Continuous Cooking

Modified Continuous Cooking (MCC®)

The concept of Modified Continuous Cooking (MCC) implies the process during

which the main part of the cooking is performed with a low alkali concentration,

while simultaneously allowing the concentration of dissolved lignin to be low. The

method is not particularly new, but has been applied in Australia since the 1960s

[141,142]. However, the conditions were far from optimal, and the process did not

attract interest for many years. It was only after theoretical and fundamental studies

at STFI and KTH in Stockholm, aimed at increasing pulping selectivity, that

this type of modified cooking process regained its attraction [16,18,143–145].

Averaging the EA concentration throughout kraft pulping of Pinus silvestris and

using a continuous two-vessel vapor/liquor-phase digester provides a more selective

delignification as compared to a conventional kraft cook [146–148]. The modified

kraft process is mainly characterized by lowering the initial EA concentration

from 1.45 mol L–1 to 0.7 mol L–1. This was achieved by a split addition of the white

liquor between the top of the impregnator and the transfer circulation line, which

takes place between the impregnator and the digester. Approximately 50% of the

total white liquor charge is sent to the chip feed system prior to the impregnation

stage. The remaining 50% of the white liquor is split into equal portions, with one

part charging to the transfer circulation which carries chips from the impregnation

vessel to the digester, and one part going to the countercurrent cooking circulation

at the top of the Hi-heat washing zone [149]. The latter EA charge keeps the

residual EA concentration at a level of 0.45 mol L–1 during the final delignification.

The EA concentration profile inside the chips is considerably leveled out by the

modifications compared with a conventional kraft cook in the digester as calculated

by a mathematical model (max 0.42 mol L–1, min 0.18 mol L–1 versus max

0.95 mol L–1, min 0.07 mol L–1, respectively) [19].

The applied temperatures in the co-current as well as countercurrent cooking

stage were typically about 165 °C.

The lignin concentration pattern is the reverse of that in conventional batch

cooking. In the latter, the lignin content increases gradually to a final concentration

of more than 100 g L–1 [150]. In MCC pulping, the highest observed lignin

concentration was about 65 g L–1 at the extraction, and this gradually decreased to

50 g L–1 at the end of the countercurrent stage. It has been reported that the concentrations

of dissolved lignin and sodium ions decreased by 40% at the end of

the cooking zone as a result of the countercurrent flow conditions [19,147].

A single-vessel hydraulic digester was modified to operate according to the modified

cooking process [151]. At a total charge of EA of approximately 24% on wood,

4.2 Kraft Pulping Processes 295

the optimum split was determined to be 57% to the feed, 10% to the upper circulation,

and 33% to the lower circulation. Under these conditions, the same residual

alkali of 8–11 g L–1 in both the downflow liquor and the upflow liquor at the

extraction screens could be maintained. The tear index was about 9% higher at

the same tensile for the MCC pulp compared to conventional pulp. Moreover,

there was less variability in pulp quality, and the pulp showed better bonding capabilities,

which resulted in better runnability on the paper machines.

Based on carbohydrate analysis of mill pulps and laboratory cooks it may be

assumed that, at an unbleached kappa number level of 25, the fully bleached pulp

yield is approximately 0.8% higher when the modified alkali profile was applied

in a continuous kraft cook [59]. The increase in yield can thus be attributed to a

better cellulose retention by simultaneously keeping the hemicellulose yield [146].

Consequently, the MCC softwood pulps show a lower hemicellulose content in

the kappa number range 21–42 as compared to conventional pulps (17.4–17.8%

versus 18.3–18.7%) [152]. The reject level from the MCC is 1.3% lower than is

experienced with conventional kraft processes (1.8% on pulp versus 3.1% on pulp

at kappa number 32 [149].

The better bleachability of MCC-cooked pulps when compared to conventional

pulps, and especially at a lower kappa number, has been explained by both the

higher residual alkali concentration (15 versus 10 g L–1) at the end of the countercurrent

cook zone, and the lower dissolved lignin concentration (50 versus 70 g L–1)

at the end of the cook. Thus, the pulp produced with the MCC technique is in

contact with liquor having about 50% higher alkali concentration and about half

the dissolved lignin concentration of the conventional cook. It has been observed

that the molecular weight of the dissolved lignin molecules increases in relation

to progress of the cook [153]. Diffusion out of the fiber will be facilitated by the

countercurrent cooking, and consequently less of the high molecular-weight lignin

should be left in the pulp fiber. Furthermore, it has been found that the ratio

of hydrogen to carbon atoms in the dissolved lignin decreases with increasing

cooking time, which might correspond to a higher degree of condensation [154].

