optimal design of hybrid msfro desalination plant

8/13/2019 Optimal Design of Hybrid MSFRO Desalination Plant

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Optimal Design of Hybrid MSF/RO

Desalination Plants

Submitted By:

Abdullah I. Al-Khudhiri

Supervised By:

Prof. Ibrahim S. Al-Mutaz

Examination Committee Members

1- Prof. Ibrahim S. Al-Mutaz 2- Dr. Malik I. Al-Ahmad

3- Dr. Osman A. Hamed

A Thesis submitted in partial fulfillment of the requirements forthe degree of Master of Science in Chemical Engineering

December 2006


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Table of Content

Table of Contents iList of Table v

List of Figures viiNomenclature ixAcknowledgment xiiAbstract xiii

CHAPTER1

Introduction 11-1 Importance of Desalination in Saudi Arabia 1

1-2 Seawater and Drinking Water Properties 3

1-3 The Current Work

CHAPTER 2

5

Major Desalination Technologies 62-1 Reverse Osmosis System (RO) 7

2-2 Multi-Stage Flash (MSF) 9

2-3 Electrodialysis 12

2-4 Vapor Compression 13

2-5 Freezing 15

2-6 Solar Desalination 16

CHAPTER 3

MSF and RO Modeling and Economics 18Multi-Stage Flash (MSF) 19

3-1-1 MSF-Once Through 19

3-1-1-1 Definition and Advantages 19

3-1-1-2 Simplified Model of MSF-Once Through 19


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3-1-1-2-1Temperature Distribution 20

3-1-1-2-2 Energy Balance On Stage i 20

3-1-1-2-3 Stage Material and Salt Balance 21

3-1-1-2-4 Brine Heater and Condensers Heat Transfer Area 233-1-1-2-5 Total Plant Areas 24

3-1-1-3 Water Production Cost Estimation of MSF-OT Plant 24

3-1-2 MSF- Brine Recirculation 27

3-1-2-1 Definition and Advantages 27

3-1-2-2 Simplified Model for MSF- Brine Recirculation 27

3-1-2-2-1 Overall Material Balance 28

3-1-2-2-2 Temperature Distribution 28

3-1-2-2-3 Energy Balance On Stage i 29

3-1-2-2-4 Stage Material and Salt Balance 30

3-1-2-2-5 Brine Heater and Condensers Heat Transfer Area 33

3-1-2-2-6 Total Plant Heat Transfer Areas 35

3-1-2-3 MSF-BR Economics 35

3-2 Reverse Osmosis (RO) 36

3-2-1 Definition and Advantages 36

3-2-2 Simplified Model of RO Design 37

3-2-3 Cost Methodology of RO 38

3-2-3-1 Water Production Cost Estimation of RO Plant 38

3-2-3-1-1 Estimation of the direct capital cost 40

3-2-3-1-2 Estimation of the water production cost 41

3-3 Comparative Study of MSF and RO 43

CHAPTER4 Hybrid MSF/RO Desalination Systems

4-1 Identification of Hybridization System

45

46

4-2 Advantages of hybrid MSF/ RO system 46

4-3 Previous Work 48

4-4 Options for Hybridization System 53


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4-4-1 Simple Hybrid Desalination Plants 53

4-4-1-1 Simplified MSF-OT/RO 53

Design Procedure 53

Cost Methodology 54 4-4-1-2 Simple MSF-BR/RO 56

Design Procedure 56 Cost Methodology 57

4-4-2 Integrated Hybrid Desalination Plants 58

4-4-2-1 Integrated Hybrid MSF-OT/RO 58

4-4-2-1-1 MSF-OT brine as a feed of RO 58


4-4-2-1-2 RO reject brine as a feed part of MSF-OT 60


4-4-2-2 Integrated Hybrid MSF-BR/RO 62

4-4-2-2-1 MSF cooling water as a feed of RO 62 Design Procedure 62 Cost Methodology 63

4-4-2-2-2 part of MSF brine and part of cooling water as

feed of RO

64


4-4-2-2-3 Mixing MSF feed with RO rejected brine 66


4-4-2-2-4 RO rejected brine is used as MSF feed 68



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CHAPTER5

5-Results and Discussions70

5-1 Results of Single Type Desalination Plants 71

5-1-1 MSF-Once Through (MSF-OT) 71

5-1-2MSF- Brine Recirculation (MSF-BR) 74

5-1-3 Reverse Osmosis (RO) 77

5-2 Simple Hybrid Desalination Plants 79

5-2-1 MSF-OT/RO 79

5-2-2 MSF-BR/RO 83

5-3 Integrated Hybrid Desalination Plants 87

5-3-1 MSF-OT brine as feed of RO 87

5-3-2 Mixing RO brine reject with MSF feed 91

5-3-3 MSF-BR cooling water as feed of RO 95

5-3-4 Part of MSF Brine and Part of Cooling Water as

feed of RO

99

5-3-5 Mixing MSF-BR feed with RO rejected brine 103

5-3-6 RO rejected brine as MSF-BR feed 107

5-4 Summary of water production and production cost 111

5-5 Discussion 112

CHAPTER6

6-Conclusion and Recommendations 115

6-1 Conclusion 116

6-2 Recommendations 117

Bibliography 118

Appendix AMATLAB Computer Programs for Desalination Plants

122


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List of Tables

Table 1-1 Saudi desalination plants Production 2

Table 1-2 Major constituents of seawater 4

Table 1-3 Drinking Water Standards 4

Table 5-1 MSF-OT Characteristic 71

Table 5-2 MATLAB Result of MSF-OT ( Modeling ) 71

Table 5-3 MATLAB Result of MSF-OT ( Economics ) 71

Table 5-4 MSF-BR Characteristic 74

Table 5-5 MATLAB Result of MSF-BR (Modeling) 74

Table 5-6 MATLAB Result of MSF-BR (Economics) 74

Table 5-7 RO Characteristic 77

Table 5-8 MATLAB Result of RO (Modeling) 77

Table 5-9 MATLAB Result of RO (Economics) 77

Table 5-10 The feed flow rate of simple hybrid MSF-OT / RO 79

Table 5-11 MATLAB Result of Simple Hybrid Desalination

Plants MSF-OT/RO(Modeling)

79

Table 5-12 MATLAB Result of Simple Hybrid DesalinationPlants MSF-OT/RO(Economics)

80

Table 5-13 The feed flow rate of simple hybrid MSF-BR/ RO 83


Plants MSF-BR/RO(Modeling)

83


Plants MSF-BR/RO(Economics)

84

Table 5-16 The feed flow rate of integrated hybrid

MSF-OT1/ RO

87

Table 5-17 MATLAB Result of integrated hybrid MSF-OT1/RO

(Modeling)

87


(Economics)

88


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Table 5-19 The feed flow rate of integrated hybrid MSF-OT2/RO

91

Table 5-20 MATLAB Result of integrated hybridMSF-OT2 /RO

(Modeling)

91


(Economics)

92

Table 5-22 The feed flow rate of integrated hybrid MSF-BR1/RO

95

Table 5-23 MATLAB Result of integrated hybrid MSF-BR1/RO

(Modeling)

95

Table 5-24 MATLAB Result of integrated hybridMSF-BR1/RO

(Economics)

96


99


(Modeling)

99

Table 5-27 MATLAB Result of integrated hybrid MSF-BR2/RO(Economics)

100


103


(Modeling)

103


(Economics)

104


107


(Modeling)

107


(Economics)

108

Table 5-34 Summary of Production & Production Cost 111


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List of Figures

Fig 2-1 Reverse Osmosis System (RO) 7

Fig 2-2 Multi-Stage Flash (MSF) 9

Fig 2-3 Electrodialysis 12

Fig 2-4 Vapor Compression 13

Fig 2-5 Freezing 15

Fig 2-6 Single Basin Solar Still 16

Fig 3-1 MSF-Once Through 19

Fig 3-2 MSF- Brine Recirculation 27

Fig 3-3 Reverse Osmosis RO 36

Fig 3-4: a simplified RO membrane 37

Fig4-1: Simplified MSF-OT/RO Design 53

Fig4-2: Simplified MSF-BR/RO Design 56

Fig4-3: MSF-OT brine as a feed of RO 58

Fig4-4: RO reject brine as a feed part of MSF-OT 60

Fig4-5: MSF cooling water as a feed of RO 62 Fig4-6: part of MSF brine and seawater as feed of RO 64

Fig4-7: Mixing MSF feed with RO rejected brine 66

Fig4-8: rejected brine as MSF feed 68

Fig 5-1: MSF-Once Through 73

Fig 5-2: MSF- Brine Recirculation 76

Fig 5-3: Reverse Osmosis RO 78

Fig 5-4: Simple Hybrid Desalination Plant (MSF-OT/RO) 82

Fig 5-5: Simple Hybrid Desalination Plant ( MSF-BR/RO ) 86

Fig 5-6: MSF-OT brine as feed of RO 90 Fig 5-7: Mixing RO brine reject with MSF feed 94

Fig 5-8: MSF Cooling Water as feed of RO 98 Fig 5-9: Part of MSF Brine and Part of Cooling Water as feed ofRO

