Effect Of Mass Transfer Limitations Research Articles

Operando characterisation of the products of Fischer-Tropsch synthesis in a fixed-bed reactor studied by magnetic resonance

Operando measurement of catalyst performance presents many difficulties as it is necessary to obtain measurements at reaction temperature and pressure and also at relevant length scales of multiple pellets to replicate the pellet-scale mass transport. Non-invasive measurement under such conditions is challenging. In this study we have developed an NMR/MRI compatible reactor that is of industrially-relevant scale and can operate at typical conditions for Fischer-Tropsch synthesis (FTS). The pilot-scale fixed-bed reactor (20 mm i.d.) used for FTS is operated at 220 °C and 37 bar and is packed with 1 wt % Ru/TiO2 catalyst pellets. Multiple magnetic resonance techniques have been developed and applied to achieve quantitative characterisation of product species including hydrocarbons, water, oxygenates and branched alkanes in pores. In addition to spatially-resolved measurements of molecular diffusion coefficients of the product species from which the carbon chain length of the saturated hydrocarbons can be determined, preliminary estimates of the relative amount of oxygenate and water species and branched saturated hydrocarbons present within the pore space of the catalyst are also determined using non spatially-resolved two-dimensional 1H T1-chemical shift and two-dimensional 1H double quantum filtered correlated spectroscopy (DQF-COSY), respectively. The concentrations of water and oxygenates detected inside the pores of catalyst pellets decrease when the feed H2/CO ratio decreases. The amount and extent of branching of saturated hydrocarbon species is observed to be higher than for the effluent wax product, most likely due to the effect of mass transfer limitation within the reactor. This work demonstrates that a toolbox of operando magnetic resonance techniques is available to characterise different product species of FTS from within catalyst pellets. The information provided by these techniques can give insight into how both catalyst and reactor can be optimised.

Controlling the microbial competition between hydrogenotrophic methanogens and homoacetogens using mass transfer and thermodynamic constraints

The reduction of CO2 allows the synthesis of platform molecules for the chemical and energy industry. Anaerobic microbial consortia contain homoacetogenic microorganisms (HAC) capable of reducing CO2 to acetate. However, one of the obstacles to their use is the understanding and management of their functional diversity. In particular, managing the competition between HAC and hydrogenotrophic methanogens (HM) that convert CO2 into methane is crucial to selectively produce acetate.This study contributes to bring new knowledge on the competition between HAC and HM. This microbial competition is encountered in numerous anaerobic systems, and it is necessary to know how to manage it. In this sense, mass transfer between the gas phase where the substrates are located, and the liquid phase which contains the microbial catalysts, as well as kinetic and thermodynamic aspects of biological reactions have been integrated in this work. The microbial competition between HM and HAC was studied in successive batches. The effect of temperature between 25 °C and 35 °C was investigated, as well as different states of mass transfer limitation in the system.A clear effect of temperature between 25 °C and 35 °C on the outcome of the competition between HM and HAC was highlighted, as well as the effect of mass transfer limitation. Enrichment of Acetobacterium homoacetogens and elimination of hydrogenotrophic methanogens was possible at 25 °C without mass transfer limitation. Acetate product selectivity was of 100% by the end of the enrichment period in successive batches. On the contrary, under mass transfer limitation, and/or at 35 °C, Methanobacterium hydrogenotrophic methanogens were promoted with 100% of methane selectivity by the end of the enrichment. This study contributes to identify specific process parameters influencing the selection of HAC over HM and should help in the design of experiments depending on the target product from H2/CO2. Furthermore, the results obtained could be applied to continuous acetate producing systems, at a larger scale, and they can also be useful in other gas fermentation systems and processes.

Investigation on the coupling effects of mass transfer limitation and the presence of ice on methane hydrate dissociation inside pore

