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1. WO2020110153 - SYSTÈME D'EXTRACTION CONTINUE D'EAU PURE À PARTIR DE CHARGE D'ALIMENTATION AVEC UNE RESATURATION ET UNE RÉUTILISATION DE L'EXTRACTION

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

TITLE OF THE INVENTION:

SYSTEM FOR CONTINUOUS EXTRACTION OF PURE WATER FROM FEEDS WITH RESATURATION AND REUSE OF DRAW

FIELD OF INVENTION

The present invention relates to a system and process for energy efficient extraction of pure water from a variety of feeds with resaturation and reuse of draw for diverse end uses. More preferably, the present invention, provides a Forward Osmosis (FO)-based system for continuous extraction of pure water directly into industrial starting materials through their use in the form of draw solutions with recycle and reuse of minor portion of the outlet draw in subsequent cycles and making available the rest for industrial processes. The invention takes advantage of the high osmotic pressures of certain saturated draw solutions such as saturated sodium chloride, potassium chloride, urea and raw sucrose to permeate pure water directly into the respective draw solutions from appropriate feed solutions such as seawater, RO reject water and sugarcane juice through the well-known process of Forward Osmosis (FO). Thereafter the under-saturated draw solution obtained at the outlet is re-saturated through dissolution of more solute online, a portion of the resultant re-saturated solution is recycled into the FO unit to maintain a continuous and constant flow of the Draw through the FO module while diverting the major portion as feedstock for diverse industrial applications.

BACKGROUND AND PRIOR ART OF THE INVENTION

NaCl is at the heart of several high volume industrial products. For example, soda ash and baking soda are produced from NaCl through the well-known Solvay process. The membrane-based chlor-alkali process, which yields caustic soda and chlorine, also utilizes NaCl as feedstock. In both cases, the NaCl is utilized in dissolved form and it is necessary to ensure that impurities such as Mg2+, Ca2+, SO42 are minimized. Thus, salt used should be as pure as possible and, similarly, presence of such impurities in the dissolution medium, i.e., in water, should also be as low as possible. In many instances plants are installed in coastal locations where the primary feedstock, namely NaCl, is readily obtainable from seawater. However, such locations are faced with the problem of fresh water shortage and the only practically available source of water is seawater. Although NaCl is the primary constituent in seawater also, there are many other dissolved salts that are co-present as would be evident from the Table 1 below.

Table 1. Major ions in seawater having total salinity of 35 g/L (https://www.soest.hawaii.edU/oceanography/courses/OCN623/Spring%202015/S alinity2015web.pdf)


It is therefore not feasible to use seawater directly as the dissolution medium since it would greatly increase the downstream purification cost. One approach that is adopted to circumvent the problem is to use desalinated seawater. Seawater can be desalinated thermally to provide condensate water that is pure. Alternatively, desalination can be effected through more energy-efficient membrane-based processes such as electrodialysis and Reverse Osmosis (RO) - and more particularly RO - but, even so, RO too is a thermodynamically uphill process that requires operation at high pressure (55-70 bar). Consequently, the power consumption in operating a seawater RO plant is >3 KWH per m3 of purified permeate water stream and more typically 5 KWH per m3 or more depending on plant size and scale of operation. It is stated, for example, by Reverter et al. of Hydronautics (www.membranes.com/does/papers/24_liirtaspl .pdf) /hat: “The operating parameters of seawater RO systems are mainly function of feed water salinity and temperature. For example, for seawater feed of about 38,000 ppm TDS salinity and water temperature in the range of 18-28 C, the RO systems are designed to operate at a recovery rate in the range of 40%-45% and with an average permeate flux in the range of 11.9-13.5 L/m2-hr. At the above operating conditions the feed pressure is in the range of 800-1000 psi (55-70 bar) and permeate salinity is in the range of 300-500 ppm TDS (total dissolved solids).’’Considering the case of a plant producing 750,000 TPA of soda ash, and further considering that 1.67 ton of NaCl is required per ton of soda ash in a conventional Solvay plant, and further that saturated brine contains 0.260 ton of NaCl and 0.740 ton of water, the daily requirement of fresh water can be computed to be of the order of 9766 m3 per day. If such water is generated by seawater RO, the total daily power requirement would be 48,832 KWH considering 5 KWH as power requirement per m3 of fresh water permeate. If one takes into consideration all such plants, the daily power consumption on account of desalination would be many times higher. The primary objective of the present invention, therefore, is to devise a more energy efficient solution to the above problem.

