Asymptotic effectiveness of a catalyst particle in the form of a hollow cylinder

An explicit formula is presented for the effectiveness factor of a catalyst particle in the form of a hollow cylinder taking into account the reactant diffusion in the fluid phase of the cylindrical core and diffusion and reaction in the outer reactive annulus. The proper normalization to be used in the definition of the generalized Thiele modulus as well as the aspect ratios that lead to the highest effectiveness are also determined. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4020–4024, 2013     

The influence of reactant adsorption on the multiplicity and stability of the steady state of a catalyst particle

The influence of a resistance to reactant adsorption on the multiplicity and stability of the steady state of a catalyst particle, promoting an exothermic reaction, is considered. For activated adsorption new regions of multiplicity are uncovered in the parameter space. It is shown that multiple solutions can arise for a surface reaction having zero activation energy. No such behaviour is observed for non-activated adsorption. The effect of non-activated adsorption is largely to diminish the regions of multiplicity.The influence of reactant adsorption resistance on the stability of the steady state is most complex and no general conclusions are drawn. A few specific cases are considered. The presence of an additional heat generation term due to adsorption has a great stabilising influence and, in addition, tends to dramatically reduce “hot-spot” temperatures which may arise during transient behaviour.     

Lyapunov stability analysis of porous catalyst particle systems: extension to interphase transfer and time-varying surface conditions

A weighted Lyanunov functional is used to derive local and global stability conditions for the porous catalyst particle system. Results are presented for finite interphase transfer, Nu and Sh ≠ _∞, and for general time-varying surface conditions. Improvements on previous results for the case of unit Lewis number and negligible interphase transfer are also developed.     

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.     

Hydrothermal stability of aluminophosphate catalyst supports

Alumina and aluminophosphate (AP) with Al/P ratio of 6 were subjected to hydrothermal treatment at 750 ° C for up to 48 h. The results indicate that AP had a higher hydrothermal stability than the -alumina. The pore size distribution and surface area of AP did not change much under hydrothermal conditions. The pore size distribution of alumina, on the other hand, shifted toward larger pore diameter accompaning with decreasing surface area upon deactivation.     

Thermal stability improvement of hopcalite catalyst

The properties of hopcalite in alumina matrix formed by rehydration of transition aluminas have been investigated in comparison with conventional hopcalite catalyst. By using X-ray diffraction, thermal analysis and activity measurements in combustion ofn-butane and air mixture it was found that the presence of alumina matrix leads to improvement of thermal stability at elevated temperatures and to limitation of spinel phase recrystallization. The modified catalyst was found to show much higher activity after thermal treatment at temperatures above 773 K than pure hopcalite at the same conditions.     

Thermal stability in a flow reactor containing a fixed catalyst bed

Biisk. Translated from Fizika Goreniya i Vzryva, Vol. 26, No. 4, pp. 68–74, July–August, 1990.     

Criterion for a uniform environment around a catalyst particle in a packed bed reactor

A criterion is presented to determine the minimum velocity required for the use of the centre-symmetric mass transfer boundary condition with a single     

Is there a correlation between catalyst particle size and CNT diameter?

Monodisperse iron oxide particles were deposited on a planar silica transmission electron microscope (TEM) grid and from TEM and Energy Filtered TEM analyses it is confirmed that the particles are involved in a series of re-arrangements during the pretreatment and initial carbon nanotube (CNT) growth. It is also shown that CNT growth occurs in two stages; an initial disordered growth, followed by a more aligned growth underneath. The planar silica TEM grid serves the dual purpose of being the catalytic support, and the tool which facilitates TEM analysis on a planar model system. The particle re-arrangements and the two modes of CNT growth may offer an explanation as to why we find no correlation between primary particle size and final CNT diameter.     

Prediction of catalyst stability in naphtha reforming

A method to predict catalyst activity (RON) and selectivity (ηc5⁺, liquid yield) of naphtha-reforming catalysts during normal operation is described. It involves performing experiments at normal operational conditions and testing accelerated deactivation. These last experiments have a severe intermediate period at lower pressure. Carbon formation is related to time and the decrease in RON and ηc5⁺ is related to carbon at normal operation. The accelerated deactivation produces a similar coke to the one at normal conditions for an equal amount of coke. Therefore for each time at normal conditions there is a corresponding pressure in the accelerated deactivation test. Power law-equations fit the data well and combining them results in the following relations: P = at^b; Δ_NRON = cP^d; Δ_Nηc5⁺ = eP^f. The coefficients a, b, c, d, e and f depend on the catalyst and are calculated from four or six experiments (half at normal conditions and half accelerated deactivation tests); b, d and f bare negative. The value of pressure for the time at which it is desired to predict catalyst activity and selectivity is calculated from the first equation. This value when applied to the second and third equations gives the activity and selectivity, respectively, that the catalyst will have after time t.     

