day desalination plants

Desalination, 64 (1987) 17-50 Elsevier Science Publishers B.V., Amsterdam -

17 Printed in The Netherlands

AN ECONOMIC COMPARISON OF 2 x 1000 ma/DAY DESALINATIOM PLANTS by

HW GREIG, C Eng, MIMechE Senior Thermal Systems Engineer Met-z and McLellan (Consulting Engineers) Newcastle upon Tyne, England

and JW WEARMOUTH, BSc, MIChemE, MIWES Associate and Chief Chemist Met-z and McLellan (Consulting Engineers) Newcastle upon Tyne, England

SYNOPSIS The paper describes an economic comparison made for a specific Middle East site to determine the most economical type of 2 x 1000 ma/day desalination plant to produce desalinated water from sea water. The results and conclusions of the comparison are presented, and the methods used in the comparison are discussed, and a method is given for making similar but aooroximate comoarisons for other sites,usinq the results of this paper and specific site information and costs peculiar-to other sites. An example of an approximate assessment made for a different Middle East site is given in the appendix. The comparison compares multi-stage flash, multiple effect, vapour Each type of plant and the variants compression and reverse osmosis plants. that are relevant to the comparison are described briefly and illustrated diagrammatically in the appendix.

INTRODUCTION The paper describes an economic comparison made for a specific Middle East site to determine the most economical type of desalination plant to produce 2000 m3/day of desalinated water from sea water using two plant units or modules each with a capacity of 1000 m'/day (2 x 1000 mj/day). The results and conclusions of the comparison are presented, and the methods used in the comparison are discussed, and a method is given for making similar but approximate comparisons for other sites, using the results of this paper and specific site information and costs peculiar to other sites.

An example

of an approximate assessment is given in the appendix. The comparison compares multi-stage flash (MSF), multiple effect (ME), vapour compression (VC) and reverse osmosis (RO) plants.

Each type of plant

and the variants that are relevant to the comparison are described briefly and illustrated diagrammatically in the appendix. The specific site for which the comparison was made has once-through MSF plants in operation, and their operational record is described qualitatively as excellent by the owner.

OOll-9164/87/$03.50

0 1987 Elsevier Science Publishers B.V.

18 THE OPTIMISATION METHODS Optimisation using Computerised Mathematical models was considered, MSF and RO models existing within the authors' firm; but the development of models for the other type of plant in the comparison, including their variants, would have taken more time than was available.

Further, as explained in the Cost

Procedure section, such models serve more purpose after the plant type has been selected. An optimisation method that uses costs and technical information from manufacturers of different types of plant was used.

Manufacturers were

requested to quote budget capital and erection costs for plants conforming in design to a specification that detailed all mandatory technical requirements relating to performance, materials, site conditions and extent of plant.

The

specification also provided costing information for design optimising purposes. To allow capitalised operational running costs to be calculated, the manufacturers were requested to give all technical parameters needed to fully describe plant operational performance including chemical, energy, andcomponent replacement requirements. Quotations were requested from at least two different manufacturers of the same type of plant, to highlight idiosyncrasies and to reduce errors to a minimum. Manufacturers were approached informally, before requesting quotations, and asked for their co-operation in the optimisation. enthusiastically.

Most responded

Some, in the course of the preparation of their quotation,

surprisingly, attested to the quality of the specification, saying that it was rare to receive a specification with a budget cost request that allowed them to optimise a plant design without having to make a single assumption. On receipt of the various quotations total capitalised costs were calculated as explained in the Cost Calculating Procedure section of this paper.

The

lowest total cost indicating the optimum or most economical type of plant for the specific site under investigation. THE SPECIFICATION Design specifications were sent with the enquiries requesting quotations from manufacturers of well proven, different types of desalination plant. They had three parts

: general, specific and requested information.

The general part that related to each type of plant is reproduced here, except for the specific site costs which are shown in the Cost Calculating Procedure section of the paper to avoid repetition. The essential requested information part (together with the information received) is shown in the Optimisation Results section of the paper.

19

To keep the paper

Each specific part related only to one type of plant.

However, the

reasonably short, the specific parts are not reproduced.

description of each type of plant in the appendix includes the essential technical requirements of the specification for that plant. GENERAL SPECIFICATION Oistillate 2

Number of streams Flow per stream

1000 m3/day

TOS

3 mg/l max

Distillate extraction pump generated head

40 m

Sea water deg C

Maximum operating temperature

90

Temperature range over year

15 to 26 deg C

Temperature range over year using power plant condenser CWoutlet

sea water

TDS

22 to 40 deg C 40 300 mg/l

Chloride

22 300 mg/l

Sulphate

2 790 mg/l

Alkalinity (P)

17 mg/l

Temporary hardness

143 mg/l

Permanent hardness

7 390 mg/l

Conductivity Suspended solids

56 000 micro mho/cm

:

normal 3 mg/l in storms rising to 300 mg/l

Steam supply and condensate return (if required) Pressure

3.5 bar g

Temperature

160 deg C

Flow available

17 tonnes/h approximately

Condensate return extraction pump generated head

32 m

Electric supply Voltage

6.6 kV, 380 V

Frequency

50 Hz

20 Compressed air supply Pressure

6.5 bar g

Flow

unlimited

Availability of plant

85 per cent minimum

Optimisation costing information The following costing information shall be used to determine the desalination plant operational costs which together with the capital costs shall be used to determine the optimum type of plant for the specific Middle East site under consideration. Interest rate

8 per cent

Interest period

30 years

Plant availability

85 per cent

Specific site costs (energy, chemical and component replacementcosts) are shown in Table 1 of the Cost Calculating Procedure section.

Major terminal points Sea water inlet connections Steam inlet connections (pressure reducing and desuperheating plant to be included) Distillate and brine heater condensate extraction connections Pump discharge connections Blowdown pump discharge connections (if required) Electrical connections Compressed air connections Requested information The costs and design parameters requested and received from the manufacturers of the various types of plant are shown in Tables 2 and 3 of the Optimisation Results section. Distillate production from condenser outlet cooling sea water As an alternative to producing distillate from raw sea water with a temperature range of 15-26X,

the production of distillate from associated

power plant condenser outlet cooling sea water with a temperature range of 22-40°C was investigated.

