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Performance investigation of advancedadsorption desalination cycle withcondenser–evaporator heat recovery scheme

Thu, Kyaw; Kim, Young Deuk; Myat, Aung; Chakraborty, Anutosh; Ng, Kim Choon

2013

Thu, K., Kim, Y. D., Myat, A., Chakraborty, A., & Ng, K. C. (2013). Performance investigation ofadvanced adsorption desalination cycle with condenser–evaporator heat recovery scheme.Desalination and Water Treatment, 51(1‑3), 150‑163.

https://hdl.handle.net/10356/100406

https://doi.org/10.1080/19443994.2012.693659

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Performance investigation of advanced adsorptiondesalination cycle with condenser–evaporator heatrecovery schemeKyaw Thu a , Young-Deuk Kim a , Aung Myat b , Anutosh Chakraborty c & Kim Choon Ng ba Water Desalination and Reuse Center, 4700 King Abdullah University of Science andTechnology , Thuwal , 23955-6900 , Kingdom of Saudi Arabiab Department of Mechanical Engineering , National University of Singapore , 10 Kent RidgeCrescent, Singapore , 117576 , Singapore Phone: Tel. +65 6516 2214c School of Mechanical and Aerospace Engineering, Nanyang Technological University , 50Nanyang Avenue, Singapore , 639798 , SingaporePublished online: 18 Jun 2012.

To cite this article: Kyaw Thu , Young-Deuk Kim , Aung Myat , Anutosh Chakraborty & Kim Choon Ng (2013) Performanceinvestigation of advanced adsorption desalination cycle with condenser–evaporator heat recovery scheme, Desalination andWater Treatment, 51:1-3, 150-163, DOI: 10.1080/19443994.2012.693659

To link to this article: http://dx.doi.org/10.1080/19443994.2012.693659

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Performance investigation of advanced adsorption desalinationcycle with condenser–evaporator heat recovery scheme

Kyaw Thua, Young-Deuk Kima, Aung Myatb, Anutosh Chakrabortyc, Kim Choon Ngb,*aWater Desalination and Reuse Center, 4700 King Abdullah University of Science and Technology, Thuwal23955-6900, Kingdom of Saudi ArabiabDepartment of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore117576, SingaporeTel. +65 6516 2214; email: mpengkc@nus.edu.sgcSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue,Singapore 639798, Singapore

Received 2 March 2012; Accepted 10 May 2012

ABSTRACT

Energy or heat recovery schemes are keys for the performance improvement of anyheat-activated cycles such as the absorption and adsorption cycles. We present two inno-vative heat recovery schemes between the condensing and evaporating units of anadsorption desalination (AD) cycle. By recovering the latent heat of condenser anddumping it into the evaporative process of the evaporator, it elevates the evaporatingtemperature and hence the adsorption pressure seen by the adsorbent. From isotherms,this has an effect of increasing the vapour uptake. In the proposed configurations, oneapproach is simply to have a run-about water circuit between the condenser and theevaporator and a pump is used to achieve the water circulation. This run-around circuitis a practical method for retrofitting purposes. The second method is targeted towards anew AD cycle where an encapsulated condenser–evaporator unit is employed. The heattransfer between the condensing and evaporative vapour is almost immediate and theprocesses occur in a fully integrated vessel, thereby minimizing the heat transfer resis-tances of heat exchangers.

Keywords: Desalination; Adsorption; Heat recovery; Solar energy

1. Introduction

Potable water is a basic and essential resource thatpermeates all human activities such as agriculture,industrial and domestic sectors [1]. With increasing

economic development and population, freshwaterdemand of the world is expected to grow 2% annuallyto 6,900 billion or Gm3 in 2030 from today’s 4,500Gm3.The sustainable water supply from the earth’s natural

*Corresponding author.

Presented at the International Conference on Desalination for the Environment, Clean Water and Energy, European Desalina-tion Society, 23–26 April 2012, Barcelona, Spain

Desalination and Water Treatmentwww.deswater.com

1944-3994/1944-3986 � 2013 Desalination Publications. All rights reserveddoi: 10.1080/19443994.2012.693659

51 (2013) 150–163

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water cycle is presently at 4,200Gm3 [2–8]. Waterscarcity is one of the most critical problems faced bythe governments in the countries around semi-aridand desert regions, as well as the land scarce andhighly populated countries. The shortfall betweendemand and supply of water in such countries couldbe met by energy-intensive desalination of the seawa-ter, brackish and recycled water. Presently, the com-mercially proven desalination methods are the multi-stage flash (MSF), the multi-effect distillation (MED),reverse osmosis (RO) and others such as vapourcompression, electro-dialysis (ED), etc., as shown inTable 1 [9].

