Batch Cooking

Cold blow

Cold blow technology can be designated as the forerunner of the modified kraft

batch cooking processes. It is characterized by one or two cooking stages and cold

displacement. Another key characteristic of this technology is that there is no

warm impregnation stage.

The cold blow technique comprises the process steps of chip filling, steaming,

charging of cooking liquor, heating to cooking temperature, cooking, filling the

digester with wash liquor to reduce the temperature below 100 °C, and finally cold

blowing [108]. After chip filling and steaming, the chip temperature will rise to

about 100 °C. White liquor is preheated in a heat exchanger and charged to the

digester together with hot black liquor. After liquor charging, the digester content

will have a temperature of ca. 130–140 °C. Raising the temperature to the target

cooking temperature (160–170 °C) will be accomplished by liquor recirculation

and indirect steam heating. The digester is then kept at cooking temperature until

the target H-factor is reached. When cooking is completed, cold washer filtrate is

introduced from the bottom of the digester to displace the cooking liquor through

strainers at the top of the digester. The displaced hot black liquor is stored in the

hot black liquor accumulator for use in a subsequent cook. In the next step, blowing

starts by opening the blow valve. The pressure at the digester top is controlled

by connecting the vapor phase of the hot black liquor accumulator with the top of

the digester. The blow is more rapid and efficient than a conventional hot blow

due to the reduced flashing in the blow line. The major advantages of the cold

blow over the standard batch technology are the 50% shorter heating time and

considerably less steam consumption. Total steam consumption during cooking

could be reduced from 4.1 GJ adt–1 to 2.4 GJ adt–1, corresponding a reduction of

274 4 Chemical Pulping Processes

41% [108,109]. Cold-blow cooking technology can also be run with two cooking

stages. Using this technology, the cooking liquor of the first cooking stage is displaced

by a mixture of weak black liquor and white liquor to reduce the dissolved

lignin concentration prior to residual delignification. After displacement, the circulation

is started again to evenly distribute the cooking liquor. The second cooking

stage is controlled by the H-factor and the EA-concentration. Cold displacement

of the cooking liquor with washer filtrate terminates the cooking stage. The

two-stage cold blow cooking technology improves the pulp quality significantly.

The results show that the kappa number could be reduced from 32 to about 25

while retaining the strength properties. This selectivity advantage can be

expressed by an increase in viscosity of approximately 100 mL g–1 at a given kappa

number in the range 25–30 [110]. Today, cold blow cooking technology has been

successfully replaced by further developed batch cooking technologies, and these

will be introduced in the following sections.

Rapid Displacement Heating (RDH) [111–113]

The digester is steam-packed with chips using a shuttle conveyor. Next, weak black

liquor from the atmospheric accumulator is used to increase chip compaction

density in the digester. Air is displaced from the digesters and the discharge valves

are closed. Preheated and pressurized weak black liquor with a temperature up to

130 °C is then introduced at the bottom until the digester is hydraulically full. The

digester is pressurized to 7 bar just as the discharge temperature during the warm

liquor fill reaches 100 °C. Liquor can enter the chips by being forced in an overpressure.

It is estimated that in RDH cooking almost all the wood is impregnated

by the pressure mechanism. Impregnation is carried out with warm black liquor;

therefore, the ratio of hydrosulfide to hydroxide ions reaches values above 3,

which ensures that cellulose is not exposed to the hydrolytic effects of high hydroxide

ion concentrations as compared to a conventional cooking pattern (see

Fig. 4.46).

As a result of a hydraulic cook, a rather high liquor-to-wood ratio of around 4.7

is achieved. This thorough impregnation as a separate stage assures first of all a

more uniform delignification within the chips, and this results in less rejects. Secondly,

it decreases alkali consumption fluctuations in the cooking phase, thereby

improving cook uniformity through pre-neutralization of the degraded carbohydrate

structures [114]. The high sulfide concentration achieved in the chip interior

during the early stage of the cooking phase is also beneficial to delignification

selectivity.

