PL241836B1 - Method of saline water pretreatment for its desalination and concentration and a hybrid membrane system for implementing the method - Google Patents

Abstract

The subject of the application is a method of pre-treatment of saline water for desalination and concentration, which consists in introducing the saline water to the first nanofiltration unit (1) equipped with nanofiltration membranes (3) with ion retention coefficients: Cl- -20% - 40% , Ca2+ 40% - 95%, Mg2+ 40% - 95% and SO42 - 40% - 99%, in which saline water is separated by means of a nanofiltration membrane (3) into two streams: permeate 5 - 95% and retentate 5 - 95 %, wherein the retentate in an amount of 0.1 - 50% by volume is discharged from the system and the remainder is combined with saline water and recycled to the first nanofiltration unit (1); permeate from the first nanofiltration node (1) is directed to the second nanofiltration node (2) equipped with nanofiltration membranes (3) with ion retention coefficients: Cl- -20% - 40%, Ca2+ 40% - 95%, Mg2+ 40% - 95 % and SO42 - 40% - 99%, in which it is separated by means of a nanofiltration membrane (3) into two streams: permeate 5 - 95% and retentate 5 - 95%, where the permeate is pre-treated saline water intended for desalination and concentration with the content of chloride ions 0.01 - 195 kg/m3 and sulphate ions 0 - 0.3 kg/m3 and calcium ions 0 - 2 kg/m3 or/and magnesium ions 0-2 kg/m3, and retentate from the second nanofiltration unit (2) combines with the retentate from the first nanofiltration node (1) before discharge from the system or combines with the remainder of the retentate from the first nanofiltration node returned to the first nanofiltration node (1). The application also includes a hybrid membrane system for purifying saline water for desalination and concentration.

Description

Description of the invention

The subject of the invention is a method for pre-treatment of saline water for desalination and concentration, and a hybrid membrane system for carrying out the method.

Salt solutions are usually concentrated by evaporative methods at high unit costs. The high investment costs of evaporation installations and high energy consumption result from the fact that the composition of saline wastewater is complicated, some components of which form hardly soluble compounds that crystallize in evaporators.

In the world, there is a tendency to develop waste-free technologies for the management of waste solutions, the so-called ZLD (zero liquid discharge); in such technologies, the water yield would have to be practically 100%. The yield in the reverse osmosis method (RO - Reverse Osmosis) is limited by the osmotic pressure of the retentate and the risk of crystallization of sparingly soluble substances; its value in the case of sea water is 35-50%. In evaporative methods, the yield is limited primarily by the risk of crystallization of sparingly soluble substances and amounts to 10-30%. The reverse osmosis retentate and the concentrate from evaporative methods are discharged into the sea, which poses serious ecological problems due to the local increase in salt concentrations and the presence of chemicals, e.g. antiscalants, used to reduce the risk of crystallization of sparingly soluble substances, and in the case of concentrate from evaporators, additionally increased temperature. Numerous methods are used to increase the yield in reverse osmosis: dosing of antiscalants; new reverse osmosis configurations such as flow reversal reverse osmosis and closed circuit desalination reverse osmosis; precipitation of sparingly soluble salts between individual stages of reverse osmosis; using advanced pre-preparation.

The world is also interested in the possibility of increasing the yield of desalinated water through the use of retentate (concentrate) and its further processing in order to obtain saturated brine or evaporated salt.

The management of waste saline solutions requires solving two fundamental problems: the need to handle concentrated salt solutions and the risk of scaling, i.e. blocking membranes with deposits of sparingly soluble salts. Reverse osmosis, due to the need to use pressure higher than the osmotic pressure of the desalted solution, is not suitable for processing solutions with a concentration of more than 70 g/dm ³ . Such highly concentrated solutions, where sodium chloride is the dominant component, can be concentrated to take advantage of the salt they contain. Concentrated sodium chloride solutions can be utilized in the form of saturated brine or evaporated salt, which avoids the discharge of salt into the environment. Saturated brine can be used for the production of sodium carbonate (soda) by the Solvay method or for the production of chlorine and sodium hydroxide by the diaphragm method; diaphragm method is one of the three methods used in the world to produce chlorine and sodium hydroxide).