The combined effect of improving the diffusion of lignin and keeping a higher

residual alkali concentration towards the end of the cook to prevent reprecipitation

should result in a pulp that is easier to bleach.

The viscosity level of the modified kraft pulp is more than 100 mL g–1 higher at

a given kappa number as compared to the conventional kraft pulp (1140 mL g–1

versus 1000 mL g–1 at kappa number 25 [147]). The unbleached kappa number

could be lowered by about eight kappa numbers with maintained strength properties

[154]. Hardwood pulps follow the same pattern as softwood pulps. The

unbleached viscosity is about 100 mL g–1 higher at the same kappa number, or

about the same viscosity at 3–4 units lower kappa number. The change to modified

continuous cooking of birch resulted in a better stability of the production

and pulp uniformity [150]. Moreover, the kappa number was lowered from 18 to

14, and the viscosity increased from 900 to 1015 mL g–1, respectively. However, it

must be stated that the performance of the batch line had previously also been on

the level of approximately 1000 mL g–1 [150].

296 4 Chemical Pulping Processes

The strength properties of MCC softwood pulps are reported to have a 10–30%

higher tear value at a given tensile level [149]. Cooking studies using Scandinavian

mixed softwood exhibit a viscosity advantage of approximately 70 SCAN units for

MCC pulps over laboratory-produced conventional pulps in the kappa number

range from 22 to 32. This viscosity advantage could be preserved after bleaching

to a brightness of 90% ISO using a sequence (C+D)EDED.

Extended Modified Cooking (EMCC®)

Pulping selectivity further improves when utilizing a prolonged countercurrent

cooking stage at a lower temperature [155]. The concept of extended modified

cooking (EMCC) comprises the addition of white liquor at the bottom of the Hi-

Heat washing zone to achieve a more even effective alkali profile and the extension

of cooking to the Hi-Heat washing zone. The EMCC process is comparable

to the ITC process, as the entire Hi-Heat zone is simultaneously used for both

cooking and washing. The ITC and EMCC processes differ only in the equipment

used for heating and circulating the white liquor in the High-Heat washing zone.

The ITC uses an additional dedicated heating circulation system.

Pinus taeda laboratory cooks confirmed that, at the same kappa number and

over the kappa number range investigated, EMCC pulps were generally found to

be superior in both viscosity and strength properties as compared to MCC and

conventional cook (CK) pulps (Fig. 4.81).

10 20 30 40

Screened Yield [%]

CK MCC EMCC

Intrinsic Viscosity [ml/g]

Kappa number

Fig. 4.81 Intrinsic viscosity and screened yield

versus kappa number of Pinus taeda kraft

cooks. Results from laboratory cooks (according

to [155]). Conventional cooks (CK):

EA-charge 19.2–21.3% on wood, 172 °C; modified

continuous cooking (MCC): EA-charge

19.6% on wood with 74:26 split addition,

171 °C; extended modified cooking (EMCC):

EA-charge 19.6% on wood with 74:26 split

addition and 160 °C.

4.2 Kraft Pulping Processes 297

The high selectivity of EMCC pulping can be explained by the low content of

dissolved lignin in the final cooking stage, combined with the low temperature

and thus prolonged cooking time. Diffusion of lignin into the aqueous phase is

improved towards the end of the cook because of the lower dissolved lignin concentration

and the longer time for diffusion. The lower pulping temperature kinetically

favors delignification over cellulose chain scission, resulting in a higher

pulp viscosity (see Section 4.2.5.2.1, Tab. 4.18, Kraft Pulping Kinetics).

Preliminary mill trials converting from CK to the EMCC cooking concept

showed an increase in intrinsic viscosity from 1160 mL g–1 to 1270 mL g–1 for a

kappa number-12 unbleached hardwood kraft pulp. In another mill trial using

northern softwoods in a single-vessel hydraulic digester, the EMCC concept was

realized by adding up to 25% of the total white liquor charge to the washing zone

which was operated at cooking temperature. Brownstock viscosity at kappa number

20 was significantly increased from 1030 mL g–1 to more than 1200 mL g–1

[155].

