External Mass Transfer Resistance Research Articles

Ofloxacin adsorption by calcined Verde-lodo bentonite clay: Batch and fixed bed system evaluation

Due to the increasing consumption of antibiotic agents and the fact that conventional effluent treatment methods are inefficient in removing these compounds, new and more effective treatment processes need to be investigated and improved. This work evaluated the adsorption of the emerging contaminant ofloxacin (OFL, antibiotic) in static and dynamic systems using calcined Verde-lodo (CVL) bentonite clay. The use of CVL bentonite clay for this purpose has not been investigated, furthermore, there are no reported studies regarding OFL adsorption in a fixed bed system. The molecular structure of ofloxacin was obtained by molecular simulation showing a stable and minimum local energy structure. The characterization study before and after the adsorption process indicated the presence of OFL in the CVL clay. The analyzes indicated that the CVL clay exhibits a lamellar structure with some roughness, with pores of 80,000 nm larger than macropores (>50 nm). In addition, the thermogravimetric analysis indicated that the clay is thermally stabile up to 1000 °C. External mass transfer resistance and pseudo-second-order models best described the kinetic data. The Dubinin-Radushkevick model was best adjusted to the equilibrium assays, indicating heterogeneous adsorbent surface. The system reached a maximum Langmuir adsorption capacity (55 °C) of 0.445 mmol g−1 (160.81 mg g−1). Fixed bed assays indicated the occurrence of multilayer adsorption. Under operating conditions of 0.4 mL min−1 and 0.05 mmol L−1, the system operated for 140 h with practically 100% removal until the breakthrough time was achieved. The clay showed high reuse capacity, with removal efficiency >93% after 3 cycles.

Reaction mechanism investigation on the esterification of rosin with glycerol over annealed Fe3O4/MOF-5 via kinetics and TGA-FTIR analysis

Reaction mechanism was investigated on the esterification of rosin with glycerol over annealed Fe3O4/MOF-5 with the annealing temperature of room temperature, 300, 500, 700 and 900 °C via the kinetics and TGA-FTIR analysis. Chemical equilibrium constant was employed in kinetic models due to reversible esterification reaction, and thermodynamic parameters were calculated. Kinetic models that considered three kinds of rate-determining steps of single reactant adsorption, single product desorption and surface reaction as well as the reaction mechanism of pseudo-homogeneous (PH), Eley-Rideal (ER) and Langmuir-Hinshelwood-Haugen-Watson (LHHW) and the different adsorption of components were proposed to fit the intrinsic kinetic data obtained by eliminating the effects of external and internal mass transfer resistance. Based on the results of linear regression and nonlinear regression, model discrimination among 38 possible kinetic models lead to the conclusion ER model with the surface reaction of adsorbed glycerol is the best appropriate to experimental results, affording a correlation coefficient above 0.981 and the F values much greater than the F0.05 values. The activation energies reduce with increasing of annealing temperature of the catalyst, from 103.28 to 89.55 kJ mol−1. Furthermore, results from TGA-FTIR characterizations further reveal that this esterification involves the exclusive adsorption of glycerol.

Study of reaction\u2013diffusion problem: modeling, exact analytical solution, and experimental verification

Nonlinear diffusion–reaction problem was investigated experimentally for the reference reaction (hydrogenation of propylene under isothermal conditions; a slab of catalyst pellet i.e., disks of large diameter/width ratio were applied). The diffusion–reaction model in the catalyst pellet with external mass-transfer resistances was solved analytically. Dependently on parameters values, two separate solutions were found: dead zone inside the pellet does not exist or it appears. In the first case, a common model is acceptable (regular model i.e., boundary value problem), in the second one, a model includes additional condition (dead zone model i.e., free boundary problem). Analysis of the solution presented indicated that either regular or “dead zone” model correctly describes the process for specific parameter values (with the only exception—multiple steady-state region—where the correct interpretation requires the combined application of the both). This result shows that the full description of the real process includes solutions of two different BVPs. Experimental research confirmed results anticipated by theory. It allowed to draw conclusions that go beyond this particular example i.e., the regular model, commonly applied in heterogeneous catalysis, does not adequately recognize dead zone problem. If “dead zone” appears, free boundary problem has to be consider, otherwise, process simulations will be incorrect. The conclusions drawn are valid also for biofilms.

