METHOD AND APPARATUS FOR EFFECTING HIGH RECOVERY DESALINATION WITH PRESSURE DRIVEN MEMBRANES

Abstract

A system and method for switching between flows of water solutions passed in groups of blocks of membrane pressure vessels arranged in parallel in a tapered flow system, wherein the system comprises a system inlet feed line, a system outlet flow line, high pressure booster pumps configured to provide a high pressure feed stream to the system; blocks of membrane pressure vessels arrayed in parallel, and a first and second bypass line each parallel to said blocks.

Claims

1. A tapered flow desalination system comprising: a system inlet feed line coupled to a first high pressure booster pump configured to provide a high pressure feed stream to the system; blocks of membrane pressure vessels arrayed in parallel, wherein the outlet of said first booster pump is coupled by means of flow lines to said blocks at first opening sides of said blocks; a second booster pump, coupled at its inlet by a first bypass line parallel to said blocks, to said first booster pump outlet, wherein said second booster pump outlet and inlet are also coupled by means of flow lines to said blocks at second opening sides of said blocks and wherein said first bypass line is also coupled by means of flow lines to said blocks at said second opening sides of said blocks; a system outlet flow line coupled to said first opening sides of said blocks, coupled to said second opening sides of said blocks and to the second booster pump outlet, wherein said system outlet flow line is coupled to said second opening sides of said blocks and to said second booster pump outlet by means of a second bypass line; wherein the first bypass line comprises a valve and wherein the second bypass line comprises a valve; wherein at least two of the lines coupling between said first booster pump outlet and said first opening sides of said blocks each comprise a valve; wherein at least two of the lines coupling between said second booster pump outlet and said second opening sides of said blocks each comprise a valve; wherein at least two of the lines coupling between said second opening sides of said blocks and said second booster pump inlet each comprise a valve; and wherein at least two of the lines coupling between said first opening sides of said blocks and said system outlet flow line each comprise a valve.

2. The system according to claim 1 wherein said system comprises three blocks, and wherein the lines coupling between said first booster pump and said first opening sides of said blocks each comprise a valve; wherein the lines coupling between said second booster pump and said second opening sides of said blocks each comprise a valve; wherein the lines coupling between said second opening sides of said blocks and said first bypass line each comprise a valve; and wherein the lines coupling between said first opening sides of said blocks and said system outlet flow line each comprise a valve.

3. The system according to claim 1, wherein the system comprises one or more control elements selected from the group of additional valves, check valves and sensors.

4. The system according to claim 1, wherein each pressure vessel block is coupled to a permeate product line.

5. A method for switching between flows of water solutions passed in groups of blocks of membrane pressure vessels arranged in parallel in a tapered flow system, wherein said method comprising the steps of: A) passing feed water solution through one or more of said system blocks in a first stage and the concentrated water solution exiting said blocks of the first stage is passed through one or more of the system blocks in a second stage and the concentrated water solution exiting the blocks of the second stage is passed through a system concentrate outlet; B) slowing the stream(s) passed in said second stage by bypassing a portion of the concentrated water solution exiting the blocks of the first stage to the system concentrate outlet; C) stopping the slowed stream(s) of a first group of blocks being of one or more of the blocks of the second stage; D) slowing the stream(s) passed in the first stage by bypassing a portion of said feed water solution, to the system concentrate outlet; E) stopping the slowed stream(s) of a second group of blocks being of one or more of the blocks of the first stage, wherein said second group of blocks comprise the same number of blocks as in said first group of blocks; and passing a portion of the feed water solution through said first group of blocks; F) stopping the bypassing of step D and passing a portion of the concentrated water solution exiting the blocks of the first stage through said second group of bocks; G) stopping the bypassing of step B.

