Desalination by solar heated membrane distillation

Desufznatron, 8 1 ( 199 1) 8 l-90 Elsevler Science Pubhshers B V., Amsterdam

DESALINATION BY SOLAR HEATED MEMBRANE DISTILLATION

P.A. Hoga#,

Sudjito”], A.G. Fane[“’ and G.L Momson[”

ill

Centre for Membrane and Separation Technology, School of Chemical Engineering and Industrial Chemistry

PI

Solar Energy Laboratory, School of Mechamcal Engmeenng, Umverslty

of New South Wales, PO Box 1, Kensington,

NSW, 2033, Australia

ABSTRACT This paper exammes the feasibility of a solar powered membrane distillanon plant for the supply of domestic drinking water in the arid/rural regions of Australia. The plant differs from conventional solar powered devices m its capacity to recover large propomons of the latent heat of vapourisation using conventional heat-exchange devices The plant has been designed and constructed using data obtamed from a computer simulation of the process. Prelimmary tests have shown the plant capable of achieving the required production capacity An economic sensmvity analysis has been used to select the optimum heat recovery. 1. INTRODUCTION Membrane Dlstlllauon (MD) has previously process but only economically feasible under 1s the cost of the energy needed to produce found to be compehtive in sltuatlons where where electnclty IS expensive

been assessed as being a techmcally viable certain conditions (1,2) The pnnclpal factor the thermal dnvmg force. Generally, MD 1s some source of waste energy IS available or

The and/rural regions of Australia provide one such semng, coupled with the need for improved water supply. These areas are charactensed by very low rainfall and an abundant source of solar energy. Families are isolated on farms often separated by hundreds of kilometres, and so large communal water punficatlon plants are not viable There IS rarely access to a central elecmaty supply and so each farm has its own generator *

Author to whom correspondence

should be duected 81

The domeshc water supplies are generally from lake or ground water and typtcally contain high levels of dissolved salts such as calcium carbonate, magnesium hydroxide and sodium chlonde as well as posstble mrcrobtal contaminants The result is a brackish water that 1s both unpleasant and unhealthy to dnnk The concept we have developed combines the MD process with a solar energy source tn a domestrc-size desalination unit Thus paper describes the design strategy, performance and econonucs of the process. 2. BACKGROUND 2.1 The MD Process MD IS a thermally driven membrane process m which a hydrophobic mlcroporous membrane separates a hot and cold stream of water The hydrophobic nature of the membrane prevents the passage of hqutd water through the pores whilst allowmg the passage of water vapour. The temperature difference produces a vapour pressure gradtent which causes water vapour to pass through the membrane and condense on the colder surface as shown in Figure 1 The result 1s a dtsnllate of very high purity which, unhke in convenaonal dtsttllauon, does not suffer from the entrainment of spectes whtch are non-volattle

Tf Warm Feed

Cool Permeate Vapour Flow

FIGURE

2.2

-

TP

1: THE MD PROCESS

The Solar Powered Membrane Distillation (SPMD) Process

A flow dtagram of the SPMD process 1s shown In Figure 2 A hot and cold stream are contacted counter-currently m the MD module, at whtch pomt mass and energy are transferred from the feed to the permeate stream. The module contams hollow frbre membranes trghtly packed rn the shell m order to enhance the heat transfer charactenstlcs of the shell side (3) The streams are recontacted in the main heat recovery heat exchanger (HXl) where energy IS transferred back from the permeate to the feed This form of energy recovery has been suggested prevrously (1,2). A plate-and-frame exchanger has been used in this study because of tts flextbtltty m the selectton of heat transfer area

83

r-

1 r

1 I ’

Pl

MD

Module

l-’

I---

Pt,PZ,P3,P4 _-_--

Cooler

Circul

pumpr

Feed Permeate Cooler

Tank

Am.

