Patent Application
20230066131
This disclosure relates to an improved forward osmotic separation system and method.
The present disclosure relates to a method and system for concentration and/or crystallization of salts from aqueous solution by means of Forward Osmosis processes, such as those implemented in industrial wastewater treatment systems, mining tails water treatment, purification of non-potable water, such as desalination of sea water, brackish water, wastewater, and in food processing, pharmaceutical industry and high purity applications. The same system may be used for de-concentration of draw solution (sea water) by permeate penetration from feed solution (wastewater).
Forward osmosis involves drawing feed water to be treated through a semi-permeable membrane 2 contained within a chamber 4 by the provision of a more concentrated draw solution on the other side of the membrane which creates a concentration gradient between the feed water and the concentrated draw solution. The membrane selectively permits water, but not salts, to pass into the concentrated solution and the water entering the concentrated solution dilutes the solution. The solutes are then removed from the dilute solution to generate potable product water, as illustrated in
The semi-permeable membranes can become clogged during normal operation through the accumulation of fouling media, such as minerals, organic particles, scaling microcrystals, bacteria and algae, on the membrane surface. A fouled membrane provides reduced separability of the dissolved salts, has a reduced flux rate and an increased pressure loss. Therefore, in order to maintain an efficient Forward Osmosis process, it is necessary to clean periodically the semi-permeable membrane.
Furthermore, forward osmosis performance is also limited by concentration polarization (CP), being the build-up of concentration gradients both inside and around the Forward Osmosis membranes during operation. In forward osmosis the feedwater solution becomes more concentrated on one side of the membrane and the draw solution becomes more diluted at the other, reducing the differential osmotic pressure and thus reducing solvent flow. These gradients reduce the effective osmotic pressure difference across the membrane and limit the attainable water flux. In this respect, when pressure is applied to the feed side of a membrane during the forward osmosis process, the solute is partially or totally retained by the membrane and will accumulate on the surface, while the solvent passes through the membrane more freely. Due to the membrane's solute retention, the concentration of the solute in the permeate is lower than the concentration in the feed. The concentration of the solute gradually increases on the surface of the membrane, due to solute accumulation from convective flow. At some point, the convective solute flow to the surface of the membrane will be balanced by the solute flux through the membrane and the diffusive flow from the membrane surface to the permeate side of the membrane. A concentration polarization profile, in which the concentration at the membrane surface (Cm) is typically higher than the permeate side of the membrane, will be established in the boundary layer.
This Concentration Polarization (CP) is the dominant component preventing effective operation of a Forward Osmosis process. In FO process CP is different in Draw solution site and in Feed solution. In feed solution CP is “Concentrative” (POrcp); in Draw solution site it is “Dilutive” (POscp). (See, e.g., Impacts of operating conditions and solution chemistry on osmotic membrane structure and performance. Mavis C. Y. Wong, Kristina Martinez, Guy Z. Ramon, Eric M. V. Hoek; Desalination 287 (2012) 340-349).
It is an object of the present disclosure is to provide an improved method and system for enhancing the efficiency of a forward osmosis process.
It is a further object of the present disclosure to provide an improved method and system for reducing fouling of membranes used in a FO process.
A first aspect of the present disclosure provides a forward osmosis separation system comprising:
a feed solution inlet for introducing a feed solution into a housing;
a draw solution distribution pipe for introducing draw solution into an internal enclosure, the draw solution having a higher salt concentration relative to the feed solution;
at least one semi-permeable membrane in the housing, said membrane being at least partially open to provide free membrane portions, the membrane forming the internal enclosure with the draw solution distribution pipe; and
at least one outlet for removal of a permeate solution from the enclosure, wherein a plurality of seeds are provided in the feed solution surrounding the enclosure and at least one of the draw solution inlet or outlet has a generator for providing at least periodically a plurality of directional pressure strokes in at least one of the draw and permeate solutions.
The combination of seeding in the feed solution, open and expanded membranes to provide free membrane portions together with the application of a plurality of directional pressure strokes in the draw or permeate solutions reduces fouling of the membrane and build-up of a concentration polarization, thereby providing a self-cleaning system having increased efficiency. Normally seeding cannot be used together with tight arranged membrane surface where distance for feed solution is less than one millimeter (feed spacer). Only when an open membrane is provided is it possible to use seeding effectively due to the larger distance between membrane. The opening and expanding of the membrane in large volume of feed solution, allows large amplitude membrane oscillation and enables the inclusion of a high seeding concentration.
Optionally, the system may further include a generator for applying a pulsed flow regime in at least one of the draw solution or the feed solution for increasing shearing force to achieve enhanced removal of concentration polarization and for preventing scale formation.
Preferably, an inlet for the draw solution is provided at a lower end of the distribution pipe and the outlet for the removal of the permeate solution is provided at an upper end of the distribution pipe. The directional pressure strokes are preferably applied via the distribution pipe. The directional pressure stokes may be provided by a generator provided in the draw solution inlet or the permeate solution outlet forming part of the distribution pipe. The generator may comprise piston or diaphragm, a pump or a valve.
