Abstract:
The invention concerns a method of and an apparatus for continuously desalinating water by reverse osmosis, in particular desalinating sea water, wherein salt water is introduced under a first pressure by means of a delivery pump into a pressure compensating device, salt water is continuously introduced from the pressure compensating device at a second increased pressure into a membrane module and separated therein by means of a membrane into desalinated water and concentrated salt water, and the concentrated salt water discharged from the membrane module is continuously introduced under approximately the second pressure into the pressure compensating device and used there for acting with approximately the second pressure on the salt water introduced into the pressure compensating device and for introducing the salt water into the membrane module. In order to avoid disturbances in operation and possibly damage to the membrane because of a reduced flow over the membrane surface, it is provided in accordance with the invention that a continuous flow of the salt water introduced into the membrane module is maintained over the surface of the membrane by means of salt water discharged from a reservoir.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention concerns a method and a corresponding apparatus for continuously desalinating water by reverse osmosis, in particular for desalinating sea water.  
         [0003]      2 . Description of the Related Art  
         [0004]     An apparatus of that kind is described for example in WO 02/41979 A1. In that apparatus the salt water is introduced under a first pressure into a pressure compensating device and from there passed under a second higher pressure into a membrane module. In the membrane module, it is separated into desalinated water and concentrated salt water. The discharged concentrated salt water which is approximately still at the second pressure is continuously introduced into the pressure compensating device again and is used therein for subjecting the salt water introduced into the pressure compensating device to approximately the second pressure and for introducing the salt water into the membrane module. More specifically the pressure compensating device described therein has two piston/cylinder devices which operate in opposite phase relationship and the pistons of which are fixedly connected together by a piston rod which is additionally driven.  
         [0005]     In desalination installations of that kind which operate on the basis of the principle of reverse osmosis, separation into concentrated salt water and desalinated water is effected at a so-called ‘crossflow’ membrane disposed in the membrane module. In the case of such a membrane, the salt water introduced flows along the surface of the membrane while a part thereof passes as desalinated water (drinking water) in a direction perpendicularly thereto through the membrane. The mutually crossing flows of water are also referred to as ‘crossflow’. In that case the flow on the surface of the membrane also flushes away unwanted foreign bodies on the surface of the membrane and accordingly therefore provides for continuously cleaning of the membrane.  
         [0006]     In the known configuration of the desalination apparatus having two piston/cylinder devices, a sufficiently high pressure is admittedly present at the moment of switching over the direction of movement of the pistons, to further press water through the membrane and thus produce desalinated water. It has been found however that the crossflow collapses at the time of switching over the direction of movement. As a result, at that moment the membrane is no longer sufficiently flushed so that the situation can involve salt molecules becoming concentrated on the surface of the membrane, and that can result in a rise in osmotic pressure and thus the operating pressure to the stage of a salt crust being formed on the surface of the membrane and operation being permanently interrupted.  
         [0007]     U.S. Pat. No. 4,187,173 and EP 0 018 128 A1 disclose a method of and an apparatus for desalinating water on the basis of reverse osmosis, wherein a respective pressure compensating container is provided both in the feed water circuit and also—in U.S. Pat. No. 4,187,173—in the concentrate circuit. Those pressure compensating containers are in the configuration therein of pulsation dampers or differential pressure dampers, in which a piston is displaceable in a cylinder and subdivides the interior of the cylinder into two chambers. For discharge of feed water disposed in a chamber, it is provided therein that pressure is applied to the piston by means of concentrate introduced into the second chamber, and a spring disposed in that chamber.  
         [0008]     FR 2 568 321 and EP 0 055 981 A1 disclose further apparatuses for and methods of reverse osmosis.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     According to principles of the present invention methods and apparatuses for continuously desalinating water by reverse osmosis which operate with a described membrane module, provide measures for avoiding the disruption of the desalination process.  
         [0010]     In accordance with the invention that object is attained by a method as set forth in claim  1 .  
         [0011]     A corresponding apparatus for resolving the problems described is set forth in claim  4 . Advantageous configurations of the method according to the invention and the apparatus according to the invention are recited in the dependent claims.  
         [0012]     In that respect, the invention is based on the realization that the problems described, in particular an interruption in operation by virtue of contamination and fouling of the membrane surface or indeed damage to the membrane can be avoided by the flow over the membrane being continuously maintained by suitable means. In accordance with the invention, provided for that purpose is a reservoir which acts on the salt water introduced into the membrane module and which, to maintain the flow over the membrane, additionally introduces water, in particular salt water, into the membrane module.  
