CROSSFLOW FILTRATION FOR CONTINUOUS DIFILTRATION

Description

[0001] The present invention relates to a crossflow filtration unit for the continuous diafiltration of a food liquid to obtain a retentate and a permeate, a corresponding method for diafiltration and the use of the crossflow filtration unit.

[0002] In crossflow filtration, which can also be referred to as transverse or tangential flow filtration, a feed liquid to be filtered flows tangentially over the surface of a filter material, which is usually a membrane, and is thereby split into a retentate (concentrate) and a permeate (filtrate) of different compositions.

[0003] The retentate flows over the surface of the filter material and can be extracted after a single pass ("single-pass" operation), or it can be recirculated, repeatedly flowing over the membrane surface. The permeate flows through the membrane perpendicular to the surface and is then removed. Target substances to be recovered can be contained in the permeate (permeate substances) and / or in the retentate (retentate substances).

[0004] Crossflow filtration units are frequently used in the form of filter cartridges, as described, for example, in DE-PS 34 41 249. Filter cartridges comprise several adjacent crossflow filtration units (filter cells), which generally consist of repeated arrangements of a retentate gap for the feed liquid or retentate to be filtered, a sheet membrane layer, and a permeate collection gap. The permeate collection gap of the preceding filter cell is separated from the retentate gap of the next filter cell by another sheet membrane layer. Each retentate gap is fluid-conducting (communicating) to an inlet for the feed liquid to be filtered and to an outlet for the retentate, and each permeate collection gap is fluid-conducting to an outlet for the permeate. US2005 / 285100 deals with diafiltration and mentions proteins.The structure of a diafiltration unit, with a diafiltration gap, a feed gap and a permeate gap, is shown.

[0005] Conventional filtration separates a food liquid into a retentate and a permeate. Diafiltration combines this separation with the addition of a diafiltration medium to the food liquid / retentate. This makes it possible, for example, to remove the other substances from a solution containing the target substance(s) and one or more other substances. Diafiltration of a protein solution, for instance, can be used to achieve buffer exchange or desalting, possibly combined with concentration.

[0006] A distinction is made between two basic types of diafiltration: variable-volume diafiltration (occasionally referred to as "discontinuous" diafiltration in the prior art) and constant-volume diafiltration (occasionally referred to as "continuous" diafiltration in the prior art).

[0007] In variable-volume diafiltration, filtration steps alternate with steps of adding a diafiltration medium. Consequently, the volume of the retentate fluctuates during this process variant, hence the term "discontinuous diafiltration." In constant-volume diafiltration, the volume of the retentate is kept constant by continuously performing both the filtration and the addition of the diafiltration medium. However, the addition of the food liquid occurs discontinuously and usually only at the beginning of the process. Therefore, both variable-volume and constant-volume diafiltration are discontinuous processes. In a discontinuous diafiltration process, a defined volume of food liquid must be completely processed in one cycle before a new cycle can begin ("batch" process).For reasons of economy and efficiency, continuous processes, in which material flows can be continuously added and removed, are preferable to batch processes. Therefore, there is a need for a continuous diafiltration process.

[0008] The present invention is therefore based on the objective of providing a crossflow filtration unit suitable for continuous diafiltration.

[0009] This problem is solved by the embodiments characterized in the claims.

[0010] In particular, the present invention relates to a crossflow filtration unit for the continuous diafiltration of a feed liquid to obtain a retentate and a permeate, comprising at least a diafiltration gap, a planar first filter material, a retentate gap, a planar second filter material, and a permeate collection gap, arranged such that the planar first filter material separates the diafiltration gap and the retentate gap from each other, and the planar second filter material separates the retentate gap and the permeate collection gap from each other, wherein the diafiltration gap is fluid-conductingly connected to at least one inlet for the diafiltration medium, the retentate gap is fluid-conductingly connected to at least one inlet for the feed liquid and to at least one outlet for the retentate, and the permeate collection gap is fluid-conductingly connected to at least one outlet for the permeate.and wherein the pore size or molecular weight exclusion limit of the planar first filter material is at least as large as the pore size or molecular weight exclusion limit of the planar second filter material. Preferably, the pore size or molecular weight exclusion limit of the planar first filter material is larger than the pore size or molecular weight exclusion limit of the planar second filter material.

[0011] Preferably, the first flat filter material has a molecular weight cut-off (MWCO) in the range of 30 kDa to 1500 kDa. The second flat filter material preferably has a molecular weight cut-off (MWCO) in the range of 5 kDa to 1500 kDa. The molecular weight cut-off can be determined according to US standard ASTM E1343-90 ("Standard test method for molecular weight cutoff evaluation of flat sheet ultrafiltration membranes").

[0012] The first, sheet-like filter material preferably has a pore size of 0.01 to 50 µm, more preferably 0.01 to 0.5 µm. The second, sheet-like filter material preferably has a pore size of less than 0.01 µm. The determination of the pore size, also referred to as "maximum pore size" or "dP", can be carried out according to US standard ASTM F316-03 TEST METHOD A ("Standard test methods for pore size characteristics of membrane filters by bubble point and mean flow pore test").

[0013] Basic procedures for characterizing membranes are described in the dissertation by Melanie Sossna "Structure formation of cellulose ester membranes", University of Hanover 2006, section 2.5 on pages 10f, in particular in Table 2-4.

[0014] According to the prior art (discontinuous diafiltration), the diafiltration medium is introduced between crossflow filtration units connected in series, or the feed liquid is diluted with diafiltration medium, combined with upstream or downstream concentration by crossflow filtration. However, the crossflow filtration unit according to the invention makes it possible to introduce a specific quantity of diafiltration medium into the retentate gap for optimal diafiltration by means of suitable pressurization. For example, according to the invention, the volume of the diafiltration medium can be 0.1 to 15 times the volume of the feed liquid. Preferably, the diafiltration medium is introduced into the retentate gap such that the entire available surface area of ​​the planar first filter material is covered with the diafiltration medium.The crossflow filtration unit according to the invention achieves an improved diafiltration result compared to discontinuous diafiltration. Furthermore, the invention leads to a reduction in process times and equipment requirements.

[0015] In continuous diafiltration, both feed liquid and diafiltration medium are added continuously, so that the process does not need to be interrupted. This allows the crossflow filtration unit according to the invention to operate efficiently and economically.

