Water tools

Henry's constant, minimum air to water ratio and stripping factor

How much air a stripper needs is fixed by the compound's Henry's constant at the water temperature. The minimum ratio comes from an operating-line argument; design runs at a few times that.

CompoundVolatile organic

Water temperature°C

Influent concentrationµg/L

Treatment objectiveµg/L

Design ratio as a multiple of the minimum3.5 minimises tower volume and power for most VOCs.

Henry's constant (dimensionless)

0.2370

Tabulated value at the nearest of 10, 15, 20, 25, 30 °C.

Minimum air to water ratio

4.01m³ air / m³ water

Design air to water ratio

14.0m³ air / m³ water

Stripping factor

3.33

Practical range about 2 to 20.

How it works

Henry's constant for a gas at temperature from its enthalpy of dissolution; for volatile organics the tabulated values at 10 to 30 °C are used directly:

H = K_{C} \exp (- \frac{Δ H_{d i s}}{R T})

(1)

With clean inlet air the operating line through the top of a countercurrent tower gives the minimum air ratio, and the stripping factor compares the design ratio with it:

{(\frac{Q_{a}}{Q})}_{m i n} = \frac{C_{0} - C_{e}}{H C_{0}} S = \frac{Q_{a}}{Q} H

(2)

where

$H$: dimensionless Henry's constant
$K_{C}$: pre-exponential constant of the gas
$Δ H_{d i s}$: enthalpy of dissolution, J/mol
$R$: gas constant, 8.314 J/mol·K
$T$: absolute temperature, K
$Q_{a} / Q$: air to water volume ratio
$C_{0}$: influent concentration
$C_{e}$: treatment objective, same unit
$S$: stripping factor

A stripping factor of 1 needs an infinite tower; below 1 the separation is equilibrium limited. About 3.5 times the minimum ratio minimises tower volume and blower power over most of the practical range. With several compounds, the one with the lowest Henry's constant sets the ratio. Ionic strength, surfactants and pH (for ionisable gases) change the effective constant.

Henry's constant, minimum air to water ratio and stripping factor

How it works

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