Desalination,76 (1989)177-187 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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OPTIMUM DESIGN FOR A HYBRID DESALTING PLANT

I.S. Al-Mutaz, M.A. Soliman and A.M. Daghthem

Chemical Engineering Department, College of Engineering, King Saud University, P.O.Box 800, Riyadh 11421, Saudi Arabia.

ABSTRACT

Dual purpose multistage flash desalination plant provides fresh water with

low cost at high desalting capacity, but requires high installment cost. Two

stage reverse osmosis desalination plant requires only half of the multistage

flash installment cost while producing water with comparable price. By com-

bining sea water reverse osmosis plant with the dual purpose multistage flash

plant, the capital and operating cost can be reduced and the excess power can

be efficiently utilized.

The design parameters for such a hybrid plant will be the applied pressure

and the recovery of the reverse osmosis plant and the number of stages and heat

transfer areas for the multistage flash plant. The objective is to minimize the

cost of water satisfying maximum total dissolved salt.

Different cost scenarios are suggested and their effect on the optimum

parameters are investigated. It is concluded from this study that the savings

obtained from scaling-up is more than that obtained from hydridization.

INTRODUCTION

About 67.6% of the total desalted capacity installed are of the multistage

flash (MSFl type. Moreover, the MSF plants accounts for over 84% of the large

size plants erected so far.

Not until 1970 when reverse osmosis (ROI plants were commercially used in

sea water deslating. RO plants account now for 23% of the total desalted capa-

city.

The total RO plant capacity is 2.3~106 m3/d (603 mgdl. The first large

municipal RO desalting plant of 570 m3/d (150,000 gpdl capacity was in

Greenfield, Iowa, U.S.A. and was built in 1973. The largest sea water RO plant

OOll-9164/89/$03.50 0 Elsevier Science Publishers B.V.

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is in Malta with a capacity of 20,000 m3/d (5.3 mgdl. A plant of twice this

capacity is being built in Bahrain. However, in recent years, RO has become a

competitor to MSF. The rate of improvements and innovations in RO desalination

processes is remarkably high with respect to other desalination processes spe-

cially MSF.

There are about 1483 desalination units operating in the Arabian Gulf coun-

tries, as illustrated in Table I. MSF accounts for 86.5% of the desalted capa-

city. RO only accounts for 10.7%. However, there is a recent trend toward the

use of more RO in sea water desalination either for new plants or inconnection

with the present MSF plants. The following is an optimization study for com-

bining RO/MSF plants for obtaining product water at lower prices.

Process Description: MSF/RO

MSF plants often use low pressure steam as an energy source. The energy

consumption in MSF plant depends on the distillate flow rate and the plant per-

formance ratio. Typically 3.7 KWh are consumed in large MSF plants per one

cubic meter of produced water.

RO plants are operated by electrical power to derive the high pressure

pumps and other plant auxiliaries, mainly the pretreatment processes. As

illustrated in Figure 1, RO power consumption depends mainly on water recovery

and the working pressure. Typically 9.7 KWh of electric power is consumed in a

30% recovery RO plant per one cubic meter of produced water without energy

recovery. If an energy recovery turbine of 80% efficiency is used, the energy

requirement will fall to 6.5 KWh/m 3(I,3).

Figure II shows the simplified MSF and RO process scheme. The following

balance equations are obtained for MSF process:

- Overall material balance

WF i Wrj = Wd + Wbd + W,o + W6I

- Overall salt balance

CF WF + Crj Wrj = Cbd Wbd + Cro Wro + WBI CF

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Table I

Desalination Inventory of the Arab Gulf Countries

Country No.of Capacity 8 of Plant type, % share

the units mgd t.d/a world MSF vc RO EO MED

Saudi Arabia 874 707.4 2.98 30.0 80.7 0.5 16.2 2.60 --

United Arab Emirates 279 288.0 1.09 11.0 95.5 1.6 1.8 0.55 0.25

Kuwait 99 269.5 1.02 10.2 98.3 - 0.9 0.50 --

Qatar 47 81.9 0.31 3.1 97.9 0.7 - - 0.90

Bahrain 143 68.7 0.26 2.6 56.7 0.8 37.2 4.9 0.40

Oman 41 26.4 0.10 1.0 91.1 1.7 1.9 0.9 --

Total 1483 1521.9 5.76 57.9 86.7 0.65 10.7 1.8 0.15

2c

W4TER RECOVERY-R -1.