Each enquiry requested manufacturers to state the

effect of using the condenser outlet cooling sea water on the plant performance

21

of their standard design plant, or the effect on the capital cost and plant performance of their special or tailor-made design plant. THE COST CALCULATING PROCEDURE The total capitalised cost of the plant is considered to be the sum of the capital and erection costs, and the capitalised operational running costs of steam, electrical power, sea water, compressed air, chemicals and replacement materials. The operational running costs are capitalised on a present worth basis. The tabulated running costs in the economic comparison tables have been calculated using a production rate of lOO%, an availability of 85%, an interest rate of 8% and a plant lifetime of 30 years. Expressed mathematically, the costs are given by the following equations

:

Ct = cc + ce + cr

(1)

C

(2)

= can1

rn

C = 8760 WnCnA an

I=[l-

C

rn

[

( l+m

c

(

(3)

i )-T3 ( i ; i (100)

)

= 8760 WnCnA I

(5)

Where Ct = plant total capitalised cost; Cc = plant capital cost; C, = plant erection cost; Cr = plant capitalised operational running cost; C,, = plant capitalised operational running cost of item n: steam or electrical power or sea water or compressed air or an individual chemical; Can = annual operating cost of item n; I = the present worth factor; i = the percentage interest rate; T = the plant lifetime or accounting lifetime; Wn = the flow rate or energy rate of item n; C, = the specific plant site cost of item n; and A = the plant availability factor:

the fraction of time that the plant is considered to be

in service. Civil, transport, operating labour and maintenance costs, and costs associated with reliability and availability are not quantified, because cost differences between different plants were anticipated to be small enough to have insignificant affect on the cost comparison.

However, these costs are

assessed qualitatively in the comparison. The specific site costs, C,, and the units of the flow rates and energy rate, Wn, used for the site under investigation in the economic comparison are given in Table

1.

22 TABLE

1

SPECIFIC SITE COSTS, Cn, AND UNITS OF FLOW AND ENERGY RATES, Wn

teln

Item

umber, n

Steam Electricity Sea water Compressed air Anti-scale (Belgard EV) Anti-foam (Belite M8) Sulphuric acid Sulphamic acid Caustic soda Sodium bi-sulphite Sodium hexametaphosphate (SHMP) Alum Polyelectrolyte Cartridge filter replacement 5

16

Flow rate or energy rate, Wn

4.64 0.08 0 05 0117 2.11 20.92 1.32 1.32 0.58 2.33 2.10 8.80 1;';:

tonnes/h kW m3/h m3/h at STP kg/h kg/h kg/h kg/h kg/h kg/h kgih kg/h kg/h ma/h of distillate m3/h of distillate m3/h o dlstl flate

$/tonne $/kWh b/m3 $/ml at STP $/kg $/kg $/kg $/kg $/kg $/kg $/kg $/kg ;;;9 J

distillate 0.20 $/mJ distillate

Membrane replacement

1

Specific site :ost, c,

Resin replacement

OdKdKte

Application of this cost comparison to other cost comparisons The capital and erection cost of a plant may be represented by equation (6)

(Cc + Ce) =

:

x (Cl + cm11 (1 + PI

Where Cc + C, = the capital and erections cost; X = the exchange rate that is used to convert the currency of the manufacturing country to the currency of the purchasing country or to a currency specified by the purchaser; Cl = the labour cost of manufacturing the plant; Cm1 = the material cost; and P = the profit and contingency factor of a manufacturer.

All the variables are

dependent on time, and they may change appreciably in small intervals of time; fluctuating exchange rates being a familiar example.

The changes in unit

values of Cl and Cm1 over definite periods of time may be calculated using published labour and material cost indicies.

Cl and Cm1 are directly

influenced by design, and therefore directly influenced by the specification that controls the design.

The specification dictates performance, materials,

specific site costs and design optimisation criteria - interest rate, lifetime and availability requirements.

The specification can appreciably influence

the cost of a design type for a given capacity. the economic comparison, wh&

This was made clear during

alternative quotations were received offering

designs to an inferior specification with price reductions of

up

to 20% of the

23 P, offers scope to manufacturers to

cost of designs to specification.

A manufacturer may

influence purchasers in a way that cannot be quantified.

reduce P to negative values in a determined effort to secure work for many reasons.

Such assertions suggest that mathematical models for optimising

economic designs should only be used when the design type is to be specified; the mean manufacturing costindicies free from the influence of P.

used in such models to generate costs being

In the economic comparison influences of P were

reduced by ensuring that at least two quotations were received from different manufacturers for any one plant design type. Equations (1) to (6) may be used in any economic comparison or assessment. If the changes in X and P, and the influence of the number of units (one, or three or more, compared with two in the comparison) are neglected, the information contained within this economic comparison may be used to form a similar but approximate cost comparison of different type of desalination plants made up from units of 1000 m3/day capacity. The capital and erection costs of a single unit of a specific type of plant These costs can be

design current in July 1986 are tabulated in Table 2.

converted to approximate current costs by using labour and material cost indicies published by BEAMA or similar organisations.

To keep the conversion

simple, yet sufficiently accurate for a first approximation, P may be taken to be 0.08, and for X = 1 the values of

the proportional

labour and material costs, may each be taken to be 0.463.

The current capital

and erection costs may then be calculated using equation (7)

C

(C + Ce)t = (C + Ce) CO.463 C

($1 1

+ 0.463 (T)ml;

:

1.08

Where (C + Ce)t = the current capital and erection cost at time t and (kt) and (T),

(kt) = the labour and material cost indicies ratios converting and (T)ml

costs over the period between July 1986 and t the current time of the new economic comparison. The capitalised operational running cost of any plant type is given by equation

(5).

By inserting values of the interest rate i, plant lifetime T,

specific site costs Cn and availability A appropriate to the new comparison, and values of Wn from Table 3 into equations (4) and (5) the plant capitalised operational running cost can easily be calculated.

