WO2019164462A1

WO2019164462A1 - Multi-stage reverse osmosis system and process for high water recovery from aqueous solutions

Info

Publication number: WO2019164462A1
Application number: PCT/TR2018/050063
Authority: WO
Inventors: Mehmet Goktug AHUNBAY
Original assignee: Istanbul Teknik Universitesi
Priority date: 2018-02-21
Filing date: 2018-02-21
Publication date: 2019-08-29
Also published as: SG11201900705PA

Abstract

The object of the invention is to provide a system and process that enables product recovery from aqueous solutions containing salts or low molecular weight solutes such as ethanol at high recovery ratio, low osmotic pressure differential (OPD) and low energy consumption compared to current methods. Particular applications of the invention include production of potable water from water sources with high salt content such as sea water, brackish water or wastewater. The system and process related to the present reduced-pressure reverse osmosis (REPRO) invention described here achieves the reduction in the SEC, a reduction in the OPD, and an increase in the potable water recovery via a novel multistage hybrid RO-NF process technology.

Description

MULTI-STAGE REVERSE OSMOSIS SYSTEM AND PROCESS FOR HIGH WATER RECOVERY FROM AQUEOUS SOLUTIONS

Technical Field of the Invention

This invention relates to a system and a process of reverse osmosis for product recovery from an aqueous solution, in particular recovery of potable water from saltwater.

Background of the Invention The World Health Organization reported in 2015 that 9% of the world population (663 million in 2010) lack access to improved sources of drinking water and the need to potable water is continuing to increase due to the expanding world population. In the absence or insufficiency of surface and underground potable water sources, oceans that contain 97% of the water on earth are a major alternative resource. However, ocean water is unsuitable for human consumption without treatment since it can contain as much as 50000 parts per million (ppm) of salt (sodium chloride). Reverse osmosis (RO) is as a major technology for producing potable water from salt water using a salt-rejecting membrane under a pressure higher than the osmotic pressure in order to allow water to permeate through the membrane to obtain desalted water while rejecting the salt and other solutes. RO can be also used to produce potable water from inland brackish water whose salt content ranges from 500 ppm to 30000 ppm.

The production of potable water from sea water by RO is more efficient compared to classical methods such as evaporation since it does not involve phase transition. However, the required pressure in RO is high in order to overcome the osmotic pressure differential (OPD), which contributes significantly to the cost of water desalination. In the case of, a pressure typically above 40 bars or more is required, whereas the recovery of potable water is low, typically 50%.

The International Patent Application Numbered WO2015152823 in the state of the art discloses an apparatus for reverse osmosis, the apparatus comprising: a single- stage reverse osmosis (SSRO) unit; and a counter-current membrane cascade with recycle (CMCR) unit comprising a plurality of stages of reverse osmosis including at least a first stage and a second stage wherein permeate from the first stage is configured to be introduced as feed to the second stage; wherein retenate from the SSRO unit is configured to be introduced as feed to the first stage, and wherein product obtained using the apparatus comprises permeate from the SSRO unit and permeate from a last stage of the CMCR unit.

Another prior art document U.S. Patent Application No. US2015014248 discloses methods and systems for generating strong brines are disclosed in which a feed stream and a draw inlet stream are passed through a forward osmosis membrane to create a concentrate and a draw outlet stream. The draw outlet stream is passed through a reverse osmosis membrane to create a reverse osmosis permeate flow and a reverse osmosis retentate flow, the reverse osmosis retentate flow is passed through a first nanofiltration membrane to create a first nanofiltration permeate flow and a first nanofiltration retentate flow; and the first nanofiltration retentate flow is passed through a second nano filtration membrane to create a second nanofiltration permeate flow and a second nanofiltration retentate flow. In some embodiments, the process is repeated through a third nanofiltration membrane. The process may be repeated through a third nanofiltration membrane.

