DEVELOPMENT OF A NOVEL SYSTEM FOR PROGRESSIVE FREEZE
CONCENTRATION PROCESS
MAZURA JUSOH
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DEVELOPMENT OF A NOVEL SYSTEM FOR PROGRESSIVE FREEZE CONCENTRATION PROCESS
MAZURA JUSOH
A thesis submitted in fulfilment of the requirements for the award the degree of Doctor of Philosophy (Chemical Engineering)
Faculty of Chemical and Natural Resources Engineering Universiti Teknologi Malaysia
DEDICATION
This thesis is especially dedicated to my ever loving wonderful husband, Zaki Yamani Zakaria and my beautiful, beloved children, Ikhwan Firdaus, Marsya Syazweena and Alya Humaira, as well as my beloved parents. You are all the
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ACKNOWLEDGEMENTS
Praise be to Allah. I wish to express my profound gratitude to my supervisors, Prof. Dato’ Dr. Rosli Mohd Yunus and Associate Professor Dr. Mohd. Ariffin Abu Hassan, for their constant comments, suggestions, guidance, encouragement and support throughout this work. May all your contributions invested on me will be paid off not only by my success indirectly, but also by blessings from the merciful Allah Azzawajalla.
A very special thanks to my ever loving and helpful husband, Mr. Zaki Yamani Zakaria, for his wisdom in offering me advices either academically or otherwise. He has always been the person I turn to in times of troubles and rough endeavour, whenever I need comfort. Your insights, knowledge, act of wisdom and love, I will cherish forever.
Grateful appreciation to En. Mohd. Rafiza Othman, Puan Noraidah Zhwal and other laboratory staffs for the assistance they offered throughout my experimental work. The hardships I encountered were certainly more or less lightened by the aid offered.
I would also like to acknowledge Universiti Teknologi Malaysia, not only for their financial support but for all other assistances in enlightening my route to this academic success.
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ABSTRACT
Freeze concentration (FC) was investigated focusing in terms of assessment towards benefiting industrial wastewater treatment. A new more productive crystalliser was aimed to be designed as the main component in a progressive freeze concentration (PFC) process, a more economical version of FC which forms ice crystals as a layer or a block on cooled surface. Subsequent analysis on its performance, a process optimisation and heat transfer study were intended following completion of design. As a result, a helical structured copper crystalliser was successfully developed and fabricated named coil crystalliser, designed in such a way to provide high surface area per coolant volume, easy scale up and practical. In the subsequent performance analysis carried out using glucose solution as simulated industrial wastewater treatment, it was found that the effective partition constant (K) was satisfactorily low at high circulation flowrates, low initial concentration and intermediate coolant temperature. Low coolant temperature of -10 °C was observed to cause high ice front growth rate which subsequently promote high solute entrapment in the ice layer formed, affecting its purity. High circulation flowrates of 1000 ml/min on the other hand resulted in low solute inclusion in solid phase thus giving out high ice purity. In terms of volume reduction, the highest achieved was 76.65 % and coolant temperature seems to influence the most where low temperatures resulted in high solution volume reduction as a consequence of high ice growth rate. A process optimisation employing Response Surface Methodology (RSM) in Statistica software to yield the optimum conditions to produce the best K, ice purity (IP) and solution volume reduction (ǻV) generated three regression models, which have been proven adequate by good R-squared, residual and Analysis of Variance (ANOVA) analysis. Interactions between process variables according to the model agree well with the fundamental theory of FC and finally optimum conditions for each response have been identified. The best K predicted was 0.17, 78.5 % for ǻV and 0.05 mg/ml for IP. In the subsequent heat transfer study standard lines for overall heat transfer coefficient (Uo) were plotted. The Uo lines then
