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1.
In this work, we present the solution of the equations that govern the reactant transport in a well mixed system that contains particles where diffusion and first-order reaction occur. The transport equations are coupled by an interfacial boundary condition that includes mass transfer resistance. The statement of the problem allows arbitrary time depending feed functions. The evaluation of the solution obtained by the Laplace method requires the solution of an eigenvalue problem. We discuss the evaluation of the solution, and typical results for three different feed functions: step, pulse and oscillatory functions are presented. The resulting equations are able to show the effect of internal and external mass transfer limitations on the particle and fluid concentrations and on kinetic experimental results.  相似文献   

2.
A study of Nusselt and Sherwood numbers in a monolith reactor   总被引:5,自引:0,他引:5  
A two-dimensional model of a single channel of a monolith reactor is used to evaluate the values of the Nusselt and Sherwood numbers under reaction conditions. The circular channel is assumed to have axisymmetry with a first-order reaction occurring at the wall. The values of the Nusselt and Sherwood numbers do not correlate uniquely with the Graetz number but rather depend on the reaction rate at the wall. Hence they depend on such variables as gas velocity, inlet temperature and reactant concentration.  相似文献   

3.
Cyclohexane oxidation is the first step in the currently used technology for production of Nylon-6 and Nylon-6,6 which employs a two-stage process. In the first stage 80% selectivity to two main products, cyclohexanol and cyclohexanone (KA oil) is obtained at 4–8% cyclohexane conversion in staged bubble columns or stirred tanks. There have been reports that increased oxygen concentration in the gas phase or pure oxygen is beneficial to cyclohexane oxidation and this was confirmed in our previous study (Jevtic et al., 2009). To fully utilize this advantage here, we present a novel, safer capillary reactor for cyclohexane oxidation with pure oxygen. The discrepancy between the experimental and modeling results was attributed to lower than expected mass transfer achieved in the capillary. With a better design for gas–liquid mixing and contacting this type of a reactor could potentially become attractive for gas–liquid reactions of similar nature.  相似文献   

4.
The wall-to-fluid mass transfer coefficient was obtained from coated wall dissolution experiments, for water flow through fixed beds of spheres with tube-to-particle diameter ratios of 2.9-11.6, and for particle Reynolds number (Re) in the range 3-200. The coefficients, in the form of dimensionless Sherwood numbers (Shwf), were shown to approach a nonzero limit as Re→0. The data were well-represented by the equation
  相似文献   

5.
Desulfurization of liquid fuels is usually obtained by heterogeneous catalyzed hydrodesulfurization (HDS). Improvement of this process is needed, particularly with respect to future low residual S-contents of about . Up to now HDS of diesel oil is performed in a trickle bed reactor equipped with an expensive H2-recycle, although the H2-supply is far beyond the amount chemically consumed, at least for deep HDS of an already hydrotreated feed. In addition, scale up of trickle bed reactors is in general problematic. In this work kinetic studies and above all a new HDS-concept using a two-phase reactor is presented. Thereby the oil is externally pre-saturated with H2 and only the liquid phase is passed over the catalytic fixed bed. The H2-recycle is then redundant, the intrinsic reaction rate can be utilized, and scale-up problems do not occur. In addition, this concept can also be used for HDS of oils with a higher content of sulfur and/or unsaturated hydrocarbons by installing a liquid recycle.  相似文献   

6.
The relationship between the geometry and the operating conditions, hydrodynamics and performance of an industrial gas-liquid stirred reactor has been studied with the help of CFD modeling and gamma ray tomography. It is seen that the interfacial area distribution is very wide. This in some areas leads to mass transfer limitations. Strategies for retrofitting the reactor were then developed (e.g. change in the operating speed, change of impeller type, change in the feed introduction, etc.) and tested by CFD for improving the reactor performance. The benefits of the implementation were in terms of the improvement in the product quality, reduction in the by-product formation, increase in the reactor throughput.  相似文献   

7.
The principle of a discontinuously operated chromatographic reactor was studied experimentally and theoretically. The heterogeneously catalysed hydrolysis of methyl formate was chosen as a model reaction. An acidic ion exchange resin was used as catalyst and adsorbent. The relevant adsorption equilibrium constants were available from a previous study. In this work the reaction rates were quantified on the basis of batch reactor experiments. Subsequently, systematic experiments were carried out using a fixed bed. The influence of temperature, residence time, feed concentration and cycle time on the reactor performance was studied. It was attempted to analyse the observations using a simplified pseudo-homogeneous cell model. Since the model was found to be capable of describing the reactor behaviour over a wide parameter range, it was applied to perform extensive parametric calculations. Besides the achievable conversion other objective functions such as recoveries and production rates were also analysed. From the results obtained a few generally applicable rules to evaluate the potential of discontinuously operated chromatographic reactors could be derived.  相似文献   

8.
G. Groppi   《Catalysis Today》2003,77(4):11101-346
A kinetic study on CH4 combustion over a very active PdO/ZrO2 catalyst with high Pd loading (10% w/w of Pd) is presented as an example of a demanding problem which requires both the development of appropriate experimental tools and a theoretical insight on surface chemistry.

