Mass-transfer Limited Conditions Research Articles

Size-Dependent Electrochemical Metal Growth Kinetics

This work explores the electrochemical deposition (ECD) of Au onto chemically preformed Au nanoparticles (NPs) as a function of Au NP size. The growth potential and current depend strongly on the size of the preformed Au NPs (seeds) due to different Au NP size-dependent kinetics of the growth process. Electrochemical reversibility depends on the Au NP size, scan rate, metal precursor concentration ([AuCl4–]), and Au NP coverage. The Au NPs need to reach a critical total surface area to have enough active area to cause the ECD over the entire electrode to be electrochemically reversible and operate under linear diffusion mass transfer-limited conditions. The smaller Au NPs require less total surface area to reach this condition because they have higher activity per surface area, which is not fully understood but could be due to a greater percentage of defects on their surface, such as edge and corner atom sites.

Efficiency in CO2-utilization strategies: The case of styrene carbonate synthesis in microdroplets conditions

The styrene oxide to styrene carbonate conversion performed in CO2 atmosphere, herein selected as a case study, was implemented in microdroplets (aerosol) reactions at the preparative scale (3.5 mmol of the starting material) and mild conditions (1 atm CO2 pressure), within a custom-made ultrasonic nebulization reactor. Upon optimization of the promoter stoichiometry (1 eq of 4.3 TEG/KI ratio) and methanol (MeOH) dilution (7.5 mL of 2.5 v/v MeOH/TEG), performances under mass transfer-limited conditions of this novel methodological paradigm have been compared at 25 °C and 50 °C with those implemented as: a) no-stirred, b) stirred, and c) sonicated bulk reactions. Complete selectivity and an apparent acceleration factor (AAF) of 1.9 was registered at both temperature for microdroplets reactions in respect with the sonicated counterparts, these latter performing better than the other bulk reactions. These significative efficiency improvements, candidate aerosol reactions as a preferred process intensification approach in the realm of effective CO2-utilization strategies and, in general, in the development and exploitation of gas-liquid two-phase reactions.

Periodic open cellular structures (POCS) as enhanced catalyst supports: Optimization of the coating procedure and analysis of mass transport

Aluminum periodic open cellular structures (POCS) of cylindrical shape (9 mm diameter and 4−25 mm length) manufactured by investment casting were investigated as potential enhanced catalyst carriers. Samples with an ideal cubic cell geometry and different cell sizes - namely 1.5, 1.75 and 2.25 mm - were tested. First, coating experiments were performed aiming at understanding the wet coating deposition process: the influence operative spin coating parameters (i.e. spinning speed, time, acceleration) on final coating thickness was studied using glycerol solutions of different viscosity. Based on this preliminary investigation, POCS were activated by depositing a Pd/CeO2 catalyst by slurry coating techniques and tested under mass-transfer limited conditions in CO oxidation, used as a test reaction for gas/solid limited heterogeneous processes. A correlation for mass transfer performances of these structures in function of the Reynolds number is herein presented and used to compare these novel materials against conventional catalyst supports.

Scenario-based modelling of mass transfer mechanisms at a petroleum contaminated field site-numerical implications

Knowledge about distribution of dissolved plumes and their influencing factors is essential for risk assessment and remediation of light non-aqueous phase liquid contamination in groundwater. Present study deals with the applicability of numerical model for simulating various hydro-geological scenarios considering non-uniform source distribution at a petroleum contaminated site in Chennai, India. The complexity associated with the hydrogeology of the site has limited scope for on-site quantification of petroleum pipeline spillage. The change in fuel composition under mass-transfer limited conditions was predicted by simultaneously comparing deviations in aqueous concentrations and activity coefficients (between Raoult's law and analytical approaches). The effects of source migration and weathering on the dissolution of major soluble fractions of petroleum fuel were also studied in relation to the apparent change in their activity coefficients and molar fractions. The model results were compared with field observations and found that field conditions were favourable for biodegradation, especially for the aromatic fraction (benzene and toluene (nearly 95% removal), polycyclic aromatic hydrocarbons (up to 65% removal) and xylene (nearly 45% removal). The results help to differentiate the effect of compositional non-ideality from rate-limited dissolution towards tailing of less soluble compounds (alkanes and trimethylbenzene). Although the effect of non-ideality decreased with distance from the source, the assumption of spatially varying residual saturation could effectively illustrate post-spill scenario by estimating the consequent decrease in mass transfer rate.

