Osmotic pressure assisted reverse osmosis membranes and modules

Abstract

The application discloses an osmosis-assisted reverse osmosis (OsRO) membrane element and a configuration method thereof. The membrane element adopts a four-port design (two in and two out) to realize independent circulation of two feed streams on both sides of the membrane. In view of the process requirements specific to OsRO, the element is significantly optimized in structure: for hollow fiber membranes, the inner diameter is increased (63-2,000 μm) and the wall thickness is optimized for a transmembrane pressure difference of 10-120 bar; for spiral wound flat sheet membranes, the uniform distribution and pressure reduction operation of the second feed stream are realized by improving the permeate-side spacer in the membrane bag. In addition, the application proposes a double active layer structure, effectively solving the pollution problem caused by the fluids on both sides of the membrane, and allowing the desalination rate to be relaxed to 30%-99.9%, and the reduction of the transmembrane osmotic pressure difference is used to significantly save the process energy consumption. The element can also be made of high-performance materials such as PBI, PEEK and PVDF to adapt to the non-environmental temperature environment in the industrial process. The application solves the technical problem that the traditional RO or FO element cannot process the double-side feed stream under high pressure conditions, and has the advantages of high flux, high pressure resistance, anti-pollution and energy saving.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a membrane element and its modular device for Osmotic Pressure Assisted Reverse Osmosis (OsRO) process. Background Technology

[0002] In existing membrane separation processes, such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), forward osmosis (FO), and pressure-delayed osmosis (PRO), conventional membrane elements mainly adopt spiral wound or hollow fiber structures. In typical reverse osmosis applications, membrane elements are designed to retain solutes in high-concentration feed solutions to produce pure water with extremely low salinity. The physical mechanism lies in the osmotic pressure generated by the solute concentration difference between the feed side and the product water side. To overcome this osmotic pressure and achieve solvent flux, a corresponding external hydraulic pressure must be applied.

[0003] However, existing commercial membrane elements have significant technical limitations when dealing with high-concentration feed solutions and extreme environmental applications: A physical bottleneck in material pressure resistance: Because existing membrane elements are typically made of fine polymer materials, their mechanical strength is limited, resulting in their ultimate operating pressure usually only being maintained at around 70 bar. In reverse osmosis processes, this value constitutes the theoretical upper limit of pressure for processing a specific feed stream; when the osmotic pressure of the feed solution exceeds this threshold, the driving pressure is insufficient to overcome the osmotic pressure, the membrane element will lose its water production capacity, leading to process failure.

[0004] Fluid dynamics and port design limitations: All existing MF, UF, NF, FO, RO, and PRO membrane modules are designed with a two-port configuration, meaning they only have a single feed and discharge channel. This design cannot achieve the complex fluid management required by the OsRO process. The core advantage of the OsRO process lies in introducing a second liquid flow (i.e., a second feed / discharge channel) inside the membrane element to regulate the concentration gradient across the membrane, thereby effectively reducing the osmotic pressure load of the system and supporting continuous water production under higher salinity conditions. Existing two-port structures cannot meet the physical requirements of the OsRO process for a four-port configuration (i.e., two independent feed / discharge channels).

[0005] Mismatch between retention performance and application scenarios: Conventional RO membrane elements are designed to achieve extremely high desalination rates for producing pure water. However, in ZLD (Zero Liquid Discharge) or specific concentration applications, OsRO processes require membrane elements to possess specific performance characteristics, namely, maintaining a moderate salt rejection rate (e.g., 60-90%) at operating pressures up to 120 bar to achieve efficient concentration of the feed solution. Currently, commercially available industrial membrane modules cannot simultaneously meet the three core technical requirements of pressure resistance, specific retention range, and four-port fluid design.

