Flow distributor for batch or half-batch ultrafiltration system

Abstract

A filtration system having: (a) a recirculation loop; (b) a pump; (c) a waste outlet; (d) a valve separating the circuit from the waste outlet; (d) a control device for the valve; (e) a membrane pressure vessel comprising a feed port, a concentrate port and a permeate port, and an ultrafiltration element in an axial direction comprising an end ultrafiltration element closest to the inlet port or the outlet port; (f) a flow distributor between the end ultrafiltration element and the fluid port, the flow distributor providing a velocity profile with a coefficient of variation less than 0.1, or a flow resistance between 10 and 200 kPasecm-1; (g) wherein the flow distributor has a proximal surface closest to the end ultrafiltration element and a distal surface closest to the fluid port; and (h) an axial positioner that positions the distal surface at least 50% of the distance between the end ultrafiltration element and the port.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority under 35 U.S.C. §365(c) to U.S. Provisional Application No. 63 / 351,594, filed on June 12, 2022, and U.S. Provisional Application No. 63 / 535,178, filed on June 17, 2022, each of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a flow distributor and its use for improving the efficiency of a batch or semi-batch ultrafiltration (reverse osmosis (RO) or nanofiltration (NF)) system. Background Art

[0004] Several patents, patent applications and publications are cited in this specification to more fully describe the state of the art to which the present invention relates. The entire disclosure of each of these patents, patent applications and publications is incorporated herein by reference.

[0005] For batch or semi-batch reverse osmosis systems (such as closed circuit reverse osmosis (CCRO)), the concentrate volume at the end of the permeation cycle is fixed by the total water hold-up volume in the feed-concentrate channels of the membrane elements, the pressure vessels in the array, and the pipelines. In combination with a larger starting volume, the permeation cycle lasts longer before the brine is replaced, which is beneficial for operating at a lower water recovery rate. In order to achieve a larger hold-up volume and longer cycle time, additional space is usually reserved in the pressure vessel in addition to the ultrafiltration elements necessary to produce the desired permeate volume. Ultrafiltration elements ("membrane elements") are filter cartridges containing reverse osmosis (RO) or nanofiltration (NF) membranes and spacers and typically have a central permeate tube, see, for example, U.S. Patent No. 10,717,050.

[0006] However, poor brine removal during the flushing step of a batch or semi-batch reverse osmosis system may cause higher salt concentrations in the system and, therefore, may lead to a higher potential for mineral scaling at the end of the filtration period. See, for example, M. Li, Desalination, 496 (2020), 114646. Higher salt concentrations within the system after the flushing step may also lead to higher operating pressures and increased concentrations of undesirable solutes in the permeate. Improved brine flushing in the system also provides incremental improvements in achievable volumetric recovery and reduced energy consumption.

[0007] Clearly, there remains a need for batch and semi-batch reverse osmosis systems with more efficient hydraulics during the flush cycle. Summary of the invention

[0008] Therefore, the present invention provides a filtration system for batch or semi-batch ultrafiltration, the filtration system comprising:

[0009] (a) Recirculation loop,

[0010] (b) a pump providing pressurized feed solution to a recirculation loop,

[0011] (c) waste outlet ducts,

[0012] (d) a valve separating the recirculation loop from a waste outlet conduit, the waste outlet conduit being adapted to provide a discontinuous flow of concentrated fluid from the recirculation loop,

[0013] (d) a control device adapted to actuate the valve,

[0014] (e) a membrane pressure vessel in the recirculation loop, the membrane pressure vessel comprising a feed port, a concentrate port and a permeate port, and containing one or more, preferably two or more, ultrafiltration elements aligned in an axial direction, the ultrafiltration elements comprising an end ultrafiltration element positioned closest to a fluid port, wherein the fluid port is selected from one of the inlet port and the outlet port, and the end ultrafiltration element has a length in the axial direction, and a first distance between the fluid port and the end ultrafiltration element is at least one third of the length,

[0015] (f) a flow distributor located between the end ultrafiltration element and the fluid port; preferably, the flow distributor is adapted to provide: (i) a velocity distribution over a cross section of the pressure vessel, wherein the velocity distribution has a coefficient of variation of less than 0.1, or (ii) a velocity distribution between 10 and 200 kPa·sec·m -1 The flow resistance between , where the flow resistance (R) is defined as:

[0016]

[0017] Where ΔP is the pressure drop across the flow distributor, v is the average velocity, and R is measured using pure water at 25°C and an average velocity of 5 cm / sec,

[0018] (g) wherein the flow distributor comprises a proximal surface closest to the end ultrafiltration element and a distal surface closest to the fluid port, and

[0019] (h) an axial locator within the container that positions the distal surface of the flow distributor a second distance from the end ultrafiltration element, wherein the second distance is at least 50% of the first distance.

