Alternative design to dual stage NF seawater desalination using high rejection brackish water membranes

Desalination 273 (2011) 391–397

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Alternative design to dual stage NF seawater desalination using high rejection brackish water membranes Ali AlTaee a, Adel O. Sharif b,⁎ a b

Collage of Civil Engineering and Computer Science Faculty, Abu Dhabi University, United Arab Emirates Centre for Osmosis Research & Application, Chemical & Process Engineering Department, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, UK

a r t i c l e

i n f o

Article history: Received 12 November 2010 Received in revised form 20 January 2011 Accepted 21 January 2011 Available online 1 March 2011 Keywords: Nanofiltration Desalination Dual stage NF Seawater Brackish water Reverse osmosis

a b s t r a c t Dual stage NF membrane desalination process was proposed as an alternative approach to RO seawater desalination. Despite of being cheaper than RO desalination, dual stage NF process is not commercially applied yet due to the low overall recovery rate. In an attempt to increase the process recovery rate and to reduce the operation complicity, brackish water RO membrane was used instead of NF in the second stage. ROSA software was used in this study to verify the applicability and cost-effectiveness of the NF–BW dual stage desalination process. Similarly to dual stage NF desalination process, an NF membrane was used in the first stage and BW membrane in the second stage. Permeate from NF membrane was used as a feed into the BW membranes. The effect of membrane type and seawater salinity on the process performance was investigated. For any given recovery rate and seawater salinity, the simulation results showed that the overall cost of NF–NF was slightly lower than the NF–BW process but that was on the cost of higher permeate TDS. For instance, at 43,000 mg/l feed salinity the difference in the specific power consumption between NF–NF and NF–BW process was 0.38 kWh/m3. The permeate TDS was 125 mg/l for NF–BW and 1030 mg/l for NF–NF process. The difference in the permeate TDS between NF–NF and NF–BW process increased with increasing the feed salinity. In dual stage NF process, a low permeate was achieved at low recovery rate. For example, at 43,000 mg/l feed salinity the permeate TDS from dual stage NF process was 359 mg/l when the overall recovery rate was 22%. It was also found that the effect of BW membrane type on the process efficiency was insignificant. Finally, the energy requirements of NF–NF and NF–BW were compared to a single stage RO desalination process. The specific power consumption at 43,000 mg/l feed salinity was 4.58 kWh/m3, 4.2 kWh/m3 and 3.86 kWh/m3 for RO, NF–BW and NF–NF process respectively. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In the middle of the last century few desalination plants were built to serve small communities and cities in water stressed area especially in the Middle East areas. Seawater desalination, later on, became a new trend of fresh water supply to small and large cities due to water scarcity or insufficient water resources. Desalination plant are also responsible for providing fresh water to small and big industries when the natural fresh water resources are not sufficient or when high quality water is required as in the case of pharmaceuticals and electronic manufacturing industries [1–3]. Currently, two main processes dominate the desalination market; i.e. thermal and membrane. Other technologies, such as electrodialysis, still exist but only in small capacities. Thermal desalination is more robust and doesn't require an intensive pretreatment but it is known for its high energy requirements [4,5] while membrane desalination is more of a

⁎ Corresponding author. Tel.: + 44 1483 686584; fax: + 44 1483 686581. E-mail address: [email protected] (A.O. Sharif). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.01.056

state-of-the art process which has received more attention due to its high efficiency and low energy requirements compared to the thermal processes. Nevertheless, membrane desalination still requires an intensive pretreatment to prevent membrane fouling. Reverse osmosis membranes were commonly, so far, used for seawater desalination. Other membranes, however, like NF membranes were proposed as an alternative to RO membrane for desalting seawaters [6]. NF membranes have intermediate rejection rate between RO and ultrafiltration. Depending on the membrane structure, NF membranes have high rejection rate to divalent ions which exceeds 98% and average to low rejection rate to monovalent ions [1,7]. The permeability of NF membrane is several times higher than RO membranes. Therefore, the main advantage of NF membranes is the lower energy consumption compared to RO membranes. Recently a novel dual stage NF membrane treatment process was suggested for seawater desalination [6,10]. Researchers claimed that dual stage NF process was able to produce potable water from seawater at a lower cost than RO. The process was able to desalt seawater by using high performance NF membranes. In the first stage seawater is pumped into the first stage NF membranes to produce a first permeate with

