experimental investigation of an adsorption desalination plant using low-temperature waste heat

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Applied Thermal Engineering 25 (2005) 2780–2789

Experimental investigation of an adsorption desalinationplant using low-temperature waste heat

Xiaolin Wang, Kim Choon Ng *

Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent,

Singapore 119260, Singapore

Received 8 December 2004; accepted 10 February 2005

Available online 28 April 2005

Abstract

Adsorption cycle is a practical and inexpensive method of desalinating the saline and brackish water to

produce potable water for both industrial and residential applications. As compared with the commercial

desalination methods, the adsorption technology has the unique advantages such as (i) the utilization of the

low-temperature waste heat, (ii) low corrosion and fouling rates on the tube materials due to the low-

temperature evaporation of saline water, (iii) and it has almost no major moving parts which renders inher-ently low maintenance cost. In addition, the adsorption cycle offers two important benefits that are not

available to the existing desalination technologies; namely, (i) a two-prong phenomenal barrier to any

‘‘bio-contamination’’ during the water generation process as compared with existing methods and (ii)

the reduction in global warming due to the utilization of low-temperature waste heat which otherwise

would have been purged to the atmosphere. This paper describes an experimental investigation and the

specific water yields from a four-bed adsorption desalination plant is presented with respect to major

assorted coolant and feed conditions.

� 2005 Published by Elsevier Ltd.

Keywords: Desalination; Adsorption process; Silica gel–water; Specific water yield

1359-4311/$ - see front matter � 2005 Published by Elsevier Ltd.doi:10.1016/j.applthermaleng.2005.02.011

* Corresponding author. Tel.: +65 68742214.

E-mail address: [email protected] (K.C. Ng).

mailto:[email protected]

X. Wang, K.C. Ng / Applied Thermal Engineering 25 (2005) 2780–2789 2781

1. Introduction

Desalination has been a practical solution to the water shortage problems encountered inmany countries of the world, in particular, in semi-arid regions as well as in countries with highpopulation density. Over the decades, numerous commercial-scale desalination plants have beendesigned, built and operated, such as (i) the multi-stage flash (MSF) [1,2] type, (ii) the multi-ef-fect desalination [3–5] type, (iii) the membrane-based reverse osmosis (RO) [6,7] plants. Morerecent improvements include the hybrid plants, which combine the RO and MSF processes[8–13], could recover higher quality and yield of water with typical dissolved solids of less than500 mg/l as required by the World Health Organization (WHO) standards. Ion exchange is an-other method whereby ions of dissolved inorganic salts are chemically replaced with the moredesirable ions, and such a process has been used to minimize the fouling and carry-over tothe water. Electro-dialysis (ED) or electro-dialysis reversal (EDR) [14] is deemed as one ofthe most promising techniques; however, the expected breakthrough has yet been realized. Ton-ner and Tonner [15] recently summarized the thermal process and analyzed the economics of allthose systems.In spite of the incremental improvements of the processes of desalination, which have contrib-

ute to the cost effectiveness and a sustainable production of fresh water, the conventional desali-nation plants are plagued by three major drawbacks [16] and they are (i) the high energy (primaryor electricity) consumption of the plant, and the associated environmental emissions, (ii) erosionand blockage of materials of membranes and mass exchangers and (iii) high maintenance costsarising from salt deposition or fouling in the outer surfaces of heat exchangers as well as the cor-rosion of the tubes. Fouling and corrosion are known to escalate at high solution temperaturesand the threshold for high salt deposition from the saline solution is known to occur when tem-peratures exceed 80 �C.Recent development on adsorption desalination cycles is aimed to mitigate the short comings of

the conventional desalination methods. The earliest patent related to adsorption-based desalina-tion was reported by Broughton [17], using an ion-retarded resin for the vapor uptake, where aprocess with a thermally-driven two-bed configuration is simulated. Similar theoretical simula-tions of adsorption desalination plant were also proposed recently by Zejli et al. [18] and Al-kha-rabsheh and Goswami [19]. Solar heat source was studied as a heat source for the desalinationplant, combined with an open-cycle adsorption heat pump using the Zeolite as the solid-vaporadsorbent.In another example, Richter [20] reported a similar solar-assisted desalination plant comprises

an initial desorbed phase during the day and follows by an adsorption phase in the night hours.The long cycle time employed in the simulation yields a high coefficient of performance (COP) butbeing solar-driven, it is weather dependent.In this paper, a unique four-bed adsorption desalination plant that employs a low-tempera-

ture waste energy was proposed and experimentally studied. The experimental study wasadapted from our previous work on a four-bed, regenerative adsorption chiller [21,22]. The spe-cific water yield is measured experimentally with respect to the key controlling parameters suchas heat source temperatures, coolant temperatures, system switching and half-cycle operationaltimes.

