Uniformity in a thin-zone multi-pulse TAP experiment: numerical analysis

The thin-zone TAP reactor (TZTR) model of a multi-pulse experiment is computationally validated based on a more general three-zone reactor model. The analysis is focused on the uniformity of gaseous and surface concentrations in the catalyst zone, which is a key property of TZTR model. It is shown that if the TZTR model is valid for the first pulse in a multi-pulse experiment then it is valid for all subsequent pulses. For a typical reactor packing (the ratio of the thin-zone thickness to the length of reactor is 1/30) and with the first pulse conversion up to 97%, the gaseous and surface concentration profiles can be considered uniform and characterized by their spatial average values only. The reaction rate in the catalyst zone may also be characterized by its spatial average value and directly related to the spatial average gaseous and surface concentrations, in the same way as an elementary rate is related to concentrations. As a result of these unique characteristics, the TZTR may be considered a “perfectly-mixed” reactor even at high conversion.     

“State defining” experiment in chemical kinetics—primary characterization of catalyst activity in a TAP experiment

This paper presents a new strategy, “state-by-state transient screening”, for kinetic characterization of states of a multicomponent catalyst as applied to TAP pulse-response experiments. The key idea is to perform an insignificant chemical perturbation of the catalytic system so that the known essential characteristics of the catalyst (e.g. oxidation degree) do not change during the experiment. Two types of catalytic substances can be distinguished: catalyst state substances, which determine the catalyst state, and catalyst dynamic substances, which are created by the perturbation. The general methodological and theoretical framework for multi-pulse TAP experiments is developed, and the general model for a one-pulse TAP experiment is solved. The primary kinetic characteristics, basic kinetic coefficients, are extracted from diffusion-reaction data and calculated as functions of experimentally measured exit-flow moments without assumptions regarding the detailed kinetic mechanism. The new strategy presented in this paper provides essential information, which can be a basis for developing a detailed reaction mechanism. The theoretical results are illustrated using furan oxidation over a VPO catalyst.     

A general formula for reactant conversion over a single catalyst particle in TAP pulse experiments

We obtain a general formula for reactant conversion in diffusion-reaction TAP systems over single non-porous catalyst particles as a product of two terms: α=P_H×K.The first term, P_H, is a purely geometric factor dependent only on the shape of the reactor and the shape and position of the catalyst particle, interpreted as the probability that an individual molecule of reactant will hit the catalyst before leaving the reactor. The second term, K=kτ_H/(1+kτ_H), is a geometrical/chemical term involving the kinetic constant k and a transport characteristic (residence time in the catalyst zone, τ_H). Both P_H and τ_H can be effectively calculated for any given reactor-particle configuration. Our formula greatly extends the validity of a formula given in Shekhtman et al. [1999. Thin-zone TAP-reactor—theory and application. Chem. Eng. Sci. 54, 4371-4378] for thin-zone TAP-systems. It is derived by a probabilistic analysis of the residence time of individual molecular trajectories in chemically active regions. Our results are based on the theory previously developed in our paper [Feres, R., Yablonsky, G.S., Mueller, A., Baernstein, A., Zheng, X., Gleaves, J., 2009. Probabilistic analysis of transport-reaction processes over catalytic particles: theory and experimental testing. Chem. Eng. Sci. 64, 568-581].     

Modeling of fast pulse responses in the Multitrack: an advanced TAP reactor

A detailed transport model for the Multitrack setup, a TAP-like system, has been developed, which allows further analysis of adsorption, diffusion and catalysis phenomena. This includes the transport in the void part between the pulse valve and the reactor inlet. The effects of viscous flow and thermal transpiration, aspects that have not been studied in detail before for this type of setup, have been analyzed. A new expression for the modeling of the output signal is proposed depending on the positioning of the MS detector used in the study. The transport parameters of the model have been estimated by the analysis of the experimental pulse responses of the empty reactor system and the reactor charged with an inert packed bed. The proposed model reproduces the experimental pulse responses very well, and therefore can be extended to study systems with reacting or adsorbing beds by including the corresponding rate equations for the processes occurring in the bed of particles.     

