WET AIR OXIDATION OF SPENT LIQUOR FROM KRAFT
PULPING PROCESS
By
Ahdab Nakhala
A Thesis submitted to the Faculty of Graduate Studies and Research in
partial fulfillment of the requirements for the Degree of
Master of Engineering
Department of Chemical Engineering
McGill University
Montreal, Canada
© Ahdab Nakhala, 2008
This thesis is dedicated
to my dear parents,
my husband, Mohamed,
my sons, Marwan and Yazan
my daughter, Jena
&
my dear family members
whose true love and endless sacrifices helped
me achieve a big part of a dream I am after
ii
ABSTRACT
The limitation of kraft recovery unit capacity in pulp and paper making can be
overcome by using wet air oxidation (WAO) as pre-treatment stage to treat spent black
liquor. WAO is a process in which organic and/or oxidizable inorganic components are
oxidized using air or pure oxygen in a liquid phase at elevated temperature (150-370 °C).
In order to keep the reaction in the liquid phase, high pressures are required (2-20 MPa).
All experiments were performed in a highly pressurized steel reactor using
oxygen to oxidize kraft black liquor. In the first part of the experimental work, the
performance of the reactor and interaction between the operating parameters were
studied. The interaction between temperature, oxygen charging pressure and initial pH of
the solution were found to be negligible. Furthermore, the effects of these operating
conditions on the degradation of the organic compounds in black liquor were
investigated. Results showed that 70 % reduction of total organic carbon amount can be
achieved even without the addition of a catalyst. As expected, the reaction temperature
has a positive effect on both the rate and the actual amount of organic removal.
Moreover, controlling the initial pH of the solution showed a great improvement
especially at the first thirty minutes. Lowering the initial pH to 4 showed a significant
improvement with reducing the chemical oxygen demand (COD), which was 88 %
reduction as compared to 76 % at same temperature and oxygen charging pressure after
an hour and a half of treatment.
iii
RÉSUMÉ
La restriction de la capacité des chaudières kraft dans la production de pâtes et
papier peut être surmontée en utilisant la technique de l’oxydation en phase aqueuse
(WAO) comme une étape de prétraitement afin de traiter la liqueur résiduaire. Le WAO
est un processus dans lequel des composantes organiques et des substances inorganiques
oxydables sont oxydées en utilisant de l'oxygène ou l’air pur dans la phase liquide à une
température élevée (entre 150-370 °C). Afin de garder la réaction dans la phase liquide,
des pressions élevées sont exigées (2-20 MPa).
Toutes les expériences ont été exécutées dans un réacteur d'acier pressurisé en
utilisant de l'oxygène pour oxyder la liqueur noire. Dans la première partie du travail
expérimental, la performance du réacteur ainsi que l'interaction entre les paramètres
d'opérations ont été étudiées. L'interaction entre la température, la pression d'oxygène et
le pH initial de la solution était négligeable. En outre, l'effet de ces conditions sur la
dégradation des substances organiques contenues dans la liqueur noire a été examiné.
Les résultats ont montré que 70 % du carbone organique total peut être atteint même sans
l'ajout d'un catalyseur. Selon toute attente, la température de la réaction a un effet positif
non seulement sur le taux d’élimination organique mais aussi sur la quantité retirée. De
plus, une étude sur le pH initial de la solution a montré qu’une amélioration significative
pouvait avoir lieu, surtout durant les premières trente minutes. En baissant le pH initial à
pH 4, 88% de la demande chimique en oxygène (DCO) fut réduite, comparé à 76 % à la
même température et pression initiale d'oxygène après une heure et demie de traitement.
iv
ACKNOWLEDGMENTS
I would like to express all my gratitude to my dear supervisors, Dr. G. Kubes and
Dr D. Berk, for their valuable guidance, support, patience, understanding and financial
support throughout the time I took to complete this thesis.
I also wish to thank all the members of the Pulp and Paper Center at McGill
University for their friendship. Also, a special thank to the Pulp and Paper Research
Institute of Canada for supplying me with kraft black liquor and allowing me using their
facility for certain analyses.
Many thanks go to the Chemical Engineering Department, especially to Ranjan
Roy for training me and helping me in the selection of certain analytical methods. Also,
many thanks to Andrew Golsztajn for always trying hard to repair the equipments during
my research and challenging me.
I want to thank as well all the people who made it possible for me to finish all
experiments during my pregnancy particularly Jill Taylor, Majda Akadiri, Atilla Hadiji
and Dezhi Chen. A special appreciation to Dr Basim Abussaud for all the advices and
help.
Lastly, and most importantly, I dedicate this thesis to my dear parents, my loving
husband, my charming kids and my sisters. I want to thank my love, Mohamed, for his
sacrifices, encouragement and moral support especially during my pregnancy. I want to
also to thank the cutest kids ever, Marwan, Yazan and my princess Jenna for being
always great and supportive when I was away from home.
v
TABLE OF CONTENTS
ABSTRACT
iii
RÉSUMÉ
iv
ACKNOWLEDGMENTS
v
TABLE OF CONTENTS
vi
LIST OF FIGURES
ix
LIST OF TABLES
x
LIST OF ABBERVIATIONS
xi
CHAPTER ONE
INTRODUCTION
1.1 General Introduction
2
1.2 Kraft Pulping and Recovery
3
1.3 Objectives
4
1.4 Thesis Structure
CHAPTER TWO
BACKGROUND AND LITERATURE REVIEW
2.1 Wet Air Oxidation
5
2.2 WAO Advantages and Disadvantages
8
2.3 WAO Reaction Mechanism
11
2.4 WAO Process Parameters
12
2.5 Previous Work
vi
CHAPTER THREE
EXPERIMENTAL MATERIALS AND METHODS
3.1 WAO Experimentation
15
3.1.1 WAO Experimental Equipments
3.1.2 Materials
3.1.3 Experimental Procedures
3.2 Analytical Methods
18
19
3.2.1 Chemical Oxygen Demand Measurement
21
3.2.2 Total Organic Carbon Measurement
23
3.2.3 Ion Chromatography
23
CHAPTER FOUR
RESULTS AND DISCUSSIONS
WET AIR OXIDATION OF KRAFT LIQUOR
4.1 Development of WAO Experiments
4.1.1 Setting the Operating Conditions
25
26
4.2 WAO Experimental Design
36
4.3 Reproducibility Assessment
38
4.4 Effect of the Oxygen Charging Pressure on Black Liquor Degradation
39
4.5 Effect of Temperature on Black Liquor Degradation
40
4.6 Effect of Initial pH of the Solution on Black Liquor Degradation
42
4.7 Effect of Operating Conditions on Intermediates Production
45
vii
CHAPTER FIVE
CONCLUSIONS & RECOMMENDATIONS
5.1 Conclusions
49
5.2 Recommendations for future work
49
REFERENCES
51
APPENDIX A : Calculation of Oxygen Pressure Needed
55
viii
LIST OF FIGURES
Page
Figure 1.2.1 Schematic drawing of the conventional Kraft pulping and
recovery
16
Figure 1.2.2 Schematic drawing of the conventional Kraft pulping and
recovery with WAO unit
17
Figure 2.3.1 Schematic of reaction pathways
17
Figure 2.4.1 (a) &(b) The effect of temperature on oxygen solubility in water at
different pressure
27
Figure 2.4.2 Measured oxygen solubility as function of effect of temperature
and three solutes
27
Figure 4.3
Repeatability of Data from Two Experiments at 260oC,
PO2= 1.38 MPa, pH=4.
28
Figure 4.4
Effect of pH on Benzene Degradation at 190oC, PO2= 1.38 MPa
29
Figure 4.5
Effect of pH on the Benzene Degradation at 220oC,
PO2= 1.38 MPa
30
Figure 4.6
Effect of pH on the Benzene Degradation at 240oC,
PO2= 1.38 MPa
31
Figure 4.7
Effect of pH on the Benzene Degradation at 260oC,
PO2= 1.38 MPa
31
Figure 4.8
Degradation of Benzene at Different Temperatures,
PO2= 1.38 MPa, pH= 6
33
Figure 4.9
Repeatability of Data from Two Experiments at 240oC,
PO2= 1.38 MPa, pH= 6
33
Figure 4.10
Degradation of Benzene at 200oC, PO2= 1.38 MPa, pH= 6
34
Figure 4.11
Degradation of Benzene at 210oC, PO2= 1.38 MPa, pH= 6.
