Desalination 401 (2017) 88–98

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Exergo-economic analysis of a seawater reverse osmosis desalinationplant with various retrofit options

Muhammad Ahmad Jamil, Bilal Ahmed Qureshi, Syed M. Zubair ⁎Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

H I G H L I G H T S

• Seawater reverse osmosis desalination plant with four different retrofit options is studied.• First- and second-law analyses are carried out to estimate energy requirements and second-law efficiency.• The product cost is compared by performing exergo-economic analysis using reliable seawater properties.• Analysis revealed that with a pressure exchanger, energy consumption can be reduced by 24%.• It is also shown that post-treatment and distribution sections increase the product cost by about 20%.

⁎ Corresponding author.E-mail address: smzubair@kfupm.edu.sa (S.M. Zubair)

http://dx.doi.org/10.1016/j.desal.2016.09.0320011-9164/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 May 2016Received in revised form 22 September 2016Accepted 28 September 2016Available online 8 October 2016

The current study is focused on carrying out exergo-economic analysis of a seawater reverse osmosis (SWRO)de-salination plant. The main objective is to compare the performance as well as the product cost of an existingSWRO plant, including post-treatment and distribution sections, for four different retrofit options made by cou-pling high-efficiency pressure exchangers (PXs) in place of conventional energy recovery turbines. For this pur-pose, first- and second-law analysis is carried out to estimate the energy requirements and second-law efficiencyfor each retrofit option. Finally, the product cost is compared by performing an exergo-economic analysis usingappropriate seawater properties for the calculations. The analysis revealed that, by introducing a PX, the specificenergy consumption (SEC) can be reduced by about 24%; thus, increasing the second-law efficiency. Besides this,it is also demonstrated that the addition of post-treatment and distribution sections enhances the product cost byalmost 20%. Furthermore, the study suggested that using a booster pumpwith a PX (as used in retrofit # 3) is bestsuited for enhancing the plant capacity compared to retrofit # 4 inwhich a PX is used in place of the pump. It hasthe least product water cost among all the options discussed.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Reverse osmosisSeawaterRetrofitPressure exchangerExergo-economic analysis

1. Introduction

Reverse osmosis (RO), a membrane-based desalination system isone of the most frequently used techniques for treatment of seawater.From 1970 till today, this technology has been widely used, studiedand improved over the time [1]. It has lower start-up time, decreasedenvironmental impacts (in terms of emissions) and easier operationandmaintenance. Energy analysis of RO systems operating under differ-ent capacities with andwithout energy recovery devices (ERDs) revealsthat their energy consumption can be greatly reduced by coupling ERDs[2–6]. Coupling of Pelton turbines (as energy recovery turbines “ERTs”)with RO systems is one of the oldest energy recoverymethods [7,8]. Iso-baric pressure exchangers (PXs) are relatively modern and better

.

devices in this regard [9]. A detailed discussion of theworking and selec-tion of ERDs is carried out by various investigators [10–15].

Besides this, exergy analysis has been used as one of themost impor-tant tools by researchers [16,17] frequently to identify the componentswith the greatest exergy destruction. Cerci [18] and Aljundi [19] ana-lyzed two different RO plants using actual plant data and reported thethrottling valves and membrane modules to be the primary locationsfor exergy destruction. Romero et al. [20] carried out a similar studyfor a complete plant includingpre-treatment, post-treatment and distri-bution sections. The above studies proposed that the second-law effi-ciency of the plants can be improved by installing pump-motorsequipped with variable frequency drives and replacing throttle valveson the brine stream with a PX.

Another useful way of analyzing the desalting systems is to combinethe exergy and cost analysis known as exergo-economic analysis.Lozano and Valero [21] presented the theory of exergetic costs whichis considered to be one of the major approaches in this field. Based on

89M.A. Jamil et al. / Desalination 401 (2017) 88–98

this theory, Romero et al. [22] carried out an exergo-economic analysisof an RO plant and reported the product cost to be 0.70 €/m3. El-Emamand Dincer [23] performed a similar analysis for different seawater sa-linities and estimated the product cost to be 2.45 $/m3 for a salinity of35 g/kg. Spiegler and El-Sayed [24,25] contributed significantly to thefield of thermo-economics by developing the correlations for the rateof fixed cost of various components of desalination systems. They sug-gested that the main focus should be on the exergy destruction whichconstitutes mainly the operating resources of any desalination systemrather than the making resources (fixed cost). Some studies [26–28]were focused on analyzing solar-powered desalination systems. Theunit product cost for a small-scale solar-poweredmembrane distillationunit was reported to be 15 $/m3 by [26] which is much higher than con-ventional systems. However, the unit product cost for a large scale PV/RO system was estimated as 1.3 $/m3 by [27], which is slightly higherthan conventional systems (0.75 $/m3) due to higher electricity cost.Penate and Rodriguez [29] proposed and analyzed four different retrofitoptions to provide upgradation opportunities for existing SWRO plantsworkingwith conventional ERTs. The results of energy, exergy and ther-mo-economic analysis for all the retrofit options were compared toidentify the best one with a minimum product cost.

