NO309398B1

NO309398B1 - Process and plant for the production of fresh water from saline water

Info

Publication number: NO309398B1
Application number: NO992949A
Authority: NO
Inventors: Bjoern Lyng; Sigbjoern Lyng; Leif S Drabloes
Original assignee: Bjoern Lyng; Sigbjoern Lyng; Leif S Drabloes
Priority date: 1999-06-16
Filing date: 1999-06-16
Publication date: 2001-01-22
Also published as: NO992949L; AU5432700A; ES2162592B1; TNSN00134A1; EG22423A; PE20010470A1; WO2000076639A1; GC0000339A; NO992949D0; ES2162592A1

Description

Foreliggende oppfinnelse angår produksjon av ferskvann fra saltholdig vann, normalt sjøvann. Det benyttes en teknikk med omvendt osmose, hvor saltvann tilføres under høyt trykk ved hjelp av et system for tilveiebringelse av det høye trykket til en beholder med en membran, heretter kallt Omvendt Osmose Membran, som slipper gjennom en viss andel av vannet som ferskvann til ferskvannsdelen, som holdes tilnærmet på atmosfæretrykk. Denne type teknikk er kjent, og benyttes særlig i de deler av verden som har rikelig tilgang på sjøvann, men lite ferskvann. Det er imidlertid et problem med denne teknikken at den normalt vil føre til et relativt høyt energiforbruk. The present invention relates to the production of fresh water from saline water, normally seawater. A reverse osmosis technique is used, where salt water is supplied under high pressure by means of a system for providing the high pressure to a container with a membrane, hereinafter called Reverse Osmosis Membrane, which lets through a certain proportion of the water as fresh water to the fresh water part , which is kept approximately at atmospheric pressure. This type of technique is known, and is used particularly in those parts of the world that have abundant access to seawater, but little fresh water. However, there is a problem with this technique that it will normally lead to a relatively high energy consumption.

For at fenomenet Omvendt Osmose skal fungere tilfredsstillende, vil det være nødvendig å tilveiebringe et relativt høyt trykk av saltvann på den ene siden av osmosemembranen, mens den andre siden av membranen holdes tilnærmet på vanlig atmosfæretrykk. Saltionene vil da over membranen søke å oppnå en likevektstilstand hvor det på lavtrykksiden befinner seg vann uten salt, dvs ferskvann. Membranen vil med andre ord slippe igjennom ferskvann dersom det opparbeides et trykk som er større enn likevektstrykket (det såkalte osmotiske trykk, hvis størrelse er avhengig av saltkonsentrasjonen), samt trykkfallet på grunn av strømning gjennom membranen. In order for the Reverse Osmosis phenomenon to work satisfactorily, it will be necessary to provide a relatively high pressure of salt water on one side of the osmosis membrane, while the other side of the membrane is kept approximately at normal atmospheric pressure. The salt ions will then seek to reach an equilibrium state above the membrane where there is water without salt on the low pressure side, i.e. fresh water. In other words, the membrane will let fresh water through if a pressure is built up that is greater than the equilibrium pressure (the so-called osmotic pressure, the size of which depends on the salt concentration), as well as the pressure drop due to flow through the membrane.

Det benyttes typisk et trykk på f.eks. 65 barg på høytrykksiden. A pressure of e.g. 65 barg on the high pressure side.

De anlegg som er i bruk, og kan kjøpes, produserer trykk med bl.annet flertrinns sentrifugalpumper, skrupumper eller stempelpumper. Saltvannet blir så ført inn til en Omvendt Osmose Membran hvor typisk 25-45% av vannstrømmen skilles ut til ferskvann. Resten av saltvannet, senere kalt returlaken, typisk 55-75% , slippes så ut i det fri, og gjennomgår således et trykkfall på ca. 65 bar, og i forbindelse med dette tapes en stor energimengde. The plants that are in use, and can be purchased, produce pressure with, among other things, multi-stage centrifugal pumps, screw pumps or piston pumps. The salt water is then fed into a Reverse Osmosis Membrane where typically 25-45% of the water flow is separated into fresh water. The rest of the salt water, later called the return brine, typically 55-75%, is then released into the open, and thus undergoes a pressure drop of approx. 65 bar, and in connection with this a large amount of energy is lost.

