1. MEMBRANE
TECHNOLOGY
Membrane processes are one of the fastest growing and fascinating
fields in separation technology.
In Separation Process
Prepared by:
Muhammed Faiq
Chra Mahmood
Muhammed Latif
Supervised by: Khadija Mirza
2. 1
Membrane technology: has become a dignified separation technology
over the past decennia. The main force of membrane technology is the
fact that it works without the addition of chemicals, with a relatively
low energy use and easy and well-arranged process conductions.
Membrane technology is a generic term for a number of different, very
characteristic separation processes. These processes are of the same
kind, because in each of them a membrane is used. Membranes are
used more and more often for the creation of process water from
groundwater, surface water or wastewater. Membranes are now
competitive for conventional techniques. The membrane separation
process is based on the presence of semi permeable membranes.
Membranes occupy through a selective separation wall. Certain
substances can pass through the membrane, while other substances
are caught.
Membrane filtration can be used as an alternative for flocculation,
sediment purification techniques, adsorption (sand filters and active
carbon filters, ion exchangers), extraction and distillation.
Membrane technology is a proven separation method used on the
molecular and ionic levels. Since the beginning of the 1970s, this
technique has been adapted for the dairy industry and developed to
many other industries.
3. 2
There are two geometry type:
Dead-end Filtration:
The most basic form of filtration is dead-end filtration. The complete
feed flow is forced through the membrane and the filtered matter is
accumulated on the surface of the membrane. The dead-end filtration is
a batch process as accumulated matter on the filter decreases the
filtration capacity, due to clogging. A next process step to remove the
accumulated matter is required. Dead-end filtration can be a very useful
technique for concentrating compounds.
Cross-flow Filtration:
With cross-flow filtration a constant turbulent flow along the membrane
surface prevents the accumulation of matter on the membrane surface.
The membranes used in this process are commonly tubes with a
membrane layer on the inside wall of the tube. The feed flow through
the membrane tube has an elevated pressure as driving force for the
filtration process and a high flow speed to create turbulent conditions.
The process is referred to as "cross-flow", because the feed flow and
filtration flow direction have a 90 degrees’ angle. Cross-flow filtration is
an excellent way to filter liquids with a high concentration of filterable
matter.
4. 3
The two main membrane modules are:
1-Tubular membranes are not self-supporting membranes. They are
located on the inside of a tube, made of a special kind of material. This
material is the supporting layer for the membrane. Because the location
of tubular membranes is inside a tube, the flow in a tubular membrane
is usually inside out. The main cause for this is that the attachment of the
membrane to the supporting layer is very weak.
Tubular membranes have a diameter of about 5 to 15 mm. Because of
the size of the membrane surface, plugging of tubular membranes is not
likely to occur. A drawback of tubular membranes is that the packing
density is low, which results in high prizes per module.
Plate and Frame membranes: spiral membranes consist of two layers of
membrane, placed onto a permeate collector fabric. This membrane
envelope is wrapped around a centrally placed permeate drain (see
picture below). This causes the packing density of the membranes to be
higher. The feed channel is placed at moderate height, to prevent
plugging of the membrane unit. Spiral membranes are only used for
Nano filtration and Reverse Osmosis (RO) applications.
5. 4
• According to the driving forces the operations are divided in the
following types:
1-Pressure:
There are four process types according to pressure:
Ultrafiltration (UF) is the process of separating extremely small
particles and dissolved molecules from fluids. The primary basis for
separation is molecular size, although in all filtration applications,
the permeability of a filter medium can be affected by the
chemical, molecular or electrostatic properties of the sample. Ultra
filtration can only separate molecules which differ by at least an
order of magnitude in size. Molecules of similar size cannot be
separated by ultrafiltration.
Microfiltration (MF) is the process of removing particles or
biological entities in the 0.025 µm to 10.0µm range from fluids by
passage through a microporous medium such as a membrane filter.
Although micron-sized particles can be removed by use of non-
membrane or depth materials such as those found in fibrous
media, only a membrane filter having a precisely defined pore size
can ensure quantitative retention. Membrane filters can be used
for final filtration or prefiltration, whereas a depth filter is generally
used in clarifying applications where quantitative retention is not
required or as a prefilter to prolong the life of a downstream
membrane. Membrane and depth filters offer certain advantages
and limitations. They can complement each other when used
together in a microfiltration process system or fabricated device.
