Study On The Mechanical Properties Of Solid Catalysts

The mechanical strength and reliability of solid catalysts and their packed beds are the important factors for the reliable and efficient performance of fixed bed converter reaction systems. In this work, the primary reason for the mechanical failure of solid catalysts, the Weibull statistics of the strength data and a concept of the mechanical reliability are elaborated. Solid catalysts are typical brittle materials, and their mechanical failure is due to brittle fracture arising from a sudden catastrophic growth of a critical flaw under tensile stress induced in the catalyst bulk phase. Variations of size, shape and orientation of the flaws, e.g. pores, defects, dislocations and discontinuations, result in a large scattering range of the strength data. It has been proposed that the strength data can be modeled well by Weibull distribution. Weibull statistics provides a method for calculating the failure probability of a catalyst particle under a specific load; a preliminary scheme, therefore, has been formulated for the prediction of the mechanical reliability of solid catalysts. The failure probability in the low loading range and Weibull modulus are both the characteristic indices for the mechanical reliability of solid catalysts. Secondly, the mechanical-strength measurement and characterization of solid catalysts are investigated. The applicability of Weibull statistics for the strength data obtained with the crushing method on spheres, tablets and extrudates, knife-edge cutting method on tablets and extrudates and three-point bending method on extrudates, respectively, is discussed. Stress analyses and experimental results reveal that, except for the crushing of extrudates, the other strength tests have a single fracture mode, and the mechanical failure of catalyst particles is due to brittle fracture. Consequently their strength data follow Weibull distribution well. The suitability of the measurement methods for differently shaped catalysts is also discussed. It is concluded that the crushing test is a satisfactory method for spheres, and crushing and cutting tests are both suitable for tablets, while cutting and bending tests are appropriate for extrudates. Thirdly, the effects of the number of testing specimens and the estimation methods on the Weibull parameters of solid catalysts are examined. A Monte Carlo simulation is used to obtain the statistical properties of the Weibull parameters estimated by the linear regression, weighted linear regression and maximum likelihood methods respectively. Results show that the maximum likelihood method results in the highest estimation precision; however, with a low safety factor. The weighted linear regression method with a weight factor2Wi = Fiand a probability estimator Pf (Fi)= (i?0.3)/(n+0.4)or Pf (Fi)= (i?3/8)/(n+1/4), which leads to a similar estimation precision and a much higher safety factor, is considered to be the best method for engineering design. It is also concluded that the precision of any estimation method increases with the increase of the number of testing specimens. It is affirmed that in the mechanical-strength measurement, 30-60 specimens are required to obtain a reasonable estimation of the Weibull parameters. The mechanical reliability of the packed bed of solid catalysts is investigated. A model is proposed for the bulk crushing strength of spherical catalysts. A conclusion is drawn that the broken percentage of spheres during the bulk crushing strength measurement follows the relationship similar to the Weibull equation. The single particle property is related to the bulk behavior of the packed bed. The applicability of the model developed is also extended to differently shaped nonspherical catalysts such as extrudates, with using another index for strength, i.e. the percentage of fine particles generated. A good correlation has been obtained between the model-predicted values and the experimental results of spherical and extruded catalysts. In addition, the pressure drop of the catalyst packed bed is also examined. It is shown that, along with the mechanical failure of the catalyst particles, there is a turning point at which the pressure drop begins to increase rapidly. It is proposed that this point is crucial for the mechanical reliability of the catalyst packed bed. Experimental results reveal that the rapid increase in the pressure drop is attributed to a mutation action, occurring as the amount of failed catalyst particles reaches a certain critical value, and that the secondary breakage of the catalyst particles is the reason for the rapid increase in the pressure drop. Finally, factors analyses of the mechanical properties of solid catalysts are carried out in the impregnating, drying, calcination and sulfidation processes of a PCoMo/Al2O3 hydrotreating catalyst. Mathematical models for the mechanical properties of the catalyst in the above processes are developed with using response surface methodology. Statistical experimental designs such as central composite rotatable design, central composite orthogonal design and Doehlert design are performed to study simultaneously the effects of the process parameters on the mean strength, Weibull modulus, pellet density and catalytic activity in benzene hydrogenation. Analysis of variance reveals that the models developed are adequate. The validity of the models is also verified by experimental data. Results reveal that the impregnating, drying, calcination and sulfidation are all important processes for the mechanical strength and reliability of the catalyst. The calcination is of great advantage to the mechanical properties. The pellet density is only dependent on the impregnating and drying processes, without reference to the calcination and sulfidation. The sulfidation process has significant effects on the catalytic activity, however, can notbe depicted adequately with a quadratic model. It is also confirmed that central composite design, Doehlert design and response surface methodology are effective techniques employed for mathematical modeling and factors analyses of the mechanical properties of solid catalysts, and are also promising for the application in optimizing other catalytic properties such as activity. In this thesis, it is also pointed out that research on the mechanical properties of solid catalysts involves a complicated multi-scale system. Its objective is to establish a mechanical reliability model of the catalyst packed bed. It is elucidated that research on each characteristic scale and the interferences between the characteristic scales are very significant.