Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 415

background concentration in the air prior to rush hour traffic is 5 mg/m 3
and the average wind velocity is 3 m/s.
6.35 3 A vent from a hydrocarbon process (process gas) contains appreciable
amounts of pentane (C5), hexane (C6), and heptane (C7), which are to
be recovered using a brine knockout condenser and a carbon adsorption
unit. The knockout condenser already existing and producing a conden-
sate has the compositions: 10, 40, and 50 mole percent C5, C6, and C7,
respectively. The temperature and pressure of the condensate and tail gas
are 10 C and 35 psia. Nitrogen is noncondensable under these conditions.
As the project engineer, your task is to recover the alkanes in the tail gas.
Assume ideal behavior. The Antoine constants for compounds are given
below:

Pentane Hexane Heptane

A 6.88 6.91 6.89

B 1076 1190 1264

C 233 226 217

(a) First calculate the composition of the tail gas that is emitted from the
knockoutcondenser.ExpressasmolefractionsofC5,C6,C7,and N 2 .
(b) Express the total concentration of hexane (C6) in kg/m 3 and define
it as C F , the inlet feed concentration to the adsorption bed. (Hint: use
ideal gas law, 14.696 psia = 101352 Pa). Based on laboratory data
for activated carbon, the mass transfer zone length and Freundlich
parameters have been calculated for the tail stream: δ = 0.1 m. The
Freundlich isotherm is C = K Freun W n , where K Freun is 10 kg/m 3
and n = 2.25. C is the concentration (kg/m 3 ) and W is the adsor-
bate concentration (kg/kg). If the adsorbent density is 384 kg/m 3 , the
actual volumetric flow rate through the bed is 1 m 3 /s, what should
be the length? The adsorber bed diameter is 2 m and breakthrough
time is 1 h. Note that it is not necessary to determine or use the per-
cent recovery for the absorber as the given mass transfer zone length
corresponds to 99% recovery.
6.36 2 What is the desired gas flow rate to obtain a 1-h breakthrough in treating
an air stream using a carbon bed of density 0.35 g/cm 3 . The inlet con-
centration of the pollutant is 0.008 kg/m 3 . The bed length is 2 m and has
a cross-sectional area of 6 m 2 . The Freundlich isotherm parameters are
K Freun = 500 kg/m 3 and n = 2. The mass transfer coefficient is 50s − 1 .
6.37 1 Answer whether the following statements are true or false. Give
explanations:
1. The mass transfer of a contaminant from a gas stream that is very
solubleinaliquid-absorbingphaseislikelytobegasphasecontrolled.
2. Particulates formed by condensation of gases (i.e., smoke) are likely
to be easily removed from a gas stream by gravity settling.
3. High ozone pollutant levels were responsible for the deaths of as
many as 4000 people in London in the early 1950s.
4. Atmospheric conditions during high winds and overcast skies would
be expected to be stable, with relatively little crosswind or vertical
pollutant dispersion.