1.1. On exergy calculation model

Fitzsimons et al. [30] examined and compared six different exergycalculationmodels and showed that thesemodels affect the final resultssignificantly. The study suggested that, among these, the electrolytemodel approach is not suitable because seawater is not an idealmixture.Similarly, the approaches used by Cerci [18] and Drioli et al. [31–33] arenot suitable for desalination system analysis because of ideal mixtureassumptions and specific separation assumptions, respectively.Sharqawy et al. [34] functions and Pitzer et al. [35,36] equations canbe used to calculate thermodynamic properties of seawater and otherelectrolytes, respectively. A similar issue regarding the definition of sec-ond-law efficiency is highlighted by various authors [17,37–39] in theirstudies. Some of them [14,40] define it as the ratio of the total exergyleaving to the total exergy entering the system, while others [16,17] asthe ratio of product to fuel exergies. Qureshi and Zubair [41] discussedthe applicability of these definitions and suggested that the secondone is more appropriate for desalination systems.

Based on the above discussion, the current study is focused onreassessing and improving the work of Penate and Rodriguez [29] byconsidering the following: (a) post-treatment and distribution sectionsin the current analysis, (b) use of reliable and updated seawater proper-ties recently compiled by Sharqawy et al. [34], (c) an appropriate defini-tion of the second-law efficiency suggested by Qureshi and Zubair [41],and (d) plant performance as a function of important input parameterssuch as unit electricity cost, feed salinity and high-pressure pump (HPP)efficiency.

2. System description and modeling

The system under consideration consists of a 10,000 m3/d seawaterreverse osmosis (SWRO) plant equipped with two membrane modulesof the same capacity coupled with two identical ERTs. The schematic forthis configuration is shown in Fig. 1, in which one HPP per train is usedto raise the feed pressure. The system is analyzed under four differentpossible retrofit options, calculations for each retrofit option are per-formed and the results are compared with the standard configuration.In the first two retrofit options, the plant capacity remains the samewhile the focus is to minimize the energy consumption by replacingERTs with a PX. The last two options are proposed to increase theplant capacity as well as minimize the energy consumption. The dataused for analysis of the plant is listed in Table 1. The analysis presentedin the paper is based on the following assumptions that are also consid-ered by [29,41]: (a) the dead state is taken as the conditions of the feed,

i.e., P0 = 101.325 kPa, T0 = 20 °C, S0 = 35 g/kg and operating temper-ature is considered constant throughout the system, (b) an overall pres-sure drop in RO modules, pipes and valves is considered to be 160 kPa,(c) feed water pressure at HPP inlet is taken as 351.325 kPa and the re-covery ratio is 45%, (d) effect of permeate back pressure, reverse salt dif-fusion, concentration polarization and system leakages are considerednegligible, (e) thermo-physical properties of seawater are based onthe correlations provided by Sharqawy et al. [34], and (f) efficienciesof the various components are, ηHPP=78%, ηBP=77%, ηFP=78%,ηDP=78%, ηMotor=92%, ηPX=90% [29].

For numerical simulation, engineering equation solver (EES) soft-ware is used with updated seawater properties compiled by Sharqawyet al. [34].

2.1. First-law analysis

To carry out the first-lawanalysis, themass balance (Eq. (1)) and thesolution balance (Eq. (2)) are applied. For a steady-state system, thesecan be expressed as,

Xin

_m ¼Xout

_m ð1Þ

Xin

_mS ¼Xout

_mS ð2Þ

Pump and turbine work is calculated using Eq. (3) and Eq. (4), re-spectively as,

WPump ¼ Q ΔPηPump

ð3Þ

WTB ¼ ηTB QΔP ð4Þ

The PX efficiency is described as [41,42]:

ηPX ¼ QB;oPB;o þ Q

F;oP F;o

QB;iPB;i þ Q

F;iP F;i

ð5Þ

Specific energy consumption (SEC) is one of the important parame-ters for comparing plants working under different capacities because itcompares the energy requirement for a unit product. It can be expressedas [41]:

SEC ¼ Win

3600Xout

Qp

ð6Þ

2.2. Second-law analysis

This analysismeasures the extent of irreversibility in terms of exergydestruction, which is calculated by applying exergy-balance on eachcomponent, separately:

Xfuel

_X−X

products

_X ¼ _XD þ _XL ð7Þ

The second law efficiency is calculated, as described in [17,41]:

ηII ¼_Wl;min

_Winð8Þ

Fig. 1. Schematic of the standard SWRO plant.

Table 1Operational data for standard configuration SWRO plant [29].