102

Fig 5-10:Mixing MSF feed with RO rejected brine 106


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Fig 5-11: RO rejected brine as MSF-BR feed 110


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Nomenclature

MHf Total hybrid seawater feed, Kg/h

Mf MSF feed, Kg/hFf RO feed, Kg/h

AW Pure water permeability, m/sec pa

AS Salt permeability, m/sec

ks Mass transfer coefficient, m/sec

p1 Operating pressure, pa

tcw Feed Temperature, C

nJ nr

Total number of rejection section stages,Total number of recovery section stages,

Tcw Intake seawater Temperature, C

Ts Steam temperature, C

To Top brine temperature, C

Tn Brine temperature in the last stage, C

cp Heat capacity of the liquid streams liquid, kJ/kg oC

Xf Salinity of the intake seawater, ppm

Xb Salinity of the rejected brine, ppm

Mcw Cooling water, Kg/h

Md Product feed (MSF-BR), kg/h

M b The rejected brine flow rate, kg/h

Mr Brine recycle flow rate, kg/h

Ms Steam flow rate, kg/h

Xr Recycled brine concentration, ppm

Ub The overall heat transfer coefficient of brineheater,

Kw/m 2 oC

Ab Brine heater area, m 2

Ur The overall heat transfer coefficient of recoverysection,

kW/m 2 oC

Ar Recovery section Area, m 2

Uj The overall heat transfer coefficient of rejection kW/m 2 oC


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section,

Aj Rejection section Area, m 2

Ac Total condenser area, m 2

PR The performance ratio,A total Total heat transfer area, m

2

cw Salt concentration at membrane wall, kg/m 3

cp permeate Concentration, kg/m 3

Fp Total Product Flow Rate (RO), kg/h

Frj Reject flow rate, kg/h

crj Concentration of reject flow rate kg/m3

Atmod Total membrane area, m 2

MHd Total product of hybrid system (MSF/ RO), kg/h

XHd Concentration of hybrid system (MSF/ RO), ppm

MHb Total BRINE of hybrid system (MSF / RO), kg/h

XHb Concentration of hybrid system, ppm

CDM Direct capital investment, $/year

CIDM Indirect capital investment, $/year

Csteam Steam cost, $/year

Cche Chemical treatment, $/year

C power Power Cost, $/year

Cspar Spares Cost, $/year

Clab Labor Cost $/year

COM The Operation & maintenance Cost $/year

CAM Annual Cost $/yearMS cost_p Production Cost of MSF, $/m

3

Rf Recovery fraction,

cost mem Membrane cost, $/year

cost civil Civil cost estimation, $/year

cost dir Direct capital cost $/year

cost 1 Fixed charges cost, $/year

cost fix Annual fixed charges, $/m3


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cost 2 Electric power cost, $/year

cost ep Annual electric power cost, $/m3

cost 3 chemicals cost, $/year

cost che Annual chemicals cost, $/m3

cost 4 Membrane replacement $/year

cost mem_rep Annual membrane replacement, $/m3

cost 5 Annual labor cost, $/year

cost labor Annual labor cost $/m3

cost RO_total RO plant cost, $/year

RO prod_cost Production Cost of RO, $/m3

Cost H1 Electromechaical equipment cost, $Cost H2 Hybrid civil work cost, $

Cost H3 Hybrid elctrochlorination, $

Cost H4 Hybrid brine disposal cost, $

cost H_total Reference hybrid intake-outfall cost $/year

COST HA annual plant intake-outfall cost, $/year

COST INT_HY Total annual cost of integrated hybrid (MSF/RO), $/year

total cost Water cost of integrated hybrid (MSF/RO), $/m3


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Acknowledgment

Praise and Thanks first and last to almighty Allah

I Would like to thank deeply Prof. Ibrahim Al-mutaz forsuggesting the topic of this thesis and for his suggestions. Prof.Ibrahim Al-mutaz alloys support me and give me great helpthroughout this thesis.Also, I Would like to thank Chemical Engineering Department fortheir cooperation and encouragement.Finally, I Would like to thank my parents and my wife.


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Abstract

This thesis addressed a very important subject in the desalination field. It is

called hybrid desalination. Hybrid desalination systems combining both

thermal and membrane desalination processes in the same site are currently

considered a good economic alternative to either of these two processes when

operated individually. The research work carried out to simulate hybrid

desalination processes is limited.

The main objective of the thesis is to carry out an in-depth and comprehensive

simulation study to determine the optimum design configuration of hybrid

desalination systems combining synergistically multi-stage flash (MSF)

desalination with seawater reverse osmosis (SWRO) process. The impact of

combining MSF and SWRO desalination processes in different hybrid

configurations on water production cost was explored.

Mathematical steady state models which incorporated mass and energy

balances equations were combined with economic relationships to predict the

unit water production cost as a targeted objective function. The simulationmodels developed in this work and the results obtained can be as an

appropriate guide by designers and operators to optimize the design and

operating conditions of hybrid desalination systems.

A total of eleven desalination systems were simulated and analyzed. For

comparison, the first three desalination arrangements include SWRO and MSF

with and without brine recirculation. The fourth and fifth systems considered a

simple hybrid configuration where The MSF and SWRO processes are

operating in parallel forming simple hybrid arrangements. The remaining six

systems include partially or totally integrated SWRO/MSF hybrid desalination

systems.

For each system mass, energy balance and heat capacity equations together

with economic relationships were developed and solved by a using MATLAB

computer programs to estimate the water production cost for a specific


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Chapter 1

Introduction


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1-Introduction

1-1 Importance of Desalination in Saudi ArabiaThe Arabian Gulf countries have about 60% of the total world desalting

capacity. Saudi Arabia has the largest capacity of desalination in the world.

Desalination in Saudi Arabia started in 1907. The first multistage flash plant,

MSF, was built in Doba and Al-Wajh on the Red Sea coast in 1969 with a

capacity of 227.1m 3/day. On the Arabian Gulf, Al-Khobar I was started in

1973 with a capacity of 28,390 m 3/day. All these plants are MSF type. The

first RO seawater desalination plant in Saudi Arabia was constructed in

Jeddah (Jeddah RO-1) in 1978 with a capacity of 12,110m 3/day. Al-Jubail

phase II is considered the world largest desalination plant. It has a design

capacity of about 960,000m 3/day and was on operation in 1983. It comprises

40 MSF units each producing 23,500m 3/day [1].

Most of the Saudi desalination plants are multistage flash type MSF. Saudi

MSF plants capacity are more than 33% of the total world MSF desalting

capacities. Saudi MSF plants accounted for more than 19% of the totalnumbers of MSF units. Reverse Osmosis, RO, has about 17.8% of the total

desalting capacity of plants producing more than 4000 m 3/day [1].

In 2004, there were 25 desalination plants in operation in Saudi Arabia with a

design total capacity of about 2.88 Mm 3/day. Nine of the 25 operated

desalination facilities in Saudi Arabia are dual system plants that generate

about 3,600 MW of electricity per day or 33%of the total power generated in

the country [2].


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Table1-1 Saudi desalination plants Production[2].

Region Plant Water production

(m 3/day)

Al-Jubail 118,447

Al-Jubail 815,185

Al-Jubail RO 78,182Al-Jubail

Total 1,011,814

AlKhobar II 191,780

AlKhobar III 240,800AlKhobarTotal 432,580

AlKhafji AlKhafji 19,682

Total Water production of East Coast 1,464,076

Jeddah II 38,916

Jeddah III 75,987

Jeddah IV 190,555

Jeddah I RO 48,848

Jeddah II RO 48,848

Jeddah

Total 402,154

Shoaibah I 191,780

Shoaibah II 390,909Shoaibah

Total 582,687Yanbu I 94,625

Yanbu II 120,069

Yanbu RO 106,904Yanbu

Total 321,625

Al Shkik Al Shkik I 91,000

Hagl 3,784


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Duba 3,784

AI-Waih 3,611

Urn Lujj 3,784

Rabig 1,978

Aziziah 3,870

AlBirk 1,952

Farasan 1,505

Small Plants

Total 24,268

Total Water production of West Coast 1,421,736

Total SWCC Production 2,885,812

1-2 Seawater and Drinking Water PropertiesSeawater is composed of about 96.57% water and 3.5% dissolved substances

of various elements. Seawater concentration in the Arabian Gulf countries

is considered to be the highest concentrations in the world, 41000-45000 ppm.

Seawater constituents can be divided into the following categories [3]:

Major elements with concentrations of more than 100 ppm, such

as chlorine, sodium, magnesium, sulfur, calcium and potassium.

Minor elements which have concentrations between 1 and 100 ppm.

These include bromine, carbon, boron and fluorine. Traces of all other elements with concentrations below 1 ppm. These

elements have a total concentration below 0.24%.

Three compounds or six elements, represent about 98% of the total mineral

concentration.


