P-factor Concept

Brasch and Free were the first to propose a prehydrolysis factor to control the prehydrolysis

step [49]. These authors calculated the relative rate of prehydrolysis for

any desired temperature by assuming that the prehydrolysis rate triples for each

10 °C rise in temperature, without measuring the activation energy of hydrolysis

and assuming that the rate constant at 100 °C is unity. Later, Lin applied the Hfactor

principle to prehydrolysis using an activation energy typical for the cleavage

of glycosidic bonds of the carbohydrate material in the wood [47]. According to

this principle, the P-factor expresses the prehydrolysis time and temperature as a

single variable and is defined as:

P _

t

t 0

krel _ dt _149_

in which krel is the relative rate of acid-catalyzed hydrolysis of glycosidic bonds.

The rate at 100 °C is chosen as unity, and all other temperatures are related to this

rate. An Arrhenius-type equation is used to express the temperature dependence

of the reaction rate. At temperature T, it is expressed as:

Ln _ kH _ T ___ Ln _ A __

EA _ H

R _

T _150_

in which kH(T) is the rate constant of xylan hydrolysis at the temperature T, A is the

pre-exponential factor, and EA,H the activation energy. The reference reaction rate

at 100 °C is then expressed in Eq. (151):

Ln kH _100_ C _ __ Ln _ A __

EA _ H

R _

373_15 _151_

The ratio of Eqs. (150) and (151) expresses the relative reaction rates at any other

temperature:

Ln

kH __ T _

k 100_ C _

EA _ H

R _ 373_15 _

EA _ H

R _ T _152_

A selection of activation energies, EA, for both bulk and residual xylan hydrolysis

is provided in Tab. 4.44. A value of 125.6 kJ mol–1 for the fast-reacting xylan, originating

from extensive investigations of xylan hydrolysis from Eucalyptus saligna,

has been successfully applied for P-factor calculation in a new Brazilian prehydrolyis

kraft mill [50,51]. Thus, it is suggested to use this activation energy for the

calculation of the P-factor. By substituting a value of 125.6 kJ mol–1 into Eq. (152),

the relative rate can be calculated according to Eq. (153):

Krel _

kH __ T _

k 100_ C _ Exp _ 40_48 _

T _ _ _153_

The relative rate is plotted against time, and the area under the curve represents

the “prehydrolysis factor” or “P-factor”:

4.2 Kraft Pulping Processes 343

P _

t

t 0

kH __ T _

k 100_ C _ dt _

t

t 0

Exp _ 40_48 _

T _ __ dt _154_

This simple concept is illustrated by calculating the P-factor for a prehydrolysis of

1 h at constant temperatures. The prehydrolysis of 1 h at 160 °C and 170 °C corresponds

to P-factors of 272 and 597, respectively. The rate constants for prehydrolysis

more than double for a 10 °C rise in temperature. The conclusion is that in

lowering the temperature from 170 °C to 160 °C, while keeping the prehydrolysis

intensity unchanged, the reaction time must be expanded from 60 min to

132 min.

The applicability of the P-factor can be examined by plotting the remaining

xylan content in the beech wood obtained by water prehydrolysis three different

temperatures against the P-factor, as shown in Fig. 4.106 [39](see also Fig. 4.97).

0 500 1000 1500 2000 2500

140.C 155.C 170.C

Xylan in residue, g/kg od wood

P-Factor

Fig. 4.106 Mass of xylan residue as a function of P-factor for

water prehydrolysis of beech wood using a liquor-to-solid

ratio of 10:1 (according to [39]).

Figure 4.106 confirms that the P-factor does indeed bear a single relationship

with the amount of residual xylan in the wood at various temperatures. In the

range of high P-factors, the results from low and high temperatures become

slightly different. This deviation can be explained by the P-factor calculation which

considers solely the activation energy of the hydrolysis of the fast-reacting xylan.

The precision of the P-factor concept can be further improved by using the activation

energies separately for the initial and the residual hydrolysis phases. In this

344 4 Chemical Pulping Processes

0 1000 2000 3000 4000

140.C 155.C 170.C

Xylan in residue, g/kg od wood

P-Factor modified

Fig. 4.107 Mass of xylan residue as a function

of the modified P-factor for water prehydrolysis

of beech wood using a liquor-to-solid ratio of

10:1 (according to [39]). The modified P-factor

considers two different activation energies.

An activation energy of 125.6 kJ mol–1 is used

during hydrolysis of the fast-reacting xylan, and

of 135.7 kJ mol–1 during hydrolysis of the resistant

xylan (see also Tab. 4.44).

particular case, the amount of resistant xylan has been determined by extrapolation

of the hydrolysis rate of the slowly-reacting xylan to time zero. Figure 4.107

illustrates the relationship between the modified P-factor and the amount of

remaining xylan in the prehydrolyzed beech wood.

