Environmental Engineering Reference
In-Depth Information
Section 1
Introduction
Adsorption
has been used for many centuries. For example, in 1500 BC
the Egyptians used charcoal to adsorb odorous vapors from wounds.
Hindu documents dating back to 450 BC report the use of charcoal to
purify water. Charcoal is one of the oldest types of adsorbents, and it is
still used today in the form of “activated charcoal,” also called activated
carbon. In modern times, activated carbon is produced from carbona-
ceous sources ranging from materials as diverse as nutshells, peat,
wood, coir, lignite, coal, and petroleum pitch. The raw carbon gets “acti-
vated,” meaning its surface area is greatly expanded. This is achieved
through various chemical and physical processes, which can yield areas
as large as 500-1,500 m
2
/g.
In daily life, you may have encountered adsorption in the form of the
little bags of silica gel that come in the pockets of new clothes. Silica
gel is very effi cient at adsorbing water, and bags of it are often added
to boxes in which moisture-sensitive materials are shipped. Many high
school chemistry courses include an experiment that shows decolor-
izing a solution using a charcoal adsorber. Baking soda is often used in
refrigerators to adsorb vapors emanating from stored foods. Of course,
in this context we are interested in separating gasses, but it is impor-
tant to realize that use of materials for adsorption is already established
in industry. Examples of popular adsorption materials are shown in
Figure 6.1.1
.
A new class of solid adsorbents is comprised of nanoporous crystal-
line materials. These crystals have pores with diameters on the order of
nanometers, which can have very large internal surface areas where
adsorption can occur. As in liquid absorption, we can increase the
capacity by increasing the amount of material. Hence, for the purpose of
CCS, the only difference between these crystalline materials and the
liquid absorbents is that one is solid and the other is liquid. So we use
the term absorption when molecules are absorbed in a liquid and adsorp-
tion if they are adsorbed in a solid. This allows us to stay as close as
possible to the nomenclature used in the literature.
Search WWH ::
Custom Search