6. A method according to claim 5, wherein the number of blocks in the first group of blocks is 1.

7. A tapered flow desalination system comprising: a system inlet feed line coupled to a first high pressure booster pump configured to provide a high pressure feed stream to the system; a first group of blocks of membrane pressure vessels arrayed in parallel, wherein the outlet of said first booster pump is coupled by means of flow lines to said first group of blocks at first opening sides of said first group of blocks; a second booster pump, coupled at its inlet by means of flow lines to said first group of blocks at second opening sides of the first group of blocks; and coupled at its outlet to a second group of blocks of membrane pressure vessels arrayed in parallel at first opening sides of the second group of blocks; a third booster pump, coupled by means of flow lines to said second group of blocks at second opening sides of the second group of blocks; and coupled by means of flow lines to said first group of blocks at said second opening sides of said first group of blocks; a system outlet flow line coupled by flow lines to said first opening sides of said first group of blocks, coupled to said second opening sides of said first group of blocks and to the third booster pump, wherein said system outlet flow line is coupled to said second opening sides of said first group of blocks and to said third booster pump by means of a bypass line; wherein the bypass line comprises a valve; wherein at least two of the lines coupling between said first booster pump and said first opening sides of said first group of blocks each comprise a valve; wherein at least two of the lines coupling between said second booster pump and said second opening sides of said first group of blocks each comprise a valve; wherein at least two of the lines coupling between said third booster pump and said second opening sides of said first group of blocks each comprise a valve; wherein at least two of the lines coupling between said first opening sides of said first group of blocks and said system outlet flow line each comprise a valve.

8. The system according to claim 7 wherein the first group of blocks comprise at least 4 blocks and the second group of blocks comprise, at most, two less blocks than the first group.

9. The system according to claim 7, wherein the system comprises one or more of the control elements selected from the group of additional valves, check valves and sensors.

10. The system according to claim 7, wherein each pressure vessel block is coupled to a permeate product line.

11. A method for switching between flows of water solutions passed in groups of blocks of membrane pressure vessels arranged in parallel in a tapered flow system, wherein said method comprising the steps of: A) passing feed water solution through one or more of said system blocks in a first stage and the concentrated water solution exiting said blocks of the first stage is passed through one or more blocks in a second stage and the concentrated water solution exiting the blocks of the second stage is passed through one or more blocks in a third stage and the concentrated water solution exiting the blocks of the third stage is passed through a system concentrate outlet; B) slowing the stream(s) passed in said third stage by bypassing a portion of the concentrated water solution exiting the blocks of the second stage to the system concentrated outlet; C) stopping the slowed stream(s) of a first group of blocks being of one or more of the blocks of the third stage; D) stopping the stream(s) of a second group of blocks being of one or more of the blocks of the first stage, wherein said second group of blocks comprise the same number of blocks as in said first group of blocks; and passing a portion of the feed water solution through said first group of blocks; E) passing a portion of the concentrated water solution exiting the blocks of the second stage through said second group of blocks; F) stopping the bypassing of step B.

12. A method according to claim 11, wherein the initial number of blocks in the third stage is at most, one less than , the number of blocks in the second stage and the number of blocks in the second stage is at most, one less than the number of blocks in the first stage.

13. A flushing loop system for periodically flushing a concentrate removal line with undersaturated solution comprising: a flushing undersaturated solution feed tank coupled to a recycle line ending back at said feed tank; wherein said recycle line comprises a recirculation pump configured to drive said undersaturated solution from said feed tank and back to said feed tank; wherein a portion said recycle line is connected in parallel with said concentrate removal line.

14. A flushing loop system for periodically flushing a concentrate removal line with undersaturated solution comprising: a flushing undersaturated solution feed tank coupled to a recirculation pump by a flow line; wherein a first portion of the concentrate removal line is coupled to a third portion of the concentrate removal line by two parallel flow lines, one being a second portion of the concentrate removal line and the other being a flush removal line; wherein said recirculation pump is coupled to the second portion of the concentrate removal line by two parallel flow lines; and wherein said feed tank is coupled to the second portion of the concentrate removal line by two parallel flow lines.

