multi effect desalination and adsorption desalination (medad): a hybrid desalination method

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Applied Thermal Engineering xxx (2014) 1e9

Contents lists avai

Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Multi effect desalination and adsorption desalination (MEDAD):A hybrid desalination method

Muhammad Wakil Shahzad a, Kim Choon Ng a,b,*, Kyaw Thu a,b, Bidyut Baran Saha c,Won Gee Chun d

aDepartment of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, SingaporebWater Desalination and Reuse Centre, King Abdullah University of Science & Technology, Thuwal 23955-6900, Saudi Arabiac Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga-shi, Fukuoka 816-8580, JapandDepartment of Nuclear and Energy Engineering, Cheju National University, 66 Jejudaehakno, Jejusi, South Korea

a r t i c l e i n f o

Article history:Received 1 December 2013Received in revised form20 March 2014Accepted 23 March 2014Available online xxx

Keywords:DesalinationMEDHybrid desalinationImproved desalination method

* Corresponding author. Department of MechaUniversity of Singapore, 9 Engineering Drive 1, STel.: þ65 65162214; fax: þ65 65161459.

E-mail addresses: [email protected], wakil.dr1

http://dx.doi.org/10.1016/j.applthermaleng.2014.03.061359-4311/� 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: M.W. Shamethod, Applied Thermal Engineering (2014

a b s t r a c t

This paper presents an advanced desalination cycle that hybridizes a conventional multi-effect distilla-tion (MED) and an emerging yet low-energy adsorption cycle (AD). The hybridization of these cycles,known as MED þ AD or MEDAD in short, extends the limited temperature range of the MED, typicallyfrom 65 �C at top-brine temperature (TBT) to a low-brine temperature (LBT) of 40 �C to a lower LBT of5 �C, whilst the TBT remains the same. The integration of cycles is achieved by having vapor uptake bythe adsorbent in AD cycle, extracting from the vapor emanating from last effect of MED. By increasing therange of temperature difference (DT) of a MEDAD, its design can accommodate additional condensation-evaporation stages that capitalize further the energy transfer potential of expanding steam. Numericalmodel for the proposed MEDAD cycle is presented and compared with the water production rates ofconventional and hybridized MEDs. The improved MEDAD design permits the latter stages of MED tooperate below the ambient temperature, scavenging heat from the ambient air. The increase recovery ofwater from the seawater feed may lead to higher solution concentration within the latter stages, but thelower saturation temperatures of these stages mitigate the scaling and fouling effects.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Fresh water is a resource which is essential for all socio-economical activities, namely the agriculture, industrial and do-mestic sectors, of the World’s countries. With an estimated of 2%annual increment, water demand of the world is expected to rise to6300 billion cubic meters (Bm3) in 2030, exceeding the supply ofwater from the natural water cycle by as much as 40% [1e3]. Thefresh water shortage between the demand and supply can bepartially addressed by measures such as water conservation, waterre-use and efficient utilization. However, all these measures canpartially reduce the water scarcity gap. Significant water shortagebetween the demand and supply has been projected and manyeconomies addressed the water deficit by engineered desalinationprocesses but they are energy intensive, as shown in Fig. 1 [4]. The

nical Engineering, Nationalingapore 117576, Singapore.

@gmail.com (K.C. Ng).

4

hzad, et al., Multi effect desal), http://dx.doi.org/10.1016/j

quest for high gross domestic product (GDP) and coupled withexponential increase in population [5] of many countries havecontributed even to higher demand for water, as shown in Fig. 2 [6e8]. In theworld, there are more than 7500 desalination plants are inoperation. Out of these plants, about 57% are installed in theMiddleEast of which Saudi Arabia has a share of 24% [9]. Although thethermodynamic limit for desalination is less than 1 kWh_electric/m3,practical desalination plants tend to have specific electricity con-sumption operating at many folds higher due to irreversibilitiesincurred in the processes, ranging from 3.5 to 10 kWh/m3. Thus,this motivates engineers and scientists to scout for more efficientdesalination processes and new material design for membranes,etc.