[0020] The advantages and features of novelty which characterize the invention are particularly pointed out in the appended claims and form a part thereof. However, for a better understanding of the invention, its advantages and objects attained by its use, reference should be made to the accompanying drawings which form another part hereof and to the accompanying descriptive matter, in which one or more preferred embodiments of the invention are shown and described. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic diagram of an embodiment of a CCRO filtration system described herein.

[0022] Figure 2 is an isometric view of the flow distributor assembly.

[0023] Figure 3 is a side view of the flow distributor assembly.

[0024] Figure 4 is a side cross-sectional view of a membrane pressure vessel.

[0025] Figure 5 is a cross-sectional view of a portion of a membrane pressure vessel.

[0026] Figure 6 An exploded cross-sectional view of a portion of a membrane pressure vessel.

[0027] Figure 7 is a plan view of the surface of a flow distributor using slots.

[0028] Figure 8 is a plan view of the flow distributor surface using a circular orifice.

[0029] Fig. 9 It is a perspective view of a flow distributor using parallel tubes. DETAILED DESCRIPTION

[0030] The configuration of batch or semi-batch reverse osmosis (RO) systems is well known, and these systems have been extensively studied and found to be more efficient than continuous RO systems. For example, see JR Werber et al., Desalination, 402 (2017) 109-122. However, incomplete brine replacement may weaken some of these advantages. We have found that brine replacement can be improved (especially when there is additional space in the pressure vessel) by using a flow distributor upstream of the empty space in the pressure vessel to evenly distribute the flow over the cross-sectional area of ​​the pressure vessel, which is perpendicular to the axis defined by the fluid flow through the pressure vessel. Ideally, the coefficient of variation of the velocity distribution downstream of the flow distributor, preferably immediately downstream, will be less than 0.1, more preferably less than 0.05, less than 0.03 or less than 0.01, where the coefficient of variation (CV) is a statistical tool defined as the ratio of the standard deviation (σ) to the mean (μ), or:

[0031]

[0032] Therefore, the filtration system preferably includes a flow distributor assembly that occupies the space between the end ultrafiltration element and the end of the pressure vessel. Preferably, the filtration system includes a flow distributor located between the end ultrafiltration element and the fluid port, the flow distributor providing a flow rate between 10 and 200 kPa·sec·m -1 The flow resistance between , where the flow resistance (R) is defined as:

[0033]

[0034] Or pressure drop (ΔP) divided by average velocity (v), where R is calculated based on ΔP using pure water at 25°C and an average velocity of 5 cm / sec. For this purpose, the average velocity can be calculated by dividing the volumetric feed flow rate into the vessel by the vessel's internal diameter.

[0035] Preferably, the flow distributor is a circular flow plate having a plurality of openings. The flow distributor assembly, the flow distributor and the flow plate are described in further detail below.

[0036] The flow distributor assembly is intended to occupy the space between the first membrane element or the last membrane element in the pressure vessel and the proximal or distal end of the pressure vessel. Therefore, the surface of the flow distributor is substantially consistent with the cross-section of the shell of the pressure vessel. Preferably, the flow distributor assembly is cylindrical, and the maximum outer diameter of the assembly is equal to or slightly less than the inner diameter of the pressure vessel. The pressure vessel assembly does not need to have a complete cylindrical surface, that is, the diameter can be equal to the maximum diameter only over a small part of the length of the assembly. Preferably, the flow distributor assembly has two or more radial locators, whose diameter is equal to or slightly less than the inner diameter of the pressure vessel, and the assembly is otherwise open at its maximum diameter, or the assembly has a cylindrical outer surface (with or without an opening) that acts as a radial locator. The radial locator is connected to the axial locator, which defines the axial position of the flow distributor in the container. Preferably, the radial locator is connected to the axial locator by a spoke, preferably with at least two spokes, preferably at least three, preferably at least four spokes. The maximum number of spokes of the radial locator and the thickness of the outer ring and the inner ring are limited only by the pressure drop caused by the combination of the radial locator and the flow distributor. A designer of ordinary technology will be able to balance the need for structural stability with the need to achieve a target pressure drop.

