Volatile
organic compounds (VOC) are air pollutants causing health and environmental
problems. VOC at low concentration in waste gas can be removed in bioreactors,
but this technique has a poor efficiency when treating hydrophobic VOC. One
possible way to remove them is to combine their absorption in an organic
solvent and their biodegradation in a two-phase partitioning bioreactor (figure
1) in order to regenerate the scrubbing liquid (Quijano et al., 2009). It was shown
that silicone oil (polydimethylsiloxane, PDMS) is an effective solvent for this purpose as it is immiscible with water, not
biodegradable, not toxic for microorganisms and has a high affinity for hydrophobic VOC (Darracq
et al., 2010).

Figure 1: Absorption-biodegradation process
for the removal of hydrophobic VOC.

The
volatility of low-viscosity PDMS may hinder the absorption process operation,
requiring the use of a PDMS of higher viscosity. Since the solvent viscosity
influences its spreading on the packing, the pressure drop in the contactor and
the VOC diffusion coefficient, the use of a high-viscosity PDMS might have a
considerable effect on the absorption performances.

The
purpose of this study was therefore to compare toluene absorption performances
using three PDMS of different viscosities (5 mm² s^-1, 20 mm²
s^-1 and 50 mm² s^-1) in a packed gas-liquid
contactor (inside diameter 0.12
m, packing height 1 m, gas flow rate 10 to 50 m³
h^-1 (0.25 to 1.2 m s^-1), liquid flow rate 0.1 to 1 m³
h^-1 (2.5 10^-2 to 2.5 10^-3 m s^-1)). Three
types of packing were used for this purpose: Raschig rings and IMTP, which are
classical and modern random packings respectively,
and Flexipac structured packing. Experimental results were compared in terms of
hydrodynamics characterization and toluene mass transfer performances.
Hydrodynamic performances obtained for each PDMS were also compared with water
(1 mm² s^-1) considered as the reference liquid.

The packed column flooded at lower gas and liquid flow
rates using Raschig rings as packing, while the hydrodynamic performances of IMTP
were slightly higher than the performances of Flexipac. This was expected
because the structures of IMTP and Flexipac cause a better distribution of gas
and liquid flows across the bed than the structure of Raschig rings.

For a given packing, the increase of the solvent
viscosity decreased the flow rates at loading and flooding points (figure 2). Viscous
liquid flow less easily in the packed colunm, thus reduce its hydrodynamic
performances.

Figure 2: Loading and flooding points of water,
PDMS 5 (viscosity = 5 mm²
s^-1), and PDMS 50 (viscosity = 50 mm² s^-1) using IMTP packing.

For all tested PDMS (viscosity = 5 and 50 mm² s^-1) absorption efficiency of toluene was much higher
than in water (figure 3); it should be related to the difference in partition
coefficient of toluene between each solvent (H_{cc water}/H_{cc PDMS}
= 271).

While an efficiency higher than 0.99 could be reached
with each PDMS, this was achieved at lower liquid flow rates when using PDMS 5
than PDMS 50. It is suspected that the toluene diffusion coefficient decreases
with increasing silicone oil viscosity. Mass transfer efficiency would follow
the same trend than the diffusion coefficient, since the diffusion coefficient
of toluene in the solvent has a direct influence on its mass tranfer
coefficient.

Figure 3: Absorption efficiency of toluene in
water, PDMS 5 (viscosity = 5 mm² s^-1), and PDMS 50 (viscosity = 50 mm² s^-1) using Flexipac packing. Gas
flow rate = 40 m³ h^-1, C₀ = 114 mg m^-3.

References:

Darracq, G., Couvert, A., Couriol, C.,
Amrane, A., Thomas, D., Dumont, E., Andres, Y., Le Cloirec, P., 2010. Silicone
oil: An effective absorbent for the removal of hydrophobic volatile organic
compounds. Journal of Chemical Technology & Biotechnology 85, 309?313.

Quijano,
G., Hernandez, M., Thalasso, F., Muñoz, R., Villaverde, S., 2009. Two-phase
partitioning bioreactors in environmental biotechnology. Applied Microbiology and Biotechnologyn 84, 829-246.