Det er tilveiebragt systemer for å gjenvinne noe av denne energien. De systemer som er tilgjengelige i dag, baserer seg i hovedsak på å gjenvinne energien i en vannturbin, av enten pelton- eller francistype, og den gjenvunnede energien blir overført til pumpeaksling eller motoraksling. Dårlige praktiske oppnådde virkningsgrader av turbin eller av pumper og turbin har hittil ført til at kun moderate andeler av den energimengden det er mulig å gjenvinne, blir gjenvunnet. Systems are provided to recover some of this energy. The systems available today are mainly based on recovering the energy in a water turbine, of either the Pelton or Francis type, and the recovered energy is transferred to the pump shaft or motor shaft. Poor practical efficiencies of turbines or of pumps and turbines have so far led to only moderate proportions of the amount of energy that can be recovered being recovered.

Andre systemer for gjenvinning baserer seg på vekslingsbeholdere/sylindre med stempler eller fleksible, tette membraner, hvor retur-saltlaken driver stempelet eller den fleksible, tette membranen som igjen presser friskt saltvann til omvendt osmose-membranen. Det er beskrevet flere systemer etter dette prinsipp i andre patentpublikasjoner, jfr. for eksempel EP-A1-0,028,913, US 5,306,428, GB-A-2,030,056. I hovedsak består slike systemer som beskrevet, av to eller flere sylindre eller beholdere og hver sylinder eller beholder har en saltvannsside og en returlakeside, og disse er da skilt ved hjelp av stempel eller fleksible, tette membraner eller sekker. Other systems for recovery are based on exchange vessels/cylinders with pistons or flexible, tight membranes, where the return brine drives the piston or the flexible, tight membrane which in turn pushes fresh salt water to the reverse osmosis membrane. Several systems according to this principle have been described in other patent publications, cf. for example EP-A1-0,028,913, US 5,306,428, GB-A-2,030,056. Essentially, such systems as described consist of two or more cylinders or containers and each cylinder or container has a salt water side and a return brine side, and these are then separated by means of a piston or flexible, tight membranes or sacks.

Hver vekslingssylinder eller -beholder har to operasjonsmodi, den ene slik at returlaken under trykk fra omvendt osmose-filteret presser friskt saltvann til omvendt osmose-membranen. Nødvendig ekstra energi for å oppveie trykkfall i hele systemet, samt energi for å trykke opp en mengde til nødvendig trykk og som tilsvarer det produserte ferskvannet, tilføres enten direkte til et eventuelt stempel i sylinderen eller i en eller flere pumper, hvorav en som kan være høytrykkspumpe, og en kan være en trykkøkningspumpe. Når sylinderen eller beholderen nærmer seg å bli tømt for sjøvann, og dermed fylt av lake, må et styresystem sørge for å styre en eller flere ventiler slik at en ny sylinder eller beholder kan starte å pumpe saltvann. Sylinderen eller beholderen som er full av returlake, og så godt som tom for friskt saltvann, må da av styresystemet trykkavlastes til friluft og fylles opp igjen med friskt saltvann. Dette er den andre operasjonsmodus. Each exchange cylinder or container has two modes of operation, one so that the return liquor under pressure from the reverse osmosis filter pushes fresh salt water to the reverse osmosis membrane. Necessary extra energy to offset pressure drop in the entire system, as well as energy to press up a quantity to the required pressure and which corresponds to the fresh water produced, is supplied either directly to a possible piston in the cylinder or to one or more pumps, one of which may be high pressure pump, and one can be a pressure boosting pump. When the cylinder or container is about to be emptied of seawater, and thus filled with brine, a control system must ensure that one or more valves are controlled so that a new cylinder or container can start pumping salt water. The cylinder or container that is full of return brine, and virtually empty of fresh salt water, must then be depressurized by the control system to open air and refilled with fresh salt water. This is the second mode of operation.

Slike systemer vil ved vellykket utførelse kunne gjenvinne en meget høy andel av den trykkenergien som ellers kan gå til spille, anslagsvis 85-98%. Such systems, if successfully implemented, will be able to recover a very high proportion of the pressure energy that might otherwise be wasted, an estimated 85-98%.

Men slike systemer har til nå ikke hatt noen suksess, og er idag ikke å få kjøpt i markedet. Årsaken til dette er at ingen hittil har fått til denne prosessen uten store variasjoner og/eller trykkslag i vannmengdene til/fra omvendt osmose-membranene. But such systems have so far not been successful, and are currently not available on the market. The reason for this is that no one has so far been able to achieve this process without major variations and/or pressure surges in the amounts of water to/from the reverse osmosis membranes.