15. The flushing loop system according to claim 14, wherein said recirculation pump is coupled to a first three-way valve by a flow line; wherein said first three way valve is coupled to a second three way valve by a flow line and to the second portion of the concentrate removal line by a flow line; wherein said second three way valve is also coupled to said second portion of the concentrate removal line; wherein said second portion of the concentrate removal line is also coupled to a third three way valve and to a fourth three way valve; wherein said third three way valve is also coupled to said fourth three way valve by a flow line; wherein said flushing removal line is coupled to said first three way valve, said second three way valve, said third three way valve and said fourth three way valve; wherein said fourth three way valve is coupled to the feed tank; wherein said third three way valve is coupled to the first portion of the concentrate removal line; wherein the second three way valve is coupled to the system third portion of the concentrate removal line; and wherein said portion of the concentrate removal line and said flushing removal line each comprise a two way valve.

16. A method for flushing a portion of a concentrate removal line, comprising: A) passing a concentrated solution through a portion of a concentrate removal line; B) redirecting the concentrated solution and passing it through a flow line parallel to said portion of the concentrate removal line; and passing a flow of undersaturated solution through said portion of the concentrate removal line; C) stopping the passing of the undersaturated solution; and redirecting the concentrated solution and passing it back through said portion of the concentrate removal line; D) periodically repeating steps B-C.

17. The method according to claim 16, wherein: step A further comprises passing an undersaturated solution through the flow line parallel to the portion of the concentrate removal line; and wherein step B further comprises redirecting the undersaturated flow of step A and passing it through the portion of the concentrate removal line; and wherein step C further comprises redirecting the undersaturated flow of step B and passing it back to the flow line parallel to the portion of the concentrate removal line.

18. A flushing loop system for periodically flushing a concentrate removal line with undersaturated solution comprising: a flushing undersaturated solution feed tank coupled to a recirculation pump by a flow line; wherein a first portion of the concentrate removal line is coupled to a third portion of the concentrate removal line by two parallel flow lines, one being a second portion of the concentrate removal line and the other being a flush removal line; wherein said recirculation pump is coupled to the second portion of the concentrate removal line by two parallel flow lines; and wherein said feed tank is coupled to the second portion of the concentrate removal line by two parallel flow lines, wherein said first portion of the concentrate removal line is the system outlet flow line of a tapered flow desalination system in accordance with claim 1.

19. A flushing loop system for periodically flushing a concentrate removal line with undersaturated solution comprising: a flushing undersaturated solution feed tank coupled to a recirculation pump by a flow line; wherein a first portion of the concentrate removal line is coupled to a third portion of the concentrate removal line by two parallel flow lines, one being a second portion of the concentrate removal line and the other being a flush removal line; wherein said recirculation pump is coupled to the second portion of the concentrate removal line by two parallel flow lines; and wherein said feed tank is coupled to the second portion of the concentrate removal line by two parallel flow lines, wherein said first portion of the concentrate removal line is the system outlet flow line of a tapered flow desalination system in accordance with claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0099] The present invention is illustrated by way of example in the accompanying drawings, in which similar references consistently indicate similar elements and in which:

[0100] FIGS. 1 and 2 illustrates a prior art inventions.

[0101] FIG. 3 illustrates the flushing of the concentrate line embodiment of the present invention.

[0102] FIGS. 4, 5A-5G illustrate the two stage with AVF bypass line embodiment of the present invention.

[0103] FIG. 6 illustrates an example of a general overview of an embodiment of the present invention.

[0104] FIGS. 7A-7F illustrate the three stage embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0105] One of the objects of the present invention is to provide technological solutions to several possible operational problems that may arise in the practice of EP1691915 and repositioning of pressure valves as taught in EP1893325, by means of a novel valve arrangement. In particular, the apparatus of the present invention is designed to prevent scaling in the lines of the downstream of the positioning valves downstream of the pressure vessels in the last stage that always see supersaturated concentrate. The present invention further provides an improved design for preventing water hammer effects or other hydraulic shock to membrane elements in the pressure vessels in the upstream stage when repositioning blocks of pressure vessels into and out of this stage. In both cases a novel valve arrangement is used in ways that would not be anticipated or practiced by those versed in the art of membrane systems.

