The present invention relates to a novel multistage membrane process technology that enables producing a potable water product from saline water feed at a high water recovery, reduced osmotic pressure differential and competitive net specific energy consumption (SECnet) while simultaneously reducing the boron concentration to the level recommended for both human consumption and crop irrigation.
There is a continuing need to improve the efficiency and reduce the cost of supplying potable water owing to the pressures of an expanding world population, changing demographics, and global climate change. Nearly 700 million people in the world lack access to safe drinking water. In January of 2015 the World Economic Forum announced that the water crisis is the number one global risk based on impact to society. Crop irrigation associated with the food sector of the global economy uses 70% of the world's freshwater. Additional sources need to be tapped to satisfy the increasing water demand for both human consumption and crop irrigation.
The oceans that contain 97% of the water on earth are a major resource. However, ocean water can contain as much as 50000 parts per million (ppm) of salt (sodium chloride) and other low molecular weight solutes that make it unfit for human consumption or irrigation without treatment. Reverse osmosis (RO) has emerged as a major technology for producing potable water from seawater as well as inland brackish water that has a salt content ranging from 500 ppm to 30000 ppm. RO uses a salt-rejecting membrane under high pressure in order to force water to permeate through the membrane while rejecting the salt and other solutes. Conventional RO technology requires a very high pressure, typically 50 bars or more, which contributes significantly to the cost of water desalination. Moreover, conventional RO technology has a limited potable water recovery, typically 50%, owing to the very high pressures required to achieve higher water recoveries. As a result, the specific energy consumption (SEC) for producing potable water from saline water is quite high. In order to reduce a salt concentration of 35000 ppm in seawater to 350 ppm in the product water, conventional single-stage reverse osmosis (SSRO) operating at a pressure of 55.5 bar and a water recovery of 50% using a membrane with a salt rejection of 0.990 requires a net SEC of 2.242 kWh/m3 (kilowatt hours of energy per cubic meter of product water). For this reason the cost of water obtained via desalination is more than twice the cost of water obtained from freshwater sources.
A concurrent problem is that typical seawater contains 10 ppm or more of boron. The World Health Organization (WHO) has recommended that the maximum boron concentration in water for human consumption should be below 2.4 ppm and for irrigation should be below 0.5 ppm, particularly for citrus and nut orchards. Boron is present in seawater as boric acid, which is only slightly larger than the clusters of hydrogen-bonded water molecules. Hence, conventional RO membranes have relatively low boron rejections, typically less than 90%, and therefore cannot reduce the boron concentration via conventional desalination technologies to a concentration of 0.5 ppm. Higher boron rejections are possible with current RO membranes if the pH (logarithm of the hydrogen ion concentration) of the feed solution to the membrane process is increased above the pK (logarithm of the dissociation constant) of 9.14 for the ionization of boric acid to borate ions, which when hydrated are sufficiently larger than the hydrogen-bonded water molecules to enable adequate rejection via commercial RO membranes. However, this process is expensive since it requires reducing the highly alkaline pH after the boron removal. Boron removal from seawater is usually done as a post-treatment process after desalination. However, this is also costly since huge volumes of water must be processed twice, one for desalination and again for boron removal. Concurrent desalination and boron removal (CDBR) is attractive but challenging owing to the poor rejections of commercially available RO membranes.
Prior art patents related with the present invention are listed as below;
No prior patents involve a process for concurrently effecting desalination and boron removal to achieve product water concentrations of less than 350 ppm of salt and 0.5 ppm of boron and that require only multistage membrane separations at a significantly reduced pressure while requiring no addition of chemicals to increase the pH.
This invention is a novel membrane technology referred to as the Concurrent Desalination Boron Removal (CDBR) process. The CDBR invention enables water desalination and boron removal to be done at the same time using membrane technology in order to achieve the desired concentrations in the product water. The SEC for conventional RO process technology is high because of the large osmotic pressure differential (OPD) between a concentrated salt solution and nearly pure water and because the water recovery is relatively low. This CDBR invention capitalizes on the recently invented energy-efficient reverse osmosis (EERO) process. The EERO process reduces the OPD and increases the water recovery by a judicious combination of single-stage reverse osmosis (SSRO) and a countercurrent membrane cascade with recycle (CMCR). However, the EERO process cannot reduce the typical boron concentration in seawater to an acceptable level in the product water.
The present invention uses membrane technology to concurrently desalinate a saline water feed and reduce the boron concentration to 0.5 ppm or lower at lower operating pressures, higher water recovery, and lower specific energy consumption. Prior art either involves desalination followed by boron removal or requires the addition of chemicals to increase the pH (logarithm of the hydrogen ion concentration) to enable adequate removal of the boron. Any chemicals added to increase the pH must be removed in the product water. Prior art that uses conventional reverse osmosis necessarily operates at higher pressures than this novel CDBR invention.