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Table1-2 Major constituents of seawater (ppm) [3]Constituent Arabian Gulf Red Sea Normal

SeawaterChloride, Cl 23,000 22,219 18,980

Sodium, Na+

15,850 14,255 10,550:Sulfate, SO 42- 3,200 3,078 2,649Magnesium, Mg 2+ 1,765 742 1,272

Calcium, Ca 2+ 500 225 400Potassium, K + 460 210 380Bicarbonate,

HCO 3 142 146 140

Strontium, Sr 2- 13Bromide, Br 80 72 65Fluoride, F 1Silicon,Si 4 1Iod1de, I

Total dissolvedsalt

45,000 41,000 34,438

Table 1-3 Drinking Water Standards (ppm)[4]Constituent Maximum

ConcentrationOptimumConcentration

Carbon dioxide 20 Carbonates(Na,K) 150 Chlorides 250 250Chlorine 1 Copper 3 Fluorides 1.5 0.5Iron 0.3 Lead 0.1 Magnesium 125 125

Zinc 15 Nitrate 10 NaCl 250 Sulfate 250 250Total DissolvedSalts

550 500


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1-3 The Current WorkThis study will focus on finding the optimum design of hybrid MSF/ RO

desalination plant. Research methodology can be summarized as follows: Review of the existing RO and MSF desalination plants in Saudi

Arabia for:- understanding the general design characteristics of these processes.- exploring the advantages of each process.

Suggestion of applicable design configurations of the hybrid MSF/ROsystems.

Modeling of each proposed design configurations of the hybridMSF/RO systems by solving the governing conservation equations ofmass and energy.

Performing an economic optimization study based on the minimumwater production cost for each proposed hybrid MSF/ROconfigurations.


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Chapter 2Major Desalination

Technologies


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2-Major Desalination Technologies

2-1 Reverse Osmosis System (RO):

Fig 2-1 Reverse Osmosis System (RO) [ 5]

All of the RO cases presented are based on the use of spiral wound membrane.

The brackish water units are operated at 400 psi and the seawater plant is

operated at about 1000 psi. It should be emphasized that feed water quality is

very important to the success of RO. The pretreatment section must be

designed and operated with due regard for the composition of the specific

water being used. For seawater, it is assumed that filtration followed by

treatment with acid and SHMP (sodium hexametaphosphate). Enough acid

should be added to reduce the pH to 6.2-6.4. Additional pretreatment would berequired for waters with high turbidity or a high content of bacteria and

algae[5].

Some of the general considerations in pretreatment for RO are[5]:

Iron and manganese must be removed to around 0.5 ppm or less,

depending on the type of membrane.

The calcium concentration should be 600 ppm or less to prevent scaling

as the concentration increases in the water brine. Injection of SHMP


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permits calcium concentrations up to 900 ppm, but for high calcium

concentrations, other means of water softening may be necessary.

Chlorine addition may be required to kill marine life or control bacterial

action.Bacterial attack can destroy the membranes. Acid addition (usually sulfuric acid) may be required to eliminate

calcium carbonate and magnesium hydroxide deposits and to reduce the

pH to 5.56.5.

Final polishing filters are required to eliminate particulate matter.

The extent of pretreatment is related to the water recovery of the plant, in as

much as the highest recoveries result in the most concentrated waste brines.

Scaling may also increase because the reject brine becomes more concentrated

and the solubility limits of scale-forming compounds such as calcium sulfate

may be exceeded.

Seawater typically is taken from a submerged inlet equipped with coarse

screens to keep out fish and trash. In most cases, concentrated brine from

seawater RO would probably be returned to the sea.

In most RO plants the high pressure brine stream is let down to a lower

pressure through a throttling valve and the energy in the high pressure stream

is not utilized. As the cost of energy rises, it becomes more and more

attractive to recover this energy, especially in large plants. A simple way to

recover some of the energy is to use hydraulic turbines connected to the high

pressure pumps or to an electric generator. A preliminary evaluation indicates

that the additional capital for power recovery is probably not justified for plants under about 10 mgd.


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MSF tubes are subjected to corrosive environments. Suitable materials vary

with the conditions of usage but they include titanium, copper-nickel alloy,

copper-nickel aluminum alloy, and aluminum brass [5].

The actual design depends on specific conditions such as the temperature andcomposition of the feed water, steam pressure available, and the cost of steam

and electricity. An actual seawater MSF plant might consist of 19-30 heat

recovery stages and 2-3 heat rejection stages.

Seawater at about 20C is screened and pumped through the condenser tubes

of the final, or heat rejection stages of the plant. With any seawater feed,

chlorination may be necessary at the source to inhibit biological growth in the

water.

After being heated in the condenser tubes, about 60% of the water is

discharged to the sea (cooling water) and the remainder is feed water for the

plant. Sulfuric acid is metered into the feed water to decompose bicarbonates,

which would otherwise cause fouling of heat transfer surfaces in the unit. The

acidified stream is pumped to a deaerator-decarbonator tower for separation of

carbon dioxide and other dissolved gases such as oxygen and nitrogen. Steam

may be admitted at the bottom of the tower to assist in separation of the gases,

which are vented to a vacuum system. A metered stream of 50% caustic may

be added to the water leaving the deaerator-decarbonator to raise the pH to a

noncorrosive level [5].

The treated seawater is mixed with concentrated brine from the last stage and

the mixture is pumped to the heat recovery section. The brine mixture containsapproximately 30% fresh feed and 70% recirculated brine. It enters the

preheater tubes and flows through the entire heat recovery section, being

preheated by the condensing product water on the outside of the tubes. When

it leaves the preheater, the brine enters a brine heater, where it is heated to its

maximum temperature, about 121C by steam from a boiler or by waste heat

from some other source. The maximum temperature is limited by the scale-

forming properties of the brine and it depends on the kind of pretreatment


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used and the concentration ratio desired. High brine temperatures are desirable

for high efficiencies and minimum cost [5].

The brine heater supplies all of the thermal energy used for vaporizing water

in the plant. From the heater, the brine passes through a flow control valveinto the first and hottest stage of the MSF unit. Because this stage is at a lower

pressure than the brine heater, part of the water in the brine flashes into vapor.

It rises through an entrainment separator and condenses on the outside of the

preheater tubes, transferring its latent heat to the brine inside the tubes. The

remaining brine at the bottom of the stage is cooled by the flashing vapor and

it passes through an orifice or flow control weir into the next stage, where the

pressure is lower and more flashing occurs. The process is repeated until the

brine leaves the last and coldest heat rejection stage at about 32C. Some of

the brine is discharged as blowdown to prevent precipitation of salts; the

remainder is recycled. The distillate that is condensed on the preheater tubes

drops into trays below the tubes and flows from stage to stage through flow

control devices much as the brine does. It is pumped from the last stage to

storage or distribution. It is essential to have a vacuum system to remove the

noncondensible gases from the unit and to maintain the reduced pressures

required. A typical system would be a three stage steam jet ejector system

with a barometric precondenser, an intercondenser, and a final condenser [5].


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2-3 Electrodialysis

Fig 2-3 Electrodialysis [6]

Electrodialysis unit ED comprises a series of alternating cation-permeable (C)

and anion-permeable (A) membranes set between two electrodes with saline

water being fed in between the membranes. As the DC current passes throughthe system the water streams become alternately diluted and concentrated

giving rise to a product and concentrate. The combination of membranes,

spacers, gaskets, electrodes, together with necessary devices for dividing,

directing and collecting the-streams of water is called an electrodialysis stack.

Each pair of membranes sealed around the edges to prevent leakage and

separated by-spacers to provide for water to flow across the membrane surface

is called a cell [7].

ED desalination cell contains two different types of ion-selective -membranes.

One of the membranes is cation-permeable allows the passage of positive ions

(cations) While the other anion-permeable membrane allows the passage of

negative ions (anions). If a direct current is established across a stream of

saline water passing between a pair of those membranes, ions acting as

carriers of electricity will migrate across the stream. The cation-permeable


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membrane will permit positive ions (e.g. Na +) to pass through while repelling

negative ions (e.g. Cl ), and the other membrane allows negative ions to go

through but not positive ions. The membranes act as one-way check valves

thus preventing the reentry of the ions they let through. Hence, the space between the membranes gets desalted while the streams on the electrode sides

become concentrated with the penetrating ions. In practical ED desalting

devices many pairs of membranes are used between a single pair of electrodes,

forming an ED stack. Plastic separators are inserted in the solution

compartment to keep the membranes apart and to promote mixing. The cells

can be stacked either horizontally or vertically. The saline water flow is

divided into many small streams in which most of the current-carrying ions

are trapped in half of these streams and desalted water in the other half. The

amount of electric current requirement varies proportionately to the amount of

dissolved salts to be removed. Increasing the number of pairs of membranes

between the electrodes increases the efficiency of current utilization [7].

2-4 Vapor Compression

Fig 2-4 Vapor Compression [5] The vapor compression (VC) distillation process is generally used for small-

and medium-scale desalting units. The heat for evaporating the water comes

from the compression of vapor rather than the direct exchange of heat from


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steam produced in a boiler. Two primary methods are used, a mechanical

compressor or a steam jet (thermo-vapor-compression).

VC is based on compression of the vapor generated by evaporating water to a

higher pressure, which allows reuse of the vapor for supplying heat for theevaporating process. Compression of the vapor may be carried out by using a

mechanical compressor (the most common way), or by mixing with small

amounts of high pressure steam (Thermal Compression). Feed water is

preheated against brine and the product leaving the system. Heat transfer

usually takes place in the form of a double falling film, which is an effective

heat transfer mechanism.

