?Equivalent absorption capacity? concept applied to the VOCs absorption in a countercurrent packed-bed column using water/silicone oil mixtures

¹E. Dumont, ^2,3A. Couvert, ^2,3A. Amrane, ^2,3C. Couriol, ⁴G. Darracq, ^2,3P. Le Cloirec

¹UMR CNRS 6144 GEPEA, L?UNAM, École des Mines de Nantes, La Chantrerie, 4 rue Alfred Kastler, B.P. 20722, 44307 Nantes Cedex 3, France, email: eric.dumont@mines-nantes.fr

²Ã‰cole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France

³Université Européenne de Bretagne, France annabelle.couvert@ensc-rennes.fr  abdeltif.amrane@ensc-rennes.fr catherine.couriol@univ-rennes1.fr pierre.le-cloirec@ensc-rennes.fr

⁴Eau de Paris ? DRDQE, 33 Avenue Jean Jaurès, 94200 Ivry-sur-Seine, France, guillaume.darracq@eaudeparis.fr

Keywords: Multiphase absorption, Mass transfer, Silicone oil, Air pollution

The removal of volatile organic compounds (VOCs) can be achieved using bioscrubbers (Daugulis, 2001). However, some of VOCs are scarcely soluble in water leading to mass transfer limitations. In response to the low solubility of some pollutants, a water-immiscible organic solvent (typically silicone oils) can be added to water in order to improve the mass transfer of pollutants from the gas phase to the liquid phase. The mass transfer in such multiphase system (gas/oil/water) is typically performed in a countercurrent gas?liquid absorber (Quijano et al., 2009). When the liquid phase consists of a mixture of an aqueous phase and a non-aqueous liquid phase, the physicochemical properties of the mixture have to be sufficiently known to enable the design to be carried out. The major problem for the design of such a multiphase absorber depends directly on the knowledge of the equilibrium solubility of the VOCs between the gas phase and the water/silicone oil mixture. To solve this issue, an ?equivalent absorption capacity? concept (EAC) has been developed for reactor design (Dumont et al., 2010, 2011, 2012). According to this concept, the absorption capacity of any water/silicone oil mixture is equivalent to that of a pseudo-homogeneous phase whose physical properties (molecular weight M_mix and density r_mix) can be expressed as:

M_mix = (1-f) M_water(H_mix/H_water) (r_mix/r_water)+ f M_oil (H_mix/H_oil)(r_mix/r_oil)

r_mix = (1-f) r_water(H_mix/H_water) + f r_oil (H_mix/H_oil)

Where H_mix is the VOC partition coefficient between air and the liquid mixture, which can be expressed as a function of the VOC partition coefficients for air/water and air/silicone oil, respectively, and as a function of the silicone volume fraction in the mixture (f):

(1/H_mix) = (1-f) /H_water + f /H_oil

The aim of this study is to experimentally apply the ?equivalent absorption capacity? concept to a countercurrent gas?liquid absorber. The absorption of two VOCs (toluene and dimethyl disulfide -DMDS-) is studied in a packed gas-liquid contactor (inside diameter 0.12 m) filled with 1/2 inch glass Raschig rings (packing height 1 m) for three gas flow rates (G = 18, 25 and 32 m³ h^-1; gas velocities 0.64, 0.88 and 1.14 m s^-1 respectively ). For each gas flow rate, VOCs absorption is carried out using three liquid solutions, i.e. (i) water, (ii) silicone oil (dimethylpolysiloxane; Rhodorsils fluids 47V5 from the BlueStar Company, France; dynamic viscosity: h = 5 mPa s) and (iii) a (90/10 - v/v) water/silicone oil mixture (f = 0.1). The liquid flow rates are ranged from 0.096 to 0.47 m³ h^-1 (molar L/G ratios from 0.09 to 36.0 and liquid velocities from 3.4 10^-3 to 16.3 10^-3 m s^-1) according to the liquid considered. The physical properties of the liquid solutions are summarized in Table 1.

The study presents the absorption efficiencies and pressure drops determined for each experiment (Table 2). In comparison with classical procedure calculation based on linearity change in VOC partition coefficient with silicone oil volume fraction, experimental results highlight that the ?equivalent absorption capacity? concept describes satisfactorily the absorption behavior of the water/silicone oil mixture (f = 0.1) in a countercurrent gas?liquid contactor. Moreover, the experimental data confirms the predicted results given in Dumont et al. (2011) showing that pure silicone oil has to be used rather than water/silicone oil mixtures for hydrophobic VOC absorption.

Table 1: Physical properties of the liquid absorbing solutions (T = 298 K).

Table 2: Experimental results (T = 298 K).

References

Daugulis A.J. Two-phase partitioning bioreactors: a new technology platform for destroying xenobiotics. Trends Biotechnol. 19, 457?462 (2001).

Dumont E., Darracq G., Couvert A., Couriol C., Amrane A., Thomas D., Andrès Y., Le Cloirec P. Determination of partition coefficients of three volatile organic compounds (dimethylsulfide, dimethyldisulfide and toluene) in water/silicone oil mixtures, Chemical Engineering Journal, 162, 927-934 (2010).

Dumont E., Darracq G., Couvert A., Couriol C., Amrane A., Thomas D., Andrès Y., Le Cloirec P. VOC absorption in a countercurrent packed-bed column using water/silicone oil mixtures: Influence of silicone oil volume fraction, Chemical Engineering Journal, 168, 241-248 (2011).

Dumont E., Darracq G., Couvert A., Couriol C., Amrane A., Thomas D., Andrès Y., Le Cloirec P., Hydrophobic VOC absorption in two-phase partitioning bioreactors; influence of silicone oil volume fraction on absorber diameter, Chemical Engineering Science, 71, 146-152 (2012).

Quijano G., Hernandez, M., Thalasso, F., Munoz, R., Villaverde, S. Two-phase partitioning bioreactors in environmental biotechnology. Appl. Microbiol. Biotechnol. 84, 829?846 (2009).