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

LIST OF FIGURES

vi

ix

LIST OF TABLES

LIST OF ABBERVIATIONS

x

xi

CHAPTER ONE

INTRODUCTION

1.1 General Introduction 2

1.2 Kraft Pulping and Recovery 3

1.3 Objectives

1.4 Thesis Structure

4

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

2.5 Previous Work

12

vi

CHAPTER THREE

EXPERIMENTAL MATERIALS AND METHODS

3.1 WAO Experimentation

3.1.1 WAO Experimental Equipments

3.1.2 Materials

15

3.1.3 Experimental Procedures 18

3.2 Analytical Methods 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 25

4.1.1 Setting the Operating Conditions 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

DCO

LMWO

TIC

TOC

TRS

WAO

Chemical oxygen demand Demande chimique en oxygène Low molecular weight organics Total inorganic carbon Total organic carbon Total reduced sulfur 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.

Liquor and Cooked Chips

Weak Black Liquor

Heavy Black Liquor

Green Liquor

Na2CO3 + Na2S

Lime

PULPWASHING

EVAPORATION

CAUSTICIZING

COMBUSTION

PULPING

CALCINATION

White Liquor

NaOH + Na2S

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

pluggin

r of the spent liquor before evaporation and

incineration (see Figure 1.2.2). Therefore, a recovery-limited mill could theoretically

increas

reduced sulfur (TRS) emissions such as hydrogen sulfide, methyl mercaptan, and

dimethylsulfide, which would otherwise be produced in the recovery boiler (Hupa, 199).

g 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 remainde

e 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

Liquor and Cooked Chips

Weak Black Liquor

Heavy Black Liquor

Green Liquor

Na2CO3 + Na2S

Lime

PULP WASHING

EVAPORATION

CAUSTICIZING

COMBUSTION

PULPING

CALCINATION

White Liquor

NaOH + Na2S

WAO

4

Figure

ecovery boiler

would offset the thermal losses in the recovery boiler due to the increased evaporator

AO, it is possible to recover inorganic

rtain reaction by-products such as acetic and formic acid.

tial pH of the solution on the reaction mechanism. The

secondary objective of this research is to characterize the improvement in the degree of

he experimental operating conditions such as temperature, oxygen

harging pressure, and initial pH.

er, the final experimental results are presented and discussed. This

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 r

duty and sulfur reduction. Also, by using W

chemicals and ce

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 ini

oxidation by varying t

c

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. Moreov

5

includes the influence of each p al efficiencies, and by-product

production. Fin ommendations

for future work.

apter contain a brief overview of wet air oxidation process and its reaction

echanisms. Moreover, the implication of the process parameters, such as temperature,

pressure, pH, oxygen solubility, and its diffusion in high pressure system are discussed in

-370 °C).

order to keep the reaction in the liquid phase, high pressures are required (2-20 MPa)

(Zimm

bon dioxide (CO2), water (H2O), ammonia (for nitrogen

containing waste), sulfate (for sulfur containing wastes). Otherwise, other products

arameter on the remov

ally, Chapter 5 comprises the general conclusions and rec

CHAPTER TWO

BACKGROUND & LITERATURE REVIEW

This ch

m

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

In

ermann, 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 car

6

would result such halogen acids (for halogenated wastes), and low molecular weight

organic compounds (LMWO).

bstitute for the purchased fuel. Flynn et al.

979) reported that by applying WAO properly about 80 % thermal efficiency can be

achieve

ns varying between 10 ppm-1000ppm;

owever, variety of technologies could be applied to control them such as scrubbing, etc.

Since

ary to remove the catalyst from the oxidized effluent streams because its

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 su

(1

d. 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 concentratio

h

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 necess

7

A + O2 C

B + O2

pathway 1

k2 k3 pathway 3

uling. Moreover, the catalysts are costly chemicals;

erefore, the recovery of these materials is recommended to turn this process

iates and further oxidized to oxidation end

roducts. Therefore, all effluents treated by WAO can be divided into three groups:

itial and unstable intermediates (A), refectory intermediates (B) (e.g. acetic acid), and

xidation end products (C) (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

action rate can be assumed as follow:

CO2, L is the oxygen concentration in the bulk liquid, and m and n are the

rders of reaction. The values for the reaction orders were reported to be first order for

presence would cause catalyst fo

th

economical.

