Mass Transfer Steps Research Articles

Transport-Optimized Nanoporous Materials for Mass Separation and Conversion as Designed by Microscopic Diffusion Measurement

Nanoporous materials find widespread application in material upgrading by separation (“molecular sieving”) and catalytic conversion. Mass transfer in these materials is a key phenomenon deciding about their technological performance. This chapter deals with the application of measurement techniques which are able to follow the diffusive fluxes of the guest molecules in such materials over “microscopic” distances, including the pulsed field gradient (PFG) technique of Nuclear Magnetic Resonance (NMR) and the techniques of microimaging by interference microscopy (IFM) and by IR microscopy (IRM). Microscopic measurement is a prerequisite for attaining unbiased information about the elementary steps of mass transfer and about their role within the overall process of technological exploitation. We dedicate this treatise to the memory of our dear and highly esteemed colleague Nicolaas Augustinus Stolwijk, notably in recognition of his manifold activities in the field of diffusion, distinguished by their impressively high standard in connecting the message of various techniques of measurement and in combining them to comprehensive views on quite intricate subjects.

Intensified and Flexible Flash Degassing in a Rotating Packed Bed

Application of flash degassing in a rotating packed bed (RPB) has been experimentally studied using oxygen removal from water as a test system. It is shown that the process can be well described by two mass transfer steps in series: an initial flash and subsequent gas–liquid mass transfer in the rotating bed. Outlet oxygen concentrations close to the flash equilibrium concentration were obtained at high angular speeds, without requiring a large degree of superheat. The rotation of the bed enhances the gas–liquid mass transfer in the packing, up to a factor 10 by increasing the angular speed from 0 to 400 rad s–1. This leads to a reduction in the required packed bed volume and possibly to an energetically more efficient operation of degassing processes. Moreover, the angular speed yields an extra degree of freedom during operating, enabling fast-responding operation at a constant outlet concentration with varying inlet conditions.

Photocatalytic treatment of petroleum industry wastewater using recirculating annular reactor: comparison of experimental and modeling

In this study, the treatment of petroleum wastewater has been investigated by applying heterogeneous photocatalytic process using a recirculating annual reactor. An attempt has been made to study the effect of operating parameters such as TiO2 load, initial concentration of the pollutant, emitted photonic flux, and pH of the solution. The degradation efficiency of toluene and benzene, as target molecules, was studied. In fact, result showed that the toluene is better degraded alone than when it is in a mixture. The rate of elimination of toluene separately was 89.5%, while it was 76.19 and 79.55% in the binary (toluene/benzene) and the ternary mixtures (toluene/benzene/xylene), respectively. Moreover, the mineralization of the solution decreased more rapidly when toluene was pure with a rate of 83.13% compared to binary and ternary mixtures. A mathematical model is proposed taking into account the parameters influencing the process performances. The mass transfer step, the degradation, and the mineralization kinetics of the pollutants were defined as model parameters. To build the model, mass balances are written in bulk region and catalyst phase (solid phase). The degradation mechanism on solid phase is divided in two stages. Firstly, the removal of toluene gives an equivalent intermediate (EI). Secondly, EI is oxidized into carbon dioxide (CO2). This approach gives a good agreement between modeling and empirical data in terms of degradation and mineralization. It also allows for the simulation of toluene kinetics without knowing the plausible chemical pathway. A satisfactory fit with experimental data was obtained for the degradation and mineralization of toluene.

Mathematical modelling applied to the rate-limiting mass transfer step determination of a herbicide biosorption onto fixed-bed columns

ABSTRACTThe herbicide removal of Diuron in a fixed-bed column packed with the Moringa oleifera bark biosorbent was investigated experimentally and through phenomenological mathematical modelling. To understand the physical phenomena involved, the steps of external mass transfer resistance, internal mass transfer resistance and the adsorption phenomenon itself were considered as possible limiting steps in the herbicide mass transfer from the liquid to the solid phase. In the developing process of the internal mass transfer resistance model, two hypotheses were considered: constant mass transfer coefficient and mass transfer coefficient as a function of the herbicide concentration in the biosorbent. The experimental breakthrough curves were obtained for different flow rates and feed concentrations, in order to evaluate the model’s predictive capacity. The mass transfer parameter values of the mathematical models were estimated using the simplex downhill optimization method. The model that considers the resistance a mass transfer internal with parameter Ks variable represented effectively the dynamic behaviour of the herbicide biosorption process in fixed-bed column, in the various evaluated conditions, indicating that this mechanism controls the biosorption process. Thus, the phenomenological mathematical modelling proved to be an analysis important tool, understanding and the herbicide adsorption systems design in a fixed-bed column.

