Gas Phase Resistance Research Articles

Comment on “Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium and nonequilibrium based approaches,” by Kathleen M. Smits, Abdullah Cihan, Toshihiro Sakaki, and Tissa H. Illangasekare

Comment on “Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium and nonequilibrium based approaches,” by Kathleen M. Smits, Abdullah Cihan, Toshihiro Sakaki, and Tissa H. Illangasekare

Simultaneous absorption of CO 2 and H 2S from biogas by capillary membrane contactor

This work presents the study on the simultaneous absorption of H 2S and CO 2 from biogas using a capillary membrane contactor. The synthetic biogas contained 250–1000 ppm H 2S, 20–40% CO 2 and CH 4. The absorbents used were water and monoethanolamine (MEA) solution. The effects of liquid and gas velocities, gas composition, on the absorption performance and selectivity of H 2S were investigated together with the detailed analysis of the mass transfer resistances of the membrane contactor system. The use of MEA solution gave much higher absorption fluxes of both CO 2 and H 2S compared to water. The absorption flux of H 2S significantly increased with increasing gas flow rate and slightly increased with liquid velocity and MEA concentration, while the absorption flux of CO 2 moderately increased with liquid velocity and was highly enhanced with increasing MEA concentration. The increase in CO 2 concentration obviously decreased the H 2S flux. The results of H 2S selectivity and the mass transfer resistance analysis of the non-wetted mode showed that the gas phase resistance played the important role on the mass transfer of H 2S. The opposite was found for the mass transfer of CO 2, i.e., the liquid phase resistance controlled the mass transfer. For the partially wetted mode, the wetted membrane resistance controlled the CO 2 absorption. On the contrary, for H 2S absorption, the wetted membrane resistance was insignificant and the gas phase resistance controlled the mass transfer. The observation of CO 2 and H 2S flux for a period of 6 h in order to investigate the effect of membrane wetting showed that the CO 2 flux dropped approximately 7.6%, while change of H 2S flux was negligible.

Mass Transfer Characteristics of Gypsum-Fly Ash Slurry for Wet Flue Gas Desulfurization

In the limestone/gypsum wet flue gas desulfurization pilot-scale test rig, key parameters such as SO2 absorption rate, mass transfer were experimentally determined．The results show that desulphurizing capacity of gypsum and fly ash is relatively weaker, which is only equivalent to fresh limestone with a content of 0.27% and 1.5% respectively. pH-t curve of slurry with different levels of fly ash could be divided into a sharply increasing stage and a steadily increasing stage. The leaching content of Mn2+ is about 9 times of Fe3+ , Mn2+ can form intermediate complex with HSO3- in the solution, which can induce catalytic reaction and accelerate SO2 absorption. Fly ash in gypsum slurry can obviously promote desulfurization. The pH value of slurry is high at the initial reaction stage, and effect of fly ash on SO2 absorption rate is less than 1.5%. when the pH value is decreased to 5.0, The leaching content of Mn2+ will grow with the decrease of pH value, better catalytic efficiency can be gained, effect of fly ash on SO2 absorption rate can increase 6.0% at most. The reaction is controlled by liquid phase resistance; the ratio of gas phase resistance to overall resistance is less than 38%. Mn2+ concentration of slurry increases with pH value decreasing and fly ash concentration increasing, which has significant effect on catalyzed oxidation of SO32-.

Influence of water vapor on hydrogen permeation through 2.5 μm Pd–Ag membranes

Hydrogen permeation experiments were performed to evaluate the influence of water vapor on hydrogen permeability in 80–20% by weight Pd–Ag membranes of 2.5 μm thickness. In particular, hydrogen flux was measured in pure hydrogen permeation tests as well as in experiments with binary mixtures containing also nitrogen or water vapor, that were performed at temperatures ranging from 473 to 723 K and at a transmembrane pressure differences up to about 3 bar. The membranes, supplied by NGK Insulator Ltd., Japan, showed a very high hydrogen permeance and lifetime, as well as virtually infinite selectivity (exceeding 10000 for H 2–N 2 mixtures). The experiments in hydrogen–nitrogen mixtures were carried out at different temperatures, hydrogen concentrations and feed flow rates and confirmed the existence of a non-negligible concentration polarization phenomenon in the experimental module. The gas phase mass transport and the module fluid dynamics were thus analyzed and the dimensionless numbers characterizing these processes were evaluated at the different operative conditions; a linear correlation was found to hold between Sherwood and Péclet numbers. Interestingly, the hydrogen permeate fluxes measured with feeds containing H 2–H 2O mixtures resulted always lower than those obtained for the nitrogen–hydrogen mixtures performed at the same hydrogen mole fraction and operative conditions: in particular, the hydrogen flux depletion increased with decreasing temperature and/or increasing the concentration of water vapor. All the experimental evidences suggest a clear interaction between water vapor and metallic layer, causing a lower hydrogen adsorption capacity of the membrane surface. That phenomenon is reversible, since the original permeance of the membrane was restored once the water vapor was removed from the feed, and is apparently due to a competitive H 2–H 2O adsorption on the Pd–Ag surface. The hydrogen flux depletion was then modeled by considering the simultaneous effects of gas phase resistance and competitive adsorption on the surface, obtaining a rather good agreement between experimental data and calculated results.

