Exergoeconomic analysis of a combined water and power plant 2

Steam Cycle Part Load Calculations 3.3.1 Gas Turbine Plant 3.3.2 Steam Turbine Plant Combined Water and Power Plant MED Mathematical Model 3.5.1 Overall Plant Calculations 3.5.2 Individu

A THESIS SUBMITTED

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Acknowledgements

ACKNOWLEDGEMENTS

The author would like to express his profound gratitude and appreciation to his supervisor and mentor Associate Professor M.N.A Hawlader for his invaluable guidance, supervision and inspiration He also would like to thank his co-supervisor Professor A S Mujumdar for his supervision and inspiration

The author would like to extend his gratitude to the FYP students working on this project, Mr Chang He, Mr Ang Boon Wee, Mr Kuan Sien He is also thankful for the technical help received from Mr Yeo Khee Ho and Mr Chew Yew Lin, Thermal Process Lab1 and Mr Tan Tiong Thiam, Energy Conversion Lab

Finally, the author would like to express his heartfelt gratitude to his family members whose inspirations have helped him to carry on studies at this level

x

xv xxiv

LITERATURE REVIEW Combined Water and Power Plant Desalination Processes

Thermal desalting processes 2.3.1 Multistage flash distillation (MSF) 2.3.2 Multi-effect distillation (MED) 2.3.3 Comparisons of MED and MSF Reverse Osmosis (RO)

Simulation of Thermal Desalting Process Experimental investigations

Waste heat utilization in MED Heat transfer augmentation Exergy Analysis of Desalination Plants Exergoeconomic Analysis

Steam Cycle Part Load Calculations 3.3.1 Gas Turbine Plant 3.3.2 Steam Turbine Plant Combined Water and Power Plant MED Mathematical Model 3.5.1 Overall Plant Calculations 3.5.2 Individual Effect Calculations Exergy Analysis of thermal desalination processes Vertical Single tube heat exchanger

3.7.1 Water Temperature variation 3.7.2 Two-phase flow: vapor generation 3.7.3 Model for Pressure drop in two phase region 3.7.4 Heat Transfer Coefficient for Tubes with Inserts 3.7.5 Experimental & Theoretical Overall Heat Transfer Coefficients

Performance Ratio Water Production Rate Specific Heat Transfer Area Mathematical Model of Reverse Osmosis (RO) system 3.11.1 Average volumetric flux

3.11.2 Average salt flux

3.11.9 Exergy Analysis for a RO module 3.11.10 Exergoeconomic Model of RO plant EXPERIMENT

The Desalination Unit 4.1.1 Operation of the desalination unit 4.1.2 Specifications of the components Design of the components

Test procedure Vertical Single tube heat exchanger Uncertainty analysis

Experiments on Reverse Osmosis system 4.6.1 Operations of the RO system 4.6.2 Calibration of measuring instruments 4.6.3 Experimental program

4.6.4 Experimental procedure 4.6.5 Water analysis

RESULTS AND DISCUSSION Combined cycle power plant: simulation results 5.1.1 Power plant simulation results

5.1.2 Power plant simulation: validation with Thermoflow®

Combined water and power plant: simulation results 5.2.1 Effect of Combined cycle load on Thermal Efficiency 5.2.2 Combined Water and Power Plant (CWPP): comparison of performance with Rensonnet et al (2007) 5.2.3 Effect of Combined cycle load on Water Production 5.2.4 Electricity cost variation with Oil Price

Experimental results of MED 5.3.1 Effectof Heating Medium Flow Rate 5.3.2 Effect of Heating Medium Temperature 5.3.3 Effect of Feed Flow Rate

5.3.4 Effect of feed temperature and flashing 5.3.5 Effect of top-brine temperature

5.3.6 Effect of variable concentration Thermal hydraulic performance comparisons for different tube profiles

5.4.1 Feed temperature distribution inside the tube 5.4.2 Variation of vapour quality along the length of the tube

5.4.3 Shell side heat transfer coefficient 5.4.4 Tube side heat transfer coefficient

MED parametric evaluation 5.5.1 Distillate production with varying number of effect 5.5.2 Effect of Top Brine Temperature on Distillate production

5.5.3 Effect of Top Brine Temperature on exergy efficiency

of effects Experimental Results for single tube vertical heat exchanger 5.6.1 Effect of Heating Medium Temperature

