External Mass Transfer Effects Research Articles

The role of reactant contact modes in droplet-based microfluidics

Undesired side reactions can significantly impact the efficiency and economic viability of chemical processes. Droplet-based microfluidics(DBM) offers precise control over reaction conditions by isolating reactants in distinct droplets. Each droplet acts as microreactor with high surface-to-volume ratio. This paper comprehensively investigates the role of two reactant contact modes that give rise to droplet flow. The first contact-mode, Conjugate Mass Transfer Mode(CMTM), is characterized by mass transfer of a reactant from Continuous phase(CP) to Dispersed phase(DP). In second mode, Single-Sphere Mode(SSM), both reactants coexist within the dispersed phase, and there is no effect of external mass transfer on performance. The Hadamard-Rybczynski flow field is used to obtain insights into the system behavior. Two reaction networks, Parallel and Series-Parallel reactions, are considered, which arise from side reactions that accompany the primary reaction.This work highlights the importance of choosing an appropriate reactant contact-mode when side reactions occur. The role of two experimentally controllable parameters, diffusivity ratio(Dr) and feed concentration ratio(M), is analyzed to identify the desirable contact mode. Notably, our study showed that, for Parallel reactions, CMTM has a superior performance compared to SSM under some conditions. Whereas for Series-Parallel reactions, SSM exhibits better performance when equal concentrations of reactants are employed.

Mass transfer studies on the solvent-free lipase-catalyzed partial hydrolysis of palm oil in packed-bed reactor

Packed-bed reactor (PBR) is widely employed for enzyme-mediated reactions at an industrial scale because the reactor configuration enables reusability of the biocatalyst with ease, thereby reducing the overall operating expenses. However, mass transfer limitation remains a key challenge in fixed-bed column systems, especially at large scale. Therefore, the present study investigated the external mass transfer effects on solvent-free lipase-catalyzed partial hydrolysis reaction and aimed to develop a mass transfer model for the operation. The influence of volumetric flow rate, one of the crucial operating parameters affecting the external mass transfer resistance, was examined. Results demonstrated increasing the flow rate led to an increase in hydrolysis reaction with the highest reaction rate observed at 5 ml min-1 . A dimensionless mathematical mass transfer model of Colburn factor, JD = K(Re)n-1 which is a function of Reynolds and Schmidt numbers, was then proposed to simulate the enzymatic partial hydrolysis reaction of palm oil in PBR. The mass transfer model was analyzed with different n values and results revealed that the mass transfer correlation of JD=0.92(Re)-0.2 was able to predict the experimental data accurately. The model developed could describe the dominance of mass transfer effects besides providing useful information for the scale-up design of industrial reactor.

Mass transfer in catalytic depolymerization: External effectiveness factors and serendipitous processivity in stagnant and stirred melts

Several heterogeneous catalysts are being developed to recycle plastics. Most operate in viscous polymer melts, where external mass transfer effects could limit the supply of co-reactants to active sites. External mass transfer can also impede the diffusion of long chain products away from the catalyst after each cut. Product egress limitations could potentially confer unintentional processivity to catalyst operation, i.e. a tendency for the catalyst to repeatedly cut the same chain after an initial encounter. We formulate reaction–diffusion equations to quantify mass transfer effects on the co-reactant transport to the catalyst and the degree of serendipitous processivity. Results are developed for catalysts in stagnant or stirred melts, with simple expressions involving Damkohler, Peclet, and Sherwood numbers, i.e. dimensionless combinations of rate constants, catalyst particle size, polymer diffusivities, and shear rates (where applicable). We estimate the impact of these effects for a spherical core–shell catalyst.