Presently, two major desalination methods are used world-wide, namely; (i) the thermal desalination methods such as themulti-effect distillation (MED), the multi-stage flashing (MSF), etc.,and (ii) the membrane-based processes that overcome the osmoticpressures of saline solution such as the reverse osmosis (RO), for-ward osmosis (FO), etc. [10e14]. Membrane-based processes em-ploys electricity for themechanical pumps to deliver saline solution

ination and adsorption desalination (MEDAD): A hybrid desalination.applthermaleng.2014.03.064

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Fig. 1. Water supply-demand gap with business as usual and with water productivity improvement.

M.W. Shahzad et al. / Applied Thermal Engineering xxx (2014) 1e92

at high water pressures up to 65e80 bars, andwith recent advancesin membrane technologies and energy recovery devices in ROprocesses, it has gained popularity in desalination applications,particularly the plants can be sited away from the coast. Today, theshare of RO plants in desalination capacity in the world is 59%whilst the thermally-driven desalination methods have only 27%[15,16]. However, in countries where the seawater feed is subjectedchanging feed quality, arising from the water salinity, silt, and theharmful algae blooms (HABs), the dominant method employed inthese sites is usually the thermally-drivenmethods such as theMSFand the MED. The seawater feed in the Gulf has salinity in excess of45,000 ppm whist the rest of the world are less than 30,000 ppm.Also, the Gulf is fed with rivers from countries where waste watertreatment is lacking and the amount of nutrients fed into the Gulfwater promotes HABs. It has been reported that during a HAB, ROplants in Oman and UAE were shut down for 6e8 weeks, a periodon no water supply in GCC economies is unthinkable as theirpotable water storage in some countries are less than 5 days. Forthese reasons, the desalination plants sited in the Gulf co-operationcountries (GCC) are usually theMSF andMED processes where theirproduction capacity shares are 80% whilst the RO plants are foundfar away from the coast or inland and they are employed for thetreatment of ground or re-use water [17,18].

As the thermal processes are the preferred options for desali-nation in GCC and MENA regions, there is a need to improve thespecific energy consumption of these processes so as to reduce the

Fig. 2. Fresh water consumption in different parts of the world and total global waterconsumption.

Please cite this article in press as: M.W. Shahzad, et al., Multi effect desalmethod, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j

consumption of oil and to lower pollution both in the air (exhaustgases) and sea (concentrated brine discharge). Presently, the MEDand MSF are consuming about 20e40 kWh_thermal/m3 due to thelimitation in the available temperature difference between the top-brine temperature (TBT) and the low-brine temperature (LBT): theTBT is constrained by the fear of hard scale formation on the tubessurfaces whilst the LBT is limited by the ambient or sea tempera-tures. With design and operation experiences of over three de-cades, improvements in materials and process design for the MEDand MSF have reached also their asymptotic limits and thus, thenew direction for thermally-driven research must lie in the ther-modynamic synergy between novel cycles and the existing provencycles, for example, the hybridization of the emerging adsorptiondesalination and the MED or MSF, etc. With the integrated cycle,there is good match for temperature cascading where the LBT canbe lowered further below the limit imposed by the ambient, dueprimarily to the continued vapor uptake from the re-generatedadsorbent of the AD cycle. We propose a hybrid desalination cy-cle called MED þ AD integrated cycle or MEDAD in short where itcan either designed as a new process with additional MED stages orretrofitted to the existing MED plant with same number of stagesbut a higher temperature difference per stage can be realized. Thecombined cycles can boost the water production by almost 3e4folds at the same TBT. In this paper, we present the numericalsimulation and the modeling of key processes.

2. MEDAD process description

Parallel feed MED system integrated with AD is considered tosimulate the process. The schematic diagram is shown in Fig. 3. TheAD cycle operation detail can be found in the literature [19e27]. ForMED system, horizontal tubes falling film evaporators are used inproposed simulation model. The feed is first de-aerated to removedissolve oxygen (using de-aeration tank with vacuum pump) andthen supplied parallel to all evaporators. Feed is passed through thebuild-in pre-heaters in evaporators prior to spray via spray nozzleson to tubes bundle. The first stage also called steam generator usedto produce initial steam. The hot water is circulated through thetubes and pressure is maintained to evaporate the seawater atrequired temperature corresponding to hot water temperature. Theproduced vapors are then directed to the tube side of next effect.The heat of condensation is used to evaporate the feed sprayed inthat stage. Similarly, vapors are cascaded in next stages up to last


Fig. 3. MEDAD detailed operational flow schematic.