Det er også nødvendig for styresystemet å kunne vite hvor i sekvensen en er, men det er pr. idag med eksisterende teknikk ingen posisjonsindikatorer eller annen anordning som klarer å registrere dette, og samtidig tåle saltvannet og de trykkvariasjonene som prosessen krever. It is also necessary for the control system to be able to know where in the sequence one is, but it is per today, with existing technology, there are no position indicators or other devices that are able to register this, and at the same time withstand the salt water and the pressure variations that the process requires.

Herværende oppfinnelse er konsipert som en fremgangsmåte og et anlegg for å få metoden med trykkgjenvinning ved hjelp av sylindre/stempler eller beholdere med fleksible, tette membraner til å fungere uten avbrudd eller vesentlige trykkslag i saltvannsstrømmene til/fra omvendt osmose-filtrene, for således å gjøre denne metoden anvendelig for industriell utnyttelse og dermed kunne produsere ferskvann ved hjelp av omvendt osmose ved lavere energiforbruk enn hva som hittil har vært oppnåelig. Dette vil kunne oppnås ved hjelp av styresystemet, kombinasjonen av styresystemet og ventiltyper og metode for å registrere når sylinder/beholder er tom. Fremgangsmåten og anlegget ifølge oppfinnelsen defineres nøyaktig i de vedføyde patentkravene. This invention is conceived as a method and a facility to make the method of pressure recovery using cylinders/pistons or containers with flexible, tight membranes work without interruption or significant pressure surges in the salt water flows to/from the reverse osmosis filters, in order to make this method applicable for industrial use and thus be able to produce fresh water by means of reverse osmosis at a lower energy consumption than has hitherto been achievable. This can be achieved with the help of the control system, the combination of the control system and valve types and method for registering when the cylinder/container is empty. The method and the plant according to the invention are precisely defined in the appended patent claims.

Ved hjelp av en utførelsesform av oppfinnelsen kan en også unngå å bruke stempler eller fleksible membraner, og dermed øke driftssikkerheten ytterligere. Dette oppnås ved å basere skillet mellom returlake og friskt saltvann på at en kontrollerer en blandingssone for de to væskene og avbryter sekvensen før blandingssonen går ut av beholderne og til omvendt osmose-membranene. Det henvises for øvrig til patentkravene både når det gjelder denne og andre utførelsesformer av oppfinnelsen. With the help of an embodiment of the invention, it is also possible to avoid using pistons or flexible membranes, thereby further increasing operational reliability. This is achieved by basing the distinction between return brine and fresh salt water on controlling a mixing zone for the two liquids and interrupting the sequence before the mixing zone exits the containers and into the reverse osmosis membranes. Reference is also made to the patent claims both in relation to this and other embodiments of the invention.

Oppfinnelsen skal nå belyses nærmere ved å se på eksempelvise utførelsesformer, og det vises samtidig til de vedføyde tegningene, hvor The invention will now be elucidated in more detail by looking at exemplary embodiments, and at the same time reference is made to the attached drawings, where

fig. 1 viser et eksempel på et totalanlegg ifølge oppfinnelsen, med en utførelsesform med membraner i vekslingsbeholdeme, og fig. 1 shows an example of a total plant according to the invention, with an embodiment with membranes in the exchange containers, and

fig. 2 viser en utførelsesform av ventil med trykkutjevnings-funksjon. fig. 2 shows an embodiment of a valve with a pressure equalization function.

Ifølge oppfinnelsen er det tilveiebragt et system for styring av trykkgjen-vinningsprosessen ved ferskvannsproduksjon fra omvendt osmose, ved hjelp av trykkgjenvinningssylindre/vekslingsbeholdere med stempler eller beholdere med fleksible/tette membraner. Et slikt styresystem kan brukes for et anlegg med for eksempel to beholdere med membraner, se fig. 1. According to the invention, a system has been provided for controlling the pressure recovery process in fresh water production from reverse osmosis, using pressure recovery cylinders/exchange containers with pistons or containers with flexible/tight membranes. Such a control system can be used for a plant with, for example, two containers with membranes, see fig. 1.

