Negligible Mass Transfer Resistance Research Articles

Experimental and Simulation Studies of TAME Synthesis in a Fixed-Bed Reactor

Experimental data of TAME synthesis in a fixed-bed reactor were obtained in an experimental setup with a laboratory-scale jacketed column packed with a macroreticular strong cation ion-exchange resin in hydrogen form (Amberlyst 15 Wet, Rohm & Haas) as catalyst. Two kinds of experiments were performed: TAME synthesis (etherification + isomerization) and only isomerization between the isoamylenes. The experiments were carried out in the temperature range 313−353 K, at 12−13 bar. A mathematical model that uses a Langmuir−Hinshelwood kinetic law previously obtained and that, for the description of the behavior of the fixed-bed reactor, considers plug flow with axial dispersion, assumes negligible mass transfer resistances and, in the adsorbed phase, considers significant only the accumulation of methanol, with the adsorption equilibrium described by a Langmuir isotherm, written in terms of activities, was developed. The experimental results, even when involving only the isomerization between the isoamylenes,...

Kinetics of batch alkylation of phenol with tert-butyl alcohol over a catalyst synthesized from coal fly ash

Abstract Alkylation of phenol with tert-butyl alcohol (TBA) was carried out in batch mode over a zeolite catalyst, synthesized from fly ash by hydrothermal treatment. The effects of various parameters, such as reaction temperature, reactant ratio (molar ratio of phenol to tert-butyl alcohol) and catalyst loading on the rate of reaction of phenol were studied with the synthesized catalyst. The alkylation reaction was found to be surface reaction controlled with negligible mass transfer resistance. An Langmuir–Hinshelwood–Hougen–Watson (L–H–H–W) surface reaction controlled kinetic model was developed and the model parameters were estimated. From the estimated kinetic constant at different temperatures, the activation energy of the phenol alkylation reaction was determined to be 37.3 kJ/mol.

Reactor Modeling of Gas‐Phase Polymerization of Ethylene

AbstractA model is developed for evaluating the performance of industrial‐scale gas‐phase polyethylene production reactors. This model is able to predict the properties of the produced polymer for both linear low‐density and high‐density polyethylene grades. A pseudo‐homogeneous state was assumed in the fluidized bed reactor based on negligible heat and mass transfer resistances between the bubble and emulsion phases. The nonideal flow pattern in the fluidized bed reactor was described by the tanks‐in‐series model based on the information obtained in the literature. The kinetic model used in this work allows to predict the properties of the produced polymer. The presented model was compared with the actual data in terms of melt index and density and it was shown that there is a good agreement between the actual and calculated properties of the polymer. New correlations were developed to predict the melt index and density of polyethylene based on the operating conditions of the reactor and composition of the reactants in feed.

Comparison between heat transfer and heat mass transfer models for transportation process in an adsorbent bed

The heat and mass transfer (HMT) model for transportation process in adsorbent bed is studied in relation to heat transfer (HT) model, which is dependent on assumptions such as the conduction dominance and negligible mass transfer resistance. Two general criteria are generated by performing an order of magnitude analysis to determine when the HT model can be applied in practical situation as a good approximation in the evaluation of the temperature field as well as dimensionless uptake and the criteria are validated with the numerical results. It is seen that the criterion for conduction dominance (CR1) is usually satisfied for all practical situations, but the criterion towards negligible mass transfer resistance (CR2) turns out to be more important for the selection of an appropriate model.

Countercurrent transport of organic and water molecules through thin film composite membranes in aqueous–aqueous extractive membrane processes. Part I: experimental characterisation

