Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization

DES-12816; No of Pages 20 Desalination xxx (2016) xxx–xxx

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Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization Senthil S., Senthilmurugan S. ⁎ Chemical Engineering Department, Indian Institute of Technology Guwahati, Assam - 781039, India

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Simulation and sensitivity analysis of integrated SWRO-PRO system configurations for eliminating brine post-treatment step. • Identification of energy efficient novel SWRO - PRO hybrid desalination process for reducing the SWRO pre-treatment cost. • Method for optimization of hybrid SWRO-PRO system design and operating parameters to achieve minimum NSEC.

a r t i c l e

i n f o

Article history: Received 14 October 2015 Received in revised form 15 January 2016 Accepted 21 January 2016 Available online xxxx Keywords: Reverse osmosis Desalination Pressure-Retarded Osmosis SWRO–PRO hybrid system Modelling Optimization

a b s t r a c t Theoretical analysis of energy harvesting from concentrated brine of Sea Water Reverse Osmosis (SWRO) system using Pressure Retarded Osmosis (PRO) is presented in this research. The mathematical model of SWRO–PRO hybrid system components such as SWRO unit, Energy Recovery Device (ERD), PRO unit and other auxiliary units were discussed. The mathematical equations were solved adapting an object oriented “Modelica language” framework in Dymola software tool. The complex flowsheet models for six different SWRO–PRO hybrid configurations were created. The performance of the SWRO–PRO hybrid system configurations was studied. The process and design parameters were optimized to reduce the Net Specific Energy Consumption (NSEC) of the system. The optimization studies were performed using SQP technique that is available in optimization library of Dymola. The possibility of using sea water (32,000 g/m3) and urban waste water (100–10,000 g/m3) as feed solution to the PRO for all the hybrid configurations were studied. Their performances were compared through simulation and optimization studies. Among the six potentially viable SWRO–PRO configurations, the one which does the direct mixing of diluted PRO draw outlet with feed water of SWRO aided to bring down the NSEC by 49% in comparison with standard SWRO desalination system. This system does not require additional ERD units and turbine at optimized process conditions, which are more expensive. © 2016 Elsevier B.V. All rights reserved.

⁎ Corresponding author. E-mail address: [email protected] (S. S.).

http://dx.doi.org/10.1016/j.desal.2016.01.027 0011-9164/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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1. Introduction The Sea Water Reverse Osmosis (SWRO) desalination is one of the pioneer technologies for desalination of sea and brackish water. As per latest reported data, desalination industry is able to produce treated water from sea water with an energy consumption of 1.8 kWh/m3 [1]. The theoretical minimum energy required for desalination of 35,000 g/m3 concentration of seawater at 50% recovery is 1.06 kWh/ m3 (i.e. multistage operation) and 1.56 kWh/m3 (i.e. single stage operation). Hence, it can be concluded that the energy demand for seawater desalination was found to be higher by 41% for multistage and 13.3% for single stage desalination than the stated theoretical minimum energy [1], which is not far off the thermodynamic limit for single stage. This was achieved with continuous research and development of fouling resistant SWRO membrane and energy efficient electrical and flow equipment. Therefore, the theoretically zero energy SWRO may be achieved by developing innovative technology which can extract the osmotic energy from SWRO brine. The Pressure-Retarded Osmosis (PRO) is one of the best technology [2] that can extract osmotic energy by controlled mixing of high and low concentrated water. In 1976, Loeb et al. published the PRO concept with experimental results for the first time [3]. When Loeb et al. proposed the concept of PRO, Dead Sea water (i.e. ≈250,000 g/m3) was considered as high salinity source. Their results were not very promising because those experiments were performed using SWRO membrane modules (i.e. power density in the range from 1.56 to 3.27 W/m2). The lower power output for the case was mainly due to severe internal concentration polarization (ICP). The estimated break even membrane power density by Gerstandta et al., [4] varies in between 4 and 6 W/m2 with respect to TDS of draw and feed solution (DS, FS). As per Statkraft (Norway), it is important to have the power density above 5 W/m2 for successful commercialization of PRO system. The continuous research on this topic resulted into improved PRO membrane with high power density [23] and stable membrane performance. Saito et al. [5] reported that Toyobo's prototype hollow fibre PRO membrane was able to achieve the maximum output power density of 7.7 W/m2 at 2.5 MPa hydraulic pressure difference having 38% permeation of pure water into brine. The use of the SWRO brine as a draw solution can improve the efficiency of PRO system and shall eliminate the necessity of SWRO brine post-treatment. Therefore, the reject water (≈ 52,000 g/m3 to 60,000 g/m3) of SWRO plant had attracted researchers as high salinity source for PRO. A conservative assessment indicates that the SWRO– PRO offers a potential energy reduction of 20–23% and total capital cost reduction of 8.7–20% compared to SWRO process [6]. Thermodynamic analysis on the feasibility of stand-alone SWRO–PRO hybrid system was done by Wang et al. [7] and Sharqawy et al. [8]. The recent theoretical analysis by Prante et al. [9] also inferred that using a wellcharacterized CTA membrane, the minimum NSEC of the modelled SWRO–PRO system was 1.2 kWh/m3 for 50% SWRO recovery. Considering a SWRO system of having specific energy consumption of 2.0 kWh/ m3, the SWRO–PRO system can theoretically achieve 40% energy reduction. The SWRO–PRO hybrid system which was considered by Prante et al. was with two ERD units to minimize the NSEC, which is more expensive and may lead to increase in the capital cost of the system. Pressure drop and concentration polarization (CP) along the length of the membrane and pressure drop and frictional loss in ERD were not considered. Therefore, reduction in the NSEC of the system may significantly reduce from 40%. Further, considering the possibility of recirculating the diluted brine water of PRO as feed water for SWRO may even reduce the NSEC. Considering the above supportive arguments, the objectives of this research work focused upon the identification of energy efficient novel SWRO–PRO hybrid desalination process to achieve reduced NSEC for desalination process. This novel hybrid system aimed at eliminating SWRO brine post-treatment and reducing sea water pre-

