Injection of additive into a unit for synthesising hydrocarbons starting from synthesis gas enabling a homogenous concentration of catalyst to be controlled and maintained

Process for synthesising hydrocarbons starting from a feedstock containing synthesis gas, in which a solid Fischer-Tropsch catalyst is employed in a three-phase reaction section operated so that said catalyst is maintained in suspension in a liquid phase by the circulation of a gaseous phase from the bottom to the top of said reaction section, characterised in that the following stages are carried out so as to control and maintain the homogeneity of the concentration of catalyst particles in the reaction section:

• (a) for all i between 1 and n−1, n representing the number of pressure measuring points disposed along the longitudinal axis of the reactor and being at least equal to 3, the differential pressures ΔPi between two consecutive pressure measuring points spaced apart by Δhi are measured and the pressure drops per metre ΔGi=ΔPi/Δhi are calculated,
• (b) for each pressure drop per metre in said catalyst suspension in the liquid phase ΔGj, a value is calculated corresponding to the difference between two consecutive pressure drops per metre divided by their mean (ΔGj+1−ΔGj)/((ΔGj+1+ΔGj)/2),
• (C) if at least one of the values calculated in the stage (b) is not less than 20%, at least one additive comprising at least one organosilicon polymer is injected at least at the bottom of said reaction section.