[0037] Preferably, the length of the flow distributor assembly is 10 to 350cm, more preferably 50 to 250cm, and more preferably 80 to 120cm. When the assembly is intended to fill a gap equal to the length of two or more membrane elements (generated by omitting more than one membrane element from the membrane pressure vessel), preferably, the length of the assembly is 100 to 300cm. Alternatively, an additional spacer without a flow distributor is added to fill the remaining space in the membrane pressure vessel. When it is intended to fill a gap in which only one membrane element is omitted, the length of the flow distributor assembly is 10 to 250cm, more preferably 50 to 200cm, and more preferably 70 to 150cm. Preferably, the flow distributor assembly includes a locking device to fix the pressure vessel flow distributor to another pressure vessel flow distributor, a pressure vessel end cap adapter, or a spiral wound membrane. Similar locking structures for connecting two adjacent membrane elements have been described (e.g., see U.S. Patent No. 6,632,356B2).

[0038] Preferably, the flow distributor assembly comprises a circular flow plate having at least one opening. Preferably, the flow plate has a diameter of 10 to 30 cm, preferably 15 to 25 cm. The exact diameter is not critical to the proper operation of the flow plate, the flow plate will be sized to fit tightly within the wall of a particular pressure vessel. In a preferred embodiment of the invention, the flow plate has a central opening to accommodate a permeate tube or an axially spaced tube or rod; preferably the central opening has a diameter of 1 to 10 cm, preferably 3 to 9 cm, preferably 4 to 8 cm. In this embodiment, the remaining area of ​​the flow plate beyond the central opening is used to modify the flow, and there are additional openings in the remaining area, preferably at least 30 openings, more preferably at least 60 openings, even more preferably at least 100 openings, even more preferably at least 130 openings. When the flow distributor comprises a fabric, a net or a screen, the number of openings may be 1000 / in 2 or more. The holes or orifices produce a pressure drop at any given flow rate and can be directly traced back to the resistance to flow. Preferably, the diameter of the flow plate is equal to or slightly less (within normal tolerances for circular cross-section objects with O-rings or other sealing devices) than the inner diameter of the pressure vessel. Preferably, the remaining area of ​​the flow plate, or the entire area when there is no central opening for the permeate tube, contains openings, preferably circular openings or slots, each slot preferably being part of an annular section of the flow plate. Preferably, the total area of ​​the openings is 1% to 80% or more preferably 1% to 50% of the total area of ​​the flow plate (or the remaining area outside the central opening if there is a permeate tube or axial spacer opening in the center), preferably at least 2%, preferably at least 3%; preferably not more than 30%, preferably not more than 20%, preferably not more than 10%. It is believed that the total area of ​​the openings is critical, not the number of openings or the size of any individual opening. Preferably, the hydraulic diameter of an individual opening (the area of ​​the opening divided by four times the circumference) is 0.5 to 10 mm, preferably at least 2 mm, preferably at least 2.5 mm; preferably no more than 7 mm, preferably no more than 5 mm. Preferably, the openings are circular, in which case the diameter is equal to the hydraulic diameter. Preferably, the openings are in a radially placed circular array. Preferably, the thickness of the flow plate is 2% to 25%, preferably 7% to 18% of its diameter. These thickness ranges do not include the thickness of optional support ribs that may be present on the flow plate.

[0039] Any material may be suitable for the flow plate, provided that the material is stable to long-term exposure to the feed solution and, further, does not mechanically deform due to the pressure differential within the pressure vessel. Suitable materials for the flow plate include, for example, polymeric materials such as polypropylene, polysulfone, polyethylene, acrylonitrile-butadiene-styrene (ABS), glass or carbon reinforced plastics, ceramics or corrosion resistant metals such as stainless steel, Hastelloy TMor titanium). Preferably, the flow plate is adjacent to the radial locator.