ABSTRAK
Pemekatan pembekuan (FC) telah dikaji untuk tujuan penggunaan di dalam olahan air sisa industri. Tujuan kajian adalah untuk merekabentuk penghablur yang lebih produktif, sebagai alat utama di dalam pemekatan pembekuan lapisan (PFC) yang lebih ekonomi di mana hablur ais terbentuk sebagai satu lapisan di atas permukaan yang disejukkan. Analisis ke atas prestasi sistem, optimasi proses dan kajian pemindahan haba juga perlu dilakukan sejurus setelah rekabentuk siap. Akhirnya, sebuah penghablur kuprum berbentuk pilin telah berjaya direkabentuk, dinamakan penghablur gulungan, yang mana boleh memberikan luas permukaan yang tinggi, memudahkan proses pembesaran skala dan praktikal. Di dalam analisis prestasi yang dilakukan menggunakan larutan glukosa sebagai air sisa buatan, pemalar pemisahan berkesan (K) didapati rendah pada kadar alir kitaran yang tinggi, kepekatan asal larutan yang rendah dan suhu cecair penyejuk medium. Suhu cecair penyejuk yang rendah pada -10°C didapati menyebabkan pertumbuhan hablur ais yang tinggi, yang mana menyebabkan penjeratan zat terlarut yang tinggi di dalam ais, mempengaruhi ketulenannya. Kadar alir kitaran yang tinggi pada 1000 ml/min pula menyebabkan penjeratan zat terlarut yang rendah, memberikan ketulenan ais yang tinggi. Dalam konteks penurunan isipadu, nilai tertinggi dicapai adalah 76.65% dan suhu cecair penyejuk dilihat paling banyak mempengaruhi di mana suhu rendah mengakibatkan penurunan yang tinggi disebabkan oleh pertumbuhan ais yang tinggi. Pengoptimuman proses menggunakan kaedah tindakbalas permukaan (RSM) dalam perisian.Statitica untuk memberikan keadaan optimum bagi menghasilkan K, ketulenan ais (IP) dan penurunan isipadu larutan (ǻV) terbaik telah menerbitkan tiga model regresi, yang telah dibuktikan kesahihannya berpandukan nilai R kuasa dua, sisa dan analisa varian (ANOVA) yang baik. Interaksi antara pembolehubah proses berpandukan model tersebut juga didapati selari dengan teori asas pemekatan pembekuan dan akhirnya keadaan yang optimum telah dikenalpasti. Ramalan nilai terbaik pada keadaan optimum untuk K adalah 0.17, 78.5 % untuk ǻV dan 0.05 mg/ml untuk IP. Dalam kajian pemidahan haba, garis-garis piawai untuk pemalar pemindahan haba keseluruhan (Uo) telah diplot. Garis-garis Uo ini membantu dalam
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT vi
ABSTRAK vii
TABLE OF CONTENTS viii
LIST OF TABLES xiii
LIST OF FIGURES xv
LIST OF PLATES xviii
LIST OF SYMBOLS AND ABBREVIATIONS xviii
LIST OF APPENDICES xxii
1 INTRODUCTION 1
1.1Introduction ... 1
1.2Problem Statement ... 4
1.3Objective of Study ... 6
1.4Scope of Research ... 7
1.5Significance of Research ... 8
1.6Outline of the Thesis ... 9
2 LITERATURE REVIEW 12 2.1Wastewater ... 12
2.1.1 Development in Wastewater Treatment ... 16
2.2.1 Evaporation of Wastewater Containing Volatile
Organic Compounds (VOCs) ... 21
2.3Wastewater Treatment by Reverse Osmosis... 21
2.4Wastewater Treatment by Freeze Concentration (FC) ... 24
2.4.1 Suspension Freeze Concentration (SFC) ... 26
2.4.1.1 Supersaturation... 27
2.4.1.2 Nucleation ... 27
2.4.1.3 Crystal Growth ... 28
2.4.2 Progressive Freeze Concentration (PFC) ... 28
2.4.2.1 Effective Partition Constant, K ... 29
2.4.3 PFC on a Stainless Steel Plate ... 34
2.4.4 PFC on Aluminium Plate Heat Exchanger ... 35
2.4.5 PFC in Vertical Aluminium Tube ... 37
2.4.6 PFC in a Balloon Flask ... 39
