Gas Emission Control - Mathematics and Statistics for the Environment - page 475

The working charge of carbon can be estimated by doubling the saturation capacity. Therefore, the
working charge is

Working charge = 2 × 2664 kg = 5328 kg

The square feet of superficial surface area is the surface area set by the maximum velocity of 0.508
m/s (100 ft/min) through the adsorber. The required surface area is

3

Q 3.78 m/s

0.508 m/s

m 2 A = = = 7.44

Maximumvelocity

For a horizontal flow adsorber this would correspond to a vessel approximately 2 m (6.6 ft) in width
and 4 m (13.1 ft) in length to give 8.0 m 2 (87 ft 2 ) surface area. This would supply more than the
required area. The flow rate is too high to be handled by a single vertical flow adsorber. An alternative
would be to use three vessels, two adsorbing while one is being regenerated. Each vessel then must be
sized to handle 1.89 m 3 is (4000 acfm). The area required for a limiting velocity of 0.508 m/s is

3

1.89 m/s
0.508 m/s

= 3.72 m 2 Area to handlehalfflow =

This cross-sectional are corresponds to a vessel diameter of

05 . 05 .

4 4

d = A = 372 . = 2.18 m(7ft)

π π

Determine the volume that the carbon would occupy in horizontal bed:

3

1
Density

m
480

11.1 m 3 Volume of carbon = Weight × = 5328 × =

kg

Note: For the three-bed vertical system, the volume of each bed is half of this, or 5.55 m 3 .

Now estimate bed depth:

3
2

Volume of carbon
Cross-secti

11.1 m
7.44 m

Depthof carbon = = = m 1.49

onalareaofadsorber

Note: The depth for the three-bed vertical system is the same, because both the volume and area
are halved.

16.4.4.5 Humidity
Activated carbon will preferentially adsorb nonpolar hydrocarbons over polar water vapor. The
water vapor molecules in the exhaust stream exhibit strong attractions for each other rather than the
adsorbent. At high relative humidity, over 50%, the number of water molecules increases such that
they begin to compete with the hydrocarbon molecule for active adsorption sites. This reduces the
capacity and the efficiency of the adsorption system. Exhaust streams with humidity greater than
50% may require installation of additional equipment to remove some of the moisture. Coolers to
remove the water are one solution. Dilution air with significantly less moisture in it than the process
stream has also been used. The contaminant stream may also be heated to reduce the humidity as
long as the increase in temperature does not greatly affect adsorption efficiency.