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Title:
APPARATUS FOR THE TREATMENT OF LIQUIDS USING AN ADJUSTABLE PISTON ACCUMULATOR WITH THROUGHPUT FLOW
Document Type and Number:
WIPO Patent Application WO/2014/040996
Kind Code:
A1
Abstract:
The invention is related to an apparatus and method for the treatment of liquids by means of filtration techniques wherein a liquid flow is separated in a permeate and a retentate. The invention is especially suited for Reverse Osmosis. According to the invention, the retentate circuit is equipped with an adjustable "piston accumulator with throughput flow" (1), which guarantees a flexible control of the functioning of the reverse osmosis equipment. The piston accumulator contains a cylinder (100), a piston (99) that can move back and forth inside the cylinder, possibly a piston rod (102) fixed to the piston, so the cylinder is divided in a back side (103) and a front side (104), and a flow-through channel (105, 201, 331, 332) through the piston, so the retentate can flow through the piston to the space (104) at the piston side of the cylinder. The piston accumulator (1) guarantees a simple control of the desired operating pressure, a stable feed (23) independent of the desired cross-flow flow rate (6), with important energy savings compared to existing systems.

Inventors:
VAN HAUDENHUYSE ROLAND (BE)
Application Number:
PCT/EP2013/068740
Publication Date:
March 20, 2014
Filing Date:
September 10, 2013
Export Citation:
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Assignee:
AR SERVICES BVBA (BE)
International Classes:
C02F1/44; B01D61/12
Foreign References:
US5401395A1995-03-28
US5096574A1992-03-17
US6174437B12001-01-16
US20110303608A12011-12-15
US4629568A1986-12-16
US6149824A2000-11-21
US5401395A1995-03-28
Attorney, Agent or Firm:
PRONOVEM-OFFICE VAN MALDEREN (Brussels, BE)
Download PDF:
Claims:
Claims:

1 . Apparatus for the treatment of a liquid , comprising the following components:

A filtration circuit comprising the following components:

o A circulation pump (5),

o Filtration means (2) configured to separate the liquid into a permeate (3) that is removed from the circuit, and a retentate (4) that continues to flow through the circuit,

o A piston accumulator with throughput flow (1 ) which comprises a cylinder (100), a piston (99) configured to move back and forth inside the cylinder, so that the cylinder is divided in a back side ( 103) and a front side ( 104), and a flow- through channel (105,201 ,331 ,332) through the piston, so that retentate can flow through the piston to the front side (104) of the cylinder,

Supply means (7,50,51 ) to bring liquid into the circuit (6),

Means to create a counter pressure at the back side (103) of the cylinder,

Means (26) to collect the retentate from the filtration circuit.

2 . Apparatus according to claim 1 , which further comprises:

- A pressure sensor (19), to measure the pressure in the filtration circuit (6),

Control equipment configured to control the counter pressure at the back side of the cylinder as a function of the pressure measured by the pressure sensor (19).

3 . Apparatus according to claim 1 or 2, wherein the filtration circuit (6) is a reverse osmosis circuit and wherein the filtration means comprise one or more reverse osmosis membranes (2).

4 . Apparatus accord i ng to any one of the claims 1 to 3 , further comprising :

Means (9) to measure the position of the piston (99) relative to the cylinder (100), Control means configured to control said supply means based on the measured position of the piston.

5 . Apparatus according to any one of the preceding claims, wherein the means to deliver a counter pressure consist of means to exert a pneumatic pressure at the back side of the cylinder.

6 . Apparatus according to claim 5, wherein said means to exert a pneumatic pressure comprise the following components:

One or more pressure bottles (12) filled with a gas under pressure,

One or more control valves (14) configured to control the flow of gas from the pressure bottles to the back side of the cylinder, Control equipment configured to control the functioning of the control valves (14) based on the pressure measured by the pressure sensor (19).

7. Apparatus according to claim 6, further comprising one or more empty pressure bottles ( 13) and one or more additional control valves ( 15) configured to regulate the flow of gas from the cylinder (100) to the empty bottles based on the measurement of the pressure by the pressure sensor (19).

8. Apparatus according to claim 6 or 7, additionally comprising a compressor (28) configured to refill the pressure bottles (12).

9. Apparatus according to any one of claims 1 to 4, wherein the means to deliver a cou nter pressure comprise an adjustable spring or adjustable spring assembly (300), mounted at the cylinder's back side .

10. Apparatus according to any one of claims 1 to 4, wherein the means to deliver a counter pressure comprise a hydraulic actuator (299) configured to exert a force on the piston (99), and a hydraulic circuit (303) configured to deliver a hydraulic pressure to said actuator.

11. Apparatus according to claim 10, wherein the actuator (299) is placed on top of or underneath the piston accumulator, with the cylinder (100) of the piston accumulator sharing its upper or lower portion (51 1 ) with the actuator (299).

12. Apparatus according to claim 10, wherein the actuator is placed adjacent to the piston accumulator, wherein the actuator is connected to the piston (99) of the accumulator via a bracket (515).

13. Apparatus according to any one of claims 1 to 4, wherein the means to deliver a counter pressure consist of a hydraulic pressure circuit (303'), configured to deliver a hydraulic pressure directly on the piston at the cylinder's back side (103).

14. Apparatus accord ing to any one of the preceding claims, wherein a flexible connection (33) is provided between the outlet of the means of filtration (2) and the inlet of the flow-through channel of the piston accumulator (1 ).

15. Apparatus accord ing to any one of the preced ing claims, wherein a piston rod (102) is fixed to said piston (99) at the back side (103) the cylinder, and wherein said flow-through channel runs through said piston rod (102) and said piston (99).

16. Apparatus according to claim 15, wherein the piston rod (102) and piston (99) are made from a solid material, penetrated by a single channel (105) that constitutes the flow-through channel.

17. Apparatus according to claim 15, wherein the piston rod is a hollow tube (200) inside of which is mounted a second tube (201 ) which also runs through the piston (99).

18. Apparatus according to claim 15, wherein a fixed connection is provided between the outlet of the filtration means (2) and the inlet of the flow-through channel, wherein this flow-through channel is formed by a fixed tube (53), which is preferably centrally placed with respect to the cylinder (100), and wherein the piston rod (200) and the piston are mounted around this fixed tube (53) and can move back and forth relative to the fixed tube, with appropriate seals (54) mounted between the piston (99) and the fixed tube (53).

19. Apparatus accord ing to any one of the preceding claims, wherein the supply means consist of a feed pump (7), preferably a volumetric feed pump.

20. Apparatus according to claim 19, wherein the apparatus further comprises a dry run protection (32), configured to prevent the feed pump from running dry.

21. Apparatus according to claim 19, wherein the apparatus further comprises a weighing means (55) configured to weigh an amount of liquid from which the feed pump (7) draws liquid, as well as means to deactivate the feed pump when this amount falls below a certain threshold.

22. Apparatus according to any one of claims 1 to 18, wherein the supply means consist of a pressure vessel (50) and a compressed gas tank (51 ), configured to feed the liquid into the reverse osmosis circuit by means of gas pressure.

23. Apparatus accord ing to any one of the preceding claims, wherein the filtration circuit further comprises one or more tanks (45) as well as the necessary valves (57) to include or exclude these tanks from the circuit, in order to raise or lower the volume of liquid circulating through the circuit.

24. Apparatus according to any one of the preceding claims, that further comprises a weighing means (47) to measure the amount of permeate, as well as control equipment to control the supply means based on this weighing.

