Introduction to Mass Transfer Operations (3 of 5)

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1. Membrane Theory
2. Industrial Processes:
1. Osmosis
2. Reverse osmosis
3. Dialysis/Electrodialysis
4. Gas permeation
5. Pervaporation
6. Microfiltration
7. Ultrafiltration/HyperFiltration
8. Liquid membranes

▪ Membrane vs. Filter
▪ Introduction to Membranes
▪ Retentate & Permeate
▪ More on Membranes
▪ Industrial Materials
▪ Pros & Cons of Membranes

▪ Both are “technically” barriers
▪ Main difference:
▪ Size of pore
▪ Chemical vs. Physical Operation

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▪ A membrane is a selective barrier that permits the
separation of certain species in a fluid
▪ It is achieved by combination of sieving and sorption
diffusion mechanism.
▪ Separation is achieved by selectively passing
(permeating) one or more components of a stream
through the membrane
▪ This occurs while retarding the passage of one or more
other components.
Permeation” is the process by which a chemical moves through a material on a molecular level.
This is different to Penetration which could be described as the process by which a chemical moves through a material on a non-molecular level.
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▪ Membranes can selectively separate components over a
wide range of:
▪ particle sizes
▪ molecular weights
▪ Examples:
▪ macromolecular materials such as starch and protein
▪ monovalent ions such as Na+ (Sodium Ion)
▪ Membranes have gained an important place in chemical
technology and are used in a broad range of applications.

▪ Membranes are fabricated mainly from natural fibers and
synthetic polymers
▪ Newer technology membranes are now being produced
from ceramics and metals.
▪ Membranes are fabricated into:
▪ flat sheets
▪ Tubes
▪ hollow fibers
▪ spiral-wound sheets
▪ Final “shape” or presentation will be either a module or
cartridge system.
▪ The membrane is almost impermeable to the solute.
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▪ The key properties determining membrane performance are:
▪ high selectivity and fluxes
▪ good mechanical
▪ chemical and thermal stability
▪ low fouling tendencies
▪ good compatibility with the operating environment
▪ cost effective
▪ defect-free production.

▪ Most common applications:
▪ Production of potable water
▪ Separation of industrial gases
▪ Other Applications growing in importance:
▪ Filtration of particulate matter from liquid suspensions
▪ Air or industrial flue gas
▪ Dehydration of ethanol azeotropes.
▪ More specialised applications:
▪ Ion separation in electrochemical processes
▪ Dialysis of blood and urine
▪ Artificial lungs
▪ Controlled release of therapeutic drugs
▪ Membrane-based sensors
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▪ Membrane processes are characterized by the fact that a feed stream is divided into
2 streams:
▪ Main Product: Permeate
▪ By-Product: Retantate

▪ The retentate is:
▪ that part of the feed that does not pass through the membrane
▪ The permeate is:
▪ that part of the feed that does pass through the membrane.
▪ The optional "sweep" is:
▪ a gas or liquid that is used to help remove the permeate.
▪ The component(s) of interest in membrane separation is known as the solute.
▪ The solute can be retained on the membrane
▪ It can be removed in the retentate
▪ or passed through the membrane in the permeate.

▪ It is important to note that there are 3 different mechanisms by which membrane can
perform separations:
▪ Size exclusion Mechanism:
▪ By having holes or pores which are of such a size that
certain species can pass through and others cannot.
▪ Pore Flow Mechanism
▪ By selective retardation by the pores when the pore
diameters are close to molecular sizes.
▪ Solution Diffusion Mechanism
▪ By dissolution into the membrane, migration by molecular
diffusion across the membrane, and re-emergence from
the other side.
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▪ As any Separation Method, it will depend on the
conditions.
▪ Membrane processes have a number of
advantages but may also encounter many type
of disadvantages

▪ Because membrane processes can separate at the
molecular scale up to a scale at which particles can
actually be seen
▪ this implies that a very large number of separation needs
might actually be met by membrane processes.

▪ Membrane processes generally do not require a phase
change to make a separation
▪ With the exception of pervaporation
▪ Therefore, energy requirements will be low
▪ BUT!
▪ This might increase the energy requirement for pressurization.
▪ Typically, we need to fix the pressure of a feed stream in order
to drive the permeating component(s) across the membrane.

▪ Membrane processes present basically a very simple setup.
▪ There are no moving parts:
▪ no complex control schemes
▪ Therefore, they can offer a:
▪ Simple installation
▪ east-to-operate
▪ low maintenance process option
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▪ Membranes can be produced with extremely high
selectivities for the components to be separated.
▪ In general, the values of these selectivities are much
higher than typical values for relative volatility for
distillation operations.
▪ This means that it might be convenient to eventually
setup a Membrane for such systems.

▪ Membrane processes are able to recover minor but
valuable components from a main stream without
substantial energy costs
▪ Membrane processes are potentially better for the
environment since the membrane approach require the
use of relatively simple and non-harmful materials
▪ (we’ll cover some materials soon)
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▪ Membrane processes seldom produce 2 pure products:
▪ one of the two streams is almost always contaminated with a
minor amount of a second component.
▪ And vice versa…
▪ In some cases, a product can only be concentrated as a
retentate because of osmotic pressure issues.
▪ In other cases:
▪ the permeate stream can contain significant amount of
materials
▪ In this: 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.
▪ 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 selectivities than would be
necessary for relative volatilities in distillation.
▪ Thus the trade-off is often:
▪ high selectivity/few stages for membrane processes
Vs.
▪ low selectivity/many stages for other processes.
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▪ Membranes can have chemical incompatibilities with process
solutions.
▪ This is especially the case in typical chemical industry solutions
which can contain high concentrations of various organic
compounds.
▪ Many polymer-based membranes* will:
▪ dissolve, or swell, or weaken
*Polymer Membranes comprise the majority of membrane materials used today