Two dominant desalination technologies are usedworldwide and they are the MSF and the RO. Interms of the shares of the total online capacity of60million cubic meters, the RO technology represents59% with more than 14,000 plants of assorted capaci-ties, whilst the MSF has a 27% share [10]. The MSFand the MED plants are widely practised in the Mid-dle East and North Africa region, and each plant hasa water production capacity up to 100,000m3 per day.The reasons for having such thermally activatedplants are that: (i) the high salinity of seawater foundin the region (>45 ppt as opposed to 32–35 ppt in otherparts of the world) and (ii) the frequent occurrence ofbacteria blooms (algae/red tides) in the seawater ofthe Gulf region. The evaporative processes in theMSFs and MEDs are less affected by the above men-tioned factors. Another key reason pertains to theavailability of cheaper fossil fuels that favours theimplementation of cogeneration in the thermal powerplants: The integration of thermal desalination meth-ods as a bottoming cycle to extract low pressure steamfrom the gas or steam-turbine power plants [11,12].Such a cogeneration concept is widely implemented,leading to high-energy utilization of the primary fossilfuels for electricity and water production. For a giventop-brine temperature from the exhaust of low pres-sure turbines, the MSF and MED designs maximizethe recovery of latent heat of condensation/evapora-

tion processes at each successive stages, leading to ahigh Gain to Output Ratio [13] for the MSFs and theMEDs.

The membrane-based RO desalination systems uti-lize a pressure differential to overcome the osmoticpressure of saline solution through a semi-permeablemembrane where suitably organized pore layers onthe membrane permit the smaller water molecules topass through. Osmotic pressure varies with the salin-ity and it is about 10–15 bar for brackish water sys-tems and 50–80 bar for seawater systems [14,15]. Therequired pressure to push less concentrated permeatethrough membrane is generated by electrically drivenpumps. Over the recent years, improved membranetechnology and better energy recovery systems haveyielded RO systems to have specific energy consump-tion between 2.5 and 4.3 kWh/m3 [16,17]. Despite thelow specific energy consumption, the drawbacks ofRO systems are the product water quality in terms ofresiduals of boron, chlorides, bromides as well as thehigh maintenance of mechanical equipment and shortmembrane lifespan [18].

All desalination (other than the natural watercycle) technologies require either electricity or thermalheat, which requires the burning of primary energyfossil fuels. The exhaust of the power plants emitsgreenhouse gases (CO2, CH4 and N2O) that have theassociated environmental issues such as global warm-ing, etc. With increasing demand for freshwater, thereis motivation for engineers and scientists to developnovel desalination technologies that are both energyefficient and yet environment-friendly. To this end,however, new emerging methods for desalinationhave been developed and tested, such as the mem-brane distillation, forward osmosis and AD. The keycriteria for evaluating such desalination methods are:(i) reducing the specific energy consumption of desalt-ing process to a level close to the thermodynamiclimit of 0.8 kWh/m3, (ii) minimizing chemical usagefor the pre- and post-treatment of water and (iii)achieving lower targets for both carbon dioxide andbrine discharge.

This paper presents the development of anadvanced adsorption desalination (AD) cycle, onetype of thermal desalination [19,20], which utilizes alow-temperature waste heat for the regeneration ofthe adsorbents, namely silica gel during every halfcycle of the batch operation. As most of the majorcomponents used in an AD plant are stationary, andonly waste heat is consumed, the specific energyneeded to the AD cycle is only 1.38 kWh/m3, and thisis almost twice that of the thermodynamic limit fordesalination. In this paper, we present an enhancedAD cycle with novel heat recovery schemes where the

Table 1Desalination method and capacity

Desalination method Total capacity(�106m3/day)

% bycapacity

RO 37 59

MSF 17 27

MED 6 9

ED 2 4

Others 1 1

Total 63 100

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latent heat of condensation of vapour is extracted anddumped into the evaporation process of saline water:such a design achieves a pressurization effect on theadsorbent and this increases the vapour uptake dur-ing the adsorption process.

We propose two possible configurations for theheat recovery process: in the first configuration, a run-around water circuit is used between the evaporatorand condenser units and a suitable flow rate of wateris pumped through the evaporator and condenser.Another approach is to encapsulate the evaporatorand condenser as an integrated unit and this saves thepumping power needed to move the transportingcoolant. The former approach is experimented in apilot plant in the laboratory of the National Universityof Singapore whilst the performance of an integratedevaporator–condenser configuration is based on pro-cess simulation. The predicted and experimentalresults of the advanced AD cycles are analysed interms of the specific daily water production (SDWP)and the performance ratio (PR).