[0106] The novel valve arrangement of the invention on the concentrate line allows flushing of the concentrate line only without losing time to production while the flushing solution flushes the whole membrane train. In the past, it has been the practice of some operators of membrane desalination plants to periodically (e.g. once every day) flush their lines with feed water or saline water (see Liberman et al.), but this involves cessation of the membrane operation and loss of production. In the case of Liberman et al. it also involves the use of saline solutions that carry both a chemical expense and an environmental penalty because of the need to dispose of these hypersaline solutions.

[0107] FIG. 6 shows a general flowsheet of the tapered flow desalination apparatus according to an embodiment of the invention. The apparatus illustrated in FIG. 3 (an expansion of a portion of the apparatus of FIG. 6) exemplifies one preferred embodiment of the invention and a method for carrying it out. The concentrate stream [4] can be directed by one or more valves (here XV-4 and XV-5) between a flushing solution recycle line (FRL) and the concentrate removal line (CRL). Both the CRL and FRL are equipped with devices (in this embodiment back pressure valves P/FV and P/FV2 respectively) to maintain the pressure in the feed/concentrate side of the membrane elements. However the pressure maintenance devices could also be pressure exchangers or other energy recovery devices. During normal operation (indicated by reference numerals 3n), the concentrate stream [4] flows through the concentrate removal line (CRL) sent out of the system while a stream of flushing solution [5] is recycled between the CIP tank and the FRL through valves XV-6 and XV-7 while P/FV2 is completely open. Periodically (for example, as frequently as once every hour up to as infrequently as the same time period as used for flow reversal or for repositioning pressure vessels between the stages to prevent scaling) the valves XV-4 and XV-5 (in this particular embodiment 3-way valves but it could be a manifold of two-way valves or slide valves) valve positions are switched to direct the concentrate stream [4] as indicated by the arrows referenced by numerals 3p so that the concentrate stream [4] is sent through the FRL, and the position of valves XV-6 and XV-7 are also switched such that the flushing solution [5] flowing in the direction of the arrows referenced by numerals 3r is sent through the CRL before returning to the CIP tank (TK-1). In this way, all the surfaces of the CRL that were exposed to supersaturated concentrate are now exposed to undersaturated flushing solution. At the same time the back pressure device P/FV is completely opened and the back pressure device P/FV2 is set to the same setting as previously held by the P/FV to maintain the same back pressure in the feed lines of the pressure vessels. In this case a limited volume of flushing solution which can be feed water or permeate is held in a tank such as a clean-in-place (CIP TK-1) tank and it is re-used over and over for periodically flushing the concentrate line. This can continue to be the case until the residual concentration of scaling ions in the flushing solution reaches their saturation limit and a new batch of flushing solution may be then introduced.

[0108] According to an embodiment of the invention, the time during which the CRL is flushed (stream [5] going in the direction of arrows 3r) can be as short as one minute or twice the hydraulic residence time in the CRL, whichever is shorter, and as long as the time that a particular block is positioned in the downstream stage or that one direction of a flow reversal cycle is being operated. Once the flushing is complete, the valve positions are returned to their original position (3n).

[0109] Some of the advantages of this approach are: the system never stops operating and producing permeate while the concentrate line is flushed; a limited amount of volume is repeatedly used (volume of the CIP tank TK-1) and therefore little permeate is wasted; and any sensors on line CRL are kept free of scaling so they will operate properly.

[0110] Other embodiments for flushing the CRL concentrate line involve briefly (for a time as little as two hydraulic residence times of the normal concentrate stream) sending all of the permeate to a point downstream of P/FV which may be used for simplicity but does involve wasting a certain amount of permeate, but does allow keeping sensors on CRL to be maintained scale-free. Another embodiment involves bypassing the 2nd stage and lowering recovery-opening P/FV (+ adding acid/AS) temporarily to get concentrate conditions to undersaturated conditions. This has the advantage of rinsing the P/FV without a separate line while maintaining production from the first stage. It has the disadvantage of losing production from the second stage. However if the time is short this may not be too bad a disadvantage.

