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Application of reverse
osmosis in concentration of
sugar solutions
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• An MPhil Thesis. Submitted in partial fulllment of the requirements
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c A. M. A. Elgaali
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APPLICATION
IN
OF
CONCENTRATION
REVERSE
OF
OSMOSIS
SUGAR
SOLUTIONS
by
A.M.A. ELGAALI·
Submitted in partial fulfilment of the
requirements for the award of the
degree of M.Phil. of
Loughborough University of Technology
Supervisor: . Professor J. Mann
Oepartment of Chemi cal Engineering
Loughborough University of· Technology
Loughborough, Le ics LE1l3TU
...
Apri 1 1981
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1't?S'6~/07..
ACKNOWLEDGEMENTS
My sincere and grateful thanks are due to my supervisor
Professor J Mann whose stimulating guidance and invaluable
suggestions are deeply appreciated.
My thanks are also due to Dr P Rice and my colleagues for
their useful discussion during the course of this study.
My thanks are due to the technical staff for their help
and services at various stages of this work.
I am very grateful to Mrs J Smith for her patience in typing
this manuscript with great efficiency.
Finally. I am indebted to my family for their patience.
tolerance and understanding throughout my study .
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CONTENTS
Page Nos
CHAPTER 1:
INTRODUCTION
1.1 Sugar Manufacturing Process
1.1.1 Cane preparation
1.1.2 Juice extraction
1.1.3 Clarification of juice
1.1.4 Concentration of juice
·1.1.5 Crystallization of sugar
1.2 Composition of Cane Juice
1.2.1 Sucrose
1 .2.2 Glucose
1 .2.3 Fructose
1.2.4 Starch ...
1.2.5 Dextran
1.2.6 Pectins
1.2.7 Organic adds
1.2.8 Proteins
1 .2.9 The colouring material
1.2.10 Inorganic constituents
1.3 Conventional Methods of Juice Concentra ti on
1.3.1 Evaporation
1.3.2 Disadvantages of evaporation
1.3.2.1 Inversion...
1.3.2.2 Destruction of sugar and
colour formation
1.3.2.3 Juice entrainment
1.3.4.2 Scale formation
CHAPTER 2:
2.1
THEORY OF REVERSE OSMOSIS' (R:O)
Definit.ions of Osmosis, Reverse Osmosis and
Ultrafiltration
2.1.1 Osmos is
2.1.2 Reverse· osmosis
2.1.3 Ultrafiltration (UF)
1
1
1
2
2
3
3
4
4
7
7
8
8
9
10
10
10
13
13
13
16
17
17
18
19
21
21
21
21
.21
Page Nos
2.2 Historical Background
2.3 Ultrafiltration and Reverse Osmosis
2.4 The Membrane
2.4.1 The fabrication of the membrane
2.4.2 The role of casting components in
membrane manufacture
2.4.3 The structure of the cellulose
acetate membrane
2.5 The Membrane Configuration
2.6 Limitations of the Membrane
2.6.1 Hydrolysis of cellulose acetate
membranes
2.6.2 Microbiological effects on cellulose
acetate membranes
2.6.3 The membrane compaction
2.6.4 Membrane fouling
2.7 Concentration Polarization
2.8 Mechanism of Transport in Reverse Osmosis
22
24
24
27
30
34
37
44
45
46
49
51
54
57
CHAPTER 3:
INOUSTRIAL USES OF REVERSE OSMOSIS
3.1 Introduction
3.2 App1 ication of Reverse Osmosis in the Food
Industry
3.2.1 Application of reverse osmosis in the
sugar industry
3.2.2 Application of reverse osmosis in
whey concentration ...
3.3 Application of Reverse Osmosis in Waste
Water Treatment
3.3.1· The application of reverse osmosis
in pulp and· paper industry
3..3.2 Application of reverse osmosis in
electroplating waste treatment
3.4 Conclusion
64
64
CHAPTER 4: APPARATUS AND EXPERIMENTAL PROCEDURE
4.1 The Pilot Plant
4.2 The Module and Membranes
80
80
80
65
67
72
73 .
75
77
78
Page Nos
4.3 The Operating Procedure
4.4 Analytical Procedure
4.4.1 Determination of the sugar concentration
4.4.2 Determination of the concentration
of calcium
4.4.3 Determination of sugar viscosity
CHAPTER 5: RESULTS AND DISCUSSION
5.1 Pure Water Permeability
5.2 The Effect of Operating Pressure on the
Flux Rate of the Solvent
5.3 The Effect of Feed Temperature on the Flux
~~
5.4 The Effect of Feed Flow Rate on the Flux
Rate
5.5 The Effect of Feed Concentration on Flux
Rate
5.6 The Effect of the Operating Pressure on
Solute Separation
5.7 The Effect of Feed Concentration on Solute
Separation
5.8) The Effect of the Operating Pressure on the
Separation of Calcium Chloride Salt in
Sugar Sol uti ons
5.9 The Effect of Feed Concentration on the
Separation of Calcium Chloride Salt
5.10.The Effect of the Presence of. Dextran on
the Flux Rate of Sugar Solution ...
CHAPTER 6 :
CONCLUSIONS
84
84
84
86
86
88
90
92
92
95
98
101
101
104
106
REFERENCES
APPENDIX A
81
,84
109
. -!.
118
,.
FIGURES
No.
Flow Sheet of a Sugar Manufacturing Process
2.1
Schematic Diagrams of Osmosis, Reverse Osmosis
and Ultrafiltration
2.2
An Exploded View of a PCI Reverse Osmosis
Tubular Module
Different Views of Spiral Wound Model
2.3
A Cut Away Drawing o~ Permasep Permator
2.4
2.5
Hydroly~is Rate Versus pH at Several Temperatures
2.6
Decrease of Flux Rate Due to Fouling
2.7
Water Transfer in Cellulose Acetate Membrane
Systematic Representation of the Preferential
2.8
Sorption-Capillary Flow Mechanism
Application of R.O. in Concentration of Desu- .
3.1
garised Molasses
3.2
Membrane Processing of Whey
Flow Diagram to Show the Application of R;O.
3.3
in Pulp Industry
4.1 . Schematic Diagram of the R.O. Plant
The Effect of Pressure on Flux Rate of Pure
5.1
Water·
5.2
The Effect of· Pressure on Flux Rate of Sugar
Solution
The Effect of Temperature on Flux Rate of Sugar
5.3
Solution The Effect of Feed Flow Rate on Flux Rate of
5.4
Sugar Solution
5.5
Variation of Flux Rate with Laminar Sublayer
Thickness
.
5-;6
The Effect of Feed Concentra tion on Flux Rate
5.7
Variation of Flux Rate with l/u
5.8
The Effect.of Pressure on Solute Separation
5.9
The Effecf·of Feed Concentration on Solute Separation of Sugar Solution
5.10 The Effect of Pressure on Salt Separation
5.11
The Effect of Feed Concentration on Salt Separation in Sugar Solution
5. i 2 The Effect of the Presence of Dextran on the F1 ux
Ra te of Sugar Sol uti on
1.1
,/
Page No
5
23
39
41
42
47
52
60
62
71
74
76
83
87
89
91
93
94
96
97
99
100
102
103
105
I
TABLES
Page Nos
No;
. 1.1
1.2
1.3
2 •.1
2.2
3.1
3.2
A.l
A.2
A.3
A.4
A.5
A.6
A.7
A.S
A.9
A.10
A.11
The Composition of Sugar and Juice Solids
Amides and Amino Acids in Raw Juice
Mineral Concentrations in Sugar Cane Juices,
Syrup and Molasses
Characteristics of R.O. and U.F.
Comparison of Types of Membrane Permeators
Industrial Applications of R.O.
Comparison of Water Removal Costs
The Effect of Pressure on Fl ux Rate of Water
The Effect of Pressure on Flux Rate of Sugar
Solution
The Effect of Temperature on.Flux Rate of
Sugar Solution
The Effect of Feed Concentration on Flux Rate
of Sugar Solution
The Effect of Sugar Viscosity on the Flux Rate
Sugar Solution
The Effect of Feed Flow Rate on Flux Rate of
Sugar Solution
.
The Effect of Pressure on Solute Separation of
Sugar Sol uti on
The Effect of· Feed Concentration on Solute
Separation of Sugar Solution
The Effect of Pressure on Salt Separation
The·Effect of Feed Concentration· on the Salt
Separation
The Effect of the Presence of Dextran on the
Flux Rate of Sugar Solution
6
11
14
25
43
66
69
l1S
119
120
121
122
123
124
125
126
127
128
1
CHAPTER
1
INTRODUCTION
In this introduction attempts are made to give a brief idea
about the sugar manufacturing process from sugar cane, the composition of cane juice and the evaporation process which is conventionally applied for the concentration of sugar solutions.
Some disadvantages of the evaporation process will be highlighted.
1 .1. The Manufacturi ng Process
The manufacture of sugar from sugar cane consists of a number
of successive operations which at the end yield the consumable
sugar.
In general these operations are similar, but they differ
in details.
1.1.1
A generAl outline of the process follows.
Cane Preparation
The cane entering the factory is always accompanied by 'trash'
and soil particles resulting from the mechanical harvesting and
loading;
If these are not removed they will affect both the·
extrac~ion
and the clarification of the juice. The cane is there-
.fore subjected to heavy jets of washing water.
The clean cane is
then chopped into small pieces using very high speed revolving
knives.
This is followed by shredding the chopped cane with a
shredder. Cane preparation irfcreases the capacity of the mills
and theil" efficiency of extracting the juice.
2
1.1.2 Juice Extraction
Juice extraction is carried out in the milling plant which
is usually formed ,in a tandem of five or six sets of mills.
Each mill comprises three horizontal rollers, two at the bottom
and the third at the top.
The fine shredded cane passes through
the rollers and the pressure applied to the top roller ruptures
the cane cells forcing the juice out.
To aid the extraction
sprays of water or 'thin juice' are directed to the 'bagasse'cane after extraction of juice - as it emerges from each mill.
This process is termed imbibition.
It hel ps to reduce the amount
of sucrose lost in the final bagasse.
After juice is extracted
from the cane the bagasse is sent to the boiler house to be used
as fuel for generation of steam and electricity.
1.1.3 Clarification of Juice
The objective of clarification is to remove the maximum amount
of .impurities associated with juice, such as the inorganic salts,
gums, proteins, and the suspended particles.
The most effective
method of clarification till now is the defecation process(lll) in'
which a sufficient amount of lime is added to the juice to neutralize the organic acids. Th:isisfollowed by heating the juice up to
r ;
l03 C which results in a heavy precipitation of calcium salts~ and
0
the coagulation of the albumens. The limed juice is then transferred
to the clarifiers where the heavy solids settle carrying with'it the
suspended particles, and giving a clear juice at the top, and muddy
juice at the bottom.
The, clear juice is decanted to the evaporators
3
while the muddy juice is pumped to the vacuum filters to separate
the juice from the mud.
The extracted juice is taken back to
the clarification process while the mud is taken away as byproduct.
1.1.4 Concentration of Juice
The clear juice delivered from the clarifiers is heated in
multi p1 e-effect evapora tors to remove the water.
The ju i ce enters
the first body with a brix of 150 Bx and leaves the last body at
65 0 Bx.
The heating is performed by the latent heat of steam.
A full description of the process is given in Section 1.3.1.
1.1.5 Crystallization of Sugar
. This is performed in single effect vacuum evaporators which are
called 'pans', the concentrated syrup handled from the evaporator
effects is introduced. into the pan and is allowed to concentrc.te·
to supersaturation.
A small amount of powder sugar is then drawn
into the. pan to·form the nuclei for crystal growth(1l2).
With
development of the crystals more syrup is introduced to feed the
growing crystals.
When these crystals reach the desired size the
mixture of crystals and their mother liquor - now known as 'massecuite' - is discharged into a crysta11izer and then pumped to
centri fuga1s to separate the suga r from the mother 1i quor.
mother liquor is now termed 'molasses'.
The
The first 'strike' dropped
is termed A-massecuite, and its ·resu1ting sugar and molasses are
known as A-sugar and A-molasses.
4
A-molasses is pumped back to the panfloor to form the start
of B-massecuite.
This is usually boiled in the same way as A-
'massecuite with the difference that it is of less purity.
B-
massecuite, when centrifuged, yields B-sugar and B-molasses.
The latter is pumped back to the pans to be boiled with syrup as
C-sugar.
This, on centrifugation gives C-sugar and the final
molasses which is of a very low purity and is taken out as a byproduct.
The C-sugar is usually used as seed for the boiling of
A- and B-sugars.
In raw sugar factories A- and B-sugars are dried
and sent to the refineries.
In refineries both A and B sugars are
melted and treated with S02 and recrystallized to yield white sugar.
Figure 1.1 shows a flow sheet of a sugar manufacturing process.
1.2 Composition of Cane Juice
Cane juice handled from the mill house contains about 15%
solids dissolved in water.
These solids are sucrose, invert sugars,
inorganic salts, organic acids, and other organic non-sugars. All
these constituents, other than ,sucrose"may be regarded as impurities which have some undesirable effects on subsequent sugar processing.
The following paragraphs discuss the characteristics of the
juice components, pointing out their effect on the process.
1.2.1
Sucrose
,As shown, in Table 1.1 sucrose is the most abundant'sugar in'
,cane juice.
It is about ,70-85% of the total sugars present.
formed by the condensation of the monosaccharides, glucose and
It is
5
Cane
I Preparat ion I
To
i3agasse ---·-Iloiler
ll~ill i ng :
Mud
IClarification
Filtrati on
• Cake
I Evafjora tion I
~irst Crystallization!
l Centrifugation}
[ Su~ar
I
.l
:lelting
Bleaching
~
Fin'!l tlolasses
I
!
[Second Crystall ization [
Molasses
J
L
To St ore
FIGURE 1.1
~
. l
Centri fuc;a tion
Drying
I
!
Flow Sheet Showing a Sugar Manufacturing Process·
6
Mi11ab1eCane
Cane (%)
Water
73 - 76
Solids
Soluble solids
Fibre (dry)
24 - 27
10 - 16
11 - 16
Juice constituents
Soluble solids (%)
Sugars
7.5 - 92
Sucrose
Glucose
Fructose
Salts
inorganic acids
organic acids
70 - 88
3.0 - 4.5
1.5 - 4.5
1.0 - 3.0
Organi c aci ds
carboxylic acids
amino acids
1.5 - 5.5
1.1-3.0
0.5 - 2.5
Other organic nonsugars
protei n
starch
gums
waxes, fats phosphatides
Other
TABLE 1.1
2 2 -
4
4
0.5 - 0.6
0.001-0.050
0.30-0.60
0.05-0.15
,
3.0 - 5.0
Showing the composition of sugar cane and juice solids01~
\
7
fructose during the growth of the cane plant
glucose
fructose
sucrose
Chemically sucrose is rather unstable being readily hydrolysed
especially in acid solution yielding equal amounts of glucose and
fructose.
At high temperature and high alkalinity in the presence
of calcium, sucrose decomposes to give calcium saccharates.
Sucrose has a molecular weight of 342.296.
It is optically· active
with specific rotation of [Cl)5 0 + 66.53 when normal weight is used.
1.2.2 Glucose
This is a monosaccharide sugar which is found in the growing
parts of the plant and on maturity it condenses with fructose to
give sucrose as described above.
It forms about 2% of the total
sugars in the juice.