[0040] The flow resistance (R) of any flow plate having holes can be measured by one skilled in the art using water at 25°C and an average velocity of 5 cm / sec in a container. One skilled in the art can identify the number and shape / size of holes to produce the desired flow resistance (R). The design of multi-hole orifice plates is well documented in the literature. See, for example, S. Bukurov M. S. Comparison of single-hole and multi-hole orifice energy consumption. Advances in Mechanical Engineering. 2022; 14(1); and Tomaszewski A, Przybylinski T, Lackowski M. Experimental and Numerical Analysis of Multi-Hole Orifice Flow Meter: Investigation of the Relationship between Pressure Drop and Mass Flow Rate. Sensors (Basel). 2020 Dec 18; 20(24): 7281.

[0041] Preferably, the RO system of the present invention is operated at a temperature of 2°C to 45°C, preferably 4°C to 40°C, preferably 5°C to 35°C. Preferably, the density of the fluid passing through the flow distributor assembly is 0.98 to 1.5 g / cm 3 , preferably 0.99 to 1.2 g / cm 3 Preferably, over a typical average velocity range for the vessel, the pressure drop (ΔP) across the flow plate is 0.344 to 10.34 kPa, more preferably 0.344 to 6.89 kPa, more preferably 0.689 to 2.76 kPa, corresponding to a flow resistance as defined herein between 10 and 200 kPa·sec·m -1 between 10 and 150 kPa·sec·m -1 between 15 and 50 kPa·sec·m -1 between.

[0042] Preferably, in a flow distributor assembly comprising a flow plate and having an overall length, the flow plate is positioned at 0% to 40% of the overall length of the pressure vessel assembly; preferably at least 4%, preferably at least 7% of the overall length; preferably no more than 30%, preferably no more than 20%, preferably no more than 15% of the overall length, as measured from the proximal end of the pressure vessel assembly. When the flow distributor assembly is part of a filtration system, an axial locator within the membrane pressure vessel positions the distal surface of the flow distributor at least 50%, preferably at least 75%, preferably at least 80%, preferably at least 85% of the distance from the end ultrafiltration element to the fluid port (preferably, the fluid port into which the liquid to be filtered is introduced); preferably no more than 99%, preferably no more than 96%.

[0043] In alternative embodiments, the flow plate comprises, for example: a porous medium, such as a mesh; a nonwoven fabric; or a sintered medium, such as plastic, metal, glass or ceramic. The resistance of this flow plate can be measured as described above. Alternatively, the flow resistance (R) defined herein can be estimated by a person skilled in the art using Darcy's law and the Kozeny-Carman equation from the particle size or pore size.

[0044] In another alternative embodiment, the flow distributor comprises a cylinder having a flow plate at each end of the cylinder; in this embodiment, the body of the cylinder serves as an axial locator and a radial locator. Preferably, in this embodiment, each orifice in the proximal flow plate is attached to a corresponding distal orifice by a tube, thereby forming a corresponding number of tubes, preferably parallel tubes, distributed over the cross-sectional area of ​​the flow plate. The friction in each parallel tube provides a flow resistance (R) that can be measured. Alternatively, in the case of multiple parallel tubes, the flow resistance can be estimated using the Hagen-Poisielle equation to provide the required flow resistance. Preferably, the number of tubes can be between 50 and 250, more preferably between 100 and 200. Those skilled in the art can identify the number and shape / size of the tubes to produce the desired flow resistance (R).

[0045] In another preferred embodiment of the present invention, the flow distributor assembly is a cylinder having a flow plate as one end of the cylinder; in this embodiment, the body of the cylinder serves as an axial locator and a radial locator. The arcuate outer surface of the cylinder can be continuous or partially open, as long as there is enough outer surface to give structural stability. The end of the cylinder opposite to the flow plate is partially or completely open.

[0046] Reference is now made to the drawings, wherein like reference numerals designate corresponding structures throughout the several views, and specific reference is made to Figure 1, a semi-batch filtration system 100 including a flow distributor assembly 4 is shown. The feed solution 17 passes through a feed pump 18, combined with a recirculation stream 23 to form a blended inlet stream 19, which enters the void volume 8 of the membrane pressure vessel 1 through the inlet port 2. The inlet stream passes through a flow distributor 7, which is adjacent to a radial positioner 6, which is connected to a radial positioner 6' through an axial positioner 5. The inlet stream then enters the end membrane element 9, where a portion of the inlet stream passes through the membrane into the permeate tube 15, becoming a permeate stream 20 that exits through the permeate end port 10 and a concentrate stream 21 that exits through the outlet port 16. In the normal filtration operating mode, the concentrate stream passes through a recirculation pump 22 to become a recirculation stream 23. During the cleaning cycle, the concentrate stream passes through a concentrate valve 24 to become a cleaned concentrate 25.

