Patent Application
20230191330
The present invention relates to design and operation of desalination facilities, and in particular to a system and method for improving water recovery and mineral byproduct production.
In the desalination industry, freshwater is produced is various processes which convert seawater, brackish water, etc., into fresh water. For convenience of reference, at most locations herein reference is made to “seawater,” “saline water” or “feedwater” as the source water. These references are not intended to be limiting, as the source water may be any saline water recognized by those of ordinary skill in the art as possible feed water to a desalination facility.
Most saline water sources contain a large number of minerals in the form of dissolved ions. In desalination a driving force is applied to remove the minerals from the seawater by means of thermal energy such as MSF (Multi Stage Flash) and MED (Multiple Effect Distillation) or pressure energy such as reverse osmosis (RO), forward osmosis and membrane distillation, or a hybrid system combined between thermal and membrane systems.
Typical desalination plants also have to manage the concentrated brine discharge remaining after separation of potable water (e.g. water with a total dissolved solids (TDS) level of approximately 300 parts per million (ppm) or less). Direct discharge of the brine in its concentrated form may potentially have an adverse impact on the marine environment. Alternative means for disposal of the concentrated brine are costly, due to the relatively large volume of this byproduct and the need to dispose of it in an environmentally safe manner.
The problems with concentrated brine may be at least partially addressed by extraction of minerals of commercial interest such as sodium, chloride, calcium and magnesium as byproducts which may be used in further applications and/or in to a zero liquid discharge system (membrane or thermal) to minimize environmental impacts. In order to utilize the dissolved ions for various applications, for example using NaCl solution as a raw material for chlor-alkali industry, it is important to increase the content of the ions selected from extraction and beneficial reuse as compared to the other ions in the saline water.
Nanofiltration (NF) is a well-known membrane-based separation method with permeate and retentate output streams (permeate being the output stream containing ions that have passed through the nanofiltration membrane, and retentate being the output stream that contains ions that have not passed through the membrane). Nanofiltration results in different ion rejections depending on the size and charge of the ions and their salt diffusion coefficient in water. In general, NF membranes have relatively higher rejection of multivalent ions and lower rejection on monovalent ions, making NF suitable for selective enhanced separation of monovalent ions where the target monovalent ions have relatively higher concentrations than the multivalent ions in the NF permeate.
Examples of differences in rejection observed in testing are illustrated in Table 1, which classes NF membranes by their respective ion rejection performance with a seawater feed source TDS concentration in the range of 35,000-47,000 ppm at approximately 17 bars of feed pressure. Most of the monovalent ions in the seawater are Sodium (Na+), Chloride (Cl—) and Potassium (K—) ions. Among divalent ions, there typically is a higher rejection of Sulfate (SO4−−) ions such as Calcium (Ca++), Magnesium (Mg++) and Bicarbonate (HCO3−) ions (while bicarbonate (HCO3−) is monovalent, it is included in the divalent portion of Table 1 because its rejection by NF is similar to that of other multivalent ions).
Although NF membranes have relatively low rate of rejection of monovalent ions as compared to the higher rejection rate of multivalent ions, Table 1 shows that the purity of monovalent ions in the permeate of the NF system might not be adequate for beneficial use after a single pass through the NF unit, particularly if the target mineral purity level is 98% or more. Accordingly, because the ion rejection rate of divalent ions is not always close to 100%, when the required minimum purity of the monovalent ions of interest is high, or the allowable “impurity level” of certain multivalent ions is very low, a single pass NF system may not be sufficient to obtain the desired product quality. Thus, additional separation processing in two or more passes may be needed to enhance the purity of the monovalent ions and/or lower the content of the multivalent ions in the NF permeate.
However, when two or more passes are considered, the total recovery of the NF system drops sharply. For example, if the recovery (R) of single pass is 70%, the recovery of a two pass NF system with R=70% of each pass will result in total R of only 49%. Alternatively, in order to maintain the same final NF permeate flow rate, the system would have to be designed for a 43% larger seawater feed to the first NF pass.