Reference may be made to the article:“FO: Principles, applications, and recent developments” by T. Cath et al. (J. Membr. Sci. 281 (2006) 70-87). It can be seen that unlike RO which is a non-spontaneous process, FO is a thermodynamically favourable process where pure water permeates spontaneously across a membrane from Feed side having lower osmotic pressure to Draw side having higher osmotic pressure. It is further stated in the above article that:“FO has been used to treat industrial wastewaters (at bench-scale), to concentrate landfill leachate (at pilot-and full-scale), and to treat liquid foods in the food industry (at bench-scale). FO is also being evaluated for reclaiming wastewater for potable reuse in life support systems (at demonstration-scale), for desalinating seawater, and for purifying water in emergency relief situations.” A related application is pressure retarded osmosis that aims to harness energy. Numerous articles have been published during the last few decades that have expanded the versatility of the technique. Reference may be made to the article:“Osmotic production of sterile oral rehydration solutions— an economic, low-technology method” by Wilson et al. (Trap Doct. 1993, 23, 69-72) wherein it is stated that a “sterile oral rehydration solution can be produced by immersing in water a semi-permeable cellulose tube containing glucose and salts. Osmotically-driven ultrafdtration excludes all microbes and particulate matter even when the immersion water contains 45 x 10(6) cfti/ml of Pseudomonas aeruginosa, 25 x 10(7) cfti/ml of Staphylococcus aureus or 20 x 10(7) cfti/ml of Escherichia coli. The process has potential for producing sterile solutions for injections and intravenous use in situations with very limited and simple resources, in emergencies and during natural disasters.” The article further states that further studies are needed to determine whether the method can be adapted to provide the large quantities of oral rehydration fluid needed in field conditions and no reference is made to continuous production of saturated solutions of important industrial raw materials such as salt, sugar, fertilizer, etc., through the proposed approach. Reference may also be made to the article“Channelizing the osmotic energy of proximate sea bittern for concentration of seawater by FO under realistic conditions to conserve land requirement for solar sea salt production” (Honmane et al. Journal of Membrane Science 567 (2018) 329- 338) which teaches the continuous dewatering of seawater for more efficient salt production utilizing sea bittern as osmotic agent. The diluted sea bittern is discarded to sea. From the utility perspective, the emphasis of the paper is on dewatering of seawater and no reference is made in the above prior art on the use of FO to continuously produce pure saturated brine from sodium chloride and seawater as it relates to the production of soda ash and chlor-alkali. Reference may be made to the article by Fi et al. (X. Fi, T. He, P. Dou, S. Zhao (2017) 2.5 Forward Osmosis and Forward Osmosis Membranes. In: Drioli, E., Giomo, F., and Fontananova, E. (eds.), Comprehensive Membrane Science and Engineering, second edition vol. 2, pp. 95-123. Oxford: Elsevier) which proposes the harnessing of fresh water from sub-soil brine for dissolution of NaCl. The article further states that a part of the fresh water requirement can be met in this manner but no reference is made to any process wherein the entire requirement of fresh water can be met in this manner through a continuous process. The above approach need not be confined to brines alone. For example, one may require fresh water to dissolve fertilizers for applications such as drip irrigation, especially in water stressed areas, and fresh water may not be available for this purpose. It is the experience of the authors that in many such locations, the groundwater quality is poor and reverse osmosis plants have been set up to provide pure water for drinking, cooking and other applications also. Now, if fresh water can be harnessed from the saline reject water generated in these plants, then that too would be of great interest. Another application that also can benefit from the above approach is the preparation of large volumes of brines of varying densities which are required in the process of oil recovery. Here again, fresh water is essential for preparation of some of the brines and such fresh water is not available in many cases, and certainly not in offshore locations. Thus if FO can be utilised to directly prepare saturated brines as required by the oil industry from various salts and seawater that would be a great advantage. The invention proposed herein can have still wider utility with certain essential further inventions. For example, one could consider many situations wherein there is a need for crystallisation of a crude product, on the one hand, and concentration of a feed, on the other hand. Take, for example, the case of production of pure NaCl through recrystallization of crude NaCl and, likewise, production of refined sugar from raw sugar. These applications too require fresh water which may be hard to come by. If the fresh water requirement can be met from the corresponding feeds, i.e., seawater and sugarcane juice, respectively, in energy efficient manner, not only would it fulfil the requirement of fresh water for the purpose of recrystallization, it would additionally reduce the feed volumes and, accordingly, lower the evaporation load for production of the crude solids therefrom. In all of the above cases, a common difficulty that is encountered during FO is that, once the solid that serves as osmotic agent or draw solution becomes diluted from the ingress of permeate water from feed side, it loses its potency and hence only a limited amount of fresh water can be conserved since the solids, to begin with, must be dissolved in fresh water to initiate a continuous FO process. These major limitations have been overcome through the system and process of the present invention described in more detail below.

OBJECTIVES OF THE INVENTION

1. Accordingly, the primary objective of the present invention is to employ solid feedstocks in the form of their saturated solutions possessing high osmotic pressure as draw to pull in pure water from feed side of FO process in energy efficient and continuous manner.

2. Another objective is to devise a system that allows for such harnessing of pure water to meet the primary objective of (1) above.

3. Another objective is to extract pure water continuously for dissolution of pure solids that can directly be used in applications such as the Solvay process.

4. Another objective is to extract pure water continuously for dissolution of crude solids, such as crude salt and crude sugar, for the purpose of producing refined products therefrom through recrystallization.

