Influence Of Mass Transfer Resistance Research Articles

Analysis Method of Oxygen Permeation Resistance of Ionomer in Cathode Catalyst Layer in PEFC

Analysis Method of Oxygen Permeation Resistance of Ionomer in Cathode Catalyst Layer in PEFC

Highly efficient selective oxidation of 2-methylnaphthalene to vitamin K3 over mesoporous Al/Ti-SBA-15 catalysts: The effect of acid sites and textural property

Catalytic oxidation of 2-methylnaphthalene to 2-methyl-1,4-naphthoquinone (vitamin K3) is a significant pathway to produce vitamin groups K. High pollution and low-productivity of the traditional method require a greener catalyst and process. In this work, Al/Ti-SBA-15 with different nSi/nAl and nSi/nTi ratios were hydrothermally prepared in mild acidic condition and tested in partial oxidation of 2-methylnaphthalene. Commercial zeolites NaY, β and ZSM-5 were applied to investigate the influence of mass transfer resistance and acidity on catalytic performance. Catalysts were characterized by XRD, N2 adsorption-desorption, TEM, NH3-TPD, FTIR and Py-FTIR. We have found that the 2-methylnaphthalene conversion and vitamin K3 selectivity depend strongly on the mesoporous structure as well as Brönsted acid concentration and strength. Irregular micropores and strong acidity will accelerate the cleavage of C = C in aromatic rings, leading to the production of phthalic anhydride. DFT calculation reveals that incorporation of aluminum in SBA-15 can reduce the energy gap, facilitating the electron transfer. And as a consequence of the substitution of Al in tetrahedral positions in SBA-15, Al-SBA-15 (10) gives the best result with 72% yield to vitamin K3. This catalyst markedly reduces the activation energy due to the modified activated center. Repeated experiments recommend that Al-SBA-15 is a steady and high-efficiency catalyst for the production of vitamin K3.

Experimental investigation on flow condensation of zeotropic mixtures of methane/ethane in a horizontal smooth tube

An experimental investigation on condensation flow pattern and heat transfer coefficient of methane/ethane mixtures (0.27/0.73, 0.54/0.46 and 0.7/0.3 by mole) in a horizontal smooth tube with inner diameter of 4 mm was carried out. The tests were conducted at saturation pressure of 1.5–2.5 MPa with mass flux of 98–257 kg m−2 s−1 and heat flux of 15.1–44.4 kW m−2 over the entire vapor quality range. The effects of concentration, saturation pressure, heat flux, mass flux and vapor quality were analyzed and discussed. A new annular/non-annular flow pattern transition criteria on condensation for zeotropic mixtures was carried out. The new transition criteria took the influence of mass transfer resistance into consideration and had satisfactory predictive ability in present study. Moreover, the experimental data were compared with many condensation heat transfer correlations of mixtures. An improved heat transfer correlation for zeotropic mixtures based on flow patterns was proposed. The new correlation combined with the equilibrium method and introduced a new correction factor (Fm). It achieves better predicting results with a mean absolute relative deviation of 8.02%.

Removal of carbon dioxide from flue gases in packed column using ionic liquids

ABSTRACTThe study of CO2 absorption in ionic liquids (ILs): [Emim] [Ac], [Bmim] [Ac] in a packed column is presented. The influence of mass transfer resistances, initial CO2 concentration, absorption temperature and 2, 5, 10% wt. water addition on CO2 removal efficiency was investigated. The resistance in series model and estimated values of enhancement factor were used to predict with good accuracy mass fluxes of absorbed carbon dioxide for both ILs. The CO2 absorption efficiency in packed column depends on temperature and initial CO2 concentration. The addition of small amounts of water to [Emim][Ac] is of minor effect on CO2 absorption.

