It is shown that the periodic state of the simulated-moving-bed (SMB) process is reproduced by a single-column chromatographic process with a recycle lag of (N - I)T time units, where N is the number of columns of the equivalent SMB unit and T is the switching time. The operation of this ideal single-column process is formulated by selecting an arbitrary column of the SMB and following its operation over a complete cycle. To implement the recycle lag in practice, a special type of plug-flow tube has been designed. It includes internal elements to make the flow as close as possible to plug flow, and a piston to compensate for the difference between the inlet and outlet flow rates. This is necessary because the recycle lag is not a multiple of the overall cycle duration. The proposed system is a more compact, less-expensive, and simpler-to-operate alternative to the SMB. It avoids the problem of packing reproducibility and the total pressure drop is 1/Nth of that of the equivalent SMB. Depending on the efficiency of the recycle tube, our single-column process can achieve the same purities as the analogous SMB unit while keeping the specific productivity constant. (c) 2005 American Institute of Chemical Engineers.