The use of an annular catalytic reactor as a tool to collect kinetic data under unusually severe conditions (high temperature and CH4 concentration) is described in comparison with the use of a conventional packed bed reactor. In particular, problems related to the biasing effects of mass, heat and momentum transfer are addressed.

Kinetic data addressing the effects of CH4, O2, H2O and CO2 concentration in a temperature range from 400 to 550 °C are analysed by means of a purely empirical power law model and of a formal kinetic model based on literature indication assuming methane dissociative adsorption as the rate controlling step.  相似文献   


9.
A centrifugal partition chromatograph (CPC) was used as a liquid-liquid catalytic reactor for the isomerisation of hexen-3-ol into ethylpropylketone with a water soluble rhodium catalyst. Global mass transfer coefficients were measured and shown to depend on both the nature of the solute and the flow rate. Liquid-liquid partition isotherms were also determined with the CPC using elution chromatography. Finally, a reactor model was derived to account for the experimental results obtained both under stationary and transient (pulse) conditions. A parameter sensitivity evaluation is also presented.  相似文献   

10.
Intuition tells us that any decrease in the catalytically active surface area should result in an equivalent decrease in the reaction yield and efficiency. Our findings counter this by showing that the active surface and hence the catalyst loading can be reduced drastically in the diffusion-limited heterogeneous reaction systems, while the conversion rate remains essentially unchanged by using fractals for spatial distribution of the catalyst load. The results of this study provide an unusual circumstance for optimal design of chemically active surfaces and can be used to drastically reduce cost of heterogeneous chemical and biological reactors, sensors, and electrodes of fuel cells. The proposed approach can be exploited to its fullest extent in chemical microsystems by utilizing the latest advances in our abilities to manipulate matter on the micro/nano scale.  相似文献   

11.
Gas and liquid velocities in laboratory scale trickle bed reactors are one or two orders of magnitude lower than those in commercial reactors. Then, the kinetic data may include the external effects. This shortcoming of laboratory scale trickle bed reactor can be resolved by diluting the catalyst bed with fine inert particles. The catalyst bed dilution increases dynamic liquid holdup, pressure drop, gas–liquid mass transfer coefficient. Hydrogenation of 2-phenylpropene on Pd/Al2O3 was performed with the trickle bed reactor diluted with fine inert particles and the coiled tubular flow-type reactor to compare the kinetics with that of the basket type batch reactor. The trickle bed reactor diluted with fine inert particles is suitable to obtain the reaction rate without external effects even if the liquid velocity is low. The coiled tubular flow-type reactor should be used at high gas velocities.  相似文献   

12.
Ethoxylation and propoxylation reactions are performed in the industry to produce mainly non-ionic surfactants and ethylene oxide(EO)–propylene oxide(PO) copolymers.Both the reactions occur in gas–liquid reactors by feeding gaseous EO,PO or both into the reactor containing a solution of an alkaline catalyst(KOH or Na OH).Non-ionic surfactants are produced by using liquid starters like fatty alcohols,fatty acids or alkyl-phenols,while when the scope is to prepare EO–PO copolymers the starter can be a mono-or multi-functional alcohol of low molecular weight.Both reactions are strongly exothermic,and EO and PO,in some conditions,can give place to runaway and also to explosive side reactions.Therefore,the choice of a suitable reactor is a key factor for operating in safe conditions.A correct reactor design requires:(i) the knowledge of the kinetic laws governing the rates of the occurring reactions;(ii) the role of mass and heat transfer in affecting the reaction rate;(iii) the solubility of EO and PO in the reacting mixture with the non-ideality of the reacting solutions considered;(iv) the density of the reacting mixture.All these aspects have been studied by our research group for different starters of industrial interest,and the data collected by using semibatch well stirred laboratory reactors have been employed for the simulation of industrial reactors,in particular Gas–Liquid Spray Tower Loop Reactors.  相似文献   

13.
The light-off curve is the conversion-temperature plot of a catalytic reaction; it is usually used in catalyst development. The possibility to deduce kinetic information such as the apparent reaction order or mass transfer limitation from the shape of the curve is examined here. The light-off plots obtained by numerical simulation with several different rate equations are compared between themselves and with experimental curves. This is illustrated with the oxidation of carbon monoxide and methane on noble metal catalysts with different degree of mass transfer resistance. The comparison is complemented with the Arrhenius plots, i.e. the logarithm of the apparent rate constant versus the reverse of temperature.From the shape of the curves, the three following situations can be discriminated: first-order kinetics, negative first-order kinetics in the absence or in the presence of external mass transfer limitation. The range of diffusional limitation is well defined for negative-order reaction. However, the mass transfer resistance is difficult to evidence for first-order reaction. It is observed that the Langmuir-Hinshelwood reaction kinetic model is not convenient to represent the effect of strong external transport limitation on a reaction in the regime of reactant inhibition (negative apparent reaction order).  相似文献   