This paper presents an experimental study on organic and water mass transfer through thin film composite (TFC) membranes, made of a 2 μm thick polydimethylsiloxane (PDMS) layer coated onto a microporous support of polyethersulfone. These were used for the extraction of various organic molecules from aqueous solutions with different ionic strengths. Independent measurements of organic and water transfer rates were performed using lab-scale single- and multi-tube mass exchangers. The rate of organic transfer across TFC membranes is shown to be independent of the affinity of the organic molecules for the PDMS layer, owing to negligible mass transfer resistance in this thin layer. As a consequence, the overall mass transfer coefficient measured for aqueous–aqueous extraction of hydrophilic compounds (i.e. with low affinity for PDMS) was up to one order of magnitude larger for TFC membranes than for 350 μm thick PDMS membranes operated under the same hydrodynamic conditions. The effect of osmotic pressure on water transport through TFC membranes was assessed by liquid–liquid extraction experiments using saline solutions. Water flux increases with increasing difference in salt concentration across the membrane, due to increased activity gradient. For the range of salt concentration and PDMS layer thickness used in this work, the osmotic water flux across the membrane has only a marginal effect on the rate of organic extraction. On the other hand, the ionic strength of feed and extractive phases has a direct influence on the organic transfer, due to the dependence of transport and equilibrium properties (i.e. diffusion and partition coefficients) on salt concentration. As a result, TFC membranes exhibit an asymmetric behaviour depending on operating mode. For the same hydrodynamic conditions and ionic strength, organic flux is higher when a saline solution is on the coated side than when it is on the porous side of the membrane. This has obvious implications in the design and operation of TFC membrane processes for the extraction of organics from saline solutions.

Mass and momentum transfer in commercial blood oxygenators

In modern blood oxygenators, microporous membranes (flat sheet or hollow fibre) are used to separate the blood and gas phases. Blood flows on one side of the membrane while gas (usually oxygen or air) flows on the other side. Since the membranes used are hydrophobic (e.g., polypropylene, Teflon), the pores are gas filled resulting in a negligible membrane mass transfer resistance. The major resistance to gas transfer is due to the blood side mass transfer boundary layer. In this study mass transfer and friction factor correlations for commercially available hollow fibre and flat sheet blood oxygenators have been determined experimentally. Water was used as a substitute for blood. The diffusion of oxygen into and out of water has been studied. Two different flow configurations used commercially have been investigated: flow outside and across woven hollow fibre bundles and flow in thin channels. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. The results obtained here can be used to further optimize the design of blood oxygenators.

Vibrofluidized- and fixed-bed photocatalytic reactors: case of gaseous acetone photooxidation

Vibrofluidized- and multiple fixed-bed photoreactors were compared under identical operating conditions. The comparison was based on the quantum efficiency for the gas-phase photocatalytic oxidation of acetone using TiO 2 (Hombikat UV 100). Multiple fixed-bed reactor configurations were used, along with a vibrofluidized-bed configuration. Quantum efficiency decreased in the following order: vibrofluidized bed (8.7%)>fixed-bed granules (6.9%)>fixed film bed (5.9%) ∼fixed powdered bed (5.8%). The increased activity of the vibrofluidized-bed could not be attributed to enhanced external mass transport, as all reactor systems used in the present study demonstrated negligible external mass transfer resistances. Instead, the increased activity is most likely credited to the effect of periodic illumination phenomenon taking place because of the random motion of catalyst granules in the fluidized-bed and higher light absorption of scattered light. The enhanced activity observed for the granular fixed bed could be related to mechanical activation of TiO 2 during the preparation of granules, as well as to increased light absorption. A maximum of acetone oxidation rate with respect to humidity level was observed. Even at high humidity levels, ultrasound did not affect the rate of gas-phase acetone oxidation. A model has been developed to quantify the absorption of light in a fluidized-bed photoreactor. The model takes into account absorption and single-light scattering and accurately describes the dependence of acetone oxidation rate on the quantity of TiO 2 granules in the vibrofluidized bed.

A poly(N-isopropylacrylamide-co-N-acryloxysuccinimide-co-2-hydroxyethyl methacrylate) composite hydrogel membrane for urease immobilization to enhance urea hydrolysis rate by temperature swing*