Continuous Hydrogenation of Aqueous Furfural Using a Metal-Supported Activated Carbon Monolith.

Continuous hydrogenation of aqueous furfural (4.5%) was studied using a monolith form (ACM) of an activated carbon Pd catalyst (∼1.2% Pd). A sequential reaction pathway was observed, with ACM achieving high selectivity and space time yields (STYs) for furfuryl alcohol (∼25%, 60–70 g/L-cat/h, 7–15 1/h liquid hourly space velocity, LHSV), 2-methylfuran (∼25%, 45–50 g/L-cat/h, 7–15 1/h LHSV), and tetrahydrofurfuryl alcohol (∼20–60%, 10–50 g/L-cat/h, <7 1/h LHSV). ACM showed a low loss of activity and metal leaching over the course of the reactions and was not limited by H2 external mass transfer resistance. Acetic acid (1%) did not significantly affect furfural conversion and product yields using ACM, suggesting Pd/ACM’s potential for conversion of crude furfural. Limited metal leaching combined with high metal dispersion and H2 mass transfer rates in the composite carbon catalyst (ACM) provides possible advantages over granular and powdered forms in continuous processing.

Use of metal mesh to assist the multi-phase reaction in a microreactor

Glycerol is a by-product from biodiesel production. Glycerol can be further converted to glycerol carbonate by several reaction routes. One of the feasible routes that have been generally reported is the reaction between glycerol and urea, while ammonia gas is generated as one of byproducts. The reaction generally requires a heterogeneous catalyst, which causes mass transfer problem. Most researchers have applied this reaction in batch, continuous-stirred tank, or fixed bed reactors, in which the external mass transfer resistance is still dominant. Due to many advantages of microreactor including high surface area-to-volume ratio, short diffusion distance, small mass and heat transfer resistances, applying the microreactor can solve the problems and enhance the reaction, Therefore, in this work, microreactor was utilized in the production of glycerol carbonate from glycerol and urea, which one side of microchannel wall was coated by commercial catalyst. In addition, Metal mesh sheet with pores is used to separate ammonia gas from the reaction. This research studies the effect of residence time, microchannel thickness, shear stress of reactants and a quantity of generated gas on a catalyst slipping, and also investigates the effect of metal mesh size on yield of glycerol carbonate from reaction in microreactor.

Microbial growth rates and local external mass transfer coefficients in a porous bed biofilm system measured by 19 F magnetic resonance imaging of structure, oxygen concentration, and flow velocity.

19 F nuclear magnetic resonance (NMR) oximetry and 1 H NMR velocimetry were used to noninvasively map oxygen concentrations and hydrodynamics in space and time in a model packed bed biofilm system in the presence and absence of flow. The development of a local oxygen sink associated with a single gel bead inoculated with respiring Escherichia coli was analyzed with a phenomenological model to determine the specific growth rate of the bacteria in situ, returning a value (0.66 hr-1 ) that was close to that measured independently in planktonic culture (0.62 hr-1 ). The decay of oxygen concentration in and around the microbiologically active bead was delayed and slower in experiments conducted under continuous flow in comparison to no-flow experiments. Concentration boundary layer thicknesses were determined and Sherwood numbers calculated to quantify external mass transfer resistance. Boundary layers were thicker in no-flow experiments compared to experiments with flow. Whereas the oxygen concentration profile across a reactive biofilm particle was symmetric in no-flow experiments, it was asymmetric with respect to flow direction in flow experiments with Sherwood numbers on the leading edge (Sh = 7) being larger than the trailing edge (Sh = 3.5). The magnitude of the experimental Sh was comparable to values predicted by a variety of correlations. These spatially resolved measurements of oxygen distribution in a geometrically complex model reveal in innovative detail the local coupling between microbial growth, oxygen consumption, and external mass transfer.

Kinetics and Gibbs Function Studies on Lipase-Catalyzed Production of Non-Phthalate Plasticizer.

