The transient response of a CSTR containing immobilized enzyme particles

A mathematical model is formulated for describing the transient substrate response in a continuous flow stirred tank reactor containing immobilized enzyme particles. The model considers the effects of internal and external substrate diffusion resistances as well as the inherent enzyme denaturation. Simulation of the model so formulated using the kinetic and physical parameters which are within the practical operating range, reveals that a substrate response minimum occurs in all the cases under consideration. Such a substrate response minumum is significantly affected by several characteristic dimensionless groups which govern the reactor performance equations. The unsteady state simulation is also extended to two CSTRs in series. It is found that the substrate response characteristics in the second reactor are similar to these in the first except for a much lower substrate concentration level.     

The transient response of a CSTR containing porous catalyst pellets: Asymptotic solutions

Transient response of a CSTR containing porous catalyst pellets is analyzed theoretically using a matched asymptotic expansion technique. This singular perturbation technique leads directly to the conditions under which the minima of reservoir concentration occur. The existence of the minima may be used to estimate some inherent parameters of the catalyst pellet.     

The transient response of a CSTR to a spherical catalyst particle with mass transfer resistance

A response minimum is shown to occur when a porous catalyst particle reacts in a CSTR under conditions of strong interfacial mass transfer resistance. It is suggested the technique may be exploited for parameter estimation purposes even under conditions of interfacial damping.     

Numerical simulation of micromixing effect on the reactive flow in a co-rotating twin screw extruder

To control the multicomponent reactions in extrusion, reactive-mixing flow in a co-rotating twin screw extruder was numerically studied in the present paper. Effects of initial species distribution, rotating speed and flow rate on a competitive-parallel reaction were investigated and the relationship between mixing and reactions was discussed from the view of chemical reaction engineering. The simulation results show the studied operational parameters, which determine residence time distribution, earliness of mixing and segregation degree of reactive-mixing flows, affect the local species concentration and reaction time and hence have significant influences on the reaction extent. Orthogonal test was adopted to clarify the significance of operational parameters. The analysis shows that initial species distribution and flow rate are the most important factors in the control of reaction extent, and effect of rotating speed is conditional depending on the micro-mixing status of the fluid.     

The transient response of ZrO2 oxygen sensors to step changes in gas composition

The transient voltage response of ZrO₂ oxygen sensors was examined following step changes in gas composition. The experiments were performed on a laboratory flow reactor at 600° C. Composition changes between (a) 100% and (b) 1% O₂ in N₂ produced response curves whose symmetry varied between composition steps (a) from low-to-high oxygen and (b) from high-to-low oxygen. This difference is due to the logarithmic dependence of sensor voltage on oxygen partial pressure. Corresponding oxygen partial pressure-time curves, derived from experimental voltage via the Nernst equation, are symmetric with respect to the direction of composition changes. Abrupt transitions are found in voltage-time curves at 600° C following step changes of reactive gases; e.g. from O₂/N₂ mixtures to CO/N₂, H₂/N₂ or D₂/N₂ mixtures. These voltage-steps represent transitions in stoichiometry of the surface boundary layer on the ZrO₂ sensor. Delay times before the transition also reflect reaction stoichiometry. Response times with O₂/CO, O₂/H₂ and O₂/D₂ follow trends predicted by the kinetic theory of gases. A limited number of experiments were performed to examine the relationships between sensor response and sensor catalytic activity. Poorer oxidation catalytic activity parallels slower response characteristics.     

Determination of the condition for the existence of composition minima in a CSTR via spectral analysis

Transient response of a CSTR containing porous catalyst pellets is analysed theoretically by a generalized integral transform, and the condition for existence of a minimum in the bulk concentration is determined via the analysis of the eigenspectrum of the governing equations. A simple unified condition for existence is found to be Vk/F (ρ_pS_gK/ε+ρ_pS_gK)<1, irrespective of the values of any other parameters, where k is the first order rate constant, V is the reactor volume excluding pellet volume, F is the volumetric flow rate, ρ_p is the pellet density, S_g is the interior surface area, ε is the pellet voidage and K is the slope of the linear adsorption isothern. Clearly, the continuous adsorber (k = 0) always satisfies this condition, therefore, a minimum in the transient bulk concentration is predicted to always occur. Simulations show that the minimum is sufficiently pronounced and the time scale for the minimum to occur is sufficiently practical that the occurrence of the minimum may be exploited for parameter estimation purposes.     

