MOLECULAR SIEVES
The three-dimensional framework of
zeolites contains a series of channels and voids. Contained
in these volumes are cations and water molecules. When
the water is removed the voids created can take in other molecules. This process is called ‘sorption,’ and the zeolites are
said to ‘sorb,’ i.e. they act as sorbents. The geometry
is the source of the ability of zeolites to separate mixtures of molecules
(gases and liquids) on the basis of their effective sizes. Zeolites are therefore described as ‘molecular sieves’.
Hydrophobic zeolite (zeolites without water) can be made by controlling the silicon to aluminum ratio. Different types of zeolites have windows from 0.3 to 3.0 nanometers. Changing the Si/Al ratio can affect the size of the window. Table 1 lists several examples of the affect that different zeolite window size has on selectively accepting various molecules.
Sorption and de-sorption methods:
Rates of sorption or de-sorption of
gaseous materials with zeolites can be determined at constant pressure or
volume using Fick’s law (10):
J = -D(c)dc/dx
J = flux (volume per area-time)
D = diffusivity
c = concentration
x represents
a coordinate axis along which diffusion takes place
This equation can be solved according
to the geometry involved in the system being studied. An
example of an equation for use under constant pressure is:
Mt/M∞ = 2A/V(Dt/ π)1/2
(ref 2)
Where Mt, M∞ are the amounts of sorbate taken up,
or released, at time intervals t=t and t=∞, A and V are the
external surface areas and volume, respectively, and D is the diffusion coefficient.
A plot of fractional attainment of
equilibrium (Mt/M∞) versus t1/2 should
give a straight line from which D can be determined. Examples
of these ‘gas transport isotherms’ are shown in figure 1 (2).
Currently applied zeolites in fixed bed reactors have very non-linear isotherms,
which increases adsorption at lower concentrations of volatile organic compounds
(VOCs). This allows a removal efficiency of 99% for
zeolites versus a carbon’s 95% efficiency (9, 3).
Chandak and Yin have shown that hydrophobic
zeolites are efficient at adsorbing and desorbing volatile organic compounds.
This is important since VOCs are known precursors to the formation of photochemical
oxidants such as ozone.
With this theoretical knowledge of
gaseous uptake by zeolites, devices have been built that concentrate volatile
organic compounds that would otherwise escape into the atmosphere.
A schematic of one commercially available
concentrator is shown in figure 2 (2).
This type of concentrator is a temperature-swing
adsorber with the zeolite bed mounted on a rotor that gets
regenerated on each rotation. The air-vapor flow
is axial through the rotor.
This particular concentrator is used
in conjunction with an incinerator, whereby the effluent of the concentrator
feeds the incinerator, minus the VOCs.
Additionally, the size and cost of
an incinerator is related to the total airflow. The
major cost savings comes from reducing the flow of air through the incinerator
that reduces the heat needed to raise the temperature of the airflow.
The biggest drawback of this method
has been in the small size of zeolite particles that lead eventually to a
very large pressure drop through reactive beds. Zeolites
now have been created in granules that allow freer airflow and are currently
be used in fixed bed adsorbers in Europe.
Other environmental applications include
the addition of hydrophobic medium and large pore zeolites upstream of
an automobile catalytic converter since typical catalytic converters do
not effectively operate until reaching very high temperature. This design would allow VOCs produced on cold start-up
to be trapped by the zeolite and eventually released into the catalytic
converter once operating temperature is reached.