Selection of small desalination plants for specific locations — reverse osmosis, flash distillation, vapour compression

Desalination, 52 (1985) 145-165 Elsevier Science Publishers B . V ., Amsterdam - Printed in The Netherlands

145

SELECTION OF SMALL DESALINATION PLANTS FOR SPECIFIC LOCATIONS - REVERSE OSMOSIS, FLASH DISTILLATION, VAPOUR COMPRESSION*

KLAUS WANGNICK Wangnick Consulting Engineers, Kuhstedtermoor 19, D-2742 Gnarrenburg (F.R .G .) Tel . 04763-8088

SUMMARY Selection criteria are given for small desalination plants (2000-3000 m 3 / d) for specific locations using RO, MSF or mechanical VC processes . Of prime economic importance to customers, consultants and manufacturers are peripheral location conditions and operating suitability . Choice by the manufacturers is especially complicated due to the fact that the decision for small plants is left to the bidder . However, based on knowledge of local conditions, manufacturers can anticipate costs. Examples given show that under the assumed conditions the MSF process cannot possibly be the most cost-effective . This applies also to RO when the required product is high-purity process water instead of drinking water . For drinking water any choice between MVC and RO is dependent on energy costs and the capitalization factor .

1 . INTRODUCTION The paper analyses criteria for the selection of small desalination plants for specific locations and for the reverse osmosis, multi stage flash distillation and mechanical vapour compression processes . Although a comparative study will show that MSF distillation units, in terms of production capacity, far outmatch any process adopted in earlier desalination plants, there are at present three rival processes which have established themselves on the world market . Even today, large-scale installations with unit capacities of over 15,000 m 3 /d are still almost exclusively constructed to operate according to the principle of MSF distillation, because running procedures for such plants are

*Presented at the Symposium on Economics of Water Desalination Processes prepared by the Working Party on Fresh Water from the Sea of the European Federation of Chemical Engineering end Dechema, Bad Soden, 8-10 October 1984 . 0011 .9164/85/$03 .30

0 1985 Elsevier Science Publishers B .V.



146

K . WANGNICK

well known, the process involved is simple, and highly specialized components are not required - to name just a few reasons . Nevertheless, there is a clear trend towards the adoption of reverse osmosis also for such highcapacity plants . The situation is different for smaller, for the most part, standardized plants with unit capacities of approx . 2,000 m 3 /d. Depending on local conditions, it is feasible for plants of this size operating on the principles of MSF distillation, mechanical vapour compression and reverse osmosis to be run economically . In fact, large numbers of such plants have been put into operation and tested in actual operating conditions (Figs 1, 2) .

V VC

HIE HI ST F

.... a 1 O 0 0 0 0

173695 97683 4269 18441 16716 13165 K978,

100000 Cyncity (m'/d)

4&6% 3.3 11 .3% 4.9% 4.5% 3.5% 2.9% 7erteitages in mletlm to all pates

Fig. 1 . Nominall output of selected seawater plants as on October 7, 1984 of 0-1000 m' /d capacity.

H3F 19 It HIE VII ST F l% rld

I

I

I

O: C3 O 0 I 0

104770

3D000

370D3

44:000

507000

6m000

7437 35M 12332 8698 35536 31934 13165 6&0

700000 Capacity (a'/d)

4.8 3 .3 &8% 6 .3 2 .5% 23% .9% 0 0.4% tx as in relatim Wall pnoaus

Fig. 2 . . Nominal output of selected seawater plants as on October 7, 1984 of 0-5000 m' /d capacity .

In terms of economic operation of the type of plant required, peripheral local conditions as well as the direct cost of the plant are a prime consideration . There then arises the problem of selecting the most suitable operating process . This choice will be of concern to the customer or his adviser and also of course to the manufacturer of desalination plants if all three processes are represented in his product range . The manufacturer's problem of making the correct selection is further complicated by the customer generally having no preference for any one process, wishing merely to buy the installation which is most economical without possessing the knowledge required to reach a decision objectively . And where installations of approx . 2,000-3,000 M3 /d are concerned, consultants,



SELECTION OF SMALL DESALINATION PLANTS

147

too, tend more and more to leave the choice of operating process to the bidder. The manufacturer is used to assessing local operating conditions at first hand in order to select the most suitable type of plant . But naturally enough, with this first-hand experience one has the advantage of being able to appreciate the costs that will actually arise . The following is a summary of the assumptions underlying such an assessment and two concrete examples are given as illustrations .