5.6.2 Effect of insertions on water production 5.6.3 Effect of heating medium flow rate on Overall heat

transfer coefficient 5.6.4 Effect of Feed Temperature 5.6.5 Effect of Feed Flow Rate 5.6.6 Effect of Insertions and Corrugated Tube Profile 5.6.7 Effect of Corrugation Pitch and Depth

RO parametric evaluation 5.7.1 Validation of the RO simulation program 5.7.2 Effect of Feed Flow rate

5.7.3 Effect of Feed pressure 5.7.4 Effect of Feed pressure and flow rate on water cost 5.7.5 Effect of Volumetric flow rate on exergy efficiency Optimization of the CWPP plant

5.8.1 Optimization study of the MED plant 5.8.2 Fuel Cost sensitivity on the water cost CONCLUSIONS

Table of Contents

REFERENCES

APPENDIX A MED simulation program

APPENDIX B Calibration graph

APPENDIX C Uncertainty analysis

APPENDIX D Spiral Wound Membrane Specification APPENDIX E MED Experimental Results APPENDIX F Property functions of Seawater

APPENDIX G Heat transfer characteristics of tubes

199

208

214

219

224

226

237

241

When MED and RO plants are coupled to the combined power plant, the thermal efficiency of the plant increases by about 15% This leads to a lower cost of water production, which is about 0.63 US$/m3 The study of the CWPP has shown that, the CC+MED+RO produce the distillate and the electricity with the lowest cost under the set conditions Also, CC+MED+RO exhibits maximum overall exergetic efficiency Experimental investigation of the MED system reveals the significant effect of flashing on the overall performance of the system For a 100C superheat of the feed water, the system produces 41.45 kg/hr of fresh water from an effective heat transfer area of 5.32 m2 with a performance ratio of 1.45 in the corrugated profile The performance ratio was found to increase significantly during the preheating of the feed temperature

Experiments were performed with various salt concentrations of 15,000-35,000 ppm

to find the sensitivity of the system to feed water concentrations Hot water was used

as the heating fluid in the shell and tube two-phase heat exchanger and the

A lab scale experimental set up for RO was designed to work with variable feed concentration of 25,000ppm – 33,000ppm The feed temperature and pressure were varied from 26oC – 32oC, 48.16 bar to 56.16 bar, respectively The performance of the system was evaluated in terms of heating medium and feed water temperature and flow rate, top brine temperature (TBT) and feed concentration

Comparison between experimental and simulation results showed good agreement The RO experimental results showed that, feed pressure had strong positive impact

on the product flow rate and exergy destruction Also, the exergetic efficiency increases rapidly with increase of the feed flow rate

Results showed that the use of a power plant coupled with MED and RO can save about 20% energy savings Moreover, it can reduce the water production cost by about 23% The simulation results for the optimization study reveals that, MED performs better with total number of effects of 11.The product salt concentration was

in the range of 20-30 ppm This experimental and simulation study indicated that, the CWPP system could effectively be used as a probable solution for arid and semi arid areas economically

C Area correction factor for different tube profile

c Salinity or salt concentration (kgm-3)

Cp Specific heat at constant pressure (kJkg-1k-1)

D Mass diffusion coefficient, (m2s-1)

∆ Effective pressure difference (Pa)

f Concentration gradient coefficient (kgm-4)

h Height (m) or enthalpy (kJ/kg)

J Average volumetric flux, (m3s-1)

J2 Average salt mass flux, (kgm-2s-1)

k Mass transfer coefficient, ms-1 or thermal conductivity, (W/ m K )

k’ Cp/Cv

k1 Water permeability coefficient, (ms-1bar-1)

k2 Solute permeability coefficient, (ms-1)

List of Symbols

xi

kf Friction parameter, (m-2)

k1m Constant in water permeability coefficient

k2m Constant in salt permeability coefficient

km Constant in mass transfer coefficient

L Membrane length (axial), (m)

LMTD Log mean temperature difference, ( K)

.

m Mass flow rate, (kgs-1)

M Molecular mass, (kgkmol-1)

P Pressure, (Pa)

PT Pitch of the tube-bundle (m)

q A constant for RO (m), and heat for other cases (W)

Q Volume flow rate, (m3s-1)

R Universal Gas Constant, (kJkg-1K-1)