Figure 1 : RO Power Consumption with Energy Recovery (2)

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LP Steam

HP steam WP b

’ Wd

Fig. 2 : Simplified MSF/RO Diagram

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Overall material and salt balances on recovery section

WR = wrj + wr + Wm = Wr + Wbd + Wd

CR WR = Crj wrj + Cr Wr + Cm Wm

Heat balance on rejection section

TI - Tc = R(T4 - T5)

R- (WR - wrj) Cp

wF CPF

Heat balance on the recovery section

T2 - T6 = T3 - T4

Heat balance on the coolant stream

Wd ld'= WF CRF (T1 - T,) + WR CPR (Tp - T1)

Heat balance on the flashing stream

wd ld = WR CPR (T3 - T5)

Heat balance on the brine heater

w, x, = WR CPR (T3 - T2)

Heat transfer equations

T1 = Tc + NJ * 8j (Ts - c"j - Tc)

Bj = 1 - exP (-Uj Aj/wF CPF)

T2 = T3 - (T3 - Tc + Al)/(NR * BR + Nj Bj + R)

BR = 1 - exP (-uR &/wR CpR)

A1 = aR NR BR + (oj Nj Bj)/R

A2 = NR BR + Nj Bj

A3 = aR NR BR R + aj Nj Bj

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T5 = T3 - ((T3 - Tc) AZ - A3II/(NR BR + Nj Bj + R)

TS = T3 + ((I - BB)/BB))(T3 - T2)

BB = 1 - exp (-UB AB/WR CPR )

T4 = ((Tl - Tc) + R T51 l/R

Distillate produced:

Wd = (a WR CpR (T3 - Tg))/&

Steam needed:

WS = (WR CPR (T3 - Tz))/~~

Gained output ratio:

GOR = _!!!?__ WS

Area economy :

AT = NJ Aj + NR AR + AB

AC = _!%_ AT

RO process has few balance equatfons. These i ncl ude :

Overal 1 matesi al bal ante

WF = W + Wbd

Overal 1 sal t bal ante

CF WF = CW + Cbd Wbd

The sol vent flux

N1 = A (AP - &r)

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- The solute flux

Np = (Dz~/KR)(CW - Cp)

- Overall materia 1 balance

Wro = Wrj + Wp

- Salt balance

Let

Cro Wro = Crj Wrj + CP WP

WP =Y --

Wro

Wrj = 1 _ y

Wro

Cro = Crj (1 - Y) + cp Y

CW = Cro + Crj

2

B = nf/AP

Concentration factor

f = CW/Cro

Osmotic Pressure of solution

lTf = O-0385 Cro

Cwo 1000 - ~ 1000

Nl = A AP (1 - fB)Ys

Y = n/(1 t (16 A U r. % n/r+4 * 1.0133 * 106))

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rl= Tanh (( A U roD.0133 * lo6 r-i)? Q/ri))

((16 A U roD.0133 * lo6 rf)?R/ri))

The objective function to be minimized consists of production costs for MSF

and RO plants+the savings obtained from hybridization.

The fixed capital cost for MSF is correlated by:

CM = K (AT)

(wd)"'15

where AT is the total heat transfer area in m2 and Wd is the distillate in m3/d.

The operating cost is taken as:

COM=0.15*CM+Ws*Cst*8000* TS-40 + 85

where W, is m3/d of steam and Cst steam price per

0.006 * WM * 8000

m3 having a temperature of

125oc. The factor (T, - 40)/85 is used to take into account that the price of

steam decreases as steam temperature Ts decreases. The steam price is taken as

$ 2/MMEItu. The third term in the above expression is the chemical price used

for sea water treatment, $ O.O06/m3 of water treated.