A composite membrane made of cross-linked poly(N-isopropylacrylamide-co-N-acryloxysuccinimide-co-2-hydroxyethyl methacrylate) (p(NIPAAm-NAS-HEMA)) hydrogel on polyester nonwoven support has been synthesized. The composite membrane shows temperature-responsive properties similar to conventional PNIPAAm hydrogels beads, which reversibly swells below and de-swells above the lower critical solution temperature of PNIPAAm (around 32 to 33 degrees C). Diffusion of urea through the membrane was temperature-dependent with the effective diffusion coefficient at 20 degrees C being 18 times that at 60 degrees C. Urease was immobilized directly to the membrane by forming covalent bonds between its amino groups and the succinimide ester groups of the membrane. Membrane prepared with NIPAAm to NAS molar ratio of 9, and then reacted in pH 7 buffer with 6 mg of urease gave the best immobilized enzyme, where 0.102 mg protein and 5.71 U activity per cm(2) membrane, and 55% relative specific activity could be obtained. There was negligible internal mass transfer resistance for this preparation judging from the calculated effectiveness factor. Urease shows enhanced thermal stability after immobilization with the first-order inactivation rate constant at 70 degrees C decreased to 1/8 of that of free urease. Membrane-immobilized urease could be utilized in a two-compartment membrane reactor with temperature swing to substantially enhance urea hydrolysis rate. The best operating condition of the membrane reactor was with temperature cycling between 60 to 20 degrees C and with temperature change every 10 min, where concentration of product ammonia after 3 h reaction increased 3.8-folds when compared with isothermal operation at 60 degrees C.

Estimation of Substrate Effective Diffusivities in Anaerobic Bioparticles

A simple and rapid method to estimate effective diffusivities (De) of substrates in bioparticles containing anaerobic sludge is proposed. The method is based on substrate mass balances under unsteady-state conditions utilizing data obtained from ‘finite-bath’ assays performed in an incubator shaker. The experiments were carried out under conditions of negligible liquid-phase mass transfer resistance and absence of biochemical reactions. The first condition was obtained by the application of high agitation speeds. The biomass was inactivated with ethanol solution 50% (v/v) to reach the second condition. Values of the effective diffusivity (De) of glucose, sucrose and acetate were obtained from the temporal variation of substrate concentration in the bulk liquid in assays utilizing alginate gel and polyurethane foam bioparticles. Additionally, the specific methanogenic activity technique was used to evaluate the effectiveness of the method proposed for biomass inactivation. The method used to estimate the substrate diffusivity coefficient was found to provide suitable values of De for substrates at very low concentrations. Values of De for glucose, sucrose and acetate in alginate gel bioparticles were estimated as 0.58 × 10−5, 1.06 × 10−5 and 1.49 × 10−5 cm2 s−1, respectively. The value of De for glucose in polyurethane foam particles was found to correspond to the value of glucose diffusion in water at infinite dilution. The method applied for cell inactivation was found to be adequate for intraparticle mass transfer experiments, making the temporary sludge inactivity long enough to avoid interference due to biochemical reactions.

Modeling of the concentration-polarization effects in a pervaporation cell with radial flow

Abstract In the present study, an analysis of the concentration-polarization effects in a cell with radial flow of the liquid is performed. The separation of chloroform from dilute aqueous solutions is taken as the test system. Experiments were performed in a flat membrane cell using a zeolite-filled polydimethylsiloxane membrane (Pervap 1070) with outward radial flow between two plates. The influence of composition, flow rate and temperature of the liquid feed was experimentally studied. The general assumption of negligible mass transfer resistance of the PDMS membrane for the separation of chloroform from water was considered. The liquid phase mass transfer coefficients were correlated with the operating variables flow rate and temperature using dimensionless groups. It was found that the Sherwood number was proportional to Re0.5 and Sc0.33.

PH gradients in immobilized amidases and their influence on rates and yields of ?-lactam hydrolysis

The pH gradients developing within immobilized biocatalysts during hydrolysis of penicillin G and glutaryl-7-aminocephalosporanic acid have been estimated both theoretically and experimentally. For the latter a fluorimetric method for the direct measurement of the average pH value within the carrier during reaction has been developed using the pH-dependent fluorescence intensity of an enzyme-bound fluorophore determined with a fiber bundle. The theoretical calculations were based on a model for the hydrolysis with immobilized enzymes using a kinetic expression with five pH-dependent, measurable kinetic and equilibrium constants. The transport reaction differential equation which considers the laminar boundary layer has been solved numerically for the key component. The calculated values agreed well with the experimental data. Under the typical reaction conditions of penicillin G hydrolysis the average pH value in the carrier was 1 and 2.5 pH units below the bulk pH (=8) with and without buffer, respectively. The corresponding changes for the hydrolysis of glutaryl-7-aminocephalosporanic acid at bulk pH 8 in the presence of buffer was 0.5. This demonstrates the existence of considerable pH gradients in carriers during hydrolytic reactions, even in buffered systems with negligible mass transfer resistance. The low pH value causes suboptimal reaction rates, reduced equilibrium conversion, and reduced enzyme stability. These pH gradients can be minimised by using buffers with pK values approximately equal to the bulk pH used for the hydrolysis. The prediction quality of the model has been tested applying it to fixed bed reactor design. The reduction in rate and yield due to concentration and pH gradients can be overcome with simple measures such as high initial pH value and pH adjustments in segmented or recycling fixed bed reactors. Thus, enzymatic conversions with high yield and high operational effectiveness are achieved.

