Influence Of Mass Transfer Limitations Research Articles

Connecting experimental degradation kinetics to theoretical models for photocatalytic reactors: The influence of mass transport limitations

Catalytic microreactors offer great opportunities to measure reaction kinetics, and parameters influencing the reaction. Although microreactors are quite useful for characterizing catalysts, it is important to understand the relative contributions of mass transport and intrinsic kinetics to the apparent reaction rate. In this paper, we demonstrate the importance of accounting for mass transport limitations in the photocatalytic degradation of Bisphenol A over titanium dioxide. Using analytical scaling laws available from literature and numerical simulations, we provide guidelines for the use of microreactors in characterizing (photo)catalysts. These guidelines identify the mass transport limited regime, or the reaction rate limited regime. The photocatalytic degradation of Bisphenol A was found to be mass transport limited at high light intensities (photon fluxes of above 25 mW/cm2). Neglecting the influence of mass transfer limitations in fitting kinetic data resulted in the exponent of reaction rate (β) with respect to light intensity to be β~0.25, while including these effects gave an exponent directly proportional to the light intensity (β~1). These findings stress the importance of a correct inclusion of mass transport limitations. A simple analysis of the transverse Péclet number and second Damköhler number, to quantify the transport and reaction rates, is presented for our laminar flow reactor to illustrate the different limiting regimes.

Effect of alkanol chain length of primary alkanolamines and alkyl chain length of secondary and tertiary alkanolamines on their CO2 capture activities

Abstract The effect of the alkanol chain length in primary alkanolamines and the alkyl chain length in secondary and tertiary alkanolamines on CO 2 absorption and desorption kinetics, equilibrium CO 2 loading, heat duty, cyclic capacity, and pKa were studied. A selection strategy developed in our earlier work was used to identify potential solvents which could be used in a blend. Based on the strategy, alkanolamines that had a combination of high absorption parameter and high desorption parameter should be selected. The results of this study showed that, for absorption parameters, longer alkanol chain lengths of primary alkanolamines and longer alkyl chain lengths of secondary and tertiary alkanolamines led to higher equilibrium CO 2 loading and pKa. However, the influence of mass transfer limitations on these positive effects resulted in a maximum trend for initial rate of CO 2 absorption for secondary and tertiary alkanolamines. On the other hand, for the desorption parameters, the increase in the chain lengths also caused the generation of larger amounts of bicarbonate ions which resulted in higher CO 2 desorption rates and cyclic capacity, but lower heat duty. However, the longer chain alkanolamines also had high viscosities which adversely modified their performance by also introducing mass transfer limitations. Based on chain length alone, BMEA came out as the best overall alkanolamine component which could be used in a blend.

How to avoid mass transfer limitations in ozonation kinetics of phenylphenol isomers?

Abstract Ozonation is a heterogeneous process of chemical absorption often controlled by a gas-liquid mass transfer rate. This paper presents the results of kinetics in a reaction between phenylphenol isomers and ozone. The degradation of phenylphenol isomers during ozonation proceeds quite fast. In order to avoid the influence of mass transfer limitation the kinetics experiments were conducted in a homogenous liquid-liquid system. The second-order rate constants were determined using classical and competition methods, which are especially recommended for fast reactions. The determined rate constants at pH 2 using the two different methods are almost the same. The increase of pH causes an increase of rate constants for the reaction of phenylphenol isomers with ozone.

