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Desalination xxx (2011) xxx–xxx

DES-10965; No of Pages 10

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Desalination

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Reverse osmosis hybrid membrane inter-stage design: A comparativeperformance assessment

Baltasar Peñate a,⁎, Lourdes García-Rodríguez b,1

a Water Department. Instituto Tecnológico de Canarias (ITC), Playa de Pozo Izquierdo, s/n. 35119 - Santa Lucía, Las Palmas, Spainb Departamento de Ingeniería Energética. Universidad de Sevilla, ETSI, Camino de Los Descubrimientos, s/n. 41092-Sevilla, Spain

⁎ Corresponding author. Tel.: +34 928727511; fax: +E-mail addresses: [email protected] (B. Peña

(L. García-Rodríguez).1 Tel.: +34 954487231; fax: +34 954487233.

0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.08.010

Please cite this article as: B. Peñate, L. Garcíassessment, Desalination (2011), doi:10.10

a b s t r a c t
a r t i c l e i n f o
Article history:Received 13 April 2011Received in revised form 9 August 2011Accepted 10 August 2011Available online xxxx

Keywords:Hybrid membrane inter-stage designReverse osmosisHigh membrane fluxLower energy consumptionCapital cost reduction

A Hybrid membrane Inter-stage Design (HID) combines membranes of different nominal flux and saltrejection – i.e. different models –, in the same pressure vessel of seawater reverse osmosis racks. The HIDexperiences reported in the literature show there are no quantitative analysis published and very fewseawater reverse osmosis plants have used it to date. The main objective of this paper is to present the resultsof a thorough computer assessment of this innovative design for conventional seawater reverse osmosisinstallations. Two types of analysis have been conducted for all representative models of membranes. Firstly,membrane replacement with a view to reduce the energy consumption due to lower working pressures whilethe plant capacity remains unchanged. Secondly, to increase plant capacity by reducing the specific energyconsumption while maintaining the power consumption of the plant. A general HID design criterion has beendefined for the three main membrane manufacturers – i.e. Filmtec, Hydranautics and Toray. Moreover, thequantitative results are useful for retrofitting plants thanks to membrane replacement.

34 928727590.te), [email protected]

l rights reserved.

a-Rodríguez, Reverse osmosis hybrid membra16/j.desal.2011.08.010

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The so-called ‘latest generation’ membranes offer high saltrejection while requiring less energy (lower working pressure),yielding high productivity (high membrane flux). These advanceshave resulted in further cost and energy reductions [1]. However, asmembrane flux increases, so does the imbalance in flow rates betweenthe lead end and tail end elements in the Pressure Vessel (PV). Thehighest flux along a PV is at the first element due to minimum osmoticpressure. In this position, the risk of fouling is greatly increased.Critical flux defines the flux at which concentration polarization leadsto severe fouling [2]. The first membrane in the PV is thus the mostprone to biological and colloidal fouling, making it essential not toexceeded the critical flux. Although it is possible to control the flow ofthe lead elements by applying back pressure on the permeate side,this is counterproductive as it increases the pressure requirement onthe feed side. Alternatively, the lead/tail flow imbalance can bereduced by using Hybrid membrane Inter-stage Design (HID),consisting of using elements of different nominal flux – i.e. differentelements in the same PV. In the HID, the effects of decreasing netdriving pressure along the PV on permeate flux can be diminished byplacing a low flux element in the lead position, followed by high flux

elements in the rest of the PV. This design concept has been tested andpatented by Dow Water Solutions Company and is known asInternally Staged Design (ISD) [3,4]. Firstly, Dow used this approachby using SW30HR-8040 elements in combination with SW30-8040elements in desalination plants as of 1986. Recently, Dow has opted touse a combination of three elements selected for their membrane fluxcharacteristics. By using elements with nominal flow rates of7500/9000/12,000 gpd, Dow has shown that Sea Water ReverseOsmosis (SWRO) plants can operate at higher recovery rates withoutexceeding pressure limits or increasing the risk of fouling due toexcessive lead element flow rate [1]. Pilot installations have beentested successfully by Dowwith recoveries of 50–60% and flux rates of18–27 l/(m2·h) per PV [3].

The HID enables both greater efficiency and energy savings andsignificant capital and operating cost savings [4,5]. To date, very fewHID-related papers have been published. Dow has contributed withpapers dealing with this issue but they only consider their Filmtecmembranes. The hybrid 7-element combination 2 SW30HRLE 400i+5SW30ULE 400i is being tested in various pilot plants. A design for SWROplantswith racks up to 26% smaller,working at pressures from2 to 6 barlower, has been available since 2008 [4].

Other references in SWRO plants have been published. Thesedescribe real experiences under specific operating conditions. Theseinclude the following:

– El Coloso SWRO plant in Chile. The plant is located at El Coloso,15 km south of the city of Antofagasta, Chile. The capacity of theplant is 45,360 m3/d. In fact, the plant is the largest SWRO

ne inter-stage design: A comparative performance

http://dx.doi.org/10.1016/j.desal.2011.08.010

mailto:[email protected]

mailto:[email protected]


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Table 1Hybrid configuration in five RO trains of Las Palmas III SWRO plant (2009) [Emalsa, 2009](1).