treatment load. Six different SWRO–PRO process flowsheets were analysed by modelling and optimization of hybrid SWRO–PRO system design and operating parameters to achieve minimum NSEC. Out of six different flowsheets studied in this work, two were synthesized in this work and others were chosen from existing literature. The effect of design and process parameters on SWRO–PRO system performance was simulated by using flowsheet models. 2. Theory In this study, six different SWRO–PRO hybrid system configurations were considered (i.e. from case II to case VII) along with the currently existing stand-alone SWRO configuration (i.e. case I) used in desalination application. Details of the system configurations are explained with process flowsheet (Fig. 1) and the particulars of those systems are given in Table 1. Throughout this study, the concentration of sea water is taken as 32,000 g/m3 and waste water concentration may vary from 100 to 10,000 g/m3. 2.1. SWRO and PRO trains In industrial practice, a SWRO plant may consist of multiple trains. Each SWRO train will consist of multiple pressure vessels connected in parallel and more than one membrane modules are connected in series to build one pressure vessel. The number of membrane modules per pressure vessels is decided based on required recovery and the number of pressure vessels per train is decided to meet the target production rate. In contrast to SWRO pressure vessel, PRO pressure vessel has two inputs (FS and DS). In this work, model development and optimization framework are limited to SWRO–PRO system consisting of single train. 2.2. SWRO–PRO configurations Alternate feasible SWRO–PRO hybrid configurations were proposed and studied by many researchers [9–13]. Kim et al. [10] identified four potential hybrid PRO–SWRO configurations and PRO is used for both extracting osmotic energy and dilution of seawater. Diluted sea water is then desalinated in SWRO to produce pure water. In brownfield projects, one of the drawbacks is integration of PRO unit before SWRO unit and it may be challenging and risky in terms of process reliability. Qureshi et al. [14] evaluated performance of case I and II process configuration (Fig. 1) for brackish water desalination using validated mathematical model. They reported that case I is found to be more energy efficient than case II for brackish water desalination. Almansoori et al. [12] studied hybrid configurations consisting of both PRO–SWRO and SWRO–PRO scenarios for sea water application. They concluded that the configuration having SWRO followed by PRO system is better than PRO followed by SWRO system. Therefore, six different possible SWRO–PRO system configurations are considered using various methods of energy recirculation and/or brine recirculation from SWRO to PRO and vice versa (Fig. 1). High pressure (HP) pump is used to pressurize SWRO feed to the desired pressure (i.e. 30–70 bar). Low pressure (LP) pump is used to pressurize the LP inlet of PX ERD such that the LP outlet can be maintained at desired pressure (i.e. LP inlet pressure should be higher than the pressure of LP outlet in order to overcome the pressure drop on LP side of PX ERD). In all hybrid systems (i.e. from case II to VII), the reject of SWRO is depressurized to the required pressure for PRO with the help of ERD. The total feed flow to the hybrid system is measured as a sum of water flow at two points, namely the HP pump and LP pump-1 inlets. Sea water is used as FS for PRO of case II (i.e. waste water is the FS in rest of the hybrid cases). The HP pump flow rate is taken to be equal to the flow rate of product water according to industrial practice. For case I, the SWRO reject is post treated and sent back to the sea. Whereas in other cases, the reject is fed as DS for PRO. In cases II and III, the entire DS outlet water is depressurized at electric turbine to generate electricity. Whereas in

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

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Fig. 1. Six different SWRO–PRO system configurations.

case IV, instead of sending the entire DS outlet through the turbine, part of it (i.e. equal to the product water flow rate of SWRO) is fed to the 2nd ERD. It is used to exchange the energy between DS outlet stream to HP pump inlet stream. In case V, the DS outlet is recirculated to both HP and

LP pump 1 by the 2nd ERD. In systems IV and V, the use of ERD-2 is expected to have a positive impact on reducing the NSEC. However, an additional ERD is used along with the one which was employed for energy recovery from SWRO reject stream. In the remaining two cases (i.e. VI

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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Table 1 Attributes of SWRO–PRO hybrid systems. System type ➔

Case I

Case II

Case III

Case IV

Case V

Case VI

Case VII

No. of PRO units No. of HP pump units No. of LP pump units No. of ERD units No. of splitters

– 1 2 1 1

1 1 3 1 1

1 1 3 1 1

1 1 4 2 2

1 1 4 2 2

1 1 3 1 2

No. of mixers

1

1

1

1

1

2

No. of Turbine PRO feed solution Brine recirculation

– – –

1 Sea water –

1 Waste water –

1 Waste water –

1 Waste water –

Energy recirculation

Yes (by ERD)

Yes, partial (by ERD)

Yes, partial (by ERD)

Yes, partial (by ERD1, 2)

Yes, maximum (by ERD1, 2)

Advantages

Simple setup

Sea water as PRO feed solution

Higher PRO recovery is possible. lesser ICP

Reducing load on HP pump

Reducing load on HP pump, reducing energy loss in turbine

1 Waste water Yes, maximum Yes, partial (by ERD, recirculation brine) Reducing load on HP pump, reducing pre-treatment and energy loss in turbine

1 1 4 1 1 1 + 1 (cum splitter) 1 Waste water Yes, maximum Yes, maximum (by ERD, recirculation brine) Reducing load on HP pump, reducing pre-treatment and energy loss in turbine

Disadvantages

NSEC is around 2 kWh/m3

Possibility of sever ICP in PRO

Availability of waste water in all the places

Use of two ERD units and four LP pumps leads to higher capital cost

Use of two ERD units and four LP pumps leads to higher capital cost

Use for two splitters and two mixers

Use of four LP pump leads to higher capital cost

and VII), the direct mixing of DS outlet with SWRO feed is enabled (similar to close circuit PRO and desalination [15]) in order to reduce the sea water pre-treatment unit capacity. Thus, the energy consumption associated with the pre-treatment of sea water can be minimized. Case VI is a kind of modified version of case IV, where the ERD-2 is removed and a fraction of the DS water equal to the flow rate of SWRO product water is directly fed to the HP pump. The remaining fraction of the DS outlet is depressurized in the turbine. In case VII, a large fraction of the DS outlet is directly supplied back as feed to SWRO after mixing with pre-treated sea water. The depressurized diluted DS outlet can be discharged into the sea without post treatment. 2.3. Model development As shown in Fig. 1, the SWRO–PRO hybrid system includes important ancillary equipment other than SWRO and PRO modules with turbine, ERD, HP pump, and low pressure pump (LP pump). Mathematical model of each process unit was developed by solving integrated mass, energy and momentum balance equations reported in literature. These individual process units were integrated to build flowsheet model of SWRO–PRO hybrid systems. 2.4. SWRO model equations The water flux is directly proportional to the difference between hydraulic pressure difference and actual osmotic pressure difference across the membrane, which can be written as Jv ¼ AðΔPRO –σ Δπact Þ

ð1Þ

The actual osmotic pressure difference across the membrane is
Δπact ¼

  iRT  Cm –Cp M

ð2Þ

where Jv, A, Δ PRO, Δ πact, σ, Cm, Cp, i, R, T and M are water flux, hydrodynamic permeability of the membrane, hydraulic pressure differences, actual osmotic pressure difference, reflection coefficient, concentration at feed side membrane, concentration of permeate,

van't Hoff factor, universal gas constant, operating temperature and molecular weight of NaCl, respectively. The permeate concentration equation is derived by combining the solute flux equation of SK model and the concentration polarization equation based on the film theory model [16] 0 1  σ  − Jv ð1−σ Þ =B 1þ 1−e B C 1−σ C f ¼ Cp @ A Jv eð k Þ

ð3Þ

where Cf, Cp, B and k are feed water concentration, permeate concentration, salt permeability and mass transfer coefficient, respectively. The concentration polarization equation based on the film theory at feed side [16] is written in equation below.
 Jv Cm −Cp ¼ ek C f −Cp

ð4Þ

Hydraulic pressure in spiral wound membrane module feed channel may vary along the length due to pressure drop and can be defined using Darcy's law [17]. Darcy's law states that the pressure drop per unit length in narrow channel is a function of the fluid flow rate. The flow rate and pressure are varying along the membrane length. Furthermore, to simplify model equations, both pressure and flow in the membrane module are calculated by taking the average between inlet and outlet condition of the streams. Therefore, the pressure drop equation can be expressed as follows Pdrop ¼ P f −Pr LD f ðQ f þ Q r Þ Pdrop ¼ 2

