V Acknowledgment Professor El-Dessouky dedicates this book to the memory of his parents. Also ...

670 Pages·2007·29.11 MB·English

V
Acknowledgment
Professor El-Dessouky dedicates this book to the
memory of his parents. Also, he dedicates the book
to his family, Fatma, Sarah, Sameh, and Sanyia.
Professor Ettouney dedicates this book to his parents,
Mohamed and Fatma, his brothers Sayed, Mohamed, and
Osama, his family, Kareema, Osama, and May, and his friends
Reem, Kareem, and Amira.
VII
Preface
Industrial desalination of sea and brackish water is becoming an essential
part in providing sustainable sources of fresh water for a larger number of
communities around the world. Desalination is a main source of fresh water in
the Gulf countries, a number of the Caribbean and Mediterranean Islands, and
several municipalities in a large number of countries. As the industry expands
there is a pressing need to have a clearly and well-written textbook that focuses
on desalination fundamentals and other industrial aspects. The book would serve
a large sector of the desalination community that includes process engineers,
designers, students, and researchers. Fundamentals of the desalination process
are based on physical principles that include mass and energy conservation,
mass, momentum, and heat transfer, and thermodynamics. The authors firmly
believe that good understanding of these fundamentals is necessary to analyze or
evaluate the performance for any of the existing and known desalination
processes. Moreover, understanding the fundamentals would allow for critical
evaluation of novel schemes or devising new schemes. Other aspects included in
the book are the historical background of the desalination process, developments
during the second half of the twentieth century, recent trends, and future
challenges. The book focuses on the processes widely used in industry, which
include the multistage flash desalination and reverse osmosis. Also, other
desalination processes with attractive features and high potential are considered.
The book includes a large number of solved examples, which are explained in
simple and careful matter that allow the reader to follow and understand the
development. The data used in development of the examples and case studies are
extracted from existing desalination plants. Also, the examples represent
practical situations for design and performance evaluation of desalination plants.
The book also includes comparison of model predictions against results reported
in literature as well as available experimental and industrial data. Although, this
textbook will target the desalination community, which may include practicing
engineers, designers, developers, graduate students, and researchers, however,
the contents can be used by engineers in other industrial disciplines. Several
industries include similar unit operation processes, i.e., evaporators, condensers,
flashing units, membrane separation, and chemical treatment. Examples of such
industries include wastewater treatment, food, petroleum, petrochemical, power
generation, and pulp and paper. Process fundamentals and design procedures of
such unit processes follow the same procedures given in this textbook.
It should be stressed that this is the first textbook on desalination that can
be used for undergraduate and graduate instruction. Although, there are a
number of books on desalination, most are of the editorial type with research and
development articles, which are not suitable for educational purposes. Other
books are descriptive and have introductory material.
VIII
This textbook includes sections on thermal desalination, membrane
desalination, associated processes, and process economics. The thermal
desalination part is focused on the single effect evaporation process, the multiple
effect evaporation, and the multistage flash desalination. It should be noted that
complete and full description and development is made for each thermal
desalination process. Because of the similarities among various thermal
desalination processes, this implied repeating some parts of the process elements
and model equations. As a result, the reader would find it simple to study each
process separately without having to search various sections for some common
features or equations. This is except for the correlations and equations for water
physical properties, thermodynamic losses, and heat transfer coefficients.
Another major feature of the analysis provided in this textbook is the inclusion of
the performance charts for each process. This data is found necessary to provide
the reader with practical limits on process operation and performance.
In Chapter 2, the single effect evaporation with submerged evaporator is
discussed. The development includes process description, mathematical model,
and performance evaluation. This is followed by discussion of various types of
evaporators. It should be stressed that coverage of this material is essential to
develop the theoretical and physical backgrounds necessary to understand the
thermal multiple effect processes. Chapter 3 includes analysis of the single effect
evaporation process combined with various types of heat pumps. These include
mechanical, thermal, adsorption, and absorption vapor compression. The analysis
includes description of processes, mathematical models, and solved examples. In
addition, comparison is made between model predictions and industrial data for
the single effect mechanical vapor compression system.
