Sustainable deep hydrodesulphurization of engine fuels is dominantly carried out in catalytic trickle bed reactors. Also testing of catalysts life cycle needs long time experimentation in pilot scale reactors to evaluate catalyst decay and activity loses. To avoid occurrence of temperature gradients, poor catalyst wetting and fluid maldistribution a dilution of bed of industry scale catalyst particles by fine inert grains is used. In contradiction to full scale reactors this dilution changes interfacial area in the bed and affects namely external mass transfer of hydrogen. Also hydrodynamics, pressure drop, gas and liquid holdups and fluid axial dispersion seems to be quite different.