was done for the production
of tons of enriched oxygen with mol% purity using
various technologies []. e comparison was based on
power requirement, capital cost, and operating cost and
the data is given in Table . As expected, the membrane
comes rst in power requirement and it can save energy
upto%and%comparedtocryogenicdistillationand
PSA, respectively. e membrane also has the lowest capital
cost of , to , per tons of oxygen compared to
cryogenic distillation and PSA. Moreover, the membrane
still has the lowest operating cost of /ton O2whereas
cryogenic distillation needs /tons O2,whichisnearly
double.
7. Gas Dehydration
One of the issues in natural gas transport is the formation of
solid hydrates. ese solids are formed due to the presence
of water and hydrocarbons at high pressure and low temper-
ature []. An example is methane hydrate with chemical
formula of CH4nH2Owhereis hydration number. is
parameter is used to determine hydrates in methane storages
and natural gas reserves []. To prevent hydrate formation,
the water content in natural gas should not exceed mg per
m3of natural gas [].
7. 1 . C u r r e nt Te ch n o l o g i e s . Physical absorption by triethylene
glycol can be used to dehydrate natural gas. However, volatile
organic compounds (VOCs) will be formed during solvent
regeneration []. Water removal by silica gel or activated
alumina is another technique where the wet gas enters a
desiccantbedandwaterwillbeadsorbed[].ebedis
simply regenerated by heating, and the adsorption process is
more eective compared to ethylene glycol.
Molecular sieve by zeolite is widely used for removal of
water from natural gas. Compared to other desiccants, zeolite
(A) can treat streams with wider range of relative humidity
[]. Furthermore, zeolite has a better chemical stability and
is capable of adsorbing hydrogen sulde and carbon dioxide,
making it a good choice for treating sour gas []. Also,
zeoliteshowsthehighestadsorptioncapacitiesofgH
2O/g
zeolite for streams having a relative humidity of % at ∘C
[]. With time, zeolite will be saturated with water, and
the bed can be regenerated by thermal regeneration (heating
to –∘C) or reducing the pressure to vacuum [].
e drawback of zeolite is the higher energy requirement
for regeneration, which is % more compared to silica and
alumina [].
Polymeric membrane not only removes water but also
separates hydrogen sulde, carbon dioxide, and heavy hydro-
carbons, all in one step []. e membrane is also expected
to run without interruption for many years. However, pre-
treatment may be necessary to remove particulates from the
feed gas. Unfortunately, the technology is not suitable for
International Journal of Polymer Science
T : Current technologies for dehydration of natural gas [, ].
Technology Advantages Disadvantages
Glycol absorption
(i) Continuous process.
(ii) Lower pressure drop compared to solid desiccants.
(iii) Better chemical stability.
(i) Dicult to achieve water dew point below −∘C.
(ii) Harmful VOCs are formed during the
regeneration of solvent.
Alumina desiccant
(i) Ability to adsorb heavy hydrocarbons.
(ii) Performance is nearly independent of the feed operating
condition.
(i) High pressure drop.
(ii) Regeneration is needed.
Zeolite molecular
sieving
(i) Ability to achieve dew point of − to ∘C.
(ii) Stable under sour gas. (i) More energy is needed for regeneration.
Polymeric
membranes
(i) Ability to separate hydrogen sulde, carbon dioxide, and
heavy hydrocarbons (C3+)inonestep.
(ii) Long life ( years).
(iii) No need for regeneration.
(i) Pretreatment may be required.
(ii) Energy requirement for compressors.
(iii) Not suitable for large volume.
T : Water permeability of hydrophilic and hydrophobic membranes.
Polymer H2O(Barrer) H2O/CH4T(∘C) Ref.
Hydrophobic membranes
Polyethylene (PE) []
Polyimide (Kapton) , []
Polycarbonate (PC) , , []
Polystyrene [, ]
Dimethyl silicone rubber []
Poly(phenylene oxide) (PPO) []
Polydimethylsiloxane (PDMS) , [, ]
Hydrophilic membranes
Poly(,-dimethylphenylene oxide) (PPO) [, ]
Polysulfone , [, ]
Cellulose acetate , , []
Ethyl cellulose , [, ]
Polyether-block-amide (Pebax) , , [, ]
Naon , ,, [, ]
treating large volume of natural gas due to economical issues
[]. Table shows the advantages and disadvantages of
each process for water removal from natural gas.
