A process for carrying out a chemical reaction comprising: supplying gaseous reactants to a first reaction zone comprising a first catalyst of a first catalyst particle equivalent diameter, expressed as 6X(volume of catalyst/surface area of particle); operating said first reaction zone such that when the reactants are contacted with the first catalyst a portion of the reactants are converted to the desired product; removing an intermediate stream comprising the unreacted reactants and the desired product and passing the stream to a second reaction zone comprising a tubular reactor comprising a plurality of tubes, wherein said tubes comprise a plurality of catalyst carriers containing a catalyst having a second catalyst particle equivalent diameter which is smaller than the first catalyst particle equivalent diameter of the first catalyst; operating said second reaction zone such that when the unreacted reactants in the stream from the first reaction zone are contacted with the second catalyst, at least some of the unreacted reactants are converted to the desired product; and recovering a product stream; wherein each catalyst carrier comprises: an annular container for holding the second reaction catalyst in use, said container having a perforated inner wall defining a tube, a perforated outer wall, a top surface closing the annular container and a bottom surface closing the annular container; a surface closing the bottom of said tube formed by the inner wall of the annular container; a skirt extending upwardly from the perforated outer wall of the annular container from a position at or near the bottom surface of said container to a position below the location of a seal; and a seal located at or near the top surface and extending from the container by a distance which extends beyond an outer surface of the skirt. supplying gaseous reactants to a first reaction zone comprising a first catalyst of a first catalyst particle equivalent diameter, expressed as 6X(volume of catalyst/surface area of particle); operating said first reaction zone such that when the reactants are contacted with the first catalyst a portion of the reactants are converted to the desired product; removing an intermediate stream comprising the unreacted reactants and the desired product and passing the stream to a second reaction zone comprising a tubular reactor comprising a plurality of tubes, wherein said tubes comprise a plurality of catalyst carriers containing a catalyst having a second catalyst particle equivalent diameter which is smaller than the first catalyst particle equivalent diameter of the first catalyst; operating said second reaction zone such that when the unreacted reactants in the stream from the first reaction zone are contacted with the second catalyst, at least some of the unreacted reactants are converted to the desired product; and recovering a product stream; an annular container for holding the second reaction catalyst in use, said container having a perforated inner wall defining a tube, a perforated outer wall, a top surface closing the annular container and a bottom surface closing the annular container; a surface closing the bottom of said tube formed by the inner wall of the annular container; a skirt extending upwardly from the perforated outer wall of the annular container from a position at or near the bottom surface of said container to a position below the location of a seal; and a seal located at or near the top surface and extending from the container by a distance which extends beyond an outer surface of the skirt. <EOS>