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5.14 CHAPTER FIVE

Design of Packed Columns
The rate at which a volatile compound is removed by air stripping in packed tower aer-
ation (PTA) depends on the following factors:
• Air-to-water ratio (A/W ratio)
• Height of packing in the column
• Available surface area for mass transfer
• Water loading rate
• Air and water temperatures
• Physical chemistry of the contaminants to be removed

The first four factors may be controlled in the design of an air stripping unit.
Airflow and Water Flow Requirements. Airflow required for a packed column depends
on the Henry's law coefficient for the compounds to be removed from the water. Pack-
ing height is a function of the required VOC removal efficiency. In general, an increase
in packing height results in higher VOC removal.
The air-to-water ratio used in a column is a function of water temperature and de-
sired level of contaminant removal. This ratio determines the size of the blower, the pri-
mary component of operating costs for PTA systems. Air-to-water ratios typically range
from 30 : 1 to 100 : 1. The water loading rate--the amount of water passing through the
column--usually ranges from 25 to 30 gpm/ft 2 [17 to 20 (L/s)/m2]. Compounds that are
more difficult to remove, such as methyl tertiary butyl ether (M'IIBE), require higher air-
to-water ratios (150:1 or greater) and are generally designed at lower liquid loading
rates (15 to 17 gprn/ft 2) to minimize air pressure drop and reduce blower energy re-
quirements. Column diameter is selected to accommodate the desired water loading on
the column.
Packed column aeration removal effectiveness usually increases with an increase in
water temperature, but it has been found that heating the influent water to increase re-
moval effectiveness is not generally cost-effective.

Column Design. The relationship between packing height and column performance is
derived from the basic mass-transfer relationship. In the following formula, packing height
Z, in feet, is related to the height of a transfer unit (HTU) in feet, and the number of trans-
fer units (NTU):
Z = (HTU)(NTU)
The HTU is a function of the liquid loading rate and KLa. This relationship is ex-
pressed in the following equation:
L
HTU -
KLaCo
where L = liquid flow, (lb mol/h)/ft 2 [(g mol/h)/m 2] and Co = molar density of water,
lb mol/ft 3 (g mol/m3).
The number of transfer units is a function of column performance and the substance
to be removed. This relationship is expressed as

NTU = R1 In (Xi]Xo)(RI - 1) + 1
R1 - 1 R1