25. Apparatus according to any one of the preceding claims, wherein the means (26,27) to collect the retentate from the filtration circuit are configured to continuously collect the retentate while it is circulating in the filtration circuit, and wherein the apparatus further comprises a first weighing means (47) to weigh the amount of permeate, and a second weighing means (48) to weigh the amount of retentate collected, as well as control equipment configured to control the supply means (7,50,51 ) based on these weighings and also control the means (27) to collect the retentate from the filtration circuit.

26. Apparatus accord ing to any one of the preceding claims, wherein the filtration means comprise multiple reverse osmosis membranes (2) mounted serially or in parallel, the apparatus further comprising one or more cleaning devices, arranged in such a manner that one or more membranes can be cleaned while the other membranes are active in the reverse osmosis circuit.

27. Apparatus accord ing to any one of the preceding claims, wherein the piston contains a ring-shaped channel (331 ) which is connected to an eccentrically placed inlet (401 ) for the passage of the liquid that comes from the filtration means towards the ring-shaped channel (331 ), and wherein the piston further contains one or more drilled conduits (332) that connect to the ring-shaped channel, for the passage of the liquid from the ring- shaped channel (331 ) to the front side of the piston accumulator.

28. Apparatus accord ing to any one of the preceding claims, wherein the supply means to bring liquid into the filtration circuit is formed by the piston accumulator (1 ) itself, said accumulator being configured to draw liquid from a supply tank (8) by upward movement of the piston (99) in the cylinder (100).

29. Apparatus according to any one of the preceding claims, further equipped with a backflow connection between the filtration circuit (6) and a supply tank (8) from where fluid is fed into the circuit, and wherein the backflow connection comprises means (61 ,62) to let an amount of liquid flow from the filtration circuit (6) back to the supply tank (8).

30. Apparatus according to claim 29, wherein said means to let an amount of liquid flow from the filtration circuit (6) back to the supply tank (8) consist of a control valve (62) and a flow regulator (61 ).

31. Method for the treatm ent of l iq u ids by reverse osmosis comprising the following steps:

Circulating the l iq u id i n a filtration ci rcu it (6) that comprises the following components:

o A circulation pump (5),

o Filtration means (2) configured to separate the liquid into a permeate (3) that is removed from the circuit, and a retentate (4) that continues to flow through the circuit,

o A piston accumulator with throughput flow (1 ) which comprises a cylinder

(100), a piston (99) configured to move back and forth inside the cylinder, so that the cylinder is divided in a back side (103) and a front side (104), and a flow-through channel (105,201 ,331 ,332) through the piston, so that retentate can flow through the piston to the front side (104) of the cylinder, - Removing an amount of permeate from the membranes (2) and adding an amount of fresh liquid to the circuit (6) via a supply means (7;50,51 ),

Exerting a counter pressure at the back side of the cylinder, to maintain the circuit at a predetermined pressure.

32. Method according to claim 31 , wherein the amount of liquid added to the filtration circuit is controlled based on the measurement of the position of the piston (99) with respect to the cylinder (100), and the control equipment maintains the position of the piston at a fixed predetermined point.

33. Method according to claim 31 or 32, wherein a certain amount of liquid is treated in the filtration circuit, after which the retentate is collected.

34. Method according to claim 31 or 32, wherein the retentate is continuously collected from the filtration circuit, while fresh liquid is added to the circuit.

35. Method according to claim 34, wherein the retentate flow that exits the filtration circuit is controlled based on a weighing of the amount of permeate collected, a weighing of the amount of retentate collected, and a predetermined concentration factor CF = HB/(HB -HP), where:

HB is equal to the amount of liquid treated,

HP is equal to the amount of permeate extracted from this amount of treated liquid.

36 . Method according to any one of the claims 31 to 35, wherein the filtration circuit (6) is a reverse osmosis circuit en wherein the filtration means comprise one or more reverse osmosis membranes (2).

37 . Method accord ing to any one of the clai ms 31 to 36, implemented in an apparatus according to any one of claims 1 to 30.

Description:
APPARATUS FOR THE TREATMENT OF LIQUIDS USING AN ADJUSTABLE PISTON ACCUMULATOR WITH THROUGHPUT FLOW

Field of the invention

[0001] The invention relates to an apparatus or installation and to a method for the treatment of liquids by means of reverse osmosis or similar filtration techniques. State of the art

[0002] The technique of 'Reverse Osmosis' (further called RO), makes use of a membrane that is semi-permeable to ions, particularly those of water and liquids with low molecular weight. Water contained in the liquid to be treated is allowed through the membrane, and the remaining elements, mostly impurities, are blocked. Mainly used to purify water, RO is also used to remove water from a liquid, and thus concentrate this liquid, or clean it, e.g. in case of solvents. RO uses a technology called "cross-flow" : the liquid to be treated continuously flows at high velocity and turbulence along one side of the membrane. This ensures that the membrane is continuously self-cleaned, and so the extremely fine pores of the membrane do not clog, except under abnormal circumstances (called "fouling"). Due to the osmotic effect a part of the liquid (mainly the water it contains) flows through the membrane. This flux is called the permeate, while the main flux, e.g. the liquid which is to be concentrated, is called the retentate. The amount of transferred permeate depends on the properties of the liquid and the type of membrane, and is influenced by the flow rate of the cross-flow, the operating pressure in the retentate circuit and the temperature of the liquid. Usually there is a closed circuit with a circulation pump working at a high flow rate and low differential pressure (the pressure drop mainly takes place across the membranes in order to realize the cross-flow) and the liquid which will disappear as permeate is continually replenished by a feed pump working at a low flow rate and high operating pressure. Therefore, such an arrangement requires 2 pumps, but the two functions (supply and circulation) are sometimes combined into a single pump, especially for low pressure applications and/or open systems.

[0003] The supply of the liquid from a storage tank to the RO equipment must be tailored to the amount of permeate leaving the RO membrane. Because the permeate flow rate can vary greatly during the process, inter alia depending on the degree of fouling of the membranes, there is a need for a specific control system for the supply pump. In addition, since the process for highly concentrated liquids usually requires high operating pressures, volumetric feed pumps are mainly used. The flow rate of these types of pumps can be adjusted with speed controls, but they are very sensitive to discordances between the requested and the delivered flow rate, which can cause overpressure and possibly breakage.

[0004] The very low flow rate of the permeate, especially at the end of the concentrating process, is hard to monitor. Accurate flow rate meters for food-safe applications are very expensive. This greatly complicates an accurate automatic control of the feed pump. Furthermore the standard speed control of a volumetric pump is limited in range (usually 1/5 to 1/8) and provides insufficient dependability for robust control. That is why usually other solutions are used, like overflow valves or pressure relief valves after the feed pump, or the purging of the excess supply over a reducing valve to the storage tank or to a special circulation tank.

[0005] This continuous purging or recirculation of liquid from the high pressure feed pump to a storage tank is associated with an important energy loss and an unwanted heating of the base liquid. For some liquids this means strong coolers have to be included , which again causes additional energy losses. Some liquids can cause strong foaming in the circulation tank. Furthermore, the previously mentioned overflow or pressure relief valves are not available or difficult to obtain for food grade applications at high pressures (24 bar and upwards) because of the difficult cleaning of the valve, especially when constructed as described in US patent 5.401.395 (see further). Applications in which the pressure needs to change during the process, because the osmotic pressure rises with higher concentration factors, make the control systems even more complex.

[0006] As the circulation tank of a standard RO system, to which the high pressure feed pump circulates liquid, has a fixed volume, the batch size of the concentrated end product directly depends on the capacity of this tank, for a given concentration factor. In addition often a third low pressure feed pump and an extra storage tank need to be used to replenish the base liquid in the closed system, if one wants to work batch-independently (i.e. not limited to treatment of batches of liquid). As the high pressure pump is fed from the circulation tank, a minimum amount of liquid needs to remain in the tank at all times, to avoid dry running of the feed pump at the end of the concentration process. This has a direct effect on the minimum batch size.