Number of pressurevessels 1st stage

Number and type of element(1st stage)

Number of pressure vessels2nd stage

Number and type of element(2nd stage)

62 (4)SWC4+ (2)SWC5 58 (6) TM820H-40051 (4)SWC4+ (2)SWC5 58 (6) SW30HR-LE400i60 (4)SW30XHR-400i+(2)HRLE – 400i 60 (4)SW30XHR-400i+(2)HRLE – 400i70 (4)SWC4+(2)SWC5 40 (6) SWC570 (4)SWC4+ (2)SWC5 40 (6) SWC5

(1) Emalsa Technical Department – Personal communication, Mr J. J. Santana. September 2009.

2 B. Peñate, L. García-Rodríguez / Desalination xxx (2011) xxx–xxx

installation in South America and has been in operation since July2006. The rack is designed to operate at a high recovery rate of 50%.The single pass includes 4 racks of 137 PVs each. Each PV is fittedwith 7elements (8-inch Filmtec) with a special HID. The first twoelements in the PVs are “high rejection” type and the next fiveelements are “high flux”. This peculiar arrangement makes itpossible to maintain a correct flow pattern throughout the PV forthe whole temperature range (11 °C to 23 °C) and for the changingfeed pressure over the year due to the high recovery rate. As aconsequence, this design offers balanced flux distribution andbetter permeate quality, without significantly affecting the feedpressure [6,7].

– The Mazarrón SWRO plant is located in Murcia (Spain) and has acapacity of 35,000 m3/d. It was built in 1995 and commissioned in2001. This plant also has two high rejection membranes in front offive standard rejection membranes – i.e. high flux – in a single PV[7].

– Las Palmas III SWRO plant – Las Palmas de Gran Canaria (Spain): InFebruary 2002 the number of PVs in the second stage wasincreased to 68, thus improving the flux distribution andconsiderably reducing the fouling rate. In October 2003, themembranes of racks A and G were replaced by new SWC1elements. Finally, all the second stages of racks A and F werereplaced by new SWC3+ elements in 2005. The first stage of rack Fwas also reconfigured to a hybrid design with 2 SWC1 elementsfollowed by four SWC3+s. The SWC3+ element has a nominalflow of 7000 gpd (≈26.5 m3/d), nominal salt rejection of 99.8%and 400 ft2 (≈37 m2) of membrane area, whereas the SWC1membrane had a nominal flow of 5000 gpd (≈18.9 m3/d),nominal salt rejection of 99.5% and a area of 315 ft2 (28.4 m2),which means around a 10% increase in element permeability [8].Table 1 shows the 2009 hybrid configuration in five racks [9].

Table 2Selection of representative SWRO elements of leading manufacturers, Filmtec, Hydranautic

Manufacturers/criteria Type of membranes Permeate flow r(flux)gpd (m3/d

Filmtec (2009a) b

High salt rejection SW30HRLE-400ia 7500 (28.4)Low energy, high flux SW30ULE-400ia 11,000 (41.6)

Hydranautics (2009a) c

High salt rejection SWC4+Max 7200 (28.5)Extra high salt rejection,Low energy, high fluxd

SWC5+Max 9900 (37.6)

High salt rejection,extra low energy,extra high flux

SWC6 12,000 (45.6)

Toray (2009a) e

High flux TM820E-400 7500 (28.4)Low energy,high flux TM820F-400 9000 (34.1)

a These membranes come with the unique iLEC™ Interlocking End caps by FILMTEC, thatquality.

b Filmtec membrane datasheets (http://www.dow.com/liquidseps/prod/prd_film.htm), 2c Hydranautics membrane datasheets (http://www.membranes.com/index.php?pagenamd C. Bartels, M. Hirose, S. Rybar, R. Franks, Optimum RO system design with high area spe Toray membrane datasheets (http://www.toraywater.com/application/page.aspx), 27th

Please cite this article as: B. Peñate, L. García-Rodríguez, Reverse osmosassessment, Desalination (2011), doi:10.1016/j.desal.2011.08.010

To sum up, the experiences reported in the literature relating tothe HID show that there is no quantitative analysis published, exceptfor Filmtec membranes, and there are no general design criteria forthis kind of rack configuration. Bartels (2008) recommends futurepublications to assess the benefits of HID by using high permeabilityelements [5]. Within this framework, the main objective of this paperis the quantitative assessment of different HID proposed using ROdesign commercial simulation software, and to identify general designcriteria to maximise the performance and operational benefits of HIDby using membranes of the three main manufacturers – i.e. Filmtec,Hydranautics and Toray.

2. Analysis of Hybrid membrane Inter-stage Design (HID)

The HID analysis performed in this paper is based on theassessment of commercial membranes, i.e. the three leading compa-nies in RO membrane technology; Filmtec, Hydranautics and Toray;have been analysed. Given the similar behaviour of some of thecommercial membranes, they are sorted by their main feature intoseveral groups. Some types of membranes were selected from eachgroup (see Table 2) as a representative element. The analysis is basedon the beach-well seawater composition of the Canary Islands(Table 3) of 37,125.92 ppm and 20 °C, and on the representativeelements (Table 2) of the leading manufacturers.