7.2. Membrane Materials. Water separation membranes are
divided into two groups: hydrophobic and hydrophilic mate-
rials. In hydrophobic membranes, natural gas permeates
while water is rejected. Examples are polyimides and silicone
rubbers particularly PDMS. e latter have a water perme-
ability of , Barrer with water-to-methane selectivity
(H2O/CH4) of [, ]. On the other hand, hydrophilic
membranes are water permeable and some examples are
polysulfone and cellulose acetate. As shown in Table ,
hydrophilic membranes have higher water permeability
and selectivity compared to hydrophobic membranes. For
example, the water-permeable Pebax has a permeability of
, Barrer, which is % higher than PDMS [, ].
Naon gives an outstanding permeability of , Barrer
and H2O/CH4selectivity of ,,. It is a copolymer
developed by DuPont and made by the copolymerization of
tetrauoroethylene and peruorovinyl with sulfonyl uoride
termination step [, ]. Actually, Naon consists of a
hydrophobic backbone (based on Polytetrauoroethylene,
PTFE)andahydrophilicsulfonatedgroupthatprovidesthe
transport path for water [].
7.3. Commercial Units and Economical Evaluation. PRISM
(Air Products) developed a water-permeable membrane for
removal of water from natural gas. A unit was successfully
installedinShellNigeriatoprocess,Nm
3/h of natural
gas []. e membrane is expected to be based on PDMS. As
discussed previously, FuelSep (MTR) is designed to remove
hydrogen sulde from natural gas but it can also permeate
carbon dioxide and water. GENERON also provides dehy-
dration membranes, and, similar to FuelSep, the membrane
permeates hydrogen sulde and carbon dioxide. e system
can work at operating condition up to bar, ∘C, and ow
rate of , Nm3/h [].
Comparing the membrane with other separation meth-
ods, glycol absorption has the lowest capital cost followed by
alumina adsorption, zeolite molecular sieve, and the mem-
brane [, ]. On the other hand, the membrane shows
International Journal of Polymer Science
T : Comparison with dierent technologies for VOCs removal [, , ].
Technology VOC content Eciency (%) Temperature (∘C) Remarks
ermal
oxidation ppm–% LEL – (i)Energyrecoveryupto%.
(ii) Chlorinated compounds can form toxic gases.
Catalytic
oxidation – –
(i)Energyrecoveryupto%.
(ii) Eciency is dependent on operating conditions.
(iii) Certain impurities can poison the catalyst.
Activated
carbon –, – < (i) Performance is greatly aected by moistures.
(ii) Unstable in ketones, aldehydes, and esters.
Membranes < ppm–% LEL – Ambient (i) Treated gas does not require further processing.
the lowest operating cost. For more details, an economical
study was made by Binci et al. to evaluate the membrane
system (PRISM) for natural gas dehydration []. e study
also included the implantation of glycol system. e feed
volume varied from , to , Nm3/h and life span
was years. e feed was at bar and ∘C. e membrane
lifetime was assumed to be years and accordingly it was
changed twice. It was concluded that the membrane was
cost eective for treating , to , Nm3/h of gas. e
system was considered uneconomical for treating more than
, Nm3/h of natural gas.
8. Removal of VOC
Volatile organic compounds are liquids having a boiling point
of to ∘C[].VOCsarecarboncompoundsthat
react with nitrogen oxides in the presence of sunlight to
form harmful ozone in the atmosphere []. erefore, from
environmental point of view, VOCs need to be removed
from air and industrial o-gases. Some VOCs are valuable
solvents, and recovery of these compounds is necessary.
Examples of VOCs are acetone, benzene, formaldehyde,
chlorouorocarbons (CFCs), and hydrochlorouorocarbons
(HCFCs) [].
8.1. Current Technologies. Activated carbon, thermal oxida-
tion, and catalytic oxidation are widely used to remove VOCs
from gases. Activated carbon is favorable to treat streams
containing –, ppm VOCs and it is based on physical
adsorption []. At high pressure, VOCs will be adsorbed
and carbon can be regenerated by reducing the pressure
to vacuum. On the other hand, thermal oxidation is more
suitable for removing VOCs with higher concentration of
ppm up to % of lower explosion limit (LEL) of the gas.
LEL is dened as the lowest concentration in which the gas
will produce re in the presence of an ignition. Going higher
than % LEL will generate excessive heat, which may result
in an explosion [].
In thermal oxidation, the gas containing VOCs will be
heated to a very high temperature of –∘CwhereVOCs
will be oxidized to carbon dioxide and water. A catalyst can
be used to reduce the temperature to –∘Candthispro-
cess is called catalytic oxidation []. e thermal/catalytic
oxidation has an advantage over activated carbon as it can
withstand streams with high humidity. However, the system
is not suitable if chlorinated compounds were presented.
isisbecausechlorinatedcompoundswillbeincompletely
combusted and this leads to formation of toxic gases [].
e membrane technology overcomes this issue due to the
high chemical stability [, ]. In addition, the membrane
can be operated under heavy moistures where activated
carbon cannot be used []. Furthermore, the membrane
works at ambient temperature where other processes need
elevated temperatures. Table compares current methods
for VOCs removal.