5. Another obj ective is to extract pure water from feeds especially suited to the context, such as seawater for brine preparations for applications such as in the Solvay process, chlor-alkali process and oil recovery process or for recrystallization of crude salt; sugarcane juice or washings or spent wash for recrystallization of crude sugar; RO reject water for preparation of salt, sugar and fertilizer solutions appropriate in a rural context, etc.

6. Another objective is to avoid use of any constituent in the draw that might endanger the FO membrane and, instead, add these constituents to the outlet draw, and more particularly to that fraction of the draw which will be used in downstream operation.

7. Another objective is to balance out permeate flux and extent of dilution of draw during FO depending on specific requirements.

8. Another objective, when utilizing seawater as feed in FO, is to match or exceed the permeate flux obtained in a standard seawater RO plant so that

membrane area requirement is comparable or lesser.

SUMMARY OF THE INVENTION

The present invention relates to a system and process for energy efficient extraction of pure water from a variety of feeds with resaturation and reuse of draw for diverse end uses. The invention takes advantage of the high osmotic pressures of certain saturated draw solutions such as saturated sodium chloride, potassium chloride, urea and raw sucrose to permeate pure water directly into the respective draw solutions from appropriate feed solutions such as seawater, RO reject water and sugarcane juice through the well-known process of Forward Osmosis (FO).

Thereafter the under-saturated draw solution obtained at the outlet is re-saturated through dissolution of more solute online, a portion of the resultant re-saturated solution is recycled into the FO unit to maintain a continuous and constant flow of the Draw through the FO module while diverting the major portion as feedstock for diverse industrial applications. The invention would be most advantageous in locations where there is dearth of pure water for required end uses such as preparation of brines for Solvay process in coastal locations, off shore oil recovery, dissolved fertilizers for drip irrigation in arid locations, raw sugar solution for recrystallization in sugar refineries confronted with shortage of fresh water, etc. It would also be very suitable in applications where the feed stands to gain from its concomitant dewatering.

The present invention relates to a system and process for energy efficient extraction of pure water from a variety of feeds with resaturation and reuse of draw for diverse end uses. Hence, the present invention provides a Forward Osmosis (FO)-based system for continuous extraction of pure water directly into industrial starting materials through their use in the form of draw solutions with recycle and reuse of minor portion of the outlet draw in subsequent cycles and making available the rest for industrial processes, said system comprising of:

- FO module consisting of semi-permeable membrane,

- inlet and outlet flow conduits connected to the FO module for contacting feed solution (FS),

- inlet and outlet flow conduits connected to FO module for contacting draw solution (DS),

- metering pumps for circulating the solutions,

-a dissolver module for contacting a diluted draw solution with a solute, preferably having external temperature-control jacket to control temperature range from 10 to 50 °C,

-a clarification module for removing the undissolved solute from saturated draw solution,

-in-line dissolved solute concentration detector,

-two conduit split gate valves for splitting the volume of outlet draw solution one for splitting before re-saturation and the other for splitting after re saturation with solute; wherein, conduit split gate valve is selected from two way flow-divider, control valve, ball valve.

An important embodiment of present invention is that, the system of present invention is functional in that the system allows continuous extraction of water from feed solution by recycling a minor part of diluted Draw solution through the FO module in subsequent cycles after re-saturation to maintain a constant flow rate while diverting the major portion of diluted draw solution for further downstream processing.

In accordance to important embodiment, the system is comprising of metering pumps are selected from centrifugal pump, diaphragm pump, gear pump, peristaltic pump, lobe pump, piston pump, meter pump, pneumatically driven pump. Whereas, dissolver module is selected from PVC, FRP, HDPE, HDPE-lined-MS, MS, SS Stirred tank Dissolver/Mixer fitted with propeller, continuously fed solute bed with up flow or down flow of outlet draw;

In accordance one more embodiment, the clarification module associated with present invention system is operated on the principle of coagulation, flocculation, sedimentation, flotation, mesh screening, cloth filtration, microfiltration, centrifugation.

The solute concentrator detector is selected from total dissolved solids meter, water activity detector, refractive index detector, density meter, spectrometer, and chromatography unit. Additionally, the turbidity of clarified stream is measured with turbidity meter;

The present invention also provides a process for continuous extraction of pure water from feed with re-saturation and reuse of draw using disclosed system and the process comprising steps of:

Step a) feeding a draw solution and a feed solution from the respective inlets into an FO module having a partitioning semi-permeable membrane between both the solutions to permeate pure water from feed solution into draw solution through osmotic pressure difference, wherein draw solution is at higher osmotic pressure than feed solution. More preferably, the feed and draw inlet gauge pressures are in the range of 0.1-2 bar,

Step b) collecting the diluted draw solution and concentrated feed solution from the respective outlets,

Step c) contacting the undersaturated draw solution with additional solute of the same type, on the one hand, and, on the other hand, contacting a minor part of the undersaturated draw solution with the same solute to make it a saturated draw solution once again while contacting the major part of the undersaturated draw solution with a different solute as per intended application,

Step d) stirring and increasing the temperature to dissolve the solute rapidly and completely, wherein temperature is selected between 10 to 50 °C,

Step e) clarifying the saturated solution to remove undissolved solute from the solution,

Step f) Recirculating the clarified re-saturated draw solution in subsequent cycles ofFO.