In fact; the present invention draws upon the features of the EERO process that enable it to reduce the OPD and increase the water recovery, but also makes a substantive addition to the process technology to permit concurrent removal of boron to an acceptable level using currently available commercial RO membranes. In one embodiment of this novel CDBR invention the retentate product from the high pressure side of an SSRO stage is introduced optimally at a point between two stages in a CMCR. The permeate from the SSRO stage is sent as the feed to a low pressure membrane stage (LPMS) to achieve further boron removal. The permeate product from the CMCR is blended with the permeate product from the LPMS to achieve the desired boron concentration in the potable water product. This novel process configuration achieves the desired boron concentration in a potable water product stream at a significantly reduced OPD, high water recovery, and competitive SEC. It accomplishes this by (i) introducing the retentate from the SSRO stage optimally as the feed to the CMCR; (ii) countercurrent retentate and permeate flow in the CMCR; (iii) permeate recycle to the retentate side in the CMCR; (iv) retentate self-recycling in at least one of the membrane stages in the CMCR; (v) introducing the permeate from the SSRO stage as the feed to an LPMS; and (vi) blending the permeate streams from the CMCR and LPMS to achieve the desired concentrations in the water product. Permeate recycle involves sending the permeate stream from a stage to the retentate (high pressure) side of the stage immediately downstream from it (i.e., in the direction of the permeate flow). Retentate self-recycling involves sending part of the retentate to the permeate side of the same stage; this can be done by using a nanofiltration (NF) membrane whose salt rejection is considerably lower than that of an RO membrane. The CDBR process configuration is energy-efficient because (i) the SSRO in combination with the CMCR reduces the OPD; (ii) the LPMS operates at very low pressure relative to an RO stage; and (iii) blending the permeate products from the LPMS and CMCR minimizes the amount of water that needs to pass through the LPMS to reduce the boron concentration.
a. The schematic of an alternative embodiment of the CDBR invention: CDBR-B, where the permeate stream out of Stage 1 is split in two fractions through a flow splitter, and one fraction is fed to the Stage 4, whereas the other fraction bypasses Stage 4 to be mixed with the permeate streams out of Stage 4 and Stage 3.
b. The schematic of an alternative embodiment of the CDBR-B invention: CDBR-BR, where the retentate from Stage 4 is totally or partially recycled as feed to Stage 1.
The present invention involves an SSRO stage (stage 1) whose retentate stream serves as the feed to a two-stage CMCR (stages 2 and 3) and whose permeate stream serves as the feed to an LPMS (stage 4).
The manner in which this CDBR invention reduces the SEC while reducing the OPD, increasing the water recovery, and achieving the desired salt and boron concentrations first will be explained in qualitative terms, after which the embodiment of this invention shown in
This CDBR invention combines SSRO with a CMCR by sending the retentate stream from the SSRO as the feed to the CMCR. In
In order to demonstrate quantitatively that this CDBR invention can achieve a boron removal down to 0.5 ppm concurrently with desalinating seawater to produce a potable water product having a salt concentration less than 350 ppm at a high water recovery, reduced OPD, and competitive SEC, the mathematical equations describing the interrelationship between the volumetric fluxes denoted by Qi in
The analysis of this 4-stage CDBR invention involves solving overall material and solute balances for each of the four stages and at the two mixing points. The balances over stage 1 constitute 3 equations involving 9 unknowns (Qf, Cfs, Cfb, Q0, C0s, C0b, Q1, C1s, C1b). The balances over stage 2 constitute 3 equations involving 9 unknowns (Q2, C2s, C2b, Q4, C4s, C4b, Q6, C6s, C6b). The balances over stage 3 constitute 3 equations involving 6 unknowns (Q3, C3s, C3b, Q5, C5s, C5b). The balances over stage 4 constitute 3 equations in 6 unknowns (Q7, C7s, C7b, Q8, C8s, C8b). The balances at the mixing point between stages 2 and 3 constitute 3 equations and 0 unknowns. The balances at the mixing point where the permeate streams from stages 3 and 4 are blended constitute 3 equations in 3 unknowns (Q9, C9s, C9b). This totals 18 equations that involve 33 unknowns. This implies 15 degrees of freedom in solving the equations for this 4-stage CDBR process.
The 15 degrees of freedom were satisfied by specifying the following quantities shown in figure above:
Specification of the 15 quantities is not unique. The values of other input parameters could be specified.