2 . PROCESS DESCRIPTION

First of all, in order to make clear the distinctions involved, the three processes will be very briefly outlined . Multistage flash evaporation, MSF (Fig . 3)

Purified brine is pumped through the evaporator condensers and preheated at the same time . The brine is heated in the brine heater to the required temperature by the admission of thermal energy . The heated brine is conducted via a flow restrictor into the 1st stage of the evaporator . The flash vapour condenses on the condenser tubes and is collected in the distillate duct. The brine is conducted via the next flow restrictor into the following stage, where the process mentioned above is repeated . The concentrated brine and the distillate that has collected are pumped out of the last stage of the evaporator .

Fig. 3 . MSF evaporation flowsheet .



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K . WANGNICK

Reverse osmosis, RO (Fig. 4) The purified and chemically conditioned feedwater is brought by means of a booster pump to the necessary pressure which is determined by the osmotic pressure of the seawater . When the water flows through the membrane modules some of it permeates through the membrane, with the result that there is separation into concentrated brine and a permeate with a considerably lower salt content . Depending on the state of the feed water and the type of membrane used, one single passage through the membrane may be enough to produce drinking water or a second process of osmosis may be necessary in another stage . RO dnataon inaa

RO-stage I

RO-stage 2

US= . Boos

's MF Dump, stage 2

Brine

Fig . 4 . Reverse osmosis desalination unit flowsheet.

Mechanical vapour compression, MVC (Fig. 5) The vapour which forms in the evaporator is drawn off by a compressor and raised to a higher pressure . This vapour which has been brought to a higher pressure level is condensed in the evaporator tubes thus providing for further evaporation of the brine sprayed over the tubes. The concentrated brine and the distillate are pumped out of the evaporator and provide preheating for the feedwater by transmitting their heat to it via heat exchangers .

Each of the processes described will have its appropriately designed systems of feedwater intake (Fig . 6), feedwater pre-treatment (Fig . 7) and product water pre-treatment (Figs . 8 and 9) where necessary. If the product is to be drinking water, post-treatment will probably not be necessary for RO but will be required for MSF or MVC in order to conform to WHO requirements . However, these processes will produce high purity process water without, or with minimal additional treatment .



149

SELECTION OF SMALL DESALINATION PLANTS

Post-treatment of the product water may require a pump and a tank of suitable size . One limit of the desalination plant is the discharge connection for the processed product . Energy supply, outfalls and civil works have not been discussed because they cannot really be standardized .

ildasalinatidn unit

1!

®~ Circul Anti-scale system dosing

Distill

Seavater pump Brine r

Distillate pump

Fig . 5 . Mechanical vapour compression unit flowsheet .

Suction strainer

Suction pipeline

intake pump

Sedimentation basin Screen Intermediate basin

Fig . 6 . Feedwater Intake with pre-purification system .

Brine pump

Brine cooler



0

a

Palyelectrolyte

Ferric sulphate

Fe2ISOa )3 or

Ferric chloride

FeCl 3

opper sulphate

or CuSO4

a(OCI) z Calcium hypochlorite

Fig . 7 . Feedwater pre-treatment .

1

sand filters

Backwash

Destabilization tanks

agitators

reactors with

Flocculation

Cartridge filter

Seawater pump

Suction strainer



Pipeline to desalination unit

Filtration

Flocculation

Flocculation aids

Flocculants

Seawater disiwecton

Discharge nozzle

Product water tank

Reainerallzatlan

pH-value control

Disinfection

Blending

Fig . 8 . Product water treatment (1) .

I

NaICO 3 Sodlua hydrogen-carbonate

Catla Calciur chloride

I

NaOH Sodium hydroxide

Ce(BCI), CaicluMYpochlorite

I

Seawater or stage I permeate

Product water plnD



SELECTION OF SMALL DESALINATION PLANTS

151

Product water pump

Cation onchanger Anion ezchingerl Mlxed bed filter

Demineralization

Product water tank

Discharge nozzle

Fig . 9 . Product water treatment (2) .