Carbon dioxide absorption in a packed column using imidazolium based ionic liquids and MEA solution

The study of CO2 absorption in ionic liquids: [Emim][Ac], [Bmim][Ac] and 15wt.% MEA solution in a packed bed column is presented. Absorption column of inner diameter 0.05m and 0.35m length was filled with Raschig rings ϕ5mm×5mm×1mm. Investigations were performed at atmospheric pressure what is important in the case of post combustion flue gases. The influence of mass transfer resistances in gas and liquid phase as well as initial CO2 concentration and absorption temperature on CO2 removal efficiency was investigated. In calculations the resistance in series model of the absorption process was applied. The estimated values of enhancement factor of the reaction between CO2 and investigated liquids were used to predict mass fluxes of absorbed carbon dioxide for both ILs and MEA solution. The comparison of experimental and calculation results show that in the same experimental conditions the investigated ILs and MEA solution have comparable CO2 absorption capacities. The liquid side mass transfer coefficients in the case of ILs are several times lower than for MEA solution. The research for new ILs characterized by chemisorption mechanism, low viscosity and price is needed to take advantages of applying ionic liquids in the process of CO2 separation in a packed column.

Kinetics of sunflower and used vegetable oil methanolysis catalyzed by CaO·ZnO

The kinetics of heterogeneous methanolysis of sunflower oil and used vegetable oil were studied at different temperatures, ranging from 60 to 96°C using CaO·ZnO as catalyst (2wt% on the basis of oil) and methanol to oil molar ratio of 10:1. Heterogeneous CaO·ZnO catalyst was synthesized by mechanochemical treatment of CaO and ZnO powder mixture with the addition of water necessary for the formation of corresponding mixed hydroxides, and their calcination at 700°C in air. It was shown that kinetics of overall process could be described as pseudo-first order reaction. For the sunflower oil methanolysis at 60 and 70°C, the rate of process at the beginning of biodiesel synthesis was limited by triglycerides mass transfer to the catalyst surface, and after that it is governed by the rate of chemical reaction at catalyst surface. At higher temperatures the influence of mass transfer resistance is almost negligible implying that the rate of chemical reaction determines the overall kinetic of biodiesel synthesis. In the case of used vegetable oil, the influence of mass transfer resistance was not observed either at higher or lower temperature. The kinetic model that describes the whole process well was proposed, and it comprises both the triglycerides mass transfer and chemical reaction controlled regime. The overall volumetric mass transfer coefficient was defined, assuming that it depends on the conversion of triglycerides.

Influence of mass transfer resistance on overall nitrate removal rate in upflow sludge bed reactors

A kinetic model with intrinsic reaction kinetics and a simplified model with apparent reaction kinetics for denitrification in upflow sludge bed (USB) reactors were proposed. USB-reactor performance data with and without sludge wasting were also obtained for model verification. An independent batch study showed that the apparent kinetic constants k′ did not differ from the intrinsic k but the apparent K s ′ was significantly larger than the intrinsic K s suggesting that the intra-granule mass transfer resistance can be modeled by changes in K s. Calculations of the overall effectiveness factor, Thiele modulus, and Biot number combined with parametric sensitivity analysis showed that the influence of internal mass transfer resistance on the overall nitrate removal rate in USB reactors is more significant than the external mass transfer resistance. The simulated residual nitrate concentrations using the simplified model were in good agreement with the experimental data; the simulated results using the simplified model were also close to those using the kinetic model. Accordingly, the simplified model adequately described the overall nitrate removal rate and can be used for process design.

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 2: investigation of mass transfer resistance effects

Detailed calculations of condensation outside a column of horizontal smooth tubes have been carried out in order to investigate the influence of mass transfer resistance for a zeotropic refrigerant mixture. Diffusive transport is calculated locally in both phases. Calculation results show that mass transfer resistance in the gas phase reduces heat transfer by 10–20% for a binary mixture with a glide similar to that of R407C. The decrease in heat transfer due to poor mixing in the condensate is from 15 to 65% for the conditions investigated; the lower the duty, the greater the decrease. Results show that if assuming mixing in the condensate by diffusion only, the degrees of mixing and the duty dependency are similar to results in earlier work where calculations were matched to experimental data. Diffusion is likely to be the dominant mixing mechanism in the condensate, and the mixing is poor under certain conditions, which might explain the drop in condenser performance for some heat pump applications.

