Principles of Desalination

465 Pages·1980·8.956 MB·English

by  K.S. Spiegler (Eds.)

Contributors
C. CALMON
KEITH S. CAMPBELL
LAWRENCE DRESNER
BELA M. FABUSS
JULIUS GLATER
JAMES S. JOHNSON, JR.
LOUIS KOENIG
GEORGE O. G. LOF
A. B. MINDLER
B. W. TLEIMAT
J. Louis YORK
Principles of
DESALINATION
Second Edition Part Β
EDITED BY
K. S. SPIEGLER
DEPARTMENT OF CHEMISTRY DEPARTMENT OF MECHANICAL
AND CHEMICAL ENGINEERING ENGINEERING
MICHIGAN TECHNOLOGICAL UNIVERSITY UNIVERSITY OF CALIFORNIA
HOUGHTON, MICHIGAN BERKELEY, CALIFORNIA
A. D. K. LAIRD
SEA WATER CONVERSION LABORATORY
UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA
1980
ACADEMIC PRESS
A Subsidiary of Harcourt Brace Jovanovich, Publishers
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COPYRIGHT © 1980, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR
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ACADEMIC PRESS, INC.
Ill Fifth Avenue, New York, New York 10003
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1 7DX
Library of Congress Cataloging in Publication Data
Spiegler, Κ S ed.
Principles of desalination.
Includes bibliographies.
1. Saline water conversion. I. Laird,
AlanD. Κ. II. Title.
TD4 79.S6 1979 628.Γ67 79-6947
ISBN 0-12-656702-6 (v. 2)
PRINTED IN THE UNITED STATES OF AMERICA
80 81 82 83 9 8 7 6 5 4 3 2 1
List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
C. CALMON* (561), Water Purification Association, Cambridge, Massa
chusetts 02142
KEITH S. CAMPBELL (627), Environmental Sciences Division, Stearns-
Roger, Incorporated, Denver, Colorado 80222
LAWRENCE DRESNER (401), Oak Ridge National Laboratory, Nuclear
Division, Oak Ridge, Tennessee 37830
BELA M. FABUSS (765), Resource Recovery Division, Raytheon Service
Company, Burlington, Massachusetts 01803
JULIUS GLATER (627), Department of Chemical, Nuclear, and Thermal
Engineering, School of Engineering and Applied Science, University of
California, Los Angeles, California 90024
JAMES S. JOHNSON, JR. (401), Oak Ridge National Laboratory, Nuclear
Division, Oak Ridge, Tennessee 37830
Louis KOENIG (725), Louis Koenig—Research, San Antonio, Texas
78216
GEORGE O. G. LOF (679), Engineering Research Center, Solar-Energy
Applications Laboratory, Colorado State University, Fort Collins,
Colorado 80523
A. B. MINDLER (561), The Permutit Company, Inc., Monmouth Junc
tion, New Jersey 08852
B. W. TLEIMAT (359), Sea Water Conversion Laboratory, University
of California, Berkeley, California 94720
J. Louis YORK (627), Environmental Sciences Division, Stearns-Roger,
Incorporated, Denver, Colorado 80217
* Present address: Consultant, Princeton, New Jersey 08540.