Transient numerical modeling of catalytic channels using a quasi-steady gas phase

This paper presents a transient model of internal catalytic combustion suitable for isolated channels and monolith reactors. Due to time-scales in the problem, the model considers a quasi-steady gas phase with a transient solid. The gas is described by axially varying bulk temperature and species. The gas includes lateral diffusion via transfer coefficients and the specification of a gas-phase species concentration at the wall; axial diffusion is neglected. The solid phase is a thermally thin shell with axially varying temperature, surface species, and surface species concentrations. The solid includes axial heat conduction and external heat loss by convection and radiation. The combustion process utilizes detailed gas and surface reaction models. The gas-phase model becomes a system of stiff ordinary differential equations with respect to axial position; the upstream (inlet) boundary conditions are specified and the axially varying solid properties are parameters in integration. The solid phase discretizes into a system of stiff ordinary differential-algebraic equations with respect to time. The time evolution of the system comes from alternating integrations of the quasi-steady gas phase and transient solid.The model is compared to two experimental cases using CO fuel: (1) steady-state conversion in an isothermal platinum tube and (2) transient propagation of a catalytic reaction inside a small platinum tube and includes external tube temperature measurements. This work presents sensitivity studies on important parameters including internal transfer coefficients, catalytic surface site density, external heat-loss, and others. Under mass-transfer limited conditions, global transfer coefficients are adequate to predict fuel conversion. Near light-off, the model predictions improve for the first case after adjusting the surface kinetics such that the net rate of CO adsorption increases compared to O2. For the second case, predictions of transient propagation speeds are good for equivalence ratios near unity and greater but require adjustment of external heat loss or kinetics to match under lean conditions.

Aldol condensation products during photocatalytic oxidation of ethanol in a photoelectrochemical cell

Electric current was produced in a photoelectrochemical cell during photocatalytic oxidation of ethanol, which was used as a representative organic compound present in wastewater from biomass processing industries. Thus, the cell behaved as a PhotoFuelCell that consumed waste material to produce electricity. The cell consisted of two compartments separated by a silica frit. The anode electrode was a Fluorine-doped Tin Oxide transparent electrode bearing nanocrystalline titania Degussa P25, applied as a paste and calcined at 550°C. The cathode electrode was a carbon-cloth bearing Pt/Carbon-Black catalyst. The electrolyte was 1.0M NaOH. When 20%v. ethanol was added in the anode compartment, the cell gave 0.75mA/cm2 short-circuit current density (calculated over 12cm2 electrode area) and 1.1V open-circuit voltage. The anode compartment of the cell was operated under three different conditions: (1) under continuous air flow, i.e. oxygen-rich conditions; (2) under Ar flow, i.e. anaerobic conditions; and (3) exposed to ambient air but without any gas flow through the cell, i.e. oxygen-deficient, mass transfer-limited conditions. In the latter two cases, the oxidation of ethanol and its photo-oxidation products was incomplete and resulted in aldol condensation reactions that produced various aldehydes, some of them of high molecular weight and insoluble. It was concluded that in order to avoid such reactions, which impede mineralization process, measures should be taken for continuous oxygen supply and for optimal combination of the quantity of the photocatalyst and the concentration of the photodegradable substance. Thus only 1%v. ethanol can give 75% of the maximum current obtained in the presence of large ethanol concentration.

