Measuring the strength of slurry phase heterogeneous catalysts

Abstract

Fischer–Tropsch (F–T) synthesis is a process used to convert coal-derived syngas to hydrocarbon liquids and waxes. A slurry phase bubble-column reactor (SBCR) is the preferred reactor type due to improved heat and mass transfer and operational simplicity in terms of catalyst loading and discharge. A potential disadvantage in the SBCR vs. a fixed-bed reactor is the attrition of the catalyst which can cause difficulty with the catalyst/wax separation, resulting in gradual loss of catalyst from the reactor. In this work, we have evaluated two approaches to measure the strength and attrition resistance of heterogeneous catalysts: uniaxial compaction and ultrasonic fragmentation. A commercial catalyst developed for F–T synthesis was tested along with a sample of an alumina support having a similar particle size distribution. It was found that the cumulative particle size mass distribution plots after ultrasonic fragmentation show significant differences in strength, whereas the same powders show small differences in strength as measured by the uniaxial compaction method. Erosion was found to be the dominant fragmentation mechanism for the alumina support whereas fracture was the dominant mechanism for the F–T catalyst. Ultrasonic fragmentation also was applied to Fe F–T catalysts containing a kaolin binder. The catalyst with the binder was very weak, comparable to the binderless catalyst. Further analysis using transmission electron microscopy (TEM) showed that kaolin and the Fe F–T catalyst occurred as two distinct phases, with plate-like structures which did not help create strong interlocking forces between them. These results provide clues for the design of attrition resistant catalysts.