2. Simulation methodology NF membranes, generally, are able to reject divalent ions up to 98% while the rejection rate of monovalent varies from 30% to 85% depending on the ion size and the type of NF membrane [1, 7–9, 11– 12]. The molecular weight cut-off for NF membranes varies from 200 to 2000 Dalton. For instance, the rejection rate of Filmtec NF membrane NF90-400 to MgSO4 is > 97% [10]. Theoretically, a dual stage NF membrane process can be used to produce potable water from seawater. In the first stage a high performance NF membrane receives seawater feed to produce a first permeate which usually has a TDS equivalent to brackish water [6,10]. Another high performance NF membrane is used in the second stage to produce potable water from the first permeate. The required feed pressure in first stage is between 28 and 35 bar while in the second stage is between 14 and 21 bar [6,13]. As described by Vuong [6], the process is capable of producing potable water from seawater only when the concentrate from second stage is recycled back to the first stage feed (Fig. 1). Pilot tests demonstrated that using a loose structure high permeability NF membrane in stage 1 increased the first permeate TDS which in turn required a tight membrane in stage 2 to guarantee the high rejection rate. This system was characterized by high recovery rate but it was not the most energy efficient configuration [13]. In contrast, if a tight membrane was used in stage 1 then the permeate flow wouldn't be enough to feed stage 2 which would increase the overall energy consumption. In consequence, a high performance NF membrane was used in stage 1 and 2 to produce potable water from seawater. One of the drawbacks of such system is the exacting operating condition and low recovery rate compared to

1.4

30

1.2

25

1

20

0.8 15 0.6 10

0.4 Es-NF Es-BW Pf-NF Pf-BW

0.2

0

0 25

30

35

40

45

50

55

HP pump 1

70

75

80

RO process. A recovery rate less than 39% was achieved to produce a second stage permeate slightly lower than 200 ppm TDS from 35,000 ppm feed salinity [13]. In order to improve the system recovery rate, the current study proposed using a slightly different system in which a high performance NF membrane is used in stage 1 and a tight brackish water RO (BWRO) membrane in stage 2. According to this design there will be no difference in the energy consumption in stage 1 but the energy requirements in stage 2 will be slightly higher than in the dual stage NF design. Therefore, the overall power consumption will be subtly higher. Once again using BWRO in stage 2 will not significantly increase the energy consumption due to the low osmotic pressure of stage 2 feed. The specific power consumption in stage 2 is affected by the membrane recovery rate which follows the normal trend observed in previous studies [14]. Energy consumption tends to increase at high and low recovery rates and drop in the middle reaching the optimal value approximately around 55% (Fig. 2). At 55% recovery rate, there was only 0.36 kWh/m3 difference in the power consumption between NF and BWRO membranes. The difference in power consumption between NF and BWRO membranes increases at high recovery rates (Fig. 2). Fig. 2 also shows that the difference in the feed pressure between NF and BWRO membranes increases with the recovery rate. At 55% recovery rate, the difference in the applied feed pressure between NF and BWRO membranes was 5.7 bar. Besides power consumption, permeate quality is the other important parameter in membrane filtration process. Similarly to specific power consumption, permeate quality is also affected by the membrane recovery rate (Fig. 3). The concentration of permeate tends to increase at low and high recovery rates due to differential diffusion of water and salt across the membrane. However, an increase in

Es (kWh/m3)

2nd stage NF

65

Fig. 2. Specific power consumption and feed pressure for different recovery rates. Simulation condition: feed TDS 4611 ppm, feed temp 25 °C, feed pH 7.6.

900

Es-NF Es-BW Cp-NF Cp-BW

1.2

1st stage NF

60

%Recovery

1.4

Freshwater tank

5

1

800 700 600

0.8

500

0.6

400 300

0.4

200

HP pump 2

0.2

100 0

0 25

2nd stage brine recycle to 1st stage feed Fig. 1. Dual stage NF desalination process (stage 1; 6 element/PV, stage 2; 5 element/ PV). Simulation condition: feed TDS 4611 ppm, feed temp 25 °C, feed pH 7.6.