ix
Contents of Part A
Chapter 1
Thermoeconomic Considerations of Sea Water Demineralization
ROBERT B. EVANS, GARY L. CRELLIN, AND MYRON TRIBUS
Chapter 2
Fundamentals of Distillation
Υ. M. EL-SAYED AND R. S. SILVER
Chapter 3
Design of Distilling Plants
A. B. STEINBRUCHEL AND R. D. RHINESMITH
Chapter 4
Vapor Reheat Distillation
T. WOODWARD
Chapter 5
Dual Purpose Plants
F. S. ASCHNER
Chapter 6
Electrodialysis
L. H. SHAFFER AND M. S. MINTZ
xi
PRINCIPLES OF DESALINATION, SECOND ED., PART Β
Chapter 7
Freezing Methods
B. W. TLEIMAT
I. Introduction 360
A. Description of Basic Freezing Processes 360
B. Some Comments on Freezing Processes 366
II. Characteristics of Saline Water as Related
to Freezing Processes 367
A. Salinity 368
B. Phase Change and Freezing Properties 368
C. Eutectic Properties 371
D. Freezing-Point Depression 372
Ε. Relationship of Original Salinity to Salinity of Brine 373
F. Thermal Conductivity of Liquid Water and Water Vapor 373
III. Thermodynamic Properties of Water, Vapor, and Ice
in the Vicinity of the Triple Point 374
A. Latent Heat and Phase Transition 374
B. Thermodynamics of the Freezing Process 376
C. Other Energy Requirements 377
IV. Crystallization 377
A. General Discussion 377
B. Crystal Purity 379
C. Entrainment of Brine in Mush 380
V. Theory of Wash and Separation 381
A. Separation-Wash Column 381
B. Wash Column Theory 383
C. Analytic Solution for a Washer of Rectangular
Cross Section 387
D. Brine Crown 389
VI. Plant Economics 394
VII. Desalting with Hydrates 394
A. Process Description 394
B. Advantages of the Hydrate Process 396
C. Application of the Hydrate Process 3%
List of Symbols 398
References 399
359
Copyright © 1980 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-656702-6
360 Β. W. Tleimat
I. Introduction
The conversion of saline water to fresh water by freezing has always
existed in nature and has been known to man for thousands of years. De­
salination of water by freezing has been practiced by the inhabitants of
Central Asia and Western Siberia for generations. Short frost periods are
utilized to collect frozen saline water in ditches where the ice is allowed to
melt in the sunlight and is used for watering cattle. Fishermen, along the
banks of the saline Aral Sea in the Kazakh Republic of the U.S.S.R., are
known to collect sea ice during the winter and store it for summer use.
All current desalination methods fall into one of two categories: those
that remove the fresh water and leave behind a concentrated brine, or
those that remove the salt and leave behind the fresh water as a residue.
The freezing process falls into the first category. It is a separation process
related to the solid-liquid phase change phenomenon. When the tempera­
ture of salt water, of limited salinity, is reduced to its freezing point,
which is a function of salinity, ice crystals of pure water are formed within
the salt solution. These ice crystals can be mechanically separated from
the concentrated salt solution and remelted to obtain pure water. All ex­
isting freezing desalination processes are based on this simple and basic
phenomenon.
A. DESCRIPTION OF BASIC FREEZING PROCESSES
In order to place the theoretical development necessary to understand
the freezing process in perspective, it is well first to describe some of the
systems considered for practical applications. Almost all freezing pro­
cesses utilize similar functional components because they utilize similar
mechanisms for forming ice and separating it from the brine. The pro­
cesses described hereafter are arranged in order of simplicity of process
and components. The indirect refrigeration method is described first as it
was the first method proposed for practical application and has the ele­
ments necessary for understanding the fundamentals of the freezing
process.
A schematic diagram of the indirect refrigeration system is shown in
Fig. 7.1. The incoming saline water is first pumped through a heat ex­
changer to reduce its temperature. It is then admitted to a freezing
chamber, in which it is cooled by means of the refrigeration coils of a sep­
arate refrigeration system to the temperature at which ice crystals are
formed. The ice and brine slurry then flow to a wash column where the ice
and brine are separated. The brine is returned to the heat exchanger to
cool the incoming feed water, and discarded. The ice is transferred to a
7. Freezing Methods 361
Compressor
Liquid Refrigerant
Freezing Melting
Chamber Unit
Fresh
Water
Separation
Unit
Incoming
Sea water
-1
Heat Exchanger
FIG. 7.1. Indirect refrigeration method.
melting unit, where the heat released by the condensation of the com
pressed refrigerant is used to melt the ice as the refrigerant is condensed.
The melted ice is then taken from the melting unit as product water, a
small part of which is bypassed to the wash column, where it is used for
washing the ice crystals, and the major part is passed through the heat ex
changer to cool the incoming feed and is then discharged for storage or
distribution. The main supply of energy (or exergy, see Chapter 1 of
Part A of this edition) is that required to drive the refrigeration com
pressor while the heat is rejected to the surroundings in the refrigerant
cooler.
