Application of Surfactants

Low molecular-weight ethoxy-based surfactants are able to accelerate oxygen

delignification of softwood kraft pulps [26]. The extent of lignin removal was

increased from 45% to 55% when 1wt.% of surfactant (15-S-5) was added to medium

consistency oxygen delignification (110 °C, 690 kPa, 60 min) using a commercial

softwood kraft pulp, kappa number 22. The study revealed that the

improved delignification efficiency can be explained rather by an accelerated

chemical reaction rate than by an increased diffusion rate. It can be assumed that

the presence of surfactants increases both the solubility of lignin and oxygen in

the liquid phase, thus increasing the intrinsic delignification rate.

7.3 Oxygen Delignification 687

7.3.4

A Model to Predict Industrial Oxygen Delignification [27]

Industrial oxygen delignification is reported to be less efficient as compared to laboratory

oxygen delignification. A study provided by Rewatkar and Bennington

revealed that industrial oxygen delignification systems operate, on average, at

about 20% below their potential [28]. The impaired efficiency of oxygen delignification

can be attributed to mass transfer limitations which occur under industrial

conditions. Oxygen delignification of pulp is a three-phase reaction system comprising

pulp fibers (solid phase), an aqueous phase, and the oxygen gas phase.

The mass transfer of oxygen to pulp fibers in medium consistency oxygen delignification

is shown schematically in Fig. 7.34.

Oxygen

gas bubble

kL

Liquid film

OHdissolved

oxygen

degraded

wood

compontens

Immobile

Liquid film

k FIBER S

Fig. 7.34 Scheme of mass transfer of oxygen to pulp fibers in

medium consistency oxygen delignification process (according

to Hsu and Hsieh [10]).

The process of oxygen delignification is described as follows:

_ Solubilization of oxygen in the alkaline pulp suspension during

high-shear mixing: first, oxygen transfer from the gas phase

through a gas film into the gas–liquid interfacial boundary takes

place. This is followed by oxygen transfer from the boundary

through a liquid film into the bulk liquid phase.

_ Diffusion of dissolved oxygen from the water surrounding the

fibers through the fiber wall, where the reaction occurs: dissolved

oxygen is transported from the bulk phase into the immobile liquid

layer surrounding the fiber by diffusion and convection, followed

by diffusion of hydroxide ions and oxygen molecules

through the immobile water layer to the fiber. Finally, the process

chemicals reach the reaction sites in the fiber through inter- and

intrafiber mass transfer.

Quite recently, van Heiningen et al. have presented a model where the effect of

mass transfer of oxygen on the efficiency of delignification in an industrial reten-

688 7Pulp Bleaching

tion tower is simulated [27]. The reaction conditions that occur at the entrance of

the oxygen reactor are determined by simulating the mass transfer and reaction

processes in a high-shear mixer. Figure 7.35 shows a simplified oxygen delignification

stage flowsheet.

MIX

NaOH

Oxygen

Steam

RETENTION

TOWER

WASH

VENT

Fig. 7.35 Schematic flowsheet of oxygen delignification.

Screened and washed pulp is pumped through one or more high-shear mixers,

where alkali, oxygen and steam are dispersed under pressure into a medium-consistency

suspension. Pulp passes through an upflow tower and is discharged from

the top of a blow tank from which gases are separated out, and the pulp finally

enters subsequent washers. The model presented by van Heiningen et al. is based

on this simplified process scheme. The main objective of this model is to calculate

the effect of the mass transfer of oxygen on the efficiency of delignification as a

function of caustic and oxygen charges, oxygen pressure, consistency and temperature.

Some minor changes and supplements have been introduced into the following

model proposed by van Heiningen et al. The model considers the following elements:

_ Oxygen solubilization during high shear mixing: the volumetric

mass transfer rate of oxygen, kLa (M), is obtained from an empiric

equation derived by Rewatkar and Bennington [29].

_ Oxygen balance through the retention tower assuming steadystate

conditions at a given pulp production rate and dimensions

of the retention tower.