Effect of membrane module arrangement of gas–liquid membrane contacting process on CO 2 absorption performance: A modeling study

Mathematical models have been developed to analyze and predict the physical absorption performance of CO 2 through hollow fiber membrane contactors with different membrane module arrangements. The membrane contactor module is operated under partially wetted and counter-current flow mode. Four membrane module arrangements including single-stage module (SM), in parallel two-stage module (IP-TM), in series two-stage module with separated liquid flow (IS-TMS) and in series two-stage module with combined liquid flow (IS-TMC) are considered. The difference in module arrangement results in different degree of membrane wetting and concentration profiles along the module. The simulation is performed at the liquid flow rates in a range of 5.67–15.75 m 3/s. At low liquid flow rate (up to 7.5 × 10 −5 m 3/s) at which the penetration of liquid into membrane pores is less than 7.6%, the total resistance is dominated by liquid phase. The absorption performance shown in term of contact area can be ranked as SM > IS-TMC > IP-TM > IS-TMS. For high liquid flow rate, the gas phase resistance becomes comparable to the liquid phase resistance. The IS-TMS shows highest absorption performance followed by IP-TM, IS-TMC and SM, respectively. The effects of important design parameters including gas composition (CO 2–CH 4), split ratio of liquid ( α), the fraction of CO 2 removal at the middle and exit of stage ( x M , x exit ), liquid temperature and number of stage on required contact area are also investigated.

A Theoretical Analysis of Dissolved-Gas Transfer to the Bulk Liquid in Gas Absorption with a First-Order Reaction

A theoretical analysis using the surface-renewal and film-penetration models, which includes gas-phase resistance to mass transfer, is presented for the rate of absorption of a gas and its transfer to the bulk liquid in the case where the solute gas undergoes a first-order chemical reaction in the liquid phase. It reveals that: a. The fraction of absorbed gas transported to the bulk liquid depends upon the Hatta number Ha in case of the surface-renewal model and on Ha as well as a dimensionless hydrodynamic parameter in case of the film-penetration model. b. The widely assumed law of addition of resistances is valid for the surface-renewal and film-penetration models. c. The reaction influences both the overall mass-transfer coefficient and the nature of the driving force, i.e. the increased rate of absorption due to the reaction is not solely due to the enhancement factor multiplying the liquid-phase mass-transfer coefficient for physical absorption as has been conventionally assumed in the literature. It is also shown that in a gas-liquid reactor the film and surface-renewal models give close predictions for both the rate of absorption and concentration of dissolved gas in the liquid leaving the reactor. For values of Ha ⩾ 0.5, the bulk-liquid concentration of dissolved gas predicted by both models is negligible compared to its interfacial concentration.

Steam Stripping of Bromine from Sea Brine: Mass-Transfer Analysis of Randomly Packed Columns

Mass transfer data have been collected on stripping of bromine from aqueous solution at different flow rates of liquid and steam using Rasching and Pall Ring packings randomly packed in glass columns (6.25 to 90 cm diameter) and tower constructed by using granite slabs having square cross-section (25 cm × 25 cm). In the present study, the model proposed by Wagner et al. [1] for distillation considering gas phase resistance as the controlling resistance is modified wherein liquid phase resistance is considered as the controlling resistance for mass-transfer. The modified model and model proposed by Castillo et al. [2] for stripping of volatile organic compounds (VOCs) from water have been successfully applied to the experimental data on debromination of sea brine. Average deviation between observed and calculated value of height of transfer unit based on the modified model of Wagner et al. [1] is 20.9 and 10.7% for bromide-bromate solution and bittern, respectively. The average deviation between observed and calculated overall volumetric liquid phase mass transfer coefficient is 16.62, 15.13 and 84.19% for debromination of bittern using pilot plant data, commercial bromine plants, and sodium bromide-bromate solution, respectively, based on correlations given by Wagner et al. [1]. Similar results based on Castillo et al. [2] model are 34.43, 49.69 and 26.80%, respectively. Volumetric liquid phase mass transfer coefficient for stripping of VOCs from water is in the range of 0.001-0.008 (1/s) whereas in the present study it varies from 0.001 to 0.017 (1/s). In other words, steam stripping of bromine from aqueous systems appears comparatively easier than stripping of VOCs from water.