[0047] Figure 1 A control device suitable for controlling valve 24 is not depicted. Any other valve that may be present in the semi-batch filtration system 100 may optionally be controlled by the same control device, or by one or more other control devices, which may be the same or different. A person skilled in the art is capable of selecting and installing appropriate (multiple) control devices, and is further capable of operating valve 24 and optionally one or more other valves that may be present in the semi-batch filtration system 100 via (multiple) control devices.

[0048] exist Figure 2 and Figure 3 In FIG. 4 , a flow distributor assembly 4 is shown, wherein a flow distributor 7 is adjacent to a radial locator 6, which is connected to an axial locator 5, which is then connected to a radial locator 6 '. As described in detail above, the flow distributor may be a flow plate or a porous sintered disk having openings.

[0049] like Figure 4As shown, the membrane pressure vessel 1 has side feed ports 2 and 2' through which the feed solution enters the void volume 8. Permeate ports 10 and 10', end cap adapters 27 and 27', and vessel end caps 3 and 3' are also shown. A flow distributor assembly 4 is also depicted here, which includes a flow distributor 7, a radial locator 6 connected to the axial locator 5, and a radial locator 6' also connected to the axial locator 5. The flow distributor 7 and the radial locator 6 together form the distal surface of the flow distributor assembly 4. Similarly, the radial locator 6' forms the proximal surface of the assembly 4. As used herein to describe the pressure vessel assembly, the term "distal" means "closest to the feed end", while the term "proximal" means "closest to the concentrate end". The flow distributor assembly 4 is connected to the end cap adapter 27 via a permeate interconnect 12. The end cap adapter 27 is connected to the permeate port 10. The flow distributor assembly 4 is also connected to the adjacent end membrane element 9 through a permeate interconnect 12', and the opposite end of the membrane element 9 is connected to another membrane element 9' through a second permeate interconnect 12", wherein the membrane element 9' is connected to the permeate port 10' through the permeate interconnect 12''. In some preferred embodiments, the membrane pressure vessel comprises more than two membrane elements, which are connected in series with additional permeate interconnects, substantially as follows: Figure 4 Depicted. Feed solution enters membrane element 9 via void space 8, with permeate entering central tube 15 and concentrate exiting through side outlet ports 16 and 16'.

[0050] Figure 5 and Figure 6 The positioning of the flow plate 7 with circular holes relative to the inlet ports 2 and 2' and the end cap 3 is shown; the rest is as described in detail above. Figure 7 A flow plate 7 is shown having slot openings 29 and a central opening 31 for incorporating a permeate tube 15 (shown in other figures). Figure 8 A flow plate 7 is shown having circular openings 30 and a central opening 31 . Fig. 9 An alternative embodiment of a flow distributor assembly 4 is shown, which includes an axial locator 5 connected to an upstream radial locator 6 and a downstream radial locator 6', wherein the axial locator is a hollow tube with a flow tube opening 11 at each end. In practice, the flow distributor assembly 4 preferably includes a plurality of parallel hollow tube axial locators 5, wherein the radial locators 6 and 6' have holes to allow flow through each axial locator 5. In another preferred embodiment, the hollow tube axial locators can be concentric, and more preferably concentric and substantially equidistantly spaced.

[0051] The following examples are provided to further describe the present invention in detail. This example, which sets forth specific embodiments and preferred modes presently considered for carrying out the present invention, is intended to illustrate, not to limit, the present invention.

[0052] Example 1

[0053] A step-by-step tracer study was conducted on a semi-batch CCRO system using a single pressure vessel 1 containing six membrane element positions to evaluate the efficiency of the flushing sequence using conductivity measurements. Five membrane elements (Y) were loaded into positions two to six in the vessel, leaving position one or the lead membrane element position empty to provide an additional system void volume of 8. When one system volume (31 gal, 117 L) of brine was displaced from the vessel, the salt retained in the system was estimated to be approximately 7% by conductivity. The data show that the residence time distribution is significantly broadened when compared to an ideal plug flow reactor due to a combination of non-ideal mixing, Taylor dispersion, and axial dispersion caused by the feed-concentrate spacers in the membrane elements. An additional 27% of flush water is required to remove 99% of the salt accumulated in the system.