34
Figure 4.12
Degradation of Benzene at Different Temperatures,
PO2= 1.38 MPa, pH=6
35
Figure 4.13
Degradation of Benzene at Different Oxygen Pressure, T= 260oC,
pH=6
36
Figure 4.14
Effect of Oxygen Pressure on the Benzene Degradation at 260oC,
pH=6
37
ix
LIST OF TABLES
Page
Table 3.1.1
Black Liquor Properties
21
Table 3.1.2
Values of Oxygen Charging Pressure
22
Table 3.1.3
Ranges of Operating Conditions
23
Table 4.1.1
Updated Ranges of Operating Conditions
28
Table 4.2.1
Preliminary Experimental Design
29
Table 4.2.2
Measured Responses of preliminary experiments and its
duplicates
34
Table 4.2.3
Final Experimental Design
35
Table 4.3.1
Reproducibility Assessment : COD Reduction
36
x
LIST OF ABBREVIATIONS
COD
Chemical oxygen demand
DCO
Demande chimique en oxygène
LMWO
Low molecular weight organics
TIC
Total inorganic carbon
TOC
Total organic carbon
TRS
Total reduced sulfur
WAO
Wet air oxidation
xi
CHAPTER ONE
INTRODUCTION
1.1 General Introduction
The concern about global pollution increases every year forcing many industries
to integrate various methods to reduce or eliminate wastes.
By treating these
toxic/hazardous effluents, not only industries meet the discharge standards forced by the
government but they also recover various chemicals, which assume high priority in
improving the profitability of their processes. Although, there are several disposal and
recovery methods such as incineration, evaporation, ultraviolet radiation treatment and
many more, the eliminations and recovery capacities are still limited.
The pulp and paper industry has been recognized as a significant source of
pollution throughout the years. The treatment of effluent streams and the recovery of
chemicals are necessary in order to meet the discharge standards.
Moreover, by
recovering these expensive pulping chemicals, the profitability of pulping operations
could be improved or maintained. The spent liquors do not only contain a considerable
amount of resources in concentrated form, they also can be an important source of
energy, which can replace purchased fuels. Although, recovery units are fully developed
processes, the mills production can be limited due to the capacity limitations of the
recovery boiler. It has been suggested that recovery-limited mills could benefit from the
use of wet air oxidation (WAO) as pre-treatment stage in the recovery cycle.
1
1.2 Kraft Pulping and Recovery
Paper is made from pulp which is produced from different types of wood by
different pulping technologies. The dominating process of pulp production worldwide is
chemical Kraft pulping. The Kraft process was developed by Carl Dahl. The pulp mill
using this technology was first implemented in Sweden in 1890. In the early 1930s, the
invention of the recovery boiler marked a great advancement of the process. It enabled
the recovery and the reuse of pulping chemicals. For that reason, the Kraft process
surpassed the sulfite process and became the dominant method for pulp production.
In Kraft pulping process (also called sulfate process), wood chips are fed into a
digester where an alkaline solution called “white liquor” is added. This solution is
typically made up of sodium sulfide (Na2S) and sodium hydroxide (NaOH). These liquor
chemicals promote the breakage of the lignin structure within the woodchips so that the
lignin becomes extractable and soluble in the liquor. Then, the pulp is sent to the
washing stage where the spent pulping liquor is separated from the pulp. The effluent
liquor is known as black liquor. Black liquor is a complex colloidal solution that consists
of white liquor residuals, lignin and other dissolved wood degredation products. This
liquor usually leaves brown stock washers with solids content between 13% and 17 %,
which is then concentrated in a muliple-effect evaporators with a maximum of 60% solid
content. The liquor is then burned in the recovery furnace to recover the inorganic
chemicals for reuse. (See Figure 1.2.1 for process)
The main objective of the recovery furnace is to recover the chemicals from spent
cooking liquor and eventually to reconstitute these chemicals to reform fresh white
liquor. The combustion is carried out so that sodium sulfate is reduced to sodium sulfide
by the organic carbon (char) in the mixture. The inorganic compounds in the liquor melt
and flow out of the furnace as a mixture of molten salts called smelt. Eventually, the
smelt is dissolved in water and then causticized with lime (CaO) to produce white liquor,
which is used again for pulping and calcium carbonate (CaCO3) is then removed and
2
converted back to calcium oxide. Moreover, because of the incineration of organic
residuals, high-pressure steam is used to generate electricity in the mill.
CALCINATION
White
Liquor
NaOH +
Na2S
PULPING
Liquor and
Cooked Chips
WASHING
Lime
PULP
Weak
Black Liquor
CAUSTICIZING
EVAPORATION
Green
Liquor
Na2CO3 +
Na2S
COMBUSTION
Heavy
Black Liquor
Figure 1.2.1: A schematic drawing of the conventional Kraft pulping and
recovery process (after Grace 1989)
The double objectives of chemical and energy recovery render the design and the
operation of the recovery boiler in evaporation stage (Figure 1.2.1) very complicated,
making it one of the most expensive process units in a pulp mill. Although the Kraft
recovery process successfully achieves its objectives, there are still several drawbacks
that researchers are trying to solve by minimizing the emission of pollutants, reducing
sodium and sulfur losses, and lowering its high fixed capital cost.
The capacity of a recovery unit is based on its ability to burn completely the dry
solids in the black liquor produced in the mill. By increasing the amount of liquor to be
burn, the furnace becomes overloaded. This leads to many problems such as lower
3
reduction efficiencies, increased production of total reduced sulphur emissions and
plugging of the fire side passages in the recovery furnace (Smook, 1992).
A recovery-limited mill would not be able to purchase and install a larger furnace
due to elevated capital cost. Taking into consideration the above limitations, Flynn et al.,
(1979) showed that up to 15 % of black liquor can be oxidized in WAO unit where the
black liquor can be mixed with the remainder of the spent liquor before evaporation and
incineration (see Figure 1.2.2). Therefore, a recovery-limited mill could theoretically
increase its capacity by approximately 15%.
Using WAO in this fashion would also have a beneficial environmental impact
since the organics treated do not contribute to air and water pollution. WAO would also
have a positive impact with respect to the conversion of sulfur to sulfate, lowering total
reduced sulfur (TRS) emissions such as hydrogen sulfide, methyl mercaptan, and
dimethylsulfide, which would otherwise be produced in the recovery boiler (Hupa, 199).
CALCINATION
White
Liquor
NaOH +
Na2S
PULPING
Liquor and
Cooked Chips
WASHING
Lime
CAUSTICIZING
PULP
WAO
Weak
Black Liquor
EVAPORATION
Green
Liquor
Na2CO3 +
Na2S
COMBUSTION
Heavy
Black Liquor
4
Figure 1.2.2: A schematic drawing of the conventional kraft pulping and recovery
process with WAO (after Grace 1989)
Overall energy efficiency, which is approximately 61% of traditional recovery
systems, is another factor to be considered (Smook, 1992). A large portion of energy
losses occur in the recovery boilers because conventional recovery boilers have a poor
heat transfer coefficient between the combustion’s products and the boiler tubes (Flynn,
1979). By applying WAO to treat a portion of weak black liquor prior the recovery boiler
would offset the thermal losses in the recovery boiler due to the increased evaporator
duty and sulfur reduction. Also, by using WAO, it is possible to recover inorganic
chemicals and certain reaction by-products such as acetic and formic acid.
1.3 Objectives
The principal objective of this project is to study the possibility of using WAO as
a pre-treatment step of Kraft spent liquor by studying the effect of initial pH in order to
further improve the degree of oxidation of weak black liquor during this process.
Moreover, explain the effect of initial pH of the solution on the reaction mechanism. The
secondary objective of this research is to characterize the improvement in the degree of
oxidation by varying the experimental operating conditions such as temperature, oxygen
charging pressure, and initial pH.
1.4 Thesis Structure
In Chapter 2, general background and literature review of wet air oxidation are
provided. Also, process parameters (e.g. temperature, pressure, and oxygen solubility)
are discussed. In Chapter 3, a detailed description of the experimental set-up, procedure,
and analytical methods are given. Chapter 4 presents the results of the preliminary
experimental work in terms of reactor set-up and interaction of operating parameters and
their relative effect on the process.
Also, the repeatability of the experiments is
discussed. Moreover, the final experimental results are presented and discussed. This
5
includes the influence of each parameter on the removal efficiencies, and by-product
production. Finally, Chapter 5 comprises the general conclusions and recommendations
for future work.
CHAPTER TWO
BACKGROUND & LITERATURE REVIEW
This chapter contain a brief overview of wet air oxidation process and its reaction
mechanisms. Moreover, the implication of the process parameters, such as temperature,
pressure, pH, oxygen solubility, and its diffusion in high pressure system are discussed in
this chapter.
2.1 Wet Air Oxidation
Wet air oxidation (WAO) is considered as a well-established technique of
importance for wastewater treatment particularly for toxic and highly organic
wastewaters.
Over the last three decades, WAO process has been the subject of
considerable studies by many researchers who continue to investigate the ability of this
technology to treat different type of effluents from wide variety of industrial waste
streams. WAO is a process in which organic and oxidizable inorganic components are
oxidized using air or pure oxygen in liquid phase at elevated temperature (150-370 °C).
In order to keep the reaction in the liquid phase, high pressures are required (2-20 MPa)
(Zimmermann, 1960) Without keeping the reaction in the liquid phase in the reactor, the
compounds are going to burn, which is simply known as combustion reaction.
WAO is an exothermic reaction where dissolved oxygen initiates a free radical
chain reaction by reacting with the weak bonds of the organic compounds, creating a
hydroxyl and organic radicals. Many organic compounds in WAO gradually degrade to
lower molecular weight compounds and finally to lower carboxylic acids such as acetic,
formic, and oxalic acids (Harmsen et al., 1997). In the case of a complete WAO,
organics are converted into carbon dioxide (CO2), water (H2O), ammonia (for nitrogen
containing waste), sulfate (for sulfur containing wastes).
