Scheme 4.30

Figure 4.99 shows the course of the xylose concentration in the solid residue of

water prehydrolysis of beech wood at 170 °C in comparison to the concentration

profiles of xylose and furfural in solution. Xylose is unstable under acidic conditions,

and dehydrates to furfural. In the early stages of prehydrolysis, the concentration

of furfural is low since the furfural formation rate depends on the furfural

concentration. Thus, with rising reaction intensity, particularly the furfural concentration

increases. Furfural production arises from a very complex group of

reactions associated with the decomposition of xylose. There are at least three

intermediates, all of which probably react with furfural at different rates. The

mechanistic scheme of furfural production and decomposition was successfully

established by Root [31].

As seen from Tab. 4.42, beechwood contains also minor amounts of glucopyranoses

(galactose and mannose) and arabinose. The development of the removal of

these sugar compounds in relation to that of xylose and glucose originating from

both glucomannan and cellulose is illustrated in Fig. 4.100.

The removal rate is largely in accordance with the conformational structure of

the carbohydrates. The arabinose structure (furanoside) decomposes instantaneously,

followed at some distance by galactose, xylose, and mannose. The

removal rate of galactose is more rapid than would have been expected from its

structural conformation. The resemblance of their degradation rates suggests the

presence of polysaccharides being composed of galactose and arabinose units.

The release of glucose units corresponds to the degradation of mannose in molar

terms which may originate from glucomannan with a glucose-to-mannose ratio

close to 1:1. With prolonged reaction time at 170 °C (>120 min), the amount of

glucose removed from the wood however exceeds this stoichiometric ratio, indicating

an additional loss from amorphous cellulose.

The monomeric and oligomeric sugars in aqueous solution are immediately

involved in secondary reactions. Removal from the wood residue to the aqueous

4.2 Kraft Pulping Processes 335

0 50 100 150 200

Total Xylose in aqueous phase Furfural as Xylose

Total Xylose removed

Xylose, g/kg od wood

Time at 170.C, min

Fig. 4.99 Course of xylose in the solid residue, in solution as

xylose (monomeric and oligomeric) and as furfural as a function

of reaction time during water prehydrolysis of beechwood

at 170 °C (according to [39]). Liquor-to-solid ratio = 10:1.

0 50 100 150 200

Glucose Xylose Arabinose Mannose Galactose

Component remaining

in the solid residue, %

Time at 170.C, min

Fig. 4.100 Course of the main carbohydrate units of beechwood

in the solid residue as a function of reaction time during

water prehydrolysis of beechwood at 170 °C (according to

[39]). Liquor-to-solid ratio = 10:1. The initial values of the single

carbohydrates are listed in Fig. 4.42.

336 4 Chemical Pulping Processes

solution and subsequent transformation to secondary products (condensation

products) can be described in the simplest form as a consecutive mechanism

according to Eq. (148):

A __

k 1 B __

k 2 C _148_

The extent of the secondary reactions of arabinose, mannose, galactose and glucose

in aqueous solution is demonstrated in Fig. 4.101 by comparing the difference

between the amount removed from the solid residue and the amount

detected in solution.

From Fig. 4.101 and Tab. 4.45 it can be seen that mannose and glucose can be

recovered in high yield from solution, whereas arabinose and galactose are heavily

involved in secondary reactions according to Scheme 4.30 and Eq. (148) after

1 hour retention time at 170 °C.

0 50 100 150 200

0 50 100 150 200

0 50 100 150 200

0 50 100 150 200

Ara

dissolved

Ara

removed

Arabinose, g/kg wood

Man

dissolved

Man

removed

Mannose, g/kg wood

Glu

dissolved

Glu

removed

Glucose, g/kg wood

Gala

dissolved

Gala

removed

Time at 170.C, min

Time at 170.C, min

Time at 170.C, min

Galactose, g/kg wood

Time at 170.C, min

Fig. 4.101 Course of arabinose, mannose, glucose, and galactose

in the solid residue and in solution as a function of reaction

time during water prehydrolysis of beech wood at 170 °C

(according to [39]). Liquor-to-solid ratio = 10:1.

4.2 Kraft Pulping Processes 337

338 4 Chemical Pulping Processes

Tab. 4.45 Course of carbohydrate removal in the solid residue of beech wood and in aqueous solution during water prehydrolysis

at 170 °C at a liquor-to-solid ratio of 10:1 (according to [39]). Values include the total amount of carbohydrates denoted as

monomers.

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