3 . COMPARISON OF ECONOMIC EFFICIENCY

Example 1 Let us now consider costs of a typical plant (Fig . 10) . These will provide the basis of a comparative study of the economic efficiency of the three processes . A plant with a daily output of 2 X 500 m 3 and all the components necessary for operation independent of general power or vapour supply or central seawater intake structures was selected as the first example . The site of installation may be taken as somewhere on the North African Coast . The individual components are as follows : The seawater intake including a conduit of the required nominal width (total feedwater requirement in relation to production output by MSF approx . 14, RO approx. 3 .4 ; MVC approx . 2 .9) measuring about 300 m in length, sedimentation basins for the feedwater requirement of approx . 30 min, a bar screen and a travelling screen plant designed for the required feedwater flow and a sump basin for a 15 min feedwater requirement .



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K. WANGNICK

The treatment of the feedwater volumes in question with 2 seawater pumps (each operating at 100% capacity) with a common backwash filter, a dosing system for disinfectants (calcium hypochlorite for MSF and MVC, coppersulphate for RO) ; for RO systems : dosing equipment for ferric chloride and polyelectrolyte as a flocculant and as a flocculant aid, flocculation reactors with agitators, destabilization tanks and gravel-bed filters as well as a transfer conduit of the appropriate nominal width, measuring approx . 100 m in length .

Operating Expenses

Capital investment

E EROY FE ED NAT ER INTAKE 6 SCREENING

OUTFALL

FEEDNATER PRE-TREATMENT

CONSIMABLES (Lubricants, replacement parts, e .g . RO madelesl

ENERGY SUPPLY

DESALINATION UNIT

PRODUCT WATER POST-TREATMEN OPERATING 6 MAINTENNICE STAFF

CAPITAL COSTS

L CIVIL WORKS FOR PLANT BUILDINGS & GENERAL STRUCTURES

Fig . 10 . Cost survey . Expenses in running of the plants.

The 2 X 500 m 3 /d desalination units, each on strip footing (MSF and MVC with canopy top, RO in containers), MSF and MVC with dosing units for a scale-preventing polymer, RO with sulphuric acid installation for the setting of the required pH value . In some instances an energy-recovery turbine is coupled to the booster pump .



SELECTION OF SMALL DESALINATION PLANTS

153

Alternative 1 for the product water post-treatment (for drinking or industrial water) involving blending the product water with seawater in the MSF and MVC processes and with the permeate from the first stage in the RO process, disinfection with calcium hypochlorite, setting of the pH value by means of caustic soda and water remineralization with calcium chloride and sodium hydrogen carbonate . Alternative 2 for processing of the product water (process water) in the cases of MSF and MVC by demineralization plants fed with water having a value of 10 ppm (after demineralization less than 1 ppm) with reservoir and mixed bed filter ; in an RO process, by demineralization plants fed with water having a value of 500 ppm (after demineralization less than 1 ppm) with cation exchanger, anion exchanger and mixed bed filter . Both alternatives include a product water tank with a capacity of 2,000 m' . Approx . 150 m long open concrete outfall channel designed to take the maximum feedwater volume if necessary (i.e. designed so as not to overflow even if the product is rejected to the channel) . Independent energy supply with diesel generators for the requisite connected loads ; i.e., 1 X 380 kVA for MSF, 2 X 575 kVA for RO and 2 X 780 kVA for MVC, including the necessary distribution systems . Supply of diesel oil by means of 7-day tanks (MSF 75 m3, RO 21 m', MVC 35 m') as well as an oil transfer and distribution station . The civil works for plant buildings and general structures comprising control rooms, switchgear spaces, day rooms for operating and maintenance staff, a small workshop, a storeroom for replacement parts and chemicals, as well as a hard approach road and fencing . 90% plant availability was assumed for the calculation of diesel fuel and chemical requirements . The results of the cost analysis are set out in Table I. No doubt many people will be surprised that the cost of the intake exceeds the cost of the desalination unit itself, but this is, in fact, the case . Of course, the cost of the intake is largely dependent on local conditions . It may be very much less or considerably more than the figures shown . The important thing to remember is, that the cost of the intake will be highest in the MSF plant because of the large feedwater requirement, and will be virtually the same in RO and MVC plants. As far as pre-treatment investment costs are concerned, the RO process is decidedly more expensive than the two others, because the surface water assumed for this study necessitates expensive flocculation. As regards the desalination units themselves, the RO plant is by far the most economical, whilst the MVC unit is slightly less expensive than the MSF equipment . There are no great differences in any of the processes as far as the two alternatives in the post-treatment of product water are concerned, apart from the production of process water by means of RO . The high costs that