Parameter Flow (m3/h) Pressure (kPa)

Total feed 926 101.325Feed per train 463 5961.325HPP inlet 463 351.325Permeate stream per train 208 101.325Total permeate 416 101.325Brine stream/Turbine inlet 255 5801.325Brine discharge/Turbine Outlet 255 101.325

90 M.A. Jamil et al. / Desalination 401 (2017) 88–98

The concept of least work of separation and the minimum of leastwork of separation is well explained by Mistry et al. [17]. The lattercan be written as:

_Wl;min ¼X

products

_X−Xfuel

_X ð9Þ

3. Retrofit options

The retrofit options proposed in this study are divided into twocategories:

Fig. 2. Schematic of re

3.1. Retrofit options for constant capacity

These options are proposed with an aim to reduce energy consump-tion when the total plant capacity is to be maintained same as in thestandard configuration. In this category, the standard plant is retrofittedwith high-efficiency ERDs as explained below.

3.1.1. Retrofit option # 1It consists of one HPP instead of two (used in the standard configu-

ration) and the two ERTs are replaced by a high-efficiency isobaric ener-gy recovery device. One HPP is sufficient for both the trains so thisconfiguration requires replacement of the HPP motor with a slightlyhigher capacity and, thus, requiresminor affordable changes in the elec-tric wiring and other protection systems. The feed pump does not re-quire any alteration because the same amount of feed water is to bepumped at the same pressure. Neither the recovery ratio nor the flowrates are modified. However, this configuration requires installation ofa booster pump (BP) for which new channeling of the feed water andbrine is required. Civil work is not required that much because theERD can be installed in the space left by ERTs. However, piping needsto bemodified because the flows are distributed over the plant in differ-ent ways as shown in Fig. 2. The feed-water is pumped to both the HPP

trofit option # 1.

Table 2Operational data for SWRO plant retrofit option # 1 [29].

Parameter Flow (m3/h) Pressure (kPa)

Total feed 926 101.325Feed per train 463 5961.325HPP inlet 416 351.325ERD feed 510 351.325Permeate stream per train 208 101.325Total permeate 416 101.325Brine stream/ERD brine inlet 510 5761.325Brine discharge/ERD brine outlet 510 221.325

Table 3Operational data for SWRO plant retrofit option # 2 [29].

Parameter Flow (m3/h) Pressure (kPa)

Total feed 926 101.325Feed per train 463 5961.325Low pressure BP inlet 416 351.325HPP inlet 463 351.325ERD feed inlet 510 351.325Permeate stream per train 208 101.325Total permeate 416 101.325Brine stream/ERD brine inlet 510 5801.325Brine discharge/ERD brine outlet 510 101.325

91M.A. Jamil et al. / Desalination 401 (2017) 88–98

and ERD inlet and its pressure is raised as it passes through these de-vices. Water from both circuits combine at state 8 and then distributedevenly to each train. Permeate fromboth the trains is obtained as shownin the figure. The high-pressure brine stream from each train is directedto the ERD where it loses its pressure energy to raise the pressure of in-coming feed water and then rejected back to the sea. The operationaldata for this configuration is given in Table 2.

3.1.2. Retrofit option # 2It consists of an isobaric pressure exchanger with a BP to raise the

pressure of the feed water and is proposed to avoid replacement ofthe existing motors coupled to the HPP. One BP is also installed priorto the HPP to maintain the required pressure, which avoids substantialelectricalmodifications by keeping the flow rates and the plant-capacityconstant. Flow arrangements remain same as discussed in the previousoption and schematic is shown in Fig. 3. Table 3 summarizes the opera-tional data for this retrofit option.

3.2. Configurations to increase the plant capacity

These options are proposedwith an aim to increase the plant capac-ity by introducing high capacity RO trainswith a higher number of pres-sure vessels. The plants are also retrofitted with high-efficiency ERDs tominimize the energy consumption. In the current study, only one trainis shown for the sake of analysis because the two trains are identical.These types of retrofits are recommended for the cases where new in-vestments can be made to upgrade the plants and sufficient space isavailable to accommodate high capacity RO trains.

3.2.1. Retrofit option # 3In this case, eachHPP is retrofitted to allow 35% reduction in the feed

flow rate because an isobaric ERD and a BP is used to manage the pres-sure of the remaining feed. The schematic for this option is shown inFig. 4. The new train consists of 23 pressure vessels and works withthe same recovery rate and the train capacity is increased from 5000m3/day to 7200 m3/day. Pipe diameters are to be modified slightly

Fig. 3. Schematic of re

and more parallel pipes are to be installed. A new pipe is to be installedfor the low- and high-pressure feed flow to the ERD and BP. No doubt,this configuration consumes more net energy but provides 40% morepermeate which reduces the SEC. The operational parameters used fordefining the new capacity are given in Table 4.

