A fixed-bed tubular reactor which extends, along a longitudinal axis (XX'), between a first end and a second end , the reactor comprising a catalyst powder bed confined in an annular space delimited by a first wall of a hollow tube and a second wall of a hollow insert , disposed in the hollow tube coaxially with the latter, the hollow insert comprising at least one distribution chamber and at least one collection chamber , separated from one another by a separating wall , and comprising, respectively, a gas intake opening at the first end and a gas discharge opening at the second end , the second wall comprising at least one distribution opening and at least one collection opening , the distribution opening allowing the distribution of a gas capable of being admitted through the intake opening of the distribution chamber to the annular space , and the collection opening allowing the collection of the gas distributed in the annular space by the collection chamber , characterized in that the second wall of the hollow insert includes, on at least a first longitudinal portion (Pl) of the insert defined along the longitudinal axis (XX'), at least one offset distribution opening and at least one offset collection opening , any offset distribution opening being contained in a transverse plane (P1) to the longitudinal axis (XX') different from any transverse plane (P2) to the longitudinal axis (XX') containing an offset collection opening such that the direction of the flow of gas distributed and/or admitted between an offset collection opening and an offset distribution opening has an axial component along the longitudinal axis (XX'). the reactor comprising a catalyst powder bed confined in an annular space delimited by a first wall of a hollow tube and a second wall of a hollow insert , disposed in the hollow tube coaxially with the latter, the hollow insert comprising at least one distribution chamber and at least one collection chamber , separated from one another by a separating wall , and comprising, respectively, a gas intake opening at the first end and a gas discharge opening at the second end , the second wall comprising at least one distribution opening and at least one collection opening , the distribution opening allowing the distribution of a gas capable of being admitted through the intake opening of the distribution chamber to the annular space , and the collection opening allowing the collection of the gas distributed in the annular space by the collection chamber , characterized in that the second wall of the hollow insert includes, on at least a first longitudinal portion (Pl) of the insert defined along the longitudinal axis (XX'), at least one offset distribution opening and at least one offset collection opening , any offset distribution opening being contained in a transverse plane (P1) to the longitudinal axis (XX') different from any transverse plane (P2) to the longitudinal axis (XX') containing an offset collection opening such that the direction of the flow of gas distributed and/or admitted between an offset collection opening and an offset distribution opening has an axial component along the longitudinal axis (XX'). <EOS>