Title: On the role of Xylan in Oxygen Delignification
Abstract: Using modern cooking concepts for the manufacture of hardwood kraft pulp, a higher pulp yield is obtained, mainly as a result of higher xylan retention. The effect of the xylan content in the pulp on the oxygen delignification is the topic of the present study. An industrial Eucalypt Urograndis kraft pulp was subjected to xylanase treatment prior to oxygen delignification in order to obtain similar pulp differing primarily in their content of xylan. The results showed that up to 30 % xylan was removed from the eucalyptus kraft pulp by xylanase but the removed xylan had no effect on the delignification in the oxygen delignification. The pulps with high xylan content showed a slightly improved selectivity in the oxygen delignification compared to pulps with low xylan content. Thus it seems that the xylan acts as a protector for the cellulose degradation. The xylanase treatment seemed to have a slightly preference to dissolve high-substituted xylan rather than xylan not attached to HexA. INTRODUCTION One efficient way to increase the overall yield of kraft pulp is to terminate the cook at a higher kappa number and continue with oxygen delignification and bleaching. The increased yield means that more carbohydrates have been retained in the pulp and in a hardwood kraft pulp it is mainly the xylan content that is higher. By terminating the cook at higher kappa number the pulp will also be less delignified and consist of a higher amount of lignin. The lignin should of course be removed in subsequent bleaching. Extending the oxygen delignification is a better alternative than extending the bleaching due to a lower cost and less environmental impact of the oxygen delignification. However, by oxygen delignification not all the lignin could be removed, at least not without a severe degradation of cellulose. The limits of oxygen delignification of hardwood kraft pulps have been explored in a number of studies, focusing on the structure of the residual lignin [1, 2], the importance of Lignin-Carbohydrate Complexes (LCC) [3], the importance of carbohydrates [4] and the importance of process parameters both in the cooking process [5] and in the oxygen delignification [6]. Recently, it has been shown by Nasman et al. [7] that the efficiency of oxygen delignification of a hardwood kraft pulp was markedly reduced as the cooking kappa number was increased. This effect was suggested to partly be a result of a higher yield of hemicellulose. The response to oxygen delignification of hardwood kraft pulp having a high hemicellulose content has also been study by Zou et al. [8]. It was then reported that pulps with a high xylan content were more difficult to delignify but showed a better selectivity compared to pulps with a low xylan content. In this study the effect of xylan content of a Eucalypt Urograndis kraft pulp on the oxygen delignification process was investigated. The xylan content was varied by means of enzymatic treatment using endoxylanase degrading parts of the xylan. Oxygen delignification trials were performed on the treated pulps. EXPERIMENTAL An industrially manufactured Eucalypt Urograndis kraft pulp (kappa number 18.1, limiting viscosity number 1400 dm3/kg) was obtained from Aracruz, Brazil. The pulp was treated with Pulpzyme HC, a 1,4endoxylanase from Novozymes. The treatment was performed at pH 7 (0.02 mol/dm phosphate buffer), 60 °C and at 4 % consistency for 2 h. Three different enzyme charges were investigated; 0.25 XU/g, 12.5 XU/g and 125 XU/g. A control without any xylanase added was also made. The xylanase treated pulps were oxygen delignified in teflon-coated stainless steel autoclaves, heated while rotating in a polyethylene glycol bath. The oxygen delignification trials were performed at 10 % consistency, at 105 °C with 0.5 MPa oxygen pressure for 90 min using an alkali charge of 20 kg sodium hydroxide/BDt [bone dry] pulp. Kappa number was determined according to ISO 302:2004 and the limiting viscosity number according to ISO 5351:2004. The carbohydrate content of the pulps was determined according to Tappi T249 cm-85, except that ion-exchange chromatography was used instead of gas chromatography for separation. The HexA content of the pulps was determined according to a method described by Gellerstedt and Li [9]. RESULTS AND DISCUSSION Xylan was removed from a Eucalypt Urograndis kraft pulp by enzymatic treatment using xylanase. The carbohydrate composition of the enzymatically treated pulps is shown in Table 1. Table 1: Carbohydrate composition in the pulps after xylanase treatment. After enzymatic treatment Xylanase Xyl Glu Man Gal XU/g Rel % Rel % Rel % Rel %
Publication Year: 2007
Publication Date: 2007-01-01
Language: en
Type: article
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Cited By Count: 1
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