It has an empirical formula of C6 H12 06
and a molecular.weight of 180.2. It is dextro-rotatory with
specific rotation of
[Cl)~O
+ 52.7.
1.2.3 Fructose
This is another monosaccharide sugar which is present in cane·
juice in the same percentage as glucose.
It has a similar mole-
cular weight and empirical formula as glucose but it is laevorotatory and has specific rotation of [Cl)20 - 92.4·.
o
Fructose and
glucose in cane juice are termed invert or reducing sugars as they
8
reduce cupric salt to cuprous oxide.
Their presence in juice has
no direct effect on the process.
1.2.4 Starch
This is a long chain polymer composed of glucose units linked
in a Cl-(1-.4) form.
It has a specific rotation of [Cll~O + 200(104).
It is present in small amounts ranging between 0.001 - 0.5% depending on the variety of the cane.
It is not soluble at room temp-
erature but when the juice is heated during clarification it becomes
soluble.
Part of the starch could be removed during the treatment.
.
.
Any remaining starch will stay in the process liquor throughout
subsequent processing.
This part has an adverse effect on the cry-
stallization of sucrose,
reducing the boiling house efficiency and·
reducing the molasses exhaustibility.
1.2.5 Dextran
This is a high molecular weight sugar polymer which is produced
by the action of the bacterium Leuconostoc mesent€rciJaes.
On the
cut cane or on the cane products in the factory:
n-sucrose dextran-sucr,se (glucose)n + n fructose(8).
Dextranconsistsofabasic straight-chain polymer of Cl-(1-6) linked
gl ucose units with some branches 1inked by Cl-( 1-3) or Cl- (1- 4)
glucosidic units.
.
It has a thread-like structure with a minimum
0 (7)
thlckness of 30-100 A
. Its molecular weight ranges between
5
10 - 107 ,at1({ibhiS<2.-: specific rotation [Cl150 ranging between
·+203 - 233.
The effects ·ofdextran on .sugar processing have
been studied by many workers.
vigayakumer(ll)
9
mentioned that the high dextrorotatory character of dextran
i nfl uences the direct polarimeter r.ead.:i nS3iv:t."[j h1.jh'arpar-ent: fJU-/-J.t;J
and thus giving misleading figures in the works laboratory,
resulting in errors in the control figures throughout the process.
In clarification, dextran is. associated with excess acids and
requires extra lime for ·neutralization.
The presence of dextran
also increases the juice viscosity causing poor settling, high.
mud volume and turbidity of clear juice.
Coll et a;(l05)
claimed
...
that dextran increases the viscosity ·of syrup significantly .causing
a decrease in the rate of
needle-shaped crystals.
increase, the
cryst~.ll
ization and the production of
Tantaoui{l06) recorded that through dextran
factory capacity may be decreased by up to about 30%.
1.2.6 Pectins
These are gel-forming water soluble polysaccharides which are
found in the plant cell.
the milling process.
They are extracted wit:l the juice during
They are very viscous:
0-1% solution of pec-
tin has a viscosity equal to that of a 10% sucrose solution(l08).
Although pectins are found in small amounts in cane juice (0.01 0.05%) they have a significant effect on the crystallization of
sucrose.
They increase the solubility of sucrose;
1 part of pec-
tin may keep 100-500 parts of sucrose in solution(l08).
They also
slow down the crystallization by colloid adsorption on the surface
of the crystals, inhibiting their growth.
Usually more crystals
are formed of smaller than the average size.
10
1.2.7 Organic Acids
Cane juice is acidic 'in nature.
It has a pH between' 5.2 - 5.4.
It contains a number of organic acids both carboxylic and aminoacids.
Their availability depen<lson the variety, age, andloca-
tion of the plant.
in cane juice.
Table 1.2 shows the percentage of these acids
The most abundant carboxylic acid is aconitic acid.
It reaches up to 2.06% of the dry solids content. Citric, malic and
oxalic acids, are also present. The most abundant amino acid is
aspartic acid (0.11% dry solids).
Q.u.t
mic~and
alanine.
The juice also contains gluta-
If these acids are not neutralized during clari\
fication they enter into complex reactions with sugars and the other
organic constituents of the juice e.g. oxalic acid and aconitic
acid are associated with the formation of scale on heating surfaces(lOB).
During heating of the juice the presence of acids
causes significant losses through the inversion of sucrose. They
are generally precipitated as calcium salts during clarification,
such as calcium oxalate(107) .
1.2.B ,Proteins
These are formed by the accumulation of free amino-acids linked
together by peptide chains. They are found in small amounts (0.01
0.05%).
They are partially removed by heating.
The remaining
,part accumulates in the molasses, ,increasing its viscosity and
slowing crystallization rate.
1.2.9 The Colouring'Material
Colouring matters in the sugar liquors occur from three different
, sources:
11
Dry solids (%)
Substance
Free
Protein
Ami des
Asparagine
Gl utamine
0.71
0.19
-
Amino acids
Aspartic
Glutamic
Alanine
Va 1ine
v-Ami nobutyri c
Threonine
Isoleucine
Glycine
Leucine
Lysine
Serine
Arginine
Phenylalanine
Tyrosine
Histidine
Proline
Total· protein
0.11
0.05
0.06
0.03
0.03
0.02
0.01
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
0.06
0.08
0.05
0.04
0.03
0.04
,
0.03
0.04
0.05
0.04
0.03
0.02
0.02
0.02
0.01
0.01
-
0.49
0.01
<
-
TABLE 1.2 Showing ami des and amino acid5 in raw juice(llG
12
i) Those colouring matters which are originally present in the
growing parts of the plant, performing certain functions in
the plant growth, such as chlorophyll,anthophyll, carotenes
and antho ·c.yanins.
milling process.
These are extracted with juice during the
Th~y
are colloidal in nature and insoluble
in water and sugar solutions, so they are removed readily
during clarification(lOg),
ii) Cane juice contains some soluble colouring materials which are
colourless in origin but they develop colour during the manufacturing process. These are represented by polyphenols and
amino compounds.
The polyphenols react with iron, coming" from
the equipment by corrosion, to form coloured compounds, especially in alkaline solutions.
The amino compounds such as
asparagine and glutamine react with the reducing sugars to
form the colour.
iii) Sugars are very heat-sensitive compounds. When they are subjected
to high temperature they caramel ize and form dark-coloured
materials.
Also when the juice is over1imed and subjected to
high temperature during clarification, the reducing sugars
decompose resulting in brown products.·
As mentioned above some of these colouring matters are precipitated during the clarification of juice but some persist and are
found in the final product.
Zaorska mentioned that .the presence of
·the colouring matters in sugar solutions causes reductions in the
,
13
crystallization rate.
1.2.10
Inorganic Constituents
These vary considerably with the variety of the cane plant
and the soil in which it grows. In general the juice contains
elements dissolved in it as ions, salts or combined with organic
compounds. The most abundant are phosphates, silica, magnesium,
potassium, sodium, c.hlorides and sulphates.
The first three are
partly removed by clarification while the others are carried over
through the process to be concentrated later on in the final molasses.
Some may be detected in the final sugar, as shown in Table
1.3.
Potassium is the most abundant element in the juice, it
reaches up to 60% of the total ash.
The juice also contains traces
of heavy metals such as. iron and aluminium.
The presence of the ash
tends to reduce the molasses exhaustibility and slows down the sugar
boil ing(llO) .
1.3 Conventional Methods of Juice Concentration
1.3.1
Evaporation
Evaporation is·the basic and essential step for the crystallization of sugar in solution.
industry early in history.
It has·been adopted in the sugar
Through evaporation about 90% of the
clarified juice water is removed. This concentrates the juice from
its initial Brlx of about 15 0 Bx· to about 65 0 Bx.
Evaporation is
carried out by the usage of the latent heat of steam to boil ·the
14
Concentration (% Solids)
Constituent
Raw Juice
Potassium (K 2O)
Sulfa te (S03)
Clarified Juice
SyruJl
Molasses
1.31
-
0.90
6.55
0.52
0.41
0.61
1.10
Chloride (Cl)
-
0.22
0.46
1.11
Ca 1ci urn (CaO)
0.29
0.30
0.35
-
Ma9nesium (MgO)
0.28
0.16
0.25
0.68
Silica (Si0 2 )
Phosphate (P 205 )
0.71
0.14
0.07
0.05
0.40
0.08
0.02
0.14
Iron (Fe 203)
Sulfated ash
-
-
0.01
-
-
-
3.43
17.73
"carbonate ash
3.64
3.55
-
13.32
TABLE 1.3 Showing 'mineral concentrations in sugar cane juices,
syrup, and mo 1asses. (116)
15
juice and the water leaves as vapour, leaving the juice more
concentrated.
For improving thermal economy, evaporation is
done in multiple-effect evaporators, usually quadruple or quintu.ple.
This will enable one pound of steam to evaporate as many
pounds of water as the number of the evaporator effect(102) . I t
will also permit the withdrawal of vapour from the effects to be
utilized in other stations, namely juice heaters and vacuum pans.
This method of steam utilization reduces the cost of steam used
for evaporation and makes the process a reasonably cheap concentration process.
Many designs of evaporators are used in the sugar industry.
The calandria type is the most generally used in cane sugar factories(102).
The evaporator body consists of a vertical cylindri-
cal .vessel which is divided into two parts, namely the calandria,
which is connected to the steam source and the body chest which is
connected to the juice source.
The body chest is formed by a large
number of tubes about 4~6 ft long and 1.25 - 2.25 in. in diameter(102).·
In the centre of the body is a large downtake which usually has a diameter·of half that of the body diameter •. The function of the downtake is to assist in the circulation of the juice during evaporation.
Above the tube surface is the vapour belt which has a height double
that of the calandria.
This serves as an entrainment separator to
reduce the· escape of the liquid droplets with the vapour.
At the
top of the body is the dome, whichhas half the diameter of the body .
. It contains the catchall, which traps the escaping. liquid droplets,
and the vapour outlet, which leads to the steam chest of the.next
body or the condenser in case of the last body.
16
The evaporation operation' is almost the same in all sugar
factories. The juice before entering the evaporator is pumped
. through the clear juice heaters to raise its temperature slightly
above the saturation temperature of the first body.
This increa-
ses the capacity of the evaporators and facilitates both heating
and evaporation as there is no sensible heat lost in heating the
juice to its boiling temperature.
The juice enters the evaporator
. at the bottom of the calandria and circulates up through the tubes
and down the central downtake. The concentrated juice usually
1eaves the body from the top of the ca 1andria to the success i ve
bodies until it is discharged from the last body.
The vapour from
the boiling juice in the first body is drawn to the steam chest of
the second body for heating its juice, and so on.
The condensing
steam or vapour in each body is collected as condensate which is
then returned to the boiler for steam generation.
1.3.2 Disadvantages of Evaporation
The'intention in evaporation is to concentrate the juice by the
removal of pure water without any substantial changes in the composition of the solids.
This appears to be. theoretical , as in
practice there are some physical and.chemical changes which take
place in the composition of the dissolved solids.
Sometimes these
changes are very dominant, which may affect the efficiency of the
process, leading to significant losses.
In the following paragraphs
some of these changes which.have direct effects on the performance
and the efficiency of the process are reviewed.
17
1.3.2.1
Inversion
This is one of the most important chemical reactions which
takes place during evaporation.
At lower pH and high temperature
sucrose hydrolyses to yield glucose and fructose:
The amount of
, sucrose inverted is proportional to the hydrogen ion concentration, the evaporation temperature and retention
time during evaporation.
At low pH and high temperature and long
retention time inversion is a maximum.
tion can be very high.
The losses due to inver-
Honig(103) mentioned that the percentage
of sucrose inverted per hour at lOOoC and pH 4.6 is 5.32%.
To
control inversion rate the juice del ivered to the evaporation sta'tion should have a pH of about 7.
The retention time should be
as 'short as possible.
1.3.2.2 Oestruction of Sugar and Colour Formation
It has been observed that the colour,of concentrated juice,
syrup, is more dense than that as the clarified juice if the syrup,
is diluted to the same density of the clarified juice. This is
,
attributed to the precipitation of non-sugars when they are subjecteq to ,the high temperature of evaporation especially in the
first two bodies.
As mentioned in Section 1.2.9 high temperature
in the evapOrators may result in the caramelization of sugar giving
dark colouring compounds. The 'destruction of reducing sugars under
18
the effect of high temperature may also lead to their combination
with other non-sugars to affect the colour and the viscosity of
the syrup.
1.3.2.3 Juice Entrainment
This is the escape of juice droplets with vapour during evaporation.
This may have two effects on the sugar process:
i) it is a direct loss of sugar which should not be underestimated,
especially in the last body of the evaporator-train where the
vacuum is high.
~ii)
the condensed water obtained from the juice vapour is usually
used as feed water to the boilers.
If it is contaminated with
juice it nay cause serious damage to the boiler tubes.
An
additional effect is that the escape of sugar-containing
droplets into the condenser waters in the case of the last
body will increase the B.O.D. in the factory effluent waters.
Many measures have been suggested for.the prevention and the
control of entrainment and their effectiveness depends on how far
they are applied.
Entrainment could be controlled by controlling
the evaporation rate, vapour velocity, juice level in the evaporator
tubes and· the vacuur,l in the last body.
The evaporators are designed
in such a way tha t the vapour space is twi ce tha t of the cahndri a
and under the effect of gravity the juice droplets usually tend.not
~to
be carried over. Juice separators and catchalls are normally
installed in the evaporators to ·restrict the escape of the juice in
thi sway.
19
1.3.2.4 Scale Formation
This is the most severe problem which may face the production
manager in a sugar factory.
As mentioned in Section 1.2.10 the
juice contains a small amount of soluble mineral salts and organic
substances which are not precipitated during the process of clarification. These substances become less soluble as the concentration
of the juice increases and they are partly deposited on the surface
of the tubes, forming a thick layer of scale.
This layer decreases
the heat transfer coefficient, resulting in a very slow evaporation
rate(llO).
Honig(103) mentioned some conditions influencing the .
rate of scaling in evaporators:
i) The presence of suspended matter.
ii) The scaling is more severe when the evaporation system has an
overcapacity in relation to the other stations of the factory.
iii) When the capacity of the mill is reduced - frequent stoppages
of short duration - the intensity of scaling increases.
iv) When the level of the juice is maintained in the recommended
levei (1/3 of the tube length), and with high rate of evaporation, scaling is minimum.
The amount and composition of scale deposited on an evaporator
surface vary from one area to another, and even in the same area it
.
.
(111)
varies from one season to another
. Generally scale .in evapora·tors consists of a mixture of inorganic and organic non-sugars.
Phosphates are more dominant in ·the first bodies while sulphates
arid silicates ar,e dominating the last bodies.
Figure 1.2 sh·ows the
relation between juice concentration and composition of scale.
20
The most familiar methods of scale removal are:
. i} Mechanical removal using hand operated scrapers or power
operated tools such as rotating scrapers or cutters which are
driven by flexible shafts.
ii} Chemical treatment:
many chemicals have been tried for the
removal of,evaporator scale. The most generally used is the
alkali-acid treatment.
Caustic soda (concentration 1.5 -
24 %) and soda ash (concentration'5-10%), are boiled in the
evaporator for about 6 hours, then followed by water washing'
and 1-2 hours boiling of hydrochloric acid (concentration,
1-5%).