BRIEF DESCRIPTION OF DRAWINGS:

For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which:

Figure 1: Schematic depiction of system and process for auto generation of solutions of industrial solutes that double up as osmotic agents and pull in continuously the required amount of pure water permeate from appropriate feed solutions

DESCRIPTION OF THE INVENTION

To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration.

Clarification of feed and draw streams was undertaken employing cloth filtration or microfiltration. Seawater was clarified initially by means of alum treatment. Peristaltic pumps were supplied by Electrolab India Pvt. Ltd. Specific gravity (p) of the feed and draw at inlet and outlet were measured at 27 °C using Anton Par DMA5000 Density Meter. Total dissolved solids was measured using a TDS meter (TDS-3, HM Digital Inc., USA) and °Brix was estimated from specific gravity data. Ionic constituents in inlet and outlet brine were characterized using an ion chromatography set up.

It is well known in the prior art that when two aqueous solutions A and B having osmotic pressure values of pi and U2, respectively - wherein ii < ni - are separated by a semi-permeable membrane, pure water permeates from A to B and the driving force is proportional to Dp (m-m). Many solutes of great

commercial importance, such as NaCl, sucrose, urea, muriate of potash, etc., exhibit low water activities (aw) when prepared as saturated solutions. It is well known in the prior art that the osmotic pressure of a solution can be related to its water activity through Equation 1.

p = -RTln(aw)/V (Equation 1)

wherein p is the osmotic pressure in atm, R is the Gas Constant (R=0.082057 L atm ihoG' K 1 ). T is the solution temperature in Kelvin, and V is the molar volume of water. NaCl solution at the saturation point having 26.5% by weight of NaCl has an aw value of 0.751-0.753 in the temperature range of 15 °C-35 °C, respectively (D. Kitic et al., Journal of Food Science, 1986, 51, 1037-1041). This translates to an osmotic pressure of 392 bar at 298 K. On the other hand, the osmotic pressure of seawater is only ca. 26 bar. Thus there is a large driving force for spontaneous supply of pure water from seawater to saturated brine. 60 °Brix sucrose solution exhibits aw of 0.8868 (Miyawaki et al. Biosci. Biotech. Biochem. 63 (1997) 466-469) which would yield an osmotic pressure of 165 bar at 298 K. Saturated solution of muriate of potash (KC1) fertilizer has aw of 0.842 (LWT -Food Science and Technology, 1996, 29, 376-378) and urea too has high solubility in water and consequently low water activity. In comparison to the above, where the osmotic pressures of draw solutions at the inlet are in excess of 100 bar in all the cases, the aw values of seawater, sugarcane juice and brackish RO reject water are in the range of 5-25 bar. Thus, these industrially important solutes would be able to readily pull out water from feed solutions of practical relevance such as those described below. And it is even feasible to operate FO at high F/D ratio if required.

As the process of FO begins and continues, the osmotic pressure on the draw side reduces while that on the feed side decreases. This limitation has been overcome in the present invention through the system and process of the present invention for dissolution of additional solid into the diluted draw solution at the outlet. Thus if one starts with an initial mass of X kg of draw and osmotic pressure of Jii bar passing through the FO module, at the outlet the mass becomes (X+Y) kg and the osmotic pressure becomes m bar, where
and Y = permeate water in kg

transported from feed side to draw side. If the outlet mass is resaturated with the solid, then its mass becomes Z kg where Z>X. Thus if X kg is recycled back for continuity of the FO process, then (Z-X) kg becomes the surplus amount which can be utilised for downstream applications. This crucial inventive step allows for continuous mining of fresh water for intended downstream applications after the initiation process. As the number of recycles becomes higher and higher, the initial requirement of fresh water becomes, as a percentage, a miniscule figure. Another invention which broadens the scope still further is the recognition that the draw solution obtained at the outlet can be bifurcated into two parts; one part could be resaturated with the same solid for the purpose of continuous FO whereas the major part, which goes for downstream application, can be reconstituted not only with the same solid but with other solids also. This approach may be especially important in certain specific cases such as when the additional constituents pose risk in the FO operation and are thus best added post-FO. Depending upon the specific requirements the invention can be practiced in a variety of other ways, for example the mass flow rates and feed to draw ratio can be manipulated to achieve higher permeate flux or higher extent of draw dilution as may be desirable in specific cases. These will be illustrated through the examples to follow.

