Biomedical Engineering Reference
In-Depth Information
The capture and separation of CO
2
is a complicated process. Currently,
the most widely employed technology involves an absorption process. The
flue gasses flow through several baths in which CO
2
is bound with amines
and chemically removed from the flue gas. This “scrubbing” technology,
however, consumes significant amounts of energy and requires expensive,
sophisticated installations, which make it very costly. It is expected that the
application of NMs for CO
2
removal would significantly reduce the costs of
infrastructure and operation. Research in the field of CO
2
separation NMs is
carried out by a number of scientific institutions, as some of it was done in
the context of the major European Sixth Framework Programme (FP6) proj-
ect NANOGLOWA (Nanostructured Membranes against Global Warming).
Until now, several novel NM types have been developed from both poly-
meric and inorganic materials. Carbon-based membranes, mesoporous
oxide membranes, and zeolite membranes are best described in the litera-
ture. Polymer membranes are relatively easy to manufacture and are suited
for low-temperature applications [50]. It is possible to control the gas perme-
ability and selectivity by just changing the morphology of the monomers
building the polymer [50]. Inorganic membranes have much greater thermal
and chemical stability than polymeric membranes [51]. Zeolite and silica
porous materials can act as molecular sieves, separating gas molecules by
their effective size [50]. Since the effective sizes of CO
2
, nitrogen, hydrogen,
and other gases present in fossil fuel conversion systems are very similar,
membrane pore spaces must be controlled on a scale comparable to the size
differences among these gas molecules [50]. Unfortunately, the gas separa-
tion NM technology is still not that developed, although active research con-
tinues in this direction.
7.4.2.3 Self-Assembled Monolayers on Mesoporous Supports (SAMMS)
The US Pacific Northwest National Laboratory (PNNL) developed the
SAMMS. SAMMS are a combination of mesoporous ceramics (with pore
diameters between 2 and 50 nm) and self-assembled chemical monolayers.
Both the monolayer and the mesoporous support can be functionalized to
remove certain contaminants (e.g., mercury, cadmium) [52]. SAMMS exhibit
faster adsorption, higher removal capacity, and better selectivity than many
other membrane and sorbent technologies (e.g., ion exchange resins, acti-
vated alumina filters, ferric oxide filters). The reason behind the rapid kinet-
ics is attributed to the rigid, open pore structure of SAMMS, which leaves all
of the binding sites available at all times to bind contaminant molecules [52].
A variety of SAMMS types exist today. The mostly used material is the
thiol-SAMMS. Other important materials are the ethylenediamine-, phos-
phonate-, hydroxypyridone-, and chelate-SAMMS. SAMMS can be assem-
bled into filters and used for the filtration of water and other liquids. SAMMS
types for a great variety of contaminants can be designed. Thiol-SAMMS
are currently actively used for the removal of mercury from contaminated
