SYSTEMS AND METHODS FOR REDUCING MAGNESIUM, CALCIUM, AND/OR SULFATE FROM SODIUM CHLORIDE BRINE DURING CONCENTRATION BY HIGH-PRESSURE NANLFILTRATION

Systems and methods for reducing at least one of magnesium, calcium and/or sulfate from sodium chloride brine are described. Systems and methods include nanofiltrating seawater to reduce calcium, magnesium, and sulfate therein. Systems and methods also include introducing permeate from the nanofiltration step as a feed to reverse osmosis (RO) followed by a progressive nanofiltration array. Systems and methods also include feeding the lower salinity permeate from the introducing permeate step to another RO system. Systems and methods also include feeding retentate from the feeding step to a progressive nanofiltration system that concentrates the brine to an appropriate salinity.

BACKGROUND

Purification and concentration of salt brines is often required for industries such as chlor-alkali. Typically, purification is achieved chemically by processes such as softening, and dewatering is performed by evaporation. These processes are energy, chemical, waste, and capital intensive. The chlor-alkali industry requires sodium chloride feed solutions in excess of 250 part per thousand (ppt) with low levels of magnesium calcium, and sulfate. Removal of calcium and magnesium from the brine typically is achieved by the addition of Na2CO3to form CaCO3solid, followed by the addition of NaOH to form Mg(OH)2solid. The solution must be filtered and acidified with HCl before use and a large amount of sludge is formed. Alternately, Ca and Mg can be removed by crystallization, but this requires expensive, energy consuming equipment, loses a portion of the NaCl, and produces a concentrated waste brine.

A membrane process for achieving high salt brines was described in U.S. Published Patent Application No. 20150014248 filed 11 Jul. 2014 to Herron, Beaudry and Lampi (Herron Membrane Process), which is incorporated herein, in its entirety, by this reference. The Herron Membrane Process is like reverse osmosis (RO) in that it uses high pressure to force water through a semipermeable membrane. RO is limited in the amount of water it can remove since commercial membrane systems are limited to pressures of at most 120 bar due to materials issues. This pressure can at most produce a 110 ppt NaCl solution because of the osmotic pressure of the solution. The membrane and process in the Herron Membrane Process uses a more permeable nanofiltration membrane that allows salt to slowly permeate through the membrane, which creates a saline permeate. The osmotic potential of salt on the permeate side allows more water to be forced from the feed solution so that the difference in osmotic pressures between the feed and the permeate is equal to the applied pressure.

It was proposed that a salt solution be dewatered as much as possible with RO then the RO retentate be fed to a series of nanofiltration elements at high pressure to produce a final retentate with osmotic pressures far above the applied pressure. The saline permeate is substantially less salty than the retentate, so the permeate can be dewatered by RO or returned to the feed of the nanofiltration elements.

The process of high pressure nanofiltration to concentrate salt brines, discussed in the Herron Membrane Process, is incorporated herein, in its entirety, by this reference. The Herron Membrane Process describes the equipment and transport equations pertinent to the concentration of single component brines to osmotic pressures higher than the applied osmotic pressure.

An additional provisional patent application No. 63/150,772 filed on 18 Feb. 2021 by Herron, Beaudry and Lampi which describes energy and equipment savings by the appropriate reinjection of nanofiltration permeate is also incorporated, in its entirety, by reference.

The high-pressure nanofiltration process can also be used to purify salt streams. Of particular interest to the chlor-alkali industry, is the separation of calcium, magnesium, and sulfate impurities from high concentration sodium chloride solutions. In the nanofiltration process, divalent cations such as Ca++, Mg++and SO4−permeate the membrane much more slowly than monovalent cations such as Na+and Cl−so during the process, the ratio of divalent to monovalent cations increases in the retentate and decreases in the permeate. This has been applied to the reduction of magnesium in brines for the solar evaporation harvesting of lithium from salar ponds. A PCT International Patent Application No. PCT/US2020/058879 filed on 4 Nov. 2020 directed to the above subject matter is incorporated herein, in its entirety, by this reference.