Otherwise, other products
6
would result such halogen acids (for halogenated wastes), and low molecular weight
organic compounds (LMWO).
2.2 WAO Advantages and Disadvantages
This process is considered advantageous in terms energy recovery since WAO is
an exothermic reaction. Moreover, the gases produced could be expanded in a turbine for
further energy recovery, which would substitute for the purchased fuel. Flynn et al.
(1979) reported that by applying WAO properly about 80 % thermal efficiency can be
achieved. Moreover, many inorganic chemicals can be recovered and reused which
would render the process cost efficient.
WAO is operated in liquid phase meaning no particulate emission occur during
the oxidation process, which leads to air pollution-free environment, eliminates
odoriferous sulfur compounds and significantly reduces any ‘difficult-to-treat’ organics
such as phenol through oxidation and conversion to readily treatable compounds.
Furthermore, these exhaust gases consist mainly of carbon dioxide, oxygen, and nitrogen,
which are known to be non-toxic. For incomplete WAO, the off-gases do contain certain
volatile LMWO compounds with concentrations varying between 10 ppm-1000ppm;
however, variety of technologies could be applied to control them such as scrubbing, etc.
Since the reaction conditions are not considered ‘harsh’, there would not be any
formation of nitrous oxides (Joshi et al., 1995).
Despite the many advantages of WAO, the defect of requiring high temperature
and oxygen pressure results in high capital and operating costs. It is also possible to end
up with an incomplete or a partial oxidation (Flynn, 1979). The above disadvantages led
the researchers to integrate a catalyst into the process so that it can be operated at
moderate conditions and the reaction rate of the process increases. This implies that
lower temperature and pressure are required to achieve the degree of oxidation desired.
This process is called catalytic wet air oxidation (CWAO). These less severe conditions
could reduce the costs significantly, which would make this technology more attractive.
It is also necessary to remove the catalyst from the oxidized effluent streams because its
7
presence would cause catalyst fouling. Moreover, the catalysts are costly chemicals;
therefore, the recovery of these materials is recommended to turn this process
economical.
2.3 WAO Reaction Mechanism
The reaction mechanism can be described by a simple reaction scheme. Organic
compounds are oxidized to unstable intermediates and further oxidized to oxidation end
products. Therefore, all effluents treated by WAO can be divided into three groups:
Initial and unstable intermediates (A), refectory intermediates (B) (e.g. acetic acid), and
oxidation end products (C) (Li et al., 1991).
pathway 1
A + O2
C
k1
k3
k2
pathway 3
pathway 2
B + O2
Figure 2.3.1 – Schematic of reaction pathways (from Li et al., 1991)
For the non-catalytic WAO, the reaction kinetics can be simplified to a global rate
expression on the removal of a general parameter (e.g. chemical oxygen demand). The
reaction rate can be assumed as follow:
rr = Ae − ( E / RT ) ∗ [C p ]m [Co2 , L ] n
(2.3.1)
where, rr is the reaction rate, A is the pre-exponential factor, E is the activation energy, R
is the gas constant, T is the reaction temperature, Cp in the pollutant concentration in the
bulk liquor, CO2, L is the oxygen concentration in the bulk liquid, and m and n are the
orders of reaction. The values for the reaction orders were reported to be first order for
pollutant concentration and zero or first order for oxygen concentration (Kolaczkowski et
al., 1997).
8
The main WAO mechanisms are not well understood.
However, Li et al.
suggested that the oxidation of organic compounds follows free radical chain reaction
mechanism. By reacting with oxygen, the C-H weak-bond of an organic compound
breaks down to produce free radicals. The free radicals eventually can be initiated by the
reaction of oxygen with the weakest O-H bond. These reaction steps reported from Li et
al., 1991.
R − H + O2 → R • + HO2•
(2.3.2)
R − H + HO2• → R • + H 2 O2
(2.3.3)
Furthermore, hydrogen peroxide will react with oxygen forming hydroxyl radicals:
H 2 O2 + O2 → 2HO2•
(2.3.4)
H 2 O2 → 2 HO •
(2.3.5)
The oxidation of organics continues with hydrogen reduction by hydroxyl radicals, which
basically means that the amount of hydrogen is reduced. The organic radical formed
reacts with oxygen to produce an organic peroxy radical (ROO-) which then reacts with
the organic compound to abstract another hydrogen from the organic.
R − H + HO • → R • + H 2 O
(2.3.6)
R • + O2 → ROO •
(2.3.7)
R − H + ROO • → ROOH + R •
(2.3.8)
The hydroperoxide formed (ROOH) is known to be unstable and it decomposes further to
form intermediates with lower carbon numbers known as low molecular weight acids
such as formic and acetic acids are obtained.
9
2.4 WAO Process Parameters
Temperature is a very important parameter to vary the degree of oxidation. High
temperatures increase the reaction rates and the free radical production. Moreover,
elevated temperatures increase the equilibrium water vapor pressure, which will rises
rapidly once operating temperature goes above 100°C. By increasing the temperature, it
will be essential to increase the operating pressure, where the minimum pressure chosen
should exceed the vapor pressure of water (to maintain a liquid phase). Without the
liquid phase, the waste treated would simply burn (combustion). The operating pressure
is basically the sum of the partial pressure of oxygen, carbon dioxide, water vapor and
inerts. Although high temperatures are required, the operating temperature should remain
below the critical temperature of the solution being oxidized (e.g. for water, the critical
temperature is 374.15°C).
The solubility and diffusion of oxygen in liquid phase are other important factors
to be considered. WAO is a heterogeneous reaction where oxygen should go through
several steps: mass transfer in the gas side phase, gas-liquid interface mass transfer, and
finally chemical reaction in the liquid phase. Assuming the gas side mass transfer
resistance is negligible; the important mass transfer would be from gas phase to liquid
phase. The above can be improved by increasing either the overall volumetric gas-liquid
mass transfer coefficient (kLa), or the oxygen solubility in the liquid phase, as described
in the following equation:
rm = k L ∗ a ∗ (C * O2 − CO2 , L )
(2.4.1)
Where, rm is the mass transfer rate of oxygen, kL is the liquid side transfer coefficient, a is
the gas-liquid interfacial area, and C*O2 is the saturated oxygen concentration
(Kolaczkowski et al., 1999).
10
Many researchers studied the solubility of oxygen in water at different
temperatures and partial pressures ranges. Tromans (1998) actually performed a study
where the results of several studies from published literature were gathered (the studies
were Broden and Simonson (1978), Pray et al (1952), Hayduka (1991), and Stephan et al
(1956)), which were all converted to experimental oxygen solubility, caq and equilibrium
k values. Temperature ranged up to 343.3 °C and partial pressures of oxygen were as
high as ~ 6.0 MPa.
Figure 2.4.1 below shows very good agreement between all
experiments showing that the solubility of oxygen decreases as the temperature increases
in the low temperature range (25° to 93.3°C). Whereas at the higher temperature range
(the range of interest for WAO), the solubility of oxygen increases along with the
temperature. It can be seen in the two figures below, Figure 2.4.1 (a) and (b).
Figure 2.4.1 (a): The effect of temperature 273K < T < 620 K on the equilibrium
constant k at PO2 101 KPa (after Tromans, 1998).
11
Figure 2.4.1 (b): The effect of temperature at different pressure on the molal solubility of
oxygen in water, 273K <T< 626K (Tromans, 1998).
All research groups concluded that the solubility of oxygen appeared to be a
linear function of pressure. The resulting linear function shows that solubility followed
Henry’s Law (kH = caq/PO2) and could be predicted over a wide range of temperature and
pressure.
The sodium content in kraft black liquor is another factor to consider since it does
affect the solubility of oxygen. Broden and Simonson (1978) also investigated the
solubility of oxygen in sodium bicarbonate and sodium hydroxide. This is interesting
since black liquor contains a significant amount of sodium cation. The conditions used
for these experiments were done at temperature ranges between 50°C and 150°C and
oxygen partial pressure varying from 1.0 MPa to 5.0 MPa. The results showed that
solubility of oxygen in aqueous salt solutions is less than in pure water, a phenomenon
known as salting-out effect (Figure 2.4.2). As it is the case in water, the solubility of
12
oxygen in an aqueous salt solution increases at temperatures above 100°C for all the
pressures examined. It was also obvious that when oxygen pressure is increased the
salting effect is more pronounced.
Figure 2.4.2: Measured oxygen solubility, caq as a function of temperature and three
solutes (Tromans, 1998)
The liquid side mass transfer coefficient (kL) is dependant on those factors as
liquid properties (e.g. gas density, liquid viscosity, etc), and diffusivity of solute in the
liquid. Although the temperature and pressure affect the gas and liquid properties, and
the diffusivity of solute in the liquid phase, a detailed discussion and research of these
influences is beyond the scope of this thesis.