Refolding of proteins in a CSTR

A model to predict refolding of proteins in a continuous stirred tank reactor (CSTR) was developed and compared to a batch refolding process with simple dilution of the protein in a stirred tank reactor. For experimental verification of the model a continuous refolding of a model protein (α-lactalbumin) was performed in a CSTR. The refolding process of denatured and fully reduced α-lactalbumin could be accurately predicted by a set of differential equations assuming a first order reaction rate for folding and a second order reaction rate for aggregation. The system composed of a CSTR with an additional diafiltration circuit for removal of denaturing agents from the feed solution and to maintain constant refolding conditions. Based on the folding kinetic the dynamic behavior of such a continuous refolding reactor was simulated under different operating conditions. It was shown that the refolding efficiency was higher compared to batch dilution under certain conditions, namely high residence times. The yield of refolded protein could further increased by recycling the outlet stream containing unfolded protein to the reactor entrance.     

Regeneration of polymeric adsorbents in a CSTR

The regeneration step is crucial in the design of cyclic fixed bed adsorbers. In order to understand the regeneration process of a polymeric adsorbent (Duolite ES861) saturated with phenol by using sodium hydroxide simple dynamic experiments in a CSTR were done. Results were explained roughly by a simple equilibrium model and more accurately by a reaction front model which will be later used in a package for the design of cyclic fixed bed adsorption processes.     

旋转床内微观混合与反应过程的特性

对旋转床内微观混合的研究有助于建立其内部微观尺度上的混合与反应机制。在以往研究工作的基础上,依据数值模拟与实验结果的对比,间接确定了旋转床内的液膜厚度范围;对液膜速度模拟进行合理修正,通过流体流动、传质及反应过程的数值模拟,得到离集指数、湍流耗散率、停留时间以及微观混合时间的变化。实验结果和数值模拟表明：不同转速下的离集指数、混合时间、湍流动能耗散率等均表现出一定的多尺度现象。研究结果丰富了旋转床内微观混合与反应的认识,将为后续研究提供坚实的基础。     

Analysis of mixing during melt-melt blending in twin screw extruders using reactive polymer tracers

Mixing during melt-melt blending of segregated polypropylene melt streams in a co-rotating twin screw extruder was experimentally investigated. The mixing limited reaction between two polymer reactive tracers, which are terminally functionalized polyolefin oligomers, was used to determine the mixing performance of a kneading block section. The selected functional groups were succinic anhydride and a primary amine, and Fourier-Transform Infrared Spectrometry (FT-IR) was used to determine the anhydride conversion. In the absence of interfacial tension, the reaction conversion was directly related to the amount of interfacial area generated. Experiments were completed to study the effects of operating conditions, kneading block design, and polymer material properties. The screw speed effect was observed to be non-linear because of competing contributions from shear rate, residence time, channel fill, and viscous heating. The mixing performance of kneading blocks backed by a reverse conveying element was observed to follow the trend of: forward > reverse > neutral. For each kneading block design, the mixing performance decreased with an increase in polymer viscosity.     

Alpha particle transient response of a polycrystalline diamond detector

We report the transient response of room temperature pulses generated from alpha particles from a chemical vapor deposition (CVD) polycrystalline diamond detector. For transient signals dominated by electron transport only prompt pulse shapes were observed with an average rise time of 160 ns limited by the preamplifier rise time. For transient signals dominated by hole transport significant slow components were observed in the majority of pulses due to thermal emission of charge from shallow hole traps. These slow pulses were observed from the device when in its as-grown state, without any previous ‘priming’. Two separate slow components were identified from the hole pulses, with average rise times of 600 ns and 1–10 μs, respectively. These data suggest that room temperature electron transport in polycrystalline CVD diamond is prompt, with no evidence for thermal de-trapping. In contrast hole transport in our sample at room temperature contains a significant delayed component due to thermal emission of holes from at least two bands of shallow defects.     

Determination of the transfer constant for copolymerization in the presence of a reactive substrate

Important equations recently presented by Bamford et al.¹, for chain transfer with reactive substrates in copolymerizing systems are analogous to the well-known relationship:

$\text{In}\text{[S]}\text{[S ]}\text{o =}\text{Cs In[M]}\text{[M] o}$

which is applicable to homopolymerization. This paper reports the first direct experimental testing of the validity of these equations. Gas/liquid chromatographic measurement of residual substrate concentration as a function of monomer removal, measured dilatometrically for the systems methyl acrylate/styrene/bromotrichloromethane and methyl methacrylate/styrene/bromotrichloromethane has been carried out. Results indicate that the equations accurately describe the course of the reaction and permit evaluation of the appropriate transfer constant.     