[0016] With the improved crossflow filtration unit according to the invention, fluids such as liquids, emulsions, suspensions, beverages like beer, wine, juice, water, milk and whey, wort, process and wastewater, and solutions in the pharmaceutical, medical, cosmetic, chemical, biotechnology, genetic engineering, environmental protection, and laboratory sectors can be used as edible liquids and diafiltered. They can be used for the recovery of valuable materials, the separation of substances such as macromolecules and biomolecules, the depyrogenation and sterilization of solutions, the removal of pollutants from fluids, the (dia)filtration and concentration of biological solutions, the separation of microorganisms such as bacteria, yeasts, viruses, and cell components, and the desalination of protein solutions and other biological media.

[0017] The crossflow filtration unit according to the invention can be used particularly advantageously for filtration, diafiltration, concentration (reduction of solvent or water content), and / or modification of the ion composition (e.g. desalting or buffer exchange) of a solution, preferably a protein solution.

[0018] The terms food liquid and retentate can be used synonymously for the liquid located in the retentate space.

[0019] The term "planar" indicates that the respective filter material lies essentially in a single plane. Preferably, all filter materials lie essentially in planes that are largely parallel to each other.

[0020] According to a preferred embodiment of the invention, the planar first filter material is a first filtration membrane. The planar second filter material is preferably a second filtration membrane. Particularly preferably, the planar first filter material is a first filtration membrane and the planar second filter material is a second filtration membrane.

[0021] A porous membrane is particularly suitable as the first filter material for ultrafiltration and microfiltration. An ultrafiltration membrane can advantageously be used as the second filter material. This configuration allows a specific quantity of diafiltration medium to be introduced into the retentate gap for optimal diafiltration by means of appropriate pressure. When pressurized, the entire available surface area of ​​the first filter material is covered with the diafiltration medium.

[0022] The ultrafiltration membranes are characterized by pore sizes of less than 0.01 µm, or by molecular weight exclusion limits in the range of approximately 5 to 1500 kDa, while the microfiltration membranes have pore sizes in the range of 0.01 to 50 µm, preferably 0.01 to 0.5 µm, or molecular weight exclusion limits of 30 to 1500 kDa. The filtration membranes can consist, for example, of polyvinylidene fluoride, cellulose and its derivatives, polyethersulfone, or polysulfone, with cross-linked cellulose hydrate being particularly preferred.

[0023] Preferably, the inlet for the feed liquid is located in a first peripheral region of the crossflow filtration unit, and the outlet for the retentate is located in a second peripheral region of the crossflow filtration unit, which is opposite the first peripheral region. This arrangement defines a largely uniform flow direction of the retentate from the feed liquid inlet as the starting point to the retentate outlet as the endpoint. The flow direction of the retentate is thus largely parallel to the flow path along the planar filter material, i.e., essentially without deflections, thereby ensuring a stable and reliable flow of the retentate through the crossflow filtration unit.Furthermore, the largely straight flow path, without deflections, loops, or similar features, minimizes the pressure drop in the filtration unit and the undesirable effects of non-linear flows on target substances contained in the feed liquid. For the aforementioned reasons, it is also preferred that the inlet for the diafiltration medium be located in the first edge region of the crossflow filtration unit. However, it is also possible to locate the inlet for the diafiltration medium in the second edge region, or in the third and / or fourth edge region.

[0024] According to a preferred embodiment of the invention, the permeate outlet is located in the second edge region of the crossflow filtration unit. Particularly preferred is the provision of at least one permeate outlet in both the first and second edge regions of the crossflow filtration unit. In a further embodiment of the invention, the permeate outlets are alternatively or additionally located in the third and / or fourth edge region of the crossflow filtration unit. When viewed from the diafiltration gap side of the crossflow filtration unit, the third edge region is located on the left side of the flow direction. The fourth edge region is located on the right side and is thus opposite the third edge region. This arrangement of the outlet(s) allows for a particularly high permeate throughput and offers design advantages.

[0025] Preferably, the first boundary region comprises the outer third of the length of the filtration unit opposite the flow direction. Similarly, the second boundary region comprises the outer third of the length of the filtration unit along the flow direction. The same applies to the third and fourth boundary regions. It is advantageous to make the first to fourth boundary regions as small as possible. Therefore, the boundary regions most preferably comprise the respective outer 20%, more preferably the respective outer 10%, and most preferably the respective outer 3%.

[0026] In principle, there are no particular restrictions regarding the placement of the inlets and outlets. For example, the inlets and outlets can be positioned so that the feed liquid enters and exits the retentate gap in the direction of flow. Similarly, the permeate outlet can be positioned so that the permeate exits the permeate collection gap in the direction of flow, and / or the diafiltration medium inlet can be positioned so that it enters the diafiltration gap in the direction of flow. Preferably, however, the inlets and outlets are positioned so that the diafiltration medium enters the diafiltration gap perpendicular to the direction of flow, and the feed liquid initially enters the retentate gap perpendicular to the direction of flow and exits it as retentate perpendicular to the direction of flow.Such a placement of the inlets and outlets facilitates the arrangement of a plurality of the filtration units according to the invention into a filter cassette.

[0027] Preferably, the crossflow filtration unit has multiple inlets for the feed liquid, multiple outlets for the retentate, and multiple outlets for the permeate.

[0028] InIn a preferred embodiment, the free volume of the diafiltration gap and / or the retentate gap (space available for the diafiltration medium / retentate, dead or empty volume) decreases in the flow direction from the feed liquid inlet to the retentate outlet. Due to the decreasing volume(s) and the surface area of ​​the filter materials, the crossflow filtration unit exhibits a low pressure drop and a substantially deflection-free flow path for the diafiltration medium and the retentate. This makes it possible to increase the surface area throughput of the crossflow diafiltration unit and to operate the crossflow diafiltration unit in "single-pass" mode (only a single pass of the retentate takes place without recirculation).

[0029] InIn a further preferred embodiment of the invention, the decrease in free volume along the flow direction is achieved by reducing the width of the diafiltration gap and / or the width of the retentate gap in the flow direction. The width runs along the planar first filter material and perpendicular to the flow direction. Particularly preferably, the width of the entire crossflow filtration unit decreases in the flow direction. Preferably, the retentate gap or the crossflow filtration unit is trapezoidal in a top view along a normal to the plane in which the planar first filter material lies. The basic trapezoidal shape of the diafiltration gap and / or the retentate gap or the crossflow filtration unit can be asymmetrical, for example, right-angled, and is preferably isosceles.

[0030] According to one embodiment, the height of the diafiltration gap and / or the retentate gap or the crossflow filtration unit can decrease in the flow direction. For example, the diafiltration gap and / or the retentate gap can be wedge-shaped. The height of the diafiltration gap and / or the retentate gap or the crossflow filtration unit is perpendicular to the planar first filter material and perpendicular to the flow direction.