The exploitation of natural gas hydrates has bottlenecks such as low gas production efficiency and poor sustainability. The scientific issues behind this bottleneck are the continuous mass transfer of methane in the water and the presence of ice phase. A model is proposed to investigate the characteristics of mass transfer limitation and ice formation/melting during methane hydrate dissociation and their effects on hydrate dissociation. A numerical simulation has been carried out on the experimental research, and the reliability of proposed model has been validated by comparing with the experiment data, and consistent results have been obtained. Cumulative methane generation, methane generation rate and volume fraction of phases at local location, distribution of pressure, temperature and phases inside pore were clarified. The findings showed that the dissociation of methane hydrate is very complicated in the presence of the ice phase, and the effect of mass transfer limitation on the hydrate dissociation has become more complicated. These two factors have a great impact on the dissociation rate of methane hydrate. The original pressure and temperature distribution changes due to the ice formation/melting, resulting in the difference in the pressure and temperature at the inner and outer layer inside the pore. Ice formation in the inner layer weakens the delaying and limiting effect of mass transfer limitation on the hydrate dissociation, and ice melting in the outer layer promotes the delaying and limiting effect of mass transfer limitation on the dissociation of the hydrate. The presence of the ice phase weakens the mass transfer limitation effect.

Biodegradation dynamics disparity between n‐alkanes and polycyclic aromatic hydrocarbons in the vadose zone: A critical review

AbstractIn the shallow terrestrial subsurface, bioavailability often controls biodegradation rates of various hydrophobic organic pollutants, such as hydrocarbons. Microorganism‐mediated depletion of n‐alkanes and polycyclic aromatic hydrocarbons (PAHs) in the vadose zone proceeds with distinctively different rates and kinetics. The observed disparity in biodegradation dynamics between these two classes of hydrocarbons are described here in terms of the Best equation, which allows for a complex process of hydrocarbon contaminant depletion to be explained in terms of the physically relevant processes. This equation combines terms describing contributions of two key elements of hydrocarbon biodegradation—the intrinsic catabolic activity of microorganisms and the mass transfer process delivering substrate to active microorganisms. If substrate supply is slow, biodegradation process is availability‐limited because microorganisms consume the substrate faster than it can be delivered. Conversely, when the mass transfer of substrate is faster than the microbial consumption, biodegradation rate tends to be controlled by metabolic kinetics.The evaluation conducted in this study ascertains that the critical factor limiting the rate and extent of PAH biodegradation is the mass transfer rather than the intrinsic microbial activity that appears to govern the rate of n‐alkane biodegradation. The specifics of mass transfer are determined by the magnitude of the substrate–soil interactions. Whereas sorption of n‐alkanes is controlled by weak hydrophobic and van der Waals forces, PAHs tend to bind to the soil matrix by much stronger electron‐donor–acceptor interactions. Stronger interactions between aromatic compounds and soil components translate into the higher activation energies for desorption of PAHs and therefore a slower mass transfer process. This aspect of the substrate–soil interaction explains a more prominent effect of mass transfer limitation on biodegradation dynamics of PAHs compared to that of the corresponding n‐alkanes.

The effect of mass transfer resistance on the kinetics of catalytic hydrodesulfurization of thiophene over Pt/Al2O3

In kinetic study of reactions carried out in the presence of heterogeneous catalyst, it is very important to study the effects of mass transfer rate firstly which can be effective in adding to chemical kinetic. The present work investigates the catalytic hydrodesulfurization (HDS) of thiophene over Pt/ Al2O3, to determine the effect of interface mass transfer resistance. The experimental work was performed in a HDS continuous flow fixed bed catalytic reactor unit consisted of a carbon steel tube (2.54 cm diameter, and 60 cm length), located in the laboratories of chemical engineering department, Baghdad University. The objective of the work was to investigate effect of mass transfer limitations (internal diffusion and external diffusion) in catalytic reaction of thiophene HDS. It was verified that mass transfer resistances has a small effect on the reaction rate.

Mass transport limitations in microchannel methanol-reforming reactors for hydrogen production

The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.

Mathematical modeling of CO oxidation on Pd(100) at near-atmospheric pressures: Effect of mass-transfer limitations

A 3D convection-diffusion-reaction model was developed to describe CO oxidation in a continuous-flow catalytic reactor containing a Pd(100) single crystal surface. The model was studied with the help of the pseudo-arclength continuation algorithm, which is based on a matrix-free Newton–Krylov method and enables a one-parameter continuation of stationary solutions of large systems. The model was used to simulate the 3D spatial distributions of CO and CO2 during “light-off” experiments and the oscillations in CO oxidation over Pd(100) detected by the planar laser-induced fluorescence (PLIF) method. With realistic values of parameters the developed model can reproduce almost quantitatively the experimental reaction rates and the PLIF images measured under steady-state conditions and during self-sustained oscillations under near-atmospheric pressure conditions. The formation of a boundary layer and the essential decrease of CO concentration near the Pd(100) single crystal surface were demonstrated after the catalytic ignition and in a high activity branch of the oscillatory cycle indicating the mass-transfer limited regime.