13
The above studies suggested that certain reactor configurations may have a
limitation with respect to mass transfer (e.g. mixing) but may not be so critical during
WAO as in other chemical processes.
The initial pH (alkaline or acidic condition) of process did show certain influence
on the oxidation of different compounds. Several researchers studied the effect of the pH
on organics removal and from the results; it appears that, the effect of pH on the reaction
rate is complex.
2.5 Previous Work
WAO had been the subject of considerable studies over the last decades as the
researchers continue to investigate the ability of this process to remove different type of
organic compounds from simple and complex (industrial) waste streams.
In 1911, the first patent for WAO system was developed and designed for the
treatment of spent sulfite pulping liquors with compressed air at 180 °C. In 1958, the
first known plant was put by Borregaard in Norway for the treatment of sulfite liquors,
which was later closed down due to its uneconomical operation. It was until early 1960’s
that WAO was applied for the treatment of industrial and municipal wastewaters
(Biermann, 1993).
The major application of WAO is the treatment of sewage sludge with
approximately 65 % of the total number of WAO applications. The remaining WAO
instillations are used for spent carbon regeneration (about 10%) and industrial wastewater
treatment (about 25%) (Kolaczkowski et al., 1999).
Numerous studies had been reported on the WAO of distillery waste, cyanide, and
nitrile containing wastes (Daga et al., 1986 and Wilhelmi et al., 1979). A great attention
is focused on the understanding of the oxidation of pure compounds such as phenol and
various carboxylic acids (Joshi et al. 1995).
14
The desire to improve the energy recovery in pulp and paper industry and to
reduce the environmental impact of chemical pulping operations had lead researchers and
engineers to examine the suitability of this technology to treat different pulping effluents.
Oxidation has proceeded on different type of pulping operations. For example, the WAO
has been investigated for the treatment of thermo-mechanical pulping (TMP) and chemithermo-mechanical pulping (CTMP) waste effluents (Kubes et al., 1994).
WAO has been investigated by many with the aim of establishing the pathways of
this process and finding suitable conditions for treatment.
Researchers focused on
decreasing the capital and operating costs due the requirement of high temperature and
pressure. Initial pH has a significant effect on the process but few thought of studying
the effect of pH as an alternative to improve in the overall WAO process of black liquor.
Verenich et al., (2000) aimed to reduce the concentration of organics in
concentrated wastewaters from TMP paper mills. The effects of temperature, pressure,
catalyst and pH were studied and all experiments were performed in a high-pressure
reactor.
Altering pH did influence the rate of oxidation visibly.
COD removal
percentage showed 37% at original pH of 5 (at 150°C and 1MPa of oxygen partial
pressure), which improved visibly when pH was reduced to pH of 2 (by adding sulfuric
acid) reaching COD removal of about 55%. The reason for pH having such a noticeable
effect is not exactly known but it may be attributed to the nature of the free radical
reactions during the process.
Merit and Kallas (2006) investigated the effect of different parameters on lignincontaining water by WAO. The solution contained about 600 mg/L lignin with an initial
COD of about 750-780 mg/L. The experiments were carried at different conditions
(temperature, pressure, and pH).
It was reported that by increasing the reaction
temperature, the oxidation rate of the process eventually increased. After 2 h, the lignin
was detected completely oxidized at the highest temperature (190°C). However, the
effect of temperature on COD reduction was lower, but detectable where about 53% of
the organics were oxidized. The effect of oxygen partial pressure did not result in large
15
changes in the process. The COD removal rate did improve by only 4 % at the highest
pressure (1.5 MPa). The experiments demonstrated that the optimum pH range for the
lignin degradation process is 12-13 (strongly alkaline). Since the focus was to find the
optimum conditions to degrade lignin, results showed that high temperature and pH
improve the process efficiency.
Lignin structure still can be difficult to handle. To look at differences in lignin
structure between species or during pulping operations, chemists have devised chemical
techniques to measure certain functional groups in wood. The major functional groups
determined are methoxyl content, phenolic hydroxyl content, and benzyl alcohol
(hydroxyl group on the alpha carbon) content. Most of researchers used phenol as lignin
model.
Suzuki et al. (2006), however, investigated WAO of lignin model not eliminate
waste but rather to produce acetic acid. In response to depletion of fossil resources, using
biomass as a source of useful chemicals and fuel has been a subject of interest. Lignin
was oxidized in a batch reactor at a temperature of 300 °C, residence time of 10-60
seconds and oxygen supplies of 50-100%. Since the focus was not COD removal, the
results were not reported in term of organic reduction. All the results were in terms of
acetic acid production. It was found that the yield of acetic acid at the highest conditions
was low (about 9%). The reason was that lignin model compounds cannot produce a
large amount of acetic acid attributed to the oxidation of phenol, which forms unsaturated
dicarboxylic acids with 4 carbon atoms that can not produce a large amount of acetic
acid.
Another recent study by Santos et al, 2005 studied the influence of pH on the wet
air oxidation of phenol with a copper catalyst. The results showed that pH is a critical
parameter able to modify the chemical stability of the catalyst, the significance of the
oxidation reaction in the liquid phase and the oxidation route of phenol. Stirred basket
and fixed bed reactors were employed at 140 °C and 16 bar (1.6MPa) of oxygen pressure.
Three initial pH values were used: pH 6 (the pH of phenol solution), pH 3.5 (by adding
sulfuric acid) and pH 8 (by adding Na2CO3). It was found that the major contribution to
16
the phenol conversion reached at acid pH by using solid catalyst was due to the catalyst
activity of the leached copper. The intermediates differed in both conditions (acidic and
basic). At basic pH, the intermediates found to be less toxic than phenol whereas at acid
pH the first intermediates were far more toxic than phenol. For industrial application
satisfying the environment conditions, it has been suggested that phenol oxidation at
basic conditions constituted a very attractive alternative.
A study done on the kinetics of WAO of phenol using Al-Fe pillared clay catalyst
studied the effect of parameters such as pH on the conversion of phenol. Air was used as
the oxidant, and pH was lowered by adding sulfuric acid into the solution to adjust initial
pH value to 3.9-4.0. As shown in their results, phenol removal rate was 2 times greater
than that without adjustment (initial pH about 5). This observation confirms the wellknown fact in phenol chemistry that the optimum pH for the maximum reaction rate is
about 4.0 (Guo and Al-Dahhan, 2003).
Past work done at McGill Pulp and Paper Research Center laboratories on
WAO of benzene examined the effect of pH. It was found that decreasing the pH of the
reaction resulted in a considerable increase in the benzene degradation rate especially at
lower temperature. When the initial pH of benzene was kept at 6, no degradation was
achieved after 5 hours of oxidation time. The initial pH was then reduced from 6 to 4 and
about 98% (at same temperature 220°C) degradation was achieved in 1 hour which
showed a significant improvement (Abussaud, 2004).
17
CHAPTER THREE
EXPERIMENTAL MATERIALS AND METHODS
3.1 WAO Experiments
This chapter focuses primarily on the experimental procedures for WAO
experiments.
Included in this chapter is a description of experimental equipment.
Furthermore, a description of the experimental procedure, experimental plan and the
analytical techniques is given.
3.1.1 WAO Experimental Equipment
WAO experiments were carried out in a 316 stainless-steel reactor (Autoclave
Engineers) with a volume of 1.0 L shown below in Figure (3.1.1). The autoclave reactor
used consists of a high-pressure vessel, a mixing mechanism, and a controller tower.
The reactor (known as EZE-Seal) is composed of a vessel placed vertically and
operates as a batch reactor. Its closure style is designed to provide the ability to operate
at high temperature and pressure. The “loose flange”, placed at the upper part of the
autoclave, allows an easy interchange of vessel. Moreover, the reactor is designed so that
the top cover is held stand and the vessel lifts up in order to be closed. The body lift
mechanism provides a mechanical assist for raising and lowering the body. On the top
head, there exist six openings (with on-off valves): two gas inlets, sparge feed line, blow
pipe, gas sampling, liquid sampling.
18
PI
SI
Rapture Disc/
Atmosphere
TI
Liquid Sample
He
O2
ICE bath/
condense
CONTROLLER
Mixing impeller
REACTION VESSEL/
Autoclave
GAS SUPPLY
HEATING JACKET
(a)
(b)
Figure 3.1.1 – (a) Schematic of experimental reactor (b) a photographic illustration of
EZE-Seal pressurized reactor
19
Once the reactor is sealed, the heating jacket is placed around the vessel where
heat transfer system is used to control the reactor’s temperature.
Temperature is
measured via a type “J” thermocouple inserted through the cap. Then a signal is sent to
the controller tower (CT-1000) where a digital temperature value is shown. The PID
controller holds the temperature within ±2 °C of the set-point. The pressure is measured
by both a Bourdon tube gauge and pressure transducer, which is designed for high
temperature procedures. The signal from the sensor is also sent to a digital display
available on the controller tower. The experimental system incorporates a rapture disk
with a burst pressure of 3,300 Psi as a safety device in case of pressure builds-up in the
reactor during experiments.