2.3 WAO Reaction Mechanism

The reaction mechanism can be described by a simple reaction scheme. Organic

compounds are oxidized to unstable intermed

p

In

o

k1

pathway 2

Figure 2.3.1 – Schematic of reaction pathways (from Li et al., 1991)

re

n

Lom

pRTE

r CCAer ][][ ,)/(

2∗= − (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,

o

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

reaks down to produce free radicals. The free radicals eventually can be initiated by the

d. These reaction steps reported from Li et

l., 1991.

(2.3.3)

Furthermore, hy th oxygen forming hydroxyl radicals:

yl radicals, which

basically means that the am n is reduced. The organic radical formed

reacts with oxyge ical (ROO-) which then reacts with

the organic compound to abstract another hydrogen from the organic.

(2.3.6)

he hydroperoxide formed (ROOH) is known to be unstable and it decomposes further to

rm intermediates with lower carbon numbers known as low molecular weight acids

ch as formic and acetic acids are obtained.

b

reaction of oxygen with the weakest O-H bon

a

•• +→+− 22 HOROHR (2.3.2)

222 OHRHOHR +→+− ••

drogen peroxide will react wi

•→+ 2222 2HOOOH (2.3.4) •→ HOOH 222 (2.3.5)

The oxidation of organics continues with hydrogen reduction by hydrox

ount of hydroge

n to produce an organic peroxy rad

OHRHOHR 2+→+− ••

•• →+ ROOOR 2 (2.3.7) •• +→+− RROOHROOHR (2.3.8)

T

fo

su

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

erts. Although high temperatures are required, the operating temperature should remain

below

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

the following equation:

here, 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).

in

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

in

)( ,*

22 LOOLm CCakr −∗∗= (2.4.1)

W

10

ature range

he 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).

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 temper

(t

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 chemi-

thermo-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 lignin-

containing 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 well-

known 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

CONTROLLER

SI

O2 He

REACTION VESSEL/ Autoclave

GAS SUPPLY HEATING JACKET

Liquid Sample

Rapture Disc/ Atmosphere

PI

TI

ICE bath/ condense

Mixing impeller

(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 [SO4

2-]) 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.

0

25

50

75

100

125

150

175

200

225

0 10 20 30 40 50 60 70 80 90Time (min)

Tem

pera

ture

(C)

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

Pres

sure

(MPa

)

Temp Pressure

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.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 10 20 30 40 50 60 70 8Time (min)

CO

Df /

CO

Di

0

500 RPM

1000 RPM

1500 RPM

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

Temperature 185 °C 250 °C

Initial pH of black liquor 4.0 12.8

Oxygen Charging Pressure (MPa) 9.6 11.0

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 (2-

1), 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.

70

80

90

0.5 1 1.5 2 2.5Temperature Level

% C

OD

red

uctio

n

[O2] 2

[O2]1

Av pH = 8.5

Figure 4.2.1 - Effect of interaction between T vs. [O2] on COD percent reduction of weak black liquor.

31

60

65

70

75

80

85

90

0.5 1 1.5 2 2.5Oxygen Level

CO

D r

educ

tion

(%)

Tav = 215°C

pH 4

pH 13

Figure 4.4 - Effect of interaction between [O2] vs. initial pH on COD percent reduction of weak black liquor.

32

60

65

70

75

80

85

90

0.5 1 1.5 2 2.5pH Level

CO

D r

educ

tion

(%) T2

T1

Av % E.O. = 30%

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) % Excess Oxygen Initial pH

1

2

3

4

5

185

205

215

225

250

20 12.8

6

7

8

225

20

30

40

8.5

9

10

11

12

13

225 30

12.8

10

8.5

6

4

14

15

16

205

20

30

40

4

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 Run # COD

Reduction (%)

Av COD Red

(%)

COD St dev

(%)

95% Confidence

Interval

(%)

10

1

2

3

4

10.2

9.7

8.7

10

9.650 0.666 8.997 ≤ µx < 10.303

30

1

2

3

4

64.6

65.7

63.8

64.2

64.575 0.818 63.773 ≤ µx < 65.377

60

1

2

3

4

71

70.6

70.4

72.8

71.200 1.095 70.126 ≤ µx < 72.274

90

1

2

3

4

74.3

73.5

73.4

75.9

77.275 1.156 73.142 ≤ µx < 75.408

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 heating-

up 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

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100Time (min)

CODf

/COD

I 20% E.O. (9.6 MPa)

40% E.O. (11.0 MPa)

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

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80 90Time (min)

TOC

f/TO

Ci

20 % E.O.(9.6 Mpa)

40% E.O.(11.0 Mpa)

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

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

Time (min)

CO

Df/C

OD

I 20% E.O. (9.6 MPa)

40% E.O. (11.0 MPa)

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.