Chapter 4 includes the multiple effect evaporation processes. The Chapter
starts with description of different process layouts. This is followed by analysis of
the forward feed system, which includes process developments, description,
detailed models, and case studies. The last part of the Chapter includes similar
treatment for the parallel feed configurations. The Chapter also includes
performance evaluation of both configurations, comparison with field data, future
challenges and trends.
In Chapter 6, the analysis of the multistage flash desalination starts with
coverage of process developments. This is followed by process synthesis, which
starts with the single stage flashing and proceeds to the multistage
configuration. In Section 6.5, details of the conventional brine circulation
multistage flashing process is given and it includes process description, model
assumptions, equations, solution method, and a number of solved examples. The
development then focuses on analysis of performance evaluation as a function of
the parameters that have the strongest effect on the unit product cost. These
parameters includes the heat transfer area for evaporators and condensers.
IX
dimensions of the flashing or evaporation stages, capacity and dimensions of the
ejectors, size and load of pumping units, and other associated processes. The
remainder of Chapter 6 covers other possible layouts for the multistage flash
desalination process, which includes the once through system, the brine mixing
process, and the thermal vapor compression configuration.
All of the thermal desalination processes can be combined with vapor
compression heat pumps, which includes thermo-compressors or steam jet
ejectors, mechanical vapor compression, adsorption vapor compression,
absorption vapor compression, and chemical vapor compression. The main
objective for using vapor compression heat pumps is to improve the process
efficiency and utilization of low grade energy that commonly discarded into the
environment. This approach conserves on the system energy and reduces the
amount of fossil fuels that upon combustion generates green house gases and
other pollutants. The analysis of the heat pumps focuses on description of the
elements forming each heat pump, mathematical and empirical models, solution
procedure, and performance analysis. The remaining sections on the combined
systems of thermal desalination and vapor compression focus on development
integrated mathematical models for the system, operating regimes, performance
evaluation, and comparison against systems with no vapor compression and field
data. The vapor compression desalination systems are given in Chapters 3 and 5,
which covers the single and the multiple effect evaporation. Section 6.5 gives
model and analysis of the multistage flash desalination combined with thermal
vapor compression system.
Chapters 7 and 8 include the reverse osmosis membrane desalination
process. Chapter 7 starts with a brief description of various membrane
separation process, separation mechanism, and recent trends. Also, description is
given for membrane modules, process layout, and staging of modules. The reverse
osmosis models include simple and short cut techniques as well as more detailed
models. The analysis of membrane systems focus on determination the required
feed pressure and membrane area to provide the desired permeate or fresh water
flow rate. Chapter 8 covers elements of feed pre treatment, membrane biofouling,
and membrane cleaning.
In Chapter 9, a number of associated processes are analyzed, which
includes the venting, steam jet ejectors, wire mesh mist eliminator, and orifices
in the MSF process. The chapter includes description of various unit process,
fundamental models as well as fitting correlations. In addition, a number of
examples are solved for each system.
Desalination economics are covered in chapter 10 and it includes elements
of economic analysis and a number of case studies. The necessary elements to
perform economic analysis of desalination processes include process capital and
amortization, operating and maintenance cost, energy cost, and inflation effects.
X
The case studies include examples for various thermal and membrane
desalination systems. In addition, all case studies include comparison with
available cost data from literature studies and existing desalination plants.
The book contents are suitable for instructing training courses for
practicing engineers. The courses could be introductory, intermediate, or
advanced. The introductory level training course would cover the first chapter,
and sections in all other chapters on description of desalination process,
associated unit processes, and performance evaluation. As for the intermediate or
advanced training courses it should include a number of the solved examples and
case studies. These may include design problems on the multistage flash
desalination, the multiple effect evaporation with/without thermal or mechanical
vapor compression, the single effect evaporation with mechanical vapor
compression, and the reverse osmosis. The difference between the intermediate
and advanced training courses comes in coverage of the descriptive part and the
type of design problems and analysis.
