Concentration of aqueous electrolyte solutions

Abstract

The present invention relates to a device for nanofiltration or reverse osmosis of aqueous electrolytes and a process for treating aqueous solutions at high osmotic pressures.

Description

[0001] Concentration of aqueous electrolyte solutions

[0002] Technical area

[0003] The present invention relates to a device for nanofiltration or reverse osmosis of aqueous electrolytes and to a method for treating aqueous solutions with high osmotic pressures.

[0004] Description of the state of the art

[0005] Nanofiltration is frequently used to remove multivalent cations such as magnesium, calcium, lead, aluminum, and iron ions, as well as anions such as nitrate and sulfate, from aqueous solutions. Its main application is water treatment, often in combination with reverse osmosis. Reverse osmosis is a common process for producing drinking water from seawater. Nanofiltration is now also used in surface treatment, e.g., for acidic or alkaline metal pickling solutions or acidic anodizing baths.

[0006] US 10,766,800 B2 relates to a process for the selective removal of nitrates from groundwater using a hybrid nanofiltration (NF)-reverse osmosis (RO) filtration system. The process produces low-nitrate product water that can be used for drinking, irrigation, and other purposes, as well as wastewater that has a relatively low salt content and can be safely disposed of into wastewater systems and recycled, for example, for irrigation purposes.

[0007] EP 2 762 61 1 B1 discloses a method for preparing a pickling and / or anodising bath and an anodising arrangement with a pickling plant and an anodising plant with nanofiltration, in which an alkaline pickling bath or an acidic anodising bath is prepared in such a way that the pickling solution or the electrolyte is returned to the process and aluminium hydroxide and possibly heavy metals are obtained as valuable materials.

[0008] CN 101 759 250 B relates to a process for the separation and recovery of heavy metal salt and inorganic acid in pickling waste by a membrane process, comprising the following concrete steps: filtering and removing suspended solids in pickling waste through an inorganic ceramic membrane; separating acid from salt in the liquid permeating the ceramic membrane by diffusion dialysis; and heating the dialysis liquid of a diffusion dialyzer, then introducing the dialysis liquid into a nanofiltration membrane and cooling and crystallizing the concentrated liquid of a nanofiltration device, and then centrifuging the precipitate of the concentrated liquid to obtain iron salt.The permeate from the nanofiltration device passes through a reverse osmosis device, the retentate from the reverse osmosis device returns to a pickling section, and the water discharged from the reverse osmosis device enters a diffusion dialysis section for recycling.

[0009] The German patent application 10 2022 104 474.2 describes the combination of nanofiltration with diffusion dialysis for bath maintenance in surface technology with very high recovery rates for the acids or alkalis used.

[0010] A challenge for the use of nanofiltration and reverse osmosis in many applications is the high osmotic pressure of the solutions to be treated (feed) due to increased ion concentrations, e.g., in bath maintenance in surface technology. Very high pressures are also required for the production of drinking water from seawater by reverse osmosis due to the high salt content. High osmotic pressures result in high costs for nanofiltration and reverse osmosis, particularly for the required pressure-stable apparatus and membrane materials, as well as for the required pumps, which must be designed for discharge heads of over 1000 m. Object of the invention

[0011] Against this background, the invention aims to provide a nanofiltration or reverse osmosis process for the treatment of aqueous solutions, in particular aqueous solutions with high osmotic pressures, which requires reduced pressures, requires less equipment and is lower in cost.

[0012] Short description of the figures

[0013] Figure 1 shows schematically an embodiment of the device according to the invention and illustrates the method according to the invention;

[0014] Figure 2 shows schematically a further embodiment of the device according to the invention.

[0015] Summary

[0016] Methods and devices for concentrating an ion-containing aqueous solution (Feed 1) are provided. The classic nanofiltration process or reverse osmosis process is extended to a countercurrent process in which another ion-containing aqueous solution (Feed 2) is fed pressurelessly to the permeate side in countercurrent to the pressure side. The retentate is a more concentrated solution than Feed 1, and the permeate is an aqueous solution that is less ion-depleted than Feed 2.

[0017] Detailed description

[0018] The invention relates to a device for concentrating an ion-containing aqueous solution, comprising a first chamber and a second chamber separated by a semipermeable membrane. The first chamber has an inlet for the ion-containing aqueous solution, which is connected to the pressure side of a pump, and an outlet for a retentate, which is connected to a reducing valve. The second chamber has an inlet for a second ion-containing aqueous solution and an outlet for a permeate. In a further embodiment, the reducing valve is replaced by a turbine or similar expansion machine for energy recovery. This leads to a further reduction in energy consumption.

[0019] In one embodiment of the device, the outlet for a retentate from the first chamber is connected via a further reducing valve to the inlet for a second ion-containing aqueous solution from the second chamber. This allows a partial flow of the retentate leaving the first chamber, or even the entire retentate, to be expanded and fed without pressure to the second chamber of the device.