ð5Þ

where Pdrop, Pr, Df, Q f, Q r, and L are pressure drop in a pressure vessel, pressure of reject stream, Darcy's law constant, flow rate of feed, flow rate of reject and total length of a pressure vessel, respectively. It is assumed that the feed channel is completely mixed, negligible pressure drop on permeate channel and permeate water pressure as atmospheric pressure, then the hydraulic pressure difference across

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

the membrane is given by
ΔPRO ¼

Pf–

 Pdrop −Patm 2

ð6Þ

Feed side mass transfer coefficient of SWRO membrane is a function of fluid velocity. The empirical equation for mass transfer coefficient is given by [18]. k¼x


 Q f þ Qr y 2

ð7Þ

Q f ¼ Qr þ Qp

ð8Þ

Q p ¼ Jv Sm

ð9Þ ð10Þ

where Q p, Sm, and Cr are flow rate of permeate (i.e. product water), total area of SWRO membrane per train, and concentration of brine respectively. SWRO recovery is defined as, RecvRO ¼


 Qp 100 Qf

ð11Þ

2.5. PRO model equations Water flux and salt flux for PRO membrane are written as below [16, 18]. Jw ¼ APRO ½Δπeff −ΔPPRO  Δπeff ¼



iRT C 1− fm Cdm Cdm M

Js ¼ BPRO ðCdm −Cfm Þ


 Q fi þ Q fo y 2


 Q di þ Q do y kd ¼ x 2

ð13Þ ð14Þ

ð15Þ

ð16Þ

where x and y are empirical constants. The equation for Dilutive External Concentration Polarization (DECP) is given by [21,22] Cdm −Cfi ¼ eð− Jw =kd Þ Cdi −Cfi

ð17Þ

Concentrative External Concentration Polarization (CECP) at feed side is given by [22] Cs −Cdi ¼ eð− Jw =k f Þ Cfi −Cdi


Cfm ¼ Cdm

Jw



 eð Jw Kp Þ þ B eð Jw   B eð Jw Kp Þ −1 þ J

Cs Cdm

Kp Þ

−1

 ð19Þ

w

ΔP f ¼ Pfi −Pfo LPRO D f;PRO ðQ fi þ Q fo Þ ΔP f ¼ 2

ð20Þ

ΔPd ¼ Pdi −Pdo LPRO Dd;PRO ðQ di þ Q do Þ ΔPd ¼ 2

ð21Þ

where Pfi, Pfo, Df,PRO, Q fi, Q fo, LPRO, Pdi, Pdo, Dd,PRO, Q di and Q do are FS inlet pressure, FS outlet pressure, FS side Darcy's law constant, FS inlet flow rate, FS outlet flow rate, length of one PRO membrane pressure vessel, DS inlet pressure, DS outlet pressure, DS side Darcy's law constant, DS inlet flow rate and DS outlet flow rate, respectively. The theoretical maximum power density is achieved when the hydraulic pressure difference is equal to half the value of the effective osmotic pressure difference [17]. The DS inlet pressure equation is given below Pdi ¼

ð12Þ

where APRO, BPRO, Cdm, Cfm, and πeff, are water permeability, salt permeability, draw side concentration at membrane–solution interface, feed side concentration at membrane–solution interface and effective osmotic pressure difference across the PRO respectively. The empirical mass transfer coefficient equations are given below [18]. kf ¼ x

Cfi, Cdi, and Cs are FS inlet concentration, DS inlet concentrations and feed side concentration at support layer, respectively. Internal concentration polarization (ICP) is a strong function of solute resistance of porous support layer and water flux [9]. The effective concentration difference across the membrane with ICP effect is predicted by equation given below

where Kp is solute resistance in porous support layer. The pressure drop across the feed and draw channel is estimated by using Darcy's law [17]

where x and y are empirical constants. The mass balance equations are given below.

Q f C f ¼ Q r Cr þ Q p Cp

5

ð18Þ


 ΔΠ eff −ΔPd þ Δp f þ Pfi 2

ð22Þ

where ΔPd and ΔPf are draw side pressure drop and feed side pressure drop, respectively. Pressure difference across the membrane is written as [17]
ΔPPRO ¼

Pdi −


 ΔPd ΔP f − Pfi − 2 2

ð23Þ

Power produced per unit area of the membrane is calculated as below (i.e. power density) [19,20] Wden ¼ Jw Pdo

ð24Þ

Mass and solute balance equations for PRO unit are written as Q do ¼ Q di þ Jw Sm;PRO

ð25Þ

Q fo ¼ Q fi −Jw Sm;PRO

ð26Þ

Cdo ¼

Q di Cdi −Js Sm;PRO Q do

ð27Þ

Cfo ¼

Q fi Cfi þ Js Sm;PRO Q fo

ð28Þ

where Sm,PRO and Cfo are total surface area per train and FS outlet concentration of PRO membrane, respectively. Recovery of PRO is given by RecvPRO ¼


 Q 1− fo 100 Q fi

ð29Þ

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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Fig. 2. SWRO sensitivity analysis.

2.6. ERD model equations The simplified Bernoulli's equation for calculating LP outlet pressure of ERD unit is given below [24].

Plo ¼

   ηERD ðQ hi Phi −Q ho Pho Þ þ Q li Pli −K ηERD Q ho ρf Vhi 2 −Q lo ρb Vli 2 Q lo ð30Þ

where ηERD, Q hi, Q li, Q ho, Q lo, Phi, Pli, Pho, Plo, Vhi, Vli, Vho, Vlo, K, and ρf are ERD efficiency, flow rate at HP inlet, flow rate at LP inlet, flow rate at HP outlet, flow rate at LP outlet, pressure at HP inlet, pressure at LP inlet, pressure at HP outlet, pressure at LP outlet, velocity at HP inlet, velocity

at LP inlet, velocity at HP outlet, velocity at LP outlet, friction loss coefficient and density of feed water, respectively. Average velocity at LP and HP inlets Vli ¼ Q li



ð31Þ

Ar

Vhi ¼ Q hi



ð32Þ

Ar

where Ar is the cross sectional area of ERD inlets. Small fraction of HP stream shall leak through piston edge to LP stream to lubricate ERD's part movement which can be written in the mathematical form as below Lub flow ¼ Q hi Lub flow ratio

ð33Þ

Table 2 Industrial SWRO operating data. Feed water flow rate of SWRO Operating pressure of SWRO Recovery Product water flow rate Pf [bar] [%] Qp [m3/s] Qf [m3/s]

0.29101 0.29105 0.29110

50.47 55.81 60.28

33.1 39.6 44.5

Product concentration CP [ppm]

Experimental

Model

Error % Experimental

0.09340 0.11603 0.12809

0.09699 −3.84 0.11534 0.59 0.1295 −1.10

44 49 52

Reject pressure PR [bar]

Model Error % Experimental Model

Error %

42.99 46.46 49.58

−0.22 0.00 −0.01

2.30 5.18 4.65

48.23 54.72 59.22

48.337 54.721 59.224

Note 1: The model parameters such A, B, σ, x, y, and Df are estimated by tuning model with experimental data. Note 2: The experimental is taken from sea water desalination plant data reported in [26]. Note 3: The sea water concentration = 32,000 g/m3, temperature = 298 K.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

Q lo ¼ Q hi −Lub flow

ð34Þ

Applying mass balance across inlets and outlets of ERD Q ho ¼ Q hi þ Q li −Q lo

where Lub_flow is the lubrication flow from HP side to LP side and Lub_flow_ratio is the ratio between lubrication flow to HP inlet flow.