The latent heat of evaporating and condensing fluids are very equal.

Therefore, the energy required to keep process in operating is only that needed

to offset the boiling point elevation on the evaporating side and provide a

small difference is 4-5 C in order to minimize energy consumption. So that,

high performance ratios are obtained from this process[4].

The mechanical vapor compression (MVC) system is the most attractive as it

is compact and dose not require an external heating source like other systems,

but it does need highly skilled operators and has a higher maintenance cost.

The operation of the system at low temperature ranges from 60-70C, and at

this low temperature the scale and corrosion are reduced. Aluminum gives the

ability to operate at low temperatures [8].

A schematic diagram of a verticaltube design VC Unit as shown above

consisting essentially of a shell and tube evaporator, a vapor compressor, anda heat exchanger for recovering heat from the effluent brine and the distillate.

In operation, a mixture of vapor and liquid ascends through the tubes at high

velocity to provide high heat transfer coefficients. The mixture is discharged

from the top of the tubes and follows a tortuous path, as shown to separate the

vapor from liquid droplets. The vapor flows to the vapor compressor and is

discharged at a higher pressure into the shell side of the evaporator, where it

yields its heat to the brine and it is condensed, forming the product water. An


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efficient heat exchanger is necessary in order to recover heat from the effluent

streams and achieve acceptable economy of operation. Titanium plates are

used in heat exchangers in typical units[ 5].

2-5 Freezing

Fig 2-5 Freezing [5]

Freezing has a number of important technical and economic advantages as a

desalination process. These advantages include low energy consumption,

especially in comparison with distillation, and freedom from corrosion,

scaling, and fouling problems. Most of these advantages stem from the use of

low temperatures. Because scaling does not occur, pretreatment is usually not

necessary [5].The seawater feed passes through a vacuum deaerator and is cooled by

countercurrent flow of product water and reject brine in an aluminum plate

type feed heat exchanger. The stream is further cooled to within -17C of its

freezing point in a refrigerated feed cooler, then it enters the freezer- absorber

unit, which operates under a pressure of about 3.3 mm Hg. A slurry of ice

crystals in brine forms in the bottom of the unit. This vapor pressure 3.3 mm

Hg is less than that of the seawater slurry and thus provides a driving force for


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vaporizing water from it. The slurry of ice crystals in the bottom of the freezer

is pumped to a wash column, where a large portion of the brine is separated

and recycled to the freezer to maintain a concentration of about 20% ice in the

freezer. In a wash column, ice crystals rise to the top, where they are washed by a portion of the product water to remove any adhering brine. Less than 5%

of the melt is lost in washing the ice, although a much larger quantity is

applied to the top of the wash column. Most of the wash water adheres to the

crystals and is recycled between the melter and wash column. When the

washed crystals reach the top of the wash column, they are conveyed into the

melter, which is combined with the generator to eliminate vapor ductwork.

The ice is melted by hot water vapor from the absorbent regenerator. The

product water is used to cool incoming feed in the feed heat exchanger [5].

2-6 Solar Desalination

Fig 2-6 Single Basin Solar Still [9]

Solar energy can be converted to heat at low temperatures by direct absorption

or to heat at high temperatures by absorption after focusing. This heat can be

used directly in desalination processes, so called passive methods. Solar still is

the simplest form of solar desalination. It uses the solar energy directly to heat

saline water up to its evaporation. The vapors formed are condensed and

collected to obtain the product. Water in these stills evaporates at a

temperature below its boiling points. Normally evaporation occurs at 50-60C.

The average still production per day is 3.3-4.1 liter/m2

[10].


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Different solar stills were designed to improve the operating efficiencies.

These designs tried to achieve low heat capacity, low air content, vapor tight

basin and good insulation around the basin. Increasing radiation and using

mltieffect thin film diffusion stills will improve the still performance.Two important problems faced the application of solar energy in desalination.

These are the low efficiency of energy conversion and the inefficient storage

of energy. So most desalination processes are limited in capacity and work

during certain hours in the day time. Solar powered desalination systems do

not differ much from conventional desalination systems. They often consist of

feed water pretreatment, solar collectors, electric power generator, a

distillation or membrane unit with an energy storage subsystem, brine disposal

subsystem and product water storage and delivery subsystem [10].


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Chapter 3MSF and ROModeling and

Economics


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3-1 Multi-Stage Flash (MSF):

3-1-1 MSF-Once Through

Fig 3-1 MSF-Once Through

3-1-1-1 Definition and AdvantagesThe once through system has the following features [11]:

-Simple plant operation with few process control since the brine recirculation

pump is omitted as well as the heat rejection section.

-The brine concentration at the exit of the brine heater is lower if compared

with the brine recirculation design.

-The cost of chemical dosing is higher than for the recirculation system.

3-1-1-2 Simplified Model of MSF-Once Through:

Basic Assumptions:

Constant and equal specific heat for all liquid streams, Cp. Equal temperature drop per stage for the flashing brine.

Equal temperature drop per stage for the feed seawater.

The latent heat of vaporization in each stage is assumed equal to the

average value for the process.

Non-condensable gases have negligible effect on the heat transfer process.


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Effects of the boiling point rise and non-equilibrium losses on the stage

energy balance are negligible; however, their effects are included in the

design of the condenser heat transfer area.

The overall material balance equations is given by:Mf = M d + M b (3-1)

Where M is the mass flow rate and the subscript b, d, and defines the brine,

distillate, and feed. The overall salt balance is given by

Xf Mf = X b M b (3-2)

Where X is the salt concentration. Equation (3-2) assumes that the distillate is

salt free.

3-1-1-2-1Temperature Distribution

The temperature drop per stage, T, is obtained from the relation

T = (T o Tn)/n (3-3)

where n is the number of stages. Therefore, the temperature in the first and

second stages are given by

T1 = To T and T 2 = T1 T

Substituting for T 1 in the T 2 equation gives

T2 = To T T = T o2 T

so the temperature of stage i

Ti = To i T (3-4)

3-1-1-2-2 Energy Balance On Stage i Di-1 Cp T vi-1 + Bi-1 Cp T i-1 Di Cp T vi Bi Cp T i = M f C p (ti-1 ti)

Assuming the temperature difference, T i-1 Tvi-1, is small and has a

negligible effect on the stage energy balance. Thus, the above equation

reduces to

(Di-1 + Bi-1) Cp T i-1 (D i + Bi) Cp T i = Mf C p (ti-1 ti)

Recalling that the sum (D i-1+Bi-1) in each stage is equal to M f , wouldsimplify the above equation to


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Mf Cp T i-1 Mf Cp T i = M f C p (ti-1 ti)

Elimination of the like terms on both sides of the equation gives the pursued

relation, thus,

Ti-1 Ti = ti-1 ti , or generally Ti=ti

The seawater temperature, which leaves the condenser of the first stage, is

then defined by

t1 = Tf + n t

The seawater temperature leaving the condenser of the second stage, t 2, is less

than t 1 by t, so

t2 = t1t

Substituting for t 1 in the above equation gives

t2 = Tf + (n 1) t

So a general equation is obtained for the condenser temperature in stage i

ti = Tf + (n (i 1)) t (3-5)

3-1-1-2-3 Stage Material and Salt BalanceThe amount of flashing vapor formed in each stage obtained by

D1 = y M f

Where

D1 is the amount of flashing vapor formed in the first stage, M f is the feed

seawater flow rate, and y is the specific ratio of sensible heat and latent heat

and is equal toy =Cp T / av (3-6)

Where Cp is the specific heat capacity and av is the average latent heat

calculated at the average temperature

Tav = (To + Tn)/2 (3-7)

The amount of distillate formed in the second stage is equal to

D2 = y (M f D1)


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Comparison of the summations of D 1+D2 and D 1+D2+D3 gives the generalform for the total summation of the distillate formed in all stages, M d, whichis given by

Md = Mf (1 (1 y)n

) (3-9)

Equation (3-9) is used to calculate the distillate flow rate, since the feed flow

rate is always specified in a design problem.

The flow rate of the brine stream leaving stage (i) is given by

==

i

1k k f i DMB (3-10)

The salt concentration in the brine stream leaving stage i is given by

Xi = Mf Xf /Bi (3-11)

The flow rate of the heating steam, Ms, is obtained the energy balanceequation for the brine heater, where

Ms s = Mf Cp (T o t1)

The above equation is arranged to calculate M s

Ms = M f Cp (T o t1) / s (3-12)

3-1-1-2-4 Brine Heater and Condensers Heat Transfer

AreaThe brine heater area is given by

A b = M s s / (U b (LMTD) b ) (3-13)

Where LMTD is given by

(LMTD) b=((Ts To) (T s t1) ) / ln ( (T s T) / (T s t1) ) (3-14)

and U b is given by [12]

U b=1.7194+3.206310 -3Ts+1.597110 -5(Ts)2 1.991810 -7(Ts)3 (3-15)

The heat transfer area for the condenser in each stage is assumed equal.