2. Description of the advanced AD cycle

In AD cycles, desalination of saline water isachieved by adsorption-triggered-evaporation anddesorption-aided-condensation processes and theplant comprises three major components, namely (i)the adsorber/desorber bed, (ii) the evaporator and (iii)the condenser. Evaporative boiling of the saline solu-tion is achieved by the high water vapour affinity ofadsorbent, namely, silica gel (SiO2 ·nH2O). Such adsor-bents are hydrophilic with surface area in excess of800m2/g and they are packed around the outer tubesurfaces of heat exchangers, housed with vacuum-rated reactors or beds. The processes are arranged tooperate in a batch-type cycle [21,22]. In an adsorptionprocess, unsaturated silica gel is exposed to the evap-orator filled with seawater. Owing to the high affinityfor water vapour, immediate vapour uptake occurs,triggering an evaporative process that achieves thedesalting phenomenon where dissolved salts remainin the solution. The drawn vapour is attached to thepores of the silica gel and remains captured by thevan der Waals forces––a process known as physicalsorption. The molecular kinetic energy is trapped andthus, the adsorption process is an exothermic and thisrapid enhanced sorption is facilitated by cooling theadsorbent. Concomitantly, the previously saturatedadsorbent is now fed with a heat source from circulat-ing hot water in the heat exchanger. The rise in theadsorbent temperature from heating increases the

vibrational energy of captured vapour moleculeswithin the pores, and until a critical level is reached,the water vapour leaves the pore surfaces to becomefree vapour molecules. This process is termed as thedesorption process and desorption continues until thepreset half-cycle time is reached. The desorbed vapourtravels to cooler tube surfaces within a condenser unitand condenses to give pure distillate as potable water.

In an advanced AD cycle, the heat recovery fromthe condensation to the evaporation of the salinewater is introduced to enhance the water productionrate. This type of heat recovery can be implementedusing two configurations: (1) with a heat recovery cir-cuit running across the evaporator and the condenserand (2) the encapsulated evaporator–condenserdevice. Fig. 1 shows the schematic diagrams of anadvanced AD cycle with the evaporator–condenserheat recovery scheme. Fig. 1(a) gives the schematics ofConfiguration_1 with heat recovery circuit betweenthe evaporator and the condenser whilst Fig. 1(b)shows the schematics of Configuration_2 with anencapsulated evaporator–condenser device. The firstconfiguration is suitable for existing AD cycles thatcan be readily configured with a slight modification tothe piping arrangement. It provides the operation flex-ibility to switch the operation modes between dual-effect (cooling and desalination) and single purpose(desalination) modes whereas the latter configurationis optimized for desalination.

The latter cycle employs an enclosed evaporator–condenser device where it can be either a shell andtube-type or plate-type heat exchanger. Here a shelland tube type with horizontal tube heat exchanger isconsidered for the evaporation of saline water in theshell side and the condensation of the water vapourin the tube side. It is noted that both the condenserand the evaporator heat transfer water circuits used inthe conventional AD cycle are eliminated and hence, asignificant saving in the pumping cost is realized inadvanced AD cycle. Condensation heat is transferreddirectly across the walls of the condenser tubes to theevaporator. This arrangement reduces the heat trans-fer resistances, leading to an improvement in theevaporation rates of water vapour from the seawatersolution.

The advantage of the advanced AD cycle with thiskind of heat recovery is the pressurization of theadsorption environment through higher vapour pres-sure in the evaporator, and this has a direct effect ofincreasing the vapour uptake by the silica gel duringthe adsorption process. Thus, the advantages of theadvanced AD cycle (with condenser–evaporator heatrecovery) are summarized as follows:

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(1) a reduction in the parasitic electrical power owingto the elimination of pumps for the chilled andcooling water circuits,

(2) an improvement in the adsorption capacity of sil-ica gel due to the pressurization effect,

(3) a lowering of the effective condensation tempera-ture in condenser and this tends to desorb morevapour during the desorption process, and

(4) the ability to operate low temperature of heatsource at 50˚C or higher.

3. Mathematical modelling

A mathematical model on an advanced AD cyclethat employs the internal heat recovery process wasdeveloped to access the performance of the cycle. Iso-

therm and kinetics properties of the silica gel–waterpair are employed to predict the uptake of the watervapour by the silica gel at specific temperature andpressure conditions. Type A++ silica gel was used asthe adsorbent and Table 2 shows the physical proper-ties of Type A++ silica gel.

A complete numerical modelling of the advancedAD cycles considering all the components involvedare listed in Table 3. These model equations coverboth configurations of heat recovery.