Advances in Infrared Spectroscopy of Catalytic Solid–Liquid Interfaces: The Case of Selective Alcohol Oxidation

Progress in the use of ATR-IR spectroscopy to improve the understanding of liquid-phase heterogeneous catalytic reactions is illustrated using the example of the oxidation of benzyl alcohol over Pd/Al2O3 and Bi–Pd/Al2O3. The in situ studies performed in both batch and continuous reactor cells provide rich information on the reaction pathway and important facets of the mechanism, such as the nature of active Pd sites and the effect of the Bi-promoter. The combination of CO site blocking prior to reaction and isotopic labeling suggests that alcohol dehydrogenation occurs uniformly over Pd nanoparticles, but only selected sites may allow desorption of the product benzaldehyde thus providing the required selectivity. Promotion of Pd/Al2O3 using bismuth produces infrared spectra free of adsorbed CO. This information demonstrates that Bi is deposited on selected adsorption sites (terraces rather than defects) and simultaneously confirms that open terraces favor product decomposition. Experiments performed in the continuous reactor cell using different catalyst film thickness show that reactions can be studied under kinetic or mass transfer limited conditions depending on catalyst film thickness. This allowed to study the alcohol oxidation under conditions of oxygen diffusion limitation, which are preferably applied in praxis in order to prevent catalyst deactivation by over-oxidation.

Simultaneous measurement of in situ bubble size and reaction rates with a heterogeneous catalytic hydrogenation reaction

The effect of solvent composition (water, 2-propanol and mixtures of the two) on the three phase, heterogeneously catalysed, two-step hydrogenation of 2-butyne-1,4-diol on Pd/alumina was investigated in a stirred reactor and for the first time, bubble sizes were measured in situ simultaneously with reaction rates. The first step forming the alkene is slow and gas–liquid mass transfer limitation is unlikely whilst the second step to alkane is much faster and may be mass transfer dominated. Overall, reaction rates displayed a complex relationship with solvent composition. The smallest bubbles where observed with 5% v/v 2-propanol in water and under gas–liquid mass transfer limited conditions (energy dissipation rates < 4 W / kg ), the overall reaction closely follows the ratio of mean bubble diameter to hydrogen solubility, both of which are a function of solvent. Overall, the solvent 5% v/v 2-propanol/water gave faster reactions combined with good selectivity compared to the others and is therefore recommended.

OXIDATION REACTIONS OF SOLID CARBON IN SUPERCRITICAL WATER

ABSTRACT Compared to the research for homogeneous phase supercritical water oxidation (SCWO), kinetic understanding of SCWO progress on solid materials is less developed. We studied the reaction progress of carbon particles under SCWO conditions, where mass transfer processes and heterogeneous reactions are involved with mutual interactions. Reaction progress under mass-transfer limited conditions was observed for solid activated carbon at 400–500°C. The overall reaction of synthetic graphite observed at 500–600°C and that of activated carbon below 400°C were controlled by the internal surface heterogeneous reaction. The reaction rate at a unit internal surface area was proportional to the oxygen concentration and the specific reaction rate was described as 1.7 × exp (− 117 kJ mol−1/RT) m s−1. The reaction product was exclusively CO2 with a small amount of CO in the case of activated carbon.

THE CURRENT DISTRIBUTION IN THROUGH-HOLE ELECTRODEPOSITION.I:MASS TRANSFER

A realistic representation of through-hole geometry consists of a cylindrical tube with an abrupt decrease in the radius in the middle section of the tube. Fluid flow, mass transfer, and geometric effects were included in modeling through-hole electrodeposition process. The current distribution under mass-transfer limited conditions was computed for both the inside and outside surfaces of a through-hole.

Nonaqueous Phase Liquid Pool Dissolution as a Function of Average Pore Water Velocity

Bench-scale reactor experiments were performed to study the dissolution of a binary naphthalene-in-nonane mixture nonaqueous phase liquid (NAPL) pool over a wide range of average pore water velocities, ν\dx(≈0.1–60 m/day). Experimental NAPL pool dissolution flux values were determined using a steady-state mass balance approach. The experimental flux data were compared to model predictions made assuming either local equilibrium or mass-transfer limited conditions. The local equilibrium model could describe the trends in the average effluent concentration and dissolution flux with 0.1 10 m/day. Data determined to be under mass-transfer limited conditions were fit to the nonequilibrium model to estimate values for an overall mass-transfer coefficient. The calculated overall mass-transfer coefficients had an average value of 0.407 m/day and showed no correlation with ν\dx, probably due to mass-transfer resistance becoming dominated by the diffusional resistance in the NAPL. These results suggest that the nonequilibrium approach is better suited for describing high velocity (ν\dx>10 m/day) dissolution of multicomponent NAPL pools, and that flushing of groundwater at very high velocities may not be an effective approach for enhancing NAPL-pool dissolution flux.

