The process of hotspot formation in the catalyst bed

Thread Content

1. The packing quality of a catalyst generally refers to the uniformity in the radial direction within the reactor bed, the compactness in the axial direction, and the gradation in the axial direction. 2. When the radial uniformity is poor, phenomena such as \"channeling\" and \"wall sticking\" occur, leading to short-circuiting of the material within the catalyst bed; this can even result in the collapse of certain sections of the bed. In conventional trickle-bed reactors, a high hydrogen-to-oil ratio is used, and the flow velocity of the gaseous phase within the reactor is much higher than that of the liquid phase. This difference in flow velocities between the gas and liquid phases can easily lead to phase separation. Once the catalyst bed becomes radially uneven, that is, when there are channels with different resistances within the bed, the gaseous stream, which consists mainly of cyclohexane, tends to flow through the channels with lower resistance and easier passage, while the liquid stream, which consists mainly of feed oil, is forced to flow through the areas of the catalyst bed where the particles are more densely packed. This leads to a separation between the gas and liquid phases, reducing the mass transfer rate between them and thereby diminishing the efficiency of the reaction. Furthermore, due to the poor heat transfer within the bed under these conditions, high-temperature \"hot spots\" occur in the bed. Once a \"hot spot\" appears, it accelerates the coking of the catalyst in that area, resulting in an increase in the pressure drop across the bed in that region. This, in turn, leads to a reduced flow rate of the gaseous material passing through that area, preventing the reaction heat from being removed in a timely manner and causing the temperature at that point to rise further, thus creating a vicious cycle.