Here, Ms;Evap is the amount of seawater in theevaporator, _ms;in is the rate of feed seawater, _md;Cond isthe mass of potable water extracted from the con-denser, and _mBrine is the mass of concentrated brinerejected from the evaporator. The feed seawater isintermittently pumped into the evaporator dependingon the amount and level of seawater, whilst brine isdischarged once the concentration in the evaporatorreaches the predetermined limit. In the concentrationbalance equation, Xs;in and Xs;Evap are the concentra-tion of the feed and seawater in the evaporator XD isthe concentration of the vapour, whilst the values h, c,and n are the flags in the equations during operationand switching processes which are listed in Table 4.

The evaporation of seawater is attributed to theuptake of the water vapour by the adsorbent and theenergy recovered from the condensation is used tomaintain the evaporation process at a certain tempera-ture level.

The first term in the right-hand side of the evapo-rator energy balance equation represents the sensibleheat by the supplied feed seawater, the second term

Table 2Physical properties of Type A++ silica gel

Property Value

Pore size (nm) 0.8–7.5

Porous volume (cm3 g�1) 0.476

Surface area (m2 g�1) 863.6

Average pore diameter (nm) 2.2

Apparent density (kgm�3) 700

pH 4.0

Specific heat capacity (kJ kg�1 k�1) 0.921

Thermal conductivity (Wm�1 k�1) 0.198

Advanced AD cycle with the

evaporator–condenser heat recovery circuit

Advanced AD cycle with encapsulated

evaporator–condenser device

(a) (b)

Fig. 1. Schematic diagrams of advanced AD cycle.

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154 K. Thu et al. / Desalination and Water Treatment 51 (2013) 150–163Tab

le3

Asu

mmaryofthenumerical

modellingofad

van

cedAD

cycles

Balan

ceeq

uation

Configuration_1

Configuration_2

Adsorptionisotherm

q�¼

q 0exp½�

ðRT ElnðP P

0ÞÞn

�Adsorptionkinetics

dq dt¼

15D

s0expð�

Ea

RTÞ

R2 p

ðq��qÞ

Isosteric

heatof

adsorption

Qst¼

h fgþEf�

lnðc c m

Þg1=nþTv gð@P @

TÞ g

c pofad

sorbed

phase

c p;a¼

c p;gþf1 T

�1 v gð@v

g

@TÞ PgQ

st�@

Qst

@T

� � P

Concentration

balan

ceM

s;Evap

dXs;Evap

dt¼

hX

s;in

_ ms;in�cX

s;Evap

_ mBrine�nX

Ddc a

ds

dtM

sg

Evap

oratoren

ergy

balan

ce½c p

;sðT

evapÞM

s;ev

apþM

HX;Evap

c pHX�dT

evap

dt¼

hhfðT

evapÞ_ ms;in�nh f

g

ðTev

apÞM

sgðdq

ads

dtÞþ

_ mev

apc p

;evapðT

chilled;in�

Tch

illed;out�ch f

ðTEvap;X

s;EvapÞ_ mBrine

½Ms;Evapc p

;sðT

Evap;X

s;EvapÞþ

MHX;Evap

c p;H

X�dT

Evap

dt¼

hh fðT

Evap;X

s;EvapÞ_ ms;in�nh f

gðT

EvapÞdq

ads

dtM

sgþ

UECAðT

Cond�TEvap�

ch fðT

Evap;X

s;EvapÞ_ mBrine

Conden

seren

ergy

balan

ce½c p

ðTcondÞM

condþM

HX;Condc p

HX�dT

cond

dt¼

�hfðT

condÞdM

d

dtþh

fgðT

condÞM

sgðdq

des

dtÞþ

_ mcondc p

;condðT

cond;in�Tcond;outÞ

½MCondc pðT

CondÞþ

MHX;Condc p

;HX�dT

Cond

dt¼

�hfðT

CondÞdM

d

dtþnh f

gðT

CondÞdq

des

dtM

sg�

UOverallAðT

Cond�TEvapÞ

Conden

serwall

temperature

–ðm

c pÞ ss

dTw2

dt¼

h CondA

CondðT

Cond�Tw2Þ�

2pkL

lnr 1 r 2

ðTw2�Tw1Þ

Evap

orator-sidewall

temperature

–ðm

c pÞ ss

dTw1

dt¼

h EvapA

EvapðT

w1�TEvapÞþ

2pkL

lnr 1 r 2

ðTw2�Tw1Þ

Uofev

aporator–

conden

serdev

ice

–U

Overall¼

ð1

h Condþ

r 2ln

r 1 r 2

kþ

r 2 r 11

h Evap�

1

hforev

aporation

Experim

entaldata

Tw�T¼

Csfh f

gPrs l

Cpf

��

q0 load

l lh f

g

ffiffiffiffiffiffiffiffiffiffiffi

rgðql�

q gÞ

p"