List of Symbols

xii

U Overall Heat transfer coefficient, (kW/m2.K)

w Membrane width (tangential), (m) or specific work, (kJkg-1)

W Power output, (kW)

x Coordinate along the membrane length, m or quality of steam

X Exergy , (kW)

y Coordinate along the membrane width, (m)

Z Capital cost, (US$)

Ψ Specific exergy (kW/kg)

Schematic of a combined water and power plant Schematic Diagram of Combined Cycle Power Plant T-s diagram of a Brayton Cycle

T-s diagram of a Rankine Cycle T-s diagram of a Rankine Cycle (for bled steam)

A schematic summarizing the exergy flows into a MED plant Schematic Diagram of Forward Feeding MED Plant

A schematic diagram of the exergy flow in the first effect

A schematic diagram of the exergy flows from the 2nd effect to the 2nd last effect

A schematic summarizing the exergy flows into the last effect

A schematic summarizing the flow entering a pump Energy balance on an element of the heat exchanger Geometry of a corrugated tube

Diagram for quality generation

Diagram for pressure variation along tube-side Smooth tube with insertions configuration dimensions

Unwound spiral wound module (1 leave) Flowchart of RO Mathematical Model

A Photograph of the desalination Rig

A Photograph of the Evaporator The layout of tubes inside the evaporator Corrugated Cu-Ni (90-10) Tube-bundle Schematic diagram of the feed water tank Schematic diagram of the heating medium tank

A photograph of the Vacuum pump

A photograph of the Blowdown pump Schematic of the single tube experimental setup

Four different evaporator tubes used Corrugated tube dimensions

Four smooth evaporator tubes of different materials Two evaporator tubes of different corrugation depths Three evaporator tubes of different corrugation pitch

A photograph of the insertions used in the experiment Hot water bath

3D view of shell tube heat exchanger Flow pattern within the shell tube heat exchanger

RTDs are connected to this data logger Flow meters to regulate the heating medium and feed flow rates Vacuum pump

Photograph of the RO experitmental set up

121

125

127

128 Figure 5.1

Exergetic efficiency of power plants for different electrical loads Variation of performance ratio and specific heat transfer area with heating medium flow rate

Variation of fresh water production rate (kg/hr) with heating medium flow rate

Variation of Overall heat transfer coefficient with heating medium flow rate

Variation of fresh water production rate (kg/hr) with heating medium temperature

Variation of concentration factor with heating medium temperature

Variation of fraction of equilibrium with the heating medium temperature

Variation of performance ratio with feed flow rate Variation of fraction of equilibrium with feed flow rate Variation of specific heat transfer area and concentration factor with feed flow rate

Variation of performance ratio with feed temperature

Variation of the feed water temperature along the length of the tube

Variation of the feed water temperature along the length of the tube

Variation of quality along the length of the tube Variation of the shell side HTC with HM flow rate Variation of the tube-side heat transfer coefficient with heating medium flow rate

Distillate productions for different No of effects at constant TBT Distillate production variations with number of effects

Graph of exergy efficiency with varying top brine temperature Graph of exergy efficiency with varying number of effect Graph of cost per unit exergy of distillate with varying number of effects

Graph of transfer heat transfer coefficient along MED plant

Graph of exergtic efficiency with varying feed flow rate Exergetic efficiency with varying feed pressure

Graph of exergy destruction with feed pressure in RO Graph of cost of water with volumetric flow rate Variation of exergtic efficiency with volumetric flow rate Optimization graph of cost of product with number of effects in MED

Impact of Fuel cost on Distillate Cost Flowchart of MED Mathematical Model Calibration of conductivity meter (0-1500 ppm) Calibration of conductivity meter (1500-5000 ppm)Calibration of conductivity meter (5000-50000 ppm) Calibration graph of Channel-1

Variation of overall heat transfer coefficient with heat medium temperature for smooth tubes with insertions

Variation of water production rate with heat medium temperature for corrugated tubes with variable corrugation pitch and depth

Variation of performance ratio with heat medium temperature for corrugated tubes with variable corrugation pitch and depth

Variation of specific heat transfer area with heat medium temperature for corrugated tubes with variable corrugation pitch and depth

Variation of overall heat transfer coefficient with heat medium flow rate for corrugated tubes with variable corrugation pitch and depth

Variation of overall heat transfer coefficient with heat medium flow rate for smooth tubes

Variation of water production rate with heat medium flow rate for corrugated tubes with variable corrugation pitch and depth

