Membrane technology covers all engineering approaches for the transport
of substances between two fractions with the help of permeable membranes. In
general, mechanical separation processes for separating gaseous or liquid streams
use membrane technology. Applications
Membrane separation processes operate without heating and therefore use less
energy than conventional thermal separation processes such as
distillation, sublimation or crystallization. The separation process
is purely physical and both fractions can be used. Cold separation using
membrane technology is widely used in the food technology, biotechnology and
pharmaceutical industries. Furthermore, using membranes enables separations to
take place that would be impossible using thermal separation methods. For
example, it is impossible to separate the constituents of azeotropic liquids
or solutes which form isomorphic crystals by distillation or
recrystallization but such separations can be achieved using membrane
technology. Depending on the type of membrane, the selective separation of
certain individual substances or substance mixtures is possible.
Important technical applications include the production of drinking water by
reverse osmosis, filtrations in the food industry, the recovery of organic
vapours such as petro-chemical vapour recovery and the electrolysis for
chlorine production. In waste water treatment, membrane
technology is becoming increasingly important. With the help of
ultra/microfiltration it is possible to remove particles, colloids and
macromolecules, so that waste-water can be disinfected in this way. This is
needed if waste-water is discharged into sensitive waters especially those
designated for contact water-sports and recreation.
About half of the market is in medical applications such as use in artificial
kidneys to remove toxic substances by hemodialysis and as artificial lung for
bubble-free supply of oxygen in the blood.
The importance of membrane technology is growing in the field of environmental
protection. Even in modern energy recovery techniques membranes are
increasingly used, for example in fuel cells and in osmotic power plants.
Current market and forecast The global demand for membrane modules
was estimated at approximately 15.6 billion USD in 2012. Driven by new
developments and innovations in material science and process technologies, global
increasing demands, new applications, and others, the market is expected to
grow around 8% annually in the next years. It is forecast to increase to
21.22 billion USD in 2016 and reach 25 billion in 2018.
Mass transfer Two basic models can be distinguished
for mass transfer through the membrane: the solution-diffusion model and
the hydrodynamic model. In real membranes, these two transport
mechanisms certainly occur side by side, especially during ultra-filtration.
= Solution-diffusion model= In the solution-diffusion model,
transport occurs only by diffusion. The component that needs to be transported
must first be dissolved in the membrane. The general approach of the
solution-diffusion model is to assume that the chemical potential of the feed
and permeate fluids are in equilibrium with the adjacent membrane surfaces such
that appropriate expressions for the chemical potential in the fluid and
membrane phases can be equated at the solution-membrane interface. This
principle is more important for dense membranes without natural pores such as
those used for reverse osmosis and in fuel cells. During the filtration
process a boundary layer forms on the membrane. This concentration gradient is
created by molecules which cannot pass through the membrane. The effect is
referred as concentration polarization and, occurring during the filtration,
leads to a reduced trans-membrane flow. Concentration polarization is, in
principle, reversible by cleaning the membrane which results in the initial
flux being almost totally restored. Using a tangential flow to the membrane
can also minimize concentration polarization.
= Hydrodynamic model= Transport through pores – in the
simplest case – is done convectively. This requires the size of the pores to
be smaller than the diameter of the two separate components. Membranes which
function according to this principle are used mainly in micro- and
ultrafiltration. They are used to separate macromolecules from solutions,
colloids from a dispersion or remove bacteria. During this process the
not-passing particles or molecules form a pulpy mass on the membrane, and this
blockage of the membrane hampers the filtration. This blockage can be reduced
by the use of the cross-flow method. Here, the liquid to be filtered flows
along the front of the membrane and is separated by the pressure difference
between the front and back of the membrane into retentate on the front and
permeate on the back. The tangential flow on the front creates a shear stress
that cracks the filter cake and reduces the fouling.
Membrane operations According to the driving force of the
operation it is possible to distinguish: Pressure driven operations
microfiltration ultrafiltration
nanofiltration reverse osmosis
Concentration driven operations dialysis
pervaporation forward osmosis
artificial lung gas separation
Operations in an electric potential gradient
electrodialysis membrane electrolysis e.g. chloralkali
process electrodeionization
electrofiltration fuel cell
Operations in a temperature gradient membrane distillation
Membrane shapes and flow geometries There are two main flow configurations
of membrane processes: cross-flow tangential flow and dead-end
filtrations. In cross-flow filtration the feed flow is tangential to the
surface of membrane, retentate is removed from the same side further
downstream, whereas the permeate flow is tracked on the other side. In dead-end
filtration the direction of the fluid flow is normal to the membrane surface.
Both flow geometries offer some advantages and disadvantages. Generally,
dead-end filtration is used for feasibility studies on a laboratory
scale. The dead-end membranes are relatively easy to fabricate which
reduces the cost of the separation process. The dead-end membrane
separation process is easy to implement and the process is usually cheaper than
cross-flow membrane filtration. The dead-end filtration process is usually a
batch-type process, where the filtering solution is loaded into the membrane
device, which then allows passage of some particles subject to the driving
force. The main disadvantage of a dead end filtration is the extensive membrane
fouling and concentration polarization. The fouling is usually induced faster at
higher driving forces. Membrane fouling and particle retention in a feed
solution also builds up a concentration gradients and particle back flow. The
tangential flow devices are more cost and labor-intensive, but they are less
susceptible to fouling due to the sweeping effects and high shear rates of
the passing flow. The most commonly used synthetic membrane devices are flat
sheets/plates, spiral wounds, and hollow fibers.