[0006] Therefore, there is an urgent need to develop a new type of membrane element that can overcome the structural limitations of existing conventional membrane elements, possess high pressure resistance and multi-port flow channel design, in order to solve engineering problems in high-concentration solution treatment and ZLD applications. Summary of the Invention

[0007] Currently, reverse osmosis (RO) membrane elements are primarily manufactured using spiral wound flat sheets or hollow fiber structures. The fundamental difference between RO and OsRO membrane elements lies in the hydrodynamic function of OsRO elements, which introduce a second fluid flow into the membrane element. In conventional RO processes, membrane elements are used to remove salt from high-concentration feed solutions, reducing the salt content on the product water side to extremely low levels. The concentration difference between the feed and product water sides generates osmotic pressure, which directly corresponds to the applied hydraulic pressure required to overcome transmembrane resistance. Because membrane elements are made of polymer materials, their structural integrity limits the ultimate operating pressure to approximately 70 bar. This physical constraint sets an upper limit on the osmotic pressure for RO membrane elements to handle a specific feed flow (i.e., the feed osmotic pressure must be below 70 bar); when the feed osmotic pressure reaches this threshold, the RO membrane element will no longer produce any permeate flux (i.e., it will no longer produce water).

[0008] OsRO elements overcome the aforementioned pressure limitations by introducing a second feed stream with a salinity equal to or lower than the feed stream. In this mode, the permeate side no longer pursues extremely low salinity but exhibits a certain intermediate salinity. Although not immediately apparent, this intermediate salinity can be used to reduce the feed concentration entering the downstream existing RO system, thereby effectively reducing operating pressure, increasing permeate flow rate, and improving permeate quality. Equally important, compared to conventional RO elements, OsRO elements can produce a higher salinity brine, thus supporting deep concentration of the feed stream to meet the process requirements of Zero Liquid Discharge (ZLD). The specific design of the four-port membrane element significantly enhances its process performance; from an application requirements perspective, these improvements have subtle and non-intuitive characteristics. These characteristics will be discussed below for hollow fiber membrane elements and spiral wound flat sheet membrane elements respectively.

[0009] Membrane Materials: Membrane Materials and Preparation Technology The membrane materials involved in this invention are not limited to specific materials; any semipermeable membrane with component-selective permeation capability is applicable. Specific material types include, but are not limited to: Polymer membranes: Phase Inversion: The phase inversion process developed by Loeb & Sourirajen can be used to prepare asymmetric membranes with porosity and pore size gradients, achieving separation through the preferential diffusion difference between solute and solvent.

[0010] Thin Film Composite (TFC): Prepared by coating a thin, semi-permeable active layer onto a porous support layer. This thin active layer can be made of the same material as the support layer or of a different material.

[0011] Inorganic membranes include ceramic membranes, glass membranes, metal membranes, carbon membranes, and zeolite membranes. These membranes are typically prepared via a sintering process, but can also be prepared using other techniques well-known in the art.

[0012] Biological membranes: Currently in the early stages of research and development. These membranes are typically composed of lipid bilayers and utilize embedded proteins to achieve selective transmembrane transport of solutes.

[0013] These membranes can be hybridized with inorganic or synthetic organic membranes to form biological-synthetic organic composite membranes. For example, **aquaporin membranes** are a typical example of biological-synthetic organic composite membranes.

[0014] Module Configuration: The OsRO membrane module's inlet / outlet port configuration is highly flexible and can be designed according to specific process requirements.

[0015] Port quantity setting: The membrane module can be configured with 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 up to 100 ports. In the OsRO process, a configuration of 4 ports is preferred.

[0016] 3-port configuration: Primarily used for traditional RO processes.

[0017] 4-port configuration: Suitable for FO (forward osmosis), PRO (pressure-delayed osmosis), RO (reverse osmosis) and OsRO (osmosis-assisted reverse osmosis) processes.

[0018] The interface architecture requirements for OsRO modules are as follows: OsRO membrane modules must have at least two independent feed inlet / outlet port sets, and these two feed streams can originate from the same or different sources.

[0019] Configuration example: 4-port configuration: 2 ingress ports + 2 egress ports.

[0020] 3-port configuration: 2 ingress ports + 1 egress port.

[0021] 5-port configuration: 3 ingress ports + 2 egress ports.

[0022] 6-port configuration: 3 ingress ports + 3 egress ports.

[0023] Those skilled in the art can construct other port combinations besides those mentioned above according to engineering application requirements.

