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

WET SCRUBBING INTENSIFICATION APPLIED TO HYDROGEN SULPHIDE REMOVAL IN WASTE WATER TREATMENT PLANT

International audienceHydrogen sulphide removal in a Waste Water Treatment Plant at semi-industrial scale in a compact wet scrubber has been investigated. The gas residence time in the scrubber was reduced to 30 ms using a NaOCl caustic scrubbing solution. The contactor is composed of a wire mesh packing structure where liquid and gas flow co-currently at high velocity (> 12 m.s-1). H2S removal percentages higher than 95% could be achieved whereas a moderate pressure drop was measured (< 4000 Pa). Both the hydrodynamic and chemical conditions can influence the efficiency of the process. Correlations were developed to predict both the pressure drop and the H2S removal efficiency at given operating conditions

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

Hydrogen sulphide removal in waste water treatment plant by compact oxidative scrubbing in Aquilair PlusTM process

International audienceRecently, the development of a high voidage contactor, named Aquilair Plus™, has demonstrated high efficiency for chemical scrubbing of hydrogen sulphide. Liquid and gas phases flow at co-current and high velocity, leading to a great dispersion of the liquid and then to an enhancement of the mass transfer rate by comparison with classical packed towers. This study focused on the results which obtained at semi-industrial scale with the Aquilair PlusTM process on a waste water treatment plant located in France. The scrubbing liquid consisted of a sodium hypochlorite alkaline solution. At once, pressure drop, H2S removal and reagents consumption were followed. The influence of the superficial gas velocity, liquid-to-gas mass ratio (L/G), pH, hypochlorite concentration of the scrubbing liquid and H2S inlet concentration was characterised. H2S removal percentages higher than 90% could be easily achieved with a moderate pressure drop (< 40 mbar). Both hydrodynamic and chemical conditions proved to influence performances. Reagents consumptions slightly higher than the predicted ones were measured

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)

A simple and timesaving method for the mass-transfer assessment of solvents used in physical absorption

International audienceA simple dynamic absorption procedure to assess the mass-transfer performances of a solvent toward a selected gaseous solute is presented. Absorption was operated semi-continuously at transient state until the equilibrium was reached without solvent recirculation. Four volatile organic compounds (VOC) more or less hydrophobic (toluene, acetone, dichloromethane, isopropanol) were absorbed in water and two heavy organic solvents (Bis(2-ethylhexyl) adipate DEHA and polydimethylsiloxane PDMS). A numerical resolution procedure was developed to simulate the gas-liquid mass-transfer and to deduce the VOC partition coefficients, expressed as the Henry’s law constants, as well as the overall liquid-phase mass-transfer coefficients. The overall liquid-phase mass-transfer coefficients were correlated to the diffusion coefficients using the Higbie penetration theory. The results confirm the high selectivity of water whereas the two organic solvents, especially DEHA, exhibit rather good affinity with all VOC even if the Henry’s law constants of the most soluble and the less soluble compounds for those solvents differ by 1 or 2 orders of magnitude. The liquid-film mass-transfer coefficients in the two organic solvents, even being more viscous, are larger than in water which confirms their good potential for hydrophobic VOC treatment

Contribution au développement d'un procédé de lavage chimique compact. Traitement du sulfure d'hydrogène par le chlore à l'échelle semi-industrielle et de COV odorants par oxydation avancée ozone/peroxyde d'hydrogène à l'échelle du laboratoire.

To increase the compactness of chemical scrubbing processes for the deodorization of waste gases, an intense gas-liquid contactor has been developed. It works at co-current with a high superficial gas velocity to obtain high mass transfer performances and reduced residence time. The first part of the study focuses on the transition of the process to the semi-industrial scale for hydrogen sulphide treatment on a waste water treatment plant. The results demonstrate interesting transfer efficiencies (until 95%). The next step was to develop and optimize a scrubbing solution suitable for the treatment of Volatile Organic Compounds (VOCs), based on the combination of ozone and hydrogen peroxide (peroxone process) using dimethyldisulfide as test pollutant. Hydroxyl radicals formed by ozone decomposition allowed to enhance DMDS mass transfer. Finally, the peroxone process was tested with the implementation of the scrubbing liquid recirculation for the treatment of several VOC with different chemical natures. The oxidation optimisation allowed to re-use the scrubbing liquid, which represents an interesting cost reduction for this process.Afin d'augmenter la compacité du lavage chimique pour la désodorisation d'effluents gazeux, un contacteur gaz-liquide intense a été développé. Celui-ci fonctionne à co-courant avec une vitesse superficielle du gaz importante, permettant d'atteindre des performances de transfert élevé et un temps de séjour réduit. La première partie de l'étude porte sur le passage à l'échelle semi-industrielle du procédé pour le traitement du sulfure d'hydrogène sur un effluent réel de station d'épuration. Les résultats démontrent des efficacités de transfert intéressantes (jusqu'à 95%). L'étape suivante a consisté à développer et optimiser une solution de lavage adaptée au traitement des Composés Organiques Volatils (COV), basée sur la combinaison de l'ozone et du peroxyde d'hydrogène (procédé peroxone), le polluant test étant le diméthyldisulfure (DMDS). Ainsi, la production de radicaux hydroxyles issus de la décomposition de l'ozone a permis d'accélérer le transfert du DMDS. Enfin, le procédé peroxone a été testé en recirculant la solution de lavage pour le traitement de COV de natures chimiques différentes. L'optimisation de l'oxydation au sein du liquide a permis le recyclage de la solution de lavage, ce qui représente une économie substantielle pour un tel procédé