30

35

40

45

50

55

60

65

70

75

Permeate concentration (mg/l)

salinity less than the seawater and concentrate or retentate of higher salinity than the seawater. Then, the first permeate from the first stage is pressurized and pumped to the second stage NF membranes to produce potable water and second concentrate of salinity less than the seawater. Optionally, but preferred, the second concentrate is recycled back to the first stage NF feed tank. It has been found that energy consumption for seawater desalination by dual stage NF membrane is proportional to the permeate salinity. The lower permeate salinity is the higher energy requirements. In this paper ROSA (Reverse Osmosis System Analysis) [15] simulation program was used to predict the performance of dual stage NF membranes. The effects of feed water salinity and membrane type were taken into account. Different type of NF and BWRO membrane were used in the second stage to produce potable water from seawater. It is envisaged here that BWRO membranes are able to produce high quality product water without significantly compromising the cost of the desalination process. Results from using different types of NF and BWRO membranes were eventually compared against single stage RO desalination in order to estimate the cost effectiveness of the process.

Feed pressure (bar)

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397

Es (kWh/m3)

392

80

%Recovery rate Fig. 3. Power consumptions and permeate concentration at different recovery rates. Simulation condition: feed TDS 4611 ppm, feed temp 25 °C, feed pH 7.6.

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397 Table 1 Membrane characteristics. Filmtec membrane

Area %Re (m2)

Table 3 Type and number of tests on each membrane or membrane combination. Aw (m3/ B (m/d) Simulation condition⁎ m2d.bar)

SW30HRLE- 37 400i

0.9975 0.025

NF90-400

0.85

0.267

0.97

0.237

37

393

BW30-440i

41

0.995

0.077

BW30-400

37

0.99

0.0797

0.00189 32,000 ppm NaCl, 5 ppm boron, 55 bar, pH 8, 25 C and 8% recovery 0.135 2000 mg/l NaCl, 4.8 bar, 25 °C, rejection 85%, and 15% recovery. 0.03 2000 mg/l MgSO4, 4.8 bar, 25 °C, rejection > 97%, and 15% recovery. 0.0053 2000 ppm NaCl, 15.5 bar, 25 °C, pH 8, rejection 90%, and 15% recovery 0.0109 2000 ppm NaCl, 15.5 bar, 25 °C, pH 8, rejection 99%, and 15% recovery

⁎ Dow membrane simulation conditions.

permeate salinity is less obvious in BWRO than NF membrane because of its higher salt rejection rate (Fig. 3). The TDS of product water at 55% recovery rate is 572 mg/l and 70 mg/l for NF and BWRO membrane respectively. Therefore, energy penalty caused by the high resistance of BWRO membrane can be initially justified by the high permeate quality. The specifications of membranes used in this study are illustrated in Table 1. SWRO membrane type SW30HRLE-400i is a high rejection rate membrane which is suitable for seawater treatment in a conventional single stage RO plants. Simulation results from SW30HRLE-400i membrane were used as a baseline for comparison purpose. Nanofiltration membrane NF90-400 is a tight structure membrane with stabilized salt rejection for NaCl between 85% and 95%. The rejection rate for divalent ions is over 97% (Table 1). The latter membrane was used in the pilot plant tests of dual stage NF desalination technology [13]. The last two membranes in Table 1 are BWRO membranes which were used in this study to demonstrate the feasibility of using a high rejection membrane in stage 2 dual stage NF desalination. BWRO membrane type BW30-400 has slightly higher water and salt permeability than BW30-440i. Both membranes have high salt rejection rate approaching 99% and they were used in stage 2 instead of NF90-400. The composition of seawater used in this study is described in Table 2 [15]. Initially, the concentration of seawater is 35,000 ppm; for simplicity the concentrations from 37,000 ppm to 43,000 ppm were estimated by equally increasing the concentrations of sodium and chlorides ions only.

Numerical experiment NF–NF 1 2 3 4 5 NF–BW 1 2 3 4 5 NF–BWRO 1 2 3 4 5 SWRO 1 2 3 4 5

Stage 1 membrane

Stage 2 membrane

Feed salinity (mg/l)