The knowledge from laboratory studies that the presence of dissolved solids during

the bulk and final delignification stages negatively influences both the rate of

delignification and pulp viscosity at a given kappa number (selectivity) led to the

development of a new continuous cooking process, the Lo-Solids™ pulping [156].

Lo-Solids™

The Low-Solids™ process is based on the ITC and EMCC technologies, and is

characterized by split white liquor additions, multiple extractions and split washer

filtrate additions to achieve both an even EA profile, minimal cooking temperatures

and minimal concentrations of dissolved lignin at the end of the cook. A

typical Lo-Solids™ digester is provided with four white liquor addition points, the

first before the impregnation, the second after the first extraction in the lower

cook circulation (LCC) zone, the third after the second and main extraction in the

modified cooking circulation (MCC) zone, and the fourth after the third and last

extraction in the washing zone [157–159]. Washing filtrate is added together with

white liquor at the final three addition points. Beneath the LCC there is a con-current

cooking zone, followed by the second extraction. Below the second extraction,

the countercurrent cooking zones start, with the MCC screens in between. Results

from mill application confirm the significant reduction in the concentration of

dissolved solids within the bulk and final phases of delignification (Fig. 4.82). The

low level of dissolved lignin concentration is most evident in the wash-cooking

zone. Simultaneously, the final bleached viscosity of the hardwood kraft pulp

increased after the transition to Lo-Solids™ pulping from 990 mL g–1 (EMCC) to

approximately 1100 mL g–1, which can be mainly attributed to the lower level of

dissolved wood components in the final stages of pulping [157].

Softwood kraft mills which have been converted to Lo-Solids from MCC or

EMCC operation have typically observed 5–10% improvements in tear strength.

Laboratory trials using northwestern softwoods and simulating the time–concentration

and time–composition profiles of dissolved wood solids in full-scale pulping

systems even report a 28% (26%) gain in tear strength in the unbleached

298 4 Chemical Pulping Processes

Blowline

Modified Cook Circ.

Lower Extraction

Upper Extraction

Lower Cook Circ.

Upper Cook Circ.

0 20 40 60

Dissolved lignin concentration [g/l]

Lo-Solids

EMCC

Fig. 4.82 Profile of dissolved lignin concentration at various

locations on the digester. The results compare profiles for

EMCC and Lo-Solids pulping (according to [157]).

(ECF bleached) pulp [160]. It has been demonstrated that pulp tear strength

increases with increasing residual EA concentration. Thus, pulp yield losses can

be avoided when a high EA concentration is maintained only towards the very end

of the cook (see Fig. 4.44).

Dissolved wood solids also consume alkali in nonproductive decomposition

reactions (e.g., retroaldol reactions, etc.). Based on laboratory results, it can be estimated

that the dissolved wood solids present in the cooking liquors consume an

equivalent of approximately 2% EA on wood due to secondary reactions [160]. The

application of the Lo-Solids pulping process can reduce approximately 30% of the

dissolved wood components. Based on these figures, it seems reasonable to

assume a decrease of about 0.5% EA on wood. The presence of dissolved solids in

the late stage of the cook also deteriorates bleachability. ECF bleaching of northern

softwood pulps which were cooked according to the Lo-Solids technique without

dissolved wood solids present in the cooking liquor, required 11% less amount of

active chlorine to attain a brightness target of 89% ISO using a DEopDD.

Since the introduction of the Lo-Solids-pulping in 1993, there are now far more

than 60 installations all over the world [161]. Fourteen rather new Lo-Solids digesters

operating on different hardwoods such as mixed southern hardwoods, eucalyptus,

birch and mixed Japanese hardwoods report a yield increase of 1–4% on

wood compared to previous operation, mainly according to the EMCC process.