Model development for sc-drying kinetics of aerogels: Part 1. Monoliths and single particles

A mass transport model for the supercritical drying of gels to aerogels in different sizes (monoliths or particles) and shapes (cylinder, sphere) was developed and evaluated. Physico-chemical data for the system CO2/ethanol from literature at relevant process conditions were analyzed for a precise description of relevant physical properties. In situ measurements of the supercritical drying kinetics of gel monoliths using Raman spectroscopy were used to fit the tortuosity factor of the gel network. Apart from the fitted tortuosity factor the presented model is predictive. The model was analyzed in detail for the case of spherical gel particles. Theoretical minimal drying times were found to range in seconds for microparticles. The mass transfer step limiting the overall drying kinetics was analyzed using the dimensionless Biot number. The transition Biot number can be used for rational selection of the drying conditions (pressure, temperature, mass flow) to achieve a fast drying with low CO2 consumption.

Model development for sc-drying kinetics of aerogels: Part 2. Packed bed of spherical particles

We apply here the previously developed mass transfer model (Part 1) for drying of aerogel monoliths to the supercritical drying of particles in a packed bed. We analyzed the influence of the operating conditions, flow rate, particle and autoclave size on the drying time and consumption of carbon dioxide. Although the model requires certain improvement to quantitatively predict the experimental drying kinetics, it captures all features of the supercritical drying: (i) transition of the limiting mass transfer step between diffusion in the gel and convection in the bulk fluid; (ii) role of CO2 density as natural variable for analysis of the CO2 consumption; (iii) influence of the autoclave dimensions on the drying process. We suggest a unified approach towards the analysis of drying process based on a dimensionless number. Practical recommendations on the rational selection of process parameters to achieve appropriate combinations of drying time and CO2 consumption are formulated.

Electrochemical behavior and localized corrosion of X65 steel in high salt concentration brines with CO2 saturated

The electrochemical characteristics and localized corrosion of X65 steel were investigated in CO2-saturated brines as a function of salt concentration employing electrochemical techniques and immersion tests. The results show that, as salt concentration increases, the corrosion mechanism changes from electrochemical steps control to the mixed control of mass transfer and electrochemical steps. The higher the salt concentration is, the more obvious the mass transfer control will be. The corrosion rate firstly increases and then decreases with salt content. There is a maximum corrosion rate somewhere between 60 and 120 g l−1 salt content. The salting-out effect may play a crucial role in steel corrosion in CO2-bearing systems with higher salt concentrations. Chloride ions account for the decrease of the surface layer thickness and make them non-adherent, thereby promoting the initiation and development of localized corrosion at salt concentration above 120 g l−1.

Enhancement in trichloroethylene dechlorination by mixed particles of iron–iron disulfide or iron–iron sulfide

A column test was carried out to clarify the effect of the mixed particles of iron (Fe) and iron disulfide (FeS2) or iron sulfide (FeS) on the trichloroethylene (TCE) degradation rate and compared with a reported batch test. The TCE degradation rate of the mixed particles of Fe and FeS2 was faster than the laminar packing of Fe and FeS2. The TCE degradation rate proceeded at the first-order reaction and had a peak value at the mass ratio of Fe/FeS2 = 7/3 and Fe/FeS = 3/7, and they showed the same trend as the batch test. The rate constant of Fe/FeS2 and Fe/FeS based on unit surface area remained constant for the mixing ratio of Fe to FeS2 or FeS, and 6.1 and 1.3 times larger than single component of Fe, respectively. The surface chemical reaction of TCE on the Fe/FeS2 and Fe/FeS particles was the rate determining step and the mass transfer steps did not affect the reaction rate. Chloride ion, ethylene and ethane were detected from the treated solution and off-gas due to the TCE degradation, and the pH value increased with the increase in the TCE degradation rate until pH ≒ 10.

Separation of platinum(IV) across hollow fiber supported liquid membrane using non-toxic diluents: Mass transfer and thermodynamics

Non-toxic diluents were studied and compared with toxic diluents for separation of platinum(IV) across hollow fiber supported liquid membrane. 91.41% extraction and 81% stripping of platinum(IV) was observed for sunflower oils at 328.15 K. Overall mass transfer resistance (R) was 146.612 × 104 s/cm. The separation process was exothermic (ΔHex0 = −975.20 kJ/mol) and spontaneous (ΔGex0 = −326.447 J/mol). The Ea of 1.673 kJ/mol indicated that the diffusion of complex species across the liquid membrane controlled the mass transfer step. Moreover, a modified Apelblat model gave an excellent match for predicting the extraction behavior of platinum(IV) at 1.2818% RMSD.