▪ Membrane modules often cannot operate at much above room
temperature.
▪ This is mostly due to the fact that membranes (polymers) do not
maintain their physical integrity at much above 100°C.
▪ This temperature limitation means that membrane processes in a
number of cases cannot be made compatible with chemical
processes conditions very easily.
*Polymer Membranes comprise the majority of membrane materials used today

▪ Membrane processes often do not scale up very well to accept
massive stream sizes.
▪ They typically consist of a number of membrane modules in
parallel
▪ This must be replicated over and over to scale to larger feed rates
▪ Fouling of the membranes is also an issue
▪ especially if it is difficult to remove, will greatly restrict the
permeation rate through the membranes
▪ This make them essentially unsuitable for such applications.
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▪ Membranes can be classified according to different viewpoints.
▪ Material: natural or synthetic
▪ Size/Morphology/Geometry: thick or thin
▪ Type of Structure: homogeneous or heterogeneous
▪ Type of transport across membrane can: active or passive
▪ Passive Transport:
▪ e.g. pressure, concentration, electrical difference), neutral or charged.

▪ The first classification is by nature/material
▪ i.e. biological or synthetic membranes.
▪ This is the clearest distinction possible.
▪ Synthetic membranes can be subdivided into:
▪ organic (polymeric or liquid)
▪ inorganic (e.g. ceramic, metal) membranes.
▪ Another means of classifying membranes is
by morphology or structure
▪ for the case of solid synthetic membranes the 2
types of membrane structures are:
▪ the symmetric
▪ asymmetric (anisotropic) membranes.
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▪ As stated, we will classify most of the membranes for their material:
▪ Natural vs. Synthetic
▪ Natural polymers include:
▪ wool, rubber (polyisoprene) and cellulose.
▪ Synthetic materials will include:
▪ Polymer → Polyamide, polystyrene and polytetrafluoroethylene (Teflon).
▪ Membranes can also be made from other non-polymeric materials.
▪ Inorganic Materials → metal, ceramic, carbon and zeolites & liquid membranes.

▪ Physical
▪ porosity, pore size and pore distribution, thickness, tortousity, thermal stability, etc.
▪ Separation
▪ permeate flow rate or permeation flux, permeability, selectivity.
▪ Surface and electrochemical
▪ streaming potential, zeta potential (or electrokinetic potential)
▪ membrane potential (voltage potential difference),
▪ surface charge density, wettability (hydrophilic or hydrophobic)
▪ Electrical
▪ impedance measurements to determine the membrane conductance and capacitance
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▪ Permeation Flux
▪ The membrane permeation flux is defined as the volume flowing through the membrane
per unit area per unit time.
▪ The SI unit used is m3/m2.s although other are often used as well.
▪ For the case of transport of gases and vapors, the volume is strongly dependent on
pressure and temperature.
▪ As such, gas fluxes are often given in terms of a "standard condition" which is defined as 0
oC and 1 atmosphere (1.0013 bar).
▪ Permeability Coefficient
▪ (P or simply the permeability) is defined as the transport flux of material through the
membrane per unit driving force per unit membrane thickness.
▪ It's value must be experimentally determined.
▪ The barrer is the commonly used unit for gas separation.
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▪ Permeance
▪ The permeance PM is defined as the ratio of the
permeability coefficient (P) to the membrane thickness (L).
▪ The permeance for a given component diffusing through a
membrane of a given thickness is analogous to a mass
transfer coefficient.

▪ Membrane Selectivity
▪ In gas separation, the membrane selectivity is used to
compare the separating capacity of a membrane for 2 (or
more) species.
▪ The membrane selectivity
▪ (also known as the permselectivity)
▪ for one component (A) over another component (B) is given by
the ratio of their permeabilities

▪ The results of the modification will yields different membrane responses to
chemical resistance, fouling, and absorption.
▪ Surfactants
▪ Coatings
▪ Chemical Grafting
▪ Polymer Blends
▪ Plasma
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▪ Surfactants
▪ Treatment with water insoluble surfactant to enhance hydrophilicity (improve wetting).
Has a potential problem for product water contamination (leaching).
▪ Coatings
▪ A thin film application of a different polymer or monomer-system to form a new surface
via composite formation. High potential for development of new chemistry polymeric
surfaces.

▪ Chemical Grafting
▪ A process of attachment of a low molecular weight active group (monomer) to a parent
polymer or membrane. Either bulk polymer or surface modification is possible. Also high
potential for development of new chemistry polymeric surfaces.
▪ Polymer Blends
▪ A mixture of two or more different polymers can improve chemical properties of
membranes and separation performance. Has limitation of polymer solution compatibility
for solvent systems required for solution cast membrane formation.

