Liquid purification or separation – Flow – fluid pressure or material level – responsive – Proportionate feed means
Reexamination Certificate
2001-12-04
2003-11-25
Walker, W. L. (Department: 1723)
Liquid purification or separation
Flow, fluid pressure or material level, responsive
Proportionate feed means
C210S650000, C210S134000, C417S404000, C417S534000, C417S313000
Reexamination Certificate
active
06652741
ABSTRACT:
The present invention relates to a method and an installation for filtering a liquid by using a membrane filter device.
The technical field of the invention is that of making semipermeable membrane filter devices.
The present invention relates more particularly to a method and apparatus for desalinating sea water or brine by reverse osmosis, and to methods and apparatuses for ultrafiltration of a liquid such as water, e.g. to produce water that is suitable for drinking or for irrigation.
A drawback of installations for filtering sea water in order to desalinate it is low efficiency: the energy consumed to obtain 1 cubic meter (m
3
) of desalinated water is about 5 kilowatt hours (kWh) to 10 kWh. In order to recover energy from the resulting supersalinated water, a turbine, such as a “Pelton” type turbine can be used, but given that the efficiency of a turbine is low, the overall efficiency of the installation is improved only a little. Furthermore, such installations fitted with centrifugal pumps and turbines are expensive, and their reliability and longevity are relatively poor.
In installations for desalinating sea water by reverse osmosis, the water to be treated is delivered to the inlet of a filter device at an inlet pressure which is greater than the osmotic pressure of water. As a general rule, the pressure with which water is fed to the inlet of the filter is not less than 25 bars, e.g. it lies in the range about 30 bars to 100 bars, and in particular in the range about 60 bars to 80 bars. The filter delivers both a concentrate of “supersalinated” water and a permeate of desalinated water (at a pressure which is close to atmospheric pressure). The pressure of the concentrate leaving the filter is generally at a pressure which is only slightly less than the feed pressure of water for desalination, e.g. it may be at a pressure which is lower by about 1 bar to 5 bars, given that the pressure drop in the filter is small.
In less powerful filter installations, in particular in nanofiltration installations for treating brine, the filter is fed with water to be treated at a pressure of about 10 bars and a concentrate is recovered at a pressure of about 4 bars to 8 bars.
U.S. Pat. No. 3,825,122 describes pumping apparatus for filtering a fluid by reverse osmosis, which apparatus comprises a plurality of cylinders in alignment defining a main fluid pumping chamber, a booster chamber for recovering energy from the concentrate, and a hydraulic chamber for actuating the apparatus by means of hydraulic fluid pressurized by a pump. Each chamber is provided with a piston that moves in reciprocating translation under the action of a piston rod which is common to all of the pistons. Although the object stated in that document is to maintain a constant flow rate of pressurized fluid, the system for reversing the travel direction of the rod by using end-of-stroke sensors controlling distributor valves placed on ducts connected to the chambers does not enable a continuous flow rate to be ensured. That is probably why that apparatus, like all other piston pumps systems, has not enjoyed effective industrial development for filtering by reverse osmosis. Filter membranes are extremely sensitive to variations in pressure and flow rate which cause them to become clogged or to break.
U.S. Pat. No. 4,432,876 describes various apparatus seeking to reduce fluctuations in water pressure and flow rate at the outlet from a pump: a device for simultaneously varying the volume of the pump chamber and the volume of an expansion chamber coupled to the pump chamber; two variants of the apparatus, one with a controlled valve and the other with two piloted check valves mounted head to tail, cause those two chambers to be put momentarily into communication while the piston is at the end of its stroke so as to clip pressure surges due to sudden opening and closing of valves located on the water ducts. That document also proposes apparatus comprising three or more pistons, driven by one or more common crank shafts, and it recommends avoiding machines having two, four, eight, or 16 pistons. In order to make the apparatus described in U.S. Pat. No. 4,432,876 more compact, and to eliminate the devices for varying chamber volume, U.S. Pat. No. 4,913,809 describes pumping apparatus having two pistons interconnected by a rod and driven by a double-acting hydraulic actuator, with the pressure controlling the position of a distributor valve provided on the water ducts being delivered with a small offset in time.
In spite of those improvements provided to piston pumps, it can be seen that present-day reverse osmosis installations essentially comprise low efficiency centrifugal pumps, since piston pump apparatuses are too complex and unsuitable for pressurizing water for delivery to membrane filters.
An object of the present invention is to provide a liquid-filtering method and installation that are improved.
An object of the present invention is to improve the overall efficiency of such filter methods and installations.
In a first aspect, the invention consists in proposing water pumping apparatus comprising at least two pumps, each pump comprising:
at least two chambers in alignment on a longitudinal axis;
at least two pistons respectively mounted to move in reciprocating translation in each of the two chambers; and
a transmission shaft for transmitting forces between the two pistons, which shaft extends in part in each of the chambers and is mounted to slide relative thereto along said longitudinal axis;
the pumping apparatus further comprising an actuator suitable for delivering the energy required for compressing the water, minus the energy recovered from the concentrate by said pistons, by causing the shaft to move—usually periodically—in reciprocating translation (sliding) together with the pistons in each of the pumps, and means for causing the shaft and the pistons of each pump to pause for a prolonged period at the end of each stroke, i.e. twice for each period of the periodic cycle, thus making it possible to avoid or greatly limit any variations in the pressure of the water at the inlet to the filter(s).
The pumping apparatus furthermore comprises means for accelerating one of said two pumps while another one of said two pumps is pausing for a prolonged period at the end of its stroke, thereby enabling the accumulated flow rate of water delivered by the pumps to the filter(s) to be maintained at a value which is substantially constant.
In the meaning of the present invention, the term “pause” means a duration during which at least one of said pistons, and in general both pistons of a pump together with the associated shaft, are substantially stationary; the duration of said pause is such that its ratio to the period of the cycle of the shaft (and of the pistons) is generally greater than 10
−3
; this ratio can rise to very high values, e.g. about 0.1 or more, in particular when said two pumps do not both have the same capacity; under such circumstances, the pause of the larger capacity pump will be of duration longer than the pause of the smaller capacity pump.
Nevertheless, in general, both pumps will have the same capacity and each will be controlled in such a manner as to perform end-of-stroke pauses of durations that are substantially identical for both pumps.
In order to control the slowing down followed by the pause at the end of a stroke, at least one of the chambers of at least one of the pumps is preferably fitted with a sensor for sensing the position of the piston (and/or the shaft) and positioned in such a manner as to issue a detection signal before said piston (and/or said shaft) reaches its end-of-stroke position; this detection signal is transmitted to an electronic control. unit which responds to receiving said signal by causing the energy delivered by said actuator of the pump in question to be stopped.
The driving energy supplied by said actuator is preferably transmitted to the water via a driving hydraulic fluid acting on a “drive” piston connected to said shaft in a manner similar
Ladas & Parry
Menon K S
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