3.2.2. Retrofit option # 4This retrofit option is amodified formof the previous one (see Fig. 5)

in which an isobaric ERD is used as a second stage HPP. A portion of thefeedwater is pressurized by the HPP while the rest of it gets pressurizedby the ERD that uses the brine (from the first stage) as theworkingfluid.This retrofit produces about 50%morewater than the standard configu-ration. It allows for the existingHPP and RO train to be usedwithout anynew installation of energy consuming devices. The operational data forthis retrofit is given in Table 5.

A sample calculation for the first- and second-law analyses (of the2nd retrofit option) is given in Appendix A.

4. Exergo-economic analysis

For exergo-economic analysis, the plant is divided into three subsys-tems for each retrofit option. Subsystem 1 consists of a feed pump and apre-treatment section and subsystem 2 consists of HPPs, BP (if needed)and ERDs. Subsystem 3 includes the post-treatment and distribution fa-cility. The schematic of subsystems for this analysis is shown in Fig. 6.The first step here is to calculate the exergy of each stream which isthe sum of physical and chemical exergies [34]. The other calculationsdepend on the model used. For instance, the model used in the currentpaper, which is based on the approach used by Romero et al. [22], re-quires the calculation of exergy destruction in each component for thecost analysis. The only difference is that, in [22], the analysis for eachcomponent is carried out separately but, in our case, the componentsare combined in the form of subsystems. The calculations are given inthe following sections.

trofit option # 2.

Fig. 4. Schematic of retrofit option # 3.

Table 4Operational data for SWRO plant retrofit option # 3 [29].

Parameter Flow(m3/h)

Pressure(kPa)

Total feed 1334 101.325Feed per train 667 5961.325HPP inlet 300 351.325ERD feed inlet 367 351.325Permeate stream per train 300 101.325Total permeate 600 101.325Brine stream/ERD brine inlet 367 5761.325Brine discharge/ERD brine outlet 367 101.325

92 M.A. Jamil et al. / Desalination 401 (2017) 88–98

4.1. Exergy costs of flow streams

The exergetic cost of any stream is based on the exergy required toproduce it. In the present study, the exergy cost calculations are basedon three assumptions [20]: (a) exergy cost of any input stream (fromthe environment) is equal to its exergy value, (b) exergy cost of any use-less flow (such as blowdown) is considered as zero, and (c) inlet andoutlet components of any fuel have the same unitary exergy cost.

The unitary exergy cost (a dimensionless parameter) of a streamrepresents the ratio of its exergetic cost to power input. It tells usabout the exergy power required to produce that exergy stream. For dif-ferent streams, the value of the exergy rate and unit exergetic cost aredifferent and can be calculated, as explained below.

The intake stream (0) is taken as the dead state so its specific exergy,exergy rate and unitary exergy cost will be zero.

Cx0 ¼ X 0 ¼ 0 ð11Þ

Fig. 5. Schematic of re

The stream leaving subsystem 1 will have certain pressure with ref-erence to the dead state, so it will have certain exergy rate. Its unitaryexergy cost can be expressed as,

Cx1 ¼ WSubsystem1

X 1ð12Þ

where WSubsystem1 represents the work supplied to the feed pump andpre-treatment unit.

Based on the assumption stated above, the exergy cost of the blow-down is taken as 0 because it has no further utility.

Cx2 ¼ 0 ð13Þ

The product stream has certain exergy rate with reference to thedead state. Its unitary exergy cost can be calculated as

Cx3 ¼ WSubsystem1 þ WSubsystem2 þ WSubsystem3

X 3

¼ WSubsystem;tot

X3ð14Þ

where WSubsystem;tot represents the total work supplied to produce thisproduct stream. This includes work input to the feed pump, HPP andBP (if required). The stream exergy (fuel, product, and losses) valuesof the subsystems (see Fig. 6) are given in Table 7.

4.2. Exergo-economic costs of the flow streams

The next step in this analysis is to calculate the exergo-economiccosts of the flow streams. Tables 8–10 show the values used in theexergo-economic analysis taken from the literature [29]. The unitexergo-economic cost or the unit cost, in c$/MJ, of a stream can be

trofit option # 4.

Table 5Operational data for SWRO plant retrofit # 4 [29].