▪ Plasma
▪ The technique of plasma modification is similar to chemical grafting in that a chemical
modification is performed on the membrane polymer material.
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▪ This Topic requires a WHOLE COURSE!
▪ Membrane Basics
▪ Membrane Characterization
▪ Membrane Shapes and Modules
▪ Industrial Membranes (Plate Frames, Tubular, spiral, Hollow Fiber)
▪ Flow Patterns in Membrane Modules
▪ Membrane Cascades → Membrane Cascade for High Stage Cut
▪ Membrane Module Selection

▪ Membrane Transport Theory
▪ The Solution-Diffusion Model
▪ The Pore Flow Model
▪ Cross-Flow vs. Dead-End (In-Line) Filtration
▪ Membrane Filtration Processes (Reverse Osmosis, Filtrations - ultra, micro, nano)
▪ Membrane Operation
▪ Flux Decline
▪ Critical Flux and Operation of Membrane Filtration
▪ Concentration Polarization
▪ Fouling

4.2 Industrial Processes:
4.2.1 Osmosis
4.2.2 Reverse osmosis
4.2.3 Dialysis/Electrodialysis
4.2.4 Gas permeation
4.2.5 Pervaporation
4.2.6 Microfiltration
4.2.7 Ultrafiltration/HyperFiltration
4.2.8 Liquid membranes
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1. Osmosis
2. Reverse Osmosis
3. Dialysis / Electrodialysis*
4. Microfiltration
5. Ultrafiltration/Hyperfiltration
6. Pervaporation
7. Gas permeation
8. Liquid membranes
*Electrodialysis → Covered in Separation by external field/gradient
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▪ Typical driving forces

▪ 4.2.1 Osmosis

▪ Osmosis, from the Greek word for ‘‘push’’
▪ Involves transfer, by a concentration gradient, of a
solvent through a membrane into a mixture of solute and
solvent.
▪ Osmosis (AKA Forward Osmosis)
▪ The membrane is almost impermeable to the solute.
▪ There is no industrial application to osmosis, rather this
is a naturally occurring phenomena.
▪ It is important to understand it in order to understand
Reverse Osmosis

▪ The change in height can be used to
calculate the osmotic pressure.
▪ Osmotic Pressure is the minimum
pressure which needs to be applied to a
solution to prevent the inward flow of its
pure solvent across a semipermeable
membrane.
▪ As you can imagine, this will require
“external” force or work
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▪ Refers to passage of a solvent, such as water, through a
membrane that is much more permeable to solvent (A) than
to solute(s) (B) (e.g., inorganic ions).
▪ As you can imagine, it is the reverse process of “Osmosis” or
“Forward Osmosis”
▪ In order to revert a naturally occurring phenomena, external
work is required.
▪ This work typically comes in the form of a Pressure Gradient
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▪ In the initial condition (a)
▪ Seawater of approximately 3.5 wt% dissolved salts and at 101.3 kPa is in cell 1
▪ Pure water at the same pressure is in cell 2.
▪ The dense membrane is permeable to water, but not to dissolved salts.
▪ By osmosis, water passes from cell 2 to the seawater in cell 1, causing dilution of the dissolved salts.

▪ Transport of solvent in the opposite direction is effected by imposing a pressure on the feed
side.
▪ It must be higher than the osmotic pressure,
▪ Using a nonporous membrane:
▪ Reverse osmosis desalts brackish water commercially.

▪ Reverse osmosis affects separation of:
▪ very small solutes
▪ Such as salts with ionic radii in the angstroms range.
▪ 4 to 8 angstroms range
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▪ The solute moves through the membrane mainly under concentration gradient forces.
▪ The solvent transport is dependent on the hydraulic pressure gradient.
▪ Pores in reverse osmosis membranes are so small they have not yet been resolved
▪ even by the most advanced microscopic techniques.

▪ At equilibrium, the condition of (b) is reached
▪ wherein some pure water still resides in cell 2 and seawater
▪ less concentrated in salt, resides in cell 1.
▪ Pressure P1, in cell 1, is now greater than pressure P2, in cell 2, with the difference, p, referred
to as the osmotic pressure.
▪ Osmosis is not a useful separation process because the solvent is transferred in the wrong
direction
▪ This results in mixing rather than separation.
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▪ However, the direction of transport of solvent through the membrane can be
reversed, as shown in (C)
▪ If we apply a pressure, P1, in cell 1, that is higher than the sum of the osmotic pressure
and the pressure, P2, in cell 2: that is, P1 P2 > p.

▪ Now water in the seawater is transferred to the pure water, and the seawater
becomes more concentrated in dissolved salts.
▪ This phenomenon, called reverse osmosis, is used to partially remove solvent from
a solute–solvent mixture.

▪ An important factor in developing a reverse-osmosis separation process is the
osmotic pressure, p, of the feed mixture, which is proportional to the solute
concentration.
▪ For pure water, p = 0.

▪ RO performs a separation without a phase change.
▪ Thus, the energy requirements are low.
▪ RO systems are compact, and space requirements are
less than with other desalting systems
▪ RO equipment is standardized:
▪ pumps, motors, valves, flowmeters, pressure gages, etc.
▪ Thus, the learning curve for unskilled labour is short.
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▪ The applied pressure must exceed the osmotic
pressure to obtain product flow and to separate the
solute from the solvent.
▪ The maximum feed pressure for seawater devices
varies from 800 - 1000 psig
▪ The limit for brackish water varies from 400 - 600 psig.
▪ RO is usually not applicable for concentrated
solutions due to the high pressure requirements

▪ All RO membranes and devices are susceptible to
fouling
▪ RO process usually cannot be applied without
pretreatment due to Fouling
▪ RO feed streams must be compatible with the
membrane and other materials of construction
used in the devices.