The U.S. Patent Application Numbered US 9427705 in the state of the art discloses a method of solvent recovery includes using a plurality of solvent recovery units to recover solvent from a dilute solution. The solvent recovery units can include a plurality of reverse osmosis or forward osmosis membrane systems arranged in series. For reverse osmosis, at least some of the concentrate in a last reverse osmosis unit of the series is recycled back to the permeate of that unit to provide a mixed permeate. The mixed permeate is then passed successively to the permeate side of each preceding reverse osmosis unit in the series. For forward osmosis, a draw solution is passed sequentially from the permeate side of each unit to the permeate side of the preceding unit. The draw solution may be prepared by concentrating part of the concentrate stream by evaporation and recycling it back as a draw solution.

Another prior art document U.S. Patent Application No. US2013118978 discloses a water treatment system combines a microfiltration or ultrafiltration membrane system with a downstream reverse osmosis membrane system. The MF or UF system has multiple trains of immersed membrane modules. The trains are connected to a common permeate pump. The permeate pump discharges directly into the inlet of an RO feed pump. The membrane trains are each subjected to the same suction. The permeate pumps are operated to provide the required flow to the RO feed pump at or above the minimum inlet pressure of the RO feed pump.

As a result, the specific energy consumption (SEC) for producing potable water from sea water is relatively high. In order to produce potable water by reducing the salt content of seawater from 35000 ppm down to 350 ppm, the theoretical gross SEC required by a conventional single-stage RO (SSRO) is 3.086 kwh/m (kilowatt hours of energy per cubic meter of product water) using a membrane with a salt rejection of 0.993 at a pressure of 55.5 bar with a water recovery of 50%. In order to increase water recovery above 65%, an SSRO requires a pressure greater than 79.3 bar, which is above the operating limits of the typical commercial membranes.

Brief Description of the Invention and its Aims

The aim of this invention is to realize a system for reverse osmosis wherein overall water recovery of the system greater than 55% can be achieved while operating each stage with a stage recovery less than or equal to 50%, that is while mantaining the safety factor, which is defined as the ratio of the retentate flow rate to the permeate flow rate for the same membrane stage, less than or equal to one. Another aim of this invention is realize a system for reverse osmosis wherein overall water recovery of the system greater than 55% can be achieved while reducing the required osmotic pressure differential below than what is required in a conventional SSRO for the same overall water recovery.

With reference to Fig. 1, the process in the present invention combines a primary water recovery (PWR) unit (1 A) with a secondary water recovery (SWR) unit (5 A) comprising a downstream RO subunit (2A), a nanofiltration (NF) subunit (3 A) and an upstream RO subunit (4A). The PWR unit (1A) and each of the subunits within the SWR unit (5A) may comprise a plurality of membrane stages in series configuration wherein each membrane stage receives the retentate of the subsequent stage as feed. The PWR unit (1A) sends its retentate stream as feed to the SWR unit (5A) by introducing into the high-pressure side of the downstream RO unit (2H).

Detailed Description of the Invention

Fig. 1 is a general schematic illustration of the present invention

Fig. 2 is a schematic illustration of four-stage embodiment of the present invention

Fig. 3 is a schematic illustration of five -stage embodiment of the present invention

Fig. 4 is a graph of net specific energy consumption (SECnet) as a function of the OPD in the downstream RO stage“R2” (Dpk₂) for 65% and 75 percent overall water recoveries in 4-stage (dashed lines) and 5-stage (solid lines) embodiments of the present invention.