[0031] There are no particular restrictions regarding the width, length, and height of the crossflow filtration unit. The length runs parallel to the flow direction and along the flat first filter material. Preferably, the crossflow filtration unit has a length of at least 50 mm, more preferably at least 150 mm, more preferably 500 mm, and more preferably 750 mm or more. Such a length can be achieved, for example, by connecting several crossflow filtration units in series, for example, at least two, at least three, or at least four. A greater length can result in particularly high efficiency.

[0032] In a preferred embodiment of the invention, the free volume of the permeate collection gap changes in the flow direction. Particularly preferably, the free volume of the permeate collection gap decreases in the flow direction. This allows, for example, the external dimensions of the filtration cassette to be maintained.

[0033] The explanations regarding the design of the retentate gap apply accordingly to the permeate collection gap and vice versa.

[0034] "Decreasing free volume in the direction of flow" means that a cross-sectional area A1, which can be flowed through by the diafiltration medium or the retentate and which lies in a plane which has a normal parallel to the direction of flow, and a corresponding cross-sectional area A2, which is parallel to A1 and is further away from the inlet for the diafiltration medium or the feed liquid than A1, exist, wherein the area A1, which can be flowed through by the diafiltration medium or the retentate, is larger than A2, and no correspondingly defined planes A1' and A2' exist for which the area of ​​A1' is smaller than that of A2'.

[0035] The decrease in free volume can be continuous (for all A₁ and A₂, A₁ ≥ A₂) or continuous (for all A₁ and A₂, A₁ > A₂). It is also possible for the volume decrease to be discontinuous, meaning that at least one discontinuous drop or jump in the cross-sectional area occurs along the flow direction.

[0036] The change in the free volume of the retentate gap in the flow direction is preferably in the range of 20:1 to 1.2:1, preferably 10:1, depending on the filtration task. The "change in the free volume of the retentate gap in the flow direction" is defined as the ratio of the cross-sectional area A1 at the inlet for the feed liquid to the cross-sectional area A2 at the outlet for the retentate.

[0037] In one embodiment of the invention, the thickness of the diafiltration gap and / or the thickness of the retentate gap and optionally the thickness of the permeate collection gap decreases in the direction of flow.

[0038] The diafiltration gap, the retentate gap, and the permeate collection gap are typically kept open for the respective media by spacers. In a preferred embodiment of the invention, a planar spacer is / are provided in the diafiltration gap and / or the retentate gap of the crossflow filtration unit, such that the free volume of the retentate gap decreases in the flow direction.

[0039] Suitable spacers for crossflow filtration units are known in the prior art and can be used in the diafiltration gap, retentate gap, and / or permeate collection gap of the crossflow filtration unit according to the invention. Preferably, the spacers are modified according to the invention such that their volume increases in the flow direction in order to achieve a decrease in the free volume available for the diafiltration medium or the retentate. Preferred spacers can be textile materials made of organic or inorganic materials, such as woven fabrics, knitted fabrics, nonwovens, or extruded nets.

[0040] Advantageously, the spacer can be a non-planar plate. The non-planar plate can be a plate having at least one non-planar primary surface. The primary surfaces of a plate are the opposing surfaces with the largest area. The at least one non-planar primary surface can have irregularities in the form of a corrugated or jagged surface. In addition, the uneven surface can have protruding elements such as truncated cones, truncated pyramids, knobs, or other geometric shapes. The non-planar plate can also be corrugated or jagged, similar to a sheet of metal, with the corrugations or jagged edges preferably extending parallel to the flow direction. Suitable materials for the non-planar plate are the same as those listed below for spacers in the form of an open-mesh matrix.

[0041] According to a preferred embodiment, the spacer consists of an open-mesh matrix or an extruded mesh. Such spacers are known in the prior art and were described, for example, in the publication of German patent application DE 100 22 259 A1. As already mentioned above, according to the invention, the spacers are preferably modified such that their volume increases in the flow direction in order to achieve a decrease in the free volume available for the diafiltration medium or the retentate. In principle, conventional spacers can also be installed in the crossflow filtration unit according to the invention, for example, in the permeate collection gap and / or in the diafiltration gap or in all gaps of the filtration unit with a width decreasing in the flow direction.As described above, a decreasing width can be achieved, for example, by a trapezoidal design of the columns or the crossflow filtration unit.

[0042] In one embodiment, the mesh size of the open-mesh matrix or extruded net can decrease in the flow direction to achieve a decrease in free volume along the flow direction. For example, the mesh size at the inlet for the diafiltration medium or the inlet for the feed liquid is 5 / cm to 15 / cm, in the middle between the inlet for the diafiltration medium or the inlet for the feed liquid and the outlet for the retentate it is 10 / cm to 30 / cm, and at the outlet for the retentate it is 20 / cm to 40 / cm.

[0043] Alternatively or additionally, the open-mesh matrix or extruded network can be composed of intersecting longitudinal and transverse threads, with the number and / or thickness of the longitudinal and / or transverse threads increasing in the flow direction. The open-mesh matrix preferably consists of an organic polymer such as polypropylene, polyethylene, polyester, polyvinyl chloride, or polyvinylidene fluoride, or blends thereof. Furthermore, the open-mesh matrix can be composed of fibers of various polymers.

[0044] In a further preferred embodiment of the invention, several layers of textile materials are arranged one above the other in the retentate gap such that the free channel volume decreases in the flow direction. This can be achieved, for example, by offsetting the superimposed layers at their starting points in the flow direction. The superimposed layers preferably extend to the second edge region. This results in an increasing volume of textile materials occupying the retentate gap in the flow direction, thus decreasing the free volume in the flow direction. The textile materials, such as woven, knitted, nonwoven, or extruded nets, can consist of organic or inorganic materials.

[0045] The configurations shown here for reducing the free volume of the diafiltration gap or retentate gap can be combined in any way.