M.W. Shahzad et al. / Applied Thermal Engineering xxx (2014) 1e9 3

effect (nth effect). Intermediate steam jet ejectors are provided topull non-condensed vapors from each stage distillate box.

The last stage of MED is connected to AD beds to adsorb thevapors on to adsorbent (silica gel) packed in the form of cakes. Thiscombination of MED and AD brings down the temperature of laststages below ambient due to high affinity of adsorbent for water

Fig. 4. MEDAD equipments arrang


vapors and help to insert more number of stages and hence morerecoveries and also reduce the corrosion chances. The heat source issupplied to desorb the vapors and condensed in the condenser. Avacuum pump is installed to maintain the pressure level in MEDstages as well as in ADmachine. It also helps to pull the air if ingressinto system due to any leak. A distillate collection tank is provided

ement and piping 3D model.


Table 1Mathematical modeling equations of hybrid MEDAD components.

Equation Remarks

Modeling equations for steam generator

½ðMhw$CphwÞ�dThwdt

¼�_mhw;hf ;Thw;in

��_mhwhf ;Thw;out

�� hin;o$Ain;i

�Thw � Ttube;i

�(1)

Energy balance for the hot waterflowing inside the tubes of SG

h�MHX;i$Cphx;i

�idTtube;idt

¼ hin;i$Ain;i�Thw;i � Ttube;i

�� hout;i$Aout;i�Ttube;i � Tv;i

�(2)

Energy balance for tube metal

dMb;i

dt¼ _mf ;i � _mb;i � _mv;i (3)

Mass balance for the seawaterinventory in the evaporator

h�Mb;i$Cpb;Tb

�þ �MHX;i$CpHX

�idTidt

¼�_mf ;ihf ;Tf

��_mb;ihf ;Tb

�� _mv;ihg;Tv

�þ Qin;i (4)Qin ¼ ho;iAi

�Tt � Tv;i

�(5)

Energy balance for the evaporatorside of the steam generator

Mb;idXb;i

dt¼�_mf ;iXf ;i

�þ � _mb;iXb;i

�� _mv;iXv;i�

(6)

Material/concentration balance

Nu ¼ hin;idin;iKtube;i

¼ 0:023 Re0:80l Pr0:40l (7)

Convective heat transfer coefficient equation [28]

Rwall;i ¼ln�dout;i

din;i

�2$p$Ktube;i$Ltube;i

(8)

Tube wall resistance

ho

�n2

k3 g

�1=3

¼ 0:0007 Re0:2 Pr0:65 q000:4 (9)

Falling film evaporation heat transfercoefficient, Han and Fletcher’s correlation [29]

ho ¼

266666640:00143$

�m2l

g$r2l $k3l

#�0:16

ðReGÞ0:45ðPrÞ3:85

h2$exp

� SSo

�� 1

i�0:38�TsatTref

!�0:89

37777775þ242:65$� q

DT

�0:84 vgvref

!�0:4735 (10)

Falling film evaporation heat transfercoefficient developed by Wakil et al. [30]

UiAi ¼ 11

hin;iAin;iþ Rwall;i þ 1

hout;iAout;i

(11)

Overall heat transfer coefficient

Modeling equations for intermediate stages

h�Ml;iþ1$Cpl;Tcond

�idTcond;iþ1

dt¼h_mv; hfg;Tv

ii� ½hin$AinðTcond � TtubeÞ�iþ1 (12)

Energy balance for the condenserside of the ith effect

h�MHX;iþ1$Cphx;iþ1

�idTtube;iþ1

dt¼ hin;iþ1$Ain;iþ1

�Tcond;iþ1 � Ttube;iþ1

�� hout;iþ1$Aout;iþ1�Ttube;iþ1 � Tv;iþ1

�(13)

