Mass Transfer Aspects Research Articles

Aeration / Stripping of n-Butyl Mercaptan in aqueous environment

Μany applications in water quality management have a common key water quality parameter, dissolved oxygen, resulting to the critical role of aeration. On the other hand, in municipal and industrial wastewater, especially where aeration is applied, the presence of volatile organic compounds (VOCs) causes several concerns including a direct threat to humans, partly due to their emission from treatment tanks. pH, temperature and Henry’s Law govern VOCs’ speciation and consequently their emission characteristics. Limited data and simplifications of available mass-transfer models pose obstacles to a realistic approach, especially in the presence of a chemical equilibrium, for example in the case of mercaptans. In the present study the importance of oxygen transfer and stripping of a VOC (n-butyl mercaptan) on aeration’s overall effectiveness are examined separately. Clean water oxygenation and stripping of mercaptan to an inert gas (nitrogen) were studied aiming to consider mass transfer aspects and to investigate the influence of chemical equilibrium between ionic and neutral form of the target compound in neutral and alkaline solutions. Using appropriate mass transfer relationships (dynamic method), experimental data were analyzed for the determination of overall mass transfer coefficient ( kOL,O2α ) of oxygen. Correlating kOL,O2 α with the corresponding mass transfer coefficient of n-butyl mercaptan in neutral solutions (calculated according the model proposed by Matter-Muller et al. [1]), a value of ratio βy of 0.566 is found, close to the reported values of other VOCs with similar values of Henry’s constant. At alkaline pH however the conventional simplified model fails to predict realistic values of mass-transfer coefficients. A coupled differential algebraic equation system, based on mass balances, taking into account dissociation of the compound to be stripped and assuming chemical non-equilibrium conditions during stripping, was developed. Reaction parameter k2 was calculated with non-linear least-squares analysis. The model predicts satisfactorily the experimental data and it provides a useful tool for the semibatch stripper design in situations where a reversible reaction is involved. At pH values below 8.5 mercaptan concentration falls exponentially whereas above 10.5 it tends to linearity. The bubble equilibrates and mercaptan transferred depends upon solubility and not diffusivity. Especially after depletion of initial neutral compound, transport depends upon neutral/ionic form speciation. The effectiveness of stripping n-butyl mercaptan, at a given pH, is mainly determined by a proportionality constant considered as “fugacity capacity” (removal effect on the process) and by a reversible reaction rate constant k2 (kinetic effect on the process). The ‘’fugacity capacity” is determined by hydrophobicity (i.e. low solubility and high limiting activity coefficient) rather than pure-component volatility (i.e. vapor pressure or boiling point). High limiting activity coefficient promotes mercaptan emission due to established vapor-liquid equilibrium, while the low reaction parameter k2, controls neutral compound quantity. At high pH, where ionic form predominates, experimental data showed that stripping was almost independent of the gas flow rate applied. A strong sensitivity of the model to uncertainty of γ∞ was found: γ∞ controls emission rate and through this the dynamic variations of neutral/ionic concentration profiles whereas reaction rate law parameter k2 controls the neutral/ionic transformation and it is the crucial quantity which governs the process at high pH values.

Kinetics of Hydrodesulfurization of Diesel: Internal Mass Transfer Aspects

Indian fuel policy aims at producing ultra low sulphur diesel diesel nationwide in the near future. Ultra deep hydrodesulfurization (UDHDS) catalysts were developed and their kinetics investigated. Experiments were conducted earlier in a continuous flow microreactor over a presulfided catalyst in the temperature range of 330–360°C, whsv in the range of 1–3 hr−1, and pressure in the range of 25–50 bar at hydrogen to hydrocarbon ratio of 750 N dm3/dm3, and its kinetic parameters estimated. In the present study, effects of physical catalyst parameters on the internal resistance were simulated and directions of improving kinetics have been suggested for optimum UDHDS catalyst performance.