▪ Reverse osmosis operation usually involves 2 components:
▪ Water (A)
▪ Salt (B).
▪ The water flux permeates the reverse osmosis membrane according to the equation:
▪ Thus, the water flux is proportional to the applied pressure.
▪ When,
▪ , water flows from the dilute to the concentrated salt solution side of the membrane by normal osmosis
▪ , no flow occurs
▪ , water flows from the concentrated to the dilute salt solution side of the membrane by reverse osmosis
( )AJ A p =  − 
: tan
Pr
Pr
A water permeability cons t
p essure change
Osmotic essure Differential
 =
 =
DP Dp
DP Dp=
DP Dp
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▪ The salt flux across a reverse osmosis membrane is given by:
▪ where:
▪ = salt permeability constant
▪ = salt concentration on the feed side of membrane
▪ = salt concentration on the permeate side of membrane
▪ The salt flux is independent of pressure.
( )o LB B BJ B C C= −
LBC
oBC
B

▪ The salt concentration on the permeate
side is usually very small compared to the
feed side, i.e.
▪ Membrane selectivity increases as the
pressure increases.
▪ Selectivity can be measured in a number
of ways, but conventionally it is measured
as the salt rejection coefficient.
o LB BC C

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▪ Desalination of brackish water
▪ Treatment of wastewater to remove impurities
▪ Treatment of surface and ground water
▪ Concentration of foodstuff
▪ Removal of alcohol from beer and wine

▪ Check out this video
▪ Reverse Osmosis Plant
▪ 26,420 GPD (gallons per day)
▪ https://www.youtube.com/watch?v=NQHI7SQIwlw
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▪ Dialysis:
▪ the transport by a concentration gradient of small solute molecules
▪ These are sometimes called crystalloids, through a porous
membrane.
▪ The molecules unable to pass through the membrane are small,
insoluble, non-diffusible particles.
▪ Microporous membranes will selectively:
▪ allow small solute molecules and/or solvents to pass through the
membrane
▪ while preventing large dissolved molecules and suspended solids from
passing through.
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▪ This is a dialysis membrane-separation process
▪ The feed is liquid at pressure P1 and contains:
▪ solvent
▪ solutes of type A
▪ solutes of type B
▪ Insoluble (but dispersed) colloidal matter
▪ A sweep liquid or wash of the same solvent is fed at pressure P2 to
the other side of the membrane.

▪ The membrane:
▪ Is thin with micropores of a size
▪ Solute of Type A:
▪ Are small enough to pass through.
▪ It is achieved by concentration-driving force alone.
▪ Solutes of Type B:
▪ Are larger in molecular size.
▪ It will not pass through the membrane

▪ This transport of solutes through the membrane is called dialysis.
▪ Colloids do not pass through the membrane.
▪ With pressure P1 = P2
▪ the solvent may also pass through the membrane
▪ BUT it requires a concentration-driving force acting in the opposite
direction.
▪ The transport of the solvent is called osmosis.
▪ By elevating P1 above P2:
▪ Solvent osmosis can be reduced or eliminated if the difference is higher
than the osmotic pressure.
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▪ The products of a dialysis unit (dialyzer) are:
▪ A liquid diffusate (permeate) containing:
▪ solvent, solutes of type A, and little or none of type B solutes
▪ A dialysate (retentate) containing:
▪ solvent, type B solutes, remaining type A solutes, and colloidal matter

▪ Ideally
▪ The dialysis unit would enable a perfect separation between solutes of
type A and solutes of type B and any colloidal matter.
▪ However:
▪ At best only a fraction of solutes of type A are recovered in the diffusate,
even when solutes of type B do not pass through the membrane.
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▪ Separation of nickel sulfate from sulphuric acid
▪ Hemodialysis:
▪ Removal of waste metabolites, excess body water, and restoration of
electrolyte balance in blood

▪ Microfiltration (MF) is the "fine" end of particle filtration
▪ Typical pore size varies:
▪ from 0.1 to 1 micron diameter
▪ perhaps up to 3 or even 10 microns
▪ MF membranes have pores two to five orders of magnitude larger
than the other classes.
▪ What is not as well known is that when the MF media is a membrane
▪ it also can be run in the cross-flow as well as normal-flow mode.
▪ This may provide lower cost operation and much longer media life.
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▪ In all cases, water must pas
▪ Suspended particles are largest
▪ Macromolecules:
▪ Sugars, Proteins, Amino Acids
▪ Multivalent salts
▪ Na+, Cl-, etc…
▪ Water

▪ Example of “micro”
▪ Proteins
▪ Sugars
▪ Amino Acids
▪ Ions
▪ Water
▪ Larger material:
▪ Particulates
▪ Colloids
▪ Bacteria
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▪ Comparison of
▪ Micro
▪ Ultra
▪ Nano
▪ Filtrations

▪ Sterilization of drugs
▪ Clarification of biological stabilization of beverages
▪ Purifications of antibiotics
▪ Separation of mammalian cells from a liquid

▪ Comparison of
▪ Micro
▪ Ultra
▪ Nano
▪ Filtrations
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▪ It’s a filtration process.
▪ It refers to the retention of molecules that range from 1 to 20
nm.
▪ To retain molecules down to 0.1 nm, nonporous membranes
can be used in.
▪ To achieve high purities, reverse osmosis requires high
pressures.

▪ Ultrafiltration membranes can separate medium to large size
dissolved molecules from the solvent
▪ This is due largely to the simple sieving mechanism.
▪ Solutes in the 5,000 to 500,000 molecular weight range are
excluded from transport based mainly on their physical size.

▪ Ultrafiltration class membranes are defined to include only
those:
▪ membranes with pores too large to reject or remove salt ions,
▪ but small enough to reject larger dissolved and colloidal species.
▪ The pores are generally accepted as ranging in size from:
▪ 20 to 500 angstroms diameter.
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▪ Pre-concentration of milk before making cheese
▪ Clarification of fruit juice
▪ Recovery of vaccines and antibiotics from fermentation broth
▪ Color removal from Kraft black liquor in paper-making

▪ Nanofiltration membranes have pores close to one
nanometer diameter and affect partial salt rejection.
▪ Typical NF membranes pass a higher percentage of
monovalent salt ions than divalent and trivalent ions.