[0007] Most often these tanks are under a slight overpressure, with a corresponding cost depending on size. In the case of hazardous liquids, like solvents, they have to comply with the ATEX directive, and the space in the tank not occupied by liquids needs to be filled with inert gasses, which demands a more stringent inspection of the equipment, especially for larger tanks.

[0008] Overflow or pressure relief valves were improved to obtain a more stable

RO process. US Patent 5.401.395 (28/3/1995) describes a "pressure limiting valve" 28 (shown in Figure 1 of US'395), controlled by the pressure in the permeate circuit through a control line 30. In the embodiment of Figure 4 of US patent 5.401.395, the "cross-flow" 24 does not go through the valve. Only the overflow (34 and/or 36) enters the valve through opening 72 but leaves again through a 'side exit' 80 without going through the piston 90 of the valve. Chamber 1 14 is indeed closed. The hole 104 is only there to compensate the counter pressure in the balance of forces within the valve. The goal of this valve is to reduce or avoid pulsations in the permeate circuit while producing pure water. Moreover, in US'395, the piston 90 in the valve 28B only moves as the result of variations in the pressure of the circuits 18 of the osmosis process. The piston 90 is not driven by an external force.

Summary of the invention

[0009] The invention aims to offer a solution for one or more of the aforementioned problems of the existing RO systems. The invention is related to an apparatus and method as described in the appended claims, for the treatment of liquids by means of filtration techniques, like micro- or nano-filtration or Reverse Osmosis, in which a liquid is separated into a permeate and a retentate. The invention is especially suitable for RO (but not limited thereto). According to the invention, the retentate circuit is equipped with an adjustable "piston accumulator with throughput flow", which ensures a smooth control of the operation of the RO equipment. According to a preferred embodiment, The piston accumulator contains a cylinder, a piston configured to move back and forth in the cylinder, and a piston rod fixed to the piston, so that the cylinder is divided into a rod side and a piston side, wherein an adjustable flow-through channel is provided through the rod and the piston, so that the retentate can flow through the piston rod and the piston, to the piston side of the cylinder. More generally, the flow-through channel is a channel which is part of the filtration circuit and which is provided from the 'rod side' to the piston side (i.e. not necessarily through the rod itself, see for example figures 8a and 8b). According to some embodiments (e.g. the embodiment of figure 13), the piston is not provided with an actual piston rod in the literal sense. The wording 'rod side' and 'piston side' are meant to indicate respectively the side that receives an external force (e.g. gas pressure, spring force, hydraulic pressure) and the side that receives the liquid to be treated, and from where the liquid further proceeds its way through the filtration circuit. The external force is applied in order to control the pressure in the retentate circuit. In the appended claims and parts of this description, the 'rod side' of the cylinder is named 'back side', while the 'piston side' is named the 'front side'. The wording 'front' and 'back' is however not to be understood as indicating the flow of the liquid through the filtration circuit. Liquid may flow in both directions in the circuit, as explained in some more detail further in this text.

[0010] The invention is thus in particular related to an apparatus for the treatment of a liquid, comprising the following components:

A filtration circuit comprising the following components:

o A circulation pump,

o Filtration means configured to separate the liq uid into a permeate that is removed from the circuit, and a retentate that continues to flow through the circuit,

o A piston accumulator with throughput flow which comprises a cylinder, a piston configured to move back and forth inside the cylinder, so that the cylinder is divided in a back side and a front side, and a flow-through channel through the piston, so that retentate can flow through the piston to the front side of the cylinder,

Supply means to bring liquid into the circuit,

- Means to create a counter pressure at the back side of the cylinder,

Means to collect the retentate from the filtration circuit.

[0011] According to an embodiment, the apparatus further comprises:

A pressure sensor, to measure the pressure in the filtration circuit,

Control equipment configured to control the counter pressure at the back side of the cylinder as a function of the pressure measured by the pressure sensor.

[0012] According to an embodiment, the filtration circuit is a reverse osmosis circuit and wherein the filtration means comprise one or more reverse osmosis membranes. According to an embodiment, the apparatus further comprises: :

Means to measure the position of the piston relative to the cylinder,

- Control means configured to control said supply means based on the measured position of the piston.

[0013] According to an embodiment, the means to deliver a counter pressure consist of means to exert a pneumatic pressure at the back side of the cylinder. Said means to exert a pneumatic pressure may comprise the following components:

- One or more pressure bottles filled with a gas under pressure,

One or more control valves configured to control the flow of gas from the pressure bottles to the back side of the cylinder,

Control equipment configured to control the functioning of the control valves based on the pressure measured by the pressure sensor.

[0014] The apparatus according to the latter embodiment may further comprise one or more empty pressure bottles and one or more additional control valves configured to regulate the flow of gas from the cylinder to the empty bottles based on the measurement of the pressure by the pressure sensor. Said apparatus may additionally comprise a compressor configured to refill the pressure bottles.

According to an embodiment, the means to deliver a counter pressure comprise an adjustable spring or adjustable spring assembly, mounted at the cylinder's back side .

[0015] According to an embodiment, the means to deliver a counter pressure comprise a hydraulic actuator configured to exert a force on the piston, and a hydraulic circuit configured to deliver a hydraulic pressure to said actuator. Said actuator may be placed on top of or underneath the piston accumulator, with the cylinder of the piston accumulator sharing its upper or lower portion with the actuator.

[0016] According to another embodiment, the actuator is placed adjacent to the piston accumulator, wherein the actuator is connected to the piston of the accumulator via a bracket (515). [0017] According to an embodiment, the means to deliver a counter pressure consist of a hydraulic pressure circuit, configured to deliver a hydraulic pressure directly on the piston at the cylinder's back side.

[0018] According to an embodiment, a flexible connection is provided between the outlet of the means of filtration and the inlet of the flow-through channel of the piston accumulator. According to an embodiment, a piston rod is fixed to said piston at the back side the cylinder, and wherein said flow-through channel runs through said piston rod and said piston. Said piston rod and piston can be made from a solid material, penetrated by a single channel that constitutes the flow-through channel. Alternatively, said piston rod may be a hollow tube inside of which is mounted a second tube which also runs through the piston.

[0019] According to an embodiment, a fixed connection is provided between the outlet of the filtration means and the inlet of the flow-through channel, wherein this flow-through channel is formed by a fixed tube, which is preferably centrally placed with respect to the cylinder, and wherein the piston rod and the piston are mounted around this fixed tube and can move back and forth relative to the fixed tube, with appropriate seals mounted between the piston and the fixed tube.

[0020] According to an embodiment, the supply means consist of a feed pump, preferably a volumetric feed pump. The apparatus may then further comprise a dry run protection, configured to prevent the feed pump from running dry.

[0021] According to an embodiment, the apparatus further comprises a weighing means configured to weigh an amount of liquid from which the feed pump draws liquid, as well as means to deactivate the feed pump when this amount falls below a certain threshold.

[0022] According to an embodiment, the supply means consist of a pressure vessel and a compressed gas tank, configured to feed the liquid into the reverse osmosis circuit by means of gas pressure.

[0023] According to an embodiment, the filtration circuit further comprises one or more tanks as well as the necessary valves to include or exclude these tanks from the circuit, in order to raise or lower the volume of liquid circulating through the circuit.

[0024] According to an embodiment, the apparatus further comprises a weighing means to measure the amount of permeate, as well as control equipment to control the supply means based on this weighing.