By adding the current

capital and erection, and the capitalised operational running costs together, the current total capitalised cost of each specific desalination plant is

24

obtained.

The most economical type of desalination plant appropriate to the

particular site, to a first approximation, then presents itself. An example of a cost comparison derived from the comparison of this paper is given in the appendix. Although an electronic calculator is all that is needed to calculate the costs, the suffix notation form of the cost equations is particularly suitable for use in simple computer programs that can be used to rapidly calculate costs, and test cost analyses for sensitivity to different interest rates and plant lifetimes or any other variables. In computer program form, the reduced form of equation (1) becomes

ctm = Ccm + cm +

:

(8760 A I) Wmn Cn

Where the further subscript m is used to identify the specific plant. The el'ements of the column matrix C, are shown in Table 1, and the elements of the rectangular matrix Wmn may be easily identified in Table 3, the quoted design parameters. THE OPTIMISATION RESULTS Total capitalised costs, budget capital and erection costs and capitalised operational running costs for the various 2 x 1000 mg/day distillate desalination plants are shown in Table 2.

The costs are for 1000 ma/day

distillate capacity plant units, and are in thousands of US Dollars, and were current in July 1986. Table 2 also indicates whether the plant is of the package or special (tailor made) type, and whether raw or condenser cooling outlet sea water is being used by the plant. Basis of costs The capitalised operational running costs have been derived in accordance with the Cost Calculating Procedure section which capitalises the running costs on a present worth basis using an interest rate of 8%, a plant lifetime of 30 years, a plant availability of 85X, the specific site costs in Table 1, and the quoted design parameters in Table 3.

Each specific capitalised

operational running cost is shown in Table 2, and commonly grouped costs denoted by supply and energy, chemical , and replacement are also shown so that attention may be focused on either individual costs or grouped costs. Design parameters Table 3 shows the quoted main design parameters which include flow rates of supply fluids and chemicals, and electrical power consumption.

25 The most economical plant The most economical or optimum 2 x 1000 ma/day desalination plant for the specific Middle East site of the comparison is, by a considerable margin, the mechanical vapour compression plant. Total capitalised cost comparison Table 2 shows that the total capitalised cost of the MVC plant is 11 102 000 US Dollars.

It is 3 890 000 US Dollars less than the cost of the

I

T

I T

Ii T I T

9 1n

1 =

-

I 248 2 161

I

10 085

s 3

442

-

p&s

26 second most economical plant, the TVC type; 6 072 000 US Dollars less than the cost of the existing once-through MSF package type; and 15 410 000 US Dollars less than the cost of the most uneconomical plant, the RO plant. Capital cost comparison The capital cost (price plus erection) of the MVC plant is 2 930 000 US Dollars which is about 26.4% of its total capitalised cost.

It is the

cheapest capital cost; and is 360 000 US Dollars, 10.9%, less than the second cheapest, the existing once-through MSF package type; and 780 000 US Dollars, 21.0%, less than the cost of the TVC plant.

21

Capitalised operational running cost comparison Thecapitalised

operational running cost of the MVC plant is 8 172 000 US

Dollars which is about 73.6% of its total capitalised cost.

It is by far the

lowest running cost; and is 3 110 000 US Dollars, 27.6%, less than the TVC running cost; and 5 712 000 US Dollars, 41.1%, less than the existing plant type running cost. Energy and supply running costs Table 2 reveals that the greatest cost advantage of the MVC plant over all the other plants except the RO plant is its relatively low energy and supply running cost.

The plant does not use steam; its heat energy is supplied in

the form of electricity.

Nor does it use compressed air for control purposes

(see description of MVC plant). quarter

Its sea water consumption is only about one

of that of its leading economical rivals.

Its electricity

consumption is about four times that of most of the other plants, but its zero steam and air costs and low sea water cost give it a considerable overall energy and supply running cost advantage of 3 110 000 US Dollars over its leading rival, the TVC plant. Chemical running costs The MSF recycle plants have the lowest chemical running cost.

The MVC

plant has the second lowest cost, but this cost advantage over its leading rivals, a maximum of 90 000 US Dollars, is not significant in comparison with the energy and supply cost advantage. The huge chemical running cost of the RO plant for this site and application is its main cost disadvantage. Replacement running costs Only the RO plant has a component and material replacement cost, and it is a further important disadvantage being over 31% of the total running cost of the MVC plant. Sensitivity to interest rate and plant lifetime Since the MVC plant has both the minimum capital and minimum running cost, there is no need to test running costs for sensitivity to interest rate and plant lifetime values; because no matter what values are chosen the WC

plant

shall always have the minimum running cost, and therefore the minimum total cost.

28 Other costs not quantified Civil, transport, operation and maintenance costs A comprison of the weights and dimensions of the various plants suggests that the MVC plant may have a small civil and transport cost advantage.

A

comparison of the operational and maintenance requirements again favour the MVC plant, but these cost advantages should be small when compared with the huge total cost advantage already discussed. Availability The only cost that could seriously reduce the total cost advantage is that associated with availability.

Sasakura who make each type of plant considered

in the comparison have recently published the following typical plant availabilities in their Desalination Handbook (March 1986) Plant type

% Availability

MVC TVC ME MSF RO

97 95

:

;; 90

These availabilities at least neutralise the statements of some TVC plant manufacturers which suggest that because the TVC plant steam compressor has no moving parts - in contrast to the rotary compressor

of the MVC plant - its

availability should be greater than that of the TVC plant.

It seems reasonable

on the basis of these availabilities to conclude that the MVC plant has, at the worst, no cost disadvantage associated with availability. Effects on plant costs of using condenser outlet cooling sea water rather than raw sea water MSF and ME plants operate more economically when using cold sea water than when using warm sea water; and for any given distillate capacity, the higher the specific design sea water temperature the higher the capital cost. Therefore these plants would be at a disadvantage using warm condenser outlet sea water. Provided the sea water is below about 40°C, the converse is true for RO plants. The TVC plants quoted were of a standard design, not specifically made to suit the application, not tailor-made.

