Abstract: Contactor-separator units of a sub-assembly in which an isotopic exchange between two immiscible phases takes place are associated in m lines and n columns. One of the phases leaving a contactor-separator unit is directly directed to the adjacent contactor-separator unit in the same line, whereas the other phase leaving the same contactor-separator unit, is directed directly to the adjacent contactor-separator unit in the same column. Each of the m lines and n columns is provided with a single pump. The instantaneous flow rates of each phase are substantially equal at the input and the output of each of the contactor-separator units.