Introduction to Cake Filtration: Analyses, Experiments and Applications

274 Pages·2006·10.38 MB·English

by  Chi Tien

Preface
The idea of writing this volume came to me almost two decades ago shortly after
I became seriously involved with cake filtration studies. By all account, cake filtration is
an important solid/fluid separation process and has been widely applied in the process,
chemical and mineral industries. It was (still is) one of the topics discussed in almost all
undergraduate, unit operations texts since the publication of the first edition of Principles
of Chemical Engineering in 1927. However, there are only a few books and monographs
devoted exclusively to the subject and most of them are aimed at applications. The
purpose of the present book is to give an introductory and yet fairly comprehensive
account of cake filtration as a physical process in a more fundamental way. Hopefully,
it will provide people who contemplate to do research and development work in cake
filtration with a source of information and get them quickly on track.
This book is divided into three parts. Part I deals with cake filtration analyses using
different approaches including the conventional theory of cake filtration, analysis based
on the solution of the volume-averaged continuity equations and treatment of cake filtra
tion as a diffusion problem. Dynamic simulation of cake filtration which examines both
filtration performance and cake structure and its evolution is also included. Descrip
tions and discussions of cake filtration experiments, the procedures used and the various
methods used for the determination of cake properties constitute Part II. In Part III, three
fluid/particle separation processes which feature cake formation and growth together
with other phenomena are discussed.
As stated earlier, I have prepared this book for the purpose of initiating those who
are interested in cake filtration research and development work including students who
plan to do their theses in this area. In order to gain a wider audience, the background
information necessary to comprehend the materials presented is kept to a minimum. The
level is consistent with what is taught at an accredited B.Sc. degree program in chemical,
civil and mechanical engineering. It should therefore be possible to adopt the book as a
text or part of text for graduate courses dealing with separation or solid/fluid separation,
even though, strictly speaking, it is not written as a text.
There is another reason for writing this book. During the past two decades, we have
seen considerable discussions and debates about the future of chemical engineering as a
profession and as a discipline. Numerous suggestions and plans on chemical engineering
education and research have been advanced for the purpose of restoring the profession
to its past glory. Somewhat overlooked in these efforts is the fact that the viability of
any profession as a field of study depends, to a large degree, upon its appeal to talented
young people on account of the intellectual challenges and practical relevancy it poses.
In this regard, while the topic of unit operations is recognized as a core subject of the
chemical engineering discipline, a search of library and publication catalogues reveals
that most of texts and monographs dealing with this topic, but not on an elementary
viii PREFACE
level, were published more than three or four decades ago, thus giving the impression
and creating the perception that the discipline has reached its maturity a long time ago.
It is therefore not surprising that as a subject of study, chemical engineering nowadays
is not able to attract a sufficiently large number of talented students as it once did. It is
hoped that writing a book such as this one may, in a very small measure, contribute to
rectify the prevailing erroneous impression.
A major part of this book is based on the studies of fluid/particle separation I conducted
during the past 20 years at Syracuse University and the National University of Singapore.
I would like to acknowledge the significant roles played by my former students and
colleagues in these studies: Professor R. Bai, Professor M.S. Chiu, Professor Y.-W. Jung,
B.V. Ramarao, Dr K. Stamatakis, Professor R.B.H. Tan, Dr S.-K. Teoh, and Professor
C.-H. Wang. I am particularly indebted to R. Bai for his tireless efforts in obtaining some
of the numerical results of cake formation and growth included in this book. I should
also add that the countless hours of stimulating discussions on cake filtration and related
problems I had with B.V. Ramarao during the past decade were certainly one of the
major rewards of writing this book.
Finally, I would like to thank my former and present publishing editors, Anouschka
Zwart and Louise Morris of Elsevier, for their efforts and assistance which made prompt
publication of this book possible, Kathy Datthyn-Madigan for her keyboard skill in
typing and assembling the manuscript and last but not the least, my wife, Julia, for all
the help and support she has given me for the past four and a half decades.