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80 90Time (min)

TOC

f /TO

Ci

20 % E.O.(9.6 Mpa)

40% E.O.(11.0 Mpa)

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

0.00

0.20

0.40

0.60

0.80

1.00

0 10 20 30 40 50 60 70 80 9Time (min)

CO

Df/C

OD

i

0

at 185°C

at 205°C

at 215°C

at 225°C

at 250°C

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.

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90Time (min)

pH

pH 4

pH 12.8

HEAT-UP Period CONSTANT TEMPERATURE Period

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.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70 80Time (min)

CO

Df/C

OD

i

90

pH12.8

pH10

pH6

pH 4

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.

0.00

0.20

0.40

0.60

0.80

1.00

0 10 20 30 40 50 60 70 80 9Time (min)

TO

Cf/T

OC

i

0

pH 4

pH 6

pH 10

pH 12.8

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 by-

products. 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.

0

2000

4000

6000

8000

10000

12000

0 10 20 30 40 50 60 70 80Time (min)

Ace

tate

Con

cent

ratio

n (m

g/L

)

90

40% E.O.

20 % E.O.

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

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

0 10 20 30 40 50 60 70 80 9Time (min)

Form

ate

Con

cent

ratio

n (m

g/L

)

0

40% E.O.

20% E.O.

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.

0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60 70 80 9Time (min)

Ace

tate

Con

cent

ratio

n (m

g/L

)

0

pH 4

pH 10

pH 12.8

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

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 10 20 30 40 50 60 70 80 9Time (min)

Form

ate

Con

cent

ratio

n (m

g/L

)

0

pH 4

pH 10

pH 12.8

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 %.

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60 70 80 9Time (min)

Oxa

late

Con

cent

ratio

n (m

g/L

)

0

pH 12.8

pH 4.0

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

REFERENCES

Abussaud, B., Kubes G., Nilgun Ulkem, Berk, D. “Wet Air Oxidation of Benzene”.

Accepted to be published by Ind. Eng. Chem (2008)

Bhargava, S.K.; Tardio, J.; Prasad, J.; Foger, K., Akolekar, D.B.; and Grocott, S.C.

“Wet Oxidation and Catalytic Wet Oxidation”, Ind. Eng. Chem. Res. 45 (4), 1221

(2006)

Biermann, C. J. “Essentials of Pulping and Papermaking”, San Diego Academic

Press, Inc (1993)

Broden, A.; Simonson, R.; and Sven, “Solubility of Oxygen. Part 2 Solubility of

Oxygen in Sodium Hydrogen Carbonate and Sodium Hydroxide Solution at

Temperatures <150 Cand Pressures < 5MPa” Papperstidn 81 (16), 487-491 (1978)

Collyer, M.J.; Kubes, G. J.; and Berk, D. “Catalytic Wet Oxidation of

Thermomechanical Pulping Sludge”, J. Pulp Pap. Sci., 23, J522 (1997)

Daga, N. S., Prassad, C.V.S., and Joshi, J. B. “Kinetics of Hydrolysis and Wet Air

Oxidation of Alchol Distillery Waste” Indian. Chem. Eng., 28(4), 22-31 (1986)

Duarte D., “Catalytic Wet Air Oxidation of Spent Kraft Pulpimg Liquors” Master

Thesis, McGill University (2004)

Flynn, B.L., and Flemmington,W. “Wet Air Oxidation of Waste Streams”. Chem Eng

Prog., 75(4), p 67-69 (1979)

Grace T., Overview of Kraft Recovery, Pulp and Paper Manufacture, Alkaline

Pulping, Montreal: Joint Textbook Committee of the Paper Industry, (vol5), 473-476

(1989)

51

Guo, J., and Al-Dahhan, M., ”Kinetics of Wet Air Oxidation of Phenol over a Novel

Catalyst”, Ind. Eng. Res. 42, p 5473-5481 (2003)

Harmsen, J.M.A, Jelemensky, L., Van Andel-Scheffer, P.J.M., Kuster, B.F.M., and

Marin, G.B. “Kinetic modeling for wet air oxidation of formic acid on carbon

supported platinum catalyst”, Elsevier Science.,165, p 499-509 (1997)

Hupa, M. “Black Liquor Droplet Burning Processes”. In Adams, T.N.(Ed.), Kraft

recovery Boiler, Atlanta: TAPPI Press, 129-160 (1997)

Joshi, J. B., Mishra, V.S., and Mahajani,V.V. “Wet Air Oxidation (Review)” Ind.