Removal of indoor air ozone using carbon-based filters: Systematic development and validation of a predictive model

Exposure to ozone can cause several adverse health effects on human beings. Carbon-based filters are usually used for the removal of ozone from indoor air. The main problem with Carbon-based filters technology is the exhaustion of carbon because of irreversible reactions with ozone. Therefore, the knowledge of the service life of filters is crucial for building engineers and designers. Since testing at low indoor concentrations is time-consuming and expensive, the existing standards suggest experimenting with high concentrations. Thus, predictive models need to be developed to estimate the service life of filters. This work provides a model to estimate the breakthrough curves of three different combined (multi-layer) filters. The model augmented mass transfer equations with a reaction rate model and divided them into interpellet mass transfer and kinetic models. First, the reaction rate parameters were measured by fitting the model onto experimental data of all filters challenged with 9 ppm or 90 ppm of ozone. This was followed by validating the model for lower concentrations. In addition, to show the model validity for real-life application, its prediction was compared with the experimental data collected using the full-scale experimental setup and higher velocity. There was excellent agreement between the model prediction and the experimental results for all filters. Furthermore, an inter-model comparison was performed to determine the importance of different mass transfer steps. The results showed internal diffusion and axial dispersion as rate-limiting steps along with reaction. Finally, a sensitivity analysis was conducted on reaction kinetic parameters, axial dispersion coefficient, external mass transfer coefficient, and activated carbon particle porosity, which indicated negligible effects of external mass transfer and porosity variations.

3D mathematical modeling of external mass transfer effect in high-rate adsorption process

The effect of external mass transfer on the high-rate adsorption system was evaluated in the present study. In the experimental section, the removal of methyl orange (MO) was carried out by using surfactant modified chitosan-clinoptilolite at specific conditions such as the optimum adsorbent dosage of 11 g L−1, solution pH of 3.5, contact time of about 3 min and constant temperature of 30 °C. The characterization of adsorbent showed the porosity value of 0.064 and the BET surface area of 10.5 m2 g−1. These features of adsorbent established the high-rate adsorption process in which the shape of decay curves is not smooth and the concentration of adsorbate falls quickly. In high-rate adsorption processes, the external mass transfer mechanism has a significant effect on total mass transfer at early time of adsorption. The pore volume and surface diffusion model (PVSDM) were applied for predicting MO concentration decay curves. This model and its parameters could not estimate properly the concentration of MO at the early time of the adsorption process. To overcome this problem, in addition to optimization of surface diffusion coefficient (Ds), the external mass transfer coefficient (ke) was optimized and the effect of those parameters was studied in three dimensions. The optimization provided a better agreement between the model and experimental data in such a way to reduce the error rate. To show better the effect of external mass transfer, Biot number (between (2.15 and 24.05) × 10−3) was applied and it revealed that the adsorption mechanism was controlled by external mass transport. The high external surface area illustrated that in all values of ke and initial concentration of MO, surface diffusion is the dominant mechanism in intraparticle diffusion and the low porosity of adsorbent limited the penetration of adsorbate species into the adsorbent cavities, whereby the pore volume diffusion could be considered negligible.

Mass transfer study of butachlor biodegradation using immobilized microbial consortium SMC1 in a packed bed bioreactor

AbstractIn the present study, the effect of external mass transfer on the rate of butachlor biodegradation was studied using calcium alginate immobilized microbial consortium SMC1 in a re‐circulated up‐flow packed bed bioreactor in a batch scale. By assuming the biodegradation kinetics to be first order, the effect of various operational parameters such as feed flow rate, substrate concentration, and the external mass transfer on the bio‐removal of butachlor from the aqueous medium has been explored. Using external film diffusion models and Colburn analogy, the mass transfer correlations at several feed rates were worked out. The correlational analysis obtained in the form of JD = 1.341 was found to predict the experimental data precisely.