The mixing mechanism is done by a Dispersimax turbine-type impeller with
variable speed “Magnedrive” stirrer (mixing speed up to 3300 RPM). This type of
impeller is well suited for gas-liquid reactions since it provides a radial flow, while it
draws the gas down a hollow shaft and disperses it through the impeller for effective
high-speed stirring. Moreover, there exists a cooling system, which provides a means of
cooling the reactor contents by circulating cold water through an internal coil. The
coolant inlet and outlet connections are located on the top cover.
3.1.2 Materials
The weak black liquor used during all the experiments was supplied by the Pulp
and Paper Research Institute of Canada (PAPRICAN). The liquor was produced using a
laboratory scale digester and was stored in 4 L containers, capped with nitrogen and
refrigerated at 3°C prior usage. The black liquor properties are presented in table 3.1.1
below. The total inorganic carbon as seen was low which can be attributed to the purity
of the chemicals used in the generation of the white liquor. In the unit operation, the
white liquor would be produced in the chemical recovery plant and it would contain
variety of inorganic chemicals, such as sodium carbonate, sulfate, thiosulfate and others.
20
The oxidant used is oxygen (99.5% minimum purity compressed cylinder). Due
to the black liquor high organic content, the only feasible method to provide the required
amount of oxygen, and remain below the pressure relief limit during WAO (since
pressure increase as the temperature rises) was to use pure oxygen. Moreover, 99% pure
helium was used during certain experiments. Both gases were obtained from BOC Gases
in Montreal, Canada. In order to lower initial pH, sulfuric acid was used, ACS grade,
Fisher Scientific.
Table 3.1.1- Black Liquor Properties
Type of wood
Soft Wood
pH
12.8
Solid content
15%
Chemical oxygen demand (COD)
114000 mg O2 /L
Total organic carbon (TOC)
59400 ppm
Total inorganic carbon (TIC)
100 ppm
Acetate concentration
1525 ppm
Formate concentration
6130 ppm
Sulfate concentration
1360 ppm
3.1.3 Experimental Procedures
Throughout experimentation, the vessel was filled with 500 ml of black liquor.
Once the reactor was sealed, the auxiliary components were well connected, and heating
jacket was placed, and finally a desired amount of oxygen was sparged in.
The
temperature and the stirrer speed were then adjusted according to the run specifications.
The operating temperature ranged between 185°C to 250°C. The heater’s temperature
had to be at least 20% higher than the operating temperature. The initial pH of the liquor
was set about 12.8, therefore; depending on the required pH, a known amount of sulfuric
acid was added. The values of pH chosen were 10, 6, 4, and 2.
21
The oxygen pressure needed is a function of the organic load, which was
calculated from the initial chemical oxygen demand of the black liquor used. The
amount of excess oxygen was expressed as to pressure (see Appendix A). Excess oxygen
levels of 20, 40, and 60 % were used during experimentation. Calculated values of
oxygen charging pressure, corresponding to excess oxygen are reported in Table 3.1.2.
Table 3.1.2- Oxygen Charging Pressure
Excess Oxygen
(%)
Oxygen Charging
Pressure
(MPa)
(Psi)
20
9.6
1430
40
11.3
1640
60
12.8
1860
Pressure and temperature were recorded at subsequent time intervals. Liquid
samples of approximately 3 mL were collected periodically from a capillary line
immersed in an ice bath. When sampling, a loss of approximately 0.25 to 0.30 MPa gage
(about 35-45 psig) in total system was experienced. The reaction was normally left for an
hour once the set-point was reached (about 24 minutes to reach it).
At the end of each experiment, when room temperature had been reached, the
excess oxygen was depressurized slowly. The vessel was disassembled and cleaned in
order to use it for another experiment. The above-mentioned procedure was repeated at
different temperatures and pressures.
Table 3.1.3 - Ranges of operating parameters
Operating Parameter
Minimum
Maximum
Temperature °C
185
250
Oxygen Charging Pressure (MPa)
9.6
12
Initial pH of Black Liquor
2.0
12.8
22
3.2 Analytical Methods
The main criterion to assess the effectiveness of operating conditions on the
degree of oxidation is organic load reduction. The analytical methods were used to
evaluate the degree of oxidation: the chemical oxygen demands (COD) and the total
organic carbon content (TOC).
Other analytical tests and measurements performed
during experimental program were:
the total inorganic carbon (TIC) content, the
concentration of formate and acetate, and the concentration of sulfur compounds (sulfite
[SO3 2-] and sulfate [SO42-]) using Ion Chromatography (IC).
3.2.1 Chemical Oxygen Demand Measurement
COD measurement is the most commonly used parameter to evaluate the degree
of oxidation during WAO process. Chemical oxygen demand is defined as the amount of
oxygen required for the oxidation of a sample susceptible to oxidation by strong chemical
oxidant. The measurement procedure requires that a sample be oxidized under acidic
conditions, by open or closed refluxing, with a known amount of potassium dichromate.
COD is a two steps process: digestion and determination. The results are expressed in
terms of oxygen equivalence, which could be determined by titration (FAS), colorimeter,
or spectrometer. The closed reflux method (standards 5220 C) was initially chosen to be
used for COD measurements during preliminary experimentation since that method is
suitable for solution with high organic load and is not too costly. The results determined
from this method were not accurate due to dilution ratio (1:1000) and the error margin
was high. Moreover, it was a time consuming method.
After investigation, the spectrometric method was found accessible, where
reacted COD reagent vials analysed using spectrophotometer. UV spectrograph was used
which accommodated the budget, the workload and much faster and more accurate
analytical method. COD was measured at wavelength 600 nm for values between 100
and 900 mg/L (higher values can be diluted). From preliminary experiments, it was
23
found by the quality control that this method is working 95% properly. Therefore, this
analytical method was used for sample analysis.
3.2.2 Total Organic Carbon Measurement
TOC is defined as the total carbon covalently bounded in organic molecules and it
is a parameter commonly used to describe the performance of wet air oxidation. TOC is
independent of the oxidation state of the organic matter and does not measure other
organically bound elements, such as nitrogen and hydrogen, and other inorganics that can
contribute to chemical oxygen demand as measured by COD.
To minimize the
interference of the inorganic content during TOC measurements, the TOC content was
obtained by separately determining the total carbon (TC) and then measuring the TIC
content (the difference between TC and TIC is TOC). The instrument used for these
measurements was the DC-80 Total Carbon Analyzer manufactured by Rosemount
Dohrmann. Prior the measurement, the TOC analyzer had to be calibrated with 2000
mg/L standard of potassium hydrogen phthalate (KHP). For sample injection into the
instrument, a 250uL syringe was used for all the samples.
3.2.3 Ion Chromatography
Ion Chromatography (IC) is an analytical method for liquids used to separate
atomic or molecular ions by the use of ion exchange resins, and it is based on
their interaction with the resin.
IC was used mainly to determine the
concentration of molecular ions for acetate, formate and sulfate.
The IC
instrument used was DX-100 manufactured by Dionex Coporation. Standards
with different concentrations for acetate, formate and sulfate were prepared, then
a calibration curve was plotted. A blank always was injected to check for any
contamination that may be present in the column. Once done, about 2 mL of
diluted sample was injected to the columns.
24
CHAPTER FOUR
RESULTS AND DISCUSSION
WAT AIR OXIDATION OF KRAFT LIQUOR
The experimental work is divided into two parts. The first is the preliminary
experimental part, in which an experimental design method was followed to investigate
the performance of the reactor, the repeatability of the experiments and the interaction
between operating parameters: temperature, oxygen charging pressure, and initial pH of
the solution. The second part contains the experimental results in terms of the influence
of operating parameters on the removal of organics in black liquor. All conclusions are
reported and discussed in this chapter.
4.1 Development of WAO Experiments
4.1.1 Setting the Operating Conditions
At the beginning, a few experiments had to be conducted in order to examine the
behaviour of the new reactor. It was found that desired temperature could be reached
within 30 ± 3 minutes and that the adjustments of the controller settings were only
needed when target temperature was changed to another value. Other factors such as
oxygen charging pressure, volume of the liquor, and mixer speed did not affect the
operation and the performance of the controller.
The temperature profile during WAO is shown in Figure 4.1.1.
Room
temperature fluctuations did affect the time necessary to attain the target temperature.
When the black liquor was at 25 °C, it took approximately 27 minutes to reach the set
temperature; whereas when the liquor was at 17 °C, heating it up took approximately 32
minutes. Once the set temperature was reached, the temperature during the reminder of
the experiment was maintained with ± 2 °C of the set point for an hour.
25
Figure 4.1.1 also shows the pressure profile where set-point temperature was
205°C and charging pressure corresponded to 40% excess oxygen. During the process,
the pressure increased with increasing temperature. Figure 4.1.1 also shows that the
pressure increased during the first 10 minutes was not as sharp as later on. The initial
profile of the pressure was likely due to oxygen consumption during oxidation of easily
oxidizable compounds. It was important to check if this profile was not due to a problem
with pressure transducer; therefore, a blank experiment using distilled water at same
operating conditions was performed.
During this run, a pressure decrease was not
observed in the initial period.