Petroleum based phthalate plasticizers encounter enormous claims to prohibit their production due to their harmful health impacts when they are mixed with plastics. That is why efforts are being done to find safer natural alternatives. We have investigated the reaction kinetics of the esterification epoxidation of oleic acid and 2-ethylhexanol in the presence of hydrogen peroxide catalyzed using Candida antarctica lipase (Novozym 435, Novozymes, Kobenhavn, Denmark). The product of this reaction is epoxidized 2-ethylhexyl oleate, a non-phthalate green plasticizer. The kinetic model for this reaction follows a multi-substrate PingPong Bi-Bi mechanism with competitive inhibition by the alcohol. The reaction's kinetic parameters were found to be 0.76 M, 0.37 M, 0.08 M, and 37.20 mM/min for Michalis-Menten constant for oleic acid (Kmo), Michalis-Menten constant for alcohol (Kma), alcohol inhibition constant (Kia), and maximum reaction velocity (Vmax), respectively. Then the Gibbs function analysis of the transition state based on the Arrhenius and Eyring equations was carried out. The internal diffusional limitations were found to be negligible as the effectiveness factor took the value of almost unity. While the external mass transfer resistance had no effect on the reaction due to operating at relatively high agitation speed and high temperature. This investigation confirms that this reaction was only kinetically controlled.

Mathematical Modeling of Batch Adsorption Kinetics of Lead Ions on Modified Natural Zeolite from Aqueous Media

Natural zeolites are evaluated for toxic heavy metal removal in water and wastewater systems. A two-mass transfer resistance model, consisting of the homogeneous solid diffusion model combined with the external mass transfer resistance, was applied to fit the experimental kinetic data of an agitated batch adsorption system, and a parabolic dependence of the driving force on the particle radius was considered. The mathematical model proposed for the batch adsorption kinetics was simulated using the finite difference method. The model has been successfully applied to simulate lead adsorption onto a modified natural zeolite, and the obtained results were well fitted to the experimental data for different initial lead concentrations. In this procedure, internal effective diffusivity as the process parameter was determined for different concentrations of the solution. Using the estimated value for the internal effective diffusivity, a parametric study has been carried out to study the effects of particle size of adsorbent, initial adsorbate concentration, solution volume and the amount of absorbent on the adsorption kinetics. The results showed that the adsorption kinetics follows the pseudo-second-order kinetic model due to its correlation coefficients (R2), suggesting that the lead adsorption process is very fast. Also, an adsorbent maximum capacity of 136.99 mg/g was found, indicating a large adsorption capacity for lead.

Box-Behnken optimization of glycerol transesterification reaction to glycerol carbonate over calcined oil palm fuel ash derived catalyst

This study aimed to optimize the synthesis of valuable glycerol carbonate (GC) over calcined oil pam fuel ash derived catalyst. A surface response methodology optimization study was conducted using the Box-Behnken optimization tool to minimize the reaction parameters and optimize GC yield and glycerol conversion. Also, the effect of intraparticle and external mass transfer resistance was conducted and the intrinsic kinetic model of the transesterification reaction was established. Overall, the transesterification temperature had significant effects on the variables of GC yield and glycerol conversion, while the catalyst loading and reaction time trivially affected the response parameters. Under optimized parameters of 80 °C reaction temperature, dimethyl carbonate (DMC)/glycerol molar ratio of 5, 5 wt% catalyst loading and 112 min reaction time, GC reached 95% with a desirability of 1. Theoretical elucidation indicates that transesterification reaction over the catalyst was unaffected by external mass transfer resistance and intraparticle diffusion above 125 μm particle size. The catalyst is heterogeneous and can be reused for five times while sustaining about 81.3% of GC yield.

