A method of operation of a fluidized bed deposition reactor for top removal of granular material comprising: providing a vertical generally cylindrical reactor of a predetermined height, said reactor having at least one gas inlet at or near the bottom thereof and at least one gas and solids outlet at or near the top thereof; providing at least one separator to separate gas from granular product and a conduit between the gas and solids outlet and said at least one separator; establishing a heated reaction zone; providing a fluidizing gas to the reaction zone at a predetermined flow rate; providing granular particles to the reaction zone to establish the desired fluidized bed, said particles having a variable fluidized height, the bed being fluidized to establish a bubbling fluidized bed with a top of a defined stable height, a disengaging space being provided above said top, the height of said disengaging space being no greater than the distance between the defined top of the bubbling bed and the gas and solids outlet of the container; providing a gas to the reaction zone while adjusting the flow rate of the fluidizing gas so that a majority of the granular particles are retained in the reactor while maintaining bubbling, the reaction gas depositing a coating on said granular particles, thus increasing the size and weight of the particles resulting in an increase in the height of the bed, and allowing the height of the bed to increase and the disengaging height to decrease until the bursting of bubbles near the surface of the bed periodically throws granular particles out of the gas and solids outlet of the reactor, along the conduit and into the gas and granular product separator, characterized by that the flow rates of the fluidizing gas and the reaction gas are controlled so as to establish a stable height of the bed. providing a vertical generally cylindrical reactor of a predetermined height, said reactor having at least one gas inlet at or near the bottom thereof and at least one gas and solids outlet at or near the top thereof; providing at least one separator to separate gas from granular product and a conduit between the gas and solids outlet and said at least one separator; establishing a heated reaction zone; providing a fluidizing gas to the reaction zone at a predetermined flow rate; providing granular particles to the reaction zone to establish the desired fluidized bed, said particles having a variable fluidized height, the bed being fluidized to establish a bubbling fluidized bed with a top of a defined stable height, a disengaging space being provided above said top, the height of said disengaging space being no greater than the distance between the defined top of the bubbling bed and the gas and solids outlet of the container; providing a gas to the reaction zone while adjusting the flow rate of the fluidizing gas so that a majority of the granular particles are retained in the reactor while maintaining bubbling, the reaction gas depositing a coating on said granular particles, thus increasing the size and weight of the particles resulting in an increase in the height of the bed, and allowing the height of the bed to increase and the disengaging height to decrease until the bursting of bubbles near the surface of the bed periodically throws granular particles out of the gas and solids outlet of the reactor, along the conduit and into the gas and granular product separator, characterized by that the flow rates of the fluidizing gas and the reaction gas are controlled so as to establish a stable height of the bed.