Abstract: An arrangement for permitting a first liquid to flow close to a second liquid, but without being intermixed therewith, includes a first plurality of plates (12) and a second plurality of plates (12), which are located between plates in the first plurality of plates but spaced therefrom by means of separating pieces. Each separating piece includes a frame formed about plate-shaped plane-parallel partial pieces, viz. an outer porous and hydrophobic plastic disphragm, an intermediate distance network of plastic and an outer thin plastic foil. The frame includes infeed holes for air and outfeed holes for water.The voids of the plastic diaphragm have a radius within the range of 2 to 5 .mu.m. The distance network has a thickness of 1 to 2 mm and consists of intersecting strips. The plastic foil has a thickness of about 0.1 mm.The plates include a frame with four through holes, of which two holes have connection to a recess in the plate, while the other two holes have no such connection.