[0025] According to an embodiment, the means to collect the retentate from the filtration circuit are configured to continuously collect the retentate while it is circulating in the filtration circuit, and wherein the apparatus further comprises a first weighing means to weigh the amount of permeate, and a second weighing means to weigh the amount of retentate collected, as well as control equipment configured to control the supply means based on these weighings and also control the means to collect the retentate from the filtration circuit.

[0026] According to an embodiment, the filtration means comprise multiple reverse osmosis membranes mounted serially or in parallel, the apparatus further comprising one or more cleaning devices, arranged in such a manner that one or more membranes can be cleaned while the other membranes are active in the reverse osmosis circuit.

[0027] According to an embodiment, the piston contains a ring-shaped channel

(331 ) which is connected to an eccentrically placed inlet for the passage of the liquid that comes from the filtration means towards the ring-shaped channel, and wherein the piston further contains one or more drilled conduits that connect to the ring-shaped channel, for the passage of the liquid from the ring-shaped channel to the front side of the piston accumulator.

[0028] According to an embodiment, the supply means to bring liquid into the filtration circuit is formed by the piston accumulator itself, said accumulator being configured to draw liquid from a supply tank by upward movement of the piston in the cylinder.

[0029] According to an embodiment, the apparatus is further equipped with a backflow connection between the filtration circuit and a supply tank from where fluid is fed into the circuit, and wherein the backflow connection comprises means to let an amount of liquid flow from the filtration circuit back to the supply tank. Said means to let an amount of liquid flow from the filtration circuit back to the supply tank may consist of a control valve and a flow regulator.

[0030] The invention is equally related to a method for the treatment of liquids by reverse osmosis comprising the following steps:

Circulating the liquid in a filtration circuit that comprises the following components: o A circulation pump,

o Filtration means configured to separate the liquid into a permeate that is removed from the circuit, and a retentate that continues to flow through the circuit,

o A piston accumulator with throughput flow which comprises a cylinder, a piston configured to move back and forth inside the cylinder, so that the cylinder is divided in a back side and a front side, and a flow-through channel through the piston, so that retentate can flow through the piston to the front side of the cylinder,

Removing an amount of permeate from the membranes and adding an amount of fresh liquid to the circuit via a supply means,

Exerting a counter pressure at the back side of the cylinder, to maintain the circuit at a predetermined pressure.

[0031] According to an embodiment of the method, the amount of liquid added to the filtration circuit is controlled based on the measurement of the position of the piston with respect to the cylinder, and the control equipment maintains the position of the piston at a fixed predetermined point.

[0032] According to an embodiment of the method, a certain amount of liquid is treated in the filtration circuit, after which the retentate is collected.

[0033] According to another embodiment of the method, the retentate is continuously collected from the filtration circuit, while fresh liquid is added to the circuit. [0034] According to an embodiment of the method, the retentate flow that exits the filtration circuit is controlled based on a weighing of the amount of permeate collected, a weighing of the amount of retentate collected, and a predetermined concentration factor CF =

HB/(HB -HP), where:

- HB is equal to the amount of liquid treated,

HP is equal to the amount of permeate extracted from this amount of treated liquid.

[0035] According to an embodiment of the method, the filtration circuit is a reverse osmosis circuit en wherein the filtration means comprise one or more reverse osmosis membranes.

[0036] The invention is related to a method according to any one of the previous paragraph, implemented in an apparatus according to the invention.

[0037] The piston accumulator ensures an easy control of the desired operating pressure and a stable supply of the base liquid, independently of the desired cross-flow flow rate, with significant energy savings compared to existing systems. Furthermore, compared with traditional RO systems, the use of components which are not allowed for food-safe applications or are difficult to clean, like pressure relief valves, can be avoided. The invention can be used at all operating pressures. It is suited for RO processes at high pressures, and when the amount of product undergoing the treatment must be controllable and independent of a fixed batch size. Specific embodiments are suited for more compact, batch operated processes. Since the additional equipment can be mounted in a completely closed system, highly volatile liquids or liquids sensitive to oxidation can be treated more easily and safely. It is clear in particular that the aim and function of the apparatus described in US patent 5.401.395, namely to reduce or avoid pulsations in the permeate circuit, is completely different from the present invention.

Brief description of the figures

[0038] Figure 1 shows an apparatus for industrial applications according to the invention, equipped with a piston accumulator, a circulating pump, an RO-membrane and a flexible connection in between. Also, it shows a counter-pressure control system with an assembly of compressed air bottles and one or more empty bottles to capture the backflow of the pressurized gas.

[0039] Figure 2 shows a detail of the piston accumulator used in figure 1.

[0040] Figure 3 shows an alternative embodiment of the piston accumulator.

[0041] Figure 4 shows a way to guide the movement of the piston rod in the piston accumulator.

[0042] Figure 5 shows an implementation of the piston accumulator without a flexible connection to the RO-membrane.

[0043] Figure 6 shows an embodiment without empty bottles in the control system . [0044] Figure 7 shows an implementation in which the compressed air bottles have been substituted by a compressor.

[0045] Figures 8a and 8b show two embodiments of the piston accumulator in which the counter pressure at the back side of the piston accumulator is provided by a spindle or worm screw (8a) or a hydraulic cylinder (8b), in both cases through an intermediate mechanic spring.

[0046] Figure 8c shows an embodiment of the piston accumulator in which the counter pressure at the rod side is provided by a hydraulic actuator with constant pressure control, omitting the need for an intermediate mechanical spring or spring arrangement (as 300 in Figure 8b). Figures 8d to 8f show specific embodiments of how the hydraulic actuator is connected to the piston accumulator. In all of the figures 8a to 8f, the piston rod of the accumulator 1 is shown as a hollow tube 200 with a tube 201 inside it. Other types of piston arrangements can be used here as well.

[0047] Figure 9 shows an implementation with two RO-membranes and two circulation pumps.

[0048] Figure 10 shows an implementation in which the feed pump is replaced by a pressure vessel.

[0049] Figure 1 1 shows an implementation in which the feed pump is controlled based on a weighing of the permeate tank and the retentate tank. This version also demonstrates the possibility to include extra tanks in the RO circuit (not necessarily combined with the weighing of permeate and retentate tanks).

[0050] Figure 12 shows an implementation with a feedback connection between the RO circuit and the supply tank.

[0051] Figure 13a/13b shows an implementation according to the invention, suitable to construct a compact version of the machine.

[0052] Figure 14 shows an embodiment wherein a hydraulic pressure is applied directly to the back side of the piston of the piston accumulator.

Detailed description of the invention

[0053] The present invention will be described with the use of a few examples and with reference to certain figures without any limitative character. The figures are schematic and not limiting. The dimensions of specific elements in the figures can be exaggerated or out of proportion for the sake of illustrative considerations. Dimensions and proportions do not necessarily have to correspond to reality. The following description applies to both the device and the method according to the invention. The description includes not only devices and methods such as described in the claims but also all the other physically possible combinations of elements described in the present specification.

[0054] An installation according to the invention, suitable for the production of large amounts of retentate, is shown in figure 1 and is described in detail below. However, the invention is not limited to this type of installation. Variants of certain components can lead to different embodiments as described below. The installation in figure 1 serves as a good example to demonstrate a few basic concepts.