7

ð35Þ

Due to high and low pressure stream mixing, the concentrations of outlet streams will be varying from the inlet streams.

Fig. 3. PRO sensitivity analysis.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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S. S., S. S. / Desalination xxx (2016) xxx–xxx

Based on solute mass balance, salinity at high pressure outlet of ERD is given by Clo ¼ ðChi −Cli ÞMix=100 þ Cli

ð36Þ

where Chi, Clo, Cli, Cho, and Mix are concentration of brine inlet (highpressure), concentration of brine outlet (low-pressure), concentration of feed inlet (low-pressure), concentration of feed outlet (high-pressure) and mixing percentage, respectively. From solute balance, the salinity at high pressure outlet of ERD is given by
Cho ¼

Chi Q hi þ Cli Q li −Clo Q lo Q ho

 ð37Þ

2.8. Mixer and splitter model equations Mixer is used to mix and distribute multiple streams. Splitter is used to split and distribute multiple streams. The pressure drop across the mixer and splitter is assumed as negligible. Mass balance for both mixer and splitter is written as Xm 1

Q out;m ¼

Xn 1

ð40Þ

Q in;n

where m and n are total number of outputs and total number of inputs respectively. 2.9. Turbine model equations

2.7. HP and LP pump model equations

Turbine is used to convert pressure head into electrical energy. Mass and momentum balance equations for turbine are given below

HP pump is used to pump sea water for SWRO at constant pressure. LP pump is working as a booster pump to pressurize the ERD's HP outlet. The mass and momentum balance equation for pump is given by

Q in ¼ Q out

ð41Þ

Pout ¼ Pin −PHU

ð42Þ

Q in ¼ Q out

ð38Þ

Pout ¼ Pin þ PHP Wconsumed ¼

PH Q in ηpump

ð39Þ

where Qin, Qout, Pin, Pout, PHP, and ηpump are the inlet flow rate, outlet flow rate, pressure of inlet stream, pressure of outlet stream, pressure head developed in the pump, and efficiency of the pump, respectively.

where Pin and Pout, PHU are pressure of inlet stream, pressure of outlet stream and pressure head utilized to generate power by turbine, respectively. The power generated by turbine is given by Wgen ¼ PHU Q in ηturbine

ð43Þ

where ηturbine is the efficiency of turbine cum generator system.

Table 3 Important input parameters of the system. Parameter name

Value

Unit

PRO Hydro dynamic permeability (APRO) Salt permeability (BPRO) Multiplying constant in mass transfer coefficient equation (x) Power constant in mass transfer coefficient equation (y) Feed channel Darcy's law constant (Df, PRO) Draw channel Darcy's law constant (Dd, PRO) Area of one membrane (SPRO) Length of one PRO membrane (lPRO) Total number of membranes per pressure vessel (mPRO) Total number of pressure vessels per train (NoP) Universal gas constant (R) Molecular weight of NaCl (M)

1.61389 × 10−6 2.44 × 10−7 0.0038 0.5 200 200 7.43224 0.9626 5 291 0.00008314 58.5

m/bar·s m/s m−0.5 s−0.5 – bar·s/m4 bar·s/m4 m2 m – – m4·bar/mol·K g/mol

SWRO Reflection coefficient (σ) Water permeability coefficient of membrane (A) Salt permeability coefficient of membrane (B) Area of one membrane (SRO) No. of membrane per pressure vessel (mRO) van't Hoff factor (i) Membrane length (l) Darcy's law constant on feed side (Df) Constant in mass transfer coefficient equation (x) Constant in mass transfer coefficient equation (y) Permeate pressure (Pp)

0.999 5.0815 × 10−7 4.34 × 10−7 7.43224 7 2 0.9626 200 0.0038 0.5 1

– m/bar·s m/s m2 – – m bar·s/m4 m−0.5 s−0.5 – bar

ERD Volumetric mixing (Mix) Lubrication flow ratio (Lub_flow_ratio) Density (ρ) Efficiency of ERD (ηERD) Friction coefficient in ERD (K)

5.2 0.01 997 96 0.0005

% – kg/m3 % –

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

3. Results and discussion

9

Table 4 Operating condition for SWRO–PRO configurations.

3.1. Simulation 3.1.1. Modelling framework Modelling and simulation were done in an object oriented, declarative and multi-domain modelling language “Modelica” with a help of a software called Dymola. The mathematical model equations of each process units were programmed in Modelica language to create individual process units (i.e. SWRO Module, PRO module and ERD) with user designed icons and graphically connected to build integrated flowsheet model. 3.1.2. Individual process unit simulation For verification purpose, the individual process unit models were simulated in appropriate operating range. So that the integrated flowsheet model can be simulated for wide range of operating conditions. The simulation results are analysed and verified with literature. 3.1.3. SWRO unit The seawater is pumped to SWRO unit by using HP pump (Fig. 2a). The SWRO model and design parameters were taken from literature [25]. As shown in Table 2, the SWRO model parameters such as A, B, σ, x, y, and Df were estimated by tuning model with experimental data used in reference [26]. For simulation study, the sea water concentration is fixed as 32,000 g/m3 and the effect of feed pressure and flow rate on SWRO performance were verified. While varying pressure or flow rate, the other variables were fixed to their default values i.e. 0.291 m3/s and 55 bar respectively. As shown in Fig. 2b, the SWRO recovery and reject concentration increases with inlet pressure as expected. For example, 40% recovery and 53,300 g/m3 reject concentration, which is essential for PRO draw solution inlet was achieved at 56 bar pressure. The SEC decreases even though the work done by the pump increases with increase in inlet pressure (Fig. 2c). However, at higher feed pressure, the rate of decrease of SEC is relatively small due to

Input variable

Unit

Default value

Simulation range

Feed to SWRO Net flow rate Pressure

m3/s bar

0.291 55

0.23–0.3 30–70

FS to PRO FS–DS inlet flow ratio Concentration (except for Case-II)

– g/m3

1 100

0.8–2 100–10,000

DS to PRO DS inlet pressure

bar

Eq. (22)

Flowsheet parameters Ratio of brine recirculation

–

1

5–15

0.8–1

increasing CP effect. Concentration of permeate water decreases till feed pressure ≈33 bar and then increases with inlet pressure (Fig. 2c) as expected for SWRO membranes [27]. The increase in inlet flow rate reduces the CP effect which results into lower product water concentration but SEC increases due to increase in pumping energy consumption (Fig. 2d). 3.1.4. PRO unit Flowsheet for PRO model is shown in Fig. 3a. Waste water (100 g/m3) was used as FS and SWRO brine (52,000 g/m3) was used as DS. The DS and FS inlet flow rates were fixed as 0.1745 m3/s by assuming 40% recovery in SWRO unit. Due to unavailability of experimental plant data for PRO unit, the PRO membrane parameters were taken from literature [28] and design parameters were calculated based on the PRO design capacity requirements as described in SWRO–PRO trains section. For example, five number of PRO modules should be connected in series per pressure vessel to achieve 67% recovery in PRO unit (for given value of membrane permeability = 1.61389 × 10−6 m/bar·s, surface area per module = 7.43224 m2 and DS inlet pressure = 10 bar). The model parameters of PRO process are given in Table 3.