Their performance measured as units

of distillate produced per unit of steam supplied remains essentially constant as the sea water temperature changes between 15-40°C which covers the raw and condenser outlet sea water temperature ranges for the site under investigation. Therefore, there is no cost benefit to be gained using the quoted TVC plants

29

with condenser outlet sea water. The performance of any TVC plant could be improved by adding control systems to it which would reduce the steam supplied as the sea water temperature increases.

None of the plants quoted have such a control system.

In a mechanical vapour compression plant, the heat input is supplied by both its rotary compressor and its electric inmersion heater.

For a constant

distillate rate the heat input by the rotary compressor is constant.

The

electric immersion heater, however, varies the heat input as the sea water feed temperature changes, by maintaining the temperature of the brine in the evaporator constant.

The lower the sea water temperature, the more the heat

input, and conversely the higher the sea water temperature, the less the heat input. The relationship between the heat input to the immersion heater and the sea water temperature depends mainly on the plants distillate capacity, and the nominal sea water feed temperature for which the evaporator has been designed (the designs are standard and cater for a range of feed temperatures). This relationship for a 1000 m3/day plant quoted by one manufacturer is given in the following table.

Capitalised savings on electrical power

running costs for one 1000 m3/day plant using different sea water feed temperatures relative to the cost using 20°C are also given in the table (the running costs given in Table 2 are based on a sea water feed temperature of 20°C). Feed temperature "C

Electrical power input kW

40 30 20

120 174 256

Difference in power input relative to power with feed water at 2O"C, kW 136 82 0

Present worth of savings in 30 years in thousands of US Dollars 912 550 0

For the plant quoted, these savings using a 40°C feed temperature represent about 53% of the total capitalised electrical power running cost at 20°C; and using 30°C feed about 32%. Additional costs to provide both raw sea water feed and sea water feed from the power plant condensers to an evaporator are estimated to be a mere 1 667 US Dollars. Therefore, the cost benefit based upon the characteristics of the quoted plant using sea water feed from the power plant condensers at a temperature between 22 and 40°C rather than using raw sea water feed at temperatures between 15-26°C would be considerable.

Taking average temperatures for

illustration, a present worth cost benefit based on a 30 year period of about

30 550 000 US Dollars could be expected.

The total capitalised costs for the

1000 m3/day mechanical vapour compression plant in Table 2 being based on operation with condenser outlet cooling sea water incorporate this cost benefit. CONCLUSION AND SUMMARY The most economical or optimum 2 x 1000 ml/day desalination plant for the specificMiddle East site in the comparison is. by a considerable margin, the mechanical vapour'compression plant. Its total capitalised cost is less than 25% of that of its leading economical rival, the thermal vapour compression plant. The mechanical vapour compression plant is also the lowest capital cost plant, being about 11% cheaper than the second lowest, an MSF once-through package plant, the tvoe existinq at the site in the comoarison. The MVC plant has-a vast energy and supply running cost advantage over all Apart from the MSF recycle plants the the other plants except the RO plant. MVC plant has the lowest chemical running‘cost, but the difference between the chemical running costs of the MVC and any leading rival plant is not significant. Table 2 shows for each type of plant in the comparison the capital and erection costs and a comprehensive list of capitalised running costs. The qualitative comparison of the civil, transport, operation, maintenance and associated availability costs which are not quantified suggests that the MVC plant would have a small but not siqnificant cost advantaae over the other plants. One manufacturer who makes each type of plant in the comparison has published typical availabilities for his plants which show that the MVC plant has the highest availability. It has an availability of 978, 2% greater than the availability of the TVC plant, the plant with the second highest availability. For optimum economical operation, the standard MVC plant should be supplied with the warm condenser outlet cooling sea water already existing at the site in the comparison, rather than with cold raw sea water. Based on the characteristics of the plant quoted, a present worth cost benefit based on a 30 year lifetime of about one third of the capital cost of the plant could be expected; the other types of plant in the comparison do not show a cost benefit when using warm condenser outlet cooling sea water rather than raw cold sea water. A method is given for making a similar but approximate comparison for other sites, using the results of this comparison and specific site information and costs peculiar to other sites. An example of an approximate comparison is given in the appendix in which actual specific site costs peculiar to a different Middle East site are used. Although some of the specific site costs differ considerably fromthoseused in the comparison, the most economical plant is again the MVC plant, and the second most economical plant is again the TVC plant. Each type of plant in the comparison is described briefly and illustrated diagrammatically in the appendix. The descriptions include the essential requirements of the technical specifications that together with the enquiries requesting quotations were sent to desalination plant manufacturers. ACKNOWLEDGEMENT The authors' desire the express their thanks to Messrs Merz and McLellan for the necessary facilities and assistance in the oreoaration of the oaoer. to their colleague Mr P McLaughlin who did the original work involving k0 plant in the comparison, and th&following desalination plant manufacturers for participating in the comparison and for giving permission to the authors to make use of their drawings : Aitons; Babcock and Wilcox, Espanola; Clarke; Hamon-Sobelco; Hitachi Zosen; Krupp; Mitsui Engineering and Shipbuilding; Sasakura; Sidem; and Weirs.

31

APPENDIX APPENDIX 1 An approximate comparison As an example, an approximate comparison is made for a different Middle East site to determine the most economical 2 x 1000 m3/day desalination plant. Results A concise summary of the costs of the various plants in the approximate comparison are tabulated in Table 4; they were derived using the Cost Calculating Procedure and Comparison Results of this paper, the specific site costs peculiar to the different site, and the BEAMA

cost indices for July 1986

32 and March 1987.

Using the nomenclature of equation (7) in the Cost Calculating k = 1.035 and $irnl

Procedure section of the paper,

= 1.031.

Table 4 is similar to Table 2, but only sumnarised costs are tabulated. Any of the sixteen individual capitalised running costs in the comparison listed in Table 2 can be simply and quickly converted to approximate comparison costs using the Cost Calculating Procedure described in the paper. To allow similar costs in the approximate comparison and the comparison to be easily compared,the summarised plant costs of the comparison are also tabulated in Table 4; and both sets of specific site costs are tabulated side by side in Table 5.