_ The gas void fraction, X g, is calculated assuming a preset and constant

gas-to-suspension linear velocity ratio.

_ The oxygen consumption rate is related to the rates of pulp

delignification and dissolved lignin (carryover) oxidation.

_ The kinetics of kappa number degradation is described by the

one-stage model proposed by Iribarne and Schroeder [12]. Due to

the lack of an appropriate kinetic model, the course of dissolved

organic carbon (DOC) oxidation is modeled by using the model

from Iribarne and Schroeder, as well considering a DOC-to-lignin

conversion factor.

7.3 Oxygen Delignification 689

_ Temperature increase in the retention tower is calculated using

published values of the heat of reactions of both kappa number

degradation and DOC oxidation, whereas heat loss through the

reactor walls is neglected.

_ The saturated oxygen concentration in the aqueous phase is

obtained from the empiric model provided by Broden and Simonson

[30].

_ Values for the mass transfer rate of oxygen, kLa (R) in the tower

are assumed in a certain range, as measured in a laboratory

equipment [28].

7.3.4.1 Theoretical Base of the van Heiningen Model [27]

The pulp suspension is assumed to pass through the oxygen bleaching tower by

plug flow. As the steady state of the process is considered, the course of all variables

through the reactor can be expressed as a function of the residence time t of

the pulp suspension. The oxygen balance is governed by Eqs. (44) and (45):

d _ O 2

dt _ kLa _ O 2_sat __ __ O 2__

_ l

_ s __1 _ con __1 _ Xg _ _ ___ rO 2 _44_

d VO 2 _ g _ dt _ _

d _ O 2

dt _ rO 2 __ _ Vl _45_

where:

t = time after entering the reactor, [s]

[O2] = oxygen concentration in the liquor, [mol L–1]

kLa = mass transfer rate of oxygen to the liquid phase, [Lliquid

–1 Lcontactor s–1]

[O2,sat ] = oxygen concentration in the liquid in equilibrium with the oxygen

pressure, [mol L–1]

ql = density of the liquor, [kg L–1]

qs = density of the suspension, [kg L–1]

con= pulp consistency, mass fraction [-]

X g = gas volume (void) fraction, [-]

r O2 = oxygen consumption rate caused by pulp delignification, [mol

O2 Lliquor

–1 s–1]

V O 2 _ g = oxygen flow in gas phase, [mol s–1]

V˙

l = liquor flow, calculated as V l = R. (1– con)/(con. ql), [L s–1]

R = rate of pulp production, [kg s–1]

It is believed that the gas void fraction, X g, is not constant throughout the reaction,

as was assumed by van Heiningen et al. due to progressive oxygen consumption

[27]. Instead, it is supposed that the gas to suspension linear velocity ratio can

be kept constant, which should be valid as long as the production rate remains

stable. The linear gas velocity in the tower is assumed to be higher than the veloci-

690 7Pulp Bleaching

ty of the suspension due to the density difference between the gas and suspension.

X g can then be calculated according to Eq. (46):

Xg _

_ V

g

_ V

s _ vgvs _ _ V _ g _ _46_

where:

V˙

g = gas flow, calculated as V˙ g= V O 2 _ g. 0.008315. Tp –1, [L s–1]

T = temperature, [K]

p = pressure, [MPa]

V˙

s = suspension flow, calculated as V˙ s = R /(con. qs) [L s–1]

v g/ v s = ratio gas to suspension velocity [-]

The oxygen consumption rate, r O2, depends on both the degradation of residual

lignin and dissolved oxidizable matter (carryover), measured as DOC according to

the following expression:

rO 2 _ _

1_5 _ d _

dt _ b 1 _ dDOC

_ dt _ b 2__ R

32 _ _ Vl _47_

where:

b1 = stoichiometric coefficient for the reaction of oxygen with the residual

lignin [g DO2/g Dlignin]. The value of b1 is taken as 1.0 [31].