Fig. 4. ERD sensitivity analysis.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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The PRO system performance is analysed by calculating following model outputs such as power density and recovery (Fig. 3). For a fixed PRO membrane surface area, the dilution water flux was increased with DS inlet flow rate that resulted into higher recovery (i.e., 1 − Q fo/Q fi). On the other hand the power density decreased due to increased pressure drop in DS membrane channel (Fig. 3b). This phenomenon was happening because the power density is highly sensitive towards pressure drop than dilutive concentration

polarization. As expected, the reverse trend is obtained for power density and recovery with respect to FS flow rate (Fig. 3c)). As reported by many researchers [23,29–31], both recovery and power density increased with respect to draw inlet concentration (Fig. 3d). The concentration of FS and DS outlets were increasing with DS inlet concentration and this is due to higher PRO recovery (Fig. 3g). Recovery decreases and power density increases while increasing draw solution inlet pressure (Fig. 3e).

Fig. 5. Effect of SWRO feed pressure.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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Fig. 6. Effect of SWRO feed pressure on concentration polarization.

As shown in Fig. 3f, the FS inlet concentration is varied to study the effect of an internal concentration polarization on recovery and power density. Due to severe ICP, the DS outlet concentration is increased

above sea water concentration with FS inlet concentration greater than 700 g/m3. Also the power density is reduced with reduced water flux.

Fig. 7. Effect of SWRO feed flow rate.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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3.1.5. ERD unit Energy Recovery Device (ERD) is an important equipment in SWRO plant, to reduce the energy consumption. The efficiency of ERD is a function of operating pressure and flow rate. As shown in Fig. 4, the HP stream is supplied from high pressure source (i.e. SWRO brine) and LP stream is supplied from low pressure source (i.e. sea water feed). The energy between HP & LP inlet streams is exchanged with minimal mixing between inlet and outlet streams. ERD model simulation results are presented in Fig. 4. In general, the ERDs are designed such that the optimal efficiency may be achieved when LP side inlet flow rate is equal to HP side inlet flow rate. Therefore, the ERD model was simulated to verify the trend of HP outlet pressure and concentration with respect to HP inlet pressure and concentration respectively. As expected, HP outlet pressure increases with HP inlet pressure (Fig. 4b) and HP outlet concentration increases with inlet concentration (Fig. 4c) [32]. 3.1.6. SWRO–PRO process flowsheet The performance of integrated flowsheet models (Fig. 1) was analysed by simulating with respect to the operating conditions mentioned in Table 4. The efficiency of HP pump, LP pump and ERD were assumed as 80%, 83% and 96% respectively. FS inlet concentration of PRO for the case II was fixed as 32,000 g/m3 (i.e. sea water). The design parameter of individual units are given in Table 3.

3.1.6.1. Effect of SWRO feed pressure on SWRO–PRO system performance. The feed pressure to SWRO unit was varied from 30 to 70 bar and corresponding variation in some of the important variables are plotted in Fig. 5. For cases I to V, the product water concentration decreases to a minimum value and then increased with feed pressure (Fig. 5a) and this is due to the CP effect at membrane surface [20]. However, for the cases VI and VII, the product water concentration increased with feed pressure. This was occurring due to increase in SWRO feed concentration with direct mixing of diluted DS outlet and SWRO feed. For all cases, SWRO and PRO recoveries were increasing with membrane feed pressure (Fig. 5b & c). The trend of SWRO and PRO recovery curves were almost same for cases I to V. However, for case II, PRO recovery was found to be too low due to severe ICP. For cases VI and VII, both PRO and SWRO recoveries were higher than the former cases at the pressure below 60 bar. Later both recoveries were found to be lesser at above 60 bar. This phenomenon was occurring due to the PRO unit capacity limitation i.e. the DS concentration can not to be reduced below the seawater concentration. For the cases III to V, The power density was increasing till a maximum value attained and then decreased (Fig. 5d). To understand this phenomenon in more detail, the effect of SWRO inlet pressure on effective osmotic pressure difference across the PRO membrane and DECP were analysed for Case V and shown in Fig. 6. The PRO power density is proportional to water flux and DS outlet pressure. The water flux is proportional to the osmotic pressure difference across the PRO membrane. As shown in Fig. 6a, the effective osmotic pressure difference

Fig. 8. Effect of DS inlet pressure.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

was found to be increasing and reached the maximum at 50 bar SWRO inlet pressure. The DS inlet flow rate was expected to decrease with SWRO recovery and this leads to severe DECP effect (Fig. 6b). The above described phenomenon was not happening for the cases VI and VII, hence no maximum found within the simulation range. As shown in Fig. 5e, in case VII, the direct mixing of DS outlet with SWRO feed aids to lowest NSEC among other cases. The optimum NSEC with respect to SWRO feed pressure for cases I to V were estimated around 38 to 45 bar. For cases VI and VII, the NSEC decreases with decrease in SWRO feed pressure. Power generation was expected to increase with power density and reduction in NSEC. But, this behaviour was not observed while comparing power density and NSEC of systems grouped under “with” and “without” brine recirculation (i.e. II–V versus VI & VII). This was due to decreased SWRO unit power consumption by minimizing SWRO feed concentration for cases VI & VII. 3.1.6.2. Effect of net SWRO feed flow rate on SWRO–PRO system performance. As shown in Fig. 7, while increasing SWRO feed flow rate from 0.23 to 0.3 m3/s, the SWRO recovery and PRO recovery are found to be decreasing due to pressure drop in membrane channel (Fig. 7a & b). On the other hand, power density was increasing due to reduced recovery in SWRO followed by increase in DS flow rate. However, for the case II, the power density was decreasing due to severe ICP. The best SWRO– PRO hybrid system should provide high SWRO & PRO recovery, low

13

NSEC and product water concentration. As shown in Fig. 7, the cases VI and VII were found to be the best by satisfying above definitions. For cases I to V, the trend of SWRO recovery, PRO recovery and power density were found to be similar and this may be possible when SWRO feed conditions are identical. 3.1.6.3. Effect of PRO DS inlet pressure on SWRO–PRO system performance. In all the previous sensitivity analysis, the DS inlet pressure was found as a function of effective osmotic pressure difference across the PRO membrane. The theoretical maximum power density may be achieved when the hydraulic pressure difference among DS and FS were equal to half the value of effective osmotic pressure difference [22]. But, this is not always true for a system when pressure and concentration vary along the length of the membrane module [19]. Therefore, the effect of DS inlet pressure on the system performance was studied (Fig. 8). As expected, the DS inlet pressure does not have any impact on either product water concentration or SWRO recovery for cases II and III, since there is no recirculation flow from PRO to SWRO. However, in other cases, recirculation flow from PRO to SWRO was enabled, which leads to variation in product water concentration and SWRO recovery with respect to DS inlet pressure. PRO recovery and NSEC were decreasing and power density was increasing with DS inlet pressure for cases III–VII. SWRO and PRO recovery of cases VI and VII were higher than all other cases due to reduced CP effect. Choosing seawater as FS for the case II resulted into severe ICP that in turn leads to decreased