Two sets of costs are tabulated for the approximate

comparison in Table 4, one set was derived using an interest rate of 8% and a

TABLE 5

SPECIFIC SITE COSTS, Cn, AN0 UNITS OF FLOW AND ENERGY RATES, Wn FOR THE COMPARISON AND APPROXIMATE COMPARISON

[tern

Item

lumber, n

Specific site cost, cn, for the comparison

Specific site cost, C,, for the approximate comparison

Flow rate or energy rate, Wn

1 :

Steam Sea water Electricity

4.64 $/tonne 0.05 $/kWh 0.08 $/m3

1.64 $/tonne 0.009 $/kWh 0.014

tonnes/h ms/h kW

4

Compressed air

0.17 S/m3 at STP

0.003 $/kWh

m3/h at STP

5

2.11 $/kg

2.62 $/kg

kg/h

15

Anti-scale (Belgard EV) Anti-foam (Belite M8) Sulphuric acid Sulphamic acid Caustic soda Sodium bi-sulphite Sodium hexametaphosphate (SHMP) Alum Polyelectrolyte Cartridge filter replacement Membrane

16

Resin replacement

6 7 8 9 10 11 12 1:

20.92 1.32 1.32 0.58

$/kg $/kg $/kg $/kg

2.33 $/kg 2.10 $/kg 8.80 $/kg 19.40 $/kg 0.06 $/m3 of distillate 0.20 $/m3 of distillate 0.01 $/m3 of distillate

21.71 0.74 1.20 0.48

$/kg $/kg $/kg $/kg

kgih kg/h kg/h kg/h

2.10 $/kg

kg/h

1.50 $/kg 1.33 $/kg 14.75 $/kg 0.06 B/m3 of distillate 0.20 $/m3 of distillate 0.01 $/m3 of distillate

kg/h kg/h kg/h m3/h of distillate m3/h of distillate ms/h of distillate

33 plant lifetime of 30 years as used in the comparison, and the other set was derived using the same interest rate of 8%, but using a plant lifetime of 20 years. Limitations of the approximate comparison The limitations of the approximate comparison method should be borne in mind:

the method does not account for site data which is different to that

used in the comparison, other than different specific site costs; the effects of using interest rates and plant lifetimes that are different from those used in the comparison on capital costs; currency exchange rates that are different from the rates used at the time of the comparison; and other effects described in the Cost Calculating Procedure section. Differences in site data could have considerable effects on capital costs and chemical flow rates, and therefore on chemical capitalised operational running costs.

Differences in interest rates and plant lifetimes could affect

capital costs of special or tailor-made plant, for these designs would be expected to be optimised using the interest rate and plant lifetime given in the specification; these differences should not alter the capital cost of standard design plants, for these designs are fixed. CONCLUSION The most economical plant is again the MVC plant, and the second most economical plant is again the TVC plant. Comparing costs in the approximate comparison with those in the comparison, on the same basis, total capitalised costs have generally fallen by greater than 505,andtotal capitalised operational running costs have generally fallen by greater than 70%, the MVC plant running costs by 80%. These large falls in cost reflect the large difference between each individual supply and energy specific site cost used in the comparison and in the approximate comparison. The different specific site costs are tabulated side by side in Table 5. The summarised costs of the approximate comparison using a plant lifetime of 20 years rather than 30 years indicate how the capitalised and relative capitalised costs reduce as the plant lifetime reduces.

34 APPENDIX 2 Descriptions and illustrations of the various types of desalination plant in the comparison The design process used in each type of plant in the comparison is briefly described and diagranatically

illustrated.

Design process implies the

designed actions and changes in form that individual fluids working in the plant undergo.

It also

implies design features.

Plant control systems are described; the purpose or effect of using each chemical is described; and the material specification for the main components are given. For each plantinthe

comparison, Table 2 gives a comprehensive list of costs,

and Table 3 gives the performance characteristics and design parameters. To save space and repetition, descriptions and explanations ccmmon to more than one plant are usually described fully once only.

After being described

once, a connnon process in each flow diagram is easily identified, and is self explanatory. THE MVC PLANT Process Figure

1 indicates the main components of the evaporator and illustrates the

process diagramnatically.

The essential difference between this and other

thermal plants is that the heat supplied to it is in the form of electricity, not in the form of steam. The evaporator is of a single-effect, multi-pass, horizontal tube design. Sea water is heated in the preheater and the evaporator spray section and partly evaporated in a film boiling process on the outside of the tubes at a temperature of about 68°C.

The evaporated vapour heats the sea water spray to

its boiling point by partly condensing. with the sea water spray.

The condensate falls over the tubes

The remaining vapour has its pressure and

temperature raised by 360 mm of water, 35 m bar, and 21'C in an electric motor driven rotary compressor, and then condenses inside the tubes to form the distilled water product.

The heat transferred from the condensed steam

evaporates part of the sea water spray and condensate. falls to the brine sump of the evaporator.

The remaining part

Neglecting the relatively small

amount of heat transferred to the sea water feed in the spray section, the heat transferred condensing a unit mass of steam evaporates a unit mass of sea water feed. Heat is removed from the evaporator by the brine and distillate leaving the preheater, and the vapour leaving the vent; and by convection and radiation from the external surfaces of the evaporator.

The difference in the heat

removed from the evaporator and the heat supplied to the evaporator by the

35

Fig. 1 TYPICAL MECHANICAL

FLOW VAPOUR

DIAGRAM

OF A

COMPRESSION

compressor is made up by the electric immersion heater.

PLANT

It maintains the

evaporator brine temperature constant at a temperature of 68°C by adding heat to the distillate at a rate depending on the sea water feed temperature, the lower the feed temperature the more the heat input rate, and vice versa. The immersion heater evaporates scme of the distillate which, in the same manner as the compressed vapour, condenses inside the tubes by transferring heat to boil the sea water,

and then returns to the distillate sump.