Høytrykkspumpen pumper friskt sjøvann opp til ca. 65 barg. Dette kan være for eksempel en flertrinns sentrifugalpumpe, en skruepumpe eller en stempelpumpe. Dette vannet går sammen med friskt saltvann fra trykkgjenvinnings-anlegget, tilveiebragt også til samme trykk , f.eks. ca. 65 barg, og dette leveres så til omvendt osmose-membranene. Her blir f.eks. 40% av inngående saltvann separert ut som ferskvann, og i det tilfelle vil da 60% av vannmengden inn gå ut som returlake med noe forhøyet saltinnhold i forhold til friskt saltvann inn. Trykket vil typisk være 1-4 bar lavere enn inngangstrykket, dvs f.eks. 62 barg. Dette vannet går så til en skifteventil hvor vannet blir dirigert til den ene beholderen, f.eks. beholder A. Her vil den tette, fleksible membranen presse friskt saltvann ut av beholder A til omtrent samme trykk som laken som strømmer inn i beholder A. Tilbakeslagsventil A1 vil forhindre at vannet presser seg tilbake mot forsyningen av det friske saltvannet, og tilbakeslags-ventil A2 vil slippe høytrykksvannet gjennom. Dette har nå et trykk på for eksempel 61-62 barg og går til en trykkøkningspumpe som pumper vannet opp til trykket som må til for at det skal kunne gå inn til omvendt osmose-membranene. The high-pressure pump pumps fresh seawater up to approx. 65 bargs. This can be, for example, a multi-stage centrifugal pump, a screw pump or a piston pump. This water goes together with fresh salt water from the pressure recovery plant, also provided at the same pressure, e.g. about. 65 barg, and this is then delivered to the reverse osmosis membranes. Here, e.g. 40% of incoming salt water is separated out as fresh water, and in that case 60% of the water quantity entering will go out as return brine with a slightly higher salt content compared to fresh salt water entering. The pressure will typically be 1-4 bar lower than the inlet pressure, i.e. e.g. 62 barges. This water then goes to a diverter valve where the water is directed to one container, e.g. container A. Here the tight, flexible membrane will push fresh salt water out of container A to approximately the same pressure as the sheet flowing into container A. Non-return valve A1 will prevent the water from pushing back towards the supply of the fresh salt water, and non-return valve A2 will let the high-pressure water through. This now has a pressure of, for example, 61-62 barg and goes to a pressure increase pump which pumps the water up to the pressure required for it to be able to enter the reverse osmosis membranes.

Trykkøkningspumpen kan også plasseres andre steder i syklusen, f.eks etter utløpet av saltlake fra omvendt osmose-membranene. Når beholder A nesten er fylt opp med saltlake/tomt for friskt saltvann, bestemt ut ifra tiden T fra fylling av Beholder A med lake startet, eller etter signal fra mengdemåler FT1, som måler hvor mye lake som er gått inn i beholder A. Tiden T bestemmes ut ifra volumet til beholderen og strømningsmengden, slik at beholder A fylles nesten, men ikke helt, med lake. Hvis en bruker mengdemåler, gir denne signal idet en har nådd en tilsvarende mengde. The pressure increase pump can also be placed elsewhere in the cycle, for example after the exit of brine from the reverse osmosis membranes. When container A is almost filled with brine/empty for fresh salt water, determined from the time T from the start of filling Container A with brine, or after a signal from quantity meter FT1, which measures how much brine has gone into container A. The time T is determined based on the volume of the container and the flow rate, so that container A is almost, but not completely, filled with brine. If a quantity meter is used, this gives a signal when a corresponding quantity has been reached.

Når tiden T eller mengden er nådd, gir styresystemet signal til sleideventilen slik at denne kjøres over for å levere lake til beholder B, og denne vil dermed umiddelbart trykkes opp og starte å presse friskt saltvann ut av den andre enden. Denne overskiftingen skal ifølge oppfinnelsen skje slik at en ikke får noe avbrudd i strømningen av lake/saltvann fra og til omvendt osmose-filteret. Dette betyr at skifteventilen under skiftingen må åpne for lake til beholder B i samme takt som den reduserer lakemengden til beholder A. When the time T or the quantity is reached, the control system gives a signal to the slide valve so that it is driven over to deliver brine to container B, and this will thus immediately be pressed up and start pushing fresh salt water out the other end. According to the invention, this transfer should take place so that there is no interruption in the flow of brine/salt water from and to the reverse osmosis filter. This means that the shift valve during the shift must open for brine to container B at the same rate as it reduces the quantity of brine to container A.