Concentration and purification of brines for the chlor-alkali industry is a useful example of the benefits of the process. In the Middle East, electricity costs are low and numerous seawater desalination installations produce large amounts of brine which are expensive to dispose of. It has been proposed to concentrate the desalination brine to use as a feed to chlor-alkali plants. Further concentration of the brine has the added benefit of recovering more water from the seawater.

Removal of calcium, magnesium and sulfate from chlor-alkali feed brine is valuable in that it reduces fouling of chlor-alkali electrocells and reduces the amount of concentrated NaCl brine that must be purged.

SUMMARY

Embodiments disclosed herein are directed to a three step combination of nanofiltration and reverse osmosis that can concentrate seawater or other mixed salt streams to high concentrations with low levels of calcium, magnesium and sulfate.

In one or more embodiments, a method of producing desalinated seawater includes nanofiltrating seawater to reduce calcium, magnesium, and sulfate therein, introducing permeate from the nanofiltration step as a feed to reverse osmosis followed by a progressive nanofiltration array, feeding the lower salinity permeate from the introducing permeate step to another reverse osmosis (RO) system, and feeding retentate from the feeding step to a progressive nanofiltration system that concentrates the brine to an appropriate salinity.

More particularly, the first step of the process includes nanofiltration of seawater to reduce calcium, magnesium, and sulfate. Numerous species such as strontium, phosphate, and silica are also reduced, but they are of less importance to the quality of the chlor-alkali brine extracted from seawater.

The second step is the introduction of the permeate from the first step as the feed to reverse osmosis followed by a progressive nanofiltration array. The second step feed is concentrated to a small volume of high strength brine with a high proportion of divalent ions. The permeate from the second step nanofiltration membranes becomes the feed to the third step. Water from the RO membranes is suitable for industrial or municipal use.

The third step feeds the lower salinity permeate from the second step nanofiltration to another RO system. The retentate from the step three RO is fed to a progressive nanofiltration system which concentrates the brine to an appropriate salinity. Permeate from the step three nanofiltration elements, as well as the high salinity permeate from step two nanofiltration, are fed to either the step three RO or to appropriate places in the step three nanofiltration train.

In an embodiment, a method of producing desalinated seawater includes nanofiltrating seawater to reduce calcium, magnesium, and sulfate therein. The method includes introducing permeate from the nanofiltration step as a feed to a first reverse osmosis system followed by a first progressive nanofiltration array, thereby forming a lower salinity permeate and a higher salinity permeate having a salinity greater than the lower salinity permeate. The method includes feeding the lower salinity permeate to a second RO system, thereby forming a retentate. The method includes feeding the retentate from the second RO system to a second progressive nanofiltration system that concentrates brine in the retentate to within at least a predetermined salinity.

In an embodiment, a system for reducing at least one of magnesium, calcium and/or sulfate from sodium chloride brine is disclosed. The system includes a first NF system positioned to receive at least filtered seawater fed by a pump, and configured to produce a first retentate and a first permeate. The system includes a first RO system positioned to be fed the first permeate from the first NF system and configured to produce at least a second retentate and desalinated water. The system includes a second NF system positioned to be fed the second retentate from the first RO system and configured to produce one or more additional permeates. The system includes a second RO system positioned to be fed at least one permeate of the one or more additional permeates from the second NF system to produce at least a fourth retentate and additional desalinated water. The system includes a third NF system positioned to be fed at least the fourth retentate from the second RO system to produce one or more further permeates and a final retentate that is substantially free of divalent ions.