[0046] According to the invention, the retentate gap is bounded by a planar first filter material and a planar second filter material. The diafiltration gap is bounded by at least one planar first filter material. The permeate collection gap is bounded by at least one planar second filter material. A diafiltration gap and a permeate collection gap are adjacent to each retentate gap. A preferred crossflow filtration unit according to the invention consists of a plurality of stacked arrangements of a diafiltration gap, a planar first filter material, a retentate gap, a planar second filter material, a permeate collection gap, a planar second filter material, a retentate gap, and a planar first filter material, preferably closed by a further diafiltration gap, such that the stacked arrangements are combined to form a filter cartridge. Suitable embodiments for filter cartridges are known in the prior art.Preferably, each diafiltration gap of these arrangements is delimited on both sides by a filter material corresponding to the first filter material, separating it from two retentate gaps. Similarly, each permeate collection gap of these arrangements is preferably delimited on both sides by a filter material corresponding to the second filter material, separating it from two retentate gaps. The first and second filter materials can be different from each other. That is, fundamentally, different types of first and second filter materials can be used. Preferably, identical first and / or identical second filter materials are used.

[0047] According to a preferred embodiment, the planar first and second filter materials each have a largely uniform thickness of preferably 50 µm to 10,000 µm, and more preferably 150 µm to 1,000 µm. If the planar boundary of the diafiltration gap formed by the planar first filter material and the further planar boundary of the diafiltration gap and / or the planar boundaries of the retentate gap formed by the first and second filter materials are not parallel to each other, the free volume of the diafiltration gap and / or the retentate gap can be wedge-shaped, so that the free volume decreases in the flow direction. It is particularly preferred that the diafiltration gap, the retentate gap, and the permeate collection gap are bounded on both sides by largely parallel surfaces.

[0048] The shape of the crossflow filtration unit is not subject to any particular restrictions. For example, the crossflow filtration unit can be cuboid or cylindrical.

[0049] In another aspect, the present invention relates to a method for diafiltration of a food liquid to obtain a retentate and a permeate, comprising the steps (A) Providing a crossflow filtration unit according to the invention; (B) Feeding a diafiltration medium into the diafiltration medium inlet; (C) Feeding the feed liquid into the feed liquid inlet; (D) Discharging the retentate from the retentate outlet; and (E) Discharging the permeate from the permeate outlet.

[0050] The explanations regarding the crossflow filtration unit and the diafiltration process are mutually applicable.

[0051] In step (A) the above-described crossflow filtration unit is preferably provided, which has a plurality of stacked arrangements of diafiltration gap, planar first filter material, retentate gap, planar second filter material, permeate collection gap, planar second filter material, retentate gap and planar first filter material, such that the stacked arrangements are combined to form a filter cassette.

[0052] The diafiltration medium used is not subject to any particular restrictions. In principle, any fluid is suitable, although water and aqueous salt solutions are preferred. An aqueous buffer solution is particularly preferred as a diafiltration medium.

[0053] Preferably, the flow rate of the supplied diafiltration medium is 0.1 to 15 times the flow rate of the supplied feed liquid. The flow rate of the removed retentate is preferably 0.05 to 10 times the flow rate of the supplied feed liquid.

[0054] In a preferred embodiment of the process, the diafiltration medium is supplied at a pressure of 0.1 to 4 bar. Particularly preferably, the diafiltration medium is supplied at a pressure greater than the retentate outlet pressure.

[0055] Preferably, the process according to the invention is operated continuously, that is, with a constant / continuous addition of the diafiltration medium and the feed liquid, thereby providing a particularly efficient and economical filtration process. According to the invention, "continuous diafiltration" is understood to mean a diafiltration process in which both the diafiltration medium and the feed liquid are added continuously.

[0056] In a preferred embodiment of the method, in step (A) several independent crossflow filtration units according to the invention are provided and connected in series such that the outlet for the retentate of the respective upstream crossflow filtration unit is fluidly connected to the inlet for the feed liquid of a downstream crossflow filtration unit. In this embodiment, in step (C) the feed liquid is also fed into the inlet for the feed liquid of the crossflow filtration unit that is not upstream of another crossflow filtration unit (first crossflow filtration unit), and in step (D) the retentate is discharged from the outlet for the retentate of the crossflow filtration unit that is not downstream of another crossflow filtration unit (last crossflow filtration unit).Thus, the retentate / feed liquid passes through the series-connected crossflow filtration units from the first to the last. Preferably, 2 to 10, and particularly preferably 2 to 5, crossflow filtration units are connected in series. In this configuration, the diafiltration medium is supplied separately to each of the series-connected crossflow filtration units. While it is possible to use different types of diafiltration media, it is preferred to supply the same diafiltration medium to each of the crossflow filtration units. Preferably, each of the series-connected crossflow filtration units, as described above, is in the form of a filter cartridge.

[0057] In a preferred embodiment of the method, several independent crossflow filtration units according to the invention are provided and connected in parallel in step (A). Parallel and series connections can be combined.

[0058] In a further preferred embodiment of the process, the retentate removed in step (D) is at least partially recycled to the feed liquid inlet. An improved result can be achieved by means of a recirculation process if a single pass through the crossflow filtration unit is insufficient. The addition of the diafiltration liquid and the removal of the permeate are carried out without recycling. If several crossflow filtration units are connected in series, each retentate stream can, in principle, be recycled to each feed liquid inlet. Preferably, the retentate from each individual crossflow filtration unit is recycled to the feed liquid inlet of the same crossflow filtration unit.

[0059] According to a further preferred embodiment of the method according to the invention, the feed liquid or the retentate in the retentate gap is set into oscillation. This superimposes an oscillation (vibration) on the flow motion in the retentate gap. This can be achieved by at least one device for generating oscillation, which may be attached to the feed liquid inlet and / or the retentate outlet. Such a device for generating oscillation moves the retentate back and forth in the retentate gap; that is, the retentate is set into an oscillating motion that is largely parallel to the planar first filter material. Preferably, the oscillation is generated by a device for generating oscillation that is attached to the feed liquid inlet and by another device for generating oscillation that is attached to the retentate outlet.A suitable device for generating oscillation according to the invention is, for example, a piston pump. Preferably, a device for generating oscillation comprises a reservoir divided into two halves by an elastic diaphragm and optionally a pressure source. The reservoir is a reservoir (storage tank) for the feed liquid or the retentate. The first half of the reservoir is connected to a pressure source, such as a compressed air source or a pump, via a valve control. The second half of the reservoir is fluidly connected to the feed liquid inlet or the retentate outlet of the filtration device. The second half also preferably contains a drain or vent valve. By counter-rotating (alternating or...By alternately pressurizing one half of each of the two reservoirs (for example, with compressed air), the retentate flow can be moved back and forth (set into oscillation). The reservoirs described above can be either a separate component of the system (not part of the crossflow filtration unit) or integrated into the housing of the crossflow filtration unit.

