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Complex separation technologies
A challenge for the process industries
For the process industries, choosing the right separation process is
essential to ensure the desired product quality and the use of
environmentally friendly production methods, and last but not least to
reduce costs. Membranes appear to be the separation technology of choice
while hybrid separation schemes are increasingly in demand.
What often stands between products that languish in research
laboratories and those that make it to commercial development is the
efficiency of the separation processes that are used to remove unwanted
substances and guarantee a final product that meets the desired purity
requirements. The efficiency, economy and environmental impact of the
separation technologies employed — whether they are used to purify
pharmaceutical compounds, make biodegradable polymers, extract chemicals
from renewable resources or clean up wastewater — can define the success
or failure of the new-product development and can directly impact the
feasibility and profitability associated with existing chemical process
operations. Most separations carried out during operations in the
chemical process industries (CPI) are achieved by classical separation
methods, such as distillation, solvent extraction, precipitation and
filtration. Others require novel technologies, such as supercritical
fluid extraction, liquid and catalytic membranes, liquid chromatography,
and electrophoresis. Demand for these newer methods is expected to
increase as organic and molecular separation methods are optimized and
scaled up to produce purified streams of water, chemicals and biological
compounds, while reducing or eliminating the generation of byproduct
waste streams and other environmental contaminants that are sometimes
produced when conventional separation methods are used. Membranes —
essentially thin layers of porous polymer through which fluid can
permeate — are expected to play an increasingly large role in the design
and configuration of today’s complex separation schemes. Membranes are
often the separation technology of choice because they can be customized
in many ways, and they offer a cost-effective way to achieve the
selectivity that is required by the process. U.S. demand for membrane
materials is forecast to grow by 7.4% from the current level to reach
$2.1 billion in 2006, according to market analysts at The Freedonia
Group (www.freedoniagroup.com). In volume terms, demand for membrane
materials is expected to increase to 1.3 billion square feet by 2006.
The total demand for membrane systems (including associated pumps, pipes
and vessels) is projected to approach $6 billion in 2006.
Hybrid separation schemes
Novel separation technologies are often used in tandem with one or
more of the classical separation methods, often in a single system. Such
combinations are called hybrid separation schemes. Pervaporation/vapour
permeation is a state-of-art technology for the separation of water –
organic and methanol organic azeotropes. Typical examples of hybrid
systems are found in the combination of distillation and
pervaporation/vapour permeation. Distillation concentrates the binary or
multi-component mixture close the respective azeotropic composition, the
vapour from the distillation column is either directly, or after
condensation as a liquid, passed over a membrane which is highly
permeable to one component (e.g. water, methanol), but nearly
impermeable to other components (organics). By the membrane system
either the specified final concentration of the organics can be
obtained, or the mixture is further separated in a second distillation
column. Thus separations can be achieved which otherwise are complex in
nature and energy comsuming. (infomembranes@sulzer.com) Much of the
recent research devoted to hybrid separation schemes has focused on
meeting the complex separation needs that are associated with the
manufacture of products and processes that support the life sciences.
For example, organic acids are typically produced by fermentation of
carbohydrate feedstocks. During the fermentation process, selected
microorganisms are used to convert the carbohydrate raw materials into
the desired organic acids. Because most of these microorganisms cannot
work efficiently under acidic conditions, a base is added to the
fermentation vessel, to maintain a high pH. The acid is then produced as
a salt, which must be converted to an organic acid outside the
fermentation vessel. Traditionally, the organic salt is converted to the
respective acid by acidification with a strong mineral acid, such as
sulfuric acid. The salt formed as a by-product must be removed,
typically via precipitation, crystallization or filtration, and then
either disposed of or sold for secondary use in another market
application. However, a newer hybrid separation scheme is now available,
and it offers some distinct advantages over the conventional route.
Eurodia Industries (www.eurodia.com) offers an electrodialysis system
with a bipolar membranes (this system is referred to as EDBM) to
directly acidify organic salts. This scheme eliminates the formation of
unwanted salt by-products. Under the driving force of an electric field,
the bipolar membrane can efficiently dissociate water into hydrogen (H+)
and hydroxyl (OH–) ions. The membrane consists of a layer of
anion-exchange membrane and a layer of cation-exchange membrane. The two
layers are bound together, either chemically or physically. A good
bipolar membrane has a very thin interface, but a very good bond between
its two layers of ion exchange membrane and a low voltage drop when
exposed to an electric field, and it allows for sufficient diffusion of
water from outside the membrane to feed the water-splitting reaction.
When a bipolar membrane is introduced in an electrodialysis stack, it is
possible to produce an acid and a base from a salt, instead of only
concentrating the salt (or desalting a product). In the case of organic
salts, this is because H+ ions, generated by the water splitting
produced by the bipolar membranes, acidify the organic salt solution.