Determination of pitting initiation of duplex stainless steel using potentiostatic pulse technique

Duplex stainless steel simultaneously comprises various phases, inclusions, precipitates and grain boundaries. It is full of academic and valuable significance for materials designing and processing optimization to ascertain the corrosion resistance of corresponding phases and various microstructures. However, up to date, efficient evaluation technique is still scare mainly due to difficulty in controlling of micropits. In this work, for the first time, optimized potentiostatic pulse technique (PPT) was established and applied to study the rule of initiation of pits of solution annealed and aged DSS in 1.0 mol/L NaCl solution. The results showed that the size of pits can be well-controlled by adjusting the condition of potential pulse. Combined with SEM/EDS system observation, it was found that for solution-annealed specimens, sulphide and oxide mixed inclusions were the preferential pit initiation sites, while for aged specimens, majority pits occurred at inclusions in area impoverished in Cr/Mo, which is vicinal to σ phase. The phenomena that growing pits is retarded by precipitation rich in Cr/Mo, i.e. σ phase, was testified and explained.     

Effect of catalyst activity in SMR-SERP for hydrogen production: Commercial vs. large-pore catalyst

In this work, we have evaluated the performance of an SMR-SERP unit (steam methane reforming sorption enhanced reaction process), using two different Ni/Al₂O₃ catalysts: commercial “Octolyst 2001” from Degussa and a large-pore catalyst (Catalyst A). The selective CO₂ sorbent was a potassium modified hydrotalcites. Several experiments were performed under different operating conditions to validate a mathematical model.Experimental results show that the Degussa catalyst is more active and more selective to CO₂ producing hydrogen with higher purity and less CO than the large-pore catalyst. Cyclic SMR-SERP experiments were also performed. The cycles comprise four different steps: reaction, depressurization, reactive regeneration and pressurization. In the cyclic experiments, conversion was 43% higher than in an SMR reactor, while H₂ purity was 75%, which is 25% higher than in normal SMR operation. Results indicate that more active catalysts also promote a better reactive regeneration optimizing the use of part of the product (H₂). The proposed mathematical model was validated in a wide range of operating conditions and in a cyclic experiment. The model was able to describe the SMR-SERP experiments without any fitting parameters.     

Rate measurement by pulse technique for individual steps involved in CO hydrogenation and its application to catalyst design

By using pulse reaction technique, the rates were measured for the individual steps involved in CO hydrogenation over transition metal catalysts with and without promoters. On all the transition metals other than Rh, the rate constant for the dissociation of C-O bond (k ₁) was much smaller than that for the hydrogenation of surface carbon species (k ₂). The oxides of V, Nb, Mo, and W added to Ru/Al₂O₃ increasedk ₁ and decreasedk ₃. The Rh catalyst was unique in the sense that there was not much difference betweenk ₁ andk ₂. The characteristic feature observed was found to be useful in designing a catalyst for the production of liquid fuel or C₂-oxygenates from syngas.     

Synthesis of esters: Development of the rate expression for the Dowex 50 Wx8-400 catalyzed esterification of propionic acid with 1-propanol