         [0013]     In accordance with the invention there is further provided a piston-cylinder device having a piston which subdivides the cylinder interior into three chambers, wherein the salt water flowing out of the pressure compensating device is present in an inlet chamber, the concentrated salt water flowing out of the membrane device is present in an outlet chamber and a medium stored in a pressure reservoir, for example also water or a hydraulic liquid, is present under a high pressure in a pressure chamber. In that respect the desired effect of maintaining the flow by the discharge of water from the reservoir preferably occurs of its own accord. It is however also possible to provide a suitable control device for controlling the piston/cylinder device in order to afford the desired pressure-assisting effect.  
         [0014]     Some possible configurations of that piston-cylinder device are recited in claims  6  and  7 .  
         [0015]     It is preferably provided that, for example at the switching-over time in the case of the known apparatus with two piston/cylinder devices, a pressure drop or flow drop is bridged over in order to maintain the continuous flow over the membrane. By way of example suitable sensors can be provided for measuring a reduction in the flow over the membrane.  
         [0016]     In accordance with the invention, there are preferably provided two piston/cylinder devices which operate in opposite phase relationship, as are known from WO 02/41979 A1. The reservoir then provides that, upon a change in the direction of movement of the pistons, that is to say in particular at the moment when the pistons are stationary, an assisting pressure is exerted on the salt water. Thus in particular at that switching-over time, a possible pressure drop is compensated and the flow is maintained over the membrane.  
         [0017]     A further advantageous configuration is provided in claim  3 . In that case, the pressure required for discharge of the water from the reservoir is produced on the one hand from the pressure of the concentrated salt water discharged from the membrane module and in addition from a pressure stored in a pressure reservoir, wherein the pressure which results overall must naturally be greater if necessary than the pressure of the salt water flowing out of the pressure compensating device. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0018]     The invention is described in greater detail hereinafter with reference to the drawing in which:  
         [0019]      FIG. 1  shows a block circuit diagram to explain the method according to the invention, and  
         [0020]      FIG. 2  shows an embodiment of an apparatus according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The block circuit diagram in  FIG. 1  shows a delivery pump  1  for introducing salt water  10  into a pressure compensating device  2  under a first pressure p 1 . The same salt water  11  which however is now subjected to a high working pressure p 2  is passed from the pressure compensating device  2  to the membrane module  3 . There a part of the salt water  11  passes through the membrane  6  which is preferably in the form of a so-called crossflow membrane, for example 25% of the salt water  11 , it is desalinated in doing so and it is discharged in the form of desalinated water  12 . The remaining part of the salt water  11 , for example 75%, cannot pass through the membrane  6  but flows along the surface of the membrane  6  into the connecting conduit  5 , by way of which it is discharged from the membrane module  3  as concentrated salt water  13 . The concentrated salt water  13  which in that case is still at a high pressure which approximately corresponds to the pressure p 2  but is somewhat lower is then passed to the pressure compensating device  2  again. There, that high pressure p 2  is used in a manner that is still to be described in detail hereinafter for the purposes of acting with pressure on the salt water introduced into the pressure compensating device  2 , and feeding it to the membrane module  3  at the inlet thereof. At the same time that pressure is used in the pressure compensating device to definitively discharge concentrated salt water  14  therein, by way of the discharge conduit  4 , and to feed unconcentrated salt water  10  to the pressure compensating device  2 . All the described procedures take place in that case simultaneously and continuously so that there is no need for a high-pressure pump for subsequently delivering the high working pressure and desalinated water  12  is continuously available.  
         [0022]     As was described hereinbefore, particularly when using a crossflow membrane  6  it is necessary for the flow of the salt water over the surface of the membrane to be maintained continuously and under a uniformly high pressure as otherwise salt molecules can be deposited at the surface of the membrane, and such molecules can result in damage to the membrane or an interruption in operation of the system. By virtue of various circumstances however it can happen that the pressure p 2  of the salt water discharged from the pressure compensating device  2  briefly falls so greatly that the flow over the surface of the membrane would be reduced or even interrupted. Desalination would then admittedly still take place; it will be noted however that the membrane could be damaged as the concentrated salt water  13  cannot flow away out of the membrane module  3 . In order in such a situation to maintain the pressure p 2  and the flow, there is therefore provided in accordance with the invention a reservoir  15  which in such a situation passes additional water into the membrane module  3  and thus ensures that the high working pressure p 2  remains maintained and the flow over the surface of the membrane is not reduced.  