[0024] Structural constraints: In the OsRO process, if a tubular hollow fiber membrane structure is used, the membrane element must ensure that both ends are open. Attached image description: Figure 1 4-port design diagram Figure 2 Diagram of NF and RO plates composed of spiral winding. Figure 3 , 4 5: Water flow direction diagram Figure 6 Flat sheet membrane blade design diagram

[0025] Figure 1 This is an example of a 4-port module. The orange arrow represents feed flow 1, and the blue arrow represents feed flow 2. Feed flows 1 and 2 are separated by an OsRO membrane.

[0026] Tubular membrane module configurations: Hollow fiber, hollow microfiber, tubular, capillary, shell-less double-opening tubular membrane bundles. Flat plate module configurations: Helically wound, plates and frames, flat plate blades suspended in the feed solution such as in bioreactors, folded flat plates placed in a chassis with low or high hydraulic pressure. Parallel blades, dynamic, tubular ceramic, oscillating, coils. Hollow fiber membrane elements: Commercially manufactured hollow fiber membranes typically use cellulose triacetate (CTA) as the material. These hollow fiber elements are mainly used in reverse osmosis (RO), brackish water reverse osmosis (BWRO), and forward osmosis (FO) applications. Because the physical properties of these processes differ significantly from those of OsRO, the key parameters of the membrane elements must be specifically optimized for OsRO applications.

[0027] In practical engineering applications of hollow fiber membranes, there is a clear design trade-off between their structural parameters (especially the inner diameter) and the maximum withstand shell-side pressure, in order to adapt to different process operation requirements.

[0028] Specifically, the technical parameters for different membrane separation applications are configured as follows: Reverse osmosis (RO): To withstand high-pressure operating environments up to 80 bar, RO membrane elements are designed with a finer fiber structure, typically with an inner diameter of 53 μm or 63 μm. This compact design aims to optimize the mechanical stability of the module under extreme pressure.

[0029] Brackish water reverse osmosis (BWRO): Given its relatively low operating pressure requirements (maximum shell-side pressure of approximately 30 bar), BWRO membrane elements employ a slightly larger inner diameter design, typically 85 μm, thereby optimizing hydrodynamic performance while ensuring a certain level of pressure resistance.

[0030] Forward osmosis (FO): Unlike pressure-driven processes, FO membranes do not require significant shell-side pressure. Therefore, their design focuses more on the efficiency of fluid transport within the cavity, and the fiber inner diameter range is relatively wide, typically between 100 μm and 230 μm.

[0031] This evolution from narrow diameter (RO / BWRO) to wide diameter (FO) reflects the essential choice between "compressive strength" and "fluid flux optimization" in membrane engineering: when the process is mainly driven by mechanical pressure (such as RO), a narrower inner diameter helps maintain the structural integrity of the shell side; while in processes mainly driven by osmotic pressure (such as FO), a larger inner diameter is more conducive to reducing fluid resistance and improving mass transfer efficiency.

[0032] In conventional applications, the optimization logic for membrane element design involves permeate (permeate) flowing out through the fiber lumen. The outflow end can be single-ended or double-ended, depending on the module's configuration. The design typically aims to minimize the fiber diameter to maximize packing density within a limited module volume, thereby increasing the effective membrane area and reducing capital expenditure per unit of permeate (CAPEX).

[0033] The unique operating mechanism of the OsRO process requires the introduction of a second feed stream from the bore at one end of the fiber in order to reduce the osmotic pressure differential.

[0034] Mass transfer process: When pressure is applied to the shell side, the pressure drives water through the membrane wall into the fiber lumen and mixes with the second feed stream in the lumen.

[0035] Process output: This process results in a decrease in the salinity of the solution flowing out of the fiber lumen (i.e., producing a medium-salinity mixture), while the salinity of the concentrate flowing out of the shell side increases further.

[0036] Unlike traditional RO processes, the core parameter optimization of OsRO needs to focus on the hydrodynamic performance of the fluid in the fiber cavity and the effect of the internal feed on the mass transfer efficiency, so as to ensure that stable separation and concentration functions can be maintained under high pressure.