For the three manufacturers, a 7-element standard PV (the samemembranes) is compared to different HID proposed in order toachieve an even distribution of permeate flow and to permit low feedpressure and high recovery rate. All designs adopted the followingcriteria to verify their suitability:

• high rejection elements in the first position (criterion A),• low energy elements in the middle positions (criterion B),

s and Toray.

ate)

Nominal activesurface area ft2

(m2)

Max. pressurebar

Stabilised saltrejection %(minimum)

400 (37.2) 82.7 99.75400 (37.2) 82.7 99.70 (99.55)

400 (37.2) 82.7 99.80 (99.70)440 (40.9) n.d. 99.85

400 (37.2) n.d. 99.80

400 (37.2) 82.7 99.75 (99.50)400 (37.2) 68.9 99.70 (99.50)

reduce system operating costs and reduce the risk of o-ring leaks that cause poor water

7th July, 2009.e=swc), 27th July, 2009.iral wound elements, Proceeding in 2008 EDS Euromed Conference, Jordan.July, 2009.

is hybrid membrane inter-stage design: A comparative performance

http://www.toraywater.com/application/page.aspx




Table 3Seawater composition (East Atlantic Ocean – Canary Islands).

East Atlantic seawater –beach well water (SDI b3)

Cations (mg/l) Anions (mg/l)

TDS 37,125.92 mg/l NH4+ 0.08 CO3

2− 5.53pH 7.50 K+ 438.00 HCO3

− 158.60Temperature 20 °C Na+ 11,080.00 NO3

− 6.10CO2 2.77 mg/l Mg2+ 1486.00 Cl− 20,926.32SiO2 34.00 mg/l Ca2+ 558.00 F− 1.71BT 5.50 mg/l Sr2+ 0.12 SO4

2− 2400.00

3B. Peñate, L. García-Rodríguez / Desalination xxx (2011) xxx–xxx

• elements with high productivity and ultra low energy (criterion C)in the final positions. The specific membranes SW30ULE-400i(Filmtec) and SWC6+ (Hydranautics) could be used in the finalpositions with excellent results [7] regarding feed flux reductionand pressure required, although the product quality is slightlyworse.

Two simulation procedures are analysed for all representativemodels of membranes. The first procedure maintains the permeateflow (3.65 m³/h) and the recovery rate (45%) constant. The secondone is carried out using the same feed flow (8.11 m³/h) and the samehigh pressure feed.

For each case, the RO process simulation software developed byeach membrane manufacturer is used. In general, the software couldbe used with success for the estimated analysis of the performance ofracks (flows and pressures required). In any event, the resultsobtained could have deviations in comparison with real operation.Several correlations and real operating data have been compared withthe software of the membrane companies [4,10,11]. The results havebeen satisfactory and the existing software is a widely accepted toolfor simulating the process.

3. Analysis of HID with Filmtec elements

3.1. Constant recovery rate and PV production

For the Filmtec membranes, simulations are gauged by usingFilmtec RO design software Rosa v.7.01 [12]. Five different hybriddesigns consisting of 7-elements operated with constant recovery

0

0,2

0,4

0,6

0,8

1

1,2

1 2 3 4 5

Per

mea

te f

lux

(m3 /h

)

S

S

in

in

m

m4

in

Fig. 1. Permeate flux distribution with different designs based on Filmtec membranes. Worproduct flux 14.02 l/m2·h.

Please cite this article as: B. Peñate, L. García-Rodríguez, Reverse osmosiassessment, Desalination (2011), doi:10.1016/j.desal.2011.08.010

(45%) and productivity (3.65 m³/h) and an average product flow of14.02 l/m2⋅h are compared with two standard PV designs. Thedesigns studied are the following:

• Standard designs (same elements): Design 1a, 7-element SW30HRLE-400i; Design 1b, 7 SW30ULE400i elements. Design 1b is not tech-nically reliable due to excessive permeate flux occurring in the firstposition, thus resulting in severe fouling. Nevertheless, it is analysed inorder to compare the results obtained.

• HID: Design 2, 2 SW30HRLE 400i elements+5 SW30ULE 400ielements; Design 3, 5 SW30HRLE 400i elements+2 SW30ULE 400ielements; Design 4, 3 SW30HRLE 400i+4 SW30ULE 400i; Design 5,2 SW30HRLE 400i+2 SW30XLE 400i+3 SW30ULE 400i; Design 6,1 SW30HRLE 400i+1 SW30XLE 400i+5 SW30ULE.

Fig. 1 shows the results of permeate flux distribution along the PV.Except for design 3, all designs exhibit a flux below 0.80 m3/h in thefirst element as well as more balanced permeated flux than thestandard design. Moreover, design 3 requires much higher pressurethan the rest (Fig. 2).

In the case of the standard design and for increasing the flow rateof the elements used, there is greater imbalance in the flow rate –withthe highest flow rate at the first element and the lowest flow rate atthe last membrane. The element recovery rate increases for theelements with high flow rates. The combined effect of high flux andrecovery leads to a significantly increased concentration of polarisa-tion, i.e. tendency to foul.

Membrane combinations with Low Energy membranes (LE) in thefirst position cause an increase in productivity imbalance. The veryhigh flux and recovery rate of the first elements make the rack proneto fouling problems.

In conclusion, Fig. 1 permits us to conclude that the LE membranesare not suitable for the lead position of the PV due to the high flux ofthe first membrane. The standard designs present the highestpermeate flux in the first element (0.88 (1a) and 1.04 m3/h (1b)) incomparison with the lower flux values of HIDs (from 0.84 to0.77 m3/h). An interesting response is identified in the 3rd positionfor designs 2 and 6. The flux in this position increases more than 20%in comparison with the standard designs. Additionally, the fluxproduced by the HIDs along the PV is more even.

