An extract component is separated from a raffinate component contained in a feed mixture by a process in which a unidirectional fluid flow system is maintained through a series of separating units through which the components travel at different rates. A component concentration distribution is established within the system of units and divided into specific zones. Feed and displacement fluid are passed into the inlets of two of the units and extract and raffinate are taken as the entire streams from outlets of two of the units all at appropriate points on the component concentration distribution. Other inlets and outlets of the various units lying in the same zone are interconnected. At the appropriate times the inlets and outlets are shifted so as to simulate movement of the units in a direction co-current with the fluid flow and thereby enable the inlets and outlets to continually lie in the appropriate zones. Zones may be combined as zone pairs, each such pair thereupon being considered a single continuous zone.