Energy balance for tube metal

dMb;iþ1

dt¼ _mf ;iþ1 � _mb;iþ1 � _mv;iþ1 (14)

Brine inventory balance

h�Mb;iþ1$Cpb

�þ �MHX;iþ1$CpHX;iþ1

�idTiþ1dt

¼�_mf ;iþ!hf ;Tf

��_mb;iþ1hf ;Tb

�� _mv;iþ1hg;Tv

�þ Qin;iþ1 (15)Qin;iþ1 ¼ hout;iþ1Aiþ1

�Tt;iþ1 � Tv;iþ1

�(16)

Energy balance for evaporator side

Mb;iþ1dXb;iþ1

dt¼�_mf ;iþ1Xf ;iþ!

�� _mb;iþ1Xb;iþ1

�� _mv;iþ!Xv;iþ!

�(17)

Material/concentration balance

Nu ¼ hin;iþ1Liþ1

Ktube;iþ1¼ 0:728

"ghfg;Tcondrl;Tcondðrl � rvÞTcondK3

l;Tcond

ml;Tconddi�Tv;iþ1 � Ttube;iþ1

�#1=4

(18)

Nusselt film condensation correlationfor the calculation of the heat transfercoefficient inside the condenser tubes

UiAi ¼ 11

hin;iAin;iþ Rwall;i þ 1

hout;iAout;i

(19)

Overall heat transfer coefficient equation.


Please cite this article in press as: M.W. Shahzad, et al., Multi effect desalination and adsorption desalination (MEDAD): A hybrid desalinationmethod, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.03.064

Table 1 (continued )

Equation Remarks

MED last stage connected with AD beds

h�Mb;n$Cpb

�þ �MHX;n$CpHX;n�idTn

dt¼�_mf ;nhf ;Tf

��_mb;nhf ;Tb

�� Msghg;Tv

�dqadsdt

þ Qin;n (20)Qin;n ¼ hout;nAn

�Tt;n � Tv;n

�(21)

Energy balance for the condenserside of the nth effect.

AD cycle modeling equations

dqdt

¼15DSO exp

��EaRT

�R2P

�q* � q

�(22)

AD transient uptake (by lineardriving force model)

q* ¼ k0 expfDHads=ðRTÞgPh1þ

nk0qN

expfDHads=ðRTÞgPoti1=t (23)

AD equilibrium uptake (by Tothisotherm model)

�MsgCP;sg þMHXCP;HX

�dTadsdt

¼ DHadsðTads; PadsÞ$Msgdqadsdt

þ _mcwCPcwðTads; PadsÞ�Tcw;in � Tcw;out

�(24)

Adsorber bed energy balance

�MsgCP;sg þMHXCP;HX

�dTdesdt

¼ �DHdesðTdes; PdesÞ$Msgdqdesdt

þ _mhwCPhwðTdes; PdesÞ�Thw;in � Thw;out

�(25)

Desorber bed energy balance

Tcw=hw;out ¼ TO þ�Tcw=hw;in � TO

�exp

24 �UA

mcw=hw Cp;TO�

35 (26)

Water outlet temperatures

Performance modeling

QSG ¼ _mhw;SGCp;Thw�Thw;SG;in � Thw;SG;out

�(27)

Heat input to MED steam generator

Qdes ¼ _mhw;bedCp;Tdes�Thw;bed;in � Thw;bed;out

�(28)

Heat input for AD bed desorption

_mdistillate;AD;cond ¼ UA

LMTDhfg;Tcond

!(29)

Distillate produced from AD condenser

_mdistillate;MED;i ¼ hout;iAi$

�Ttube � Tv;i

�hfg;Tvi

!(30)

Distillate produced from MED stages

_mdistillate;total ¼Xni¼1

_mMED;i þ _mAD;cond (31)

Total distillate produced

PR ¼_mdistillate;totalhfg

TPE(32)

Hybrid MEDAD performance ratio

TPE ¼�Qthermal;payable

hboiler

�þ Qelectrichpp

!(33)

Total primary energy


to collect the distillate from all stages of MED and from ADcondenser. The conceptual 3D piping model is shown in Fig. 4. Themathematical modeling used for simulation of the system is dis-cussed in below sections.