6 7Element position

W30HRLE-400i

W30ULE-400i

terstage vessel (2 elements SW30HRLE 400i + 5 SW30ULE 400i)


ulti interstage vessel (2 elements SW30HRLE 400i + 2 SW30XLE400i + 3 SW30ULE 400i)

ulti interstage vessel (1 elements SW30HRLE 400i + 1 SW30XLE 00i + 5 SW30ULE 400i)


king conditions: constant recovery (45%) and productivity (3.65 m³/h) and an average

s hybrid membrane inter-stage design: A comparative performance


50 51 52 53 54 55 56 57 58 59

1a

1b

2

3

4

5

6

Des

ign

s

Feed pressure (bar)

Fig. 2. Feed pressure in different pressure vessel designs (FILMTEC) at constant recovery(45%) and productivity (3.65 m³/h) and an average product flux 14.02 l/(m2·h).

0 50 100 150 200 250 300 350

1a

1b

2

3

4

5

6

Des

ign

s

Product water quality (mg/l)

Fig. 3. Product water salinity in different pressure vessel designs (FILMTEC) at constantrecovery (45%) and productivity (3.65 m³/h) and an average product flux 14.02 l/(m2·h).


Furthermore, excluding design 1b, which is not reliable, designs 2and 6 exhibit the best flux balance and lowest feed pressure required.This may confirm the suitability of the aforementioned design criteriaA+B+C for HID.Moreover, design 6 (1 SW30HRLE 400i+1 SW30XLE

0

0,2

0,4

0,6

0,8

1

1 2 3 4 5

Per

mea

te f

lux

(m3 /

h)

Fig. 4. Flux distribution along the pressure vessel in different designs with Filmtec memb


400i +5 SW30ULE 400i) offers further key advantages in relation tofouling trends:

• the lowest flux, 0.77 m3/h, at the first element,• gradual flux distribution over the rest of the elements.

Moreover, the permeate quality has to be taken into account. Fig. 3shows the freshwater salinity obtained in the combinations simulated. Arelatively lower product water quality is obtained with the HIDsanalyseddue to the high rejectionelements only beingplaced in thefirstpositions. In spite of exhibiting the best hybrid design, designs 2 and 6produce the poorest water quality. Nevertheless, the product waterquality still complies with the international regulation limitations(b500 μS/cm) and the boron concentration also meets the Europeanstandards since the ‘latest generation’ membranes, used in the lastpositions of the PVs, reject 93% of the boron.

Finally, Figs. 1–3 quantify the advantages of a suitable HID, such asdesign 2 or 6, in comparison to a conventional design (1a). They alsoshow how the LE and Ultra-LE membranes require less pressure toproduce the same amount of water at the expense of maintaining highflux over the first elements of the PV and thus causing a reduction inthe quality of the water produced.

3.2. Constant feed flow and feed pressure

The second HID analysis using Filmtec elements is presentedbelow. The seawater composition (Tab.3), feed flow (8.11 m3/h) andfeed pressure (57.84 bar) are constant in all the simulations per-formed, thus the variables are permeate production and recoveryrates. The criteria for selecting the best design are: the lowest workingpressure, the best distribution of the permeate flux over the PV andthe lowest permeate flux in the first membrane.

According to the analysis presented in Section 3.1, the best designsare selected for this analysis, the best two hybrid inter-stage com-binations (design 2, 2 SW30HRLE 400i+5 SW30ULE 400i, and design6, 1 SW30HRLE 400i+1 SW30XLE 400i+5 SW30ULE 400i) and thebest standard design (design 1a, 7 SW30HRLE 400i elements) forassessing the improvements of HID.

Figs. 4–6 show the comparative results. Fig. 4 outlines the behaviourof the permeate flux distribution over the 7 elements of the PV. Incomparison with the standard design, in the HIDs analysed, theproduction is significantly increased from the 3rd to the 5th elementswhile maintaining the same acceptable productivity and recovery rates

6 7 Element position

SW30HRLE-400i

interstage vessel (2 elements SW30HRLE 400i + 5 SW30ULE 400i)

multi interstage vessel (1 elements SW30HRLE 400i + 1 SW30XLE 400i + 5 SW30ULE 400i)

ranes. Working conditions: constant feed flow (8.11 m3/h) and pressure (57.84 bar).



3,5 3,6 3,7 3,8 3,9 4 4,1

1a

2

6

Des

ign

s

Product flow (m3/h)

Fig. 5. Product flow with different pressure vessel designs (FILMTEC) at constant feedflow (8.11 m3/h) and pressure (57.84 bar).

COMBINATIONS

40,00% 41,00% 42,00% 43,00% 44,00% 45,00% 46,00% 47,00% 48,00% 49,00% 50,00%

1a

2

6

Des

ign

s

Recovery rate (%)

Fig. 6. Recovery rate with different pressure vessel designs (FILMTEC) at constant feedflow (8.11 m3/h) and pressure (57.84 bar).