[0060] Preferably, the crossflow filtration unit according to the invention comprises at least one (preferably two) device(s) for generating an oscillation of the feed liquid or the retentate in the retentate gap. It is preferred according to the invention that a device for generating oscillation comprises a reservoir with a pressure source. Particularly preferably, the crossflow filtration unit according to the invention comprises a first reservoir, optionally with a first pressure source, and a second reservoir, optionally with a second pressure source, wherein the first reservoir is fluidly connected to the inlet for the feed liquid and the second reservoir is fluidly connected to the outlet for the retentate, provided that at least one of the first and second pressure sources is present. Preferably, both the first and the second reservoir each have a pressure source.Each reservoir is preferably divided into two halves by an elastic and fluid-impermeable (gas- and liquid-impermeable) membrane, the first half being connected to a pressure source. The second half of the first reservoir is preferably fluid-conducting and connected to the inlet for the feed liquid. The second half of the second reservoir is preferably fluid-conducting and connected to the outlet for the feed liquid.

[0061] According to a preferred embodiment, the inventive method further comprises step (C0) of separating a starter liquid into a pre-retentate and a pre-permeate. This upstream step allows a liquid to first be concentrated and / or (partially) freed from impurities, for example by filtration or diafiltration, in order to then be subjected to subsequent diafiltration using the crossflow filtration unit according to the invention. If step (C0) is a filtration or diafiltration step, both the pre-retentate and the pre-permeate can, in principle, be used as the starter liquid; however, the pre-retentate from step (C0) is preferably used.

[0062] Step (C0) is advantageously carried out using a crossflow filtration unit (C0 unit) to separate a pre-retentate and a pre-permeate from a pre-retentate. Such crossflow filtration units are known in the prior art. The CO unit can be connected upstream of the first crossflow filtration unit, as described above for the series connection of the crossflow filtration units according to the invention.

[0063] A particularly advantageous CO unit is a crossflow filtration unit comprising a pre-retentate gap, a planar filter material, and a pre-permeate collection gap, arranged such that the planar filter material separates the pre-retentate gap and the pre-permeate collection gap from each other, wherein the pre-retentate gap is fluid-conductingly connected to at least one inlet for the feed liquid and to at least one outlet for the pre-retentate, and the pre-permeate collection gap is fluid-conductingly connected to at least one outlet for the pre-permeate, the inlet for the feed liquid being located in a first edge region of the crossflow filtration unit, and the outlet for the pre-retentate being located in a second edge region of the crossflow filtration unit, which is opposite the first edge region.and preferably the free volume of the pre-retentate gap decreases in the flow direction from the inlet for the starter liquid to the outlet for the pre-retentate. By decreasing the free volume in the flow direction, a CO unit with low pressure loss and high surface area utilization can be provided.

[0064] The design of the CO unit, and in particular the pre-retentate gap and the pre-permeate collection gap, is governed by the provisions relating to the crossflow filtration unit for diafiltration according to the invention, and in particular the provisions relating to its retentate gap and permeate collection gap. The planar filter material of the CO unit can be a microfiltration membrane and preferably an ultrafiltration membrane. Like the crossflow filtration unit according to the invention, the CO unit can be configured as a filter cartridge.

[0065] The method according to the invention is preferably operated under the following conditions: P DF ≥ P Retentat ; x = V DF / V Feed , preferably x ≥ 1, particularly preferably 3 to 10; and k = V Feed / V Retentate , preferably k ≥ 1; where P DF is the pressure at which the diafiltration medium is added, P Retentate is the retentate outlet pressure, i.e., the pressure at which the retentate leaves the filtration device, V Retentate is the volume flow rate of the retentate, V DF is the volume flow rate of the diafiltration medium, V Feed is the volume flow rate of the feed liquid, x is the so-called DF ratio, and k is the so-called concentration factor.

[0066] The method according to the invention is particularly suitable for the filtration, diafiltration, concentration and / or modification of the ion composition of a protein solution or combinations thereof.

[0067] The process according to the invention can be part of a more comprehensive process sequence. For example, the diafiltration process can be performed downstream of a pretreatment and / or upstream of a posttreatment. Examples of suitable pretreatments and posttreatments include the conversion of reactants to products by biological or chemical means, thermal and mechanical separation processes, and chemical analysis methods.

[0068] The present invention relates in a further aspect to an apparatus for carrying out a chemical or biological process, which includes the crossflow diafiltration unit described above. Such an apparatus can, for example, comprise a bioreactor, a cell separation unit, a diafiltration unit with the crossflow diafiltration unit according to the invention, and a chromatography unit. Suitable chemical and biological processes include, for example, the production of vaccines or biopharmaceuticals.

[0069] Furthermore, the present invention relates to the use of the crossflow filtration unit for continuous diafiltration within a chemical or biological process, wherein the continuous diafiltration step is preceded by at least one preparation step for the feed liquid and / or followed by at least one post-treatment step for the retentate. Suitable pre-treatment and post-treatment steps include, for example, conversion in a bioreactor, optionally followed by cell separation, chromatography, filtration, concentration, and dilution (see [reference]). Figure 4 ).

[0070] The present invention is explained by the following examples, without being limited thereto. Example 1

[0071] A solution of albumin and NaCl (5.2 wt% albumin in 0.9 wt% aqueous NaCl solution) was subjected to diafiltration. The albumin concentration was determined photometrically at a wavelength of 280 nm. The conductivity of the initial solution was 14 mS. Demineralized water was used as the diafiltration medium.

[0072] The diafiltration unit was designed as a filter cassette, with 13 stacked arrangements of diafiltration gap, flat first filter material, retentate gap, flat second filter material, permeate collection gap, flat second filter material, retentate gap, and flat first filter material, supplemented by an additional diafiltration gap. A Hydrosart® membrane, type 10 kDa, from Sartorius Stedim Biotech GmbH was used as the flat second filter material. A polyethersulfone membrane, type 30 kDa, from Sartorius Stedim Biotech GmbH, was used as the flat first filter material. The DF ratio (volume of diafiltration medium / volume of feed liquid) was approximately 5.5:1.

[0073] During the test, the pump and valve settings were not readjusted. The diafiltration unit operated with stable performance. The low conductivity of the retentate (2.2 mS) indicates that excellent diafiltration performance was achieved.