[0037] 1. Retention Rate Index and Energy Efficiency Optimization of OsRO Process In reverse osmosis (RO) and brackish water reverse osmosis (BWRO) applications, membrane elements are typically designed to have extremely high salt rejection rates (BWRO typically exceeds 99%, and seawater reverse osmosis SWRO can even reach 99.8%), in order to desalinate the feed water to a level that meets drinking water standards.

[0038] In OsRO applications, the rejection criterion of over 99% can be relaxed. In the OsRO process, a certain degree of mixing between the two feed streams is allowed as they pass through the membrane medium. This mechanism reduces the transmembrane osmotic pressure differential, thereby increasing transmembrane flux. By reducing the rejection rate of the target salt, the external pressure required to regulate salinity can be reduced, thus achieving energy savings. Depending on the specific salt composition and application scenario, the rejection rate can be relaxed to a range of 30%–99.9%; the preferred range is 40%–95%, and the more preferred range is 60%–90%.

[0039] 2. Design Differences and Parameter Optimization of OsRO and FO Membrane Elements: In forward osmosis (FO) hollow fiber membrane elements, the fiber inner diameter is typically larger (e.g., 230 μm) to accommodate the draw solution flow rate. However, in the FO process, the shell side does not need to withstand significant pressure because its process driving force comes from the osmotic pressure of the draw solution. Therefore, FO membranes have thinner walls to increase water flux, and there is no need to consider the risk of fiber collapse under high pressure.

[0040] In contrast, OsRO fibers require the shell side to withstand intermediate pressures of 10 to 120 bar (preferably 30 to 80 bar) and the bore side pressure of 0 to 20 bar during operation. Therefore, the wall thickness of the OsRO membrane must be specifically optimized according to the operating pressure differential required for the application. The suitable inner diameter range for OsRO membrane elements is at least 63 μm to 2,000 μm, preferably 85 μm to 230 μm, and more preferably 100 μm to 150 μm, with the wall thickness adjusted accordingly for the operating pressure differential (10 to 120 bar).

[0041] 3. "Activated Layer" Configuration and Antifouling Design: Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), brackish water reverse osmosis (BWRO), seawater reverse osmosis (SWRO), and forward osmosis (FO) membrane elements typically have only one "active layer" on the membrane surface. Here, the "active layer" refers to the membrane structure layer responsible for retaining target substances (such as particles, organic / inorganic matter, and microorganisms). In commercial hollow fiber membranes, the active layer typically faces the feed liquid to utilize its smooth surface properties for better retention of contaminants in the water.

[0042] In contrast, the feed flow of an OsRO membrane element acts on both sides of the hollow fiber, and the fluids on both sides may contain components that cause membrane fouling. Therefore, OsRO membrane elements can be designed with active layers on both sides (one layer on the inner surface and one layer on the outer surface). Of course, if the feed liquid undergoes rigorous pretreatment, the OsRO membrane can also be configured with only a single active layer.

[0043] 4. Material Selection and Temperature Resistance: RO and FO elements typically operate within a narrow temperature range near ambient temperature. However, in some industrial processes, OsRO elements must be used in conditions where fluid heating or cooling is not feasible. For such applications, suitable membrane materials include, but are not limited to: polybenzimidazole (PBI), polyethersulfone / polysulfone (PES / PS), polyetherimide (PEI), polyamide-imide (PAI), polyetheretherketone (PEEK), polyphenylsulfone (PPSU), polyvinylidene fluoride (PVDF), perfluoroalkoxy resin (PFA), regenerated cellulose, and fluorinated ethylene propylene (FEP). Module Configuration: Inlet / Outlet Configuration: 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 to 100 ports. Four ports are preferred.

[0044] The 3-port configuration is used for reverse osmosis (RO); the 4-port configuration is used for forward osmosis (FO), pressure delayed osmosis (PRO), reverse osmosis (RO), and osmosis-assisted reverse osmosis (OsRO).

