Water tools

Packed tower air stripper: transfer units and packing height

The packing height is the product of how hard the separation is (transfer units) and how effective the packing is (height per unit). The same relation rates an existing tower at a new duty.

Water flowm³/h

Tower cross-sectionm²Standard shells 1.22, 1.83, 2.44, 3.05, 3.66 m diameter.

Overall mass transfer coefficient KLa1/sFrom pilot data or a correlation with a 0.7 safety factor for packings over 2.5 cm; typically 0.004 to 0.03.

Stripping factor

Influent concentrationµg/L

Treatment objectiveµg/L

Existing packing height to rate (optional)m

Packing height

7.82m

Height of a transfer unit

3.04m

Number of transfer units

2.57

Liquid loading

822L/m²·min

How it works

H T U = \frac{Q}{A K_{L} a} N T U = \frac{S}{S - 1} \ln [\frac{1 + (C_{0} / C_{e}) (S - 1)}{S}] Z = H T U \cdot N T U

(1)

Rating an existing tower inverts the same relation for the effluent:

C_{e} = \frac{C_{0} (S - 1)}{S \exp [\frac{Z K_{L} a (S - 1)}{(Q / A) S}] - 1}

(2)

where

$H T U$: height of a transfer unit, m
$Q$: water flow, m³/s
$A$: tower cross-section, m²
$K_{L} a$: overall mass transfer coefficient, 1/s
$N T U$: number of transfer units
$S$: stripping factor
$C_{0}, C_{e}$: influent and effluent concentration
$Z$: packing height, m

Assumes steady countercurrent plug flow in both phases, clean inlet air, linear equilibrium and no reaction. Smaller packing raises the mass transfer coefficient and lowers the height per unit; the transfer units measure only the difficulty of the separation. With several compounds compute each and build to the tallest, then confirm the others at that height. Pilot confirmation is standard before construction.

Packed tower air stripper: transfer units and packing height

How it works

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