The soda reacts with oxa1ates, silicates, silica and
the organic scale while the acid reacts with the carbonates
and phosphates(1l1}.
In some practices an inhibitor is added
to prevent the corrosion of the tubes by the acid.
ZT
CHAPTER 2
THEORY OF REVERSE OSMOSIS (R.O)
2.1
Oefinitions of Osmosis, Reverse Osmosis and Ultrafiltration
:2.1.1
Osmosis
This is a natural phenomenon of solutions in which the solvent
passes from a dilute solution to a concentrated one through a
semipermeable membrane.
The solvent flow occurs to attempt to
equalise the chemical potential of the solvent on each side of the
membrane.
As a result of this flow a pressure build-up will occur
on the concentrated solution side of the membrane.
This pressure
is termed 'Osmotic Pressure'.
2.1.2 Reverse Osmosis
In the process described above if a pressure equal to the
osmotic pressure of the concentrated solution is applied to the
concentrated solution side of the membrane, the osmosis process
will stop:
If the applied pressure is further increased to over-
come that of the osmotic pressure, the process will be reversed
and'the solvent will pass from the concentrated solution to the
diluted one.
2.1.3
This process is termed 'Reverse Osmosis'.
Ultrafiltration (UF)
This is a similar membrane process in which, under the effect
of applied pressure, both the solvent and any microsolute (S) of the
22
concentrated solution are forced to flow through the membrane and
only the macromolecules are completely retained by the membrane.
Schematic diagrams of osmosis, reverse osmosis and ultrafiltration
are shown in Figure 2.1.
2.2 Historical Background
The phenomenon of Osmosis was discovered by Abbe No11et in
1748(75), who observed that when alcohol and water were separated
by an animal bladder membrane, the water passed through into the
alcohol while the alcohol did not pass through to the water.
He
concluded that the membrane could allow the passage of the solvent,
but not that of the solute.
This encouraged many scientists to
investigate this behaviour.
In '1861 Schmilt(74) published the fact
that a bovine pericarium membrane could partially retain dissolved
gum arabic.
In 1864 M Traube discovered a method for the prepara-
tion of artificial membranes from copper ferrocyanide.
Later on,
in 1877 Pfeffer was able to measure the osmotic pressure of many
solutions.
Martin(74) in 1896 was able to make an artificial membrane
which was completely impermeable to protein;
Bechho1d in 1906 intro-
duced the use of flat reinforced collodion membranes which were made
of cellu1·ose nitrate and he was the first to use the term 'ultrafi ltra t ion '.
Asheshov demonstra ted tha t the addit ion of acetone
to cellulose nitrate solution would result in collodion membranes
of varied porosity.
The next important advance was made by E1ford
in 1930 who established a .standard technique for preparing a series
of membranes of varying porosity;
Ferry(76) in 1936 gave a complete
23
a)
Osmosis:
normal flow from low to hi~h concentration
11. SOLUTIO
SOLVENT
b)
•.
"tI
Reverse Osmosis: flow is reversed by application of pressure
to the more highly concentrated solution
I
'PP I i ,d P""""
• I'
SOLVENT I:SOLUT ION
..
c)
11
Ultrafiltration:. flow of solvent and microso1ute
applied pressure
iI
•
solvent
solution
'lrnacrosolutE
mic ro solute
FIGURE 2.1
Schematic diagrams of osmosis, reverse osmosis and
u1 trafi 1tration
24
review of all developments in ultrafiltration up to 1936.
Due to the performance of membranes and the difficulty of
their reproducibility coupled with the discovery of new and
better procedures, ultrafiltration tended to receive less attention.
This continued until the 1950's when Reid and co-workers(17)
discovered that cellulose acetate membranes exhibit a remarkable
selectivity between salt and water.
A few years later Sourirajan
and Loeb(l) were able to develop very successful ce11o1use acetate
membranes, which transferred the reverse osmosis and u1trafi1tration processes from laboratory applications to wide industrial
uses.
2.3 Ultrafiltration and Reverse Osmosis
From the definitions of the processes made in Section (2.1)
there is no great distinction between the two processes.
In both
pressure is used in conjunction with a semipermeable membrane to
affect separation.
In practice the separation achieved, the
membranes, and operating conditions are quite different.
Table
(2.1) highlights the major differences between the two processes.
2.4
The Membrane
As mentioned in. Section 2.2, the
basic principles of reverse.
osmosis were known earlier in this century, but its wide scale
application was hindered by the poor performance of the membranes
25
TABLE 2.1
Characteristics of RO and UF
R.O.
U. F.
solute molecular
weight generally
1ess than 1000
solute mol. wt.
generally greater
than 1000
2. Osmotic pressure
of feed
Osmotic pressure
of feed can be up
to 1000 psi
Osmotic pressure
of feed is
generally negligible
3. Operating pressure
operating pressure
up to 2000 ps i
opera ti ng pressure
up to 100 psi
4. Nature of the
membranes
the chemical nature
of the membrane is
important in affecting transport
properties
the chemi ca 1
nature of the membrane is relatively unimportant
l. Size of solute
retained
26
manufactured at'that time. Reid and co-workers(l?) in the early
1950's at the University of Florida, examining some membranes
'to measure their semipermeability, found that cellulose acetate
membranes exhibit a remarkable selectivity between salt and
water.
These membranes, known as homogeneous membranes, suffer
from uneconomically low water flux rates.
During the period
1958-1960, Loeb and Sourirajan(l), testing various commercially
available plastic films, discovered the desalination .characteristics of the cellulose acetate filters supplied by Schleicher and
Schuell.
Through their trials they were able'to develop a tech-,
nique for the fabrication of a porous cellulose acetate membrane
which could satisfy the essential requirements of a good desalinating membrane, namely:
1.
High permeability to water, and
2.
Low permeability to water soluble compounds.
These new membranes were then termed 'asymmetrically modified'
membranes.
Although other polymeric membranes have been manufactured,
still the cellulose acetate membranes are the most commercially
available membranes.
In the following sections, discussions will
be centered on the methods of fabri cat ion of these membranes,'
the role played by each component of the casting solution, and the
membrane configurations.
27
2.4.1
The Fabrication of the Membrane:
The membranes made by Sourirajan and Loeb were prepared. from
a quaternary mixture of cellulose acetate, acetone, magnesium
perch10rate, and water, in the ratio 22.2, 66.7, 10.0 and 1.1% wt.
The fabrication process was performed in four steps which were
carefully controlled to give the desired membrane performance.
i)
Casting
The membrane is cast in a box at a temperature between -5 -
100C, on a glass plate having 0.025 cm side runners to give this
as cast thickness to the membrane. To ensure the uniformity of
the membrane an inclined knife is passed across the top of the
plate, resting on the side runners only. The use of the glass plate
is to give the. membrane a smooth and rigid structure.
Membranes
cast o'n rough surfaces showed lower performance characteristics,
while membranes cast on a filter paper surface have been found
:to give similar results to thosecaston glass(l).
ii) Evaporation
After casting, the casting solution and all the casting components are kept in ice cold water for 3-4 minutes.
period, the solvent starts to evaporate.
During this
Evaporation is strongest
at the surface' of the viscous solution which increases the concentration of the polymer at the polymer air interface.
As soon as
it is formed this layer hinders further evaporation of the solvent
through it, thus leaving the remaining underlying solution 1arge'lY
\
28
unchanged.
The purpose of the low temperature is to control the
rate of evaporation of the solvent from the mixture.
Kunst et a1(22)
found that a too rapid evaporation rate resulted in a membrane of
large size pores while too slow evaporation resulted in even
bigger pores with a decrease in the number of these pores.
The
length of ·the evaporation period is also an important factor in
the membrane performance.
Pi10n et a1(21) suggested that the
evaporation time had an effect on the pore size and its uniformity.
Short evaporation times resulted in smaller and less uniform
pores while Loeb(52) claimed that extending the evaporation period
resulted in serious deterioration of membrane performance.
He
attributed that to be due to the increase in membrane thickness.
iii) Gelation
In this step the membrane plate assembly is immersed in ice
water for a period of one hour.
The water diffuses rapidly into
the interior of the film and its concentration soon exceeds
the limit of cellulose acetate solubility.
This will cause the
gelation of the film by the coagulation of the cellulose acetate(66)
resulting in formation of the hard
brane.
porous structure of the mem-
During this period the remaining solvent, acetone and
the additive, magnesium perch10rate, will be 1eached'out.
The
role of water in gelation has been studied by many rese~rchers.
Manjikian(33) postulated that the waterof gelation has, a specific
and crucial role to play;, diffusion water displaces both the
sol verit and the addi tive and becomes a permanent structural component of the finished membrane.
Loeb(52) claimed that attempts to
29
replace water by other substituents were not successful.
On
the other hand sourirajan(l) claimed that methanol has 'the same
effect as that of water in gelation step and he concluded that
the surface layer was not formed during the gelation period.
Carter(31) suggested that water can be substituted by waterethanol mixture.
iv)
Annealing
It was found that in the as-cast state the performance of
the membrane is poor. The salt rejection is only about 50% and
water permeability is high.
This was due to the extensive physi'-
cal crosslinks and macropores of the film.
The membrane contains
a large amount of water both in the surface layer and in the
spongy layer.
If the membrane is subjected to temperature
treatment, the salt rejection increases and the water permeability
decreases whi'le the mechanical strength of the membrane is improved
greatly.
Wasilewaski(32) claimed that heating of the membrane
initiates the formation of intermolecular bonds between the neighbouring cellulose acetate molecules instead of the intra-mo,lecular
bonds already present there.
Thus the molecules of cellulose
acetate are brought closer.
This will result in decreasing the
pore size of the membrane.
This was supported by Vincent et al(S)
who postulated that there is a decrease of 27.0% of the 'membrane
bound water if the temperature treatment is increased from 2S oC '85 0 C, this results in bringing the polymer chains closer to each
other. 'Carter(31) recorded that microwave and infrared studies
showed that these changes in the membrane structure cannot occur
30
until the heat treatment reaches 6S oC.
From what is stated
above it is clear that treatment temperature is necessary for
pore size adjustment, and by adjusting the temperature of annealing different membrane properties can be tailored to give.
different levels of solute separation and flux rates.
Generally
the membranes are annealed in hot water for one hour(2). Metha
et al(24) found that the cellulose acetate membrane characteristics are also dependent on the composition of the annealing
medium.
Membranes annealed in a glycerine-water mixture yielded
higher flux rates at the same level of salt rejection, than those
annealed in water.
Kunst and Sourirajan(22) found that membrane performance will
.be improved significantly if it is subjected to a low pressure
treatment before being used in a reverse osmosis application.
The applied pressure may squeeze more of the bound and capillary
water present in the membrane film.
2.4.2 The Role of Casting Components in Membrane Manufacture
Extensive research has been done in this subject to optimize
the fabrication of the cellulose acetate membranes.
Each component
of the casting mixture has been found to play a significant role
in the membrane performance.
i)
The cellulose acetate
This is the major component of the.membrane mixture.
It gives'
·the membrane its maximum·uniformity and optimum physical properties
e~g. flexibility. and strength(Sl).
Membranes prepared from other
31
cellulose derivatives as membrane materials, gave poor performance(25).
The recommended degree of acetylation for good
me~brane
performance
is between
75-88~.
37.5-40%.
The upper limit is determined by the solubility of
This will given an acetyl content of about
cellulose acetate in the solvent while the lower limit is determined by the membrane performance.
If the acetyl content is
increased the salt rejection increases while the flux rate decreases(l?) .
ii)
The solvent, acetone
Manjikian(33) mentioned that a good solvent for reverse osmosis
membranes should satisfy certain requirements:
1.
It should not react with the other components of the casting
solution.
2.
It should be miscible with water and the additive salt.
3.
It should have the ability to leach out at a faster rate'
than the additive salt during evaporation and gelation.
·up to now acetone is the main solvent which satisfies the
above-mentioned requirements.
Glacial acetic acid has been tried
as a substitute for acetone.
It gave good results as far as flux
rates· and salt rejections were concerned.
It was even found
possible to omit the addition of the swell ing agent and the annealing process, but it was also found that the membranes were physically poor and had poor tear strengths(52).
Other solvents have
been tested for complete or partial substitution of acetone, but
the performance of the membranes thus prepa red were found to· be
. f er10r.
.
t 0 th ose prepare duS1ng
'
1n
ace t one.
Ke1. l'1n (31) ment10ne
. d
32
that p-dioxane can substitute for acetone in the casting solution
with the result that casting can be performed at room temperature,
'as its volatility is much lower than that of acetone.
The use
of p-dioxane has been limited as it is expensive and toxic.
It
,cannot be used in reverse osmosis applications where the product
is needed for human consumption.
iii) The additive salt (swelling agent)
The addition of an additive for the preparation of membranes
was first introduced by Dorby in 1936(29) when she was testing the
misCibility of cellulose acetate in magnesium perchlorate for the
preparation of ultrafiltration membranes.
This idea prompted
Loeb and Sourirajan to add magnesium perchlorate to the casting
solution for the fabrication of their asymmetric membranes.
The
resulting membranes were found to have high flux rates compared to
the homogeneous membranes of Reid.
It was then believed that the
additive was the flux inducing component in the fabrication of the
semipermeable membranes.
It helped to reduce the resistance to,
the flow of water through the membrane. Kesting(26) mentioned that
the additive was active as a hydrophilic agent, which caused the
membrane to swell during the water immersion period, leading to
the-formation of pores in the membrane film.
This was confirmed
by Manjikian(33) who suggested that the additive served to keep,
the polymer chains apart until gelation of the membrane 'structure
was complete.
There' are certain requirements which are looked for
in successful additives:
1.
Th~
additive should be miscible with the other
component~
of the
\
33
casting solutions giving a homogeneous mixture as insoluble
particles in the casting solution might tend to leave
macroscopic holes in the membrane after the additive is
leached out.
2.
It should not be reactive with the other components of the
casting solution.
3.
It must be possible to be .leached out from the membrane film
by washing.
4.
The ion of the electrolyte should not form ion-pairing in
solution.
Extensive studies were made to investigate which part of the
electrolyte additive was responsible for
brane.
the swelling of the mem-
Kelin(36) suggested that the cation is the important part
which causes the swelling of the membrane during the immersion
period.
On the other hand Loeb and Nagaraj (37) claimed that the
anion is. the effe"ttive part of the additive.
This was supported
by the experiments of Loeb and Milstein(34) in which successful
'results were obtained when membranes prepared from perchlorates
of cati ons other than ,magnes i um.
When subst i tuti ng the perch-
lorates with other ions however the membranes gave poor results.
Manjikian(31) testing the role of water in the casting solu·tion substituted it with formamide solution.
The membranes thus
made gave reasonable results. This prompted him to replace both
the water and the inorganic salt.
The resulting membranes were
,
34
even better than those prepared by the addition of the electrolyte
additive.
The new casting solution was made in the following
proporti ons:
'Cellulose acetate
25%
Acetone
30%
Formamide
45%
The use of formamide as an additive has simplified the fabrication
of membranes and eliminated the need for casting temperature res"
trictions i.e. the membranes may be prepared at room temperature.
2.4.3 The Structure of the Cellulose Acetate Membrane
The structure of the asymmetric cellulose acetate membrane
has been examined by Riley et al(39) in 1964, using electron
mi croscopy techn i ques.
They reported that the membrane cons i s ts
ofa fine-pored matrix with a very thin layer which has a thickness
of about 0.25).1 compared with the total membrane thickness of 100\l.'
