Dynamic Absorption Method Research Articles

Absorption of hydrophobic volatile organic compounds in renewable vegetable oils and esterified fatty acids: Determination of gas-liquid partitioning coefficients as a function of temperature

Air scrubbing is an effective technology to treat industrial polluted air loaded with VOCs. Water is the most widely used absorbent liquid, but when hydrophobic pollutants are treated, mass transfer between the gas phase and the liquid phase is limited. Therefore, organic solvents are an attractive option to improve the removal efficiency of hydrophobic VOCs. In this study, four esterified fatty acids (isopropyl isostearate, isopropyl laurate, isopropyl myristate and methyl oleate) and three vegetable oils (corn oil, peanut oil and sunflower oil) were evaluated as suitable absorption liquids for seven hydrophobic VOCs, i.e., toluene, m-xylene, ethylbenzene, butane, pentane, hexane and heptane. In addition, one synthetic silicone oil (polydimethylsiloxane) was used as a reference absorption liquid. Dimensionless gas-liquid partitioning coefficients (KGL, 1.3×10−4 – 2.4×10−2) were determined as a function of temperature (25–50 °C) by an optimized dynamic absorption method (DynAb) using Selected-Ion Flow Tube Mass Spectrometry (SIFT-MS). Our experimental results show that, in all the absorption liquids tested, the lowest KGL coefficients were obtained for m-xylene + ethylbenzene, followed by toluene. Isopropyl myristate was found to be the most efficient liquid to absorb the target VOCs, except for heptane where methyl oleate was better. Knowledge of the KGL coefficients of hydrophobic VOCs in these absorption liquids is crucial for process optimization and modeling. Moreover, the effect of temperature is relevant for industrial applications where KGL temperature dependence relations are often lacking. This study evaluates for the first time the use of three renewable vegetable oils and four esterified fatty acids as pure solvents for the absorption of industrially widely used hydrophobic VOCs, where except for toluene, very little or no research had been carried out.

Gas–liquid mass transfer using advanced optical probe in a mimicked FT slurry bubble column

Abstract Gas-liquid volumetric liquid-phase mass transfer coefficient (k L a) was studied in a slurry bubble column at the conditions mimicking Fischer–Tropsch synthesis. To avoid the hydrodynamic disturbances due to the gas switching, oxygen enriched air dynamic absorption method was used. Influence of reactor models (CSTR, ADM and RCFD) on the volumetric mass transfer coefficient was investigated. Effect of operating pressure, superficial gas velocity and solids loading were investigated. From the reactor models investigated, it is recommended to use ADM model for k L a study. If the CSTR model is used, applicability of the model should be checked. With increase in the superficial gas velocity and operating pressure, volumetric liquid-phase mass transfer coefficient increases, while it decreases with the solids loading corroborating with the literature.

Determination of the gas-to-liquid partitioning coefficients using a new dynamic absorption method (DynAb method)

The determination of partitioning coefficients which describes the behaviour of a pollutant between phases (gas, liquid, solid) is of fundamental importance in different scientific areas (fate and behaviour of pollutants, design of abatement technologies, flavour release). A new and fast method, based on selected ion flow tube mass spectrometry, was developed to determine the gas-to-liquid partitioning coefficients (KAW) of volatile organic compounds (VOC). By using the dynamic absorption method (DynAb method), a gas stream with a known and constant compound concentration is bubbled through a liquid volume. The outlet gas concentration is continuously measured and results in a breakthrough curve. From the breakthrough curve of the measured gas concentration, KAW can be calculated. In contrast to the existing dynamic methods, KAW obtained by DynAb method does not assume equilibrium of the target compound between the liquid and the air bubble leaving the liquid and therefore avoids erroneous evaluation of KAW.The developed method was applied to determine KAW for five odorous VOCs (dimethyl sulphide, dimethyl disulphide, 2-methylpropanal, 3-methylbutanal and hexanal). KAW values can be determined in short time (less than 15 minutes) with a reproducibility of less than 5%. Measured KAW (mol m−3gas/mol m−3liquid) at 25°C are 8.61±0.20×10−2 for dimethyl sulphide, 5.90±0.23×10−2 for dimethyl disulphide, 1.27±0.06×10−2 for 2-methylpropanal, 1.43±0.05×10−2 for 3-methylbutanal and 1.41±0.03×10−2 for hexanal. The dependence of the partitioning coefficients on temperature (4 to 25°C) and ammonium sulphate concentration (0 to 300gL−1) were described by Van’t Hoff and Setchenow equations, respectively.