2.4.7 PFC in Square Pillars ... 41
2.4.8 PFC Using a Stainless Steel Cylindrical Vessel ... 43
2.4.9 PFC in a Tubular Ice System ... 45
2.5Factors Affecting a PFC) System ... 49
2.5.1 Effect of Circulation Flowrate on the Process Efficiency ... 49
2.5.2 Effect of Initial Concentration on the Process Efficiency ... 50
2.5.3 Effect of Coolant Temperature on the Process Efficiency ... 51
2.5.4 Effect of the Initial Temperature on Process Efficiency ... 52
2.6Design of Experiment (DOE) and Optimisation ... 54
2.6.1 Response Surface Methodology (RSM) ... 55
2.6.2 Central Composite Design (CCD) ... 57
2.6.3 Analysis of Variance ... 58
3 RESEARCH METHODOLOGY 59 3.1General Research Activity ... 59
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3.2.1 Preliminary Crystalliser ... 63
3.2.2 Actual Crystalliser ... 64
3.3Experimental Set-up ... 64
3.4Raw Materials ... 67
3.5Construction of Calibration Curves ... 68
3.6Experimental Procedure ... 70
3.6.1 Sampling ... 71
3.7Efficiency Assessment ... 72
3.8Investigations Effects of the Operating Conditions ... 74
3.8.1 Effects of Coolant Temperature ... 75
3.8.2 Effects of Circulation Flowrate ... 76
3.8.3 Effects of Initial Concentration ... 77
3.8.4 Effects of Circulation Time ... 78
3.9Optimisation Process ... 79
3.9.1 Design of Experiment (DOE): Experimental Planning .. 80
3.10 Temperature Profiling on the Heat Transfer ... 83
4 DESIGN OF THE COIL CRYSTALLISER 85 4.1Basic Shape and Dimensions of the CC ... 85
4.1.1 The Actual CC ... 87
4.2Main Features of the CC ... 89
4.2.1 Construction Material ... 90
4.2.2 Solution Movement ... 91
4.2.2.1 Effect of Types of Pump ... 91
4.2.3 Large Surface Area for Ice Formation... 93
4.2.4 Ice Visibility... 95
4.2.5 Accessibility for Ice Sampling ... 96
4.2.6 Temperature Profiling Feature ... 97
5 PERFORMANCE OF COIL CRYSTALLISER C IN THE TREATMENT OF SIMULATED WASTEWATER 98 5.1Introduction ... 98
5.2Calibration Curves ... 99
5.4Effect of Circulation Flowrate ... 106
5.5Effect of Coolant Temperature... 112
5.5.1 Growth Rate Based on Coolant Temperature ... 116
5.6Effect of Initial Concentration of Solution ... 118
6 OPTIMISATION OF PARAMETERS FOR THE FREEZE CONCENTRATION PROCESS 125 6.1Introduction ... 125
6.2Experimental Design ... 126
6.3Optimisation of the System Efficiency According to the Effective Partition Constant (K) Value... 127
6.3.1 Model Adequacy Check ... 129
6.3.2 Response Surface Contour Plots Analysis ... 135
6.3.2.1 Effects and Interactions of CTemp (X1) and FL (X2) ... 136
6.3.2.2 Effects and Interactions of CTemp (X1) and ISC (X3) ... 138
6.3.2.3 Effects and Interactions of CTemp (X1) and CT (X4) ... 139
6.3.2.4 Effects and Interactions of FL (X2) and ISC (X3) ... 141
6.3.2.5 Effects and Interactions of FL (X2) and CT (X4) ... 143
6.3.2.6 Effects and Interactions of ISC (X3) and CT (X4) ... 145
6.3.3 Optimum Operating Condition ... 147
6.4Optimisation of the System Efficiency According to Solution Volume Reduction ... 148
6.4.1 Model Adequacy Check ... 150
6.4.2 Response Surface Contour Plots Analysis ... 155
6.4.3 Effect and Interaction Model ... 161
6.4.4 Optimum Values ... 162
6.5Optimisation of the Process Efficiency Based on Ice Purity ... 162
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6.5.2 Response Surface Contour Plot Analysis ... 168
6.5.3 Effect and Interaction Model for Ice Purity ... 171
6.5.4 Optimum Operating Conditions for High Ice Purity .... 172
6.6Validation Experiments ... 173
7 HEAT TRANSFER MODEL 174 7.1Introduction ... 174
7.2Temperature Distribution ... 175
7.3Overall Heat Transfer Coefficient ... 176
7.4Model for Prediction of Ice Crystal Mass ... 179
7.4.1 Error Analysis ... 183
8 CHAPTER 8 186 8.1General Conclusions ... 186
8.2Recommendations for Future Research ... 190
REFERENCES 191
LIST OF TABLES
TABLE TITLE PAGE
NO.