The present invention addresses these and other problems, providing for a two or more-unit NF system with recirculation of the NF retentate rejected from the second and/or subsequent passes to the feed entering the first NF unit. The recirculation of the second and/or subsequent NF reject both increases the overall recovery and increases the purity of monovalent ions in the final NF permeate.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
In the following descriptions, calculations of mass and ion balances in the
A simplified schematic illustration of an embodiment of a conventional two-pass nanofiltration system 100 is shown in
The effluents from the first nanofiltration unit 120 include a portion of the saline water 101 which entered the nanofiltration unit 120 and passed through the separation membrane 111 (i.e., NF #1 permeate stream 102), and a portion of the saline water 101 which does not pass through the nanofiltration membrane 111 (i.e., NF #1 reject stream 103). As shown in the second column of Table 1 the permeate NF #1 permeate water 102 yield is 70% (i.e., 700 tons), with the NF #1 permeate stream concentrations being TDS at 34,916 ppm, Cl− at 21,963 ppm, Na+ at 12,028 ppm, SO4−2 at 35 ppm, Mg−2 at 228 ppm, Ca+2 at 171 ppm, K— at 425 ppm, HCO3−. at 66 ppm. The NF #1 reject stream 103, at 300 tons, has higher concentrations of dissolved solids as shown in the third column of Table 2, with the TDS of the NF #1 reject stream 103 having increased to 68,505 ppm and corresponding increases in the constituents, i.e., Cl− at 31,766 ppm, Na+ at 18,145 ppm, SO4−2, at 11,299 ppm, Mg+2 at 4,990 ppm, Ca+2 at 1,276 ppm, K+ at 615 ppm, and 415 at ppm HCO3− at 415 ppm.
The NF #1 reject stream 103 is removed from the nanofiltration system 100 for subsequent further processing and/or disposal in an environmentally appropriate manner. The NF #1 product stream 102 is introduced to a second nanofiltration unit 130 as the NF #2 feed stream (the NF #2 feed stream contains the same concentrations as the NF #1 permeate stream). Similar to the first nanofiltration unit 120, the second nanofiltration unit 130 includes a separation membrane 121. The fourth column of Table 1 lists the NF #2 permeate stream 104 concentrations, with a TDS at 30,295 ppm, Cl− at 19,370 ppm, Na+ at 10,435 ppm, SO4−2 at 0 ppm, Mg+2 at 32 ppm, Ca+2 at 58 ppm, K+ at 375 ppm, and HCO3− at 25 ppm. The NF #2 retentate discharge (reject) stream 105 concentrations shown in Table 1, column 5 are TDS at 45,698 ppm, Cl− at 28,025 ppm, Na+ at 15,743 ppm, SO4−2 at 117 ppm, Mg+2 at 688 ppm, Ca+2 433 ppm, K+ at 542 ppm, and HCO3− at 160 ppm.
As the source saline water is processed through the conventional nanofiltration system's first NF subsystem (NF #1 120), the impurity index (Mg/TDS) is lowered from 3.683% to 0.654%, with the impurity level being further lowered to 0.104% after processing through the second NF unit (NF #2 130), as shown in the fifth column of Table 1 (NF #2 permeate). However, the overall recovery of the whole NF system is reduced to only 49%, which means a larger intake system will be needed to meet desired production targets.
Among the advantages resulting from recirculation of the highly concentrated brine of reject stream 205 into the feed stream 201 are increasing of the overall recovery of the nanofiltration system 200 and enhanced concentration of monovalent ions in the NF #1 reject stream 203. Moreover, this recirculation process increases energy efficiency, as little to no additional energy is required to significantly raise the pressure of the recirculated NF #2 reject stream 204.
Tables 3 illustrates the improved concentration performance with the present invention's reject stream recirculation, using a specific example in which the
As a consequence of the NF #2 reject recirculation, the feed flow rate of NF #1 220 increases, which in turn increases the production of permeate from NF #1 220. The recirculation of the NF #2 reject stream 205 also can lead to a desirable increase the concentration of monovalent ions in the NF #2 permeate stream 204 relative to the concentration of multivalent ions. thereby increasing the purity of the final permeate and its beneficial use for target applications.
The effects of the improved system in the
The higher overall recovery may also be accompanied by an increase in the content of monovalent ions and a relatively lower amount of increase in divalent ion content (content=flow rate times concentration). This is because the modified NF #1 feed stream is a mixture of the fixed flow rate of saline water 201 and the recirculation flow of NF #2 reject stream 205. The ion concentration in the NF #1 feed stream therefore is a function of the efficiencies of the nanofiltration units NF #1 220 and NF #2 230 (i.e., the ion rejection rates as well as the recovery rates) that, combined, result in the ion concentrations of the NF #2 reject stream 205. Thus, because the ion rejection rate of monovalent ions of a typical NF membrane is lower than the rejection rate of multivalent ions, the ratio of multivalent ion concentration to monovalent ion concentration (a measure of impurity of the permeate in terms of content of multivalent ions) in the NF #1 feed stream will be lower with the present invention's NF #2 reject stream 205 recirculation.