NF90-400 NF90-400 NF90-400 NF90-400 NF90-400

NF90-400 NF90-400 NF90-400 NF90-400 NF90-400

35,076 37,077 39,077 41,078 43,078

NF90-400 NF90-400 NF90-400 NF90-400 NF90-400

BW30-400 BW30-400 BW30-400 BW30-400 BW30-400

35,076 37,077 39,077 41,078 43,078

NF90-400 NF90-400 NF90-400 NF90-400 NF90-400

BW30-440i BW30-440i BW30-440i BW30-440i BW30-440i

35,076 37,077 39,077 41,078 43,078

SW30HRLE-400i SW30HRLE-400i SW30HRLE-400i SW30HRLE-400i SW30HRLE-400i

– – – – –

35,076 37,077 39,077 41,078 43,078

3. Results and discussion Table 3 represents the case studies simulated by the Filmtec ROSA6.1 program. Four types of membrane or/and membrane combinations were investigated in the current study; for each membrane type five tests were conducted (Table 3). The SWRO membrane tests were used as a baseline for comparison purposes. In the latter experiments the feed pressure and specific energy consumption were recorded and plotted for different seawater TDS (Fig. 4). As expected, specific power consumption increased with seawater salinity as a result of higher feed pressure required to overcome feed osmotic pressure. It is worth to be noted here that a 2 stage RO system is very common in the seawater desalination process. Typically, RO membrane is used in stage 1 while BWRO membrane is used in stage 2. Unlike stage 1, stage 2 is often a low pressure process to produce a high quality permeate with low TDS. For the other experiments, a high performance NF membrane was used in stage 1 while three different types of membrane were used in stage 2. Option 1 which is a high performance NF membrane, similar to that in stage 1, was used in stage 2. Alternatively, high rejection rate BWRO membranes were used in stage 2. As mentioned earlier, the purpose from using a high rejection rate BWRO in stage 2 was to

Table 2 Seawater composition. Concentration (mg/L)

70

Calcium Magnesium Sodium Potassium Barium Strontium Iron Manganese Silica Chloride Sulfate Fluoride Bromide Bicarbonate Boron TDS pH

410 1310 10,900 390 0.05 13 b0.02 b0.01 0.04–8 19,700 2740 1.4 65 152 2–5 35,000 8.1

60

5 4.5 4

50

3.5

40

3 2.5

30

2 1.5

20

1

Feed pressure Es

10

Es (kWh/m3)

Feed pressure (bar)

Ion

0.5

0

0 35000

37000

39000

41000

43000

Seawater TDS (mg/l) Fig. 4. SW30HRLE-400i feed pressure and specific power consumption. Simulation condition: Recovery rate 45%, pH 8.1, 7 elements/PV, and feed temperature 25 °C.

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397

Feed

Permeate

0 63.53 1646.16 59.79 18.82 0.36 0 0.25 8.35 0 2753.29 0.24 61.22 0 0 0.05 4612.03 17.42

0 0.95 24.1 0.48 0.15 0 0 0 0.22 0 36.36 0 0.32 0 0 0.02 65.59

Feed 0 63.51 1645.51 59.77 18.81 0.36 0 0.25 8.35 0 2752.38 0.24 61.2 0 0 0.05 4610.52 16.37

NF90-400 Permeate 0 1.04 26.32 0.52 0.16 0 0 0 0.23 0 42.98 0 0.35 0 0 0.02 71.6

Feed 0 63.36 1642.89 60.15 18.93 0.36 0 0.25 8.4 0 2748.98 0.24 61.68 0 0 0.05 4605.27 10.7

Permeate 0 8.62 217.68 3.13 0.96 0.2 0 0 0.57 0 353.03 0.04 1.28 0 0 0.02 585.33

70 60

35 50

30

40

25 NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400 NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400

30 20

Recovery rate (%)

Feed pressure 1st pass (bar)

6 4 2

30 20 10 0

0 37000

39000

41000

43000

70

60

ð1Þ

40

5

NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400 NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400

8

65

56

45

10

40

10

Fig. 6. Feed pressures in pass 2. Simulation conditions: feed temp 25 °C, feed pH 8.1, 6 elements/PV.

Where Pf is feed pressure (bar), Qf is feed flow rate (m3/h), Qp is permeate flow rate (m3/h), and η is the pump efficiency. Therefore,

15

50

12

58

Pf Qf Qp η

20

14

Seawater TDS (mg/l)

achieve a high recovery rate desalination process with high permeate quality. The composition of feed and permeate stream for NF and BWRO membranes used in this study are shown in Table 4. As predicted by ROSA6.1 program, the rejection rate of BWRO was much higher than NF membrane especially for monovalent ions. The permeate TDS produced by the BWRO membrane was almost 9 times lower than the NF permeate. Using the same NF membrane in stage1 guaranteed an identical feed to the membranes in stage 2 (Table 4). The applied feed pressure in pass1 is shown in Fig. 5. As expected, the required feed pressure was the same in all experiments. However, this was not the case in stage 2 in which different membranes were used. The required feed pressure for BWRO filtration was higher than in NF membrane due to the higher rejection rate and resistance of the former membrane (Fig. 6). Also there was a slight drop in feed pressure with increasing feed salinity (Fig. 6). This was because of limited feed permeate from stage 1 to stage 2. In pressure driven membrane the specific power consumption, Es, is function of feed pressure, feed flow and permeate flow as in the following equation:

Es =

60

16

35000

10

0

0 35000

37000

39000

41000

43000

Seawater TDS (mg/l) Fig. 5. Feed pressure in stage 1. Simulation conditions: feed temp 25 °C, feed pH 8.1, 6 elements/PV.