DOC = dissolved organic carbon, kg t–1 pulp

b2 = stoichiometric coefficient for the reaction of oxygen with the dissolved

black liquor [kg DO2/kg DDOC]; as no experimental values are available,

it is assumed that only the dissolved lignin fraction reacts with

oxygen: 50% of the DOC can be assigned to lignin compounds, and

1kg lignin relates to 0.63 kg DOC, then 0.5/0.63 = 0.79 kg lignin kg–1

DOC; therefore, b2 can be taken as 0.79 kg DO2/kg DDOC.

The kinetics of kappa number degradation is described by the model obtained

by Iribarne and Schroeder [12] (Tab. 7.17), as proposed by van Heiningen et al.

[27]. Any other kinetic model, as introduced in Chapter 4.2.3 (Mass transfer and

kinetics) may also be used for illustration. The validity of the model from Iribarne

and Schroeder is limited to softwoods (preferably Pinus taeda) in the kappa number

range 20–58 (see Tabs. 7.14 and 7.17):

_

d _

dt _

3_0 _ 106

60 _ Exp _

8_315 _ T __ OH _ _ 0_7

__ O 20_7__2_0 _48_

where T is the temperature, °K after residence time t, and [OH– ] (mol L–1) is the

hydroxide ion concentration in the liquor. The change in hydroxide ion concentration

can be calculated as follows:

d OH _ _ dt _ _

1_5 _ d _

_ dt _ b 3__ R

17 _ _ Vl _49_

7.3 Oxygen Delignification 691

where b3 = stoichiometric coefficient of hydroxide ion consumption by the residual

lignin of the pulp, given as kg hydroxide ions, OH–, consumed per kg lignin

removed; b3 is taken as 0.9. 17/40, based on recent measurements by Violette

[32].

To the present authors’ knowledge, a kinetic expression for DOC degradation

during oxygen delignification is not yet available. In order to estimate the effect of

dissolved lignin, measured as DOC, on the course of oxygen delignification, a

similar kinetic expression as depicted in Eq. (48) is considered.

The heat of reaction is estimated by a value of 14 MJ per ton of pulp and

removed kappa number [33]. Thus, the temperature increase caused by the oxidation

reactions during oxygen delignification may be obtained from Eq. (50):

dT

dt _

D HL _ d _

dt _ 1_5 _ D HDOC _ dDOC

_ dt _

cpulp _ cH 2 O _ 1

_ _ con _ 1__ mO 2_ g _ cO 2_ _50_

where:

DHL= heat of reaction of residual lignin oxidation [9.3 MJ kg–1 lignin], assuming

that one kappa number unit, j represents 1.5 kg of lignin in 1 t of

pulp.

DHDOC = heat of reaction dissolved lignin oxidation [7.4 MJ kg–1 DOC], assuming

that 1kg DOC contains 0.79 kg of dissolved lignin.

mO2_g = oxygen in gas phase, [kg t–1 pulp]

cpulp = specific heat capacity of pulp, 1550 kJ t–1 K–1

cH2O = specific heat capacity of water, 4187 kJ t–1 K–1

cO2 = specific heat capacity of oxygen, 0.93 kJ kg–1 K–1.

The pressure drop across the reactor can be calculated by Eq. (51):

dp

dt _ _0_00981 _ _ s _ vs _ _0_00981 _ _ s _

_ V

s _ H

1 _ Xg _ __ V _51_

where H and V are height (m) and volume (m3) of the reactor.

The model of Broden and Simonson was used to estimate the solubility of oxygen

in equilibrium conditions, [O2,sat], as a function of oxygen pressure, temperature

and hydroxide ion concentration [34]. A minimum in solubility is obtained at

a temperature of about 100 °C. The presence of dissolved sodium hydroxide

induces a salting-out effect which leads to a decrease in the oxygen solubility. The

dissolved oxygen concentration as a function of temperature and pressure for two

different sodium hydroxide concentrations is expressed by Eq. (52):

_ O 2_ sat_a1 _ a2 _ T _ a3 _ p _ a4 _ p _ T 2 _ _ a5 _ p _ T __ 0_001 _52_

where [O2,sat] = oxygen concentration in the liquid in equilibrium with the oxygen

pressure, [mol L–1].