2782 X. Wang, K.C. Ng / Applied Thermal Engineering 25 (2005) 2780–2789

2. Experimental facility

Fig. 1 shows a pictorial view of our experimental adsorption desalination plant that comprisestwo major sections; namely (i) the rating facility that conditioned the temperatures of the heatsource and cooling water, and (ii) the adsorption water production system where saline or brack-ish water is desalinated. Fluctuations of the test condition, due either to system perturbations orto the external environment changes, would be dampened by the purposed built rating facility.The experimental set-up of the saline water desalination system, as shown in Fig. 2, provides adetailed flow diagram to the various sub-components in the plant. Although the operation schemeof the system is similar to that of the four-bed an adsorption chiller [21,22], the main difference inthe two systems lies in the condensed water where it is removed as pure water using a 10 m highliquid filled tubing or a vacuum pump.

Fig. 1. A pictorial view of the four-bed adsorption desalination plant.

Fig. 2. Schematic of a four-bed adsorption desalination plant.


The tests are conducted under steady state conditions of the coolant temperatures (hot watersource and cooling water source temperatures) which are accurately conditioned by the ratingfacility and hence, the test facility is weather independent. The cooling water, returning fromthe beds undergoing the adsorption processes and the condenser, is first mixed in a cooling waterstorage tank to minimize its temperature fluctuations and secondly, it is pumped serially throughthe cooling tower and an evaporator coil of a mechanical chiller, dropping the coolant tempera-ture to a level below that of the set-point temperature of the heater controller. In the heater tankof the rating facility, the cooling water temperature is now fine-tuned by an electrical heat inputusing a cascaded two-loop PID controller. Excellent control of the outlet water temperatures isachieved by this control arrangement as the time constant of electrical heating is faster than thatof the cooling process of the cooling tower and the mechanical chiller.The control strategy of the hot water is also similar to that of the cold water loop where a hot

water storage tank is used to damp any temperature fluctuation of the hot water returning fromthe desorption processes. The hot water from the mixed tank is then fed into the hot water heatertank where its temperature is also fine-tuned by the electrical heat input with a similar PID con-troller. Similar to the hot water circuit, the evaporator in the adsorption plant is maintained by aconstant chilled water return which is electrically controlled at the chilled water heater tank. Thisheater also simulates the cooling load. All the above-mentioned circuits have a controlled accu-racy of ±0.3 �C.


In the adsorption desalination plant, the saline water, which is fed to the evaporator, is evap-orated by the thermal load supplied by the circulating chilled water loop. The process of vaporuptake is maintained by the adsorbent in the designated adsorption processes and the heat gen-erated in the process is removed by the circulating cool water. Concomitantly, hot water suppliedto the desorption beds drives the vapor out and is condensed in the condenser to produce the purewater which is collected in a collection tank and intermittently pumped out to the ambient by acondensate pump.It is noted in the adsorption desalination cycle, the saline water is kept in the evaporator

where the evaporation occurs at low temperatures, typically less than 25 �C. Hence, the corro-sion and fouling rates, caused by the presence of salts in the solution, on the external surfaces ofheat exchanger is substantially lowered. Only the evaporator of the plant would be fabricated ofstainless steel to withstand the corrosion of the saline or brackish water whilst other componentsrequire only mild steel. A saline concentration controller is used to control the level of the salineor brackish water feed in the evaporator and it is noted that the evaporator operates continu-ously despite the batch cycle for the adsorption and desorption beds. The water productionin this experiment was measured using a flow meter. All temperature measurements employsthe 5 kX type thermistors with a 3 s time constant (±0.2 �C, YSI) that each sensor captures accu-rately the transient swings during the switching period (either pre-heating or -cooling of beds).Otherwise, significant cumulative error may be introduced if a sluggish temperature sensor isused. Electromagnetic flow transmitters for our flow rate measurements (±0.5% of read-ing + ±0.05 l/min, Krohne) are used and the absolute pressure sensors with an accuracy of±3.5% of the reading are also employed.Energy balance of the individual component inside the adsorption desalination plant can be

calculated from the following equations. The instantaneous energy input dQhot to the desorp-tion beds and the heat released dQrel during the desorption process could be respectively ex-pressed as