Emulsion polymerization of dimethylcyclosiloxane in cationic emulsion: mechanism study utilizing two phase liquid–liquid reaction kinetics

Abstract Dimethylcyclosiloxane emulsion polymerization (EP) was studied using heterogeneous two phase liquid–liquid kinetic theory combined with particle size measurements during reaction. A typical octamethylcyclotetrasiloxane EP was found to have a reaction half-life of about 1400 sec; much greater than the time (less than about 10 sec) for the cyclosiloxane to diffuse through the droplet surface into the aqueous phase. Previously published research proposing a mechanism where the cyclosiloxane EP begins by opening the cyclosiloxane ring at the droplet surface appears improbable because a reaction half-life less than a few seconds would be required. The aqueous solubility of D4, D5 and D6 was determined as a function of temperature. Most significantly; the rate of reaction of these cyclosiloxanes in EP was then determined to be a linear function of their aqueous solubility at the reaction temperature. Dimethylcyclosiloxane EP was determined to fully conform to the four characteristics of two phase heterogeneous liquid–liquid reaction kinetics wherein the reaction occurs in the aqueous phase. The kinetic rate equation was developed and the effect of cyclosiloxane mass transfer limitations on reaction rate was shown. (The influence of mass transfer limitations can give an erroneous appearance that reaction occurs at the droplet surface.) Evidence is presented that polymerization of cyclosiloxane in micelles is not favorable. The mechanism of cyclosiloxane emulsion polymerization was concluded to be coagulative homogeneous nucleation in the aqueous phase.

The impact of mass transfer limitations on size distributions of particle associated SVOCs in outdoor and indoor environments

Semi-volatile organic compounds (SVOCs) partition between the gas phase and airborne particles. The size distribution of particle-associated SVOCs impacts their fate in outdoor and indoor environments, as well as human exposure to these compounds and subsequent health risks. Allen et al. (1996) previously proposed that the rate of mass transfer can impact polycyclic aromatic hydrocarbon (PAH) partitioning among different sized particles, especially for time scales relevant to urban aerosols. The present study quantitatively builds on this idea, presenting a model that incorporates dynamic SVOC/particle interaction and applying this model to typical outdoor and indoor scenarios. The model indicates that the impact of mass transfer limitations on the size distribution of a particle-associated SVOC can be evaluated by the ratio of the time to achieve gas–particle equilibrium relative to the residence time of particles. The higher this ratio, the greater the influence of mass transfer limitations on the size distribution of particle-associated SVOCs. The influence of such constraints is largest on the fraction of particle-associated SVOCs in the coarse mode (>2μm). Predictions from the model have been found to be in reasonable agreement with size distributions measured for PAHs at roadside and suburban locations in Japan. The model also quantitatively explains shifts in the size distributions of particle associated SVOCs compared to those for particle mass, and the manner in which these shifts vary with temperature and an SVOC's molecular weight.

In situ Monitoring of Coke Deposits during Coking and Regeneration of Solid Catalysts by Electrical Impedance‐based Sensors

AbstractThe activity of solid catalysts is often substantially reduced by the formation of coke. The in situ monitoring of the coke load of catalyst particles could therefore be a controlling instrument of high value. Such sensors could be useful to measure the axial coke profile in a fixed bed reactor, both during coke formation and regeneration by coke burn‐off. As a contribution to the development of such sensors, single particles of an alumina oxide catalyst were used as a model system. The particles were electrically contacted and characterized by electrical impedance spectroscopy. The coke formation/burn‐off and the impedance were simultaneously measured in a magnetic suspension balance. A clear relationship between the coke loading and the respective electrical impedance signal could be observed, both during coke formation and during the regeneration by coke burn‐off. Kinetic studies were conducted to verify the influence of mass transfer limitations on the impedance signal.

Influence of Mass-Transfer Limitations on Carbon Isotope Fractionation during Microbial Dechlorination of Trichloroethene