[0111] The present invention further provides a two-way valve arrangement for effecting repositioning of pressure vessels between the stages of a membrane desalination plant, wherein the valve arrangement employs a bypass valve on the first stage to prevent sudden changes in hydraulic flows on pressure vessels in the first stage. One preferred embodiment of this solution of the invention is shown in FIG. 4. In this embodiment, the apparatus 40 the pairs of 2-way valves (AFI/ARO, BFI/BRO, CFI/CRO) have replaced the two-way upstream valves V.sub.1A, V.sub.1B, V.sub.1C, and three-way valves V.sub.Af, V.sub.Bf, and V.sub.Cb, shown in FIG. 2, and the pairs of two-way valves (ARI/AFO, BRI/BFO, CRI/CFO) have replaced two-way valves V.sub.2A, V.sub.2B, V.sub.2C, and three-way valves V.sub.Ab, V.sub.Bb and V.sub.Cf shown in FIG. 2. In addition, an additional bypass valve AVF has been added to link the outlet of the high pressure pump to the inlet of the interstage booster pump to take the excess flow from the high pressure feed pump when only one block of pressure vessels is operative in the first stage, and to reduce the initial flow on re-introducing a block of pressure vessels into the first stage after repositioning. A particular embodiment can include partially opening the proportional back pressure valve P/FV when the flow rate increases as a result of using the bypass AV and/or AVF. By doing this, an increase in pressure can be prevented from the increase in flow to the P/FV when feed flows through the second stage and/or first stage bypass line.

[0112] According to an embodiment of the present invention, the tapered flow desalination system and method are as follows. The system comprises two stages in which the concentrate exiting the first stage becomes the feed entering the second stage. The system comprises I membrane block pressure vessels having feed water pass through them during the first stage and J membrane block pressure vessels having feed (concentrate exiting the I first stage blocks) pass through them during the second stage, wherein preferably I>J. A feed high pressure pump is connected to the flow line at a portion of the line wherein the incoming inlet feed flows through a single line before being split into the lines of the blocks into the next stage.

[0113] An interstage booster pump is connected between the blocks of both stages. The flow direction in block(s) of the second stage can be reversed and become part of the first stage and the flow direction in block(s) of the first stage can be reversed and become part of the second stage.

[0114] Additional elements (e.g. valves or control elements such as check valves or sensors) are connected to the system. Each pressure vessel is coupled to a permeate product line wherein the permeate filtered exits the pressure vessel through it. The permeate product lines coming out of the pressure vessels are coupled to one main system outlet permeate product line.

[0115] According to an embodiment, an AV bypass line (with an AV valve) couples between the concentrate outlet of the system and the second stage booster pump. The bypass line AVF line (with the AVF valve) couples between the first stage booster pump and the second stage booster pump.

[0116] FIG. 4 shows an embodiment with two vessel blocks in the first stage (A and B), and one vessel block (C) in the second stage. The first booster pump is connected to the system incoming feed inlet line 60.

[0117] The blocks of pressure vessels A,B, and C have been connected to the flow manifold so that they can either operate in parallel and as part of stage one (the bottom stage) or as part of stage two (the top stage). When the valves connected to a block and labeled with the symbol FI and FO are open (open symbols) then that block is parallel to and part of the first stage. When the valves connect to a block and labeled with the symbols RI and RO are open (open symbols) then that block is part of the second stage.

[0118] The valves AFI, BFI and CFI are each coupled by flow lines to the first booster pump 51, to the valves ARO, BRO and CRO respectively, to the inlets/outlets of blocks A, B and C respectively, to the AVF valve 54 and to one another.

[0119] The valves ARO, BRO and CRO are also coupled to the inlets/outlets of each of the blocks A, B and C respectively, to the AV valve 55, to the system outlet concentrate outlet line 70 (of the system) and to one another.

[0120] The valves AFO, BFO and CFO are each coupled by flow lines to the second booster pump 52, to the valves AM, BRI and CRI respectively, to the inlets/outlets of blocks A, B and C respectively, to the AVF valve 54 and to one another.

[0121] The valves AM, BRI and CRI are also coupled to the inlets/outlets of blocks A, B and C respectively, to the second booster pump 52, to the AV valve 55 and to one another.

[0122] The first booster pump 51 is also coupled to the AVF valve 54.