Increasingly, nanofiltration (NF) is used as a method of pre-preparation of saline solutions. Typical nanofiltration membranes are characterized by a high degree of removal of polyvalent ions (over 70% for SO4 ^2- ) and a lower degree of removal of monovalent ions, e.g. Na+, Cl ^- (below 60%). In nanofiltration, the permeate has a significant salinity (compared to a typical reverse osmosis permeate), due to this, the value of the working pressure in nanofiltration can be much lower than in reverse osmosis for the same feed: the non-absolute value of the osmotic pressure must be overcome (assuming that reverse osmosis permeate is close to zero) but the difference in the osmotic pressure of the feed/retentate and the nanofiltration permeate.

The literature describes the use of nanofiltration for the treatment of production water (partial water desalination, decarbonization, softening), cooling water, recovery of water or raw materials from post-production streams and wastewater treatment. Nanofiltration is also used as a method of initial brine preparation: nanofiltration - reverse osmosis system with a yield in the nanofiltration node of 65%, nanofiltration - reverse osmosis - distillation system (MSF - Multi-Stage Flash Distillation) the yield in the nanofiltration node was 64%.

From the literature, the use of single-stage nanofiltration in a system combined with reverse osmosis carried out on nanofiltration permeate and crystallization of calcium carbonate from nanofiltration retentate is known (F. Macedonio, E. Curcio, E. Drioli, Integrated membrane systems for seawater desalination: energetic and exergetic analysis, economic evaluation, experimental study, Desalination 203 (2007) 260-276).

PL 241 836 B1

The use of a three-stage membrane system is known from the literature. The retentate from the first stage of the membrane system is directed to the second stage with the addition of antiscalants. The retentate from the second stage of the membrane system is ozonated to remove antiscalants, and then sparingly soluble salts are precipitated from it. After removing the sediments, the solution goes to the third stage of membrane filtration. Permeates from all stages are not recycled and are combined into one stream (L.F. Greenlee, F. Testa, D.F. Lawler, B.D. Freeman, P. Moulin, Effect of antisealant degradation on salt precipitation and solid/liquid separation of RO concentrate, J. Membr Sci. 366 (2011) 48-61).

From the literature, it is known to use single-stage nanofiltration as a pre-treatment before reverse osmosis for the production of salt from seawater. Crystallization does not occur after the nanofiltration process (M. Turek, Seawater desalination and salt production in a hybrid membrane-thermal process, Desalination 153 (2003) 173-177).

The use of single-stage nanofiltration to remove calcium carbonate and magnesium sulfate is known from the literature. Magnesium sulphate is removed in the process of membrane crystallization (E. Drioli, E. Curcio, A. Criscuoli, G. Di Profio, Integrated system for recovery of CaCO3, NaCl and MgSO4-7H2O from nanofiltration retentate, J. Membr. Sci. 239 ( 2004) 27-38).

The use of single-stage nanofiltration for the production of salt without crystallization of salt from the retentate is known from the literature (US6783682B1, Salt water desalination process using ion selective membranes).

From the American patent description US8647509B2, the use of nanofiltration for the production of salt from sea water is known. Reverse osmosis retentate from seawater desalination is used as feed in nanofiltration, and nanofiltration retentate is not recycled.

Another US patent US8366924B2 describes the use of two-stage nanofiltration for desalination of sea water and production of drinking water. The second stage of nanofiltration is supplied with permeate from the first stage of nanofiltration. The membranes used in this solution have very high retention coefficients for monovalent and polyvalent ions.

From the literature, the use of single-stage nanofiltration for seawater desalination with salt, boron and gypsum is known (US 2013/0020259 A1, Membrane and electrodialysis based seawater desalination with salt, boron and gypsum recovery).

The object of the invention is to prepare saline water for desalination or concentration in a hybrid membrane system.

This goal was achieved using a hybrid membrane system in which the ions of sparingly soluble salts, such as calcium, sulphate and magnesium, are removed. In order to achieve a high salt yield in the nanofiltration process, a membrane with a low degree of chloride retention (below 40%) and a high degree of sulphate, calcium and magnesium retention (over 40%) should be used.