3. Mathematical model

To make the model closer to actual practical operational plant,some assumptions are considered. The main features of MEDADmodel are:

� The heat transfer area of evaporators is maintained constant justlike common practice in commercial plants.

� The pipe pressure losses for vapor flow are considered for eacheffect.


� Liquid properties are calculated as a function of salinity, tem-perature and pressure of respective effect to take in to accountthe effect of these parameters like actual plant.

� Overall heat transfer coefficient is calculated for each effectindividually according to actual conditions during operation.

The following are the assumptions for MEDAD modeling:

� The heat loss from effects to surrounding is considered negli-gible because of low operating temperatures (TBT z 50 �C)

� The salt concentration in distillate is considered as zero.

The detailed mathematical modeling of the MEDAD system iscompleted by using a) mass conversation, b) energy conservationand c) salt conservation equations. Table 1 provides the detail


Table 2MED simulation parameters.

MED steam generator

Design parametersArea of heat transfer 4.0 m2

Tube outer diameter (corresponding saturation pressure) 16.0 mmTube thickness 0.7 mmSingle tube length 1300 mmTube matrix detail (tubes in a row � rows) 8 � 8Evaporator shell inside diameter 500 mmEvaporator shell length 1300 mmOperation parametersHot water flow rate 48 LPMHot water inlet temperature 50 CFeed water salinity 35,000 ppmMED stage design parametersArea of heat transfer 4.0 m2

Tube outer diameter (corresponding saturation pressure) 25.4 mmTube thickness 0.7 mmSingle tube length 1300 mmTube matrix detail (tubes in a row � rows) 8 � 4Evaporator shell inside diameter 500 mmEvaporator shell length 1300 mm

Table 3AD simulation parameters.

AD simulation parameters

Heat of adsorption (DHads) 2693.0 kJ/kgMaximum adsorbed amount (qN) 0.45 kg/kg of SGConstant (Ko) 7.3 � 10�13 kg/kgPaConstant (t) 12Kinetic constant (Dso) 2.54 � 10�4 m2/sActivation energy (Ea) 4.2 � 10�4 kJ/kgAverage radius of SG particles (Rp) 0.40 mmSpecific heat of SG (Cp,sg) 921 J/kg kMass of SG per bed (Msg) 100 kgThermal mass of beds (MHx) 284.1 kJ/kWater flow rate (mcw/hw) 1.52 kg/sec


modeling equations of the system that can be used to 100 �Csaturation temperature. Tables 2 and 3 shows parameters used forsimulation of MED and AD systems. The saline water properties arecalculated by using the formulation developed by Wagner and

Fig. 5. Typical temperature profiles of parallel fe


Kruse [31], Sieder and Peters [32] and Lienhard [33]. All propertiesare the functions of temperature, pressure and salinity. The steamproperties are also calculated by using the same formulation as afunction of pressure and temperature of system.

4. Results and discussion

MED and AD modeling equations are written in user definedsub-routine of FORTRAN. The last MED evaporator is combinedwith AD beds. The IMSL is used to solve the equations simulta-neously. The tolerance 1 � 10�9 is used to converge the solution.

Fig. 5 shows the conventional MED stages temperature profiles.It can be seen that in traditional MED system the temperaturedifference between stages is varies from 0.8 to 1 �C. The sametemperature is also observed in operational MED plants reported inthe literature [34,35]. It can also be seen that overall operationaltemperature gap is limited (50 �Ce38 �C) that limits the totalnumber of MED stage, in other words, the total number of vaporheat recoveries with same heat input.

Fig. 6 shows temperature profiles of hybrid MEDAD stages. Forhybrid system, there are two main drivers namely; MED steamgenerator at upstream and AD reactor at downstream. Three mainpoints observed here are: Firstly, the temperature of the last stage ofMED is below ambient (z3 �C) as compared to conventional MED(z40 �C) and this is only possible by addition of AD. Secondly, cyclictemperature profile of MED stages is observed and it is because ofAD cyclic operation. These temperature fluctuations can be seenclearly in last 3-4 stages and then it is damping down due to up-stream driver effect. Lastly, the inter stage temperature difference isvaries from 2 �C to 3 �C as compared to 1 �C in conventional MEDsystem.