Heat Effects in ZLC Experiments

The problem of nonisothermal desorption in a zero length column (ZLC) experiment is considered theoretically. Simple analytical expressions for the ZLC desorption curve are derived for certain limiting situations in which the governing equations reduce to a linear form. More general numerical solutions are calculated for a wide range of experimental conditions assuming both negligible mass transfer resistance and finite mass transfer resistance controlled by intraparticle diffusion. A simple criterion for negligible thermal effects is developed. It is shown that when the ZLC technique is applied to the measurement of diffusion in unaggregated zeolite crystals, as originally intended, heat effects are generally insignificant. However, when applied to the measurement of macropore diffusion in relatively large adsorbent particles heat effects can become important and may cause major modification of both the desorption rate and the shape of the desorption curve. A recent experimental ZLC study carried out with commercial adsorbent particles, under conditions of macropore diffusion control, showed an anomalous dependence of the desorption rate on both temperature and particle size. These effects can be qualitatively explained by the nonisothermal model. A more precise quantitative representation of these experiments will require a more refined model incorporating a nonlinear equilibrium isotherm as well as intraparticle diffusional resistance.

Optimal distribution of catalyst for a case involving heterogeneous and homogeneous reactions

The effect of axial distribution of a fixed amount of catalyst on the performance of an isothermal reactor, in which both heterogeneous and homogeneous reactions occur, is investigated. Analytical solutions for first-order reactions reveal that a uniform axial distribution of catalyst is optimal, except for the case of negligible mass transfer resistance when all catalyst distributions give the same result. In cases where the catalyst enhances the rate of homogeneous reaction, the competition between the heterogeneous reaction and the enhanced homogeneous reaction leads to an optimal step-width, whose size and location depend on the values of the physicochemical parameters.

Estimation of intrinsic kinetic parameters in immobilised cell systems for anaerobic wastewater treatment

Intrinsic kinetic parameters for anaerobic sludge immobilised in polyurethane foam matrices treating a glucose based substrate were estimated under negligible mass transfer resistance in shaker incubator experiments. Micro-granules detachment due to high rotational agitation speeds was found to affect the results.

Oxygen diffusivity in calcium alginate gel beads containing Gluconobacter suboxydans.

Gaining insight into the mass transfer characteristics in immobilized enzyme and microorganism systems bears much importance because of their widely increased use in industrial scale production as well as for analysis purposes. In this study, the effective diffusion coefficient of oxygen in calcium alginate gel with and without cells has been determined by the Moment Analysis Method for the first time in literature. When the gel concentration was increased from 1 to 3 %, De values of oxygen varied between 2.54 x 10(-5) cm2/s and 2.58 x 10(-5) cm2/s indicating almost no dependency on gel concentration. However, a decrease in effective diffusion coefficient (from 2.55 cm2/s to 2.47 cm2/s) was observed with increased immobilized Gluconobacter suboxydans concentration in the range of 0 to 6 %. Experimental results on liquid-particle mass transfer coefficient revealed a negligible external mass transfer resistance. Effectiveness factor for the bioreactor system was also calculated and found to be 0.39 and 0.21 for gel beads of 0.1 and 0.2 cm radius respectively. It is concluded therefore that the use of smaller gel beads could substantially improve the production efficiency in similar bioreactors.

Modeling NaCl and KCl Movement in Fynbo Cheese during Salting

ABSTRACTA model considering a ternary diffusion in axial and radial directions of a finite cylindrical solid was developed for predicting NaCl and KCl concentrations in cheese during brining. The generalized Fick's law form, negligible external mass transfer resistance, and an equilibrium between cheese and brine solution were assumed. Cheese samples (12 × 6 cm) were salted in a KCl‐NaCl) brine solution. NaCl and KCl concentrations were determined experimentally and theoretically at 5, 10 and 15 hr. The average relative error of the model was higher for KCl than for NaCl and increased with brining time. The model can be applied to systems of similar geometry and conditions provided diffusion constants are known.