With respect to the trend when the temperature was constant at the latter stage, the
pressure decreased slightly about 4% below the pressure at target temperature. This
decrease can be attributed to the sample collection.
225
16.0
200
15.0
175
14.0
Pressure (MPa)
150
Temperature (C)
125
100
13.0
12.0
75
11.0
50
10.0
25
Temp
Pressure
0
9.0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.1.1 – Temperature and pressure profiles during wet air oxidation of weak black
liquor CODi =105,000 mg O2/L, T= 205° and excess oxygen = 25 %.
26
The overall oxidation process is controlled by two important steps: (i) oxygen
mass transfer from the gas phase to the liquid phase, and (ii) reaction between dissolved
oxygen and black liquor. In order to ensure that the resistance of the first step is
eliminated, the impeller speed was set at specific value. In Figure 4.1.2, it was found the
degradation of organic compounds in black liquor was independent of the mixing rate
above 1000 RPM. Therefore, the mixing speed was fixed for all experiments at this
level.
1.20
500 RPM
1000 RPM
1.00
1500 RPM
CODf / CODi
0.80
0.60
0.40
0.20
0.00
0
10
20
30
40
50
60
80
70
Time (min)
Figure 4.1.2 – Degradation profile of organics in weak black liquor at different impeller
speeds. CODi: 105,000 mg O2/L, T:225°C, pHi: 12.8, and excess oxygen: 40%
Eight experiments were needed for the experimental design in order to minimize
the total number of experiments and to give a good idea about the experimental space. All
the runs were conducted for a 90 minute period. To test repeatability all experiments
were conducted in duplicates under identical conditions.
The ranges of operating
conditions given in Chapter 3, Table 3.1.3, were modified slightly at this stage as seen in
27
Table 4.1.1. The lowest value for initial pH changed to pH 4. The reason is that to
operate at pH 2, harsh conditions, would damage the stainless steel reactor. One reason is
pitting corrosion that would affect the reactor walls. It is so serious that once a pit is
initiated there is a strong tendency for it to continue to grow, even if the majority of the
surrounding is not affected.
The oxygen charging pressure represent the theoretical amount of oxygen plus an
known excess amount of oxygen. For example, 20% excess oxygen is equivalent to 1.2
multiplied by theoretical amount of oxygen required. The range between 20% to 60%
excess oxygen was chosen based on the work done previously on CWAO in our
laboratories. The maximum designed pressure is 20 MPa. The experiments with 60%
excess oxygen would reach 19 MPa during the heating period. Therefore, 60% E.O. was
eliminated to ensure a safe environment. Table 4.1.1 presents the ranges of the operating
conditions.
Table 4.1.1 – Ranges of Operating Conditions
Operating Parameter
Minimum
Maximum
185 °C
250 °C
Initial pH of black liquor
4.0
12.8
Oxygen Charging Pressure (MPa)
9.6
11.0
Temperature
4.2 WAO Experimental Design
The purpose of the exploratory experiments presented in this section was to
examine the performance of the process, the interaction between the operating
parameters, the reliability of the experimental set-up, and repeatability of experimental
results. The experimental parameters investigated were: (i) temperature, (ii) oxygen
charging pressure, and (iii) the initial pH of the solution.
28
The interaction between the parameters have been studied by performing an
experimental design of eight experiments at two levels described by Taguchi (Taguchi
and Konishi, 1987; Taguchi and Yokoyama, 1994), which minimize the total number of
experiments. The response of each experiment was determined in terms of percent COD
removal. The goal of Taguchi’s method is not only to optimize a process, but also reduce
the sensitivity of a design to noise or uncontrollable factors. That moves the design
targets toward the middle of the design space so that any external variation affects the
design’s behaviour as little as possible.
Table 4.2.1 represents the preliminary set of experiments needed to be done.
Values 1 and 2 correspond to lowest and highest of each parameter previously shown in
Table 4.1.1. The COD removal reduction is defined as the ratio of the final over the
initial COD. Based on the method by Taguchi, the following conditions were applied to
obtain a balanced design:
1. Each parameter has to appear 4 times at each level for the whole set of experiments.
2. Two parameters have to meet together at the same combination two times in the
entire set of experiments (as follow 1-1, 1-2, 2-1, 2-2).
Table 4.2.1 – Preliminary Experimental Design
Run No.
T
PO2
pH
P1
1
1
1
P2
1
1
2
P3
1
2
1
P4
1
2
2
P5
2
1
1
P6
2
1
2
P7
2
2
1
P8
2
2
2
29
Table 4.2.2 presents the experimental results of the eight designed experiments
stated earlier in Table 4.2.1. The results show that the percentage error between any
duplicate experiments was always less than 5 %, which gives a good indication that the
experiments are repeatable. The percent reduction of organics in black liquor has been
plotted as a function of the operating parameters in Figure 4.2.1 to Figure 4.2.3 in
accordance to Taguchi method. Each point in those figures was calculated by taking the
average of the two-measured responses where two parameters are set together at the same
level combination according to those in Table 4.2.1. Each figure shows two lines, one
between the averaged response of the parameters at the level combinations (1-1) and (21), and the second line is between (1-2) and (2-2). The lines aid in understanding the
interaction between any parameters while the third one is set at its average level.
Table 4.2.2 – Measured response of preliminary experiments and its duplicates.
Run No.
% COD Reduction
% COD Reduction
P1
84.2
83.9
P2
67.3
68.0
P3
83.8
83.2
P4
69.5
69.9
P5
87.3
88.5
P6
74.3
74.3
P7
86.0
86.5
P8
75.3
74.9
In all the cases shown in the figure, changing the level of any parameter did not
change the influence of the parameter. Also, as seen from the plots, the two lines in each
30
plot always have the same slope, and in most of them were close to be parallel. These
are indications that the interaction between parameters is negligible.
Figure 4.2.1 showed that an increasing temperature resulted in a higher percent
COD reduction because more organics were oxidized. This can be observed at any levels
of O2 or initial pH as can be seen in Figure 4.2.2 and 4.2.3. The oxidation of the organic
compounds was influenced by the initial pH of the solution. A greater reduction in COD
resulted at lower initial pH value at any level of O2 or temperature as shown in Figures
4.2.1 and 4.2.3. On the other hand, the oxygen content in the reactor did not show a
dramatic effect. However, from Figures 4.2.1 and 4.2.2, it can be observed that a lower
amount of oxygen showed a slightly higher COD reduction at any temperature or initial
pH levels meaning that charging the system with a higher amount of excess oxygen does
not necessary lead to better organics degradation.
90
% COD reduction
Av pH = 8.5
[O2]1
80
[O2] 2
70
0.5
1
1.5
Temperature Level
2
2.5
Figure 4.2.1 - Effect of interaction between T vs. [O2] on COD percent reduction of weak
black liquor.
31
90
Tav = 215°C
COD reduction (%)
85
pH 4
80
75
pH 13
70
65
60
0.5
1
1.5
2
2.5
Oxygen Level
Figure 4.4 - Effect of interaction between [O2] vs. initial pH on COD percent reduction
of weak black liquor.
32
90
Av % E.O. = 30%
COD reduction (%)
85
T2
80
T1
75
70
65
60
0.5
1
1.5
pH Level
2
2.5
Figure 4.5 - Effect of interaction between initial pH vs. T on COD percent reduction of
weak black liquor.
Once the first set of experiments gave a clear indication about the interaction of
the operating parameters and the repeatability of the results, it was decided to study the
effect of each parameter individually while keeping the other two parameters constant at
their medium level (average between minimum and maximum levels).
The final
experimental conditions are presented in Table 4.2.3. The temperature effect was studied
at five levels between 185°C and 250°C. The effect of oxygen was investigated at three
levels between 20% to 40% excess oxygen. Lastly, the effect of initial pH was conducted
at five values between 4 and 12.8. Each run was repeated at least once in order to
validate the results.
33
Table 4.2.3 - Final Experimental Design
Run #
Temperature (°C)
1
185
2
205
3
215
4
225
5
250
6
7
% Excess Oxygen
Initial pH
20
12.8
20
225
8
30
8.5
40
9
12.8
10
10
11
225
30
8.5
12
6
13
4
14
15
16
20
205
30
4
40
34
4.3 Reproducibility Assessment
In order to ensure that experimental procedure was repeatable, a reproducibility
assessment was performed. Four replicates were performed at target temperature 225 °C,
charging pressure at 20 % excess oxygen and initial pH of solution at 12.8. In Table
4.3.1, the COD reduction results from the replicate experiments are presented. The
relevant statistics (average, standard deviations, and 95% confidence interval are also
shown. According to the 95% confidence interval, the results of most of experiments fall
in acceptable ranges.
Table 4.3.1 – Reproducibility Assessment: COD Reduction
Sample
10
30
60
90
Run #
95% Confidence
COD
Av COD Red
COD St dev
Reduction (%)
(%)
(%)
9.650
0.666
8.997 ≤ µx < 10.303
64.575
0.818
63.773 ≤ µx < 65.377
71.200
1.095
70.126 ≤ µx < 72.274
77.275
1.156
73.142 ≤ µx < 75.408
1
10.2
2
9.7
3
8.7
4
10
1
64.6
2
65.7
3
63.8
4
64.2
1
71
2
70.6
3
70.4
4
72.8
1
74.3
2
73.5
3
73.4
4
75.9
Interval
(%)
35
Various errors contributed to the observed variance in the experimental results.