The wood yield is determined either by the measurement of the wood consumption

during a longer period of time, or according to a straight-line correlation of

the logarithm of TAPPI viscosity (V) divided by the square of the cellulose content

fraction (G′), log(V)/G′2, with the lignin-free yield [162]. The reasons for this really

4.2 Kraft Pulping Processes 299

significant yield advantage over conventional and even EMCC pulping processes

have been mainly attributed to the even and very low EA concentration throughout

the cook. This never exceeds values of ca. 15 g L–1, except at the very beginning

of the impregnation zone (Fig. 4.83). After impregnation, and immediately before

the start of bulk delignification, the residual EA is as low as 4 g L–1. At the same

time, the ratio of [HS– ]ion to [OH– ]ion exceeds 3, which almost achieves the values

optimized in laboratory operation [33]. The concept of Lo-Solids pulping comprises

a two-stage continuous kraft process. The pre-steamed wood chips are

impregnated with black liquor with a maximum ratio of [HS– ]/[OH– ]. The digester

is divided into two sections, one co-current and one countercurrent. The upper

half is devoted to the high sulfidity stage in a co-current flow. The sulfidity can be

selectively increased by extracting the black liquor after the first treatment zone

and reintroducing the withdrawn liquor with dilution liquor. In a subsequent second

treatment zone a kraft cooking liquor is introduced having a higher sulfidity

as compared to the first treatment zone [163].

Almost 75% of the lignin is removed in the first stage under highly selective

conditions. In the lower part of the digester, the second cooking stage is performed

in a countercurrent procedure. The cooking temperature can be kept at a

low level, at about 150–155 °C due to the fairly long retention time in the later

stage of the cook.

Blow

Wash cook zone

Main Extr.

Lower cook circ.

Upper cook circ.

Con-curr. impregn

0 5 10 15 20 25 30

Molar ratio [HS-] / [OH-]

EA-profile

EA-concentration [g/l]

0 1 2 3 4

[HS-] / [OH-]

Fig. 4.83 Profiles of effective alkali (EA) and the molar ratio

[HS– ]/[OH– ]through Lo-Solids pulping (according to Refs.

[161,164]) (mill data).

300 4 Chemical Pulping Processes

The low level of EA concentration, in combination with the low cooking temperature

of 153 °C, decreases the extent of carbohydrate degradation reactions according

to random alkaline hydrolysis and secondary peeling reactions.

Laboratory studies on birch wood were conducted to elucidate the origin of the

yield advantage of the Lo-Solids operation [165]. Three different EA-profiles (A, B,

and C) were applied to the impregnation stages and during early bulk delignification.

The A-profile represents a high EA-level between 8–18 g EA L–1, the B-profile

an intermediate EA-concentration in the range 4–18 g L–1, and the C-profile a low

EA-level in the range 2–9 g L–1. The final bulk and residual cooking phases were

conducted at comparable conditions by varying the initial EA-concentration in the

range 5 to 25 g L–1. The cooking temperature was kept constant at 153 °C during

both cooking stages. The influence of temperature on the cooking performance

was investigated in the final cooking stage, where selected trials were run at

165 °C. The cooking time or H-factor were adjusted accordingly to attain the target

kappa number 18. The kappa number of the selected samples showed only minor

deviations from the target kappa number (16–22), so that the kappa number can

be assumed to be constant. The graph in Fig. 4.84 illustrates that the total yield is

highly dependent upon the amount of EA-charge in both the early stages and the

final stages of the cook.

The comparable slopes of the curves in Fig. 4.84 suggest that the yield loss is

similar in magnitude for all three EA profiles investigated. A lower EA concentration

at the final cooking stage is certainly favorable for xylan reprecipitation, as

0 10 20 30

EA-Profiles in the early cooking stage:

High EA (A) Intermediate-EA (B) Low-EA (C)

Total Yield [%]

Residual EA concentration [g/l]

Fig. 4.84 Influence of different effective alkali

(EA)-profiles in the early stages (EA-profiles A–

C) and final stages of the cook (residual EA in

the Figure) on the total yield of birch Lo-Solids

laboratory cooks (according to [165]). All trials

were conducted at 153 °C in both cooking

stages. The kappa numbers of all pulps were

on average about 18 (minimum 16, maximum

22).

