This invention relates to an apparatus for the separation of a liquid into two fractions (in the following referred to as a permeate fraction and a concentrate fraction, respectively) by filtration through semi-permeable membranes, the apparatus comprising a stack of semi-permeable membranes and elongated membrane support plates arranged so as to form flow passages extending along the surfaces of the membranes, each membrane support plate comprising at least two spaced holes extending therethrough so as to form two main flow passages extending longitudinally of the stack, means for supplying liquid to be fractionated to one of the main flow passages, means for discharging a permeate fraction and means for discharging a concentrate fraction from the stack.
The prior art apparatuses of the above-mentioned type may be designed in a manner such that the liquid to be fractionated flows through the flow passages extending parallel to the membrane surfaces in a series flow pattern, a parallel flow pattern or a combination of a series flow and a parallel flow pattern.
A prior art apparatus comprises elliptical membrane support plates and the centres of the holes provided in the membrane support plates are located in or adjacent to the focal points of the ellipses.
Such a prior art apparatus suffers from the drawback that the liquid flowing along the membrane surfaces from one main flow passage towards the other is not uniformly distributed over the membrane surfaces and consequently an optimum filtration efficiency cannot be achieved.
It has been attempted to obtain an improved distribution of the liquid to be fractionated over the membrane surfaces by using membrane support plates having continuous ribs projecting from the surfaces of the plates. When assembled in a stack of membranes and membrane support plates, the ribs cause the membranes located between two membrane support plates to be pressed together within narrow zones, thus forming several separate flow passages extending from one main flow passage to the other one. Similar flow passages may be formed in case only one membrane is located in the space between two adjacent membrane support plates.
These measures have not provided a fully satisfactory solution to the problem discussed above, i.e. because of the fact that the flow resistance of the relatively long flow passages is considerably higher than that of the relatively short flow passages.