[0054] To evaluate the improvement in residence time distribution of the present invention, a computational fluid dynamics (CFD) model was developed in ANSYS FLUENT software commercially available from ANSYS, Inc. of Canonsburg, Pennsylvania using the internal dimensions of a commercially available reverse osmosis pressure vessel, the fluid path dimensions of the feed-concentrate flow path of a commercially available 8" brackish water reverse osmosis membrane and known membrane pressure drop correlations, and typical flow rates in a single pressure vessel for displacement flushing of a semi-batch CCRO system. All modeling was performed using the physical properties of water at 25°C. CFD simulations were performed for the above-described tracer study with a spacer element downstream of the side port inlet of the pressure vessel to provide void volume, followed by five commercially available 400 square feet (37.2 square meters) membrane elements. To evaluate the effect of a flow distributor installed far upstream of the lead membrane element location (110 mm downstream of the centerline of the side port inlet connection), the model was modified to add a 7.95 inch (202 mm) diameter and 1 inch thick spacer element. A sintered porous distributor with a diameter of 25.4 mm and a pressure drop of 0.15 psi (1.034 kPa) was used for initial modeling. This is essentially equivalent to the pressure drop produced by using a multi-orifice plate with a diameter of 7.95 inches (202 mm), a thickness of 1 inch (25.4 mm), and a center hole (nominal diameter of 2.6 inches (66 mm)) to accommodate the axial locator, wherein the plate has 174 orifices (Y) evenly distributed on the distributor plate 7 outside the center hole, each orifice is 1 / 8 inch (3.17 mm) in diameter, and the flow resistance of the plate is 19.8 kPa·sec·m -1The estimated pressure vessel system volume was 33.2 gal (125.6 L) and the flow rate during the displacement step was 26.8-gpm (6.1 m 3 Table 1 summarizes the results of CFD modeling.

[0055] Table 1. Results of CFD modeling

[0056]

[0057] Due to the improved flow distribution into the void volume 8, the volume of water required to remove 99% of the accumulated salt during the CCRO displacement flush was reduced by 6.1%.

[0058] Although certain preferred embodiments of the present invention have been described and specifically illustrated above, the present invention is not intended to be limited to such embodiments. Rather, it should be understood that although many of the characteristics and advantages of the present invention have been set forth in the foregoing description, together with the details of the structure and function of the present invention, the present disclosure is illustrative only, and changes may be made in detail, especially in the shape, size and arrangement of parts, within the principles of the present invention to the full extent indicated by the broad general meaning of the terms expressing the appended claims.

Claims

1. A filtration system for batch or semi-batch ultrafiltration, the filtration system comprising: (a) Recirculation loop, (b) a pump providing pressurized feed solution to the recirculation loop, (c) waste outlet ducts, (d) a valve separating the recirculation loop from a waste outlet conduit, the waste outlet conduit being adapted to provide a discontinuous flow of concentrated fluid from the recirculation loop, (d) a control device adapted to actuate the valve, (e) a membrane pressure vessel in the recirculation loop, the membrane pressure vessel comprising a feed port, a concentrate port and a permeate port, and containing two or more ultrafiltration elements aligned in an axial direction, the ultrafiltration elements including an end ultrafiltration element positioned closest to the fluid port, wherein The fluid port is selected from one of the inlet port and the outlet port, and the end ultrafiltration element has a length in the axial direction, and a first distance between the fluid port and the end ultrafiltration element is at least one third of the length, (f) a flow distributor, the flow distributor being located between the end ultrafiltration element and the fluid port; (g) wherein the flow distributor comprises a proximal surface closest to the end ultrafiltration element and a distal surface closest to the fluid port, and (h) an axial locator within the container, the axial locator positioning the distal surface of the flow distributor a second distance from the end ultrafiltration element, wherein the second distance is at least 50% of the first distance.