[0055] The installation shown in figure 1 comprises the following components :

A su pply tank 8 (for example, under atmospheric pressure, and possibly under slight overpressure, with or without an inert gas) for the base liquid, i.e. the starting product;

A feed pump 7 that pumps the liquid 23 from the supply tank 8 into the Reverse Osmosis circuit 6;

A Reverse Osmosis circuit 6, with a circulation pump 5, the RO membrane 2 (or possibly multiple membranes, placed parallel or serial) and a "piston accumulator with throughput flow" 1. Figure 2 shows this component in detail. It contains a cylinder 100, a piston 99 configured to move up and down inside cylinder 100 (with the necessary seals 101 between the piston and the cylinder wall, possibly equipped with a leak detection between the seals) and a piston rod 102 fixed to the piston. The piston divides the cylinder in a rod side 103 and a piston side 104. In the implementation shown in the figure, the piston rod and the piston are solid parts (whether or not in one piece) provided with a drilled channel 105 (one or more) that, together with the part of the cylinder at piston side 104, is a part of the Reverse Osmosis circuit 6 (in other words, the liquid flows through channel 105 to cylinder part 104 on the piston side). Seals 106 close the cylinder part 103 at rod side, to enable the development of pressure in this space by a gas fed through a supply line 107, from an external control circuit (see below). On the bottom of the cylinder there is an exit channel 108 through which the liquid returns to circulation pump 5;

A flexible feed pipe 33 connected to connector 109, to enable the connection to the channel 105 of the piston accumulator 1 ;

An accurate sensor 9 to determine the position of the piston 99 inside of the piston accumulator 1 , measured relative to the cylinder (e.g. a "Magnetostrictive Position Sensor" or an LDT, a "Linear Displacement Transducer");

Optionally, a cooler can be included into the RO circuit 6 (not shown in the figure); a permeate tank 10, to catch and collect the permeate;

a retentate tank 1 1 , connected to the piston accumulator through valve 26 and flow regulator 27;

a counter-pressure control circuit 30 with a switching valve 22 and control valves 14 and 15 through which an assembly of full 12 and empty gas bottles 13 (preferably compressed air), optionally completed with a compressor 28, can generate a counter pressure at rod side, to create or maintain a certain pressure in the RO circuit 6. This counter pressure at the rod side of the cylinder can also be realized through means of another mechanism, e.g. a mechanical jack or hydraulic cylinder whether or not equipped with an intermediate spring (see below); - A pressure sensor 19 to measure the pressure in RO circuit 6, which is a consequence of the forces generated on the piston;

Electronic control equipment (not shown in figures but schematically represented by the dotted lines in figure 1 ) for the management of the supply pump 7 and possibly the switching valve 22 (see below) based on the measured position of piston 99 inside the piston accumulator 1 , and for the control of valves 14 and 15 based on the pressure delivered by the feed pump 7 and measured by pressure sensor 19.

[0056] The implementation in figure 1 is designed for operation in so-called

'batch' mode, i.e. the concentration of a certain amount of liquid that is supplied to the RO circuit 6 by the feed pump 7, whereby the liquid is separated into a permeate and a retentate by the RO-membrane 2, and the retentate is circulated inside the RO circuit 6 so it flows multiple times along the RO membrane (which is called cross-flow, referred to above). The feed pump 7 adds fresh liquid depending on the quantity of permeate being extracted , in order to maintain a predetermined high pressure in RO circuit 6. This process continues until the batch of fresh liquid in supply tank 8 is completely used. The process then continues with valve 26 closed. At the end of the procedure the retentate is collected through this valve. During the concentration process, feed pump 7 is controlled based on the position of the piston rod detected by the measurement sensor 9 (see below). The circulating of the retentate in the RO circuit happens at a predetermined liquid pressure, maintained using the piston accumulator, as described below.

[0057] The flow rate of the circulation pump 5 is adapted to the maximum cross- flow 6 that the selected RO membrane can manage. Preferably the circulation pump 5 is a traditional centrifugal pump. The discharge head of the circulation pump 5 must be able to cover the pressure drop in the pipes of the closed circuit 6 and the piston accumulator 1 at maximum flow rate, plus the maximum allowable differential pressure across the RO membrane. The circulation pump 5 is slowed down as the permissible differential pressure across the membrane 2 is exceeded. Preferably a control system is provided (see the dotted line between the membrane 2 and pump 5) for control of the circulation pump based on a measurement of the differential pressure across the membrane. Preferably, when the maximum allowable differential pressure across the membrane is exceeded, the revolution speed of the circulation pump and hence the cross flow rate is reduced until the pressure drops below the maximum differential pressure. The circulation pump 5, the lines of the circuit 6, the flexible conduit 33 and the piston accumulator 1 should be designed so they can tolerate the high pressure 19.

[0058] As stated, a "piston accumulator with throughput flow" is part of the RO circuit 6. This is a kind of hydraulic cylinder with a moving piston rod but no hydraulic force development. The position of piston 99 inside the cylinder is determined by measuring the length over which the piston rod extends upward, with a linear measuring instrument 9.

[0059] At the rod side the counter-pressure is set via a control valve 14, from an assembly of compressed gas bottles 12 at a given pressure (the assembly pressure, e.g . between 120 and 300bar), depending on the application filled with (food-grade) air or another gas. The pressure after the pressure controller 14 depends, among other things, on the piston side / rod side ratio (i.e. the ratio of the piston surface that receives a pressure on the piston side to the piston surface that receives a pressure on the rod side; in practice this is D 2 /D 2 -d 2 when the piston and the rod have diameters D and d respectively) , the weight of the piston rod and the piston, and the requested operational pressure in the RO circuit. The requested pressure of the liquid circuit at piston side is measured by the pressure sensor 19.

[0060] The "compressed gas bottle assembly" 12 is filled by a compressor 28, or the bottles can be pre-filled replaceable gas bottles. The gas can move from the assembly 12 to the cylinder 1 via control valve 14, when the piston is going down. Depending on the pressure the gas can return to the full assembly 12 via check valve 20 or to the empty bottles 13 via control valve 15 when the piston rod is going up. Valves 14 and 15 are appropriately controlled based on the liquid pressure detected by sensor 19 to ensure that the predetermined operating pressure in RO circuit 6 is maintained. Control valve 15 is always set slightly higher than control valve 14, which determines the spread of the desired operating pressure. For instance, at a counter-pressure of 80bar at rod side, valve 14 will open when the pressure drops to 79bar (because the piston is going down) and valve 15 will open when the pressure rises to 82bar (when the gas at rod side is compressed due to the piston going up). This enables a simple and broad control of the operating pressure in the RO circuit 6. When the flow rate of the permeate rises so strongly that the corresponding pressure drop in the RO circuit cannot be compensated by adjusting the feed pump 7, the pressure in the RO circuit can still be maintained using the counter-pressure control. Compressor 28 is optional and can be used to bring the gas bottle assembly 12 back to the required pressure when the batch process is finished. This is mainly environmentally and energetically interesting when inert gas is used, a gas which for example one does not want to be lost because of the cost. The process according to the invention can be executed at a predetermined constant pressure in the RO circuit, or at a variable pressure. For instance, if the operating pressure (the pressure in circuit 6) has to be maintained at e.g. 30bar at the beginning of the batch process, and at the end of the process e.g. 60bar is required because due to the higher concentration of the liquid, the osmotic pressure has risen and the permeate flow rate has dropped. Raising the pressure can then increase the permeate flow.