[0045] The OsRO component requires at least two independent sets of inlet and outlet ports, and the feed streams can come from the same or different sources. For example: 2 inlets and 2 outlets represent a 4-port configuration; 2 inlets and 1 outlet represent a 3-port configuration; 3 inlets and 2 outlets represent a 5-port configuration; or 3 inlets and 3 outlets represent a 6-port configuration. Those skilled in the art can combine these to devise any further configuration schemes.

[0046] Tubular membrane module configurations include, but are not limited to: hollow fiber, hollow fine fiber, tubular, capillary, and double open tubular membrane bundle without housing.

[0047] Considerations for Flat Sheet OsRO: A spiral-wound flat sheet membrane consists of an "active" skin for desalination, a dense support layer, and a third porous support layer. A membrane spacer is introduced beneath this porous support layer, and a mirror-structured membrane sheet is placed on the other side of the spacer. The entire assembly is then sealed (glued) around a central feed tube into a bag-like structure and spirally wound for insertion into a pressure vessel tube. The inside of the vessel tube is pressurized by the feed flow, which then permeates through the outer surface of the membrane into the interior of the membrane bag / leaf. Desalinated water flows out through the central tube. This is a common membrane configuration, as shown in Figure 2. Although such flat sheet membrane assemblies may have four ports, their central tube is only used to discharge permeate and cannot be easily used to introduce new liquid flows.

[0048] Figure 2. Structural diagram of spiral-wound nanofiltration (NF) and reverse osmosis (RO) sheet membranes. The reason a second fluid stream cannot be introduced in this configuration is that the edges of the membrane bags / leaf are glued together with adhesive strips. Pressurizing the inside of the membrane bag / leaf would cause it to expand (similar to an air bladder), tearing the seal. Even a few pounds (psi) of pressure is sufficient to break the seal. Because liquid flows on both sides of the membrane in an OsRO element, the pressure inside the membrane bag / leaf must never be greater than the pressure outside. Another challenge that sheet membranes must overcome is that the second feed stream in an OsRO membrane can also contain contaminants that easily become trapped between the two support layers. Another reason for including a fourth port in the element is to allow for the introduction and discharge of a second feed stream.

[0049] For OsRO elements, the following design changes are required: In reverse osmosis (RO) and brackish water reverse osmosis (BWRO) applications, membrane element design typically requires extremely high desalination rates: BWRO needs to achieve... Seawater reverse osmosis (SWRO) typically achieves a desalination rate of over 99%, often reaching 99.8%, to ensure the desalinated water meets drinking water standards. However, in OsRO applications, this desalination rate standard can be appropriately relaxed. In the OsRO process, two fluid streams are allowed to mix through transmembrane migration, thereby reducing the transmembrane osmotic pressure difference and increasing the transmembrane flux. By lowering the desalination rate parameter of the target salt component, the effect of increasing or decreasing salinity can be reduced. This reduces the required pressure, thereby saving energy. Depending on the specific salt composition and application scenario, the desalination rate can be relaxed to a range of 30%-99.9%; the preferred range is 40%-95%, and the more preferred range is 60%-90%.

[0050] Component configuration: Import / export configuration: 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 to 100 ports. 4 ports are preferred.

[0051] 3 ports are used for reverse osmosis (RO); 4 ports are used for forward osmosis (FO), pressure delayed osmosis (PRO), reverse osmosis (RO), and osmosis-assisted reverse osmosis (OsRO).

[0052] The OsRO component requires at least two independent sets of inlet and outlet ports, and the feed streams can come from the same or different sources. For example: 2 inlets and 2 outlets represent a 4-port configuration; 2 inlets and 1 outlet represent a 3-port configuration; 3 inlets and 2 outlets represent a 5-port configuration; or 3 inlets and 3 outlets represent a 6-port configuration. Those skilled in the art can combine these to devise any further configuration schemes.

[0053] Flat sheet membrane assembly configurations include, but are not limited to: spiral wound, plate and frame, flat sheet membranes suspended in the feed liquid (such as bioreactors), and folded flat sheet membranes placed in a housing with a low-pressure or high-pressure hydraulic environment.