2. The filtration system according to claim 1, wherein: The flow distributor is adapted to provide: (i) a velocity distribution across a cross section of the pressure vessel, wherein the velocity distribution has a coefficient of variation of less than 0.1, or (ii) a velocity distribution between 10 and 200 kPa·sec·m -1 The flow resistance between , where the flow resistance (R) is defined as: Where ΔP is the pressure drop across the flow distributor, v is the average velocity, and R is measured using pure water at 25° C. and an average velocity of 5 cm / sec.

3. The filtration system according to claim 1 or claim 2, wherein: The end ultrafiltration element has a length in the axial direction, and the second distance is greater than one third of the length.

4. The filtration system according to any one of claims 1, 2 or 3, wherein: The flow distributor comprises: (i) a flow plate having at least one opening, wherein the total area of ​​the at least one opening is 1% to 50% of the total area of ​​the flow plate; (ii) a sintered porous medium; (iii) a mesh, screen or fabric; or (iv) a set of parallel or concentric tubes.

5. A filtration system as claimed in any preceding claim, wherein: The flow plate has a central opening for receiving a permeate tube, and the remaining area of ​​the flow plate has at least one additional opening, wherein the total area of ​​the at least one additional opening is 2% to 10% of the remaining area of ​​the flow plate.

6. A flow distributor assembly for use in a pressure vessel of a batch or semi-batch ultrafiltration system, the flow distributor assembly comprising: (a) a flow distributor, said flow distributor preferably having an outer circular surface, (b) an axial positioner, and (c) Radial positioner.

7. The flow distributor assembly of claim 6, wherein: The flow distributor is adapted to provide: (i) a velocity distribution across a cross section of the pressure vessel, wherein the velocity distribution has a coefficient of variation of less than 0.1, or (ii) a velocity distribution between 10 and 200 kPa·sec·m -1 The resistance to flow between , where resistance (R) is defined as: Where ΔP is the pressure drop across the flow distributor, v is the average velocity, and R is measured using pure water at 25° C. and an average velocity of 5 cm / sec.

8. The flow distributor assembly according to claim 6 or claim 7, wherein: The flow distributor comprises: (i) a flow plate having at least one opening, wherein the total area of ​​the at least one opening is 1% to 50% of the total area of ​​the flow plate; (ii) a porous plate; (iii) a mesh, screen or fabric; or (iv) a set of parallel or concentric tubes.

9. The flow distributor assembly according to any one of claims 6, 7 or 8, wherein: The flow distributor comprises a flow plate having a central opening for receiving a permeate tube, and a remaining area of ​​the flow plate having at least one additional opening, wherein a total area of ​​the at least one additional opening is 2% to 10% of the remaining area of ​​the flow plate.

10. A flow distributor assembly for insertion into a pressure vessel, the pressure vessel being used in a batch or semi-batch reverse osmosis or nanofiltration system, the flow distributor assembly comprising: (a) flow distributor, (b) an axial positioner, and (c) Radial positioner.

11. The flow distributor assembly of claim 10, wherein: The flow distributor comprises: (i) a flow plate having at least one opening, wherein the total area of ​​the at least one opening is 1% to 50% of the total area of ​​the flow plate; (ii) a porous plate; (iii) a mesh, screen or fabric; or (iv) a set of parallel or concentric tubes.

12. A flow distributor assembly as claimed in claim 10 or claim 11, wherein: The flow distributor comprises a flow plate having at least one opening, wherein a total area of ​​the at least one opening is 1% to 50% of a total area of ​​the flow plate.

13. The flow distributor assembly of any one of claims 10, 11 or 12, wherein: The flow plate has a central opening for receiving a permeate tube, and the remaining area of ​​the flow plate has at least one additional opening, wherein the total area of ​​the at least one additional opening is 2% to 10% of the remaining area of ​​the flow plate.

14. The flow distributor assembly according to any one of claims 10 to 13, wherein: The axial distributor comprises a plurality of hollow tubes which can have different axes parallel to the axial positioner or can be concentric.

15. The flow distributor assembly according to any one of claims 10 to 14, wherein: The flow resistance of the flow distributor is between 10 and 200 kPa·sec·m -1 between; the flow resistance (R) is defined as: Where ΔP is the pressure drop across the flow distributor, v is the average velocity, and R is measured when using pure water at 25°C and an average linear velocity of 5 cm / sec.

16. A pressure vessel assembly comprising the flow distributor assembly according to any one of claims 6 to 15.

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