This higher temperature difference increases the water pro-duction due to higher heat flux. It can also be observed clearly thatthe overall operating temperature range is increased from 50 �C to38 �C in conventional MED system to 50 �Ce3 �C in hybrid systemwith same operating parameters. This larger overall operationaltemperature gap helps to insert more number of stages with sameheat input and these extra heat recoveries also increase the waterproduction and hence the system performance.

Fig. 7 shows the water production from hybrid MEDAD system.For comparison purpose water production of conventional 8-stages

ed MED components at heat source 50 �C.


Fig. 6. Hybrid MEDAD components temperature profiles: condenser, tube surface and evaporator side temperatures at heat source 50 �C.

Fig. 7. Hybrid MEDAD cycle and conventional MED system water production comparison.

Fig. 8. Hybrid MEDAD total primary energy consumption and performance ratio.


Please cite this article in press as: M.W. Shahzad, et al., Multi effect desalination and adsorption desalination (MEDAD): A hybrid desalinationmethod, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.03.064


MED system is also presented. It can be seen clearly that there is aremarkable production improvement (more than three folds) byhybridization with same heat input temperature. The adsorptioneffect can be seen clearly in last stages of MED in hybrid cycle as itwas expected. Adsorbent effect is damping down towards first ef-fect or steam generator as can be seen clearly. It can also be seenthat water production from AD evaporator is higher at the start ofdesorption process and decreasing with time due to decrease indesorption rate.

Fig. 8 shows the total primary energy consumption, perfor-mance ratio and total water production of hybrid 8-stage MEDADsystem. Total primary energy is calculated by considering thethermal power plant and boiler efficiencies as 42% and 95%respectively. This is total payable energy for hybrid MEDAD desa-lination cycle while the thermal energy required for adsorbentregeneration is considered as a non-payable energy such as in-dustrial process waste heat or solar energy.

5. Conclusion

To fulfill the water demand especially in semi arid area a newthermal desalination system is proposed. This cycle is a combina-tion of traditional MED and AD cycle. Simulation results are pre-sented by using FORTRAN. In MEDAD cycle, the last stages of MEDoperating below ambient temperatures that not only break thetemperature limit barrier as in traditional MED (40 �C to 3 �C) aswell as help to harness the ambient energy in last stages. This hy-bridization of MED þ AD increases the production more than 3folds with same TBT as compared to traditional MED plants.

Acknowledgements

The authors wish to thanks to National Research Foundation(NRF) Singapore (grant WBS no. R-265-000-399-281), KingAbdullah University of Science & Technology (KAUST) (Project no.7000000411) and World Class University (WCU) program of Korea,R-33-2009-000-10101660, Jeju National University (JNU) forfinancial support.

Nomenclature

_m mass flow rateh enthalpy or heat transfer coefficientT temperatureX brine concentrationK material conductivityRe Reynolds numberR tube wall resistanceU overall heat transfer coefficientq adsorbent uptakem viscosityS feed water salinitySo reference seawater salinity (30,000 ppm)v vapor specific volume (vref ¼ 52.65 m3/kg at 295 K)DT temperature difference (Tch,out � Tevap)Cp specific heat capacityA area of heat transferQ heat inputd tube diameterNu Nusselt numberPr Prandtl numberL tube lengthhfg latent heatt timer density


Subscriptshw hot waterv vapor/gasf liquid/feedb brinesat saturationcond condenserads adsorptioncw cooling waterin inside/tube sideout outside/vapor spaceg gasHX heat exchangerref referenceg gasdes desorptionSG steam generator

AbbreviationsMED multi effect desalinationAD adsorption desalinationTBT top brine temperatureBm3 billion cubic metersGCC Gulf co-operation councilMENA Middle East and North AfricaGDP gross domestic productMSF multi stage flashRO reverse osmosisTVC thermal vapor compressionHAB hazards algae bloomsTPE total primary energyWP water productionPR performance ratio

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multi effect desalination and adsorption desalination (medad): a hybrid desalination method

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