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 2: investigation of mass transfer resistance effects

Detailed calculations of condensation outside a column of horizontal smooth tubes have been carried out in order to investigate the influence of mass transfer resistance for a zeotropic refrigerant mixture. Diffusive transport is calculated locally in both phases. Calculation results show that mass transfer resistance in the gas phase reduces heat transfer by 10–20% for a binary mixture with a glide similar to that of R407C. The decrease in heat transfer due to poor mixing in the condensate is from 15 to 65% for the conditions investigated; the lower the duty, the greater the decrease. Results show that if assuming mixing in the condensate by diffusion only, the degrees of mixing and the duty dependency are similar to results in earlier work where calculations were matched to experimental data. Diffusion is likely to be the dominant mixing mechanism in the condensate, and the mixing is poor under certain conditions, which might explain the drop in condenser performance for some heat pump applications.

Influence of mass transfer resistance on detection of nonaqueous phase liquids with partitioning tracer tests

Partitioning interwell tracer tests (PITTs) are a relatively new technique for measuring the amount of nonaqueous phase liquid (NAPL) within saturated porous media. In this work we examined the influence of mass transfer limitations on the accuracy of measured NAPL from PITTs. Two mathematical models were used along with laboratory column experiments to explore the influence of tracer partition coefficient, tracer detection limit, and injected tracer mass on NAPL measurements. When dimensionless mass transfer coefficients were small, NAPL measurement errors decreased with decreasing tracer partition coefficient, decreasing tracer detection limit, and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. Although transport in a single stream tube was used in the mathematical models and laboratory experiments, the results from this simplified domain were supported by data taken from a three-dimensional computational experiment, where the NAPL resided as large pool. Based on these results, we suggest guidelines for interpreting tracer breakthrough data to ascertain the importance of mass transfer limitations on NAPL measurements.

Process kinetics of UASB reactors treating non‐inhibitory substrate

AbstractThe degradation of a non‐inhibitory substrate (sucrose) in upflow anaerobic sludge bed (UASB) reactors with different superficial flow velocites (us) was performed to generate experimental data. Additionally, a kinetic model accounting for the mass fraction of methanogens (f) and granule size distribution in UASB reactors is also proposed. At the volumetric loadings of 2.65–21.16 g COD dm−3 day−1, both the COD removal efficiency and granule size of the UASB reactors increase with increasing us. The f values determined experimentally increase from 0.13–0.24 to 0.27–0.43 if the volumetric loading is increased from 2.65 to 5.29 g COD dm−3 day−1. With a further increase in volumetric loading, the f values decline because of the accumulation of volatile fatty acids (VFAs). The predicted residual concentrations of VFAs and COD are in fairly good agreement with the experimental data. From the calculated effectiveness‐factor values, the influence of mass transfer resistance of the substrate sucrose on the overall substrate removal rate should not be neglected. From parametric sensitivity analyses together with the simulated concentration profiles, methanogenesis is the rate‐limiting step. Copyright © 2003 Society of Chemical Industry

Performance of an Endless-Belt Counter-Current Contactor for Vapor-Phase Adsorption: Dispersion and Residence Time Distribution

The residence time distribution and axial dispersion characteristics of an endless-belt counter-current adsorbent contactor have been studied experimentally and theoretically using, as a model system, the adsorption of CO2 from a N2 carrier on a silicalite adsorbent. Expressions for the Taylor dispersion coefficient, and hence the height equivalent to a theoretical plate (HETP), as a function of the gas flow rate and belt velocity have been derived (from the general Aris model) and are shown to provide an approximate indication of the system performance. However, dispersion in the experimental system is increased by mixing in the dead space within the apparatus, making it difficult to test the theory quantitatively. Although diffusion of CO2 in silicalite is rapid and the adsorbent layer is thin, the influence of mass transfer resistance is still appreciable. The experimental system (length of 50 cm) is shown to be equivalent to 5−10 theoretical stages (HETP 5−10 cm), depending on flow rate and belt velocity.

