Prehydrolysis

Prehydrolysis is generally used to remove selectively the hemicelluloses from the

biomass by hydrolysis in water at 160–180 °C [1], in dilute acid (0.3–0.5% H2SO4 at

120–140 °C) [2–4], or in concentrated acid (20–30% HCl at 40 °C) [3,5]. In water

prehydrolysis, acetyl groups are cleaved from the b-(1–4)-linked xylan backbone

and the acetic acid released acts as a catalyst for the hydrolysis of glycosidic bonds.

The resulting pH in the prehydrolyzate ranges between 3 and 4. The addition of a

mineral acid catalyst will, of course, greatly increase the rate of solubilization of

the xylan. Hydrolysis with dilute or concentrated aqueous mineral acids is mainly

applied for wood saccharification, which emphasizes the yield and quality of the

released sugars. In terms of xylose production, prehydrolysis with an aqueous solution

of sulfuric acid (0.4% at 170 °C) gave a higher yield of monomeric xylose as

4.2 Kraft Pulping Processes 325

compared to pure-water prehydrolysis [1]. The more intense degradation of xylose

in the water prehydrolysis is explained by the higher hydroxyl ion concentration at

a pH of 3.5 typical for water prehydrolysis. The hydroxyl ions are known to be a

very effective catalyst for sugar degradation. On the other hand, glycosidic linkages,

such as those in the xylan backbone, are rather stable to hydroxyl ions, but

are easily cleaved in the presence of hydrogen ions [6]. The rate of hydrolysis of

the glycosidic linkages decreases proportionally as solution acidity decreases,

whereas the rate of xylose degradation decreases to a lesser extent due to the hydroxyl

ion catalysis resulting in an overall greater xylose destruction at low acidities.

The addition of mineral acids to water prehydrolysis prior to kraft pulping

impairs – in the case of sulfuric acid catalysis (hydrochloric acid catalysis is not appropriate

due to uncontrollable corrosion problems) – the sulfur-to-sodiumratio and renders

the lignin fraction more soluble and reactive for undesirable condensation

reactions, thus making the subsequent alkaline delignification more difficult.

As autohydrolysis generally is unsatisfactory for softwoods, more severe conditions

must be applied in order to obtain the target purity level. Softwoods contain

a higher amount of lignin which also has a greater tendency to acid condensations.

In some cases, mineral acid catalysis may be appropriate for softwood prehydrolyis

kraft cooking to achieve the optimum with regard to economic and pulp

quality requirements.

To date, the prehydrolysis step in connection with kraft pulping is solely carried

out without the addition of mineral acid. The use of water prehydrolysis with a

typical liquor-to-wood ratio of 3–4:1, however, produces enormous amounts of prehydrolyzates

(8–12 t adt–1 pulp) containing large quantities of xylose and its oligomers.

Over the years, much effort has been applied with regard to the recovery

and utilization of dissolved substances. Initially, it was proposed to isolate pure

xylose by crystallization from mild acid hydrolysis of beechwood hemicelluloses,

using a multi-stage procedure [7–9], or to recover a pure syrup which can be used

as fodder or for other purposes in the food industry [10]. The technology to recover

xylose and its oligomers from the water prehydrolyzate remains a challenge for

optimization. With progressive prehydrolysis, highly reactive secondary products

are created as soon as the pressure is released to drain the prehydrolyzate. These

hydrolysis products show a high tendency for precipitation and the formation of

resinous agglutinations that are very difficult to control.

Both high yield of the released carbohydrate compounds and the production of

high-value products are prerequisites for an economically feasible process. Hydrolysis

products from wood are considered to be a source for chemicals, fodder, food

additives, and even pharmaceutical products. To date, a wide range of research

investigations are being undertaken to develop new products based on xylose and

its oligomers, with the main focus on food additives [11–19]. However, at present

there is no commercial utilization of the water prehydrolyzate during the course

of a prehydrolysis-kraft operation for dissolving pulp manufacture. The running

of a water prehydrolysis step, without utilizing the degraded wood by-products, is

economically not feasible because recovery of the large amounts of prehydrolyzate

requires additional equipment and evaporation capacity.

326 4 Chemical Pulping Processes

The breakthrough for cheap but low-efficiency vapor-phase prehydrolysis came

with the application of displacement technology to the prehydrolysis kraft process.

With the development of the Visbatch® process, the advantages of both steam

hydrolysis and displacement technology were combined [20]. The efficiency of

steam prehydrolysis can be significantly increased by subsequently charging a

mixture of white and black liquor to the digester to further degrade and extract

the acidic reaction products formed during prehydrolysis. The neutralization

liquor, which contains the degraded hemicelluloses, is finally displaced by the

cooking liquor to limit the effective alkali (EA) consumption and to improve the

purification efficiency. This technology is presented in detail in Section 4.2.7.2,

Prehydrolysis-Kraft cooking.

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