A novel reactive 4-diethylamino-2-butanol solvent for capturing CO2 in the aspect of absorption capacity, cyclic capacity, mass transfer, and reaction kinetics

In the present work, the performance of a novel reactive solvent, 4-diethylamino-2-butanol (DEAB), was comprehensively investigated and compared with the conventional solvents (e.g., MEA and MDEA) in terms of CO2 absorption capacity, cyclic capacity, mass transfer performance, and absorption kinetics. The results showed that DEAB can be considered as alternative promising solvent for capturing CO2 because its outstanding performance on much higher absorption capacity, higher cyclic capacity, and higher mass transfer performance; comparing with those of MEA and MDEA. In addition, the CO2 absorption kinetics of DEAB was found to be faster than that of conventional tertiary MDEA, but slower than that of conventional primary MEA.

A two-interface transport model with pore-size distribution for predicting the performance of direct contact membrane distillation (DCMD)

We investigated fundamental aspects of heat and mass transfer of direct contact membrane distillation. Molar flux of water vapor through a membrane pore was analytically obtained by solving Fick's law in the original differential form. Axial variation of the temperature profile was derived as exponentially decreasing, and was found to be linear due to small membrane thickness and dominant heat conduction through the solid part of the membrane. An alternative expression of water vapor pressure at a constant temperature was developed using experimental data of water latent heat for evaporation, and was used to calculate the concentration of water vapor in the membrane pore. The effective diffusion coefficient was obtained by combining Knudsen and Brownian diffusion coefficients with Bosanquet's assumption. The effective diffusivity and mean free path of water vapor slowly decrease in the axial direction, and the vapor concentration increases along the membrane pore primarily due to the linearly decreasing temperature. We found that the required heat flux monotonously increases with the vapor flux through membrane pores. Finite variance of a pore size distribution provides less vapor flux than that of mono-dispersed pores. This is because a number of smaller pores than the average pore size significantly hinders the vapor transport across the porous membrane. Theoretical prediction of permeate flux agrees very well with experimental observations reported in the literature.

Kinetic and Equilibrium Profiles of Adsorptive Recovery of Thorium(IV) from Aqueous Solutions Using Poly(methacrylic acid) Grafted Cellulose/Bentonite Superabsorbent Composite

The removal and recovery of thorium(IV) [Th(IV)] ions from aqueous solutions were investigated using poly(methacrylic acid)-grafted-cellulose/bentonite (PMAA-g-Cell/Bent) superabsorbent composite through batch adsorption experiments. Surface characterizations of the adsorbent were investigated. The adsorbent showed significant Th(IV) removal (>99.7%) at pH 5.0. The influence of coexisting ions on the adsorption of Th(IV) was studied. Mass transfer aspects of Th(IV) adsorption onto PMAA-g-Cell/Bent were evaluated. The Sips adsorption isotherm described the adsorption data very well, with a maximum adsorption capacity of 188.1 mg/g. Thermodynamic parameters such as standard enthalpy (ΔH°), standard entropy (ΔS°), standard free energy (ΔG°), activation energy (Ea), isosteric enthalpy (ΔHx), and entropy (ΔSx) were calculated. Tests with a seawater sample revealed the effectiveness of PMAA-g-Cell/Bent for adsorptive removal of Th(IV) from aqueous solutions. Desorption experiments showed that 0.1 M HNO3 can eff...

Glucose Mass Transfer Under Substrate Inhibition Conditions in a Stationary Basket Bioreactor with Immobilized Yeast Cells

The work is dedicated to the study on the external and internal mass transfers of glucose for alcoholic fermentation under glucose inhibition limitation using a bioreactor with stationary basket beds of immobilized Saccharomyces cerevisiae cells. By means of the substrate mass balance for a single particle of biocatalysts, considering the kinetic model adapted for the inhibitory effect of glucose, specific mathematical models have been developed for describing the profiles of the substrate concentration in the outer and inner regions of biocatalysts and, implicitly, for estimating its mass flows in the liquid boundary layer surrounding the particle and inside the particle. The values of the mass flows are significantly influenced by the internal diffusion velocity of substrate and rate of the biochemical reaction of substrate consumption, but also by the position inside the basket bed. These influences cumulated led to the appearance of biological inactive region near the particle centre, its magnitude varying from 0.34 to 31% from the overall volume of particles. The presented results are the first unitary approach of the aspects of substrate mass transfer in a basket bed of immobilized biocatalysts, the proposed models being original and complex.