I Heat Exohanger II

*

Heater

j

“eatE.hanger:(~____j \ ~~~Fpl~~Tp,ANT

To provide process contmulty the feed and permeate must be returned to their ongmal temperatures before being recycled to the MD module The feed stream IS reheated using energy from a solar collector which, in this case, 1s isolated from the feed circuit Water IS taken from the feed tank, IS heated through the collector network and returns to the tank allowing the flow conditions through the collector to be set independently of those chosen for the MD Circuit The tank enables the system to store energy by acting as a thermal capacl.cltance The temperature of the permeate stream IS reduced by heat exchange with coolmg water in another plate-and-frame heat exchanger (HX2) The coohng water also acts as the feed make-up, and so this energy IS also substantially recovered In practice this water would come from the mam water storage tank of the farm As the process continues the accumulating product water IS removed from the permeate stream The feed stream 1s depleted of water by the dlsullation, and becomes more concentrated m the retained impunties This build-up must be hmlted, and this 1s controlled by a bleed stream 3. PILOT PLANT DEVELOPMENT 3.1 Simulation A FORTRAN program has been wntten to simulate the MD process, based on theory previously developed by Schofield et al (4) The membrane length IS dlvlded mto segments to calculate the mass transfer, temperature and pressure gradients along the fibres An iterative solution procedure is used to solve the mass and energy balances m the dlrectlon of the feed stream As the feed and permeate are counter current, the pressure and temperature dlsmbunons along the permeate stream must also be solved lteratlvelv in order to match the specified inlet condmons

84 3.2 Module Performance The simulation has been used to provtde desrgn mformatlon with respect to the MD module, by predrctmg the performance of the system under vanous condltrons Ftgure 3 shows the benefits of operating at htgher feed temperatures and flowrates with a constant membrane area. This mformation IS based on the pilot plant which has a 0 17m2 module consisting of 1100 fibres (0.17m length, 0.3mm 1.d and 0.6mm o.d.) with a pore stze of 0.22 micron and 70% porosity The non-hnear increase m flux with mcreasmg temperature reflects the exponential increase m the vapour pressure whrch provides the dnvmg force. The effect of a higher flowrate 1s to increase the heat transfer coefficient, and thus reduce the effect of temperature polanzatton Thus means that the temperatures at the membrane surface more closely approximate that of the bulk streams, and thus the trans-membrane temperature difference 1s greater This produces a greater dnvmg force, and consequently enhances the flux Accompanying these increases m flux is a decrease m the heat loss factor This represents the fraction of the total heat transferred across the membrane that does not contnbute to the flux. Thus occurs primarily by conductron of heat through the membrane structure, and through the air and water vapour in the pores Heat transfer by conductton increases approximately linearly with temperature gradrent, unlike the vapour pressure dnvmg force and thus flux Thts means that although more heat IS lost by conductron at higher temperature difference, rt 1s less as a proportton of the total heat transfer Thts trend IS also deptcted m Figure 3 Figure 4 shows the effect of increasing the membrane area wrth constant stream Inlet temperatures and flowrates. The increase in area tn this case 1s achreved by mcreasmg the length with the number of fibres being set at 1100. The overall effect of larger membrane area 1s that a greater amount of heat 1s transferred from feed to permeate As the approach temperatures (inlet or outlet temperature differences) become closer the dnvmg force across the membrane 1s reduced, and so the flux drops This drop In flux 1s offset by the increase in area and so the productton rate (defined as Productron Rate = Flux x Area = kgIhr) IS observed to climb. As the area becomes larger the rate of increase in the productton rate dtmnushes unul the point where a further increase tn area has negligible effect One of the mam advantages of the process is its ability to recover the latent heat of vaponsatron for re-use The proportion of heat transferred dunng dtstrllatton that can be re-used depends pnmarily on the approach temperatures of the streams m the MD module. The membrane area and flowrates both affect this, as shown in Frgure 5 under condmons of constant feed inlet temperature The heat recovery factor 1s defined as the maximum possible heat recoverable in the main heat recovery exchanger divided by the heat transferred m the membrane module (H R F = Qnx,/Q& Larger membrane areas and lower flowrates both lead to increased contact time in the module, and this gives nse to closer approach temperatures and thus more recoverable heat Companson of Figures 3 and 5 reveals one of the inherent trade-offs in MD That IS whilst the higher flowrates and smaller membrane areas yield higher flux, the posstbtltty of heat recovery and re-use is reduced The condmons of chorce then become a function of the apphcatton.

40

06 FLUX

= 30 r cv

-

0 5 llmln

-

2 I/mln

1

E g

X 3 A IL

I/mln

HEAT LOSS

20

05l/mln

_

1 I/m&n -

2 I/mln

10 i

i 40

30

50

60

FEED

FIGURE

70

60

90

02

100

TEMPERATURE

3: FLUX

AND HEAT

LOSS

VS FEED TEMPERATURE ,5-

I

L

FLUX

c

I

i5

45 3a

?