At the same time, salt cations, either potassium, sodium or ammonium,
can combine with the hydroxyl ions generated in the bipolar membranes
into the base compartment. The base, which can be reused during the
fermentation process to control the pH, is recycled for reuse.
Meanwhile, Nizo Food Research (www.nizo.nl), and the University of
Twente (Twente, Netherlands), use membrane filtration and
electrodialysis to isolate bioactive ingredients from natural products.
The process, called electromembrane filtration, is more selective than
membrane filtration alone, and is said to be less expensive than
conventional column chromatography. The setup for electromembrane
filtration is similar to that for a conventional plate-in-frame
electrodialysis system, but the system employs an additional porous
membrane filter between the ion exchange membranes. The cell operates
with current densities of about 100 A/m2. The electric field transports
the cationic peptides through the membrane to the negative electrode,
thus separating them from the similar-sized neutral and anionic
peptides.
Separating hydrocarbons
Some of the earliest applications for hybrid separations have been
in enhanced oil recovery (EOR), where the carbon dioxide (CO2) content
is high — 70% or more. Such CO2–rich natural gas streams are good
candidates for membrane separation for removal of all or part of the
unwanted acid gas, according to UOP LLC (www.uop.com), which has done
extensive work in this field. Besides acid gas separation, membrane
processes for the separation of steam and higher hydrocarbons for the
dew-point span setting of natural gas are about to be launched
commercially. A further application is setting the methane number of
petroleum gas so that this gas can be used as fuel for gas engines. The
proportion of higher hydrocarbons in petroleum gas impairs the knock
resistance of the fuel, making it imperative to reduce it to a viable
level. This is where membrane processes provide an economic alternative
to condensation processes. Membranes have been used in industrial
applications for organic vapour separation since around 1990. The first
plants were used to separate petrol vapour caused by fuel transfer.
Currently over 60% of all petrol stations in Germany are equipped with
membrane plants for vapour recovery (www.borsig.de, www.vacono.com,
www.gkss.de). Membrane processes for organic vapour separations are
being increasingly applied for the treatment of exhaust air and process
gas streams in chemistry, pharmaceutics and petrochemistry. The majority
of prevalent solvents can be separated cost-effectively by membrane
techniques. Moreover, from both a technical and an economic point of
view hybrid procedures, coupling membrane and adsorption techniques,
have established themselves as the processes of choice for petrol vapour
recovery and clean-up of solvent-polluted waste air streams with minimal
residual pollutants (www.sterlingsat.com, www.gkss.de). Two examples of
monomer recovery in polymer production are the separation of vinyl
chloride in PVC production and the recovery of propylene and
polyethylene in the production of these two products
(www.sterlingsat.com, www.borsig.de, www.mtrinc.com ).
Membranes for gas separation
The use of cryogenic separation systems to produce industrial gases
from air and to separate other gas streams has an economic advantage in
applications where high purity or high flowrates are required. However,
membrane-based separation provides a lower-cost alternative to cryogenic
air separation for many applications, particularly those that can
tolerate slightly lower purity levels. For example, cryogenic separation
systems for separating nitrogen from air routinely produce nitrogen at
purities to 99.9999999% (1 part per billion) and flowrates to 900,000
ft3/hour. By comparison, commercial membrane-separation systems are
typically designed to produce nitrogen at flowrates to 50,000 ft3/day,
and purity levels to 99.99%. However, because membranes operate at
ambient temperatures, they consume much less energy than cryogenic
systems. All of the major industrial gas suppliers — Messer Group
(www.messergroup.com), Linde AG (www.linde.com), Air Liquide
(www.airliquide.com), BOC Gases (www.boc.com), Praxair (www.praxair.com)
and Air Products and Chemicals (www.airproducts.com) — have
membrane-based systems in various stages of commercialization.
Separation and recovery of hydrogen from purge gas streams in the
production of ammonia is a state-of- art process since several years.
Increasingly such membranes are used in refineries to recover hydrogen
from off-gases and concentrate it to more than 95 % purity for reuse.
Membranes become increasingly competitive with cryogenic processes if a
high level of portability is required. For example, membrane modules for
producing nitrogen and drying compressed air are widely used by
small-volume users, who typically require nitrogen or dry air to be
produced at the point of use. Suppliers of plant for drying compressed
air by cryogenic or adsorption processes often additionally offer
membrane driers. Ultratroc in Flensburg (www.ultratroc.com) deploys
membranes and modules jointly developed with GKSS Research Centre in
Geesthacht (www.gkss.de).
Besides nitrogen production from air, membranes are also employed in
oxygen enrichment techniques. The latter require membranes that are
highly oxygen-permeable. The Chinese have opened up a new market
specializing in the enrichment of combustion air from steam boilers and
of the oxygen content of room air in vehicles at heights exceeding 2000
m. These systems have been launched by Leader Science & Technology in
Dalian (www.gkss.de).