The most significant source of error is the starting temperature; the time to-target
temperature increased as the starting temperature was reduced, and vice-versa. A major
source of error would be the black liquor.
Due to the nature of the solution, the
consistency of black liquor might not be the same for each run. Examples of less
significant source of errors are the volume of black liquor and the amount of oxygen
sparged to the reactor. The errors of the volume of black liquor would affect the initial
organic load measurement, and eventually the theoretical amount of oxygen needed.
4.4 Effect of Oxygen Charging Pressure
Based on the results obtained from the preliminary experimental phase, to
evaluate the effect of oxygen charging pressure, three excess oxygen levels were
investigated: 20%, 30% and 40%. During these experiments, the target temperature was
fixed at 225 °C and pHi 12.8. Multiple sample experiments were performed at both
maximum and minimum excess oxygen values.
Each run took 90 minutes which
includes the heating-up period. The COD and TOC reductions as a function of oxygen
partial pressure are presented in Figures 4.4.1 and 4.4.2.
These two Figures, show that increasing the initial oxygen charging pressure
improved slightly the degree of oxidation at the first thirty minutes, which is the heatingup period. As the reaction proceeded to 90 minute, there was not any improvement
noticed for the different charging pressure used. For the samples recovered immediately
upon reaching target temperature (225 °C), there was not a significant improvement in
the degree of oxidation when the excess oxygen was increased from 20 % to 40% (i.e. the
increase in the degree of the oxidation was negligible considering the quantity of oxygen
used).
36
1.2
20% E.O. (9.6 MPa)
40% E.O. (11.0 MPa)
1
CODf/CODI
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
100
Time (min)
Figure 4.4.1 - The effect of oxygen charging pressure on the residual COD of weak black
liquor during WAO. CODi: 105,000 mg O2/L, target temperature: 225°C, and pHi:12.8.
Figure 4.4.2 shows the effect of charging pressure on TOC residual at 225°C and
pHi 12.8. The results gave similar pattern to that of COD residual where again a higher
oxygen loading did achieve the same degree of oxidation as the lower value at the end of
the reaction. Moreover, during the heating-up period, organic carbon reduction was
slightly higher at higher oxygen charging pressure. Excess oxygen means more oxygen
found in the reactor more precisely at the liquid phase, which means more oxygen
available to degrade more organics found in the black liquor. The above explains the
behaviour at the beginning of the WAO reaction. Since results between 20 % and 40%
excess oxygen did not give an appreciable difference in term of organic compounds
degradation, experiments at 30 % excess oxygen were not performed since the objective
of this research is to find optimum conditions.
37
1
20 % E.O.(9.6 Mpa)
40% E.O.(11.0 Mpa)
0.8
TOCf/TOCi
0.6
0.4
0.2
0
0
10
20
30
40
50
Time (min)
60
70
80
90
Figure 4.4.2 - The effect of oxygen charging pressure on the residual TOC of weak black
liquor during WAO. CODi: 105,000 mg O2/L, target temperature: 225°C and pHi: 12.8.
To ensure the conclusions obtained from previous set of experiments about
oxygen charging pressure were reasonable, experiments at 205°C at both 20% and 40 %
excess oxygen were performed. Figures 4.4.3 and 4.4.4 show the effect of oxygen
charging pressure on COD and TOC residuals of black liquor during WAO at 205° C and
pHi 12.8. Similar profiles were obtained for both oxygen charging pressure values when
the target temperature was 225°C. The profile again showed slightly a better oxidation
of black liquor at higher oxygen charging pressure during the thirty minutes heating-up.
38
1.2
20% E.O. (9.6 MPa)
1
40% E.O. (11.0 MPa)
CODf/CODI
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
90
100
Time (min)
Figure 4.4.3 - The effect of oxygen charging pressure on the residual COD of
weak black liquor during WAO. CODi: 105,000 mg O2/L at T= 205°C and pHi
=12.8.
1
20 % E.O.(9.6 Mpa)
40% E.O.(11.0 Mpa)
0.8
TOCf /TOCi
0.6
0.4
0.2
0
0
10
20
30
40
50
Time (min)
60
70
80
90
Figure 4.4.4 The effect of oxygen charging pressure on the residual TOC of weak
black liquor during WAO. CODi: 105,000 mg O2/L at T= 205°C and pHi =12.8.
39
4.5 Effect of Temperature
As mentioned earlier in the chapter, experiments were carried out at five different
target temperatures: 185°C, 205°C, 215 °C, 225°C, and 225°C. Based on the results from
the previous section, the oxygen charging pressure was chosen to be at 20%. The reason
is that from the previous section it was found out that between the highest and the lowest
excess pressure, the overall degradation of black liquor was similar. Since the objective
of this section is to examine the effect of the temperature on WAO, 20% excess oxygen
presents a reasonable choice.
Figure 4.5.1, shows the effect of temperature on the residual COD as a function of
reaction time. The results shown in this figure are the averages of duplicate experiments.
The results showed that as the target temperature increases, a higher degradation of black
liquor occurs.
The pattern of reaction for each target temperature illustrated that
hydroxyl radicals formed and reacted as soon as they accumulated.
The reaction
proceeded quickly since reactor was supplied with excess oxygen. After sixty minutes,
most of the reactants were oxidized which is shown with a plateau.
From Figure 4.5.1, it is seen that WAO at 205°C and 215°C showed 72%
degradation of black liquor followed by 225°C, which gave about 76% degradation.
Target temperature 250°C gave the most in term of degree of oxidation, which is about
78% degradation of weak black.
However, at 225°C, a 25 °C decrease of temperature
led to about 76% degradation as mentioned earlier, which is only 2 % lower than at
250°C. The 2 % is not considered significant value. Therefore, WAO experiment at
225°C did show the most significant improvement compared to all other values. At 50
minutes, 72 % COD reduction, which is equal to that obtained during the WAO
experiments performed at 215°C and 205°C after 90 minutes. Finally, as seen 185°C was
the lowest temperature, hence, the lowest degradation rate, which was only 67 % COD
reduction.
40
1.00
at 185°C
at 205°C
0.80
at 215°C
at 225°C
CODf/CODi
0.60
at 250°C
0.40
0.20
0.00
0
10
20
30
40
50
Time (min)
60
70
80
90
Figure 4.5.1 - The effect of target temperature on the residual COD of weak black
liquor CODi: 105,000 mg O2/L, excess oxygen: 20 % E.O., and pHi =12.8.
4.6 Effect of Initial Solution pH
The initial pH of the black liquor used for this set of experiments was at pH 12.8.
To adjust the initial pH to the four values reported earlier, sulfuric acid was used.
It is been noticed that pH of the solution fluctuated during WAO. The pH profile,
presented in Figure 4.6.1, shows the variation of pH for two different initial pH values:
pH 12.8 (the original pH of the liquor) and pH 4. In the case of pHi 12.8, pH reached its
minimum value at approximately 30 minutes and as the reaction progressed, pH value
increased. The decrease in the pH of the solution can be explained by the generation of
organic acids as a result of the oxidation of organic compounds at the beginning of the
reaction.
As the experiment proceeded, these larger acids oxidized to low molecular
41
weight acids (LMWA) at thirty minutes. These LMWA such as acetic acid were not
destroyed in the solution, which is reflected by constant pH.
To lower the initial pH of the solution to pH 4, a known amount of sulfuric acid
was added prior to the start of the reaction. Since the original solution started in an acidic
range, organic acids generated during WAO reaction did not influence greatly the overall
pH value of the solution, which is reflected by the very small variation in pH line in
Figure 4.6.1.
Initial pH 10 showed similar profile as pH 12.8, where again the pH of the
solution reaches a minimum value and then remained constant as the acetic acid
accumulated in the system.
14
pH 4
12
pH 12.8
10
pH
8
6
4
2
HEAT-UP Period
CONSTANT TEMPERATURE Period
0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.6.1 - pH profile during WAO at two different values. CODi: 105,000 mg O2/L,
excess oxygen 20 % and target temperature 225°C.
The initial pH of the solution was investigated at four values: pH 4, pH 6, pH 10
and pH 12.8. Figure 4.6.2 reports COD residuals values as a function of initial solution
42
pH variations. The experiments were all done at target temperature 225°C and 20%
excess oxygen corresponds to 9.6 MPa. It is noticeable that lowering the initial pH of the
solution affected remarkably the degradation of organic compounds during heating period
to about 2 to 3 times greater than that without pH adjustment (pH 12.8). For example, the
CODf/CODi at pH 12.8 and pH 4 were 0.72 and 0.31 respectably, where at pH 4 the value
was about 2.5 times lower than that at pH 12.8. Moreover, it is found that about 88 % of
organics were degraded after 90 minutes at pHi 4, which shows a significant amount
compared to 74% COD reduction at pHi 12.8.