Fig. 9. Effect of FS–DS ratio.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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power density and increased NSEC with DS inlet pressure. It is evident from the Fig. 5 that if the same study was done at the pressure above 60 bar, then the trend of SWRO recovery, PRO recovery, power density and NSEC would have been reversed. 3.1.6.4. Effect of PRO FS/DS inlet ratio on SWRO–PRO system performance. The ratio between FS and DS inlet flow rate is very important. Altaee et al. reported that the DS flow rate has more impact on power density than the feed flow rate [33]. The relationship between DS/FS flow rate and power density are not linear. Increasing FS/DS ratio will decrease the effect of concentration polarization but cannot eliminate it, because very high flow rates would increase the overall power consumption instead of improving power density. Therefore, optimal ratio is very important to achieve maximum power output from PRO. In this simulation study, since DS flow rate was fixed based on SWRO unit operating conditions, the FS/DS ratio was varied from 0.8 to 2 and corresponding hybrid system output variables are plotted in Fig. 9. As expected, FS/DS ratio does not have much impact on product water concentration for all the cases. For cases IV to VII, SWRO recovery was increasing steadily, whereas PRO recovery was decreasing and power density was increasing with FS/DS ratio as reported by Altaee et al. [33]. For the case II, both power density and PRO recovery were almost constant compared to other cases. Therefore, the NSEC increased with FS/DS ratio for case II. For other cases, NSEC reaches to minimum value then again increasing with FS/DS ratio.

3.1.6.5. Effect of PRO FS concentration on SWRO–PRO system performance. As reported by Altaee et al. [34], the PRO power density and recovery can be influenced by FS concentration. In this simulation, the FS inlet concentration was varied from 100 g/m3 to 10,000 g/m3 and this may be the case when waste water is considered as a source. As shown in Fig. 10b & c, both PRO recovery and power density were decreasing with FS inlet concentration and may reach zero asymptotically when both FS and DS concentrations were equal. Therefore, NSEC was increasing while FS inlet concentration was increased and reaches asymptotic value for cases III to V (Fig. 10d). But for cases VI and VII, the NSEC was expected to increase due to the interaction between PRO and SWRO streams by direct mixing. For example, in case VII the NSEC is lesser than other systems with FS inlet concentration lesser than 1300 g/m3, but for higher FS inlet concentration NSEC increased rapidly. 3.1.6.6. Effect of back mixing ratio on SWRO–PRO system performance. As shown in Fig. 11, back mixing ratio is applicable for cases VI and VII only. The effect of back mixing was studied for two different SWRO inlet pressures (i.e. 55 bar and 65 bar) by keeping all other operating condition as in Table 4. The flow ratio between DS flow to SWRO and DS flow at point P1 is defined as back mixing ratio (refer Fig. 1f & g) and will vary from 0 to 1. For the case VI, DS recirculation flow to HP pump was kept equal to the product water flow rate. As shown in Fig. 11a, the DS outlet concentration was decreasing with back mixing ratio at 55 bar, and it was vice versa at 65 bar SWRO

Fig. 10. Effect of FS inlet concentration.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

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Fig. 11. Effect of back mixing ratio.

feed pressure. When DS outlet concentration is realized below sea water concentration, the SWRO feed is diluted and that leads to decreased DS inlet concentration. Similarly, recovery of both PRO and SWRO units was increasing at 55 bar and decreased at 65 bar of SWRO feed pressure. The above phenomenon was in compliance with Fig. 5b. For a fixed FS inlet concentration, the power density was proportional to DS concentration. Therefore, power density was also expected to perform similar to DS outlet concentration profile (Fig. 11d). The power density and SWRO recovery profiles were found to be opposite in nature with

respect to back mixing ratio. NSEC was found to be decreasing while increasing back mixing ratio for case VII (i.e. independent of SWRO feed pressure variation). On the other hand, for case VI, the NSEC was marginally decreasing at 55 bar and it was contrast at 65 bar. 3.2. Optimization Reducing the SWRO–PRO system NSEC is one of the main objectives of this work. As presented in the SWRO–PRO process flowsheet section,

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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the NSEC is a function of process parameters. The relative impact of process parameters on NSEC are presented in Table 5. The relative impact on NSEC is defined based on the gain between NSEC and process parameter and linearity between NSEC and process parameter. The impact levels are defined as follows: (i) high means gain is larger and nonlinear, (ii) medium means gain is larger and linear and (iii) low means gain lesser and linear. The target production for SWRO was fixed as 0.1164 m3 /s. The NSEC was optimized for two scenarios i.e. with and without target production at five different FS inlet concentrations (i.e. 100 g/m3, 500 g/m3, 1000 g/m3, 5000 g/m3 and 32,000 g/m3). The performance of cases III–VII were compared with case I (i.e. existing SWRO system). The case II was also reported under case III with FS inlet concentration of 32,000 g/m3, since both cases were similar with respect to optimization problem formulation. 3.3. Objective function The NSEC is defined as the amount of energy consumed by all pumps minus the energy generated in turbine per unit quantity of pure water produced from SWRO.  Xj minimize NSEC ¼

1

X1

WPump −

0

, WTurb

ðQ p 3600Þ

ð44Þ

where NSEC, j, WPump , W Turb , and Q P are net specific energy consumption (kWh/m3), number of pumps, power consumption in pump (kW), power generation by turbine (kW) and product water flow rate (m3/s) respectively. 3.4. Constraints Based on the target production rate and other operational limitations, constraints and bounds for decision variable are given in Table 5. The decision variables which are not applicable for given hybrid system were marked as not applicable (NA). The significance of decision variable on NSEC was marked with three levels (i.e. high, medium, low). If required, the variable with low significance may be excluded from optimization decision variable for reducing complexity in optimization. However, in this study, all the applicable decision variables were considered for optimizing NSEC. 3.5. Optimization results and discussion The optimum NSEC for case I was obtained to be 2.079 and 1.661 kWh/m3 for the production rate of 0.1164 (i.e. target) and 0.0448 m3/s respectively. For the fixed production rate, the optimum NSEC for the cases III–VII are presented in the bar chat with respect to five different FS inlet concentrations Fig. 12. The energy saving with respect to the existing SWRO system is also reported. The positive values of energy saving represent that the SWRO–PRO hybrid system is efficient than SWRO system. As shown in Fig. 12, for lower FS inlet concentration, all the SWRO–PRO hybrid configurations were found to be energy efficient than SWRO system. But at higher FS concentration,