Overall and solute mass balances for stage 1 are given by the following:
Q
f
=Q
0
+Q
1 (1)
Q
f
C
fs
=Q
0
C
0s
+Q
i
C
1s (2)
Q
f
C
fb
=Q
0
C
0b
+Q
1
C
1b (3)
Overall and solute mass balances for stage 2 are given by the following:
Q
6
=Q
2
+Q
4 (4)
Q
6
C
6s
=Q
2
C
2s
+Q
4
C
4s (5)
Q
6
C
6b
=Q
2
C
2b
+Q
4
C
4b (6)
Overall and solute mass balances for stage 3 are given by the following:
Q
4
=Q
3
+Q
5 (7)
Q
4
C
4s
=Q
3
C
3s
+Q
5
C
5s (8)
Q
4
C
4b
=Q
3
C
3b
+Q
5
C
5b (9)
Overall and solute mass balances for stage 4 are given by the following:
Q
0
=Q
7
+Q
8 (10)
Q
0
C
0s
=Q
7
C
7s
+Q
8
C
8s (11)
Q
0
C
0b
=Q
7
C
7b
+Q8C8b (12)
Overall and solute mass balances at the mixing point between stages 2 and 3 are given by the following:
Q
6
=Q
1
+Q
5 (13)
Q
6
C
6s
=Q
1
C
1s
+Q
5
C
5s (14)
Q
6
C
6b
=Q
1
C
1b
+Q
5
C
5b (15)
Overall and solute mass balances at the mixing point where the permeate streams from stages 3 and 4 are blended are given by the following:
Q
9
=Q
3
+Q
7 (16)
Q
9
C
9s
=Q
3
C
3s
+Q
7
C
7s (17)
Q
9
C
9b
=Q
3
C
3b
+Q
7
C
7b (18)
The additional equations that relate the volumetric fluxes and concentrations are given by the following:
Δπ1=K(C1s−C0s) OPD is specified in stage 1 (19)
Δπ2=Δπ3⇒C2s−C4s=C5s−C3s OPDs set equal in CMCR (20)
Solving these equations gives the following for the volumetric fluxes:
Solving for the salt concentrations gives the following:
Solving for the boron concentrations gives the following:
The pressure required in stage 4 is given by the following:
where P4 is the permeability coefficient of the membrane in stage 4. The overall water recovery from this 4-stage CDBR process is given by the following:
Y=Q9 (59)
The net specific energy consumption (SECnet), which is the energy required per unit of water produced allowing for the recovery of the pressure energy in the retentate via an energy-recovery device (ERD), is given by the following:
where p is the efficiency of the pumps and ERD is the efficiency of the ERD.
The predictions of Equations (29)-(60) will be used to establish the proof-of-concept for this CDBR invention. The performance of the CDBR invention will be assessed in terms of the OPD and SECnet required to produce a potable water product containing 0.5 ppm of boron and no more than 350 ppm of salt from a saline water feed containing 35000 ppm of salt and 10 ppm of boron. The fractional water recovery values for stages 2 and 3 are input parameters in solving the model equations, which were chosen to be 0.3 and 0.7, respectively.
Running stage 2 at a lower recovery increases the safety factor (ratio of retentate to permeate flow) in this stage, thereby helping to mitigate concentration polarization and fouling in this stage that has a feed containing a high concentration of divalent salts. Running stage 3 at a higher recovery is possible since the feed to this stage has passed through both stage 1 and stage 2, thereby removing all the divalent salts that could cause scaling. The feed to stage 4 is nearly pure water since it has passed through stage 1, an RO stage; hence, the OPD in stage 4 is negligible. Moreover, the foulants have been removed in the feed to stage 4. Hence, stage 4 can be run at a very high water recovery or equivalently a very low safety factor. The only requirement is that there be sufficient retentate flow to remove the small amount of boron rejected by the membrane in stage 4. Hence, stage 4 is assumed to have a water recovery of 95%. Pump and ERD efficiencies of 85% and 90%, respectively, are assumed, which are consistent with commercially available devices. The performance of the CDBR invention will be assessed in terms of the OPD and SECnet required to achieve the specified boron and salt concentrations in the product water for a range of overall water recoveries. The implications on the CDBR invention of using membranes having a range of salt rejections and a range of boron rejections also will be assessed. Whereas the salt and boron rejections are specified input parameters for stage 1, the salt rejections are predicted quantities in stages 2 and 3, and the boron rejection is a predicted quantity in stage 4. For stages 2 and 3 the boron rejection is scaled to the predicted salt rejection, whereas in stage 4 the salt rejection is scaled to the predicted boron rejection; that is, the ratio of the boron rejection to the salt rejection is assumed to be the same as that attainable via currently available commercial membranes that can achieve rejections of 90.0% and 99.7% for boron and salt, respectively.