The retention boundary defined by a membrane filter can also be
used as an analytical tool to validate the integrity and efficiency of
a system.
6. 5
Nano filtration. (NF) is essentially a liquid phase one, because it
separates a range of inorganic and organic substances from
solution in a liquid – mainly, but by no means entirely, water. This
is done by diffusion through a membrane, under pressure
differentials that are considerable less than those for reverse
osmosis, but still significantly greater than those for ultrafiltration.
It was the development of a thin film composite membrane that
gave the real impetus to Nano filtration as a recognized process,
and its remarkable growth since then is largely because of its
unique ability to separate and fractionate ionic and relatively low
molecular weight organic species.
Reverse osmosis (RO) separates salts and small molecules from
low molecular weight solutes (typically less than 100 daltons) at
relatively high pressures using membranes with NMWLs of 1 kDa
or lower. RO membranes are normally rated by their retention of
sodium chloride while ultrafiltration membranes are characterized
according to the molecular weight of retained solutes. Millipore
water purification systems employ both reverse osmosis
membranes as well as ultrafiltration membranes. Reverse osmosis
systems are primarily used to purify tap water to purities that
exceed distilled water 2 quality. Ultrafiltration systems ensure that
ultrapure water is free from endotoxins as well as nucleases for
critical biological research.
7. 6
2-Concentration:
There are following main process types according to concentration:
• dialysis
• pervaporation
• forward osmosis
• artificial lung
• gas separation
Gas separation can refer to any of a number of techniques used to
separate gases, either to give multiple products or to purify a single
product.
Membrane technologies are not as well developed as other gas
separation techniques and as a result they are less widely used.
Manufacturing challenges mean the units are better suited for small to
mid-scale operations.
The use partially permeable membranes which allow "fast" gases to pass
through and be removed, while "slow" gases remain in the airstream and
emerge without the original contaminants. Membrane technology is
most often used for moisture removal, hydrogen removal and nitrogen
enrichment.
3-Electric potential gradient
• Electro dialysis
• Electrode ionization
8. 7
4-Operations in a temperature gradient
Membrane distillation (MD) is a separation process where a micro-
porous hydrophobic membrane separates two aqueous solutions at
different temperatures. The hydrophobicity of the membrane prevents
mass transfer of the liquid, whereby a gas-liquid interface is created. The
temperature gradient on the membrane results in a vapour pressure
difference, whereby volatile components in the supply mix evaporate
through the pores (10 nm – 1 µm) and, via diffusion and/or convection
of the compartment with high vapour pressure, are transported to the
compartment with low vapour pressure where they are condensated in
the cold liquid/vapour phase. For supply solutions that only contain non-
volatile substances, such as salts, water vapour will be transported
through the membrane whereby demineralized water is obtained on the
distillation-side and a further concentrated salt flow on the supply-side.
The manner in which the vapour pressure difference is generated across
the membrane is determined by the specific module configuration. In the
most commonly used configuration, direct contact membrane
distillation (DCMD), the permeate-side consists of a condensation liquid
(often clean water) that is in direct contact with the membrane.
Alternatively, the evaporated solvent can be collected on a condensation
surface that can be separated from the membrane via an air gap (AGMD)
or a vacuum (VMD), or can be discharged via a cold, inert sweep gas
(SGMD). In the latter two cases, condensation of vapour molecules takes
place outside the membrane module. Theoretically, the type of driving
force, together with the water-repelling nature of the membrane,
permits full retention of non-volatile components, such as ions, macro-
molecules and colloidal particles.
9. 8
Refinery and Syngas
For hydrogen recovery, liquefied petroleum gas (LPG) recovery, syngas
upgrading, or carbon dioxide removal, MTR has the membrane for you.
MTR offers a variety of solutions tailored to your specific separation
needs.
Hydrogen Purification in Refineries.
Hydrogen membranes are an economical method to recover and purify
hydrogen from a refinery's own waste gases and reactor purges. MTR's
hydrogen-permeable VaporSep-H2 membranes can provide 90% to 99%
hydrogen purity and greater than 90% recovery. Refinery hydrogen
requirements are growing due to the increased use of hydrotreating (to
remove sulfur) and hydrocracking (to convert heavy hydrocarbons to
lighter, higher-value fuels). Residual gas from these processes contains a
significant amount of unused hydrogen at pressure, and membranes
provide an economical recovery method. MTR’s hydrogen-permeable
VaporSep-H2™ membranes can provide 90% to 99% pure hydrogen and
greater than 90% recovery.