Chi Tien
- 1-
INTRODUCTION
Notation
ki empirical constant of Equation (1.1) (t^'^m'^"^)
^2 empirical constant of Equation (1.1) (-)
t time (s)
V cumulative filtrate volume (m)
Cake filtration as a process is used for separating the two phases (solid and liquid) of a
suspension from each other. The specific purpose of the separation varies from case to
case, including the recovery of the solid (discarding the suspending liquid), clarifying
the Hquid (discarding the solid) or recovering both. It is a long-standing engineering
practice and has been widely used in the chemical, process and mineral industries. The
principle of filter press operation can be traced to the ancient practice of squeezing juice
through cloth in sugar manufacturing (Wakeman, 1972). Similarly, many of the filtration
devices used nowadays may claim their origins of more than a century ago. At the
same time, new development and inventions of cake filtration systems (both hardware
and software) continue and abound because of the importance of solid/liquid separation
technology to our manufacturing economy.
The critical role of solid/liquid separation in industrial applications can be seen from
the examples shown in Figs 1.1 and 1.2, which give the flow sheets of producing raw
sugar and sugar refining (King, 1980). As shown in these figures, there are altogether
11 different classes of separation steps: sedimentation (clarifier), filtration (scum filter
and pressure filter), centrifugation (centrifuges for both raw and refined sugar), screening
(classification by crystal size), expression (milling rolls for dewatering), washing and
leaching, precipitation (lime tanks), evaporation (evaporator), crystallization (vacuum
pans), adsorption (char filters) and drying (granulators); among which four classes -
sedimentation, filtration, centrifugation and expression - belong to the category of
solid/liquid separation. This example, by no means, is an exception. The importance
of and the reliance on solid/liquid separation technology can be found in a number
of industries including mineral processing, paper making, and water and waste water
treatment.
INTRODUCTION TO CAKE FILTRATION
W^sh Sugar cane Water vapor Water vapor
water from fields
Clarified _L
Cane juice^
CHOPPING EVAPORATOR
CLARIFIERS
\r ' N/" Steam CRYSTALIZER
Steam
Water + debris v_ _y
CRUSHING
' ' Y
Water MILLING LIME TANKS FILTER Raw
ROLLS Juice Juice sugar
k
' ' 1
Y
Bagasse (pulp) Milk of lime Solids Blackstrap
to fuel (calcium hydroxide) to fields for molasses
fertilizer
Figure 1.1 Flow sheet of raw sugar production. (King, 1980. Reprinted by permission of
McGraw-Hill Inc.)
BULK
RAW SUGAR
BINS
Figure 1.2 Flow sheet of sugar refining. (King, 1980. Reprinted by permission of
McGraw-Hill Inc.)
1.1 CAKE FILTRATION AS A SEPARATION PROCESS
A simple schematic representation illustrating the working of a separation process
is shown in Fig. 1.3. Through the application of a separating agent which may be
either energy or matter or both, a feed stream is split into several streams of different
INTRODUCTION
Separating agent
(nnatter or energy)
'
Feed stream Separation Product streams
(one or more) device (different in composition)
^
Figure 1.3 A general representation of the separation process. (King, 1980. Reprinted by per
mission of McGraw-Hill Inc.)
compositions. With a different concept, Giddings advanced the premise that separation
of a mixture is caused by the relative displacement of the various components involved
(1991). Accordingly, cake filtration may be viewed as a process employing an agent
consisting of energy (which causes the flow of suspension) and matter (filter media).
The relative displacement between suspending liquid and suspended particles of a cake
filtration process results from the particle-exclusion effect of the filter medium used.
Similar selective displacement may also be caused by gravity (in sedimentation), rotation
(in centrifugation) and mechanical force (in consolidation).
Over the years, a large number of solid/liquid separation processes have been devel
oped and they are too numerous to be mentioned individually. Generally speaking, the
most commonly used ones include cake filtration, depth (deep bed) filtration, cycloning,
thickening, flocculation, and consolidation. The relationship among these processes can
be seen from the classification scheme proposed by Tiller (1974). This scheme shown in
Fig. 1.4 is based on Tiller's idea that solid/liquid separation can be viewed as a system
consisting of one or more subsystems: (1) pretreatment to facilitate subsequent process
ing, (2) soHd concentration to increase particle content, (3) solid separation, and (4)
post-treatment to further enhance the degree of separation and improve product quality.
Based on this classification scheme, cake filtration is applied mainly for the separation
or recovery of suspended particles from suspensions of relatively high solid content.