         [0023]      FIG. 2  shows a specific configuration of an apparatus according to the invention. It has two identical piston/cylinder devices  401 ,  402  with two aligned mutually opposite cylinders which each have a respective inlet chamber  201 ,  202  for receiving the salt water and a respective outlet chamber  101 ,  102  for receiving the concentrated salt water  13 . Arranged within each of the piston/cylinder devices  401 ,  402  is a respective special piston  301 ,  302  which subdivides the piston interior into the above-mentioned chambers and which in the Figure is displaceable in the horizontal direction within the piston/cylinder device.  
         [0024]     From the delivery pump  1 , a respective feed conduit with a passive non-return valve  7  leads to the inlet chambers  201 ,  202 . The non-return valves  7  in that case are of such a configuration that they open and permit a through flow when the pressure in the feed conduit is greater than in the inlet chambers  201 ,  202 . Comparable non-return valves  8  which however involve a different through-flow direction are disposed in the feed conduits from the inlet chambers  201 ,  202  to the membrane module  3 .  
         [0025]     In contrast, actively switchable main valves V 3 , V 6  and V 1 , V 4  respectively are arranged in the feed conduits  5  from the membrane module  3  to the outlet chambers  101 ,  102  and in the discharge conduits  4  from the outlet chambers  101 ,  102 ; the feed flow of the concentrated salt water  13  from the membrane module  3  and the discharge flow of the concentrated salt water  14  out of the pressure compensating device  2  respectively can be controlled by way of the above-mentioned main valves.  
         [0026]     The pistons  301 ,  302  are fixedly connected together by means of a piston rod  30 . Pinions  40  which for example can be driven by electric gear motors and which engage into a tooth arrangement provided on the piston rod  30  can drive the piston rod  30  and by way thereof the pistons  301 ,  302  in order to compensate for pressure losses.  
         [0027]     The pistons are arranged in such a way that they operate in opposite phase relationship. When therefore one piston is disposed in a position in which the volume of the inlet chamber  202  is at a maximum and the volume of the outlet chamber  102  is at a minimum, then the other piston which is connected by way of the piston rod  30  is in a position in which the volume of the inlet chamber  201  is at a minimum and the volume of the outlet chamber  101  is at a maximum (see  FIG. 2 ). In that situation the inlet chamber  202  is filled with water and the outlet chamber  101  is filled with concentrated salt water. The valves V 1 , V 3 , V 4  and V 6  which are illustrated here as switches are controlled in such a way that V 3  and V 4  are now closed while V 1  and V 6  are opened.  
         [0028]     In this connection opening a valve signifies producing a flow communication in order to allow a through-flow, for which purpose the valve is purely mechanically opened. Similarly closing a valve signifies interrupting a flow communication in order to prevent a through-flow, for which purpose the valve is purely mechanically closed.  
         [0029]     By virtue of the main valve V 1  being open, firstly the pressure of the concentrated salt water in the outlet chamber  101  escapes. By virtue of opening of the main valve V 6  the outlet chamber  102  is subjected to the effect of pressure (for example about 65 bars) and the concentrated salt water flows into that chamber. At the same time the salt water disposed in the inlet chamber  202  is pressed to the membrane module  3  by the piston subjected to pressure.  
         [0030]     As the pistons are arranged in such a way that they operate in opposite phase relationship, introduction of the concentrate which is subjected to pressure (for example 65 bars) into the outlet chamber  102  by the piston rod  30  causes movement of the other piston  301  which as a result empties the pressure-less outlet chamber  101 . At the same time a reduced pressure is produced in the inlet chamber  201  and sucks in the salt water and fills that chamber.  
         [0031]     When the outlet chamber  102  is filled the main valves are suitably controlled and the opposite procedure takes place.  
         [0032]     As the membrane module is preferably operated at about 70 bars in order to provide for a sufficiently high level of fresh water, and at a maximum about 5-10 bars occur as a pressure loss at the membrane, at least the above-mentioned pressure of about 65 bars of the concentrated salt water is still available at the concentrate discharge  5  of the membrane module  3 .  
         [0033]     In order to maintain the flow of the water along the surface of the membrane  6  during the operation of switching over the direction of movement of the pistons  301 ,  302 , in particular at the moment when the pistons  301 ,  302  are stopped, in accordance with the invention there is provided an additional piston/cylinder device  403 , referred to hereinafter as the piston reservoir. It has three chambers, namely a feed water chamber, inlet chamber  203 , which is connected to the feed conduit for the salt water  11  which is fed in, a concentrate chamber, outlet chamber  103 , connected to the concentrate conduit  5  and a pressure chamber  503 . In that arrangement the pressure chamber  503  is connected on the one hand by way of an active valve V 7  to the feed conduit  11  and on the other hand directly to a pressure reservoir  20 , preferably a bladder reservoir. During operation the valve V 7  is always closed, it only serves to be able to fill up the circuit comprising the pressure chamber  503  and the pressure reservoir  20  again with the pressure fluid, for example a hydraulic fluid, after an interruption in operation, and to restore the required high pressure in the pressure reservoir  20 .  