放射線グラフト重合法によるアフィニティ膜の開発

Membrane-based affinity chromatography has advantages over conventional bead-based chromatography for the following two reasons : (1) the module charged with hollow fibers requires a much lower operating pressure than a bead-packed bed ; and (2) as the biomolecule can be transported by convection to the ligand immobilized on the inner surface of the microporous membrane, faster adsorption onto the affinity membrane can be attained. Hollow-fiber affinity membranes containing phenylalanine and tryptophan as ligands were prepared by the radiation-induced grafting of glycidyl methacrylate onto a porous polyethylene hollow fiber, followed by coupling of the epoxide group produced with Phe and Trp. The remaining epoxide group is quantitatively converted into a diol group. The diol group renders the polymer surface hydrophilic and prevents nonselective adsorption. To evaluate the adsorption behavior of the membrane, the gamma-globulin-containing buffer solution was permeated from the inside to the outside of the Trp-containing hollow-fiber affinity membrane. The breakthrough curves as a function of effluent volume coincided irrespective of the flow-rate, i. e., the residence time of the solution across the membrane, as a result of negligible diffusional mass transfer resistance.

Adsorption and elution of bovine γ-globulin using an affinity membrane containing hydrophobic amino acids as ligands

A hollow-fibre affinity membrane containing hydrophobic amino acids as ligands was prepared by the radiation-induced grafting of glycidyl methacrylate onto a porous polyethylene hollow fibre and subsequent coupling with phenylalanine (Phe) or tryptophan (Trp). The densities of the Phe and Trp ligand of the resulting affinity membrane were 0.4 and 0.4 mol/kg, respectively. The Trp-containing affinity membrane exhibited a higher amount of adsorbed bovine γ-globulin (BGG) than the Phe-containing membrane. To evaluate the adsorption behaviour of the membrane, the BGG-containing buffer solution was permeated from the inside to the outside of the Trp-containing hollow-fibre affinity membrane through the ligand-immobilized pores. The breakthrough curves as a function of effluent volume coincided irrespective of the flow-rate, i.e. the residence time (55–220 s) of the solution across the membrane (thickness 0.83 mm), as a result of negligible mass transfer resistance. A series of chromatographic procedures, (adsorption—washing—elution) was repeated twice and a satisfactory quantitative elution was attained. The reproducible profile of the flux and the protein concentration assured a quantitative cycle of chromatography using the affinity membrane containing Trp as a ligand.

Diffusion of carbon dioxide in tomato fruits during cold storage in modified atmosphere

SummaryTreatments with 20% CO2 of tomato fruit produced a beneficial extension of storage life. The diffusion of CO2 in tomato (Lycopersicon esculentum Mill) cv. Platense was measured chromatographically and the concentration profiles in the internal gaseous atmosphere of the fruit were determined for non‐steady state conditions to establish minimum treatment times that can ensure adequate penetration of the gas.The effective diffusivity of CO2 in the tissue was determined by fitting concentration data, obtained from experiments with whole fruit without skin, to the non‐steady state solution of the diffusion equation, assuming negligible external mass transfer resistance. The diffusivity was modelled using Maxwell‐Eucken expressions, taking account of the heterogeneous structure of the tissue.The resistance of the fruit skin was evaluated using whole fruits with the stem scar covered with paraffin and fitting experimental data to the solution in terms of mass Biot number; the specific resistance of the peel was some 200 times greater than that of the stem scar.

Continuous extraction loop reactor (CELR): Alcoholic fermentation by fluidized entrapped biomass

For the study of the continuous fermentation in a loop bioreactor, the entrapment and the activity of cells into a Ca-alginate matrix and the solvent extractive fermentation were considered. These proposals had already been presented as single ideas and sometimes supported by theoretical approaches. The initial tests, performed for more then a month of work, seem indicate that the extractive fermentation increases the specific production rate, but the alginate matrix shows many serious problems: a low mechanical resistance, the leakage of viable cells, a not negligible internal mass transfer resistance and the decrease in viability of the entrapped microorganisms. The reported results concern only the initial part of the research and need to be integrated with data of further research.