The system described in the invention comprises a primary water recovery (PWR) unit (1A) with a secondary water recovery (SWR) unit (5 A) as shown in Figure 1; wherein the PWR unit (1A) comprises one single RO stage or a plurality of RO stages in series configuration wherein the retentate of the preceding RO stage is fed to the high-pressure side of the subsequent RO stage; wherein the SWR unit (5A) comprises one downstream RO subunit (2A), one NF subunit (3A) and one upstream RO subunit (4A), wherein downstream is defined as the direction of the retentate flow from the NF subunit (3 A) and upstream is defined as the opposite direction of the retentate flow from the NF subunit (3 A); wherein the rententate of the PWR unit (1A) is introduced to the SWR unit (5A) as feed to the high-pressure side of the downstream RO subunit (2H); wherein the rententate of the downstream RO subunit (2A) is introduced as feed to the high-pressure side of the NF subunit (3H); wherein the rententate of the NF subunit (3A) is discharged as concentrate stream and the permeate of the NF subunit (3 A) is recycled to be introduced as feed to the high-pressure side of the upstream RO subunit (4H); wherein the permeate of the upstream RO subunit (4A) is collected as product water and the rententate of the upstream RO subunit (4A) is introduced together with the retentate of the PWR unit (1A) as feed to the high-pressure side of the downstream RO subunit (2H); wherein each subunit within the SWR unit (5A) comprises one single membrane stage or a plurality of membrane stages in series configuration wherein the retentate of the preceding membrane stage is fed to the high- pressure side of the subsequent membrane stage and wherein the permeates of the membrane stages are combined; wherein the PWR unit (1A) comprises one single RO membrane stage or a plurality of RO membrane stages in series configuration wherein the retentate of the preceding membrane stage is fed to the high-pressure side of the subsequent membrane stage and the retentate of the final RO stage is fed to the SWR unit (5A); wherein the permeates of all RO stages are collected as product water. The process provided in the present invention comprises the steps of: introducing a feed of aqueous solution into a PWR unit (1A) comprising at least a single RO stage; introducing retentate from the PWR unit (1A) into high-pressure side of a downstream RO subunit (2H) within a SWR unit (5 A) comprising a downstream RO subunit (2A), an NF subunit (3A) and an upstream RO subunit (4A); introducing retentate from the downstream RO subunit (2A) into high-pressure side of the NF subunit (3H); introducing permeate from the NF subunit into high- pressure side of the upstream RO subunit (4H); introducing retentate from the upstream RO subunit (4A) as feed to the high-pressure side of the downstream RO subunit (2H); and collecting as product permeates from all RO stages.

One embodiment of this invention involves operating the NF subunit (3A) at the same pressure as the retentate stream leaving the downstream RO subunit (2A) and not employing any interstage pumping on the high-pressure (retentate) side of the NF subunit (3H). This is done by using NF membranes with decreasing salt rejection in the direction of retentate flow. i.e. by using a membrane in stage Nl that passes more salt than the highly salt rejecting membranes used in the downstream RO subunit (2A); and by using a membrane with lower salt rejection than the preceding stage in each of the subsequent NF stages. The OPD in PWR unit (1A) is significantly lower than OPD in the downstream RO subunit (2A). Since the recycling permeate leaving the NF subunit is blended with the saltwater feed to be introduced into the downstream RO subunit (2A). Pumping requirement for the recycling permeate will be significantly reduced, resulting in a further lower SEC.

Fig. 2 shows a four-stage embodiment of the invention, wherein the PWR unit comprises one single RO stage (Rl); and the downstream RO subunit comprises one RO stage (R2), the NF subunit comprises one NF stage (Nl) and the upstream RO subunit comprises one RO stage (R3), within the SWR unit. The feed of aqueous solution is fed to the high-pressure (retentate) side (R1H) of stage Rl . The retentate stream leaving stage Rl is introduced into the high-pressure side (R2H) of RO stage R2. The retentate stream leaving stage R2 is introduced into the high- pressure side (N1H) of NF stage Nl, wherein the rententate of the stage Nl is discharged as concentrate stream and the permeate of stage Nl is recycled to be introduced as feed to the high-pressure side (R3H) of the upstream RO stage R3, wherein the retentate stream leaving stage R3 is blended with the feed stream at mixing point MP1 to be introduced into the high-pressure side (R2H) of stage R2. The permeate of RO stages Rl, R2 and R3 are combined at mixing point MP2 to be collected as product water.