Diffusion mechanism and effect of mass transfer limitation during the adsorption of CO2 by polyaspartamide in a packed-bed unit

ABSTRACTA systematic study of the diffusion mechanism and effect of mass transfer limitation during the adsorption of CO2 onto polyaspartamide is presented using a differential adsorption bed method, carried out in a 100 × 60 × 40 mm packed-bed adsorption unit. The rate-limiting step where mass transfer limitation is dominant was studied using diffusion models. It was observed that intraparticle diffusion mechanism is the major contributor to the resistance offered to the transport of gas molecule through polyaspartamide. The behaviour of polyaspartamide, based on the intraparticle diffusion rate parameter derived from the plots of CO2 adsorbed versus the square root of time, signified that the adsorption mechanism involved both film and intraparticle diffusion. The intraparticle diffusion parameter (kid) obtained was dependent on temperature as well as intraparticle convection effects and ranged from 1.24 × 10−4 to 2.13 × 10−4 ms−1. The Biot number (Bi) values were all greater than 10 (ranged from 17.80 – 30.74), confirming that the intraparticle diffusion was the rate-limiting step and heat transfer is more by conduction from the gas film layer than convection within the pores of polyaspartamide. Results from this study provide an important basis for future scale-up and optimisation of CO2 capture process using polyaspartamide.

Hydrolytic enzymes in sewage sludge treatment: A mini-review

Biological wastewater treatment processes can be classified as either aerobic or anaerobic. These two biological treatment processes are each characterised by groups of micro-organisms and their associated enzymes. Hydrolytic enzymes secreted by these micro-organisms are vital for the rate-limiting step of hydrolysis in the treatment of highly polymeric substrates present in sewage sludge. In this mini-review, the effects of mass transfer limitation, metabolic intermediates, extracellular polymeric substances (EPS), electron acceptor conditions and pH and temperature on the activity of these enzymes are summarised. The most salient and current perspectives of the significance and the role that hydrolytic enzymes play in sewage sludge treatment are highlighted.Keywords: EPS, floc, hydrolases, pH, sewage, sludge, temperature

Hygroscopic behavior of atmospheric aerosols containing nitrate salts and water-soluble organic acids

Abstract. While nitrate salts have critical impacts on environmental effects of atmospheric aerosols, the effects of coexisting species on hygroscopicity of nitrate salts remain uncertain. The hygroscopic behaviors of nitrate salt aerosols (NH4NO3, NaNO3, Ca(NO3)2) and their internal mixtures with water-soluble organic acids were determined using a hygroscopicity tandem differential mobility analyzer (HTDMA). The nitrate salt ∕ organic acid mixed aerosols exhibit varying phase behavior and hygroscopic growth depending upon the type of components in the particles. Whereas pure nitrate salt particles show continuous water uptake with increasing relative humidity (RH), the deliquescence transition is still observed for ammonium nitrate particles internally mixed with organic acids such as oxalic acid and succinic acid with a high deliquescence point. The hygroscopicity of submicron aerosols containing sodium nitrate and an organic acid is also characterized by continuous growth, indicating that sodium nitrate tends to exist in a liquid-like state under dry conditions. It is observed that in contrast to the pure components, the water uptake is hindered at low and moderate RH for calcium nitrate particles containing malonic acid or phthalic acid, suggesting the potential effects of mass transfer limitation in highly viscous mixed systems. Our findings improve fundamental understanding of the phase behavior and water uptake of nitrate-salt-containing aerosols in the atmospheric environment.

Identifying and overcoming the effect of mass transfer limitation on decreased yield in enzymatic hydrolysis of lignocellulose at high solid concentrations

Cellulose conversion decreases significantly with increasing solid concentrations during enzymatic hydrolysis of insoluble lignocellulosic materials. Here, mass transfer limitation was identified as a significant determining factor of this decrease by studying the hydrolysis of delignified corncob residue in shake flask, the most used reaction vessel in bench scale. Two mass transfer efficiency-related factors, mixing speed and flask filling, were shown to correlate closely with cellulose conversion at solid loadings higher than 15% DM. The role of substrate characteristics in mass transfer performance was also significant, which was revealed by the saccharification of two corn stover substrates with different pretreatment methods at the same solid loading. Several approaches including premix, fed-batch operation, and particularly the use of horizontal rotating reactor were shown to be valid in facilitating cellulose conversion via improving mass transfer efficiency at solid concentrations higher than 15% DM.