_0_

0 5 I/mln

-

1 I/mln

-

2 llmin

a PRODUCTION _0_

0 5 limln

-

2 I/mfn

1 Ilmln

0 01

02

03

MEMBRANE

FIGURE

04

AREA

05

06

(m2)

4: FLUX AND PRODUCTION VS MEMBRANE AREA

RATE 1

30

0 75 :

20

ix

FLUX

g-i-

0 5 Ilmin

8

1 Ilmln 2 llmln

ii-=-

E 05

Z

t Y

y

LL

10

0 25

a 3=k

0

0 01

02

05

MEMBRANE

FIGURE

AR”E4A (m2)

5: FLUX AND HEAT RECOVERY VS MEMBRANE AREA

06

2

MHR _6_

0 5 Ilmln 1 I/mln 2 Ilmln

86

3.3 The Integrated System The pilot plant (Figure 2) has been modelled by combmmg the module slmulanon program with the TRNSYS solar simulatton program (5) and tts extensions (6) The operating conditions, module specifications, and solar collector type were kept constant during the senstttvtty study The module spectficattons chosen were as for Ftgure 3 Heat exchanger nze, defined by the product of the heat transfer coefficient and area (UA), was vat-ted from 100 to 2000 kJ/hrK for I-IX1 The performance measure chosen was the productivity, defined as the kg water produced per unit of external energy provided (kg of permeate per MJ energy from the solar collector) Figure 6 indicates that mcreasmg the size of the mam heat recovery exchanger reduces the need for external energy, as would be expected. Thts trend 1s shown for two sizes of the cooling heat exchanger, HX2 The effect of I-IX2 1s srmrlar where the larger size increases the mternal heat recovery In each case there reaches a pomt where a further increase in the stze of I-IX1 has little effect on the productivtty The capacity of the plant 1s affected by the efficiency of the collector m convertmg the incident radiation into sensible heat. These efficiences are m the order of 20-30% for a flat plate and 60-70% for an evacuated tube collector at 70°C but vary with the collector type and the chosen operating temperature. Figure 7 shows the system performance using a flat plate and evacuated tube collector over a range of feed temperatures. The first trend to note is that the performance of the system with the flat plate collector drops off at around 7O”C, after which heat losses become sigmficant enough to reduce the producttvtty In contrast, the evacuated tube increases m efficiency past 100°C A smaller collector area implies less external energy input and thus the need for greater heat recovery if the feed temperature 1s to be mamtamed With the capactty of the other process units held constant this is accomplished by lowenng the flowrate The net effect 1s that proportionally more energy is transferred to, and recovered from, the permeate stream The trade-off to this 1s a lower flux and hence a lower production rate 3.4 Simulated Operation In the field the plant will not have the steady energy input of an elecmcal heatmg device Rather, the feed temperature will depend to a large extent on the diurnal vanatton in insolation Figure 8 shows the simulated vat-ration in production rate over an eight hour day-time operation, using statistically collected radiation data for Sydney m summer This mformation is based on a module with the same number and type of fibres as described previously Comparative data for the actual pilot plant under the same condmons are also graphed (with energy input from a heating element) The chosen operatmg condmons were for an average feed temperature of 70°C and flowrates of 30kg/hr. The simulation gives a reasonably accurate picture of the productton rate trend, although the figure shows that it typically predrcts 10% higher values than those determined experimentally This may be due to the heat losses expenenced in practice, or to tmperfect fluid dynamics in the membrane module.

;

7-

z 2

6-

* I5 F 0

54-

HEAT EXCHANGER 2

3 g

3-

-__t_

UA=lOOO UA= 2000

P r)-

L

0

1000

HEAT

FIGURE

2000

EXCHANGER

3000

1 (kJ/hr)

6: PRODUCTIVITY VS LOAD ON HEATRECOVERYEXCHANGERS

05 5m2 g

04

3m2

03

51-t-12

,” z

1

6

5m2

3m2

i= 0 a

02

;

01

1

n 00 30

40

50

FEED

FIGURE

7:

60

70

60

TEMPERATURE

PRODUCTIVITY

AND

SOLAR

90

100

(C)

VS

FEED

COLLECTOR

TEMPS

TYPE

1.4 1.2l.O0.8 -

0.6 0.4 -

-

SIMULATION

-

EXPERIMENTAL

0.2

J 6

FIGURE

10

12

TIME

14

(24hrs)

16

8: DIURNAL VARIATION PRODUCTION RATE

._

11)

OF

5m2

88

3.5 Economic Sensitivity Analysis Since the energy source is free, the optimum configuranon of process units 1s largely based on capttal cost That IS, the unit giving the required dally production with the lowest capital outlay becomes the prefemed option A senstttvtty analysis was camed out on the capital cost of a 50 kg/day plant using a computer. In this analysis the feed Inlet and exit temperatures from the membrane module were kept constant while the approach temperatures of the streams entenng the module and HXl were varied from 1 to 5 degrees The flowrate was maintained at 120kg/hr and ratio of the feed to bleed stream was 5 1 The duty of the membrane essenaally remained constant, since the diuly production was not vaned The estimates of costs of mdlvldual items are summansed m Table 1, and the total cost 1s plotted against the collector area in Figure 9 The heat exchangers are the most costly items and calculations have been made on a maximum and mmimum basis