8.2. Membrane Materials. Silicone rubbers like PDMS are
widely studied for removal of organic vapors from air. ese
rubbery polymers were tested for many VOCs like acetone,
benzene, toluene, and xylene. For acetone removal from air,
PDMS has a selectivity of to while, for removal of
toluene, PDSM has a higher selectivity of , as given in
Table .
Glassy polymers like polyimide were also evaluated for
VOCs recovery. Polyimide type PI (developed by
Upjohn and based on condensation of ,,,-benzophe-
none tetracarboxylic dianhydride, BDTA) was tested for
dierent VOCs such as methanol, ethanol, hexane, toluene,
andxylene[].PIhasatoluene-to-airselectivitymore
than double compared to PDMS. Furthermore, xylene-to-air
selectivity is times more in PI in comparison with
PDMS.
8.3. Commercial Units and Economical Evaluation. MTR
started installing VOC-recovery membranes for reneries
and petrochemical industries in . e process was fea-
sible for removal of VOCs in the range of to ppm
containing carbon tetrachloride. First, air containing VOCs
is compressed to bar to condense water and some of VOCs.
Aer that, the stream enters two-stage membrane system,
andVOCspermeateintheliquidformduetotheuseof
vacuum pump []. Content of VOCs in the treated air will
have less than ppm. GKSS also developed a spiral-wound
membrane for VOCs removal and it is based on PDMS with
polyetherimide support [].
Unfortunately, there are some economical issues for
selecting the membrane system for VOCs recovery and this
is related to high capital and operating costs. A study was
done on the removal of ppm VOCs from air with
capacity of Nm3/h, and it showed that the membrane
International Journal of Polymer Science
T : Selectivity of various membranes from VOC separation from air (or N2if stated).
Membrane VOC Selectivity Ref.
Silicone
Acetone/N2 []
Ethylbenzene/N2 []
Toluene/N2 []
Xylen e/N2 []
Freon-/N2 []
PDMS
Acetone – []
Toluene []
p-Xylene []
,-Dichloromethane []
,-Dichloroethane []
Polyimide (PI )
Methanol []
Ethanol []
Hexane []
Benzene []
Toluene [ ]
p-Xylene []
requires a capital cost of , whereas thermal/catalytic
oxidation needs only , []. e activated carbon
is also expected to have a capital cost less than , .
ermal/catalytic oxidation achieved the lowest operating
cost of , /month, and it increased to , /month
when the membrane system was used. e activated carbon
has slightly higher operating cost of , /month. e
study is given in Table .
Despite the excellent capital and operating costs of
thermal/catalytic oxidation, the technology is not suitable to
treat gases with volume less than Nm3/h. In this case,
activatedcarbonormembranesystemshouldbeselected.
Activated carbon is a better choice for treating low quantity
of VOCs (e.g., ppm), but if the stream contains higher
than ,ppm VOCs, the membrane is the winner because
activated carbon cannot be operated at these concentrations.
9. LPG Recovery
Liqueed petroleum gas (LPG) contains mainly propane (C3)
and butane (C4). e mixture is in the gas state at normal
pressure but it becomes a liquid at moderate pressures [].
LPG is generally used as a source of heating and cooking
and a fuel for vehicles []. It is found in natural gas or
producedfromcrudeoil.LPGcanalsoberecoveredfrom
renery o-gases such as FCC overhead gas and PSA tail gas
[]. Furthermore, are gases can have valuable amounts of
LPG.
9.1. Current Technologies. e dominant method to recover
LPG is by the combination of cryogenic cooling and gas
expansion (also known as turbo-expander) of natural gas.
First,thegasiscompressedandcooledtoaverylow
temperature of −∘C resulting in a partial condensation (cold
box process). e gas stream is then sent to a turbo-expander
in which the pressure is reduced and the temperature is
further decreased to −∘C. e liquid stream (from the cold
box process) passes through a throttle valve to decrease the
temperature to −∘C. Aer that, both streams are sent to a
demethanizer unit to produce natural gas liquids (C2+)and
recover methane by distillation [, ].
Before the invention of turbo-expander method in s,
LPG was separated from natural gas by an absorption plant.
e process uses a hydrocarbon solvent to physically remove
LPG at low temperature of −∘C. Due to the intensive
manpower and complexity of the technology, the process was
replaced with turbo-expander [].
e membrane technology is recently applied for LPG
recovery. Unlike turbo-expander, the membrane is more
energy-ecient because it operates at ambient temperature.
