Environmental Engineering Reference
In-Depth Information
E XAMPLE 6.33 E FFICIENCY OF A H AZARDOUS W ASTE I NCINERATOR
A chlorinated hydrocarbon waste is to be incinerated in a rotary kiln at a temperature of
1500 F. The activation energy for the reaction was observed to be 50 kcal/mol and the
pre-exponential factor was 3 × 10 7 P/s. Laboratory work has shown that the combustion
gas flow rate required was 500 SCF/lb at 70 F. If the feed rate is 1000 lb/h at a face
velocity of 30 ft/s and a kiln residence time of 1 s is desired, what should be the length
and diameter of the incinerator. What is the efficiency of the reactor if it were a PFR?
From the given gas flow rate and feed rate we get feed flow rate at 70 F.
Q g = ( 500 ft 3 /lb)(1000 lb/h) (1/60 h/min ) = 8333 ft 3 /min. Feed flow rate at 1500 F
is given by Q g = 8333 ( T F /T i ) = ( 8333 )( 1500 + 460 / 70 + 460 ) = 30, 817 ft 3 /min.
We know that U g = Q g /A = X s / τ R , where X s is the length of the incinerator and
τ R is the mean residence time. If the kiln is cylindrical, A = π (D 2 / 4 ) . Hence
Q g = π (D 2 / 4 )(X s / τ R ) or D = 4 Q g τ R / π X s . Since U g τ R = X s = ( 30 )( 1 ) = 30 ft,
we have D = ( 4 )( 30817 )( 1 )/( 60 )( 3.14 )( 30 ) =
4.7 ft and X s =
30 ft. Efficiency of a PFR is E = 1 exp ( k τ R ) . We need k at 1500 F. K =
( 3 × 10 7 ) exp [− 50, 000 /( 1.98 )( 1960 ) ]= 76 s 1 . Hence E = 1 exp [− ( 76 )( 2 ) ]= 1.
Complete destruction of the material is possible.
4.7 ft. Thus D =
6.5 APPLICATIONS OF CHEMICAL KINETICS IN
ENVIRONMENTAL BIOENGINEERING
Our world is teeming with various biological species. Microorganisms lie at the base
of our evolutionary chain and have played a critical role in changing the earth's
environment. They possess the ability to transform complex molecules. It has long
been known that microorganisms are at work in composting waste into manure. They
are also critical in treating sewage and wastewater. Table 6.15 lists typical examples
where microorganisms play a role in environmental engineering.
Soils, sediments, and groundwater contain a milieu of naturally occurring microor-
ganisms that can break down toxic compounds. By providing sufficient nutrients and
TABLE 6.15
Examples of Bioengineering in Environmental Processes
Subsurface bioremediation
Addition of microbes to waste materials (groundwater,
soils, and sediments) mostly in situ
Wastewater treatment
Removal of organic compounds from industrial
wastewaters, activated sludge, and trickling filters
Land farming
Petroleum exploration and production waste treatment
(above ground)
Enhanced NAPL recovery
Introduction of bacterial cultures in subsurface soils for
simulation of surfactant production
Oil spill cleanup
Microorganisms for oil spill biodegradation
Engineered microorganisms
To treat special waste materials (highly toxic and
refractory compounds)
 
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