Environmental Engineering Reference
In-Depth Information
8.7 Very wet manure containing only 10 wt% dry matter (the rest can be considered
to be water) is spray dried in a continuous-flow chamber. Small droplets of 1 mm
radius with a density of 1000 kg
m −3 are introduced in the spray chamber with a
s −1 . A large flow of hot air is introduced at 130 C in cross-flow
mode and has a relative humidity of 25%. Calculate the temperature of the
droplets in the initial drying phase. Use steam tables and property tables for
air for the calculations.
Can the Chilton
velocity of 5 m
-
Colburn analogy be used in this case?
8.8 For a wet sludge of algae residue, a spray dryer is operated at steady state. The
slurry, containing 30 wt% solids, enters the spray dryer through a spray head.
Dry air enters at 177 C and 1 atm and moist air exits at 85 C and 1 atm, with
a relative humidity of 21% and a volume flow of 310 m 3
min −1 . Dried particles
are separated from the moist airstream.
Calculate: (i) The volume flow of dry air that enters in m 3
min −1
(ii) The mass flow of leaving dried particles in kg
min −1
8.9 A wood drying system consists of boiler in which dried wood (with 25 wt%
moisture content) is burned with ambient air (25 C) and hot water is generated.
The flue gas available at 130 C (immediately after the boiler) is used to dry the
wood from 55 wt% moisture content in an integrated manner. The wood enters
the dryer at 25 C. The initial LHV of the wood is 9.5 MJ
kg −1 (ar) and the wet
h −1 . Consider a combustion system in which combus-
tion with 25% excess air is applied. Use the wood composition for pellets
(daf basis) presented in Table 2.3. The boiler and dryer may be assumed to
have no heat losses. The c p value of wood can be assumed to be constant at
1200 kJ
wood feed rate is 250 kg
kg −1
K −1 .
a. Which type of dryer do you prefer for this system and why?
b. What must be the capacity of the air fan (in kg
h −1 )?
c. What is the amount of heat transferred per unit of time to the water system in
the boiler?
d. What is the end temperature of the flue gas after the dryer?
e. Is the temperature at the dryer exit above the dew point of the water?
PROJECTS
P8.1 Visit a plant where biomass (also, e.g., food/feed) is processed, and identify
the different pretreatment techniques. Which techniques are most energy con-
suming and how much energy is consumed?
P8.2 Use a lab-scale ball mill (or develop one) and determine the HGI for some
types of biomass. Study the impact of the moisture content and thermal
pretreatment conditions (torrefaction; see Chapter 12).
 
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