NSEC of all the hybrid systems were found to be higher than the SWRO system. The lowest possible NSEC was achieved for the case VII when the FS inlet concentration was 100 g/m3. The calculated optimal NSEC was 1.120 and 0.842 kWh/m3 for the production rate of 0.1164 (i.e. target) and 0.054 m3/s. The corresponding energy saving with respect to case I were 41.7% and 49.33% respectively. The optimized process operating conditions for best cases are reported in Table 6. The optimized decision variables such as FS/DS ratio, DS inlet pressure and DS back mixing ratio were found to be at their bounds for case VII. The SWRO–PRO system does not require turbine for energy generation since all the brine water was recirculated at optimized process condition. The PRO operating pressure was hitting to lower bound i.e. PRO operation conditions are moving towards Forward osmosis process operating condition. Similarly, for higher FS concentration, it was found that the existing SWRO system (case I) is energy efficient compared to hybrid SWRO–PRO configurations. The optimization results with relaxed target product water flow were presented in Fig. 13. For all the cases, the calculated optimum NSEC without target production was lesser than NSEC with target production. None of the system was able to meet the target production (0.1164 m3/s) at optimal NSEC and their optimal product water flow rate was not equal. The optimal product water flow rate for SWRO system was 0.045 m3/s. For benchmarking purpose, the hybrid systems having production rate greater than 0.045 m3/s were compared among them with respect to NSEC. The comparisons of SWRO–PRO hybrid systems from lowest to highest NSEC for different FS inlet concentration were given below. 100 g/m3 is given as VII N VI N IV N III N I 500 g/m3 is given as VII N VI N IV N I N III 1000 g/m3 is given as VII N VI N IV N I N III 5000 g/m3 is given as V N I N IV N III 32,000 g/m3 is given as I N V N IV N VII N III N VI Above observations shows that, if low concentrated waste water is fed as a FS to PRO, then the hybrid SWRO–PRO system which does the back mixing was found to be energy efficient solution for desalination. For higher FS concentration, it was found that the existing SWRO system (case I) is energy efficient compared to hybrid SWRO–PRO configurations. The optimum operating conditions for the best performing case at five different FS inlet concentrations were given in Table 6. The estimated optimal operating condition of PRO system concluded that the PRO was operated at high pressure (i.e. PRO mode) for all configurations while target water production constraint was relaxed. As concluded by Straub et al. [35], the above analysis was also identified energy saving prospective for hybrid SWRO–PRO system after including energy losses in membrane, pumps and energy recovery devices. However, it will be feasible only when large amount of pretreated waste water is available at locations nearby SWRO desalination plant. The following important practical constraints has to be addressed before implementation of such a hybrid SWRO–PRO system: (i) bio fouling will be the likely biggest technical challenge for the combined SWRO–PRO system due to use of waste water as FS and this issue may be resolved by implementing proper pre-treatment system; (ii) integrated operation of the hybrid SWRO–PRO system requires large size storage facility for waste water storage; and (iii) validation of overall

Table 5 Constraints for optimization and their relative impact on NSEC. Effect on NSEC Sl. no.

Variable

Bounds

1 2 3 6 4 5 7

Product water flow rate SWRO feed pressure SWRO feed flow rate DS inlet pressure FS/DS flow ratio Concentration of waste water Back mixing ratio

Qp ≥ 0.1164 m3/s 30 bar ≤ Pf ≥ 70 bar 0.234 m3/s ≤ Qf ≤ 0.291 m3/s 5 bar ≤ Pdi ≤ 15 bar 0.8 ≤ FS/DS ≤ 2 100 g/m3 ≤ Cfi ≤ 10,000 g/m3 0 ≤ nmix ≤ 1

Case I

Case II

Case III

Case IV

Case V

Case VI

Case VII

High High Medium NA NA NA NA

High High Medium High High NA NA

High High Medium Medium Low Medium NA

High High Medium High Medium Medium NA

High High Medium High Medium Medium NA

High High Low High High High High

High High Medium High High High High

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

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Fig. 12. Optimum NSEC and the percentage energy saving (with target).

Table 6 Optimal process parameters. FS inlet Best Optimum value of decision variables concentration case PRO DS inlet Brine back Product water Feed water Operating FS/DS ratio (g/m3) pressure of PRO inlets pressure flow rate mixing ratio flow ratea of SWRO Pdi [bar] of SWRO nmix Q p [m3/s] Pf [bar] Qf [m3/s]

Values of other important variables at optimum point Flow rate Flow rate at at BP pump LP pump-1 Q HPP [m3/s] Q LPP-1 [m3/s]

Flow rate at LP pump-2 Q LPP-2 [m3/s]

Flow rate at Flow rate LP pump-3 at FS outlet Q LPP-3 Q fo [m3/s] [m3/s]

0.12239 0.14066 0.15495 0.17705 0.17464

0.24479 0.28133 0.30991 0.18421 NA

0.00105 0.00107 0.00109 0.291 NA

0.12945 0.16602 0.19469 0.12595 NA

0.18003 0.18411 0.18830 0.19141 0.18913

0.14402 0.14729 0.15064 0.15313 NA

0.00164 0.00192 0.00159 0.234 NA

0.09170 0.09903 0.10653 0.11348 NA

For targeted production rate of 0.1164 m3/s 100 VII 0.239 35.303 500 VII 0.257 41.005 1000 VII 0.271 47.934 5000 V 0.291 56.479 I 0.291 56.120 32,000b

2 2 2 1.0404 NA

5 5 5 15 NA

1 1 1 NA NA

0.1164

0.12363 0.14209 0.15652 0.17884 0.17641

For non-targeted production rate 100 VII 0.234 500 VII 0.234 1000 VII 0.234 5000 V 0.234 b I 0.234 32,000

0.8 0.8 0.8 0.8 NA

15 15 15 15 NA

1 1 1 NA NA

0.05396 0.04988 0.04569 0.04490 0.04486

0.18185 0.18597 0.19020 0.19334 0.19105

a b

30 30 30 38.750 38.755

The SWRO and SWRO–PRO systems were designed such that the feed flow rate through HP pump is always equal to permeate water produced. None of the hybrid system is better than the existing system when the sea water (32,000 g/m3) is used as FS for PRO.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

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Fig. 13. Optimum NSEC and the percentage energy saving (without target).

economic benefit considering both capital and operating expenditure of PRO unit in place of SWRO brine post treatment unit. 4. Conclusion Mathematical model for SWRO–PRO hybrid system was developed and implemented in Modelica language using Dymola software tool. The verified individual process unit models are used for SWRO–PRO flowsheet simulation studies and the following observations were made: • When sea water was used as feed solution for PRO, the SWRO–PRO hybrid system is found to be an unsustainable system with respect to SWRO system. • This is mainly due to severe ICP in PRO unit. Unless the ICP is nullified with help of innovative PRO membranes, the self-sustainable SWRO– PRO configuration may not be viable if sea water is used as a FS solution. • When water waster was used as FS for PRO, the SWRO–PRO hybrid system was found to be energy efficient solution for desalination application.

• Among the six SWRO–PRO configurations presented in this work, the one (i.e. case VII) which does the direct mixing of diluted DS outlet with SWRO feed was found to be energy efficient solution for desalination. • For optimal case VII, the calculated NSEC is reduce by 41% and 16% with respect to SWRO desalination system for FS inlet concentration of 100 g/m3 and 1000 g/m3 respectively. • The identified best configuration (i.e. case VII) in this work does not require additional ERD and turbine at optimized process conditions.

5. Future work Further studies on improved return on investment (ROI) for hybrid SWRO–PRO system may help us to verify whether the proposed hybrid system can provide improved ROI than the SWRO with directly reclaiming wastewater.

Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027

S. S., S. S. / Desalination xxx (2016) xxx–xxx

Nomenclature A water permeability of SWRO the membrane (m/bar·s) water permeability of PRO membrane (m/bar·s) APRO Ar area of ERD's inlet and outlet pipes (m) B salt permeability of SWRO the membrane (m/s) salt permeability PRO membrane (m/s) BPRO concentration at SWRO feed side membrane (g/m3) Cm concentration of permeate (g/m3) Cp SWRO feed water concentration (g/m3) Cf SWRO concentration of reject (g/m3) Cr draw side concentration at membrane–solution interface Cdm (g/m3) feed side concentration at membrane–solution interface Cfm (g/m3) FS inlet concentration (g/m3) Cfi DS inlet concentrations (g/m3) Cdi feed side concentration at support layer (g/m3) Cs FS outlet concentration (g/m3) Cfo concentration of high-pressure inlet (g/m3) Chi concentration of low-pressure outlet (g/m3) Clo concentration of low-pressure inlet (g/m3) Cli concentration of high-pressure outlet (g/m3) Cho Darcy's law constant (bar·s/m4) Df FS side Darcy's law constant (bar·s/m4) Df,PRO DS side Darcy's law constant (bar·s/m4) Dd,PRO i van't Hoff factor j number of pumps water flux in SWRO (m3/m2·s) Jv k mass transfer coefficient of SWRO membrane (m/s) DS side mass transfer coefficients (m/s) kd FS side mass transfer coefficients (m/s) kf solute resistance in porous support layer (s/m) Kp K friction loss coefficient l length of one SWRO membrane (m) L total length of a SWRO pressure vessel (m) length of one PRO membrane (m) lPRO length of one PRO membrane pressure vessel (m) LPRO M molecular weight of NaCl (g/m3) Mix mixing percentage (%) number of membranes per one SWRO pressure vessel mRO number of membranes per one PRO pressure vessel mPRO m, n total number of outputs and inputs of mixer/splitter NoP number of pressure vessels per train efficiency of ERD ηERD efficiency of pump ηpump ηturbine efficiency of turbine cum generator system pressure of reject stream (bar) Pr hydraulic pressure differences in SWRO (bar) ΔPRO FS inlet pressure (bar) Pfi FS outlet pressure (bar) Pfo actual osmotic pressure difference across the SWRO memΔπact brane (bar) effective osmotic pressure difference across the PRO memΔπeff brane (bar) pressure drop in a pressure vessel (bar) Pdrop DS inlet pressure (bar) Pdi DS outlet pressure (bar) Pdo pressure at HP inlet (bar) Phi pressure at LP inlet (bar) Pli pressure at LP outlet (bar) Plo pressure at HP outlet (bar) Pho draw side pressure drop (bar) ΔPd feed side pressure drop (bar) ΔPf pressure of inlet stream (bar) Pin pressure of outlet stream (bar) Pout pressure head utilized to generate power by turbine (bar) PHU

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PHP pressure head developed in pump (bar) density of feed water (kg/m3) ρf flow rate of feed (m3/s) Qf flow rate of reject (m3/s) Qr flow rate of permeate (m3/s) Qp flow rate at HP inlet (m3/s) Qhi flow rate at LP inlet (m3/s) Qli flow rate at LP outlet (m3/s) Qlo flow rate at HP outlet (m3/s) Qho FS inlet flow rate (m3/s) Qfi FS outlet flow rate (m3/s) Qfo DS inlet flow rate (m3/s) Qdi DS outlet flow rate (m3/s) Qdo inlet flow rate (m3/s) Qin outlet flow rate (m3/s) Qout R universal gas constant (m4·bar/mol·K) RecvRO recovery of SWRO (%) RecvPRO recovery of PRO (%) σ reflection coefficient S area of one SWRO membrane (m2) total membrane area of a SWRO train (m2) Sm area of one PRO membrane (m2) SPRO total membrane area of a PRO train (m2) Sm,PRO T operating temperature (K) velocity at HP inlet (m/s) Vhi velocity at LP inlet (m/s) Vli velocity at LP outlet (m/s) Vlo velocity at HP outlet (m/s) Vho power produced per unit area of the membrane (W/m2) Wden power consumption in pump (kW) WPump power generation by turbine (kW) WTurb x, y empirical constants References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717, http://dx.doi.org/10.1126/ science.1200488. [2] Z. Jia, B. Wang, S. Song, Y. Fan, Blue energy: current technologies for sustainable power generation from water salinity gradient, Renew. Sust. Energ. Rev. 31 (2014) 91–100, http://dx.doi.org/10.1016/j.rser.2013.11.049. [3] S. Loeb, F. Van Hessen, D. Shahaf, Production of energy from concentrated brines by pressure-retarded osmosis, J. Membr. Sci. 1 (1976) 249–269, http://dx.doi.org/10. 1016/S0376-7388(00)82271-1. [4] K. Gerstandt, K.-V. Peinemann, S.E. Skilhagen, T. Thorsen, T. Holt, Membrane processes in energy supply for an osmotic power plant, Desalination 224 (2008) 64–70, http://dx.doi.org/10.1016/j.desal.2007.02.080. [5] K. Saito, M. Irie, S. Zaitsu, H. Sakai, H. Hayashi, A. Tanioka, Power generation with salinity gradient by pressure retarded osmosis using concentrated brine from SWRO system and treated sewage as pure water, Desalin. Water Treat. 41 (2012) 114–121, http://dx.doi.org/10.1080/19443994.2012.664696. [6] V. Sim, Q. She, T. Chong, C. Tang, A. Fane, W. Krantz, Strategic co-location in a hybrid process involving desalination and pressure retarded osmosis (PRO), Membranes (Basel) 3 (2013) 98–125, http://dx.doi.org/10.3390/membranes3030098. [7] W. He, Y. Wang, A. Sharif, M.H. Shaheed, Thermodynamic analysis of a stand-alone reverse osmosis desalination system powered by pressure retarded osmosis, Desalination 352 (2014) 27–37, http://dx.doi.org/10.1016/j.desal.2014.08.006. [8] M.H. Sharqawy, S.M. Zubair, J.H. Lienhard, Second law analysis of reverse osmosis desalination plants: an alternative design using pressure retarded osmosis, Energy 36 (2011) 6617–6626, http://dx.doi.org/10.1016/j.energy.2011.08.056. [9] J.L. Prante, J.A. Ruskowitz, A.E. Childress, A. Achilli, RO–PRO desalination: an integrated low-energy approach to seawater desalination, Appl. Energy 120 (2014) 104–114, http://dx.doi.org/10.1016/j.apenergy.2014.01.013. [10] J. Kim, M. Park, S.A. Snyder, J.H. Kim, Reverse osmosis (RO) and pressure retarded osmosis (PRO) hybrid processes: model-based scenario study, Desalination 322 (2013) 121–130, http://dx.doi.org/10.1016/j.desal.2013.05.010. [11] D. Inhyuk, J. Kim, H. Kyong, S. Hong, Pressure retarded osmosis (PRO) for integrating seawater desalination and wastewater reclamation : energy consumption and fouling, J. Membr. Sci. 483 (2015) 34–41, http://dx.doi.org/10.1016/j. memsci.2015.02.025. [12] A. Almansoori, Y. Saif, Structural optimization of osmosis processes for water and power production in desalination applications, Desalination 344 (2014) 12–27, http://dx.doi.org/10.1016/j.desal.2014.03.002. [13] L.G. Palacin, F. Tadeo, C. De Prada, K. Touati, Evaluation of the recovery of osmotic energy in desalination plants by using pressure retarded osmosis, Desalin. Water Treat. 51 (2013) 360–365, http://dx.doi.org/10.1080/19443994.2012.715130.

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Please cite this article as: S. S., S. S., Reverse Osmosis–Pressure Retarded Osmosis hybrid system: Modelling, simulation and optimization, Desalination (2016), http://dx.doi.org/10.1016/j.desal.2016.01.027