[0123] The second booster pump 52 is also coupled to the AV valve 55 and to the AVF valve 54.

[0124] The AV valve 55 is also coupled to the concentrate outlet line 70 of the system. (which according to an embodiment is line [4] of FIG. 3).

[0125] A particular method for operating apparatus 40 is illustrated in FIGS. 5A to 5G comprising the same structure as in FIG. 4. Though not shown in the figures, check valves may be installed on each of the lines to prevent short-circuiting and bypassing during the transition process. Dotted lines refer to lines in which feed solution is flowing between the feed high pressure pump and the interstage booster pump. Dashed lines refer to lines in which interstage concentrate or final concentrate is flowing between the interstage booster pump and the back pressure valve or device (P/FV) and onward to the concentrate outlet line. Thin lines refer to process lines in which no flow is occurring. Apparatus 40 comprises pairs of 2-way valves on each high pressure port of the pressure vessels to effect repositioning of membrane blocks A B and C of pressure vessels between a first and second membrane stages. The sequence to effect this change is illustrated in FIGS. 5A to 5G. Table 1 summarizes a sequence of steps needed to reposition block C from the second stage to the first stage, and block B from the first stage to the second stage. In particular, this highlights the role of the bypass valve AVF in the first stage.

[0126] In yet another preferred embodiment of the invention, the apparatus is configured to switch blocks by placing all three blocks in the first stage to slow the flow rate to each one in preparation for switching. Then one of the block's valving is changed so that it is moved into the second stage.

[0127] In yet another preferred embodiment of the invention, the apparatus is configured to switch one of the two blocks in the first stage with the block of the second stage, slowing the flow rate to each one in preparation for switching. The steps performed in this embodiment are demonstrated in table 2. This embodiment has the advantage of eliminating the need for the first stage bypass valve, AVF. On the other hand it could lead to too high recovery being obtained in the first stage unless applied feed pressures are adjusted during the transition.

[0128] In both embodiments (with AFV and without), there is an advantage to increasing the time to effect the opening and closing of the two-way valves to between 5 and 30 seconds (and preferably between 5 and 15 seconds) in order to reduce the increase in pressure even further.

[0129] In the method, as explained in FIGS. 5A-5G, the initial working mode (FIG. 5A) is wherein at a first stage incoming feed water is passed through blocks A, B. The concentrated liquid coming out of those four blocks is passed in the second stage though block C, and from there out of the system. The method comprises switching the roles between block B and block C. After the switching, block C is connected to the feed line 60 (at its opposite side), and block B becomes the second stage vessel block. This is done by opening/closing the appropriate valves. The working modes are switched periodically.

[0130] Thus in FIG. 5A, block C is in the third stage and blocks A and B are in the first stage. In FIG. 5G, block B is in the second stage and blocks A and C are in the first stage.

[0131] The following table clarifies the switching procedure step by step.

TABLE-US-00001 TABLE 1 Valve steps in moving from configuration with blocks A and B in stage I to blocks A and C in stage I (switching blocks B and C between stages), as illustrated in FIGS. 5A to 5G. STEPS Start End (FIG. 5A) Iv (FIG. 5G) A, B 1.sup.st Iii (FIG. 5E) V A, C 1.sup.st stage C I Ii (FIG. 5D) B (FIG. 5F) stage, 2.sup.nd (FIG. 5B) (FIG. 5C) Stage 1 stopped, B slowed, B 2.sup.nd Valves stage C slowed C stopped slowed C forward C forward stage BLOCK AFI On On On On On On On A AFO On On On On On On On ARI Off Off Off Off Off Off Off ARO Off Off Off Off Off Off Off BLOCK BFI On On On On Off Off Off B BFO On On On On Off Off Off BRI Off Off Off Off Off On On BRO Off Off Off Off Off On On BLOCK CFI Off Off Off Off On On On C CFO Off Off Off Off On On On CRI On On Off Off Off Off Off CRO On On Off Off Off Off Off Auxiliary AVF Off Off Off On On Off. Off Bypass AV Off On On On On On Off Valves Comments A, B in C C Stage I B B rev., A, C 1.sup.st 1.sup.st stage slowed stopped Slowed stopped slow stage C, in 2nd C C for., B, 2nd stage forward fast stage slow