An improved perm-selectivity prediction of forward osmosis membrane by incorporating the effect of the surface charge on the solute partitioning

Perm-selectivity consisting of water flux Jw and solute flux Js or in form of Js/Jw ratio is an important parameter of designing Forward Osmosis (FO) membrane as it indicates the membrane performance and how much solute replenishment over the extracted pure water from the feed solution. Parameter Js/Jw ratio is dependent on hydrodynamic condition i.e cross-flow velocity (CFV), solute type i.e. diffusivity, trans-membrane surface potential. This study employed Cellulose Triacetate (CTA) membrane for representing low-charge membrane and Polyamide-Thin Film Composite (PA-TFC) for the membrane of highly negative surface charge. Six (6) different models were used to quantify the effect of external mass transfer, the ideal solution, non-ideal solution, trans-membrane surface potential on the transport of solute across the membrane. Our new model was proven to improve the prediction of perm-selectivity. The improvement was attributed to the application of trans-membrane potential-dependent solute partitioning for solute permeability correction and the application of experimentally obtained mass transfer coefficient. By the incorporation of the Donnan effect in determining the transmembrane potential, it was found that in the low charge membrane namely CTA, the dominant transport mechanism was diffusion, while in a highly negative surface charged membrane namely TFC, the partition of solute to be the dominant mechanism. Operation at a low CFV posed less impact of the membrane charge. The newly developed model provided a good foundation for FO process design under different CFVs and modified surface charges.

Kinetic Modelling and Simulation Studies for the Esterification Process with Amberlyst 16 Resin

The reaction of methanol with acetic acid in a simple isothermal batch reactor was carried out to produce methyl acetate and water with and without catalyst. Amberlyst 16 used as the solid resin catalyst. The reaction mixture temperature was adjusted for 313.15 K - 353.15 K with and without Amberlyst 16. The concentration for catalyst changed from 0.048 mol H + /L - 0.24 mol H + /L based on volume of mixture. The reactant mole ratio for acid to alcohol was varied from 1:1 - 1:4. The influence of catalyst loading, reaction mixture temperature, initial reactant mole ratio, catalyst size, agitation speed on acetic acid conversion has been investigated. An experimental results show that reaction is kinetically controlled when compared to internal and external mass transfer effects. Kinetic rate equation of second order is used to fit experimental data. The reaction rate constants and activation energies were calculated from Arrhenius plot with and without catalyst. This equation can be used in the modelling and simulation of reactive distillation process.

A study of external mass transfer effect on biodegradation of phenol using low‐density polyethylene immobilized Bacillus flexus GS1 IIT (BHU) in a packed bed bioreactor

AbstractThe impact of external mass transport on the biodegradation rate of phenol in a packed bed bioreactor (PBBR) was studied. A potential bacterial species, Bacillus flexus GS1 IIT (BHU), was isolated from the petroleum‐contaminated soil. Low‐density polyethylene (LDPE) immobilized with the B. flexus GS1 IIT (BHU) was used as packing material in the PBBR. The PBBR was operated by varying the inlet feed flow rate from 4 to 10 mL/min, and the corresponding degradation rate coefficients were found to be in the range of 0.119–0.157 L/g h. In addition, the highest removal rate of phenol was obtained to be 1.305 mg/g h at an inlet feed rate of 10 mL/min. The external mass transfer was studied using the model . A new empirical correlation for the biodegradation of phenol in the PBBR was developed after the evaluation at various values of K and n.

Evaporation and crystallization process for sessile saline droplets during depressurization.

The evaporation and crystallization process for sessile saline droplets during depressurization is experimentally studied. The relationship between ambient pressure and the crystallization pattern is primarily discussed. When the ambient pressure is low, salt particles are easily formed at the droplet contact line. In contrast, when the ambient pressure is similar to atmospheric pressure, it is more likely for cubic crystals to be formed inside the droplet. By analysing the contact angle fluctuation during crystallization, the experimental results show that the growth of a cubic salt crystal under high ambient pressure or low salt concentration leads to a greater deformation of the liquid-gas interface and a larger contact angle fluctuation. Finally, the Peclet number Pe is introduced to reflect the ratio of the rate of ion advection to the rate of diffusion. The Pe number is larger at lower ambient pressure, which means that the external mass transfer and convection effect is more significant under low pressure, with salt particles easily formed at the droplet contact line. The effect of concentration diffusion inside the droplet increases at higher ambient pressure, thereby, making it easy for cubic crystals to be formed inside the droplet.