The rest of the membrane is a spongy mass having a pore size of
the order of O.l\l.
characteristics.
The surface thin layer is devoid of structure
Later in 1966 using improved techniques they
confirmed their previous work and ,affirmed that the dense layer
showed no evidence of pores greater than 100 ~ whereas the in'terior
s'ubstructure, showed larger pores (0.4\l).
They claimed that there
./
,is no sharp 1ine of demarcation between the porous part of the
membrane and the skin layer, but rather the pores become progressively
35
smaller towards the air-dried surface.
The idea of absence of pores in the surface layer mentioned
by Riley, was confirmed by Burghoff et al(44) who claimed that his
electron microscopy study did not recognise any pores, neither
in the homogeneous membranes nor in the dense layer of the asymm,etric membranes.
In contradiction to the idea that the membrane is formed of
only two layers, Gittens(45) suggested that the membrane is made
up of three mutually distinct layers, namely, a dense surface
layer of only a few molecule layers in thickness, a comparatively
thicker transitional layer underneath the surface layer, and a
very much thicker spongy supporting layer constituting the bulk
of the membrane material.
He concluded that the transitional
layer may be e'xpected to play a dominant role on the pressure
stability of the porous structure of the membrane.
Kesting(51)
confirmed the presence of this transitional layer.
He postulated
that this l.ayer was not a universal feature.
It is formed in mem-
branes prepared'from casting solutions containing low to intermediate,concentration of the swelling agent.
He added that this
transition layer consisted of closed voids which tend to decrease
permeab il ity by;
,i)
Offering high resistance to water flow.
ii)
Undergoing compaction into a'dense film thereby increasing
the effective thickness of the overlying skin layer.
According to Sourirajan(l ,3,4) the modified membrane is formed
of two asymmetric layers.
36
1.
The active skinned layer:
this is very thin, and has got
very fine pores, identified as the air-dried surface.
It
is the layer which has been away from the glass at the time
of casting and
it will be in contact with the feed solu-
tion during the reverse osmosis process.
This layer offers.
most of the resistance to fluid flow through the membrane. .
The flow rate of the solvent varies inversely with the thickness of the layer, while the solute rejection is independent
of its thickness. If this layer is damaged the performance of
the membrane deteriorates seriously.
2.
The substructure layer:
this layer is spongy and it is macro-
porous. It has relatively little resistance to the flow of
the solvent.
It contains most of the bound water held by the
membrane.
The presence of pores in the surface layer
su~sested
by
Sourirajan was confirmed by the analysis ofGlueckanf et a1 (47) who
recognised that the surface layer is microporous.
However more
recent electron microscopy studies suggested the presence of these
pores.
This was further supported by the work of Stern(43) who
estimates the pore'diameter to be in,·the range of 1"x"10'-~c50 x 10- 8 cm.
Frommer et a1(23) discussing the formation of pores in the
cellulose acetate membranes concluded that the pore structure is,
dependent on the activity of water in the gelling medium and the
characteristics of the membrane. can be controlled by adjusting the
37
concentration of the swelling agent in the gelling water.
On
the other hand Pagean et al(46) claimed that the concentration
of the solvent in the casting solution and the casting atmosphere
is responsible for the generation of pores in the membrane, and
that by increasing the concentration of the solvent in the casting
solution, numerous pores of small size can be obtained.
2.5 The Membrane Configuration
Reverse osmosis membranes are thin polymer sheets which can
not withstand the high operating pressure needed by the process.
They therefore require a backing material as a support to provide
this mechanical strength.
These backing materials are made in
such a way that:
1.
They can enclose very large areas of membrane in a small
volume to reduce the size of the permeators.
2.
They should be designed in such a way that there is a minimum
pressure drop during the reverse osmosis process.
3;
They can be simple to fabricate and assemble.
Many membrane configurations have been designed, but it should
be recognised that no single configuration will be optimum for all
.applications and each design has its advantages and disadvantages.
A brief idea concerning these designs and their characteristics,
advantages, and disadvantages is given below.
38
i)
Plate and frame configuration
This configuration takes its shape from the conventional
plate and frame filter press.
stainless steel or resin.
The supporting material is normally
It is mainly used as an analytical or
preparatory technique an a laboratory scale.
of design problems.
It has a number
The unit size is normally large which is
difficult to clean and maintain.
of isolation and replacement.
It also faces the difficulties
But this design was used until
recently by D.D.S.
ii)
The tubular configuration
This was first introduced by Havens Industries in 1964(1).
The membrane sheet is rolled onto a tube having a diameter of
about half an inch which is then inserted into a supporting tube
to give the necessary mechanical strength.
Figure 2.2 shows an
exploded view of a PCI,tubular module of this form.
The membrane
may be cast directly inside the supporting tube, or cast in a
casting tube and then transferred into the backing tube.
In this
configuration the membrane's surface area/unit volume of module is'
small, which may be considered a disadvantage and a large number
of tubes are usually placed in modules if a 'large application is
needed.
Unlike the plate and frame design, there is no problem of
large sealing areas, as the sealing is only confined to ,the tube
ends.
Cl eani ng, i so 1ating and repl acement of the defecti v~ tubes
is not difficult when compared with plate and frame design.
Cleaning can be performed' by water and chemical flushing or foam
swabs, without dismantling the e'quipment.
•
•
A.
IND CAP NUT.
w
Ou
PIN
'"
A
I
•
FIGURE 2.2 An exploded view of a PCI reverse osmosis tubular module
_.....I
-
....
40
iii) The spirally wound configuration
This design consists of two sheets of membrane placed over
a porous backing material with a perforated pipe at the end.
A line of glue along the edge of the membrane seals the porous
backing material inside'the membrane envelope. The membrane is
then covered with a mesh of spacing material on one side, and
rolled around the plastic tube to produce a compact multi layer
\
unit.
The whole is then placed in a tubular pressure vessel.
The pressurised feed is passed through the mesh spacer passages
into the porous backing material and the permeate passes through
into the central plastic pipe. The concentrate flows out through
the mesh spacing as in Figure 2.3.
This design, like the plate
frame design faces the difficulty of cleaning and maintenance,
coupled with high pressure losses during the,flow of the feed
through the small flow passage. The' advantage of this module is
that the membrane surface area/unit volume is high, which can be
three times that of the tubular module.
iv)
The hollow fibre module
This design is formed of very fine fibres with an outer diameter
of 45\1 and inner diameter of 24\1(14): The fibres are placed in
bundles whi ch are self supporting and can ,be potted s imply and
inexpensively into standard size pipes and fittings.
They are
completely compact and the membrane surface area/unit volume is
comparatively high.
The 'high-pressure fluid passes through the
fibre to the hollow interior, Figure 2.4 shows a cut-away 'drawing
41
P[q,M[AT[ nOW
I AFTER. PASSAGE
THROUGH H[MBRINE)
P£IHifAT£ GUT
PERHEATE SIDE BACKING
HATERIAl WITH MEHBRANE ON
[ACH SIDE AND GLUED AROUND
[DGES AND TO tEtHU TUBE
"",
,
\
FEED.
PBI
~~~~~::~~M:_EMBRANE
,
FEED
PSI
5.5.
ENTRANCE
MEMBRANE
PIPE
FIGURE 2.3
Showing different views of spiral wound model (3).
42
[POlY
Sk[U
TUB(
SNA.P RIHCi
CONCENTRATE
OUTLET
POROUS
uP DISC
flOW
rEED
SHELL
F([O
[NO ptAT[
OlSTRISUTOR TuM
CUT AWAY DRAWING OF PERMASEp· PERMEATOR
FIGURE 2.4
Showing a cut'away drawing of Permasep permeator (19)
. TABLE 2.2
Comparison of Types of Membrane·Permeators (5)
Type
Plate-and-frame
Tubular
Spi ra l-wound
Holl ow-fi bre
Advantages
Low holdup per unit membrane area.
Much operating experience
Low floor space per sq. ft.
Disadvantages
Can plug at points of solution stagnation.
May be hard to clean acceptably for food uses
Expensive at present
(ca. $lOO/sq. ft).
Status
Commercial
Easily cleaned; accepted for
processing food products.
Much operating experience
Individual tubes replaceable
High holdup per unit membrane
area.
Relatively expensive (ca
$lO-$20/sq ft.
Requires moderately large
floor space per sq. ft.
although tube modules can be
placed separately in and
around existing equipment in
special cases.
Low in cost (ca $3/sq. ft)
Compact; low floor space per
sq. ft.
Low holdup.per unit membrane area
Long operating experience
Easily plugged.
Hard to clean acceptably for
processing food products
Commercial
Very low in cost
Very compact
Low holdup
Plugs easily
Very hard to clean
Commerci a1for reverse
osmosis only
Commercial
,
I
44
of a hollow fibre permeator.
Although these modules are adyan-
tangeous in that they are highly productive and cheap to manufacture, they suffer from a number of disadvantages:
1.
They have the tendency to be fouled by the suspended matter
of the feed and this necessitates a very high degree of feed
pretreatment.
2.
There is a problem of permeate pressure drop through the
hollow fibre which restricts the length of these fibres to
only 3-4 ft in length(lO).
3.
They cannot withstand very high pressure·, as the fibres tend
to rupture.
To overcome this problem the wall thickness of
the fibre is kept relatively high which in turn has its effect
on the permeate flux. Table 2.2 shows a comparison between
different types of confi gura tion di scussed above.
2.6
Limitations of the Membrane
As mentioned in Section 2.4.3 cellulose acetate membranes are
thin polymer sheets which consist of a fine-pored matrix with
a very thin layer at the top and a spongy porous substructure.
.
,.
Due
to this delicate structure and its polymer properties the membranes
suffer from a number·of limitations.
extreme conditions of operation.
They are unable to withstand
They undergo structural changes
at elevated temperatures .which alters their transport characteristics.
pH of'the feed is restricted to. the range 3-8 as .higher or lower pH
.
45
will lead to the hydrolytic degradation of the membrane.
Many
organic solvents either dissolve the membrane or plasticize
it sufficiently to cause it to collapse.
Most of the membranes
must be kept under water or in aqueous alcohol solution, or
just in moist conditions.
If they dry out the structure tightens
and they become impermeable.
In the following pages the effects of some of these limitations will be highlighted.
2.6.1
Hydrolysis of Cellulose Acetate Membranes
Hydrolysis is one of the most serious limitations of cellulose
acetate membranes
\~hi
ch restricts its appl i cation s igni ficantly.
The membrane material is an ester which is unstable to water at
higher and lower pH, reacting, to give alcohol and acid. The
occurrence of hydrolysis is detected by an increase in water flux
and decrease in salt rejection.
In the beginning the water flux.
changes slowly and the salt rejection decreases more rapidly,
then water flux.increases more quickly and salt rejection drops
to zero and the weakened porous structure then collapses completely.
Lonsdale(66) mentioned that X-ray diffraction and infrared-spectroscopy studies demonstrated that the failure was accompanied by an .
increase in molecular orientation and increased hydroxyl content.
Riley et al(68) studied a membrane which lost its permeability
through hydrolysis, he found that not all the membrane was hydrolysed equally but partial hydrolysis took place and the hydrolysed
part became insoluble in acetone. indicating that it was not
cellulose acetate.
46
Vos et al (58) have studied the hydrolysis process of cellulose aceta te membrane extens i ve ly.
They found that the 1ife-
. time of the membrane was dependent on the feed pH. When the
membrane was supplied with feed solution of pH B.9, the salt
rejection deteriorated from 95% to 49% within only four days.
However when the pH was lowered to 5.0 - 6.5, the membrane
showed no membrane deterioration in salt rejection.
In another
study(5B) they investigated the hydrolysis rate as a function
of both pH and temperature over the range of pH 2.2 - 10 and
temperature ranging between 23 - 95 0 C.
They found that the rate
of hydrolysis increases with temperature. The pH at which the
reaction is minimum is in the range of 4.5 - 5.5 and the life of
the membrane is substantially increased by operating close to this.
range - as shown in Figure 2.5.
They calculated that at tempera-
ture 23 0 C and pH 4.B the lifetime of the membrane reached up to
4.3 years,while if the pH is 6 the membrane lifetime will be
reduced to only 2.5 years. If the pH is 1 or g the membrane will
.collapse within a few days.
Most of the commercially available
membranes are working within the range of pH 3-B and the temperature
in the range of 5 - 35 0 C.
2.6.2
Microbiological Effects on Cellulose Acetate Memc
branes
This is another problem which faces the application of cellulose acetate membranes in the food industry.
In general food prp- .
.
ducts are good nutrients. for the growth of microorganisms, and the
processing conditions are alsofavour~ble for their multiplication(72).
Many workers investigated the effect of microorganisms on the perfor-
47
I
ID·"
0
9~·C
6
.. ·C
7Z·C
D
~U
•
.,
10
•
·C
HOC
....,
...
'u 10·-
lii
0:
.,
10·r
in
~
0
0:
a
>-
00"
:J:
00"
10- 10
2
FIGURE 2;5
•
•
OH
•
10
Hydrolysis rate versus pH at several temperatures(66)
48'
mance of cellulose acetate membranes.
Lonsdale(16) claimed that
the organisms can cause deterioration in the membrane performance
as a result of enzymatic digestion of the membrane.
This agreed
with the findings of Cantor and Mechalas(73). in which they have
shown that performance of cellulose acetatemembranes, and especially those with degrees of substitution from 2.3 - 2.5, can be
completely damaged by enzynBtic hydrolysis.
On the other hand Vos
et al(70), who studied the effect of the storage of cellulose acetate membranes, found that there were no changes observed in the
transport properties of the membrane, and there was no biological
attack on the membrane by the organisms.
This was further confirmed
by Kissinger et al (71) who were concentrating maple sap in a EURO
concentrator.
They found that the microorganisms had no destruc-
tive action on the membrane, but that slimy layers were usually
formed on the surface of the membrane which caused a marked decrease
in the flux rate of water through the membrane.
This encouraged
them to devise a means of reducing the microbial
population in
the sap to the lowest possible level. They used ultraviolet germicidical lamps in the maple sap feed supply line, which showed an
effective result in reducing the microbial growth.
Many chemicals and detergents have been examined as sanitizing
agents.
Chlorine was proved to be the most effective.
It has ·the
advantage .of being permeated through the membrane so· that it can
disinfect both sides of the membrane, and the area between the
membrane and the backing material. The disadvantage of chlorine· js
that at concentrations hjgher than 50 ppm it may cause significant
degradation in the membrane strength and reduce the salt rejection.
49
It should only be used for short periods.
If continuous disinfec-
tion is needed, then the concentration should be lowered to
1 ppm.
It is generally agreed that in reverse osmosis systems
designed for food processing, consideration should be taken to
meet certain sanitary requirements.
The system should be designed
in such a manner that there are no stagnant regions in the feed
path.
The equipment should be designed in such a way that visual
inspection is possible and easy, and in a manner that parts of
the system can be isolated for cleaning without affecting the
process.
2.6.3 The Membrane Compaction
This is one of the major limitations of modified cellulose
acetate membranes. When the membranes are subjected to a relatively high pressure, the membrane porosity decreases irreversibly from about 60% to 10_5%(55) resulting in the decrease of the
permeation rate.