The kinetics of the esterification reaction of propionic acid with 1-propanol over the ion-exchange resin Dowex 50Wx8-400 has been studied in this investigation. Kinetic experiments were conducted using a 1 L Lab-Max system at a stirrer speed of 900 rpm over a temperature range of 303.15 -333.15 K. The catalyst loading was varied from 10 to 60 g dry cat/L and acid to alcohol molar ratios of 1:1, 1:2, 1:4, 2:1 and 4:1 were employed. The equilibrium constants for this reaction were determined in separate experiments at 303.15, 313.15 and 323.15 K. The values were equal to 33.18, 30.62 and 28.37, respectively, with a standard enthalpy change of reaction of 6.4 kJ/mol. These values show the reaction to be mildly exothermic. It was found that both external and internal diffusion limitations did not affect the overall reaction rate. The conversion of propionic acid increased with increasing temperature and catalyst loading and decreased with increasing initial mole fraction of acid. The increase in chain length of acid or alcohol or branching had a retarding effect on the conversion. Several kinetic models were tested to correlate the kinetic data, the pseudo-homogeneous (P-H) model, the Eley-Rideal (E-R) model, the Langmuir-Hinshelwood (L-H) model, the modified Eley-Rideal (M-E-R) model and the modified Langmuir-Hinshelwood (M-L-H) model. In all models, the activity coefficients were estimated using UNIFAC to account for the non-ideal thermodynamic behavior of reactants and products. A correction factor for the resin affinity for water (α) was used in both M-E-R and M-L-H models. The above models predicted the kinetic behavior of the studied system with an overall error ranging from 1.65% to 13.32%. Water was found to be more strongly adsorbed than other species present in the system. The M-E-R model between adsorbed 1-propanol and non-adsorbed propionic acid which assumes surface reaction as the rate controlling step, with α equal to 2, was found to be the best model with the least overall error (1.65%). The activation energy for the esterification was estimated to be 67.3 kJ/mol by this model.     

When the final catalyst activity profile depends only on the total amount of admitted substance: Theoretical proof

It is shown, based on pulse‐response experiments, that under special conditions the activity profile of a prepared catalytic system depends only on the total amount of admitted substance. This property, previously found computationally, is here established mathematically for porous and nonporous catalysts in different pulse reactors. This result can be used as a theoretical guidance for the design of systems or materials with an optimal activity profile, in particular a catalyst bed or a catalyst particle. Consequently, it can be used for understanding and developing the different diffusion‐reaction processes, e.g., wet impregnation, deactivation of active materials, and so forth. © 2014 American Institute of Chemical Engineers AIChE J, 61: 31–34, 2015     

Pillared smectite modified with carbon and manganese as catalyst for SCR of NOx with NH3. Part I. General characterization and catalyst screening

Carbon- and manganese-modified zirconia-pillared smectites were prepared, characterized (XRD, BET and pore analysis, XPS) and tested in selective catalytic reduction of NO_x with NH₃. Both untreated and acidic pretreated smectites were used. The acid pretreatment increased NO conversion and influenced the extent of carbon introduction into the porous system. The carbon deposit improved selectivity of the catalytic reduction to N₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optimization of the activity of CaO/Al2O3 catalyst for biodiesel production using response surface methodology

In this work the response surface methodology (RSM) in conjunction with the central composite design (CCD) were used to optimize the activity of CaO/Al₂O₃ solid catalysts for the production of biodiesel. In order to measure the catalyst activity, we used palm oil as a representative raw material for the conversion to biodiesel. The biodiesel production was carried out in a batch laboratory scale reactor. The results showed that both the calcination temperature and the amount of calcium oxide loaded on the support had significant positive effects on the biodiesel yield. The maximum basicity and biodiesel yield obtained were about 194 μmol/g and 94%, respectively. Overall, the catalyst showed high performance at moderate operating conditions and its activity was maintained after two cycles.     

Synthesis, characterization and catalytic activity of an ultrafine Pd/C catalyst for formic acid electrooxidation

An ultrafine Pd/C catalyst with a uniformly sized and highly dispersed nanostructure was synthesized by an improved liquid phase reduction method; in this process, a complexone (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, C_yDTA) was used as an alternative stabilizer for the first time. Physicochemical characterizations indicated that the resulting Pd nanoparticles possessed ideal structural characteristics, including an average diameter of 2.1 nm, narrow size distribution ranging from 0.5 to 4.0 nm, no visible agglomerations, and no residual C_yDTA. Electrochemical tests showed that the catalytic activity of the obtained Pd/C catalyst for formic acid electrooxidation was 2.2 times greater than that of Pd/C catalyst prepared in the absence of C_yDTA. This improvement in the electrocatalytic performance was attributed to the uniformly sized and highly dispersed nanostructure, which provided a larger overall electrochemical active surface area.     