Evaluation of mass transfer effect and response surface optimization for abatement of phenol and cyanide using immobilized carbon alginate beads in a fixed bio‐column reactor

AbstractCoke oven sectors dispense phenol and cyanide into the circumferential ecosystem, which becomes a serious concern to the subsistence of the flora and fauna. The current study investigates phenol–cyanide treatment using carbon alginate beads immobilized with mixed bacterial consortium. Response surface using central composite design was contrived for the batch and packed bed bio‐column optimization study. The optimal removal conditions obtained in batch study were 89.77% and 82.33% for phenol and cyanide, respectively, with 10‐g/L adsorbent dosage, time 2 hr, and particle diameter 0.3 cm, whereas 87.22% and 90.97% with 22‐cm column height, column diameter 3 cm, 10‐ml/min flow rate, and 1‐hr operation time. The actual exposure time of the pollutants in the bio‐column reactor was calculated to be 22.15 min. Analysis of variance and model statistics predicted a high coefficient of determination for column operation with R2 = .9950 (phenol), R2 = .9976 (cyanide), and p values < .0001 stating significant model. The quantitative estimation of the combined external mass transfer and biodegradation effect was performed to evaluate correlation as (phenol) and (cyanide) with km = 0.052 and km = 0.055 cm/hr, respectively. The surface morphological study was executed by field emission scanning electron microscopy and Brunauer–Emmett–Teller surface area analysis depicting bacterial film development on the porous carbon matrix for effective treatment of binary system.

Modeling of 2-phenylethanol adsorption onto polymeric resin from aqueous solution: Intraparticle diffusion evaluation and dynamic fixed bed adsorption

Adsorption of 2-phenylethanol from an aqueous solution onto hyper-crossed-linked adsorption resin Macronet MN270 was investigated in a fixed bed column. Adsorption equilibrium was obtained in batch experiments and the results were fitted with the Langmuir isotherm model. A batch adsorption experiment was performed to determine the intraparticle diffusion mechanism. It was found that pore volume diffusion cannot solely govern the intraparticle mass transfer. According to the results of the batch experiment, surface diffusion plays an important role in the adsorption of 2-phenylethanol. Diffusion coefficients obtained in the batch experiment were further verified by comparison of the experimental data with the results of the mathematical model of fixed bed adsorption which was able to predict experimental breakthrough curves obtained under various conditions. A better description of the experimental data was obtained using the mathematical model which considered pore volume diffusion together with surface diffusion. The varying parameters in the measurements of breakthrough curves were: particle diameter, column diameter, volumetric flow rate and the inlet column concentration. Fixed bed adsorption experiments showed that smaller particles together with lower volumetric flow rate produce sharper breakthrough curves. The effect of external mass transfer and axial dispersion was not very significant. Lastly, fixed bed desorption of 2-phenylethanol with water was tested and was shown to be ineffective. Therefore, different solvents for 2-phenylethanol desorption were recommended.

Facile synthesis of shell-core structured Fe3O4@ACS as recyclable magnetic adsorbent for methylene blue removal

A series of magnetic activated carbon nanospheres (MACSs) were synthesized via a simple one-step hydrothermal and pyrolysis method. The characterization results confirm that the shell-core structured MACSs were formed because of Fe3O4 nanoparticles (Fe3O4NPs) mostly dispersed on the surface of activated carbon nanospheres. This structure can not only effectively prevent the agglomeration of activated carbon, but can provide a sufficient magnetism for recycle via the magnetic assisted separation technology. Batch tests for methylene blue (MB) removal show that an excellent adsorption capacity of 192.64 mg g−1 at 298 K can be obtained on MACS-700, which can be kept up to 156.4 mg g−1 even after five cycles. The adsorption isotherm and kinetics results show that the adsorption behavior can be well described by Langmuir isotherm model and the experimental data fit better for the pseudo-second-order kinetic model. It demonstrate that the adsorption rate of MB mainly controlled by chemical-adsorption, meanwhile, effects of external mass transfer and intra particle diffusion also can be found.