[0061] Hereafter the operation of the apparatus of Figure 1 (with piston accumulator of Figure 2) is presented from start-up, in batch mode. At the start of the process, the piston 99is in its lowest position. There is no pressure on any circuit, including rod side of the piston accumulator 1. When the feed pump 7 is started, the conduits of the circuit 6 and the piston accumulator 1 are filled with the base liquid from the supply tank 8 via the shut-off valve 31 and the check valve 17. As the feed pump 7 pumps more and more liquid into the RO circuit, the piston 99 rises within the piston accumulator. When the piston 99 reaches its desired position, feed pump 7 stops pumping. The gas present in the cylinder at rod side can escape through vent 21 while the piston is rising. The initial position of piston 99 at the start of a batch process determines the volume of the RO circuit 6 and thus the size of the batch, and as a consequence determines the concentration factor(*) one wants to obtain and the quantity one wants to be treated. The circulation pump 5 starts and the flow is brought to the desired cross flow rate. Via the switching valve 22 and the control valve 14, the rod side is slowly pressurized, causing the RO circuit 6 to be charged to the desired pressure. As the pressure rises, the RO process is launched. At the start, the permeate will start to flow as soon as a minimum RO pressure is reached (e.g. 5bar). The permeate will reach its maximum flow rate as the pressure approaches its maximum value.

(*) Concentration factor: the extent to which permeate 3 is withdrawn from the base liquid 23.

The concentration factor is calculated as follows:

CF = HB/(HB -HP)

CF: Concentration factor

HB: quantity of base liquid to be treated

HP: quantity of permeate produced from the quantity of base liquid to be treated.

[0062] During filling of the RO circuit and the piston accumulator by the feed pump 7, there is no counter pressure at the rod side of the cylinder, which causes piston 99 to rise to a position corresponding to the desired "set point". This "set point" depends on the desired concentration factor. When this set point is reached , the feed pump stops. Next the circulation pump 5 starts and is set to a cross-flow flow rate that does not exceed the permissible differential pressure. The control valve 14 opens. The pressure at rod side, and therefore also at piston side, will rise in the piston accumulator. The "Set point" of control valve 14 is determined by the desired pressure in the RO circuit. Control valve 14 will be open until the requested pressure, measured by sensor 19, is reached. Due to the effect of the realized RO pressure the RO process begins, and permeate 3 exits the RO circuit. As a consequence, the piston will descend, away from its set point position. This is detected by the position sensor 9, which will activate the feed pump until the original set point of the piston is reached again.

The pressure at rod side will in the mean time be kept at least at the set level by control valve 14. However, if the piston rises so much that the pressure at rod side exceeds the requested value, the overpressure is vented through control valve 15 to the empty bottles 13. The revolution speed of the feed pump 7 is eventually controlled by a PID control in such a way that the flow rate delivered by the pump corresponds to the permeate that is being produced , causing the piston to remain at its set point.

[0063] While circulating in the RO circuit 6, the retentate 4 flows from the circulation pump 5 along the RO membrane 2 and flexible tube 33, through the drilled conduit 105 along rod 102 and piston 99 to the opening 108 in the bottom of the cylinder of the piston accumulator 1 , where it is drawn in again by the circulation pump 5. The feed pump 7 adds liquid in the suction line of the circulation pump 5, to compensate for the liquid which disappears as permeate 3 in the RO membrane 2. In case the feed pump 7 delivers insufficient fresh liquid, the piston 99 will descend within the cylinder 1 . In case the feed pump 7 delivers too much flow, the piston 99 will rise. By measuring the position of the piston rod with measuring instrument 9, it is possible to accurately control the feed pump 7. Small differences between the removed amount of permeate 3 and added quantity of base liquid 23 via the feed pump 7 will cause the piston 99 to be displaced slightly so no overpressure can appear on the liquid circuit. This way, the presence of the piston accumulator 1 simplifies the flow rate control of the feed pump 7. It is nevertheless preferable to include a rupture disk 24 and pressure relief valve 29 on the gas circuit and a rupture disk 25 on the liqu id circuit as ultimate safety features. According to specific embodiments, an extremely high or low position of the piston rod may trigger additional control signals to stop or start the feed pump 7.

[0064] When supply tank 8, comprising the base liquid, is empty, the feed pump

7 stops. This can be monitored for example by weighing the supply tank with a weighing means (any suitable type of scales or the like can be used) 55 (as shown in figure 1 1 ) or with a dry run protection 32 (figure 1 ). As permeate is still disappearing, the piston 99 will continue to descend because of the pressure present at the rod side, until the requested position, corresponding to the desired concentration factor, is reached. Then the circulation pump 5 stops. The batch process has thereby come to its end. Before emptying the RO circuit 6 using valve 26 to the retentate tank 1 1 , the RO circuit 6 must be made pressure free. Both pumps 7 and 5 are stopped. Switching valve 22 slowly lets the gas in the cylinder at rod side escape towards the atmosphere through vent 21. The remaining pressure in the RO circuit 6 will be determined by the weight of the piston 99 and piston rod 102, and will be sufficient to fill the retentate tank through the flow control 27. The small amount of gas lost at the end of the batch process can be replenished in the gas bottle assembly 12, for the next batch. For instance, the compressor 28 can recuperate this gas from the now more or less filled 'empty' gas bottle assembly 13.

[0065] Above, the switching valve 22 was mentioned several times. Preferably this is a directly controlled electro valve, more specifically a 3/3 way valve with closed mid- position. In the first position 'straight' the rod side of the piston accumulator is connected to the gas batteries, and depending on the pressure at the rod side, compressed gas will flow from the full bottles to the piston accumulator, or from the piston accumulator to the empty bottles (i.e. the normal process operation). In the second position 'middle' the piston accumulator is isolated from the compressed gas bottle assembly. In the third position 'crossed' the rod side can evacuate gas through control valve 21.

[0066] In the implementation shown in the figures, the switching valve 22 is controlled based on the position measurement of the piston. For example, at the beginning of the process the valve is in the third position so the air at the rod side can be vented when the piston is raised. Once the piston has reached the required position the valve switches to the first position for the normal process operation.

[0067] As mentioned before, the implementation as shown in figure 1 is only an example of an apparatus according to the invention. Possible alternatives for specific components or parts are described below.

[0068] Instead of a massive piston rod with a central bore hole, one could use a piston rod made from a hollow tube 200 in which a second tube 201 is mounted, that continues through the piston, see figure 3. The measuring device 9 to determine the position of the piston could possibly be mounted inside of the hollow piston rod, also see figure 3.

[0069] As the flexible tube 33 exerts a certain lateral force, because of its weight and stiffness, on the piston rod and thus on the seals of the piston, one or more separate exterior guides could be used to compensate for this lateral force (see figure 4). To completely avoid the use of the flexible connection 33, the implementation shown in figure 5 could be used, in which a fixed supply tube 53 is used. The piston 99 glides over the supply tube 53 on appropriate seals 54. The tube 53 is preferably placed concentrically relative to the piston and piston rod. The piston rod 200 is preferably hollow and moves up and down with the piston relative to the fixed supply tube 53.

[0070] In the embodiment shown in figure 6, the apparatus is equipped with a compressed gas bottle assembly 12 and a control valve 14, but no empty gas bottles 13. This setup can for example be used when the compressed gas bottle assembly 12 is at a pressure that corresponds to a specific operating pressure. In this case the compressed gas at the rod side can be pushed back into the compressed gas bottle assembly by the rising piston via the check valve 20 due to the pressure exerted by the feed pump 7.

[0071] The compressed gas bottle assembly 12 can also be replaced by a compressor 38, as shown in figure 7. In another embodiment (not shown), the counter pressure at the rod side is provided by an external hydraulic circuit instead of a pneumatic circuit (i.e. by building up a hydraulic pressure in the part of the cylinder at rod side). Specific examples of the latter embodiment will be given further in this description.