The heat-exchange surface between the saline water and refrigerant in
the freezing chamber is a disadvantage because it represents a resistance
to heat flow from the water to the refrigerant resulting in higher cost due
to the cost of the surface and the additional cost of the compression en
ergy to overcome this resistance. Therefore, a method which permits
direct exchange of heat between the feed water and the refrigerant is de
sirable. This is the direct refrigeration technique (State of Israel and
Zarchin, 1958).
One technique for effecting direct heat exchange is to use the water it
self as a refrigerant. By introducing the saline water to be purified into a
vacuum chamber, which is maintained at a water vapor pressure equal to
or below the triple point of the water, some of the water immediately
flashes into vapor. The formation of this vapor removes heat from the
water equal to the total latent heat of vaporization for the mass of evapo-
362 Β. W. Tleimat
rated water. With proper design this will reduce the saline water to a
temperature at which nucleation of ice crystals will begin.
Figure 7.2 is a schematic diagram of a direct refrigeration method which
uses water as a refrigerant and then mechanically compresses the re­
sulting water vapor. This method is called the vacuum-freezing vapor-
compression method. As before, the incoming saline water is cooled in a
heat exchanger and then sprayed into a freezing chamber. The slurry of
ice and brine is fed to the wash column, where the ice and brine are sepa­
rated. The ice is then transferred to a melting unit. The water vapor
originating in the freezing chamber is compressed (and thus heated) and
discharged to the melting unit. The compressed water vapor transfers
heat to the ice crystals within the melting unit where the ice is melted and
the vapor is condensed to form the product water. An auxiliary refriger­
ation coil is necessary in the system to remove the equivalent of the me­
chanical energy supplied and heat influx due to heat leakage. Again, some
of the product water from the melter is by-passed for washing. The main
energy supplied is that required to drive the compressor while the heat
discarded is extracted by means of the auxiliary refrigeration system (not
shown). The system design is simple and requires a minimum of acces­
sory equipment. On the other hand, the compressor design is difficult,
owing to the large specific volume of the water vapor at this low tempera­
ture.
To avoid the difficulties of compressor design, a direct refrigeration
method has been devised which absorbs the water vapors and then re­
claims the vapors from the absorbent. Figure 7.3 is a schematic diagram
Compressed Vapor
Compressor
Freezing Separation
Chamber Unit
Heat Exchanger
Incoming
Sea water
FIG. 7.2. Vacuum-freeze vapor-compression method.
7. Freezing Methods 363
Water Water
Vapor Vapor
1_±
Melting and Cooling
Freezing Absorption I Absorbent j Vapor 'Water
Chamber Unit J Generator J Condenser
I
Fresh
Ice& Water
Brine
Fresh
Water
Separation Absorbent
Unit Heat
Exchanger
Incoming
Sea water
Heat Exchanger
FIG. 7.3. Vapor-absorption method.
of a method called vapor-absorption method similar to the previous one,
except that the vapor produced is absorbed rather than compressed. As in
the vapor-compression method, saline water is refrigerated in the freezing
chamber, and the resulting brine is drained and pumped through the heat
exchanger to discharge. Some of the ice removed from the wash column is
pumped through coils in the melting unit. The water vapor leaving the
freezing chamber is absorbed into the concentrated absorbent, usually
concentrated lithium bromide solution, which is sprayed over the cooling
coils in the melting unit. The absorbent, now cold and dilute, passes
through a heat exchanger on its way to the absorbent generator. Here the
water vapor which had been absorbed is driven out of the absorbent by
heating and is led to the vapor condenser, where it passes over cooling
coils and is condensed. The condensed vapor becomes part of the product
water, the remainder of which is made up of melted ice which has been
piped from the wash column through the heat exchanger to discharge. The
main energy supplied in this system is provided as latent heat by the
steam in the absorbent generator while the heat discarded is carried away
by the cooling water in the vapor condenser.
Another interesting variation of the direct refrigeration method is the
direct evaporation and condensation of an immiscible refrigerant in con
tact with the saline water. This process is called the secondary refrigerant
process. The refrigerant used must be practically insoluble in water. Such
a process was originally proposed by H. F. Wiegandt (1963) at Cornell
University. This method uses isobutane as the refrigerant and is schemat-