When conventional polymer membranes are used to separate unwanted CO2
from industrial exhaust streams (to make them suitable for discharge),
the gas stream typically has to be cooled to below 150°C. This cooling
requirement consumes energy, and increases the cost of separation.
However, Los Alamos National Laboratory (www.lanl.gov) is developing a
high-temperature membrane that can be used to separate and capture CO2
from industrial exhaust streams without the need for cooling.
LANL’s new membrane — a thin-film composite that consists of a polymer
film, polybenzimidazole (PBI), on a porous metallic support — is said to
be resistant to chemicals and can tolerate operating temperatures up to
370°C. Its most promising application, in terms of capturing [removing?]
CO2, is the separation of CO2 from synthesis gas. The combination of a
polymer with a metallic support allows this new membrane to be more
effective at higher pressures than conventional membranes, according to
researchers at Los Alamos.
Hot-gas filtration
Filtration involves the separation of particles from a fluid (liquid
or gas) by passage through a permeable medium. By tailoring
characteristics such as pore size, shape and uniformity, and type and
thickness of the filter media, a given filtration system can be designed
to retain specific contaminants. To clean up or recover products from
hot gas streams, ceramic- and metal-based filter elements are available
to operate at temperatures of 250–1,600°C. These new gas-cleaning
systems operate under high pressures and in chemical environments that
would destroy fabric bags. Because these systems operate at elevated
temperatures, they avoid or reduce some of the incidentals associated
with using fiber bags, such as cooling the hot gas by diluting it with
air. The condensation and sublimation that cause fouling are also
avoided, and in cases of incineration, there is no dioxin formation.
Higher cleaning intensities with less cleaning pressure than is typical
(0.5–1 Mpa) for hot gas filter systems that use jet-pulse cleaning is
available in the coupled pressure-pulse (CPP) system, developed by
Forschungszentrum Karlsruhe (www.fzk.de) and USF Schumacher Umwelt- und
Trenntechnik GmbH (www.usf-schumacher.de). CPP requires a cleaning-gas
pressure that is only 0.05–0.1 Mpa higher than system operating
pressure. The treatment vessel is divided into inlet and clean-gas
sides, using a tube sheet to which the filter elements are attached. The
CPP system does not suffer from high-pressure losses or large pressure
differentials across the filter and it does not cool the gas stream,
which could lead to condensation on the clean side of the filter.
Virus removal
Viruses and bacteria pose a constant threat to products and
processes. Unwanted microorganisms can be introduced during the
production of biological and bio-therapeutic compounds, destroying
thousands of dollars worth of recombinant product. Routine processing
and purification methods provide numerous opportunities to clear viruses
from biologically produced pharmaceuticals and other products. Many
methods that are used primarily for protein purification also do a good
job at removing viruses. These include precipitation, ion exchange, gel
filtration, hydrophobic interaction, affinity and mixed-mode-exchange
chromatography, and low-pH buffer elution. These separation methods
either inactivate the virus particles, or physically separate them from
the product based on size, charge, density, binding affinities, and
other differences between the virus and the product. In addition, other
physical and chemical methods are under development to render viruses
inactive. However, viral-inactivation treatments can cause a number of
side effects, such as protein denaturation (i.e. unwanted modification
of the protein structure) and subsequent loss of biological activity.
Furthermore, many inactivation methods are only partially effective, due
to the presence of resistant viruses or resistant fractions within a
population of viruses. Size-exclusion filtration is relatively
independent of product or process conditions. As a result, it is
considered to be relatively robust because its effectiveness is
independent of changing production parameters. And, because it does not
compromise the biological integrity of a product, size exclusion is less
likely to induce adverse biological and immunological reactions. In
addition, filtration based on size exclusion does not require the use of
stabilizers or chemical agents (or the subsequent removal of these
agents later). Pall Corp.’s Life Sciences Div. (www.pall.com) offers
several systems for virus filtration. One is its Ultipor VF Grade DV50,
a virus-retentive membrane filter designed for efficient removal of
viruses by size exclusion from such fluids as biopharmaceuticals, plasma
derivatives, diagnostic reagents, tissue culture media and buffers. Made
of modified polyvinylidene fluoride, the pleated filter features
extremely narrow pore-size distribution and ultra-low protein binding
properties. Another manufacturer is Millipore Corp. (www.millipore.com)
which markets the Viresolve family of virus-removal for monoclonal
antibody processing. The line includes Viresolve NFP capsules and
cartridges, and Viresolve NFR, an asymmetric, void-free membrane for
higher flowrates and fast operation. <<
Source: Dechema
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