Beholder A er nå tilnærmet full av lake/tom for friskt saltvann, og når skifteventilen er kommet helt over, mottar dumpeventil A signal fra styresystemet om å åpne, og beholder A trykkavlastes dermed til atmosfæretrykk. Som en ytterligere forbedring kan det monteres små trykkavlastnings- og oppbyggings-ventiler som åpner en viss tid før skifteventil eller / og dumpeventil skifter / åpner, f.eks 1 -5 sek. før. Denne trykkavlastningsventilen kan typisk ha et strømnings-tverrsnitt på 1 -15 mm, og vil foreta trykkavlastning / trykkoppbygging på en skånsom måte og unngå de reaksjonskrefter en kan få ved plutselig å skifte / åpne en stor ventil med et differansetrykk på opp til 65 barg. Container A is now almost full of brine/empty of fresh salt water, and when the changeover valve has come all the way over, dump valve A receives a signal from the control system to open, and container A is thus depressurized to atmospheric pressure. As a further improvement, small pressure relief and build-up valves can be installed which open a certain time before the changeover valve or / and dump valve changes / opens, e.g. 1 -5 sec. for. This pressure relief valve can typically have a flow cross-section of 1-15 mm, and will carry out pressure relief / pressure build-up in a gentle way and avoid the reaction forces you can get by suddenly changing / opening a large valve with a differential pressure of up to 65 barg.

Som en ytterligere forbedring kan skifte- og/eller dumpeventilene konstrueres slik at det er innebygget en egen trykkavlastnings/oppbyggings-mekanisme i dem, og slik at ventilene ikke åpner/skifter hovedporten før trykkutjevning tilnærmet er foretatt. Dette kan oppnåes gjennom konstruksjon av ventilen slik at kraften ventilmotoren utøver på ventilen, ikke er større enn at det må være tilnærmet trykk utjevnet før den lar seg bevege. Ventilen må da være konstruert slik at trykkene på begge sider av ventilstempelet i kombinasjon med flatene som trykkene arbeider mot, gir en nettokraft på ventilstempelet som oppfyller det ovennevnte forhold. Et eksempel på en slik ventil er vist i fig. 2. As a further improvement, the shift and/or dump valves can be designed so that a separate pressure relief/build-up mechanism is built into them, and so that the valves do not open/change the main port until pressure equalization is approximately done. This can be achieved through construction of the valve so that the force exerted by the valve motor on the valve is not greater than that the pressure must be approximately equalized before it is allowed to move. The valve must then be designed so that the pressures on both sides of the valve piston, in combination with the surfaces against which the pressures work, produce a net force on the valve piston that meets the above-mentioned condition. An example of such a valve is shown in fig. 2.

Forsyningen av friskt saltvann, som forutsettes å kunne bli levert ved et ønsket lavt trykk på 0.5-5 bar, vil dermed strømme inn i beholder A på den andre siden av den tette, fleksible membranen gjennom tilbakeslagsventil A1. Tilbakeslagsventil A2 vil forhindre at høytrykks-saltvann som nå presses ut av beholder B, vil strømme tilbake til beholder A. Det friske saltvannet vil dermed via den fleksible, tette membranen presse laken ut av beholder A og ut gjennom dumpeventil A. Det er ifølge oppfinnelsen avgjørende at trykket til det friske saltvannet er slik innstillt at det presser laken ut raskere i denne beholderen enn laken presser friskt saltvann under høyt trykk ut av den andre. Dette fordi systemet må rekke å tømme laken i den ene beholderen (i dette sekvenstrinn beholder A) dvs. åpne dumpeventil, tømme beholderen for lake og stenge dumpeventil, før tiden T er gått ut, eller at mengden målt ved FT1 har nådd sin forhåndsbestemte verdi, slik at beholder A er klar, full av friskt saltvann når skifteventilen får signal om å skifte over til beholder A igjen. The supply of fresh salt water, which is assumed to be able to be delivered at a desired low pressure of 0.5-5 bar, will thus flow into container A on the other side of the tight, flexible membrane through non-return valve A1. Non-return valve A2 will prevent high-pressure salt water, which is now being pushed out of container B, from flowing back to container A. The fresh salt water will thus, via the flexible, tight membrane, push the sheet out of container A and out through dump valve A. It is according to the invention crucial that the pressure of the fresh salt water is set so that it pushes the sheet out faster in this container than the sheet pushes fresh salt water under high pressure out of the other. This is because the system must manage to empty the brine in one container (in this sequence step, container A), i.e. open the dump valve, empty the container for brine and close the dump valve, before the time T has expired, or that the quantity measured at FT1 has reached its predetermined value , so that container A is ready, full of fresh salt water when the changeover valve receives the signal to change over to container A again.