In an embodiment, a method for reducing at least one of magnesium, calcium and/or sulfate from sodium chloride brine is disclosed. The method includes feeding at least filtered seawater to a first NF system, thereby producing a first retentate and a first permeate. The method includes feeding the first permeate from the first NF system to a first RO system, thereby producing at least a second retentate and desalinated water. The method includes feeding the second retentate from the first RO system to a second NF system, thereby producing one or more additional permeates. The method includes feeding at least one permeate of the one or more additional permeates from the second NF system to a second RO system, thereby producing at least a fourth retentate and additional desalinated water. The method includes feeding at least the fourth retentate from the second RO system to a third NF system, thereby producing one or more further permeates and a final retentate that is substantially free of divalent ions.

DETAILED DESCRIPTION

Embodiments disclosed herein are related to systems and methods for reducing one or more (e.g., all) of magnesium (Mg), calcium (Ca), and/or sulfate (SO4) from sodium chloride (NaCl) brine during concentration by high-pressure nanofiltration. In at least one, some, or all embodiments, a combined, three-stage, nanofiltration (NF)/reverse osmosis (RO) system separates a mixed salt solution (e.g. feed solution) into water, streams of combined mixed salts, and a concentrated salt solution substantially free of divalent ions. The feed solution is seawater, according to an embodiment. The concentrated salt solution that is substantially free of divalent ions results in the technical of effect of providing feedstock for industrial processes and/or the chlor-alkali industry.

In at least one, some, or all embodiments, a method of producing desalinated seawater includes nanofiltrating seawater to reduce calcium, magnesium, and sulfate therein. The method also may include introducing permeate from the nanofiltration step as a feed to reverse osmosis followed by a progressive nanofiltration array. The method also may include feeding the lower salinity permeate from the introducing permeate step to another RO system. The method also may include feeding retentate from the feeding step to a progressive nanofiltration system that concentrates the brine to an appropriate salinity.

FIGS.1-3are schematics of a system and method for reducing one or more (e.g., all) of magnesium, calcium, and/or sulfate from sodium chloride brine during concentration by high-pressure nanofiltration, according to an embodiment.FIGS.1-3are based on standard modeling of RO systems, such as the RO system analysis (ROSA) model and internal models of the performance of high-pressure nanofiltration elements developed by Fluid Technology Solutions (FTS) of Albany, Oregon USA. In some embodiments, the high-pressure nanofiltration elements may include a range of sodium ion permeabilities, but the ratio of permeabilities of different species may remain relatively constant for all membranes. The species approximate relative ratios of permeability for membranes in the high-pressure nanofiltration elements is shown below in Table 1, according to at least one, some, or all embodiments. Chloride is poorly rejected, and in the model, it is assumed to move to maintain electrical neutrality.

The model of the behavior of multiple species can predict the behavior of only two species (along with chloride). The modeling procedure was to first to estimate the removal of sulfate by treating all cations as sodium. This analysis showed sulfate is largely removed in the first step and the modeling was performed by ignoring sulfate, lumping sodium and potassium, calcium and magnesium, and modeling the water as a two-cation solution. The model also assumes that nanofiltration elements are selected with permeabilities that provide a constant flux of 10 liters/m2/hr (lmh). It was assumed that all elements are 8040 spiral wound design with 40 m2membrane.

FIG.1is a schematic of a first step100of nanofiltration (NF) of prefiltered Arabian Gulf seawater, according to an embodiment. In some embodiments, the first step100includes nanofiltration of seawater to reduce calcium, magnesium, and sulfate. Numerous species such as strontium, phosphate, and silica are also reduced, but they may be of less importance to the quality of the chlor-alkali brine extracted from seawater.

In at least one, some, or all embodiments, an antiscalant is added to the seawater, and the resulting initial solution is fed to banks of NF elements operating at 40 bar pressure in a first NF system110that produces a retentate (e.g., a concentrate) and a first permeate (A). The antiscalant may include a dianionic polyelectrolyte (DAPE), such as poly[disodium 3-(N,N-diallylamino)propanephosphonate. The retentate from the first NF system110is high in divalent ions and is disposed of. The first permeate (A) from the first NF system110passes on to a second step200of the process, shown inFIG.2.