Using the book for instruction of an undergraduate course depends on the
level of instruction, where a simple freshmen or sophomore course would include
most of the descriptive part and simple problems that include basic material and
energy balances and evaluation of the stream physical properties. On the other
hand, a desalination course being taught on the junior or senior levels would
allow for more detailed design in addition to the process description section.
Finally, the teaching material for a graduate course should involve a larger
number of the design problems for various desalination processes. In the
graduate course, many of the descriptive part should be assigned for reading
ahead of the class period and its coverage should be instructed in an interactive
discussion between students and instructor.
The book includes a comprehensive computer package. The computer
package is written in visual basic. The package allows for flow sheet as well as
design displays. The results of the package are displayed on the screen and are
also written to text files. The screen displays as well as the text files can be
printed. The flow sheet display includes the unit processes forming the desired
desalination process. Upon completion of the calculations each unit process in the
flow sheet becomes active, where pressing a specific unit process gives a small
display with the stream properties of the unit. The design display gives more a
comprehensive list of the design parameters, i.e., thermal conductivity of the
evaporator/condenser tubes, fouling factors, etc. Completion of the design
calculations gives the major design results, i.e., heat transfer areas, properties of
outlet streams, and thermal performance ratio.
Hisham T. El-Dessouky ([email protected])
Hisham M. Ettouney ([email protected])
XI
Symbols
A Area, m^.
BPE Boiling point elevation, ^C
Cp Specific heat at constant pressure, kJ/kg ^C.
Cr Compression ratio defined as pressure of compressed vapor to
pressure of entrained vapor,
d Vapor flow rate formed in flashing boxes, kg/s.
D Vapor flow rate formed by flashing or boiling, kg/s.
Er Expansion ratio defined as pressure of compressed vapor to pressure
of entrained vapor,
h Heat transfer coefficient, kW/m^ ^C.
H Enthalpy of liquid water, kJ/kg.
H" Enthalpy of vapor phase, kJ/kg.
k Thermal conductivity, kW/m ^C.
L Length, m.
LMTD Logarithmic temperature difference, ^C.
M Mass flow rate, kg/s.
M2 Mass of adsorbing solids, kg.
P Pressure, kPa.
PR Performance ratio, dimensionless.
AP Pressure drop, kPa.
Q Heat transfer rate, kJ/s.
r Tube radius, m.
R Universal gas constant, kJ/kg°C.
Rf Fouling resistance, m^ oC/kW
Rs Load ratio, mass flow rate of motive steam to mass flow rate of
entrained vapor.
s Salt concentration, mg/i.
sA Specific heat transfer area, m2/(kg/s).
sMpw Specific cooling water flow rate, dimensionless.
T Temperature, ^C.
AT Temperature drop, ^C.
U Overall heat transfer coefficient, kW/m^ ^C.
V Vapor specific volume, m^/kg.
V Vapor velocity, m/s
w Entrainment ratio, mass flow rate of entrained vapor to mass flow
rate of motive steam.
Xj^C Mass fraction of non-condensable gases.
X Salinity, ppm.
XII
Z Length, m.
Greek Letters
a Adsorption capacity, kg water/kg zeolite
^ efficiency.
X Latent heat, kJ/kg.
P Density, kg/m^
8 Void fraction
Y Compressibility ratio.
Subscripts
b Brine.
c Condenser.
cw Cooling water.
d Distillate product.
e Evaporator
ev Entrained vapor.
f Feed seawater
i Inlet stream or inner diameter
m Motive steam.
n Nozzle.
0 Outlet stream or outer diameter.
P Demister.
s Compressed vapor or heating steam.
t Throat of the ejector nozzle.
V Formed vapor.
w Tube wall.
z Solid bed.
