An exothermic reaction of a gas with a gas permeable material is performed in a retention vessel. Temperature control of the exothermic reaction is obtained by recirculating the gas from the top of the vessel downwardly through and co-current to the gas permeable material and removing the downwardly flowing gas at a point above the bottom of the reactor and simultaneously recirculating gas which has been cooled into the bottom of the reactor, upwardly through and counter-current to the gas permeable material and removing the gas at a point above the bottom of the reactor.