         [0034]     If the effective piston area of the piston  303  in the concentrate chamber  103  is about three quarters of the piston area in the feed water chamber  203  and the piston area in the pressure chamber  503  is about a quarter of that area, the pressure distributions are as follows. The feed water chamber  203  is subjected to a pressure of about 70 bars in operation. That results in up to 280 bars in the circuit comprising the pressure chamber  104  and the pressure reservoir  20 . They are however not attained in operation. The operating pressure in that region is between about 200 and 210 bars.  
         [0035]     At the time of switching over the direction of movement of the pistons  301 ,  302 , a pressure of about 70 bars acts on the piston  303 , from the feed water chamber  203 . The pressure in the reservoir  20  is only 160 bars. Then, from there, because of the smaller piston area in the pressure chamber  503 , the pressure acting is about 160/4, that is to say about 40 bars. The pressure in the concentrate circuit, that is to say the pressure of the concentrated salt water  13  discharged from the membrane module  3 , is about 68 bars. That pressure acts on an area which includes three quarters of the piston area. Consequently a pressure of about 51 bars acts here. Those two pressures act in the same direction and are thus added to give a total of about 91 bars. Only the approximately 70 bars in the feed water chamber  203  acts against that resulting pressure. Accordingly there is a sufficiently high pressure to press the piston  203  downwardly in the illustrated position and thus to maintain the flow over the membrane  6 .  
         [0036]     Even if it is only a pressure of about 60 bars that is taken as the basis for the concentrate circuit, that still affords a proportion of 45 bars in the concentrate chamber  103 . Even if the pressure in the pressure reservoir  20  is only 120 bars, that results in a further 30 bars, so that there is still an overall pressure of 75 bars, which allows the flow over the membrane  6  to be maintained.  
         [0037]     The piston reservoir  403  can be controlled in such a way that it is only in the case of a pressure drop in the connecting conduit between the inlet chambers  201 ,  202  and the membrane module  3  or a reduction in the flow over the membrane  6 , that an additional pressure is exerted on that connecting conduit. For that purpose it is possible to provide for example suitable sensors which detect such a pressure drop or a reduction in flow and which trigger the appropriate pressure control procedure. In addition it is also possible to provide valves which are suitably controlled for that purpose in the concentrate conduit  5  between the membrane module  3  and the piston/cylinder device  403 , which are opened if necessary, in order to produce the described movement of the piston  303  downwardly by the introduction of a pressure into the concentrate chamber  103 . If in contrast such pressure assistance is not required, such a valve can also be closed again so that, because of the higher pressure in the feed water chamber  203  in relation to the pressure chamber  503 , the piston  303  is moved upwardly again and thus remains virtually in the readiness position.  
         [0038]     In the case of the piston reservoir  403  according to the invention however such a control can be omitted as it can automatically set the specified pressure conditions in operation and the desired effect is achieved by separate control. On the one hand then feed water can flow out of the chamber  203  and on the other hand concentrate can flow out of the membrane module  3 , into the chamber  103 , so that the flow over the membrane  6  is maintained.  
         [0039]     In addition it is also possible to additionally provide secondary or bypass valves parallel to the described main valves V 1 , V 3 , V 4 , V 6  in order to reduce the loading on the main valves and thus to increase the service life thereof. In addition it is also possible to provide one or more quantitative flow limiting devices which are intended to prevent an abrupt pressure compensation effect insofar as they limit the maximum quantitative through-flow and thus contribute to a gradual compensation of pressure and slow changes in pressure, instead of abrupt pressure fluctuations. Elements of that kind and further elements are described and illustrated in above-mentioned WO 02/41979 A1 to which reference is hereby expressly directed and the description of which is to be deemed to be included herein. The basic mode of operation of such an apparatus with two piston/cylinder devices is also discussed in detail therein, and reference is also made thereto.  
         [0040]     The invention can also be used in relation to apparatuses of a different configuration for the desalination of water by reverse osmosis, which for example, instead of the illustrated two piston/cylinder devices, have another number of such devices, for example one or three piston/cylinder devices. They can in principle also be of a different configuration. The configuration of the reservoir in the form of the piston/cylinder device with three chambers, as is shown in  FIG. 2 , is also not absolutely necessary but in principle can also be of a different nature.  
         [0041]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0042]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.