dQhot ¼ _mhot � CpðtÞ � ðT hot-in � T hot-outÞ ð1Þ

dQrel ¼ dqðtÞ � DadsH ð2Þ

where _mhot, hot water flow rate to the beds. Cp(t), specific heat at instantaneous temperature.Thot-in, Thot-out, hot water temperatures at system inlet and outlet, respectively. DadsH is the heatsof adsorption and dq(t) represents instantaneous desorbed vapor amount which is determined bythe linear driving force equation as

dqðtÞ ¼ 15Dsoe�Ea=RT

R2pðq� � qÞ ð3Þ

where Dso, Ea, Rp are constants and R is the gas constant. q is the adsorbate amount inside thebed. The saturated vapor uptake q*, at a given P and T, is calculated by the Toth equation,

q� ¼ K � expðDadsH=RT Þ � P1þ K0=qm � expðDadsH=RT Þ � P½ t� �1=t ð4Þ

where qm is the maximum adsorption amount and K0, t are Toth constant.


The Eqs. (1) and (2) are also used to calculate the instantaneous energy rejection dQcooling andheat dQgen generated in the adsorption process in the adsorber respectively. All the parametersmust be replaced with cooling water parameters as shown

dQcooling ¼ _mcooling � CpðtÞ � ðT cooling-in � T cooling-outÞ ð5Þ

dQgen ¼ dqðtÞ � DadsH ð6Þ

dq(t) is computed using Eqs. (3) and (4), noting that the pressure and temperature must be re-placed with the parameter of the adsorber.The cooling capacity, dQevap, is calculated according to the chilled water inlet/outlet tempera-

tures, Tchilled-in, Tchilled-out and chilled water flow rate _mchilled

dQevap ¼ _mchilled � CpðtÞ � ðT chilled-in � T chilled-outÞ ð7Þ

Over a period of a day, the specific water production, SWP is estimated from the measurement ofthe cooling capacity at the evaporator as

SWP ¼Z3600dQevaphfgðT evapÞ

� ðhrÞ=msg dt ð8Þ

where hr is the working hours per day and hfg is the latent heat of water at the evaporating tem-perature Tevap. msg is the amount of the silica gel in the system. The energy conversion efficiency(COP) defined here is

COP ¼RdQevap dtRdQhot dt

ð9Þ

3. Results and discussion

Presently, there is no available standard rating conditions for the testing of adsorption desali-nation plants. Based on the test experience carried out in adsorption cycles as well as for a faircomparison of performance, it is proposed that the adsorption desalination plants are to be testedat the following rating conditions:

(i) The inlet temperatures of hot, cooling and chilled water supplied to the beds and heatexchangers of the plant are set at 85 �C, 29.4 �C and 12.2 �C respectively.

(ii) The coolant flow rates for the evaporator is 0.0455 l/(s kW), where the kW is based on themaximum cooling capacity across the evaporator.

(iii) The cooling water flow rate is set to 2.5 times of the evaporator flow rate, where the ratio ofwater supplied to the designated adsorption beds and the condenser is 1–1.5, respectively.

(iv) The hot water flow rate supplied to the beds (in series flow configuration) is set to have thesame flow rate as that of the evaporator.

Based on the above-mentioned rating conditions, the performance of the experimental desali-nation plant is examined by changing the half-cycle time of the batch cycle, typically from 120 s to


600 s whilst the switching time is held constant at 40 s. Fig. 3 shows the variation of specific waterproductions (SWP) and COPs of the adsorption desalination plant at assorted half-cycle time onthe abscissa axis. Maximum specified water production of 4.7 kg/kg silica gel occurs at an oper-ating cycle time of 180 s, whilst increasing the half-cycle time of the adsorption cycle leads to itsreduction. Owing to the increasing trend of COP, a practical half-cycle time for the desalinationplant is about 250 s without sacrificing much of the SWP yield. The increasing trend of COP ispartially attributed to maximization of adsorbent potential and the asymptotic reduction of thetemperature difference between the heat source and the adsorbent when the cycle time islengthened.Fig. 4 shows the effects of the cooling and chilled water temperature on the SWP and the COP

of the desalination plant. It is observed that the SWP is sensitive to both cooling and chilled watertemperatures: for example, a drop of 1.6 �C could increase the SWP by as much as 10% or a rise inthe chilled water temperature of 1.8 �C would have the same effect. However, the effect of thewater temperatures on the COP is lesser typically about 3% (also within the same order of mag-nitude of experimental uncertainty) for the above-mentioned temperature changes. This impliesthat increasing the chilled water temperature to improve SWP is a realistic proposition. Further-more, if the chilled water is used to further cool the cooling water circuit, further enhancement ofthe SWP is possible.The effect of the heat source temperature on the desalination plant performance has been dem-

onstrated in the Fig. 5 with two half-cycle times of 180 s and 300 s. It is observed that both SWPand COP increase with increasing heat source temperatures but the system COP is substantiallymore sensitive with the heat source temperature. Water production from the desalination plantcould still function at very low-temperature waste heat such as 65 �C into the useful water produc-tion and this is the unique feature of the plant.From these tests, it is obvious that the COP of a desalination plant is obstinately low, about