Mass-transfer enhancement by static mixers in a wall-coated catalytic reactor

Using CO oxidation in a wall-coated tubular reactor, the enhancement of radial mass transfer by a static mixer was investigated. Experiments were conducted at various flow rates, CO concentrations and temperatures. Flow rates were chosen such that the flow remains in the laminar regime. Several effects were observed: light-off occurred at higher temperatures for higher flow rates and higher CO concentrations. Lower conversions were obtained for higher flow rates. In general, measured conversions were higher with the static mixer, compared to an empty tube under mass-transfer limited conditions. An expression for the Sherwood number, Sh=2+0.059 Re, was calculated for the system with static mixers. Our study demonstrates that the performance of wall-coated reactors can be significantly enhanced using static mixers. This reactor may be a new tool for reaction engineers, e.g., to replace catalyst-pellet-filled tubes in multi-tubular reactors to suppress hot-spot formation or for heterogeneously catalyzed viscous liquid-phase reactions.

Design and characterisation of a close-concentric annular reactor for kinetic studies at high temperatures

A novel annular reactor for kinetic studies at high temperature and flow conditions has been designed to keep eccentricity tolerances below 10%. In a previous work, we have shown that it is very important to keep such low eccentricity values in order to collect reliable kinetic data from this type of reactors. As proposed in this study, a modified reactor with the use of a spacer could guarantee an annular duct with low levels of eccentricity. Manufacturing tolerances or deformation effects giving rise to eccentricity can be significantly minimised when using this apparatus. The reactor has been both experimentally and theoretically characterised. Carbon monoxide oxidation was used as a model reaction under mass-transfer limited conditions revealing an eccentricity of ∼5%. With such small eccentricity levels, a concentric annular form can be assumed in the reactor analysis. Simple 1D or 2D models can therefore be inexpensively used in the evaluation of the kinetic data. Also, prior to the design of the annular reactor, a numerical investigation was carried out to clarify the effects of eccentricity, physical properties of the carrier gas and the annular aspect ratio on mass-transfer limitations. Contrary to expectations, a considerable increase in the fuel mass-diffusivity by carrier gas substitution did not change the mass-transfer rates for cases when eccentricity and aspect ratios were high.

Development of a process model to describe the synthesis of (R)-mandelonitrile by Prunus amygdalus hydroxynitrile lyase in an aqueous-organic biphasic reactor.

A process model for the enzymatic synthesis of (R)-cyanohydrins in an aqueous-organic biphasic-stirred tank reactor was developed. The conversion of benzal-dehyde into (R)-mandelonitrile, catalyzed by Prunus amygdalus hydroxynitrile lyase at 5 degrees C and pH 5.5, was chosen as a model system with methyl tert-butyl ether as the organic phase. The process model consisted of a description of the reaction kinetics, mass transfer kinetics, and the mass balances for both the aqueous and the organic phase. Values for the enzyme kinetic parameters, according to ordered bi-uni kinetics, the lumped mass transfer coefficient for benzaldehyde, and the partition coefficients were determined separately. The process model is validated by using 11 experimental data sets of batch conversions in the aqueous-organic biphasic-stirred tank reactor. In these 11 experiments, different enzyme concentrations and phase volume ratios were used. The model was found to be valid with respect to both the conversion and the enantiomeric excess. To synthesize cyanohydrins with a high enantiomeric excess, the enzyme is required to work at mass transfer limited conditions. The developed process model will be used to investigate other process concepts and other substrates.