[0074] The result of the above example is shown in Table 1 below. Table 1 Time (min) P Feed (bar) P Retentat (bar) P DF (bar) V Feed (ml / min) V Retentate (ml / min) V Permeate ml / min Protein C (wt.%) x (Vol / Vol) LF Permeate (mS) LF Retentat (mS) T (°C) 0 (Start) 2,42 0,77 1,8 14,6 12,8 80,8 5,6 5,5 : 1 2.5 mS 2.25 mS approx. 20°C 30 min 2,40 0,77 1,8 14,6 14,3 82,0 4,96 5,5 : 1 2.4 mS 2.18 mS approx. 20°C 60 min 2,39 0,77 1,8 14,6 14,6 81 5,1 5,5 : 1 2.4 mS 2.18 mS approx. 20°C P Feed Pressure at which the feed liquid is added P Retentate Retentate outlet pressure P DF Pressure at which the diafiltration medium is added V Feed Flow rate of the feed liquid V Retentate Flow rate of the retentate V Permeate Flow rate of the permeate C Protein Concentration of albumin (weight percent) xDF ratio (volume of diafiltration medium / volume of feed liquid) LF Permeate Conductivity of the permeate LF Retentate Conductivity of the retentate TTemperature Example 2

[0075] A solution of approximately 22 g / L albumin (1 g / L corresponds to 0.1 wt%) in 10 mmol KPI buffer (10 mmol / L potassium dihydrogen phosphate and 10 mmol / L dipotassium hydrogen phosphate) and 0.9 wt% NaCl was subjected to diafiltration. The albumin concentration was determined photometrically at a wavelength of 280 nm. The conductivity of the initial solution was 15.9 mS. A 10 mmol KPI solution with a conductivity of 2.05 mS was used as the diafiltration medium (DF).

[0076] The diafiltration units used were designed as filter cassettes, with each diafiltration unit consisting of eight stacked arrangements of diafiltration gap, flat first filter material, retentate gap, flat second filter material, permeate collection gap, flat second filter material, retentate gap, and flat first filter material, supplemented by an additional diafiltration gap. A Hydrosart® membrane, type 10 kDa, from Sartorius Stedim Biotech GmbH was used as the flat second filter material. A Hydrosart® membrane, type 30 kDa, from Sartorius Stedim Biotech GmbH was used as the flat first filter material.

[0077] In this example, three of the diafiltration units described above were connected in series using diverter plates for the feed liquid, such that the feed liquid was passed serially through all three cassettes. In contrast, the diafiltration fluid was passed in parallel to all three diafiltration units.

[0078] The total filter area of ​​all three planar secondary filter materials was 0.2 m². The DF ratio (volume of diafiltration medium / volume of feed liquid) was approximately 4.5:1.

[0079] The result of the above example is shown in Table 2 below.

[0080] During the test, the pump and valve settings were not readjusted. The diafiltration unit operated with constant and stable performance for a period of 2 hours.

[0081] The effectiveness of the diafiltration was calculated from the decrease in conductivity (CV) in the feed stream and is labeled "Clearance" in the table. The clearance was calculated as a percentage using the formula... Clearance = 100 − LF Retentat − LF DF / LF Feed − LF DF * 100 , where LF is the conductivity of the respective medium.

[0082] The calculated clearance value was compared with the theoretically achievable value.

[0083] The theoretically achievable clearance was calculated using the following formula: theoretisch erreichbare Clearance = 100 − 1 / e n ⋅ 100 where e is Euler's number and n represents the DF ratio.

[0084] Example 2 shows that with the device according to the invention, extremely effective continuous diafiltration is possible, which can achieve the theoretical (maximum possible) clearance value of 99%.

[0085] It should be noted that, according to the prior art, dilution with the 4.5-fold amount of diafiltration solution mentioned in the example, followed by concentration of the protein solution to its original volume, only achieves a clearance value of 78%. Even successive addition in five individual steps, each followed by concentration to the initial volume, would yield a clearance value of only 96%. This is still significantly below the clearance of 99% achievable according to the invention. Table 2 Time (min) P Feed (bar) P Retentat (bar) P DF V Feed (ml / min) V Retentate (ml / min) V Permeate ml / min c Protein (g / l) x (Vol / Vol) LF Retentat (mS) Clearance % Theoretical clearance T (°C) (bear) % 5 min 1,07 1,08 1,2 20 20 90 21,3 4,5 2,28 99 99 approx. 19°C 15 min 1,09 1,02 1,2 22 20 90 22,4 4,5 2,18 99 99 approx. 19°C 30 min 1,07 1,01 1,19 22 20 88 21,5 4,5 2,17 99 99 approx. 19°C 60 min 1,03 0,97 1,14 21,5 20 88 23,6 4,5 2,14 99 99 approx. 19°C 90 min 1,01 0,95 1,12 21 20 88 25 4,5 2,16 99 99 approx. 19°C 120 min 1,00 0,95 1,12 22 20 88 22,5 4,5 2,16 99 99 approx. 19°C P Feed Pressure at which the feed liquid is added P Retentate Discharge pressure P DF Pressure at which the diafiltration medium is added V Feed Flow rate of the feed liquid V Retentate Flow rate of the retentate V Permeate Flow rate of the permeate c Protein Concentration of albumin (g / l) xDF ratio (volume of diafiltration medium / volume of feed liquid) LF Retentate Conductivity of the retentate Clearance Percentage decrease in conductivity (100% = maximum possible decrease in conductivity) Clearance Theoretically maximum possible decrease in conductivity in percent TTemperature

[0086] Figure 1Figure 1 shows a possible configuration of a crossflow diafiltration unit (1) according to the invention, comprising a planar second filter material in the form of an ultrafiltration membrane (6) and a planar first filter material in the form of a microfiltration membrane (4), wherein the flows of diafiltration medium (3), feed liquid (5), retentate (7), and permeate (2) are illustrated by arrows. The diafiltration gap, the retentate gap, and the permeate collection gap are kept open for the respective media by spacers (8).

[0087] Figure 2Figure 1 schematically illustrates the implementation of the diafiltration process according to the invention using a crossflow diafiltration unit designed as a diafiltration cassette (1). The feed liquid (5) is supplied to the feed liquid inlet via a pump (14). The pressure of the diafiltration medium (3) and the retentate (7) is measured by pressure gauges (16) upstream of the inlet and downstream of the outlet for the retentate. The respective composition of the retentate (7) and the permeate (2) is monitored by a measuring device, for example, a conductivity meter (19). The volumetric flow rate of the retentate (7) is controlled by a throttle valve (17).