[0072] Instead of a pneumatic circuit 30, the counter pressure at the rod side can be provided by an adjustable spring or spring assembly 300, as illustrated in figure 8a/8b. In the embodiment shown in figure 8a, a spring is mounted that can be pre-tensioned to the desired force with a mechanical actuator in the form of a worm screw 301 . Alternatively, this mechanical worm screw can be replaced by a hydraulic or electrical actuator 302, see figure 8b. In both cases the spring tension is controlled through the actuator 301/302 based on the measurement of the pressure by sensor 19, to bring the pressure in the RO circuit to a certain level or maintain it there, analogously as described in the case of a pneumatic pressure control system 30 (see figure 1 ).

[0073] In the aforementioned embodiment, the spring can be replaced by a hydraulic actuator 299 with constant pressure or force control, as shown in Figure 8c. This is for example possible by making use of an (electronic) pressure control apparatus 303. Th e pressure control apparatus 303 may be an apparatus known in the art and available from known suppliers (e.g. the Sytronix® system from Rexroth®). The basic components of such a system are indicated in Figure 8c : hydraulic storage tank 304, pump 305 driven by a precise frequency controlled variable speed motor 306, the latter receiving an electrical input signal from a pressure sensor 307. The hydraulic liquid is supplied to one or the other side of the hydraulic actuator 299 or disconnected from said actuator via a three-way control valve 308. The lowest of the three positions of the valve 308 is applied during operation of the RO circuit, with a downward force applied on the piston 99. The upper position reverses the hydraulic flow and serves to pu ll the piston u pwards at the end of a batch process . The m idd le position disconnects the RO and hydraulic circuits. A safety valve 309 and filter 310 are further present, as well as a pressure vessel 31 1.

[0074] Figure 8d shows a practical arrangement wherein the actuator 299 is mounted on a support structure 500, to which also the cylinder 100 is fixed. The actuator 299 is configured to put pressure on the piston 99. A guide rod 501 is further mounted on said support structure 500 and held in a fixed position. The sensor 9 moves along this ruler, thereby measuring the piston's position. Figure 8e shows a compact serial arrangement, wherein the hydraulic actuator 299 is mounted on top of the piston accumulator 1 , with the bottom portion 51 1 of the actuator forming also the top portion of the cylinder 100 of the piston accumulator, i.e. the cylinder 100 of the piston accumulator shares its top portion 51 1 with the actuator 299 . The piston 510 of the actuator is connected directly to the accumulator's piston rod 200. The piston rod 200 moves through the bottom portion 51 1 of the actuator, for which adequate packings 512 are required, in order to avoid leaking of the actuator's hydraulic liquid into the RO circuit. The hydraulic actuator 299 could also be positioned underneath the accumulator 1 , which is more advantageous in that leakage of the pressure liquid cannot lead to contamination of the RO circuit. In the latter case, the cylinder 100 shares its bottom portion with the actuator 299. Figure 8f shows a compact parallel arrangement with the actuator 299 mounted adjacent the accumulator 1 , with a bracket 515 connecting the actuator's piston 510 with the piston 200 of the accumulator. This version also doesn't suffer from possible contamination of the RO circuit by the actuator's pressure liquid.

[0075] In some implementations multiple RO membranes are used, in serial or parallel arrangement. More than a single circulation pump can be used, and/or more than one feed pump. Figure 9 shows an embodiment with 2 membranes in a parallel arrangement, each one equipped with its own circulation pump 5. The means for delivering a counter pressure at the rod side is not shown in detail in this drawing but can be brought into practice according to any one of the embodiments described in the present description.

[0076] As the small movements of the piston 99, caused by the pulsations of the volumetric feed pump 7, can cause premature wear and tear on the seals, the feed pump 7 can be replaced by a supply through a pressure vessel 50 which will feed the base liquid to the system by gas pressure, as a function of the position of piston 99 in the piston accumulator 1 , as shown in figure 10. The gas delivering the pressure, for example air, nitrogen or C0 2 , comes from a pressure bottle 51 connected to the pressure vessel 50 with a regulating valve 56 that is controlled based on the measurement of the position of the piston by position sensor 9. In this embodiment the pressure vessel 50 can possibly be replaced by a hydraulic cylinder.

[0077] According to another implementation shown in figure 1 1 , one or more tanks 45 with fixed volume can be added to the RO circuit in order to adapt the batch size. Using valves 57 these containers can be included in or excluded from the circuit. Possibly multiple tanks can be mounted parallel or in series to be able to adapt the batch size flexibly through activating or deactivating a certain number of these tanks.

[0078] According to an embodiment, the feed pump control can be made dependent of the measured flow rate of the permeate, for example using accurate flow meters, or by weighing the amount of permeate 3 that is produced on a weighing means 47, see figure 1 1.

[0079] In another implementation, also shown in figure 1 1 , use is made of a measurement signal produced by weighing of the permeate, via weighing means 47. The same signal, combined with the weighing of the produced retentate by the weighing means 48 and the desired concentration factor, allows the building of a control circuit that enables the continuous draining of the concentrate via flow rate control 27. After all, once the desired concentration factor is reached, which happens when the amount of permeate produced corresponds to the amount of retentate present in the RO circuit, multiplied by the desired concentration factor minus one, a continuous flow of retentate can be drained equal to the feed flow rate of the feed pump minus the extra retentate produced.

[0080] Figure 1 1 shows a combination of a number of implementations that can be used separately. In particular: the version with additional tanks 45 may be used without the weighing means 47 and 48. The version with weighing means 47 for the permeate can be used without the additional tanks 45 and the weighing means 48, but with the necessary control systems for the feed pump 7 based on the weighing with means 47 of the permeate tank. The version with both weighing means 47/48 can be used without the additional tanks 45.

[0081] Figure 12 shows an embodiment of the apparatus in figure 1 with an additional controlled valve 62 with flow control 61. This version can be combined with every other implementation described above, and is important for example when high concentration factors are desired when treating certain liquids. With this implementation the retentate within the piston accumulator can be partly or completely fed back to the supply tank 8 through flow control 61 by opening the controlled valve 62. This is especially important when the productivity of the apparatus has lowered due to a lower flow rate of permeate, because of the concentration factor already attained within the RO circuit. By mixing the retentate with the base liquid, the piston accumulator can be refilled with a liquid that has a lower deg ree of concentration than before. Because of the lower degree of concentration within the piston accumulator and thus within the whole RO circuit the TDS value (Total Dissolved Solids) of the liquid will go down, and therefore also the osmotic pressure will decrease accordingly, which will cause the permeate flow rate to rise again. Possibly the control of valve 62 can be automated with the aid of a TDS measurement.

[0082] The apparatus can be easily equipped with CIP ('Cleaning In Place') (not shown in the figures). One could even choose to clean one membrane while producing on another that is mounted serially or in parallel. Also, the system has no components that would hinder a thorough cleaning. [0083] In an apparatus according to the invention, the piston 99 can be set at any position within the cylinder while filling the RO circuit at the start of the batch process, whereby the free cylinder volume at piston side can be adjusted with a control range from 0 to 100% of the maximum content. This allows to set the desired content of the closed retentate circuit 6 and therefore the final batch size of the retentate in a wide range, from the minimum value, the total capacity of the pipes of the circuit, to the maximum value, the capacity of the pipes plus the maximum capacity of the extended cylinder. In case a volumetric feed pump is used that does not provide a continuous flow rate, like a plunger pump, the piston accumulator 1 will also partly function as a damper for the occurring pulsations.

[0084] The components of an apparatus according to the invention, specifically the piston accumulator and the membranes, can be mounted vertically as well as horizontally. When multiple membranes are used, these can be arranged serially as well as parallel.

[0085] When in the text above 'control equipment' is mentioned, this can refer to any suitable equipment known to the skilled person for regulating and directing a variable in a process, for example a PI or PID controller.