Therefore, the calculated heat transfer area for the first stage is used to obtain

the total heat transfer area in the plant. The condenser heat transfer area in the

first stage is obtained from


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Ac = M f Cp (t 1 t2)/(Uc (LMTD) c) (3-16)

Where is given by [12]

Uc=1.7194+3.206310 -3Tv1+1.597110-5Tv12 1.991810 -7Tv1

3 (3-17)

Tv1 = T1 BPE 1 NEA 1 (3-18)

and

(LMTD) c =((T v1 t1) (T v1 t2) ) /ln ( (T v1 t1) / (T v1 t2) ) (3-19)

In the above equations (BPE) is the boiling point elevation, (NEA) is the non-

equilibrium allowance, (T v) is the condensing vapor temperature and (U c) is

the condenser overall heat transfer coefficient .BPE can be estimated from the following correlation [13]:

( ) ( )( ) 31 1 1 10 BPE X B X C = + where values of B and C in the correlation for the boiling point elevation are[12]:

( ) ( )22 51 1(6.71 6.34 10 9.74 10 )10 B x T x T = + + 38

( ) ( )23 51 1(22.238 9.59 10 9.42 10 )10C x T x T = + + The non-equilibrium allowance, NEA 1, in the first stage is calculated from the

gate height, GH 1, the height of the brine pool, H 1, the stage width, W, the

stage pressure drop, P 1-P2, and the brine density. For a good approximation

NEA 1= 0.213 oC [14].

3-1-1-2-5 Total Heat Transfer AreasThe total heat transfer in the plant is obtained by summing the heat transfer

area for all condensers and the brine heaterA = A b + n A c (3-20)

3-1-1-3 Water Production Cost Estimation of MSF-OT

PlantThe capital needed to supply the necessary manufacturing and plant facilities

is called the fixed-capital investment, while that necessary for the operation of

the plant is termed the working capital. In other words, we can say The


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Capital investment is the total amount of money needed to supply the

necessary plant and manufacturing facilities plus the amount of money

required as working capital for operation of the facilities. The sum of the

fixed-capital investment and the working capital is known as the total capitalinvestment. The fixed-capital portion may be further subdivided into

manufacturing fixed-capital investment (Direct Costs) and nonmanufacturing

fixed-capital investment (Indirect Costs) . Manufacturing fixed-capital

investment represents the capital necessary for the installed process equipment

with all auxiliaries that are needed for complete process operation. Expenses

for piping, instruments, insulation, foundations, and site preparation are

typical examples of costs included in the manufacturing fixed-capital

investment [15].

Fixed capital required for construction overhead and for all plant components

that are not directly related to the process operation is designated as the

nonmanufacturing fixed-capital investment. The construction overhead cost

consists of field-office and supervision expenses, home-office expenses,

engineering expenses, miscellaneous construction costs, contractors fees, and

contingencies [15].

The approximate formula of MSF, Once-through type can be summarized as

follow [16]:

direct capital investment, C DM is given by

CDM =0.0963 A total / Md0.27 (3-21)

where = has value between 5000-9000

A total = total heat transfer area, m 2

Md = Distillate flow rate, kg/h

Indirect capital investment, C IDM can be expressed

CIDM = 0.1 C DM (3-22)

Steam cost, C steam in $/year is given by

C steam = 8000 M s [(TS 40)/85](0.00415) (3-23)


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Chemical treatment, C che in $/year is obtained from

C che = 8000*(M f /rj) 0.024 (3-24)

where

rj = density of rejection stream, kg/m 3

Power Cost, C power in $/year is expressed as

C power = 8000 (M d/ d)*0.109 (3-25)

Where

d = density of distillate stream, kg/m 3

Spares Cost, C spar in $/year is given by

C spar = 8000 (M d / d )*0.082 (3-26)

Labor Cost, C lab in $/year is given by

C lab =8000(M d / d )*0.1 (3-27)

So, the Operation & maintenance Cost, C OM in $/year is obtained

COM = C steam + C che + C power + C spar + C lab (3-28)

Annual Cost, C AM in $/year is expressed as

CAM = C DM + C IM + COM (3-28)

Then the water production cost is found by dividing the total annual cost by

the total water product.


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3-1-2 Simplified Model of MSF- Brine Recirculation

Fig 3-2 MSF- Brine Recirculation

3-1-2-1 Definition and AdvantagesThe recirculation system has the following characteristics [11]:

Limited make-up flow rate which means small amounts of antiscalant

chemicals to be injected.

The heat rejection section acts as a deaerator and degasifier for the

make up seawater thus help to minimize the corrosion problems in the

evaporator.

Higher concentration of flashing brine compared with the once-through

arrangement.

3-1-2-2 Simplified Model for MSF- Brine RecirculationModel assumptions:

-Constant and equal specific heat for all liquid streams, Cp.

-Equal temperature drop per stage for the flashing brine.

-Equal temperature drop per stage for the feed seawater.

-The latent heat of vaporization in each stage is assumed equal to the average

value for the process.


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-The non-condensable gases have negligible effect on the heat transfer

process.

-Effects of the boiling point rise and non-equilibrium losses on the stage

energy balance are negligible; however, their effects are included in thedesign of the condenser heat transfer area.

-The temperature of the feed seawater leaving the rejection section is equal to

the brine temperature in the last stage.

3-1-2-2-1 Overall Material Balance

The overall material balance equations is given by

Mf = M d + M b (3-29)

where M is the mass flow rate and the subscript b, d, and defines the brine,

distillate, and feed. The overall salt balance is given by

Xf M f = X b M b (3-30)

where X is the salt concentration. Equation (3-30) assumes that the distillate is

salt free. Equations (3-29) and (3-30) can be rearranged to obtain the

expression for the total feed flow rate in terms of the distillate flow rate; this is

Md = M f (1- X f /X b ) (3-31)

Equation (3-31) is used to calculate M d, since the values of X b , X f , and M f

are known.

3-1-2-2-2 Temperature DistributionThe temperature drop per stage, T, is obtained from the relation

T = (T o Tn)/n (3-32)

where n is the number of recovery and rejection stages. Therefore, the

temperature in the first stage is given by

T1 = T o T

As for the second stage temperature it is equal to

T2 = T 1 T

Substituting for T 1 in the above equation gives


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T2 = T o T T = T o 2 T

The same procedure is repeated for subsequent stages and a general

expression is developed for the temperature of stage i

Ti = T o i T (3-33)

3-1-2-2-3 Energy Balance On Stage iAn energy balance on stage i in the heat recovery section gives

Di-1CpT vi-1+ B i-1Cp T i-1 D iCpT vi B iCpT i = M r C p(Tr i-1 T r i)

Assuming the temperature difference, T i-1 T vi-1, is small and has a negligible

effect on the stage energy balance. Thus, the above equation reduces to

(D i-1 + B i-1) Cp T i-1 (D i + B i) Cp T i = Mr C p (Tr i-1 T r i)

Recalling that the sum (D i-1 + B i-1) in each stage is equal to M r , would

simplify the above equation to

Mr Cp T i-1 Mr Cp T i = Mr C p (T r i-1 T r i)


relation, thus,Ti-1 T i = T r i-1 T r i , or Ti= Tri

The seawater temperature, which leaves the condenser of the first stage, is

then defined by

Tr 1= Tn +( n j) T

The seawater temperature leaving the condenser of the second stage, T r 2, is

less than T r 1 by T, so T r 2 = T r 1 T

Substituting for T r 1 in the above equation gives

Tr 2 = T n + (n j) T T

Similar to Equation (3-25), a general equation is obtained for the condenser

temperature in stage i

Tr i = T n + (n j) T (i 1) T (3-34)


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As for the temperature drop of the seawater in the condensers of the heat

rejection section, it is obtained from the stage energy balance. This is

Di-1 Cp T vi-1 +B i-1 Cp T i-1 D i Cp T vi B i Cp T i =(M f +Mcw) C p (Tf j T f j+1)

Assuming the small temperature difference, T i-1Tvi-1, has a negligible effect

on the stage energy balance. Thus, the above equation reduces to

(D i-1 + B i-1) Cp T i-1 (D i + B i) Cp T i = (M f +M cw) C p (T f i-1 T f i)

Recalling that the sum (D i-1+B i-1) in each stage is equal to M r , would simplify

the above equation to

Mr Cp T i-1 M r Cp T i = (M f +M cw) C p (T ji-1 T ji)


relation, thus,

(T ji-1 T ji) = (T i-1 T i) (M r /(M f +M cw) ) , or generally

( T ji) = Ti (M r /(M f +M cw) )

Since the temperature profile is assumed linear, the above relation can also be

obtained from the following simple relation( T ji) = (T n-Tcw) / j

The seawater temperature, which leaves the condenser of the last stage, is then

defined by

T jn = T cw+ (T ji)

This gives the general relation for the seawater temperature in the rejection

sectionT ji = T cw + (n-i+1)( T ji) (3-35)

3-1-2-2-4 Stage Material and Salt BalanceThe amount of flashing vapor formed in each stage obtained by conservation

of energy within the stage, where the latent consumed by the flashing vapor is

set equal to the decrease in the brine sensible heat. This is

D1 = y M r


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where D 1 is the amount of flashing vapor formed in the first stage, M r is the

recycle brine flow rate, and y is the specific ratio of sensible heat and latent

heat and is equal to

y = Cp T/ av (3-36)

where Cp is the specific heat capacity and av is the average latent heat

calculated at the average temperature

Tav = (T o+Tn)/ 2 (3-37)