合成醇醚羧酸的Pt/C催化剂催化活性研究及表征

通过浸渍法结合液相还原法将金属Pt,Ni和Sn负载在活性炭上制备了多组分Pt/C催化剂,并应用于催化氧化直接制备醇醚羧酸的反应中,考察了催化剂组分不同对脂肪醇聚氧乙烯醚(AEO9)氧化法制备醇醚羧酸(AE9C)过程中催化性能的影响;采用TEM,XRD和XPS对催化剂进行了表征。结果表明,在Pt/C催化剂中加入Ni和Sn后,Pt的颗粒粒径变小,分散性更好,Pt0的含量增加,从而提高了催化剂的活性。多组分Pt/C催化剂与单一组分Pt/C催化剂相比,AE9C产率提高了27.54百分点。     

Complex physico-chemical characterization and kinetic studies on the reduction and activity of catalyst for ammonia synthesis regenerated by electric-arc plasma

A comprehensive physico-chemical characterization is carried out and the reduction kinetic and activity are studied of samples of CA-1-type catalyst for ammonia synthesis regenerated in quasi-equilibrium electric-arc low-temperature plasma. The experimental conditions are described for the plasma-chemical regeneration of a spent catalyst for ammonia synthesis. The samples regenerated are characterized by means of determining: the specific surface (using the Klyatchko-Gurvich technique); the size of the finely-dispersed particles (electron microscope analysis); and the phase content (X-ray phase analysis, Möessbauer spectroscopy, chemical analysis, derivatographic analysis). A mechanism is proposed for the plasma-chemical regeneration of the samples. The reduction process is studied and the activity, the relative degree of transformation, the activation energies, the rate constants, the relative activity and the degree of thermal deactivation of the regenerated samples are determined. It is found that the samples are reduced three to five times as fast as the standard ones. The increased reduction rate and the high catalytic activity are due to the increased pre-exponent factor in the Arrhenius equation at constant activation energy. The causes for the high activity and thermal stability of the samples are explained. The advantages of the plasma-chemical technique are outlined as compared with the conventional methods for activation of spent catalysts for ammonia synthesis.     

Optimization of explosion puffing drying for high-value yellow-fleshed peach crisps using response surface methodology

Response surface methodology (RSM) was used to comprehensively evaluate the pilot scale nonfried explosion puffing drying (EPD) conditions for developing high-value yellow-fleshed peach crisps. The independent variables were different levels of vacuum drying temperature, vacuum drying time, and puffing pressure difference. The responses were crispness, total color difference (ΔE), total carotenoid content (TC), and DPPH radical scavenging capability (AEAC). Statistical analysis revealed that vacuum drying temperature significantly affected all the responses, puffing pressure difference showed maximum influences on TC, AEAC, and crispness, while vacuum drying time showed influences on TC and AEAC. The optimum EPD conditions obtained by RSM were vacuum drying temperature, vacuum drying time, and puffing pressure difference of 73?°C, 144?min, and 0.05?MPa, respectively. At this optimal condition, crispness, ΔE, TC, and AEAC value were found to be 7.3?N, 18.3, 53.5?µg/mg d.w., and 865.4?mg AA/100g, respectively. The EPD-dried yellow-fleshed peach crisps contained higher amounts of carotenoids and AEAC values when compared to hot-air dried samples, they were also found to be crispier than vacuum freeze-dried samples.     