Displacement and Mass Transfer of CO2/Brine in Sandstone

The displacement and fluid/fluid mass transfer between CO2 and brine in Berea sandstone have been investigated by unsteady-state core-flood experiments combined with x-ray computed tomography. Relative permeability and capillary pressure saturation functions of primary drainage for mutually saturated fluid phases have been determined from production data and the saturation profiles by history matching and have been benchmarked against standard SCAL. The displacement stability of the CO2/brine drainage process has been investigated by numerical modeling, and the consequences for experimental procedures and for geological storage of CO2 are discussed. Aspects of mass transfer during drainage and imbibition that are relevant for nonequilibrated fluid phases were studied by core flooding with unsaturated CO2 and brine phases.

Assessment of mass transfer and hydraulic aspects of CO2 absorption in packed columns

The paper evaluates, using modeling and simulation techniques, CO2 capture process from flue gases produced by power generation sector (post-combustion capture) in aqueous solution of mono-ethanolamine (MEA). The aim of the present article is to validate the absorber model as well as to understand the dynamic behavior of CO2 absorption process. The mathematical model of carbon dioxide absorption includes differential equations, e.g. mass and heat transfer models to study the coupled effect of temperature and concentration on the absorption rate, reaction kinetics, vapor–liquid equilibrium (VLE), hydrodynamic aspects, etc. Determinant parts of the absorption model are the effective interfacial area, the mass transfer coefficient, pressure drop and liquid hold-up models. The simulation results have shown that the column operating conditions highly influence the accuracy of the absorber model and the importance of the mass transfer and hydraulic model selection for minimize the deviation of the model results from experimental data value. The present absorber model can be applied to study column operability during dynamic operation.

Experimental and theoretical study of LDPE: Evaluation of different food simulants and temperatures

The mass transfer process of additives from plastic packaging to food is of great importance and interest because of the possible harmful effects on the human health. In this work the effect of different food simulants (EtOH 10–50%) and temperatures (28–60 °C) on the specific migration of Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate (I-1076) from LDPE films is experimentally and theoretically studied, considering also the antioxidant stability in the food simulants studied at different temperatures. On the other hand, a phenomenological model based on a resistances-in-series which was resolved through the Regula Falsi algorithm according to a routine that considers the income of the structural parameters of the system and experimental conditions in which migration testing was performed. Diffusion coefficients of I-1076 in LDPE were ranged between 8.0E10–14 and 1.2E10–12 (m 2 s − 1 ) for temperatures of 28 to 60 °C, respectively. The coupled effect of mass transfer and thermodynamical aspects was analyzed through the parameter Bi/K P/FS proposed in this work, estimated in order to identify the combined contribution of the transport rate and the availability of the migrant compound in the food phase.

Review of mass transfer aspects for biological gas treatment

This contribution reviews the mass transfer aspects of biotechnological processes for gas treatment, with an emphasis on the underlying principles and technical feasible methods for mass transfer enhancements. Understanding of the mass transfer behavior in bioreactors for gas treatment will result in improved reactor designs, reactor operation, and modeling tools, which are important to maximize efficiency and minimize costs. Various methods are discussed that show the potential for a more effective treatment of compounds with poor water solubility.

ChemInform Abstract: Two‐Phase Partitioning Bioreactors in Environmental Biotechnology

AbstractReview: 113 refs.

Evaluation of model parameters for simulating TiO2 coated UV reactors

A CFD-based model for simulating TiO(2) coated photocatalytic reactors used in drinking water treatment applications was preliminarily evaluated. The model includes aspects of hydrodynamics, mass transfer, UV-radiation field, and surface chemical reactions. Appropriate models for each of the associated physicochemical phenomena were experimentally or analytically examined. Once defined and evaluated, the individual models were integrated into a CFD-based model for simulating photocatalytic reactor performance, which was experimentally evaluated.