4.2 Kraft Pulping Processes 301

0 10 20 30

EA-Profiles in the early cooking stage:

High EA (A) Intermediate-EA (B) Low-EA (C)

Intrinsic Viscosity [ml/g]

Residual EA-concentration [g/l]

Fig. 4.85 Influence of both different effective

alkali (EA)-profiles in the early stages (EA-profiles

A–C) and final stages of the cook (residual

EA in the Figure) on the pulp viscosity of birch

Lo-Solids

laboratory cooks (according to [165]). All trials

were conducted at 153 °C in both cooking

stages. The kappa numbers of all pulps were

on average about 18 (minimum 16, maximum

22).

has been confirmed by carbohydrate analysis. The cellulose content is particularly

preserved at lower EA concentration in the early stages of the cook. A high concentration

of EA in the early bulk delignification phase also deteriorates pulp viscosity,

as shown in Fig. 4.85. The lower cellulose content might be an explanation

for the decrease in viscosity. The influence of the EA concentration in the final

cooking stages, however, has an even more pronounced influence on pulp viscosity

(Fig. 4.85). High viscosity levels are attained at very low residual EA concentration,

despite the high extent of xylan reprecipitation. With an increasing EA

charge, pulp viscosity passes through a minimum at a residual EA concentration

of 5 g L–1 for all three EA-profiles investigated.

Higher EA charges in the residual delignification phase promote higher pulp

viscosities due to unfavorable conditions for xylan retention on the fiber, while

largely preserving the cellulose fraction.

By increasing the temperature from 153 °C to 165 °C in the final cooking stages,

the brownstock yield is decreased by 1.2% at a given kappa number. It has been

calculated that 28% of the yield loss is due to a lower cellulose yield, and 72% to a

lower hemicellulose yield [165]. Hardwood kraft pulping is associated with the formation

of the HexA. Kraft pulping offers only limited possibilities to reduce the

HexA content prior to bleaching; however, the HexA content was shown to

decrease slightly, from approximately 73 lmol g–1 to about 65 lmol g–1 with a

higher EA concentration in both early stages and the last cooking stage.

302 4 Chemical Pulping Processes

0 100 200 300

Temperature [. C]

CK Low EA High EA

EA-concentration [g/l]

Cooking time [min]

Temperature

Fig. 4.86 Effective alkali and temperature profiles (according to [166]).

In another study, where a conventional batch cook was included for comparative

purposes, it was again shown that a higher EA concentration at the start of bulk

delignification tends to decrease mainly the cellulose yield, while a higher EA concentration

towards the end of the cook decreases mainly the yield of xylans [139].

The corresponding alkali profiles are shown in Fig. 4.86.

For the low-EA profile, the cellulose yield is 37.5% on wood, and thus 1.5% on

wood higher as compared to the conventional EA profile, whereas the xylan yield

amounts to 13.7% on wood for both profiles. For the high-EA profile, the cellulose

yield is 35.9% on wood and thus the same as for the conventional, but the xylan

yield is 1.4% on wood lower than for the conventional and low- EA profiles.

The concentrations of dissolved xylan in the black liquor throughout the cooks

reflect the amount of xylan reprecipitation onto the fiber. In the case of the conventional

EA profile, the xylan concentration increases steadily to 15 g L–1 as it dissolves

into the liquor. After having achieved maximum temperature, the xylan

concentration decreases again as it reprecipitates onto the fiber. Interestingly, the

dissolved xylan concentration remains constant at a very low level of about 4 g L–1

when cooking with a low-EA profile (see Figs. 4.86 and 4.87).

The dissolved xylan content is exactly the same after both conventional and low-

EA cooks, reflecting equal xylan yields for the two cooks. As shown in Fig. 4.87,

the dissolved xylan content after the high-EA cook remains at a high level when

the residual EA concentration is high. This implies a direct correlation between

the residual EA concentration and the xylan content of the pulp.