Variation of performance ratio with heat medium flow rate for corrugated tubes with variable corrugation pitch and depth

Variation of specific heat transfer area with heat medium flow rate for corrugated tubes with variable corrugation pitch and depth

Variation of overall heat transfer coefficient with feed temperature for smooth tubes

Variation of performance ratio with feed temperature for corrugated tubes with variable corrugation pitch and depth Variation of performance ratio with feed temperature for smooth tubes with insertions

Variation of specific heat transfer area with feed temperature for corrugated tubes with variable corrugation pitch and depth

Variation of specific heat transfer area with feed temperature for smooth tubes and smooth tubes with insertions

Variation of water production rate with feed flow rate for corrugated tubes with variable corrugation pitch and depth

Variation of water production rate with feed flow rate for smooth tubes

Variation of water production rate with feed flow rate for corrugated tubes with variable corrugation pitch and depth Variation of overall heat transfer coefficient with feed flow rate for smooth tubes

252

253

253

List of table

List of Tables

Table 2.1 Comparison Summary of Desalination Processes 16

Table 4.2 Specification of Corrugated Cu-Ni (90-10) tube profile 104 Table 4.3 Technical Specification of PTFE Coating 105 Table 4.4 Operating variables and their ranges during the

Table 4.6 Specifications of the twelve evaporator tubes 114

Table 4.9 Range of experimental parameters 129

Table 5.1 Different input parameters for combined cycle power

plant for Siemens V94.2

132

Table 5.2 First law Power plant efficiencies and comparison with

Franco et al (2002)

133

Table 5.3 Comparison of results with thermoflow for combined

cycle power plant

Table E1 Experimental results for the variation of heating medium

flow rate (Single-fluted Aluminum Tube bundle)

226

Table E2 Experimental results for the variation of heating

medium flow rate (Smooth Cu-Ni (90-10) Tube-bundle)

226

List of table

Table E3 Experimental results for the variation of heating

medium flow rate (Corrugated Cu-Ni (90-10) bundle)

Tube-227

Table E4 Experimental results for the variation of heating medium

flow rate (PTFE-Coated Aluminum Tube-bundle)

227

Table E5 Experimental results for the variation of heating medium

temperature (Single-fluted Aluminum Tube bundle)

228

Table E6 Experimental results for the variation of heating medium

temperature (Corrugated Cu-Ni (90-10) Tube-bundle)

228

Table E7 Experimental results for the variation of heating medium

temperature (Smooth Cu-Ni (90-10) Tube-bundle)

229

Table E8 Experimental results for the variation of heating medium

temperature (PTFE Coated Al Tube-bundle)

229

Table E9 Experimental results for the variation of feed flow rate

(Single-fluted Aluminum Tube bundle)

230

Table E10 Experimental results for the variation of feed flow rate

(Smooth Cu-Ni (90-10) Tube-bundle)

230

Table E11 Experimental results for the variation of feed flow rate

(Corrugated Cu-Ni (90-10) Tube-bundle)

231

Table E12 Experimental results for the variation of feed flow rate

(PTFE Coated Aluminum Tube-bundle)

231

Table E13 Experimental results for the variation of feed water

temperature (Single-fluted Aluminum Tube bundle)

232

Table E14 Experimental results for the variation of feed water

temperature (Smooth Cu-Ni (90-10) tube-bundle)

232

Table E15 Experimental results for the variation of feed water

temperature (Corrugated Cu-Ni (90-10) Tube bundle)

233

Table E16 Experimental results for the variation of feed water

temperature (PTFE-Coated Al Tube bundle)

234

Table E17 Experimental results for the variation of feed water 234

Table E19 Experimental results for the variation of feed water

concentration (Corrugated Cu-Ni (90-10) Tube bundle)

235

Table E20 Experimental results for the variation of feed water

concentration (PTFE coated Al Tube bundle)

236

Introduction

CHAPTER 1

INTRODUCTION

At present, desalination of sea water is one of the effective methods to produce a large supply of

fresh water Multi-Stage Flash Distillation (MSF) and Multi-Effect Distillation (MED) and