[0020] In one embodiment of the device, the semipermeable membrane is a nanofiltration membrane. Nanofiltration membranes have a pore size of 0.1 nm to 10 nm, in particular of 0.5 nm to 2 nm, and an exclusion limit in the range of 180 to 2000 Da. In another embodiment of the device, the semipermeable membrane is a reverse osmosis membrane. Reverse osmosis membranes have an exclusion limit in the range of 100 to 200 Da, for example, in the range of 100 to 150 Da.

[0021] In one embodiment of the device, the semipermeable membrane comprises a bundle of tubular membranes or hollow fiber membranes, the ends of which are embedded in end walls that separate the second space on the outside of the membranes from the first space comprising the lumen of the membranes.

[0022] In one embodiment of the device, baffles are arranged in the second space between the tubular membranes or hollow-fiber membranes. The use of baffles intensifies mass transfer, analogous to tube-bundle heat exchangers. Examples of other suitable devices for countercurrent flow include tube-bundle devices with nanofiltration membranes (NF membranes) or reverse osmosis membranes (RO membranes), or plate stacks with NF or RO membranes. Spiral devices with countercurrent flow, as described in WO 2021 / 123052 A1, or wound membrane modules, as known from WO 2017 / 001060 A1, can also be used.

[0023] Suitable membranes are commercially available and can be purchased in rolls for stacked plate or countercurrent spiral apparatus, for example, from Microdyn-Nadir GmbH, 65203 Wiesbaden, or from DuPont de Nemours (Deutschland) GmbH, 63263 Neu-Isenburg. Ceramic tubular membranes can also be used for tube bundle apparatus (available, for example, from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS, 07629 Hermsdorf).

[0024] Suitable pumps for processes with corrosive aqueous media are produced, for example, by Danfoss GmbH, 63073 Offenbach am Main (APP models), and suitable reducing valves or overflow valves are produced, for example, by Swagelok Company, Solon, Ohio 44139, USA.

[0025] The invention also relates to a process for concentrating a first ion-containing aqueous solution (Feed 1), in which the first solution is passed under increased pressure on one side of a semipermeable membrane past the semipermeable membrane and a second ion-containing aqueous solution (Feed 2) is passed at normal pressure on the other side of the semipermeable membrane in countercurrent past the semipermeable membrane, wherein water from Feed 1 passes through the semipermeable membrane into Feed 2 and a retentate which has a higher ion concentration than Feed 1 and a permeate which has a lower ion concentration than Feed 2 are obtained.

[0026] According to the invention, the classic nanofiltration process or reverse osmosis process is extended to a countercurrent process in which another ion-containing aqueous solution (Feed 2) is fed pressurelessly in countercurrent to the pressure side on the permeate side. Due to different diffusion rates, the retentate is a solution that is more concentrated in ions than Feed 1, and the permeate is an aqueous solution that is less ionic than Feed 2.

[0027] In one embodiment of the process, the second ion-containing aqueous solution (Feed 2) has the same composition as the first ion-containing aqueous solution (Feed 1).

[0028] In another embodiment of the process, the second ion-containing aqueous solution (Feed 2) is a partial stream of the retentate.

[0029] A mixture of feed 1 and retentate is also possible, as is the addition or use of other aqueous electrolytes, for example other wastewater streams or solutions from other electroplating baths, to optimize the second feed stream.

[0030] In one embodiment of the process, the semipermeable membrane is a nanofiltration membrane. In another embodiment of the process, the semipermeable membrane is a reverse osmosis membrane.

[0031] In one embodiment of the process, the pressure on the overpressure side is greater than the difference between the osmotic pressures of the solutions on the overpressure side and the normal pressure side. In another embodiment of the process, the pressure (overpressure) on the overpressure side is less than 40 bar. In another embodiment of the process, the pressure on the overpressure side is in the range from 30 bar to 40 bar.

[0032] Particularly advantageous applications of the invention are nanofiltration and reverse osmosis processes, which require high pressures. Examples include the removal of metal ions from coating baths, e.g., the removal of aluminum ions from an anodizing bath, or the removal of metal ions from metal pickling baths, e.g., iron, copper, aluminum, or nickel pickling baths, where the concentration of the retentate stream is paramount. Another example is the generation of a low-salt and a high-salt solution from high-salt aqueous solutions, e.g., as a precursor in the production of drinking water from seawater, with a subsequently reduced-pressure conventional reverse osmosis process to produce drinking water from the low-salt permeate. The primary focus here is on reducing the ion concentration in the permeate stream.By using permeate, which has a lower salt content than seawater, in conventional reverse osmosis for drinking water production, the process can also be optimized so that the salt concentration in the retentate corresponds to the salt concentration of the seawater, so that no salinization of the coastal waters occurs when the retentate is returned to the sea.