Rickles(15) estimated the loss in flux rate due
to compaction to be between 23-25%. Sourirajan(97) attributed the
reduction in flux rate to be caused by the progressive increase
in the thi·ckness of the spongy layer which offers more resistance
to the fluid flow. From his experiments(l) he concluded that the
temperature treatment in the manufacturing process has an effect
'on the rate of the membrane compaction.
t·lembranes shrunk at temp-
eratures higher than 85 0 C exhibit stable surface structure. This
idea was supported by Osborn(56) who postulated that compaction is'
50
generally more severe in membranes cured at 82 0 C than membranes
cured at a higher temperature. Reid(17) found that the more the
membrane is compressed more crosslinking will occur in the poly-
mer material. This can be explained by the fact that the pressure
will remove more of the_ bound water present in the membrane
resulting in bringing the molecules of the polymer more closer.
This was confirmed by the experiments of Vincent(S) who found that
pressure reduces the bound water content and also forces a large
amount of the non-bound capillary water out, which results in the
formation of additional crosslinks in the polymer material. Lons.( 55)
dale
mentioned that the spongy layer of the membrane decreases
in thickness while the thickness of the thin skin surface layer
.increases as the very fine pores just beneath it close. This is
accompanied by a decrease in both the water flux rate and the
solute permeability.
He also claimed that compaction is strongly
temperature dependent. At 1500 psi the rate of compaction increased
by a factor of two for each 10-15 0 C increase in temperature.
Many attempts have been made to eliminate or reduce the effects
of compaction.
Osborn(56) postulated that compaction can be mini-
mized by precompacting the new membrane to·a higher pressure than
that used in any subsequent future experiment for a long period,
for example 18 hours.
Another approach to conquer compaction. is
by the preparation of cellulose acetate membranes of higher degrees
of an acetylation permitted by the manufacturing procedure and this
i·s possible by blending cellulose triacetate with the usual acetate.
Numerous workers suggested that the flux decline due"to compaction
could be. greatly .\reduced by crosslinking the cellulose acetate.
membrane with chemical agents such ·as formaldehyde and diisocyanate.
51
2.6.4 Membrane Fouling
Membrane foul ing can be defined as the state in which the
reverse osmosis membrane undergoes plugging or coating by one
of the constituents of the feed solution in such a way that the
flux rate is reduced significantly.
Effects of fouling can be
observed in decreased production rates and in decreased product
quality.
It differs from hydrolysis in that fouling tends to
decrease both the product rate and the. quality, while hydrolysis
increases the product rate and decreases the product quality.
The flux decline rate in fouling exceeds the decline rate due to
compaction.
In fact the exact mechanism of fouling is not well
known but all reverse osmosis membranes and all membrane configurations are subjected to fouling.
It may be considered as a charac-
teristic of each application depending on the constituents of the
feed. Peri et al(63) found that fouling of the membrane at the
start of the process is principally due to pores plugging.
causes a rapid decrease in the permeation rate.
This
The plugging of
pores then tauses the gradual accumulation of a gel like layer or
cake which increases the resistance to hydraulic flow.
\
In water desalination fouling arises from the precipitation
of iron and magnesium. oxides, calcium carbonate and sulphate, or·.
.
from attachment of organic molecules
(17)
.
tn the food industry
fouling becomes the dominant controlling factor.
Figure 2.6 shows
a dramatic decrease of flux rate in whey ultrafiltation plants
due to fouling.
Le~ et al(62), studying the membrane deposits
from whey ultrafiltration by scanning electron microscope found
.
.
.
that a thin film of the fouling material covers the membrane in
52
'"
...
~
...
..,
...
•
..
u
.,
"
,
"
. . . .
TIME
FIGURE 2.6.
h
r.
u
"
Decrease of Fl ux Rate Due to Foul i ng (62)
53
less than ten minutes of the operation, closing even the large
pores of the membrane.
The larger molecules in fluid mixture
then act as anchor points for film-forming constituents and consequently cause a thicker layer to build up on the membrane.
They observed that deposits were heaviest near the entrance of
the flow, and decrease towards the outlet.
They concluded that
it is difficult to assign the cause of the membrane fouling to
any single chemical constituent or physical structure but rather
to the composite interactions of these moieties under changing
concentrations, pressures and ionic microenvironment near the
membrane.
Short term experiments were made(57) on cooled clarified
sugar juice of about 80% purity and pH 7.6. The flux rate started
at about 500/1;"t/m2/24 hours, but dropped within a few minutes to
below 200 lit/m2/24 hours, and within 2 hours to below 100 ml/m2/
24 hours.
Inspection of the membranes showed a yellowish-red
deposit which could h~lVe been wax.
Remedies for the elimination or reduction of membrane fouling
were suggested by many authors, dependent on the field of appl ication.
In general high feed velocities and low membrane flux
reduce but do not eliminate the problem of membrane fouling(16) ..
The chemical treatment of the feed has been proven to be successful.
In water desal ination the feed is normally treated by the
addition of some chemicals to remove the dissolved
oxygen~
so as
to maintain iron and magnesium oxides in the reduced forms.
Precipitation inhibitors are used to prevent the precipitation of
calcium and magnesium sulphates which tend t'o build up on the
54
surface of the membranes as scale, and that reduce the efficiency
of the equipment.
In the ultrafiltration of whey, Lee et al(65) claimed that
membrane fouling was reduced and an increase in ultrafiltration
rate was obtained by changing the chemical environment of the
foul ants.
By the treatment of the feed with acid and calcium
sequestering agents the protein-foul ant - underwent marked
structural changes which retarded the formation of protein sheets
on the membrane surface.
Another approach to conquer fouling is the mechanical removal
of the large particles of the fouling materials.
Lee et al(65)
were able to remove the large particles by prefi1tration of the
feed on large pored u1trafiltation membranes. As a conclusion it
should be emphasized that there will be no possible chance of
success for reverse osmosis processes in the concentration of
sugar juices unless a proper method for the removal of the fouling
matter is found.
Sugar juice is normally characterized by having
a large amount of fibre and soil particles, which would have a
drastic effect on the·membrane. Added to this, the juice also contains some amounts of high molecular weight constituents which may
deposit on the surface· of the membrane.
This was seen in the work
of Madsen(57) mentioned earlier.
2.7
Concentration Polarization
One of the most serious problems which may severely 1imit
performance in reverse osmosis applications, is known as "concentration polarization".
This is the term given to the bui 1d up of
55
a solute layer adjacent to the membrane surface, which is more
concentrated than the rest of the feed solution.
The degree of
concentration polarization is determined by the balance of two
opposing factors.
1.
The transport of solute towards the membrane, caused by the
driving force of the applied pressure and the rejection of
the solute at the membrane surface, and
2.
The removal of the solute to the bulk solution by diffusion
or by any hydrodynamic motion.
When these above-mentioned factors are not balanced then a continous build-up of the solute on the surface length of the membrane occurs.
a)
This build up has several effects on reverse osmosis:
The osmotic pressure of the feed solution increases with solute
concentration at the membrane surface. This will lead to a
sharp .decrease in the water flux, so the operating pressure
must be increased to produce a given flux rate, and consequently the operating costs increase'.
b)
The solute flux in reverse osmosis may be considered to be
proportional to the solute ·concentratio·n at the interface(2).
So an. increase in concentration polarization will result in
an increase in solute flux(16).
When this is coupled with the
decrease in solvent flux rate then the quality of the product
deteriorates significantly.
----------------------------------------56
c)
Excessive concentration polarization increases the probability of precipitating dissolved components which are
insoluble at higher concentrations.
This may have dama-
ging effects on the membrane and may accelerate the chemical deterioration of the membrane.
,
d)
In the food industry, where the viscosity of food is high,
the presence of concentrated layer at the membrane surface
may lead to the precipitation" of gelatinous materials, forming
a thicker layer which reduces the effective surface area of
the membrane.
Many workers have studied methods to eliminate or reduce the
effects of concentration polarization.
Brall et al(49) suggested
that concentration polarization can be minimized in 1aminar flow
regimes by:
1.
Decreasing the channel height.
2.
Increasing the inlet velocity, and
3.
Decreasing the channel length.
f10nge(19) suggested that concentration polarization can be
reduced
by heating the boundary 1ayerhext to the membrane
'wall.
This can be performed by circulating hot water through a concentric
jacket surrounding the membrane support.
He found that on heating
the membrane at 45 0 (, while the feed solution eritered the system,
at2S 0 C, the permeation rate increased by 54%.
Through heating the
viscosity of the boundary layer, decreases resulting in more diffu-
57
sion of solute molecules to the bulk solution and thus the concentration polarization decreased, resulting in higher flux·
rates.
Lowe et al(18), studying the effect of dynamic turbulence
promotion in reverse osmosis processing of food, concluded that
.the boundary layer can be disrupted by the introduction of plastic
spheres into the feed channel.
These spheres were kept in motion
by pulsingthe feed flow at the ends of the channel. They recorded
that flux rates could be improved up to 3-fold by this method.
Short term investigation showed that the sphere movement has no
effect on the membrane surface.
2.8. Mechanism' of Transport in Reverse Osmosis
There are different schools of thought to explain
anism of transport in· the reverse osmosis process.
th~
mech-
Extensive
work has been done to demonstrate the way in which discrimination
between the solvent and solute molecules in membrane processes
takes ·place.
Several theories have been put forward.
At the beg-
inning, it was believed that the operation of the membrane was a
simple filtration action .. The molecules.were.thought·to be··discriminated on the basis of molecular dimensions.
Thus the membrane
would allow the passage of small molecules and retain those of the
large molecular size.
This ,may be true in the case of ultra-
filtration, where there is a large difference between the molecular size of the solvent and that of the solute, but 'it cannot
explain the separation of reverse osmosis membranes, where there
58
is no large difference between the molecular size of the solvent
and that of the solute.
Reid and Breton(17,59) proposed that two different mechanisms of diffusion occur in reverse osmosis.
Ions and molecules
that can combine with carbonyl oxygen in the membrane with
hydro~
gen bonding were said to be transported across the membrane by
alignment of hydrogen bonds with the membrane and thus they would
migrate across the membrane by transferring from one hydrogen
bonding site to another until they would finally be discharged
from the other side of the membrane.
In the second mechanism
,the ions and molecules that cannot enter into hydrogen bonding
are transported across the membrane by hole diffusion. This diffusion depends on the'possibility ofhole formation in the membrane.
The latter mechanism is not probable in cellulose acetate membranes,
where the pores are filled with tightly
bound water, and the
chances for any transport by this mechanism are unlikely.
Accor-
ding to the above mechanisms the semipermeability of cellulose
acetate to water is on the basis of hydrogen bonding formation
while the rejection of the salt is justified by its inability to
form hydrogen bonding and the absence of holes for the hole type
diffusion.
The Reid and Breton theory is supported
by the high
semipermeability of cellulose acetate membranes to ammonia and
the fluoride ion, which are able to enter into hydrogen bonding
with cellulose acetate molecules. Further support for the mechanis"1
is
th~
work of Baumgartl(77) on
~1traSOniCs,
which suggested that
,strong electrolytes could migrate in cellophane by hole-type,
diffusion, which can be doubled by the action of ultrasonic forces.
59
The ultrasonic forces were assumed to increase the vibration of
cellulose polymer and thus increase the concentration of the
holes in the polymers leading to the transfer of electrolytes
through the membranes.
Vincent ,," et al(5) in their study of semipermeability of
cellulose acetate, reinforced the theory of Reid and
Breton,
suggesting that the bound water in the membrane was responsible
for the formation of the hydrogen bonding sites. The solvents
which can form hydrogen bonding are able to transport through the
membrane whil e the salts can only be transported by hole forma tion.
. They explained the transport of the small amount of salt being due
to a diffusive permeation by participation of the salt ion with
the membrane's bound water.
Lacey(61) confirmed the theory of the
hydrogen bonding mechanism and he suggested that the carbonyl
groups in the membrane matrix are responsible for the hydrogen
bonding with the solvent molecules, and then under the effect of
the applied pressure the solvent molecules jump from one site to
another. until they are finally discharged out of the membrane ..
This can ·be demonstrated clearly in Figure 2: 7.
Lonsdale et al(25) postulated the solution diffusion mechanism
in which they suggested that the membrane surface is non-porous. '
The solvent to be transported was said to dissolve in the membrane
and then under· the effect of the applied pressure it diffuses
through the membrane to be rejected at the other side of the membrane.
The solute ions are rejected because they ,cannot dissolve
in the membrane.
60
Ha O
FIGURE 2.7
HaO
H,o
Water transfer in cellulose acetate membrane· (5)
,
61
Sourirajan(52,l) has proposed the preferential sorptioncapillary mechanism.
According to this mechanism reverse osmosis
membranes are microporous and heterogeneous at all levels of
solute separation.
Appropriate chemical nature of the membrane
surface in contact with the solution and the existence of pores
of appropriate size and number on the surface of the membrane, are
the most important necessities of the successful reverse osmosis
process.
If the surface of the membrane in contact with the
solution is of such a chemical nature that it has a preferential
sorption for one of the constituents of the fluid mixture, then a
steep concentration gradient will occur and hence a layer of the
component would form at the membrane solution interface.
Under
the effect of the operating pressure this layer would be forced
out through the membrane.
Figure 2.8 shows a systematic repre-
sentation of the preferential sorption-capillary flow mechanism.
This mechanism initiates the idea of critical pore diameter for
maximum separation and permeability.
The pore size has to be
twice the thickness of the interfacial solvent layer . .
Th'is idea of separation on the basis of the chemical nature
of the solution and the membrane surface is supported by the work
of Glueckanf(9), who demonstrated that the electrostatic free
energy of an ion in a pore filled with wate·r surrounded by a
material of low dielectric constant, is much larger than in the
bulk solution.
Thus the equilibrium concentration of ions in the
·mouth of the pores is much lower than the adjacent solution.
If a single ion enters the pores, it will experience an electrostatic force which ensures that it is more 1 ikely' to jump out of
62
\
THE
BULK ·.OF
SOLUTION
FIGURE 2. 8A
1
Systematic Representation of the Preferential Sorption
~ Capillary Flow Mechanism
(1)
DEM'NERAUSED
WJlTER
AT
THE
T~~·::~~~~~~;~·~~Y(lb~~~~~~~~~~;
FILM
::::::
FILM
"------1.:
I.
~
::::1" ':a;------'
:'2:('-
~ CRITICAL PORE
ON
AT
FIGURE 2.88
THE
THE
DIAMETER
AREA OF THE FILM
INTERFACE
Critical Pore Diamete·r for Maximum Separation and PermeaJ>ility
63
the pore than going in. By this force the ions of the solute are
'rejected away of the pores and thus forced to diffuse back to the
bulk of the solution.
From the above mentioned hypotheses one may be able to
. conclude that there is no single hypothesis which can fully explain
the transport mechanism in reverse osmosis.
The following factors
may have some significance:
i) Thickness of the membrane.
ii) The chemical nature of the membrane surface, and the solution to be processed,
iii) .The ability of the solvent to form hydrogen bonding during
its transfer through the membrane.
iv) The size, number and the distribution of pores in the membrane,
64
CHAPTER 3
INDUSTRIAL USES OF REVERSE OSMOSIS
3.1
Introduction
In the past two decades reverse osmosis has emerged as a
new chemical engineering unit operation, which has a wide range
of known and potential industrial applications.
time it has shown some advantages
(83)
During this
.
over other methods of
separation.
1.
It involves no 'phase change, and can be applied at ambient
temperature.