Fig 3. shows a five-stage embodiment of the invention, wherein the PWR unit comprises one single RO stage (Rl); and the downstream RO subunit comprises one RO stage (R2), the NF subunit comprises two NF stages in series (Nl and N2) and the upstream RO subunit comprises one RO stage (R3), within the SWR unit. The feed of aqueous solution is fed to its high-pressure (retentate) side (R1H) of stage Rl. The retentate stream leaving the stage Rl is introduced into the high- pressure side (R2H) of RO stage R2. The retentate stream leaving stage R2 is introduced into the high-pressure side (N 1H) of NF stage N 1 , wherein the rententate of the stage Nl is introduced as feed to the high-pressure side (N2H) of the second NF stage N2, wherein the rententate of the stage N2 is discharged as concentrate stream. The permeate streams of the stages Nl and N2 are combined at mixing point MP3 to be introduced as feed to the high-pressure side (R3H) of the upstream RO stage R3, wherein the retentate stream leaving stage R3 is blended with the feed stream at mixing point MP1 to be introduced into the high-pressure side (R2H) of stage R2. The permeate of RO stages Rl, R2 and R3 are combined at mixing point MP2 to be collected as product water.

In order to demonstrate that the invention can achieve desalinating seawater to produce a potable water product having a salt concentration less than or equal to 350 ppm at a high water recovery, reduced OPD, and competitive SEC, the qualitative analysis is carried out for the five-stage embodiment shown in Figure 3. The mathematical equations describing the interrelationship between the volumetric flowrates denoted by Qi and the salt concentrations expressed as mass per unit volume and denoted by Ci in Figure 3, where the subscript denotes the location of the particular stream or concentration will be solved analytically. The solution to this system of algebraic equations will permit determining the recovery, OPD, SEC, and initially unspecified salt rejections in each stage of the invention.

The analysis of this five-stage embodiment of the invention involves solving overall material and solute balances for each of the five stages and at three mixing points. The balances over stage Rl constitute 2 equations involving 6 unknowns (Q_F, C_F, Qo, Co, Qi, Ci) The balances over stage R2 constitute 2 equations involving 6 unknowns (Q₂, C₂, Q₃, C₃, Q₄, C₄) The balances over stage Nl constitute 2 equations involving 4 unknowns (Q₅, C₅, Q₆, C₆). The balances over stage N2 constitute 2 equations involving 4 unknowns (Q₇, C₇, Q_D, C_D) The balances over stage R3 constitute 2 equations involving 6 unknowns (Q₈, C₈, Q₉, C₉, Q₁₀, C₁₀). The balances at the mixing point (MP1) constitute 2 equations and no unknowns. The balances at the mixing point (MP2), combining permeates of stages Rl, R2 and R3 constitute 2 equations and 2 unknowns (Qp, Cp). The balances at the mixing point (MP3) combining NF permeates constitute 2 equations and no unknowns. This totals 16 equations that involve 28 unknowns. This implies 12 degrees of freedom in solving the equations for this five-stage embodiment of the invention process.

The 12 degrees of freedom were satisfied by specifying the following quantities shown below:

Q_F, flow rate of saline water feed to RO stage Rl

C_F, salt concentration in the feed to RO stage Rl

Cp, salt concentration in the water product

Y, overall water recovery of the process

Dpϋ₂ = Dp_Nΐ, equal OPDs in RO stage R2 and NF stage Nl

Dp_Nΐ = Dp_N2, equal OPDs in NF stage N 1 and NF stage N2 YNI, recovery in NF stage N 1

YN2, recovery in NF stage N2

C₃ = Cp, salt concentration in the permeate from RO stage R2 C₅ = Cp, salt concentration in the permeate from RO stage R3

DpM, OPD in RO stage Rl

Dpr3, OPD in RO stage R3

The system of equations that describe 5 -stage embodiment in Figure 3 is given below. An overall mass balance and a component mass balance on stages Rl, R2, Nl, N2, R3 and at the mixing points are given by the following equations:

QF = Qo + Qi equation (1)

Q2 = Q3 + Q4 equation (2) Q4 = Q5 + Q6 equation (3) Q6 = Q7 + QD equation (4) Qs = Q9 + Q10 equation (5) Q2 = Qi + Q10 equation (6) Qs = Qs + Q₇ equation (7) Qp = Qo + Q3 + Q9 equation (8)