# n �P

Patm�� m

Awetted

Abase

�� a

hforconden

sation

Experim

entaldata

h¼

Fq lgk

3ðq

l�qvÞ½h

fgþ3 8c

plðT

�Tw�

DlðT�T

wÞ

hi 0:25

Adsorber

ðMsgc p

;sgþM

HX;A

dsc p

;HXþM

abec p

;aÞdT

ads=des

dt¼

�QstðT

ads=des;Pev

ap=condÞn

�

Msg

dqad

s=des

dt�

_ mcw

=hwc p

;cw=hwðT

ads=desÞð

Tcw

=hw;in�Tcw

=hw;outÞ

½Msgc p

;sgþM

HXc p

;HXþM

abec p

;a�dT

ads=des

dt¼

�

n:Q

stM

sgdq

ads=des

dt�

_ mcw

=hwc pðT

cw=hw;in�Tcw

=hw;outÞ

Outlet

water

temperature

Tout¼

T0þðT

in�T0Þe

xpð

�UA

_ mc pðT

0ÞÞ

Adsorber

heat

Qad

s¼

_ mcw

c p;cwðT

cw;out�Tcw

;inÞ

Desorber

heat

Qad

s¼

_ mcw

c p;cwðT

cw;out�Tcw

;inÞ

Evap

oratorheat

QEvap

¼_ mEvapc p

;EvapðT

EvapÞð

Tch

illed;in�Tch

illed;outÞ

QEvap

¼h E

vapA

EvapðT

w1�TEvapÞ

Conden

serheat

Qcond¼

_ mcondc p

;cwðT

condÞð

Tcond;out�Tcond;inÞ

QCond¼

h CondA

CondðT

Cond�Tw2Þ

SDW

PSDW

P¼

R t cycle

0Q

Conds

h fgðT

CondÞM

sgdt

PR

PR¼

1t cycle

R t cycle

0

_ md;Condh f

gðT

CondÞ

Qdes

dt

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stands for the heat removal by the uptake of adsor-bent, the third and fourth terms denote the energysupplied for the evaporation through heat recoveryfrom the condensation process and the sensible heatremoval by the brine discharge. The specific heat ðcp;sÞand enthalpy (hf) of seawater are calculated as func-tions of temperature, pressure and salinity.

As the evaporated seawater is associated onto thesolid adsorbent by the flow of external cooling fluidduring adsorption period, whilst the desorbed watervapour is dissociated from the solid adsorbent by theflow of heating fluid during desorption period, theheat energy is exchanged between the cooling/heatingfluid and the adsorption/desorption bed. Here isoster-ic heat, Qst, and the specific heat cp;a of the adsorbedphase are considered for better prediction [23].

Using the heat transfer Equation, the outlet tem-perature of water from each heat exchanger is esti-mated using the log mean temperature differencemethod.

The adsorption/desorption rate of water vapourby the silica gel is calculated from the knowledge ofadsorption equilibrium and kinetics and is given bythe conventional linear driving force equation, whereDso defines a pre-exponential factor of the efficientwater diffusivity in the adsorbent, Ea represents theactivation energy, R is the universal gas constant andRp is the average radius of the adsorbent grains.Kinetic data were taken from [24,25]. The adsorptionuptake at equilibrium condition is expressed as afunction of pressure (P) and temperature (T). Theseexperimentally measured data are fitted using theDubinin–Astakhov equation.

After desorption, the desorbed water vapours aredelivered to the condenser as latent heat and inadvanced AD cycle, this energy is internally recoveredfor the evaporation process. In modelling, it is

assumed that the condenser tube bank surface is ableto hold a certain maximum amount of condensate.Beyond this, the condensate would flow into thewater collection tank via a U-tube.

For Configuration_2 i.e. the integrated evaporator–condenser design, the heat transfer coefficient of theboiling of the saline solution is predicted using themodified Rohsenow correlation for subatmosphericpressures [26] whilst Nusselt film condensation corre-lation is applied to calculate the heat transfer coeffi-cient for condensation of the water vapour inside thecondenser. The values of all the coefficients and theparameters used in the simulation are listed in Table 5.

For the desorption process, the energy required todesorb water vapour from the silica gels, Qdes; is cal-culated using the inlet and outlet temperatures of theheat source supplied to the reactors. Concomitantly,the energy rejected to the cooling water during theadsorption process is estimated by the inlet and outlettemperatures of the cooling fluid that is supplied tothe other reactor.

The performance of the AD cycle is given bySDWP and PR, defined here as the ratio of usefuleffects to the energy input to the cycle.