0

0,2

0,4

0,6

0,8

1

1 2 3 4 5

Per

mea

te f

lux

(m3 /

h)

Fig. 7. Permeate flux distribution with different designs based on Hydranautics membranobtaining an average product flux 12.8 l/(m2·h).



at the first elements. However, the three hybrid stage combinationsobtain greater production at the 2ndposition. Furthermore, Figs. 5 and 6show the total product flow and the recovery rate for the selecteddesigns in comparison with the best standard PV. The use of high-productivity, low-pressure membranes increases the total recovery of arack up to 8% over the standard rack.Moreover, it also allows for greaterproduction per element.

To sum up Sections 3.1–3.2, there are two HIDs (2 SW30HRLE 400ielements+5 SW30ULE 400i and 1 SW30HRLE 400i element+1SW30XLE 400i+5 SW30ULE 400i), which guarantee greater andmore balanced flux inside the PV together with higher production atlower energy costs. Both designs are able towork at lower pressure forthe same flux. Therefore, there is increased flux over the rack bydecreasing the contribution of the first element and increasing theglobal recovery rate to 8% with a feed pressure from 3 to 5 bar lower.These improvements have a direct impact in the form of reducingcapital and operating costs and allowing for a simpler pre-treatmentto decrease the fouling effect. This is simpler and less expensive whileincreasing the useful life of the membranes.

In this study, design 6 (1 SW30HRLE 400i+1 SW30XLE 400i+5SW30ULE 400i) offers slight improvements over design 2 and shouldnot be ruled out in the near future due to a possible increase in thepermeate flux of the Ultra-LE elements, over 41.6 m3/d (11,000 gpd).

4. Analysis of HID with Hydranautics elements


In the case of simulations with Hydranautics membranes, twodifferent 7-element HIDs are compared with one standard PV designwhenoperatingat constant recovery (45%) andproductivity (3.65 m³/h),thus resulting in an average product flux of 12.8 l/(m2⋅h). All thesesimulations are carried out by using IMSDesign RO software designv.2008 [13]. The designs analysed are as follows:

• Standard designs (one membrane model): Design 1c, 7 SWC4maxelements.

• HID: Design 7, 2 SWC4+max elements+5 SWC6 elements; Design8, 1 SWC4+max+1 SWC5max+SWC6.

The results obtained – see Fig. 7 – show permeate flux at the firstmembrane in hybrid configurations of 0.75 m³/h, which is much


SWC4max

interstage vessel (2 elements SWC4+max + 5 SWC6)

multi interstage vessel (1 element SWC4+max + 1 SWC5max + 5SWC6)

es. Working conditions: constant recovery (45%) and productivity (3.65 m³/h), thus



54 55 56 57 58 59 60 61 62

1c

7

8

Des

ign

s

Feed pressure (bar)

Fig. 8. Feed pressure in different pressure vessel designs (HYDRANAUTICS) at constantrecovery (45%) and productivity (3.65 m³/h) and an average product flux 12.8 l/(m2·h).

0 50 100 150 200 250 300 350 400 450 500

1c

7

8

Des

ign

s


Fig. 9. Product water salinity in different pressure vessel designs (HYDRANAUTICS) atconstant recovery (45%) and productivity (3.65 m³/h) and an average product flux12.8 l/(m2·h).


lower than the flux obtained in the standard PV (0.89 m³/h), plus amore balanced flux along the PV. Moreover, the two HIDs requirelower operating pressure – see Fig. 8, although, design 8 exhibits

0

0,2

0,4

0,6

0,8

1

1 2 3 4 5

Per

mea

te f

lux

(m3 /

h)

Fig. 10. Flux distribution in a pressure vessel with several designs based on Hydran


slightly higher membrane flux (19.6 l/(m2⋅h)) than the nominalvalue recommended by the manufacturer.

Conclusions from the comparative analysis of conventional designand HID are similar for Hydranautic and Filmtec membranes, fromFig. 7:

• lower flow in the lead position is obtained with HID,• permeate flux is distributed more evenly in HID,• the maximum flux is lower in HID.

Moreover, HID offer further key advantages:

• THE highest flux is obtained over the 2nd (design 8, 0.81 m³/h) or3rd element (design 7, 0.78 m³/h).

• THE flux is distributed more gradually in the rest of the elements.• THE above advantages reduce the fouling tendency considerably.• No element exhibits a permeate flux higher than the maximum fluxdefined by the manufacturer (1.1 m³/h).

Figs. 8 and 9 show the comparative results for feed pressure andproduct water quality. The lowest feed pressure appears in design8 (55.7 bar). This value is very similar for design 7,which is considerablylower (6 bar lower) than the figure for the standard design. Finally, thepermeate salinity is shown in Fig. 9. The results obtained are similar tothe results shown in Fig. 6 (Filmtec membranes). Poorer product waterquality is obtained with HID than with the standard design due to thefact that the high rejection elements are placed only in the firstpositions. A standard PV obtains water with 219 mg/l but the poorestpermeate water quality is obtained in design 8(358 mg/l). This productwater quality does however remain within the limits set by interna-tional regulations.


As in the case of the Filmtec elements, a second HID analysis wasconducted, this time using Hydranautics elements. Seawater compo-sition (Tab.3), feed flow (8.11 m3/h) and feed pressure (61.2 bar) areconstant in the simulation performed, thus resulting in variablepermeate production and recovery rate. The standard PV (design 1c)was compared with HIDs 7 and 8. The main objective of the analysiswas to ascertain how the PV performed and to identify the highest-productivity design.