In one embodiment, the system may include multiple internal enclosures provided by multiple semi-permeable membranes, the multiple internal enclosures being provided in a single housing with a common distribution pipe providing a feed solution inlet and permeate solution outlet. Preferably, the draw solution inlet introduces draw solution to a first enclosure, preferably via a lower end of the distribution pipe and the outlet of the first enclosure, preferably being an upper end of the pipe line for the first enclosure introduces solution into the next enclosure as the feed solution inlet. Again, a plurality of seeds is provided in the feed solution in the housing surrounding all the enclosures and a plurality of directional pressure strokes are applied, at least periodically, to the draw solution and/or permeate solution. The generator for application of the pressure strokes may be provided in a first and/or last draw solution inlet or permeate outlet or may be provided in the individual inlets and outlets of each internal enclosure.
A second aspect of the present disclosure provides a method for increasing driving force and water flux across a semi-permeable membrane during a forward osmosis process, the method comprising:
introducing a feed solution stream into a housing containing a semi-permeable membrane, the membrane being at least partially open and having free membrane portions, the membrane forming an internal enclosure wherein the feed solution surrounds the internal enclosure and includes a plurality of seeds;
introducing a draw solution stream into the internal enclosure, the draw solution stream having a higher salt concentration relative to the feed solution stream;
removing a permeate stream from the enclosure; and
at least periodically applying a plurality of directional pressure strokes in at least one of the draw or permeate streams.
Preferably, the draw solution stream is introduced into the internal enclosure through a lower end of a distribution pipe forming part of the enclosure. The permeate stream is preferably removed from an upper end of the distribution pipe. More preferably, the application of directional pressure strokes occurs in the distribution pipe.
The plurality of seeds may be continuously or intermittently introduced into the housing with the feed solution. Alternatively, the seeds may be introduced during initiation of the process only. The process may also comprise formation of seed particles in the feed solution stream.
Optionally, the process may further include applying a pulsed flow regime in at least one of the draw solution or the feed solution for increasing shearing force to achieve enhanced removal of concentration polarization and for preventing scale formation.
Embodiments of the disclosure shall now be described, by way of example only, with reference to the accompanying drawings in which:
The present disclosure relates to an improved method and system for reducing and/or preventing concentration polarization and/or membrane fouling in a forward osmosis process.
The efficiency of this process is reduced both by membrane fouling (i.e. salt accumulation 18 on the feed side of the membrane—see
CP “Dilutive” POscp takes place on the internal surface of enclosure 11 which faces the draw solution 19. Mechanism of CP “Dilutive” relates to dilution of draw solution by permeate 13 in internal membrane vicinity. It is known that concentration polarization very significantly diminishes the driving force that allows for permeate 13 penetration. Some distance from membrane feed surface the osmotic pressure of the Feed solution is POr but on the membrane it may be almost twice this concentration POrcp. Similarly, some distance from internal surface, the osmotic pressure of Draw solution is POs but, on the membrane, it can be almost half this concentration POscp.
The present disclosure provides means to diminish fouling and scaling on the membrane, as well as moving the concentration polarization layers and seeding particles away from the membrane, thereby reducing the need for membrane cleaning and increasing the efficiency of the FO process. This is achieved by the introduction of seed particles 17 into the module 10 surrounding the free membrane portions that form the enclosure 11 and combining this with the application, at least periodically, of a plurality of directional gauge pressure strokes more the 1 mm amplitude PGs, directed from the draw solution inlet 20 and/or the draw solution outlet 21 thereby effecting mechanical oscillation of the free membrane portions 11 from position 12 to position 22 to detach foulant from the membrane and reduce the concentration polarization at the membrane surface (see
The application of periodical directional pressure strokes to a reverse osmosis process is described in the Applicant's earlier publication WO 2016/024179, the entire contents of which are incorporated herein by reference thereto. The generation of water pressure strokes causes oscillation of thousands of displaceable free membrane portions of the semi-permeable membrane, serving to reduce concentration polarization across the membrane support layer and prevent membrane fouling. In WO 2016/024179, the tight arrangement of the membrane limits the amplitude of oscillation to a few microns. In contrast, embodiments of the present disclosure use open and expanded membranes to allow oscillation of a few centimeters. An innovation step is in opening and expand membrane in large volume of feed solution, allowing large amplitude membrane oscillation and the implementation of a high concentration of seed particles.
The system and process of embodiments of the disclosure may also include, optionally applying during at least part of the FO process, a pulsed flow regime in at least one of the draw solution and/or feed solution stream, thereby increasing shearing force for achieving enhanced removal of concentration polarization seeding particles from the membrane surface and for preventing scale formation, thereby enhancing the Forward Osmosis process.
Any appropriate seed material may be introduced into the module 10 surrounding the membrane enclosure 11. It is to be appreciated that the seed particles may be introduced continuously, periodically or at initiation only. The seeding process prevents crystallization of the salts at the most concentrated part of the feed solution, instead causing crystallization of the salts on the seed particles.
In this manner, embodiments of the present disclosure serve to diminish concentration polarization on both sides of the membrane thereby increasing the driving force (NDFcp) and the FO flux 13 (see
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003685.1 | Mar 2020 | GB | national |
This application is a National Phase entry of International Application No. PCT/IB2021/052043 under § 371 and claims the benefit of GB Patent Application No. 2003685.1, filed Mar. 13, 2020, which is hereby incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/052043 | 3/11/2021 | WO |