These sets of energy and mass balance equationsare solved by Adams–Moulton’s method in the DIV-PAG subroutine of the IMSL FORTRAN math librarysubroutines. A double-precision accuracy was appliedand the tolerance was set to 1� 10�12. The code runson the platform of FORTRAN PowerStation. As theAD cycle is operated in a batch manner, the connec-tion or pointers for linking one subroutine to anotherhave to be carefully directed so that the correctboundary conditions are maintained during the com-putation. The cyclic steady-state condition can beachieved in 4 to 5 cycles after the commencement ofcomputation.

4. Results and discussion

The advanced AD cycle with internal heat recov-ery between the condenser and the evaporator isinvestigated at various operation conditions such asdifferent cycle times, hot and cooling water tempera-tures, different hot and cooling water flow rates.Fig. 2 depicts the pictorial view of the advanced ADplant showing the condenser–evaporator heat recov-ery circuit.

The transient temperature profiles of the majorcomponents of the advanced AD cycle that incorpo-rates the heat recovery scheme between the condenserand the evaporator are shown in Fig. 3. In this analy-sis, the hot water inlet temperature was maintained at70˚C whilst the cycle time (tcycle) and the switching

Table 4Values of the indicators for changing the operation modeof the AD cycle

Mode Parameter Value

Operation n 2

h 1 (charging sea water)

0 (otherwise)

c 1 (brine discharge)

0 (otherwise)

Switching n 2

h 1 (charging sea water)

0 (otherwise)

c 1 (brine discharge)

0 (otherwise)

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time (tswitching) are kept 600 s with 40 s, respectively. Itis noted that the experimental temperature measure-ments are subjects to the time constant and accuracyof the sensors. As can be observed from Fig. 3, thesimulated results of transient temperature agree wellwith those of the experimental data, thus the simula-tion is being validated by the experiment. The hyster-etic found in the condenser temperature resulted fromthe location of the sensor that is placed near thedesorber vapour pipe. Thus, it fails to capture thetemperature drop by the cold front from the evapora-tor at the beginning of the cycle operation.

Fig. 4 gives the SDWP and the PR of the advancedAD cycle whilst operating at different hot water inlettemperatures. The experimental results show that theSDWP linearly increases with the hot water inlet tem-perature and is about 9.24m3/tonne of silica gel perday at 70˚C hot water inlet temperature, whilst theSDWP reaches 14.2m3/tonne of silica gel per day at85˚C hot water inlet temperature. The increase in theSDWP with the increase in the hot water temperatureis attributed to the better desorption–condensationprocess at higher regeneration temperatures resultingin the increase in SDWP. It should be noted that the

Table 5Values of the parameters used in the simulation program

Parameter Value Unit

q0 0.592 kg/kg of silica gel

E 3.105 kJmole�1

n 1.1 –

Dso 2.54� 10�4 m2 s�1

Ea (kJ/kg) 4.2� 10�4 kJ kg�1

Rp 0.4 mm

Hot water inlet temperature 50–85 ˚C

Cooling water inlet temperature 30 ˚C

Mass of silica gel per bed, Msg 36 kg

Adsorber/desorber bed heat transfer area 41.7 m2

Tube length 0.576 m

MHXcp;HX 184.1 kJK�1

Uads 250 [27] Wm�2 K�1

Udes 240 [27] Wm�2 K�1

Configuration_1 (Evaporator–condenser heat recovery circuit)

Uevap 1,715.2 Wm�2 K�1

Ucond 2,657.5 Wm�2 K�1

Aevap 3.5 m2

Acond 5.08 m2

Thermal mass of the evaporator including fins and support 25.44 kJK�1

Mass of refrigerant in the evaporator 250 kg

Thermal mass of the condenser including fins and support 18.61 kJK�1

Mass of condensate in the condenser 10 kg

Configuration_2 (Integrated evaporator–condenser)

Ltube 2.0 m

Number of tubes (stainless steel) 60 –

r1 7.1 mm

Awetted 9.35 m2

Abase 4.67 m2

ACond 5.35 m2

cp 500 J kg�1 K�1

k 16.2 Wm�1 K�1

Operation conditions

tcycle 300–600 s

tswtiching 20 s

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advanced AD cycle is capable of operating at a sub-stantially low hot water inlet temperature, i.e. 50˚C atwhich conventional AD cycle fails to operate and theproduced SDWP is 5.2m3 of potable water per tonneof silica gel per day.

The effect of the cooling water inlet temperatureon the performance of the advanced AD cycle isshown in Fig. 5. The SDWP of the cycle for two setsof hot water inlet temperatures, i.e. 50 and 70˚C, ispresented whilst varying the temperature of the cool-ant from 26 to 32˚C. A higher water production rate is

achieved at lower coolant temperatures whilst the pro-duction rate decreases as the coolant temperature isincreased.

