A process of hydrogenation of a silicon tetrahalide and silicon to a trihalosilane comprising: providing a vessel with at least one inlet and one outlet with a plurality of silicon particles located in a bed inside the vessel and feeding a mixture of gases consisting primarily of a silicon tetrahalide and hydrogen. A calculational procedure is provided where the flow rate of the gases and the incoming size of the silicon particulates are chosen so that at least 90% of the volume of the bed is free of bubbles after allowance for decrease in particle size due to the reaction and attrition resulting in a higher yield of the trihalosilane.