It has therefore gained a wide acceptance 'in
the concentration, purification and fractionation of food
materials especially for heat sensitive solutions, such as
those containing proteins or desirable natural aromas and
flavours.
2.
Reverse osmosis 'is a selective process and by using suitable
membranes it is capable of· carrying out more than one function simultaneously, such as purification and concentration
e.g. removal of salts in the concentration of sugar solutions(87) .
3.
It has lower energy requi rements, so it might be expected to
offer an economic advantage over other processes, e.g. evaporation.
This is more noticeable in small and medium size plants,
but for larger plants the reverse is usually true.
Evaporation
is highly competitive and it can generally produce higher. concentrations with relatively low cost.
65
4.
Although reverse osmosis is not successful in achieving
higher concentrations, a combination of reverse osmosis
and another concentration process may be more economic than
using just one process.
Up to date reverse osmosis has shown many successes in
industry.
Desalination of sea-water and brackish water for the
production of potable water, has been the first field for its
application and it has been proven that reverse osmosis is a successful alternative to the conventional methods of desalination.
In food
processing it has gained increasing attention as it is now applied
in concentration of many" foodstuffs, such as orange juice and
maple syrup. Another area where it has found wider application is
in the field of pollution control and waste water treatment.
Some
of the industrial uses of reverse osmosis are shown in Table 3.1.
In the followlng sections some of these applications will be dis-
cussed in more detail.
3.2 Application of Reverse Osmosis in the Food Industry
Food raw materials are generally aqueous solutions of
carbo~
hydrates"," proteins and fatty" acids';whicli' "contain a large amount
of water, ranging between 75-99% wt.
This water has nocontribu-
tion to the nutritional value of the food.
If it is not removed
the food deteriorates due to changes resulting from chemical reactions which accompany microbiological growth in the food.
The remo-
val of water facilitates further processing of food, preservation,
I
66
TABLE 3.1
Industrial Applications of Reverse Osmosis*(9)
A.
Water Conservation
o.
Pollution Control (continued)
1.
Photographic processing waste
5.
Pu1 p and paper. industry was te
2.
Primary and secondary sewage
eff1 uent.
6.
Paint primer waste
7.
Acid mine drainage.
B.
Food Concentration
E.
Water Conservation
1.
Tomato, orange, lemon, grape,
and apple juices.
1.
Boiler feed make-up water.
2.
Coffee, tea.
3.
Blood.
F.
Industrial Molecular Separation
Process
4.
Maple syrup(79)
1.
Pharmaceutical preparation.
5.
Skimmed milk.
2.
Water soluble paint 1atexes(81)
6.
Whole milk.
3.
Enzyme production(82).
7.
Egg white.
C.
Food Industry Waste Treatment
1.
Cheese whey.
2.
Soy bean whey
3.
Potato processing waste(86).
0; . Pollution Control
1.
Cannery wastes.
2.
Tanning liquor waste.
3.
Plating waste
4 .. Radioactive waste
(Continued at top of page)
* Some of these applications may be done by ultrafiltration
67
storage and transportation of food.
Many techniques have been
applied to the removal of the water such as evaporation and.
freeze drying. In many cases the attractiveness of the food is
reduced as in concentration by evaporation, proteins may be
denatured, vitamins may be destroyed and flavour volatiles may
be stripped from the product.
Some of the available techniques
are expensive so the search for alternative processes continues.
3.2.1
Application of Reverse Osmosis in the Sugar Industry
Although the use of reverse osmosis has been accelerating in
other food processing industries, its application in the sugar
industry has been very limited.
This has partly. been due to the
fact that the sugar industry is an old and traditional one. Any ,
change from one process to an alternative one needs some time for
acceptance to become widespread.
Most of the reported work has
been done on an experimental basis.
Sourirajan(97) in his study of reverse osmosis as a separation
and concentration technique for sugar solutions, concluded that
reverse osmosis would be expected to have a significant role in
partial replacement of. vacuum. concentration .. This .. was.supported by
T Baloh(96) who suggested that a pure sucrose s~lution c~uld be
obtained from raw. juice by removing the high molecular weight non.sugars using ultrafiltration and then by applying reverse osmosis
for concentration.
In this way thermal energy usage would be reduced
by up to about 29%.
Porter(98) claimed that ultrafiltation could be
68
used for the treatment of diffusion juice in beet sugar factories
for the recovery of protein for human consumption.
The extractable
protein content of sugar beet is about 0.6% by weight.
If a
factory is processing 8000 T/day then it could produce about
45 tons of protein.
In. the same time a clear juice would be
produced which would need little further treatment.
S Harrison(95) and Zanto et a1(99) suggested a two stage process for treatment of beet or cane juices.
In the first stage a
low pressure reverse osmosis process would be used to produce a.
clear colourless solution.
In the second stage the sugar solution
could be concentrated up to 50% weight with the concurrent removal
of inorganic salts and low molecular weight sugars.
G W Vane(100)
discussing the app1 icabi1 ity of membrane processes in the sugar
industry mentioned that reverse osmosis could be applied for concentration of dilute sugar solutions up to concentrations of 40% wt
with the. probability of. removal of ash from the juice stream.
He
concluded that the opportunity for using both reverse osmosis and
ultrafiltration in the sugar industry does exist and that these
opportunities would be increased by the production of more durab1e
membranes.
Madsen(101) has done extensive experim·enta1 work to test the
application of ultrafiltration and reverse osmosis in cane juice
treatment.
He found that there is a good. chance for success of both
processes if a satisfactory procedure for the ·remova1 of clay and
small bagci110 particles could be developed.
He made a comparison
between the operating co·sts for the removal of 1000 tons of water
by reverse osmosis and evaporation in cane factories using oil as
69
fue 1. - He found reverse osmos i s to be cheaper.
TABLE 3.2
Comparison of Water Removal Costs
Oil 22 tons
@
El ectri city kW
Evaporation Costs/
1000 tons water removed
Reverse Osmosis
Costs/1000 tons water
removed
220
-
£10
@£0.006
6
26.4
Membrane Cost
-
72.0
Evaporator Cleaning
3
-
TOTAL:
£229
£98.4
In practice reverse osmosis has been applied on small scales.
The Finnish Sugar Co. Ltd. has been using reverse osmosis for preconcentration of desugared molasses since 1975.
The diluted molasses
which were desugarised by chromatographic separation, were passed
through tubular reverse osmosis plants where it was concentrated to
20-25% solids . . It was then further concentrated by evaporation.
Thi 5 is shown in Fi gure 3.1. They found that reverse osmosi s was
more economic in evaporating dilute sugar solutions
Peter Paul Chocolate Candy plant in Frankfurt(102) started to
use reverse osmosis in 1972 for the treatment of the process water
. coming from the coconut cutting machines.
They were able to recover
about 75 gall/day of sugar syrup and they were. able also to reduce
their waste handling problem.
-
70
G WVane
(100)
.
mentioned thatcandy manufacturers were able
to concentrate dilute sugar streams up to 25-30% solids by using
reverse osmosis systems. The process was proved to be economic
and successful and the equipment recovered its cost in less than
one year.
From the above discussion one may conclude that the sugar
industry is a potential field for the application of u1trafi1tration and reverse osmosis.
There are three areas in the sugar
industry-for the application of these processes:
1.
Ultrafiltration could be used in juice clarification for the
removal of the high molecular weight impurities such as protein,
pectins and colouring material. This would give a clear,
colourless juice which could be used for the production of
white sugar.
Here problems may arise from the fouling of the
membranes by the suspended matter associated with juice, such
as bagaci110 and soil particles.
If an appropriate procedure
for the pretreatment of the juice to remove the suspended
matter were to be adopted. and a good cleaning method for
membrane were to be found, the fouling problem might be overcome.
2.
Reverse osmosis can be applied successfully in the concentration of dilute sugar streams and molasses, where concentration
is possible up to 25% wt - with the concurrent removal of ash.·
·3.
Reverse osmosis could be used in the concentration of juice.
Here there.are some problems to be sorted out, such as the
71
Molasses
Hater -'.--------,~
r---.:....-------,
Chroma tographi c
separation
I
. I
I
I
Oesugarised Molasses
Sucrose
~
•
Reverse
Evaporation
Osmosis
Ua ter - - - - - - - - - - - '
Sucrose to crystall i zation
Evaporation
' - - - - - - - - - \la ter
Concentrated
Residual Molasses
FIGURE 3.1
. Appl ication of RO in concentration of desugarised
molasses(72)
72
hydrolysis of the membrane, and the problem of viscosity and
,concentration polarization.
The former could only be solved by
improvement of the membrane characteristics and the development
of durable membranes which could withstand extremes of pH and
higher temperatures.
The problem of concentration polarization
has been tackled by many workers and suggestions for its reduction have been discussed in Chapter 2.
3.2.2 Application of Reverse Osmosis in Whey Concentration
Whey is discarded as a by-product in the manufacture of
cheese. It contains between 5.5-6.5% dissolved solids that have
an approximate composition of 70-75%, lactose, 12%, protein 8-10%,
inorganics, 1.0% fat and 0.1-1.0% lactic acid(61).
In practice
whey represents a serious waste disposal problem.
It cannot be
disposed directly to the sewage system as it has a high B.O.D.
(50 000 mg/1)(89).
Fenton-May(9) mentioned that 100 1bs of whey
has a B.O.D. equivalent to the waste produced by 21 persons in 24
hours.
Generally whey is concentrated by evaporation up to 40% and
then part of it is used as animal feed and the rest is disposed.
The process of concentration is very expensive and cannot
be afforded by some cheese manufacturers. In addition it presents
a wastage of a potentially valuable food material. This encouraged
manufacturers to look for an easier and more economic operation which
could ,solve the problem of whey disposal and at the same time
. recover the 'va1uab1e nutrients.
In the .1ast decade reverse osmosis
. has proven that it has the potentiality of satisfying these needs.
73
The raw whey is first fractionated by ultrafiltration with open
membranes which retain mainly the protein and pass the water,
salt, and lactose in the permeate.
This is further concentrated
by reverse osnrosis using tighter membranes to retain the lactose
.and salts and pass only the water in the permeate.
By this method
both the protein and lactose can be retained as'food or as a food
ingredient.
The water which has a very low biological oxygen
demand (B.O.D) can be sent to sewage.
This is illustrated in Figure 3.2.
3.3 Application of Reverse Osmosis in Waste Water Treatment
In the past the industry paid little attention to the treatment
of used water.
It was readily disposed of in rivers and esturine
waters without regard to its effect on the environment.
Nowadays
our world is more conscious with pollution and the conservation
of the envi.ronment.
Adequate waste treatment becomes an integral
part·of· the costs of industry.
This has gone further even to the
consideration of the re-use of the water 'itself and to extraction
of the valuable components in the effluent.
Many processes have
been 'established for treating waste water effluent, based on chemfcal, physical and biological methods, and a choice of a method of
treatment depends on the nature of the effl uent, economi c cons i derationand the degree of treatment desired.
Recently reverse osmosis'and ultrafiltration 'have been introduced to this field and they have shown substantial financia'l savings.
Their at.tractiveness comes from their ability to give a very high
~
74
-Protein
6.7
-Lactose 5.0
-Ash
0.5
-Other solids 0.33
-Water
93.5
High Protein
fraction
1000 1bs of Raw Hhey
Protein
6.7
Lactose 50.0
Ash
5.0
Other solids 3.3
Water
935.0
Con Lactose -Lactose
44.1
4.5
~---S~t-r-e-a-m----~-Ash
- Other soli ds 2.85
-ilater
168.31
-Lactose
95.0
~P~e~rm~e~a~t~e____~~-Ash
4.5
-Other solids 2.97
-Water
841.5
L.__::.Ef:...:f~l..:u:.::e:..:.n.:.t___-Lac tos e
-Ash
0.91
0,45
-Other sol ids 0.15
-Hater
673.5
. FIGURE 3.2
Membrane Processing of Whey
75
quality of water.
At the same time the pollutants are removed
nondestructively i.e. no chemical or biological degradation(91).
This can be clearly shown in the waste water treatment of the
paper and pulp industry and electroplating industry, discussed
below.
3.3.1
The Application of Reverse Osmosis in Pulp and
Paper Industry
The pulp and paper industry uses a huge amount of water.
An average size pulp mill may commonly use from 5-50 million
gallons per day{l).
A large portion of this comes out as effluent
containing a high content of organic and inorganic substances, as
well as other suspended materials. These waters are high in
biological oxygen demand (B.O.D), chemical oxygen demand (C.O.D)
and total dissolved solids (T.D.S).
Thus restrictions are placed
on its direct disposal to sewage systems.
The biological treatment
and concentration of these wastes by evaporation are rather expensive and paper and pulp mills are faced with high capital investment
for their removal.
In the last decade reverse osmosis has offered a new and efficient approach to the problem. ·The organic and the inorganic subs tances can be recovered with a lower cos t of Sl /l 000 gall compared
with.that.of the conventional evaporation method of $2-6/1000 gall(92).
The reductiori of the B.O.D. and C.O.D. is as high .as 92% and there
is excellent removal of colouring materials (Figure 3.3).
The
most significant roje of reverse osmosis in the pulp industry waste
76
FIGURE 3.3
flow Diagram to show the application of reverse osmosis
,
in the pulp industry (90)
a) Without Reverse Osmosis
9
I
WASHER
r~--...
50-500%
EVAPORATOR
RECOVERY
•
RIVER
SOLIDS
~
[
b)
1
[
SALES
1
With Reverse Osmosis
~1-1--".
[
REUSE
WASHER
8-15%
EVAPORATOR
r-__5_0_-5_5_%_ _~
0.5-2%1
/
RECOVERY ,
REUSE
[
SALE
77
water treatment is its ability to recover about 80% of a highly
purified colour free water which can be recycled wi.thin the mill
system. Thus reverse osmosis provides a combination of benefits
such as removing the pollutants, producing concentrates of
dissolved solids which can be dried to be sold or burned for their
ash, and recovery of clean re-usable water.
3.3.2 Application of Reverse Osmosis in Electroplating
Waste Treatment
The electroplating industry disposes a large amount of
effluent waters which contain a high level of heavy metals, such
as chromium, copper, aluminium, zinc, silver and even gold(93) .
These together with contaminants, such as paint, make a serious
pollution problem to the metal finishing industry.
These conta-
minants are toxic to human beings and wild life, as well as being
poisonous to the bacteria that carry out the processing of organic
matter in waste water treatment plants (lagoons) usually used.
Moreover tlie waters conta in substantial quantities of potentially
recoverable plating salts - 25-1000 mg/l (93) wh i ch represents a
considerable loss of valuable resource materials.
Conventional
procedures for the treatment of plating waste are often awkward,.
expensive and" unsatisfactory(93),;".
The introduction of reverse osmosis offered an effective,
easy and economi ca 1 method for the recovery of both the va 1uab,l e
metals and the water which can then be re-used.
this time is from a Japanese company
window frames.
w~ich
Our example(94)
is painting aluminium
They were involved in a troublesome' waste disposal
problem which cost them a large amount of money.
When they installed
78
a reverse osmosis system to treat the rinse water from the e1ectro- plating line they were able to cut their cost of painting by 30%.
The system separates the acrylic .paint and accompanying solvent
from the water so effectively that both can be recycled to the
line for re-use.
This enabled the company to save about $90000/·
year.
3.4 Conclusion
Although the economics of reverse osmosis are not yet well
established the future for its application in industry seems to be
bright.
