Overview of mass transfer enhancement factor determination for acidic and basic compounds absorption in water

International audienceAbsorption or gas-liquid mass transfer is a fundamental unit operation useful in many fields, particularly gas treatment (wet scrubbing). Absorption of basic or acidic compounds, even hydrophobic, in water can be achieved successfully due to the mass transfer enhancement linked to proton transfer reactions in the liquid film. The absorption rate takes this phenomenon into account through the enhancement factor E, which depends on many parameters: nature (irreversible or reversible), kinetics and stoichiometry of the reaction, reagents and products diffusion coefficients and concentrations. This article gives an overview of the enhancement factor determination for acidic and basic compounds transfer in water. Modeling is performed for three compounds of interest, hydrogen sulfide H2S, methyl mercaptan CH3SH and ammonia NH3, for different scenarii to assess the influence of the pH. The results demonstrate that recombination with HO- and protonation reactions are respectively the two preponderant reactions for respectively acidic and basic compounds. They enable to reach large values of the enhancement factor at appropriated pH and to reduce the mass transfer resistance in the liquid film. Furthermore, the simulations highlight that, in many cases, knowledge of the reaction kinetics is not necessary since the reaction can be considered as instantaneous compared to mass transfer

Enzymatic degradation of Congo Red by turnip (Brassica rapa) peroxidase.

Pas de DOI. Info bibliogr : http://www.znaturforsch.com/ac/v67c/67c0429.pdfInternational audienceThe enzyme peroxidase is known for its capacity to remove phenolic compounds and aromatic amines from aqueous solutions and also to decolourize textile effluents. This study aims at evaluating the potential of a turnip (Brassica rapa) peroxidase (TP) preparation in the discolouration of textile azo dyes and effluents. An azo dye, Congo Red (CR), was used as a model pollutant for treatment by the enzyme. The effects of various operating conditions like pH value, temperature, initial dye and hydrogen peroxide concentrations, contact time, and enzyme concentration were evaluated. The optimal conditions for maximal colour removal were at pH 2.0, 40 degrees C, 50 mM hydrogen peroxide, 50 mg/l CR dye, and TP activity of 0.45 U/ml within 10 min of incubation time. Analysis of the by-products from the enzymatic treatment by UV-Vis and IR spectroscopy showed no residual compounds in the aqueous phase and a precipitate of polymeric nature

Competitive kinetics study of sulfide oxidation by chlorine using sulfite as reference compound

International audienceTo design and optimize hydrogen sulfide scrubbers working with chlorine, the knowledge of the kinetics of the hydrogen sulfide oxidation is necessary. In this work, the kinetics of the hydrogen sulfide oxidation by sodium hypochlorite was experimentally investigated using a reactor without headspace (100 mL gas-tight syringe) and the competitive kinetics method. The sulfite ion was selected as the reference compound. First, the apparent stoichiometries of sulfite anion and hydrogen sulfide chlorinations were determined performing single-compound experiments. Then, the kinetics of the hydrogen sulfide chlorination was studied in the pH range 6-12 performing simultaneous sulfite and sulfide chlorinations. The results demonstrated that sulfide and sulfite oxidation kinetic rates have the same order of magnitude, which validates the choice of the sulfite anion as the reference compound. Kinetic simulations emphasized that the kinetic rates of the oxidation of both compounds were limited by acid-base reactions. The sulfide oxidation in the pH range 6-12 is mainly due to the hydrosulfide (HS−) oxidation by the hypochlorous acid (ClOH) with an associated kinetic constant of 1.2 × 109 L mol−1 s−1 at 25 °C

Traitement de composés organiques volatils par lavage chimique compact: recyclage de la solution de lavage par oxydation avancée O3/H2O2