Study on the distillation rates of LiCl–KCl eutectic salt under different vacuum conditions

A study on the distillation rate of LiCl–KCl eutectic salt under different vacuums from 0.5 to 50 Torr was performed by using thermogravimetric (TG) method. A distillation rate of the order of 10 −4–10 −5 mol cm −2 s −1 was obtainable at temperatures of 1200–1300 K and vacuums of 5–50 Torr. Based on the non-isothermal TG data, model distillation flux equations could be derived as a function of temperature. Pure gas-phase and gas–liquid interfacial resistances at different vacuum conditions were evaluated from the comparison of experimental vaporization fluxes with the maximum flux obtained from the kinetic theory of gas. The difference between interfacial mass transfer coefficients and gas-phase ones increases with the temperature. Gas-phase resistance is much greater than that of the phase transition between condensed and gas phases at tested vacuum conditions of 0.5–50 Torr.

Influence of the gas phase resistance on hydrogen flux through thin palladium–silver membranes

Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773 K and at pressure differences up to 6 bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd–Ag layer (20 wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts’ law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance.

Corrigendum to: Estimating the internal conductance to CO2 movement.

The concentration of CO2 in the chloroplast is less than atmospheric owing to a series of gas-phase and liquid-phase resistances. For a long time it was assumed that the concentration of CO2 in the chloroplasts is the same as in the intercellular spaces (e.g. as measured by gas exchange). There is mounting evidence that this assumption is invalid and that CO2 concentrations in the chloroplasts are significantly less than intercellular CO2. It is now generally accepted that internal conductance (gi) is a significant limitation to photosynthesis, often as large as that due to stomata. Internal conductance describes this decrease in CO2 concentration between the intercellular spaces and chloroplasts as a function of net photosynthesis [gi = A/(Ci - Cc)]. Internal conductance is commonly estimated by simultaneous measurements of gas exchange and chlorophyll a fluorescence or instantaneous discrimination against 13CO2. These common methods are complemented by three alternative methods based on (a) the difference between intercellular and chloroplastic CO2 photocompensation points, (b) the curvature of an A/Ci curve, and (c) the initial slope of an A/Ci curve v. the estimated initial slope of an A/Cc curve. The theoretical basis and protocols for estimating internal conductance are described. The common methods have poor precision with relative standard deviations commonly > 10%; much less is known of the precision of the three alternative methods. Accuracy of the methods is largely unknown because all methods share some common assumptions and no truly independent and assumption-free method exists. Some assumptions can and should be tested (e.g. the relationship of fluorescence with electron transport). Methods generally require knowledge of either the kinetic parameters of Rubisco, or isotopic fractionation by Rubisco. These parameters are difficult to measure, and thus are generally assumed a priori. For parameters such as these a sensitivity analysis is recommended. One means of improving confidence in gi estimates is by using two or more methods, but it is essential that the methods chosen share as few common assumptions as possible. All methods require accurate and precise measurements of A and Ci - these are best achieved by minimising leaks, maximising the signal-to-noise ratio by using a large leaf area and moderate flow rate, and by taking into account cuticular and boundary layer conductances.

Gas-phase mass-transfer resistance at PEMFC electrodes: Part 2. Effects of the flow geometry and the related pressure field

It has been demonstrated that it is possible to have a better understanding of the mass-transfer phenomena occurring at PEMFC electrodes by distinguishing the various diffusive regimes taking place inside the porous layer close to the electrodes themselves. In each regime the interactions between diffusive and forced flows have been expressed in terms of Peclet numbers and the overall diffusive resistances have been expressed in terms of Sherwood numbers. Now the comparison of traditional and non-traditional geometrical arrangements can be more fully discussed and the geometrical optimisation of the cell can be more clearly determined by explicitly considering the role of the geometrical arrangement of the cell channels and the related pressure field. Both interdigitated and serpentine cells can be operated in such a way that high Sherwood numbers and, then, a high limit current are attained. Both types of cells are penalised by higher head losses than traditional cells, but these appear to be much greater in the serpentine arrangement. The ultimate goal of reaching high, almost uniform Sherwood numbers and low head losses is still problematic. A partially interdigitated configuration might be a step in the right direction.