TABLE 1 COST COMPONENTS

FOR SPMD PLANT

COST BASIS

ITEM solar collector membrane area heat exchanger piping pumps control devices tanks frame

$300 + $150/m2 $100 + $140/m2 $150 + $1000/m2 (max) $150 + $500/m2 (mm) $10/m 4 x $150 $300 $5/lltre

$300

Figure 9 indicates that the high internal heat recovery required by the use of a small collector area results in a steep nse in the capital cost, This is due to the large increases in heat exchanger areas required to obtain the closer approach temperatures As the collector area increases there IS less need for heat recovery and the capital cost passes through a tnmtmum value, only to nse again as the heat recovery IS further reduced The result IS an optimum value for a a-ans-membrane temperature difference of around 3-4”C, at this feed temperature This corresponds to a heat recovery of around 75% of that transferred m dlsallation Figure 9 also shows design shifts towards summanses the areas the optima m Figure

that by reducing the unit cost for heat exchangers the optimum using more heat recovery and less solar collector area Table 2 of collector, heat exchanger and membrane and the capital costs for 9.

89

4000

2000

-

I 1

0

.

-

HX = $1 OOO/m2

_t_

HX = $5OO/m2

1 2

.

I 3

*

1 4

’

05

1

.

1 6

.

7

SOLAR COLLECTOR AREA (m2) FIGURE

9: CAPITAL COST FOR 50KGIDAY PLANT VS SOLAR COLLECTOR AREA

TABLE 2: CAPITAL

4.

TMD “C

SOLAR m2

1 2 3 4 5

0.09 1.48 2 88 4.27 5 67

COST FOR 5Okg/day PLANT

MEMBRANE m2 4.63 2.60 174 1 26 091

HXl m2

HX2 m2

COST $ (max)

COST S (max)

2 13 0.98 0.61 0.42 0.31

002 0.03 004 0.05 0.05

4425 3430 3355 3524 3785

3350 2920 303 1 3292 3606

CONCLUSIONS

Solar powered membrane distillation has been found to be technically feasible A simulation model has been developed which succesfully combines programs for the simulaaon of the separate MD and solar components. This simulator predicts actual performances reasonably well The simulator has also been used to analyse the effect of design strategies on actual producttvity. The capital cost of the unit is very sensmve to the extent to which heat is recovered, especially above a heat recovery factor of about 0 8 The muumum capital cost requires heat recovery in the region between 60-80% To obtain this level of energy conservation the process condnions that must be used are generally those that result in fairly low MD fluxes. These are low flowrates and large heat tranfer areas, which lower the dnvmg force The excepaon is high feed temperatures which result m mcreased flux and proportionally less energy lost by conduction For the domesttc sized plant of 50kg/day the optimum configuration appears to be a solar collector area of around 3 m2, a membrane area of 1.8 m2 and a total heat exchange are? of 0.7 m2. The capital cost for this unit is conservatively estimated at $3500 (Aust )

90 5. ACKNOWLEDGEMENTS The authors would like to thank Memtec Ltd, for matenal support and advice, and the National Energy Research Development and Demonstrauon Counctl for financial support

REFERENCES 1. KSchneider ~~514-521.

and T.S.Van Gassel, “Membrandesallatton”,

Chem.Ing Tech. (1984), No 7,

2 R.W.Schofield, A.G.Fane and C.J.D.Fell, “The Effictent Use of Energy in Membrane Dtstillation”, Desalination. 64( 1987) pp23 l-243. 3 R.W.Schofield, P.A.Hogan, A.G.Fane and C J.D Fell, “Developments Dtsttllation”, Proceedings ICOM ‘90, ~728-730

in Membrane

4 R.W.Schofield, A.G.Fane, and C J.D.Fell, “Heat and Mass Transfer Distillation”, Journal of Membrane Science, 33( 1987), ~~299-3 13

m Membrane

5. S.A.Kline et al., “TRNSYS 12.1 Users Manual”, Umversity of Wisconsm Solar Energy Laboratory, 1983. 6 G.L.Morrison, “Extension for the TRNSYS Simulation Program (TRNAUS Version 9.0)“, Report No. 199O/FMT/l, School of Mechanical and Manufactunng Engmeenng, University of N S.W., 1990.