The method of the invention consists in introducing saline water into the first nanofiltration node equipped with nanofiltration membranes with ion retention coefficients: Cl ^- -20% - 40%, Ca ²⁺ 40% - 95%, Mg ²⁺ 40% - 95% and SOa ² - 40% - 99%, in which saline water is separated by means of a nanofiltration membrane into two streams: permeate 5-95% and retentate 5-95%, with retentate in the amount of 0.1-50% by volume being discharged from the system, and the remainder is combined with saline water and recycled to the first nanofiltration node; permeate from the first nanofiltration node is directed to the second nanofiltration node equipped with nanofiltration membranes with ion retention coefficients: Cl ^- -20% - 40%, Ca ²⁺ 40% - 95%, Mg ²⁺ 40% - 95% and SO4 ^2- 40% - 99%, in which it is separated by means of a nanofiltration membrane into two streams: permeate 5-95% and retentate 5-95%, where the permeate is pre-treated saline water intended for desalination and concentration with a chloride ion content of 0.01 - 195 kg/m ³ and sulphate ions 0 - 0.3 kg/m ³ and calcium ions 0 - 2 kg/m ³ or/and magnesium ions 0 - 2 kg/m ³ , and the retentate from the second nanofiltration unit is combined with retentate from the first nanofiltration node before being discharged from the system or is combined with the remainder of the retentate from the first nanofiltration node being recycled to the first nanofiltration node.

Preferably, it comprises two nanofiltration nodes.

Preferably, the calcium sulfate saturation in the retentate from the first nanofiltration unit is below 160%.

The hybrid membrane system is characterized by the fact that nanofiltration nodes are equipped with membrane modules containing nanofiltration membranes with ion retention coefficients: Cl -20% - 40%, Ca²⁺ 40% - 95%, Mg²⁺ 40% - 95% and SOa²' 40% - 99%; outlet of the retentate from the first

EN 241 836 B1 of the nanofiltration node is connected to the saline water inlet of the first nanofiltration node, the retentate outlet of the second nanofiltration node is connected to the retentate outlet of the first nanofiltration node or is connected to the saline water inlet of the first nanofiltration node.

The advantage of the solution according to the invention is the reduction of: the risk of scaling in devices for desalination/concentration of saline waters, energy consumption of the process, costs of vacuum salt production, environmental pollution and the amount of generated waste.

The subject of the invention is explained in diagrams showing variants of preparation of saline water for desalination or concentration, depending on the composition of saline water and retentates, where Fig. 1 shows a two-stage nanofiltration system in which retentate from the first and second nanofiltration stage after mixing are partially recycled to the first nanofiltration stage, Fig. 2 shows a two-stage nanofiltration system in which the retentate from the first nanofiltration stage is partially recycled to the first nanofiltration stage and the retentate from the second nanofiltration stage is directly recycled to the first nanofiltration stage.

Based on the results obtained in laboratory tests, computer modeling of two-stage nanofiltration was performed. Calculations were made for different yields: 5%-95%, for nanofiltration membranes (3) with different ion retention coefficients: Cl ^- -20% - 40%, Ca ²⁺ 40% - 95%, Mg ²⁺ 40% - 95% and SO4 ^{2 -} 40% - 99%.

Example 1

1 m is introduced into the first nanofiltration node (1).³ saline water with composition Cl 33.2 kg/m³, on⁺ 15.8 kg/m³, ca²⁺ 2.8 kg/m³, Mg²⁺ 1.3 kg/m³, SO4^2- 0.015 kg/m³. In the first nanofiltration unit (1) under the applied pressure (57.7 bar), saline water is separated by means of a nanofiltration membrane (3) with Cl retention coefficients^- 30%, Ca²⁺ 85%, Mg²⁺ 90% and SO4^2- 99% for two streams: permeate and retentate, in proportions of 60% and 40% respectively. The retentate from the first stage of nanofiltration (1) with the composition Cl^- 68.1 kg/m³, on⁺ 3.47 kg/m³, ca²⁺ 19.6 kg/m³, Mg²⁺9.49 kg/m³, SO4^2- 0.114 kg/m³ with a relative calcium sulphate saturation of 15% is divided into a stream of 0.65 m³ solution returned to the first nanofiltration node (1) and 0.132 m³ solution discharged from the system. The permeate from the first nanofiltration node (1) is fed to the second nanofiltration node (2), where it is separated under the applied pressure (56.0 bar) by means of a nanofiltration membrane (3) with retention coefficients: Cl^- 15%, Ca²⁺ 80%, Mg²⁺ 85% and SO4^2- 95% for two streams: permeate and retentate, in proportions of 88% and 12%, respectively. The permeate from the second nanofiltration node (2) is the desired product, i.e. 0.868 m³ pre-treated saline water intended for desalination and concentration with a composition of Cl^- 28 kg/m³, on⁺ 17.7 kg/m³, ca²⁺ 0.258 kg/m³, Mg²⁺ 0.061 kg/m³, SO4^2- 0.000 kg/m³. The retentate from the second nanofiltration node (2) is recycled to the first nanofiltration node (1).