Refinery hydrogen requirements are growing due to the increased use of
hydro treating (to remove sulfur) and hydrocracking (to convert heavy
hydrocarbons to lighter, higher-value fuels). Residual gas from these
processes contains a significant amount of unused hydrogen at pressure,
and membranes provide an economical recovery method. MTR’s
hydrogen-permeable VaporSep-H2™ membranes can provide 90% to
99% pure hydrogen and greater than 90% recovery.
10. 9
VaporSep-H2™ offers a simple method for recovering hydrogen from
refinery streams. Hydrogen permeates preferentially through the
membrane, producing a purified hydrogen "permeate" stream and a
hydrocarbon-enriched "residue" stream. The available pressure for the
purified hydrogen depends on the feed conditions, but can be as high as
1500 psi. The hydrocarbon-enriched "residue" is recovered at close to
the feed pressure, and can be sent to directly to fuel, or first treated for
liquefied petroleum gas (LPG) recovery if these components have value.
Refinery Gas Upgrading: LPG and/or H2 Recovery from
Fuel/Waste/Flare Streams
Refineries often produce low-pressure gas streams containing hydrogen,
methane, ethane, and other light hydrocarbons. This gas is typically used
as fuel or even flared, but MTR's hydrocarbon-permeable VaporSep
membranes make it economical to recover the C3+hydrocarbons as
LPG. If required, residual gas can be further processed to recover a
hydrogen-rich stream.
11. 10
Recovery of heavy hydrocarbons (C3+) or LPG from refinery purge and
fuel gas streams is more profitable than sending these high-value
components to fuel. LPG components are produced in many refinery
operations. Traditionally, absorption and cryogenic systems have been
used for the recovery of LPG. However, these technologies require
numerous moving parts and/or external chemicals, and have high capital
and operating costs. VaporSep offers a simple alternative for recovering
LPG from refinery waste streams.
VaporSep® Solution
The flow diagram above shows how MTR’s two different membrane
types can be combined to recover LPG and hydrogen from fuel gas. The
first membrane, VaporSep-H2™, permeates purified hydrogen from the
fuel gas. The gas is then passed to a VaporSep™ membrane which
preferentially permeates LPG components. The permeate is compressed
and LPG is recovered as a liquid in the condenser.
Hydrogen Separations in Syngas Processes
Syngas produced in gasifiers or steam-methane reformers must be
treated to remove impurities (such as acid gases and methane), to adjust
12. 11
the H2/CO ratio to suit the downstream process requirements, or to
recover purified H2 or CO for use in other processes. VaporSep-
H2 membranes offer a simple method for separating and recovering H2 in
these applications.
Syngas Ratio Adjustment
Syngas (H2 + CO) is required to make a variety of products. Each of these
syngas derivatives has a specific ratio of H2 to CO in the feed syngas that
is optimal for its production. However, the H2:CO ratio produced is a
function of the syngas process and the hydrocarbon feed. This ratio will
not necessarily be optimal for downstream products. Therefore, some
method for reducing the amount of hydrogen in the syngas is needed.
VaporSep-H2™ Solution
VaporSep-H2™ offers a simple method for separating H2 from syngas to
adjust the H2:CO ratio. In a typical system, the feed gas is first cooled to
remove condensable hydrocarbons, and then heated before entering the
membrane. Hydrogen preferentially permeates through the membrane,
producing a purified hydrogen "permeate" stream and a hydrogen-
depleted "ratio-adjusted syngas" stream. This ratio adjustment can even
produce high-purity CO for processes requiring it as a feed.
13. 12
Hydrogen Recovery from Ammonia Plant Purge Gas
When syngas is reacted to produce ammonia, inert gases have to be
purged from the synthesis loop. VaporSep-H2 membranes provide a
simple and effective way to recover valuable hydrogen from these purge
streams.