In accordance to one of embodiment, the present invention provides a system for energy efficient extraction of pure water from a variety of feeds with resaturation and reuse of draw for diverse end uses. The said system and process of the present invention that enables the above objectives to be achieved is depicted schematically in Figure 1 with the numbering scheme as follows:

1 - FO Membrane module

2 - Feed source

3 - Inlet feed with Metering Pump to pump 2 through lumen side of 1 with continuous flow

4 - Saturated Draw (entire amount used as inlet draw for initiation and subsequently minor fraction used for recycle as inlet draw and major fraction for downstream use)

5 - 2-way Control valve for bifurcation of 4

6 - Inlet draw with Metering Pump to pump 4 through shell side of Module 1 with continuous flow

7 - Pressure gauge for Feed at inlet

8 - Pressure gauge for Draw at inlet

9 - Concentrated Feed at outlet

10 - Under-saturated Draw at outlet

11 - Pump for circulation of 10 for 12-19

12 - 2-way Control valve for bifurcation of under-saturated outlet draw (10) prior to resaturation with solute used in FO process.

13 - Surplus 10 for direct use or after addition of other constituents

14 - Solute addition for re saturation of a part or all of 10

15 - Solute dissolver

16 - Re-saturation sensor

17 - Clarifier

18 - Turbidity sensor

19 - For downstream use of 4

Accordingly, the present invention provides a Forward Osmosis (FO)-based system for continuous extraction of pure water directly into industrial starting materials through their use in the form of draw solutions with recycle and reuse of minor portion of the outlet draw in subsequent cycles and making available the rest for industrial processes,

The present invention also provides a process for continuous extraction of pure water from feed with re-saturation and reuse of draw using disclosed system and the process comprising steps of:

Step a) feeding a draw solution and a feed solution from the respective inlets into an FO module having a partitioning semi-permeable membrane between both the solutions to permeate pure water from feed solution into draw solution through osmotic pressure difference, wherein draw solution is at higher osmotic pressure than feed solution. More preferably, the feed and draw inlet gauge pressures are in the range of 0.1-2.0 bar,

Step b) collecting the diluted draw solution and concentrated feed solution from the respective outlets,

Step c) contacting the undersaturated draw solution with additional solute of the same type, on the one hand, and, on the other hand, contacting a minor part of the undersaturated draw solution with the same solute to make it a saturated draw solution once again while contacting the major part of the undersaturated draw solution with a different solute as per intended application,

Step d) stirring and increasing the temperature to dissolve the solute rapidly and completely, wherein temperature is selected between 10 to 50 °C,

Step e) clarifying the saturated solution to remove undissolved solute from the solution,

Step f) Recirculating the clarified re-saturated draw solution in subsequent cycles ofFO.

The various processes that can be carried out with the system as described above are now illustrated through the following examples. Examples 1-4 pertain to applications of the present invention that are related to brine preparation and salt recrystallization. Examples 5-6 pertain to applications of the present invention that are related to supply of pure water for recrystallization of raw sugar with concomitant dewatering of sugarcane juice. Examples 7-8 relate to harnessing pure water from RO reject water feed for the express purpose of making solutions of the solutes salt, sugar, fertilizer that are relevant in the local context with simultaneous dewatering of the feed to facilitate the process of evaporating it further to dryness in a drying bed.

Example 1: Applications of the present invention to NaCl solution as an industrial starting materials:

NaCl solution that is used in soda ash and chlor-alkali production must be saturated and have minimal impurities. Such salt is normally obtained from seawater through evaporative crystallization. It is then subjected to further purification in a twin screw washery to achieve purity levels in excess of 98% on dry weight basis. The salt is then re-dissolved in pure water obtained naturally or through desalination and traces of impurities that still remain are removed after dissolution through well-known techniques such as lime-soda treatment. In situations such as the modified Solvay process - also known as the Dual Process -the salt needs to be of highest purity since downstream purification after dissolution is not feasible in this case. In that case and all other applications requiring extremely high purity of salt, recrystallization is an important means of refinement. Although recrystallization has been performed using even seawater (see for example US Patent 9090478), it is generally the norm that such aqueous recrystallization is effected utilizing pure water as dissolution medium.

An experiment was conducted in continuous single pass mode on a 2.3 m2 hollow fibre FO membrane module from M/s Aquaporin, Denmark with natural seawater collected from Arabian sea as feed and near-saturated NaCl solution (25% NaCl w/w prepared by dissolving pure NaCl as obtained above in deionized water) as draw, with both feed and draw maintained at room temperature. A high feed to draw ratio was selected in the present example, the rationale for the same being the following: Considering that 1 ton of saturated NaCl solution would have 0.22 tons of NaCl and further that the production of 0.22 tons of NaCl would entail processing of ca. 8-12 ton of seawater depending on its salinity, the FO experiment was conducted using high (7: 1-10: 1) mass flow rate ratio of seawater to saturated NaCl.