%R 1st pass

NH4 K Na Mg Ca Sr Ba CO3 HCO3 NO3 Cl F SO4 SiO4 Boron CO2 TDS Pf Recovery

BW30-440i

70

18

54

60

52 50

55

48 50

46 44

Recovery rate 1 st pass

42

Recovery rate 2 nd pass

%R 2nd pass

BW30-400

45

40

40 35000

37000

39000

41000

43000

Seawater TDS (mg/l) Fig. 7. Recovery rates in pass 1 and 2. Simulation conditions: feed temp 25 °C, feed pH 8.1, 1st 6 elements/PV, 2nd pass 5 elements/PV, Pf 40 bar.

the specific power consumption decreases with the in feed flow rate decrease. To increase the permeate feed to stage 2 a higher recovery rate was required in stage 1 but to achieve that the maximum feed pressure in stage 1 would be exceeded (Fig. 5). The osmotic pressure in pass1 feed increased with feed salinity which resulted in a lower recovery rate to maintain the maximum feed pressure limit in NF membrane as recommended by the manufacturing company (Fig. 7). The recovery rate of stage 1 and 2 decreased with increasing feed salinity which substantiated the fact that the performance of stage 2 was directly affected by the performance of stage 1. The higher the feed salinity is the lower the stage 1 recovery rate. Fig. 7 also shows that the drop of stage 1 recovery rate was twice that in stage 2; this was mainly due to the higher feed osmotic pressure in stage 1.

Permeate TDS (Mg/l)

Ions

20

Recovery rate (%)

Table 4 Stage 2 feed and permeate TDS.

Feed pressure 2nd pass (bar)

394

1100 1000 900 800 700 600 500 400 300 200 100 0 34000

NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400 NF90-400/NF90-400 low %R

36000

38000

40000

42000

44000

Seawater TDS (mg/l) Fig. 8. Stage 2 permeate TDS. Simulation conditions: feed temp 25 °C, feed pH 8.1, 6 elements/PV, (%R shown in Fig. 7).

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397

1

NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400 RO-SW30HRLE-400i NF90-400/NF90-400 low %R

4.5

Es 2nd pass (kWh/m3)

Es total (kWh/m3)

5

4 3.5 3 2.5 34000

36000

38000

395

40000

42000

0.9 0.8 0.7 0.6 0.5 0.4 0.3 NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400

0.2 0.1 0 30000

44000

32000

Seawater TDS (mg/l)

34000

36000

38000

40000

42000

44000

Seawater TDS (mg/l)

In the current study, same feed salinities and recovery rates were used in stage 1 and 2. Although the permeate TDS of stage 1 was identical in all experiments, simulation results showed that the permeate TDS of stage 2 was different for different membranes (Fig. 8). A lower permeate TDS was achieved when BWRO membrane were used in stage 2. The higher rejection rate of BWRO membrane, especially to monovalent ions, resulted in a better product water quality than NF membrane. Monovalent ions was poorly rejected by stage 1 NF membrane and when NF membrane was used in stage 2 the product water quality decreased sharply with increasing feed TDS. However, the product water quality was less affected by the feed salinity when BWRO membrane was used in stage 2 due its high rejection rate to monovalent ions. For instance, at feed salinity 35,000 ppm the permeate TDS from stage 2 was 585 ppm for NF membrane and between 67 ppm and 72 ppm for BWRO membranes. A sharp increase in the permeate TDS was observed with increasing the feed salinity when NF membrane was used in stage 2. While the increase in permeate TDS with feed salinity was less noticeable when BWRO membrane was used in stage 2 (Fig. 8). At feed salinity 43,000, the permeate TDS for the stage 2 NF membrane increased to 1030 ppm and for the BWRO membrane was between 125 ppm and 136 ppm. The high recovery rate and better product water quality when BWRO membrane was used in stage 2 couldn't be achieved without some energy penalty. As expected, the energy consumption in dual stage NF/BWRO was higher than in dual stage NF/NF membrane (Fig. 9). The higher energy consumption in the former case was realized in stage 2 BWRO membrane. Figs. 10 and 11 show the energy consumption in stage I and 2. A quick examination of energy consumption results showed that there was no difference in energy consumption in stage 1 as a result of using the same type of NF membrane (Fig. 10). This was not the case, however, in stage 2 when different NF and BWRO membrane were used (Fig. 11). Results