Parameter Flow(m3/h)

Pressure(kPa)

Total feed 1436 101.325Feed per train 1 463 5961.325Feed per train 2 255 5711HPP inlet 463 351.325ERD feed inlet 255 351.325Permeate stream per train 1 208 101.325Permeate stream per train 2 112 101.325Total permeate 640 101.325Brine train 1/ERD brine inlet 255 5761.325Brine train 1/ERD brine outlet 255 101.325Brine train 2 143 101.325

93M.A. Jamil et al. / Desalination 401 (2017) 88–98

calculated by applying the general formula which states that the unitexergo-economic cost of any product stream is equal to sum of thecosts of fuel streams and fixed cost of the components producing it.This is expressed as [22]:

Cp ¼ ∑fC f

X f

Xpþ Z

Xpð15Þ

The rate of exergo-economic cost C p(in c$/s) is calculated as [22]:

C p ¼ C i þ Celectricity X � �

þ Z ð16Þ

The cost of fresh (desalted)water, in c$/m3, for each retrofit option iscalculated as:

γp ¼ Cp

Qp

ð17Þ

A sample calculation for the exergo-economic analysis is given inAppendix B.

Fig. 6. Schematic for exerg

5. Results and discussion

5.1. First and second law analysis

This section compares the power requirements, specific energy con-sumptions and second law efficiencies of the systems that are discussedin the previous section.

5.1.1. Standard configuration plantReferring to Table 6, it is obvious that, for the standard plant, the net

energy requirement reduces to 1573 kW from 2251 kW after installa-tion of ERTs. SEC for the standard configuration is 3.78 kWh/m3. Thisis expected to be reduced after installation of high-efficiency ERDs. Itis important to emphasize that selection of any retrofit option requiresthe following parameters to be analyzed critically:

(a) overall plant efficiency,(b) new operational data and recovery rate,(c) civil works and hydraulic network,(d) capital investment associated with retrofitting, and(e) management of the new system in terms of operation andmain-

tenance, etc.

Keeping the above facts in mind, four different possible retrofit op-tions are analyzed for comparison purpose.

5.1.2. Retrofit option # 1Table 6 illustrates that, for the same plant capacity, retrofit option #

1 shows a considerable reduction in energy requirements compared tothe original plant. The table shows there is no change in the feedpump work because this section is same for all the retrofit options hav-ing same plant capacity. However, coupling of an isobaric PX reducesoverall energy consumption by 20.63% compared to the original plant.Additionally, second-law efficiency is also increased from 19.47% to24.53% for this option. Therefore, one can say that this retrofit is moreefficient from second-law viewpoint compared to the standard plantbecause of better energy recovery.

o-economic analysis.

Table 6First law analysis results.

Parameter Standard plant Retrofit option #1 Retrofit option #2 Retrofit option #3 Retrofit option #4

Feed pump work (kW) 82.44 82.44 82.44 118.8 127.8HPP work (kW) 925 816.1 594.1 1201 1850BP work (kW) n/a 57.03 297.13 47.66 n/aProduct pump (kW) 209.4 209.4 209.4 302.1 322.2Pelton turbine work (kW) −339.2⁎ n/a n/a n/a n/aTotal energy requirement (kW) 1573 1248 1268 1788 2472SEC (kWh/m3) 3.78 3.00 3.05 2.98 3.86Energy saving (%) n/a 20.63 19.31 21.15 −2.18⁎⁎

ηII (%) 19.51 24.53 24.15 24.70 19.06

n/a stands for not applicable; * represents energy produced by the system; and ** represents increase in SEC compared to the base configuration.

Table 8Data used in economic analysis [29].

Parameter Value

Taxes 0.35Amortization period 8 yearsLifetime 15 yearsLoan Interest rate 0.06Mean inflation rate 0.02Annual increasing of capital goodsabove or below inflation rate

0.00

Years in which the devices should bereplaced

Intake and pumping once during lifetime,while membrane every five years

Electricity cost 0.1344 ($/kWh)Annual increasing of O & M costsabove or below inflation rate

0.00

Annual increasing of product costabove or below inflation rate

0.00

Annual availability 0.95

Table 9Input data costs for each retrofit options analyzed.

94 M.A. Jamil et al. / Desalination 401 (2017) 88–98

5.1.3. Retrofit option # 2This retrofit is obtained by slightly modifying the previous one with

an aim to avoid major replacements. The input power is distributed dif-ferently in this retrofit because of an additional BP. From Table 6, it canbe seen that SEC for this option is reduced by 19.31% compared to thestandard plant. The second-law efficiency, for this case, is 24.15%,which is slightly lower than the previous retrofit but higher than thestandard one. So, we may say that the energy consumption for both ofthese retrofit options lies in the same range because of the same plantcapacity. Hence, overall cost for modification is the only deciding factoramong these two.

5.1.4. Retrofit option # 3The energy requirements for components like feed-pump, HPP and

distribution pump are higher for this retrofit option because it hashigher plant capacity compared to the standard configuration. Whenwe compare the results (refer to Table 6), this retrofit has the lowestSEC value of 2.98 kWh/m3 and highest second-law efficiency of 24.7%.It gives energy saving of 21.15% compared to all other retrofits. There-fore, this retrofit is recommended where higher capacity RO trains canbe accommodated.