QFCF = QOCO + Q1C1 equation (9) Q₂CF = Q3C3 + Q4C4 equation (10) Q4CF = Q5C5 + QeCe equation (11) Q₆CF = Q7C7 + QDCD equation (12) QSCF = Q₉C₉ + Q1₀C1₀ equation (13) Q2CF = Q1C1 + Q10C10 equation (14) QSCF = Q5C5 + QTC₇ equation (15) QPCF = QOFO + Q3C3 + Q9C9 equation (16)

The overall water recovery and the fractional recovery at each stage in the process are defined as

Y = Qp / QF equation (17) YRI = QO / QF equation (18) YR2 = Q3 / Q2 equation (19) YNI = Q5 / Q4 equation (20) YN2 = Q7 / Q6 equation (21) YR3 = Q9 / Qe equation (22)

When Equations (1-16) are combined with Equations (17-12), the overall recovery of the system and the recoveries in each stage can be expressed as follows: Y = YRI + Y_R2 + YR₃ equation (23)

YRI = (Ci - C_F) / (Ci - Co) equation (24) YR₂ = (C₄ - C₂) / (C₄ - C₃) equation (25) Y_R3 = (Cio - C₈) / (CIO - C₉) equation (26) YNI = (C_{6 -} C₄) / (C_{6 -} Cs) equation (27) Y_N2 = (C_{D -} C₆) / (C_{D -} C₇) equation (28) Apki = K(Ci - Co) equation (29)

Apki = K(C_{4 -} C₃) equation (30)

Dp_Ni = K(C_{6 -} Cs) equation (31)

DpN2 = K(CD - C₇) equation (32) Dpϋ3 = K(Cio - C9) equation (33)

Where K = 0.801 L-bar/g is an empirical constant. When all stages R2, Nl and N2 operate at the same OPD (Dpr₂ = DpN_ΐ = Dp\2),

C_{4 -} C₃ = C_{6 -} C5 = CD - Ci equation (34) For a specified salt concentration of the product water (Cp), specified overall water recovery (Y), the above equations can be solved analytically as a function of the OPDs in stages Rl and R3 (Dpio and Dpr and fractional water recoveries in stages Nl and N2 (YNI, YN2, respectively), that gives the following

Qo = QFYRI equation (35) Qi = QF - Qo equation (36) Q2 = Qi + Q10 equation (37) Q₄ = Q₆ / (1 - Y_N1) equation (38)

Q₅ = Q₆YNI / (1- YNI) equation (39)

Q₆ = Q_F(l - Y) / (1- Y_N2) equation (40) Q₇ = Q₆YN2 equation (41) Qs = Qs + Q₇ equation (42) Q9 = QSYR3 equation (43) Q10 = Qs - Q9 equation (44) QD = QFY equation (45) Solving for the salt concentrations gives the following: Co = Cp equation (46)

Ci = Dp_bi / K + Co equation (47) C₂ = (C_{F -} YCp) / (1 - Y) equation (48)

C₃ = C_P equation (49) C₄ = ((YNI + Y_N2)C₃ + C_D) / (1+ YNI + Y_N2) equation (50) C₅ = C₆ + C_{7 -} C_D equation (51) C₆ = CD - YN₂(CD - C₇) equation (52)

C₇ = C_D + C_{3 -} C₄ equation (53) C₈ = (Q5C5 + Q₇C₇) / Qs equation (54) C₉ = C_P equation (55)

C10 = Dpr₃ / K + C9 equation (56) C_D = (CF - YC_P) / (l - Y) equation (57)

Then the required salt rejection of

stage can be calculated as:

ORI = 1 - Co / CF equation (58)

OR2 = 1 - C3 / C2 equation (59)

O_R3 = 1 - C9 / C₈ equation (60)

ONI = 1 - C5 / C4 equation (61) sN2 = 1 - C₇ / C₆ equation (62) and Dt¾2, Dp_Nΐ and Art_N2can be calculated through Equations (30-32). The gross specific energy consumption (SEC), which is the energy required per unit of water produced is given by the following:

