Abstract: A continuous flow of particulate solids to a reactor is accomplished by first forming a heavy slurry in a storage tank, then transferring the heavy slurry to a mixing tank, where the heavy slurry is diluted to form a dilute slurry of desired density which can be continuously pumped to the reactor. Control of the particulate solids flow to the reactor is accomplished by calculating the flow rate of solid particles contained in the dilute slurry in an on-line computer, and comparing the calculated value to a desired value. The on-line calculation of particulate flow is based on the measured density and flow rate of the dilute slurry, and the predetermined densities of the dry particulate solids and the liquid diluent consituting the dilute slurry. The flow rate of the dilute slurry is continuously manipulated so as to maintain a desired flow rate of solid particles contained in the dilute slurry.