Simulation of continuous stirred rotating disk-membrane module: An approach based on surface renewal theory

A rigorous surface renewal model has been developed describing the aspects of mass transfer in a rotating disk-membrane (RDM) ultrafiltration cell. The model takes into consideration of two distribution functions of random surface elements, one with respect to their point of origin and the other related to the corresponding residence time on the membrane surface. The back transport flux and the permeate flux are evaluated at the membrane surface in order to develop a surface component balance equation. The component balance equation and a flux-rejection relationship arising from irreversible thermodynamics are solved simultaneously to develop a dynamic simulation. The simulation elucidates on permeate flux, membrane surface concentration and the permeate concentration under various operating conditions of transmembrane pressure, bulk concentration, membrane and stirrer speeds. For validation of the proposed model, experiments were conducted with bovine serum albumin (BSA)/water as feed in a standard rotating disk membrane module fitted with polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and permeate concentration was found to be in good agreement with the experimental data, and the maximum absolute deviation for both cases was found to be well within ±5%.

Ex situ bioremediation of Cr(VI) contaminated soil by Bacillus sp.: Batch and continuous studies

This study presents the ex situ batch and continuous bioremediation of tannery effluent contaminated soil using Bacillus sp. isolated from hexavalent chromium (Cr(VI)) contaminated environment. The remediation of Cr(VI) contaminated soil by conventional methods is challenging and hence, the present study aims to address this issue by examining ex situ bioremediation of Cr(VI) contaminated soil. The elemental composition of the soil was analyzed by SEM–EDX. The leachate of the soil was remediated using Bacillus sp. and the effect of temperature, pH and inoculum volume on the process performance was explored. The influence of electron donors such as glucose, fructose, sucrose and bagasse extract on Cr(VI) reduction was also enumerated. The optimum pH and temperature were found to be 7 and 37 °C respectively and glucose was found to offer higher Cr(VI) reduction compared to other electron donors. The reduction was found to follow Arrhenius equation and the enzyme responsible for the reduction was found to be chromium reductase. The continuous reduction of Cr(VI) was carried out in a packed bed reactor using sand as the inert support for Bacillus sp. The analysis of kinetics and mass transfer aspects of the process was done with simple models.

Biosorption of lead by the immobilised fungus Phanerochaete chrysosporium in a packed bed column

Biosorption of lead by immobilised Phanerochaete chrysosporium over two successive cycles of sorption-desorption was investigated in a continuously operated packed bed column. The breakthrough curves in each sorption cycle were measured as a function of different bed depths in the column at an inlet lead concentration of 40 mg/L. The values of various breakthrough parameters obtained in the study indicated good potential of the system in removing lead from constituent wastewaters. Kinetic and mass transfer aspects of lead biosorption in the study were also evaluated by fitting the experimental data to several breakthrough models found in the literature. And, compared to the other models, Clark model showed good predictability of the entire breakthrough data.

Nitration of nitrobenzene at high‐concentrations of sulfuric acid: Mass transfer and kinetic aspects

AbstractThis article reports studies on mass transfer and kinetics of nitration of nitrobenzene at high concentrations of sulfuric acid in a batch reactor at different temperatures. The effects of concentration of sulfuric acid, speed of stirring, and temperature on mass transfer coefficient were investigated. The kinetics of nitration under homogenized conditions was studied at different sulfuric acid concentrations at these temperatures. The reaction rate constants were determined. The variation of rate constant with sulfuric acid concentration was explained by the Mc function. The activation energies of the reactions were determined from the Arrhenius plots. The regimes of the reactions were determined using the values of the mass transfer coefficients and the reaction rate constants. A model was developed for simultaneous mass transfer and chemical reaction in the aqueous phase. The yields of the three isomers of dinitrobenzene were determined, and the variation of isomer distribution with sulfuric acid concentration and temperature was analyzed. This work demonstrates that more than 90% conversion of nitrobenzene is possible at high‐sulfuric acid concentrations resulting in high yield of the product even at moderate temperatures and at low speeds of stirring. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Two-phase partitioning bioreactors in environmental biotechnology

Two-phase partitioning bioreactors (TPPBs) in environmental biotechnology are based on the addition of a non-aqueous phase (NAP) into a biological process in order to overcome both mass-transfer limitations from the gas to aqueous phase and pollutant-mediated inhibitions. Despite constituting a robust and reliable technology in terms of pollutant biodegradation rates and process stability in wastewater, soil, and gas treatment applications, this superior performance only applies for a restricted number of pollutants or contamination events. Severe limitations such as high energy requirements, high costs of some NAPs, foaming, or pollutant sequestration challenge the full-scale application of this technology. The introduction of solid NAPs into this research field has opened a promising pathway for the future development of TPPBs. Finally, this work reviews fundamental aspects of NAP selection and mass transfer and identifies the niches for future research: low energy-demand bioreactor designs, experimental determination of partial mass transfers, and solid NAP tailoring.