Example 2

1 m ³ of saline water with the composition Cl ^- 0.415 kg/m ³ , Na ⁺ 0.165 kg/m ³ , Ca 2 ⁺ 0.312 kg/m ³ , Mg 2 ⁺ 0.139 kg/m ³ , is fed into the first nanofiltration node (1). SO4 ^{2 -} 1.09 kg/m ³ . In the first nanofiltration unit (1) under the influence of applied pressure (27 bar), saline water is separated by means of a nanofiltration membrane (3) with retention coefficients of Cl ^- 20%, Ca ²⁺ 80%, Mg ²⁺ 85% and SO4 ^2- 90 % for two streams: permeate and retentate, in proportions of 30% and 70%, respectively. Retentate from the first stage of nanofiltration (1) with the composition Cl ^- 0.547 kg/m ³ , Na ⁺ 0.155 kg/m ³ , Ca ²⁺ 0.814 kg/m ³ , Mg ²⁺ 0.373 kg/m ³ , SO4 ^{2 -} 3.03 kg/m ³ with a relative calcium sulphate saturation of 121% is divided into a stream of 0.354 m ³ of solution recycled to the first nanofiltration node (1) and 0.872 m ³ of solution discharged from the system. The permeate from the first nanofiltration node (1) is fed to the second nanofiltration node (2), where, under the applied pressure (25.2 bar), it is separated using a nanofiltration membrane (3) with retention coefficients: Cl ^- 15%, Ca ²⁺ 70%, Mg ²⁺ 75% and SO4 ^2- 90% into two streams: permeate and retentate, in proportions of 85% and 15%, respectively. The permeate from the second nanofiltration unit (2) is the desired product, i.e. 0.646 m ³ of pre-treated saline water intended for desalination and concentration with the composition Cl ^- 0.343 kg/m ³ , Na ⁺ 0.171 kg/m ³ , Ca 2 ⁺ 0.036 kg/m ³ , Mg2 ⁺ 0.010 kg/ ^m3 , ^SO42- 0.022 kg/ ^m3 . The retentate from the second nanofiltration node (2) is recycled to the first nanofiltration node (1).

Example 3 ''

1 m ³ of saline water with the composition Cl ^- 33.2 kg/m ³ , Na ⁺ 15.8 kg/m ³ , Ca ²⁺ 2.8 kg/m ³ , Mg ²⁺ is introduced into the first nanofiltration unit (1). 1.3 kg/ ^m3 , SO42 ^- 0.015 kg/ ^m3 . In the first nanofiltration unit (1) under the influence of applied pressure (47.1 bar), saline water is separated by means of a nanofiltration membrane (3) with retention coefficients of Cl ^- 30%, Ca ²⁺ 85%, Mg ²⁺ 90% and SO4 ^{2 -} 99%