Det er også viktig å detektere et endepunkt for hver fylling/tømming, for å unngå at sekvensen driver i startpunktet, noe som kan medføre at den fleksible, tette membranen kan tømme beholderen helt for friskt saltvann i det sekvenstrinn hvor den leverer friskt saltvann under trykk til omvendt osmose-membranen, noe som vil medføre brudd i leveringen av vann til omvendt osmose-membranen, og fare for ødeleggelse av omvendt osmose-membraner og den fleksible, tette membranen i beholderen. It is also important to detect an end point for each fill/empty, to avoid the sequence drifting at the start point, which could cause the flexible, tight membrane to completely empty the container of fresh salt water in the sequence step where it delivers fresh salt water under pressure to the reverse osmosis membrane, which will cause a break in the supply of water to the reverse osmosis membrane, and the risk of destruction of reverse osmosis membranes and the flexible, tight membrane in the container.

Detekteringen av et slikt endepunkt gjøres ifølge oppfinnelsen ved å registrere, måle, mengden av friskt sjøvann som strømmer inn i beholderen i det sekvenstrinn hvor lake dumpes til friluft. Når beholderen er helt tom for lake/full av friskt saltvann, vil den tette, fleksible membranen legge seg mot utløps-anordningen og forhindre at mer friskt sjøvann strømmer inn. Dette vil bli registrert av mengdemåleren FT2 som da gir signal til styresystemet om å stenge dumpeventilen. Dette forutsetter at utløpsanordningen er laget slik, og at membranen er så solid at den tåler dette uten å ødelegges. Trykket her er lavt, fra 0.5 til 5 barg. The detection of such an end point is done according to the invention by recording, measuring, the amount of fresh seawater that flows into the container in the sequence step where the brine is dumped into the open air. When the container is completely empty of brine/full of fresh salt water, the tight, flexible membrane will lie against the outlet device and prevent more fresh seawater from flowing in. This will be registered by the quantity meter FT2, which then gives a signal to the control system to close the dump valve. This assumes that the outlet device is made in this way, and that the membrane is so solid that it can withstand this without being destroyed. The pressure here is low, from 0.5 to 5 barg.

Disse prinsippene, alle samlet eller enkeltvis, for styring av slike trykkgjenvinnings-prosesser kan brukes på alle prosesser som baserer seg på fylling/tømming av beholdere eller sylindre. These principles, all together or individually, for managing such pressure recovery processes can be applied to all processes based on the filling/emptying of containers or cylinders.

Som en ytterligere forbedring kan en ha en tilsvarende prosess, men hvor vekslings-beholderne eller sylindrene har hverken membraner eller stempler som skiller lake og friskt saltvann, men baserer seg på at disse væskene ikke blander seg i stort omfang. Dette kan oppnås bl.a. ved hjelp av en anordning i hver ende av sylindrene / beholderne hvor en bremser ned innløpsstrømmen og fordeler væsken gjennom f.eks. en hullplate som dekker hele eller store deler av tverrsnittet til beholderen. Videre at en avbryter sekvenstrinnet hvor en presser friskt saltvann ved høyt trykk til omvendt osmose-membranene, før blandingssonen kommer til utløpet. Dette kan bestemmes ut i fra en tid, eller ved å måle ledningsevnen, og en kan fortsette til neste sekvenstrinn når ledningsevnen kommer over en forutbestemt verdi. As a further improvement, one can have a similar process, but where the exchange containers or cylinders have neither membranes nor pistons that separate brine and fresh salt water, but are based on the fact that these liquids do not mix to a large extent. This can be achieved i.a. by means of a device at each end of the cylinders / containers where the inlet flow is slowed down and the liquid is distributed through e.g. a perforated plate that covers all or large parts of the cross-section of the container. Furthermore, that one interrupts the sequence step where one presses fresh salt water at high pressure to the reverse osmosis membranes, before the mixing zone reaches the outlet. This can be determined from a time, or by measuring the conductivity, and one can proceed to the next sequence step when the conductivity exceeds a predetermined value.