Eng. Chem. Res., 34, 2-48 (1995)

Kolaczkowski, S.T.; Plucinski, P.; Beltran,F.J.; Rivas,F. J.; and McLaurgh, D.B.,

“Wet Air Oxidation: A Review of Process Technologies and Aspects in Reactor

Design”, Chem. Eng. J., 73 (2), 143-160 (1999)

Kolaczkowski, S.T.; Beltran , F.J.; McLaurgh, D.B., “Wet Air Oxidation of Phenol:

Factors That May Influence Global Kinetics”, Trans IChemE. 75 part B 257-265

(1997)

Kubes,G. J.; Lunn, R.; and Keskin-Schneider; A. “Wet Air Oxidation of Waste

Effluents from Thermomechanical Pulping”, Published by Paperican miscellaneous

report MR-285 (1994).

Li, L; Chen, P.; and Gloyna, E.F.’ “Generalized Kinetic Model for Wet Air

Oxidation of Organic Compounds”, AICh E., J., 37 (11), 1687-1697 (1991)

Merit K.; and Kallas J., “Degredation of Lignin by Wet Oxidation: Model Water

Solution”, Proc. Acad. Sci. Chem, 55, 132-144 (2006)

52

Miguelez, J.R.; Lopez Bernal; J., Sanz, E.; and Martinez de la Ossa, E., “ Kinetics of

Wet Air Oxidation of Phenol”, Chemical Engineering Journal, 67, 115-121 (1997)

Pray, H.A.; Schweickert, CE.; Minnich, B.H., “Solubility of Hydrogen, Oxygen,

Nitrogen and Helium in Water at Elevated Temperatures”, Ind. Chem. Eng., 44,

1146-1151 (1952)

Santos, A.; Yustos, P.; and Quintanilla, A.,“ Influence of pH on the Wet Oxidation of

Phenol with Copper Catalyst”, Topics in Catalyst, 33, 181-191 (2005)

Smook G.A., “Handbook for Pulp and Paper Technologists”, 2nd edition, Angus

Wilde Publications (1992)

Standard Methods Committee for the Examination of Water and Waste Water,

Chemical Oxygen Demand, Closed Reflux: Titrimetric Method, standard (5220.C)

(1998)

Suzuki, H.; Cao, J.; Jin, F.; and Enomoto, H. “Wet Oxidation of Lignin Model

Compounds and Acetic Acid Production”, Mater Sci, 41, 1591-1597 (2006)

Taguchi, G., “Systenm of Experimental Design: Engineering Methods to Optimize

Quality and Minimize Costs”, Volumes 1 & 2, UNIPUB/Kraus International

Publications, White Plains, NY (1987)

Taguchi, G.; and Konishi, S., “Taguchi Methods: Orthogonal Arrays and Linear

Graphs”, American Supplier Institute, Michigan (1987)

Tromans D., “Temperature and pressure dependent solubility of oxygen in water: a

thermodynamic analysis”, Hydrommetallurgy, 48, 327-342 (1998)

Verenich, S.; Laari A.; and Kallas, J. “Wet Air Oxidation of Concentrated

Wastewaters of Pulp Mills for Water Cycle”, Waste Management, 20, 287-293 (2000)

53

Wilhelmi, A. and Knopp, P., “Wet Air Oxidation: Alternative to Incineration”, Chem.

Eng. Prog, 60 (8), 46-52 (1979)

Zimmermann, F.J. and Diddams, D.G., “The Zimmerman Process and its

Applications in the Pulp and Paper Industry” Tappi, 43 (8): 710-715 (1960)

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

o

bo M

VVv

−=

Where, is expressed in units of L/mg Oov 2 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

o

roomto v

TRP

×=

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:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛+×=

100..1 OEPP toco

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