Flat plates are usually constructed as circular thin flat membrane surfaces to
be used in dead-end geometry modules. Spiral wounds are constructed from
similar flat membranes but in the form of a “pocket” containing two membrane
sheets separated by a highly porous support plate. Several such pockets are
then wound around a tube to create a tangential flow geometry and to reduce
membrane fouling. hollow fiber modules consist of an assembly of
self-supporting fibers with dense skin separation layers, and a more open
matrix helping to withstand pressure gradients and maintain structural
integrity. The hollow fiber modules can contain up to 10,000 fibers ranging from
200 to 2500 μm in diameter; The main advantage of hollow fiber modules is
very large surface area within an enclosed volume, increasing the
efficiency of the separation process. Membrane performance and governing
equations The selection of synthetic membranes for
a targeted separation process is usually based on few requirements. Membranes
have to provide enough mass transfer area to process large amounts of feed
stream. The selected membrane has to have high selectivity properties for
certain particles; it has to resist fouling and to have high mechanical
stability. It also needs to be reproducible and to have low
manufacturing costs. The main modeling equation for the dead-end filtration at
constant pressure drop is represented by Darcy’s law:
where Vp and Q are the volume of the permeate and its volumetric flow rate
respectively, μ is dynamic viscosity of permeating fluid, A is membrane area, Rm
and R are the respective resistances of membrane and growing deposit of the
foulants. Rm can be interpreted as a membrane resistance to the solvent
permeation. This resistance is a membrane intrinsic property and is
expected to be fairly constant and independent of the driving force, Δp. R
is related to the type of membrane foulant, its concentration in the
filtering solution, and the nature of foulant-membrane interactions. Darcy’s
law allows for calculation of the membrane area for a targeted separation
at given conditions. The solute sieving coefficient is defined by the equation:
where Cf and Cp are the solute concentrations in feed and permeate
respectively. Hydraulic permeability is defined as the inverse of resistance and
is represented by the equation: where J is the permeate flux which is
the volumetric flow rate per unit of membrane area. The solute sieving
coefficient and hydraulic permeability allow the quick assessment of the
synthetic membrane performance. Membrane separation processes
Membrane separation processes have a very important role in the separation
industry. Nevertheless, they were not considered technically important until
the mid-1970s. Membrane separation processes differ based on separation
mechanisms and size of the separated particles. The widely used membrane
processes include microfiltration, ultrafiltration, nanofiltration, reverse
osmosis, electrolysis, dialysis, electrodialysis, gas separation, vapor
permeation, pervaporation, membrane distillation, and membrane contactors.
All processes except for pervaporation involve no phase change. All processes
exceptdialysis are pressure driven. Microfltration and ultrafiltration is
widely used in food and beverage processing, biotechnological
applications and pharmaceutical industry, water purification and
wastewater treatment, the microelectronics industry, and others.
Nanofiltration and reverse osmosis membranes are mainly used for water
purification purposes. Dense membranes are utilized for gas separations and
sometimes in membrane distillation. The later process helps in the separation of
azeotropic compositions reducing the costs of distillation processes.
Pore size and selectivity The pore sizes of technical membranes
are specified differently depending on the manufacturer. One common distinction
is by nominal pore size. It describes the maximum pore size distribution and
gives only vague information about the retention capacity of a membrane. The
exclusion limit or "cut-off" of the membrane is usually specified in the
form of NMWC. It is defined as the minimum molecular weight of a globular
molecule that is retained to 90% by the membrane. The cut-off, depending on the
method, can by converted to so-called D90, which is then expressed in a metric
unit. In practice the MWCO of the membrane should be at least 20% lower
than the molecular weight of the molecule that is to be separated.
Filter membranes are divided into four classes according to pore size:
The form and shape of the membrane pores are highly dependent on the
manufacturing process and are often difficult to specify. Therefore, for
characterization, test filtrations are carried out and the pore diameter refers
to the diameter of the smallest particles which could not pass through
the membrane. The rejection can be determined in
various ways and provides an indirect measurement of the pore size. One
possibility is the filtration of macromolecules, another is measurement
of the cut-off by gel permeation chromatography. These methods are used
mainly to measure membranes for ultrafiltration applications. Another
testing method is the filtration of particles with defined size and their
measurement with a particle sizer or by laser induced breakdown spectroscopy. A
vivid characterization is to measure the rejection of dextran blue or other
colored molecules. The retention of bacteriophage and bacteria, the
so-called "bacteriachallenge test", can also provide information about the pore
size. To determine the pore diameter, physical
methods such as porosimetry are also used, but a certain form of the pores is
assumed. Such methods are used for membranes whose pore geometry does not
match the ideal, and we get "nominal" pore diameter, which characterizes the
membrane, but does not necessarily reflect its actual filtration behavior
and selectivity. The selectivity is highly dependent on
the separation process, the composition of the membrane and its electrochemical
properties in addition to the pore size. With high selectivity, isotopes can be
enriched in nuclear engineering or industrial gases like nitrogen can be
recovered. Ideally, even racemics can be enriched with a suitable membrane.
When choosing membranes selectivity has priority over a high permeability, as
low flows can easily be offset by increasing the filter surface with a
modular structure. In gas phase filtration different deposition
mechanisms are operative, so that particles having sizes below the pore
size of the membrane can be retained as well.
See also Artificial membrane
Particle deposition Notes
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