This is due to its simplicity and low operational costs.
For the process to·be more competitive research should be directed
to the following:
1.
The membrane technology for the production of more durable
and stable membranes which have higher flux rates and higher
resistance to the extreme conditions of
2.
~he
operation.
The mechanical .design of the membrane configuration will have
to be improved to withstand the higher· pressures needed in
some applications.
3.
The design and manufacture of the subsidiary equipment such
as pumps and instrumentation to guarantee ·troub1e.-free
tion and reduce the.operational costs.
oper~­
--c------------
79
- =,- - ---
4.
Development
of good procedures for the pretreatment of the
feed' solutions and the cleaning and sanitation of the membranes.
80
CHAPTER 4
APPARATUS AND EXPERIMENTAL PROCEDURE
4.1
The Pilot Plant
This study was carried out using a Paterson Candy Internat- .
ional Ltd Type Bl pilot plant reverse osmosis unit.
The unit is
self contained with a special 4-piston membrane pump, which
can deliver up to 14.7 lit/min of the feed solution.
The equip-
ment is also supplied with feed and concentrate flow indicators
and all necessary hydraulic and electronic controls.
The main
advantage of the unit is its flexibility in adjustment of the·
feed flow rate by the use of the variable speed pulley system
supplied, and the ability to increase the capacity by adding
additional membrane modules connected either in series or in
parallel.
The unit is designed in such a way that it can give
continuous operation with minimum supervision and it is fitted
with safety controls·which can stop the equipment automatically in
the event of feed failure or
over.~pressurization.
A feed filter is fitted in the feed line to prevent any foreign
particles from passing to the membrane pores.
The unit was also
supplied with a heat exchanger for heating the feed if necessary.
The pressure in the module pack is maintained by a hand operated.
spring loaded valve. The maximum pressure recommended by the manufacturer is 700 psi .. ·
4.2 The Module and Membranes
The equipment was supplied with a type Bl tubular module which
81
consists of a number of perforated stainless steel tubes. Each
tube is lined with a membrane element. The membrane used was
cellulose acetate type T2/15 which has a length of 2400 mm and
an internal diameter of 12.5 mm.
The membrane was prepared by the
loeb-Sourirajan techniques with nominal 93% rejection of 0.5% salt.
Each membrane module was made up of eighteen tubes which provide
2
a total membrane module area of 1.7.m.
The turbulent flow of
,
the feed through the tube is effected by connecting the eighteen
tubes in series by especially designed end caps.
The pH range of
0
the feed is limited to 3-6 at a temperature up to 30 C and 2-8 at a
temperature of 15 0 C.
The sanitation of the system was performed by
the addition of sodium hypochlorite to the feed.
Continuous sani-
tation of the feed was carried out by the addition of 1 ppm free
chlorine.
Sanitation of the equipment was done every weekend by
flushing the membrane with water containing 50 ppm free chlorine
for half an hour, after which the equipment was washed with water
for another half hour.
4.3 The Operating Procedure
The flow scheme used in this study is demonstratec:l in Figure
4.1.
The feed solution was stored in a feed tank at the top of the
equipment- from which it could be pumped by'the priming pump through
a heat exchanger for heating if necessary.
There is a provision for
circulating the feed solution from the heat exchanger to the feed
tank again .. A 50 micron strainer with stainless steel mesh filter
. was placed over the heat exchanger to prevent any 'foreign particles
\
82
from passing to the membrane.
The solution was then passed to
a high pressure pump which fed the module. Following the module
, is a flushing valve which is followed by a hand operated pressure
valve which maintained the pressure in the module pack.
The
module has two outlets, one for the concentrate and the other for
the pe rmea te .
At the beginning of the study the membrane was flushed with
•
water at a pressure of 350 psi for a long period - 3 hours. This
was done to remove the preservative material and to improve the
performance of the membrane, as mentioned in Section 2.4.1.
The flow rate of the feed sol ution through the tube corresponding to a given setting of the vari-speed drive on the pump
was determined by measuring the amount of solution flowing through
the tube during an interval of one minute.
The flow rate was
measuredi n terms of 1it/mi n and then converted to ml /sec.
The sugar solutions were prepared using commercially available
granulated sugar and distilled water.
When the feed solution was
prepared a sample was taken for analysis.
It was then circulated
through the module for ten minutes using the priming pump.
The
main feed pump was then switched on and the feed was circulated
for about two minutes to allow air to clear'the system, after which
the by-pass flushing valve was closed slowly.
This was followed by
pressurising the module to 350 psi and the feed solution was circulated for ten minutes. The resultant permeate was remixed with the
bulk solution and the desired experiments commenced.
FIGURE 4.1
Schematic Diagram of· the Reverse Osmosis Plant
FEED
TANK
t
~,
_AJl;1i\.
AL
f~~~----------hYVVV4-------~----~
~--------~,
,
A
-'"
I
~~
T
Priming Pump
Fi l-ters
Heat Exchanger
00
w
Reject
Flow
Rotameter
Inlet
Flow
Rotameter
Flushing
Valve
'--
Permeate
Pressure ~,
Control A~
Valve
Feed Pump
Modules
,
84
After each ·experiment the system was flushed with water
for ten minutes to remove any trace of sugar solution.
4.4 Analytical Procedure
4.4.1
Determination of the sugar concentration
Sugar concentrations were determined using a Bellingham and
Stanley Abbe Co. refractometer, and supplied by Analytical Supplies
Ltd., Long Eaton, Derby.
Sodium D line 589.3
nm
The readings were read relative to
and with the bath temperature at 20 oC.
4.4.2 Determination of the concentration of calcium
Calcium concentrations were determined with an atomic absorption spectrophotometer EEL l40/Evans Electroselenium Ltd.
A set
of standard solutions of calcium chloride was prepared for which
the absorption was determined by the spectrophotometer.
tion for a calibration curve was calculated.
The equa-
The absorption rea-
dings for different samples were found and their concentrations were
worked out from the calibration curve equation.
4.4.3 Determination of sugar viscosity
The viscosity of sugar solution was determined with an Ostwald
Viscometer type BS/U si ze A. The following steps were followed to
find the viscosity:
i)
ii)
The water bath was adjusted at 25 0 C.
The viscometer was filled with sugar solution to the indicated
marks.
85
iii) The viscometer was placed in the water bath for five minutes.
iv) The flow time of the sugar solution through the capillary
tube was determined.
v) The above step was repeated three times and the average time
was found.
vi) The viscosity of the solution was calculated according to the
following equation:
Kinematic viscosity
c
= constant
t
=
flow time
(~)
=
ct mm 2/sec
=
0.003484
86
CHAPTER 5
RESULTS AND DISCUSSION
As the objective of this project is to study the application
of reverse osmosis in the concentration of sugar solutions,
attempts were made to investigate the parameters which affect
the process of concentration.
Experiments were run to see the
effects of pressure, temperature, flow rate and feed concentration on the flux rate of the membrane.
Then the effects of
pressure and concentration on solute separation were also evalua-.
ted.
In nature cane juice contains small amounts of inorganic
salts and as it is
des~rable
to remove these salts during concen-
tration, some experiments were made to investigate their behaviour
during the concentration process.
The cane juice also contains
some impurities which may affect the performance of the membrane
such as dextran and pectin. The effect of dextran on the flux
rate of the membrane was measured.
5.1
Pure Water Permeability
The flux rate of water was measured at·different pressures
ranging between 300-600 psi.
As seen in Table Al and Figure 5.1
it is clear that the flux rate increased as the operating pressure
was increased.
This increase in flux rate may be attributed to
the increase of the driving force, i.e. the effective pressure
.which increases the mass ·transfer coefficient of the. water.
In
the case of water the line will intercept the axis at the origin
s
87
FIGURE 5.1
The effect of pressure on flux rate of pure water.
/·1embrane type:
Flow rate:
T2/15 type cellulose acetate (PCI)
12.3 1 i t/mi n
Tempera ture:
23°C
50
40
I
'(\J
~
"-
-.l
30
w
~
c:::(
a::
x
::>
-.l
LL
20
10
100
200
300
OPERATING
400
500
PRESSURE
600
PS I
700
88
as the water has no osmotic pressure to resist the flow of the
water through the membrane.
5.2 The Effect of Operating Pressure on the Flux Rate of the
Solvent
In this experiment the flux rate was measured at different
operating pressures, while the flow rate temperature and feed
concentration were kept constant. As shown in Figure 5.2 and
Table A2, within the pressure range 300-600 psi the flux rate
was directly proportional to the operating pressure.
I
It increased
linearly with ,increasing pressure. At a feed concentration of
12% increasing the pressure from 300-400 psi resulted in an
increase in the flux rate of about 58.4%.
On increasing the
pressure to 600 psi the increase of the flux rate reached 191.1%.
As seen in the graph it will be noticed that on increasing the feed
concentration, the slope of the line decreases resulting in lower
flux rate.
It was also observed that the X-intercept of the lines
will give the osmotic pressures of the solutions.
The increase in flux rate with increase in operating pressure,
may be explained on the grounds that increasing the pressure will
increase the drivi ng force of the sol uti on and hence the, mass
transfer.
It will be worthwhile to mention that at higher pressures
the flux rate decreases with increasing pressure(l). ' This is attributed to the closure of the smaller membranes and the resistance to,
the solvent flow increases.
This cannot be verified in these
ex~eri­
ments as the maximum pressure recommended by the manufacturer is
700 psi.
j
89
FIGURE 5.2
50
The effect of operating pressure on flux rate of
sugar solution.
o
feed concentration 12%
•
feed concentration 14.7%
Flow rate
12.3 lit/min
Tempera ture
23 0 C
I40
........
N
2:
........
-1
30
W
Ic::(
0::
x
::>
-.J
La..
20
10
Osmoti c pressure
12%
Osmotic pressure 14.7
100
200
300
400
OPERATING
500
600
PRESSURE
700
PS I
90
5.3 The Effect of Feed Temperature on the Flux Rate
The flux rate was investigated at constant pressure, feed
concentration and feed flow rate, between the temperatures 21 0 C
and 330 C.
The fl ux rate was found to increase 1i nearly with,
increase in temperature', as seen in Figure 5.3 and Table A3.
Raising the temperature of the feed from 21 to 25 0 C resulted
in increasing the flux rate about 12.4%.
An increase of 24.8%
was obtained when the feed temperature was raised to 27°C.
When the temperature was ,further increased to 330 C the increase
in flux rate reached 43:::8%.
This increase of flux rate can be
attributed to three parameters:
i)
Increase of temperature increases the solubility of the
solvent in the membrane and thus increases permeations(l).
ii)
Increasing the temperature increases the diffusion of solvent
in the feed through the increase in mass transfer and activation energy(77).
It also increases the diffusion of solute
from the membrane solution interface to the bulk solution and
'consequently reducing concentration polarization(19).
iii)
Increasing the temperature results in reduction in the viscosity of the feed which facilitates the flow of the feed'
along the meinbrane resulting in 'higher permeation rates(35).
The overall result of all these factors is a substantial
increase in flux rate at higher temperatures.
This increase in
,operating temperature is 'limited by thermal damage to ,the membrane.
91
. FIGURE 5.3
The effect of temperature on flux rate of ·sugar solution·
Feed concentration
Pressure
14.5%
600 psi
Flow rate
12.3 1 it/mi n
35
o
25
20
20
25
30
OPERATING TEMPERATURE
33
~
92
5.4 The Effect of Feed Flow Rate on The Flux Rate
The intention in this experiment was to investigate the
effect of the flow rate at constant operating pressure, operating
temperature, and feed concentration, on the flux rate of sugar
solutions.
The feed was pumped at five different flow rates
7.9, 9.2, 10.8, 12.3 and 14.7 lit/min. As shown in Table A6 and
Figure 5.4 the flux rate increased with increasing flow rate.
There was an increase of about 15.2% when the flow rate was
increased from 7.9 - 10.8 lit/min, and the flux rate increased
up to 26.9% on increasing the flow rate to 14.7 lit/min.
This
increase can be explained by the fact that at higher flow rates
there is a better mixing of solution at the membrane surface with
a resultant increase in mass transfer.
The mass transfer is depen-
dent on the thickness of the sub-laminar boundary layer.
It
increases with decrease in the thickness of the layer. This can
be shown in Figure 5.5.
The increase in flow rate also results in
a decrease in concentration polarization at the membrane-solution
interface (77) .
In the reverse osmosis process it is advisable to operate
at high feed flow rate and the upper 1imit should be determined
by- the economic optimi zation, of the capi tal- and operating- cos ts
required for the pumping to overcome the osmotic pressure of the,
sol ution _'
5.5 The Effect of Feed Concentra ti on on Fl ux Rate
The results shown iri Figure 5.6 and Table A4 have already
demonstrated that flux rate decreases with increasing feed concen-
93
FIGURE 5.4
30
The effect of feed flow rate on flux rate of sugar ·solutionPressure
600 psi
Concentration
14.5%
Temperature
:r:
N
"L
"-
---.J
w 25
~
c:!
er:
X
::::>
---.J
l.L.
0
20
8
_9
11
FEED FLO W
13
RATE
-L/M1N
- 15
94
FIGURE 5.5-
Va:riation of flux rate with -boundarylayer*thickness
29
28
27
25
w
re(
a:::
24
22
21
20
15
10
20
BOUNDARY· LAYER* THICKNESS
*
Laminar sublayer
MM
95
tration when all other variables were kept constant. This is
consistent with- the findings of Wor1e/ 50 ) and Kimura et a1(114).
Increasing the feed concentration from 15.5% to 27.3% resulted
in a _decrease of about 69% in flux rate.
When the feed concen-
tration was further increased to 34.2% the flux rate decreased
about 84.5%.
The increase in feed concentration resulted in an
increase in the osmotic pressure of the solution and consequently
the driving force was reduced giving lower flux rate.
Also the
solution viscosity is a function of the feed concentration and
hi~.u-
it is higher the. the feed concentration.
Increase in viscosity
results in lower mass transfer and lower flux rates(22).
In Figure 5.7 the viscosity of the sugar solution was plotted
against the flux rate, showing that the relationship between the
flux rate and the viscosity reciprocal is linear.
To maintain a constant permeate flow rate the operating pressure
must be increased with the increase of the feed concentration.
5.6 -The- Effect-of the Operating Pressure on- Solute Separation
In this experiment the solute separation was investigated at
different pressures, while keepi ng the feed fl ow rate, the feed
concentration-and -the feed--temperature" constant:
As--shown in--"
Table A7 and Figure 5.8, the solute separation increased with
increasing the operating pressure.
At higher pressures the curve
-reached an asymptote and solute separation seemed to become ,independent of the- pressure. It is Clear from the graph that the separation is also dependent on the concentration of the feed.
In the
96
FIGURE 5.6
The effect of feed concentration on flux rate
Pressure
600 psi
Fl ow rate
12.3 1it/min
Temperature
23°C'
25
20
I
"N
2:
"..J
15
,
w
r-
e::(
0::
. x::::> 10
..J
lL.
5
5
10
15
20
25
FEED CONCENTR·ATION
30
'!. W T
35
'
97
FIGURE 5.7
Variation of flux rate with 1'/\1*
25
20
:r:
"N
L
"-.l
w 15
~
c:{
0::
X
::::>
-.l
1.L
10
0.1
0.2
*Kinematic viscosity
0.3
0.4
1!",u'
0.5
0.6
S/MM
2
0.7
-
-
-- -
-
------------
98
case of a feed concentration of 14.7% the total increase of
separation for the increase of pressure between 300-600 psi is
about 2.8% while when the concentration was raised to 23.60%,
the solute separation increased about 18.8%. , The increase in
solute separation with
~he
increase in pressure could be attribu-
ted to the partial closure of the larger pores of the membrane.