Application of Kinetic Models for Heavy Metal Adsorption in the Single and Multicomponent Adsorption System

The overall rate of heavy metal adsorption on the porous sorbent consists of three separate steps: external mass transport, intraparticle diffusion, and adsorption on active sites. The pseudo-first-order model, pseudo-second-order model, and film-pore volume diffusion model are three types of kinetic models that have developed based on these steps. In this research, studies for predicting these kinetic models from the concentration decay curves of Pb2+, Cd2+, Ni2+, and Zn2+ were done on a batch adsorption process. Evaluation of the models prediction was performed by two separate reported experimental data. There is limited literature on the application of film-pore volume diffusion model for multicomponent adsorption systems in previous modeling studies; hence, the performance of this kinetic model was evaluated in this study using data from concentration decay curves of multicomponent adsorption systems. Investigation showed that for different adsorption systems, the application of the film-pore volume diffusion model has decreased the average relative error between 1–83% and 1–40% in comparison to the pseudo-first-order and pseudo-second-order model, respectively. More accurate prediction of the film-pore volume diffusion model is related to consideration of external mass transfer resistance on prediction. Effective pore volume diffusivity of different metal ions was calculated between 0.004 × 10−6 cm2 s−1 and 2.2 × 10−6 cm2 s−1 for a different adsorption system.

Experimental and numerical study into the effects of passive flow disturbance on conversion and C7+ yield of the packed bed Fischer Tropsch reaction

This experimental and numerical study investigates the effect of flow field alteration via bluff body flow disturbers in the packed bed Fischer-Tropsch (FT) reaction, which is known to be limited by heat and mass transfer and chemical kinetics as prescribed by the reactor geometry, flow and temperature fields, and catalyst properties. Experiments were carried out using a 188 cm3 reactor with 30.8 aspect (length-to-diameter) ratios loaded with 20% cobalt/Al2O3 catalyst and controlled at 200 °C at the exterior wall. Numerical simulation was carried out in the commercial ANSYS Fluent package. Experimental results show that the bluff bodies increased the CO (carbon monoxide) conversion and C7+ yields (hydrocarbons with 7 carbon chains or longer) across the factorial design space by as much as 1.8× and 3.3× of the baseline values, respectively. Numerical simulations suggest that these increases can be attributed to higher thermal conductivities and local velocities for greater heat rejection and lower external mass transfer resistance, respectively. Results suggest that the use of internal flow disturbers in the packed bed had significant influence in lowering the heat and mass transfer resistances of the FT process, and can improve the cost-competitiveness of FT liquids production in the small scale.

Stability of pore-plated membranes for hydrogen production in fluidized-bed membrane reactors

Pd-based membranes prepared by pore-plating technique have been investigated for the first time under fluidization conditions. A palladium thickness around 20 μm was achieved onto an oxidized porous stainless steel support. The stability of the membranes has been assessed for more than 1300 h in gas separation mode (no catalyst) and other additional 200 h to continuous fluidization conditions. Permeances in the order of 5·10−7 mol s−1 m−2 Pa−1 have been obtained for temperatures in a range between 375 and 500 °C. During fluidization, a small decrease in permeance is observed, as consequence of the increased external (bed-to-wall) mass transfer resistances. Moreover, water gas shift (WGS) reaction cases have been carried out in a fluidized bed membrane reactor. It has been confirmed that the selective H2 separation through the membranes resulted in CO conversions beyond the thermodynamic equilibrium (of conventional systems), showing the benefits of membrane reactors in chemical conversions.

Kinetics of Hydrogenation of Acetic Acid over Supported Platinum Catalyst

Petroleum is nonrenewable and contributes to environmental pollution, thus bio-oil can be substituted as a potential alternative. However, bio-oil in its crude form cannot be used directly as fuel since it contains a high proportion of oxygenated, acidic, and reactive compounds such as carboxylic acids. These are known to cause corrosion of vessels and pipework, instability, and phase separation. The heating value of bio-oil can be improved through hydrodeoxygenation (HDO). In this study, the HDO of acetic acid is presented, being a typical model compound found in bio-oil. Kinetic data were obtained over the range of temperature (175–210 °C), hydrogen pressure (20–50 bar), initial acetic acid concentration (0.16–0.521 M), and catalyst loading (0.2–0.5 g), in a 100 mL batch reactor using 4% Pt/TiO2. It was found that catalyst particle sizes < 65 μm and a stirring speed of 1000 min–1 were sufficient to overcome internal and external mass transfer resistances and ensure that the reaction is within the kineti...