The amount of distillate formed in the second stage is equal to

D2 = y (M r D 1)

Substituting the value of D 1 in the above equation gives

D2 = y (M r y M r )

Which simplifies to

D2 = M r y (1 y)

The D 3 balance is

D3 = y(M r D 1 D 2)

Substituting for the values of D 1 and D 2 in the above equation gives

D3 = y (M r M r y M r y (1 y) )

Taking M r as a common factor in the above equation gives

D3 = M r y (1 y y + y2

)

This simplifies to

D3= M r y (1 y)2

The balance equations for D 2 and D 3 will reveal the general form for the

formula of D i Accordingly, the resulting general formula for D i is

Di = M r y (1 y)(i 1) (3-38)

The total distillate flow rate is obtained by summing the values of D i for all

stages. The summation is performed in steps in order to obtain a closed form

equation. Therefore, the summation of D 1 and D 2 gives


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D1 + D 2 = M r (y + y (1 y) )

= M r (2 y y2 )

= M r (1 (1 y)2

)

Addition of D 3 to the above gives

D1 + D 2 + D 3 = M r ((2 y y2) + y(1 y)

2 )

This simplifies to

D1 + D 2 + D 3 = M r (2 y y2 + y 2 y

2 + y

3 )

= M r (3 y 3 y2 + y 3 )

= M r (3 y 3 y2 + y 3 )

= M r (1 (1 y)3

)

Comparison of the summations of D 1+D2 and D 1+D2+D3 gives the general

form for the total summation of the distillate formed in all stages, M d, which

is given by

Md = M r (1 (1 y)

n

) (3-39)Equation (3-39) is used to calculate the brine recycle flow rate, since the

distillate flow is calculated.

The salt concentration in the recycle stream, X r , is obtained by performing salt

balance as follows

Xr M r + M b X b = X f M f + (M r M d) Xn

The above balance is arranged toXr = (X f M f + (M r M d) Xn M b X b)/M r

Assuming that X n = X b, simplifies the above equation to

Xr = (X f M f + (M r M d) X b M b X b)/M r

Since M f = M b + M d, then,

Xr = ((X f X b) M f + M r X b)/M r (3-40)

The flow rate of the brine stream leaving stage i is given by


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==

i

1k k r i DMB (3-41)

The salt concentration in the brine stream leaving stage i is given by

i

i

1k k r iBDMX

==

(3-42)

The determination of the cooling water flow rate, M cw required to obtain the

specific cooling water flow rate, M cw,which affects the process economics.

This flow rate is obtained from an overall energy balance around the

desalination plant. The intake seawater temperature, T cw, is used as the

reference temperature in the energy balance. This gives

Ms s = M cw C p (Tn T cw) + M b C p(Tn T cw) + M d C p (Tn Tcw)The above equation is arranged to obtain an expression for M cw

Mcw = (M s s M f C p(Tn T cw))/ (C p (Tn T cw)) (3-43)

3-1-2-2-5 Brine Heater and Condensers Heat Transfer Area

The motive steam provides the brine heater with the necessary energy to

increase the feed seawater temperature from T f 1 to the top brine temperature,To. This requires calculation of the motive steam flow, which is obtained from

the brine heater energy balance

Ms s = M r Cp (T o T f1)

The above equation is arranged to calculate M s

Ms = M r Cp (T o T f1)/ s (3-44)

The brine heater area is given byA b = M s s /(U b (LMTD) b) (3-45)

where LMTD is given by

(LMTD) b=((T s To) (Ts Tf1))/ln((T s To)/(T s Tf1)) (3-46)

and U b is given by [12]

U b=1.7194+3.2063 10-3

Ts+1.5971 10-5

Ts2 1.9918 10

-7Ts

3 (3-47)


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The heat transfer area for the condenser in each stage in the heat recovery

section is assumed equal. The same assumption is made for the condenser heat

transfer area in the heat rejection section. Therefore, the calculated heat

transfer area for the first stage is used to obtain the total heat transfer area inthe heat recovery section. The condenser heat transfer area in the first stage is

obtained from

Ar = M r Cp (T r 1 T r 2)/(U r (LMTD) r ) (3-48)

Where U r [12]

Ur = 1.7194+3.2063 10-3

Tv1+1.5971 10-5

Tv12 1.9918 10

-7Tv1

3 (3-49)

Tv1 = T1 BPE1 NEA 1 (3-50)

and

(LMTD) r =((T v1 Tr 1)

(Tv1 Tr 2))/ln((T v1

Tr 1)/(Tv1 Tr 2)) (3-51)

In the above equations (BPE) is the boiling point elevation, (NEA) is the non-

equilibrium allowance, (T v) is the condensing vapor temperature and (U c) is

the condenser overall heat transfer coefficient.

BPE can be estimated from the following correlation [13]:

( ) ( )( ) 31 1 1 10 BPE X B X C = + where values of B and C in the correlation for the boiling point elevation are:

( ) ( )22 51 1(6.71 6.34 10 9.74 10 )10 B x T x T = + + 38

( ) ( )23 51 1(22.238 9.59 10 9.42 10 )10C x T x T = + +

The non-equilibrium allowance, NEA1

, in the first stage is calculated from the

gate height, GH 1, the height of the brine pool, H 1, the stage width, W, the

stage pressure drop, P 1-P2, and the brine density. For a good approximation in

brine recirculation MSF plant, the non-equilibrium allowance is equal 0.213 oC

for heat recovery section and 1.217 oC heat rejection section [14].

The same procedure is applied to the stages in the heat rejection section,

where the condenser area in rejection stages is given by

A j = (M f + M r ) Cp (T jn T cw)/(U j (LMTD) j) (3-52)


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3-2 Reverse Osmosis (RO)

Fig 3-3 Reverse Osmosis RO

3-2-1 Definition and AdvantagesRO process consists of three main steps: (a) pre-treatment. (b) membrane

passage, and (c) post treatment. In the post-treatment step, product water

passes through a decarbonation system, pH adjustment system. Chlorine

injection to comply with the required quality and use of the product water.

The purpose of the pretreatment step is to avoid any risk of clogging, fouling

or scaling the membrane. Pre-treatment is an important aspect of RO system.

All RO devices required pretreatment to remove the suspended solids,

sealants, foulants, and colloidal matters.

The benefits of RO against MSF can be stated in the following points [11]:

Limited make-up flow rate which means small amounts of antiscalantchemicals to be injected.

The flexibilities in RO in meeting various water and power ratio while

maintaining maximum process efficiency.

Corrosion problems are much less in RO than MSF.

Energy consumption is low in RO than MSF.


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3-2-2 Simplified Model of RO Design

Fig 3-4: a simplified RO membrane

Figure 3-4 shows a simplified spiral wound RO module. The mathematical

formula of this process as follows:

Pure water flux, N w in (kg/m 2.sec) is given by:

Nw= Aw ( P ) (3-57)

While the salt flux, Ns in (kg/m 2.sec) is given by

Ns= A s ( cw cp) (3-58)

Where

Aw = water permeability coefficient, m/sec.pa

As = salt permeability coefficient, m/sec

P = net applied pressure, Pa

= differential osmotic pressure, Pa

cw = salt concentration at membrane wall, kg/m 3

cp = permeate concentration, kg/m 3

The salt flux can be expressed based on the permeate concentration as Ns = N w cp (3-59)

The osmotic pressure of seawater can be expressed as [17]

= (0.6955 + 0.0025 t) 10 8 ( ci/i) (3-60)

where

ci = concentration, kg/m 3

t = temperature, C

The density of seawater, in (kg/m 3) is calculated by the formula [17]


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= 498.4 m + ( 248,400 m 2 + 752.4 m c ) (3-61)

where m is constant and equal to

m=1.0069 2.757 10 -4 t (3-62)

The net applied pressure, P in pa isP = P 2 P 1 (3-63)

where

P2 = operated pressure, Pa

P1 = atmospheric pressure, Pa

The permeate flux, N p in (kg/m 2. sec) is given by

N p = Nw + N s (3-64)

Salt concentration at membrane wall,cw in (kg/m 3)

cw = cp + (cf cp) exp [Nw/ks*1000] (3-65)

where

cf = seawater concentration, kg/m 3

ks = mass transfer coefficient, m/sec

mass transfer coefficient, ks can be calculated by[18]

ks = 1.10110 -4 u b0.5 (3-66)

Where

u b = velocity of brine, m/sec

3-2-3 Cost Methodology of RO

3-2-3-1 Water Production Cost Estimation of RO PlantEstimation of the water product cost depends on the plant capacity, site

characteristics, and design features. Plant capacity specifies sizes for various

process equipment, pumping units, and membrane area. Site characteristics

have a strong effect on the type of pretreatment and postreatment equipment,

and rates of chemicals. The electric power consumption, steam requirement

and chemicals are also design dependant. Water product cost depend on

several design and operational variables which includes [19]:


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Salinity and quality of feed water: Lower feed salinity allows for higher

conversion rates. As a result, the plant can operate with lower specific

power consumption and dosing of antiscalent chemicals. Also, downtime

related to chemical scaling is considerably reduced. Plant capacity: Larger plant capacity reduces the capital cost for unit

product. Although, the increase in the plant capacity implies higher capital.