The OPD is a specified input parameter used in solving Equations (1) to (28) for the volumetric fluxes and concentrations in the CDBR invention. The overall water recovery is determined from Equation (59) using the volumetric fluxes determined from Equations (29) to (38).
It is of interest to determine the minimum value of the boron rejection required for the CDBR invention to produce product water that contains no more than 350 ppm of salt and a specified boron concentration of 0.5 ppm and to determine the implications for the CDBR invention if membranes with boron rejections higher than 90% could be obtained.
Table 3 compares the OPD and SECnet for conventional SSRO for just desalination and the novel CDBR invention for achieving a water product having a salt concentration of 350 ppm and a boron concentration of 0.5 ppm for overall water recoveries of 50%, 65% and 75%. Note that conventional SSRO cannot reduce the boron concentration to 0.5 ppm for a typical saline water feed containing 10 ppm of boron using commercially available RO membranes. The CDBR invention can achieve the same overall water recovery as conventional SSRO at a substantially reduced OPD. The CDBR invention reduces the OPD required for just desalination via SSRO by 10%, 18%, and 20% at overall water recovery values of 50%, 65%, and 75%, respectively. The CDBR invention results in an increase in the SECnet of 8%, 4%, and 2% for overall water recovery values of 50%, 65%, and 75%, respectively, relative to using conventional SSRO for just desalination. Since the CDBR invention can desalinate and reduce the boron concentration to 0.5 ppm at a substantially reduced OPD, it will translate to a significant reduction in the fixed costs for the pumps, piping, and pressure vessels relative to using SSRO for just desalination. Moreover, operation at lower pressure via the CDBR invention will reduce the maintenance costs for desalination and boron removal.
The proof-of-concept for the CDBR invention has been shown in detail for the four-stage embodiment involving sending the retentate from an SSRO stage to a 2-stage CMCR and sending the permeate from the SSRO stage to an LPMS after which the permeate streams from the CMCR and LPMS are blended to achieve the desired salt and boron concentrations. The CDBR invention has been shown to capable of producing a water product having a salt concentration equal to or less than 350 ppm and a specified boron concentration of 0.5 ppm, which meets WHO recommendations for potable and irrigation water. The CDBR invention has been shown to achieve the specified water product concentrations at substantially lower pressures than required for just desalination via conventional SSRO for the same overall water recovery. Moreover, the CDBR invention can achieve the specified water product concentrations at a SECnet only slightly higher than for just desalination via conventional SSRO at moderate recoveries of 50% and at nearly the same values as conventional SSRO for recoveries of 65% and 75%. Since the CDBR invention substantially reduces the pressure required for desalination and concurrent boron removal, it will reduce the fixed costs of construction associated with the pumps, piping, and pressure vessels and will reduce the maintenance costs associated with continuous operation at high pressure. These additional cost reductions are not included in the proof-of-concept analysis.
The proof-of-concept for this CDBR invention has been shown based on maintaining the same OPD in stages 1, 2, and 3. This embodiment of the EERO invention is advantageous since it avoids any interstage pumping on the high pressure side of the CMCR. However, another embodiment of this CDBR invention is to allow for a reduced OPD in one or more of the stages in the CMCR while at the same time avoiding any interstage pumping on the high pressure side of the CMCR membrane cascade. This will reduce the pumping costs at the expense of a reduced potable water recovery. For some applications this embodiment of the CDBR invention could be desirable. The CDBR invention may be also implemented in two additional embodiments that are illustrated in
The process conditions for the CDBR-B invention to produce a product water that contains no more than 350 ppm of salt and a specified boron concentration of 0.5 ppm are summarized in Table 4. It yields the OPD and SECnet lower than SSRO at all recoveries.
It is also of interest to determine the performance of the proposed invention for desalination only. It would be possible to obtain a water product with 0.350 ppm salt concentration at lower OPD and SECnet values than SSRO when the CDBR-BR invention is used. In Table 5, the performance of the CDBR-BR invention with a split ratio of 0.95 and a complete recycle of the retentate from Stage 4 is compared to that of SSRO for desalination for 65% and 75% water recoveries.
Furthermore, instead of the SSRO stage in the embodiments described in
This application is the national phase entry of International Application No. PCT/TR2016/050387, filed on Oct. 19, 2016, the entire contents of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/TR2016/050387 | 10/19/2016 | WO | 00 |