0.38, but it could be improved using a passive heat recovery scheme for the adsorption/desorptionbeds. The detailed energy recovery schemes have been reported in a recent publication [23] where

3.0

3.5

4.0

4.5

5.0

50 150 250 350 450 550 650

Cycle time, s

Spec

ific

dai

ly w

ater

pro

dcut

ion,

kg/

kgsi

lica

gel

0.20

0.24

0.28

0.32

0.36

0.40

CO

P

Fig. 3. Performance of the four-bed adsorption desalination system under standard operation condition.

2.5

3.0

3.5

4.0

4.5

5.0

50 150 250 350 450 550 650

Cycle time, s

Spec

ific

dai

ly w

ater

pro

duct

ion,

kg/

kg s

ilica

gel

0.20

0.24

0.28

0.32

0.36

0.40

CO

P

Th=85 oC, T c=31oC, Tch =12.2 oC

Th=85 oC, T c=29.4oC, Tch =12.2oC

Th=85 oC, Tc=31 oC, Tch =14 oC Error bar

Fig. 4. Effects of the cooling water temperature and chilled water temperature on the system performance.

0.5

1.5

2.5

3.5

4.5

5.5

60 65 70 75 80 85 90Temperature, oC

Spec

ific

dai

ly w

ater

pro

duct

ion,

kg/

kg

silic

a ge

l

0.2

0.3

0.4

0.5

CO

P

water production at 180s water production at 300sCOP at180s COP at 300s

Fig. 5. Effects of the hot water temperature on the system performance.


the timing for the water valves, controlling the cold and hot water that emanate from the beds, areswitched in a manner such that their respective ‘‘hot’’ and ‘‘cold’’ fronts are correctly directed tothe hot or cold water mixing tanks, eliminating any unnecessary temperature fluctuation in thesetanks.The temperature profiles before and after the activation of passive heat recovery are shown in

the Fig. 6. The shaded areas bounded by these temperatures represent the portion of energy saved,typically up to 26% of the original heat input and a reduction in the energy rejection. The corre-sponding COP improvement due to heat recovery scheme is shown in the Fig. 7, and an improve-ment from 0.34 to 0.43 has been attained by such a scheme.

25

30

35

40

45

50

55

60

65

70

75

80

85

90

0 50 100 150 200 250 300 350 400 450 500

Time, s

Tem

pera

ture

, o C

Input energy saving

Reduction of energy rejection

Fig. 6. Temperature profile of the system hot and cooling water under standard operation scheme and passive heat

recovery scheme. (–h–) Hot water inlet temperature using two different operation schemes. (–m–) Hot water outlet

temperature using standard operation scheme. (–j–) Hot water outlet temperature using passive heat recovery scheme.

(–�–) Cooling water inlet temperature using two different operation schemes. (–�–) Cooling water outlet temperature

using passive heat recovery scheme. (–n–) Cooling water outlet temperature using standard operation scheme.

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

110 150 190 230 270 310

Cycle time, s

CO

P

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

Spec

ific

dai

ly w

ater

pro

duct

ion,

kg/

kg s

ilica

gel

Fig. 7. Comparison of the system performance under two different operation schemes.


4. Conclusions

An adsorption desalination plant has been investigated using a four-bed regeneration scheme.Based on the proposed standard rating conditions, the optimal specific daily water production(SWP) of 4.7 kg/kg silica gel has been obtained. The SWP yield from the plant may be furtherboosted up by adopting a higher chilled water temperature supply to the evaporator and a lowercooling water temperature to the designated adsorption beds. It has been demonstrated that thedesalination plant is also functional when the heat source temperature is lowered to 65 �C. The


other controlling parameter, namely, the switching time has been optimally determined at 40 s. Ithas been found that by simple implementation of the valve timing delay (with no hardware alter-ation) during the switching interval, a passive heat recovery scheme improves the COP by a sig-nificant margin. Lastly, it is emphasized that an adsorption desalination plant guarantees nopossibility of ‘‘bio-contamination’’ in the potable water and this is attributed to the both the evap-orative and desorption processes of the adsorption cycle.

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experimental investigation of an adsorption desalination plant using low-temperature waste heat

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