2.1 Levels of wastewater treatment 17
2.2 Unit operations and processes used to remove constituents found in
wastewater 17
2.3 Environmental Quality (Industrial Effluents) Regulation, 2009 20
3.1 Specifications of waterbath 66
3.2 Specifications of pump 66
3.3 Specifications of USB TC-08 temperature data acquisition tool 66 3.4 Freezing point of ethylene glycol based water solutions 68 3.5 Glucose solution preparation for 1 – 10 mg/ml range 69
3.6 Specification of the refractrometer 69
3.7 Specifications of Spectrophotometer for COD Analysis 70
3.8 Range of Values for the Parameters 72
3.9 Operating conditions to study the effect of coolant temperature 75 3.10 Operating conditions to study the effect of circulation flowrate 76 3.11 Operating Conditions to study the effect of initial concentration 77
3.12 Specifications of DSC 78
3.13 Operating conditions to study the effect of circulation time 79 3.14 Arrangement of experiment using Central Composite Design 81 3.15 Range of process parameters for the PFC process 82
4.1 Conductivity of different materials 90
4.2 Specification of the temperature probe 97
5.1 Assessment according to the coolant temperature 125
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6.1 Experimental design with coded and actual value 127
6.2 Response (K) for each run 128
6.3 Experimental and predicted K for each run 130 6.4 ANOVA results for the model relating K to the operating
parameters 132
6.5 Regression analysis for K 134
6.6 Optimum range for operating conditions 148
6.7 Volume reduction for each run 149
6.8 Experimental and predicted ǻV for each run 151 6.9 ANOVA results for the model relating ǻV to the operating
parameters 153
6.10 Regression analysis for ǻV 154
6.11 Summary of the model for effect and interaction between two
operating parameters 161
6.12 Response (Ice Purity) for each run 163
6.13 Experimental, predicted and residual values for each run 164 6.14 ANOVA results for the model relating IP to the operating
parameters 166
6.15 Regression analysis for ice purity model 167
6.16 Optimum range of optimum conditions 171
6.17 Effect and interaction model for each combination 172 6.18 Summary of Optimum Value for Parameters and Validation
Experiments 173
7.1 Calculation of Uo 178
7.2 Calculation of ice crystal mass 182
LIST OF FIGURES
FIGURE TITLE PAGE
NO.
1.1 Different type of FC: (a) SFC and (b) PFC 5
2.1 Schematic diagram of a wastewater management infrastructure 13
2.2 Osmosis illustration 22
2.3 Reverse osmosis illustration 23
2.4 Conventional vertical vessel PFC system 33
2.5 Schematic diagram of the experimental apparatus by Chen et al.,
(1998) 34
2.6 Experimental apparatus for freeze wastewater treatment 36
2.7 FC set-up 38
2.8 Laboratory scale PFC 39
2.9 Schematic system flow diagram of freeze wastewater treatment 41 2.10 Schematic model of ice maker structure: up, arrangement of ice
making cylinder; left, agitation case; right, aeration case 42 2.11 Diagram of the apparatus for freeze-concentration of Andes berry
juice 44
2.12 Tubular ice system for scale-up of PFC 45
2.13 Basic schematic of the experimental freezing unit 47 2.14 Effect of flowrate on the contamination of glucose into ice 50 2.15 Effect of coolant temperature on contamination on organics in the 52 2.16 Effect of initial solution temperture on contamination of organics in
the formed ice layer 53
2.17 Example of 3-D Graphic Surface Optimisation 56
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3.1 Flowchart of the research methodology 60
4.1 The structure of the helical copper crystalliser 88 4.2 Temperatures of fluid affected by different types of pump 92
5.1 Glucose calibration curve (RI) 100
5.2 Glucose calibration curve (COD vs RI) 101
5.3 Concentration of glucose in the solid phase at different circulation
time 103
5.4 Graphical representation of calculation of K 104 5.5 COD of melted ice against circulation time 105 5.6 Liquid phase volume reduction at different circulation time 105 5.7 Effect of circulation flowrate on the purity of water produced 106 5.8 Concentration polarization at the ice front in PFC 107 5.9 Effect of circulation flowrate on the effective partition constant, K 108