Mass transfer and thermodynamic aspects of sodium desorption from eucalyptus kraft pulp by acidification using carbon dioxide

The thermodynamics and mass transfer aspects associated with the use of CO 2 for the acidification, particularly for the removal of Na + from pulp fibres, were studied. The acidifications with CO 2 and with H 2SO 4 were carried out in a stirred tank reactor under different temperatures, agitation speeds, concentrations of CO 2 and H 2SO 4 and treatment times. The CO 2 acidification of pulp was found to be very fast at Re > 10 4 and N p of 4, and reached equilibrium within 5 min. The volumetric mass transfer coefficient, k L a, of the CO 2 acidification was found to be in the range of 0.0005–0.0083 s −1. Thermodynamically, the CO 2 acidification was achievable only at pH > 4. The extent of the resultant Na + exchange increased with the acidification time, agitator speed and CO 2 flow rate. Under similar experimental conditions, the extent of Na + desorption by CO 2 acidification was found to be slightly lower than that achieved by sulfuric acidification. The equilibrium distribution of Na + in the solution and the fibre phase can be estimated using a model based on the Donnan theory.

Mass transfer in a small scale post-combustion flue gas absorber, experiment and modelling

Abstract Carbon dioxide can be removed from flue gas streams with aqueous solutions of alkanolamines. In the absorber carbon dioxide is contacted with the solvent and due to several chemical reactions the carbon dioxide is converted into ionic species. These species are non-volatile and remain in the liquid phase until the carbon dioxide is released in the stripper (by adding heat). For the design of gas treating processes a sound fundamental process model is required in which all relevant mass transfer aspects, thermodynamics and kinetics are incorporated. However, the quality of a process model should always be determined by the validation with experimental data. In this study a rate-based model of a flue gas absorber is compared quantitatively with experimental data derived from a continuous absorber–stripper pilot plant which was operated at typical post-combustion flue gas conditions.

Effect of impeller type and mechanical agitation on the mass transfer and power consumption aspects of ASBR operation treating synthetic wastewater

The effect of flow type and rotor speed was investigated in a round-bottom reactor with 5 L useful volume containing 2.0 L of granular biomass. The reactor treated 2.0 L of synthetic wastewater with a concentration of 800 mgCOD/L in 8-h cycles at 30 °C. Five impellers, commonly used in biological processes, have been employed to this end, namely: a turbine and a paddle impeller with six-vertical-flat-blades, a turbine and a paddle impeller with six-45°-inclined-flat-blades and a three-blade-helix impeller. Results showed that altering impeller type and rotor speed did not significantly affect system stability and performance. Average organic matter removal efficiency was about 84% for filtered samples, total volatile acids concentration was below 20 mgHAc/L and bicarbonate alkalinity a little less than 400 mgCaCO 3/L for most of the investigated conditions. However, analysis of the first-order kinetic model constants showed that alteration in rotor speed resulted in an increase in the values of the kinetic constants (for instance, from 0.57 h −1 at 50 rpm to 0.84 h −1 at 75 rpm when the paddle impeller with six-45°-inclined-flat-blades was used) and that axial flow in mechanically stirred reactors is preferable over radial-flow when the vertical-flat-blade impeller is compared to the inclined-flat-blade impeller (for instance at 75 rpm, from 0.52 h −1 with the six-flat-blade-paddle impeller to 0.84 h −1 with the six-45°-inclined-flat-blade-paddle impeller), demonstrating that there is a rotor speed and an impeller type that maximize solid–liquid mass transfer in the reaction medium. Furthermore, power consumption studies in this reduced reactor volume showed that no high power transfer is required to improve mass transfer (less than 0.6 kW/10 3 m 3).