Contribution au développement d un procédé de lavage chimique compact (traitement du sulfure d hydrogène par le chlore à l échelle semi-industrielle et de COV odorants par oxydation avancée ozone/peroxyde d hydrogène à l échelle du laboratoire)

Afin d augmenter la compacité du lavage chimique pour la désodorisation d effluents gazeux, un contacteur gaz-liquide intense a été développé. Celui-ci fonctionne à co-courant avec une vitesse superficielle du gaz importante, permettant d atteindre des performances de transfert élevés et un temps de séjour réduit. La première partie de l étude porte sur le passage à l échelle semi-industrielle du procédé pour le traitement du sulfure d hydrogène sur un effluent réel de station d épuration. Les résultats démontrent des efficacités de transfert intéressantes (jusqu à 95%). L étape suivante a consisté à développer et optimiser une solution de lavage adaptée au traitement des Composés Organiques Volatils (COV), basée sur la combinaison de l ozone et du peroxyde d hydrogène (procédé peroxone), le polluant test étant le diméthyldisulfure (DMDS). Ainsi, la production de radicaux hydroxyles issus de la décomposition de l ozone a permis d accélérer le transfert du DMDS. Enfin, le procédé peroxone a été testé en recirculant la solution de lavage pour le traitement de COV de natures chimiques différentes. L'optimisation de l'oxydation au sein du liquide a permis le recyclage de la solution de lavage, ce qui représente une économie substantielle pour un tel procédé.To increase the compactness of chemical scrubbing processes for the deodorization of waste gases, an intense gas-liquid contactor has been developed. It works at co-current with a high superficial gas velocity to obtain high mass transfer performances and reduced residence time. The first part of the study focuses on the transition of the process to the semi-industrial scale for hydrogen sulphide treatment on a waste water treatment plant. The results demonstrate interesting transfer efficiencies (until 95%). The next step was to develop and optimize a scrubbing solution suitable for the treatment of Volatile Organic Compounds (VOCs), based on the combination of ozone and hydrogen peroxide (peroxone process) using dimethyldisulfide as test pollutant. Hydroxyl radicals formed by ozone decomposition allowed to enhance DMDS mass transfer. Finally, the peroxone process was tested with the implementation of the scrubbing liquid recirculation for the treatment of several VOC with different chemical natures. The oxidation optimisation allowed to re-use the scrubbing liquid, which represents an interesting cost reduction for this process.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Intensification of volatile organic compounds mass transfer in a compact scrubber using the O3/H2O2 advanced oxidation process: kinetic study and hydroxyl radical tracking.

International audienceThis study assesses the potential of ozonation and advanced oxidation process O(3)/H(2)O(2) to enhance the dimethyldisulfide (DMDS) mass transfer in a compact chemical scrubber developed for air treatment applications. Theoretical calculations, through Hatta number and enhancement factor evaluations for two parallel irreversible reactions, were compared to experimental data and enabled the description of the mass transfer mechanisms. These calculations required the determination of the kinetic constant of the DMDS oxidation by molecular ozone ( [Formula: see text] ) and the measurement of the hydroxyl radical concentration within the scrubber. The competitive kinetic method using the 1,2-dihydroxybenzene (resorcinol) enabled to determine a value of the kinetic constant [Formula: see text] of 1.1×10(6)M(-1)s(-1) at 293K. Then, experiments using para-chlorobenzoic acid in solution allowed measuring the average hydroxyl concentration in the scrubber between the inlet and the outlet depending on the chemical conditions (pH and inlet O(3) and H(2)O(2) concentrations). High hydroxyl radical concentrations (10(-8)M) and ratio of the HO°-to-O(3) exposure (R(ct)≈10(-4)) were put in evidence