Fig. 10 gives the permeate flux for each position for the threedesigns analysed. Design 1c has the lowest water productivity and


SWC4+max

interstage vessel (2 elements SWC4+max + 5 SWC6)

multi interstage vessel (1 elements SWC4+max + 1 SWC5max + 5 SWC6)

autics membranes at constant feed flow (8.11 m3/h) and pressure (61.2 bar).



3,5 3,6 3,7 3,8 3,9 4 4,1 4,2

1c

9

10

Des

ign

s

Product flow (m3/h)

Fig. 11. Product flow with different pressure vessel designs (HYDRANAUTICS) atconstant feed flow (8.11 m3/h) and pressure (61.2 bar).

COMBINATIONS

40,00% 42,00% 44,00% 46,00% 48,00% 50,00% 52,00%

1c

9

10

Des

ign

s

Recovery rate (%)

Fig. 12. Recovery rate with different pressure vessel designs (HYDRANAUTICS) atconstant feed flow (8.11 m3/h) and pressure (61.2 bar).


flux differential of all these cases. In these inter-stage cases, muchmore water is produced from 2nd to 5th elements than in thestandard design with the same acceptable productivity and recovery

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1 2 3 4 5

Per

mea

te f

lux

(m3 /

h)

Fig. 13. Permeate flux distribution in different pressure vessels with Toray membranes. Workan average product flux 14.03 l/m2·h.


rates at the first element (up to 1.1 m³/h recommended by themanufacturer [13]. Figs. 11 and 12 show the total product flow and therecovery rate of the selected designs in comparison with the beststandard PV. In this case, the HID makes it possible to increase therecovery of the PV up to 9% compared with a non-hybrid design. Thisguarantees greater water production per element used.

In summary, attention should be drawn to the fact that simulatedhybrid configurations (2 SWC4+max elements+5 SWC6; 1 SWC4maxelement+1 SWC5max+5 SWC6) guarantee greater and more bal-anced flux over the PV in comparison to conventional designs. More-over, greater production can be achieved at lower energy costs or lowerworking pressure is required to obtain the same flux. The PV flux in HIDis increased and the contribution of the first element is decreased, thusincreasing the global recovery rate up to 9% and reducing the feedpressures (5–6 bar). This represents an improvement over the HIDsbased on Filmtec elements. In general, these improvements benefit theinstallations since they bring down the capital and operating costswhileincreasing the useful life of the membranes.

As a general conclusion for Sections 4.1 and 4.2, excellent results areobtained in HID by combining high-productivity elements, 45.6 m3/d(12,000 gpd) and low-permeability elements, 28.5 m3/d (7.200 gpd) incomparison to the standard design 1c.

5. Analysis of HID with Toray elements


Firstly, the analysis is carried out with constant recovery andproductivity. All the PV designs consist of 7 elements and are simulatedwith constant recovery rate (45%) and productivity (3.65 m³/h), thusresulting in an average flux inside the PV of 14.03 l/(m2·h). All thesesimulationswere calculatedwith TorayDS software design v.1.1.48 [14].The TM820E-400 element was selected for the 7-element standard PV(design 1d) and the high permeability TM820F-400 element for thehybrid design (design 11, 2 TM820E-400+5 TM820F-400).

As in thepreviousdesigns basedonothermembrane brands, theHIDwas seen to offer better balance of permeate flux inside the PV incomparison with the conventional design (Fig. 13). In this case, the HIDdid not offer a significant advantage over the conventional PV butgreater productivity is obtained over the intermediate elements of thePV. On the contrary, designers should bear in mind that the flux of thetwo first elements of the HID is higher than the average flux value

6 7Element position

TM820E-400

interstage vessel (2 elements TM820E-400 + 5 TM820F-400)

ing conditions: constant recovery (45%) and productivity (3.65 m³/h), thus resulting in



58575655

1.d

11

Des

ign

s

Feed pressure (bar)

Fig. 14. Feed pressure in different pressure vessel designs (TORAY) at constant recovery(45%) and productivity (3.65 m³/h) and an average product flux 14.03 l/m2·h.

0 50 100 150 200 250 300 350 400

1.d

11

Des

ing

s


Fig. 15. Product water salinity in different pressure vessel designs (TORAY) at constantrecovery (45%) and productivity (3.65 m³/h) and an average product flux 14.03 l/m2⋅h.


recommended by themanufacturer for the rack (b20 l/m2·h), althoughit is not over the limit value for the element (35 l/m2·h) [14].

Regarding the high feed pressure (Fig. 14), a reduction of 3.6% isachieved in the feed pressure required (from 57.7 bar for the standarddesign to 56.6 bar in the hybrid one). Finally, the permeate salinity is

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1 2 3 4 5

Per

mea

te f

lux

(m3 /

h)

Fig. 16. Flux distribution with different pressure vessels – Toray membranes.W


shown in Fig. 15. As in all the previous analyses, the water quality ispoorer using HID thanwith the conventional design. This is attributableto the fact that high rejection elements have only been placed at thefirstpositions.