In an example, the schematic modeled inFIG.1was initially run with 238 m3/hr feed with 36 parts per thousand (ppt) NaCl and 4 ppt Na2SO4to estimate the amount of sulfate which permeated the membrane. It was assumed that the system was operated at 40 bar. The model showed little sulfate crossed the membrane, so sulfate was ignored in the subsequent membrane processes.

The schematic modeled inFIG.1was run a second time with about 238 m3/hr feed, about 33 ppt NaCl, and about 7 ppt MgCl2to simulate the combined effect of calcium and magnesium. In this implementation, the first retentate inFIG.1is concentrated by the nanofiltration process to a flow (e.g., of about 72 m3/hr) and a salinity of about 89 parts per thousand (ppt), according to an embodiment. The lumped Ca and Mg concentration rose from about 1.7 in the initial solution to about 5.1 ppt in the first retentate, and the sulfate rose from about 3 in the initial solution to about 9.2 ppt in the first retentate, according to an embodiment. The first permeate (A) from the first NF system110had a salinity of 22.2 ppt, a flow of about 166 m3/hr, about 0.2 ppt divalent cations (e.g., of Mg and Ca), and negligible sulfate, according to an embodiment. The first retentate leaves the process and the first permeate (A) continues to a second step200, shown inFIG.2.

Turning now toFIG.2, a schematic of the second step200of systems and methods includes a first RO system250and a subsequent second NF system210for processing the first permeate (A) formed according toFIG.1. In some embodiments, the second step200may include the introduction of the permeate (A) from the first step100as the feed to reverse osmosis followed by a progressive nanofiltration array. The second step200feed may be concentrated to a small volume of high strength brine with a high proportion of divalent ions. The permeate from the second step200nanofiltration membranes may become the feed to the third step. Water from the RO membranes is suitable for industrial or municipal use.

FIG.2picks up the first permeate (A) from the first NF system110inFIG.1, and concentrates the first permeate (A) as much as possible with the first RO system250to form a second retentate. The second retentate from the first RO system250of the second step200is then passed through successive arrays or banks of NF membranes in the second NF system210until a small volume of third retentate is left with high divalent ion concentrations. The third retentate is disposed of and a second permeate (B) and a third permeate (C) is passed on to a third step300shown inFIG.3.

More specifically, the second step200starts with the first permeate (A) from the first step100pumped (e.g., at about 70 bar) to the first RO system250, which produces water or other solution (e.g., about 114 m3/hr of water), according to an embodiment. The flow of the second retentate from the first RO system250may be about 51.4 m3/hr and have salinity higher (e.g., 71.6 ppt) than the first permeate (A) fed to the first RO system250, according to an embodiment. The second NF system210of the second step200passes the second retentate from the first RO system250through multiple (e.g., three) arrays of NF elements, according to an embodiment. The first array211of NF elements in the second NF system210may include 7 banks in parallel with 8 elements per bank. The second array212of NF elements in the second NF system210may include three banks in parallel, each having 16 elements in series. The third array213of NF elements in the second NF system210may include a single bank of 16 elements in series.

The first array211of NF elements in the second NF system210produces a retentate that leaves (e.g., at about 29 m3/hr) the first array211of NF elements in the second NF system210having a higher salinity (e.g. about 107 ppt) than the second retentate fed into the first array211of NF elements in the second NF system210. The second array212of NF elements in the second NF system210produces a retentate that leaves (e.g., at about 9.8 m3/hr) the second array212of NF elements in the second NF system210having a higher salinity (e.g., about 170 ppt) than the retentate from the first array211of NF elements in the second NF system210that is fed to the second array212of NF elements in the second NF system210. The third array213of NF elements in the second NF system210produces a third retentate that leaves (e.g., at about 3.4 m3/hr) the third array213of NF elements in the second NF system210having a salinity (e.g., 230 ppt) higher than the retentate from the second array212of NF elements in the second NF system210that is fed to the third array213of NF elements in the second NF system210. The level of Mg (e.g., 7 ppt) in the third retentate may be at least 5 times, at least 10 times, or at least 15 times greater than the level of Mg (0.5 ppt) in the second retentate fed into the second NF system210. Substantially all of the sulfate (e.g., at least about 75%, at least about 90%, or at least about 99% of the sulfate) which permeated the first NF system110(e.g., in the first permeate) in the first step100is in the third retentate.