Reverse Osmosis (RO) are the widely used desalting processes for large scale production of fresh

water

All desalting systems require energy to operate As the demand for greater efficiency of using

fuel increases, studies had been carrying out on the above mentioned systems to enhance their

efficiencies Producing both power and desalted water by a combined water and power plant has

better efficient use of fuel as compared to separate power and desalting plants Therefore,

cogeneration of electricity and water from a single fuel has been gaining an increasing interest to

produce water more energy-efficiently and economically In recent years, as a result of the

continuing energy crisis, much research effort has been directed towards developing

energy-efficient plants Recent advances of desalination technologies and its contemporaries are

discussed before the research objectives are described

Combined heat and power (CHP) cycles have been deployed to produce electricity and heat for

heating purpose At present, combined-cycle power plants meet the growing energy demand with

least fuel consumption It is, therefore, needed to develop strategies for the optimization of these

Introduction

systems The efficiency of an energy conversion system is determined according to the first law

of the thermodynamics

The second law of thermodynamics, on the other hand, deals with the quality of energy and

determines the maximum amount of work obtainable from an energy resource Therefore, the

application of the exergy analysis in the context of the second law of thermodynamics to the

thermodynamic processes has become a necessity.Dincer (2002) considered the design of more

efficient power systems by reducing the exergy losses Shin et al (2002) pointed out that the

combined-cycle power plants have several advantages including high efficiency and low

emission Miyazaki et al (2000) considered a combined-cycle power plant using waste energy

and liquefied natural gas (LNG) to determine the conditions for the highest performance of the

system

E1-Nashar (1999) investigated the performances of different configurations of Combined Water

and Power plant focusing on integrating a power plant with Multi Stage Flash (MSF) Among the

systems considered was one with back-pressure steam turbines (BP-ST) and the discharge steam

directed to desalination; controlled extraction-condensing steam turbines (EC-ST), where the

steam for desalination is bled for Multi Stage Flash (MSF) It discussed a wide variety of options

available for combining conventional power plants with desalination plants Also, the influence

of the technical and economic performance parameters of each combination was discussed This

study concluded that the optimum cogeneration option depends strongly on the load variation

throughout the year Both monthly electrical and water production loads should be input

parameters to the computer model, as hinted by E1-Nashar (1999)

Kamal and Tusel (2003) in a recent study has hinted on the synergies in integrating power plants

and sea water reverse osmosis (SWRO) with MSF The theoretical analysis presented in the

Introduction

study concluded that stand alone MSF plants should be retrofitted or integrated with RO to

remain competitive Almost all the available information in the literature shows that optimization

for MSF/MED plants has been done using the thermodynamic model based on the First Law

coupled with economical model (Al-Sulaimman et al., 1995, El-Sayed et al., 1998; Al-Mutaz,

2003)

Although MSF has been widely used in most of the Middle Eastern countries, but recent

developments in MED in the last few years have brought this process to compete technically and

economically with the MSF process MED is reportedly superior to MSF in terms of

performance ratio and usage of low grade heat resources So, given the technical advantages over

MSF, MED promises a better potential for next generation Combined water and power plant The

part load condition is utilized, i.e when the power plant is running at part load, RO starts

operation to increase the electricity load demand Consequently, the fuel efficiency will increase

as well as the water demand is fulfilled In view of the above mentioned scenario, to improve the

efficiency a number of studies have been undertaken (Al-Mutaz I., 2003, Sayed E., 1998,

El-Sayed, 2000) mainly dealing with MSF and RO integrated with Combined cycle power plant

But very few research works were performed that dealt with the application of exergy on the

combined water and power plants

In the last two or three decades, the usefulness of exergoeconomic concept has been

acknowledged compared to First Law method of analyses and the developments are still

continuing today The most important advantages are: the capability of an exergy analysis to

highlight and quantify energy inefficiencies; the possibility of systematically generating insight

towards improvement; and the relative ease of its application to automated optimal searches

Fiorini et al (2005) described a combined power and water plant with Multi Stage Flash (MSF)