154

K. WANGNICK

TABLE I COST SURVEY -EXAMPLE 1 (DM) MSF

RC

Drinking

Process

MVC

Drinking

Process

Drinking

Process

Investment costs Intake with prepurification Feedwater pretreatment Desalination unit

6,039,000

6,039,000

5,183,000

5,183,000

6,157,000

5,167,000

306,000

305,000

568,000

566,000

237,000

237,000

4,987,000

4,987,000

3,076,000

3,076,000

4,651,000

4,651,000

Product water treatment

825,000

659,000

873,000

1,299,000

826,000

659,000

Outfall

314 .000

314,000

282,000

282,000

274,000

274,000

Energy supply

445,000

445,000

1,068,000

1,068,000

1,322,000

1 .322,000

Civil works for general structures

1,523,000

1,523,000

1,523,000

1,523,000

1,623,000

1,523,000

14,438,000

14,272,000

12 .571,000

12,997,000

13,989,000

13,823,000

1 .709,000

1,709,000

494,000

494,000

737,000

737,000

126 .000

55,000

210,000

588,000

89,000

15,000

71,000

70,000

410,000

415,000

72,000

71,000

Labour

514 .000

514,000

514,000

514,000

514,000

514,000

Capital

1,585,000

1,567,000

1,380,000

1,427,000

1,536,000

1,518,000

Total

4,005,000

3,915,000

3,008,000

3,418,000

2 .948 .000

2,855,000

Total

Running costs Energy Chemicals Consumables

will be met with here are the result of the expensive process of demineralization . The cost of the outfall has been neglected in the cost analysis of the three different types of plant . Since most of the energy required in a MSF plant is thermal, costs in connection with the energy-supply plant the MSF process involves will be considerably lower than for RO and MVC plants . Apart from this, the high output of individual units in RO and MVC plants, with the resulting high starting current level necessitate increased connection ratings which constitute a significant factor in the cost analysis . The civil works for general facilities will be virtually the same for all three types of plants . It can be seen from the above that the total investment costs for the RO plant will always be less than for the thermal plants, of which the MSF plant is the more expensive . The picture is rather different when it comes to run-



SELECTION OF SMALL DESALINATION PLANTS

155

ning costs . In the MSF plant energy costs (always the cost of diesel oil, in this study) will not only include the oil consumption of the diesel generators but also the oil consumption of the burners for the hot water boilers . Bearing in mind the high capital outlay, the running costs of the MSF plant are so high, even if diesel is estimated at a relatively low price, that it cannot be considered the most economical plant in this kind of set-up . Further comparison between RO and MVC shows that energy consumption costs are higher for MVC than for RO . However, this is compensated for by the minimal consumption of chemicals (of crucial importance, particularly in the production of process water) and the minimal cost of expendable materials in an MVC plant . Especially the replacement required for the modules (1/3 of the modules every year) constitute a major cost factor in RO plants. It should also be noted that 150 (for drinking water production) or 476 (for process water production) tonnes of chemicals per annum are required for RO, as compared to 20 and 53 tonnes per annum for MVC . European prices have been assumed for the costs of the chemicals ; freight rates are for North Africa. Any further transport of the chemicals would raise the running costs of an RO plant considerably, since even in this example transport costs are of the same order as purchasing costs . The cost of employing staff for the running and maintenance of all three types of plant can be taken as the same in all cases examined . The assessment of capital costs is based on a 15-year period of depreciation at an interest rate of 7% (capitalization factor : 0 .1098) . This example shows that, in spite of the higher investment required, the MVC plant produces drinking and industrial water more economically than the others, although the RO unit comes a very close second . However, as far as high-purity water is concerned, the MVC far outmatches the other systems in terms of production costs . Given the values shown in Table I, further alternatives can now be investigated relatively easily and quickly by varying individual items . Leaving aside the cost of employing staff, which in practice is not affected by the particular process adopted, the decisive factors in an assessment of running costs are capital outlay and energy costs . In Fig . 11 these two parameters are varied . Each process appears as a plane in this three-dimensional diagram . Running costs are set out vertically, in ascending order from top to bottom, and so the most economical process is always the one whose plane is above those of the others in the appropriate area . The diagram shows clearly that, under the assumed conditions, the MSF process cannot possibly be the most cost-effective . The same applies also to RO when the required product is high-purity process water . As far as the production of the drinking water is concerned, any choice between MVC and RO is dependent on energy costs and the capitalization factor . In Fig . 12 the respective areas of the MVC and RO planes lying uppermost are projected onto the base . A very simple, quantitative comparison