[0088] Figure 3Figure 1 schematically illustrates the implementation of the diafiltration process according to the invention, wherein three diafiltration cassettes (1) are connected in series. A preliminary step involves separating a liquid into a pre-retentate and a pre-permeate using a conventional crossflow filtration cassette (11). The feed liquid (5) is supplied via a pump (14). The diafiltration medium (3) is supplied to the three diafiltration cassettes (1) via a pump (12). The pre-retentate from each upstream filtration unit is supplied as feed liquid to each downstream filtration unit via channel (10). The pressure of the diafiltration medium (3) and the retentate (7) is measured by pressure gauges (16) before the inlet for the diafiltration medium and after the outlet for the retentate, respectively. The composition of the retentate (7) is monitored by a measuring device, for example, a conductivity meter (19).The volume flow of the retentate (7) is controlled by a throttle valve (17). The permeate (2) from all modules (11, 1) is discharged via the collecting line (20).

[0089] Figure 4Figure 1 schematically illustrates the implementation of the diafiltration process according to the invention, as well as a crossflow diafiltration unit according to the present invention, wherein the feed liquid / retentate (5) is / are oscillated in the retentate gap. The feed liquid (5) is introduced into a reservoir (30a) via a pump (14) and subsequently into the retentate gap. The feed liquid / retentate (5) then passes through a second oscillation-generating device or a second reservoir (30b). The diafiltration medium (3) is introduced into the diafiltration gap via a pump (12). The illustrated crossflow diafiltration unit has two oscillation-generating devices, the first device comprising a first reservoir (30a) and the second device comprising a separate second reservoir (30b). Reservoir (30a) is fluidly connected to the inlet for the feed liquid (5).Reservoir (30b) is fluid-conductingly connected to the outlet for the retentate (7). Each reservoir is separated into two halves by an elastic and fluid-impermeable membrane. The first half of the first reservoir (30a) (in . Figure 4 (not shown hatched) is connected to a pressure source via a valve control. The second half of the first reservoir (30a) (in Figure 4 The reservoir (30b) is fluid-conducting and connected to the inlet for the feed liquid. The second reservoir (30b) is divided into two halves by an elastic and fluid-impermeable membrane. The first half of the second reservoir (30b) (in Figure 4 (not shown hatched) is connected to a pressure source via a valve control. The second half of the second reservoir (30b) (in Figure 4The section shown in hatched areas) is fluid-conducting and connected to the outlet for the retentate. By controlling or regulating the pressure level of the compressed air (by applying compressed air in opposite directions to the first halves of 30a and 30b), an oscillating movement of the feed fluid / retentate is generated in the retentate gap. A portion of the retentate is continuously removed to ensure a continuous process.

[0090] Figure 5 Figure 1 shows exemplary excerpts A to C from process diagrams for carrying out a method for the production of biopharmaceuticals, wherein the processes each comprise the provision of at least one crossflow diafiltration unit (1) according to the invention. The dashed square brackets and the preceding and following arrows indicate that further process steps may be added upstream or downstream.

[0091] The in Figure 5AThe illustrated embodiment comprises a crossflow diafiltration unit (1) according to the invention, wherein a bioreactor (41) and a cell separation unit (42) are arranged upstream. A chromatography unit / chromatography step (44) is arranged downstream of the crossflow diafiltration unit (1). The flow of the product solution from the bioreactor (1) is illustrated by arrows.

[0092] The in Figure 5B The illustrated embodiment comprises a crossflow diafiltration unit (1) according to the invention between two chromatography steps (44), i.e. a chromatography step is placed upstream of the diafiltration and a chromatography step is placed downstream.

[0093] The in Figure 5C The illustrated embodiment comprises a crossflow diafiltration unit (1) according to the invention prior to a final filtration step (45).

[0094] The in Figures 5A to 5CThe process steps shown can be extended by further process steps or combined as desired. The crossflow filtration according to the invention is preferably used for filtration, diafiltration, concentration and / or modification of the composition of a solution. Reference symbol list

[0095] 1 Crossflow diafiltration unit / diafiltration cassette 2 Permeate 3 Diafiltration medium 4 First filter material 5 Feed liquid 6 Second filter material 7 Retentate 8 Spacer 10 Feed liquid diversion channel 11 Standard crossflow cassette 12 Diafiltration pump 14 Feed liquid pump 16 Pressure gauge 17 Throttle valve 19 Conductometer 20 Manifold 30a, 30b Reservoir 41 Bioreactor 42 Cell separation 44 Chromatography 45 Filtration step / Filter

[0096] Furthermore, the present invention relates to the following points 1 to 13: 1. Crossflow filtration unit (1) for the continuous diafiltration of a feed liquid (5) to obtain a retentate (7) and a permeate (2), comprising at least a diafiltration gap, a planar first filter material (4), a retentate gap, a planar second filter material (6) and a permeate collection gap, arranged such that the planar first filter material (4) separates the diafiltration gap and the retentate gap from each other and the planar second filter material (6) separates the retentate gap and the permeate collection gap from each other, wherein the diafiltration gap is fluid-conducting with at least one inlet for the diafiltration medium (3),The retentate gap is fluid-conducting and connected to at least one inlet for the feed liquid (5) and to at least one outlet for the retentate (7), and the permeate collection gap is fluid-conducting and connected to at least one outlet for the permeate (2), wherein the pore size or the molecular weight exclusion limit of the planar first filter material (4) is at least as large as the pore size or the molecular weight exclusion limit of the planar second filter material (6). 2. Crossflow filtration unit (1) according to point 1, wherein the planar first filter material (4) is a first filtration membrane and / or the planar second filter material (6) is a second filtration membrane. 3. Crossflow filtration unit (1) according to point 2,wherein the first filtration membrane is a micro- or ultrafiltration membrane and / or the second filtration membrane is an ultrafiltration membrane. 4. Crossflow filtration unit (1) according to any one of points 1 to 3, wherein the crossflow filtration unit (1) comprises a plurality of stacked arrangements of diafiltration gap, planar first filter material (4), retentate gap, planar second filter material (6), permeate collection gap, planar second filter material (6), retentate gap and planar first filter material (4), such that the stacked arrangements are combined to form a filter cassette. 5. Crossflow filtration unit (1) according to any one of points 1 to 4, wherein the free volume of the diafiltration gap and / or the retentate gap decreases in the flow direction from the feed liquid inlet (5) to the retentate outlet (7). 6. Crossflow filtration unit (1) according to point 5,wherein several layers of textile materials are arranged one above the other in the retentate gap such that the free volume of the retentate gap decreases in the flow direction. 7. Method for diafiltration of a food liquid (5) to obtain a retentate (7) and a permeate (2), comprising the steps (A) providing a crossflow filtration unit (1) according to any one of points 1 to 6; (B) supplying a diafiltration medium (3) to the diafiltration medium inlet (3); (C) supplying the food liquid (5) to the food liquid inlet (5); (D) removing the retentate (7) from the retentate outlet (7); and (E) removing the permeate (2) from the permeate outlet (2). 8. Method for diafiltration according to point 7, wherein the diafiltration medium (3) and the food liquid (5) are supplied continuously. 9. Method for diafiltration according to point 7 or 8, wherein in step (A) several crossflow filtration units (1),each according to one of points 1 to 6, and are connected in series such that the outlet for the retentate (7) of the respective upstream crossflow filtration unit (1) is fluid-conductingly connected to the inlet for the feed liquid (5) of a downstream crossflow filtration unit (1), and in step (C) the feed liquid (5) is fed into the inlet for the feed liquid (5) of the crossflow filtration unit (1) to which no other crossflow filtration unit (1) is connected upstream, and in step (D) the retentate (7) is discharged from the outlet for the retentate (7) of the crossflow filtration unit (1) to which no other crossflow filtration unit (1) is connected downstream. 10. Method for diafiltration according to one of points 7 to 9,wherein the retentate (7) removed in step (D) is at least partially recycled to the feed liquid inlet (5). 11. Method for diafiltration according to any one of points 7 to 10, further comprising the step: (C0) separation of a liquid into a pre-retentate and a pre-permeate, wherein the pre-retentate or the pre-permeate is used as the feed liquid (5). 12. Use of the crossflow filtration unit (1) according to any one of points 1 to 6 for the filtration, diafiltration, concentration and / or modification of the ionic composition of a solution. 13. Use of the crossflow filtration unit (1) according to any one of points 1 to 6 for continuous diafiltration within a chemical or biological process, wherein the continuous diafiltration step is preceded by at least one feed liquid (5) preparation step and / or followed by at least one retentate (7) preparation step.