[0132] The following items indicated in FIGS. 5A-5G refer to:

[0133] FWFeed Water

[0134] C1-LPLow Pressure Concentrate 1st stage

[0135] C1-HPHigh Pressure Concentrate 1st stage

[0136] C11st stage Concentrate

[0137] C22.sup.nd stage

[0138] PPPermeate Product

TABLE-US-00002 TABLE 2 Alternate way to switch blocks B and C between stages I and II eliminating need of valve AVF STEPS Valves Start I ii Iii Iv v End BLOCK AFI On On On On On On On A AFO On On On On On On On ARI Off Off Off Off Off Off Off ARO Off Off Off Off Off Off Off BLOCK BFI On On On On Off Off Off B BFO On On On On Off Off Off BRI Off Off Off Off Off On On BRO Off Off Off Off Off On On BLOCK CFI Off Off Off On On On On C CFO Off Off Off On On On On CRI On On Off Off Off Off Off CRO On On Off Off Off Off Off Auxiliary AVF Off Off Off Off Off Off Off Bypass AV Off On On On On On Off Valves Comments A, B 1.sup.st C C All B B Rev., A, C 1st stage slowed stopped blocks Stopped slow B, 2nd st C, 2nd on stage A, C For. stage I and Fast slow

[0139] According to another embodiment of the present invention, the tapered flow desalination system and method are as follows. The system comprises three stages in which the concentrate exiting one stage becomes the feed entering the next stage. The system comprises K membrane block pressure vessels having feed water pass through them during the first stage, L membrane block pressure vessels having feed (concentrate exiting the K first stage blocks) pass through them during the second stage and M membrane block pressure vessels having feed (concentrate exiting the L second stage blocks) pass through them during the third stage, wherein preferably K>L>M. A booster pump is preferably connected to the flow lines at a portion of the line wherein the incoming feed at each stage flows through a single line before being split into the lines of the blocks of the next stage. The flow direction in block(s) of the third stage can be reversed and become part of the first stage and the flow direction in block(s) of the first stage can be reversed and become part of the third stage.

[0140] Additional elements (e.g. valves or control elements such as check valves or sensors) are connected to the system in a similar manner as explained hereinabove regarding the 2 stage embodiment. Each pressure vessel is coupled to a permeate product line wherein the permeate filtered exits the pressure vessel through it. The permeate product lines coming out of the pressure vessels are coupled to one main system outlet permeate product line.

[0141] According to an embodiment, the AV line (with the AV valve) couples between the concentrate outlet of the system and the third stage booster pump, in a similar manner as explained hereinabove regarding the 2 stage booster pump. Optionally, the AVF line (with the AVF valve) couples between the first stage booster pump and the second stage booster pump, as explained hereinabove regarding the second stage embodiment.

[0142] FIG. 7A-7F show an embodiment with six vessel blocks in the first stage (B, C and four more), three vessel blocks in the second stage and one vessel block (A) in the third stage. The system valves are connected to the system in a similar manner as in FIGS. 4-5.

[0143] The first booster pump is connected to the system incoming feed line 60.

[0144] The valves AFI, BFI and CFI are each coupled by flow lines to the first booster pump 51, to the valves ARO, BRO and CRO respectively, to the inlets/outlets of blocks A, B and C respectively, to the inlet of the four additional first stage blocks and to one another.

[0145] The valves ARO, BRO and CRO are also coupled to the inlets/outlets of each of the blocks A, B and C respectively, to the AV valve 55, to the system outlet concentrate outlet line 70 (of the system) and to one another.

[0146] The valves AFO, BFO and CFO are each coupled by flow lines to the second booster pump 52, to the valves ARI, BRI and CRI respectively, to the inlets/outlets of blocks A, B and C respectively, to the outlets of the four additional first stage blocks and to one another.

[0147] The valves ARI, BRI and CRI are also coupled to the inlets/outlets of blocks A, B and C respectively, to the third booster pump 53, to the AV valve 55 and to one another.

