Process and apparatus for production of granular polycrystalline silicon

In a process for producing granular polycrystalline silicon in a fluidized-bed reactor comprising a reactor tube and a heater outside the reactor tube, in which silicon nucleus particles (seed) are fluidized in the reactor tube by a gas stream in a fluidized bed which is heated by heater, and polycrystalline silicon is deposited by pyrolysis on the hot silicon nucleus particles by introduction of a silicon-containing reaction gas into the fluidized bed and granular polycrystalline silicon formed in this way is removed from the reactor tube, where the reactor tube has a fluidized bed region and an unheated region above the fluidized bed, an intermediate space between an interior wall of a vessel and an outer wall of the reactor tube containing an insulation material and the reactor tube has a silicon coating on its interior wall, the improvement comprising:

• reducing the deposition of silicon on an interior of the unheated region of the reactor tube above the fluidized bed by establishing a wall temperature of the unheated region of the reactor tube which is lower than the wall temperature of the reactor tube in the region of the fluidized bed, where, owing to the lower wall temperature, the deposition of silicon above the fluidized bed is reduced, and setting a defined axial temperature gradient in the reactor tube in the region above the fluidized bed of from 300 K/m to 5000 K/m, whereby the silicon coating in the total reactor tube has a maximum silicon layer thickness which has a ratio to the average silicon layer thickness in the fluidized bed region of from 7:1 to 1.5:1 after a period of operation of from 15 to 500 days, wherein the axial temperature gradient is achieved by employing the fluidized bed reactor comprising:
• the vessel having the interior wall into which the reactor tube having the outer wall is inserted, the reactor having the intermediate space between the interior wall of the vessel and the outer wall of the reactor tube, the intermediate space containing the insulation material, wherein the insulation material in the intermediate space in the reactor tube in the region of the unheated region above the fluidized bed is arranged such that the outside of the reactor tube above the fluidized bed is not insulated to the same degree as the region of the reactor tube adjoining the fluidized bed within the reactor tube.