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.

Novel study on biodegradation of malathion and investigation of mass transfer correlation using alginate beads immobilized Bacillus sp. S4 in bioreactor

In this study the external mass transfer effects on the biodegradation of malathion were studied. The experiments were performed in continuous packed bed bioreactor packed with immobilized alginate beads. The isolated bacterial species Bacillus sp. S4 was encapsulated with alginate beds. The effect of inlet flow rate (5–20 mL/h) on biodegradation of malathion (300 mg/L) was examined. At different flow rates the biodegradation rate constants were calculated by assuming the first-order biodegradation kinetics. The possible degradation routes were proposed for malathion biodegradation. The external mass transfer coefficients for biodegradation of malathion were also investigated for different values of flow rate. The mass transfer effect was correlated with the model JD = K NRe−(1−n). The novel mass transfer correlation was found as JD = 5.7 NRe−0.40 in order to predict the experimental data accurately and the model were studied with different values of n and K. The developed model would be useful for the scale up of useful industrial bioreactors.

Photocatalytic degradation of aniline using an autonomous rotating drum reactor with both solar and UV-C artificial radiation

The aim of this work was to evaluate the performance of a novel self-autonomous reactor technology (capable of working with solar irradiation and artificial UV light) for water treatment using aniline as model compound. This new reactor design overcomes the problems of the external mass transfer effect and the accessibility to photons occurring in traditional reaction systems. The UV-light source is located inside the rotating quartz drums (where TiO2 is immobilized), allowing light to easily reach the water and the TiO2 surface. Several processes (UV, H2O2, Solar, TiO2, Solar/TiO2, Solar/TiO2/H2O2 and UV/Solar/H2O2/TiO2) were tested. The synergy between Solar/H2O2 and Solar/TiO2 processes was quantified to be 40.3% using the pseudo-first-order degradation rate.The apparent photonic efficiency, ζ, was also determined for evaluating light utilization. For the Solar/TiO2/H2O2 process, the efficiency was found to be practically constant (0.638–0.681%) when the film thickness is in the range of 1.67–3.87 μm. However, the efficiency increases up to 2.67% when artificial UV light was used in combination, confirming the efficient design of this installation. Thus, if needed, lamps can be switched on during cloudy days to improve the degradation rate of aniline and its mineralization. Under the optimal conditions selected for the Solar/TiO2/H2O2 process ([H2O2] = 250 mg/L; pH = 4, [TiO2] = 0.65–1.25 mg/cm2), 89.6% of aniline is degraded in 120 min. If the lamps are switched on, aniline is completely degraded in 10 min, reaching 85% of mineralization in 120 min. TiO2 was re-used during 5 reaction cycles without apparent loss in activity (<2%). Quantification of hydroxyl radicals and dissolved oxygen allows a chemical-based explanation of the process. Finally, the UV/Solar/TiO2/H2O2 process was found to have lower operation costs than other systems described in literature (0.67 €/m3).