SEC = (QFA:TI;RI + z⁾i(Dti¾2-Dpri) + z⁾io(Dp¾2-D7ΐ¾3) + z⁾ ₄DpNΐ + Q AKS2) / r|pQp equation (63) where hr is the efficiency of the pumps. Then, the net specific energy consumption (SEC_net), 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:

SECnet = SEC - T|ERDQDA:TI;N2 / Qp equation (64) where T|ERD is the efficiency of the ERD. The predictions of Equations (35-57) will be used to establish the proof-of-concept for this reduced -pres sure reverse osmosis (REPRO) invention through the following examples. The performance of the REPRO invention will be assessed in terms of the OPD and SEC_net required to produce a potable water product containing no more than 350 ppm of salt from a saltwater feed. It must be noted that the reduction in OPD and specific energy consumption will also reduce the fixed costs since the pumps, membrane modules, and piping will not need to sustain the high pressures required for conventional SSRO as well as the equipment maintenance costs. In the following examples, it is assumed that the pump and ERD efficiencies are 85% and 90%, respectively, which are consistent with commercially available devices.

One benefit of the REPRO invention is that overall water recoveries greater than 55% can be achieved while operating each stage with a stage recovery less than or equal to 50% and maintaining the safety factor in each stage at a value less than or equal to 1.

Another benefit is that the present invention allows the maximum OPD (OPD_max) to be varied over a wide range in order to achieve a specif ed overall water recovery and specified salt concentration of the product water. The maximum OPD_max is determined by the salt concentration difference between the retentate of the downstream RO subunit (2A) that is introduced to the NF subunit (3A) and the permeate of the downstream RO subunit (2A). For example, in the 5-stage embodiment of the REPRO invention shown in Figure 3, the OPD_max is determined by the salt concentration difference between streams 4 and 3, hence the OPD_max is equal to Dp ₂. This allows an additional flexibility in designing the REPRO system for a desired overall water recovery by setting optimally the water recoveries in the NF stages and determining required salt rejections of the NF membranes and the OPD_max in order to minimize the overall cost of the desalination process.

On the other hand, in a conventional SSRO system, the OPD_max is the same as the OPD of the SSRO stage; and in a two-stage TSRO system, the OPD_max is equal to the OPD of the second RO stage. Once the salt concentration of the product water is specified, the OPD_max is determined by the target overall water recovery, since it is a function of the salt concentration difference between the discharged concentrate stream and the permeate stream leaving the same stage as the discharge; hence it is not possible to vary OPD_max while keeping the overall water recovery constant.

Example 1:

In order to produce a potable water product containing no more than 350 ppm of salt from a saltwater feed with 35000 ppm salt concentration, the specific energy consumption and OPD required to obtain overall water recoveries of 65% and 75% are evaluated for the 4-Stage and 5-Stage embodiments of the REPRO inventions. The input parameters are OPD in RO stage“Rl” (Dpki) and fractional water recovery in NF stage“Nl” (YNI). The NF stages Nl and N2 have the same OPD as stage Rl. The OPD in the upstream RO stage“R3” (Dpk₃) is set to have the same OPD as stage Rl in both embodiments, hence the retentate of stage R3 can be combined with the retentate of stage Rl in order to be pressurized using a single pump to be introduced to the high-pressure side of stage R2 (R2H). The NF stage “N3” is set to have fractional water recovery (YN2) equal to YNI in stage“Nl” in the 5-stage embodiment. The value of YNI is varied between the minimum and maximum given in Table 1 for each case to show the flexibility of the embodiment in setting the OPD in the downstream RO stage“R2” (Dpk₂) for a target overall water recovery. The values of Dpio are also provided in Table 1 for each of the cases. The table includes also the required salt rejections of the NF membranes corresponding to minimum and maximum stage recoveries.