In ammonia production, hydrogen and nitrogen are reacted at high
pressure to form ammonia. Since the conversion per pass is not 100%,
the reactor is operated in loop mode. Ammonia is continuously
condensed out of the loop and fresh synthesis gas is added. Because the
synthesis gas contains small quantities of methane and argon, these
impurities build up in the loop and must be continuously purged to
prevent them from exceeding a certain concentration. Although this
purge stream can be used to supplement reformer fuel gas, it contains
valuable hydrogen which is lost from the ammonia synthesis loop.
VaporSep-H2™ Solution
14. 13
The VaporSep-H2™ unit consists of a single-stage membrane system that
recovers most of the hydrogen from the purge gas stream. The hydrogen
permeating the membrane, almost free of methane and argon, is
recycled to the synthesis gas compressor suction, pressurized and
returned to the synthesis loop. The hydrogen depleted residue,
containing the purged methane and argon, is sent to reformer fuel.
VaporSep-H2™ systems recover more than 80% of purge gas hydrogen.
Installation of such a unit will increase ammonia production by 4-5%,
without increasing gas feed to the reformer.
Hydrogen Recovery from Methanol Plant Purge Gas
VaporSep-H2 membranes offer a simple way to recover hydrogen from
methanol plant purge gas. This hydrogen can be used to adjust the
syngas hydrogen to carbon ratio.
In methanol production, synthesis gas is generated by reforming natural
gas with steam. The synthesis gas is compressed and then reacted to
form methanol. Unreacted syngas is recycled to the reactor. Since the
normal ratio of hydrogen to carbon in the synthesis gas is greater than 2
(the value required for methanol synthesis), excess hydrogen is removed
by purging a hydrogen rich stream from the reactor loop and sending it
to fuel.
VaporSep-H2™ Solution
15. 14
If there is a use for hydrogen elsewhere in the facility, MTR’ s VaporSep-
H2™ membranes can be used to recover relatively pure hydrogen from
this purge gas.
CO2 Removal from Syngas
CO2 separation from syngas, refinery, and power plant streams has taken
on increased importance since its role as a greenhouse gas has become
accepted. MTR’s Polaris™ membrane is unique because it is highly
permeable to CO2, but retains hydrogen. With this advance, it is now
possible to use membranes to remove CO2 from streams containing
hydrogen such as gasifier streams, PSA tail gas, and various
petrochemical process streams.
Syngas is produced via partial oxidation of a carbon source (coal, natural
gas, or biomass) using oxygen (or air) and steam. Syngas produced from
gasification or steam methane reforming (SMR) contains mostly
16. 15
hydrogen and carbon monoxide, desirable constituents that are used as
feedstock in refineries, chemical processes and power generation.
However, a significant amount of CO2, a greenhouse gas, is also produced
as a by-product. Removal of CO2 is desired due to downstream process
requirements or to reduce CO2 emissions. Until recently, membranes
could not be used in these applications because previously available
membranes cannot separate CO2 from syngas.
Polaris™ Solution
MTR’s unique Polaris™ membrane is the first commercially available
membrane that separates CO2 from syngas. The Polaris™ membrane is
much more permeable to CO2 than to other syngas constituents and can
be used to recover and purify CO2 for sequestration, enhanced oil
recovery (EOR), or for use in chemical and industrial applications. The
resulting CO2 enriched stream can be produced in gas or liquid form,
depending on the final use for CO2.
17. 16
Fouling in membranes
Membrane fouling is characterized by the accumulation of feed stream
components onto the surface of or within the pores of a membrane,
resulting in an increase in hydraulic resistance and accompanied by a
decrease in permeate flux. There are several different means by which
fouling can occur. Static membrane fouling can take place in the absence
of a permeate flux and occurs due to the adsorption of feed components
onto the membrane itself, which is driven by both physical and chemical
interactions. Dynamic membrane fouling, on the other hand, occurs
when a permeate flux is generated and occurs through one or more
mechanisms. One of these is through the partial or full pore blockage
caused by the accumulation of foul ants within the pores of a membrane.
Another is through the development of a cake layer via the continuous
deposition of feed particulates on the membrane surface. Similarly, a gel
layer may form near the membrane surface depending on the extent of
concentration polarization. Membrane fouling can be characterized as
reversible or irreversible depending on both the extent and type of
fouling observed.