The permeate from the first array211of NF elements in the second NF system210and the permeate from the second array212of NF elements in the second NF system210are combined to produce an third permeate (C) having a flow (e.g., of about 41.6 m3/hr) and a salinity (e.g., about 48.8.ppt) higher than the salinity (about 22.2 ppt) of the first permeate fed into the first RO system250but lower than the salinity (about 71.6 ppt) fed into the first array211of NF elements of the second NF system210. The permeate from the first array211of NF elements in the second NF system210may have a lower salinity (e.g., 26 ppt) than the salinity (e.g., about 75.3) of the permeate from the second array212of NF elements to which the permeate from the first array211of NF elements is combined. The second permeate (B) from the third array213of NF elements of the second NF system210is produced (e.g., about 6.4 m3/hr) having a higher salinity (e.g., 139 ppt) than the third permeate, as well as a higher salinity than the second retentate fed into the first array211of NF elements in the second NF system210. Both the second permeate (B) and the third permeate (C) streams are passed to the third step300the process, shown inFIG.3.

Turning now toFIG.3, a schematic of the third step300of systems and methods includes a second RO system350and a subsequent third NF system310for processing the second permeate (B) and the third permeate (C) formed according toFIG.2. In some embodiments, the third step300feeds the lower salinity permeate from the second step200nanofiltration to another RO system. The retentate from the step three300RO may be fed to a progressive nanofiltration system which concentrates the brine to an appropriate salinity. Permeate from the step three300nanofiltration elements, as well as the high salinity permeate from step two nanofiltration, are fed to either the step three300RO or to appropriate places in the step three nanofiltration train, according to an embodiment.

More specifically,FIG.3shows second RO system350followed by the third NF system310for producing a final retentate or concentrate (e.g., at 250 ppt) that is low in divalent ions (e.g., about or less than 0.1 ppt Mg). In some embodiments, the concentration of divalent ions in the initial solution may be at least 5 times, at least 10 times, or at least 15 times greater than the concentration of divalent ions in the final retentate. Feed water to the second RO system350comes from the third permeate (C) that has a lower salinity (e.g., 48.8) than the salinity (e.g., 139 ppt) of the second permeate (B). Feed water to the second RO system350also may come from another permeate from the third NF system310, as described in greater detail below, and also may have a lower salinity (e.g., about 39.4 ppt) than the second permeate (B). A fourth retentate from the second RO system350in the third step300is passed through a series of NF banks of the third NF system310, and the permeate streams from this third NF system310may be reintroduced to one or more of the second RO system350and/or the third NF system310at appropriate or preselected places. The second permeate (B) from the second step200has more saline than the third permeate (C) and is also injected into the third NF system, according to an embodiment. The systems and methods described herein result in the technical effect of producing water produced in the second step200and a third step300ofFIG.3that is high quality desalinated seawater.

In some embodiments, the third step300ofFIG.3combines the low salinity, third permeate (C) from the second step200with a low salinity fourth permeate from the third NF system310(e.g., from the first array311of NF elements in the third NF system310) in the third step300nanofiltration to form a stream (e.g., a 99.2 m3/hr stream) having a salinity (e.g., about 43.3 ppt). This stream may be pressurized to 70 bar and fed to the second RO system350, which produces water (e.g., at a rate of 36.2 m3/hr) and the fourth retentate (e.g., at a rate of 63 m3/hr) and having a higher salinity (e.g., about 68 ppt) than the combined third permeate and fourth permeate fed to the second RO system350.