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.     

金超微粒子催化剂的新进展

金超微粒子催化剂有许多独特的性能而受到人们的广泛关注,世界各国都在进行开发研究。介绍了金超微粒子催化剂的制备方法、特性及应用实例,指出了当前研究的主要方向。     

Exterior differential forms in stability analysis with application to catalyst stability

Stability theorems for distributed parameter dynamical systems are stated in terms of exterior differential forms. It is shown that these n-forms can be used to develop a relatively systematic procedure for constructing Liapunov functionals. The procedure is demonstrated on linear and nonlinear slab catalyst stability problems, including Le ≠ 1. The examples illustrate several advantages of the construction procedure. Nonquadratic Liapunov functionals can be developed for nonlinear problems. Systematic corrections to the functionals follow directly from the construction procedure. The technique is applicable to both single and systems of partial differential equations. Numerical calculations are presented showing that improvements up to 8-fold can be realized by starting with previously published results and generating corrections using the formalism of exterior differential forms.     

Dynamic behaviour and stability of non-porous catalyst particles

The stability and dynamic behaviour of a model of an exothermic chemical reaction occurring on a non-porous catalyst particle are studied. The model accounts for the effects of external film heat and mass transfer resistances and intraparticle heat conduction.An orthogonal collocation technique is shown to be superior to the more conventional finite difference methods in obtaining solutions of the system equations.It is found that the thermal conductivity as well as the heat capacity of the particle can have a profound effect on stability and dynamic behaviour. Two simpler models which neglect the effects of thermal conductivity and accumulation of reactants are shown to give very limited insight into the dynamic behaviour of the system.     

Correlation between catalyst particle and single-walled carbon nanotube diameters

Single-walled carbon nanotubes (CNTs) were synthesised at different conditions by a novel aerosol method based on the introduction of pre-formed iron catalyst particles into conditions leading to CNT formation. The results of statistical measurements of individual CNT dimensions based on high-resolution TEM images showed the effects of the residence time and temperature in the reactor. The ratio between catalyst particle and CNT diameters was close to 1.6 and independent of the experimental conditions, thus revealing an astonishing “universality” in the growth process. A proposed geometric model of heptagon defect formation, which initiates and maintains the CNT growth, allowed us to theoretically explain the phenomenon.     

Some uniqueness conditions for the symmetric steady state of a catalyst particle with surface resistances

Sufficient conditions for the uniqueness of the symmetric temperature and concentration profiles in a porous catalyst slab are obtained, for the case in which there are non-negligible heat and mass transfer resistances at the boundaries. The general results are used to obtain explicit uniqueness criteria for a single irreversible reaction.     

Ion exchange resin catalyst stability in in-situ epoxidation

Summary The highly oxidizing conditions of expoxidation cause decross-linking of the ion exchange resin catalyst, resulting in swelling of the resin with its eventual dissolution if uncontrolled. Minimum metal contamination (particularly iron), low hydrogen peroxide concentration, low reaction temperature, high cross-linked resin catalyst, and the addition of a complexing agent all contribute to maintaining resin catalyst stability. Metal contamination is by far the most detrimental factor to resin stability. Operating with essentially a metal-free system is most important. This can be obtained by using a resin with less than 0.005% metal contamination and by avoiding metal contamination from raw materials and equipment being used for epoxidation. A minimum of metal contamination and perhaps a lower operating temperature provide the most practical methods for controlling resin breakdown and for improving the efficiency of the ion exchange resin catalyst inin-situ epoxidation processes. Avoiding resin breakdown is particularly essential for the fixed resin catalyst bed and for the resin recycle batch processes. Even for the expendable resin process where resin stability is not too critical, control of metal contamination and operating temperature should result in a trouble-free and a more economical operation. Presented at the fall meeting, American Oil Chemists’ Society, Cincinnati, O., September 30, October 1–2, 1957.