Claims

1. Method for diafiltration of a feed fluid (5) to obtain a retentate (7) and a permeate (2), the method comprising the steps of: (A) providing a crossflow filtration unit (1) for continuous diafiltration, at least comprising: a diafiltration channel, a flat first filter material (4), a retentate channel, a flat second filter material (6) and a permeate collection channel, arranged such that the flat first filter material (4) delimits the diafiltration channel and the retentate channel from one another, and the flat second filter material (6) delimits the retentate channel and the permeate collection channel from one another, wherein the diafiltration channel is connected in a fluid-conducting manner to at least one inlet for the diafiltration medium (3), the retentate channel is connected in a fluid-conducting manner to at least one inlet for the feed fluid (5) and to at least one outlet for the retentate (7), and the permeate collection channel is connected in a fluid-conducting manner to at least one outlet for the permeate (2), and wherein the pore size or the molecular weight cut-off of the flat first filter material (4) is at least as large as the pore size or the molecular weight cut-off of the flat second filter material (6); (B) feeding a diafiltration medium (3) into the inlet for the diafiltration medium (3); (C) feeding the feed fluid (5) into the inlet for the feed fluid (5); (D) discharging the retentate (7) from the outlet for the retentate (7); and (E) discharging the permeate (2) from the outlet for the permeate (2), wherein the method is operated under the following conditions: P DF ≥ P Retentate ; x ≥ 1, where x = VDF / VFeed; and k ≥ 1, where k = VFeed / VRetentate; where PDF is the pressure at which the diafiltration medium is fed in step (B), PRetentate is the retentate output pressure, i.e. the pressure at which the retentate leaves the filtration unit (1) in step (D), VRetentate is the volume flow of the retentate, VDF is the volume flow of the diafiltration medium, and VFeed is the volume flow of the feed fluid.

2. Method for diafiltration according to claim 1, wherein the flat first filter material (4) of the crossflow filtration unit (1) is a first filtration membrane, and / or the flat second filter material (6) of the crossflow filtration unit (1) is a second filtration membrane.

3. Method for diafiltration according to claim 2, wherein the first filtration membrane of the crossflow filtration unit (1) is a microfiltration membrane or an ultrafiltration membrane, and / or the second filtration membrane of the crossflow filtration unit (1) is an ultrafiltration membrane.

4. Method for diafiltration according to any one of claims 1 to 3, wherein the crossflow filtration unit (1) comprises a plurality of stacked arrays of diafiltration channel, flat first filter material (4), retentate channel, flat second filter material (6), permeate collection channel, flat second filter material (6), retentate channel, and flat first filter material (4), such that the stacked arrays are combined to form a filter cassette.

5. Method for diafiltration according to any one of claims 1 to 4, wherein the free volume of the diafiltration channel and / or the retentate channel of the crossflow filtration unit (1) decreases in the flow direction from the inlet for the feed fluid (5) to the outlet for the retentate (7).

6. Method for diafiltration according to claim 5, wherein a plurality of layers of textile materials are arranged one above another in the retentate channel of the crossflow filtration unit (1) such that the free volume of the retentate channel decreases in the flow direction.

7. Method for diafiltration according to any one of claims 1 to 6, wherein the diafiltration medium (3) and the feed fluid (5) are fed in continuously.

8. Method for diafiltration according to any one of claims 1 to 7, wherein in step (A) a plurality of crossflow filtration units (1) are provided and are connected in series such that the outlet for the retentate (7) of the respective upstream crossflow filtration unit (1) is connected in a fluid-conducting manner to the inlet for the feed fluid (5) of a downstream crossflow filtration unit (1), and in step (C) the feed fluid (5) is fed into the inlet for the feed fluid (5) of that crossflow filtration unit (1) which is not preceded by any other crossflow filtration unit (1) and in step (D) the retentate (7) is discharged from the outlet for the retentate (7) of that crossflow filtration unit (1) which is not followed by any other crossflow filtration unit (1) .

9. Method for diafiltration according to any one of claims 1 to 8, wherein the retentate (7) discharged in step (D) is recycled at least partially into the inlet for the feed fluid (5).

10. Method for diafiltration according to any one of claims 1 to 9, further comprising the step of: (C0) separating a fluid into a pre-retentate and a pre-permeate, the pre-retentate or the pre-permeate being used as the feed fluid (5).

11. Method for diafiltration according to any one of claims 1 to 10, wherein the method filters, diafilters, concentrates a solution and / or changes the ion composition of the solution.

12. Method for diafiltration according to any one of claims 1 to 11, wherein the method follows a pre-treatment and / or precedes an aftertreatment.

13. Method for diafiltration according to any one of claims 1 to 12, wherein x is 3 to 10.