H2S adsorption on NaY zeolite

Abstract In this work, high-pressure H2S adsorption data in equilibrium and fixed bed conditions using NaY zeolite as adsorbent were determined. Adsorption and desorption isotherms for different temperatures and H2S partial pressures up to 1.2 bar were obtained experimentally and modelled with Toth and Dubinin-Astakhov equations in order to give information about adsorbate/adsorbent and adsorbate/adsorbate interactions. H2S adsorption in a fixed bed column was achieved for different inlet volumetric flow rates and breakthrough curve data were evaluated considering adsorbed amount, length of mass transfer zone, contributions of internal and external mass transfer resistances and were modelled with Linear Driving Force (LDF) model to estimate adsorbent particle's effective mass transfer coefficient. Equilibrium results indicate maximum uptake capacities are above 6.0 mol kg−1 for highest partial pressures. Isotherms' modeling suggests the predominance of physisorption of H2S in NaY zeolite. However, a hysteresis effect was observed for desorption curves and attributed to H2S chemisorption on zeolite surface, as pointed out by strongly favorable isotherms obtained. Fixed bed adsorption data showed breakthrough curves were nonsymmetrical, which indicated the presence of internal resistance mainly, probably due to the microporosity of adsorbent particle. In accordance, mass transfer zones did not vary significantly with inlet flow rate. Effective intraparticle mass transfer coefficients obtained with LDF model increases as flow rate increases, due to a higher availability of H2S molecules at adsorbent surface, which increases the driven force necessary for mass transfer in the intraparticle region.

Characterization of kinetic parameters and mass transfer resistance in an aerobic fixed-bed reactor by in-situ respirometry

An airlift reactor filled with Raschig rings in the downcomer section only was designed to allow for the characterization of biofilm biomass at several superficial liquid velocities. The reactor was inoculated with activated sludge and operated with synthetic wastewater containing acetate. After steady state was reached, the kinetic parameters of the process were determined at each superficial liquid velocity by an in-situ pulse respirometry method. Subsequently, the biofilm contained in the reactor was disaggregated, resuspended in a mineral medium, and reintroduced in the reactor, without Raschig rings, i.e., a standard airlift reactor. The kinetic parameters of the bioprocess were determined again, by the same respirometry method. That strategy allowed for the estimation of apparent and intrinsic kinetic parameters; i.e., with and without mass transfer limitation, respectively. Moreover, the method also allowed for the determination of the internal and external effectiveness factors and mass transfer resistances. The results shown that the hydrodynamic conditions had a strong effect on the kinetic parameters but none on the stoichiometric parameters. The external mass transfer accounted for up to 70% of the total mass transfer resistance. It is concluded that respirometry is a useful tool for the characterization of biofilms, in actual random packing reactors.

Study on mass transfer of ethyl acetate in polymer adsorbent by experimental and theoretical breakthrough curves

Abstract A new polymeric adsorbent with highly hypercrosslinked structure was developed for the removal of VOCs from polluted air. The purpose of this work is to obtain the intraparticle mass transfer coefficient of the adsorbent particles. Adsorption experiments for obtaining breakthrough curves were carried out with a fixed bed system. A dynamic mathematical model for the fixed bed adsorption system was developed. By model fitting, the overall mass transfer coefficient was determined when the deviation error was minimum. Then, the intraparticle mass transfer coefficient of the adsorbent was determined when the external mass transfer resistance was eliminated at higher velocities. Furthermore, a linear relationship of the intraparticle mass transfer coefficient and equilibrium coefficient at lower inlet gas concentrations range was correlated. Moreover, an equation for predicting external mass transfer coefficient at low Reynolds number range at room temperature was obtained.