National audienceThis study focuses on Volatile Organic Compounds (VOC) treatment in a compact chemical scrubber composed of a structured packing operating at co-current and high gas superficial velocity (> 10 m.s-1). The scrubbing liquid is composed of ozone and hydrogen peroxide to generate very reactive hydroxyl radicals. Results demonstrate a high mineralisation rate of the transferred VOC, even with low oxidant concentrations. Therefore, the scrubbing liquid can be recycled and recirculated at the top of the scrubber without efficiency drop over time. Neutral VOC removal efficiency increases with their solubility in water, from 14% for dimethyldisulfide to 86% for butanol using a mass flow rates ratio (L/G) of 2.5 and residence time of 20 ms. For trimethylamine, a basic VOC, the increased apparent solubility at pH = 8 leads to a high removal efficiency of 80%. Major by-products were identified and quantified. An absence of stripping in the treated gas emphasizes their innocuity.Cette étude porte sur le traitement de Composés Organiques Volatils (COV) dans un laveur chimique compact composé d’un garnissage structuré fonctionnant à co-courant et à vitesse superficielle du gaz élevée (> 10 m.s-1). La solution aqueuse de lavage se compose d’un mélange d’ozone et de peroxyde d’hydrogène afin de générer des radicaux hydroxyles très réactifs. Les résultats démontrent une minéralisation poussée des COV transférés même avec des concentrations en oxydants modestes. La solution de lavage peut donc être régénérée et réinjectée en tête du laveur sans diminution d’efficacité au cours du temps. L’abattement en phase gazeuse des COV neutres, observés pour un rapport des débits massiques L/G de 2,5, augmente avec leur solubilité en phase aqueuse, démarrant à 14% pour le dimethyldisulfide jusqu’à 86% pour le butanol en 20 ms de temps de contact. Pour la triméthylamine, composé basique, l’augmentation de la solubilité apparente à pH 8 permet d’atteindre un abattement élevé de 80%. Les sous-produits majoritaires ont été identifiés et quantifiés. L’absence de stripping dans le gaz traité met en évidence leur innocuité

Removal of Hydrophobic Volatile Organic Compounds in an Integrated Process Coupling Absorption and Biodegradation -- Selection of an Organic Liquid Phase

International audienceSince usual processes involve water as ab- sorbent, they appear not always really efficient for the treatment of hydrophobic volatile organic compound (VOC). Recently, absorption and biodegradation cou- pling in a two-phase partitioning bioreactor (TPPB) proved to be a promising technology for hydrophobic compound treatment. The choice of the organic phase, the non-aqueous phase liquid (NAPL) is based on various parameters involved in both steps of the pro- cess, hydrophobic VOC absorption in a gas - liquid contactor, and biodegradation in the TPPB. VOC sol- ubility and diffusivity in the selected NAPL, as well as NAPL viscosity, seems to be the main parameters during the absorption step, while biocompatibility, namely the absence of toxic effect of the NAPL to- wards microorganisms, non-biodegradability and VOC partition coefficient between NAPL and water were revealed as the key factors during the biodegra- dation step. The screening of the various NAPL avail- able in the literature highlighted two families of compounds matching the required conditions for the proposed integrated process, silicone oils and ionic liquids

Absorption of toluene in silicone oil: effect of the solvent viscosity on hydrodynamics and mass transfer

International audienceThe purpose of this study was to compare toluene absorption performances using two PDMS of different viscosities (η = 5 and 50 mPas respectively) in terms of hydrodynamics and mass transfer. Three types of packing were tested in a counter-current packed gas-liquid contactor, Raschig rings, IMTP®, and Flexipac®. Hydrodynamic results showed that the viscosity of PDMS 50 is not a hurdle to be used with IMTP® or Flexipac®; the pressure drop being around 288 Pa m−1 for Flexipac® at the flooding point (FG = 0.99 Pa1/2; L’ = 4.95 kg m−2 s−1), which corresponds to an acceptable value for an industrial application. The determination of the loading zones showed that Raschig rings were not suitable for solvents having a high viscosity, such as PDMS 50; they resulted in high pressure drops if compared to IMTP® and Flexipac®. The mass transfer study showed that 100% removal efficiency can be reached for both PDMS using Flexipac®. For IMTP®, the removal efficiency is limited to 88% in the presence of PDMS 50. As a result, PDMS 50 should be a relevant absorbing liquid for the removal of toluene in the absorption–biodegradation process, and Flexipac® packing should be preferred to IMTP

