A Fischer Tropsch ("FT") unit (115) that includes an FT tube (110) that is packed with a catalyst. The catalyst is designed to catalyze an FT reaction to produce a hydrocarbon. An insert (100) that is positioned within the FT tube (110). The insert (100) comprises at least one cross-piece (118) that contacts an inner surface (126) of the FT tube (110) and at least one cross-fin (127) extending from the cross-piece (118). There may be a corresponding second cross-fin adjacent each cross-fin (127). Both the cross-fins and the second cross-fins may be disposed radially outwardly such that the edge (131) of the cross-fins (127) are closer to the inner surface (126) of the FT tube (110) than is the base (133) of the cross-fins (127).