Determination of gas film mass transfer coefficient in a packed bed reactor for the catalytic combustion of hydrogen

The overall kinetics of catalytic combustion of hydrogen in air is decided by the contribution of the different resistances offered in individual steps such as external diffusion, internal diffusion and surface reaction. To estimate the contribution of external mass transfer resistance, empirical correlations in terms of dimensionless numbers viz., Sherwood number, particle Reynold's number, and Schmidt number are used in general. The Schmidt number for hydrogen in air is approximately 0.2 and most of the mass transfer correlations in packed bed reactors are valid for Schmidt number higher than 0.6, which corresponds to that of water vapour in the air. The external mass transfer correlations applicable for hydrogen-air system are scarce. In the present study, the external mass transfer effects were decoupled from internal diffusion and intrinsic reaction by conducting experiments with different particle Reynolds numbers in a packed bed batch reactor with complete recycle. Further, the system is theoretically modeled and the external film mass transfer coefficients were estimated at different particle Reynold's numbers using experimental data. Based on the experimental data, a new mass transfer correlation has been proposed to determine the gas film mass transfer coefficient of hydrogen in the air in packed bed reactors, in terms of conventional dimensionless numbers i.e., Sherwood number, Schmidt number, and particle Reynolds number. Further, the contribution of external mass transfer resistance on the overall reaction kinetics was also estimated.

Hierarchically structured TiO2-based composites for Fenton-type oxidation processes

A novel hierarchically structured composite aimed as a stable catalyst for the heterogeneous Fenton-type (HFT) oxidation process was developed by using a cost-effective and versatile technique. Prussian Blue nanoparticles (PBNP) were dispersed onto aligned macroporous TiO2 (rutile) monoliths prepared via directional freezing of aqueous dispersions of TiO2 nanoparticles. The catalytic performance was evaluated in the HFT oxidation of an azo dye frequently used as a model contaminant, Orange G (OG). Experiments were carried out in a liquid batch-recycle reactor, in which the liquid flow rate was set to ensure negligible external mass transfer resistance. The catalyst exhibited good activity to form highly oxidative radicals from hydrogen peroxide decomposition, which readily discolored OG. Significant reduction of the time required to attain complete discoloration and improvement in TOC removal were achieved by adjusting operating conditions and oxidant dosage strategies. Almost complete OG conversion at around 90 min and 34.4% of TOC removal after 4 h were achieved by using the best evaluated strategy. The catalyst activity was tested under specific operating conditions and remained unaltered during 42 cycles of 4 h each (total 168 h). The fresh and used PBNP/TiO2 catalysts and the support were thoroughly characterized by several techniques. Results supported the excellent stability exhibited by the catalyst in the OG HFT oxidation.

Dynamic analysis of the anomalous diffusion in catalyst particles considering chemical reactions with non-linear kinetics

In this work, a generalized reaction-diffusion model useful to study the interactions between anomalous diffusion and chemical reactions in catalyst particles is presented. The fractional reaction-diffusion model considers the linearization of the non-linear reaction kinetic terms, which allows the solution of the mathematical model through the Fourier transform. The proposed methodology can be used for the dynamic analysis of the concentration profiles in the catalytic pellets, including internal and external mass transfer resistances, as well as for the estimation of effectiveness factor (EF). These findings revealed that the analysis of dynamic reaction-diffusion linearized models where the mass transfer is described by the anomalous transport can be analysed in the frequency domain. Two kinetic models, a general power-law and Langmuir-Hinshelwood, were considered. The results demonstrated the effect of use the traditional Fick’s law and the anomalous transport (i.e., subdiffusion or superdiffusion phenomena) in a porous media system, particularly on the space-time concentration profile predictions and the influence on the effectiveness factor of the catalytic particle.

Removal of 2,4-dichlorophenol in a micro-packed bed adsorber by activated carbon from &lt;i&gt;Coffea arabica&lt;/i&gt; fruit endocarp

Activated carbon, obtained by phosphoric acid activation of Coffea arabica fruit endocarp, was used to remove 2,4-dichlorophenol from aqueous solutions in a micro-packed bed adsorber. In batch experiments, effects such as initial concentration, solution's pH, and temperature on adsorption were investigated. It was found that acidic pH is favourable and that the adsorption isotherm is best represented by the Langmuir model. In continuous experiments, the effect of inlet concentration, bed length, and flow rate on the adsorption capacity of the micro-packed bed adsorber were evaluated. Adsorption capacities were higher than those obtained in batch experiments. Inlet concentration and bed length show a significant effect on the adsorption capacity. Breakthrough curves were very well fitted by axial dispersion model. Additionally, breakthrough profiles indicate a particle-controlled process, which suggest a negligible external mass transfer resistance.