In addition, it does not need the distillation step, especially if
the feedstock does not contain signicant amount of heavier
hydrocarbon (C5+).
9.2. Membrane Materials. e concept of using the mem-
brane for LPG recovery from renery o-gases was intro-
ducedbyExxonMobilin[].emembranewas
basedonarubberypolymer,whichpermeatespropaneand
heavier hydrocarbons (C3+) but rejects hydrogen, methane,
and ethane []. Polymers like polysiloxane and polybutadi-
enearesuitableforLPGseparationduetothehighsorption
of C3+ compounds []. Unfortunately, few materials were
tested for LPG removal and some of them are given in
Table . PDMS membrane gives propane and butane perme-
abilitiesofand,Barrer,respectively[,].On
the other hand, poly[-(trimethylsilyl)--propyne] (PTMSP)
shows interesting permeabilities of , and , Barrer
for propane and butane [, ].
9.3. Commercial Units and Economical Evaluation. MTR
developed a membrane system called LPG-SEP to recover
International Journal of Polymer Science
T : Economical study for removal of VOCs ( ppm) from air to treat Nm3/h by dierent technologies [].
Technology Capacity (Nm3/h) Capital cost () Operating costs (per month, )
ermal/catalytic oxidation –, , ,
Activated carbon –, <, ,
Membranes – , ,
T : Performance of polymeric membranes for LPG removal.
Polymer C3H8(Barrer) C4H10 (Barrer) C3H8/CH4C4H10/CH4(∘C) Ref.
Polyvinyl-allyl-dimethylsilane (PVADMS) . . . . []
Dimethyl silicon rubber . . []
Poly(-methyl--pentyne) (PMP) , . . []
Polydimethylsiloxane (PDMS) , . . [, ]
Poly[-(trimethylsilyl)--propyne] (PTMSP) , , . . [, ]
LPG from natural gas containing heavy hydrocarbons (asso-
ciated petroleum gas) [, ]. is stream sometimes needs
to be ared thus wasting valuable products and causing
increase in carbon dioxide emissions. In LPG-SEP process,
associatedgasiscompressedtobarandthencooledto
∘C
to condense hydrocarbons of propane and above (C3+). ese
hydrocarbons are then sent to a fractionator (distillation
column)toseparateLPG.ecompressedassociatedgaswill
enter a membrane that permeates methane to recover natural
gas. is membrane system can handle –, Nm3/h
of gas with natural gas content of to mol%. LPG recovery
can reach % with payback of to months [].
MTRalsodevelopedamembranecalledVaporSep,which
canbeusedtoseparateLPGfromaregas,FCCoverhead
gas, and PSA tail gas []. As a case study, a Texas renery
hadanissuewithexcessaregasthatcontainsvaluable
amounts of hydrogen and LPG. e problem was evaluated by
the installation of a compression-condensation-membrane
combination system. e are gas was rst compressed and
condensed to recover some of LPG. Aer that, the gas enters
a membrane system to separate LPG from hydrogen. e unit
wasdesignedtohandle.Nm
3/h of LPG, and payback was
less than a year [].
As discussed, the membrane technology needs to be
integrated with conventional methods if the stream contains
signicant amounts of C5+. is is because the membrane
permeates C3and above and the permeability increases with
carbon number. erefore, it is not possible to produce
LPG from a stream containing C3to C5+, and therefore a
distillation column will be required to separate C3and C4
from C5+. However, the membrane will be a good separation
technique if the stream contains LPG only with other gases
such as hydrogen or carbon dioxide.
10. Conclusion
In this paper, applications of polymeric membranes in
the renery were discussed. e membranes are currently
implemented for hydrogen sulde separation, carbon dioxide
capture, hydrogen recovery, air separation, gas dehydration,
VOCs removal, and LPG recovery. For hydrogen sulde
separation, cellulose acetate is widely used as a membrane
material,andtheprocessingcostfornaturalgastreatment
was lower compared to amine scrubbing to treat natural
gas with mol% of hydrogen sulde. For carbon dioxide
capture, polyimide membrane has an advantage over other
technologies as it can remove hydrogen sulde and water
in one step. e membrane also shows lower capital costs
compared to conventional methods. For hydrogen recovery,
polyimide membrane can be used to recover hydrogen from
natural gas and renery o-gases. However, the process is
considered economical only if hydrogen content is higher
than mol% in the waste gas. In air separation, use of
enriched oxygen can improve the capacity of Claus and
FCC units. Polysulfone membranes were used to produce
mol% oxygen, and the technology has reduced the power
requirement by % compared to cryogenic distillation. For
gas dehydration, water needs to be removed from natural
gas to avoid solid hydrates formation. is is usually done
by glycol absorption but the process results in formation of
toxic VOC. e membrane not only eliminates this issue
but also removes other natural gas impurities. Furthermore,
PDMS membrane was proven to be cost eective compared
to glycol absorption for treating , to , Nm3/h of
natural gas. VOCs are usually found in waste gases and some
of VOCs are expensive solvents. Recovery of these VOCs
is a must due to environmental and economical issues. e
membrane technology is unique for that application as it can
deal with feeds containing halogens and moistures. However,
highcapitalandoperatingcostsnegativelyaecttheselection
of this technology compared to thermal/catalytic oxidation.