5.10 Effect of flowrate on K and CL 109
5.11 COD of ice phase against circulation flowrate 110
5.12 A snapshot of Sigmaplot11 solver 110
5.13 Volume reduction of solution against circulation flowrate 111
5.14 Effect of coolant temperature on K 112
5.15 Electron density and location in a water molecule 113
5.16 Hexagonal structure of ice 114
5.17 Three dimensional structure of ice 114
5.18 Illustration of ice layer formed inside the copper tube of the CC 117 5.19 Ice growth as affected by coolant temperature 118
5.20 Effect of initial concentration on K 119
5.21 Ice crystal lattice 120
5.22 Solid-liquid equilibrium temperature at the interphase 120 5.23 Ice contamination in terms of COD according to initial solution
concentration 122
5.24 Effects of initial solution concentration on volume reduction of
solution 123
5.25 A sample of DSC curve 123
6.3 Pareto chart of effects of variable on K 135 6.4 A contour plot of K as a function of CTemp and FL 136
6.5 Fitted response profile for CTemp and FL 137
6.6 Surface plot for K against ISC and CTemp 138
6.7 A contour plot representing the effect of ISC and CTemp
towards K 139
6.8 Surface plot of K against CT and CTemp 140
6.9 A contour plot of CT against CTemp 141
6.10 Surface plot of K against ISC and FL 142
6.11 Contour plot of ISC against FL 143
6.12 Surface plot of K against CT and FL 144
6.13 Contour plot of CT against FL 145
6.14 Surface plot of K against CT and ISC 146
6.15 Contour plot of CT against ISC 147
6.16 Predicted vs experimental values for ǻV 152
6.17 Residuals vs predicted values for ǻV. 152
6.18 Pareto chart of effects of variable on ǻV 155
6.19 Effects of FL and CTemp on ǻV 156
6.20 Effects of ISC and CTemp on ǻV 156
6.21 Effects of CT and CTemp on ǻV 157
6.22 Effects of ISC and FL on ǻV 157
6.23 Effects of CT and FL on ǻV 158
6.24 Effects of CT and ISC on ǻV 158
6.25 2D contour plot for all possible combinations of independent
variable to affect ǻV 160
6.26 Predicted vs observed value of Ice Purity (IP) 165
6.27 Residual vs predicted value of IP 166
6.28 Pareto chart of effect of variables on ice purity 168 6.29 3D surface plot for all possible combinations of two variables 169 6.30 2D contour plots for all possible combinations of two variables 170 7.1 Online PICOlog recorder temperature profile 175
7.2 Uo for each coolant temperature applied 179
7.3 Temperature distribution in the CC 180
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LIST OF PLATES
PLATE NO. TITLE PAGE
3.1 Photograph of the PFC experimental set-up during running time 65
4.1 Preliminary CC 87
4.2 Actual CC 88
4.3 Top view of the CC 89
4.4 Detached CC 95
5.1 Ice layer formed at the inner wall of CC 101
LIST OF SYMBOLS AND ABBREVIATIONS
IWW - Industrial wastewater
SSFE - Scrapped surface heat exchangers
FC - FC
PFC - Progressive Freeze Concentration DOE - Design of experiment
CCD - Central composite design RSM - Response surface methodology CC - Coil Crystalliser
COD - Chemical oxygen demand RI - Refractive index
DSC - Differential scanning calorimetric IP - Ice purity
RO - Reverse osmosis
K - Effective Partition constant
FL - Flowrate
CT - Circulation time
ISC - Initial solution concentration CTemp - Coolant temperature
¨V - Volume reduction
Uo - Overall heat transfer coefficient
CL - Concentration of liquid phase
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BOD - Biochemical oxygen demand
BTEX - Benzene, toluene, ethylbenzene and xylene TCE - Trichloroethylene
PCE - Perchloroethylene μ - Chemical potential
μ1 - Chemical potential of the liquid
μs - Chemical potential of the solid
T - Temperature
CS - Concentration of ice/solid phase
CL - Concentration of liquid phase
VL - Volume of liquid phase
Vo - Initial volume of glucose concentration
Co - Initial solution concentration
υ
ice - Average ice growthm2 - Meter squared
kg - Kilogram
ȡice - Mass of melted ice
IJ - Length of time taken for ice growth A - Area covered by ice on the plate surface Ȧs, - Mass fraction of the solute
w/w - Weight/weight
μms-1 - Micrometer per second us, - Velocity of ice front
ppm - Part per million
F - Feed flow