Membrane fouling is characterized by the accumulation of feed stream
components onto the surface of or within the pores of a membrane,
resulting in an increase in hydraulic resistance and accompanied by a
decrease in permeate flux. There are several different means by which
fouling can occur. Static membrane fouling can take place in the absence
of a permeate flux and occurs due to the adsorption of feed components
onto the membrane itself, which is driven by both physical and chemical
interactions. Dynamic membrane fouling, on the other hand, occurs
when a permeate flux is generated and occurs through one or more
mechanisms. One of these is through the partial or full pore blockage
18. 17
caused by the accumulation of foul ants within the pores of a membrane.
Another is through the development of a cake layer via the continuous
deposition of feed particulates on the membrane surface. Similarly, a gel
layer may form near the membrane surface depending on the extent of
concentration polarization. Membrane fouling can be characterized as
reversible or irreversible depending on both the extent and type of
fouling observed.
There are three main factors that contribute to membrane fouling:
1. Feed stream properties. This includes characteristics such as
concentration, pH and ionic strength, dissolved solid content,
suspended solid content, etc.
2. Membrane material and its physical-chemical properties.
Membrane pore size, porosity and pore distribution, and
membrane surface charge and hydrophobicity in certain pH and
ionic strength conditions can contribute to fouling.
3. Processing variables. Transmembrane pressure, temperature, and
cross-flow velocity have a big impact on membrane fouling.
Several major approaches can be taken to minimize membrane fouling:
1. Optimize pH and ionic strength of the feed solution to minimize the
adsorption or deposition of the feed materials.
2. Select an appropriate pre-filtration procedure or other means to
remove large molecules, since the presence of larger molecules or
particles could cause a steric hindrance to the passage of smaller
molecules through the membrane.
3. Select a membrane with an optimum pore size to result in good
separation performance as well as optimized permeate flux.
4. Optimize the operating conditions. This includes increasing
transmembrane pressure to maximize flux without introducing
19. 18
more fouling potential and increase the cross-flow velocity, which
generally results in an improvement in permeate flux.
The disadvantages include:
Membrane processes seldom produce 2 pure products, that is, one of
the 2 streams is almost always contaminated with a minor amount of a
second component. In some cases, a product can only be concentrated
as a retentate because of osmotic pressure problems. In other cases,
the permeate stream can contain significant amount of materials which
one is trying to concentrate in the retentate because the membrane
selectivity is not infinite.
Membrane processes cannot be easily staged compared to processes
such as distillation, and most often membrane processes have only one
or sometimes two or three stages. This means that the membrane
being used for a given separation must have much higher selectivity’s
than would be necessary for relative volatilities in distillation. Thus the
trade-off is often high selectivity/few stages for membrane processes
versus low selectivity/many stages for other processes.
Membrane modules often cannot operate at much above room
temperature. This is again related to the fact that most membranes are
polymer-based, and that a large fraction of these polymers do not
maintain their physical integrity at much above 100 oC. This
temperature limitation means that membrane processes in a number
of cases cannot be made compatible with chemical processes
conditions very easily.
Membrane processes often do not scale up very well to accept massive
stream sizes. Membrane processes typically consist of a number of
membrane modules in parallel, which must be replicated over and over
to scale to larger feed rates.
20. 19
There are many applications of membrane technology in various
industrial uses.
Commons are:
Concentration: The desired component is present in a low
concentration and solvent has to be removed.
Fractionation: A mixture must be separated into two or more
desired components.
Purification: Undesirable impurities have to be removed.
Distillation: A hydrophobic membrane displays a barrier for the
liquid phase, allowing the vapor phase to pass through the
membrane's pores.
References:
• Baker, R.W., 2000. Membrane technology. John Wiley & Sons, Inc.
• Cui, Z.F. and Muralidhara, H.S., 2010. Membrane technology: a
practical guide to membrane technology and applications in food and
bioprocessing. Elsevier.
• http://nmrc-sut.ir/wp-content/uploads/2016/04/Membrane-Technology-
and-applications.pdf
• http://synderfiltration.com/learning-center/articles/membranes/methods-
to-reduce-membrane-fouling/
• http://synderfiltration.com/learning-
center/articles/membranes/membrane-processes/
• http://www.mtrinc.com/refinery_and_syngas.html
• http://www.separationprocesses.com/Membrane/MT_Chp01c.htm
