Method for forming carbon filters

In a process for preparing carbon fibers in a gas phase reaction, the improvement comprising introducing a gaseous mixture comprising a gaseous carbonaceous compound and a decomposable iron-containing compound into a reactor having an inlet, an outlet and a passageway therebetween defining first and second flow paths in which the first flow path is co-extensive with at least a portion of the second flow path and is encompassed by at least a portion of the second flow path;

• flowing the mixture in the passageway in the reactor through the first flow path and then through the second flow path;
• changing the velocity of the mixture from a first velocity in the first flow path to a second velocity in the second flow path where the second velocity is less than the first velocity;
• and heating the reactor to establish a hot zone within the reactor and a relatively cool zone adjacent the reactor outlet to create a first temperature adjacent the inlet, a second temperature greater than the first temperature within the hot zone to heat the first and second flow paths, decompose the iron-containing compound to provide particles of an iron-containing decomposed product and to sustain fiber growth, and a third temperature less than the second temperature adjacent the outlet to terminate fiber growth or to minimize convective currents within the reactor;
• and intermittently interrupting the flow of the gaseous mixture and flowing a purge gas stream through the reactor at a pressure sufficient to forcibly move at least a portion of any carbonaceous material clogging the reactor passageway out of the reactor;
• and then during the interruption, flowing an oxygen-containing gas through the reactor to oxidize any remaining carbonaceous material in the reactor.