Transient Thermal Analysis of an FCC Catalyst Cooling Silo and Hopper

An engineering company supplying catalyst-handling equipment for a refinery's fluid catalytic cracking (FCC) unit needed to know how quickly spent catalyst would cool once it left the process. The catalyst arrives very hot, collects in a cooling silo and is then discharged to a catalyst hopper where it is held for several days before disposal. Both vessels had to be designed around realistic temperatures, and the client wanted to know whether extra cooling air would be needed in the hopper.

Shirsh Technosolutions carried out a transient thermal analysis of the catalyst cooling silo and the catalyst hopper. We modelled heat flow by conduction through the catalyst bed and the steel shell, natural convection and radiation from the outer surface, and the effect of injecting cooling air through air pads in the hopper cone.

Axisymmetric transient thermal model. Both vessels are bodies of revolution, so we built 2D axisymmetric finite element models of the silo and the hopper with the catalyst bed and the steel shell. This kept the mesh fine through the bed while allowing long transient runs at modest cost.

All three heat transfer paths. Conduction through the catalyst and the shell followed Fourier's law, with bulk density, thermal conductivity and specific heat assigned separately to the catalyst and to the steel. Natural convection from the shell used a film coefficient that varies with surface temperature, and radiation used the Stefan-Boltzmann law with an emissivity for oxidised steel. The governing equation was solved in the time domain, so the heat stored in the bed is captured as it cools.

Silo filling and cooling. The silo model was run through the filling and cooling period with the catalyst level rising over time. Temperature contours at successive time steps showed how each new charge of hot catalyst sits above layers that are already cooling, and where the coolest catalyst collects near the outlet. From these results we set a conservative discharge temperature as the starting condition for the hopper.

Hopper cooling options. Three hopper cases were compared: natural cooling only, cooling air injected through one air pad at the bottom of the cone, and air injected through two air pads at the bottom and the side. For each case we tracked the bed temperature along with the heat removed by convection and by radiation over time.

Clear direction for the design. With natural cooling alone a hot core stayed in the middle of the hopper bed. Air injection cooled the bed much faster, and the two-pad arrangement brought the whole bed close to ambient within the simulated period, giving the client a sound basis for specifying the hopper cooling arrangement.

Common questions

Why use transient thermal analysis for a catalyst cooling silo? The catalyst bed stores a large amount of heat and the silo keeps filling, so temperatures change with time and position. A transient analysis follows that history and gives the actual discharge temperature, which a steady-state calculation cannot.

How do convection and radiation enter a thermal FEA of a hot vessel? Both are applied as surface boundary conditions on the outer shell. Convection uses a film coefficient that rises with surface temperature, and radiation uses the Stefan-Boltzmann law with the shell emissivity, so heat loss is highest when the vessel is hottest and falls as it cools.