PL 241 836 B1 into two streams: permeate and retentate, in proportions of 45% and 55%, respectively. The permeate from the first nanofiltration node (1) is introduced to the second nanofiltration node (2), where, under the applied pressure (41.6 bar), it is separated using a nanofiltration membrane (3) with retention coefficients: Cl ^- 15%, Ca ²⁺ 70%, Mg ²⁺ 75% and SO4 ^2- 90% into two streams: permeate and retentate, in proportions of 75% and 25%, respectively. The permeate from the second nanofiltration unit (2) is the desired product, i.e. 0.850 m ³ of pre-treated saline water intended for desalination and concentration with the composition Cl ^- 28.8 kg/m ³ , Na ⁺ 17.8 kg/m ³ , Ca ²⁺ 0.480 kg/ ^m3 , Mg2 ⁺ 0.132 kg/ ^m3 , ^SO42- 0.000 kg/ ^m3 . Retentate from the second nanofiltration unit (2) with the composition Cl ^- 49.1 kg/m ³ , Na ⁺ 22.8 kg/m ³ , Ca ²⁺ 4.96 kg/m ³ , Mg ²⁺ 1.71 kg/m 3 ³ , SO4 ^{2 -} 0.002 kg/m ³ is combined with the retentate from the first stage of nanofiltration (1) with the composition Cl ^- 60.2 kg/m ³ , Na ⁺ 0.77 kg/m ³ , Ca 2 ⁺ 18.1 kg /m ³ , Mg ²⁺ 9.13 kg/m ³ , SO4 ^{2 -} 0.119 kg/m ³ , obtaining 1.668 m ³ of a solution with the composition CI ^- 58.3 kg/m ³ , Na ⁺ 4.497 kg/m ³ , Ca ^{2 +} 15.86 kg/m ³ , Mg ²⁺ 7.87 kg/m ³ , SO4 ^{2 -} 0.099 kg/m ³ with a relative calcium sulphate saturation of 11% divided into a stream of 1.518 m ³ of solution recycled to the first nanofiltration unit (1 ) and 0.150 m ³ of solution discharged from the system.

Claims (4)

Patent claims 1. The method of pre-treatment of saline water for desalination and concentration consists in the use of saline water with a chloride ion content of 0.01 - 195 kg/m as the feed³ and sulfate ions 0.01 - 30 kg/m³ and calcium ions 0.02 - 20 kg/m³ and/or magnesium ions 0.02 - 20 kg/m³, which is introduced into two nanofiltration nodes (1), (2), in which the permeate from the first nanofiltration node (1) is the feed for the second nanofiltration node (2), characterized in that saline water is introduced into the first nanofiltration node ( 1) equipped with nanofiltration membranes (3) with ion retention coefficients: Cl^- -20% - 40%, Ca²⁺ 40% - 95%, Mg²⁺ 40% - 95% and SO4^2- 40% - 99%, in which saline water is separated by means of a nanofiltration membrane (3) into two streams: permeate 5-95% and retentate 5-95%, with retentate in the amount of 0.1-50% by volume being discharged from system and the remainder is combined with saline water and recycled to the first nanofiltration unit (1); permeate from the first nanofiltration node (1) is directed to the second nanofiltration node (2) equipped with nanofiltration membranes (3) with ion retention factors: Cl -20% - 40%, Ca²⁺ 40% - 95%, Mg²⁺ 40% - 95% and SO4^2- 40% - 99%, in which it is separated by means of a nanofiltration membrane (3) into two streams: permeate 5-95% and retentate 5-95%, where the permeate is pre-treated saline water intended for desalination and concentration with chloride ion content 0.01 - 195 kg/m³ and sulfate ions 0 - 0.3 kg/m³ and calcium ions 0 - 2 kg/m³ and/or magnesium ions 0 - 2 kg/m³; and the retentate from the second nanofiltration node (2) is combined with the retentate from the first nanofiltration node (1) prior to being discharged from the system or is combined with the remainder of the retentate from the first nanofiltration node being recycled to the first nanofiltration node (1). 2. The method of claim The device according to claim 1, characterized in that it comprises two nanofiltration nodes (1), (2). 3. The method of claim The method of claim 1, characterized in that the calcium sulphate saturation in the retentate from the first nanofiltration unit (1) is below 160%. 4. A hybrid membrane system for purifying saline water for desalination and concentration, consisting of two nanofiltration nodes (1), (2), characterized in that the nanofiltration nodes (1), (2) are equipped with membrane modules containing nanofiltration membranes (3 ) with ion retention coefficients: CI ^- -20% - 40%, Ca ²⁺ 40% - 95%, Mg ²⁺ 40% - 95% and SO4 ^{2 -} 40% - 99%; the retentate outlet from the first nanofiltration node (1) is connected to the saline water inlet to the first nanofiltration node (1), the retentate outlet from the second nanofiltration node (2) is connected to the retentate outlet from the first nanofiltration node (1) or is connected to the water inlet to the first nanofiltration node (1).