Chiral separation by SMB chromatography

The paper deals with chiral separation by simulated moving bed (SMB) chromatography. A model for the prediction of the cyclic steady-state performance of the SMB, based on the analogy with the true moving bed (TMB), is developed assuming multicomponent adsorption equilibria, axial dispersion flow and the linear driving force approximation to describe the intraparticle mass transfer rate. The simulation package is used to predict the effect of operating variables on the process performance and to define the regions for enantiomers separation. The influence of mass transfer resistance in the separation region is analyzed. A practical tool for choosing the better SMB operating conditions as a function of the feed flow-rate is proposed. The experimental operation of a pilot unit of SMB, Licosep 12-26 (Novasep, France), is carried out for the separation of enantiomers and illustrated with two systems: the bi-naphthol and the chiral epoxide enantiomers. Experimental results and model predictions are compared in terms of process performance and internal concentration profiles.

Catalyst fragmentation during propylene polymerization. III: Bulk polymerization process simulation

AbstractFragmentation of support/catalyst particles during propylene bulk polymerization is analyzed by means of a mathematical model including energy and mass balances, with chemical reaction. The rupture phenomenon is specifically considered by the model and analyzed as it proceeds along time. Model predictions concerning the effects of fragmentation on polymerization are discussed. The influence of mass‐transfer resistances at the macroparticle and microparticle level, as well as the microparticle nucleus‐size effects over the polymerization process, are analyzed. Macroparticle mass‐transfer resistance affects both the rate of fragmentation and temperature excursions. Microparticle nucleus‐size exerts a strong influence over the whole polymerization process. A small micronucleus‐size produces both a delay in the fragmentation process and a greater value of she final catalyst yield. The effects of major critical parameters are evaluated via model simulation, and the results are discussed. The analysis shows that fragmentation depends on the combined effect of the parameters studied. Modeling of the process considering all parameters simultaneously is the proper way of predicting the fragmentation sequence for a given support/catalyst particle. Crystallinity of the produced polymer affects the rate of fragmentation, either increasing or decreasing the rupture rate depending on macroparticle porosity and compactness. Heat transfer conditions in the liquid‐phase system make the temperature runaway problem easy to predict and control, in spite of high polymer yields. The design of “tailor‐made” support/catalyst macroparticles in accordance with catalytic activity is necessary in order to obtain high yields and controlled process temperatures.

Dynamics of linear interactions in heterogeneous media: A systems approach

A general method is presented for modelling linear reversible interactions between a flowing fluid and a porous medium, taking into account non-ideal flow and mass transfer resistance. It is shown that the overall transfer function of the system may be obtained by combining the transfer function characterizing the flow pattern, and that characterizing interactions at the local level. Several mechanisms are considered: one single retention layer, several layers in series, several retention sites exchanging in parallel, diffusion into a porous solid. It is also possible to account for distributed exchange times and first order chemical reactions in both phases. Examples of simulations are presented. Deformation of the response to a pulse injection of tracer under the influence of mass transfer resistance, case of two sites of different activity exchanging in parallel, existence of a transfer time distribution. Different mechanisms may lead to identical shapes. Complex networks may be easily represented. Finally, simple hints are provided for interpreting peak shapes after the proposed models.

Maltodextrin hydrolysis in a fluidized‐bed immobilized enzyme reactor

The present work deals with maltodextrin hydrolysis by glucoamylase immobilized onto corn stover in a fluidized bed reactor. An industrial enzyme preparation was covalently grafted onto corn stover, yielding an activity of up to 372 U/g and 1700 U/g for support particle sizes of 0.8 and 0.2 mm, respectively. A detailed kinetic study, using a differential reactor, allowed the characterization of the influence of mass transfer resistance on the reaction catalyzed by immobilized glucoamylase. A simple and general mathematical model was then developed to describe the experimental conversion data and found to be valid.

Kinetics of methane formation by granular sludge at low substrate concentrations

Apparent K m values for methanogenesis from volatile fatty acids by various sludge granules were determined from progress curve experiments. The influence of mass transfer resistance on the kinetics of methane formation in biofilms was demonstrated with the apparent K m values thus obtained. It was shown that the effects of mass transfer resistance depend on the maximum specific activity of the biolayers and on their thickness. the data indicate that mass transfer resistance in methanogenic biolayers becomes only of significance at low substrate concentrations and in thick biolayers with high methanogenic activities.