1.0
0.9
pH12.8
0.8
pH10
pH6
CODf/CODi
0.7
pH 4
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
10
20
30
40
Time (min)
50
60
70
80
90
Figure 4.6.2 – Effect of initial solution pH on the residual COD of weak black liquor
during WAO. CODi: 105,000 mg O2/L, excess oxygen 20 % and target temperature
225°C.
This behaviour is probably because at lower pH, the reaction mechanism changes
and eventually affects the formation of hydroxyl radicals which are easily oxidized. In
43
Chapter 2, an overview of WAO reaction mechanism was given and reported from
literature reviews. Hence, at low pH, there is a possibility of a faster formation of more
degradable compounds at the early stage of the reaction.
1.00
pH 4
pH 6
0.80
pH 10
pH 12.8
TOCf/TOCi
0.60
0.40
0.20
0.00
0
10
20
30
40
Time (min)
50
60
70
80
90
Figure 4.6.3-- The effect of initial solution pH on the residual TOC of weak black liquor
during WAO. CODi: 105,000 mg O2/L, TOCi: 59400, target temperature: 225°C, and
excess oxygen: 20 %.
Figure 4.6.3 represents TOC residuals during WAO also at different initial pH
values at target temperature 225 °C and 20% excess oxygen. Overall profile again shows
how lowering the pH of the solution gave a better organic carbon degradation It shows
again that at pH 4, about 80% TOC reduction occurred after 90 minutes of reaction
whereas about 60% TOC reduction occurred without adjusting the initial pH.
The
influence of initial pH of the solution on the degradation of organics obtained is in a good
agreement with those reported in literature.
It is been indicated that lowering pH
44
increases free radical formations i.e. organic acids were both produced and degraded
much faster than at higher pH value.
4.7 Effect of Operating Conditions on Intermediate Products
From above figures, it was apparent that degree of degradation varied according
to the changes in the operating conditions: oxygen charging pressure, target temperature
and initial pH of the solution.
According to the analysis used, the by products found were acetate, formate and
oxalate. Varying the operating conditions of the reaction did affect the formation of byproducts. Generally, it was found that as the time of reaction increased, accumulation of
acetate occurred, implying that WAO reaction did not go to completion. Unlike acetate,
oxalate and formate were initially formed and then oxidized as reaction time increased.
Figures 4.7.1 and 4.7.2 show the effect of charging pressure on the formation of
the by-products. All runs were done at 225 °C and initial pH of 12.8. When a higher
amount of oxygen was present in the reactor, a larger amount of by-products were
formed. The higher amount of oxygen enhanced the degradation of black liquor and
resulted of course in a higher production of acids. Moreover, oxalate and formate were
not completely oxidized at the end of the reaction time. The presence of oxalate as an
intermediate could be caused by the excessive amount of oxygen found in the reactor
when initial pH of the solution was not lowered. It is been noticeable that at the pH 4,
oxalate only appeared at the first twenty minutes then disappeared completely.
Figures 4.7.3 to 4.7.5 illustrate the concentrations of acetate, formate, and oxalate
at different initial pH. In all cases, the concentration of acetate increased as reaction time
increased and remained constant indicating that this anion is difficult to oxidize by WAO.
The accumulation of acetate is more pronounced at the two high pH values (pH 10 and
pH 12.8). Regarding formate, Figure 4.7.4 shows that the decrease in concentration
started immediately upon the addition of the acid to lower the pH of black liquor from
12.8 to 4. Subsequently, a decrease in the formate concentration is observed at the latter
45
stage of the reaction. In contrast, there was not any oxalate in the original black liquor.
At high pH, oxalate is produced very quickly as shown in Figure 4.7.5, as reaction
continued oxalate concentration decreased slightly. At pH 4, however, the concentration
of oxalate was very low and finally it was reduced to zero as soon as reaction reached
target temperature 225 °C.
12000
Acetate Concentration (mg/L)
10000
8000
6000
4000
40% E.O.
2000
20 % E.O.
0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.7.1 - Acetate concentrations as a function of oxygen charging pressure.
CODi: 105,000 mg O2/L, target temperature: 225°C, and initial pH of the
solution: pH 12.8.
46
Formate Concentration (mg/L)
7000
6500
40% E.O.
6000
20% E.O.
5500
5000
4500
4000
3500
3000
2500
2000
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.7.2- Formate concentrations as a function of oxygen charging pressure.
CODi: 105,000 mg O2/L, target temperature: 225°C, and initial pH of the solution: pH
12.8.
14000
pH 4
12000
Acetate Concentration (mg/L)
pH 10
pH 12.8
10000
8000
6000
4000
2000
0
0
10
20
30
40
Time (min)
50
60
70
80
90
Figure 4.7.3 - Acetate concentrations as a function of initial pH of solution. CODi:
105,000 mg O2/L, target temperature: 225°C, and excess oxygen: 20 %.
47
5000
4500
Formate Concentration (mg/L)
4000
3500
3000
2500
2000
1500
pH 4
1000
pH 10
pH 12.8
500
0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.7.4- Formate concentrations as a function of initial pH of solution.
CODi: 105,000 mg O2/L, target temperature: 225°C, and excess oxygen: 20 %.
2500
pH 12.8
2000
Oxalate Concentration (mg/L)
pH 4.0
1500
1000
500
0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 4.7.5- Oxalate concentrations as a function of initial pH of solution.
CODi: 105,000 mg O2/L, target temperature: 225°C, and excess oxygen: 20 %.
48
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
From this research, it was concluded that the degradation of organics in Kraft
black liquors depends on the operating conditions, which were temperature, oxygen
charging pressure, and initial pH of the solution.
It was found that at fixed temperature, the lower the initial pH of the solution, the
faster the degradation of organics due to the influence of pH on the type of free radical
reactions and stability of intermediates. The main intermediates were found to be acetate,
formate, and oxalate. Acetic acid was the most stable product and was not oxidized
completely.
The optimum conditions for black liquor degradation were found to be 225oC and
oxygen partial pressure of 9.8 MPa in the pH range of 4, which gave a total degradation
of 88% after 90 minute. Moreover, at these conditions, at 50 minute, the reduction of
organic load is similar to the value resulted after 90 minute at Ph 12.8, which was 74 %
COD reduction. Finally, the earlier stage of the reaction, during the heating-up period,
did show great improvement when original pH was lowered to pH 4, which is a great
indication that the behaviour of the reaction changed, so the free radical reaction.
5.2 Recommendations for future work
For further work, it is really recommended to use the High Pressure Liquid
Chromatography (HPLC) to analyze the samples that were periodically collected to
identify if other intermediates were produced in addition to those reported in this
research.
49
Investigate other possible strategies to improve the degradation of organics such
as study the effect of adding a catalyst or free radical initiator at the lower initial pH. It is
recommended to try different additives, which might enhance the black liquor
degradation such as hydroquinone. Moreover, it is been known that stainless steel walls
do have an effect on free radical reactions, therefore, adding a Pyrex liner to the reactor
could possibly eliminate the above effect. Finally, try another oxidizing agent other than
oxygen could be ozone or hydrogen peroxide.
50
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54
Appendix A
As oxidant, pure oxygen was used throughout the experiments. As mentioned, oxygen
loading is based on the organic load as determined by the chemical oxygen demand of the
kraft black liquor COD [i].
The theoretical mass of oxygen required can be calculated as follow:
Mo = COD [i] × V
Where COD
i
is expressed in mg O2/L and V is the volume of black liquor used in the
experiment.
The specific volume of oxygen must be determined so that the ideal gas law can be used
to calculate the required amount of oxygen
vo =
Vb − V
Mo
Where, vo is expressed in units of L/mg O2 and Vb is the volume of the reactor 1.0 L.
Once the specific volume is determined, the theoretical oxygen chargig pressure (P to) can
be computed
Pto =
R × Troom
vo
where Pto is expressed in MPa, R is the gas constant, i.e., 0.008315 (MPa.L/g-molK), and
T room is the room temperature in degree Kelvin.
55
The oxygen charging pressure is then determined as follows:
⎛ ⎛ E.O. ⎞ ⎞
Pco = Pto × ⎜⎜1 + ⎜
⎟ ⎟⎟
⎝ ⎝ 100 ⎠ ⎠
where Pco is also expressed in MPa, and the degree of excess oxygen (E.O) is expressed
as a percentage. For this thesis, the percentages 20, 40, 60% will be used.
Sample calculation:
•
COD I = 104,930 mg O2/L
•
V = 0.5 L
•
Vb = 1.0 L
•
Troom = 17°C (290.15 K)
i) Mo = CODi * V = 104,930 mg O2/ L * 0.5 L= 52465 mg O2
ii) vo = (1-0.5)L / 52465 * 1000mg O2/ 1 g O2 * 32g O2 /1 g-mmol O2
= 0.29 L/g-mol O2
iii)
P O2 = (R*T)/ vo = 8.042 MPa
iv)
Finally, the actual value of charging pressure assuming E.O. 40%
Pco = Pto (1+(40/100)) = 11.3 MPa
56