Promotion of the catalytic activity of a Ag/Al2O3 catalyst for the N2O + CO reaction by the addition of Rh a comparative activity tests and kinetic study

Preliminary activity tests show a synergic effect on the yield of the N₂O+CO reaction by the addition of small quantities of rhodium in a Ag/Al₂O₃ catalyst. An analytical comparative kinetic study over Rh/Al₂O₃, Ag/Al₂O₃ and Rh-Ag/Al₂O₃ was performed in order to explain this effect. The reaction kinetics seems to follow a L–H mechanism with competitive adsorption of N₂O and CO over the rhodium catalyst, where a strong CO inhibition effect was obvious. On the two other catalysts (silver and mixed) a L–H mechanism with the reactants adsorbed in different active sites seems to be followed. The kinetic, adsorption and thermodynamic constants were calculated and compared. From the results it seems that the synergic effect is connected with an increase of the activity of rhodium since silver offers more active sites for CO adsorption and removes the inhibition effect, while rhodium offers more active sites for both reactants to be adsorbed. The above findings show that there are no strong interactions (e.g. alloying) between silver and rhodium for the catalyst prepared by the method and active constituents concentrations of this study.     

Computer-aided experiment of using real-time small angle light scattering image processing technique for visual characterization flow field of polymer melts

The phenomenon of small angle light scattering (SALS) has been applied to the actual extrusion molding process. The current study utilizes a self-designed mold with built-in windows for observation of polymer melts within a slit die. A high-speed charge-coupled-device (CCD) camera is used to record the SALS images in real-time with different process conditions for subsequent analysis. Modification algorithm has been proposed to eliminate the effect of multiple scattering. Flow behavior of polymer melts is simulated and analyzed by real-time SALS image processing technique. Visualization is performed via a high-performance computer-aided analysis software which allows on-line data acquisition and characterization the flow field of polymer melts.     

Optimization of process conditions using response surface methodology for the microwave-assisted transesterification of Jatropha oil with KOH impregnated CaO as catalyst

A transesterification reaction of Jatropha curcas oil with methanol in the presence of KOH impregnated CaO catalyst was performed in a simple continuous process. The process variables such as methanol/oil molar ratio (X₁), amount of catalyst (X₂) and total reaction time (X₃) were optimized through response surface methodology, using the Box–Behnken model. Within the range of the selected operating conditions, the optimal ratio of methanol to oil, amount of catalyst and total reaction time were found to be 8.42, 3.17% and 67.9 min, respectively. The results showed that the amount of catalyst and total reaction time have significant effects on the transesterification reaction. For the product to be accepted as a biodiesel fuel, its purity must be above 96.5% of alkyl esters. Based on the optimum condition, the predicted biodiesel conversion was 97.6% while the actual experimental value was 97.1%. The above mentioned results demonstrated that the response surface methodology (RSM) based on Box–Behnken model can well predict the optimum condition for the biodiesel production.     

Preparation of Efavirenz resinate by spray drying using response surface methodology and its physicochemical characterization for taste masking

Abstract

In the present study, taste masked drug–resin complex (DRC) of efavirenz (EFV) was prepared by spray drying technique. The DRC was then incorporated in to a fast dissolving tablet dosage form. EFV is antiretroviral agent of very bitter taste and low oral bioavailability. To investigate the influence of the independent variables on encapsulation efficiency (EE) as well as on the bitter taste of EFV, response surface methodology was employed. Ion-exchange resin (tulsion-335) and amount of solvent were selected as the independent variables while the EE was selected as the dependent variable. DRC was characterized for EE, X-ray diffraction, particle size distribution, Fourier transform infrared spectroscopy, surface morphology, and in vitro dissolution study. The spectrophotometric method was used for the evaluation of bitter taste of EFV. The EE was found to be 37–84%. The compressed tablets were evaluated for the hardness, Friability, wetting time, disintegration time and in vitro drug release studies. This study confirmed that complexation of the drug with an ion-exchange resin can effectively mask the bitter taste of the drug in combination with production of fast dissolving tablets.