The partially dewatered seawater, 6, can be put back in salt pans for production of NaCl in those cases where salt production and downstream application are integrated. If, on the other hand, the outlet draw 6 in Figure 1 is discharged to sea - this would be the case, for example, in the offshore FO assembly for application in oil recovery. In that case the feed to draw ratio may be selected considering other criteria such as pumping cost, etc. In one such experiment, the feed and draw flow rates through the inlet lumen and shell sides, respectively were maintained at a steady 0.4223 kgmin 1 and 0.0581 kgmin 1, respectively. Flow rates were controlled with the help of peristaltic pumps. 4.223 kg of feed and 0.581 kg of draw were passed over 10 minutes. The mass of the feed at the outlet reduced to 2.985 kg while the mass of the Draw at the outlet increased to 1.802 kg. Data on mass balance and associated parameters, particularly the permeate flux, are provided in Table 2. When deionised water was added into the outlet feed to the same extent that permeated out, the original density was restored. Thus, the back diffusion of solute from draw solution to feed solution appeared to be not so significant.

Table 3 provides ion chromatographic analyses of inlet and outlet draw solutions. It can be seen that no additional constituents were present at the outlet. Moreover, the observed change in concentration at the outlet matched reasonably the expected lowering of concentration of draw due to dilution by permeate from feed.

Table 2. Relevant data of mass balance and associated parameters pertaining to the experiment of Example 1



This example teaches pure water permeation from seawater feed to a saturated NaCl solution which can thereby provide in cost-effective manner pure water required for brine preparation.

Table 3. Ion chromatographic data of constituents in inlet and out Draw solutions pertaining to the Experiment in Example 1.



Example 2: Effect of change in the flow rates and F/D ratio:

The experiment of Example 1 was repeated, albeit with small changes in the flow rates and F/D ratio (see data in Table 4). The mass of outlet Draw, which was prepared from 0.212 kg of NaCl and 0.636 kg of deionized water, increased from 0.848 kg to 2.554 kg due to permeation of 1.706 kg of water from seawater feed over 10 min of continuous single pass FO. Thus the fraction of pure water in the outlet draw which was harnessed from seawater by the process of FO was 73%. When the entire amount of outlet draw was re-saturated by adding solid NaCl and clarified in salt dissolver 15 and clarifier 17, respectively, of Figure 1 the total mass of draw increased to 3.122 kg. This was then distributed using the 2-way control valve, 5. The minor portion measuring 0.770 kg was recycled in cycle 2 of FO to maintain continuity of operation, while the major portion of 2.352 kg became available as pure brine for downstream processes such as the Solvay process. The draw amount at the outlet increased to 2.426 kg in cycle 2, i.e., the amount of permeate was 1.656 kg.

Thus, after the second cycle of FO, the fraction of pure water originating from seawater in the brine used as draw increased to 91%. As in the case of Example 1,

it was verified that the specific gravity of seawater used as Feed did not change at all when the concentrated seawater outlet Feed was diluted with deionized water equivalent in amount to the pure water that permeated out from seawater through the FO membrane process (compare the specific gravity values of 1.02796 for original seawater and 1.02746 for make-up seawater in Table 4).

Table 4. Mass balance and related data for the Experiments described under

Example 2.



This example teaches us that re-saturation of outlet Draw with NaCl - followed by recycle of a minor portion into the FO process, while making available the rest for downstream use - enables pure water to be continuously extracted from seawater in cost-effective manner for the purpose of brine preparation and that after a few successive cycles, the fraction of pure water from deionised water used to initiate the process would become negligibly small.

Example 3: Applications of the present invention in NaCl solution as an industrial starting materials with flow diversion at valve number 12 :

The outlet Draw from Example 1 was divided into two portions at step 12 of Figure 1. The minor portion was resaturated with NaCl and recycled for FO, maintaining the same amount of Draw as in the first cycle. Solid calcium chloride was added into the major portion (55 g of outlet Draw plus 21 g of CaCh) till saturation point. The value of p increased to 1.292. This was substantially higher than the p value of 1.177 of saturated NaCl.

This example teaches us the bifurcation of the outlet Draw, with resaturation of the minor part with NaCl for continuity of FO operation and utilisation of the major part for various other applications which may necessitate addition of constituents other than NaCl. In this particular case, the application of interest is the preparation of high density brines for oil recovery. The Example further teaches that although it may be undesirable to pass calcium chloride solution as draw in an FO process involving seawater as Feed - this is due to the risk of gypsum precipitation - one can confine the FO operation to compatible draw solutions such as saturated NaCl and add incompatible constituents into the major part of outlet draw.

Example 4: Comparative assessment of seawater and 35,000 ppm NaCl as Feeds:

Two experiments were conducted on 2.3 m2 membrane module under similar conditions, one with seawater as feed and the other with 35,000 ppm NaCl solution (this is similar to the total dissolved solids in seawater). The results obtained were almost identical as can be seen from columns A and B in Table 5 below. When a similar experiment with 35,000 ppm NaCl as Feed was run on a 0.6 m2 module, the permeate flux rose from 4.23 L.m 2h_l to 8.04 L.m 2h_l (compare columns B and C). It increased further to 10.51 L.m 2h_ l when the F/D ratio reduced to 7.91 (column D). This Example teaches us that lower F/D ratio and higher area normalised flow rate (kgmirf'm 2) help increase the permeate flux. Note that the permeate flux of 10.51 L.m 2h_l is comparable to the flux of 11-13 L.m 2h_l reported typically for seawater desalination by RO at 50-70 bar. Suffice it to say that further enhancement of FO permeate flux is feasible through further increases in area normalised inlet flow rates of feed and draw and decrease of F/D ratio.