This

distillate is merelya recycled heat carrier, none of it leaves the evaporator as product. Maintaining the brine temperature constant implies maintaining the pressure above the brine, the sea water boiling pressure, the pressure at inlet to the compressor constant.

Under all sea water temperature conditions, the immersion

heater evaporates distillate at a rate which maintains the sum of the distillate vapour and generated blower pressures at the sea water boiling pressure corresponding to the controlled brine temperature. Because the distillate pressure is at a greater pressure than the brine, tube or tubeplate leakage does not contaminate the distillate product. Chemicals Anti-scale is injected continuously into the sea water, according to manufacturers as a precautionary measure, to inhibit deposits from precipitating on the tubes and to reduce deposits to a loose form easily swept away by the' various fluids.

36 Control If

desired, the distillate output rate can be controlled within a range of

about 60-120% of nominal by adjusting the controlled brine temperature.

A

temperature switch maintains the selected brine temperature, controlling by direct on or off switching the rate of heat supplied from the immersion heater to the sea water in the evaporator. ,+ conductivity detector monitors the product and causes it to be dumped if it becomes contaminated. Evaporator water levels are maintained automatically without any control devices by pumps working in a cavitation mode.

Specified materials Components

Evaporator

Preheater

Shell

Mild steel lined with neoprene rubber Aluminium brass Mild steel casings epoxy coated internally and stainless steel impellers Stainless steel 90-10 Cupronickel

90-10 Cupronickel

Tubes Compressor Pumps Pipework

Aluminimum brass

THE TVC PLANT Process Figure 2 indicates the main components of the four effect evaporator and illustrates the process diagrammatically. The essential difference between this compression plant and the mechanical type is that the vapour is compressed thermally by a steam jet type compressor, rather than by an electric motor driven compressor.

The steam supplied to the

jet compressor also supplies the total heat input into the system. There are two other major differences; the number of effects in the TVC plant evaporator is four; and the heat in the fluids leaving the evaporator in the TVC plant is partially recovered in a condenser, not in an all liquid preheater. The sea water evaporated in one effect condenses inside the tubes of the next lower pressure effect as distillate.

Unevaporated sea water feed sprayed

into an effect falls to the brine sump of the effect as brine as in the MVC plant, and then flows into the brine sump of the next effect where part of it is flashed to vapour.

Distillate from one effect flows into the distillate

sump of the next effect where part of it is flashed to vapour which condenses inside the tubes of the effect. each effect and between effects.

The same flashing processes are repeated in

THERMAL

v

DIAGRAM

WATER

PLANT

OUTPUT

FRESH

OF A COMPRESSION

FLOW

BLOWDOWN

VAPOUR

TYPICAL

BRINE

IO

n p

(

RAW

WATER

FEED

WATER

REMOVAL

COOLING

AIR

b- REJECT

D

38 The maximum brine temperature is only about 60°C. The thermal compressor raises the pressure and temperature of part of the vapour evaporated in the last effect by 200 m bar and 16°C.

In contrast, the

MVC plant compressor raises the pressure and temperature of the evaporated vapour by only 35 m bar and 21°C.

With such a small temperature rise it is

found more economical to have only one effect.

The small generated pressure

permits a low speed and reliable rotary compressor to be used.

It also

reduces the loss of electrical energy supplied because of motor inefficiency to a minimum by minimising the electrical power input. The rest of the vapour evaporated in the last effect is condensed as distillate in the condenser.

To recover heat from the distillate leaving the

evaporator, the vapour is flashed from the distillate and condensed.

Sea

water flowing through the condenser tubes condenses the vapour, but unlike the sea water of the MVC plant which is all used as feed, only part of it is used as feed, the rest together with its heat is rejected to the sea. Chemicals The sea water is dosed continuously with the anti-scale inhibitor sodium hexametaphosphate. Control The plant is essentially a constant output device that has a constant steam demand.

As the sea water temperature changes, pressures and temperatures

adjust to new values automatically, unaided by control devices. Specified materials Components

Evaporator

Condenser

Shell

External : mild steel Internal : 316 L stainless steel Aluminium brass 304 L stainless steel

Mild steel

Tubes Therm0 compressor Pumps Pipework

90-10 Cupronickel

Stainless steel Raw water and brine 90-10 Cupronickel Product : 316 L stainless steel Steam : mild steel

THE MSF PLANTS The MSF Once-Through Plants Process Figure 3 indicates diagramaatically the main components of ,the plant and illustrates the process diagraasnatically.

39

Fig. 3

I

I

40 The evaporator is divided into stages; each stage has a sea water condenser, brine flash chamber, demister and a distillate collecting and transfer system. The sea water is heated as it flows in series through the condenser tubes of each evaporator stage and brine heater, being heated to a temperature greater than the saturation vapour temperature in the final stage.

It then flows into

the first stage inlet box and is evenly distributed across the width of the evaporator.

It enters the first stage through an orifice and weir system

which control its flow rate, and flashing contour characteristics, causing it to flash to vapour in the flash chamber in a predetermined manner.

The

flashed vapour flows through a demister which removes any entrained brine, then over a condenser that condenses it.

The condensate, referred to as

distillate, is collected in a distillate collecting and transfer system. The sea water, now referred to as brine, because its salinity has increased, then flows through the evaporator stages in turn, releasing flashed vapour in each stage in the same manner.

The brine is rejected from the last stage

evaporator by a brine blowdown pump to the sea. The distillate formed in each stage passes through the transfer system into the succeeding lower temperature stage, where a proportion is flashed to vapour similarly to the sea water.

This vapour flows over the condenser of the stage,

together with the vapour flashed from the sea water and is condensed and transferred to the next stage.

The distillate accumulates as it flows

through succeeding stages and is discharged from the last stage to a product water tank by the product or distillate pump. The sea water, therefore, apart from a small loss of vent steam, is converted totally into distillate product and rejected brine. Non-condensable gases liberated from the flashing brine together with air that leaks into the sub-atmospheric stages of the evaporator are vented to the air extraction plant. The heat is supplied to the plant in the form of steam which transfers heat to the sea water by condensing in the brine heater.

