The influence of non-uniform catalytic activity on the performance of a single spherical pellet

This work investigates the influence of a non-uniform distribution of the intrinsic catalytic activity of a single spherical pellet on its effectiveness factor, selectivity and deactivation. In most cases the performance of non-uniformly active catalysts is superior to that of uniformly active catalysts which have the same volume averaged activity. The specific deactivation mechanisms and rate as well as the diffusional resistance have an important influence on the most desired activity profile. Concentrating the catalytic activity in the exterior shell most often yields the maximal selectivity but it also yields minimal resistance to deactivation when compared with catalysts with the same volume averaged activity and different activity profiles. For this reason, when it is important to maintain the catalytic activity for long periods it may be desirable not to use catalysts in which the activity is concentrated in the exterior shell.It is also shown that the maximal temperature rise during regeneration of a catalyst, in which coke is deposited non-uniformly, may largely exceed that obtained for a uniformly coked pellet.     

Non-isothermal adsorption in a bidisperse adsorbent pellet

The kinetics of adsorption in bidisperse pellets has been largely studied under isothermal condition. But due to large heat of adsorption, the isothermal models may lead to erroneous results. The present paper proposes a non-isothermal bidisperse model, taking into account two scale mass diffusion (macro—micropore diffusion) and one scale heat diffusion (the temperature within the microparticles is assumed uniform). This model is solved numerically, yielding the sorption uptake and temperature evolution. It turns out that considering the simultaneous macro—micropore diffusion is only necessary for 10⁻⁶ < α < 10⁻¹ (α is the ratio of macropore and micropore diffusional resistances). The macropore diffusion should be rate limiting if the parameter α > 10⁻¹ and the micropore diffusion become rate limiting if α < 10⁻⁶. On the other hand, heat effect may make the isothermal assumption quite erroneous when the diffusion in the macropores and micropores is rapid and the heat transfer between the adsorbent and the ambient sorbate is small.     

Asymmetric temperature profiles in a single catalyst pellet

For rapid reactions the differences in local coefficients of heat and mass transfer around the surface of a catalyst particle lead to a distribution of surface temperatures and an asymmetric internal profile. The nature of these distributions and their changes with flow rate were predicted for the hydrogenation of ethylene on a single spherical pellet of nickel on silica alumina. The measured surface and internal temperature were in good agreement with the predictions.     

Non-isothermal effects in a single pellet diffusion reactor

A theoretical analysis of an adiabatic non-isothermal single-pellet diffusion reactor shows that non-isothermal effects can alter the value of the centerplane concentration. Deviation from isothermal behavior is significant under exothermic conditions where multiplicity of solutions can lead to reactor instabilities or falsification of reaction parameters. Conversely, it is shown that the internal isothermal model is a good approximation to detect non-isothermal pellet behavior from centerplane concentration measurements.     

Dynamic response of a single catalyst pellet

It has been shown previously that pulse-response experiments on single catalyst pellets can provide a simple, rapid method for determining effective diffusivities and adsorption equilibrium constants. Assumptions were made about boundary conditions in the earlier work in order to simplify the mathematical treatment. In this paper a more rigorous development is presented in order to evaluate the magnitude of likely errors in the simplified approach.By using large flow rates, small chamber volumes, and for catalyst pellets of moderate or low porosity, the error in using the approximate equations should be small (less than 10%). For short diffusion paths (short pellets) of highly porous catalysts, the error can be large.     

Isothermal drying rate of ethylbenzene from a spherical polystyrene pellet

Desorption behaviour of ethylbenzene from a spherical polystyrene pellet was investigated. The predicted drying rate using an average diffusion coefficient differed substantially from that simulated with concentration dependent diffusivity. As the drying temperature decreased and the initial solute content of the pellet increased, the difference between the two calculations became more significant.     