14.
15.
The catalytic performance of cermets made of 10% nickel or nickel oxide supported on YSZ (yttria-stabilized zirconia) for chemical looping combustion (CLC) and steam reforming (SR) of methane at 700 °C is investigated. Steam reforming of methane over the reduced catalyst resulted in a syngas containing more than 70% hydrogen and about 15% carbon monoxide. Chemical looping combustion of methane with insufficient lattice oxygen could potentially lead to 40–65% hydrogen rich gas products. Prolonged induction period (e.g. 30–80 min) in reduction of nickel oxide by methane has been observed in the presence of steam. The span of induction period increases by increasing steam partial pressure. It is hypothesized that the delayed reduction of nickel oxide is related to the retarding effect of steam on autocatalytic reactions of methane and hydrogen with lattice oxygen of nickel oxide and the subsequent reforming reactions.  相似文献   

16.
Particle growth kinetics of calcium fluoride in a fluidized bed reactor   总被引:1,自引:0,他引:1  
Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L0, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.  相似文献   

17.
It is shown that various multicomponent diffusion equations can be written in the generalized Maxwell-Stefan (MS) form by using a simple generalization of the inversion method presented in a publication by Merk [1959. Applied Scientific Research A 8, 73-99]. The new approach can be considered a reformulation of the Curtiss-Bird inversion [1999. Industrial & Engineering Chemistry Research 38(7), 2515-2522], but it is simpler and more versatile—it can be used to invert the Onsager-Fuoss, Darken, Nernst-Planck, and generalized Fick equations; and to provide expressions for the MS diffusivities without making assumptions about the diagonal MS diffusivities. This approach complements the reversion described by Taylor and Krishna, which changes the MS equations to the form of the generalized Fick equations. As a detailed example, the Lattice Density Functional Theory (LDFT) equations for diffusion in a quaternary system are inverted to MS form by using the new approach; in addition, the resultant MS equations are reverted to the generalized Fick form by using the approach of Taylor and Krishna; last, the original LDFT equations are retrieved by using a priori information from the new approach. This example demonstrates the straightforwardness of switching between various forms of any diffusion equation. Additionally, the example ascertains that the MS diffusivities are symmetric and always positive, at least when obtained from the near-equilibrium LDFT equations at low density—this builds on the works of Curtiss and Bird [1999. Industrial & Engineering Chemistry Research 38(7), 2515-2522] and Condiff [1969. Journal of Chemical Physics 51(10) 4209] that demonstrate only the symmetry, and it offers an alternative explanation for the observance of negative MS diffusivities.  相似文献   

18.
By combining endothermic and exothermic reactions in one reactor, a mutual utilization of thermal energy involved in reactions is expected to produce a saving energy and a cost-down for running in industrial reaction process. In this case, a wall-type reaction system is thought to be suitable because such reaction system is good at exchangeability of thermal energy by conductive heat transfer. This study supposed a wall-type reaction system consisting of endothermic and exothermic reaction channels stacked up and a fixed-bed reaction system of the same configuration, and compared them by numerical simulation in the case where endothermic and exothermic reactions progress simultaneously.

In the fixed-bed reaction system, heat transfer in the catalyst bed takes place by convection, and this transfer becomes the rate-limiting process. Accordingly, occurrence of hot spot in the exothermic channel and shortage of thermal energy in the endothermic channel were predicted. This trend became distinct by making the feed gas directions flowing in the two channels countercurrent and by stacking the channels in multiple tiers. In the wall-type reaction system, however, the temperature distributions in the exothermic and endothermic channels almost conformed to the set temperatures, and the temperature difference between channels was small. Even if the feed gases flowed in countercurrent and even if the channels were stacked several deep, this trend did not change. In the wall-type reaction system, the exchange of thermal energy would take place efficiently by conductive heat transfer between the endothermic and exothermic channels. Furthermore, it was inferred that the wall-type reaction system would provide a stable operation in mutual utilization of thermal energy.  相似文献   


19.
In this work, nano-CaCO3 particles with tunable size have been synthesized via CO2/Ca(OH)2 precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO2 and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (KLa) of CO2 absorption into Ca(OH)2 slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)2 content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO3 particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles.  相似文献   

20.
Continuous biodiesel production from a waste pig-roasting lard, methanol and KOH was carried out in a reciprocating plate reactor (RPR) using a factorial design containing three process factors, namely methanol/lard molar ratio, catalyst loading, and normalized height of the reactor. The main goals were to optimize the influential process factors with respect to biodiesel purity using the response surface methodology and to model the kinetics of the transesterification reaction in order to describe the change of triacylglycerols (TAG) and fatty acid methyl esters (FAME) concentrations along the RPR height. The first-order rate law was proved for both the reaction and the mass transfer. The model of the changing reaction mechanism and mass transfer of TAG was also applicable. Both kinetic models agreed with the experimental concentrations of TAG and FAME determined along the RPR height.  相似文献   

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