Mass transfer of organic contaminants from nonaqueous phase liquids to the aqueous phase can significantly modulate the observable carbon isotope fractionation behavior associated with contaminant transformation. We evaluated the effects of kinetic interphase mass transfer between tetradecane and water on the observable (13)C enrichment factor, epsilon(obs), pertinent to the reductive dechlorination of trichloroethene (TCE) by Sulfurospirillum sp. in laboratory batch model systems containing organic, aqueous and gaseous phases. We propose a conceptual model, which includes the kinetics of tetradecane-water and gas-water mass transfer, microbial growth, and isotope-sensitive parameters describing dehalorespiration, for quantifying variable (13)C enrichment factors. While the C isotope fractionation of TCE reduction to cis-dichloroethene (cDCE) in the absence of phase-transfer effects can be characterized by a constant epsilon-value of -18.8 +/- 0.6 per thousand, mass-transfer limitations impede describing this process with a constant enrichment factor typically used in Rayleigh equations. Owing to the masking of kinetic isotope effects by the transfer of TCE from tetradecane to the aqueous phase, (obs)-values gradually changed from -18.4 per thousand to -5.9 per thousand. Such variations may complicate the interpretation of compound-specific isotope analysis in the assessment of chloroethene biodegradation in field applications.

Influence of mass transfer limitations on the enzymatic synthesis of β-lactam antibiotics catalyzed by penicillin G acylase immobilized on glioxil-agarose

Mass transfer effects were investigated for the synthesis of ampicillin and amoxicillin, at pH 6.5 and 25 degrees C, catalyzed by penicillin G acylase immobilized on agarose. The influence of external mass transfer was analysed using different stirring rates, ranging form 200 to 800 rpm. Above 400 rpm, the film resistance may be neglected. Intra-particle diffusion limitation was investigated using biocatalysts prepared with different enzyme loads and agarose with different mean pore diameters. When agarose with 6, 8 and 10% of crosslinking were used, for the same enzyme load, substrates and products concentration profiles presented no expressive differences, suggesting pore diameter is not important parameter. An increase on enzyme load showed that when more than 90 IU of enzyme activity were used per mL of support, the system was influenced by intra-particle mass transfer. A reactive-diffusive model was used to estimate effective diffusivities of substrates and products.

Influence of mass transfer resistance on detection of nonaqueous phase liquids with partitioning tracer tests

Partitioning interwell tracer tests (PITTs) are a relatively new technique for measuring the amount of nonaqueous phase liquid (NAPL) within saturated porous media. In this work we examined the influence of mass transfer limitations on the accuracy of measured NAPL from PITTs. Two mathematical models were used along with laboratory column experiments to explore the influence of tracer partition coefficient, tracer detection limit, and injected tracer mass on NAPL measurements. When dimensionless mass transfer coefficients were small, NAPL measurement errors decreased with decreasing tracer partition coefficient, decreasing tracer detection limit, and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. Although transport in a single stream tube was used in the mathematical models and laboratory experiments, the results from this simplified domain were supported by data taken from a three-dimensional computational experiment, where the NAPL resided as large pool. Based on these results, we suggest guidelines for interpreting tracer breakthrough data to ascertain the importance of mass transfer limitations on NAPL measurements.

Influence of mass transfer on regio- and enantioselectivity in hydrogenation of 1-phenyl-1,2-propanedione over modified Pt catalysts

The influence of mass transfer limitations on the enantio- and regioselectivity in hydrogenation of 1-phenyl-1,2-propanedione was studied. Gas–liquid mass transfer limitation as well as liquid–solid mass transfer limitation did not influence the enantio- and regioselectivity of the reaction, compared to the performance of the system under kinetic control. Under various regimes, the enantiomeric excess of the ( R)-1-hydroxy-1-phenylpropanone was about 55% and regioselectivity remained at about 10, which makes process scale-up with respect to enantio- and regioselectivity straightforward.

Modeling the performance of immobilized α-chymotrypsin catalyzed peptide synthesis in acetonitrile medium

A model was developed which describes simultaneous reaction and internal diffusion for kinetically controlled, immobilized α-chymotrypsin-catalyzed, oligopeptide synthesis in acetonitrile medium. The model combines the equations that describe the intrinsic kinetics of four different reactions and the physical characteristics of three different support materials, as determined experimentally, to predict the apparent initial activity and nucleophile selectivity of the immobilized biocatalyst. The model is able to predict reasonably well the experimentally observed initial rate and nucleophile selectivity vs. enzyme loading profiles. The reduction in observed initial rate with enzyme loading when fast reactions are carried out with α-chymotrypsin immobilized on celite, and the larger influence of mass transfer limitations on the initial reaction rates than on nucleophile selectivities are correctly predicted by the numerical calculations. The model is general in terms of its application to other systems — enzymes, reactions, support materials and/or kinetic schemes — as long as the intrinsic kinetics and the characteristics of the enzyme and support material are known.