Fig. 11. Power consumption in stage 2. Simulation conditions: feed temp 25 °C, feed pH 8.1, 5 elements/PV, (%R shown in Fig. 7).

50

Total recovery rate (%)

Fig. 9. Total power consumption in dual stage membrane filtration. Simulation conditions: feed temp 25 °C, feed pH 8.1 For dual stage (1st 6 elements/PV, 2nd pass 5 elements/PV)%R in Fig. 7: for RO 7 elements/PV, %R 45.

NF90-400/BW30-400 NF90-400/NF90-400 high recovery NF90-400/NF90-400 low recovery

45 40 35 30 25

20 35000 36000 37000 38000 39000 40000 41000 42000 43000 44000

Feed TDS (mg/l) Fig. 12. Overall system recovery rate.

showed that the energy consumption was higher in case of BWRO membrane because of its higher rejection rate and resistance to permeate flow compared to NF membrane. But the difference in power consumption between stage 2 BWRO and NF membrane was insignificant and decreased with increasing feed water TDS. Typically, stage 2 is a low pressure process due to the low osmotic pressure of feed solution and high recovery rates that could be achieved in this stage. Consequently, the energy consumption was low. This is, probably, one advantage of using tight BWRO in stage 2 rather than NF membrane. For instance, at feed salinity 43,000 ppm, the difference in stage 2 power consumption between NF and BWRO was 0.32 kWh/m3. Furthermore, results revealed that the difference in power consumption between using BW40-400 and BW30-440i was insignificant (Fig. 11). In general, the total energy requirements of dual stage NF desalination was lower than that in RO as shown in Fig. 9. The other advantage of using BWRO membrane in stage 2 was the high recovery rate that can be achieved without compromising the end product quality. The overall system recovery rate, which is stage 2

3.4

Es 1st pass

3 2.8 2.6 NF90-400/BWRO30-400 NF90-400/BWRO30-440i NF90-400/NF90-400

2.4 2.2 30000

Permeate TDS (mg/l)

600

3.2

500 400 300 200 100

Stage 2 premeate TDS

0 32000

34000

36000

38000

40000

42000

44000

Seawater TDS (mg/l) Fig. 10. Power consumption in stage 1. Simulation conditions: feed temp 25 °C, feed pH 8.1, 6 elements/PV, (%R shown in Fig. 7).

15

17

19

21

23

Feed flow (m3/h) Fig. 13. Effect of feed flow rate on permeate TDS. Simulation conditions: feed temp 25 °C, feed pH 8.1, 6 elements/PV, feed TDS 35,000 mg/l.

396

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397

1.7

Cost (million $/year)

Total recovery rate (%)

60 50 40 30 20 Dual Stage NF/NF-NF/BW RO

10

1.6 1.5 1.4 1.3 1.2

NF90-400/BW30-400

1.1 1 35000

NF90-400/NF90-400 low %R

37000

41000

43000

45000

Fig. 15. Energy cost per annum.

Feed TDS (mg/l) Fig. 14. Total recovery rate in RO and dual stage NF process.

permeate flow rate divided by the raw water flow rate, was 48% at feed TDS 35,000 mg/l and dropped to 30% when feed TDS increased to 43,000% (Fig. 12). Fig. 12 also shows the total recovery rate in conventional low recovery dual stage NF process. As such the recovery rate in the latter process was lower than that could be achieved in NF/ BW process but the permeate TDS was better than that in high recovery dual stage NF process (Fig. 8). Typically, the permeate TDS from low recovery NF was of a better quality than that achieved from high recovery NF process as evidently shown in Fig. 12. There are several measures that could be applied to maintain a high product quality in dual stage NF process including increasing stage 2 concentrate recycle rate to stage 1 feed, increasing the recovery rate of stage 2, increasing feed flow rate, and stage 1 permeate recycle to stage 1 feed. For instance the effect of feed flow rate on the permeate quality is shown in Fig. 13. A lower permeate TDS could be obtained at higher feed flow in stage 1. Mathematically this can be demonstrated according the following equation: Cp =