[0033] Examples

[0034] Exemplary embodiments of the invention are illustrated in the following examples and the accompanying drawings and are further described with reference to the drawings.

[0035] Figure 1 schematically shows an embodiment of a device 1 according to the invention with a countercurrent module 10 comprising an overpressure region 11 and a normal pressure region 12, which are separated from each other by a semipermeable membrane. A pump 21 is located in the inlet of the overpressure region 11, which generates an overpressure in the overpressure region 11. A reducing valve 23 is located in the outlet of the overpressure region 11, which is designed to expand a liquid stream 200 leaving the overpressure region 11 to normal pressure. In the illustrated embodiment, a further reducing valve 22 is arranged between the outlet of the overpressure region 11 and the inlet of the normal pressure region 12, which is designed to expand a partial flow of a liquid stream leaving the overpressure region 11 to normal pressure and feed it to the inlet of the normal pressure region 12. Figure 2 schematically shows an embodiment of a countercurrent module 10.

[0036] The semipermeable membrane 13 comprises a plurality of membrane tubes or hollow fiber membranes, the ends of which are embedded in end walls 14. The end walls separate the normal pressure region 12 on the outside of the membranes from the overpressure region 11, which defines the space between the inlet 16 and one end wall 14, the lumen of the membranes 13 and the

[0037] Space between the other end wall 14 and the drain 17. The

[0038] Normal pressure zone 12 has an inlet 18 and an outlet 19. Baffles 15 are arranged between the individual membranes, which act as static mixing elements to intensify the mass transport.

[0039] Comparison example 1

[0040] Conventional nanofiltration of an anodizing bath is carried out without countercurrent using the device 1 shown in Figure 1. The first feed stream 100 contains approximately 65 g / kg Al2(SO4)3 and 20 wt.% sulfuric acid. The total ion concentration of the feed solution 100 is approximately 7 mol / l, resulting in a total osmotic pressure of the feed solution of approximately 170 bar at 293 K. For simplification, 90% acid transport and 10% Al ion transport with the diffusing water are assumed. These rates are achieved, for example, with a membrane 13 of the type NADIR® NP 030 (Microdyn Nadir GmbH) in a conventional nanofiltration process. The process parameters are summarized in Table 1. A value of 1,689,110.045 mm / h was used for ln(Ap(H2O)), 1.53E-02 mm / h for water diffusion, and 3.26E-02 mm / h for beta*A (H2O).

[0041] As can be seen from Table 1, conventional nanofiltration without countercurrent at 50% permeate flow 300 of feed stream 100 requires pressures greater than 70 bar. At 110 l / h feed stream 100, the ideal pumping power of pump 21 is 226.1 W, with an actual pressure difference in the nanofiltration module of 75.43 bar. Table 1: Conventional nanofiltration without countercurrent

[0042] Example 1

[0043] In the process according to the invention, a second feed stream 110 with the same composition as the first feed stream 100 is then added to the permeate side 12. The inlet stream 110 on the permeate side 12 is also the solution from the anodizing bath. Table 2 shows the results for the same retentate stream 200 as for the conventional nanofiltration in Comparative Example 1, i.e., with the same amount of Al ions removed.

[0044] To avoid increasing the membrane area compared to conventional NF, the pump pressure was adjusted to achieve approximately the same beta*A value for water diffusion (this value can be interpreted as the inverse of the transport resistance of the membrane module) as in the conventional process in Comparative Example 1. The second feed stream 110 on the unpressurized permeate side 12 was 25% of the first feed stream 100. A value of 1,720,688.454 mm / h for ln(Ap(H2O)), a value of 1.53E-02 mm / h for water diffusion, and a value of 3.20E-02 mm / h for beta*A (H2O) were used. Table 2: Countercurrent nanofiltration with bath solution as additional input

[0045] As can be seen from Table 2, the osmotic pressure differences AFI(H2O) are approximately 24 bar on the permeate 300 outlet side and 35 bar on the retentate 200 outlet side. The required ideal pumping power of pump 21 is reduced from 226.1 W to 141.5 W compared to Comparative Example 1, and the pressure difference in the nanofiltration module is only approximately 47 bar. The energy requirement is reduced by approximately 40% compared to Comparative Example 1. The countercurrent nanofiltration module 10, pump 21, and the overflow valve or reducing valve 23 are significantly more cost-effective than conventional nanofiltration.

