Environmental Engineering Reference
In-Depth Information
Ultrafiltration, in turn, has become the method of choice for protein concentration,
replacing size-exclusion chromatography in this application [19]. Plasmid DNA [20] and
virus- like particles [21] can also be purified using UF. UF membranes are typically
composed of polysulfone, polyethersulfone, or regenerated cellulose, and experience fouling
problems analogous to those that plague MF membranes. Newly-developed composite
cellulose membranes are less susceptible to fouling than are many synthetic polymers, and are
more easily cleaned, but still possess excellent mechanical strength, thus outperforming other
membrane materials. Nevertheless, cellulose has had a much longer development history than
have synthetic polymers in membrane applications, and promising developments in the latter
are still expected [22].
2.2.3. Membrane chromatography
. Recent studies are generating renewed interest in
membrane chromatography [23]. Diffusion limitations are less pronounced in membranes
than in conventional bead packings, thus providing, in principle, a binding capacity
independent of flow rate. Although the high, internal surface area provided by small pores in
membranes is often offset by the reduction in convective flow, and it is challenging to achieve
binding capacities competitive with bead packings, research into optimization of membrane
chromatography continues [24, 25]. Especially encouraging are manifold designs with high
membrane density and low retention volumes that can accomplish up to 100-fold
concentrations in a single stage [5].
Flow distribution within membrane modules is an important consideration for optimal
performance, especially in manifold designs, and careful design of entrance and exit regions
is essential to provide even, well-distributed flow. In addition, sensing and maintenance of
constant concentration of the retained species at the membrane surface is becoming a new
robust control strategy, enhancing product yield and providing greater operational robustness
with respect to variations in feed quality [26]. Finally, additional efforts are directed toward
improving binding capacity, membrane selectivity, flow distribution, and flow rate, through
adjustments in pore size, membrane chemistry, and membrane morphology [27, 28].
2.3. Microbe Engineering
Sometimes bioprocessing procedures can be simplified greatly by engineering an
organism itself to produce a more convenient product, as in the development of DuPont's bio-
based 1,3-propanediol (3G) process [29]. In nature, two separate microorganisms are required
to convert glucose to glycerol and then to convert glycerol to 3G. To avoid the difficulty of
purifying the glycerol, a genetically-engineered microorganism was developed that converted
glucose directly to 3G. With increasing advances in metabolic pathway engineering (see
section II.A.2.7), such avenues are expected to become increasingly available.
3. Research Priorities
Separation technologies to facilitate commercial success of biomass conversions include
those suitable for low-molecular-weight organic acids, organic esters, diacids, and alcohols;
gases such as H2; and biobased oils such as biodiesel and biolubricants. Among these,
advances in membrane technologies and in processes utilizing environmentally-benign
solvents promise especially great benefits.