A second analysis has also been conducted, using an HID with Torayelements. The seawater composition is given in Tab.3, feed flow(8.11 m3/h) and feed pressure (57.7 bar) are constant, thus resultingin variable permeate production and recovery rate. According toSection 5.1, the standard PV (design 1 d) is compared with the besthybrid combination (design 12). Fig. 16 gives the permeate flux alongthe PV, design 1 d has the lowest water productivity and fluxdifferential. In the case of HID, much more water is produced betweenthe 3rd and 5th elements. As in simulation 11, the first elements of thiscombination produce more average flux than the rack recommenda-tions offered by the manufacturer (b20 l/m2⋅h) [14]. Finally, the use ofhigh-productivity/low-pressure membranes together with high rejec-tionmembranesmake it possible to attain a 6% increase in the total rackrecovery with a decline in water quality up to 30% compared with non-hybrid PV (Figs. 17 and 18).

5.3. Final selection of the best HIDs

In previous sections, the product water quality proved to besuitable as it met the standards for human consumption (after post-treatment) and designs with the lowest permeate flux in the firstelements have been selected for every membrane model. The mostimportant parameters for selecting the right HID are as follows:

• Recovery rate. This parameter is significant because of the consump-tion of chemical products and for auxiliary energy consumption. Highproductivity elements in the middle and rear positions improve therecovery rate of the PV. This selection does not create a risk of foulingas they operate far below the average recovery rate of the PV.

• Specific energy consumption (kWh per m3 of permeate). Thisparameter not only affects the operating costs, it also affects capitalcosts since it defines the power required by the high pressure pump.

• Specific capital cost (capital cost per m3/d of nominal production).Capital cost of PVs, namely membranes and PVs, is one of the mainitems of the capital cost of a RO plant.


TM820E-400

interstage vessel (2 elements TM820E-400 + 5 TM820F-400)

orking conditions: constant feed flow (8.11 m3/h) and pressure (57.7 bar).



43,93,83,73,63,5

1.d

12

Des

ign

s

Product flow (m3/h)

Fig. 17. Product flow in different pressure vessel designs (TORAY) at constant feed flow(8.11 m3/h) and pressure (57.7 bar).

40% 41% 42% 43% 44% 45% 46% 47% 48% 49% 50%

1.d

12

Des

ign

s

Recovery rate (%)

Fig. 18. Recovery rate in different pressure vessel designs (TORAY) at constant feed flow(8.11 m3/h) and pressure (57.7 bar).


Table 4 shows results obtained for the aforementioned parametersfrom the analysis performed to obtain a preliminary comparisonbetween a seawater 7-element standard PV with a similar PV using anHID at constant feed flow and feed pressure without energy recovery.A comparison is made of the specific energy consumption (kWh/m3),the total recovery rate obtained in the PV (%), the specific capital costper PV (€/m3) and the capital cost savings (%) of both PVs.

Table 4Standard pressure vessel and hybrid inter-stage pressure vessel comparison using Filmtec,

Manufacturer/design of the PV Specific ene(kWh/m3)a

FILMTECb

Design 1.a: Standard vessel 7-element SW30HRLE 400i 4.49Design 2: 2 SW30HRLE 400i+5 SW30ULE 400i 4.15Design 6: 1 SW30HRLE 400i+1 SW30XLE 400i+5 SW30XLE 400i 4.11

HYDRANAUTICSc

Design 1.c: Standard vessel 7-element SWC4max 4.75Design 7: 2 SWC4+max+5 SWC6 4.33Design 8: 1 SWC4+max+1 SWC5max+5 SWC6 4.32

TORAYd

Design 1.d: Standard vessel 7-element TM820E-400 4.48Design 11: 2 TM820E-400+5 TM820F-400 4.19

a Seawater of 37,125.92 mg/l, pH: 7.50 and temperature of 20 °C, 80% efficient high pressupressure were neglected.

b Feed flow (8.11 m3/h) and feed pressure (57.84 bar) are constants.c Feed flow (8.11 m3/h) and feed pressure (61.2 bar) are constants.d Feed flow (8.11 m3/h) and feed pressure (57.7 bar) are constants.


This comparison confirms the advantages of the HID in compar-ison with standard design and offers a final decision on the mostsuitable HID (brands and membrane models) in the event of retro-fitting a desalination plant with a view to increasing the total pro-duction capacity.

The sales departments of the three manufacturers in Spainprovided the Instituto Tecnológico de Canarias (ITC) with usefulinformation about the cost of membranes used in the analysisperformed. In general, all brands have similar membranes costs.Even the new generation membrane is sold at approximately thesame price as the older elements, which have poorer productionperformance. So, there is no significant cost increase if an HID PV isinstalled instead of a standard PV rack.

According to the table, there is a significant reduction in thepermeate costs in the different HIDs in comparison with standarddesigns. In general, the capital costs could be reduced because theHIDs require less PVs and membranes. From the energy perspective,energy consumption of HIDs is lower than the standard PV design butsimilar for different HIDs/brands analysed.

Filmtec HIDs present the lowest specific energy consumption andcapital cost savings. Hydranautics HIDs present the highest recoveryrate per PV and the lowest specific capital cost per PV. Finally, thebehaviour of Toray HIDs is similar to Filmtec ones.

Obviously, the maintenance tasks in the SWRO desalination plantswith HID will be stricter due to the fact that they operate with two orthree types of elements. On the contrary, this design offers significantcapital and operating cost savings [Bartels et al., 2008] [García-Molinaet al., 2008] and a considerably increased lifetime of the elements.

