A hydrogen-processing assembly, comprising: an enclosure, comprising: a body defining an internal volume and the body having an internal perimeter, wherein the internal volume includes a mixed gas region and a permeate region; at least one input port extending through the body and through which a fluid stream is delivered to the enclosure; at least one product output port extending through the body and through which a permeate stream is removed from the permeate region; and at least one byproduct output port extending through the body and through which a byproduct stream is removed from the mixed gas region; and a hydrogen-separation assembly positioned within the internal volume in a spaced relation to at least a portion of the internal perimeter of the body of the enclosure, the hydrogen-separation assembly including at least one planar or generally planar hydrogen-selective membrane and having an outer perimeter measured in the plane of the at least one hydrogen-selective membrane, wherein the hydrogen-separation assembly is adapted to receive a mixed gas stream containing hydrogen gas and other gases and to separate the mixed gas stream into the permeate stream and the byproduct stream, wherein the permeate stream has at least one of a greater concentration of hydrogen gas and a lower concentration of the other gases than the mixed gas stream, and further wherein the byproduct stream contains at least a substantial portion of the other gases; wherein the permeate region of the internal volume is defined between at least a majority of the outer perimeter of the hydrogen-separation assembly and at least a portion of the internal perimeter of the body of the enclosure, optionally wherein the hydrogen-processing assembly is in combination with a fuel cell stack adapted to receive at least a portion of the permeate stream. an enclosure, comprising: a body defining an internal volume and the body having an internal perimeter, wherein the internal volume includes a mixed gas region and a permeate region; at least one input port extending through the body and through which a fluid stream is delivered to the enclosure; at least one product output port extending through the body and through which a permeate stream is removed from the permeate region; and at least one byproduct output port extending through the body and through which a byproduct stream is removed from the mixed gas region; and a body defining an internal volume and the body having an internal perimeter, wherein the internal volume includes a mixed gas region and a permeate region; at least one input port extending through the body and through which a fluid stream is delivered to the enclosure; at least one product output port extending through the body and through which a permeate stream is removed from the permeate region; and at least one byproduct output port extending through the body and through which a byproduct stream is removed from the mixed gas region; and a hydrogen-separation assembly positioned within the internal volume in a spaced relation to at least a portion of the internal perimeter of the body of the enclosure, the hydrogen-separation assembly including at least one planar or generally planar hydrogen-selective membrane and having an outer perimeter measured in the plane of the at least one hydrogen-selective membrane, wherein the hydrogen-separation assembly is adapted to receive a mixed gas stream containing hydrogen gas and other gases and to separate the mixed gas stream into the permeate stream and the byproduct stream, wherein the permeate stream has at least one of a greater concentration of hydrogen gas and a lower concentration of the other gases than the mixed gas stream, and further wherein the byproduct stream contains at least a substantial portion of the other gases; wherein the permeate region of the internal volume is defined between at least a majority of the outer perimeter of the hydrogen-separation assembly and at least a portion of the internal perimeter of the body of the enclosure, optionally wherein the hydrogen-processing assembly is in combination with a fuel cell stack adapted to receive at least a portion of the permeate stream.