Videre at en i den fasen hvor den ene beholderen er trykkavlastet og hvor friskt saltvann presser returlaken ut til avløp, sørger for at dette ikke avbrytes før en har friskt saltvann ut i avløpet. Dette endepunktet kan bestemmes ut i fra en forhåndsbestemt tid, eller ved å måle ledningsevnen, og sekvenstrinnet avbrytes når ledningsevnen faller under en forhåndsbestemt verdi. Ved å benytte stående beholdere, med returlaken nederst, vil tetthetsforskjellen mellom returlaken og friskt sjøvann bevirke at blandingssonens utstrekning minimaliseres. Furthermore, in the phase where one container is depressurized and where fresh salt water pushes the return brine out to the drain, you ensure that this is not interrupted until you have fresh salt water out into the drain. This endpoint can be determined from a predetermined time, or by measuring the conductivity, and the sequence step is interrupted when the conductivity falls below a predetermined value. By using upright containers, with the return sheet at the bottom, the difference in density between the return sheet and fresh seawater will mean that the extent of the mixing zone is minimized.

Claims

1. Method for the production of fresh water from salty water, especially sea water, using a reverse osmosis plant, where fresh salt water is fed into the reverse osmosis plant at the necessary high pressure by at least one high-pressure pump in combination with an energy recovery plant which utilizes the pressure in the high-pressure return liquor from the reverse osmosis plant, to pressurize salt water in a sequential process with a forward sequence stage and a return stage, through two parallel exchange vessels which will mainly be in opposite stages, characterized in that the sequential the process is controlled so that the return step is carried out so much faster than the forward step, that the exchange container that is in the return step has time to exchange contents before the forward step is finished for the other exchange container, whereby salt water is made to flow uninterrupted to the reverse osmosis plant, and return liquor uninterrupted from the reverse osmosis plant.

2. Method according to claim 1, where each exchange container contains a sliding separation piston or a flexible, tight membrane that separates return brine and fresh salt water, characterized in that the speed of the return stage is controlled by regulating the inlet pressure of the fresh salt water, with the return brine being dumped to atmospheric pressure and fresh salt water being topped up at the same time.

3. Method according to claim 1 or 2, characterized in that changing the inflow of return liquor from one container to the other is carried out before said one container is completely full of return liquor.

4. Method according to claim 3, characterized in that changing the inflow of return liquor from one container to the other is controlled by a predetermined time being used for return liquor flow to the container in question, which time is determined based on the volume of the container and the return liquor flow quantity.

5. Method according to claim 3, characterized in that the change of inflow of return liquor from one container to the other is controlled on the basis of a measurement signal from a quantity meter which measures the amount of return liquor that flows into the container in question after the last change.

6. Method according to claim 1 or 2, characterized in that the end position for the return stage, i.e. when dumping return brine into the open air and filling up fresh salt water in the container in question, is determined by measuring the amount of fresh salt water in the container, and it is recorded when this amount falls, which indicates that the container is full of fresh salt water and empty foF return brine.

7. Method according to claim 1, where an interface or a mixing zone is established between return brine and fresh salt water in direct contact in the exchange containers, characterized in that the control is based on control of the interface/mixing zone's position, extent and movement, with the forward sequence step ending before the interface/mixing zone reaches the outlet/inlet on the saltwater side of the container in question.

8. Method according to claim 7, characterized in that the inlet flow at each end of the containers is slowed down by means of at least one plate with holes or mesh to create a certain pressure drop and distribute the flow over the entire cross section of the container.

9. Method according to claim 7 or 8, characterized by the fact that the forward sequence step is interrupted after a pre-calculated time, where quantity and container volume are included in the calculation and a good margin is given for the extent of the mixing zone, and that the return step is controlled so that all return brine and a certain amount of fresh salt water is pushed to the drain.

10. Method according to claim 7 or 8, characterized in that the sequence steps are controlled on the basis of signals from conductivity meters in the containers for determining the passage of the interface/mixing zone past given positions in the containers.