Effectiveness factors for non-uniform catalytic activity of a spherical pellet

The effectiveness factor for catalysts with non-uniform activity of different intrinsic activity profiles was derived as functions of a modified Weisz modulus and a modified Thiele modulus. The modified moduluses were chosen so as to take into consideration the different diffusion lengths in catalysts with non-uniform activity.     

等温或非等温颗粒吸附动力学模型:应用于吸附热泵(英文)

Understanding the interaction between a fluid and a solid phase is of fundamental importance to the design of an adsorption process.Because the heat effects associated with adsorption are comparatively large,the as-sumption of isothermal behavior is a valid approximation only when uptake rates are relatively slow.In this article,we propose to determine when it is needed to choose the isothermal or non-isothermal assumption according to two physical parametersα(ratio convection/capacity) andβ(quantity of energy/capacity) .The proposed problem is solved by a mathematical method in the Laplace domain.Whenα→∞(infinitely high heat transfer coefficient) or β→0(infinitely large heat capacity) ,the limiting case is isothermal.When the diffusion is rapid(α10) the kinetics of sorption is controlled entirely by heat transfer.If the adsorption process is to be used as a heat pump,it shall be represented by an isotherm model withαandβas high as possible.     

Experimental observation of temperature gradients occurring in a single zeolite pellet

An experimental study has been made of temperature profiles incident within a zeolite adsorbent particle in the course of adsorption of n-heptan. Using thermocouple wires 0.1 mm in diameter it has been possible to measure simultaneously the temperatures of the surface and in the centre. It was observed that temperature differences between center of the pellet and ambient stagnant gas may be more than 20°C. The results of this work indicate that the overheating of an adsorbent particle is a very rapid process which is followed by a slow cooling of the hot particle. Experimental data are compared with predictions calculated from the theoretical model.     

Preparation of modified styrene-divinylbenzene copolymers having different physical structures and their adsorption characteristics of nonionic surfactants

Several α-oxoethylated and α-hydroxyethylated styrene-divinylbenzene (St-DVB) resins having different degrees of crosslinking and porosity were prepared and their adsorption behaviour toward a nonionic surfactant, decaethyleneglycol nonylphenyl monoether, was investigated. The increment of the adsorption capacity caused by the introduction of the α-hydroxyethyl groups was remarkable in the less crosslinked resin. The α-hydroxyethylated resin derived from the St-DVB resin synthesized with 10 vol.-% DVB and 100 – 120 vol.-% of 2,2,4-trimethylpentane showed the highest capacity among the resins tested. The significance of both macroporosity and gel porosity in the effective adsorption of the nonionic surfactant was elucidated. A column operation using proposed resins was carried out and versatility was evaluated in terms of the breakthrough capacity.     

Experimental study of the poisoning of a single catalyst pellet in a diffusion reactor

Catalyst poisoning experiments were carried out in an isothermal single-pellet reactor under conditions where both kinetic and diffusion processes contribute to the overall reaction rate (Thiele parameter between about 0·5 and 5). The concentration of the reacting species was monitored at the pellet's external surface and at its center plane.Impurity poisoning and parallel, series and triangular self-poisoning processes could be qualitatively discriminate by a sequence of three experiments, once the order of the main reaction was established.It was found that the hydrogenolysis of cyclopropane to propane between 50 and 75°C over an ν-Al₂O₃ supported Pt catalyst is simultaneously poisoned by the reactant cyclopropane and the product propane (triangular self-poisoning).Quantitative values of the poisoning kinetic exponents were evaluated by decoupling the time variable from the poisoning process, using relative overall rate versus normalized center-plane concentration data. Superimposing time on the results (center-plane concentration versus time and relative overall rate versus time data) detected an initial time-delay in the poisoning process. This time-delay was shown to depend upon the hydrogen-pretreatment of the catalyst. Conventional bulk rate-time studies would have been seriously hampered by the complicated time-behavior of the system, while the approach presented in this paper allowed the resolution of the mechanism and kinetics of the poisoning process.