The condensate is returned

to the plant steam supply system by the brine heater condensate pump. Neglecting small heat losses, the heat supplied by the steam to the sea water is equal to the heat removed by the brine and distillate leaving the evaporator. The MSF Recirculation Plants Process The essential difference between the brine recycle and the once-through type of plant is that the chemical cost of running the recycle plant is much lower, typically less than 30% of the once-through cost.

41 Figure 4 indicates diagrammatically the main components and illustrates the process. To avoid chemically treating all of the sea water, the brine is recirculated. It has a flow of about twice

Only make-up sea water is chemically treated.

that of the distillate, which is small compared to the sea water flow of a once-through plant that is usually more than eight times the distillate flow. The evaporator now consists of two sections: heat rejection section.

a heat recovery section and a

Although drawn separately to show the process, the

two sections are constructionally integral.

Recirculating brine flows through

the condensers of the heat recovery section, and recovers the heat lost by the flashing brine that flows through the flash chambers of the heat recovery section. Heat is rejected from the flashing brine to the sea water which flows through the condensers of the heat rejection section to maintain an economical last stage brine temperature.

Brine is blown down from the last stage to

avoid a high salt concentration in the recirculating brine.

The blowdown and

the distillate product are replaced by heated sea water, known as make-up, which mixes with the last stage brine after blowdown has been removed. brine is pumped by the brine recirculating plmptothelaststagecondenser

The of the

heat recovery section. The flashing brine and distillate processes are as described for the oncethrough plant. Neglecting small heat losses, the heat supplied by the steam to the recirculating brine in the brine heater is equal to the heat removed by the sea water, brine and distillate leaving the heat rejection section of the evaporator. Chemicals Anti-scale and anti-foam chemicals are injected continuously into all of the sea water entering the once-through plants, but only into the sea water make-up entering the recirculating brine of the recycle plants.

The anti-scale

inhibits deposits from precipitating inside the bores of the heat transfer surface tubes in the hotter stages, and reduces deposits to a soft or loose foam which is easily swept away by the sea water or brine.

Ouring the flashing

processes, the anti-foam prevents the brine foaming which would cause carry over of sea water droplets into the distillate. In the brine recirculating plants, the higher the brine salinity, the lower the sea water make-up, and therefore the lower the chemical cost. Consequently, the brine salinity is maintained as high as scaling considerations permit:

just below the value at which hard scales begin to precipitate.

The

sea water make-up is also deaerated either in a separate deaerator or in the

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43 One recycle plant in the comparison

last stage of the heat rejection section.

doses the deaerated make-up with sodium bi-sulphite to scavenge any remaining oxygen. Ball cleaning is used in most MSF plants to remove soft scale deposits from the bore of the tubes. Specified materials Components

Material

Shell. tubes. tubeolate, oipework Oemisier Pumps Steam and condensate system

90-10 Cupronickel Stainless steel or monel Copper alloys Steel

Controls To preserve stability of levels, flows and pressures, MSF plants are usually operated constantly at full distillate load.

Operation at a distillate load

of 60% is possible but load changes have to be made slowly.

A load change

from lOO-60% and vice versa takes between 1 and 2 hours. The following design parameters are controlled at selected values

:

Maximum brine or sea water and sea water inlet temperatures. Steam pressure and temperature. Sea water, sea water make-up and recirculating brine flows, and the sea water make-up to distillate flow ratio. Last stage brine and distillate, and brine heater condensate levels. Distillate and condensate purities (conductivities). Under steady flow conditions which are implied by a constant last stage brine level, the maximum salinity of the recirculating brine for a given distillate production and sea water salinity is dependent on the make-up to distillate flow ratio. THE MULTI EFFECT (ME) PLANTS The ME horizontal tube plant Process Figure 5 indicates the main components of the plant and diagrammatically illustrates the process. Sea water feed is pumped through the preheater in each effect and through the spray nozzles, from where it sprays onto the outside of the brine heater tubes at the top of the plant.

Steam supplies heat to the plant by

condensing in the brine heater which forms part of the first effect. condensate returns to the steam supply system.

The

Some of sea water spray is

evaporated by the heat transferred from the condenser steam.

Most of the

evaporated sea water flows through the demister, over the preheater where it heats the feed by partly condensing, and into the tubes of the second effect where the rest of it condenses; the condensate forms distillate product.

44

Fig. 5

load he Stf

preheater

&&]b

product

sulphuric acid

reject seawater

blowdown sea

TYPICAL FLOW DIAGRAM OF A MULTIPLE EFFECT HORIZONTAL TUBE PLANT

45 The rest of the evaporated sea water, a relatively small amount, heats the feed spray until it reaches boiling point, by condensing, and falls over the tubes as part of the spray.

The unevaporated spray, now brine, falls to the

bottom of the effect, flows through the spray nozzles, and flashes as it enters the lower pressure second effect.

Part of the remaining brine

evaporates on the outside of the tubes, and together with the flashed vapour flows through the demisters into the next effect.

The unevaporated brine

falls to the bottom of the effect. The flashing, evaporating and condensing processes are repeated in each effect.

The raw sea water cools the vapour generated in the last effect by

condensing it in the heat rejection condenser.

The concentrated brine inside

the last effect flashes in the lower pressure section of the condenser and the flashed vapour condenses, producing more distillate and heating the sea water. By flashing, the product from each effect cascades down the plant into the condenser. About 25% of the raw sea water leaving the condenser after being dosed with sulphuric acid forms the plant feed.