Chapter 1
Introduction
2 Chapter 1 Introduction
Objectives
The main objective of this chapter is to give an overview of various
desalination processes, developments, and the needs for industrial desalination.
This is made through discussion of the following topics:
- Water resources
- Classification of salt water
- History of industrial desalination
- Definition and classification of industrial desalination processes
- Market status for desalination processes
1.1 Resources and Need for Water Desalination
The earth contains about 1.4x10^ km^ of water, which covers
approximately 70% of the planet surface area; the percentage of salt water in this
large amount is 97.5%. The remaining 2.5% is fresh water with 80% of this
amount frozen in the icecaps or combined as soil moisture. Both forms are not
easily accessible for human use. The remaining quantity, about 0.5%, is believed
to be adequate to support all life on Earth. Unfortunately, this water is not
distributed evenly throughout the plant and it is not available in sufficient
quantities either when or where it is needed. Table 1 gives a summary for
distribution of various water resources across the globe. The global daily average
of rainfall is 2xl0ll m^. This amount is poorly distributed across the globe.
The solar energy is the main driver for formation of fresh water from
oceans. The thermal energy absorbed by the earth surface generates sufficient
temperature gradients that drive water evaporation from the large surfaces of
ocean water. The water vapor rises through the ambient air and forms a cloud
cover at various elevations. The clouds are formed of fine water droplets with an
average diameter of 10 |im. The clouds are transported over land, where
precipitation takes place. The form of precipitation depends on the surrounding
air temperature. Snow is formed in clod climates and higher elevations; while,
rain is formed in warmer climates and lower elevations. On the other hand,
mixtures of ice, snow, and rain are formed during spring time of clod climates.
Precipitation depends on the wind direction and speed, which have fixed
patterns that varies subject to location and seasonal temperature variations.
Also, precipitation is affected by geographical conditions, i.e., presence of
mountains, flat land, as well as local ambient conditions, i.e., temperature, and
humidity. The wind pattern, geographical forms, and ambient conditions
generates zones of constant water precipitation, monsoon seasons, and areas of
very low precipitation.
1.1 Resources and Need for Water Desalination
Precipitated water forms lakes, rivers, underground surface water, deep
aquifers, or massive flood areas. These forms could be seasonal or permanent.
For example, rapid increase in the ambient temperature during the spring season
could result in the melting of large amounts of winter snow over a short period of
time. Such events are experience in several regions in Europe, Russia, and the
US. Also, the monsoon rain in the Indian content results in precipitation of large
amounts of water over a very short period of time. Both forms of water
precipitation give rise to destruction of property and loss of life. Permanent rivers
form the life line to several regions around the globe, where it transports water
from high precipitant area to dessert land. The best example for this situation is
the river Nile, which originates in the high mountains of Ethiopia and Kenya and
travels more than 2000 km. Through the ages the river Nile gave life to the Nile
valley and supported the development of Egyptian civilization.
A major part of water precipitation ends up as ground moisture in the form
of sub-surface water or deep aquifers. Deep aquifers proved to be viable source for
bottled drinking water. This is because of regulated and limited rates of water
use from these sources insure sufficient natural replenishment of the source.
Also, the natural process through various rock formations provides the water
with natural minerals and keeps its pH at acceptable levels.
Table 1
Distribution of water resources across the globe
Resource Volume Percent of Percent of
km^ total water Fresh Water
Atmospheric Water 12900 0.001 0.01
Glaciers 24064000 1.72 68.7
Ground Ice 300000 0.021 0.86
Rivers 2120 0.0002 0.006
Lakes 176400 0.013 0.26
Marshes 11470 0.0008 0.03
Soil Moisture 16500 0.0012 0.05
Aquifers 10530000 0.75 30.1
Lithosphere 23400000 1.68
Oceans 1338000000 95.81
Total 1396513390
Classification of various types of water is based on the purpose for which
the water is used. The first water grade is set for safe drinking, household
purposes, and a number of industrial applications. This water category has a