▪ Most NF membrane polymers carry formal
charges which exclude higher valence ions
more than monovalents from passing
through the membrane with the solvent
water.
▪ Nanofiltration membranes span the gap
between RO and UF classes.
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▪ Permeation (also called imbuing) is the penetration of a permeate
(such as a liquid, gas, or vapor) through a solid.
▪ Gas Permeation → Gas through a Solid
▪ It is directly related to the concentration gradient of the permeate
▪ This is a material's intrinsic permeability
▪ It is also dependent on the materials' mass diffusivity.
▪ Permeation is modeled by equations such as Fick's laws of diffusion
Permeation of A >> Permeation of B
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▪ Nonporous polymer membranes are employed to enrich mixtures containing H2
▪ These recover hydrocarbons from gas streams, and produce O2-enriched air.

▪ Gas Permeation (GP) occurs through a thin film, where:
▪ Feed gas, at high pressure P1, contains some low molecular
weight species (MW < 50)
▪ These are to be separated from small amounts of higher-
molecular-weight species.
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▪ Usually a sweep gas is not needed
▪ BUT
▪ The other side of the membrane is maintained at a much lower pressure (P2)
▪ Often near-ambient to provide an adequate driving force.
▪ The membrane must be:
▪ Dense
▪ Microporous
▪ Perm-selective for the low molecular weight species A.

▪ If the membrane is dense:
▪ these species are absorbed at the surface and then transported through the
membrane by one or more mechanisms.
▪ Then, perm-selectivity depends on both membrane absorption and
transport rate.
▪ Mechanisms are formulated in terms of a partial-pressure or fugacity
driving force
▪ These are modeled using the solution-diffusion model.

▪ The products are:
▪ A permeate enriched in A
▪ A retentate enriched in B.
▪ A near-perfect separation is generally not achievable.

▪ If the membrane is microporous
▪ pore size is extremely important because it is necessary to block the passage
of species B
▪ Otherwise:
▪ unless molecular weights of A and B differ appreciably
▪ only a very modest separation is achievable
▪ Since the early 1980s, applications of GP with dense polymeric
membranes have increased dramatically.
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▪ Gas permeation competes with:
▪ Absorption, pressure-swing adsorption, and cryogenic distillation.
▪ Advantages of gas permeation:
▪ Low capital investment
▪ Ease of installation
▪ Ease of operation
▪ Absence of rotating parts
▪ High process flexibility
▪ Low weight and space requirements
▪ Low environmental impact.
▪ In addition, if the feed gas is already high pressure, a gas
compressor is not needed, and thus no utilities are required.

▪ Major applications include:
▪ separation of hydrogen from methane;
▪ adjustment of H2-to-CO ratio in synthesis gas;
▪ O2 enrichment of air;
▪ N2 enrichment of air;
▪ removal ofCO2;
▪ drying of natural gas and air;
▪ removal of helium; and
▪ removal of organic solvents from air.

▪ Pervaporation is a processing method for the separation of mixtures
of liquids by partial vaporization through a:
▪ non-porous
▪ porous membrane.
▪ This method, which is used to separate azeotropic mixtures:
▪ uses much lower pressures than reverse osmosis
▪ BUT → the heat of vaporization must be supplied.
▪ Essentially, you are changing ESA → Pressure to Heat
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▪ Pervaporation differs from:
▪ dialysis, reverse osmosis, and gas permeation
▪ in that the phase on one side of the pervaporation membrane is
different from that on the other.
▪ Feed to the membrane module is a liquid mixture at pressure
P1
▪ This Pressure is high enough to maintain a liquid phase as the
feed is depleted of species A and B to produce liquid
retentate.

▪ A composite membrane is used that is:
▪ selective for species A
▪ but with some finite permeability for species B.
▪ The dense, thin-film side of the membrane is in contact with the
liquid side.
▪ The retentate is enriched in species B.
▪ Generally:
▪ A sweep fluid is not used on the other side of the membrane
▪ but a pressure P2, which may be a vacuum, is held at or below the dew
point of the permeate, making it vapor.
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▪ Vaporization may occur near the downstream face.
▪ The membrane operates with two zones:
▪ a liquid- phase zone
▪ a vapor-phase zone

▪ Alternatively, the vapor phase may exist only on the permeate
side of the membrane.
▪ The vapor permeate is enriched in species A.
▪ Overall permeabilities of species A and B depend on solubilities
and diffusion rates.
▪ Generally:
▪ solubilities cause the membrane to swell.
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▪ Pervaporation is an efficient energy conserving alternative to processes such as
distillation and evaporation.
▪ It allows the exchange of two phases without direct contact.
▪ Examples of Industrial applications:
▪ Dehydration of ethanol-water azeotropes
▪ Removal of water from organic solvents
▪ Removal of organics from water

▪ Check out this website:
▪ https://pervaporation-membranes.com/
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▪ A liquid membrane (LM) is literally a membrane made of liquid.
▪ It consists of a liquid phase (e.g. a thin oil film) existing either in
supported or unsupported form
▪ It serves as a membrane barrier between two phases of aqueous
solutions or gas mixtures.
▪ One of the benefits of using a liquid membrane is that:
▪ LMs are highly selective
▪ If there is use of carriers for the transport mechanism
▪ specific molecular recognition can be achieved.
▪ LMs are relatively high in efficiency
▪ LM are being looked into for industrial applications.
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▪ Typically:
▪ Only a few molecules thick, can be formed from surfactant-
containing mixtures at the interface between two fluid phases.
▪ With liquid membranes, aromatic/paraffinic hydrocarbons can
be separated.
▪ Also,
▪ A liquid membrane can be formed by imbibing the micro-pores with
liquids doped with additives to facilitate transport of solutes such as
CO2 and H2S.

