Geoscience Reference
In-Depth Information
16.4.4 F
aCtors
a
FFeCting
a
dsorption
A number of factors or system variables influence the performance of an adsorption system, includ-
ing the following (USEPA, 1981, p. 5-18)
• Temperature
• Pressure
• Gas velocity
• Bed depth
• Humidity
• Contamination
We discuss these variables and their effects on the adsorption process in this subsection.
16.4.4.1 Temperature
For physical adsorption processes, the capacity of an adsorbent decreases as the temperature of the
system increases. As the temperature increases, the vapor pressure of the adsorbate increases, rais-
ing the energy level of the adsorbed molecules. Adsorbed molecules now have sufficient energy to
overcome the van der Waal's attraction and migrate back to the gas phase. Molecules already in the
gas phase tend to stay there due to their high vapor gas phase. As a general rule, adsorber tempera-
tures are kept below 130°F (54°C) to ensure adequate bed capacities. Temperatures above this limit
can be divided by cooling the exhaust stream to be treated.
16.4.4.2 Pressure
Adsorption capacity increases with an increase in the partial pressure of the vapor. The partial
pressure of a vapor is proportional to the total pressure of the system. Any increase in pressure will
increase the adsorption capacity of a system. The increase in capacity occurs because of a decrease
in the mean free path of vapor at higher pressures. Simply, the molecules are packed more tightly
together. More molecules have a chance to hit the available adsorption sites, increasing the number
of molecules adsorbed.
16.4.4.3 Gas Velocity
The gas velocity through the adsorber determines the contact or residence time between the con-
taminant stream and adsorbent. The residence time directly affects capture efficiency. The slower
the contaminant stream flows through the adsorbent bed, the greater the probability of a contami-
nant molecule hitting an available site. Once a molecule has been captured, it will stay on the
surface until the physical conditions of the system are changed. In order to achieve 90% + capture
efficiency, most carbon adsorption stems are designed for a maximum gas flow velocity of 30 m/
min (100 ft/min) through the adsorber. A lower limit of at least 6 m/min (20 ft/min) is maintained
to avoid flow distribution problems, such as channeling.
16.4.4.4 Bed Depth
Providing a sufficient depth of adsorbent is very important in achieving efficient gas removal. If the
adsorber bed depth is shorter than the required mass transfer zone, breakthrough will occur imme-
diately, rendering the system ineffective. Computing the length of the mass transfer zone (MTZ) is
very difficult since it depends upon six factors: adsorbent particle size, gas velocity, adsorbate con-
centration, fluid properties of the gas stream, temperature, and pressure. The relationship between
breakthrough capacity and MTZ is
0.
CMTZ
(
)
+
CD MTZ
D
(
−
)
s
s
C
=
(16.15)
B
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