Gas–Liquid Mass Transfer Characteristics in a Gas–Liquid–Solid Bubble Column under Elevated Pressure and Temperature

The volumetric mass transfer coefficient kLa of gases (H2, CO, CO2) and mass transfer coefficient kL on liquid paraffin side were studied using the dynamic absorption method in slurry bubble column reactors under elevated temperature and elevated pressure. Meanwhile, gas-holdup and gas–liquid interfacial area a were obtained. The effects of temperature, pressure, superficial gas velocity and solid concentration on the mass transfer coefficient were discussed. Experimental results show that the gas–liquid volumetric mass transfer coefficient kLa and interfacial area a increased with the increase of pressure, temperature, and superficial gas velocity, and decreased with the slurry concentration. The mass transfer coefficient kL increased with increasing superficial gas velocity and temperature and decreased with higher slurry concentration, while it changed slightly with pressure. According to analysis of experimental data, an empirical correlation is obtained to calculate the values of kLa for H2 (CO, CO2) in the gas–paraffin–quartz system in a bubble column under elevated temperature and elevated pressure.

Mass Transfer Performance for Low SO2 Absorption into Aqueous N,N′-Bis(2-hydroxypropyl)piperazine Solution in a θ-Ring Packed Column

The mass transfer performance of SO2 absorption in an aqueous solution of a newly proposed organic amine absorbent, N,N′-bis(2-hydroxypropyl)piperazine (HPP), was studied by the dynamic absorption method in a θ-ring packed column. The absorption performance was interpreted in terms of the overall mass transfer coefficient, KGa. The experimental results showed that the KGa value increased with an increase in the liquid flow rate, concentration, and initial pH of HPP absorbent. However, it decreased with an increase in temperature, SO2 partial pressure, and gas flow rate. An empirical correlation for the overall mass transfer coefficient is subsequently developed as a function of the process parameters. The obtained KGa from empirical correlation is found to be in relatively good agreement with the experimental results with a mean relative deviation of 6.6%, which could be used for engineering design.

Mass transfer coefficients of styrene into water/silicone oil mixtures: New interpretation using the “equivalent absorption capacity” concept

The physical absorption of styrene into water/silicone oil systems at a constant flow rate for mixtures of different compositions (silicone oil volume fraction ϕ=0%, 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10%) was investigated in laboratory-scale bubble reactors using a dynamic absorption method. Experimental results previously analyzed assuming no contact between gas and silicone oil [10] were reconsidered by applying the “equivalent absorption capacity” concept characterized by the value of the styrene partition coefficient between air and the mixture (Hmix). The results indicate that silicone oil addition slightly hinders the styrene mass transfer rate compared to the air/water system. Moreover, a dramatic decrease in KLa values due to silicone oil addition is observed. In comparison with similar measurements available in the literature, it is noted that this decrease in KLa value could be related to the change in the partition coefficient ratio mR=Hwater/Hoil. Two explanations concerning the relationship between the change in a hydrodynamic parameter (KLa) and that in a thermodynamic parameter (mR) are proposed. Finally, it appears questionable to study the change in KLa in terms of silicone oil addition.

Volumetric mass transfer coefficients characterising VOC absorption in water/silicone oil mixtures

The physical absorption of three Volatile Organic Compounds (dimethyldisulphide (DMDS), dimethylsulphide (DMS) and toluene) in “water/silicone oil” systems at a constant flow rate for mixtures of different compositions (ϕ=0%, 5%, 10%, 15%, 20%, and 100%) was investigated using a dynamic absorption method. The results indicate that silicone oil addition leads to a dramatic decrease in KLa which can be related to the change in the partition coefficient (Hmix). They confirm the results obtained for styrene absorption using another measurement technique [15]. The interpretation of the results using dimensionless ratios KLa(ϕ)/KLa(ϕ=0%) and KLa(ϕ)/KLa(ϕ=100%) versus ϕ also confirms the importance of the partition coefficient ratio mR=Hwater/Hoil in the KLa change. Moreover, the results obtained for toluene absorption in “air/water/silicone oil” systems (ϕ=10%, 15%, and 20%) suggest that the mass transfer pathway is in the order gas→water→oil for these operating conditions.

Styrene absorption in water/silicone oil mixtures

The absorption of styrene in water/silicone oil systems at a constant flow rate for emulsion compositions ranging from 0% to 20% was investigated using a dynamic absorption method. It was found that the mass transfer for air/water/silicone oil systems is roughly double than those determined for air/water systems whatever the silicone oil percentage (ϕ). This result, mainly due to the high value of the partition coefficient ratio (mR=Hwater/Hoil=257), was in agreement with the enhancement factor models proposed in the literature. Considering the volumetric mass transfer coefficient, it was shown that change in KLa versus ϕ depended on the mass transfer model used for its determination. By using the “equivalent absorption capacity” concept developed by Dumont et al. [18] (CEJ 162 (2010) 927–934; doi:10.1016/j.cej.2010.06.045), a dramatic decrease in the KLa with increasing silicone oil volume fraction was observed in relation to the decrease in the value of the partition coefficient Hmix. Conversely, considering a styrene mass transfer pathway in series (gas→water→oil), the KLa values for gas/water/silicone oil systems were roughly double the KLa for gas/water systems and did not depend on the mixture composition. The styrene mass transfer performances were also analyzed using the modeling framework proposed by Hernandez et al. [17] (CEJ 172 (2011) 961–969; doi:10.1016/j.cej.2011.07.008). This model confirmed the ability of a water/silicone oil mixture to increase the styrene mass transfer by a factor of 2 and verified that the change in Hmix versus ϕ followed the trend predicted by the theory (although the Hmix values determined from this model were significantly lower than the theoretical values).