Prediction of complete bioremediation periods for PAH soil pollutants in different physical states by mechanistic models

Mass-transfer models and biodegradation models were developed for three theoretical physical states of polycyclic aromatic hydrocarbons (PAHs) in soil. These mechanistic models were used to calculate the treatment periods necessary for complete removal of the PAH pollutants from the soil under batch conditions. Results indicate that the bioremediation of PAHs in such systems is mainly mass-transfer limited. The potential for bioremediation as a treatment technique for PAH contaminated soils is therefore mainly determined by the mass-transfer dynamics of PAHs. Under mass-transfer limited conditions simplified mathematical models, based on the assumption of a zero dissolved PAH concentrations, can be used to predict the period of time needed for complete bioremediation.

Mathematical model of a monolith catalytic incinerator

A set ofl-dimensional mathematical models were developed to simulate both the steady state and transient performance of monolithic catalytic incinerators for VOC abatement. In modelling transient performance, quasi-steady state gas phase was assumed since transient response time is determined primarily by the thermal inertia of the monolith. Higher inlet gas temperatures and lower gas velocities were predicted to give higher conversion and faster response times. VOC concentration had little influence on the performance within the concentration ranges used. A catalytic incinerator is shown to operate typically under mass transfer limited conditions, and monolith channel density and shape have significant influence on the conversion and monolith heating time. The metallic monolith was predicted to show superior steady state and transient responses due to its lower thermal inertia generated by higher cell density and thinner wall.

The dissolution of benzene, toluene, m-xylene and naphthalene from a residually trapped non-aqueous phase liquid under mass transfer limited conditions

The results of dissolution experiments for benzene, toluene, m-xylene and naphthalene (BTXN) from a relatively insoluble oil phase (tridecane), residually trapped in a non-sorbing porous medium, are described. This mixture was chosen to simulate dissolution of soluble aromatic compounds from a petroleum hydrocarbon mixture, e.g., crude oil, for which a large fraction of the mixture is relatively insoluble. The experiments were carried out at a small source length to interstitial velocity ratio, L/ v, so that dissolution would be mass transfer limited (MTL). When fitted to data for toluene, a multiregion mass transfer model was found to predict the experimental data satisfactorily for the other components without adjustment of the mass transfer rate parameters. These results indicate that the dissolution process can be generalized for various hydrophobic organic compounds present in a multicomponent non-aqueous phase liquid (NAPL) when mass transfer limitations are present. This also suggests that dissolution data obtained for one compound can be useful for predicting the dissolution histories for other compounds from petroleum hydrocarbon mixtures.

Influence of hydrodynamic conditions on naphthalene dissolution and subsequent biodegradation.

The influence of hydrodynamic conditions on the dissolution rate of crystalline naphthalene as a model polycyclic aromatic hydrocarbon (PAH) was studied in stirred batch reactors with varying impeller speeds. Mass transfer from naphthalene melts of different surface areas to the aqueous phase was measured and results were modeled according to the film theory. Results were generalized using dimensionless numbers (Reynolds, Schmidt, and Sherwood). In combined mass transfer and biodegradation experiments, the effect of hydrodynamic conditions on the degradation rate of naphthalene by Pseudomonas 8909N was studied. Experimental results were mathematically described using mass-transfer and microbiological models. The experiments allowed determination of mass-transfer and microbiological parameters separately in a single run. The biomass formation rate under mass transfer limited conditions, which is related to the naphthalene biodegradation rate, was correlated to the dimensionless Reynolds number, indicating increased bioavailability at increased mixing in the reactor liquid. The methodology presented in which mass transfer processes are quantified under sterile conditions followed by a biodegradation experiment can also be adapted to more complex and realistic systems, such as particulate, suspended PAH solids or soils with intrapartically sorbed contaminants when the appropriate mass-transfer equations are incorporated.

Digital simulation via the hopscotch algorithm of a microelectrode-based channel flow-through amperometric detector

The current flowing at a band ultramicroelectrode in a rectangular channel under mass transfer-limited conditions is computed numerically. The procedure is based on the “hopscotch” algorithm and on an appropriate space transformation, δ = tanh( x/α). The edge effect is found to influence the response depending on the flow rate. The polarization times necessary to reach the steady-state current are also computed under different operating conditions.