Pore‐scale network modeling of microbially induced calcium carbonate precipitation: Insight into scale dependence of biogeochemical reaction rates

AbstractThe engineering of microbially induced calcium carbonate precipitation (MICP) has attracted much attention in a number of applications, such as sealing of CO2 leakage pathways, soil stabilization, and subsurface remediation of radionuclides and toxic metals. The goal of this work is to gain insight into pore‐scale processes of MICP and scale dependence of biogeochemical reaction rates. This will help us develop efficient field‐scale MICP models. In this work, we have developed a comprehensive pore‐network model for MICP, with geochemical speciation calculated by the open‐source PHREEQC module. A numerical pseudo‐3‐D micromodel as the computational domain was generated by a novel pore‐network generation method. We modeled a three‐stage process in the engineering of MICP including the growth of biofilm, the injection of calcium‐rich medium, and the precipitation of calcium carbonate. A number of test cases were conducted to illustrate how calcite precipitation was influenced by different operating conditions. In addition, we studied the possibility of reducing the computational effort by simplifying geochemical calculations. Finally, the effect of mass transfer limitation of possible carbonate ions in a pore element on calcite precipitation was explored.

Design of an Air-Sparged Tubular Photocatalytic Reactor for the Degradation of Methylene Blue: Mass-Transfer Limitation Studies

An alternative process for the removal of organic pollutants in aqueous systems is photocatalysis. The challenges hindering its industrial use are electron-hole recombination and mass-transfer limitations. In order to address these problems, the objective of this study is to introduce air by sparging, and design an air-sparged photocatalytic reactor using titanium dioxide immobilized on borosilicate glass. The performance of the reactor on the removal of the model pollutant, methylene blue (MB), was evaluated and compared against the reactor operated without sparging. The effect of mass-transfer limitations on reactor performance was also investigated by regression using a Langmuir-type model equation. The sparged photocatalytic reactor was able to degrade 57% MB in 2 hours, an improvement of 40% compared to no sparging, and is comparable to similar reactors in literature, but with the advantage of using less expensive materials of construction and simpler immobilization technique. Mass-transfer limitation studies showed a good fitting of the initial reaction rate r , with r = 0.1399 Q / (0.6120 + Q ) for the sparged operation, and Q is the volumetric flowrate of water (L/min). The model also shows that the reactor operates near the reaction-limited regime, and that the extent of mass-transfer limitation effects was reduced by the present reactor.

On the upscaling of mass transfer rate expressions for interpretation of source zone partitioning tracer tests

AbstractAnalysis of partitioning tracer tests conducted in dense nonaqueous phase liquid (DNAPL) source zones relies on conceptual models that describe mass exchange between the DNAPL and aqueous phases. Such analysis, however, is complicated by the complex distribution of entrapped DNAPL mass and formation heterogeneity. Due to parameter uncertainty in heterogeneous regions and the desire to reduce model complexity, the effect of mass transfer limitations is often neglected, and an equilibrium‐based model is typically used to interpret test results. This work explores the consequences of that simplifying assumption on test data interpretation and develops an alternative upscaled modeling approach to quantify effective mass transfer rates. To this end, a series of partitioning tracer tests is numerically simulated in heterogeneous two‐dimensional PCE‐DNAPL source zones, representative of a range of hydraulic conductivity and DNAPL mass distribution characteristics. The effective mass transfer coefficient corresponding to each test is determined by fitting an upscaled model to the simulated data, and regression analysis is performed to explore the correlation between various source zone metrics and the effective mass transfer coefficient. Results suggest that vertical DNAPL spreading, Reynolds number, pool fraction, and the effective organic phase saturation are the most significant parameters controlling tracer partitioning rates. Finally, a correlation for prediction of the effective (upscaled) mass transfer coefficient is proposed and verified using existing experimental data. The developed upscaled model incorporates the influence of physical heterogeneity on the rate of tracer partitioning and, thus, can be used for the estimation of source zone mass distribution characteristics from tracer test results.