[0046] If the volume flow of the second feed stream 110 with the same composition as the first feed stream 100 is significantly increased, the osmotic pressure difference AFI(H2O) on the outlet side of the permeate 300 is reduced to very low values, so that at the same pressure of the second feed solution 110, the average driving pressure difference for water transport through the membrane is increased, and at the same diffusing flow, the membrane area can be reduced accordingly, thereby realizing further cost reductions. Example 2

[0047] Instead of the second feed stream 110, a portion of the retentate stream 200 was released via the valve 22 and recirculated without pressure to the permeate side 12 into the countercurrent nanofiltration apparatus 10. Thus, the osmotic pressure difference on this side is 0, and the generated pressure is fully available for water transport.

[0048] Table 3 shows the results again for the same discharged retentate stream 200 as for the conventional nanofiltration in Comparative Example 1, i.e., with the same amount of Al ions removed. A value of 2,055,036.687 mm / h was used for ln(Ap(H2O)), a value of 1.91 E-02 mm / h for water diffusion, and a value of 3.35 E-02 mm / h for beta*A (H2O).

[0049] Table 3: Countercurrent nanofiltration with retentate as additional input

[0050] The osmotic pressure differences AF1(H2O) are approximately 23.5 bar on the outlet side of permeate 300 and 0 bar on the outlet side of retentate 200. The ideal pumping power of pump 21 is only 129.2 W and the pressure difference in the nanofiltration module is now only approximately 34.5 bar.

[0051] In further embodiments, a combination of the variants listed in the examples can also be selected, and with a second feed stream 110 mixed from bath solution and retentate partial stream, countercurrent nanofiltration can be optimized for the respective application. The addition or use of additional aqueous electrolyte solutions, for example from additional electroplating baths, to optimize feed stream 110 is also possible. In further embodiments, a cross-countercurrent apparatus with two feed solutions can also be used.

[0052] In surface technology, the combination of countercurrent nanofiltration with downstream diffusion dialysis leads to even higher recovery rates of bath acids or alkalis compared to the combination of conventional nanofiltration and diffusion dialysis, with reduced investment costs for nanofiltration and minimized wastewater quantities.

[0053] List of reference symbols

[0054] I Device

[0055] 10 countercurrent module

[0056] II Overpressure range 12 Normal pressure range

[0057] 13 Membran

[0058] 14 End wall

[0059] 15 Deflection plate

[0060] 16 Inlet Feed 1 17 Outlet Retentate

[0061] 18 Inlet Feed 2

[0062] 19 Permeate outlet

[0063] 21 Pump

[0064] 22 Reducing valve 23 Reducing valve

[0065] 100 Feed stream 1

[0066] 110 Feed stream 2

[0067] 200 retentate stream

[0068] 300 permeate stream

Claims

Claims: Device (1) for concentrating an aqueous solution (100) containing ions, comprising a module (10) with a first chamber (11) and a second chamber (12) separated by a semipermeable membrane (13), wherein the first chamber (11) has an inlet (16) for the aqueous solution (100) containing ions, which is connected to the pressure side of a pump (21), and the first chamber (11) has an outlet (17) for a retentate (200) connected to a reducing valve (23), and wherein the second chamber (12) has an inlet (18) for an aqueous solution (110) containing a second ion and an outlet (19) for a permeate (300). Device (1) according to claim 1, wherein the outlet (17) for a retentate (200) of the first chamber (11) is connected to the inlet via a reducing valve (22). (18) for a second ion-containing aqueous solution (110) of the second space (12). Device (1) according to claim 1 or 2, wherein the semipermeable membrane (13) is a nanofiltration membrane. Device (1) according to claim 1 or 2, wherein the semipermeable membrane (13) is a reverse osmosis membrane. Device (1) according to any one of claims 1 to 4, wherein the semipermeable membrane (13) comprises a bundle of tubular membranes or hollow fiber membranes, the ends of which are embedded in end walls (14) that form the second space (12) on the outside of the membranes. separating the first chamber (11) comprising the lumen of the membranes. Device (1) according to claim 5, wherein deflecting plates (15) are arranged in the second chamber (12) between the tubular membranes or hollow fiber membranes. Method for concentrating a first ion-containing aqueous solution (100), in which the first solution (100) is passed over the semipermeable membrane (13) under increased pressure on one side and a second ion-containing aqueous solution (110) is passed over the semipermeable membrane (13) at normal pressure on the other side of the semipermeable membrane (13) in countercurrent flow, whereby water from the first solution (100) passes through the semipermeable membrane (13) into the second solution (110) and a retentate (200) having a higher ion concentration than the first solution (100) and a permeate (300) having a lower ion concentration than the second solution (110) are obtained.A method according to claim 7, wherein the aqueous solution (110) containing the second ion has the same composition as the aqueous solution (100) containing the first ion. A method according to claim 7, wherein the aqueous solution (110) containing the second ion is a partial stream of the retentate (200). A method according to any one of claims 7 to 9, wherein the pressure in the aqueous solution (100) containing the first ion is in the range of 30 to 40 bar.

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