Multi-cylinder reverse osmosis apparatus and method

A method of membrane separation of a feed fluid into permeate fluid and concentrate fluid fractions which respectively are permeated and rejected by selective membrane means, the membrane means being exposed to pressurized feed fluid supplied by a reciprocating feed pump having at least three feed cylinders in a cooperating set, each cylinder having a displacer means which is reciprocable within the respective feed cylinder, the displacer means dividing the respective cylinder into a pumping chamber in which, during a pumping stroke, feed fluid is pressurized and an expansion chamber in which concentrate fluid is depressurized, each cylinder having a displacer rod for reciprocating the respective displacer means, each cylinder cooperating with valve means in conduit means which communicate with the membrane means, the valve means including an expansion chamber valve means which controls flow relative to the corresponding expansion chamber;

• the method, which occurs in each cylinder, including the steps of:
• inducting feed fluid into a particular pumping chamber and simultaneously exhausting concentrate fluid from a corresponding expansion chamber during an induction stroke, followed by pressurizing the feed fluid in the pumping chamber and simultaneously admitting pressurized concentrate fluid into the corresponding expansion chamber to supplement energy supplied to the displacer means during the pumping stroke, the method being further characterized by:
• (a) actuating directly with each displacer rod a valve portion of an associated expansion chamber valve means of each cylinder so as to control flow relative to an expansion chamber of a cylinder other than that directly associated with the displacer rod,
• (b) actuating the displacer rods of the cylinders so that actuation of the expansion chamber valve means is phased in such a way that when a particular displacer rod of a particular cylinder reaches an end of each stroke thereof, another displacer rod associated directly with the expansion chamber valve means controlling that particular cylinder is approaching a mid stroke position of said other displacer rod, so that the expansion chamber valve means directly associated with said other displacer rod is entering a closed intermediate position thereof during which no fluid passes therethrough,
• (c) permitting initial travel of the displacer rod of each cylinder following beginning of each respective displacer rod stroke while the corresponding expansion chamber valve means controlling that particular cylinder is in the closed intermediate position thereof.