An apparatus and method for producing chlorine dioxide includes a reactor for reacting an aqueous reaction solution including an aqueous acid solution and a solution of an alkali metal salt of a chlorite ion to form a product solution. The reactor includes a substantially cylindrical inner column for receiving the aqueous reaction solution and a substantially cylindrical outer column positioned in coaxial surrounding relation to at least a portion of the inner column. The outer column is for containing a temperature-controlled fluid for maintaining solution flowing through the inner column at a predetermined temperature. A stripper is in fluid communication with an outlet of the inner column for stripping chlorine dioxide from the product solution into a gas to provide a product gas and a stripped product solution. An absorber is provided for absorbing chlorine dioxide from the product gas to provide a substantially byproduct-free aqueous chlorine dioxide solution.
