NOVELTY - Device (I) comprises reactors having first and second flow paths to enable the reactants from the second reactant flow path to be continuously introduced to at least a portion of the first reactant flow path.    USE - (I) is useful to remove carbon monoxide from a reactant stream (claimed).    ADVANTAGE - The invention deals with carbon-monoxide clean-up subsystems that are more efficient.    DETAILED DESCRIPTION - Device (I) comprises a reactor defined by at least one first reactant flow path (A), where: (A) comprises a length; the reactor comprises at least one second reactant flow path (B) proximate to (A); the reactor further comprises at least one catalyst disposed along (A); (A) is configured to enable a first reactant stream to flow along the length of (A) from a beginning of the length of (A) to an end of the length of (A); (A) is configured to enable the first reactant stream to contact the catalyst; (B) is configured to enable a second reactant stream to flow proximate to the first reactant stream along at least a portion of (A) from the beginning of the length of (A) to the end of the length of (A); and (B) is configured to enable a second reactant from the second reactant stream to be continuously introduced to (A) along at least a portion of the length of (A).    INDEPENDENT CLAIMS are also included for    (1) a device (II) comprising a preferential oxidation reactor defining at least one channel, at least one first reactant flow path, and at least one second reactant flow path, where: the first reactant flow path comprises hydrogen and carbon monoxide; the second reactant flow path comprises oxygen; the channel comprises a length, an inlet, and an outlet; the first reactant flow path extends along the length between the channel inlet and the channel outlet; the channel comprises a catalyst selected to alter a rate of reaction of carbon monoxide and oxygen; and the channel is configured such that the second reactant flow path is placed in communication with the portion of the first reactant flow path via a partition that is configured to define a substantially continuous introduction of the oxygen to the first reactant flow path along at least a portion of the length of the channel;    (2) a method for removing carbon monoxide from a reactant stream, comprising: providing the first reactant flow path comprising a length; providing at least one catalyst along the first reactant flow path; providing the second reactant flow path proximate to the first reactant flow path; flowing a first reactant stream comprising carbon monoxide and hydrogen along the first reactant flow path from a beginning of the length to an end of the length, where the first reactant stream is defined by a carbon monoxide concentration, and the first reactant stream contacts the catalyst; and flowing a second reactant stream along the second reactant flow path, where: the second reactant stream comprises oxygen; the second reactant flow path is configured such that the oxygen from the second reactant stream is continuously introduced to at least a portion of the first reactant flow path along the length of the first reactant flow path; and the carbon monoxide concentration of the first reactant stream is reduced between the beginning of the length of the first reactant flow path to the end of the length of the first reactant flow path; and    (3) a device, comprising a reactor defined by at least one channel where: the channel comprises a length, an inlet, and an outlet; the reactor comprises at least one first reactant flow path along the channel between the inlet and the outlet; the reactor comprises at least one second reactant flow path proximate to the channel; at least a portion of the first reactant flow path and at least a portion of the second reactant flow path are separated by an oxygen porous membrane (yttria stabilized bismuth oxide, silver, ETS-4 zeolite material, solid oxide fuel cell (SOFC) electrolyte material and/or polyimide and titanium oxide/polyimide composite), the reactor further comprises at least one preferential oxidation catalyst disposed along the channel; the first reactant flow path is configured to enable a first reactant stream to flow along the length of the channel from the inlet to the outlet; the first reactant flow path is configured to enable the first reactant stream to contact the preferential oxidation catalyst; the second reactant flow path is configured to enable a second reactant stream comprising oxygen to flow proximate to the channel along the length of the reactor from the inlet to the outlet; and the second reactant flow path is configured to enable the oxygen from the second reactant flow stream to permeate the membrane and be continuously introduced to at least a portion of the first reactant flow path along the length of the channel from the inlet to the outlet.