In the next step, hot black and hot white liquors are charged to the digester, displacing

the warm liquor to the atmospheric accumulator Soap is separated there

and excess weak liquor is pumped to the evaporators. Digesters are thus brought

to approximately 160 °C before any steam is added. Hot black liquor is continuously

passed through a heat exchanger to heat the incoming white liquor, which

is then stored in an accumulator for the next cook. Since liquors entering the

digester are already hot, the heating-up time is very short. The amount of steam

required to achieve the target cooking temperature (160–170 °C) is added directly

4.2 Kraft Pulping Processes 275

to the digester circulation line. After having reached the final cooking temperature,

the circulation is stopped. The cooking phase is short compared to that of a

conventional cook since the charging liquors are close to the cooking temperature,

thus enabling an accelerated temperature elevation.

As soon as the target H-factor is reached, hot black liquor is displaced through

the upper strainers with washer filtrate to immediately stop the cook. Hot black

liquor with a temperature above 130 °C is displaced into the hot black liquor tank.

Approximately 70% of the hot liquor displaced remains at the cooking temperature

[115,116]. The displacement continues until all excess filtrate from the

brownstock washing is pumped through the digester. Displaced liquors at temperatures

below 130 °C pass to a second hot black liquor tank at lower temperature

for reuse in a subsequent impregnation cycle. Finally, liquors with a temperature

level below 93 °C are displaced into the atmospheric accumulator. The ideal displacement

continues until approximately half of the free volume of digester has

been pumped out. The proportion of wash liquor in the displaced liquor increases

steadily.

At this point, the cook has ended and the digesters can be blown. The cooled

and delignified “chips” are discharged with compressed air stored in the air receiver

at significantly shorter time than conventional cooks due to the absence of

flash steam. It is reported that the sulfur emissions are reduced by about 98% as

compared to conventional hot blow. The TRS emissions are only constituted by

methyl sulfides and very little methyl mercaptan. They are basically noncondensibles,

with no affinity for water or alkali [117].

As a result of the sequence of liquor charging and displacing, the RDH-cook

reveals a rather smooth alkali profile (Fig. 4.67). At the end of the cook, the residual

alkali concentration remains at a higher level, in the range 0.42 to 0.5 mol L–1.

The alkali concentration falls by 30% during the first 15 min, which is less when

compared to conventional cooking. This can be explained by the higher liquid-towood

ratio and the already reacted alkali during black liquor impregnation [118].

The alkali profile is also determined by the wood source applied, depending on its

carbohydrate composition. Softwood galactoglucomannan (GGM) begins to react

at temperatures around 100 °C according to the peeling mechanism, whereas

hardwood arabinoglucuronoxylan (AX) will react only at temperatures exceeding

140 °C [118]. As a net result, the major alkali consumption in softwood pulping

starts with the introduction of the hot liquor, whereas hardwood hemicelluloses

are more resistant towards peeling reactions, thus keeping the alkali profile flat

and at a higher level.

A reduction in total alkali consumption from 18 to 17.5% on o.d. wood has

been reported at the Joutseno mill [116], and this can be assumed to be connected

to the less alkali-consuming peeling reactions that take place with RDH cooking.

In the white liquor, the total amount of sulfide charged is present as free hydrogen

sulfide ions, HS–. With the beginning of the cook, part of the HS– ions

becomes immobilized due to both physical bonds to wood (loosely bound) and

organic material in the cooking liquor and chemical bonds to lignin and lignin

degradation products (chemically bound) [119,120]. The chips absorb HS– ions

276 4 Chemical Pulping Processes

0 20 40 60 80 100

0.0

0.5

1.0

1.5

Cold Displacement

Cook

WL Injection

Hot Fill

Warm Fill

Conventional Batch Superbatch

Effective alkali [mol/l]

Time [min]