In the renery, LPG is recovered from natural gas and
waste gases. Combination of cryogenic distillation and gas
expansion is widely used to separate LPG. e membrane
still cannot substitute the current technology but it can be
integrated to eliminate the cryogenic step as it operates at
ambient temperature and this will greatly reduce the energy
requirement.
One issue of the membrane technology is the sensitivity
to impurities in the feedstock. Cellulose acetate can be used
for many applications such as acid gas removal, hydrogen
recovery, and air separation, but presence of water and
International Journal of Polymer Science
T : Summary of gas separation processes in the renery and advantages of using membranes.
Process Separation Applications Current technologies Advantages of membranes Membrane materials
Hydrogen
sulde
separation
CH4/H2S NG sweetening
Amine scrubbing
PEG absorption
K2CO3absorption
Methanol absorption
PSA
(i) Does not need a solvent.
(ii) Can treat feeds with wider
range of H2S.
(iii) Low NG processing cost for
feeds with <mol%H
2S.
Cellulose acetate (UOP)
Polyether-block-amide
Polyamide
Polyether-urethane-urea
Carbon
dioxide
capture
CO2/CH4
CO2/N2
NG sweetening
Tre a t ment of
o-gases
Amine scrubbing
Water absorption
PEG absorption
K2CO3absorption
Methanol absorption
PSA
Cryogenic distillation
(i) Can separate CO2with other
impurities such as H2SandH
2O.
(ii) Can be operated continuously
for more than years.
(iii) Low NG processing cost for
feed with < mol% CO2.
Cellulose triacetate (Cynara)
Cellulose acetate (UOP)
Polyimide (UBE)
Polyether-block-amide
Polysulfone
Polyamide
Polyether-urethane-urea
Hydrogen
recovery
H2/CH4
H2/CO
H2/N2
H2recovery from
NG
Syngas adjustment
Ammonia purge
gas
Cryogenic distillation
PSA
(i) Ability to treat feeds with
wider range of H2.
(ii) Better turndown.
(iii) Higher reliability.
Cellulose acetate (Separex)
Polysulfone (PRISM)
Polyimide (UBE)
Polyetherimide
Air
separationO2/N2Oxygen
enrichment
Cryogenic distillation
PSA
(i) Can be operated at ambient
temperature.
(ii) Does not need regeneration.
(iii) Low capital and operating
costs.
Cellulose acetate
Polysulfone (UOP)
Poly(-methyl--pentene)
(GENERON)
Polydimethylsiloxane (PRISM)
Polyimide
Polyamide
Polyetherimide
Wat er
removal H2O/CH4NG dehydration
TEG absorption
Silica bed
Activated alumina
Zeolite molecular
sieve
(i) Can be run for more than
years without interruption.
(ii) Ability to remove H2S, CO2,
and C3+ compounds.
Polydimethylsiloxane (PRISM)
Cellulose acetate
Polysulfone
Polyether-block-amide
Polyimide
VOC
recovery
VOC/air
VOC/N2
Tre a t ment of
o-gases
Recovery of
solvents
ermal oxidation
Catalytic oxidation
Activated carbon
(i) Works at ambient
temperature.
(ii) Better chemical stability.
Polydimethylsiloxane (GKSS)
Polyimide (Upjohn)
LPG (C3-C4)/CH4Recovery of LPG
from NG
Cryogenic distillation
and gas expansion
(i) Process integration to reduce
energy requirement.
Polydimethylsiloxane
Poly[-(trimethylsilyl)--propyne]
NG: natural gas.
hydrocarbons can negatively aect the membrane perfor-
mance. erefore, the membrane should be tested under
real feeds to insure the membrane stability for long-term
operation. Summary of this paper is given in Table .
Competing Interests
e authors declare that they have no competing interests.
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