▪ The major problem restricting widespread application is stability.
▪ LM, liquid membranes require stability in order to be effective
▪ If they are pushed out of the pores or ruptured in some way due to pressure differentials or
turbulence, then they just do not work.

▪ There are 2 basic types of liquid membranes:
▪ the Emulsion Liquid Membrane (ELM)
▪ the Immobilized Liquid Membrane (ILM)
▪ also called a Supported Liquid Membrane.

▪ An Emulsion Liquid Membrane, ELM
can be visualized as consisting of a
"bubble within a bubble".
▪ The inner most bubble is the receiving
phase
▪ The outer bubble is the separation
"skin" containing the carriers.
▪ Anything outside the bubble is the source phase.
▪ In an ELM set-up there would be huge numbers of these bubbles.
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▪ ELM

▪ This system has several disadvantages, all having to do with the formation of the
emulsion.
▪ Anything effecting emulsion stability must be controlled. ie.ionic strengths, pH, etc.
▪ If, for any reason, the membrane does not remain intat duringoperation, the separation
achieved to that point is destroyed.
▪ In order to recover the receiving phase, and in order toreplenish the carrier phase, you have to
break down theemulsion.
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▪ An the Immobilized Liquid Membrane (ILM) is much simpler to
visualize.
▪ It is made of some kind of rigid polymer membrane
▪ The membrane must have lots of microscopic pores in it which are
filled with organic liquid.
▪ In the liquid, there will be carriers that perform the required
separation.

▪ What happens then is that:
▪ the ILM takes things from one side of the rigid membrane (the source
phase)
▪ Then it carries it to the other side (the receiving phase) through this
liquid phase.

▪ Metal Ion Extraction:
▪ Cu from dilute aqueous solutions
▪ Recovery of U from wet process phosphoric acid using TOPO
▪ Removal of weak acids/bases Weak acids like phenol and cresol
and weak bases like ammonium and amines have been
successfully removed from wastewater.
▪ Separation of inorganic species Apart from ammonia, some other
inorganic species
▪ Recovery of zinc from wastewater
▪ Recovery of nickel from electroplating solutions
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▪ 5.1 Adsorption Columns
▪ 5.2 Ion Exchange
▪ 5.3 Chromatography

▪ Adsorption (also known as adsorptive separation) can be simply
defined as the concentration of a solute, which may be
molecules in a gas stream or a dissolved or suspended substance
in a liquid stream, on the surface of a solid.
▪ The major applications had been:
▪ the separation of solutes from liquid streams
▪ removal of impurities from gas streams.
▪ Important Streams:
▪ the adsorbed solute is called the adsorbate
▪ the solid material is the adsorbent.
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▪ In an adsorption process:
▪ Molecules or atoms or ions in a gas or liquid diffuse to the surface of a solid, where they
bond with the solid surface or are held there by weak inter-molecular forces.

▪ During adsorption, the solid adsorbent becomes saturated or nearly saturated with
the adsorbate.
▪ To recover the adsorbate and allow the adsorbent to be reused, it
is regenerated by desorbing the adsorbed substances (i.e. the adsorbates).

▪ We see that a feed stream containing a
contaminated component is passed through a solid
adsorbent, and the contaminant component is
retained in the adsorbent.
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▪ Now:
▪ If a second liquid feed (known as the desorbent)
containing a different component that also has an
affinity for adsorbing to the solid adsorbent…
▪ There will be competition between the contaminant
and this component for the limited number of
adsorption sites available.
▪ This will result in the removal or "desorption" of the
contaminant component from the solid.

▪ If sufficient quantity of desorbent is fed to the solid
adsorbent:
▪ the contaminant component can be completely
removed.
▪ Because the contaminant is removed and the solid
adsorbent:
▪ the adsorbent is said to have been "regenerated" and
so can be used again to adsorb and concentrate more
contaminant from fresh liquid streams.
▪ Removal of adsorbates can also be achieved by
changing:
▪ Pressure
▪ Temperature.
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▪ Adsorption processes can be divided into 2 groups:
▪ Bulk separation
▪ which involves the separation of up to half of the
components from a process stream
▪ Purification
▪ a process in which a small amount of impurity is removed
from the gas stream.

▪ Because regeneration is conducted periodically:
▪ Two or more vessels are used
▪ One desorbing while the other(s) adsorb(s)
▪ If the vessel is vertical:
▪ Gas flow is best employed downward.
▪ With upward flow:
▪ jiggling can cause particle attrition, pressure-drop increase, and
loss of material.
▪ However, for liquid mixtures:
▪ upward flow achieves better flow distribution
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▪ Regeneration occurs by one of four methods
▪ vaporization of the adsorbate with a hot purge gas (thermal-swing
adsorption),
▪ reduction of pressure to vaporize the adsorbate (pressure-swing
adsorption),
▪ inert purge stripping without change in temperature or pressure, and
▪ displacement desorption by a fluid containing a more strongly
adsorbed species.

▪ To achieve a very large surface area for adsorption per unit volume, highly porous
solid adsorbents with small diameter inter-connected pores are used.
▪ The adsorbents are less than 5-mm in diameter:
▪ Pore sizes of the order of 0.01- m in diameter.
▪ The porous structures can account for up to 50% of the volume of the material.
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▪ Major types of adsorbents in use are:
▪ Activated alumina
▪ Silica gel
▪ Activated carbon
▪ Molecular sieve carbon
▪ Molecular sieve zeolites
▪ Polymeric adsorbents.
▪ Most adsorbents are manufactured (such as activated carbons), but a few, such as
some zeolites, occur naturally.
▪ Each material has its own characteristics such as porosity, pore structure and nature
of its adsorbing surfaces.