[0054] In OsRO sheet membrane elements, the design of the permeate spacer must not only allow the permeate flowing from the outside across the active layer into the membrane bag / sheet, but also ensure that the second feed stream is uniformly distributed within the membrane sheet and flows close to the active layer, thereby minimizing concentration polarization with minimal pressure drop. OsRO sheet membranes require moderate pressures of 10 to 120 bar (preferably 30 to 80 bar) on the outside. Depending on the specific application, the required feed pressure inside the membrane sheet is between 0 and 20 bar (preferably 0 to 10 bar). The permeate spacer must have channels that allow uniform, bidirectional diffusion of the second feed stream, while allowing easy drainage of the permeate from the first feed stream across the active layer. Figure 3 shows various membrane sheet designs for OsRO sheet membrane elements. Note: Blue arrows indicate water flow inside the membrane sheet, and gray arrows indicate water flow outside the membrane sheet.

[0055] : Figure 3 4, 5, OsRO flat sheet film design scheme 1.

[0056] Figure 6 OsRO flat sheet membrane design scheme 2 (open channel design): The high-pressure flow is distributed through the central tube, flows through the outside of the membrane sheet, and exits the membrane module through the opening formed by the winding. The low-pressure flow enters the interior of the membrane sheet through the adhesive seal edge, is distributed along the guide mesh inside the membrane sheet, and finally exits from the other side of the adhesive seal. The two fluids are isolated from each other inside the membrane shell.

[0057] Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), brackish water reverse osmosis (BWRO), seawater reverse osmosis (SWRO), and forward osmosis (FO) membrane elements typically have only one "active skin" on the membrane surface, which can be located on the inside or outside of the membrane element. The term "active" as used here refers to the portion of the membrane responsible for retaining target substances. These substances can include, for example, particulate matter, organic and inorganic materials, and microorganisms. For commercial hollow fiber membranes, the active layer typically faces the feed liquid to better retain pollutants present in the water using a smooth "active" skin. In contrast, OsRO membrane elements introduce feed streams on both sides of the hollow fiber, and both streams may contain compounds that could foul the membrane element. Therefore, OsRO elements can have two active layers, one on the inside and one on the outside of the membrane. However, if the feed stream is pretreated, OsRO can also have only one active layer.

[0058] Reverse osmosis (RO) and forward osmosis (FO) elements typically operate within a narrow temperature range (around ambient temperature). In some cases, OsRO elements can be used in industrial processes where heating or cooling process fluids is not always feasible. Materials better suited for these conditions include, but are not limited to: polybenzimidazole (PBI), polyethersulfone / polysulfone (PES / PS), polyetherimide (PEI), polyamide-imide (PAI), polyetheretherketone (PEEK), polyphenylene sulfone (PPSU), polyvinylidene fluoride (PVDF), perfluoroalkoxy resins (PFA), regenerated cellulose, and fluorinated ethylene propylene copolymer (FEP).

Claims

1. A permeation-assisted reverse osmosis (OsRO) membrane module, characterized in that, include: Membrane elements; The assembly includes a housing and at least four fluid ports configured to form two independent feed / discharge channels for two fluid streams to act on both sides of the membrane element. The assembly is configured to operate at a pressure of 10 bar to 120 bar and to have a rejection rate of 30% to 99.9% for the target salt.

2. The membrane module as claimed in claim 1, characterized in that, The membrane element is a hollow fiber membrane or a flat sheet membrane; when it is a hollow fiber membrane, its inner diameter is 63 μm to 2,000 μm; and the membrane element has at least one active layer on its inner or outer surface.

3. The membrane module as described in claim 1, characterized in that, The material of the membrane element is selected from at least one of polymers, inorganic materials, or bio-based materials.

4. A method for OsRO treatment using a membrane module as described in any one of claims 1-3, characterized in that, include: A first feed stream and a second feed stream are introduced into the two sets of independent flow channels, respectively, wherein the salinity of the second feed stream is equal to or lower than that of the first feed stream; Applying shell-side pressures of 10 bar to 120 bar drives mass transfer across the membrane, thereby achieving salinity regulation of the product stream.

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