Fig. 6 shows the temperature–time history of theadsorber, desorber, evaporator and condenser of theadvanced AD cycle with an evaporator–condenserdevice at the cyclic steady-state conditions. Thetemperatures of the heat source and cooling waterare maintained at 85 and 30˚C, respectively, whilstthe half-cycle time is set at 300 s and the switchingtime 20 s. Owing to the integral evaporator–con-

Fig. 2. Pictorial view of the AD cycle with the condenser–evaporator heat recovery circuit.

Fig. 3. The temperature profiles of the major components of the advanced AD cycle.

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denser design, the pressurization effect on theadsorption–evaporation process is observed. This isachieved by bringing about an increase in theevaporation temperature, which is about 42˚Cthrough heat recovery from condensation. From theisotherm properties of silica gel–water pair, thehigher adsorption pressure enhances the adsorptioncapacity by the adsorbent.

The improvement in the uptake of the adsorbenttends to a higher rejection of adsorption heat and sub-sequently the adsorption temperature. It is observedthat the adsorber bed temperature is relatively high at

44˚C whilst the condensation temperature is found tobe about 48˚C. The pressure profile of the cycle isgiven in Fig. 7 and it is noted that the operating pres-sure region of the advanced AD cycle is elevated to 7–12 kPa in contrast to the conventional cycle where thepressure ranges from 3 to 7 kPa. The performance ofthe evaporator–condenser device presented is given inFig. 8 where the maximum temperature differencebetween the evaporator and the condenser wall isobserved to be 1.3˚C.

The performance of the evaporator–condenserdevice implemented in the advanced AD cycle is alsoanalysed in terms of the overall heat transfer coeffi-cient. The overall heat transfer coefficient of the evap-orator–condenser unit is evaluated using the ModifiedRohsenow correlation for subatmospheric conditionsfor evaporation and Nusselt film correlation for con-densation. The thermal resistance imposed by thestainless steel tubes is also accounted for. The overallheat transfer coefficient (U) of the evaporator–con-denser device is shown in Fig. 9.

The predicted data showed that the U value ishigher at the beginning of every cycle and graduallylower with the cycle time. This is because the uptakeand desorption amount by the silica gel is higher atthe beginning of the cycle operation resulting in rapidboiling and condensation. Thus, the U value is higherat that period. The uptake amount gets diminished asthe silica gel gets saturated as well as the desorptionrate becomes slower as the cycle operation advances.However, during the switching period, the U value

Fig. 4. The SDWP and PR of the advanced AD cycle with assorted hot water inlet temperatures ranging from 50˚C to70˚C.

Fig. 5. The performance of the advanced AD cycle atdifferent cooling water inlet temperatures.

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drops to a value less than 500Wm�2 K�1. This isbecause there are no adsorption and desorption phe-nomena in this period and the resultant small value ofU is due to the exchange of heat between the thermalmasses of the evaporator and the condenser. The aver-age overall heat transfer coefficient (UOverall) is found

to be about 2,300Wm�2 K�1, which is quite a goodvalue across the vapour condensing in the condenserto the vapour leaving the saline solution falling overthe surface of tubes of evaporator.

A parametric analysis on the advanced AD cyclewas conducted to study the performance of the cycle

Fig. 6. Predicted temperature–time histories of the adsorber, desorber, evaporator, and condenser of the advanced ADcycle.

Fig. 7. The temporal pressure profiles of the advanced AD cycle.

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at various operating conditions such as different cycletimes, different hot and cooling water inlet tempera-tures to the adsorber beds.

Fig. 10 represents the SDWP of the advanced ADcycle for assorted regeneration temperatures from 50to 85˚C. The results show that the water production

rate of the advanced cycle linearly varies with the hotwater temperature. This is due to a better regenerationprocess for higher hot water temperatures.

The effect of cooling water temperature on the per-formance of the advanced AD cycle is also investi-gated. The temperature range between 25 and 35˚C is

Fig. 8. Temperature profiles of the evaporator–condenser device in advanced AD cycle.

Fig. 9. Overall heat transfer coefficient of the evaporator–condenser device.

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selected as this is a practical cooling water temperaturerange readily available using a conventional coolingtower. The effect of the cooling water temperature onthe cycle is investigated at assorted hot water inlettemperatures between 50 and 85˚C. The performanceof the cycle in terms of SDWP is presented in Fig. 11in which the results show that the cycle performs moreefficiently at a lower cooling water temperature givinga higher SDWP. As the cooling water temperatureincreases, the SDWP of the cycle becomes lower. Thisphenomenon can be attributed to the isotherm behav-iour of the silica gel where the lower-temperature

adsorption environment promotes uptake capacity bythe adsorbent.