[0148] The second booster pump 52 is also coupled to the outlets of the four additional first stage blocks and to the inlets of the three second stage blocks.

[0149] The third booster pump 53 is also coupled to the outlets of the three second stage blocks and to the AV valve 55.

[0150] The AV valve 55 is also coupled to the concentrate outlet line 70 of the system.

[0151] Each pressure vessel is coupled to a permeate product line wherein the permeate filtered exits the pressure vessel through it. The permeate product lines coming out of the pressure vessels are coupled to one main system outlet permeate product line 80.

[0152] The method of this embodiment comprises switching roles between two pressure vessel blocks, one in the first stage and the other in the third stage. Thus the third stage block which is most exposed to scaling/membrane fouling because the feed entering it is highly concentrated (supersaturated) after passing two vessel blocks (of the first stage and second stage). Therefore this method of switching is a very efficient because after switching, the former third stage vessel is exposed to under saturated feed water which washes out all the minerals and materials that accumulated on the former third stage vessel, and this is done without water hammering.

[0153] The initial working mode is wherein at a first stage incoming feed water is passed through blocks C, B and through 4 additional blocks. The concentrated liquid coming out of those four blocks is passed in the second stage through three blocks. The concentrated liquid coming out of those three blocks is passed in the third stage though block A, and from there out of the system. The method comprises switching the roles between block A and block B. After the switching, block A is connected to the feed line 60 (at its opposite side), and block B becomes the third stage vessel block. This is done by opening/closing the appropriate valves. In this example a further switch can be made in which block C becomes the third stage vessel block and the vessel blocks A and B are part of the 6 blocks of vessels in the first stage. The working modes are switched periodically. Alternatively there can be only blocks A and B which are switched between the first and third stages, without any block C being connected to a manifold for switching as described in the previous paragraph.

[0154] The blocks of pressure vessels A,B, and C have been connected to the flow manifold so that they can either operate in parallel and as part of the first stage or as part of the third stage . When the valves connected to a block and labeled with the symbol FI and FO are open (open symbols) then that block is parallel to and part of the first stage. When the valves connect to a block and labeled with the symbols RI and RO are open (open symbols) then that block is part of the third stage. Thus in FIG. 7A, block A is in the third stage and blocks B and C are in the first stage. In FIG. 7F, block B is in the third stage and blocks A and C are in the first stage parallel to the rest of the pressure vessels in the first stage. This arrangement has the advantage that because the blocks are being switched into and out of the first and last stage, the change in the number of pressure vessels in parallel in the first stage during the switching is relatively minor and this minimizes any hydraulic upset. The steps described hereinabove that are used in switching between the first and second stage, are similar to those taken here as well to minimize any hydraulic shocks.

[0155] The following table clarifies the switching procedure step by step.

TABLE-US-00003 TABLE 3 Valve steps in moving from configuration with block A in stage III (while blocks B and C are together with rest of stage I) to block B in stage III (while blocks A and C are together with rest of stage I), as illustrated in FIGS. 7A to 7F. (C does not change stages) STEPS Start (I) II III IV V End (VI) Valves (FIG. 6A) (FIG. 6B) (FIG. 6C) (FIG. 6D) (FIG. 6E) (FIG. 6F) BLOCK AFI Off Off Off On On On A AFO Off Off Off On On On ARI On On Off Off Off Off ARO On On Off Off Off Off BLOCK BFI On On On Off Off Off B BFO On On On Off Off Off BRI Off Off Off Off On On BRO Off Off Off Off On On BLOCK CFI On On On On On On C CFO On On On On On On CRI Off Off Off Off Off Off CRO Off Off Off Off Off Off Bypass AV Off On On On On Off Valves Comments B, C 1.sup.st A slowed A A in 1.sup.st B B in 3.sup.rd stage stopped stage reversed stage A, 3rd and B in 3.sup.rd and stage stopped stage regular and slow speed A, C 1.sup.st A, C 1.sup.st stage stage Fast Fast

[0156] While some of the embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of a person skilled in the art, without departing from the spirit of the invention, or the scope of the claims.