Mechanism of high contaminant removal performance in the expanded granular sludge blanket (EGSB) reactor involved with granular activated carbon for low-strength wastewater treatment

The sludge granulation in an anaerobic bioreactor is rather difficult for treating low-strength wastewater. In this study, an expanded granular sludge blanket (EGSB) reactor involved with granular activated carbon (GAC) was employed to treat wastewater at different operational conditions. In general, a stable COD removal performance was operated at the HRT condition of 8, 6, 5, and 4 h, and Vup ranges from 1.09 to 2.44 m h−1. However, the NH3-N removal efficiency was at a relatively low level resulted from the nitrification efficiency was limited to the low dissolved oxygen concentration in anaerobic bioreactor. Adding GAC to the raw sludge, the average size of granular sludge and enzymatic [dehydrogenase activity (DHA) and specific methanogenic activity (SMA)] activities presented an increase trend with the experiment grows. In addition, the promotion of organic load rate (OLR) and Vup might stimulate the external mass transfer effect in EGSB reactor, which resulted in the increase of extracellular polymeric substances (EPS) content. DNA sequencing analysis demonstrated that the growth of Aeromonas genus was the main reason for the unstable distribution of acetoclastic and hydrogenotrophic methanogens in HRT condition of 4 h.

External Mass Transfer Model for Hydrogen Peroxide Decomposition by Terminox Ultra Catalase in a Packed-Bed Reactor

Abstract It is known that external diffusional resistances are significant in immobilized enzyme packed-bed reactors, especially at large scales. Thus, the external mass transfer effects were analyzed for hydrogen peroxide decomposition by immobilized Terminox Ultra catalase in a packed-bed bioreactor. For this purpose the apparent reaction rate constants, kP, were determined by conducting experimental works at different superficial velocities, U, and temperatures. To develop an external mass transfer model the correlation between the Colburn factor, JD, and the Reynolds number, Re, of the type JD = K Re(n-1) was assessed and related to the mass transfer coefficient, kmL. The values of K and n were calculated from the dependence (am kp-1 - kR-1) vs. Re-1 making use of the intrinsic reaction rate constants, kR, determined before. Based on statistical analysis it was found that the mass transfer correlation JD = 0.972 Re-0.368 predicts experimental data accurately. The proposed model would be useful for the design and optimization of industrial-scale reactors.

Effect of mechanical activation on the hydrogen reduction kinetics of magnetite concentrate

The effect of mechanical activation on the reduction kinetics of magnetite concentrate by hydrogen was studied. The magnetite concentrate was milled for 8 h in a planetary ball mill. After 8 h of milling, the average particle size was reduced from 14 to 4.4 µm, resulting in a lattice microstrain of 0.3. Isothermal reduction experiments were conducted by thermogravimetry to focus on the chemical reaction as the rate-controlling factor by eliminating external mass transfer effects and using a thin layer of particles to remove interstitial diffusion resistance. Thus, about 2 mg of magnetite powder was reduced at different temperatures under a sufficient flow of hydrogen. The magnetite concentrate and reduction products were analyzed by SEM and XRD. It was found that the onset reduction temperature decreased from 587 to 500 K (314–227 °C) due to the mechanical activation. The activation energy for hydrogen reduction of the activated concentrate decreased about 10% compared with the as-received concentrate. In view of the results, a reaction rate expression was established based on the nucleation and growth model with an Avrami parameter n = 2.5.

Production of isoamyl acetate by enzymatic reactions in batch and packed bed reactors with supercritical CO2

Batch reactors (BRs) are probably the most widely used reactor designs for biocatalysis in supercritical fluids, but for industrial scale applications packed bed reactors (PBRs) are frequently preferred. The objective of this work was to investigate and compare BR and PBR in the synthesis of isoamyl acetate through enzymatic reactions in SCCO2 media. Experiments were performed to determine the kinetic parameters and to evaluate the internal and external mass transfer effects. The results indicate that the reaction is kinetically controlled, independently of particle diameter and agitation, therefore intra-particle diffusion and external mass transfer resistance are unimportant. The kinetic Ping-Pong Bi–Bi model described well the esterification of isoamyl alcohol with acetic anhydride at the evaluated conditions, indicating that the affinity of the enzyme to acetic anhydride is larger than to isoamyl alcohol. The PBR achieved the highest productivity of isoamyl acetate, although its conversion was lower than that obtained by the BR. In summary, PBR is the best option to synthesize isoamyl acetate in SC-CO2 media.