Cp - Specific heat of the feed dș - Differential increase in time dW - Amount of ice crystallized ¨H - Heat of fusion of water
U - Overal heat transfer coefficient Am - Mean area for heat transfer
¨Tlog - Mean logarithmic temperature difference between feed and
hamb - Ambient difference between the outside surface of the
freezer and the environment
¨Tamb - Temperature difference between the outside surface of the
freezer and the environment RF - Freezing ratio
VS - Volume of the solid produced
Vi - Initial solution volume
ID - Immersion distance IR - Immersion rate t - Time of process
Bx - Brixx
Re - Reynolds number De - Representative diameter
u - Average flowrate uo - Fluid viscosity
De - Diameter
S - Cross sectional area of the flow
lp - Wetted perimeter
ANOVA - Analysis of variance K2Cr2O7 - Potassium dichromate
Y - Predicted response value
Ǻ - Regression coefficient
X - Experimental factor influencing the process Į - Extreme values
R - Total thermal resistance from inside to outside flow oC/W hi, ho - heat transfer coefficient for inside and outside flow,
respectively W/(m2.oC)
k - Thermal conductivity of tube material W/(m.oC) x - Thickness of medium wall
Kexp - K experimental
Kp - Model predicted K
¨Vexp - Experimental value for ¨V
R2 - R-squared
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Tsi - Temperature of the solution at the inlet Twm - Temperature of the wall at the middle point Tsm - Temperature of the solution at the middle point Tcm - Temperature of the coolant at the middle point Two - Temperature of the wall at the outlet
Tso - Temperature of the solution at the outlet Tco - Temperature of the coolant at the outlet Ao - Outside surface area
Ai - Inside surface areas of tube
hg - Heat transfer coefficient for glucose solution
ho - Heat transfer coefficients for ethylene glycol (50%)
ki - Thermal conductivity of ice
x - Thickness of ice layer Am - Logarithmic mean area
R - Radius of copper tube for the CC Tb - Bulk solution temperature
ǻTb - Bulk solution temperature difference, ˚C
Tc - Bulk coolant temperature
LIST OF APPENDICES
APPENDIX TITLE PAGE
A MSDS of Ethylene Glycol 203
B Procedure of COD Measurement 212
C Extreme Values Calculation for RSM 224 D RI Calibration Curve Regression Report 227 E COD Calibration Curve Regression Report 230 F Regression Report for Plate 5.13 233
G DSC Curves 236
H Standard F-Table 245
I Experimental Data 247
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1 CHAPTER 1
INTRODUCTION
1.1 Introduction
Solution concentration is a process that would normally separate relatively pure water and leaves behind concentrates in a smaller volume. There are basically three methods available which are evaporation, reverse osmosis (RO) and freeze concentration (FC). Evaporation, a concentration method which produces pure water in terms of condensed vapour would leave behind a concentrated solution by applying heat to reach the solvent boiling point (Geankoplis, 1993). It is the simplest and commonest method but large amount of energy is needed to achieve the boiling point of solvent (in most cases is water) and consuming a large amount of fuel (Ramteke et al., 1993, Keshani et al., 2010).
compatibility of the dissolved substances with the membrane. The high osmotic pressure involved is also very expensive to attain thus making RO not favourable to be applied in solution concentration (Rodriguez et al., 2000).
FC, a process that freezes or crystallises water molecules out as ice crystals and leaves behind a highly concentrated solution has become more and more favourable these days. The purging of highly pure water is enabled by the nature of the crystal lattice build up by pure water from an aqueous solution or suspension at temperatures lower than its melting point, rejecting all impurities which remain in the mother liquor (Halde, 1980). Approximately 80% of the dissolved compounds can be concentrated in 25% of the original volume through FC (Maurer et. al, 2006) and the process uses less energy compared to evaporation as the heat of crystallization of water (330 kJ/kg) is lower than heat of vaporisation (2200 kJ/kg) (Lorain et al., 2001).