Site conditions: Installation of new units as an addition to existing sites,

would eliminate cost associated with facilities for feed water intake, brine

disposal, and feed water pretreatment.

Qualified manpower: Availability of qualified operators, engineers, andmanagement would result in higher plant availability, production capacity,

and lower down time caused by trips of devices.

Energy cost: Availability of inexpensive sources for low cost electric

power and heating steam have strong impact on the unit product cost.

Plant life and amortization: Increase in plant life reduces the capital

product cost.

The production cost of desalted water is made up of two major components;

the capital cost and the operating cost. Capital cost covers the direct and the

indirect cost. The direct capital cost covers purchasing cost of various types of

equipment, auxiliary equipment, land cost, construction, and buildings.

Indirect capital costs include the freight and insurance, construction overhead

and contingency. Indirect capital costs are expressed as percentage of the total

direct capital cost. Operating cost covers all expenditure incurred after plantcommissioning and during actual operation. These items include labor,

energy, chemical, spare parts, and miscellaneous.

The most critical parameters in cost evaluation are the fixed charges

(amortization) and the energy cost. Other parameters that have lesser effect on

the unit product cost include the cost of chemicals and labor.

Amortization or fixed charges defines the annual payments that cover the totaldirect and indirect cost. This cost is obtained by multiplying the total direct


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and indirect cost by the amortization factor, which is defined by the following

relation [20]

1)i1(

)i1(i

a n

n

+

+=

where i is the annual interest rate and n is the plant life. Accumulated

experience in the desalination industry indicates that an amortization life of 30

years is adequate. As for the interest rate, its average value is equal 5%,

however, a range of 3-8% should be considered in economics analysis.

The following is a general procedure to estimate the water production cost in

Reverse osmosis plants:

3-2-3-1-1 Estimation of the direct capital costAs mentioned before the direct capital cost consist mainly of membrane

purchase cost (cost_mem), civil work cost (cost_civil) and pumping and

energy recover systems cost (cost_pump). These items can be evaluated as

[16]:

Membrane cost in ($) is given by

cost_mem = cost mem(A mem /Amodule ) (3-67)

where

A mod = area of one module, m 2

A mem = total Area of membrane, m 2

Civil work cost in ($) is obtained fromcost_civil = 2390 Q ref 0.8 (3-68)

where

Q ref = reference RO plant capacity, m 3/day

Pumping and energy recovery system cost in $ is given by

cost_pump = 0.0141 [(Q ref 101.32 P)/R f ] (3-69)

where

R f = recovery fraction


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power cost = 0.04 $/m 3

power consumption = 5 kWh/m 3 as a typical RO power consumption

value.

RO plant availability = 0.90The annual chemicals cost in $/m 3 is approximately give as [20]:

cost_chem = 0.03 $/m 3

The annual membrane replacement cost is normally taken as 10% of the

membrane purchase cost:

cost_mem_rep = 0.10 cost_mem (3-74)

The annual labor cost is approximately equal to 0.05 $/m 3 [19]:

cost_labor = 0.05 $/m 3

Then the water production cost is found by dividing the total annual costs by

the total water product.


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3-3 Comparative Study of MSF & ROThe applications of MSF distillation have been mainly large seawater

desalting plants producing water for municipal, industrial, or power generating

uses mostly on the Middle East.

An important characteristic of MSF plants is that scaling has little effect on

their production capacity. It reduces the thermal efficiency and thus more

energy is required per unit of fresh water, but as long as the required quantity

of water can be flashed through the design temperature range, the design

output can be maintained. MSF plants have therefore been favored where

reliability of water quantity has been of major importance.Comparison of the two design arrangements MSF clearly indicate that the

once-through option is more economical particularly for MSF units of large

capacities due to its simplicity of construction and lower pumping power

requirements.

RO is a recent technology for seawater desa1ting, although it has been

successfully used for many years to purify brackish waters. It reduces the

seawater TDS from approximately 41,000 ppm to less than 1,000 ppm.

Indications are that the RO plant has significantly lower capital and operating

costs than do MSF plants in the same area, even though the MSF plants are

much larger. The plant will be important in assessing the long-term reliability

of large scale RO in comparison with MSF in a difficult application. The

reliability of an RO plant is not so closely related to plant capacity as it is in a

distillation plant, where equipment size increases and design problems tend to

multiply as capacity is increased. Large capacity in an RO plant is obtained by

using a large number of standard units rather than by increasing the unit size.

Reliability can therefore be adequately demonstrated with a small plant. RO

has major advantages over MSF and other distillation processes in terms of

capital and operating costs.

RO also has significantly lower absolute energy requirements per unit of water produced than does MSF, although this may not translate directly into a cost


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benefit if MSF uses a low-cost, degraded steam. A large part of the energy

used for RO is consumed in raising the feed water to operating pressure,

which in a seawater plant may be 800 to 1500 psig.

At low fuel cost MSF will continue to be the optimum choice for largedesalting plants. For moderate capacities, RO offers low water cost. RO would

be the correct selection if fuel costs were increased. In any case an RO/MSF

hybrid plant is worth considering. It gives the lowest possible water cost if it

is associated with a dual plant.

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

and other plant auxiliaries, mainly the pretreatment processes. RO power

consumption depends mainly on water recovery and the working pressure.

The following can summarize some of the technical differences between RO

and MSF [11]:

1- Seawater intake in MSF is twice that of RO.

2- Energy consumption per cubic meters in MSF is about three times that

of RO.

3- Volume and area required for MSF are large compared to those required

for RO.

4- Pumping energy in RO is about 25% of that required for MSF. A

possible decrease in pumping consumption in RO might be while using

energy recovery systems.

5- RO has no thermal energy consumption. In 2,750 m 3/d MSF about 8 MW

of thermal energy is consumed. This can be very expensive if not extractedfrom the steam turbine.

6- Heavy foundation and extensive civil work is required by MSF due to its

heavy weight.

The main benefits of RO against MSF are Limiting make-up flow rate which

means small amounts of antiscalant chemicals to be injected, Corrosion

problems are much less in RO than MSF and energy consumption is low in

RO than MSF [11].


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Chapter 4Hybrid

DesalinationSystems


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4-1 Identification of Hybridization SystemHybridization System is a combination of two or more processes in order to

provide a better cost effective product than either alone can provide. In

desalination, there are membrane and thermal (distillation) processes. The

effective integration of membrane / thermal desalination technology is to

reduce the cost of desalination and electrical power consumption. There are

two Possible Options for Hybridization System, Simple Hybridization and

Integrated Hybridization.

In the simple hybrid MSF/ RO desalination process, the seawater RO plant is

combined with either a new or existing dual-purpose MSF plant but theintegrated MSF/RO desalination process is much more advance than simple

hybrid such as:

The RO feed water temperature is optimized and controlled by using

cooling water from the heat-reject section of the MSF plant.

Brine discharged from RO plant is combined with brine recycle in

MSF.

4-2 Advantages of hybrid MSF/ RO systemHybrid RO MSF desalination-power process has the following advantages

[16,21]:

The capital cost of the combined RO/MSF plant can be reduced.

A common seawater intake is used.

Product waters from the RO and MSF plants are blended to obtain

suitable product water quality.

A single-stage RO process can be used.

RO membrane life can be extended by blending the high purity

distillate of the MSF plant with the RO permeate


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MSF plant provides better control of the feed water temperature to

the RO plant by using the warm reject coolant water from the MSF

heat rejection section.

The low-pressure steam from the power plant can be used todeaerate and warm up the feed to the RO plant this minimizes

corrosion hazard by eliminating residual chlorine and dissolved

oxygen.

A common post-treatment plant can handle the combined product

of the two plants.

Electric power production from the MSF plant can be efficientlyutilized in the RO plant, thereby reducing net export power

production.

Blending MSF product with RO product water reduces the

temperature of the MSF product water. RO for high-pressure brine

without energy recovery can be used to cool the MSF product

water.

Preheat RO feed water increases the recovery significantly.

The RO reject brine can be combined with the recycle stream in

the MSF plant.


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Layyah plant, Sharjah, UAE. They found the hybridization of sweater reverse

osmosis (SWRO) and the multi stage flash (MSF) technology was considered

to improve the performance of latter and reduce the cost of the produced

water. Integrating MSF with RO has been implemented recently in Fujairahdesalination plant with a total capacity of 100 MIGD of which 37.5 MIGD are

produced by RO and the balance is produced by MSF.

They studied the data search and collection, technology selection, water

demand, power demand, and fuel availability were all used to come up with

optimum scenarios to reduce cost of power and water generation and to meet

increasing demand of water and power.