156

K . WANGNICK

can now be made between the two processes . Of course, the dividing line between them will not be absolutely clearcut . The two processes must be considered equally efficient over a relatively broad area . A final choice between them will only be possible after exact analysis of the actual peripheral conditions .

I 0 2 4

WAI

Fig . 11 . Three-dimensional diagram of running costs .



SELECTION OF SMALL DESALINATION PLANTS

157

Fig . 12 . Product : drinking water .

Fig . 13 shows the projection of the planes which appear when high-purity process water is to be produced at identical conditions . In the circumstances delineated, the MVC plant then represents the most cost-effective solution .

0

1000 I

0.20

1500 0.70

0.10

MVC

SF 1 -

DO 0

500

I 1000 Diesel fuel DM/t

0.0 1500

Fig. 13 . Product : process water .

Figs . 14 and 15 illustrate comparable cases, but here it was assumed that a very simple, i .e . negligible cost intake system can be adopted . MSF plants, too, become economical now, though in a very narrow region . In Figs . 16 and 17 an energy-recovering turbine driving the booster pumps of an RO plant is taken into account, in addition to an expensive intake system . As compared to the alternative represented by Fig . 12 the cost-effectiveness region for RO plants producing drinking water is broader here . Even when producing process water, the RO plant occupies a sector . These condi-



K . WANGNICK

158 o

1000 l

0.20

MSF"

1

1500 0.20

I I I i

0,10

MVC

RO

010

I 1 q0

10

0

w0

1000 Diesel

fuel

1500

DM/t

Fig . 14 . Product : drinking water .

000

two

010

0.20

MSF 7 I I 1

0 0

I 0

-0.10

MVC

I

I I I 1 OA

"u l AO

0,0 500 Diesel

1000 fuel Dm/t

Fig . 16 . Product : process water .

S0o

1000

0

c

C)

u

0.0 0

500

1o00 Diesel fuel

Fig . 16 . Product : drinking water.

Dm/t

1500



SELECTION OF SMALL DESALINATION PLANTS 0

500 1

159

1000 1

0.0 f 0 5oo

a0

l 1000 Diesel, fuel DM/t

1500

Fig. 17 . Product : process water.

a0 U 0.0 1

500

1000 Diesel fuel DM/t

150p

Fig. 18 . Product : drinking water .

0

500 I

0,20

100D I

1500 0,20

MSF

MVC

0 .10

0,10

RO

K

U 0,0

nv ro

I 1 500 1000 Diesel fuel DM/t

Fig . 19 . Product : process water .

0,0 1500



160

K . WANGNICK

tions change but slightly when the costs for an intake are neglected (Figs . 18 and 19) .

Example 2 The second example is along the same lines as the first, but deals with a plant which is annexed to a power station . This means that independent power supply is no longer required and that bleed steam from the turbine installation can provide the thermal energy for the MSF plant . The cost of providing electric energy is offset by the estimated prime cost, whilst 10% of the production costs are assumed for the supply of steam . A cost analysis is given in Table II . In this case all three processes are almost equally cost-effective in the production of drinking or industrial water, with the MSF plant coming out slightly better than the other two . If the required product is process water, reverse osmosis can be passed over as a cost-effective process for the reason already given (the high cost of demineralization and chemicals for regenera-