This effect is discernable in Table 4, which illustrates the relative ion concentrations in the NF #1 feed stream, and associated relative ion rejection and recovery rates for various amounts of NF #2 reject stream recirculation as compared to no recirculation (i.e., NF #1 feed=1.000). Table 4 shows that ions which are rejected to a greater degree by nanofiltration membranes (typically, multivalent ions) are concentrated less (left columns), and ions rejected poorly by the NF membranes (typically monovalent ions) are concentrated at a relatively higher rate (right columns). Therefore, the ratio of multivalent ion concentration to monovalent ion concentration in the permeate streams will be proportionally reduced. When the resultant ion concentration in the NF #1 feed is less than 1,000 ppm, then there is benefit to recirculation (in Table 4, the underlined entries). For example, using the example of Table 2 of, R_individual=70% and rejection of Mg=between 80% and 90%, from Table 3, we can find the relative concentration is 0.845˜0.898 (i.e., less than 1.000), and thus recirculation is desirable. The advantages of the present invention are particularly manifested when the recovery of individual NF subsystems (R) is 50% or lower, and, when R is greater than 50%, the interested ion rejection of an individual NF subsystem (Rej) is between approximately Rej (in %)≥1.4×R (%)−40%.
Accordingly, selection of the amount of recirculation may be used to alter the concentrations in the respective permeate and reject streams to tailor the present invention's operations to targeted stream output concentrations.
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0.919
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The increase in overall recovery achieved by the present invention also permits cost and energy efficiency improvements. For example, the recirculation of the NF #2 reject stream 205 may permit the size of the seawater intake and pumping station to be reduced. Further, because the reject stream 205 from NF #2 is typically pressurized, the recirculation of the NF #2 reject stream 205 may require little or no boosting of its pressure to be introduced into the seawater feed stream 201, where the feed pressure to NF #1 220 is lower than the feed pressure to NF #2 230. Thus, the present invention's NF #2 recirculation approach recovers pressure energy in the NF #2 reject stream 205 to increase system efficiency.
The amount of benefit from the present invention is dependent on the specific recovery and ion rejection capacities of the particular NF subsystems, which in turn are a function of NF membrane type and operating conditions. Such selective concentration of desirable ions allows for designing the NF system of the present invention in a manner that it suitable for a large number of practical applications. For example, Table 5 illustrates the results of testing of various nanofiltration units, which were found to fall within three broad categories based on their separation performance:
The highly concentrated brine from the NF effluent streams, enriched with sodium and chloride and of low content of calcium and magnesium impurity, may be generated as a raw source material for various industrial uses in which calcium and magnesium impurities must be reduced below an allowable target concentration (e.g., such as chlora-alkali).
The present invention may also be used with reverse osmosis and brine concentrator systems installed downstream of the present invention's NF subsystem arrangements. As most of RO and brine concentrator systems remove both the monovalent and the divalent ions with similar high rejection rates from the NF permeate, it is important to minimize the content of impurities in the NF permeate to be fed to these downstream subsystems, in order to minimize these impurities in the concentrated brine and minimize further processing costs (e.g., further brine purification) before the brine is suitable for use in other industrial production processes. In the example of chlor-alkali industry, magnesium is considered the main impurity that requires removal before the use of the brine in the downstream industrial chemical production process, and therefore minimization of this ion in the NF permeate minimizes further purification needs.
The present invention is not limited to the use of only two nanofiltration units, but may include multiple nanofiltration units arranged with one or more of the downstream reject streams being recirculated to the first nanofiltration inlet and/or to the inlets of one or more upstream nanofiltration units, depending on the target permeate stream quality and/or quantity targets, target reject stream quality and/or quantity targets, and other factors such as cost and suitability of the component arrangements to a particular installation environment.
For example, in the
The recirculation of reject streams to solely the immediately upstream nanofiltration unit is not required, and other recirculation routes or combination of routes are possible. For example,
For example, assuming that a first type of nanofiltration unit (“NF type-A”) rejects less of certain multivalent ions (“Z” ions) relative to a second type of nanofiltration unit (NF type-B”), i.e., higher ionic rejection of Z ions for NF type-B than type-A), a system may use a NF type-A unit for the initial pass(es) in an NF #1 to #i−1 sub-system, and use an NF type-B unit for the downstream passes (i.e., NF #i+1 to n sub-system). In such a system the NF #1 unit reject stream would have higher purity of other multivalent ions, but not the multivalent Z ions, while the NF #i unit reject stream would have higher purity of multivalent Z ions (other multivalent ions having are already removed by the upstream nanofiltration units. Thus, multivalent ions Z may be selectively separated by appropriate choice and arrangement of separation membranes. Similarly, appropriate choices would allow separation of two Depending on the NF membrane types and their ion rejection characteristics, separation of more than two types of multivalent ions.
Another embodiment of the present invention is shown in
With the combination of nanofiltration unit types and arrangements typified by
An example of method for increasing nanofiltration system performance in accordance with the present invention includes introducing a saline water source stream to the inlet of a first nanofiltration unit, supply of at least a portion of a permeate stream from the first nanofiltration unit to the inlet of a second nanofiltration unit, and recirculation of at least a portion of a reject stream from the second nanofiltration unit to the inlet of the first nanofiltration unit.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Because such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