5.1.5. Retrofit option # 4It is another retrofit option that can also be used to enhance the plant

capacity. However, the analysis shows, (refer to Table 6) that this optionis not as efficient energetically as the previous ones. It can be seen fromthis table that SEC for this retrofit is 3.86 kWh/m3, which is the highestamong all, including the standard configuration. It shows an increase ofabout 2% in the energy requirement compared to the base system. In ad-dition, second-law efficiency has the least value of about 19%. So, we cansay that this retrofit is not suitable both from first- and second-lawconsiderations.

5.2. Exergo-economic analysis

Table 11 summarizes both exergy values and unitary exergy costs offlow streams. The exergy rates of streams leaving subsystem 1 and 2 aresame for the first two retrofit options because of their same capacity,while these are different for the last two because of different capacities.It is important to emphasize that unitary cost of stream 1 is same for allthe retrofit options since subsystem1 consists of the same equipment inall cases. The difference, however, in work supplied due to differentmass flow rates is compensated by the product stream exergy and

Table 7Stream exergies of the subsystems.

Stream Subsystem 1 Subsystem 2

Fuel (f) _X0 þ _X4_X1 þ _X5

Product (p) _X1_X3

Losses (L) 0 X2

their ratio remains same. However, the unitary exergy cost of productstream is different for all retrofit options since different types of equip-ment are attached to each system with dissimilar capacities. Table 12summarizes the final product cost for all retrofit options. It can beseen that retrofit option # 3 has the least product cost of 70.34 c$/m3

followed by option # 2 with a value of 72.24 c$/m3. Among the firsttwo retrofits, the second one has lower cost compared to the first one.This is because no change in HPP or any other component is requiredand only a BP is introduced to meet the demand. We note that option# 4 has the highest product cost of 83.07 c$/m3. This is primarily dueto the fact that it needs larger modifications and has the highest energyconsumption and irreversible losses in the system components,resulting in the lowest second-law efficiency.

5.3. Comparison with literature

To assess the importance and effectiveness of the modifications in-vestigated in the currentwork, it is necessary to compare the present re-sults with the one published in literature [29]. Figs. 7 to 9 compare theSEC, second-law efficiency and product cost for all the plant configura-tions with and without, post-treatment and distribution sections. The

Parameter Retrofitoption #1

Retrofitoption #2

Retrofitoption #3

Retrofitoption #4

Subsystem 1 cost 0.2274 M$ 0.2274 M$ 0.4558 M$ 0.4872 M$Subsystem 2 cost 1.1268 M$ 1.167 M$ 1.467 M$ 1.596 M$Specific O & M Cost(insurance, labor,overheads,breakdowns, fuelexcluded)

0.1512 $/m3 0.1512 $/m3 0.1456 $/m3 0.1568 $/m3

Table 10Effective rate of fixed costs for different subsystems [29].

Retrofit option #1Subsystem (in c$/s)

Retrofit option #2Subsystem (in c$/s)

Retrofit option #3Subsystem (in c$/s)

Retrofit option #4Subsystem (in c$/s)

(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)0.3523 1.932 1.364 0.375 1.818 1.364 0.772 2.386 1.790 0.829 2.614 1.960

Table 11Stream exergy values and unitary exergy costs of streams.

Parameter

_X0

(kW)

_X1

(kW)

_X2

(kW)

_X3

(kW)Cx0

(--)Cx1(--)

Cx2

(--)Cx3

(--)

Retrofit option # 1 0 64.31 0 306.2 0 1.282 0 3.369Retrofit option # 2 0 64.31 0 306.2 0 1.282 0 3.433Retrofit option # 3 0 92.4 0 441.6 0 1.282 0 3.342Retrofit option # 4 0 99.72 0 473.3 0 1.282 0 4.54

Table 12Product costs for the retrofit options.

Parameter Retrofit option # 1 Retrofit option # 2 Retrofit option # 3 Retrofit option # 4

Unit exergo-economic cost, C (c$/MJ) 35.85 35.90 37.31 42.07

Rate of exergo-economic cost, _Cp (c$/s) 8.37 8.34 11.73 14.78

Fresh water cost, γp (c$/m3) 72.48 72.24 70.34 83.07

95M.A. Jamil et al. / Desalination 401 (2017) 88–98

major outcomes of the comparisons can be summarized in the followingparagraphs.

Fig. 7 shows that SEC results for the current analysis (without addi-tional sections) are in excellent agreement with thework of Penate andRodriguez [29] and confirms themodel validity. It can be seen that afterintroducing post-treatment and distribution sections, SEC values showan increase of 15 to 20% due to additional energy consuming compo-nents. It is, however, important to note that second-law efficiency(refer to Fig. 8) values, calculated by using updated seawater propertiesand an appropriate definition of the efficiency, show a 50 to 60% de-crease compared to the values reported in the literature for all thecases invesitgated. In addition, second-law efficiency values decreaseby 13 to 16% after incorporation of the post-treatment and distributionsections.