Kinetic study of hydrogen sulfide absorption in aqueous chlorine solution

International audienceHydrogen sulfide (H2S) is currently removed from gaseous effluents by chemical scrubbing using water. Chlorine is a top-grade oxidant, reacting with H2S with a fast kinetic rate and enhancing its mass transfer rate. To design, optimize and scale-up scrubbers, knowledge of the reaction kinetics and mechanism is requested. This study investigates the H2S oxidation rate by reactive absorption in a mechanically agitated gas-liquid reactor. Mass transfer (gas and liquid sides mass transfer coefficients) and hydrodynamic (interfacial area) performances of the gas-liquid reactor were measured using appropriated physical or chemical absorption methods. The accuracy of these parameters was checked by modeling the H2S absorption in water without oxidant. A sensitivity analysis confirmed the robustness of the model. Finally, reactive absorption of H2S in chlorine solution for acidic or circumneutral pH allowed to investigate the kinetics of reaction. The overall oxidation mechanism could be described assuming that H2S is oxidized irreversibly by both hypochlorite anion ClO- (k = 6.75 106 L mol-1 s-1) and hypochlorous acid ClOH (k = 1.62 105 L mol-1 s-1)

Equivalent Absorption Capacity (EAC) concept applied to the absorption of hydrophobic VOCs in a water/PDMS mixture

International audienceAbsorption of hydrophobic Volatile Organic Compounds (VOCs) in multiphase system gas water PolyDiMethylSiloxane (PDMS, i.e. silicone oils) was considered. Absorption experiments were carried out in a countercurrent gas-liquid absorber filled with Raschig rings as packing. Three absorbing liquids, water, PDMS and a mixture of water/PDMS (90/10 v/v) were used to transfer toluene and dimethyl disulfide (DMDS) selected as VOC targets. Moreover, the Equivalent Absorption Capacity (EAC) concept previously developed to characterize the mixtures of water/PDMS was applied to the experimental data obtained at three different gas flow rates (18, 25 and 32 m3 h-1). Experimental measurements showed that absorption efficiencies (E) were low for water (around 2-8% for toluene and around 12-25% for DMDS) and higher for PDMS (from 88% to 98% according to the operating conditions). For the water/PDMS mixture, it was shown that the PDMS addition increased significantly the absorption of pollutants (E values in the range 25 to 65% according to the operating conditions). Besides, it was emphasized that the EAC concept describes satisfactorily the absorption behavior of the water/PDMS mixture. Finally, results confirmed that pure PDMS has to be used rather than a (90/10 v/v) water/PDMS mixture for hydrophobic VOC absorption. Using pure PDMS as absorbing liquid (dynamic viscosity of 5 mPa s), high absorption efficiencies (up to 98%) were obtained for L’/G’ value around 5 (L’/G’ corresponding to the ratio between the specific flow of the liquid and the specific flow of the gas). Under such conditions, pressure drops (ΔP) in the packed column and overall mass transfer coefficients (KLa) were around 1000 Pa m-1 (G’ = 1.06 kg m-2 h-1) and 5 10-3 s-1, respectively

Biofiltration of high concentration of H2S in waste air under extreme acidic conditions

International audienceRemoval of high concentrations of hydrogen sulfide using a biofilter packed with expanded schist under extreme acidic conditions was performed. The impact of various parameters such as H2S concentration, pH changes and sulfate accumulation on the performances of the process was evaluated. Elimination efficiency decreased when the pH was lower than 1 and the sulfate accumulation was more than 12 mg S-SO42-/g dry media, due to a continuous overloading by high H2S concentrations. The influence of these parameters on the degradation of H2S was clearly underlined, showing the need for their control, performed through an increase of watering flow rate. A maximum elimination capacity (ECmax) of 24.7 g m−3 h−1 was recorded. As a result, expanded schist represents an interesting packing material to remove high H2S concentration up to 360 ppmv with low pressure drops. In addition, experimental data were fitted using both Michaelis–Menten and Haldane models, showing that the Haldane model described more accurately experimental data since the inhibitory effect of H2S was taken into accoun