Al-Mutaz [21] investigated the Desalination plants required. He found that,

the Desalination plants require significant amounts of energy in the form of

heat and/or electricity (power). This energy can be supplied by nuclear

reactors since nuclear reactors are used mainly for the production of either

heat or power. Coupling desalination plants with nuclear reactors gives many

economical and technical advantages. The hybrid RO MSF desalination plant

coupled to nuclear power plant gives high overall availability factor. He

suggested that, the CANDU PHWR is the appropriate type of nuclear reactor

for coupling with hybrid RO MSF desalination plants. This hybrid system has

potential advantages of a low power demand, improved water quality and

possible lower running cost.

Al-Mutaz et al. [23] reviewed the hybrid plant concept. They found that the

hybrid plant is only useful to bring the salinity of the water produced from RO plants to an acceptable limit. If high pressure membranes(higher than 80 atm)

are developed and become reliable and fuel cost increases, the reverse osmosis

plants would certainly replace the MSF Plants Al-Mutaz and his team

concluded that more efforts should be given to scaling up MSF and RO plants

and for developing high pressure membrane.

Jacques Andrianne and Felix Alardin [24] found that the reduction in cost and

the improved economics of desalination plants are essential elements for the


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development of communities. Energy, capital, and operating costs are key

issues of water desalination economics. The choice will depend on the specific

conditions prevailing on site, such as existing facilities, power and water

demand increase, land availability, raw water quality, quality of water to be produced, ratio between power and water production, ratio between thermal

and membrane desalination.

Essam El-Sayed et al. [25] made a experimental testing of RO performance in

an MSF/RO hybrid model for 1,800 h. The RO feed water temperature

ranging from 24 to 31C. They found a significant increases in the RO product

water flow rate when the data from before and after hybridization were

compared. 42-48% gain in the product water flow rate for RO plant operating

at temperature of 33C, over that operating at 15C. The energy consumption

of RO can be reduced to the level of 5.2 kWh/m 3 using a simple MSF/RO

hybrid arrangement in which the RO plant is fed the preheated seawater

rejected from the MSF heat rejection section.

M. Turek and P. Dydo [26] determined the highly concentrated solutions

cannot be treated by RO as a consequence of a physical limit imposed by their

osmotic pressure value as well as the scaling phenomenon. Scaling

phenomenon can be diminished with partial softening of RO feed with

nanofiltration membranes. They found the integration of NF, RO and MSF

makes it also possible to overcome these limits. The performance of

desalination and concentration in hybrid membrane-thermal systems seems to

be higher than the performance of simple membranes. They determined thethermal methods seem to be more effective than membrane methods in term

of Production of highly concentrated brines.

Neil M. Wade et al.[27] studied the cost of water production .They found the

cost of water produced by RO plant is less sensitive to increase in fuel cost

than MSF plant in dual purpose schemes. At fuel costs above $35 per barrel

there would be substantial cost savings for RO plant compared with

conventional dual purpose plant.


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Also, Al-Mutaz and Alabdula'aly [28] reviewed the issue of water production

cost for RO and MSF in Saudi Arabia. The results indicate that the RO has

lower capital and production costs. At high membrane replacement rate and

low energy cost. MSF offers lower water production cost specially at largecapacity plants.

Van Dijk et al. [29] discussed the influence of design parameters, such as raw

water temperature and total dissolved solids content (TDS), design pressure,

and recovery, on total unit cost for the production of desalinated water from

the Arabian Gulf in Saudi Arabia. They found the total water costs are

influenced significantly by raw water TDS. Also, they found the cost of

seawater RO increase from 1.62 $/m 3 at 35,000 ppm TDS to 2.31 $/m 3 at

44,000 ppm TDS to 3.84 $/m 3 at 55,000 ppm TDS. Operation at a pressure of

900 psi rather than 800 psi will lead to a significant drop in total water costs

down to 2.05 $/m 3 at 44,000 ppm TDS, and to 3.09 $/m 3 at 55,000 ppm TDS.

As the effects of this increased pressure are yet unknown, it is recommended

to study these effects. Total water costs may decrease significantly when

membrane costs are lower.

Braj M. Misra et al. [30] reviewed Hybrid system (MSF/RO). There is

significant reduction in the operation and maintenance (O&M) cost of

desalted water by taking the advantage of producing both process and drinking

quality water, common pretreatment to a considerable extent and possibility of

using reject streams from one plant to the other. Preliminary investigations

also, have been carried out in terms of using ultrafiltration (UF) asapretreatment for RO and nanofiltration (NF) to improve the performance of

MSF and RO. UF will reduce the fouling and scaling of RO giving longer life

of the membrane elements. NF will remove the hardness and scaling

constituent improving the recovery for RO and increasing the top brine

temperature for MSF.

Osman A.Hamed [31] studied the simple and fully integrated hybrid

desalination systems. He found the hybrid desalination systems combining


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with power generation systems are currently considered a good economic

alternative to dual purpose evaporation plants. Hybrid (membrane /thermal

/power) configurations are characterized by flexibility in operation, less

specific energy consumption, low construction cost, high plant availability and better power and water matching.


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4-4 Options for Hybridization System

4-4-1 Simple Hybrid Desalination Plants

4-4-1-1 Simplified MSF-OT/RO

Fig4-1: Simplified MSF-OT/RO Design

Design Procedure

In the simple hybrid multistage flashreverse osmosis (MSF/RO) desalination

power process, a seawater RO plant is combined with either a new or an

existing dual purpose MSF plant in the simplest possible manner. In this case,

the seawater feed is split to two streams. One of them to MSF-OT unit and the

other is to RO unit. RO product water is combined with MSF product water.

The overall material balance equations is given by

MHf = M Hd + M H b (4-1)

Where M is the mass flow rate and the subscript Hb, Hd, and Hf defines the

brine, distillate, and feed.

where

Mass balance for hybrid product

MHd=M d + F p (4-2)

Salt balance


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XHd MHd=c p FP (4-3)

Assuming the product from MSF is salt free.

The overall salt balance is given by

X Hf M Hf = X Hd M Hd + X Hb MHb (4-4)

The mass balance and the energy balance for the MSF-OT will be the same as

the stand alone MSF-OT. The RO also is at the same condition.

Cost MethodologyThe water production cost of Simple Hybrid Desalination Plants equal to the

summation of annual water production cost of MSF section and annual water production cost of RO section and also the total annual plant intake-outfall

cost of hybrid plant divided by the summation of water production of MSF

section and RO section.

The annual Cost of MSF-OT procedure calculation and Annual Cost of RO

procedure calculation will be the same as before.

Annual Cost of MSF-OT, C AM in $/year isCAM = C DM + C IM + C OM

The water production cost of RO is calculated from the total annual cost. The

total annual cost has following major cost items:

- the annual fixed charges,

- the annual electric power cost,

- the annual chemicals cost,

- the annual membrane replacement cost,

- the annual labor cost,

RO_prod_cost = cost_fix + cost_ep + cost_chem + cost_mem_rep +

cost_labor (4-5)

The total annual plant intake-outfall cost of hybrid plant has the following

[11]:

hybrid electromechaical equipment cost


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cost_H1=44000+70*F_ref (4-6) hybrid civil work cost

cost_H2=150000+375*F_ref (4-7)

hybrid elctrochlorinationcost_H3=22000+12000*F_ref (4-8)

hybrid brine disposal cost

cost_H4=25000+10*B_ref (4-9)

Where

F_ref = reference feed flowrate = has value between 200-1000 m 3/h

B_ref= reference blowdown flowrate = has value between 40-1000 m 3/h

The reference hybrid intake-outfall cost:

cost_H_total=cost_H1+cost_H2+cost_H3+cost_H4 (4-10)

the annual plant intake-outfall cost in $/year:

COST_HA=0.0963*(cost_H_total)*(f_ratio 0.9) (4-11)

Then, the total annual cost of SIMPLE HYBRID (MSF-OT/RO)

COST_SIM_HY=(C_AM + cost_RO_total + 1.1*COST_HA) (4-12)

WATER COST OF SIMPLE HYBRID (MSF-OT/RO) in $/m 3

total_cost=(C_AM+cost_RO_total+1.1*COST_HA)/M_Hd1 (4-13)

Where

M_Hd1= the summation of water production of MSF section and RO section

in m 3/year .


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4-4-1-2 Simple MSF-BR/RO

MSF- BR

RO

M Hf

Mf

Frj

Mb

M Hb

Fp

MdM Hd

M cw

Fig4-2: Simple MSF-BR/RO

Design Procedure Same as before, the seawater feed is split to two stream. One is to MSF-BR

unit and the other is to RO unit and RO product water is combined with MSF

product water to become one steam.The overall material balance equations is given by

MHf = M Hd + M H b+M cw (4-14)

Where M is the mass flow rate and the subscript Hb, Hd, and Hf defines the

brine, distillate, and feed.

where

Mass balance for hybrid product

MHd=M d + F p

Salt balance

XHd MHd=c p FP

Assuming the product from MSF is salt free.

The overall salt balance is given by

X Hf M Hf = X Hd M Hd + X Hb MHb+X f Mcw (4-15)


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The mass balance and the energy balance for the MSF-OT will be the same as

the stand alone MSF. The RO also is at the same condition.

Cost MethodologyThe same procedure was used for evaluating the water production cost in

MSF-BR/RO as used in MSF-OT/RO .

optimal design of hybrid msfro desalination plant

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