The rest of the sea water is returned to

the sea. Brine by-pass loops provided between effects prevent brine levels becoming too deep during load changes which cause unstable conditions and levels. Control The distillate output can be controlled between 30 and 100% full load by altering the controlled pressure of the steam entering the brine heater. The pH of the sea water feed is controlled at a selected value which may be adjusted to cater for varying sea water conditions; it may also be adjusted for cleaning purposes. Figure 5 clearly indicates these and the other plant control systems. Chemicals The sea water feed is continuously dosed with sulphuric acid to destroy scale forming compounds, by decomposing them into gases which are liberated together with any dissolved air in the process and removed from the plant by the vent ejector system. Specified materials Components

Material

Shells

Stainless steel clad or 90-10 Cupronickel clad mild steel 90-10 Cupronickel and Titanium Stainless steel Stainless steel or monel Stainless steel

Tubes Tubeplates Demisters Pumps

46

i

2 3 v)

Fig. 6

The ME vertical tube Process Figure 6 indicates the components and diagrammatically illustrates the process. The vertical tube type evaporator differs from the horizontal tube type in the following ways

: the evaporator tubes are arranged vertically;

evaporation takes place inside the tubes , not on the outside, and the brine does not flow in a flashing cascade between effects, it cascades between effect and associated feed heater stage and part of it is pumped to the evaporator tubes of the next effect, the other part flashes into the next heater stage. The brine vapour flashed from the brine heats the feed and condenses to form distillate.

The distillate similarly flows in a flashing cascade from effect

to feed heater and down the feed heater stages, the flashed distillate vapour heating the feed by condensing then recombining with the distillate. The steam from the last effect flows to the heat rejection condenser and to the deaerator-decarbonator.

Some condenses by heating the feed in the

condenser to form distillate; which combines with the distillate from the last stage of the feed heater and heats the feed in the product cooler.

The rest

of the steam deaerates and decarbonates the feed in the deaerator-decarbonator. The feed is dosed with sulphuric acid before it enters the deaerator and with sodium bi-sulphite and caustic soda as it leaves the deaerator. Control By adjusting the steam supply and the sea water feed flows to correspond to a selected distillate load, the plant load controller controls the distillate load automatically within a 40-100% load range.

Other controls are similar to

those used on the horizontal tube type plant. Chemicals The sea water feed is dosed with sulphuric acid to a pH of about 4.5, before it enters the deaerator-decarbonator,

to decompose its scale forming elements;

with sodium bi-sulfite, as it leaves the deaerator, to scavenge any remaining oxygen from it; with the alkaline caustic soda, before it enters the feed heater, to neutralise it; and with anti-foam, before it enters the feed heater, to prevent foaming during the brine flashing processes in the feed heater. Carbon dioxide and air are released from the sea water in the deaeratordecarbonator and removed by the vent system.

48 Specified materials Components

Evaporator

Feed heater

Shell

Mild steel clad or lined with stainless steel. Mild steel in lower temperature staoes: clad or lined where flashing takes place. Mild steel clad with 90-10 Cuoronickel. Sacrificial anode plates installed. 70-30 Cupronickel, fluted. Aluminium brass, fluted. Titanium in heat rejection section. Mild steel clad with 70-30

Mild steel clad or lined with stainless steel. Mild steel in lower temperature stages; clad or lined where flashing takes place. Mild steel clad with 90-10 Cupronickel. Sacrificial anode plates installed. 70-30 Cupronickel, spirally indented.

Water boxes Tubes

Tubeplates

Cupronickel or 90-10 Cupronickel clad or stainless steel.

Mild steel clad with 70-30 Cupronickel.

REVERSE OSMOSIS Process description The reverse osmosis process works by applying an external pressure greater than the osmotic pressure to a salt solution in the presence of a semipermeable membrane.

The pressure reverses the natural flow of the salt solution and

pure water flows through the membrane. The process indicated in Figure 7 requires the feed water entering the membranes to be treated in a pretreatment section which removes any suspended solids or organic material.

To provide optimum conditions for the membranes,

the water is also chemically treated and then pumped, using high and low pressure pumps, through the membrane modules and product water is produced. The suspended solids concentrated in the sea water in the area of the intake is normally 3 mg/l, but during the storm season the concentration can rise to 300 mg/l.

The maximum suspended solids concentration that can be practically

handled by pressure, gravity or multimedia filters is about 30-50 mg/l.

To

cope with concentrations in the 50-500 mg/l range a clarifier is required. The pretreatment plant therefore consists of a sludge blanket clarifier followed by a rapid gravity filter.

To assist in the removal of suspended

solids, the sea water is dosed with alum before the clarifier and polyelectrolyte in the clarifier.

Acid and sodium hexametaphosphate dosing is

carried out after filtration to prevent scale formation on the membranes. The operating limits of the chlorine content of the feed water at the sea water reverse osmosis membranes depends on the membrane manufacturer; and it may be necessary to add sodium bisulphite to remove any chlorine present in the filtered water.

!‘iblJ-------•ka

ALUM DOSING

CLARIFIER

POLYE&cC~lYTE

OF

R. 0. MEMBRANE

PUMP

LOW PRESSURE

LOW

.--ii,,.

PRESSURE

cJ

PLANT

DIAGRAM OSMOSIS

FLOW

A REVERSE

TYPICAL

R.O. tlEHBRANE

PUHP

HIGH PRESSURE

HIGH PRESSURE

SAND

+---cT

FILTERS

AC10 AN0 SODIUM tIEXAMtTAPtlOSPHATE DOSING

HIXEO

BE0

OEMINERALISEO

c:

‘p q

11 _.

PROOUC 1 rwa;ER

SUlPHURIC ACID & CAUSTIC SODA DOSING

50 To provide product with a TDS of 3 mg/l, the process requires two stages of reverse osmosis followed by one stage of mixed bed ion exchange units. 5000 m3/h of sea water at a pressure of about 65 bar (depending on sea water temperature) flowsinto

the first stage RO membrane.

1500 m3/h of product

water with a TDS of about 450 mg/l and a pressure of 1 bar leaves the first stage, and is pumped through the second stage RO membrane at a pressure of 40 bar.

1050 ma/h of product water leaves the second stage with a TDS of about

35 mg/l, and is denionized in mixed bed ion exchange units to the required TDS of 3 mg/l. The overall conversion ratio of the sea water to distillate is 20%.

This is

low for a reverse osmosis plant, but necessarily low, because of the very low specified 3 mg/l TDS of the product.

Specified materials Component

Material

Sea water and high and low pressure pumps Pipework Membrane modules (casings) Membranes

316 L stainless steel 317 L stainless steel Fibreglass Cellulose acetate and thin film composite cross-linked polyamide