FC could be engaged in many fields of applications, and among them is in liquid food concentration. Liquid food such as fruit juices are semi finished products for use in further production of fruit juice beverages and powder. The concentration process provides microbiological stability, and also leads to economical packaging, transportation and distribution of final products (Keshani et al., 2010). Because FC does not involve heating, it could successfully provide a concentration process with low loss of volatile components in the juice, thus preserving its original taste and aroma. This dewatering nature makes it the best concentration method in avoiding quality loss of liquid food and dairy products (Nakagawa et al., 2009).
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variety of applications on earth, where the polluted and/or contaminated fractions termed wastewater (Mark, 2004).
Although some kinds of water pollution can occur through natural processes, this phenomenon is mostly and frequently a result of human activities. Apart from generation sourced by unavoidable utilisation domestically, the industries are playing a major role in generating and increasing the amount of wastewater generated. Plants and factories are continuously expanded, established and operated as urged by modern living, to produce goods and materials to equip for the lifestyle. In most of the operations water is used as a very important utility, which is consequently converted into industrial wastewater (IWW) usually tainted with various kinds of substances. IWW should rightfully be treated according to the local municipal council or environmental department standard before being disposed off to the environment (Tchobanoglous et al., 1991).
IWW can contain various types of pollutant ranging from chemicals to suspended matters and the type of treatment depends on the type of the pollutants. Wastewater treatment can be divided into three major categories, which are biological, physical and chemical. In short, biological treatments involve the usage of microorganisms in the treatment, chemical treatments involve addition of chemicals to the wastewater to treat it and physical treatments usually engaged mechanical means (Tchobanoglous et al., 1991).
The three concentration methods mentioned earlier can provide a treatment for IWW through water removal which would yield concentrated wastewater in a smaller volume and almost pure water that can be readily disposed off to the environment. Despite the simplicity of evaporation process, it is very dangerous when hazardous volatile organic compounds (VOCs) constitute the IWW, (Rogers, 1999) which is one of the major setbacks to the method. With high operation and maintenance cost for RO, FC is seen as an appealing alternative to other wastewater treatment by water removal. It is safe to be applied to wastewater containing hazardous VOCs as it does not involve heating.
1.2 Problem Statement
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Figure 1.1: Different type of FC: (a) SFC and (b) PFC (Miyawaki et al., 2005)
Researches have been carried out on FC in order to tackle all the limitations of SFC mentioned above and eventually PFC was discovered, which produces ice crystals in forms of a layer or a block on cooled surface. This ice block is not produced as a suspension in its mother liquor but formed at the wall where refrigeration is supplied as illustrated in Figure 1.1 (b). This eliminates the need of a complicated separation mechanism to remove the ice crystal block because the mother liquor can simply be drained out of the system to separate the two phases. It also eliminates the usage of SSHE that is used in producing small ice crystal and only utilizes the refrigeration system to obtain ice layers on the crystalliser surface.
Many kinds of design have been investigated for PFC other than the basic conventional design of PFC, which is just a cylinder with metal bottom supplied with refrigeration and the ice is formed layer by layer vertically. Other designs include ice maker channels in square pillars by Wakisaka et al. (2001), tubular PFC system by Miyawaki et al. (2004) and a multi-plate cryoconcentrator by Raventos et al. (2006) and Hernandez et al. (2009) to name a few.
Coolant Agitator
Ice block Mother Liquor/concentrate
Ice crystal
Metal
PFC was proven to be a promising alternative to be applied in treating and concentrating wastewater based on its capability of lowering the capital cost involved in SFC through elimination of several unit operations such as the scraped surface heat exchanger (which is the ice crystal producer and usually contributes 30% of the capital cost) and the washing column. However, as previous related researches revealed, the productivity of such a system is very much lower compared to the conventional SFC. This is mainly due to the nature of the formation of ice layer which is normally very slow. The slow ice layer formation is a vital obstructive factor in making the PFC favourable commercially. Even with the lower capital cost that can be proposed by such a system, it would not be the choice for the industry if the process would not yield a good productivity and efficiency.