Fig. 4.67 Effective alkali profiles of RDH and conventional

cooking procedures (according to [117]).

from the cooking liquor in an amount of 0.1–0.2 mol S kg–1 wood. It is assumed

that the loosely bound sulfide is not available for delignification reactions. At the

end of the initial cooking phase, the loosely bound sulfide is transformed to free

HS– ions again. A deficiency of the hydrogen sulfide concentration during the initial

pulping phase decreases the delignification rate in the bulk phase and promotes

the formation of enol ether structures in the lignin, which results in a residual

lignin that is difficult to bleach [40,121]. In the RDH cooking process, black

liquor is displaced through the chips before cooking, and this counteracts the lack

of available hydrogen sulfide ions. A mill study of softwood cooking according to

the RDH process revealed that the content of free HS– ions during the impregnation

and charging/displacement phases was approximately four-fold higher when

compared to a conventional kraft cook of the same sulfidity [119]. The graph in

Fig. 4.68 shows that the initial concentration of free HS– ions in the RDH cook is

comparable to that of a conventional cook, with a sulfidity of about 46%. At the

start of the bulk delignification (after 60 min cooking time), the content of free available

hydrogen sulfide is approximately 10% higher in the RDHcook compared to the

reference cook at the same sulfidity level (32%), although at the end of the bulk

phase the levels are equal.

The measurements confirm that the displacement of black liquor through softwood

chips prior to cooking promotes a sulfide accumulation in the chips, and a

higher concentration of free available HS– ions in the cooking liquor during the

initial delignification phase. There is a strong indication that the better selectivity

towards delignification of the RDH process can be traced back to a higher concentration

of available HS– ions in the transition from initial to bulk delignification.

4.2 Kraft Pulping Processes 277

0 50 100 150 200

RDH - 32% sulfidity Conventional - 32% sulfidity

Conventional - 46 % sulfidity

Free Sulfide [%]

Cooking time [min]

Fig. 4.68 Concentration of free sulfide ions versus

cooking time in impregnation and cooking

liquors in the RDH-process (Joutseno mill) and

in the cooking liquor during conventional

batch cooks at laboratory scale according to

[119]. The method to determine the concentration

of free hydrogen sulfide is described in

Ref. [122]. The concentration of free sulfide

ions is expressed in relative terms: 100% corresponds

to the total amount of sulfide charged

with white and black liquor in the reference

batch cook with 32% sulfidity.

The lignin concentration in the initial phase of the RDH cook is high, and

remains high throughout the whole cooking process. It increases rather rapidly

following the hot liquor charges, and reaches a lignin concentration at the end of

the cook which is quite comparable to that during conventional cooking [114].

RDH pulps from radiata pine contained about one-third less HexA as compared

to conventional pulps of similar kappa number (Tab. 4.35). The lower content of

HexA may be attributed to the greater overall alkali charge necessary to obtain the

same kappa number level. Based on carbohydrate analysis, it has been determined

that HexA substitution along the arabinoxylan chain was 4.0 HexA per 100 xylose for

the RDH pulps, compared to 4.7 HexA per 100 xylose units for conventional pulp.

The major advantage over a conventional batch kraft process is the heat savings

of more than 70%, comprising about 2.5–3.0 GJ admtp–1 [124–127]. A mill-scale

trial of the RDH process at the Georgia Pacific mill in Port Hudson, Louisiana,

resulted in a reduction in steam consumption by 61%, from 2.32 t odt–1 to

0.90 t odt–1 as compared to conventional batch cooking [128]. The steam reduction

is verified by a comparison of starting temperatures of 63 °C for conventional

cooks, and 141 °C for RDH cooks.

Further advantages comprise a reduction in cover-to-cover time by 12.5% and

15%, respectively [126].

278 4 Chemical Pulping Processes

Tab. 4.35 Hexenuronic acid (HexA) content of conventional and

RDH pulps (according to [123]).

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