Estimating the internal conductance to CO2 movement.

The concentration of CO2 in the chloroplast is less than atmospheric owing to a series of gas-phase and liquid-phase resistances. For a long time it was assumed that the concentration of CO2 in the chloroplasts is the same as in the intercellular spaces (e.g. as measured by gas exchange). There is mounting evidence that this assumption is invalid and that CO2 concentrations in the chloroplasts are significantly less than intercellular CO2. It is now generally accepted that internal conductance (gi) is a significant limitation to photosynthesis, often as large as that due to stomata. Internal conductance describes this decrease in CO2 concentration between the intercellular spaces and chloroplasts as a function of net photosynthesis [gi = A / (Ci - Cc)]. Internal conductance is commonly estimated by simultaneous measurements of gas exchange and chlorophyll a fluorescence or instantaneous discrimination against 13CO2. These common methods are complemented by three alternative methods based on (a) the difference between intercellular and chloroplastic CO2 photocompensation points, (b) the curvature of an A / Ci curve, and (c) the initial slope of an A / Ci curve v. the estimated initial slope of an A / Cc curve. The theoretical basis and protocols for estimating internal conductance are described. The common methods have poor precision with relative standard deviations commonly > 10%; much less is known of the precision of the three alternative methods. Accuracy of the methods is largely unknown because all methods share some common assumptions and no truly independent and assumption-free method exists. Some assumptions can and should be tested (e.g. the relationship of fluorescence with electron transport). Methods generally require knowledge of either the kinetic parameters of Rubisco, or isotopic fractionation by Rubisco. These parameters are difficult to measure, and thus are generally assumed a priori. For parameters such as these a sensitivity analysis is recommended. One means of improving confidence in gi estimates is by using two or more methods, but it is essential that the methods chosen share as few common assumptions as possible. All methods require accurate and precise measurements of A and Ci - these are best achieved by minimising leaks, maximising the signal-to-noise ratio by using a large leaf area and moderate flow rate, and by taking into account cuticular and boundary layer conductances.

Gas-phase mass-transfer resistance at PEMFC electrodes: Part 1. Diffusive and forced migration through a porous medium

The mass-transfer phenomena at the electrodes and, in particular, the diffusion of oxygen at the cathode significantly affect the limit performance of PEMFCs. Some particular geometric arrangements, such as the interdigitated or serpentine, have demonstrated their effectiveness in lowering diffusive resistances. It is possible to have a better understanding of these phenomena by determining the various possible diffusive regimes taking place inside the porous layer close to the electrodes. In each regime the interaction between diffusive and forced flows can be expressed in terms of Peclet numbers and the overall diffusive resistance in terms of Sherwood numbers. In this way, the comparison of traditional and non-traditional geometric arrangements can be more deeply studied, so that the problems relating to the simulation and optimisation of the cell can be more efficiently dealt with.

Removal of Benzene/Toluene from Water by Vacuum Membrane Distillation in a PVDF Hollow Fiber Membrane Module

Asymmetric polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by a phase inversion method using dimethylacetamide (DMAc) and a mixture of water/LiCl as solvent and a nonsolvent additive, respectively. The prepared membranes were characterized by scanning electron microscopy (SEM) for observing its microstructures and by a gas permeation method for measuring its surface porosity, pore size, and pore size distribution. Wetting pressures of the dry hollow fiber membranes were also measured. Using the prepared PVDF hollow fiber membranes, a membrane module was fabricated for removal of benzene/toluene from water. Effects of various operating parameters such as downstream vacuum levels, feed temperatures, and feed flow rates on performances of the module were investigated experimentally. The benzene/toluene removal was achieved over 99% under an optimal operating condition. Mass transfer of benzene or toluene removal is controlled not only by the liquid phase resistance but also by the membrane and gas phase resistances. Benzene and toluene can be removed from water simultaneously with no adverse coupling effects.

Water Requirements and Drought Tolerance of Bedding Plants

Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO2 concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m3·m-3 for all species and did not differ between a θ of 0.15 and 0.22 m3·m-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m3·m-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m3·m-3. Based on these results, a θ of 0.22 m3·m-3 for salvia and impatiens and a slightly lower θ of 0.15 m3·m-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m3·m-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO2 transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.