Table 5. Comparative assessment of seawater and 35,000 ppm NaCl as Feeds against saturated NaCl as Draw.



By combining the preaching of Example 4 with the preaching of Examples 1-3, it is feasible to extract pure water from seawater with high permeate flux in continuous manner employing the system of Figure 1. It will further be evident that if instead of pure NaCl one were to take crude NaCl, one can effect recrystallization of the crude salt - as is well known in the prior art - making use of the permeate water from FO as the dissolution medium. Considering further that 1 kg of salt can yield around 4 kg of saturated brine, and that 1 kg of salt is typically obtainable from 40 kg of seawater, a 10: 1 F/D ratio - this is close to the high F/D ratios used in the examples 1 -4 - would yield sufficient outlet seawater to produce the required amount of salt. Moreover, since the seawater is dewatered to the extent of 15-30%, the evaporation load for salt production would be considerably reduced as already taught in the prior art.

Example 5: Applications of the present invention for processing a sugar based industrial starting materials:

It is well known in the prior art that all or part of the raw sugar in a sugar refinery is converted into refined white sugar, and that water is required for this purpose to re-dissolve the raw sugar and carry out evaporative recrystallization. In a manner akin to Example 2 above, a saturated solution of raw sugar (60% w/w sugar) in deionized water was prepared and taken as draw solution while sugarcane juice was taken as feed solution. The data are summarized in Table 6. If one considers the raw sugar content in saturated Draw at the inlet, 1 kg of draw contains 0.6 kg of sugar. This amount of sugar can be obtained from around 5 kg of sugarcane juice. Accordingly a 5: 1 F/D ratio was chosen. In the FO cycle 1, the mass of inlet draw

- which was prepared from 0.201 kg of raw sugar and 0.134 kg of deionized water

- increased from 0.335 kg to 0.594 kg at the outlet due to permeation of 0.259 kg of water from sugarcane juice feed over 3 min of continuous single pass FO operation. Thus the fraction of pure water in the outlet draw which was harnessed from sugarcane juice by the process of FO was 66%. When the entire amount of outlet draw was re -saturated by adding additional amount of raw sugar in the solute dissolver 15 and clarified in the clarifier 17 of Figure 1, the total mass of draw increased to 0.986kg. This was then distributed using the 2-way control valve, 5 (Figure 1). 0.337 kg was recycled in FO cycle 2 to maintain continuity of operation, while the major fraction of 0.649 kg became available as saturated raw sugar solution for downstream recrystallization as practiced in the prior art. The draw amount at the outlet in FO cycle 2 increased to 0.610 kg, i.e., the amount of permeate was 0.292 kg. It can be shown that after the second cycle of FO, the fraction of pure water supplied from sugarcane juice for intended recrystallization of raw sugar increased to 89%. This example teaches us that re-saturation of outlet draw with raw sugar - followed by recycle of a minor portion in the FO process, while making available the rest for downstream recrystallization following known prior art - enables pure water to be continuously extracted from sugarcane juice in cost-effective manner for the purpose of preparation of raw sugar solution and that after a few successive cycles, the fraction of pure water from deionised water used to initiate the process becomes negligibly small.

Table 6. Mass balance and related data for the Experiments described under Example 5.




from the FO cycles 1 and 2 in Table 6 that the extent of dewatering of the juice was approximately 19%, this example teaches us that the sugarcane juice evaporation load to produce sugar would reduce by 19% while continuously supplying pure water for recrystallization of raw sugar into refined sugar.

Example 6: Effect of area normalised flow rates of Feed and Draw at the inlet:

A similar FO experiment as in Example 5 was conducted with 15% w/w sucrose solution as feed in place of sugarcane juice, maintaining other conditions the same including the use of 60% w/w saturated raw sugar solution as draw. Similar results were obtained which indicated that 15 °Brix sugarcane juice can be substituted with 15% w/w sucrose solution. An experiment was conducted subsequently as in Example 5 maintaining all conditions the same except that sugarcane juice was substituted with 15% w/w sucrose solution and the area normalised flow rates of the feed and the draw were increased by 3.65 times, i.e., the feed and draw area normalised flow rates were 0.892 kgmirf'm"2 and 0.173 kgmirf'm 2. respectively. It was observed that compared to the Permeate Flux of 2.4-2.5 L.m 2l ' and extent of dewatering of 19% in Example 5, the corresponding figures in this case were 6.430 L.m 2l ' and 14%, respectively. This example teaches us that by increasing the area normalised flow rates of Feed and Draw at the inlet, the Permeate Flux can be significantly enhanced which would reduce the size of the FO plant and lower the investment cost. The absolute evaporation load of sugarcane juice would also reduce.