Determination of partition coefficients of three volatile organic compounds (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone oil mixtures

The main objective of this work was to propose a model able to predict the partition coefficient of odorous or toxic gaseous pollutants (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone oil mixtures. Experimental measurements using a static headspace method were carried out for pure water ( H voc,water), for pure silicone oil ( H voc,solvent) and for mixtures of varying composition ( H voc,mixture). The dramatic decrease in the partition coefficient ( H voc,mixture) with oil addition clearly showed a deviation from linearity, which was more pronounced for increasing H voc,water/ H voc,solvent ratios. Moreover, experiments using a dynamic absorption method underlined that the absorption capacity of a biphasic water/silicone oil mixture can be classed as the absorption capacity of a pseudo-homogeneous phase whose physical properties (molecular weight and density) can be calculated from the physical properties of water and solvent, and balanced using the “equivalent absorption coefficients” H voc,mixture/ H voc,water and H voc,mixture/ H voc,solvent. An “equivalent absorption capacity” concept is then proposed, which should be useful to design absorption units using two-phase liquid mixtures for the treatment of industrial air loaded with volatile organic compounds.

Mass transfer coefficients of styrene and oxygen into silicone oil emulsions in a bubble reactor

The absorption of oxygen and styrene in water–silicone oil emulsions was independently studied in laboratory-scale bubble reactors at a constant gas flow rate for the whole range of emulsion compositions (0–10% v/v). The volumetric mass transfer coefficients to the emulsions were experimentally measured using a dynamic absorption method. It was assumed that the gas phase contacts preferentially the water phase. In the case of oxygen absorption, it was found that the addition of silicone oil hinders oxygen mass transfer compared to an air–water system. Decreases in k L a oxygen of up to 25% were noted. Such decreases in the oxygen mass transfer coefficient, which imply longer aeration times to transfer oxygen, could represent a limiting step in biotechnological processes strongly dependent on oxygen concentration. Nevertheless, as the large affinity of silicone oil for oxygen enables greater amounts of oxygen to be transferred from the gas phase, it appears that the addition of more than 5% silicone oil should be beneficial to increase the oxygen transfer rate. In the case of styrene absorption, it was established that the volumetric mass transfer coefficient based on the emulsion volume is roughly constant with the increase in the emulsion composition. In spite of the relatively high cost of silicone oil, water–silicone oil emulsions remain relevant to treat low-solubility volatile organic compounds, such as styrene, in low-concentration gas streams.

Gas-Liquid mass transfer in a bubble column with organic liquids

Water, methanol, toluene and ligroin were aerated in bubble column of 0.15 m I.D. and a height of 4.3 m . Using a gas disengagement technique, the holdups of large were found to be similar in all liquids whereas the holdups of small bubbles differed significally. Oxygen mass transfer studies with a dynamic absorption method were impaired by height-and time-dependent oxygen depletion in the gas phase. Only at low dispersion heights, there was agreement with the results of a steady state method. Photographic determination of the small bubbles' diameters and knowledge of their holdup fraction allowed to estimate their potential contributions to the mass transfer rate at gas flow rates up to 0.2m/s. These contribution were generally high and in ligroin the estimates even exceeded the observed total oxygen transfer rate. It is conclude that the finite rates of coalescence and break-up reduce the driving force for mass transfer from the small bubble fraction.

Resistance of Permethrin to Weathering in Fabrics Treated for Protection Against Mosquitoes (Diptera: Culicidae)

Two different methods of treating cotton and nylon-cotton fabrics with permethrin were evaluated for protection from mosquito bites after laboratory weathering. Cotton fabric treated by the individual dynamic absorption method provided consistently better protection than cotton fabric treated by the aerosol method. The nylon-cotton fabric provided similar protection regardless of the treatment method. After weathering, the toxic effects of both types of permethrin-treated fabrics treated by both methods diminished much more rapidly than did the repellent effect. Low residual amounts of permethrin in the fabrics provided 85% protection from bites against Aedes aegypti (L.) and 93% protection against Anopheles stephensi Liston. Permethrin-treated fabrics were effective in providing protection from mosquito bites for long periods, even after exposure to weathering, and appear to be an effective means of reducing nuisance effects and disease transmission by mosquitoes.