Simulation of petcoke gasification in slagging moving bed reactors

A mathematical model for simulation of moving bed petcoke gasifiers was developed. The model introduces a new feed characterization method, gas-phase resistance and volatilization models. The model is validated using reported data for a slagging gasifier. Effect of feed oxygen-to-coke and steam-to-coke ratios and feed coke rates on gasification performance was examined. Slagging zone moving bed gasifier operation with very high petcoke fluxes of over 4000 kg/m 2/h was possible with high petcoke conversion. Peak gas temperatures exceeded 1500 °C. Fluxes higher than 5000 kg/m 2/h are limited by an approach to fluidization of small particles in the combustion zone. The moving bed gasifier performance was found superior to performance of an entrained flow gasifier (EFG) with respect to energy efficiency and oxygen consumption.

Research on convective condensation heat transfer for a gas mixture in a vertical tube

AbstractThis paper discusses the convective condensation of a gas mixture in a vertical tube. A mathematical model was derived by combining a modified film model and Nusselt's condensation theory. The effect of wall temperature on film thickness and interfacial temperature was predicted and film thickness was calculated. When compared with the gas phase resistance method, the film model is better. The phenomenon of SO2 absorption into condensate is illustrated and discussed. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 219–228, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20011

Selective absorption of H2S from gas streams containing H2S and CO2 into aqueous solutions of N-methyldiethanolamine and 2-amino-2-methyl-1-propanol

Selective absorption of H2S from N2 streams containing H2S and CO2 into aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) as well as N-methyldiethanolamine (MDEA) was investigated in a 2.81×10−2 m o.d. stainless steel wetted-wall column at atmospheric pressure and constant feed gas ratio. In the range of gas flow rates studied (90–180×10−6 m3/s), the effect of gas-phase resistance on the absorption of H2S was significant. The rates of absorption of H2S and the selectivity factor decreased with the contact time for both alkanolamine solutions. With increasing amine concentration in the range 2.0–3.0 kmol/m3, the rates of absorption of both CO2 and H2S increased, but relatively more for CO2, resulting in a consequent decrease in the selectivity factor. In the temperature range 293–313 K, the rates of absorption of CO2 increased marginally with the increase in temperature while the rates of absorption of H2S and the selectivity factor decreased. The maximum selectivity observed in this work was 17.57 and 23.02 for AMP and MDEA, respectively. The acid gas mass transfer has been modelled using equilibrium-mass-transfer-kinetics-based combined model for CO2 and gas-phase transport equation-based approximate model for H2S considering negligible interaction between CO2 and H2S in the liquid phase. The experimental and model results have been found to be in good agreement.

Modeling of absorption of NO2 with chemical reaction in a falling raindrop

A model has been proposed for the scavenging of NO2 in a falling raindrop. After absorption, aqueous NO, undergoes a second order reaction to form various ions such as NO 2 - , NO 3 - and H+. The model is based on the unsteady state convective diffusion equation, which was solved for given boundary conditions by using implicit alternate direction (ADI) method. The circulation of fluid inside and outside the raindrop has been taken into account to realistically describe the flow field in the numerical domain. The model predictions indicate that the pH of a raindrop is a direct function of the drop size and bulk concentration of NO,. The model predicted a pH of about 4.9 for a 100-micron raindrop falling through a 20-ppb ambient concentration of NO2. For the same ambient concentration of NO2, a 10-micron raindrop would have a pH of about 4.75. The predictions also suggested that for all practical purposes the gas phase resistance may be taken as the rate-controlling step. The predicted values of gas-side mass transfer coefficient compared well with the estimated values using standard mass transfer correlations.

Investigation of the permeability and selectivity of gases and volatile organic compounds for polydimethylsiloxane membranes

Selectivity of oxygen, carbon dioxide, ethylene, dimethyl sulphide, trichloroethylene and toluene from air and their permeability ( P) and diffusivity ( D m) are reported for a polydimethylsiloxane (PDMS) membrane at 30 °C. All are constants within the studied feed concentration interval (0.04–233 g m −3). The presence of oxygen in the vapour/air mixtures has no significant influence on the vapour permeation. All resistances for mass transfer are determined in the PDMS membrane while gas phase resistances are negligible. Increasing feed concentration or decreasing membrane thickness results in a linear increase of the compound’s permeation. Computer simulations of the permeation tests indicate that the concentration in the permeate stream along the membrane sheet can be approximated by a logarithmic concentration profile. The highest permeability and lowest diffusivity are found for toluene, followed by trichloroethylene, dimethyl sulphide, ethylene, carbon dioxide and oxygen. Ratios between lowest and highest value for P and D m were 300 and 11, respectively. The relative high selectivity of vapours compared to oxygen can hinder an optimal bioreactor operation for breaking down gaseous pollutants when a PDMS membrane is used as gas/liquid contactor. In some cases, it is expected that unsufficient amounts of oxygen permeate for complete metabolisation of the pollutants.