Page 313 - Chemical Process Equipment - Selection and Design

9.1 1. THEORY OF AIR-WATER INTERACTION IN PACKED TOWERS
TABLE 9.18. Effects of Variables on Operation of Spray Dryers

Variable Increased Factors Increased Factors Decreased
Chamber inlet temperature Feed rate and thus: bulk density (b)
product rate,
particle size (b),
product moisture content,
chamber wall build-up (a)
Chamber cutlet temperature product thermal degradation feed rate and thus:
(a) product rate
particle size (b)
product moisture content
chamber wall build-up
Gas volume rate feed rate and thus: residence time
product rate,
particle size (b),
product moisture content,
chamber wall build-up (a)
Feed concentration product rate,
bulk density (b),
particle size (b)
Atomizer speed
Atomizer disc diameter
For stable lattices bulk density particle size and thus:
product moisture content
chamber wall build-up
For unstable lattices coagulation (a) and thus:
particle size,
product moisture content,
chamber wall build-up
Atomizer vane depth bulk density (b) particle size (b) and thus:
Atomizer vane number product moisture content,
chamber wall build-up
Atomizer vane radial length For unstable lattices
particle size
chamber wall build-up
Feed surface tension bulk density (b) particle size (b)
Chamber inlet gas humidity product moisture content,
chamber wall build-up (a)
‘This factor will only occur if a critical value of the variable is exceeded.
Mot for suspensions.
(Nonhebel and Moss, 1971)

The smallest pilot unit supplied by Bowen Engineering has a Analysis of the interaction of air and water involves the making
diameter of 30 in. and straight side of 29 in., employs parallel flow, of material and enthalpy balances. These are made over a
up to 25 ACFM, 150-10OOoF, particle sizes 30-40 pm average, differential section of the tower shown on Figure 9.15(a) and are
either pneumatic nozzle or spray wheel. The performance of this subsequently integrated to establish the size of equipment for a
unit is given in Table 9.3.9. The magnitude of the “product number” given performance. In terms of empirical heat, k,, and mass, k,,
is arrived at by pilot plant work and experience; it increases with transfer coefficients, these balances are
increased difficulty of drying or thermal sensitivity or both.
Although much useful information can be obtained on this small Gdh=LC,dT=LdT (9.21)
scale, Williams-Gardner (1971) states that data on at least a 7 ft dia = k,(h, - h) dz (9.22)
dryer be obtained for final design of large capacity units. = k,(T - T,) dz. (9.23)

9.11. THEORY OF AIR-WATER INTERACTION IN PACKED In Eq. (9.21) the heat capacity of water has been taken as unity.
TOWERS The approximations that are involved in making an enthalpy
difference a driving force are discussed for example by Foust et al.
The key properties of mixtures of air and water vapor are described (1980). Rearrangement and integration leads to the results
in Section 9.1. Here the interactions of air and water in packed
towers under steady flow conditions will be analyzed. The primary
objectives of such operations may be to humidify or dehumidify the (9.24)
air as needed for particular drying processes or other processes, or
to cool process water used for heat transfer elsewhere in the plant. = $dr
Humidification-dehumidification usually is accomplished in spray h, - h (9.25)
towers, whereas cooling towers almost invariably are filled with
some type of packing of open structure to improve contacting but (9.26)
with minimum pressure drop of air.