Effect of mass transfer limitations on catalyst performance during reduction and carburization of iron based Fischer-Tropsch synthesis catalysts

Existence of intraparticle mass transfer limitations under typical Fischer-Tropsch synthesis has been reported previously, but there is no suitable study on the existence of intraparticle diffusion limitations under pretreatment steps (reduction and activation) and their effect on catalytic performance for iron based catalysts. In this study, Fe-Cu-La-SiO2 catalysts were prepared by co-precipitation method. To investigate the intraparticle mass transfer limitation under reduction, activation and reaction steps, and its effect on catalytic performance, catalyst pellets with different sizes of 6, 3, 1 and 0.5 mm have been prepared. All catalysts were calcined, pretreated and tested under similar conditions. The catalysts were activated in hydrogen (5% H2 in N2) at 450 °C for 3 h and exposed to syngas (H2/CO = 1) at 270 °C and atmospheric pressure for 40 h. Afterwards, FTS reaction tests were performed for approximately 120 h to reach steady state conditions at 290 °C, 17 bar and a feed flow (syngas H2/CO= 1) rate of 3 L/h (STP). Using small pellets resulted in higher CO conversion, FT reaction rate and C5+ productivity as compared with larger pellets. The small pellets reached steady state conditions just 20 h after starting the reaction. Whereas for larger pellets, CO conversion, FT reaction rate and C5+ productivity increased gradually, and reached steady state and maximum values after 120 h of operation. The results illustrate that mass transfer limitations exist not only for FTS reaction but also for the reduction and carburization steps which lead to various phase formation through catalyst activation. Also the results indicate that some effects of mass transfer limitations in activation step, can be compensated in the reaction step. The results can be used for better design of iron based catalyst to improve the process economy.

Comparison of Novozyme 435 and Purolite D5081 as heterogeneous catalysts for the pretreatment of used cooking oil for biodiesel production

The catalytic performance of two types of catalysts, an ion-exchange resin, Purolite D5081 and an immobilised enzyme, Novozyme 435, was compared for the esterification pretreatment of used cooking oil (UCO) for the preparation of biodiesel. The reactions were carried out using a jacketed batch reactor with a reflux condenser. The effect of mass transfer limitations was investigated and it was shown that internal and external mass transfer limitations were negligible. An immobilised enzyme, Novozyme 435, was investigated because it has been shown to give high free fatty acids (FFAs) conversion. This catalyst has been compared to an ion-exchange resin, Purolite D5081, which was developed for the esterification of UCO for the production of biodiesel. It was found that a conversion of 94% was achieved using Purolite D5081 compared to 90% conversion with Novozyme 435. However, the optimum methanol to FFA ratio for Purolite D5081 was 98:1 compared to 6.2:1 for Novozyme 435. In addition, it has been found that with Novozyme 435 there are side reactions which result in the formation of additional fatty acid methyl esters (FAMEs) and FFAs at longer reaction times.

The influence of diffusion phenomena on catalysis: A study at the single particle level using fluorescence microscopy

Mass transfer phenomena are known to play a crucial rule in porous heterogeneous catalysts. When intraparticle diffusion is too slow to provide a sufficient flux of reactants to the inner parts of the catalyst particles, substrate depletion occurs and only the outer parts of the particles are efficiently used for catalysis. Quantifying the degree of intraparticle diffusion limitation by means of the Thiele modulus and the related effectiveness factor remains a big challenge. Only few techniques combine the necessary spatial resolution and sensitivity to measure in situ the concentration profiles of organic products and/or reagents inside the catalytic particles. In this contribution a brief overview is given of in situ techniques that allow such space-resolved activity profiling at the single particle level and their strengths and weaknesses are highlighted. The superior sensitivity of fluorescence microscopy is exploited to study the effects of mass transfer limitations on the overall catalytic efficiency in a spatially resolved fashion. The epoxidation of bulky substrates over mesoporous Ti-MCM-41 was chosen as a model case. It is shown how individual reaction events inside the Ti-MCM-41 particles are visualized and localized and how the obtained reactivity maps yield direct information on the Thiele modulus and effectiveness factor of this specific intraparticle diffusion limited catalytic process. Moreover, a value for the intraparticle diffusion constant and the intrinsic rate constant of the catalytic reaction could be estimated based on the data obtained by one in situ experiment. The presented method is versatile and applicable to a variety of catalytic systems, provided that a suitable fluorogenic probe molecule is used, which preferably is detectable at the single molecule level after catalytic conversion.