6. Conclusions

This paper presents a comprehensive computer analysis of theHybrid Inter-stage Design (HID) of pressure vessels (PVs) in conven-tional seawater reverse osmosis (SWRO) plants. This consists of acombination of different RO element types in the same PV. The resultsobtained in this paper quantify the estimated advantages of suchdesigns for operation, maintenance and capital cost savings. Thesequantitative results are useful for designing new plants or retrofittingplants thanks to membrane replacement. Moreover, general designcriteria have been set for the threemainmembranemanufacturers. Themain conclusions can be summarised as follows.

1. The results obtained validate the suitability of the criteria sequenceA–B–C for the three membrane brands:

• Criterion A: high rejection elements in the first positions.• Criterion B: high productivity – low energy elements in the mid-final positions.

Hydranautics and Toray elements.

rgy consumption Recoveryrate (%)

Specific capital costper PV ($/m3)

Capital costsavings (%)

45.0% 1102.73 n/a48.7% 1018.99 7.6%49.1% 1011,31 8.3%

45.0% 997.26 n/a49.3% 947.50 5.3%49.4% 947.40 5.4%

45.0% 1179.45 n/a48.1% 1103.85 6.4%

re pump, energy consumption of one 7-elements PV without energy recovery, low feed




• Criterion C: high productivity and ultra-low energy elements inthe final positions.

2. A general simple design criterion for the three membrane brandscould be: to apply criterion A at the lead positions and criterion Cfor the last 5 positions. This could be useful in the case of very slightdifferences between criterion A or B at the 2nd position.

3. The following specific HIDs are recommended for 7-element PVs• Filmtec: A–B :(2) SW30HRLE 400i – (5) SW30ULE 400iA–B–C: (1) SW30HRLE 400i – (1) SW30XLE 400i – (5) SW30ULE400i.

• Hydranautics: A–B: (2) SWC4+max – (5) SWC6A–B–C: (1) SWC4+max – (1) SWC5max – (5) SWC6

• Toray: A–B: (2) TM820E-400 – (5) TM820F-400

In the case of having to maintain the feed flow and the pressureconstant, the recovery rate reached can be from 6 to 8% higher. It allowsfor capital cost savings from 5 to 8% since the installation will requirefewer PVs and membranes. If the recovery and permeate flux are heldconstant, the best preliminary design obtained allows for a reduction inthe energy consumption of the installation due to working at lowerpressures – i.e. reduction in feed pressure from 3 to 6 bar.

There is a significant reduction in the permeate costs in thedifferent HIDs in comparison with standard designs. Concerningenergy, the specific energy consumption of HIDs is lower than thestandard PV design but similar for the different HIDs/brands analysed.

Acknowledgements

The authors wish to thank the European Commission for theirfinancial assistance with the POWERSOL project [15] – Mechanicalpower generation based on solar heat engines – FP6-INCO2004-MPC3-032344) within the International Cooperation Activities Programme.


They would also like to give special thanks to V. Subiela and J. A. de laFuente for their help in developing the POWERSOL 640 and 660 tasks.

References

[1] W.E. Michols, A novel design approach for seawater plants, Presented at IDAWorld Congress on Desalination and Water Reuse, SP05-052 Singapore, 2005.

[2] H. Winters, Twenty years experience in seawater reverse osmosis and howchemicals in pretreatment affect fouling of membranes, Desalination 110 (1997)93–96.

[3] M. Busch, V.G. Molina, Y. Maeda, W.E. Mickols, J. Tonner, Higher, more balancedflux reduces costs, Desalin. Water Reuse 18 (2009) 29–32.

[4] V. García-Molina, M. Busch, P. Sehn, Cost savings by novel seawater reverseosmosis elements and design concepts, Desalin. Water Treat. 7 (2009) 160–177.

[5] C. Bartels, M. Hirose, S. Rybar, R. Franks, Optimum RO system design with higharea spiral wound elements, presented at EuroMed 2008, Desalination for CleanWater and Energy Cooperation among Mediterranean Countries of Europe andthe MENA Region, November 9–13 2008, (Jordan).

[6] M. Pétry, V. Bonnelye, F. Beltrán, E. Trauman, El Coloso: An Innovative Design forthe Largest Copper Mine in the World, presented at IDA World Congress,Maspalomas, Spain, 2007.

[7] F. Muddassir, Unique Design Features of Commercial SWRO Plants, IDA WorldCongress, Maspalomas, Spain, 2007.

[8] S. Talo, J. Suárez, C. Bartels, M. González, J. Curbelo, A. Abanades, M.Wilf, Evolutionof Configuration and Operation Regime at the Las Palmas III, presented at IDAWorld Congress, Maspalomas, Spain, 2007.

[9] Emalsa Company Technical Department – Personal communication, Mr J. J.Rodríguez. September 2009.

[10] M. Alahmad, Prediction of performance of sea water reverse osmosis units,Desalination 261 (2010) 131–137.

[11] H. Oh, T. Hwang, S. Lee, A simplified simulation model of RO systems for seawaterdesalination, Desalination 238 (2009) 128–139.

[12] Filmtec RO design software Rosa v.7.01 (Dow Water Solutions Company). July –

August 2009.[13] Hydranautics RO design software IMSDesign v.2008 (Nitto Denko Corporation).

July – August 2009.[14] D.S. Toray, Toray RO software design v.1.1.48, Toray Corporation, July 2009.[15] L. García-Rodríguez, J. Blanco, Solar-heated Rankine cycles for water and

electricity production: POWERSOL project, Desalination 212 (2007) 311–318.



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