Mass Transfer Model of Desulfurization in the Electroslag Remelting Process

Experimental and theoretical studies have been carried out to investigate the effects of the slag on desulfurization during the electroslag remelting (ESR) process with a focus of developing a mass transfer model to understand the mechanism of desulfurization. Stainless steel 1Cr21Ni5Ti was used as the electrode and remelted with two different kinds of slags using a 50-kg ESR furnace. The contents of sulfur along the axial direction of product ingots were analyzed. It was found that the sulfur content of 350 ppm in the electrode is reduced to 71 to 95 ppm in the ingot by remelting with the slag containing 5 wt pct of CaO, and lowered more to 47 to 59 ppm with another slag having 20 wt pct CaO. On the basis of the penetration and film theories, the theoretical model developed in this work well elucidates the kinetics of desulfurization revealing the mechanism of sulfur transfer during the ESR process. The calculation results obtained from the model agree well with the experimental results. The model indicates that when sulfur content in electrode is given, there is a corresponding minimum value of sulfur content in the ingot due to the kinetics limit. This lowest sulfur content cannot be further reduced even with increasing L S (sulfur distribution coefficient between metal and slag phases) or decreasing sulfur content in the slag. Constant addition of extra amount of CaO to the molten slag with the increase of sulfur content in the slag during the remelting process can improve the macrosegregation of sulfur distributed along the axial direction of ESR ingots. Since the rate-determining steps of the sulfur mass transfer lie in the metal phase, adding calcium as deoxidizer can change mass transfer of sulfur and thus promote desulfurization further during the ESR process.

Functional Species Encapsulated in Nitrogen-Doped Porous Carbon as a Highly Efficient Catalyst for the Oxygen Reduction Reaction.

The scarcity, high cost, and poor stability of precious metal-based electrocatalysts have stimulated the development of novel non-precious metal catalysts for the oxygen reduction reaction (ORR) for use in fuel cells and metal-air batteries. Here, we fabricated in situ a hybrid material (Co-W-C/N) with functional species (tungsten carbide and cobalt nanoparticles) encapsulated in an N-doped porous carbon framework, through a facile multi-constituent co-assembly method combined with subsequent annealing treatment. The unique structure favors the anchoring active nanoparticles and facilitates mass transfer steps. The homogenously distributed carbide nanoparticles and adjacent Co-N-C sites lead to the electrocatalytic synergism for the ORR. The existence of Co and W can promote the graphitization of the carbon matrix. Benefiting from its structural and material superiority, the Co-W-C/N electrocatalyst exhibits excellent electrocatalytic activity (with a half-wave potential of 0.774 V vs. reversible hydrogen electrode (RHE)), high stability (96.3 % of the initial current remaining after 9000 s of continuous operation), and good immunity against methanol in alkaline media.

Biosorption of binary heavy metal systems: Phenomenological mathematical modeling

A phenomenological mathematical modeling approach was applied in order to describe the biosorption of binary heavy metal systems. The mathematical modeling here proposed was verified with the biosorption of Cu(II)–Ni(II) systems onto residue of alginate extraction from Sargassum filipendula. Equilibrium studies were carried out in order to elucidate the competition between the ions in the process. The Cu(II) ions showed a higher affinity with the biosorbent, while the Ni(II) removal was highly dependent on the presence of Cu(II) ions (i.e., the Cu(II) ions inhibit the removal of Ni(II)). According to the Akaike criterion, the equilibrium experimental data was better described by the Langmuir-Freundlich model, which considers the competition among ions for the active sites. The phenomenological mathematical modeling of the kinetics of biosorption was based on binary equilibrium equations, material balances and possible limiting mass transfer steps instead of the empirical models usually applied. The model was then adjusted to the experimental kinetic data and validated by a simulation in a different condition. According to the model, the internal mass transfer resistance (intra-particle diffusion) is the limiting step that controls the kinetics of the process. Therefore, due to its high predictive capacity, the phenomenological approach described in this work can be applied as a tool for design and optimization of binary heavy metal adsorption systems.