39000

Feed TDS (mg/l)

0 35000 36000 37000 38000 39000 40000 41000 42000 43000 44000

Js JA = s Qf Jw

ð2Þ

Where Cp is the permeate concentration mg/l, Js is salt flux across the membrane mg/m2.h, Jw is water flux through the membrane l/m2.h, Qf is feed flow m3/h, and A the membrane area m2. From Eq. (2), the permeate concentration, Cp, decreases with increasing the feed flow rate. Most importantly here is to underline the fact that the recovery rate of dual stage NF desalination process tended to decrease sharply with the feed salinity (Fig. 14); this is due to the feed pressure constraint of

stage 1 NF process. To maintain feed pressure within the design limits recommended by the manufacturing company, the recovery rate of stage 1 NF had to be reduced with feed salinity and hence the overall recovery rate was reduced. As to the RO process, the recovery rate was less affected by the feed salinity because the operating pressure in RO was almost twice that in NF membrane. However, the permeate TDS tended to increase with feed salinity in all pressure driven membranes including RO membranes. Increasing permeate TDS with feed salinity can be, to some extent, controlled when BWRO membrane was employed in stage 2 of dual stage NF/BW desalination (Fig. 8). Finally, a comparison study was made between the conventional NF–NF and NF–BW process to determine the number of membrane elements and pressure vessels required for each process at different feed salinities (Table 5). The estimated plant capacity was 20,000 m3/d. As a rule of thumb, the number of membrane elements and hence the pressure vessels increased with the feed salinities; this was mainly due to the lower membrane flux at higher feed salinities. At any feed salinity, it was also observed, that the NF–NF process required less membrane elements than NF–BWRO. This was because of the higher flux of the NF membrane compared to BWRO. On the other hand, the foot print and capital cost of NF–NF process would be slightly cheaper than NF–BWRO process. This is may be one of the advantages of NF–NF desalination process when the plant construction area is an issue. But the permeate TDS in dual stage NF desalination was almost four times higher than in NF–BWRO process (Fig. 8, Table 5). Results also showed that the energy requirements in conventional low recovery rate dual stage NF desalination were less than in NF–BWRO process (Fig. 15). Difference in the energy cost between dual stage NF and NF–BWRO process increased with feed TDS; this was due to the higher feed TDS to stage 2 in the latter process.

Table 5 Number of membrane elements and pressure vessels in dual stage membrane desalination. Process

Unit

NF90-400/NF90-400

Stage 1_no. of elements Stage 2_no. of elements Stage 1_no. elements per PV Stage 2_no. elements per PV Stage 1_no. of PV Stage 2_no. of PV Permeate TDS (mg/l) Es total (kWh/m3) Energy cost ($/year) Stage 1_no. of elements Stage 2_no. of elements Stage 1_no. of elements per PV Stage 2_no. of elements per PV Stage 1_no. of PV Stage 2_no. of PV Permeate TDS (mg/l) Es total (kWh/m3) Energy cost ($/year)

NF90-400/BW30-400

Feed TDS 35,000

37,000

39,000

41,000

43,000

1866 778 6 6 311 130 254 3.35 1,222,750 2095 873 6 5 349 175 67 1,186,250 3.25

1980 825 6 6 330 138 263 3.64 1,328,600 2377 990 6 5 396 198 84 1,259,250 3.45

2098 874 6 6 350 146 295 3.84 1,401,600 2699 1123 6 5 450 225 99 1,317,650 3.61

2417 1007 6 6 403 168 325 3.99 1,456,350 3025 1260 6 5 504 252 111 1,368,750 3.75