According to Wade(z) the operating cost for RO is given by

C0R = 0.15 * (10.111 M ) + 2.39 * M0-8 + 0.808 * (M) + 1.41 * 1O-3 MP)

MS Y Y

* 1000 + 0.2 * 1000 * 10.11(__!!.._) + CRR + CWR

MS

where M = Wp and

MS = module capacity in - m3

day

The first term represents fixed investment related cost. The second term

represents the cost of membrane replacement. The third term is the electric

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power cost given by

CPR = 0.06 (( Op8 ) - (l-Y)* 0.8 (?___ (0.9 P-10) + Wrj - wr * 0.9 P)) . Wrj Wrj

* 0.0011 * M * fJ()oo

Y

The electric power is priced at $10.06 kWh. The above expression is used

when part of the brine refected from the RO is fed to the MSF. If more sea

water is added to the MSF than that obtained from the RO, the following

expression is used:

CPR = 0.06( p - (1-Y) * 0.8 *((0.9 P-10) + (Wr - wrj) * Y * lo * 1.25)) 0.08 M

* M * 8000 * o.0011 Y

The fourth term is for the cost of sea water treatment. The chemicals used

are priced at S O.O18/m3 water treated. Thus

CUR = 0.018 * WRO * 8000

Thus the objective function I to be minimized is

1 = c()M + COR

The following constraints should be

a) The TDS in the product water should

WP * CP + 50 * Wd <

satisfied:

not exceed 500 ppm,

500 (Wp + Wd)

b) The salt concentration in the MSF blowdown should not exceed 72,000 ppm,

(Wf - Wbl) * Cf < 72,000 * Wbd

c) The salt concentration in the brine stream from the RO should not exceed

63,000 ppm,

Cf Wro - Cp Wp Q 63,000 Wrj

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Parameters used in this study are as follows: The plant capacity 15 f+l gal

per day of fresh water, sea water inlet temperature 270C concentration 43,000

ppm.

For MSF plant:

Heat transfer coefficient in the rejection section = 450 Btu/hr ft2 oF

Heat transfer coefficient in the recovery section = 530 Btu/hr ft2 oF

Heat transfer coefficient in the brine heater = 460 Btu/hr ft2 oF

For reverse osmosis plant:

A = 8 * 10-6 cm/sec.atm

( D2M ) = 2 * 10-6 cm/set KS

membrane length = 75.0 cm

membrane seal length = 7.5 cm

fiber outside radius = 42 * 10-4 cm

fiber inside radius = 21 * 10-4 cm

u1 = brine viscosity = 3.54 * 10-3 gm/cm.sec

S = membrane area per module = 1.696 * 106 cm2

RESULTS

The parameter K in the capital cost formula of the MSF plant has been

changed to reflect the uncertainty about the relative cost of MSF plants with

respect to RO plants. For a maximum pressure in the RO plant of 80 atm and at

a value of K = 5770, the optimal solution was to build a MSF plant with a water

cost of S l.O8/m3. At a value of K = 6000, the solution was to build a RO

plant with a recovery of 0.32 and water cost of $ l.l/ms. In no case the model

suggests a hybrid plant. Thus it is clear that the savings obtained from

scaling-up the plants is more than that obtained from hydridization. When the

maximum pressure in the RO plant is dropped to 60 atm, the RO plant is not able

to produce water with a salinity less than 500 ppm. Thus the solution was

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either to have an MSF plant or a hybrid plant in which the function of the MSF

unit is just to bring the salinity to the 500 ppm limit. Thus for the case of

K = 7200, an MSF plant is to be erected with a water cost of $ 1.35/m3. If K =

9600, a hybrid plant is suggested with 7.8 FMG/day of RO capacity and 7.2 iWG

per day of MSF giving a water of $ 1.5/m3.

CONCLUSIONS

The hybrid plant concept is only useful to bring the salinity of the water

produced from RO plants to an acceptable limit. If high pressure membranes

(that can stand a pressure higher than 80 atm) are developed and become

reliable and as fuel cost increases, the reverse osmosis plants would certainly

replace the MSF plants. Wade(2) has already indicated that at a fuel cost of $

18/barrel water produced from MSF is cheaper whereas at a fuel cost of $ 27 per

barrel, water from RO plants is marginally cheaper. Thus we conclude that more

efforts should be given to scaling up MSF and RO plants and for developing high

pressure membranes.

REFERENCES

1. N.M. Wade, "Comparison of MSF and RO in Dual Purpose Plant", Paper present

at Saline Water Conversion Corporation, Riyadh, Saudi Arabia, 1986.

2. N.M. Wade, "RO Design Optimization", Desalination, 64, 3-16 (1987).

3. M.A. Al-Sofi, "Desalination Industry", Presented at Water Science and

Technology Association Seminar Held in Nov. 10, 1988 in Bahrain.