How the conditioning and storage of samples for SEM-EDX analysis may influence the interpretation of diffusion mechanisms in the sorption of metals ions by extractant impregnated resins

BACKGROUND Modeling mass transfer steps is important for optimizing the use of sorbents such as extractant impregnated resins (EIR). However, mathematical fit of kinetic profiles requires physical characterization for validating mechanism hypotheses. SEM-EDX is a very useful tool for achieving this objective. RESULTS In this work a special attention has been focused on the simultaneous modeling of the diffusion mechanisms (using homogeneous diffusion and shrinking core models associated to resistance to film diffusion, to intraparticle diffusion and to chemical reaction rate) and the SEM-EDX analysis of the diffusion profiles for gold sorption on ionic liquid/impregnated resins (tetraalkylphosphonium IL immobilized in Amberlite XAD resins). Some discrepancies in the conclusions raised by the two approaches led to carefully investigate the effect of conditioning and storage of resins before SEM-EDX analysis. The self-diffusion of the metal ion in the EIR causes misinterpretation of diffusion profiles when inappropriate conditions are used. CONCLUSION SEM-EDX can be efficiently used for improving the understanding of diffusion mechanisms in the study of metal uptake kinetics in EIRs providing that the analysis of the samples is carried out within 24 h and the sample is freeze-dried (to limit metal-ion self-diffusion in the extractant phase). This study can obviously be extended to other sorption systems.

Effects of the magnetic field on the corrosion dissolution of the 304 SS│FeCl3 system

The effects of the magnetic field (MF, 450mT) on the corrosion dissolution of the 304 stainless steel (304 SS) in FeCl3 solution have been studied. The dynamic processes at the electrode│electrolyte interface have been observed by using the in-line digital holography. Results show that the MF effects are different at different potentials because they vary with the elements in 304 SS: MF enhances the corrosion dissolution of Fe and Ni, but inhibits that of Cr. At low potentials, the low FL accelerates the corrosion dissolution of Fe, leading to the enhanced dissolution of 304 SS, as Fe is its main component. However, at high potentials, the high FL enhances the mass-transfer step to drive the aggressive ions away from the electrode│electrolyte interface, inhibiting the occurrence of pitting corrosion. Moreover, MF increases Cr% on the electrode surface as shown by the XPS results, and a relatively stable oxide film is formed rather easily on the surface of 304 SS. Thus, the corrosion dissolution of 304 SS is inhibited by MF.

Methanol synthesis in a three‐phase catalytic bed under nonwetted condition

To overcome the heat removal problem encountered in methanol synthesis at high syngas concentrations in the gas phase, a three‐phase nonwetted catalytic system was established by introducing an inert liquid medium into a fixed‐bed reactor. To form a repellent interface between the liquid and the catalyst, the catalyst was modified into hydrophobic, while the liquid medium was chosen as a room temperature ionic liquid with hydroxyl groups. The liquid‐solid contact angle was measured to be 115°, and only 20% of the catalyst external surface was wetted by the liquid. Under three‐phase condition, the reaction rate was measured to be 60%–70% of gas‐phase reaction, while it was merely 10%–20% for the fully wetted catalyst. From the resistance analysis on the mass transfer and reaction steps, the overall reaction rate is expected to increase further if the surface could be more wet proofed. © 2016 American Institute of Chemical Engineers AIChE J, 63: 226–237, 2017

Performance of mesoporous silicas and carbon in adsorptive removal of phenanthrene as a typical gaseous polycyclic aromatic hydrocarbon

Adsorptive removal of the predominant polycyclic aromatic hydrocarbons (PAHs), represented by phenanthrene, from hot gases using mesoporous adsorbents (mesosilicas MCM-41 and SBA-15, and mesocarbon CMK-3) has been studied through tests of column adsorption and temperature programmed desorption (TPD). Adsorption equilibria on MCM-41 and CMK-3, and on SBA-15 were well described by Langmuir and Freundlich models, respectively, showing the order of adsorption capacity of CMK-3 > SBA-15 > MCM-41 originated from different geometrical structures and texture properties. Adsorption dynamics based on breakthrough fitting by the constant-pattern wave propagation model showed that mesoporosity contributed to fast overall adsorption kinetics by reducing the internal resistance as being the predominant mass-transfer step, particularly on SBA-15 with a 2-D cross-linked structure. The TPD tests revealed the order of ease of desorption of SBA-15 ≈ MCM-41 > CMK-3, where the structural advantage on SBA-15 dominated the strong site binding as indicated by the intrinsic activation energy and exponential factor obtained by TPD models. Results from different TPD models showed that re-adsorption effect during desorption increased with decreasing dimension of adsorbent structure and that the utilization of the simplified linear TPD model could be violated by the strong sorbate-sorbent binding. Comparisons with traditional adsorbents and with the removal of naphthalene by these mesoporous adsorbents were made, showing higher applicability of CMK-3 for phenanthrene regarding stronger adsorption affinity and capacity as well as decreased disadvantages in regenerability relative to mesosilicas.