11. Method according to claim 1, characterized in that pressure equalization is carried out by pressure increase and pressure decrease between the sequence steps, by using small, actuator-controlled pressure relief and pressure build-up valves that open a given time before dump or shift valves are to change, in order to avoid shocks and vibrations when the latter, large valves open or change .

12. Method according to claim 1, characterized in that pressure equalization is carried out by pressure increase and pressure decrease between the sequence steps, by using valves with a built-in pressure equalization mechanism, arranged so that the valve cannot change or open until approximate pressure equalization has been carried out.

13. Installations for the production of fresh water from saline water, especially sea water, extensive a reverse osmosis plant, at least one high-pressure pump for feeding fresh salt water to reverse the osmosis system with the necessary high pressure, an energy recovery plant for utilizing the pressure in high-pressure return liquor from the reverse osmosis plant, for pressurizing fresh salt water on the reverse osmosis plant's inlet side in combination with the high-pressure pump, where the energy recovery plant includes two parallel exchange containers where return brine and fresh salt water meet in each of these, valves for distribution of return brine to the containers and for control of fresh salt water to and from the containers and for control of return brine to dumping, as well as a pressure boosting pump for providing high pressure for fresh salt water between the containers and the input side of the reverse osmosis plant, and where the energy recovery plant works in a sequential process with a forward sequence step and a return step for the exchange containers, the two containers being essentially in opposite stages, characterized by a control system for the process which is arranged to carry out the return step so much faster than the forward step, that the exchange container which is in the return stage, has time to exchange contents before the forward stage is finished for the second exchange vessel, whereby salt water is brought to flow uninterruptedly to the reverse osmosis plant, and return brine uninterruptedly from the reverse osmosis plant.

14. Plant according to claim 13, characterized in that each exchange container contains a sliding separating piston or a flexible, tight membrane that separates return brine and fresh salt water, and that the control system is designed to control the speed of the return stage by regulating the inlet pressure of the fresh salt water, which causes dumping of return brine to atmospheric pressure and at the same time refilling with fresh salt water.

15. Installation according to claim 13 or 14, characterized by at least one changeover valve for changing the return brine flow to the containers, from one container to the other, which changeover valve has a sufficiently large available flow cross-section during the entire changeover so that an interruption in or large variation of flow quantities is avoided during the changeover.

16. Installation according to claim 13 or 14, characterized by at least one quantity meter for measuring the amount of return liquor flowing into a container, for sending a signal to the control system.

17. Installation according to claim 13 or 14, characterized by at least one flow meter for measuring the amount of fresh salt water flowing into a container, for sending a signal to the control system.

18. Plant according to claim 14, characterized in that the container's outlet/inlet end on the return brine side is designed so that the pistons or membranes can withstand the differential pressure between fresh salt water that is supplied in the return stage, and the return brine's atmospheric pressure during dumping, when the container is completely emptied of return brine and filled completely with fresh salt water.

19. Plant according to claim 13, characterized in that each exchange container is arranged to allow return brine and fresh salt water to meet directly in an interface or mixing zone, and that the control system is arranged to control the position, extent and movement of the interface/mixing zone in each container, as the forward sequence step must be terminated before the interface /the mixing zone reaches the outlet/inlet on the saltwater side of the container in question.

20. Plant according to claim 19, characterized by at least one plate with holes, possibly mesh, placed at each end of the containers to create a certain pressure drop and distribute the flow over the entire cross section of the container.

21. Plant according to claim 19, characterized by conductivity meters placed in the containers for determining the passage of the interface/mixing zone past given positions in the containers, and for sending a signal to the control system.

22. Plant according to claim 19, characterized in that the exchange containers are arranged standing mainly vertically and with the return sheet at the bottom.

23. Plant according to claim 13, characterized by small, actuator-controlled pressure relief and pressure build-up valves placed in connection with dumping and switching valves between the reverse osmosis plant and the return liquor side of the containers, for pressure equalization and avoidance of shocks and vibrations when switching between sequence steps, which small valves are opened a given time before dumping - or the shift valves must be changed.

24. Plant according to claim 13, characterized by the fact that dump and shift valves between the reverse osmosis plant and the return liquor side of the containers have a built-in pressure equalization mechanism, designed so that the valve cannot shift or open until approximate pressure equalization has been carried out, whereby shocks and vibrations can be avoided during pressure increases and pressure decreases at sequence step transitions .