A comparison on the water production rates by dif-ferent adsorption cycles at normalized cycle times andsame operating conditions (Thot = 85˚C and Tcw = 30˚C)is given in Fig. 12. A conventional AD cycle and an ADcycle with heat recovery between condenser and evapo-rator using an evaporator–condenser heat recovery cir-cuit are selected for the comparison [20–30]. It is notedthat experimental data for the aforementioned cyclesare used here. The results show that the advanced ADcycle can produce potable water up to threefold of the

Fig. 10. The predicted SDWPs and overall heat transfercoefficients of the advanced AD cycle at assorted hotwater inlet temperatures.

Fig. 11. The predicted potable water production of theadvanced AD cycle for different cooling water inlettemperatures.

Fig. 12. Comparison on the water production rates by different AD cycles at typical operating conditions.

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conventional cycle whilst it produces twice that of theAD cycle with an evaporator–condenser heat recoverycircuit. Another significant advantage of the advancedcycle is that it can be operational at hot water inlet tem-peratures as low as 50˚C whilst a conventional cycle isunable to do so at such low temperatures. This low-grade regeneration temperature is readily availablefrom the solar thermal sources or waste heat from pro-cesses. The SDWP at 50˚C hot water inlet temperatureis found to be about 8.1m3 per day that is comparablewith that of the conventional cycle even at this low heatsource temperature.

5. Conclusions

We have presented an advanced AD cycle fordesalination where the internal latent heat betweenthe evaporator and condenser units is recovered. Thisis equivalent to having a compressor to pressurize theadsorbent during adsorption, and thus making theadsorbent have a higher water vapour uptake. Twoconfigurations have been described and they havebeen proven to be feasible and practical. Retrofittingusing a run-around loop was readily implemented onan existing AD plant and an increase in the water pro-duction was measured. In the NUS’s pilot plant, therun-around heat recovery procedure increases theSDWP to 9.24m3/tonne-day, despite a low heatsource inlet of 70˚C. At an inlet hot water of 85˚C, thewater production of the retrofitted AD cycle attains aSWDP of 14.2m3/tonne-day. On the other hand, thesimulated results from an integrated condenser–evap-orator design show higher yields, up to a threefoldincrease in SWDP: at 55˚C hot water inlet to the ADcycle, a SWDP of 8.1m3/tonne-day could be achievedwhilst SWDP of 26m3/tonne-day is attained at 85˚C.Such increases are attributed to the higher heat trans-fer coefficient of the integrated design, typicallyaround 2,300W/m2K.

Symbols

r –– surface tension, Nm�1

a –– constant in modified Rohsenow correlation,–

q –– density, kgm�3

c –– flag which governs the mode of operation,–

h –– flag which governs the mode of operation,–

q00 –– heat flux, Wm�2

_m –– mass flow rate, kg s�1

s –– number of cycles per day, –

l –– viscosity, kgm�1 s�1

A –– area, m2

COP –– the coefficient of performance, –

cp –– specific heat capacity, J kg�1K�1

Csf –– a constant of Rohsenow correlation, –

Dso –– a kinetic constant for the silica gel watersystem, m2 s�1

Ea –– activation energy of surface diffusion,kJmol�1

g –– gravitational acceleration, m s�2

hf –– sensible heat, kJ kg�1

hfg –– latent heat, kJ kg�1

k –– the thermal conductivity, Wm�1 K�1

L –– length, m

m –– constant in modified Rohsenow correlation,–

M –– mass, kg

n –– number of beds/constant in modifiedRohsenow correlation, –

P –– pressure, Pa

P0 –– reference pressure, Pa

PR –– performance ratio, –

q –– concentration, kg kg�1

Q –– the total heat or energy, W or J

q⁄ –– the equilibrium uptake, kg kg�1

qo –– the limiting uptake, kg kg�1

Qst –– isosteric heat of adsorption, kJ kg�1

r –– radius, m

R –– Universal gas constant, J K�1mol�1

Rp –– average radius of silica gel, m

SDWP –– specific daily water production, m3/tonne-day�1

T –– temperature, K

t –– time, s

T0 –– reference temperature, K

UOverall –– overall heat transfer coefficient, Wm�2K�1

v –– specific volume, m3 kg�1

vp –– the pore volume, cm3 g�1

X –– salt concentration, mgL�1

Subscripts

a –– adsorbate

ads –– adsorption

Brine –– concentrated brine

ch –– chilled water

Cond –– condenser

cw –– cooling water

d –– distillate

Evap –– evaporator

g –– gaseous phase

hw –– hot water

HX –– heat exchanger

in –– inlet

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l –– liquid

des –– desorption

out –– outlet

reg –– regeneration

s –– salt/adsorbent

sg –– silica gel

ss –– stainless steel

v –– vapour

w –– wall

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