[0086] An apparatus and method according to the invention can be implemented as an industrial installation for the treatment of large quantities of liquid. To obtain a retentate based on beer, one could for instance treat batches of 1000 or 2000 litres with concentration factors of 5 to 12 and higher, at operating pressures of 20 to 80 bar or higher. Many liquids are suited for treatment with this apparatus and method according to the invention, for instance liquid foods like beer, wine, milk, fruit and vegetable juices, but also chemical liquids like solvents, residual products from the pharmaceutical or cosmetic industry, waste water, in short: all liquids from which for some reason or other water or another liquid with low molecular weight needs to be removed.

[0087] The supply tank 8, retentate tank 1 1 and permeate tank 10 may or may not form part of the machine itself. The same goes for the flow rate controller 27 and gas bottle batteries 12 and 13. These components can be part of an apparatus according to the invention, or the apparatus can be connectable to these components.

[0088] The invention also relates to devices on a smaller scale, for example for use in bakeries or other food processing companies, as well as appliances intended for domestic use. Regardless of the scale, these devices contain the components which are characteristic of the invention, in particular the piston accumulator. Depending on the scale and application, one or more of the variations of specific components described above can be better suited. For example, for devices on a small scale the implementations with spring or spring assembly and hydraulic cylinder or mechanical jack as shown in figures 8a/b or the solutions with a hydraulic power source without a spring as shown in figures 8c to 8f will be better suited than the implementation with the pneumatic pressure circuit 30.

[0089] A specific compact implementation is the small batch application (1 to 10 litres) illustrated in figures 13a and 13b. The reverse osmosis membrane 2, the flexible connection 33 to the piston accumulator 1 and the circulation pump 5 are clearly shown. Also shown are the drain for the permeate 3 and the supply tank 8. The piston accumulator 1 is built according to a specific embodiment shown in more detail in figure 13b. The cylinder 100 and the piston 99 are indicated. The liquid flows through a hollow tube 400 which is placed eccentrically relative to the piston 99 and the cylinder 100 and is connected to an eccentric channel 401 in the piston. In this case the piston comprises two halves and between these there is a ring- shaped chamber 331 and several (radially) drilled conduits 332 through which the liquid enters the piston chamber. Because of this assembly the cleaning liquid of the CIP can reach all the difficultly accessible cavities of the piston for a thorough cleaning. This implementation of the piston can also be used in a larger apparatus such as is shown in figure 1 . The counter pressure at rod side is provided by a spring 300, which is adjusted by a worm screw 301 (or alternatively a hydraulic cylinder) driven by an electromotor 310 and controlled by pressure sensor 19. The position of the piston is measured by position meter 9. The piston accumulator of figure 13b illustrates that a piston 'rod' in the literal sense is not always required . In this embodiment, the throughput channel is formed by an eccentrically placed inlet channel 401 combined with the radial conduits 332, hence there is no piston rod with a channel inside the rod. The embodiments of Figure 2 and figure 13b are however fully equivalent in terms of their function as a 'piston accumulator' within an apparatus and method according to the invention.

[0090] The embodiment of Figure 13a does not comprise a separate feed pump.

When the piston of the piston accumulator rises, driven by the worm screw 301 and electromotor 310, fresh liquid can be drawn from the supply tank 8 through the check valve 17. The size of a batch is then determined by the volume of the cylinder with the piston in the highest position. Once the cylinder is full, the process starts as described above. During the process the piston position lowers as permeate is drained. The spring force is adjusted based on the pressure sensor 19 to maintain a constant pressure in the circuit, but not with the piston in a fixed position. In this embodiment, the force of the spring will push the piston down as more liquid is removed from the circuit. The measurement of the piston position in this embodiment is not meant as input for the controller of the feed pump, but rather to detect when the piston has reached its lowest position, after which the cylinder can be refilled by raising the piston to its highest position. Actually, position sensor 9 is optional in this embodiment : it can be replaced by another way to detect the piston has reached its lowest position, or one could purely visually monitor this and at that time shut down the machine. Possibly (but not necessarily) a control valve 62 and a flow rate controller 61 can be present between the RO circuit and the supply tank 8. The retentate can be sent back to the supply tank via valve 62 and the flow rate controller 61. The assembly is controlled by a small pre-programmed PLC.

[0091] Also there is the possibility to include a compact steam generator (similar as used in cappuccino machines) for sterilization purposes. The whole can be built into a compact appliance with a small display and some control buttons for the automated operation. The appliance would be the size of a large microwave or food processor. [0092] This compact implementation for small batch applications can also be equipped with a hydraulic power source : a hydraulic actuator with constant (electronic) pressure control as described above with reference to figures 8c-8f.

[0093] Figure 14 shows an embodiment wherein a hydraulic pressure liquid is introduced directly on the rod side of the cylinder 100 and acts directly on the piston 99 itself. In other words, the cylinder of the piston accumulator 1 serves also as a hydraulic actuator with hydraulic liquid being introduced only on one side of the piston 99, via the inlet 600. This embodiment requires the use of food grade hydraulic pressure liquid (e.g. food grade oil), given that a 100% separation between the RO circuit and the hydraulic liquid is not possible. In the hydraulic circuit 303' that can be used with this embodiment, the building blocks (tank 304, pump 305 etc) are the same as in the embodiment of figure 8c, except for the control valve 312 which has three positions : decoupled, forward flow (for applying pressure on top of the piston

99 during the RO process) and reverse flow. The latter position is applied at the end of the RO process, when the piston has moved to its lowest position. By re-introducing liquid into the RO circuit via a feed pump (for example pump 7 in the circuit of Figure 1 ), the hydraulic liquid is pushed back into the tank 304. This embodiment is suitable for making a very compact batch- operating apparatus, for the treatment of a batch of liquid equal to the contents of the cylinder

100 when the piston is in its highest position. The piston is then not kept at a constant position but moves gradually downward as the RO process proceeds. A measurement of the piston position is then not required, except optionally for detecting when the piston reaches its lowest position. The embodiment of figure 14 is however also suitable for treatment of larger batches in the manner of the circuit of Figure 1 (in which case a position sensor 9 is required).

[0094] In any of the embodiments described above, the pressure measurement by the sensor 19 is preferable but not strictly necessary for operation of the apparatus. The force (e.g. gas pressure, spring force, hydraulic pressure) applied to the rod side of the piston accumulator 1 is predefined in order to keep the RO circuit at a predefined pressure during the RO process (not necessarily a pressure that remains constant throughout the process, but in any case a pressure that is controlled throughout said process). The maintaining of the 'force' on the rod side is preferably done by controlling this force on the basis of a measurement of the pressure in the RO circuit (such as the pressure measurement with sensor 19), but may also be done on the basis of the measurement of the 'force' itself, or a parameter related to said force, such as the hydraulic pressure in the embodiment of Figures 8c to 8g, or the spring force in the embodiment of Figures 8a and 8b (in the latter case for example by using a load sensor or measuring the spring elongation).

[0095] In any of the embodiments described above, the direction of the liquid flow in the filtration circuit 6 can be as shown in the drawings, or it can be the reverse. The flow through the piston accumulator is then also reversed, i.e. for example in the case of Figures 1 and 2 : liquid arrives at the front side 104 of the piston accumulator and flows through the throughput channel 105. The operation and function of the means for applying a counter pressure at the piston accumulator's back side 103 remains unchanged. [0096] All embodiments mentioned above are applicable to other cross-flow filtration technologies besides Reverse Osmosis, like for instance nano-filtration or ultrafiltration, techniques that also separate a liquid into a permeate and a retentate.

[0097] All the embodiments of an apparatus of the invention described above are applicable also in the equivalent embodiments of the method of the invention, as described in its most general form in the appended independent method claim.

[0098] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.