Structural properties of SOFC anodes and reactivity

Fuel cells convert electrical energy directly into electricity and are thus an alternative to conventional energy conversion techniques. A detailed knowledge of the different processes in the fuel cell is necessary to develop fuel cells with high efficiency. One aspect is the internal reforming of methane in the anode of SOFC fuel cells. The anode structure is investigated by gas adsorption, diffusion and permeation measurements. The reforming reactivity is determined as a function of the anode thickness to characterize the influence of mass transfer limitations. The preparation of the SOFC anode material ends up in a sintered mix of NiO and YSZ. Temperature programmed reduction experiments are carried out in order to define the condition for complete NiO reduction at a low temperature level.

Influence of gas phase mass transfer limitations on molten carbonate fuel cell cathodes

The purpose of this paper is to elucidate to what extent mass transfer limitations in the gas phase affect the performance of porous molten carbonate fuel cell cathodes. Experimental data from porous nickel oxide cathodes and calculated data are presented. One and two-dimensional models for the current collector and electrode region have been used. Shielding effects of the current collector are taken into account. The mass balance in the gas phase is taken into account by using the Stefan-Maxwell equation. For standard gas composition and normal operating current density, the effect of gas-phase diffusion is small. The diffusion in the gaseous phase must be considered at operation at higher current densities. For low oxygen partial pressures, the influence of mass transfer limitations is large, even at low current densities. To eliminate the influence of conversion on polarization curves recorded on laboratory cell units, measurements should always be performed with a five to tenfold stoichiometric excess of oxygen. Two-dimensional calculations show rather large concentration gradients in directions parallel to the current collector. However, the influence on electrode performance is still small, which is explained by the fact that most of the current is produced close to the electrolyte matrix.

Influence of mass transfer limitations on determination of the half saturation constant for hydrogen uptake in a mixed‐culture CH4‐producing enrichment

There is strong evidence in the literature supporting the existence of significant mass transfer limitations on the kinetics of exogenous H(2) consumption by methanogens. The half saturation constant for H (2) uptake by a mixed-culture, CH(4) producing enrichment was measured using an experimental protocol that avoided internal mass transfer limitations. The value obtained was two orders of magnitude smaller than any other previously reported. A mathematical model for acetogenic syntrophic associations was developed to check the capacity of H(2) as electron transporter between syntrophic partners. It was found that H(2) diffusion could account for the rate of transport of electrons between the syntrophic microorganisms and that formate is not a necessary intermediate. The possibility that formate may be an intermediate in this system was not ruled out. A Monod-type kinetic equation was modified to include the observed H(2) threshold effect. This modified equation was used to predict the CH(4)-production rate in a batch-fed digester. The results show that the external and internal H(2) pools are kinetically coupled.

Chemical composition and kinetic properties of granular methanogenic sludge grown on propionate

The chemical composition and kinetic properties of propionate-grown methanogenic granules were analyzed as a function of the diameter of the granules (range: 0.1–3.0 mm in diameter). No significant variation in the chemical composition of the granules was observed. All granules contained large amounts of ash (between 53 and 59%). Extracellular polymers extracted from the granules were equivalent to 86–99 mg/g mixed-liquor volatile suspended solids (MLVSS), in which proteins, carbohydrates, and nucleic acids were the main constituents. The apparent substrate saturation constant (K s) value for propionate utilization was clearly influenced by the diameter of granules, although the maximum rate of propionate utilization was virtually constant, irrespective of the diameter. This indicates that the influence of mass transfer limitation was more pronounced in larger granules, especially at lower propionate concentrations.