Stochastic Simulation of Imperfect Mixing in Free Radical Polymerization

AbstractA modified algorithm for the stochastic simulation of chemical reactions subject to mass transfer limitation (imperfect mixing) is presented. This algorithm takes into account the mixing by diffusion of the reacting species between two consecutive reactions. The method is used to simulate the effect of mass transfer limitation in free‐radical polymerization. Since this is a stiff reaction network, a hybrid stochastic‐deterministic approach is considered. The hybrid stochastic algorithm under imperfect mixing (HSSA‐IM) is applied to the bulk polymerization of methyl methacrylate up to high conversions. The accuracy of the algorithm relies on the precise determination of diffusion coefficients during the reaction.

Optimized interesterification of virgin olive oil with a fully hydrogenated fat in a batch reactor: Effect of mass transfer limitations

AbstractThe lipase‐catalyzed interesterification of virgin olive oil and fully hydrogenated palm oil (FHPO) was studied in a batch reactor operating at 75 °C. The reactions between olive oil {rich in OOO (32.36%), OPO (21.7%) and OLO (11.6%) [L = linoleic; O = oleic; P = palmitic acid]} and the fully hydrogenated fat {(36.5% PSP, 28.8% PPP, 23.2% SPS) [S = stearic acid]} produced semi‐solid fats. For an initial weight ratio of olive oil to FHPO of 60 : 40, the reaction product is a complex mixture of triacylglycerol (TAG) species. The TAG profile of the fat product is time dependent. Because of the high viscosity of the liquid reagent phase, it was important to determine if mass transfer effects were significant. Hence, the reaction was optimized with respect to the type and speed of agitation employed, temperature, use of solvent, and the type of biocatalyst. Three immobilized lipases [from Thermomyces lanuginosus (TL IM), Rhizomucor miehei (RM IM) and Candida antarctica B (Novozym 435)] were compared as catalysts for the interesterification reaction. Equilibrium is reached four times faster (in 1–4 h) with a magnetic stirrer to provide agitation than when agitation is not sufficient, i.e. when orbital agitation is employed. Equilibrium was reached faster with Lipozyme TL IM than with the other two lipases. The effects of all the factors investigated on the composition of the products have also been determined. Semi‐solid fats obtained with the non‐specific Novozym 435 contain levels of unsaturated fatty acid residues on sn‐2 sites that are similar to the products obtained with the 1(3)‐regiospecific enzymes Lipozyme TL IM and RM IM. The chemical properties of the product semi‐solid fat were characterized. The fat prepared using optimal reaction conditions contained 17.20% OPO, 13.61% OOO, 11.09% POP, and 10.35% OSP isomers as the primary products. The induction time obtained in the assay of the oxidative stability of the fat product was 21 h at 98 °C. The lipases Lipozyme TL IM and Novozym 435 were very stable with residual activities of 90 and 100%, respectively, after 15 batch reaction cycles.

Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen

The conceptual feasibility of a packed bed membrane reactor for the autothermal reforming (ATR) of methane for the production of ultrapure hydrogen was investigated. By integrating H 2 permselective Pd-based membranes under autothermal conditions, a high degree of process integration and intensification can be accomplished which is particularly interesting for small scale H 2 production units. A two-dimensional pseudo-homogeneous packed bed membrane reactor model was developed that solves the continuity and momentum equations and the component mass and energy balances. In adiabatic operation, autothermal operation can be achieved; however, large axial temperature excursions were seen at the reactor inlet, which are disadvantageous for membrane life and catalyst performance. Different operation modes, such as cooling the reactor wall with sweep gas or distributive feeding of O 2 along the reactor length to moderate the temperature profile, are evaluated. The concentration polarisation because of the selective hydrogen removal along the membrane length was found to become significant with increasing membrane permeability thereby constraining the reactor design. To decrease the negative effects of mass transfer limitations to the membrane wall, a small membrane tube diameter needs to be selected. For a relatively small ratio of the membrane tube diameter to the particle diameter, the porosity profile needs to be taken into account to prevent overestimation of the H 2 removal rate. It is concluded that autothermal production of H 2 in a PBMR is feasible, provided that the membranes are positioned outside the inlet region with large temperature gradients.