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Reverse osmosis, particularly for water desalination, uses semipermeable membranes which selectively permeate purified water from a feed solution pressurized by a reciprocating feed pump driven by rotating crankshaft. Pump action is assisted by depressurizing concentrate fluid within the pump cylind

Reverse osmosis, particularly for water desalination, uses semipermeable membranes which selectively permeate purified water from a feed solution pressurized by a reciprocating feed pump driven by rotating crankshaft. Pump action is assisted by depressurizing concentrate fluid within the pump cylinder, which has an internal displacer separating feed and concentrate fluids. Directional valves controlling concentrate fluid flow to and from pump cylinder are actuated mechanically by crankshaft. A dwell interval is provided in displacer motion to permit completion of directional valve actuation while displacer is stationary and displacer rod is providing compression or decompression action to approximately equalize pressure differences across valve ports prior to valve actuation. Detrimental concentration polarization is reduced by reducing pressure and flow fluctuations across membranes by using a differential surge absorber, a multi-feed cylinder pump in which the feed cylinders are phased apart, or a single feed cylinder pump with one or more recirculating cylinders phased equally from the feed cylinder to supply pulses of recycled concentrate fluid. In the multi-cylinder pumps, actuation of the piston rod of one cylinder can be used to actuate a directional valve of another cylinder, which can be a spool valve integral with displacer rod.

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A method of membrane separation of a feed fluid into permeate fluid and concentrate fluid fractions which are respectively permeated and rejected by selective membrane means, the membrane means being exposed to pressurized feed fluid supplied by a reciprocating feed pump means having a feed cylinder

A method of membrane separation of a feed fluid into permeate fluid and concentrate fluid fractions which are respectively permeated and rejected by selective membrane means, the membrane means being exposed to pressurized feed fluid supplied by a reciprocating feed pump means having a feed cylinder, a feed displacer and a feed displacer rod, the displacer dividing the cylinder into a pumping chamber in which feed fluid is pressurized and an expansion chamber in which concentrate fluid is depressurized, valve means in conduit means associated with the chambers determining fluid flow directions to and from the membrane means, the method being characterized by steps of: (a) as the displacer rod initiates an induction stroke, permitting yielding between portions of the displacer and the displacer rod so that there is relative movement therebetween, (b) whilst inducting feed fluid into the pumping chamber, simultaneously exhausting depressurized concentrate fluid from the expansion chamber, (c) after the end of the induction stroke of the feed displacer, shifting the valve means across a closed intermediate position thereof during an interval of zero fluid transfer from the cylinder by the displacer, and when pressure difference across the valve means is relatively low, (d) as a displacer rod initiates a pumping stroke, permitting yielding between portions of the displacer and the diplacer rod so that there is relative movement therebetween, (e) whilst pressurizing feed fluid in the pumping chamber to force the pressurized feed fluid to the membrane means, simultaneously admitting into the expansion chamber pressurized concentrate fluid fraction from the membrane means to depressurize the concentrate fluid fraction to supplement energy supplied to the feed displacer in the pumping stroke, so that pressure differences across the displacer are relatively low, (f) after the end of the pumping stroke of the feed displacer, shifting the valve means during an interval of zero fluid transfer in the expansion chamber and when pressure difference across the valve means is relatively low.