With the elimination of the ice crystal producer (SSHE) in PFC, the crystalliser as the equipment where the main heat transfer to occur must be of the right design and carefully outlaid. The ice layer is to be produced at a high rate and simultaneously resulting in ice block with high purity, thus requiring a high surface area per coolant volume ratio. A careful selection process of the suitable non-corrosive metal with high thermal conductivity to be used as the crystallisation surface has to be carried out in order to ensure high productivity. The shape of the crystalliser should also be competent in facilitating future scale of the process. Several heat transfer aspect of the crystallisation chamber design should be investigated as well as the operating condition of the process. An investigation leading to construction of a PFC system of satisfactory productivity and efficiency in order to compete with costly SFC system would be very much beneficial.
1.3 Objective of Study
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i. To design/develop a new crystalliser for PFC of wastewater;
ii. To study the effect of circulation flowrate on the performance of the crystallisation chamber;
iii. To investigate the effect of coolant temperature on the partition constant (K), solution volume reduction and ice purity;
iv. To explore the efficiency of the crystallisation chamber design by varying the initial concentration of solution;
v. To study the effect of circulation time/period on the performance of the crystallisation chamber;
vi. To determine the optimum process conditions (parameters) using Response Surface Methodology (RSM);
vii. To profile the temperature distribution in order to calculate the overall heat transfer coefficient, Uo, for the PFC process;
1.4 Scope of Research
i. The designing of the new crystalliser took into account new material of construction, shape to increase surface area and equipped with all relevant features for a successful PFC process;
ii. The range of circulation flowrate studied was 400 to 1000ml/min, to investigate its effect on K, ice purity and solution volume reduction;
iii. The coolant temperature range applied was -4 to -10˚C, investigated on the K, solution volume reduction and ice purity;
iv. Initial solution concentration was investigated in the range of 4 to 10mg/ml to explore the efficiency of the crystalliser design;
v. The effect of circulation time/period on the performance of the crystalliser was investigated between 5 to 20 minutes of circulation;
vi. RSM was carried out using STATISTICA software to generate an experimental design, followed by generation of a prediction model with subsequent investigations on the model’s adequacy. Interactions between the process variable were also observed and the optimum conditions for the process were generated;
vii. Overall heat transfer coefficient, Uo, was calculated for the PFC process
according to different coolant temperature and time, followed by a development of a model predicting the ice crystal mass produced during operation.
1.5 Significance of Research
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treatment by water removal. This PFC system should highly be in demand in industries producing IWW with high VOCs, where heating would cause an alarming danger. It is also suitable for removing water from IWW with high total suspended solids (TSS) and heavy metals as most particles are rejected during ice crystal lattice formation, making it a more advantageous solution to RO that would have a high rate of fouling in this situation. This research would also provide an understanding in how to design a PFC system with high productivity and also provide references for future application of PFC in concentration of food and pharmaceutical compounds.
1.6 Outline of the Thesis
Below is how the thesis is arranged in order to report the research carried out:
Chapter 1 introduces the research and highlights the objective and the scopes implemented in order to achieve it.
Chapter 3 provides explanations of the methodology of the experiment. A research methodology flow chart attached in the beginning of the chapter provides an overview of how the research is undertaken. Later in the chapter, the methods or experimental steps taken through the research are elaborated. Operating parameters investigated are circulation flowrate, coolant temperature, initial concentration of glucose solution and circulation time/period.
Chapter 4 presents the resulted design of the new crystalliser, which is termed, multi-layer crystalliser (CC). Material used and the geometry of the design is elaborated, equipped with all necessary chemical and physical characteristics. The designing work is carried out cautiously, considering the necessary aspects to result in good FC process. It also includes justifications of why each materials or equipment is used in the experimental rig that were used in the investigation.
Chapter 5 reports on the findings and theories to explain the effects of all the operating parameters investigated, which are the process fluid circulation flowrate, coolant temperature, initial concentration of solution and circulation time, on the performance of the PFC system. The performance was indicated by three variables, which are the partition constant, K, ice purity and volume reduction of the process fluid.
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Chapter 7 reports on the heat transfer activity occurring during the FC process. The overall heat transfer coefficient, Uo, was calculated for each coolant
temperature employed, generating standard lines as reference. A model was also proposed in order to predict the crystal mass produced at any time of the FC operation.
