If stored for long periods of time, catalyst fines may settle out of the fuel and build up as sediment in storage tanks. If the tanks are not drained regularly, this sludge can be disturbed in heavy weather and enter the fuel system.
The figure shows the distribution of combined aluminium and silicon in residual fuel worldwide. It may be seen that over 90% of the samples have a combined aluminium value less than 40 mg/kg. ISO 8127:2010 contains a limit for aluminium and silicon combined of 60mg/kg for residual fuel categories RMG and RMK. There are lower limits for lower viscosity fuels.
Reduction of catalyst fines to an acceptable level for inlet to the engine takes place in the settling tank and the centrifuge. The extent of this reduction depends on the water content of the fuel, as catalyst fines are “hydrophilic”, in that they attract water and become
contained in a water shell. Inclusion in the fuel of significant volumes of used lube oil may also limit the effective removal of fines.
The rate of settling is determined by Stokes’ Law, which takes account of the particle size, difference in density of the catalyst fine and the fuel, and the viscosity of the fuel. Various values are quoted for the density of catalyst fines, but in reality they may be likened to honeycombed structures, which retard the rate of separation. This is further hindered by the outer shell of water by virtue of the close proximity of the density of water to that of the fuel.
The extent of the removal also depends on the height of the tank (fixed) and the size of the particles (variable). As far as the centrifuge is concerned, the critical factor is the relationship between the actual viscosity of the fuel and that for which the centrifuge was sized. If there is a difference in viscosity, the residence time of the fuel in the centrifuge will be greater than the design value; hence directionally the centrifuge should be able to remove fines of a smaller size. Whilst this approach is theoretically correct, the operational result is totally dependent on the size distribution of the fines. With the introduction of modern centrifuges without gravity discs the recommendation is now to operate all available in parallel, which enables the flow through each to be reduced to the minimum practical level. The fuel is afforded the longest residence time in the centrifuges and the highest separation efficiency can be achieved. Combined output should be equal to the consumption. The temptation of using a higher rate so the daily service tank overflows back to the settling tank and is re-circulated should be avoided.
Cat fines will damage fuel injection equipment. The fines are particles of spent aluminium and silicon catalyst that arise from the catalytic cracking process in the refinery. The fines are in a form of complex alumino-silicates and, depending on the catalyst used, vary both in size and hardness. If not reduced by suitable treatment, the abrasive nature of these fines will damage the engine, particularly fuel pumps, injectors, piston rings and liners.