A solid product resulting from the nucleated growth of the product on solid material of either the same or different composition and having a density higher than the reaction medium is formed from one or more liquid phase reactants by a method which comprises tangentially introducing the liquid phase reaction medium into the lower, smaller end of an inverted, frusto-conical reactor-separator, thereby imparting an upward swirling motion to the reaction medium in the reactor-separator, the horizontal velocity at the bottom of the reactor-separator being sufficiently large to cause fluidization of larger, solid product particles and concentration of them in the central lower portion of the reactor-separator and the vertical velocity at the top of the reactor-separator being sufficiently small to avoid carry-over of the smaller solid particles but sufficiently large to concentrate them in the upper portion of the reactor-separator; at least periodically recovering the larger, solid product particles in spherical form from the bottom of the reactor-separator; and recovering fluid products from the top of the reactor-separator. The method described is useful in a variety of reactions wherein a solid product forms by a nucleated growth mechanism. Examples of such reactions include thermal upgrading of petroleum derived feedstocks and coal liquids, synthesis of zeolites and Ziegler-Natta polymerization of olefins.