▪ Pore sizes in adsorbents may be distributed throughout the solid.
▪ Pore sizes are classified generally into 3 ranges:
▪ macropores have "diamaters" in excess of 50-nm
▪ mesopores (also known as transitional pores) have "diameters" in the range 2 - 50-nm,
▪ micropores have "diameters" which are smaller than 2-nm.
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▪ Many adsorbent materials are amorphous and contain complex networks of inter-
connected micropores, mesopores and macropores.
▪ Examples:
▪ Carbons
▪ silica gels
▪ Aluminas
▪ In contrast:
▪ pores in zeolitic adsorbents have
precise dimensions.

www.ChemicalEngineeringGuy.com http://www.separationprocesses.com/Adsorption/AD_Chp01a.htm
▪ Typical applications of commercial adsorbents:

▪ The phenomenon of adsorption is essentially an attraction of:
▪ Adsorbate molecules to an adsorbent surface.
▪ The preferential concentration of molecules in the proximity of a surface
arises because the surface forces of an adsorbent solid are unsaturated.
▪ Both repulsive and attractive forces become balanced when adsorption
occurs.
▪ Adsorption is nearly always an exothermic process.
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▪ Physical adsorption
▪ van der Waals adsorption
▪ The individuality of the adsorbate and the adsorbent are preserved.
▪ Physical adsorption occurs quickly and may be:
▪ Mono-molecular (unimolecular) layer
▪ Monolayer, or 2, 3 or more layers thick (multi-molecular).
▪ As physical adsorption takes place:
▪ it begins as a monolayer.
▪ it can then become multi-layer
▪ If the pores are close to the size of the molecules:
▪ more adsorption occurs until the pores are filled with adsorbate.

▪ Chemisorption (activated adsorption)
▪ depending on the type of forces between the adsorbate and the adsorbent.
▪ there is a transfer or sharing of electron
▪ OR breakage of the adsorbate into atoms
▪ OR radicals which are bound separately.
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▪ Chemisorption involves:
▪ the formation of chemical bonds between the adsorbate and adsorbent is
a monolayer
▪ These often with a release of heat much larger than the heat of condensation.
▪ Chemisorption from a gas generally takes place only at temperatures
greater than 200 oC, and may be slow and irreversible.
▪ Most commercial adsorbents rely on physical adsorption; while catalysis
relies on chemisorption.

▪ Stirred Tank Slurry Adsorption
▪ Fixed Bed Adsorption (Percolation)
▪ Pressure Swing Adsorption
▪ Temperature Swing Adsorption
▪ Displacement Purge Adsorption (DPA)
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▪ Stirred Tank Slurry Adsorption
▪ In the stirred tank, slurry operation, a batch of
liquid is added to a powdered adsorbent (such as
activated carbon) in an agitated vessel to form a
slurry.
▪ The main application of this mode of operation is:
▪ the removal of very small amounts of dissolved, and
relatively large molecules, such as colouring agents,
from water.

▪ The required residence time of the operation is mainly determined by how fast equilibrium is
approached.
▪ Generally the spent adsorbent is removed from the slurry by filtration or sedimentation, and
is discarded.

▪ One way of reducing the total amount of adsorbent required is to carry out the batch processing
in 2 steps.
▪ The feed is first contacted with a fresh batch of adsorbent.
▪ After separation of the fluid from the adsorbent, the fluid is contacted with a further fresh batch
of adsorbent.

▪ Fixed Bed Adsorption (Percolation)
▪ The cyclic-batch operating mode using fixed bed
▪ It is widely used with both gas and liquid feeds.
▪ Separation in a fixed bed is typically:
▪ an unsteady state rate-controlled process.
▪ This means that conditions at any particular point
within the fixed bed vary with time.
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▪ Adsorption only occurs in a particular region of the
bed, known as the mass transfer zone (MTZ), which
moves through the bed.
▪ Applications of fixed bed adsorption, also called
percolation, include the removal of dissolved organic
compounds from water.

▪ The factors which determine the number and
arrangement of fixed beds include:
▪ total feed flow rate
▪ allowable pressure drop
▪ energy demands
▪ length of the MTZ
▪ method of adsorbent regeneration
▪ capital investment.
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▪ In order to achieve a steady flow of product:
▪ most applications include at least 2 beds.
▪ One is in the adsorption mode while,
▪ The other is in the regeneration mode

▪ Multiple beds in parallel would be used with a
relatively high flow rate and a short MTZ length
while multiple beds in series would be used if the
MTZ were long.
▪ For high flow rates and large MTZ lengths the
choice is likely to be multiple beds in series and
parallel.

▪ Pressure Swing Adsorption
▪ Regeneration in a PSA process is achieved by
reducing the partial pressure of the adsorbate.
▪ There are 2 ways in which this can be achieved:
▪ a reduction in the system total pressure,
▪ introduction of an inert gas while maintaining the
total system pressure.