2762 1151 6 6 460 192 359 4.2 1,533,000 3276 1365 6 5 546 273 125 1,423,500 3.9

A. AlTaee, A.O. Sharif / Desalination 273 (2011) 391–397

4. Conclusions The current study proposed another approach in which stage 2 NF membrane was replaced by BWRO high salt rejection membrane. The simulation program ROSA was used to predict the feasibility of replacing NF in stage 2 by BWRO membrane. Although BWRO membrane demanded a higher energy for feed filtration but it can produce a high quality permeate compared to NF membrane. The energy penalty due to the use of BWRO in stage 2 varied between 0.1 kWh/m3 and 0.3 kWh/ m3; depending on the feed salinity. The higher energy demands was corresponding to the high feed salinity which varied from 35,000 mg/ l to 43,000 mg/l. for all feed salinities, results from ROSA showed that the permeate quality was much better when BW was used in stage 2 instead of NF membrane. The subtle energy increase due to BW was justified by the high permeate quality. However, the specific power consumption of NF/BWRO system remained lower than that in the conventional RO system as shown in Fig. 9. Using BWRO membrane in stage 2 reduced the system operating complicity and offered a better control of the permeate TDS over a wide range of feed salinities (Fig. 8) and the overall system recovery rate was also higher especially at high feed salinity. Most importantly the increase in the recovery rate of NF/BW system didn't compromise the permeate salinity and the system flexibility at reasonable energy increase. The effect of membrane area should also be investigated in future experimental work. Finally, the results of this study may be useful to conduct further investigations about the benefits of dual stage NF/BWRO desalination and its applicability in commercial desalination plants. It should be mentioned here that there will be some difference between the experimental and simulation results from ROSA. However, these results from ROSA are still reliable but bench/pilot plant study is required to confirm them. References [1] X.L. Wang, C.H. Zhang, J. Zhao, Separation mechanism of nanofiltration membranes and its applications in food and pharmaceutical industries, Membrane Science and Technology 20 (1) (2000) 29–30.

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[2] H.S. Alkhatim, M.I. Alcaina, E. Soriano, M.I. Iborra, J. Lora, J. Arnal, Treatment of whey effluents from dairy industries by nanofiltration membranes, Desalination 119 (1–3) (20 September 1998) 177–183. [3] Li Xu, Shiyong Wang, The Ginkgo biloba extract concentrated by nanofiltration, Desalination 184 (1–3) (1 November 2005) 305–313. [4] Jacques Andrianne, Félix Alardin, Thermal and membrane processe economics: optimized selection for seawater desalination, Desalination 153 (1–3) (10 February 2003) 305–311. [5] Sulaiman Al-Obaidani, Efrem Curcio, Francesca Macedonio, Gianluca Di Profio, Hilal Al-Hinai, Enrico Drioli, Potential of membrane distillation in seawater desalination: thermal efficiency, sensitivity study and cost estimation, Journal of Membrane Science 323 (1) (1 October 2008) 85–98. [6] Diem Xuan Vuong, Two stage nanofiltration seawater desalination system, United State Patent 7, 144, 511 B2, December 2006. [7] J.S. Zhou, G.W. Chen, Development of nanofiltration membrane, Membrane Science and Technology 19 (4) (1999) 1–116. [8] Mohammad A.K. Al-Sofi, Ata M. Hassan, Ghulam M. Mustafa, Abdul Ghani I. Dalvi, Mohammad N.M. Kither, Nanofiltration as a means of achieving higher TBT of ≥120 °C in MSF, Desalination 118 (1–3) (20 September 1998) 123–129. [9] B. Van der Bruggen, A. Koninckx, C. Vandecasteele, Separation of monovalent and divalent ions from aqueous solution by electrodialysis and nanofiltration, Water Research 38 (5) (March 2004) 1347–1353. [10] Mohan Noronha, Valko Mavrov, Horst Chmiel, Efficient design and optimisation of two-stage NF processes by simplified process simulation, Desalination 145 (1–3) (10 September 2002) 207–215. [11] Serena Bandini, Jennifer Drei, Daniele Vezzani, The role of pH and concentration on the ion rejection in polyamide nanofiltration membranes, Journal of Membrane Science 264 (1–2) (1 November 2005) 65–74. [12] Maria Diná Afonso, Maria Norberta de Pinho, Transport of MgSO4, MgCl2, and Na2SO4 across an amphoteric nanofiltration membrane, Journal of Membrane Science 179 (1–2) (15 November 2000) 137–154. [13] Yann A. Le Gouellec, David A. Cornwell, Robert C. Cheng, A Novel Approach to Seawater Desalination Using Dual-Staged Nanofiltration (Awwarf Report), IWA Publishing, 2006. [14] Mark Wilf, Craig Bartels, Optimization of seawater RO systems design, Desalination 173 (1) (1 March 2005) 1–12. [15] Filmtec website, http://dow-answer.custhelp.com/cgi-bin/dow_answer.cfg/php/ enduser/std_adp.php?p_faqid=265&p_created=1043797236&p_topview=1, 26/4/2010.