Introduction

only with varying number of stages and opined that capital and steam costs are equally important

in determining the optimal operative conditions

Mehdizadeh (2006) outlined an exergy analysis for a nano-filtration reverse osmosis plant with a

view to minimize the energy requirement Parametric studies have been performed by Alsafour et

al (2005) to locate the source of major inefficiency for different types of thermal desalination

technologies e.g multi effect distillation (MED), thermal vapor compression (TVC) According

to them, if the steam is directly fed from a boiler then the main exergy destruction occurs in the

first effect With the emergence of energy crisis, the exeregoeconomic concept is getting more

attention from the research community in a bid to improve the performance of plants So, it is

quite clear from the above discussion that, to improve the efficiency of the combined power and

water plant, it is imperative to produce an exergoeconomic analysis to locate the major sources

of inefficiencies, and introduce measures to overcome it

1.2 Research Objectives

The main idea of this project was to propose a combined system that can fulfill both water and

power demand at a reasonable cost and efficiency Most of the power plants run at part load

condition and only occasionally it runs at peak load (usually 8-10 hours maximum per day) As a

result, there is a lot of waste heat that are thrown to the environment On the other hand,

desalination plants are very much energy intensive, if they are run as a stand alone plant The

thermal desalination plants can be easily fed by the exhaust waste heat from the power plant

Moreover, when the electricity demand is low, mainly electricity driven process eg RO can

fulfill part of the water demand Consequently, improve the efficiency of the combined water and

power plant

Introduction

The present study aims at optimizing the proper product mix and minimizes the water production

cost using exergoeconomy Exergoeconomy analysis investigates the exergy consumption of a

fuel plant while taking into consideration its investment cost Therefore, it provides the user with

a simple yet comprehensive overview of the plant before initiating any technical feasibility

study As a result, it will obviate the expensive experimental study to some extent and facilitate

the design of next generation desalination plant

The following objectives have been set for the present research investigation:

1 To improve efficiency of a desalination plant combined with a power plant using the

waste heat during full load operation and exploit the part load condition (idle power)

2 Develop a simulation tool for Multi Effect Distillation (MED) and Reverse Osmosis

(RO) for both technical and economic analyses

3 Design and construct experimental systems to validate the simulation results

4 To conduct a series of experiments of the subsystems (i.e MED, RO) to evaluate the system

performance and validate the simulation results

5 A comprehensive exergy analysis of the proposed hybrid plant to make it more energy

efficient

6 Application of the exergoeconomic methodology to optimize the combined power and

water plant

7 To develop a software that can be used as a design tool for the next generation Combined

water and power plant

Introduction

1.3 The Scope

A detailed background of the research intended and the objectives is given in chapter 1 Chapter

2 contains an extensive literature review on the various existing technologies along with articles

dealing with Combined Water and Power Plant (CWPP) The mathematical model developed for

simulating the combined cycle power and water plant is described in Chapter 3 Description of

experiment and experimental setup has been included in Chapter 4 Results and discussion are

included in Chapter 5 Projection of performance of the system in CWPP mode has been made as

well Lastly, conclusions from the present research investigation have been presented in the

concluding chapter 6 Chapter 7 contains some recommendations on further development

an exerergoeconomic analysis of the CWPP has been made to outline the current problem In the section that follows, a review of the simulation and experimental investigations of different possible configurations of the CWPP have been made In addition, a brief review of the two-phase flow heat transfer research investigations related

to this area has been made in this section

The possibility to combine different desalination processes in view of a synergetic effect has been suggested over a decade ago (Kamal et al., 1989; Awerbuch et al., 1989) The benefits of RO, in particular, could be used in combination with other distillation plants (usually MSF, possibly also MED or Thermal Vapor Compression (TVC)) This should allow greater flexibility in Combined power and water plants for the cogeneration of water and electricity, because the RO facilities can cover the water demand when the

Literature Review

electricity needs are low (El-Sayed et al., 1998) Moreover the study revealed that, the

RO operates at maximum permeability for the temperature range of 25 oC -32oC, because

of the positive influence of preheating the seawater (optimization of energy reuse – a flux increase of 2.5% per degree Celsius temperature increase is to be expected) In practice, the water flux has an upper limit because of fouling considerations The desired flux at the aforesaid temperatures is obtained by decreasing the trans-membrane pressure, so that energy consumption is lower at the same production level

Desalination or desalting is a process to obtain fresh water from seawater or brackish water through removing excess salt and other minerals Several desalting methods have been developed over the years, and based on their commercial success, they can be classified into thermal and membrane desalting processes This is shown in Figure 2.1

Figure 2.1 Various Types of Desalting Processes (Source: El-dessouky et al (1999)) According to IDA’s 1998 Worldwide Desalting Plants Inventory, the total capacity of desalting plants in the world is 22.7 million m3/d Desalting systems are now used in over

100 countries Almost half of this desalting capacity is used to desalt seawater in the