The results show an improvement of the relationship between P-factor and the

remaining xylan content after prehydrolysis at different temperature levels. However,

the rather insignificant improvements do not justify use of the more complex

modified P-factor concept.

It can be concluded that the P-factor concept is applicable to control the extent

of purification during the course of a prehydrolysis-kraft process for the production

of a dissolving pulp. Of course, it must be borne in mind that the removal of

hemicelluloses from wood is controlled by many additional parameters, and these

will be described in the next section.

4.2.7.2 Prehydrolysis: Kraft Pulping

The prehydrolysis step is followed by a simple kraft or Soda-anthraquinone (AQ)

process for the manufacture of a high-purity dissolving pulp. The conventional

approach comprises the introduction of cold white liquor from the causticizing

plant, heating-up and cooking until the desired degree of delignification is

reached, after which the digester’s contents are emptied to a blow tank by digester

4.2 Kraft Pulping Processes 345

pressure. This conventional concept has several serious disadvantages. First, water

prehydrolysis may adversely affect the process behavior due to the formation of

highly reactive intermediates undergoing condensation products. As a result,

pitch-like compounds are formed, which separate from the aqueous phase with

drainage of the prehydrolyzate, and deposit on any surface available. Moreover,

the deposition of these compounds on the chip surface may affect the diffusion-controlled

mass transfer. The prehydrolyzate must then to be evaporated and burned together

with the black liquor, which again impairs the economy of the process. Second,

the two heating-up phases, prior to prehydrolysis and cooking, require large

amounts of steam, and this leads to a significant prolongation of the cook.

In order to reduce the high energy costs incurred in evaporation of the prehydrolyzates,

attempts have been made to replace water by steam prehydrolysis.

However, this apparently simple change resulted in very poor delignification,

bleachability and reactivity of the dissolving pulps. To prevent extensive condensation

reactions occurring prior to the cooking stage, two measures must be undertaken.

First, a pressure release must to be avoided in the transition to the cooking

stage. Second, the reactive hydrolysis products must be immediately neutralized,

extracted and displaced prior to cooking. These requirements have been fulfilled

by applying the known displacement technology to this two-stage process. The

process which has been developed to overcome the described problems is known

as the Visbatch® process, and it combines the advantages of displacement technology

and steam prehydrolysis [20].

The prehydrolysis step is terminated by adding hot white liquor (HWL) and hot

black liquor (HBL) to the digester. In this way, the organic acids are neutralized

and the oligo- and monosaccharides (as well as their reactive intermediates) are

subjected to extensive fragmentation reactions and solubilized in the neutralization

liquor. Due to the high [OH– ]ion and the elevated temperature, delignification

begins parallel to extraction of the acidic constituents of the prehydrolyzate.

In the next step, neutralization liquor is displaced from bottom to top to ensure

that acid-volatile substances are completely expelled from the digester. During

this hot displacement step, the alkali requirement is supplied for subsequent

cooking by further addition of HWL and HBL.

In a further development of the Visbatch® technology, the displacement process

is continued until the predetermined H-factor is reached. This process, denoted

as the VisCBC process, is based on CBC technology where the cooking liquor (CL)

is prepared outside the digester (in the CL tank) by adjusting the preset alkali concentration

and temperature. Cooking is terminated by displacing the cooking

liquor by means of the washing filtrates. As soon as the temperature falls to less

than 100 °C, the unbleached pulp suspension is pumped into the blow tank. The

temperature, pressure and [OH– ]ion profiles throughout a typical prehydrolysiskraft

procedure according to the VisCBC technology is shown in Fig. 4.108. No

abrupt pressure or temperature drops are detected during transition from the prehydrolysis

to the cooking step; indeed, both pressure and temperature profiles

develop very smoothly. The same can be observed for the [OH– ]ion profiles. The

[OH– ]ion in the ingoing cooking liquor is kept constant at between 0.6 and

346 4 Chemical Pulping Processes

00:00 01:30 03:00 04:30

Pressure, bar / [OH-], mol/l

Temperature

Temperature,.C

Time, hh:mm

0.0

0.2

0.4

0.6

0.8

Pressure [OH-]

IN

[OH-]

OUT

Fig. 4.108 Temperature, pressure, and [OH– ]ion profiles during

the course of a hardwood VisCBC cooking process

(according to [52]). P-factor 700, H-factor 400; unbleached

pulp: kappa number 6.6, intrinsic viscosity 1015 mL g–1, R18

97.0%.

1.0 mol L–1, the objective being to avoid excessive cellulose degradation and an

uneven pulp quality. With continuous cooking, the difference in [OH– ]ion between

the ingoing and outgoing cooking liquors gradually diminishes.

The high [OH– ]ion concentration in the later stage of the cook ensures a high

delignification rate, and also prevents significant xylan reprecipitation.

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