Fig. 9 shows that the unit product cost (in c$/m3) calculated by thecurrent approach is higher than the one reported in [29]. It is, however,close to the one reported in [22,43]. The possible reasons for this differ-ence include the use of updated seawater properties and additionalpost-treatment and distribution sections that are considered in thepresent investigations. It should be noted from Fig. 9 that coupling ofthe post-treatment and distribution sections increases the final productcost by 25 to 35% for all the cases.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Base System Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4

SEC

(kW

h/m

3 )

Plant TypeLiterature Current analysis Current analysis with post-treatment & distribution

Fig. 7. Specific energy consumption for different plant configurations.

5.4. Parametric investigation

In this section, the effect of input parameters like feed salinity,pump efficiency and input energy cost on the plant performanceare discussed. All the operating conditions that are used in thisstudy are mentioned in the corresponding figures.

Fig. 10 shows that, by introducing high-efficiency HPPs, SEC of theplant can be greatly reduced. The base system shows an abrupt re-duction in SEC because two HPPs are involved and have the highestenergy consumption. While in the retrofit options, due to the pres-ence of PXs, the energy consumption of HPPs is already lower thanthe base system. Thus, the increase in their efficiency does not re-duce the SEC as abruptly as in the base system. Similarly, Fig. 11shows a significant rise in second-law efficiency for the base systemwith an increase in HPP efficiency compared to other retrofit optionsfor the same reason. Though it is obvious that by introducing high-ef-ficiency components, lower SEC, and higher second-law efficiencycan be obtained, the current analysis is carried out to give an ideathat how and how much the performance of a plant changes withthese parameters.

0

10

20

30

40

50

60

Base System Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4

η II (%

)

Plant TypeLiterature Current analysis Current analysis with post-treatment & distribution

Fig. 8. Comparison of second-law efficiency for various plant configurations.

0102030405060708090

Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4Spec

ific

prod

uct

cost

, γγp

(c$/

m3 )

Plant Type

Literature Current analysis Current analysis with post-treatment & distribution

Fig. 9. Comparison of specific product cost for various plant configurations.

Fig. 10. Specific energy consumption vs high-pressure pump efficiency.

Fig. 12. Second-law efficiency vs feed salinity.

96 M.A. Jamil et al. / Desalination 401 (2017) 88–98

Feed salinity is another parameter that affects the second-law ef-ficiency of the plants as shown in Fig. 12.When the feed salinity is in-creased, the recovery ratio decreases which is inversely proportionto the input energy. The second-law efficiency is obtained by divid-ing the minimum of least work of separation by input energy so it in-creases with the feed salinity.

It is important to understand the variation of the product costagainst the input energy cost because the price of electricity is differentin every locality. It can be seen from Fig. 13 that the final product costincreases linearlywith the electricity cost. However, thefirst and secondretrofit options show an interesting shift at higher electricity cost. Forexample, at an electricity cost ≤0.22 $/kWh, the second retrofit optionhas lower product cost while, for higher values of unit electricity cost,the first option gives better results from an economic standpoint.

ηmotor = 92%, SF = 35g/kg, TF B = 65g/kg

Fig. 11. Second-law efficiency vs high-pressure pump efficiency.

6. Concluding remarks

A seawater reverse osmosis plant discussed by Penate andRodriguez [29] is re-evaluated by using updated seawater propertiesand an appropriate definition of the second law efficiency for fourdifferent retrofit options along with the base system. Furthermore,the study is also updated by adding the post-treatment and distribu-tion sections that were not considered in the previous investigation.The study provides reliable information about improving theexisting SWRO plant in terms of energy consumption by introducinghigh-efficiency PXs as well as upgrading the plant capacity. Themajor findings of the present study can be summarized as:

• Compared to conventional energy recovery turbines, modern PXsare more efficient and their installation resulted in 18 to 23%reduction in SEC of the plant.

• About 50 to 60% reduction in second-law efficiency was observedby using the updated and reliable seawater properties as well asby using an appropriate relation for second-law efficiency.

• The installation of post-treatment and distribution sections in-creases the energy consumption as well as the product cost and de-creases the second-law efficiency of the plant. About 15 to 20%increase in SEC, 25 to 30% increase in product cost and 13 to 16% re-duction in second-law efficiency is observed by introducing thesesections. This fact suggests that these should not be neglected fora reliable design and analysis of desalination plants.

• Among all the retrofit options discussed in the paper, options 2 and3 present better results compared to options 1 and 4 in terms of theproduct cost.

Fig. 13. Variation of the specific product cost vs electricity cost.

97M.A. Jamil et al. / Desalination 401 (2017) 88–98

• For a constant plant capacity, option 2 is recommended for an elec-tricity cost of ≤0.22 ($/kWh) since it has slightly lower product costcompared to the first option. While at higher electricity costs, thefirst option is more favorable.