Using a novel adsorbent macrocyclic compound cucurbit[8]uril for Pb2 + removal from aqueous solution

In this study, cucurbit[8]uril (CB[8]) was utilized as a kind of new adsorbent to remove Pb2+ ions from aqueous solution. With the solution pH increased from 2 to 6, the removal efficiency of adsorption increased from 55.6% to 74.5%correspondingly. The uptake of Pb2+ increased rapidly in the initial 30min, and then the adsorption rate became slower. The Pseudo-second order model could be used to interpret the adsorption kinetics satisfactorily; and the rate determining step in Pb2+ adsorption onto CB[8] was the external mass transfer step. Equilibrium isotherm study reveals that the Langmuir model gave a better fitting result than Freundlich model. The maximum adsorption capacity calculated by the Langmuir model was 152.67mg/g for 298K, 149.70mg/g for 313K and 136.42mg/g for 323K, respectively. The adsorption is a spontaneous process of exothermic nature. The effect of the adsorbent dosage and the influences of solution pH and co-existing cations were also investigated. The CB[8] was synthesized and characterized by 1H NMR, IR, ESI-MS spectra, SEM-EDAX, Zeta-potential and BET-analysis. The adsorption mechanism was due to the coordination between CB[8] molecule and Pb2+ ions.

Modeling of corrosion of steel tubing in CO2 storage

AbstractIn this work, a mechanistic model was developed to predict the corrosion rate of steel tubing under carbon storage conditions. The model integrates a number of sub‐models that quantify contributions of interrelated steps to the corrosion process. The water chemistry sub‐model determines the solution pH and the concentrations of species. The electrochemical corrosion sub‐model assesses both charge‐transfer and mass‐transfer steps and their effects on corrosion. The scale formation and its role in corrosion are considered. The finite difference method is used to solve numerical equations, and a MetLab program was written for computation of the corrosion rate. The modeling results are validated by both laboratory testing and literature data. The parametric effects, including temperature, CO2 partial pressure, solution salinity, pH, time, etc., on corrosion are predicted. The limitations of the model are also discussed. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

Understanding Crystal Growth Kinetics in the Absence and Presence of a Polymer Using a Rotating Disk Apparatus

Polymer inhibition of crystal growth from supersaturated solutions is an important area of study, particularly in the pharmaceutical field. Polymers can be used to modify crystal morphology or to slow crystal growth with the goal of enhancing oral drug absorption. In order to fully understand the effect of polymers on crystal growth, it is important to understand the kinetics of the integration step. In this study, a rotating disk apparatus is used to isolate the integration step from the mass transfer step so that the integration kinetics can be determined for growth rates in the absence and presence of polymers. The results indicate that the presence of polymers in solution greatly impacts the integration step and has no impact on the mass transfer kinetics.

Synthesis of aminated glycidyl methacrylate grafted rice husk and investigation of its anion‐adsorption properties

ABSTRACTIn this study, we focused on the synthesis, characterization, and adsorption capacity testing of aminated glycidyl methacrylate grafted rice husk (RH‐g‐GMA–Am). Our goal was to obtain a high‐performance surface for the adsorption of various anions. Glycidyl methacrylate grafted rice husk (RH‐g‐GMA) was prepared by the graft copolymerization of glycidyl methacrylate with rice husk; the product was further subjected to an amination reaction. The surface properties, sorption characteristic functional groups, isotherm and kinetic studies, pore diffusion models, and effects of the temperature and pH on the material properties were studied under batch conditions. The IR spectroscopy results show additional surface functional groups for RH‐g‐GMA–Am. The adsorptions of and on RH‐g‐GMA–Am were found to follow pseudo‐second‐order kinetics; this indicated a possible dominant role played by chemisorption. The rate‐limiting step for mass transfer was found to be boundary layer diffusion. Furthermore, the sorption isotherms for and fit the Langmuir model. The amination of RH‐g‐GMA drastically increased the removal efficiency from 3 to 82% and from 6 to 93% for and , respectively. Moreover, RH‐g‐GMA–Am exhibited a better removal efficiency in the pH range of 4–6.5. Regeneration studies revealed that the surface of RH‐g‐GMA–Am could be regenerated repetitively by simple acid washing with an insignificant decrease in the active surface for consecutive adsorptions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43002.