The effects of the alkali profile during Lo-Solids cooking of eucalyptus chips has

been described in a recent study [167]. After the impregnation stage, a first displacement

stage simulates a countercurrent heating stage by displacing part of

the cooking liquor with white liquor; this is then followed by a 60-min co-current

4.2 Kraft Pulping Processes 303

0 100 200 300

CK Low EA High EA

Dissolved Xylan [g/l]

Cooking Time [min]

Fig. 4.87 Dissolved xylan concentration in the cooking liquor

for cooks with different effective alkali (EA) profile (according

to [139]).

cooking phase. Finally, the second displacement simulates the countercurrent

cooking/washing stage at the bottom zone of a Lo-Solids digester. Again, the cooking

liquor is displaced by white liquor. The maximum cooking temperature is varied

in the range from 147 °C to 153 °C. Conventional batch cooks with a total EA

charge of 20% NaOH on o.d. wood and a maximum temperature of 164 °C were

conducted as a reference. Lo-Solids cooks were performed from both E. urophylla

and mixed eucalypts from New Zealand, using different alkali profiles that were

adjusted by adding alkali to each cooking stage. The conditions were selected such

that the unbleached kappa number ranges between 14 and 17. The results indicate

that pulping selectivity in terms of both yield and viscosity is clearly associated

with a low and even EA profile throughout the whole cook (Fig. 4.88). Only

small deviations from the optimum alkali profile would lead to a reduced pulping

selectivity. The lower alkali profile (as shown in Fig. 4.88) contributes to a yield

gain of 3.7% compared to the conventional batch cooks.

The Lo-Solids cooks of the mixed eucalypts showed, however, only a yield gain

of 1.7% compared to the reference cook, probably due to a suboptimal EA profile.

In both cases the yield gain resulted from a better retention of both cellulose

(1.5–2.7% higher glucan yield) and xylan (1–1.2 % higher xylan yield) in comparison

to the conventional batch cooks. Maintaining an optimum alkali

profile and a low cooking temperature also improves the selectivity in terms of

viscosity at a given kappa number. The viscosity/kappa number ratio for the optimized

E. urophylla Lo-Solids cook was 82.4 (1400 mL g–1 at kappa number 17)

304 4 Chemical Pulping Processes

0 100 200 300 400

E. urophylla Mixed eucalypt

EA concentration, g/l

Cooking time, min

Fig. 4.88 Effective alkali (EA) profiles for Lo-Solids cooks of

both mixed eucalypts originating from New Zealand and E.

urophylla at the same cooking temperature of 147 °C (according

to [167].)

with only 72.4 (1100 mL g–1 at kappa number 15.2) for the conventional batchcooked

pulp.

A further development of the continuous cooking concept is based on the

results from numerous laboratory trials in which the combination of low EA concentration

at the beginning of a cook and a high EA concentration in the late stage

turned out to be favorable with respect to cooking time or temperature, pulp yield,

pulp viscosity, bleachability and HexA content [24]. This new continuous cooking

process – denoted as Enhanced Alkali Profile Cooking (EAPC) – allows the EA

profiles to be controlled in kraft cooking, without increasing white liquor consumption

[29]. The first zone in the digester comprises a co-current pretreatment

stage where most of the alkali introduced into the feed system is allowed to be

consumed. The spent liquor with the low alkali concentration is extracted to recovery.

The chips then pass the countercurrent impregnation zone where fresh alkali

is added. The subsequent cooking is divided between co-current and countercurrent

zones. After the first cooking zone, the cooking liquor is extracted and

replaced by white liquor before entering the countercurrent cooking zone. To

maintain a high EA concentration during residual delignification, white liquor is

added at the end of the countercurrent cooking zone to the upflow liquor. Because

of the high EA concentration, the cooking liquor from the lower extraction (of the

final cooking stage) is recycled to the chip feed system to utilize the residual alkali

for the subsequent cook. In this process, all black liquor to recovery is extracted

from the digester between the end of the pretreatment and beginning of the

4.2 Kraft Pulping Processes 305

impregnation stage. The black liquor extracted from the cooking zones is to be

reused in the previous stages. Thus, the continuous EAPC cooking process facilitates

control of the EA profile during kraft cooking to consider the principles of

modified cooking. Consequently, a wide range of alkali profiles in the cooking

phase can be adjusted to partly control the unbleached pulp properties (bleachability,

tear strength, etc.). Mill trials at the Enso Varkaus mill in 1996 revealed an

increase in tear strength (bulk) at a given tensile strength of about 20% (10%),

which confirmed the predicted improvements of unbleached pulp properties.

Moreover, chlorine dioxide consumption was decreased by about 10% in a subsequent

ECF bleaching sequence, suggesting an improved bleachability of the

EAPC-produced softwood kraft pulps [168].

Дата добавления: 2015-10-21