▪ In the majority of pressure swing separations a
combination of the 2 methods is employed.
▪ Use of a purge fluid alone is unusual.
▪ Here, the effect of partial pressure on
equilibrium loading for:
▪ Type I isotherm at a temperature of T1.
▪ Reducing the partial pressure from p1 to p2
causes the equilibrium loading to be reduced
from q1 to q2.
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▪ Temperature Swing Adsorption
▪ Regeneration of adsorbent in a TSA process is
ahieved by an increase in temperature.
▪ The effect of temperature on the adsorption
equilibrium (Type I isotherm) of a single
adsorbate.
▪ For any given partial pressure of the adsorbate in
the gas phase (or concentration in the liquid
phase):
▪ An increase in temperature leads to a decrease in
the quantity adsorbed.
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▪ If the partial pressure remains constant at p1:
▪ increasing the temperature from T1 to T2 will decrease the
equilibrium loading from q1 to q2.
▪ A relatively modest increase in temperature can effect a
relatively large decrease in loading.
▪ It is therefore generally possible to desorb any components
provided that the temperature is high enough.
▪ However:
▪ it is important to ensure that the regeneration temperature does
not cause degradation of the adsorbents.

▪ Displacement Purge Adsorption (DPA)
▪ Adsorbates can be removed from the adsorbent surface by
replacing them with a more preferentially adsorbed species.
▪ This displacement fluid can be:
▪ A gas
▪ A Vapour
▪ A Liquid
▪ It should adsorb about as strongly as the components which
are to be desorbed.
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▪ The mechanism for desorption of the original adsorbate
involves 2 aspects:
▪ Partial pressure (or concentration) of original adsorbate in the
gas phase surrounding the adsorbent is reduced
▪ There is competitive adsorption for the displacement fluid.
▪ The displacement fluid is present on the adsorbent and
thus will contaminate the product.

▪ One advantage of the displacement fluid method of
regeneration is that:
▪ the net heat generated or consumed in the adsorbent will be
close to zero.
▪ This is due to the heat of adsorption of the displacement fluid
is likely to be close to that of the original adsorbate.
▪ Thus the temperature of the adsorbent should remain
more or less constant throughout the cycle.
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▪ Isolation of proteins
▪ Dehumidification
▪ Odor/Color/Taste Removal in Food Industry
▪ Gas Pollutant removal

▪ Water softening and de-ionization
▪ Hydrocarbon fractionation
▪ Pharmaceutical purification
▪ Adsorption is used to cool water for air
conditioning units.

▪ Aquarium filtration and home water filtration.
▪ Silica gel is used to prevent moisture from damaging electronics and
clothing.
▪ Adsorbents are used to increase the capacity of carbide-derived
carbons.
▪ Adsorbents are used to produce non-stick coatings on surfaces.
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▪ Adsorption may be used to extend the exposure time of
specific drugs.
▪ Zeolites are used to:
▪ remove carbon dioxide from natural gas
▪ remove carbon monoxide from reforming gas
▪ for catalytic cracking
▪ The process is used in chemistry labs for ion-exchange and
chromatography.

▪ Ion exchange usually describes a processes of purification of aqueous
solutions using solid polymeric ion exchange resin.
▪ More precisely:
▪ the term encompasses a large variety of processes where:
▪ ions are exchanged between two electrolytes.
▪ Aside from its use to purify drinking water:
▪ the technique is widely applied for purification and separation of a variety of
industrially and medicinally important chemicals.
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▪ Ion exchangers are either:
▪ Cation exchangers
▪ which exchange positively charged ions (cations)
▪ Anion exchanger
▪ which exchange negatively charged ions (anions).
▪ Amphoteric exchangers
▪ are able to exchange both cations and anions simultaneously.

▪ Typical ion exchangers are:
▪ ion-exchange resins (functionalized porous or gel polymer)
▪ Zeolites
▪ Montmorillonite
▪ Clay
▪ Soil humus.

▪ When operating an Ion Exchange Column:
▪ Ions of positive charge (cations) or negative charge (anions) in a
liquid solution, usually aqueous:
▪ Will be replaced by dissimilar and displaceable ions
▪ These ions are called counter-ions
▪ They contain the same charge
▪ These are in the solid ion exchanger
▪ which also contains:
▪ immobile, insoluble, and permanently bound co-ions of the opposite
charge.

▪ Typical examples of ions that can bind to ion exchangers are:
▪ H+ (proton) and OH− (hydroxide).
▪ Singly charged monatomic ions
▪ Na+, K+, and Cl−.
▪ Doubly charged monatomic ions:
▪ Ca2+ and Mg2+.
▪ Polyatomic inorganic ions:
▪ SO42− and PO43−.
▪ Organic bases:
▪ usually molecules containing the amine functional group −NR2H+.
▪ Organic acids:
▪ often molecules containing −COO− (carboxylic acid) functional groups.
▪ Biomolecules that can be ionized:
▪ amino acids, peptides, proteins, etc.
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▪ Water softening by ion exchange involves a cation exchanger,
in which a reaction replaces calcium ions with sodium ions:
▪ Ca2+, 2NaR, CaR2, NaR “R” is the ion exchanger.
▪ The exchange of ions is reversible and does not cause any
permanent change to the solid ion-exchanger structure.
▪ Thus, it can be used and reused

▪ The ion-exchange concept can be extended to the removal of
essentially all inorganic salts from water by a two-step
demineralization process or deionization.
▪ Step 1:
▪ A cation resin exchanges hydrogen ions for cations such as
calcium, magnesium, and sodium.
▪ Step 2:
▪ An anion resin exchanges hydroxyl ions for strongly and weakly
ionized anions such as sulfate, nitrate, chloride, and bicarbonate.
▪ The hydrogen and hydroxyl ions combine to form water.
▪ Regeneration of the cation and anion resins is usually
accomplished with sulfuric acid and sodium hydroxide.
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▪ Ion Exchange - Process

▪ Go to this website:
▪ https://www.novasep.com/technologies/ion-
exchange-and-adsorption-technologies-for-large-
scale-bio-industrial-applications.html
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Introduction to Mass Transfer Operations (3 of 5)

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