This is consistent with what is shown in the flow sheet of sugar manufacturing (see
Figs 1.1 and 1.2).
1.2 CAKE FILTRATION VS. DEEP BED FILTRATION
Cake filtration and deep bed filtration share a common feature of using the same kind
of separating agent. However, the roles played by filter medium in these two processes
are different. In cake filtration, the filter medium acts as a screen so that particles of
the suspension to be treated are retained by the medium, resulting in the formation of
filter cakes. In contrast, in deep bed filtration, separation is effected through particle
deposition throughout the entire depth of the medium. In other words, the individual
INTRODUCTION TO CAKE FILTRATION
Chemical
Flocculation
Coagulation
PRETREATMENT
Physical
Crystal growth
Freezing and other physical changes
Filter aid addition
Thickening
Hydrocycloning
SOLIDS
CONCENTRATION
•— Clarification Batch
PRESS, VACUUM,
Recovery of solid GRAVITY FILTERS"
particles
Continuous
CAKE FORMATION
FILTERING
SOLIDS
Solid bowl
SEPARATION
CENTRIFUGES H
SEDIMENTING
Clarification Perforated bowl
No cake formed
Deep granular beds
Cartridges
Filtrate— Polishing
Membranes
Ultrafiltration
POST-TREATMENT H
Cake — Washing - Deliquoring
Displacement Drainage
Reslurry Mechanical
Hydraulic
Figure 1.4 Stages of solid/liquid separation according to Tiller.
entities constituting the medium act as particle collectors. Consequently, cake filtration
is also known as surface filtration while deep bed filtration is often referred to as depth
filtration. A schematic illustration of this difference is shown in Fig. 1.5.
Qualitatively speaking, the most important factor in determining cake formation is
the relative medium pore size to particle size. The empirical 1/3 law suggests that cake
formation commences if the particle size is 1/3 of the size of the medium pore. While
this law may not be exact, sufficient empirical evidence exists indicating the occurrence
of cake formation if the particle size and the medium pore size are of the same order of
magnitude.
In terms of applications, cake filtration is used to treat suspensions of relatively
high solid content while deep bed filtration is applied to clarify suspensions of low
INTRODUCTION
(a)
O
O
• Particle
O o
o o o
o
o o o
O o
o
S o o o OQ o"o °
°. O ° ...Q Q ° °
Cake
Medium
Filtrate
(b)
Particle
O O O
O O
O O O
O O O
o
ZWM-S^
^S^T-^
DO
IVIedium
°o
f^o °o^o ^§o
Filtrate
Figure 1.5 Cake filtration (a) vs. deep bed filtration (b).
6 INTRODUCTION TO CAKE FILTRATION
particle concentration.^ However, this difference has become blurred with the advent of
cross-flow membrane filtration. Taking advantage of the relatively thin cakes formed
and the capability of fabricating membranes of small thickness, clarification by mem
brane filtration has become increasingly popular especially in food and beverage
industries.
Over the years, investigators have speculated the mechanism and the manner of
cake formation. Beginning with Hermans and Bredee (1935), the so-called "laws of
filtration" were advanced. Based on the manner in which particle deposition takes place,
cake filtration was classified into four different types: complete blocking, intermediate
blocking, bridging and standard blocking. The dynamics of filtration is given as
d2^_ /_d^y
(1.1)
where V denotes the cumulative filtrate volume, t the time, and k^ and k2 are empirical
constants.
It was suggested that the value of k2 characterizes the types of cake formation
with A: = 0, 1, 1.5 and 2 corresponding to bridging (proper cake filtration), intermediate
blocking, standard blocking and complete blocking, respectively.
Hermans of Bredee's formulation was based entirely on intuitive argument with some
arbitrary assumptions. Both complete blocking and bridging lead to cake formation.
Furthermore, since in practical situations, the medium pores and particles are likely not
uniform in sizes, different types of deposition may take place simultaneously. On a
more fundamental level, cake formation, in principle, can be examined in detail through
simulation studies (see Chapter 5). Besides some historical interests, the significance of
the so-called laws of filtration is rather limited.