• Retrofit option 3 is the best possible choice because of the lowerSEC and product cost. However, it requires higher capacity ROtrains which may not be affordable in every case.

• Retrofit option 4 shows the worst performance. It has the highestproduct cost among all the retrofit options because of higher SECand lower second-law efficiency values.

The present study clearly shows that reliable seawater properties,method of calculation and plant layout must carefully be selectedwhile analyzing any desalination system as they can affect the finalresults significantly.

NomenclatureC unit exergo-economic cost (c$/MJ)Cx unitary exergy costC p rate of exergo-economic cost (c$/s)ESaving energy saving (%)_m mass flow rate (kg/s)P pressure (kPa)Q volume flow rate (m3/s)S salinity (g/kg)T temperature (°C)W power requirement (kW)X exergy rate (kW)Z rate of fixed costs (c$/s)

Greek lettersΔ change in quantityη efficiencyγ specific cost (c$/m3)

Subscripts0 dead stateB brineD destroyedF feedi inletII second lawin inputl leastL lossmin minimumo outletp productRf# retrofit optionTB turbineTot total

AbbreviationsBP booster pumpDP distribution pumpEES engineering equation solverERT energy recovery turbineERD energy recovery deviceHPP high-pressure pumpLP low-pressure pumpPVs pressure vesselsPX pressure exchangerRO reverse osmosisSWRO seawater reverse osmosisSEC specific energy consumption, (kWh/m3)

Acknowledgement

The authors acknowledge the support provided by King Fahd Uni-versity of Petroleum & Minerals through the project IN151001.

Appendix A. First- and second-law analysis calculations

For sample calculations, retrofit option # 2 is presented here becauseit includes almost all the components that are discussed in this paper.

The pump work is given by,

WPump ¼ QΔPηPump

ðA� 1Þ

Now, we will calculate the individual pump works.Feed pump work:

_WFP ¼ 0:2572� 351:325−101:325ð Þ0:78

¼ 82:44 kW

Low-pressure BP work:

_WLP;BP ¼ 0:1155� 1951:325−101:325ð Þ0:77

¼ 240 kW

BP work:

_WBP ¼ 0:14167� 5961:325−5651:325ð Þ0:77

¼ 57:03 kW

HPP work:

_WHPP ¼ 0:1156� 5961:325−1951:325ð Þ0:78

¼ 594:3 kW

Distribution pump work:

_WDP ¼ 0:1155� 1402−101:325ð Þ0:78

¼ 192:599 kW

With a motor efficiency of 0.92, the total pump work comes out tobe,

_Wtot ¼ 82:435þ 240þ 57:03þ 594:3þ 192:5990:92

¼ 1267:8 kW

The SEC is given by,

SEC ¼_W ˙

in

3600�Xout

_Qp

ðA� 2Þ

For retrofit option # 2, it is found to be,

SEC ¼ 1267:8416

¼ 3:05 kWh=m3

The energy saving is then calculated as:

ESaving ¼ 3:78−3:053:78

� �� 100 ¼ 19:31%

The second-law efficiency is given as,

ηII ¼Wl;min

WinðA� 3Þ

98 M.A. Jamil et al. / Desalination 401 (2017) 88–98

For retrofit option # 2, it is found to be,

ηII ¼306:21267:4

� 100 ¼ 24:15%

Appendix B. Sample exergo-economics calculation

The sample calculations for exergo-economic analysis are given inthis appendix. In this regard, retrofit option # 2 is presented here ac-cording to the procedure given in Section 4.

The unitary exergy cost of streams is expressed as,

Cx# ¼WSubsystem#

X#ðB� 1Þ

For stream 1,

Cx1 ¼ 89:60364:31

¼ 1:393

For stream 3,

Cx3 ¼ 1267:8306:8

¼ 4:13

Now, the unit exergo-economic cost, in c$/MJ, is described as

Cp ¼ ∑fC f

X f

Xpþ Z

XpðB� 2Þ

Cp ¼ 3:7931� 10−5 � 100� 89:60364:31

þ 0:363664:31

þ 3:7931� 10−5 � 100� 968:51306:2

þ1:818306:2

þ 3:7931� 10−5 � 100� 209:34306:2

þ 1:3636306:2

¼ 35:92 c$=MJ

Now, the rate of product cost, in c$/s, is described as,

C p ¼ C i þ Celectricity X � �

þ Z ðB� 3Þ

where C i represents the cost of inlet stream to that subsystem. Thisgives,

C p ¼ 8:35 c$=s:

The fresh water cost, in c$/m3, is given by,

γRf#2;3 ¼ Cp

Qp

ðB� 4Þ

Therefore,

γRf#2;p ¼ γRf#2;3 ¼ 8:35� 3600416

¼ 72:25 c$=m3

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