The coefficients ai are presented in Tab. 7.19.

692 7Pulp Bleaching

Tab. 7.19 Numerical values of the coefficients ai in Eq. (52) for

the calculation of oxygen solubility as a function of temperature,

oxygen partial pressure and hydroxide ion concentration (as

determined by Broden and Simonson [34]).

Parameter 0.01 M [OH– ] 0.1 M [OH– ]

a1 3.236 9.582

a2 –0.00747 –0.02436

a3 –56.02 –94.77

a4 0.00016 0.00025

a5 15421 24610

Solubilization of oxygen in the alkaline pulp suspension is accomplished by

high shear mixing. The volumetric mass transfer rate of oxygen to the liquid

phase, kLa, for the mixer can be calculated by an empirical equation determined

by Rewatkar and Bennington [29], considering the specific power dissipation,

e[Wm–3], the gas void fraction, X g, and the pulp consistency, con:

kLa _ 1_7 _ 10_4 _ e1_0_ Xg _2_6

_ Exp __0_386 _ con _ _53_

where e = power dissipation per unit volume of the mixer, [W m–3].

The power dissipation of the mixer largely determines the achieved level of dissolved

oxygen concentration at the entrance of the retention tower. So far, only

limited data are available concerning the mass transfer rate, kLa, in the tower.

Based on laboratory measurements, Rewatkar and Bennington reported kLa values

in the tower as being in the range between 0.002 and 0.01s –1 [28]. In their chemical

reactor analysis, van Heiningen et al. have not considered any relationship between

the efficiency of a high-shear mixer in terms of the extent of dissolution of

oxygen in the aqueous phase and the mass transfer rate, kLa, in the tower [27].

Based on our own industrial experience, we believe that the efficiency of a highshear

mixer also determines the kLa in the tower to a certain extent [35]. It was

shown that the increase of both power dissipation and residence time in a highshear

mixer significantly improved the degree of delignification of a beech acid

sulfite dissolving pulp. Therefore, we are quite convinced that there should be a

relationship between the efficiency of a high-shear mixer and the mass transfer

rate in an oxygen delignification tower. As bubbles of oxygen gas tend to coalesce

during their transport through the tower, the kLa would rather follow a gradient to

lower values. Due to lack of information, the effect of different kLa values in the

tower on the extent of delignification is evaluated in a case study.

The second step of the mass transfer of oxygen to pulp fibers is the diffusion of

dissolved oxygen from the water surrounding the fibers through the fiber wall

where reaction occurs. It has been estimated by considering the ratio between the

rate of oxygen consumption by reaction to oxygen diffusion into the fiber that the

7.3 Oxygen Delignification 693

liquid–fiber transfer resistance is negligible in comparison with the apparent

intrinsic reaction rate [10,27]. Therefore, the intra-fiber diffusion resistance is considered

insignificant for oxygen delignification. Quite recently, measurements

revealed that oxygen is able to diffuse at least a distance of some 4–6 mm within

the pulp suspension in a 60-min retention time of a typical pressurized retention

tower at 786 kPa [36].

The effect of mass transfer of oxygen on the course of delignification through

the mixer and the bleaching tower can be calculated by solving the equations

numerically.

Although the model equations can be solved by any method suited for solving

ordinary differential equations (ODE), we use a simple scheme which exploits the

structure of the equations to yield accurate and reliable results. The tower is

divided into a large number of layers, each of volume DV. A total of 500 layers was

used for the examples discussed below, and this resulted in an error lower than

0.0001kappa units at the outlet of the reactor. The retention time Dt in the volume

element DV is calculated by the following expression:

D t _

1 _ Xg _ __ D V

_ V

s _54_

The calculation in a layer consists of two steps. In the first step, an approximation

for the variables at layer outlet is obtained, while the second step applies the

midpoint rule to improve the approximation.

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