Sourirajan(l) claimed that the increase in solute separation with
increase in pressure might be due to a decrease in the average pore
size on the membrane surface and/or an increase in the preferential sorption'of the membrane for water at higher pressures.
5.7 The Effect of Feed Concentration on Solute Separation
The effect of feed concentration on solute separation was
investigated between the feed concentration of 10.83 to 26.5% wt.
Results shown in Table A8 and Figure 5.9 demonstrate that unlike
the effect of pressure on solute separation, an increase in ,feed
concentration resulted in a decrease in the solute separation.
This decrease was more pronounced at higher feed concentrations.
Increasing,the feed concentration from 10.85% to 14.95% the
solute separation decreased only about 0.67%, while on increasing
the concentration from 23.40% to 26.50% the, decrease in solute
separation was about 5.05%.
The decrease in solute sep>-i-aiia'HTli1;'jbe,expla1necl.
by the fact that on increasing the feed concentration the solute
concentration at the membrane surface will increase i.e. concentration polarization develops, resulting in solute flow through
, the memb ra ne.
,"
99
FIGURE 5.8
The effect of operating pressure on solute separation
o feed concentrati on . 14.7% ,
•
feed concentration
23.60%
Flow rate
12.3 1it/min
Temperature 230 C
100
90
z
o
f-
er
a:::
80
~
i
w
If)
W
f-
::::> 7 0
-1
o
If)
60
100
200
300
400
.500
OPERATING PRESSURE
600
700
PSI·
100
. FIGURE-5~9- The· effect of feed concentration on, solute separation
of sugar solution
Pressure
400 psi
100
Flow rate
Temperature
12.3 1 i t/mi n
23 0 C
~90
I-
er
0:::
~80
w
If)
w
1-70
::>
---1
o
If)
60
50
40
30
20
10
10
20
FE EO CONCE NTRATION
30
.0;. WT
101
5.8 The Effect of the Operating Pressure on the Separation of
Calclum ChJorlde Salt in Sugar Solutlons
The intention of this experiment was to detect the behaviour
of inorganic salts in sugar solution during reverse osmosis.
The
separation of salt was investigated at different operating pressures,
while the concentration of sugar, the concentration of salt, the
flow rate and the operating temperature were kept constant. As
shown in Figure 5.10 and Table A9 the separation of salt increased
as the pressure was increased. Increasing the pressure from
300-400 psi resulted in about 21.8% increase in salt separation,
while increasing the pressure from 400 to 600 psi resulted in
about 13.4% increase.
This indicated that the rate of increase
was less at higher pressures. This may be explained by the fact
that at higher pre'ssures there was a reduction in the pore size of
the membrane.
The overall increase in separation may be attributed
to the decrease in the average pore size on the membra (le surface
resulting from the increase in the operating pressure.
5.9 The 'Effect of Feed Concentration on the Separation-of Calcium
Chloride Salt
The effect of feed concentration on the separation of calcium
chlor.ide salt was examined at constant operating pressure" flow
rate and operating temperature. 'Figure 5.11 and Table AIO show
that the salt separation decreased with increase in feed concen:tration.This decrease was more pronounced at higher feed con, centrations.
Increasi ng the feed concentra tion fr'om 10.85% to
14.95% decreased the salt separation about 3.4%., Increasing the
feed concentration to 19.50% resulted in decreasing the separation
102
FIGURE 5.10 - The effect of operating pressure. on the separation
. of salt in sugar solution
Feed concentration
23.60%
Flow rate
12.3 1it/min
100
Temperature
23 0 C
90
80
70
z:
o
~ 60
Cl:
c:(
a.. .
~ 50
~
~
. c:(
40
(f)
30
20
10
100
200
300
400
500
600
OPERA TING PR ESS URE
700
PSI
103
FIGURE 5.11
The effect ,of feed concentrationonsa1t separation
in sugar solution.
Pressure
Flow rate
Temperature
400 ps;
12.3 1it/mi n
23°C
100
90
80
z
o
.-
~70
c::(
Cl..
W
V)
.-
60
-'
c::(
V)
50
40
o
30 '
20
10
20
30
CONCENTRATION OF SUGAR Sa...UTION ·1. WT
104
about 13.79%. When the feed concentration was 26.5% the separation
decreased about 54.01%.
This decrease in the salt separation with
feed concentration was not expected as the increase in feed concentration should form a layer at the solution-membrane interface
which hinders the salt flow through the membrane.
Pereia et a1(22)
justified this decrease by saying that the salt in sucrose solution
may not exist as independent ions and it may be bound to the sucrose
molecule to form a complex compound.
5.10 The Effect of the Presence of Dextran on the F1 ux Rate of
Sugar Solution
In this experiment the flux rates of sugar solution (14.7%),
sugar·so1ution +0.3%, dextran,and sugar solution +0.5% dextran
were determined at different operating pressures.
As shown in
Figure 5.12 and Table A.11 the flux rate decreased with the addition of dextran. At a pressure of 550 psi the decrease in flux
rate was 5.5% in the solution containing 0.3% dextran.
The addition
of more dextran (0.5%) resulted in a reduction of flux rate by about
7.5%.
This decrease in flux rate with the addition of dextran
was expected as dextran increases the osmotic pressure of the feed
solution, as seen in Figure 5.12, resulting in lower flux rates.
Dextran also increases the viscosity·of the sugar solution which
reduces the mass transfer coefficient and ·consequent1y reduces the
flux rate.
105
FIGURE 5.12
The effect of the presence of Dextran on the fl ux rate
of sugar solution
o
X
sugar
•
sugar
Flow rate
Temperature
25
14.7%
feed concetnration
+ 0.3% Dextran
+ 0.5% Dextran
12.3 lit/min
23 0 C
w
~15
er
10
100
200
300
OPERATING
400
500
PRESSURE
600
PSI
700
106
CHAPTER 6
CONCLUSIONS
In this research project the factors affecting the concentration of sugar solutions by reverse osmosis were studied.
The experimental results revealed that:
1.
The flux rate of the solvent increased linearly with
increasing operating pressure in the range 300-600 psi.
i) At a feed concentration of 12% wt increasing the pressure
from 300 psi to 400 psi resulted in an increase of
58.4% in the flux rate.
ii) On increasing the pressure from 300 to 600 psi the increase
in the flux rate reached up to 191.1% of the initial flux
rate,
2.
as seen in Figure 5.2.
The flux rate also increased linearly with the operating
temperature.
As shown in Figure 5.3, increasing the tempera-
ture from 21 0 C to 25 0 C the flux rate increased about 12.4%.
Increasing the temperature to 33 0 C the increase in flux rate
reached about 43.8% of the flux rate at 21 0 C.
3.
The flux rate increased with increasing feed flow rate .. Figure
5.4 demonstrates that on increasing the flow rate from 7.9 l/min
to 14.7 l/min the increase in flux rate reached about 26.9% .
. 4.
Increasing feed concentration was found to result in a decrease
in. the flux.
As shown in Figure 5.6 this decrease in flux rate
107
was about 89.5% when the feed concentration was increased
from 15.5% to 34.2% wt.
5.
Increase in operating pressure was found to increase the
solute separation.
Figure 5.8 illustrates that at feed
concentrations of 14.7%'· increasing the pressure from 300 psi
to 600 psi resulted in an increase of 2.8% of the solute
separation from 92.38% to 95.18%.
6.
Unlike the operating pressure Figure 5.9 showed that an
increase in the feed concentration resulted in a decrease
in the solute separation.
The decrease in flux rate due to
the increase in feed concentration was more pronounced at
higher concentrations.
7;
With the type of membrane used in this study it was found that
it was not possible to remove the inorganic salt from the feed
stream as intended.
the membrane.
This may be due to the lower cut-off of
Nevertheless the separation of the salt increased
with the increase in the operating p.ressure and it decreased
with increasing the feed concentration, as shown in Figure
5.10 and Figure
8.
5~11.
As shown in Figure 5.12 the presence of dextran in the feed
solution resulted in a decrease in the flux rate of the
solvent.
108
From what has been stated above, it is obvious that it is
advisable to operate at high operating pressure, high operating
temperature and high feed flow rate.
The upper limits of these
should be determined by the economic optimization of the capital
and the operating costs:, within the tolerance of the membrane
selected.
As mentioned in the literature survey and confirmed by these
experimental' results, reverse osmosis is more successful in the
concentration of dilute solutions, and can be used for partial concentration of solutions, as at high concentration both flux rate
and the solute separation are decreased greatly, resulting in
high operating costs and poor product quality, and the effectiveness of the process is then questionable.
,
'
109
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P HornG, "Chemical Tech. of the Purification Process". in
Principles of Sugar Technology,.Vol. Ill.
109;
M J KORT, "Colour in Sugar Industry". in "Sugar:
Science and
Technology", ed. by G G Birch and K J Park·, Applied Science
Publishing Co. (1979).
117
110. D N STEVENSON and J DANIELS, Int. Sugar J. Vol. 73 (1971).
111. G H JENKINS, in Introduction to Cane Sugar Technology,'
Elsevier Publishing Co. (1966).
112. J C ABRAM and J T Remage, "Sugar Refining: Present Tech. and
Future Develop", in Sugar, Sci ence and Technology.
113 .. W L THAYER, L PAGEAN, S SOURIRAJAN, The Canadian Journal of
Chem. Eng. Vol. 53 (1975).
114. SHOJI KIMURA, and S SOURlRAJAN, Ind. and Eng. Ch. Process
Design and Development, Vol. 7, No. 4 (1968).
115. W EYKAMP, "Fouling of Membrane in Food Processing", A.I. Ch.
Symposium Series, No. 172, Vol. 74 (1977).
116 •.MEADE-CHEN, "Cane Sugar Handbook", 10th Edition, (1977).
118
APPENDIX A
TABLE A-1
The effect of operating pressure on flux rate of water
No.
Pressure psi
Fl ux Rate
m/sec.
1
300
14.18
2
350
16.44
3
400
18.44
4
450
20.29
5
500
22.22
6
550
24.72
7
600
27.13
Temperature' 23°C
The flow rate was 12.3 l/min
119
TABLE A.2
The effect of operating pressure on flux rate of sugar solution
Run.
No.
Pressure·
psi
Conc.
Flux Rate (ml/sec)
12.0% wt.
Conc. 14.7% wt.
1
300
5.31
-
2
350
6.56
4.2
3
400
8.41
5.33
4
450
10.4
7.25
5
500
12.06
8.8
6
550
13.5
10.16
7
600
15.46
12.34
Feed flow rate 12.3 l/min
Temperature. 23 0 C
.
120
.TABLE A.3
The effect of operating temperature on flux rate ,of sugar
solution
Run
No.
Temperature
oC
F1 ux Rate
m1/sec
1
21
11.88
2
25
13.35
3
28
14.83
4
33
17.08
Feed concentration
14.5% wt
Operating pressure
600 psi
Feed flow rate
12.3 l/min
121
TABLE A.4
The effect of feed concentration on the' flux rate of sugar
solution
Run
No.
Feed Concentration
% wt
Vi scos i ty
nrn 2 /sec
F1 ux Rate
m1/sec
1
15.5
1.49
10.83
2
20.5
1.72
7.50
3
24.2
2.08
4.58
4
27.3
2.46
3.33
5
31.3
2.62
L80
6
34.2
2.93
1. 13
Operating pressure
600 psi
Flow rate
12.3 1 it/min
Temperature
23°C
122
TABLE A.5
. The effect of viscosity on flux rate of sugar solution
Run
No.
Viscosity
mm 2 /sec
l/u mm- 2 /sec
Flux Rate
m1/sec
1
1.42
0.70
10.83
2
1.72
0.58
7.50
3
2.08
0.48
4.58
4
2.46
0.40
3.33
5
2.62
0.38
1.80
6
2.93
0.34
1.13
.
Feed concentration
14.5% wt
Operating pressure
600 psi
Feed flow ra te
12.3 l/min
123
..
TABLE A.6
Effect of feed flow rate on flux rate of sugar solution
Run
No.
Flow Rate
1it/min
Flux Rate
m1/sec
1
14.750
13.40
2
12.30
12.75
3
10.80
12.21
4
9.20
11.30
5
7.92
10.60
Feed concentration
14.5% wt
Operating pressure
600 psi
Tempera t ure 2.30C
124
TABLE A.7
Effect of operating pressure on solute separation of sugar
solution
Pressure
psi
Run No.
SeDaration
Conc . 14.760
Conc. 23.60
.
1
350
92.38
75.84
2
400
92.97
83.89
3
450
94.32
87.96
4
500
94.83
91.84
5
550
95.18
93.85
6
600
95.18
94.65
.
0
Temperature 23 C
Flow rate:
Separation
12.3 lit/min
=
100 (conc. of feed - conc. of product)
. . concentratlon of feed
125
TABLE A.8
The effect of feed concentration on solute separation of sugar
solution
.
Run No.
Feed Concentration
% wt
Permeate
Conc. % wt
Separation
1
10.83
0.787
92.73
2
14.95
1 .187
92.06
3
19.5
1.950
90.00
4
23.5
3.066
86.89
5
26.50
4.812
81.84
. Operati ng pressure:
%
400 psi
Flow rate:
12.3 1it/mi n
Separation:
100 (conc. of feed - conc. of product)
concen.tratlon of feed
Temperature 23 0 C.
126
TABLE A.9
The effect of operating pressure on salt separation
,
j
Run
No.
Pressure
psi
1
300
2
350
3
400
4
450
5
500·
6
600
Penneate
Conc.
66.89
77.84
5.1826 x 10- 4
3.6489 x 10- 4
81.5
3.1378 x 10- 4
2.1156 x 10- 4
88.8
Salt concentration in feed
.Separation % =
%
4
9.2711 x 106.2045 x 10- 4
Sugar solution concentration
Flow rate
Separation
87.00
82.44
23.60
0.0028
12.3 1it/min
100 concentration of salt in feed -
Temperature 23 0C
concentratl0n
0
sa
rodu~
127
TABLE A.l0
The effect of feed concentration on the salt separation
Run No.
Feed Concentration
% wt.
Conc. of Salt
in Permeate
1
. 10.85
2
14.95
3
19.50
2.7731 x 10- 4
. -4
3.3776 x 10
5.1875 x 10- 4
4
23.40
5
26.50
Concentration of salt in feed
400 psi
Pressure
2.· F.low rate
.
12.31/min
0
3.. Temperature 23 C
86.32
83.33
74.41
52.09
39.69
,
= 2.0276x 10- 3
Separation
%
9.7140 x 10- 4
1.2228 x 10- 3
,
1.
I
128
TABLE A.ll
The effect of the presence of Dextran on the flux rate of sugar
solution
Flux rate ml/sec
14.30 + .3%
Dextran
Pressure
psi
Conc-. 14.760
350
4.2
3.70
-
400
5.33
5.14
5.01
450
7.25
6.68
-
500
8.80
8.40
8.13
550
10.16
9.60
9.39
600
12.23
11.38
11 .00
Feed flow rate
= 12.3
14.3 + .5%
Dextran
lit/min
Temperature 23°C
."
."