Literature Review

Middle East and North Africa

The report indicates the market share of various desalting processes From Figure 2.2, the MSF, MED and RO processes make up about 90% of the total capacity For that reason, the work of this project will focus on these 3 major desalting processes

Figure 2.2 Installed Desalination Capacity by Process (Source: El-dessouky et al (1999))

In the thermal desalting process, the seawater is heated producing water vapour that is in turn condensed to form fresh water The overall scheme of a thermal desalting process, as described by Malek et al (1992), is shown in the figure below:

Literature Review

Figure 2.3 Overall Scheme of seawater distillation Two most popular thermal desalting processes are:

• Multistage flash distillation process (MSF)

• Multi-effect distillation (MED)

2.3.1 Multistage flash distillation (MSF)

Semiat (2000) described the MSF process where the pressurized sea water flows through closed pipes exchanging heat with vapor condensing in the upper sections of the flash chambers Water is then heated to top brine temperature (maximum temperature), using burnt fuel or external steam, and this allows flashing along the lower part of the chambers, from chamber to chamber under reduced pressure conditions Vapour generated is allowed to flow through a mist eliminator to meet the condensing tubes, where heat is transferred to the heating feed seawater The condensate drips into collectors and is pumped out as the plant product Exhaust brine, concentrated in salt, is pumped out and rejected to the sea Part of the brine is re-circulated with the feed in order

Handling

Literature Review

to increase water recovery

The important controlling parameters of MSF are:

• Temperatures drop in each stage

• Total flash range (difference between the top brine temperature (TBT) and the brine reject temperature)

• Stage heat transfer coefficient

Figure 2.4 Schematic presentation of a Multi-Stage Flash desalination plant

2.3.2 Multi-effect distillation (MED)

Semiat (2000) in a review paper described MED which is considered to be one of the most promising evaporation techniques available today The process has been used for seawater desalination for the last 25 years

Excess Seawater

To Vacuum System

Distillate Seawate Treated

Reject Brine

Recirculation Stream Heat Recovery Section

Condensate return

Literature Review

Figure 2.5 Schematic presentation of a Multi-effect distillation (MED) plant

Basically, the method can use low-temperature, low-pressure steam as the main energy source In MED, two or more effects are employed for the production of water Each effect operates at a successively lower temperature and pressure The first (highest temperature) effect is heated by low-pressure steam Vapors are generated from feed water in the first effect through evaporation and flashing These vapors are directed to the second (low temperature) effect So, vapors from one effect are used as heat input to the next effect for heating and evaporating the brine Vapors produced in the first effect pass through demisters before going to the second effect tube bundle Some of the vapors produced in each effect are sent to the associated pre-heater, where they heat incoming feed water and are condensed The remaining vapors pass to the next effect MED usually operates either in horizontal or vertical modes where steam condenses on one side of the heat transfer surface while seawater evaporates on the other side

P1>P2>P3 T1>T2>T3

To Vacuum System

Literature Review

2.3.3 Comparisons of MED and MSF

In MSF technique, latent heat of vapor is used in preheater for heating the incoming seawater feed However, in MED the latent heat of vapor is used for evaporation system The efficiency of production that may be obtained from a unit feed water is essentially higher than that in the case of MSF distillation

Semiat (2000) elaborated the MED process that operates usually on horizontal or vertical modes where steam condenses on one side of the heat transfer surface while seawater evaporates on the other Usually, 8 to 16 stages are common in such operations which allows good performance ratio The ratio in MED can be as high as 15 while the corresponding ratio for MSF unit is limited to 10 The MED specific power consumption

is below 1.9 kWh/m3 of distillate, much lower than 4-6 kWh/m3 for MSF The effect design has the advantage of permitting changes in water output to respond to a varying demand simply by varying the steam flow rate, the maximum temperature, or both Water production may be varied from 20% to 120% of the design rate, a range not attainable with the MSF design

multi-Kronenberg (1995) and multi-Kronenberg et al., (2001) discussed the advantages of MED over MSF as follows:

a The Performance Ratio of MED is higher than that of the MSF, i.e it can produce more water per kg of steam used

b Higher heat transfer co-efficient can be obtained

c It can run on low temperature below 70o

C, so that it can be easily integrated to waste heat recovery boilers of power plants

The ability of a Low Temperature (LT) plant to make effective use of low-cost, low-grade