1.3 CAKE FILTRATION VS. CROSS-FLOW FILTRATION
Traditionally, cake filtration is carried out with the direction of the feed (suspension)
flow coinciding with that of the filtrate flow and cake growth taking place along the
opposite direction. However, one may carry out cake filtration by passing the suspension
to be treated along the filter medium such that the direction of the filtrate flow is normal
to that of the suspension flow. This type of operation may therefore be referred to as
cross-flow filtration. In contrast, the term "dead end filtration" is often used to describe
the traditional operation in which both the suspension and filtrate flow along the same
direction (see Fig. 1.6).
Significant and successful developments of membrane technology during the past three
decades have made available classes of materials (polymeric, ceramic and metal) suitable
^ For using deep bed filtration in water treatment, the particle concentration of the feed stream is
often limited to 100 parts per million.
INTRODUCTION
(a)
Suspension
o
O o o
o
o o o^ o o^ o o o
o
0° ° °°>°oo o \ : °:
_ ° o_ ° JO a, °. °
Filtrate
(b)
O O o
o o
o o o o o o o
o o
Suspension Cross-flow
o
o o
..,_ Q .....Q. 9
-^ oxyoo^x)Ty) cvo o o
Medium
Filtrate
Figure 1.6 Dead-end filtration (a) vs. cross-flow filtration (b).
as filtration media and with them, various types of devices for filtration applications. The
very nature of membrane modules developed so far has made it practical to carry out
filtration in the cross-flow mode. These operations have found applications in removing
particles from liquid streams. The size of the particles removed ranges from submicrons
(as low as 10 nm) to microns depending upon the types of membranes used (ultrafiltration
vs. microfiltration membranes).
8 INTRODUCTION TO CAKE FILTRATION
In both dead-end and cross-flow filtration, particle separation leads to the formation of
filter cakes which contribute resistance to filtrate flow. There are, however, significant
differences between the two types of operations. Regular pressure cake filtration (dead
end) may operate under relatively high pressure (10^ kPa) with filtration velocity of the
order of lO'^^-lO"^ m s"^. The thickness of the cake formed is of the magnitude of 10"^ m.
In contrast, the operating pressure (the so-called transmembrane pressure) in cross-flow
membrane filtration, in most cases, is not more than 100 kPa (often much lower). The
thickness of the cake formed is thin (less than 10~^m) and the filtration velocity is
below 10~'^ms~^ Equally significant is the difference of the hydraulic resistance of
the filter medium used. For medium used in traditional cake filtration equipment (belt
filters and diaphrange filters), R^ is of the order of 10^^ m~^ while for micromembrane
filtration, R^^ is of the order of 10^^ m~^ Because of these differences, the assumption
commonly used in cake filtration of neglecting medium resistance is no longer valid in
cross-flow membrane filtration.^ Accordingly, the need of properly accounting for the
effect of medium surface clogging is imperative. These problems will be discussed later
(see Chapter 8, Section 8.2.).
1.4 FILTRATION CYCLE
Actual operation of cake filtration equipment may be divided into several phases includ
ing filtration, consolidation, washing, deliquoring and cake discharge. The exact number
of the phases involved depends upon the type of the equipment used, the kind of sus
pension to be treated and the specific purpose of the operation. Three examples of filter
cycles given by Wakeman and Tarleton (1999) are shown in Fig. 1.7.
The physics of these different phases of operation may be similar or different. The
phenomena of filtration and consolidation can be described on a common basis embracing
problems such as liquid flow through saturated porous medium undergoing growth
(in filtration) or reduction (consolidation). They are different from cake washing, which
is largely a problem of mass transfer (for example, to reduce the entrained liquid or
other impurities). Similarly, deliquoring by air or compressed gas flow reduces filter
cake from the saturated state to unsaturated state and is governed by laws different from
those of filtration and consolidation. It is, however, safe to say that the filtration phase
represents the essential part of the operation of all types of filtration cycles. It is this
part of the operation which has occupied the major interest of investigators during the
past several decades.
The present monograph is intended to provide an introduction to the analysis and study
of the formation and growth of filter cakes in cake filtration. Simple (conventional) and
more exact (complex) analyses of cake growth are described and outlined in the first part
^ Fradin and Field (1999) found from their microfiltration experiments that the medium resistance
was always greater than the cake resistance by a factor of 2.