Example 7: Applications of the present invention for reuse of waste RO reject water for rural communities (Role in drought relief):

Similar FO experiments as described in the examples above were conducted with saturated NaCl solution and saturated sucrose solution as draw but the feed in these experiments was RO reject water having 3100 ppm total dissolved solids generated in a community RO plant in Ausa, Fatur. The motivation behind these experiments was to recover maximum utilisable water from underground feed water but the RO reject water obtained as a result has a salinity which makes it difficult to dispose the water easily. On the other hand, if too little water is recovered in the RO operation then there is high wastage of water. The proposed solution is to maximise the fraction of utilisable water and then subject the reject water to drying in a drying bed so that the solid residue obtained would be easier to dispose. However, this necessitates that the reject water volume be reduced further. Attempt to recover additional amount of water by RO led to fouling of the RO membrane module due to scale formation and lodging of foulants on membrane surface due to the high pressure applied. The aim of the invention thus was to overcome the above

drawbacks by removing water from RO reject water through the ambient FO process and to make good use of the permeated water. Since most households require salt, sugar and water for edible purposes, the objective was to provide such pure salt and sugar solutions to the community. These can also be used for preparation of soft beverages such as sweet and salty lemon juice which is common in rural areas. Results of experiments conducted with saturated salt and sugar solutions as draw and RO reject water as feed are shown in Table 7.

The FO module had 2.3 m2 membrane area and the run time was 10 min in continuous single pass mode. Both draw solutions were found to efficiently pull in pure water from the RO reject water. In the case of saturated NaCl as draw, the draw amount increased from 0.501 kg at the inlet to 1.837 kg at the outlet due to permeation of pure water from RO reject feed whereas in the case of saturated sugar solution, the draw amount increased from 0.540 kg at the inlet to 1.242 kg at the outlet. The outlet draw solutions can be re-saturated by adding the respective solutes in the solute dissolver 15 of Figure 1 and thereafter clarified in clarifier 17. The minor portion of re-saturated draw can be recycled in cycle 2 of FO to maintain continuity of operation, while the major portion can become available as pure brine or pure sugar solution for intended applications.

Table 7. Mass balance and related data for the Experiments described under

Example 7.



In this manner, there can be a continuous supply of pure permeate for production of pure salt and sugar solutions with 20-32% dewatering of the RO reject with permeate flux ranging from 13.36-19.99 Lm^h 1. Besides making more water available from the groundwater for beneficial uses, the area of the evaporation bed would also reduce.

Example 8: Applications of the present invention for reuse of waste RO reject water for farmers:

The study of Example 7 was repeated with saturated draw solutions prepared from agriculturally important solutes, particularly, urea and KC1 (MOP). The

RO reject TDS was lower at 1760 ppm and the F/D ratio was reduced to the range of 5-6 compared to 7.5-8.5 in Example 7. The FO module had 2.3 m2 membrane area and the run time was 9 min in continuous single pass mode. In the case of saturated KC1 as draw, 79% dewatering with permeate flux of 13.2 L.m _2h_l was obtained whereas in the case of saturated urea solution, the corresponding values were 50% and 7.4 L.m^h 1.

Table 8. Mass balance and related data for the Experiments described under

Example 8.



Thus there would be substantial dewatering of RO reject water through use of solutions of KC1 and urea as draw while at the same time meeting the requirement of pure water to dissolve these fertilizers especially for drip irrigation application. Suffice it to say that the outlet draw solution can alternatively be supplemented and saturated with one or more other nutrients. In this case, the bifurcation of the streams would be carried out before resaturation with the solute used for draw preparation.

Advantages and Inventive Steps:

1. Recognising that saturated solutions of many industrial solids that are used in solution form exert high osmotic pressure when used as draw solution and thereby can extract pure water from relevant feed solutions.

2. Recognising that when pure solids are employed for draw solution preparation, the undersaturated draw at the outlet can be once again resaturated generating more saturated draw solution than to begin with, and that this helps to sustain FO operation while making available the surplus for industrial production.

3. Recognising that in those situations wherein industrial solids in crude form are recrystallized to obtain pure solids, the crude form of the solid can be used for draw preparation and the water extracted by the FO process can be utilised to effect recrystallization.

4. Recognising that some industrial solids may be incompatible for FO and that these can be added into the undersaturated draw at outlet, especially where the residual solute in the outlet draw poses no harm.

5. Providing an energy efficient alternative to processes such as RO, and with reduced fouling problem, where the ultimate objective of RO is to supply pure water as dissolution medium.

6. The simultaneous dewatering of the feed solution would be advantageous in those situations where it helps to reduce the evaporation load as well known in the prior art.

7. FO does not require high pressure pumps or heavy duty steel vessels as flow and thus the overall weight of equipment can be considerably reduced, which would be a another advantage especially in offshore operations such as brine preparation for oil recovery.