In some embodiments, the fourth retentate from the second RO system350is combined with a higher salinity fifth permeate from another portion of the third NF system310(e.g., from the second array312of NF elements in the third NF system310) to form a feed (e.g., about 127.8 m3/hr) for the third NF system310having a higher salinity (e.g., about 83.7 ppt) than the salinity (e.g., about 48.8 ppt) of the third permeate, the salinity (e.g., about 43.3 ppt) of the feed for the second RO system350, and/or the salinity (e.g., about 68 ppt) of the fourth retentate output by the second RO system350. The third NF system310may include multiple (e.g., three) arrays of NF elements. In some embodiments, the first array311of NF elements of the third NF system310has 18 banks of elements with 8 elements in series per bank. This first array311of NF elements in the third NF system310may produce (e.g., at about 57.6 m3/hr) the fourth permeate having a salinity (e.g., about 39.4 ppt) lower than the feed for the first array of NF elements311in the third NF system310. This first array311of NF elements in the third NF system310also may produce (e.g., about 70.2 m3/hr) a retentate having a salinity (e.g., about 120 ppt) higher than the salinity (e.g., about 48.8 ppt) of the third permeate, the salinity (e.g., about 43.3 ppt) of the feed for the second RO system350, the salinity (e.g., about 68 ppt) of the fourth retentate output by the second RO system350, and/or the salinity (e.g., about 83.7 ppt) of the feed for the first array311of NF elements in the third NF system310.

The retentate from the first array311of NF elements in the third NF system310may be combined with the (high salinity) second permeate (B) from the second step200to feed the second array312of NF elements in the third NF system310. A sixth permeate from the third array313of NF elements of the third NF system310also may be combined with at least one (e.g., both) of second permeate (B) and the retentate from the first array311of NF elements of the third NF system310to create a feed (e.g., about 98 m3/hr) for the second array312of NF elements in the third NF system310having a higher salinity (e.g., about 131 ppt) than the retentate first array311of NF elements in the third NF system310. The second array312of NF elements in the third NF system310may include 9 banks of NF in parallel with 18 elements in series per bank. The second array312of NF elements of the third NF system310may produce (e.g., about 64.8 m3/hr) a fifth permeate having a salinity (e.g., about 99 ppt) less than the salinity (e.g., about 131 ppt) of the feed for the second array312of NF elements in the third NF system310. The second array312of NF elements of the third NF system310also may produce (e.g., about 33.2 m3/hr) a retentate having a salinity (e.g., about 194 ppt) that is greater than the salinity (e.g., about 131 ppt) of the feed for the second array312of NF elements in the third NF system310.

The retentate from the second array312of NF elements of the third NF system310may be fed to the third (e.g., last) array313of NF elements in the third NF system310, which may include 3 banks in parallel and with each bank having 18 elements in series. The third array313of NF elements in the third NF system310may produce (e.g., about 21.6 m3/hr) a sixth permeate having a salinity (e.g., about 164 ppt) that is less than the salinity (e.g., about 194 ppt) of the feed for the third array313of NF elements in the third NF system310, but greater than the salinity (e.g. about 139 ppt) of the second permeate (B) and greater than the salinity (e.g., about 48.8) of the third permeate (C). The third array313of NF elements in the third NF system310also may produce (e.g., about 11.6 m3/hr) a fifth (or final) retentate having a salinity (e.g. about 250 ppt) at least about four times or five times greater than the salinity of the initial solution, at least about 1.5 times greater than the salinity (e.g. about 139 ppt) of the second permeate, and/or at least about four or five times greater than the salinity (e.g., about 48.8) of the third permeate (C). The magnesium concentration in the final retentate may be about 0.1 ppt or less.

As used herein, the term “about” or “substantially” refers to an allowable variance of the term modified by “about” or “substantially” by ±10% or ±5%. Further, the terms “less than,” “or less,” “greater than,” “more than,” or “or more” include, as an endpoint, the value that is modified by the terms “less than,” “or less,” “greater than,” “more than,” or “or more.”