TABLE II COST SURVEY - EXAMPLE 2 (DM) MSF

MVC

RO

Drinking

Process

Drinking

Process

Drinking

Process

Investment costs Intake with prepurification Feedwater pretreatment

6,040,000

6,040,000

5,183,000

6,183,000

5,157,000

6,157,000

305,000

305,000

566,000

568,000

237,000

237,000

4,987,000

4,987,000

3,076,000

3,076,000

4,651,000

4,651,000

Product water treatment

825,000

659,000

873,000

1,299,000

825,000

659,000

Outfall

314,000

314,000

282,000

282,000

274,000

274,000

Desalination unit

Energy supply Civil works for general structures

-

-

-

-

-

-

1,523,000

1,523,000

1,523,000

1,523,000

1,623,000

1,623,000

13,994,000

13,828,000

11,603,000

11,929,000

12,667,000

12,501,000

Energy

256,000

256 .000

359 .000

359,000

536,000

536.000

Chemicals

126,000

86,000

210,000

668,000

89,000

15,000

67,000

66,000

400,000

405,000

80,000

59,000

Labour

513,000

513,000

513,000

513,000

613,000

613,000

Capital

1,536 .000

1,518 .000

1 .263,000

1,310,000

1,391,000

1,373,000

Total

2,498,000

2,439,000

2,746,000

3,195,000

2,589,000

2,496,000

Total

Running costs

Consumables



SELECTION OF SMALL DESALINATION PLANTS

161

0 20

Steam DM/t Electric energy DM/K'wn Fig . 20 . Three-dimensional diagram of running costs as a function of energy costs .

tion), whilst for the thermal plants the same applies here as for the production of drinking water . In this example too, the three-dimensional representation of running costs as a function of energy costs has been chosen (Fig . 20) . The parameters are the cost of electric energy and vapour, respectively. Cost of an intake is included . As in the previous example, the RO process can be passed-over in favour of the thermal processes, as far as the production of process water is concerned (Fig. 22) . However, as shown in Fig . 21, the most cost-effective process for the production of drinking water will in any case be decided by the cost of energy alone .



162

K . WANGNICK

MSF 0 .2-

/ RO

IJ

0,0

1D

0

Steam cost Dm/t

Fig . 21 . Product : drinking water. 0

Fig. 22 . Product : process water .

10

10 1

0,3

MSF 0 .2-

X

VC / /

S

MVC

W

/

D,0 ' 0

1 10 Steam cost ors/,

Fig . 23 . Product : drinking water .

0,0 20

0,0 0

1 10 Steam cost Dm/t

Fig . 24 . Product : process water.

0,0 20



SELECTION OF SMALL DESALINATION PLANTS

163

If a capital cost factor of 0 .2 (e .g . 10 years depreciation, 15% interest) is assumed for both alternatives, conditions will be those shown in Figs . 23 and 24 . The sector for RO plants expands owing to the comparatively low capital investment . This still applies when the RO plant is equipped with an energyrecovering turbine (Figs . 25 and 26) . The RO region becomes even larger, if a capital cost factor of 0 .2 is applied (Figs . 27 and 28) . In conclusion it may be said that the peripheral conditions of operation will dictate which of the three processes is the most economical . In order to make the correct choice, it is necessary to investigate what costs will actually arise . In many cases where typical peripheral conditions are obvious, a preliminary selection can be made with the aid of the information provided in this work . Of course, further development of the processes is likely to entail some changes to the situation . Mention can be made of the further development of modules or the use of energy-recovery plants - even in comparatively small standard reverse osmosis units . The preference that MSF still enjoys in all applications considered, is certainly due to the high reputation as a mature and reliable process it has acquired in the past 26 years since its introduction . 10

0

20 0,3

0.3

MSF

RO

3 a 0

0,2 -

-0.2

u

r

0'1- M c

0,1

--

.

w

/

MVC

/

0,0

r /& /

i 10 Steam cost

0,0 20 or/t

Fig . 25 . Product : drinking water .

Fig . 26 . Product : process water .

T

4 4

MVC 0,0

0,0 1

0

10 Steam cast cm/t

Fig . 27 . Product : drinking water .

20



SELECTION OF SMALL DESALINATION PLANTS

0,0

0.0 0

10 Steam cost

20 DMA

Fig. 28 . Product : process water.

Fig. 29 . MSF desalination plant with 30 000 m'Id capacity at Ez-Zuetina, Libya .

165