Experimental evaluation of a model-based degree of segregation in a CSTR

Conversions measured at the outlet of a CSTR, in which sodium bromoacetate reacted isothermally with sodium thiosulfate, were found to lie between the macro and microfluid conversion limits.On the basis of a reactor model, these data led to evaluations of a degree of segregation within the reacting mixture. This degree of segregation decreases linearly with increasing propeller speed. This rate of decrease is greater the larger the propeller diameter and the higher the propeller height above the reactor bottom. Feed jet velocity had no significant effect on this rate.This degree of segregation is inversely proportional to Reynolds' number to the $\text{1}\text{6}$th power.     

An analytical solution for the transient response in a diffusion cell of the Wicke-Kallenbach type

An analytical solution is developed for the problem of transient diffusion and first order reaction in a solid flanked by two well mixed fluid compartments. This solution is necessary and useful in describing unsteady-state measurements in a Wicke-Kallenbach diffusion cell or reaction studies in a single pellet reactor. It is shown that a proper choice of an inner product vector space, following the methodology developed by Ramkrishna and Amundson [14, 15], leads readily to the desired solution and guarantees its completeness. This solution is valid for the equivalent heat transfer problem also.     

Characterization of transient response curves in heterogeneous catalysis—II Estimation of the reaction mechanism in the oxidation of ethylene over a silver catalyst from the mode of the transient response curves

The reaction mechanism and the kinetic structure of the oxidation of ethylene over a silver catalyst at 91°C have roughly been elucidated from the mode of the transient response curves of ethylene oxide, carbon dioxide and water, due to the concentration jump of reactants. The typical overshoot mode with an instantaneous maximum for C₂H₄O clearly indicates two things. First that the surface reaction controlling in the reaction between adsorbed diatomic oxygen species and gaseous C₂H₄ (an Eley—Rideal type mechanism) with the rapid desorption of C₂H₄O formed and, second, that the slower regeneration of the adsorbed oxygen species. The typical S-shape mode with an overshoot for CO₂ strongly suggests the existence of an intermediate (In) and, the presence of the (In) has been actually confirmed by the transient response of its decomposition in an O₂-He stream. The (In) can desorb as acetic acid only when the surface is reduced by H₂. Much transient data seems to suggest that the (In) is formed from the reaction of adsorbed monoatomic oxygen species with C₂H₄ and, is decomposed by the reaction with diatomic oxygen species. The false start mode for the response curve of (In) decomposition indicates the inhibitory effect of adsorbed ethylene on the reaction.     

Deep level transient spectroscopy of CVD diamond: the observation of defect states in hydrogenated films

Hydrogenated polycrystalline CVD diamond films support a number of defect states within the range 0.03–1.0 eV, as determined by charge-based deep level transient spectroscopy (Q-DLTS). The observation of a 30-meV state is direct evidence for such a shallow level in this material, and is most likely to be the acceptor state that gives rise to the p-type character of these films. Deeper levels, at 0.11 eV, 0.39 eV, 0.65 eV and 0.70 eV–1.0 eV can also be observed and again appear to be associated with the hydrogenation level within the near surface region of the CVD diamond film. The loss of the 0.11 eV level at temperatures greater than 417 K is most easily explained if adsorbates are being removed from the surface at this temperature.     

Floc breakage: The dynamic response of the particle size distribution in a flocculated suspension to a step change in turbulent energy dissipation

Floc samples taken from batches prepared on a bench scale flocculator are repeatedly driven through a tube to and from a reservoir at a controlled rate of flow. The Floc Size Distribution (FSD) of a sample from the reservoir is determined using a light obscuration instrument for a sequence of completed strokes in approximately geometric progression. The response of the FSD to a step change in flowrate is examined by consideration of the mean and standard deviation of the distribution and the variation in total floc population. Correlations between the parameters of the FSD and the microscales of turbulence are presented. Observations on the relationships governing the mechanisms of floc disruption are reported. The splitting of flocs is not defined simply by a frequency since the mechanism fails once the remaining flocs are resistant to the most severe disruptive forces available in the system. Either detailed information regarding the floc density distribution or knowledge of the history of a particular suspension will be required to fully predict the response of a floc population to a given flow field.