Figure 4 shows the tradeoff between the net SEC and Dpk₂ required to produce a potable water product containing no more than 350 ppm of salt from a saltwater feed with 35000 ppm salt concentration, as a function of Y_NI . The specific energy consumption and OPD required in SSRO are 79.3 bar and 2.922 kWh/m³, respectively, for 65% overall water recovery, and are 111 bar and 3.915 kWh/m³, respectively, for 75% overall water recovery. The specific energy consumption required in TSRO is 2.256 kWh/m³ for 65% overall water recovery, and is 2.705 kWh/m³ for 75% overall water recovery, while the OPD requirements are the same as in SSRO.

It is of interest to compare both embodiments of the REPRO of the present invention with conventional SSRO and two-stage RO (TSRO) based on the specific energy consumption and OPD requirement. The TSRO comprises two RO stages in series wherein the retentate of the first stage is introduced into the high-pressure side of a second RO stage wherein the OPD in the second stage (Dpi^) is higher than the OPD in the first stage (Dpio). The OPD requirements (Dpk₂₎ in 4-Stage and 5-Stage REPRO embodiments are <60.5 and <55.1 bar, respectively, for 65% overall water recovery; and <87.2 and <79.3 bar, respectively, for 75% overall water recovery, with comparable net SEC values. It is of interest to note that commercial membranes such as DOW® FILMTEC™ SW30 allows a maximum operating pressure of 69 bar, while special membranes such as DOW® FILMTEC™ SW30HR may allow a maximum operating pressure of 84 bar. Hence, achieving overall water recoveries above 65% using an SSRO or TSRO process is not practically viable with current commercial membranes.

Table 1. Input parameters (Dp_M and YNI) and required membrane rejections for Example 1

Example 2:

It is of interest to compare 4-Stage and 5-Stage embodiments of the REPRO of the present invention with the SSRO based on the metrics of specific energy consumption and OPD required to produce a potable water product containing no more than 350 ppm of salt from salt water feed with different salt concentrations (CF = 10000 ppm, CF = 35000 ppm and CF = 45000 ppm). The fractional water recovery of stages Nl (YNI) and the OPD of the first RO stage (Dpki are input parameters in solving the model equations. In order to determine the minimum OPD required in the downstream RO stage Rl, YNI, and if applicable YN2 are set to 0.50. The NF stages N 1 and N2 have the same OPD as stage Rl . Table 2 summarizes the OPD, net SEC and stage recoveries for selected overall water recoveries for the two embodiments in comparison to SSRO. The comparisons show that the embodiments of the REPRO of the present invention allow production of potable water with substantially lower OPD values relative to SSRO. Table 2. Comparison of the OPD, net SEC and stage rejections for desalination using SSRO and the 4-Stage and 5-Stage embodiments of the REPRO invention for producing a water product with a salt concentration equal to 350 ppm for a range of saltwater feed concentrations.

Example 3:

It is of interest to compare the 4-Stage and 5-Stage embodiments of the present invention with the two-stage TSRO based on the metrics of specific energy consumption and OPD required to produce a potable water product containing no more than 350 ppm of salt from saltwater feed with a salt concentrations of 35000 ppm. The OPD in the upstream RO stage“R3” (Dpk₃₎ is set to have a different OPD than the OPD in stage Rl in both embodiments in order to further reduce the net SEC requirement. The fractional water recovery of stages Nl (YNI) and the OPDs of the RO stage Rl (Dpia) ^and upstream RO stage R2 (Dpk₂₎ are input parameters in solving the model equations as given in Table 3. The NF stages Nl and N2 have the same OPD as stage Rl. The NF stage“N3” is set to have fractional water recovery (YN2) equal to YNI in stage“Nl” in the 5-stage embodiment.

Table 3. Comparison of the OPD, net SEC and stage rejections for desalination using TSRO and the 4-Stage and 5-Stage embodiments of the REPRO invention for producing a water product with a salt concentration equal to 350 ppm for a saltwater feed with 35000 ppm salt concentration.

In the comparisons above, the operating practice for RO that is to maintain the safety factor at a value greater than or equal to one in order to minimize membrane scaling, is taken into account. However, since the feed to the upstream RO subunit (4A) is the permeate of the NF subunit (3A) that is relatively free from scale forming ions, it will be possible to operate the upstream RO subunit (4 A) at a safety factor less than one, which corresponds to a fractional stage recovery greater than 0.50.

Alternatively, fraction of a retentate stream from any NF stage, which needs to be choses optimally, can be recycled to the high-pressure side of that stage in order to maintain the salt concentration in the feed stream to the NF stage at the optimum value if commercial membranes with higher salt rejection are used in that NF stage.

Claims

1. A system for reverse osmosis, the system comprising;

a primary water recovery (PWR) unit (1A) comprising one single reverse osmosis (RO) stage or a plurality of reverse osmosis (RO) stages in series configuration wherein the retentate of the preceding reverse osmosis (RO) stage is fed to the high-pressure side of the subsequent reverse osmosis (RO) stage,

a secondary water recovery (SWR) unit (5A); comprising nanofiltration (NF) subunit (3 A) wherein the rententate of the nanofiltration (NF) subunit (3 A) is discharged as concentrate stream, downstream reverse osmosis (RO) subunit (2A) wherein downstream is defined as the direction of the retentate flow from the NF subunit (3 A), and upstream reverse osmosis (RO) subunit (4 A) wherein upstream is defined as the opposite direction of the retentate flow from the NF subunit (3 A)

characterized in that

the rententate of the primary water recovery (PWR) unit (1A) is introduced to the secondary water recovery (SWR) unit (5 A) as feed to the high-pressure side of the downstream reverse osmosis (RO) subunit, (2H) wherein the rententate of the downstream reverse osmosis (RO) subunit (2A) is introduced as feed to the high-pressure side of the nanofiltration (NF) subunit (3H).

2. The system for reverse osmosis of claim 1, wherein the rententate of the nanofiltration (NF) subunit (3 A) is discharged as concentrate stream and the permeate of the nanofiltration (NF) subunit (3 A) is recycled to be introduced as feed to the high-pressure side of the upstream reverse osmosis (RO) subunit (4H).

3. The system for reverse osmosis of claim 1, wherein the permeate of the upstream reverse osmosis (RO) subunit (4A) is collected as product water and the rententate of the upstream reverse osmosis (RO) subunit (4A) is introduced together with the retentate of the primary water recovery (PWR) unit (1A) as feed to the high-pressure side of the downstream reverse osmosis (RO) subunit (2H).

4. The system for reverse osmosis of claim 1, wherein each subunit within the secondary water recovery (SWR) unit (5 A) comprises one single membrane stage or a plurality of membrane stages in series configuration wherein the retentate of the preceding membrane stage is fed to the high-pressure side of the subsequent membrane stage and wherein the permeates of the membrane stages are combined.

5. A process of reverse osmosis, the process comprising the steps of:

introducing a feed of aqueous solution into a primary water recovery (PWR) unit (1A) comprising at least one single reverse osmosis (RO) stage;

introducing retentate from the primary water recovery (PWR) unit (1A) into high-pressure side of a downstream reverse osmosis (RO) subunit (2H) within a secondary water recovery (SWR) unit (5A) comprising a downstream reverse osmosis (RO) subunit (2A), a nanofiltration (NF) subunit (3A) and an upstream reverse osmosis (RO) subunit (4A);

introducing retentate from the downstream reverse osmosis (RO) subunit (2A) into high-pressure side of the nanofiltration (NF) subunit (3H);

introducing permeate from the nanofiltration (NF) subunit (3 A) into high-pressure side of the upstream reverse osmosis (RO) subunit (4H);

introducing retentate from the upstream reverse osmosis (RO) subunit (4A) as feed to the high-pressure side of the downstream reverse osmosis (RO) subunit (2H);

collecting as product permeates from all reverse osmosis (RO) stages.

6. The process of reverse osmosis of claim 5, wherein safety factor, which is defined as the ratio of the retentate flow rate to the permeate flow rate for the same membrane stage, in each stage at a value less than or equal to 1.